Actin- and Myosin-Dependent Vesicle Loading of Presynaptic Docking Sites Prior to Exocytosis

Phospholipid Scramblase 1 Controls Efficient Neurotransmission and Synaptic Vesicle Retrieval at Cerebellar Synapses

Phospholipids (PLs) are asymmetrically distributed at the plasma membrane. This asymmetric lipid distribution is transiently altered during calcium-regulated exocytosis, but the impact of this transient remodeling on presynaptic function is currently unknown. As phospholipid scramblase 1 (PLSCR1) randomizes PL distribution between the two leaflets of the plasma membrane in response to calcium activation, we set out to determine its role in neurotransmission. We report here that PLSCR1 is expressed in cerebellar granule cells (GrCs) and that PLSCR1-dependent phosphatidylserine egress occurred at synapses in response to neuron stimulation. Synaptic transmission is impaired at GrC Plscr1 -/- synapses, and both PS egress and synaptic vesicle (SV) endocytosis are inhibited in Plscr1 -/- cultured neurons from male and female mice, demonstrating that PLSCR1 controls PL asymmetry remodeling and SV retrieval following neurotransmitter release. Altogether, our data reveal a novel key role for PLSCR1 in SV recycling and provide the first evidence that PL scrambling at the plasma membrane is a prerequisite for optimal presynaptic performance.

Physiological roles of endocytosis and presynaptic scaffold in vesicle replenishment at fast and slow central synapses

After exocytosis, release sites are cleared of vesicular residues to replenish with transmitter-filled vesicles. Endocytic and scaffold proteins are thought to underlie this site-clearance mechanism. However, the physiological significance of this mechanism at diverse mammalian central synapses remains unknown. Here, we tested this in a physiologically optimized condition using action potential evoked EPSCs at fast calyx synapse and relatively slow hippocampal CA1 synapse, in post-hearing mice brain slices at 37°C and in 1.3 mM [Ca2+]. Pharmacological block of endocytosis enhanced synaptic depression at the calyx synapse, whereas it attenuated synaptic facilitation at the hippocampal synapse. Block of scaffold protein activity likewise enhanced synaptic depression at the calyx but had no effect at the hippocampal synapse. At the fast calyx synapse, block of endocytosis or scaffold protein activity significantly enhanced synaptic depression as early as 10 ms after the stimulation onset. Unlike previous reports, neither endocytic blockers nor scaffold protein inhibitors prolonged the recovery from short-term depression. We conclude that the release-site clearance by endocytosis can be a universal phenomenon supporting vesicle replenishment at both fast and slow synapses, whereas the presynaptic scaffold mechanism likely plays a specialized role in vesicle replenishment predominantly at fast synapses.

Parallel processing of quickly and slowly mobilized reserve vesicles in hippocampal synapses

Vesicles within presynaptic terminals are thought to be segregated into a variety of readily releasable and reserve pools. The nature of the pools and trafficking between them is not well understood, but pools that are slow to mobilize when synapses are active are often assumed to feed pools that are mobilized more quickly, in a series. However, electrophysiological studies of synaptic transmission have suggested instead a parallel organization where vesicles within slowly and quickly mobilized reserve pools would separately feed independent reluctant- and fast-releasing subdivisions of the readily releasable pool. Here, we use FM-dyes to confirm the existence of multiple reserve pools at hippocampal synapses and a parallel organization that prevents intermixing between the pools, even when stimulation is intense enough to drive exocytosis at the maximum rate. The experiments additionally demonstrate extensive heterogeneity among synapses in the relative sizes of the slowly and quickly mobilized reserve pools, which suggests equivalent heterogeneity in the numbers of reluctant and fast-releasing readily releasable vesicles that may be relevant for understanding information processing and storage.

Different states of synaptic vesicle priming explain target cell type-dependent differences in neurotransmitter release

Pronounced differences in neurotransmitter release from a given presynaptic neuron, depending on the synaptic target, are among the most intriguing features of cortical networks. Hippocampal pyramidal cells (PCs) release glutamate with low probability to somatostatin expressing oriens-lacunosum-moleculare (O-LM) interneurons (INs), and the postsynaptic responses show robust short-term facilitation, whereas the release from the same presynaptic axons onto fast-spiking INs (FSINs) is ~10-fold higher and the excitatory postsynaptic currents (EPSCs) display depression. The mechanisms underlying these vastly different synaptic behaviors have not been conclusively identified. Here, we applied a combined functional, pharmacological, and modeling approach to address whether the main difference lies in the action potential-evoked fusion or else in upstream priming processes of synaptic vesicles (SVs). A sequential two-step SV priming model was fitted to the peak amplitudes of unitary EPSCs recorded in response to complex trains of presynaptic stimuli in acute hippocampal slices of adult mice. At PC-FSIN connections, the fusion probability (Pfusion) of well-primed SVs is 0.6, and 44% of docked SVs are in a fusion-competent state. At PC-O-LM synapses, Pfusion is only 40% lower (0.36), whereas the fraction of well-primed SVs is 6.5-fold smaller. Pharmacological enhancement of fusion by 4-AP and priming by PDBU was recaptured by the model with a selective increase of Pfusion and the fraction of well-primed SVs, respectively. Our results demonstrate that the low fidelity of transmission at PC-O-LM synapses can be explained by a low occupancy of the release sites by well-primed SVs.

Synaptotagmin 7 docks synaptic vesicles to support facilitation and Doc2α-triggered asynchronous release

Despite decades of intense study, the molecular basis of asynchronous neurotransmitter release remains enigmatic. Synaptotagmin (syt) 7 and Doc2 have both been proposed as Ca2+ sensors that trigger this mode of exocytosis, but conflicting findings have led to controversy. Here, we demonstrate that at excitatory mouse hippocampal synapses, Doc2α is the major Ca2+ sensor for asynchronous release, while syt7 supports this process through activity-dependent docking of synaptic vesicles. In synapses lacking Doc2α, asynchronous release after single action potentials is strongly reduced, while deleting syt7 has no effect. However, in the absence of syt7, docked vesicles cannot be replenished on millisecond timescales. Consequently, both synchronous and asynchronous release depress from the second pulse onward during repetitive activity. By contrast, synapses lacking Doc2α have normal activity-dependent docking, but continue to exhibit decreased asynchronous release after multiple stimuli. Moreover, disruption of both Ca2+ sensors is non-additive. These findings result in a new model whereby syt7 drives activity-dependent docking, thus providing synaptic vesicles for synchronous (syt1) and asynchronous (Doc2 and other unidentified sensors) release during ongoing transmission.

Structure, biophysics, and circuit function of a "giant" cortical presynaptic terminal

The hippocampal mossy fiber synapse, formed between axons of dentate gyrus granule cells and dendrites of CA3 pyramidal neurons, is a key synapse in the trisynaptic circuitry of the hippocampus. Because of its comparatively large size, this synapse is accessible to direct presynaptic recording, allowing a rigorous investigation of the biophysical mechanisms of synaptic transmission and plasticity. Furthermore, because of its placement in the very center of the hippocampal memory circuit, this synapse seems to be critically involved in several higher network functions, such as learning, memory, pattern separation, and pattern completion. Recent work based on new technologies in both nanoanatomy and nanophysiology, including presynaptic patch-clamp recording, paired recording, super-resolution light microscopy, and freeze-fracture and "flash-and-freeze" electron microscopy, has provided new insights into the structure, biophysics, and network function of this intriguing synapse. This brings us one step closer to answering a fundamental question in neuroscience: how basic synaptic properties shape higher network computations.

Pre- versus Post-synaptic Forms of LTP in Two Branches of the Same Hippocampal Afferent

There has been considerable controversy about pre- versus postsynaptic expression of memory-related long-term potentiation (LTP), with corresponding disputes about underlying mechanisms. We report here an instance in male mice, in which both types of potentiation are expressed but in separate branches of the same hippocampal afferent. Induction of LTP in the dentate gyrus (DG) branch of the lateral perforant path (LPP) reduces paired-pulse facilitation, is blocked by antagonism of cannabinoid receptor type 1, and is not affected by suppression of postsynaptic actin polymerization. These observations are consistent with presynaptic expression. The opposite pattern of results was obtained in the LPP branch that innervates the distal dendrites of CA3: LTP did not reduce paired-pulse facilitation, was unaffected by the cannabinoid receptor blocker, and required postsynaptic actin filament assembly. Differences in the two LPP termination sites were also noted for frequency facilitation of synaptic responses, an effect that was reproduced in a two-step simulation by small adjustments to vesicle release dynamics. These results indicate that different types of glutamatergic neurons impose different forms of filtering and synaptic plasticity on their afferents. They also suggest that inputs are routed to, and encoded by, different sites within the hippocampus depending upon the pattern of activity arriving over the parent axon.

RIM-BP2 regulates Ca2+ channel abundance and neurotransmitter release at hippocampal mossy fiber terminals

Synaptic vesicles dock and fuse at the presynaptic active zone (AZ), the specialized site for transmitter release. AZ proteins play multiple roles such as recruitment of Ca2+ channels as well as synaptic vesicle docking, priming, and fusion. However, the precise role of each AZ protein type remains unknown. In order to dissect the role of RIM-BP2 at mammalian cortical synapses having low release probability, we applied direct electrophysiological recording and super-resolution imaging to hippocampal mossy fiber terminals of RIM-BP2 knockout (KO) mice. By using direct presynaptic recording, we found the reduced Ca2+ currents. The measurements of excitatory postsynaptic currents (EPSCs) and presynaptic capacitance suggested that the initial release probability was lowered because of the reduced Ca2+ influx and impaired fusion competence in RIM-BP2 KO. Nevertheless, larger Ca2+ influx restored release partially. Consistent with presynaptic recording, STED microscopy suggested less abundance of P/Q-type Ca2+ channels at AZs deficient in RIM-BP2. Our results suggest that the RIM-BP2 regulates both Ca2+ channel abundance and transmitter release at mossy fiber synapses.

A maximum of two readily releasable vesicles per docking site at a cerebellar single active zone synapse

Recent research suggests that in central mammalian synapses, active zones contain several docking sites acting in parallel. Before release, one or several synaptic vesicles (SVs) are thought to bind to each docking site, forming the readily releasable pool (RRP). Determining the RRP size per docking site has important implications for short-term synaptic plasticity. Here, using mouse cerebellar slices, we take advantage of recently developed methods to count the number of released SVs at single glutamatergic synapses in response to trains of action potentials (APs). In each recording, the number of docking sites was determined by fitting with a binomial model the number of released SVs in response to individual APs. After normalization with respect to the number of docking sites, the summed number of released SVs following a train of APs was used to estimate of the RRP size per docking site. To improve this estimate, various steps were taken to maximize the release probability of docked SVs, the occupancy of docking sites, as well as the extent of synaptic depression. Under these conditions, the RRP size reached a maximum value close to two SVs per docking site. The results indicate that each docking site contains two distinct SV-binding sites that can simultaneously accommodate up to one SV each. They further suggest that under special experimental conditions, as both sites are close to full occupancy, a maximal RRP size of two SVs per docking site can be reached. More generally, the results validate a sequential two-step docking model previously proposed at this preparation.

Interpretation of presynaptic phenotypes of synaptic plasticity in terms of a two-step priming process

Studies on synaptic proteins involved in neurotransmitter release often aim at distinguishing between their roles in vesicle priming (the docking of synaptic vesicles to the plasma membrane and the assembly of a release machinery) as opposed to the process of vesicle fusion. This has traditionally been done by estimating two parameters, the size of the pool of fusion-competent vesicles (the readily releasable pool, RRP) and the probability that such vesicles are released by an action potential, with the aim of determining how these parameters are affected by molecular perturbations. Here, it is argued that the assumption of a homogeneous RRP may be too simplistic and may blur the distinction between vesicle priming and fusion. Rather, considering priming as a dynamic and reversible multistep process allows alternative interpretations of mutagenesis-induced changes in synaptic transmission and suggests mechanisms for variability in synaptic strength and short-term plasticity among synapses, as well as for interactions between short- and long-term plasticity. In many cases, assigned roles of proteins or causes for observed phenotypes are shifted from fusion- to priming-related when considering multistep priming. Activity-dependent enhancement of priming is an essential element in this alternative view and its variation among synapse types can explain why some synapses show depression and others show facilitation at low to intermediate stimulation frequencies. Multistep priming also suggests a mechanism for frequency invariance of steady-state release, which can be observed in some synapses involved in sensory processing.

Recently recycled synaptic vesicles use multi-cytoskeletal transport and differential presynaptic capture probability to establish a retrograde net flux during ISVE in central neurons

Presynapses locally recycle synaptic vesicles to efficiently communicate information. During use and recycling, proteins on the surface of synaptic vesicles break down and become less efficient. In order to maintain efficient presynaptic function and accommodate protein breakdown, new proteins are regularly produced in the soma and trafficked to presynaptic locations where they replace older protein-carrying vesicles. Maintaining a balance of new proteins and older proteins is thus essential for presynaptic maintenance and plasticity. While protein production and turnover have been extensively studied, it is still unclear how older synaptic vesicles are trafficked back to the soma for recycling in order to maintain balance. In the present study, we use a combination of fluorescence microscopy, hippocampal cell cultures, and computational analyses to determine the mechanisms that mediate older synaptic vesicle trafficking back to the soma. We show that synaptic vesicles, which have recently undergone exocytosis, can differentially utilize either the microtubule or the actin cytoskeleton networks. We show that axonally trafficked vesicles traveling with higher speeds utilize the microtubule network and are less likely to be captured by presynapses, while slower vesicles utilize the actin network and are more likely to be captured by presynapses. We also show that retrograde-driven vesicles are less likely to be captured by a neighboring presynapse than anterograde-driven vesicles. We show that the loss of synaptic vesicle with bound molecular motor myosin V is the mechanism that differentiates whether vesicles will utilize the microtubule or actin networks. Finally, we present a theoretical framework of how our experimentally observed retrograde vesicle trafficking bias maintains the balance with previously observed rates of new vesicle trafficking from the soma.

Fully-primed slowly-recovering vesicles mediate presynaptic LTP at neocortical neurons

Pre- and postsynaptic forms of long-term potentiation (LTP) are candidate synaptic mechanisms underlying learning and memory. At layer 5 pyramidal neurons, LTP increases the initial synaptic strength but also short-term depression during high-frequency transmission. This classical form of presynaptic LTP has been referred to as redistribution of synaptic efficacy. However, the underlying mechanisms remain unclear. We therefore performed whole-cell recordings from layer 5 pyramidal neurons in acute cortical slices of rats and analyzed presynaptic function before and after LTP induction by paired pre- and postsynaptic neuronal activity. LTP was successfully induced in about half of the synaptic connections tested and resulted in increased synaptic short-term depression during high-frequency transmission and a decelerated recovery from short-term depression due to an increased fraction of a slow recovery component. Analysis with a recently established sequential two-step vesicle priming model indicates an increase in the abundance of fully-primed and slowly-recovering vesicles. A systematic analysis of short-term plasticity and synapse-to-synapse variability of synaptic strength at various types of synapses revealed that stronger synapses generally recover more slowly from synaptic short-term depression. Finally, pharmacological stimulation of the cyclic adenosine monophosphate and diacylglycerol signaling pathways, which are both known to promote synaptic vesicle priming, mimicked LTP and slowed the recovery from short-term depression. Our data thus demonstrate that LTP at layer 5 pyramidal neurons increases synaptic strength primarily by enlarging a subpool of fully-primed slowly-recovering vesicles.

An antagonism between Spinophilin and Syd-1 operates upstream of memory-promoting presynaptic long-term plasticity

We still face fundamental gaps in understanding how molecular plastic changes of synapses intersect with circuit operation to define behavioral states. Here, we show that an antagonism between two conserved regulatory proteins, Spinophilin (Spn) and Syd-1, controls presynaptic long-term plasticity and the maintenance of olfactory memories in Drosophila. While Spn mutants could not trigger nanoscopic active zone remodeling under homeostatic challenge and failed to stably potentiate neurotransmitter release, concomitant reduction of Syd-1 rescued all these deficits. The Spn/Syd-1 antagonism converged on active zone close F-actin, and genetic or acute pharmacological depolymerization of F-actin rescued the Spn deficits by allowing access to synaptic vesicle release sites. Within the intrinsic mushroom body neurons, the Spn/Syd-1 antagonism specifically controlled olfactory memory stabilization but not initial learning. Thus, this evolutionarily conserved protein complex controls behaviorally relevant presynaptic long-term plasticity, also observed in the mammalian brain but still enigmatic concerning its molecular mechanisms and behavioral relevance.

Intersectin and Endophilin condensates prime synaptic vesicles for release site replenishment

Neurotransmitter is released from dedicated sites of synaptic vesicle fusion within a synapse. Following fusion, the vacated sites are replenished immediately by new vesicles for subsequent neurotransmission. These replacement vesicles are assumed to be located near release sites and used by chance. Here, we find that replacement vesicles are clustered around this region by Intersectin-1. Specifically, Intersectin-1 forms dynamic molecular condensates with Endophilin A1 near release sites and sequesters vesicles around this region. In the absence of Intersectin-1, vesicles within 20 nm of the plasma membrane are reduced, and consequently, vacated sites cannot be replenished rapidly, leading to depression of synaptic transmission. Similarly, mutations in Intersectin-1 that disrupt Endophilin A1 binding result in similar phenotypes. However, in the absence of Endophilin, this replacement pool of vesicles is available but cannot be accessed, suggesting that Endophilin A1 is needed to mobilize these vesicles. Thus, our work describes a distinct physical region within a synapse where replacement vesicles are harbored for release site replenishment.

Mechanistic insights into cAMP-mediated presynaptic potentiation at hippocampal mossy fiber synapses

Presynaptic plasticity is an activity-dependent change in the neurotransmitter release and plays a key role in dynamic modulation of synaptic strength. Particularly, presynaptic potentiation mediated by cyclic adenosine monophosphate (cAMP) is widely seen across the animals and thought to contribute to learning and memory. Hippocampal mossy fiber-CA3 pyramidal cell synapses have been used as a model because of robust presynaptic potentiation in short- and long-term forms. Moreover, direct presynaptic recordings from large mossy fiber terminals allow one to dissect the potentiation mechanisms. Recently, super-resolution microscopy and flash-and-freeze electron microscopy have revealed the localizations of release site molecules and synaptic vesicles during the potentiation at a nanoscale, identifying the molecular mechanisms of the potentiation. Incorporating these growing knowledges, we try to present plausible mechanisms underlying the cAMP-mediated presynaptic potentiation.

Cav2.2 Channels Sustain Vesicle Recruitment at a Mature Glutamatergic Synapse

The composition of voltage-gated Ca2+ channel (Cav) subtypes that gate action potential (AP)-evoked release changes during the development of mammalian CNS synapses. Cav2.2 and Cav2.3 lose their function in gating-evoked release during postnatal synapse maturation. In mature boutons, Cav2.1 currents provide the almost exclusive trigger for evoked release, and Cav2.3 currents are required for the induction of presynaptic long-term potentiation. However, the functional significance of Cav2.2 remained elusive in mature boutons, although they remain present at active zones and continue contributing significantly to presynaptic Ca2+ influx. Here, we addressed the functional significance of Cav2.2 and Cav2.3 at mature parallel-fiber (PF) to Purkinje neuron synapses of mice of either sex. These synapses are known to exhibit the corresponding developmental Cav subtype changes in gating release. We addressed two hypotheses, namely that Cav2.2 and Cav2.3 are involved in triggering spontaneous glutamate release and that they are engaged in vesicle recruitment during repetitive evoked release. We found that spontaneous miniature release is Ca2+ dependent. However, experiments with Cav subtype-specific blockers excluded the spontaneous opening of Cavs as the Ca2+ source for spontaneous glutamate release. Thus, neither Cav2.2 nor Cav2.3 controls spontaneous release from PF boutons. Furthermore, vesicle recruitment during brief bursts of APs was also independent of Ca2+ influx through Cav2.2 and Cav2.3. However, Cav2.2, but not Cav2.3, currents significantly boosted vesicle recruitment during sustained high-frequency synaptic transmission. Thus, in mature PF boutons Cav2.2 channels are specifically required to sustain synaptic transmission during prolonged neuronal activity.SIGNIFICANCE STATEMENT At young CNS synapses, action potential-evoked release is gated via three subtypes of voltage-gated Ca2+ channels: Cav2.1, Cav2.2, and Cav2.3. During postnatal maturation, Cav2.2 and Cav2.3 lose their function in gating evoked release, such that at mature synapses Cav2.1 provides the almost exclusive source for triggering evoked release. Cav2.3 currents are required for the induction of presynaptic long-term potentiation. However, the function of the still abundant Cav2.2 in mature boutons remained largely elusive. Here, we studied mature cerebellar parallel-fiber synapses and found that Cav2.2 does not control spontaneous release. However, Ca2+ influx through Cav2.2 significantly boosted vesicle recruitment during trains of action potentials. Thus, Cav2.2 in mature parallel-fiber boutons participate in sustaining synaptic transmission during prolonged activity.

Rate-limiting recovery processes in neurotransmission under sustained stimulation

At active zones of chemical synapses, an arriving electric signal induces the fusion of vesicles with the presynaptic membrane, thereby releasing neurotransmitters into the synaptic cleft. After a fusion event, both the release site and the vesicle undergo a recovery process before becoming available for reuse again. Of central interest is the question which of the two restoration steps acts as the limiting factor during neurotransmission under high-frequency sustained stimulation. In order to investigate this problem, we introduce a non-linear reaction network which involves explicit recovery steps for both the vesicles and the release sites, and includes the induced time-dependent output current. The associated reaction dynamics are formulated by means of ordinary differential equations (ODEs), as well as via the associated stochastic jump process. While the stochastic jump model describes the dynamics at a single active zone, the average over many active zones is close to the ODE solution and shares its periodic structure. The reason for this can be traced back to the insight that recovery dynamics of vesicles and release sites are statistically almost independent. A sensitivity analysis on the recovery rates based on the ODE formulation reveals that neither the vesicle nor the release site recovery step can be identified as the essential rate-limiting step but that the rate-limiting feature changes over the course of stimulation. Under sustained stimulation, the dynamics given by the ODEs exhibit transient changes leading from an initial depression of the postsynaptic response to an asymptotic periodic orbit, while the individual trajectories of the stochastic jump model lack the oscillatory behavior and asymptotic periodicity of the ODE-solution.

Lytic granule exocytosis at immune synapses: lessons from neuronal synapses

Regulated exocytosis is a central mechanism of cellular communication. It is not only the basis for neurotransmission and hormone release, but also plays an important role in the immune system for the release of cytokines and cytotoxic molecules. In cytotoxic T lymphocytes (CTLs), the formation of the immunological synapse is required for the delivery of the cytotoxic substances such as granzymes and perforin, which are stored in lytic granules and released via exocytosis. The molecular mechanisms of their fusion with the plasma membrane are only partially understood. In this review, we discuss the molecular players involved in the regulated exocytosis of CTL, highlighting the parallels and differences to neuronal synaptic transmission. Additionally, we examine the strengths and weaknesses of both systems to study exocytosis.

Morphofunctional changes at the active zone during synaptic vesicle exocytosis

Synaptic vesicle (SV) fusion with the plasma membrane (PM) proceeds through intermediate steps that remain poorly resolved. The effect of persistent high or low exocytosis activity on intermediate steps remains unknown. Using spray-mixing plunge-freezing cryo-electron tomography we observe events following synaptic stimulation at nanometer resolution in near-native samples. Our data suggest that during the stage that immediately follows stimulation, termed early fusion, PM and SV membrane curvature changes to establish a point contact. The next stage-late fusion-shows fusion pore opening and SV collapse. During early fusion, proximal tethered SVs form additional tethers with the PM and increase the inter-SV connector number. In the late-fusion stage, PM-proximal SVs lose their interconnections, allowing them to move toward the PM. Two SNAP-25 mutations, one arresting and one disinhibiting spontaneous release, cause connector loss. The disinhibiting mutation causes loss of membrane-proximal multiple-tethered SVs. Overall, tether formation and connector dissolution are triggered by stimulation and respond to spontaneous fusion rate manipulation. These morphological observations likely correspond to SV transition from one functional pool to another.

Presynaptic Plasticity Is Associated with Actin Polymerization

Modulation of presynaptic short-term plasticity induced by actin polymerization was studied in rat hippocampal slices using the paired-pulse paradigm. Schaffer collaterals were stimulated with paired pulses with a 70-ms interstimulus interval every 30 s before and during perfusion with jasplakinolide, an activator of actin polymerization. Jasplakinolide application resulted in the increase in the amplitudes of CA3-CA1 responses (potentiation) accompanied by a decrease in the paired-pulse facilitation, suggesting induction of presynaptic modifications. Jasplakinolide-induced potentiation depended on the initial paired-pulse rate. These data indicate that the jasplakinolide-mediated changes in actin polymerization increased the probability of neurotransmitter release. Less typical for CA3-CA1 synapses responses, such as a very low paired-pulse ratio (close to 1 or even lower) or even paired-pulse depression, were affected differently. Thus, jasplakinolide caused potentiation of the second, but not the first response to the paired stimulus, which increased the paired-pulse ratio from 0.8 to 1.0 on average, suggesting a negative impact of jasplakinolide on the mechanisms promoting paired-pulse depression. In general, actin polymerization facilitated potentiation, although the patterns of potentiation differed depending on the initial synapse characteristics. We conclude that in addition to the increase in the neurotransmitter release probability, jasplakinolide induced other actin polymerization-dependent mechanisms, including those involved in the paired-pulse depression.

Increased vesicle fusion competence underlies long-term potentiation at hippocampal mossy fiber synapses

Presynaptic long-term potentiation (LTP) is thought to play an important role in learning and memory. However, the underlying mechanism remains elusive because of the difficulty of direct recording during LTP. Hippocampal mossy fiber synapses exhibit pronounced LTP of transmitter release after tetanic stimulation and have been used as a model of presynaptic LTP. Here, we induced LTP by optogenetic tools and applied direct presynaptic patch-clamp recordings. The action potential waveform and evoked presynaptic Ca²⁺ currents remained unchanged after LTP induction. Membrane capacitance measurements suggested higher release probability of synaptic vesicles without changing the number of release-ready vesicles after LTP induction. Synaptic vesicle replenishment was also enhanced. Furthermore, stimulated emission depletion microscopy suggested an increase in the numbers of Munc13-1 and RIM1 molecules within active zones. We propose that dynamic changes in the active zone components may be relevant for the increased fusion competence and synaptic vesicle replenishment during LTP.

Different priming states of synaptic vesicles underlie distinct release probabilities at hippocampal excitatory synapses

A stunning example of synaptic diversity is the postsynaptic target cell-type-dependent difference in synaptic efficacy in cortical networks. Here, we show that CA1 pyramidal cell (PC) to fast spiking interneuron (FSIN) connections have 10-fold larger release probability (P_v) than those on oriens lacunosum-moleculare (O-LM) interneurons. Freeze-fracture immunolabeling revealed that different nano-topologies and coupling distances between Ca²⁺ channels and release sites (RSs) are not responsible for the distinct P_v. Although [Ca²⁺] transients are 40% larger in FSINs innervating boutons, when [Ca²⁺] entry is matched in the two bouton populations, EPSCs in O-LM cells are still 7-fold smaller. However, application of a phorbol ester analog resulted in a ∼2.5-fold larger augmentation at PC - O-LM compared to PC - FSIN synapses, suggesting incomplete docking or priming of vesicles. Similar densities of docked vesicles rule out distinct RS occupancies and demonstrate that incompletely primed, but docked, vesicles limit the output of PC - O-LM synapses.

A sequential two-step priming scheme reproduces diversity in synaptic strength and short-term plasticity

Central nervous system synapses are diverse in strength and plasticity. Short-term plasticity has traditionally been evaluated with models postulating a single pool of functionally homogeneous fusion-competent synaptic vesicles. Many observations are not easily explainable by such simple models. We established and experimentally validated a scheme of synaptic vesicle priming consisting of two sequential and reversible steps of release–machinery assembly. This sequential two-step priming scheme faithfully reproduced plasticity at a glutamatergic model synapse. The proposed priming and fusion scheme was consistent with the measured mean responses and with the experimentally observed heterogeneity between synapses. Vesicle fusion probability was found to be relatively uniform among synapses, while the priming equilibrium at rest of mature versus immature vesicle priming states differed greatly.

Glutamatergic synapses display variable strength and diverse short-term plasticity (STP), even for a given type of connection. Using nonnegative tensor factorization and conventional state modeling, we demonstrate that a kinetic scheme consisting of two sequential and reversible steps of release–machinery assembly and a final step of synaptic vesicle (SV) fusion reproduces STP and its diversity among synapses. Analyzing transmission at the calyx of Held synapses reveals that differences in synaptic strength and STP are not primarily caused by variable fusion probability (p_fusion) but are determined by the fraction of docked synaptic vesicles equipped with a mature release machinery. Our simulations show that traditional quantal analysis methods do not necessarily report p_fusion of SVs with a mature release machinery but reflect both p_fusion and the distribution between mature and immature priming states at rest. Thus, the approach holds promise for a better mechanistic dissection of the roles of presynaptic proteins in the sequence of SV docking, two-step priming, and fusion. It suggests a mechanism for activity-induced redistribution of synaptic efficacy.

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.

Novel types of frequency filtering in the lateral perforant path projections to dentate gyrus

Despite its evident importance to learning theory and models, the manner in which the lateral perforant path (LPP) transforms signals from entorhinal cortex to hippocampus is not well understood. The present studies measured synaptic responses in the dentate gyrus (DG) of adult mouse hippocampal slices during different patterns of LPP stimulation. Theta (5 Hz) stimulation produced a modest within-train facilitation that was markedly enhanced at the level of DG output. Gamma (50 Hz) activation resulted in a singular pattern with initial synaptic facilitation being followed by a progressively greater depression. DG output was absent after only two pulses. Reducing release probability with low extracellular calcium instated frequency facilitation to gamma stimulation while long-term potentiation, which increases release by LPP terminals, enhanced within-train depression. Relatedly, per terminal concentrations of VGLUT2, a vesicular glutamate transporter associated with high release probability, were much greater in the LPP than in CA3-CA1 connections. Attempts to circumvent the potent gamma filter using a series of short (three-pulse) 50 Hz trains spaced by 200 ms were only partially successful: composite responses were substantially reduced after the first burst, an effect opposite to that recorded in field CA1. The interaction between bursts was surprisingly persistent (>1.0 s). Low calcium improved throughput during theta/gamma activation but buffering of postsynaptic calcium did not. In all, presynaptic specializations relating to release probability produce an unusual but potent type of frequency filtering in the LPP. Patterned burst input engages a different type of filter with substrates that are also likely to be located presynaptically. KEY POINTS: The lateral perforant path (LPP)-dentate gyrus (DG) synapse operates as a low-pass filter, where responses to a train of 50 Hz, γ frequency activation are greatly suppressed. Activation with brief bursts of γ frequency information engages a secondary filter that persists for prolonged periods (lasting seconds). Both forms of LPP frequency filtering are influenced by presynaptic, as opposed to postsynaptic, processes; this contrasts with other hippocampal synapses. LPP frequency filtering is modified by the unique presynaptic long-term potentiation at this synapse. Computational simulations indicate that presynaptic factors associated with release probability and vesicle recycling may underlie the potent LPP-DG frequency filtering.

Mechanisms of Synaptic Vesicle Exo- and Endocytosis

Within 1 millisecond of action potential arrival at presynaptic terminals voltage-gated Ca²⁺ channels open. The Ca²⁺ channels are linked to synaptic vesicles which are tethered by active zone proteins. Ca²⁺ entrance into the active zone triggers: (1) the fusion of the vesicle and exocytosis, (2) the replenishment of the active zone with vesicles for incoming exocytosis, and (3) various types of endocytosis for vesicle reuse, dependent on the pattern of firing. These time-dependent vesicle dynamics are controlled by presynaptic Ca²⁺ sensor proteins, regulating active zone scaffold proteins, fusion machinery proteins, motor proteins, endocytic proteins, several enzymes, and even Ca²⁺ channels, following the decay of Ca²⁺ concentration after the action potential. Here, I summarize the Ca²⁺-dependent protein controls of synchronous and asynchronous vesicle release, rapid replenishment of the active zone, endocytosis, and short-term plasticity within 100 msec after the action potential. Furthermore, I discuss the contribution of active zone proteins to presynaptic plasticity and to homeostatic readjustment during and after intense activity, in addition to activity-dependent endocytosis.

Transient docking of synaptic vesicles: Implications and mechanisms

As synaptic vesicles fuse, they must continually be replaced with new docked, fusion-competent vesicles to sustain neurotransmission. It has long been appreciated that vesicles are recruited to docking sites in an activity-dependent manner. However, once entering the sites, vesicles were thought to be stably docked, awaiting calcium signals. Based on recent data from electrophysiology, electron microscopy, biochemistry, and computer simulations, a picture emerges in which vesicles can rapidly and reversibly transit between docking and undocking during activity. This "transient docking" can account for many aspects of synaptic physiology. In this review, we cover recent evidence for transient docking, physiological processes at the synapse that it may support, and progress on the underlying mechanisms. We also discuss an open question: what determines for how long and whether vesicles stay docked, or eventually undock?

Three small vesicular pools in sequence govern synaptic response dynamics during action potential trains

Short-term changes in the strength of synaptic connections underlie many brain functions. The strength of a synapse in response to subsequent stimulation is largely determined by the remaining number of synaptic vesicles available for release. We developed a methodological approach to measure the dynamics of various vesicle pools following synaptic activity. We find that the readily releasable pool, which comprises vesicles that are docked or tethered to release sites, is fed by a small-sized pool containing approximately one to four vesicles per release site at rest. This upstream pool is significantly depleted even after a short stimulation train. Therefore, regulation of the size of the upstream pool emerges as a key factor in determining synaptic strength during and after sustained stimulation.

During prolonged trains of presynaptic action potentials (APs), synaptic release reaches a stable level that reflects the speed of replenishment of the readily releasable pool (RRP). Determining the size and filling dynamics of vesicular pools upstream of the RRP has been hampered by a lack of precision of synaptic output measurements during trains. Using the recent technique of tracking vesicular release in single active zone synapses, we now developed a method that allows the sizes of the RRP and upstream pools to be followed in time. We find that the RRP is fed by a small-sized pool containing approximately one to four vesicles per docking site at rest. This upstream pool is significantly depleted by short AP trains, and reaches a steady, depleted state for trains of >10 APs. We conclude that a small, highly dynamic vesicular pool upstream of the RRP potently controls synaptic strength during sustained stimulation.

Quantal analysis estimates docking site occupancy determining short-term depression at hippocampal glutamatergic synapses

Before fusion, synaptic vesicles (SVs) pause at discrete release/docking sites. During repetitive stimulation, the probability of site occupancy changes following SV fusion and replenishment. The occupancy probability is considered to be one of the crucial determinants of synaptic strength, but it is difficult to estimate separately because it usually blends with other synaptic parameters. Thus, the contribution of site occupancy to synaptic function, particularly to synaptic depression, remains elusive. Here, we directly estimated the occupancy probability at the hippocampal mossy fibre-CA3 interneuron synapse showing synaptic depression, using statistics of counts of vesicular events detected by deconvolution. We found that this synapse had a particularly high occupancy (∼0.85) with a high release probability of a docked SV (∼0.8) under 3 mm external calcium conditions. Analyses of quantal amplitudes and SV counts indicated that quantal size reduction decreased the amplitudes of all responses in a train to a similar degree, whereas release/docking site number was unchanged during trains, suggesting that quantal size and release/docking site number had little influence on the extent of synaptic depression. Model simulations revealed that the initial occupancy with high release probability and slow replenishment determined the time course of synaptic depression. Consistently, decreasing external calcium concentration reduced both the occupancy and release probability, and the reductions in turn produced less depression. Based on these results, we suggest that the occupancy probability is a crucial determinant of short-term synaptic depression at glutamatergic synapses in the hippocampus. KEY POINTS: The occupancy probability of a release/docking site by a synaptic vesicle at presynaptic terminals is considered to be one of the crucial determinants of synaptic strength, but it is difficult to estimate separately from other synaptic parameters. Here, we directly estimate the occupancy probability at the hippocampal mossy fibre-interneuron synapse using statistics of vesicular events detected by deconvolution. We show that the synapses have particularly high occupancy (0.85) with high release probability (0.8) under high external calcium concentration ([Ca²⁺ ]_o ) conditions, and that both parameter values change with [Ca²⁺ ]_o , shaping synaptic depression. Analyses of the quantal amplitudes and synaptic vesicle counts suggest that quantal sizes and release/docking site number have little influence on the extent of synaptic depression. The results suggest that the occupancy probability is a crucial determinant of short-term synaptic depression at glutamatergic synapses in the hippocampus.

Topographical distance between presynaptic Ca2+ channels and exocytotic Ca2+ sensors contributes to differential facilitatory actions of roscovitine on neurotransmitter release at cerebellar glutamatergic and GABAergic synapses

Calcium influx into presynaptic terminals through voltage-gated Ca²⁺ channels triggers univesicular or multivesicular release of neurotransmitters depending on the characteristics of the release machinery. However, the mechanisms underlying multivesicular release (MVR) and its regulation remain unclear. Previous studies showed that in rat cerebellum, the cyclin-dependent kinase inhibitor roscovitine profoundly increases excitatory postsynaptic current (EPSC) amplitudes at granule cell (GC)-Purkinje cell (PC) synapses by enhancing the MVR of glutamate. This compound can also moderately augment the amplitude and prolong the decay time of inhibitory postsynaptic currents (IPSCs) at molecular layer interneuron (MLI)-PC synapses via MVR enhancement and GABA spillover, thus allowing for persistent activation of perisynaptic GABA receptors. The enhanced MVR may depend on the driving force for Ca_v 2.1 channel-mediated Ca²⁺ influx. To determine whether the distinct spatiotemporal dynamics of presynaptic Ca²⁺ influence MVR, we compared the effects of slow and fast Ca²⁺ chelators, that is, EGTA and BAPTA, respectively, on roscovitine-induced actions at GC-PC and MLI-PC synapses. Membrane-permeable EGTA-AM decreased GC-PC EPSC and MLI-PC IPSC amplitudes to a similar extent but suppressed the roscovitine-induced enhancement of EPSCs. In contrast, BAPTA-AM attenuated the effects of roscovitine on IPSCs. These results suggest that roscovitine augmented glutamate release by activating the release machinery located distally from the Ca_v 2.1 channel clusters, while it enhanced GABA release in a manner less dependent on those at distal sites. Therefore, the spatial relationships among Ca²⁺ channels, buffers, and sensors are critical determinants of the differential facilitatory actions of roscovitine on glutamatergic and GABAergic synapses in the cerebellar cortex.

Calcium dependence of neurotransmitter release at a high fidelity synapse

The Ca²⁺-dependence of the priming, fusion, and replenishment of synaptic vesicles are fundamental parameters controlling neurotransmitter release and synaptic plasticity. Despite intense efforts, these important steps in the synaptic vesicles’ cycle remain poorly understood due to the technical challenge in disentangling vesicle priming, fusion, and replenishment. Here, we investigated the Ca²⁺-sensitivity of these steps at mossy fiber synapses in the rodent cerebellum, which are characterized by fast vesicle replenishment mediating high-frequency signaling. We found that the basal free Ca²⁺ concentration (<200 nM) critically controls action potential-evoked release, indicating a high-affinity Ca²⁺ sensor for vesicle priming. Ca²⁺ uncaging experiments revealed a surprisingly shallow and non-saturating relationship between release rate and intracellular Ca²⁺ concentration up to 50 μM. The rate of vesicle replenishment during sustained elevated intracellular Ca²⁺ concentration exhibited little Ca²⁺-dependence. Finally, quantitative mechanistic release schemes with five Ca²⁺ binding steps incorporating rapid vesicle replenishment via parallel or sequential vesicle pools could explain our data. We thus show that co-existing high- and low-affinity Ca²⁺ sensors mediate priming, fusion, and replenishment of synaptic vesicles at a high-fidelity synapse.

Calcium-dependent docking of synaptic vesicles

The concentration of calcium ions in presynaptic terminals regulates transmitter release, but underlying mechanisms have remained unclear. Here we review recent studies that shed new light on this issue. Fast-freezing electron microscopy and total internal reflection fluorescence microscopy studies reveal complex calcium-dependent vesicle movements including docking on a millisecond time scale. Recordings from so-called 'simple synapses' indicate that calcium not only triggers exocytosis, but also modifies synaptic strength by controlling a final, rapid vesicle maturation step before release. Molecular studies identify several calcium-sensitive domains on Munc13 and on synaptotagmin-1 that are likely involved in bringing the vesicular and plasma membranes closer together in response to calcium elevation. Together, these results suggest that calcium-dependent vesicle docking occurs in a wide range of time domains and plays a crucial role in several phenomena including synaptic facilitation, post-tetanic potentiation, and neuromodulator-induced potentiation.

Myosin V Regulates Spatial Localization of Different Forms of Neurotransmitter Release in Central Synapses

Synaptic active zone (AZ) contains multiple specialized release sites for vesicle fusion. The utilization of release sites is regulated to determine spatiotemporal organization of the two main forms of synchronous release, uni-vesicular (UVR) and multi-vesicular (MVR). We previously found that the vesicle-associated molecular motor myosin V regulates temporal utilization of release sites by controlling vesicle anchoring at release sites in an activity-dependent manner. Here we show that acute inhibition of myosin V shifts preferential location of vesicle docking away from AZ center toward periphery, and results in a corresponding spatial shift in utilization of release sites during UVR. Similarly, inhibition of myosin V also reduces preferential utilization of central release sites during MVR, leading to more spatially distributed and temporally uniform MVR that occurs farther away from the AZ center. Using a modeling approach, we provide a conceptual framework that unites spatial and temporal functions of myosin V in vesicle release by controlling the gradient of release site release probability across the AZ, which in turn determines the spatiotemporal organization of both UVR and MVR. Thus myosin V regulates both temporal and spatial utilization of release sites during two main forms of synchronous release.

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.

Synaptic vesicles transiently dock to refill release sites

Synaptic vesicles fuse with the plasma membrane to release neurotransmitter following an action potential, after which new vesicles must ‘dock’ to refill vacated release sites. To capture synaptic vesicle exocytosis at cultured mouse hippocampal synapses, we induced single action potentials by electrical field stimulation then subjected neurons to high-pressure freezing to examine their morphology by electron microscopy. During synchronous release, multiple vesicles can fuse at a single active zone. Fusions during synchronous release are distributed throughout the active zone, whereas fusions during asynchronous release are biased toward the center of the active zone. After stimulation, the total number of docked vesicles across all synapses decreases by ~40%. Within 14 ms, new vesicles are recruited and fully replenish the docked pool, but this docking is transient and they either undock or fuse within 100 ms. These results demonstrate that recruitment of synaptic vesicles to release sites is rapid and reversible.

Nanoscale Organization of Vesicle Release at Central Synapses

Presynaptic boutons support neurotransmitter release with nanoscale precision at sub-millisecond timescales. Studies over the past two decades have revealed a rich tapestry of molecular players governing synaptic vesicle fusion at highly specialized release sites in the active zone (AZ). However, the spatiotemporal organization of release at active synapses remains elusive, in part owing to the extremely small size of the AZ and the limited resolution of conventional approaches. Recent advances in fluorescence nanoscopy have revolutionized direct investigation of presynaptic release organization and dynamics. We discuss here recent nanoscopy-based studies of the molecular architecture, the spatial organization and dynamic regulation of release sites, and the mechanisms of release site replenishment. These findings have uncovered previously unknown levels of structural and functional organization at central synapses, with important implications for synaptic transmission and plasticity.

Post-tetanic potentiation lowers the energy barrier for synaptic vesicle fusion independently of Synaptotagmin-1

Previously, we showed that modulation of the energy barrier for synaptic vesicle fusion boosts release rates supralinearly (Schotten, 2015). Here we show that mouse hippocampal synapses employ this principle to trigger Ca²⁺-dependent vesicle release and post-tetanic potentiation (PTP). We assess energy barrier changes by fitting release kinetics in response to hypertonic sucrose. Mimicking activation of the C2A domain of the Ca²⁺-sensor Synaptotagmin-1 (Syt1), by adding a positive charge (Syt1^D232N) or increasing its hydrophobicity (Syt1^4W), lowers the energy barrier. Removing Syt1 or impairing its release inhibitory function (Syt1^9Pro) increases spontaneous release without affecting the fusion barrier. Both phorbol esters and tetanic stimulation potentiate synaptic strength, and lower the energy barrier equally well in the presence and absence of Syt1. We propose a model where tetanic stimulation activates Syt1-independent mechanisms that lower the energy barrier and act additively with Syt1-dependent mechanisms to produce PTP by exerting multiplicative effects on release rates.

A Slow Short-Term Depression at Purkinje to Deep Cerebellar Nuclear Neuron Synapses Supports Gain-Control and Linear Encoding over Second-Long Time Windows

Modifications in the sensitivity of neural elements allow the brain to adapt its functions to varying demands. Frequency-dependent short-term synaptic depression (STD) provides a dynamic gain-control mechanism enabling adaptation to different background conditions alongside enhanced sensitivity to input-driven changes in activity. In contrast, synapses displaying frequency-invariant transmission can faithfully transfer ongoing presynaptic rates enabling linear processing, deemed critical for many functions. However, rigid frequency-invariant transmission may lead to runaway dynamics and low sensitivity to changes in rate. Here, I investigated the Purkinje cell to deep cerebellar nuclei neuron synapses (PC_DCNs), which display frequency invariance, and yet, PCs maintain background activity at disparate rates, even at rest. Using protracted PC_DCN activation (120 s) to mimic background activity in cerebellar slices from mature mice of both sexes, I identified a previously unrecognized, frequency-dependent, slow STD (S-STD), adapting IPSC amplitudes in tens of seconds to minutes. However, after changes in activation rates, over a behavior-relevant second-long time window, S-STD enabled scaled linear encoding of PC rates in synaptic charge transfer and DCN spiking activity. Combined electrophysiology, optogenetics, and statistical analysis suggested that S-STD mechanism is input-specific, involving decreased ready-to-release quanta, and distinct from faster short-term plasticity (f-STP). Accordingly, an S-STD component with a scaling effect (i.e., activity-dependent release sites inactivation), extending a model explaining PC_DCN release on shorter timescales using balanced f-STP, reproduced the experimental results. Thus, these results elucidates a novel slow gain-control mechanism able to support linear transfer of behavior-driven/learned PC rates concurrently with background activity adaptation, and furthermore, provides an alternative pathway to refine PC output.SIGNIFICANCE STATEMENT The brain can adapt to varying demands by dynamically changing the gain of its synapses; however, some tasks require ongoing linear transfer of presynaptic rates, seemingly incompatible with nonlinear gain adaptation. Here, I report a novel slow gain-control mechanism enabling scaled linear encoding of presynaptic rates over behavior-relevant time windows, and adaptation to background activity at the Purkinje to deep cerebellar nuclear neurons synapses (PC_DCNs). A previously unrecognized PC_DCNs slow and frequency-dependent short-term synaptic depression (S-STD) mediates this process. Experimental evidence and simulations suggested that scaled linear encoding emerges from the combination of S-STD slow dynamics and frequency-invariant transmission at faster timescales. These results demonstrate a mechanism reconciling rate code with background activity adaptation and suitable for flexibly tuning PCs output via background activity modulation.

Direct imaging of rapid tethering of synaptic vesicles accompanying exocytosis at a fast central synapse

A high rate of synaptic vesicle (SV) release is required at cerebellar mossy fiber terminals for rapid information processing. As the number of release sites is limited, fast SV reloading is necessary to achieve sustained release. However, rapid reloading has not been observed directly. Here, we visualize SV movements near presynaptic membrane using total internal reflection fluorescence (TIRF) microscopy. Upon stimulation, SVs appeared in the TIRF-field and became tethered to the presynaptic membrane with unexpectedly rapid time course, almost as fast as SVs disappeared due to release. However, such stimulus-induced tethering was abolished by inhibiting exocytosis, suggesting that the tethering is tightly coupled to preceding exocytosis. The newly tethered vesicles became fusion competent not immediately but only 300 ms to 400 ms after tethering. Together with model simulations, we propose that rapid tethering leads to an immediate filling of vacated spaces and release sites within <100 nm of the active zone by SVs, which serve as precursors of readily releasable vesicles, thereby shortening delays during sustained activity.

Roscovitine differentially facilitates cerebellar glutamatergic and GABAergic neurotransmission by enhancing Cav 2.1 channel-mediated multivesicular release

Synaptic vesicle exocytosis is triggered by Ca²⁺ influx through several subtypes of voltage-gated calcium channels in the presynaptic terminal. We previously reported that paired-pulse stimulation at brief intervals increases Ca_v 2.1 (P/Q-type) channel-mediated multivesicular release (MVR) at glutamatergic synapses between granule cells (GCs) and molecular layer interneurons (MLIs) in rat cerebellar slices. However, it has yet to be determined how Ca_v 2 channel subtypes take part in MVR in single axon terminal. This study therefore aimed at examining the effects of roscovitine on different types of cerebellar synapses that make contacts with Purkinje cells (PCs), because this compound has been shown to enhance Ca_v 2.1 channel-mediated MVR at GC-MLI synapses. Bath application of roscovitine profoundly increased the amplitude of excitatory postsynaptic currents (EPSCs) at GC-PC synapses by a presynaptic mechanism as previously observed at GC-MLI synapses, whereas it caused a marginal effect on climbing fiber-mediated EPSCs in PCs. At MLI-PC synapses, roscovitine increased both the amplitude and decay time of inhibitory postsynaptic currents (IPSCs) by enhancing multivesicular GABA release. When extracellular Ca²⁺ concentration ([Ca²⁺ ]_e ) decreased, roscovitine became less effective in increasing GC-PC EPSCs. By contrast, roscovitine was able to augment MLI-PC IPSCs in the low [Ca²⁺ ]_e . The Ca_v 2.1 channel blocker ω-agatoxin IVA suppressed the roscovitine-induced facilitatory actions on both GC-PC EPSCs and MLI-PC IPSCs. These results demonstrate that roscovitine enhances MVR at the GC-PC excitatory synapses in a manner dependent on the driving force of Ca_v 2.1 channel-mediated Ca²⁺ influx into the nerve terminal, while it also facilitates MLI-PC inhibitory transmission via Ca²⁺ -insensitive mechanisms.

Differentially poised vesicles underlie fast and slow components of release at single synapses

In several types of central mammalian synapses, sustained presynaptic stimulation leads to a sequence of two components of synaptic vesicle release, reflecting the consecutive contributions of a fast-releasing pool (FRP) and of a slow-releasing pool (SRP). Previous work has shown that following common depletion by a strong stimulation, FRP and SRP recover with different kinetics. However, it has remained unclear whether any manipulation could lead to a selective enhancement of either FRP or SRP. To address this question, we have performed local presynaptic calcium uncaging in single presynaptic varicosities of cerebellar interneurons. These varicosities typically form "simple synapses" onto postsynaptic interneurons, involving several (one to six) docking/release sites within a single active zone. We find that strong uncaging laser pulses elicit two phases of release with time constants of ∼1 ms (FRP release) and ∼20 ms (SRP release). When uncaging was preceded by action potential-evoked vesicular release, the extent of SRP release was specifically enhanced. We interpret this effect as reflecting an increased likelihood of two-step release (docking then release) following the elimination of docked synaptic vesicles by action potential-evoked release. In contrast, a subthreshold laser-evoked calcium elevation in the presynaptic varicosity resulted in an enhancement of the FRP release. We interpret this latter effect as reflecting an increased probability of occupancy of docking sites following subthreshold calcium increase. In conclusion, both fast and slow components of release can be specifically enhanced by certain presynaptic manipulations. Our results have implications for the mechanism of docking site replenishment and the regulation of synaptic responses, in particular following activation of ionotropic presynaptic receptors.

Incomplete vesicular docking limits synaptic strength under high release probability conditions

Central mammalian synapses release synaptic vesicles in dedicated structures called docking/release sites. It has been assumed that when voltage-dependent calcium entry is sufficiently large, synaptic output attains a maximum value of one synaptic vesicle per action potential and per site. Here we use deconvolution to count synaptic vesicle output at single sites (mean site number per synapse: 3.6). When increasing calcium entry with tetraethylammonium in 1.5 mM external calcium concentration, we find that synaptic output saturates at 0.22 vesicle per site, not at 1 vesicle per site. Fitting the results with current models of calcium-dependent exocytosis indicates that the 0.22 vesicle limit reflects the probability of docking sites to be occupied by synaptic vesicles at rest, as only docked vesicles can be released. With 3 mM external calcium, the maximum output per site increases to 0.47, indicating an increase in docking site occupancy as a function of external calcium concentration.

Endophilin-A coordinates priming and fusion of neurosecretory vesicles via intersectin

Endophilins-A are conserved endocytic adaptors with membrane curvature-sensing and -inducing properties. We show here that, independently of their role in endocytosis, endophilin-A1 and endophilin-A2 regulate exocytosis of neurosecretory vesicles. The number and distribution of neurosecretory vesicles were not changed in chromaffin cells lacking endophilin-A, yet fast capacitance and amperometry measurements revealed reduced exocytosis, smaller vesicle pools and altered fusion kinetics. The levels and distributions of the main exocytic and endocytic factors were unchanged, and slow compensatory endocytosis was not robustly affected. Endophilin-A’s role in exocytosis is mediated through its SH3-domain, specifically via a direct interaction with intersectin-1, a coordinator of exocytic and endocytic traffic. Endophilin-A not able to bind intersectin-1, and intersectin-1 not able to bind endophilin-A, resulted in similar exocytic defects in chromaffin cells. Altogether, we report that two endocytic proteins, endophilin-A and intersectin-1, are enriched on neurosecretory vesicles and regulate exocytosis by coordinating neurosecretory vesicle priming and fusion.

Endophilins-A are conserved membrane-associated proteins required for endocytosis. Here, the authors report that endophilins-A also promote exocytosis of neurosecretory vesicles by coordinating priming and fusion through intersectin-1, independently of their roles in different types of endocytosis.

Rapid regulation of vesicle priming explains synaptic facilitation despite heterogeneous vesicle:Ca2+ channel distances

Chemical synaptic transmission relies on the Ca²⁺-induced fusion of transmitter-laden vesicles whose coupling distance to Ca²⁺ channels determines synaptic release probability and short-term plasticity, the facilitation or depression of repetitive responses. Here, using electron- and super-resolution microscopy at the Drosophila neuromuscular junction we quantitatively map vesicle:Ca²⁺ channel coupling distances. These are very heterogeneous, resulting in a broad spectrum of vesicular release probabilities within synapses. Stochastic simulations of transmitter release from vesicles placed according to this distribution revealed strong constraints on short-term plasticity; particularly facilitation was difficult to achieve. We show that postulated facilitation mechanisms operating via activity-dependent changes of vesicular release probability (e.g. by a facilitation fusion sensor) generate too little facilitation and too much variance. In contrast, Ca²⁺-dependent mechanisms rapidly increasing the number of releasable vesicles reliably reproduce short-term plasticity and variance of synaptic responses. We propose activity-dependent inhibition of vesicle un-priming or release site activation as novel facilitation mechanisms.

Cells in the nervous system of all animals communicate by releasing and sensing chemicals at contact points named synapses. The ‘talking’ (or pre-synaptic) cell stores the chemicals close to the synapse, in small spheres called vesicles. When the cell is activated, calcium ions flow in and interact with the release-ready vesicles, which then spill the chemicals into the synapse. In turn, the ‘listening’ (or post-synaptic) cell can detect the chemicals and react accordingly.

When the pre-synaptic cell is activated many times in a short period, it can release a greater quantity of chemicals, allowing a bigger reaction in the post-synaptic cell. This phenomenon is known as facilitation, but it is still unclear how exactly it can take place. This is especially the case when many of the vesicles are not ready to respond, for example when they are too far from where calcium flows into the cell. Computer simulations have been created to model facilitation but they have assumed that all vesicles are placed at the same distance to the calcium entry point: Kobbersmed et al. now provide evidence that this assumption is incorrect.

Two high-resolution imaging techniques were used to measure the actual distances between the vesicles and the calcium source in the pre-synaptic cells of fruit flies: this showed that these distances are quite variable – some vesicles sit much closer to the source than others.

This information was then used to create a new computer model to simulate facilitation. The results from this computing work led Kobbersmed et al. to suggest that facilitation may take place because a calcium-based mechanism in the cell increases the number of vesicles ready to release their chemicals.

This new model may help researchers to better understand how the cells in the nervous system work. Ultimately, this can guide experiments to investigate what happens when information processing at synapses breaks down, for example in diseases such as epilepsy.

Developmental Increase of Neocortical Presynaptic Efficacy via Maturation of Vesicle Replenishment

The efficacy of neocortical synapses to transmit during bursts of action potentials (APs) increases during development but the underlying mechanisms are largely unclear. We investigated synaptic efficacy at synapses between layer 5 pyramidal neurons (L5PNs) during development, using paired recordings, presynaptic two-photon Ca²⁺ imaging, and numerical simulations. Our data confirm a developmental increase in paired-pulse ratios (PPRs). Independent of age, Ca²⁺ imaging revealed no AP invasion failures and linear summation of presynaptic Ca²⁺ transients, making differences in Ca²⁺ signaling an unlikely reason for developmental changes in PPR. Cumulative excitatory postsynaptic current (EPSC) amplitudes indicate that neither the size of the readily-releasable pool (RRP) nor replenishment rates were different between age groups, while the time-courses of depression differed significantly. At young synapses, EPSCs depressed rapidly to near steady-state during the first four APs, and synaptic failures (F_syn) increased from 0 to 30%. At mature synapses this drop was significantly slower and strongly biphasic, such that near steady-state depression was reached not before 18 APs with F_syn remaining between 0 and 5%. While young synapses reliably transmitted during pairs of APs, albeit with strong depression, mature synapses maintained near 100% transfer efficacy with significantly less depression during high-frequency bursts of APs. Our analysis indicates that at mature synapses a replenishment pool (RepP) is responsible for their high efficacy during bursting activity, while this RepP is functionally immature at young synapses. Hence, our data provide evidence that the functional maturation of a RepP underlies increasing synaptic efficacy during the development of an excitatory cortical synapse.

Spatiotemporal dynamics of multi-vesicular release is determined by heterogeneity of release sites within central synapses

A synaptic active zone (AZ) can release multiple vesicles in response to an action potential. This multi-vesicular release (MVR) occurs at most synapses, but its spatiotemporal properties are unknown. Nanoscale-resolution detection of individual release events in hippocampal synapses revealed unprecedented heterogeneity among vesicle release sites within a single AZ, with a gradient of release probability decreasing from AZ center to periphery. Parallel to this organization, MVR events preferentially overlap with uni-vesicular release (UVR) events at sites closer to an AZ center. Pairs of fusion events comprising MVR are also not perfectly synchronized, and the earlier event tends to occur closer to AZ center. The spatial features of release sites and MVR events are similarly tightened by buffering intracellular calcium. These observations revealed a marked heterogeneity of release site properties within individual AZs, which determines the spatiotemporal features of MVR events and is controlled, in part, by non-uniform calcium elevation across the AZ.

Considerations for Measuring Activity-Dependence of Recruitment of Synaptic Vesicles to the Readily Releasable Pool

The connection strength of most chemical synapses changes dynamically during normal use as a function of the recent history of activity. The phenomenon is known as short-term synaptic plasticity or synaptic dynamics, and is thought to be involved in processing and filtering information as it is transmitted across the synaptic cleft. Multiple presynaptic mechanisms have been implicated, but large gaps remain in our understanding of how the mechanisms are modulated and how they interact. One important factor is the timing of recruitment of synaptic vesicles to a readily-releasable pool. A number of studies have concluded that activity and/or residual Ca²⁺ can accelerate the mechanism, but alternative explanations for some of the evidence have emerged. Here I review the methodology that we have developed for isolating the recruitment and the dependence on activity from other kinds of mechanisms that are activated concurrently.

Microtubule and Actin Differentially Regulate Synaptic Vesicle Cycling to Maintain High-Frequency Neurotransmission

Cytoskeletal filaments such as microtubules (MTs) and filamentous actin (F-actin) dynamically support cell structure and functions. In central presynaptic terminals, F-actin is expressed along the release edge and reportedly plays diverse functional roles, but whether axonal MTs extend deep into terminals and play any physiological role remains controversial.

Cytoskeletal filaments such as microtubules (MTs) and filamentous actin (F-actin) dynamically support cell structure and functions. In central presynaptic terminals, F-actin is expressed along the release edge and reportedly plays diverse functional roles, but whether axonal MTs extend deep into terminals and play any physiological role remains controversial. At the calyx of Held in rats of either sex, confocal and high-resolution microscopy revealed that MTs enter deep into presynaptic terminal swellings and partially colocalize with a subset of synaptic vesicles (SVs). Electrophysiological analysis demonstrated that depolymerization of MTs specifically prolonged the slow-recovery time component of EPSCs from short-term depression induced by a train of high-frequency stimulation, whereas depolymerization of F-actin specifically prolonged the fast-recovery component. In simultaneous presynaptic and postsynaptic action potential recordings, depolymerization of MTs or F-actin significantly impaired the fidelity of high-frequency neurotransmission. We conclude that MTs and F-actin differentially contribute to slow and fast SV replenishment, thereby maintaining high-frequency neurotransmission.

SIGNIFICANCE STATEMENT The presence and functional role of MTs in the presynaptic terminal are controversial. Here, we demonstrate that MTs are present near SVs in calyceal presynaptic terminals and that MT depolymerization specifically prolongs the slow-recovery component of EPSCs from short-term depression. In contrast, F-actin depolymerization specifically prolongs fast-recovery component. Depolymerization of MT or F-actin has no direct effect on SV exocytosis/endocytosis or basal transmission, but significantly impairs the fidelity of high-frequency transmission, suggesting that presynaptic cytoskeletal filaments play essential roles in SV replenishment for the maintenance of high-frequency neurotransmission.

Doc2b Ca2+ binding site mutants enhance synaptic release at rest at the expense of sustained synaptic strength

Communication between neurons involves presynaptic neurotransmitter release which can be evoked by action potentials or occur spontaneously as a result of stochastic vesicle fusion. The Ca²⁺-binding double C₂ proteins Doc2a and –b were implicated in spontaneous and asynchronous evoked release, but the mechanism remains unclear. Here, we compared wildtype Doc2b with two Ca²⁺ binding site mutants named DN and 6A, previously classified as gain- and loss-of-function mutants. They carry the substitutions D218,220N or D163,218,220,303,357,359A respectively. We found that both mutants bound phospholipids at low Ca²⁺ concentrations and were membrane-associated in resting neurons, thus mimicking a Ca²⁺-activated state. Their overexpression in hippocampal primary cultured neurons had similar effects on spontaneous and evoked release, inducing high mEPSC frequencies and increased short-term depression. Together, these data suggest that the DN and 6A mutants both act as gain-of-function mutants at resting conditions.