The Role of Protein Kinase C in Short-Term Synaptic Plasticity

Short-term synaptic plasticity results from use-dependent activity, lasts on the timescale of milliseconds to minutes, and is thought to underlie working memory and neuronal information processing. Here, we focus on two forms of short-term plasticity: 1) post-tetanic potentiation (PTP), which is induced by high-frequency stimulation, and 2) presynaptic ionotropic receptoractivated synaptic enhancement, which can be produced by the activation of presynaptic glycine receptors. Potentiation of evoked and spontaneous responses is thought to arise from elevations in presynaptic residual Ca, which activates one or more molecular targets to increase neurotransmitter release. However, the Ca sensor protein has not yet been identified. The overall goal of this work is to elucidate the Ca-dependent mechanisms of short-term plasticity. Pharmacological studies have implicated protein kinase C (PKC) in short-term plasticity. We overcame the limitations of problematic PKC pharmacology by using knockout (ko) animals to examine the roles of Ca-dependent PKC (PKCCa) isoforms (α, β, γ) in short-term plasticity at the calyx of Held synapse. We found that PKCαβ isoforms predominantly mediate PTP and glycine-induced potentiation by increasing the size of the readily releasable pool (RRP) of vesicles in animals after hearing onset. Specifically, the PKCβ isoform appeared to mediate the bulk of PTP. However, PKCγ mediates PTP by increasing the release probability (p) of vesicles in pre-hearing animals. Whether PTP is induced by an increase in p or RRP is important from a functional perspective, because it has very different effects on prolonged responses during firing iv of action potentials. Our results are the first to show that different PKC isoforms can perform specialized roles in mediating synaptic plasticity. To definitively demonstrate that PKCβ functions as the Ca-sensor for PTP, we performed rescue experiments by infecting PKCαβ double ko neurons with mutant PKCβ where the amino acids required for Ca-coordination were mutated. We found that mutant PKCβ could not rescue PTP, but could rescue phorbol ester-induced synaptic potentiation, because the phorbol ester-activated domain remains fully functional. These results suggest that Cadependent PKCβ isoforms act as a Ca-sensor protein in PTP. Together our findings provide insight into the molecular mechanisms underlying short-term synaptic plasticity.