Control of the anterior pituitary cell lineage regulator POU1F1 by the stem cell determinant Musashi
The high energetic cost of reproduction requires that female fertility be closely linked to metabolic status. However, the mechanisms by which the metabolic and reproductive systems communicate is still largely undefined.  We have previously reported that signaling through the major metabolic cytokine leptin controls levels of the pituitary maturation determinant POU1F1 (also referred to as Pit1) that is required for optimal reproductive function. These studies further indicated that leptin increases POU1F1levels in females by promoting Pou1f1 mRNA translation, although the mechanism ofPou1f1 mRNA translation control is unknown.  In this study, we report that the stem cell marker and mRNA translational control protein, Musashi, represses translation of the Pou1f1 mRNA. In FACS-sorted purified female somatotropes, Msi1 mRNA and Musashi protein levels are increased in the mouse model that lacks leptin signaling (Gh-CRE leprLepr-null), coincident with the observed attenuation of Pou1f1 mRNA translation. Single-cell RNA sequencing of pituitary cells from control female animals indicates that Msi1 and Pou1f1 mRNAs are co-expressed in Gh-expressing cells and immunocytochemistry analyses of these cells confirms that Musashi protein is present in the GH-containing somatotrope population. We demonstrate that Musashi interacts directly with the Pou1f1 mRNA 3' UTR and exerts translational repression of a Pou1f1 mRNA translation reporter in vitro, and interacts with the Pou1f1 mRNA in vivo.  These findings indicate a critical role for Musashi-mediated mRNA translational regulation in the coordination between metabolic status and the maturation of pituitary somatotropes that is required for optimized reproductive function . Overall design: single cell RNA sequencing (scRNA-seq) on pooled pituitary samples from 8-week old, virgin, diestrous female mice. Three animals were pooled to generate two scRNA-seq control samples for sequencing.  After sequencing and bioinformatics clustering using the 10x Genomics cellular barcodes and unique molecular identified (UMI) counts (see Methods), the predominant pituitary cell types (somatotropes, lactotropes, thyrotropes, gonadotropes, corticotropes and melanotropes) were identified primarily based on established pituitary endocrine expression markers (Pou1f1, Gh, Prl, Tshb, Lhb, Fshb, Pomc, Pax7),