Mitochondria are small in size to m and have double
Mitochondria are small in size (0.75 to 3 μm), and have double membranes; while the outer membrane is porous with no membrane potential, the inner membrane has barriers to all ions and molecules and is the site of oxidative phosphorylation . Structurally, mitochondria are highly dynamic, and depending on the energy demand and supply balance, mitochondria constantly fuse and change size and shape. Fragmentation of mitochondria is associated with a decline in ATP production, and fusion mixes the contents of two mitochondria and dilutes injured mitochondrial proteins and DNA [, , ]. Disruption in fusion results in mitochondria to fragment into short rods or spheres, and in accumulation of mtDNA point mutations and deletions. Mitochondrial fusion is a two-step process, which uses mitofusin 1 and 2 (Mfn1 and Mfn2) to control the outer mitochondrial membrane fusion and optic atrophy 1 (Opa1) for the inner mitochondrial membrane fusion . Although Mfn1 and Mfn2 share some common functions, Mfn2 itself is necessary and sufficient to modulate mitochondrial metabolism by regulating mitochondrial fuel oxidation, membrane potential and oxidative phosphorylation [14,15]. In diabetes, expression of Mfn2 (gene and protein) is decreased in the retina [16,17]. However, the role of Mfn2 in diabetic retinopathy remains to be explored.
Gene expression is also regulated by epigenetic modifications without altering the DNA sequence , and in diabetes, enzymes responsible for these epigenetic modifications are altered in the retina [3,19]. Enzyme machinery for DNA methylation, one of the epigenetic modifications which suppresses gene expression, is activated in the retina and its vasculature in diabetes, and mtDNA is hypermethylated, resulting in impaired transcription of mtDNA-encoded genes [3,20]. The mechanism responsible for decreased retinal Mfn2 expression in diabetes is, however, not clear.
The aim of this study was to investigate the importance of mitochondrial fusion in the development of diabetic retinopathy, and illustrate the molecular mechanism responsible for Mfn2 suppression. Using human retinal endothelial DETA NONOate (HRECs), manipulated for Mfn2 expression, we have investigated the role of mitochondrial fusion in increased mitochondrial free radical production and mitochondrial damage in diabetic retinopathy. To understand the mechanism responsible for Mfn2 suppression in diabetes, DNA methylation status of its promoter was evaluated. Key parameters were confirmed in the retinal microvessels from rat model of diabetic retinopathy, and also from human donors with documented diabetic retinopathy.
Discussion Mitochondria, the primary source of cellular energy, are one of the most important subcellular organelle, and their homeostasis is critical for cell survival. They are considered to play central role in diabetic microvascular complications . In diabetic retinopathy, retinal mitochondria are swollen, their membranes integrity is impaired, superoxide levels are elevated, mtDNA is damaged and the transcription of mtDNA is decreased, and compromised electron transport system continues to propagate a futile free radical cycle. Mitochondria are dynamic organelles that continuously undergo fission and fusion, and this process is critical for adaptation of a cell to changing energy demand and cell survival [, , ]. A dynamic regulatory fusion protein, Mfn2, is considered critical for the maintenance of normal mitochondrial function in mammalian cells . Using in vivo and in vitro models of diabetic retinopathy, here we show that overexpression of Mfn2 in retinal endothelial cells prevents them from hyperglycemia-induced mitochondrial structural and functional damage, and ameliorates mtDNA damage and its transcription. In addition, our study has also identified a novel mechanism responsible for decreased Mfn2 expression in diabetes; due to DNA hypermethylation of Mfn2 promoter in hyperglycemic milieu, the binding of the transcription factors is decreased, suppressing its gene transcription. Results from the experimental models of diabetic retinopathy are further supported by significant reduction in Mfn2 and increase in its promoter DNA methylation in the retinal microvasculature from human donors with documented diabetic retinopathy. Taken together, this study shows a significant role of Mfn2 in mitochondrial homeostasis in diabetic retinopathy, and epigenetic modification, especially DNA methylation, in its suppression.