Hans Schoofs: Regulation of Lymphatic Development and (Dys)Function: A Matter of Cellular Competition and Dynamics

Abstract

 Lymphatic vessels are essential for maintaining fluid homeostasis, immune cell trafficking and lipid absorption in the gut. Postnatal expansion of the lymphatic vasculature occurs through sprouting lymphangiogenesis from pre-existing lymphatic networks, which is regulated primarily by vascular endothelial growth factor C (VEGF-C) and its receptors, VEGFR2 and VEGFR3. While the role of VEGFR3 in lymphangiogenesis is well established, the function of VEGFR2 remains less understood. In Paper I, we use high-fidelity conditional genetics for VEGFR2 deletion and adeno-associated viruses (AAVs) overexpressing selective VEGFR2 and VEGFR3 ligands to reveal a critical role of VEGFR2 in lymphatic biology. In Paper II, we extend our studies to the mature lymphatic vasculature, composed of specialized lymphatic capillaries and collecting vessels. Fluid absorption occurs in lymphatic capillaries, which are composed of oak leaf shaped lymphatic endothelial cells (LECs) connected by discontinuous junctions. However, it is unclear how these capillaries maintain endothelial integrity while taking up fluid from the interstitial space. We show that capillary LECs dynamically remodel their shape during homeostasis and in response to increased interstitial fluid in a process driven by cytoskeletal actin remodelling. We further identify isotropic stretch as an upstream regulator of LEC cell shape and use mathematical modelling to show that the oak leaf cell shape provides increased resilience, preventing luminal collapse upon increased pressure on the vessel wall. While the development of blood and lymphatic vasculature is tightly controlled, certain pathologies are associated with aberrant expansion of these vessels. In Paper III and IV, we investigate the mechanisms underlying vascular malformations, which are a spectrum of diseases characterised by focal lesions of malformed blood or lymphatic vessels. The majority of vascular malformations are caused by somatic activating mutations in genes involved in (lymph-)angiogenesis, leading to ectopic growth of endothelial cells. Using genetic mouse models of vascular malformations, Paper III characterized organ-specific responses of LECs driving lymphatic malformations, while Paper IV identified a venous-specific feedback loop that amplifies upstream growth factor signalling, promoting venous malformations. These results illustrate that the same activating mutations can elicit distinct responses in endothelial cells depending on the organs or vessel type involved. In summary, by using various in vivo genetic models coupled with advanced imaging techniques, this thesis work uncovers critical new molecular mechanisms and the underlying cellular dynamics involved in the development, maintenance and pathological expansion of the blood and lymphatic vasculature.