Byproducts of normal mitochondrial metabolism and homeostasis include the buildup of potentially damaging levels of reactive oxygen species (ROS), Ca2+, etc., which must be normalized. Evidence suggests that brief mitochondrial permeability transition pore (mPTP) openings play an important physiological role maintaining healthy mitochondria homeostasis. Adaptive and maladaptive responses to redox stress may involve mitochondrial channels such as mPTP and inner membrane anion channel (IMAC). Their activation causes intra- and intermitochondrial redox-environment changes leading to ROS release. This regenerative cycle of mitochondrial ROS formation and release was named ROS-induced ROS release (RIRR). Brief, reversible mPTP opening-associated ROS release apparently constitutes an adaptive housekeeping function by the timely release from mitochondria of accumulated potentially toxic levels of ROS (and Ca2+). At higher ROS levels, longer mPTP openings may release a ROS burst leading to destruction of mitochondria, and if propagated from mitochondrion to mitochondrion, of the cell itself. The destructive function of RIRR may serve a physiological role by removal of unwanted cells or damaged mitochondria, or cause the pathological elimination of vital and essential mitochondria and cells. The adaptive release of sufficient ROS into the vicinity of mitochondria may also activate local pools of redox-sensitive enzymes involved in protective signaling pathways that limit ischemic damage to mitochondria and cells in that area. Maladaptive mPTP- or IMAC-related RIRR may also be playing a role in aging. Because the mechanism of mitochondrial RIRR highlights the central role of mitochondria-formed ROS, we discuss all of the known ROS-producing sites (shown in vitro) and their relevance to the mitochondrial ROS production in vivo.

Mitochondrial Reactive Oxygen Species (ROS) and ROS-Induced ROS Release

Inflammasomes are key signalling platforms that detect pathogenic microorganisms and sterile stressors, and that activate the highly pro-inflammatory cytokines interleukin-1β (IL-1β) and IL-18. In this Review, we discuss the complex regulatory mechanisms that facilitate a balanced but effective inflammasome-mediated immune response, and we highlight the similarities to another molecular signalling platform - the apoptosome - that monitors cellular health. Extracellular regulatory mechanisms are discussed, as well as the intracellular control of inflammasome assembly, for example, via ion fluxes, free radicals and autophagy.

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Activation and regulation of the inflammasomes

NLRP3 (NOD-, LRR- and pyrin domain-containing protein 3) is an intracellular sensor that detects a broad range of microbial motifs, endogenous danger signals and environmental irritants, resulting in the formation and activation of the NLRP3 inflammasome. Assembly of the NLRP3 inflammasome leads to caspase 1-dependent release of the pro-inflammatory cytokines IL-1β and IL-18, as well as to gasdermin D-mediated pyroptotic cell death. Recent studies have revealed new regulators of the NLRP3 inflammasome, including new interacting or regulatory proteins, metabolic pathways and a regulatory mitochondrial hub. In this Review, we present the molecular, cell biological and biochemical bases of NLRP3 activation and regulation and describe how this mechanistic understanding is leading to potential therapeutics that target the NLRP3 inflammasome.

The NLRP3 inflammasome: molecular activation and regulation to therapeutics

Immunometabolism is emerging an important area of immunological research, but for many immunologists the complexity of the field can be daunting. Here, the authors provide an overview of six key metabolic pathways that occur in immune cells and explain what is known (and what is still to be uncovered) concerning their effects on immune cell function. In recent years a substantial number of findings have been made in the area of immunometabolism, by which we mean the changes in intracellular metabolic pathways in immune cells that alter their function. Here, we provide a brief refresher course on six of the major metabolic pathways involved (specifically, glycolysis, the tricarboxylic acid (TCA) cycle, the pentose phosphate pathway, fatty acid oxidation, fatty acid synthesis and amino acid metabolism), giving specific examples of how precise changes in the metabolites of these pathways shape the immune cell response. What is emerging is a complex interplay between metabolic reprogramming and immunity, which is providing an extra dimension to our understanding of the immune system in health and disease.

A guide to immunometabolism for immunologists

Ischaemia-reperfusion injury occurs when the blood supply to an organ is disrupted and then restored, and underlies many disorders, notably heart attack and stroke. While reperfusion of ischaemic tissue is essential for survival, it also initiates oxidative damage, cell death and aberrant immune responses through the generation of mitochondrial reactive oxygen species (ROS). Although mitochondrial ROS production in ischaemia reperfusion is established, it has generally been considered a nonspecific response to reperfusion. Here we develop a comparative in vivo metabolomic analysis, and unexpectedly identify widely conserved metabolic pathways responsible for mitochondrial ROS production during ischaemia reperfusion. We show that selective accumulation of the citric acid cycle intermediate succinate is a universal metabolic signature of ischaemia in a range of tissues and is responsible for mitochondrial ROS production during reperfusion. Ischaemic succinate accumulation arises from reversal of succinate dehydrogenase, which in turn is driven by fumarate overflow from purine nucleotide breakdown and partial reversal of the malate/aspartate shuttle. After reperfusion, the accumulated succinate is rapidly re-oxidized by succinate dehydrogenase, driving extensive ROS generation by reverse electron transport at mitochondrial complex I. Decreasing ischaemic succinate accumulation by pharmacological inhibition is sufficient to ameliorate in vivo ischaemia-reperfusion injury in murine models of heart attack and stroke. Thus, we have identified a conserved metabolic response of tissues to ischaemia and reperfusion that unifies many hitherto unconnected aspects of ischaemia-reperfusion injury. Furthermore, these findings reveal a new pathway for metabolic control of ROS production in vivo, while demonstrating that inhibition of ischaemic succinate accumulation and its oxidation after subsequent reperfusion is a potential therapeutic target to decrease ischaemia-reperfusion injury in a range of pathologies.

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Ischaemic accumulation of succinate controls reperfusion injury through mitochondrial ROS.

scitranslmed.aaa0835 “CD4+ T cell metabolism is hyperactive in lupus,” says Laurence Morel, lead author of a study now published in Science Translational Medicine. She states, “This study is a proof of principle—the first I think— that metabolism can be targeted to reverse a complex spontaneous autoimmune disease.” T‐cell function is known to be dependent on cellular metabolism as helper T cells are more glycolytic than regulatory T cells, and naive T cells activate glycolysis and mitochondrial metabolism in response to T‐cell receptor activation. Morel also coauthored an earlier study that identified Esrrg, a regulator of mitochondrial oxidative phosphorylation, as a lupus susceptibility gene in mice (locus Sle1c2). Armed with this knowledge, she says her group hypothesized “that lupus T cells are sustained by an enhanced metabolism and that metabolic inhibitors should alleviate disease by normalizing T‐cell metabolism.” The extracellular acidification rate (ECAR) and oxygen consumption rate (OCR) of ex vivo CD4+ T cells were higher in 2‐month old (preclinical disease) spontaneous‐lupus‐prone mice (B6.Sle1. Sle2.Sle3.) than in wildtype mice, and this difference was even more pronounced in 9‐month old (clinical signs of disease) mice. Polyclonal stimulation had a similar response in vitro, increasing mTOR complex 1 activity and IFN‐γ production, and reducing IL‐2 production, by the CD4+ T cells from the lupus‐prone mice. These cells also had higher expression of glycolytic genes, such as Hif1a, and lower expression of the pyruvate oxidation inhibitor Pdk1. Confirming the mouse data, CD4+ T cells from patients with SLE also had higher ECAR and OCR than cells from healthy individuals, even though patients are treated with immunosuppressive drugs that reduce these rates when applied in vitro. Importantly, treatment of lupus‐prone mice with metabolic inhibitors metformin and 2‐deoxyglucose reduced ECAR, OCR and early signs of lupus such as splenomegaly and the production of anti‐double‐stranded‐DNA IgG and anti‐nuclear autoantibodies. “Although metabolism of other immune cells was probably affected by the in vivo treatment,” notes Morel “our study shows that a normalization of the T‐cell phenotypes was associated with disease reversal.” Her hope now is that human T‐cell metabolism could be targeted “with existing drugs repurposed for the treatment of immune‐related diseases.”

Connective tissue diseases: Can SLE be treated by altering T-cell metabolism?

Rheumatoid arthritis: where there's smoke, there's altered transcription.

mevalonate kinase deficiency (MKD; n = 74), TNF-receptor-associated periodic fever syndrome (TRAPS; n = 86) or cryopyrin-associated periodic syndromes (CAPS; n = 67). Gold-standard was defined as confirmatory genetic results of two MEFV mutations (≥1 in exon 10) for FMF, two MVK mutations (excluding variants with an unknown role in pathogenesis) for MKD, heterozygous TNFRSF1A mutations (excluding low-penetrance or unknown mutations) for TRAPS, and heterozygous NLRP3 mutations (excluding functional polymorphisms and variants with either low-penetrance or an unknown role in pathogenesis) for CAPS. Patients with chronic (nonperiodic) disease were excluded. To establish the classification criteria, the researchers compared the four goldstandard groups with a control group of patients with periodic fever, apthosis, pharyngitis and adenitis (PFAPA; n = 199), a non-monogenic disease. The groups were then split randomly into two sets, a training set (n = 412), to establish the classification criteria, and a validation set (n = 305). Univariate and multivariate analysis of clinical variables between the four diseases in the training set enabled the formulation of weighted classification scores totalling 100–110 points for each disease. For example, variables included absence of chest pain (11) and presence of painful lymph nodes (13) for MKD (cut-off ≥42), absence of vomiting (14) and presence of perorbital oedema (21) in TRAPS (cut-off ≥43), absence of adominal pain (15) and presence of urticarial rash (25) for CAPS (cut-off ≥52), and the absence of urticarial rash (15) and presence of abdominal pain (9) in FMF (cut-off ≥60). All criteria had high sensitivity and specificity in the gold-standard groups. Importantly, the criteria also performed well when applied to the group of patients who had been excluded from the testing set owing to nonconfirmatory genetic results (specificity: FMF, 87%; TRAPS, 84%; CAPS, 95%; MKD, 89%; sensitivity: FMF, 68%; TRAPS, 59%; CAPS, 70%; MKD, 53%). However, in their paper, the authors admit these data are overstated as selection from the registry probably enriched the dataset for patients already suspected of having an autoinflammatory disease. The true test of the new criteria will, therefore, be in daily practice. “Many patients have mutations in more than one gene,” says Gattorno on the difficulty of diagnosing periodic fevers, “so nonconfirmatory genetic tests can overestimate the importance of genetic variants in patients with an unclear inflammatory phenotype. This problem will be exacerbated by next-generation sequencing. Studies to correlate and validate data from molecular and clinical analysis, like ours, will be critical for classification and understanding the effect of genetic variants.” In reference to developing the criteria beyond provisional status, Gattorno adds, “We are working to establish a consensus among clinicians and geneticists to identify definitive classification criteria by combining clinical criteria and results from genetic analysis.” Hoffman’s assessment of the provisional criteria is that the benefit to experts might be limited but, he says, “This scorebased approach may help physicians who are not familiar with these diagnoses.” He predicts, “An approach like this could be adapted to develop a user-friendly web or computer-based diagnostic algorithm for use by patients or physicians.”

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Inflammation: New classification criteria for autoinflammatory periodic fevers.

The researchers also administered the compound orally to ovariectomized mice and to mice with RANKL-induced osteoporosis. Femurs were scanned by microcomputed tomography to measure bone density parameters, including trabecular number, thickness, separation and spacing. Delphinidin treatment (figure, right panel) prevented cancellous bone degradation (figure, middle panel) in both mouse models. In response to Niida’s publication, Mone Zaidi, an osteoporosis expert at the Icahn School of Medicine at Mount Sinai (NY, USA) notes the “renewed interest in BONE

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Nicholas J. Bernard

Papers

Connective tissue diseases: Can SLE be treated by altering T-cell metabolism?

Rheumatoid arthritis: where there's smoke, there's altered transcription.

Inflammation: New classification criteria for autoinflammatory periodic fevers.

Bone: Fighting osteoporosis black and blue.

TLR7 drives human lupus