What glowing zebrafish can tell us about how the eye gets its shape
How do organs and organisms form? A new study led by the Instituto Gulbenkian de Ciência and the Max Planck Institute for Cell Biology and Genetics in Dresden got us a step closer to answering this question by combining innovative analysis methods and experimental tools. Using glowing zebrafish, the researchers were able to show how important processes are regulated during development.
Many of our cells are surrounded by a complex network of molecules they secrete, the extracellular matrix. Besides giving cells support, this matrix plays an important role in their movement, being important for organismal growth and function. In particular, some of its properties, such as stiffness and organization, can influence the migration of cells cultured in the laboratory. But it is not yet clear if the same is true when cells move in the developing organism to form different tissues and organs.
To clarify this issue, researchers from the Instituto Gulbenkian de Ciência (IGC) and the Max Planck Institute for Cell Biology and Genetics created genetically modified zebrafish with fluorescent extracellular matrix. Zebrafish are ideal to study development, given that their embryos are transparent and develop fast. This is the model chosen by Caren Norden’s group at the IGC to study how the eye is taking shape. “When we looked at the extracellular matrix in this region, the complexity and beauty of its structure totally draw us in”, recalls Karen Soans, PhD student at the lab and first author of their most recent study. This piqued their curiosity and led them to zoom in and look at it in more detail.
The images the researchers captured from this region showed that, in the initial phases of eye development, cells move collectively over the extracellular matrix through projections that extend beneath the cell immediately in front, along the direction of migration. But this dynamic is not constant. Since they used zebrafish as a model, the researchers could track cell-matrix interactions and record videos in real-time. This life-imaging data showed that in the beginning and at the end of the migration path, cells slide through the matrix, whereas in the middle they seem to transiently detach from their surface, taking small steps towards the finish line.
But the work went beyond this qualitative observation of the cells’ movements. The collaborations established with five different research centers in Germany allowed them to “push the project in a more quantitative direction”, which was challenging but also exciting, Karen says. Part of this work involved advanced image analysis pipelines to quantify the dynamics of cell-matrix interactions. With this, the researchers concluded that during initial phases of migration cells moved in a more directed manner while in the end they barely changed their position, eventually stopping when they reached their destination.
The use of high-resolution microscopes allowed the researchers to realize that, curiously, the matrix in which the cells moved had a different organization throughout the various crossed regions. In the final part, the fibrils that make up the matrix were more aligned than in the rest of the path, which was also more porous. So, the researchers thought that the properties of the matrix could be influencing the contacts with the cells and, consequently, their migration.
To test this hypothesis, they triggered breaks of various sizes in the matrix, first recurring to theoretical models, and then to zebrafish, by altering the expression of an important component of this network. In the cases in which the matrix was most disturbed, instead of extending protrusions in the direction of the movement, cells presented unregular protuberances in their bodies which frequently got stuck in the matrix breaks, preventing their migration. In less serious cases, cells did not get stuck, but their movements were less directed and efficient. This revealed that the physical properties of the matrix, in particular, its porosity, affect the interaction with cells necessary for efficient cell migration and the correct development of the embryo.
Besides giving support and shaping cell behavior during embryonic development, as shown here in shaping the vertebrate eye, matrix organization influences important processes, such as wound healing, and is disturbed in disease states, such as cancer. “With more tools being developed to visualize the extracellular matrix molecules, namely in fruit flies and zebrafish, we can begin to dissect how matrix dynamics influence organisms, not only when they develop, but also in adult stages”, Caren Norden explains. These results, published in Current Biology, could also help create better laboratory conditions to develop organoids, that is, mini-organ replicas created from cells, that are useful to understand the development of the human body and regeneration.