**8. Leptin regulation of pituitary musashi**

Our studies of animal models in which LEPR was ablated in gonadotropes or somatotropes opened the door to the discovery that leptin may regulate some of its target gene products by post-transcriptional pathways. The post-transcriptional targets included *Gnrhr* mRNA in gonadotropes [1, 7, 8] and the mRNA encoding the POU1F1 transcription factor in somatotropes [2, 60, 70].

The concept that Musashi would be involved in the translational regulation of either *Gnrhr* or *Pou1f*1 mRNA was novel, as both transcripts are important in the function and differentiation of somatotropes and gonadotropes. The expression and involvement of Musashi in differentiated hormone-producing cell lineages was surprising as Musahsi is typically implicated in stem and progenitor cell self-renewal. Nonetheless, while *Msi1* and *Msi2* are expressed in pituitary stem cells as expected, our scRNA sequencing clearly demonstrated that *Msi1* and *Msi2* mRNAs were also expressed in all hormone-producing cell lineages of the anterior pituitary [2].

Since our findings indicated that Musashi was involved in the repression of translation of *Gnrhr* or *Pou1f*1 mRNAs, we hypothesized that normal signals from leptin were needed to reverse this repression [2, 7]. This was based on the fact that the loss of leptin signals resulted in a reduction in the proteins (but not the mRNA). We were able to demonstrate a role for leptin in regulating the actions of Musashi s in reporter assays, where leptin mediated the reversal of Musashi-dependent repression [2].

Our studies also showed that leptin may directly reduce expression of Musashi in its target cells. In pituitaries from proestrous female mice lacking LEPR in gonadotropes, *Msi* mRNA expression is higher. Furthermore, leptin treatment of normal pituitaries from proestrous females resulted in reduced levels of *Msi1* mRNA [8]. More specifically, leptin treatment of normal pituitaries reduced Musashi1 immunolabeling in gonadotropes, identified by their expression of binding to biotinylated GnRH [8].

Similarly, Musashi1 protein and *Msi1* mRNA levels were increased in pure somatotropes lacking LEPR [2]. Furthermore, leptin treatment of pure somatotropes significantly reduced their expression of Musashi1 proteins. Collectively these findings point to a post-transcriptional pathway for leptin, which would reverse repression of translation of key target molecules in somatotropes or gonadotropes by regulating the function and expression of the Musashi family of translational regulatory proteins.

Our *in silico* analyses have identified other potential Musashi targets in the anterior pituitary, which may be regulated by leptin as well. Notably, there are MBEs in the *Fshb, Tshb*, *Prl* and *Pomc* mRNAs. However, no MBEs are found in *Lhb, Gh, or Ghrhr* mRNAs. It is interesting to speculate that this differential targeting may reflect specific roles for Musashi in regulating differentiated pituitary cells. Musashi could act in multihormonal cells by repressing translation of one set of hormones but not another.

Furthermore, our studies of leptin regulation of Musashi, GnRHR, and Pou1f1 protein levels suggest that leptin may use this Musashi pathway to promote selective differentiation of a given cell type depending on the body's needs.
