Mechanical communication of endothelial cells and its influence on the organization of cellular structures

The formation of new blood vessels from existing ones is called angiogenesis and plays an important role in tissue development, healing and homeostasis as well as in pathological processes, such as tumor growth. During this process, coordinated cell movement is essential for which endothelial cells use a chemical communication via the vascular endothelial growth factor (VEGF). The extracellular matrix (ECM) regulates many tissue processes and influences cell behavior. The dynamic cell-ECM interaction enables a mechanical communication between cells. This thesis aims to validate, whether endothelial cells also use a mechanical communication for coordinated cell behavior. For the investigation of the endothelial cell behavior, tube formation assays on Matrigel were performed. Cell communication was examined with a focus on the initial “finding phase” of the cells. It was shown that cells sense each other to align and cluster over a distance of at least 106 µm. However, the finding phase was not dependent on chemical communication via growth factors, neither soluble nor matrix-bound. This led to the hypothesis that endothelial cells use a mechanical communication pathway for initial cell connection during tube formation assay. For further analysis, Matrigel or collagen I printing on polydimethylsiloxane (PDMS) with different stiffness values was applied. This could show that stiffness had an influence on tube formation but was not a key factor, revealing the high mechanical homogeneity of Matrigel compared to collagen I to be a crucial factor. The destruction of the laminin network in Matrigel by netrin-4 or the blocking of integrins for laminin led to an inhibition of tube formation, whereas the presence of laminin supported tube formation. Thus, laminin was found to be a master regulator. Furthermore, the process of mechanical interaction was shown to be dependent on cell contraction and independent of proteolytic processes or protein secretion. Cell forces due to cellular contraction ranges in the same radius as the maximum distance of cell communication, as shown by traction force microscopy, and led to a plastic irreversible remodeling of the Matrigel matrix. Atomic force microscopy (AFM) measurements proved that the remodeling results in a stiffness gradient, a process called strain stiffening, and the interaction of the cells occurred via the remodeled stiffened fibers. In conclusion, it was shown that endothelial cells use a mechanical as well as a chemical interaction for communication and coordination