Endothelin-1 mediated decline in mitochondrial function contributes to neurodegeneration in glaucoma
Chaphalkar, Renuka M.
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Glaucoma is an optic neuropathy with multifactorial etiologies, commonly associated with elevated intraocular pressure (IOP) and characterized by degeneration of the optic nerve, loss of retinal ganglion cells (RGC), cupping of optic disc and visual field deficits, which could ultimately lead to vision loss. In most cases, glaucoma is a chronic, asymptomatic and gradually progressing neurodegenerative disease, sometimes referred to as the "silent thief of sight," hence, routine eye examinations by an ophthalmologist are critical to determine if there is a likelihood of developing the disease. Elevated IOP is a primary and the only modifiable risk factor in glaucoma. Currently, reducing IOP remains the only proven treatment to delay the progression of RGC death; however, some patients continue to have neurodegenerative effects despite lowering IOP. Therefore, development of novel neuroprotection strategies as an adjunct therapy to IOP-lowering agents will provide a valuable therapeutic strategy in glaucoma. One of the promising targets for neuroprotection is the endothelin system of peptides and their receptors. The endothelin (ET) system comprises of three vasoactive peptides (ET-1, ET-2 and ET-3), which act through two types of G-protein coupled receptors, namely, ETA and ETB receptors. Originally discovered in the cardiovascular system, the diverse expression pattern of endothelin peptides and their receptors implicate their involvement in a variety of physiological processes in the body. A growing body of evidence suggests that endothelins and their receptors are associated with neurodegeneration in glaucoma. Previous studies have demonstrated that ET-1 levels are elevated in aqueous humor (AH) and plasma of glaucoma patients. Our lab previously demonstrated that in an ocular hypertension model in rats, there was an increase in ETB as well as ETA receptor expression primarily in RGCs compared to contralateral eyes. Following IOP elevation, RGC loss was significantly attenuated in the ETB receptor-deficient rats, pointing to a causative role of the ETB receptor in glaucomatous neurodegeneration. However, the precise cellular and molecular mechanisms by which ET-1 promotes neurodegeneration through its actions on the endothelin receptors are not completely understood. Previous studies have shown that ETB receptor stimulation increases the oxidative stress and production of superoxide anions, in sympathetic neurons. Several studies point to the role of mitochondrial dysfunction and oxidative stress as contributors to glaucomatous damage in animal models of glaucoma. To investigate various molecular events contributing to the ET-1 mediated RGC loss in glaucoma, we carried out RNA-seq analysis of the translatome in rat primary RGCs following ET-1 treatment. We identified several key mitochondrial and neurodegenerative gene candidates including Atp5h, Cox17, Foxo1, Moap1 and Map3k11 that were differentially expressed in the translatome by ET-1 treatment in RGCs. Based on our RNA-seq findings, we hypothesized that ET-1 causes an increase in reactive oxygen species (ROS) by acting through the ETB receptor that produces a subsequent decline in mitochondrial function and bioenergetics ultimately predisposing RGCs to cell death. To test this hypothesis, we used an in vitro approach by utilizing rat primary culture of RGCs treated with ET-1 as well as an in vivo approach by intravitreal ET-1 injections in rodents and the Morrison's model of glaucoma in rats. Our data showed that there is a significant decrease in the expression of cytochrome c oxidase 17 copper chaperone (COX17) and ATP synthase, H+ transporting, mitochondrial F0 complex, subunit D (ATP5H), both of which are critical components of the electron transport chain and oxidative phosphorylation pathway. Using a Seahorse mitostress assay, we also found a significant decline of several mitochondrial parameters following ET-1 treatment in primary RGCs, which indicated the possibility of a disruption in the mitochondrial quality control machinery. Hence, we also explored the effect of the ET-1 treatment on the mitophagy pathway, specifically in RGCs. Our findings suggest that there is a decrease in mitophagosome formation in RGCs in the Morrison ocular hypertensive model as well as in GFP-LC3 mice injected with ET-1, indicating an impairment in the mitochondrial quality control mechanism. Our studies reveal several novel candidates that could be targeted for the development of neuroprotective approaches to treat glaucoma.