Acute brain injuries trigger microglia as an additional source of the proteoglycan NG2
Acute brain injuries trigger microglia as an additional source of the proteoglycan NG2
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DOI：
10.1186/s40478-020-01016-2
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发表时间：
2020-08-26
影响因子：
7.1
通讯作者：
Scheller, Anja
中科院分区：
文献类型：
--
作者：
Huang, Wenhui;Bai, Xianshu;Scheller, Anja
NG 2是一种I型跨膜糖蛋白，称为硫酸软骨素蛋白聚糖4（CSPG 4）。在健康的中枢神经系统中，NG 2仅由少突胶质细胞祖细胞和血管周细胞表达。大量的免疫组织化学研究表明，在病理条件下，如急性脑损伤和实验性自身免疫性脑脊髓炎（EAE），许多活化的小胶质细胞是NG 2免疫阳性，表明NG 2在这些细胞中的表达。小胶质细胞NG 2标记的替代解释考虑NG 2的生化特性或活化的小胶质细胞的吞噬活性。据报道，跨膜NG 2蛋白聚糖可被多种蛋白酶切割，以将NG 2胞外域存款到细胞外基质中。然而，胞外域也可以粘附在小胶质细胞表面。由于小胶质细胞是吞噬垂死细胞碎片的吞噬细胞，因此很难鉴定NG 2的真正表达。最近的研究表明（1）中风后周细胞产生小胶质细胞，和（2）在EAE模型中缺乏NG 2表达的NG 2-EYFP敲入小鼠的免疫细胞对急性脑损伤后小胶质细胞中NG 2的从头表达产生了怀疑。在本研究中，我们利用三个基因敲入小鼠品系（NG 2-CreERT 2，CX 3CR 1-EGFP和NG 2-EYFP）来研究转基因荧光蛋白在tMCAO（短暂大脑中动脉闭塞）或皮质刺伤损伤（SWI）后小胶质细胞中的表达。我们提供了强有力的证据表明，NG 2表达细胞，包括OPCs和周细胞，在急性脑损伤后没有分化成小胶质细胞，而活化的小胶质细胞确实以疾病依赖性的方式表达NG 2。一个小胶质细胞亚群至少在tMCAO后第一周内持续激活NG 2基因，而SWI后3天内，病变部位有限数量的小胶质细胞瞬时表达NG 2。免疫组织化学研究表明，这些小胶质细胞与NG 2基因活性也合成NG 2蛋白，这表明激活的小胶质细胞作为一个额外的来源NG 2蛋白聚糖急性脑损伤后。
NG2 is a type I transmembrane glycoprotein known as chondroitin sulfate proteoglycan 4 (CSPG4). In the healthy central nervous system, NG2 is exclusively expressed by oligodendrocyte progenitor cells and by vasculature pericytes. A large body of immunohistochemical studies showed that under pathological conditions such as acute brain injuries and experimental autoimmune encephalomyelitis (EAE), a number of activated microglia were NG2 immuno-positive, suggesting NG2 expression in these cells. Alternative explanations for the microglial NG2 labeling consider the biochemical properties of NG2 or the phagocytic activity of activated microglia. Reportedly, the transmembrane NG2 proteoglycan can be cleaved by a variety of proteases to deposit the NG2 ectodomain into the extracellular matrix. The ectodomain, however, could also stick to the microglial surface. Since microglia are phagocytic cells engulfing debris of dying cells, it is difficult to identify a genuine expression of NG2. Recent studies showing (1) pericytes giving rise to microglial after stroke, and (2) immune cells of NG2-EYFP knock-in mice lacking NG2 expression in an EAE model generated doubts for the de novo expression of NG2 in microglia after acute brain injuries. In the current study, we took advantage of three knock-in mouse lines (NG2-CreERT2, CX3CR1-EGFP and NG2-EYFP) to study NG2 expression indicated by transgenic fluorescent proteins in microglia after tMCAO (transient middle cerebral artery occlusion) or cortical stab wound injury (SWI). We provide strong evidence that NG2-expressing cells, including OPCs and pericytes, did not differentiate into microglia after acute brain injuries, whereas activated microglia did express NG2 in a disease-dependent manner. A subset of microglia continuously activated the NG2 gene at least within the first week after tMCAO, whereas within 3 days after SWI a limited number of microglia at the lesion site transiently expressed NG2. Immunohistochemical studies demonstrated that these microglia with NG2 gene activity also synthesized the NG2 protein, suggesting activated microglia as an additional source of the NG2 proteoglycan after acute brain injuries.