Harmonin Mutations Cause Mechanotransduction Defects in Cochlear Hair Cells

Abstract

In hair cells, mechanotransduction channels are gated by tip links, the extracellular filaments that consist of cadherin 23 (CDH23) and protocadherin 15 (PCDH15) and connect the stereocilia of each hair cell. However, which molecules mediate cadherin function at tip links is not known. Here we show that the PDZ-domain protein harmonin is a component of the upper tip-link density (UTLD), where CDH23 inserts into the stereociliary membrane. Harmonin domains that mediate interactions with CDH23 and F-actin control harmonin localization in stereocilia and are necessary for normal hearing. In mice expressing a mutant harmonin protein that prevents UTLD formation, the sensitivity of hair bundles to mechanical stimulation is reduced. We conclude that harmonin is a UTLD component and contributes to establishing the sensitivity of mechanotransduction channels to displacement.

The structure of the harmonin/sans complex reveals an unexpected interaction mode of the two Usher syndrome proteins

“…The stable α-helical N-domain of harmonin, together with its second PDZ domain, bind to two separate fragments of the cadherin23 cytoplasmic domain, providing a structural explanation for the stable harmonin/cadherin23 complex found in the tip links of hair cells (11,16,18). In this study, we first set out to characterize the structure of the harmonin PDZ1, as the domain has been shown to bind canonical carboxyl peptide ligands (e.g., the C-terminal tails of Sans, Usherin, and VLGR1) as well as the SAM domain of Sans and the tail domain of Myosin VIIa (7,8,19).…”

“…At embryonic stages before formation of stereocilia, harmonin and Sans colocalize with each other at the apical membranes of hair cells where stereocilia will emerge at later stage during development (8). After formation of stereocilia, Sans is restricted to the cuticular plate beneath the apical membrane of hair cells (7), whereas harmonin is concentrated specifically at the upper tip-linker density together with cadherin23 (16). In contrast, the protocadherin15-resident lower tip-linker density is devoid of harmonin (16).…”

“…After formation of stereocilia, Sans is restricted to the cuticular plate beneath the apical membrane of hair cells (7), whereas harmonin is concentrated specifically at the upper tip-linker density together with cadherin23 (16). In contrast, the protocadherin15-resident lower tip-linker density is devoid of harmonin (16). It is possible that harmonin binds to a distinct set of proteins during the development of and in the mature hair cells.…”

“…Harmonin b also contains a Pro, Ser, Th-rich sequence (PST) domain preceding the C-terminal PDZ domain that is known to be responsible for binding to F-actin (8). The N-terminal domain, together with the second PDZ domain of harmonin, binds to the cytoplasmic portion of cadherin23 (8,9,18) and anchors the cadherin23 at the upper linker density of hair cell stereocilia (16). It was also reported that the first PDZ domain of harmonin may bind to protein modules, including the globular tail domain of Myosin VIIa and the SAM domain from Sans (7), although such noncanonical PDZ/target interactions are conceptually intriguing.…”

Harmonine est un composant de la machinerie de mécanotransduction auditive

“…Afin de tester si Harmonine est un authentique composant de la machinerie de mécanotransduction, nous avons tout d'abord entrepris une analyse de sa localisation au sein du faisceau stéréociliaire adulte, en utilisant la résolu-tion la plus élevée possible par MET. De nouveaux anticorps dirigés contre Harmonine ont été détectés grâce à leur couplage à des billes d'or, opaques au faisceau d'électron [11]. Cette analyse a montré une concentration des billes au niveau de la densité supérieure du tiplink, suggérant que Harmonine en est un composant.…”