The gut microbiome restores intrinsic and extrinsic nerve function in germ-free mice accompanied by changes in calbindin

Feeding-dependent VIP neuron-ILC3 circuit regulates the intestinal barrier

The intestinal mucosa serves both as a conduit for the uptake of food-derived nutrients and microbiome-derived metabolites, and as a barrier that prevents tissue invasion by microorganisms and tempers inflammatory responses to the myriad contents of the lumen. How the intestine coordinates physiological and immune responses to food consumption to optimize nutrient uptake while maintaining barrier functions remains unclear. Here we show in mice how a gut neuronal signal triggered by food intake is integrated with intestinal antimicrobial and metabolic responses that are controlled by type-3 innate lymphoid cells (ILC3)1-3. Food consumption rapidly activates a population of enteric neurons that express vasoactive intestinal peptide (VIP)4. Projections of VIP-producing neurons (VIPergic neurons) in the lamina propria are in close proximity to clusters of ILC3 that selectively express VIP receptor type 2 (VIPR2; also known as VPAC2). Production of interleukin (IL)-22 by ILC3, which is upregulated by the presence of commensal microorganisms such as segmented filamentous bacteria5-7, is inhibited upon engagement of VIPR2. As a consequence, levels of antimicrobial peptide derived from epithelial cells are reduced but the expression of lipid-binding proteins and transporters is increased8. During food consumption, the activation of VIPergic neurons thus enhances the growth of segmented filamentous bacteria associated with the epithelium, and increases lipid absorption. Our results reveal a feeding- and circadian-regulated dynamic neuroimmune circuit in the intestine that promotes a trade-off between innate immune protection mediated by IL-22 and the efficiency of nutrient absorption. Modulation of this pathway may therefore be effective for enhancing resistance to enteropathogens2,3,9 and for the treatment of metabolic diseases.

The gut microbiota to the brain axis in the metabolic control

The regulation of glycemia is under a tight neuronal detection of glucose levels performed by the gut-brain axis and an efficient efferent neuronal message sent to the peripheral organs, as the pancreas to induce insulin and inhibit glucagon secretions. The neuronal detection of glucose levels is performed by the autonomic nervous system including the enteric nervous system and the vagus nerve innervating the gastro-intestinal tractus, from the mouth to the anus. A dysregulation of this detection leads to the one of the most important current health issue around the world i.e. diabetes mellitus. Furthemore, the consequences of diabetes mellitus on neuronal homeostasis and activities participate to the aggravation of the disease establishing a viscious circle. Prokaryotic cells as bacteria, reside in our gut. The strong relationship between prokaryotic cells and our eukaryotic cells has been established long ago, and prokaryotic and eukaryotic cells in our body have evolved synbiotically. For the last decades, studies demonstrated the critical role of the gut microbiota on the metabolic control and how its shift can induce diseases such as diabetes. Despite an important increase of knowledge, few is known about 1) how the gut microbiota influences the neuronal detection of glucose and 2) how the diabetes mellitus-induced gut microbiota shift observed participates to the alterations of autonomic nervous system and the gut-brain axis activity.

The Microbiota-Gut-Brain Axis

The importance of the gut-brain axis in maintaining homeostasis has long been appreciated. However, the past 15 yr have seen the emergence of the microbiota (the trillions of microorganisms within and on our bodies) as one of the key regulators of gut-brain function and has led to the appreciation of the importance of a distinct microbiota-gut-brain axis. This axis is gaining ever more traction in fields investigating the biological and physiological basis of psychiatric, neurodevelopmental, age-related, and neurodegenerative disorders. The microbiota and the brain communicate with each other via various routes including the immune system, tryptophan metabolism, the vagus nerve and the enteric nervous system, involving microbial metabolites such as short-chain fatty acids, branched chain amino acids, and peptidoglycans. Many factors can influence microbiota composition in early life, including infection, mode of birth delivery, use of antibiotic medications, the nature of nutritional provision, environmental stressors, and host genetics. At the other extreme of life, microbial diversity diminishes with aging. Stress, in particular, can significantly impact the microbiota-gut-brain axis at all stages of life. Much recent work has implicated the gut microbiota in many conditions including autism, anxiety, obesity, schizophrenia, Parkinson’s disease, and Alzheimer’s disease. Animal models have been paramount in linking the regulation of fundamental neural processes, such as neurogenesis and myelination, to microbiome activation of microglia. Moreover, translational human studies are ongoing and will greatly enhance the field. Future studies will focus on understanding the mechanisms underlying the microbiota-gut-brain axis and attempt to elucidate microbial-based intervention and therapeutic strategies for neuropsychiatric disorders.

Relationship between the gut microbiome and brain function

It has become increasingly evident in recent years that the gut microbiome and the brain communicate in a bidirectional manner, with each possibly affecting the other's functions. Substantial research has aimed to understand the mechanisms of this interaction and to outline strategies for preventing or treating nervous system-related disturbances. This review explores the evidence demonstrating how the gut microbiome may affect brain function in adults, thereby having an impact on stress, anxiety, depression, and cognition. In vitro, in vivo, and human studies reporting an association between a change in the gut microbiome and functional changes in the brain are highlighted, as are studies outlining the mechanisms by which the brain affects the microbiome and the gastrointestinal tract. Possible modes of action to explain how the gut microbiome and the brain functionally affect each other are proposed. Supplemental probiotics to combat brain-related dysfunction offer a promising approach, provided future research elucidates their mode of action and possible side effects. Further studies are warranted to establish how pre- and probiotic interventions may help to balance brain function in healthy and diseased individuals.

The gut microbiota and the brain-gut-kidney axis in hypertension and chronic kidney disease

Crosstalk between the gut microbiota and the host has attracted considerable attention owing to its involvement in diverse diseases. Chronic kidney disease (CKD) is commonly associated with hypertension and is characterized by immune dysregulation, metabolic disorder and sympathetic activation, which are all linked to gut dysbiosis and altered host-microbiota crosstalk. In this Review, we discuss the complex interplay between the brain, the gut, the microbiota and the kidney in CKD and hypertension and explain our brain-gut-kidney axis hypothesis for the pathogenesis of these diseases. Consideration of the role of the brain-gut-kidney axis in the maintenance of normal homeostasis and of dysregulation of this axis in CKD and hypertension could lead to the identification of novel therapeutic targets. In addition, the discovery of unique microbial communities and their associated metabolites and the elucidation of brain-gut-kidney signalling are likely to fill fundamental knowledge gaps leading to innovative research, clinical trials and treatments for CKD and hypertension.

The role of the gut microbiota in development, function and disorders of the central nervous system and the enteric nervous system

The gut microbiota has emerged as an environmental factor that modulates the development of the central nervous system (CNS) and the enteric nervous system (ENS). Before obtaining its own microbiota, eutherian foetuses are exposed to products and metabolites from the maternal microbiota. At birth, the infants are colonised by microorganisms. The microbial composition in early life is strongly influenced by the mode of delivery, the feeding method, the use of antibiotics and the maternal microbial composition. Microbial products and microbially produced metabolites act as signalling molecules that have direct or indirect effects on the CNS and the ENS. An increasing number of studies show that the gut microbiota can modulate important processes during development, including neurogenesis, myelination, glial cell function, synaptic pruning and blood-brain barrier permeability. Furthermore, numerous studies indicate that there is a developmental window early in life during which the gut microbial composition is crucial and perturbation of the gut microbiota during this period causes long-lasting effects on the development of the CNS and the ENS. However, other functions are readily modulated in adult animals, including microglia activation and neuroinflammation. Several neurobehavioural, neurodegenerative, mental and metabolic disorders, including Parkinson disease, autism spectrum disorder, schizophrenia, Alzheimer's disease, depression and obesity, have been linked to the gut microbiota. This review focuses on the role of the microorganisms in the development and function of the CNS and the ENS, as well as their potential role in pathogenesis.

Glutamatergic Signaling Along The Microbiota-Gut-Brain Axis

A complex bidirectional communication system exists between the gastrointestinal tract and the brain. Initially termed the “gut-brain axis” it is now renamed the “microbiota-gut-brain axis” considering the pivotal role of gut microbiota in maintaining local and systemic homeostasis. Different cellular and molecular pathways act along this axis and strong attention is paid to neuroactive molecules (neurotransmitters, i.e., noradrenaline, dopamine, serotonin, gamma aminobutyric acid and glutamate and metabolites, i.e., tryptophan metabolites), sustaining a possible interkingdom communication system between eukaryota and prokaryota. This review provides a description of the most up-to-date evidence on glutamate as a neurotransmitter/neuromodulator in this bidirectional communication axis. Modulation of glutamatergic receptor activity along the microbiota-gut-brain axis may influence gut (i.e., taste, visceral sensitivity and motility) and brain functions (stress response, mood and behavior) and alterations of glutamatergic transmission may participate to the pathogenesis of local and brain disorders. In this latter context, we will focus on two major gut disorders, such as irritable bowel syndrome and inflammatory bowel disease, both characterized by psychiatric co-morbidity. Research in this area opens the possibility to target glutamatergic neurotransmission, either pharmacologically or by the use of probiotics producing neuroactive molecules, as a therapeutic approach for the treatment of gastrointestinal and related psychiatric disorders.

Antibiotics and the nervous system: More than just the microbes?

The use of antibiotics has recently risen to prominence in neuroscience due to their potential value in studying the microbiota-gut-brain axis. In this context they have been largely employed to illustrate the many influences of the gut microbiota on brain function and behaviour. Much of this research is bolstered by the abnormal behaviour seen in germ-free animals and other well-controlled experiments. However, this literature has largely failed to consider the neuroactive potential of antibiotics themselves, independent from, or in addition to, their microbicidal effects. This is problematic, as clinical as well as experimental literature, largely neglected through the past decade, has clearly demonstrated that broad classes of antibiotics are neuroactive or neurotoxic. This is true even for some antibiotics that are widely regarded as not absorbed in the intestinal tract, and is especially concerning when considering the highly-concentrated and widely-ranging doses that have been used. In this review we will critically survey the clinical and experimental evidence that antibiotics may influence a variety of nervous system functions, from the enteric nervous system through to the brain and resultant behaviour. We will discuss substantial evidence which clearly suggests neuro-activity or -toxicity by most classes of antibiotics. We will conclude that, while evidence for the microbiota-gut-brain axis remains strong, clinical and experimental studies which employ antibiotics to probe it must consider this potential confound.

Intestinal Sensing by Gut Microbiota: Targeting Gut Peptides

There are more than 2 billion overweight and obese individuals worldwide, surpassing for the first time, the number of people affected by undernutrition. Obesity and its comorbidities inflict a heavy burden on the global economies and have become a serious threat to individuals' wellbeing with no immediate cure available. The causes of obesity are manifold, involving several factors including physiological, metabolic, neural, psychosocial, economic, genetics and the environment, among others. Recent advances in genome sequencing and metagenomic profiling have added another dimension to this complexity by implicating the gut microbiota as an important player in energy regulation and the development of obesity. As such, accumulating evidence demonstrate the impact of the gut microbiota on body weight, adiposity, glucose, lipid metabolism, and metabolic syndrome. This also includes the role of microbiota as a modulatory signal either directly or through its bioactive metabolites on intestinal lumen by releasing chemosensing factors known to have a major role in controlling food intake and regulating body weight. The importance of gut signaling by microbiota signaling is further highlighted by the presence of taste and nutrient receptors on the intestinal epithelium activated by the microbial degradation products as well as their role in release of peptides hormones controlling appetite and energy homeostasis. This review present evidence on how gut microbiota interacts with intestinal chemosensing and modulates the release and activity of gut peptides, particularly GLP-1 and PYY.

Neuroimmunophysiology of the gut: advances and emerging concepts focusing on the epithelium

The epithelial lining of the gastrointestinal tract serves as the interface for digestion and absorption of nutrients and water and as a defensive barrier. The defensive functions of the intestinal epithelium are remarkable considering that the gut lumen is home to trillions of resident bacteria, fungi and protozoa (collectively, the intestinal microbiota) that must be prevented from translocation across the epithelial barrier. Imbalances in the relationship between the intestinal microbiota and the host lead to the manifestation of diseases that range from disorders of motility and sensation (IBS) and intestinal inflammation (IBD) to behavioural and metabolic disorders, including autism and obesity. The latest discoveries shed light on the sophisticated intracellular, intercellular and interkingdom signalling mechanisms of host defence that involve epithelial and enteroendocrine cells, the enteric nervous system and the immune system. Together, they maintain homeostasis by integrating luminal signals, including those derived from the microbiota, to regulate the physiology of the gastrointestinal tract in health and disease. Therapeutic strategies are being developed that target these signalling systems to improve the resilience of the gut and treat the symptoms of gastrointestinal disease.

Enteric nervous system development: what could possibly go wrong?

The gastrointestinal tract contains its own set of intrinsic neuroglial circuits - the enteric nervous system (ENS) - which detects and responds to diverse signals from the environment. Here, we address recent advances in the understanding of ENS development, including how neural-crest-derived progenitors migrate into and colonize the bowel, the formation of ganglionated plexuses and the molecular mechanisms of enteric neuronal and glial diversification. Modern lineage tracing and transcription-profiling technologies have produced observations that simultaneously challenge and affirm long-held beliefs about ENS development. We review many genetic and environmental factors that can alter ENS development and exert long-lasting effects on gastrointestinal function, and discuss how developmental defects in the ENS might account for some of the large burden of digestive disease.

Gut microbiota, cannabinoid system and neuroimmune interactions: New perspectives in multiple sclerosis

The gut microbiota plays a fundamental role on the education and function of the host immune system. Immunological dysregulation is the cause of numerous human disorders such as autoimmune diseases and metabolic disorders frequently associated with inflammatory processes therefore is critical to explore novel mechanisms involved in maintaining the immune system homeostasis. The cannabinoid system and related bioactive lipids participate in multiple central and peripheral physiological processes that affect metabolic, gastrointestinal and neuroimmune regulatory mechanisms displaying a modulatory role and contributing to the maintenance of the organism's homeostasis. In this review, we gather the knowledge on the gut microbiota-endocannabinoids interactions and their impact on autoimmune disorders such as inflammatory bowel disease, rheumatoid arthritis and particularly, multiple sclerosis (MS) as the best example of a CNS autoimmune disorder. Furthermore, we contribute to this field with new data on changes in many elements of the cannabinoid system in a viral model of MS after gut microbiota manipulation by both antibiotics and probiotics. Finally, we highlight new therapeutic opportunities, under an integrative view, targeting the eCBS and the commensal microbiota in the context of neuroinflammation and MS.

Host-microbe interaction in the gastrointestinal tract

The gastrointestinal tract is a highly complex organ in which multiple dynamic physiological processes are tightly coordinated while interacting with a dense and extremely diverse microbial population. From establishment in early life, through to host-microbe symbiosis in adulthood, the gut microbiota plays a vital role in our development and health. The effect of the microbiota on gut development and physiology is highlighted by anatomical and functional changes in germ-free mice, affecting the gut epithelium, immune system and enteric nervous system. Microbial colonisation promotes competent innate and acquired mucosal immune systems, epithelial renewal, barrier integrity, and mucosal vascularisation and innervation. Interacting or shared signalling pathways across different physiological systems of the gut could explain how all these changes are coordinated during postnatal colonisation, or after the introduction of microbiota into germ-free models. The application of cell-based in-vitro experimental systems and mathematical modelling can shed light on the molecular and signalling pathways which regulate the development and maintenance of homeostasis in the gut and beyond.

Gut microbiota regulates maturation of the adult enteric nervous system via enteric serotonin networks

The gut microbiota affects several physiological processes, including gut motility. Here we observed that germ-free mice have an immature enteric nervous system (ENS) that is normalized upon colonization with a normal microbiota. We identified the mechanism of communication between the microbiota and enteric neurons as the initiation of serotonin release and subsequent activation of the 5-HT₄ receptor. This demonstrates a strong interaction between the microbiota and the ENS and indicates potential mechanisms linking microbial dysbiosis to gastrointestinal disorders. The ability to modulate the microbiota, e.g., by diet, will open new perspectives of research in neurogastroenterology.

The enteric nervous system (ENS) is crucial for essential gastrointestinal physiologic functions such as motility, fluid secretion, and blood flow. The gut is colonized by trillions of bacteria that regulate host production of several signaling molecules including serotonin (5-HT) and other hormones and neurotransmitters. Approximately 90% of 5-HT originates from the intestine, and activation of the 5-HT₄ receptor in the ENS has been linked to adult neurogenesis and neuroprotection. Here, we tested the hypothesis that the gut microbiota could induce maturation of the adult ENS through release of 5-HT and activation of 5-HT₄ receptors. Colonization of germ-free mice with a microbiota from conventionally raised mice modified the neuroanatomy of the ENS and increased intestinal transit rates, which was associated with neuronal and mucosal 5-HT production and the proliferation of enteric neuronal progenitors in the adult intestine. Pharmacological modulation of the 5-HT₄ receptor, as well as depletion of endogenous 5-HT, identified a mechanistic link between the gut microbiota and maturation of the adult ENS through the release of 5-HT and activation of the 5-HT₄ receptor. Taken together, these findings show that the microbiota modulates the anatomy of the adult ENS in a 5-HT–dependent fashion with concomitant changes in intestinal transit.

Altered fecal microbiota composition in all male aggressor-exposed rodent model simulating features of post-traumatic stress disorder

The bidirectional role of gut-brain axis that integrates the gut and central nervous system activities has recently been investigated. We studied "cage-within-cage resident-intruder" all-male model, where subject male mice (C57BL/6J) are exposed to aggressor mice (SJL albino), and gut microbiota-derived metabolites were identified in plasma after 10 days of exposure. We assessed 16S ribosomal RNA gene from fecal samples collected daily from these mice during the 10-day study. Alpha diversity using Chao indices indicated no change in diversity in aggressor-exposed samples. The abundance profile showed the top phyla were Firmicutes and Bacteroidetes, Tenericutes, Verrucomicrobia, Actinobacteria and Proteobacteria, respectively. The phyla Firmicutes and Bacteroidetes are vulnerable to PTSD-eliciting stress and the Firmicutes/Bacteroidetes ratio increases with stress. Principal coordinate analysis showed the control and aggressor-exposed samples cluster separately where samples from early time points (day 1-3) clustered together and were distinct from late time points (day 4-9). The genus-based analysis revealed all control time points clustered together and aggressor-exposed samples had multiple clusters. The decrease in proportion of Firmicutes after aggressor exposure persisted throughout the study. The proportion of Verrucomicrobia immediately decreased and was significantly shifted at most of the later time points. The genus Oscillospira, Lactobacillus, Akkermansia and Anaeroplasma are the top four genera that differed between control and stressor-exposed mice. The data showed immediate effect on microbiome composition during a 10 day time period of stress exposure. Studying the longitudinal effects of a stressor is an important step toward an improved mechanistic understanding of the microbiome dynamics.

Development of an ex Vivo Method for Multi-unit Recording of Microbiota-Colonic-Neural Signaling in Real Time

Background and Objectives: Bidirectional signaling between the gastrointestinal tract and the brain is vital for maintaining whole-body homeostasis. Moreover, emerging evidence implicates vagal afferent signaling in the modulation of host physiology by microbes, which are most abundant in the colon. This study aims to optimize and advance dissection and recording techniques to facilitate real-time recordings of afferent neural signals originating in the distal colon.

New Protocol: This paper describes a dissection technique, which facilitates extracellular electrophysiological recordings from visceral pelvic, spinal and vagal afferent neurons in response to stimulation of the distal colon.

Examples of Application: Focal application of 75 mM KCl to a section of distal colon with exposed submucosal or myenteric nerve cell bodies and sensory nerve endings evoked activity in the superior mesenteric plexus and the vagal nerve. Noradrenaline stimulated nerve activity in the superior mesenteric plexus, whereas application of carbachol stimulated vagal nerve activity. Exposure of an ex vivo section of distal colon with an intact colonic mucosa to peptidoglycan, but not lipopolysaccharide, evoked vagal nerve firing.

Discussion: Previous studies have recorded vagal signaling evoked by bacteria in the small intestine. The technical advances of this dissection and recording technique facilitates recording of afferent nerve signals evoked in extrinsic sensory pathways by neuromodulatory reagents applied to the distal colon. Moreover, we have demonstrated vagal afferent activation evoked by bacterial products applied to the distal colonic mucosa. This protocol may contribute to our understanding of functional bowel disorders where gut-brain communication is dysfunctional, and facilitate real-time interrogation of microbiota-gut-brain signaling.

Calbindin D28k-Immunoreactivity in Human Enteric Neurons

Calbindin (CALB) is well established as immunohistochemical marker for intrinsic primary afferent neurons in the guinea pig gut. Its expression by numerous human enteric neurons has been demonstrated but little is known about particular types of neurons immunoreactive for CALB. Here we investigated small and large intestinal wholemount sets of 26 tumor patients in order to evaluate (1) the proportion of CALB⁺ neurons in the total neuron population, (2) the colocalization of CALB with calretinin (CALR), somatostatin (SOM) and vasoactive intestinal peptide (VIP) and (3) the morphology of CALB⁺ neurons. CALB⁺ neurons represented a minority of myenteric neurons (small intestine: 31%; large intestine: 25%) and the majority of submucosal neurons (between 72 and 95%). In the submucosa, most CALB⁺ neurons co-stained for CALR and VIP (between 69 and 80%) or for SOM (between 20 and 3%). In the myenteric plexus, 85% of CALB⁺ neurons did not co-stain with the other markers investigated. An unequivocal correlation between CALB reactivity and neuronal morphology was found for myenteric type III neurons in the small intestine: uniaxonal neurons with long, slender and branched dendrites were generally positive for CALB. Since also other neurons displayed occasional CALB reactivity, this protein is not suited as an exclusive marker for type III neurons.

Functional Abdominal Pain and Related Syndromes

Functional gastrointestinal disorders are very common. They result from dysfunctional interaction in the brain-gut axis. Although the nature is benign, symptoms may be debilitating. The etiology is multifactorial; therefore, the diagnosis should be approached in a bio-psychosocial model. There are no biomarkers to characterize these conditions, but a solid understanding of the pathophysiology allows providers to present these disorders as a positive clinical diagnosis, rather than a diagnosis of exclusion. Effective management entails close collaboration between the medical and mental health providers.

Impact of Intestinal Peptides on the Enteric Nervous System: Novel Approaches to Control Glucose Metabolism and Food Intake

The gut is one of the most important sources of bioactive peptides in the body. In addition to their direct actions in the brain and/or peripheral tissues, the intestinal peptides can also have an impact on enteric nervous neurons. By modifying the endogenousproduction of these peptides, one may expect modify the "local" physiology such as glucose absorption, but also could have a "global" action via the gut-brain axis. Due to the various origins of gut peptides (i.e., nutrients, intestinal wall, gut microbiota) and the heterogeneity of enteric neurons population, the potential physiological parameters control by the interaction between the two partners are multiple. In this review, we will exclusively focus on the role of enteric nervous system as a potential target of gut peptides to control glucose metabolism and food intake. Potential therapeutic strategies based on per os administration of gut peptides to treat type 2 diabetes will be described.

Visceral Inflammation and Immune Activation Stress the Brain

Stress refers to a dynamic process in which the homeostasis of an organism is challenged, the outcome depending on the type, severity, and duration of stressors involved, the stress responses triggered, and the stress resilience of the organism. Importantly, the relationship between stress and the immune system is bidirectional, as not only stressors have an impact on immune function, but alterations in immune function themselves can elicit stress responses. Such bidirectional interactions have been prominently identified to occur in the gastrointestinal tract in which there is a close cross-talk between the gut microbiota and the local immune system, governed by the permeability of the intestinal mucosa. External stressors disturb the homeostasis between microbiota and gut, these disturbances being signaled to the brain via multiple communication pathways constituting the gut-brain axis, ultimately eliciting stress responses and perturbations of brain function. In view of these relationships, the present article sets out to highlight some of the interactions between peripheral immune activation, especially in the visceral system, and brain function, behavior, and stress coping. These issues are exemplified by the way through which the intestinal microbiota as well as microbe-associated molecular patterns including lipopolysaccharide communicate with the immune system and brain, and the mechanisms whereby overt inflammation in the GI tract impacts on emotional-affective behavior, pain sensitivity, and stress coping. The interactions between the peripheral immune system and the brain take place along the gut-brain axis, the major communication pathways of which comprise microbial metabolites, gut hormones, immune mediators, and sensory neurons. Through these signaling systems, several transmitter and neuropeptide systems within the brain are altered under conditions of peripheral immune stress, enabling adaptive processes related to stress coping and resilience to take place. These aspects of the impact of immune stress on molecular and behavioral processes in the brain have a bearing on several disturbances of mental health and highlight novel opportunities of therapeutic intervention.

Postinfection Irritable Bowel Syndrome: The Links Between Gastroenteritis, Inflammation, the Microbiome, and Functional Disease

Postinfection irritable bowel syndrome (PI-IBS) is a diarrheal disease that develops after infectious gastroenteritis (IGE). Profound alterations in the microbiota accompany IGE yet only 10% of IGE patients progress to PI-IBS. This review explores research linking IGE severity, psychological comorbidity, PI-IBS, and the microbiome in various patient populations. Selective pressures caused by inflammation and increased gastrointestinal motility during gastroenteritis can alter intestinal bacterial phyla including Bacteroidetes, Firmicutes, and Proteobacteria. More specifically, classes such as Bacteroides and Clostridia are differentially abundant in many PI-IBS patients. Altered microbiota may perpetuate a cycle of enteric and systemic inflammation, potently activating neural afferent signaling in the enteric nervous system and causing pain and diarrhea in PI-IBS patients. Altered production of microbial metabolites, for example short chain fatty acids, may have enteric and systemic effects on the host. Longitudinal sampling to characterize changes in the microbiota's genetic, metabolic, and transcriptional activities over time from IGE to PI-IBS may enable improved diagnosis and classification of PI-IBS cases into subtypes, allowing for targeted antibiotic, probiotic, and prebiotic treatments. PI-IBS is a heterogenous and largely organic disease marked by specific alterations in functions of the microbiota and is an important model for studying microbial influences on intestinal, neurological, and psychological host functions.

Recent developments in understanding the role of the gut microbiota in brain health and disease

There is a growing appreciation of the role of the gut microbiota in all aspects of health and disease, including brain health. Indeed, roles for the bacterial commensals in various psychiatric and neurological conditions, such as depression, autism, stroke, Parkinson's disease, and Alzheimer's disease, are emerging. Microbiota dysregulation has been documented in all of these conditions or in animal models thereof. Moreover, depletion or modulation of the gut microbiota can affect the severity of the central pathology or behavioral deficits observed in a variety of brain disorders. However, the mechanisms underlying such effects are only slowly being unraveled. Additionally, recent preclinical and clinical evidence suggest that targeting the microbiota through prebiotic, probiotic, or dietary interventions may be an effective "psychobiotic" strategy for treating symptoms in mood, neurodevelopmental disorders, and neurodegenerative diseases.

A Specific Gut Microbiota Dysbiosis of Type 2 Diabetic Mice Induces GLP-1 Resistance through an Enteric NO-Dependent and Gut-Brain Axis Mechanism

Glucagon-like peptide-1 (GLP-1)-based therapies control glycemia in type 2 diabetic (T2D) patients. However, in some patients the treatment must be discontinued, defining a state of GLP-1 resistance. In animal models we identified a specific set of ileum bacteria impairing the GLP-1-activated gut-brain axis for the control of insulin secretion and gastric emptying. Using prediction algorithms, we identified bacterial pathways related to amino acid metabolism and transport system modules associated to GLP-1 resistance. The conventionalization of germ-free mice demonstrated their role in enteric neuron biology and the gut-brain-periphery axis. Altogether, insulin secretion and gastric emptying require functional GLP-1 receptor and neuronal nitric oxide synthase in the enteric nervous system within a eubiotic gut microbiota environment. Our data open a novel route to improve GLP-1-based therapies.

The pathogenesis of proventricular dilatation disease

Bornaviruses cause neurologic diseases in several species of birds, especially parrots, waterfowl and finches. The characteristic lesions observed in these birds include encephalitis and gross dilatation of the anterior stomach — the proventriculus. The disease is thus known as proventricular dilatation disease (PDD). PDD is characterized by extreme proventricular dilatation, blockage of the passage of digesta and consequent death by starvation. There are few clinical resemblances between this and the bornaviral encephalitides observed in mammals. Nevertheless, there are common virus-induced pathogenic pathways shared across this disease spectrum that are explored in this review. Additionally, a review of the literature relating to gastroparesis in humans and the control of gastric mobility in mammals and birds points to several plausible mechanisms by which bornaviral infection may result in extreme proventricular dilatation.

Extracerebral Dysfunction in Animal Models of Autism Spectrum Disorder

Genetic factors might be largely responsible for the development of autism spectrum disorder (ASD) that alone or in combination with specific environmental risk factors trigger the pathology. Multiple mutations identified in ASD patients that impair synaptic function in the central nervous system are well studied in animal models. How these mutations might interact with other risk factors is not fully understood though. Additionally, how systems outside of the brain are altered in the context of ASD is an emerging area of research. Extracerebral influences on the physiology could begin in utero and contribute to changes in the brain and in the development of other body systems and further lead to epigenetic changes. Therefore, multiple recent studies have aimed at elucidating the role of gene-environment interactions in ASD. Here we provide an overview on the extracerebral systems that might play an important associative role in ASD and review evidence regarding the potential roles of inflammation, trace metals, metabolism, genetic susceptibility, enteric nervous system function and the microbiota of the gastrointestinal (GI) tract on the development of endophenotypes in animal models of ASD. By influencing environmental conditions, it might be possible to reduce or limit the severity of ASD pathology.

Development of Calbindin- and Calretinin-Immunopositive Neurons in the Enteric Ganglia of Rats

Calbindin D28 K (CB) and calretinin (CR) are the members of the EF-hand family of calcium-binding proteins that are expressed in neurons and nerve fibers of the enteric nervous system. CB and CR are expressed differentially in neuronal subpopulations throughout the central and peripheral nervous systems and their expression has been used to selectively target specific cell types and isolate neuronal networks. The present study presents an immunohistochemical analysis of CB and CR in the enteric ganglia of small intestine in rats of different ages (newborn, 10-day-old, 20-day-old, 30-day-old, 60-day-old, 1-year-old, and 2-year-old). The data obtained suggest a number of age-dependent changes in CB and CR expression in the myenteric and submucous plexuses. In the myenteric plexus, the lowest percentage of CB-immunoreactive (IR) and CR-IR neurons was observed at birth, after which the number of IR cells increased in the first 10 days of life. In the submucous plexus, CB-IR and CR-IR neurons were observed from 10-day-old onwards. The percentage of CR-IR and CB-IR neurons increased in the first 2 months and in the first 20 days, respectively. In all animals, the majority of the IR neurons colocalized CR and CB. From the moment of birth, the mean of the cross-sectional area of the CB-IR and CR-IR neuronal profiles was larger than that of CB- and CR-negative cells.

Neuroimmune Cross Talk in the Gut. Neuroendocrine and neuroimmune pathways contribute to the pathophysiology of irritable bowel syndrome

Irritable bowel syndrome (IBS) is a common disorder characterized by recurrent abdominal pain, bloating, and disturbed bowel habit, symptoms that impact the quality of life of sufferers. The pathophysiological changes underlying this multifactorial condition are complex and include increased sensitivity to luminal and mucosal factors, resulting in altered colonic transit and visceral pain. Moreover, dysfunctional communication in the bidirectional signaling axis between the brain and the gut, which involves efferent and afferent branches of the peripheral nervous system, circulating endocrine hormones, and local paracrine and neurocrine factors, including immune and perhaps even microbial signaling molecules, has a role to play in this disorder. This minireview will examine recent advances in our understanding of the pathophysiology of IBS and assess how cross talk between hormones, immune, and microbe-derived factors and their neuromodulatory effects on peripheral nerves may underlie IBS symptomatology.

Gene-environment interactions and the enteric nervous system: Neural plasticity and Hirschsprung disease prevention

Intestinal function is primarily controlled by an intrinsic nervous system of the bowel called the enteric nervous system (ENS). The cells of the ENS are neural crest derivatives that migrate into and through the bowel during early stages of organogenesis before differentiating into a wide variety of neurons and glia. Although genetic factors critically underlie ENS development, it is now clear that many non-genetic factors may influence the number of enteric neurons, types of enteric neurons, and ratio of neurons to glia. These non-genetic influences include dietary nutrients and medicines that may impact ENS structure and function before or after birth. This review summarizes current data about gene-environment interactions that affect ENS development and suggests that these factors may contribute to human intestinal motility disorders like Hirschsprung disease or irritable bowel syndrome.

Growing up in a Bubble: Using Germ-Free Animals to Assess the Influence of the Gut Microbiota on Brain and Behavior

There is a growing recognition of the importance of the commensal intestinal microbiota in the development and later function of the central nervous system. Research using germ-free mice (mice raised without any exposure to microorganisms) has provided some of the most persuasive evidence for a role of these bacteria in gut-brain signalling. Key findings show that the microbiota is necessary for normal stress responsivity, anxiety-like behaviors, sociability, and cognition. Furthermore, the microbiota maintains central nervous system homeostasis by regulating immune function and blood brain barrier integrity. Studies have also found that the gut microbiota influences neurotransmitter, synaptic, and neurotrophic signalling systems and neurogenesis. The principle advantage of the germ-free mouse model is in proof-of-principle studies and that a complete microbiota or defined consortiums of bacteria can be introduced at various developmental time points. However, a germ-free upbringing can induce permanent neurodevelopmental deficits that may deem the model unsuitable for specific scientific queries that do not involve early-life microbial deficiency. As such, alternatives and complementary strategies to the germ-free model are warranted and include antibiotic treatment to create microbiota-deficient animals at distinct time points across the lifespan. Increasing our understanding of the impact of the gut microbiota on brain and behavior has the potential to inform novel management strategies for stress-related gastrointestinal and neuropsychiatric disorders.

Neuroimmunomodulation in the Gut: Focus on Inflammatory Bowel Disease

Intestinal immunity is finely regulated by several concomitant and overlapping mechanisms, in order to efficiently sense external stimuli and mount an adequate response of either tolerance or defense. In this context, a complex interplay between immune and nonimmune cells is responsible for the maintenance of normal homeostasis. However, in certain conditions, the disruption of such an intricate network may result in intestinal inflammation, including inflammatory bowel disease (IBD). IBD is believed to result from a combination of genetic and environmental factors acting in concert with an inappropriate immune response, which in turn interacts with nonimmune cells, including nervous system components. Currently, evidence shows that the interaction between the immune and the nervous system is bidirectional and plays a critical role in the regulation of intestinal inflammation. Recently, the maintenance of intestinal homeostasis has been shown to be under the reciprocal control of the microbiota by immune mechanisms, whereas intestinal microorganisms can modulate mucosal immunity. Therefore, in addition to presenting the mechanisms underlying the interaction between immune and nervous systems in the gut, here we discuss the role of the microbiota also in the regulation of neuroimmune crosstalk involved in intestinal homeostasis and inflammation, with potential implications to IBD pathogenesis.

Microbe-host interactions: Influence of the gut microbiota on the enteric nervous system

The enteric nervous system (ENS), considered a separate branch of the autonomic nervous system, is located throughout the length of the gastrointestinal (GI) tract as a series of interconnected ganglionated plexi. Given the proximity of the intestinal microbiota to the ENS, it is perhaps not surprising that the gut microbiota can influence its development and function. However, these interactions are complex and may be either direct or indirect, often involving signalling initiated by microbe-derived components, metabolites or host-derived intermediaries which subsequently affect enteric nerve excitability and GI function. Individual microbes and strains can differentially influence ENS activity and neurochemistry. In this review we will briefly summarise the role of the microbiota on ENS development, and, in some more detail, explore the mechanisms by which the microbiota can influence ENS activity and function.

Enteric nervous system assembly: Functional integration within the developing gut

Co-ordinated gastrointestinal function is the result of integrated communication between the enteric nervous system (ENS) and "effector" cells in the gastrointestinal tract. Unlike smooth muscle cells, interstitial cells, and the vast majority of cell types residing in the mucosa, enteric neurons and glia are not generated within the gut. Instead, they arise from neural crest cells that migrate into and colonise the developing gastrointestinal tract. Although they are "later" arrivals into the developing gut, enteric neural crest-derived cells (ENCCs) respond to many of the same secreted signalling molecules as the "resident" epithelial and mesenchymal cells, and several factors that control the development of smooth muscle cells, interstitial cells and epithelial cells also regulate ENCCs. Much progress has been made towards understanding the migration of ENCCs along the gastrointestinal tract and their differentiation into neurons and glia. However, our understanding of how enteric neurons begin to communicate with each other and extend their neurites out of the developing plexus layers to innervate the various cell types lining the concentric layers of the gastrointestinal tract is only beginning. It is critical for postpartum survival that the gastrointestinal tract and its enteric circuitry are sufficiently mature to cope with the influx of nutrients and their absorption that occurs shortly after birth. Subsequently, colonisation of the gut by immune cells and microbiota during postnatal development has an important impact that determines the ultimate outline of the intrinsic neural networks of the gut. In this review, we describe the integrated development of the ENS and its target cells.

Moody microbes or fecal phrenology: what do we know about the microbiota-gut-brain axis?

INTRODUCTION

The microbiota-gut-brain axis is a term that is commonly used and covers a broad set of functions and interactions between the gut microbiome, endocrine, immune and nervous systems and the brain. The field is not much more than a decade old and so large holes exist in our knowledge.

DISCUSSION

At first sight it appears gut microbes are largely responsible for the development, maturation and adult function of the enteric nervous system as well as the blood brain barrier, microglia and many aspects of the central nervous system structure and function. Given the state of the art in this exploding field and the hopes, as well as the skepticism, which have been engendered by its popular appeal, we explore recent examples of evidence in rodents and data derived from studies in humans, which offer insights as to pathways involved. Communication between gut and brain depends on both humoral and nervous connections. Since these are bi-directional and occur through complex communication pathways, it is perhaps not surprising that while striking observations have been reported, they have often either not yet been reproduced or their replication by others has not been successful.

CONCLUSIONS

We offer critical and cautionary commentary on the available evidence, and identify gaps in our knowledge that need to be filled so as to achieve translation, where possible, into beneficial application in the clinical setting.

Posttraumatic Stress Disorder: Does the Gut Microbiome Hold the Key?

Gut bacteria strongly influence our metabolic, endocrine, immune, and both peripheral and central nervous systems. Microbiota do this directly and indirectly through their components, shed and secreted, ranging from fermented and digested dietary and host products to functionally active neurotransmitters including serotonin, dopamine, and γ-aminobutyric acid. Depression has been associated with enhanced levels of proinflammatory biomarkers and abnormal responses to stress. Posttraumatic stress disorder (PTSD) appears to be marked in addition by low cortisol responses, and these factors seem to predict and predispose individuals to develop PTSD after a traumatic event. Dysregulation of the immune system and of the hypothalamic-pituitary-adrenal axis observed in PTSD may reflect prior trauma exposure, especially early in life. Early life, including the prenatal period, is a critical time in rodents, and may well be for humans, for the functional and structural development of the immune and nervous systems. These, in turn, are likely shaped and programmed by gut and possibly other bacteria. Recent experimental and clinical data converge on the hypothesis that imbalanced gut microbiota in early life may have long-lasting immune and other physiologic effects that make individuals more susceptible to develop PTSD after a traumatic event and contribute to the disorder. This suggests that it may be possible to target abnormalities in these systems by manipulation of certain gut bacterial communities directly through supplementation or indirectly by dietary and other novel approaches.

Zebrafish as a model for understanding enteric nervous system interactions in the developing intestinal tract

The enteric nervous system (ENS) forms intimate connections with many other intestinal cell types, including immune cells and bacterial consortia resident in the intestinal lumen. In this review, we highlight contributions of the zebrafish model to understanding interactions among these cells. Zebrafish is a powerful model for forward genetic screens, several of which have uncovered genes previously unknown to be important for ENS development. More recently, zebrafish has emerged as a model for testing functions of genes identified in human patients or large-scale human susceptibility screens. In several cases, zebrafish studies have revealed mechanisms connecting intestinal symptoms with other, seemingly unrelated disease phenotypes. Importantly, chemical library screens in zebrafish have provided startling new insights into potential effects of common drugs on ENS development. A key feature of the zebrafish model is the ability to rear large numbers of animals germ free or in association with only specific bacterial species. Studies utilizing these approaches have demonstrated the importance of bacterial signals for normal intestinal development. These types of studies also show how luminal bacteria and the immune system can contribute to inflammatory processes that can feedback to influence ENS development. The excellent optical properties of zebrafish embryos and larvae, coupled with the ease of generating genetically marked cells of both the host and its resident bacteria, allow visualization of multiple intestinal cell types in living larvae and should promote a more in-depth understanding of intestinal cell interactions, especially interactions between other intestinal cell types and the ENS.

Neuro-immune Interactions Drive Tissue Programming in Intestinal Macrophages

Proper adaptation to environmental perturbations is essential for tissue homeostasis. In the intestine, diverse environmental cues can be sensed by immune cells, which must balance resistance to microorganisms with tolerance, avoiding excess tissue damage. By applying imaging and transcriptional profiling tools, we interrogated how distinct microenvironments in the gut regulate resident macrophages. We discovered that macrophages exhibit a high degree of gene-expression specialization dependent on their proximity to the gut lumen. Lamina propria macrophages (LpMs) preferentially expressed a pro-inflammatory phenotype when compared to muscularis macrophages (MMs), which displayed a tissue-protective phenotype. Upon luminal bacterial infection, MMs further enhanced tissue-protective programs, and this was attributed to swift activation of extrinsic sympathetic neurons innervating the gut muscularis and norepinephrine signaling to β2 adrenergic receptors on MMs. Our results reveal unique intra-tissue macrophage specialization and identify neuro-immune communication between enteric neurons and macrophages that induces rapid tissue-protective responses to distal perturbations.

Epigenetic Regulation of Enteric Neurotransmission by Gut Bacteria

The Human Microbiome Project defined microbial community interactions with the human host, and provided important molecular insight into how epigenetic factors can influence intestinal ecosystems. Given physiological context, changes in gut microbial community structure are increasingly found to associate with alterations in enteric neurotransmission and disease. At present, it is not known whether shifts in microbial community dynamics represent cause or consequence of disease pathogenesis. The discovery of bacterial-derived neurotransmitters suggests further studies are needed to establish their role in enteric neuropathy. This mini-review highlights recent advances in bacterial communications to the autonomic nervous system and discusses emerging epigenetic data showing that diet, probiotic and antibiotic use may regulate enteric neurotransmission through modulation of microbial communities. A particular emphasis is placed on bacterial metabolite regulation of enteric nervous system function in the intestine.

Neuroimmune pharmacological approaches

Intestinal inflammation is a major health problem which impairs the quality of life, impacts mental health and is exacerbated by stress and psychiatric disturbances which, in turn, can affect disease prognosis and response to treatment. Accumulating evidence indicates that the immune system is an important interface between intestinal inflammation and the enteric, sensory, central and autonomic nervous systems. In addition, the neuroimmune interactions originating from the gastrointestinal tract are orchestrated by the gut microbiota. This article reviews some major insights into this complex homeostatic network that have been achieved during the past two years and attempts to put these advances into perspective with novel opportunities of pharmacological intervention.

Could the beneficial effects of dietary calcium on obesity and diabetes control be mediated by changes in intestinal microbiota and integrity?

Evidence from animal and human studies has associated gut microbiota, increased translocation of lipopolysaccharide (LPS) and reduced intestinal integrity (II) with the inflammatory state that occurs in obesity and type 2 diabetes mellitus (T2DM). Consumption of Ca may favour body weight reduction and glycaemic control, but its influence on II and gut microbiota is not well understood. Considering the impact of metabolic diseases on public health and the role of Ca on the pathophysiology of these diseases, this review critically discusses possible mechanisms by which high-Ca diets could affect gut microbiota and II. Published studies from 1993 to 2015 about this topic were searched and selected from Medline/PubMed, Scielo and Lilacs databases. High-Ca diets seem to favour the growth of lactobacilli, maintain II (especially in the colon), reduce translocation of LPS and regulate tight-junction gene expression. We conclude that dietary Ca might interfere with gut microbiota and II modulations and it can partly explain the effect of Ca on obesity and T2DM control. However, further research is required to define the supplementation period, the dose and the type of Ca supplement (milk or salt) required for more effective results. As Ca interacts with other components of the diet, these interactions must also be considered in future studies. We believe that more complex mechanisms involving extraintestinal disorders (hormones, cytokines and other biomarkers) also need to be studied.