Acute kidney injury can lead to chronic renal failure, which causes fluid and electrolyte imbalances in the blood that require dialysis. Such injuries commonly involve ischaemia–reperfusion events, in which the blood supply to the kidney is temporarily restricted but then restored; this process generates toxic oxygen radicals that can cause renal inflammation and damage. Writing in Nature, Zhou et al.1 report that the signalling molecule nitric oxide2,3 reprograms a metabolic pathway, and thereby limits ischaemic injury and protects renal function.
Nitric oxide is synthesized by a family of enzymes called nitric oxide synthases (NOS), which fall into three groups: neuronal NOS, inducible NOS and endothelial NOS (eNOS). The molecule signals through several distinct mechanisms4. For example, it can interact with transition metals such as those in the haem group of guanylyl cyclase enzymes, which produce cyclic GMP — a messenger molecule involved in many biological processes. It can also combine with oxygen molecules to produce reactive nitrogen oxide species that, in turn, react with cysteine amino-acid residues on target proteins5, forming modifications called S-nitrosothiols. Nitric oxide regulates a variety of physiological processes, including dilation of blood vessels (vasodilation), communication between neurons and the killing of disease-causing agents by the immune system.
Zhou and colleagues now show that nitric oxide protects kidneys from ischaemic damage. In particular, they observed that renal injury after ischaemia and reperfusion was worse in mice genetically engineered to lack eNOS than in wild-type mice. This result is consistent with previous findings that nitric oxide — not only nitric oxide produced in the body, but also that introduced from an external source6 — can limit ischaemic injury in the kidneys7, heart8, brain9 and other organs. The role of nitric oxide in these protective effects was not fully understood, but it has been proposed to act variously as an antioxidant10, an anti-inflammatory agent11 or a vasodilator7.
The authors of the current study set out to identify the pathways by which nitric oxide protects against ischaemia. Using mass spectrometry, they discovered that one of the proteins most commonly modified by the molecule is pyruvate kinase M2, an enzyme that catalyses glycolysis (the metabolic pathway by which glucose is converted into energy). In a clever set of biochemical studies, they showed that nitric oxide modifies specific cysteine residues of pyruvate kinase M2. These modifications block the assembly of the active form of the enzyme, thereby inhibiting glycolysis. This is one of the key findings of the study: pyruvate kinase M2 is a target of nitric oxide.
Zhou et al. next genetically engineered mice so that their kidneys did not produce pyruvate kinase. The authors found that ischaemia causes less damage in these mice than in wild-type mice, consistent with the idea that pyruvate kinase mediates the protective effects of nitric oxide. But how?
The researchers used a technique called metabolic profiling to show that the kidney cells of mice lacking pyruvate kinase have high levels of products of the pentose phosphate pathway12 — a metabolic pathway parallel to glycolysis that produces sugars called pentoses and the enzyme cofactor NADPH. NADPH acts in antioxidant systems to restore the function of proteins that have been damaged by oxidative stress in ischaemia13. The authors therefore conclude that nitric oxide inhibits pyruvate kinase and glycolysis, causing glucose levels to increase. The excess glucose spills over into the pentose phosphate pathway, generating high levels of NADPH, which shores up the antioxidant defences that limit renal injury (Fig. 1). This reprogramming of metabolism represents a major new aspect of nitric oxide biology.
How is nitric oxide conveyed to its renal-protein targets? Workers from the same group as Zhou and colleagues had previously identified14 a two-component system that controls the availability of nitrosothiol groups in yeast. The first component is S-nitroso-coenzyme A, a molecule that donates nitric oxide groups to target proteins. The second component is an enzyme called S-nitroso-coenzyme A reductase, which removes nitric oxide from S-nitroso-coenzyme A. But does this binary system have any relevance to mammals?
To answer this question, Zhou et al. studied the impact of S-nitroso-coenzyme A reductase in mice during renal ischaemia and reperfusion. As expected, genetic deletion of the enzyme increased levels of S-nitrosylated proteins, protected mice from renal damage and prolonged survival compared with results in wild-type mice. Kidney levels of NADPH were also increased compared with levels of its oxidized form, NADP+, as were levels of the antioxidant glutathione relative to its oxidized form, glutathione disulfide, confirming that protection occurs through the action of antioxidant defences. These exciting results show that S-nitroso-coenzyme A reductase acts in vivo in mammals to control nitric oxide signalling, which is the third major discovery of the study.
This work highlights important questions for further research. The authors’ identification of a two-component system for regulating S-nitrosylation levels in renal injury raises the issue of what effect this system has on such regulation in normal physiological processes. How does this system function during other disorders, such as inflammation and cancer, which are also characterized by oxidant stress? And could pyruvate kinase M2 be a target for anti-ischaemic therapies?
Further work is also needed to identify how modification of pyruvate kinase M2 by nitric oxide protects cells — through inhibition of the enzyme’s metabolic activity, or by inhibiting its other functions15 (such as protein kinase activity and transcriptional co-activation)? Finally, Zhou et al. show that nitric oxide inhibits glycolysis in the setting of renal ischaemia, but it has previously been shown that it increases glycolysis in other settings16. Perhaps the activity of the newly discovered two-component regulatory system can explain previously puzzling aspects of nitric oxide biology, and might open up approaches for treating ischaemic injury in the kidney and other organs.
Nature 565, 33-34 (2018)