The Musashi RNA binding proteins direct the translational activation of key pituitary mRNAs

The pituitary functions as a master endocrine gland that secretes hormones critical for regulation of a wide variety of physiological processes including reproduction, growth, metabolism and stress responses. The distinct hormone-producing cell lineages within the pituitary display remarkable levels of cell plasticity that allow remodeling of the relative proportions of each hormone-producing cell population to meet organismal demands. The molecular mechanisms governing pituitary cell plasticity have not been fully elucidated. Our recent studies have implicated a role for the Musashi family of sequence-specific mRNA binding proteins in the control of pituitary hormone production, pituitary responses to hypothalamic stimulation and modulation of pituitary transcription factor expression in response to leptin signaling. To date, these actions of Musashi in the pituitary appear to be mediated through translational repression of the target mRNAs. Here, we report Musashi1 directs the translational activation, rather than repression, of the Prop1, Gata2 and Nr5a1 mRNAs which encode key pituitary lineage specification factors. We observe that Musashi1 further directs the translational activation of the mRNA encoding the glycolipid Neuronatin (Nnat) as determined both in mRNA reporter assays as well as in vivo. Our findings suggest a complex bifunctional role for Musashi1 in the control of pituitary cell function.

Anterior Pituitary Transcriptomics Following a High-Fat Diet: Impact of Oxidative Stress on Cell Metabolism

Anterior pituitary cell function requires a high level of protein synthesis and secretion which depend heavily on mitochondrial adenosine triphosphate production and functional endoplasmic reticula. Obesity adds stress to tissues, requiring them to adapt to inflammation and oxidative stress, and adding to their allostatic load. We hypothesized that pituitary function is vulnerable to the stress of obesity. Here, we utilized a 10- to 15-week high-fat diet (HFD, 60%) in a thermoneutral environment to promote obesity, testing both male and female FVB.129P mice. We quantified serum hormones and cytokines, characterized the metabolic phenotype, and defined changes in the pituitary transcriptome using single-cell RNA-sequencing analysis. Weight gain was significant by 3 weeks in HFD mice, and by 10 weeks all HFD groups had gained 20 g. HFD females (15 weeks) had increased energy expenditure and decreased activity. All HFD groups showed increases in serum leptin and decreases in adiponectin. HFD caused increased inflammatory markers: interleukin-6, resistin, monocyte chemoattractant protein-1, and tumor necrosis factorα. HFD males and females also had increased insulin and increased TSH, and HFD females had decreased serum prolactin and growth hormone pulse amplitude. Pituitary single-cell transcriptomics revealed modest or no changes in pituitary cell gene expression from HFD males after 10 or 15 weeks or from HFD females after 10 weeks. However, HFD females (15 weeks) showed significant numbers of differentially expressed genes in lactotropes and pituitary stem cells. Collectively, these studies reveal that pituitary cells from males appear to be more resilient to the oxidative stress of obesity than females and identify the most vulnerable pituitary cell populations in females.

Musashi Exerts Control of Gonadotrope Target mRNA Translation During the Mouse Estrous Cycle

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.

RF01 | PMON46 Regulation of the Pituitary Cell Lineage Regulator Prop1 by the Stem Cell Determinant Musashi

The pituitary gland controls responses to changing physiological requirements during growth, pregnancy, puberty, and stress. Synthesis and secretion of appropriate pituitary hormones in response to changing organismal demands is mediated through a remarkable capacity for cellular plasticity of the distinct hormone-producing cells. The mechanisms governing pituitary cellular plasticity are not fully understood. We hypothesize that plasticity in the adult tissue is mediated through similar mechanisms to those that occur during embryonic and postnatal pituitary development. The Prophet of Pit1(Prop1), a paired-like homeodomain transcription factor, is the earliest pituitary-specific transcription factor and is highly expressed in pituitary progenitor cells where it controls lineage commitment to the distinct hormone-producing cell types. Our recent work has identified post-transcriptional regulation of pituitary gene expression in the adult tissue via the stem cell determinant, mRNA binding protein, Musashi. The full-length mouse Prop1 (mProp1) mRNA 3' untranslated region (UTR) contains 24 consensus binding elements for Musashi (MBEs) and demonstrates Musashi1-dependent translational activation in vitro. Specifically, the terminal mProp1 3' UTR MBE directs 40% of Musashi1-mediated translation activation, and mutational disruption of this terminal MBE abolishes all Musashi1-dependent translational activation. Immunoprecipitation of Musashi-mRNA complexes from adult mouse pituitaries confirmed Musashi1-mProp1 mRNA association in adult tissue. The gene sequence data for the human Prop1 (hPROP1) mRNA 3' UTR is incomplete in the Ensembl and RefSeq databases. Using published RNA sequencing datasets, we identified an expressed full-length hPROP1 mRNA 3' UTR of 6996 nucleotides with 54 predicted consensus MBEs, suggesting that the human PROP1 mRNA, like the murine Prop1 mRNA, may be subject to Musashi-dependent translational control. Our findings are consistent with the hypothesis that the Musashi protein exerts evolutionarily conserved control of Prop1 mRNA translation to facilitate plasticity of anterior pituitary function.

Presentation: Saturday, June 11, 2022 1:31 p.m. - 1:36 p.m., Monday, June 13, 2022 12:30 p.m. - 2:30 p.m.

Single and double modified salinomycin analogs target stem-like cells in 2D and 3D breast cancer models

Breast cancer remains one of the leading cancers among women. Cancer stem cells (CSCs) are tumor-initiating cells which drive progression, metastasis, and reoccurrence of the disease. CSCs are resistant to conventional chemo- and radio-therapies and their ability to survive such treatment enables tumor reestablishment. Metastasis is the main cause of mortality in women with breast cancer, thus advances in treatment will depend on therapeutic strategies targeting CSCs. Salinomycin (SAL) is a naturally occurring polyether ionophore antibiotic known for its anticancer activity towards several types of tumor cells. In the present work, a library of 17 C1-single and C1/C20-double modified SAL analogs was screened to identify compounds with improved activity against breast CSCs. Six single- and two double-modified analogs were more potent (IC50 range of 1.1 ± 0.1-1.4 ± 0.2 µM) toward the breast cancer cell line MDA-MB-231 compared to SAL (IC50 of 4.9 ± 1.6 µM). Double-modified compound 17 was found to be more efficacious than SAL against the majority of cancer cell lines in the NCI-60 Human Tumor Cell Line Panel. Compound 17 was more potent than SAL in inhibiting cell migration and cell renewal properties of MDA-MB-231 cells, as well as inducing selective loss of the CD44+/CD24/low stem-cell-like subpopulation in both monolayer (2D) and organoid (3D) culture. The present findings highlight the therapeutic potential of SAL analogs towards breast CSCs and identify select compounds that merit further study and clinical development.

The Cell Fate Determinant Musashi Is Controlled Through Dynamic Protein:Protein Interactions

The Musashi RNA-binding protein functions as a gatekeeper of cell maturation and plasticity through the control of target mRNA translation. It is understood that Musashi promotes stem cell self-renewal and opposes differentiation. While Musashi is best characterized as a repressor of target mRNA translation, we have shown that Musashi can activate target mRNA translation in a cell context specific manner via regulatory phosphorylation on two evolutionarily conserved C-terminal serine residues. Our recent work has found that Musashi is expressed in pituitary stem cells as well as in differentiated hormone producing cell lineages in the adult pituitary. We hypothesize that Musashi maintains cell fate plasticity in the adult pituitary to allow the gland to modulate hormone production in response to changing organismal needs. Here, we seek to understand the regulation of Musashi function. Both Musashi isoforms (Musashi1 and Musashi2) contain two RNA-recognition motifs (RRMs) that bind to specific sequences in the 3’-UTR of target mRNA transcripts; however, neither isoform has enzymatic properties and thus functions through interactions with other proteins to regulate translational outcomes, but the identity and role of Musashi partner proteins is largely unknown. In this study, we have identified co-associated partner proteins that functionally contribute to Musashi-dependent mRNA translational activation during the maturation of Xenopus oocytes. Using mass spectrometry, we identified 29 co-associated proteins that interact specifically with Musashi1 during oocyte maturation and determined that the Musashi co-associated proteins ePABP, PABP4, LSM14A/B, CELF2, PUM1, ELAV1, ELAV2, and DDX6 attenuated oocyte maturation through individual antisense DNA oligo knockdowns. An assessment of the role of these cofactors in the control of Musashi-dependent target mRNA translation is in progress. In addition to studying co-associated proteins, we have created a computational 3D model of the Musashi1 molecule to assist in our investigation Musashi dimerization. This model has indicated that both Musashi1 dimerization and Musashi1:Musashi2 heterodimerization are energetically favorable, and co-pulldown studies have verified both Musashi1 homo-dimerization and Musashi1:Musashi2 heterodimerization in vivo. Computational modeling of Musashi dimer complexes has also identified the key amino acids necessary for these interactions. The contribution of each co-associated protein’s influence on Musashi-dependent translation, relative to the requirement for Musashi:Musashi dimerization, is expected to provide unparalleled insight into regulation of Musashi action. Moreover, cell type specific regulation of association of Musashi co-factors would directly influence Musashi target mRNA translation in oocyte maturation and during pituitary cell plasticity.

The Musashi1 RNA-Binding Protein Functions as a Leptin-Regulated Enforcer of Pituitary Cell Fate and Hormone Production

The pituitary gland is the major endocrine organ that produces and secretes hormones in response to hypothalamic signals to regulate important processes like growth, reproduction, and stress. The anterior pituitary adapts to metabolic and reproductive needs by exhibiting cellular plasticity, resulting in altered hormone production and secretion. The adipokine, leptin, serves a critical role to couple energy status to pituitary function. We have recently reported that the cell fate determinant, Musashi, functions as a post-transcriptional regulator of target mRNA translation in the mouse pituitary and have speculated that Musashi may modulate pituitary cell plasticity. However, the underlying mechanisms governing such pituitary plasticity are not fully understood. Musashi is an mRNA binding protein that is required for self-renewal, proliferation, and to control the differentiation of stem and progenitor cells. We have recently shown that Musashi is expressed in Sox2+ pituitary stem cells and surprisingly, we also found Musashi expression in all differentiated hormone expressing cell lineages in the adult anterior pituitary. The role of Musashi in these mature differentiated cells is unknown. We have observed that a range of critical pituitary mRNAs, including the lineage specification transcription factors Prop1 and Pou1f1, as well as hormone mRNAs including Tshb, Prl, and Gnrhr, all contain consensus Musashi binding elements (MBEs) in their 3’ untranslated regions (3’ UTRs). Using RNA electrophoretic mobility shift assays (EMSAs) and luciferase mRNA translation reporter assays we show that Musashi binds to these mRNAs and exerts inhibitory control of mRNA translation. Moreover, we determined that leptin stimulation opposes the ability of Musashi to exert translational repression of the Pou1f1 and Gnrhr 3’ UTRs. This de-repression does not require regulatory phosphorylation of Musashi on two conserved C-terminal serine residues. Interestingly in the same cell assay system, Musashi exerts translational activation of the Prop1 3’ UTR. We observed that this translational activation requires Musashi phosphorylation on the two regulatory C-terminal serine residues, consistent with the requirement for regulatory phosphorylation to drive translational activation of Musashi target mRNAs during Xenopus oocyte cell maturation. The distinction between MBEs in 3’ UTRs that exert repression (Pou1f1, Prl, Tshb, and Gnrhr) and the Prop1 3’ UTR that directs translational activation is under investigation. We propose that Musashi acts as a bifunctional regulator of pituitary hormone production and lineage specification and may function to maintain pituitary hormone plasticity in response to changing organismal needs.

Musashi as a Regulator of the Follicle-Stimulating Hormone in the Gonadotropes

The cyclic expression of gonadotropin releasing-hormone receptors (GnRHR), luteinizing hormone (LH), and follicle-stimulating hormone (FSH) by pituitary gonadotropes is critical in the female reproductive process. We have shown that the translational regulator Musashi (MSI) binds to Gnrhr mRNA and inhibits its translation, and the gonadotrope-specific deletion of Msi1 and Msi2 (Gon-Msi-null) leads to increased pituitary GnRHR protein levels. An in silico analysis of gonadotropin mRNAs revealed 5 different MSI binding elements in the 3’UTR of Fshb mRNA. We hypothesize that, in addition to Gnrhr, MSI may also bind and repress Fshb mRNA translation in the gonadotropes. To test if MSI does target the Fshb transcript in the pituitary, we performed RNA immunoprecipitation (IP) on pooled control female mouse pituitaries using a MSI1 antibody and measured Fshb mRNA by qRT-PCR. To study the in vivo effects of MSI on Fshb, we harvested the pituitaries of the Gon-Msi-null (MUT) female mice and their littermate controls (CTL) during the estrous cycle. We collected serum and protein for EIAs to measure the levels of FSH and LH, and RNA for Fshb qRT-PCR. We harvested proestrous ovaries and fixed them for embedding, sectioning, and H&E staining. Our RNA IP experiments show a 7-fold enrichment for Fshb with the MSI1 antibody. The Gon-Msi-null females have significantly higher pituitary FSH protein content than controls on estrous morning (MUT: 4.8±1.3 vs. CTL: 1.8±2.6 ng/ml/μg protein, p<0.0001, n=9-10/group). These mice also have increased serum FSH levels (MUT: 56.9±6.4 vs. CTL: 44.5±9.6 ng/ml, p=0.0147, n=9-10/group). No changes at the Fshb mRNA level were detected. Analysis of Gon-Msi-null ovaries revealed a 50% decrease in the number of follicles, with significant decreases in the average numbers of maturing follicles (p<0.0175) and corpora lutea (p<0.0215). Interestingly, the LH levels in these mice were also altered. The Gon-Msi-null females show a decrease in the pituitary LH protein content in the evening of proestrus (MUT: 11.8±1.4 vs. CTL: 15.1±2.0 ng/ml/μg protein, p=0.0333, n=7/group), in addition to a delayed and blunted LH surge (MUT: 2.6±1.9 vs. CTL: 7.3±3.5 ng/ml, p=0.0089, n=7-11/group). Taken together, our data indicate that Fshb is a Musashi target in the gonadotropes. By deleting MSI from the pituitary gonadotropes, we observe an increase in FSH protein content and serum levels. These Gon-Msi-null female mice have significantly fewer maturing follicles and corpora lutea, which might suggest lower levels of estrogens and progesterone. This, together with the increased GnRHR pituitary protein content, affects LH secretion, leading to a blunted LH surge in the Gon-Msi-null females. Our studies thus reveal a novel translational regulatory mechanism to govern levels of critical reproductive hormones in the pituitary.

Control of the Anterior Pituitary Cell Lineage Regulator POU1F1 by the Stem Cell Determinant Musashi

The adipokine leptin regulates energy homeostasis through ubiquitously expressed leptin receptors. Leptin has a number of major signaling targets in the brain, including cells of the anterior pituitary (AP). We have previously reported that mice lacking leptin receptors in AP somatotropes display growth hormone (GH) deficiency, metabolic dysfunction, and adult-onset obesity. Among other targets, leptin signaling promotes increased levels of the pituitary transcription factor POU1F1, which in turn regulates the specification of somatotrope, lactotrope, and thyrotrope cell lineages within the AP. Leptin's mechanism of action on somatotropes is sex dependent, with females demonstrating posttranscriptional control of Pou1f1 messenger RNA (mRNA) translation. Here, we report that the stem cell marker and mRNA translational control protein, Musashi1, exerts repression of the Pou1f1 mRNA. In female somatotropes, Msi1 mRNA and protein levels are increased in the mouse model that lacks leptin signaling (Gh-CRE Lepr-null), coincident with lack of POU1f1 protein, despite normal levels of Pou1f1 mRNA. Single-cell RNA sequencing of pituitary cells from control female animals indicates that both Msi1 and Pou1f1 mRNAs are expressed in Gh-expressing somatotropes, and immunocytochemistry confirms that Musashi1 protein is present in the somatotrope cell population. We demonstrate that Musashi interacts directly with the Pou1f1 mRNA 3' untranslated region and exerts translational repression of a Pou1f1 mRNA translation reporter in a leptin-sensitive manner. Musashi immunoprecipitation from whole pituitary reveals coassociated Pou1f1 mRNA. These findings suggest a mechanism in which leptin stimulation is required to reverse Musashi-mediated Pou1f1 mRNA translational control to coordinate AP somatotrope function with metabolic status.

Abstract PS18-46: Inhibition of breast cancer stem cells in 2- and 3-dimensional culture by novel salinomycin analogs

Breast cancer remains one of the leading cancers among women and an estimated 90% of breast cancer deaths are due to metastasis. Cancer stem cells (CSCs) are a sub-population of cancer cells which are responsible for its initiation, progression and metastasis. CSCs are resistant to conventional chemo- and radio-therapies and therefore therapeutic strategies targeting CSCs hold great potential for novel advances in cancer treatment. Salinomycin (SAL) is a naturally occurring polyether ionophore antibiotic which was shown to effectively target breast CSCs. A library of 17 novel SAL analogs was synthesized and screened to identify compounds with improved selectivity against breast cancer stem cells. SAL analogs were either single modified esters or amides in the C1 position or double-modified C20-oxo derivatives. Eight single- and two double-modified analogs were more potent (IC50 range of 1.13 ± 0.19 to 3.93 ± 0.39 µM) towards the breast cancer cell line MDA-MB-231 compared to parent SAL (IC50 of 4.90 ± 1.60 µM). These analogs induced DNA fragmentation suggestive of apoptotic cell death. Compounds 2 (butyl ester analog of SAL) and 17 (double-modified C20-oxosalinomycin with benzhydroxamic acid) have been chosen for follow-up screening due to their improved activity and selectivity versus parent SAL. This included clonogenic assays to assess the ability of the compounds to affect cell renewal, and wound healing assays to assess cell migratory properties. In both assays, compound 17 showed superior properties over SAL. Furthermore, analog 17 showed improved targeting of breast CSCs in both cell monolayer and organoid culture as assessed in assays measuring the CD44+/CD24- stem cell sub-population. Analogs and parent compound were further studied examining (ADP-ribose) polymerase (PARP) cleavage and Bcl-2 levels by immunoblotting. All three compounds induced loss of 116 kDa PARP expression within 48h, with the highest effect induced by analog 17. In addition, treatment with compound 17 caused a decrease in Bcl-2 expression as early as 24 h. Select analogs were next screened against the NCI-60 Human Tumor Cell Line Panel. The described above double-modified analog was found to be more potent than SAL towards all 6 breast cancer cell lines in the panel as well as other tumor types. The present findings highlight the therapeutic potential of SAL analogs towards breast stem cells and support further research and clinical development of these compounds.

Funding: The present study was funded by grants (to AM and TCC) from the Arkansas Breast Cancer Research Program. Testing was performed by the Developmental Therapeutics Program, Division of Cancer Treatment and Diagnosis, National Cancer Institute, http://dtp.cancer.gov.

Citation Format: Alicja Joanna Urbaniak, Megan R. Reed, Michał Antoszczak, Michał Sulik, Adam Huczyński, Robert L. Eoff, Melanie C. MacNicol, Timothy C. Chambers, Angus M. MacNicol. Inhibition of breast cancer stem cells in 2- and 3-dimensional culture by novel salinomycin analogs [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PS18-46.

The Importance of Leptin to Reproduction

A healthy nutritional state is required for all aspects of reproduction and is signaled by the adipokine leptin. Leptin acts in a relatively narrow concentration range: too much or too little will compromise fertility. The leptin signal timing is important to prepubertal development in both sexes. In the brain, leptin acts on ventral premammillary neurons which signal kisspeptin (Kiss1) neurons to stimulate gonadotropin releasing hormone (GnRH) neurons. Suppression of Kiss1 neurons occurs when agouti-related peptide neurons are activated by reduced leptin, because leptin normally suppresses these orexigenic neurons. In the pituitary, leptin stimulates production of GnRH receptors (GnRHRs) and follicle-stimulating hormone at midcycle, by activating pathways that derepress actions of the messenger ribonucleic acid translational regulatory protein Musashi. In females, rising estrogen stimulates a rise in serum leptin, which peaks at midcycle, synchronizing with nocturnal luteinizing hormone pulses. The normal range of serum leptin levels (10-20 ng/mL) along with gonadotropins and growth factors promote ovarian granulosa and theca cell functions and oocyte maturation. In males, the prepubertal rise in leptin promotes testicular development. However, a decline in leptin levels in prepubertal boys reflects inhibition of leptin secretion by rising androgens. In adult males, leptin levels are 10% to 50% of those in females, and high leptin inhibits testicular function. The obesity epidemic has elucidated leptin resistance pathways, with too much leptin in either sex leading to infertility. Under conditions of balanced nutrition, however, the secretion of leptin is timed and regulated within a narrow level range that optimizes its trophic effects.

Sex differences in somatotrope response to fasting: biphasic responses in male mice

Anterior pituitary somatotropes are important metabolic sensors responding to leptin by secreting growth hormone (GH). However, reduced leptin signals caused by fasting have not always correlated with reduced serum GH. Reports show that fasting may stimulate or reduce GH secretion, depending on the species. Mechanisms underlying these distinct somatotrope responses to fasting remain unknown. To define the somatotrope response to decreased leptin signaling we examined markers of somatotrope function over different time periods of fasting. Male mice were fasted for 24 and 48 h, with female mice fasted for 24 h compared to fed controls ad libitum. Body weight and serum glucose were reduced in both males and females, but, unexpectedly, serum leptin was reduced only in males. Furthermore, in males, serum GH levels showed a biphasic response with significant reductions at 24 h followed by a significant rise at 48 h, which coincided with the rise in serum ghrelin levels. In contrast, females showed an increase in serum GH at 24 h. We then explored mechanisms underlying the differential somatotrope responses seen in males and observed that pituitary levels of Gh mRNA increased, with no distinction between acute and prolonged fasting. By contrast, the Ghrhr mRNA (encoding GH releasing hormone receptor) and the Ghsr mRNA (encoding the ghrelin receptor) were both greatly increased at prolonged fasting times coincident with increased serum GH. These findings show sex differences in the somatotrope and adipocyte responses to fasting and support an adaptive role for somatotropes in males in response to multiple metabolic signals.

Metabolic signalling to somatotrophs: Transcriptional and post-transcriptional mediators

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.

SAT-284 Musashi Exerts Translational Control Within Anterior Pituitary Cells of the POU1F1 Lineage

The activation of transcription factor Pou1f1 at embryonic day 13 gives rise to the pituitary populations of somatotropes, lactotropes, and thyrotropes and these populations maintain expression of Pou1f1 throughout life. The Musashi family of RNA regulatory proteins is known to regulate stem cell fate by repressing translation of target mRNAs needed for differentiation. Previously our lab has shown that female Lepr-null somatotropes have reduced POU1F1 protein levels but do not have changes in Pou1f1 mRNA expression. Stimulation with leptin increased the POU1F1 protein levels 3-fold, but did not change Pou1f1 mRNA suggesting a post-transcriptional mechanism for leptin’s regulation of Pou1f1. An in silico analysis indicated the presence a number of potential regulatory elements (MBEs) within the Pou1f1 mRNA 3’ UTR, including 8 consensus Musashi binding elements. Interestingly, we found musashi mRNA and protein levels were increased in Lepr-null somatotropes. This suggested that leptin regulates the expression of musashi in somatotrope populations and may be a candidate translational regulator of the Pou1f1 mRNA. We verified that MSI binds directly to the Pou1f1 mRNA 3’ UTR MBEs by EMSA in vitro and exerts translational repression (using reporter mRNA assays in transfected cell populations). Single cell RNA sequencing of pituitary cells from control male and female mice indicates that MSI and Pou1f1 mRNAs are co-expressed in somatotropes, lactotropes as well as thyrotropes. Immunocytochemical analyses confirmed that Musashi protein is present in mixed and purified somatotrope populations. Furthermore, immunoprecipitation with Musashi1 antibody showed a 5-fold enrichment of Pou1f1 mRNA in control female mouse pituitaries. These findings point to a critical in vivo role for Musashi-mediated mRNA translational regulation within the Pou1f1 lineage and specifically in the control of somatotrope maturation and response to metabolic cues.

SAT-292 Musashi: A Novel Regulator of the Gonadotrope Transcriptome

Sufficient nutrition is critical for reproduction. We have previously shown that leptin, a circulating indicator of fat stores, signals to pituitary gonadotropes to maintain gonadotropin releasing hormone receptor (GnRHR) protein levels in female mice. We hypothesized that this process is post-transcriptional, happening primarily through regulation of the RNA-binding protein Musashi (MSI). We showed that MSI binds to Gnrhr and inhibits translation, and a gonadotrope-specific deletion of Msi1 and Msi2 (Gon-Msi1/2-null) leads to increased GnRHR protein levels. This culminates in dysregulated luteinizing hormone (LH) and follicle-stimulating hormone (FSH). We have recently identified other gonadotrope and pituitary targets of MSI. We therefore suspected that MSI plays a role in both the maturation of gonadotropes and the normal cyclic regulation of gonadotropes. We hypothesized that the deletion of MSI would lead to downstream effects on (1) the composition of the gonadotrope population and (2) the molecular landscape of these cells. Using our adult, diestrous Gon-Msi1/2-null females, we performed single-cell RNA-sequencing on methanol-fixed dispersed pituitary cells. Libraries were made from two control pools and two mutant pools (n=3 pituitaries/pool) using 10x Genomics v3.1 Single-Cell Gene Expression technology and initially sequenced on an Illumina Next-seq mid-output flow-cell, yielding 5,000 reads/cell. Subsequent high-output sequencing obtained 25,000 reads/cell. We recovered single-cell mRNA transcript information from 18,206 control pituitary cells and 16,255 Gon-Msi1/2-null cells. Our analyses revealed that the Gon-Msi1/2-null pools had a higher % of cells expressing Fshb, as well as an expected significant drop in Msi2-expressing gonadotropes and no change in Lhb-expressing cells. We have recently identified Fshb as an MSI target in silico, and qRT-PCR of female pituitary lysate immunoprecipitated with anti-MSI1 shows a 7-fold enrichment in Fshb mRNA. We identified differentially expressed genes comparing the control and Gon-Msi1/2-null gonadotrope clusters. Using Gene Ontology analyses, the Gon-Msi1/2-null gonadotrope cluster appears to have aberrant expression of mRNAs involved in protein folding and cellular responses to nutrients. Our high-output sequencing has allowed us to achieve 25,000 reads/cell and will provide greater resolution of the role of Musashi in control of gonadotrope function. Taken together, our data indicate that Musashi influences the molecular landscape and subsequent physiology of the female gonadotrope. We have identified potential gonadotrope-specific MSI targets, including pathways that may underlie the dysregulated gonadotropin production and secretion seen in our Gon-Msi1/2-null females. Future studies will compare pubertal and adult females, as well as females from different estrous cycle stages.

Molecular Mechanisms of Pituitary Cell Plasticity

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.

Musashi interaction with poly(A)-binding protein is required for activation of target mRNA translation

The Musashi family of mRNA translational regulators controls both physiological and pathological stem cell self-renewal primarily by repressing target mRNAs that promote differentiation. In response to differentiation cues, Musashi can switch from a repressor to an activator of target mRNA translation. However, the molecular events that distinguish Musashi-mediated translational activation from repression are not understood. We have previously reported that Musashi function is required for the maturation of Xenopus oocytes and specifically for translational activation of specific dormant maternal mRNAs. Here, we employed MS to identify cellular factors necessary for Musashi-dependent mRNA translational activation. We report that Musashi1 needs to associate with the embryonic poly(A)-binding protein (ePABP) or the canonical somatic cell poly(A)-binding protein PABPC1 for activation of Musashi target mRNA translation. Co-immunoprecipitation studies demonstrated an increased Musashi1 interaction with ePABP during oocyte maturation. Attenuation of endogenous ePABP activity severely compromised Musashi function, preventing downstream signaling and blocking oocyte maturation. Ectopic expression of either ePABP or PABPC1 restored Musashi-dependent mRNA translational activation and maturation of ePABP-attenuated oocytes. Consistent with these Xenopus findings, PABPC1 remained associated with Musashi under conditions of Musashi target mRNA de-repression and translation during mammalian stem cell differentiation. Because association of Musashi1 with poly(A)-binding proteins has previously been implicated only in repression of Musashi target mRNAs, our findings reveal novel context-dependent roles for the interaction of Musashi with poly(A)-binding protein family members in response to extracellular cues that control cell fate.

Association of Gnrhr mRNA With the Stem Cell Determinant Musashi: A Mechanism for Leptin-Mediated Modulation of GnRHR Expression

The cyclic expression of pituitary gonadotropin-releasing hormone receptors (GnRHRs) may be an important checkpoint for leptin regulatory signals. Gonadotrope Lepr-null mice have reduced GnRHR levels, suggesting these receptors may be leptin targets. To determine if leptin stimulated GnRHR directly, primary pituitary cultures or pieces were exposed to 1 to 100 nM leptin. Leptin increased GnRHR protein levels and the percentages of gonadotropes that bound biotinylated analogs of gonadotropin-releasing hormone (bio-GnRH) but had no effect on Gnrhr messenger RNA (mRNA). An in silico analysis revealed three consensus Musashi (MSI) binding elements (MBEs) for this translational control protein in the 3' untranslated region (UTR) of Gnrhr mRNA. Several experiments determined that these Gnrhr mRNA MBE were active: (1) RNA electrophoretic mobility shift assay analyses showed that MSI1 specifically bound Gnrhr mRNA 3'-UTR; (2) RNA immunoprecipitation of pituitary fractions with MSI1 antibody pulled down a complex enriched in endogenous MSI protein and endogenous Gnrhr mRNA; and (3) fluorescence reporter assays showed that MSI1 repressed translation of the reporter coupled to the Gnrhr 3'-UTR. In vitro, leptin stimulation of pituitary pieces reduced Msi1 mRNA in female pituitaries, and leptin stimulation of pituitary cultures reduced MSI1 proteins selectively in gonadotropes identified by binding to bio-GnRH. These findings show that leptin's direct stimulatory actions on gonadotrope GnRHR correlate with a direct inhibition of expression of the posttranscriptional regulator MSI1. We also show MSI1 interaction with the 3'-UTR of Gnrhr mRNA. These findings now open the door to future studies of leptin-modulated posttranscriptional pathways.

Leptin Regulation of Gonadotrope Gonadotropin-Releasing Hormone Receptors As a Metabolic Checkpoint and Gateway to Reproductive Competence

The adipokine leptin signals the body's nutritional status to the brain, and particularly, the hypothalamus. However, leptin receptors (LEPRs) can be found all throughout the body and brain, including the pituitary. It is known that leptin is permissive for reproduction, and mice that cannot produce leptin (Lep/Lep) are infertile. Many studies have pinpointed leptin's regulation of reproduction to the hypothalamus. However, LEPRs exist at all levels of the hypothalamic-pituitary-gonadal axis. We have previously shown that deleting the signaling portion of the LEPR specifically in gonadotropes impairs fertility in female mice. Our recent studies have targeted this regulation to the control of gonadotropin releasing hormone receptor (GnRHR) expression. The hypotheses presented here are twofold: (1) cyclic regulation of pituitary GnRHR levels sets up a target metabolic checkpoint for control of the reproductive axis and (2) multiple checkpoints are required for the metabolic signaling that regulates the reproductive axis. Here, we emphasize and explore the relationship between the hypothalamus and the pituitary with regard to the regulation of GnRHR. The original data we present strengthen these hypotheses and build on our previous studies. We show that we can cause infertility in 70% of female mice by deleting all isoforms of LEPR specifically in gonadotropes. Our findings implicate activin subunit (InhBa) mRNA as a potential leptin target in gonadotropes. We further show gonadotrope-specific upregulation of GnRHR protein (but not mRNA levels) following leptin stimulation. In order to try and understand this post-transcriptional regulation, we tested candidate miRNAs (identified with in silico analysis) that may be binding the Gnrhr mRNA. We show significant upregulation of one of these miRNAs in our gonadotrope-Lepr-null females. The evidence provided here, combined with our previous work, lay the foundation for metabolically regulated post-transcriptional control of the gonadotrope. We discuss possible mechanisms, including miRNA regulation and the involvement of the RNA binding protein, Musashi. We also demonstrate how this regulation may be vital for the dynamic remodeling of gonadotropes in the cycling female. Finally, we propose that the leptin receptivity of both the hypothalamus and the pituitary are vital for the body's ability to delay or slow reproduction during periods of low nutrition.

Evasion of regulatory phosphorylation by an alternatively spliced isoform of Musashi2

The Musashi family of RNA binding proteins act to promote stem cell self-renewal and oppose cell differentiation predominantly through translational repression of mRNAs encoding pro-differentiation factors and inhibitors of cell cycle progression. During tissue development and repair however, Musashi repressor function must be dynamically regulated to allow cell cycle exit and differentiation. The mechanism by which Musashi repressor function is attenuated has not been fully established. Our prior work indicated that the Musashi1 isoform undergoes site-specific regulatory phosphorylation. Here, we demonstrate that the canonical Musashi2 isoform is subject to similar regulated site-specific phosphorylation, converting Musashi2 from a repressor to an activator of target mRNA translation. We have also characterized a novel alternatively spliced, truncated isoform of human Musashi2 (variant 2) that lacks the sites of regulatory phosphorylation and fails to promote translation of target mRNAs. Consistent with a role in opposing cell cycle exit and differentiation, upregulation of Musashi2 variant 2 was observed in a number of cancers and overexpression of the Musashi2 variant 2 isoform promoted cell transformation. These findings indicate that alternately spliced isoforms of the Musashi protein family possess distinct functional and regulatory properties and suggest that differential expression of Musashi isoforms may influence cell fate decisions.

Functional Integration of mRNA Translational Control Programs

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.

Neural stem and progenitor cell fate transition requires regulation of Musashi1 function

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, p21^WAF1/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.

Ringo/cyclin-dependent kinase and mitogen-activated protein kinase signaling pathways regulate the activity of the cell fate determinant Musashi to promote cell cycle re-entry in Xenopus oocytes

Cell cycle re-entry during vertebrate oocyte maturation is mediated through translational activation of select target mRNAs, culminating in the activation of mitogen-activated protein kinase and cyclin B/cyclin-dependent kinase (CDK) signaling. The temporal order of targeted mRNA translation is crucial for cell cycle progression and is determined by the timing of activation of distinct mRNA-binding proteins. We have previously shown in oocytes from Xenopus laevis that the mRNA-binding protein Musashi targets translational activation of early class mRNAs including the mRNA encoding the Mos proto-oncogene. However, the molecular mechanism by which Musashi function is activated is unknown. We report here that activation of Musashi1 is mediated by Ringo/CDK signaling, revealing a novel role for early Ringo/CDK function. Interestingly, Musashi1 activation is subsequently sustained through mitogen-activated protein kinase signaling, the downstream effector of Mos mRNA translation, thus establishing a positive feedback loop to amplify Musashi function. The identified regulatory sites are present in mammalian Musashi proteins, and our data suggest that phosphorylation may represent an evolutionarily conserved mechanism to control Musashi-dependent target mRNA translation.

Context-dependent regulation of Musashi-mediated mRNA translation and cell cycle regulation

Musashi-mediated mRNA translational control has been implicated in the promotion of physiological and pathological stem cell proliferation. During self-renewal of mammalian stem cells, Musashi has been proposed to act to repress the translation of mRNAs encoding inhibitors of cell cycle progression. By contrast, in maturing Xenopus oocytes Musashi activates translation of target mRNAs that encode proteins promoting cell cycle progression. The mechanisms directing Musashi to differentially control mRNA translation in mammalian stem cells and Xenopus oocytes is unknown. In this study, we demonstrate that the mechanisms defining Musashi function lie within the cellular context. Specifically, we show that murine Musashi acts as an activator of translation in maturing Xenopus oocytes while Xenopus Musashi functions as a repressor of target mRNA translation in mammalian cells. We further demonstrate that within the context of a primary mammalian neural stem/progenitor cell, Musashi can be converted from a repressor of mRNA translation to an activator of translation in response to extracellular stimuli. We present current models of Musashi-mediated mRNA translational control and discuss possible mechanisms for regulating Musashi function. An understanding of these mechanisms presents exciting possibilities for development of therapeutic targets to control physiological and pathological stem cell proliferation.

Developmental timing of mRNA translation--integration of distinct regulatory elements

Targeted mRNA translation is emerging as a critical mechanism to control gene expression during developmental processes. Exciting new findings have revealed a critical role for regulatory elements within the mRNA untranslated regions to direct the timing of mRNA translation. Regulatory elements can be targeted by sequence-specific binding proteins to direct either repression or activation of mRNA translation in response to developmental signals. As new regulatory elements continue to be identified it has become clear that targeted mRNAs can contain multiple regulatory elements, directing apparently contradictory translational patterns. How is this complex regulatory input integrated? In this review, we focus on a new challenge area-how sequence-specific RNA binding proteins respond to developmental signals and functionally integrate to regulate the extent and timing of target mRNA translation. We discuss current understanding with a particular emphasis on the control of cell cycle progression that is mediated through a complex interplay of distinct mRNA regulatory elements during Xenopus oocyte maturation.

Enforcing temporal control of maternal mRNA translation during oocyte cell-cycle progression

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.

Mos 3' UTR regulatory differences underlie species-specific temporal patterns of Mos mRNA cytoplasmic polyadenylation and translational recruitment during oocyte maturation

The Mos proto-oncogene is a critical regulator of vertebrate oocyte maturation. The maturation-dependent translation of Mos protein correlates with the cytoplasmic polyadenylation of the maternal Mos mRNA. However, the precise temporal requirements for Mos protein function differ between oocytes of model mammalian species and oocytes of the frog Xenopus laevis. Despite the advances in model organisms, it is not known if the translation of the human Mos mRNA is also regulated by cytoplasmic polyadenylation or what regulatory elements may be involved. We report that the human Mos 3' untranslated region (3' UTR) contains a functional cytoplasmic polyadenylation element (CPE) and demonstrate that the endogenous Mos mRNA undergoes maturation-dependent cytoplasmic polyadenylation in human oocytes. The human Mos 3' UTR interacts with the human CPE-binding protein and exerts translational control on a reporter mRNA in the heterologous Xenopus oocyte system. Unlike the Xenopus Mos mRNA, which is translationally activated by an early acting Musashi/polyadenylation response element (PRE)-directed control mechanism, the translational activation of the human Mos 3' UTR is dependent on a late acting CPE-dependent process. Taken together, our findings suggest a fundamental difference in the 3' UTR regulatory mechanisms controlling the temporal induction of maternal Mos mRNA polyadenylation and translational activation during Xenopus and mammalian oocyte maturation.

A novel mRNA 3' untranslated region translational control sequence regulates Xenopus Wee1 mRNA translation

Cell cycle progression during oocyte maturation requires the strict temporal regulation of maternal mRNA translation. The intrinsic basis of this temporal control has not been fully elucidated but appears to involve distinct mRNA 3' UTR regulatory elements. In this study, we identify a novel translational control sequence (TCS) that exerts repression of target mRNAs in immature oocytes of the frog, Xenopus laevis, and can direct early cytoplasmic polyadenylation and translational activation during oocyte maturation. The TCS is functionally distinct from the previously characterized Musashi/polyadenylation response element (PRE) and the cytoplasmic polyadenylation element (CPE). We report that TCS elements exert translational repression in both the Wee1 mRNA 3' UTR and the pericentriolar material-1 (Pcm-1) mRNA 3' UTR in immature oocytes. During oocyte maturation, TCS function directs the early translational activation of the Pcm-1 mRNA. By contrast, we demonstrate that CPE sequences flanking the TCS elements in the Wee1 3' UTR suppress the ability of the TCS to direct early translational activation. Our results indicate that a functional hierarchy exists between these distinct 3' UTR regulatory elements to control the timing of maternal mRNA translational activation during oocyte maturation.

Early expression of p107 is associated with 3T3-L1 adipocyte differentiation

In response to hormonal stimulation quiescent 3T3-L1 preadipocyte cells reenter the cell cycle and undergo a mitotic expansion phase prior to terminal differentiation. The cell cycle regulatory proteins p130 and p107 undergo dramatic changes in protein levels within 24 h of differentiation. The role of these proteins in regulating adipocyte mitotic clonal expansion and/or differentiation are unclear. It has recently been demonstrated that adipocyte proliferation can be uncoupled from adipocyte differentiation through the use of the pharmacological MEK inhibitor PD98059 or the tyrosine phosphatase inhibitor, sodium vanadate. We examined the expression of p130 and p107 in stimulated 3T3-L1 cells in the presence of either PD98059, U0126 or sodium vanadate. While inhibition of MEK blocked proliferation, the cells underwent differentiation normally. In contrast, vanadate blocked differentiation without affecting proliferation. Inhibition of MEK did not affect the increase in p107 expression in stimulated cells indicating that induction of p107 is independent of MAP kinase signaling. Vanadate treatment caused a significant delay in p107 expression in the first 24 h following stimulation. Under these conditions, p130 expression was relatively unchanged. Our results indicate that a rapid increase in p107 expression correlates with a commitment to undergo adipocyte differentiation. The data further suggest that the rapid induction of p107 is not required for cellular proliferation during the mitotic clonal expansion phase. Taken together, these findings provide correlative data that implicate p107 in the terminal differentiation, but not proliferation, of quiescent preadipocytes following hormonal stimulation.

A novel regulatory element determines the timing of Mos mRNA translation during Xenopus oocyte maturation

Progression through vertebrate oocyte maturation requires that pre-existing, maternally derived mRNAs be translated in a strict temporal order. The mechanism that controls the timing of oocyte mRNA translation is unknown. In this study we show that the early translational induction of the mRNA encoding the Mos proto-oncogene is mediated through a novel regulatory element within the 3' untranslated region of the Mos mRNA. This novel element is responsive to the MAP kinase signaling pathway and is distinct from the late acting, cdc2-responsive, cytoplasmic polyadenylation element. Our findings suggest that the timing of maternal mRNA translation is controlled through signal transduction pathways targeting distinct 3' UTR mRNA elements.

Disruption of the 14-3-3 binding site within the B-Raf kinase domain uncouples catalytic activity from PC12 cell differentiation

A number of Raf-associated proteins have recently been identified, including members of the 14-3-3 family of phosphoserine-binding proteins. Although both positive and negative regulatory functions have been ascribed for 14-3-3 interactions with Raf-1, the mechanisms by which 14-3-3 binding modulates Raf activity have not been fully established. We report that mutational disruption of 14-3-3 binding to the B-Raf catalytic domain inhibits B-Raf biological activity. Expression of the isolated B-Raf catalytic domain (B-Rafcat) induces PC12 cell differentiation in the absence of nerve growth factor. By contrast, the B-Rafcat 14-3-3 binding mutant, B-Rafcat S728A, was severely compromised for the induction of PC12 cell differentiation. Interestingly, the B-Rafcat 14-3-3 binding mutant retained significant in vitro catalytic activity. In Xenopus oocytes, the analogous full-length B-Raf 14-3-3 binding mutant blocked progesterone-stimulated maturation and the activation of endogenous mitogen-activated protein kinase kinase and mitogen-activated protein kinase. Similarly, the full-length B-Raf 14-3-3 binding mutant inhibited nerve growth factor-stimulated PC12 cell differentiation. We conclude that 14-3-3 interaction with the catalytic domain is not required for kinase activity per se but is essential to couple B-Raf catalytic activity to downstream effector activation.

Nerve growth factor-stimulated B-Raf catalytic activity is refractory to inhibition by cAMP-dependent protein kinase

The cAMP-dependent protein kinase (PKA) exhibits both inhibitory and stimulatory effects upon growth factor signaling mediated by the mitogen-activated protein kinase signaling pathway. PKA has been demonstrated to inhibit Raf-1-mediated cellular proliferation. PKA can both prevent Ras-dependent Raf-1 activation and directly inhibit Raf-1 catalytic activity. In contrast to the inhibitory effect of PKA on Raf-1-dependent processes, PKA potentiates nerve growth factor-stimulated PC12 cell differentiation, a B-Raf mediated process. This potentiation, rather than inhibition, of PC12 cell differentiation is curious in light of the ability of PKA to inhibit Raf-1 catalytic activity. The kinase domains of Raf-1 and B-Raf are highly conserved, and it has been predicted that B-Raf catalytic activity would also be inhibited by PKA. In this study we examined the ability of PKA to regulate the kinase activity of the B-raf proto-oncogene. We report that nerve growth factor-stimulated B-Raf activity is not inhibited by PKA. By contrast, an N-terminally truncated, constitutively active form of B-Raf is inhibited by PKA both in vitro and in transfected PC12 cells. These results suggest that the N-terminal regulatory domain interferes with the ability of PKA to modulate B-Raf catalytic activity and provide an explanation for the observed resistance of B-Raf-dependent processes to PKA inhibition.

pXen, a utility vector for the expression of GST-fusion proteins in Xenopus laevis oocytes and embryos

We describe a plasmid, pXen, designed for the optimized expression of proteins fused to glutathione-S-transferase (GST) in Xenopus laevis oocytes and embryos. The Xenopus model system permits the biochemical analysis of signaling pathways and analysis of embryo phenotype in response to manipulation of proto-oncogene expression. pXen is a modified pSP64T vector which contains an SP6 RNA polymerase promoter followed by the translational initiation sequence of Xenopus beta-globin and the glutathione binding domain of GST. The Xenopus 3' beta-globin untranslated region and polyadenylation site immediately follow the multiple cloning site to permit the efficient translation of in vitro transcribed RNA in oocytes and embryos. The utility of pXen is demonstrated by cloning the catalytic domain of the serine/threonine kinase proto-oncogene Raf-1 into this vector and injecting the corresponding in vitro transcribed RNA into oocytes. Catalytically active GST-vRaf fusion protein was expressed in the injected oocytes and induced oocyte maturation. Moreover, the GST-vRaf fusion protein could be readily purified from Xenopus extracts using glutathione Sepharose. We demonstrate that the Raf-1 catalytic domain retains activity when fused with the N-terminal GST moiety and is subject to negative regulation by the cyclic AMP-dependent protein kinase (PKA). The pXen vector will be useful for an in vivo analysis of the physiological role and regulation of a wide variety of signaling molecules when expressed in Xenopus oocytes and embryos.

Regulation of Raf-1-dependent signaling during early Xenopus development

The Raf-1 gene product is activated in response to cellular stimulation by a variety of growth factors and hormones. Raf-1 activity has been implicated in both cellular differentiation and proliferation. We have examined the regulation of the Raf-1/MEK/MAP kinase (MAPK) pathway during embryonic development in the frog Xenopus laevis. We report that Raf-1, MEK, and MAPK activities are turned off following fertilization and remain undetectable up until blastula stages (stage 8), some 4 h later. Tight regulation of the Raf-1/MEK/MAPK pathway following fertilization is crucial for embryonic cell cycle progression. Inappropriate reactivation of MAPK activity by microinjection of oncogenic Raf-1 RNA results in metaphase cell cycle arrest and, consequently, embryonic lethality. Our findings demonstrate an absolute requirement, in vivo, for inactivation of the MAPK signaling pathway to allow normal cell cycle progression during the period of synchronous cell divisions which occur following fertilization. Further, we show that cytostatic factor effects are mediated through MEK and MAPK.