The Role of the Epicardium During Heart Development and Repair

Abstract

The heart is lined by a single layer of mesothelial cells called the epicardium that provides important cellular contributions for embryonic heart formation. The epicardium harbors a population of progenitor cells that undergo epithelial-to-mesenchymal transition displaying characteristic conversion of planar epithelial cells into multipolar and invasive mesenchymal cells before differentiating into nonmyocyte cardiac lineages, such as vascular smooth muscle cells, pericytes, and fibroblasts. The epicardium is also a source of paracrine cues that are essential for fetal cardiac growth, coronary vessel patterning, and regenerative heart repair. Although the epicardium becomes dormant after birth, cardiac injury reactivates developmental gene programs that stimulate epithelial-to-mesenchymal transition; however, it is not clear how the epicardium contributes to disease progression or repair in the adult. In this review, we will summarize the molecular mechanisms that control epicardium-derived progenitor cell migration, and the functional contributions of the epicardium to heart formation and cardiomyopathy. Future perspectives will be presented to highlight emerging therapeutic strategies aimed at harnessing the regenerative potential of the fetal epicardium for cardiac repair.

Keywords: fibrosis; growth and development; myocardial ischemia; paracrine communication; regeneration.

Figure 1.. Epicardium During Cardiac Development.

Epicardium-derived progenitor cells (EPDCs) emerge from the fetal epicardium through the process of epithelial-to-mesenchymal transition (EMT) and contribute to various cardiac lineages such as cardiac fibroblasts (CFs), smooth muscle cells (SMCs) and pericytes (PCs). The ability of EPDCs to differentiate into endothelial cells (ECs) in vivo is limited, and CMs are not generally thought to derive from EPDCs. In addition to cellular contributions, the epicardium participates in reciprocal paracrine signaling (dashed arrows) to stimulate CM proliferation, macrophage (MC) recruitment and coronary vessel growth and maturation. epi = epicardium, sub-epi = sub-epicardium, myo = myocardium. (Illustration credit: Ben Smith)

Figure 2.. Molecular Control of Epicardial EMT.

Intrinsic and extrinsic molecular programs regulate epicardial epithelial-to-mesenchymal transition in mice, which include the regulation of transcription factors and molecular signaling. Cytokine mediated signaling between the epicardium and cardiomyocytes is reported to stimulate heart growth, and epicardium-derived signals support the growing coronary vasculature.

Figure 3.. Resident Epicardium-Derived Fibroblasts Contribute to Fibrosis Following Myocardial Infarction.

Evaluation of epicardium-derived cell distribution in left ventricles of mice subjected to sham or myocardial infarction (MI). Epicardial cells are visualized (GFP⁺=green) by crossing constitutive (Wt1^CreBAC/+) or tamoxifen-inducible (Wt1^CreERT2/+) Cre lines to the Rosa26^mTmG Cre-dependent fluorescent lineage reporter mouse line. (A) Epicardial cells of developmental origin (Wt1^CreBAC/+; Rosa26^mTmG/+) were observed on the epicardial surface (Epi), and in both perivascular and interstitial areas of the left ventricle of sham-operated hearts. Upon cardiac injury, pre-existing GFP⁺ cells contribute to epicardial thickening and the formation of a cellularized scar. Green= GFP⁺ EPDCs, Red = cardiac troponin T or cTNT, Blue = DAPI to visualize nuclei. (B) When tamoxifen was administered to 8-week old Wt1^CreERT2/+; Rosa26^mTmG/+ mice, GFP⁺ cells were observed primarily on the epicardial surface of the left ventricle in sham operated animals. After MI, adult Wt1-lineage derived cells increase in number at the epicardial border surrounding the borderzone region of the left ventricle, but they do not directly contribute to scar formation. Green= GFP⁺ EPDCs, Red = membrane tethered tdTomato (non-recombined cells), Blue = DAPI to visualize nuclei. Scale bars for (A) and (B) = 50μm.

Figure 4.. Epicardium-Derived Cells in the Cardiac Injury Response.

Pre-existing epicardium-derived resident cardiac fibroblasts are the primary source of MyoFbs that generate scar tissue during pressure overload or ischemic remodeling through the secretion ECM. Epicardium-derived pericytes (PC) or endothelial cells (EC) have also been reported to exhibit mesenchymal cell characteristics and produce ECM in response to injury. The adult epicardium is reactivated and expands in response to cardiac injury. A small proportion of epicardium-derived progenitor cells (EPDCs) undergo epithelial-to-mesenchymal transition (EMT) to become myofibroblasts (MyoFbs) that secrete extracellular matrix (ECM). In limited cases, EPDCs can become adipocytes (AC). The activated epicardium also promotes the recruitment of inflammatory cells (Neutrophils or NT, T-regulatory cells or Treg⁺, and Macrophages or MC) and stabilizes coronary vasculature through the secretion of chemokines (dashed arrows). The epicardium may also promote cardiomyocyte (CM) survival and/or cell growth. epi = epicardium, sub-epi = sub-epicardium, myo = myocardium. (Illustration credit: Ben Smith)

Figure 5.. Epicardium in Cardiac Regeneration.

Cardiac regeneration occurs in response to a number of stimuli including apical resection, neonatal myocardial infarction, cryoablation or targeted ablation of the epicardium in both zebrafish and the early postnatal mammalian heart. The epicardium actively regenerates through the migration of “leader” binucleated epicardial cells initiating wound closure, and contributes to the formation of perivascular cells (smooth muscle cells or SMCs and fibroblasts or Fbs). Epicardium-derived fibroblasts have been shown to acquire myofibroblast phenotypes (MyoFb and ECM), which may be reversible during the resolution phase of the scar. The epicardium promotes macrophage (MC) recruitment, neovascularization as well as cardiomyocyte (CM) cell cycle re-entry via secretion of mitogens (dashed arrows). epi = epicardium, sub-epi = sub-epicardium, myo = myocardium. (Illustration credit: Ben Smith)