Derangements in synaptic transmission are an important part of the pathology of several neurological mental diseases including epilepsy, schizophrenia, depression, and perhaps Alzheimer's disease. Much of delicate regulation of synaptic strength that is important for information processing and storage occurs through biochemical regulation of the postsynaptic membrane. The signaling protein complexes that carry out this regulation are associated with a postsynaptic structure called the postsynaptic density (PSD), a large, fibrous specialization of the submembrane cytoskeleton that adheres to the postsynaptic membrane opposite presynaptic terminals. Previous work on this application has focused on the identification of proteins that make the PSD. Here we propose to extend our studies by determining the regulatory roles of two prominent proteins that we discovered in the PSD, whose functions are still uncertain. One of the proteins, synGAP, is a ras GTPase activating protein that accelerates the inactivation of ras. We have found that synGAP is tightly associated with the NMDA-receptor signaling complex, and is inactivated by phosphorylation by CaMKII CaMKII is, in turn, activated by the calcium ion that flows through active NMDA receptors. We postulate based on our preliminary data, that inactivation of synGAP triggered by activation of NMDA receptors potentiate the actions of neurotrophins such as BDNF at glutamatergic synapses. We will carry experiments to test this hypothesis, comparing neuronal cultures from mutant mice that are missing synGAP protein to cultures from wild type littermates. We will also investigate the effect of the synGAP deletion on assembly of the NMDA receptor-associated signaling complex during synaptic development. The second PSD protein that we propose to study, densin-180, is a member of a family of proteins that contribute to the formation of polarized membrane domains. We have found that densin forms a ternary complex with the actin-associated protein alpha-actinin and CaMKII. We will test the hypothesis that densin, in addition to NR2 subunits of the NMDA receptor, is an anchoring site for CaMKII in the PSD. We will also determine whether densin contributes to the translocation of CAMKII to the PSD that occurs upon stimulation of hippocampal neurons with glutamate.
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