Contrasting Defects in Lymphangiogenic Remodeling and Lymphangiogenesis Revealed in Angiopoietin-2 Deficient and Vegfc Hemizygous Mice

Abstract

Despite recent advances in lymphology, the molecular mechanisms regulating the development of the lymphatic system have not been fully delineated. Here I show that the growth factors Angiopoietin-2 (Ang2) and Vascular endothelial growth factor-c (Vegfc) serve distinct roles in the development of the lymphatic system.Adult Ang2-/- mice exhibit dermal lymphatic hypoplasia, abnormal smooth muscle cell (SMC) coverage of initial lymphatic vessels, and a severe deficiency of collecting lymphatic vessels. To determine whether these abnormalities were due to defects in the remodeling of the lymphatic vasculature, I characterized this process in Ang2-/- mice. Indeed, lymphatic vessels in the skin of Ang2-/- pups prematurely recruited SMCs and did not mature into collecting vessels during the remodeling stage of lymphatic development. In contrast, Ang2 knockout Ang1 knock-in mice did develop a hierarchal lymphatic vasculature, suggesting that activation of Tie-2 is required for normal lymphatic development. To further delineate the molecular mechanisms regulating lymphatic development, I also characterized Chylous ascites-3 (Chy-3) mice, a strain missing cytobands 8A4 to 8B3 from one copy of chromosome 8. Real-time PCR using genomic DNA demonstrated that the lymphangiogenesis gene, Vegfc, was included in the deleted region. All of the key components of a normal lymphatic network had developed in Chy-3 mice; however, the number of lymphatic vessels was reduced. Although the patterning of lymphatic vasculature in Chy-3 mice was altered, the architecture of the blood vasculature appeared normal. Ang2-/- and Chy-3 mice are grossly indistinguishable from one another; however, their underlying lymphatic defects are dramatically different. Ang2-/- mice exhibit defects in lymphangiogenic remodeling and lymphangiogenesis, whereas Chy-3 mice show a reduced capacity for lymphangiogenesis. I propose that Ang2 maintains lymphatic vessel plasticity by preventing SMC-lymphatic vessel associations so maturation can occur and facilitates lymphangiogenesis in the presence of Vegfc. This is analogous to Ang2's function on the blood vasculature where it is thought to destabilize SMC-blood vessel interactions and facilitate hemangiogenesis in the presence of Vegfa.Taken together, these findings further delineate the development of the lymphatic system and could provide translational clues relevant to the pathogenesis and treatment of clinical disorders of the lymphatic system.