ACTIVITY-DRIVEN PLASTICITY OF THE HAIR CELL CYTOSKELETON

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PROJECT SUMMARY/ABSTRACT The mechanosensitivity of the inner ear hair cells depends on cellular projections known as stereocilia, organized in rows of increasing height, with mechano-electrical transduction (MET) channels located at the tips of shorter row stereocilia. The core of stereocilia consists of a highly crosslinked paracrystalline array of actin filaments. While crosslinker proteins are constantly renewed, the renewal of actin is limited to the stereocilia tips. We previously reported that the stereocilia actin core exhibits activity-dependent plasticity (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 lead to the selective shortening of transducing stereocilia (i.e. the stereocilia that harbor MET channels), while the non-transducing tallest row stereocilia remain unaffected. Once the MET blockage is removed or the tip links regenerate, the stereocilia regrow. Our preliminary data also show that this MET-dependent stereocilia remodeling can affect the resting tension within the MET machinery in seconds. Thus, this process may dynamically regulate the sensitivity of hair cells to sound-induced vibrations and, hence, the sensitivity of our hearing. Yet, the exact mechanisms of MET-dependent stereocilia remodeling are still obscure. It is unknown even whether the activity-dependent plasticity of the stereocilia cytoskeleton is limited to the regions of active actin renewal or can expand beyond this region into the “stable” part of the stereocilia shaft. Here, we hypothesize that the MET activity regulates the extent of the stereocilia cytoskeleton undergoing active actin remodeling. To test this, Aim 1 will evaluate MET-dependent changes in actin dynamics within the stereocilia and the cuticular plate, an actin-rich structure supporting the stereocilia bundle. Aim 2 will evaluate MET-driven changes in the ultrastructural organization of stereocilia actin with transmission electron microscopy tomography. Since the MET-dependent stereocilia remodeling was studied so far only in young postnatal hair cells, Aim 3 will assess whether this phenomenon is present also in the mature adult auditory hair cells. In Aim 4, we begin to explore the molecular players involved in the MET-driven stereocilia remodeling, by evaluating the expression of so-called “stereocilia row identity proteins” in a mutant mouse model that exhibits MET-dependent actin remodeling not only in transducing stereocilia but also, unexpectedly, in non-transducing stereocilia. The study is significant, because it may clarify how exactly a hair cell performs fine adjustments of the architecture of the stereocilia bundle, thereby maintaining the sensitivity of our hearing throughout a lifetime. In addition, stereocilia shortening—and perhaps their eventual 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.