Abstract: Previous studies have demonstrated that the mechanisms underlying the exquisite sensitivity and frequency selectivity of the cochlea rely partly on the voltage-dependent hair bundle motility and outer hair cell (OHC) lateral wall electromotility (eM). Several gene products involved in cochlear sound amplification have been identified, and their mutations have been shown to result in hearing loss in human and mouse models. For example, mutations of K+ channels (Kv), such as Kv7.4 (critical in controlling OHC membrane excitability) result in profound progressive hearing loss (PHL: DFNA2). While the global expanse of families with DFNA2 has been identified, the mechanism of the disease is largely unknown. Additionally, the activity of OHCs is transmitted to the brain via the scarce (~5%) and small diameter, unmyelinated type II auditory neurons (spiral ganglion neurons (SGNs). These features have made it impractical to isolate and to determine their functional properties. We hypothesize that the properties of Kv7.4 currents in OHCs are achieved by the interaction of Kv7.4 with KCNE4, the Ca2+ binding protein 2 (CaBP2) and their ability to form clusters. For the first time, we have developed innovative and painstaking strategies that allow robust assessment of type II auditory neuron functions. We will deploy innovative molecular biology, electrophysiology, imaging techniques, and gene-targeted mouse models to unravel the fundamental and newly accessible arena of type II SGN/OHC physiology. Aim 1 will identify the molecular determinants for the unique low-voltage-activation properties of Kv7.4 currents in OHCs. In Aim 2 we will determine the in vivo functions of KCNE4 and CaBP2 in the inner ear. Finally, in Aim 3 we will identify the mechanisms underlying type II neuronal modulation of OHCs. The proposed studies will reveal critical missing links of OHC functions and for the first time, determine features of the scarce type II SGNs that innervate OHCs: therefore, shifting the prevailing monolithic type I SGN-centric physiology (that is known) to comprehensive understanding of distinct afferent auditory neurons, information essential for the treatment of sensorineural hearing loss (SNHL).