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Specification of arteries and veins is a key process for establishing functional vasculature during embryogenesis and involves distinctly different signaling mechanisms. Vascular endothelial growth factor-A (VEGFA) is required for differentiation of arteries; however, the upstream angiogenic factor for vein specification is unknown. Klippel–Trenaunay syndrome (KTS) is a congenital vascular disease associated with capillary and venous malformations (VMs), but not with arterial defects. We have previously reported that upregulation of angiogenic factor AGGF1 is associated with KTS, but the molecular mechanism is not clear. Here, we show that  AGGF1  is involved in establishing venous identity in zebrafish embryos. Overexpression of  AGGF1  led to increased angiogenesis and increased lumen diameter of veins, whereas knockdown of  AGGF1  expression resulted in defective vasculogenesis and angiogenesis. Overexpression of  AGGF1  increased expression of venous markers (e.g.  flt4 ), but had little effect on arterial markers (e.g.  notch5 ). Knockdown of  AGGF1  expression resulted in a loss of venous identity (loss of expression of  flt4 ,  ephb4  and  dab2 ), but had no effect on the expression of arterial development. We further show that AGGF1 activates AKT, and that decreased  AGGF1  expression inhibits AKT activation. Overexpression of constitutively active AKT rescues the loss of venous identity caused by  AGGF1  downregulation. Our study establishes AGGF1 as an angiogenic factor with an important role in the specification of vein identity and suggests that AGGF1-mediated AKT signaling is responsible for establishing venous cell fate. We propose that increased  AGGF1  expression leads to increased vein differentiation by inducing activation of AKT signaling, resulting in VMs s in KTS patients.

human molecular genetics

Functional characterization of Klippel–Trenaunay syndrome gene  AGGF1  identifies a novel angiogenic signaling pathway for specification of vein differentiation and angiogenesis during embryogenesis