Short chain fatty acids: Microbial metabolites for gut-brain axis signalling

Critical windows of early-life microbiota disruption on behaviour, neuroimmune function, and neurodevelopment

Numerous studies have emphasised the importance of the gut microbiota during early life and its role in modulating neurodevelopment and behaviour. Epidemiological studies have shown that early-life antibiotic exposure can increase an individual's risk of developing immune and metabolic diseases. Moreover, preclinical studies have shown that long-term antibiotic-induced microbial disruption in early life can have enduring effects on physiology, brain function and behaviour. However, these studies have not investigated the impact of targeted antibiotic-induced microbiota depletion during critical developmental windows and how this may be related to neurodevelopmental outcomes. Here, we addressed this gap by administering a broad-spectrum oral antibiotic cocktail (ampicillin, gentamicin, vancomycin, and imipenem) to mice during one of three putative critical windows: the postnatal (PN; P2-9), pre-weaning (PreWean; P12-18), or post-weaning (Wean; P21-27) developmental periods and assessed the effects on physiology and behaviour in later life. Our results demonstrate that targeted microbiota disruption during early life has enduring effects into adolescence on the structure and function of the caecal microbiome, especially for antibiotic exposure during the weaning period. Further, we show that microbial disruption in early life selectively alters circulating immune cells and modifies neurophysiology in adolescence, including altered myelin-related gene expression in the prefrontal cortex and altered microglial morphology in the basolateral amygdala. We also observed sex and time-dependent effects of microbiota depletion on anxiety-related behavioural outcomes in adolescence and adulthood. Antibiotic-induced microbial disruption had limited and subtle effects on social behaviour and did not have any significant effects on depressive-like behaviour, short-term working, or recognition memory. Overall, this study highlights the importance of the gut microbiota during critical windows of development and the subtle but long-term effects that microbiota-targeted perturbations can have on brain physiology and behaviour.

The gut microbiota is important for the maintenance of blood-cerebrospinal fluid barrier integrity

The gut microbiota communicates with the brain through several pathways including the vagus nerve, immune system, microbial metabolites and through the endocrine system. Pathways along the humoral/immune gut microbiota-brain axis are composed of a series of vascular and epithelial barriers including the intestinal epithelial barrier, gut-vascular barrier, blood-brain barrier and blood-cerebrospinal fluid barrier. Of these barriers, the relationship between the gut microbiota and blood-cerebrospinal fluid barrier is yet to be fully defined. Here, using a germ-free mouse model, we aimed to assess the relationship between the gut microbiota and the integrity of the blood-cerebrospinal fluid barrier, which is localized to the choroid plexus epithelium. Using confocal microscopy, we visualized the tight junction protein zonula occludens-1, an integral aspect of choroid plexus integrity, as well as the choroid plexus fenestrated capillaries. Quantification of tight junction proteins via network analysis led to the observation that there was a decrease in the zonula occludens-1 network organization in germ-free mice; however, we did not observe any differences in capillary structure. Taken together, these data indicate that the blood-cerebrospinal fluid barrier is another barrier along the gut microbiota-brain axis. Future studies are required to elucidate its relative contribution in signalling from microbiota to the brain.

HPLC-MS-MS quantification of short-chain fatty acids actively secreted by probiotic strains

Introduction

Short-chain fatty acids (SCFAs) are the main by-products of microbial fermentations occurring in the human intestine and are directly involved in the host's physiological balance. As impaired gut concentrations of acetic, propionic, and butyric acids are often associated with systemic disorders, the administration of SCFA-producing microorganisms has been suggested as attractive approach to solve symptoms related to SCFA deficiency.

Methods

In this research, nine probiotic strains (Bacillus clausii NR, OC, SIN, and T, Bacillus coagulans ATCC 7050, Bifidobacterium breve DSM 16604, Limosilactobacillus reuteri DSM 17938, Lacticaseibacillus rhamnosus ATCC 53103, and Saccharomyces boulardii CNCM I-745) commonly included in commercial formulations were tested for their ability to secrete SCFAs by using an improved protocol in high-performance liquid chromatography coupled to tandem mass spectrometry (HPLC-MS-MS).

Results

The developed method was highly sensitive and specific, showing excellent limits of detection and quantification of secreted SCFAs. All tested microorganisms were shown to secrete acetic acid, with only B. clausii and S. boulardii additionally able to produce propionic and butyric acids. Quantitative differences in the secretion of SCFAs were also evidenced.

Discussion

The experimental approach described in this study may contribute to the characterization of probiotics as SCFA-producing organisms, a crucial stage toward their application to improve SCFA deficiency.

Antibiotics and mental health: The good, the bad and the ugly

Antibiotics are recognised as, on occasion, producing psychiatric side effects, most notably depression and anxiety. Apart from antimicrobial activity, antibiotics have multiple off-target effects. The brain-gut-microbiota axis has multiple sites for off-target activity, which may produce either positive or negative antibiotic effects. Here we review how antibiotics impact mental health by acting through the brain-gut-microbiota axis. Microbes in the gut influence brain function by acting through the vagus nerve or by altering the production of short-chain fatty acids or the amino acid tryptophan, the building block of serotonin. Not all antimicrobial actions of antibiotics have a negative impact. The first antidepressant discovered was actually an antibiotic: isoniazid is an antibacterial drug developed for treating tuberculosis. Minocycline, which enters the brain and mediates its effects through microglia, shows antidepressant activity. Some antibiotics bring about a significant decrease in gut microbial diversity, and this is viewed as a risk factor for depression. Other risk factors induced by antibiotics include altered gut barrier function, activation of the hypothalamic-pituitary-adrenal axis, reducing levels of brain-derived neurotrophic factor or oxytocin and alteration of vagal tone. Although most patients taking antibiotics do not suffer from an iatrogenic psychiatric disorder, some do. As clinicians, we need to keep this in mind. The development of new antibiotics is primarily focused on antibiotic resistance, but efforts should be made to reduce off-target brain-gut-microbiota effects resulting in mental health problems.

Carrot-based fermentation juice rich in sleep-promoting components improved sleep in mice

The impact of fermentation by Levilactobacillus brevis YSJ3 on sleep-promoting components (SPCs) of carrot juice was evaluated. The contents of acetic acid, isovaleric acid, butyric acid, and γ-aminobutyric acid (GABA) significantly increased after fermentation. The beneficial effects of fermented carrot juice (FCJ) on sleep were evaluated in animal experiments. Behavioral test reveal SPCs-enriched FCJ could effectively relieve anxiety. The sleep duration in the FCJ group were extended compared to the control (NC) group and the unfermented carrot juice (UCJ) group. Moreover, the relative abundances of Ruminiclostridium and Akkermansia in the FCJ group and PC group, respectively, increased significantly, compared to the NC group the UCJ group. The contents of gut short-chain fatty acids in the FCJ group were significantly higher than that in the NC group and the UCJ group. The levels of GABA and 5-hydroxytryptamine in the brain for the FCJ group also increased significantly, compared to the NC group and the UCJ group. It indicated that SPCs-enriched FCJ effectively improved sleep in mice, which might be related to the fermentation of carrot juice and the compounds produced during the fermentation.

Microbial-derived metabolites induce actin cytoskeletal rearrangement and protect blood-brain barrier function

The gut microbiota influences host brain function, but the underlying gut-brain axis connections and molecular processes remain unclear. One pathway along this bidirectional communication system involves circulating microbially derived metabolites, such as short-chain fatty acids (SCFAs), which include butyrate and propionate. Brain endothelium is the main interface of communication between circulating signals and the brain, and it constitutes the main component of the blood-brain barrier (BBB). Here, we used a well-established in vitro BBB model treated with physiologically relevant concentrations of butyrate and propionate with and without lipopolysaccharide (LPS) to examine the effects of SCFAs on the actin cytoskeleton and tight junction protein structure. Both SCFAs induced distinct alterations to filamentous actin directionality. SCFAs also increased tight junction protein spikes and protected from LPS-induced tight-junction mis-localization, improved BBB integrity, and modulated mitochondrial network dynamics. These findings identify the actin cytoskeletal dynamics as another target further illuminating how SCFAs can influence BBB physiology.

Diet and the microbiota - gut - brain-axis: a primer for clinical nutrition

PURPOSE OF REVIEW

Diet is an essential modulator of the microbiota - gut - brain communication in health and disease. Consequently, diet-induced microbiome states can impact brain health and behaviour. The integration of microbiome into clinical nutrition perspectives of brain health is sparse. This review will thus focus on emerging evidence of microbiome-targeted dietary approaches with the potential to improve brain disorders.

RECENT FINDINGS

Research in this field is evolving toward randomized controlled trials using dietary interventions with the potential to modulate pathways of the microbiota - gut - brain-axis. Although most studies included small cohorts, the beneficial effects of Mediterranean-like diets on symptoms of depression or fermented foods on the immune function of healthy individuals shed light on how this research line can grow. With a clinical nutrition lens, we highlight several methodological limitations and knowledge gaps, including the quality of dietary intake information, the design of dietary interventions, and missing behavioural outcomes.

SUMMARY

Findings in diet - microbiome - brain studies can have groundbreaking implications in clinical nutrition practice and research. Modulating brain processes through diet via the gut microbiota raises numerous possibilities. Novel dietary interventions targeting the microbiota - gut - brain-axis can offer various options to prevent and treat health problems such as mental disorders. Furthermore, knowledge in this field will improve current nutritional guidelines for disease prevention.

The in vitro fermentation of compound oral liquid by human colonic microbiota altered the abundance of probiotics and short-chain fatty acid production

Compound oral liquid (COL), made from functional herbal foods, has gained immense popularity in China for healthcare. However, the interaction between the nutrients in COL and gut microbiota is still unclear. In our study, the content of total flavonoids, polyphenols, and proteins was increased and the total sugar reduced by crushing raw ingredients to 10 mesh (COL-C). After 24 h incubation with supplemented COL by human gut microbiota, the results of 16S rRNA high-throughput sequencing revealed that Faecalibacterium, Collinsella, Bifidobacterium, Megamonas, Lactobacillus, Phascolarctobacterium, and Dialister were enriched by COL. In particular, the latter three genera were observed to be significantly enriched after incubation with COL-C. Meanwhile, the abundance of Dorea, Clostridium XIVa, and Escherichia/Shigella was inhibited by COL. Moreover, the increased levels of acetate, propionate, and butyrate in COL were jointly contributed by supplementary carbohydrates and the enrichment of short-chain fatty acid (SCFA)-producing bacteria. In summary, our results indicated that the optimized extraction facilitated the nutrients to be dissolved out and enhanced the potential prebiotic effects for promoting the abundance of probiotics, suggesting that the nutrients in COL-C might improve the microbial structure by strengthening the metabolism of beneficial bacteria and restricting the conditioned pathogens more efficiently.

The crushing pretreatment before extraction facilitated the nutrients to dissolve in compound oral liquid and enhanced the prebiotic effects for promoting the abundance of probiotics and short-chain fatty acid synthesis.

Maternal gut microbiota mediate intergenerational effects of high-fat diet on descendant social behavior

Dysbiosis of the maternal gut microbiome during pregnancy is associated with adverse neurodevelopmental outcomes. We previously showed that maternal high-fat diet (MHFD) in mice induces gut dysbiosis, social dysfunction, and underlying synaptic plasticity deficits in male offspring (F₁). Here, we reason that, if HFD-mediated changes in maternal gut microbiota drive offspring social deficits, then MHFD-induced dysbiosis in F₁ female MHFD offspring would likewise impair F₂ social behavior. Metataxonomic sequencing reveals reduced microbial richness among female F₁ MHFD offspring. Despite recovery of microbial richness among MHFD-descendant F₂ mice, they display social dysfunction. Post-weaning Limosilactobacillus reuteri treatment increases the abundance of short-chain fatty acid-producing taxa and rescues MHFD-descendant F₂ social deficits. L. reuteri exerts a sexually dimorphic impact on gut microbiota configuration, increasing discriminant taxa between female cohorts. Collectively, these results show multigenerational impacts of HFD-induced dysbiosis in the maternal lineage and highlight the potential of maternal microbiome-targeted interventions for neurodevelopmental disorders.

Central and peripheral regulations mediated by short-chain fatty acids on energy homeostasis

The human gut microbiota influences obesity, insulin resistance, and the subsequent development of type 2 diabetes (T2D). The gut microbiota digests and ferments nutrients resulting in the production of short-chain fatty acids (SCFAs), which generate various beneficial metabolic effects on energy and glucose homeostasis. However, their roles in the central nervous system (CNS)-mediated outputs on the metabolism have only been minimally studied. Here, we explore what is known and future directions that may be worth exploring in this emerging area. Specifically, we searched studies or data in English by using PubMed, Google Scholar, and the Human Metabolome Database. Studies were filtered by time from 1978 to March 2022. As a result, 195 studies, 53 reviews, 1 website, and 1 book were included. One hundred and sixty-five of 195 studies describe the production and metabolism of SCFAs or the effects of SCFAs on energy homeostasis, glucose balance, and mental diseases through the gut-brain axis or directly by a central pathway. Thirty of 195 studies show that inappropriate metabolism and excessive of SCFAs are metabolically detrimental. Most studies suggest that SCFAs exert beneficial metabolic effects by acting as the energy substrate in the TCA cycle, regulating the hormones related to satiety regulation and insulin secretion, and modulating immune cells and microglia. These functions have been linked with AMPK signaling, GPCRs-dependent pathways, and inhibition of histone deacetylases (HDACs). However, the studies focusing on the central effects of SCFAs are still limited. The mechanisms by which central SCFAs regulate appetite, energy expenditure, and blood glucose during different physiological conditions warrant further investigation.

Influence of the Gut Microbiota on the Development of Neurodegenerative Diseases

Neurodegenerative disorders are marked by neuronal death over time, causing a variety of cognitive and motor dysfunctions. Protein misfolding, neuroinflammation, and mitochondrial and protein clearance system dysfunction have all been identified as common pathways leading to neurodegeneration in recent decades. An altered microbiome of the gut, which is considered to play a central role in diseases as well as health, has recently been identified as another potential feature seen in neurodegenerative disorders. An array of microbial molecules that are released in the digestive tract may mediate gut-brain connections and permeate many organ systems, including the nervous system. Furthermore, recent findings from clinical as well as preclinical trials suggest that the microbiota of the gut plays a critical part in gut-brain interplay and that a misbalance in the composition of the gut microbiome may be linked to the etiology of neurological disorders (majorly neurodegenerative health problems); the underlying mechanism of which is still unknown. The review aims to consider the association between the microbiota of the gut and neurodegenerative disorders, as well as to add to our understanding of the significance of the gut microbiome in neurodegeneration and the mechanisms that underlie it. Knowing the mechanisms behind the gut microbiome's role and abundance will provide us with new insights that could lead to novel therapeutic strategies.

The Microbiota–Gut–Brain Axis in Psychiatric Disorders

Modulating the gut microbiome and its influence on human health is the subject of intense research. The gut microbiota could be associated not only with gastroenterological diseases but also with psychiatric disorders. The importance of factors such as stress, mode of delivery, the role of probiotics, circadian clock system, diet, and occupational and environmental exposure in the relationship between the gut microbiota and brain function through bidirectional communication, described as “the microbiome–gut–brain axis”, is especially underlined. In this review, we discuss the link between the intestinal microbiome and the brain and host response involving different pathways between the intestinal microbiota and the nervous system (e.g., neurotransmitters, endocrine system, immunological mechanisms, or bacterial metabolites). We review the microbiota alterations and their results in the development of psychiatric disorders, including major depressive disorder (MDD), schizophrenia (SCZ), bipolar disorder (BD), autism spectrum disorder (ASD), and attention-deficit hyperactivity disorder (ADHD).

The double-edged sword of probiotic supplementation on gut microbiota structure in Helicobacter pylori management

As Helicobacter pylori management has become more challenging and less efficient over the last decade, the interest in innovative interventions is growing by the day. Probiotic co-supplementation to antibiotic therapies is reported in several studies, presenting a moderate reduction in drug-related side effects and a promotion in positive treatment outcomes. However, the significance of gut microbiota involvement in the competence of probiotic co-supplementation is emphasized by a few researchers, indicating the alteration in the host gastrointestinal microbiota following probiotic and drug uptake. Due to the lack of long-term follow-up studies to determine the efficiency of probiotic intervention in H. pylori eradication, and the delicate interaction of the gut microbiota with the host wellness, this review aims to discuss the gut microbiota alteration by probiotic co-supplementation in H. pylori management to predict the comprehensive effectiveness of probiotic oral administration.Abbreviations: acyl-CoA- acyl-coenzyme A; AMP- antimicrobial peptide; AMPK- AMP-activated protein kinase; AP-1- activator protein 1; BA- bile acid; BAR- bile acid receptor; BCAA- branched-chain amino acid; C2- acetate; C3- propionate; C4- butyrate; C5- valeric acid; CagA- Cytotoxin-associated gene A; cAMP- cyclic adenosine monophosphate; CD- Crohn's disease; CDI- C. difficile infection; COX-2- cyclooxygenase-2; DC- dendritic cell; EMT- epithelial-mesenchymal transition; FMO- flavin monooxygenases; FXR- farnesoid X receptor; GPBAR1- G-protein-coupled bile acid receptor 1; GPR4- G protein-coupled receptor 4; H2O2- hydrogen peroxide; HCC- hepatocellular carcinoma; HSC- hepatic stellate cell; IBD- inflammatory bowel disease; IBS- irritable bowel syndrome; IFN-γ- interferon-gamma; IgA immunoglobulin A; IL- interleukin; iNOS- induced nitric oxide synthase; JAK1- janus kinase 1; JAM-A- junctional adhesion molecule A; LAB- lactic acid bacteria; LPS- lipopolysaccharide; MALT- mucosa-associated lymphoid tissue; MAMP- microbe-associated molecular pattern; MCP-1- monocyte chemoattractant protein-1; MDR- multiple drug resistance; mTOR- mammalian target of rapamycin; MUC- mucin; NAFLD- nonalcoholic fatty liver disease; NF-κB- nuclear factor kappa B; NK- natural killer; NLRP3- NLR family pyrin domain containing 3; NOC- N-nitroso compounds; NOD- nucleotide-binding oligomerization domain; PICRUSt- phylogenetic investigation of communities by reconstruction of unobserved states; PRR- pattern recognition receptor; RA- retinoic acid; RNS- reactive nitrogen species; ROS- reactive oxygen species; rRNA- ribosomal RNA; SCFA- short-chain fatty acids; SDR- single drug resistance; SIgA- secretory immunoglobulin A; STAT3- signal transducer and activator of transcription 3; T1D- type 1 diabetes; T2D- type 2 diabetes; Th17- T helper 17; TLR- toll-like receptor; TMAO- trimethylamine N-oxide; TML- trimethyllysine; TNF-α- tumor necrosis factor-alpha; Tr1- type 1 regulatory T cell; Treg- regulatory T cell; UC- ulcerative colitis; VacA- Vacuolating toxin A.

Sex, pain, and the microbiome: The relationship between baseline gut microbiota composition, gender and somatic pain in healthy individuals

BACKGROUND AND AIM

Relative to men, women present with pain conditions more commonly. Although consistent differences exist between men and women in terms of physiological pain sensitivity, the underlying mechanisms are incompletely understood and yet could inform the development of effective sex specific treatments for pain. The gut microbiota can modulate nervous system functioning, including pain signaling pathways. We hypothesized that the gut microbiota and critical components of the gut-brain axis might influence electrical pain thresholds. Further, we hypothesized that sex, menstrual cycle, and hormonal contraceptive use might account for inter-sex differences in pain perception.

METHODS

Healthy, non-obese males (N = 15) and females (N = 16), (nine of whom were using hormonal contraceptives), were recruited. Male subjects were invited to undergo testing once, whereas females were invited three times across the menstrual cycle, based on self-reported early follicular (EF), late follicular (LF), or mid-luteal (ML) phase. On test days, electrical stimulation on the right ankle was performed; salivary cortisol levels were measured in the morning; levels of lipopolysaccharide-binding protein (LBP), soluble CD14 (sCD14), pro-inflammatory cytokines were assessed in plasma, and microbiota composition and short-chain fatty acids (SCFAs) levels were determined in fecal samples.

RESULTS

We observed that the pain tolerance threshold/pain sensation threshold (PTT/PST) ratio was significantly lesser in women than men, but not PST or PTT alone. Further, hormonal contraceptive use was associated with increased LBP levels (LF & ML phase), whilst sCD14 levels or inflammatory cytokines were not affected. Interestingly, in women, hormonal contraceptive use was associated with an increase in the relative abundance of Erysipelatoclostridium, and the relative abundances of certain bacterial genera correlated positively with pain sensation thresholds (Prevotella and Megasphera) during the LF phase and cortisol awakening response (Anaerofustis) during the ML phase. In comparison with men, women displayed overall stronger associations between i) SCFAs data, ii) cortisol data, iii) inflammatory cytokines and PTT and PST.

DISCUSSION AND CONCLUSION

Our findings support the hypothesis that the gut microbiota may be one of the factors determining the physiological inter-sex differences in pain perception. Further research is needed to investigate the molecular mechanisms by which specific sex hormones and gut microbes modulate pain signaling pathways, but this study highlights the possibilities for innovative individual targeted therapies for pain management.

Making Sense of Quorum Sensing at the Intestinal Mucosal Interface

The gut microbiome can produce metabolic products that exert diverse activities, including effects on the host. Short chain fatty acids and amino acid derivatives have been the focus of many studies, but given the high microbial density in the gastrointestinal tract, other bacterial products such as those released as part of quorum sensing are likely to play an important role for health and disease. In this review, we provide of an overview on quorum sensing (QS) in the gastrointestinal tract and summarise what is known regarding the role of QS molecules such as auto-inducing peptides (AIP) and acyl-homoserine lactones (AHL) from commensal, probiotic, and pathogenic bacteria in intestinal health and disease. QS regulates the expression of numerous genes including biofilm formation, bacteriocin and toxin secretion, and metabolism. QS has also been shown to play an important role in the bacteria–host interaction. We conclude that the mechanisms of action of QS at the intestinal neuro–immune interface need to be further investigated.

Microbial Metabolites in Multiple Sclerosis: Implications for Pathogenesis and Treatment

Metabolites produced by the gut microbiota have been shown to play an important role in numerous inflammatory, neuropsychiatric, and neurodegenerative diseases. Specifically, microbial metabolites have been implicated in the modulation of innate and adaptive immunity, especially in the generation of regulatory T cells (Tregs), which are key regulators of multiple sclerosis (MS) pathogenesis. Furthermore, they affect processes relevant to MS pathophysiology, such as inflammation and demyelination, which makes them attractive molecules to be explored as therapeutics in MS. In this review, we discuss the importance of these metabolites as factors contributing to disease pathogenesis and as therapeutic targets in MS. Establishing an improved understanding of these gut-microbiota derived metabolites may provide new avenues for the treatment of MS.

Blood-Based Biomarkers for Alzheimer's Disease Diagnosis and Progression: An Overview

Alzheimer’s Disease (AD) is a progressive neurodegenerative disease characterized by amyloid-β (Aβ) plaque deposition and neurofibrillary tangle accumulation in the brain. Although several studies have been conducted to unravel the complex and interconnected pathophysiology of AD, clinical trial failure rates have been high, and no disease-modifying therapies are presently available. Fluid biomarker discovery for AD is a rapidly expanding field of research aimed at anticipating disease diagnosis and following disease progression over time. Currently, Aβ_1–42, phosphorylated tau, and total tau levels in the cerebrospinal fluid are the best-studied fluid biomarkers for AD, but the need for novel, cheap, less-invasive, easily detectable, and more-accessible markers has recently led to the search for new blood-based molecules. However, despite considerable research activity, a comprehensive and up-to-date overview of the main blood-based biomarker candidates is still lacking. In this narrative review, we discuss the role of proteins, lipids, metabolites, oxidative-stress-related molecules, and cytokines as possible disease biomarkers. Furthermore, we highlight the potential of the emerging miRNAs and long non-coding RNAs (lncRNAs) as diagnostic tools, and we briefly present the role of vitamins and gut-microbiome-related molecules as novel candidates for AD detection and monitoring, thus offering new insights into the diagnosis and progression of this devastating disease.