Lipid bilayer regulation of cochlear hair cells involves transmembrane channel-like proteins

Auditory mechanotransduction occurs in the hair bundle, an organelle composed of rows of linked stereocilia that increase in height in a staircase-like manner. Hair bundle deflection exerts force onto the tip link that is translated to mechanically gated (MET) ion channels located at the tops of the shorter stereocilia. There is a limited but growing body of data suggesting that stereocilia membrane properties modulate MET channels. Transmembrane channel-like proteins (TMCs) are considered part of the MET channel machinery, but also may have membrane scramblase activity that regulates membrane homeostasis in hair cells. To further investigate the role of the membrane in modulating MET channels, we used a novel viscosity sensor BODIPY1c. The lifetime changes of BODIPY1c correlate with viscosity allowing precise spatial and dynamic monitoring of membrane properties within live hair cells. We found that BODIPY1c can enter hair cells through MET channels and fluorescently label the cytoplasmic membranes, thus allowing identification of hair cells with functional MET channels. We show that both stereocilia and soma membrane viscosity of mammalian cochlear hair cells vary during development. Membrane viscosity decreases and strongly correlates with the onset of MET. TMIE and TMC1 mutant mice, both of which lack TMC1 in stereocilia, have significantly higher membrane viscosity compared to litter mate controls at P10. In TMC1 mutants, the stereocilia membrane viscosity strongly correlates to the intracellular intensity of BODIPY1c. Together these data suggest that the membrane viscosity of the stereocilia and hair cell soma undergoes developmental changes that correlated strongly with the onset of MET. Lack of TMC1 and TMIE inhibits these developmental changes to the membrane. Further studies are ongoing to determine the functional relevance of TMCs in regulating membrane mechanics.