Arginase 1 promotes retinal neurovascular protection from ischemia through suppression of macrophage inflammatory responses

Abdelrahman Y Fouda,Paulo C Rodriguez,S Priya Narayanan,Sylvia B Smith,Xuezhi Cui,Rebekah Tritz,Brian K Stansfield,Jijun Chen,Esraa Shosha,Yuqing Huo,Tahira Lemtalsi,Ruth B Caldwell,Zhimin Xu,R William Caldwell,Haroldo A Toque

doi:10.1038/s41419-018-1051-6

Abstract

The lack of effective therapies to limit neurovascular injury in ischemic retinopathy is a major clinical problem. This study aimed to examine the role of ureohydrolase enzyme, arginase 1 (A1), in retinal ischemia-reperfusion (IR) injury. A1 competes with nitric oxide synthase (NOS) for their common substrate l-arginine. A1-mediated l-arginine depletion reduces nitric oxide (NO) formation by NOS leading to vascular dysfunction when endothelial NOS is involved but prevents inflammatory injury when inducible NOS is involved. Studies were performed using wild-type (WT) mice, global A1+/− knockout (KO), endothelial-specific A1 KO, and myeloid-specific A1 KO mice subjected to retinal IR injury. Global as well as myeloid-specific A1 KO mice showed worsened IR-induced neuronal loss and retinal thinning. Deletion of A1 in endothelial cells had no effect, while treatment with PEGylated (PEG) A1 improved neuronal survival in WT mice. In addition, A1+/− KO mice showed worsened vascular injury manifested by increased acellular capillaries. Western blotting analysis of retinal tissue showed increased inflammatory and necroptotic markers with A1 deletion. In vitro experiments showed that macrophages lacking A1 exhibit increased inflammatory response upon LPS stimulation. PEG-A1 treatment dampened this inflammatory response and decreased the LPS-induced metabolic reprogramming. Moreover, intravitreal injection of A1 KO macrophages or systemic macrophage depletion with clodronate liposomes increased neuronal loss after IR injury. These results demonstrate that A1 reduces IR injury-induced retinal neurovascular degeneration via dampening macrophage inflammatory responses. Increasing A1 offers a novel strategy for limiting neurovascular injury and promoting macrophage-mediated repair.

Highlights

Ischemia-induced retinal neurovascular injury is a primary contributor in blinding diseases that affect neonates, working age adults, and the elderly
WT IR injured retinas showed a large number of acellular capillaries (≈150/mm[2] empty basement membrane sleeves, red arrows, Fig. 1c, d) at 14 days after IR injury and this was further increased by ≈50% in arginase 1 (A1)+/− mice
In this study, we present evidence on the important role of A1 in macrophage polarization toward a reparative phenotype leading to neurovascular recovery after retinal IR injury

Summary

Introduction

Ischemia-induced retinal neurovascular injury is a primary contributor in blinding diseases that affect neonates (retinopathy of prematurity), working age adults (diabetic retinopathy), and the elderly (branch vein occlusion). Fouda et al Cell Death and Disease (2018)9:1001. A1, the cytosolic isoform, is strongly expressed in the liver, where it is the central player in the urea cycle[7]. The mitochondrial isoform, A2, is expressed in extrahepatic tissues, especially the kidney[8]. Both isoforms are expressed in the retina and brain[9], and have been linked to CNS diseases[10]. A1 has been reported to be strongest in myeloid cells with less expression in astrocytes[11,12]. A1 and nitric oxide synthase (NOS) enzyme compete for their common substrate the semi-essential amino acid L-arginine[13]. A1 upregulation can lead to suppression of nitric oxide (NO) formation by endothelial NOS (eNOS)

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