Introduction of synaptotagmin 7 promotes facilitation at the climbing fiber to Purkinje cell synapse

Minimal presynaptic protein machinery governing diverse kinetics of calcium-evoked neurotransmitter release

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

Synaptotagmins 3 and 7 mediate the majority of asynchronous release from synapses in the cerebellum and hippocampus

Neurotransmitter release consists of rapid synchronous release followed by longer-lasting asynchronous release (AR). Although the presynaptic proteins that trigger synchronous release are well understood, the mechanisms for AR remain unclear. AR is sustained by low concentrations of intracellular Ca2+ and Sr2+, suggesting the involvement of sensors with high affinities for both ions. Synaptotagmin 7 (SYT7) partly mediates AR, but substantial AR persists in the absence of SYT7. The closely related SYT3 binds Ca2+ and Sr2+ with high affinity, making it a promising candidate to mediate AR. Here, we use knockout mice to study the contribution of SYT3 and SYT7 to AR at cerebellar and hippocampal synapses. AR is dramatically reduced when both isoforms are absent, which alters the number and timing of postsynaptic action potentials. Our results confirm the long-standing prediction that SYT3 mediates AR and show that SYT3 and SYT7 act as dominant mechanisms for AR at three central synapses.

Region-Related Differences in Short-Term Synaptic Plasticity and Synaptotagmin-7 in the Male and Female Hippocampus of a Rat Model of Fragile X Syndrome

Fragile X syndrome (FXS) is an intellectual developmental disorder characterized, inter alia, by deficits in the short-term processing of neural information, such as sensory processing and working memory. The primary cause of FXS is the loss of fragile X messenger ribonucleoprotein (FMRP), which is profoundly involved in synaptic function and plasticity. Short-term synaptic plasticity (STSP) may play important roles in functions that are affected by FXS. Recent evidence points to the crucial involvement of the presynaptic calcium sensor synaptotagmin-7 (Syt-7) in STSP. However, how the loss of FMRP affects STSP and Syt-7 have been insufficiently studied. Furthermore, males and females are affected differently by FXS, but the underlying mechanisms remain elusive. The aim of the present study was to investigate possible changes in STSP and the expression of Syt-7 in the dorsal (DH) and ventral (VH) hippocampus of adult males and females in a Fmr1-knockout (KO) rat model of FXS. We found that the paired-pulse ratio (PPR) and frequency facilitation/depression (FF/D), two forms of STSP, as well as the expression of Syt-7, are normal in adult KO males, but the PPR is increased in the ventral hippocampus of KO females (6.4 ± 3.7 vs. 18.3 ± 4.2 at 25 ms in wild type (WT) and KO, respectively). Furthermore, we found no gender-related differences, but did find robust region-dependent difference in the STSP (e.g., the PPR at 50 ms: 50.0 ± 5.5 vs. 17.6 ± 2.9 in DH and VH of WT male rats; 53.1 ± 3.6 vs. 19.3 ± 4.6 in DH and VH of WT female rats; 48.1 ± 2.3 vs. 19.1 ± 3.3 in DH and VH of KO male rats; and 51.2 ± 3.3 vs. 24.7 ± 4.3 in DH and VH of KO female rats). AMPA receptors are similarly expressed in the two hippocampal segments of the two genotypes and in both genders. Also, basal excitatory synaptic transmission is higher in males compared to females. Interestingly, we found more than a twofold higher level of Syt-7, not synaptotagmin-1, in the dorsal compared to the ventral hippocampus in the males of both genotypes (0.43 ± 0.1 vs. 0.16 ± 0.02 in DH and VH of WT male rats, and 0.6 ± 0.13 vs. 0.23 ± 0.04 in DH and VH of KO male rats) and in the WT females (0.97 ± 0.23 vs. 0.31 ± 0.09 in DH and VH). These results point to the susceptibility of the female ventral hippocampus to FMRP loss. Importantly, the different levels of Syt-7, which parallel the higher score of the dorsal vs. ventral hippocampus on synaptic facilitation, suggest that Syt-7 may play a pivotal role in defining the striking differences in STSP along the long axis of the hippocampus.

Synaptotagmin 7 Sculpts Short-Term Plasticity at a High Probability Synapse

Synapses with high release probability (Pr ) tend to exhibit short-term synaptic depression. According to the prevailing model, this reflects the temporary depletion of release-ready vesicles after an initial action potential (AP). At the high-Pr layer 4 to layer 2/3 (L4-L2/3) synapse in rodent somatosensory cortex, short-term plasticity appears to contradict the depletion model: depression is absent at interstimulus intervals (ISIs) <50 ms and develops to a maximum at ∼200 ms. To understand the mechanism(s) underlying the biphasic time course of short-term plasticity at this synapse, we used whole-cell electrophysiology and two-photon calcium imaging in acute slices from male and female juvenile mice. We tested several candidate mechanisms including neuromodulation, postsynaptic receptor desensitization, and use-dependent changes in presynaptic AP-evoked calcium. We found that, at single L4-L2/3 synapses, Pr varies as a function of ISI, giving rise to the distinctive short-term plasticity time course. Furthermore, the higher-than-expected Pr at short ISIs depends on expression of synaptotagmin 7 (Syt7). Our results show that two distinct vesicle release processes summate to give rise to short-term plasticity at this synapse: (1) a basal, high-Pr release mechanism that undergoes rapid depression and recovers slowly (τ = ∼3 s) and (2) a Syt7-dependent mechanism that leads to a transient increase in Pr (τ = ∼100 ms) after the initial AP. We thus reveal how these synapses can maintain a very high probability of neurotransmission for multiple APs within a short time frame. Key words : depression; facilitation; short-term plasticity; synaptotagmin 7.

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.

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?