The Role of Adenosine in Retinal Ischemia

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Significant visual loss may result from retinal ischemia in retinal arterial or venous occlusion, glaucoma, atherosclerosis, or in systemic disorders such as diabetes mellitus. The pathogenesis involves changes in cellular biochemistry and energy level, blood flow, and gene expression. During the past 11 years of this project, we documented extensive biochemical, functional, structural, and hemo-'dynamic evidence for the complex, but major involvement of the purine nucleoside adenosine in retinal ischemia-reperfusion injury. More recently, we also discovered the closely related and dramatic finding of complete functional and histological protection from ischemic damage in the in vivo retina conferred by a brief period of non damaging ischemia, i.e., ischemic preconditioning (IPC). Other significant accompanying protective mechanisms of IPC include the attenuation of hypoperfusion, protein phosphorylation, and apoptosis. We demonstrated that adeno-'sine is a trigger for IPC, and we began to uncover the roles of downstream signal transduction factors, including mitochondrial KATP channels, PKC, mitogen-activated protein kinase p38, nitric oxide, and reactive oxygen species, in this neuroprotection. These exciting results extend our earlier findings of the remarkable functional and histological protection from ischemic damage afforded by IPC, indicating that IPC has a profound influence upon cell signaling and survival. Examination of the mechanisms responsible for IPC in our established retinal ischemia model provides a unique and innovative window into the retina's endogenous ability to counter ischemic injury. The first aim will characterize the signaling pathwaysfor IPC involving mitochondrial KATP channels and the associated signal transduction factors. The second aim will characterize the involvement of NOS and PKC subtypes as essential signaling intermediaries in IPC. The third aim will examine the mechanisms of the paradoxical effect whereby transient MAPK p38 expression protects the retina, while its blockade prior to ischemia protects against ischemic damage. Our experiments will definitively examine major mechanisms of IPC and should bring us closer to understanding molecular events underlying this robust, intriguing, and clinically relevant neuroprotection.