Modulation of neurotransmitter release by the second messenger-activated protein kinases: implications for presynaptic plasticity.

Activity-dependent modulation of synaptic function and structure is emerging as one of the key mechanisms underlying synaptic plasticity. Whereas over the past decade considerable progress has been made in identifying postsynaptic mechanisms for synaptic plasticity, the presynaptic mechanisms involved have remained largely elusive. Recent evidence implicates that second messenger regulation of the protein interactions in synaptic vesicle release machinery is one mechanism by which cellular events modulate synaptic transmission. Thus, identifying protein kinases and their targets in nerve terminals, particularly those functionally regulated by synaptic activity or intracellular [Ca2+], is critical to the elucidation of the molecular mechanisms underlying modulation of neurotransmitter release and presynaptic plasticity. The phosphorylation and dephosphorylation states of synaptic proteins that mediate vesicle exocytosis could regulate the biochemical pathways leading from synaptic vesicle docking to fusion. However, functional evaluation of the activity-dependent phosphorylation events for modulating presynaptic functions still represents a considerable challenge. Here, we present a brief overview of the data on the newly identified candidate targets of the second messenger-activated protein kinases in the presynaptic release machinery and discuss the potential impact of these phosphorylation events in synaptic strength and presynaptic plasticity.