Defects in Angiopoietin-Tie2 signaling lead to dose-dependent glaucoma in mice

Abstract

Purpose : Glaucoma is a devastating, progressive optic neuropathy often associated with elevated intraocular pressure (IOP). Despite the high prevalence of disease, few therapies exist as events leading to neurological damage in glaucoma remain obscure. Research into critical mechanisms has been hindered by lack of small animal glaucoma models which develop rapid and reproducible disease. We have shown that disruption of the Angiopoietin (Angpt)-Tie2 signaling pathway in mice results in loss of Schlemm’s canal (SC), elevated IOP and an extreme glaucomatous phenotype. Based on these results, we hypothesized that incomplete disruption of this pathway may lead to milder defects in SC development, less severe ocular hypertension and a progressive glaucoma closely resembling human disease.

Methods : Mice lacking Angpt1 or heterozygous for the Tie2 receptor were generated and aged 8-20 weeks. IOP was measured using a rebound tonometer before euthanasia and tissue harvest. Schlemm’s canal and retina morphology was then assessed using histological methods.

Results : Mice lacking Angpt1 or heterozygous for the Tie2 receptor exhibit a hypoplastic Schlemm’s canal phenotype, with focal narrowing, gaps and convolutions. This misshapen SC is insufficient for normal aqueous humor drainage and Angpt1 KO /Tie2 heterozygous mice exhibit elevated IOP (Angpt1 KO 21.09 mmHg, p<0.001, Tie2 heterozygote 15.39 mmHg p=0.0046) compared to control (12.30 mmHg), with no apparent buphthalmos. Histological analysis revealed progressive glaucoma reminiscent of human disease. As SC defects and glaucoma were caused by partial loss of Tie2 signaling, we attempted a genetic rescue by parallel deletion of the Tie2-regulating phosphatase VE-PTP. Unlike Tie2 heterozygous littermates, Tie2;VE-PTP double-heterozygous mice had normal IOP and did not develop disease, demonstrating that Tie2 activation through inhibition of a negative regulator prevents glaucoma pathogenesis.

Conclusions : Angpt/Tie2 signaling has a dose-dependent effect on SC development and IOP. Using inducible knockout systems we have created several novel mouse glaucoma models with variable onset and severity. In addition, we have identified a potential therapeutic target, VE-PTP, which can directly impact canal function. Pharmacological inhibition of VE-PTP leads to increased Angpt/Tie2 signaling activity and may provide an effective tool for increasing SC function in patients with glaucoma.

This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.