Sigma 1 receptor regulates the oxidative stress response in primary retinal Muller glial cells via NRF2 signaling and system xc-, the Na+-independent glutamate-cystine exchanger

Oxidative stress figures prominently in retinal diseases, including diabetic retinopathy, and glaucoma. Ligands for sigma 1R, a unique transmembrane protein localized to the endoplasmic reticulum, mitochondria, and nuclear and plasma membranes, have profound retinal neuroprotective properties in vitro and in vivo. Studies to determine the mechanism of sigma 1R-mediated retinal neuroprotection have focused mainly on neurons. Little is known about the effects of sigma 1R on Muller cell function, yet these radial glial cells are essential for homeostatic support of the retina. Here we investigated whether sigma 1R mediates the oxidative stress response of Muller cells using wild type (WT) and sigma 1R-knockout (sigma 1RKO) mice. We observed increased endogenous reactive oxygen species (ROS) levels in 6 1RKO Muller cells compared to WT, which was accompanied by decreased expression of Sod1, catalase, Nqo1, Hmox1, Cstm6, and Gpx1 The protein levels of SOD1, CAT, NI:201, and GPX1 were also significantly decreased. The genes encoding these antioxidants contain an antioxidant response element (ARE), which under stress is activated by NRF2, a transcription factor that typically resides in the cytoplasm bound by KEAP1. In the sigma 1RKO Muller cells Nrf2 expression was decreased significantly at the gene (and protein) level, whereas Keap1 gene (and protein) levels were markedly increased. NRF2-ARE binding affinity was decreased markedly in sigma 1RKO Muller cells. We investigated system x(c)(-), the cystine-glutamate exchanger important for synthesis of glutathione (GSH), and observed decreased function in sigma 1RKO Muller cells compared to WT as well as decreased GSH and GSH/GSSG ratios. This was accompanied by decreased gene and protein levels of xCT, the unique component of system We conclude that Muller glial cells lacking sigma 1R manifest elevated ROS, perturbation of antioxidant balance, suppression of NRF2 signaling, and impaired function of system x(c)(-). The data suggest that the oxidative stress-mediating function of retinal Muller glial cells may be compromised in the absence of sigma 1R. The neuroprotective role of sigma 1R may be linked directly to the oxidative stress-mediating properties of supportive glial cells. (C) 2015 Elsevier Inc. All rights reserved.