Author + information
- Mark A. Sussman, PhD∗ ()
- San Diego State University, Department of Biology and Integrated Regenerative Research Institute, San Diego, California
- ↵∗Address for correspondence:
Dr. Mark A. Sussman, San Diego State University, 5500 Campanile Drive, NLS426, San Diego, California 92182.
“Diversity: the art of thinking independently together.”
—Malcolm Forbes (1)
Cardiac regenerative medicine has been steadily advancing for well over a decade with a seemingly ever-expanding catalog of both cellular participants and opinions regarding specific role or roles of each stem cell type in the process of mediating myocardial healing. The incessant forward momentum of stem cell studies in the myocardial context is fueled in large part through use of selective surface markers that define subpopulations, thus allowing for identification of cells in situ as well as for selective enrichment. The first such marker to be applied in the myocardial context was c-Kit, a membrane receptor well documented as a progenitor population in hematopoietic and embryonic tissues (2,3). Subsequent studies have forwarded other candidate cell surface markers including Sca-1, platelet-derived growth factor receptor α (PDGFRα), and a host of molecules collectively referred to as cluster of differentiation (CD) antigens. However, these cell surface markers reveal only the tip of the proverbial iceberg of stem cell biology because there are undoubtedly more defining markers of subpopulations lurking within the stem cells themselves sequestered away from surface immunophenotyping protocols. One such molecule, named PW1, exists as a transcription factor of interstitial nonsatellite progenitor cell populations in early postnatal (juvenile) skeletal muscle, also appearing within stem cells of multiple adult tissue types (4,5).
Published reports on PW1 have accumulated a fair amount of detail regarding this factor in skeletal muscle differentiation, but the biology of PW1 in the heart remained obscure until now, as described in the article by Yaniz-Galende et al. (6) in this issue of the Journal (6). There is indeed an interstitial PW1+ cell population in the myocardium that identifies a subpopulation with a preference for fibroblast-type cell commitment. Moreover, the story of PW1 supports the premise that diversity of stem cell populations, as identified with selective markers, is likely to provide important clues regarding myocardial remodeling and repair.
Involvement of PW1 in the context of human heart disease was implicated by expression in explanted tissue of myocardial infarction patients, where the PW1+ cells accumulate in the region of damage. To facilitate investigation of PW1 functional cellular biology, Yaniz-Galende et al. (6) turned to a murine reporter mouse line that expresses β-galactosidase (β-gal) to tag PW1+ cells. PW1 reporter mice possess a resident population of tagged cells that overlap to varying degrees with other canonical stem cell markers such as c-Kit, Sca-1, and PDGFRα, thereby supporting the premise of heterogeneity within the PW1 subpopulation. Using a substrate that fluoresces on exposure to β-gal, tagged PW1+ cells were selected by flow cytometry and characterized in vitro, where they exhibited multipotency and gave rise to multiple cardiovascular and mesenchymal lineages. Transcriptional profiling of PW1+ cells was consistent with a role in tissue development and remodeling. These initial findings were with isolated and cultured cardiac-derived PW1+ cells, but when reporter mice were subjected to ischemic injury by myocardial infarction the PW1+ cell numbers increased and appeared to be proliferating. Colocalization of the PW1+ cells with markers typical for fibroblasts suggested that these cells were adopting a fibrogenic fate following myocardial infarction. Although it would have been easy to conclude that the PW1 cells likely contribute to scar formation in the wake of pathological damage, Yaniz-Galende et al. (6) chose to look a bit deeper into the population and found that the story became a bit more complicated than just a scar.
Coincidence of PW1 with c-Kit in a normal murine heart was relatively low, but it increased significantly by 3-fold following infarction injury. To unravel an explanation for expansion of the c-Kit marker on the PW1 population, cells were sorted into populations expressing PW1 alone, c-Kit alone, or both PW1 and c-Kit together. Interestingly, although PW1+/c-Kit- cells were more prevalent in normal heart samples, the population of double-positive PW1+/c-Kit+ cells increased dramatically following infarction. Subsequent characterization showed that PW1 expression correlated with differentiation capacity of the c-Kit+ cells, meaning that each of the 3 subpopulations exhibited distinct profiles of cell commitment markers. Even more provocatively, although the majority of PW1 cells did indeed run toward fibrogenic or myofibroblast commitment, only 1 population exhibited predilection toward cardiogenic differentiation evidenced by expression of α-sarcomeric actin: the c-Kit+/PW1− cells. The punch line from the study is that PW1 could very well serve as a marker to discriminate between stem cells destined to contribute to fibrosis and those contributing to myogenesis.
Through elegant and logical experimental design, the emerging role of PW1+ cells in the heart appears to be, at least in the context of myocardial infarction injury, mediators of fibrotic deposition. Having another marker to delineate fibroblast precursors in the heart would be worthwhile because PW1 could potentially be exploited to modulate post-infarction remodeling and scar formation. Of course, the current study by Yaniz-Galende et al. (6) does not shed much insight into the molecular role of PW1 as a regulator of stem cell biology, and hopefully future investigations will provide more insight into how PW1 operates on a transcriptional or signaling level within the precursor cells of the heart, as had been previously published for skeletal muscle (4). So, too, we are left to ponder the possibilities of manipulating PW1 expression to affect precursor cell fate. Perhaps by repressing the expression of PW1 in the expanding population following infarction injury we could direct the cellular response away from maladaptive fibroblast or myofibroblast proliferation that, although clearly necessary to reinforce ventricular structure after damage, also can prove deleterious if provoked for extended periods of time.
Moreover, the tantalizing possibility also exists that by redirecting the double-positive PW1+/c-Kit+ cells into single-positive c-Kit+/PW1− cells we may be able to tilt the cellular response to infarction toward production of more myogenic precursor cells to replace and rebuild damaged myocardial tissue. Such in vivo reprogramming using transcription factors has been explored for fibroblast conversion to myocytes (7,8), and the advent of PW1 as a novel marker of the expanding fibroblast precursor cell population will provide another avenue to narrow in on a relevant target population for manipulating myocardial injury responses in pursuit of more beneficial outcomes.
The study from Yaniz-Galende et al. (6) illustrates the promise and the pitfalls of cardiovascular regenerative research, serving as a cautionary example for those who pursue a reductionist view of stem cell biology. Clearly, most PW1+ cells contribute to fibrogenic activity following infarction damage; scar formation is one of the myocardial responses to injury that the heart seems highly competent to perform. However, the allure of findings from Yaniz-Galende et al. (6) may reside within the relatively rare PW1−/c-Kit+ cell population possessing presumptive cardiogenic potential. Although the role of c-Kit cells as mediators or myocardial repair has been attacked (9) and the rationale for clinical stem cell therapy has been challenged (10), the basis for these assaults could be explained by reductionist thinking and a lack of consideration for diversity and cooperativity in the cardiac stem cell population. Indeed, the findings of Yaniz-Gallende et al. (6) are in perfect harmony with the conclusion that c-Kit+ cells are poor mediators of cardiogenesis because most of the c-Kit+ cells identified in the infarcted heart are also PW1+ and lead toward fibrotic remodeling. However, to focus on this straightforward outcome misses the nuanced point that the c-Kit+ cell population is heterogeneous, with a minority subpopulation that lacks PW1. Therefore negative selection to PW1 could be just as important, if not more so, in understanding how cellular responses could be reoriented and improved for cardiomyogenesis. Because our adult myocardial biology is clearly distinct from that of lower vertebrates (11) or neonatal mice (12) capable of robust cardiomyogenesis, perhaps our focus should be on trying to identify rare and phenotypically unusual cells residing within the adult myocardium that retain myogenic potential.
The field of myocardial regenerative research is only recently embracing concepts of combinatorial cell therapy (13), “rejuvenation” of cardiac stem cells (14), and diversity of resident stem cell populations (15). Transcriptional analyses such as that performed by Yaniz-Galende et al. (6) need to be implemented on a single cell level with population-based analyses to reveal inherent subtle diversity of the PW1+ cell population that is hinted at by cell surface discrimination using multiple markers such as c-Kit, Sca-1, and PDGFRα. As appreciation of cooperativity and diversity of adult cardiac progenitor cells grows, our ability to predict cell behavior accurately and to select for optimal cell-based regenerative therapeutic approaches thoughtfully will concomitantly increase. When it comes down to choosing the right stem cell or cells, I concur with Malcom Forbes on another of his observations: “Being right half the time beats being half-right all the time” (16).
↵∗ Editorials published in the Journal of the American College of Cardiology reflect the views of the author and do not necessarily represent the views of JACC or the American College of Cardiology.
Dr. Sussman is supported by National Institutes of Health grants R01HL067245, R37HL091102, R01HL105759, R01HL113647, R01HL117163, P01HL085577, and R01HL122525, as well as an award from the Fondation Leducq. Dr. Sussman is Chief Science Officer and Founding member of CardioCreate, Inc., a regenerative medicine company.
- 2017 American College of Cardiology Foundation
- ↵BrainyQuote. Malcom Forbes quotes. Available at: https://www.brainyquote.com/quotes/quotes/m/malcolmfor151513.html. Accessed June 23, 2017.
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