Children’s Hospital Boston researchers regenerate zebrafish heart muscle

Work has implications for repairing human heart muscle after heart attacks

Experiments on zebrafish provide important clues that could eventually lead to the ability to regenerate damaged human heart muscle, say researchers from the Howard Hughes Medical Institute at Children’s Hospital Boston. Reporting in the Dec. 13 issue of Science, a team led by HHMI investigator Mark T. Keating, MD, senior associate, department of Cardiology, showed for the first time that zebrafish can regenerate heart muscle within two months after a severe injury. The team also identified a possible genetic and molecular model for regeneration in zebrafish that could help direct further research in humans. The findings, while still in an early stage, might someday benefit millions of people who suffer heart attacks or experience other forms of cardiac injury.

The zebrafish is the subject of active study because of its ability to regenerate spinal cord, retina, and fins. This finding points to the study of zebrafish heart regeneration as a means to understand and reduce cardiac injury in humans. When a human heart is injured, it cannot “grow back” the damaged muscle, which is instead replaced by scar tissue. Too much scarring can impair the heart’s ability to pump and can lead to life-threatening arrhythmias. Keating and his colleagues believe that zebrafish, unlike humans, have especially vigorous development of new heart-muscle cells, or cardiomyocytes. This proliferation of cells regenerates the heart muscle with little or no scarring.

“We think that there’s a balance between scar formation and regeneration — if you regenerate, you don’t scar, but if you can’t regenerate, you scar,” Keating said. “If you inhibit cell proliferation in the zebrafish, the heart doesn’t regenerate and instead scars, like in humans. The implication, which isn’t proven, but is quite exciting, is that by enhancing human cardiomyocyte proliferation after heart injury, one may be able to enhance cardiac regeneration and reduce cardiac scarring.”

In the experiments, Keating and co-authors Kenneth D. Poss and Lindsay G. Wilson, also of HHMI and Children’s Hospital Boston, injured the hearts of adult zebrafish by surgically removing 20 percent of the muscle from the heart’s lower chamber, or ventricle. They then returned the animals to water (most survived and resumed swimming) and examined their hearts at regular intervals. They compared their observations with those of a control group of zebrafish that had their hearts removed and returned to their bodies uninjured.

Within a week, the zebrafish with heart injuries were as active and swam as well as the zebrafish whose hearts were uninjured. Within two months, most injured ventricles had regained their size and shape and appeared to be contracting normally. Digitally aided measurements of the ventricles’ surface area showed complete recovery of heart muscle. Cardiomyocytes rapidly increased in number at the edge of the wound, first forming a layer to restore the ventricle wall, and then expanding this new layer of muscle.

Using techniques of cell biology, Keating’s team also showed that zebrafish with a genetic mutation affecting the enzyme mitotic checkpoint kinase (Mps1) no longer had rapid heart-cell formation and lost their ability to regenerate heart muscle. Instead, they had muscle scarring, as humans do. However, Keating emphasized this mutation is not necessarily the mechanism that inhibits cell proliferation in humans.

The next research steps will be to further investigate what genes are required for cardiomyocyte proliferation and heart regeneration in zebrafish, and to seek their counterparts in humans. Two major questions must then be answered. “Can you enhance cardiomyocyte proliferation in humans? That’s not known,” Keating said. “And, if you enhance proliferation, will it induce cardiac regeneration?”

Many amphibians, such as newts, lizards, and salamanders, are also able to regenerate organs and tissues, whereas mammals, including humans, are not. But the zebrafish is an especially valuable model for studying the molecular mechanisms that trigger regeneration, because its genome shares many characteristics with the human genome. The small tropical fish also reproduces rapidly, facilitating genetic studies over multiple generations. A scientific push to sequence the zebrafish genome is under way at Children’s Hospital Boston and elsewhere.

“The field of regeneration has been of interest for thousands of years, but it hasn’t really entered the mainstream of molecular biology,” said Keating. “The reason, in my opinion, is that the tools haven’t been there. We decided to create the field of zebrafish regeneration genetics.”

Children’s Hospital Boston is home to the world’s largest research enterprise based at a pediatric medical center, where its discoveries have benefited both children and adults for over 100 years. More than 500 scientists, including seven members of the National Academy of Sciences, nine members of the Institute of Medicine and nine members of the Howard Hughes Medical Institute comprise Children’s research community. Founded in 1869 as a 20-bed hospital for children, Children’s Hospital Boston today is a 300-bed comprehensive center for pediatric and adolescent health care grounded in the values of excellence in patient care and sensitivity to the complex needs and diversity of children and families. It is also the primary pediatric teaching affiliate of Harvard Medical School. For more information about the hospital visit: www.childrenshospital.org.

This research was funded by the Helen Hay Whitney Foundation and grants from the National Heart, Lung, and Blood Institute.