Jeffrey R. Holt

The Snell’s waltzer mouse ( Myo6 ^sv/sv ) serves as a model for human deafness and vestibular behavioral impairment, caused by a spontaneous 130 bp recessive deletion in the Myo6 gene. In this study, we characterized the auditory and vestibular phenotypes of Myo6 ^sv/sv mice. These mice exhibit profound hearing loss, with cochlear hair cell stereocilia beginning to fuse soon after birth, ultimately leading to disorganization of hair bundles and degeneration of hair cells. Mice also exhibit behavioral phenotypes characterized by severe imbalance, hyperactivity with bouts of circling, and delayed spatial learning of a novel environment, but preserved normal behavioral circadian rhythms. These behaviors emerge in association with the loss of the characteristic staircase morphology of vestibular hair cell stereocilia soon after birth and the subsequent profound elongation of the stereocilia. Adeno-associated virus (AAV) gene replacement therapy, delivered on the day of birth or one day after, failed to restore auditory or vestibular function. Our findings underscore the essential role of Myo6 in the auditory and vestibular systems and imply prenatal intervention may be required for effective therapy.

Adeno-associated virus-based gene therapy offers a promising treatment paradigm for inner ear diseases; however, the genetic heterogeneity of hereditary deafness requires gene-specific strategies and optimization of current approaches to identify the range of treatable conditions and improve therapeutic outcomes. To consider the therapeutic potential for a hearing loss gene not previously explored, we investigated the gene encoding the chloride intracellular channel protein CLIC5, mutations in which lead to DFNB103 in humans and deafness and circling behavior in a Clic5 -deficient mouse model. In this study, we utilized two constructs to deliver the wild-type Clic5 coding sequence into Clic5 -deficient mice: single-stranded and self-complementary adeno-associated virus, the latter known for rapid onset of transgene expression. We report a robust restoration of CLIC5 expression using either construct, including prevention of morphological degeneration and preserving auditory and vestibular function. Interestingly, the self-complementary construct achieved comparable functional recovery to single-stranded construct but at a lower titer. These findings highlight the potential of self-complementary adeno-associated virus to reduce dose requirements, minimize toxicity and broaden clinical utility for inner ear therapies.

Pathogenic variants in the gene TMPRSS3 are a common cause of hearing loss in humans, although the causal mechanisms remain unknown. Previous work has shown that Tmprss3 ^Y260X/Y260X mice exhibit normal hair cell development, mechanosensory transduction, and spiral ganglion patterning, but experience rapid hair cell death from P12 to P14 at the onset of hearing. Here, we demonstrate that Tmprss3 ^Y260X/Y260X mice display an early and temporary spike in endocochlear potential (EP) prior to the onset of hair cell death. In vitro experiments with cochlear explants from Tmprss3 ^Y260X/Y260X mice and in vivo studies with Tmprss3 ^Y260X/Y260X mice crossed with 2 different mutant models that lacked EP generation promoted hair cell survival. Furthermore, systemic administration of furosemide, a drug that reduces EP in vivo, reduced hair cell death in Tmprss3 ^Y260X/Y260X mice. These findings suggest that extracellular factors, including EP, play a role in TMPRSS3-related hair cell survival and hearing loss, and suggest that modulating EP could be a therapeutic strategy.

This paper, on the role of calcium and integrin-binding protein 2 and 3 in the hair-cell in the mechano-electrical transduction (MET) apparatus, is a mix of confirmatory studies with new and potentially important data. Some parts, such as zebrafish studies, the modelling and simulations, are regarded as necessary and convincing. Other parts of the paper do not have the same novelty. Both Liang et al. (2021) and Wang et al. (2023) had previously demonstrated a role for CIB2/CIB3 in auditory and vestibular cells in mice. Moreover, there are also data in Riazuddin et al. (2012) paper that demonstrates the importance of CIB2 in zebrafish and Drosophila. Breaking the manuscript up to focus on specific aspects of the problem might alleviate the limitations of this multi-faceted study.

In this Viewpoint, Holt, Fettiplace, and Müller weigh the evidence supporting a role for PIEZO and TMC channels in mechanosensory transduction in inner ear hair cells.

Tmc1 and Tmc2 are essential pore-forming subunits of mechanosensory transduction channels localized to the tips of stereovilli in auditory and vestibular hair cells of the inner ear. To investigate expression and function of Tmc1 and Tmc2 in vestibular organs, we used quantitative polymerase chain reaction (qPCR), fluorescence in situ hybridization – hairpin chain reaction (FISH-HCR), immunostaining, FM1-43 uptake and we measured vestibular evoked potentials (VsEPs) and vestibular ocular reflexes (VORs). We found that Tmc1 and Tmc2 showed dynamic developmental changes, differences in regional expression patterns, and overall expression levels which differed between the utricle and saccule. These underlying changes contributed to unanticipated phenotypic loss of VsEPs and VORs in Tmc1 KO mice. In contrast, Tmc2 KO mice retained VsEPs despite the loss of the calcium buffering protein calretinin, a characteristic biomarker of mature striolar calyx-only afferents. Lastly, we found that neonatal Tmc1 gene replacement therapy is sufficient to restore VsEP in Tmc1 KO mice for up to six months post-injection.