Thyroid hormone helped our ancestors survive but left us vulnerable

Unlike many cells in the body, heart cells do not regenerate once they are damaged. According to an American study, this vulnerability comes from the evolution of our mammalian ancestors, and particularly from the thyroid hormone.

Every year in the United States, 735,000 people survive an infarction. However, they still have sequelae because the heart tissue often remains irreparably damaged. Heart cells, unlike others in our body, do not regenerate. American researchers in San Francisco believe they understand why. According to them, our mammal ancestors would have lost, in order to survive, this capacity. And this, in exchange for endothermia, colloquially called "hot blood". Their study is published in the journal Science.

Regulate your own body temperature, the key to survival

The first mammals, a kind of small rodent, emerged in a world where animals were cold-blooded (fish, amphibians, reptiles). The latter could not regulate their own body temperature, so they were condemned to live in temperate climates. Mammals have developed a strategy called endothermia. With their "warm blood", they have managed to live in colder climates. A survival technique that ultimately costs humanity. Indeed over time mammals have lost their ability to regenerate parts of the body and organs, such as the heart. At first glance, the link between the ability to regulate one's body temperature and the inability to regenerate heart damage is not obvious. Still, San Francisco researchers seem to have found it: thyroid hormones.

Thyroid hormones prevent regeneration of heart cells

The thyroid gland, in humans, is located in the neck. It produces thyroid hormones, known to regulate body temperature and heart function. The thyroid hormones can generate heat, so they are supposed to have a role when the blood goes from "cold" to "hot". The scientists, led by Guo Huang, a research scientist at the University of California, San Francisco, Institute of Cardiovascular Research, found that thyroid hormones prevent heart tissue from repairing when damaged, as they stop cell division. A first in the world of science.

Mammalian cells divide less

Huang's team compared the "ploidy" of cardiac cells from 41 different vertebrate species. Ploidy refers to the number of copies of each pair of chromosomes in a cell. It is closely related to a cell's ability to divide and replicate. Virtually all animal cells are diploid, that is, they contain only one pair of each chromosome (a copy inherited from the father and the mother). On the other hand, there are also polyploid cells, which contain several copies of each pair and can not usually divide.

According to the researchers, cold-blooded animals have largely diploid cells, which respond to cardiac injury by increasing cell division. Warm-blooded mammals have polyploid cells, which rarely divide after cardiac injury. "This led us to hypothesize that thyroid hormones responsible for regulating body temperature could also be responsible for the diploid polyploid transition and the arrest of cardiac cell division," says Huang.

When you change your thyroid hormone levels, the heart repairs itself

Final step in the research: the San Francisco team conducted laboratory experiments with mice - warm-blooded mammals - and a zebrafish, a cold-blooded animal known for its ability to completely repair its heart, even though large pieces are removed surgically. The researchers injected newly born mice with a drug to block the thyroid hormone receptors. As a result, two weeks later, they found four times more diploid heart muscle cells than mice that did not receive the drug. The researchers also genetically modified mice, whose cardiac cells did not have a functional receptor for thyroid hormones. The latter also had a large number of diploid cardiac cells.

In addition, when scientists limited blood flow to the heart, a condition that usually causes permanent damage to the heart tissue, they observed a ten-fold increase in the number of heart cells. Cardiac function also improved by 11%. For the zebrafish, the researchers did the opposite. They increased thyroid hormone levels while imputing part of the heart. The repair of the heart, normally fast, has been considerably slowed down. The number of polyploid cardiac cells has increased.

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