Despite a long appreciation for the role of nonsense-mediated mRNA decay (NMD) in destroying faulty, disease-causing mRNAs and maintaining normal, physiologic mRNA abundance, additional effectors that regulate NMD activity in mammalian cells continue to be identified. Here, we describe a haploid-cell genetic screen for NMD effectors that has unexpectedly identified 13 proteins constituting the AKT signaling pathway. We show that AKT supersedes UPF2 in exon-junction complexes (EJCs) that are devoid of RNPS1 but contain CASC3, defining an unanticipated insulin-stimulated EJC. Without altering UPF1 RNA binding or ATPase activity, AKT-mediated phosphorylation of the UPF1 CH domain at T151 augments UPF1 helicase activity, which is critical for NMD and also decreases the dependence of helicase activity on ATP. We demonstrate that upregulation of AKT signaling contributes to the hyperactivation of NMD that typifies Fragile X syndrome, as exemplified using FMR1-KO neural stem cells derived from induced pluripotent stem cells.

尽管长期以来人们一直认识到无义介导的 mRNA 衰变 (NMD) 在破坏有缺陷、致病的 mRNA 和维持正常的生理 mRNA 丰度方面的作用，但调节哺乳动物细胞中 NMD 活性的其他效应子仍在不断被发现。在这里，我们描述了针对 NMD 效应子的单倍体细胞遗传筛选，该筛选意外地鉴定出了构成 AKT 信号通路的 13 种蛋白质。我们发现，AKT 在不含 RNPS1 但含有 CASC3 的外显子连接复合物 (EJC) 中取代了 UPF2，定义了一种意想不到的胰岛素刺激 EJC。在不改变 UPF1 RNA 结合或 ATP 酶活性的情况下，AKT 介导的 UPF1 CH 结构域 T151 磷酸化会增强 UPF1 解旋酶活性，这对于 NMD 至关重要，并且还会降低解旋酶活性对 ATP 的依赖性。我们证明，AKT 信号传导的上调会导致脆性 X 综合征典型特征的 NMD 过度激活，如使用源自诱导多能干细胞的FMR1 -KO 神经干细胞进行的例证。