Atsushi Asakura and his colleagues at the University of Minnesota Stem Cell Institute have extended some of their earlier findings in a paper that appeared in PLoS One last year. This paper is almost a year old by now, but its results are fascinating and are definitely worth examining.
In 2007, Asakura published a paper with the Canadian researcher Michael A. Rudnicki in the Proceedings of the National Academy of Sciences. In this paper, Asakura and his colleagues examined the ability of muscle satellite cells from MyoD- mice to integrate into injured muscle. I realize that last sentence just sounded like gobbledygook, to some of you, but I will try to put the cookies on a lower shelf.
Satellite cells constitute a stem cell population within skeletal muscle. They are a small population of muscle-making stem cells found in skeletal muscle and they express a whole host of muscle-specific genes (e.g., desmin, Pax7, MyoD, Myf5, and M-cadherin). Satellite cells are responsible for muscle repair, but previous work has shown that there are at least two populations of satellite cells in skeletal muscle. One population rapidly contributes to muscle repair, whereas the other population is more stem cell-like and remains longer in an undifferentiated state in the recipient muscle (see Beauchamp JR , et al (1999) J Cell Biol 144:1113–1122; Kuang S , et al (2007) Cell 129:999–1010). Presently, it is not clear which population is more efficient in repairing continuously degenerating muscle.
MyoD is a gene that encodes a protein that binds to DNA and activates the expression of particular genes. It plays a vital role in regulating muscle differentiation, and belongs to a family of proteins known as myogenic regulatory factors or MRFs. All MRFs are bHLH or basic helix loop helix transcription factors, and they act sequentially in muscle differentiation. MRF family members include MyoD, Myf5, myogenin, and MRF4 (Myf6). MyoD is one of the earliest genes that indicates a cell has committed to become a muscle cell. MyoD is expressed in activated satellite cells, but not in quiescent (sleeping) satellite cells. Strangely, even though MyoD marks myoblast commitment, muscle development is not dramatically prevented in mouse mutants that lack the MyoD gene. However, this is likely to result from functional redundancy from Myf5. Nevertheless, the combination of MyoD and Myf5 is vital to the success of muscle production.
Therefore, Asakura and his crew decided to isolated muscle satellite cells from mice that lacked functional copies of the MyoD gene. Making such mice is labor intensive, but doable with mouse embryonic stem cell technology. When such MyoD- mice were made, Asakura and others isolated the satellite cells from these mice and characterized them. They discovered in their 2007 paper, that the satellite cells from the MyoD- mice were much more stem cell-like than satellite cells from MyoD+ mice. The MyoD- satellite cells grew better in culture, integrated into injured muscles better and survived better than their MyoD+ counterparts.
Why is this important? Because when it comes to treating degenerative muscle diseases like muscular dystrophy, finding the best cell is crucial. MyoD+ satellite cells have been used, but they are limited in the amount of muscle repair they provide. MyoD- cells might be a better option for treating a disease like muscular dystrophy.
Or for that matter, what about the heart? Finding the right cell to treat the heart after a heart attack has proven difficult. There are some things bone marrow cells do well, and other things they do not do well when it comes to regenerating the heart. Likewise, there are some things mesenchymal do well and other things they do not do well when placed in a damaged heart. Can MyoD- satellite cells do a better job than either of these types of stem cells?
That was the question addressed in the 2012 Nakamura paper that was published in PLoS One. Clinical trials that have treated heart attack patients with injections of MyoD+ satellite cells into the heart have shown that such treatments can improve heart function, but usually only transiently. They also prevent remodeling of the heart after a heart attack. However, two larger studies failed to produce significant improvements in heart function compared to the placebo, and patients who received the satellite cell transplants were also susceptible to very fast heart beats (tachycardia). Because of these downsides, the excitement for transplanting muscle satellite cells into the heart has waned.
So how did MyoD- satellite cells do? All the laboratory animals used in this experiment (BALB/c mice) were given heart attacks, and injected with either MyoD+ or MyoD- satellite cells. The hearts of animals injected with MyoD- satellite cells were compared with animals whose hearts were injected with MyoD+ satellite cells.
In culture, the MyoD- satellite cells grew better than the MyoD+ cells. When injected into the heart, the MyoD- cells integrated into the heart muscle and spread throughout the heart muscle much more robustly than the MyoD+ cells. The MyoD- cells were also much less susceptible to cell death and survived better than their MyoD+ counterparts.
Functionally speaking for the heart, animals that had received transplantations of MyoD- satellite cells had higher ejection fractions, small areas of dead heart tissue, lower end systolic and end diastolic volumes, and more normal echocardiograms. Even though MyoD- cells differentiated into skeletal muscle and not heart muscle (no surprise there), the MyoD- cells induced a very substantial quantity of new blood vessels to sprout in the scar area.
From these experiments, it seems that the MyoD- satellite cells are superior to the MyoD+ satellite cells for treating heart after a heart attack. These cells secrete a whole host of factors that aid the heart in healing and also structurally support the heart and prevent remodeling.
Might it be possible to use such cells in human trials? Asakura notes that engineering MyoD- satellite cells would be impractical for human clinical purposes, but it might be possible to downregulate MyoD expression with drugs (bromodeoxyuridine) or other reagents (RNAi or Id protein transformation).
This work shows that there is a better way to use muscle satellite cells for heart treatments. It simply requires you to remove MyoD function, and the cells will grow and spread throughout the heart better, and more robustly augment heart function and healing.
See NakamuraY, et al PLoS One 7(7) 2012:e41736.