RF25 | PMON50 Insight into Integrated Control of Pituitary Function Revealed Through Analysis of Musashi Target mRNAs

The anterior pituitary controls growth, metabolism, reproduction and stress responses through the synthesis and secretion of specific hormones. Distinct cell lineages within the anterior pituitary generate specific hormones. Interestingly, the anterior pituitary exhibits a high level of adult cell plasticity that allows committed hormone-producing cells to alter their cell fate and produce different hormones to meet altered organismal demands. The molecular mechanisms underlying this adult cell plasticity have not been fully characterized. Our recent work has implicated the stem cell determinant and sequence-specific mRNA binding protein, Musashi, as a regulator of pituitary cell plasticity. The mechanisms by which Musashi controls pituitary cell plasticity are unknown. In this study, we have sought to identify the mRNA regulatory targets of Musashi in the adult mouse pituitary. Through the use of Musashi-RNA-immunoprecipitation we report a cohort of 1192 pituitary mRNAs that specifically interact with Musashi. mRNAs specific to each of the hormone-producing cell lineages were identified, as well as mRNAs specific to immature stem and progenitor cell lineages. The biological pathways encoded by the Musashi-enriched mRNAs include cell adhesion, cellular homeostasis, unfolded protein responses, protein trafficking, endocrine processes and female pregnancy. Functional analysis of validated mRNAs reveals both activating and inhibitory control of mRNA translation by Musashi. Together, our findings indicate a broad and complex role for Musashi in the post-transcriptional regulation of adult pituitary function.

Presentation: Monday, June 13, 2022 12:30 p.m. - 2:30 p.m., Monday, June 13, 2022 12:58 p.m. - 1:03 p.m.

OR08-2 Functional Association of the Stem Cell Protein Musashi With LSM14B in Control of mRNA Translation

The Musashi RNA-binding protein functions as a gatekeeper of cell maturation and maintains stem cell plasticity by regulating the translation of target mRNAs. The adult anterior pituitary tissue expresses a high level of Musashi and also demonstatres a high level of cell plasticity, indicating a Musashi-dependent function in maintaining endocrine homeostasis in the anterior pituitary. Towards understanding the mechanism(s) by which Musashi functions to control cell plasticity, we have identified co-associated proteins necessary for Musashi function. The two Musashi isoforms (Musashi1 and Musashi2) are conserved across species and are structurally characterized by two N-terminal RNA-recognition motifs (RRMs, that associate with specific sequences in the 3' untranslated region of mRNAs targets) and a disordered C-terminus. Musashi uses these domains to selectively repress, or to activate, mRNA translation in a manner specific to target mRNA and cell context. Both Musashi isoforms contain two C-terminal regulatory serine residues that require phosphorylation to enable mRNA translational activation. However, the Musashi proteins do not have inherent mRNA translation function and utilize co-associated protein complexes to mediate mRNA translation. We have previously identified Musashi1 binding protein partners through mass spectrometry. Here, we have identified the protein interactions that are required for Musashi1-dependent translational activation. Knockdown of target protein levels in Xenopus laevis oocytes indicated that the PABP4, CELF2, LSM14A/B, ELAVL1, ELAVL2, ELAVL4, and PUM1 proteins are required for Musashi target mRNA translational activation. We further determined that the effect of LSM14A/B dual knockdown was in fact specific to LSM14B alone. LSM14B is an RNA-binding protein that is required for RNA granule formation, RNA transcript metabolism, and mRNA translation. Studies in mouse oocytes have shown that LSM14B is required to maintain transcript levels of the Musashi1 target Cyclin B1 (Ccnb1) mRNA prior to mRNA translation. In the adult murine pituitary, LSM14B is ubiquitously expressed in both hormone-producing cells and in stem cells as indicated through single cell RNA sequencing. These findings indicate that LSM14B and Musashi1 may function cooperatively to regulate pituitary hormone production and cell differentiation in response to changes in physiological demand.

Presentation: Saturday, June 11, 2022 11:45 a.m. - 12:00 p.m.

PMON29 A Comparative Analysis of Musashi-Dependent Control of Mouse and Human Pituitary mRNA Translation

Specialized cell lineages within the anterior pituitary produce specific hormones which, in turn, regulate diverse biological processes ranging from growth, metabolism, and reproduction to stress. Interestingly, these specialized cell lineages exhibit a high degree of cellular plasticity, readily altering their differentiation state to produce different hormones in response to organismal demand. Underlying molecular mechanisms controlling cellular plasticity in the pituitary are not fully defined but appear to involve post-transcriptional regulation exerted by the Musashi family of sequence-specific RNA binding proteins. Musashi is a bifunctional regulator of target mRNAs and can exert either repression or activation of translation in a context-dependent manner. Our recent work has implicated a role for Musashi-dependent repression of key pituitary mRNAs in the mouse, including a lineage-specific transcription factor (Pou1f1), the gonadotropin releasing hormone receptor (Gnrhr), and two anterior pituitary hormones (Prl and Tshb). In this study, we look to determine the potential relevance of Musashi-dependent mRNA translational control to the human pituitary. Reflecting our findings in the adult mouse, a re-analysis of published human fetal pituitary single cell RNA sequencing data revealed Musashi1 (MSI1) and Musashi2 (MSI2) expression in both stem/progenitor cell populations as well as in all hormone-producing cell lineages of the anterior pituitary. We further report that the human homologs of identified Musashi-target mRNAs in the mouse pituitary also possess Musashi binding elements (MBEs) in their 3' untranslated regions (UTRs). In the case of the human Pou1f1 3' UTR, we were able to verify Musashi-dependent mRNA translational repression in reporter assays as seen with the murine Pou1f1 3' UTR. Interestingly, a germline Pou1f1 mutation identified in human patients, disrupts a consensus MBE in the human Pou1f1 3' UTR and abolishes Musashi-dependent repression. We developed a bioinformatic pipeline to characterize pathogenic human mutations catalogued in the Geno2MP database for disruption or creation of consensus MBEs in pituitary-specific mRNA 3' UTRs. Our findings suggest an evolutionarily conserved role for Musashi in the post-transcriptional regulation of pituitary mRNAs in both mice and humans and identify several human mutations which may perturb Musashi regulation of human pituitary function.

Presentation: Monday, June 13, 2022 12:30 p.m. - 2:30 p.m.

OR24-3 Persistence of Progenitor Cell Markers Following the Selective Ablation of Musashi in Somatotropes

Metabolic and reproductive demands are met and coordinated through the complex control of hormone synthesis and secretion exerted by the anterior pituitary. While pituitary cells are known to possess remarkable plasticity to change their cell fate and alter hormone production in response to ever changing environmental cues, the underlying molecular control of this plasticity has not been established. Our lab has previously introduced the Musashi (MSI) family of RNA binding proteins as important players in control of pituitary hormone levels. Musashi typically governs stem cell fate and promotes self-renewal by repressing translation of target mRNAs needed for differentiation. However, we found that MSI1 is expressed in most differentiated cells of the adult pituitary and can specifically bind to the 3’ UTRs of Prl, Tsh, and Pou1f1 and exert translational control in reporter assays. Confirmation of the requirement for MSI was demonstrated through in vivo analyses where MSI1 and MSI2 were selectively ablated in somatotropes. The mutant animals were subfertile. The mutant males showed reduced serum and pituitary content of LH and FSH, and significant decreases in serum GH and PRL despite 2-fold increases in pituitary protein content of PRL and GH. To further assess the role and downstream pathways regulated by MSI in the pituitary, we collected somatotrope MSI-null pituitaries from males and females for qPCR analysis of common somatotrope target genes. In correlation with our previous findings described above (low serum GH and high pituitary GH content), we found that GHRHR mRNA levels were reduced by 2-fold in male mutants. RNA-seq analysis followed by qPCR validation shows that Prop1 mRNA was significantly increased in male mutants, with no significant change in Pou1f1 mRNA. These data suggest that MSI may be involved in the regulation of progenitor cells giving rise to the somatotrope lineage and that MSI ablation may have caused retention of these progenitors and/or a failure to fully differentiate somatotropes. Our studies of MSI-null somatotrope function support this interpretation because the mutant somatotropes clearly stored GH proteins, but could not secrete them, as demonstrated by the low serum GH values, and the 50% reduction in GHRHR mRNA. RNA-seq evaluation of males revealed a change in the expression of 720 genes between controls and mutants (FDR <0.05). When we examined female MSI-null somatotrope animals, a much smaller cohort of genes showed a change in expression (153 genes, FDR <0.05). Interestingly, 38 genes were altered in both mutant males and females suggesting shared regulation by MSI. Further characterization of the NGS datasets will elucidate additional downstream targets and effector pathways of MSI-dependent control of anterior pituitary cell differentiation and function.

SAT-406 Deletion of Musashi in Gonadotropes Leads to Increased GnRHR Protein Levels and Gonadotrope Dysfunction

Leptin is a critical mediator of metabolic regulation of the hypothalamic-pituitary gonadal (HPG) axis. We have previously shown that leptin is responsible for the optimal expression of GnRHR, a rate-limiting component of the reproductive process. Female mice lacking leptin receptors (Lepr-null) specifically on gonadotropes are sub-fertile. Their reduced GnRHR proteins and normal Gnrhr mRNA levels suggest leptin’s actions on gonadotropes are post-transcriptional. A clue about a candidate translational regulator is seen in our studies showing that Lepr-null gonadotropes have increased expression of Musashi (MSI), which has binding sites within the Gnrhr 3’ UTR. Furthermore, leptin reduced MSI expression specifically in gonadotropes and increased GnRHR expression. We hypothesized that MSI may repress Gnrhr mRNA translation and that leptin alleviates this repression. To determine the effects of MSI on the HPG axis, we developed a gonadotrope-Msi1/2-null mouse line and compared mutant females (MUT) to their littermate controls (CTL) on the morning of diestrus, when the pituitary GnRHR protein levels should reach a peak. The levels of GnRHR proteins (measured by EIA) are significantly increased (1.6X) in the pituitary of the mutant females (CTL: 1.355 ± 0.11 ng/ml vs MUT: 2.202 ± 0.16 ng/ml, p=.0006), with no change in mRNA. The gonadotrope-Msi1/2-null females are subfertile, with litter sizes of 3 ± 0.4 pups, with the first litter at around day 40 and an average of 41 day delay between litters. To understand the downstream effect of the MSI knockout on gonadotropin levels, we measured pituitary and serum LH and FSH protein levels (Luminex EIA) and mRNA (qPCR). Serum FSH levels are decreased by >50% in mutant females (CTL: 0.947 ± 0.14 ng/mL vs MUT: 0.406 ± 0.05 ng/ml, p=.0049), but FSH stores and Fsh mRNAs are unchanged. Additionally, we observed a 2.2X increase in the pituitary LH protein content in mutant females (CTL: 0.328 ± 0.11 ng/ml vs MUT: 0.729 ± 0.09 ng/ml, p=.0174), but no changes in serum LH or Lh mRNA levels. These studies thus show that, as a repressor of GnRHR translation, Musashi can also regulate expression of gonadotropins. In the gonadotrope Lepr-null model, we hypothesized that leptin signals were needed to de-repress Musashi actions and allow GnRHR translation. In contrast, the gonadotrope-MSI-null mice over-express GnRHR, which confirms this role for MSI as a GnRHR regulator. We propose that the increased expression of GnRHR in the gonadotrope MSI-null animals causes the increased LH content, and disrupted FSH secretion, resulting in a higher serum LH:FSH ratio and subfertility. Collectively these data suggest that MSI regulation is necessary for optimal fertility in the adult female mouse. Future studies will determine the impact of loss of MSI on gonadotrope function throughout the estrous cycle.

SAT-417 The Gonadotrope Leptin Signal Is Critical for the Early-Morning Estrus Rise in FSH in Female Mice

The importance of peripheral nutritional signals to the functioning of the reproductive axis is appreciated, but the mechanisms are not well understood. The three tiers of the hypothalamic-pituitary-gonadal axis are receptive to many of these nutritional signals, including leptin. We have previously shown that leptin communicates with pituitary gonadotropes to maintain numbers of gonadotropin-releasing-hormone receptors (GnRHrs), and that loss of the leptin signal (LEPR) in gonadotropes results in subfertility in females. Using our current model of gonadotrope leptin resistance (GnRHR-cre, floxed Lepr exon 1) which targets the deletion of all LEPRs specifically to gonadotropes expressing Gnrhr, we investigated the role of leptin in the follicle-stimulating hormone (FSH) surge in female mice. Pituitaries and serum from 2-3 month-old control (CTL) and gonadotrope-Lepr-null (MUT) females were collected at midnight between proestrus and estrus, and at 0400 and 0900 on the morning of estrus. The samples were assayed for LH and FSH, and pituitaries were also used to determine GnRHr levels (ELISA) and Lh, Fsh, and Gnrhr mRNA levels (qPCR). Our gonadotrope-Lepr-null females showed a dramatic decrease in serum FSH during the peak of FSH secretion (0400 estrus, CTL: 25.8 ng/mL ± 5.7, n=6, MUT: 6.9 ng/mL ± 0.8, n=5, p<0.03). This result is significant, despite the large natural variability in gonadotropin surges. MUT FSH secretion also trended down at midnight by about 40%, although this effect was not significant. Interestingly, stores of FSH at 0400 were significantly increased in MUT pituitaries compared to controls. Control FSH stores dropped by ~74% from midnight to 0400, whereas mutant stores only dropped about 14% in the same time period, suggesting that the mutants have insufficient FSH secretion beginning at midnight. Analyses of ovarian sections from diestrous or proestrous MUT females showed a significant 38-43% decline in the average number of antral follicles (ANOVA, p<0.05 n=7-8), which correlates with the low FSH secretion. We also discovered a major drop in Fsh mRNA in our MUT pituitaries at 0400 (CTL RQ: 0.68 ± 0.14, n=5; MUT RQ: 0.10 ± 0.04, n=6, p<0.02). MUT Fsh mRNA remained low at 0900 (NS), at about 40% of control mRNA. Importantly, despite finding GnRHR proteins decreased at other stages in the cycle, no differences were seen between controls and mutants with regards to GnRHR (protein or mRNA) during these specific FSH surge time points. Our study indicates that leptin signals to the reproductive axis through control of FSH secretion specifically during the time of the FSH surge in female mice. We hypothesize that leptin may also be able to regulate Fsh transcription. Future studies will test the ability of leptin to stimulate Fsh transcription and secretion from normal, early AM estrous pituitaries and will investigate the pathways that mediate these effects.

MON-482 Sex-Dependent Biphasic Response by Somatotropes to Fasting and Loss of Leptin Signaling

Fasting results in a well-established rise in serum growth hormone (GH) in mice and humans. In mice, this coincides with increases in Gh and growth hormone releasing hormone receptor (Ghrhr) mRNA, as well as serum ghrelin. However, fasting also results in a dramatic decline in serum leptin and we have shown that pituitary somatotropes are dependent on circulating leptin to maintain sufficient stores of GH and GHRHR proteins. This study was designed to determine the mechanisms by which GH increases in the presence of reduced serum leptin in fasted mice. We hypothesized that the rise in GH was due to ghrelin-stimulated pathways and therefore developed a fasting model to test this. However, a review of the literature revealed a study by Steyn et al. (2011) that demonstrated an acute decline in GH pulses in male mice after 12-18 hours of fasting, which suggest that somatotropes may display a biphasic response to fasting, with the early or acute phase driven by the absence of leptin signals. We tested this hypothesis in 3 groups of male and female mice: group 1 was fasted for 24 hours; group 2 was also fasted for 24 hours and received 10% glucose water, which is known to normalize serum leptin; and group 3 mice were fed ad libitum. Following a 24 h fast, leptin levels declined significantly from 5.5±2 ng/ml to 0.37±0.1 ng/ml (Student’s t; p=0.01) along with significant decreases in body weight, serum glucose and insulin. Fasted mice receiving glucose water showed a significant recovery in leptin to 6.9±2.7 ng/ml (Student’s t; p=0.03 n=8) along with a recovery in serum glucose and insulin. In agreement with Steyn et al. (2011), serum GH in male mice showed a significant decline from 10.7±3 ng/ml (fed) to 3±0.6 ng/ml after the 24 h fast (p=0.04 ANOVA). Glucose water and the recovery in serum glucose, leptin and insulin were unable to normalize GH levels in fasted male. In contrast, fasted female mice had a 2.2 fold (p= 0.0427) increase in serum GH and no change with glucose water. There was a significant increase in pituitary Ghrhr and Gh mRNA in fasted male mice but not in fasted female mice. Assays of serum or mRNA levels of most pituitary hormones showed no significant changes following a 24 h fast. These studies have thus uncovered an acute response to fasting by GH cells in male mice, which correlates to reduced serum GH resulting from a loss of leptin signals. The data extends findings reported previously showing that male somatotropes are more dependent on leptin signals than female somatotropes. Whereas the high Gh and Ghrhr mRNA support somatotrope stimulation, the somatotropes are unable to secrete normal levels of GH in the male, suggesting defects at the level of GH or GHRHR protein expression.

Sex-specific changes in postnatal GH and PRL secretion in somatotrope LEPR-null mice

The developing pituitary is a rapidly changing environment that is constantly meeting the physiological demands of the growing organism. During early postnatal development, the anterior pituitary is refining patterns of anterior hormone secretion in response to numerous genetic factors. Our laboratory previously developed a somatotrope leptin receptor (LEPR) deletion mouse model that had decreased lean body mass, disrupted metabolism, decreased GH stores and was GH deficient as an adult. To understand how deletion of LEPR in somatotropes altered GH, we turned our attention to postnatal development. The current study examines GH, PRL, TSH, ACTH, LH and FSH secretion during postnatal days 4, 5, 8, 10 and 15 and compares age and sex differences. The LEPR mutants have dysregulation of GH (P < 0.03) and a reduced developmental prolactin peak in males (P < 0.04) and females (P < 0.002). There were no differences in weight between groups, and the postnatal leptin surge appeared to be normal. Percentages of immunolabeled GH cells were reduced in mutants compared with controls in all age groups by 35-61% in males and 41-44% in females. In addition, we measured pituitary expression of pituitary transcription factors, POU1F1 and PROP1. POU1F1 was reduced in mutant females at PND 10 (P < 0.009) and PND 15 (P < 0.02) but increased in males at PND 10 (P < 0.01). PROP1 was unchanged in female mutants but showed developmental increases at PND 5 (P < 0.02) and PND 15 (P < 0.01). These studies show that the dysfunction caused by LEPR deletion in somatotropes begins as early as neonatal development and involves developing GH and prolactin cells (somatolactotropes).