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09_20_05

Staircase to Hearing: The Role of Myosin 15a in the Stereocilia

Background: As they develop, stereocilia organize into bundles of precisely specified rows of increasing heights forming a characteristic staircase pattern. The movement of the stereocilia initiates the complex molecular signals that stimulate the auditory nerve. The auditory nerve carries information to the brain, eventually becoming the perception of sound. Hearing loss, balance defects, or both occurs if the stereocilia do not develop properly into the staircase bundle. Several years ago, scientists showed that mutations in the MYO15A gene, which encodes the protein myosin XVa, are responsible for profound, congenital deafness in humans and mice. The type of myosin found in stereocilia performs work within cells such as moving molecules or vesicles to specific locations.

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Advances: Recently, NIDCD intramural scientists, in collaboration with scientists at the University of Michigan, have shown that myosin XVa is essential for the development of the normal length and elongation into the staircase-shaped bundle of stereocilia. Myosin XVa is localized at the tops of hair cell stereocilia in the inner ear of normal hearing mice, and the amount of this motor protein appears to be directly correlated with the length of a stereocilium. Mice with mutations in Myosin XV have hair cell stereocilia bundles that are abnormally short.

Implications: Scientists have shown that mutations of myosin XVa in humans and mice cause a form of profound congenital deafness. They believe that in individuals with dysfunctional myosin XVa, the loss of hearing is due to a failure of hair cell stereocilia to elongate into their characteristic staircase-shaped bundles. Insight into the biology of the sensory hair cells in the inner ear will foster the design of different therapies and preventative strategies for hearing loss.