Identification of a Post-transcriptional Maturation Switch Underlying Regenerative and Pathologic Growth Programs in the Heart

Abstract

A central obstacle in the treatment of cardiac disease is that the adult mammalian heart cannot regenerate muscle lost to injury. This lack of organ plasticity is due to postnatal terminal differentiation of cardiomyocytes. In neonatal mammals, cardiomyocytes are capable of remarkable state plasticity, where they can partially reverse their terminally differentiated characteristics, undergo clonal expansion, and contribute to near-complete heart regeneration following injury. On the other hand, adult mammalian cardiomyocytes are largely post-mitotic, unable to dedifferentiate following injury, and become dysfunctional following similar insults. In this dissertation, I identify the RNA binding protein Muscleblind-like 1 (MBNL1) as an endogenous promoter of cardiomyocyte terminal differentiation initiation and maintenance. Using MBNL1 gain- and loss-of-function mouse models, I show that MBNL1 acts as a postnatal maturation switch which underlies cardiomyocyte growth programs in development, regeneration, and pathologic stress contexts. Mechanistic interrogation at multiple levels of transcriptional and proteomic regulation revealed MBNL1 acts through coordinated transcript stabilization of key transcriptional promoters of cardiomyocyte terminal differentiation as well as inhibition of transcriptional elongation; these mechanistic studies represent a significant advancement in our understanding of MBNL1, which is traditionally regarded as a regulator of alternative splicing in the heart. Furthermore, experimental control of MBNL1 dose in regenerative and pathologic growth settings was used as a tool to perturb cardiomyocyte preferred growth strategies following pathologic stress, allowing us to better understand the role of terminal differentiation maintenance in the heterogeneity and flexibility of cardiomyocytes following injury. These data suggest that ablating cardiomyocyte terminal differentiation is sufficient to promote cardiac regeneration beyond the normal neonatal regeneration window, but that additional stimuli are needed to promote adult cardiac regeneration. However, forced maintenance of cardiomyocyte terminal differentiation following pathologic stress significantly improves cardiac remodeling.