Negative Guidance System

Science Signaling  22 Sep 2009:
Vol. 2, Issue 89, pp. ec313
DOI: 10.1126/scisignal.289ec313

Vascular endothelial growth factor A (VEGF-A) is required for the de novo formation of blood vessels as well as the formation of vascular networks that connect the original (or parent) blood vessels, a process known as sprouting angiogenesis. Flt-1 (Fms-related tyrosine kinase 1) is a VEGF-A receptor that has a membrane-localized form (mFlt-1) and a soluble form (sFlt-1). sFlt-1 had been previously shown to inhibit angiogenesis, leading Chappell et al. to investigate its role in sprouting angiogenesis. In an in vitro model of vessel formation that used mouse embryonic stem (ES) cells, wild-type cells formed sprouts that were oriented 50° to 90° relative to the parent vessel, were located 75 μm or farther from other sprouts, and had tip filopodia that were in line with the sprout axis. Sprouts from flt-1–/– cells, in contrast, were oriented at more acute angles, formed closer to other sprouts, and had tip filopodia that were randomly directed. The guidance defects in sprouts derived from flt-1–/– ES cells were rescued by endothelial cell reexpression of sFlt-1 but not mFlt-1. Injection of a Flt-1–neutralizing antibody into mouse retinas increased the number of bifurcated sprouts, filopodia, and randomly oriented filopodia from sprouting retinal vessels. flt-1 expression (as assessed by activity of a β-galactosidase reporter linked to the flt-1 locus) and Flt-1 protein abundance were higher in the cells immediately adjacent to sprouts compared with that in cells within the sprout. Mosaic vessels were generated from mixtures of wild-type and flt-1–/– ES cells, which were differentiated by β-galactosidase activity. Sprouts that were derived from wild-type ES cells and were surrounded by flt-1–/– cells displayed signs of aberrant sprout directionality, effects that were rescued by expression of a transgene encoding sFlt-1 but not one encoding mFlt-1. The authors propose that by acting as a ligand sink for VEGF-A in areas adjacent to the sprout path, sFlt-1 is a key component of the local guidance system for sprouting blood vessels.

J. C. Chappell, S. M. Taylor, N. Ferrara, V. L. Bautch, Local guidance of emerging vessel sprouts requires soluble Flt-1. Dev. Cell 17, 377–386 (2009). [PubMed]