MDH2 is a metabolic switch rewiring the fuelling of respiratory chain and TCA cycle

Abstract

The mitochondrial respiratory chain (RC) enables many metabolic processes by regenerating both mitochondrial and cytosolic NAD+ and ATP. In contrast to ADP, NADH metabolically produced in the cytosol is not transported across the inner mitochondrial membrane and must be indirectly transferred inside mitochondria through the malate-aspartate shuttle (MAS) to fuel RC with electrons. MAS is the major pathway maintaining cytosolic NADH/NAD+ redox balance in mammalian tissues such as liver and heart and its activity is crucial for cell metabolism, division and survival. However, the specific metabolic regulations allowing mitochondrial respiration to prioritize NADH oxidation in response to high NADH/NAD+ redox stress have not been elucidated. The recent discovery that complex I (NADH dehydrogenase), and not complex II (Succinate dehydrogenase), can assemble with other RC complexes to form functional entities called respirasomes, led to the assumption that this supramolecular organisation would favour NADH oxidation. Surprisingly, our bioenergetic characterization of liver and heart mitochondria demonstrates that the RC systematically favours electrons provided by complex II. However, mitochondrial malate dehydrogenase (MDH2) mediated metabolic regulation can rewire respiratory chain electrons flow from succinate toward NADH oxidation in response to increase MAS activity. Interestingly, this new regulatory mechanism synergistically increases the NADH oxidative capacity of the RC and rewires MDH2 driven anaplerosis of the TCA, preventing malate production from succinate to favor oxidation of cytosolic malate. This discovery demonstrates that MAS does not only passively balance cytosolic and mitochondrial NADH but instead, in response to cytosolic redox stress, MAS actively rewires fuelling of the RC, inhibiting complex II to prioritize cytosolic NADH oxidation and increase complex I oxidative capacity.

Circadian rhythm disruption results in visual dysfunction

“…To validate targets in Figure 7B, mRNA analysis was performed on mapped targets, sharing functions, and having a role either in TCA cycle (Mdh2, Idh3b), glucose uptake (Ppm1a), mitochondrial function (Anxa6 23 ), or photoreceptor function (Espn 24 ). In mRNA validation, none of the above targets exhibited a statistical significance (Figure S9) except Mdh2, a vital member of the TCA cycle and malate aspartate shuttle 25,26 ; thus, a critical enzyme of mitochondrial respiration showed a statistically significant decrease between different behaviors and the control group (Figure 7C), suggesting a crucial role of TCA cycle in our studies. Because Bmal1 ablation in other tissues such as the heart results in mitochondrial defects and a decrease in expression of genes for fatty acid oxidative pathway, TCA cycle, and mitochondrial respiratory chain, including MDH2, 27 we reasoned to test the mRNA expression of clock gene Bmal1.…”

“…Our studies highlight that Mdh2 was downregulated irrespective of the behavior. MDH2 is a critical enzyme in a malate‐aspartate shuttle and is involved in maintaining cytosolic NADH/NAD redox balance, mitochondrial respiration, and electron transfer 25,26 . The highest concentration of mitochondria is observed in retinal photoreceptors, and its dysfunction is associated with oxidative stress and inflammation in the retina 30,46 .…”

“…MDH2 is a critical enzyme in a malate-aspartate shuttle and is involved in maintaining cytosolic NADH/ NAD redox balance, mitochondrial respiration, and electron transfer. 25,26 The highest concentration of mitochondria is observed in retinal photoreceptors, and its dysfunction is associated with oxidative stress and inflammation in the retina. 30,46 While we observed an increase in mitochondrial oxidative stress genes, only Sod1 did reach a statistical significance for the zigzagging group; we speculate there is a potential involvement of mitochondrial dysfunction with implications for future studies in this direction.…”