Stereocilin-deficient mice reveal the origin of cochlear waveform distortions.

Although the cochlea is an amplifier and a remarkably sensitive and finely tuned detector of sounds, it also produces conspicuous mechanical and electrical waveform distortions. These distortions reflect nonlinear mechanical interactions within the cochlea. By allowing one tone to suppress another (masking effect), they contribute to speech intelligibility. Tones can also combine to produce sounds with frequencies not present in the acoustic stimulus. These sounds compose the otoacoustic emissions that are extensively used to screen hearing in newborns. Because both cochlear amplification and distortion originate from the outer hair cells-one of the two types of sensory receptor cells-it has been speculated that they stem from a common mechanism. Here we show that the nonlinearity underlying cochlear waveform distortions relies on the presence of stereocilin, a protein defective in a recessive form of human deafness. Stereocilin was detected in association with horizontal top connectors, lateral links that join adjacent stereocilia within the outer hair cell's hair bundle. These links were absent in stereocilin-null mutant mice, which became progressively deaf. At the onset of hearing, however, their cochlear sensitivity and frequency tuning were almost normal, although masking was much reduced and both acoustic and electrical waveform distortions were completely lacking. From this unique functional situation, we conclude that the main source of cochlear waveform distortions is a deflection-dependent hair bundle stiffness resulting from constraints imposed by the horizontal top connectors, and not from the intrinsic nonlinear behaviour of the mechanoelectrical transducer channel.

Stereocilin connects outer hair cell stereocilia to one another and to the tectorial membrane.

Stereocilin is defective in a recessive form of deafness, DFNB16. We studied the distribution of stereocilin in the developing and mature mouse inner ear and analyzed the consequences of its absence in stereocilin-null (Strc(-/-)) mice that suffer hearing loss starting at postnatal day 15 (P15) and progressing until P60. Using immunofluorescence and immunogold electron microscopy, stereocilin was detected in association with two cell surface specializations specific to outer hair cells (OHCs) in the mature cochlea: the horizontal top connectors that join the apical regions of adjacent stereocilia within the hair bundle, and the attachment links that attach the tallest stereocilia to the overlying tectorial membrane. Stereocilin was also detected around the kinocilium of vestibular hair cells and immature OHCs. In Strc(-/-) mice the OHC hair bundle was structurally and functionally normal until P9. Top connectors, however, did not form and the cohesiveness of the OHC hair bundle progressively deteriorated from P10. The stereocilia were still interconnected by tip links at P14, but these progressively disappeared from P15. By P60 the stereocilia, still arranged in a V-shaped bundle, were fully disconnected from each other. Stereocilia imprints on the lower surface of the tectorial membrane were also not observed in Strc(-/-) mice, thus indicating that the tips of the tallest stereocilia failed to be embedded in this gel. We conclude that stereocilin is essential to the formation of horizontal top connectors. We propose that these links, which maintain the cohesiveness of the mature OHC hair bundle, are required for tip-link turnover.