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.

Enteric helminth coinfection enhances host susceptibility to neurotropic flaviviruses via a tuft cell-IL-4 receptor signaling axis.

Although enteric helminth infections modulate immunity to mucosal pathogens, their effects on systemic microbes remain less established. Here, we observe increased mortality in mice coinfected with the enteric helminth Heligmosomoides polygyrus bakeri (Hpb) and West Nile virus (WNV). This enhanced susceptibility is associated with altered gut morphology and transit, translocation of commensal bacteria, impaired WNV-specific T cell responses, and increased virus infection in the gastrointestinal tract and central nervous system. These outcomes were due to type 2 immune skewing, because coinfection in Stat6-/- mice rescues mortality, treatment of helminth-free WNV-infected mice with interleukin (IL)-4 mirrors coinfection, and IL-4 receptor signaling in intestinal epithelial cells mediates the susceptibility phenotypes. Moreover, tuft cell-deficient mice show improved outcomes with coinfection, whereas treatment of helminth-free mice with tuft cell-derived cytokine IL-25 or ligand succinate worsens WNV disease. Thus, helminth activation of tuft cell-IL-4-receptor circuits in the gut exacerbates infection and disease of a neurotropic flavivirus.

pH-Gated Succinate Secretion Regulates Muscle Remodeling in Response to Exercise.

In response to skeletal muscle contraction during exercise, paracrine factors coordinate tissue remodeling, which underlies this healthy adaptation. Here we describe a pH-sensing metabolite signal that initiates muscle remodeling upon exercise. In mice and humans, exercising skeletal muscle releases the mitochondrial metabolite succinate into the local interstitium and circulation. Selective secretion of succinate is facilitated by its transient protonation, which occurs upon muscle cell acidification. In the protonated monocarboxylic form, succinate is rendered a transport substrate for monocarboxylate transporter 1, which facilitates pH-gated release. Upon secretion, succinate signals via its cognate receptor SUCNR1 in non-myofibrillar cells in muscle tissue to control muscle-remodeling transcriptional programs. This succinate-SUCNR1 signaling is required for paracrine regulation of muscle innervation, muscle matrix remodeling, and muscle strength in response to exercise training. In sum, we define a bioenergetic sensor in muscle that utilizes intracellular pH and succinate to coordinate tissue adaptation to exercise.

A Metabolite-Triggered Tuft Cell-ILC2 Circuit Drives Small Intestinal Remodeling.

The small intestinal tuft cell-ILC2 circuit mediates epithelial responses to intestinal helminths and protists by tuft cell chemosensory-like sensing and IL-25-mediated activation of lamina propria ILC2s. Small intestine ILC2s constitutively express the IL-25 receptor, which is negatively regulated by A20 (Tnfaip3). A20 deficiency in ILC2s spontaneously triggers the circuit and, unexpectedly, promotes adaptive small-intestinal lengthening and remodeling. Circuit activation occurs upon weaning and is enabled by dietary polysaccharides that render mice permissive for Tritrichomonas colonization, resulting in luminal accumulation of acetate and succinate, metabolites of the protist hydrogenosome. Tuft cells express GPR91, the succinate receptor, and dietary succinate, but not acetate, activates ILC2s via a tuft-, TRPM5-, and IL-25-dependent pathway. Also induced by parasitic helminths, circuit activation and small intestinal remodeling impairs infestation by new helminths, consistent with the phenomenon of concomitant immunity. We describe a metabolic sensing circuit that may have evolved to facilitate mutualistic responses to luminal pathosymbionts.

Molecular dynamics-based identification of binding pathways and two distinct high-affinity sites for succinate in succinate receptor 1/GPR91.

SUCNR1 is an auto- and paracrine sensor of the metabolic stress signal succinate. Using unsupervised molecular dynamics (MD) simulations (170.400 ns) and mutagenesis across human, mouse, and rat SUCNR1, we characterize how a five-arginine motif around the extracellular pole of TM-VI determines the initial capture of succinate in the extracellular vestibule (ECV) to either stay or move down to the orthosteric site. Metadynamics demonstrate low-energy succinate binding in both sites, with an energy barrier corresponding to an intermediate stage during which succinate, with an associated water cluster, unlocks the hydrogen-bond-stabilized conformationally constrained extracellular loop (ECL)-2b. Importantly, simultaneous binding of two succinate molecules through either a "sequential" or "bypassing" mode is a frequent endpoint. The mono-carboxylate NF-56-EJ40 antagonist enters SUCNR1 between TM-I and -II and does not unlock ECL-2b. It is proposed that occupancy of both high-affinity sites is required for selective activation of SUCNR1 by high local succinate concentrations.

Succinate Dehydrogenase Supports Metabolic Repurposing of Mitochondria to Drive Inflammatory Macrophages.

Activated macrophages undergo metabolic reprogramming, which drives their pro-inflammatory phenotype, but the mechanistic basis for this remains obscure. Here, we demonstrate that upon lipopolysaccharide (LPS) stimulation, macrophages shift from producing ATP by oxidative phosphorylation to glycolysis while also increasing succinate levels. We show that increased mitochondrial oxidation of succinate via succinate dehydrogenase (SDH) and an elevation of mitochondrial membrane potential combine to drive mitochondrial reactive oxygen species (ROS) production. RNA sequencing reveals that this combination induces a pro-inflammatory gene expression profile, while an inhibitor of succinate oxidation, dimethyl malonate (DMM), promotes an anti-inflammatory outcome. Blocking ROS production with rotenone by uncoupling mitochondria or by expressing the alternative oxidase (AOX) inhibits this inflammatory phenotype, with AOX protecting mice from LPS lethality. The metabolic alterations that occur upon activation of macrophages therefore repurpose mitochondria from ATP synthesis to ROS production in order to promote a pro-inflammatory state.

Tuft-Cell-Derived Leukotrienes Drive Rapid Anti-helminth Immunity in the Small Intestine but Are Dispensable for Anti-protist Immunity.

Helminths, allergens, and certain protists induce type 2 immune responses, but the underlying mechanisms of immune activation remain poorly understood. In the small intestine, chemosensing by epithelial tuft cells results in the activation of group 2 innate lymphoid cells (ILC2s), which subsequently drive increased tuft cell frequency. This feedforward circuit is essential for intestinal remodeling and helminth clearance. ILC2 activation requires tuft-cell-derived interleukin-25 (IL-25), but whether additional signals regulate the circuit is unclear. Here, we show that tuft cells secrete cysteinyl leukotrienes (cysLTs) to rapidly activate type 2 immunity following chemosensing of helminth infection. CysLTs cooperate with IL-25 to activate ILC2s, and tuft-cell-specific ablation of leukotriene synthesis attenuates type 2 immunity and delays helminth clearance. Conversely, cysLTs are dispensable for the tuft cell response induced by intestinal protists. Our findings identify an additional tuft cell effector function and suggest context-specific regulation of tuft-ILC2 circuits within the small intestine.

SUCLG1 restricts POLRMT succinylation to enhance mitochondrial biogenesis and leukemia progression.

Mitochondria are cellular powerhouses that generate energy through the electron transport chain (ETC). The mitochondrial genome (mtDNA) encodes essential ETC proteins in a compartmentalized manner, however, the mechanism underlying metabolic regulation of mtDNA function remains unknown. Here, we report that expression of tricarboxylic acid cycle enzyme succinate-CoA ligase SUCLG1 strongly correlates with ETC genes across various TCGA cancer transcriptomes. Mechanistically, SUCLG1 restricts succinyl-CoA levels to suppress the succinylation of mitochondrial RNA polymerase (POLRMT). Lysine 622 succinylation disrupts the interaction of POLRMT with mtDNA and mitochondrial transcription factors. SUCLG1-mediated POLRMT hyposuccinylation maintains mtDNA transcription, mitochondrial biogenesis, and leukemia cell proliferation. Specifically, leukemia-promoting FMS-like tyrosine kinase 3 (FLT3) mutations modulate nuclear transcription and upregulate SUCLG1 expression to reduce succinyl-CoA and POLRMT succinylation, resulting in enhanced mitobiogenesis. In line, genetic depletion of POLRMT or SUCLG1 significantly delays disease progression in mouse and humanized leukemia models. Importantly, succinyl-CoA level and POLRMT succinylation are downregulated in FLT3-mutated clinical leukemia samples, linking enhanced mitobiogenesis to cancer progression. Together, SUCLG1 connects succinyl-CoA with POLRMT succinylation to modulate mitochondrial function and cancer development.

Cancer-Derived Succinate Promotes Macrophage Polarization and Cancer Metastasis via Succinate Receptor.

Macrophages form a major cell population in the tumor microenvironment. They can be activated and polarized into tumor-associated macrophages (TAM) by the tumor-derived soluble molecules to promote tumor progression and metastasis. Here, we used comparative metabolomics coupled with biochemical and animal studies to show that cancer cells release succinate into their microenvironment and activate succinate receptor (SUCNR1) signaling to polarize macrophages into TAM. Furthermore, the results from in vitro and in vivo studies revealed that succinate promotes not only cancer cell migration and invasion but also cancer metastasis. These effects are mediated by SUCNR1-triggered PI3K-hypoxia-inducible factor 1α (HIF-1α) axis. Compared with healthy subjects and tumor-free lung tissues, serum succinate levels and lung cancer SUCNR1 expression were elevated in lung cancer patients, suggesting an important clinical relevance. Collectively, our findings indicate that the secreted tumor-derived succinate belongs to a novel class of cancer progression factors, controlling TAM polarization and promoting tumorigenic signaling.

Detection of Succinate by Intestinal Tuft Cells Triggers a Type 2 Innate Immune Circuit.

In the small intestine, type 2 responses are regulated by a signaling circuit that involves tuft cells and group 2 innate lymphoid cells (ILC2s). Here, we identified the microbial metabolite succinate as an activating ligand for small intestinal (SI) tuft cells. Sequencing analyses of tuft cells isolated from the small intestine, gall bladder, colon, thymus, and trachea revealed that expression of tuft cell chemosensory receptors is tissue specific. SI tuft cells expressed the succinate receptor (SUCNR1), and providing succinate in drinking water was sufficient to induce a multifaceted type 2 immune response via the tuft-ILC2 circuit. The helminth Nippostrongylus brasiliensis and a tritrichomonad protist both secreted succinate as a metabolite. In vivo sensing of the tritrichomonad required SUCNR1, whereas N. brasiliensis was SUCNR1 independent. These findings define a paradigm wherein tuft cells monitor microbial metabolites to initiate type 2 immunity and suggest the existence of other sensing pathways triggering the response to helminths.

Lactobacillus Firm-5-derived succinate prevents honeybees from having diabetes-like symptoms.

The gut microbiome plays an important role in honeybee hormonal regulation and growth, but the underlying mechanisms are poorly understood. Here, we showed that the depletion of gut bacteria resulted in reduced expression of insulin-like peptide gene (ilp) in the head, accompanied by metabolic syndromes resembling those of Type 1 diabetes in humans: hyperglycemia, impaired lipid storage, and decreased metabolism. These symptoms were alleviated by gut bacterial inoculation. Gut metabolite profiling revealed that succinate, produced by Lactobacillus Firm-5, played deterministic roles in activating ilp gene expression and in regulating metabolism in honeybees. Notably, we demonstrated that succinate modulates host ilp gene expression through stimulating gut gluconeogenesis, a mechanism resembling that of humans. This study presents evidence for the role of gut metabolite in modulating host metabolism and contributes to the understanding of the interactions between gut microbiome and bee hosts.

Alternative oxidase blunts pseudohypoxia and photoreceptor degeneration due to RPE mitochondrial dysfunction.

Loss of mitochondrial electron transport complex (ETC) function in the retinal pigment epithelium (RPE) in vivo results in RPE dedifferentiation and progressive photoreceptor degeneration, and has been implicated in the pathogenesis of age-related macular degeneration. Xenogenic expression of alternative oxidases in mammalian cells and tissues mitigates phenotypes arising from some mitochondrial electron transport defects, but can exacerbate others. We expressed an alternative oxidase from Ciona intestinalis (AOX) in ETC-deficient murine RPE in vivo to assess the retinal consequences of stimulating coenzyme Q oxidation and respiration without ATP generation. RPE-restricted expression of AOX in this context is surprisingly beneficial. This focused intervention mitigates RPE mTORC1 activation, dedifferentiation, hypertrophy, stress marker expression, pseudohypoxia, and aerobic glycolysis. These RPE cell autonomous changes are accompanied by increased glucose delivery to photoreceptors with attendant improvements in photoreceptor structure and function. RPE-restricted AOX expression normalizes accumulated levels of succinate and 2-hydroxyglutarate in ETC-deficient RPE, and counteracts deficiencies in numerous neural retinal metabolites. These features can be attributed to the activation of mitochondrial inner membrane flavoproteins such as succinate dehydrogenase and proline dehydrogenase, and alleviation of inhibition of 2-oxyglutarate-dependent dioxygenases such as prolyl hydroxylases and epigenetic modifiers. Our work underscores the importance to outer retinal health of coenzyme Q oxidation in the RPE and identifies a metabolic network critical for photoreceptor survival in the context of RPE mitochondrial dysfunction.

TCA cycle off, ATF4 on for metabolic homeostasis.

Tricarboxylic acid (TCA) cycle is a major hub for catabolic and anabolic reactions, yet cellular metabolic adaptations following its inhibition are largely unknown. Using multi-tiered omics approaches, Ryan et al. have shown convergent activation of the integrated stress response (ISR) through ATF4-mediated rewiring of cellular amino acid and redox metabolic pathways.

Tuft cells mediate commensal remodeling of the small intestinal antimicrobial landscape.

Succinate produced by the commensal protist Tritrichomonas musculis (T. mu) stimulates chemosensory tuft cells, resulting in intestinal type 2 immunity. Tuft cells express the succinate receptor SUCNR1, yet this receptor does not mediate antihelminth immunity nor alter protist colonization. Here, we report that microbial-derived succinate increases Paneth cell numbers and profoundly alters the antimicrobial peptide (AMP) landscape in the small intestine. Succinate was sufficient to drive this epithelial remodeling, but not in mice lacking tuft cell chemosensory components required to detect this metabolite. Tuft cells respond to succinate by stimulating type 2 immunity, leading to interleukin-13-mediated epithelial and AMP expression changes. Moreover, type 2 immunity decreases the total number of mucosa-associated bacteria and alters the small intestinal microbiota composition. Finally, tuft cells can detect short-term bacterial dysbiosis that leads to a spike in luminal succinate levels and modulate AMP production in response. These findings demonstrate that a single metabolite produced by commensals can markedly shift the intestinal AMP profile and suggest that tuft cells utilize SUCNR1 and succinate sensing to modulate bacterial homeostasis.

Complex II ambiguities-FADH2 in the electron transfer system.

The prevailing notion that reduced cofactors NADH and FADH2 transfer electrons from the tricarboxylic acid cycle to the mitochondrial electron transfer system creates ambiguities regarding respiratory Complex II (CII). CII is the only membrane-bound enzyme in the tricarboxylic acid cycle and is part of the electron transfer system of the mitochondrial inner membrane feeding electrons into the coenzyme Q-junction. The succinate dehydrogenase subunit SDHA of CII oxidizes succinate and reduces the covalently bound prosthetic group FAD to FADH2 in the canonical forward tricarboxylic acid cycle. However, several graphical representations of the electron transfer system depict FADH2 in the mitochondrial matrix as a substrate to be oxidized by CII. This leads to the false conclusion that FADH2 from the ß-oxidation cycle in fatty acid oxidation feeds electrons into CII. In reality, dehydrogenases of fatty acid oxidation channel electrons to the Q-junction but not through CII. The ambiguities surrounding Complex II in the literature and educational resources call for quality control, to secure scientific standards in current communications of bioenergetics, and ultimately support adequate clinical applications. This review aims to raise awareness of the inherent ambiguity crisis, complementing efforts to address the well-acknowledged issues of credibility and reproducibility.

Blood progenitor redox homeostasis through olfaction-derived systemic GABA in hematopoietic growth control in Drosophila.

The role of reactive oxygen species (ROS) in myeloid development is well established. However, its aberrant generation alters hematopoiesis. Thus, a comprehensive understanding of events controlling ROS homeostasis forms the central focus of this study. We show that, in homeostasis, myeloid-like blood progenitor cells of the Drosophila larvae, which reside in a specialized hematopoietic organ termed the lymph gland, use TCA to generate ROS. However, excessive ROS production leads to lymph gland growth retardation. Therefore, to moderate blood progenitor ROS, Drosophila larvae rely on olfaction and its downstream systemic GABA. GABA internalization and its breakdown into succinate by progenitor cells activates pyruvate dehydrogenase kinase (PDK), which controls inhibitory phosphorylation of pyruvate dehydrogenase (PDH). PDH is the rate-limiting enzyme that connects pyruvate to the TCA cycle and to oxidative phosphorylation. Thus, GABA metabolism via PDK activation maintains TCA activity and blood progenitor ROS homeostasis, and supports normal lymph gland growth. Consequently, animals that fail to smell also fail to sustain TCA activity and ROS homeostasis, which leads to lymph gland growth retardation. Overall, this study describes the requirement of animal odor-sensing and GABA in myeloid ROS regulation and hematopoietic growth control.

Trailblazing in adjuvant research: succinate's uncharted territory with neutrophils.

The purpose of this study is to investigate the previously unknown connection that succinate has with neutrophils in the setting of adjuvant-mediated immunological enhancement. It has been discovered that succinates stimulate the recruitment of neutrophils in immunization sites, which in turn induces the expression of what is known as neutrophil-derived B cell-activating factor (BAFF). Further amplification of vaccine-induced antibody responses is provided via the succinate receptor 1-interferon regulatory factor 5 (SUCNR1-IRF5)-BAFF signaling pathway, which provides insights into a unique mechanism for immunological enhancement.NEW & NOTEWORTHY This study explores the role of succinate as a vaccine adjuvant, revealing its capacity to enhance neutrophil recruitment at immunization sites, which boosts B cell activation through the succinate receptor 1-interferon regulatory factor 5-B cell-activating factor (SUCNR1-IRF5-BAFF) signaling pathway. Results demonstrate succinate's potential to amplify vaccine-induced antibody responses, highlighting its significance in immunological enhancement and offering new insights into the adjuvant mechanisms of action, particularly in neutrophil-mediated immune responses.

Type 2 diabetes and succinate: unmasking an age-old molecule.

Beyond their conventional roles in intracellular energy production, some traditional metabolites also function as extracellular messengers that activate cell-surface G-protein-coupled receptors (GPCRs) akin to hormones and neurotransmitters. These signalling metabolites, often derived from nutrients, the gut microbiota or the host's intermediary metabolism, are now acknowledged as key regulators of various metabolic and immune responses. This review delves into the multi-dimensional aspects of succinate, a dual metabolite with roots in both the mitochondria and microbiome. It also connects the dots between succinate's role in the Krebs cycle, mitochondrial respiration, and its double-edge function as a signalling transmitter within and outside the cell. We aim to provide an overview of the role of the succinate-succinate receptor 1 (SUCNR1) axis in diabetes, discussing the potential use of succinate as a biomarker and the novel prospect of targeting SUCNR1 to manage complications associated with diabetes. We further propose strategies to manipulate the succinate-SUCNR1 axis for better diabetes management; this includes pharmacological modulation of SUCNR1 and innovative approaches to manage succinate concentrations, such as succinate administration and indirect strategies, like microbiota modulation. The dual nature of succinate, both in terms of origins and roles, offers a rich landscape for understanding the intricate connections within metabolic diseases, like diabetes, and indicates promising pathways for developing new therapeutic strategies.

An evolving view of complex II-noncanonical complexes, megacomplexes, respiration, signaling, and beyond.

Mitochondrial complex II is traditionally studied for its participation in two key respiratory processes: the electron transport chain and the Krebs cycle. There is now a rich body of literature explaining how complex II contributes to respiration. However, more recent research shows that not all of the pathologies associated with altered complex II activity clearly correlate with this respiratory role. Complex II activity has now been shown to be necessary for a range of biological processes peripherally related to respiration, including metabolic control, inflammation, and cell fate. Integration of findings from multiple types of studies suggests that complex II both participates in respiration and controls multiple succinate-dependent signal transduction pathways. Thus, the emerging view is that the true biological function of complex II is well beyond respiration. This review uses a semichronological approach to highlight major paradigm shifts that occurred over time. Special emphasis is given to the more recently identified functions of complex II and its subunits because these findings have infused new directions into an established field.

Huanglian Jiedu Wan intervened with "Shi-Re Shanghuo" syndrome through regulating immune balance mediated by biomarker succinate.

With increasing stress in daily life and work, subhealth conditions induced by "Shi-Re Shanghuo" syndrome was gradually universal. "Huanglian Jiedu Wan" (HLJDW) was the first new syndrome Chinese medicine approved for the treatment of "Shi-Re Shanghuo" with promising clinical efficacy. Preliminary small-sample clinical studies have identified some notable biomarkers (succinate, 4-hydroxynonenal, etc.). However, the correlation and underlying mechanism between these biomarkers of HLJDW intervention on "Shi-Re Shanghuo" syndrome remained ambiguous. Therefore, this study was designed as a randomized, double-blind, multicenter, placebo-controlled Phase II clinical trial, employing integrated analysis techniques such as non-targeted and targeted metabolomics, salivary microbiota, proteomics, parallel peaction monitoring, molecular docking and surface plasmon resonance (SPR). The results of the correlation analysis indicated that HLJDW could mediate the balance between inflammation and immunity through succinate produced via host and microbial source to intervene "Shi-Re Shanghuo" syndrome. Further through the HIF1α/MMP9 pathway, succinate regulated downstream arachidonic acid metabolism, particularly the lipid peroxidation product 4-hydroxynonenal. Finally, an animal model of recurrent oral ulcers induced by "Shi-Re Shang Huo" was established and HLJDW was used for intervention, key essential indicators (succinate, glutamine, 4-hydroxynonenal, arachidonic acid metabolism) essential in the potential pathway HIF1α/MMP9 discovered in clinical practice were validated. The results were found to be consistent with our clinical findings. Taken together, succinate was observed as an important signal that triggered immune responses, which might serve as a key regulatory metabolic switch or marker of "Shi-Re Shanghuo" syndrome treated with HLJDW.