In embryos and induced pluripotent stem cells, Nanog sets the ground state of pluripotency
Nanog, named after the fountain of eternal youth, is considered the key to pluripotency, but its role is puzzling: it can be deleted from embryonic stem cells without causing them to differentiate, and it is not among the collection of genes that can induce specialized cells to become pluripotent. Nonetheless, it does help human cells reprogram and can be used to separate incompletely reprogrammed cells from fully reprogrammed ones. A series of experiments led by José Silva and Austin Smith of the University of Cambridge has shed light on Nanog's role1. “Nanog is not a trigger of the reprogramming process,” explains Silva, “but it is absolutely required for the acquisition of pluripotency. We found that both in vitro and in the embryo.”
The researchers showed that cells that could not produce Nanog could still undergo the early stages of the reprogramming process but did not transition to full pluripotency. However, if Nanog expression was restored to incompletely reprogrammed cells via genetic engineering, reprogramming continued. For both reprogramming cells and early-stage embryos, Nanog expression was also essential for the reactivation of the X chromosome, a process closely associated with pluripotency.
Next, the researchers tried to derive mouse embryonic stem cells from developing mouse embryos that cannot make Nanog. They used culture conditions and techniques that are reliable, but they were unable to generate embryonic stem cells. “If there were pluripotent cells in a Nanog-null embryo, we would expect these cells to expand,” says Silva. Instead, he says, the embryos did not make the appropriate cells. “It is an embryo without an inner cell mass; the embryo never generated the pluripotent compartment.”
The next steps, says Silva, are to figure out exactly what Nanog is doing. He believes that OCT4 (also called POU5f) and other pluripotency factors begin to trigger a cascade of events necessary for pluripotency. Nanog functions later in the process to establish a self-sustaining state. Figuring out the underlying mechanism will be important, he says, but that information should be tied to an understanding of Nanog's biological role. “It does help us to define what the true pluripotent cell is, and this type of knowledge will help us to create better and safer iPS [induced pluripotent stem] cells.”
Silva, J. et al. Nanog is the gateway to the pluripotent ground state. Cell 138, 722–737 (2009).