LIN28 is an RNA binding protein with important roles in early embryo development, stem cell differentiation/reprogramming, tumorigenesis and metabolism. Previous studies have focused mainly on its role in the cytosol where it interacts with Let-7 microRNA precursors or mRNAs, and few have addressed LIN28’s role within the nucleus. Here, we show that LIN28 displays dynamic temporal and spatial expression during murine embryo development. Maternal LIN28 expression drops upon exit from the 2-cell stage, and zygotic LIN28 protein is induced at the forming nucleolus during 4-cell to blastocyst stage development, to become dominantly expressed in the cytosol after implantation. In cultured pluripotent stem cells (PSCs), loss of LIN28 led to nucleolar stress and activation of a 2-cell/4-cell-like transcriptional program characterized by the expression of endogenous retrovirus genes. Mechanistically, LIN28 binds to small nucleolar RNAs and rRNA to maintain nucleolar integrity, and its loss leads to nucleolar phase separation defects, ribosomal stress and activation of P53 which in turn binds to and activates 2C transcription factor Dux. LIN28 also resides in a complex containing the nucleolar factor Nucleolin (NCL) and the transcriptional repressor TRIM28, and LIN28 loss leads to reduced occupancy of the NCL/TRIM28 complex on the Dux and rDNA loci, and thus de-repressed Dux and reduced rRNA expression. Lin28 knockout cells with nucleolar stress are more likely to assume a slowly cycling, translationally inert and anabolically inactive state, which is a part of previously unappreciated 2C-like transcriptional program. These findings elucidate novel roles for nucleolar LIN28 in PSCs, and a new mechanism linking 2C program and nucleolar functions in PSCs and early embryo development.

LIN28 是一种 RNA 结合蛋白，在早期胚胎发育、干细胞分化/重编程、肿瘤发生和代谢中具有重要作用。以前的研究主要集中在它在与Let-7相互作用的细胞质中的作用microRNA 前体或 mRNA，很少有人提到 LIN28 在细胞核中的作用。在这里，我们表明 LIN28 在小鼠胚胎发育过程中表现出动态的时空表达。母体 LIN28 表达从 2 细胞阶段退出后下降，在 4 细胞至囊胚阶段发育过程中，在形成的核仁处诱导合子 LIN28 蛋白，在植入后主要在细胞质中表达。在培养的多能干细胞 (PSC) 中，LIN28 的缺失导致核仁应激和以内源性逆转录病毒基因表达为特征的 2 细胞/4 细胞样转录程序的激活。从机制上讲，LIN28 与小核仁 RNA 和 rRNA 结合以维持核仁完整性，其丢失导致核仁相分离缺陷，杜克斯。LIN28 还存在于含有核仁因子 Nucleolin (NCL) 和转录阻遏物 TRIM28 的复合物中，LIN28 的丢失导致Dux和 rDNA 基因座上 NCL/TRIM28 复合物的占有率降低，因此去抑制Dux并降低 rRNA 表达. 具有核仁应激的Lin28敲除细胞更有可能呈现缓慢循环、翻译惰性和合成代谢失活状态，这是以前未被重视的 2C 样转录程序的一部分。这些发现阐明了核仁 LIN28 在 PSC 中的新作用，以及将 2C 程序与 PSC 中的核仁功能和早期胚胎发育联系起来的新机制。