There’s a paper just out in Cell today moving researchers closer to reprogramming without adding oncogenes. Sure, it starts with a cell type that’s not readily accessed in humans, but it does indicate that the cell type could matter. Also of interest should be a paper that was published in Cell last week, which compared where the four Yamanaka factors are binding in fully reprogrammed cells, partially reprogrammed cells, and fibroblasts. (See how the four factors reprogram)
Cells that behave like embryonic stem cells can be made from cultured skin, liver, and stomach cells. All techniques so far require the addition of at least two pluripotency genes, which renders the cells much less attractive for cell therapy and drug screening. Now, researchers led by Hans Schöler of the Max Planck Institute for Molecular Biomedicine in Münster, Germany show that cells can be reprogrammed to pluripotency using just one of the standard four genes.1 “With only one “switch,” the gene Oct4, we have turned adult somatic cells into stem cells that are very similar to embryonic stem cells,” he says.
Schöler’s team began not with fibroblasts, the cultured skin cells most frequently used to make so-called induced pluripotent stem cells (iPS cells), but with mouse neural stem cells, which naturally express three of the four standard transcription factors. They were able to induce pluripotency by adding the gene for the missing factor, Oct4. Oct4, which is officially called Pou5F1, is expressed in embryonic stem cells and germ cells, and has long been considered a key regulator of pluripotency. The team had previously been able to reprogram neural stem cells using two of the four factors. The trick to using just one was waiting longer for cells to reprogram. Reprogramming generally takes about three weeks, but Schöler and his colleagues cultured the Oct4 infected cells for four to five weeks. The resulting cells passed several tests of pluripotency, including germline transmission in chimeric mice. The reprogramming efficiency was similar to that of reprogramming mouse embryonic fibroblasts with all four factors, about 0.014%.
Practical implications may be a ways off. Unlike skin cells, brain cells cannot be obtained readily from a human biopsy. However, Schöler says these cells present a good model for reprogramming not only because they can be transformed readily but also because they can be grown easily in pure cultures, so researchers can be certain what type of cells are being reprogrammed.
“The study sets the basis to understand, at a mechanistic level, whether Oct4 alone, in the absence of other oncogenes, could be used to reprogram different adult stem cells,” says Juan Carlos Izpisua Belmonte of the Salk Institute in La Jolla, California, whose work has shown that cells from plucked human hair reprogram much more swiftly and efficiently than fibroblasts. If such an approach could be made to work with more easily obtained cell types, the therapeutic implications would be “extraordinary,” he says. In the meantime, understanding what cell types are most susceptible to reprogramming “will surely help at unveiling the nuts and bolts of the process.”
Other techniques will also be helpful. Schöler and other researchers and other researchers previously showed that fetal neural stem cells could be reprogrammed without requiring the insertion of the Oct4 gene, though doing so required insertion of the other pluripotency genes plus a small molecule that inhibits an enzyme known as G9a histone methyltransferase. However, Schöler says that since this molecule turns on many genes, it requires the addition of the other three factors to focus the “crucial action” of Oct4.
[[Author Affiliation]] Monya Baker is editor of Nature Reports Stem Cells
1. Kim, J.B. et al. Oct4-induced pluripotency in adult neural stem cells. Cell 136, 411–419 (2009)