Synapsins Contribute to the Dynamic Spatial Organization of Synaptic Vesicles in an Activity-Dependent Manner

Abstract

The precise subcellular organization of synaptic vesicles (SVs) at presynaptic sites allows for rapid and spatially restricted exocytotic release of neurotransmitter. The synapsins (Syns) are a family of presynaptic proteins that control the availability of SVs for exocytosis by reversibly tethering them to each other and to the actin cytoskeleton in a phosphorylation-dependent manner. Syn ablation leads to reduction in the density of SV proteins in nerve terminals and increased synaptic fatigue under high-frequency stimulation, accompanied by the development of an epileptic phenotype. We analyzed cultured neurons from wild-type and Syn I,II,III Ϫ/Ϫ triple knock-out (TKO) mice and found that SVs were severely dispersed in the absence of Syns. Vesicle dispersion did not affect the readily releasable pool of SVs, whereas the total number of SVs was considerably reduced at synapses of TKO mice. Interestingly, dispersion apparently involved exocytosis-competent SVs as well; it was not affected by stimulation but was reversed by chronic neuronal activity blockade. Altogether, these findings indicate that Syns are essential to maintain the dynamic structural organization of synapses and the size of the reserve pool of SVs during intense SV recycling, whereas an additional Syn-independent mechanism, whose molecular substrate remains to be clarified, targets SVs to synaptic boutons at rest and might be outpaced by activity.

Early Exposure to General Anesthesia Disrupts Spatial Organization of Presynaptic Vesicles in Nerve Terminals of the Developing Rat Subiculum

“…A tightly organized and finely orchestrated trafficking of synaptic vesicles is fundamental to guaranteeing adequate exchange of information between synapses and is reflected in the precisely regulated topography of presynaptic vesicles within nerve terminals [28]. In fact, it is becoming increasingly evident that the morphological structure of the presynaptic (as well as the postsynaptic) terminal is inseparable from synaptic function and that there is a tight link between the ability of a synapse to maintain an efficient compartmentalization among functionally diverse synaptic vesicle pools and its ability to function properly [28][29][30]. For example, it is generally accepted that the vesicles docked at the active zone constitute the readily releasable pool of synaptic vesicles, as assessed by electrophysiology, i.e., vesicles that are primed, partially fused to the plasma membrane and ready to be released into the synaptic cleft as soon as calcium enters the nerve terminal [28].…”

“…For example, at the level of the presynaptic terminal, the actin cytoskeleton has a crucial role in maintaining and regulating the spatial organization of synaptic vesicles [10][11][12]. In particular, the actin cytoskeleton is associated with a family of phosphoproteins called synapsins (synapsin I, II, III) which, in turn, are linked to synaptic vesicles [29,30]. The actinsynapsin scaffold is believed to sequester synaptic vesicles into the reserve pool and when vesicle recycling capacity is exceeded, phosphorylation of synapsins releases the reserve vesicles from the actin-synapsin scaffold, so they can buffer the depletion of the readily releasable pool.…”

“…The actinsynapsin scaffold is believed to sequester synaptic vesicles into the reserve pool and when vesicle recycling capacity is exceeded, phosphorylation of synapsins releases the reserve vesicles from the actin-synapsin scaffold, so they can buffer the depletion of the readily releasable pool. Consequently, regulation of the dynamic equilibrium between reserve and readily releasable pools by synapsins underlies important forms of synaptic transmission and plasticity [27][28][29][30]. Interestingly, general anesthetics can modulate the state of phosphorylation of synapsins in mouse brain [31] and the impairment of synapsin function has been associated with many behavioral and learning deficits [29,30].…”

“…Consequently, regulation of the dynamic equilibrium between reserve and readily releasable pools by synapsins underlies important forms of synaptic transmission and plasticity [27][28][29][30]. Interestingly, general anesthetics can modulate the state of phosphorylation of synapsins in mouse brain [31] and the impairment of synapsin function has been associated with many behavioral and learning deficits [29,30]. Several studies have demonstrated reduced numbers of vesicles after synapsin deletion in various types of synapses [32,33].…”

A system for studying mechanisms of neuromuscular junction development and maintenance

“…Additionally, we investigated whether the NMJ bore active zones at presynaptic terminals. To this end, we stained for synapsin I, a synaptic vesicle protein; synaptotagmin, a calcium sensor also important for the vesicle docking process; and bassoon, a scaffolding protein believed to guide the synaptic vesicles to the active zones (Fornasiero et al, 2012;Reist et al, 1998;Südhof, 2012;Willig et al, 2006;Zhai et al, 2001;Ziv and Garner, 2004). We found that all these proteins were enriched specifically at the pre-synaptic terminals of the NMJ (Fig.…”