Codeficiency of Lysosomal Mucolipins 3 and 1 in Cochlear Hair Cells Diminishes Outer Hair Cell Longevity and Accelerates Age-Related Hearing Loss

Abstract

Acquired hearing loss is the predominant neurodegenerative condition associated with aging in humans. Although mutations on several genes are known to cause congenital deafness in newborns, few genes have been implicated in age-related hearing loss (ARHL), perhaps because its cause is likely polygenic. Here, we generated mice lacking lysosomal calcium channel mucolipins 3 and 1 and discovered that both male and female mice suffered a polygenic form of hearing loss. Whereas mucolipin 1 is ubiquitously expressed in all cells, mucolipin 3 is expressed in a small subset of cochlear cells, hair cells (HCs) and marginal cells of the stria vascularis, and very few other cell types. Mice lacking both mucolipins 3 and 1, but not either one alone, experienced hearing loss as early as at 1 month of age. The severity of hearing impairment progressed from high to low frequencies and increased with age. Early onset of ARHL in these mice was accompanied by outer HC (OHC) loss. Adult mice conditionally lacking mucolipins in HCs exhibited comparable auditory phenotypes, thereby revealing that the reason for OHC loss is mucolipin codeficiency in the HCs and not in the stria vascularis. Furthermore, we observed that OHCs lacking mucolipins contained abnormally enlarged lysosomes aggregated at the apical region of the cell, whereas other organelles appeared normal. We also demonstrated that these aberrant lysosomes in OHCs lost their membrane integrity through lysosomal membrane permeabilization, a known cause of cellular toxicity that explains why and how OHCs die, leading to premature ARHL. Presbycusis, or age-related hearing loss (ARHL), is a common characteristic of aging in mammals. Although many genes have been identified to cause deafness from birth in both humans and mice, only a few are known to associate with progressive ARHL, the most prevalent form of deafness. We have found that mice lacking two lysosomal channels, mucolipins 3 and 1, suffer accelerated ARHL due to auditory outer hair cell degeneration, the most common cause of hearing loss and neurodegenerative condition in humans. Lysosomes lacking mucolipins undergo organelle membrane permeabilization and promote cytotoxicity with age, revealing a novel mechanism of outer hair cell degeneration and ARHL. These results underscore the importance of lysosomes in hair cell survival and the maintenance of hearing.

Advances in understanding of presbycusis

“…The increasing availability of modern genome sequencing techniques has allowed for identification of genetic factors that predispose to ARHL, including Nrp1. CD, mucolipin 3 and GRM7, among many others (Dai et al, ; Friedman et al, ; Salehi et al, ; Wiwatpanit et al, ). The recognition of major pathway involvement such as IGF‐1 and homocysteine in ARHL highlight the likely roles of hormonal and dietary factors (Lassale et al, ; Manche et al, ).…”

“…Properly functioning substrate and ion transport is crucial to maintenance of normal hearing. Ion transport is necessary for supporting hair cell development, sustaining organelle function, and maintaining the endolymphatic potential (EP) (Gratton, Smyth, Lam, Boettcher, & Schmiedt, ; Di Palma et al, ; Schulte & Schmiedt, ; Wiwatpanit et al, ; Zanini & Gopfert, ). Genetic studies have identified numerous mutations in membrane transport proteins that result in age‐progressive hearing loss in animal models.…”

“…Mucolipin channels are recognized for their key roles in the function of several mammalian sensory systems, including hearing (Clapham, ; Montell, ). Within this family both mucolipin 1 and mucolipin 3 are present on inner ear hair cell lysosomes, where they assist in calcium regulation (Wiwatpanit et al, ).…”

“…Mouse models lacking both mucolipin 1 and mucolipin 3 demonstrated a form of polygenic, early onset ARHL with associated hair cell loss (Wiwatpanit et al, ). It should be noted that the quality of this mouse model is questionable, however, as even juvenile double knockout mice demonstrated a mild hearing loss compared to wildtype mice.…”

“…In Va mice, mucolipin 3 channels have a higher probability of opening, leading to a constitutive cationic current capable of causing lysosomal breakdown. Ultimately, lysosomal content leakage inflicts lethal hair cell damage and early onset ARHL (Atiba‐Davies & Noben‐Trauth, ; Wiwatpanit et al, ; Zanini & Gopfert, ). While these data illuminate the mechanism by which mucolipin mutations contribute to hearing loss, the Va mouse exhibits early‐onset hearing loss, vestibular defects, pigmentation abnormalities, and perinatal lethality (Di Palma et al, ), and is therefore, a poor animal model for ARHL.…”