FRI291 Musashi1 And Musashi2 Mark Distinct Pituitary Stem Cell Populations

Disclosure: Y. Rahmatallah: None. J. Banik: None. K. Bronson: None. M.C. MacNicol: None. The cell plastic state of the pituitary is required to meet the changing demands upon pituitary function and may also result in these cells being more vulnerable to malignant transformation. The mechanisms that control pituitary cell plasticity are unknown. The RNA binding protein Musashi (MSI) is expressed in embryonic stem cells, and in cancer stem cells, where MSI functions to promote stem cell plasticity and to oppose premature cell maturation. MSI is also expressed at high levels in adult gonad and adult pituitary. The function of MSI in adult tissues is not fully understood, and the mechanisms that control the pro-proliferative and pro-malignant aspects of MSI function in adult tissues are not known. The two evolutionarily conserved MSI proteins, Musashi1 (MSI1) and Musashi2 (MSI2), are predicted to function redundantly, however, regulatory distinctions have been reported for MSI1 and MSI2. Towards a characterization of MSI1 and MSI2 in pituitary, we analyzed transcriptomics data from public datasets of single-nucleus and single-cell and bulk-tissue gene expression data from physiological and pathophysiological human pituitary tissue. We performed bioinformatics analysis utilizing the single-cell genomics analysis tool, Seurat in R and the pseudotime trajectory analysis tool Monocle and additional genomics and statistics analyses tools in R. We identify the MSI1-expressing pituitary cell population as immature stem cells and the distinct MSI2-expressing pituitary cell population as more mature, committed progenitor cells. Pituitary from human fetal tissue contains multiple distinct MSI1-expressing cell populations while, in contrast, pituitary from human adult tissue contains multiple distinct MSI2-expressing cell populations. Pituitary from human pediatric tissue demonstrates an intermediate state with both distinct MSI1-expressing and distinct MSI2-expressing cell populations. We identify increased MSI1 expression but not increased MSI2 expression in adult pituitary adenoma, and a high level of variation in the MSI1 to MSI2 expression ratio between distinct pituitary adenomas. Based upon this re-analysis of reposited datasets from other pituitary investigators, we hypothesize that the increase that we observe, in MSI1 to MSI2 expression ratio in adult pituitary tumors is consistent with a reversion of mature cells to an earlier immature state. Adult pituitary may contain the mechanisms that regulate MSI2 function but may not be equipped to properly control high levels of re-expressed MSI1. An understanding of the physiological functions and mechanisms of regulation of the distinct MSI proteins in pituitary may provide insight into potential mechanisms that could be recruited to increase physiological pituitary cell growth and differentiation in regenerative therapy and to oppose pathological cell growth in pituitary tumor. Presentation: Friday, June 16, 2023