Here are some recently published papers that caught my eye: a pair of papers showing that small molecules boost the success of turning regular-old cells into embryonic-like cells (called induced pluripotent cells); one paper even shows that the ubiquitous pluripotency gene Oct4 is not required. These are in Nature Biotechnology and Cell Stem Cell, and they’ll be up on my site as formal research highlights on July 3. You can see them now below. The author of the Cell Stem Cell paper recently spoke to us about how small molecules can control stem cells.
To try to understand what’s going on, try a review in Nature Reviews Molecular Cell Biology by Kevin Eggan and others describes how the machinery controlling gene expression has to be re-established with each cell division, and how oocytes and reprogramming methods can reset this expression. It’s part of a series of article on stem cells.
Next, some papers characterizing stem cells in the body. A couple Nature papers show heart stem cells with surprising flexibility in surprising places. That highlights will also be on the site on July 3rd, and it’s posted below. A few months ago, our editor at large, Natalie DeWitt, attended a meeting where scientists debated what cells in the heart could heal. Now there’s more to talk about.
And one more: Nance Beyer Nardi of the Universidade Federal do Rio Grande do Sul in Brazil and colleagues have published a paper in Stem Cells characterizing the natural niche of mesenchymal stem cells, which are being tested clinically for a variety of indications. It says these cells hang out around blood vessels, stabilizing them and helping the immune system respond to homeostasis. “This view,” the researchers write “connects the MSC to the immune and vascular systems, emphasizing its role as a physiological integrator and its importance in tissue repair/regeneration.” It’s one of the Open Access articles this week.
Small molecules boost reprogramming rates
Two teams of scientists have identified small molecules that can dramatically boost the rates at which mouse cells can be reprogrammed to an embryonic-like state. Several labs have already shown that cultured mouse and human skin cells (fibroblasts) can be induced to pluripotency if transfected with a set of viruses carrying four genes that are highly expressed in embryonic stem cells. However, very few cells successfully reprogram, and the extra genes inserted into the genome make the cells less predictable and more likely to form tumours. Now, two labs show that small molecules can boost efficiency and replace at least some of the genes.
Sheng Ding of The Scripps Research Institute, in La Jolla, California, Hans Scholer of the Max Planck Institute for Molecular Biomedicine, in Münster, Germany, and others show that some cells require less genetic manipulation than others to be reprogrammed; they also show that a small molecule can replace one of the quintessential reprogramming factors1. They worked on mouse fetal neural progenitor cells, a cell type that can be grown in culture and that already expresses high levels of Sox2, one of the genes used for reprogramming. These cells could be reprogrammed, albeit at a very low rate, with the addition of only two genes, Oct4 and Klf4, The researchers also found that adding a small molecule, the MEK inhibitor PD0325901, could inhibit growth of nonreprogrammed cells while boosting growth of reprogrammed cells.
When the team screened combinations of small molecules and genes, they found that a molecule known as BIX-01294 could boost reprogramming rates to the same levels as if all four genes from the original reprogramming recipe were used. BIX01294 inhibits the G9a histone methyltransferase, an enzyme that regulates how genes on spooled DNA are expressed, including the pluripotency gene Oct4. In fact, this inhibitor can be used instead of the Oct4 gene to generate induced pluripotent stem cells in neural progenitor cells, even though such cells do not naturally express the gene. Though reprogramming recipes vary somewhat in the set of genes they use, this is the first report of a recipe that does not require Oct4.
Another team, led by Doug Melton and colleagues at Harvard University, in Cambridge, Massachusetts, applied lessons learned from cloning to their work in reprogramming mouse embryonic fibroblasts2. Enzymes known as histone deacetylases (HDACs) regulate the expression of genes on the DNA-spooling structures called histones. Small molecules that inhibit HDACs increase success rates of cloning through nuclear transfer up to fivefold. After testing several molecules, Melton found one in particular, valproic acid, that boosted reprogramming efficiencies for fibroblasts by more than 100-fold. Other inhibitors of epigenetic regulators (DNA methyltransferases) also boosted reprogramming rates. Further, the cells could be reprogrammed efficiently without the addition of c-Myc, a tumorigenic component of reprogramming recipes.
Further analysis showed that valproic acid may help to create a state of gene expression closer to that of embryonic stem cells. For example, nearly 1,000 genes are upregulated at least tenfold in embryonic stem cells. When valproic acid was added to fibroblasts, some two-thirds of these genes were also upregulated at least twofold.
Researchers across the world are racing to find methods that can reprogram differentiated cells obtained from adult tissues. Neither study achieves that, but both encourage additional exploration of small molecules and cell types to accomplish that goal.
Related articles
References
1. Shi, Y. et al. A combined chemical and genetic approach for the generation of induced pluripotent stem cells. Cell Stem Cell 2, 525–528 (2008).
2. Huangfu, D. et al. Induction of pluripotent stem cells by defined factors is greatly
improved by small-molecule compounds. Nature Biotechnol. advance online publication, doi:10.1038/nbt1418 (22 June 2008).
The heart’s protective shield shows muscle
The outermost layer of the heart wall does more than just protect life’s most vital organ — it holds a new type of heart stem cell, according to a pair of studies published in Nature. Two teams — one led by William Pu of the Harvard Stem Cell Institute, in Cambridge, Massachusetts, and the other led by Sylvia Evans of the University of California, San Diego — independently discovered the new stem cells in the epicardium, the epithelial coating the envelops the heart. The findings potential new routes to mending a broken heart.
Epicardial cells are known to give rise to smooth muscle cells, fibroblasts and the endothelial cells that line coronary blood vessels, but they had never been shown to contribute to cardiac muscle cells, also known as cardiomyocytes. “Before, it wasn’t thought that the epicardium could make cardiomyocytes,” says Pu. “But now it appears they have a natural ability” to do so. In fact, “they can turn into all major cell types in the heart,” he says.
Each research group generated mice expressing marker genes to trace the lineages of the epicardial stem cells. Pu’s team used the transcription factor Wt1 and showed that Wt1-expressing epicardial cells contribute around 5% of the cardiac muscle cells across all four heart chambers1. Evans’ team, however, used a different marker — the Tbox transcription factor, Tbx18. Similar to Pu’s results, Evans’ team found that Tbx18-expressing epicardial progenitors contributed a substantial portion of the heart muscle tissue2.
The two studies are “simple lineage-tracing studies [that] provide good in vivo evidence for the origin of a subset of cardiomyocytes,” says Deepak Srivastava, of the Gladstone Institute of Cardiovascular Disease at the University of California, San Francisco, who was not involved in either study. “They’re good for what they are, but they don’t tell us about how the progenitors are regulated or how decisions are made” on what fates the cells adopt.
The cells identified by each group showed some subtle differences. Both sets of epicardial precursors differentiated into cardiac and smooth muscle, but although a minority of Pu’s Wt1-expressing cells became endothelial cells, Evans’ Tbx18-expressing cells did not do so at all. The Tbx18-marked cells did give rise to fibroblasts though — something that Pu’s team did not investigate. Srivastava says this difference is probably just a matter of when the markers were expressed within the same pool of cells. So, are both researchers studying the same epicardial heart stem cells? “I think they’re the same,” Srivastava says. “They’re just marked by different markers.”
The authors themselves take more reserved stances. “You can’t say they’re the exact same population, but if they’re not, there’s probably a huge overlap,” says Jody Martin of UC San Diego, a first author on the Evans study. Pu agrees. “I think it’s safe to say that there is likely considerable overlap,” he says.
Christine Mummery, of the Leiden University Medical Center in the Netherlands, describes the papers as “very elegant genetic studies,” but cautions that the cardiac muscle–producing cells might not even be epicardial cells at all. “The question is whether they’ve used only epicardial cells, or, because of the timing and the area they’ve taken, whether they’ve included mesenchymal cells [as well].” It remains to be seen if the newfound epicardial cells are truly multipotent precursors, she says. Only a clonal analysis might be able to strike at the heart of the epicardium’s potential.
References
1. Zhou, B. et al. Epicardial progenitors contribute to the cardiomyocyte lineage in the developing heart. Nature advance online publication, doi:10.1038/nature07060 (22 June 2008). | Article |
2. Cai, C.-L. et al. A myocardial lineage derives from Tbx18 epicardial cells. Nature advance online publication, doi:10.1038/nature06969 (14 May 2008). | Article |
The heart highlight was written by Elie Dolgin, a Canadian science writer currently residing in Milwaukee, Wisconsin.