Von Willebrand Factor Deficiency Impairs Vascular Morphogenesis via Angiopoietin-2: Relevance for Gut Angiodysplasia
Management of recurrent gastrointestinal (GI) bleeding is a clinical unmet need for patients with Von Willebrand disease (VWD) and is linked to the presence of gut vascular malformations (angiodysplasia). We previously demonstrated that von Willebrand factor (VWF) regulates angiogenesis and vascular integrity, the likely mechanism underlying angiodysplasia. VWF controls the storage of the angiogenesis regulator Angiopoietin-2 (Angpt-2) in endothelial cells (EC), suggesting a candidate for the genesis of angiodysplasia; however, no direct evidence of the role of Angpt-2 in VWF-dependent angiogenesis is available. Here we use VWF-deficient HUVEC, and endothelial colony forming cells (ECFCs) from severe VWD patients and find that loss of VWF in EC results in increased Angpt-2 expression through a positive feedback loop via the Angpt-2-TIE2-AKT-FOXO1 pathway. We also show an imbalance of the Angpt/Tie2 pathway in vivo. In the gut of VWF-deficient mice, Angpt-2 expression is increased whilst Angpt-1 expression is decreased; this correlates with reduced expression of the pericyte marker NG2. These data suggest that VWF regulates the Angpt/Tie2 balance in the gut. To investigate the functional defects caused by loss of VWF, we use a fibrin bead assay and show that VWF-deficient HUVEC present increased sprouting. We develop a microfluidic model of 3D vasculogenesis/angiogenesis and find that ECFCs from VWD patients exhibit defective remodeling and abnormal lumen formation compared to healthy controls. Importantly, inhibition of Angpt-2 reduces sprouting in VWF-deficient HUVEC and normalises vascular networks in ECFCs from severe VWD, suggesting Angpt-2 inhibitors may be effective in VWD patients with GI bleeding and angiodysplasia. Key pointsO_LIEndothelial VWF regulates multiple steps of angiogenesis, including sprouting and lumen formation. C_LIO_LIVWF regulates Angpt-2 storage and expression, and Angpt-2 blockade normalises defective angiogenesis in VWD ECFCs. C_LI Visual Abstract O_FIG O_LINKSMALLFIG WIDTH=170 HEIGHT=200 SRC="FIGDIR/small/675051v1_ufig1.gif" ALT="Figure 1"> View larger version (19K): org.highwire.dtl.DTLVardef@150eb15org.highwire.dtl.DTLVardef@179e6beorg.highwire.dtl.DTLVardef@1bcadborg.highwire.dtl.DTLVardef@a30fb8_HPS_FORMAT_FIGEXP M_FIG VWF deficiency in HUVECs results in increased Angpt-2 release and expression via the Tie2-Akt-FOXO1 pathway. A. Model pathway for the regulation of Angpt-2 levels by VWF. Loss of VWF results in increased Angpt-2 release from endothelial cells; Angpt-2 binds to and inhibits the Tie2 receptor, decreasing its phosphorylation as well as the downstream phosphorylation of Akt and FOXO1; this leads to FOXO1 activation and an increase expression of Angpt-2, generating a feedback loop. B. Model for the pathogenesis of angiodysplasia in VWD. In the absence of VWF, the increase in Angpt-2 disrupts vascular morphogenesis through multiple mechanisms: increased sprouting, impaired vascular remodelling and an imbalance between Angpt-1 and Angpt-2 in the gut. Figure generated with Biorender. C_FIG