Myosin-I Isozymes in Neonatal Rodent Auditory and Vestibular Epithelia

Myosin isozymes are essential for hair cells, the sensory cells of the inner ear. Because a myosin-I subfamily member may mediate adaptation of mechanoelectrical transduction, we examined expression of all eight myosin-I isozymes in rodent auditory and vestibular epithelia. Using RT-PCR, we found prominent expression of three isozymes, Myo1b (also known as myosin-Ia or myr 1), Myo1c (myosin-Ib or myr 2), and Myo1e (myr 3). By contrast, Myo1a (brush-border myosin-I), Myo1d (myosin lg or myr 4), Myo1f, Myo1g, and Myo1h were less readily amplified. Because sequence analysis demonstrated that the RT-PCR products encoded the appropriate isozymes, this represents the first demonstration of expression of all eight mouse myosin-I genes. Using immunocytochemistry with isozyme-selective antibodies, we found that Myo1b was located at apical surfaces of supporting cells that surround hair cells in auditory epithelia of postnatal rats. In vestibular epithelia, Myo1b was present in a ring within the apical pole of the hair cell. In both cases, expression was prominent only immediately after birth. Myo1e was found in hair cells of the auditory and vestibular epithelia; this isozyme was enriched in the cuticular plate, the actin meshwork that anchors the stereocilia. Myo1c was found in hair-cell stereocilia, concentrated towards their tips; we confirmed this localization by using adenovirus vectors to direct expression of a GFP-Myo1c tail fusion protein; this fusion protein localized to plasma membranes, often concentrating at stereociliary tips. Myo1c therefore remains the myosin isozyme best localized to carry out transducer adaptation.