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," said
Eduardo Marban, professor and chief of
cardiology at Johns 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 Johns Hopkins findings, 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 Johns Hopkins
approach stand out, said Hee Cheol Cho, a postdoctoral
cardiology research fellow, 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
be the safest and most convenient so far," he said.
Other biopacemaker technologies, Cho said, 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 said.
In his guinea pig studies, Cho, along with others at
Johns 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 Johns Hopkins
biopacemaker showed that muscle cells had incorporated the
pacing gene into their own cytoplasm (the material inside
the cell membrane but outside 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 said.
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, Cho said. 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 cardiology research
fellow Yuji Kashiwakura, the Johns 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.
The research, which took one year to complete, was
supported by the Donald W. Reynolds Foundation and the
Heart Rhythm Society.