Neurotrophins stimulate phosphorylation of synapsin I by MAP kinase and regulate synapsin I-actin interactions.

TLDR

Neurotrophins modulate neuronal survival, differentiation, and synaptic transmission through Trk/MAP kinase signaling, and synapsins—neuron‑specific phosphoproteins—regulate neurotransmitter release, axonal growth, and synapse formation. This study demonstrates that synapsin I functions as a downstream effector of the neurotrophin/Trk/MAP kinase pathway. MAP kinase stoichiometrically phosphorylates synapsin I at Ser‑62, Ser‑67, and Ser‑549 in purified systems. Phosphorylation at these sites occurs in rat brain, cultured neurons, and presynaptic terminals, is enhanced by BDNF and NGF, diminished by KCl depolarization, and reduces synapsin I’s ability to promote G‑actin polymerization and actin bundling, implying a role in neurotrophin‑mediated synaptic plasticity.

Abstract

The ability of neurotrophins to modulate the survival and differentiation of neuronal populations involves the Trk/MAP (mitogen-activated protein kinase) kinase signaling pathway. More recently, neurotrophins have also been shown to regulate synaptic transmission. The synapsins are a family of neuron-specific phosphoproteins that play a role in regulation of neurotransmitter release, in axonal elongation, and in formation and maintenance of synaptic contacts. We report here that synapsin I is a downstream effector for the neurotrophin/Trk/MAP kinase cascade. Using purified components, we show that MAP kinase stoichiometrically phosphorylated synapsin I at three sites (Ser-62, Ser-67, and Ser-549). Phosphorylation of these sites was detected in rat brain homogenates, in cultured cerebrocortical neurons, and in isolated presynaptic terminals. Brain-derived neurotrophic factor and nerve growth factor upregulated phosphorylation of synapsin I at MAP kinase-dependent sites in intact cerebrocortical neurons and PC12 cells, respectively, while KCl- induced depolarization of cultured neurons decreased the phosphorylation state at these sites. MAP kinase-dependent phosphorylation of synapsin I significantly reduced its ability to promote G-actin polymerization and to bundle actin filaments. The results suggest that MAP kinase-dependent phosphorylation of synapsin I may contribute to the modulation of synaptic plasticity by neurotrophins and by other signaling pathways that converge at the level of MAP kinase activation.