Maintenance of stereocilia and apical junctional complexes by Cdc42 in cochlear hair cells

Cdc42 is a key regulator of dynamic actin organization. However, little is known about how Cdc42-dependent actin regulation influences steady-state actin structures in differentiated epithelia. We employed inner ear hair cell (HC)-specific conditional knockout to analyze the role of Cdc42 in HCs possessing highly elaborate stable actin protrusions (stereocilia). HCs of Atoh1–Cre;Cdc42flox/flox mice developed normally but progressively degenerated after maturation, resulting in progressive hearing loss particularly at high frequencies. Cochlear HC degeneration was more robust in inner HCs than in outer HCs, and began as stereocilia fusion and depletion, accompanied by a thinning and waving circumferential actin belt at apical junctional complexes (AJCs). Adenovirus-encoded GFP-Cdc42 expression in HCs and fluorescence resonance energy transfer (FRET) imaging of HCs from transgenic mice expressing Cdc42-FRET biosensor indicated Cdc42 presence/activation at stereociliary membranes and AJCs in cochlear HCs. Cdc42-knockdown in MDCK cells produced phenotypes similar to those of Cdc42-deleted HCs, including abnormal microvilli, disrupted AJCs, and downregulated actin-turnover represented by enhanced phospho-cofilin levels. Thus, Cdc42 influenced maintenance of stable actin structures through elaborate tuning of actin-turnover and maintained function and viability of cochlear HCs.