How does signaling induce human primordial germ cells?

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Primordial germ cells (PGCs) are the cells in an embryo that later become egg and sperm in adults, and they form in the early stages of an embryo's development. Understanding the molecular/genetic mechanism that controls PGC development is important for assisted reproduction, technologies, understanding the origins germ line cancers, and for regenerative medicine. However, to date little is known about how PGCs development in humans is controlled. Indeed, understanding how cells in an embryo decide to become PGCs, in vertebrates, generally, has posed a unique challenge to biologists for reasons that are only now becoming clear. At its heart is the process of evolution, and how evolutionary forces have affected the mechanisms for PGC development. Because, while many mechanisms that control development have been worked out in embryos of simpler animals, like those of frogs or fish, it has not been possible to use these species as "models" for human PGC development. This is because each of the model organisms typically studied in laboratories has evolved a unique mechanism for producing PGCs. Humans, in contrast, employ what is apparently the original mechanism that evolved in vertebrates to produce PGCs, the so-called conserved mechanism. We recognized this problem several years ago, and to explain it we developed a novel theory of evolution concerning the relationship between PGCs and the other cells in an embryo, known as somatic cells. We hypothesized that human embryos retain the mechanism for PGC development that originally evolved in vertebrates, the so-called conserved mechanism. A major component of this theory is, also, that PGCs are derived from the same cells as somatic cells, not from specialized cells. To test this theory, the MRC funded development of an experimental system using embryos from axolotls, a salamander. Axolotls were chosen because they resemble the first vertebrates to move onto land, in other words, the amphibian ancestor to mammals. We predicted that axolotls and humans would share the same mechanism for PGC development, and axolotl embryos could therefore be used as an experimental model to unpick the mechanisms driving this process. We determined the signals that govern PGC development in axolotls, and then considered a an established genetic pathway known to act downstream of these signals in other cell types. From this we identified a principle role for the transcription factor Elk-1, and its functional partner Med23, in PGC development. Elk-1 was discovered over 25 years ago, but its role in embryos has never been clearly determined because commonly studied animal models, including mice, evolved genetic circuits that circumvent its ancient role in embryos. We showed that the pathway discovered in axolotls also controls development of PGCs in pigs, whose embryos accurately model those of humans, strongly suggesting that the role for Elk-1/Med23 that we discovered also directs development of human PGCs. Our proposal is designed to use axolotl embryos to define the biochemical and genetic mechanisms controlling the conserved pathway for vertebrate PGC development. We propose to reduce the activity of Elk-1 or Med23, and replace these proteins with mutant molecules lacking specific biochemical functions. We will also identify all of the genes either up or down-regulated by Elk-1/Med23 that control the distinction of PGCs from somatic cells. We will also test the function of Elk-1/Med23 using newly developed methods to induce PGC-like cells from human embryonic stem cells (hESC), and we will use this hESC system to identify the genetic elements responsible for switching-on genes that regulate human of PGC development. This will be a step towards defining the conserved network of gene required for vertebrate PGC development, enhancing our ability to understand and manipulate germ cells to address issues concerning human health.