Ageing evokes an intrinsic pro-apoptotic signalling pathway in rat cochlea

Abstract

These findings support the hypothesis that age-related apoptosis evokes an intrinsic pathway of pro-apoptotic signalling within the rat cochlea. The aim of this study was to explore the effects of ageing on cochlear apoptosis in rats, as well as the different signalling pathways involved. Female Sprague Dawley rats of different ages were used (mean age 7.72+/-1.93 months, n=100). Luciferase assays were used to determine the different caspase activities and ATP levels in rat cochlear protein extracts. Protein and gene expression was examined by Western blotting and real-time RT-PCR assays, respectively. Caspase-3/7 activity, as well as caspase-3 gene expression, were statistically higher in early-mature rats (EM, 276,139+/-13 669 RLUs (relative light units), p<0.001; and 390+/-50 arbitrary units, p=0.0017, respectively) or in aged-mature rats (AM, 371,020+/-26,457, p<0.0001; and 1510+/-90, p<0.0001, respectively) than younger rats (YR 147,129+/-8485 and 0.14+/-0.004, respectively). An increased caspase-9 activity with ageing was also observed (YR 49,932+/-2046 RLUs vs EM 260,890+/-5939, p<0.0001 or AM 118,241+/-12,423, p<0.0001). Caspase-8 activity was not affected significantly by age. Bax protein expression also increased by age (YR 38,200+/-1790 arbitrary units vs EM 76,549+/-5450, p<0.05), in contrast to Bcl-xL protein expression (YR 27,000+/-5000 arbitrary units vs EM 10,200+/-5000, p<0.005).