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(1f/2f) mice (loxP sites flanking exons 4-6) and wild-type mice were injected with 2x10(7) PFU adenovious-cre recombinase and 2x10(9) genome copies of adeno-associated virus-vascular endothelial growth factor to induce focal homozygous Alk1 deletion (in Alk1(1f/2f) mice) and angiogenesis. Brain vessels were analyzed 8 weeks later. Compared with wild-type mice, the Alk1-deficient brain had more fibrin (99 +/- 30x10(3) pixels/mm(2) versus 40 +/- 13x10(3); P=0.001), iron deposition (508 +/- 506 pixels/mm2 versus 6 +/- 49; P=0.04), and Iba1(+) microglia/macrophage infiltration (888 +/- 420 Iba1(+) cells/mm(2) versus 240 +/- 104 Iba1(+); P=0.001) after vascular endothelial growth factor stimulation. In the angiogenic foci, the Alk1-deficient brain had more alpha-smooth muscle actin negative vessels (52 +/- 9% versus 12 +/- 7%, P<0.001), fewer vascular-associated pericytes (503 +/- 179/mm(2) versus 931 +/- 931 +/- 115, P<0.001), and reduced platelet-derived growth factor receptor-beta expression. Conclusion-Reduction of mural cell coverage in response to vascular endothelial growth factor stimulation is a potential mechanism for the impairment of vessel wall integrity in hereditary hemorrhagic telangiectasia type 2-associated brain arteriovenous malformation. (Arterioscler Thromb Vasc Biol. 2013;33:305-310.)