Mitochondria are double membrane organelle
Mitochondria are double membrane organelle, and they share proteins, membranes, solutes and metabolites through dynamic fusion-fission process. Fusion is a two-step process; the outer mitochondrial membrane fusion is controlled by Mfn1 and Mfn2, and the inner membrane by Opa1 1 . Mfn1 and Mfn2 share approximately 80% similarity , and Mfn1-dependent mitochondrial dynamics is required for glucose sensing and insulin release in neurons , Mfn2 is a multifunctional protein and its expression is ubiquitous in mammalian tissues. Furthermore, tissues enriched in mitochondria including neuronal tissues have higher levels of Mfn2 compared to less metabolic tissues [35,38]. In hyperglycemic milieu, while Mfn2 is decreased in the retina and its mitochondria, Mfn1 levels remain unchanged . Here, we show that Mfn2 is also decreased in the retinal microvessels in hyperglycemia, human donors with documented diabetic retinopathy also have decreased Mfn2 in their microvasculature compared to the age-matched nondiabetic donors. In addition, mitochondria are fragmented, and overexpression of Mfn2 prevents mitochondrial fragmentation, confirming an integral role of Mfn2 in mitochondrial structural damage. In diabetes, retinal mitochondria become swollen and their membrane potential is decreased, superoxide levels are elevated, and complex III activity is decreased . Here we show that overexpression of Mfn2 alleviates diabetes-induced increase in superoxide radicals and mitochondrial dysfunction. It prevents alteration in mitochondrial membrane potential and complex III activity. In support, Mfn2 is also considered essential in maintaining mitochondrial Epoprostenol metabolism and coenzyme Q levels, and Mfn2 deficiency is implicated with increased superoxide levels and impaired mitochondrial permeability and respiration and decrease ATP production [35,39,40]. Impairment in mitochondrial fusion is implicated in release of cytochrome c from mitochondria and accelerating the apoptotic machinery . In the pathogenesis of diabetic retinopathy, mitochondrial dysfunction-accelerated capillary cell apoptosis precedes the development of histopathology characteristic of diabetic retinopathy [42,43]. Our results demonstrate that overexpression of Mfn2 also attenuates increase in capillary cell apoptosis experienced by cells in hyperglycemic milieu, further supporting the role of Mfn2 in diabetic retinopathy. Mitochondrial fusion, which allows the exchange of contents including DNA and metabolites between neighboring mitochondria, also preserves mtDNA function; abnormalities in mitochondrial fusion are implicated in mtDNA instability . Loss in mitofusins is shown to cause mtDNA mutations and deletions in muscle atrophy . Mfn2 mutations are also shown to affect mtDNA replication, and alter mitochondrial oxidative phosphorylation . Damaged mtDNA results in impaired transcription , and mitochondrial fusion helps maintain mtDNA , and a link between Mfn2 and mtDNA-encoded electron transport chain components is seen in neuropathy . Our results demonstrate that overexpression of Mfn2 prevents hyperglycemia-induced mtDNA damage, and also ameliorates decrease in expression of mtDNA-encoded genes including CytB of complex III of the electron transport chain. We recognize that both Mfn1 and Mfn2 play role in outer mitochondrial membrane fusion, these two Mfns share many common functions and Mfn1 has higher GTPase activity . However, while Mfn1 is implicated in increased susceptibility to certain pathogen infections, its mutations have not been associated with diseases . In contrast, mutations in Mfn2 are associated with many diseases, and Mfn2 itself is necessary and sufficient to modulate mitochondrial metabolism by regulating mitochondrial fuel oxidation, membrane potential and oxidative phosphorylation [15,36]. Furthermore, our initial mitochondrial PCR array data in rat retina, and retina from human donors, have shown significant decrease in Mfn2 expression in diabetes . However, in the same samples, decrease in Mfn1 was >15% and was not significant compared to their age-matched nondiabetic controls, further supporting the role of Mfn2 in diabetic retinopathy.