Protein kinase a-mediated synapsin I phosphorylation is a central modulator of Ca2+-dependent synaptic activity

Protein kinase A (PKA) modulates several steps of synaptic transmission. However, the identification of the mediators of these effects is as yet incomplete. Synapsins are synaptic vesicle (SV)-associated phosphoproteins which represent the major presynaptic targets of PKA. We show that in hippocampal neurons cAMP-dependent pathways affect SV exocytosis and that this effect is largely brought about through synapsin I phosphorylation. Phosphorylation by PKA, by promoting dissociation of synapsin I from SVs, enhances the rate of SV exocytosis upon stimulation. This effect becomes relevant when neurons are challenged with sustained stimulation, since it appears to counteract synaptic depression and accelerate recovery from depression by fostering the supply of SVs from the reserve pool to the readily-releasable pool. In contrast, synapsin phosphorylation appeared to be dispensable for the effects of cAMP on the frequency and amplitude of spontaneous synaptic currents and on the amplitude of evoked synaptic currents. The modulation of depolarization-evoked SV exocytosis by PKA phosphorylation of synapsin I is largely caused by calmodulin (CaM)-dependent activation of cAMP pathways, rather than by direct activation of CaM kinases. These data define a hierarchical cross-talk between cAMP- and CaM-dependent cascades and point to synapsin as a major effector of PKA in the modulation of activity-dependent SV exocytosis.