Lawrence Huang1, Weiwei Xiang1, Matthew A. Nystoriak2, Xitao Wang1, Weiya Jiang1, Kevin S. Hou1, Jiayi Zhang1, Manuel F. Navedo2, Rong A. Wang1*
1Laboratory for Accelerated Vascular Research, Department of Surgery, University of California, San Francisco, San Francisco, CA; 2Department of Pharmacology, University of California, Davis, Davis, CA.
Arteriovenous (AV) malformations (AVMs) are characterized by a nidus of enlarged, tangled vessels that shunt blood directly from arteries to veins, displacing intervening capillaries. Mechanisms underlying AVM pathogenesis are poorly understood, hindering therapeutic development. Endothelial expression of constitutively active Notch4 (Notch4*) initiates brain AVMs in mice through enlargement of capillary-like vessels without an increase in endothelial cell number or proliferation. Instead, initial enlargement of AV shunts correlates with area expansion of individual endothelial cells, suggesting that aberrant vasodilation may play a role in early stages of AV shunting. We hypothesized that Notch4* disrupts endothelial nitric oxide synthase (eNOS) signaling, permitting vessel enlargement and AV shunting. Consistent with this, pharmacological inhibition of nitric oxide synthase (NOS) by administering the NOS inhibitor NG-nitro-L-arginine (L-NNA) or
eNOS gene deletion decreased brain AV shunt diameter, severity of brain AVM-associated pathologies, and illness progression in mice expressing endothelial Notch4*. Furthermore, pial arteries isolated from Notch4* mice exhibited decreased arterial tone compared to controls, and this was abolished by L-NNA. Interestingly, endothelial Notch4* expression did not result in detectable changes in nitric oxide (NO) production by 4-Amino-5-Methylamino-2',7'-Difluorofluorescein (DAF-FM) Diacetate in the brain or by cyclic guanosine monophosphate (cGMP) production, a surrogate for aortic NO production. Instead, NOS-dependent superoxide production was elevated in Notch4* brains at the initial stages of AV shunting formation, as assessed by dihydroethidium (DHE). Administering the superoxide dismutase mimetic 4-Hydroxy-2,2,6,6-tetramethylpiperidine 1-oxyl (Tempol) decreased brain AV shunt diameter, severity of brain AVM-associated pathologies, and illness progression in mice expressing endothelial Notch4*, mirroring the effects of NOS inhibition and eNOS deletion. Our data suggest that endothelial Notch4*-induced brain AVM involves an eNOS-dependent molecular mechanism that upregulates superoxide production.
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