Stem Cell Fixes for the Heart

Two recent papers have provided very good evidence that pluripotent stem cells can help heal a heart that has experienced a heart attack. One of these papers used induced pluripotent stem cells from rats, and the other used embryonic stem cells.

The first paper comes from the laboratory of Yoshiki Sawa, who is a professor in the Department of Surgery at the Osaka University Graduate School of Medicine in Osaka, Japan. In this paper, Sawa’s group made induced pluripotent stem cells (iPSCs) from mice and cultured them under conditions known to induce differentiation into heart muscle cells. Beating cells were detected and grown on gelatin-coated plates with Delbecco’s medium. When these cells were tested for gene expression, they made all the same genes as those found in a mouse heart.

To get the cells to form sheets of heart muscle cells, Sawa and his team plated his iPSCs on UpCell plates that are coated with a chemical that causes the cells to adhere to it at normal temperatures, but when the temperature is dropped, the cells detach from the plate. Sawa used another innovation with this culture system; he grew cell without any sugar. This caused all the non-heart cells to die off. The result was a sheet of heart muscle cells that contracted in unison.

Next, the Sawa team took induced heart attacks in a Japanese rat strain. 2 weeks after suffering the heart attack, the sheet of heart muscle cells were placed on the heart scar in half of the rats and the other half received no implants.

Four weeks after implantation of the heart muscle sheet, the differences in heart function were stark. The ejection fraction in the hearts of the animals that had received the iPSC-derived heart muscle sheets increased almost 10%. The fractional shortening, which is the degree to which the heart muscle shortens when it contracts, also increased more than 5%. Also, the amount of stretching during pumping decreased, which indicates that the heart is pumping more efficiently.

When the heart muscle from the implants were examined, they were also filled with molecules associated with the production of new blood vessels. Thus the implanted heart muscle sheets also helped heal the heart by inducing the formation of new blood vessels.

A danger of using iPSC-derived heart muscle cells is the tendency to miss undifferentiated cells and have undifferentiated cells that cause tumors. In this experiment, they noticed tumors if they only grew the cells in the sugar-free medium for a little while. However, if they grew the iPSC-heart muscle cells in sugar-free media for at least three days, all the tumor-causing cells died and implants from these sheets never formed any tumors.

This paper demonstrated the efficacy and plausibility of using patient-specific iPSCs to treat a heart that has had a heart attack some time ago.

The second paper comes from the laboratory of Marisa Jaconi in Geneva, Switzerland. In this paper, Jaconi and her gang of stem cell scientists at the Geneva University Hospitals and the Ecole Polytechnique Fédérale de Lausanne used a “cardiopatch” seeded with cardiac-committed embryonic stem cells to treat a heart attack in rats.

Because the injection of stem cells can induce arrhythmias (irregular heart beats), narrowing of blood vessels, blood vessel obstruction, and other types of damage, these two papers tried to use sheets of cells or cells embedded in biodegradable patches to treat the heart. In this paper, Jacobi and others used a hydrogel made from fibrin, which is the same material found in blood clots. Into that fibrin hydrogel, they placed mouse embryonic stem cells that had been treated with a protein called BMP-2, which drives pluripotent stem cells toward a heart cell fate.

To use these cardiopatches, Jacobi and her group induced heart attacks in a French rat strain and then applied the patch to the heart. They had two groups of rats; those that had been given heart attacks and those that had not. The sham group received either a patch with cells, a patch with iron particles (for detection with MRI) or not patch. The heart attack group received the same.

The results are a little hard to interpret, but the patch + cells definitely improved heart function. First, the hearts that had received patches with cells showed in increase in small blood vessels and blood vessel-making (CD31+) cells. Therefore the patches + cells improved heart circulation. Second, the hearts with the patch + cells showed the presence of new heart muscle cells and much mess thinning of the walls of the heart. Third, the heart functional parameters were better preserved in the patch + cells hearts. The ejection fraction decreased substantially in the hearts that did not receive cells, but in the hearts that received patch + cells, the amount of blood left in the heart after pumping and at rest did not increase nearly as much as in the other groups. These parameters are in indication of the efficiency with which the heart is pumping. The fact that the heart + cells hearts did not decrease in efficiency nearly as precipitously as the others shows that the stem cells are healing the heart.

While these results may not seem terribly robust, we must remember that the cardiopatch was only placed over a small portion of the heart. Therefore, we would not expect to see large increased in function. The fact that we do see new heart muscle cells, new blood vessels, and an arrest in the functional free fall of the heart is significant, given the small area of the heart that was cover with the cells.

The cardiopatch is a new technology and this experiment showed that the patch biodegrades quickly and without incident. It also showed that embedding cells in the patch is feasible, and that the patch is a plausible vehicle to deliver cells to the heart. This procedure also induced the formation of new heart muscle cells in the heart scar and new blood vessels too. Perhaps even more encouraging is the absence of tumors reported in this paper. Even though the ESCs were not differentiated completely into heart muscle cells, the cardiac-directed cells were differentiated enough to form either blood vessels, smooth muscle, or heart muscle. This seems to be enough to prevent the cells from forming tumors. Also, the fibrin scaffold was not deleterious to the heart, even though some studies have used other scaffolds that are damaging to the heart.

Thus cardiopatches and cardiac muscle sheets are perfectly good strategies for treating heart with stem cells. More work needs to be done, but the results are encouraging.