Long-term changes in synaptic strength are widely believed to underlie memory storage. Thus one approach to understanding memory storage is to investigate long-term synaptic plasticity, particularly a persistent synaptic enhancement induced by strong afferent stimulation, long-term potentiation (LTP). The mechanisms of LTP can be divided into 2 phases: induction, triggering the potentiation, and maintenance, sustaining the potentiation over time. Although inducing persistent LTP requires new protein synthesis and involves complex signaling mechanisms, the functionally important, newly synthesized proteins maintaining LTP, and whether they play similar roles in memory storage, had previously been unknown. Recently, we found that a constitutively active PKC isoform, PKMzeta (PKM6), is both necessary and sufficient for maintaining LTP. Produced from a PKM6 mRNA, PKM6 is the independent catalytic domain of PKC6, which, lacking an autoinhibitory regulatory domain, persistently phosphorylates without second messenger stimulation. Once synthesized in LTP induction, persistent PKM6 activity maintains synaptic potentiation because inhibiting PKM6 reverses established LTP, even when the inhibition begins hours after induction. Moreover, inhibiting PKM6 disrupts behavioral long-term memory retention, even days after learning, without affecting subsequent memory storage. Thus, the overall hypothesis of this application is that persistent increases in PKM6 at synapses may be a common molecular mechanism of information storage for both LTP maintenance and behavioral long-term memory. This application therefore focuses on the physiological mechanisms for initially encoding and persistently maintaining PKM6 at synapses.
Our first aim i s to examine the mechanisms of PKM6 synthesis in the phases of LTP, focusing on a positive feedback loop that might increase PKM6 levels through dendritic protein synthesis. Second is to examine changes in the distribution of PKM6 in neurons during LTP. We will examine if PKM6 activity maintains its targeting to spines by its autonomous activity and through the same trafficking mechanism by which the kinase also potentiates synaptic strength. Third, we will determine if these mechanisms of synthesis and synaptic compartmentalization are the same in behavioral memory storage. Preliminary data indicate that PKM6 in the hippocampus increases after learning and correlates with the extent of memory retention. Thus our 3 aims will provide fundamental new knowledge on how PKM6 maintains activity- dependent information within neurons, which might be a common mechanism for synaptic and behavioral memory storage that may be relevant to disorders of memory.
How long-term memories are stored is a fundamental question in neuroscience with important clinical implications for neurology and psychiatry. Our laboratory has identified a molecular component of the storage mechanism of long-term memory, a protein kinase termed PKMzeta, by studying the maintenance mechanism of a persistent form of synaptic plasticity, long-term potentiation (LTP). This application focuses on the physiological mechanisms for initially encoding and persistently maintaining PKMzeta at synapses that might be common mechanisms for information storage during both LTP and long-term memory, and thus relevant to the pathophysiology of amnestic disorders.
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