# Pathological Role of High Sugar in Mitochondrial Respiratory Chain Defect-Augmented Mitochondrial Stress

## TL;DR

This study shows that high sugar levels worsen mitochondrial stress in cells with a specific genetic mutation, leading to increased cell damage and death.

## Contribution

The study reveals that mitochondrial calcium overload mediates high glucose toxicity in cells with defective respiratory chain mutations.

## Key findings

- NARP cells with T8993G mutations show increased reactive oxygen species formation under high glucose.
- High glucose enhances mitochondrial membrane depolarization in NARP cells.
- NARP cells exhibit higher cell death when exposed to hydrogen peroxide and high glucose.

## Abstract

High glucose levels lead to the production of reactive oxygen species, which, according to some research, is the link between high glucose and the toxicity observed at cellular levels. At normal physiological levels, mitochondrial calcium serves as a second messenger; however, mitochondrial calcium overload leads to the production of reactive oxygen species, which has lethal effects on the cells. The mitochondria have complexes, identified as I, II, III, IV, and V, and any anomaly to these complexes also leads to reactive oxygen species production. Complex V is the enzyme responsible for catalyzing the final step of oxidative phosphorylation by coupling the translocation of protons in the mitochondria for adenosine triphosphate (ATP) synthesis to take place. Mitochondrial complex V T8993G mutation blocks the translocation of protons, hence blocking the production of ATP, leading to a condition called Neuropathy, Ataxia, and Retinitis Pigmentosa (NARP) syndrome. In this study, we explore the possibility of mitochondrial calcium overload mediating the toxicity of high glucose levels in defective respiratory chain-mediated mitochondrial stress. The NARP cells harbor 98% of mitochondrial DNA T8993G mutations, and 143B osteosarcoma cell lines were the parental normal cells used for comparison. Using MTT assays and confocal microscopic techniques, we observed that NARP cells enable reactive oxygen species formation and enhance the depolarization of the mitochondrial membrane potential.

According to many research groups, high glucose induces the overproduction of superoxide anions, with reactive oxygen species (ROS) generally being considered the link between high glucose levels and the toxicity seen at cellular levels. Respiratory complex anomalies can lead to the production of ROS. Calcium [Ca2+] at physiological levels serves as a second messenger in many physiological functions. Accordingly, mitochondrial calcium [Ca2+]m overload leads to ROS production, which can be lethal to the mitochondria through various mechanisms. F1F0-ATPase (ATP synthase or complex V) is the enzyme responsible for catalyzing the final step of oxidative phosphorylation. This is achieved by F1F0-ATPase coupling the translocation of protons in the mitochondrial intermembrane space and shuttling them to the mitochondrial matrix for ATP synthesis to take place. Mitochondrial complex V T8993G mutation specifically blocks the translocation of protons across the intermembrane space, thereby blocking ATP synthesis and, in turn, leading to Neuropathy, Ataxia, and Retinitis Pigmentosa (NARP) syndrome. This study seeks to explore the possibility of [Ca2+]m overload mediating the pathological roles of high glucose in defective respiratory chain-mediated mitochondrial stress. NARP cybrids are the in vitro experimental models of cells with F1FO-ATPase defects, with these cells harboring 98% of mtDNA T8993G mutations. Their counterparts, 143B osteosarcoma cell lines, are the parental cell lines used for comparison. We observed that NARP cells mediated and enhanced the death of cells (apoptosis) when incubated with hydrogen peroxide (H2O2) and high glucose, as depicted using the MTT assay of cell viability. Furthermore, using fluorescence probe-coupled laser scanning confocal imaging microscopy, NARP cells were found to significantly enable mitochondrial reactive oxygen species (mROS) formation and enhance the depolarization of the mitochondrial membrane potential (ΔΨm). Elucidating the mechanisms of sugar-enhanced toxicity on the mitochondria may, in the future, help to alleviate the symptoms of patients with NARP syndromes and other neurodegenerative diseases.