Scientists from the laboratory of Jianyi Zhang at the University of Minnesota Medical School, in collaboration with researchers from the Stem Cell Institute and several other departments at the University of Minnesota have used heart cells from heart surgery patients to make induced pluripotent stem cells. These cells were then differentiated into sheets of heart muscle cells that were used to treat mice that had suffered heart attacks. This work was published in the journal Circulation: Heart Failure, which is a journal of the American Heart Association.
Induced pluripotent stem cells (iPSCs) are made from adult cells by means of genetic engineering and cell culture techniques. They share most, though not all, of the features for embryonic stem cells even though they are not made from the destruction of human embryos. iPSCs have the disadvantage of retaining (at least to some degree in most cases) the epigenetic fingerprint of the cells from which they were derived, which limits the efficiency with which some iPSC lines can be differentiated into various adult cell types. This characteristic of iPSCs, however, is line specific to some degree, and these statements are generalities that hold true for most, though not all iPSC lines.
With this in mind, Zhang and his colleagues sought to make iPSCs from heart cells taken from patients who were undergoing open heart surgery. This way, the epigenetic fingerprint of the iPSCs would be a heart-specific one, and this would enhance rather than detract from the capabilities of the heart cells derived from this iPSC line.
A second novelty of this paper was to use a sheet of heart muscle cells that had been grown in culture and apply this sheet to the damaged area of the heart.
Therefore, Zhang and his collaborators, made what he called hciPSCs or human, cardiac, induced pluripotent stem cells from fibroblasts taken from heart patients. In fact his coworkers made three such lines, one from a male patient and two from female patients. All three iPSc lines had normal numbers of structurally normal-looking human chromosomes. These cells were then differentiated into heart muscle cells using a well characterized protocol that yielded beating heart muscle cells in eight days. These hciPSC-derived heart muscle cells were then grown into sheeting of beating heart muscle cells in culture.
One of the advantages of using a cell sheet, is that it obviates the need for injecting single cells into the heart. Such a procedure causes the death of the majority of the cells that are injected because the heart after a heart attack is a very inhospitable place for newly implanted cells. A cell sheet, however, can be overlaid onto the dead or moribund part of the heart and the cell sheet will adhere (with a few tricks), and integrate onto the surface of the myocardium. The heart muscle cells will orient themselves in the same direction as the native myocardium, and beat in sync with it.
In this study, Zhang and his crew induced heart attacks in laboratory mice, and then took the cell sheets straight from the culture dishes and overload them onto the damaged heart tissue. To get the sheets to stick to the heart, a few small pinprick in the heart were made, and the clotted blood served as a kind of glue that held the sheet to the heart. The heart muscle sheets were also suspended fibrinogen – the protein that is clipped to form fibrin, which composes clots.
In culture the sheets of heart muscle cells derived from hciPSCs handled calcium like actual heart cells than heart muscle cells derived from other types of iPSCs. Thus, this procedure does seems to actually make better heart muscle cells.
When it came to the hearts of the mice treated with the sheets of hciPSC-derived heart muscle cells, the cells in the sheet definitely engrafted into the mouse hearts. Cell death in the treated mice was half that of the control mice that had suffered a heart attack, but were not treated. Furthermore, the overlaid cell sheets were also filled with new blood vessels. These blood vessels were not made by the cells in the sheets, but were the result of native vessels making new branches that grew into the sheets. Heart function also was significantly improved in the mice treated with the heart muscle cell sheets; their ejection fractions were over 10% higher on average, and the fractional shortening (the degree to which the heart physically contracts) was also much higher.
Taken together, these results support the potential of hciPSC-heart muscle cell sheet transplantation for the treatment of heart failure. However, this is a laboratory experiment in a rodent model. I have noted this before, but rodent heart tend to beat very fast and are not a terribly good model system for human hearts. Therefore, this experiment will need to be replicated in a larger animal model system, such as pigs, which have larger hearts that beat much slower. If this protocol works well in pigs, then it might be time to consider adapting this procedure to human patients for clinical trials.