Restarting cell division helps the heart to heal itself

Scientists have discovered a protein that works with others during development to put the brakes on cell division in the heart. Published in the journal Nature, the findings could be used to reverse this developmental block and help heart cells regenerate, offering a new way to treat a variety of conditions in which heart muscle becomes damaged.

As explained by Professor Hesham A Sadek, a physician-researcher at UT Southwestern Medical Center, current pharmaceutical treatments for heart failure — including ACE inhibitors and beta blockers — centre on trying to stop a vicious cycle of heart muscle loss as strain further damages remaining heart muscle, causing more cells to die. There are no existing treatments to rebuild heart muscle.

Nine years ago, Prof Sadek and his colleagues discovered that mouse hearts can regenerate if they’re damaged in the first few days of life, spurred by the division of cardiomyocytes — the cells responsible for a heart’s contractile force. However, this capacity is completely lost by seven days old, at which point division of these cells dramatically slows and the cells themselves enlarge.

Prof Sadek and his team later discovered that a protein called Meis1, which falls into a category known as transcription factors that regulate the activity of genes, plays a key role in stopping heart cell division. However, although deleting this gene in mice extends the window of heart cell division, this effect is transient — heart cells missing this gene eventually slow and stop their multiplication.

Consequently, the researchers wondered whether there were redundant mechanisms in place that stop heart cell division even when Meis1 is absent. Seeking other transcription factors that might track activity with Meis1 in heart cells as they rapidly divide and then slow to a halt in the days after birth, they quickly discovered one called Hoxb13 that fits the bill. Other proteins in the Hox family have been shown to act as chaperones to Meis1 in other types of cells, ferrying Meis1 into the cell nucleus.

To better understand Hoxb13’s role in heart cells, the researchers genetically engineered mice in which the gene that codes for Hoxb13 was deleted. These mice behaved much like those in which just the gene for Meis1 was deleted — the window for heart cell rapid division was increased but still closed within a few weeks. When the researchers shut off Hoxb13 in adult mouse hearts, their cell division had a brief resurgence — enough to prevent progressive deterioration after an induced heart attack but not enough to promote significant recovery.

When the researchers deleted both the genes for Meis1 and Hoxb13, heart cells in these mice appeared to revert to an earlier stage in development, both decreasing in size and multiplying more. After an induced heart attack, these mice had a rapid improvement in the amount of blood each beat could expel from the heart. Their heart function had almost returned to normal.

With clear evidence that Meis1 and Hoxb13 work together to stop heart cell division in the days after birth, Prof Sadek and his colleagues looked for what might in turn regulate these proteins. Their experiments suggest that the answer is calcineurin — a protein that’s responsible for regulating the activity of other proteins by removing their phosphate groups. And because calcineurin plays a key role in a variety of diseases and other medical conditions, such as rheumatic arthritis, schizophrenia, diabetes and organ transplant, several drugs already exist on the market that target this protein.

It is thus conceivable that other drugs could be developed to directly target Meis1 and Hoxb13, Prof Sadek said. Indeed, researchers may eventually be able to develop strategies to restart heart cell division through a single drug or combinations that target any part of this regulatory pathway, he added.

Pictured: In this image of adult mouse hearts in which the genes Hoxb13 and Meis1 were knocked out, cardiomyocytes display features normally just seen in newborn mouse hearts.

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