A Mutation in Tmem135 Causes Progressive Sensorineural Hearing Loss.

Transmembrane protein 135 (TMEM135) is a 52 kDa protein with five predicted transmembrane domains that is highly conserved across species. Previous studies have shown that TMEM135 is involved in mitochondrial dynamics, thermogenesis, and lipid metabolism in multiple tissues; however, its role in the inner ear or the auditory system is unknown. We investigated the function of TMEM135 in hearing using wild-type (WT) and Tmem135 FUN025/FUN025 ( FUN025 ) mutant mice on a CBA/CaJ background, a normal-hearing mouse strain. Although FUN025 mice displayed normal auditory brainstem response (ABR) at 1 month, we observed significantly elevated ABR thresholds at 8, 16, and 64 kHz by 3 months, which progressed to profound hearing loss by 12 months. Consistent with our auditory testing, 13-month-old FUN025 mice exhibited a severe loss of outer hair cells and spiral ganglion neurons in the cochlea. Our results using BaseScope in situ hybridization indicate that TMEM135 is expressed in the inner hair cells, outer hair cells, and supporting cells. Together, these results demonstrate that the FUN025 mutation in Tmem135 causes progressive sensorineural hearing loss, and suggest that TMEM135 is crucial for maintaining key cochlear cell types and normal sensory function in the aging cochlea.

Loss of Esr1 Does Not Affect Hearing and Balance.

Although estrogen affects the structure and function of the nervous system and brain and has a number of effects on cognition, its roles in the auditory and vestibular systems remain unclear. The actions of estrogen are mediated predominately through two classical nuclear estrogen receptors, estrogen receptor 1 (ESR1) and estrogen receptor 2 (ESR2). In the current study, we investigated the roles of ESR1 in normal auditory function and balance performance using 3-month-old wild-type (WT) and Esr1 knockout (KO) mice on a CBA/CaJ background, a normal-hearing strain. As expected, body weight of Esr1 KO females was lower than that of Esr1 KO males. Body weight of Esr1 KO females was higher than that of WT females, while there was no difference in body weight between WT and Esr1 KO males. Similarly, head diameter was higher in Esr1 KO vs. WT females. Contrary to our expectations, there were no differences in auditory brainstem response (ABR) thresholds, ABR waves I-V amplitudes and ABR waves I-V latencies at 8, 16, 32, and 48 kHz, distortion product otoacoustic emission (DPOAE) thresholds and amplitudes at 8, 16, and 32 kHz, and rotarod balance performance (latency to fall) between WT and Esr1 KO mice. Furthermore, there were no sex differences in ABRs, DPOAEs, and rotarod balance performance in Esr1 KO mice. Taken together, our findings show that Esr1 deficiency does not affect auditory function or balance performance in normal hearing mice, and suggest that loss of Esr1 is likely compensated by ESR2 or other estrogen receptors to maintain the structure and function of the auditory and vestibular systems under normal physiological conditions.

Sex differences in body composition, voluntary wheel running activity, balance performance, and auditory function in CBA/CaJ mice across the lifespan.

Hearing loss is the third most prevalent chronic health condition affecting older adults and age-related hearing loss (ARHL) is the most common form of hearing impairment. Significant sex differences in hearing have been documented in humans and rodents. In general, the results of these studies show that men lose their hearing more rapidly than women. However, the cellular mechanism underlying sex differences in hearing or hearing loss remains largely unknown, and to our knowledge, there is no well-established animal model for studying sex differences in hearing. In the current study, we examined sex differences in body composition, voluntary wheel running activity, balance performance, auditory function, and cochlear histology in young, middle-age, and old CBA/CaJ mice, a model of age-related hearing loss. As expected, body weight of young females was lower than that of males. Similarly, lean mass and total water mass of young, middle-age, and old females were lower than those of males. Young females showed higher voluntary wheel running activity during the dark cycle, an indicator of mobility, physical activity, and balance status, compared to males. Young females also displayed higher auditory brainstem response (ABR) wave I amplitudes at 8 kHz, wave II, III, V amplitudes at 8 and 48 kHz, and wave IV/I and V/I amplitude ratios at 48 kHz compared to males. Collectively, our findings suggest that the CBA/CaJ mouse strain is a useful model to study the cellular mechanisms underlying sex differences in physical activity and hearing.