December 13, 2002
Zebrafish May Point the Way to Mending a Broken Heart
Researchers have found that the secret to mending a broken
heart— at least at the molecular level— resides within the
two-chambered heart of a fish commonly found in household aquariums.
The scientists showed that the zebrafish can regenerate its heart after
injury, and their studies suggest that understanding cardiac
regeneration in this fish may lead to specific strategies to repair
damaged human hearts.
Many studies have documented that various invertebrates can
regenerate vital organs. But most vertebrates and all mammals develop
scarring in response to cardiac injury, with minimal regeneration of
heart muscle. In an article published in the December 13, 2002, issue
of the journal Science, Howard Hughes Medical Institute
investigator Mark T. Keating and colleagues at Harvard Medical School
and Children's Hospital in Boston report that the zebrafish is one
vertebrate that is capable of cardiac regeneration. Two months after
Keating and his colleagues removed 20 percent of the heart in
zebrafish, the fish had fully regenerated the excised portion of the
heart. Keating's group also reported that zebrafish with an induced
mutation in a specific gene failed to regenerate heart tissue and
instead developed scarring.
“The zebrafish could take this field of research out of the Dark Ages.”
Mark T. Keating
Improved understanding of heart regeneration and the associated
problem of cardiac scarring have lagged behind "for want of a
genetically manipulable system to study the problem," Keating said.
Very few genetically well-characterized organisms regenerate damaged
heart muscle, and most scientists who study regeneration focus on fully
regenerating invertebrates including planarians, or flatworms, and the
Hydra polyps, he said. In addition, the complex and poorly
understood genomes of these organisms have proven difficult for
scientists to manipulate in experiments.
Researchers have studied organ regeneration in vertebrates, such as
newts and salamanders, but their hearts scar when damaged. "One
vertebrate organism that is different," said Keating, "is the
zebrafish. It is manipulable and its genetics and genomics are better
known. And it can regenerate just about every type of organ. The
zebrafish could take this field of research out of the Dark Ages."
In the zebrafish studied, Keating and his colleagues made surgical
incisions in the ventral side of adult zebrafish, pushed out the
two-chambered heart and then clipped off a chunk at its apex equaling
about 20 percent of the total organ. After staunching the bleeding, the
scientists put the hearts back in the fish. Some 80 percent of the fish
survived the procedure.
To follow the healing process, Keating and his colleagues examined
the hearts of the fish in the study over a two-month period. They
examined the hearts under a microscope, and began to see before their
own eyes how a heart begins the process of regeneration.
According to Keating, regeneration commences with a clot of
erythrocytes that forms at the site of the wound. Over the next week,
fibrin, the thick, static tissue that forms a mature clot. Starting a
little over a week after the surgery, though, Keating observed a
startling process: the heart started to grow back into its previous
form.
In the first phase of this process, cardiac myofibers, or
cardiomyocytes, the tissue that composes heart muscle and wall, begin
to infiltrate and replace the fibrin over the wound. At one month after
the surgery, a completely new heart wall forms and, based on
observations of chemically labeled cells, the scientists could see that
its leading edge then proliferates and expands rapidly outward until it
completely reforms the missing apex of the heart. Two months after the
surgery, all evidence of scarring is gone, and the size, shape and
cellular activity of the zebrafish hearts appear to be no different
from typical zebrafish. The regenerated heart beats just like a normal,
healthy one.
Keating says that his team's observations indicate that the process
of regeneration in the heart appears similar to that seen in other
organs. The specialized heart muscle cells nearest the wound
dedifferentiate, that is, they lose some of their specialized
characteristics and are capable of cell division and migration. These
stem cells then proliferate as cardiomyocytes until they have rebuilt a
completed heart.
Although most studies of cardiac regeneration in vertebrates
indicate that scarring is a major problem, the zebrafish continued to
regenerate heart tissue with little or no scarring. Keating's theory is
that while both regeneration and scarring are possible in the
zebrafish, a competition takes place between the two. The regenerative
mechanism rapidly overwhelms the formation of scar tissue in this case,
Keating says.
To test the theory, he and his colleagues studied the process of
cardiomyocyte proliferation to see what would happen when it was
inhibited. The investigators looked at zebrafish with a mutation in
Mps1, a gene that encodes a mitotic checkpoint kinase protein
critical in cells that regenerate zebrafish fins. At temperatures above
33 degrees Celsius the mutant form of Mps1 can no longer function
properly and the cells stop proliferating. When the temperature is
raised to that level, the fish with the mutant form of the kinase
protein cannot regenerate hearts. Instead, they produced scars at the
site of the wound.
"That tells us two things," said Keating. "Cell proliferation is
essential for regeneration, and there is a competition between the
regeneration potential of the organism and scarring." In normal
zebrafish, regeneration wins.
Keating is excited about this finding because it provides a hint
that there may be a similar competitive situation at work in human
hearts, which show a minimal regenerative capacity following heart
trauma such as a heart attack. "If one enhances the regenerative
potential in humans," he said, "perhaps one can overcome the fibrotic
potential."
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