Our overall objective is to understand the role of norrin/frizzled4 signaling as regulator of the blood-retina barrier (BRB) in human disease, using mice as model system. Impairment of the BRB causes edema and is associated with diabetic retinopathy, age-related macular degeneration, retinopathy of prematurity, familial exudative vitreoretinopathy (FEVR), and uveitis. BRB induction and maintenance are controlled by the critical b-catenin-dependent (canonical) norrin/frizzled4 signaling pathway. Disrupted norrin/frizzled4 signaling causes FEVR. How the impairment of norrin/frizzled4 signaling causes pathological changes, how the pathway is regulated under stress, and if activation of norrin/frizzled4 signaling is a valid therapeutic approach to restore BRB dysfunction, are all not understood. These knowledge gaps need to be addressed to exploit the enormous basic research progress on norrin/frizzled4 signaling for therapeutic intervention.
Aim1 : We showed that endothelial cell-specific inactivation of norrin signaling in developed mice provides a new model to study pathological consequences of BRB breakdown. The new model does not display compound pathologies observed in existing models of BRB disruption characterized by pericyte defects, perfusion defects, and hypoxia, allowing us to study pathological consequences of BRB defects per se. Endothelial cell-specific disruption of norrin/frizzled4 signaling in developed mice causes immunoglobulin extravasation, complement activation, cystoid edema, and reduced b-wave in electroretinography (ERG). We will use mouse genetics to test the role of the classical complement pathway in mediating long-term pathological consequences of BRB breakdown in an otherwise intact vasculature. Fluorescein angiography and ERG will be performed at multiple time points, and histopathology at the 1-year end-point. Successful completion of this aim will determine roles of the classical complement pathway in retinal disease beyond the known roles in age-related macular degeneration.
Aim2 : Preliminary data show that increased levels of a stress sensor protein in endothelial cells strongly inhibit canonical signaling and negatively regulate BRB function. We will use conditional mouse genetics to activate the stress sensor in endothelial cells, determine BRB function, and test for genetic interactions with increased or reduced levels of b-catenin signaling. Cell-based assays will be performed to determine the mechanistic basis of this powerful regulation. Successful completion of this aim will provide novel insights into the regulation of the BRB under stress. These mechanisms may be exploited for modulating the BRB, e.g., in the context of restoring the barrier in disease, or for drug delivery.
Aim3 : Current efforts to target norrin/frizzled4 signaling for therapeutic intervention have focused on anti-angiogenesis approaches in proliferative retinopathies. We will determine if the activation of b-catenin in endothelial cells restores BRB function in a transient hypoxia disease model. Successful completion of this aim is expected to provide a model to test agonists of the pathway for their ability to restore a dysfunctional BRB (aim3).

Public Health Relevance

The blood-retina barrier protects the retina and is disrupted in ocular disease. Norrin signaling induces and maintains the blood-retina barrier but the roles of this pathway in ocular disease remain poorly understood. This project aims to understand the pathological consequences of barrier defects, mechanisms of barrier regulation, and if activation of Norrin signaling restores the barrier in disease models. !

National Institute of Health (NIH)
National Eye Institute (NEI)
Research Project (R01)
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Diseases and Pathophysiology of the Visual System Study Section (DPVS)
Program Officer
Shen, Grace L
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University of Minnesota Twin Cities
Schools of Medicine
United States
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Zhang, Chi; Lai, Maria B; Pedler, Michelle G et al. (2018) Endothelial Cell-Specific Inactivation of TSPAN12 (Tetraspanin 12) Reveals Pathological Consequences of Barrier Defects in an Otherwise Intact Vasculature. Arterioscler Thromb Vasc Biol 38:2691-2705
Wang, Yanshu; Cho, Chris; Williams, John et al. (2018) Interplay of the Norrin and Wnt7a/Wnt7b signaling systems in blood-brain barrier and blood-retina barrier development and maintenance. Proc Natl Acad Sci U S A 115:E11827-E11836
Lai, Maria B; Zhang, Chi; Shi, Jianli et al. (2017) TSPAN12 Is a Norrin Co-receptor that Amplifies Frizzled4 Ligand Selectivity and Signaling. Cell Rep 19:2809-2822
Junge, Harald J (2017) Ligand-Selective Wnt Receptor Complexes in CNS Blood Vessels: RECK and GPR124 Plugged In. Neuron 95:983-985
Zhang, Chi; Lai, Maria B; Khandan, Lavan et al. (2017) Norrin-induced Frizzled4 endocytosis and endo-lysosomal trafficking control retinal angiogenesis and barrier function. Nat Commun 8:16050
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Johnson, Verity; Xiang, Mengqing; Chen, Zhe et al. (2015) Neurite Mistargeting and Inverse Order of Intraretinal Vascular Plexus Formation Precede Subretinal Vascularization in Vldlr Mutant Mice. PLoS One 10:e0132013