Hélène Boudin

Beside behavioral symptoms, many patients with autism spectrum disorder (ASD) present gastrointestinal disorders and gut microbiota dysbiosis. In order to better understand the mechanisms underlying microbiota-gut-brain interactions in ASD, we isolated fecal extracellular vesicles (EVs), as a source of microbiota-derived EVs, from control individuals (f-EV-CTL) and patients with ASD (f-EV-ASD) to study in mice their impact on brain and gut. The f-EV-ASD induced hypersocial and anxiolytic-like behavior, and reduced gastrointestinal transit time compared to mice treated with f-EV-CTL. In the colon, f-EV-ASD altered motility through the enteric neuronal nitric oxide (NO)-sensitive component. f-EV-ASD potentiated NO signaling and disrupted intracellular calcium dynamics in enteric neurons. This effect was mediated by butyrate and valerate through FFAR3 receptor. Our findings establish fecal EVs as critical mediators of microbiota-host communication, linking microbial metabolites to altered behavior and gut function and identifying a potential mechanism underlying ASD pathophysiology.

Autism spectrum disorder (ASD) is a neurodevelopmental disorder characterized by impairments in social interaction, restricted interests, and repetitive behaviors. In addition to these core behavioral symptoms, gastrointestinal (GI) disorders are frequently reported, ranging from severe constipation to diarrhea. The links between ASD and gut dysfunction are further supported by alterations in gut microbiota composition and in bacteria‐derived metabolites. As the intrinsic nervous system of the digestive tract, the enteric nervous system (ENS) plays a central role in gut physiology by exerting neuronal control on several gut functions, including motility. Located at the interface between the gut microbiota and intestinal function, the ENS may play a key role in the GI symptoms associated with ASD, as demonstrated in several mouse models. This review aims to summarize the interconnected alterations of the gut microbiota and the ENS in ASD, with a particular focus on the microbiota‐derived mediators that are altered in ASD, among patients and animal models, and that may affect ENS development and function. By highlighting these interactions, this review seeks to provide new insights on the digestive pathophysiology of ASD, which may also contribute to the severity of behavioral symptoms.

Secondary bile acids (BAs) are metabolites produced by the gut microbiota and shown to impact digestive functions, at least in part through the enteric nervous system (ENS). In the ENS, enteric neurons express the BA receptor Takeda G protein‐coupled receptor 5 (TGR5), making them potential direct cellular targets of secondary BAs, although their effects on enteric neuronal functions remain poorly understood. Enteric neuronal activity and connectivity form the basis of the regulatory control exerted by the ENS on gut functions. Yet, the influence of microbiota‐derived metabolites, such as secondary BAs, on enteric neuron connectivity and synaptic activity remains largely unexplored. To address this question, we studied the effects of secondary BAs on neuronal connectivity using a model of rat primary culture of enteric neurons. We found that exposure to deoxycholic acid (DCA) increased the expression of key presynaptic proteins, synapsin‐1 and synaptophysin, and enhanced synaptic density in enteric neurons. Moreover, DCA enhanced synaptic activity by increasing synaptic vesicle exocytosis upon KCl depolarisation, potentially through amplified phosphorylation of synapsin‐1 at the Ser62–67 sites. In addition, we found that DCA modulated the intracellular Ca ²⁺ response induced by acetylcholine, a major excitatory neurotransmitter in enteric neurons, through a mechanism mediated by the TGR5 receptor. Overall, this study identifies DCA as a microbiota‐derived compound capable of reshaping the enteric neuronal functional network. These findings highlight the potential of bacterial metabolites like DCA to link the microbiome with modulation of enteric neuronal activity and connectivity, supporting the relevance of secondary BAs in digestive physiology and their possible roles in gastrointestinal disorders. image

Autism Spectrum Disorders (ASDs) are neurodevelopmental disorders defined by impaired social interactions and communication with repetitive behaviors, activities, or interests. Gastrointestinal (GI) disturbances and gut microbiota dysbiosis are frequently associated with ASD in childhood. However, it is not known whether microbiota dysbiosis in ASD patients also occurs in adulthood. Further, the consequences of altered gut microbiota on digestive functions and the enteric nervous system (ENS) remain unexplored. Therefore, we studied, in mice, the ability offecal supernatant (FS) from adult ASD patients to induce GI dysfunctions and ENS remodeling. First, the analyses of the fecal microbiota composition in adult ASD patients indicated a reduced α-diversity and increased abundance of three bacterial 16S rRNA gene amplicon sequence variants compared to healthy controls (HC). The transfer of FS from ASD patients (FS–ASD) to mice decreased colonic barrier permeability by 29% and 58% compared to FS–HC for paracellular and transcellular permeability, respectively. These effects are associated with the reduced expression of the tight junction proteins JAM-A, ZO-2, cingulin, and proinflammatory cytokines TNFα and IL1β. In addition, the expression of glial and neuronal molecules was reduced by FS–ASD as compared to FS-HC in particular for those involved in neuronal connectivity (βIII-tubulin and synapsin decreased by 31% and 67%, respectively). Our data suggest that changes in microbiota composition in ASD may contribute to GI alterations, and in part, via ENS remodeling.

Most of the gut functions are controlled by the enteric nervous system (ENS), a complex network of enteric neurons located throughout the wall of the gastrointestinal tract. The formation of ENS connectivity during the perinatal period critically underlies the establishment of gastrointestinal motility, but the factors involved in this maturation process remain poorly characterized. Here, we examined the role of Semaphorin 3A (Sema3A) on ENS maturation and its potential implication in Hirschsprung disease (HSCR), a developmental disorder of the ENS with impaired colonic motility. We found that Sema3A and its receptor Neuropilin 1 (NRP1) are expressed in the rat gut during the early postnatal period. At the cellular level, NRP1 is expressed by enteric neurons, where it is particularly enriched at growth areas of developing axons. Treatment of primary ENS cultures and gut explants with Sema3A restricts axon elongation and synapse formation. Comparison of the ganglionic colon of HSCR patients to the colon of patients with anorectal malformation shows reduced expression of the synaptic molecule synapsin 1 in HSCR, which is inversely correlated with Sema3A expression. Our study identifies Sema3A as a critical regulator of ENS connectivity and provides a link between altered ENS connectivity and HSCR.

Ulcerative colitis (UC) is a highly frequent inflammatory bowel disease (IBD), which causes are unknown. We have shown that the offspring gestated under thyroxine (T4) deficiency, condition named hypothyroxinemia (HTX), shows inflammatory features at the intestine. Moreover, the offspring gestated under HTX (HTX-offspring) suffers strong experimental autoimmune disease. Suggesting that gestational HTX could imprint their offspring with a high immune response after an inflammatory challenge. Thus, we think that the HTX-offspring will be more prone to suffer IBD. To analyze this hypothesis C57BL6 pregnant mice were induced with gestational HTX during E10 to E15 by giving them methimazole in the tap-drinking water. Then, UC was induced to the HTX-offspring or control at postnatal P55 by giving them dextran sodium sulphate (DSS) in the tap-drinking water for six days. Body weight, pathological score of IBD and hidden blood in stool were analyzed daily. Then, mice were sacrificed at day 10 to evaluate their inflammatory response. For that histological analysis were performed in the intestine; the content of cytokines in serum and intestine by ELISA and the populations of innate and adaptive immune cells at the intestine and spleen by flow cytometry. Our results shown that the male and female offspring developed an opposite immune response to UC. The female HTX-offspring were more resistant and the male HTX-offspring were more sensitive to UC compared to their respective female and male control groups. Thus, our results suggest that the immune response of an individual can be set up during gestation and that the normal level of maternal T4 during gestation is fundamental for the proper function of the offspring immune system.

Le cerveau et l’intestin représentent deux organes clés dans l’évolution et la survie des espèces. Le système nerveux du tube digestif, le système nerveux entérique (SNE), assure la régulation des fonctions digestives et présente de nombreuses similitudes avec le cerveau dans les grands principes de son développement et de son fonctionnement. Le SNE et le cerveau sont particulièrement sensibles au microbiote intestinal, qui représente un véritable écosystème à l’interface de notre corps et de notre environnement. L’installation du microbiote dans le tube digestif dès la naissance joue un rôle majeur dans le développement et la maturation fonctionnelle de l’intestin et du cerveau. Cet axe de communication microbiote–intestin–cerveau, opérant par l’intermédiaire de voies nerveuses et humorales, semble être également particulièrement impliqué dans des pathologies neurologiques et psychiatriques, avec troubles digestifs associés, comme la maladie de Parkinson et les troubles du spectre de l’autisme. Les similitudes structurelles et les voies d’interactions liant étroitement l’intestin et le cerveau sont probablement à la base de la capacité du microbiote à moduler en « miroir » ces deux organes. Ces découvertes ouvrent de nouvelles perspectives de prévention et/ou de traitement de nombreuses pathologies digestives et cérébrales fondées sur la modulation du microbiote intestinal. The brain and gut are the two organs that had played key roles during the evolution and in species’ survival. The nervous system of the digestive tract, enteric nervous system (ENS), regulates digestive functions and shares several similarities with the brain, especially regarding the main principles of development and function. The ENS and brain are particularly sensitive to the intestinal microbiota, which represents a true ecosystem at the interface between our body and the environment. The establishment of the microbiota in the digestive tract at birth plays a critical role in the development and functional maturation of both the gut and brain. This microbiota–gut–brain axis, which communicates through neural and humoral pathways, appears to be particularly involved in a group of neurological and psychiatric diseases with gastrointestinal comorbidities, such as Parkinson’s disease and autism spectrum disorder. The structural analogies and the communication pathways resulting in close interactions between the gut and brain are likely the basis for a mirrored modulation of these two organs by the gut microbiota. These discoveries open new avenues for the prevention and/or treatment of several digestive and brain disorders by targeting the intestinal microbiota.