Abstract
Sirtuin 3 (SIRT3) is a mitochondrial deacetylase that regulates redox balance and cellular stress resistance. Its role in hearing maintenance under basal conditions is controversial. Here, we investigated the effect of Sirt3 deficiency on auditory function and studied whether honokiol, a small-molecule SIRT3 activator, provides protection in the cochlea. Male and female Sirt3+/+, Sirt3+/-, and Sirt3-/- mice were assessed for auditory brainstem response thresholds at 6, 8, and 12 weeks of age. Mice were given honokiol via intraperitoneal injection at weeks 6, 8, and 10. Synapse integrity, hair cell survival, antioxidant enzyme activity, hydrogen peroxide levels, and gene expression were studied in the cochlea. Generalized linear mixed-effects models were used to investigate the interaction between genotype, frequency, age, and treatment on auditory brainstem response thresholds. Male Sirt3-/- mice showed early-onset, progressive hearing loss, with reduced postsynaptic densities. Honokiol treatment preserved hearing thresholds, increased SIRT3 signal in wild-type mice, and increased SIRT5 signal in male Sirt3-/- mice. In male Sirt3-/- mice, honokiol did not restore presynaptic or postsynaptic puncta numerically, but restored presynaptic morphology. Honokiol treatment improved superoxide dismutase 2 enzymatic activity, decreased cochlear hydrogen peroxide levels, and upregulated Sirt5 mRNA expression in male Sirt3-/- cochleae. Linear mixed-effects modeling identified genotype, frequency, and interactions of fixed effects with honokiol treatment as significant predictors of auditory brainstem response thresholds. This study demonstrates that SIRT3 is required for hearing function and that honokiol attenuates hearing loss and oxidative stress in male Sirt3-/- mice.