Lin28-mediated promotion of protein synthesis is critical for neural progenitor cell maintenance and brain development in mice

Neural progenitor cells (NPCs) undergo rapid proliferation during neurulation in early development. This rapid growth generates a high demand for mRNA translation in a timing-dependent manner, but its underlying mechanism and functional importance remain poorly understood. Lin28 is an RNA-binding protein with two paralogs, Lin28a and Lin28b, in mammals. Lin28b deletion exhibited no developmental defects in mice, while we previously reported that Lin28a deletion led to microcephaly. Here we found that Lin28a/b double knockout (dKO) mice displayed neural tube defects (NTDs) coupled with reduced proliferation and precocious differentiation of NPCs. We used ribosomal protein 24 hypomorphic mice (Rpl24Bst/+) as a genetic tool to dampen global protein synthesis. In support of the importance of Lin28-mediated translation promotion, Lin28a-/-;Rpl24Bst/+ compound mutants exhibited NTDs resembling those seen in Lin28a/b dKO mice. Furthermore, increased NPC numbers and brain sizes in Lin28a-overexpressing mice were rescued by Rpl24Bst/+ heterozygosity. Mechanistically, RNA-sequencing of polysome sucrose gradient fractions revealed a reduced translation of genes involved in the regulation of cell cycle, ribosome biogenesis, and translation in mutants. Lin28a localizes in the nucleoli of NPCs, and ribosome biogenesis was reduced in dKO and increased in Lin28a-overexpressing NPCs. Together, these results suggest that Lin28-mediated promotion of protein synthesis is essential for NPC maintenance and early brain development. Examination of 3 groups of transcriptomes and polysome profilings in mouse brain神经前体细胞（Neural progenitor cells, NPCs）在胚胎早期发育的神经胚形成过程中会经历快速增殖。这种快速增殖以时间依赖性方式对mRNA翻译提出极高需求，但其背后的分子机制与功能重要性仍有待深入阐明。Lin28是一类RNA结合蛋白，在哺乳动物中存在两个旁系同源蛋白：Lin28a与Lin28b。在小鼠模型中，单独敲除Lin28b不会引发发育缺陷；而我们此前的研究表明，单独敲除Lin28a会导致小头畸形。本研究发现，Lin28a/b双敲除（double knockout, dKO）小鼠会出现神经管缺陷（neural tube defects, NTDs），同时伴随神经前体细胞增殖能力下降并发生过早分化。我们将核糖体蛋白24低功能型小鼠（Rpl24Bst/+）作为遗传工具，以抑制整体蛋白质合成。为验证Lin28介导的翻译促进作用的重要性，Lin28a-/-与Rpl24Bst/+的复合突变体小鼠表现出的神经管缺陷与Lin28a/b双敲除小鼠的表型一致。此外，Lin28a过表达小鼠中出现的神经前体细胞数量增多与脑体积增大的表型，可通过Rpl24Bst/+的杂合状态得以挽救。从分子机制层面来看，对多聚核糖体蔗糖梯度分级组分进行RNA测序的结果显示，突变体中参与细胞周期调控、核糖体生物发生以及翻译过程的基因的翻译水平出现下调。Lin28a定位于神经前体细胞的核仁中；双敲除小鼠的神经前体细胞内核糖体生物发生水平下降，而Lin28a过表达的神经前体细胞内该水平则有所上升。综合以上结果，我们认为Lin28介导的蛋白质合成促进作用对于神经前体细胞的维持以及早期脑发育至关重要。本研究对小鼠脑组织中的3组转录组与多聚核糖体谱进行了检测分析。