Abstract
BackgroundAlthough hyperglycemia is the main instigator in the development of diabetic retinopathy, elevated circulating levels of a non-protein amino acid, homocysteine, are also associated with an increased risk of retinopathy. Homocysteine is recycled back to methionine by methylenetetrahydrofolate reductase (MTHFR) and/or transsulfurated by cystathionine β-synthase (CBS) to form cysteine. CBS and other transsulfuration enzyme cystathionine-γ-lyase (CSE), through desulfuration, generates H2S. Methionine cycle also regulates DNA methylation, an epigenetic modification associated with the gene suppression. The aim of this study was to investigate homocysteine and its metabolism in diabetic retinopathy.MethodsHomocysteine and H2S levels were analyzed in the retina, and CBS, CSE and MTHFR in the retinal microvasculature from human donors with established diabetic retinopathy. Mitochondrial damage was evaluated in retinal microvessels by quantifying enzymes responsible for maintaining mitochondrial dynamics (fission-fusion-mitophagy). DNA methylation status of CBS and MTHFR promoters was examined using methylated DNA immunoprecipitation technique. The direct effect of homocysteine on mitochondrial damage was confirmed in human retinal endothelial cells (HRECs) incubated with 100 μM L-homocysteine.ResultsCompared to age-matched nondiabetic control human donors, retina from donors with established diabetic retinopathy had ~ 3-fold higher homocysteine levels and ~ 50% lower H2S levels. The enzymes important for both transsulfuration and remethylation of homocysteine including CBS, CSE and MTHFR, were 40–60% lower in the retinal microvasculature from diabetic retinopathy donors. While the mitochondrial fission protein, dynamin related protein 1, and mitophagy markers optineurin and microtubule-associated protein 1A/1B-light chain 3 (LC3), were upregulated, the fusion protein mitofusin 2 was downregulated. In the same retinal microvessel preparations from donors with diabetic retinopathy, DNA at the promoters of CBS and MTHFR were hypermethylated. Incubation of HRECs with homocysteine increased reactive oxygen species and decreased transcripts of mtDNA-encoded CYTB.ConclusionsCompromised transsulfuration and remethylation processes play an important role in the poor removal of retinal homocysteine in diabetic patients. Thus, regulation of their homocysteine levels should ameliorate retinal mitochondrial damage, and by regulating DNA methylation status of the enzymes responsible for homocysteine transsulfuration and remethylation, should prevent excess accumulation of homocysteine.
Highlights
Diabetic retinopathy remains the leading cause of vision loss in working age adults
Experimental models of diabetic retinopathy have clearly documented the role of mitochondrial homeostasis in the development of diabetic retinopathy; retinal mitochondria were damaged in diabetes, their copy numbers were decreased, and while the mitochondrial fusion marker, mitofusin 2 (Mfn2), was downregulated, mitophagy markers were upregulated, and capillary cell apoptosis was accelerated [3, 4, 13, 14]
Our results show that the donors with established diabetic retinopathy have higher homocysteine levels in their retinal microvasculature, the site of retinal histopathology characteristic of diabetic retinopathy
Summary
Diabetic retinopathy remains the leading cause of vision loss in working age adults. Many molecular mechanisms have been implicated in its development, but despite ongoing cutting edge research in the field, the molecular mechanism of this multi-factorial disease is still not clear [1]. In the pathogenesis of diabetic retinopathy, oxidative stress is increased in the retina and its vasculature, mitochondria are damaged and have impaired homeostasis, gene transcription associated with oxidative stress are altered, and apoptosis of capillary cells are accelerated [2,3,4,5]. Experimental and clinical studies have documented that diabetic patients and animal models have elevated circulating levels of homocysteine, a sulfur-containing amino acid [6]. Experimental models of diabetic retinopathy have clearly documented the role of mitochondrial homeostasis in the development of diabetic retinopathy; retinal mitochondria were damaged in diabetes, their copy numbers were decreased, and while the mitochondrial fusion marker, mitofusin 2 (Mfn2), was downregulated, mitophagy markers were upregulated, and capillary cell apoptosis was accelerated [3, 4, 13, 14]. The aim of this study was to investigate homocysteine and its metabolism in diabetic retinopathy
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