Central to the focus of this grant is the AMPA receptor (AMPAR), which drives most fast synaptic transmission in the brain. Remarkably the AMPAR is the final common path for at least three mechanistically distinct forms of plasticity. This grant will compare and contrast these forms of plasticity. To accomplish these goals we use a combination of techniques. Central to our studies is electrophysiology, since this is the most critical and quantitative way to measure the functional consequences of our molecular manipulations. Most of the experiments involve a molecular replacement strategy that we recently developed, in which endogenous receptor subunits are genetically deleted and replaced with mutated receptors. The three forms of activity- dependent trafficking of AMPARs that will be studied are: 1) Long-term potentiation (LTP). Using a molecular replacement strategy we have recently found that LTP can be normally elicited without the C-tails of GluA1 or GluA2 and is also normal when exogenous kainate receptors replace AMPARs. This raises many issues. Two types of experiments are planned. First, we will determine if there are shared features of the non-NMDARs, such as the N-Terminal Domain (NTD), that are required. Second, we will test whether in more intact conditions TARPs/NETOs do play a role. 2) Long term depression (LTD). A substantial literature has argued that the C-tails of both GluA1 and GluA2 are required for LTD. We will use the same molecular replacement strategy that was used to study LTP to determine the minimal requirements for LTD. It has also been reported that, although LTD requires glutamate binding to the NMDAR, it does not requires calcium entry, a finding that we have confirmed. Using a molecular replacement strategy for NMDARs we will determine the critical domain(s) of the NMDAR that is/are required for this metabotropic action. 3) Homeostatic synaptic scaling/distance-dependent dendritic scaling. Homeostasis involves the global change in synaptic strength following prolonged changes in activity, whereas distance-dependent dendritic scaling is a phenomenon in which distant synapses have twice as many AMPARs as proximal synapses. We have found that the GluA2 subunit is absolutely required for both homeostasis and for distance-dependent dendritic scaling, raising the possibility that these two forms of plasticity use similar mechanisms. We will define which domains of GluA2 are required for both forms of plasticity. Given the central role that AMPARs plays in synaptic plasticity it is anticipated that findings from these studies will have direct clinical impact.
Learning and memory is one of the most important functions of the brain and yet we know extraordinarily little about the underlying mechanisms. The elucidation of the cellular and molecular mechanisms will provide a platform for the development of a rational therapeutic approach for such diseases as Alzheimer's Disease.
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