It is of crucial importance that cells stick tightly together where their function requires them to do so, for example in organs such as the heart and the liver, to name but two examples. However, it is equally crucial that cells start to migrate at some stage during embryonic development in order to form such organs. Prof. Dr. Wolfgang Driever from the Institute of Biology I at the University of Freiburg and his team have elucidated the molecular mechanisms that control the initial movement of cells of the zebrafish (Danio rerio) embryo. These findings are of major biomedical relevance, for example for the understanding of wound healing and the development of cancer.
Imagine an organism where cells have never migrated. The result would be a simple ball. All of us start life as small spheres, because the organisation of the early stages of embryogenesis is characterised by the fact that all cells stick closely to their neighbouring cells, and continue to do so for some time after cell division. E-cadherins (calcium adhering), transmembrane adhesion proteins that stabilise cell-cell contacts, ensure that the cells remain connected with each other as long as is required. Reduced quantities of E-cadherin molecules cause the blastomers to fall apart and the embryo to die. The presence of slightly less than normal quantities of E-cadherin in the early embryo induces the development of a secondary embryonic axis, with the result that two zebrafish have a common body, just like Siamese twins. “It is worth pointing out that the adhesion of the cells must be tightly regulated in order for the cells to be able to migrate away from their original site,” said Prof. Dr. Wolfgang Driever, chair of developmental biology at the Institute of Biology I at the University of Freiburg.
As an animal develops from a simple spherical ball of cells into a multi-layered organism (gastrulation), the cells have to start migrating in order to build the germ layers (ectoderm, endoderm and mesoderm), which would otherwise be impossible. These three germ layers eventually give rise to all of an animal’s tissues and organs. “In order for the spherical structure to develop into germ layers and form a long body axis, the cells need to migrate, elongate and change location,” Driever says.
Growth and regeneration depends on cell migration. The tight connections between the cells need to be resolved in order for the cells to be able to migrate into areas where they are needed. “Cell movement results in the reorganisation of the embryo, many of the cells at/near the surface of the embryo move to a more interior location where they have properties different from their original ones,” said the embryologist. Zebrafish embryos are popular models for studying cell movement, as their development enables easy visualisation of the process. Driever’s team discovered a fundamental molecular mechanism that leads to the reprogramming of static cells and enables them to move around. It is noteworthy that the first large cell migration in zebrafish embryos is not controlled by gene regulation. The embryos’ mothers put all the necessary factors into the cradle, i.e. the egg, so that they are present when the cells start to migrate around two hours after fertilisation. This ensures that there is no delay in embryonic development. During the movement of the cells, a process known as epiboly, the cells of the developing fish migrate around the yolk cell until they have completely engulfed it and the body axis forms. This process involves three components: the E-cadherins, the transcription factor Oct4 and the epidermal growth factor EGF.
E-cadherins are transmembrane glycoproteins that interact with neighbouring cells, thereby mediating cell-cell adhesion. But what happens when the cells start to detach from each other and to migrate? E-cadherins move through recycling endosomes (lipid vesicles) which transport the molecules into the cell interior and back into the membrane. This leads to a dynamic balance during which the E-cadherins sometimes appear inside the cells and sometimes on their surface.
Driever and his team made an unexpected discovery: “We expected the quantity of E-cadherin to decrease to enable the cells to migrate. However, what we actually found was that although the quantity of E-cadherin decreases, this was not the major effect of cell movement.” Driever’s team were able to show that the stem cell factor Oct4 controls the synthesis of the epidermal growth factor (EGF), which in turn controls and even speeds up the endosomal transport of E-cadherin from the cell membrane to the interior of the cell. They also found that the quantity of static E-cadherins only decreases on the cell surface.
These mechanisms enable cells to dynamically form new connections and start to move. “In order to be able to move, cells first need to detach from neighbouring cells before they can then form new connections,” said Driever going on to explain, “they adhere with their ‘E-cadherin feet’ to neighbouring cells and pull themselves along them. This is why they need E-cadherins. Although the overall sum of cellular adhesion proteins, i.e. E-cadherins, remains approximately the same, the resulting dynamics enable the cells to move around efficiently.
The mechanism that leads to the reprogramming of originally static cells and enables them to migrate is of huge importance in many areas. The collaborative research centre (SFB) 850, in which Driever’s project is embedded, is particularly focused on the “control of cell motility in morphogenesis, cancer invasion and metastasis”. Driever and his colleagues are interested in the factors that cause cancer cells to move away from the primary tumour and form metastases. Driever’s results are therefore relevant for potential mechanisms involved in cancer metastasis. Research with zebrafish mutants with defective EGF receptors (EGFR) has shown that these EGFRs are active without having to bind to ligands. The researchers therefore assume that the constant activation of the EGFRs induces vesicular trafficking, thereby contributing to the cells’ motility. “In fact, in many tumour cells, EGFR mutation correlates with aggressive invasion,” said Driever assuming that, “the same factors that induce cell migration in embryos, might also contribute to the aggressive nature of some cancers.”The researchers have also shown that the stem cell factor Oct4 needs to be present in any cocktail that is used to produce induced pluripotent stem cells (iPS). Hence the paramount importance of this factor in the field of regeneration medicine. However, an in-depth understanding of the underlying processes is still required. “Pluripotent stem cells are valuable and important, but they can also be dangerous,” said the zebrafish expert. “They constantly need to be kept in check.” While tissue-specific stem cells, which can only develop into a specific type of cells, always remain in controlled stem cell niches, embryonic and induced pluripotent stem cells do not need such niches in order to grow and thrive, which is why they have the potential to form tumour cells. Driever hopes that in-depth insights into the regulatory mechanisms of pluripotent stem cells will at some stage in the future enable the researchers “to tame the beasts”.
Further information:Prof. Dr. Wolfgang DrieverInstitute of Biology IDevelopmental BiologyUniversity of FreiburgHauptstr.179104 FreiburgTel.: +49 (0)761/ 203-2587Fax.: +49 (0)761/ 203-2597E-mail: driever(at)biologie.uni-freiburg.de