title: A claudin-9-based ion permeability barrier is essential for hearing.
abstract: Hereditary hearing loss is one of the most common birth defects, yet the majority of genes required for audition is thought to remain unidentified. Ethylnitrosourea (ENU)-mutagenesis has been a valuable approach for generating new animal models of deafness and discovering previously unrecognized gene functions. Here we report on the characterization of a new ENU-induced mouse mutant (nmf329) that exhibits recessively inherited deafness. We found a widespread loss of sensory hair cells in the hearing organs of nmf329 mice after the second week of life. Positional cloning revealed that the nmf329 strain carries a missense mutation in the claudin-9 gene, which encodes a tight junction protein with unknown biological function. In an epithelial cell line, heterologous expression of wild-type claudin-9 reduced the paracellular permeability to Na+ and K+, and the nmf329 mutation eliminated this ion barrier function without affecting the plasma membrane localization of claudin-9. In the nmf329 mouse line, the perilymphatic K+ concentration was found to be elevated, suggesting that the cochlear tight junctions were dysfunctional. Furthermore, the hair-cell loss in the claudin-9-defective cochlea was rescued in vitro when the explanted hearing organs were cultured in a low-K+ milieu and in vivo when the endocochlear K+-driving force was diminished by deletion of the pou3f4 gene. Overall, our data indicate that claudin-9 is required for the preservation of sensory cells in the hearing organ because claudin-9-defective tight junctions fail to shield the basolateral side of hair cells from the K+-rich endolymph. In the tight-junction complexes of hair cells, claudin-9 is localized specifically to a subdomain that is underneath more apical tight-junction strands formed by other claudins. Thus, the analysis of claudin-9 mutant mice suggests that even the deeper (subapical) tight-junction strands have biologically important ion barrier function.
authors.affiliation: Department of Anatomy and Cell Biology, University of Iowa, Iowa City, Iowa, USA.
authors.forename: Yoko
authors.initials: Y
authors.lastname: Nakano
authors.forename: Sung H
authors.initials: SH
authors.lastname: Kim
authors.forename: Hyoung-Mi
authors.initials: HM
authors.lastname: Kim
authors.forename: Joel D
authors.initials: JD
authors.lastname: Sanneman
authors.forename: Yuzhou
authors.initials: Y
authors.lastname: Zhang
authors.forename: Richard J H
authors.initials: RJ
authors.lastname: Smith
authors.forename: Daniel C
authors.initials: DC
authors.lastname: Marcus
authors.forename: Philine
authors.initials: P
authors.lastname: Wangemann
authors.forename: Randy A
authors.initials: RA
authors.lastname: Nessler
authors.forename: Botond
authors.initials: B
authors.lastname: Bánfi
mesh_terms: D000818:Animals; D001692:Biological Transport; D057167:Claudins; D003051:Cochlea / Q000737:chemistry / Q000378:metabolism; D004195:Disease Models, Animal; D005260:Female; D006198:Hair Cells, Auditory / Q000737:chemistry / Q000378:metabolism; D034381:Hearing Loss / Q000235:genetics / Q000378:metabolism*; D006801:Humans; D007477:Ions / Q000737:chemistry / Q000378:metabolism*; D008297:Male; D008565:Membrane Proteins / Q000235:genetics / Q000378:metabolism*; D051379:Mice; D008805:Mice, Inbred A; D008822:Mice, Transgenic; D016296:Mutagenesis; D010539:Permeability; D019108:Tight Junctions / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*
chemical_list: D057167:Claudins; C515808:Cldn9 protein, mouse; D007477:Ions; D008565:Membrane Proteins
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