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Molecular Details of Sound Detection

Molecular Details of Sound Detection

Background: In humans, sound is detected by the sensory hair cells located within the cochlea of the inner ear. Hair cells have bundles of finger-like protrusions. A cluster of these precisely organized protrusions on a single hair cell is called a stereocilia bundle. The stereocilia bend, setting off a series of chemical and electrical signals within the cells. These signals are ultimately carried to the brain by the auditory nerve and interpreted as sound. Although scientists understand the basic principles of sound detection, the molecular details underlying hair cell function are much less well known.

Schematic secondary structure of cadherin-23 and mutations in the waltzer mouse mutant.

Figure 3: Schematic structure of cadherin 23.
The 27 ectodomains (orange), transmembrane region, intracellular portion (blue), and the location of ten loss-of-function alleles in otocadherin are shown.
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Advances: NIDCD-funded scientists have described two new molecules expressed in the tips of hair cell stereocilia. One group identified a cell surface adhesion molecule (Cadherin 23, or CDH23) that may participate in opening and closing an ion channel to transmit the message that a sound has been detected. Mutations in the gene that makes CDH23 cause hearing loss and deafness in mice and humans. The protein produced by the gene is the expected size of the tip link protein that joins adjacent hair cells together at their tips. Preliminary results from a second group of researchers suggest that another protein, Myosin-X (Myo10) is also located in hair cell projections. Their experiments demonstrate that Myo10 is responsible for moving proteins into growing axons of nerve cells. These proteins are important for helping nerve cells connect with other nerve cells. Thus, mutations in Myo10 could disrupt the cellular connections within the stereocilia that are vital to hearing.

Implications: These studies provide insight into how hair cells detect sound on a molecular level. As scientists gain a clearer understanding of molecules important for hearing, they can design improved therapies to treat hearing loss caused by molecular defects in the ear.