Endothelial cell–specific postnatal deletion of Nos3 preserves intraocular pressure homeostasis via macrophage recruitment and NOS2 upregulation

Polymorphisms in Nos3 increase risk for glaucoma, the leading cause of irreversible blindness worldwide. A key modifiable risk factor for glaucoma is intraocular pressure (IOP), which is regulated by NO - a product of nitric oxide synthase 3 (encoded by Nos3) - in Schlemm's canal of the conventional outflow pathway. We studied the effects of a conditional, endothelial cell-specific postnatal deletion of Nos3 (Endo-SclCre-ERT;Nos3fl/fl) on tissues of the outflow pathway. We observed that Cre-ERT expression spontaneously and gradually increased with time in vascular endothelia including in Schlemm's canal, beginning at P10, with complete Nos3 deletion occurring around P90. Whereas outflow resistance was reduced in global Nos3-KO mice, outflow resistance and IOP in Endo-SclCre-ERT;Nos3fl/fl mice were normal. We observed - coincident with Nos3 deletion - recruitment of macrophages to and induction of both ELAM1 and NOS2 expression by endothelia in the distal portion of the outflow pathway, which increased vessel diameter. These adjustments reduced outflow resistance to maintain IOP in these Endo-SclCre-ERT;Nos3fl/fl mice. Selective inhibition of iNOS by 1400W resulted in narrowing of distal vessels and IOP elevation. Together, the results emphasize the pliability of the outflow system and the importance of NO signaling in IOP control, and imply an substantial role for macrophages in IOP homeostasis.