Photolysis of a caged peptide reveals rapid action of N-ethylmaleimide sensitive factor before neurotransmitter release

The role of synaptic protein NSF in the development and progression of neurological diseases

This document provides a comprehensive examination of the pivotal function of the N-ethylmaleimide-sensitive factor (NSF) protein in synaptic function. The NSF protein directly participates in critical biological processes, including the cyclic movement of synaptic vesicles (SVs) between exocytosis and endocytosis, the release and transmission of neurotransmitters, and the development of synaptic plasticity through interactions with various proteins, such as SNARE proteins and neurotransmitter receptors. This review also described the multiple functions of NSF in intracellular membrane fusion events and its close associations with several neurological disorders, such as Parkinson's disease, Alzheimer's disease, and epilepsy. Subsequent studies should concentrate on determining high-resolution structures of NSF in different domains, identifying its specific alterations in various diseases, and screening small molecule regulators of NSF from multiple perspectives. These research endeavors aim to reveal new therapeutic targets associated with the biological functions of NSF and disease mechanisms.

Organization of a functional glycolytic metabolon on mitochondria for metabolic efficiency

Glucose, the primary cellular energy source, is metabolized through glycolysis initiated by the rate-limiting enzyme hexokinase (HK). In energy-demanding tissues like the brain, HK1 is the dominant isoform, primarily localized on mitochondria, and is crucial for efficient glycolysis-oxidative phosphorylation coupling and optimal energy generation. This study unveils a unique mechanism regulating HK1 activity, glycolysis and the dynamics of mitochondrial coupling, mediated by the metabolic sensor enzyme O-GlcNAc transferase (OGT). OGT catalyses reversible O-GlcNAcylation, a post-translational modification influenced by glucose flux. Elevated OGT activity induces dynamic O-GlcNAcylation of the regulatory domain of HK1, subsequently promoting the assembly of the glycolytic metabolon on the outer mitochondrial membrane. This modification enhances the mitochondrial association with HK1, orchestrating glycolytic and mitochondrial ATP production. Mutation in HK1's O-GlcNAcylation site reduces ATP generation in multiple cell types, specifically affecting metabolic efficiency in neurons. This study reveals a previously unappreciated pathway that links neuronal metabolism and mitochondrial function through OGT and the formation of the glycolytic metabolon, providing potential strategies for tackling metabolic and neurological disorders.

Transient Synaptic Enhancement Triggered by Exogenously Supplied Monocarboxylate in Drosophila Motoneuron Synapse

Current evidence suggests that glial cells provide C3 carbon sources to fuel neuronal activity; however, this notion has become challenged by biosensor studies carried out in acute brain slices or in vivo, showing that neuronal activity does not rely on the import of astrocyte-produced L-lactate. Rather, stimulated neurons become net lactate exporters, as it was also shown in Drosophila neurons, in which astrocyte-provided lactate returns as lipid droplets to be stored in glial cells. In this view, we investigate whether exogenously supplied monocarboxylates can support Drosophila motoneuron neurotransmitter release (NTR). By assessing the excitatory post-synaptic current (EPSC) amplitude under voltage-clamp as NTR indicative, we found that both pyruvate and L-lactate, as the only carbon sources in the synapses bathing-solution, cause a large transient NTR enhancement, which declines to reach a synaptic depression state, from which the synapses do not recover. The FM1-43 pre-synaptic loading ability, however, is maintained under monocarboxylate, suggesting that SV cycling should not contribute to the synaptic depression state. The NTR recovery was reached by supplementing the monocarboxylate medium with sucrose. However, monocarboxylate addition to sucrose medium does not enhance NTR, but it does when the disaccharide concentration becomes too reduced. Thus, when pyruvate concentrations become too reduced, exogenously supplied L-lactate could be converted to pyruvate and metabolized by the neural mitochondria, triggering the NTR enhancement. SIGNIFICANCE STATEMENT: The question of whether monocarboxylic acids can fuel the Drosophila motoneuron NTR was challenged. Our findings show that exogenously supplied monocarboxylates trigger a large transient synaptic enhancement just under extreme glycolysis reduction but fail to maintain NTR under sustained synaptic demand, still at low frequency stimulation, driven to the synapses to a synaptic depression state. Glycolysis activation, by adding sucrose to the monocarboxylate bath solution, restores the motoneuron NTR ability, giving place to a hexoses role in SV recruitment. Moreover these results suggest exogenously supplied C3 carbon sources could have an additional role beyond providing energetic support for neural activity.

Mitochondrial Mutations Can Alter Neuromuscular Transmission in Congenital Myasthenic Syndrome and Mitochondrial Disease

Congenital myasthenic syndromes (CMS) are a group of rare, neuromuscular disorders that usually present in childhood or infancy. While the phenotypic presentation of these disorders is diverse, the unifying feature is a pathomechanism that disrupts neuromuscular transmission. Recently, two mitochondrial genes-SLC25A1 and TEFM-have been reported in patients with suspected CMS, prompting a discussion about the role of mitochondria at the neuromuscular junction (NMJ). Mitochondrial disease and CMS can present with similar symptoms, and potentially one in four patients with mitochondrial myopathy exhibit NMJ defects. This review highlights research indicating the prominent roles of mitochondria at both the pre- and postsynapse, demonstrating the potential for mitochondrial involvement in neuromuscular transmission defects. We propose the establishment of a novel subcategorization for CMS-mitochondrial CMS, due to unifying clinical features and the potential for mitochondrial defects to impede transmission at the pre- and postsynapse. Finally, we highlight the potential of targeting the neuromuscular transmission in mitochondrial disease to improve patient outcomes.

A multivesicular body-like organelle mediates stimulus-regulated trafficking of olfactory ciliary transduction proteins

Stimulus transduction in cilia of olfactory sensory neurons is mediated by odorant receptors, Gαolf, adenylate cyclase-3, cyclic nucleotide-gated and chloride ion channels. Mechanisms regulating trafficking and localization of these proteins in the dendrite are unknown. By lectin/immunofluorescence staining and in vivo correlative light-electron microscopy (CLEM), we identify a retinitis pigmentosa-2 (RP2), ESCRT-0 and synaptophysin-containing multivesicular organelle that is not part of generic recycling/degradative/exosome pathways. The organelle's intraluminal vesicles contain the olfactory transduction proteins except for Golf subunits Gγ13 and Gβ1. Instead, Gβ1 colocalizes with RP2 on the organelle's outer membrane. The organelle accumulates in response to stimulus deprivation, while odor stimuli or adenylate cyclase activation cause outer membrane disintegration, release of intraluminal vesicles, and RP2/Gβ1 translocation to the base of olfactory cilia. Together, these findings reveal the existence of a dendritic organelle that mediates both stimulus-regulated storage of olfactory ciliary transduction proteins and membrane-delimited sorting important for G protein heterotrimerization.

Membrane Traffic and Ca2+ -Signals in Ciliates

A Paramecium cell has as many types of membrane interactions as mammalian cells, as established with monoclonal antibodies by R. Allen and A. Fok. Since then, we have identified key-players, such as SNARE-proteins, Ca²⁺ -regulating proteins, including Ca²⁺ -channels, Ca²⁺ -pumps, Ca²⁺ -binding proteins of different affinity etc. at the molecular level, probed their function and localized them at the light and electron microscopy level. SNARE-proteins, in conjunction with a synaptotagmin-like Ca²⁺ -sensor protein, mediate membrane fusion. This interaction is additionally regulated by monomeric GTPases whose spectrum in Tetrahymena and Paramecium has been established by A. Turkewitz. As known from mammalian cells, GTPases are activated on membranes in conjunction with lumenal acidification by an H⁺ -ATPase. For these complex molecules we found in Paramecium an unsurpassed number of 17 a-subunit paralogs which connect the polymeric head and basis part, V1 and V0. (This multitude may reflect different local functional requirements.) Together with plasmalemmal Ca²⁺ -influx-channels, locally enriched intracellular InsP₃ -type (InsP₃ R, mainly in osmoregulatory system) and ryanodine receptor-like Ca²⁺ -release channels (ryanodine receptor-like proteins, RyR-LP), this complexity mediates Ca²⁺ signals for most flexible local membrane-to-membrane interactions. As we found, the latter channel types miss a substantial portion of the N-terminal part. Caffeine and 4-chloro-meta-cresol (the agent used to probe mutations of RyRs in man during surgery in malignant insomnia patients) initiate trichocyst exocytosis by activating Ca²⁺ -release channels type CRC-IV in the peripheral part of alveolar sacs. This is superimposed by Ca²⁺ -influx, i.e. a mechanism called "store-operated Ca²⁺ -entry" (SOCE). For the majority of key players, we have mapped paralogs throughout the Paramecium cell, with features in common or at variance in the different organelles participating in vesicle trafficking. Local values of free Ca²⁺ -concentration, [Ca²⁺ ]_i , and their change, e.g. upon exocytosis stimulation, have been registered by flurochromes and chelator effects. In parallel we have registered release of Ca²⁺ from alveolar sacs by quenched-flow analysis combined with cryofixation and x-ray microanalysis.

Does GEC1 Enhance Expression and Forward Trafficking of the Kappa Opioid Receptor (KOR) via Its Ability to Interact with NSF Directly?

We reported previously that GEC1 (glandular epithelial cell 1), a member of microtubule-associated proteins (MAPs), interacted directly with the C-tail of KOR (KCT) and tubulin and enhanced cell surface expression of KOR in CHO cells by facilitating its trafficking along the export pathway. Two GEC1 analogs (GABARAP and GATE16) were also shown to increase KOR expression. In addition, to understand the underlying mechanism, we demonstrated that N-ethylmaleimide-sensitive factor (NSF), an essential component for membrane fusion, co-immunoprecipitated with GEC1 from brain extracts. In this study, using pull-down techniques, we have found that (1) GEC1 interacts with NSF directly and prefers the ADP-bound NSF to the ATP-bound NSF; (2) D1 and/or D2 domain(s) of NSF interact with GEC1, but the N domain of NSF does not; (3) NSF does not interact with KCT directly, but forms a protein complex with KCT via GEC1; (4) NSF and/or α-SNAP do not affect KCT-GEC1 interaction. Thus, GEC1 (vs the α-SNAP/SNAREs complex) binds to NSF in distinctive ways in terms of the ADP- or ATP-bound form and domains of NSF involved. In conclusion, GEC1 may, via its direct interactions with KOR, NSF, and tubulin, enhance trafficking and fusion of KOR-containing vesicles selectively along the export pathway, which leads to increase in surface expression of KOR. GABARAP and GATE16 may enhance KOR expression in a similar way.

Intersectin-Mediated Clearance of SNARE Complexes Is Required for Fast Neurotransmission

The rapid replenishment of release-ready synaptic vesicles (SVs) at a limiting number of presynaptic release sites is required to sustain high-frequency neurotransmission in CNS neurons. Failure to clear release sites from previously exocytosed material has been shown to impair vesicle replenishment and, therefore, fast neurotransmission. The identity of this material and the machinery that removes it from release sites have remained enigmatic. Here we show that the endocytic scaffold protein intersectin 1 clears release sites by direct SH3 domain-mediated association with a non-canonical proline-rich segment of synaptobrevin assembled into the SNARE complex for neuroexocytosis. Acute structure-based or sustained genetic interference with SNARE complex recognition by intersectin 1 causes a rapid stimulation frequency-dependent depression of neurotransmission due to impaired replenishment of release-ready SVs. These findings identify a key molecular mechanism that underlies exo-endocytic coupling during fast neurotransmitter release at central synapses.

Transendothelial transport of lipoproteins

The accumulation of low-density lipoproteins (LDL) in the arterial wall plays a pivotal role in the initiation and pathogenesis of atherosclerosis. Conversely, the removal of cholesterol from the intima by cholesterol efflux to high density lipoproteins (HDL) and subsequent reverse cholesterol transport shall confer protection against atherosclerosis. To reach the subendothelial space, both LDL and HDL must cross the intact endothelium. Traditionally, this transit is explained by passive filtration. This dogma has been challenged by the identification of several rate-limiting factors namely scavenger receptor SR-BI, activin like kinase 1, and caveolin-1 for LDL as well as SR-BI, ATP binding cassette transporter G1, and endothelial lipase for HDL. In addition, estradiol, vascular endothelial growth factor, interleukins 6 and 17, purinergic signals, and sphingosine-1-phosphate were found to regulate transendothelial transport of either LDL or HDL. Thorough understanding of transendothelial lipoprotein transport is expected to elucidate new therapeutic targets for the treatment or prevention of atherosclerotic cardiovascular disease and the development of strategies for the local delivery of drugs or diagnostic tracers into diseased tissues including atherosclerotic lesions.

Impaired Cognitive Function and Altered Hippocampal Synaptic Plasticity in Mice Lacking Dermatan Sulfotransferase Chst14/D4st1

Chondroitin sulfate (CS) and dermatan sulfate (DS) proteoglycans (PGs) are major extracellular matrix (ECM) components of the central nervous system (CNS). A large body of evidence has shown that CSPGs/DSPGs play critical roles in neuronal growth, axon guidance, and plasticity in the developing and mature CNS. It has been proposed that these PGs exert their function through specific interaction of CS/DS chains with its binding partners in a manner that depends on the sulfation patterns of CS/DS. It has been reported that dermatan 4-O-sulfotransferase-1 (Chst14/D4st1) specific for DS, but not chondroitin 4-O-sulfotransferase-1 (Chst11/C4st1) specific for CS, regulates proliferation and neurogenesis of neural stem cells (NSCs), indicating that CS and DS play distinct roles in the self-renewal and differentiation of NSCs. However, it remains unknown whether specific sulfation profiles of DS has any effect on CNS plasticity. In the present study, Chst14/D4st1-deficient (Chst14^−/−) mice was employed to investigate the involvement of DS in synaptic plasticity. First, behavior study using Morris Water Maze (MWM) showed that the spatial learning and memory of Chst14^−/− mice was impaired when compared to their wild type (WT) littermates. Corroborating the behavior result, long-term potentiation (LTP) at the hippocampal CA3-CA1 connection was reduced in Chst14^−/− mice compared to the WT mice. Finally, the protein levels of N-Methyl-D-aspartate (NMDA) receptor, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor, postsynaptic density 95 (PSD95), growth associated protein 43 (GAP-43), synaptophysin (SYN) and N-ethylmaleimide sensitive factor (NSF) which are important in synaptic plasticity were examined and Chst14/D4st1 deficiency was shown to significantly reduce the expression of these proteins in the hippocampus. Further studies revealed that Akt/mammalian target rapamycin (mTOR) pathway proteins, including protein kinase B (p-Akt), p-mTOR and p-S6, were significantly lower in Chst14^−/− mice, which might contribute to the decreased protein expression. Together, this study reveals that specific sulfation of DS is critical in synaptic plasticity of the hippocampus and learning and memory, which might be associated with the changes in the expression of glutamate receptors and other synaptic proteins though Akt/mTOR pathway.

Light-triggered release of photocaged therapeutics - Where are we now?

The current available therapeutics face several challenges such as the development of ideal drug delivery systems towards the goal of personalized treatments for patients benefit. The application of light as an exogenous activation mechanism has shown promising outcomes, owning to the spatiotemporal confinement of the treatment in the vicinity of the diseased tissue, which offers many intriguing possibilities. Engineering therapeutics with light responsive moieties have been explored to enhance the bioavailability, and drug efficacy either in vitro or in vivo. The tailor-made character turns the so-called photocaged compounds highly desirable to reduce the side effects of drugs and, therefore, have received wide research attention. Herein, we seek to highlight the potential of photocaged compounds to obtain a clear understanding of the mechanisms behind its use in therapeutic delivery. A deep overview on the progress achieved in the design, fabrication as well as current and possible future applications in therapeutics of photocaged compounds is provided, so that novel formulations for biomedical field can be designed.

Local protein dynamics during microvesicle exocytosis in neuroendocrine cells

Calcium-triggered exocytosis is key to many physiological processes, including neurotransmitter and hormone release by neurons and endocrine cells. Dozens of proteins regulate exocytosis, yet the temporal and spatial dynamics of these factors during vesicle fusion remain unclear. Here we use total internal reflection fluorescence microscopy to visualize local protein dynamics at single sites of exocytosis of small synaptic-like microvesicles in live cultured neuroendocrine PC12 cells. We employ two-color imaging to simultaneously observe membrane fusion (using vesicular acetylcholine ACh transporter tagged to pHluorin) and the dynamics of associated proteins at the moments surrounding exocytosis. Our experiments show that many proteins, including the SNAREs syntaxin1 and VAMP2, the SNARE modulator tomosyn, and Rab proteins, are preclustered at fusion sites and rapidly lost at fusion. The ATPase N-ethylmaleimide–sensitive factor is locally recruited at fusion. Interestingly, the endocytic Bin-Amphiphysin-Rvs domain–containing proteins amphiphysin1, syndapin2, and endophilins are dynamically recruited to fusion sites and slow the loss of vesicle membrane-bound cargo from fusion sites. A similar effect on vesicle membrane protein dynamics was seen with the overexpression of the GTPases dynamin1 and dynamin2. These results suggest that proteins involved in classical clathrin-mediated endocytosis can regulate exocytosis of synaptic-like microvesicles. Our findings provide insights into the dynamics, assembly, and mechanistic roles of many key factors of exocytosis and endocytosis at single sites of microvesicle fusion in live cells.

Munc13-1 and Munc18-1 together prevent NSF-dependent de-priming of synaptic vesicles

Synaptic transmission requires a stable pool of release-ready (primed) vesicles. Here we show that two molecules involved in SNARE-complex assembly, Munc13-1 and Munc18-1, together stabilize release-ready vesicles by preventing de-priming. Replacing neuronal Munc18-1 by a non-neuronal isoform Munc18-2 (Munc18-1/2SWAP) supports activity-dependent priming, but primed vesicles fall back into a non-releasable state (de-prime) within seconds. Munc13-1 deficiency produces a similar defect. Inhibitors of N-ethylmaleimide sensitive factor (NSF), N-ethylmaleimide (NEM) or interfering peptides, prevent de-priming in munc18-1/2SWAP or munc13-1 null synapses, but not in CAPS-1/2 null, another priming-deficient mutant. NEM rescues synaptic transmission in munc13-1 null and munc18-1/2SWAP synapses, in acute munc13-1 null slices and even partially in munc13-1/2 double null synapses. Together these data indicate that Munc13-1 and Munc18-1, but not CAPS-1/2, stabilize primed synaptic vesicles by preventing NSF-dependent de-priming.

The molecular mechanism underlying the generation and maintenance of the readily releasable pool composed of primed synaptic vesicles is only partially known. Here the authors show that in mouse primary neurons, Munc13-1 and Munc18-1 stabilize primed synaptic vesicles by preventing NSF-dependent de-priming.

Phenotype analysis of male transgenic mice overexpressing mutant IGFBP-2 lacking the Cardin-Weintraub sequence motif: Reduced expression of synaptic markers and myelin basic protein in the brain and a lower degree of anxiety-like behaviour

Brain growth and function are regulated by insulin-like growth factors I and II (IGF-I and IGF-II) but also by IGF-binding proteins (IGFBPs), including IGFBP-2. In addition to modulating IGF activities, IGFBP-2 interacts with a number of components of the extracellular matrix and cell membrane via a Cardin-Weintraub sequence or heparin binding domain (HBD1). The nature and the signalling elicited by these interactions are not fully understood. Here, we examined transgenic mice (H1d-hBP2) overexpressing a mutant human IGFBP-2 that lacks a specific heparin binding domain (HBD1) known as the Cardin-Weintraub sequence. H1d-hBP2 transgenic mice have the genetic background of FVB mice and are characterized by severe deficits in brain growth throughout their lifetime (p<0.05). In tissue lysates from brain hemispheres of 12-21day old male mice, protein levels of the GTPase dynamin-I were significantly reduced (p<0.01). Weight reductions were also found in distinct brain regions in two different age groups (12 and 80weeks). In the younger group, impaired weights were observed in the hippocampus (-34%; p<0.001), cerebellum (-25%; p<0.0001), olfactory bulb (-31%; p<0.05) and prefrontal cortex (-29%; p<0.05). At an age of 12weeks expression of myelin basic protein was reduced (p<0.01) in H1d-BP-2 mice in the cerebellum but not in the hippocampus. At 80weeks of age, weight reductions were similarly present in the cerebellum (-28%; p<0.001) and hippocampus (-31; p<0.05). When mice were challenged in the elevated plus maze, aged but not younger H1d-hBP2 mice displayed significantly less anxiety-like behaviour, which was also observed in a second transgenic mouse model overexpressing mouse IGFBP-2 lacking HBD1 (H1d-mBP2). These in vivo studies provide, for the first time, evidence for a specific role of IGFBP-2 in brain functions associated with anxiety and risk behaviour. These activities of IGFBP-2 could be mediated by the Cardin-Weintraub/HBD1 sequence and are altered in mice expressing IGFBP-2 lacking the HBD1.

Two-photon fluorescence lifetime imaging of primed SNARE complexes in presynaptic terminals and β cells

It remains unclear how readiness for Ca(2+)-dependent exocytosis depends on varying degrees of SNARE complex assembly. Here we directly investigate the SNARE assembly using two-photon fluorescence lifetime imaging (FLIM) of Förster resonance energy transfer (FRET) between three pairs of neuronal SNAREs in presynaptic boutons and pancreatic β cells in the islets of Langerhans. These FRET probes functionally rescue their endogenous counterparts, supporting ultrafast exocytosis. We show that trans-SNARE complexes accumulated in the active zone, and estimate the number of complexes associated with each docked vesicle. In contrast, SNAREs were unassembled in resting state, and assembled only shortly prior to insulin exocytosis, which proceeds slowly. We thus demonstrate that distinct states of fusion readiness are associated with SNARE complex formation. Our FRET/FLIM approaches enable optical imaging of fusion readiness in both live and chemically fixed tissues.

Cortical Granule Exocytosis Is Mediated by Alpha-SNAP and N-Ethilmaleimide Sensitive Factor in Mouse Oocytes

Cortical granule exocytosis (CGE), also known as cortical reaction, is a calcium- regulated secretion that represents a membrane fusion process during meiotic cell division of oocytes. The molecular mechanism of membrane fusion during CGE is still poorly understood and is thought to be mediated by the SNARE pathway; nevertheless, it is unkown if SNAP (acronym for soluble NSF attachment protein) and NSF (acronym for N-ethilmaleimide sensitive factor), two key proteins in the SNARE pathway, mediate CGE in any oocyte model. In this paper, we documented the gene expression of α-SNAP, γ-SNAP and NSF in mouse oocytes. Western blot analysis showed that the expression of these proteins maintains a similar level during oocyte maturation and early activation. Their localization was mainly observed at the cortical region of metaphase II oocytes, which is enriched in cortical granules. To evaluate the function of these proteins in CGE we set up a functional assay based on the quantification of cortical granules metaphase II oocytes activated parthenogenetically with strontium. Endogenous α-SNAP and NSF proteins were perturbed by microinjection of recombinant proteins or antibodies prior to CGE activation. The microinjection of wild type α-SNAP and the negative mutant of α-SNAP L294A in metaphase II oocytes inhibited CGE stimulated by strontium. NEM, an irreversibly inhibitor of NSF, and the microinjection of the negative mutant NSF D1EQ inhibited cortical reaction. The microinjection of anti-α-SNAP and anti-NSF antibodies was able to abolish CGE in activated metaphase II oocytes. The microinjection of anti-γ SNAP antibody had no effect on CGE. Our findings indicate, for the first time in any oocyte model, that α-SNAP, γ-SNAP, and NSF are expressed in mouse oocytes. We demonstrate that α-SNAP and NSF have an active role in CGE and propose a working model.

The role of mitochondrially derived ATP in synaptic vesicle recycling

Synaptic mitochondria are thought to be critical in supporting neuronal energy requirements at the synapse, and bioenergetic failure at the synapse may impair neural transmission and contribute to neurodegeneration. However, little is known about the energy requirements of synaptic vesicle release or whether these energy requirements go unmet in disease, primarily due to a lack of appropriate tools and sensitive assays. To determine the dependence of synaptic vesicle cycling on mitochondrially derived ATP levels, we developed two complementary assays sensitive to mitochondrially derived ATP in individual, living hippocampal boutons. The first is a functional assay for mitochondrially derived ATP that uses the extent of synaptic vesicle cycling as a surrogate for ATP level. The second uses ATP FRET sensors to directly measure ATP at the synapse. Using these assays, we show that endocytosis has high ATP requirements and that vesicle reacidification and exocytosis require comparatively little energy. We then show that to meet these energy needs, mitochondrially derived ATP is rapidly dispersed in axons, thereby maintaining near normal levels of ATP even in boutons lacking mitochondria. As a result, the capacity for synaptic vesicle cycling is similar in boutons without mitochondria as in those with mitochondria. Finally, we show that loss of a key respiratory subunit implicated in Leigh disease markedly decreases mitochondrially derived ATP levels in axons, thus inhibiting synaptic vesicle cycling. This proves that mitochondria-based energy failure can occur and be detected in individual neurons that have a genetic mitochondrial defect.

Circadian systems biology in Metazoa

Systems biology, which can be defined as integrative biology, comprises multistage processes that can be used to understand components of complex biological systems of living organisms and provides hierarchical information to decoding life. Using systems biology approaches such as genomics, transcriptomics and proteomics, it is now possible to delineate more complicated interactions between circadian control systems and diseases. The circadian rhythm is a multiscale phenomenon existing within the body that influences numerous physiological activities such as changes in gene expression, protein turnover, metabolism and human behavior. In this review, we describe the relationships between the circadian control system and its related genes or proteins, and circadian rhythm disorders in systems biology studies. To maintain and modulate circadian oscillation, cells possess elaborative feedback loops composed of circadian core proteins that regulate the expression of other genes through their transcriptional activities. The disruption of these rhythms has been reported to be associated with diseases such as arrhythmia, obesity, insulin resistance, carcinogenesis and disruptions in natural oscillations in the control of cell growth. This review demonstrates that lifestyle is considered as a fundamental factor that modifies circadian rhythm, and the development of dysfunctions and diseases could be regulated by an underlying expression network with multiple circadian-associated signals.

Family based genome-wide copy number scan identifies complex rearrangements at 17q21.31 in dyslexics

Developmental dyslexia (DD) is a complex heritable disorder with unexpected difficulty in learning to read and spell despite adequate intelligence, education, environment, and normal senses. We performed genome-wide screening for copy number variations (CNVs) in 10 large Indian dyslexic families using Affymetrix Genome-Wide Human SNP Array 6.0. Results revealed the complex genomic rearrangements due to one non-contiguous deletion and five contiguous micro duplications and micro deletions at 17q21.31 region in three dyslexic families. CNVs in this region harbor the genes KIAA1267, LRRC37A, ARL17A/B, NSFP1, and NSF. The CNVs in case 1 and case 2 at this locus were found to be in homozygous state and case 3 was a de novo CNV. These CNVs were found with at least one CNV having a common break and end points in the parents. This cluster of genes containing NSF is implicated in learning, cognition, and memory, though not formally associated with dyslexia. Molecular network analysis of these and other dyslexia related module genes suggests NSF and other genes to be associated with cellular/vesicular membrane fusion and synaptic transmission. Thus, we suggest that NSF in this cluster would be the nearest gene responsible for the learning disability phenotype.

Distinct Initial SNARE Configurations Underlying the Diversity of Exocytosis

The dynamics of exocytosis are diverse and have been optimized for the functions of synapses and a wide variety of cell types. For example, the kinetics of exocytosis varies by more than five orders of magnitude between ultrafast exocytosis in synaptic vesicles and slow exocytosis in large dense-core vesicles. However, in all cases, exocytosis is mediated by the same fundamental mechanism, i.e., the assembly of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins. It is often assumed that vesicles need to be docked at the plasma membrane and SNARE proteins must be preassembled before exocytosis is triggered. However, this model cannot account for the dynamics of exocytosis recently reported in synapses and other cells. For example, vesicles undergo exocytosis without prestimulus docking during tonic exocytosis of synaptic vesicles in the active zone. In addition, epithelial and hematopoietic cells utilize cAMP and kinases to trigger slow exocytosis of nondocked vesicles. In this review, we summarize the manner in which the diversity of exocytosis reflects the initial configurations of SNARE assembly, including trans-SNARE, binary-SNARE, unitary-SNARE, and cis-SNARE configurations. The initial SNARE configurations depend on the particular SNARE subtype (syntaxin, SNAP25, or VAMP), priming proteins (Munc18, Munc13, CAPS, complexin, or snapin), triggering proteins (synaptotagmins, Doc2, and various protein kinases), and the submembraneous cytomatrix, and they are the key to determining the kinetics of subsequent exocytosis. These distinct initial configurations will help us clarify the common SNARE assembly processes underlying exocytosis and membrane trafficking in eukaryotic cells.

Light-controlled tools

Spatial and temporal control over chemical and biological processes plays a key role in life, where the whole is often much more than the sum of its parts. Quite trivially, the molecules of a cell do not form a living system if they are only arranged in a random fashion. If we want to understand these relationships and especially the problems arising from malfunction, tools are necessary that allow us to design sophisticated experiments that address these questions. Highly valuable in this respect are external triggers that enable us to precisely determine where, when, and to what extent a process is started or stopped. Light is an ideal external trigger: It is highly selective and if applied correctly also harmless. It can be generated and manipulated with well-established techniques, and many ways exist to apply light to living systems--from cells to higher organisms. This Review will focus on developments over the last six years and includes discussions on the underlying technologies as well as their applications.

Requirements for the catalytic cycle of the N-ethylmaleimide-Sensitive Factor (NSF)

The N-ethylmaleimide-Sensitive Factor (NSF) was one of the initial members of the ATPases Associated with various cellular Activities Plus (AAA(+)) family. In this review, we discuss what is known about the mechanism of NSF action and how that relates to the mechanisms of other AAA(+) proteins. Like other family members, NSF binds to a protein complex (i.e., SNAP-SNARE complex) and utilizes ATP hydrolysis to affect the conformations of that complex. SNAP-SNARE complex disassembly is essential for SNARE recycling and sustained membrane trafficking. NSF is a homo-hexamer; each protomer is composed of an N-terminal domain, NSF-N, and two adjacent AAA-domains, NSF-D1 and NSF-D2. Mutagenesis analysis has established specific roles for many of the structural elements of NSF-D1, the catalytic ATPase domain, and NSF-N, the SNAP-SNARE binding domain. Hydrodynamic analysis of NSF, labeled with (Ni(2+)-NTA)(2)-Cy3, detected conformational differences in NSF, in which the ATP-bound conformation appears more compact than the ADP-bound form. This indicates that NSF undergoes significant conformational changes as it progresses through its ATP-hydrolysis cycle. Incorporating these data, we propose a sequential mechanism by which NSF uses NSF-N and NSF-D1 to disassemble SNAP-SNARE complexes. We also illustrate how analytical centrifugation might be used to study other AAA(+) proteins.

Vesicle priming in a SNAP

In this issue of Neuron, Burgalossi et al. investigate synaptic vesicle priming by using presynaptic Ca(2+) uncaging at a small, glutamatergic, central synapse. Combining this technique with mouse genetics, the authors demonstrate that vesicle priming during ongoing neural activity can be limited by the recycling of recently used SNARE complexes.

SNARE protein recycling by αSNAP and βSNAP supports synaptic vesicle priming

Neurotransmitter release proceeds by Ca(2+)-triggered, SNARE-complex-dependent synaptic vesicle fusion. After fusion, the ATPase NSF and its cofactors α- and βSNAP disassemble SNARE complexes, thereby recycling individual SNAREs for subsequent fusion reactions. We examined the effects of genetic perturbation of α- and βSNAP expression on synaptic vesicle exocytosis, employing a new Ca(2+) uncaging protocol to study synaptic vesicle trafficking, priming, and fusion in small glutamatergic synapses of hippocampal neurons. By characterizing this protocol, we show that synchronous and asynchronous transmitter release involve different Ca(2+) sensors and are not caused by distinct releasable vesicle pools, and that tonic transmitter release is due to ongoing priming and fusion of new synaptic vesicles during high synaptic activity. Our analysis of α- and βSNAP deletion mutant neurons shows that the two NSF cofactors support synaptic vesicle priming by determining the availability of free SNARE components, particularly during phases of high synaptic activity.

Identification of phosphoproteins associated with human neutrophil granules following chemotactic peptide stimulation

Regulated exocytosis of neutrophil intracellular storage granules is necessary for neutrophil participation in the inflammatory response. The signal transduction pathways that participate in neutrophil exocytosis are complex and poorly defined. Several protein kinases, including p38 MAPK and the nonreceptor tyrosine kinases, Hck and Fgr, participate in this response. However, the downstream targets of these kinases that regulate exocytosis are unknown. The present study combined a novel inhibitor of neutrophil exocytosis with proteomic techniques to identify phosphopeptides and phosphoproteins from a population of gelatinase and specific granules isolated from unstimulated and fMLF-stimulated neutrophils. To prevent loss of granule-associated phosphoproteins upon exocytosis, neutrophils were pretreated with a TAT-fusion protein containing a SNARE domain from SNAP-23 (TAT-SNAP-23), which inhibited fMLF-stimulated CD66b-containing granule exocytosis by 100±10%. Following TAT-SNAP-23 pretreatment, neutrophils were stimulated with the chemotactic peptide fMLF for 0 min, 1 min, and 2 min. Granules were isolated by gradient centrifugation and subjected to proteolytic digestion with trypsin or chymotrypsin to obtain peptides from the outer surface of the granule. Phosphopeptides were enriched by gallium or TiO2 affinity chromatography, and phosphopeptides and phosphorylation sites were identified by reversed phase high performance liquid chromatography-electrospray ionization-tandem MS. This resulted in the identification of 243 unique phosphopeptides corresponding to 235 proteins, including known regulators of vesicle trafficking. The analysis identified 79 phosphoproteins from resting neutrophils, 81 following 1 min of fMLF stimulation, and 118 following 2 min of stimulation. Bioinformatic analysis identified a potential Src tyrosine kinase motif from a phosphopeptide corresponding to G protein coupled receptor kinase 5 (GRK5). Phosphorylation of GRK5 by Src was confirmed by an in vitro kinase reaction and by precursor ion scanning for phospho-tyrosine specific immonium ions containing Tyr251 and Tyr253. Immunoprecipitation of phosphorylated GRK5 from intact cells was reduced by a Src inhibitor. In conclusion, targets of signal transduction pathways were identified that are candidates to regulate neutrophil granule exocytosis.

SNARE conformational changes that prepare vesicles for exocytosis

When cells release hormones and neurotransmitters through exocytosis, cytosolic Ca(2+) triggers the fusion of secretory vesicles with the plasma membrane. It is well known that this fusion requires assembly of a SNARE protein complex. However, the timing of SNARE assembly relative to vesicle fusion--essential for understanding exocytosis--has not been demonstrated. To investigate this timing, we constructed a probe that detects the assembly of two plasma membrane SNAREs, SNAP25 and syntaxin-1A, through fluorescence resonance energy transfer (FRET). With two-photon imaging, we simultaneously measured FRET signals and insulin exocytosis in beta cells from the pancreatic islet of Langerhans. In some regions of the cell, we found that the SNARE complex was preassembled, which enabled rapid exocytosis. In other regions, SNARE assembly followed Ca(2+) influx, and exocytosis was slower. Thus, SNARE proteins exist in multiple stable preparatory configurations, from which Ca(2+) may trigger exocytosis through distinct mechanisms and with distinct kinetics.

Proteomic analysis reveals the role of synaptic vesicle cycling in sustaining the suprachiasmatic circadian clock

The central circadian pacemaker of the suprachiasmatic nucleus (SCN) is characterized as a series of transcriptional/posttranslational feedback loops. How this molecular mechanism coordinates daily rhythms in the SCN and hence the organism is poorly understood. We conducted the first systematic exploration of the "circadian intracellular proteome" of the SCN and revealed that approximately 13% of soluble proteins are subject to circadian regulation. Many of these proteins have underlying nonrhythmic mRNAs, so they have not previously been noted as circadian. Circadian proteins of the SCN include rate-limiting factors in metabolism, protein trafficking, and, intriguingly, synaptic vesicle recycling. We investigated the role of this clock-regulated pathway by treating organotypic cultures of SCN with botulinum toxin A or dynasore to block exocytosis and endocytosis. These manipulations of synaptic vesicle recycling compromised circadian gene expression, both across the SCN as a circuit and within individual SCN neurons. These findings reveal how basic cellular processes within the SCN are subject to circadian regulation and how disruption of these processes interferes with SCN cellular pacemaking. Specifically, we highlight synaptic vesicle cycling as a novel point of clock cell regulation in mammals.

New photochemical tools for controlling neuronal activity

Neurobiology has entered a new era in which optical methods are challenging electrophysiological techniques for their value in measuring and manipulating neuronal activity. This change is occurring largely because of the development of new photochemical tools, some synthesized by chemists and some provided by nature. This review is focused on the three types of photochemical tools for neuronal control that have emerged in recent years. Caged neurotransmitters, including caged glutamate, are synthetic molecules that enable highly localized activation of neurotransmitter receptors in response to light. Natural photosensitive proteins, including channelrhodopsin-2 and halorhodopsin, can be exogenously expressed in neurons and enable rapid photocontrol of action potential firing. Synthetic small molecule photoswitches can bestow light-sensitivity on native or exogenously expressed proteins, including K(+) channels and glutamate receptors, allowing photocontrol of action potential firing and synaptic events. At a rapid pace, these tools are being improved and new tools are being introduced, thanks to molecular biology and synthetic chemistry. The three families of photochemical tools have different capabilities and uses, but they all share in enabling precise and noninvasive exploration of neural function with light.

Molecular mechanisms determining conserved properties of short-term synaptic depression revealed in NSF and SNAP-25 conditional mutants

Current models of synaptic vesicle trafficking implicate a core complex of proteins comprised of N-ethylmaleimide-sensitive factor (NSF), soluble NSF attachment proteins (SNAPs), and SNAREs in synaptic vesicle fusion and neurotransmitter release. Despite this progress, major challenges remain in establishing the in vivo functions of these proteins and their roles in determining the physiological properties of synapses. The present study employs glutamatergic adult neuromuscular synapses of Drosophila, which exhibit conserved properties of short-term synaptic plasticity with respect to mammalian glutamatergic synapses, to address these issues through genetic analysis. Our findings establish an in vivo role for SNAP-25 in synaptic vesicle priming, and support a zippering model of SNARE function in this process. Moreover, these studies define the contribution of SNAP-25-dependent vesicle priming to the detailed properties of short-term depression elicited by paired-pulse (PP) and train stimulation. In contrast, NSF is shown here not to be required for WT PP depression, but to be critical for maintaining neurotransmitter release during sustained stimulation. In keeping with this role, disruption of NSF function results in activity-dependent redistribution of the t-SNARE proteins, SYNTAXIN and SNAP-25, away from neurotransmitter release sites (active zones). These findings support a role for NSF in replenishing active zone t-SNAREs for subsequent vesicle priming, and provide new insight into the spatial organization of SNARE protein cycling during synaptic activity. Together, the results reported here establish in vivo contributions of SNAP-25 and NSF to synaptic vesicle trafficking and define molecular mechanisms determining conserved functional properties of short-term depression.

Illuminating the chemistry of life: design, synthesis, and applications of "caged" and related photoresponsive compounds

Biological systems are characterized by a level of spatial and temporal organization that often lies beyond the grasp of present day methods. Light-modulated bioreagents, including analogs of low molecular weight compounds, peptides, proteins, and nucleic acids, represent a compelling strategy to probe, perturb, or sample biological phenomena with the requisite control to address many of these organizational complexities. Although this technology has created considerable excitement in the chemical community, its application to biological questions has been relatively limited. We describe the challenges associated with the design, synthesis, and use of light-responsive bioreagents; the scope and limitations associated with the instrumentation required for their application; and recent chemical and biological advances in this field.

Radiolytic one-electron reduction characteristics of tyrosine derivative caged by 2-oxopropyl group

We employed X-irradiation to activate a caged amino acid with a 2-oxoalkyl group. We designed and synthesized tyrosine derivative caged by a 2-oxoalkyl group (Tyr(Oxo)) to evaluate its radiolytic one-electron reduction characteristics in aqueous solution. Upon hypoxic X-irradiation, Tyr(Oxo) released a 2-oxopropyl group to form the corresponding uncaged tyrosine. In addition, radiolysis of dipeptides containing Tyr(Oxo) revealed that the efficiency of radiolytic removal of 2-oxopropyl group increased significantly by the presence of neighboring aromatic amino acids.

Synaptic vesicle endocytosis: fast and slow modes of membrane retrieval

Several modes of synaptic vesicle release, retrieval and recycling have been identified. In a well-established mode of exocytosis, termed 'full-collapse fusion', vesicles empty their neurotransmitter content fully into the synaptic cleft by flattening out and becoming part of the presynaptic membrane. The fused vesicle membrane is then reinternalized via a slow and clathrin-dependent mode of compensatory endocytosis that takes several seconds. A more fleeting mode of vesicle fusion, termed 'kiss-and-run' exocytosis or 'flicker-fusion', indicates that during synaptic transmission some vesicles are only briefly connected to the presynaptic membrane by a transient fusion pore. Finally, a mode that retrieves a large amount of membrane, equivalent to that of several fused vesicles, termed 'bulk endocytosis', has been found after prolonged exocytosis. We are of the opinion that both fast and slow modes of endocytosis co-exist at central nervous system nerve terminals and that one mode can predominate depending on stimulus strength, temperature and synaptic maturation.