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The anterior pituitary controls key biological processes, including growth, metabolism, reproduction, and stress responses through distinct cell types that each secrete specific hormones. The anterior pituitary cells show a remarkable level of cell type plasticity that mediates the shifts in hormone-producing cell populations that are required to meet organismal needs. The molecular mechanisms underlying pituitary cell plasticity are not well understood. Recent work has implicated the pituitary stem cell populations and specifically, the mRNA binding proteins of the Musashi family in control of pituitary cell type identity. In this study we have identified the target mRNAs that mediate Musashi function in the adult mouse pituitary and demonstrate the requirement for Musashi function in vivo. Using Musashi RNA immunoprecipitation, we identify a cohort of 1184 mRNAs that show specific Musashi binding. Identified Musashi targets include the Gnrhr mRNA, which encodes the gonadotropin-releasing hormone receptor (GnRHR), and the Fshb mRNA, encoding follicle-stimulating hormone (FSH). Reporter assays reveal that Musashi functions to exert repression of translation of the Fshb mRNA, in addition to the previously observed repression of the Gnrhr mRNA. Importantly, mice engineered to lack Musashi in gonadotropes demonstrate a failure to repress translation of the endogenous Gnrhr and Fshb mRNAs during the estrous cycle and display a significant heterogeneity in litter sizes. The range of identified target mRNAs suggests that, in addition to these key gonadotrope proteins, Musashi may exert broad regulatory control over the pituitary proteome in a cell type–specific manner. [ABSTRACT FROM AUTHOR]

In normal individuals, pituitary somatotrophs optimise body composition by responding to metabolic signals from leptin. To identify mechanisms behind the regulation of somatotrophs by leptin, we used Cre‐LoxP technology to delete leptin receptors (LEPR) selectively in somatotrophs and developed populations purified by fluorescence‐activated cell sorting (FACS) that contained 99% somatotrophs. FACS‐purified, Lepr‐null somatotrophs showed reduced levels of growth hormone (GH), growth hormone‐releasing hormone receptor (GHRHR), and Pou1f1 proteins and Gh (females) and Ghrhr (both sexes) mRNAs. Pure somatotrophs also expressed thyroid‐stimulating hormone (TSH) and prolactin (PRL), both of which were reduced in pure somatotrophs lacking LEPR. This introduced five gene products that were targets of leptin. In the present study, we tested the hypothesis that leptin is both a transcriptional and a post‐transcriptional regulator of these gene products. Our tests showed that Pou1f1 and/or the Janus kinase/signal transducer and activator of transcription 3 transcriptional regulatory pathways are implicated in the leptin regulation of Gh or Ghrhr mRNAs. We then focused on potential actions by candidate microRNAs (miRNAs) with consensus binding sites on the 3' UTR of Gh or Ghrhr mRNAs. Somatotroph Lepr‐null deletion mutants expressed elevated levels of miRNAs including miR1197‐3p (in females), miR103‐3p and miR590‐3p (both sexes), which bind Gh mRNA, or miRNA‐325‐3p (elevated in both sexes), which binds Ghrhr mRNA. This elevation indicates repression of translation in the absence of LEPR. In addition, after detecting binding sites for Musashi on Tshb and Prl 3' UTR, we determined that Musashi1 repressed translation of both mRNAs in in vitro fluc assays and that Prl mRNA was enriched in Musashi immunoprecipitation assays. Finally, we tested ghrelin actions to determine whether its nitric oxide‐mediated signalling pathways would restore somatotroph functions in deletion mutants. Ghrelin did not restore either GHRH binding or GH secretion in vitro. These studies show an unexpectedly broad role for leptin with respect to maintaining somatotroph functions, including the regulation of PRL and TSH in subsets of somatotrophs that may be progenitor cells. [ABSTRACT FROM AUTHOR]

The mechanisms that mediate plasticity in pituitary function have long been a subject of vigorous investigation. Early studies overcame technical barriers and challenged conceptual barriers to identify multipotential and multihormonal cell populations that contribute to diverse pituitary stress responses. Decades of intensive study have challenged the standard model of dedicated, cell type-specific hormone production and have revealed the malleable cellular fates that mediate pituitary responses. Ongoing studies at all levels, from animal physiology to molecular analyses, are identifying the mechanisms underlying this cellular plasticity. This review describes the findings from these studies that utilized state-of-the-art tools and techniques to identify mechanisms of plasticity throughout the pituitary and focuses on the insights brought to our understanding of pituitary function. [ABSTRACT FROM AUTHOR]

Regulated mRNA translation plays a key role in control of cell cycle progression in a variety of physiological and pathological processes, including in the self-renewal and survival of stem cells and cancer stem cells. While targeting mRNA translation presents an attractive strategy for control of aberrant cell cycle progression, mRNA translation is an underdeveloped therapeutic target. Regulated mRNAs are typically controlled through interaction with multiple RNA binding proteins (RBPs) but the mechanisms by which the functions of distinct RBPs bound to a common target mRNA are coordinated are poorly understood. The challenge now is to gain insight into these mechanisms of coordination and to identify the molecular mediators that integrate multiple, often conflicting, inputs. A first step includes the identification of altered mRNA ribonucleoprotein complex components that assemble on mRNAs bound by multiple, distinct RBPs compared to those recruited by individual RBPs. This review builds upon our knowledge of combinatorial control of mRNA translation during the maturation of oocytes from Xenopus laevis, to address molecular strategies that may mediate RBP diplomacy and conflict resolution for coordinated control of mRNA translational output. Continued study of regulated ribonucleoprotein complex dynamics promises valuable new insights into mRNA translational control and may suggest novel therapeutic strategies for the treatment of disease. [ABSTRACT FROM AUTHOR]

Background: There is increasing evidence of a pivotal role for regulated mRNA translation in control of developmental cell fate transitions. Physiological and pathological stem and progenitor cell self-renewal is maintained by the mRNA-binding protein, Musashi1 through repression of translation of key mRNAs encoding cell cycle inhibitory proteins. The mechanism by which Musashi1 function is modified to allow translation of these target mRNAs under conditions that require inhibition of cell cycle progression, is unknown. Results: In this study, we demonstrate that differentiation of primary embryonic rat neural stem/progenitor cells (NSPCs) or human neuroblastoma SH-SY5Y cells results in the rapid phosphorylation of Musashi1 on the evolutionarily conserved site serine 337 (S337). Phosphorylation of this site has been shown to be required for cell cycle control during the maturation of Xenopus oocytes. S337 phosphorylation in mammalian NSPCs and human SH-SY5Y cells correlates with the de-repression and translation of a Musashi reporter mRNA and with accumulation of protein from the endogenous Musashi target mRNA, p21WAF1/CIP1. Inhibition of Musashi regulatory phosphorylation, through expression of a phospho-inhibitory mutant Musashi1 S337A or over-expression of the wild-type Musashi, blocked differentiation of both NSPCs and SH-SY5Y cells. Musashi1 was similarly phosphorylated in NSPCs and SH-SY5Y cells under conditions of nutrient deprivation-induced cell cycle arrest. Expression of the Musashi1 S337A mutant protein attenuated nutrient deprivation-induced NSPC and SH-SY5Y cell death. Conclusions: Our data suggest that in response to environmental cues that oppose cell cycle progression, regulation of Musashi function is required to promote target mRNA translation and cell fate transition. Forced modulation of Musashi1 function may present a novel therapeutic strategy to oppose pathological stem cell self-renewal. [ABSTRACT FROM AUTHOR]

Meiotic cell-cycle progression in progesterone-stimulated Xenopus oocytes requires that the translation of pre-existing maternal mRNAs occur in a strict temporal order. Timing of translation is regulated through elements within the mRNA 3′ untranslated region (3′ UTR), which respond to cell cycle-dependant signalling. One element that has been previously implicated in the temporal control of mRNA translation is the cytoplasmic polyadenylation element (CPE). In this study, we show that the CPE does not direct early mRNA translation. Rather, early translation is directed through specific early factors, including the Musashi-binding element (MBE) and the MBE-binding protein, Musashi. Our findings indicate that although the cyclin B5 3′ UTR contains both CPEs and an MBE, the MBE is the critical regulator of early translation. The cyclin B2 3′ UTR contains CPEs, but lacks an MBE and is translationally activated late in maturation. Finally, utilizing antisense oligonucleotides to attenuate endogenous Musashi synthesis, we show that Musashi is critical for the initiation of early class mRNA translation and for the subsequent activation of CPE-dependant mRNA translation. [ABSTRACT FROM AUTHOR]