Excitability and Excitotoxicity in Type-I Cochlear Afferents: Synapse Structure and Function

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Project Summary: This project investigates the cochlear AMPA-type glutamate receptors (AMPARs) that are necessary for hearing, overactivation of which leads to excitotoxic synapse loss and hearing disorders. Each cochlear afferent synapse expresses many hundreds to a few thousand of these AMPARs, of both the Ca2+-permeable subtype (CP-AMPARs, lacking subunit GluA2) and the Ca2+-impermeable subtype (CI-AMPARs, containing subunit GluA2). The combination of pore-forming GluA subunits and auxiliary subunits of the AMPAR complex, influenced by transsynaptic adhesion factors, determine its physiological properties and pharmacological sensitivities. The cochlear AMPAR complex has properties that make it unique in the nervous system, for example, the absence of GluA1. However, the precise complement of cochlear AMPAR subunits in not known. This proposal uses mouse genetics, in vivo and ex vivo cochlear electrophysiology, proteomics, and ultrastructural molecular anatomy to investigate the subunit composition, pharmacological sensitivity, and functional significance of the cochlear AMPAR complex. We will determine the influence of auxiliary subunit TARP-2 (Stargazin) on cochlear function, synaptic transmission, and GluA subunit expression. We will test the hypothesis that synaptopathy in GluA3KO mice results from an increase in Ca2+-permeability of the AMPAR complex. We will determine how Neuroligin1 and 3 affect AMPAR subunit expression and auditory nerve fiber physiology. We will determine the influence of GluA3, TARP-2, Nlgn1, and Nlgn3 on the intrasynaptic distribution of AMPAR subunits. With recombinant expression of different combinations of GluA pore-forming and auxiliary subunits in HEK cells (with or without GluA3, with or without TARP-2), we will challenge our understanding of the cochlear AMPAR complex by comparing changes in pharmacological sensitivity with those changes observed for the native cochlear synapses (GluA3WT vs GluA3KO, TARP-2WT vs TARP-2KO). The gain of this basic knowledge will inform design of small molecules to target cochlear AMPARs. With chronic systemic administration of the tool compound (CP-AMPAR blocker IEM-1925), we will measure synaptic adaptation and resistance to noise-induced synaptopathy. With acute systemic dosing, we will ask if noise trauma can be prevented if IEM-1925 is given only during, not before, the noise exposure and if IEM- 1925 + antioxidant combination therapy can protect cochlear function from more intense noise exposures. The long-term goal of this line of investigation is to develop systemic drugs to target CP-AMPARs of the inner ear while allowing hearing function to be maintained through CI-AMPARs, and while avoiding unwanted CNS side effects. The successful completion of this collaborative project will determine the precise subunit composition of the cochlear AMPAR complex and its influence on pharmacological sensitivity.