Cell‐specific caspase expression by different neuronal phenotypes in transient retinal ischemia

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Abstract
Emerging evidence supports an important role for caspases in neuronal death following ischemia‐reperfusion injury. This study assessed whether cell specific caspases participate in neuronal degeneration and whether caspase inhibition provides neuroprotection following transient retinal ischemia. We utilized a model of transient global retinal ischemia. The spatial and temporal pattern of the active forms of caspase 1, 2 and 3 expression was determined in retinal neurons following ischemic injury. Double‐labeling with cell‐specific markers identified which cells were expressing different caspases. In separate experiments, animals received various caspase inhibitors before the induction of ischemia. Sixty minutes of ischemia resulted in a delayed, selective neuronal death of the inner retinal layers at 7 days. Expression of caspase 1 was not detected at any time point. Maximal expression of caspase 2 was found at 24 h primarily in the inner nuclear and ganglion cell layers of the retina and localized to ganglion and amacrine neurons. Caspase 3 also peaked at 24 h in both the inner nuclear and outer nuclear layers and was predominantly expressed in photoreceptor cells and to a lesser extent in amacrine neurons. The pan caspase inhibitor, Boc‐aspartyl fmk, or an antisense oligonucleotide inhibitor of caspase 2 led to significant histopathologic and functional improvement (electroretinogram) at 7 days. No protection was found with the caspase 1 selective inhibitor, Y‐vad fmk. These observations suggest that ischemia‐reperfusion injury activates different caspases depending on the neuronal phenotype in the retina and caspase inhibition leads to both histologic preservation and functional improvement. Caspases 2 and 3 may act in parallel in amacrine neurons following ischemia‐reperfusion. These results in the retina may shed light on differential caspase specificity in global cerebral ischemia.