Spatially restricted patterning cues provided by heparin-binding VEGF-A control blood vessel branching morphogenesis

Abstract

Branching morphogenesis in the mammalian lung and Drosophila trachea relies on the precise localization of secreted modulators of epithelial growth to select branch sites and direct branch elongation, but the intercellular signals that control blood vessel branching have not been previously identified. We found that VEGF 120/120 mouse embryos, engineered to express solely an isoform of VEGF-A that lacks heparin-binding, and therefore extracellular matrix interaction domains, exhibited a specific decrease in capillary branch formation. This defect was not caused by isoform-specific differences in stimulating endothelial cell proliferation or by impaired isoform-specific signaling through the Nrp1 receptor. Rather, changes in the extracellular localization of VEGF-A in heparin-binding mutant embryos resulted in an altered distribution of endothelial cells within the growing vasculature. Instead of being recruited into additional branches, nascent endothelial cells were preferentially integrated within existing vessels to increase lumen caliber. The disruption of the normal VEGF-A concentration gradient also impaired the directed extension of endothelial cell filopodia, suggesting that heparin-binding VEGF-A isoforms normally provide spatially restricted stimulatory cues that polarize and thereby guide sprouting endothelial cells to initiate vascular branch formation. Consistent with this idea, we found opposing defects in embryos harboring only a heparin-binding isoform of VEGF-A, including excess endothelial filopodia and abnormally thin vessel branches in ectopic sites. We conclude that differential VEGF-A isoform localization in the extracellular space provides a control point for regulating vascular branching pattern.

Improved growth factor directed vascularization into fibrin constructs through inclusion of additional extracellular molecules

“…This approach is based on observations that growth factor spatial patterns are important for biological developmental as well as tissue regeneration and wound healing (Gerhardt et al, 2003;Ruhrberg et al, 2002;Singer and Clark, 1999;Tabata, 2001) and the idea that these patterns could be exploited to create improved therapies. In our first model studies we have been investigating CAM blood vessel ingrowth into fibrin constructs containing the simplest growth factor pattern, a uniform concentration of immobilized fibroblast growth factor-2 (FGF-2).…”

Multi-scale Computational Models of Pro-angiogenic Treatments in Peripheral Arterial Disease

“…VEGF gradients are important for chemotactic guidance of angiogenic sprouts. It is likely that both increased concentrations and gradients that the cell can sense 15,52 are required for the efficient vascularization of ischemic or hypoxic tissue.…”

“…15,52 Cells do not typically proliferate and migrate simultaneously, and therefore both absolute concentration and relative gradient may play a role in the cellularlevel decision-making process. Both values are difficult or impossible to measure in vivo at the microenvironmental level, and we report predicted values for both in this study.…”

“…31 We have also shown that increased cell-surface receptor expression can significantly increase the magnitude of VEGF gradients. 24,31,32 The size of the gradient over 10 lm can be compared with the typical length of endothelial tip cells, approximately 30-50 lm, 15,52 to determine whether the gradient is steep enough to be sensed by the cell.…”

“…In in vitro studies, they increase survival, proliferation and migration of endothelial cells; 55 in vivo, they act as a chemoattractant and direct capillary growth. 15,20,52 In this study, we use the term VEGF to refer to the two most common protein products of the rat VEGF-A gene, VEGF 120 , and VEGF 164 (VEGF 121 and VEGF 165 , respectively, in humans). The difference between these two proteins is the ability of the longer isoform to bind to both glycosaminoglycans (GAGs) in the interstitial space and Neuropilin co-receptors on endothelial cells.…”