Apparent calcium dependence of vesicle recruitment

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.

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.

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.

Fast resupply of synaptic vesicles requires synaptotagmin-3

Sustained neuronal activity demands a rapid resupply of synaptic vesicles to maintain reliable synaptic transmission. Such vesicle replenishment is accelerated by submicromolar presynaptic Ca²⁺ signals by an as-yet unidentified high-affinity Ca²⁺ sensor^1,2. Here we identify synaptotagmin-3 (SYT3)^3,4 as that presynaptic high-affinity Ca²⁺ sensor, which drives vesicle replenishment and short-term synaptic plasticity. Synapses in Syt3 knockout mice exhibited enhanced short-term depression, and recovery from depression was slower and insensitive to presynaptic residual Ca²⁺. During sustained neuronal firing, SYT3 accelerated vesicle replenishment and increased the size of the readily releasable pool. SYT3 also mediated short-term facilitation under conditions of low release probability and promoted synaptic enhancement together with another high-affinity synaptotagmin, SYT7 (ref. ⁵). Biophysical modelling predicted that SYT3 mediates both replenishment and facilitation by promoting the transition of loosely docked vesicles to tightly docked, primed states. Our results reveal a crucial role for presynaptic SYT3 in the maintenance of reliable high-frequency synaptic transmission. Moreover, multiple forms of short-term plasticity may converge on a mechanism of reversible, Ca²⁺-dependent vesicle docking.

Calretinin-Expressing Synapses Show Improved Synaptic Efficacy with Reduced Asynchronous Release during High-Rate Activity

Calretinin (CR) is a major calcium binding protein widely expressed in the CNS. However, its synaptic function remains largely elusive. At the auditory synapse of the endbulb of Held, CR is selectively expressed in different subtypes. Combining electrophysiology with immunohistochemistry, we investigated the synaptic transmission at the endbulb of Held synapses with and without endogenous CR expression in mature CBA/CAJ mice of either sex. Two synapse subtypes showed similar basal synaptic transmission, except a larger quantal size in CR-expressing synapses. During high-rate stimulus trains, CR-expressing synapses showed improved synaptic efficacy with significantly less depression and lower asynchronous release, suggesting more efficient exocytosis than non-CR-expressing synapses. Conversely, CR-expressing synapses had a smaller readily releasable pool size, which was countered by higher release probability and faster synaptic recovery to support sustained release during high-rate activity. EGTA-AM treatment did not change the synaptic transmission of CR-expressing synapses, but reduced synaptic depression and decreased asynchronous release at non-CR-expressing synapses, suggesting that CR helps to minimize calcium accumulation during high-rate activity. Both synapses express parvalbumin, another calcium-binding protein with slower kinetics and higher affinity than CR, but not calbindin. Furthermore, CR-expressing synapses only express the fast isoform of vesicular glutamate transporter 1 (VGluT1), while most non-CR-expressing synapses express both VGluT1 and the slower VGluT2, which may underlie their lagged synaptic recovery. The findings suggest that, paired with associated synaptic machinery, differential CR expression regulates synaptic efficacy among different subtypes of auditory nerve synapses to accomplish distinctive physiological functions in transmitting auditory information at high rates.SIGNIFICANCE STATEMENT CR is a major calcium-binding protein in the brain. It remains unclear how endogenous CR impacts synaptic transmission. We investigated the question at the large endbulb of Held synapses with selective CR expression and found that CR-expressing and non-CR-expressing synapses had similar release properties under basal synaptic transmission. During high-rate activity, however, CR-expressing synapses showed improved synaptic efficacy with less depression, lower asynchronous release, and faster recovery. Furthermore, CR-expressing synapses use exclusive VGluT1 to refill synaptic vesicles, while non-CR-expressing synapses use both VGluT1 and the slower isoform of VGluT2. Our findings suggest that CR may play significant roles in promoting synaptic efficacy during high-rate activity, and selective CR expression can differentially impact signal processing among different synapses.

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.

Munc13-1 is a Ca2+-phospholipid-dependent vesicle priming hub that shapes synaptic short-term plasticity and enables sustained neurotransmission

During ongoing presynaptic action potential (AP) firing, transmitter release is limited by the availability of release-ready synaptic vesicles (SVs). The rate of SV recruitment (SVR) to release sites is strongly upregulated at high AP frequencies to balance SV consumption. We show that Munc13-1-an essential SV priming protein-regulates SVR via a Ca2+-phospholipid-dependent mechanism. Using knockin mouse lines with point mutations in the Ca2+-phospholipid-binding C2B domain of Munc13-1, we demonstrate that abolishing Ca2+-phospholipid binding increases synaptic depression, slows recovery of synaptic strength after SV pool depletion, and reduces temporal fidelity of synaptic transmission, while increased Ca2+-phospholipid binding has the opposite effects. Thus, Ca2+-phospholipid binding to the Munc13-1-C2B domain accelerates SVR, reduces short-term synaptic depression, and increases the endurance and temporal fidelity of neurotransmission, demonstrating that Munc13-1 is a core vesicle priming hub that adjusts SV re-supply to demand.

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.

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.

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.

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.

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.

Ca-dependence of synaptic vesicle exocytosis and endocytosis at the hippocampal mossy fibre terminal

KEY POINTS

We used presynaptic capacitance measurements at the hippocampal mossy fibre terminal at room temperature to measure Ca-dependence of exo- and endocytotic kinetics. The readily releasable pool (RRP) of synaptic vesicles was released with a time constant of 30-40 ms and was sensitive to Ca buffers, BAPTA and EGTA. Our data suggest that recruitment of the vesicles to the RRP was Ca-insensitive and had a time constant of 1 s. In addition to the RRP, the reserve pool of vesicles, which had a similar size to RRP, was depleted during repetitive stimulation. Our data suggest that synaptic vesicle endocytosis was also Ca-insensitive.

ABSTRACT

Hippocampal mossy fibre terminals comprise one of the cortical terminals, which are sufficiently large to be accessible by patch clamp recordings. To measure Ca-dependence of exo- and endocytotic kinetics quantitatively, we applied presynaptic capacitance measurements to the mossy fibre terminal at room temperature. The time course of synaptic vesicle fusion was slow, with a time constant of tens of milliseconds, and was sensitive to Ca buffers EGTA and BAPTA, suggesting a loose coupling between Ca channels and synaptic vesicles. The size of the readily-releasable pool (RRP) of synaptic vesicles was relatively insensitive to Ca buffers. Once the RRP was depleted, it was recovered by a single exponential with a time constant of ∼1 s independent of the presence of Ca buffers, suggesting Ca independent vesicle replenishment. In addition to the RRP, the reserve pool of vesicles was released slowly during repetitive stimulation. Endocytosis was also insensitive to Ca buffers and had a slow time course, excluding the involvement of rapid vesicle cycling in vesicle replenishment. Although mossy fibre terminals are known to have various forms of Ca-dependent plasticity, some features of vesicle dynamics are robust and Ca-insensitive.

What We Can Learn From Cumulative Numbers of Vesicular Release Events

Following action potential invasion in presynaptic terminals, synaptic vesicles are released in a stochastic manner at release sites (docking sites). Since neurotransmission occurs at frequencies up to 1 kHz, the mechanisms underlying consecutive vesicle releases at a docking site during high frequency bursts is a key factor for understanding the role and strength of the synapse. Particularly new vesicle recruitment at the docking site during neuronal activity is thought to be crucial for short-term plasticity. However current studies have not reached a unified docking site model for central synapses. Here I review newly developed analyses that can provide insight into docking site models. Quantal analysis using counts of vesicular release events provide a wealth of information not only to monitor the number of docking sites, but also to distinguish among docking site models. The stochastic properties of cumulative release number during bursts allow us to estimate the total number of releasable vesicles and to deduce the features of vesicle recruitment at docking sites and the change of release probability during bursts. This analytical method may contribute to a comprehensive understanding of release/replenishment mechanisms at a docking site.

Underpinning heterogeneity in synaptic transmission by presynaptic ensembles of distinct morphological modules

Synaptic heterogeneity is widely observed but its underpinnings remain elusive. We addressed this issue using mature calyx of Held synapses whose numbers of bouton-like swellings on stalks of the nerve terminals inversely correlate with release probability (Pr). We examined presynaptic Ca2+ currents and transients, topology of fluorescently tagged knock-in Ca2+ channels, and Ca2+ channel-synaptic vesicle (SV) coupling distance using Ca2+ chelator and inhibitor of septin cytomatrix in morphologically diverse synapses. We found that larger clusters of Ca2+ channels with tighter coupling distance to SVs elevate Pr in stalks, while smaller clusters with looser coupling distance lower Pr in swellings. Septin is a molecular determinant of the differences in coupling distance. Supported by numerical simulations, we propose that varying the ensemble of two morphological modules containing distinct Ca2+ channel-SV topographies diversifies Pr in the terminal, thereby establishing a morpho-functional continuum that expands the coding capacity within a single synapse population.

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.