The Chloride Intracellular Channel Protein CLIC5 Is Expressed at High Levels in Hair Cell Stereocilia and Is Essential for Normal Inner Ear Function

Abstract

Although CLIC5 is a member of the chloride intracellular channel protein family, its association with actin-based cytoskeletal structures suggests that it may play an important role in their assembly or maintenance. Mice homozygous for a new spontaneous recessive mutation of the Clic5 gene, named jitterbug ( jbg), exhibit impaired hearing and vestibular dysfunction. The jbg mutation is a 97 bp intragenic deletion that causes skipping of exon 5, which creates a translational frame shift and premature stop codon. Western blot and immunohistochemistry results confirmed the predicted absence of CLIC5 protein in tissues of jbg/jbg mutant mice. Histological analysis of mutant inner ears revealed dysmorphic stereocilia and progressive hair cell degeneration. In wild-type mice, CLIC5-specific immunofluorescence was detected in stereocilia of both cochlear and vestibular hair cells and also along the apical surface of Kolliker's organ during cochlear development. Refined immunolocalization in rat and chicken vestibular hair cells showed that CLIC5 is limited to the basal region of the hair bundle, similar to the known location of radixin. Radixin immunostaining appeared reduced in hair bundles of jbg mutant mice. By mass spectrometry and immunoblotting, CLIC5 was shown to be expressed at high levels in stereocilia of the chicken utricle, in an approximate 1:1 molar ratio with radixin. These results suggest that CLIC5 associates with radixin in hair cell stereocilia and may help form or stabilize connections between the plasma membrane and the filamentous actin core.

Progressive hearing loss and vestibular dysfunction caused by a homozygous nonsense mutation in CLIC5

“…By 7 months of age, their hearing loss had progressed to complete deafness due to progressive hair bundle degeneration and a reduced density of spiral ganglion cells. 9 The vestibular hair cells of jbg mice also showed a progressive degeneration. In the crista ampullaris, the number of vestibular hair cells was reduced at 3 months and hair cells were nearly absent at 12 months of age.…”

“…In the crista ampullaris, the number of vestibular hair cells was reduced at 3 months and hair cells were nearly absent at 12 months of age. 9 Besides inner ear dysfunction, the phenotype in humans and mice with Clic5 mutations may well overlap with respect to the renal phenotype. The jbg mice have abnormalities in the foot processes of the kidney podocytes leading to proteinuria.…”

“…The third gene, Clic5, was described to underlie progressive sensorineural hearing loss and vestibular dysfunction in the jbg mouse mutant. 9 Therefore, CLIC5 represented an excellent candidate gene.…”

“…The orthologous mouse gene, Clic5, was described to be mutated in the jitterbug (jbg) mouse exhibiting congenital progressive hearing impairment and vestibular dysfunction due to progressive hair cell degeneration. 9 …”

Ion Channel Gene Expression in the Inner Ear

“…Other members identified include CLIC5 and CLCNKA and CLCNKB. CLIC5 is found in hair cell stereocilia, its loss leads to hearing deficiency in mouse knockouts (Gagnon et al 2006). The latter two are functionally expressed in non-sensory tissues of the inner ear and the kidney (Ando and Takeuchi 2000;Hebert 2003;Maehara et al 2003;Qu et al 2006), although, there is some evidence for expression of the Clc-K1 channel (CLCNKA) in the outer hair cells (Kawasaki et al 2000).…”