Two transcription factors and a chromatin remodeller help make mouse cardiomyocytes
Ever since researchers turned cultured cells into muscle, scientists have been searching for ways to do something similar to make heart cells.1 That's because, at least in the developed world, heart disease kills more people than anything else — in part because adult hearts are not able to replace damaged cells. Now, Jun Takeuchi and Benoit Bruneau at the Gladstone Institute of Cardiovascular Disease in San Francisco have found that adding cardiac-specific genes to developing mouse embryos can make even some extra-embryonic parts become beating cardiomyocytes2.
Several other researchers have successfully transformed cells using the appropriate transcription factors — Thomas Graf with blood, Doug Melton with insulin-producing cells and Shinya Yamanaka with induced pluripotent stem cells, to name a few. Still, no group had previously been able to find a combination of transcription factors to make heart cells. "It's a great advance. This is something that many folks in the field have been trying to do for many years," says Robert Schwartz of Texas A&M Health Science Center in Houston, who has worked out several essential components of cardiac differentiation. "A lot of folks have been trying this experiment, but they were missing the rate-limiting step."
That step, explains Bruneau, was adding the gene for a different type of protein to the mix, one for a chromatin remodeller called Baf60c that Bruneau had previously identified as important in cardiac differentiation3. That protein helps reset the way that DNA on chromosomes is coiled up "to allow the transcription factors to bind and open up chromatin and make things go".
Baf60c (also called Smarcd3), along with the cardiac transcription factors Gata4 and Tbx5, was able to form cardiomyocytes even in a region of the embryo called the amnion. Though it is comprised in part of mesoderm, the same broad category of germ layer to which heart cells belong, it grows away from the embryo and never becomes part of the body. Getting cardiomyocytes to form in that region demonstrates that the researchers weren't preferentially hitting cardiac progenitors that just hadn't reached their proper location, explains Bruneau. There is other evidence that the technique will work for more cell types, he says. "All these experiments were done prior to the endogenous heart being properly formed. That was a bit of an exciting surprise; we were able to not only turn it on but turn it on ahead of schedule." Bruneau is now trying the technique on cells derived from adult mice because it would more readily translate to heart therapies.
Though it would not solve issues of getting healthy cells to integrate into diseased hearts, the technique might provide a robust way to generate cardiomyocytes from induced pluripotent stem cells, Schwartz says. An important next question is to figure out how the factors induce cardiac differentiation programs. For example, a transcription factor called serum response factor has been shown to be essential for the formation of cardiomyocyte-like cells as evolutionarily far back as the worm Caenorhabditis elegans, he explains4. Does this new combination of proteins activate that and other proteins, or does it override them? Schwartz and Bruneau suspect the former to be true.
What's more, the use of chromatin remodelling proteins might make transformation techniques for other cell types more powerful, says Schwartz. "You can have all the transcription factors you want, but until you change the situation of genes that are off and activate them by chromatin remodelling, you can't drive the differentiation."