Mice with conditional deletion of Cx26 exhibit no vestibular phenotype despite secondary loss of Cx30 in the vestibular end organs

Abstract

Connexins are components of gap junctions which facilitate transfer of small molecules between cells. One member of the connexin family, Connexin 26 (Cx26), is prevalent in gap junctions in sensory epithelia of the inner ear. Mutations of GJB2, the gene encoding Cx26, cause significant hearing loss in humans. The vestibular system, however, does not usually show significant functional deficits in humans with this mutation. Mouse models for loss of Cx26 function demonstrate hearing loss and cochlear pathology but the extent of vestibular dysfunction and organ pathology are less well characterized. To understand the vestibular effects of Cx26 mutations, we evaluated vestibular function and histology of the vestibular sensory epithelia in a conditional knockout (CKO) mouse with Cx26 loss of function. Transgenic C57BL/6 mice, in which cre-Sox10 drives excision of the Cx26 gene from non-sensory cells flanking the sensory epithelium of the inner ear (Gjb2-CKO), were compared to age-matched wild types. Animals were sacrificed at ages between 4 and 40 weeks and their cochlear and vestibular sensory organs harvested for histological examination. Cx26 immunoreactivity was prominent in the peripheral vestibular system and the cochlea of wild type mice, but absent in the Gjb2-CKO specimens. The hair cell population in the cochleae of the Gjb2-CKO mice was severely depleted but in the vestibular organs it was intact, despite absence of Cx26 expression. The vestibular organs appeared normal at the latest time point examined, 40 weeks. To determine whether compensation by another connexin explains survival of the normal vestibular sensory epithelium, we evaluated the presence of Cx30 in the Gjb2-CKO mouse. We found that Cx30 labeling was normal in the cochlea, but it was decreased or absent in the vestibular system. The vestibular phenotype of the mutants was not different from wild-types as determined by time on the rotarod, head stability tests and physiological responses to vestibular stimulation. Thus presence of Cx30 in the cochlea does not compensate for Cx26 loss, and the absence of both connexins from vestibular sensory epithelia is no more injurious than the absence of one of them. Further studies to uncover the physiological foundation for this difference between the cochlea and the vestibular organs may help in designing treatments for GJB2 mutations.

Vestibular dysfunction in the adult CBA/CaJ mouse after lead and cadmium treatment

“…Our study has revealed vestibular deficits in CBA mice exposed to occupationally‐relevant doses of lead. These deficits, which are highly relevant for human pathophysiology, were unmasked via a newly developed multi‐axis test of head stability (Godin et al, ; Lee et al, ), but not by a common but less specific test of vestibular function, the rotarod. The deficits observed in the head stability tests were greater among animals that received higher Pb exposures, and correlated with individual bone Pb levels.…”

“…Head motion was then sampled at 1 kHz for five minutes. The amount and direction of measured head motion was considered indicative of VCR function (Godin et al, ; Lee et al, ).…”

“…Vestibular testing was performed before and after the treatment period to assess potential changes in vestibular function. We opted to exclude a positive control treatment group to confirm the sensitivity of the head stability test compared to the rotarod based on the results of previously published studies that used the VCR assessment to evaluate vestibular function (Godin et al, ; Lee et al, ), and which determined that vestibular dysfunction corresponds to significant changes in head motion.…”

GJB2 Mutations Linked to Hearing Loss Exhibit Differential Trafficking and Functional Defects as Revealed in Cochlear-Relevant Cells

“…The expression of Cx26 within the epithelial gap junction network begins to occur around embryonic day 16 in mice (Frenz and Van De Water, 2000) and continues for approximately two weeks after birth as mouse hearing matures. As revealed in mouse studies, improper cochlear development is a pathological outcome of Cx26 mutant expression or Cx26 ablation as noted by the deformation of hair cells and disrupted formation of the tunnel of Corti, which is formed by supporting cells (Wang et al, 2009;Mese et al, 2011;Schutz et al, 2011;Inoshita et al, 2014;Anzai et al, 2015;Lee et al, 2015;Zhu et al, 2015;Chen et al, 2018b). A few rare mutations in GJC3 (Cx30.2/Cx29) and GJB3 (Cx31) have also been linked to hearing loss but it is unclear what role these connexins play and even where these connexins are localized in the auditory tract (Wingard and Zhao, 2015).…”

Treatment of peripheral vestibular dysfunction using photobiomodulation

“…The inner ear comprises the cochlea (hearing organ) and the vestibular organ (balancing organ), which are located adjacent to one another and form a fluid-filled cavity with a bony wall. Several recent studies have shown promising effects from photobiomodulation of the cochlea; [8][9][10][11][12][13] however, the effect of photobiomodulation on the vestibular organ is not well understood.…”