Arginase2 Deficiency Reduces Hyperoxia-induced Retinal Neurodegeneration through the Regulation of Polyamine Metabolism

Abstract

Purpose: : Retinopathy of prematurity (ROP) is a major cause of visual impairment in premature infants. Using an oxygen-induced retinopathy (OIR) mouse model for ROP, we have shown that deletion of the arginase2 (Arg2) significantly reduced neuro-glial injury and improved retinal function (Narayanan et al 2011). The current study was undertaken to study the mechanism of Arg2 -mediated neuroprotection in the OIR model.

Methods: : Newborn wild-type (WT) and mice deficient in arginase2 (Arg2-/-) were used. Pups were exposed 70% oxygen from postnatal day 7 to 12 and were sacrificed at different stages of hyperoxia. Retinal cryostat sections were prepared for immunohistochemical analysis and TUNEL labeling of apoptotic cells. Fresh frozen retinal samples were used for Western blotting of signaling molecules or Mass Spectrometry analysis of polyamines.

Results: : During the hyperoxic phase of OIR, a significant increase in the number of apoptotic cells was observed in the WT retina compared to normoxic controls. Number of TUNEL positive cells was significantly reduced in Arg2-/- OIR retina, as early as 24 hours of hyperoxia, compared to WT OIR (p< 0.01, N=5). Further, we investigated the involvement of polyamines in the hyperoxia mediated retinal cell death. Mass spectrometric analysis showed a significant increase in the level of spermidine in WT OIR retina at P9 compared to normoxic controls (p< 0.05, N=10). Arg2-/- OIR retina showed significantly reduced spermidine levels (p< 0.01, N=10) along with significantly decreased levels of N-Acetyl spermine and N-Acetyl spermidine, compared with WT OIR (p< 0.05, N=10). These results suggest reduced oxidation of spermine and spermidine in Arg2-/-OIR retina compared to WT OIR. This was further confirmed by the increased expression levels of polyamine oxidase in WT OIR retinal samples at P8 and P9, relative to Arg2-/- OIR and normoxic controls. Analysis of signaling pathways showed the involvement of the P38 MAPK pathway in the Arg2-mediated neuroprotection during the hyperoxia phase of OIR.

Conclusions: : Our data suggest that retinal arginase is involved in the hyperoxia-induced neuronal degeneration in the OIR model, through P38 MAPK mediated polyamine metabolism.

Keywords: neuroprotection • retinopathy of prematurity • signal transduction