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A/J and C57BL/6 J (B6) mice share a mutation in Cdh23 (ahl allele) and are characterized by age-related hearing loss. However, hearing loss occurs much earlier in A/J mice at about four weeks of age. Recent study has revealed that a mutation in citrate synthase (Cs) is one of the main contributors, but the mechanism is largely unknown. In the present study, we showed that A/J mice displayed more severe degeneration of hair cells, spiral ganglion neurons, and stria vascularis in the cochleae compared with B6 mice. Moreover, messenger RNA accumulation levels of caspase-3 and caspase-9 in the inner ears of A/J mice were significantly higher than those in B6 mice at 2 and 8 weeks of age. Immunohistochemistry localized caspase-3 expression mainly to the hair cells, spiral ganglion neurons, and stria vascularis in cochleae. In vitro transfection with Cs short hairpin RNA (shRNA) alone or cotransfection with Cs shRNA and Cdh23 shRNA significantly increased the levels of caspase-3 in an inner ear cell line (HEI-OC1). Finally, a pan-caspase inhibitor Z-VAD-FMK could preserve the hearing of A/J mice by lowering about 15 decibels of the sound pressure level for the auditory-evoked brainstem response thresholds. In conclusion, our results suggest that caspase-mediated apoptosis in the cochleae, which may be related to a Cs mutation, contributes to the early onset of hearing loss in A/J mice.

Monosialoganglioside (GM1) has been considered to have a neurotrophic factor-like activity. Nerve growth factor (NGF), a member of the neurotrophin family, is essential for neuronal survival, differentiation and maturation. The aim of the present study was to investigate whether co-administration of GM1 and NGF reverses glutamate (Glu) neurotoxicity in primary cultured rat embryonic dorsal root ganglion (DRG) neurons. DRG neurons were exposed to Glu (2 mmol/1), Glu (2 mmol/1) plus GM1 (10 microg/ml), Glu (2 mmol/l) plus NGF (10 ng/ml), Glu (2 mmol/l) plus GM1 (5 microg/ml) and NGF (5 ng/ml) and then processed for detecting intracellular concentrations of Ca2+ ([Ca2+] i) by confocal laser scanning microscopy and growth-associated protein 43 (GAP43) mRNA by RT-PCR. The fluorescent intensity in Glu plus GM1 and NGF incubated neurons was the lowest as compared with that in other groups. The expression of GAP43 mRNA in Glu plus GM1 and NGF incubated neurons was the highest as compared with that in other groups. These results implicated that GM1 and NGF have synergistic neuroprotective effects on DRG neurons with excitotoxicity induced by Glu in vitro.