Whirlin complexes with p55 at the stereocilia tip during hair cell development

Abstract

Hearing in mammals depends upon the proper development of actin-filled stereocilia at the hair cell surface in the inner ear. Whirlin, a PDZ domain-containing protein, is expressed at stereocilia tips and, by virtue of mutations in the whirlin gene, is known to play a key role in stereocilia development. We show that whirlin interacts with the membrane-associated guanylate kinase (MAGUK) protein, erythrocyte protein p55 (p55). p55 is expressed in outer hair cells in long stereocilia that make up the stereocilia bundle as well as surrounding shorter stereocilia structures. p55 interacts with protein 4.1R in erythrocytes, and we find that 4.1R is also expressed in stereocilia structures with an identical pattern to p55. Mutations in the whirlin gene (whirler) and in the myosin XVa gene (shaker2) affect stereocilia development and lead to early ablation of p55 and 4.1R labeling of stereocilia. The related MAGUK protein Ca 2؉ -calmodulin serine kinase (CASK) is also expressed in stereocilia in both outer and inner hair cells, where it is confined to the stereocilia bundle. CASK interacts with protein 4.1N in neuronal tissue, and we find that 4.1N is expressed in stereocilia with an identical pattern to CASK. Unlike p55, CASK labeling shows little diminution of labeling in the whirler mutant and is unaffected in the shaker2 mutant. Similarly, expression of 4.1N in stereocilia is unaltered in whirler and shaker2 mutants. p55 and protein 4.1R form complexes critical for actin cytoskeletal assembly in erythrocytes, and the interaction of whirlin with p55 indicates it plays a similar role in hair cell stereocilia. mutant analysis ͉ stereocilia development A ctin cytoskeleton remodeling is fundamental to a variety of cellular processes, including morphological alterations at the cell surface. In the mammalian inner ear, hair cells develop highly organized bundles of actin-filled stereocilia whose most prominent feature is an extraordinary staircase arrangement. The anatomical process of stereocilia development from microvilli on the surface of hair cells in the vicinity of the kinocilium has been well described both in vivo and in organotypic culture (1-5). At around embryonic day (E)17.5 in mice, microvilli begin to elongate. As stereocilia thicken and continue to elongate, the staircase pattern emerges from around postnatal day (P)1 in mice, with morphological development complete around P5-7. Stereocilia in adjacent rows are connected by tip links that attach the tips of shorter stereocilia to the sides of neighboring taller stereocilia. Tip-link attachment sites are thought to harbor mechanotransduction channels, such that sound stimulation and stereocilia deflection lead to channel opening, cation influx, hair cell depolarization, and auditory transduction (6).Until recently, the critical molecules for development of the stereocilia bundle were unknown. However, recent work has established that whirlin, a PSD-95͞SAP90 Discs-large ZO-1 homologous (PDZ) protein, localizes to the stereocilia tips and, by virtue of mutat...

Clarin-1, Encoded by the Usher Syndrome III Causative Gene, Forms a Membranous Microdomain

“…A link between CLRN1 and Usher II gene products can be established through the interaction of CLRN1 with protein 4.1. Protein 4.1 binds indirectly to whirlin, an Usher type IID protein, along with the membrane-associated MAGUS family protein, p55 (53). Our studies support and extend the previously proposed hypothesis that Usher proteins interact together to form an intricate network wherein deficiencies can lead to similar disease phenotypes of variable severity (reviewed in Refs.…”

Erythrocyte scaffolding protein p55/MPP1 functions as an essential regulator of neutrophil polarity

“…MAGUKs, including the Drosophila Dlg tumor suppressor, function as regulators of cell polarity (26). Recent findings have shown that p55 forms a complex with whirlin and regulates cell polarity during hair cell development (27). Mutations in the whirlin gene cause deafness and Usher syndrome, and p55 also interacts with MPP5 to link the Usher protein network with the Crumbs protein complex in the retina (28).…”

Development of the hair bundle and mechanotransduction

“…Recent evidence suggests the membrane-associated guanylate kinase (MAGUK) protein, p55 and protein 4.1R also interact with whirlin and may together form a complex responsible for mediating actin polymerisation and stereocilia development (Mburu et al, 2006). The PDZ1 domain of whirlin can also interact with the cytoplasmic tails of usherin and Vlgr1, both of which contain a C-terminal class I PBM and it has been suggested that whirlin may be involved in the anchoring of the ankle links through its associations with usherin and Vlgr1 and also with myosin VIIa Adato et al, 2005b;Kremer et al, 2006).…”