A retroviral link to vertebrate myelination

Myelin, the insulating sheath that surrounds neuronal axons, is produced by oligodendrocytes in the central nervous system (CNS). This evolutionary innovation, which first appears in jawed vertebrates, enabled rapid transmission of nerve impulses, more complex brains and greater morphological diversity. Here we report that RNA level expression of RNLTR12-int, a retrotransposon of retroviral origin, is essential for myelination. We show RNLTR12-int-encoded non-coding RNA binds to the transcription factor SOX10 to regulate transcription of myelin basic protein (Mbp, the major constituent of myelin) in rodents. RNLTR12-int like sequences (which we nameRetroMyelin) are found in all jawed-vertebrates and we further demonstrate their function in regulating myelination in two different vertebrate phyla (zebrafish and frogs). Our study therefore suggests that retroviral endogenization was a key step in the emergence of vertebrate myelin.包裹神经元轴突的髓磷脂（Myelin）由中枢神经系统（CNS）内的少突胶质细胞生成。这一首次出现于有颌脊椎动物的演化革新，使得神经冲动得以快速传导，同时助力形成更复杂的大脑与更丰富的形态多样性。本研究发现，逆转录病毒起源的逆转录转座子（retrotransposon）RNLTR12-int的RNA水平表达对髓鞘形成至关重要。我们在啮齿类动物中证实，RNLTR12-int编码的非编码RNA（non-coding RNA）可结合转录因子（transcription factor）SOX10，从而调控髓鞘主要成分髓鞘碱性蛋白（Mbp）的转录。所有有颌脊椎动物体内均存在RNLTR12-int同源序列，我们将其命名为RetroMyelin；并进一步在两类不同脊椎动物类群（斑马鱼与蛙类）中证实了其调控髓鞘形成的功能。因此本研究表明，逆转录病毒内源化是脊椎动物髓鞘形成演化过程中的关键步骤。