Calpain inhibition restores basal autophagy and suppresses MNU-induced photoreceptor cell death in mice

Aim: A single systemic administration of N-methyl-N-nitrosourea (MNU) causes retinal degeneration involving photoreceptor cell death within 7 days. MNU-induced photoreceptor cell death is due to apoptosis, and is a reliable animal model for human retinitis pigmentosa. The purpose of this study was to elucidate the involvement of calpain-mediated autophagy, as well as apoptosis on the cell death cascade caused by MNU and to evaluate the efficacy of calpain inhibitor SNJ-1945. Materials and Methods: Seven-week-old BALBIc mice were left untreated or received an intraperitoneal (IP) injection of MNU. The MNU-exposed mice received an IP injection of SNJ-1945 or vehicle alone (distilled water containing 0.5% carboxymethyl cellulose) 3 h prior to MNU and once daily thereafter until sacrifice. Eyes were examined histologically, histochemically, and morphometrically to analyze the photoreceptor cell ratio and retinal damage ratio. The retinal expression of caspase-3, microtubule-associated protein light chain 3 (LC3), autophagy-related protein 5 (Atg5), and α-spectrin was determined by Western blot analysis. Results: During the 72-h period after MNU exposure, the caspase-3 expression increased and the LC3 and Atg5 expression decreased, indicating increased levels of apoptosis and decreased levels of autophagy, as compared with the MNU-unexposed control mouse retina. MNU-induced photoreceptor cell death was caused by increased calpain activation as measured by α-spectrin proteolysis products, while SNJ-1945 ameliorated photoreceptor cell death by blocking calpain activation and restoring basal autophagy. Conclusion: Calpain activation is involved in MNU-induced photoreceptor cell death, and calpain inhibition effectively restored photoreceptor cell autophagy and photoreceptor cell death in mice.