Synapsin I bundles F-actin in a phosphorylation-dependent manner

Abstract

Synapsin I is a neuron-specific phosphoprotein localized to the cytoplasmic surface of synaptic vesicles. This phosphoprotein is a major substrate for cyclic AMP-dependent and calcium/calmodulin-dependent protein kinases. Its state of phosphorylation can be altered both in vivo and in vitro by a variety of physiological and pharmacological manipulations known to affect synaptic function. Recent direct evidence suggests that it may be involved in the regulation of neurotransmitter release from the nerve terminal. In the nerve terminal, synaptic vesicles are embedded in a cytoskeletal network, consisting in part of actin. We report here the ability of the dephospho-form of synapsin I to bundle F-actin. This bundling activity is reduced when synapsin I is phosphorylated by cAMP-dependent protein kinase and virtually abolished when it is phosphorylated by calcium/calmodulin-dependent protein kinase II or by both kinases. These results, demonstrating an interaction of synapsin I with actin in vitro, support the possibility that synapsin I is involved in clustering of synaptic vesicles at the presynaptic terminal and that the phosphorylation of synapsin I may be involved in regulating the translocation of synaptic vesicles to their sites of release.

Detection by chemical cross-linking of bovine brain synapsin I self-association

“…Preliminary evidence of synapsin I polymer formation has been reported by several groups Bahler & Greengard, 1987;Steiner et al, 1987b), but without any experimental details. Furthermore, it has been suggested that formation of synapsin I polymers could explain its actin-or microtubule-bundling activity (Petrucci et al, 1988).…”

“…Utilization of different cross-linkers to reveal synapsin I self-association It has been reported Bahler & Greengard, 1987;Petrucci & Morrow, 1987) that some of the effects of synapsin I could be explained by synapsin I self-association. In the present experiments, we have tested the ability of various cross-linking agents to disclose the association of synapsin with itself.…”

“…Previous studies have shown that synapsin I is able to bind to small synaptic vesicles (Schiebler et al, 1986), and to brain spectrin (Baines & Bennett, 1985;Krebs et al, 1987a) with high affinity, to associate with membrane proteins and neurofilaments (Steiner et al, 1987a), and to bundle F-actin (Bahler & Greengard, 1987;Petrucci & Morrow, 1987) and microtubules . All these interactions could be of physiological significance, since they are modulated by the phosphorylation state of the protein.…”

Administration of the calcineurin inhibitor cyclosporine modulates cocaine-induced locomotor activity in rats

“…In its dephosphorylated form, synapsin I binds to both synaptic vesicles and the actin cytoskeleton (Bahler and Greengard 1987;Huttner et al 1983). In the current work, we demonstrate that repeated cocaine exposure leads to increased synapsin I phosphorylation (Fig 3B), which in turn, would be expected to decrease its affinity for both actin and synaptic vesicles, thus releasing these vesicles from the actin cytoskeleton and allowing them to move to the readily releasable pool (Bahler and Greengard 1987;Greengard et al 1993;Huttner et al 1983). …”

“…Synapsin I normally tethers synaptic vesicles to the pre-synaptic cytoskeleton to limit vesicular neurotransmitter release (Greengard et al 1993). When phosphorylated, synapsin I has a decreased affinity for both actin and synaptic vesicles, shifting these vesicles to the readily releasable pool and allowing for greater neurotransmitter release upon stimulation (Bahler and Greengard 1987;Greengard et al 1993;Huttner et al 1983). In addition, the synapsin family of proteins is thought to contribute to cocaine-mediated dopamine release in the striatum as this release is decreased in synapsin knockout mice (Venton et al 2006).…”