Reduction of Photoreceptor Cell Death by Calpain Inhibitor SNJ–1945 in N–Methyl–N–Nitrosourea Treated Rats

Abstract

Purpose: : Photoreceptor cell death induced by the DNA–alkylating agent N–methyl–N–nitrosourea (MNU) is relevant to retinitis pigmentosa. We previously reported involvement of the protease calpain in photoreceptor cell death in rats treated with MNU. Calpain inhibitor SNJ–1945 was recently synthesized and can be administered orally. The purpose of the present experiments was to determine if calpain inhibitor SNJ–1945 reduced photoreceptor cell death induced by MNU in rats.

Methods: : Photoreceptor cell death was induced by a single intraperitoneal injection of MNU (60 mg/kg body weight) to Sprague–Dawley rats. Just after MNU injection, calpain inhibitor SNJ–1945 was administered orally as a single dose of 200 mg/kg body weight and thereafter once daily. Photoreceptor cell death was evaluated in histological sections of retinas stained by H&E and by TUNEL staining. Activation of calpain isoforms and proteolysis of calpain substrates were analyzed by casein zymography and immunoblotting.

Results: : TUNEL–positive nuclei in outer nuclear layer were observed 1 day after MNU injection. Severe loss of photoreceptor cells was observed 7 days after MNU injection. Calpain inhibitor SNJ–1945 significantly reduced photoreceptor cell death induced by MNU in rats. Several presumptive biochemical indicators of calpain activation accompanied the morphologic changes.

Conclusions: : Our results showed that orally administered calpain inhibitor SNJ–1945 reduced photoreceptor cell death induced by MNU in rats. If future studies show that calpain is involved in photoreceptor cell death in humans, inhibition of calpain could be tested in optimal drug therapy against conditions showing photoreceptor cell death, such as retinitis pigmentosa.

Keywords: proteolysis • calcium • pathology: experimental