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 one of the genes.

Sheng Ding of The Scripps Research Institute, in La Jolla, California, Hans Schöler r 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.

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