Differential effects of TARPs on AMPA receptor subunits

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DESCRIPTION (provided by applicant): Excitatory synaptic transmission underlies all cognition, and changes in this transmission are thought to play a role in many neurological disorders. The fast component of excitatory transmission is mainly carried by AMPA-type glutamate receptors. These receptors are tetramers assembled in neuron- specific stoichiometries from four types of subunits (GluR1-4). It has recently been discovered that AMPA receptors in neurons are also associated with auxiliary subunits called TARPs (transmembrane AMPA-R regulatory proteins), a family of four proteins of which stargazin (STG) has been most widely characterized. STG has two distinct effects on AMPA receptors. One is to promote trafficking to the plasma membrane and to anchor AMPA receptors to the synapse via PSD-95. The other one is to modulate the kinetics of AMPA receptors by lowering the EC50 for glutamate and by slowing receptor deactivation and desensitization, effects which have immediate impact on the waveform of synaptic transmission. However, most studies have been carried out by coexpressing STG with GluR1 and 2, and much less is known about STG's effects on other receptor subunits and about the actions of the other three TARPs which are far more prevalent in cortex and hippocampus. We have recently discovered that the receptor subunit GluR3, unlike GluR1 and GluR2, exhibits almost no kinetic modulation by STG. The TARPs gamma-4 and gamma-8, on the other hand, caused a slowing of deactivation and desensitization in GluR3 that was larger than in GluR1 and 2. Interestingly, surface expression of GluR3 was strongly promoted by STG, which indicates (i) that STG did associate with GluR3 and (ii) that modulation of receptor kinetics and trafficking are independently regulated. In this project we will determine how and why the four TARPs differ in their effect on receptor subunits. In Specific Aim 1, we will complete our analysis of their impact on GluR3 and extend our analysis to include GluR2 and GluR4. Moreover, we will examine the effects of the four TARPs on heteromeric subunit combinations that are prevalent in brain. These data will have immediate relevance for understanding the waveform of synaptic transmission in different brain areas. In Specific Aim 2 we will then determine which amino acids in GluR3 are responsible for the differential effects of the TARPs. For this we will systemically exchange segments between GluR2 and GluR3, starting with the S1-S2 domain thought to be central, and then use point mutations to identify the critical amino acids. These experiments should allow us to identify the contact points between AMPA receptor subunits and TARPs that are important for kinetic modulation. Recent evidence suggests that both GluR and TARP expression is altered in disorders such as schizophrenia. Thus, the results of this study will be relevant also to understand aberrations in excitatory transmission that are thought to underlie many neurological and psychiatric disorders. Much evidence suggests that disturbances in excitatory synaptic transmission contribute to the pathogenesis of various neurological and psychiatric disorders. Such disturbances in synaptic transmission may arise from changes in AMPA-type glutamate receptors, but recent studies have found that proteins associated with AMPA receptors may similarly contribute to pathology. For instance, proteins called TARPs which modulate AMPA receptor function have been shown to be altered in the brains of schizophrenic patients. In this project we will study the impact of TARPs on different types of AMPA receptors. Given that AMPA receptors vary across neurons, the results from these studies are expected to offer new explanations as to why synaptic transmission varies in different parts of the brain, and they may also provide new insights about aberrations in excitatory transmission in various brain disorders.