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Engineering cardiac muscle: new ways to refurbish old hearts?

Myocardial infarction leads to irreversible cell loss and scar formation, which result in impaired cardiac function, cardiac hypertrophy and, eventually, heart failure. As the intrinsic regenerative capacity of the human heart is very limited, organ transplantation or ventricular assist devices remain the only end-stage therapy options to date. For at least two decades, alternative therapeutic concepts of heart repair based either on direct cell injection or on the transplantation of in vitro engineered tissue constructs have been considered potential approaches to overcome the shortage of donor organs. However, the multitude of recent experimental and clinical studies applying various adult stem cells have thus far not been able to fulfil the initial high hopes vested in adult stem cell-based therapies. As summarized in a recent meta study, the vast majority of these studies resulted either in no or in only minor functional improvement [1], most likely due to paracrine effects, including accelerated revascularization, enhanced myocyte survival in the infarct border zone and the modulation of scar formation, resulting in improved mechanical properties [2]. Despite sporadic myocyte formation, a significant generation of de novo myocardium could not be demonstrated [3]. Interestingly, very recent studies have demonstrated the possibility of targeted trans-differentiation of cardiac fibroblasts into functional cardiomyocytes (CMs). Inspired by a recent approach utilizing the transcription factor MyoD to drive transdifferentiation of fibroblasts into skeletal muscle [4], a set of cardiac transcription factors was overexpressed for the targeted differentiation of mouse cardiac and tail-tip fibroblasts (Fbs) into cardiomyocytes in vitro [5]. Although trans-differentiation of cardiac Fbs in vivo could be demonstrated [6], the efficiency of such direct conversion experiments is still controversial [7, 8]. At this point, the usefulness of this kind of cell conversion remains uncertain and will critically depend not only on a much higher efficiency, but also on the functional similarity of these artificially converted cells into normal cardiomyocytes. For many years, embryonic stem cells (ESCs) were considered the only cell source suitable to supply the huge numbers of cardiomyocytes lost after myocardial infarction. However, a potential clinical use of ESCs is highly controversial, since their generation requires the destruction of human embryos and, if not produced through therapeutic cloning, no autologous ESCs are available for cellular therapies. In 2006, these limitations were overcome by the groundbreaking development of induced pluripotent stem cells (iPSCs) by S. Yamanaka [9], who was awarded the Nobel Prize in Medicine in 2012. In the meantime, the generation of human iPSCs has become a standard procedure in many laboratories and it is now clear that these cells are almost indistinguishable from ESCs with respect to their phenotype, culture characteristics and potential for proliferation and differentiation. Unfortunately, the availability of human iPSCs and CMs derived from these cells (Figure 1) has solved only one of the most severe limitations for current concepts for myocardial repair. Further critical hurdles are low cell survival and the lack of functional integration of the cellular transplants. In particular, the functional coupling and formation of well-organized myocardium appears to be difficult to achieve. The direct injection of cells to improve heart function was investigated in different animal models with a variety of mouse and human cell sources, including undifferentiated cells as well as cells that had been differentiated to cardiac progenitors or cardiomyocytes. Regardless of the cell type or application method (intramyocardial or intracoronary injection), only up to 5% of the cells remained in the heart, and neither significant integration nor long-term survival could be shown ([10], reviewed by ref. [11]). Although survival and cardiovascular in vitro differentiation was demonstrated, the injection of iPSC-derived cardiovascular progenitors did not result in the formation of structured myocardium [12]. In contrast, a very recent study was the first to report the formation of relatively large electrically coupled and well-structured muscle islands from injected human ESC-derived myocytes in a guinea pig model of myocardial infarction [13]. These data are considered to be very promising, although it was necessary to inject extremely high numbers of myocytes, (i.e. 10 cells, which correlates to 2 × 10 or approximately four times the total number of myocytes in the adult human left ventricle, when projected from the guinea pig to the human heart, which is ca. two hundred times larger). Even more remarkable is the fact that the formation of large contractile human ESC-derived muscle islands have evidently been confirmed by the same group in a