Precursor Pinpointed that Generates Heart Cells

Studies Agree that Heart Arises from Fewer Progenitors than Previously Believed

The Wizard of Oz could only offer a placebo heart to the Tin Woodman, but two teams of HMS researchers have found cardiac progenitor cells that may give rise to the real thing in a flesh-and-blood animal model.

Their papers, published side by side in the Dec. 15 Cell, report that a “master” cell can morph into the two or three major kinds of mouse heart cell.

Photo by Graham Ramsay

From left, Kenneth Chien, Leslie Caron, Atsushi (Austin) Nakano, and their colleagues identified a single heart “master” cell in mice embryos that differentiates into three major cardiac cell types—cardiomyocytes, smooth muscle cells, and endothelial cells. An independent study led by Sean Wu (not pictured) reported similar cardiac differentiation from a neighboring progenitor heart cell.

The findings challenge the classic view of cardiac development, in which the cardiac building blocks stream in from several sources, including the neural crest, at different times in the growing embryo. The results provide hope that scientists may one day be able to identify stemlike cardiac cells in people. The potential benefits of stemlike human heart cells could range from repairing or replacing damaged or diseased heart tissue to mass screening for new drugs.

The authors of both papers urge caution in interpreting their latest studies. For one thing, differentiated heart cells have been grown only on lab dishes. The researchers have not yet shown that the progenitor cells can develop into integrated working heart tissue in an animal.

“There is not a family in this country that is not affected by cardiovascular disease, but these are early days,” said Kenneth Chien, senior author of one of the papers, director of cardiovascular research at Massachusetts General Hospital, and the Charles Addison and Elizabeth Ann Sanders professor of basic science. “We’ve known for decades that embryonic cells of a mouse or human can easily differentiate into beating cardiac muscle, but they can also go in many other different directions. It’s like having kids—you hope they go to Stockholm, but you worry they will end up in Guantanamo.”

The scientific wariness comes from the rush to human clinical trials that followed optimistic reports from other animal studies, since shown to be false, that bone marrow cells could recharge cardiac function after heart attacks by generating new heart muscle or blood vessels. To date, the only proven medical stem cell therapy happens within the blood system. Bone marrow stem cells can regenerate an impressively diverse eight to 10 specialized blood and immune cells—but not solid heart tissue, as far as anyone can yet demonstrate.

Even as that promising basic-science rationale disappeared, randomized placebo-controlled clinical trials have proliferated and will likely continue. So far, injection of bone marrow cells in people with acute myocardial infarction appears to be little or no better than the modern standard of care, according to the latest results of three studies published together in the Sept. 21 New England Journal of Medicine. The experimental therapy appears to be safe, but clinical researchers have yet to identify which—if any—cellular components are beneficial to the heart.

Narrowing the Line of Descent
For basic researchers, the paradigm set by bone marrow stem cells and their progeny has more to do with the meticulous molecular definitions at each stage within a single organ system. The genealogy of the blood looks like half of a basketball tournament bracket in reverse: the champion hematopoietic stem cell on the left branches into progenitor cells, which in turn diverge into the different players on team blood.

The two new Cell papers may provide the start of an equivalent line of descent for heart development. “Both works support the notion that the heart arises from a more limited number of cells than one might have guessed and that these give rise to multiple lineages,” said Stuart Orkin, senior author of the second paper, a Howard Hughes investigator, and the David G. Nathan professor of pediatrics at Dana–Farber Cancer Institute.

For both papers, researchers tapped into the cardiac progenitor cells that form a rainbow-shaped structure during early development in mouse embryos. Single cells from the top rim of the crescent, known as the first heart field, differentiated into both cardiomyocytes (including conduction system cells) and smooth muscle cells in the hands of Sean Wu, first author of the study conducted in the Orkin lab and now a cardiologist and an instructor in medicine at Massachusetts General Hospital.

Single cells destined for the second heart field (the second band of the rainbow) gave rise to three major heart cells—cardiomyocytes (including conduction system cells), smooth muscle cells, and endothelial cells, in the Chien group study, led by postdoctoral fellows Alessandra Moretti, Leslie Caron, and Atsushi (Austin) Nakano.

“The novelty of the story is that there are multipotent cells, like in the hematopoietic system, giving rise to major cell types that form the heart,” said Moretti, currently an instructor at the Technical University in Munich, Germany. “Now it will be interesting to figure out how the cell decides to become one lineage and not the other.”

Blazing the Development Trail
Mature heart cells cannot divide, and so cannot patch up any damage. Nearly two years ago, the Chien lab, then based in San Diego, reported the discovery of remnants of a cardiac progenitor cell in the postnatal hearts of rats, mice, and people. Cultured, the mouse cells grew into differentiated cardiomyocytes with all the signs of being functional. Those cells uniquely sported the islet-1 transcription factor.

Precursor cells in the blood and the brain can give rise to more than one cell type. In the new study, researchers in the Chien lab tracked the islet-1–expressing cells back to their embryonic origins. By tagging the islet-1–expressing cells with a genetic blue marker, they showed these cells gave rise to 90 percent of the right ventricle and lesser portions of other components.

A malleable mouse embryonic stem cell line enabled the researchers to recap some developmental processes. They found that different islet-1 progenitors led to different fates. Progenitors with the additional transcriptional markers Nkx2.5 and flk1, known to mark cells destined for the heart and other tissues, could differentiate into all three major heart cell types. Those with only islet-1 and Nkx2.5 could turn into two cell types, myocytes and smooth muscle cells.

Then experiments moved into cells extracted from the more tightly regulated mouse embryo. In another important advance, the team showed that a lawn of mesenchymal cells isolated from the heart of adult animals could both select for and amplify embryonic cardiac progenitor cells harvested 8.25 days postcoital, while they were transiently expressing islet-1. Once again, cell colonies with a triple marker grew into cardiomyocytes, endothelial cells, and smooth muscle cells.

Meanwhile, Wu had deliberately approached the same cardiac questions from a hematopoietic point of view: beginning with cells sporting the earliest known developmental signpost for heart-only cells. He selected Nkx2.5, in part because the absence of its homologue in fruit flies, called tinman, results in a fly with no heart.

First in embryonic stem cells and then in cells isolated from mouse embryos, Wu and his colleagues found the combination of Nkx2.5- and c-kit-defined precursor cells that could differentiate into both cardiomyoctyes and smooth muscle cells.

“We have both found the cell populations that are the precursors for all these lineages,” Wu said. “These two populations are known to lie side by side during heart formation.” It may be that the cells they sampled from the two bands of the crescents are the same cells at staggered stages of differentiation. So far, the two teams have not figured out exactly how their two slightly different differentiation maps relate to each other, but they will begin to collaborate more closely in Chien’s department at MGH and explore the potential of this approach to benefit people, tapping into the resources and human embryonic stem cell lines of the Harvard Stem Cell Institute.

—Carol Cruzan Morton

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