In guinea pig experiments, Johns Hopkins scientists fused common connective tissue cells taken from lungs with heart muscle cells to create a safe and effective biological pacemaker whose cells can fire on their own and naturally regulate the muscle's rhythmic beat.
"This work with fibroblasts could pave the way to an alternative to implanted electronic pacemakers," says Eduardo Marbán, M.D., Ph.D., professor and chief of cardiology at Hopkins and its Heart Institute. "Such a 'biopacemaker' is a potentially important option for patients at too high a risk for infection or who are physically too small to accommodate mechanical pacemakers."
Two sets of tiny electroactive "pacing cells" give rise to the heart's normal rhythm by stimulating other cells to contract in certain sequences. Potentially fatal arrythmias occur when these pacing cells are damaged or die, and implanted pacers have been lifesaving for the estimated 250,000 Americans a year who can tolerate them.
The Hopkins findings, to be presented Nov. 16 at the American Heart Association's annual Scientific Sessions in Dallas, are among several approaches scientists are taking to develop biopacemakers. What makes the Hopkins approach stand out, says Hee Cheol Cho, Ph.D., a postdoctoral cardiology research fellow at Hopkins, is that the fibroblasts are found throughout the body, even in skin. "They proliferate well and grow fast and when fused with heart muscle, form cells that are very stable. Thus, our method would seem to the safest and most convenient so far," he says.
Other biopacemaker technologies, Cho notes, use adenoviruses as part of gene therapy to carry pacing genes into the heart, or use combinations of gene- and stem-cell therapies that may cause cardiac inflammation or uncontrolled cell growth that cause arrhythmias instead of stopping them.
"It is very difficult to guide stem cells into forming exactly the kind of cell needed, but not so with fibroblasts," he says.
In his guinea pig studies, Cho, along with others at Hopkins, successfully combined regular heart muscle cells having no pacing abilities with fibroblasts taken from the animals' lungs. The fibroblasts had been altered by adding HCN1, a gene that codes for potassium ion channels, and another gene, If, which produces proteins involved in electrical signaling, called pacemaker channels. Such channels are protein structures that permit electrical signals, the ions, to pass in and out of cells.
Within three minutes of fusion, the cells showed signs of forming their own potassium ion channels and began generating their very own electrical current, one much like the heart's natural pacing cells would. The effect lasted at least two weeks. The team also fused heart muscle cells with control fibroblasts that had not been genetically altered, but no pacemaker activity developed.
Subsequent tissue analysis of the Hopkins biopacemaker showed that muscle cells had incorporated the pacing gene into their own cytoplasm (the material inside the cell membrane but outside of the nucleus) and were capable of generating an electrical current, effectively turning them into pacing cells. Indeed, If was expressed only in the biopacemaker cells - not in heart muscle cells alone or in the heart muscle cells fused with control fibroblasts.
In a second experiment, when the genetically altered fibroblasts were injected into the animals' hearts - which had been chemically slowed - they fused with heart muscle cells and quadrupled heart rates to nearly half-normal levels. Tests performed to record the electrical activity of the hearts showed that the pacemaker channel fibroblasts, after fusing with heart muscle, were helping guide the heartbeat, while control fibroblasts injected into other guinea pigs' hearts showed no increase in electrical activity.
"These animals' heart rates were headed for a shut-down, but the biopacemakers took over," Cho says. These cells quadrupled the animals' heart rates, from one beat every two seconds to two beats per second. "This shows that the fused cells can fire spontaneously, the hallmark of pacing cells."
While electronic pacemakers work, they have limitations, notes Cho. The device's battery must be changed periodically, a permanent catheter tube must be implanted in the chest to allow access to the pacemaker, and diodes that carry electric current must be embedded in the heart, creating infection risks.
In another experiment, led by Hopkins cardiology research fellow Yuji Kashiwakura, Ph.D., the Hopkins team showed that an alternate potassium ion channel in the muscle cells could be converted to a pacing ion channel, a backup mechanism that could protect the heart from triggering rejection of the biopacemaker.