Axonal protection by Nmnat3 overexpression with involvement of autophagy in optic nerve degeneration

Y Kitaoka,S Ueno,A Hirano,H Takagi,K Kojima,Y Munemasa

doi:10.1038/cddis.2013.391

Abstract

Axonal degeneration often leads to the death of neuronal cell bodies. Previous studies demonstrated the crucial role of nicotinamide mononucleotide adenylyltransferase (Nmnat) 1, 2, and 3 in axonal protection. In this study, Nmnat3 immunoreactivity was observed inside axons in the optic nerve. Overexpression of Nmnat3 exerts axonal protection against tumor necrosis factor-induced and intraocular pressure (IOP) elevation-induced optic nerve degeneration. Immunoblot analysis showed that both p62 and microtubule-associated protein light chain 3 (LC3)-II were upregulated in the optic nerve after IOP elevation. Nmnat3 transfection decreased p62 and increased LC3-II in the optic nerve both with and without experimental glaucoma. Electron microscopy showed the existence of autophagic vacuoles in optic nerve axons in the glaucoma, glaucoma+Nmnat3 transfection, and glaucoma+rapamycin groups, although preserved myelin and microtubule structures were noted in the glaucoma+Nmnat3 transfection and glaucoma+rapamycin groups. The axonal-protective effect of Nmnat3 was inhibited by 3-methyladenine, whereas rapamycin exerted axonal protection after IOP elevation. We found that p62 was present in the mitochondria and confirmed substantial colocalization of mitochondrial Nmnat3 and p62 in starved retinal ganglion cell (RGC)-5 cells. Nmnat3 transfection decreased p62 and increased autophagic flux in RGC-5 cells. These results suggest that the axonal-protective effect of Nmnat3 may be involved in autophagy machinery, and that modulation of Nmnat3 and autophagy may lead to potential strategies against degenerative optic nerve disease.

Highlights

Autophagy is involved in a cell-protective process and has a role in cell death.[1]
We previously demonstrated that intravitreal injection of tumor necrosis factor (TNF), which is involved in certain types of glaucoma,[8,9,10,11,12,13] induces progressive optic nerve degeneration with slow retinal ganglion cells (RGCs) body death.[14]
We examined the localization of Nmnat[3] in the retina and optic nerve and evaluated whether Nmnat[3] overexpression exerts axonal protection against optic nerve degeneration induced by TNF and intraocular pressure (IOP) elevation

Summary

Introduction

Autophagy is involved in a cell-protective process and has a role in cell death.[1]. In the retinal neurons, it has been reported that transient increases in microtubule-associated protein light chain 3 (LC3)-II, an autophagic marker, occur in retinal ganglion cells (RGCs) after optic nerve transection.[2]. Autophagy was reported to have a cytoprotective role in RGCs after traumatic injury.[3] In contrast, it was demonstrated that the inhibition of autophagy resulted in an attenuation of RGC death in a hypertensive glaucoma model.[4] Those studies focused on RGC body death, the axonal degeneration pathway in the optic nerve has not been well documented in spite of evidence that the mechanisms of degeneration of neuronal cell bodies and their axons differ.[5,6,7]. We examined the involvement of autophagy in axonal protection by Nmnat[3] in the optic nerve in the hypertensive glaucoma model

Methods

Results

Conclusion