Control of the anterior pituitary cell lineage regulator POU1F1 by the stem cell determinant Musashi

<p><span><span><span><span><span><span><span><span><span><span><span>The cytokine 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.  Amongst other targets, leptin signaling promotes increased levels of the pituitary transcription factor POU1F1 (also referred to as Pit1) which, in turn, regulates the specification of somatotrope, lactotrope and thyrotrope cell lineages within the anterior pituitary.  Leptin's mechanism of action on somatotropes is sex-dependent, with females demonstrating post-transcriptional control of <i>Pou1f1 </i>mRNA translation.  In this study, we have investigated the mechanism that regulates <i>Pou1f1 </i>mRNA translational control and report that the stem cell marker and mRNA translational control protein, Musashi1, exerts repression of the <i>Pou1f1</i> mRNA. In FACS-purified female somatotropes, <i>Msi1</i> mRNA levels and Musashi1 protein levels are increased in the mouse model that lacks leptin signaling (<i>Gh</i>-CRE <i>Lepr</i>-null), coincident with lack of POU1f1 protein, despite normal levels of <i>Pou1f1</i> mRNA. Single-cell RNA sequencing of pituitary cells from control female animals indicates that both <i>Msi1</i> and <i>Pou1f1 </i>mRNAs are expressed in <i>Gh</i>-expressing somatrotropes and immunocytochemistry analyses of these cells confirms that Musashi1 protein is present in the somatotrope population. We demonstrate that Musashi interacts directly with the <i>Pou1f1</i> mRNA 3' untranslated region and exerts translational repression of a <i>Pou1f1 </i>mRNA translation<i> </i>reporter in a leptin-sensitive manner.<i> </i> Musashi immunoprecipitation from whole pituitary reveals co-associated <i>Pou1f1 </i>mRNA.  These findings suggest a mechanism in which leptin stimulation is required to reverse Musashi-mediated <i>Pou1f1 </i>mRNA translational control to coordinate AP somatotrope function with metabolic status.</span></span></span></span></span></span></span></span></span></span></span></p>