Epicardium-derived cells (EPDCs) invade the myocardium and differentiate into fibroblasts and vascular smooth muscle (SM) cells, which support the coronary vessels. The transcription factor Pod1 (Tcf21) is expressed in subpopulations of the epicardium and EPDCs in chicken and mouse embryonic hearts, and the transcription factors WT1, NFATC1, and Tbx18 are expressed in overlapping and distinct subsets of Pod1-expressing cells. Expression of Pod1 and WT1, but not Tbx18 or NFATC1, is activated with all-trans-retinoic acid (RA) treatment of isolated chick EPDCs in culture. In intact chicken hearts, RA inhibition leads to decreased Pod1 expression while RA treatment inhibits SM differentiation. The requirements for Pod1 in differentiation of EPDCs in the developing heart were examined in mice lacking Pod1. Loss of Pod1 in mice leads to epicardial blistering, increased SM differentiation on the surface of the heart, and a paucity of interstitial fibroblasts, with neonatal lethality. Epicardial epithelial-to-mesenchymal transition (EMT) and endothelial differentiation of coronary vessels are relatively unaffected. On the surface of the myocardium, expression of multiple SM markers is increased in Pod1-deficient EPDCs, demonstrating premature SM differentiation. Increased SM differentiation also is observed in Pod1-deficient lung mesenchyme. Together, these data demonstrate a critical role for Pod1 in controlling mesenchymal progenitor cell differentiation into SM and fibroblast lineages during cardiac development.
During embryonic heart development, the transcription factors Tcf21, Wt1, and Tbx18 regulate activation and differentiation of epicardium-derived cells, including fibroblast lineages. Expression of these epicardial progenitor factors and localization of cardiac fibrosis was examined in mouse models of cardiovascular disease and in human diseased hearts. Following ischemic injury in mice, epicardial fibrosis is apparent in the thickened layer of subepicardial cells that express Wt1, Tbx18, and Tcf21. Perivascular fibrosis with predominant expression of Tcf21, but not Wt1 or Tbx18, occurs in mouse models of pressure overload or hypertensive heart disease, but not following ischemic injury. Areas of interstitial fibrosis in ischemic and hypertensive hearts actively express Tcf21, Wt1, and Tbx18. In all areas of fibrosis, cells that express epicardial progenitor factors are distinct from CD45-positive immune cells. In human diseased hearts, differential expression of TCF21, WT1, and TBX18 also is detected with epicardial, perivascular, and interstitial fibrosis, indicating conservation of reactivated developmental mechanisms in cardiac fibrosis in mice and humans. Together, these data provide evidence for distinct fibrogenic mechanisms that include Tcf21, separate from Wt1 and Tbx18, in different fibroblast populations in response to specific types of cardiac injury.
SUMMARYEpicardium-derived cells (EPDCs) contribute to formation of coronary vessels and fibrous matrix of the mature heart. Nuclear factor of activated T-cells cytoplasmic 1 (NFATC1) is expressed in cells of the proepicardium (PE), epicardium and EPDCs in mouse and chick embryos. Conditional loss of NFATC1 expression in EPDCs in mice causes embryonic death by E18.5 with reduced coronary vessel and fibrous matrix penetration into myocardium. In osteoclasts, calcineurin-mediated activation of NFATC1 by receptor activator of NFkB ligand (RANKL) signaling induces cathepsin K (CTSK) expression for extracellular matrix degradation and cell invasion. RANKL/NFATC1 pathway components also are expressed in EPDCs, and loss of NFATC1 in EPDCs causes loss of CTSK expression in the myocardial interstitium in vivo. Likewise, RANKL treatment induces Ctsk expression in PE-derived cell cultures via a calcineurin-dependent mechanism. In chicken embryo hearts, RANKL treatment increases the distance of EPDC invasion into myocardium, and this response is calcineurin dependent. Together, these data demonstrate a crucial role for the RANKL/NFATC1 signaling pathway in promoting invasion of EPDCs into the myocardium by induction of extracellular matrixdegrading enzyme gene expression.
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