Myosin 7a is required for maintaining the transducing stereocilia and for force transmission to the MET channel during cochlear hair cell development

Shaker-1 mice carry a spontaneous missense mutation in Myo7a (Myo7a^Sh1) that interferes with the motor function of MYO7A. Mutation in the orthologous gene in humans causes syndromic (Usher 1B) or non-syndromic forms of deafness. In hair cells, MYO7A is expressed throughout the stereocilia, where it has been implicated in tip-link tensioning required for gating the mechanoelectrical transducer (MET) channel and setting its resting open probability (P_o). The Myo7a^Sh1 mutation progressively dysregulated the height of shorter stereocilia rows from the end of the first postnatal week onwards, associated with reduced MET current amplitude and hearing loss. Noise exposure exacerbated stereocilia dysfunction in Myo7a^Sh1/Sh1 mice. Following the onset of maturation, hair cells from Myo7a^Sh1/Sh1 mice showed normal resting P_o and calcium sensitivity of the MET channel. In immature Myo7a^Sh1/Sh1 hair cells, the resting P_o was very small or absent in comparison to control cells, but it was restored by changing the membrane lipid bilayer fluidity or thickness by depleting cholesterol or PIP₂. Bundle stiffness in immature IHCs was not affected by the absence of functional MYO7A but decreased after their onset of maturation in both genotypes. Expression of a subset of genes was affected similarly in immature Myo7a^Sh1/Sh1 mice and in adult Myo7a conditional knockout mice, indicating a common response pathway in Myo7a-deficient mice. This study reveals that MET channel gating might differ depending on hair cell developmental stage, and MYO7A is likely to influence, albeit indirectly, force transmission via the lipid bilayer to the MET channel and maintenance of the shorter rows of transducing stereocilia. KEY POINTS: Shaker-1 mice carry a spontaneous missense mutation in the shaker-1 gene (Myo7a^Sh1) that interferes with the motor function of MYO7A, a protein expressed in hair-cell stereocilia. The absence of functional MYO7A (Myo7a^Sh1/Sh1 mice) caused a progressive dysregulation in the height of the shortest two rows of stereocilia and the consequent loss of mechanoelectrical transduction (MET) current. Although immature hair cells from Myo7a^Sh1/Sh1 mice exhibited a markedly reduced resting open probability of their MET channels, this was restored upon maturation or following depletion of cholesterol or PIP₂ from the lipid bilayer. Hair-bundle stiffness was affected in immature inner hair cells from Myo7a^Sh1/Sh1 mice, suggesting that MYO7A is not required for establishing the resting tension of the tip links gating the MET channels. We conclude that MYO7A is crucial for the structural integrity of the MET complex and transport of key proteins required to transfer forces efficiently from the lipid bilayer to the MET channel.