Graded oligodendrocyte vulnerability is regulated by the transcription factor DNA-damage inducible transcript 3 (DDIT3)

Oligodendrocytes are integral to efficient neuronal signaling. Loss of myelinating oligodendrocytes is a central feature of many neurological diseases, among multiple sclerosis. Results of neuropathological studies suggest that oligodendrocytes react with differing sensitivity to toxic insults, with some cells dying early during lesion development and some cells being resistant for even weeks. Such a proposed graded vulnerability has never been demonstrated but provides an attractive window for therapeutic interventions. Beyond, biochemical pathways associated with graded oligodendrocyte vulnerability are not well explored. By immunohistochemistry and serial block-face scanning electron microscopy (3D-EM) we show that cuprizone-induced metabolic stress results in an “out of phase” degeneration of oligodendrocytes. While the expression induction of stress response transcriptions factors, specifically in oligodendrocytes, occurs within days, subsequent oligodendrocyte apoptosis is a process going on for weeks. In line with the idea of an out of phase degeneration of oligodendrocytes, detailed ultrastructural reconstructions of the axon-myelin unit demonstrate demyelination of single internodes. Genome wide array analysis revealed an active unfolded protein response early after initiation of cuprizone intoxication. Besides cytoprotective pathways, the pro-apoptotic transcription factor DNA-damage-inducible transcript 3 (DDIT3) was induced selectively in the vulnerable oligodendrocyte population. Besides the amelioration of toxin-induced oligodendrocyte apoptosis, demyelination, microgliosis, astrocytosis and acute axonal damage were less intense in Ddit3-null mutants. This study identifies activation of an endoplasmic reticulum stress cascade in oligodendrocytes as an important regulator of a graded vulnerability of this cell population. Interference with such endoplasmic reticulum stress cascades offers a promising therapeutic approach for demyelinating disorders.少突胶质细胞（Oligodendrocytes）是高效神经元信号传导不可或缺的核心组分。髓鞘形成性少突胶质细胞的丢失是诸多神经系统疾病的核心病理特征，多发性硬化（multiple sclerosis）即为典型代表之一。神经病理学研究结果表明，少突胶质细胞对毒性损伤的反应敏感性存在显著差异：部分细胞在病变发生早期即发生死亡，而另一些细胞甚至可耐受数周之久。这种被提出的分级易感性假说尚未得到实验证实，却为治疗干预提供了极具吸引力的切入点。此外，与少突胶质细胞分级易感性相关的生化通路仍未得到充分探索。本研究通过免疫组织化学与连续块面扫描电子显微镜（serial block-face scanning electron microscopy, 3D-EM）技术，证实双环己酮草酰二腙（cuprizone）诱导的代谢应激可导致少突胶质细胞出现"失同步"变性。尽管仅在少突胶质细胞中发生的应激反应转录因子表达诱导可在数天内完成，但后续的少突胶质细胞凋亡过程却可持续数周之久。与少突胶质细胞失同步变性的假说相符，轴突-髓鞘单元的超微结构精细重建结果显示，单个结间体可发生脱髓鞘改变。全基因组表达芯片分析显示，在双环己酮草酰二腙染毒早期，细胞即激活了未折叠蛋白反应（unfolded protein response）。除细胞保护通路外，促凋亡转录因子DNA损伤诱导转录蛋白3（DNA-damage-inducible transcript 3, DDIT3）可在易感少突胶质细胞群中选择性被诱导表达。与野生型相比，Ddit3基因敲除（Ddit3-null）小鼠中，毒素诱导的少突胶质细胞凋亡、脱髓鞘、小胶质细胞活化、星形胶质细胞增生以及急性轴突损伤均显著减轻。本研究证实，少突胶质细胞内内质网应激级联反应的激活，是调控该细胞群分级易感性的关键机制。靶向干预此类内质网应激级联反应，可为脱髓鞘疾病提供极具前景的治疗策略。