To the Editor:
In the recently published review Branching Morphogenesis,1 we inadvertently overlooked an important and highly relevant study. This study, by Ruhrberg et al,2 analyzed the differential role of vascular endothelial growth factor (VEGF)-A splice variants in shaping the vascular branching pattern during mouse development. The study compared the branching pattern in the hindbrain and somites of wild-type (WT) and mutant mice expressing either only VEGF-A120 or only VEGF-A188. These splice variants are at opposite ends in terms of their capacity to bind the heparan sulfate (HS) moiety in the glycosaminoglycan chains of proteoglycans: VEGF-A188, the longest VEGF-A splice variant, contains the full ensemble of 2 HS-binding domains, whereas the VEGF-A120 variant contains none. Consequently, the former is immobilized onto cell surface and extracellular matrix proteoglycans, whereas the latter is soluble and diffusive. Ruhrberg et al observed that the hindbrain vasculature of the VEGF-A120/120 mice contained 60% fewer branching points than that of WT mice. Moreover, the vessels of the VEGF-A120/120 mice were larger in diameter and contained ≈3 endothelial cells per capillary cross-section compared to ≈2 cells in WT capillaries. The hindbrain vasculature of mice expressing only the VEGF-A188 splice variant exhibited an opposite phenotype to that of VEGF-A120/120 mice: its vessels were narrower than those of WT mice, and there was an excessive number of branches. The study of Ruhrberg et al suggests that angiogenesis requires the combinatorial action of HS-binding and nonbinding VEGF-A splice variants: whereas the diffusible VEGF-A120 variant provides long-range proliferative and migratory signals that promote vascular growth, the immobilized VEGF-A188 variant produces steep local gradients that induce vessel branching. VEGF-A164 contains a single HS-binding domain, and hence its HS-binding affinity is intermediate between those of VEGF-A120 and VEGF-A188. Therefore, the expression of VEGF-A164 alone or of VEGF-A120 and VEGF-A188 is sufficient for the development of normal vasculature.2 The steep gradient formed by immobilized HS-binding VEGF-A is required for stimulating the growth of endothelial cell filopodia. The emerging filopodia appear to act as sensory structures that track the VEGF-A gradient and lure proliferating endothelial cells away from preexisting vessels and into new branches.
Sources of Funding
Horowitz A, Simons M. Branching morphogenesis. Circ Res. 2008; 103: 784–795.
Ruhrberg C, Gerhardt H, Golding M, Watson R, Ioannidou S, Fujisawa H, Betsholtz C, Shima DT. Spatially restricted patterning cues provided by heparin-binding VEGF-A control blood vessel branching morphogenesis. Genes Dev. 2002; 16: 2684–2698.