Shedding New Light on the Mechanism Underlying Stem Cell Therapy for the Heart
Authors: Christine Mummery, Marie Jose Goumans
Journal: Molecular Therapy (2011)

It is widely accepted that the adult mammalian heart is a terminally differentiated organ with little capacity to regenerate after damage, such as that caused by a myocardial infarction (MI). Nevertheless, mounting evidence suggests that mature cardiomyocytes do slowly turn over (up to 1% per year).1Hsieh PC Segers VF Davis ME Mac Gillivray C Gannon J Molkentin JD et al.Evidence from a genetic fate-mapping study that stem cells refresh adult mammalian cardiomyocytes after injury.Nat Med. 2007; 13: 970-974Crossref PubMed Scopus (603) Google Scholar,2Bergmann O Bhardwaj RD Bernard S Zdunek S Barnabé-Heider F Walsh S et al.Evidence for cardiomyocyte renewal in humans.Science. 2009; 324: 98-102Crossref PubMed Scopus (2270) Google Scholar In addition, there are many claims that the heart contains cell populations expressing stem cell markers that could in principle contribute to repair.3Passier R van Laake LW Mummery CL Stem-cell-based therapy and lessons from the heart.Nature. 2008; 453: 322-329Crossref PubMed Scopus (469) Google Scholar,4Smits AM Ramkisoensing AA Atsma DE Goumans MJ Young at heart. An update on cardiac regeneration.Minerva Med. 2010; 101: 255-270PubMed Google Scholar However, whether proliferating cardiomyocytes or endogenous stem or progenitor cells actually contribute to cardiac repair after injury has remained unclear – until now. In a study recently published in Cell Stem Cell,5Loffredo FS Steinhauser ML Gannon J Lee RT Bone marrow-derived cell therapy stimulates endogenous cardiomyocyte progenitors and promotes cardiac repair.Cell Stem Cell. 2011; 8: 389-398Abstract Full Text Full Text PDF PubMed Scopus (330) Google Scholar Lee and colleagues used a clever lineage-tracing approach in mice1Hsieh PC Segers VF Davis ME Mac Gillivray C Gannon J Molkentin JD et al.Evidence from a genetic fate-mapping study that stem cells refresh adult mammalian cardiomyocytes after injury.Nat Med. 2007; 13: 970-974Crossref PubMed Scopus (603) Google Scholar to identify two potential sources of new cardiomyocytes in the adult heart: those arising from the division of existing cardiomyocytes and those formed by differentiation of the endogenous progenitors. Using their transgenic mouse model, they were able to distinguish these two sources of cardiomyocytes. More strikingly, the authors show that some stem cells currently being tested for the ability to improve heart function in cell transplantation trials can enhance the differentiation of endogenous progenitors, whereas others cannot. The study provides new insight into how cell therapy for the heart might work and suggests ways in which optimal cell types for therapy might be chosen – as well as how the controversial outcomes of many clinical trials might be resolved. Whether or not stem cell transplantation benefits heart function in patients after MI is hotly debated, despite hundreds of reports in animals and the inclusion of thousands of patients in clinical studies. A meta-analysis of several large controlled clinical trials using bone marrow (BM) derivatives6Abdel-Latif A Bolli R Tleyjeh IM Montori VM Perin EC Hornung CA et al.Adult bone marrow-derived cells for cardiac repair: a systematic review and meta-analysis.Arch Intern Med. 2007; 167: 989-997Crossref PubMed Scopus (820) Google Scholar has indicated modest long-term positive effects on ejection fraction (the amount of blood squeezed out of the heart during one contraction), whereas the results of individual studies show greater variability, reporting either no effects,7Hirsch A Nijveldt R van der Vleuten PA Tijssen JG van der Giessen WJ Tio RA et al.Intracoronary infusion of mononuclear cells from bone marrow or peripheral blood compared with standard therapy in patients after acute myocardial infarction treated by primary percutaneous coronary intervention: results of the randomized controlled HEBE trial.Eur Heart J. 2002; (e-pub ahead of print 10 December 2010)Google Scholar effects restricted to patients with large infarcts,8Herbots L D'hooge J Eroglu E Thijs D Ganame J Claus P et al.Improved regional function after autologous bone marrow-derived stem cell transfer in patients with acute myocardial infarction: a randomized, double-blind strain rate imaging study.Eur Heart J. 2009; 30: 662-670Crossref PubMed Scopus (91) Google Scholar or significant benefits mainly to patients with low ejection fraction.9Assmus B Rolf A Erbs S Elsässer A Haberbosch W Hambrecht R et al.Clinical outcome 2 years after intracoronary administration of bone marrow-derived progenitor cells in acute myocardial infarction.Circ Heart Fail. 2010; 3: 89-96Crossref PubMed Scopus (244) Google Scholar There is no systematic correlation with the type or number of transplanted cells or their means of delivery. Moreover, it is unclear whether the function of stem or progenitor cells is adversely affected by predisposing factors for cardiovascular disease, such as diabetes, obesity, or smoking. Animal experiments have previously shown that noncardiac cells are not retained in the heart for more than a few weeks after transplantation and that most accumulate in the lung capillaries.10Hansson EM Lindsay ME Chien KR Regeneration next: toward heart stem cell therapeutics.Cell Stem Cell. 2009; 5: 364-377Abstract Full Text Full Text PDF PubMed Scopus (149) Google Scholar Despite early claims, noncardiac cells are no longer thought to transdifferentiate into cardiomyocytes.3Passier R van Laake LW Mummery CL Stem-cell-based therapy and lessons from the heart.Nature. 2008; 453: 322-329Crossref PubMed Scopus (469) Google Scholar,4Smits AM Ramkisoensing AA Atsma DE Goumans MJ Young at heart. An update on cardiac regeneration.Minerva Med. 2010; 101: 255-270PubMed Google Scholar This probably explains why transplantation of, for example, BM cells in patients with acute MI is considered safe, with no reported incidence of arrhythmias that might occur if (beating) cardiomyocytes were introduced into a human heart.11Mummery CL Davis RP Krieger JE Challenges in using stem cells for cardiac repair.Sci Transl Med. 2010; 2: 27ps17Crossref PubMed Scopus (85) Google Scholar It is unclear what the transplanted cells do in the heart, and it is to this question that Lee and colleagues have now provided some answers. In an elegant paper in Nature Medicine in 2007, Lee et al. described a double-transgenic mouse in which green fluorescent protein (GFP) was expressed conditionally in cardiomyocytes to trace the origin of new cardiomyocytes in the adult heart.1Hsieh PC Segers VF Davis ME Mac Gillivray C Gannon J Molkentin JD et al.Evidence from a genetic fate-mapping study that stem cells refresh adult mammalian cardiomyocytes after injury.Nat Med. 2007; 13: 970-974Crossref PubMed Scopus (603) Google Scholar When a STOP sequence flanked by loxP sites in the transgenic construct was removed genetically in the mice using Cre recombinase under an inducible cardiac promoter, GFP was expressed only in cardiomyocytes (Figure 1a). Put more simply, 80% of the cardiomyocytes permanently expressed GFP and were green. The other 20% expressed b-galactosidase and were blue. If new cardiomyocytes were derived from existing ones, they would be green; if they were derived from other cells (noncardiomyocytes), they would not. Using this method of lineage tracing, the authors demonstrated that after an experimental MI only ∼65% of the cardiomyocytes still expressed GFP instead of the original 80%. This meant that 15% new cardiomyocytes had formed – not from the existing (green) cardiomyocytes but from the nonfluorescent progenitor cell population. This was an important finding because it showed unequivocally that de novo cardiomyocyte differentiation was possible in the adult heart in addition to cardiomyocyte division, although it did not identify the responsible stem cells or their source. However, in terms of clinical relevance, perhaps the more fascinating results using these mice were yet to come. These subsequent findings show the effects of transplanting ectopic BM stem cells into the heart after MI on the differentiation of progenitors to cardiomyocytes. This is what was reported in the Cell Stem Cell article.5Loffredo FS Steinhauser ML Gannon J Lee RT Bone marrow-derived cell therapy stimulates endogenous cardiomyocyte progenitors and promotes cardiac repair.Cell Stem Cell. 2011; 8: 389-398Abstract Full Text Full Text PDF PubMed Scopus (330) Google Scholar BM stem cells were used because various stem cell populations from BM have been transplanted in many of the clinical trials on patients with MI. Among the stem or progenitor cells that have been tested as potential cell therapies for the heart are the c-kit+ subpopulation and mesenchymal stem cells (MSCs), which are derived from BM (or adipose tissue) as a cell population that attaches to tissue culture plastic and can differentiate into bone, fat, or cartilage in response to the appropriate stimuli. In the new study, transplantation of the c-kit+ BM cells into the hearts of the double-transgenic mice after experimental MI reduced the proportion of GFP-expressing cardiomyocytes from 80% to 50%, whereas in the mice that received no cells – just the MI – the reduction was from 80% to ∼60%, as reported in the earlier study (Figure 1b). The injection of c-kit+ BM cells had therefore resulted in an additional 10% of new cardiomyocytes from the endogenous progenitors in the “border zone” of the infarct, with none of these new cardiomyocytes derived directly from the c-kit+ population. This is thus further confirmation of the previous findings that transdifferentiation of BM cells to cardiomyocytes is most unlikely and probably impossible. Intriguingly, though, injection of the MSC population from BM had no effect whatsoever; new cardiomyocyte formation remained at the level of control mice in which no ectopic cells were transplanted. This provides essential clues as to why some cells appear to benefit cardiac function after MI and others do not – they may have different capacities to stimulate endogenous progenitor differentiation depending on their intrinsic phenotype and the underlying cardiovascular disease. The lineage-tracing model, albeit labor-intensive, provides a unique opportunity to compare a specific aspect of stem cell behavior in the infarct zone in vivo and, once fully exploited, should lead us to the underlying molecular mechanisms. This is something the study does not address, however; it remains to be seen whether a paracrine or inflammatory mechanism underlies the observed effects. In addition, the identity of the progenitor population remains to be determined. An exciting future perspective might be that patients could be selected for inclusion in stem cell therapy trials after MI based on the intrinsic properties of their own stem cells, perhaps reducing or eliminating “nonresponders” (patients in whom there appears to be no effect above controls) who have been observed in multiple trials. Even better, if the mechanism underlying the effect were known, it might be possible to mimic or enhance it in vivo, obviating the need for any exogenous stem cells for transplantation to the heart. The principle of lineage tracing in the heart developed by these authors might also be applicable to other organs, so that in the future appropriate stem cells could be identified that mediate, for example, liver or muscle repair. Indeed, rather than a one-size-fits-all approach to stem cell therapy, it may actually become possible to tailor cell therapy treatments to individual patients and diseases. Although there is much work yet to do, the door has been cracked and a bit more light has been shed on the mechanisms underlying organ repair by endogenous progenitor cells in the heart. Research in the laboratory of C.M. is supported by the Netherlands Heart Foundation, EU FP7 (“InduStem” PIAP-GA-2008–230675), ZonMW (114000101), the Netherlands Institute of Regenerative Medicine, and the Netherlands Proteomics Consortium (050–040–250). Research in the laboratory of M.J.G. is supported by the Netherlands Heart Foundation, the Netherlands Institute of Regenerative Medicine, a VIDI grant (016.056.319) from the Netherlands Organization for Scientific Research (NWO), and the SmartCare project of the Biomedical Materials Program (BMM).