Abstract
Mouse Tmc1 and Tmc2 are required for sensory transduction in cochlear and vestibular hair cells. Homozygous Tmc1∆/∆ mice are deaf, Tmc2∆/∆ mice have normal hearing, and double homozygous Tmc1∆/∆; Tmc2∆/∆ mice have deafness and profound vestibular dysfunction. These phenotypes are consistent with their different spatiotemporal expression patterns. Tmc1 expression is persistent in cochlear and vestibular hair cells, whereas Tmc2 expression is transient in cochlear hair cells but persistent in vestibular hair cells. On the basis of these findings, we hypothesized that persistent Tmc2 expression in mature cochlear hair cells could restore auditory function in Tmc1∆/∆ mice. To express Tmc2 in mature cochlear hair cells, we generated a transgenic mouse line, Tg[PTmc1::Tmc2], in which Tmc2 cDNA is expressed under the control of the Tmc1 promoter. The Tg[PTmc1::Tmc2] transgene slightly but significantly restored hearing in young Tmc1∆/∆ mice, though hearing thresholds were elevated with age. The elevation of hearing thresholds was associated with deterioration of sensory transduction in inner hair cells and loss of outer hair cell function. Although sensory transduction was retained in outer hair cells, their stereocilia eventually degenerated. These results indicate distinct roles and requirements for Tmc1 and Tmc2 in mature cochlear hair cells.
Highlights
The primary sensory cells of the auditory organ, known as hair cells (HCs), are arranged as one row of inner HCs and three rows of outer HCs along the cochlear sensory epithelium
In vestibular type II HCs Tg[PTmc1::Tmc2]; Tmc1∆/∆; Tmc2∆/∆ mice had robust sensory transduction currents that were significantly larger than those of Tmc1+/∆; Tmc2∆/∆ HCs (Supplementary Fig. 3, paired t-test, P < 0.001). These results show that transgenic expression of Tmc[2] can preserve sensory transduction currents in vestibular type II cells and cochlear inner HCs but only partially restores currents in cochlear outer HCs of postnatal Tmc1∆/∆; Tmc2∆/∆ mice during the first postnatal week
We could not detect any responses to 90 dB sound pressure level (SPL) stimuli at any frequency in Tg[Tmc1∆Ex8_9]; Tmc1∆/∆ mice at P16 (Fig. 5E), indicating there was little or no functional expression of Tmc[1] in Tg[Tmc1∆Ex8_9]; Tmc1∆/∆ mice. These results indicated that transgenic Tmc[2] expressed in juvenile cochlear HCs can slightly but significantly restore hearing at stimulus frequencies corresponding to the apical to middle turns of the cochlea but is unable to compensate for loss of endogenous Tmc[1] by P25
Summary
The primary sensory cells of the auditory organ, known as hair cells (HCs), are arranged as one row of inner HCs and three rows of outer HCs along the cochlear sensory epithelium. Sensory receptor potentials in inner HCs induce neurotransmitter release to 95% of the auditory afferent fibers[1,2]. Depolarization and hyperpolarization of outer HCs induces contraction and elongation of outer HC somata[5,6] This property, known as electromotility, contributes to amplitude and frequency selectivity of sound vibrations in a process known as cochlear amplification. Inner and outer HCs have distinct functions and it remains unknown whether this reflects differences in the molecules that mediate sensory transduction. Receptor potentials which lead to the activation of voltage-gated calcium channels, neurotransmitter release and activation of glutamatergic auditory nerve fibers whose afferent terminals contact the basolateral membrane of the inner HCs10. The mouse genes and proteins required for auditory function show high similarities of sequence and functions with their human orthologs[15]
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