Gut microbiota depletion delays somatic peripheral nerve development and impairs neuromuscular junction maturation.

Gut microbiota is responsible for essential functions in human health. Several communication axes between gut microbiota and other organs via neural, endocrine, and immune pathways have been described, and perturbation of gut microbiota composition has been implicated in the onset and progression of an emerging number of diseases. Here, we analyzed peripheral nerves, dorsal root ganglia (DRG), and skeletal muscles of neonatal and young adult mice with the following gut microbiota status: a) germ-free (GF), b) gnotobiotic, selectively colonized with 12 specific gut bacterial strains (Oligo-Mouse-Microbiota, OMM12), or c) natural complex gut microbiota (CGM). Stereological and morphometric analyses revealed that the absence of gut microbiota impairs the development of somatic median nerves, resulting in smaller diameter and hypermyelinated axons, as well as in smaller unmyelinated fibers. Accordingly, DRG and sciatic nerve transcriptomic analyses highlighted a panel of differentially expressed developmental and myelination genes. Interestingly, the type III isoform of Neuregulin1 (NRG1), known to be a neuronal signal essential for Schwann cell myelination, was overexpressed in young adult GF mice, with consequent overexpression of the transcription factor Early Growth Response 2 (Egr2), a fundamental gene expressed by Schwann cells at the onset of myelination. Finally, GF status resulted in histologically atrophic skeletal muscles, impaired formation of neuromuscular junctions, and deregulated expression of related genes. In conclusion, we demonstrate for the first time a gut microbiota regulatory impact on proper development of the somatic peripheral nervous system and its functional connection to skeletal muscles, thus suggesting the existence of a novel 'Gut Microbiota-Peripheral Nervous System-axis.'

The Genetic Background Is Shaping Cecal Enlargement in the Absence of Intestinal Microbiota.

Germ-free (GF) rodents have become a valuable tool for studying the role of intestinal microbes on the host physiology. The major characteristic of GF rodents is an enlarged cecum. The accumulation of mucopolysaccharides, digestion enzymes and water in the intestinal lumen drives this phenotype. Microbial colonization normalizes the cecum size in ex-GF animals. However, whether strain genetics influences the cecal enlargement is unknown. Here we investigated the impact of mouse genetic background on the cecal size in five GF strains frequently used in biomedical research. The cecal weight of GF mice on B6 background (B6J and B6N) represented up to 20% of total body weight. GF NMRI and BALBc mice showed an intermediate phenotype of 5-10%, and those on the C3H background of up to 5%. Reduced cecal size in GF C3H mice correlated with decreased water content, increased expression of water transporters, and reduced production of acidic mucins, but was independent of the level of digestive enzymes in the lumen. In contrast, GF B6J mice with greatly enlarged cecum showed increased water content and a distinct metabolic profile characterized by altered amino acid and bile acid metabolism, and increased acidic mucin production. Together, our results show that genetic background influences the cecal enlargement by regulating the water transport, production of acidic mucins, and metabolic profiles.

Approaches to discern if microbiome associations reflect causation in metabolic and immune disorders.

Our understanding of microorganisms residing within our gut and their roles in the host metabolism and immunity advanced greatly over the past 20 years. Currently, microbiome studies are shifting from association and correlation studies to studies demonstrating causality of identified microbiome signatures and identification of molecular mechanisms underlying these interactions. This transformation is crucial for the efficient translation into clinical application and development of targeted strategies to beneficially modulate the intestinal microbiota. As mechanistic studies are still quite challenging to perform in humans, the causal role of microbiota is frequently evaluated in animal models that need to be appropriately selected. Here, we provide a comprehensive overview on approaches that can be applied in addressing causality of host-microbe interactions in five major animal model organisms (Caenorhabditis elegans, Drosophila melanogaster, zebrafish, rodents, and pigs). We particularly focused on discussing methods available for studying the causality ranging from the usage of gut microbiota transfer, diverse models of metabolic and immune perturbations involving nutritional and chemical factors, gene modifications and surgically induced models, metabolite profiling up to culture-based approached. Furthermore, we addressed the impact of the gut morphology, physiology as well as diet on the microbiota composition in various models and resulting species specificities. Finally, we conclude this review with the discussion on models that can be applied to study the causal role of the gut microbiota in the context of metabolic syndrome and host immunity. We hope this review will facilitate important considerations for appropriate animal model selection.

Synthetic Microbiomes on the Rise-Application in Deciphering the Role of Microbes in Host Health and Disease.

The intestinal microbiota conveys significant benefits to host physiology. Although multiple chronic disorders have been associated with alterations in the intestinal microbiota composition and function, it is still unclear whether these changes are a cause or a consequence. Hence, to translate microbiome research into clinical application, it is necessary to provide a proof of causality of host-microbiota interactions. This is hampered by the complexity of the gut microbiome and many confounding factors. The application of gnotobiotic animal models associated with synthetic communities allows us to address the cause-effect relationship between the host and intestinal microbiota by reducing the microbiome complexity on a manageable level. In recent years, diverse bacterial communities were assembled to analyze the role of microorganisms in infectious, inflammatory, and metabolic diseases. In this review, we outline their application and features. Furthermore, we discuss the differences between human-derived and model-specific communities. Lastly, we highlight the necessity of generating novel synthetic communities to unravel the microbial role associated with specific health outcomes and disease phenotypes. This understanding is essential for the development of novel non-invasive targeted therapeutic strategies to control and modulate intestinal microbiota in health and disease.

Monitoring and contamination incidence of gnotobiotic experiments performed in microisolator cages.

With the increased interest in the microbiome research, gnotobiotic animals and techniques emerged again as valuable tools to investigate functional effects of host-microbe and microbe-microbe interactions. The increased demand for gnotobiotic experiments has resulted in the greater need for housing systems for short-term maintenance of gnotobiotic animals. During the last six years, the gnotobiotic facility of the Hannover Medical School has worked intensively with different housing systems for gnotobiotic animals. Here, we report our experience in handling, contamination incidence, and monitoring strategies that we apply for controlling gnotobiotic experiments. From our experience, the risk of introducing contaminants to animals housed in microisolator cages is higher than in isolators. However, with strict operating protocols, the contamination rate in these systems can be minimized. In addition to spore-forming bacteria and fungi from the environment, spore-forming bacteria from defined bacterial communities used in experiments represent the major risk for contamination of gnotobiotic experiments performed in microisolator cages. The presence/absence of contaminants in germ-free animals can be easily monitored by preparation of wet mounts and Gram staining of fecal samples. Contaminants in animals colonized with specific microorganisms need to be tracked with methods such as next-generation sequencing. However, when using PCR-based methods it is important to consider that relatively small amounts of bacterial DNA detected likely originates from food, bedding, or reagents and is not to be interpreted as true contamination.

Lymph Node Stromal Cells From Different Draining Areas Distinctly Regulate the Development of Chronic Intestinal Inflammation.

The balance between the responsiveness of the intestinal immune system and the gut environment is fundamental for the maintenance of intestinal homeostasis, which is required for an adequate recognition of entering antigens. The disruption of this homeostasis by exaggerated immune response to harmless antigens can lead to the development of intestinal disorders such as inflammatory bowel disease. Stromal cells are sessile non-hematopoietic cells that build the backbone of the lymph node, an important site for the immune response induction, but also contribute to immune response and tolerance induction. However, the knowledge about the role of stromal cells in the regulation of inflammatory responses is still limited. Therefore, in this study we analyzed the influence of stromal cells on the development of chronic intestinal inflammation. Here, we show that intestinal inflammation alters the immune activation of the mesenteric lymph node-derived stromal cells. Podoplanin+ and CD21/35+ stromal cells showed increased expression of MHC class II molecules, but CD106 expression on CD21/35+ cells was reduced. Stromal cells secreted cytokines and chemokines such as CCL7 and CXCL16 influenced the gut-homing phenotype and proliferation of CD4+ and CD8+ T cells. Furthermore, stromal cells of peripheral lymph nodes transplanted into the mesentery attenuated colitis severity in B6-Il10-/- mice. The reduced colitis severity in these mice was associated with increased expression of IL4 and distinct activation pattern of stromal cells derived from transplanted peripheral lymph nodes. Altogether, our results demonstrate that lymph node stromal cells impact development of chronic colitis via T cell induction. Moreover, lymph node stromal cells from different draining area due to neonatally imprinted processes distinctly regulate the induction of immune responses.

Composition of the Intestinal Microbiota Determines the Outcome of Virus-Triggered Colitis in Mice.

The intestinal microbiota is a complex ecosystem implicated in host health and disease. Inflammatory bowel disease (IBD) is a multifactorial chronic disorder of the gastrointestinal mucosa. Even though the exact mechanisms are still unknown, the intestinal microbiota is crucial in IBD development. We previously showed that murine norovirus (MNV) induces colitis in the Il10-deficient (Il10-/-) mouse model of IBD in a microbiota-dependent manner. Thus, in this study we analyzed whether distinct minimal bacterial consortia influence the outcome of MNV-triggered colitis in Il10-/- mice. Gnotobiotic Il10-/- mice associated with Oligo-Mouse-Microbiota 12 (OMM12) or Altered Schaedler Flora (ASF) developed little to no inflammatory lesions in the colon and cecum. MNV infection exacerbated colitis severity only in ASF-colonized mice, but not in those associated with OMM12. Four weeks after MNV infection, inflammatory lesions in ASF-colonized Il10-/- mice were characterized by epithelial hyperplasia, infiltration of inflammatory cells, and increased barrier permeability. Co-colonization of ASF-colonized Il10-/- mice with segmented filamentous bacteria (SFB) abolished MNV-induced colitis, whereas histopathological scores in SFB-OMM12-co-colonized mice stayed unchanged. Moreover, SFB only colonized mice associated with ASF. The SFB-mediated protective effects in ASF-colonized mice involved enhanced activation of intestinal barrier defense mechanisms and mucosal immune responses in the chronic and acute phase of MNV infection. SFB colonization strengthened intestinal barrier function by increasing expression of tight junction proteins, antimicrobial peptides and mucus. Furthermore, SFB colonization enhanced the expression of pro-inflammatory cytokines such as Tnfα, Il1ß, and Il12a, as well as the expression of the regulatory cytokine Tgfß. Altogether, our results showed that MNV-triggered colitis depends on the microbial context.