Long-term modifications in the efficiency of specific synapses are believed to contribute to the physiological basis of memory. A leading hypothesis for the molecular mechanism of this sustained modification is that the signal transduction events which modulate synaptic strength in the short-term can be extended into long-term changes. For example, a persistent activation of protein kinase C (PKC) may participate in the long-term increase in synaptic efficacy (long-term potentiation, LTP) that follows a high-frequency afferent stimulation at the Schaffer collateral/commissural-CA1 synapse in the hippocampus. Recent studies, however, have suggested that activity-dependent synaptic plasticity is complex. LTP appears to evolve through several temporal phases, which may have distinct, but perhaps related, molecular mechanisms. In addition, other patterns of afferent stimulation may produce either short-term potentiation (STP), lasting but a few minutes, or long-term depression (LTD) of synaptic transmission. Both the early induction and the later maintenance phases of LTP may involve PKC, a heterogeneous family of multiple isoforms, each with subtly different enzymatic characteristics. Our overall hypothesis is that the various types of synaptic plasticity may involve specific forms of PKC. In preliminary data, we show that in the induction phase of LTP in CA1 of rat hippocampal slices, multiple PKC isozymes translocate to membrane transiently. In addition to detecting 8 isoforms of PKC, we have identified in rat hippocampus the free, constitutively-active catalytic fragment, PKM, of a single isozyme, zeta (PKMzeta). We find that PKMzeta increases specifically in the maintenance phase of LTP, and decreases in LTD. The first specific aim of this proposal is the structural and biochemical characterization of PKMzeta, which appears to be selectively expressed in neural tissue.
Our second aim i s to analyze the stimulus dependency and temporal phases of the regulation of all the PKC isozymes and PKMzeta during synaptic plasticity. Finally, we plan to characterize the signal transduction pathways that regulate these persistent changes in PKC, particularly the bidirectional control of PKMzeta. We will examine the contributions of glutamate receptor subtypes, synaptic inhibition, and other second messenger pathways.
These specific aims are designed to characterize the molecular mechanisms of persistence of PKC, which are likely to prove important in understanding both the normal formation and clinical perturbations of memory.
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