Excitatory synapses in the CNS release glutamate, which acts on two types of ionotropic receptors: AMPA receptors (AMPARs) and NMDA receptors (NMDARs). Evidence indicates that AMPARs, in contrast to NMDARs, are highly mobile and that activity can rapidly change the number of receptors at the synapse. To begin to understand the molecular basis underlying the regulation of synaptic AMPARs, we have focused on the ataxic and epileptic mouse stargazer. Cerebellar granule cells in this mouse lack functional AMPARs, although NMDARs are normal and excitatory synapses release normal amounts of glutamate. During the current Conte grant we have carried out a series of studies on the role of stargazin (v-2), the mutated gene in the stargazer mouse. We have found that stargazin is an auxiliary subunit of AMPARs, not only controlling their trafficking to the cell surface and to the synapse, but also controlling their biophysical properties. We have identified a total of three additional proteins (v-3, y-4, v-8) that are expressed throughout the CNS and can rescue the AMPAR defect in cerebellar granule cells. We refer to these proteins as transmembrane AMPAR regulatory proteins (TARPs). We have succeeded in deleting the gene for each of the TARPs in mice. These mutant mice will form the basis for many of the proposed experiments in this RO1 grant, which is a continuation of the work carried out on the Conte Grant. There are 4 Specific Aims. (1) Determine the role of v-3 in the mouse brain, (2) determine the role of y-4 in the mouse brain, (3) determine the functional differences among TARPs, and (4) determine whether TAPRs may have AMPAR-independent roles in the CNS. The role of TARPs in AMPAR trafficking, synaptic transmission and plasticity will be studied primarily in the hippocampus. Preliminary studies indicate that y-8 plays an important role in AMPAR trafficking in the hippocampus, but substantial AMPAR transmission remains. We will also compare the ability of various TARPs to modify the deactivation of AMPARs and the kinetics of synaptic transmission. We have evidence that each of the roles TARPs play (i.e., surface delivery, synaptic targeting and receptor gating) all vary for each of the TARPs. We have found that the Y-2/Y-3A/-4, triple KO is lethal and the newborn pups are completely immobile. We will determine why the spinal cord is nonfunctional. These studies may uncover novel roles for TARPs in the nervous system. Given the critical role that receptor trafficking plays in synaptic plasticity and, by implication in certain aspects of learning and memory, it is anticipated that findings from these studies will have direct clinical impact. Indeed, clinically promising AMPAkines exert their effect, in part, by controlling the kinetics of AMPAR gating similar to TARPs. In addition, TARPs modify the pharmacological properties of AMPAkines and, thus, represent a novel target for drug design.
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