Exceptionally tight membrane-binding may explain the key role of the synaptotagmin-7 C2A domain in asynchronous neurotransmitter release

G-Quadruplex-Mediated Transcriptional Regulation of SYT7: Implications for Tumor Progression and Therapeutic Strategies

Synaptotagmin 7 (SYT7), a member of the synaptotagmin family, exhibits high expression in various tumors and is closely associated with patient prognosis. The tight regulation of SYT7 expression assumes paramount significance in the progression of tumorigenesis. In this study, we detected a high GC content in the first 1000 bp of the promoter region of SYT7, suggesting a potential role of the G-quadruplex in its transcriptional regulation. Circular dichroism spectroscopy results showed that -187 to -172 bp sequence can form a typical parallel G-quadruplex structure, and site mutation revealed the critical role of the ninth guanine in its formation. Then, treatment of two ligands of G-quadruplex (TMPyP4 and Pyridostatin) reduced both the expression of SYT7 and subsequent tumor proliferation, demonstrating the potential of the G-quadruplex as a targeted therapy for tumors. By shedding light on the pivotal role of the G-quadruplex in regulating SYT7 transcription, our study not only advances our comprehension of this intricate regulatory mechanism but also emphasizes the significance of SYT7 in tumor proliferation. These findings collectively contribute to a more comprehensive understanding of the interplay between G-quadruplex regulation and SYT7 function in tumor development.

A minimal presynaptic protein machinery mediating synchronous and asynchronous exocytosis and short-term plasticity

Neurotransmitters are released from synaptic vesicles with remarkable precision in response to presynaptic Ca2+ influx but exhibit significant heterogeneity in exocytosis timing and efficacy based on the recent history of activity. This heterogeneity is critical for information transfer in the brain, yet its molecular basis remains poorly understood. Here, we employ a biochemically-defined fusion assay under physiologically-relevant conditions to delineate the minimal protein machinery sufficient to account for different modes of Ca2+-triggered vesicle fusion and short-term facilitation. We find that Synaptotagmin-1, Synaptotagmin-7, and Complexin, synergistically restrain SNARE complex assembly, thus preserving vesicles in a stably docked state at rest. Upon Ca2+ activation, Synaptotagmin-1 induces rapid vesicle fusion, while Synaptotagmin-7 mediates delayed fusion. Competitive binding of Synaptotagmin-1 and Synaptotagmin-7 to the same SNAREs, coupled with differential rates of Ca2+-triggered fusion clamp reversal, govern the kinetics of vesicular fusion. Under conditions mimicking sustained neuronal activity, the Synaptotagmin-7 fusion clamp is destabilized by the elevated basal Ca2+ concentration, thereby enhancing the synchronous component of fusion. These findings provide a direct demonstration that a small set of proteins is sufficient to account for how nerve terminals adapt and regulate the Ca2+-evoked neurotransmitter exocytosis process to support their specialized functions in the nervous system.

Complexin has a dual synaptic function as checkpoint protein in vesicle priming and as a promoter of vesicle fusion

The presynaptic SNARE-complex regulator complexin (Cplx) enhances the fusogenicity of primed synaptic vesicles (SVs). Consequently, Cplx deletion impairs action potential-evoked transmitter release. Conversely, though, Cplx loss enhances spontaneous and delayed asynchronous release at certain synapse types. Using electrophysiology and kinetic modeling, we show that such seemingly contradictory transmitter release phenotypes seen upon Cplx deletion can be explained by an additional of Cplx in the control of SV priming, where its ablation facilitates the generation of a "faulty" SV fusion apparatus. Supporting this notion, a sequential two-step priming scheme, featuring reduced vesicle fusogenicity and increased transition rates into the faulty primed state, reproduces all aberrations of transmitter release modes and short-term synaptic plasticity seen upon Cplx loss. Accordingly, we propose a dual presynaptic function for the SNARE-complex interactor Cplx, one as a "checkpoint" protein that guarantees the proper assembly of the fusion machinery during vesicle priming, and one in boosting vesicle fusogenicity.

Mechanism and cellular function of direct membrane binding by the ESCRT and ERES-associated Ca2+-sensor ALG-2

Apoptosis linked Gene-2 (ALG-2) is a multifunctional intracellular Ca2+ sensor and the archetypal member of the penta-EF hand protein family. ALG-2 functions in the repair of damage to both the plasma and lysosome membranes and in COPII-dependent budding at endoplasmic reticulum exit sites (ERES). In the presence of Ca2+, ALG-2 binds to ESCRT-I and ALIX in membrane repair and to SEC31A at ERES. ALG-2 also binds directly to acidic membranes in the presence of Ca2+ by a combination of electrostatic and hydrophobic interactions. By combining giant unilamellar vesicle-based experiments and molecular dynamics simulations, we show that charge-reversed mutants of ALG-2 at these locations disrupt membrane recruitment. ALG-2 membrane binding mutants have reduced or abrogated ERES localization in response to Thapsigargin-induced Ca2+ release but still localize to lysosomes following lysosomal Ca2+ release. In vitro reconstitution shows that the ALG-2 membrane-binding defect can be rescued by binding to ESCRT-I. These data thus reveal the nature of direct Ca2+-dependent membrane binding and its interplay with Ca2+-dependent protein binding in the cellular functions of ALG-2.

Synaptotagmin 7 C2 domains induce membrane curvature stress via electrostatic interactions and the wedge mechanism

Synaptotagmin 7 (Syt-7) is part of the synaptotagmin protein family that regulates exocytotic lipid membrane fusion. Among the family, Syt-7 stands out by its membrane binding strength and stabilization of long-lived membrane fusion pores. Given that Syt-7 vesicles form long-lived fusion pores, we hypothesize that its interactions with the membrane stabilize the specific curvatures, thicknesses, and lipid compositions that support a metastable fusion pore. Using all-atom molecular dynamics simulations and FRET-based assays of Syt-7's membrane-binding C2 domains (C2A and C2B), we found that Syt-7 C2 domains sequester anionic lipids, are sensitive to cholesterol, thin membranes, and generate lipid membrane curvature by two competing, but related mechanisms. First, Syt-7 forms strong electrostatic contacts with the membrane, generating negative curvature stress. Second, Syt-7's calcium binding loops embed in the membrane surface, acting as a wedge to thin the membrane and induce positive curvature stress. These curvature mechanisms are linked by the protein insertion depth as well as the resulting protein tilt. Simplified quantitative models of the curvature-generating mechanisms link simulation observables to their membrane-reshaping effectiveness.

The release of inhibition model reproduces kinetics and plasticity of neurotransmitter release in central synapses

Calcium-evoked release of neurotransmitters from synaptic vesicles (SVs) is catalysed by SNARE proteins. The predominant view is that, at rest, complete assembly of SNARE complexes is inhibited ('clamped') by synaptotagmin and complexin molecules. Calcium binding by synaptotagmins releases this fusion clamp and triggers fast SV exocytosis. However, this model has not been quantitatively tested over physiological timescales. Here we describe an experimentally constrained computational modelling framework to quantitatively assess how the molecular architecture of the fusion clamp affects SV exocytosis. Our results argue that the 'release-of-inhibition' model can indeed account for fast calcium-activated SV fusion, and that dual binding of synaptotagmin-1 and synaptotagmin-7 to the same SNARE complex enables synergistic regulation of the kinetics and plasticity of neurotransmitter release. The developed framework provides a powerful and adaptable tool to link the molecular biochemistry of presynaptic proteins to physiological data and efficiently test the plausibility of calcium-activated neurotransmitter release models.

Mechanism and cellular function of direct membrane binding by the ESCRT and ERES-associated Ca2+-sensor ALG-2

Apoptosis Linked Gene-2 (ALG-2) is a multifunctional intracellular Ca2+ sensor and the archetypal member of the penta-EF hand protein family. ALG-2 functions in the repair of damage to both the plasma and lysosome membranes and in COPII-dependent budding at endoplasmic reticulum exit sites (ERES). In the presence of Ca2+, ALG-2 binds to ESCRT-I and ALIX in membrane repair and to SEC31A at ERES. ALG-2 also binds directly to acidic membranes in the presence of Ca2+ by a combination of electrostatic and hydrophobic interactions. By combining GUV-based experiments and molecular dynamics simulations, we show that charge-reversed mutants of ALG-2 at these locations disrupt membrane recruitment. ALG-2 membrane binding mutants have reduced or abrogated ERES localization in response to Thapsigargin-induced Ca2+ release but still localize to lysosomes following lysosomal Ca2+ release. In vitro reconstitution shows that the ALG-2 membrane-binding defect can be rescued by binding to ESCRT-I. These data thus reveal the nature of direct Ca2+-dependent membrane binding and its interplay with Ca2+-dependent protein binding in the cellular functions of ALG-2.

Transmembrane Membrane Readers form a Novel Class of Proteins That Include Peripheral Phosphoinositide Recognition Domains and Viral Spikes

Membrane proteins are broadly classified as transmembrane (TM) or peripheral, with functions that pertain to only a single bilayer at a given time. Here, we explicate a class of proteins that contain both transmembrane and peripheral domains, which we dub transmembrane membrane readers (TMMRs). Their transmembrane and peripheral elements anchor them to one bilayer and reversibly attach them to another section of bilayer, respectively, positioning them to tether and fuse membranes while recognizing signals such as phosphoinositides (PIs) and modifying lipid chemistries in proximity to their transmembrane domains. Here, we analyze full-length models from AlphaFold2 and Rosetta, as well as structures from nuclear magnetic resonance (NMR) spectroscopy and X-ray crystallography, using the Membrane Optimal Docking Area (MODA) program to map their membrane-binding surfaces. Eukaryotic TMMRs include phospholipid-binding C1, C2, CRAL-TRIO, FYVE, GRAM, GTPase, MATH, PDZ, PH, PX, SMP, StART and WD domains within proteins including protrudin, sorting nexins and synaptotagmins. The spike proteins of SARS-CoV-2 as well as other viruses are also TMMRs, seeing as they are anchored into the viral membrane while mediating fusion with host cell membranes. As such, TMMRs have key roles in cell biology and membrane trafficking, and include drug targets for diseases such as COVID-19.

On the difficulties of characterizing weak protein interactions that are critical for neurotransmitter release

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.

Allosteric stabilization of calcium and phosphoinositide dual binding engages several synaptotagmins in fast exocytosis

Synaptic communication relies on the fusion of synaptic vesicles with the plasma membrane, which leads to neurotransmitter release. This exocytosis is triggered by brief and local elevations of intracellular Ca²⁺ with remarkably high sensitivity. How this is molecularly achieved is unknown. While synaptotagmins confer the Ca²⁺ sensitivity of neurotransmitter exocytosis, biochemical measurements reported Ca²⁺ affinities too low to account for synaptic function. However, synaptotagmin’s Ca²⁺ affinity increases upon binding the plasma membrane phospholipid PI(4,5)P₂ and, vice versa, Ca²⁺ binding increases synaptotagmin’s PI(4,5)P₂ affinity, indicating a stabilization of the Ca²⁺/PI(4,5)P₂ dual-bound state. Here, we devise a molecular exocytosis model based on this positive allosteric stabilization and the assumptions that (1.) synaptotagmin Ca²⁺/PI(4,5)P₂ dual binding lowers the energy barrier for vesicle fusion and that (2.) the effect of multiple synaptotagmins on the energy barrier is additive. The model, which relies on biochemically measured Ca²⁺/PI(4,5)P₂ affinities and protein copy numbers, reproduced the steep Ca²⁺ dependency of neurotransmitter release. Our results indicate that each synaptotagmin engaging in Ca²⁺/PI(4,5)P₂ dual-binding lowers the energy barrier for vesicle fusion by ~5 k_BT and that allosteric stabilization of this state enables the synchronized engagement of several (typically three) synaptotagmins for fast exocytosis. Furthermore, we show that mutations altering synaptotagmin’s allosteric properties may show dominant-negative effects, even though synaptotagmins act independently on the energy barrier, and that dynamic changes of local PI(4,5)P₂ (e.g. upon vesicle movement) dramatically impact synaptic responses. We conclude that allosterically stabilized Ca²⁺/PI(4,5)P₂ dual binding enables synaptotagmins to exert their coordinated function in neurotransmission.

For our brains and nervous systems to work properly, the nerve cells within them must be able to ‘talk’ to each other. They do this by releasing chemical signals called neurotransmitters which other cells can detect and respond to.

Neurotransmitters are packaged in tiny membrane-bound spheres called vesicles. When a cell of the nervous system needs to send a signal to its neighbours, the vesicles fuse with the outer membrane of the cell, discharging their chemical contents for other cells to detect. The initial trigger for neurotransmitter release is a short, fast increase in the amount of calcium ions inside the signalling cell. One of the main proteins that helps regulate this process is synaptotagmin which binds to calcium and gives vesicles the signal to start unloading their chemicals.

Despite acting as a calcium sensor, synaptotagmin actually has a very low affinity for calcium ions by itself, meaning that it would not be efficient for the protein to respond alone. Synpatotagmin is more likely to bind to calcium if it is attached to a molecule called PIP₂, which is found in the membranes of cells The effect also occurs in reverse, as the binding of calcium to synaptotagmin increases the protein’s affinity for PIP₂. However, how these three molecules – synaptotagmin, PIP₂, and calcium – work together to achieve the physiological release of neurotransmitters is poorly understood.

To help answer this question, Kobbersmed, Berns et al. set up a computer simulation of ‘virtual vesicles’ using available experimental data on synaptotagmin’s affinity with calcium and PIP₂. In this simulation, synaptotagmin could only trigger the release of neurotransmitters when bound to both calcium and PIP₂. The model also showed that each ‘complex’ of synaptotagmin/calcium/PIP₂ made the vesicles more likely to fuse with the outer membrane of the cell – to the extent that only a handful of synaptotagmin molecules were needed to start neurotransmitter release from a single vesicle.

These results shed new light on a biological process central to the way nerve cells communicate with each other. In the future, Kobbersmed, Berns et al. hope that this insight will help us to understand the cause of diseases where communication in the nervous system is impaired.

Synaptotagmin-13 orchestrates pancreatic endocrine cell egression and islet morphogenesis

During pancreas development endocrine cells leave the ductal epithelium to form the islets of Langerhans, but the morphogenetic mechanisms are incompletely understood. Here, we identify the Ca2+-independent atypical Synaptotagmin-13 (Syt13) as a key regulator of endocrine cell egression and islet formation. We detect specific upregulation of the Syt13 gene and encoded protein in endocrine precursors and the respective lineage during islet formation. The Syt13 protein is localized to the apical membrane of endocrine precursors and to the front domain of egressing endocrine cells, marking a previously unidentified apical-basal to front-rear repolarization during endocrine precursor cell egression. Knockout of Syt13 impairs endocrine cell egression and skews the α-to-β-cell ratio. Mechanistically, Syt13 is a vesicle trafficking protein, transported via the microtubule cytoskeleton, and interacts with phosphatidylinositol phospholipids for polarized localization. By internalizing a subset of plasma membrane proteins at the front domain, including α6β4 integrins, Syt13 modulates cell-matrix adhesion and allows efficient endocrine cell egression. Altogether, these findings uncover an unexpected role for Syt13 as a morphogenetic driver of endocrinogenesis and islet formation.

Analysis of the MCTP Amino Acid Sequence Reveals the Conservation of Putative Calcium- and Lipid-Binding Pockets Within the C2 Domains In Silico

MCTPs (Multiple C2 Domains and Transmembrane region Proteins) are evolutionarily and structurally related to other C2 proteins, which are central to exocytosis and membrane trafficking; however, their specific function has been little studied. MCTPs are associated with endosomes and the endoplasmic reticulum and possess three C2 domains (C2A-C2C) and two transmembrane regions (TMRs) well conserved in different species. Here, we generated structural models of the MCTP C2 domains of C. elegans and analyzed their putative function by docking, which revealed that these domains possess Ca²⁺- and lipid-binding pockets, suggesting that MCTPs play a significant, calcium-dependent role in membrane physiology.

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.

5-IP7 is a GPCR messenger mediating neural control of synaptotagmin-dependent insulin exocytosis and glucose homeostasis

5-diphosphoinositol pentakisphosphate (5-IP₇) is a signalling metabolite linked to various cellular processes. How extracellular stimuli elicit 5-IP₇ signalling remains unclear. Here we show that 5-IP₇ in β cells mediates parasympathetic stimulation of synaptotagmin-7 (Syt7)-dependent insulin release. Mechanistically, vagal stimulation and activation of muscarinic acetylcholine receptors triggers G_αq-PLC-PKC-PKD-dependent signalling and activates IP6K1, the 5-IP₇ synthase. Whereas both 5-IP₇ and its precursor IP₆ compete with PIP₂ for binding to Syt7, Ca²⁺ selectively binds 5-IP₇ with high affinity, freeing Syt7 to enable fusion of insulin-containing vesicles with the cell membrane. β-cell-specific IP6K1 deletion diminishes insulin secretion and glucose clearance elicited by muscarinic stimulation, whereas mice carrying a phosphorylation-mimicking, hyperactive IP6K1 mutant display augmented insulin release, congenital hyperinsulinaemia and obesity. These phenotypes are absent in mice lacking Syt7. Our study proposes a new conceptual framework for inositol pyrophosphate physiology in which 5-IP₇ acts as a GPCR second messenger at the interface between peripheral nervous system and metabolic organs, transmitting G_q-coupled GPCR stimulation to unclamp Syt7-dependent, and perhaps other, exocytotic events.

The neuronal calcium sensor Synaptotagmin-1 and SNARE proteins cooperate to dilate fusion pores

All membrane fusion reactions proceed through an initial fusion pore, including calcium-triggered release of neurotransmitters and hormones. Expansion of this small pore to release cargo is energetically costly and regulated by cells, but the mechanisms are poorly understood. Here, we show that the neuronal/exocytic calcium sensor Synaptotagmin-1 (Syt1) promotes expansion of fusion pores induced by SNARE proteins. Pore dilation relied on calcium-induced insertion of the tandem C2 domain hydrophobic loops of Syt1 into the membrane, previously shown to reorient the C2 domain. Mathematical modelling suggests that C2B reorientation rotates a bound SNARE complex so that it exerts force on the membranes in a mechanical lever action that increases the height of the fusion pore, provoking pore dilation to offset the bending energy penalty. We conclude that Syt1 exerts novel non-local calcium-dependent mechanical forces on fusion pores that dilate pores and assist neurotransmitter and hormone release.

Presynaptic store-operated Ca2+ entry drives excitatory spontaneous neurotransmission and augments endoplasmic reticulum stress

Store-operated calcium entry (SOCE) is activated by depletion of Ca²⁺ from the endoplasmic reticulum (ER) and mediated by stromal interaction molecule (STIM) proteins. Here, we show that in rat and mouse hippocampal neurons, acute ER Ca²⁺ depletion increases presynaptic Ca²⁺ levels and glutamate release through a pathway dependent on STIM2 and the synaptic Ca²⁺ sensor synaptotagmin-7 (syt7). In contrast, synaptotagmin-1 (syt1) can suppress SOCE-mediated spontaneous release, and STIM2 is required for the increase in spontaneous release seen during syt1 loss of function. We also demonstrate that chronic ER stress activates the same pathway leading to syt7-dependent potentiation of spontaneous glutamate release. During ER stress, inhibition of SOCE or syt7-driven fusion partially restored basal neurotransmission and decreased expression of pro-apoptotic markers, indicating that these processes participate in the amplification of ER-stress-related damage. Taken together, we propose that presynaptic SOCE links ER stress and augmented spontaneous neurotransmission, which may, in turn, facilitate neurodegeneration.

Synaptotagmin-7 places dense-core vesicles at the cell membrane to promote Munc13-2- and Ca2+-dependent priming

Synaptotagmins confer calcium-dependence to the exocytosis of secretory vesicles, but how coexpressed synaptotagmins interact remains unclear. We find that synaptotagmin-1 and synaptotagmin-7 when present alone act as standalone fast and slow Ca²⁺-sensors for vesicle fusion in mouse chromaffin cells. When present together, synaptotagmin-1 and synaptotagmin-7 are found in largely non-overlapping clusters on dense-core vesicles. Synaptotagmin-7 stimulates Ca²⁺-dependent vesicle priming and inhibits depriming, and it promotes ubMunc13-2- and phorbolester-dependent priming, especially at low resting calcium concentrations. The priming effect of synaptotagmin-7 increases the number of vesicles fusing via synaptotagmin-1, while negatively affecting their fusion speed, indicating both synergistic and competitive interactions between synaptotagmins. Synaptotagmin-7 places vesicles in close membrane apposition (<6 nm); without it, vesicles accumulate out of reach of the fusion complex (20-40 nm). We suggest that a synaptotagmin-7-dependent movement toward the membrane is involved in Munc13-2/phorbolester/Ca²⁺-dependent priming as a prelude to fast and slow exocytosis triggering.

Asynchronous Glutamate Release at Autapses Regulates Spike Reliability and Precision in Mouse Neocortical Pyramidal Cells

Autapses are self-synapses of a neuron. Inhibitory autapses in the neocortex release GABA in 2 modes, synchronous release and asynchronous release (AR), providing precise and prolonged self-inhibition, respectively. A subpopulation of neocortical pyramidal cells (PCs) also forms functional autapses, activation of which promotes burst firing by strong unitary autaptic response that reflects synchronous glutamate release. However, it remains unclear whether AR occurs at PC autapses and plays a role in neuronal signaling. We performed whole-cell recordings from layer-5 PCs in slices of mouse prefrontal cortex (PFC). In response to action potential (AP) burst, 63% of PCs showed robust long-lasting autaptic AR, much stronger than synaptic AR between neighboring PCs. The autaptic AR is mediated predominantly by P/Q-type Ca2+ channels, and its strength depends on the intensity of PC activity and the level of residual Ca2+. Further experiments revealed that autaptic AR enhances spiking activities but reduces the temporal precision of post-burst APs. Together, the results show the occurrence of AR at PC autapses, the delayed and persistent glutamate AR causes self-excitation in individual PCs but may desynchronize the autaptic PC population. Thus, glutamatergic autapses should be essential elements in PFC and contribute to cortical information processing.

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.

Neurotransmitter Release Site Replenishment and Presynaptic Plasticity

An action potential (AP) triggers neurotransmitter release from synaptic vesicles (SVs) docking to a specialized release site of presynaptic plasma membrane, the active zone (AZ). The AP simultaneously controls the release site replenishment with SV for sustainable synaptic transmission in response to incoming neuronal signals. Although many studies have suggested that the replenishment time is relatively slow, recent studies exploring high speed resolution have revealed SV dynamics with milliseconds timescale after an AP. Accurate regulation is conferred by proteins sensing Ca²⁺ entering through voltage-gated Ca²⁺ channels opened by an AP. This review summarizes how millisecond Ca²⁺ dynamics activate multiple protein cascades for control of the release site replenishment with release-ready SVs that underlie presynaptic short-term plasticity.

Synaptotagmin-7 enhances calcium-sensing of chromaffin cell granules and slows discharge of granule cargos

Synaptotagmin-7 (Syt-7) is one of two major calcium sensors for exocytosis in adrenal chromaffin cells, the other being synaptotagmin-1 (Syt-1). Despite a broad appreciation for the importance of Syt-7, questions remain as to its localization, function in mediating discharge of dense core granule cargos, and role in triggering release in response to physiological stimulation. These questions were addressed using two distinct experimental preparations-mouse chromaffin cells lacking endogenous Syt-7 (KO cells) and a reconstituted system employing cell-derived granules expressing either Syt-7 or Syt-1. First, using immunofluorescence imaging and subcellular fractionation, it is shown that Syt-7 is widely distributed in organelles, including dense core granules. Total internal reflection fluorescence (TIRF) imaging demonstrates that the kinetics and probability of granule fusion in Syt-7 KO cells stimulated by a native secretagogue, acetylcholine, are markedly lower than in WT cells. When fusion is observed, fluorescent cargo proteins are discharged more rapidly when only Syt-1 is available to facilitate release. To determine the extent to which the aforementioned results are attributable purely to Syt-7, granules expressing only Syt-7 or Syt-1 were triggered to fuse on planar supported bilayers bearing plasma membrane SNARE proteins. Here, as in cells, Syt-7 confers substantially greater calcium sensitivity to granule fusion than Syt-1 and slows the rate at which cargos are released. Overall, this study demonstrates that by virtue of its high affinity for calcium and effects on fusion pore expansion, Syt-7 plays a central role in regulating secretory output from adrenal chromaffin cells.

Cytotoxic Granule Trafficking and Fusion in Synaptotagmin7-Deficient Cytotoxic T Lymphocytes

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.

Drosophila Synaptotagmin 7 negatively regulates synaptic vesicle release and replenishment in a dosage-dependent manner

Synchronous neurotransmitter release is triggered by Ca2+ binding to the synaptic vesicle protein Synaptotagmin 1, while asynchronous fusion and short-term facilitation is hypothesized to be mediated by plasma membrane-localized Synaptotagmin 7 (SYT7). We generated mutations in Drosophila Syt7 to determine if it plays a conserved role as the Ca2+ sensor for these processes. Electrophysiology and quantal imaging revealed evoked release was elevated 2-fold. Syt7 mutants also had a larger pool of readily-releasable vesicles, faster recovery following stimulation, and intact facilitation. Syt1/Syt7 double mutants displayed more release than Syt1 mutants alone, indicating SYT7 does not mediate the residual asynchronous release remaining in the absence of SYT1. SYT7 localizes to an internal membrane tubular network within the peri-active zone, but does not enrich at active zones. These findings indicate the two Ca2+ sensor model of SYT1 and SYT7 mediating all phases of neurotransmitter release and facilitation is not applicable at Drosophila synapses.

Calcium Channels, Synaptic Plasticity, and Neuropsychiatric Disease

Voltage-gated calcium channels couple depolarization of the cell-surface membrane to entry of calcium, which triggers secretion, contraction, neurotransmission, gene expression, and other physiological responses. They are encoded by ten genes, which generate three voltage-gated calcium channel subfamilies: Ca_V1; Ca_V2; and Ca_V3. At synapses, Ca_V2 channels form large signaling complexes in the presynaptic nerve terminal, which are responsible for the calcium entry that triggers neurotransmitter release and short-term presynaptic plasticity. Ca_V1 channels form signaling complexes in postsynaptic dendrites and dendritic spines, where their calcium entry induces long-term potentiation. These calcium channels are the targets of mutations and polymorphisms that alter their function and/or regulation and cause neuropsychiatric diseases, including migraine headache, cerebellar ataxia, autism, schizophrenia, bipolar disorder, and depression. This article reviews the molecular properties of calcium channels, considers their multiple roles in synaptic plasticity, and discusses their potential involvement in this wide range of neuropsychiatric diseases.

Presynaptic Calcium Channels

Presynaptic Ca²⁺ entry occurs through voltage-gated Ca²⁺ (Ca_V) channels which are activated by membrane depolarization. Depolarization accompanies neuronal firing and elevation of Ca²⁺ triggers neurotransmitter release from synaptic vesicles. For synchronization of efficient neurotransmitter release, synaptic vesicles are targeted by presynaptic Ca²⁺ channels forming a large signaling complex in the active zone. The presynaptic Ca_V2 channel gene family (comprising Ca_V2.1, Ca_V2.2, and Ca_V2.3 isoforms) encode the pore-forming α1 subunit. The cytoplasmic regions are responsible for channel modulation by interacting with regulatory proteins. This article overviews modulation of the activity of Ca_V2.1 and Ca_V2.2 channels in the control of synaptic strength and presynaptic plasticity.

Membrane-Binding Cooperativity and Coinsertion by C2AB Tandem Domains of Synaptotagmins 1 and 7

Synaptotagmin-1 (Syt-1) and synaptotagmin-7 (Syt-7) contain analogous tandem C2 domains, C2A and C2B, which together sense Ca²⁺ to bind membranes and promote the stabilization of exocytotic fusion pores. Syt-1 triggers fast release of neurotransmitters, whereas Syt-7 functions in processes that involve lower Ca²⁺ concentrations such as hormone secretion. Syt-1 C2 domains are reported to bind membranes cooperatively, based on the observation that they penetrate farther into membranes as the C2AB tandem than as individual C2 domains. In contrast, we previously suggested that the two C2 domains of Syt-7 bind membranes independently, based in part on measurements of their liposome dissociation kinetics. Here, we investigated C2A-C2B interdomain cooperativity with Syt-1 and Syt-7 using directly comparable measurements. Equilibrium Ca²⁺ titrations demonstrate that the Syt-7 C2AB tandem binds liposomes lacking phosphatidylinositol-4,5-bisphosphate (PIP₂) with greater Ca²⁺ sensitivity than either of its individual domains and binds to membranes containing PIP₂ even in the absence of Ca²⁺. Stopped-flow kinetic measurements show differences in cooperativity between Syt-1 and Syt-7: Syt-1 C2AB dissociates from PIP₂-free liposomes much more slowly than either of its individual C2 domains, indicating cooperativity, whereas the major population of Syt-7 C2AB has a dissociation rate comparable to its C2A domain, suggesting a lack of cooperativity. A minor subpopulation of Syt-7 C2AB dissociates at a slower rate, which could be due to a small cooperative component and/or liposome clustering. Measurements using an environment-sensitive fluorescent probe indicate that the Syt-7 C2B domain inserts deeply into membranes as part of the C2AB tandem, similar to the coinsertion previously reported for Syt-1. Overall, coinsertion of C2A and C2B domains is coupled to cooperative energetic effects in Syt-1 to a much greater extent than in Syt-7. The difference can be understood in terms of the relative contributions of C2A and C2B domains toward membrane binding in the two proteins.

Synaptotagmin Ca2+ Sensors and Their Spatial Coupling to Presynaptic Cav Channels in Central Cortical Synapses

Ca²⁺ concentrations drop rapidly over a distance of a few tens of nanometers from an open voltage-gated Ca²⁺ channel (Ca_v), thereby, generating a spatially steep and temporally short-lived Ca²⁺ gradient that triggers exocytosis of a neurotransmitter filled synaptic vesicle. These non-steady state conditions make the Ca²⁺-binding kinetics of the Ca²⁺ sensors for release and their spatial coupling to the Ca_vs important parameters of synaptic efficacy. In the mammalian central nervous system, the main release sensors linking action potential mediated Ca²⁺ influx to synchronous release are Synaptotagmin (Syt) 1 and 2. We review here quantitative work focusing on the Ca²⁺ kinetics of Syt2-mediated release. At present similar quantitative detail is lacking for Syt1-mediated release. In addition to triggering release, Ca²⁺ remaining bound to Syt after the first of two successive high-frequency activations was found to be capable of facilitating release during the second activation. More recently, the Ca²⁺ sensor Syt7 was identified as additional facilitation sensor. We further review how several recent functional studies provided quantitative insights into the spatial topographical relationships between Syts and Ca_vs and identified mechanisms regulating the sensor-to-channel coupling distances at presynaptic active zones. Most synapses analyzed in matured cortical structures were found to operate at tight, nanodomain coupling. For fast signaling synapses a developmental switch from loose, microdomain to tight, nanodomain coupling was found. The protein Septin5 has been known for some time as a developmentally down-regulated “inhibitor” of tight coupling, while Munc13-3 was found only recently to function as a developmentally up-regulated mediator of tight coupling. On the other hand, a highly plastic synapse was found to operate at loose coupling in the matured hippocampus. Together these findings suggest that the coupling topography and its regulation is a specificity of the type of synapse. However, to definitely draw such conclusion our knowledge of functional active zone topographies of different types of synapses in different areas of the mammalian brain is too incomplete.

Mechanism of neurotransmitter release coming into focus

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.

The high-affinity calcium sensor synaptotagmin-7 serves multiple roles in regulated exocytosis

MacDougall et al. review the structure and function of the calcium sensor synaptotagmin-7 in exocytosis.

Synaptotagmin (Syt) proteins comprise a 17-member family, many of which trigger exocytosis in response to calcium. Historically, most studies have focused on the isoform Syt-1, which serves as the primary calcium sensor in synchronous neurotransmitter release. Recently, Syt-7 has become a topic of broad interest because of its extreme calcium sensitivity and diversity of roles in a wide range of cell types. Here, we review the known and emerging roles of Syt-7 in various contexts and stress the importance of its actions. Unique functions of Syt-7 are discussed in light of recent imaging, electrophysiological, and computational studies. Particular emphasis is placed on Syt-7–dependent regulation of synaptic transmission and neuroendocrine cell secretion. Finally, based on biochemical and structural data, we propose a mechanism to link Syt-7’s role in membrane fusion with its role in subsequent fusion pore expansion via strong calcium-dependent phospholipid binding.

The synaptotagmin C2B domain calcium-binding loops modulate the rate of fusion pore expansion

In chromaffin cells, the kinetics of fusion pore expansion vary depending on which synaptotagmin isoform (Syt-1 or Syt-7) drives release. Our recent studies have shown that fusion pores of granules harboring Syt-1 expand more rapidly than those harboring Syt-7. Here we sought to define the structural specificity of synaptotagmin action at the fusion pore by manipulating the Ca2+-binding C2B module. We generated a chimeric Syt-1 in which its C2B Ca2+-binding loops had been exchanged for those of Syt-7. Fusion pores of granules harboring a Syt-1 C2B chimera with all three Ca2+-binding loops of Syt-7 (Syt-1:7C2B123) exhibited slower rates of fusion pore expansion and neuropeptide cargo release relative to WT Syt-1. After fusion, this chimera also dispersed more slowly from fusion sites than WT protein. We speculate that the Syt-1:7 C2B123 and WT Syt-1 are likely to differ in their interactions with Ca2+ and membranes. Subsequent in vitro and in silico data demonstrated that the chimera exhibits a higher affinity for phospholipids than WT Syt-1. We conclude that the affinity of synaptotagmin for the plasma membrane, and the rate at which it releases the membrane, contribute in important ways to the rate of fusion pore expansion.

Hypothesis - buttressed rings assemble, clamp, and release SNAREpins for synaptic transmission

Neural networks are optimized to detect temporal coincidence on the millisecond timescale. Here, we offer a synthetic hypothesis based on recent structural insights into SNAREs and the C2 domain proteins to explain how synaptic transmission can keep this pace. We suggest that an outer ring of up to six curved Munc13 ‘MUN’ domains transiently anchored to the plasma membrane via its flanking domains surrounds a stable inner ring comprised of synaptotagmin C2 domains to serve as a work‐bench on which SNAREpins are templated. This ‘buttressed‐ring hypothesis’ affords straightforward answers to many principal and long‐standing questions concerning how SNAREpins can be assembled, clamped, and then released synchronously with an action potential.

Single-molecule force spectroscopy of protein-membrane interactions

Many biological processes rely on protein–membrane interactions in the presence of mechanical forces, yet high resolution methods to quantify such interactions are lacking. Here, we describe a single-molecule force spectroscopy approach to quantify membrane binding of C2 domains in Synaptotagmin-1 (Syt1) and Extended Synaptotagmin-2 (E-Syt2). Syts and E-Syts bind the plasma membrane via multiple C2 domains, bridging the plasma membrane with synaptic vesicles or endoplasmic reticulum to regulate membrane fusion or lipid exchange, respectively. In our approach, single proteins attached to membranes supported on silica beads are pulled by optical tweezers, allowing membrane binding and unbinding transitions to be measured with unprecedented spatiotemporal resolution. C2 domains from either protein resisted unbinding forces of 2–7 pN and had binding energies of 4–14 k_BT per C2 domain. Regulation by bilayer composition or Ca²⁺ recapitulated known properties of both proteins. The method can be widely applied to study protein–membrane interactions.