The role of dystroglycan in hearing and hearing loss

The transduction of sound waves into electrical signals in the hair cells is entirely dependent on the stereociliary hair bundles that reside on their apical surface. These hair-like projections are organised into three or more rows in a staircase structure, with the height of each stereocilium being similar within each row. The current theory is that this height disparity between the rows is entirely established and maintained via the differential localisation of several actin effector proteins. Epidermal growth factor receptor pathway substrate 8 (EPS8) selectively localises to the tips of the tallest row of stereocilia, on the pillar side of the hair cell, with only small amounts of the protein detectable at the second tallest row. Conversely, EPS8 like protein 2 (EPS8L2) selectively localises to the shorter two rows of stereocilia. Each of these two interacts with a distinct complex of actin effector proteins and isoforms of the myosin-15 (MYO15) specialised motor protein, but the exact mechanism by which this differential localisation is established and maintained remains unknown.

Here, we investigate two proteins which were hypothesised to modulate the EPS8/EPS8L2 differential localisation axis. Dystroglycan, a specialised cell adhesion receptor best known for the role it plays in the pathogenesis of Duchenne’s muscular dystrophy, was identified as a potential interactor of EPS8L2. The inverse bin-amphiphysin-rvs (I-BAR) family member BAR/IMD domain containing adaptor protein 2 like 2 (BAIAP2L2) was identified as a potential interactor of EPS8, and reported as important in hearing loss by the IMPC mutagenesis screen.

Dystroglycan expression could not be visualised within the organ of Corti with any of the antibodies at our disposal, and a hair cells specific knock-out of the protein did not result in elevated hearing thresholds. Single-cell whole-cell patch clamping experiments indicated that the basolateral profile of inner hair cells matured as normal. Conversely, the global knock-out of BAIAP2L2 resulted in deficits in hair cell mechanotransduction which became apparent at postnatal day 11 (P11). Hearing thresholds were significantly elevated at all ages tested and appeared to show a progressive phenotype reminiscent of a more severe version of the EPS8L2 knock-out. Scanning electron microscopy revealed that the shortest row of stereocilia had almost entirely degenerated by P54. Furthermore, immunostaining data indicated that EPS8 is not localised to the tips of the tallest stereocilia by BAIAP2L2 as originally hypothesised, and in fact BAIAP2L2 is dependent on the presence of EPS8 for proper localisation to the tips of the shorter two rows of stereocilia.