Effects of PKA-Mediated Phosphorylation of Snapin on Synaptic Transmission in Cultured Hippocampal Neurons

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Abstract
Use-dependent activation of protein kinase A (PKA) modulates transmitter release, contributing to synaptic plasticity. Snapin, a PKA substrate in neurons, associates with the solubleN-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex, and its phosphorylation leads to increased binding of synaptotagmin to the SNARE complex. We investigated the role of PKA-dependent phosphorylation of Snapin in hippocampal neurons. Overexpression of Snapin S50D, a mutant mimicking the phosphorylated state, resulted in a decreased number of readily releasable vesicles. In addition, both the release probability of individual vesicles and the depression rate during high-frequency stimulation were increased. Overexpression of Snapin S50A, a mutant that cannot be phosphorylated, did not alter the size of the pool or the probability of release. Furthermore, dialysis of Sp-cAMPS, a nonhydrolyzable analog of cAMP that will promote phosphorylation by PKA, also led to increased synaptic depression in cells overexpressing wild-type Snapin. These results establish Snapin as an important target of PKA in CNS synapses and indicate a role for Snapin in the plasticity of transmitter release.