doi: 10.1161/01.RES.0000117524.76792.6B
© 2004 American Heart Association, Inc.
Editorials |
Some Like It Plastic
From the Department of Medicine, Cardiovascular Research Institute, New York Medical College, Valhalla, NY.
Correspondence to Annarosa Leri, MD, or Piero Anversa, MD, Cardiovascular Research Institute, Vosburgh Pavilion, Room 302, New York Medical College, Valhalla, NY 10595. E-mail annarosa_leri{at}nymc.edu; piero_anversa{at}nymc.edu
Key Words: transdifferentiation • fusion • cardiac repair • regenerative medicine
The artist,..., like the thinker or the scientist, seeks the truth and makes his appeal."1 The artist, however, enjoys a superior level of freedom and in doing so he "must strenuously aspire to the plasticity of sculpture, to the color of painting, and to the magic suggestiveness of music." Plasticity is the foundation of the Manifesto of the Futurist Sculpture2: "the Futurist sculptor perceives the body and its parts as plastic zones, and will introduce into the sculptural composition twenty different materials, provided that the plastic emotion requires it." As in the case of the radical position of Boccioni, the discovery that the body contains plastic cells, ie, cells with the ability to change into another cell type, has created skepticism, excitement, surprise, and disbelief. Because of controversial findings, which are at times impossible to reconcile,3–7 the painful process of understanding and reaching a consensus has been difficult and is not completed yet. Cells during prenatal development undergo a hierarchical progressive restriction of developmental options. This process was thought to be irreversible and inviolable in adulthood. However, several examples of transition from one cell type to another or, even more unexpectedly, from one cell lineage to a different lineage have challenged this paradigm.8–11 Such a condition is described in the study by Planat-Bénard and collaborators in this issue of Circulation Research,12 in which multipotent stromal cells isolated from adipose tissue differentiate spontaneously into cardiac myocytes in vitro.
Some confusion in terminology has certainly contributed to the heated debate about stem cell plasticity. Originally, transdifferentiation was defined as an irreversible switch from one differentiated cell to another differentiated cell type.13 Transdifferentiation belongs to a broader class of cell transformation defined as metaplasia14 that also includes cases in which stem cells of one tissue become cells of another tissue. With the recent explosion of the field of regenerative medicine and stem cell therapy, the terms "plasticity" and "transdifferentiation" are used as synonyms and the more accurate nomenclature of metaplasia has been abandoned. Moreover, the reintroduction of the notion of cellular fusion, extremely popular in the 1980s,15,16 has created further uncertainty about stem cell plasticity. Cellular and/or nuclear fusion requires the merge of two distinct cells with formation of a hybrid. The growth of the heterokaryon seems to depend on the nucleus of the more undifferentiated cell that dominates the nucleus of the somatic cell by transferring its replication properties while the destiny of the heterokaryon is regulated by the differentiated cell.17,18 Whether the twist in fate occurs by transdifferentiation or fusion, reprogramming of chromatin configuration is required, mostly through activation of transcription factors driving the formation of specific progeny. In both cases, the mechanism is slow and limited in efficiency.19 Moreover, in the event of cell fusion, the bulky burden of the high nuclear DNA content implies genetic instability and reduced replicative potential. In this state of confusion, rigorous criteria have to be met in any study involving alternative differentiation to avoid that misleading artifacts are interpreted as scientific facts.
The gold standard concerning the determination of the ancestor-descendant relationship during in vitro transdifferentiation has been satisfied in the study by Planat-Bénard et al12: purity of the preparation, clonal analysis, assessment of the differentiated phenotype, and functional assays. With respect to the identity of the primitive cells, the stroma vascular fraction (SVF) of the adipose tissue includes a population of spindle-shaped cells with multipotent characteristics.12 The procedure of isolation of these cells is similar to that commonly used for mesenchymal stem cells that are present in several organs of mesodermic origin.20 The progenitor properties of the subset of SVF cells used by Planat-Bénard et al12 have been validated through the documentation of their capability of generating preadipocytes, adult adipocytes, and other cell types.21 The use of a semisolid matrix of methylcellulose has allowed the authors to obtain by proliferation of single progenitors distinct colonies containing morphologically recognizable progenies. This approach has been used for colony assay of hematopoietic stem cells and for growth and cloning of satellite muscle cells and neural stem cells.22–24 In the present work,12 individual founder cells gave rise to clusters of adipocytes and groups of rounded cells mixed with myotube-like structures. The latter represents a clone that comprises self-renewing primitive cells and differentiating cells. The low cloning efficiency detected here is typical of stem cells from various organs.
A thorough morphological and functional analysis of the clones was performed at different time points in vitro by using polymerase chain reaction, immunohistochemistry, electron microscopy, and patch-clamp techniques. Transcription factors, such as MEF2C, GATA4, and Nkx2.5, which are early markers of embryonic cardiac development, were documented. Cells with the characteristics of atrial and ventricular myocytes were identified; atrial myocytes contained ANF granules and expressed MLC-2a while ventricular myocytes exhibited MLC-2v. Importantly, clonogenic cells were spontaneously beating. The electrical activity of individual cells changed with time in a manner identical to that found during the commitment and maturation of embryonic stem cells in adult myocytes. The response of 30-day-old cells to chronotropic agents was consistent with the behavior of cardiomyocytes: a dose-dependent increase and decrease in the beating rate were observed when myocytes were exposed to isoproterenol or carbamylcholine, respectively. The sensitivity of the cells to these molecules was heterogeneous, most likely reflecting different developmental stages of myocytes in vitro.
Because of the rigorous methodology, the study of Planat-Bénard and collaborators12 is an important proof of stem cell plasticity. The commitment of adipocyte progenitors toward the myogenic lineage represents an example of multipotency within the same germ layer. The confounding possibility of cell fusion can be excluded here because of the growth potential of a single cell. To the best of our knowledge, the present study provides the first demonstration of spontaneous stem cell differentiation in contracting myocytes without coculture with embryonic, fetal, or adult myocytes or the exposure to specific differentiating media. Although the in vitro differentiation of adipose tissue-derived stem cells is beyond question, their efficacy in vivo remains to be shown. Moreover, an unanswered relevant issue is whether these cells can also form endothelial and vascular smooth muscle cells and thereby constitute new functioning myocardium in vivo.
The significance of this study12 lies more on its biological impact than on its clinical importance. Once again, the three fundamental properties of stemness—clonogenicity, self-renewal, and multipotentiality—have been unequivocally documented in adult stem cells. These intrinsic features of stem cells exclude cell fusion as a necessary requirement for growth and differentiation and point to the plasticity of adult stem cells. In a clinical perspective, it might be more efficient and powerful for tissue reconstitution to use stem cells that reside in the same organ that has been damaged, although adipose tissue stem cells are easily accessible and expandable for therapeutic purposes. The identification of cardiac stem cells clustered in niches25–28 indicates that the heart is a self-renewing organ and possesses an intrinsic growth reserve capable of responding, at least in part, to the physiological and pathological demands of the myocardium. The presence of stem cells that reside in the heart (Figure) and are therefore predestined to become cardiac cells overcomes the need for the more complex and time-consuming process of chromatin reorganization involved in the commitment to cell lineages different from the organ of origin.19 A more rapid and efficient regeneration of lost or irreversibly altered myocardium is often crucial for the survival of the organ and organism. This clinical necessity has its dramatic overtone in patients with large myocardial infarcts in which the immediate reduction of infarct size is critical for survival. However, this desperate setting does not diminish the importance of the fact that the organism resembles a futurist sculpture, which corresponds to "a pure construction of completely renewed plastic elements."2
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This is why some like it plastic.
Acknowledgments
This work was supported by NIH Grants HL-38132, AG-15756, HL-65577, HL-66923, HL-65573, AG-17042, and AG-023071.
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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