Outer hair cell stereocilia bundle stiffness and mechanical coherence by horizontal top connectors determined using acoustic frequency modulation atomic force microscopy

Authors

• AX Cartagena-Rivera
• E Verpy
• C Petit
• RS Chadwick

Abstract

Outer hair cell (OHC) stereocilia bundle deflection results in mechanical opening of mechanoelectrical transduction (MET) channels at the tip of the stereocilia and alters the elastic energy in horizontal top connectors, the lateral links that connect adjacent stereocilia. Previous studies showed the stereocilin-null mice, that lack horizontal top connectors in OHC stereocilia bundles, become progressively deaf from P15. However, at P14, cochlea sensitivity and frequency tuning are intact, but interestingly suppressive masking and cochlear waveform distortions are absent. This phenotype suggest that the main source of waveform distortions may be horizontal top connector-mediated MET channel cooperativity or cohesive-dependent hair bundle stiffness resulting from constraints imposed by the presence of top connectors, not only from canonical MET nonlinear behavior. Here we describe a noninvasive, acoustic, and quantitative method to investigate the stiffness and coherence of stereocilia bundles. We developed a mathematical theory for relating bundle stiffness to frequency shifts of cantilevers with attached microspheres oscillating at acoustic frequencies. We use the atomic force microscope to measure cantilever frequency shifts at nanometer distances from the stereocilia bundle. Stereocilia bundle stiffness determination of P9-P14 stereocilin-deficient mice with detached tectorial membrane (Strc-/-/Tecta-/-) were used and compared with heterozygous littermate controls (Strc+/-/Tecta-/-). Interestingly, significant decrease in bundle stiffness was measured when top connectors were absent. Therefore, the bundle mechanical coherence is affected by the absence of top connectors. These finding suggest that interconnected stereocilia by horizontal top connectors provide bundle stability and coherence maintenance in response to mechanical deflection, a critical requirement for hearing integrity.

Scientific Focus Area: Biomedical Engineering and Biophysics

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