Primary cilium-autophagy-Nrf2 (PAN) axis links the cell cycle to neuroectoderm fate in human embryonic stem cells

Under defined differentiation conditions human embryonic stem cells (hESCs) can be directed toward a mesendodermal (ME) or neuroectoderm (NE) fate, the first decision during hESC differentiation. Coupled with G1 lengthening a divergent ciliation pattern emerged within the first 24 hours of induced lineage specification and these changes heralded a neuroectoderm decision before any neural precursor markers were expressed. By day 2, increased ciliation in NE precursors induced autophagy that resulted in the inactivation of Nrf2. Nrf2 binds directly to upstream regions of the OCT4 and NANOG genes to promote their expression and represses NE derivation. Nrf2 suppression was sufficient to rescue poorly neurogenic iPSC lines. Only after these events have been initiated do neural precursor markers get expressed at day 4. Thus we have identified a primary cilium-autophagy-Nrf2 (PAN) axis coupled to cell cycle progression that directs hESCs toward NE.

在特定分化条件下，人类胚胎干细胞（human embryonic stem cells, hESCs）可被定向诱导为中内胚层（mesendodermal, ME）或神经外胚层（neuroectoderm, NE）命运，这是hESC分化过程中的首个细胞命运抉择节点。伴随G1期延长，诱导谱系特化后的最初24小时内便出现了差异化的纤毛形成模式，且这些变化可在神经前体细胞标志物表达之前，预示细胞将走向神经外胚层命运。诱导分化至第2天时，神经外胚层前体细胞中纤毛形成的增强会诱导自噬发生，进而导致核因子E2相关因子2（Nuclear factor erythroid 2-related factor 2, Nrf2）失活。Nrf2可直接结合八聚体结合转录因子4（OCT4）及Nanog同源盒基因（NANOG）的上游调控区域，以促进二者的表达，同时抑制神经外胚层的分化进程。抑制Nrf2的活性足以挽救神经发生能力低下的诱导多能干细胞（induced pluripotent stem cells, iPSC）株系。直至上述事件均被触发后，神经前体细胞标志物才会在分化第4天开始表达。综上，本研究鉴定出一条与细胞周期进程耦联的初级纤毛-自噬-Nrf2（primary cilium-autophagy-Nrf2, PAN）轴，该轴可引导人类胚胎干细胞定向分化为神经外胚层细胞。