When heart muscle cells are made from embryonic stem cells, they integrate into the heart and form proper connections with other heart muscle cells. Such experiments have been conducted in mice, guinea pigs, and nonhuman primates (i.e. monkeys). Chong and others earlier this year (Nature (2014) 510, 273-277) implanted heart muscle cells produced from embryonic stem cells into the hearts of nonhuman primates that had suffered from heart attacks. There was extensive evidence of engraftment of these cells, remuscularization of the heart, and electrical synchronization 2 to 7 weeks after transplantation. However, despite these successes, the hearts of some of these animals also showed abnormal heart beat patterns (known as arrhythmias). Such a problem has also been observed in other laboratory animals as well (see my book The Stem Cell Epistles), and this problem has to be addressed before derivatives of pluripotent stem cells can be used to treat damaged hearts (pluripotent means capable of differentiating into all the mature adult cell types).
Jianyi Zhang and his colleagues at the University of Minnesota have used induced pluripotent stem cells made from human skin cells to produce heart muscle cells that were used to treat pigs that had suffered from induced heart attacks. Their results differed slightly from those of Chong and others.
Zhang and others noted that implanted heart muscle cells typically survive better if they are implanted with blood vessel cells (endothelial cells or ECs). This was first shown in culture by Xiong and others in 2012 (Circulation Research 111, 455-468), but other work has confirmed this. That is, Zhang’s coworkers in his laboratory co-transplanted heart muscle cells made from induced pluripotent stem cells with endothelial cells and smooth muscle cells (which are also a part of blood vessels), and saw that the co-transplanted cells survived much better than heart muscle cells that were transplanted without these other cell types.
On the basis of these experiments, Zhang and his crew decided that implanted heart muscle cells would do much better if they were implanted into pig hearts if they were implanted with endothelial and smooth muscle cells. This was the hypothesis that Zhang and others wanted to test in this paper (which was published in Cell Stem Cell, Dec 4, 2014, 750-761).
Skin biopsies from human volunteers were used as a source of skin cells that were then genetically engineered and then cultured to form human induced pluripotent stem cells (hiPSCs). These cultured hiPSCs were differentiated into heart muscle cells by means of the “Sandwich method,” which yielded beating heart muscle cells in about 30 days. Additionally, their hiPSC lines were differentiated into smooth muscle and endothelial cells as well.
Next, Zhang and his colleagues and collaborators used 92 pigs and subjected them to experimentally-induced heart attacks. Why pigs? Pigs are a larger animal than rodents, and their hearts are larger and beat much slower than the hearts of rats and mice. Therefore, they are a more expensive, but better experimental model system for the human heart. Nevertheless, these pigs were divided into six different groups (3 pigs died from the procedure, so there were 89 pigs involved in this experiment). Animals in the first group or SHAM group underwent the surgery to induce a heart attack, but no heart attack was induced. The second group was called the MI group and this group received no other interventions after surgery. The Patch group received a fibrin patch over the site of injury, but no cells. The CM + EC + SMC group received injections of 2 million heart muscle cells, two million endothelial cells, and two million smooth muscle cells directly into the injured portion of the heart. The Cell + Patch group received all three cell types in a fibrin patched that was imbued with a growth factor called Insulin-like growth Factor-1 (IGF-1) that had been loaded into microspheres. This causes the growth factor to be released gradually and exert its effects over a much greater period of time.
That’s a lot of information so let’s review – six groups: 1) SHAM (no heart attack; 2) MI (heart attack and no treatment); 3) Patch (just the fibrin patch); 4) Cells + Patch (fibrin patch with the three cell types); 5) Cells (cells, but no patch), and a final group cells Patch + CM (just heart muscle cells in the patch).
Animals were evaluated one week after the heart attack and four weeks after their heart attacks. I am uncertain how soon after the heart attack the treatments were given, but in the paper it reads to me as though the treatments were given right after the heart attacks had been induced. Because all implanted cells were engineered to glow in the dark, the number of surviving cells could be counted and tracked.
Only 4.2% of the cell survived in the Cells group, up to 9% of the cells in the Cell + Patch group survived. 32% of the cells in the CM + Patch group survived. Thus, it seemed as though the presence of the other cell types did increase the survival of the heart muscle cells and the patch also increased cell survival rates. Secondly, the heart function of all the treated groups was better than the MI group, but the hearts treated with Cells + Patch were clearly superior to all the others, with the exception of the SHAM group. The hiPSC-derived heart muscle cells also clearly engrafted into the hearts of the pigs, but the big surprise in this paper is that THERE WERE NO INDICATIONS OF ARRHYTHMIAS!!! Apparently the manner in which these hiPSC-derived heart muscle cells integrated and adapted to the native heart in such as way as to preclude irregular electrical activity. Another indicator measured was ratio of phosphocreatine to ATP. If that sounds like a language from outer space, it simply means a measurement of the efficiency of muscle mitochondria (the part of the cell that makes all the energy). Again the Cells + Patch hearts had significantly more efficient mitochondria, and, hence, better energy production than the other hearts. Damage to mitochondria also tends cause cells to up and die, which means that these cells were in better health that those from the MI group.
This paper shows that an ingenious tissue engineering innovation that uses a fibrin patch and a a combination of cells, not just heart muscle cells can significantly increase the healing after a heart attack. Also, even though neither embryonic stem cell-derived cells nor iPSC-derived cells are ready for clinical trials, this paper shows that iPSCs are not as far behind iPSCs as some authors have suggested. Furthermore, because iPSCs would not be subject to immunological rejection, they have an inherent superiority over embryonic stem cells. The problem comes with the time required to make iPSCs and then derived heart muscle cells from them, which might put it outside the time window for treat of an acute heart attack.