Diverse injury responses of human oligodendrocyte to mediators implicated in multiple sclerosis
Early multiple sclerosis lesions feature relative preservation of oligodendrocyte cell bodies with dying back retraction of their myelinating processes. Cell loss occurs with disease progression. Putative injury mediators include metabolic stress (low glucose/nutrient), pro-inflammatory mediators (interferon ? and tumor necrosis factor a), and excitotoxins (glutamate). Our objective was to compare the impact of these disease relevant mediators on the injury responses of human mature oligodendrocytes. In the current study, we determined the effects of these mediators on process extension and survival of human brain derived mature oligodendrocytes in vitro and used bulk RNA sequencing to identify distinct effector mechanisms that underlie the responses. All mediators induced significant process retraction of the oligodendrocytes in dissociated cell culture. Only metabolic stress (low glucose/nutrient) conditions resulted in delayed (4-6 days) non-apoptotic cell death. Metabolic effects were associated with induction of the integrated stress response, which can be protective or contribute to cell injury dependent on its level and duration of activation. Addition of Sephin1, an agonist of the integrated stress response induced process retraction under control conditions and further enhanced retraction under metabolic stress conditions. The antagonist ISRIB restored process outgrowth under stress conditions, and if added to already stressed cells, reduced delayed cell death and prolonged the period in which recovery could occur.  Inflammatory cytokine functional effects were associated with activation of multiple signaling pathways (including Jak/Stat-1) that regulate process outgrowth, without integrated stress response induction. Glutamate application produced limited transcriptional changes suggesting a contribution of effects directly on cell processes. Overall design: Investigation of the stress response mechanims of injured oligodendrocytes in situ and in vitro. We used fresh frozen brain tissue sections from MS patients and identified the implication of the integrated stress response in MS lesions in oligodendrocyte cells. We further investigated the function of this pathway in vitro in human primary oligodendrocyte cells under stress related conditions: metabolic, pro-inflammatory and excitotoxic insults. We finally modulated the integrated stress response using agonist and antagonist drugs and assessed the functional outcome in oligodendrocyte stress response.