Molecular and Spatial Regulators of Cardiac Fibroblast Cell State and Plasticity in Myocardial Infarction Injury

Abstract

A central obstacle in heart disease is the replacement of healthy kinetic muscle with stiff fibrotic scarring. Fibrotic deposition represents a major clinical burden because it rapidly progresses the heart towards failure and current antifibrotic therapies are ineffective. It has been proposed that myofibroblasts are the major cellular source of the fibrotic response, yet the molecular mechanisms governing the formation and maintenance of the myofibroblast cell state are poorly understood. Here it was found that infarct scar alignment acts as a positive regulator of the myofibroblast cell state through focal adhesion sensation and downstream p38-YAP-TEAD signaling. Furthermore, is has been suggested that the RNA binding protein Muscleblind-Like1 (MBNL1) binds to and post-transcriptionally regulates many targets in the p38 signaling axes, suggesting a post-transcriptional control point for mediating fibrosis. Using fibroblast specific gain and loss of function models it was shown that MBNL1 is not only necessary for mediating the fibrotic response, but that MBNL1 acts as a post-transcriptional switch required for cells to transit between mesenchymal, quiescent, and activated cell states. Collectively, these data deepen our understanding of fibroblast to myofibroblast cell state transitions, where MBNL1 is upregulated after the early fibroproliferative window following myocardial infarction (MI) to stabilize pro-differentiation transcripts providing sufficient activation energy for myofibroblast formation. Although MBNL1 expression remains high through the first 2 weeks of MI, it is suggested that ECM alignment helps to maintain the activation energy necessary for maintaining the myofibroblast cell state until remodeling has resolved. Finally, these data suggest that antifibrotic therapies must not only consider the reduction in overall fibrotic quantity but also the topography and content of the matrix being laid down by fibroblasts with altered function.