Exploring the Cell-Specific Role of Serotonin 2B in Regulating Fibrosis Following Myocardial Infarction
Snider, John Caleb
Myocardial infarction (MI) induces an intense injury response which ultimately generates a collagen-dominated scar. While required to prevent ventricular rupture, the fibrotic process is often sustained in a manner detrimental to optimal recovery. Cardiac myofibroblasts are the cells tasked with depositing and remodeling collagen and are a prime target to limit the fibrotic process post-MI. Myofibroblast overactivity results in scar expansion, tissue stiffening, and ultimately, heart failure. Serotonin 2B receptor (5-HT2B) signaling has been shown to be harmful in a variety of cardiopulmonary pathologies through mediating fibrotic remodeling of valves and pulmonary arteries, and could play an important role in mediating scar formation after MI. To test this hypothesis, we utilized two pharmacologic agents to explore the effect of 5-HT2B inhibition on outcomes post-MI and characterized the histological and microstructural changes involved in tissue remodeling. We demonstrated that 5-HT2B antagonism preserved cardiac structure and function by facilitating a less fibrotic scar, indicated by decreased scar thickness. We mathematically defined the border zone between scar tissue and uninjured myocardium, and quantified the rate of transition as a metric of scar expansion. We found 5-HT2B antagonism decreased border zone area, revealing a less expansive scar and decreased interruption of cardiomyocyte contraction. 5-HT2B antagonism resulted in collagen fiber redistribution to thinner collagen fibers which were more anisotropic, enhancing left ventricular contractility, while fibrotic tissue stiffness was decreased, limiting the hypertrophic response of uninjured cardiomyocytes. We genetically ablated 5-HT¬2B from Tcf21-lineage resident cardiac fibroblasts and saw similar improvements to the pharmacological approach. Left ventricle function, systolic and diastolic structure, and heart weight were all improved with cardiac fibroblast-specific 5-HT2B ablation. Tamoxifen-inducible, Cre-mediated ablation of 5-HT2B after injury in Postn-lineage myofibroblasts achieved identical outcomes as the resident cardiac fibroblast model. RNA sequencing of infarct myofibroblasts and subsequent in vitro analyses corroborate a decrease in fibroblast proliferation, migration, and remodeling capabilities. Together, our findings illustrate that inhibiting 5-HT2B limits structural and functional damages after MI through myofibroblast-mediated alterations in scar formation.