The heart of young children might possess untapped potential for regeneration, according to new research. For decades, scientists believed that after a child’s first few days of life, cardiac muscle cells did not divide. Heart growth was thought to occur by means of enlargement of muscle cells.
This view, however, has been seriously challenged in the last few years. New findings in mice that have recently been published in the journal Cell seriously question this dogma. These results have serious implications for the treatment of congenital heart disorders in humans.
Researchers at Emory University School of Medicine have discovered that in 15-day old mice, cardiac muscle cells undergo a precisely timed spurt of cell division that lasts about a day. The total number of cardiac muscle cells in the heart increases by about 40% during this time when the child’s body is growing rapidly. To give you some perspective, a 15-day-old mouse is roughly comparable to a child in kindergarten, and puberty occurs at day 30-35 in mice.
This burst of cell division is driven by a surge of thyroid hormone. This suggests that thyroid hormone might be able to aid in the treatment of children with congenital heart defects. Small trials have even tested thyroid hormone in children with congenital heart defects.
These findings also have broader hints for researchers who are developing regenerative heart therapies. Activating the regenerative potential of the muscle cells themselves is a strategy that is an alternative to focusing on the heart’s stem cells, according to senior author Ahsan Husain, PhD, professor of medicine (cardiology) at Emory University School of Medicine.
“It’s not as dramatic as in fish or amphibians, but we can show that in young mice, the entire heart is capable of regeneration, not just the stem cells,” he says.
This Emory group collaborated with Robert Graham, MD, executive director of the Victor Change Cardiac Research Institute in Australia.
One test conducted by these groups was to determine how well 15-day old mice can recover from the blockage of a coronary artery. Consistent with previous research, newborn (day 2) mice showed a high level of repair after such an injury, but at day 21, endogenous heart repair was quite poor. The 15-day old mice recovered better than the day 21 mice, indicating that some repair is still possible at day 15.
This discovery was an almost accidental finding while Naqvi and Husain were investigating the role of the c-kit gene. The c-kit gene is an important marker for stem cells in cardiac muscle growth. Adult mice that lack a functional c-kit gene in the heart have more cardiac muscle cells. When do these differences appear?
“We started counting the cardiomyocyte cell numbers from birth until puberty,” Naqvi says. “It was a fascinating thing, to see the numbers increasing so sharply on one day.” According to Naqvi, c-kit-deficient and wild-type mice both have a spurt of proliferation early in life but the differences in cardiac muscle cells between the c-kit+ and c-kit- mice appear later.
“Probably, previous investigators did not see this burst of growth because they were not looking for it,” Husain says. “It occurs during a very limited time period.” Even if in humans, the proliferation of cardiac muscle cells does not take place in such a tight time period as it does in mice, the finding is still relevant for human medicine, he says. “Cardiomyocyte proliferation is happening long after the immediate postnatal period,” Husain says. “And cells that were once thought incapable of dividing are the ones doing it.”
Naqvi and Husain plan to continue to investigate the relationships between thyroid hormone, nutrition during early life, and cardiac muscle growth.
Reference: N. Naqvi et al. A Proliferative Burst during Preadolescence Establishes the Final Cardiomyocyte Number. Cell 157, 795-807, 2014.