Stem cells in the heart were first identified in 2003 and since then there has been a great deal of excitement about their potential for use in regenerative therapy. Two clinical trials have been conducted with heart-specific stem cells, SCIPIO and CADUCEUS. Both of these clinical trials were very successful, and further trials might, hopefully, bring this work to the operating room. In the meantime, further work with cardiac stem cells in animals models is necessary in order to clarify the mechanism by which these cells improve the function of an ailing heart.
In light of these findings, Carolyn Carr in Kieran Clarke’s lab at the University of Oxford has published an interesting paper in collaboration with Georgina Ellison at the University La Sapienza in Rome. This paper used magnetic resonance imaging (MRI) to examine rats hearts that had suffered a heart attack, and then had been implanted with cardiosphere-derived stem cells (CDCs). The study showed that CDCs not only improve the function of the heart, but do so for an extended period of time.
Cardiospheres are small balls of cells that have been grown from stem cells that are harvested from the heart. The cells are easily isolated from the upper chambers of the heart (atria; see Messina et al., Circulation Research 95 (2004): 911-24), and when the cells grow, they beat in synchrony and strongly attach to each other. The strong attachment of the cells for each other causes them to grow into small balls of cells that can be implanted into a living heart.
When the CDCs are grown in culture, they generate a mixed population of cells that include progenitor cells that express a gene called “c-kit,” and heart-specific mesenchymal cells that express a surface proteins called “CD90.” Additionally, there are other cells in cultured cardiac cells called explant derived cells (EDCs), which are round and refractory. There is a fair amount of debate as to the nature of these EDCs. Three main papers have demonstrated that EDCs neither form heart muscle nor survive when they are implanted into a living heart (see Shenje LT,et al., Lineage tracing of cardiac explant derived cells. PLoS One. 3(4), 2008:e1929; & Li Z, et al., Imaging survival and function of transplanted cardiac resident stem cells. J Am Coll Cardiol. 53(14), 2009 :1229-40; & Andersen DC,et al., Murine “cardiospheres” are not a source of stem cells with cardiomyogenic potential, Stem Cells. 27(7) 2009, 1571-81). However, other papers have established that EDCs and CDCs can grow in culture and renew themselves (Davis et al., PLoS ONE 4 (2009): e7195; & Chimenti et al., Circulation Research 106 (2010): 971-80; & Davis et al., Journal of Molecular and Cellular Cardiology 49 (2010): 312-21). Other studies have even shown that implantation of EDCs can improve the function of the heart after a heart attack (Shintani, et al., Journal of Molecular and Cell Cardiology 47 (2009): 288-295).
If you are confused by all these conflicting data, join the club because I am too. The clinical studies say that CDCs work to fix a heart, but these animals studies leave us asking if the whole thing is not just a ponzi scheme. This shows us why more animal studies are necessary. Just because something works, does not mean that we know how it works.
In the Carr study, CDCs were isolated from rats and cultured. The cultured tissue yielded CDCs and EDCs that grew well in culture. The CDCs were then labeled with magnetic microspheres to make them easier to detect after transplantation. The CDCs were able to differentiate into heart muscle in culture as determined by their expression of proteins and genes that are specific to heart muscle. Then the rats were given heart attacks, and seven of them received CDC infusions and another seven rats did not. All rats underwent perfusion of the heart, and two days after perfusion, all rats were given CDCs in the tail vein (4 x 10 cells). MRI was used to view the heart and assess its function, and to view the CDCs in the heart.
EDCs in the hands of these researchers not only grew and renewed themselves in culture, but they also formed cardiospheres. Most of the cells were mesenchymal stem cells, but others were fibroblasts and a few others were stem cells (3%). Cardiosphere-derived cells expressed several heart-specific genes. It was also clear that labeling the cells with iron microspheres did not affect the cells.
MRI of the heart showed that the untreated hearts were in functional decline, but those that had been treated with CDCs had better functional readings, an a much smaller scar, and had less problems with heart wall that did not move. MRI also showed that the CDCs had integrated into the heart muscle and were a part of the heart muscle wall. They did not have the maturity of adult heart muscle cells in that they were poorly connected with the neighboring cells. However, they did not die but were maintained in the heart for at least 16 weeks. Blood vessel density was also increased in the hearts of the rats that had received the CDC implantations.
Thus, this CDC population, which far more CD90-expressing mesenchymal cells (30%) and fewer c-Kit cells (12%) homed to the site of heart damage and retained for at least 16 weeks. Furthermore, a large proportion of CDCs formed heart muscle, reduced scarring and increased blood vessel synthesis. These features improved the function of the rat hearts and prevented them from deteriorating further. These cells also survived in the heart and did not die. This shows that the EDCs can differentiate into heart muscle cells and preserve long-term function in the infarcted heart. Therefore, we have evidence that the cells in cardiac stem cells can contribute to a damaged heart and help regenerate dead heart muscle.