Nanog (red) and Eomes (green) trigger opposing responses in ESCs, respectively maintaining pluripotency and stimulating differentiation into endoderm. Nanog expression also plays an important role in switching on the gene EOMES (yellow cells).
Early in development, embryonic stem cells (ESCs) abandon their undefined ‘pluripotent’ state in order to participate in the formation of three germ layers: the ectoderm, mesoderm and endoderm. These layers will in turn each give rise to distinct subsets of cell types and tissues in the mature organism: the ectoderm develops into the nervous system and epidermis, the mesoderm into connective tissue and bone, and the endoderm the stomach and lungs.
Understanding the molecular and genetic basis for this transition is more than an academic exercise. “We are interested in the derivation of definitive endoderm from human ESCs,” explains Ray Dunn at the A*STAR Institute of Medical Biology. He and his co-workers have now gained new insights into definitive endoderm (DE) formation. As key organs, such as the pancreas and liver, originate from DE, the findings will have important implications in regenerative medicine.
The investigation began with a mystery: the signaling factor activin or Nodal can block stem-cell differentiation by stimulating the production of pluripotency-maintaining factors such as Nanog, yet can also promote differentiation into DE. The researchers resolved this apparent contradiction through a series of experiments using human ESCs, revealing that the protein eomesodermin, or Eomes, is a ‘master switch’ for DE formation and a modulator of the effects of activin/Nodal signaling.
Previous studies have flagged Eomes as a regulator of endodermal development, and recent studies have demonstrated that this protein directly controls the activity of thousands of genes that favor DE formation while inhibiting mesodermal differentiation. The gene EOMES, which encodes the protein Eomes, is initially inhibited by various pluripotency-promoting factors. However, as levels of these inhibitory proteins begin to decrease at sites of future endoderm development, Nanog changes sides, so to speak, and collaborates with Smad2 or Smad3, downstream components of the activin/Nodal signaling pathway, to turn on EOMES (see image). “Contrary to its widely touted role as a pluripotency factor and general blocker of differentiation, Nanog actually plays an active role in germ layer formation,” says Dunn.
The researchers determined that a high level of Eomes inhibits the production of Nanog. In addition, the Eomes protein works with Smad2 or Smad3 to aggressively switch on genes that commit embryonic cells to the endoderm developmental pathway. “This helps us bridge a hole in our conceptual framework about how activin or Nodal effectively plays a double role,” says Dunn. He and his co-workers are now looking to further explore this process by characterizing some of the target genes in greater detail.
The A*STAR-affiliated researchers contributing to this research are from the Institute of Medical Biology.
