Reduced Mural Cell Coverage and Impaired Vessel Integrity After Angiogenic Stimulation in the Alk1-deficient Brain

OBJECTIVE—Vessels in brain arteriovenous malformations are prone to rupture. The underlying pathogenesis is not clear. Hereditary hemorrhagic telangiectasia type 2 patients with activin receptor-like kinase 1 (Alk1) mutation have a higher incidence of brain arteriovenous malformation than the general population. We tested the hypothesis that vascular endothelial growth factor impairs vascular integrity in the Alk1-deficient brain through reduction of mural cell coverage. METHODS AND RESULTS—Adult Alk1 mice (loxP sites flanking exons 4–6) and wild-type mice were injected with 2×10 PFU adenovious-cre recombinase and 2×10 genome copies of adeno-associated virus-vascular endothelial growth factor to induce focal homozygous Alk1 deletion (in Alk1 mice) and angiogenesis. Brain vessels were analyzed 8 weeks later. Compared with wild-type mice, the Alk1-deficient brain had more fibrin (99±30×10 pixels/mm versus 40±13×10; P=0.001), iron deposition (508±506 pixels/mm versus 6±49; P=0.04), and Iba1 microglia/macrophage infiltration (888±420 Iba1 cells/mm versus 240±104 Iba1; P=0.001) after vascular endothelial growth factor stimulation. In the angiogenic foci, the Alk1-deficient brain had more α-smooth muscle actin negative vessels (52±9% versus 12±7%, P