Endothelial - mural cell interplay in regulation of blood- and lymph vessel development and function

Abstract: Blood- and lymphatic vascular development and homeostasis depend on correct endothelial- mural cell interaction, and dysregulation thereof is apparent in multiple human diseases. However the mechanisms controlling the formation of these respective vascular systems are not fully understood. Previously, the binding of EC-derived Platelet derived growth factor B (PDGFB) to its receptor, PDGFRβ on mural cells, was shown to play an important role in recruitment of mural cells to blood capillaries. Whether PDGFB carries similar functions in the lymphatic vasculature is still unclear. In addition, potential regulation of PDGFRβ signalling by other PDGFs, in the context of endothelial - mural cell interaction, is not fully understood. This thesis focuses on PDGFRβ signalling, mediated by PDGFB and PDGFD, in regulation of endothelial - mural cell interplay in blood and lymphatic vessel development. By generation of several genetic modified mouse models, including inducible targeting of PDGFB in the lymphatic endothelium, we reveal that PDGFB is required for recruitment of smooth muscle cells (SMCs) to collecting lymphatic vessels. In addition we show that SMCs play no major role in the establishment of lymphatic vessel identities but that these cells are responsible for the recorded pulsatile contraction of dermal collecting vessels. Furthermore, our data suggest that it is unlikely that pathological SMC recruitment to capillaries is caused by altered PDGFB expression alone, but that it also relies on extra cellular matrix composition. Besides PDGFB, we also demonstrated a potential involvement of PDGFD in regulation of EC-pericyte interplay. We found that, although PDGFB and PDGFD evoked similar PDGFRβ activation, these ligands promoted differential pericyte behavioural responses in 3-dimesional angiogenesis assays. This may be related to our discovery of an interaction between PDGFD and Neuropilin-1 (NRP1). A part of this thesis was also dedicated to record vascular development from a new level of both imaging and time resolution. By using the wounded mouse cornea as a live imaging site, we developed an in vivo imaging approach that allows for documentation of vascular morphogenesis over time, at subcellular resolution. We used this method to analyse EC migratory behaviour and highlighted directional migration against blood flow. We also characterized vessel patterning with respect to mural cell distribution during sprouting angiogenesis in the inflamed cornea. Furthermore, we recorded temporal and spatial aspects of VEGFA-induced vessel permeability by intra vital live imaging and revealed distinct artery-venous properties. Taken together, this thesis contributes to the understanding of the roles of endothelial- and mural cells in the context of blood and lymphatic vascular development. Our findings shed new light on mechanisms regulating cardiovascular homeostasis in development and disease.