Synaptotagmin: Mechanisms of an electrostatic switch

Mechanistic Insights into Synaptotagmin-1 Mediated Membrane Fusion and Interactions

We present two innovative approaches to investigate the dynamics of membrane fusion and the strength of protein-membrane interactions. The first approach employs pore-spanning membranes (PSMs), which allow for the observation of protein-assisted fusion processes. The second approach utilizes colloidal probe microscopy with membrane-coated probes with reconstituted proteins. PSMs enable one to obtain detailed information about the fusion process with particular emphasis on fusion intermediates and fusion pore formation. We demonstrate the potential of the PSM system using SNARE-mediated fusion. Accompanied by colloidal probe microscopy, molecular information can be gathered on how full-length synaptotagmin-1 (syt-1) contributes to the fusion process. We propose that syt-1 engages with anionic bilayers, significantly modifying the adhesion between membranes. The introduction of Ca2+ transforms these interactions, shifting from a state of minimal interaction force between bilayers to one of pronounced strength. This syt-1 interaction facilitates fusion in the presence of Ca2+ with a significant reduction in the occurrence of stalled intermediate fusion states. Moreover, the presence of Ca2+ significantly accelerates the fusion process, an effect that is further amplified by the addition of multivalent anions such as ATP.

Vesicle docking and fusion pore modulation by the neuronal calcium sensor Synaptotagmin-1

Synaptotagmin-1 (Syt1) is a major calcium sensor for rapid neurotransmitter release in neurons and hormone release in many neuroendocrine cells. It possesses two tandem cytosolic C2 domains that bind calcium, negatively charged phospholipids, and the neuronal SNARE complex. Calcium binding to Syt1 triggers exocytosis, but how this occurs is not well understood. Syt1 has additional roles in docking dense-core vesicles (DCVs) and synaptic vesicles to the plasma membrane and in regulating fusion pore dynamics. Thus, Syt1 perturbations could affect release through vesicle docking, fusion triggering, fusion pore regulation, or a combination of these. Here, using a human neuroendocrine cell line, we show that neutralization of highly conserved polybasic patches in either C2 domain of Syt1 impairs both DCV docking and efficient release of serotonin from DCVs. Interestingly, the same mutations resulted in larger fusion pores and faster release of serotonin during individual fusion events. Thus, Syt1's roles in vesicle docking, fusion triggering, and fusion pore control may be functionally related.

Pre- and Post-Synaptic protein in the major depressive Disorder: From neurobiology to therapeutic targets

Major depressive disorder (MDD) has demonstrated its negative impact on various aspects of the lives of those affected. Although several therapies have been developed over the years, it remains a challenge for mental health professionals. Thus, understanding the pathophysiology of MDD is necessary to improve existing treatment options or seek new therapeutic alternatives. Clinical and preclinical studies in animal models of depression have shown the involvement of synaptic plasticity in both the development of MDD and the response to available drugs. However, synaptic plasticity involves a cascade of events, including the action of presynaptic proteins such as synaptophysin and synapsins and postsynaptic proteins such as postsynaptic density-95 (PSD-95). Additionally, several factors can negatively impact the process of spinogenesis/neurogenesis, which are related to many outcomes, including MDD. Thus, this narrative review aims to deepen the understanding of the involvement of synaptic formations and their components in the pathophysiology and treatment of MDD.

Vesicle docking and fusion pore modulation by the neuronal calcium sensor Synaptotagmin-1

Synaptotagmin-1 (Syt1) is a major calcium sensor for rapid neurotransmitter release in neurons and hormone release in many neuroendocrine cells. It possesses two tandem cytosolic C2 domains that bind calcium, negatively charged phospholipids, and the neuronal SNARE complex. Calcium binding to Syt1 triggers exocytosis, but how this occurs is not well understood. Syt1 has additional roles in docking dense core vesicles (DCV) and synaptic vesicles (SV) to the plasma membrane (PM) and in regulating fusion pore dynamics. Thus, Syt1 perturbations could affect release through vesicle docking, fusion triggering, fusion pore regulation, or a combination of these. Here, using a human neuroendocrine cell line, we show that neutralization of highly conserved polybasic patches in either C2 domain of Syt1 impairs both DCV docking and efficient release of serotonin from DCVs. Interestingly, the same mutations resulted in larger fusion pores and faster release of serotonin during individual fusion events. Thus, Syt1's roles in vesicle docking, fusion triggering, and fusion pore control may be functionally related.

Schisandrin A Alleviates Spatial Learning and Memory Impairment in Diabetic Rats by Inhibiting Inflammatory Response and Through Modulation of the PI3K/AKT Pathway

Clinical and epidemiological research shows that people with diabetes mellitus frequently experience diabetic cognitive impairment. Schisandrin A (SchA), one of the lignans found in the dried fruit of Schisandra chinensis, has a variety of pharmacological effects on immune system control, apoptosis suppression, anti-oxidation and anti-inflammation. The goal of the current investigation was to clarify the probable neuro-protective effects of SchA against streptozotocin-induced diabetes deficiencies of the spatial learning and memory in rats. The outcomes show that SchA therapy effectively improved impaired glucose tolerance, fasting blood glucose level and serum insulin level in diabetic rats. Additionally, in the Morris water maze test, diabetic rats showed deficits in spatial learning and memory that were ameliorated by SchA treatment. Moreover, giving diabetic rats SchA reduced damage to the hippocampus structure and increased the production of synaptic proteins. Further research revealed that SchA therapy reduced diabetic-induced hippocampus neuron damage and the generation of Aβ, as demonstrated by the upregulated phosphorylation levels of insulin signaling pathway connected proteins and by the decreased expression levels of inflammatory-related factors. Collectively, these results suggested that SchA could improve diabetes-related impairments in spatial learning and memory, presumably by reducing inflammatory responses and regulating the insulin signaling system.

Tirzepatide ameliorates spatial learning and memory impairment through modulation of aberrant insulin resistance and inflammation response in diabetic rats

Background: One of the typical symptoms of diabetes mellitus patients was memory impairment, which was followed by gradual cognitive deterioration and for which there is no efficient treatment. The anti-diabetic incretin hormones glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) were demonstrated to have highly neuroprotective benefits in animal models of AD. We wanted to find out how the GLP-1/GIP dual agonist tirzepatide affected diabetes's impairment of spatial learning memory. Methods: High fat diet and streptozotocin injection-induced diabetic rats were injected intraperitoneally with Tirzepatide (1.35 mg/kg) once a week. The protective effects were assessed using the Morris water maze test, immunofluorescence, and Western blot analysis. Golgi staining was adopted for quantified dendritic spines. Results: Tirzepatide significantly improved impaired glucose tolerance, fasting blood glucose level, and insulin level in diabetic rats. Then, tirzepatide dramatically alleviated spatial learning and memory impairment, inhibited Aβ accumulation, prevented structural damage, boosted the synthesis of synaptic proteins and increased dendritic spines formation in diabetic hippocampus. Furthermore, some aberrant changes in signal molecules concerning inflammation signaling pathways were normalized after tirzepatide treatment in diabetic rats. Finally, PI3K/Akt/GSK3β signaling pathway was restored by tirzepatide. Conclusion: Tirzepatide obviously exerts a protective effect against spatial learning and memory impairment, potentially through regulating abnormal insulin resistance and inflammatory responses.

Role of synaptotagmin 13 (SYT13) in promoting breast cancer and signaling pathways

PURPOSE

Breast cancer is one of the leading causes of tumor death worldwide in female, and the five-year overall survival of breast cancer patients remains poor. It is an urgent need to seek novel target for its treatment. Synaptotagmin 13 (SYT13) is a synaptic vesicle transporting protein that regulates the malignant phenotypes of various cancers. However, its role in breast cancer is still unclear. The current study aimed to investigate the effects of SYT13 on the progression of breast cancer.

METHODS

Twenty-five pairs of breast cancer tissues and non-tumor tissues were obtained to assess the expression of SYT13. We manually modified the expression of SYT13 in MCF-7 and MDA-MB-231 cells. CCK-8 assay, EdU staining, and cell cycle analysis were carried out to measure the proliferated ability of cells. Annexin V/PI and TUNEL assays were used to detect the apoptotic ability of cells. Wound healing and transwell assays were employed to evaluate the migrated and invasive ability of breast cancer cells.

RESULTS

The results revealed that the mRNA and protein levels of SYT13 were higher in breast cancer tissues and cell lines. Knockdown of SYT13 inhibited the cell proliferation and induced cell cycle arrest in G1 phase of MCF-7 cells by downregulating cyclin D1 and CDK4, as well as upregulating p21. The migration and invasion of MCF-7 cells were repressed by the loss of SYT13 via the gain of E-cadherin and the loss of vimentin. Overexpression of SYT13 in MDA-MB-231 cells led to the opposite effects. Silencing of SYT13 induced the apoptosis ability of MCF-7 cells by the upregulation of bax and the downregulation of bcl-2. Moreover, we found that SYT13 depletion suppressed the FAK/AKT signaling pathway. PF573228 (a FAK inhibitor) and MK2206 (an AKT inhibitor) reversed the SYT13 overexpression-induced promotion of proliferation, migration, and invasion of MDA-MB-231 cells.

CONCLUSION

The results indicated that SYT13 promoted the malignant phenotypes of breast cancer cells by the activation of FAK/AKT signaling pathway.

Genetic disorders of neurotransmitter release machinery

Synaptic neurotransmitter release is an evolutionarily conserved process that mediates rapid information transfer between neurons as well as several peripheral tissues. Release of neurotransmitters are ensured by successive events such as synaptic vesicle docking and priming that prepare synaptic vesicles for rapid fusion. These events are orchestrated by interaction of different presynaptic proteins and are regulated by presynaptic calcium. Recent studies have identified various mutations in different components of neurotransmitter release machinery resulting in aberrant neurotransmitter release, which underlie a wide spectrum of psychiatric and neurological symptoms. Here, we review how these genetic alterations in different components of the core neurotransmitter release machinery affect the information transfer between neurons and how aberrant synaptic release affects nervous system function.

Functional regulation of syntaxin-1: An underlying mechanism mediating exocytosis in neuroendocrine cells

The fusion of the secretory vesicle with the plasma membrane requires the assembly of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) protein complexes formed by synaptobrevin, syntaxin-1, and SNAP-25. Within the pathway leading to exocytosis, the transitions between the "open" and "closed" conformations of syntaxin-1 function as a switch for the fusion of vesicles with the plasma membranes; rapid assembly and disassembly of syntaxin-1 clusters on the plasma membrane provide docking and fusion sites for secretory vesicles in neuroendocrine cells; and the fully zippered trans-SNARE complex, which requires the orderly, rapid and accurate binding of syntaxin-1 to other SNARE proteins, play key roles in triggering fusion. All of these reactions that affect exocytosis under physiological conditions are tightly regulated by multiple factors. Here, we review the current evidence for the involvement of syntaxin-1 in the mechanism of neuroendocrine cell exocytosis, discuss the roles of multiple factors such as proteins, lipids, protein kinases, drugs, and toxins in SNARE complex-mediated membrane fusion, and present an overview of syntaxin-1 mutation-associated diseases with a view to developing novel mechanistic therapeutic targets for the treatment of neuroendocrine disorders.

Potential roles of synaptotagmin family members in cancers: Recent advances and prospects

With the continuous development of bioinformatics and public database, more and more genes that play a role in cancers have been discovered. Synaptotagmins (SYTs) are abundant, evolutionarily conserved integral membrane proteins composed of a short N-terminus, a variable linker domain, a single transmembrane domain, and two C2 domains, and they constitute a family of 17 isoforms. The synaptotagmin family members are known to regulate calcium-dependent membrane fusion events. Some SYTs play roles in hormone secretion or neurotransmitter release or both, and much evidence supports SYTs as Ca²⁺ sensors of exocytosis. Since 5 years ago, an increasing number of studies have found that SYTs also played important roles in the occurrence and development of lung cancer, gastric cancer, colon cancer, and other cancers. Down-regulation of SYTs inhibited cell proliferation, migration, and invasion of cancer cells, but promoted cell apoptosis. Growth of peritoneal nodules is inhibited and survival is prolonged in mice administrated with siSYTs intraperitoneally. Therefore, most studies have found SYTs serve as an oncogene after overexpression and may become potential prognostic biomarkers for multiple cancers. This article provides an overview of recent studies that focus on SYT family members’ roles in cancers and highlights the advances that have been achieved.

Synaptic Secretion and Beyond: Targeting Synapse and Neurotransmitters to Treat Neurodegenerative Diseases

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.

Revisiting the molecular basis of synaptic transmission

Measurements of the time elapsed during synaptic transmission has shown that synaptic vesicle (SV) fusion lags behind Ca²⁺-influx by approximately 60 microseconds (µsec). The conventional model cannot explain this extreme rapidity of the release event. Synaptic transmission occurs at the active zone (AZ), which comprises of two pools of SV, non-releasable "tethered" vesicles, and a readily-releasable pool of channel-associated Ca²⁺-primed vesicles, "RRP". A recent TIRF study at cerebellar-mossy fiber-terminal, showed that subsequent to an action potential, newly "tethered" vesicles, became fusion-competent in a Ca²⁺-dependent manner, 300-400 milliseconds after tethering, but were not fused. This time resolution may correspond to priming of tethered vesicles through Ca²⁺-binding to Syt1/Munc13-1/complexin. It confirms that Ca²⁺-priming and Ca²⁺-influx-independent fusion, are two distinct events. Notably, we have established that Ca²⁺ channel signals evoked-release in an ion flux-independent manner, demonstrated by Ca²⁺-impermeable channel, or a Ca²⁺ channel in which Ca²⁺ is replaced by impermeable La³⁺. Thus, conformational changes in a channel coupled to RRP appear to directly activate the release machinery and account for a µsec Ca²⁺-influx-independent vesicle fusion. Rapid vesicle fusion driven by non-ionotropic channel signaling strengthens a conformational-coupling mechanism of synaptic transmission, and contributes to better understanding of neuronal communication vital for brain function.

Molecular Mechanisms Underlying Neurotransmitter Release

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.

Expanding the genotype and phenotype spectrum of SYT1-associated neurodevelopmental disorder

PURPOSE

Synaptotagmin-1 (SYT1) is a critical mediator of neurotransmitter release in the central nervous system. Previously reported missense SYT1 variants in the C2B domain are associated with severe intellectual disability, movement disorders, behavioral disturbances, and electroencephalogram abnormalities. In this study, we expand the genotypes and phenotypes and identify discriminating features of this disorder.

METHODS

We describe 22 individuals with 15 de novo missense SYT1 variants. The evidence for pathogenicity is discussed, including the American College of Medical Genetics and Genomics/Association for Molecular Pathology criteria, known structure-function relationships, and molecular dynamics simulations. Quantitative behavioral data for 14 cases were compared with other monogenic neurodevelopmental disorders.

RESULTS

Four variants were located in the C2A domain with the remainder in the C2B domain. We classified 6 variants as pathogenic, 4 as likely pathogenic, and 5 as variants of uncertain significance. Prevalent clinical phenotypes included delayed developmental milestones, abnormal eye physiology, movement disorders, and sleep disturbances. Discriminating behavioral characteristics were severity of motor and communication impairment, presence of motor stereotypies, and mood instability.

CONCLUSION

Neurodevelopmental disorder-associated SYT1 variants extend beyond previously reported regions, and the phenotypic spectrum encompasses a broader range of severities than initially reported. This study guides the diagnosis and molecular understanding of this rare neurodevelopmental disorder and highlights a key role for SYT1 function in emotional regulation, motor control, and emergent cognitive function.

Expression and distribution of synaptotagmin family members in the zebrafish retina

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.

Dynamics of Neuromuscular Transmission Reproduced by Calcium-Dependent and Reversible Serial Transitions in the Vesicle Fusion Complex

Neuromuscular transmission, from spontaneous release to facilitation and depression, was accurately reproduced by a mechanistic kinetic model of sequential maturation transitions in the molecular fusion complex. The model incorporates three predictions. First, calcium-dependent forward transitions take vesicles from docked to preprimed to primed states, followed by fusion. Second, prepriming and priming are reversible. Third, fusion and recycling are unidirectional. The model was fed with experimental data from previous studies, whereas the backward (β) and recycling (ρ) rate constant values were fitted. Classical experiments were successfully reproduced with four transition states in the model when every forward (α) rate constant had the same value, and both backward rate constants were 50–100 times larger. Such disproportion originated an abruptly decreasing gradient of resting vesicles from docked to primed states. By contrast, a three-state version of the model failed to reproduce the dynamics of transmission by using the same set of parameters. Simulations predict the following: (1) Spontaneous release reflects primed to fusion spontaneous transitions. (2) Calcium elevations synchronize the series of forward transitions that lead to fusion. (3) Facilitation reflects a transient increase of priming following the calcium-dependent maturation transitions. (4) The calcium sensors that produce facilitation are those that evoke the transitions form docked to primed states. (5) Backward transitions and recycling restore the resting state. (6) Depression reflects backward transitions and slow recycling after intense release. Altogether, our results predict that fusion is produced by one calcium sensor, whereas the modulation of the number of vesicles that fuse depends on the calcium sensors that promote the early transition states. Such finely tuned kinetics offers a mechanism for collective non-linear transitional adaptations of a homogeneous vesicle pool to the ever-changing pattern of electrical activity in the neuromuscular junction.

Forces, Kinetics, and Fusion Efficiency Altered by the Full-Length Synaptotagmin-1 -PI(4,5)P2 Interaction in Constrained Geometries

A mechanism for full-length synaptotagmin-1 (syt-1) to interact with anionic bilayers and to promote fusion in the presence of SNAREs is proposed. Colloidal probe force spectroscopy in conjunction with tethered particle motion monitoring showed that in the absence of Ca²⁺ the binding of syt-1 to membranes depends on the presence and content of PI(4,5)P₂. Addition of Ca²⁺ switches the interaction forces from weak to strong, eventually exceeding the cohesion of the C2A domain of syt-1 leading to partial unfolding of the protein. Fusion of single unilamellar vesicles equipped with syt-1 and synaptobrevin 2 with planar pore-spanning target membranes containing PS and PI(4,5)P₂ shows an almost complete suppression of stalled intermediate fusion states and an accelerated fusion kinetics in the presence of Ca²⁺, which is further enhanced upon addition of ATP.

Involvement of Rare Mutations of SCN9A, DPP4, ABCA13, and SYT14 in Schizophrenia and Bipolar Disorder

Rare mutations associated with schizophrenia (SZ) and bipolar disorder (BD) usually have high clinical penetrance; however, they are highly heterogeneous and personalized. Identifying rare mutations is instrumental in making the molecular diagnosis, understanding the pathogenesis, and providing genetic counseling for the affected individuals and families. We conducted whole-genome sequencing analysis in two multiplex families with the dominant inheritance of SZ and BD. We detected a G327E mutation of SCN9A and an A654V mutation of DPP4 cosegregating with SZ and BD in one three-generation multiplex family. We also identified three mutations cosegregating with SZ and BD in another two-generation multiplex family, including L711S of SCN9A, M4554I of ABCA13, and P159L of SYT14. These five missense mutations were rare and deleterious. Mutations of SCN9A have initially been reported to cause congenital insensitivity to pain and neuropathic pain syndromes. Further studies showed that rare mutations of SCN9A were associated with seizure and autism spectrum disorders. Our findings suggest that SZ and BD might also be part of the clinical phenotype spectra of SCN9A mutations. Our study also indicates the oligogenic involvement in SZ and BD and supports the multiple-hit model of SZ and BD.

Effects and Mechanisms of Synaptotagmin-7 in the Hippocampus on Cognitive Impairment in Aging Mice

Aging is an irreversible biological process that involves oxidative stress, neuroinflammation, and apoptosis, and eventually leads to cognitive dysfunction. However, the underlying mechanisms are not fully understood. In this study, we investigated the role and potential mechanisms of Synaptotagmin-7, a calcium membrane transporter in cognitive impairment in aging mice. Our results indicated that Synaptotagmin-7 expression significantly decreased in the hippocampus of D-galactose-induced or naturally aging mice when compared with healthy controls, as detected by western blot and quantitative reverse transcriptase-polymerase chain reaction analysis. Synaptotagmin-7 overexpression in the dorsal CA1 of the hippocampus reversed long-term potentiation and improved hippocampus-dependent spatial learning in D-galactose-induced aging mice. Synaptotagmin-7 overexpression also led to fully preserved learning and memory in 6-month-old mice. Mechanistically, we demonstrated that Synaptotagmin-7 improved learning and memory by elevating the level of fEPSP and downregulating the expression of aging-related genes such as p53 and p16. The results of our study provide new insights into the role of Synaptotagmin-7 in improving neuronal function and overcoming memory impairment caused by aging, suggesting that Synaptotagmin-7 overexpression may be an innovative therapeutic strategy for treating cognitive impairment.

ATR regulates neuronal activity by modulating presynaptic firing

Ataxia Telangiectasia and Rad3-related (ATR) protein, as a key DNA damage response (DDR) regulator, plays an essential function in response to replication stress and controls cell viability. Hypomorphic mutations of ATR cause the human ATR-Seckel syndrome, characterized by microcephaly and intellectual disability, which however suggests a yet unknown role for ATR in non-dividing cells. Here we show that ATR deletion in postmitotic neurons does not compromise brain development and formation; rather it enhances intrinsic neuronal activity resulting in aberrant firing and an increased epileptiform activity, which increases the susceptibility of ataxia and epilepsy in mice. ATR deleted neurons exhibit hyper-excitability, associated with changes in action potential conformation and presynaptic vesicle accumulation, independent of DDR signaling. Mechanistically, ATR interacts with synaptotagmin 2 (SYT2) and, without ATR, SYT2 is highly upregulated and aberrantly translocated to excitatory neurons in the hippocampus, thereby conferring a hyper-excitability. This study identifies a physiological function of ATR, beyond its DDR role, in regulating neuronal activity.

Function of Drosophila Synaptotagmins in membrane trafficking at synapses

The Synaptotagmin (SYT) family of proteins play key roles in regulating membrane trafficking at neuronal synapses. Using both Ca²⁺-dependent and Ca²⁺-independent interactions, several SYT isoforms participate in synchronous and asynchronous fusion of synaptic vesicles (SVs) while preventing spontaneous release that occurs in the absence of stimulation. Changes in the function or abundance of the SYT1 and SYT7 isoforms alter the number and route by which SVs fuse at nerve terminals. Several SYT family members also regulate trafficking of other subcellular organelles at synapses, including dense core vesicles (DCV), exosomes, and postsynaptic vesicles. Although SYTs are linked to trafficking of multiple classes of synaptic membrane compartments, how and when they interact with lipids, the SNARE machinery and other release effectors are still being elucidated. Given mutations in the SYT family cause disorders in both the central and peripheral nervous system in humans, ongoing efforts are defining how these proteins regulate vesicle trafficking within distinct neuronal compartments. Here, we review the Drosophila SYT family and examine their role in synaptic communication. Studies in this invertebrate model have revealed key similarities and several differences with the predicted activity of their mammalian counterparts. In addition, we highlight the remaining areas of uncertainty in the field and describe outstanding questions on how the SYT family regulates membrane trafficking at nerve terminals.

Disorders of synaptic vesicle fusion machinery

The revolution in genetic technology has ushered in a new age for our understanding of the underlying causes of neurodevelopmental, neuromuscular and neurodegenerative disorders, revealing that the presynaptic machinery governing synaptic vesicle fusion is compromised in many of these neurological disorders. This builds upon decades of research showing that disturbance to neurotransmitter release via toxins can cause acute neurological dysfunction. In this review, we focus on disorders of synaptic vesicle fusion caused either by toxic insult to the presynapse or alterations to genes encoding the key proteins that control and regulate fusion: the SNARE proteins (synaptobrevin, syntaxin-1 and SNAP-25), Munc18, Munc13, synaptotagmin, complexin, CSPα, α-synuclein, PRRT2 and tomosyn. We discuss the roles of these proteins and the cellular and molecular mechanisms underpinning neurological deficits in these disorders.

Membrane Association and Functional Mechanism of Synaptotagmin-1 in Triggering Vesicle Fusion

Upon Ca²⁺ influx, synaptic vesicles fuse with the presynaptic plasma membrane (PM) to release neurotransmitters. Membrane fusion is triggered by synaptotagmin-1, a transmembrane protein in the vesicle membrane (VM), but the mechanism is under debate. Synaptotagmin-1 contains a single transmembrane helix (TM) and two tandem C2 domains (C2A and C2B). This study aimed to use molecular dynamics simulations to elucidate how Ca²⁺-bound synaptotagmin-1, by simultaneously associating with VM and PM, brings them together for fusion. Although C2A stably associates with VM via two Ca²⁺-binding loops, C2B has a propensity to partially dissociate. Importantly, an acidic motif in the TM-C2A linker competes with VM for interacting with C2B, thereby flipping its orientation to face PM. Subsequently, C2B readily associates with PM via a polybasic cluster and a Ca²⁺-binding loop. The resulting mechanistic model for the triggering of membrane fusion by synaptotagmin-1 reconciles many experimental observations.