Metabolic dysfunction promoted by mitochondrial DNA mutation burden drives retinal degeneration

Abstract

Retinal degenerative diseases, such as age-related macular degeneration (AMD), retinitis pigmentosa, and glaucoma, have been linked to mitochondrial dysfunction. However, the impact of mitochondrial DNA (mtDNA) mutation accumulation in the context of these retinopathies has yet to be thoroughly explored. Our previous studies focused on the retinal phenotype observed in the PolgD257A mutator mice (D257A), revealing the effects of aging and mtDNA mutation accumulation in the retina. We have reported that this model exhibited significant morphological and functional deficits in the retina by 6 months of age, with notable alterations in the retinal pigment epithelium (RPE) occurring as early as 3 months, including changes in the cristae density and reduction in length of mitochondria. This study investigated how mtDNA mutations affect the metabolic interaction between the retina and RPE in young (3 months) and old (12 months) wild-type (WT) and D257A mice. We assessed cellular energy production using freshly dissected retina samples from both groups through Seahorse analysis, immunofluorescence, and Western blot experiments. The analysis of aged D257A retina punches revealed significantly reduced basal and maximal mitochondrial respiration, along with increased mitochondrial reserve capacity compared to WT. However, glycolytic flux, measured as a function of extracellular acidification rate (ECAR), did not differ between WT and D257A mice. Both D257A retina and RPE exhibited decreased expression of essential electron transport proteins involved in oxidative phosphorylation. Additionally, we observed a reduction in the expression of glucose transporter 1 (GLUT-1) and lactate transporter (MCT1) at the apical surface of the RPE. Enzymes associated with glycolysis, including hexokinase II and lactate dehydrogenase A, were significantly lower in the aged D257A retina, while hexokinase I and pyruvate kinase 2 were upregulated in the RPE. These findings indicate that the accumulation of mtDNA mutations leads to impaired metabolism in both the retina and RPE. Furthermore, it suggests that glucose from the choroidal blood supply is being utilized by the RPE rather than being transported to the neural retina. Mitochondrial dysfunction in RPE promotes a glycolytic state in these cells, leading to reduced availability of metabolites and, consequently, diminished overall retinal function. These results are essential for advancing our understanding of the mechanisms underlying retinal degeneration and provide a new perspective on the role of mtDNA mutations in these diseases.