Succinate Receptor 1: An Emerging Regulator of Myeloid Cell Function in Inflammation

Succinate activates UCP2 to suppress neuroinflammation and confer protection following intracerebral hemorrhage

AIMS

Succinate, a metabolite in the tricarboxylic acid cycle, is increasingly recognized to play essential roles in inflammation by functioning either as an intracellular or extracellular signaling molecule. However, the role and mechanisms of succinate in inflammation remain elusive. Here, we investigated the mechanism underlying the effects of succinate on neuroinflammation in intracerebral hemorrhage (ICH) models.

RESULTS

We unexpectedly found that succinate robustly inhibited neuroinflammation and conferred protection following ICH. Mechanistically, oxidation of succinate by succinate dehydrogenase (SDH) drove reverse electron transport (RET) at mitochondrial complex I, leading to mitochondrial superoxide production in microglia. Complex I-derived superoxide, in turn, activated uncoupling protein 2 (UCP2). By using mice with specific deletion of UCP2 in microglia/macrophage, we showed that UCP2 was needed for succinate to inhibit neuroinflammation, confer protection, and activate downstream AMP-activated protein kinase (AMPK) following ICH. Moreover, knockdown of SDH, complex I or AMPK abolished the therapeutic effects of succinate following ICH.

INNOVATION AND CONCLUSION

We provide evidence that driving complex I RET to activate UCP2 is a novel mechanism of succinate intracellular signaling and a mechanism underlying the inhibition of neuroinflammation by succinate.

KEY WORDS

succinate; uncoupling protein 2; microglia; neuroinflammation; intracerebral hemorrhage.

Itaconate as a key player in cardiovascular immunometabolism

Cardiovascular diseases (CVDs) are the leading cause of death globally, resulting in a major health burden. Thus, an urgent need exists for exploring effective therapeutic targets to block progression of CVDs and improve patient prognoses. Immune and inflammatory responses are involved in the development of atherosclerosis, ischemic myocardial damage responses and repair, calcification, and stenosis of the aortic valve. These responses can involve both large and small blood vessels throughout the body, leading to increased blood pressure and end-organ damage. While exploring potential avenues for therapeutic intervention in CVDs, researchers have begun to focus on immune metabolism, where metabolic changes that occur in immune cells in response to exogenous or endogenous stimuli can influence immune cell effector responses and local immune signaling. Itaconate, an intermediate metabolite of the tricarboxylic acid (TCA) cycle, is related to pathophysiological processes, including cellular metabolism, oxidative stress, and inflammatory immune responses. The expression of immune response gene 1 (IRG1) is upregulated in activated macrophages, and this gene encodes an enzyme that catalyzes the production of itaconate from the TCA cycle intermediate, cis-aconitate. Itaconate and its derivatives have exerted cardioprotective effects through immune modulation in various disease models, such as ischemic heart disease, valvular heart disease, vascular disease, heart transplantation, and chemotherapy drug-induced cardiotoxicity, implying their therapeutic potential in CVDs. In this review, we delve into the associated signaling pathways through which itaconate exerts immunomodulatory effects, summarize its specific roles in CVDs, and explore emerging immunological therapeutic strategies for managing CVDs.

Cellular succinate metabolism and signaling in inflammation: implications for therapeutic intervention

Succinate, traditionally viewed as a mere intermediate of the tricarboxylic acid (TCA) cycle, has emerged as a critical mediator in inflammation. Disruptions within the TCA cycle lead to an accumulation of succinate in the mitochondrial matrix. This excess succinate subsequently diffuses into the cytosol and is released into the extracellular space. Elevated cytosolic succinate levels stabilize hypoxia-inducible factor-1α by inhibiting prolyl hydroxylases, which enhances inflammatory responses. Notably, succinate also acts extracellularly as a signaling molecule by engaging succinate receptor 1 on immune cells, thus modulating their pro-inflammatory or anti-inflammatory activities. Alterations in succinate levels have been associated with various inflammatory disorders, including rheumatoid arthritis, inflammatory bowel disease, obesity, and atherosclerosis. These associations are primarily due to exaggerated immune cell responses. Given its central role in inflammation, targeting succinate pathways offers promising therapeutic avenues for these diseases. This paper provides an extensive review of succinate's involvement in inflammatory processes and highlights potential targets for future research and therapeutic possibilities development.

Pulmonary succinate receptor 1 elevation in high-fat diet mice exacerbates lipopolysaccharides-induced acute lung injury via sensing succinate

BACKGROUND

Individuals with obesity have higher level of circulating succinate, which acts as a signaling factor that initiates inflammation. It is obscure whether succinate and succinate receptor 1 (SUCNR1) are involved in the process of obesity aggravating acute lung injury (ALI).

METHODS

The lung tissue and blood samples from patients with obesity who underwent lung wedgectomy or segmental resection were collected. Six-week-old male C57BL/6J mice were fed a high-fat diet for 12 weeks to induce obesity and lipopolysaccharides (LPS) were injected intratracheally (100 μg, 1 mg/ml) for 24 h to establish an ALI model. The pulmonary SUCNR1 expression and succinate level were measured. Exogenous succinate was supplemented to assess whether succinate exacerbated the LPS-induced lung injury. We next examined the cellular localization of pulmonary SUCNR1. Furthermore, the role of the succinate-SUCNR1 pathway in LPS-induced inflammatory responses in MH-s macrophages and obese mice was investigated.

RESULT

The pulmonary SUCNR1 expression and serum succinate level were significantly increased in patients with obesity and in HFD mice. Exogenous succinate supplementation significantly increased the severity of ALI and inflammatory response. SUCNR1 was mainly expressed on lung macrophages. In LPS-stimulated MH-s cells, knockdown of SUCNR1 expression significantly inhibited pro-inflammatory cytokines' expression, the increase of hypoxia-inducible factor-1α (HIF-1α) expression, inhibitory κB-α (IκB-α) phosphorylation, p65 phosphorylation and p65 translocation to nucleus. In obese mice, SUCNR1 inhibition significantly alleviated LPS-induced lung injury and decreased the HIF-1α expression and IκB-α phosphorylation.

CONCLUSION

The high expression of pulmonary SUCNR1 and serum succinate accumulation at least partly participate in the process of obesity aggravating LPS-induced lung injury.

Real-time OXPHOS capacity analysis in wounded skin from diabetic mice: A pilot study

INTRODUCTION

Diabetes mellitus (DM) impairs wound healing. The aim was to determine whether DM influences mitochondrial respiration in wounded skin (WS) and non-wounded skin (NWS), in a pre-clinical wound healing model of streptozotocin (STZ)-induced diabetes.

METHODS

Six weeks after diabetes induction, two wounds were created in the back of C57BL/J6 mice. Using high-resolution respirometry (HRR), oxygen flux was measured, in WS and NWS, using two substrate-uncoupler-inhibitor titration protocols, at baseline (day 0), day 3 and 10 post-wounding, in STZ-DM and non-diabetic (NDM) mice. Flux control ratios for the oxidative phosphorylation (OXPHOS) capacity were calculated.

RESULTS

A significant increase in mitochondrial respiration was observed in STZ-DM skin compared to control skin at baseline. The OXPHOS capacity was decreased in WS under diabetes at day 3 post-wounding (inflammation phase). However, at day 10 post-wounding (remodeling phase), the OXPHOS capacity was higher in WS from STZ-DM compared to NDM mice, and compared to NWS from STZ-DM mice. A significant relative contribution of pyruvate, malate and glutamate (PMG) oxidation to the OXPHOS capacity was observed in WS compared to NWS from STZ-DM mice, at day 10, while the relative contribution of fatty acid oxidation to the OXPHOS capacity was higher in NWS. The OXPHOS capacity is altered in WS from STZ-DM compared to NDM mice across the healing process, and so is the substrate contribution in WS and NWS from STZ-DM mice, at each time point.

CONCLUSION

HRR may be a sensitive tool to evaluate the underlying mechanisms of tissue repair during wound healing.

Methylmalonic acid in aging and disease

Metabolic byproducts have conventionally been disregarded as waste products without functions. In this opinion article, we bring to light the multifaceted role of methylmalonic acid (MMA), a byproduct of the propionate metabolism pathway mostly commonly known as a clinical biomarker of vitamin B12 deficiency. MMA is normally present at low levels in the body, but increased levels can come from different sources, such as vitamin B12 deficiency, genetic mutations in enzymes related to the propionate pathway, the gut microbiota, and aggressive cancers. Here, we describe the diverse metabolic and signaling functions of MMA and discuss the consequences of increased MMA levels, such as during the aging process, for several age-related human pathologies.

Mechanisms of Gills Response to Cadmium Exposure in Greenfin Horse-Faced Filefish (Thamnaconus septentrionalis): Oxidative Stress, Immune Response, and Energy Metabolism

Cadmium (Cd) pollution has become a global issue due to industrial and agricultural developments. However, the molecular mechanism of Cd-induced detrimental effects and relevant signal transduction/metabolic networks are largely unknown in marine fishes. Here, greenfin horse-faced filefish (Thamnaconus septentrionalis) were exposed to 5.0 mg/L Cd up to 7 days. We applied both biochemical methods and multi-omics techniques to investigate how the gills respond to Cd exposure. Our findings revealed that Cd exposure caused the formation of reactive oxygen species (ROS), which in turn activated the MAPK and apoptotic pathways to alleviate oxidative stress and cell damage. Glycolysis, protein degradation, as well as fatty acid metabolism might assist to meet the requirements of nutrition and energy under Cd stress. We also found that long-term (7 days, "long-term" means compared to 12 and 48 h) Cd exposure caused the accumulation of succinate, which would in turn trigger an inflammatory response and start an immunological process. Moreover, ferroptosis might induce inflammation. Overall, Cd exposure caused oxidative stress, energy metabolism disturbance, and immune response in greenfin horse-faced filefish. Our conclusions can be used as references for safety risk assessment of Cd to marine economic fishes.

Succinate metabolism: a promising therapeutic target for inflammation, ischemia/reperfusion injury and cancer

Succinate serves as an essential circulating metabolite within the tricarboxylic acid (TCA) cycle and functions as a substrate for succinate dehydrogenase (SDH), thereby contributing to energy production in fundamental mitochondrial metabolic pathways. Aberrant changes in succinate concentrations have been associated with pathological states, including chronic inflammation, ischemia/reperfusion (IR) injury, and cancer, resulting from the exaggerated response of specific immune cells, thereby rendering it a central area of investigation. Recent studies have elucidated the pivotal involvement of succinate and SDH in immunity beyond metabolic processes, particularly in the context of cancer. Current scientific endeavors are concentrated on comprehending the functional repercussions of metabolic modifications, specifically pertaining to succinate and SDH, in immune cells operating within a hypoxic milieu. The efficacy of targeting succinate and SDH alterations to manipulate immune cell functions in hypoxia-related diseases have been demonstrated. Consequently, a comprehensive understanding of succinate's role in metabolism and the regulation of SDH is crucial for effectively targeting succinate and SDH as therapeutic interventions to influence the progression of specific diseases. This review provides a succinct overview of the latest advancements in comprehending the emerging functions of succinate and SDH in metabolic processes. Furthermore, it explores the involvement of succinate, an intermediary of the TCA cycle, in chronic inflammation, IR injury, and cancer, with particular emphasis on the mechanisms underlying succinate accumulation. This review critically assesses the potential of modulating succinate accumulation and metabolism within the hypoxic milieu as a means to combat various diseases. It explores potential targets for therapeutic interventions by focusing on succinate metabolism and the regulation of SDH in hypoxia-related disorders.

Environmental and behavioral regulation of HIF-mitochondria crosstalk

Reduced oxygen availability (hypoxia) can lead to cell and organ damage. Therefore, aerobic species depend on efficient mechanisms to counteract detrimental consequences of hypoxia. Hypoxia inducible factors (HIFs) and mitochondria are integral components of the cellular response to hypoxia and coordinate both distinct and highly intertwined adaptations. This leads to reduced dependence on oxygen, improved oxygen supply, maintained energy provision by metabolic remodeling and tapping into alternative pathways and increased resilience to hypoxic injuries. On one hand, many pathologies are associated with hypoxia and hypoxia can drive disease progression, for example in many cancer and neurological diseases. But on the other hand, controlled induction of hypoxia responses via HIFs and mitochondria can elicit profound health benefits and increase resilience. To tackle pathological hypoxia conditions or to apply health-promoting hypoxia exposures efficiently, cellular and systemic responses to hypoxia need to be well understood. Here we first summarize the well-established link between HIFs and mitochondria in orchestrating hypoxia-induced adaptations and then outline major environmental and behavioral modulators of their interaction that remain poorly understood.

Extracellular Succinate: A Physiological Messenger and a Pathological Trigger

When tissues are under physiological stresses, such as vigorous exercise and cold exposure, skeletal muscle cells secrete succinate into the extracellular space for adaptation and survival. By contrast, environmental toxins and injurious agents induce cellular secretion of succinate to damage tissues, trigger inflammation, and induce tissue fibrosis. Extracellular succinate induces cellular changes and tissue adaptation or damage by ligating cell surface succinate receptor-1 (SUCNR-1) and activating downstream signaling pathways and transcriptional programs. Since SUCNR-1 mediates not only pathological processes but also physiological functions, targeting it for drug development is hampered by incomplete knowledge about the characteristics of its physiological vs. pathological actions. This review summarizes the current status of extracellular succinate in health and disease and discusses the underlying mechanisms and therapeutic implications.

Protective effects of the succinate/SUCNR1 axis on damaged hepatocytes in NAFLD

OBJECTIVE

Succinate and succinate receptor 1 (SUCNR1) are linked to fibrotic remodeling in models of non-alcoholic fatty liver disease (NAFLD), but whether they have roles beyond the activation of hepatic stellate cells remains unexplored. We investigated the succinate/SUCNR1 axis in the context of NAFLD specifically in hepatocytes.

METHODS

We studied the phenotype of wild-type and Sucnr1^-/- mice fed a choline-deficient high-fat diet to induce non-alcoholic steatohepatitis (NASH), and explored the function of SUCNR1 in murine primary hepatocytes and human HepG2 cells treated with palmitic acid. Lastly, plasma succinate and hepatic SUCNR1 expression were analyzed in four independent cohorts of patients in different NAFLD stages.

RESULTS

Sucnr1 was upregulated in murine liver and primary hepatocytes in response to diet-induced NASH. Sucnr1 deficiency provoked both beneficial (reduced fibrosis and endoplasmic reticulum stress) and detrimental (exacerbated steatosis and inflammation and reduced glycogen content) effects in the liver, and disrupted glucose homeostasis. Studies in vitro revealed that hepatocyte injury increased Sucnr1 expression, which when activated improved lipid and glycogen homeostasis in damaged hepatocytes. In humans, SUCNR1 expression was a good determinant of NAFLD progression to advanced stages. In a population at risk of NAFLD, circulating succinate was elevated in patients with a fatty liver index (FLI) ≥60. Indeed, succinate had good predictive value for steatosis diagnosed by FLI, and improved the prediction of moderate/severe steatosis through biopsy when added to an FLI algorithm.

CONCLUSIONS

We identify hepatocytes as target cells of extracellular succinate during NAFLD progression and uncover a hitherto unknown function for SUCNR1 as a regulator of hepatocyte glucose and lipid metabolism. Our clinical data highlight the potential of succinate and hepatic SUCNR1 expression as markers to diagnose fatty liver and NASH, respectively.

Succinate at the Crossroad of Metabolism and Angiogenesis: Roles of SDH, HIF1α and SUCNR1

Angiogenesis is an essential process by which new blood vessels develop from existing ones. While adequate angiogenesis is a physiological process during, for example, tissue repair, insufficient and excessive angiogenesis stands on the pathological side. Fine balance between pro- and anti-angiogenic factors in the tissue environment regulates angiogenesis. Identification of these factors and how they function is a pressing topic to develop angiogenesis-targeted therapeutics. During the last decade, exciting data highlighted non-metabolic functions of intermediates of the mitochondrial Krebs cycle including succinate. Among these functions is the contribution of succinate to angiogenesis in various contexts and through different mechanisms. As the concept of targeting metabolism to treat a wide range of diseases is rising, in this review we summarize the mechanisms by which succinate regulates angiogenesis in normal and pathological settings. Gaining a comprehensive insight into how this metabolite functions as an angiogenic signal will provide a useful approach to understand diseases with aberrant or excessive angiogenic background, and may provide strategies to tackle them.

Metabolite G-protein coupled receptor signaling: Potential regulation of eicosanoids

Eicosanoids are a family of bioactive compounds derived from arachidonic acid (AA) that play pivotal roles in physiology and disease, including inflammatory conditions of multiple organ systems. The biosynthesis of eicosanoids requires a series of catalytic steps that are controlled by designated enzymes, which can be regulated by inflammatory and stress signals via transcriptional and translational mechanisms. In the past decades, evidence have emerged indicating that G-protein coupled receptors (GPCRs) can sense extracellular metabolites, and regulate inflammatory responses including eicosanoid production. This review focuses on the recent advances of metabolite GPCRs research, their role in regulation of eicosanoid biosynthesis, and the link to pathophysiological conditions.

Tumor cells dictate anti-tumor immune responses by altering pyruvate utilization and succinate signaling in CD8+ T cells

The tumor microenvironment (TME) is a unique metabolic niche that can inhibit T cell metabolism and cytotoxicity. To dissect the metabolic interplay between tumors and T cells, we establish an in vitro system that recapitulates the metabolic niche of the TME and allows us to define cell-specific metabolism. We identify tumor-derived lactate as an inhibitor of CD8+ T cell cytotoxicity, revealing an unexpected metabolic shunt in the TCA cycle. Metabolically fit cytotoxic T cells shunt succinate out of the TCA cycle to promote autocrine signaling via the succinate receptor (SUCNR1). Cytotoxic T cells are reliant on pyruvate carboxylase (PC) to replenish TCA cycle intermediates. By contrast, lactate reduces PC-mediated anaplerosis. The inhibition of pyruvate dehydrogenase (PDH) is sufficient to restore PC activity, succinate secretion, and the activation of SUCNR1. These studies identify PDH as a potential drug target to allow CD8+ T cells to retain cytotoxicity and overcome a lactate-enriched TME.

Nutritional and metabolic signalling through GPCRs

Deregulated metabolism is a well-known feature of several challenging diseases, including diabetes, obesity and cancer. Besides their important role as intracellular bioenergetic molecules, dietary nutrients and metabolic intermediates are released in the extracellular environment. As such, they may achieve unconventional roles as hormone-like molecules by activating cell-surface G-protein-coupled receptors (GPCRs) that regulate several pathophysiological processes. In this review, we provide an insight into the role of lactate, succinate, fatty acids, amino acids, ketogenesis-derived and β-oxidation-derived intermediates as extracellular signalling molecules. Moreover, the mechanisms by which their cognate metabolite-sensing GPCRs integrate nutritional and metabolic signals with specific intracellular pathways will be described. A better comprehension of these aspects is of fundamental importance to identify GPCRs as novel druggable targets.

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.

Targeting oncometabolism to maximize immunotherapy in malignant brain tumors

Brain tumors result in significant morbidity and mortality in both children and adults. Recent data indicates that immunotherapies may offer a survival benefit after standard of care has failed for malignant brain tumors. Modest results from several late phase clinical trials, however, underscore the need for more refined, comprehensive strategies that incorporate new mechanistic and pharmacologic knowledge. Recently, oncometabolism has emerged as an adjunct modality for combinatorial treatment approaches necessitated by the aggressive, refractory nature of high-grade glioma and other progressive malignant brain tumors. Manipulation of metabolic processes in cancer and immune cells that comprise the tumor microenvironment through controlled targeting of oncogenic pathways may be utilized to maximize the efficacy of immunotherapy and improve patient outcomes. Herein, we summarize preclinical and early phase clinical trial research of oncometabolism-based therapeutics that may augment immunotherapy by exploiting the biochemical and genetic underpinnings of brain tumors. We also examine metabolic pathways related to immune cells that target tumor cells, termed ‘tumor immunometabolism’. Specifically, we focus on glycolysis and altered glucose metabolism, including glucose transporters, hexokinase, pyruvate dehydrogenase, and lactate dehydrogenase, glutamine, and we discuss targeting arginase, adenosine, and indoleamine 2,3-dioxygenase (IDO), and toll-like receptors. Lastly, we summarize future directions targeting metabolism in combination with emerging therapies such as oncolytic virotherapy, vaccines, and chimeric antigen receptor T cells.

SUCNR1 Mediates the Priming Step of the Inflammasome in Intestinal Epithelial Cells: Relevance in Ulcerative Colitis

Intestinal epithelial cells (IECs) constitute a defensive physical barrier in mucosal tissues and their disruption is involved in the etiopathogenesis of several inflammatory pathologies, such as Ulcerative Colitis (UC). Recently, the succinate receptor SUCNR1 was associated with the activation of inflammatory pathways in several cell types, but little is known about its role in IECs. We aimed to analyze the role of SUCNR1 in the inflammasome priming and its relevance in UC. Inflammatory and inflammasome markers and SUCNR1 were analyzed in HT29 cells treated with succinate and/or an inflammatory cocktail and transfected with SUCNR1 siRNA in a murine DSS model, and in intestinal resections from 15 UC and non-IBD patients. Results showed that this receptor mediated the inflammasome, priming both in vitro in HT29 cells and in vivo in a murine chronic DSS-colitis model. Moreover, SUNCR1 was also found to be involved in the activation of the inflammatory pathways NFкB and ERK pathways, even in basal conditions, since the transient knock-down of this receptor significantly reduced the constitutive levels of pERK-1/2 and pNFкB and impaired LPS-induced inflammation. Finally, UC patients showed a significant increase in the expression of SUCNR1 and several inflammasome components which correlated positively and significantly. Therefore, our results demonstrated a role for SUCNR1 in basal and stimulated inflammatory pathways in intestinal epithelial cells and suggested a pivotal role for this receptor in inflammasome activation in UC.

Dietary succinate supplementation to maternal mice improves fetal brown adipose tissue development and thermogenesis of female offspring

Succinic acid widely exists in foods and is used as a food additive. Succinate not only serves as an energy substrate, but also induces protein succinylation. Histone succinylation activates gene transcription. The brown adipose tissue (BAT) is critical for prevention of obesity and metabolic dysfunction, and the fetal stage is pivotal for BAT development. Up to now, the role of maternal succinate supplementation on fetal BAT development and offspring BAT function remains unexamined. To test, female C57BL/6J mice (2-month-old) were separated into 2 groups, received with or without 0.5% succinic acid in drinking water during gestation and lactation. After weaning, female offspring were challenged with high fat diet (HFD) for 12 weeks. Newborn, female weanling, and HFD female offspring mice were analyzed. For neonatal and weaning mice, the BAT weight relative to the whole body weight was significantly increased in the succinate group. The expression of PGC-1α, a key transcription co-activator promoting mitochondrial biogenesis, was elevated in BAT of female neonatal and offspring born to succinate-fed dams. Consistently, maternal succinate supplementation enhanced thermogenesis and the expression of thermogenic genes in offspring BAT. Additionally, maternal succinate supplementation protected female offspring against HFD-induced obesity. Furthermore, in C3H10T1/2 cells, succinate supplementation promoted PGC-1α expression and brown adipogenesis. Mechanistically, succinate supplementation increased permissive histone succinylation and H3K4me3 modification in the Ppargc1a promoter, which correlated with the higher expression of Ppargc1a. In conclusion, maternal succinate supplementation during pregnancy and lactation enhanced fetal BAT development and offspring BAT thermogenesis, which prevented HFD-induced obesity and metabolism dysfunction in offspring.

Metabolite G-Protein Coupled Receptors in Cardio-Metabolic Diseases

G protein-coupled receptors (GPCRs) have originally been described as a family of receptors activated by hormones, neurotransmitters, and other mediators. However, in recent years GPCRs have shown to bind endogenous metabolites, which serve functions other than as signaling mediators. These receptors respond to fatty acids, mono- and disaccharides, amino acids, or various intermediates and products of metabolism, including ketone bodies, lactate, succinate, or bile acids. Given that many of these metabolic processes are dysregulated under pathological conditions, including diabetes, dyslipidemia, and obesity, receptors of endogenous metabolites have also been recognized as potential drug targets to prevent and/or treat metabolic and cardiovascular diseases. This review describes G protein-coupled receptors activated by endogenous metabolites and summarizes their physiological, pathophysiological, and potential pharmacological roles.

The role of neural stem cells in regulating glial scar formation and repair

Glial scars are a common pathological occurrence in a variety of central nervous system (CNS) diseases and injuries. They are caused after severe damage and consist of reactive glia that form a barrier around the damaged tissue that leads to a non-permissive microenvironment which prevents proper endogenous regeneration. While there are a number of therapies that are able to address some components of disease, there are none that provide regenerative properties. Within the past decade, neural stem cells (NSCs) have been heavily studied due to their potent anti-inflammatory and reparative capabilities in disease and injury. Exogenously applied NSCs have been found to aid in glial scar healing by reducing inflammation and providing cell replacement. However, endogenous NSCs have also been found to contribute to the reactive environment by different means. Further understanding how NSCs can be leveraged to aid in the resolution of the glial scar is imperative in the use of these cells as regenerative therapies. To do so, humanised 3D model systems have been developed to study the development and maintenance of the glial scar. Herein, we explore the current work on endogenous and exogenous NSCs in the glial scar as well as the novel 3D stem cell–based technologies being used to model this pathology in a dish.

Integrative Analysis of Gut Microbiota and Fecal Metabolites in Rats after Prednisone Treatment

Prednisone (PRED) is a synthetic glucocorticoid (GC) widely used in immune-mediated diseases for its immunosuppressive and anti-inflammatory properties. The effects of GC are achieved by genomic and nongenomic mechanisms. However, the nongenomic effects are largely unknown. Thus, we aimed to investigate how long-term prednisone therapy changes the composition of the gut microbiota and fecal metabolites in rats. Male Sprague-Dawley rats were randomly assigned to a control (CON) group and a PRED group, which received prednisone treatment daily for 6 weeks by gavage. The V3 to V4 regions of bacterial 16S rRNA genes were amplified and sequenced after the total bacterial DNA was extracted from fecal samples. The alpha and beta diversities were calculated. The compositional alteration of the gut microbiota at different taxonomic levels was analyzed using the Metastats method. Meanwhile, the fecal metabolites were quantitated in an ultra-performance liquid chromatography system. Similar microbial richness and diversity between the CON and PRED groups were indicated by the alpha diversity results. The gut microbial communities differed significantly between two groups. The relative abundances of the genera Eisenbergiella, Alistipes, and Clostridium XIVb decreased, whereas that of Anaerobacterium increased significantly in rats after the 6-week prednisone treatment. In total, 11 downregulated and 10 upregulated fecal metabolites were identified. Differential fecal metabolites were enriched in the pathways, including phenylalanine metabolism, butanoate metabolism, and propanoate metabolism. The lowered production of short-chain fatty acids was associated with the decreased relative abundance of the genera Alistipes and Clostridium XIVb and increased abundance of the genus Anaerobacterium. The composition of the gut microbiota and fecal metabolites was changed after long-term prednisone treatment. This may help us to understand the pharmacology of prednisone. IMPORTANCE Prednisone is widely used in chronic glomerular diseases, immunological disorders, and rheumatic diseases for its anti-inflammatory and immunosuppressive properties. It is a synthetic glucocorticoid (GC) that shows therapeutic effects after conversion to prednisolone by the liver. Prolonged GC therapy causes anti-inflammatory effects; it also results in a variety of adverse events, including obesity, hypertension, psychiatric symptoms, and dyslipidemia. The therapeutic effects and adverse events of GCs may be associated with changes in the gut microbiota, as the host might be affected by the metabolites generated by the altered gut microbes. Thus, we investigated how long-term prednisone therapy changed the composition of the gut microbiota and fecal metabolites in rats. This study may shed new light on the pharmacology of prednisone.

Untargeted metabolomics identifies succinate as a biomarker and therapeutic target in aortic aneurysm and dissection

AIMS

Aortic aneurysm and dissection (AAD) are high-risk cardiovascular diseases with no effective cure. Macrophages play an important role in the development of AAD. As succinate triggers inflammatory changes in macrophages, we investigated the significance of succinate in the pathogenesis of AAD and its clinical relevance.

METHODS AND RESULTS

We used untargeted metabolomics and mass spectrometry to determine plasma succinate concentrations in 40 and 1665 individuals of the discovery and validation cohorts, respectively. Three different murine AAD models were used to determine the role of succinate in AAD development. We further examined the role of oxoglutarate dehydrogenase (OGDH) and its transcription factor cyclic adenosine monophosphate-responsive element-binding protein 1 (CREB) in the context of macrophage-mediated inflammation and established p38αMKOApoe-/- mice. Succinate was the most upregulated metabolite in the discovery cohort; this was confirmed in the validation cohort. Plasma succinate concentrations were higher in patients with AAD compared with those in healthy controls, patients with acute myocardial infarction (AMI), and patients with pulmonary embolism (PE). Moreover, succinate administration aggravated angiotensin II-induced AAD and vascular inflammation in mice. In contrast, knockdown of OGDH reduced the expression of inflammatory factors in macrophages. The conditional deletion of p38α decreased CREB phosphorylation, OGDH expression, and succinate concentrations. Conditional deletion of p38α in macrophages reduced angiotensin II-induced AAD.

CONCLUSION

Plasma succinate concentrations allow to distinguish patients with AAD from both healthy controls and patients with AMI or PE. Succinate concentrations are regulated by the p38α-CREB-OGDH axis in macrophages.

Rethinking succinate: an unexpected hormone-like metabolite in energy homeostasis

There has been an explosion of interest in the signaling capacity of energy metabolites. A prime example is the Krebs cycle substrate succinate, an archetypal respiratory substrate with functions beyond energy production as an intracellular and extracellular signaling molecule. Long associated with inflammation, emerging evidence supports a key role for succinate in metabolic processes relating to energy management. As the natural ligand for SUCNR1, a G protein-coupled receptor, succinate is akin to hormones and likely functions as a reporter of metabolism and stress. In this review, we reconcile new and old observations to outline a regulatory role for succinate in metabolic homeostasis. We highlight the importance of the succinate-SUCNR1 axis in metabolic diseases as an integrator of macrophage immune response, and we discuss new metabolic functions recently ascribed to succinate in specific tissues. Because circulating succinate has emerged as a promising biomarker in chronic metabolic diseases, a better understanding of the physiopathological role of the succinate-SUCNR1 axis in metabolism might open new avenues for clinical use in patients with obesity or diabetes.

Metabolic Control of Smoldering Neuroinflammation

Compelling evidence exists that patients with chronic neurological conditions, which includes progressive multiple sclerosis, display pathological changes in neural metabolism and mitochondrial function. However, it is unknown if a similar degree of metabolic dysfunction occurs also in non-neural cells in the central nervous system. Specifically, it remains to be clarified (i) the full extent of metabolic changes in tissue-resident microglia and infiltrating macrophages after prolonged neuroinflammation (e.g., at the level of chronic active lesions), and (ii) whether these alterations underlie a unique pathogenic phenotype that is amenable for therapeutic targeting. Herein, we discuss how cell metabolism and mitochondrial function govern the function of chronic active microglia and macrophages brain infiltrates and identify new metabolic targets for therapeutic approaches aimed at reducing smoldering neuroinflammation.

Succinate Mediates Tumorigenic Effects via Succinate Receptor 1: Potential for New Targeted Treatment Strategies in Succinate Dehydrogenase Deficient Paragangliomas

Paragangliomas and pheochromocytomas (PPGLs) are chromaffin tumors associated with severe catecholamine-induced morbidities. Surgical removal is often curative. However, complete resection may not be an option for patients with succinate dehydrogenase subunit A-D (SDHx) mutations. SDHx mutations are associated with a high risk for multiple recurrent, and metastatic PPGLs. Treatment options in these cases are limited and prognosis is dismal once metastases are present. Identification of new therapeutic targets and candidate drugs is thus urgently needed. Previously, we showed elevated expression of succinate receptor 1 (SUCNR1) in SDHB PPGLs and SDHD head and neck paragangliomas. Its ligand succinate has been reported to accumulate due to SDHx mutations. We thus hypothesize that autocrine stimulation of SUCNR1 plays a role in the pathogenesis of SDHx mutation-derived PPGLs. We confirmed elevated SUCNR1 expression in SDHx PPGLs and after SDHB knockout in progenitor cells derived from a human pheochromocytoma (hPheo1). Succinate significantly increased viability of SUCNR1-transfected PC12 and ERK pathway signaling compared to control cells. Candidate SUCNR1 inhibitors successfully reversed proliferative effects of succinate. Our data reveal an unrecognized oncometabolic function of succinate in SDHx PPGLs, providing a growth advantage via SUCNR1.