Involvement of Calpain in Retinal Degeneration Induced By N–Methyl–N–Nitrosourea in the Rats

Abstract

Abstract: : Purpose: N–methyl–N–nitrosourea (MNU) is a direct–acting, DNA–alkylating agent. Administration of MNU in a variety of animal species specifically targets photoreceptor cells, causing apoptosis and cell loss. MNU is useful for investigating the mechanism of photoreceptor degeneration, relevant to retinitis pigmentosa. The purpose of the present experiments was to investigate the involvement of calpains in MNU–induced retinal degeneration in rats. Methods: Retinal degeneration was induced by a single intraperitoneal injection of MNU to Sprague–Dawley rats. Cell damage was evaluated in histological sections of retinas stained with H&E and TUNEL reagents. Total calcium was measured by atomic absorption spectrophotometry. Activation of calpain and proteolysis of calpain substrates were analyzed by immunoblotting. Results: TUNEL–positive photoreceptor cells were observed 24 hours after MNU injection, followed by loss of photoreceptor cells by 7 days. These morphologic changes were accompanied by several presumptive biochemical indicators of calpain activation. Total calcium was increased, and proteolysis of α–spectrin (a sensitive substrate for calpains) was observed. Conclusions: Our results suggested that calpain was involved in photoreceptor degeneration induced by MNU in rats. Inhibition of calpains may be a possible therapeutic approach to slow progression of photoreceptor degeneration in such conditions as retinitis pigmentosa.

Keywords: photoreceptors • proteolysis • calcium