How does signaling induce human primordial germ cells?

原始生殖细胞(PGCs)是胚胎中的细胞，成年后变成卵子和精子，在胚胎发育的早期形成。了解控制PGC发育的分子/遗传机制对于辅助生殖、技术、了解生殖系癌症的起源以及再生医学都很重要。然而，到目前为止，人们对人类前列腺癌的发育是如何控制的知之甚少。事实上，理解胚胎中的细胞是如何决定成为脊椎动物中的PGC的，通常对生物学家构成了一个独特的挑战，原因直到现在才变得清晰。其核心是进化的过程，以及进化力量如何影响PGC的发展机制。因为，尽管许多控制发育的机制已经在更简单的动物的胚胎中被研究出来，比如青蛙或鱼的胚胎，但还不可能将这些物种作为人类PGC发育的“模型”。这是因为通常在实验室中研究的每一种模式生物都进化出了一种独特的产生PGC的机制。相比之下，人类使用的显然是脊椎动物进化的原始机制来产生PGC，即所谓的保守机制。我们早在几年前就认识到了这个问题，为了解释这个问题，我们发展了一种新的进化理论，涉及胚胎中PGCs和其他细胞(称为体细胞)之间的关系。我们假设人类胚胎保留了最初在脊椎动物中进化的PGC发育机制，即所谓的保守机制。这一理论的一个主要组成部分也是，原生殖细胞与体细胞来自相同的细胞，而不是来自特殊的细胞。为了验证这一理论，医学研究中心出资开发了一种实验系统，该系统使用了一种火蜥蜴的胚胎。之所以选择Axolotls，是因为它们类似于第一批迁徙到陆地上的脊椎动物，换句话说，它们是哺乳动物的两栖祖先。我们预测，axolotl和人类将分享相同的PGC发育机制，因此axolotl胚胎可以作为一个实验模型来揭示这一过程的驱动机制。我们确定了控制PGC发育的信号，然后考虑了已知的在其他细胞类型中作用于这些信号下游的已建立的遗传途径。由此，我们确定了转录因子Elk-1及其功能伙伴Med23在PGC发育中的主要作用。Elk-1是在25年前被发现的，但它在胚胎中的作用从未被明确确定，因为通常研究的动物模型，包括小鼠，进化出了绕过它在胚胎中的古老作用的遗传电路。我们发现在Axolotls中发现的途径也控制着猪的PGCs的发育，猪的胚胎准确地模拟了人类的胚胎，这有力地表明我们发现的Elk-1/Med23的作用也指导着人类PGCs的发育。我们的建议旨在利用axolotl胚胎来确定控制脊椎动物PGC发育的保守途径的生化和遗传机制。我们建议降低Elk-1或Med23的活性，并用缺乏特定生化功能的突变分子取代这些蛋白。我们还将确定所有由ELK-1/Med23上调或下调的基因，这些基因控制着PGCs与体细胞的区别。我们还将使用新开发的方法从人胚胎干细胞(HESC)诱导PGC样细胞来测试Elk-1/Med23的功能，并将使用这个hESC系统来鉴定负责启动调控人类PGC发育的基因的遗传元件。这将是朝着确定脊椎动物PGC发育所需的保守基因网络迈出的一步，增强我们理解和操纵生殖细胞的能力，以解决与人类健康有关的问题。

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.