Interactions between mural cells and endothelial cells stabilize the developing zebrafish dorsal aorta.

Mural cells (vascular smooth muscle cells and pericytes) play an essential role in the development of the vasculature, promoting vascular quiescence and long-term vessel stabilization through their interactions with endothelial cells. However, the mechanistic details of how mural cells stabilize vessels are not fully understood. We have examined the emergence and functional role of mural cells investing the dorsal aorta during early development using the zebrafish. Consistent with previous literature, our data suggest that cells ensheathing the dorsal aorta emerge from a sub-population of cells in the adjacent sclerotome. Inhibition of mural cell recruitment to the dorsal aorta through disruption of pdgfr signaling leads to a reduced vascular basement membrane, which in turn results in enhanced dorsal aorta vessel elasticity and failure to restrict aortic diameter. Our results provide direct in vivo evidence for a functional role for mural cells in patterning and stabilization of the early vasculature through production and maintenance of the vascular basement membrane to prevent abnormal aortic expansion and elasticity.

Chemokine mediated signalling within arteries promotes vascular smooth muscle cell recruitment.

The preferential accumulation of vascular smooth muscle cells (vSMCs) on arteries versus veins during early development is a well-described phenomenon, but the molecular pathways underlying this polarization are not well understood. In zebrafish, the cxcr4a receptor (mammalian CXCR4) and its ligand cxcl12b (mammalian CXCL12) are both preferentially expressed on arteries at time points consistent with the arrival and differentiation of the first vSMCs during vascular development. We show that autocrine cxcl12b/cxcr4 activity leads to increased production of the vSMC chemoattractant ligand pdgfb by endothelial cells in vitro and increased expression of pdgfb by arteries of zebrafish and mice in vivo. Additionally, we demonstrate that expression of the blood flow-regulated transcription factor klf2a in primitive veins negatively regulates cxcr4/cxcl12 and pdgfb expression, restricting vSMC recruitment to the arterial vasculature. Together, this signalling axis leads to the differential acquisition of vSMCs at sites where klf2a expression is low and both cxcr4a and pdgfb are co-expressed, i.e. arteries during early development.

Anti-angiogenic effects of VEGF stimulation on endothelium deficient in phosphoinositide recycling.

Anti-angiogenic therapies have generated significant interest for their potential to combat tumor growth. However, tumor overproduction of pro-angiogenic ligands can overcome these therapies, hampering success of this approach. To circumvent this problem, we target the resynthesis of phosphoinositides consumed during intracellular transduction of pro-angiogenic signals in endothelial cells (EC), thus harnessing the tumor's own production of excess stimulatory ligands to deplete adjacent ECs of the capacity to respond to these signals. Using zebrafish and human endothelial cells in vitro, we show ECs deficient in CDP-diacylglycerol synthase 2 are uniquely sensitive to increased vascular endothelial growth factor (VEGF) stimulation due to a reduced capacity to re-synthesize phosphoinositides, including phosphatidylinositol-(4,5)-bisphosphate (PIP2), resulting in VEGF-exacerbated defects in angiogenesis and angiogenic signaling. Using murine tumor allograft models, we show that systemic or EC specific suppression of phosphoinositide recycling results in reduced tumor growth and tumor angiogenesis. Our results suggest inhibition of phosphoinositide recycling provides a useful anti-angiogenic approach.