Role of Staircase Hair Bundle Morphology in Auditory Mechanotransduction

ABSTRACT While number of proteins responsible for structural integrity of the mechanosensory bundle of the inner ear hair cells has been identified, how most of these proteins contribute to mechano-electrical transduction is still unknown. Also unknown is whether the lack or modification of MET contributes to the development of deafness and/or vestibular disorders that result from mutations of hair bundle proteins. The goal of the current project is to determine the role of the myosin XVa-based stereocilia elongation complex in mechanotransduction. In homozygous shaker 2 mice (Myol5~'~'2), a recessive point mutation in the motor domain of myosin XVa prevents normal localization of this protein to the tips of stereocilia, resulting in abnormally short stereocilia. According to our preliminary data, cochlear outer hair cells of young postnatal Myol5"°-'~"2 mice possess numerous obliquely oriented "tip links" and apparently normal mechanotransduction. In contrast, Myol&'°"~"2 inner hair cells have equally short stereocilia without any obliquely oriented tip links, but with numerous lop-to-top" links perpendicular to the core of stereocilia. In spite of their abnormal morphology, Myol5S~~~~2 inner hair cells have a prominent transduction current with "wild type" nanoampere-scale amplitude but abnormal directional sensitivity and no rapid Ca2tdependent deactivation, known as "fast adaptation". The central hypothesis of the proposal is that the myosin XVa- based stereocilia elongation complex is not required for mechanosensitivity of hair cells but may affect directional sensitivity and adaptation of mechanotransduction. This study will determine: 1) the role of myosin XVa in directional sensitivity of the hair bundle; 2) localization of known molecular components of stereocilia links in the myosin XVa-deficient hair bundles; 3) the role of myosin XVa and its molecular partner, whirlin in the fast adaptation of the transduction current. This study represents a step toward my long-term goal to understand how the hair cells acquire and maintain mechanosensitivity in normal and pathological conditions. Apart from being important for understanding the basic mechanisms of hair cell mechanotransduction, this study will establish how developmental abnormalities of stereocilia growth may affect the transduction machinery. This project will also be the first to study mechanotransduction during restoration of the hair bundle morphology and/or tip links in mammalian cochlear hair cells. Finally, our study will provide a wealth of data on hair cell function in shaker 2 and whirler mice, the animal models for DFNB3 and DFNB31 hereditary deafness in humans.