JBC：阿尔茨海默症中神经元营养细胞的复仇

研究人员发现，当阿尔茨海默症患者的大脑细胞开始渗出过多淀粉样蛋白amyloid时，平时为神经元提供营养和保护的星形胶质细胞一反常态，对神经元发动自杀式袭击。该研究发表在Journal of Biological Chemistry杂志上。

　　

生物通报道：当阿尔茨海默症患者的大脑细胞开始渗出过多淀粉样蛋白amyloid时，平时为神经元提供营养和保护的星形胶质细胞astrocyte一反常态，对神经元发动自杀式袭击。

所有的神经元都分泌Amyloid，并且分泌量随着年龄的增长而增加，而在阿尔茨海默症中Amyloid会大量累积。在这种情况下，过多的Amyloid会激活为神经元清除垃圾并输送血液、氧和营养物质的星形胶质细胞并引起发炎。

目前研究人员发现了星形胶质细胞的另一种回应方式，即将神经酰胺和PAR-4蛋白打包。这两种物质各自都是有害的，但如果凑在一起，就会成为“死亡组合，”Georgia健康大学医学院的生化学家Erhard Bieberich博士说。

该研究发表在Journal of Biological Chemistry杂志上。该文章的共同作者Bieberich形象的说：“如果神经元产生了有毒物质并丢在你家门口，你会怎么做？你多半会做些什么来保护自己吧。”

研究人员认为，星形胶质细胞产生的这种脂膜包裹最终会将两种细胞都杀死，这就能解释阿尔茨海默症中的脑细胞死亡和萎缩。“如果星形胶质细胞死亡，那么神经元也会死，”Bieberich说。研究显示过多的淀粉样蛋白并不能单独杀死脑细胞，Bieberich认为“必然存在使amyloid具有毒性的另一过程；不然神经元就会在产生一个大斑块前被自身毒害，”他说。“神经元会先死亡。”

这项研究发现amyloid能激活caspase 3并诱导原代培养的星形胶质细胞凋亡，而抑制caspase 3能避免该过程。用shRNA下调PAR-4（一种使细胞对sphingolipid ceramide敏感的蛋白）也能阻止上述细胞凋亡过程。与此一致的是，5xFAD小鼠大脑（AD 患者大脑）中amyloid斑块周围的星形胶质细胞caspase 3被激活，而当PAR-4和神经酰胺同时表达时这些星形胶质细胞会发生凋亡。研究者并未在缺乏nSMase2的星形胶质细胞中观察到上述凋亡现象，说明nSMase2生成的神经酰胺对于amyloid诱导的凋亡是至关重要的。

在体内和体外试验中，PAR-4和神经酰胺抗体都能阻止amyloid诱导的细胞凋亡，说明凋亡是由外源性的PAR-4和神经酰胺介导的，而这可能与细胞分泌的脂质小囊泡有关。正常情况下神经酰胺和Par-4并不在小囊泡中，而是分别位于细胞的两个独立部分。当遇到amyloid时神经酰胺会促使两者结合。

神经酰胺和Par-4被包入的脂质膜小囊泡，被称为胞外体exosomes；细胞分泌上千种这样的囊泡，而科学家才开始了解这些囊泡的一般功能。他们将致死的胞外体称为apoxosomes。

通过对条件培养基上的脂质体小囊泡进行分析，证实amyloid能诱导含PAR-4和神经酰胺的胞外体分泌。而nSMase2缺陷型的星形胶质细胞不分泌这种胞外体，说明神经酰胺在该过程中必不可少。研究人员在nSMase2缺陷型的星形胶质细胞中添加神经酰胺，能使胞外体分泌恢复。星形胶质细胞能吸收分离得到的含PAR-4和神经酰胺的胞外体，在这种情况下即使不存在amyloid细胞也会凋亡。

据阿尔茨海默症协会消息，阿尔茨海默症的风险因素包括衰老、家族史和遗传。越来越多的证据显示，阿尔茨海默症与心血管疾病具有许多共同的风险因素，如高胆固醇、高血压和缺乏运动等。

（生物通编辑：叶予）

生物通推荐原文摘要：

Astrocytes secrete exosomes enriched with pro-apoptotic ceramide and prostate apoptosis response 4 (PAR-4): a potential mechanism of apoptosis induction in Alzheimer's disease (AD)

Amyloid protein is well known to induce neuronal cell death, while only little is known about its effect on astrocytes. We found that amyloid peptides activated caspase 3 and induced apoptosis in primary cultured astrocytes, which was prevented by caspase 3 inhibition. Apoptosis was also prevented by shRNA-mediated down-regulation of PAR-4, a protein sensitizing cells to the sphingolipid ceramide. Consistent with a potentially pro-apoptotic effect of PAR-4 and ceramide astrocytes surrounding amyloid plaques in brain sections of the 5xFAD mouse (and AD patient brain) showed caspase 3 activation and were apoptotic when co-expressing PAR-4 and ceramide. Apoptosis was not observed in astrocytes with deficient neutral sphingomyelinase 2 (nSMase2), indicating that ceramide generated by nSMase2 is critical for amyloid-induced apoptosis. Antibodies against PAR-4 and ceramide prevented amyloid-induced apoptosis in vitro and in vivo, suggesting that apoptosis was mediated by exogenous PAR-4 and ceramide, potentially associated with secreted lipid vesicles. This was confirmed by the analysis of lipid vesicles from conditioned medium showing that amyloid peptide induced the secretion of PAR-4 and C18 ceramide-enriched exosomes. Exosomes were not secreted by nSMase2-deficient astrocytes, indicating that ceramide generated by nSMase2 is critical for exosome secretion. Consistent with the ceramide composition in amyloid-induced exosomes, exogenously added C18 ceramide restored PAR-4-containing exosome secretion in nSMase2-deficient astrocytes. Moreover, isolated PAR-4/ceramide-enriched exosomes were taken up by astrocytes and induced apoptosis in the absence of amyloid peptide. Taken together, we report a novel mechanism of apoptosis induction by PAR-4/ceramide-enriched exosomes, which may critically contribute to AD.