doi: 10.1161/01.RES.0000147619.96113.51
© 2004 American Heart Association, Inc.
Editorials |
Cardiac Regeneration
Self-Service at the Pump
From the Departments of Internal Medicine (D.J.G., C.M.M.) and Molecular Biology (D.J.G.), and The Donald W. Reynolds Cardiovascular Clinical Research Center (D.J.G.), University of Texas Southwestern Medical Center, Dallas.
Correspondence to Daniel J. Garry, MD, PhD, NB11.118A, 5323 Harry Hines Blvd, University of Texas Southwestern Medical Center, Dallas, TX 75390-8573. E-mail daniel.garry{at}utsouthwestern.edu
See related article, pages 911–921
Key Words: heart failure • stem cells • heart • cardiospheres • c-kit • Sca-1 • Abcg2
Despite current pharmacological and whole organ transplantation strategies, advanced heart failure remains a common and deadly disease.1 Limited availability of donor organs for use in orthotopic heart transplantation has prompted examination of alternative therapeutic strategies directed toward patients with advanced heart failure. Intense interest has recently focused on cell transfer strategies or the mobilization of resident stem cells as strategies that may enhance the repair or regenerative capacity of the myopathic heart.
Cell Transfer Strategies as a Platform for Myocardial Repair
Cell transfer therapies have been used successfully to treat chronic and life-threatening diseases. An effective cell transfer therapy is bone marrow transplantation (BMT); allogeneic and autologous stem cell sources have been used successfully for BMT for >35 years.2 An enhanced understanding of immunosuppression therapy, immunological barriers, and stem cell biology have collectively resulted in >17 000 BMT procedures performed last year by 
200 US centers for the successful treatment of otherwise lethal diseases. The proven efficacy of this therapy using somatic stem cells underscores the potential of adult stem cells and provides further rationale for the applications of cell transfer strategies in disease treatment.2 Unlike BMT, use of cell transfer strategies for treatment of other debilitating and terminal diseases, such as muscular dystrophy, neurodegenerative disorders, diabetes, and advanced heart failure, has had limited or no success.3–5 This lack of success may be multifactorial and may be attributable to patient selection, cell source, timing of delivery (with regard to the onset of disease), decreased viability of cells after delivery (>90% cell death during the 72-hour period after delivery), milieu of the diseased tissue, or inability of the transplanted cells to form a functional syncytium with the host tissue.4–6 Although initial studies suggested that cell transfer therapies for regeneration of myocardial tissue might be effective,7 considerable controversy exists regarding the ability of extracardiac stem cells for transdifferentiation to cardiomyocytes and the long-term viability of these cells after delivery into the injured heart.8,9 These results and controversies raise further challenges and will require future studies that will mechanistically examine the feasibility of cell transfer strategies for the treatment of advanced heart failure. An alternative therapeutic strategy for debilitating diseases is the mobilization of resident tissue–specific stem cell populations that will repopulate diseased tissues.
Somatic Stem Cell Populations Resident in Adult Tissues
The cardinal features of a stem cell population are the ability for self-renewal, increased plasticity, and an increased proliferative capacity.4–6 Virtually all somatic adult tissues have a resident stem cell or progenitor cell population that functions in the maintenance and regeneration of the respective tissues.4,5 The prototypic somatic stem cell population is the hematopoietic stem cell, but other lineages, such as skeletal muscle (ie, satellite cells), liver (oval cells), skin, and brain (neural stem cells), also contain stem cells that function in repair and regeneration.4,5,10,11
The identification of somatic stem cell populations has relied in part on the expression (or the absence) of specific markers including c-kit (CD117, a tyrosine kinase receptor that binds steel factor), CD34, Sca-1 (stem cell antigen-1), Abcg2 (an ATP-binding cassette transporter), etc.4,5,11 Expression of these cell surface markers allows for the convenient identification and isolation of the respective stem cell populations using magnetic cell sorting or flow cytometry.
Cardiac Stem/Progenitor Cells Resident in the Adult Heart
Unlike other somatic tissues, the heart has been viewed as an organ composed of terminally differentiated cardiomyocytes and incapable of regeneration. Recent studies challenge these preexisting notions regarding cardiac repair/regeneration and suggest that the heart is capable of limited regeneration through the activation and recruitment of a stem/progenitor cell population that is resident in the adult heart. Three separate studies have independently described a cardiac stem cell or progenitor cell population that may participate in limited myocardial regeneration in response to an injury (Figure 1).12–15 The article by Messina et al in this issue of Circulation Research further describes the isolation of a population of cells derived from the murine and human heart biopsy samples that display stem/progenitor cell characteristics.16 In this study, the authors describe the culture methodology in which cellular aggregates or cardiospheres (CSs) are obtained from biopsy specimens. These sphere-forming cells could be isolated at repeated intervals from the same explant (ie, biopsy specimen), and once they were established, the CSs (20 to 150 µm in size) would beat in culture spontaneously (ie, murine CSs) or after coculture with rat neonatal cardiomyocytes (ie, human CSs). These CS-forming cells displayed characteristics of a clonal cell population as single cells could form CSs (1% to 10% efficiency), and the subcloned spheres displayed the same functional and phenotypic behavior as the original CS. Periodical dissociation and partial media substitution resulted in considerable expansion of the CSs (log-phase expansion for the human CSs), resulting in >1 million human CSs within a 30-day period. Interestingly, the addition of thrombin to the culture media resulted in a >7-fold increase in the number of CSs. Although the authors did not further examine or discuss this thrombin-mediated effect, thrombin interacts with a number of pathways. Futhermore, studies undertaken in lower models, such as urodele amphibians, have shown that thrombin induces a dedifferentiation of cells to form a progenitor cell population.17 Future studies that define this thrombin-mediated expansion of the CSs in culture will be important for continued characterization of this cell population.
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In addition, the authors use genetic mouse models to label cell populations to complement their flow cytometry and immunohistochemical studies. Collectively, these studies establish that human and murine CSs at the 2- to 10-cell stage were largely positive for KDR/flk-1, CD31 (pecam-1), c-kit, and Sca-1. In contrast, the larger, more mature CSs were described as having a central core of proliferating (5-bromodeoxyuridine–positive) c-kit–positive cell population enveloped by a circumferential layer of differentiated cardiomyocytes. C-kit was the only conserved marker in the development and maturation of the CSs, and these data were interpreted as suggesting the c-kit–positive cells were contributing to the maintenance of proliferation. To confirm this finding, the authors analyzed the CSs isolated from a double heterozygote mouse model (green fluorescent protein [GFP]-cKit/MLC3F-nLacZ or GFP-cKit/TnI-nLacZ), and no double-positive cells were observed in the CSs. Having preliminarily defined the characteristics of the CSs population, the authors evaluated their response after the subcutaneous or intramyocardial delivery of CSs from mouse and human, respectively, into the SCID mouse model. Admittedly, the interpretation of these studies is limited and the results are preliminary. Although the delivery of human CSs into the injured heart of the SCID mouse resulted in improved left ventricular function, it is important to note that the control group received PBS and not an alternative cell source. Use of an alternative cell source (eg, fibroblasts from the CSs) as a control is important because studies have shown that the delivery of cell sources that do not form cardiomyocytes into the injured heart improves cardiac function.18,19 Presumably, these cell sources that do not form cardiomyocytes function to prevent remodeling and limit infarct extension in the injured heart. Furthermore, the labeling of the human CSs with GFP before delivery into the injured heart of the SCID mouse would have allowed the authors to evaluate cell survival and the differentiation capacity of the grafts. These and other future studies will be necessary to decipher the contribution of these cells for myocardial regeneration and rule out the fusion of these cells with the host myocardial cell population.
Despite these limitations, the study by Messina et al describes a culture methodology in which a large number of spheres may be obtained over a relatively short period from a single biopsy sample in mouse and human. Although it is possible to use these spheres as vehicles for gene delivery in various genetic diseases (such as muscular dystrophy), the broader question is why don’t these resident stem/progenitor cells participate in myocardial regeneration after a severe myocardial injury? Do they function only in the maintenance of the myocardium? Are there barriers for recruitment and regeneration of the injured myocardium by these stem/progenitor cells that are resident in the adult heart?
Stem Cells and Heart Failure: A Paradox
An apparent paradox exists when considering the availability of stem cells in the heart and the prevalence of heart failure. Stem cells provide regenerative potential and should influence the status of the heart, thereby decreasing the incidence of heart failure. So why is heart failure so prevalent? Several competing hypotheses may, in part, explain this paradox, and they are schematized in Figure 2. These include but are not limited to an inadequate number of resident myocardial stem cells to repopulate injured tissue after a large myocardial infarction. Alternatively, the fibroproliferative response after a myocardial injury produces a fibrotic scar. This scar may function to limit the access of resident stem cells to the area of injury, or it may limit the release (or serve as an anatomical barrier) of signals (growth factors such as hepatocyte growth factor, insulin growth factors, stromal derived growth factor, etc.) that recruit stem cells to the site of injury. Additionally, the milieu of the injured myocardium may have a negative (ie, inflammatory environment or lack of vascular supply) effect on stem cell viability and differentiation. Finally, an active working contracting heart may have a limited regenerative response compared with a resting, unloaded heart. Future experiments that test each of these hypotheses will be necessary for this field to decipher the mechanisms that regulate these cardiac stem cell populations as a prelude for cardiovascular therapies.
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In summary, the results of recent studies including those of Messina et al suggest we should reconsider the regenerative capacity of the mammalian heart and direct our efforts toward an enhanced understanding of the signals that recruit and expand these stem/progenitor cell populations in vivo. Perhaps the key to cardiac regeneration is in coaxing the pump to service itself.
Footnotes
The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association.
References
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