After a heart attack, the cells within the damaged part of the heart stop beating and become ensconced in scar tissue. Not only does this region not beat, it does not conduct the signal to beat either and that can not only lead to a slow, sluggish heartbeat, it can also cause irregular heart rates or arrhythmias.
Now, however, scientists have demonstrated that this damage to the heart muscle need not be permanent. Instead there is a way to transform those cells that form the human scar tissue into cells that closely resemble beating heart cells.
Last year, researchers from the laboratory of Deepak Srivastava, MD, the director of Cardiovascular and Stem Cell Research at the Gladstone Institute, transformed scar-forming heart cells (fibroblasts) into beating heart-muscle cells in live mice. Now they report doing the same to human cells in a culture dishes.
“Fibroblasts make up about 50 percent of all cells in the heart and therefore represent a vast pool of cells that could one day be harnessed and reprogrammed to create new muscle,” said Dr. Srivastava, who is also a professor at the University of California, San Francisco. “Our findings here serve as a proof of concept that human fibroblasts can be reprogrammed successfully into beating heart cells.”
In 2012, Srivastava and his team reported that fibroblasts could be reprogrammed into beating heart cells by injecting just three genes (collectively known as GMT, which is short for Gata4, Mef2c, and Tbx5), into the hearts of live mice that had been damaged by a heart attack (Qian L, et al., Nature. 2012 31;485(7400):593-8). From this work, they reasonably concluded that the same three genes could have the same effect on human cells.
“When we injected GMT into each of the three types of human fibroblasts (fetal heart cells, embryonic stem cells and neonatal skin cells) nothing happened—they never transformed—so we went back to the drawing board to look for additional genes that would help initiate the transformation,” said Gladstone staff scientist Ji-dong Fu, Ph.D., the study’s lead author. “We narrowed our search to just 16 potential genes, which we then screened alongside GMT, in the hopes that we could find the right combination.”
The research team began by injecting all candidate genes into the human fibroblasts. They then systematically removed each one to see which were necessary for reprogramming and which were dispensable. In the end, they found that injecting a cocktail of five genes—the 3-gene GMT mix plus the genes ESRRG and MESP1—were sufficient to reprogram the fibroblasts into heart-like cells. They then found that with the addition of two more genes, called MYOCD and ZFPM2, the transformation was even more complete.
To help things along, the team used a growth factor known as Transforming Growth Factor-Beta (TGF-Beta) to induce a signaling pathway during the early stages of reprogramming that further improved reprogramming success rates.
“While almost all the cells in our study exhibited at least a partial transformation, about 20 percent of them were capable of transmitting electrical signals—a key feature of beating heart cells,” said Dr. Fu. “Clearly, there are some yet-to-be-determined barriers preventing a more complete transformation for many of the cells. For example, success rates might be improved by transforming the fibroblasts within living hearts rather than in a dish—something we also observed during our initial experiments in mice.”
The immediate next steps are to test the five-gene cocktail in hearts of larger mammals. Eventually, the team hopes that a combination of small, drug-like molecules could be developed to replace the cocktail, which would offer a safer and easier method of delivery.
This latest study was published online August 22 in Stem Cell Reports.