Activity-Driven Plasticity of the Hair Cell Cytoskeleton

PROJECT SUMMARY/ABSTRACT The mechanosensitivity of the inner ear hair cells depends on actin-filled cellular projections known as stereocilia, organized in a bundle of rows of increasing heigh in staircase pattern. The stereocilia cytoskeleton shows actin renewal limited to the stereocilia tips. We recently found that this stability of the stereocilia cytoskeleton depends on the constant influx of calcium ions through the mechano-electrical transduction (MET) channels that are located at the tips of stereocilia (Velez-Ortega, et al., eLife 2017). We showed that the blockage of MET channels or the breakage of the tip links that gate these channels leads to the selective thinning and shortening of transducing stereocilia in the bundle (i.e. only the middle and short row stereocilia harbor MET channels). Once the MET blockage is removed or the tip links regenerate, the stereocilia get thicker and regrow. An increase in intracellular calcium buffering also led to the selective remodeling of transducing stereocilia, indicating that calcium ions are the main component of the MET current regulating the stereocilia actin stability. Stereocilia remodeling in response to variations in the resting MET current represents an activity-dependent plasticity of the stereocilia actin cytoskeleton, which may regulate hair cell sensitivity and/or impact the morphology of the hair bundle over the lifetime of the cell. This project will explore several molecular mechanisms involved in this activity-driven cytoskeleton plasticity. Aim 1 will evaluate MET-dependent changes in actin dynamics within stereocilia and between stereocilia and the cuticular plate. Aim 2 will evaluate MET-driven ultrastructural changes to the stereocilia actin filament organization via electron tomography. Aim 3 will assess whether this MET-dependent stereocilia cytoskeleton plasticity is a feature unique to developing hair cells or still present in adult/mature mammalian hair cells. Lastly, given that the long isoform of myosin XV is required for the maintenance of the hair bundle, Aim 4 will evaluate the impact of deficiencies in the long isoform of myosin XV on stereocilia row identity proteins and susceptibility to noise-induced hearing loss in heterozygous mice. This study is significant, because it may uncover the molecular mechanisms of the fine adjustments and maintenance of the staircase architecture of the hair cell bundles. Moreover, stereocilia shortening—and perhaps eventual stereocilia disappearance—could occur after noise exposure (when the MET current is reduced due to tip link breakage) or in certain cases of congenital deafness (due to impaired MET current). Therefore, this study will expand our knowledge of the molecular mechanisms of various types of hearing loss.