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Molecular Treadmill Keeps Stereocilia Healthy

Background: Stereocilia, in their staircase pattern, are key cellular organelles located in the inner ear that are responsible for hearing and balance. Although stereocilia are exquisitely sensitive to mechanical vibration and easily damaged by over-stimulation, they must maintain themselves to properly function for an entire lifetime to prevent hearing loss.

Advances: In the past year, NIDCD intramural scientists have made significant advances towards elucidating the mechanisms that underlie the formation, regulation, renewal, and lifespan of stereocilia. The length of stereocilia is highly dependent on the bundles of protein filaments constituting their core foundation. Scientists have now discovered that the entire core, consisting of the protein actin, continuously self–renews and the renewal process is based on a molecular treadmill that operates at rates that are precisely matched to the length of each individual stereocilium. Other studies revealed that the myosin XV protein is necessary for filament elongation and the proper regulation of stereocilia length. Myosin XVa was observed at the tips of the stereocilia. Mutations in the myosin 15a gene result in profound deafness in humans and mice. Another actin-related protein called espin also plays a role in regulating stereocilia length and mutations in the espin gene also results in deafness.

Implications: Assembly of actin proteins by myosin XVa and espin may dynamically form the functional shape of stereocilia. Stereocilia undergo continuous self-renewal, the renewal follows a treadmill mechanism. This process together with myosins may also determine the shape of the stereocilia. This new dynamic view of stereocilia renewal is essential to understanding the development, repair and maintenance of normal sensory function and may provide a new basis to understanding the mechanisms of genetic and age-related hearing and balance disorders. Damage to these stereocilia may be one of the mechanisms responsible for noise-induced hearing loss.