Sirtuin3 Regulates Mitochondrial Fusion by Reversible Optic Atrophy1 Deacetylation

Abstract

Purpose: Aging increases the risk for eye diseases. Mitochondrial dysfunction is central to aging and eye diseases. Mitochondria undergo fission and fusion. Excessive fission without fusion causes mitochondrial fragmentation, oxidative stress and neurodegeneration. Optic atrophy1 (OPA1) is required for mitochondrial fusion. Rare inherited OPA1 mutations cause retinal ganglion cell death, optic nerve degeneration and vision loss. It is unknown whether OPA1 becomes sporadically inactivated in aging and common eye diseases. Sirtuins are NAD+-dependent protein deacetylases and enhance energy metabolism and cell survival. Sirtuin-3 (SIRT3) is present in mitochondria. SIRT3 is activated by stress. As a result, enzymes of the energy metabolism and antioxidant defenses become activated. The role of SIRT3 in the retina is unknown. SIRT3 and OPA1 localize to the mitochondrial cristae and their deletions show similar defects. The purpose of this study was to test whether SIRT3 regulates OPA1.

Methods: SIRT3 knockout and control mice were used. Aged DBA/2J mice were used as glaucoma models. For in vitro studies, OPA1 and SIRT3 proteins were purified. Acetyl/deacetyl mimetic OPA1 mutants were generated by site-directed mutagenesis. Antibodies for acetyl-lysine-OPA1 were produced. Tissue lysates were immune stained or immune precipitated.The effects of SIRT3 and OPA1 on mitochondria and neuronal survival were tested by gene transfer.

Results: We show that OPA1 is acetylated at specific lysine residues close to the GTPase domain and a hot spot for pathogenic OPA1 mutations. Purified SIRT3 deacetylates OPA1 and stimulates its GTPase activity and mitochondrial fusion. SIRT3 deletion in mice causes OPA1 hyperacetylation in the retina and mitochondrial fragmentation. SIRT3 activation during fasting causes OPA1 deacetylation. By contrast, SIRT3 decrease during high-fat diet feeding or aging results in OPA1 hyperacetylation. Deacetylated OPA1 has enhanced GTPase activity, promotes mitochondrial fusion, and inhibits cell death while an acetyl-mimetic OPA1 mutant causes the opposite. Collectively, we provide evidence of a novel SIRT3-mediated OPA1 activation and the erosion of this protective pathway in aging and neurodegeneration.

Conclusions: Here, we identified a previously unknown link between SIRT3 and OPA1. The data have implications for the treatment of neurodegeneration and vision loss.

Keywords: 600 mitochondria • 413 aging • 615 neuroprotection