Insight into integrated control of pituitary function revealed through analysis of Musashi target mRNAs

The anterior pituitary is comprised of distinct cell types that each secrete specific hormones to control a variety of biological processes including growth, metabolism, reproduction and stress responses. The anterior pituitary shows a remarkable level of cell type plasticity that allows shifts in hormone producing populations to meet organismal demands. Pituitary cell plasticity is tightly regulated and both deficiency in cell plasticity and excessive cell plasticity are associated with common pituitary pathologies. The molecular mechanisms underlying this plasticity are not well characterized but recent work has implicated the stem cell determinants and sequence-specific mRNA binding proteins of the Musashi family as regulators of adult pituitary hormone production. In this study we have sought to identify the full range of Musashi target mRNAs in the adult mouse pituitary. Using Musashi RNA immunoprecipitation we identify a cohort of 1192 mRNAs that show specific Musashi binding. These include mRNAs restricted to discrete hormone-producing cell lineages as well as mRNAs associated with stem and progenitor cells. The processes influenced by the proteins encoded by the Musashi-associated mRNAs include cellular homeostasis, protein trafficking and secretion, unfolded protein response, endocrine processes and female pregnancy. Functional analysis of validated mRNA regulatory 3' untranslated regions (3’ UTRs) reveals UTR-specific positive or negative control by the Musashi proteins within the same cellular context. Together, our findings indicate a broad role for Musashi proteins in the control of pituitary function. RIP-seq

垂体前叶由多种不同的细胞类型构成，各类细胞均分泌特异性激素，以调控生长、代谢、生殖及应激反应等多种生理过程。垂体前叶具备显著的细胞类型可塑性，可通过改变激素产生细胞群的组成比例以满足机体需求。垂体细胞可塑性受到严格调控，可塑性缺失或过度增强均与常见的垂体病理状态密切相关。尽管该可塑性背后的分子机制尚未得到充分阐明，但近期研究表明，Musashi家族的干细胞决定因子与序列特异性mRNA结合蛋白可作为成年垂体激素生成的调控因子。本研究旨在鉴定成年小鼠垂体中Musashi蛋白的全部靶mRNA。通过Musashi RNA免疫沉淀（RNA Immunoprecipitation, RIP）技术，我们共鉴定出1192个具有特异性Musashi结合活性的mRNA群体，其中既包括局限于特定激素产生细胞谱系的转录本，也包含与干细胞及祖细胞相关的转录本。Musashi结合mRNA所编码的蛋白所参与的生物学过程涵盖细胞稳态、蛋白质运输与分泌、未折叠蛋白反应、内分泌过程以及雌性妊娠。对经过验证的受Musashi调控的mRNA 3'非翻译区（3' untranslated regions, 3' UTRs）进行功能分析发现，在同一细胞环境中，Musashi蛋白可对UTR序列产生特异性的正向或负向调控作用。综上，本研究结果表明Musashi蛋白在垂体功能调控中发挥广泛作用。RIP-seq