The Role of RNA-Binding Protein, Muscleblind-Like 1, in Cardiac Wound Healing and Remodeling

Abstract

Myocardial infarction (MI) triggers cardiomyocyte necrosis and a reparative response that involves deposition of a collagenous, fibrotic scar. Persistent scar formation leads to adverse remodeling of the heart structure and progression to heart failure, which is the leading cause of death worldwide. Although advancements in treatments have allowed patients to live longer, the underlying fibrotic scar is not remedied. The primary effector of cardiac fibrosis is the myofibroblast, which is a specialized, heterogeneous cell type that secretes extracellular matrix (ECM) and functionally contracts to help maintain ventricular wall integrity and prevent heart rupture after acute MI. Our lab previously identified the RNA-binding protein, Muscleblind-Like 1 (MBNL1), to be robustly upregulated during myofibroblast transformation. MBNL1 (a) stabilizes mRNA transcripts, (b) activates nodal signaling axes that transition fibroblasts into a myofibroblast cell fate and (c) augments fibrosis in the heart after MI. However, there are still several unanswered questions: (1) Which cardiac cell type contributes to MBNL1-mediated fibrotic remodeling after MI? (2) Could genetically dosing MBNL1 in the heart improve cardiac function post-infarct? (3) What other protein complexes associate with MBNL1 to regulate mRNA maturation and cell differentiation? In order to answer these questions, we either genetically knocked out or overexpressed Mbnl1 selectively in two different cells within the heart, fibroblasts and cardiomyocytes, to determine if Mbnl1 is a global mediator in the fibrotic response. We hypothesized fibroblasts were the main contributor to MBNL1-dependent fibrosis after MI. Indeed, we demonstrated that genetic deletion of Mbnl1 in resident cardiac fibroblasts diminishes the formation of a fibrotic scar and protects the heart from cardiac dysfunction post-MI. Mbnl1 depletion in resident cardiac fibroblasts inhibits TGFβ-mediated myofibroblast differentiation, which can be rescued by reintroduction of functional pro-fibrogenic transcripts serum response factor (Srf) and calcineurin (CnA) that are bound and regulated by MBNL1. A decrease in fibrosis post-MI was also observed in mice with cardiomyocyte-specific Mbnl1 loss of function, suggesting Mbnl1 function in cardiomyocytes also contributes to post-infarction wound healing and fibrotic remodeling after MI. Mechanistically, we show for the first time that MBNL1 binds with RNA processing machinery in fibroblasts to promote myofibroblast differentiation. When Mbnl1 is overexpressed in cardiac fibroblasts and cardiomyocytes, we observed diastolic dysfunction phenotypes in the absence of injury, highlighting MBNL1’s regulatory role in maintaining homeostasis in the cell. Collectively, this dissertation addresses the cell-specific role of MBNL1 in cardiac remodeling after MI by genetically manipulating Mbnl1 levels in both resident cardiac fibroblasts and cardiomyocytes. This work is significant because it will build upon a basic scientific understanding of transcriptome changes during cardiac wound healing and may provide new approaches to functionally repair and heal the myocardium post-MI.