Activation of mitochondrial calpain and release of apoptosis-inducing factor from mitochondria in RCS rat retinal degeneration

The present study was performed to investigate changes of cytosolic and mitochondrial calpain activities, and effects of intravitreously injected calpain inhibitor on photoreceptor apoptosis in Royal College of Surgeon’s (RCS) rats. Time courses of activities for both cytosolic and mitochondrial calpains and amount of calpastatin in RCS rat retina were analyzed by subcellular fractionation, calpain assay and western blotting. Calpain assay was colorimetrically performed using Suc-LLVY-Glo as substrate. Effects of intravitreously injected calpain inhibitor (ALLN and PD150606) on RCS rat retinal degeneration were analyzed by TUNEL staining. Effects of mitochondrial calpain activity on activation and translocation of apoptosis-inducing factor (AIF) were analyzed by western blotting. Mitochondrial calpain started to be significantly activated at postnatal (p) 28 days in RCS rat retina, whereas cytosolic μ-calpain was activated at p 35 days, although specific activity of mitochondrial calpain was 13% compared to cytosolic μ-calpain. Intravitreously injected ALLN and PD150606 effectively inhibited photoreceptor apoptosis only when injected at p 25 days, but did not inhibit photoreceptor apoptosis when injected at p 32 days. Parts of AIF were truncated/activated by mitochondrial calpains and translocated to the nucleus. These results suggest that 1), calpain presents not only in the cytosolic fraction but also in the mitochondrial fraction in RCS rat retina; 2), mitochondrial calpain is activated earlier than cytosolic calpain during retinal degeneration in RCS rats; 3), photoreceptor apoptosis may be regulated by not only calpain systems but also other mechanisms; 4), mitochondrial calpain may activate AIF to induce apoptosis; and 5), calpain inhibitors may be partially effective to inhibit photoreceptor apoptosis in RCS rats. The present study provides new insights into the molecular basis for photoreceptor apoptosis in RCS rats and the future possibility of new pharmaceutical treatments for retinitis pigmentosa.