LIGHT-INDUCED APOPTOSIS - DIFFERENTIAL TIMING IN THE RETINA AND PIGMENT-EPITHELIUM

Apoptosis is a genetically regulated form of cell death. Individual ce lls show condensed nuclear chromatin and cytoplasm, and biochemical an alysis reveals fragmentation of the DNA. Ensuing cellular components, apoptotic bodies. are removed by macrophages or neighboring cells. Gen es involved in the regulation of apoptosis as well as stimuli and sign al transduction systems, are only beginning to be understood in the re tina. Therefore, we developed a new in vivo model system for the inves tigation of events leading to apoptosis in the retina and the pigment epithelium. We induced apoptosis in retinal photoreceptors and the pig ment epithelium of albino rats by exposure to 3000 lux of diffuse, coo l white fluorescent light for short time periods of up to 120 minutes. Animals were killed at different time intervals during and after ligh t exposure. The eyes were enucleated and the lower central retina was processed for light- and electron microscopy. DNA fragmentation was an alysed in situ by TdT-mediated dUTP nick-end labeling (TUNEL) or by ge l electrophoresis of total retinal DNA. We observed that the timing of apoptosis in the photoreceptors and pigment epithelium was remarkably different, the pigment epithelium showing a distinct delay of several hours before the onset of apoptosis. In photoreceptors. apoptosis was induced within 90 minutes of light exposure. with the morphological a ppearance of apoptosis preceding the fragmentation of DNA. In the pigm ent epithelium, the morphological appearance of apoptosis and DNA frag mentation were coincident, Different regulative mechanisms may lead to apoptotic cell death in the retinal photoreceptors and pigment epithe lium. This in vivo model system will allow measurement of dose-respons es, a potential spectral dependence and the molecular background of ap optotic mechanisms in the retina. (C) 1997 Academic Press Limited.