Both Endothelin A (ET_A) and Endothelin B (ET_B) Receptors Are Involved in Neurodegeneration in a Rodent Model of Glaucoma.

Abstract

Purpose : Ongoing studies in our laboratory have demonstrated a key role of the ET_B receptor in causing optic nerve axonal degeneration and retinal ganglion cell (RGC) loss in a rodent model of ocular hypertension. The current study examined the involvement of the ET_A receptor in the Morrison’s model of IOP elevation in rats. Additional studies were carried out to determine if the dual ET_A/ET_B receptor antagonist, macitentan could attenuate neurodegenerative changes following IOP elevation in Brown Norway rats.

Methods : IOP was elevated in one eye of adult male retired breeder Brown Norway rats using the Morrison’s method, while the contralateral eye served as control. Following IOP elevation, rats were maintained for 2 and 4 weeks and sacrificed. Retinal sections were stained with an antibody to the ET_A receptor and co-stained with β-tubulin III (a selective marker of the RGC layer) and immunofluorescence analysis was carried out. Primary RGCs were infected with an adeno-associated virus encoding the ET_A cDNA following which an immunocytochemical analysis of the ET_B receptor and a cell viability assay were carried out. Another set of Brown Norway rats were fed for 1 month with macitentan (10 mg/kg/body weight) during IOP elevation (100 to 120 mm Hg-days). Retinal flat mounts obtained from the rats were assessed for RGC survival by counting surviving RBPMS positive-RGCs.

Results : IOP elevation for 2 weeks produce a modest increase in immunostaining for the ET_A receptor in the RGC layer, inner plexiform layer and outer plexiform layer compared to those of wild type rats. Significantly increased immunostaining for the ET_A receptor was observed at 4 weeks of IOP elevation in the RGCs and inner plexiform layer. AAV-2 mediated upregulation of the ET_A produced an increase expression of the ET_B receptor and a decrease in viability of cultured RGCs. Rats fed with macitentan displayed increased RGC survival by 25 to 42% following IOP elevation, compared to untreated rats.

Conclusions : Both ET_A and ET_B receptors levels are increased following IOP elevation in Brown Norway rats. ET_A receptor activation contributes to neurodegeneration possibly by its ability to increase ET_B receptor expression. Blocking both ET_A and ET_B receptors could have neuroprotective effects by enhancing RGC survival without affecting IOP.

This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.