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Julia Santos, Renu Kowluru; Why does diabetes result in impaired mitochondria DNA biogenesis in the retina?. Invest. Ophthalmol. Vis. Sci. 2013;54(15):3686.
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© ARVO (1962-2015); The Authors (2016-present)
Retinal mitochondria are dysfunctional in diabetes, and their DNA (mtDNA) is damaged and biogenesis is impaired. In the regulation of transcription and replication of mtDNA, mitochondrial transcription factor A (TFAM) plays an important role. TFAM is encoded by nuclear DNA, and is transported into the mitochondria to facilitate biogenesis. Gene transcripts of TFAM are increased in diabetes, but its mitochondrial expression is decreased. Furthermore, the levels of cytosolic chaperone Hsp70 and the binding of TFAM with Hsp70 are also decreased. Since diabetes milieu favors post-translational modifications of various proteins, the aim is to investigate if decrease in mitochondrial TFAM is due to its post-translational modification.
Ubiquination of TFAM was determined in bovine retinal endothelial cells by co-immunoprecipitation and fluorescence microscopy techniques. The effect of overexpression of TFAM on glucose-induced alterations in its mitochondrial accumulation and the transcription of mtDNA-encoded genes, cytochrome b (Cytb) and NADH dehydrogenase subunit 1 and 6 (ND1 and ND6), were determined. To investigate the role of Hsp70 in mitochondrial translocation of TFAM, cells overexpressing Hsp70 were used. The results were confirmed in retina from streptozotocin-induced diabetic rats.
High glucose increased the ubiquination of TFAM by over 2 fold, and significantly decreased the transcripts of Cytb, ND1 and ND6 compared to the values obtained from the cells in normal glucose. Overexpression of TFAM prevented glucose-induced decrease in Cytb, ND1 and ND6. In contrast, despite subnormal transcripts of Hsp70 in high glucose conditions, its overexpression did not ameliorate decrease in mtDNA-encoded genes. Consistent with the in vitro results, ubiquination of TFAM was also increased and the transcripts of Cytb, ND1 and ND6 decreased in the retina from diabetic rats compared to age-matched normal rats.
Ubiquination of TFAM impairs its translocation to the mitochondria; biogenesis of mtDNA becomes subnormal resulting in a dysfunctional electron transport system. Regulation of post-translational modification has a potential to help TFAM reach its target and prevent abnormalities in mtDNA biogenesis, and ultimately inhibit the development of diabetic retinopathy.
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