Noddy, a Mouse Harboring a Missense Mutation in Protocadherin-15, Reveals the Impact of Disrupting a Critical Interaction Site between Tip-Link Cadherins in Inner Ear Hair Cells

Abstract

In hair cells of the inner ear, sound or head movement increases tension in fine filaments termed tip links, which in turn convey force to mechanosensitive ion channels to open them. Tip links are formed by a tetramer of two cadherin proteins: protocadherin 15 (PCDH15) and cadherin 23 (CDH23), which have 11 and 27 extracellular cadherin (EC) repeats, respectively. Mutations in either protein cause inner ear disorders in mice and humans. We showed recently that these two cadherins bind tip-to-tip in a “handshake” mode that involves the EC1 and EC2 repeats of both proteins. However, a paucity of appropriate animal models has slowed our understanding both of the interaction and of how mutations of residues within the predicted interface compromise tip link integrity. Here, we present noddy, a new mouse model for hereditary deafness. Identified in a forward genetic screen, noddy homozygotes lack inner ear function. Mapping and sequencing showed that noddy mutant mice harbor an isoleucine-to-asparagine (I108N) mutation in the EC1 repeat of PCDH15. Residue I108 interacts with CDH23 EC2 in the handshake and its mutation impairs the interaction in vitro. The noddy mutation allowed us to determine the consequences of blocking the handshake in vivo: tip link formation and bundle morphology is disrupted, and mechanotransduction channels fail to remain open at rest. These results offer new insights into the interaction between PCDH15 and CDH23, and help explain the etiology of human deafness linked to mutations in the tip-link interface.

Usher protein functions in hair cells and photoreceptors

“…The noddy mutation in the PCDH15 gene is in the region of interaction between the amino terminal domains of protocadherin 15 and cadherin 23 and disrupts the formation of tip links. This mouse has significant stereocilia dysmorphology (Geng et al, 2013). Another missense mutation in a calcium binding motif of the CDH23 gene of the salsa mouse model is the same mutation associated with DFNB12 (non syndromic hearing loss).…”

Structural determinants of adhesion by Protocadherin-19 and implications for its role in epilepsy

“…However, PCDH19 along with the rest of the non-classical cadherins lack these critical tryptophan residues and must mediate adhesion by an alternative mechanism (Emond et al, 2011; Sotomayor et al, 2014; Biswas et al, 2010). In the case of the non-classical protocadherin-15 and cadherin-23 proteins, an adhesive interface is formed by overlapping, antiparallel interactions of their EC1 and EC2 tips (Elledge et al, 2010; Sotomayor et al, 2010; 2012; Geng et al, 2013). For clustered protocadherins, recent binding assays and structures suggest that adhesion is mediated by an antiparallel interaction of fully overlapping EC1 to EC4 domains (Rubinstein et al, 2015; Nicoludis et al, 2015; Goodman et al, 2016).…”

A Partial Calcium-Free Linker Confers Flexibility to Inner-Ear Protocadherin-15

“…Unlike classical cadherins that typically have five EC repeats, CDH23 and PCDH15 are considerably longer and have 27 and 11 EC repeats respectively. The mature tip link is made of CDH23 and PCDH15 homodimers interacting at their N-terminal tips through a unique “handshake” interface distinct from that of classical cadherins (Geng et al, 2013; Indzhykulian et al, 2013; Kazmierczak et al, 2007; Sotomayor et al, 2012). …”