Ready to pull : force transmission during vascular development

Abstract: The transmission of mechanical forces controls a multitude of cellular processes such as migration, the formation of new organs, muscle contraction, and resistance to the continuous stress that blood flow exerts on the blood vessel wall. These forces are sensed and replayed at the sites of cellular adhesion to the basal membrane as well as cell-cell junctions, where protein complexes connect to the cellular cytoskeleton to relay force into the cell. During vascular development, endothelial cells (ECs) continuously sense and respond to mechanical cues from the microenvironment to form a functional and hierarchical vessel network. Uncovering the underlying mechanisms, by which the mechanical tension is generated, sensed, and relayed into the cell is important in advancing our understanding of the pathogenesis of vascular disease. In this thesis, evidence is presented for a role of the Angiomotin (Amot) scaffold protein family as essential mediators in the vascular endothelium. As stated in Paper I, deletion of Amot in the endothelium inhibits the expansion of the physiological and pathological vascular network. Furthermore, we provide evidence that Amot is a novel component of the integrin adhesome important for linking fibronectin (Fn) in the extra-cellular matrix (ECM) to the intercellular actin filaments. Fn is a major component in the ECM, which is sensitive to mechanical forces resulting in extensive molecular elongation. In Paper II, the force-sensitive “integrin switch” of Fn-binding sites is experimentally confirmed, and an engineered scFv named H5 is developed, which specifically binds to that force-induced conformational change of Fn. We provide further experimental evidence both in vitro and ex vivo for the H5 ability to detect and target the early molecular signatures of cell contractile forces in vivo. Transmission of mechanical force occurs not only via cell-matrix adhesions but also cell-cell junctions. As described in Paper III, AmotL1 forms a complex with N-cadherin presented on both ECs and pericytes. Exploiting endothelial-specific knockout mice, we demonstrate that AmotL1 is essential for normal establishment of vascular networks in the post-natal mouse retina and in a transgenic breast cancer model. In Paper IV, we indicate that AmotL2 connects junctional VE-cadherin and radial actin filaments to the LINC complex in the nuclear membrane. Deletion of AmotL2 impairs the formation of radial actin filaments and the flow-induced alignment of aortic ECs and affects nuclear shape and positioning. Moreover, the absence of AmotL2 in ECs provokes a pro-inflammatory response and abdominal aortic aneurysms in the aortae of adult mice that are consuming a normal diet. Overall, these findings provide a conceptual framework regarding force mechanotransduction via cell-matrix adhesions and cell-cell junctions that are associated with vascular physiology and relevant diseases.