Nature 515, 431–435 (2014)

Credit: NATURE

Ischemia is caused by a lack of oxygen due to the blockage of blood flow during a heart attack or stroke. Subsequent restoration of blood flow (reperfusion) is associated with the accumulation of mitochondrial reactive oxygen species (ROS), which cause oxidative damage. Although the role of ROS in ischemic damage has been validated, it was not known what factors mediate the elevation in ROS production and whether this is a direct or indirect response to ischemic conditions. Chouchani et al. hypothesized that a metabolic signal might stimulate ROS formation. To test this hypothesis, they first performed LC/MS-based metabolomics analysis of four tissues (kidney, liver, heart and brain) under ischemic conditions. The authors found that the TCA cycle intermediate succinate was consistently elevated in all examined tissues during ischemia and was rapidly oxidized during reperfusion. Although succinate is generated by the citric acid cycle using either glucose, fatty acids or glutamate or by the GABA shunt, none of these sources contributed to the elevated succinate levels. Instead, in silico flux analysis suggested that the source of the increased succinate was from the reversed activity of succinate dehydrogenase (SDH), reducing fumarate to succinate. The excess fumarate was thought to derive from two major pathways: the malate/aspartate shuttle and the purine nucleotide cycle. Additional flux analysis predicted that succinate is oxidized during reperfusion and promotes superoxide formation through mitochondrial complex I–mediated reverse electron transport (RET). Using a cell-permeable derivative of succinate and an inhibitor of complex I RET, the authors demonstrated that high levels of succinate along with a strong proton motive force were sufficient to induce ROS formation. Finally, the addition of dimethylmalonate, a cell-permeable precursor of the SDH competitive inhibitor malonate, reduced succinate and mitochondrial ROS levels during ischemia and exhibited protective effects in a cardiac and brain ischemia model. These findings may inspire new ways to modulate succinate metabolism for potential treatments of cellular damage caused by ischemia-reperfusion injury.