"String theory" of c-kit(pos) cardiac cells: a new paradigm regarding the nature of these cells that may reconcile apparently discrepant results

Abstract

Although numerous preclinical investigations have consistently demonstrated salubrious effects of c-kit(pos) cardiac cells administered after myocardial infarction, the mechanism of action remains highly controversial. We and others have found little or no evidence that these cells differentiate into mature functional cardiomyocytes, suggesting paracrine effects. In this review, we propose a new paradigm predicated on a comprehensive analysis of the literature, including studies of cardiac development; we have (facetiously) dubbed this conceptual construct "string theory" of c-kit(pos) cardiac cells because it reconciles multifarious and sometimes apparently discrepant results. There is strong evidence that, during development, the c-kit receptor is expressed in different pools of cardiac progenitors (some capable of robust cardiomyogenesis and others with little or no contribution to myocytes). Accordingly, c-kit positivity, in itself, does not define the embryonic origins, lineage capabilities, or differentiation capacities of specific cardiac progenitors. C-kit(pos) cells derived from the first heart field exhibit cardiomyogenic potential during development, but these cells are likely depleted shortly before or after birth. The residual c-kit(pos) cells found in the adult heart are probably of proepicardial origin, possess a mesenchymal phenotype (resembling bone marrow mesenchymal stem/stromal cells), and are capable of contributing significantly only to nonmyocytic lineages (fibroblasts, smooth muscle cells, and endothelial cells). If these 2 populations (first heart field and proepicardium) express different levels of c-kit, the cardiomyogenic potential of first heart field progenitors might be reconciled with recent results of c-kit(pos) cell lineage tracing studies. The concept that c-kit expression in the adult heart identifies epicardium-derived, noncardiomyogenic precursors with a mesenchymal phenotype helps to explain the beneficial effects of c-kit(pos) cell administration to ischemically damaged hearts despite the observed paucity of cardiomyogenic differentiation of these cells.

Keywords: muscle development; myocytes, cardiac; regeneration.

Figure 1

Proposed position of c-kit^pos intermediates in the hierarchy of cardiac progenitors in the fetal heart. A and B. Arising first during cardiac development from a common pre-cardiac mesodermal progenitor (Bry+/Mesp1+/Eomes+/KDR+), endocardial (Bry+/KDR+/Nkx2.5+/Isl-1+) and FHF (Bry+/Nkx2.5+/KDR-/c-kit^pos) progenitors diverge early; the latter have been shown to have dedicated cardiomyocyte and smooth muscle cell bipotential differentiation capacity and to include c-kit^pos intermediates. The expression of c-kit is postulated to be “low” because although these progenitors have been found to be c-kit^pos, the van Berlo study failed to demonstrate their lineage contribution to the cardiomyocyte compartment in the adult heart, and it has been proposed that recombination in the van Berlo model is less effective in the presence of low levels of c-kit. Endocardial progenitors have not been shown to express c-kit. C. Subsequently arising SHF progenitors (Isl-1+/Nkx2.5+/c-kit^neg) are c-kit negative and contribute to cardiomyocytes, smooth muscle cells, and vascular endothelium. D. Proepicardial progenitors (with diverging Wt1+/Tbx18+ and Sema3D/Scx+ populations) arise later from a mesodermal or SHF progenitor, giving rise to vasculogenic lineages and nearly all of cardiac adventitia including fibroblasts (lineages identified as having partially come from c-kit^pos cells in the van Berlo study). Proepicardial progenitors expressing WT1 and Tbx18 undergo epithelial-to-mesenchymal transition (EMT) characterized by an upregulation of c-kit, resulting in c-kit^pos intermediate phenotypes. The same is likely true for Sema3D/Scx expressing progenitors. The expression of c-kit in these proepicardial progenitors is postulated to be “high” because, in the van Berlo study, it was sufficient to induce recombination in adventitial lineages (which do not arise for FHF or SHF progenitors) and vascular lineages, in contrast to that of known c-kit^pos FHF progenitors, which remained unlabeled. The hierarchy illustrated herein shows that c-kit expression is not limited to one cardiac progenitor and does not in itself define one specific cardiac precursor population. Shown at the bottom of the figure are the relative contributions of epicardium-derived cells to each cardiac lineage in fetal development according to the evidence outlined above.

Figure 2

Proposed origin of c-kit^pos cells in the adult heart. The figure illustrates the concept that, in the adult heart, c-kit^pos cells are intermediates derived from c-kit^neg epicardial progenitors that undergo EMT. (EMT is known to be associated with expression of c-kit.) These c-kit^pos intermediates have a mesenchymal phenotype and give rise to c-kit^neg EDPCs with differentiation potential limited to noncardiomyocytic lineages. A. WT-1+/Tbx18+ epicardial progenitors contribute predominantly to smooth muscle and cardiac adventitial fibroblasts, with minimal endothelial cell formation. B. Sema3D+/Scx+ epicardial progenitors (distinct from WT-1+/Tbx18+ cells) form vascular endothelium and cardiac fibroblasts and contribute minimally to smooth muscle cells. Both of these epicardium-derived c-kit^pos intermediates express CD105 and possess a mesenchymal phenotype with canonical MSC markers; neither of these two populations has shown any significant ability to form myocytes. The expression of ckit in these epicardium-derived intermediates is postulated to be “high” relative to that of c-kit^pos intermediates from the FHF (shown in Fig. 1), which have bipotent cardiomyocyte and smooth muscle differentiation capacity. This differential c-kit expression among cardiac progenitors is currently a conjecture and has not been demonstrated experimentally; nevertheless, it is inferred from the results of the van Berlo study, which did not detect significant myogenesis from FHF c-kit^pos progenitors but detected robust evidence of a c-kit^pos progenitor of fibroblasts, smooth muscle cells, and endothelial cells (all of which are of epicardial origin), and from the proposed insensitivity of the van Berlo model to low levels of c-kit expression, which could theoretically result in underestimation of “low” expressers of c-kit. Shown below the figure is the relative contribution of mesenchymally transitioned EPDCs to cardiac lineages of the adult heart; this pattern is consistent with studies of adult c-kit^pos cells, which have shown preferential adventitial and vasculogenic differentiation and a paucity of direct cardiomyogenic potential.