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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.

ABSTRACT: Oxidative stress figures prominently in retinal diseases, including diabetic retinopathy, and glaucoma. Ligands for ?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 ?1R-mediated retinal neuroprotection have focused mainly on neurons. Little is known about the effects of ?1R on Müller cell function, yet these radial glial cells are essential for homeostatic support of the retina. Here we investigated whether ?1R mediates the oxidative stress response of Müller cells using wild-type (WT) and ?1R-knockout (?1RKO) mice. We observed increased endogenous reactive oxygen species (ROS) levels in ?1RKO Müller cells compared to WT, which was accompanied by decreased expression of Sod1, catalase, Nqo1, Hmox1, Gstm6, and Gpx1. The protein levels of SOD1, CAT, NQO1, 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 ?1RKO Müller 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 ?1RKO Müller cells. We investigated system xc(-), the cystine-glutamate exchanger important for synthesis of glutathione (GSH), and observed decreased function in ?1RKO Müller 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 xc(-). We conclude that Müller glial cells lacking ?1R manifest elevated ROS, perturbation of antioxidant balance, suppression of NRF2 signaling, and impaired function of system xc(-). The data suggest that the oxidative stress-mediating function of retinal Müller glial cells may be compromised in the absence of ?1R. The neuroprotective role of ?1R may be linked directly to the oxidative stress-mediating properties of supportive glial cells.

SUBMITTER: Wang J

PROVIDER: S-EPMC4554890 | biostudies-literature | 2015 Sep

REPOSITORIES: biostudies-literature

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

Wang Jing J Shanmugam Arul A Markand Shanu S Zorrilla Eric E Ganapathy Vadivel V Smith Sylvia B SB

Free radical biology & medicine 20150425

Oxidative stress figures prominently in retinal diseases, including diabetic retinopathy, and glaucoma. Ligands for σ1R, 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 σ1R-mediated retinal neuroprotection have focused mainly on neurons. Little is known about the effects of σ1R on Müller cell function, yet these radial ...[more]

PMID: 25920363

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Project description:This study evaluated the effects of elevated homocysteine (Hcy) on the oxidative stress response in retinal Müller glial cells. Elevated Hcy has been implicated in retinal diseases including glaucoma and optic neuropathy, which are characterized by retinal ganglion cell (RGC) loss. To understand the mechanisms of Hcy-induced RGC loss, in vitro and in vivo models have been utilized. In vitro isolated RGCs are quite sensitive to elevated Hcy levels, while in vivo murine models of hyperhomocysteinemia (HHcy) demonstrate a more modest RGC loss (∼20%) over a period of many months. This differential response to Hcy between isolated cells and the intact retina suggests that the retinal milieu invokes mechanisms that buffer excess Hcy. Oxidative stress has been implicated as a mechanism of Hcy-induced neuron loss and NRF2 is a transcription factor that plays a major role in regulating cytoprotective responses to oxidative stress. In the present study we investigated whether HHcy upregulates NRF2-mediated stress responses in Müller cells, the chief retinal glial cell responsible for providing trophic support to retinal neurons. Primary Müller cells were exposed to L-Hcy-thiolactone [50μM-10mM] and assessed for viability, reactive oxygen species (ROS), and glutathione (GSH) levels. Gene/protein levels of Nrf2 and levels of NRF2-regulated antioxidants (NQO1, CAT, SOD2, HMOX1, GPX1) were assessed in Hcy-exposed Müller cells. Unlike isolated RGCs, isolated Müller cells are viable over a wide range of Hcy concentrations [50 μM - 1 mM]. Moreover, when exposed to elevated Hcy, Müller cells demonstrate decreased oxidative stress and decreased ROS levels. GSH levels increased by ∼20% within 24 h exposure to Hcy. Molecular analyses revealed 2-fold increase in Nrf2 expression. Expression of antioxidant genes Nqo1, Cat, Sod2, Hmox1, Gpx1 increased significantly. The consequences of Hcy exposure were evaluated also in Müller cells harvested from Nrf2-/- mice. In contrast to WT Müller cells, in which oxidative stress decreased upon exposure to Hcy, the Nrf2-/- Müller cells showed a significant increase in oxidative stress. Our data suggest that at least during early stages of Hhcy, a cytoprotective response may be in place, mediated in part by NRF2 in Müller cells.

Project description:The cystine-glutamate antiporter (system xc-) is a Na+-independent amino acid transporter that exchanges extracellular cystine for intracellular glutamate. It is thought to play a critical role in cellular redox processes through regulation of intracellular glutathione synthesis via cystine uptake. In gliomas, system xc- expression is universally up-regulated while that of glutamate transporters down-regulated, leading to a progressive accumulation of extracellular glutamate and excitotoxic cell death of the surrounding non-tumorous tissue. Additionally, up-regulation of system xc- in activated microglia has been implicated in the pathogenesis of several neurodegenerative disorders mediated by excess glutamate. Consequently, system xc- is a new drug target for brain cancer and neuroinflammatory diseases associated with excess extracellular glutamate. Unfortunately no potent and selective small molecule system xc- inhibitors exist and to our knowledge, no high throughput screening (HTS) assay has been developed to identify new scaffolds for inhibitor design. To develop such an assay, various neuronal and non-neuronal human cells were evaluated as sources of system xc-. Human glioma cells were chosen based on their high system xc- activity. Using these cells, [14C]-cystine uptake and cystine-induced glutamate release assays were characterized and optimized with respect to cystine and protein concentrations and time of incubation. A pilot screen of the LOPAC/NINDS libraries using glutamate release demonstrated that the logistics of the assay were in place but unfortunately, did not yield meaningful pharmacophores. A larger, HTS campaign using the 384-well cystine-induced glutamate release as primary assay and the 96-well 14C-cystine uptake as confirmatory assay is currently underway. Unexpectedly, we observed that the rate of cystine uptake was significantly faster than the rate of glutamate release in human glioma cells. This was in contrast to the same rates of cystine uptake and glutamate release previously reported in normal human fibroblast cells.

Dataset Information

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.

Publications

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

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