A novel therapeutic target for treating neurodegeneration and vascular permeability in Diabetic Retinopathy

Abstract

Purpose: The p75^NTR neurotrophin receptor promotes neuronal prunning and death, and recently was implicated in cardiovascular disorders. The main p75^NTR ligand is proNGF. In the healthy adult retina p75^NTR and proNGF are expressed at very low levels, but are up-regulated in many disease states( e.g. in Muller cells during glaucoma and retinitis pigmentosa, and in neovasculature during retinopathy of prematurity). For that reason, we studied p75^NTR in the vascular and the neurodegenerative pathologies associated with Diabetic Retinopathy.

Methods: We used drug-like small molecule antagonists of p75^NTR or biological antagonists of proNGF (neutralizing anti-proNGF mAb), administered after disease onset. We studied the mechanism of action of p75^NTR and its role in diabetes pathology, and modulation of the pathology by the inhibitors in the Streptozotocin (STZ) animal model. Drug delivery was by 1x intravitreal injection after disease onset. Endpoints: FD-OCT quantification of retinal structres, biochemical quantification of p75^NTR signals, kinetics of expression of receptor and ligand during disease progression by immunohistochemistry and biochemical analyses, and neuronal survival was quantified by TUNEL assay and by counting BRN3 labeled cells in whole retina. Vascular permeability and leakage was quantified by Evans Blue extravasation. Time-dependent kinetic studies were done from 1 week to 6 weeks of diabetes.

Results: We show that p75^NTR is up-regulated in Muller glial cells, and it is responsible for promoting glial production of neurotoxic cytokines TNF-α and α₂M, which in turn kill RGCs. As well, p75^NTR is up-regulated in vasculature causing vascular dysfunction. The kinetics of p75^NTR and proNGF expression correlate with disease progression. Pharmacological inhibition of p75^NTR or of the proNGF ligand normalizes levels of neurotoxic cytokines and prevents neuronal fiber loss, prevents RGC death and reduces vascular permeability.

Conclusions: The p75^NTR mechanisms are paracrine by regulation of glia and vascular biology, impacting on RGC health and vascular peremeability. Using p75^NTR antagonists or proNGF antagonists, it was possible to ameliorate the neuronal component as well as the vascular component of the pathology in diabetes. These studies validate p75^NTR as a druggable therapeutic target for diabetic retinopathy; and potentially it could be a target for other diiseases.