HARVARD GAZETTE ARCHIVES

Douglas Cowan and his team used muscle cells from the backs of rats to restore a normal heart beat. The technique could be used in infants with disrupted heart rhythms. (Staff photo Jon Chase/Harvard News Office)

Muscle cells have been used successfully to restore life-sustaining rhythms to ailing hearts, a first step toward developing natural pacemakers. Placed in a tiny raft of collagen implanted into the hearts of rats, these cells survived for the entire lifespan of the animals.

"Our experiments provide proof that engineered tissue can function as an electric conduit in the heart and, ultimately, may offer a substitute for artificial (electronic) devices," says Douglas Cowan. He is an assistant professor of anesthesiology at Harvard Medical School who led a team of biologists, cardiologists, and surgeons at Children's Hospital Boston to create a biological substitute for the tissue that keeps the heart beating regularly.

When birth defects, heart disease, surgery, or other causes block that vital function, people can suffer heart failure or other life-threatening disruptions of rhythm. Standard treatment is to implant an electronic device to provide a strong, regular beat. Cowan estimates that hundreds of thousands of these pacemakers are installed each year. But, often, this isn't a lifetime fix. Patients must undergo re-operations every 4-5 years to replace internal batteries and wires. And, recently, the media has carried reports of thousands of pacemakers recalled because they are defective.

The problem is particularly heart breaking for children. "Over the course of their lives, many more battery-replacement operations are required for a 2-year-old than, say, a 55-year-old who gets a pacemaker," Cowan points out. "Add to this the risk of infection, perforation of the small heart by the wires, and other complications. Thirty years ago, many infants with disrupted heart rhythms did not survive. But new surgical techniques now make it possible to keep them alive."

Muscle cells from the backs of rats (green) were implanted into their hearts, cells of which are stained red and green. The implanted cells were able to develop into electrical nodes, which kept the hearts beating for the life of the animals. (Courtesy Douglas Cowan)

To fill what they see as an accelerating need, Cowan and his team decided to build a biological replacement for the node of tissue that helps synchronize the beating of a heart.

Heartening results

Electrical impulses that collect at the node excite muscles in the two atria or upper chambers of the heart, squeezing blood into the lower chambers or ventricles. These impulses then travel quickly to the ventricles, exciting them to contract and pump blood out of the heart. The left ventricle sends blood that is rich in oxygen into the body's arteries, while the right ventricle sends "spent" or deoxygenated blood to the lungs. Then the cycle starts again when "fresh," oxygen-laden blood from the lungs fills the left atrium, and deoxygenated blood goes into the right atrium.

To work efficiently, this atrioventricular (AV) node must coordinate atrial and ventricular contractions in a precise rhythm that allows the chamber to completely fill, then empty. Any disruption in the electrical impulses that travel through the AV node can lead to heart failure and death.

Cowan's team, which included Yeong-Hoon Choi and Christof Stamm, heart surgeons from Children's Hospital Boston, decided to use muscle cells from the backs of rats to power their tissue-engineered device. The cells chosen were young or precursor cells not yet fully developed into working muscles. The hope was that they could be "trained" to take over the work of the AV node.

Such cells have several advantages. "They are renewable," Cowan notes. "You can get more of them if needed, unlike heart cells." Also, they do not trigger the body's immune defense system to reject them as foreign invaders.

These cells were "seeded" into tiny, flexible scaffolds of collagen, a tough, fibrous protein that holds together various cells and tissues in the body. The resulting bit of three-dimensional living tissue was then surgically implanted into the rats' hearts, between the right atrium and right ventricle. In the case of human infants, muscle cells could come from their legs or thighs, and the collagen from a small spot of skin.

The results were heartening. The muscle cells easily integrated with nearby heart cells. They coupled with them electrically and began transmitting signals that stimulated contractions of the chambers.

Nearly a third of the engineered hearts grew new electrical pathways, and these stayed functional through the lives of the rodents, about three years. Details of these experiments and their results appear in the July issue of the American Journal of Pathology.

Muscle cells embedded into a collagen scaffold were sewn into the hearts of rats where they coupled with heart cells and began transmitting electric signals to keep the heart beating in a precise rhythm. (Courtesy Douglas Cowan)

First step taken

It's a first step toward helping an estimated 14,000 infants and young children whose hearts need to be re-timed in the U.S. each year. But that's not going to happen soon.

The hardest part of the experiment, Cowan says, involved measuring just how much good the implants did. The rats kept their natural AV node, so it was difficult to determine which electric node did what. In an effort to separate the two pathways, the researchers used a special device that actually shows how electric impulses move along the surface of the heart. Only a small number of such instruments exist, and they are all hand-made.

With its help, the team found that the implants did not operate exactly like natural heart timers. Human AV nodes pause between contractions to allow the chambers to fill up. The implants, on the other hand, did not come to a full stop.

The researchers intend to work on this glitch with the help of larger experimental animals. Newborn lambs, approximately the size of human infants, will be used, and their natural AV node will be knocked out to determine how well the tissue-engineered implants work.

Other types of cells for AV nodes also will be tried. The precursor cells used in the experiments so far can only develop into muscles and fat. It may be better to use cells in an earlier state of development, such as stem cells, which can form a broader variety of tissue types, including heart cells.

However they are made, Cowan does not see biological pacemakers completely replacing electronic ones. "I envision saving lives with a combination of both," he says. "The biological implant would be dominant, but if it should fail, an electronic device would take over." This would eliminate the need for a lifetime of battery-replacement operations and extend the usefulness of an electronic device.

The Harvard group is not alone in working on this problem. "Other researchers are injecting stem cells and genes cells directly into animal hearts," Cowan says. With all these efforts, "I believe a technique will eventually be developed to more efficiently treat infants and young children with these serious heart disruptions."