Tyrosine phosphorylation of synapsin by src regulates synaptic vesicle trafficking

Synapsins are synaptic vesicle (SV)-associated phosphoproteins involved in the regulation of neurotransmitter release. Synapsins reversibly tether SVs to the cytoskeleton and their phosphorylation by serine/threonine kinases increases SV availability for exocytosis by impairing their association with SVs and/or actin. We recently showed that synapsin I, through SH3/SH2-mediated interactions, activates and is phosphorylated by c-Src at Tyr301. Here we demonstrate that, opposite to serine phosphorylation, c-Src phosphorylation of synapsin I increased the binding of synapsin I to SVs and actin and the formation of synapsin oligomers potentially involved in SV clustering. Synapsin phosphorylation by c-Src was physiologically regulated in brain slices and increased in response to depolarization and BDNF and affected SV dynamics. An increase in the sizes of the readily-releasable and recycling pools of SVs was observed upon expression of the non-phosphorylatable (T301F) synapsin I mutant in primary synapsin knockout neurons with respect to the wild-type form, consistent with an increased availability of recycled SVs for exocytosis. The data provide a mechanism for the effects of Src on SV trafficking and indicate that tyrosine phosphorylation of synapsins, opposite to serine phosphorylation, stimulates the reclustering of recycled SVs and their recruitment to the reserve pool.