Short-chain fatty acids activate GPR41 and GPR43 on intestinal epithelial cells to promote inflammatory responses in mice

Exploring the interaction and impact of probiotic and commensal bacteria on vitamins, minerals and short chain fatty acids metabolism

There is increasing evidence that probiotic and commensal bacteria play a role in substrate metabolism, energy harvesting and intestinal homeostasis, and may exert immunomodulatory activities on human health. In addition, recent research suggests that these microorganisms interact with vitamins and minerals, promoting intestinal and metabolic well-being while producing vital microbial metabolites such as short-chain fatty acids (SCFAs). In this regard, there is a flourishing field exploring the intricate dynamics between vitamins, minerals, SCFAs, and commensal/probiotic interactions. In this review, we summarize some of the major hypotheses beyond the mechanisms by which commensals/probiotics impact gut health and their additional effects on the absorption and metabolism of vitamins, minerals, and SCFAs. Our analysis includes comprehensive review of existing evidence from preclinical and clinical studies, with particular focus on the potential interaction between commensals/probiotics and micronutrients. Finally, we highlight knowledge gaps and outline directions for future research in this evolving field.

Molecular mechanisms of gut microbiota in diabetic nephropathy

Diabetic nephropathy is a common complication of diabetes and a considerable contributor to end-stage renal disease. Evidence indicates that glucose dysregulation and lipid metabolism comprise a pivotal pathogenic mechanism in diabetic nephropathy. However, current treatment outcomes are limited, as they only provide symptomatic relief without preventing disease progression. The gut microbiota is a group of microorganisms that inhabit the human intestinal tract and play a crucial role in maintaining host energy balance, metabolism, and immune activity. Patients with diabetic nephropathy exhibit altered gut microbiota, suggesting its potential involvement in the onset and progression of the disease. However, how a perturbed microbiota induces and promotes diabetic nephropathy remains unelucidated. This article summarizes the evidence of the impact of gut microbiota on the progression of diabetic nephropathy, with a particular focus on the molecular mechanisms involved, aiming to provide new insights into the treatment of diabetic nephropathy.

From Gut to Brain: Unraveling the Intricate Link Between Microbiome and Stroke

Stroke, a neurological disorder, is intricately linked to the gut microbiota, influencing microbial composition and elevating the risk of ischemic stroke. The neuroprotective impact of short-chain fatty acids (SCFAs) derived from dietary fiber fermentation contrasts with the neuroinflammatory effects of lipopolysaccharide (LPS) from gut bacteria. The pivotal role of the gut-brain axis, facilitating bidirectional communication between the gut and the brain, is crucial in maintaining gastrointestinal equilibrium and influencing cognitive functions. An in-depth understanding of the interplay among the gut microbiota, immune system, and neurological outcomes in stroke is imperative for devising innovative preventive and therapeutic approaches. Strategies such as dietary adjustments, probiotics, prebiotics, antibiotics, or fecal transplantation offer promise in modulating stroke outcomes. Nevertheless, comprehensive research is essential to unravel the precise mechanisms governing the gut microbiota's involvement in stroke and to establish effective therapeutic interventions. The initiation of large-scale clinical trials is warranted to assess the safety and efficacy of interventions targeting the gut microbiota in stroke management. Tailored strategies that reinstate eubiosis and foster a healthy gut microbiota hold potential for both stroke prevention and treatment. This review underscores the gut microbiota as a promising therapeutic target in stroke and underscores the need for continued research to delineate its precise role and develop microbiome-based interventions effectively.

Butyrate as a potential therapeutic agent for neurodegenerative disorders

Maintaining an optimum microbial community within the gastrointestinal tract is intricately linked to human metabolic, immune and brain health. Disturbance to these microbial populations perturbs the production of vital bioactive compounds synthesised by the gut microbiome, such as short-chain fatty acids (SCFAs). Of the SCFAs, butyrate is known to be a major source of energy for colonocytes and has valuable effects on the maintenance of intestinal epithelium and blood brain barrier integrity, gut motility and transit, anti-inflammatory effects, and autophagy induction. Inducing endogenous butyrate production is likely to be beneficial for gut-brain homeostasis and for optimal neuronal function. For these reasons, butyrate has gained interest as a potential therapy for not only metabolic and immunological disorders, but also conditions related to the brain, including neurodegenerative diseases. While direct and indirect sources of butyrate, including prebiotics, probiotics, butyrate pro-drugs and glucosidase inhibitors, offer a promising therapeutic avenue, their efficacy and dosage in neurodegenerative conditions remain largely unknown. Here, we review current literature on effects of butyrate relevant to neuronal function, the impact of butyrate in a range of neurodegenerative diseases and related treatments that may have potential for the treatment of neurodegenerative diseases.

GPR41 and GPR43: From development to metabolic regulation

G-protein-coupled receptors are a diverse class of cell surface receptors that orchestrate numerous physiological functions. The G-protein-coupled receptors, GPR41 and GPR43, sense short-chain fatty acids (SCFAs), which are metabolites of dietary fermentation by the host's intestinal bacteria. These receptors have gained attention as potential therapeutic targets against various diseases because of their SCFA-mediated beneficial effects on the host's intestinal health. Mounting evidence has associated the activity of these receptors with chronic metabolic diseases, including obesity, diabetes, inflammation, and cardiovascular disease. However, despite intensive research using various strategies, including gene knockout (KO) mouse models, evidence about the precise roles of GPR41 and GPR43 in disease treatment remains inconsistent. Here, we comprehensively review the latest findings from functional studies of the signaling mechanisms that underlie the activities of GPR41 and GPR43, as well as highlight their multifaceted roles in health and disease. We anticipate that this knowledge will guide future research priorities and the development of effective therapeutic interventions.

Revolutionizing lung health: Exploring the latest breakthroughs and future prospects of synbiotic nanostructures in lung diseases

The escalating prevalence of lung diseases underscores the need for innovative therapies. Dysbiosis in human body microbiome has emerged as a significant factor in these diseases, indicating a potential role for synbiotics in restoring microbial equilibrium. However, effective delivery of synbiotics to the target site remains challenging. Here, we aim to explore suitable nanoparticles for encapsulating synbiotics tailored for applications in lung diseases. Nanoencapsulation has emerged as a prominent strategy to address the delivery challenges of synbiotics in this context. Through a comprehensive review, we assess the potential of nanoparticles in facilitating synbiotic delivery and their structural adaptability for this purpose. Our review reveals that nanoparticles such as nanocellulose, starch, and chitosan exhibit high potential for synbiotic encapsulation. These offer flexibility in structure design and synthesis, making them promising candidates for addressing delivery challenges in lung diseases. Furthermore, our analysis highlights that synbiotics, when compared to probiotics alone, demonstrate superior anti-inflammatory, antioxidant, antibacterial and anticancer activities. This review underscores the promising role of nanoparticle-encapsulated synbiotics as a targeted and effective therapeutic approach for lung diseases, contributing valuable insights into the potential of nanomedicine in revolutionizing treatment strategies for respiratory conditions, ultimately paving the way for future advancements in this field.

The Gut Microbiome and Joint Microbiome Show Alterations in Patients with Knee Osteoarthritis Versus Controls: A Systematic Review

PURPOSE

To assess the current scientific literature on the microbiome's relationship with knee osteoarthritis (OA), with specific focuses on the gut microbiome-joint axis and joint microbiome-joint axis.

METHODS

A systematic review was conducted following the Preferred Reporting Items for Systematic Reviews and Meta-analyses guidelines, searching PubMed, Embase, and Cochrane databases for relevant English-language clinical studies on the gut and/or joint microbiomes' association with knee OA in humans. Bias was evaluated using the methodological index for non-randomized studies score.

RESULTS

Thirty-five thousand bacterial species comprise the gut microbiome; approximately 90% are members of the phyla Bacteroides and Firmicutes. Symbiosis between the gut microbiome and host under normal physiological conditions positively affects host growth, development, immunity, and longevity. Gut microbiome imbalance can negatively influence various physiological processes, including immune response, inflammation, metabolism, and joint health including development of knee OA. In addition, next generation gene sequencing suggests the presence of microorganisms in the synovial fluid of osteoarthritic knees, and distinct microbiome profiles detected are presumed to play a role in the development of OA. With regard to the gut microbiome, consistent alterations in microbial composition between OA patients and controls are noted, in addition to several associations between certain gut bacteria with OA-related knee pain, patient-reported outcome measure performance, imaging findings, and changes in metabolic and inflammatory pathways. Regarding the joint microbiome, studies revealed increased levels of lipopolysaccharide (LPS) and LPS-binding protein in synovial fluid are associated with activated macrophages, and correlated with worsened osteophyte severity, joint space narrowing, and pain scores in knee OA patients. In addition, studies demonstrated various microbial composition differences in OA patients compared to control, with certain joint microbes directly associated with OA pathogenesis, inflammation, and metabolic dysregulation.

CONCLUSIONS

The gut microbiome-joint axis and joint microbiome shows alterations in microbial composition between osteoarthritic patients and controls. These alterations are associated with perturbations of metabolic and inflammatory pathways, imaging findings, osteoarthritis-related pain, and patient reported outcome measure performance.

LEVEL OF EVIDENCE

Systematic Review; Level III.

Effects of Saponins on Lipid Metabolism: The Gut-Liver Axis Plays a Key Role

Unhealthy lifestyles (high-fat diet, smoking, alcohol consumption, too little exercise, etc.) in the current society are prone to cause lipid metabolism disorders affecting the health of the organism and inducing the occurrence of diseases. Saponins, as biologically active substances present in plants, have lipid-lowering, inflammation-reducing, and anti-atherosclerotic effects. Saponins are thought to be involved in the regulation of lipid metabolism in the body; it suppresses the appetite and, thus, reduces energy intake by modulating pro-opiomelanocortin/Cocaine amphetamine regulated transcript (POMC/CART) neurons and neuropeptide Y/agouti-related peptide (NPY/AGRP) neurons in the hypothalamus, the appetite control center. Saponins directly activate the AMP-activated protein kinase (AMPK) signaling pathway and related transcriptional regulators such as peroxisome-proliferator-activated-receptors (PPAR), CCAAT/enhancer-binding proteins (C/EBP), and sterol-regulatory element binding proteins (SREBP) increase fatty acid oxidation and inhibit lipid synthesis. It also modulates gut-liver interactions to improve lipid metabolism by regulating gut microbes and their metabolites and derivatives-short-chain fatty acids (SCFAs), bile acids (BAs), trimethylamine (TMA), lipopolysaccharide (LPS), et al. This paper reviews the positive effects of different saponins on lipid metabolism disorders, suggesting that the gut-liver axis plays a crucial role in improving lipid metabolism processes and may be used as a therapeutic target to provide new strategies for treating lipid metabolism disorders.

Gut microbiota as a key regulator of intestinal mucosal immunity

Gut microbiota is a complex microbial community with the ability of maintaining intestinal health. Intestinal homeostasis largely depends on the mucosal immune system to defense external pathogens and promote tissue repair. In recent years, growing evidence revealed the importance of gut microbiota in shaping intestinal mucosal immunity. Therefore, according to the existing findings, this review first provided an overview of intestinal mucosal immune system before summarizing the regulatory roles of gut microbiota in intestinal innate and adaptive immunity. Specifically, this review delved into the gut microbial interactions with the cells such as intestinal epithelial cells (IECs), macrophages, dendritic cells (DCs), neutrophils, and innate lymphoid cells (ILCs) in innate immunity, and T and B lymphocytes in adaptive immunity. Furthermore, this review discussed the main effects of gut microbiota dysbiosis in intestinal diseases and offered future research prospects. The review highlighted the key regulatory roles of gut microbiota in intestinal mucosal immunity via various host-microbe interactions, providing valuable references for the development of microbial therapy in intestinal diseases.

Gut Microbiota-Derived 3-Hydroxybutyrate Blocks GPR43-Mediated IL6 Signaling to Ameliorate Radiation Proctopathy

Radiation proctopathy (RP) is a common complication of radiotherapy for pelvic malignancies with high incidence. RP accompanies by microbial dysbiosis. However, how the gut microbiota affects the disease remains unclear. Here, metabolomics reveals that the fecal and serous concentrations of microbiota-derived 3-hydroxybutyrate (3HB) are significantly reduced in RP mice and radiotherapeutic patients. Moreover, the concentration of 3HB is negatively associated with the expression of proinflammatory IL6 that is increased along with the severity of radiation damage. 3HB treatment significantly downregulates IL6 expression and alleviates IL6-mediated radiation damage. Irradiated cell-fecal microbiota co-culture experiments and in vivo assays show that such a radioprotection of 3HB is mediated by GPR43. Microbiome analysis reveals that radiation leads to a distinct bacterial community compared to untreated controls, in which Akkermansia muciniphila is significantly reduced in RP mice and radiotherapeutic patients and is associated with lower 3HB concentration. Gavage of A. muciniphila significantly increases 3HB concentration, downregulates GPR43 and IL6 expression, and ameliorates radiation damage. Collectively, these results demonstrate that the gut microbiota, including A. muciniphila, induce higher concentrations of 3HB to block GPR43-mediated IL6 signaling, thereby conferring radioprotection. The findings reveal a novel implication of the gut-immune axis in radiation pathophysiology, with potential therapeutic applications.

Microecological regulation in HCC therapy: Gut microbiome enhances ICI treatment

The exploration of the complex mechanisms of cancer immunotherapy is rapidly evolving worldwide, and our focus is on the interaction of hepatocellular carcinoma (HCC) with immune checkpoint inhibitors (ICIs), particularly as it relates to the regulatory role of the gut microbiome. An important basis for the induction of immune responses in HCC is the presence of specific anti-tumor cells that can be activated and reinforced by ICIs, which is why the application of ICIs results in sustained tumor response rates in the majority of HCC patients. However, mechanisms of acquired resistance to immunotherapy in unresectable HCC result in no long-term benefit for some patients. The significant heterogeneity of inter-individual differences in the gut microbiome in response to treatment with ICIs makes it possible to target modulation of specific gut microbes to assist in augmenting checkpoint blockade therapies in HCC. This review focuses on the complex relationship between the gut microbiome, host immunity, and HCC, and emphasizes that manipulating the gut microbiome to improve response rates to cancer ICI therapy is a clinical strategy with unlimited potential.

The emergence of inflammatory microglia during gut inflammation is not affected by FFAR2 expression in intestinal epithelial cells or peripheral myeloid cells

Gut inflammation can trigger neuroinflammation and is linked to mood disorders. Microbiota-derived short-chain fatty acids (SCFAs) can modulate microglia, yet the mechanism remains elusive. Since microglia do not express free-fatty acid receptor (FFAR)2, but intestinal epithelial cells (IEC) and peripheral myeloid cells do, we hypothesized that SCFA-mediated FFAR2 activation within the gut or peripheral myeloid cells may impact microglia inflammation. To test this hypothesis, we developed a tamoxifen-inducible conditional knockout mouse model targeting FFAR2 exclusively on IEC and induced intestinal inflammation with dextran sodium sulfate (DSS), a well-established colitis model. Given FFAR2's high expression in myeloid cells, we also investigated its role by selectively deleting it in these populations of cells. In an initial study, male and female wild-type mice received 0 or 2% DSS for 5d and microglia were isolated 3d later to assess inflammatory status. DSS induced intestinal inflammation and upregulated inflammatory gene expression in microglia, indicating inflammatory signaling via the gut-brain axis. Despite the lack of significant effects of sex in the intestinal phenotype, male mice showed higher microglial inflammatory response than females. Subsequent studies using FFAR2 knockout models revealed that FFAR2 expression in IECs or immune myeloid cells did not affect DSS-induced colonic pathology (i.e. clinical and histological scores and colon length), or colonic expression of inflammatory genes. However, FFAR2 knockout led to an upregulation of several microglial inflammatory genes in control mice and downregulation in DSS-treated mice, suggesting that FFAR2 may constrain neuroinflammatory gene expression under healthy homeostatic conditions but may permit it during intestinal inflammation. No interactions with sex were observed, suggesting sex does not play a role on FFAR2 potential function in gut-brain communication in the context of colitis. To evaluate the role of FFAR2 activated by microbiota-derived SCFAs, we employed the same knockout and DSS models adding fermentable dietary fiber (0 or 2.5% inulin for 8 wks). Despite no genotype or fiber main effects, contrary to our hypothesis, inulin feeding augmented DSS-induced inflammation and signs of colitis, suggesting context-dependent effects of fiber. These findings highlight microglial involvement in colitis-associated neuroinflammation and advance our understanding of FFAR2's role in the gut-brain axis. Although not integral, we observed that the role of FFAR2 differs between homeostatic and inflammatory conditions, underscoring the need to consider different inflammatory conditions and disease contexts when investigating the role of FFAR2 and SCFAs in the gut-brain axis.

Gut-liver axis: Recent concepts in pathophysiology in alcohol-associated liver disease

The growing recognition of the role of the gut microbiome's impact on alcohol-associated diseases, especially in alcohol-associated liver disease, emphasizes the need to understand molecular mechanisms involved in governing organ-organ communication to identify novel avenues to combat alcohol-associated diseases. The gut-liver axis refers to the bidirectional communication and interaction between the gut and the liver. Intestinal microbiota plays a pivotal role in maintaining homeostasis within the gut-liver axis, and this axis plays a significant role in alcohol-associated liver disease. The intricate communication between intestine and liver involves communication between multiple cellular components in each organ that enable them to carry out their physiological functions. In this review, we focus on novel approaches to understanding how chronic alcohol exposure impacts the microbiome and individual cells within the liver and intestine, as well as the impact of ethanol on the molecular machinery required for intraorgan and interorgan communication.

In vitro and in vivo fermentation models to study the function of dietary fiber in pig nutrition

The importance of dietary fiber (DF) in animal diets is increasing with the advancement of nutritional research. DF is fermented by gut microbiota to produce metabolites, which are important in improving intestinal health. This review is a systematic review of DF in pig nutrition using in vitro and in vivo models. The fermentation characteristics of DF and the metabolic mechanisms of its metabolites were summarized in an in vitro model, and it was pointed out that SCFAs and gases are the important metabolites connecting DF, gut microbiota, and intestinal health, and they play a key role in intestinal health. At the same time, some information about host-microbe interactions could have been improved through traditional animal in vivo models, and the most direct feedback on nutrients was generated, confirming the beneficial effects of DF on sow reproductive performance, piglet intestinal health, and growing pork quality. Finally, the advantages and disadvantages of different fermentation models were compared. In future studies, it is necessary to flexibly combine in vivo and in vitro fermentation models to profoundly investigate the mechanism of DF on the organism in order to promote the development of precision nutrition tools and to provide a scientific basis for the in-depth and rational utilization of DF in animal husbandry. KEY POINTS: • The fermentation characteristics of dietary fiber in vitro models were reviewed. • Metabolic pathways of metabolites and their roles in the intestine were reviewed. • The role of dietary fiber in pigs at different stages was reviewed.

Short-Chain Fatty Acids and Human Health: From Metabolic Pathways to Current Therapeutic Implications

The gastrointestinal tract is home to trillions of diverse microorganisms collectively known as the gut microbiota, which play a pivotal role in breaking down undigested foods, such as dietary fibers. Through the fermentation of these food components, short-chain fatty acids (SCFAs) such as acetate, propionate, and butyrate are produced, offering numerous health benefits to the host. The production and absorption of these SCFAs occur through various mechanisms within the human intestine, contingent upon the types of dietary fibers reaching the gut and the specific microorganisms engaged in fermentation. Medical literature extensively documents the supplementation of SCFAs, particularly butyrate, in the treatment of gastrointestinal, metabolic, cardiovascular, and gut-brain-related disorders. This review seeks to provide an overview of the dynamics involved in the production and absorption of acetate, propionate, and butyrate within the human gut. Additionally, it will focus on the pivotal roles these SCFAs play in promoting gastrointestinal and metabolic health, as well as their current therapeutic implications.

Catabolism of phenolics from grape peel and its effects on gut microbiota during in vitro colonic fermentation

BACKGROUND

Grape peels, the main by-products of wine processing, are rich in bioactive ingredients of phenolics, including proanthocyanidins, flavonoids and anthocyanins. Phenolics have the function of regulating intestinal microbiota and promoting intestinal health. From the perspective of the dietary nutrition of grape peel phenolics (GPP), the present study aimed to investigate the influence of GPP on the composition and metabolism of human gut microbiota during in vitro fermentation.

RESULTS

The results indicated that GPP could decrease pH and promote the production of short-chain fatty acids. ACE and Chao1 indices in GPP group were lower than that of the Blank group. GPP enhanced the levels of Lachnospiraceae UCG-004, Bacteroidetes and Roseburia, but reduced the Firmicutes/Bacteroidetes ratio. Kyoto Encyclopedia of Proteins and Genome enrichment pathways related to phenolic acid metabolism mainly included flavonoid, anthocyanin, flavone and flavonol biosynthesis. Gut microbiota could accelerate the release and breakdown of phenolic compounds, resulting in a decrease in the content of hesperetin-7-O-glucoside, delphinidin-3-O-glucoside and cyanidin-3-rutinoside etc. In vitro antibacterial test found that GPP increased the diameters of the inhibition zones of Escherichia coli and Staphylococcus aureus in a dose-dependent manner.

CONCLUSION

The results of the present study revealed that GPP might be a potential prebiotic-like to prevent diseases by improving gut health. The findings could provide a theoretical basis for the potential to exploit GPP as dietary nutrition to maintain intestinal function. © 2024 Society of Chemical Industry.

Nutritional strategies to reduce intestinal cell apoptosis by alleviating oxidative stress

The gut barrier is the first line of defense against harmful substances and pathogens in the intestinal tract. The balance of proliferation and apoptosis of intestinal epithelial cells (IECs) is crucial for maintaining the integrity of the intestinal mucosa and its function. However, oxidative stress and inflammation can cause DNA damage and abnormal apoptosis of the IECs, leading to the disruption of the intestinal epithelial barrier. This, in turn, can directly or indirectly cause various acute and chronic intestinal diseases. In recent years, there has been a growing understanding of the vital role of dietary ingredients in gut health. Studies have shown that certain amino acids, fibers, vitamins, and polyphenols in the diet can protect IECs from excessive apoptosis caused by oxidative stress, and limit intestinal inflammation. This review aims to describe the molecular mechanism of apoptosis and its relationship with intestinal function, and to discuss the modulation of IECs' physiological function, the intestinal epithelial barrier, and gut health by various nutrients. The findings of this review may provide a theoretical basis for the use of nutritional interventions in clinical intestinal disease research and animal production, ultimately leading to improved human and animal intestinal health.

Short chain fatty acids inhibit corneal inflammatory responses to TLR ligands via the ocular G-protein coupled receptor 43

PURPOSE

Short chain fatty acids (SCFAs) produced by gut microbiota are known to play primary roles in gut homeostasis by immunomodulation partially through G-protein coupled receptors (GPR) 43. Using mouse models of TLR ligand induced keratitis, we investigated whether SCFAs and GPR43 play any regulatory roles in the pathogenesis of inflammatory responses in the eye.

METHODS

Both human and mouse eyes were labeled with a specific antibody for GPR43 and imaged by a laser scanning confocal microscope. Corneal cups from naïve C57BL/6J (B6) and GPR43 knockout (KO) mice were stimulated with TLR ligands in the presence or absence of sodium butyrate overnight and then processed for RT-PCR assay for expression of GPR43 and cytokines. Keratitis was induced by Poly I:C in wild type (WT) B6, GPR43KO and chimeric mice and the disease severity was evaluated by the corneal fluorescein staining test, and infiltrating cell staining and calculating in corneal whole mount.

RESULTS

GPR43 is expressed in both human and mouse eyes and the expression is bidirectionally regulated by TLR ligands and butyrate. Butyrate significantly inhibited inflammation caused by several TLR ligands such as Poly I:C, Flagellin, and CpG-ODN (TLR-3, 5 and 9 agonists, respectively) in WT, but not GPR43KO, mice. Butyrate inhibition of TLR-induced keratitis is mediated by the GPR43 expressed in tissue but not hematopoietic, cells.

CONCLUSIONS

This is the first report to demonstrate of the protective effect of SCFAs on microbial keratitis, and the dynamic expression and anti-inflammatory function of GPR43 in the eye. SCFAs can modulate inflammation and immunity in the eye through GPR43.

Harnessing and delivering microbial metabolites as therapeutics via advanced pharmaceutical approaches

Microbial metabolites have emerged as key players in the interplay between diet, the gut microbiome, and host health. Two major classes, short-chain fatty acids (SCFAs) and tryptophan (Trp) metabolites, are recognized to regulate inflammatory, immune, and metabolic responses within the host. Given that many human diseases are associated with dysbiosis of the gut microbiome and consequent reductions in microbial metabolite production, the administration of these metabolites represents a direct, multi-targeted treatment. While a multitude of preclinical studies showcase the therapeutic potential of both SCFAs and Trp metabolites, they often rely on high doses and frequent dosing regimens to achieve systemic effects, thereby constraining their clinical applicability. To address these limitations, a variety of pharmaceutical formulations approaches that enable targeted, delayed, and/or sustained microbial metabolite delivery have been developed. These approaches, including enteric encapsulations, esterification to dietary fiber, prodrugs, and nanoformulations, pave the way for the next generation of microbial metabolite-based therapeutics. In this review, we first provide an overview of the roles of microbial metabolites in maintaining host homeostasis and outline how compromised metabolite production contributes to the pathogenesis of inflammatory, metabolic, autoimmune, allergic, infectious, and cancerous diseases. Additionally, we explore the therapeutic potential of metabolites in these disease contexts. Then, we provide a comprehensive and up-to-date review of the pharmaceutical strategies that have been employed to enhance the therapeutic efficacy of microbial metabolites, with a focus on SCFAs and Trp metabolites.

The effect of oral butyrate on colonic short-chain fatty acid transporters and receptors depends on microbial status

Butyrate, a metabolite produced by gut bacteria, has demonstrated beneficial effects in the colon and has been used to treat inflammatory bowel diseases. However, the mechanism by which butyrate operates remains incompletely understood. Given that oral butyrate can exert either a direct impact on the gut mucosa or an indirect influence through its interaction with the gut microbiome, this study aimed to investigate three key aspects: (1) whether oral intake of butyrate modulates the expression of genes encoding short-chain fatty acid (SCFA) transporters (Slc16a1, Slc16a3, Slc16a4, Slc5a8, Abcg2) and receptors (Hcar2, Ffar2, Ffar3, Olfr78, Olfr558) in the colon, (2) the potential involvement of gut microbiota in this modulation, and (3) the impact of oral butyrate on the expression of colonic SCFA transporters and receptors during colonic inflammation. Specific pathogen-free (SPF) and germ-free (GF) mice with or without DSS-induced inflammation were provided with either water or a 0.5% sodium butyrate solution. The findings revealed that butyrate decreased the expression of Slc16a1, Slc5a8, and Hcar2 in SPF but not in GF mice, while it increased the expression of Slc16a3 in GF and the efflux pump Abcg2 in both GF and SPF animals. Moreover, the presence of microbiota was associated with the upregulation of Hcar2, Ffar2, and Ffar3 expression and the downregulation of Slc16a3. Interestingly, the challenge with DSS did not alter the expression of SCFA transporters, regardless of the presence or absence of microbiota, and the effect of butyrate on the transporter expression in SPF mice remained unaffected by DSS. The expression of SCFA receptors was only partially affected by DSS. Our results indicate that (1) consuming a relatively low concentration of butyrate can influence the expression of colonic SCFA transporters and receptors, with their expression being modulated by the gut microbiota, (2) the effect of butyrate does not appear to result from direct substrate-induced regulation but rather reflects an indirect effect associated with the gut microbiome, and (3) acute colon inflammation does not lead to significant changes in the transcriptional regulation of most SCFA transporters and receptors, with the effect of butyrate in the inflamed colon remaining intact.

Human-derived bacterial strains mitigate colitis via modulating gut microbiota and repairing intestinal barrier function in mice

BACKGROUND

Unbalanced gut microbiota is considered as a pivotal etiological factor in colitis. Nevertheless, the precise influence of the endogenous gut microbiota composition on the therapeutic efficacy of probiotics in colitis remains largely unexplored.

RESULTS

In this study, we isolated bacteria from fecal samples of a healthy donor and a patient with ulcerative colitis in remission. Subsequently, we identified three bacterial strains that exhibited a notable ability to ameliorate dextran sulfate sodium (DSS)-induced colitis, as evidenced by increased colon length, reduced disease activity index, and improved histological score. Further analysis revealed that each of Pediococcus acidilactici CGMCC NO.17,943, Enterococcus faecium CGMCC NO.17,944 and Escherichia coli CGMCC NO.17,945 significantly attenuated inflammatory responses and restored gut barrier dysfunction in mice. Mechanistically, bacterial 16S rRNA gene sequencing indicated that these three strains partially restored the overall structure of the gut microbiota disrupted by DSS. Specially, they promoted the growth of Faecalibaculum and Lactobacillus murinus, which were positively correlated with gut barrier function, while suppressing Odoribacter, Rikenella, Oscillibacter and Parasutterella, which were related to inflammation. Additionally, these strains modulated the composition of short chain fatty acids (SCFAs) in the cecal content, leading to an increase in acetate and a decrease in butyrate. Furthermore, the expression of metabolites related receptors, such as receptor G Protein-coupled receptor (GPR) 43, were also affected. Notably, the depletion of endogenous gut microbiota using broad-spectrum antibiotics completely abrogated these protective effects.

CONCLUSIONS

Our findings suggest that selected human-derived bacterial strains alleviate experimental colitis and intestinal barrier dysfunction through mediating resident gut microbiota and their metabolites in mice. This study provides valuable insights into the potential therapeutic application of probiotics in the treatment of colitis.

Achieving healthy aging through gut microbiota-directed dietary intervention: Focusing on microbial biomarkers and host mechanisms

BACKGROUND

Population aging has become a primary global public health issue, and the prevention of age-associated diseases and prolonging healthy life expectancies are of particular importance. Gut microbiota has emerged as a novel target in various host physiological disorders including aging. Comprehensive understanding on changes of gut microbiota during aging, in particular gut microbiota characteristics of centenarians, can provide us possibility to achieving healthy aging or intervene pathological aging through gut microbiota-directed strategies.

AIM OF REVIEW

This review aims to summarize the characteristics of the gut microbiota associated with aging, explore potential biomarkers of aging and address microbiota-associated mechanisms of host aging focusing on intestinal barrier and immune status. By summarizing the existing effective dietary strategies in aging interventions, the probability of developing a diet targeting the gut microbiota in future is provided.

KEY SCIENTIFIC CONCEPTS OF REVIEW

This review is focused on three key notions: Firstly, gut microbiota has become a new target for regulating health status and lifespan, and its changes are closely related to age. Thus, we summarized aging-associated gut microbiota features at the levels of key genus/species and important metabolites through comparing the microbiota differences among centenarians, elderly people and younger people. Secondly, exploring microbiota biomarkers related to aging and discussing future possibility using dietary regime/components targeted to aging-related microbiota biomarkers promote human healthy lifespan. Thirdly, dietary intervention can effectively improve the imbalance of gut microbiota related to aging, such as probiotics, prebiotics, and postbiotics, but their effects vary among.

Effects of dietary arsenic exposure on liver metabolism in mice

Arsenic, a ubiquitous environmental toxicant with various forms and complex food matrix interactions, can reportedly exert differential effects on the liver compared to drinking water exposure. To examine its specific liver-related harms, we targeted the liver in C57BL/6 J mice (n=48, 8-week-old) fed with arsenic-contaminated food (30 mg/kg) for 60 days, mimicking the rice arsenic composition observed in real-world scenarios (iAsV: 7.3%, iAsIII: 72.7%, MMA: 1.0%, DMA: 19.0%). We then comprehensively evaluated liver histopathology, metabolic changes, and the potential role of the gut-liver axis using human hepatocellular carcinoma cells (HepG2) and microbiota/metabolite analyses. Rice arsenic exposure significantly altered hepatic lipid (fatty acids, glycerol lipids, phospholipids, sphingolipids) and metabolite (glutathione, thioneine, spermidine, inosine, indole-derivatives, etc.) profiles, disrupting 33 metabolic pathways (bile secretion, unsaturated fatty acid biosynthesis, glutathione metabolism, ferroptosis, etc.). Pathological examination revealed liver cell necrosis/apoptosis, further confirmed by ferroptosis induction in HepG2 cells. Gut microbiome analysis showed enrichment of pathogenic bacteria linked to liver diseases and depletion of beneficial strains. Fecal primary and secondary bile acids, short-chain fatty acids, and branched-chain amino acids were also elevated. Importantly, mediation analysis revealed significant correlations between gut microbiota, fecal metabolites, and liver metabolic alterations, suggesting fecal metabolites may mediate the impact of gut microbiota and liver metabolic disorders. Gut microbiota and its metabolites may play significant roles in arsenic-induced gut-liver injuries. Overall, our findings demonstrate that rice arsenic exposure triggers oxidative stress, disrupts liver metabolism, and induces ferroptosis.

GPR41 and GPR43 regulate CD8+ T cell priming during herpes simplex virus type 1 infection

Naïve CD8+ T cells need to undergo a complex and coordinated differentiation program to gain the capacity to control virus infections. This not only involves the acquisition of effector functions, but also regulates the development of a subset of effector CD8+ T cells into long-lived and protective memory cells. Microbiota-derived metabolites have recently gained interest for their influence on T cells, but much remains unclear about their role in CD8+ T cell differentiation. In this study, we investigated the role of the G protein-coupled receptors (GPR)41 and GPR43 that can bind microbiota-derived short chain fatty acids (SCFAs) in CD8+ T cell priming following epicutaneous herpes simplex virus type 1 (HSV-1) infection. We found that HSV-specific CD8+ T cells in GPR41/43-deficient mice were impaired in the antigen-elicited production of interferon-gamma (IFN-γ), tumour necrosis factor-alpha (TNF-α), granzyme B and perforin, and failed to differentiate effectively into memory precursors. The defect in controlling HSV-1 at the site of infection could be restored when GPR41 and GPR43 were expressed exclusively by HSV-specific CD8+ T cells. Our findings therefore highlight roles for GPR41 and GPR43 in CD8+ T cell differentiation, emphasising the importance of metabolite sensing in fine-tuning anti-viral CD8+ T cell priming.

Current updates on metabolites and its interlinked pathways as biomarkers for diabetic kidney disease: A systematic review

Diabetic kidney disease (DKD) is a major microvascular complication of diabetes mellitus (DM) that poses a serious risk as it can lead to end-stage renal disease (ESRD). DKD is linked to changes in the diversity, composition, and functionality of the microbiota present in the gastrointestinal tract. The interplay between the gut microbiota and the host organism is primarily facilitated by metabolites generated by microbial metabolic processes from both dietary substrates and endogenous host compounds. The production of numerous metabolites by the gut microbiota is a crucial factor in the pathogenesis of DKD. However, a comprehensive understanding of the precise mechanisms by which gut microbiota and its metabolites contribute to the onset and progression of DKD remains incomplete. This review will provide a summary of the current scenario of metabolites in DKD and the impact of these metabolites on DKD progression. We will discuss in detail the primary and gut-derived metabolites in DKD, and the mechanisms of the metabolites involved in DKD progression. Further, we will address the importance of metabolomics in helping identify potential DKD markers. Furthermore, the possible therapeutic interventions and research gaps will be highlighted.

Associations between rheumatoid arthritis and intestinal flora, with special emphasis on RA pathologic mechanisms to treatment strategies

BACKGROUND

Rheumatoid arthritis (RA) is an autoimmune inflammatory disease that primarily affects the joints. Individuals at risk for RA and people with RA develop intestinal dysbiosis. The changes in intestinal flora composition in preclinical and confirmed RA patients suggest that intestinal flora imbalance may play an important role in the induction and persistence of RA.

METHODS

Based on the current research on the interaction between RA and intestinal microbiota, intestinal microbiota metabolites and intestinal barrier changes. This paper systematically summarized the changes in intestinal microbiota in RA patients, the metabolites of intestinal flora, and the influence mechanism of intestinal barrier on RA, and further discussed the influence of drugs for RA on intestinal flora and its mechanism of action.

RESULTS

Compared with healthy controls, α diversity analysis of intestinal flora showed no significant difference, β diversity analysis showed significant differences. The intestinal flora produces bioactive metabolites, such as short-chain fatty acids and aromatic amino acids, which have anti-inflammatory effects. Abnormal intestinal flora leads to impaired barrier function and mucosal immune dysfunction, promoting the development of inflammation. Traditional Chinese medicine (TCM) and chemical drugs can also alleviate RA by regulating intestinal flora, intestinal flora metabolites, and intestinal barrier. Intestinal flora is closely related to the pathogenesis of RA and may become potential biomarkers for the diagnosis and treatment of RA.

CONCLUSIONS

Intestinal flora and its metabolites play an important role in the pathogenesis of autoimmune diseases such as RA, and are expected to become a new target for clinical diagnosis and treatment, providing a new idea for targeted treatment of RA.

Immune regulation of the gut-brain axis and lung-brain axis involved in ischemic stroke

Local ischemia often causes a series of inflammatory reactions when both brain immune cells and the peripheral immune response are activated. In the human body, the gut and lung are regarded as the key reactional targets that are initiated by brain ischemic attacks. Mucosal microorganisms play an important role in immune regulation and metabolism and affect blood-brain barrier permeability. In addition to the relationship between peripheral organs and central areas and the intestine and lung also interact among each other. Here, we review the molecular and cellular immune mechanisms involved in the pathways of inflammation across the gut-brain axis and lung-brain axis. We found that abnormal intestinal flora, the intestinal microenvironment, lung infection, chronic diseases, and mechanical ventilation can worsen the outcome of ischemic stroke. This review also introduces the influence of the brain on the gut and lungs after stroke, highlighting the bidirectional feedback effect among the gut, lungs, and brain.

Biomolecular Actions by Intestinal Endotoxemia in Metabolic Syndrome

Metabolic syndrome (MetS) is a combination of metabolic disorders that concurrently act as factors promoting systemic pathologies such as atherosclerosis or diabetes mellitus. It is now believed to encompass six main interacting conditions: visceral fat, imbalance of lipids (dyslipidemia), hypertension, insulin resistance (with or without impairing both glucose tolerance and fasting blood sugar), and inflammation. In the last 10 years, there has been a progressive interest through scientific research investigations conducted in the field of metabolomics, confirming a trend to evaluate the role of the metabolome, particularly the intestinal one. The intestinal microbiota (IM) is crucial due to the diversity of microorganisms and their abundance. Consequently, IM dysbiosis and its derivate toxic metabolites have been correlated with MetS. By intervening in these two factors (dysbiosis and consequently the metabolome), we can potentially prevent or slow down the clinical effects of the MetS process. This, in turn, may mitigate dysregulations of intestinal microbiota axes, such as the lung axis, thereby potentially alleviating the negative impact on respiratory pathology, such as the chronic obstructive pulmonary disease. However, the biomolecular mechanisms through which the IM influences the host's metabolism via a dysbiosis metabolome in both normal and pathological conditions are still unclear. In this study, we seek to provide a description of the knowledge to date of the IM and its metabolome and the factors that influence it. Furthermore, we analyze the interactions between the functions of the IM and the pathophysiology of major metabolic diseases via local and systemic metabolome's relate endotoxemia.

Multi-omics revealed rumen microbiota metabolism and host immune regulation in Tibetan sheep of different ages

The rumen microbiota and metabolites play an important role in energy metabolism and immune regulation of the host. However, the regulatory mechanism of rumen microbiota and metabolite interactions with host on Tibetan sheep's plateau adaptability is still unclear. We analyzed the ruminal microbiome and metabolome, host transcriptome and serum metabolome characteristics of Tibetan sheep at different ages. Biomarkers Butyrivibrio, Lachnospiraceae_XPB1014_group, Prevotella, and Rikenellaceae_RC9_gut_group were found in 4 months, 1.5 years, 3.5 years, and 6 years Tibetan sheep, respectively. The rumen microbial metabolites were mainly enriched in galactose metabolism, unsaturated fatty acid biosynthesis and fatty acid degradation pathways, and had significant correlation with microbiota. These metabolites further interact with mRNA, and are co-enriched in arginine and proline metabolism, metabolism of xenobiotics by cytochrome P450, propanoate metabolism, starch and sucrose metabolism, gap junction pathway. Meanwhile, serum metabolites also have a similar function, such as chemical carcinogenesis - reactive oxygen species, limonene and pinene degradation, and cutin, suberine and wax biosynthesis, thus participating in the regulation of the body's immune and energy-related metabolic processes. This study systematically revealed that rumen microbiota, metabolites, mRNA and serum metabolites of Tibetan sheep were involved in the regulation of fermentation metabolic function and immune level of Tibetan sheep at different ages, which provided a new perspective for plateau adaptability research of Tibetan sheep at different ages.

Effect of GVHD on the gut and intestinal microflora

Graft-versus-host disease (GVHD) is one of the most important cause of death in patients undergoing allogeneic hematopoietic stem cell transplantation (allo-HSCT). The gastrointestinal tract is one of the most common sites affected by GVHD. However, there is no gold standard clinical practice for diagnosing gastrointestinal GVHD (GI-GVHD), and it is mainly diagnosed by the patient's clinical symptoms and related histological changes. Additionally, GI-GVHD causes intestinal immune system disorders, damages intestinal epithelial tissue such as intestinal epithelial cells((IEC), goblet, Paneth, and intestinal stem cells, and disrupts the intestinal epithelium's physical and chemical mucosal barriers. The use of antibiotics and diet alterations significantly reduces intestinal microbial diversity, further reducing bacterial metabolites such as short-chain fatty acids and indole, aggravating infection, and GI-GVHD. gut microbe diversity can be restored by fecal microbiota transplantation (FMT) to treat refractory GI-GVHD. This review article focuses on the clinical diagnosis of GI-GVHD and the effect of GVHD on intestinal flora and its metabolites.

A cross talk between microbial metabolites and host immunity: Its relevance for allergic diseases

BACKGROUND

Allergic diseases, including respiratory and food allergies, as well as allergic skin conditions have surged in prevalence in recent decades. In allergic diseases, the gut microbiome is dysbiotic, with reduced diversity of beneficial bacteria and increased abundance of potential pathogens. Research findings suggest that the microbiome, which is highly influenced by environmental and dietary factors, plays a central role in the development, progression, and severity of allergic diseases. The microbiome generates metabolites, which can regulate many of the host's cellular metabolic processes and host immune responses.

AIMS AND METHODS

Our goal is to provide a narrative and comprehensive literature review of the mechanisms through which microbial metabolites regulate host immune function and immune metabolism both in homeostasis and in the context of allergic diseases.

RESULTS AND DISCUSSION

We describe key microbial metabolites such as short-chain fatty acids, amino acids, bile acids and polyamines, elucidating their mechanisms of action, cellular targets and their roles in regulating metabolism within innate and adaptive immune cells. Furthermore, we characterize the role of bacterial metabolites in the pathogenesis of allergic diseases including allergic asthma, atopic dermatitis and food allergy.

CONCLUSION

Future research efforts should focus on investigating the physiological functions of microbiota-derived metabolites to help develop new diagnostic and therapeutic interventions for allergic diseases.

Polysaccharide from Artocarpus heterophyllus Lam. (Jackfruit) Pulp Ameliorates Dextran Sodium Sulfate-Induced Enteritis in Rats

A polysaccharide from Artocarpus heterophyllus Lam. (jackfruit) pulp (JFP-Ps) is known for its excellent bioactivities. However, its impact on small intestinal barrier function is still largely unexplored. The study aimed to examine the protection effect of JFP-Ps against dextran sodium sulfate-induced enteritis and its underlying mechanism. This research revealed that JFP-Ps mitigated small intestinal tissue damage by reducing the expression of pro-inflammatory cytokines and promoting the expression of the anti-inflammatory cytokine interleukin-10 in the small intestine. JFP-Ps diminished oxidative stress by bolstering the activity of antioxidant enzymes and reducing the concentration of malondialdehyde in the small intestine. In addition, JFP-Ps may restore the mechanical barrier and inhibit intestinal structure damage by augmenting the expression of short-chain fatty acids (SCFAs) receptors (GPR41/43) and up-regulating the expression of tight junction proteins (occludin). In conclusion, JFP-Ps may positively influence intestinal health by relieving oxidative stress in the small intestine, improving mechanical barrier function, activating the SCFA-GPR41/GPR43 axis, and inhibiting TLR4/MAPK pathway activation. The results augment our comprehension of the bioactivities of JFP-Ps, corroborating its great potential as a functional food.

Short-chain fatty acids (SCFAs) from gastrointestinal disorders, metabolism, epigenetics, central nervous system to cancer - A mini-review

Short-chain fatty acids (SCFAs), generated through microbial fermentation of dietary fibers and proteins in the gut, play a pivotal role in maintaining intestinal integrity, cellular function, and the immune response. SCFAs, including butyrate, acetate, and propionate, are absorbed in the colon or excreted through feces, contributing to essential physiological processes. Butyrate, a primary energy source for colonocytes, exhibits anti-inflammatory properties and regulates key pathways, such as nuclear factor-κB (NF-κB) inhibition. SCFAs' impact extends beyond the intestines, influencing the gut-brain axis, systemic circulation, and folate metabolism. A decline in colonic SCFAs has been linked to gastrointestinal diseases, emphasizing their clinical relevance, while their effects on immune checkpoints, such as ipilimumab, provide intriguing prospects for cancer therapy. This mini-review explores SCFAs' diverse roles, shedding light on their significance in health and potential implications for disease management. Understanding SCFAs' intricate mechanisms enhances our knowledge of their therapeutic potential and highlights their emerging importance in various physiological contexts.

Free fatty acid 3 receptor agonist AR420626 reduces allergic responses in asthma and eczema in mice

Free fatty acid 3 receptor (FFA3; previously GPR41) is a G protein-coupled receptor that senses short-chain fatty acids and dietary metabolites produced by the gut microbiota. FFA3 deficiency reportedly exacerbates inflammatory events in asthma. Herein, we aimed to determine the therapeutic potential of FFA3 agonists in treating inflammatory diseases. We investigated the effects of N-(2,5-dichlorophenyl)-4-(furan-2-yl)-2-methyl-5-oxo-1,4,5,6,7,8-hexahydroquinoline-3-carboxamide (AR420626), an FFA3 agonist, in in vivo models of chemically induced allergic asthma and eczema in BALB/c mice. Administration of AR420626 decreased the number of immune cells in the bronchoalveolar lavage fluid and skin. AR420626 suppressed inflammatory cytokine expression in the lung and skin tissues. Histological examination revealed that AR420626 suppressed inflammation in the lungs and skin. Treatment with AR420626 significantly suppressed the enhanced lymph node size and inflammatory cytokine levels. Overall, FFA3 agonist AR420626 could suppress allergic asthma and eczema, implying that activation of FFA3 might be a therapeutic target for allergic diseases.

Comparison of the efficacy of fish oil and probiotic supplementation on glucose and lipid metabolism in patients with type 2 diabetes: a systematic review and network meta-analysis

BACKGROUND

Abnormalities in glucose and lipid metabolism contribute to the progression and exacerbation of type 2 diabetes mellitus (T2DM). Fish oil and probiotics are dietary supplements that have the potential to improve glucose and lipid metabolism. However, their efficacy remains unclear in T2DM patients.

METHODS

PubMed, Embase, and the Cochrane Library were retrieved to collect randomized controlled trials (RCTs) on the efficacy of fish oil or probiotic supplementation in T2DM patients from the database inception to December 13, 2023. Primary outcome indicators encompassed glycated hemoglobin (HbA1c), homeostatic model assessment for insulin resistance (HOMA-IR) and blood lipid profile (triglyceride (TG) and total cholesterol (TC). Secondary outcome indicators included inflammatory markers such as tumor necrosis factor -α (TNF-α) and adipocytokine (including leptin and adiponectin). The R software was used for statistical analysis, and GraphPad Prism was used for figure rendering.

RESULTS

A total of 60 RCTs involving 3845 T2DM patients were included in the analysis. The results showed that the probiotics (Bifidobacterium, Lactobacillus, Lactococcus, Propionibacterium, etc.) were more effective in reducing HOMA-IR than fish oil (Surca = 0.935). Bifidobacterium demonstrated the highest efficacy in reducing HbA1c levels (Surca = 0.963). Regarding lipid metabolism, fish oil was superior to probiotics in lowering TG and TC levels (Surca values of 0.978 and 0.902, respectively). Furthermore, fish oil outperformed probiotics in reducing TNF-α (Surca = 0.839) and leptin (Surca = 0.712), and increasing adiponectin levels (Surca = 0.742). Node-splitting analysis showed good consistency (P > 0.05 for direct, indirect, and network comparison across various interventions).

CONCLUSIONS

In T2DM patients, fish oil was more effective than probiotics in regulating lipid metabolism. Probiotics outperformed fish oil in regulating glucose metabolism particularly; specifically, Bifidobacterium showed higher efficacy in reducing blood glucose.

Microbial metabolites as modulators of host physiology

The gut microbiota is increasingly recognised as a key player in influencing human health and changes in the gut microbiota have been strongly linked with many non-communicable conditions in humans such as type 2 diabetes, obesity and cardiovascular disease. However, characterising the molecular mechanisms that underpin these associations remains an important challenge for researchers. The gut microbiota is a complex microbial community that acts as a metabolic interface to transform ingested food (and other xenobiotics) into metabolites that are detected in the host faeces, urine and blood. Many of these metabolites are only produced by microbes and there is accumulating evidence to suggest that these microbe-specific metabolites do act as effectors to influence human physiology. For example, the gut microbiota can digest dietary complex polysaccharides (such as fibre) into short-chain fatty acids (SCFA) such as acetate, propionate and butyrate that have a pervasive role in host physiology from nutrition to immune function. In this review we will outline our current understanding of the role of some key microbial metabolites, such as SCFA, indole and bile acids, in human health. Whilst many studies linking microbial metabolites with human health are correlative we will try to highlight examples where genetic evidence is available to support a specific role for a microbial metabolite in host health and well-being.

Structural basis for the ligand recognition and signaling of free fatty acid receptors

Free fatty acid receptors 1 to 4 (FFA1 to FFA4) are class A G protein-coupled receptors (GPCRs). FFA1 to FFA3 share substantial sequence similarity, whereas FFA4 is unrelated. However, FFA1 and FFA4 are activated by long-chain fatty acids, while FFA2 and FFA3 respond to short-chain fatty acids generated by intestinal microbiota. FFA1, FFA2, and FFA4 are potential drug targets for metabolic and inflammatory conditions. Here, we determined the active structures of FFA1 and FFA4 bound to docosahexaenoic acid, FFA4 bound to the synthetic agonist TUG-891, and butyrate-bound FFA2, each complexed with an engineered heterotrimeric Gq protein (miniGq), by cryo-electron microscopy. Together with computational simulations and mutagenesis studies, we elucidated the similarities and differences in the binding modes of fatty acid ligands to their respective GPCRs. Our findings unveiled distinct mechanisms of receptor activation and G protein coupling. We anticipate that these outcomes will facilitate structure-based drug development and underpin future research on this group of GPCRs.

Short Chain Fatty Acids: Essential Weapons of Traditional Medicine in Treating Inflammatory Bowel Disease

Inflammatory bowel disease (IBD) is a chronic and recurrent intestinal inflammatory disease, mainly including Crohn's disease (CD) and ulcerative colitis (UC). In recent years, the incidence and prevalence of IBD have been on the rise worldwide and have become a significant concern of health and a huge economic burden on patients. The occurrence and development of IBD involve a variety of pathogenic factors. The changes in short-chain fatty acids (SCFAs) are considered to be an important pathogenic mechanism of this disease. SCFAs are important metabolites in the intestinal microbial environment, which are closely involved in regulating immune, anti-tumor, and anti-inflammatory activities. Changes in metabolite levels can reflect the homeostasis of the intestinal microflora. Recent studies have shown that SCFAs provide energy for host cells and intestinal microflora, shape the intestinal environment, and regulate the immune system, thereby regulating intestinal physiology. SCFAs can effectively reduce the incidence of enteritis, cardiovascular disease, colon cancer, obesity, and diabetes, and also play an important role in maintaining the balance of energy metabolism (mainly glucose metabolism) and improving insulin tolerance. In recent years, many studies have shown that numerous decoctions and natural compounds of traditional Chinese medicine have shown promising therapeutic activities in multiple animal models of colitis and thus attracted increasing attention from scientists in the study of IBD treatment. Some of these traditional Chinese medicines or compounds can effectively alleviate colonic inflammation and clinical symptoms by regulating the generation of SCFAs. This study reviews the effects of various traditional Chinese medicines or bioactive substances on the production of SCFAs and their potential impacts on the severity of colonic inflammation. On this basis, we discussed the mechanism of SCFAs in regulating IBD-associated inflammation, as well as the related regulatory factors and signaling pathways. In addition, we provide our understanding of the limitations of current research and the prospects for future studies on the development of new IBD therapies by targeting SCFAs. This review may widen our understanding of the effect of traditional medicine from the view of SCFAs and their role in alleviating IBD animal models, thus contributing to the studies of IBD researchers.

Bifidobacterium longum promotes postoperative liver function recovery in patients with hepatocellular carcinoma

Timely liver function recovery (LFR) is crucial for postoperative hepatocellular carcinoma (HCC) patients. Here, we established the significance of LFR on patient long-term survival through retrospective and prospective cohorts and identified a key gut microbe, Bifidobacterium longum, depleted in patients with delayed recovery. Fecal microbiota transfer from HCC patients with delayed recovery to mice similarly impacted recovery time post hepatectomy. However, oral gavage of B. longum improved liver function and repair in these mice. In a clinical trial of HCC patients, orally administering a probiotic bacteria cocktail containing B. longum reduced the rates of delayed recovery, shortened hospital stays, and improved overall 1-year survival. These benefits, attributed to diminished liver inflammation, reduced liver fibrosis, and hepatocyte proliferation, were associated with changes in key metabolic pathways, including 5-hydroxytryptamine, secondary bile acids, and short-chain fatty acids. Our findings propose that gut microbiota modulation can enhance LFR, thereby improving postoperative outcomes for HCC patients.

Rattan Pepper Polysaccharide Regulates DSS-Induced Intestinal Inflammation and Depressive Behavior through Microbiota-Gut-Brain Axis

Inflammatory bowel disease (IBD) is a chronic and recurrent disease. Increasing evidence suggests a higher incidence of depression in IBD patients compared with the general population, but the underlying mechanism remains uncertain. Rattan pepper polysaccharide (RPP) is an important active ingredient of rattan pepper, yet its effects and mechanisms on intestinal inflammation and depression-like behavior remain largely unknown. This study aims to investigate the ameliorating effect of RPP on dextran sulfate sodium salt (DSS)-induced intestinal inflammation and depression-like behavior as well as to reveal its mechanism. Our results indicate that RPP effectively ameliorated intestinal microbiota imbalance and metabolic disorders of short-chain fatty acids (SCFAs) and bile acids in mice with DSS-induced inflammation, contributing to the recovery of intestinal Th17/Treg homeostasis. Importantly, RPP effectively alleviated brain inflammation caused by intestinal inflammatory factors entering the brain through the blood-brain barrier. This effect may be attributed to the inhibition of the TLR4/NF-κB signaling pathway, which alleviates neuroinflammation, and the activation of the CREB/BDNF signaling pathway, which improves synaptic dysfunction. Therefore, our findings suggest that RPP may play a role in alleviating DSS-induced gut inflammation and depression-like behavior through the microbiota-gut-brain axis.

The functional roles of short chain fatty acids as postbiotics in human gut: future perspectives

The significance of gut microbiome and their metabolites (postbiotics) on human health could be a promising approach to treat various diseases that includes inflammatory bowel diseases, colon cancer, and many neurological disorders. Probiotics with potential mental health benefits (psychobiotics) can alter the gut-brain axis via immunological, humoral, neuronal, and metabolic pathways. Recently, probiotic bacteria like Lactobacillus and Bifidobacterium have been demonstrated for SCFAs production, which play a crucial role in a variety of diseases. These acids could enhance the production of mucins, antimicrobial proteins (bacteriocins and peptides), cytokines (Interleukin 10 and 18) and neurotransmitters (serotonin) in the intestine to main the gut microbiota, intestinal barrier system and other immune functions. In this review, we discuss about two mechanisms such as (i) SCFAs mediated intestinal barrier system, and (ii) SCFAs mediated gut-brain axis to elucidate the therapeutic options for the treatment/prevention of various diseases.

A tailored series of engineered yeasts for the cell-dependent treatment of inflammatory bowel disease by rational butyrate supplementation

Intestinal microbiota dysbiosis and metabolic disruption are considered essential characteristics in inflammatory bowel disorders (IBD). Reasonable butyrate supplementation can help patients regulate intestinal flora structure and promote mucosal repair. Here, to restore microbiota homeostasis and butyrate levels in the patient's intestines, we modified the genome of Saccharomyces cerevisiae to produce butyrate. We precisely regulated the relevant metabolic pathways to enable the yeast to produce sufficient butyrate in the intestine with uneven oxygen distribution. A series of engineered strains with different butyrate synthesis abilities was constructed to meet the needs of different patients, and the strongest can reach 1.8 g/L title of butyrate. Next, this series of strains was used to co-cultivate with gut microbiota collected from patients with mild-to-moderate ulcerative colitis. After receiving treatment with engineered strains, the gut microbiota and the butyrate content have been regulated to varying degrees depending on the synthetic ability of the strain. The abundance of probiotics such as Bifidobacterium and Lactobacillus increased, while the abundance of harmful bacteria like Candidatus Bacilloplasma decreased. Meanwhile, the series of butyrate-producing yeast significantly improved trinitrobenzene sulfonic acid (TNBS)-induced colitis in mice by restoring butyrate content. Among the series of engineered yeasts, the strain with the second-highest butyrate synthesis ability showed the most significant regulatory and the best therapeutic effect on the gut microbiota from IBD patients and the colitis mouse model. This study confirmed the existence of a therapeutic window for IBD treatment by supplementing butyrate, and it is necessary to restore butyrate levels according to the actual situation of patients to restore intestinal flora.

The olfactory receptor Olfr78 promotes differentiation of enterochromaffin cells in the mouse colon

The gastrointestinal epithelium constitutes a chemosensory system for microbiota-derived metabolites such as short-chain fatty acids (SCFA). Here, we investigate the spatial distribution of Olfr78, one of the SCFA receptors, in the mouse intestine and study the transcriptome of colon enteroendocrine cells expressing Olfr78. The receptor is predominantly detected in the enterochromaffin and L subtypes in the proximal and distal colon, respectively. Using the Olfr78-GFP and VilCre/Olfr78flox transgenic mouse lines, we show that loss of epithelial Olfr78 results in impaired enterochromaffin cell differentiation, blocking cells in an undefined secretory lineage state. This is accompanied by a reduced defense response to bacteria in colon crypts and slight dysbiosis. Using organoid cultures, we further show that maintenance of enterochromaffin cells involves activation of the Olfr78 receptor via the SCFA ligand acetate. Taken together, our work provides evidence that Olfr78 contributes to colon homeostasis by promoting enterochromaffin cell differentiation.

The Role of Gut Microbiota and Leaky Gut in the Pathogenesis of Food Allergy

Food allergy (FA) is a growing public health concern, with an increasing prevalence in Western countries. Increasing evidence suggests that the balance of human gut microbiota and the integrity of our intestinal barrier may play roles in the development of FA. Environmental factors, including industrialization and consumption of highly processed food, can contribute to altering the gut microbiota and the intestinal barrier, increasing the susceptibility to allergic sensitization. Compositional and functional alterations to the gut microbiome have also been associated with FA. In addition, increased permeability of the gut barrier allows the translocation of allergenic molecules, triggering Th2 immune responses. Preclinical and clinical studies have highlighted the potential of probiotics, prebiotics, and postbiotics in the prevention and treatment of FA through enhancing gut barrier function and promoting the restoration of healthy gut microbiota. Finally, fecal microbiota transplantation (FMT) is now being explored as a promising therapeutic strategy to prevent FA in both experimental and clinical studies. In this review article, we aim to explore the complex interplay between intestinal permeability and gut microbiota in the development of FA, as well as depict potential therapeutic strategies.

Faecal microbiota and fatty acids in feline chronic enteropathy

BACKGROUND

Feline chronic enteropathy is a set of disorders defined as the presence of clinical signs of gastrointestinal disease for at least three weeks. The most common final diagnoses are inflammatory bowel disease and alimentary small cell lymphoma. The etiopathogenesis of these diseases is incompletely understood; however, it is hypothesised that they involve a combination of factors, including altered composition and/or functionality of the intestinal microbiome. An important factor in the interplay of the microbiome and host is the production of short- and branched-chain fatty acids.  The aim of this study was to evaluate the possible differences in faecal microbiota diversity, composition and fatty acid production between cats suffering from chronic enteropathy and healthy cats. Sixteen cats suffering from chronic enteropathy and fourteen healthy control cats were enrolled in the study. The microbiota compositions of faecal samples were analysed by using next-generation amplicon sequencing of the V3V4 fragment of the 16S rRNA gene. Fatty acids were evaluated by high-performance liquid chromatography.

RESULTS

Both the alpha and beta diversities were significantly lower in samples obtained from cats with chronic enteropathy. The relative abundance of the phylum Proteobacteria, orders Lactobacillales and Enterobacterales, family Enteriobacteriaceae and genus Escherichia Shigella were higher in diseased cats, whereas the abundance of the phylum Bacteroidota and order Peptococcales were higher in control cats. The faecal concentrations of short-chain fatty acids were higher in cats with chronic enteropathy, with lower propionate proportions and higher butyrate proportions.

CONCLUSION

The study revealed alterations in microbiota compositions and short-chain fatty acid concentration in cats suffering from chronic enteropathy, which is an important finding both for research on the pathogenesis of the disease and for potential therapeutic interventions in the form of faecal microbiota transplantation and/or probiotic supplementation.

Multidirectional associations between the gut microbiota and Parkinson's disease, updated information from the perspectives of humoral pathway, cellular immune pathway and neuronal pathway

The human gastrointestinal tract is inhabited by a diverse range of microorganisms, collectively known as the gut microbiota, which form a vast and complex ecosystem. It has been reported that the microbiota-gut-brain axis plays a crucial role in regulating host neuroprotective function. Studies have shown that patients with Parkinson's disease (PD) have dysbiosis of the gut microbiota, and experiments involving germ-free mice and fecal microbiota transplantation from PD patients have revealed the pathogenic role of the gut microbiota in PD. Interventions targeting the gut microbiota in PD, including the use of prebiotics, probiotics, and fecal microbiota transplantation, have also shown efficacy in treating PD. However, the causal relationship between the gut microbiota and Parkinson's disease remains intricate. This study reviewed the association between the microbiota-gut-brain axis and PD from the perspectives of humoral pathway, cellular immune pathway and neuronal pathway. We found that the interactions among gut microbiota and PD are very complex, which should be "multidirectional", rather than conventionally regarded "bidirectional". To realize application of the gut microbiota-related mechanisms in the clinical setting, we propose several problems which should be addressed in the future study.

Gut feelings in the islets: The role of the gut microbiome and the FFA2 and FFA3 receptors for short chain fatty acids on β-cell function and metabolic regulation

Short-chain fatty acids (SCFAs) are biosynthesized via fermentation of polysaccharides by gastrointestinal microbiota and have been shown to have wide-reaching effects on almost all tissues, including the pancreatic islets. Historically, the effects of SCFAs have been attributed to their intracellular metabolism and function as energy sources, but the discovery of free fatty acid G protein-coupled receptors (GPCRs) in the 2000s suggested that many functional outcomes of SCFAs are receptor-mediated. The SCFA receptors FFA2/GPR43 and FFA3/GPR41 are expressed on β-cells, where they regulate glucose-dependent insulin secretion, making them attractive targets for treatment of diabetes and other metabolic disorders. Here, we provide an update on the current evidence regarding regulation of FFA2/FFA3 receptors by specific probiotic bacterial species within the gut microbiome that synthesize SCFAs. We also review the body of research regarding the FFA2- and FFA3 receptor-specific function of SCFAs on β-cells and discuss the somewhat controversial and opposing findings within these studies.

Microbial sensing in the intestine

The gut microbiota plays a key role in host health and disease, particularly through their interactions with the immune system. Intestinal homeostasis is dependent on the symbiotic relationships between the host and the diverse gut microbiota, which is influenced by the highly co-evolved immune-microbiota interactions. The first step of the interaction between the host and the gut microbiota is the sensing of the gut microbes by the host immune system. In this review, we describe the cells of the host immune system and the proteins that sense the components and metabolites of the gut microbes. We further highlight the essential roles of pattern recognition receptors (PRRs), the G protein-coupled receptors (GPCRs), aryl hydrocarbon receptor (AHR) and the nuclear receptors expressed in the intestinal epithelial cells (IECs) and the intestine-resident immune cells. We also discuss the mechanisms by which the disruption of microbial sensing because of genetic or environmental factors causes human diseases such as the inflammatory bowel disease (IBD).

Complex interplay of gut microbiota between obesity and asthma in children

Obesity is an important risk factor and common comorbidity of childhood asthma. Simultaneously, obesity-related asthma, a distinct asthma phenotype, has attracted significant attention owing to its association with more severe clinical manifestations, poorer disease control, and reduced quality of life. The establishment of the gut microbiota during early life is essential for maintaining metabolic balance and fostering the development of the immune system in children. Microbial dysbiosis influences host lipid metabolism, triggers chronic low-grade inflammation, and affects immune responses. It is intimately linked to the susceptibility to childhood obesity and asthma and plays a potentially crucial transitional role in the progression of obesity-related asthma. This review article summarizes the latest research on the interplay between asthma and obesity, with a particular focus on the mediating role of gut microbiota in the pathogenesis of obesity-related asthma. This study aims to provide valuable insight to enhance our understanding of this condition and offer preliminary evidence to support the development of therapeutic interventions.

Chrono-Gerontology: Integrating Circadian Rhythms and Aging in Stroke Research

Stroke is a significant public health concern for elderly individuals. However, the majority of pre-clinical studies utilize young and healthy rodents, which may result in failure of candidate therapies in clinical trials. In this brief review/perspective, the complex link between circadian rhythms, aging, innate immunity, and the gut microbiome to ischemic injury onset, progression, and recovery is discussed. Short-chain fatty acids and nicotinamide adenine dinucleotide+ (NAD+ ) production by the gut microbiome are highlighted as key mechanisms with profound rhythmic behavior, and it is suggested to boost them as prophylactic/therapeutic approaches. Integrating aging, its associated comorbidities, and circadian regulation of physiological processes into stroke research may increase the translational value of pre-clinical studies and help to schedule the optimal time window for existing practices to improve stroke outcome and recovery.