Taking a Back Seat: Synaptic Vesicle Clustering in Presynaptic Terminals

Abstract

Central inter-neuronal synapses employ various molecular mechanisms to sustain neurotransmitter release during phases of high-frequency synaptic activity. One of the features ensuring this property is the presence of a pool of synaptic vesicles (SVs) in the presynaptic terminal. At rest and low rates of stimulation, most of the vesicles composing this pool remain in a tight cluster. They are actively utilized when neurons fire action potentials at higher rates and the capability of the recycling machinery is limited. In addition, SV clusters are capable of migrating between release sites and reassemble into clusters at neighboring active zones (AZs). Within the cluster, thin “tethers” interconnect SVs. These dynamic filamentous structures are reorganized during stimulation thereby releasing SVs from the cluster. So far, one protein family, the synapsins, which bind actin filaments and vesicles in a phosphorylation-dependent manner, has been implicated in SV clustering in vertebrate synapses. As evident from recent studies, many endocytic proteins reside in the SV cluster in addition to synapsin. Here we discuss alternative possible mechanisms involved in the organization of this population of SVs. We propose a model in which synapsins together with other synaptic proteins, a large proportion of which is involved in SV recycling, form a dynamic proteinaceous “matrix” which limits the mobility of SVs. Actin filaments, however, do not seem to contribute to SV crosslinking within the SV cluster, but instead they are present peripherally to it, at sites of neurotransmitter release, and at sites of SV recycling.

Dual pools of actin at presynaptic terminals

“…It is also possible that a more stable pool of actin provides a framework for myoglobin, which may also play an important role in vesicle recycling (Mochida 1995;Srinivasan et al 2008;Takagishi et al 2005). However, our proposal would also agree with the recent concept of a matrix of filaments within the active zone that might alter vesicular release and motility (Pechstein and Shupliakov 2010). We conclude that there exist, at minimum, dual pools of actin at the presynaptic terminal.…”

The reserve pool of synaptic vesicles acts as a buffer for proteins involved in synaptic vesicle recycling

“…Previous concepts focused, for example, on clusters releasing molecules when high activity disintegrates them (8); this phenomenon is unlikely to happen in vivo in view of our findings that suggest that clusters stay intact (3). A different hypothesis is that the proteins themselves form a matrix whose function is to retain vesicles in the cluster, to provide a reservoir of vesicles near active zones (8,26). Our data cannot support this, given that (i) no reservoir of vesicles is actually needed, because only a few mobile vesicles function in release, and (ii) it is difficult to explain how most of the proteins we investigated could act as vesicle cross-linkers.…”

A Preferentially Segregated Recycling Vesicle Pool of Limited Size Supports Neurotransmission in Native Central Synapses

“…We showed that stabilizing actin with jasplakinolide disrupted the preferential distribution of recycling vesicles, indicating that remodeling actin is important in facilitating the repositioning of recycling vesicles toward the active zone after endocytosis. These findings are broadly compatible with the current model for actin function in the presynaptic terminal as a scaffolding element, guiding vesicle-associated components to their destination during repeated cycles of activity (Sankaranarayanan et al., 2003; Shupliakov et al., 2002) (also see Pechstein and Shupliakov, 2010). Importantly, we show that actin stabilization, and by association the abolition of preferential recycling pool distribution, does not prevent vesicle turnover but does affect the rate of release; experiments measuring FM dye loss show clear stimulation-evoked destaining but notably the timecourse of exocytosis is significantly slower compared to controls.…”