Abstract
BackgroundElevated levels of oncostatin M (OSM), an interleukin-6 cytokine family member, have been observed in HIV-1-associated neurocognitive disorders (HAND) and Alzheimer’s disease. However, the function of OSM in these disease conditions is unclear. Since deficient glutamate uptake by astrocytes is instrumental in HAND-associated neurotoxicity, we hypothesized that OSM impairs glutamate uptake in astrocytes and thereby promotes neuronal excitotoxicity.MethodsPrimary cultures of mouse cortical astrocytes, neurons, microglia, and BV2 cell line were used. The expression of glutamate transporters (GLAST/EAAT1 and GLT-1/EAAT2) was investigated using real-time PCR and Western blot, and their activity was assessed by measuring 3H-d-aspartate uptake. Neuronal toxicity was measured using the colorimetric MTT (3-(4,5-dimethylthiazol-2-yl-) 2,5-diphenyltetrazolium bromide) assay and immunocytochemistry. A chimeric HIV-1 that infects murine cells (EcoHIV/NL4-3-GFP virus (EcoHIV)) was used to investigate whether the virus induces OSM, OSM receptor (OSMR)-β, glycoprotein 130 (gp130), GLT-1, GLAST (mRNA and protein), and OSM release (ELISA) in cultured BV2 cells, primary microglia, or astrocytes. Statistical analyses of the data were performed using one-way ANOVA (to allow multiple comparisons) and two-tailed Student’s t test.ResultsOSM treatment (10 ng/mL) time-dependently reduced GLAST and GLT-1 expression and inhibited 3H-d-aspartate uptake in cultured astrocytes in a concentration-dependent manner, an effect prevented by the Janus kinase (JAK)/signal transducers and activators of transcription (STAT)3 inhibitor AG490. Down-regulation of astrocytic glutamate transport by OSM resulted in NMDA receptor-dependent excitotoxicity in cortical neurons. Infection with EcoHIV induced OSM gene expression and protein release in BV2 cells and microglia, but not in astrocytes. Conversely, EcoHIV caused a fivefold increase in OSMR-β mRNA (but not gp130) and protein in astrocytes, but not in microglia, which did not express OSMR-β protein. Finally, astrocytic expression of GLAST gene was unaffected by EcoHIV, whereas GLT-1 mRNA was increased by twofold.ConclusionsWe provide first evidence that activation of JAK/STAT3 signaling by OSM inhibits glutamate uptake in astrocytes, which results in neuronal excitotoxicity. Our findings with EcoHIV suggest that targeting OSMR-β signaling in astrocytes might alleviate HIV-1-associated excitotoxicity.Electronic supplementary materialThe online version of this article (doi:10.1186/s12974-016-0613-8) contains supplementary material, which is available to authorized users.
Highlights
Elevated levels of oncostatin M (OSM), an interleukin-6 cytokine family member, have been observed in HIV-1-associated neurocognitive disorders (HAND) and Alzheimer’s disease
Treatment with OSM (10 ng/mL for 24 h) of microglia-depleted astrocyte cultures reduced both glutamate transporter-1 (GLT-1) and glutamate aspartate transporter (GLAST) messenger ribonucleic acid (mRNA) (p < 0.001, n = 3) (Additional file 2: Figure S2B), and this effect of OSM was comparable to the observations made from astrocyte cultures containing 5 % microglia (Fig. 1b) further establishing that OSM receptors are primarily expressed in astrocytes
OSM is a pleiotropic cytokine belonging to the IL-6 family that may differentially affect neuroinflammatory processes associated with disease conditions of the central nervous system (CNS)
Summary
Elevated levels of oncostatin M (OSM), an interleukin-6 cytokine family member, have been observed in HIV-1-associated neurocognitive disorders (HAND) and Alzheimer’s disease. Astrocytes are the most abundant cell type of the brain [1], where one of their major functions is to support neurons They ensure optimal conditions by maintaining local ion and pH homeostasis, regulating neurotransmitters, and clearing metabolic waste [2]. One major cause of neuronal death in the central nervous system (CNS) is excessive release of glutamate and subsequent activation of neuronal N-methyl Daspartate (NMDA) receptors [4, 5]. Astrocytes prevent this neuronal excitotoxicity by sequestration of extracellular glutamate, which is mediated mainly by two Na+-dependent transporters: glutamate aspartate transporter (GLAST/ EAAT1) and glutamate transporter-1 (GLT-1/EAAT2) [6, 7]. In order to develop therapeutic interventions against excitotoxicity associated with these CNS disorders, it is critical to identify factors that regulate astrocytic glutamate transporter expression and activity
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