The fate of primordial germ cells
When an embryo develops, single cells acquire specific fates that allow them to perform specific tasks in the adult organism. The primordial germ cells are formed very early in embryonic development and migrate within the embryo to the developing testis or the ovary, where they give rise to sperm and egg cells. During their migration, the germ cells pass through tissues and interact with cells that acquire other specific fates such as muscle, bone or nerve cells in response to different cues. Primordial germ cells, however, ignore those signals and maintain their fate. What are the mechanisms behind this process? Researchers at the Cells-in-Motion (CiM) Cluster of Excellence at the University of Münster have now discovered that a certain protein expressed within the progenitor germ cells is responsible for their fate maintenance: the Dead End protein. “For the first time, we have been able to demonstrate that germ cells lacking the protein undergo differentiation into other cell types during their migration,” says Theresa Gross-Thebing, lead author of the study and a PhD student at the Cluster of Excellence’s Graduate School. As a consequence, in embryos lacking the Dead End protein the progenitor germ cells do not give rise to cells critical for reproduction and the adult organism becomes infertile. These findings are also relevant for the understanding of the development of certain germ cell tumours. The study appears in the latest issue of the journal “Developmental Cell”.
The detailed story:
The new results were obtained by studying primordial germ cells in zebrafish embryos. These embryos are transparent and develop rapidly outside the body of the female, allowing the visualization of such processes within the live organism. The researchers in the CiM group headed by Prof. Erez Raz observed progenitor germ cells in which the level of the Dead end protein was reduced. In previous studies, researchers had already discovered that germ cells lacking the Dead End protein had disappeared after one day. It was therefore presumed that these germ cells died during their migration such that they did not arrive at their destination. Following this assumption the protein was named “Dead end”.
To be able to observe the primordial germ cells over a longer period of time than in earlier studies, the researchers from Münster labelled germ cells with fluorescent proteins and employed different microscopy techniques. A type of microscopy that was especially useful in this study is “light-sheet fluorescence microscopy”. This type of microscopy scans the tissue very rapidly layer by layer, while a camera records the fluorescence signal. Next, a composite image of individual layers is generated allowing observation of the three-dimensional tissue structure and the position of cells within it.
By genetically deactivating certain signalling cues, that normally guide primordial germ cells to the correct place, the researchers made these cells migrate into foreign tissues. After one day they were able to recognize that primordial germ cells lacking the Dead End protein obviously changed their shape: the characteristic round shape and the migratory behaviour could not be observed anymore. Instead, the cells displayed shapes typical of the neighbouring somatic cells – for example, an elongated form of a muscle cell or long processes characteristic of nerve cells. In each case, the shape, behaviour and molecular features were matching the tissue in which the cells resided.
After one day only 20 percent of all primordial germ cells lacking Dead End still showed their original shape. After two days, the researchers could no longer observe any cells displaying the shape of a progenitor germ cell. While some of the germ cells were dying, as had been observed in earlier studies, most of them were transformed into other types of cells based on their shape. Importantly, in addition to the morphological changes, germ cells lacking Dead End function developed into other types of cells as judged by the expression of specific molecules. In contrast with wildtype cells, in the absence of Dead end germ cells started expressing proteins characteristic of muscle or nerve cells. “These results enable us to show for the first time that Dead End as a protein is responsible for maintaining the fate of primordial germ cells,” says Prof. Erez Raz.
These findings are relevant for research concerning certain germ cell tumours in humans, named teratomas. These tumours occur in large part in ovaries or testicles and contain, for example, tissues like teeth or hair. “Previous studies in mice suggest that germ cells lacking Dead End can initiate tumours and that within these tumours somatic differentiation occurs,” says Theresa Gross-Thebing. In future, the researchers want to investigate how Dead end functions in maintenance of germ cell fate and in inhibition of transformation of the cells into cancer cells that can develop into different types of somatic cells. Further studies will determine if the results of this basic research study can find their way into any possible medical applications.
The study received funding from the Cells-in-Motion Cluster of Excellence, the European Research Council (ERC) and the German Research Foundation.
Gross-Thebing T, Yigit S, Pfeiffer J, Reichman-Fried M, Bandemer J, Ruckert C, Rathmer C, Goudarzi M, Stehling M, Tarbashevich K, Seggewiss J, Raz E. The vertebrate protein Dead end maintains primordial germ cell fate by inhibiting somatic differentiation. Dev Cell 2017, DOI: 10.1016/j.devcel.2017.11.019