Reduced mitophagy in the cochlea of aged C57BL/6J mice.

An increase in mitochondrial damage has been associated with a decline in the ability to mitigate damage through mitophagy in age-related pathologies. The present study aimed to investigate the changes of mitophagy in a mouse model with age-related hearing loss. C57BL/6J mice were divided into two groups: young (1 month) and aged (12 months). Hearing tests were conducted through the measurement of auditory brainstem response (ABR). Mitochondrial DNA copy number, the level of mitochondrial DNA damage, mitochondrial biogenesis, and mitophagy-related genes and proteins were investigated using real-time PCR and western blot analysis. Coexpression of mitophagosomes and lysosomes in the cochlea was investigated through immunofluorescence imaging analysis. Major players of mitophagy, Parkin and BNIP3, were also investigated through immunohistochemical staining in the cochlea. Hearing thresholds were observed to have increased in the aged group. The mitochondrial DNA copy number, PGC-1α, and PGC-1β significantly decreased in the cochlea of mice in the aged group. The mRNA levels of PINK1, Parkin, MUL1, Atg5, Atg12, Atg13, NIX, and BNIP3 significantly decreased in the cochlea of the mice in the aged group. The level of mitochondrial DNA damage significantly increased in the cochlea of mice in the aged group. Protein levels of PINK1, Parkin, BNIP3, COX4, LC3B, and all OXPHOS subunits significantly decreased in the cochlea of the mice in the aged group. Immunofluorescence imaging analysis of mitophagosomes and lysosomes revealed a decrease in the colocalization in the cochlea of mice in the aged group. Immunohistochemical imaging analysis of Parkin and BNIP3 revealed their decreased expression in aged cochlea. Our results indicate that reduced mitophagy with aging might be attributed to the cellular changes that occur in aged cochlea in the development of age-related hearing loss.