Abstract 299: Nitric Oxide Synthase Inhibition Attenuates the Formation of Notch-mediated Brain Arteriovenous Malformations

Abstract

Background: Arteriovenous (AV) malformations (AVMs) are characterized by abnormal AV shunts that displace intervening capillaries. Brain AVMs (BAVMs) may cause life-threatening strokes and have limited treatment options. Mechanisms underlying AVM pathogenesis remain poorly understood, hindering therapeutic development. Rationale: We reported that endothelial expression of constitutively active Notch4 (Notch4*) in mice initiates BAVMs de novo through enlargement of microvessels without an increase in endothelial cell number or proliferation. Thus, we hypothesized that Notch4* initiates BAVMs by disrupting normal vasodilation and vascular tone. Nitric oxide (NO) regulates vasodilation and vascular tone, and Notch activation induces NO synthesis. Thus, we further hypothesized that Notch4* disrupts NO signaling, thereby permitting vessel enlargement and AV shunting. Results: Cerebral arteries isolated from Notch4* mutant mice were less constricted and exhibited decreased arterial tone than that of controls. Administering the NO synthase (NOS) inhibitor NG-nitro-L-arginine (L-NNA, 30 mg/kg, 1x/day) attenuated Notch4*-induced AV shunt initiation: AV connection diameter in L-NNA-treated Notch4* mutants was decreased compared to that of saline-treated Notch4* mutants (7.0±1.1 vs. 15.1±2.3 μm) at postnatal day 12, when vessel enlargement and AV shunting first become apparent. Although L-NNA treatment attenuated AV shunt initiation, it did not prevent AV shunt formation or its progression; LNNA-treated Notch4* mutants still exhibited BAVM-associated pathologies and mortality. In moribund L-NNA-treated Notch4* mutants, AV connection diameter was decreased compared to that of saline-treated Notch4* mutants (18.9±6.5 vs. 30.6±8.2 μm), but remained ~4 fold that of genetic controls (~5 μm). A reduction in diameter in L-NNA-treated Notch4* mutants was accompanied by an increase in median survival time (30 vs. 25 days in saline-treated Notch4* mutants). Conclusions: Our results suggest that arterial dysfunction may contribute to Notch4*-mediated BAVM formation, and that inhibiting NOS attenuates BAVM formation. These data support a role for NO pathway in Notch4*-mediated BAVM formation.