The Musashi1 RNA-binding protein is a leptin-regulated enforcer of pituitary cell fate and hormone production Jewel Banik, Juchan Lim, Linda L. Hardy, Angela K. Odle, Gwen V. Childs, Melanie C. MacNicol and Angus M. MacNicol Department of Neurobiology & Developmental Sciences, University of Arkansas for Medical Sciences, Little Rock, AR 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 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. Interestingly in the same cell assay system, Musashi exerts translational activation of the Prop1 3’ UTR. 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.