Neuroscience PhD Program, Mayo Clinic Graduate School of Biomedical Sciences, Rochester, MN, 55905, USA.
Translational Neuroimmunology Lab, Mayo Clinic, Rochester, MN, 55905, USA.
J Neuroinflammation. 2021 Dec 27;18(1):305. doi: 10.1186/s12974-021-02360-3.
Microglia are the primary phagocytes of the central nervous system and are responsible for removing damaged myelin following demyelination. Previous investigations exploring the consequences of myelin phagocytosis on microglial activation overlooked the biochemical modifications present on myelin debris. Such modifications, including citrullination, are increased within the inflammatory environment of multiple sclerosis lesions.
Mouse cortical myelin isolated by ultracentrifugation was citrullinated ex vivo by incubation with the calcium-dependent peptidyl arginine deiminase PAD2. Demyelination was induced by 6 weeks of cuprizone (0.3%) treatment and spontaneous repair was initiated by reversion to normal chow. Citrullinated or unmodified myelin was injected into the primary motor cortex above the cingulum bundle at the time of reversion to normal chow and the consequent impact on remyelination was assessed by measuring the surface area of myelin basic protein-positive fibers in the cortex 3 weeks later. Microglial responses to myelin were characterized by measuring cytokine release, assessing flow cytometric markers of microglial activation, and RNAseq profiling of transcriptional changes.
Citrullinated myelin induced a unique microglial response marked by increased tumor necrosis factor α (TNFα) production both in vitro and in vivo. This response was not induced by unmodified myelin. Injection of citrullinated myelin but not unmodified myelin into the cortex of cuprizone-demyelinated mice significantly inhibited spontaneous remyelination. Antibody-mediated neutralization of TNFα blocked this effect and restored remyelination to normal levels.
These findings highlight the role of post-translation modifications such as citrullination in the determination of microglial activation in response to myelin during demyelination. The inhibition of endogenous repair induced by citrullinated myelin and the reversal of this effect by neutralization of TNFα may have implications for therapeutic approaches to patients with inflammatory demyelinating disorders.
小胶质细胞是中枢神经系统的主要吞噬细胞，负责在脱髓鞘后清除受损的髓磷脂。先前研究髓磷脂吞噬对小胶质细胞激活的影响时，忽略了髓磷脂碎片上存在的生化修饰。这种修饰，包括瓜氨酸化，在多发性硬化症病变的炎症环境中会增加。
通过超速离心从鼠皮质中分离髓鞘，然后用钙依赖性肽基精氨酸脱亚氨酶 PAD2 将其体外瓜氨酸化。通过 6 周的 cuprizone（0.3%）处理诱导脱髓鞘，并在恢复正常饮食时开始自发修复。在恢复正常饮食时，将瓜氨酸化或未修饰的髓鞘注射到扣带束上方的初级运动皮层中，3 周后通过测量皮层中髓鞘碱性蛋白阳性纤维的表面积来评估对再髓鞘化的影响。通过测量细胞因子释放、评估小胶质细胞激活的流式细胞术标志物以及对转录变化的 RNAseq 分析来表征小胶质细胞对髓鞘的反应。
瓜氨酸化的髓鞘在体外和体内均可诱导独特的小胶质细胞反应，表现为肿瘤坏死因子 α（TNFα）的产生增加。未修饰的髓鞘不会引起这种反应。将瓜氨酸化的髓鞘而非未修饰的髓鞘注射到 cuprizone 脱髓鞘的小鼠皮层中，可显著抑制自发的再髓鞘化。TNFα 的抗体中和阻断了这种作用，并将再髓鞘化恢复到正常水平。
这些发现强调了翻译后修饰（如瓜氨酸化）在脱髓鞘过程中小胶质细胞对髓磷脂的反应中决定小胶质细胞激活的作用。瓜氨酸化的髓鞘抑制内源性修复，以及 TNFα 中和逆转这种作用，可能对炎症性脱髓鞘疾病患者的治疗方法有影响。
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