Myosin translocation in retinal pericytes during free-radical induced apoptosis

Vascular pathologies induced by ischemia/reperfusion involve the production of reactive oxygen species (ROS) that in part cause tissue injury. The production of ROS that occurs upon reperfusion activates specific second messenger pathways. In diabetic retinopathy there is a characteristic loss of the microvascular pericyte. Pericytes are more sensitive than endothelial cells to low concentrations of ROS, such as hydrogen peroxide (H(2)O(2)) when tested in vitro. Whether the pericyte loss is due to toxic cell death triggered by the noxious H(2)O(2) or apoptosis, due to activation of specific second messenger pathways, is unknown. During apoptosis, a cell's nucleus and cytoplasm condense, the cell becomes fragmented, and ultimately forms apoptotic bodies. It is generally assumed that apoptosis depends on nuclear signaling, but cytoplasmic morphological processes are not well described. We find that exposing cultured retinal pericytes to 100 microM H(2)O(2) for 30 min leads to myosin heavy chain translocation from the cytosol to the cytoskeleton and a significant decrease in cell surface area. Pericyte death follows within 60-120 min. Exposing cells to 150 mJ/cm(2) ultraviolet radiation, an alternate free radical generating system, also causes pericyte myosin translocation and apoptosis. Proteolytic cleavage of actin is not observed in pericyte apoptosis. 3-aminobenzamide, a pharmacological inhibitor of the cleavage and activation of the DNA-repairing enzyme poly (ADP-ribose) polymerase (PARP) inhibits pericyte apoptosis, and prevents myosin translocation. Deferoxamine, an iron chelator known to interfere with free radical generation, also inhibits pericyte myosin translocation, contractility, and cell death. Myosin translocation to the cytoskeleton may be an early step in assembly of a competent contractile apparatus, which is involved in apoptotic cell condensation. These results suggest that pericyte loss associated with increased free radical production in diabetic retina may be by an apoptotic phenomenon.