Neurovascular Injury after Retinal Ischemia Reperfusion Insult: Contrasting Roles of Arginase Enzyme Isoforms

Abstract

Purpose : Our previous studies have demonstrated involvement of the ureohydrolase enzyme arginase in potentially blinding disease conditions characterized by retinal inflammation and neurovascular injury. Whereas the cytosolic isoform, arginase 1 (A1) is implicated in inflammation and vascular dysfunction in models of uveitis and diabetic retinopathy, the mitochondrial isoform arginase 2 (A2) is involved in neurovascular injury in oxygen-induced retinopathy model. The present study was undertaken to determine and compare the distinct roles of A1 and A2 in neurovascular damage following ischemia/reperfusion (I/R) injury.

Methods : Studies were performed with mice lacking one copy of arginase 1 or both copies of arginase 2 and wild type (WT) controls (C57BL6J). I/R insult was conducted on the right eye and the left eye underwent sham surgery. Retinas were collected for analysis at different times (3h-4wk post injury). Neuronal and microvascular degeneration were evaluated using NeuN staining and vascular digests, respectively. Glial activation was evaluated by GFAP expression. Microglial activation was measured by Iba-1 staining and confocal microscopy. Necrotic cell death was studied by propidium iodide (PI) labelling and western blot for RIP-3. Arginase expression was determined by western blot and quantitative RT-PCR. Retinal function was determined by electroretinography (ERG).

Results : I/R injury caused significant increases in A2 expression along with thinning of the neural retina, decreases in NeuN+ GCL neurons and formation of acellular capillaries. Increases in PI labeling and RIP-3 expression showed that cell death occurred by necroptosis. Neurovascular injury was accompanied by microglial activation along with increased expression of GFAP and impairment of the ERG. Neuronal cell loss, capillary degeneration, necroptosis, gliosis and ERG impairment were all significantly reduced by deletion of A2, whereas A1 expression was significantly increased. On the other hand, A1 deletion exacerbated I/R-induced neuronal and vascular injury and further increased necroptosis and gliosis as compared with WT retinas.

Conclusions : This study shows for the first time that arginase isoforms play different roles in retinal neurovascular injury and repair after I/R insult. I/R-induced necrotic cell death and gliosis are mediated by A2, whereas upregulation of A1 may play a role in limiting the pathology.

This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.