Abstract

Myocardial infarctioninduced heart failure is a prevailing cause of death in the United States and most developed countries. The cardiac tissue has extremely limited regenerative potential, and heart transplantation for reconstituting the function of damaged heart is severely hindered mainly due to the scarcity of donor organs. To that end, stem cells with their extensive proliferative capacity and their ability to differentiate toward functional cardiomyocytes may serve as a renewable cellular source for repairing the damaged myocardium. Here, we review recent studies regarding the cardiogenic potential of adult progenitor cells and embryonic stem cells. Although large strides have been made toward the engineering of cardiac tissues using stem cells, important issues remain to be addressed to enable the translation of such technologies to the clinical setting.

Heart disease is a significant cause of morbidity and mortality worldwide. In the United States, heart failure is ranked number one as a cause of death, affecting over 5 million people and with more than 500,000 new cases diagnosed each year.1 The health care expenditures associated with heart failure were $26.7 billion in 2004 and are estimated to $33.2 billion in 2007. Although significant progress has been made in mechanical devices and pharmacological interventions, more than half of the patients with heart failure die within 5 years of initial diagnosis. Wide application of heart transplantation is severely hindered by the limited availability of donor organs. To this end, cardiac cell therapy may be an appealing alternative to current treatments for heart failure.

Recent investigations focusing on engineering cells and tissues to repair or regenerate damaged hearts in animal models and in clinical trials have yielded promising results. Considering the limited regenerative capacity of the heart muscle, renewable sources of cardiomyocytes are highly sought. Cells suitable for myocardial engineering should be nonimmunogenic, should be easy to expand to large quantities, and should differentiate into mature, fully functional cardiomyocytes capable of integrating to the host tissue. Adult progenitor cells (APCs) and embryonic stem cells (ESCs) have extensive proliferative potential and can adopt different cell fates, including that of heart cells. The recent advances in the fields of stem cell biology and heart tissue engineering have intensified efforts toward the development of regenerative cardiac therapies. In this article, we review findings pertaining to the cardiogenic potential of major APC populations and of ESCs (). We also discuss significant challenges in the way of realizing stem cellbased therapies aiming to reconstitute the normal function of heart.

Potential sources of stem/progenitor cells for cardiac repair. ESCs derived from the inner cell mass of a blastocyst can be manipulated ex vivo to differentiate toward heart cells. APCs residing in various tissues such as the BM and skeletal muscle may ...

Bone marrow (BM) is a heterogeneous tissue comprising of multiple cell types, including minute fractions of mesenchymal stem cells (MSCs; 0.0010.01% of total cells2) and hematopoietic stem cells (HSCs; 0.71.5cells/108 nucleated marrow cells3). The heterogeneity of BM makes challenging the identification of a subpopulation of cells capable of cardiogenesis, and studies of BM celltocardiac cell transdifferentiation should be examined through this prism.

The notion that BM-derived cells may contribute to the regeneration of the heart was first illustrated when dystrophic (mdx) female mice received BM cells from male wild-type mice.4 More than 2 months after the transplantation, tissues of the recipient mice were histologically examined for the presence of Y-chromosome+ donor cells. Besides the skeletal muscle, donor cells were identified in the cardiac region, suggesting that circulating BM cells contribute to the regeneration of cardiomyocytes.

Further supporting evidence was provided by Jackson et al.5 in studies using a side population (SP) of cells characterized by their intrinsic capacity to efflux Hoechst 33342 dye through the ATP-binding Bcrp1/ABCG2 transporter. The cells were isolated from the BM fraction of HSCs of Rosa26 mice constitutively expressing the -galactosidase reporter gene (LacZ). After SP cells were injected into mice with coronary occlusioninduced ischemia, cells coexpressing LacZ and cardiac -actinin were identified around the infarct region with a frequency of 0.02%. Endothelial engraftment was more prevalent (3.3%). The observed improvement in myocardial function may thus be attributed to the potential of BM cells to give rise to a rather endothelial progeny. This may be a parallel to cardiovascular progenitors from differentiating ESCs giving rise to cardiomyocytes, and endothelial and vascular smooth muscle lineages.6,7

Orlic et al.8 also reported the regeneration of infarcted myocardium after transplantation of lineage-negative (LIN)/C-KIT+ BM cells from transgenic mice constitutively expressing enhanced green fluorescent protein (eGFP). Cells were injected in the contracting wall close to the infarct area. Nine days after transplantation, an impressive 68% of the infarct was occupied by newly formed myocardium with eGFP+ cells displaying cardiomyocyte markers such as troponin, MEF2, NKX2.5, cardiac myosin, GATA-4, and -sarcomeric actin. Similar outcomes were reported by the same group9 when mouse C-KIT+ (but not screened for LIN) BM cells were transplanted.

Although these findings led to the conclusion that BM cells can repopulate a damaged heart, work by other investigators has casted doubt on this assertion. Balsam et al.10 noted that mice with infarcts receiving BM LIN/C-KIT+, C-KIT-enriched or THY1.1low/LIN/stem cell antigen-1 (SCA-1+) cells exhibited improved ventricular function. However, donor cells expressed granulocyte but not heart cell markers 1 month after injection. In another study,11 HSCs carrying a nuclear-localized LacZ gene flanked by the cardiac -myosin heavy chain promoter were delivered into the periinfarct zone of mice 5h after coronary artery occlusion. One to 4 weeks later, LacZ+ cells were absent in heart tissue sections from 117 mice that received HSCs. Similarly, no eGFP+ cells were detected in the infarcted hearts of mice infused with BM cells constitutively expressing eGFP. Finally, Nygren et al.12 in similar transplantation experiments observed only blood cells (mainly leukocytes) originating from BM HSCs in the infarcted myocardium without evidence of transdifferentiation of donor cells to cardiomyocytes.

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