Hair Bundle Length and Functional Maturation of Mammalian Auditory Hair Cells Are Regulated by Eps8

Mammalian cochlear hair cells are specialized for the dynamic coding of sound. The transduction of sound waves into electrical signals depends upon mechanosensitive hair bundles that project from the cell’s apical surface. Each stereocilium within a hair bundle is composed of uniformly polarized and tightly packed actin filaments. Several stereociliary proteins have been shown to be associated with hair bundle development and function, and are known to cause deafness in mice and humans when mutated (Petit, Richardson 2009 NatNeurosciRev 12:703-10). The growth of the stereociliar actin core is dynamically regulated by elongation at the actin filament barbed ends in the stereociliary tip. However, the control of actin dynamics in stereocilia is still largely unknown. We used a combination of single-cell electrophysiology, immunolabelling, electron microscopy and in vivo physiology to investigate the role of Eps8, a protein with actin binding, bundling and barbed-end-capping activities (Di Fiore, Scita 2002 IntJBiochemCellBiol 34:1178-83), in the cochlea. Using control and Eps8 knockout mice we show that this protein is a novel component of the hair cell hair bundle. Eps8 was localized predominantly at the tip of the tallest row of stereocilia and was essential for their normal growth and for maintaining the normal mechano-sensitivity of the transducer apparatus. Moreover, we found that Eps8 knockout mice are profoundly deaf and that IHCs, but not OHCs, fail to develop the adult-like characteristics required to become functional sensory receptors. We propose that Eps8 directly regulates actin dynamics in hair cell stereocilia and also plays a crucial role in the physiological maturation of mammalian cochlear IHCs.Together, our results indicate that the multifunctional nature of Eps8 is critical for coordinating the maturation and functionality of mammalian auditory hair cells.