Caspases and Calpains in Light Induced Retinal Degeneration

Abstract

Purpose:: We had previously shown that caspase inhibitors protected the retina against light-induced damage. Therefore, we tested the effect of a calpain inhibitor and then studied caspases and calpains activation in light-induced retinal degeneration.

Methods:: Wistar rats were raised in dim cyclic light. In a first set of experiments, rats were uninjected or injected intravitreally with the calpain inhibitor Mu-hPhe-Phe-FMK (2.2 mM) or DMSO 2% (vehicle). Thereafter, they were placed in the dark for 18 hours before being unexposed or exposed for 24 hours to a 3400 lux light. Electroretinograms were recorded before treatment and/or exposure, and one day after light exposure (D1). The b-wave sensitivity curves were fitted to calculate the maximal b-wave amplitude (B_max). Rats were sacrificed at D1 for apoptotic nuclei detection in the outer nuclear layer (ONL) or fifteen days later to measure the ONL thickness. In a second set of experiments, uninjected rats were sacrificed at 0, 2, 4, 6, 12, 24 hours of light-exposure or at D1, to evaluate DNA fragmentation, and to measure caspase-1, -3, -7, -9, and calpain activities.

Results:: In unexposed rats, Mu-hPhe-Phe-FMK had no toxic effect on the retina. In uninjected retina, light-exposure induced a decrease of B_max by 89%, an increase of apoptotic nuclei by 54% and a reduction of the ONL thickness by 95%. To inject Mu-hPhe-Phe-FMK or DMSO 2% did not protect retinal function or structure and did not reduce photoreceptor apoptosis compared to DMSO2%. DNA fragmentation was observed at 4 hours of light exposure and increased up to D1. Caspase-1, -3 and -9 activities were decreased to 53% at 2 and 4 hours of light exposure, and progressively increased to reach 111 % at D1. No caspase-7 activity was detected at 0h, during light exposure or at D1. Calpain activity increased to 200% at 2 hours and then progressively decreased to reach 117% at D1.

Conclusions:: While caspases are down regulated during the first hours of light exposure, calpains are up-regulated. However, calpain inhibition had no effect against light-induced retinal degeneration suggesting that calpains do not play a major role in the degenerative process induced by light.

Keywords: retinal degenerations: cell biology • apoptosis/cell death • neuroprotection