Angiogenesis, the sprouting of new blood vessels from pre‐existing ones, and the permeability of blood vessels are regulated by vascular endothelial growth factor (VEGF) via its two known receptors Flt1 (VEGFR‐1) and KDR/Flk‐1 (VEGFR‐2). The Flt4 receptor tyrosine kinase is related to the VEGF receptors, but does not bind VEGF and its expression becomes restricted mainly to lymphatic endothelia during development. In this study, we have purified the Flt4 ligand, VEGF‐C, and cloned its cDNA from human prostatic carcinoma cells. While VEGF‐C is homologous to other members of the VEGF/platelet derived growth factor (PDGF) family, its C‐terminal half contains extra cysteine‐rich motifs characteristic of a protein component of silk produced by the larval salivary glands of the midge, Chironomus tentans. VEGF‐C is proteolytically processed, binds Flt4, which we rename as VEGFR‐3 and induces tyrosine autophosphorylation of VEGFR‐3 and VEGFR‐2. In addition, VEGF‐C stimulated the migration of bovine capillary endothelial cells in collagen gel. VEGF‐C is thus a novel regulator of endothelia, and its effects may extend beyond the lymphatic system, where Flt4 is expressed.

Vascular endothelial growth factor (VEGF) is a key regulator of blood vessel development in embryos and angiogenesis in adult tissues. Unlike VEGF, the related VEGF-C stimulates the growth of lymphatic vessels through its specific lymphatic endothelial receptor VEGFR-3. Here it is shown that targeted inactivation of the gene encoding VEGFR-3 resulted in defective blood vessel development in early mouse embryos. Vasculogenesis and angiogenesis occurred, but large vessels became abnormally organized with defective lumens, leading to fluid accumulation in the pericardial cavity and cardiovascular failure at embryonic day 9.5. Thus, VEGFR-3 has an essential role in the development of the embryonic cardiovascular system before the emergence of the lymphatic vessels.

Primary human lymphedema (Milroy's disease), characterized by a chronic and disfiguring swelling of the extremities, is associated with heterozygous inactivating missense mutations of the gene encoding vascular endothelial growth factor C͞D receptor (VEGFR-3). Here, we describe a mouse model and a possible treatment for primary lymphedema. Like the human patients, the lymphedema (Chy) mice have an inactivating Vegfr3 mutation in their germ line, and swelling of the limbs because of hypoplastic cutaneous, but not visceral, lymphatic vessels. Neuropilin (NRP)-2 bound VEGF-C and was expressed in the visceral, but not in the cutaneous, lymphatic endothelia, suggesting that it may participate in the pathogenesis of lymphedema. By using virus-mediated VEGF-C gene therapy, we were able to generate functional lymphatic vessels in the lymphedema mice. Our results suggest that growth factor gene therapy is applicable to human lymphedema and provide a paradigm for other diseases associated with mutant receptors.