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

Abstract

BSTRACTCdc42 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-specific conditional knockout to analyze the role of Cdc42 in hair cells possessing highly elaborate stable actin protrusions (stereocilia). Hair cells of Atoh1-Cre;Cdc42 flox/flox mice developed normally but progressively degenerated after maturation, resulting in progressive hearing loss particularly at high frequencies. Cochlear hair cell degeneration was more robust in inner hair cells than in outer hair cells, 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 hair cells and fluorescence resonance energy transfer (FRET) imaging of hair cells from transgenic mice expressing a Cdc42-FRET biosensor indicated Cdc42 presence and activation at stereociliary membranes and AJCs in cochlear hair cells. Cdc42-knockdown in MDCK cells produced phenotypes similar to those of Cdc42-deleted hair cells, including abnormal microvilli and disrupted AJCs, and downregulated actin turnover represented by enhanced levels of phosphorylated cofilin. Thus, Cdc42 influenced the maintenance of stable actin structures through elaborate tuning of actin turnover, and maintained function and viability of cochlear hair cells.

Severe hearing loss and outer hair cell death in homozygous Foxo3 knockout mice after moderate noise exposure

“…Fused stereocilia were also reported in early studies of noise damage 61 . We note that the damaged outer hair cells phenocopy a conditional loss-of-function Cdc42 mutant 65 . Cdc42 is a member of the Rho sub-family of small GTPases that regulates the assembly of actin-rich cellular structures 66 .…”

Constitutive activation of DIA 1 ( DIAPH 1) via C‐terminal truncation causes human sensorineural hearing loss

“…We previously reported inner ear HC‐specific Cdc42‐knockout (KO) mice, which show progressive deafness in parallel with HC loss with stereocilia degradation, including short, elongated, fused stereocilia (Ueyama et al , ). Furthermore, we found that the amount of active RhoA (GTP‐form) is increased in Cdc42‐knockdown (KD) cells (Ueyama et al , ).…”

Stereocilia morphogenesis and maintenance through regulation of actin stability

“…Stereocilia bundle formation follows the boundary of these domains, with the vertex of the bundle attached to the kinocilia and the tallest stereocilia forming closest to the lateral side of the cell in a decreasing height gradient to the medial side. Phenotypes similar to mutants lacking kinocilia also arise when non-ciliary proteins are lost that regulate basal body positioning through organization of the microtubule network, including the small GTPases Cdc42 and Rac1 [48–51]. Thus, proper cortical-microtubule interactions are critical for positioning kinocilia in their correct location, and ultimately orienting the newly formed bundles properly along the medial-lateral axis.…”