Inhibition of exocytosis or endocytosis blocks activity‐dependent redistribution of synapsin

Abstract

Synaptic vesicles are pre-synaptic organelles which release their neurotransmitter content onto the post-synaptic neuron by exocytosis, followed by their local reconstitution by endocytosis, in what is known as the synaptic vesicle cycle (Südhof 2004). A minority of vesicles constitute the readily releasable pool (RRP) of vesicles which are immediately ready for exocytosis. The rest are either recycling vesicles, which can be recruited rapidly, or they belong to the reserve or resting pool, meaning that mobilizing them for eventual use requires the investment of substantially more time and effort (Denker and Rizzoli 2010). The interplay between the pools and the kinetics of the cycle determine the overall release dynamics of the terminal (Pan and AbstractThe synaptic vesicle cycle encompasses the pre-synaptic events that drive neurotransmission. Influx of calcium leads to the fusion of synaptic vesicles with the plasma membrane and the release of neurotransmitter, closely followed by endocytosis. Vacated release sites are repopulated with vesicles which are then primed for release. When activity is intense, reserve vesicles may be mobilized to counteract an eventual decline in transmission. Recently, interplay between endocytosis and repopulation of the readily releasable pool of vesicles has been identified. In this study, we show that exo-endocytosis is necessary to enable detachment of synapsin from reserve pool vesicles during synaptic activity. We report that blockage of exocytosis in cultured mouse hippocampal neurons, either by tetanus toxin or by the deletion of munc13, inhibits the activity-dependent redistribution of synapsin from the pre-synaptic terminal into the axon. Likewise, perturbation of endocytosis with dynasore or by a dynamin dominant-negative mutant fully prevents synapsin redistribution. Such inhibition of synapsin redistribution occurred despite the efficient phosphorylation of synapsin at its protein kinase A/CaMKI site, indicating that disengagement of synapsin from the vesicles requires exocytosis and endocytosis in addition to phosphorylation. Our results therefore reveal hitherto unidentified feedback within the synaptic vesicle cycle involving the synapsin-managed reserve pool.

Overexpression of synapsin Ia in the rat calyx of Held accelerates short-term plasticity and decreases synaptic vesicle volume and active zone area

“…The accelerated recovery from depression due to synapsin Ia overexpression suggests that this isoform fulfills an additional function within the recycling pool, possibly related to Ca 2+ -modulated SV endocytosis or recycling (Bloom et al, 2003; Coleman et al, 2008). In fact, a direct connection between endocytosis and synapsin dispersion within the presynaptic terminal has been shown previously (Orenbuch et al, 2012). Hence, synapsin Ia may potentiate an existing mechanism that translates the Ca 2+ signal into faster SV retrieval and recycling.…”

Tissue Multicolor STED Nanoscopy of Presynaptic Proteins in the Calyx of Held

“…We found three classes of immunosignals: isolated VGluT1, isolated synapsin and colocalized VGluT1/synapsin immunoreactivity (Figure 7). We interpret this as synaptic vesicles lacking synapsin, dispersed synapsin unbound to synaptic vesicles [16], [17] and synaptic vesicles with bound synapsin, respectively. The latter interpretation is constrained by the limited axial resolution of our imaging approach, providing the possibility that two or even more vesicles may be stacked vertically within the imaging volume.…”

“…Nevertheless, vesicles devoid of vesicular glutamate transporters are unlikely to exist because the transporters are essential for presynaptic function and are expressed at a high abundance at the calyx of Held [19]. If either VGluT1 or synapsin can be detected in the volume sampled by STED microscopy, the conclusion seems warranted that either synaptic vesicles lacking synapsin or dispersed synapsin [16], [17] are present, respectively. However, in this case another limitation arsing from using antibodies for protein localization needs to be considered: steric hindrance and inaccessibiliy of the epitope may prevent binding of one of the antibodies.…”

Huntingtin‐associated protein‐1 is a synapsin I‐binding protein regulating synaptic vesicle exocytosis and synapsin I trafficking

“…Interestingly, we and others have detected a defect in vesicle docking in the absence of HAP1 and a decrease in the RRP in endocrine cells (Cape et al 2012;Mackenzie et al 2014). More importantly, a blockage of exocytosis or endocytosis inhibits activity-dependent redistribution of synapsin I (Orenbuch et al 2012) and synapsin I is implicated in a post-endocytic role where it functions in vesicle trafficking from perisynaptic endocytic sites back to the vesicle clusters (Bloom et al 2003). Future studies are needed to elucidate the biological significance of the HAP1-synapsin I association in vivo with respect to neuronal development and neurological disorders.…”

“…More importantly, a blockage of exocytosis or endocytosis inhibits activity‐dependent redistribution of synapsin I (Orenbuch et al . ) and synapsin I is implicated in a post‐endocytic role where it functions in vesicle trafficking from peri‐synaptic endocytic sites back to the vesicle clusters (Bloom et al . ).…”