Cochlear vibrations in mice with altered stereociliary mechanics

The mechanical properties of outer hair cell (OHC) stereocilia are critical for the transduction currents that drive OHC force generation. Stereociliary properties therefore play a prominent role in shaping "active" cochlear mechanics. However, it is less clear whether OHC stereocilia also directly influence passive cochlear mechanics due to contributions to the overall stiffness or coupling within the cochlear partition. To examine this, we used volumetric optical coherence tomography vibrometry to measure vibrations from the cochlear apex of wild type (WT) mice and mice with genetically-Altered stereociliary properties. The latter included salsa mice, which have a Cdh23 missense mutation and progressively lose their stereociliary tip links, and TRIOBP-4/5 deficient Triobp^Δex8/Δex8 mice, which lack stereociliary rootlets and suffer progressive stereociliary degeneration. Consistent with the absence of normal mechanotransduction in salsa and Triobp^Δex8/Δex8 mice, cochlear vibrations from these mice exhibited no nonlinear gain and largely resembled vibrations from dead WT mice. However, examination of vibrations in dead salsa and Triobp^Δex8/Δex8 mice also revealed a downward shift in the high-frequency portions of the vibratory gain and phase curves, as well as a steeper phase slope in Triobp^Δex8/Δex8 mice, relative to dead WT mice. Though subtle, these effects suggest that OHC stereociliary bundles contribute to longitudinal coupling within the cochlear partition, likely via their interactions with the tectorial membrane.