Inhibition of cAMP/PKA Pathway Protects Optic Nerve Head Astrocytes against Oxidative Stress by Akt/Bax Phosphorylation-Mediated Mfn1/2 Oligomerization.

Won-Kyu Ju,Keun-Young Kim,Tae Lim Park,Genea Edwards,Robert N Weinreb,Sangphil Ahn,Myoung Sup Shim

doi:10.1155/2019/8060962

Abstract

Glaucoma is characterized by a progressive optic nerve degeneration and retinal ganglion cell loss, but the underlying biological basis for the accompanying neurodegeneration is not known. Accumulating evidence indicates that structural and functional abnormalities of astrocytes within the optic nerve head (ONH) have a role in glaucomatous neurodegeneration. Here, we investigate the impact of activation of cyclic adenosine 3′,5′-monophosphate (cAMP)/protein kinase A (PKA) pathway on mitochondrial dynamics of ONH astrocytes exposed to oxidative stress. ONH astrocytes showed a significant loss of astrocytic processes in the glial lamina of glaucomatous DBA/2J mice, accompanied by basement membrane thickening and collagen deposition in blood vessels and axonal degeneration. Serial block-face scanning electron microscopy data analysis demonstrated that numbers of total and branched mitochondria were significantly increased in ONH astrocytes, while mitochondrial length and volume density were significantly decreased. We found that hydrogen peroxide- (H2O2-) induced oxidative stress compromised not only mitochondrial bioenergetics by reducing the basal and maximal respiration but also balance of mitochondrial dynamics by decreasing dynamin-related protein 1 (Drp1) protein expression in rat ONH astrocytes. In contrast, elevated cAMP by dibutyryl-cAMP (dbcAMP) or isobutylmethylxanthine treatment significantly increased Drp1 protein expression in ONH astrocytes. Elevated cAMP exacerbated the impairment of mitochondrial dynamics and reduction of cell viability to oxidative stress in ONH astrocytes by decreasing optic atrophy type 1 (OPA1), and mitofusin (Mfn)1/2 protein expression. Following combined treatment with H2O2 and dbcAMP, PKA inhibition restored mitochondrial dynamics by increasing mitochondrial length and decreasing mitochondrial number, and this promoted cell viability in ONH astrocytes. Also, PKA inhibition significantly promoted Akt/Bax phosphorylation and Mfn1/2 oligomerization in ONH astrocytes. These results suggest that modulation of the cAMP/PKA signaling pathway may have therapeutic potential by activating Akt/Bax phosphorylation and promoting Mfn1/2 oligomerization in glaucomatous ONH astrocytes.

Highlights

Glaucoma is a complex, multifactorial optic neuropathy characterized by a slow and progressive degeneration of the optic nerve, including retinal ganglion cell (RGC) axons, leading to visual impairment [1, 2]
Quantitative analysis of mitochondrial morphology showed that numbers of total mitochondria and branched mitochondria were significantly increased in glaucomatous optic nerve head (ONH) astrocytes compared with D2-Gpnmb+ mice (Figure 1(f))
We demonstrated that glaucomatous ONH astrocytes showed unbalanced mitochondrial dynamics by increasing excessive mitochondrial fission and loss, and this was accompanied by the compromised blood vessels and degenerative axons

Summary

Introduction

Multifactorial optic neuropathy characterized by a slow and progressive degeneration of the optic nerve, including retinal ganglion cell (RGC) axons, leading to visual impairment [1, 2]. Accumulating evidence indicates that the increased level of cAMP is associated with the unstimulated glaucomatous ONH astrocytes from patients with POAG [21] as well as experimental glaucoma [7, 20]. The precise cellular and molecular mechanism(s) of the activation of the cAMP/PKA signaling pathway underlying astrocyte dysfunction in glaucomatous ONH degeneration remains to be determined. In this regard, mitochondria have been suggested to have a key role

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Results

Conclusion