Mechanism of VEGF-C activation and effect on lymphatic vessel growth and regeneration

Lymphangiogenesis, the growth of the lymphatic vasculature, is a crucial process during embryonic development, and - if compromised by genetic damage - can lead to hereditary lymphedema. Although the molecular mechanisms that regulate the growth, development, and maintenance of the lymphatic vasculature have been researched with increasing intensity over the last 25 years, the therapeutic regeneration of lymphatic vessels is still a work in progress in the treatment of conditions such as lymphedema. Vascular endothelial growth factor-C (VEGF-C) is the primary growth factor responsible for the growth and development of the lymphatic vasculature. VEGF-C is activated by a complex process, which is indispensable for its ability to induce lymphangiogenesis via its primary receptor, VEGFR-3. The understanding of this process is a key factor for the development of VEGF-C as a drug target. The goal in my studies has been to increase our insights into VEGF-C activation at the molecular level, to identify its regulatory factors, and to establish its role in the lymphangiogenic process. Absence of the collagen- and calcium-binding EGF domains 1 (CCBE1) protein interrupts the lymphangiogenic process at about the same developmental stage when VEGF-C is first required. We utilized cell-based assays and adeno-associated viral-based gene transduction to investigate the role of CCBE1 on VEGF-C activation. In study I, we identified A disintegrin and metalloprotease with thrombospondin motifs-3 (ADAMTS3) as a protease that cleaves and activates VEGF-C, resulting in the major mature form of VEGF-C. We showed that CCBE1 acts as a cofactor in this process by enhancing the ability of ADAMTS3 to activate VEGF-C. Correspondingly, CCBE1 augmented the lymphangiogenic potential of VEGF-C in vivo. The presence of N- and C- terminal domains and their proteolytic cleavage characterize both CCBE1 and VEGF-C. In study II, we investigated the role of these domains for VEGF-C activation and the lymphangiogenic process. Our study demonstrated a requirement for the C-terminal domain of VEGF-C for the robust activation of VEGF-C both in vitro and in vivo. Moreover, we identified that the N- and C-terminal domains of CCBE1 have independent roles in the process of VEGF-C activation. The C-terminal domain accelerates the proteolytic cleavage, while the N-terminal domain aids in the assembly of the VEGF-C/ADAMTS3/CCBE1 cleavage complex by mobilizing VEGF-C to the endothelial cell surface. In study III, we searched for additional proteases that can cleave VEGF-C. We identified kallikrein-related peptidase 3 (KLK3) in seminal plasma and cathepsin D in saliva as proteases that cleave and activate VEGF-C. In human seminal plasma, we found substantial amounts of VEGF-C, which became activated concurrently with the semen liquefaction process. The newly identified VEGF-C cleavage sites are conserved in VEGF-D and we found that KLK3 and cathepsin D were able to activate VEGF-D as well. We also found that cleaved forms of VEGF-C and VEGF-D differ in their abilities to activate VEGFR-2 and VEGFR-3. When their N-termini were progressively shortened, the ability of VEGF-D to bind to and activate VEGFR-3 was decreased, while VEGF-C lost preferentially its ability to bind to and activate VEGFR-2. These findings contribute to the existing knowledge on the mechanisms of VEGF-C activation and the functional consequences thereof, and provide new opportunities to target VEGF-C for therapeutic purposes.