Two-stage metabolic remodelling in macrophages in response to lipopolysaccharide and interferon-γ stimulation

Itaconate controls its own synthesis via feedback-inhibition of reverse TCA cycle activity at IDH2

Macrophages undergo extensive metabolic reprogramming during classical pro-inflammatory polarization (M1-like). The accumulation of itaconate has been recognized as both a consequence and mediator of the inflammatory response. In this study we first examined the specific functions of itaconate inside fractionated mitochondria. We show that M1 macrophages produce itaconate de novo via aconitase decarboxylase 1 (ACOD1) inside mitochondria. The carbon for this reaction is not only supplied by oxidative TCA cycling, but also through the reductive carboxylation of α-ketoglutarate by isocitrate dehydrogenase (IDH). While macrophages are capable of sustaining a certain degree of itaconate production during hypoxia by augmenting the activity of IDH-dependent reductive carboxylation, we demonstrate that sufficient itaconate synthesis requires a balance of reductive and oxidative TCA cycle metabolism in mouse macrophages. In comparison, human macrophages increase itaconate accumulation under hypoxic conditions by augmenting reductive carboxylation activity. We further demonstrated that itaconate attenuates reductive carboxylation at IDH2, restricting its own production and the accumulation of the immunomodulatory metabolites citrate and 2-hydroxyglutarate. In line with this, reductive carboxylation is enhanced in ACOD1-depleted macrophages. Mechanistically, the inhibition of IDH2 by itaconate is linked to the alteration of the mitochondrial NADP⁺/NADPH ratio and competitive succinate dehydrogenase inhibition. Taken together, our findings extend the current model of TCA cycle reprogramming during pro-inflammatory macrophage activation and identified novel regulatory properties of itaconate.

Immunolipidomics Reveals a Globoside Network During the Resolution of Pro-Inflammatory Response in Human Macrophages

Toll-like receptor 4 (TLR4)-mediated changes in macrophages reshape intracellular lipid pools to coordinate an effective innate immune response. Although this has been previously well-studied in different model systems, it remains incompletely understood in primary human macrophages. Here we report time-dependent lipidomic and transcriptomic responses to lipopolysaccharide (LPS) in primary human macrophages from healthy donors. We grouped the variation of ~200 individual lipid species measured by LC-MS/MS into eight temporal clusters. Among all other lipids, glycosphingolipids (glycoSP) and cholesteryl esters (CE) showed a sharp increase during the resolution phase (between 8h or 16h post LPS). GlycoSP, belonging to the globoside family (Gb3 and Gb4), showed the greatest inter-individual variability among all lipids quantified. Integrative network analysis between GlycoSP/CE levels and genome-wide transcripts, identified Gb4 d18:1/16:0 and CE 20:4 association with subnetworks enriched for T cell receptor signaling (PDCD1, CD86, PTPRC, CD247, IFNG) and DC-SIGN signaling (RAF1, CD209), respectively. Our findings reveal Gb3 and Gb4 globosides as sphingolipids associated with the resolution phase of inflammatory response in human macrophages.

Research Progress on the Mechanism of Itaconate Regulating Macrophage Immunometabolism

The field of immunology is undergoing rapid and dramatic changes. Immunometabolism, a change in metabolic pathways within immune cells, is a key determinant in the activation of immune cells, and intermediates of immunometabolic processes which can influence inflammatory gene expression and play a role in inflammation. Itaconate is one of the most representative metabolites, produced in the tricarboxylic acid cycle (TCA cycle), which links macrophage metabolism, oxidative stress response and immune response to regulate macrophage activity, playing an important role in the function of macrophages. In this paper, we review the mechanisms of the metabolite itaconate and its derivatives in the regulation of macrophage immune metabolism, intending to gain further insight into the role and mechanisms of this metabolite in macrophages and provide new ideas for the mechanisms and treatment of clinical diseases.

Coronavirus disease 2019 (COVID-19) update: From metabolic reprogramming to immunometabolism

The field of immunometabolism investigates and describes the effects of metabolic rewiring in immune cells throughout activation and the fates of these cells. Recently, it has been appreciated that immunometabolism plays an essential role in the progression of viral infections, cancer, and autoimmune diseases. Regarding COVID‐19, the aberrant immune response underlying the progression of diseases establishes two major respiratory pathologies, including acute respiratory distress syndrome (ARDS) or pneumonia‐induced acute lung injury (ALI). Both innate and adaptive immunity (T cell‐based) were impaired in the course of severe acute respiratory syndrome coronavirus 2 (SARS‐CoV‐2) infection. Current findings have deciphered that macrophages (innate immune cells) are involved in the inflammatory response seen in COVID‐19. It has been demonstrated that immune system cells can change metabolic reprogramming in some conditions, including autoimmune diseases, cancer, and infectious disease, including COVID‐19. The growing findings on metabolic reprogramming in COVID‐19 allow an exploration of metabolites with immunomodulatory properties as future therapies to combat this hyperinflammatory response. The elucidation of the exact role and mechanism underlying this metabolic reprograming in immune cells could help apply more precise approaches to initial diagnosis, prognosis, and in‐hospital therapy. This report discusses the latest findings from COVID‐19 on host metabolic reprogramming and immunometabolic responses.

The Pyruvate Dehydrogenase Complex Mitigates LPS-Induced Endothelial Barrier Dysfunction by Metabolic Regulation

ABSTRACT

Sepsis is a fatal health issue induced by an aberrant host response to infection, and it correlates with organ damage and a high mortality rate. Endothelial barrier dysfunction and subsequent capillary leakage play major roles in sepsis-induced multiorgan dysfunction. Anaerobic glycolysis is the primary metabolic mode in sepsis and the pyruvate dehydrogenase complex (PDHC) serves as a critical hub in energy regulation. Therefore, it is important to understand the role of PDHC in metabolic regulation during the development of sepsis-induced endothelial barrier dysfunction.In present study, human umbilical vein endothelial cells (HUVECs) and C57 BL/6 mice were treated with lipopolysaccharide (LPS) as models of endotoxemia. LPS increased basal glycolysis, compensatory glycolysis, and lactate secretion, indicating increased glycolysis level in endothelial cells (ECs). Activation of PDHC with dichloroacetate (DCA) reversed LPS-induced glycolysis, allowing PDHC to remain in the active dephosphorylated state, thereby preventing lactic acid production and HUVECs monolayers barrier dysfunction, as assessed by transendothelial electrical resistance and Fluorescein Isothiocyanate-labeled dextran. The in vivo study also showed that the lactate level and vascular permeability were increased in LPS-treated mice, but pretreatment with DCA attenuated these increases. The LPS-treated HUVEC model showed that DCA reversed LPS-induced phosphorylation of pyruvate dehydrogenase E1α Ser293 and Ser300 to restore PDHC activity. Immunoprecipitation results showed that LPS treatment increased the acetylation level of PDH E1α in HUVECs.Our study suggested that activation of PDHC may represent a therapeutic target for treatment of LPS-induced endothelial barrier dysfunction.

A matter of time: temporal structure and functional relevance of macrophage metabolic rewiring

The response of macrophages to stimulation is a dynamic process which coordinates the orderly adoption and resolution of various immune functions. Accumulating work over the past decade has demonstrated that during the immune response macrophage metabolism is substantially rewired to support important cellular processes, including the production of bioactive molecules, intercellular communication, and the regulation of intracellular signaling and transcriptional programming. In particular, we discuss an important concept emerging from recent studies - metabolic rewiring during the immune response is temporally structured. We review the regulatory mechanisms that drive the dynamic remodeling of metabolism, and examine the functional implications of these metabolic dynamics.

Mitochondrial electron transport chain is necessary for NLRP3 inflammasome activation

The NLRP3 inflammasome is linked to sterile and pathogen-dependent inflammation, and its dysregulation underlies many chronic diseases. Mitochondria have been implicated as regulators of the NLRP3 inflammasome through several mechanisms including generation of mitochondrial reactive oxygen species (ROS). Here, we report that mitochondrial electron transport chain (ETC) complex I, II, III and V inhibitors all prevent NLRP3 inflammasome activation. Ectopic expression of Saccharomyces cerevisiae NADH dehydrogenase (NDI1) or Ciona intestinalis alternative oxidase, which can complement the functional loss of mitochondrial complex I or III, respectively, without generation of ROS, rescued NLRP3 inflammasome activation in the absence of endogenous mitochondrial complex I or complex III function. Metabolomics revealed phosphocreatine (PCr), which can sustain ATP levels, as a common metabolite that is diminished by mitochondrial ETC inhibitors. PCr depletion decreased ATP levels and NLRP3 inflammasome activation. Thus, the mitochondrial ETC sustains NLRP3 inflammasome activation through PCr-dependent generation of ATP, but via a ROS-independent mechanism.

Epigenetic and transcriptional regulation of innate immunity in cancer

Innate immune cells participate in the detection of tumor cells via complex signaling pathways mediated by pattern-recognition receptors, such as Toll-like receptors (TLR) and NOD-like receptors (NLR). These pathways are finely tuned via multiple mechanisms, including epigenetic regulation. It is well established that hematopoietic progenitors generate innate immune cells that can regulate cancer cell behavior, and the disruption of normal hematopoiesis in pathologic states may lead to altered immunity and the development of cancer. In this review, we discuss the epigenetic and transcriptional mechanisms that underlie the initiation and amplification of innate immune signaling in cancer. We also discuss new targeting possibilities for cancer control that exploit innate immune cells and signaling molecules, potentially heralding the next generation of immunotherapy.

Glucocorticoids coordinate macrophage metabolism through the regulation of the tricarboxylic acid cycle

OBJECTIVES

Glucocorticoids (GCs) are one of the most widely prescribed anti-inflammatory drugs. By acting through their cognate receptor, the glucocorticoid receptor (GR), GCs downregulate the expression of pro-inflammatory genes and upregulate the expression of anti-inflammatory genes. Metabolic pathways have recently been identified as key parts of both the inflammatory activation and anti-inflammatory polarization of macrophages, immune cells responsible for acute inflammation and tissue repair. It is currently unknown whether GCs control macrophage metabolism, and if so, to what extent metabolic regulation by GCs confers anti-inflammatory activity.

METHODS

Using transcriptomic and metabolomic profiling of macrophages, we identified GC-controlled pathways involved in metabolism, especially in mitochondrial function.

RESULTS

Metabolic analyses revealed that GCs repress glycolysis in inflammatory myeloid cells and promote tricarboxylic acid (TCA) cycle flux, promoting succinate metabolism and preventing intracellular accumulation of succinate. Inhibition of ATP synthase attenuated GC-induced transcriptional changes, likely through stalling of TCA cycle anaplerosis. We further identified a glycolytic regulatory transcription factor, HIF1α, as regulated by GCs, and as a key regulator of GC responsiveness during inflammatory challenge.

CONCLUSIONS

Our findings link metabolism to gene regulation by GCs in macrophages.

The role of itaconate in host defense and inflammation

Macrophages exposed to inflammatory stimuli including LPS undergo metabolic reprogramming to facilitate macrophage effector function. This metabolic reprogramming supports phagocytic function, cytokine release, and ROS production that are critical to protective inflammatory responses. The Krebs cycle is a central metabolic pathway within all mammalian cell types. In activated macrophages, distinct breaks in the Krebs cycle regulate macrophage effector function through the accumulation of several metabolites that were recently shown to have signaling roles in immunity. One metabolite that accumulates in macrophages because of the disturbance in the Krebs cycle is itaconate, which is derived from cis-aconitate by the enzyme cis-aconitate decarboxylase (ACOD1), encoded by immunoresponsive gene 1 (Irg1). This Review focuses on itaconate’s emergence as a key immunometabolite with diverse roles in immunity and inflammation. These roles include inhibition of succinate dehydrogenase (which controls levels of succinate, a metabolite with multiple roles in inflammation), inhibition of glycolysis at multiple levels (which will limit inflammation), activation of the antiinflammatory transcription factors Nrf2 and ATF3, and inhibition of the NLRP3 inflammasome. Itaconate and its derivatives have antiinflammatory effects in preclinical models of sepsis, viral infections, psoriasis, gout, ischemia/reperfusion injury, and pulmonary fibrosis, pointing to possible itaconate-based therapeutics for a range of inflammatory diseases. This intriguing metabolite continues to yield fascinating insights into the role of metabolic reprogramming in host defense and inflammation.

Divergent Genetic Regulation of Nitric Oxide Production between C57BL/6J and Wild-Derived PWD/PhJ Mice Controls Postactivation Mitochondrial Metabolism, Cell Survival, and Bacterial Resistance in Dendritic Cells

Dendritic cell (DC) activation is characterized by sustained commitment to glycolysis that is a requirement for survival in DC subsets that express inducible NO synthase (Nos2) due to NO-mediated inhibition of mitochondrial respiration. This phenomenon primarily has been studied in DCs from the classic laboratory inbred mouse strain C57BL/6J (B6) mice, where DCs experience a loss of mitochondrial function due to NO accumulation. To assess the conservation of NO-driven metabolic regulation in DCs, we compared B6 mice to the wild-derived genetically divergent PWD/PhJ (PWD) strain. We show preserved mitochondrial respiration and enhanced postactivation survival due to attenuated NO production in LPS-stimulated PWD DCs phenocopying human monocyte-derived DCs. To genetically map this phenotype, we used a congenic mouse strain (B6.PWD-Chr11.2) that carries a PWD-derived portion of chromosome 11, including Nos2, on a B6 background. B6.PWD-Chr11.2 DCs show preserved mitochondrial function and produce lower NO levels than B6 DCs. We demonstrate that activated B6.PWD-Chr11.2 DCs maintain mitochondrial respiration and TCA cycle carbon flux, compared with B6 DCs. However, reduced NO production by the PWD Nos2 allele results in impaired cellular control of Listeria monocytogenes replication. These studies establish a natural genetic model for restrained endogenous NO production to investigate the contribution of NO in regulating the interplay between DC metabolism and immune function. These findings suggest that reported differences between human and murine DCs may be an artifact of the limited genetic diversity of the mouse models used, underscoring the need for mouse genetic diversity in immunology research.

3D printed submicron patterns orchestrate the response of macrophages

The surface topography of engineered extracellular matrices is one of the most important physical cues regulating the phenotypic polarization of macrophages. However, not much is known about the ways through which submicron (i.e., 100-1000 nm) topographies modulate the polarization of macrophages. In the context of bone tissue regeneration, it is well established that this range of topographies stimulates the osteogenic differentiation of stem cells. Since the immune response affects the bone tissue regeneration process, the immunomodulatory consequences of submicron patterns should be studied prior to their clinical application. Here, we 3D printed submicron pillars (using two-photon polymerization technique) with different heights and interspacings to perform the first ever systematic study of such effects. Among the studied patterns, the highest degree of elongation was observed for the cells cultured on those with the tallest and densest pillars. After 3 days of culture with inflammatory stimuli (LPS/IFN-γ), sparsely decorated surfaces inhibited the expression of the pro-inflammatory cellular marker CCR7 as compared to day 1 and to the other patterns. Furthermore, sufficiently tall pillars polarized the M1 macrophages towards a pro-healing (M2) phenotype, as suggested by the expression of CD206 within the first 3 days. As some of the studied patterns are known to be osteogenic, the osteoimmunomodulatory capacity of the patterns should be further studied to optimize their bone tissue regeneration performance.

Sphingolipid metabolism during Toll-like receptor 4 (TLR4)-mediated macrophage activation

Macrophage activation in response to stimulation of Toll-like receptor 4 (TLR4) provides a paradigm for investigating energy metabolism that regulates the inflammatory response. TLR4-mediated pro-inflammatory macrophage activation is characterized by increased glycolysis and altered mitochondrial metabolism, supported by selective amino acid uptake and/or usage. Fatty acid metabolism remains as a highly complex rewiring that accompanies classical macrophage activation. TLR4 activation leads to de novo synthesis of fatty acids, which flux into sphingolipids, complex lipids that form the building blocks of eukaryotic cell membranes and regulate cell function. Here, we review the importance of TLR4-mediated de novo synthesis of membrane sphingolipids in macrophages. We first highlight fatty acid metabolism during TLR4-driven macrophage immunometabolism. We then focus on the temporal dynamics of sphingolipid biosynthesis and emphasize the modulatory role of some sphingolipid species (i.e. sphingomyelins, ceramides and glycosphingolipids) on the pro-inflammatory and pro-resolution phases of LPS/TLR4 activation in macrophages.

Integration of transcriptional and metabolic control in macrophage activation

Macrophages react to microbial and endogenous danger signals by activating a broad panel of effector and homeostatic responses. Such responses entail rapid and stimulus-specific changes in gene expression programs accompanied by extensive rewiring of metabolism, with alterations in chromatin modifications providing one layer of integration of transcriptional and metabolic regulation. A systematic and mechanistic understanding of the mutual influences between signal-induced metabolic changes and gene expression is still lacking. Here, we discuss current evidence, controversies, knowledge gaps, and future areas of investigation on how metabolic and transcriptional changes are dynamically integrated during macrophage activation. The cross-talk between metabolism and inflammatory gene expression is in part accounted for by alterations in the production, usage, and availability of metabolic intermediates that impact the macrophage epigenome. In addition, stimulus-inducible gene expression changes alter the production of inflammatory mediators, such as nitric oxide, that in turn modulate the activity of metabolic enzymes thus determining complex regulatory loops. Critical issues remain to be understood, notably whether and how metabolic rewiring can bring about gene-specific (as opposed to global) expression changes.

Macrophage Responses to Environmental Stimuli During Homeostasis and Disease

Work over the last 40 years has described macrophages as a heterogeneous population that serve as the frontline surveyors of tissue immunity. As a class, macrophages are found in almost every tissue in the body and as distinct populations within discrete microenvironments in any given tissue. During homeostasis, macrophages protect these tissues by clearing invading foreign bodies and/or mounting immune responses. In addition to varying identities regulated by transcriptional programs shaped by their respective environments, macrophage metabolism serves as an additional regulator to temper responses to extracellular stimuli. The area of research known as "immunometabolism" has been established within the last decade, owing to an increase in studies focusing on the crosstalk between altered metabolism and the regulation of cellular immune processes. From this research, macrophages have emerged as a prime focus of immunometabolic studies, although macrophage metabolism and their immune responses have been studied for centuries. During disease, the metabolic profile of the tissue and/or systemic regulators, such as endocrine factors, become increasingly dysregulated. Owing to these changes, macrophage responses can become skewed to promote further pathophysiologic changes. For instance, during diabetes, obesity, and atherosclerosis, macrophages favor a proinflammatory phenotype; whereas in the tumor microenvironment, macrophages elicit an anti-inflammatory response to enhance tumor growth. Herein we have described how macrophages respond to extracellular cues including inflammatory stimuli, nutrient availability, and endocrine factors that occur during and further promote disease progression.

Nitric Oxide Modulates Metabolic Processes in the Tumor Immune Microenvironment

The metabolic requirements and functions of cancer and normal tissues are vastly different. Due to the rapid growth of cancer cells in the tumor microenvironment, distorted vasculature is commonly observed, which creates harsh environments that require rigorous and constantly evolving cellular adaption. A common hallmark of aggressive and therapeutically resistant tumors is hypoxia and hypoxia-induced stress markers. However, recent studies have identified alterations in a wide spectrum of metabolic pathways that dictate tumor behavior and response to therapy. Accordingly, it is becoming clear that metabolic processes are not uniform throughout the tumor microenvironment. Metabolic processes differ and are cell type specific where various factors promote metabolic heterogeneity within the tumor microenvironment. Furthermore, within the tumor, these metabolically distinct cell types can organize to form cellular neighborhoods that serve to establish a pro-tumor milieu in which distant and spatially distinct cellular neighborhoods can communicate via signaling metabolites from stroma, immune and tumor cells. In this review, we will discuss how biochemical interactions of various metabolic pathways influence cancer and immune microenvironments, as well as associated mechanisms that lead to good or poor clinical outcomes.

Effect of pH on the antimicrobial activity of the macrophage metabolite itaconate

The production of itaconate by macrophages was only discovered in 2011. An increasing number of studies have since revealed essential biological functions for this small molecule, ranging from antimicrobial to immunomodulator. The antibacterial role of itaconate has however been questioned because the estimated concentration of itaconate in macrophages (low-millimolar) is lower than the minimum inhibitory concentration (MIC) of itaconate reported for several bacterial strains (low-to-mid-millimolar). We note that some of these investigations have tended to ignore the high acidity of this small diacid (pKas 3.85 and 5.45), thereby potentially biassing activity measurements. We measured the MIC of itaconate in Escherichia coli (not known to metabolize itaconate) and in Salmonella enterica serovar Typhimurium (known to metabolize itaconate) at varying pH values to probe the effect that pH has on itaconate toxicity. Herein, we demonstrate that the antimicrobial effect of itaconate is dependent upon the pH of the media and that itaconate does have antimicrobial activity at biologically relevant pH and concentrations. Under nutrient-poor conditions, the antimicrobial activity of itaconate in both E. coli and S. Typhimurium increased approximately 200-fold when the pH was dropped by one unit, whereas itaconate was not found to be toxic under nutrient rich conditions. Our results also reveal that the activity of itaconate is synergistic with acidity, yet is not a function of increased permeability with protonation. Similar experiments performed with succinate (a pKa-matched diacid) yielded drastically different results, consistent with a target-based mechanism of action for itaconate. Overall, our work shows the importance of controlling the pH when performing experiments with itaconic acid.

Discovery of Protein Modifications Using Differential Tandem Mass Spectrometry Proteomics

Recent studies have revealed diverse amino acid, post-translational, and noncanonical modifications of proteins in diverse organisms and tissues. However, their unbiased detection and analysis remain hindered by technical limitations. Here, we present a spectral alignment method for the identification of protein modifications using high-resolution mass spectrometry proteomics. Termed SAMPEI for spectral alignment-based modified peptide identification, this open-source algorithm is designed for the discovery of functional protein and peptide signaling modifications, without prior knowledge of their identities. Using synthetic standards and controlled chemical labeling experiments, we demonstrate its high specificity and sensitivity for the discovery of substoichiometric protein modifications in complex cellular extracts. SAMPEI mapping of mouse macrophage differentiation revealed diverse post-translational protein modifications, including distinct forms of cysteine itaconatylation. SAMPEI's robust parametrization and versatility are expected to facilitate the discovery of biological modifications of diverse macromolecules. SAMPEI is implemented as a Python package and is available open-source from BioConda and GitHub (https://github.com/FenyoLab/SAMPEI).

SIGNAL: A web-based iterative analysis platform integrating pathway and network approaches optimizes hit selection from genome-scale assays

Hit selection from high-throughput assays remains a critical bottleneck in realizing the potential of omic-scale studies in biology. Widely used methods such as setting of cutoffs, prioritizing pathway enrichments, or incorporating predicted network interactions offer divergent solutions yet are associated with critical analytical trade-offs. The specific limitations of these individual approaches and the lack of a systematic way by which to integrate their rankings have contributed to limited overlap in the reported results from comparable genome-wide studies and costly inefficiencies in secondary validation efforts. Using comparative analysis of parallel independent studies as a benchmark, we characterize the specific complementary contributions of each approach and demonstrate an optimal framework to integrate these methods. We describe selection by iterative pathway group and network analysis looping (SIGNAL), an integrated, iterative approach that uses both pathway and network methods to optimize gene prioritization. SIGNAL is accessible as a rapid user-friendly web-based application (https://signal.niaid.nih.gov). A record of this paper's transparent peer review is included in the Supplemental information.

Malonate Promotes Adult Cardiomyocyte Proliferation and Heart Regeneration

BACKGROUND

Neonatal mouse cardiomyocytes undergo a metabolic switch from glycolysis to oxidative phosphorylation, which results in a significant increase in reactive oxygen species production that induces DNA damage. These cellular changes contribute to cardiomyocyte cell cycle exit and loss of the capacity for cardiac regeneration. The mechanisms that regulate this metabolic switch and the increase in reactive oxygen species production have been relatively unexplored. Current evidence suggests that elevated reactive oxygen species production in ischemic tissues occurs as a result of accumulation of the mitochondrial metabolite succinate during ischemia via succinate dehydrogenase (SDH), and this succinate is rapidly oxidized at reperfusion. Mutations in SDH in familial cancer syndromes have been demonstrated to promote a metabolic shift into glycolytic metabolism, suggesting a potential role for SDH in regulating cellular metabolism. Whether succinate and SDH regulate cardiomyocyte cell cycle activity and the cardiac metabolic state remains unclear.

METHODS

Here, we investigated the role of succinate and SDH inhibition in regulation of postnatal cardiomyocyte cell cycle activity and heart regeneration.

RESULTS

Our results demonstrate that injection of succinate into neonatal mice results in inhibition of cardiomyocyte proliferation and regeneration. Our evidence also shows that inhibition of SDH by malonate treatment after birth extends the window of cardiomyocyte proliferation and regeneration in juvenile mice. Remarkably, extending malonate treatment to the adult mouse heart after myocardial infarction injury results in a robust regenerative response within 4 weeks after injury via promoting adult cardiomyocyte proliferation and revascularization. Our metabolite analysis after SDH inhibition by malonate induces dynamic changes in adult cardiac metabolism.

CONCLUSIONS

Inhibition of SDH by malonate promotes adult cardiomyocyte proliferation, revascularization, and heart regeneration via metabolic reprogramming. These findings support a potentially important new therapeutic approach for human heart failure.

Dwellers and Trespassers: Mononuclear Phagocytes at the Borders of the Central Nervous System

The central nervous system (CNS) parenchyma is enclosed and protected by a multilayered system of cellular and acellular barriers, functionally separating glia and neurons from peripheral circulation and blood-borne immune cells. Populating these borders as dynamic observers, CNS-resident macrophages contribute to organ homeostasis. Upon autoimmune, traumatic or neurodegenerative inflammation, these phagocytes start playing additional roles as immune regulators contributing to disease evolution. At the same time, pathological CNS conditions drive the migration and recruitment of blood-borne monocyte-derived cells across distinct local gateways. This invasion process drastically increases border complexity and can lead to parenchymal infiltration of blood-borne phagocytes playing a direct role both in damage and in tissue repair. While recent studies and technical advancements have highlighted the extreme heterogeneity of these resident and CNS-invading cells, both the compartment-specific mechanism of invasion and the functional specification of intruding and resident cells remain unclear. This review illustrates the complexity of mononuclear phagocytes at CNS interfaces, indicating how further studies of CNS border dynamics are crucially needed to shed light on local and systemic regulation of CNS functions and dysfunctions.

Immunometabolism in the Tumor Microenvironment

Advances in immunotherapy have underscored the importance of antitumor immune responses in controlling cancer. However, the tumor microenvironment (TME) imposes several obstacles to the proper function of immune cells, including a metabolically challenging and immunosuppressive microenvironment. The increased metabolic activity of tumor cells can lead to the depletion of key nutrients required by immune cells and the accumulation of byproducts that hamper antitumor immunity. Furthermore, the presence of suppressive immune cells, such as regulatory T cells and myeloid-derived suppressor cells, and the expression of immune inhibitory receptors can negatively impact immune cell metabolism and function. This review summarizes the metabolic reprogramming that is characteristic of various immune cell subsets, discusses how the metabolism and function of immune cells are shaped by the TME, and highlights how therapeutic interventions aimed at improving the metabolic fitness of immune cells and alleviating the metabolic constraints in the TME can boost antitumor immunity.

More than just protein building blocks: how amino acids and related metabolic pathways fuel macrophage polarization

Macrophages represent the first line of defence in innate immune responses and additionally serve important functions for the regulation of host inflammation and tissue homeostasis. The M1/M2 model describes the two extremes of macrophage polarization states, which can be induced by multiple stimuli, most notably by LPS/IFN‐γ and IL‐4/IL‐13. Historically, the expression of two genes encoding for enzymes, which use the same amino acid as their substrate, iNOS and ARG1, has been used to define classically activated M1 (iNOS) and alternatively activated M2 (ARG1) macrophages. This ‘arginine dichotomy’ has recently become a matter of debate; however, in parallel with the emerging field of immunometabolism there is accumulating evidence that these two enzymes and their related metabolites are fundamentally involved in the intrinsic regulation of macrophage polarization and function. The aim of this review is to highlight recent advances in macrophage biology and immunometabolism with a specific focus on amino acid metabolism and their related metabolic pathways: iNOS/ARG1 (arginine), TCA cycle and OXPHOS (glutamine) as well as the one‐carbon metabolism (serine, glycine).

Itaconate Alters Succinate and Coenzyme A Metabolism via Inhibition of Mitochondrial Complex II and Methylmalonyl-CoA Mutase

Itaconate is a small molecule metabolite that is endogenously produced by cis-aconitate decarboxylase-1 (ACOD1) in mammalian cells and influences numerous cellular processes. The metabolic consequences of itaconate in cells are diverse and contribute to its regulatory function. Here, we have applied isotope tracing and mass spectrometry approaches to explore how itaconate impacts various metabolic pathways in cultured cells. Itaconate is a competitive and reversible inhibitor of Complex II/succinate dehydrogenase (SDH) that alters tricarboxylic acid (TCA) cycle metabolism leading to succinate accumulation. Upon activation with coenzyme A (CoA), itaconyl-CoA inhibits adenosylcobalamin-mediated methylmalonyl-CoA (MUT) activity and, thus, indirectly impacts branched-chain amino acid (BCAA) metabolism and fatty acid diversity. Itaconate, therefore, alters the balance of CoA species in mitochondria through its impacts on TCA, amino acid, vitamin B₁₂, and CoA metabolism. Our results highlight the diverse metabolic pathways regulated by itaconate and provide a roadmap to link these metabolites to potential downstream biological functions.

JAK2V617F Mutation Promoted IL-6 Production and Glycolysis via Mediating PKM1 Stabilization in Macrophages

A substitution mutation of valine to phenylalanine at codon encoding position 617 of the Janus kinase 2 (JAK2) gene (JAK2^V617F) has been detected in myeloid cells of some individuals with higher levels of proinflammatory cytokine production such as interleukin (IL)-6. However, the mechanisms by which JAK2^V617F mutation mediating those cytokines remain unclear. We, therefore, established JAK2^V617F-expressing murine macrophages (JAK2^V617F macrophages) and found that the levels of p-STAT3 were markedly elevated in JAK2^V617F macrophages in association with an increase in IL-6 production. However, inhibition of STAT3 by C188-9 significantly decreased the production of IL-6. Furthermore, the JAK2^V617F mutation endowed macrophages with an elevated glycolytic phenotype in parallel with aberrant expression of PKM1. Interestingly, silencing of PKM1 inactivated STAT3 in parallel with reduced IL-6 production. In contrast, ectopic expression of PKM1 elevated IL-6 production via STAT3 activation. Importantly, the JAK2^V617F mutation contributed to PKM1 protein stabilization via blockade of lysosomal-dependent degradation via chaperone-mediated autophagy (CMA), indicating that the JAK2^V617F mutation could protect PKM1 from CMA-mediated degradation, leading to activation of STAT3 and promoting IL-6 production.

Reprogramming of Central Carbon Metabolism in Myeloid Cells upon Innate Immune Receptor Stimulation

Immunometabolism is a relatively new field of research that aims at understanding interconnections between the immune system and cellular metabolism. This is now well-documented for innate immune cells of the myeloid lineage such as macrophages and myeloid dendritic cells (DCs) when they engage their differentiation or activation programs. Several studies have shown that stimulation of DCs or macrophages by the binding of pathogen-associated molecular patterns (PAMPs) to pattern recognition receptors (PRRs) leads to increased glycolytic activity and rewiring of central carbon metabolism. These metabolic modulations are essential to support and settle immunological functions by providing energy and immunoregulatory metabolites. As the understanding of molecular mechanisms progressed, significant differences between cell types and species have also been discovered. Pathways leading to the regulation of central carbon metabolism in macrophages and DCs by PRR signaling and consequences on cellular functions are reviewed here.

The versatile biochemistry of iron in macrophage effector functions

Macrophages are mononuclear phagocytes with remarkable polarization ability that allow them to have tissue-specific functions during development, homeostasis, inflammatory and infectious disease. One particular trophic factor in the tissue environment is iron, which is intimately linked to macrophage effector functions. Macrophages have a well-described role in the control of systemic iron levels, but their activation state is also depending on iron-containing proteins/enzymes. Haemoproteins, dioxygenases and iron-sulphur (Fe-S) enzymes are iron-binding proteins that have bactericidal, metabolic and epigenetic-related functions, essential to shape the context-dependent macrophage polarization. In this review, I describe mainly pro-inflammatory macrophage polarization focussing on the role of iron biochemistry in selected haemoproteins and Fe-S enzymes. I show how iron, as part of haem or Fe-S clusters, participates in the cellular control of pro-inflammatory redox reactions in parallel with its role as enzymatic cofactor. I highlight a possible coordinated regulation of haemoproteins and Fe-S enzymes during classical macrophage activation. Finally, I describe tryptophan and α-ketoglutarate metabolism as two essential effector pathways in macrophages that use diverse iron biochemistry at different enzymatic steps. Through these pathways, I show how iron participates in the regulation of essential metabolites that shape macrophage function.

Exploring the evolutionary roots and physiological function of itaconate

New small molecules are continuing to emerge as metabolically derived regulators of cell function. Itaconate is a recent example where endogenous mammalian synthesis was demonstrated only seven years ago. Since then, interest in the biochemistry and therapeutic potential of itaconate has grown dramatically. Itaconate is an unsaturated dicarboxylic acid that has antimicrobial properties and modulates metabolic pathways throughout the cell. Naturally occurring mutations of enzymes involved in human itaconate synthesis and degradation pathways are associated with disease susceptibility and immunity. Here, we highlight recent discoveries on itaconate metabolism and discuss the relevance of its evolutionary origin to its function in mammals. We also consider the therapeutic relevance of itaconate metabolism and its derivatives for treating metabolic and inflammatory diseases.

BCAT1 affects mitochondrial metabolism independently of leucine transamination in activated human macrophages

In response to environmental stimuli, macrophages change their nutrient consumption and undergo an early metabolic adaptation that progressively shapes their polarization state. During the transient, early phase of pro-inflammatory macrophage activation, an increase in tricarboxylic acid (TCA) cycle activity has been reported, but the relative contribution of branched-chain amino acid (BCAA) leucine remains to be determined. Here, we show that glucose but not glutamine is a major contributor of the increase in TCA cycle metabolites during early macrophage activation in humans. We then show that, although uptake of BCAAs is not altered, their transamination by BCAT1 is increased following 8 h lipopolysaccharide (LPS) stimulation. Of note, leucine is not metabolized to integrate into the TCA cycle in basal or stimulated human macrophages. Surprisingly, the pharmacological inhibition of BCAT1 reduced glucose-derived itaconate, α-ketoglutarate and 2-hydroxyglutarate levels without affecting succinate and citrate levels, indicating a partial inhibition of the TCA cycle. This indirect effect is associated with NRF2 (also known as NFE2L2) activation and anti-oxidant responses. These results suggest a moonlighting role of BCAT1 through redox-mediated control of mitochondrial function during early macrophage activation.

How could we forget immunometabolism in SARS-CoV2 infection or COVID-19?

SARS-CoV2 infection or COVID-19 has created panic around the world since its first origin in December 2019 in Wuhan city, China. The COVID-19 pandemic has infected more than 46.4 million people, with 1,199,727 deaths. The immune system plays a crucial role in the severity of COVID-19 and the development of pneumonia-induced acute lung injury (ALI) or acute respiratory distress syndrome (ARDS). Along with providing protection, both innate and T cell-based adaptive immune response dysregulate during severe SARS-CoV2 infection. This dysregulation is more pronounced in older population and patients with comorbidities (Diabetes, hypertension, obesity, other pulmonary and autoimmune diseases). However, COVID-19 patients develop protective antibodies (Abs) against SARS-CoV2, but they do not long for last. The induction of the immune response against the pathogen also requires metabolic energy that generates through the process of immunometabolism. The change in the metabolic stage of immune cells from homeostasis to an inflammatory or infectious environment is called immunometabolic reprogramming. The article describes the cellular immunology (macrophages, T cells, B cells, dendritic cells, NK cells and pulmonary epithelial cells (PEC) and vascular endothelial cells) and the associated immune response during COVID-19. Immunometabolism may serve as a cell-specific therapeutic approach to target COVID-19.

Nitric Oxide in Macrophage Immunometabolism: Hiding in Plain Sight

Nitric Oxide (NO) is a soluble endogenous gas with various biological functions like signaling, and working as an effector molecule or metabolic regulator. In response to inflammatory signals, immune myeloid cells, like macrophages, increase production of cytokines and NO, which is important for pathogen killing. Under these proinflammatory circumstances, called “M1”, macrophages undergo a series of metabolic changes including rewiring of their tricarboxylic acid (TCA) cycle. Here, we review findings indicating that NO, through its interaction with heme and non-heme metal containing proteins, together with components of the electron transport chain, functions not only as a regulator of cell respiration, but also a modulator of intracellular cell metabolism. Moreover, diverse effects of NO and NO-derived reactive nitrogen species (RNS) involve precise interactions with different targets depending on concentration, temporal, and spatial restrictions. Although the role of NO in macrophage reprogramming has been in evidence for some time, current models have largely minimized its importance. It has, therefore, been hiding in plain sight. A review of the chemical properties of NO, past biochemical studies, and recent publications, necessitates that mechanisms of macrophage TCA reprogramming during stimulation must be re-imagined and re-interpreted as mechanistic results of NO exposure. The revised model of metabolic rewiring we describe here incorporates many early findings regarding NO biochemistry and brings NO out of hiding and to the forefront of macrophages immunometabolism.

Autofluorescence Imaging of 3D Tumor-Macrophage Microscale Cultures Resolves Spatial and Temporal Dynamics of Macrophage Metabolism

Macrophages within the tumor microenvironment (TME) exhibit a spectrum of protumor and antitumor functions, yet it is unclear how the TME regulates this macrophage heterogeneity. Standard methods to measure macrophage heterogeneity require destructive processing, limiting spatiotemporal studies of function within the live, intact 3D TME. Here, we demonstrate two-photon autofluorescence imaging of NAD(P)H and FAD to nondestructively resolve spatiotemporal metabolic heterogeneity of individual macrophages within 3D microscale TME models. Fluorescence lifetimes and intensities of NAD(P)H and FAD were acquired at 24, 48, and 72 hours poststimulation for mouse macrophages (RAW264.7) stimulated with IFNγ or IL4 plus IL13 in 2D culture, confirming that autofluorescence measurements capture known metabolic phenotypes. To quantify metabolic dynamics of macrophages within the TME, mouse macrophages or human monocytes (RAW264.7 or THP-1) were cultured alone or with breast cancer cells (mouse polyoma-middle T virus or primary human IDC) in 3D microfluidic platforms. Human monocytes and mouse macrophages in tumor cocultures exhibited significantly different FAD mean lifetimes and greater migration than monocultures at 24, 48, and 72 hours postseeding. In cocultures with primary human cancer cells, actively migrating monocyte-derived macrophages had greater redox ratios [NAD(P)H/FAD intensity] compared with passively migrating monocytes at 24 and 48 hours postseeding, reflecting metabolic heterogeneity in this subpopulation of monocytes. Genetic analyses further confirmed this metabolic heterogeneity. These results establish label-free autofluorescence imaging to quantify dynamic metabolism, polarization, and migration of macrophages at single-cell resolution within 3D microscale models. This combined culture and imaging system provides unique insights into spatiotemporal tumor-immune cross-talk within the 3D TME. SIGNIFICANCE: Label-free metabolic imaging and microscale culture technologies enable monitoring of single-cell macrophage metabolism, migration, and function in the 3D tumor microenvironment.

β-Hydroxybutyrate Oxidation Promotes the Accumulation of Immunometabolites in Activated Microglia Cells

Metabolic regulation of immune cells has arisen as a critical set of processes required for appropriate response to immunological signals. While our knowledge in this area has rapidly expanded in leukocytes, much less is known about the metabolic regulation of brain-resident microglia. In particular, the role of alternative nutrients to glucose remains poorly understood. Here, we use stable-isotope (¹³C) tracing strategies and metabolomics to characterize the oxidative metabolism of β-hydroxybutyrate (BHB) in human (HMC3) and murine (BV2) microglia cells and the interplay with glucose in resting and LPS-activated BV2 cells. We found that BHB is imported and oxidised in the TCA cycle in both cell lines with a subsequent increase in the cytosolic NADH:NAD⁺ ratio. In BV2 cells, stimulation with LPS upregulated the glycolytic flux, increased the cytosolic NADH:NAD⁺ ratio and promoted the accumulation of the glycolytic intermediate dihydroxyacetone phosphate (DHAP). The addition of BHB enhanced LPS-induced accumulation of DHAP and promoted glucose-derived lactate export. BHB also synergistically increased LPS-induced accumulation of succinate and other key immunometabolites, such as α-ketoglutarate and fumarate generated by the TCA cycle. Finally, BHB upregulated the expression of a key pro-inflammatory (M1 polarisation) marker gene, NOS2, in BV2 cells activated with LPS. In conclusion, we identify BHB as a potentially immunomodulatory metabolic substrate for microglia that promotes metabolic reprogramming during pro-inflammatory response.

Selective inhibition of mitochondrial respiratory complexes controls the transition of microglia into a neurotoxic phenotype in situ

Microglia are tissue resident macrophages (innate immunity) and universal sensors of alterations in CNS physiology. In response to pathogen or damage signals, microglia feature rapid activation and can acquire different phenotypes exerting neuroprotection or neurotoxicity. Although transcriptional aspects of microglial phenotypic transitions have been described, the underlying metabolic reprogramming is widely unknown. Employing postnatal organotypic hippocampal slice cultures, we describe that microglia transformed into a mild reactive phenotype by single TLR4 stimulation with lipopolysaccharide (LPS), which was boosted into a severe neurotoxic phenotype by IFN-γ (LPS + INF-γ). The two reactive phenotypes associated with reduction of microglial homeostatic "surveillance" markers, increase of cytokine release (IL-6, TNF-α) as well as enhancement of tissue energy demand and lactate production. These reactive phenotypes differed in the pattern of inhibition of the respiratory chain in mitochondria, however. TLR4 stimulation induced succinate dehydrogenase (complex II) inhibition by the metabolite itaconate. By contrast, TLR4 + IFN-γ receptor stimulation mainly resulted in complex IV inhibition by nitric oxide (NO) that also associated with severe oxidative stress, neuronal dysfunction and death. Notably, pharmacological depletion of microglia or treatment with itaconate resulted in effective neuroprotection reflected by well-preserved cytoarchitecture and electrical network activity, i.e., neuronal gamma oscillations (30-70 Hz) that underlie higher cognitive functions in vivo. Our findings provide in situ evidence that (i) proinflammatory microglia can substantially alter brain energy metabolism and (ii) fine-tuning of itaconate and NO metabolism determines microglial reactivity, impairment of neural network function and neurodegeneration. These data add mechanistic insights into microglial activation, with relevance to disorders featuring neuroinflammation and to drug discovery.

Fever-Inducible Lipid Nanocomposite for Boosting Cancer Therapy through Synergistic Engineering of a Tumor Microenvironment

A fever-mimic response capable of recruiting reprogrammed macrophages holds great potential in the engineering of the tumor microenvironment (TME). Low-temperature photothermal therapy (LT-PTT) can maintain tumors at a fever-like temperature (<45 °C) temporarily; however, it still faces several challenges in efficient regulation of TME because of reciprocal cross-talk between the bypass pathways. Here, we report a synergistic engineering of TME through an enhanced activation of a fever-mimic response based on both LT-PTT and tumor vascular normalization. Such engineering is achieved by a fever-inducible lipid nanocomposite (GNR-T/CM-L), which produces mild heat (∼43 °C) and sequentially releases multicomponents to cooperatively upregulate interferon-gamma under NIR irradiation, forming a bidirectionally closed loop for downstream M1 tumor-associated macrophage polarization and promoting the inhibition of the tumor growth. In proof-of-concept studies, GNR-T/CM-L demonstrated efficient tumor ablation in breast tumor xenograft-bearing mice and significantly prolonged their survival period. It paves an avenue to precisely reprogram TME for efficient cancer therapy through synergistic pathways of creating fever-like responses in the tumor.

The Metabolic Basis of Immune Dysfunction Following Sepsis and Trauma

Critically ill, severely injured and high-risk surgical patients are vulnerable to secondary infections during hospitalization and after hospital discharge. Studies show that the mitochondrial function and oxidative metabolism of monocytes and macrophages are impaired during sepsis. Alternatively, treatment with microbe-derived ligands, such as monophosphoryl lipid A (MPLA), peptidoglycan, or β-glucan, that interact with toll-like receptors and other pattern recognition receptors on leukocytes induces a state of innate immune memory that confers broad-spectrum resistance to infection with common hospital-acquired pathogens. Priming of macrophages with MPLA, CPG oligodeoxynucleotides (CpG ODN), or β-glucan induces a macrophage metabolic phenotype characterized by mitochondrial biogenesis and increased oxidative metabolism in parallel with increased glycolysis, cell size and granularity, augmented phagocytosis, heightened respiratory burst functions, and more effective killing of microbes. The mitochondrion is a bioenergetic organelle that not only contributes to energy supply, biosynthesis, and cellular redox functions but serves as a platform for regulating innate immunological functions such as production of reactive oxygen species (ROS) and regulatory intermediates. This review will define current knowledge of leukocyte metabolic dysfunction during and after sepsis and trauma. We will further discuss therapeutic strategies that target leukocyte mitochondrial function and might have value in preventing or reversing sepsis- and trauma-induced immune dysfunction.

A small molecule G6PD inhibitor reveals immune dependence on pentose phosphate pathway

Glucose is catabolized by two fundamental pathways, glycolysis to make ATP and the oxidative pentose phosphate pathway to make NADPH. The first step of the oxidative pentose phosphate pathway is catalyzed by the enzyme glucose-6-phosphate dehydrogenase (G6PD). Here we develop metabolite reporter and deuterium tracer assays to monitor cellular G6PD activity. Using these, we show that the most widely cited G6PD antagonist, dehydroepiandosterone (DHEA), does not robustly inhibit G6PD in cells. We then identify a small molecule (G6PDi-1) that more effectively inhibits G6PD. Across a range of cultured cells, G6PDi-1 depletes NADPH most strongly in lymphocytes. In T cells but not macrophages, G6PDi-1 markedly decreases inflammatory cytokine production. In neutrophils, it suppresses respiratory burst. Thus, we provide a cell-active small molecule tool for oxidative pentose phosphate pathway inhibition, and use it to identify G6PD as a pharmacological target for modulating immune response.

Krebs Cycle Reborn in Macrophage Immunometabolism

A striking change has happened in the field of immunology whereby specific metabolic processes have been shown to be a critical determinant of immune cell activation. Multiple immune receptor types rewire metabolic pathways as a key part of how they promote effector functions. Perhaps surprisingly for immunologists, the Krebs cycle has emerged as the central immunometabolic hub of the macrophage. During proinflammatory macrophage activation, there is an accumulation of the Krebs cycle intermediates succinate and citrate, and the Krebs cycle–derived metabolite itaconate. These metabolites have distinct nonmetabolic signaling roles that influence inflammatory gene expression. A key bioenergetic target for the Krebs cycle, the electron transport chain, also becomes altered, generating reactive oxygen species from Complexes I and III. Similarly, alternatively activated macrophages require α-ketoglutarate-dependent epigenetic reprogramming to elicit anti-inflammatory gene expression. In this review, we discuss these advances and speculate on the possibility of targeting these events therapeutically for inflammatory diseases.

Metabolic flux analysis and fluxomics-driven determination of reaction free energy using multiple isotopes

Metabolite concentrations, fluxes, and free energies constitute the basis for understanding and controlling metabolism. Mass spectrometry and stable isotopes are integral tools in quantifying these metabolic features. For absolute metabolite concentration and flux measurement, ¹³C internal standards and tracers have been the gold standard. In contrast, no established methods exist for comprehensive thermodynamic quantitation under physiological environments. Recently, using high-resolution mass spectrometry and multi-isotope tracing, flux quantitation has been increasingly adopted in broader metabolism. The improved flux quantitation led to determination of Gibbs free energy of reaction (ΔG) in central carbon metabolism using a relationship between reaction reversibility and thermodynamic driving force. Here we highlight recent advances in multi-isotope tracing for metabolic flux and free energy analysis.

Itaconate modulates tricarboxylic acid and redox metabolism to mitigate reperfusion injury

OBJECTIVES

Cerebral ischemia/reperfusion (IR) drives oxidative stress and injurious metabolic processes that lead to redox imbalance, inflammation, and tissue damage. However, the key mediators of reperfusion injury remain unclear, and therefore, there is considerable interest in therapeutically targeting metabolism and the cellular response to oxidative stress.

METHODS

The objective of this study was to investigate the molecular, metabolic, and physiological impact of itaconate treatment to mitigate reperfusion injuries in in vitro and in vivo model systems. We conducted metabolic flux and bioenergetic studies in response to exogenous itaconate treatment in cultures of primary rat cortical neurons and astrocytes. In addition, we administered itaconate to mouse models of cerebral reperfusion injury with ischemia or traumatic brain injury followed by hemorrhagic shock resuscitation. We quantitatively characterized the metabolite levels, neurological behavior, markers of redox stress, leukocyte adhesion, arterial blood flow, and arteriolar diameter in the brains of the treated/untreated mice.

RESULTS

We demonstrate that the "immunometabolite" itaconate slowed tricarboxylic acid (TCA) cycle metabolism and buffered redox imbalance via succinate dehydrogenase (SDH) inhibition and induction of anti-oxidative stress response in primary cultures of astrocytes and neurons. The addition of itaconate to reperfusion fluids after mouse cerebral IR injury increased glutathione levels and reduced reactive oxygen/nitrogen species (ROS/RNS) to improve neurological function. Plasma organic acids increased post-reperfusion injury, while administration of itaconate normalized these metabolites. In mouse cranial window models, itaconate significantly improved hemodynamics while reducing leukocyte adhesion. Further, itaconate supplementation increased survival in mice experiencing traumatic brain injury (TBI) and hemorrhagic shock.

CONCLUSIONS

We hypothesize that itaconate transiently inhibits SDH to gradually "awaken" mitochondrial function upon reperfusion that minimizes ROS and tissue damage. Collectively, our data indicate that itaconate acts as a mitochondrial regulator that controls redox metabolism to improve physiological outcomes associated with IR injury.

Macrophage activation as an archetype of mitochondrial repurposing

Mitochondria are metabolic organelles essential not only for energy transduction, but also a range of other functions such as biosynthesis, ion and metal homeostasis, maintenance of redox balance, and cell signaling. A hallmark example of how mitochondria can rebalance these processes to adjust cell function is observed in macrophages. These innate immune cells are responsible for a remarkable breadth of processes including pathogen elimination, antigen presentation, debris clearance, and wound healing. These diverse, polarized functions often include similarly disparate alterations in the metabolic phenotype associated with their execution. In this chapter, mitochondrial bioenergetics and signaling are viewed through the lens of macrophage polarization: both classical, pro-inflammatory activation and alternative, anti-inflammatory activation are associated with substantive changes to mitochondrial metabolism. Emphasis is placed on recent evidence that aims to clarify the essential - rather than associative - mitochondrial alterations, as well as accumulating data suggesting a degree of plasticity within the metabolic phenotypes that can support pro- and anti-inflammatory functions.

Metabolic regulation of epigenetic remodeling in immune cells

Immune cells are capable of sensing various signals in the microenvironment and turning on specific immune functions in response. The appropriate transition of immune cells into diverse functional states, which is crucial for immunity, involves complex and well-regulated changes in transcriptional program. Accumulating evidence shows that epigenetic remodeling plays a central role in mediating the transcriptional program for immune cell activation and immunological memory. Concurrently, immune cells undergo significant metabolic reprogramming during immune response. Here we review recent studies that demonstrate shifts in metabolic state can orchestrate immune cell functions through its impact on epigenetic remodeling, and the microenvironment can exert its influence on immune cells through the metabolic regulation of epigenetics. We also discuss the systems biology approaches that enabled these discoveries.