OR08-2 Functional Association of the Stem Cell Protein Musashi With LSM14B in Control of mRNA Translation

The Musashi RNA-binding protein functions as a gatekeeper of cell maturation and maintains stem cell plasticity by regulating the translation of target mRNAs. The adult anterior pituitary tissue expresses a high level of Musashi and also demonstatres a high level of cell plasticity, indicating a Musashi-dependent function in maintaining endocrine homeostasis in the anterior pituitary. Towards understanding the mechanism(s) by which Musashi functions to control cell plasticity, we have identified co-associated proteins necessary for Musashi function. The two Musashi isoforms (Musashi1 and Musashi2) are conserved across species and are structurally characterized by two N-terminal RNA-recognition motifs (RRMs, that associate with specific sequences in the 3' untranslated region of mRNAs targets) and a disordered C-terminus. Musashi uses these domains to selectively repress, or to activate, mRNA translation in a manner specific to target mRNA and cell context. Both Musashi isoforms contain two C-terminal regulatory serine residues that require phosphorylation to enable mRNA translational activation. However, the Musashi proteins do not have inherent mRNA translation function and utilize co-associated protein complexes to mediate mRNA translation. We have previously identified Musashi1 binding protein partners through mass spectrometry. Here, we have identified the protein interactions that are required for Musashi1-dependent translational activation. Knockdown of target protein levels in Xenopus laevis oocytes indicated that the PABP4, CELF2, LSM14A/B, ELAVL1, ELAVL2, ELAVL4, and PUM1 proteins are required for Musashi target mRNA translational activation. We further determined that the effect of LSM14A/B dual knockdown was in fact specific to LSM14B alone. LSM14B is an RNA-binding protein that is required for RNA granule formation, RNA transcript metabolism, and mRNA translation. Studies in mouse oocytes have shown that LSM14B is required to maintain transcript levels of the Musashi1 target Cyclin B1 (Ccnb1) mRNA prior to mRNA translation. In the adult murine pituitary, LSM14B is ubiquitously expressed in both hormone-producing cells and in stem cells as indicated through single cell RNA sequencing. These findings indicate that LSM14B and Musashi1 may function cooperatively to regulate pituitary hormone production and cell differentiation in response to changes in physiological demand. Presentation: Saturday, June 11, 2022 11:45 a.m. - 12:00 p.m.