Upregulation of the endothelin A (ET) receptor and its association with neurodegeneration in a rodent model of glaucoma.

BACKGROUND

Primary open angle glaucoma is a heterogeneous group of optic neuropathies that results in optic nerve degeneration and a loss of retinal ganglion cells (RGCs) ultimately causing blindness if allowed to progress. Elevation of intraocular pressure (IOP) is the most attributable risk factor for developing glaucoma and lowering of IOP is currently the only available therapy. However, despite lowering IOP, neurodegenerative effects persist in some patients. Hence, it would be beneficial to develop approaches to promote neuroprotection of RGCs in addition to IOP lowering therapies. The endothelin system is a key target for intervention against glaucomatous neurodegeneration. The endothelin family of peptides and receptors, particularly endothelin-1 (ET-1) and endothelin B (ET) receptor, has been shown to have neurodegenerative roles in glaucoma. The purpose of this study was to examine changes in endothelin A (ET) receptor protein expression in the retinas of adult male Brown Norway rats following IOP elevation by the Morrison's model of ocular hypertension and the impact of ET receptor overexpression on RGC viability in vitro.

RESULTS

IOP elevation was carried out in one eye of Brown Norway rats by injection of hypertonic saline through episcleral veins. After 2 weeks of IOP elevation, immunohistochemical analysis of retinal sections from rat eyes showed an increasing trend in immunostaining for ET receptors in multiple retinal layers including the inner plexiform layer, ganglion cell layer and outer plexiform layer. Following 4 weeks of IOP elevation, a significant increase in immunostaining for ET receptor expression was found in the retina, primarily in the inner plexiform layer and ganglion cells. A modest increase in staining for ET receptors was also found in the outer plexiform layer in the retina of rats with IOP elevation. Cell culture studies showed that overexpression of ET receptors in 661W cells as well as primary RGCs decreases cell viability, compared to empty vector transfected cells. Adeno-associated virus mediated overexpression of the ET receptor produced an increase in the ET receptor in primary RGCs.

CONCLUSIONS

Elevated IOP results in an appreciable change in ET receptor expression in the retina. Overexpression of the ET receptor results in an overall decrease in cell viability, accompanied by an increase in ET receptor levels, suggesting the involvement of both ET and ET receptors in mediating cell death. These findings raise possibilities for the development of ET/ET dual receptor antagonists as neuroprotective treatments for glaucomatous neuropathy.