Failure Of Hearing Acquisition in Mice With Reduced Expression of Connexin 26 Correlates With the Abnormal Phasing of Apoptosis Relative to Autophagy and Defective ATP-Dependent Ca2+ Signaling in Kölliker's Organ.

Lianhua Sun,Fabio Mammano,Dekun Gao,Jun Yang,Shule Hou,Junmin Chen,Yuyu Huang,Jianyong Chen,Yue Li

doi:10.3389/fncel.2022.816079

Abstract

Mutations in the GJB2 gene that encodes connexin 26 (Cx26) are the predominant cause of prelingual hereditary deafness, and the most frequently encountered variants cause complete loss of protein function. To investigate how Cx26 deficiency induces deafness, we examined the levels of apoptosis and autophagy in Gjb2loxP/loxP; ROSA26CreER mice injected with tamoxifen on the day of birth. After weaning, these mice exhibited severe hearing impairment and reduced Cx26 expression in the cochlear duct. Terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) positive cells were observed in apical, middle, and basal turns of Kölliker’s organ at postnatal (P) day 1 (P1), associated with increased expression levels of cleaved caspase 3, but decreased levels of autophagy-related proteins LC3-II, P62, and Beclin1. In Kölliker’s organ cells with decreased Cx26 expression, we also found significantly reduced levels of intracellular ATP and hampered Ca2+ responses evoked by extracellular ATP application. These results offer novel insight into the mechanisms that prevent hearing acquisition in mouse models of non-syndromic hearing impairment due to Cx26 loss of function.

Highlights

The sense of hearing originates in a portion of the cochlear sensory epithelium, the organ of Corti, which comprises two types of mechanosensory hair cells, the inner and outer hair cells (IHCs and OHCs), which do not express connexins, and at least six types of associated supporting cells, all of which express connexins
To investigate how connexin 26 (Cx26) deficiency induces deafness, we examined the levels of apoptosis and autophagy in Gjb2loxP/loxP; ROSA26CreER mice injected with tamoxifen on the day of birth
As mentioned in the introduction, prior work with mouse models indicate that Cx26 expression has a profound impact on the development of the cochlear sensory epithelium through a complex interplay between Ca2+ signaling, autophagy, and apoptosis; reviewed in Mammano and Bortolozzi (2018) and Mammano (2019)

Summary

Introduction

The sense of hearing originates in a portion of the cochlear sensory epithelium, the organ of Corti, which comprises two types of mechanosensory hair cells, the inner and outer hair cells (IHCs and OHCs), which do not express connexins, and at least six types of associated supporting cells, all of which express connexins. Connexin 26 (Cx26, encoded by the GJB2 gene) and the closely related connexin 30 (Cx30, encoded by GJB6) are the prevailing isoforms expressed in non-sensory cells of both the epithelial and connective tissue of the developing and mature cochlea (Forge et al, 2003; Cohen-Salmon et al, 2005). Interruption of the potassium ion recycling pathway via gap junction systems in the mammalian cochlea has been postulated as the cause of hereditary non-syndromic deafness (Kikuchi et al, 2000). This hypothesis lacks experimental proof and is contradicted by different studies (Beltramello et al, 2005; Jagger and Forge, 2015; Zhao, 2017). The available evidence from mouse models points to a fundamental role played by connexins, connexin hemichannels, during the crucial phases of postnatal cochlear development that lead to hearing acquisitions; reviewed in (Mammano, 2013, 2019)

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