Mechanotransduction-Dependent Control of Stereocilia Dimensions and Row Identity in Inner Hair Cells

Abstract

Actin-rich structures like stereocilia and microvilli are assembled with precise control of length, diameter, and relative spacing. We found that developmental widening of the second-tallest stereocilia rank (row 2) of mouse inner hair cells correlated with the appearance of mechanotransduction. Correspondingly, Tmc1 KO/KO ;Tmc2 KO/KO or Tmie KO/KO hair cells, which lack transduction, have significantly altered stereocilia lengths and diameters. EPS8 and the short splice isoform of MYO15A, identity markers for row 1 (tallest), lost their row exclusivity in transduction mutants, a result that was mimicked by block of transduction channels. Likewise, the heterodimeric capping protein subunit CAPZB and its partner TWF2 lost their row 2 tip localization in mutants, and GNAI3 failed to accumulate at row 1 tips. Redistribution of marker proteins was accompanied by increased variability in stereocilia height. Transduction channels thus specify and maintain row identity and control addition of new actin filaments to increase stereocilia diameter.

Loss of Baiap2l2 destabilizes the transducing stereocilia of cochlear hair cells and leads to deafness

“…We quantified stereocilia dimensions of P0.5 to P21 IHCs and of P5.5 to P21 OHCs from Baiap2l2 Δ16 mice by measuring the apparent length and width of the phalloidin‐stained actin cores (Krey et al . 2020). Although pairwise comparisons for row dimensions between genotypes at each age yielded several significant differences, no consistent trend in the direction of the differences was observed for either IHCs (Fig.…”

“…The full‐width at half‐maximum of the point‐spread function (PSF) in the x – y plane was 186 nm; resolution in the z ‐axis is ∼3‐fold poorer (Krey et al . 2020).…”

“…Because these measurements are deconvolved with the PSF of an objective, they are approximations of stereocilia dimensions rather than absolute dimensions with errors of ∼40% for width and ∼15% for length (Krey et al . 2020). However, the measurements obtained as described nonetheless demonstrate the relative changes in stereociliary shape that occur throughout development.…”

New Tmc1 Deafness Mutations Impact Mechanotransduction in Auditory Hair Cells

“…Each immunolabeling run was performed on three cochleas from different animals, and the process was repeated two times, so six animals in total were used for each labelling procedure. FM1-43 staining was performed on apical turns of unfixed cochlear explants of P15, P20 and P30 mice using labeling procedures described previously (Velez-Ortega et al, 2017;Krey et al, 2020). Animals were anesthetized with isoflurane then decapitated according to the animal protocol approved by the Institutional Animal Care and Use Committee at the University of Wisconsin-Madison.…”

Rat Auditory Inner Hair Cell Mechanotransduction and Stereociliary Membrane Diffusivity Are Similarly Modulated by Calcium

“…MET channels are only present in the shorter stereocilia rows ( Beurg et al., 2009 ). The upper tip-link insertion point has a density with selective molecules like harmonin, whirlin, and cadherin 23, while the lower insertion point is linked via protocadherin 15, CIB2 (Calcium And Integrin Binding Family Member 2), LHFPL5 (Lipoma HMGIC fusion partner-like 5), TMIE (Transmembrane Inner Ear), and TMC (Transmembrane channel-like) molecules ( Kazmierczak et al., 2007 ; Xiong et al., 2012 ; Kurima et al., 2015 ; Giese et al., 2017 ; Pan et al., 2018 ; Cunningham et al., 2020 ; Krey et al., 2020 ). The MET channel at the lower end of the tip links permeates calcium and monovalent ions into this small volume.…”