MARIO NOTARI, CMRBNewborn mice are able to repair damaged heart tissue better than animals injured just a few days later in their lives. What accounts for this regenerative capacity, and exactly when and why it disappears, have been unanswered questions. A report in Science Advances today (May 2) posits that the extracellular matrix gets in the way of heart tissue renewal.

The investigators also found that scarring was minimal in mice injured on their first day of life, but damage occurring after that, even just a day later, led to large fibrotic scars. “I thought this was an intriguing paper that was well done,” says stem cell and regeneration biologist Richard Lee of Harvard University who was not involved with the work. “It pinpoints the timeline [of neonatal heart regeneration] in a manner that’s more precise than what others have done.”

That result is important and really relevant to human disease.—Joshua Hare,

University of Miama Miller School of Medicine

Other scientists are skeptical that what the researchers observed is true regeneration. Clinical biochemist Ditte Andersen of the University of Southern Denmark who has removed small portions of heart tissue from hundreds of newborn mice but never observed regeneration argues that Raya’s team did not actually show the growth of new muscle. “There is a problem in this research field that we [rely on] this fibrosis hallmark because the [extent of] ventricle outgrowth is very hard to determine,” she says. “If fibrosis is absent, people are very eager to conclude, ‘OK, this is regeneration.’ . . . But it is not evidence of myocardial regrowth.”

Because the adult mammalian heart cannot regenerate to any significant degree, an injury, such as that caused by a heart attack, damages the muscle irrevocably and can ultimately lead to heart failure and death. Following a 2011 paper that showed newborn mice could regenerate their hearts after having a chunk removed, some scientists began speculating that if they could figure out the mechanisms behind this renewal and recapitulate them in human heart attack victims, they might be able to prevent heart failure.

Although Andersen and other scientists have been unable to reproduce these 2011 findings, Ángel Raya of the Center for Regenerative Medicine in Barcelona and colleagues is among the researchers who have observed neonatal regeneration. He reasoned that determining precisely when in the first week of life this capacity ceases might enable the identification of the factors involved. It was known that heart muscle cells continue to copy their DNA for a few days after birth, so one idea was that the heart’s renewal capacity might be linked to this replication.

Raya and colleagues cut out the apical tips from the hearts of newborn mice on day 1, 2, 3, 4, or 9 after birth. Three weeks later, the researchers sacrificed the mice and reexamined their hearts. Animals whose hearts were resected on day 1 showed minimal scarring and the hearts were approximately the same size and shape as those of control animals. By contrast, animals who underwent heart surgery on day 2, 3, 4, or 9 exhibited large fibrotic scars in place of regrowth.

Given the different recoveries of day 1 and day 2 mice, the team looked for differences between the animals’ transcriptomes. “We were actually expecting to find differences in cell cycle [genes], but that was not the case,” says Raya. “The main difference that we found was in genes related to the extracellular matrix [ECM].” The group saw a general upregulation of genes for ECM components and went on to show that the ECMs of day 2 mouse hearts were approximately 50 percent stiffer than those of day 1 hearts. This was “surprising,” Raya says.

It’s critical to show there is new muscle creation.—Mark Mercola,

Stanford University School of Medicine

To determine whether ECM stiffness and regeneration were causally linked, Raya and his colleagues disrupted ECM formation in developing pups. They treated the pups with β-aminopropionitrile (BAPN)—an inhibitor of the ECM cross-linking enzyme LOX—during pregnancy (via the mothers’ drinking water) and for three days after birth (through the mothers’ milk). As a result, three-day-old pups were able to regenerate their hearts with significantly reduced fibrosis compared with controls whose ECMs were intact.

“That result is important and really relevant to human disease,” says cardiologist and stem cell researcher Joshua Hare of the University of Miami Miller School of Medicine who was not involved in the work, “and may offer important insights . . . into potential ways to manipulate regeneration in the adult.”

But Stanford University School of Medicine’s Mark Mercola says the proof that these mice are actually regenerating heart tissue wasn’t provided. “It’s critical to show there is new muscle creation,” he says, and argues that lineage tracing experiments would have been able to identify that.

Raya is in no doubt that regrowth is occurring. “When there is no regeneration, you can see that the heart ventricular apex is missing a bit, and is replaced by a white patch (a scar),” he writes in an email to The Scientist. By contrast, “it is not possible to distinguish, morphologically, a heart that completely regenerated from one that was not amputated.”

Despite the controversy, “we all want to see scarless healing in the adult heart,” says Lee, and with each paper “we’re learning more and more and inching along closer to potentially someday being able to do this.”

M. Notari et al., “The local microenvironment limits the regenerative potential of the mouse neonatal heart,” Science Advances, 4:eaao5553, 2018.