Cadherin 23 and protocadherin 15 interact to form tip-link filaments in sensory hair cells

Abstract

Hair cells of the inner ear are mechanosensors that transduce mechanical forces arising from sound waves and head movement into electrochemical signals to provide our sense of hearing and balance. Each hair cell contains at the apical surface a bundle of stereocilia. Mechanoelectrical transduction takes place close to the tips of stereocilia in proximity to extracellular tip-link filaments that connect the stereocilia and are thought to gate the mechanoelectrical transduction channel. Recent reports on the composition, properties and function of tip links are conflicting. Here we demonstrate that two cadherins that are linked to inherited forms of deafness in humans interact to form tip links. Immunohistochemical studies using rodent hair cells show that cadherin 23 (CDH23) and protocadherin 15 (PCDH15) localize to the upper and lower part of tip links, respectively. The amino termini of the two cadherins co-localize on tip-link filaments. Biochemical experiments show that CDH23 homodimers interact in trans with PCDH15 homodimers to form a filament with structural similarity to tip links. Ions that affect tip-link integrity and a mutation in PCDH15 that causes a recessive form of deafness disrupt interactions between CDH23 and PCDH15. Our studies define the molecular composition of tip links and provide a conceptual base for exploring the mechanisms of sensory impairment associated with mutations in CDH23 and PCDH15.

Three-dimensional Architecture of Hair-bundle Linkages Revealed by Electron-microscopic Tomography

“…Our results are in agreement with the recent demonstration that the mammalian tip link comprises a head-to-head heterodimer of paired cadherin 23 and protocadherin 15 filaments (Kazmierczak et al 2007). In particular, this structure would be expected to yield links about 170 nm in total length, a value that accords with the longer tip links in the present study.…”

“…The tip link may consist of glycosylated protein, for polycationic compounds label it (Neugebauer and Thurm 1987;Tsuprun and Santi 2000), and in avian preparations, it is severed by proteases only after deglycosylation (Kachar et al 2000). Genetic and immunological evidence localizes cadherin 23 to the tip link Sollner et al 2004), where it evidently constitutes the structure's upper two thirds (Kazmierczak et al 2007). The balance of the link is thought to consist of protocadherin 15 (Ahmed et al 2006).…”

Crystal structures of Drosophila N-cadherin ectodomain regions reveal a widely used class of Ca 2+ -free interdomain linkers

“…Notably, among the few exceptions (less than 1% of the stretches containing 10 EC domains or more), we found cadherin-23, whose 27 EC domains remain "uninterrupted" by missing or partial Ca 2þ -binding sites. In good agreement with our hypothesis, this cadherin does not function in a regular intercellular space, but instead associates with protocadherin-15 (11 EC domains) to form the tip link of the inner ear's stereocilia (34), where a large distance (150 to 200 nm) must be spanned by extended conformations of both cadherins.…”

Distinct capacity for differentiation to inner ear cell types by progenitor cells of the cochlea and vestibular organs

“…Differentiated progenitors and native tissues all expressed myosin 1c, which plays a key role in transducer adaptation in vestibular HCs (Gillespie, 2004;Holt et al, 2002), and the tip-link proteins, cadherin 23 and protocadherin 15, which are crucial for transduction (Kazmierczak et al, 2007). Differentiated progenitor cells from both vestibular and OC tissues expressed two isoforms of espin, espin1 and espin4, which are associated with stereociliary elongation.…”