Abstract

The notion of the adult heart as terminally differentiated organ without self-renewal potential has been undermined by the existence of a subpopulation of replicating myocytes in normal and pathological states. The origin and significance of these cells has remained obscure for lack of a proper biological context. We report the existence of Lin c-kitPOS cells with the properties of cardiac stem cells. They are self-renewing, clonogenic, and multipotent, giving rise to myocytes, smooth muscle, and endothelial cells. When injected into an ischemic heart, these cells or their clonal progeny reconstitute well-differentiated myocardium, formed by blood-carrying new vessels and myocytes with the characteristics of young cells, encompassing 70% of the ventricle. Thus, the adult heart, like the brain, is mainly composed of terminally differentiated cells, but is not a terminally differentiated organ because it contains stem cells supporting its regeneration. The existence of these cells opens new opportunities for myocardial repair.

Until recently, the accepted paradigm in cardiac biology considered the adult mammalian heart to be a postmitotic organ without regenerative capacity. It has been assumed that from shortly after birth to adulthood and senescence the heart has a relatively stable but slowly diminishing number of myocytes. This static view of the myocardium implied that both myocyte death and myocyte regeneration had little role in cardiac cellular homeostasis. Although stem cells have been isolated from many adult tissues including the blood, skin, central nervous system, liver, gastrointestinal tract, and skeletal muscle (see Rosenthal, 2003), the search for a cardiac stem cell has been considered futile given the accepted lack of regenerative potential of this tissue.

Evidence challenging the accepted wisdom has been slowly accumulating McDonnell and Oberpriller 1984andRumyantsev and Broisov 1987. In the past few years, we have documented the existence of cycling ventricular myocytes in the normal and pathologic adult mammalian heart of several species, including humans Kajstura et al. 1998, Beltrami et al. 2001andQuaini et al. 2002. Although these data provided an alternative view of cardiac homeostasis, they also raised questions because it required reconciliation of two apparent contradictory bodies of evidence: the well-documented irreversible withdrawal of cardiac myocytes from the cell cycle soon after birth on one hand MacLellan and Schneider 2000andChien and Olson 2002, and the presence of cycling myocytes undergoing mitosis and cytokinesis on the other. These results raised the question as to the origin of the cycling myocytes and their dramatic increase in response to an acute work overload.

In cases of sex-mismatched cardiac transplants in humans, the female hearts in the male hosts had a significant number of Y positive myocytes and coronary vessels (Quaini et al., 2002). Most likely due to technical differences (Anversa and Nadal-Ginard, 2002a), there are some discrepancies among groups about the degree of cardiac chimerism Muller et al. 2002, Glaser et al. 2002andLaflamme et al. 2002. It is likely that these male cells colonized the female heart after the transplant and subsequently differentiated, although alternative explanations have been raised. These male cells in the female heart presuppose the existence of mobile stem-like cells able to differentiate into the three main cardiac cell types: myocytes, smooth, and endothelial vascular cells.

Primitive cells of donor and recipient origin that express stem cell-related surface antigensc-kit, Sca-1, and MDR1were identified in the recipient hearts. More importantly, identical cells were found in human control hearts Quaini et al. 2002andAnversa and Nadal-Ginard 2002b. It is well known that in early fetal life, c-kitPOS cells colonize the yolk sack, liver, and probably other organs. The colonized organs express stem cell factor (SCF), the ligand of the c-kit receptor (Teyssier-Le Discorde et al., 1999); SCF mRNA is also present in fetal and neonatal myocardium (Kunisada et al., 1998), raising the possibility that stem-like cells could have been in the heart from fetal life. The rapid induction of SCF during myocardial ischemia (Frangogiannis et al., 1998) could be involved in the activation of these cells and explain the significant increase in new myocyte formation (Beltrami et al., 2001). However, the origin of these primitive cells, their presence in normal and pathological hearts, together with the identification of some of them having initiated the cardiomyocyte gene expression program, is suggestive that they might be true cardiac stem cells that give rise to the cycling myocytes detected in the adult heart. If this were the case, their manipulation might provide the opportunity to stimulate myocardial regeneration with endogenous cells. For this reason, we endeavored to establish a precursor-product relationship between these primitive cells and the fully differentiated cardiac cells and to determine, in vitro and in vivo, whether they behave like true adult cardiac stem cells.

To determine whether the putative cardiac stem cells detected in human heart transplants and their controls are bona fide stem cells with cardiogenic potential, we isolated them to test their differentiation potential in vivo and in vitro. For experimental convenience, we chose the rat as the animal model system. We first analyzed whether cells with the cell surface markers commonly expressed by other stem cells could be identified in the adult rat myocardium. Based on the postulated higher number of proliferating stem and precursor cells with age (Morrison et al., 1996), we analyzed the myocardium from older animals. Histological sections of myocardium from Fisher rats 2023 months of age were examined by confocal microscopy for the presence of cells negative for the expression of blood lineage markers (Lin) but positive for the common stem cell markers c-kit (Kondo et al., 2003), Sca-1 (Morrison et al., 1997), and MDR-1 (Sellers et al., 2001). Small Lin cells with a very high nucleus/cytoplasm ratio and positive for each of the above markers were distributed throughout the ventricular and atrial myocardium with a higher density in the atria and the ventricular apex. Because of the role of bone marrow-derived Lin c-kitPOS cells in myocardial regeneration (Orlic et al., 2001), the mesodermal origin of both the heart and the bone marrow, and the use of c-kit as a hematopoietic stem cell marker Morrison et al. 1997, Weissman et al. 2001andKondo et al. 2003, we decided to concentrate on the cardiac cells expressing this marker, the receptor for SCF. Although the density of these cells varied among different regions of the heart, on average we identified one Lin c-kitPOS cell every 1 104 myocytes. It should be noted that most, if not all, of the detected c-kitPOS cells were negative for the pan leukocyte marker CD45 and the endothelial/hematopoietic progenitor marker CD34.

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Adult Cardiac Stem Cells Are Multipotent and Support ...

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