C-kit expression identifies cardiac precursor cells in neonatal mice

Chasing c-Kit through the heart: Taking a broader view

Stem cell mediated cardiac repair is an exciting and controversial area of cardiovascular research that holds the potential to produce novel, revolutionary therapies for the treatment of heart disease. Extensive investigation to define cell types contributing to cardiac formation, homeostasis and regeneration has produced several candidates, including adult cardiac c-Kit+ expressing stem and progenitor cells that have even been employed in a Phase I clinical trial demonstrating safety and feasibility of this therapeutic approach. However, the field of cardiac cell based therapy remains deeply divided due to strong disagreement among researchers and clinicians over which cell types, if any, are the best candidates for these applications. Research models that identify and define specific cardiac cells that effectively contribute to heart repair are urgently needed to resolve this debate. In this review, current c-Kit reporter models are discussed with respect to myocardial c-Kit cell biology and function, and future designs imagined to better represent endogenous myocardial c-Kit expression.

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

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.

Cardiac stem cells and their roles in myocardial infarction

Myocardial infarction leads to loss of cardiomyocytes, scar formation, ventricular remodeling and eventually deterioration of heart function. Over the past decade, stem cell therapy has emerged as a novel strategy for patients with ischemic heart disease and its beneficial effects have been demonstrated by substantial preclinical and clinical studies. Efficacy of several types of stem cells in the therapy of cardiovascular diseases has already been evaluated. However, repair of injured myocardium through stem cell transplantation is restricted by critical safety issues and ethic concerns. Recently, the discovery of cardiac stem cells (CSCs) that reside in the heart itself brings new prospects for myocardial regeneration and reconstitution of cardiac tissues. CSCs are positive for various stem cell markers and have the potential of self-renewal and multilineage differentiation. They play a pivotal role in the maintenance of heart homeostasis and cardiac repair. Elucidation of their biological characteristics and functions they exert in myocardial infarction are very crucial to further investigations on them. This review will focus on the field of cardiac stem cells and discuss technical and practical issues that may involve in their clinical applications in myocardial infarction.

Nox2 and Nox4 influence neonatal c-kit(+) cardiac precursor cell status and differentiation

Redox status has emerged as critical in modulating stemness and lineage commitment in several precursor cell types. However, a role for redox genes, specifically NADPH oxidases (Nox), in cardiac precursor cells (CPCs) has not been established. We tested whether CPCs marked by type III receptor tyrosine kinase c-kit (c-kit(+)) exhibit a unique NADPH oxidase signature that confers precursor status and whether alterations in this profile are functionally linked to changes in lineage specification. Dihydroethidium (DHE) microfluorography indicated reduced basal reactive oxygen species (ROS) formation within early postnatal c-kit(+) CPCs. Real-time quantitative PCR revealed downregulation of ROS generator Nox2 and its subunit p67(phox) in c-kit(+) CPCs under basal conditions but upregulation of Nox2 and Nox4 over the course of differentiation. Adenoviral silencing of Nox2 and Nox4 increased expression of CPC markers c-kit and Flk-1 and blunted smooth and cardiac muscle differentiation, respectively, while overexpression of Nox2 and Nox4 significantly reduced c-kit expression. These changes were accompanied by altered expression of transcription factors regulating cardiac lineage commitment, Gata6 and Gata4, and cytokine transforming growth factor (TGF)-β1. Similar to other precursor cell types, RT(2)Profiler PCR Arrays revealed that c-kit(+) CPCs also exhibit enhanced antioxidant capacity at the mRNA level. In conclusion, we report that c-kit(+) CPCs demonstrate reduced Nox2 expression and ROS levels and that increases in Nox2 and Nox4 influence their differentiation into mature cells. We speculate that ROS generators Nox2 and Nox4, along with the antioxidant genes identified by PCR Arrays, may be novel targets in CPCs that could prove useful in cell-based therapy of the heart.

Progenitor cells identified by PDGFR-alpha expression in the developing and diseased human heart

Platelet-derived growth factors (PDGFs) and their tyrosine kinase receptors play instrumental roles in embryonic organogenesis and diseases of adult organs. In particular, platelet-derived growth factor receptor-alpha (PDGFRα) is expressed by multipotent cardiovascular progenitors in mouse and human embryonic stem cell systems. Although cardiac PDGFRα expression has been studied in multiple species, little is known about its expression in the human heart. Using immunofluorescence, we analyzed PDGFRα expression in both human fetal and diseased adult hearts, finding strong expression in the interstitial cells of the epicardium, myocardium, and endocardium, as well as the coronary smooth muscle. Only rare endothelial cells and cardiomyocytes expressed PDGFRα. This pattern was consistent for both the fetal and adult diseased hearts, although more PDGFRα+ cardiomyocytes were noted in the latter. In vitro differentiation assays were then performed on the PDGFRα+ cell fraction isolated from the cardiomyocyte-depleted human fetal hearts. Protocols previously reported to direct differentiation to a cardiomyocyte (5-azacytidine), smooth muscle (PDGF-BB), or endothelial cell fates (vascular endothelial growth factor [VEGF]) were used. Although no significant cardiomyocyte differentiation was observed, PDGFRα+ cells generated significant numbers of smooth muscle cells (smooth muscle-α-actin+ and smooth muscle myosin+) and endothelial cells (CD31+). These data suggest that a subfraction of the cardiac PDGFRα+ populations are progenitors contributing predominantly to the vascular and mesenchymal compartments of the human heart. It may be possible to control the fate of these progenitors to promote vascularization or limit fibrosis in the injured heart.