Long-term changes of synaptic strength are thought to contribute to the underlying physiological substrate of memory. The molecular mechanisms mediating this plasticity can be divided into 2 phases: induction, triggering the change, and maintenance, sustaining it over time. One model to study these mechanisms is a long-term increase in synaptic strength induced by afferent tetanic stimulation, called long-term potentiation (LTP). Whereas the signaling pathways of LTP induction are exceedingly complex, much less is known about LTP maintenance. These maintenance mechanisms, however, may underlie memory storage. Our laboratory has focused on protein kinase C (PKC) in LTP maintenance. By examining the complete PKC isoform family, we identified a new PKC isoform, called PKMzeta, which persistently increases in LTP maintenance. Most PKCs consist of an N-terminal autoinhibitory regulatory domain and a C-terminal catalytic domain;second messengers activate PKC by releasing this intramolecular autoinhibition. PKMzeta, in contrast, is the independent catalytic domain of the atypical PKCzeta isoform, and, lacking a regulatory domain, is constitutively active. In Work Accomplished, we found whole-cell perfusion of PKMzeta into CA1 pyramidal cells potentiates AMPA receptor-mediated synaptic transmission. Furthermore, PKMzeta inhibitors reverse established LTP. Thus our overall goal now is to elucidate the mechanisms of PKMzeta function. Our 1st Aim is to determine the receptors mediating PKMzeta enhancement of excitatory synaptic transmission. AMPARs are composed of subunits, GluR1-4;we will use knock-out mice for each subunit to determine the subunit targets of AMPAR potentiation by PKMzeta. Our 2nd aim is to examine the molecular mechanisms of PKMzeta-mediated synaptic enhancement. Preliminary data indicate that PKMzeta increases the number of postsynaptic AMPARs through interaction between the GluR2 subunit and a critical AMPAR- trafficking protein called NSF (N-ethylmaleimide sensitive fusion protein). Our 3rd aim is to define the phase of LTP maintained by synaptically activated PKMzeta and to begin to determine the kinase's role in hippocampus-dependent spatial memory. These 3 aims will elucidate the function of PKMzeta and might provide core molecular mechanisms for maintaining long-term synaptic plasticity. These mechanisms may be important for both normal memory storage and memory dysfunction in amnestic disorders.
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