The VEGF receptor flt-1 (VEGFR-1) is a positive modulator of vascular sprout formation and branching morphogenesis

Abstract

Sprouting angiogenesis is critical to blood vessel formation, but the cellular and molecular controls of this process are poorly understood. We used time-lapse imaging of green fluorescent protein (GF-P)-expressing vessels derived from stem cells to analyze dynamic aspects of vascular sprout formation and to determine how the vascular endothelial growth factor (VEGF) receptor flt-1 affects sprouting. Surprisingly, loss of flt-1 led to decreased sprout formation and migration, which resulted in reduced vascular branching. This phenotype was also seen in vivo, as flt-1 ؊/؊ embryos had defective sprouting from the dorsal aorta. We previously showed that loss of flt-1 increases the rate of endothelial cell division. However, the timing of division versus morphogenetic effects suggested that these phenotypes were not causally linked, and in fact mitoses were prevalent in the sprout field of both wild-type and flt-1 ؊/؊ mutant vessels. Rather, rescue of the branching defect by a soluble flt-1 (sflt- 1 IntroductionThe first step in the formation of most blood vessels is the production of an intricately branched vascular plexus. [1][2][3] This plexus is subsequently pruned and remodeled and in some cases coalesced to form larger vessels. The primary branched plexus forms by several processes, including the initial assembly of vascular precursor cells called vasculogenesis, and the subsequent migration of endothelial cells from the parent vessel called sprouting angiogenesis. The sprouts that form migrate until they reach another sprout or vessel, whereupon they most often form connections that are essential for elaborating and expanding the branched network. Although the formation of vascular sprouts has been described historically, 4-6 surprisingly little is known of the cellular processes and molecular controls of sprout formation, and even less is known about how these processes are integrated with other ongoing cellular events such as cell division.The vascular sprouts that form during embryonic development extend from primitive vessels. Individual endothelial cells send out filopodia, then migrate away from the parent vessel without breaking all contacts with surrounding cells, so that eventually multiple cells comprise the sprout. Cell numbers in the sprout are also increased by cell division behind the leading tip of the sprout. 6,7 Formation of a branched vascular plexus depends on the regulated expression of vascular endothelial growth factor-A (VEGF-A) by nonendothelial cells. This signal modulates intracellular signaling pathways that regulate endothelial cell division, migration, and survival. 8 This regulation is dose-dependent, as modest changes in the amount of available VEGF-A in either direction compromise vascular development, and loss of even one copy of the Vegf-A gene leads to vascular disruption and embryonic lethality. 9-13 Availability also seems to be regulated by the production of different isoforms of VEGF-A via alternative splicing. These isoforms have differing affinities for matrix componen...

Immunolocalization of vascular endothelial growth factor in rat condylar cartilage during postnatal development

“…However, recently Flt-1 has been focused on as an important mediator of hematopoietic and endothelial stem cell recruitment, mobilization, angiogenesis, and inflammation. Kearney et al (2004) reported that loss of Flt-1 led to decreased sprout formation and migration, which result in reduced vascular branching. Furthermore, it is evident that osteoclasts predominantly express Flt-1, and VEGF signal mediated through Flt-1 enhances osteoclast recruitment, survival, and bone resorption (Niida et al 1999;Nakagawa et al 2000).…”

Decay-accelerating Factor Induction on Vascular Endothelium by Vascular Endothelial Growth Factor (VEGF) Is Mediated via a VEGF Receptor-2 (VEGF-R2)- and Protein Kinase C-α/ϵ (PKCα/ϵ)-dependent Cytoprotective Signaling Pathway and Is Inhibited by Cyclosporin A

“…In contrast, the role of VEGF-R1 on adult ECs remains less well understood, although recent studies have demonstrated a role in VEGF-induced NO synthesis (14), in release of soluble mediators such as interleukin-6 by liver sinusoidal ECs (56), and in the regulation of vascular sprouting (57). The pleiotropic effects associated with ligation of VEGF receptors are reflected in the complexity of their downstream signaling pathways, with c-Src, PLC␥, PI3K, calcineurin, and PKC all implicated in VEGF-R2 signaling (7).…”

RACK1 Regulates VEGF/Flt1-mediated Cell Migration via Activation of a PI3K/Akt Pathway

“…In addition to promoting weak signals for VEGF-deprived cell growth and survival, Flt1 is also involved in regulating cell movement in both endothelial cells and macrophage-lineage cells. Loss of Flt1 expression in endothelial cells led to a decrease in sprout formation and cell migration, which resulted in reduced vascular branching (20). VEGF induces the migration and activation of macrophage-lineage cells into tumor tissue or inflamed areas by binding to Flt1 (11,(21)(22)(23)(24).…”