Electronic pacemakers are used in children and adults with certain heart conditions that interfere with a normal heartbeat. However, these life-saving devices can't react the way the heart's own pacemaker normally does -- for example, raising the heart rate to help us climb stairs or react to a scary movie.
In the researchers' experiments, described in the Dec. 20 advance online edition of Circulation, human ES cells were genetically engineered to make a green protein, grown in the lab and then encouraged to become heart cells. The researchers then selected clusters of the cells that beat on their own accord, indicating the presence of pacemaking cells. These clusters triggered the unified beating of heart muscle cells taken from rats, and, when implanted into the hearts of guinea pigs, triggered regular beating of the heart itself.
"These implanted cells also responded appropriately to drugs used to slow or speed the heart rate, which electronic pacemakers can't do," says study leader Ronald Li, Ph.D., assistant professor of medicine. "But many challenges remain before this technique could be used for patients. We want to bring this to the clinic as fast as possible, but we need to be extremely careful. If this process isn't done properly, it could jeopardize a very promising field."
The genetic engineering of the ES cells, accomplished by Tian Xue, Ph.D., a postdoctoral fellow at the School of Medicine, inserted a gene (for green fluorescence protein) so that the human cells would be easily distinguished from animal cells in the experiments. Since the engineered cells survived and worked properly, other more clinically important genetic engineering of the cells also will probably not interfere with the cells' fate, say the researchers.
"To our knowledge, these are the first genetically engineered heart cells derived from human ES cells," notes Xue. "We're now using genetic engineering to customize the pacing rate of these cells, for example. For any future clinical applications, you want to make sure that the beating rate is what you want it to be."
First isolated at the University of Wisconsin, the human ES cells used by the researchers have the natural ability to become any type of cell found in the human body, and therefore they hold the potential to replace damaged cells. But such applications await proof that the desired type of cells can be obtained, isolated and controlled, because expected risks include primitive cells developing into tumors or implanted cells being rejected.
In the researchers' experiments, clusters of beating human heart cells derived from ES cells were injected into the heart muscle of six guinea pigs. A few days later, the researchers destroyed each animal's own pacemaking cells, located near the point of injection, by freezing them. Careful electrical measurements on the hearts revealed a new beat, coordinated by the implanted human cells and slower than the animals' normal heart rate -- likely reflecting humans' lower heart rate.
To prove that the human heart cells were controlling the beat of the guinea pigs' hearts, colleagues Fadi Akar, Ph.D., and Gordon Tomaselli, M.D., conducted careful experiments that showed exactly where the electrical signal originated and followed the signal's conduction across the heart's surface. Sure enough, the signal started from the transplanted human cells, easy to locate because of their fluorescence.
"We've answered three very important questions," says Xue. "We've shown that these human cells survived when we put them into the animals, they were able to combine functionally with the animal's heart muscle, and they didn't create tumors for as long as we have watched."
But new questions have come up because of these promising results, notes Li. For instance, the researchers don't know why the animal's immune system didn't attack and kill the human cellular "invaders " -- that was a surprise. One possibility is that the cluster of cells didn't connect enough with the animal's circulatory system to trigger an immune response, but more experiments will be necessary to see whether that's the case and, if so, how that might affect the implanted cells' long-term survival.
The researchers weren't too surprised that no tumors formed over the course of a few months of observation, however, since they had selected beating heart cells and left behind any cells that weren't adequately specialized.
The stem cell approach isn't the first Hopkins research to create a biological pacemaker, but it is likely to be a better choice if the heart is very damaged. In 2002, Hopkins scientists reported that inserting a particular gene into existing heart muscle cells in a guinea pig allowed the cells to create a pacemaking signal. If heart damage is extensive, however, it might be preferable to introduce new pacemaking cells, rather than to convert existing cells into pacemakers, notes Li.
The research was funded by the National Heart, Lung and Blood Institute, the Blaustein Pain Research Center, the Croucher Foundation, and the Cardiac Arrhythmias Research and Education Foundation. Authors are Xue, Li, Akar, Tomaselli, Eduardo Marbán, Heecheol Cho, Suk-ying Tsang and Steven Jones, all of Johns Hopkins.