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.

中文翻译：

---

急性脑损伤触发小胶质细胞作为蛋白聚糖NG2的额外来源。

NG2是一种I型跨膜糖蛋白，称为硫酸软骨素蛋白聚糖4（CSPG4）。在健康的中枢神经系统中，NG2仅由少突胶质祖细胞和脉管周细胞表达。大量的免疫组织化学研究表明，在急性脑损伤和实验性自身免疫性脑脊髓炎（EAE）等病理条件下，许多活化的小胶质细胞均为NG2免疫阳性，表明NG2在这些细胞中表达。小胶质NG2标记的替代解释考虑了NG2的生化特性或活化的小胶质细胞的吞噬活性。据报道，跨膜NG2蛋白聚糖可以被多种蛋白酶切割，以将NG2胞外域沉积到细胞外基质中。但是，外部域 也可能粘在小胶质细胞表面。由于小胶质细胞是吞噬垂死细胞碎片的吞噬细胞，因此很难鉴定出真正的NG2表达。最近的研究表明（1）中风后周细胞产生小胶质细胞；（2）在EAE模型中缺乏NG2表达的NG2-EYFP敲入小鼠的免疫细胞引起了对急性脑损伤后小胶质细胞NG2从头表达的怀疑。 。在本研究中，我们利用三种敲入小鼠系（NG2-CreERT2，CX3CR1-EGFP和NG2-EYFP）来研究转基因荧光蛋白在tMCAO（短暂性中脑动脉闭塞）或皮质后在小胶质细胞中表达的NG2。刺伤（SWI）。我们提供有力证据表明，表达NG2的细胞（包括OPC和周细胞）急性脑损伤后未分化为小胶质细胞，而活化的小胶质细胞却以疾病依赖的方式表达NG2。小胶质细胞的一个子集至少在tMCAO后的第一周内持续激活NG2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NG2。免疫组织化学研究表明，这些具有NG2基因活性的小胶质细胞也合成了NG2蛋白，这表明活化的小胶质细胞是急性脑损伤后NG2蛋白聚糖的另一种来源。而SWI后3天内，病变部位的少量小胶质细胞瞬时表达NG2。免疫组织化学研究表明，这些具有NG2基因活性的小胶质细胞也合成了NG2蛋白，这表明活化的小胶质细胞是急性脑损伤后NG2蛋白聚糖的另一种来源。SWI后3天内，病变部位的少量小胶质细胞瞬时表达NG2。免疫组织化学研究表明，这些具有NG2基因活性的小胶质细胞也合成了NG2蛋白，这表明活化的小胶质细胞是急性脑损伤后NG2蛋白聚糖的另一种来源。