CRL4^Mahj E3 ubiquitin ligase promotes neural stem cell reactivation

The ability of neural stem cells (NSCs) to transit between quiescence and proliferation is crucial for brain development and homeostasis. Drosophila Hippo pathway maintains NSC quiescence, but its regulation during brain development remains unknown. Here, we show that CRL4Mahj, an evolutionarily conserved E3 ubiquitin ligase, is essential for NSC reactivation (exit from quiescence). We demonstrate that damaged DNA-binding protein 1 (DDB1) and Cullin4, two core components of Cullin4-RING ligase (CRL4), are intrinsically required for NSC reactivation. We have identified a substrate receptor of CRL4, Mahjong (Mahj), which is necessary and sufficient for NSC reactivation. Moreover, we show that CRL4Mahj forms a protein complex with Warts (Wts/large tumor suppressor [Lats]), a kinase of the Hippo signaling pathway, and Mahj promotes the ubiquitination of Wts. Our genetic analyses further support the conclusion that CRL4Mahj triggers NSC reactivation by inhibition of Wts. Given that Cullin4B mutations cause mental retardation and cerebral malformation, similar regulatory mechanisms may be applied to the human brain.神经干细胞（neural stem cells, NSCs）在静止状态与增殖状态间相互转换的能力，对于大脑发育及内稳态维持至关重要。果蝇的Hippo信号通路可维持神经干细胞的静止状态，但其在大脑发育过程中的调控机制尚不清楚。本研究显示，进化保守的E3泛素连接酶CRL4Mahj对神经干细胞再激活（即退出静止状态）是必需的。我们证实，Cullin4-RING泛素连接酶（Cullin4-RING ligase, CRL4）的两个核心组分——损伤DNA结合蛋白1（damaged DNA-binding protein 1, DDB1）与Cullin4，是神经干细胞再激活的内在必需因子。我们鉴定出CRL4的一种底物受体Mahjong（Mahj），其对于神经干细胞再激活既是必要的也是充分的。此外，我们发现CRL4Mahj可与Hippo信号通路的激酶Warts（Wts/大肿瘤抑制因子[Lats]）形成蛋白复合物，且Mahj能够促进Wts的泛素化。我们的遗传分析进一步支持了CRL4Mahj通过抑制Wts来触发神经干细胞再激活的结论。鉴于Cullin4B突变可引发智力障碍与大脑畸形，类似的调控机制或许也可应用于人类大脑。