Cardiac stem cell research has a turbulent history. Studies revealing the presence of regenerative progenitors in adult rodents’ hearts formed the basis of numerous clinical trials, but several experiments have cast doubt on these cells’ ability to produce new tissue. Some scientists are now lauding the results of a report published in April in Circulation as “undeniable evidence” against the idea that resident stem cells can give rise to new cardiomyocytes.
“The concept of [many] clinical trials arose from the basic science in labs of a few individuals more than 15 years ago, and that basic science is what’s now being called into question,” says Jeffery Molkentin, a cardiovascular biologist at Cincinnati Children’s Hospital who penned an editorial about the latest work.
The first evidence supporting the notion of cardiac stem cells in adults emerged in the early 2000s, when researchers reported that cells derived from bone marrow or adult heart expressing the protein c-kit could give rise to new muscle tissue when injected into damaged myocardium in rodents. These studies “caused some controversy right from the start,” Molkentin says. “The main reason that this struck a raw nerve with people is because we already know that heart, in human patients, doesn’t regenerate itself after an infarct.”
Early skepticism arose in 2004, when two separate groups of researchers published back-to-back papers refuting the claims that bone marrow–derived c-kit cells could regenerate damaged heart tissue. Still, the concept of endogenous cardiac stem cells remained a mainstream idea until Molkentin and his colleagues published a study in 2014 reporting that c-kit cells in the adult mouse heart almost never produced new cardiomyocytes, says Bin Zhou, a cell biologist at the Chinese Academy of Sciences and a coauthor of the new study.
Although Molkentin’s findings were replicated shortly afterwards by two independent groups (including Zhou’s), some researchers held fast to the idea that cardiac progenitors could regenerate injured heart tissue. Earlier this year, a team of researchers—including Bernardo Nadal-Ginard and Daniele Torella of Magna Graecia University in Italy and several other scientists who conducted the early work on c-kit cells—published a paper reporting the flaws in the cell lineage tracing technique employed by Molkentin, Zhou, and their colleagues. For example, they noted that the method, which involved tagging c-kit–expressing cells and their progeny with a fluorescent marker, compromised the gene required to express the c-kit protein, impairing the progenitors’ regenerative abilities.
In the new Circulation study, Zhou and his colleagues used a different approach to examine endogenous stem cell populations in mice. Instead of tagging c-kit cells, the team applied a technique that would fluorescently label nonmyocytes and newly generated muscle cells a different color from existing myocytes. This method allowed the researchers to investigate all proposed stem cell populations, rather than specifically addressing c-kit cells. “We wanted to ask the broader question of whether there are any stem cells in the adult heart,” Zhou says.
These experiments revealed that, while nonmyocytes generate cardiomyocytes in mouse embryos, they do not give rise to new muscle cells in adult rodents’ hearts. The results also address the concerns raised about c-kit lineage tracing, Zhou tells The Scientist. “We think our system can conclude that nonmyocytes cannot become myocytes in adults in homeostasis and after injury.”
Torella says that he’s not convinced by Zhou’s evidence. The main issue, he explains, is that the researchers did not explicitly test whether cardiac stem cells were indeed labeled as nonmyocytes to ensure that they were not inadvertently tagging them as myocytes instead.
Molkentin disagrees with this critique, stating that the only way the system would label a myocyte progenitor as a myocyte is “if it was no longer a true stem cell, but instead an immature myocyte.” Zhou’s group uses an “exhausting and very rigorous genetic approach,” he adds. “My opinion is that we need to go back to the bench and conduct additional research to truly understand the mechanisms at play to better inform how we design the next generation of clinical trials.”
Other scientists note that stem cells may not need to become new myocytes to help repair the injured heart. According to Phillip Yang, a cardiologist at Stanford University who did not take part in the work, many scientists now agree that stem cells are not regenerating damaged cardiomyocytes. Instead, he explains, a growing body of research now supports an alternative theory, which posits that progenitor cells secrete small molecules called paracrine factors that help repair injured heart cells. (Yang is involved in several stem cell clinical trials).
“When you inject these stem cells, it’s pretty incontrovertible that they help heart function in a mouse injury model,” Yang says. “But the truth is, most of these cells are dead upon arrival [to the site of injury]. So the question is: Why is heart function still improving if these cells are dying?”
Y. Li et al., “Genetic lineage tracing of nonmyocyte population by dual recombinases,” Circulation, 138:793-805, 2018.