DESCRIPTION: Cognition, behavior, activity of neural circuits, synaptic plasticity, and neuronal survival relate to proper functioning of ligand gated ion channels of the ionotropic glutamate receptors (iGluRs). AMPA-type iGluRs (AMPA-Rs) contribute to the majority of excitatory synaptic transmission in mammalian brain and their dysfunction involves a variety of neurological and psychiatric disorders. For example, positive modulators of AMPA-Rs alleviate major depression disorder (MDD) symptoms, synaptic AMPA-Rs are reduced in mouse models of Alzheimer's disease, and auto-antibodies that bind to AMPA-Rs cause Rasmussen's encephalitis and a subset of limbic encephalitis. In particular, existing drugs for MDD targeting AMPA-Rs have limitations and mechanistically novel AMPA-R modulators are needed for therapeutic development and understanding disease mechanism. AMPA-Rs are protein complexes made of ? and ? subunits. The ? subunits are known as GluA1-4 and construct the tetrameric core of the ligand gated ion channel pore, whereas ? subunits contribute to functional modulation of the receptors without being part of the pore structure. The ? subunits are also known as the auxiliary factors and may be regarded as a set of endogenous modulators of AMPA-Rs developed by nature during evolution. Auxiliary factors modulate the magnitude and shape of postsynaptic responses mediated by AMPA-Rs. Mechanistically, modulating trafficking relates to changing the number and mobility of AMPA-Rs at synapses, whereas altering the gating kinetics of the ion channel will directly modify the time course of membrane depolarization. By changing trafficking and gating parameters influencing postsynaptic currents, auxiliary subunits impact coincident detection and dendritic integration. Ultimately such synaptic modulation is believed to affect the activity of neural circuits and behavior. It is thus conceivable that intervening with endogenous modulators of AMPA-Rs would have strong physiological effects. It is unclear, however, whether ? subunits of iGluR would be effective drug targets for manipulating ligand gated ion channel function. To test this hypothesis, we plan to combine HTS with new cell based assays we developed in order to screen for chemical compounds that specifically act on auxiliary factor dependent modulation of AMPA-Rs. Conventional drugs developed against AMPA-Rs focus on pore forming ? subunits. If new probes are identified from our proposed screening, auxiliary factors of iGluR will become a mechanistically new target for drug development. It is important to note that ? subunits in potassium channels are already established drug targets. New endogenous auxiliary factors of AMPA-Rs are continuously being identified, providing broader spectrum of molecular targets. Each auxiliary factor has distinct expression patters, indicating that targeted drugs will have cel-type specific effects. Using the proposed HTS screening approach, the identification of compounds that modulate AMPA-R activity through interaction with the auxiliary subunits will provide a paradigm shift in developing new drugs against AMPA-Rs, in particular for MDD.

PUBLIC HEALTH RELEVANCE: Major depression disorder, Alzheimer's disease, and Rasmussen's encephalitis are potential target diseases for drugs modulating the function of AMPA-type glutamate receptors (AMPA-Rs), ion channels mediating the majority of excitatory synaptic transmission in our brain. Because existing drugs have limitation due to side effects, we propose to develop new drugs that act in mechanistically novel ways and in a brain region specific manner. Specifically, we will conduct a high-throughput drug screening using our new assay in order to identify new chemical compounds that; (1) target auxiliary subunits of AMPA-Rs and (2) enhance or decrease the ion channel function of AMPA-Rs.