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

Succinate Regulates Exercise-Induced Muscle Remodelling by Boosting Satellite Cell Differentiation Through Succinate Receptor 1

BACKGROUND

Skeletal muscle remodelling can cause clinically important changes in muscle phenotypes. Satellite cells (SCs) myogenic potential underlies the maintenance of muscle plasticity. Accumulating evidence shows the importance of succinate in muscle metabolism and function. However, whether succinate can affect SC function and subsequently coordinate muscle remodelling to exercise remains unexplored.

METHODS

A mouse model of high-intensity interval training (HIIT) was used to investigate the effects of succinate on muscle remodelling and SC function by exercise capacity test and biochemical methods. Mice with succinate receptor 1 (SUCNR1)-specific knockout in SCs were generated as an in vivo model to explore the underlying mechanisms. RNA sequencing of isolated SCs was performed to identify molecular changes responding to succinate-SUCNR1 signalling. The effects of identified key molecules on the myogenic capacity of SCs were investigated using gain- and loss-of-function assays in vitro. To support the translational application, the clinical efficacy of succinate was explored in muscle-wasting mice.

RESULTS

After 21 days of HIIT, mice supplemented with 1.5% succinate exhibited striking gains in grip strength (+0.38 ± 0.04 vs. 0.26 ± 0.03 N, p < 0.001) and endurance (+276.70 ± 55.80 vs. 201.70 ± 45.31 s, p < 0.05), accompanied by enhanced muscle hypertrophy and neuromuscular junction regeneration (p < 0.001). The myogenic capacity of SCs was significantly increased in gastrocnemius muscle of mice supplemented with 1% and 1.5% succinate (+16.48% vs. control, p = 0.008; +47.25% vs. control, p < 0.001, respectively). SUCNR1-specific deletion in SCs abolished the modulatory influence of succinate on muscle adaptation in response to exercise, revealing that SCs respond to succinate-SUCNR1 signalling, thereby facilitating muscle remodelling. SUCNR1 signalling markedly upregulated genes associated with stem cell differentiation and phosphorylation pathways within SCs, of which p38α mitogen-activated protein kinase (MAPK; fold change = 6.7, p < 0.001) and protein kinase C eta (PKCη; fold change = 12.5, p < 0.001) expressions were the most enriched, respectively. Mechanistically, succinate enhanced the myogenic capacity of isolated SCs by activating the SUCNR1-PKCη-p38α MAPK pathway. Finally, succinate promoted SC differentiation (1.5-fold, p < 0.001), ameliorating dexamethasone-induced muscle atrophy in mice (p < 0.001).

CONCLUSIONS

Our findings reveal a novel function of succinate in enhancing SC myogenic capacity via SUCNR1, leading to enhanced muscle adaptation in response to exercise. These findings provide new insights for developing pharmacological strategies to overcome muscle atrophy-related diseases.

Phenyllactic acid modulates the gut microbiota, enhances intestinal health, and alleviates physical frailty in aging mice

Phenyllactic acid (PLA) is a natural antibiotic-like compound derived from certain foods and probiotics. PLA levels have been associated with age-related sarcopenia and provide benefits to metabolic health when derived from probiotics. However, the specific regulatory effects of PLA in aging remain largely unexplored. In this study, aging mice were administered PLA via gavage, followed by fecal 16S rRNA sequencing, measurements of targeted metabolites, glucose metabolism monitoring, and physical performance assessments. Our results indicate that PLA administration significantly altered gut microbiota composition, increased the abundance of short-chain fatty acids (SCFAs) and succinate producing microbiota, and enhanced gut integrity in aging mice. Furthermore, PLA treatment raised fasting blood glucose levels and improved physical activity. Mechanistically, PLA intake elevated the levels of circulating SCFAs and succinate, promoting glycogen metabolic homeostasis and maintaining skeletal muscle oxidative capacity. This study provides evidence that PLA modulates the gut microbiota in aging mice, supports intestinal health, promotes glucose homeostasis, and enhances physical activity.

Gut Bifidobacterium pseudocatenulatum protects against fat deposition by enhancing secondary bile acid biosynthesis

Gut microbiome is crucial for lipid metabolism in humans and animals. However, how specific gut microbiota and their associated metabolites impact fat deposition remains unclear. In this study, we demonstrated that the colonic microbiome of lean and obese pigs differentially contributes to fat deposition, as evidenced by colonic microbiota transplantation experiments. Notably, the higher abundance of Bifidobacterium pseudocatenulatum was significantly associated with lower backfat thickness in lean pigs. Microbial-derived lithocholic acid (LCA) species were also significantly enriched in lean pigs and positively correlated with the abundance of B. pseudocatenulatum. In a high-fat diet (HFD)-fed mice model, administration of live B. pseudocatenulatum decreased fat deposition and enhances colonic secondary bile acid biosynthesis. Importantly, pharmacological inhibition of the bile salt hydrolase (BSH), which mediates secondary bile acid biosynthesis, impaired the anti-fat deposition effect of B. pseudocatenulatum in antibiotic-pretreated, HFD-fed mice. Furthermore, dietary LCA also decreased fat deposition in HFD-fed rats and obese pig models. These findings provide mechanistic insights into the anti-fat deposition role of B. pseudocatenulatum and identify BSH as a potential target for preventing excessive fat deposition in humans and animals.

Proceedings of the 5th Meeting of Translational Hepatology, organized by the Spanish Association for the Study of the Liver (AEEH)

This is the summary report of the 5th Translational Hepatology Meeting, endorsed by the Spanish Association for the Study of the Liver (AEEH) and held in Seville, Spain, in October 2023. The meeting aimed to provide an update on the latest advances in the field of basic and translational hepatology, covering different molecular, cellular, and pathophysiological aspects of the most relevant clinical challenges in liver pathologies. This includes the identification of novel biomarkers and diagnostic tools, the understanding of the relevance of immune response and inflammation in liver diseases, the characterization of current medical approaches to reverse liver diseases, the incorporation of novel molecular insights through omics techniques, or the characterization of the impact of toxic and metabolic insults, as well as other organ crosstalk, in liver pathophysiology.

The Warburg Effect: Is it Always an Enemy?

The Warburg effect, also known as 'aerobic' glycolysis, describes the preference of cancer cells to favor glycolysis over oxidative phosphorylation for energy (adenosine triphosphate-ATP) production, despite having high amounts of oxygen and fully active mitochondria, a phenomenon first identified by Otto Warburg. This metabolic pathway is traditionally viewed as a hallmark of cancer, supporting rapid growth and proliferation by supplying energy and biosynthetic precursors. However, emerging research indicates that the Warburg effect is not just a strategy for cancer cells to proliferate at higher rates compared to normal cells; thus, it should not be considered an 'enemy' since it also plays complex roles in normal cellular functions and/or under stress conditions, prompting a reconsideration of its purely detrimental characterization. Moreover, this review highlights that distinguishing glycolysis as 'aerobic' and 'anaerobic' should not exist, as lactate is likely the final product of glycolysis, regardless of the presence of oxygen. Finally, this review explores the nuanced contributions of the Warburg effect beyond oncology, including its regulatory roles in various cellular environments and the potential effects on systemic physiological processes. By expanding our understanding of these mechanisms, we can uncover novel therapeutic strategies that target metabolic reprogramming, offering new avenues for treating cancer and other diseases characterized by metabolic dysregulation. This comprehensive reevaluation not only challenges traditional views but also enhances our understanding of cellular metabolism's adaptability and its implications in health and disease.

The association of objective daytime sleepiness with impaired glucose metabolism in patients with obstructive sleep apnea: a multi-omics study

STUDY OBJECTIVES

To examine the joint effect of obstructive sleep apnea (OSA) and objective excessive daytime sleepiness (EDS) on glucose metabolism and the underlying mechanisms.

METHODS

We included 127 patients with OSA. The multiple sleep latency test (MSLT) and Epworth sleepiness scale (ESS) were used to assess objective and subjective EDS, respectively. Disordered glucose metabolism was defined as either a physician diagnosis or having fasting blood glucose levels ≥ 5.6 mmol/L. Values of fasting insulin and homeostasis model assessment of insulin resistance (HOMA-IR) higher than the median values of our sample were defined as high fasting insulin and insulin resistance. Serum metabolomics and fecal microbiota were used to explore underlying mechanisms.

RESULTS

Lower MSLT values were associated with higher levels of fasting blood glucose, fasting insulin, and HOMA-IR. Furthermore, objective EDS was associated with increased odds of disordered glucose metabolism, elevated fasting insulin, and insulin resistance. Dysregulation of serum valine degradation and dysbiosis of fecal Bacteroides thetaiotaomicron were associated with impaired glucose metabolism in OSA with objective EDS. No association between subjective EDS and impaired glucose metabolism was observed.

CONCLUSION

OSA with objective, but not subjective, EDS is associated with an increased risk of disordered glucose metabolism and insulin resistance. Dysregulation of valine degradation and dysbiosis of Bacteroides thetaiotaomicron appear to link objective EDS and disordered glucose metabolism in OSA.

Succinic Acid Improves the Metabolism of High-Fat Diet-Induced Mice and Promotes White Adipose Browning

Succinic acid plays a crucial role as an essential intermediate in the mitochondrial tricarboxylic acid cycle in mitochondria. In recent years, growing evidence has supported the the important role of succinic acid in fat metabolism. Therefore, we aimed to investigate the effects of succinic acid on adipose tissue metabolism and insulin sensitivity in high-fat diet (HFD)-induced obese mice and try to explore its potential mechanism. We found that the addition of succinic acid (40 mM) to drinking water inhibited the hypertrophy of inguinal white adipose tissue (iWAT) in HFD-induced mice. Furthermore, succinic acid supplementation enhanced insulin sensitivity and improved their glucose tolerance in obese mice. Interestingly, succinic acid supplementation improved lipid metabolism in HFD-fed mice, as shown by decreased serum levels of TG, TC, LDL-C, and increased HDL-C. In addition, succinic acid supplementation increased the expression of browning markers and mitochondria-related genes in iWAT. Further studies showed that the addition of succinic acid to drinking water promotes the browning of iWAT by activating the PI3K-AKT/MAPK signaling pathway. These results suggest that succinic acid has the potential to be used as an effective component for dietary intervention and may, therefore, play an important role in ameliorating and preventing obesity and associated metabolic diseases caused by HFD.

Neuregulin 4 Downregulation Alters Mitochondrial Morphology and Induces Oxidative Stress in 3T3-L1 Adipocytes

Neuregulin 4 (Nrg4) is an adipokine that belongs to the epidermal growth factor family and binds to ErbB4 tyrosine kinase receptors. In 3T3-L1 adipocytes, the downregulation of Nrg4 expression enhances inflammation and autophagy, resulting in insulin resistance. Here, we searched for the causes of this phenotype. Nrg4 knockdown (Nrg4 KD) adipocytes showed a significant reduction in mitochondrial content and elongation, along with a lower content of the mitochondria fusion protein mitofusin 2 (MFN2), and increased H2O2 production compared to the control scrambled cells (Scr). The antioxidant N-acetylcysteine reversed the oxidative stress and reduced the gene expression of the pro-inflammatory cytokine tumor necrosis factor α (TNFα). Nrg4 KD adipocytes showed enhanced lipolysis and reduced lipogenesis, in addition to a significant reduction in several intermediates of the Krebs cycle. In summary, Nrg4 downregulation in adipocytes affects mitochondrial content and functioning, causing impaired cellular metabolism, which in turn results in oxidative stress, inflammation, and insulin resistance.

Succinate Activates Uncoupling Protein 2 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, the 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 superoxides, in turn, activated uncoupling protein 2 (UCP2). By using mice with specific deletion of UCP2 in microglia/macrophages, we showed that UCP2 was needed for succinate to inhibit neuroinflammation, confer protection, and activate downstream 5'-adenosine monophosphate-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-mediated intracellular signaling and a mechanism underlying the inhibition of neuroinflammation by succinate.

A New Target for Hepatic Fibrosis Prevention and Treatment: The Warburg Effect

Hepatic fibrosis is a major public health problem that endangers human wellbeing. In recent years, a number of studies have revealed the important impact of metabolic reprogramming on the occurrence and development of hepatic fibrosis. Among them, the Warburg effect, as an intracellular glucose metabolism reprogramming, can promote the occurrence and development of hepatic fibrosis by promoting the activation of hepatic stellate cells (HSCs) and inducing the polarization of liver macrophages (KC). Understanding the Warburg effect and its important role in the progression of hepatic fibrosis will assist in developing new strategies for the prevention and treatment of hepatic fibrosis. This review focuses on the Warburg effect and the specific mechanism by which it affects the progression of hepatic fibrosis by regulating HSCs activation and KC polarization. In addition, we also summarize and discuss the related experimental drugs and their mechanisms that inhibit the Warburg effect by targeting key proteins of glycolysis in order to improve hepatic fibrosis in the hope of providing more effective strategies for the clinical treatment of hepatic fibrosis.

Associations between TCA cycle plasma metabolites and fatigue in black females with systemic lupus erythematosus: An untargeted metabolomics pilot study

OBJECTIVE

In this pilot study, we used untargeted metabolomics to identify biochemical mechanisms or biomarkers potentially underlying SLE-related fatigue.

METHODS

Metabolon conducted untargeted metabolomic plasma profiling using ultrahigh performance liquid chromatography/tandem mass spectrometry on plasma samples of 23 Black females with systemic lupus erythematosus (SLE) and 21 no SLE controls. Fatigue phenotypes of general fatigue, physical fatigue, mental fatigue, reduced activity, and reduced motivation were measured with the reliable and valid Multidimensional Fatigue Inventory (MFI).

RESULTS

A total of 290 metabolites were significantly different between the SLE and no SLE groups, encompassing metabolites related to glycolysis, TCA cycle activity, heme catabolism, branched chain amino acids, fatty acid metabolism, and steroids. Within the SLE group, controlling for age and co-morbidities, TCA cycle metabolites of alpha-ketoglutarate (AKG) and succinate were statistically significantly associated (p < .05) with physical and general fatigue.

CONCLUSION

While pervasive perturbations in the entire TCA cycle have been implicated as a potential mechanism for fatigue, our results suggest individual metabolites of AKG and succinate may be potential biomarkers or targets of intervention for fatigue symptom management in SLE. Additionally, perturbations in heme metabolism in the SLE group provide additional insights into mechanisms that promote systemic inflammation.

Gut microbe-derived metabolites and the risk of cardiovascular disease in the METSIM cohort

Background

An association between gut microbes and cardiovascular disease (CVD) has been established, but the underlying mechanisms remain largely unknown.

Methods

We conducted a secondary analysis of the cross-sectional data obtained from the Metabolic Syndrome in Men (METSIM) population-based cohort of 10,194 Finnish men (age = 57.65 ± 7.12 years). We tested the levels of circulating gut microbe-derived metabolites as predictors of CVD, ischemic cerebrovascular accident (CVA), and myocardial infarction (MI). The Kaplan-Meier method was used to estimate the time from the participants' first outpatient clinic visit to the occurrence of adverse outcomes. The associations between metabolite levels and the outcomes were assessed using Cox proportional hazard models.

Results

During a median follow-up period of 200 months, 979 participants experienced CVD, 397 experienced CVA, and 548 experienced MI. After adjusting for traditional risk factors and correcting for multiple comparisons, higher plasma levels of succinate [quartile 4 vs. quartile 1; adjusted hazard ratio, aHR = 1.30, (confidence interval (CI), 1.10-1.53) p = 0.0003, adjusted p = 0.01] were significantly associated with the risk of CVD. High plasma levels of ursodeoxycholic acid (UDCA) (quartile 3 vs. quartile 1); [aHR = 1.68, (CI, 1.26-2.2); p = 0.0003, adj. p = 0.01] were associated with a higher risk of CVA. Furthermore, as a continuous variable, succinate was associated with a 10% decrease in the risk of CVD [aHR = 0.9; (CI, 0.84-0.97); p = 0.008] and a 15% decrease in the risk of MI [aHR = 0.85, (CI, 0.77-0.93); p = 0.0007].

Conclusion

Gut microbe-derived metabolites, succinate, and ursodeoxycholic acid were associated with CVD, MI, and CVA, respectively. Regulating the gut microbes may represent a potential therapeutic target for modulating CVD and CVA.

Structural insights into ligand recognition and activation of the succinate receptor SUCNR1

Succinate, a citric acid cycle intermediate, serves important functions in energy homeostasis and metabolic regulation. Extracellular succinate acts as a stress signal through succinate receptor (SUCNR1), a class A G protein-coupled receptor. Research on succinate signaling is hampered by the lack of high-resolution structures of the agonist-bound receptor. We present cryoelectron microscopy (cryo-EM) structures of SUCNR1-Gi complexes bound to succinate and its non-metabolite derivative cis-epoxysuccinate. Key determinants for the recognition of succinate in cis conformation include R2817.39 and Y832.64, while Y301.39 and R993.29 participate in the binding of both succinate and cis-epoxysuccinate. Extracellular loop 2, through F175ECL2 in its β-hairpin, forms a hydrogen bond with succinate and caps the binding pocket. At the receptor-Gi interface, agonist binding induces the rearrangement of a hydrophobic network on transmembrane (TM)5 and TM6, leading to TM signaling through TM3 and TM7. These findings extend our understanding of succinate recognition by SUCNR1, aiding the development of therapeutics for the succinate receptor.

Metabolic profiling of peri-implant crevicular fluid in peri-implantitis

OBJECTS

This study aims to explore the etiology of peri-implantitis by comparing the metabolic profiles in peri-implant crevicular fluid (PICF) from patients with healthy implants (PH) and those with peri-implantitis (PI).

MATERIALS AND METHODS

Fifty-six patients were enrolled in this cross-sectional study. PICF samples were collected and analyzed using both non-targeted and targeted metabolomics approaches. The relationship between metabolites and clinical indices including probing depth (PD), bleeding on probing (BOP), and marginal bone loss (MBL) was examined. Additionally, submucosal microbiota was collected and analyzed using 16S rRNA gene sequencing to elucidate the association between the metabolites and microbial communities.

RESULTS

Significant differences in metabolic profiles were observed between the PH and PI groups, with 179 distinct metabolites identified. In the PI group, specific amino acids and fatty acids were significantly elevated compared to the PH group. Organic acids including succinic acid, fructose-6-phosphate, and glucose-6-phosphate were markedly higher in the PI group, showing positive correlations with mean PD, BOP, and MBL. Metabolites that increased in the PI group positively correlated with the presence of Porphyromonas and Treponema and negatively with Streptococcus and Haemophilus.

CONCLUSIONS

This study establishes a clear association between metabolic compositions and peri-implant condition, highlighting enhanced metabolite activity in peri-implantitis. These findings open avenues for further research into metabolic mechanisms of peri-implantitis and their potential therapeutic implications.

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.

SUCNR1 regulates insulin secretion and glucose elevates the succinate response in people with prediabetes

Pancreatic β-cell dysfunction is a key feature of type 2 diabetes, and novel regulators of insulin secretion are desirable. Here we report that the succinate receptor (SUCNR1) is expressed in β-cells and is up-regulated in hyperglycemic states in mice and humans. We found that succinate acts as a hormone-like metabolite and stimulates insulin secretion via a SUCNR1-Gq-PKC-dependent mechanism in human β-cells. Mice with β-cell-specific Sucnr1 deficiency exhibit impaired glucose tolerance and insulin secretion on a high-fat diet, indicating that SUCNR1 is essential for preserving insulin secretion in diet-induced insulin resistance. Patients with impaired glucose tolerance show an enhanced nutritional-related succinate response, which correlates with the potentiation of insulin secretion during intravenous glucose administration. These data demonstrate that the succinate/SUCNR1 axis is activated by high glucose and identify a GPCR-mediated amplifying pathway for insulin secretion relevant to the hyperinsulinemia of prediabetic states.

A novel SUCNR1 inhibitor alleviates dysbiosis through inhibition of host responses without direct interaction with host microbiota

Type 2 diabetes (T2D) is a chronic metabolic disorder in which insulin resistance and impaired insulin secretion result in altered metabolite balance, specifically elevated levels of circulating glucose and succinate, which increases the risk of many pathologies, including periodontitis. Succinate, a tricarboxylic acid (TCA) cycle intermediate, can be produced and metabolized by both host cells and host microbiota, where elevated levels serve as an inflammation and pathogen threat signal through activating the succinate G protein-coupled receptor, SUCNR1. Modulating succinate-induced SUCNR1 signaling remains a promising therapeutic approach for pathologies resulting in elevated levels of succinate, such as T2D and periodontitis. Here, we demonstrate hyperglycemia and elevated intracellular succinate in a T2D mouse model and determine gut microbiome composition. Drawing on previous work demonstrating the ability of a novel SUCNR1 antagonist, compound 7a, to block inflammation and alleviate dysbiosis in a mouse model, we examined if compound 7a has an impact on the growth and virulence gene expression of bacterial and fungal human microbiota in vitro, and if 7a could reduce bone loss in a periodontitis-induced mouse model. T2D mice harbored a significantly different gut microbiome, suggesting the altered metabolite profile of T2D causes shifts in host-microbial community structure, with enrichment in succinate producers and consumers and mucin-degrading bacteria. Bacterial and fungal cultures showed that 7a did not influence growth or virulence gene expression, suggesting the therapeutic effects of 7a are a direct result of 7a interacting with host cells and that alterations in microbial community structure are driven by reduced host SUCNR1 signaling. This work further suggests that targeting SUCNR1 signaling is a promising therapeutic approach in metabolic, inflammatory, or immune disorders with elevated succinate levels.

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.

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.

Colon-targeted piperine-glycyrrhizic acid nanocrystals for ulcerative colitis synergetic therapy via macrophage polarization

Ulcerative colitis (UC) is a chronic inflammatory disease that affects the gastrointestinal tract and is characterized by immune dysregulation. Oral administration of nanoformulations containing immunomodulators is a desirable approach to treating UC. However, low drug-loading (<10%, typically), premature drug release, and systemic absorption of these nanoformulations continue to be significant challenges restricting clinical applications. Herein, we developed colon-targeted piperine-glycyrrhizic acid nanocrystals (ES100-PIP/GA NCs) to treat UC through the regulation of macrophages. The ES100-PIP/GA NCs exhibited ultra-high drug loading and colon-specific drug release. In vitro studies demonstrated that the ES100-PIP/GA NCs could effectively be internalized by lipopolysaccharide (LPS)-induced RAW 264.7 and Caco-2 cells. More importantly, the ES100-PIP/GA NCs could downregulate pro-inflammatory factors (IL-1β, IL-17A), upregulate anti-inflammatory factors (TGF-β1), and repair the intestinal mucosal barrier. In a murine model of acute colitis induced by dextran sodium sulfate (DSS), ES100-PIP/GA NCs could protect PIP and GA from gastric acid destruction, reach the colon, and significantly inhibit colitis. Surprisingly, ES100-PIP/GA NCs enhance M2 macrophages by increasing the mammalian target of rapamycin (mTOR), and inhibit M1 macrophages by reducing hypoxia-inducible factor-1α (HIF-1α). Overall, this study shows that ES100-PIP/GA NCs have synergistic immunotherapy capabilities with macrophage regulation, which offers a promising blueprint for the oral delivery of multicomponent drugs in UC therapy.

Extracellular succinate derived from ectopic milieu drives adhesion and implantation growth of ectopic endometrial stromal cells via the SUCNR1 signal in endometriosis

BACKGROUND

As a dual-function metabolite, succinate has emerged in cell function and plays a key signaling role in linking mitochondrial function to other cellular functions. Succinate accumulation in the cytoplasm is commonly associated with hypoxia in the microenvironment and immune cell activation. Extracellular succinate released into the microenvironment is considered an inflammatory alarm that can be sensed by its membrane receptor SUCNR1, which boosts proinflammatory responses and acts akin to classical hormones and cytokines. Succinate plays an important role in the development of inflammatory diseases. Whether succinate facilitates the progression of endometriosis (EMs), characterized by chronic inflammation and peritoneal adhesion, is worth exploring.

OBJECTIVE

We mimicked the ectopic milieu in vitro and in vivo to evaluate the main source and potential role of succinate in endometriosis. We assessed the molecular and functional effects of succinate on macrophages and peritoneal mesothelial cells in peritoneal cavity. The effect of succinate/SUCNR1 signaling on ectopic endometrial stromal cells (ESCs) was further explored in this study.

METHODS

In this study, we used targeted organic acid metabolomics analysis and in vitro assays to assess the potential accumulation of succinate in the peritoneal fluid of EMs patients. We examined its correlation with disease severity, Visual Analogue Scale, and the Endometriosis Fertility Index. Flow cytometry, enzyme linked immunosorbent assay, western blot assay, quantitative real-time PCR, and other molecular biology techniques were used to explore the potential mechanisms.

RESULTS

By mimicking the ectopic milieu, we constructed an in vitro co-culture system and found that M1 polarized macrophages and that the peritoneal mesothelial cell line (HMrSV5) mainly released succinate into their microenvironment and activated the succinate receptor (SUCNR1) signal, which further polarized the macrophages and significantly enhanced the invasive survival of ESCs, and the adhesion to the peritoneum. We further investigated the pathological effects of extracellular succinate in vivo using a xenograft mouse models of endometriosis.

CONCLUSIONS

Succinate-SUCNR1 signaling facilitates the creation of inflammatory cells and plays a vital role in EMs progression and peritoneal adhesion. Our work on the molecular mechanisms underlying succinate accumulation and function will help elucidate the phenotypic mysteries of pain and infertility in EMs. Video Abstract.

Succinate Enhances Lipolysis and Decreases Adipocytes Size in Both Subcutaneous and Visceral Adipose Tissue from High-Fat-Diet-Fed Obese Mice

Obesity is a risk factor for many chronic diseases related to the overexpansion of adipose tissue during obesity, leading to metabolic dysfunction and ectopic lipids. Previous studies reported a close relationship between succinate and obesity and its co-morbidities, and studies have also reported on its anti-obesity potential. To confirm its efficacy in obesity interventions, we supplemented mice with obesity induced by a high-fat diet with succinate (1.5% m/v in drinking water) for 11 weeks without changing the diet. After succinate supplementation, the changes in body weight, adipose tissue deposition, glucose tolerance, energy expenditure and lipid metabolism were evaluated. It was found that succinate supplementation significantly decreased subcutaneous adipose tissue (HFD: 4239.3 ± 211.2 mg; HFD-SA: 3268.9 ± 265.7 mg. p < 0.05), triglyceride contents (decreased by 1.53 mmol/g and 0.39 mmol/g in eWAT and ingWAT, respectively, p < 0.05) and NEFA (decreased by 1.41 μmol/g and 1.31 μmol/g in eWAT and ingWAT, respectively, p < 0.05). The adipocytes' sizes all significantly decreased in both subcutaneous and visceral adipose tissue (the proportion of adipocytes with diameters larger than 100 μm in eWAT and ingWAT decreased by 16.83% and 11.96%, respectively. p < 0.05). Succinate significantly enhanced lipolysis in adipose tissue (eWAT: Adrb3, Hsl and Plin1; ingWAT: Hsl and CPT1a; p < 0.05), whereas the expression of lipogenesis-related genes remained unchanged (p > 0.05). Succinate supplementation also enhanced the activity of BAT by stimulating the expression of Ucp1 and Cidea (p < 0.05). Our results reported that succinate has a potential beneficial effect on obesity pathogenesis but cannot efficiently decrease bodyweight.

The hepatic GABAergic system promotes liver macrophage M2 polarization and mediates HBV replication in mice

Macrophages display functional phenotypic plasticity. Hepatitis B virus (HBV) infection induces polarizations of liver macrophages either to M1-like pro-inflammatory phenotype or to M2-like anti-inflammatory phenotype. Gamma-aminobutyric acid (GABA) signaling exists in various non-neuronal cells including hepatocytes and some immune cells. Here we report that macrophages express functional GABAergic signaling components and activation of type A GABA receptors (GABAARs) promotes M2-polarization thus advancing HBV replication. Notably, intraperitoneal injection of GABA or the GABAAR agonist muscimol increased HBV replication in HBV-carrier mice that were generated by hydrodynamical injection of adeno-associated virus/HBV1.2 plasmids (pAAV/HBV1.2). The GABA-augmented HBV replication in HBV-carrier mice was significantly reduced by the GABAAR inhibitor picrotoxin although picrotoxin had no significant effect on serum HBsAg levels in control HBV-carrier mice. Depletion of liver macrophages by liposomal clodronate treatment also significantly reduced the GABA-augmented HBV replication. Yet adoptive transfer of liver macrophages isolated from GABA-treated donor HBV-carrier mice into the liposomal clodronate-pretreated recipient HBV-carrier mice restored HBV replication. Moreover, GABA or muscimol treatment increased the expression of "M2" cytokines in macrophages, but had no direct effect on HBV replication in the HepG2.2.15 cells, HBV1.3-transfected Huh7, HepG2, or HepaRG cells, or HBV-infected Huh7-NTCP cells. Taken together, these results suggest that increasing GABA signaling in the liver promotes HBV replication in HBV-carrier mice by suppressing the immunity of liver macrophages, but not by increasing the susceptibility of hepatocytes to HBV infection. Our study shows that a previously unknown GABAergic system in liver macrophage has an essential role in HBV replication.

Succinate signaling attenuates high-fat diet-induced metabolic disturbance and intestinal barrier dysfunction

Succinate is a vital signaling metabolite produced by the host and gut microbiota. Succinate has been shown to regulate host metabolic homeostasis and inhibit obesity-associated inflammation in macrophages by engaging its cognate receptor, SUCNR1. However, the contribution of the succinate-SUCNR1 axis to intestinal barrier dysfunction in obesity remains unclear. In the present study, we explored the effects of succinate-SUCNR1 signaling on high-fat diet (HFD)-induced intestinal barrier dysfunction. Using a SUCNR1-deficient mouse model under HFD feeding conditions, we identified the effects of succinate-SUCNR1 axis on obesity-associated intestinal barrier impairment. Our results showed that HFD administration decreased goblet cell numbers and mucus production, promoted intestinal pro-inflammatory responses, induced gut microbiota composition imbalance, increased intestinal permeability, and caused mucosal barrier dysfunction. Dietary succinate supplementation was sufficient to activate a type 2 immune response, trigger the differentiation of barrier-promoting goblet cells, suppress intestinal inflammation, restore HFD-induced mucosal barrier impairment and intestinal dysbiosis, and eventually exert anti-obesity effects. However, SUNNR1-deficient mice failed to improve the intestinal barrier function and metabolic phenotype in HFD mice. Our data indicate the protective role of the succinate-SUCNR1 axis in HFD-induced intestinal barrier dysfunction.

Metabolism disturbance by light/dark cycle switching depends on the rat health status: the role of grape seed flavanols

Changes in light/dark cycles and obesogenic diets are related to the disruption of circadian rhythms and metabolic disorders. Grape seed flavanols have shown beneficial effects on metabolic diseases and, recently, a circadian system modulation has been suggested to mediate their health-enhancing properties. Therefore, the aim of this study was to evaluate the grape seed (poly)phenol extract (GSPE) effects in healthy and obese rats after a light/dark cycle disruption. Forty-eight rats were fed a standard (STD) or cafeteria (CAF) diet for 6 weeks under STD conditions of a light/dark cycle (12 h light per day, L12). Then, animals were switched to a long (18 h light per day, L18) or short (6 h light per day, L6) photoperiod and administered a vehicle (VH) or GSPE (25 mg kg-1) for 1 week. The results showed changes in serum lipids and insulin and metabolomic profiles dependent on the photoperiod and animal health status. GSPE administration improved serum parameters and increased the Nampt gene expression in CAF rats and modified the metabolomic profile in a photoperiod-dependent manner. Metabolic effects of light/dark disturbance depend on the health status of the rats, with diet-induced CAF-induced obese rats being more affected. Grape seed flavanols improve the metabolic status in a photoperiod-dependent manner and their effects on the circadian system suggest that part of their metabolic effects could be mediated by their action on biological rhythms.

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 pretreatment attenuates intestinal ischemia-reperfusion injury by inhibiting necroptosis and inflammation via upregulating Klf4

Intestinal ischemia-reperfusion (I/R) injury is a common pathophysiological process in various diseases, and the disruption of the intestinal barrier composed of tight junction proteins is the initiating factor, which then leads to a large number of bacteria and endotoxins in the intestine into the bloodstream causing stress and distant organ damage. The release of inflammatory mediators and abnormal programmed death of intestinal epithelial cells are important factors of intestinal barrier damage. Succinate is an intermediate product of the tricarboxylic acid cycle with anti-inflammatory and pro-angiogenic activities, but its role in the maintenance of intestinal barrier homeostasis after I/R has not been fully elucidated. In this study, we explored the effect of succinate on intestinal ischemia-reperfusion injury and the possible mechanism of its role by flow cytometry, western blotting, real-time quantitative PCR and immunostaining. The results of pretreatment with succinate in the mouse intestinal I/R model and IEC-6 cells hypoxia-reoxygenation (H/R) model revealed a reduction in tissue damage, necroptosis and associated inflammation due to ischemia-reperfusion. Furthermore, it was found that the protective effect of succinate pretreatment may be associated with the transcriptional upregulation of the inflammatory protein KLF4 and the protective effect of intestinal barrier of succinate was diminished after inhibition of KLF4. Thus, our results suggest that succinate can exert a protective effect in intestinal ischemia-reperfusion injury through upregulation of KLF4 and also demonstrate the potential therapeutic value of succinate pretreatment in acute I/R injury of the intestine.

Chronic Vitamin E Deficiency Dysregulates Purine, Phospholipid, and Amino Acid Metabolism in Aging Zebrafish Skeletal Muscle

Muscle wasting occurs with aging and may be a result of oxidative stress damage and potentially inadequate protection by lipophilic antioxidants, such as vitamin E. Previous studies have shown muscular abnormalities and behavioral defects in vitamin E-deficient adult zebrafish. To test the hypothesis that there is an interaction between muscle degeneration caused by aging and oxidative damage caused by vitamin E deficiency, we evaluated long-term vitamin E deficiency in the skeletal muscle of aging zebrafish using metabolomics. Zebrafish (55 days old) were fed E+ and E- diets for 12 or 18 months. Then, skeletal muscle samples were analyzed using UPLC-MS/MS. Data were analyzed to highlight metabolite and pathway changes seen with either aging or vitamin E status or both. We found that aging altered purines, various amino acids, and DHA-containing phospholipids. Vitamin E deficiency at 18 months was associated with changes in amino acid metabolism, specifically tryptophan pathways, systemic changes in the regulation of purine metabolism, and DHA-containing phospholipids. In sum, while both aging and induced vitamin E deficiency did have some overlap in altered and potentially dysregulated metabolic pathways, each factor also presented unique alterations, which require further study with more confirmatory approaches.

SUCNR1 signaling in adipocytes controls energy metabolism by modulating circadian clock and leptin expression

Adipose tissue modulates energy homeostasis by secreting leptin, but little is known about the factors governing leptin production. We show that succinate, long perceived as a mediator of immune response and lipolysis, controls leptin expression via its receptor SUCNR1. Adipocyte-specific deletion of Sucnr1 influences metabolic health according to nutritional status. Adipocyte Sucnr1 deficiency impairs leptin response to feeding, whereas oral succinate mimics nutrient-related leptin dynamics via SUCNR1. SUCNR1 activation controls leptin expression via the circadian clock in an AMPK/JNK-C/EBPα-dependent manner. Although the anti-lipolytic role of SUCNR1 prevails in obesity, its function as a regulator of leptin signaling contributes to the metabolically favorable phenotype in adipocyte-specific Sucnr1 knockout mice under standard dietary conditions. Obesity-associated hyperleptinemia in humans is linked to SUCNR1 overexpression in adipocytes, which emerges as the major predictor of adipose tissue leptin expression. Our study establishes the succinate/SUCNR1 axis as a metabolite-sensing pathway mediating nutrient-related leptin dynamics to control whole-body homeostasis.

Transcriptomic Analysis of Diethylstilbestrol in Daphnia Magna: Energy Metabolism and Growth Inhibition

With the widespread use of diethylstilbestrol (DES), it has become a common contaminant in the aquatic environment. It is toxic to a wide range of aquatic organisms, disrupting the water flea growth and further interfering with several ecosystem services. Nevertheless, the molecular mechanism of DES in water fleas is still unexplicit. In this study, the 21-day chronic test showed that a negative effect of growth and reproduction can be observed with DES exposure. Subsequently applied transcriptomic analysis illustrated the molecular mechanism in mode freshwater invertebrate Daphnia magna (D. magna) exposed to 2, 200, and 1000 μg·L^-1 of DES for 9 days. Meanwhile, exposure to DES at 200 and 1000 μg·L^-1 significantly restrains the growth (body length) and reproduction (first spawning time) of D. magna. Identified differentially expressed genes (DEGs) are majorly enriched relative to energy metabolism, lipid metabolism, the digestive system, transport and catabolism pathways which were remarkably changed. These repressed and up-regulated pathways, in relation to energy synthesis and metabolism, may be the reasons for the reduced body length and delayed first spawning time. Taken together, this study revealed that DES is a threat to D. magna in the aquatic environment and clarifies the molecular mechanism of the toxicity.

Succinate level is increased and succinate dehydrogenase exerts forward and reverse catalytic activities in lipopolysaccharides-stimulated cardiac tissue: The protective role of dimethyl malonate

This study aimed to investigate the alterations of myocardial succinate and fumarate levels with or without succinate dehydrogenase (SDH) inhibitor dimethyl malonate during 24 h of lipopolysaccharides (LPS) challenge, as well as the effects of dimethyl malonate on the impaired cardiac tissue. Myocardial succinate and fumarate levels were increased in the initial 9 h of LPS challenge. During this time, dimethyl malonate increased the succinate level, decreased the fumarate level, aggravated the cardiac dysfunction, reduced the oxidative stress, had little effect on interleukin-1β production, promoted interleukin-10 production and bothered the ATP production. Co-treatment with exogenous succinate significantly increased interleukin-1β production in this period. After 12 h of LPS challenge, myocardial the succinate level increased sharply, while the fumarate level gradually decreased. During 12-24 h of LPS challenge, dimethyl malonate effectively reduced the succinate level, increased the fumarate level, improved cardiac dysfunction, inhibited interleukin-1β production, and had little effect on oxidative stress, interleukin-10 production, and ATP production. LPS challenge also significantly increased the myocardial succinate receptor 1 expression and circulating succinate level. Inhibition of succinate receptor 1 significantly reduced the mRNA expression of interleukin-1β. In conclusion, the current study suggests that myocardial succinate accumulates during LPS challenge, and that SDH activity may be transformed (from forward to reversed) and involved in a line of stress response. Dimethyl malonate inhibits SDH and, depending on the time of treatment, reduces LPS-induced cardiac impairment. Furthermore, accumulated succinate exerts pro-inflammatory effects partly via succinate receptor 1 signaling.

Succinate-SUCNR1 induces renal tubular cell apoptosis

Succinate has long been known to be only an intermediate product of the tricarboxylic acid cycle until identified as a natural ligand for SUCNR1 in 2004. SUCNR1 is widely expressed throughout the body, especially in the kidney. Abnormally elevated succinate is associated with many diseases, including obesity, type 2 diabetes, nonalcoholic fatty liver disease, and ischemia injury, but it is not known whether succinate can cause kidney damage. This study showed that succinate induced apparent renal injury after treatment for 12 wk, characterized by a reduction in 24 h urine and the significant detachment of the brush border of proximal tubular epithelial cells, tubular dilation, cast formation, and vacuolar degeneration of tubular cells in succinate-treated mice. Besides, succinate caused tubular epithelial cell apoptosis in kidneys and HK-2 cells. Mechanistically, succinate triggered cell apoptosis via SUCNR1 activation. In addition, succinate upregulated ERK by binding to SUCNR1, and inhibition of ERK using PD98059 abolished the proapoptotic effects of succinate in HK-2 cells. In summary, our study provides the first evidence that succinate acts as a risk factor and contributes to renal injury, and further research is required to discern the pathological effects of succinate on renal functions.

Impact of combined chronic exposure to low-dose bisphenol A and fructose on serum adipocytokines and the energy target metabolome in white adipose tissue

Background: Adipose tissue is a dynamic endocrine organ that plays a key role in regulating metabolic homeostasis. Previous studies confirmed that bisphenol A (BPA) or fructose can interfere with the function of adipose tissue. Nonetheless, knowledge on how exposure to BPA and fructose impacts energy metabolism in adipose tissue remains limited.Purpose: To determine impact of combined chronic exposure to low-dose bisphenol A and fructose on serum adipocytokines and the energy target metabolome in white adipose tissue.Method: 57 energy metabolic intermediates in adipose tissue and 7 adipocytokines in serum from Sprague Dawley rats were examined after combined exposure to two levels of BPA (lower dose: 0.25, and higher dose: 25 μg/kg every other day) and 5% fructose for 6 months.Results: combined exposure to lower-dose BPA and fructose significantly increased omentin-1, pyruvic acid, adenosine triphosphate (ATP), adenosine monophosphate (AMP), inosine monophosphate (IMP), inosine, and l-lactate; however, these parameters were not significantly affected by higher-dose BPA combined with fructose. Interestingly, the level of succinate (an intermediate of the citric acid cycle) increased dose-dependently in adipose tissue, and the level of apelin 13 (a versatile adipocytokine) decreased dose-dependently in serum after combined exposure to BPA and fructose. Phosphoenolpyruvic acid, phenyl-lactate, and ornithine were significantly correlated with asprosin, omentin-1, apelin, apelin 13, and adiponectin, while l-tyrosine was significantly correlated with irisin and a-FABP under combined exposure to BPA and fructose.Conclusions: these findings indicated that lower-dose BPA combined with fructose could amplify the impact on glycolysis, energy storage, and purine nucleotide biosynthesis in adipose tissue, and adipocytokines, such as omentin-1 and apelin 13, may be related to metabolic interference induced by BPA and fructose exposure.

Metabokines in the regulation of systemic energy metabolism

Metabolism consists of life-sustaining chemical reactions involving metabolites. Historically, metabolites were defined as the intermediates or end products of metabolism and considered to be passive participants changed by metabolic processes. However, recent research has redefined how we view metabolism. There is emerging evidence of metabolites which function to mediate cellular signalling and interorgan crosstalk, regulating local metabolism and systemic physiology. These bioactive metabolite signals have been termed metabokines. Metabokines regulate diverse energy metabolism pathways across multiple tissues, including fatty acid β-oxidation, mitochondrial oxidative phosphorylation, lipolysis, glycolysis and gluconeogenesis. There is increasing impetus to uncover novel metabokine signalling axes to better understand how these may be perturbed in metabolic diseases and determine their utility as therapeutic targets.

Trypanosoma cruzi pathogenicity involves virulence factor expression and upregulation of bioenergetic and biosynthetic pathways

The molecular repertoire of Trypanosoma cruzi effects its virulence and impacts the clinical course of the resulting Chagas disease. This study aimed to determine the mechanism underlying the pathogenicity of T. cruzi. Two T. cruzi cell lines (C8C3hvir and C8C3lvir), obtained from the clone H510 C8C3 and exhibiting different virulence phenotypes, were used to evaluate the parasite's infectivity in mice. The organ parasite load was analysed by qPCR. The proteomes of both T. cruzi cell lines were compared using nLC-MS/MS. Cruzipain (Czp), complement regulatory protein (CRP), trans-sialidase (TS), Tc-85, and sialylated epitope expression levels were evaluated by immunoblotting. High-virulence C8C3hvir was highly infectious in mice and demonstrated three to five times higher infectivity in mouse myocardial cells than low-virulence C8C3lvir. qPCR revealed higher parasite loads in organs of acute as well as chronically C8C3hvir-infected mice than in those of C8C3lvir-infected mice. Comparative quantitative proteomics revealed that 390 of 1547 identified proteins were differentially regulated in C8C3hvir with respect to C8C3lvir. Amongst these, 174 proteins were upregulated in C8C3hvir and 216 were downregulated in C8C3lvir. The upregulated proteins in C8C3hvir were associated with the tricarboxylic acid cycle, ribosomal proteins, and redoxins. Higher levels of Czp, CRP, TS, Tc-85, and sialylated epitopes were expressed in C8C3hvir than in C8C3lvir. Thus, T. cruzi virulence may be related to virulence factor expression as well as upregulation of bioenergetic and biosynthetic pathways proteins.

Artemisia sphaerocephala Krasch polysaccharide promotes adipose thermogenesis and decreases obesity by shaping the gut microbiota

This study was designed to investigate the underlying mechanism of Artemisia sphaerocephala Krasch polysaccharide (ASKP) against obesity. Here, our results showed that ASKP considerably reduced body weight gain and metabolic disorders in high fat diet (HFD)-fed mice. 16S rRNA gene sequencing revealed that ASKP relieved the gut microbiota disorder caused by HFD and promoted the proliferation of probiotics such as Lactobacillus, Bifidobacterium and Blautia. Interestingly, the fecal levels of succinate, a microbial metabolite associated with adipose thermogenesis, were dramatically elevated by ASKP treatment in obese mice. Accordingly, ASKP promoted thermogenesis of brown adipose tissue (BAT) and browning of inguinal white adipose tissue (iWAT) of mice fed with a HFD, as revealed by the elevated expression of thermogenic marker genes (UCP1, CIDEA and PGC1α) in BAT and iWAT. Importantly, antibiotic treatment significantly decreased the ASKP-elevated fecal levels of succinate and further abolished the adipose thermogenesis effects of ASKP. Taken together, our results show that ASKP prevents obesity through iWAT browning and BAT activation, a mechanism that is dependent on the gut microbiota metabolism.

Temperature induces metabolic reprogramming in fish during bacterial infection

Water temperature elevation as a consequence of global warming results in increased incidence of bacterial disease, such as edwardsiellosis, in fish farming. Edwardsiellosis is caused by the bacterial pathogen Edwardsiella tarda and affects many farmed fish including flounder (Paralichthys olivaceus). Currently, the effect of temperature on the metabolic response of flounder to E. tarda infection is unclear. In this study, we found that compared to low temperature (15°C), high temperature (23°C) enhanced E. tarda dissemination in flounder tissues. To examine the impact of temperature on the metabolism of flounder induced by E. tarda, comparative metabolomics were performed, which identified a large number of metabolites responsive to E. tarda invasion and temperature alteration. During E. tarda infection, the metabolic profile induced by elevated temperature was mainly featured by extensively decreased amino acids and TCA intermediates such as succinate, a proven immune regulator. Further, 38 potential metabolite markers of temperature effect (MMTE) in association with bacterial infection were identified. When used as exogenous supplements, two of the MMTE, i.e., L-methionine and UDP-glucose, effectively upregulated the expression of pro-inflammatory cytokines and suppressed E. tarda infection in flounder leukocytes. Taken together, the results of this study indicate an important influence of temperature on the metabolism of flounder during bacterial infection, which eventually affects the survivability of the fish.

Circulating pyruvate is a potent prognostic marker for critical COVID-19 outcomes

Background

Coronavirus-19 (COVID-19) disease is driven by an unchecked immune response to the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) virus which alters host mitochondrial-associated mechanisms. Compromised mitochondrial health results in abnormal reprogramming of glucose metabolism, which can disrupt extracellular signalling. We hypothesized that examining mitochondrial energy-related signalling metabolites implicated in host immune response to SARS-CoV-2 infection would provide potential biomarkers for predicting the risk of severe COVID-19 illness.

Methods

We used a semi-targeted serum metabolomics approach in 273 patients with different severity grades of COVID-19 recruited at the acute phase of the infection to determine the relative abundance of tricarboxylic acid (Krebs) cycle-related metabolites with known extracellular signaling properties (pyruvate, lactate, succinate and α-ketoglutarate). Abundance levels of energy-related metabolites were evaluated in a validation cohort (n=398) using quantitative fluorimetric assays.

Results

Increased levels of four energy-related metabolites (pyruvate, lactate, a-ketoglutarate and succinate) were found in critically ill COVID-19 patients using semi-targeted and targeted approaches (p<0.05). The combined strategy proposed herein enabled us to establish that circulating pyruvate levels (p<0.001) together with body mass index (p=0.025), C-reactive protein (p=0.039), D-Dimer (p<0.001) and creatinine (p=0.043) levels, are independent predictors of critical COVID-19. Furthermore, classification and regression tree (CART) analysis provided a cut-off value of pyruvate in serum (24.54 µM; p<0.001) as an early criterion to accurately classify patients with critical outcomes.

Conclusion

Our findings support the link between COVID-19 pathogenesis and immunometabolic dysregulation, and show that fluorometric quantification of circulating pyruvate is a cost-effective clinical decision support tool to improve patient stratification and prognosis prediction.

Orally administered Odoribacter laneus improves glucose control and inflammatory profile in obese mice by depleting circulating succinate

BACKGROUND

Succinate is produced by both human cells and by gut bacteria and couples metabolism to inflammation as an extracellular signaling transducer. Circulating succinate is elevated in patients with obesity and type 2 diabetes and is linked to numerous complications, yet no studies have specifically addressed the contribution of gut microbiota to systemic succinate or explored the consequences of reducing intestinal succinate levels in this setting.

RESULTS

Using germ-free and microbiota-depleted mouse models, we show that the gut microbiota is a significant source of circulating succinate, which is elevated in obesity. We also show in vivo that therapeutic treatments with selected bacteria diminish the levels of circulating succinate in obese mice. Specifically, we demonstrate that Odoribacter laneus is a promising probiotic based on its ability to deplete succinate and improve glucose tolerance and the inflammatory profile in two independent models of obesity (db/db mice and diet-induced obese mice). Mechanistically, this is partly mediated by the succinate receptor 1. Supporting these preclinical findings, we demonstrate an inverse correlation between plasma and fecal levels of succinate in a cohort of patients with severe obesity. We also show that plasma succinate, which is associated with several components of metabolic syndrome including waist circumference, triglycerides, and uric acid, among others, is a primary determinant of insulin sensitivity evaluated by the euglycemic-hyperinsulinemic clamp.

CONCLUSIONS

Overall, our work uncovers O. laneus as a promising next-generation probiotic to deplete succinate and improve glucose tolerance and obesity-related inflammation. Video Abstract.

Inulin reduces visceral adipose tissue mass and improves glucose tolerance through altering gut metabolites

AIM

Inulin, a soluble dietary fiber, is a source of energy for the host while the metabolites, such as short-chain fatty acids (SCFAs), produced in the gut through bacterial fermentation exerts the anti-obesity effect. In this study, we aimed to apply a metabolomics approach and clarify the role of this soluble dietary fiber on glucose and lipid metabolism under the calorie-matched condition.

MATERIALS AND METHODS

Eight-week-old male C57BL/6J mice were fed a high-fat/high-sucrose based diet containing maltodextrin or inulin for 12 weeks through calorie-matched pair feeding. We evaluated glucose tolerance, and energy expenditure using indirect calorimetry, comprehensive metabolites in the content of jejunum, feces, and portal vein serum using gas chromatography-mass spectrometry, and histological changes in the adipose tissue.

RESULTS

The inulin group exhibited reduced visceral adipose tissue and smaller size of visceral adipocyte. It also exhibited improved glucose tolerance and an increase in energy expenditure. Reflecting the results of fermentation, the metabolomics analysis revealed an increase in the succinic acid and SCFA contents in both feces and portal vein serum in the inulin group.

CONCLUSIONS

Inulin altered the gut metabolites and reduced visceral adipose tissue, thereby resulting in improved glucose tolerance.

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.

Biological responses of Eisenia fetida towards the exposure and metabolism of tris (2-butoxyethyl) phosphate

The toxicity of various organophosphorus flame retardants (OPFRs) is of increasing concern. However, there is still a lack of research on the toxicity of OPFRs to terrestrial invertebrates and its metabolism in vivo. Herein, earthworms (Eisenia fetida) were exposed to soil spiked with 0, 0.05, 0.5, and 5 mg/kg tris(2-butoxyethyl) phosphate (TBOEP, a typical alkyl OPFRs) for 28 d to study the biological responses to the exposure and metabolism of TBOEP. TBOEP exposure inhibited the activity of acetyl-cholinesterase (64.4-68.6% of that in the control group), increased the energy consumption level, and affected calcium-dependent pathways of E. fetida, which caused a 3.6-12.4% reduction in the weight gain rate (developmental toxicity), a 10.6-69.4% reduction in the number of juveniles (reproduction toxicity), and neurotoxicity to E. fetida. The 5 mg/kg TBOEP exposure caused a significant accumulation of malondialdehyde (1.68 times higher than that in the control group) in E. fetida, which indicated that the balance of oxidation and anti-oxidation of E. fetida was broken. Meanwhile, E. fetida maintained the absorption and metabolic abilities to TBOEP under the environmental condition. The removal rate of soil TBOEP was increased by 25.1-35.5% by the presence of E. fetida. Importantly, TBOEP could accumulate in E. fetida (0.09-76.0 μg/kg) and the activation of cytochrome P450 and glutathione detoxification pathway promoted the metabolism of TBOEP in E. fetida. These findings link the biological responses and metabolic behavior of earthworms under pollution stress and provide fundamental data for the environmental risk assessment and pollution removal of OPFRs in soil.

Activation of ectopic olfactory receptor 544 induces GLP-1 secretion and regulates gut inflammation

Olfactory receptors are ectopically expressed in extra-nasal tissues. The gut is constantly exposed to high levels of odorants where ectopic olfactory receptors may play critical roles. Activation of ectopic olfactory receptor 544 (Olfr544) by azelaic acid (AzA), an Olfr544 ligand, reduces adiposity in mice fed a high-fat diet (HFD) by regulating fuel preference to fats. Herein, we investigated the novel function of Olfr544 in the gut. In GLUTag cells, AzA induces the cAMP-PKA-CREB signaling axis and increases the secretion of GLP-1, an enteroendocrine hormone with anti-obesity effects. In mice fed a HFD and orally administered AzA, GLP-1 plasma levels were elevated in mice. The induction of GLP-1 secretion was negated in cells with Olfr544 gene knockdown and in Olfr544-deficient mice. Gut microbiome analysis revealed that AzA increased the levels of Bacteroides acidifaciens and microbiota associated with antioxidant pathways. In fecal metabolomics analysis, the levels of succinate and trehalose, metabolites correlated with a lean phenotype, were elevated by AzA. The function of Olfr544 in gut inflammation, a key feature in obesity, was further investigated. In RNA sequencing analysis, AzA suppressed LPS-induced activation of inflammatory pathways and reduced TNF-α and IL-6 expression, thereby improving intestinal permeability. The effects of AzA on the gut metabolome, microbiome, and colon inflammation were abrogated in Olfr544-KO mice. These results collectively demonstrated that activation of Olfr544 by AzA in the gut exerts multiple effects by regulating GLP-1 secretion, gut microbiome and metabolites, and colonic inflammation in anti-obesogenic phenotypes and, thus, may be applied for obesity therapeutics.

Organokines and Exosomes: Integrators of Adipose Tissue Macrophage Polarization and Recruitment in Obesity

The prevalence of obesity is escalating and has become a worldwide health challenge coinciding with the development of metabolic diseases. Emerging evidence has shown that obesity is accompanied by the infiltration of macrophages into adipose tissue, contributing to a state of low-grade chronic inflammation and dysregulated metabolism. Moreover, in the state of obesity, the phenotype of adipose tissue macrophages switches from the M2 polarized state to the M1 state, thereby contributing to chronic inflammation. Notably, multiple metabolic organs (adipose tissue, gut, skeletal muscle, and the liver) communicate with adipose tissue macrophages via secreting organokines or exosomes. In this review, we systematically summarize how the organokines (adipokines, gut microbiota and its metabolites, gut cytokines, myokines, and hepatokines) and exosomes (adipocyte-, skeletal muscle-, and hepatocyte-derived exosomes) act as important triggers for macrophage recruitment in adipose tissue and adipose tissue macrophage polarization, thus providing further insight into obesity treatment. In addition, we also highlight the complex interaction of organokines with organokines and organokines with exosomes, revealing new paths in understanding adipose tissue macrophage recruitment and polarization.

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.