Enteric short-chain fatty acids: microbial messengers of metabolism, mitochondria, and mind: implications in autism spectrum disorders

An approach to gut microbiota profile in children with autism spectrum disorder

In recent years, there has been an increase in studies on the implications of gut microbiota (GM) on the behaviour of children with autism spectrum disorders (ASD) due to a dysbiosis in GM that can trigger onset, development or progression of ASD through the microbiota-gut-brain axis. The aim of this study is to carry out a systematic review of articles from the last 6 years that analyse GM in children with ASD compared to GM in control groups. Children with ASD showed a higher abundance of Roseburia and Candida genera, and lower abundance of Dialister, Bilophila, Veillonella, Streptococcus, Coprococcus and Prevotella genera. Those differences can be attributed to factors such as different nationalities, nature of control groups, place where the sample was taken, gastrointestinal (GI) problems or bacterial detection methods. It is still too early to define a specific GM profile of children with ASD, and future studies should focus on homogenizing the characteristics of samples and control groups. Furthermore, new multicentre studies should also focus on the impact of GM on GI physiology, neurophysiology and behaviour of children with ASD, and on performing psychometric analyses of the correlation between the severity of ASD behavioural symptoms and GM profiles.

Behavioural and brain ultrastructural changes following the systemic administration of propionic acid in adolescent male rats. Further development of a rodent model of autism

Short chain fatty acids, produced as gut microbiome metabolites but also present in the diet, exert broad effects in host physiology. Propionic acid (PPA), along with butyrate and acetate, plays a growing role in health, but also in neurological conditions. Increased PPA exposure in humans, animal models and cell lines elicit diverse behavioural and biochemical changes consistent with organic acidurias, mitochondrial disorders and autism spectrum disorders (ASD). ASD is considered a disorder of synaptic dysfunction and cell signalling, but also neuroinflammatory and neurometabolic components. We examined behaviour (Morris water and radial arm mazes) and the ultrastructure of the hippocampus and medial prefrontal cortex (electron microscopy) following a single intraperitoneal (i.p.) injection of PPA (175 mg/kg) in male adolescent rats. PPA treatment showed altered social and locomotor behaviour without changes in learning and memory. Both transient and enduring ultrastructural alterations in synapses, astro- and microglia were detected in the CA1 hippocampal area. Electron microscopic analysis showed the PPA treatment significantly decreased the total number of synaptic vesicles, presynaptic mitochondria and synapses with a symmetric active zone. Thus, brief systemic administration of this dietary and enteric short chain fatty acid produced behavioural and dynamic brain ultrastructural changes, providing further validation of the PPA model of ASD.

Oxiracetam and Zinc Ameliorates Autism-Like Symptoms in Propionic Acid Model of Rats

Autism spectrum disorder (ASD) is a complex neurodevelopmental disorder characterized by restrictive behaviour, deficit in social skills and interaction. The multifactorial etiology, complex pathophysiology and different combination of symptoms (unusual speech patterns, frequent repetition of phrases) make it difficult to treat. Thus, present study aimed to find the protective effects of oxiracetam alone and in combination with zinc on brain behavioral, biochemical, pro-inflammatory cytokines and neurotransmitters level. Rats were administered with propionic acid (250 mg/kg p.o.) for 3 days and immediately on next day treatment were given with oxiracetam (25, 50 mg/kg i.p), zinc (4 mg/kg) as well as oxiracetam (25 mg/kg i.p) in combination with zinc (4 mg/kg p.o). Behavioral parameters were performed from 22th to 28th day. On 29th day, all the animals were sacrificed by cervical dislocation and the brain was preserved for biochemical (LPO, GSH, nitrite, mitochondrial complex I, IV and cAMP), neuroinflammatory (TNF-α, IL-1β, IL-6) and neurotransmitters (5-HT, GABA, glutamate and acetylcholine) analysis. The propionic acid administration showed memory impairment, restrictive behavior, increased proinflammatory cytokines level, biochemical and neurotransmitters alteration. However, treatment with oxiracetam alone and in combination with zinc significantly attenuated behavioral, biochemical, inflammatory cytokines and restored neurotransmitters level. The finding of present study demonstrated that oxiracetam alone and in combination with zinc afforded superior anti-autistic effect through antioxidant, anti-inflammatory and anti-excitotoxic mechanisms and could serve as attractive strategy in managing autism.

Photoperiod modulates the gut microbiome and aggressive behavior in Siberian hamsters

Seasonally breeding animals undergo shifts in physiology and behavior in response to changes in photoperiod (day length). Interestingly, some species, such as Siberian hamsters (Phodopus sungorus), are more aggressive during the short-day photoperiods of the non-breeding season, despite gonadal regression. While our previous data suggest that Siberian hamsters employ a 'seasonal switch' from gonadal to adrenal regulation of aggression during short-day photoperiods, there is emerging evidence that the gut microbiome, an environment of symbiotic bacteria within the gastrointestinal tract, may also change seasonally and modulate social behaviors. The goal of this study was to compare seasonal shifts in the gut microbiome, circulating levels of adrenal dehydroepiandrosterone (DHEA) and aggression in male and female Siberian hamsters. Hamsters were housed in either long-day (LD) or short-day (SD) photoperiods for 9 weeks. Fecal samples were collected and behaviors were recorded following 3, 6 and 9 weeks of housing, and circulating DHEA was measured at week 9. SD females that were responsive to changes in photoperiod (SD-R), but not SD-R males, displayed increased aggression following 9 weeks of treatment. SD-R males and females also exhibited distinct changes in the relative abundance of gut bacterial phyla and families, yet showed no change in circulating DHEA. The relative abundance of some bacterial families (e.g. Anaeroplasmataceae in females) was associated with aggression in SD-R but not LD or SD non-responder (SD-NR) hamsters after 9 weeks of treatment. Collectively, this study provides insight into the complex role of the microbiome in regulating social behavior in seasonally breeding species.

Autism and Gut-Brain Axis: Role of Probiotics

Characterized by a wide range of behavioural, social and language problems, autism is a complex developmental disability that affects an individual's capacity to communicate and interact with others. Although the real causes that lead to the development of autism are still unclear, the gastrointestinal tract has been found to play a major role in the development of autism. Alterations in macrobiotic compositions have been reported in autistic children. Irregularities in carbohydrate digestion and absorption could also explain some of the gastrointestinal problems reported in autistic patients, although their role in the neurological and behavioural problems remains uncertain. A relationship between improved gut health and decrease of symptoms in autism has been reported as well. Studies done to evaluate the gluten-free diets, casein-free diets, pre- and probiotic and multivitamin supplementation have shown promising results. Probiotics have been thought to alleviate the progression of autism and reduce cognitive and behavioural deficits.

Enteric short-chain fatty acids promote proliferation of human neural progenitor cells

Short-chain fatty acids (SCFAs) are a group of fatty acids predominantly produced during the fermentation of dietary fibers by the gut anaerobic microbiota. SCFAs affect many host processes in health and disease. SCFAs play an important role in the 'gut-brain axis', regulating central nervous system processes, for example, cell-cell interaction, neurotransmitter synthesis and release, microglia activation, mitochondrial function, and gene expression. SCFAs also promote the growth of neurospheres from human neural stem cells and the differentiation of embryonic stem cells into neural cells. It is plausible that maternally derived SCFAs may pass the placenta and expose the fetus at key developmental periods. However, it is unclear how SCFA exposure at physiological levels influence the early-stage neural cells. In this study, we explored the effect of SCFAs on the growth rate of human neural progenitor cells (hNPCs), generated from human embryonic stem cell line (HS980), with IncuCyte live-cell analysis system and immunofluorescence. We found that physiologically relevant levels (µM) of SCFAs (acetate, propionate, butyrate) increased the growth rate of hNPCs significantly and induced more cells to undergo mitosis, while high levels (mM) of SCFAs had toxic effects on hNPCs. Moreover, no effect on apoptosis was observed in physiological-dose SCFA treatments. In support, data from q-RT PCR showed that SCFA treatments influenced the expression of the neurogenesis, proliferation, and apoptosis-related genes ATR, BCL2, BID, CASP8, CDK2, E2F1, FAS, NDN, and VEGFA. To conclude, our results propose that SCFAs regulates early neural system development. This might be relevant for a putative 'maternal gut-fetal brain-axis'. Cover Image for this issue: doi: 10.1111/jnc.14761.

Fecal Short-Chain Fatty Acids Levels Were Not Associated With Autism Spectrum Disorders in Chinese Children: A Case-Control Study

Evidence from animal models supports a link between short-chain fatty acids (SCFAs), a key subset of gut microbial metabolites, and autism spectrum disorders (ASD). However, findings from human studies on this topic are unclear. We aimed to investigate whether fecal SCFAs are associated with ASD in Chinese children aged 6–9 years old. A total of 45 ASD children aged 6–9 years and 90 sex- and age-matched neurotypical controls were enrolled. High-performance liquid chromatography was applied to quantify 10 SCFA subtypes in feces. Dietary and other socio-demographic information were obtained via face-to-face interview using questionnaires. After adjustment for multiple comparisons, paired t-test analysis indicated that the fecal total and subtype SCFA concentrations were comparable in autistic children and the controls. Conditional logistic regression analysis showed that there was no significant relationship between the fecal concentration of SCFAs and the risk of ASD after adjustment for age, sex, BMI, breastfeeding, mode of delivery, parental education level, and daily energy, protein, fat, and fiber intake. In conclusion, our results did not support the hypothesis that fecal SCFA levels might be associated with the presence of ASD. However, SCFA measurement was based on a single stool sample test, so this conclusion should be treated with caution. Further studies with measurement of long-term bodily SCFA concentrations are needed to examine this relationship.

Autism, Gastrointestinal Symptoms and Modulation of Gut Microbiota by Nutritional Interventions

Autism spectrum disorder (ASD) is a complex behavioral syndrome that is characterized by speech and language disorders, intellectual impairment, learning and motor dysfunctions. Several genetic and environmental factors are suspected to affect the ASD phenotype including air pollution, exposure to pesticides, maternal infections, inflammatory conditions, dietary factors or consumption of antibiotics during pregnancy. Many children with ASD shows abnormalities in gastrointestinal (GI) physiology, including increased intestinal permeability, overall microbiota alterations, and gut infection. Moreover, they are "picky eaters" and the existence of specific sensory patterns in ASD patients could represent one of the main aspects in hampering feeding. GI disorders are associated with an altered composition of the gut microbiota. Gut microbiome is able to communicate with brain activities through microbiota-derived signaling molecules, immune mediators, gut hormones as well as vagal and spinal afferent neurons. Since the diet induces changes in the intestinal microbiota and in the production of molecules, such as the SCFA, we wanted to investigate the role that nutritional intervention can have on GI microbiota composition and thus on its influence on behavior, GI symptoms and microbiota composition and report which are the beneficial effect on ASD conditions.

The Potential Influence of the Bacterial Microbiome on the Development and Progression of ADHD

The latest research cumulates staggering information about the correlation between the microbiota-gut-brain axis and neurodevelopmental disorders. This review aims to shed light on the potential influence of the microbiome on the development of the most prevalent neurodevelopmental disease, attention-deficit-hyperactive disorder (ADHD). As the etiology and pathophysiology of ADHD are still unclear, finding viable biomarkers and effective treatment still represent a challenge. Therefore, we focused on factors that have been associated with a higher risk of developing ADHD, while simultaneously influencing the microbial composition. We reviewed the effect of a differing microbial makeup on neurotransmitter concentrations important in the pathophysiology of ADHD. Additionally, we deduced factors that correlate with a high prevalence of ADHD, while simultaneously affecting the gut microbiome, such as emergency c-sections, and premature birth as the former leads to a decrease of the gut microbial diversity and the latter causes neuroprotective Lactobacillus levels to be reduced. Also, we assessed nutritional influences, such as breastfeeding, ingestion of short-chain fatty acids (SCFAs) and polyunsaturated fatty acids (PUFAs) on the host′s microbiome and development of ADHD. Finally, we discussed the potential significance of Bifidobacterium as a biomarker for ADHD, the importance of preventing premature birth as prophylaxis and nutrition as a prospective therapeutic measurement against ADHD.

Plasma peroxiredoxin changes and inflammatory cytokines support the involvement of neuro-inflammation and oxidative stress in Autism Spectrum Disorder

BACKGROUND

It has been established that children with Autism Spectrum Disorders (ASD) are affected by oxidative stress, the origin of which is still under investigation. In the present work, we evaluated inflammatory and pro-oxidant soluble signature in non-syndromic ASD and age-matched typically developing (TD) control children.

METHODS

We analyzed leukocyte gene expression of inflammatory cytokines and inflammation/oxidative-stress related molecules in 21 ASD and 20 TD children. Moreover, in another-comparable-group of non-syndromic ASD (N = 22) and TD (N = 21) children, we analyzed for the first time the protein expression of the four members of the antioxidant enzyme family of peroxiredoxins (Prx) in both erythrocyte membranes and in plasma.

RESULTS

The gene expression of IL6 and of HSP70i, a stress protein, was increased in ASD children. Moreover, gene expression of many inflammatory cytokines and inflammation/oxidative stress-related proteins correlated with clinical features, and appeared to be linked by a complex network of inter-correlations involving the Aryl Hydrocarbon Receptor signaling pathway. In addition, when the study of inter-correlations within the expression pattern of these molecules was extended to include the healthy subjects, the intrinsic physiological relationships of the inflammatory/oxidative stress network emerged. Plasma levels of Prx2 and Prx5 were remarkably increased in ASD compared to healthy controls, while no significant differences were found in red cell Prx levels.

CONCLUSIONS

Previous findings reported elevated inflammatory cytokines in the plasma of ASD children, without clearly pointing to the presence of neuro-inflammation. On the other hand, the finding of microglia activation in autoptic specimens was clearly suggesting the presence of neuro-inflammation in ASD. Given the role of peroxiredoxins in the protection of brain cells against oxidative stress, the whole of our results, using peripheral data collected in living patients, support the involvement of neuro-inflammation in ASD, and generate a rational for neuro-inflammation as a possible therapeutic target and for plasma Prx5 as a novel indicator of ASD severity.

The Role of Gut Microbiota in Gastrointestinal Symptoms of Children with ASD

Background and objectives: Autism spectrum disorder (ASD) is a neurodevelopmental disorder characterized by impaired communication, social interaction disorder, and repetitive behavior. Dysbiotic gut microbiota (GM) could be a contributing factor to the appearance of ASD, as gastrointestinal (GI) symptoms are comorbidities frequently reported in ASD. As there is a lack of reviews about the role played by GM in the GI symptoms of ASD, this work aimed to carry out a systematic review of current studies comparing the GM of children with ASD and GI symptoms with those of healthy controls in the last six years. Materials and Methods: The systematic review was performed following the PRISMA guidelines. The databases chosen were Web of Science, Scopus, PubMed, and PsycINFO, and the keywords were (gut* OR intestine* OR bowel* OR gastrointestinal*) AND (microbiota* OR microflora* OR bacteria* OR microbiome* OR flora* OR bacterial* OR bacteria* OR microorganism* OR feces* OR stool*) AND (autistic* OR autism* OR ASD*). Results: A total of 16 articles were included. Ten articles performed correlations analysis between GI symptoms and ASD. Among those 10 articles, 7 found differences between the GI symptoms present in children with ASD and healthy controls. The most common GI symptom was constipation. Among the seven articles that found differences, three performed correlations analysis between GI symptoms and gut microbe abundance. Candida, Prevotella, Streptococcus, and Veillonella showed higher and lower abundance, respectively, in children with ASD and GI symptoms in more than one article. Bacteroidetes, Firmicutes, Actinomyces, Dorea, Lactobacillus, Faecalibacterium prausnitzii, and Bacteroidetes/Firmicutes ratios showed abundance discrepancies. Conclusions: It is still too early to draw a conclusion about the gut microbes involved in GI symptoms of ASD. Future research should consider the relationship between ASD behavior, GM, and GI symptoms in a multidisciplinary way and homogenize sample characteristics.

Microbiome programming of brain development: implications for neurodevelopmental disorders

During the last decade, research on germ-free mice has discovered that the gut microbiome (i.e. the normal bacteria colonizing the gastrointestinal tract) can programme brain function and behaviour during early development. At the same time a growing number of clinical studies have shown altered gut microflora in children with autism spectrum disorder (ASD), in combination with altered bacterial metabolites and inflammatory cytokines being part of the gut-brain axis. This review covers the concept of the microbiome; how it is established during childhood; how it is affected by malnutrition; how it can programme the development of the brain through epigenetic mechanisms; which pathways are used from the gut to the brain; and assesses findings that suggest the gut microbiome may be involved in ASD and other neurodevelopmental disorders. This is a new research field with a number of exciting, but so far fragmented, findings indicating the important role of the normal microbiome in shaping the brain. Research also suggests that disruptions of the microbiome may be involved in the aetiology of neurodevelopmental disorders. WHAT THIS PAPER ADDS: The gut microbiome shapes the brain via the gut-brain axis. The microbiome may play a role in neurodevelopmental disorders.

Antidepressants affect gut microbiota and Ruminococcus flavefaciens is able to abolish their effects on depressive-like behavior

Accumulating evidence demonstrates that the gut microbiota affects brain function and behavior, including depressive behavior. Antidepressants are the main drugs used for treatment of depression. We hypothesized that antidepressant treatment could modify gut microbiota which can partially mediate their antidepressant effects. Mice were chronically treated with one of five antidepressants (fluoxetine, escitalopram, venlafaxine, duloxetine or desipramine), and gut microbiota was analyzed, using 16s rRNA gene sequencing. After characterization of differences in the microbiota, chosen bacterial species were supplemented to vehicle and antidepressant-treated mice, and depressive-like behavior was assessed to determine bacterial effects. RNA-seq analysis was performed to determine effects of bacterial treatment in the brain. Antidepressants reduced richness and increased beta diversity of gut bacteria, compared to controls. At the genus level, antidepressants reduced abundances of Ruminococcus, Adlercreutzia, and an unclassified Alphaproteobacteria. To examine implications of the dysregulated bacteria, we chose one of antidepressants (duloxetine) and investigated if its antidepressive effects can be attenuated by simultaneous treatment with Ruminococcus flavefaciens or Adlercreutzia equolifaciens. Supplementation with R. flavefaciens diminished duloxetine-induced decrease in depressive-like behavior, while A. equolifaciens had no such effect. R. flavefaciens treatment induced changes in cortical gene expression, up-regulating genes involved in mitochondrial oxidative phosphorylation, while down-regulating genes involved in neuronal plasticity. Our results demonstrate that various types of antidepressants alter gut microbiota composition, and further implicate a role for R. flavefaciens in alleviating depressive-like behavior. Moreover, R. flavefaciens affects gene networks in the brain, suggesting a mechanism for microbial regulation of antidepressant treatment efficiency.

The profiles of dysbiotic microbial communities

Alterations in the human gut microbiota play an important role in disease pathogenesis. Although next-generation sequencing has provided observational evidence linking shifts in gut microbiota composition to alterations in the human host, underlying mechanisms remain elusive. Metabolites generated within complex microbial communities and at the crossroads with host cells may be able to explain the impact of the gut microbiome on human homeostasis. Emerging technologies including novel culturing protocols, microfluidic systems, engineered organoids, and single-cell imaging approaches are providing new perspectives from which the gut microbiome can be studied paving the way to new diagnostic markers and personalized therapeutic interventions.

Autism Spectrum Disorders and the Gut Microbiota

In recent years, there has been an emerging interest in the possible role of the gut microbiota as a co-factor in the development of autism spectrum disorders (ASDs), as many studies have highlighted the bidirectional communication between the gut and brain (the so-called "gut-brain axis"). Accumulating evidence has shown a link between alterations in the composition of the gut microbiota and both gastrointestinal and neurobehavioural symptoms in children with ASD. The aim of this narrative review was to analyse the current knowledge about dysbiosis and gastrointestinal (GI) disorders in ASD and assess the current evidence for the role of probiotics and other non-pharmacological approaches in the treatment of children with ASD. Analysis of the literature showed that gut dysbiosis in ASD has been widely demonstrated; however, there is no single distinctive profile of the composition of the microbiota in people with ASD. Gut dysbiosis could contribute to the low-grade systemic inflammatory state reported in patients with GI comorbidities. The administration of probiotics (mostly a mixture of Bifidobacteria, Streptococci and Lactobacilli) is the most promising treatment for neurobehavioural symptoms and bowel dysfunction, but clinical trials are still limited and heterogeneous. Well-designed, randomized, placebo-controlled clinical trials are required to validate the effectiveness of probiotics in the treatment of ASD and to identify the appropriate strains, dose, and timing of treatment.

Anorexia Nervosa and Autism Spectrum Disorders: Future Hopes Linked to Mucosal Immunity

Mental health is becoming a public health priority worldwide. Anorexia nervosa and autism spectrum disorders are 2 important types of childhood disorders with a bad prognosis. They share cognitive impairments and, in both cases, the microbiota appears to be a crucial factor. Alteration of the microbiota-gut-brain axis is an appealing hypothesis to define new pathophysiological mechanisms. Mucosal immunity plays a key role between the microbiota and the brain. The mucosal immune system receives and integrates messages from the intestinal microenvironment and the microbiota and then transmits the information to the nervous system. Abnormalities in this sensorial system may be involved in the natural history of mental diseases and might play a role in their maintenance. This review aims to highlight data about the relationship between intestinal mucosal immunity and these disorders. We show that shared cognitive impairments could be found in these 2 disorders, which both present dysbiosis. This literature review provides details on the immune status of anorexic and autistic patients, with a focus on intestinal mucosal factors. Finally, we suggest future research hypotheses that seem important for understanding the implication of the gut-brain-axis in psychiatric diseases.

A Subset of Patients With Autism Spectrum Disorders Show a Distinctive Metabolic Profile by Dried Blood Spot Analyses

Autism spectrum disorder (ASD) is currently diagnosed according to behavioral criteria. Biomarkers that identify children with ASD could lead to more accurate and early diagnosis. ASD is a complex disorder with multifactorial and heterogeneous etiology supporting recognition of biomarkers that identify patient subsets. We investigated an easily testable blood metabolic profile associated with ASD diagnosis using high throughput analyses of samples extracted from dried blood spots (DBS). A targeted panel of 45 ASD analytes including acyl-carnitines and amino acids extracted from DBS was examined in 83 children with ASD (60 males; age 6.06 ± 3.58, range: 2-10 years) and 79 matched, neurotypical (NT) control children (57 males; age 6.8 ± 4.11 years, range 2.5-11 years). Based on their chronological ages, participants were divided in two groups: younger or older than 5 years. Two-sided T-tests were used to identify significant differences in measured metabolite levels between groups. Näive Bayes algorithm trained on the identified metabolites was used to profile children with ASD vs. NT controls. Of the 45 analyzed metabolites, nine (20%) were significantly increased in ASD patients including the amino acid citrulline and acyl-carnitines C2, C4DC/C5OH, C10, C12, C14:2, C16, C16:1, C18:1 (P: < 0.001). Näive Bayes algorithm using acyl-carnitine metabolites which were identified as significantly abnormal showed the highest performances for classifying ASD in children younger than 5 years (n: 42; mean age 3.26 ± 0.89) with 72.3% sensitivity (95% CI: 71.3;73.9), 72.1% specificity (95% CI: 71.2;72.9) and a diagnostic odds ratio 11.25 (95% CI: 9.47;17.7). Re-test analyses as a measure of validity showed an accuracy of 73% in children with ASD aged ≤ 5 years. This easily testable, non-invasive profile in DBS may support recognition of metabolic ASD individuals aged ≤ 5 years and represents a potential complementary tool to improve diagnosis at earlier stages of ASD development.

A systematic review of gut-immune-brain mechanisms in Autism Spectrum Disorder

Despite decades of research, the etiological origins of Autism Spectrum Disorder (ASD) remain elusive. Recently, the mechanisms of ASD have encompassed emerging theories involving the gastrointestinal, immune, and nervous systems. While each of these perspectives presents its own set of supporting evidence, the field requires an integration of these modular concepts and an overarching view of how these subsystems intersect. In this systematic review, we have synthesized relevant evidences from the existing literature, evaluating them in an interdependent manner and in doing so, outlining their possible connections. Specifically, we first discussed gastrointestinal and immuno-inflammation pathways in-depth, exploring the relationships between microbial composition, bacterial metabolites, gut mucosa, and immune system constituents. Accounting for temporal differences in the mechanisms involved in neurodevelopment, prenatal and postnatal phases were further elucidated, where the former focused on maternal immune activation (MIA) and fetal development, while the latter addressed the role of immune dysregulation in contributing to atypical neurodevelopment. As autism remains, foremost, a neurodevelopmental disorder, this review presents an integration of disparate modules into a "Gut-Immune-Brain" paradigm. Existing gaps in the literature have been highlighted, and possible avenues for future research with an integrated physiological perspective underlying ASD have also been suggested.

Microbiome-microglia connections via the gut-brain axis

A role for the gut microbiome in facilitating microglial maturation and shaping microglial physiology has emerged in recent years. This review highlights evidence demonstrating the various mechanisms by which the gut microbiota can influence microglia in both homeostatic and disease conditions.

Microglia, the resident immune cells in the brain, are essential for modulating neurogenesis, influencing synaptic remodeling, and regulating neuroinflammation by surveying the brain microenvironment. Microglial dysfunction has been implicated in the onset and progression of several neurodevelopmental and neurodegenerative diseases; however, the multitude of factors and signals influencing microglial activity have not been fully elucidated. Microglia not only respond to local signals within the brain but also receive input from the periphery, including the gastrointestinal (GI) tract. Recent preclinical findings suggest that the gut microbiome plays a pivotal role in regulating microglial maturation and function, and altered microbial community composition has been reported in neurological disorders with known microglial involvement in humans. Collectively, these findings suggest that bidirectional crosstalk between the gut and the brain may influence disease pathogenesis. Herein, we discuss recent studies showing a role for the gut microbiome in modulating microglial development and function in homeostatic and disease conditions and highlight possible future research to develop novel microbial treatments for disorders of the brain.

The Gut-Brain Axis: Influence of Microbiota on Mood and Mental Health

The gut-brain axis is a bidirectional communication network that links the enteric and central nervous systems. This network is not only anatomical, but it extends to include endocrine, humoral, metabolic, and immune routes of communication as well. The autonomic nervous system, hypothalamic-pituitary-adrenal (HPA) axis, and nerves within the gastrointestinal tract, all link the gut and the brain, allowing the brain to influence intestinal activities, including activity of functional immune effector cells; and the gut to influence mood, cognition, and mental health.

Gut Microbiota and Fecal Metabolome Perturbation in Children with Autism Spectrum Disorder

The brain-intestinal axis concept describes the communication between the intestinal microbiota as an ecosystem of a number of dynamic microorganisms and the brain. The composition of the microbial community of the human gut is important for human health by influencing the total metabolomic profile. In children with autism spectrum disorder (ASD), the composition of the fecal microbiota and their metabolic products has a different configuration of the healthy child. An imbalance in the metabolite derived from the microbiota in children with ASD affect brain development and social behavior. In this article, we review recent discoveries about intestinal metabolites derived from microbiota based on high-yield molecular studies in children with ASD as part of the "intestinal brain axis"

The role of the gut microbiota on animal model reproducibility

The gut microbiota is composed of approximately 1010-1014 cells, including fungi, bacteria, archaea, protozoa, viruses, and bacteriophages; their genes and their various metabolites were found throughout the gastrointestinal tract. It has co-evolved with each species to assist with day to day bodily functions, such as digestion, metabolism of xenobiotics, development of mucosal immunity and immunomodulation, and protection against invading pathogens. Because of the significant beneficial impact that gut microbiota may have, there is interest in learning more about it and translating these findings into clinical therapies. Results from recent studies characterizing the gut microbiota of various species have demonstrated the range of influences that may affect gut microbiota diversity, including animal strain, obesity, types of enrichment used, bedding and housing methods, treatment with antimicrobials, vendor source, specific animal housing, diet, and intercurrent disease. Relatively little is known about the functional consequences of alterations of the gut microbiota and exactly how changes in richness and diversity of the microbiota translate into changes in health and susceptibility to disease. Furthermore, questions have been raised as to whether germ-free or even ultraclean, barrier-raised mice are relevant models of human disease, given their significantly reduced gut microbiota diversity and complexity compared with conventionally housed mice. In addition, evidence suggests that the specific anatomical location selected for assessing the gut microbiota has a highly significant effect on study outcomes, in that bacterial phyla change significantly along the gastrointestinal tract. This paper will explore animal model reproducibility in light of this information about the gut microbiota.

The Ramazzini Institute 13-week pilot study on glyphosate and Roundup administered at human-equivalent dose to Sprague Dawley rats: effects on the microbiome

BACKGROUND

Glyphosate-based herbicides (GBHs) are broad-spectrum herbicides that act on the shikimate pathway in bacteria, fungi, and plants. The possible effects of GBHs on human health are the subject of an intense public debate for both its potential carcinogenic and non-carcinogenic effects, including its effects on microbiome. The present pilot study examines whether exposure to GBHs at doses of glyphosate considered to be "safe" (the US Acceptable Daily Intake - ADI - of 1.75 mg/kg bw/day), starting from in utero, may modify the composition of gut microbiome in Sprague Dawley (SD) rats.

METHODS

Glyphosate alone and Roundup, a commercial brand of GBHs, were administered in drinking water at doses comparable to the US glyphosate ADI (1.75 mg/kg bw/day) to F0 dams starting from the gestational day (GD) 6 up to postnatal day (PND) 125. Animal feces were collected at multiple time points from both F0 dams and F1 pups. The gut microbiota of 433 fecal samples were profiled at V3-V4 region of 16S ribosomal RNA gene and further taxonomically assigned and assessed for diversity analysis. We tested the effect of exposure on overall microbiome diversity using PERMANOVA and on individual taxa by LEfSe analysis.

RESULTS

Microbiome profiling revealed that low-dose exposure to Roundup and glyphosate resulted in significant and distinctive changes in overall bacterial composition in F1 pups only. Specifically, at PND31, corresponding to pre-pubertal age in humans, relative abundance for Bacteriodetes (Prevotella) was increased while the Firmicutes (Lactobacillus) was reduced in both Roundup and glyphosate exposed F1 pups compared to controls.

CONCLUSIONS

This study provides initial evidence that exposures to commonly used GBHs, at doses considered safe, are capable of modifying the gut microbiota in early development, particularly before the onset of puberty. These findings warrant future studies on potential health effects of GBHs in early development such as childhood.

Dietary Considerations in Autism Spectrum Disorders: The Potential Role of Protein Digestion and Microbial Putrefaction in the Gut-Brain Axis

Children with autism spectrum disorders (ASD), characterized by a range of behavioral abnormalities and social deficits, display high incidence of gastrointestinal (GI) co-morbidities including chronic constipation and diarrhea. Research is now increasingly able to characterize the “fragile gut” in these children and understand the role that impairment of specific GI functions plays in the GI symptoms associated with ASD. This mechanistic understanding is extending to the interactions between diet and ASD, including food structure and protein digestive capacity in exacerbating autistic symptoms. Children with ASD and gut co-morbidities exhibit low digestive enzyme activity, impaired gut barrier integrity and the presence of antibodies specific for dietary proteins in the peripheral circulation. These findings support the hypothesis that entry of dietary peptides from the gut lumen into the vasculature are associated with an aberrant immune response. Furthermore, a subset of children with ASD exhibit high concentrations of metabolites originating from microbial activity on proteinaceous substrates. Taken together, the combination of specific protein intakes poor digestion, gut barrier integrity, microbiota composition and function all on a background of ASD represents a phenotypic pattern. A potential consequence of this pattern of conditions is that the fragile gut of some children with ASD is at risk for GI symptoms that may be amenable to improvement with specific dietary changes. There is growing evidence that shows an association between gut dysfunction and dysbiosis and ASD symptoms. It is therefore urgent to perform more experimental and clinical research on the “fragile gut” in children with ASD in order to move toward advancements in clinical practice. Identifying those factors that are of clinical value will provide an evidence-based path to individual management and targeted solutions; from real time sensing to the design of diets with personalized protein source/processing, all to improve GI function in children with ASD.

Postnatal colonization with human "infant-type" Bifidobacterium species alters behavior of adult gnotobiotic mice

Accumulating studies have defined a role for the intestinal microbiota in modulation of host behavior. Research using gnotobiotic mice emphasizes that early microbial colonization with a complex microbiota (conventionalization) can rescue some of the behavioral abnormalities observed in mice that grow to adulthood completely devoid of bacteria (germ-free mice). However, the human infant and adult microbiomes vary greatly, and effects of the neonatal microbiome on neurodevelopment are currently not well understood. Microbe-mediated modulation of neural circuit patterning in the brain during neurodevelopment may have significant long-term implications that we are only beginning to appreciate. Modulation of the host central nervous system by the early-life microbiota is predicted to have pervasive and lasting effects on brain function and behavior. We sought to replicate this early microbe-host interaction by colonizing gnotobiotic mice at the neonatal stage with a simplified model of the human infant gut microbiota. This model consortium consisted of four “infant-type” Bifidobacterium species known to be commensal members of the human infant microbiota present in high abundance during postnatal development. Germ-free mice and mice neonatally-colonized with a complex, conventional murine microbiota were used for comparison. Motor and non-motor behaviors of the mice were tested at 6–7 weeks of age, and colonization patterns were characterized by 16S ribosomal RNA gene sequencing. Adult germ-free mice were observed to have abnormal memory, sociability, anxiety-like behaviors, and motor performance. Conventionalization at the neonatal stage rescued these behavioral abnormalities, and mice colonized with Bifidobacterium spp. also exhibited important behavioral differences relative to the germ-free controls. The ability of Bifidobacterium spp. to improve the recognition memory of both male and female germ-free mice was a prominent finding. Together, these data demonstrate that the early-life gut microbiome, and human “infant-type” Bifidobacterium species, affect adult behavior in a strongly sex-dependent manner, and can selectively recapitulate the results observed when mice are colonized with a complex microbiota.

Differential immune responses and microbiota profiles in children with autism spectrum disorders and co-morbid gastrointestinal symptoms

OBJECTIVES

Many studies have reported the increased presence of gastrointestinal (GI) symptoms in children with autism spectrum disorders (ASD). Altered microbiome profiles, pro-inflammatory responses and impaired intestinal permeability have been observed in children with ASD and co-morbid GI symptoms, yet few studies have compared these findings to ASD children without GI issues or similarly aged typical developing children. The aim of this study was to determine whether there are biological signatures in terms of immune dysfunction and microbiota composition in children with ASD with GI symptoms.

METHODS

Children were enrolled in one of four groups: ASD and GI symptoms of irregular bowel habits (ASDGI), children with ASD but without current or previous GI symptoms (ASDNoGI), typically developing children with GI symptoms (TDGI) and typically developing children without current or previous GI symptoms (TDNoGI). Peripheral blood mononuclear cells (PBMC) were isolated from the blood, stimulated and assessed for cytokine production, while stool samples were analyzed for microbial composition.

RESULTS

Following Toll-Like receptor (TLR)-4 stimulation, the ASDGI group produced increased levels of mucosa-relevant cytokines including IL-5, IL-15 and IL-17 compared to ASDNoGI. The production of the regulatory cytokine TGFβ1 was decreased in the ASDGI group compared with both the ASDNoGI and TDNoGI groups. Analysis of the microbiome at the family level revealed differences in microbiome composition between ASD and TD children with GI symptoms; furthermore, a predictive metagenome functional content analysis revealed that pathways were differentially represented between ASD and TD subjects, independently of the presence of GI symptoms. The ASDGI also showed an over-representation of the gene encoding zonulin, a molecule regulating gut permeability, compared to the other groups.

CONCLUSIONS

Overall our findings suggest that children with ASD who experience GI symptoms have an imbalance in their immune response, possibly influenced by or influencing metagenomic changes, and may have a propensity to impaired gut barrier function which may contribute to their symptoms and clinical outcome.

The Impact of Carnitine on Dietary Fiber and Gut Bacteria Metabolism and Their Mutual Interaction in Monogastrics

Carnitine has vital roles in the endogenous metabolism of short chain fatty acids. It can protect and support gut microbial species, and some dietary fibers can reduce the available iron involved in the bioactivity of carnitine. There is also an antagonistic relationship between high microbial populations and carnitine bioavailability. This review shows the interactions between carnitine and gut microbial composition. It also elucidates the role of carnitine bacterial metabolism, mitochondrial function, fiber fermentability, and short chain fatty acids (SCFAs).

Propionate enters GABAergic neurons, inhibits GABA transaminase, causes GABA accumulation and lethargy in a model of propionic acidemia

Propionic acidemia is the accumulation of propionate in blood due to dysfunction of propionyl-CoA carboxylase. The condition causes lethargy and striatal degeneration with motor impairment in humans. How propionate exerts its toxic effect is unclear. Here, we show that intravenous administration of propionate causes dose-dependent propionate accumulation in the brain and transient lethargy in mice. Propionate, an inhibitor of histone deacetylase, entered GABAergic neurons, as could be seen from increased neuronal histone H4 acetylation in the striatum and neocortex. Propionate caused an increase in GABA (γ-amino butyric acid) levels in the brain, suggesting inhibition of GABA breakdown. In vitro propionate inhibited GABA transaminase with a Ki of ∼1 mmol/l. In isolated nerve endings, propionate caused increased release of GABA to the extracellular fluid. In vivo, propionate reduced cerebral glucose metabolism in both striatum and neocortex. We conclude that propionate-induced inhibition of GABA transaminase causes accumulation of GABA in the brain, leading to increased extracellular GABA concentration, which inhibits neuronal activity and causes lethargy. Propionate-mediated inhibition of neuronal GABA transaminase, an enzyme of the inner mitochondrial membrane, indicates entry of propionate into neuronal mitochondria. However, previous work has shown that neurons are unable to metabolize propionate oxidatively, leading us to conclude that propionyl-CoA synthetase is probably absent from neuronal mitochondria. Propionate-induced inhibition of energy metabolism in GABAergic neurons may render the striatum, in which >90% of the neurons are GABAergic, particularly vulnerable to degeneration in propionic acidemia.

Pathophysiological and neurobehavioral characteristics of a propionic acid-mediated autism-like rat model

Autism spectrum disorder (ASD) is induced by complex hereditary and environmental factors. However, the mechanisms of ASD development are poorly understood. The purpose of this study was to identify standard indicators of this condition by comparing clinical, pathophysiological, and neurobehavioral features in an autism-like animal model. A total of 22 male Sprague-Dawley rats were randomly divided into control and 500 mg/kg propionic acid (PPA)-treated groups. Rats were subjected to behavioral tests, gene expression analyses, and histological analyses to detect pathophysiological and neurobehavioral alterations. Exploratory activity and non-aggressive behavior were significantly reduced in PPA-treated rats, whereas enhanced aggressive behavior during adjacent interactions was observed on day 14 after PPA administration. To evaluate gene expression after PPA administration, we analyzed hippocampal tissue using reverse transcription PCR. Glial fibrillary acidic protein was augmented in the PPA-treated group on day 14 after appearance of ASD-like behaviors by PPA administration, whereas octamer-binding transcription factor 4 expression was significantly decreased in the PPA-treated group. Histological evaluation revealed significantly reduced diameter and layer thickness of granule cells in PPA-treated rats compared with control rats. We conclude that PPA administration induced abnormal neural cell organization, which may have led to autism-like neurobehaviors, including increased aggressive behavior, reduced exploratory activity, and isolative and passive behaviors.

Butyrate enhances mitochondrial function during oxidative stress in cell lines from boys with autism

Butyrate (BT) is a ubiquitous short-chain fatty acid (SCFA) principally derived from the enteric microbiome. BT positively modulates mitochondrial function, including enhancing oxidative phosphorylation and beta-oxidation and has been proposed as a neuroprotectant. BT and other SCFAs have also been associated with autism spectrum disorders (ASD), a condition associated with mitochondrial dysfunction. We have developed a lymphoblastoid cell line (LCL) model of ASD, with a subset of LCLs demonstrating mitochondrial dysfunction (AD-A) and another subset of LCLs demonstrating normal mitochondrial function (AD-N). Given the positive modulation of BT on mitochondrial function, we hypothesized that BT would have a preferential positive effect on AD-A LCLs. To this end, we measured mitochondrial function in ASD and age-matched control (CNT) LCLs, all derived from boys, following 24 and 48 h exposure to BT (0, 0.1, 0.5, and 1 mM) both with and without an in vitro increase in reactive oxygen species (ROS). We also examined the expression of key genes involved in cellular and mitochondrial response to stress. In CNT LCLs, respiratory parameters linked to adenosine triphosphate (ATP) production were attenuated by 1 mM BT. In contrast, BT significantly increased respiratory parameters linked to ATP production in AD-A LCLs but not in AD-N LCLs. In the context of ROS exposure, BT increased respiratory parameters linked to ATP production for all groups. BT was found to modulate individual LCL mitochondrial respiration to a common set-point, with this set-point slightly higher for the AD-A LCLs as compared to the other groups. The highest concentration of BT (1 mM) increased the expression of genes involved in mitochondrial fission (PINK1, DRP1, FIS1) and physiological stress (UCP2, mTOR, HIF1α, PGC1α) as well as genes thought to be linked to cognition and behavior (CREB1, CamKinase II). These data show that the enteric microbiome-derived SCFA BT modulates mitochondrial activity, with this modulation dependent on concentration, microenvironment redox state, and the underlying mitochondrial function of the cell. In general, these data suggest that BT can enhance mitochondrial function in the context of physiological stress and/or mitochondrial dysfunction, and may be an important metabolite that can help rescue energy metabolism during disease states. Thus, insight into this metabolic modulator may have wide applications for both health and disease since BT has been implicated in a wide variety of conditions including ASD. However, future clinical studies in humans are needed to help define the practical implications of these physiological findings.

Harnessing Gut Microbes for Mental Health: Getting From Here to There

There has been an explosion of interest in the study of microorganisms inhabiting the gastrointestinal tract (gut microbiota) and their impact on host health and physiology. Accumulating data suggest that altered communication between gut microbiota and host systems could participate in disorders such as obesity, diabetes mellitus, and autoimmune disorders as well as neuropsychiatric disorders, including autism, anxiety, and major depressive disorders. The conceptual development of the microbiome-gut-brain axis has facilitated understanding of the complex and bidirectional networks between gastrointestinal microbiota and their host, highlighting potential mechanisms through which this environment influences central nervous system physiology. Communication pathways between gut microbiota and the central nervous system could include autonomic, neuroendocrine, enteric, and immune systems, with pathology resulting in disruption to neurotransmitter balance, increases in chronic inflammation, or exacerbated hypothalamic-pituitary-adrenal axis activity. However, uncertainty remains regarding the generalizability of controlled animal studies to the more multifaceted pattern of human pathophysiology, especially with regard to the therapeutic potential for neuropsychiatric health. This narrative review summarizes current understanding of gut microbial influence over physiological function, with an emphasis on neurobehavioral and neurological impairment based on growing understanding of the gut-brain axis. Experimental and clinical data regarding means of therapeutic manipulation of gut microbiota as a novel treatment option for mental health are described, and important knowledge gaps are identified and discussed.

Gut microbiota changes in the extreme decades of human life: a focus on centenarians

The gut microbiota (GM) is a complex, evolutionarily molded ecological system, which contributes to a variety of physiological functions. The GM is highly dynamic, being sensitive to environmental stimuli, and its composition changes over the host's entire lifespan. However, the basic question of how much these changes may be ascribed to variables such as population, diet, genetics and gender, and/or to the aging process per se is still largely unanswered. We argue that comparison among studies on centenarians-the best model of healthy aging and longevity-recruited from different geographical areas/populations (different genetics and dietary habits) can help to disentangle the contribution of aging and non-aging-related variables to GM remodeling with age. The current review focuses on the role of population, gender and host genetics as possible drivers of GM modification along the human aging process. The feedback impact of age-associated GM variation on the GM-brain axis and GM metabolomics is also discussed. We likewise address the role of GM in neurodegenerative diseases such as Parkinson's and Alzheimer's, and its possible therapeutic use, taking advantage of the fact that centenarians are characterized by an extreme (healthy) phenotype versus patients suffering from age-related pathologies. Finally, it is argued that longitudinal studies combining metagenomics sequencing and in-depth phylogenetic analysis with a comprehensive phenotypic characterization of centenarians and patients using up-to-date omics (metabolomics, transcriptomics and meta-transcriptomics) are urgently needed.

Modulation of Immunological Pathways in Autistic and Neurotypical Lymphoblastoid Cell Lines by the Enteric Microbiome Metabolite Propionic Acid

Propionic acid (PPA) is a ubiquitous short-chain fatty acid which is a fermentation product of the enteric microbiome and present or added to many foods. While PPA has beneficial effects, it is also associated with human disorders, including autism spectrum disorders (ASDs). We previously demonstrated that PPA modulates mitochondrial dysfunction differentially in subsets of lymphoblastoid cell lines (LCLs) derived from patients with ASD. Specifically, PPA significantly increases mitochondrial function in LCLs that have mitochondrial dysfunction at baseline [individuals with autistic disorder with atypical mitochondrial function (AD-A) LCLs] as compared to ASD LCLs with normal mitochondrial function [individuals with autistic disorder with normal mitochondrial function (AD-N) LCLs] and control (CNT) LCLs. PPA at 1 mM was found to have a minimal effect on expression of immune genes in CNT and AD-N LCLs. However, as hypothesized, Panther analysis demonstrated that 1 mM PPA exposure at 24 or 48 h resulted in significant activation of the immune system genes in AD-A LCLs. When the effect of PPA on ASD LCLs were compared to the CNT LCLs, both ASD groups demonstrated immune pathway activation, although the AD-A LCLs demonstrate a wider activation of immune genes. Ingenuity Pathway Analysis identified several immune-related pathways as key Canonical Pathways that were differentially regulated, specifically human leukocyte antigen expression and immunoglobulin production genes were upregulated. These data demonstrate that the enteric microbiome metabolite PPA can evoke atypical immune activation in LCLs with an underlying abnormal metabolic state. As PPA, as well as enteric bacteria which produce PPA, have been implicated in a wide variety of diseases which have components of immune dysfunction, including ASD, diabetes, obesity, and inflammatory diseases, insight into this metabolic modulator may have wide applications for both health and disease.

Mitochondrial dysfunction in the gastrointestinal mucosa of children with autism: A blinded case-control study

Gastrointestinal (GI) symptoms are prevalent in autism spectrum disorder (ASD) but the pathophysiology is poorly understood. Imbalances in the enteric microbiome have been associated with ASD and can cause GI dysfunction potentially through disruption of mitochondrial function as microbiome metabolites modulate mitochondrial function and mitochondrial dysfunction is highly associated with GI symptoms. In this study, we compared mitochondrial function in rectal and cecum biopsies under the assumption that certain microbiome metabolites, such as butyrate and propionic acid, are more abundant in the cecum as compared to the rectum. Rectal and cecum mucosal biopsies were collected during elective diagnostic colonoscopy. Using a single-blind case-control design, complex I and IV and citrate synthase activities and complex I-V protein quantity from 10 children with ASD, 10 children with Crohn’s disease and 10 neurotypical children with nonspecific GI complaints were measured. The protein for all complexes, except complex II, in the cecum as compared to the rectum was significantly higher in ASD samples as compared to other groups. For both rectal and cecum biopsies, ASD samples demonstrated higher complex I activity, but not complex IV or citrate synthase activity, compared to other groups. Mitochondrial function in the gut mucosa from children with ASD was found to be significantly different than other groups who manifested similar GI symptomatology suggesting a unique pathophysiology for GI symptoms in children with ASD. Abnormalities localized to the cecum suggest a role for imbalances in the microbiome, potentially in the production of butyrate, in children with ASD.

Elimination diets' efficacy and mechanisms in attention deficit hyperactivity disorder and autism spectrum disorder

Nutrition plays an important role in neurodevelopment. This insight has led to increasing research into the efficacy of nutrition-related interventions for treating neurodevelopmental disorders. This review discusses an elimination diet as a treatment for attention deficit hyperactivity disorder and autism spectrum disorder, with a focus on the efficacy of the food additives exclusion diet, gluten-free/casein-free diet and oligoantigenic diet. Furthermore, we discuss the potential mechanisms of elimination diets' effects in these neurodevelopmental disorders. The main candidate mechanism is the microbiome-gut-brain axis possibly involving complex interactions between multiple systems, including the metabolic, immune, endocrine, and neural system. We conclude with practical implications and future directions into the investigation of an elimination diet's efficacy in the treatment of attention deficit hyperactivity disorder and autism spectrum disorder.

Brain carnitine deficiency causes nonsyndromic autism with an extreme male bias: A hypothesis

Could 10-20% of autism be prevented? We hypothesize that nonsyndromic or "essential" autism involves extreme male bias in infants who are genetically normal, but they develop deficiency of carnitine and perhaps other nutrients in the brain causing autism that may be amenable to early reversal and prevention. That brain carnitine deficiency might cause autism is suggested by reports of severe carnitine deficiency in autism and by evidence that TMLHE deficiency - a defect in carnitine biosynthesis - is a risk factor for autism. A gene on the X chromosome (SLC6A14) likely escapes random X-inactivation (a mixed epigenetic and genetic regulation) and could limit carnitine transport across the blood-brain barrier in boys compared to girls. A mixed, common gene variant-environment hypothesis is proposed with diet, minor illnesses, microbiome, and drugs as possible risk modifiers. The hypothesis can be tested using animal models and by a trial of carnitine supplementation in siblings of probands. Perhaps the lack of any Recommended Dietary Allowance for carnitine in infants should be reviewed. Also see the video abstract here: https://youtu.be/BuRH_jSjX5Y.

The Gut, Its Microbiome, and Hypertension

PURPOSE OF THE REVIEW

Evidence is rapidly accumulating implicating gut dysbiosis in hypertension (HTN). However, we are far from understanding whether this is a cause or consequence of HTN, and how to best translate this fundamental knowledge to advance the management of HTN. This review aims to summarize recent advances in the field, illustrate the connections between the gut and hypertension, and establish that the gut microbiota (GM)-gut interaction is centrally positioned for consideration as an innovative approach for HTN therapeutics.

RECENT FINDINGS

Animal models of HTN have shown that gut pathology occurs in HTN, and provides some clues to mechanisms linking the dysbiosis, gut pathology, and HTN. Circumstantial evidence links gut dysbiosis and HTN. Gut pathology, apparent in animal HTN models, has not been fully investigated in hypertensive patients. Objective evidence and an understanding of mechanisms could have a major impact for new antihypertensive therapies and/or improved applications of current ones.

The Obesity-Impulsivity Axis: Potential Metabolic Interventions in Chronic Psychiatric Patients

Pathological impulsivity is encountered in a broad range of psychiatric conditions and is thought to be a risk factor for aggression directed against oneself or others. Recently, a strong association was found between impulsivity and obesity which may explain the high prevalence of metabolic disorders in individuals with mental illness even in the absence of exposure to psychotropic drugs. As the overlapping neurobiology of impulsivity and obesity is being unraveled, the question asked louder and louder is whether they should be treated concomitantly. The treatment of obesity and metabolic dysregulations in chronic psychiatric patients is currently underutilized and often initiated late, making correction more difficult to achieve. Addressing obesity and metabolic dysfunction in a preventive manner may not only lower morbidity and mortality but also the excessive impulsivity, decreasing the risk for aggression. In this review, we take a look beyond psychopharmacological interventions and discuss dietary and physical therapy approaches.

The bacterial peptidoglycan-sensing molecule Pglyrp2 modulates brain development and behavior

Recent studies have revealed that the gut microbiota modulates brain development and behavior, but the underlying mechanisms are still poorly understood. Here, we show that bacterial peptidoglycan (PGN) derived from the commensal gut microbiota can be translocated into the brain and sensed by specific pattern-recognition receptors (PRRs) of the innate immune system. Using expression-profiling techniques, we demonstrate that two families of PRRs that specifically detect PGN (that is, PGN-recognition proteins and NOD-like receptors), and the PGN transporter PepT1 are highly expressed in the developing brain during specific windows of postnatal development in both males and females. Moreover, we show that the expression of several PGN-sensing molecules and PepT1 in the developing striatum is sensitive to manipulations of the gut microbiota (that is, germ-free conditions and antibiotic treatment). Finally, we used the PGN-recognition protein 2 (Pglyrp2) knockout mice to examine the potential influence of PGN-sensing molecules on brain development and behavior. We demonstrate that the absence of Pglyrp2 leads to alterations in the expression of the autism risk gene c-Met, and sex-dependent changes in social behavior, similar to mice with manipulated microbiota. These findings suggest that the central activation of PRRs by microbial products could be one of the signaling pathways mediating the communication between the gut microbiota and the developing brain.

May the Force Be With You: The Light and Dark Sides of the Microbiota-Gut-Brain Axis in Neuropsychiatry

The role of the gut microbiota in health and disease is becoming increasingly recognized. The microbiota-gut-brain axis is a bi-directional pathway between the brain and the gastrointestinal system. The bacterial commensals in our gut can signal to the brain through a variety of mechanisms, which are slowly being resolved. These include the vagus nerve, immune mediators and microbial metabolites, which influence central processes such as neurotransmission and behaviour. Dysregulation in the composition of the gut microbiota has been identified in several neuropsychiatric disorders, such as autism, schizophrenia and depression. Moreover, preclinical studies suggest that they may be the driving force behind the behavioural abnormalities observed in these conditions. Understanding how bacterial commensals are involved in regulating brain function may lead to novel strategies for development of microbiota-based therapies for these neuropsychiatric disorders.

The Significance of the Enteric Microbiome on the Development of Childhood Disease: A Review of Prebiotic and Probiotic Therapies in Disorders of Childhood

Recent studies have highlighted the fact that the enteric microbiome, the trillions of microbes that inhabit the human digestive tract, has a significant effect on health and disease. Methods for manipulating the enteric microbiome, particularly through probiotics and microbial ecosystem transplantation, have undergone some study in clinical trials. We review some of the evidence for microbiome alteration in relation to childhood disease and discuss the clinical trials that have examined the manipulation of the microbiome in an effort to prevent or treat childhood disease with a primary focus on probiotics, prebiotics, and/or synbiotics (ie, probiotics + prebiotics). Studies show that alterations in the microbiome may be a consequence of events occurring during infancy and/or childhood such as prematurity, C-sections, and nosocomial infections. In addition, certain childhood diseases have been associated with microbiome alterations, namely necrotizing enterocolitis, infantile colic, asthma, atopic disease, gastrointestinal disease, diabetes, malnutrition, mood/anxiety disorders, and autism spectrum disorders. Treatment studies suggest that probiotics are potentially protective against the development of some of these diseases. Timing and duration of treatment, the optimal probiotic strain(s), and factors that may alter the composition and function of the microbiome are still in need of further research. Other treatments such as prebiotics, fecal microbial transplantation, and antibiotics have limited evidence. Future translational work, in vitro models, long-term and follow-up studies, and guidelines for the composition and viability of probiotic and microbial therapies need to be developed. Overall, there is promising evidence that manipulating the microbiome with probiotics early in life can help prevent or reduce the severity of some childhood diseases, but further research is needed to elucidate biological mechanisms and determine optimal treatments.

Modulation of mitochondrial function by the microbiome metabolite propionic acid in autism and control cell lines

Propionic acid (PPA) is a ubiquitous short-chain fatty acid, which is a major fermentation product of the enteric microbiome. PPA is a normal intermediate of metabolism and is found in foods, either naturally or as a preservative. PPA and its derivatives have been implicated in both health and disease. Whereas PPA is an energy substrate and has many proposed beneficial effects, it is also associated with human disorders involving mitochondrial dysfunction, including propionic acidemia and autism spectrum disorders (ASDs). We aimed to investigate the dichotomy between the health and disease effects of PPA by measuring mitochondrial function in ASD and age- and gender-matched control lymphoblastoid cell lines (LCLs) following incubation with PPA at several concentrations and durations both with and without an in vitro increase in reactive oxygen species (ROS). Mitochondrial function was optimally increased at particular exposure durations and concentrations of PPA with ASD LCLs, demonstrating a greater enhancement. In contrast, increasing ROS negated the positive PPA effect with the ASD LCLs, showing a greater detriment. These data demonstrate that enteric microbiome metabolites such as PPA can have both beneficial and toxic effects on mitochondrial function, depending on concentration, exposure duration and microenvironment redox state with these effects amplified in LCLs derived from individuals with ASD. As PPA, as well as enteric bacteria, which produce PPA, have been implicated in a wide variety of diseases, including ASD, diabetes, obesity and inflammatory diseases, insight into this metabolic modulator from the host microbiome may have wide applications for both health and disease.

The Importance of Diet and Gut Health to the Treatment and Prevention of Mental Disorders

The departure from traditional lifestyles and the rising disease burden of mental disorders are increasing global health concerns. Changes in diet around the world mean that populations are now increasingly reliant on highly processed, poor quality foods, which have been linked to increased risk for mental disorder. Conversely, a nutrient-rich diet is understood to be protective of mental health, and researchers are now aiming to understand the biological underpinnings of this relationship. The gut microbiota has been proposed as a key mediator of this link, given its association with both diet and mental health. Importantly, several critical "windows of opportunity" for prevention and intervention have been identified, particularly early life and adolescence; these are periods of rapid development and transition that provide a foundation for future health. Strategies that promote overall diet quality, high in fiber and nutrients, have been linked to increased microbial diversity and gut health. Improving diet quality and subsequent gut health may have benefits for individuals' mental health, as well as the mental health of future generations. Here we discuss specific, targeted dietary and gut focused strategies for the prevention and treatment of mental disorder.

Communicating systems in the body: how microbiota and microglia cooperate

Microglia are tissue macrophages of the central nervous system (CNS). Their key tasks are immune surveillance as well as responding to infections or other pathological states such as neurological diseases or injury. In recent years it has been discovered that microglia are additionally crucial for the maintenance of brain homeostasis during development and adulthood by adjusting the neuronal network and phagocytosing neuronal debris. Microglia persist in the CNS throughout the life of the organism and self-renew without engraftment of bone-marrow-derived cells. Until recently it remained unknown what controls their maturation and activation under homeostatic conditions. In this review we discuss new aspects of the interaction between host microbiota and brain function with special focus on the brain-resident innate immune cells, the microglia.

Altered gut microbiota in Rett syndrome

BACKGROUND

The human gut microbiota directly affects human health, and its alteration can lead to gastrointestinal abnormalities and inflammation. Rett syndrome (RTT), a progressive neurological disorder mainly caused by mutations in MeCP2 gene, is commonly associated with gastrointestinal dysfunctions and constipation, suggesting a link between RTT's gastrointestinal abnormalities and the gut microbiota. The aim of this study was to evaluate the bacterial and fungal gut microbiota in a cohort of RTT subjects integrating clinical, metabolomics and metagenomics data to understand if changes in the gut microbiota of RTT subjects could be associated with gastrointestinal abnormalities and inflammatory status.

RESULTS

Our findings revealed the occurrence of an intestinal sub-inflammatory status in RTT subjects as measured by the elevated values of faecal calprotectin and erythrocyte sedimentation rate. We showed that, overall, RTT subjects harbour bacterial and fungal microbiota altered in terms of relative abundances from those of healthy controls, with a reduced microbial richness and dominated by microbial taxa belonging to Bifidobacterium, several Clostridia (among which Anaerostipes, Clostridium XIVa, Clostridium XIVb) as well as Erysipelotrichaceae, Actinomyces, Lactobacillus, Enterococcus, Eggerthella, Escherichia/Shigella and the fungal genus Candida. We further observed that alterations of the gut microbiota do not depend on the constipation status of RTT subjects and that this dysbiotic microbiota produced altered short chain fatty acids profiles.

CONCLUSIONS

We demonstrated for the first time that RTT is associated with a dysbiosis of both the bacterial and fungal component of the gut microbiota, suggesting that impairments of MeCP2 functioning favour the establishment of a microbial community adapted to the costive gastrointestinal niche of RTT subjects. The altered production of short chain fatty acids associated with this microbiota might reinforce the constipation status of RTT subjects and contribute to RTT gastrointestinal physiopathology.

Emerging Technologies for Gut Microbiome Research

Understanding the importance of the gut microbiome on modulation of host health has become a subject of great interest for researchers across disciplines. As an intrinsically multidisciplinary field, microbiome research has been able to reap the benefits of technological advancements in systems and synthetic biology, biomaterials engineering, and traditional microbiology. Gut microbiome research has been revolutionized by high-throughput sequencing technology, permitting compositional and functional analyses that were previously an unrealistic undertaking. Emerging technologies, including engineered organoids derived from human stem cells, high-throughput culturing, and microfluidics assays allowing for the introduction of novel approaches, will improve the efficiency and quality of microbiome research. Here, we discuss emerging technologies and their potential impact on gut microbiome studies.