Probiotics and the microbiota-gut-brain axis in neurodegeneration: Beneficial effects and mechanistic insights

Neurodegenerative diseases (NDs) are a group of heterogeneous disorders with a high socioeconomic burden. Although pharmacotherapy is currently the principal therapeutic approach for the management of NDs, mounting evidence supports the notion that the protracted application of available drugs would abate their dopaminergic outcomes in the long run. The therapeutic application of microbiome-based modalities has received escalating attention in biomedical works. In-depth investigations of the bidirectional communication between the microbiome in the gut and the brain offer a multitude of targets for the treatment of NDs or maximizing the patient's quality of life. Probiotic administration is a well-known microbial-oriented approach to modulate the gut microbiota and potentially influence the process of neurodegeneration. Of note, there is a strong need for further investigation to map out the mechanistic prospects for the gut-brain axis and the clinical efficacy of probiotics. In this review, we discuss the importance of microbiome modulation and hemostasis via probiotics, prebiotics, postbiotics and synbiotics in ameliorating pathological neurodegenerative events. Also, we meticulously describe the underlying mechanism of action of probiotics and their metabolites on the gut-brain axis in different NDs. We suppose that the present work will provide a functional direction for the use of probiotic-based modalities in promoting current practical treatments for the management of neurodegenerative-related diseases.

Indole-3-pyruvate: Analysis and Control of Tautomerism and Reactivity

It is well-known in biochemistry that structure confers function, meaning that chemical structural elucidation is critical to truly understanding the function of a given metabolite. Indole-3-pyruvate (IPyA) exists in an equilibrium between the keto and enol tautomeric forms. IPyA is suggested to play a role in immune function; however, determining whether the tautomeric forms function differently can only be studied if an analytical method is capable of distinguishing between the two forms. Herein, we describe the use of UHPLC-HRMS to gain insight into the physical variables that govern IPyA tautomer equilibrium, reactivity, and detection limit. We use hydrogen-deuterium exchange (HDX) to identify enol and keto peaks, and we show that tautomers exhibit a valley of fronting followed by a tailing peak shape (though separation is still attainable) and identical MS/MS spectra. We observed drastically different ratios of keto and enol forms in different solvents, which is an important consideration for in vitro studies. IPyA was found to be highly unstable with accelerated reactivity in peroxides. Through in vitro reactivity studies, IPyA produced a myriad of known and unknown metabolites via nonenzymatic processes, many of which were mapped in vivo via the analysis of human plasma. Finally, we show that vitamin C (ascorbic acid) can slow this reactivity and enable sensitive detection in whole blood.

The influence of indole propionic acid on molecular markers of steroidogenesis, ER stress, and apoptosis in rat granulosa cells exposed to high glucose conditions

Hyperglycemia is known as one of the main causes of infertility in human societies. Indole propionic acid (IPA) is produced by intestinal microbiota and has antioxidant and anti-inflammatory properties. This study aims to investigate the effects of IPA on molecular indices of steroidogenesis, ER stress, and apoptosis induced by high glucose (HG) in granulosa cells. Primary GCs, isolated from ovarian follicles of Rats were cultured in 5 mM (control) and 30 mM (HG) of glucose and in the presence of 10 and 20 µM of IPA for 24 h. The cell viability was assessed by MTT. The gene expression of P450SCC, 3βHSD, CYP19A, BAX, BCL2, and STAR was evaluated by Real-Time PCR. Protein expression of ATF6, PERK, GRP78, and CHOP determined by western blot. Progesterone, estradiol, IL-1β, and TNF-α were measured by ELISA. HG decreased the viability, and expression of P450SCC, 3βHSD, CYP19A, BCL2, STAR, and increased BAX. 10 and 20 µM of IPA increased cell viability, expression of P450SCC, 3βHSD, CYP19A, BCL2 and STAR and decreased BAX compared to the HG group. The expression of ATF6, PERK, GRP78, and CHOP proteins increased by HG and IPA decreased the expression of these proteins compared to the HG group. Also, HG decreased progesterone and estradiol levels and increased IL-1β and TNF-α. IPA significantly increased progesterone and estradiol and decreased IL-1β and TNF-α compared to the HG group. IPA can improve the side effects of HG in GCs of rats, as responsible cells for fertility, by improving steroidogenesis, regulation of ER-stress pathway, suppression of inflammation, and apoptosis.

Indole-3-propionic acid attenuates high glucose induced ER stress response and augments mitochondrial function by modulating PERK-IRE1-ATF4-CHOP signalling in experimental diabetic neuropathy

OBJECTIVES

We aimed to evaluate the neuroprotective effect of Indole-3-propionic acid (IPA) against streptozotocin (STZ) induced diabetic peripheral neuropathy (DPN) in rats and in high glucose (HG) induced neurotoxicity in neuro2a (N2A) cells.

METHODS

Diabetes was induced in male SD rats STZ (55 mg/kg, i.p.) and IPA (10 and 20 mg/kg, p.o.) was administered for two weeks, starting from sixth week after diabetes induction. Neurobehavioral, functional assessments were made, and various molecular studies were performed to evaluate the effect of IPA on HG induced ER stress and mitochondrial dysfunction in sciatic nerves, DRGs and in N2A cells.

RESULTS

Diabetic rats and high glucose exposed N2A cells showed marked increase in oxidative damage accompanied by ER stress and mitochondrial dysfunction along with increased apoptotic markers. IPA treatment for two weeks markedly alleviated these changes and attenuated pain behaviour.

CONCLUSION

IPA exhibited neuroprotective activity against hyperglycaemic insults.

Investigation of Effects of Novel Bifidobacterium longum ssp. longum on Gastrointestinal Microbiota and Blood Serum Parameters in a Conventional Mouse Model

Representatives of the genus Bifidobacterium are widely used as probiotics to modulate the gut microbiome and alleviate various health conditions. The action mechanisms of probiotics rely on their direct effect on the gut microbiota and the local and systemic effect of its metabolites. The main purpose of this animal experiment was to assess the biosafety of the Bifidobacterium longum strain BIOCC1719. Additional aims were to characterise the influence of the strain on the intestinal microbiota and the effect on several health parameters of the host during 15- and 30-day oral administration of the strain to mice. The strain altered the gut microbial community, thereby altering luminal short-chain fatty acid metabolism, resulting in a shift in the proportions of acetic, butyric, and propionic acids in the faeces and serum of the test group mice. Targeted metabolic profiling of serum revealed the possible ability of the strain to positively affect the hosts' amino acids and bile acids metabolism, as the cholic acid, deoxycholic acid, aspartate, and glutamate concentration were significantly higher in the test group. The tendency to increase anti-inflammatory polyamines (spermidine, putrescine) and neuroprotective 3-indolepropionic acid metabolism and to lower uremic toxins (P-cresol-SO4, indoxyl-SO4) was registered. Thus, B. longum BIOCC1719 may exert health-promoting effects on the host through modulation of the gut microbiome and the host metabolome via inducing the production of health-promoting bioactive compounds. The health effects of the strain need to be confirmed in clinical trials with human volunteers.

Expression and metabolism profiles of CVT associated with inflammatory responses and oxygen carrier ability in the brain

AIM

As the main type of stroke, the incidence of cerebral venous thrombosis (CVT) has been rising. However, the comprehensive mechanisms behind it remain unclear. Thus, the multi-omics study is required to investigate the mechanism after CVT and elucidate the characteristic pathology of venous stroke and arterial stroke.

METHODS

Adult rats were subjected to CVT and MCAO models. Whole-transcriptome sequencing (RNA-seq) and untargeted metabolomics analysis were performed to construct the transcriptome and metabolism profiles of rat brains after CVT and also MCAO. The difference analysis, functional annotation, and enrichment analysis were also performed.

RESULTS

Through RNA-seq analysis, differentially expressed genes (DEGs) were screened. 174 CVT specific genes including Il1a, Ccl9, Cxxl6, Tnfrsf14, etc., were detected. The hemoglobin genes, including both Hba and Hbb, were significantly downregulated after CVT, compared both to the MCAO and Sham groups. Metabolism analysis showed that CVT had higher heterogeneity of metabolism compared to MCAO. Metabolites including N-stearoyltyrosine, 5-methoxy-3-indoleaceate, Afegostat, pipecolic acid, etc. were specially regulated in CVT. Through the immune infiltration analysis, it was found that CVT had a higher immune response, with the abundance of certain types of immune cells increased, especially T helper cells. It was important to find the prevalence of the activation of inflammatory chemokine, cytokine, NOD-like pathway, and neutrophil extracellular trap.

CONCLUSION

We explored and analyzed the gene expression and metabolomic characteristics of CVT, revealed the specific inflammatory reaction mechanism of CVT and found the markers in transcriptome and metabolism levels. It points out the direction for CVT early diagnosis and treatment.

Update on the gut microbiome in health and diseases

The Human Microbiome Project, Earth Microbiome Project, and next-generation sequencing have advanced novel genome association, host genetic linkages, and pathogen identification. The microbiome is the sum of the microbes, their genetic information, and their ecological niche. This study will describe how millions of bacteria in the gut affect the human body in health and disease. The gut microbiome changes in relation with age, with an increase in Bacteroidetes and Firmicutes. Host and environmental factors affecting the gut microbiome are diet, drugs, age, smoking, exercise, and host genetics. In addition, changes in the gut microbiome may affect the local gut immune system and systemic immune system. In this study, we discuss how the microbiome may affect the metabolism of healthy subjects or may affect the pathogenesis of metabolism-generating metabolic diseases. Due to the high number of publications on the argument, from a methodologically point of view, we decided to select the best papers published in referred journals in the last 3 years. Then we selected the previously published papers. The major goals of our study were to elucidate which microbiome and by which pathways are related to healthy and disease conditions.

Update on the reciprocal interference between immunosuppressive therapy and gut microbiota after kidney transplantation

Gut microbiota is often modified after kidney transplantation. This principally happens in the first period after transplantation. Antibiotics and, most of all, immunosuppressive drugs are the main responsible. The relationship between immunosuppressive drugs and the gut microbiota is bilateral. From one side immunosuppressive drugs modify the gut microbiota, often generating dysbiosis; from the other side microbiota may interfere with the immunosuppressant pharmacokinetics, producing products more or less active with respect to the original drug. These phenomena have influence over the graft outcomes and clinical consequences as rejections, infections, diarrhea may be caused by the dysbiotic condition. Corticosteroids, calcineurin inhibitors such as tacrolimus and cyclosporine, mycophenolate mofetil and mTOR inhibitors are the immunosuppressive drugs whose effect on the gut microbiota is better known. In contrast is well known how the gut microbiota may interfere with glucocorticoids, which may be transformed into androgens. Tacrolimus may be transformed by micro biota into a product called M1 that is 15-fold less active with respect to tacrolimus. The pro-drug mycophenolate mofetil is normally transformed in mycophenolic acid that according the presence or not of microbes producing the enzyme glu curonidase, may be transformed into the inactive product.

Indole-3-acetic acid ameliorates dextran sulfate sodium-induced colitis via the ERK signaling pathway

Microbiota-derived catabolism of nutrients is closely related to ulcerative colitis (UC). The level of indole-3-acetic acid (IAA), a microbiota-dependent metabolite of tryptophan, was decreased significantly in the feces of UC patients. Thus supplementation with IAA could be a potential therapeutic method for ameliorating colitis. In this work, the protective effect of supplementation with IAA on dextran sulfate sodium (DSS)-induced colitis was evaluated, and the underlying mechanism was elucidated. The results indicated that the administration of IAA significantly relieved DSS-induced weight loss, reduced the disease activity index (DAI), restored colon length, alleviated intestinal injury, and improved the intestinal tight junction barrier. Furthermore, IAA inhibited intestinal inflammation by reducing the expression of proinflammatory cytokines and promoting the production of IL-10 and TGF-β1. In addition, the ERK signaling pathway is an important mediator of various physiological processes including inflammatory responses and is closely associated with the expression of IL-10. Notably, IAA treatment induced the activation of extracellular signal-regulated kinase (ERK), which is involved in the progression of colitis, while the ERK inhibitor U0126 attenuated the beneficial effects of IAA. In summary, IAA could attenuate the clinical symptoms of colitis, and the ERK signaling pathway was involved in the underlying mechanism. Supplementation with IAA could be a potential option for preventing or ameliorating UC.

Effects of biodegradable and conventional microplastics on the intestine, intestinal community composition, and metabolic levels in tilapia (Oreochromis mossambicus)

Despite growing interest in conventional microplastics (CMPs) and their toxicological effects on aquatic species, little is known about biodegradable microplastics (BMPs) and their corresponding implications for aquatic life. Here, tilapia (Oreochromis mossambicus) were semi-statically exposed for 14 days to the bio-based plastic polylactic acid (PLA, 100 μg/L, 2.52 ± 0.46 μm) and the petroleum-based plastic polyvinyl chloride (PVC, 100 μg/L, 1.58 ± 0.36 μm). The results showed that ingesting the above two types of microplastics (MPs) led to oxidative stress in the fish gut, and damage to gut tissues and organelles, and PLA resulted in more obvious gut tissue edema than PVC. Furthermore, PLA caused increased levels of gut microbiota dysbiosis and a decrease in the abundance of the genus Cetobacterium, which is linked to vitamin B-12 synthesis, whereas an opposite relationship was observed on PVC. Metabolomic analysis indicated that PVC caused a significant down-regulation of orotic acid, co-metabolite of folic acid with vitamin B-12, while PLA did not affect orotic acid, which may lead to the accumulation of folic acid in fish. The joint analysis found that MPs disturbed gut metabolism homeostasis, implying that abnormal gut microbiota metabolites may be a key mechanism for MPs to induce tissue damage and oxidative stress in the gut. Overall, this study systematically illustrates the differential toxic effects of BMPs and CMPs on tilapia through gut microbiota and metabolite interactions, which will contribute to assessing the risks of BMPs to organismal health.

The role of the indoles in microbiota-gut-brain axis and potential therapeutic targets: A focus on human neurological and neuropsychiatric diseases

At present, a large number of relevant studies have suggested that the changes in gut microbiota are related to the course of nervous system diseases, and the microbiota-gut-brain axis is necessary for the proper functioning of the nervous system. Indole and its derivatives, as the products of the gut microbiota metabolism of tryptophan, can be used as ligands to regulate inflammation and autoimmune response in vivo. In recent years, some studies have found that the levels of indole and its derivatives differ significantly between patients with central nervous system diseases and healthy individuals, suggesting that they may be important mediators for the involvement of the microbiota-gut-brain axis in the disease course. Tryptophan metabolites produced by gut microbiota are involved in multiple physiological reactions, take indole for example, it participates in the process of inflammation and anti-inflammatory effects through various cellular physiological activities mediated by aromatic hydrocarbon receptors (AHR), which can influence a variety of neurological and neuropsychiatric diseases. This review mainly explores and summarizes the relationship between indoles and human neurological and neuropsychiatric disorders, including ischemic stroke, Alzheimer's disease, Parkinson's disease, multiple sclerosis, cognitive impairment, depression and anxiety, and puts forward that the level of indoles can be regulated through various direct or indirect ways to improve the prognosis of central nervous system diseases and reverse the dysfunction of the microbiota-gut-brain axis. This article is part of the Special Issue on "Microbiome & the Brain: Mechanisms & Maladies".

Microbiota-indole 3-propionic acid-brain axis mediates abnormal synaptic pruning of hippocampal microglia and susceptibility to ASD in IUGR offspring

BACKGROUND

Autism spectrum disorder (ASD) has been associated with intrauterine growth restriction (IUGR), but the underlying mechanisms are unclear.

RESULTS

We found that the IUGR rat model induced by prenatal caffeine exposure (PCE) showed ASD-like symptoms, accompanied by altered gut microbiota and reduced production of indole 3-propionic acid (IPA), a microbiota-specific metabolite and a ligand of aryl hydrocarbon receptor (AHR). IUGR children also had a reduced serum IPA level consistent with the animal model. We demonstrated that the dysregulated IPA/AHR/NF-κB signaling caused by disturbed gut microbiota mediated the hippocampal microglia hyperactivation and neuronal synapse over-pruning in the PCE-induced IUGR rats. Moreover, postnatal IPA supplementation restored the ASD-like symptoms and the underlying hippocampal lesions in the IUGR rats.

CONCLUSIONS

This study suggests that the microbiota-IPA-brain axis regulates ASD susceptibility in PCE-induced IUGR offspring, and supplementation of microbiota-derived IPA might be a promising interventional strategy for ASD with a fetal origin. Video Abstract.

Microbiota-derived tryptophan metabolism: Impacts on health, aging, and disease

The intricate interplay between gut microbiota and the host is pivotal in maintaining homeostasis and health. Dietary tryptophan (TRP) metabolism initiates a cascade of essential endogenous metabolites, including kynurenine, kynurenic acid, serotonin, and melatonin, as well as microbiota-derived Trp metabolites like tryptamine, indole propionic acid (IPA), and other indole derivatives. Notably, tryptamine and IPA, among the indole metabolites, exert crucial roles in modulating immune, metabolic, and neuronal responses at both local and distant sites. Additionally, these metabolites demonstrate potent antioxidant and anti-inflammatory activities. The levels of microbiota-derived TRP metabolites are intricately linked to the gut microbiota's health, which, in turn, can be influenced by age-related changes. This review aims to comprehensively summarize the cellular and molecular impacts of tryptamine and IPA on health and aging-related complications. Furthermore, we explore the levels of tryptamine and IPA and their corresponding bacteria in select diseased conditions, shedding light on their potential significance as biomarkers and therapeutic targets.

Early-life noise exposure causes cognitive impairment in a sex-dependent manner by disrupting homeostasis of the microbiota-gut-brain axis

Epidemiological investigations show that noise exposure in early life is associated with health and cognitive impairment. The gut microbiome established in early life plays a crucial role in modulating developmental processes that subsequently affect brain function and behavior. Here, we examined the impact of early-life exposure to noise on cognitive function in adolescent rats by analyzing the gut microbiome and metabolome to elucidate the underlying mechanisms. Chronic noise exposure during early life led to cognitive deficits, hippocampal injury, and neuroinflammation. Early-life noise exposure showed significant difference on the composition and function of the gut microbiome throughout adolescence, subsequently causing axis-series changes in fecal short-chain fatty acid (SCFA) metabolism and serum metabolome profiles, as well as dysregulation of endothelial tight junction proteins, in both intestine and brain. We also observed sex-dependent effects of microbiota depletion on SCFA-related beneficial bacteria in adolescence. Experiments on microbiota transplantation and SCFA supplementation further confirmed the role of intestinal bacteria and related SCFAs in early-life noise-exposure-induced impairments in cognition, epithelial integrity, and neuroinflammation. Overall, these results highlight the homeostatic imbalance of microbiota-gut-brain axis as an important physiological response toward environmental noise during early life and reveals subtle differences in molecular signaling processes between male and female rats.

Scientific Landscape of Oxidative Stress in Stroke: From a Bibliometric Analysis to an in-Depth Review

Stroke is an acute cerebrovascular disease resulting from either obstruction or rupture of a blood vessel in the brain. Oxidative stress (OS), referred to a status where cellular oxidative capacities overwhelm antioxidative defenses, is involved in the pathophysiology of stroke. The bibliometric analysis and in-depth review aim to depict the research trend of OS in stroke. Relevant scientific publications were acquired from the Web of Science Core Collection database. Scientific landscape of OS in stroke was illustrated by general quantitative trend, impactful journals, and co-authorship of various academic units (i.e., countries/regions, organizations, and authors). Furthermore, theme analysis predicting the hot research issues and frontiers was performed. 15,826 documents regarding OS in stroke were obtained over a time span of more than 20 years from 1992 to 2021. The overall tendency of publication counts was continuously on the rise. Bibliometric analysis indicated China and the United States were predominant in this study field, as reflected by their high publication counts and intensive collaboration with other countries. Current key research areas of OS in stroke may lie in the investigation of neuroinflammation, and interaction among multiple cell death mechanisms including apoptosis, autophagy, and ferroptosis to search for effective treatments. Moreover, another hot topic could be the association between air pollution and stroke, and its underlying mechanisms. As the exploration of OS in stroke is speculated to be a continuous hot spot in the future, this article may be helpful for researchers to conduct future studies with the understanding of influential academic forces and research highlights.

Discrete interplay of gut microbiota L-tryptophan metabolites in host biology and disease

The gut microbiota and the host maintain a conjoint relationship and together achieve optimal physiology via a multitude of interactive signalling cues. Dietary-derived L-tryptophan (L-trp) is enzymatically metabolized by the resident symbiotic gut microbiota to indole and various indole derivatives. Indole and indole metabolites secreted by the gut bacteria act locally in the intestinal cells as well as distally and modulate tissue-specific functions which are beneficial to the host. Functions attributed to these microbial indole metabolites in the host include regulation of intestinal permeability, immunity and mucosal roles, inflammation, and insulin sensitivity. On the other hand, dysregulation of gut microbiota L-trp metabolism compromises the optimal availability of indole and indole metabolites and can induce the onset of metabolic disorders, inflammation, liver steatosis, and decrease gut barrier integrity. Gut dysbiosis is regarded as one of the prime reasons for this deregulated microbial-derived indole metabolites. A number of indole metabolites from the gut bacteria have been identified recently displaying variable affinity towards xenobiotic nuclear receptors. Microbial metabolite mimicry concept can be used to design and develop novel indole-moiety-containing compounds with higher affinity towards the receptors and efficacy in preclinical studies. Such compounds may serve as therapeutic drugs in clinical trials in the future. In this article, I review L-trp metabolism in the host and gut microbiota and the various physiological functions, patho-physiologies associated with the microbial-released indole metabolites in the host, including the metabolite mimicry-based concept to develop tailored indole-containing novel experimental drugs.

Determination of tryptophan and its indole metabolites in follicular fluid of women with diminished ovarian reserve

Tryptophan (TRP) and its indole metabolites exhibit numerous biological effects, especially their antioxidant properties. This study used untargeted metabolomics in conjunction with targeted metabolomics to investigate the differential expression of tryptophan and its indole metabolites in follicular fluid (FF) of diminished ovarian reserve (DOR) and normal ovarian reserve (NOR) populations. This study included patients with DOR (n = 50) and females with NOR (n = 35) who received in vitro fertilization and embryo transfer. Untargeted metabolomics suggests that diminished ovarian reserve affects the metabolic profile of FF, TRP and indole metabolites were significantly down-regulated in the DOR group. Targeted metabolomics quantification revealed that the levels of TRP, IPA and IAA in the FF of the DOR group were significantly lower than those of the NOR group (P < 0.01). The concentration of TRP in FF is positively correlated with the available embryo rate in NOR females. These results provide data support to explore the pathogenesis of DOR and to look for new biomarkers and ovarian protectors. Additionally, alterations in TRP and its indole metabolites in FF may indirectly reflect the interaction between intestinal flora and the follicular microenvironment.

Maternal PBDE exposure disrupts gut microbiome and promotes hepatic proinflammatory signaling in humanized PXR-transgenic mouse offspring over time

Developmental exposure to the persistent environmental pollutant, polybrominated diphenyl ethers (PBDEs), is associated with increased diabetes prevalence. The microbial tryptophan metabolite, indole-3-propionic acid (IPA), is associated with reduced risk of type 2 diabetes and lower-grade inflammation and is a pregnane X receptor (PXR) activator. To explore the role of IPA in modifying the PBDE developmental toxicity, we orally exposed humanized PXR-transgenic (hPXR-TG) mouse dams to vehicle, 0.1 mg/kg/day DE-71 (an industrial PBDE mixture), DE-71+IPA (20 mg/kg/day), or IPA, from 4 weeks preconception to the end of lactation. Pups were weaned at 21 days of age and IPA supplementation continued in the corresponding treatment groups. Tissues were collected at various ages until 6 months of age (n = 5 per group). In general, the effect of maternal DE-71 exposure on the gut microbiome of pups was amplified over time. The regulation of hepatic cytokines and prototypical xenobiotic-sensing transcription factor target genes by DE-71 and IPA was age- and sex-dependent, where DE-71-mediated mRNA increased selected cytokines (Il10, Il12p40, Il1β [both sexes], and [males]). The hepatic mRNA of the aryl hydrocarbon receptor (AhR) target gene Cyp1a2 was increased by maternal DE-71 and DE-71+IPA exposure at postnatal day 21 but intestinal Cyp1a1 was not altered by any of the exposures and ages. Maternal DE-71 exposure persistently increased serum indole, a known AhR ligand, in age- and sex-dependent manner. In conclusion, maternal DE-71 exposure produced a proinflammatory signature along the gut-liver axis, including gut dysbiosis, dysregulated tryptophan microbial metabolism, attenuated PXR signaling, and elevated AhR signaling in postweaned hPXR-TG pups over time, which was partially corrected by IPA supplementation.

Host-Microbiome Interactions: Tryptophan Metabolism and Aromatic Hydrocarbon Receptors after Traumatic Brain Injury

Traumatic brain injury refers to the damage caused to intracranial tissues by an external force acting on the head, leading to both immediate and prolonged harmful effects. Neuroinflammatory responses play a critical role in exacerbating the primary injury during the acute and chronic phases of TBI. Research has demonstrated that numerous neuroinflammatory responses are mediated through the "microbiota-gut-brain axis," which signifies the functional connection between the gut microbiota and the brain. The aryl hydrocarbon receptor (AhR) plays a vital role in facilitating communication between the host and microbiota through recognizing specific ligands produced directly or indirectly by the microbiota. Tryptophan (trp), an indispensable amino acid in animals and humans, represents one of the key endogenous ligands for AhR. The metabolites of trp have significant effects on the functioning of the central nervous system (CNS) through activating AHR signalling, thereby establishing bidirectional communication between the gut microbiota and the brain. These interactions are mediated through immune, metabolic, and neural signalling mechanisms. In this review, we emphasize the co-metabolism of tryptophan in the gut microbiota and the signalling pathway mediated by AHR following TBI. Furthermore, we discuss the impact of these mechanisms on the underlying processes involved in traumatic brain injury, while also addressing potential future targets for intervention.

Gut microbiota indole-3-propionic acid mediates neuroprotective effect of probiotic consumption in healthy elderly: A randomized, double-blind, placebo-controlled, multicenter trial and in vitro study

BACKGROUND & AIMS

The beneficial effects of probiotic consumption on age-related decline in cerebral function have been previously reported in the literature; however, the mechanistic link between gut and brain interactions has not yet been fully elucidated. Therefore, this study aimed to identify the role of gut microbiota-derived metabolites in gut-brain interactions via blood metabolomic profiling analysis in clinical trials and in vitro mechanistic studies.

METHODS

A randomized, double-blind, placebo-controlled, multicenter clinical trial was conducted in 63 healthy elderly individuals (≥65 years of age). Participants were administered either placebo (placebo group, N = 31) or probiotic capsules (Bifidobacterium bifidum BGN4 and Bifidobacterium longum BORI; probiotics group, N = 32) for 12 weeks. Global and targeted metabolomic profiling analyses of their blood samples were then performed using ¹H nuclear magnetic resonance and liquid chromatography-mass spectrometry methods, both at baseline and at the end of the trial. Gut microbial analysis was conducted using the 16S ribosomal ribonucleic acid gene sequencing method. Subsequently, microglial BV2 cells were treated in vitro with indole-3-propionic acid (IPA) following lipopolysaccharide stimulation, and neuronal SH-SY5Y cells were treated with conditioned media from the BV2 cells. Finally, the levels of pro-inflammatory cytokines in BV2 cells and neurotrophins in SH-SY5Y cells were quantified using a real-time polymerase chain reaction or enzyme-linked immunosorbent assay.

RESULTS

The metabolomic profiling analyses showed that probiotic consumption significantly altered the levels of metabolites involved in tryptophan metabolism (P < 0.01). Among these metabolites, gut microbiota-produced IPA had a 1.91-fold increase in the probiotics group (P < 0.05) and showed a significant relation to gut bacterial profiles (P < 0.01). Elevated IPA levels were also positively associated with the level of serum brain-derived neurotropic factor (BDNF) in the probiotics group (r = 0.28, P < 0.05), showing an inverse trend compared to the placebo group. In addition, in vitro treatment with IPA (5 μM) significantly reduced the concentration of proinflammatory TNF-α in activated microglia (P < 0.05), and neuronal cells cultured with conditioned media from IPA-treated microglia showed a significant increase in BDNF and nerve growth factor production (P < 0.05).

CONCLUSIONS

These results show that gut microbiota-produced IPA plays a role in protecting the microglia from inflammation, thus promoting neuronal function. Therefore, this suggests that IPA is a significant mediator linking the interaction between the gut and the brain in the elderly with probiotic supplementation.

Determination of Indolepropionic Acid and Related Indoles in Plasma, Plasma Ultrafiltrate, and Saliva

The microbial metabolite indolepropionic acid (IPA) and related indolic metabolites, including indolecarboxylic acid (ICA), indolelactic acid (ILA), indoleacetic acid (IAA), indolebutyric acid (IBA), indoxylsulfate (ISO4), and indole, were determined in human plasma, plasma ultrafiltrate (UF), and saliva. The compounds were separated on a 150 × 3 mm column of 3 μm Hypersil C18 eluted with a mobile phase of 80% pH 5 0.01 M sodium acetate containing 1.0 g/L of tert-butylammonium chloride/20% acetonitrile and then detected fluorometrically. Levels of IPA in human plasma UF and of ILA in saliva are reported for the first time. The determination of IPA in plasma UF enables the first report of free plasma IPA, the presumed physiologically active pool of this important microbial metabolite of tryptophan. Plasma and salivary ICA and IBA were not detected, consistent with the absence of any prior reported values. Observed levels or limits of detection for other indolic metabolites usefully supplement limited prior reports.

Understanding the ADHD-Gut Axis by Metabolic Network Analysis

Attention deficit hyperactivity disorder (ADHD) is a neurodevelopmental disorder diagnosed with hyperactivity, impulsivity, and a lack of attention inconsistent with the patient's development level. The fact that people with ADHD frequently experience gastrointestinal (GI) dysfunction highlights the possibility that the gut microbiome may play a role in this condition. The proposed research aims to determine a biomarker for ADHD by reconstructing a model of the gut-microbial community. Genome-scale metabolic models (GEM) considering the relationship between gene-protein-reaction associations are used to simulate metabolic activities in organisms of gut. The production rates of dopamine and serotonin precursors and the key short chain fatty acids which affect the health status are determined under three diets (Western, Atkins', Vegan) and compared with those of healthy people. Elasticities are calculated to understand the sensitivity of exchange fluxes to changes in diet and bacterial abundance at the species level. The presence of Bacillota (genus Coprococcus and Subdoligranulum), Actinobacteria (genus Collinsella), Bacteroidetes (genus Bacteroides), and Bacteroidota (genus Alistipes) may be possible gut microbiota indicators of ADHD. This type of modeling approach taking microbial genome-environment interactions into account helps us understand the gastrointestinal mechanisms behind ADHD, and establish a path to improve the quality of life of ADHD patients.

New Indole-3-Propionic Acid and 5-Methoxy-Indole Carboxylic Acid Derived Hydrazone Hybrids as Multifunctional Neuroprotectors

In light of the known neuroprotective properties of indole compounds and the promising potential of hydrazone derivatives, two series of aldehyde-heterocyclic hybrids combining those pharmacophores were synthesized as new multifunctional neuroprotectors. The obtained derivatives of indole-3-propionic acid (IPA) and 5-methoxy-indole carboxylic acid (5MICA) had good safety profiles: Hemolytic effects < 5% (200 μM) and IC₅₀ > 150 µM were found in the majority of the SH-SY5Y and bEnd3 cell lines. The 2,3-dihydroxy, 2-hydroxy-4-methoxy, and syringaldehyde derivatives of 5MICA exhibited the strongest neuroprotection against H₂O₂-induced oxidative stress in SH-SY5Y cells and 6-OHDA-induced neurotoxicity in rat-brain synaptosomes. All the compounds suppressed the iron-induced lipid peroxidation. The hydroxyl derivatives were also the most active in terms of deoxyribose-degradation inhibition, whereas the 3,4-dihydroxy derivatives were able to decrease the superoxide-anion generation. Both series of compounds showed an increased inhibition of hMAO-B, with greater expression detected in the 5MICA hybrids. The in vitro BBB model with the bEnd3 cell line showed that some compounds increased the permeability of the endothelial monolayer while maintaining the tight junctions. The combined results demonstrated that the derivatives of IPA and 5MICA showed strong neuroprotective, antioxidant, MAO-B inhibitory activity and could be considered as prospective multifunctional compounds for the treatment of neurodegenerative disorders.

Activation of aryl hydrocarbon receptor (AhR) in Alzheimer's disease: role of tryptophan metabolites generated by gut host-microbiota

Gut microbiota in interaction with intestinal host tissues influences many brain functions and microbial dysbiosis has been linked with brain disorders, such as neuropsychiatric conditions and Alzheimer's disease (AD). L-tryptophan metabolites and short-chained fatty acids (SCFA) are major messengers in the microbiota-brain axis. Aryl hydrocarbon receptors (AhR) are main targets of tryptophan metabolites in brain microvessels which possess an enriched expression of AhR protein. The Ah receptor is an evolutionarily conserved, ligand-activated transcription factor which is not only a sensor of xenobiotic toxins but also a pleiotropic regulator of both developmental processes and age-related tissue degeneration. Major microbiota-produced tryptophan metabolites involve indole derivatives, e.g., indole 3-pyruvic acid, indole 3-acetaldehyde, and indoxyl sulfate, whereas indoleamine and tryptophan 2,3-dioxygenases (IDO/TDO) of intestine host cells activate the kynurenine (KYN) pathway generating KYN metabolites, many of which are activators of AhR signaling. Chronic kidney disease (CKD) increases the serum level of indoxyl sulfate which promotes AD pathogenesis, e.g., it disrupts integrity of blood-brain barrier (BBB) and impairs cognitive functions. Activation of AhR signaling disturbs vascular homeostasis in brain; (i) it controls blood flow via the renin-angiotensin system, (ii) it inactivates endothelial nitric oxide synthase (eNOS), thus impairing NO production and vasodilatation, and (iii) it induces oxidative stress, stimulates inflammation, promotes cellular senescence, and enhances calcification of vascular walls. All these alterations are evident in cerebral amyloid angiopathy (CAA) in AD pathology. Moreover, AhR signaling can disturb circadian regulation and probably affect glymphatic flow. It seems plausible that dysbiosis of gut microbiota impairs the integrity of BBB via the activation of AhR signaling and thus aggravates AD pathology. KEY MESSAGES: Dysbiosis of gut microbiota is associated with dementia and Alzheimer's disease. Tryptophan metabolites are major messengers from the gut host-microbiota to brain. Tryptophan metabolites activate aryl hydrocarbon receptor (AhR) signaling in brain. The expression of AhR protein is enriched in brain microvessels and blood-brain barrier. Tryptophan metabolites disturb brain vascular integrity via AhR signaling. Dysbiosis of gut microbiota promotes inflammation and AD pathology via AhR signaling.

Extensive Summary of the Important Roles of Indole Propionic Acid, a Gut Microbial Metabolite in Host Health and Disease

Increasing evidence suggests that metabolites produced by the gut microbiota play a crucial role in host-microbe interactions. Dietary tryptophan ingested by the host enters the gut, where indole-like metabolites such as indole propionic acid (IPA) are produced under deamination by commensal bacteria. Here, we summarize the IPA-producing bacteria, dietary patterns on IPA content, and functional roles of IPA in various diseases. IPA can not only stimulate the expression of tight junction (TJ) proteins to enhance gut barrier function and inhibit the penetration of toxic factors, but also modulate the immune system to exert anti-inflammatory and antioxidant effects to synergistically regulate body physiology. Moreover, IPA can act on target organs through blood circulation to form the gut-organ axis, which helps maintain systemic homeostasis. IPA shows great potential for the diagnosis and treatment of various clinical diseases, such as NAFLD, Alzheimer's disease, and breast cancer. However, the therapeutic effect of IPA depends on dose, target organ, or time. In future studies, further work should be performed to explore the effects and mechanisms of IPA on host health and disease to further improve the existing treatment program.

Indole-3-propionic Acid Attenuates HI-Related Blood-Brain Barrier Injury in Neonatal Rats by Modulating the PXR Signaling Pathway

The blood-brain barrier (BBB) is an important physiological barrier of the human body contributing to maintaining brain homeostasis and normal function. Hypoxic-ischemic (HI)-related brain injury is one of the main causes of neonatal acute morbidity and chronic disability. The previous research of our group confirmed that there was serious BBB destruction during HI brain injury. However, at present, the protection strategy of BBB is very limited, and further research on the protection mechanism is warranted. Indole-3-propionic acid (IPA) is a bacterial metabolism with anti-inflammatory and antioxidant properties, having neuroprotective effects and protective effects on the mucosal barrier. However, the role of IPA in BBB is not clear. In this research, we demonstrated the protective effect of IPA on BBB disruption from HI brain injury and hypothesized that it involves the amelioration of inflammation, oxidative stress, and MMP activation, thereby inhibiting apoptosis of rat brain microvascular endothelial cells (rBMECs). We demonstrated that expression levels of several inflammatory markers, including iNOS, TNF-α, IL-6, and IL-1β, were significantly increased from HI damage or OGD injury. However, IPA treatment inhibited the increase significantly. Moreover, we demonstrated that IPA reduced intracellular ROS levels and MMP activation in rBMECs from OGD injury. Further research on the underlying detailed molecular mechanisms suggested that IPA attenuates inflammation by inhibiting NF-κB signaling. Finally, we investigated the mechanism of the relationship between PXR activation and NF-κB inhibition. The results suggested overexpression of PXR in rBMECs could significantly counteract the decrease of junction proteins and downregulate the increased p-IκB-α and p-NF-κB from OGD injury. However, the protective effects of IPA were reversed by antagonists of the PXR. Taken together, IPA might mitigate HI-induced damage of the BBB and the protective effect may be exerted through modulating the PXR signaling pathway.

Bifidobacterium breve MCC1274 Supplementation Increased the Plasma Levels of Metabolites with Potential Anti-Oxidative Activity in APP Knock-In Mice

Background: We previously reported the effects of a probiotic strain, Bifidobacterium breve MCC1274, in improving cognitive function in preclinical and clinical studies. Recently, we demonstrated that supplementation of this strain led to decreased amyloid-β production, attenuated microglial activation, and suppressed inflammation reaction in the brain of APP knock-in (App NL - G - F ) mice. Objective: In this study, we investigated the plasma metabolites to reveal the mechanism of action of this probiotic strain in this Alzheimer’s disease (AD)-like model. Methods: Three-month-old mice were orally supplemented with B. breve MCC1274 or saline for four months and their plasma metabolites were comprehensively analyzed using CE-FTMS and LC-TOFMS. Results: Principal component analysis showed a significant difference in the plasma metabolites between the probiotic and control groups (PERMANOVA, p = 0.03). The levels of soy isoflavones (e.g., genistein) and indole derivatives of tryptophan (e.g., 5-methoxyindoleacetic acid), metabolites with potent anti-oxidative activities were significantly increased in the probiotic group. Moreover, there were increased levels of glutathione-related metabolites (e.g., glutathione (GSSG)_divalent, ophthalmic acid) and TCA cycle-related metabolites (e.g., 2-Oxoglutaric acid, succinic acid levels) in the probiotic group. Similar alternations were observed in the wild-type mice by the probiotic supplementation. Conclusion: These results suggest that the supplementation of B. breve MCC1274 enhanced the bioavailability of potential anti-oxidative metabolites from the gut and addressed critical gaps in our understanding of the gut-brain axis underlying the mechanisms of the probiotic action of this strain in the improvement of cognitive function.

Oral administration of asparagine and 3-indolepropionic acid prolongs survival time of rats with traumatic colon injury

BACKGROUND

Traumatic colon injury (TCI) is a common disease during wartime. Prolongation of posttraumatic survival time is an effective approach to patient outcome improvement. However, there is a lack of basic research in this field. This study aimed to elucidate the mechanisms underlying TCI progression and to develop novel regimens to buy time for TCI patients on the battlefield.

METHODS

A total of 669 Sprague-Dawley rats were used in this study. Surgical colon incision was performed to generate the TCI rat model. The landscape of colon microbiota compositions was depicted using 16S rRNA sequencing and metabolites in the intestinal contents were detected by metabolomics profiling. The signaling transduction in the intestinal epithelium was investigated using antibody microarrays and Western blotting. The enzyme-linked immunosorbent assay was conducted to measure the levels of interleukin-6 and tumor necrosis factor-α in intestines and plasma for the detection of inflammatory responses. Diamine oxidase, D-lactate and endotoxin in plasma and protein expression of zonula occludens 1 and occludin were selected as the indicators of intestinal barrier permeability. To investigate alterations of microbiota symbiosis, the relative abundances of specific bacterial genera were detected using quantitative real-time PCR.

RESULTS

As a type of lethal injury, TCI induced acute disruption of intestinal homeostasis, characterized by inflammatory responses, intestinal barrier hyperpermeability and microbiota dysbiosis (P < 0.05). Significant alterations in bacterial metabolic patterns were detected with decreases in many metabolites. After a series of screenings, we found that oral administration of asparagine (Asn) and 3-indolepropionic acid (IPA) effectively prolonged posttraumatic survival time [Asn plus IPA vs. Vehicle: hazard ratio (HR) = 0.105, 95% CI 0.031-0.356, P = 0.0003] and restored intestinal homeostasis in TCI rats (P < 0.05). Mechanistically, this combinational strategy protected the rats against TCI through synergistic activation of Akt signaling in the intestinal epithelium (P < 0.05).

CONCLUSIONS

Abrupt dysregulation of intestinal homeostasis plays a critical role in the progression toward TCI-induced death. Oral administration of Asn plus IPA may serve as an effective regimen to restore intestinal functions and prolong the posttraumatic survival time.

Indole-3-Propionic Acid as a Potential Therapeutic Agent for Sepsis-Induced Gut Microbiota Disturbance

The effects of using gut microbiota metabolites instead of live microorganisms to modulate sepsis-induced gut dysbiosis remain largely unknown. We assessed the effects of microbiota metabolite indole-3-propionic acid (IPA) on gut microbiota in mice during sepsis. Sepsis models were constructed by cecal ligation and puncture (CLP) methods. Fecal microbiota composition analysis was performed to characterize the gut microbiota composition. Fecal microbiota transplantation was performed to validate the roles of gut microbiota on sepsis progression. IPA-treated mice exhibited lower serum inflammatory mediator levels and a higher survival rate than those of saline-treated mice after modeling of sepsis, which were negated in the presence of antibiotics. Compared with saline-treated mice after modeling, IPA-treated mice showed a markedly different intestinal microbiota composition, with an enrichment of Bifidobacteriaceae family and a depletion of Enterobacteriaceae family. Mice gavaged with postoperative feces from IPA-treated animals displayed better survival than mice gavaged with feces from saline-treated animals. Overall, these data suggest that IPA offers a microbe-modulated survival advantage in septic mice, indicating that some microbiota metabolites could replace live microorganisms as potential options for regulation of sepsis-induced gut dysbiosis. IMPORTANCE The role of gut microbiota in the pathophysiology of sepsis is gaining increasing attention and developing effective and safe sepsis therapies targeting intestinal microorganisms is promising. Given the safety of probiotic supplementation or fecal microbiota transplantation in critically ill patients, identifying an abiotic agent to regulate the intestinal microbiota of septic patients is of clinical significance. This study revealed that IPA, a microbiota-generated tryptophan metabolite, ameliorated sepsis-induced mortality and decreased the serum levels of proinflammatory cytokines by modulating intestinal microbiota. Although IPA did not increase the abundance and diversity of the microbiota of septic mice, it significantly decreased the number of Enterobacteriaceae family. These findings indicate that a specific microbiota metabolite (e.g., IPA) can mediate the intestinal microbiota apart from FMT or probiotics.

Gut Flora Mediates the Rapid Tolerance of Electroacupuncture on Ischemic Stroke by Activating Melatonin Receptor through Regulating Indole-3-Propionic Acid

Electroacupuncture (EA) is commonly used to treat cerebrovascular diseases. This study aimed to clarify the mechanisms of action of treatments of cerebral ischemic stroke from the perspective of gut microecology. We used a mouse model and cell cultures to investigate the effects of EA on the intestinal microflora in mice models of middle cerebral artery occlusion (MCAO) and the mechanisms underlying the antioxidant activities of metabolites. Fecal microbiota transplantation (FMT) was used to validate the roles of gut microbiota. Metabolomic analysis was performed to characterize the metabolic profile differences between the mice in the EA + MCAO and MCAO groups. Gavaging with feces relieved brain damage in mice that received EA (EA mice) more than in mice that did not (non-EA [NEA] mice). The gut microbial composition and metabolic profiles of the EA and NEA mice were different. In particular, the microbiota from the mice in the EA or EA-FMT groups generated more indole-3-propionic acid (IPA) than the microbiota from the mice in the MCAO or NEA-FMT groups. We confirmed that IPA binds to specific melatonin receptors (MTRs) in target cells and exerts antioxidant effects by adding MTR inhibitors or knocking out the MTR1 gene in vivo and in the oxygen and glucose deprivation/reperfusion models of N2a cell experiments. EA can prevent ischemic stroke by improving the composition of intestinal microbiota in MCAO mice. Moreover, this study reveals a new mechanism of intestinal flora regulation of stroke that differs from inflammation/immunity, namely gut microbiota regulates stroke by affecting IPA levels.

Multi-Omic Analyses Reveal Bifidogenic Effect and Metabolomic Shifts in Healthy Human Cohort Supplemented With a Prebiotic Dietary Fiber Blend

Dietary fiber, a nutrient derived mainly from whole grains, vegetables, fruits, and legumes, is known to confer a number of health benefits, yet most Americans consume less than half of the daily recommended amount. Convenience and affordability are key factors determining the ability of individuals to incorporate fiber-rich foods into their diet, and many Americans struggle to access, afford, and prepare foods rich in fiber. The objective of this clinical study was to test the changes in microbial community composition, human metabolomics, and general health markers of a convenient, easy to use prebiotic supplement in generally healthy young participants consuming a diet low in fiber. Twenty healthy adults participated in this randomized, placebo-controlled, double-blind, crossover study which was registered at clinicaltrials.gov as NCT03785860. During the study participants consumed 12 g of a prebiotic fiber supplement and 12 g of placebo daily as a powder mixed with water as part of their habitual diet in randomized order for 4 weeks, with a 4-week washout between treatment arms. Fecal microbial DNA was extracted and sequenced by shallow shotgun sequencing on an Illumina NovaSeq. Plasma metabolites were detected using liquid chromatography–mass spectrometry with untargeted analysis. The phylum Actinobacteria, genus Bifidobacterium, and several Bifidobacterium species (B. bifidum, B. adolescentis, B. breve, B. catenulatum, and B. longum) significantly increased after prebiotic supplementation when compared to the placebo. The abundance of genes associated with the utilization of the prebiotic fiber ingredients (sacA, xfp, xpk) and the production of acetate (poxB, ackA) significantly changed with prebiotic supplementation. Additionally, the abundance of genes associated with the prebiotic utilization (xfp, xpk), acetate production (ackA), and choline to betaine oxidation (gbsB) were significantly correlated with changes in the abundance of the genus Bifidobacterium in the prebiotic group. Plasma concentrations of the bacterially produced metabolite indolepropionate significantly increased. The results of this study demonstrate that an easy to consume, low dose (12 g) of a prebiotic powder taken daily increases the abundance of beneficial bifidobacteria and the production of health-promoting bacteria-derived metabolites in healthy individuals with a habitual low-fiber diet.

Sepsis-Induced Gut Dysbiosis Mediates the Susceptibility to Sepsis-Associated Encephalopathy in Mice

Sepsis-associated encephalopathy (SAE) is common in septic patients and is associated with adverse outcomes. The gut microbiota has been recognized as a key mediator of neurological disease development. However, the exact role of the gut microbiota in regulating SAE remains elusive. Here, we investigated the role of the gut microbiota in SAE and its underlying mechanisms. Cecal ligation and puncture (CLP) was conducted to induce sepsis in mice. Neurological scores were recorded to distinguish SAE-resistant (SER) (score of >6 at 36 h postoperatively) from SAE-susceptible (SES) (score of ≤6 at 36 h postoperatively) mice. 16S rRNA gene sequencing and metabolomics analyses were used to characterize the gut microbiota in the two groups. Fecal microbiota transplantation was performed to validate the role of the gut microbiota in SAE progression. The gut microbiota was more severely disrupted in SES mice than in SER mice after sepsis modeling. Interestingly, mice receiving postoperative feces from SES mice exhibited more severe cortical inflammation than mice receiving feces from SER mice. Indole-3-propionic acid (IPA), a neuroprotective molecule, was more enriched in feces from SER mice than in feces from SES mice. IPA alleviated CLP-induced anxiety and spatial memory impairment in septic mice. Moreover, IPA markedly inhibited NLRP3 inflammasome activation and interleukin-1β (IL-1β) secretion in lipopolysaccharide-stimulated microglia. These responses were attenuated after antagonizing the aryl hydrocarbon receptor. Our study indicates that the variability in sepsis-induced gut dysbiosis mediates the differential susceptibility to SAE in CLP-induced experimental sepsis mice, and microbially derived IPA is possibly involved in SAE development as a neuroprotective compound.

IMPORTANCE The bidirectional interactions between the gut microbiota and sepsis-associated encephalopathy (SAE) are not well characterized. We found that the gut microbiota was more severely disturbed in SAE-susceptible (SES) mice than in SAE-resistant (SER) mice after sepsis modeling. Mice gavaged with postoperative feces from SES mice exhibited more severe neuroinflammation than mice gavaged with feces from SER mice. The gut microbiota from SER mice enriched a neuroprotective metabolite, IPA, which appeared to protect mice from SAE. The potential underlying mechanism of the protective effect of IPA may be mediated via the inhibition of NLRP3 inflammasome activation and IL-1β secretion in microglia. These anti-inflammatory effects of IPA may be regulated by aryl hydrocarbon receptors. These results enhance our understanding of the role of the intestinal microbiota in sepsis. In particular, gut microbiota-derived IPA may serve as a potential therapeutic agent to prevent neuroinflammation in SAE.

NMR based metabolic profiling of patients with potential celiac disease elucidating early biochemical changes of gluten-sensitivity: A pilot Study

BACKGROUND

The patients with positive celiac disease (CeD) specific serology, but no evidence of intestinal inflammation are defined as potential celiac disease (PCeD) patients. About one-third of PCeD patients develop intestinal inflammation over time. The present study investigated the metabolome of small intestinal biopsies, blood plasma, and urine of patients with PCeD to understand the biochemical changes underlying the CeD.

METHODS

The metabolic profiles of small intestinal biopsies, blood plasma, and urine of patients with PCeD (n=7) were compared with CeD (n=64) and controls (n=15) [disease controls (DC) and healthy controls (HC)] using ¹H NMR spectroscopy.

RESULTS

The intestinal mucosa of PCeD showed lower levels of histidine, glycine, tyrosine, and tryptophan compared to DC. Altered levels of 6 metabolites (glucose, acetate, acetoacetate, β-hydroxybutyrate, pyruvate, arginine) in blood plasma and two metabolites (succinate and aminohippurate) in urine were observed in PCeD compared to HC. The PLS-DA model built on the concentration of blood plasma showed separate clustering for PCeD and CeD patients.

CONCLUSION

Altered metabolic profile of PCeD suggested that gluten intolerance was evident at the metabolic level before the intestinal damage. Altered energy metabolism and lower cytoprotective activity (histidine, glycine, arginine) indicated vulnerability to develop intestinal inflammation in PCeD over time. Our study may provide an insight into early biochemical processes of the progression of PCeD to CeD.

Modifying the diet and gut microbiota to prevent and manage neurodegenerative diseases

The global prevalence of Alzheimer's disease and Parkinson's disease is steadily increasing due to the aging population. The lack of effective drugs against these neurodegenerative disorders makes it imperative to identify new strategies for their prevention and treatment. Recent studies have revealed that harnessing the power of the gut microbiota through modification of diet may be a valuable approach for reducing the risk, modulating the symptoms, and ameliorating the pathophysiological aspects of neurodegenerative diseases. Consuming specific dietary components can alter the prevalence of bacterial communities within the gut to a healthy enterotype, which can influence the production of beneficial metabolites by microbiota. This article focuses on several dietary components, which have been demonstrated to affect the gut microbiota-brain axis and therefore could lead to attenuation of specific pathological processes in neurodegenerative diseases. Published evidence indicates that fermented foods, including kefir, and foods that are high in bioactive polyphenols and complex carbohydrates, such as grapes, pomegranates, and seaweed, may be effective at reducing neuroinflammation, oxidative stress, neurotransmitter dysfunction, and neuronal death associated with Alzheimer's and Parkinson's diseases. Even though experimental evidence supporting the protective properties of the above dietary components in these diseases is emerging, it is evident that further human clinical studies are required to conclusively establish the benefits of any suggested dietary interventions. The translational potential of such research is illustrated by the clinical success of the recently developed Alzheimer's drug, GV-971, which is a seaweed derivative that works by modulating the gut microbiota-brain axis.

What If Not All Metabolites from the Uremic Toxin Generating Pathways Are Toxic? A Hypothesis

The topic of uremic toxicity has received broad attention from the nephrological community over the past few decades. An aspect that is much less often considered is the possibility that the metabolic pathways that generate uremic toxins also may produce molecules that benefit body functions. Here, we discuss this dualism based on the example of tryptophan-derived metabolites, which comprise elements that are mainly toxic, such as indoxyl sulfate, kynurenine and kynurenic acid, but also beneficial compounds, such as indole, melatonin and indole-3-propionic acid, and ambivalent (beneficial for some aspects and harmful for others) compounds such as serotonin. This dualism can also be perceived at the level of the main receptor of the tryptophan-derived metabolites, the aryl hydrocarbon receptor (AHR), which has also been linked to both harm and benefit. We hypothesize that these beneficial effects are the reason why uremic toxin generation remained preserved throughout evolution. This duality is also not unique for the tryptophan-derived metabolites, and in this broader context we discuss the remote sensing and signaling theory (RSST). The RSST proposes that transporters (e.g., organic anion transporter 1—OAT1; ATP-binding cassette transporter G—ABCG2) and drug metabolizing enzymes form a large network of proteins interacting to promote small molecule remote communication at the inter-organ (e.g., gut–liver–heart–brain–kidney) and inter-organismal (e.g., gut microbe–host) levels. These small molecules include gut microbe-derived uremic toxins as well as beneficial molecules such as those discussed here. We emphasize that this positive side of uremic metabolite production needs more attention, and that this dualism especially needs to be considered when assessing and conceiving of therapeutic interventions. These homeostatic considerations are central to the RSST and suggest that interventions be aimed at preserving or restoring the balance between positive and negative components rather than eliminating them all without distinction.

Xenobiotic receptors and the regulation of intestinal homeostasis: harnessing the chemical output of the intestinal microbiota

The commensal bacteria that reside in the gastrointestinal tract exist in a symbiotic relationship with the host, driving the development of the immune system and maintaining metabolic and tissue homeostasis in the local environment. The intestinal microbiota has the capacity to generate a wide array of chemical metabolites to which the cells of the intestinal mucosa are exposed. Host cells express xenobiotic receptors, such as the aryl hydrocarbon receptor (AhR) and the pregnane X receptor (PXR), that can sense and respond to chemicals that are generated by nonhost pathways. In this review, we outline the physiological and immunological processes within the intestinal environment that are regulated by microbial metabolites through the activation of the AhR and the PXR, with a focus on ligands generated by the stepwise catabolism of tryptophan.

Biological Effects of Indole-3-Propionic Acid, a Gut Microbiota-Derived Metabolite, and Its Precursor Tryptophan in Mammals' Health and Disease

Actions of symbiotic gut microbiota are in dynamic balance with the host’s organism to maintain homeostasis. Many different factors have an impact on this relationship, including bacterial metabolites. Several substrates for their synthesis have been established, including tryptophan, an exogenous amino acid. Many biological processes are influenced by the action of tryptophan and its endogenous metabolites, serotonin, and melatonin. Recent research findings also provide evidence that gut bacteria-derived metabolites of tryptophan share the biological effects of their precursor. Thus, this review aims to investigate the biological actions of indole-3-propionic acid (IPA), a gut microbiota-derived metabolite of tryptophan. We searched PUBMED and Google Scholar databases to identify pre-clinical and clinical studies evaluating the impact of IPA on the health and pathophysiology of the immune, nervous, gastrointestinal and cardiovascular system in mammals. IPA exhibits a similar impact on the energetic balance and cardiovascular system to its precursor, tryptophan. Additionally, IPA has a positive impact on a cellular level, by preventing oxidative stress injury, lipoperoxidation and inhibiting synthesis of proinflammatory cytokines. Its synthesis can be diminished in the presence of different risk factors of atherosclerosis. On the other hand, protective factors, such as the introduction of a Mediterranean diet, tend to increase its plasma concentration. IPA seems to be a promising new target, linking gut health with the cardiovascular system.

The mechanism of microglia-mediated immune inflammation in ischemic stroke and the role of natural botanical components in regulating microglia: A review

Ischemic stroke (IS) is one of the most fatal diseases. Neuroimmunity, inflammation, and oxidative stress play important roles in various complex mechanisms of IS. In particular, the early proinflammatory response resulting from the overactivation of resident microglia and the infiltration of circulating monocytes and macrophages in the brain after cerebral ischemia leads to secondary brain injury. Microglia are innate immune cells in the brain that constantly monitor the brain microenvironment under normal conditions. Once ischemia occurs, microglia are activated to produce dual effects of neurotoxicity and neuroprotection, and the balance of the two effects determines the fate of damaged neurons. The activation of microglia is defined as the classical activation (M1 type) or alternative activation (M2 type). M1 type microglia secrete pro-inflammatory cytokines and neurotoxic mediators to exacerbate neuronal damage, while M2 type microglia promote a repairing anti-inflammatory response. Fine regulation of M1/M2 microglial activation to minimize damage and maximize protection has important therapeutic value. This review focuses on the interaction between M1/M2 microglia and other immune cells involved in the regulation of IS phenotypic characteristics, and the mechanism of natural plant components regulating microglia after IS, providing novel candidate drugs for regulating microglial balance and IS drug development.

Indole-3-propionic acid mitigates chlorpyrifos-mediated neurotoxicity by modulating cholinergic and redox-regulatory systems, inflammatory stress, apoptotic responses and DNA damage in rats

This study probed the neuroprotective influence of indole-3-propionic acid (IPA) in rats exposed to chlorpyrifos (CPF) alone at 5 mg/kg body weight or co-administered with IPA at 12.5 and 25 mg/kg for 14 days. Behavioral data indicated that IPA significantly (p < 0.05) abated CPF-mediated anxiogenic-like behaviors with concomitant improvement in the locomotor and exploratory behaviors as substantiated by track plots and heat maps data. Also, IPA mitigated CPF-mediated diminution in cholinergic and antioxidant defense systems whereas it markedly improved thioredoxin level and thioredoxin reductase activity in cerebral and cerebellar tissues of the animals. Co-administration of IPA significantly enhanced anti-inflammatory cytokine, interleukin-10 but suppressed oxidative and inflammatory stress, caspase-9 and caspase-3 activation with concomitant reduction in 8-hydroxy-2'-deoxyguanosine (8-OHdG) level and histological damage. Collectively, IPA-mediated neuroprotection involves modulation of cholinergic and redox-regulatory systems, inflammatory stress, apoptotic responses and DNA damage in cerebrum and cerebellum of rats.

"The quantitative determination of indolic microbial tryptophan metabolites in human and rodent samples: A systematic review"

Concentrations reported for indolic microbial metabolites of tryptophan in human and rodent brain, cerebrospinal fluid, plasma, saliva and feces were compiled and discussed. A systematic review of the literature was accomplished by key word searches of Pubmed, Google Scholar and the Human Metabolome Data Base (HMDB), and by searching bibliographies of identified publications including prior reviews. The review was prompted by the increasing appreciation of the physiological importance of the indolic compounds in human health and disease. The compounds included were indoleacetic acid (IAA), indole propionic acid (IPA), indoleacrylic acid (IACR), indolelactic acid (ILA) indolepyruvic acid (IPY), indoleacetaldehyde (IAALD), indolealdehyde (IALD), tryptamine (TAM), indole (IND) and skatole (SKT). The undertaking aimed to vet and compare existing reports, to resolve apparent discrepancies, to draw biological inferences from the consideration of multiple analytes across sample types, to survey the analytical methodologies used, and to point out areas in need of greater attention.

The Potential Utility of Prebiotics to Modulate Alzheimer's Disease: A Review of the Evidence

The gut microbiome has recently emerged as a critical modulator of brain function, with the so-called gut-brain axis having multiple links with a variety of neurodegenerative and mental health conditions, including Alzheimer's Disease (AD). Various approaches for modulating the gut microbiome toward compositional and functional states that are consistent with improved cognitive health outcomes have been documented, including probiotics and prebiotics. While probiotics are live microorganisms that directly confer beneficial health effects, prebiotics are oligosaccharide and polysaccharide structures that can beneficially modulate the gut microbiome by enhancing the growth, survival, and/or function of gut microbes that in turn have beneficial effects on the human host. In this review, we discuss evidence showing the potential link between gut microbiome composition and AD onset or development, provide an overview of prebiotic types and their roles in altering gut microbial composition, discuss the effectiveness of prebiotics in regulating gut microbiome composition and microbially derived metabolites, and discuss the current evidence linking prebiotics with health outcomes related to AD in both animal models and human trials. Though there is a paucity of human clinical trials demonstrating the effectiveness of prebiotics in altering gut microbiome-mediated health outcomes in AD, current evidence highlights the potential of various prebiotic approaches for beneficially altering the gut microbiota or gut physiology by promoting the production of butyrate, indoles, and secondary bile acid profiles that further regulate gut immunity and mucosal homeostasis, which are associated with beneficial effects on the central immune system and brain functionality.

Hemp and buckwheat are valuable sources of dietary amino acids, beneficially modulating gastrointestinal hormones and promoting satiety in healthy volunteers

Purpose

This study evaluated the postprandial effects following consumption of buckwheat, fava bean, pea, hemp and lupin compared to meat (beef); focussing on biomarkers of satiety, gut hormones, aminoacids and plant metabolites bioavailability and metabolism.

Methods

Ten subjects (n = 3 men; n = 7 women; 42 ± 11.8 years of age; BMI 26 ± 5.8 kg/m²) participated in six 1-day independent acute interventions, each meal containing 30 g of protein from buckwheat, fava bean, pea, hemp, lupin and meat (beef). Blood samples were collected during 24-h and VAS questionnaires over 5-h.

Results

Volunteers consumed significantly higher amounts of most amino acids from the meat meal, and with few exceptions, postprandial composition of plasma amino acids was not significantly different after consuming the plant-based meals. Buckwheat meal was the most satious (300 min hunger scores, p < 0.05).Significant increase in GLP-1 plasma (AUC, iAUC p = 0.01) found after hemp compared with the other plant-based meals. Decreased plasma ghrelin concentrations (iAUC p < 0.05) found on plant (hemp) vs. meat meal. Several plasma metabolites after hemp meal consumption were associated with hormone trends (partial least squares-discriminant analysis (PLS-DA): 4-hydroxyphenylpyruvic acid, indole 3-pyruvic acid, 5-hydoxytryptophan, genistein and biochanin A with GLP-1, PYY and insulin; 3-hydroxymandelic acid and luteolidin with GLP-1 and ghrelin and 4-hydroxymandelic acid, benzoic acid and secoisolariciresinol with insulin and ghrelin. Plasma branched-chain amino acids (BCAAs), (iAUC, p < 0.001); and phenylalanine and tyrosine (iAUC, p < 0.05) were lower after buckwheat comparison with meat meal.

Conclusion

Plants are valuable sources of amino acids which are promoting satiety. The impact of hemp and buckwheat on GLP-1 and, respectively, BCAAs should be explored further as could be relevant for aid and prevention of chronic diseases such as type 2 diabetes.

Study registered with clinicaltrial.gov on 12th July 2013, study ID number: NCT01898351.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00394-021-02711-z.

Indole-3-Propionic Acid, a Gut-Derived Tryptophan Metabolite, Associates with Hepatic Fibrosis

Background and Aims: Gut microbiota-derived metabolites play a vital role in maintenance of human health and progression of disorders, including obesity and type 2 diabetes (T2D). Indole-3-propionic acid (IPA), a gut-derived tryptophan metabolite, has been recently shown to be lower in individuals with obesity and T2D. IPA’s beneficial effect on liver health has been also explored in rodent and cell models. In this study, we investigated the association of IPA with human liver histology and transcriptomics, and the potential of IPA to reduce hepatic stellate cell activation in vitro. Methods: A total of 233 subjects (72% women; age 48.3 ± 9.3 years; BMI 43.1 ± 5.4 kg/m²) undergoing bariatric surgery with detailed liver histology were included. Circulating IPA levels were measured using LC-MS and liver transcriptomics with total RNA-sequencing. LX-2 cells were used to study hepatoprotective effect of IPA in cells activated by TGF-β1. Results: Circulating IPA levels were found to be lower in individuals with liver fibrosis compared to those without fibrosis (p = 0.039 for all participants; p = 0.013 for 153 individuals without T2D). Accordingly, levels of circulating IPA associated with expression of 278 liver transcripts (p < 0.01) that were enriched for the genes regulating hepatic stellate cells (HSCs) activation and hepatic fibrosis signaling. Our results suggest that IPA may have hepatoprotective potential because it is able to reduce cell adhesion, cell migration and mRNA gene expression of classical markers of HSCs activation in LX-2 cells (all p < 0.05). Conclusion: The association of circulating IPA with liver fibrosis and the ability of IPA to reduce activation of LX-2 cells suggests that IPA may have a therapeutic potential. Further molecular studies are needed to investigate the mechanisms how IPA can ameliorate hepatic fibrosis.

In silico insights into potential gut microbial modulation of NAD+ metabolism and longevity

Recent evidence has prompted the notion of gut-microbial signatures as an indirect marker of aging and aging-associated decline in humans. However, the underlying host-symbiont molecular interactions contributing to these signatures remain poorly understood. In this study, we address this gap using cheminformatic analyses to elucidate potential gut microbial metabolites that may perturb the longevity-associated NAD+ metabolic network. In silico ADMET, KEGG interaction analysis, molecular docking, molecular dynamics simulation, and molecular mechanics calculation predict a large number of safe and bioavailable microbial metabolites to be direct and/or indirect activators of NAD+-dependent sirtuin proteins. Our simulation results suggest dihydropteroate, phenylpyruvic acid, indole-3-propionic acid, phenyllactic acid, all-trans-retinoic acid, and multiple deoxy-, methyl-, and cyclic nucleotides from intestinal microbiota as the best-performing regulators of NAD+ metabolism. Retracing these molecules to their source microorganisms also suggest commensal Escherichia, Bacteroides, Bifidobacteria, and Lactobacilli to be associated with the highest number of pro-longevity metabolites. These findings from our early-stage study, therefore, provide an informatics-based context for previous evidence in the area and grant novel insights for future clinical investigation intersecting anti-aging drug discovery, probiotics, and gut microbial signatures.

New Approaches to Profile the Microbiome for Treatment of Neurodegenerative Disease

Progressive neurodegenerative diseases represent some of the largest growing treatment challenges for public health in modern society. These diseases mainly progress due to aging and are driven by microglial surveillance and activation in response to changes occurring in the aging brain. The lack of efficacious treatment options for Alzheimer's disease (AD), as the focus of this review, and other neurodegenerative disorders has encouraged new approaches to address neuroinflammation for potential treatments. Here we will focus on the increasing evidence that dysbiosis of the gut microbiome is characterized by inflammation that may carry over to the central nervous system and into the brain. Neuroinflammation is the common thread associated with neurodegenerative diseases, but it is yet unknown at what point and how innate immune function turns pathogenic for an individual. This review will address extensive efforts to identify constituents of the gut microbiome and their neuroactive metabolites as a peripheral path to treatment. This approach is still in its infancy in substantive clinical trials and requires thorough human studies to elucidate the metabolic microbiome profile to design appropriate treatment strategies for early stages of neurodegenerative disease. We view that in order to address neurodegenerative mechanisms of the gut, microbiome and metabolite profiles must be determined to pre-screen AD subjects prior to the design of specific, chronic titrations of gut microbiota with low-dose antibiotics. This represents an exciting treatment strategy designed to balance inflammatory microglial involvement in disease progression with an individual's manifestation of AD as influenced by a coercive inflammatory gut.

Microbial-derived indoles inhibit neutrophil myeloperoxidase to diminish bystander tissue damage

During episodes of acute inflammation, polymorphonuclear leukocytes (PMNs) are actively recruited to sites of inflammation or injury where they provide anti-microbial and wound-healing functions. One enzyme crucial for fulfilling these functions is myeloperoxidase (MPO), which generates hypochlorous acid from Cl^- and hydrogen peroxide. The potential exists, however, that uncontrolled the extracellular generation of hypochlorous acid by MPO can cause bystander tissue damage and inhibit the healing response. Previous work suggests that the microbiota-derived tryptophan metabolites 1H-indole and related molecules ("indoles") are protective during intestinal inflammation, although their precise mechanism of action is unclear. In the present work, we serendipitously discovered that indoles are potent and selective inhibitors of MPO. Using both primary human PMNs and recombinant human MPO in a cell-free system, we revealed that indoles inhibit MPO at physiologic concentrations. Particularly, indoles block the chlorinating activity of MPO, a reliable marker for MPO-associated tissue damage, as measured by coulometric-coupled HPLC. Further, we observed direct interaction between indoles and MPO using the established biochemical techniques microscale thermophoresis and STD-NMR. Utilizing a murine colitis model, we demonstrate that indoles inhibit bystander tissue damage, reflected in decreased colon 3-chlorotyrosine and pro-inflammatory chemokine expression in vivo. Taken together, these results identify microbiota-derived indoles that acts as endogenous immunomodulatory compounds through their actions on MPO, suggesting a symbiotic association between the gut microbiota and host innate immune system. Such findings offer exciting new targets for future pharmacological intervention.

Metabolic Host-Microbiota Interactions in Autophagy and the Pathogenesis of Inflammatory Bowel Disease (IBD)

Inflammatory bowel disease (IBD) is a family of conditions characterized by chronic, relapsing inflammation of the gastrointestinal tract. IBD afflicts over 3 million adults in the United States and shows increasing prevalence in the Westernized world. Current IBD treatments center on modulation of the damaging inflammatory response and carry risks such as immunosuppression, while the development of more effective treatments is hampered by our poor understanding of the molecular mechanisms of IBD pathogenesis. Previous genome-wide association studies (GWAS) have demonstrated that gene variants linked to the cellular response to microorganisms are most strongly associated with an increased risk of IBD. These studies are supported by mechanistic work demonstrating that IBD-associated polymorphisms compromise the intestine's anti-microbial defense. In this review, we summarize the current knowledge regarding IBD as a disease of defects in host-microbe interactions and discuss potential avenues for targeting this mechanism for future therapeutic development.