Protein kinase M (PKM) is a newly described form of PKC that is necessary and sufficient for the maintenance of hippocampal long term potentiation (LTP) and the persistence of memory in Drosophila. PKM is the independent catalytic domain of the atypical PKC isoform and produces long term effects at synapses because it is persistently active, lacking autoinhibition from the regulatory domain of PKC. PKM has been thought of as a proteolytic fragment of PKC. Here we report that brain PKM is a new PKC isoform, synthesized from a PKM mRNA encoding a PKC catalytic domain without a regulatory domain. Multiple -specific antisera show that PKM is expressed in rat forebrain as the major form of in the near absence of full-length PKC. A PKC knockout mouse, in which the regulatory domain was disrupted and catalytic domain spared, still expresses brain PKM, indicating that this form of PKM is not a PKC proteolytic fragment. Furthermore, the distribution of brain PKM does not correlate with PKC mRNA but instead with an alternate RNA transcript thought incapable of producing protein. In vitro translation of this RNA, however, generates PKM of the same molecular weight as that in brain. Metabolic labeling of hippocampal slices shows increased de novo synthesis of PKM in LTP. Because PKM is a kinase synthesized in an autonomously active form and is necessary and sufficient for maintaining LTP, it serves as an example of a link coupling gene expression directly to synaptic plasticity. LTP1 is a persistent enhancement of synaptic transmission widely studied as a physiological model of memory (1). LTP can be divided into two phases: induction, which triggers the potentiation, and maintenance, which sustains it over time. Many molecules have been implicated in LTP induction, which is initiated by the activation of N-methyl-D-aspartate (NMDA) receptors and involves several protein kinases (2). In contrast, very little is known about the molecular mechanism of maintenance. Recently, however, a specific, autonomously active form of the atypical PKC isozyme (3, 4), PKM, has been found both necessary and sufficient for maintaining LTP (5-7). Overexpression of PKM also prolongs memory in Drosophila melanogaster, suggesting it is part of an evolutionarily conserved molecular mechanism for memory storage (8).The unique role of PKM in LTP maintenance is due, in part, to its unusual structural and enzymatic properties as an autonomously active kinase. PKM consists of the independent catalytic domain of a PKC isoform (5). PKC isoforms are divided into three classes: conventional, novel, and atypical (reviewed in Refs. 9 -11). Each isoform is a single polypeptide consisting of an N-terminal regulatory domain and a C-terminal catalytic domain linked by a hinge (Fig. 1A, left). The regulatory domain contains binding sites for second messengers and an autoinhibitory pseudosubstrate sequence, which interacts with and blocks the active site of the catalytic domain. Second messengers stimulate PKC by binding to the regulatory domain, translocating th...
Synaptic stimulation activates signal transduction pathways, producing persistently active protein kinases. PKMzeta is a truncated, persistently active isoform of atypical protein kinase C-zeta (aPKCzeta), which lacks the N-terminal pseudosubstrate regulatory domain. Using a Pavlovian olfactory learning task in Drosophila, we found that induction of the mouse aPKMzeta (MaPKMzeta) transgene enhanced memory. The enhancement required persistent kinase activity and was temporally specific, with optimal induction at 30 minutes after training. Induction also enhanced memory after massed training and corrected the memory defect of radish mutants, but did not improve memory produced by spaced training. The 'M' isoform of the Drosophila homolog of MaPKCzeta (DaPKM) was present and active in fly heads. Chelerythrine, an inhibitor of PKMzeta, and the induction of a dominant-negative MaPKMzeta transgene inhibited memory without affecting learning. Finally, induction of DaPKM after training also enhanced memory. These results show that atypical PKM is sufficient to enhance memory in Drosophila and suggest that it is necessary for normal memory maintenance.
Long-term potentiation (LTP) and long-term depression (LTD) are persistent modifications of synaptic strength that have been implicated in learning, memory, and neuronal development. Despite their opposing effects, both forms of plasticity can be triggered by the activation of NMDA receptors. One mechanism proposed for this bidirectional response is that the specific patterns of afferent stimulation producing LTP and LTD activate to different degrees a uniform receptor population. A second possibility is that these patterns activate separate receptor subpopulations composed of different NMDA receptor (NR) subunits. To test this hypothesis we examined the inhibition of LTP and LTD by a series of competitive NMDA receptor antagonists that varied in their affinities for NR2A/B and NR2C/D subunits. The potency for the inhibition of LTP compared with inhibition of LTD varied widely among the agents. Antagonists with higher affinity for NR2A/B subunits relative to NRC/D subunits showed more potent inhibition of LTP than of LTD. D-3-(2-carboxypiperazine-4-yl)-1-propenyl-1-phosphonic acid, which binds to NR2A/B with very high affinity relative to NR2C/D, showed an approximately 1000-fold higher potency for LTP than for LTD. These results show that distinct subpopulations of NMDA receptors characterized by different NR2 subunits contribute to the induction mechanisms of potentiation and depression.
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