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Bykhovskaia M. Dynamic Formation of the Protein-Lipid Pre-fusion Complex. BIORXIV : THE PREPRINT SERVER FOR BIOLOGY 2024:2024.04.17.589983. [PMID: 38659925 PMCID: PMC11042276 DOI: 10.1101/2024.04.17.589983] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 04/26/2024]
Abstract
Synaptic vesicles (SVs) fuse with the presynaptic membrane (PM) to release neuronal transmitters. The SV protein Synaptotagmin 1 (Syt1) serves as a Ca 2+ sensor for evoked fusion. Syt1 is thought to trigger fusion by penetrating into PM upon Ca 2+ binding, however the mechanistic detail of this process is still debated. Syt1 interacts with the SNARE complex, a coiled-coil four-helical bundle that enables the SV-PM attachment. The SNARE-associated protein Complexin (Cpx) promotes the Ca 2+ -dependent fusion, possibly interacting with Syt1. We employed all-atom molecular dynamics (MD) to investigate the formation of the Syt1-SNARE-Cpx complex interacting with the lipid bilayers of PM and SV. Our simulations demonstrated that the PM-Syt1-SNARE-Cpx complex can transition to a "dead-end" state, wherein Syt1 attaches tightly to PM but does not immerse into it, as opposed to a pre-fusion state, which has the tips of the Ca 2+ -bound C2 domains of Syt1 inserted into PM. Our simulations unraveled the sequence of Syt1 conformational transitions, including the simultaneous Syt1 docking to the SNARE-Cpx bundle and PM, followed by the Ca 2+ chelation and the penetration of the tips of Syt1 domains into PM, leading to the pre-fusion state of the protein-lipid complex. Importantly, we found that the direct Syt1-Cpx interactions are required to promote these transitions. Thus, we developed the all-atom dynamic model of the conformational transitions that lead to the formation of the pre-fusion PM-Syt1-SNARE-Cpx complex. Our simulations also revealed an alternative "dead-end" state of the protein-lipid complex that can be formed if this pathway is disrupted. Statement of Significance Neurons communicate by releasing transmitter molecules. Transmitters are packed in synaptic vesicles (SVs) and released by the fusion of SVs with the presynaptic membrane (PM). This process is regulated by a dynamic complex of fusion proteins, including the coil-coiled SNARE bundle that attaches SV to PM, Synaptotagmin that serves as a Ca 2+ sensor for the release process, and Complexin that attaches to the SNARE bundle and promotes the Ca 2+ -dependent release. To understand how these proteins interact dynamically with the lipid bilayers of SV and PM, we employed molecular dynamics, a computational approach that enables simulating the behavior of proteins and lipids at the atomistic resolution. Our simulations enabled us to delineate the stages of the formation of prefusion protein-lipid complex.
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Lawrence SS, Kirschbaum C, Bennett JL, Lutomski CA, El-Baba TJ, Robinson CV. Phospholipids Differentially Regulate Ca 2+ Binding to Synaptotagmin-1. ACS Chem Biol 2024; 19:953-961. [PMID: 38566504 PMCID: PMC11040605 DOI: 10.1021/acschembio.3c00772] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/14/2023] [Revised: 03/20/2024] [Accepted: 03/21/2024] [Indexed: 04/04/2024]
Abstract
Synaptotagmin-1 (Syt-1) is a calcium sensing protein that is resident in synaptic vesicles. It is well established that Syt-1 is essential for fast and synchronous neurotransmitter release. However, the role of Ca2+ and phospholipid binding in the function of Syt-1, and ultimately in neurotransmitter release, is unclear. Here, we investigate the binding of Ca2+ to Syt-1, first in the absence of lipids, using native mass spectrometry to evaluate individual binding affinities. Syt-1 binds to one Ca2+ with a KD ∼ 45 μM. Each subsequent binding affinity (n ≥ 2) is successively unfavorable. Given that Syt-1 has been reported to bind anionic phospholipids to modulate the Ca2+ binding affinity, we explored the extent that Ca2+ binding was mediated by selected anionic phospholipid binding. We found that phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) and dioleoylphosphatidylserine (DOPS) positively modulated Ca2+ binding. However, the extent of Syt-1 binding to phosphatidylinositol 3,5-bisphosphate (PI(3,5)P2) was reduced with increasing [Ca2+]. Overall, we find that specific lipids differentially modulate Ca2+ binding. Given that these lipids are enriched in different subcellular compartments and therefore may interact with Syt-1 at different stages of the synaptic vesicle cycle, we propose a regulatory mechanism involving Syt-1, Ca2+, and anionic phospholipids that may also control some aspects of vesicular exocytosis.
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Affiliation(s)
- Sophie
A. S. Lawrence
- Department
of Chemistry, University of Oxford, South Parks Road, Oxford OX1 3QZ, U.K.
- The
Kavli Institute for Nanoscience Discovery, University of Oxford, South Parks Road, Oxford OX1 3QU, U.K.
| | - Carla Kirschbaum
- Department
of Chemistry, University of Oxford, South Parks Road, Oxford OX1 3QZ, U.K.
- The
Kavli Institute for Nanoscience Discovery, University of Oxford, South Parks Road, Oxford OX1 3QU, U.K.
| | - Jack L. Bennett
- Department
of Chemistry, University of Oxford, South Parks Road, Oxford OX1 3QZ, U.K.
- The
Kavli Institute for Nanoscience Discovery, University of Oxford, South Parks Road, Oxford OX1 3QU, U.K.
| | - Corinne A. Lutomski
- Department
of Chemistry, University of Oxford, South Parks Road, Oxford OX1 3QZ, U.K.
- The
Kavli Institute for Nanoscience Discovery, University of Oxford, South Parks Road, Oxford OX1 3QU, U.K.
| | - Tarick J. El-Baba
- Department
of Chemistry, University of Oxford, South Parks Road, Oxford OX1 3QZ, U.K.
- The
Kavli Institute for Nanoscience Discovery, University of Oxford, South Parks Road, Oxford OX1 3QU, U.K.
| | - Carol. V. Robinson
- Department
of Chemistry, University of Oxford, South Parks Road, Oxford OX1 3QZ, U.K.
- The
Kavli Institute for Nanoscience Discovery, University of Oxford, South Parks Road, Oxford OX1 3QU, U.K.
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Mehta N, Mondal S, Watson ET, Cui Q, Chapman ER. The juxtamembrane linker of synaptotagmin 1 regulates Ca 2+ binding via liquid-liquid phase separation. Nat Commun 2024; 15:262. [PMID: 38177243 PMCID: PMC10766989 DOI: 10.1038/s41467-023-44414-5] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/11/2023] [Accepted: 12/12/2023] [Indexed: 01/06/2024] Open
Abstract
Synaptotagmin (syt) 1, a Ca2+ sensor for synaptic vesicle exocytosis, functions in vivo as a multimer. Syt1 senses Ca2+ via tandem C2-domains that are connected to a single transmembrane domain via a juxtamembrane linker. Here, we show that this linker segment harbors a lysine-rich, intrinsically disordered region that is necessary and sufficient to mediate liquid-liquid phase separation (LLPS). Interestingly, condensate formation negatively regulates the Ca2+-sensitivity of syt1. Moreover, Ca2+ and anionic phospholipids facilitate the observed phase separation, and increases in [Ca2+]i promote the fusion of syt1 droplets in living cells. Together, these observations suggest a condensate-mediated feedback loop that serves to fine-tune the ability of syt1 to trigger release, via alterations in Ca2+ binding activity and potentially through the impact of LLPS on membrane curvature during fusion reactions. In summary, the juxtamembrane linker of syt1 emerges as a regulator of syt1 function by driving self-association via LLPS.
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Affiliation(s)
- Nikunj Mehta
- Howard Hughes Medical Institute, Department of Neuroscience, University of Wisconsin-Madison, Madison, WI, 53705, USA
| | - Sayantan Mondal
- Department of Chemistry, Boston University, Boston, MA, 02215, USA
| | - Emma T Watson
- Howard Hughes Medical Institute, Department of Neuroscience, University of Wisconsin-Madison, Madison, WI, 53705, USA
| | - Qiang Cui
- Department of Chemistry, Boston University, Boston, MA, 02215, USA
| | - Edwin R Chapman
- Howard Hughes Medical Institute, Department of Neuroscience, University of Wisconsin-Madison, Madison, WI, 53705, USA.
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4
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Mehta N, Mondal S, Watson ET, Cui Q, Chapman ER. The juxtamembrane linker of synaptotagmin 1 regulates Ca 2+ binding via liquid-liquid phase separation. BIORXIV : THE PREPRINT SERVER FOR BIOLOGY 2023:2023.08.11.551903. [PMID: 37609296 PMCID: PMC10441399 DOI: 10.1101/2023.08.11.551903] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 08/24/2023]
Abstract
Synaptotagmin (syt) 1, a Ca2+ sensor for synaptic vesicle exocytosis, functions in vivo as a multimer. Syt1 senses Ca2+ via tandem C2-domains that are connected to a single transmembrane domain via a juxtamembrane linker. Here, we show that this linker segment harbors a lysine-rich, intrinsically disordered region that is necessary and sufficient to mediate liquid-liquid phase separation (LLPS). Interestingly, condensate formation negatively regulates the Ca2+-sensitivity of syt1. Moreover, Ca2+ and anionic phospholipids facilitate the observed phase separation, and increases in [Ca2+]i promote the fusion of syt1 droplets in living cells. Together, these observations suggest a condensate-mediated feedback loop that serves to fine-tune the ability of syt1 to trigger release, via alterations in Ca2+ binding activity and potentially through the impact of LLPS on membrane curvature during fusion reactions. In summary, the juxtamembrane linker of syt1 emerges as a regulator of syt1 function by driving self-association via LLPS.
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Affiliation(s)
- Nikunj Mehta
- Howard Hughes Medical Institute, Department of Neuroscience, University of Wisconsin–Madison, Madison, WI 53705, United States
| | - Sayantan Mondal
- Department of Chemistry, Boston University, Boston, MA 02215, United States
| | - Emma T. Watson
- Howard Hughes Medical Institute, Department of Neuroscience, University of Wisconsin–Madison, Madison, WI 53705, United States
| | - Qiang Cui
- Department of Chemistry, Boston University, Boston, MA 02215, United States
| | - Edwin R. Chapman
- Howard Hughes Medical Institute, Department of Neuroscience, University of Wisconsin–Madison, Madison, WI 53705, United States
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5
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Bykhovskaia M. Molecular Dynamics Simulations of the Proteins Regulating Synaptic Vesicle Fusion. MEMBRANES 2023; 13:307. [PMID: 36984694 PMCID: PMC10058449 DOI: 10.3390/membranes13030307] [Citation(s) in RCA: 1] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 01/23/2023] [Revised: 02/11/2023] [Accepted: 03/01/2023] [Indexed: 06/18/2023]
Abstract
Neuronal transmitters are packaged in synaptic vesicles (SVs) and released by the fusion of SVs with the presynaptic membrane (PM). An inflow of Ca2+ into the nerve terminal triggers fusion, and the SV-associated protein Synaptotagmin 1 (Syt1) serves as a Ca2+ sensor. In preparation for fusion, SVs become attached to the PM by the SNARE protein complex, a coiled-coil bundle that exerts the force overcoming SV-PM repulsion. A cytosolic protein Complexin (Cpx) attaches to the SNARE complex and differentially regulates the evoked and spontaneous release components. It is still debated how the dynamic interactions of Syt1, SNARE proteins and Cpx lead to fusion. This problem is confounded by heterogeneity in the conformational states of the prefusion protein-lipid complex and by the lack of tools to experimentally monitor the rapid conformational transitions of the complex, which occur at a sub-millisecond scale. However, these complications can be overcome employing molecular dynamics (MDs), a computational approach that enables simulating interactions and conformational transitions of proteins and lipids. This review discusses the use of molecular dynamics for the investigation of the pre-fusion protein-lipid complex. We discuss the dynamics of the SNARE complex between lipid bilayers, as well as the interactions of Syt1 with lipids and SNARE proteins, and Cpx regulating the assembly of the SNARE complex.
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Affiliation(s)
- Maria Bykhovskaia
- Neurology Department, Wayne State University, Detroit, MI 48202, USA
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6
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Role of calcium-sensor proteins in cell membrane repair. Biosci Rep 2023; 43:232522. [PMID: 36728029 PMCID: PMC9970828 DOI: 10.1042/bsr20220765] [Citation(s) in RCA: 2] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/09/2022] [Revised: 01/20/2023] [Accepted: 02/01/2023] [Indexed: 02/03/2023] Open
Abstract
Cell membrane repair is a critical process used to maintain cell integrity and survival from potentially lethal chemical, and mechanical membrane injury. Rapid increases in local calcium levels due to a membrane rupture have been widely accepted as a trigger for multiple membrane-resealing models that utilize exocytosis, endocytosis, patching, and shedding mechanisms. Calcium-sensor proteins, such as synaptotagmins (Syt), dysferlin, S100 proteins, and annexins, have all been identified to regulate, or participate in, multiple modes of membrane repair. Dysfunction of membrane repair from inefficiencies or genetic alterations in these proteins contributes to diseases such as muscular dystrophy (MD) and heart disease. The present review covers the role of some of the key calcium-sensor proteins and their involvement in membrane repair.
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7
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Jaczynska K, Esquivies L, Pfuetzner RA, Alten B, Brewer KD, Zhou Q, Kavalali ET, Brunger AT, Rizo J. Analysis of tripartite Synaptotagmin-1-SNARE-complexin-1 complexes in solution. FEBS Open Bio 2023; 13:26-50. [PMID: 36305864 PMCID: PMC9811660 DOI: 10.1002/2211-5463.13503] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/01/2022] [Revised: 10/19/2022] [Accepted: 10/27/2022] [Indexed: 01/07/2023] Open
Abstract
Characterizing interactions of Synaptotagmin-1 with the SNARE complex is crucial to understand the mechanism of neurotransmitter release. X-ray crystallography revealed how the Synaptotagmin-1 C2 B domain binds to the SNARE complex through a so-called primary interface and to a complexin-1-SNARE complex through a so-called tripartite interface. Mutagenesis and electrophysiology supported the functional relevance of both interfaces, and extensive additional data validated the primary interface. However, ITC evidence suggesting that binding via the tripartite interface occurs in solution was called into question by subsequent NMR data. Here, we describe joint efforts to address this apparent contradiction. Using the same ITC approach with the same C2 B domain mutant used previously (C2 BKA-Q ) but including ion exchange chromatography to purify it, which is crucial to remove polyacidic contaminants, we were unable to observe the substantial endothermic ITC signal that was previously attributed to binding of this mutant to the complexin-1-SNARE complex through the tripartite interface. We were also unable to detect substantial populations of the tripartite interface in NMR analyses of the ITC samples or in measurements of paramagnetic relaxation effects, despite the high sensitivity of this method to detect weak protein complexes. However, these experiments do not rule out the possibility of very low affinity (KD > 1 mm) binding through this interface. These results emphasize the need to develop methods to characterize the structure of synaptotagmin-1-SNARE complexes between two membranes and to perform further structure-function analyses to establish the physiological relevance of the tripartite interface.
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Affiliation(s)
- Klaudia Jaczynska
- Department of BiophysicsUniversity of Texas Southwestern Medical CenterDallasTXUSA
- Department of BiochemistryUniversity of Texas Southwestern Medical CenterDallasTXUSA
- Department of PharmacologyUniversity of Texas Southwestern Medical CenterDallasTXUSA
| | - Luis Esquivies
- Department of Molecular and Cellular PhysiologyStanford UniversityCAUSA
- Department of Neurology and Neurological SciencesStanford UniversityCAUSA
- Department of Structural BiologyStanford UniversityCAUSA
- Department of Photon ScienceStanford UniversityCAUSA
- Howard Hughes Medical InstituteStanford UniversityCAUSA
| | - Richard A. Pfuetzner
- Department of Molecular and Cellular PhysiologyStanford UniversityCAUSA
- Department of Neurology and Neurological SciencesStanford UniversityCAUSA
- Department of Structural BiologyStanford UniversityCAUSA
- Department of Photon ScienceStanford UniversityCAUSA
- Howard Hughes Medical InstituteStanford UniversityCAUSA
| | - Baris Alten
- Department of PharmacologyVanderbilt UniversityNashvilleTNUSA
- Vanderbilt Brain InstituteVanderbilt UniversityNashvilleTNUSA
- Present address:
Department of NeurologyMassachusetts General HospitalBostonMAUSA
- Present address:
Department of NeurologyBrigham and Women's HospitalBostonMAUSA
- Present address:
Harvard Medical SchoolBostonMAUSA
| | - Kyle D. Brewer
- Department of BiophysicsUniversity of Texas Southwestern Medical CenterDallasTXUSA
- Department of BiochemistryUniversity of Texas Southwestern Medical CenterDallasTXUSA
- Department of PharmacologyUniversity of Texas Southwestern Medical CenterDallasTXUSA
- Present address:
ETTA BiotechnologyPalo AltoCAUSA
| | - Qiangjun Zhou
- Vanderbilt Brain InstituteVanderbilt UniversityNashvilleTNUSA
- Department of Cell and Developmental BiologyVanderbilt UniversityNashvilleTNUSA
| | - Ege T. Kavalali
- Department of PharmacologyVanderbilt UniversityNashvilleTNUSA
- Vanderbilt Brain InstituteVanderbilt UniversityNashvilleTNUSA
| | - Axel T. Brunger
- Department of Molecular and Cellular PhysiologyStanford UniversityCAUSA
- Department of Neurology and Neurological SciencesStanford UniversityCAUSA
- Department of Structural BiologyStanford UniversityCAUSA
- Department of Photon ScienceStanford UniversityCAUSA
- Howard Hughes Medical InstituteStanford UniversityCAUSA
| | - Josep Rizo
- Department of BiophysicsUniversity of Texas Southwestern Medical CenterDallasTXUSA
- Department of BiochemistryUniversity of Texas Southwestern Medical CenterDallasTXUSA
- Department of PharmacologyUniversity of Texas Southwestern Medical CenterDallasTXUSA
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Zhou Q. Calcium Sensors of Neurotransmitter Release. ADVANCES IN NEUROBIOLOGY 2023; 33:119-138. [PMID: 37615865 DOI: 10.1007/978-3-031-34229-5_5] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 08/25/2023]
Abstract
Calcium (Ca2+) plays a critical role in triggering all three primary modes of neurotransmitter release (synchronous, asynchronous, and spontaneous). Synaptotagmin1, a protein with two C2 domains, is the first isoform of the synaptotagmin family that was identified and demonstrated as the primary Ca2+ sensor for synchronous neurotransmitter release. Other isoforms of the synaptotagmin family as well as other C2 proteins such as the double C2 domain protein family were found to act as Ca2+ sensors for different modes of neurotransmitter release. Major recent advances and previous data suggest a new model, release-of-inhibition, for the initiation of Ca2+-triggered synchronous neurotransmitter release. Synaptotagmin1 binds Ca2+ via its two C2 domains and relieves a primed pre-fusion machinery. Before Ca2+ triggering, synaptotagmin1 interacts Ca2+ independently with partially zippered SNARE complexes, the plasma membrane, phospholipids, and other components to form a primed pre-fusion state that is ready for fast release. However, membrane fusion is inhibited until the arrival of Ca2+ reorients the Ca2+-binding loops of the C2 domain to perturb the lipid bilayers, help bridge the membranes, and/or induce membrane curvatures, which serves as a power stroke to activate fusion. This chapter reviews the evidence supporting these models and discusses the molecular interactions that may underlie these abilities.
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Affiliation(s)
- Qiangjun Zhou
- Department of Cell and Developmental Biology, Vanderbilt Brain Institute, Vanderbilt University, Nashville, TN, USA.
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9
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Ali Moussa HY, Park Y. Electrostatic regulation of the cis- and trans-membrane interactions of synaptotagmin-1. Sci Rep 2022; 12:22407. [PMID: 36575295 PMCID: PMC9794720 DOI: 10.1038/s41598-022-26723-9] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/15/2022] [Accepted: 12/19/2022] [Indexed: 12/28/2022] Open
Abstract
Synaptotagmin-1 is a vesicular protein and Ca2+ sensor for Ca2+-dependent exocytosis. Ca2+ induces synaptotagmin-1 binding to its own vesicle membrane, called the cis-interaction, thus preventing the trans-interaction of synaptotagmin-1 to the plasma membrane. However, the electrostatic regulation of the cis- and trans-membrane interaction of synaptotagmin-1 was poorly understood in different Ca2+-buffering conditions. Here we provide an assay to monitor the cis- and trans-membrane interactions of synaptotagmin-1 by using native purified vesicles and the plasma membrane-mimicking liposomes (PM-liposomes). Both ATP and EGTA similarly reverse the cis-membrane interaction of synaptotagmin-1 in free [Ca2+] of 10-100 μM. High PIP2 concentrations in the PM-liposomes reduce the Hill coefficient of vesicle fusion and synaptotagmin-1 membrane binding; this observation suggests that local PIP2 concentrations control the Ca2+-cooperativity of synaptotagmin-1. Our data provide evidence that Ca2+ chelators, including EGTA and polyphosphate anions such as ATP, ADP, and AMP, electrostatically reverse the cis-interaction of synaptotagmin-1.
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Affiliation(s)
- Houda Yasmine Ali Moussa
- grid.418818.c0000 0001 0516 2170Neurological Disorders Research Center, Qatar Biomedical Research Institute (QBRI), Hamad Bin Khalifa University (HBKU), Qatar Foundation, Doha, Qatar
| | - Yongsoo Park
- grid.418818.c0000 0001 0516 2170Neurological Disorders Research Center, Qatar Biomedical Research Institute (QBRI), Hamad Bin Khalifa University (HBKU), Qatar Foundation, Doha, Qatar ,grid.418818.c0000 0001 0516 2170College of Health and Life Sciences (CHLS), Hamad Bin Khalifa University (HBKU), Qatar Foundation, Doha, Qatar
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Cui L, Li H, Xi Y, Hu Q, Liu H, Fan J, Xiang Y, Zhang X, Shui W, Lai Y. Vesicle trafficking and vesicle fusion: mechanisms, biological functions, and their implications for potential disease therapy. MOLECULAR BIOMEDICINE 2022; 3:29. [PMID: 36129576 PMCID: PMC9492833 DOI: 10.1186/s43556-022-00090-3] [Citation(s) in RCA: 11] [Impact Index Per Article: 5.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/25/2022] [Accepted: 07/12/2022] [Indexed: 11/10/2022] Open
Abstract
Intracellular vesicle trafficking is the fundamental process to maintain the homeostasis of membrane-enclosed organelles in eukaryotic cells. These organelles transport cargo from the donor membrane to the target membrane through the cargo containing vesicles. Vesicle trafficking pathway includes vesicle formation from the donor membrane, vesicle transport, and vesicle fusion with the target membrane. Coat protein mediated vesicle formation is a delicate membrane budding process for cargo molecules selection and package into vesicle carriers. Vesicle transport is a dynamic and specific process for the cargo containing vesicles translocation from the donor membrane to the target membrane. This process requires a group of conserved proteins such as Rab GTPases, motor adaptors, and motor proteins to ensure vesicle transport along cytoskeletal track. Soluble N-ethyl-maleimide-sensitive factor (NSF) attachment protein receptors (SNARE)-mediated vesicle fusion is the final process for vesicle unloading the cargo molecules at the target membrane. To ensure vesicle fusion occurring at a defined position and time pattern in eukaryotic cell, multiple fusogenic proteins, such as synaptotagmin (Syt), complexin (Cpx), Munc13, Munc18 and other tethering factors, cooperate together to precisely regulate the process of vesicle fusion. Dysfunctions of the fusogenic proteins in SNARE-mediated vesicle fusion are closely related to many diseases. Recent studies have suggested that stimulated membrane fusion can be manipulated pharmacologically via disruption the interface between the SNARE complex and Ca2+ sensor protein. Here, we summarize recent insights into the molecular mechanisms of vesicle trafficking, and implications for the development of new therapeutics based on the manipulation of vesicle fusion.
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Rizo J, David G, Fealey ME, Jaczynska K. On the difficulties of characterizing weak protein interactions that are critical for neurotransmitter release. FEBS Open Bio 2022; 12:1912-1938. [PMID: 35986639 PMCID: PMC9623538 DOI: 10.1002/2211-5463.13473] [Citation(s) in RCA: 4] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/08/2022] [Revised: 08/09/2022] [Accepted: 08/18/2022] [Indexed: 01/25/2023] Open
Abstract
The mechanism of neurotransmitter release has been extensively characterized, showing that vesicle fusion is mediated by the SNARE complex formed by syntaxin-1, SNAP-25 and synaptobrevin. This complex is disassembled by N-ethylmaleimide sensitive factor (NSF) and SNAPs to recycle the SNAREs, whereas Munc18-1 and Munc13s organize SNARE complex assembly in an NSF-SNAP-resistant manner. Synaptotagmin-1 acts as the Ca2+ sensor that triggers exocytosis in a tight interplay with the SNAREs and complexins. Here, we review technical aspects associated with investigation of protein interactions underlying these steps, which is hindered because the release machinery is assembled between two membranes and is highly dynamic. Moreover, weak interactions, which are difficult to characterize, play key roles in neurotransmitter release, for instance by lowering energy barriers that need to be overcome in this highly regulated process. We illustrate the crucial role that structural biology has played in uncovering mechanisms underlying neurotransmitter release, but also discuss the importance of considering the limitations of the techniques used, including lessons learned from research in our lab and others. In particular, we emphasize: (a) the promiscuity of some protein sequences, including membrane-binding regions that can mediate irrelevant interactions with proteins in the absence of their native targets; (b) the need to ensure that weak interactions observed in crystal structures are biologically relevant; and (c) the limitations of isothermal titration calorimetry to analyze weak interactions. Finally, we stress that even studies that required re-interpretation often helped to move the field forward by improving our understanding of the system and providing testable hypotheses.
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Affiliation(s)
- Josep Rizo
- Department of BiophysicsUniversity of Texas Southwestern Medical CenterDallasTXUSA,Department of BiochemistryUniversity of Texas Southwestern Medical CenterDallasTXUSA,Department of PharmacologyUniversity of Texas Southwestern Medical CenterDallasTXUSA
| | - Guillaume David
- Department of BiophysicsUniversity of Texas Southwestern Medical CenterDallasTXUSA,Department of BiochemistryUniversity of Texas Southwestern Medical CenterDallasTXUSA,Department of PharmacologyUniversity of Texas Southwestern Medical CenterDallasTXUSA
| | - Michael E. Fealey
- Department of BiophysicsUniversity of Texas Southwestern Medical CenterDallasTXUSA,Department of BiochemistryUniversity of Texas Southwestern Medical CenterDallasTXUSA,Department of PharmacologyUniversity of Texas Southwestern Medical CenterDallasTXUSA
| | - Klaudia Jaczynska
- Department of BiophysicsUniversity of Texas Southwestern Medical CenterDallasTXUSA,Department of BiochemistryUniversity of Texas Southwestern Medical CenterDallasTXUSA,Department of PharmacologyUniversity of Texas Southwestern Medical CenterDallasTXUSA
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12
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Dominguez MJ, McCord JJ, Sutton RB. Redefining the architecture of ferlin proteins: Insights into multi-domain protein structure and function. PLoS One 2022; 17:e0270188. [PMID: 35901179 PMCID: PMC9333456 DOI: 10.1371/journal.pone.0270188] [Citation(s) in RCA: 7] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/31/2022] [Accepted: 06/07/2022] [Indexed: 11/18/2022] Open
Abstract
Ferlins are complex, multi-domain proteins, involved in membrane trafficking, membrane repair, and exocytosis. The large size of ferlin proteins and the lack of consensus regarding domain boundaries have slowed progress in understanding molecular-level details of ferlin protein structure and function. However, in silico protein folding techniques have significantly enhanced our understanding of the complex ferlin family domain structure. We used RoseTTAFold to assemble full-length models for the six human ferlin proteins (dysferlin, myoferlin, otoferlin, Fer1L4, Fer1L5, and Fer1L6). Our full-length ferlin models were used to obtain objective domain boundaries, and these boundaries were supported by AlphaFold2 predictions. Despite the differences in amino acid sequence between the ferlin proteins, the domain ranges and distinct subdomains in the ferlin domains are remarkably consistent. Further, the RoseTTAFold/AlphaFold2 in silico boundary predictions allowed us to describe and characterize a previously unknown C2 domain, ubiquitous in all human ferlins, which we refer to as C2-FerA. At present, the ferlin domain-domain interactions implied by the full-length in silico models are predicted to have a low accuracy; however, the use of RoseTTAFold and AlphaFold2 as a domain finder has proven to be a powerful research tool for understanding ferlin structure.
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Affiliation(s)
- Matthew J. Dominguez
- Department of Cell Physiology and Molecular Biophysics, Texas Tech University Health Sciences Center, Lubbock, TX, United States of America
| | - Jon J. McCord
- Department of Cell Physiology and Molecular Biophysics, Texas Tech University Health Sciences Center, Lubbock, TX, United States of America
| | - R. Bryan Sutton
- Department of Cell Physiology and Molecular Biophysics, Texas Tech University Health Sciences Center, Lubbock, TX, United States of America
- Center for Membrane Protein Research, Texas Tech University Health Sciences Center, Lubbock, TX, United States of America
- * E-mail:
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13
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Synaptic Secretion and Beyond: Targeting Synapse and Neurotransmitters to Treat Neurodegenerative Diseases. OXIDATIVE MEDICINE AND CELLULAR LONGEVITY 2022; 2022:9176923. [PMID: 35923862 PMCID: PMC9343216 DOI: 10.1155/2022/9176923] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 02/10/2022] [Revised: 05/16/2022] [Accepted: 06/04/2022] [Indexed: 11/17/2022]
Abstract
The nervous system is important, because it regulates the physiological function of the body. Neurons are the most basic structural and functional unit of the nervous system. The synapse is an asymmetric structure that is important for neuronal function. The chemical transmission mode of the synapse is realized through neurotransmitters and electrical processes. Based on vesicle transport, the abnormal information transmission process in the synapse can lead to a series of neurorelated diseases. Numerous proteins and complexes that regulate the process of vesicle transport, such as SNARE proteins, Munc18-1, and Synaptotagmin-1, have been identified. Their regulation of synaptic vesicle secretion is complicated and delicate, and their defects can lead to a series of neurodegenerative diseases. This review will discuss the structure and functions of vesicle-based synapses and their roles in neurons. Furthermore, we will analyze neurotransmitter and synaptic functions in neurodegenerative diseases and discuss the potential of using related drugs in their treatment.
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14
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Rizo J, Sari L, Qi Y, Im W, Lin MM. All-atom molecular dynamics simulations of Synaptotagmin-SNARE-complexin complexes bridging a vesicle and a flat lipid bilayer. eLife 2022; 11:76356. [PMID: 35708237 PMCID: PMC9239685 DOI: 10.7554/elife.76356] [Citation(s) in RCA: 19] [Impact Index Per Article: 9.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/14/2021] [Accepted: 06/15/2022] [Indexed: 12/14/2022] Open
Abstract
Synaptic vesicles are primed into a state that is ready for fast neurotransmitter release upon Ca2+-binding to Synaptotagmin-1. This state likely includes trans-SNARE complexes between the vesicle and plasma membranes that are bound to Synaptotagmin-1 and complexins. However, the nature of this state and the steps leading to membrane fusion are unclear, in part because of the difficulty of studying this dynamic process experimentally. To shed light into these questions, we performed all-atom molecular dynamics simulations of systems containing trans-SNARE complexes between two flat bilayers or a vesicle and a flat bilayer with or without fragments of Synaptotagmin-1 and/or complexin-1. Our results need to be interpreted with caution because of the limited simulation times and the absence of key components, but suggest mechanistic features that may control release and help visualize potential states of the primed Synaptotagmin-1-SNARE-complexin-1 complex. The simulations suggest that SNAREs alone induce formation of extended membrane-membrane contact interfaces that may fuse slowly, and that the primed state contains macromolecular assemblies of trans-SNARE complexes bound to the Synaptotagmin-1 C2B domain and complexin-1 in a spring-loaded configuration that prevents premature membrane merger and formation of extended interfaces, but keeps the system ready for fast fusion upon Ca2+ influx.
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Affiliation(s)
- Josep Rizo
- Department of Biophysics, The University of Texas Southwestern Medical Center, Dallas, United States.,Department of Biochemistry, University of Texas Southwestern Medical Center, Dallas, United States.,Department of Pharmacology, University of Texas Southwestern Medical Center, Dallas, United States
| | - Levent Sari
- Department of Biophysics, The University of Texas Southwestern Medical Center, Dallas, United States.,Green Center for Systems Biology, The University of Texas Southwestern Medical Center, Dallas, United States
| | - Yife Qi
- Department of Medicinal Chemistry, School of Pharmacy, Fudan University, Shanghai, China
| | - Wonpil Im
- Department of Biological Sciences, Lehigh University, Bethlehem, United States.,Department of Chemistry, Lehigh University, Bethlehem, United States.,Department of Bioengineering, Lehigh University, Bethlehem, United States.,Department of Computer Science and Engineering, Lehigh University, Bethlehem, United States
| | - Milo M Lin
- Department of Biophysics, The University of Texas Southwestern Medical Center, Dallas, United States.,Green Center for Systems Biology, The University of Texas Southwestern Medical Center, Dallas, United States
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15
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Laghaei R, Meriney SD. Microphysiological Modeling of the Structure and Function of Neuromuscular Transmitter Release Sites. Front Synaptic Neurosci 2022; 14:917285. [PMID: 35769072 PMCID: PMC9236679 DOI: 10.3389/fnsyn.2022.917285] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/11/2022] [Accepted: 05/12/2022] [Indexed: 11/13/2022] Open
Abstract
The general mechanism of calcium-triggered chemical transmitter release from neuronal synapses has been intensely studied, is well-known, and highly conserved between species and synapses across the nervous system. However, the structural and functional details within each transmitter release site (or active zone) are difficult to study in living tissue using current experimental approaches owing to the small spatial compartment within the synapse where exocytosis occurs with a very rapid time course. Therefore, computer simulations offer the opportunity to explore these microphysiological environments of the synapse at nanometer spatial scales and on a sub-microsecond timescale. Because biological reactions and physiological processes at synapses occur under conditions where stochastic behavior is dominant, simulation approaches must be driven by such stochastic processes. MCell provides a powerful simulation approach that employs particle-based stochastic simulation tools to study presynaptic processes in realistic and complex (3D) geometries using optimized Monte Carlo algorithms to track finite numbers of molecules as they diffuse and interact in a complex cellular space with other molecules in solution and on surfaces (representing membranes, channels and binding sites). In this review we discuss MCell-based spatially realistic models of the mammalian and frog neuromuscular active zones that were developed to study presynaptic mechanisms that control transmitter release. In particular, these models focus on the role of presynaptic voltage-gated calcium channels, calcium sensors that control the probability of synaptic vesicle fusion, and the effects of action potential waveform shape on presynaptic calcium entry. With the development of these models, they can now be used in the future to predict disease-induced changes to the active zone, and the effects of candidate therapeutic approaches.
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Affiliation(s)
- Rozita Laghaei
- Biomedical Applications Group, Pittsburgh Supercomputing Center, Carnegie Mellon University, Pittsburgh, PA, United States
| | - Stephen D. Meriney
- Department of Neuroscience, Center for Neuroscience, University of Pittsburgh, Pittsburgh, PA, United States
- *Correspondence: Stephen D. Meriney
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16
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Abstract
Major recent advances and previous data have led to a plausible model of how key proteins mediate neurotransmitter release. In this model, the soluble N-ethylmaleimide-sensitive factor (NSF) attachment protein (SNAP) receptor (SNARE) proteins syntaxin-1, SNAP-25, and synaptobrevin form tight complexes that bring the membranes together and are crucial for membrane fusion. NSF and SNAPs disassemble SNARE complexes and ensure that fusion occurs through an exquisitely regulated pathway that starts with Munc18-1 bound to a closed conformation of syntaxin-1. Munc18-1 also binds to synaptobrevin, forming a template to assemble the SNARE complex when Munc13-1 opens syntaxin-1 while bridging the vesicle and plasma membranes. Synaptotagmin-1 and complexin bind to partially assembled SNARE complexes, likely stabilizing them and preventing fusion until Ca2+ binding to synaptotagmin-1 causes dissociation from the SNARE complex and induces interactions with phospholipids that help trigger release. Although fundamental questions remain about the mechanism of membrane fusion, these advances provide a framework to investigate the mechanisms underlying presynaptic plasticity.
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Affiliation(s)
- Josep Rizo
- Departments of Biophysics, Biochemistry, and Pharmacology, University of Texas Southwestern Medical Center, Dallas, Texas, USA;
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17
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Henry D, Joselevitch C, Matthews GG, Wollmuth LP. Expression and distribution of synaptotagmin family members in the zebrafish retina. J Comp Neurol 2022; 530:705-728. [PMID: 34468021 PMCID: PMC8792163 DOI: 10.1002/cne.25238] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/15/2021] [Revised: 08/21/2021] [Accepted: 08/24/2021] [Indexed: 11/10/2022]
Abstract
Synaptotagmins belong to a large family of proteins. Although various synaptotagmins have been implicated as Ca2+ sensors for vesicle replenishment and release at conventional synapses, their roles at retinal ribbon synapses remain incompletely understood. Zebrafish is a widely used experimental model for retinal research. We therefore investigated the homology between human, rat, mouse, and zebrafish synaptotagmins 1-10 using a bioinformatics approach. We also characterized the expression and distribution of various synaptotagmin (syt) genes in the zebrafish retina using RT-PCR, qPCR, and in situhybridization, focusing on the family members whose products likely underlie Ca2+ -dependent exocytosis in the central nervous system (synaptotagmins 1, 2, 5, and 7). Most zebrafish synaptotagmins are well conserved and can be grouped in the same classes as mammalian synaptotagmins, based on crucial amino acid residues needed for coordinating Ca2+ binding and determining phospholipid binding affinity. The only exception is synaptotagmin 1b, which lacks 34 amino acid residues in the C2B domain and is therefore unlikely to bind Ca2+ there. Additionally, the products of zebrafish syt5a and syt5b genes share identity with mammalian class 1 and 5 synaptotagmins. Zebrafish syt1, syt2, syt5, and syt7 paralogues are found in the zebrafish brain, eye, and retina, excepting syt1b, which is only present in the brain. The complementary expression pattern of the remaining paralogues in the retina suggests that syt1a and syt5a may underlie synchronous release and syt7a and syt7b may mediate asynchronous release or other Ca2+ -dependent processes in different retinal neurons.
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Affiliation(s)
- Diane Henry
- Department of Neurobiology & Behavior, Stony Brook University, Stony Brook, NY 11794-5230,Center for Nervous System Disorders, Stony Brook University, Stony Brook, NY 11794-5230
| | - Christina Joselevitch
- Department of Neurobiology & Behavior, Stony Brook University, Stony Brook, NY 11794-5230,Center for Nervous System Disorders, Stony Brook University, Stony Brook, NY 11794-5230
| | - Gary G. Matthews
- Department of Neurobiology & Behavior, Stony Brook University, Stony Brook, NY 11794-5230,Center for Nervous System Disorders, Stony Brook University, Stony Brook, NY 11794-5230
| | - Lonnie P. Wollmuth
- Department of Neurobiology & Behavior, Stony Brook University, Stony Brook, NY 11794-5230,Department of Biochemistry & Cell Biology, Stony Brook University, Stony Brook, NY 11794-5230,Center for Nervous System Disorders, Stony Brook University, Stony Brook, NY 11794-5230
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18
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Xia X, Wang Y, Qin Y, Zhao S, Zheng JC. Exosome: A novel neurotransmission modulator or non-canonical neurotransmitter? Ageing Res Rev 2022; 74:101558. [PMID: 34990846 DOI: 10.1016/j.arr.2021.101558] [Citation(s) in RCA: 27] [Impact Index Per Article: 13.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/15/2021] [Revised: 08/13/2021] [Accepted: 12/30/2021] [Indexed: 02/08/2023]
Abstract
Neurotransmission is the electrical impulse-triggered propagation of signals between neurons or between neurons and other cell types such as skeletal muscle cells. Recent studies point out the involvement of exosomes, a type of small bilipid layer-enclosed extracellular vesicles, in regulating neurotransmission. Through horizontally transferring proteins, lipids, and nucleic acids, exosomes can modulate synaptic activities rapidly by controlling neurotransmitter release or progressively by regulating neural plasticity including synapse formation, neurite growth & removal, and axon guidance & elongation. In this review, we summarize the similarities and differences between exosomes and synaptic vesicles in their biogenesis, contents, and release. We also highlight the recent progress made in demonstrating the biological roles of exosome in regulating neurotransmission, and propose a modified model of neurotransmission, in which exosomes act as novel neurotransmitters. Lastly, we provide a comprehensive discussion of the enlightenment of the current knowledge on neurotransmission to the future directions of exosome research.
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19
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Voleti R, Bali S, Guerrero J, Smothers J, Springhower C, Acosta GA, Brewer KD, Albericio F, Rizo J. Evaluation of the tert-butyl group as a probe for NMR studies of macromolecular complexes. JOURNAL OF BIOMOLECULAR NMR 2021; 75:347-363. [PMID: 34505210 PMCID: PMC9482097 DOI: 10.1007/s10858-021-00380-y] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/28/2021] [Accepted: 08/23/2021] [Indexed: 05/04/2023]
Abstract
The development of methyl transverse relaxation optimized spectroscopy has greatly facilitated the study of macromolecular assemblies by solution NMR spectroscopy. However, limited sample solubility and stability has hindered application of this technique to ongoing studies of complexes formed on membranes by the neuronal SNAREs that mediate neurotransmitter release and synaptotagmin-1, the Ca2+ sensor that triggers release. Since the 1H NMR signal of a tBu group attached to a large protein or complex can be observed with high sensitivity if the group retains high mobility, we have explored the use of this strategy to analyze presynaptic complexes involved in neurotransmitter release. For this purpose, we attached tBu groups at single cysteines of fragments of synaptotagmin-1, complexin-1 and the neuronal SNAREs by reaction with 5-(tert-butyldisulfaneyl)-2-nitrobenzoic acid (BDSNB), tBu iodoacetamide or tBu acrylate. The tBu resonances of the tagged proteins were generally sharp and intense, although tBu groups attached with BDSNB had a tendency to exhibit somewhat broader resonances that likely result because of the shorter linkage between the tBu and the tagged cysteine. Incorporation of the tagged proteins into complexes on nanodiscs led to severe broadening of the tBu resonances in some cases. However, sharp tBu resonances could readily be observed for some complexes of more than 200 kDa at low micromolar concentrations. Our results show that tagging of proteins with tBu groups provides a powerful approach to study large biomolecular assemblies of limited stability and/or solubility that may be applicable even at nanomolar concentrations.
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Affiliation(s)
- Rashmi Voleti
- Department of Biophysics, University of Texas Southwestern Medical Center, Dallas, TX, 75390, USA
- Department of Biochemistry, University of Texas Southwestern Medical Center, Dallas, TX, 75390, USA
- Department of Pharmacology, University of Texas Southwestern Medical Center, Dallas, TX, 75390, USA
| | - Sofia Bali
- Center for Alzheimer's and Neurodegenerative Diseases, University of Texas Southwestern Medical Center, Dallas, TX, 75390, USA
| | - Jaime Guerrero
- Department of Biophysics, University of Texas Southwestern Medical Center, Dallas, TX, 75390, USA
| | - Jared Smothers
- Department of Molecular Genetics, University of Texas Southwestern Medical Center, Dallas, TX, 75390, USA
| | - Charis Springhower
- Department of Biophysics, University of Texas Southwestern Medical Center, Dallas, TX, 75390, USA
- Alicat Scientific, Tucson, AZ, 85743, USA
| | - Gerardo A Acosta
- CIBER-BBN, Networking Centre on Bioengineering, Biomaterials and Nanomedicine, and Department of Organic Chemistry, University of Barcelona, 08028, Barcelona, Spain
- Institute of Advanced Chemistry of Catalonia (IQAC-CSIC), Spanish National Research Council (CSIC), Jordi Girona 18-26, 08034, Barcelona, Spain
| | - Kyle D Brewer
- Department of Biophysics, University of Texas Southwestern Medical Center, Dallas, TX, 75390, USA
- Department of Biochemistry, University of Texas Southwestern Medical Center, Dallas, TX, 75390, USA
- Department of Pharmacology, University of Texas Southwestern Medical Center, Dallas, TX, 75390, USA
| | - Fernando Albericio
- CIBER-BBN, Networking Centre on Bioengineering, Biomaterials and Nanomedicine, and Department of Organic Chemistry, University of Barcelona, 08028, Barcelona, Spain
- Institute of Advanced Chemistry of Catalonia (IQAC-CSIC), Spanish National Research Council (CSIC), Jordi Girona 18-26, 08034, Barcelona, Spain
- Peptide Science Laboratory, School of Chemistry and Physics, University of KwaZulu-Natal, Durban, 4001, South Africa
| | - Josep Rizo
- Department of Biophysics, University of Texas Southwestern Medical Center, Dallas, TX, 75390, USA.
- Department of Biochemistry, University of Texas Southwestern Medical Center, Dallas, TX, 75390, USA.
- Department of Pharmacology, University of Texas Southwestern Medical Center, Dallas, TX, 75390, USA.
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20
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Sauvola CW, Littleton JT. SNARE Regulatory Proteins in Synaptic Vesicle Fusion and Recycling. Front Mol Neurosci 2021; 14:733138. [PMID: 34421538 PMCID: PMC8377282 DOI: 10.3389/fnmol.2021.733138] [Citation(s) in RCA: 11] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/29/2021] [Accepted: 07/20/2021] [Indexed: 01/01/2023] Open
Abstract
Membrane fusion is a universal feature of eukaryotic protein trafficking and is mediated by the soluble N-ethylmaleimide sensitive factor attachment protein receptor (SNARE) family. SNARE proteins embedded in opposing membranes spontaneously assemble to drive membrane fusion and cargo exchange in vitro. Evolution has generated a diverse complement of SNARE regulatory proteins (SRPs) that ensure membrane fusion occurs at the right time and place in vivo. While a core set of SNAREs and SRPs are common to all eukaryotic cells, a specialized set of SRPs within neurons confer additional regulation to synaptic vesicle (SV) fusion. Neuronal communication is characterized by precise spatial and temporal control of SNARE dynamics within presynaptic subdomains specialized for neurotransmitter release. Action potential-elicited Ca2+ influx at these release sites triggers zippering of SNAREs embedded in the SV and plasma membrane to drive bilayer fusion and release of neurotransmitters that activate downstream targets. Here we discuss current models for how SRPs regulate SNARE dynamics and presynaptic output, emphasizing invertebrate genetic findings that advanced our understanding of SRP regulation of SV cycling.
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Affiliation(s)
- Chad W Sauvola
- The Picower Institute for Learning and Memory, Department of Brain and Cognitive Sciences, Massachusetts Institute of Technology, Cambridge, MA, United States
| | - J Troy Littleton
- The Picower Institute for Learning and Memory, Department of Brain and Cognitive Sciences, Massachusetts Institute of Technology, Cambridge, MA, United States.,Department of Biology, Massachusetts Institute of Technology, Cambridge, MA, United States
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21
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Piekut T, Wong YY, Walker SE, Smith CL, Gauberg J, Harracksingh AN, Lowden C, Novogradac BB, Cheng HYM, Spencer GE, Senatore A. Early Metazoan Origin and Multiple Losses of a Novel Clade of RIM Presynaptic Calcium Channel Scaffolding Protein Homologs. Genome Biol Evol 2021; 12:1217-1239. [PMID: 32413100 PMCID: PMC7456537 DOI: 10.1093/gbe/evaa097] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 05/11/2020] [Indexed: 12/18/2022] Open
Abstract
The precise localization of CaV2 voltage-gated calcium channels at the synapse active zone requires various interacting proteins, of which, Rab3-interacting molecule or RIM is considered particularly important. In vertebrates, RIM interacts with CaV2 channels in vitro via a PDZ domain that binds to the extreme C-termini of the channels at acidic ligand motifs of D/E-D/E/H-WC-COOH, and knockout of RIM in vertebrates and invertebrates disrupts CaV2 channel synaptic localization and synapse function. Here, we describe a previously uncharacterized clade of RIM proteins bearing domain architectures homologous to those of known RIM homologs, but with some notable differences including key amino acids associated with PDZ domain ligand specificity. This novel RIM emerged near the stem lineage of metazoans and underwent extensive losses, but is retained in select animals including the early-diverging placozoan Trichoplax adhaerens, and molluscs. RNA expression and localization studies in Trichoplax and the mollusc snail Lymnaea stagnalis indicate differential regional/tissue type expression, but overlapping expression in single isolated neurons from Lymnaea. Ctenophores, the most early-diverging animals with synapses, are unique among animals with nervous systems in that they lack the canonical RIM, bearing only the newly identified homolog. Through phylogenetic analysis, we find that CaV2 channel D/E-D/E/H-WC-COOH like PDZ ligand motifs were present in the common ancestor of cnidarians and bilaterians, and delineate some deeply conserved C-terminal structures that distinguish CaV1 from CaV2 channels, and CaV1/CaV2 from CaV3 channels.
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Affiliation(s)
| | | | - Sarah E Walker
- Department of Biological Sciences, Brock University, St. Catharines, Ontario, Canada
| | - Carolyn L Smith
- National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, Maryland
| | | | | | | | | | | | - Gaynor E Spencer
- Department of Biological Sciences, Brock University, St. Catharines, Ontario, Canada
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22
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Synaptotagmin-1-, Munc18-1-, and Munc13-1-dependent liposome fusion with a few neuronal SNAREs. Proc Natl Acad Sci U S A 2021; 118:2019314118. [PMID: 33468652 DOI: 10.1073/pnas.2019314118] [Citation(s) in RCA: 19] [Impact Index Per Article: 6.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/29/2022] Open
Abstract
Neurotransmitter release is governed by eight central proteins among other factors: the neuronal SNAREs syntaxin-1, synaptobrevin, and SNAP-25, which form a tight SNARE complex that brings the synaptic vesicle and plasma membranes together; NSF and SNAPs, which disassemble SNARE complexes; Munc18-1 and Munc13-1, which organize SNARE complex assembly; and the Ca2+ sensor synaptotagmin-1. Reconstitution experiments revealed that Munc18-1, Munc13-1, NSF, and α-SNAP can mediate Ca2+-dependent liposome fusion between synaptobrevin liposomes and syntaxin-1-SNAP-25 liposomes, but high fusion efficiency due to uncontrolled SNARE complex assembly did not allow investigation of the role of synaptotagmin-1 on fusion. Here, we show that decreasing the synaptobrevin-to-lipid ratio in the corresponding liposomes to very low levels leads to inefficient fusion and that synaptotagmin-1 strongly stimulates fusion under these conditions. Such stimulation depends on Ca2+ binding to the two C2 domains of synaptotagmin-1. We also show that anchoring SNAP-25 on the syntaxin-1 liposomes dramatically enhances fusion. Moreover, we uncover a synergy between synaptotagmin-1 and membrane anchoring of SNAP-25, which allows efficient Ca2+-dependent fusion between liposomes bearing very low synaptobrevin densities and liposomes containing very low syntaxin-1 densities. Thus, liposome fusion in our assays is achieved with a few SNARE complexes in a manner that requires Munc18-1 and Munc13-1 and that depends on Ca2+ binding to synaptotagmin-1, all of which are fundamental features of neurotransmitter release in neurons.
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23
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Bourgeois-Jaarsma Q, Miaja Hernandez P, Groffen AJ. Ca 2+ sensor proteins in spontaneous release and synaptic plasticity: Limited contribution of Doc2c, rabphilin-3a and synaptotagmin 7 in hippocampal glutamatergic neurons. Mol Cell Neurosci 2021; 112:103613. [PMID: 33753311 DOI: 10.1016/j.mcn.2021.103613] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/24/2020] [Revised: 03/09/2021] [Accepted: 03/13/2021] [Indexed: 11/28/2022] Open
Abstract
Presynaptic neurotransmitter release is strictly regulated by SNARE proteins, Ca2+ and a number of Ca2+ sensors including synaptotagmins (Syts) and Double C2 domain proteins (Doc2s). More than seventy years after the original description of spontaneous release, the mechanism that regulates this process is still poorly understood. Syt-1, Syt7 and Doc2 proteins contribute predominantly, but not exclusively, to synchronous, asynchronous and spontaneous phases of release. The proteins share a conserved tandem C2 domain architecture, but are functionally diverse in their subcellular location, Ca2+-binding properties and protein interactions. In absence of Syt-1, Doc2a and -b, neurons still exhibit spontaneous vesicle fusion which remains Ca2+-sensitive, suggesting the existence of additional sensors. Here, we selected Doc2c, rabphilin-3a and Syt-7 as three potential Ca2+ sensors for their sequence homology with Syt-1 and Doc2b. We genetically ablated each candidate gene in absence of Doc2a and -b and investigated spontaneous and evoked release in glutamatergic hippocampal neurons, cultured either in networks or on microglial islands (autapses). The removal of Doc2c had no effect on spontaneous or evoked release. Syt-7 removal also did not affect spontaneous release, although it altered short-term plasticity by accentuating short-term depression. The removal of rabphilin caused an increased spontaneous release frequency in network cultures, an effect that was not observed in autapses. Taken together, we conclude that Doc2c and Syt-7 do not affect spontaneous release of glutamate in hippocampal neurons, while our results suggest a possible regulatory role of rabphilin-3a in neuronal networks. These findings importantly narrow down the repertoire of synaptic Ca2+ sensors that may be implicated in the spontaneous release of glutamate.
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Affiliation(s)
- Quentin Bourgeois-Jaarsma
- Department of Functional Genomics, Faculty of Science, Center for Neurogenomics and Cognitive Research, VU University, De Boelelaan 1085, 1081HV Amsterdam, the Netherlands
| | - Pablo Miaja Hernandez
- Department of Functional Genomics, Faculty of Science, Center for Neurogenomics and Cognitive Research, VU University, De Boelelaan 1085, 1081HV Amsterdam, the Netherlands
| | - Alexander J Groffen
- Department of Functional Genomics, Faculty of Science, Center for Neurogenomics and Cognitive Research, VU University, De Boelelaan 1085, 1081HV Amsterdam, the Netherlands; Department of Clinical Genetics, VU Medical Center, De Boelelaan 1085, 1081HV Amsterdam, the Netherlands.
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24
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Calcium binds and rigidifies the dysferlin C2A domain in a tightly coupled manner. Biochem J 2021; 478:197-215. [DOI: 10.1042/bcj20200773] [Citation(s) in RCA: 6] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/28/2020] [Revised: 12/03/2020] [Accepted: 12/07/2020] [Indexed: 11/17/2022]
Abstract
The membrane protein dysferlin (DYSF) is important for calcium-activated plasma membrane repair, especially in muscle fibre cells. Nearly 600 mutations in the DYSF gene have been identified that are causative for rare genetic forms of muscular dystrophy. The dysferlin protein consists of seven C2 domains (C2A–C2G, 13%–33% identity) used to recruit calcium ions and traffic accessory proteins and vesicles to injured membrane sites needed to reseal a wound. Amongst these, the C2A is the most prominent facilitating the calcium-sensitive interaction with membrane surfaces. In this work, we determined the calcium-free and calcium-bound structures of the dysferlin C2A domain using NMR spectroscopy and X-ray crystallography. We show that binding two calcium ions to this domain reduces the flexibility of the Ca2+-binding loops in the structure. Furthermore, calcium titration and mutagenesis experiments reveal the tight coupling of these calcium-binding sites whereby the elimination of one site abolishes calcium binding to its partner site. We propose that the electrostatic potential distributed by the flexible, negatively charged calcium-binding loops in the dysferlin C2A domain control first contact with calcium that promotes subsequent binding. Based on these results, we hypothesize that dysferlin uses a ‘calcium-catching’ mechanism to respond to calcium influx during membrane repair.
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25
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Bykhovskaia M. SNARE complex alters the interactions of the Ca 2+ sensor synaptotagmin 1 with lipid bilayers. Biophys J 2021; 120:642-661. [PMID: 33453271 DOI: 10.1016/j.bpj.2020.12.025] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/29/2020] [Revised: 12/19/2020] [Accepted: 12/24/2020] [Indexed: 12/24/2022] Open
Abstract
Release of neuronal transmitters from nerve terminals is triggered by the molecular Ca2+ sensor synaptotagmin 1 (Syt1). Syt1 is a transmembrane protein attached to the synaptic vesicle (SV), and its cytosolic region comprises two domains, C2A and C2B, which are thought to penetrate into lipid bilayers upon Ca2+ binding. Before fusion, SVs become attached to the presynaptic membrane (PM) by the four-helical SNARE complex, which is thought to bind the C2B domain in vivo. To understand how the interactions of Syt1 with lipid bilayers and the SNARE complex trigger fusion, we performed molecular dynamics (MD) simulations at a microsecond scale. We investigated how the isolated C2 modules and the C2AB tandem of Syt1 interact with membranes mimicking either SV or PM. The simulations showed that the C2AB tandem can either bridge SV and PM or insert into PM with its Ca2+-bound tips and that the latter configuration is more favorable. Surprisingly, C2 domains did not cooperate in penetrating into PM but instead mutually hindered their insertion into the bilayer. To test whether the interaction of Syt1 with lipid bilayers could be affected by the C2B-SNARE attachment, we performed systematic conformational analysis of the C2AB-SNARE complex. Notably, we found that the C2B-SNARE interface precludes the coupling of C2 domains and promotes their insertion into PM. We performed the MD simulations of the prefusion protein complex positioned between the lipid bilayers mimicking PM and SV, and our results demonstrated in silico that the presence of the Ca2+ bound C2AB tandem promotes lipid merging. Altogether, our MD simulations elucidated the role of the Syt1-SNARE interactions in the fusion process and produced the dynamic all-atom model of the prefusion protein-lipid complex.
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26
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Roberts RE, Vervliet T, Bultynck G, Parys JB, Hallett MB. EPIC3, a novel Ca 2+ indicator located at the cell cortex and in microridges, detects high Ca 2+ subdomains during Ca 2+ influx and phagocytosis. Cell Calcium 2020; 92:102291. [PMID: 33099169 DOI: 10.1016/j.ceca.2020.102291] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/07/2020] [Revised: 09/11/2020] [Accepted: 09/17/2020] [Indexed: 11/15/2022]
Abstract
The construction of a low affinity Ca2+-probe that locates to the cell cortex and cell surface wrinkles, is described called. EPIC3 (ezrin-protein indicator of Ca2+). The novel probe is a fusion of CEPIA3 with ezrin, and is used in combination with a Ca2+-insensitive probe, ezrin-mCherry, both of which locate at the cell cortex. EPIC3 was used to monitor the effect of Ca2+ influx on intra-wrinkle Ca2+ in the macrophage cell line, RAW 264.7. During experimentally-induced Ca2+influx, EPIC3 reported Ca2+ concentrations at the cell cortex in the region of 30-50 μM, with peak locations towards the tips of wrinkles reaching 80 μM. These concentrations were associated with cleavage of ezrin (a substrate for the Ca2+ activated protease calpain-1) and released the C-terminal fluors. The cortical Ca2+ levels, restricted to near the site of phagocytic cup formation and pseudopodia extension during phagocytosis also reached high levels (50-80 μM) during phagocytosis. As phagocytosis was completed, hotspots of Ca2+ near the phagosome were also observed.
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Affiliation(s)
- Rhiannon E Roberts
- Neutrophil Signalling Group, Cardiff University Medical School, Cardiff, CF14 4XN, UK
| | - Tim Vervliet
- KU Leuven, Laboratory for Molecular and Cellular Signaling, Department of Cellular and Molecular Medicine, Leuven, Belgium
| | - Geert Bultynck
- KU Leuven, Laboratory for Molecular and Cellular Signaling, Department of Cellular and Molecular Medicine, Leuven, Belgium
| | - Jan B Parys
- KU Leuven, Laboratory for Molecular and Cellular Signaling, Department of Cellular and Molecular Medicine, Leuven, Belgium
| | - Maurice B Hallett
- Neutrophil Signalling Group, Cardiff University Medical School, Cardiff, CF14 4XN, UK.
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27
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Synaptotagmin-1 membrane binding is driven by the C2B domain and assisted cooperatively by the C2A domain. Sci Rep 2020; 10:18011. [PMID: 33093513 PMCID: PMC7581758 DOI: 10.1038/s41598-020-74923-y] [Citation(s) in RCA: 11] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/09/2020] [Accepted: 07/23/2020] [Indexed: 11/08/2022] Open
Abstract
Synaptotagmin interaction with anionic lipid (phosphatidylserine/phosphatidylinositol) containing membranes, both in the absence and presence of calcium ions (Ca2+), is critical to its central role in orchestrating neurotransmitter release. The molecular surfaces involved, namely the conserved polylysine motif in the C2B domain and Ca2+-binding aliphatic loops on both C2A and C2B domains, are known. Here we use surface force apparatus combined with systematic mutational analysis of the functional surfaces to directly measure Syt1-membrane interaction and fully map the site-binding energetics of Syt1 both in the absence and presence of Ca2+. By correlating energetics data with the molecular rearrangements measured during confinement, we find that both C2 domains cooperate in membrane binding, with the C2B domain functioning as the main energetic driver, and the C2A domain acting as a facilitator.
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28
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Voleti R, Jaczynska K, Rizo J. Ca 2+-dependent release of synaptotagmin-1 from the SNARE complex on phosphatidylinositol 4,5-bisphosphate-containing membranes. eLife 2020; 9:57154. [PMID: 32808925 PMCID: PMC7498268 DOI: 10.7554/elife.57154] [Citation(s) in RCA: 22] [Impact Index Per Article: 5.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/23/2020] [Accepted: 08/12/2020] [Indexed: 11/13/2022] Open
Abstract
The Ca2+ sensor synaptotagmin-1 and the SNARE complex cooperate to trigger neurotransmitter release. Structural studies elucidated three distinct synaptotagmin-1-SNARE complex binding modes involving 'polybasic', 'primary' and 'tripartite' interfaces of synaptotagmin-1. We investigated these interactions using NMR and fluorescence spectroscopy. Synaptotagmin-1 binds to the SNARE complex through the polybasic and primary interfaces in solution. Ca2+-free synaptotagmin-1 binds to SNARE complexes anchored on PIP2-containing nanodiscs. R398Q/R399Q and E295A/Y338W mutations at the primary interface, which strongly impair neurotransmitter release, disrupt and enhance synaptotagmin-1-SNARE complex binding, respectively. Ca2+ induces tight binding of synaptotagmin-1 to PIP2-containing nanodiscs, disrupting synaptotagmin-1-SNARE interactions. Specific effects of mutations in the polybasic region on Ca2+-dependent synaptotagmin-1-PIP2-membrane interactions correlate with their effects on release. Our data suggest that synaptotagmin-1 binds to the SNARE complex through the primary interface and that Ca2+ releases this interaction, inducing PIP2/membrane binding and allowing cooperation between synaptotagmin-1 and the SNAREs in membrane fusion to trigger release.
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Affiliation(s)
- Rashmi Voleti
- Department of Biophysics, University of Texas Southwestern Medical Center, Dallas, United States.,Department of Biochemistry, University of Texas Southwestern Medical Center, Dallas, United States.,Department of Pharmacology, University of Texas Southwestern Medical Center, Dallas, United States
| | - Klaudia Jaczynska
- Department of Biophysics, University of Texas Southwestern Medical Center, Dallas, United States.,Department of Biochemistry, University of Texas Southwestern Medical Center, Dallas, United States.,Department of Pharmacology, University of Texas Southwestern Medical Center, Dallas, United States
| | - Josep Rizo
- Department of Biophysics, University of Texas Southwestern Medical Center, Dallas, United States.,Department of Biochemistry, University of Texas Southwestern Medical Center, Dallas, United States.,Department of Pharmacology, University of Texas Southwestern Medical Center, Dallas, United States
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29
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Sleiman M, Stevens DR, Chitirala P, Rettig J. Cytotoxic Granule Trafficking and Fusion in Synaptotagmin7-Deficient Cytotoxic T Lymphocytes. Front Immunol 2020; 11:1080. [PMID: 32547563 PMCID: PMC7273742 DOI: 10.3389/fimmu.2020.01080] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/13/2020] [Accepted: 05/05/2020] [Indexed: 11/13/2022] Open
Abstract
Granules of cytotoxic T lymphocytes (CTL) are derived from the lysosomal compartment. Synaptotagmin7 (Syt7) appears to be the calcium sensor triggering fusion of lysosomes in fibroblasts. Syt7 has been proposed to control cytotoxic granule (CG) fusion in lymphocytes and mice lacking Syt7 have reduced ability to clear infections. However, fusion of CG persists in the absence of Syt7. To clarify the role of Syt7 in CTL function, we have examined the fusion of cytotoxic granules of CD8+ T-lymphocytes from Syt7 knock-out mice. We have recorded granule fusion in living CTL, using total internal reflection microscopy. Since Syt7 is considered a high affinity calcium-sensor specialized for fusion under low calcium conditions, we have compared cytotoxic granule fusion under low and high calcium conditions in the same CTL. There was no difference in latencies or numbers of fusion events per CTL under low-calcium conditions, indicating that Syt7 is not required for cytotoxic granule fusion. A deficit of fusion in Syt7 KO CTL was seen when a high-calcium solution was introduced. Expressing wild type Syt7 in Syt7 KO lymphocytes reversed this deficit, confirming its Syt7-dependence. Mutations of Syt7 which disrupt calcium binding to its C2A domain reduced the efficacy of this rescue. We counted the cytotoxic granules present at the plasma membrane to determine if the lack of fusion events in the Syt7 KO CTL was due to a lack of granules. In low calcium there were no differences in fusion events per CTL, and granule numbers were similar. In high calcium, granule number was similar though wild type CTL exhibited significantly more fusion than Syt7 KO CTL. The modest differences in granule counts do not account for the lack of fusion in high calcium in Syt7 KO CTL. In Syt7 KO CTL expressing wild type Syt7, delivery of cytotoxic granules to the plasma membrane was comparable to that of wild type CTL. Syt7 KO CTL expressing Syt7 with deficient calcium binding in the C2A domain had significantly less fusion and fewer CG at the plasma membrane. These results indicate that Syt7 is involved in trafficking of CG to the plasma membrane.
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Affiliation(s)
- Marwa Sleiman
- Cellular Neurophysiology, Center for Integrative Physiology and Molecular Medicine (CIPMM), Saarland University, Homburg, Germany
| | - David R Stevens
- Cellular Neurophysiology, Center for Integrative Physiology and Molecular Medicine (CIPMM), Saarland University, Homburg, Germany
| | - Praneeth Chitirala
- Cellular Neurophysiology, Center for Integrative Physiology and Molecular Medicine (CIPMM), Saarland University, Homburg, Germany
| | - Jens Rettig
- Cellular Neurophysiology, Center for Integrative Physiology and Molecular Medicine (CIPMM), Saarland University, Homburg, Germany
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30
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Shields MC, Bowers MR, Kramer HL, Fulcer MM, Perinet LC, Metz MJ, Reist NE. The role of the C2A domain of synaptotagmin 1 in asynchronous neurotransmitter release. PLoS One 2020; 15:e0232991. [PMID: 32407359 PMCID: PMC7224543 DOI: 10.1371/journal.pone.0232991] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/19/2020] [Accepted: 04/24/2020] [Indexed: 11/22/2022] Open
Abstract
Following nerve stimulation, there are two distinct phases of Ca2+-dependent neurotransmitter release: a fast, synchronous release phase, and a prolonged, asynchronous release phase. Each of these phases is tightly regulated and mediated by distinct mechanisms. Synaptotagmin 1 is the major Ca2+ sensor that triggers fast, synchronous neurotransmitter release upon Ca2+ binding by its C2A and C2B domains. It has also been implicated in the inhibition of asynchronous neurotransmitter release, as blocking Ca2+ binding by the C2A domain of synaptotagmin 1 results in increased asynchronous release. However, the mutation used to block Ca2+ binding in the previous experiments (aspartate to asparagine mutations, sytD-N) had the unintended side effect of mimicking Ca2+ binding, raising the possibility that the increase in asynchronous release was directly caused by ostensibly constitutive Ca2+ binding. Thus, rather than modulating an asynchronous sensor, sytD-N may be mimicking one. To directly test the C2A inhibition hypothesis, we utilized an alternate C2A mutation that we designed to block Ca2+ binding without mimicking it (an aspartate to glutamate mutation, sytD-E). Analysis of both the original sytD-N mutation and our alternate sytD-E mutation at the Drosophila neuromuscular junction showed differential effects on asynchronous release, as well as on synchronous release and the frequency of spontaneous release. Importantly, we found that asynchronous release is not increased in the sytD-E mutant. Thus, our work provides new mechanistic insight into synaptotagmin 1 function during Ca2+-evoked synaptic transmission and demonstrates that Ca2+ binding by the C2A domain of synaptotagmin 1 does not inhibit asynchronous neurotransmitter release in vivo.
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Affiliation(s)
- Mallory C. Shields
- Department of Biomedical Sciences, Colorado State University, Fort Collins, Colorado, United States of America
- Molecular, Cellular, and Integrative Neuroscience Program, Colorado State University, Fort Collins, Colorado, United States of America
| | - Matthew R. Bowers
- Department of Biomedical Sciences, Colorado State University, Fort Collins, Colorado, United States of America
- Molecular, Cellular, and Integrative Neuroscience Program, Colorado State University, Fort Collins, Colorado, United States of America
| | - Hannah L. Kramer
- Department of Biomedical Sciences, Colorado State University, Fort Collins, Colorado, United States of America
| | - McKenzie M. Fulcer
- Department of Biomedical Sciences, Colorado State University, Fort Collins, Colorado, United States of America
| | - Lara C. Perinet
- Department of Biomedical Sciences, Colorado State University, Fort Collins, Colorado, United States of America
| | - Marissa J. Metz
- Department of Biomedical Sciences, Colorado State University, Fort Collins, Colorado, United States of America
- Molecular, Cellular, and Integrative Neuroscience Program, Colorado State University, Fort Collins, Colorado, United States of America
| | - Noreen E. Reist
- Department of Biomedical Sciences, Colorado State University, Fort Collins, Colorado, United States of America
- Molecular, Cellular, and Integrative Neuroscience Program, Colorado State University, Fort Collins, Colorado, United States of America
- * E-mail:
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31
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Bowers MR, Reist NE. Synaptotagmin: Mechanisms of an electrostatic switch. Neurosci Lett 2020; 722:134834. [DOI: 10.1016/j.neulet.2020.134834] [Citation(s) in RCA: 13] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/03/2020] [Revised: 02/06/2020] [Accepted: 02/09/2020] [Indexed: 02/09/2023]
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32
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Bowers MR, Reist NE. The C2A domain of synaptotagmin is an essential component of the calcium sensor for synaptic transmission. PLoS One 2020; 15:e0228348. [PMID: 32032373 PMCID: PMC7006929 DOI: 10.1371/journal.pone.0228348] [Citation(s) in RCA: 10] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/22/2019] [Accepted: 01/13/2020] [Indexed: 01/27/2023] Open
Abstract
The synaptic vesicle protein, synaptotagmin, is the principle Ca2+ sensor for synaptic transmission. Ca2+ influx into active nerve terminals is translated into neurotransmitter release by Ca2+ binding to synaptotagmin’s tandem C2 domains, triggering the fast, synchronous fusion of multiple synaptic vesicles. Two hydrophobic residues, shown to mediate Ca2+-dependent membrane insertion of these C2 domains, are required for this process. Previous research suggested that one of its tandem C2 domains (C2B) is critical for fusion, while the other domain (C2A) plays only a facilitatory role. However, the function of the two hydrophobic residues in C2A have not been adequately tested in vivo. Here we show that these two hydrophobic residues are absolutely required for synaptotagmin to trigger vesicle fusion. Using in vivo electrophysiological recording at the Drosophila larval neuromuscular junction, we found that mutation of these two key C2A hydrophobic residues almost completely abolished neurotransmitter release. Significantly, mutation of both hydrophobic residues resulted in more severe deficits than those seen in synaptotagmin null mutants. Thus, we report the most severe phenotype of a C2A mutation to date, demonstrating that the C2A domain is absolutely essential for synaptotagmin’s function as the electrostatic switch. The postulated role of synaptotagmin’s C2A domain in triggering neurotransmitter release has fluctuated wildly over the years. Early biochemical experiments suggested that the C2A domain was essential, while the C2B domain was superfluous. Then, functional experiments measuring neurotransmitter release in vivo following disruptions in Ca2+ binding suggested that C2B was essential, while C2A was superfluous. Subsequently, the use of more refined mutations to disrupt Ca2+ binding indicated that C2A played a facilitatory role. Here we show two hydrophobic residues of the C2A domain are absolutely required for synaptotagmin-triggered neurotransmitter release. Thus, after over twenty years of research, we now demonstrate that the C2A domain of synaptotagmin is an essential component of the Ca2+ sensor for triggering synaptic transmission in vivo.
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Affiliation(s)
- Matthew R. Bowers
- Department of Biomedical Sciences, Molecular, Cellular, Integrative Neurosciences Program, Colorado State University, Fort Collins, CO, United States of America
| | - Noreen E. Reist
- Department of Biomedical Sciences, Molecular, Cellular, Integrative Neurosciences Program, Colorado State University, Fort Collins, CO, United States of America
- * E-mail:
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33
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Katti S, Nyenhuis SB, Her B, Cafiso DS, Igumenova TI. Partial Metal Ion Saturation of C2 Domains Primes Synaptotagmin 1-Membrane Interactions. Biophys J 2020; 118:1409-1423. [PMID: 32075747 DOI: 10.1016/j.bpj.2020.01.032] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/01/2019] [Revised: 01/15/2020] [Accepted: 01/27/2020] [Indexed: 12/29/2022] Open
Abstract
Synaptotagmin 1 (Syt1) is an integral membrane protein whose phospholipid-binding tandem C2 domains, C2A and C2B, act as Ca2+ sensors of neurotransmitter release. Our objective was to understand the role of individual metal-ion binding sites of these domains in the membrane association process. We used Pb2+, a structural and functional surrogate of Ca2+, to generate the protein states with well-defined protein-metal ion stoichiometry. NMR experiments revealed that binding of one divalent metal ion per C2 domain results in loss of conformational plasticity of the loop regions, potentially pre-organizing them for additional metal-ion and membrane-binding events. In C2A, a divalent metal ion in site 1 is sufficient to drive its weak association with phosphatidylserine-containing membranes, whereas in C2B, it enhances the interactions with the signaling lipid phosphatidylinositol-4,5-bisphosphate. In full-length Syt1, both Pb2+-complexed C2 domains associate with phosphatidylserine-containing membranes. Electron paramagnetic resonance experiments show that the extent of membrane insertion correlates with the occupancy of the C2 metal ion sites. Together, our results indicate that upon partial metal ion saturation of the intra-loop region, Syt1 adopts a dynamic, partially membrane-bound state. The properties of this state, such as conformationally restricted loop regions and positioning of C2 domains in close proximity to anionic lipid headgroups, "prime" Syt1 for cooperative binding of a full complement of metal ions and deeper membrane insertion.
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Affiliation(s)
- Sachin Katti
- Department of Biochemistry and Biophysics, Texas A&M University, College Station, Texas
| | - Sarah B Nyenhuis
- Department of Chemistry and Biophysics Program, University of Virginia, Charlottesville, Virginia
| | - Bin Her
- Department of Biochemistry and Biophysics, Texas A&M University, College Station, Texas
| | - David S Cafiso
- Department of Chemistry and Biophysics Program, University of Virginia, Charlottesville, Virginia
| | - Tatyana I Igumenova
- Department of Biochemistry and Biophysics, Texas A&M University, College Station, Texas.
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34
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Resolving kinetic intermediates during the regulated assembly and disassembly of fusion pores. Nat Commun 2020; 11:231. [PMID: 31932584 PMCID: PMC6957489 DOI: 10.1038/s41467-019-14072-7] [Citation(s) in RCA: 30] [Impact Index Per Article: 7.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/12/2019] [Accepted: 12/10/2019] [Indexed: 11/08/2022] Open
Abstract
The opening of a fusion pore during exocytosis creates the first aqueous connection between the lumen of a vesicle and the extracellular space. Soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) mediate the formation of these dynamic structures, and their kinetic transitions are tightly regulated by accessory proteins at the synapse. Here, we utilize two single molecule approaches, nanodisc-based planar bilayer electrophysiology and single-molecule FRET, to address the relationship between SNARE complex assembly and rapid (micro-millisecond) fusion pore transitions, and to define the role of accessory proteins. Synaptotagmin (syt) 1, a major Ca2+-sensor for synaptic vesicle exocytosis, drove the formation of an intermediate: committed trans-SNARE complexes that form large, stable pores. Once open, these pores could only be closed by the action of the ATPase, NSF. Time-resolved measurements revealed that NSF-mediated pore closure occurred via a complex ‘stuttering’ mechanism. This simplified system thus reveals the dynamic formation and dissolution of fusion pores. SNAREs mediate the formation of a fusion pore during exocytosis which connects the lumen of a vesicle with the extracellular space. Here, authors use single molecule approaches to define the role of synaptotagmin 1 and NSF in synaptic pore formation and dissolution.
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35
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Quantitation and Simulation of Single Action Potential-Evoked Ca 2+ Signals in CA1 Pyramidal Neuron Presynaptic Terminals. eNeuro 2019; 6:ENEURO.0343-19.2019. [PMID: 31551250 PMCID: PMC6800293 DOI: 10.1523/eneuro.0343-19.2019] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/27/2019] [Accepted: 09/10/2019] [Indexed: 01/07/2023] Open
Abstract
Presynaptic Ca2+ evokes exocytosis, endocytosis, and synaptic plasticity. However, Ca2+ flux and interactions at presynaptic molecular targets are difficult to quantify because fluorescence imaging has limited resolution. In rats of either sex, we measured single varicosity presynaptic Ca2+ using Ca2+ dyes as buffers, and constructed models of Ca2+ dispersal. Action potentials evoked Ca2+ transients with little variation when measured with low-affinity dye (peak amplitude 789 ± 39 nM, within 2 ms of stimulation; decay times, 119 ± 10 ms). Endogenous Ca2+ buffering capacity, action potential-evoked free [Ca2+]i, and total Ca2+ amounts entering terminals were determined using Ca2+ dyes as buffers. These data constrained Monte Carlo (MCell) simulations of Ca2+ entry, buffering, and removal. Simulations of experimentally-determined Ca2+ fluxes, buffered by simulated calbindin28K well fit data, and were consistent with clustered Ca2+ entry followed within 4 ms by diffusion throughout the varicosity. Repetitive stimulation caused free varicosity Ca2+ to sum. However, simulated in nanometer domains, its removal by pumps and buffering was negligible, while local diffusion dominated. Thus, Ca2+ within tens of nanometers of entry, did not accumulate. A model of synaptotagmin1 (syt1)-Ca2+ binding indicates that even with 10 µM free varicosity evoked Ca2+, syt1 must be within tens of nanometers of channels to ensure occupation of all its Ca2+-binding sites. Repetitive stimulation, evoking short-term synaptic enhancement, does not modify probabilities of Ca2+ fully occupying syt1’s C2 domains, suggesting that enhancement is not mediated by Ca2+-syt1 interactions. We conclude that at spatiotemporal scales of fusion machines, Ca2+ necessary for their activation is diffusion dominated.
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36
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Harsini FM, Bui AA, Rice AM, Chebrolu S, Fuson KL, Turtoi A, Bradberry M, Chapman ER, Sutton RB. Structural Basis for the Distinct Membrane Binding Activity of the Homologous C2A Domains of Myoferlin and Dysferlin. J Mol Biol 2019; 431:2112-2126. [PMID: 31004665 DOI: 10.1016/j.jmb.2019.04.006] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/01/2019] [Revised: 04/04/2019] [Accepted: 04/04/2019] [Indexed: 02/03/2023]
Abstract
Dysferlin has been implicated in acute membrane repair processes, whereas myoferlin's activity is maximal during the myoblast fusion stage of early skeletal muscle cell development. Both proteins are similar in size and domain structure; however, despite the overall similarity, myoferlin's known physiological functions do not overlap with those of dysferlin. Here we present for the first time the X-ray crystal structure of human myoferlin C2A to 1.9 Å resolution bound to two divalent cations, and compare its three-dimensional structure and membrane binding activities to that of dysferlin C2A. We find that while dysferlin C2A binds membranes in a Ca2+-dependent manner, Ca2+ binding was the rate-limiting kinetic step for this interaction. Myoferlin C2A, on the other hand, binds two calcium ions with an affinity 3-fold lower than that of dysferlin C2A; and, surprisingly, myoferlin C2A binds only marginally to phospholipid mixtures with a high fraction of phosphatidylserine.
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Affiliation(s)
- Faraz M Harsini
- Department of Cell Physiology and Molecular Biophysics, Texas Tech University Health Sciences Center, Lubbock, TX, 79430, USA; Center for Membrane Protein Research, Texas Tech University Health Sciences Center, Lubbock, TX, 79430, USA
| | - Anthony A Bui
- Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, TX, 79409, USA
| | - Anne M Rice
- Department of Biophysics, Johns Hopkins University, Baltimore, MD, 21205, USA
| | - Sukanya Chebrolu
- Department of Cell Physiology and Molecular Biophysics, Texas Tech University Health Sciences Center, Lubbock, TX, 79430, USA
| | - Kerry L Fuson
- Department of Cell Physiology and Molecular Biophysics, Texas Tech University Health Sciences Center, Lubbock, TX, 79430, USA
| | - Andrei Turtoi
- Tumor Microenvironment and Resistance to Treatment Lab, Institut de Recherche en Cancrologie de Montpellier, 34090 Montpellier, France; Institut du Cancer Montpellier, 34090 Montpellier, France; Universit Montpellier, 34298 Montpellier, France
| | - Mazdak Bradberry
- Department of Neuroscience, University of Wisconsin-Madison, Madison, WI, 53705, USA
| | - Edwin R Chapman
- Howard Hughes Medical Institute, University of Wisconsin-Madison, Madison, WI, 53705, USA; Department of Neuroscience, University of Wisconsin-Madison, Madison, WI, 53705, USA
| | - R Bryan Sutton
- Department of Cell Physiology and Molecular Biophysics, Texas Tech University Health Sciences Center, Lubbock, TX, 79430, USA; Center for Membrane Protein Research, Texas Tech University Health Sciences Center, Lubbock, TX, 79430, USA.
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37
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Prinslow EA, Stepien KP, Pan YZ, Xu J, Rizo J. Multiple factors maintain assembled trans-SNARE complexes in the presence of NSF and αSNAP. eLife 2019; 8:38880. [PMID: 30657450 PMCID: PMC6353594 DOI: 10.7554/elife.38880] [Citation(s) in RCA: 44] [Impact Index Per Article: 8.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/03/2018] [Accepted: 01/17/2019] [Indexed: 11/13/2022] Open
Abstract
Neurotransmitter release requires formation of trans-SNARE complexes between the synaptic vesicle and plasma membranes, which likely underlies synaptic vesicle priming to a release-ready state. It is unknown whether Munc18-1, Munc13-1, complexin-1 and synaptotagmin-1 are important for priming because they mediate trans-SNARE complex assembly and/or because they prevent trans-SNARE complex disassembly by NSF-αSNAP, which can lead to de-priming. Here we show that trans-SNARE complex formation in the presence of NSF-αSNAP requires both Munc18-1 and Munc13-1, as proposed previously, and is facilitated by synaptotagmin-1. Our data also show that Munc18-1, Munc13-1, complexin-1 and likely synaptotagmin-1 contribute to maintaining assembled trans-SNARE complexes in the presence of NSF-αSNAP. We propose a model whereby Munc18-1 and Munc13-1 are critical not only for mediating vesicle priming but also for precluding de-priming by preventing trans-SNARE complex disassembly; in this model, complexin-1 also impairs de-priming, while synaptotagmin-1 may assist in priming and hinder de-priming.
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Affiliation(s)
- Eric A Prinslow
- Department of Biophysics, University of Texas Southwestern Medical Center, Dallas, United States.,Department of Biochemistry, University of Texas Southwestern Medical Center, Dallas, United States.,Department of Pharmacology, University of Texas Southwestern Medical Center, Dallas, United States
| | - Karolina P Stepien
- Department of Biophysics, University of Texas Southwestern Medical Center, Dallas, United States.,Department of Biochemistry, University of Texas Southwestern Medical Center, Dallas, United States.,Department of Pharmacology, University of Texas Southwestern Medical Center, Dallas, United States
| | - Yun-Zu Pan
- Department of Biophysics, University of Texas Southwestern Medical Center, Dallas, United States.,Department of Biochemistry, University of Texas Southwestern Medical Center, Dallas, United States.,Department of Pharmacology, University of Texas Southwestern Medical Center, Dallas, United States
| | - Junjie Xu
- Department of Biophysics, University of Texas Southwestern Medical Center, Dallas, United States.,Department of Biochemistry, University of Texas Southwestern Medical Center, Dallas, United States.,Department of Pharmacology, University of Texas Southwestern Medical Center, Dallas, United States
| | - Josep Rizo
- Department of Biophysics, University of Texas Southwestern Medical Center, Dallas, United States.,Department of Biochemistry, University of Texas Southwestern Medical Center, Dallas, United States.,Department of Pharmacology, University of Texas Southwestern Medical Center, Dallas, United States
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38
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Park Y, Ryu JK. Models of synaptotagmin-1 to trigger Ca 2+ -dependent vesicle fusion. FEBS Lett 2018; 592:3480-3492. [PMID: 30004579 DOI: 10.1002/1873-3468.13193] [Citation(s) in RCA: 39] [Impact Index Per Article: 6.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/03/2018] [Revised: 07/02/2018] [Accepted: 07/06/2018] [Indexed: 11/08/2022]
Abstract
Vesicles in neurons and neuroendocrine cells store neurotransmitters and peptide hormones, which are released by vesicle fusion in response to Ca2+ -evoking stimuli. Synaptotagmin-1 (Syt1), a Ca2+ sensor, mediates ultrafast exocytosis in neurons and neuroendocrine cells. After vesicle docking, Syt1 has two main groups of binding partners: anionic phospholipids and the SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptors) complex. The molecular mechanisms by which Syt1 triggers vesicle fusion remain controversial. This Review introduces and summarizes six molecular models of Syt1: (a) Syt1 triggers SNARE unclamping by displacing complexin, (b) Syt1 clamps SNARE zippering, (c) Syt1 causes membrane curvature, (d) membrane bridging by Syt1, (e) Syt1 is a vesicle-plasma membrane distance regulator, and (f) Syt1 undergoes circular oligomerization. We discuss important conditions to test Syt1 activity in vitro and attempt to illustrate the possible roles of Syt1.
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Affiliation(s)
- Yongsoo Park
- Department of Molecular Biology and Genetics, Koç University, Istanbul, Turkey
| | - Je-Kyung Ryu
- Department of Bionanoscience, Kavli Institute of Nanoscience Delft, Delft University of Technology, The Netherlands
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39
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Rizo J. Mechanism of neurotransmitter release coming into focus. Protein Sci 2018; 27:1364-1391. [PMID: 29893445 DOI: 10.1002/pro.3445] [Citation(s) in RCA: 125] [Impact Index Per Article: 20.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/04/2018] [Accepted: 05/10/2018] [Indexed: 12/11/2022]
Abstract
Research for three decades and major recent advances have provided crucial insights into how neurotransmitters are released by Ca2+ -triggered synaptic vesicle exocytosis, leading to reconstitution of basic steps that underlie Ca2+ -dependent membrane fusion and yielding a model that assigns defined functions for central components of the release machinery. The soluble N-ethyl maleimide sensitive factor attachment protein receptors (SNAREs) syntaxin-1, SNAP-25, and synaptobrevin-2 form a tight SNARE complex that brings the vesicle and plasma membranes together and is key for membrane fusion. N-ethyl maleimide sensitive factor (NSF) and soluble NSF attachment proteins (SNAPs) disassemble the SNARE complex to recycle the SNAREs for another round of fusion. Munc18-1 and Munc13-1 orchestrate SNARE complex formation in an NSF-SNAP-resistant manner by a mechanism whereby Munc18-1 binds to synaptobrevin and to a self-inhibited "closed" conformation of syntaxin-1, thus forming a template to assemble the SNARE complex, and Munc13-1 facilitates assembly by bridging the vesicle and plasma membranes and catalyzing opening of syntaxin-1. Synaptotagmin-1 functions as the major Ca2+ sensor that triggers release by binding to membrane phospholipids and to the SNAREs, in a tight interplay with complexins that accelerates membrane fusion. Many of these proteins act as both inhibitors and activators of exocytosis, which is critical for the exquisite regulation of neurotransmitter release. It is still unclear how the actions of these various proteins and multiple other components that control release are integrated and, in particular, how they induce membrane fusion, but it can be expected that these fundamental questions can be answered in the near future, building on the extensive knowledge already available.
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Affiliation(s)
- Josep Rizo
- Departments of Biophysics, Biochemistry and Pharmacology, University of Texas Southwestern Medical Center, Dallas, Texas, 75390
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40
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Meese S, Cepeda AP, Gahlen F, Adams CM, Ficner R, Ricci AJ, Heller S, Reisinger E, Herget M. Activity-Dependent Phosphorylation by CaMKIIδ Alters the Ca 2+ Affinity of the Multi-C 2-Domain Protein Otoferlin. Front Synaptic Neurosci 2017; 9:13. [PMID: 29046633 PMCID: PMC5632675 DOI: 10.3389/fnsyn.2017.00013] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/21/2017] [Accepted: 09/11/2017] [Indexed: 01/20/2023] Open
Abstract
Otoferlin is essential for fast Ca2+-triggered transmitter release from auditory inner hair cells (IHCs), playing key roles in synaptic vesicle release, replenishment and retrieval. Dysfunction of otoferlin results in profound prelingual deafness. Despite its crucial role in cochlear synaptic processes, mechanisms regulating otoferlin activity have not been studied to date. Here, we identified Ca2+/calmodulin-dependent serine/threonine kinase II delta (CaMKIIδ) as an otoferlin binding partner by pull-downs from chicken utricles and reassured interaction by a co-immunoprecipitation with heterologously expressed proteins in HEK cells. We confirmed the expression of CaMKIIδ in rodent IHCs by immunohistochemistry and real-time PCR. A proximity ligation assay indicates close proximity of the two proteins in rat IHCs, suggesting that otoferlin and CaMKIIδ also interact in mammalian IHCs. In vitro phosphorylation of otoferlin by CaMKIIδ revealed ten phosphorylation sites, five of which are located within C2-domains. Exchange of serines/threonines at phosphorylated sites into phosphomimetic aspartates reduces the Ca2+ affinity of the recombinant C2F domain 10-fold, and increases the Ca2+ affinity of the C2C domain. Concordantly, we show that phosphorylation of otoferlin and/or its interaction partners are enhanced upon hair cell depolarization and blocked by pharmacological CaMKII inhibition. We therefore propose that otoferlin activity is regulated by CaMKIIδ in IHCs.
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Affiliation(s)
- Sandra Meese
- Department of Molecular Structural Biology, Institute for Microbiology and Genetics, and Collaborative Research Center 889, University of Göttingen, Göttingen, Germany.,Göttingen Graduate School for Neurosciences, Biophysics, and Molecular Biosciences, Göttingen, Germany
| | - Andreia P Cepeda
- Göttingen Graduate School for Neurosciences, Biophysics, and Molecular Biosciences, Göttingen, Germany.,Molecular Biology of Cochlear Neurotransmission Group, Department of Otorhinolaryngology, University Medical Center Göttingen, and Collaborative Research Center 889, University of Göttingen, Göttingen, Germany
| | - Felix Gahlen
- Department of Otorhinolaryngology, Head and Neck Surgery, Ruhr-University Bochum, Bochum, Germany
| | - Christopher M Adams
- Stanford University Mass Spectrometry, Stanford University, Stanford, CA, United States
| | - Ralf Ficner
- Department of Molecular Structural Biology, Institute for Microbiology and Genetics, and Collaborative Research Center 889, University of Göttingen, Göttingen, Germany
| | - Anthony J Ricci
- Department of Otolaryngology, Head and Neck Surgery, Stanford School of Medicine, Stanford, CA, United States
| | - Stefan Heller
- Department of Otolaryngology, Head and Neck Surgery, Stanford School of Medicine, Stanford, CA, United States
| | - Ellen Reisinger
- Molecular Biology of Cochlear Neurotransmission Group, Department of Otorhinolaryngology, University Medical Center Göttingen, and Collaborative Research Center 889, University of Göttingen, Göttingen, Germany
| | - Meike Herget
- Department of Otolaryngology, Head and Neck Surgery, Stanford School of Medicine, Stanford, CA, United States
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41
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Exceptionally tight membrane-binding may explain the key role of the synaptotagmin-7 C 2A domain in asynchronous neurotransmitter release. Proc Natl Acad Sci U S A 2017; 114:E8518-E8527. [PMID: 28923929 DOI: 10.1073/pnas.1710708114] [Citation(s) in RCA: 32] [Impact Index Per Article: 4.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Synaptotagmins (Syts) act as Ca2+ sensors in neurotransmitter release by virtue of Ca2+-binding to their two C2 domains, but their mechanisms of action remain unclear. Puzzlingly, Ca2+-binding to the C2B domain appears to dominate Syt1 function in synchronous release, whereas Ca2+-binding to the C2A domain mediates Syt7 function in asynchronous release. Here we show that crystal structures of the Syt7 C2A domain and C2AB region, and analyses of intrinsic Ca2+-binding to the Syt7 C2 domains using isothermal titration calorimetry, did not reveal major differences that could explain functional differentiation between Syt7 and Syt1. However, using liposome titrations under Ca2+ saturating conditions, we show that the Syt7 C2A domain has a very high membrane affinity and dominates phospholipid binding to Syt7 in the presence or absence of l-α-phosphatidylinositol 4,5-diphosphate (PIP2). For Syt1, the two Ca2+-saturated C2 domains have similar affinities for membranes lacking PIP2, but the C2B domain dominates binding to PIP2-containing membranes. Mutagenesis revealed that the dramatic differences in membrane affinity between the Syt1 and Syt7 C2A domains arise in part from apparently conservative residue substitutions, showing how striking biochemical and functional differences can result from the cumulative effects of subtle residue substitutions. Viewed together, our results suggest that membrane affinity may be a key determinant of the functions of Syt C2 domains in neurotransmitter release.
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42
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Jackman SL, Regehr WG. The Mechanisms and Functions of Synaptic Facilitation. Neuron 2017; 94:447-464. [PMID: 28472650 DOI: 10.1016/j.neuron.2017.02.047] [Citation(s) in RCA: 212] [Impact Index Per Article: 30.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/06/2016] [Revised: 02/23/2017] [Accepted: 02/28/2017] [Indexed: 12/22/2022]
Abstract
The ability of the brain to store and process information relies on changing the strength of connections between neurons. Synaptic facilitation is a form of short-term plasticity that enhances synaptic transmission for less than a second. Facilitation is a ubiquitous phenomenon thought to play critical roles in information transfer and neural processing. Yet our understanding of the function of facilitation remains largely theoretical. Here we review proposed roles for facilitation and discuss how recent progress in uncovering the underlying molecular mechanisms could enable experiments that elucidate how facilitation, and short-term plasticity in general, contributes to circuit function and animal behavior.
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Affiliation(s)
- Skyler L Jackman
- Department of Neurobiology, Harvard Medical School, Boston, MA 02118, USA
| | - Wade G Regehr
- Department of Neurobiology, Harvard Medical School, Boston, MA 02118, USA.
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43
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Rao TC, Santana Rodriguez Z, Bradberry MM, Ranski AH, Dahl PJ, Schmidtke MW, Jenkins PM, Axelrod D, Chapman ER, Giovannucci DR, Anantharam A. Synaptotagmin isoforms confer distinct activation kinetics and dynamics to chromaffin cell granules. J Gen Physiol 2017; 149:763-780. [PMID: 28687607 PMCID: PMC5560776 DOI: 10.1085/jgp.201711757] [Citation(s) in RCA: 26] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/16/2017] [Revised: 05/15/2017] [Accepted: 06/12/2017] [Indexed: 01/09/2023] Open
Abstract
Chromaffin cells release transmitters from populations of granules to which synaptotagmin-1 and synaptotagmin-7 are selectively sorted. Rao et al. characterize the functional properties of these granules and show that synaptotagmin-7 confers fast kinetics and high efficacy to the exocytotic event. Adrenomedullary chromaffin cells respond to sympathetic nervous system activation by secreting a cocktail of potent neuropeptides and hormones into the circulation. The distinct phases of the chromaffin cell secretory response have been attributed to the progressive fusion of distinct populations of dense core granules with different activation kinetics. However, it has been difficult to define what distinguishes these populations at the molecular level. Functional segregation of granule pools may depend on selective sorting of synaptotagmin-1 (Syt-1) and synaptotagmin-7 (Syt-7), which our previous work showed are rarely cosorted to the same granule. Here we assess the consequences of selective sorting of Syt isoforms in chromaffin cells, particularly with respect to granule dynamics and activation kinetics. Upon depolarization of cells expressing fluorescent Syt isoforms using elevated K+, we find that Syt-7 granules fuse with faster kinetics than Syt-1 granules, irrespective of stimulation strength. Pharmacological blockade of Ca2+ channels reveals differential dependence of Syt-1 versus Syt-7 granule exocytosis on Ca2+ channel subtypes. Syt-7 granules also show a greater tendency to fuse in clusters than Syt-1 granules, and granules harboring Syt-1 travel a greater distance before fusion than those with Syt-7, suggesting that there is spatial and fusion-site heterogeneity among the two granule populations. However, the greatest functional difference between granule populations is their responsiveness to Ca2+. Upon introduction of Ca2+ into permeabilized cells, Syt-7 granules fuse with fast kinetics and high efficacy, even at low Ca2+ levels (e.g., when cells are weakly stimulated). Conversely, Syt-1 granules require a comparatively larger increase in intracellular Ca2+ for activation. At Ca2+ concentrations above 30 µM, activation kinetics are faster for Syt-1 granules than for Syt-7 granules. Our study provides evidence for functional specialization of chromaffin cell granules via selective expression of Syt isoforms with different Ca2+ sensitivities.
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Affiliation(s)
- Tejeshwar C Rao
- Department of Pharmacology, University of Michigan, Ann Arbor, MI
| | | | - Mazdak M Bradberry
- Howard Hughes Medical Institute, Department of Neuroscience, University of Wisconsin, Madison, WI
| | | | - Peter J Dahl
- Department of Pharmacology, University of Michigan, Ann Arbor, MI
| | | | - Paul M Jenkins
- Department of Pharmacology, University of Michigan, Ann Arbor, MI
| | - Daniel Axelrod
- Department of Pharmacology, University of Michigan, Ann Arbor, MI
| | - Edwin R Chapman
- Howard Hughes Medical Institute, Department of Neuroscience, University of Wisconsin, Madison, WI
| | - David R Giovannucci
- Department of Neurosciences, University of Toledo Medical School, Toledo, OH
| | - Arun Anantharam
- Department of Pharmacology, University of Michigan, Ann Arbor, MI
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44
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Katti S, Nyenhuis SB, Her B, Srivastava AK, Taylor AB, Hart PJ, Cafiso DS, Igumenova TI. Non-Native Metal Ion Reveals the Role of Electrostatics in Synaptotagmin 1-Membrane Interactions. Biochemistry 2017; 56:3283-3295. [PMID: 28574251 DOI: 10.1021/acs.biochem.7b00188] [Citation(s) in RCA: 16] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
C2 domains are independently folded modules that often target their host proteins to anionic membranes in a Ca2+-dependent manner. In these cases, membrane association is triggered by Ca2+ binding to the negatively charged loop region of the C2 domain. Here, we used a non-native metal ion, Cd2+, in lieu of Ca2+ to gain insight into the contributions made by long-range Coulombic interactions and direct metal ion-lipid bridging to membrane binding. Using X-ray crystallography, NMR, Förster resonance energy transfer, and vesicle cosedimentation assays, we demonstrate that, although Cd2+ binds to the loop region of C2A/B domains of synaptotagmin 1 with high affinity, long-range Coulombic interactions are too weak to support membrane binding of individual domains. We attribute this behavior to two factors: the stoichiometry of Cd2+ binding to the loop regions of the C2A and C2B domains and the impaired ability of Cd2+ to directly coordinate the lipids. In contrast, electron paramagnetic resonance experiments revealed that Cd2+ does support membrane binding of the C2 domains in full-length synaptotagmin 1, where the high local lipid concentrations that result from membrane tethering can partially compensate for lack of a full complement of divalent metal ions and specific lipid coordination in Cd2+-complexed C2A/B domains. Our data suggest that long-range Coulombic interactions alone can drive the initial association of C2A/B with anionic membranes and that Ca2+ further augments membrane binding by the formation of metal ion-lipid coordination bonds and additional Ca2+ ion binding to the C2 domain loop regions.
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Affiliation(s)
- Sachin Katti
- Department of Biochemistry and Biophysics, Texas A&M University , 300 Olsen Boulevard, College Station, Texas 77843, United States
| | - Sarah B Nyenhuis
- Department of Chemistry and Biophysics Program, University of Virginia , Charlottesville, Virginia 22904, United States
| | - Bin Her
- Department of Biochemistry and Biophysics, Texas A&M University , 300 Olsen Boulevard, College Station, Texas 77843, United States
| | - Atul K Srivastava
- Department of Biochemistry and Biophysics, Texas A&M University , 300 Olsen Boulevard, College Station, Texas 77843, United States
| | - Alexander B Taylor
- Department of Biochemistry and Structural Biology and the X-ray Crystallography Core Laboratory, University of Texas Health Science Center at San Antonio , San Antonio, Texas 78229, United States
| | - P John Hart
- Department of Biochemistry and Structural Biology and the X-ray Crystallography Core Laboratory, University of Texas Health Science Center at San Antonio , San Antonio, Texas 78229, United States
| | - David S Cafiso
- Department of Chemistry and Biophysics Program, University of Virginia , Charlottesville, Virginia 22904, United States
| | - Tatyana I Igumenova
- Department of Biochemistry and Biophysics, Texas A&M University , 300 Olsen Boulevard, College Station, Texas 77843, United States
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45
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Guo T, Duan Z, Chen J, Xie C, Wang Y, Chen P, Wang X. Pull-down combined with proteomic strategy reveals functional diversity of synaptotagmin I. PeerJ 2017; 5:e2973. [PMID: 28194317 PMCID: PMC5301975 DOI: 10.7717/peerj.2973] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/18/2016] [Accepted: 01/10/2017] [Indexed: 12/05/2022] Open
Abstract
Synaptotagmin I (Syt I) is most abundant in the brain and is involved in multiple cellular processes. Its two C2 domains, C2A and C2B, are the main functional regions. Our present study employed a pull-down combined with proteomic strategy to identify the C2 domain-interacting proteins to comprehensively understand the biological roles of the C2 domains and thus the functional diversity of Syt I. A total of 135 non-redundant proteins interacting with the C2 domains of Syt I were identified. Out of them, 32 and 64 proteins only bound to C2A or C2B domains, respectively, and 39 proteins bound to both of them. Compared with C2A, C2B could bind to many more proteins particularly those involved in synaptic transmission and metabolic regulation. Functional analysis indicated that Syt I may exert impacts by interacting with other proteins on multiple cellular processes, including vesicular membrane trafficking, synaptic transmission, metabolic regulation, catalysis, transmembrane transport and structure formation, etc. These results demonstrate that the functional diversity of Syt I is higher than previously expected, that its two domains may mediate the same and different cellular processes cooperatively or independently, and that C2B domain may play even more important roles than C2A in the functioning of Syt I. This work not only further deepened our understanding of the functional diversity of Syt I and the functional differences between its two C2 domains, but also provided important clues for the further related researches.
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Affiliation(s)
- Tianyao Guo
- Key Laboratory of Protein Chemistry and Developmental Biology of Ministry of Education, College of Life Sciences, Hunan Normal University , Changsha , Hunan , P. R. of China
| | - Zhigui Duan
- Key Laboratory of Protein Chemistry and Developmental Biology of Ministry of Education, College of Life Sciences, Hunan Normal University , Changsha , Hunan , P. R. of China
| | - Jia Chen
- Key Laboratory of Protein Chemistry and Developmental Biology of Ministry of Education, College of Life Sciences, Hunan Normal University , Changsha , Hunan , P. R. of China
| | - Chunliang Xie
- Key Laboratory of Protein Chemistry and Developmental Biology of Ministry of Education, College of Life Sciences, Hunan Normal University , Changsha , Hunan , P. R. of China
| | - Ying Wang
- Key Laboratory of Protein Chemistry and Developmental Biology of Ministry of Education, College of Life Sciences, Hunan Normal University , Changsha , Hunan , P. R. of China
| | - Ping Chen
- Key Laboratory of Protein Chemistry and Developmental Biology of Ministry of Education, College of Life Sciences, Hunan Normal University , Changsha , Hunan , P. R. of China
| | - Xianchun Wang
- Key Laboratory of Protein Chemistry and Developmental Biology of Ministry of Education, College of Life Sciences, Hunan Normal University , Changsha , Hunan , P. R. of China
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46
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Pinheiro PS, Houy S, Sørensen JB. C2-domain containing calcium sensors in neuroendocrine secretion. J Neurochem 2016; 139:943-958. [DOI: 10.1111/jnc.13865] [Citation(s) in RCA: 44] [Impact Index Per Article: 5.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/29/2016] [Revised: 09/17/2016] [Accepted: 10/05/2016] [Indexed: 12/11/2022]
Affiliation(s)
- Paulo S. Pinheiro
- Center for Neuroscience and Cell Biology; University of Coimbra; Coimbra Portugal
| | - Sébastien Houy
- Department of Neuroscience and Pharmacology; Faculty of Health and Medical Sciences; University of Copenhagen; Copenhagen Denmark
| | - Jakob B. Sørensen
- Department of Neuroscience and Pharmacology; Faculty of Health and Medical Sciences; University of Copenhagen; Copenhagen Denmark
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47
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Drmota Prebil S, Slapšak U, Pavšič M, Ilc G, Puž V, de Almeida Ribeiro E, Anrather D, Hartl M, Backman L, Plavec J, Lenarčič B, Djinović-Carugo K. Structure and calcium-binding studies of calmodulin-like domain of human non-muscle α-actinin-1. Sci Rep 2016; 6:27383. [PMID: 27272015 PMCID: PMC4895382 DOI: 10.1038/srep27383] [Citation(s) in RCA: 20] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/02/2016] [Accepted: 05/16/2016] [Indexed: 12/11/2022] Open
Abstract
The activity of several cytosolic proteins critically depends on the concentration of calcium ions. One important intracellular calcium-sensing protein is α-actinin-1, the major actin crosslinking protein in focal adhesions and stress fibers. The actin crosslinking activity of α-actinin-1 has been proposed to be negatively regulated by calcium, but the underlying molecular mechanisms are poorly understood. To address this, we determined the first high-resolution NMR structure of its functional calmodulin-like domain (CaMD) in calcium-bound and calcium-free form. These structures reveal that in the absence of calcium, CaMD displays a conformationally flexible ensemble that undergoes a structural change upon calcium binding, leading to limited rotation of the N- and C-terminal lobes around the connecting linker and consequent stabilization of the calcium-loaded structure. Mutagenesis experiments, coupled with mass-spectrometry and isothermal calorimetry data designed to validate the calcium binding stoichiometry and binding site, showed that human non-muscle α-actinin-1 binds a single calcium ion within the N-terminal lobe. Finally, based on our structural data and analogy with other α-actinins, we provide a structural model of regulation of the actin crosslinking activity of α-actinin-1 where calcium induced structural stabilisation causes fastening of the juxtaposed actin binding domain, leading to impaired capacity to crosslink actin.
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Affiliation(s)
- Sara Drmota Prebil
- Department of Chemistry and Biochemistry, Faculty of Chemistry and Chemical Technology, University of Ljubljana, Večna pot 113, SI-1000 Ljubljana, Slovenia
| | - Urška Slapšak
- Slovenian NMR Centre, National Institute of Chemistry, Hajdrihova 19, SI-1000 Ljubljana, Slovenia
| | - Miha Pavšič
- Department of Chemistry and Biochemistry, Faculty of Chemistry and Chemical Technology, University of Ljubljana, Večna pot 113, SI-1000 Ljubljana, Slovenia
| | - Gregor Ilc
- Slovenian NMR Centre, National Institute of Chemistry, Hajdrihova 19, SI-1000 Ljubljana, Slovenia.,EN-FIST Centre of Excellence, Trg Osvobodilne fronte 13, SI-1000 Ljubljana, Slovenia
| | - Vid Puž
- Department of Chemistry and Biochemistry, Faculty of Chemistry and Chemical Technology, University of Ljubljana, Večna pot 113, SI-1000 Ljubljana, Slovenia
| | - Euripedes de Almeida Ribeiro
- Department of Structural and Computational Biology, Max F. Perutz Laboratories (MFPL), University of Vienna, Campus Vienna Biocenter 5, A-1030 Vienna, Austria
| | - Dorothea Anrather
- Mass Spectrometry Service Facility, Max F. Perutz Laboratories (MFPL), University of Vienna, Vienna Biocenter (VBC), Dr. Bohr-Gasse 3, A-1030 Vienna, Austria
| | - Markus Hartl
- Mass Spectrometry Service Facility, Max F. Perutz Laboratories (MFPL), University of Vienna, Vienna Biocenter (VBC), Dr. Bohr-Gasse 3, A-1030 Vienna, Austria
| | - Lars Backman
- Department of Chemistry, Umeå University, Linnaeus väg 10, SE-90187 Umeå, Sweden
| | - Janez Plavec
- Department of Chemistry and Biochemistry, Faculty of Chemistry and Chemical Technology, University of Ljubljana, Večna pot 113, SI-1000 Ljubljana, Slovenia.,Slovenian NMR Centre, National Institute of Chemistry, Hajdrihova 19, SI-1000 Ljubljana, Slovenia.,EN-FIST Centre of Excellence, Trg Osvobodilne fronte 13, SI-1000 Ljubljana, Slovenia
| | - Brigita Lenarčič
- Department of Chemistry and Biochemistry, Faculty of Chemistry and Chemical Technology, University of Ljubljana, Večna pot 113, SI-1000 Ljubljana, Slovenia.,Department of Biochemistry, Molecular Biology and Structural Biology, Jožef Stefan Institute, Jamova 39,SI-1000 Ljubljana, Slovenia
| | - Kristina Djinović-Carugo
- Department of Chemistry and Biochemistry, Faculty of Chemistry and Chemical Technology, University of Ljubljana, Večna pot 113, SI-1000 Ljubljana, Slovenia.,Department of Structural and Computational Biology, Max F. Perutz Laboratories (MFPL), University of Vienna, Campus Vienna Biocenter 5, A-1030 Vienna, Austria
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48
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Fealey ME, Mahling R, Rice AM, Dunleavy K, Kobany SEG, Lohese KJ, Horn B, Hinderliter A. Synaptotagmin I's Intrinsically Disordered Region Interacts with Synaptic Vesicle Lipids and Exerts Allosteric Control over C2A. Biochemistry 2016; 55:2914-26. [PMID: 27191789 DOI: 10.1021/acs.biochem.6b00085] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/04/2023]
Abstract
Synaptotagmin I (Syt I) is a vesicle-localized integral membrane protein that senses the calcium ion (Ca(2+)) influx to trigger fast synchronous release of neurotransmitter. How the cytosolic domains of Syt I allosterically communicate to propagate the Ca(2+) binding signal throughout the protein is not well understood. In particular, it is unclear whether the intrinsically disordered region (IDR) between Syt I's transmembrane helix and first C2 domain (C2A) plays an important role in allosteric modulation of Ca(2+) binding. Moreover, the structural propensity of this IDR with respect to membrane lipid composition is unknown. Using differential scanning and isothermal titration calorimetry, we found that inclusion of the IDR does indeed allosterically modulate Ca(2+) binding within the first C2 domain. Additionally through application of nuclear magnetic resonance, we found that Syt I's IDR interacts with membranes whose lipid composition mimics that of a synaptic vesicle. These findings not only indicate that Syt I's IDR plays a role in regulating Syt I's Ca(2+) sensing but also indicate the IDR is exquisitely sensitive to the underlying membrane lipids. The latter observation suggests the IDR is a key route for communication of lipid organization to the adjacent C2 domains.
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Affiliation(s)
- Michael E Fealey
- Department of Chemistry and Biochemistry, University of Minnesota-Duluth , Duluth, Minnesota 55812, United States.,Department of Biochemistry, Molecular Biology, and Biophysics, University of Minnesota , Minneapolis, Minnesota 55455, United States
| | - Ryan Mahling
- Department of Chemistry and Biochemistry, University of Minnesota-Duluth , Duluth, Minnesota 55812, United States
| | - Anne M Rice
- Department of Chemistry and Biochemistry, University of Minnesota-Duluth , Duluth, Minnesota 55812, United States
| | - Katie Dunleavy
- Department of Chemistry and Biochemistry, University of Minnesota-Duluth , Duluth, Minnesota 55812, United States
| | - Stephanie E G Kobany
- Department of Chemistry and Biochemistry, University of Minnesota-Duluth , Duluth, Minnesota 55812, United States
| | - K Jean Lohese
- Department of Chemistry and Biochemistry, University of Minnesota-Duluth , Duluth, Minnesota 55812, United States
| | - Benjamin Horn
- Department of Chemistry and Biochemistry, University of Minnesota-Duluth , Duluth, Minnesota 55812, United States
| | - Anne Hinderliter
- Department of Chemistry and Biochemistry, University of Minnesota-Duluth , Duluth, Minnesota 55812, United States.,Department of Biochemistry, Molecular Biology, and Biophysics, University of Minnesota , Minneapolis, Minnesota 55455, United States
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49
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Calcium binding promotes conformational flexibility of the neuronal Ca(2+) sensor synaptotagmin. Biophys J 2016; 108:2507-2520. [PMID: 25992729 DOI: 10.1016/j.bpj.2015.04.007] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/07/2015] [Revised: 04/07/2015] [Accepted: 04/09/2015] [Indexed: 01/12/2023] Open
Abstract
Synaptotagmin 1 (Syt1) is a synaptic vesicle protein that serves as a calcium sensor of neuronal secretion. It is established that calcium binding to Syt1 triggers vesicle fusion and release of neuronal transmitters, however, the dynamics of this process is not fully understood. To investigate how Ca(2+) binding affects Syt1 conformational dynamics, we performed prolonged molecular dynamics (MD) simulations of Ca(2+)-unbound and Ca(2+)-bound forms of Syt1. MD simulations were performed at a microsecond scale and combined with Monte Carlo sampling. We found that in the absence of Ca(2+) Syt1 structure in the solution is represented by an ensemble of conformational states with tightly coupled domains. To investigate the effect of Ca(2+) binding, we used two different strategies to generate a molecular model of a Ca(2+)-bound form of Syt1. First, we employed subsequent replacements of monovalent cations transiently captured within Syt1 Ca(2+)-binding pockets by Ca(2+) ions. Second, we performed MD simulations of Syt1 at elevated Ca(2+) levels. All the simulations produced Syt1 structures bound to four Ca(2+) ions, two ions chelated at the binding pocket of each domain. MD simulations of the Ca(2+)-bound form of Syt1 revealed that Syt1 conformational flexibility drastically increased upon Ca(2+) binding. In the presence of Ca(2+), the separation between domains increased, and interdomain rotations became more frequent. These findings suggest that Ca(2+) binding to Syt1 may induce major changes in the Syt1 conformational state, which in turn may initiate the fusion process.
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Evans CS, He Z, Bai H, Lou X, Jeggle P, Sutton RB, Edwardson JM, Chapman ER. Functional analysis of the interface between the tandem C2 domains of synaptotagmin-1. Mol Biol Cell 2016; 27:979-89. [PMID: 26792839 PMCID: PMC4791141 DOI: 10.1091/mbc.e15-07-0503] [Citation(s) in RCA: 18] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/15/2015] [Accepted: 01/12/2016] [Indexed: 11/20/2022] Open
Abstract
Synaptotagmin (syt)-1 is a Ca2+ sensor that triggers rapid synaptic vesicle exocytosis. Mutations that disrupt physical interactions between the tandem Ca2+-sensing modules (C2 domains) of syt-1 disrupt regulated membrane fusion in reconstituted fusion reactions and in neurons. Hence contacts between these domains are important for function. C2 domains are widespread motifs that often serve as Ca2+-binding modules; some proteins have more than one copy. An open issue is whether these domains, when duplicated within the same parent protein, interact with one another to regulate function. In the present study, we address the functional significance of interfacial residues between the tandem C2 domains of synaptotagmin (syt)-1, a Ca2+ sensor for neuronal exocytosis. Substitution of four residues, YHRD, at the domain interface, disrupted the interaction between the tandem C2 domains, altered the intrinsic affinity of syt-1 for Ca2+, and shifted the Ca2+ dependency for binding to membranes and driving membrane fusion in vitro. When expressed in syt-1 knockout neurons, the YHRD mutant yielded reductions in synaptic transmission, as compared with the wild-type protein. These results indicate that physical interactions between the tandem C2 domains of syt-1 contribute to excitation–secretion coupling.
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Affiliation(s)
- Chantell S Evans
- Howard Hughes Medical Institute, University of Wisconsin-Madison, Madison, WI 53705-2275 Department of Neuroscience, University of Wisconsin-Madison, Madison, WI 53705-2275 Molecular and Cellular Pharmacology Program, University of Wisconsin-Madison, Madison, WI 53705-2275
| | - Zixuan He
- Howard Hughes Medical Institute, University of Wisconsin-Madison, Madison, WI 53705-2275 Department of Neuroscience, University of Wisconsin-Madison, Madison, WI 53705-2275
| | - Hua Bai
- Howard Hughes Medical Institute, University of Wisconsin-Madison, Madison, WI 53705-2275 Department of Neuroscience, University of Wisconsin-Madison, Madison, WI 53705-2275
| | - Xiaochu Lou
- Howard Hughes Medical Institute, University of Wisconsin-Madison, Madison, WI 53705-2275 Department of Neuroscience, University of Wisconsin-Madison, Madison, WI 53705-2275
| | - Pia Jeggle
- Department of Pharmacology, University of Cambridge, Cambridge CB2 1PD, UK
| | - R Bryan Sutton
- Department of Cell Physiology and Molecular Biophysics, Texas Tech University Health Sciences Center, Lubbock, TX 79430
| | | | - Edwin R Chapman
- Howard Hughes Medical Institute, University of Wisconsin-Madison, Madison, WI 53705-2275 Department of Neuroscience, University of Wisconsin-Madison, Madison, WI 53705-2275 Molecular and Cellular Pharmacology Program, University of Wisconsin-Madison, Madison, WI 53705-2275
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