Hyperglycemia has been found experimentally to be sufficient to initiate the development of diabetic retinopathy, but the mechanism by which hyperglycemia causes the retinal disease remains unclear. Cells of the retina and its microvasculature die at an accelerated rate in diabetes by a process consistent with apoptosis. Caspases are a family of proteolytic enzymes, and participate in one of the two distinct signaling pathways: (1) activation of pro-inflammatory cytokines, and (2) promotion of apoptotic cell death. Caspase-1 is primarily involved in production of the pro-inflammatory cytokines interleukin-1beta and interleukin-18, but has also been associated with apoptosis induction. Our results have shown that caspases are activated in the retinas of diabetic and galactose-fed mice and diabetic patients. Interestingly, common to both diabetes and galactosemia was the early activation of caspase-1 and increased levels of interleukin-1beta. Inhibition of either caspase-1 or interleukin-1beta signaling prevents the formation of diabetes-induced acellular capillaries raising the possibility that interleukin-1beta production and signaling might play an important role in the formation of histologic lesions observed in the two models for diabetic retinopathy. The finding of caspase-1 activation in retinas of diabetic patients strengthens this idea and is consistent with a growing awareness that diabetic retinopathy has several characteristics of a chronic inflammatory disease. How caspase-1 becomes activated in diabetes is not known. Our preliminary results indicate that retinal Muller cells might be the source for active caspase-1 and interleukin-1beta production. We propose to assess (1) the function of caspase-1/interleukin-1beta in the development of diabetic retinopathy in vivo, (2) the potential role of P2X7 receptor in hyperglycemia-induced caspase-1 activation in vitro and in vivo, (3) the effect of hyperglycemia on caspase-1 activation and interleukin-1beta production in Muller cells, and (4) the effect of hyperglycemia- induced interleukin-1beta production by Muller cells on activation and survival of retinal endothelial cells. The experiments are to be conducted in vivo and in vitro using diabetic, galactosemic, and caspase-1 and P2X7 knock-out mice, cultured retinal endothelial and Muller cells. These studies will clarify if the caspase-1/interleukin-1beta signaling pathway is essential for the development of diabetes-induced retinal pathology.
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