PROJECT ABSTRACT/SUMMARY. Cardiomyocyte cell state is dramatically altered during postnatal development. During this period, cardiomyocytes terminally differentiate, fundamentally changing their energetics, functional machinery, and mechanism of cell growth. Although we have a good understanding of the factors controlling embryonic heart development, we still have a poor understanding of the mechanisms that establish cardiomyocyte terminal differentiation during postnatal development and maintain this cell state in the adult. Critically, cardiomyocyte terminal differentiation is integrally linked to cardiac remodeling and regeneration. While a terminally differentiated transcriptome is necessary for cardiomyocytes to accommodate adult circulatory demands, this transcriptional program also suppresses cardiomyocyte proliferation, underlying the inability of the mammalian heart to meaningfully regenerate after injury. Indeed, in model systems of cardiac regeneration, cardiomyocytes must first de-differentiate and return to a fetal-like transcriptomic state in order to proliferate. Few studies examine the factors responsible for maintaining cardiomyocyte terminal differentiation, but it stands to reason that targeted disruption of cardiomyocyte terminal differentiation could unmask latent pro-proliferative pathways in the adult heart and promote endogenous cardiac regeneration. In this study, we will examine the role of the RNA-binding protein Muscleblind-like protein 1 (MBNL1) in controlling cardiomyocyte terminal differentiation and cardiac regeneration. Although MBNL1 expression increases during cardiomyocyte terminal differentiation and MBNL1 is known to promote fetal-to-adult isoform switching of a number of developmentally regulated genes, it has never been examined directly for controlling cardiomyocyte terminal differentiation or integrated into regulatory mechanisms surrounding cardiac plasticity. Specifically, this proposal will use a variety of in vivo and in vitro genetic approaches to address the following aims: (1) to determine the role of MBNL1 in maintaining cardiomyocyte terminal differentiation in the adult mammalian heart and (2) to determine the role of MBNL1 in controlling cardiac regenerative potential. Insight gleaned from these aims will characterize MBNL1-dependent post-transcriptional regulatory mechanisms governing cardiomyocyte terminal differentiation and will determine if MBNL1 could be used as a novel therapeutic target to promote endogenous cardiac regeneration.

PROJECT NARRATIVE. This project seeks to define how the protein Muscleblind-like protein 1 (MBNL1) regulates cardiomyocyte differentiation and heart regeneration. Heart disease is the number one cause of death globally because the adult heart is incapable of any meaningful regeneration after injury due to a postnatal developmental switch during which cardiomyocytes transition from an immature, proliferative state to a mature, terminally differentiated state largely incapable of proliferation. MBNL1 has never been studied in this context, but understanding its role in controlling cardiomyocyte differentiation, cardiac regeneration, and the mechanisms by which it acts will allow us to determine if modulation of MBNL1 could be used therapeutically for the creation of novel regenerative medicine approaches to treating cardiovascular disease.