Cell fate decisions are made in the early mouse embryo when it is nothing more than a spherical mass of cells. A molecule known as eomesodermin determines whether pluripotent stem cells become cardiac or intestinal progenitor cells. Dr. Sebastian Arnold and his research group at the Freiburg University Medical Centre have recently discovered why one single molecule can have a twofold effect. These findings provide Arnold and his team with greater insights into how a living embryo develops. It might also be possible to use this knowledge to make the in vitro production of cardiac tissue from stem cells more efficient.
In mouse embryos, the process of gastrulation starts around 6.5 days after fertilisation. The gastrula looks like a cup in which the central cavity (which ultimately forms the gut) is surrounded by three layers of cells. Each of the three layers gives rise to specific tissue and organs in the developing embryo. The innermost layer – the ectoderm – gives rise to the nervous system and the skin; the outermost layer – the endoderm – gives rise to the epithelium of the digestive system and inner organs such as the liver, lungs and spleen; the layer between the ectoderm and the endoderm – the mesoderm – gives rise to skeletal muscles and heart muscle cells. Stem cells play a key role during these early processes; originally able to turn into any type of cell whatsoever, their fate is determined by the activity of specific genes. Considering the complexity of the molecular processes, it seems miraculous that a trilaminar cell cluster can develop into an embryo and subsequently into a mouse.
“Primarily, we see ourselves as embryologists,” said Dr. Sebastian Arnold, who is the head of one of four Emmy Noether research groups in the Department of Nephrology (director: Prof. Dr. Gerd Walz) at the Freiburg University Medical Centre. “We would like to understand how an embryo develops. Molecular methods such as gene knockouts are important tools for our work. We are not particularly interested in defective development processes; what we are primarily interested in is the complex development of healthy animals,” said Dr. Arnold, who is a medical doctor by training so, for him, as with pure biologists, everything starts with observations. State-of-the-art methods like real-time imaging using fluorescence microscopes are able to provide astonishing new insights into the dynamic reorganisation processes on molecular, cellular and organ levels. In addition, these methods have brought the field of developmental biology into the Twenty First Century. Molecular biology research also helps the researchers gain insights into defective mechanisms that help them understand developmental disorders and associated human diseases. On the other hand, insights into developmental disorders and diseases can be used to gain further insights into healthy embryonic development.
Stem cell research also has a concrete application that many people are talking about: the regeneration of human tissue in Petri dishes. It will most likely take quite a long time before medical doctors in transplantation hospitals will be able to grow organ tissue reliably and efficiently in vitro. However, a paper that was recently published by Dr. Arnold and his team in the renowned journal Nature Cell Biology reports on the researchers’ success in increasing the number of cardiac progenitor cells that can be produced from stem cell tissue in cell culture dishes. The stumbling block that the researchers were able to overcome should not be underestimated. Although stem cell researchers previously managed to grow such cardiomyocytes from embryonic stem cells, they did not manage to grow a large enough number of cardiomyocytes as the embryonic stem cells can develop into mesoderm and hence into cardiac progenitor cells as well as into endoderm and hence into intestinal progenitor cells. This is why embryonic stem cell cultures usually contain a mixture of these two types of cells.
Arnold and his team have now shed significant light on the molecular relationships of these processes. Back in 2008 when he was postdoc in Oxford, Arnold genetically engineered mice in which he eliminated the gene eomesodermin in specific stem cell types using a technique known as conditional gene knockout. These mutants display early developmental disorders; they are not able to form either mesoderm or endoderm. “For us this was proof that eomesodermin is a key factor in initiating mesoderm and endoderm differentiation processes,” said Arnold. The protein is a cue for the embryos, telling them which genes to activate and hence determining the fate of the cells. However, the researchers were unable to solve the key question as to how a group of cells knows that they are to become cardiac progenitor cells and while others know that they have to turn into intestinal cell progenitors although all of them have received the same signal.
Arnold and his team subsequently showed that this decision depended on the interaction between eomesodermin and the signalling molecule TGF-ß, which is a key molecule used in signalling research. TGF-ß is also involved in other cellular processes and controls many functions in cells, including proliferation and apoptosis. The decision whether a stem cell turns into a cardiomyocyte or an intestinal cell progenitor depends on the strength of the TGF-ß signal a stem cell receives. A high concentration of TGF-ß induces the formation of endoderm and hence intestinal progenitor cells while TGF-ß deficiency in the medium makes stem cells turn into cardiomyocytes. “By dynamically changing the TGF-ß concentration in the culture medium, we succeeded in increasing the quantity of cardiomyocytes considerably; fewer intestinal cell progenitors were produced,” said Arnold summarising his findings.Medical doctors who are looking for ways to regenerate the heart tissue of cardiac infarction patients are likely to be very interested in these results. However, for Arnold these findings mean a lot more; he and his team are fascinated by the highly dynamic and complex interactions of the key signals and regulators involved in embryonic development, and they will continue to work in this field. Arnold is particularly interested in the early migration of cells that ultimately form the mesoderm as these cells detach from the ectoderm during gastrulation. Which molecular and cellular processes cause the cells to detach from the cell layer? How and why do the cells detach and then migrate away from their original site? The ability of embryos to change one tissue type into another, which is known as epithelial mesenchymal transition (EMT), is also of great importance for our understanding of cancer cells, which detach from solid tumour tissue, migrate through the body and form metastases elsewhere in the body. Arnold pointed out that it is thanks to the department’s director, Prof. Gerd Walz, that he is able to focus his research activities on any area he would like to, and that he is very grateful to Prof. Walz for having great faith in his four Emmy Noether research group leaders.
Further information:Dr. Sebastian ArnoldFreiburg University Medical Centre, Department of NephrologyCentre for Clinical ResearchBreisacher Straße 6679106 FreiburgPhone: +49 (0)761/ 270 63 120 (Office)Fax: +49(0)761/ 270 63 240E-mail: sebastian.arnold(at)uniklinik-freiburg.de