Israeli Researchers at the Weizmann Institute of Science has discovered a molecule in newborn hearts that appears to control the renewal process of heart tissue suffered damage.
Mammalian hearts can regenerate and repair damage – but only up to around the time of birth. Afterward, that ability disappears, seemingly forever.
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When the new molecules injected into adult mouse hearts injured by heart attacks, this molecule, called Agrin, seems to “unlock” that renewal process and enable heart muscle repair.
Heart disease remains the leading cause of death worldwide, yet the few available treatments are still mostly unsuccessful once the heart tissue has suffered damage. The findings, published in Nature, are pointing to new directions for research on restoring the function of damaged hearts.
Prof. Eldad Tzahor, who led the study together with team members, explains that following a heart attack in humans, the healing process is long and inefficient.
Once damaged, muscle cells called cardiomyocytes replaced by scar tissue, which is unable of contracting and thus cannot take part, in pumping. This leads to further stress on the remaining muscle and eventual heart failure.
Heart renewal, into maturity, does exist in some of our fellow vertebrates. Fish, for example, can effectively invigorate damaged hearts. Mice, the fish closer relative on the evolutionary tree are born with this ability but lose it after a week of life.
That week provides Tzahor and his lab a time window in which to explore the cues that promote heart regeneration.
Tzahor and doctoral student Elad Bassat believed that part of the secret might lay outside of the heart cells themselves – in the surrounding supportive tissue known as the extracellular matrix, or ECM. Many cell-to-cell messages are passed through this matrix, while others stored within its fibrous structure. So the team began to experiment with ECM from both newborn and week-old mice, clearing away the cells until only the surrounding material was left, and then observing what happened when bits of the ECM added to cardiac cells in culture. The researchers found that the younger ECM, in contrast to the older, elicited cardiomyocyte proliferation.
Many cell-to-cell messages are passed through this matrix, while others stored within its fibrous structure. So the team began to experiment with ECM from both newborn and week-old mice, clearing away the cells until only the surrounding material was left, and then observing what happened when bits of the ECM added to cardiac cells in culture. The researchers found that the younger ECM, in contrast to the older, elicited cardiomyocyte proliferation.
A screening of ECM proteins identified several candidate molecules for regulating this response, among them Agrin. Agrin was already known for its effects on other tissues – particularly in the neuromuscular junction, where it helps settle the signals passed from nerves to muscles. In mouse hearts, levels of this molecule drop over the first seven days of life, suggesting a possible role in heart regeneration. The researchers then added Agrin to cell cultures and noted that it caused the cells to divide.
Next, the researchers found that following a single injection of Agrin into mouse hearts were almost entirely healed and fully functional, although the scientists were surprised to find that it took over a month for the treatment to impart its full impact on cardiac function and regeneration. At the end of the recovery period, however, the scar tissue was dramatically reduced, replaced by living heart tissue that restored the heart’s pumping function.
At the end of the recovery period, however, the scar tissue was dramatically reduced, replaced by living heart tissue that restored the heart’s pumping function.
In other words, Tzahor speculates that Agrin somehow affects the body’s inflammatory and immune responses to a heart attack, as well as the pathways involved in suppressing the fibrosis, or scarring, which leads to heart failure. The length of the recovery process, however, is still a mystery, as the Agrin, itself, disappears from the body within a few days of the injection. “Clearly this molecule sets a chain of events in
The length of the recovery process, however, is still a mystery, as the Agrin, itself, disappears from the body within a few days of the injection. “Clearly this molecule sets a chain of events in motion,” he says.
“We discovered that it attaches to a previously unstudied receptor on the heart muscle cells, and this binding takes the cells back to a slightly less mature state – closer to that of the embryo – and releases signals that may, among other things, initiate cell division.” Experiments with mice that were genetically engineered to lack Agrin in their hearts further support this idea: In its absence, newborn mice could not properly regenerate heart tissue following injury. Because mice cannot live without the other functions of Agrin, this was a technically challenging experiment to perform, adds Tzahor.
The team then proved that Agrin has a similar effect on human heart cells grown in culture. He and his team are now working to understand what happens in the period between the injection of Agrin and the return of full cardiac functionality.
