Direct and indirect effects of microbiota-derived metabolites on neuroinflammation in multiple sclerosis

New therapeutic avenues in multiple sclerosis: is there a place for gut microbiota-based treatments?

The bidirectional interaction between the gut and the central nervous system (CNS), the so-called gut microbiota-brain axis, is reported to influence brain functions, thus having a potential impact on the development or the progression of several neurodegenerative disorders. Within this context, it has been documented that multiple sclerosis (MS), an autoimmune inflammatory, demyelinating, and neurodegenerative disease of the CNS, is associated with gastrointestinal symptoms, including constipation, dysphagia, and faecal incontinence. Moreover, some evidence suggests the existence of an altered gut microbiota (GM) composition in MS patients with respect to healthy individuals, as well as the potential influence of GM dysbiosis on typical MS features, including increased intestinal permeability, disruption of blood-brain barrier integrity, chronic inflammation, and altered T cells differentiation. Starting from these assumptions, the possible involvement of GM alteration in MS pathogenesis seems likely, and its restoration could represent a supplemental beneficial strategy against this disabling disease. In this regard, the present review will explore possible preventive approaches (including several dietary interventions, the administration of probiotics, prebiotics, synbiotics, and postbiotics, and the use of faecal microbiota transplantation) to be pursued as prophylaxis or in combination with pharmacological treatments with the aim of re-establishing a proper GM, thus helping to prevent the development of this disease or to manage it by alleviating symptoms or slowing down its progression.

Modulation of multiple sclerosis risk and pathogenesis by the gut microbiota: Complex interactions between host genetics, bacterial metabolism, and diet

Multiple sclerosis (MS) is an autoimmune disease of the central nervous system, affecting nearly 2 million people worldwide. The etiology of MS is multifactorial: Approximately 30% of the MS risk is genetic, which implies that the remaining ~70% is environmental, with a number of factors proposed. One recently implicated risk factor for MS is the composition of the gut microbiome. Numerous case-control studies have identified changes in gut microbiota composition of people with MS (pwMS) compared with healthy control individuals, and more recent studies in animal models have begun to identify the causative microbes and underlying mechanisms. Here, we review some of these mechanisms, with a specific focus on the role of host genetic variation, dietary inputs, and gut microbial metabolism, with a particular emphasis on short-chain fatty acid and tryptophan metabolism. We put forward a model where, in an individual genetically susceptible to MS, the gut microbiota and diet can synergize as potent environmental modifiers of disease risk and possibly progression, with diet-dependent gut microbial metabolites serving as a key mechanism. We also propose that specific microbial taxa may have divergent effects in individuals carrying distinct variants of MS risk alleles or other polymorphisms, as a consequence of host gene-by-gut microbiota interactions. Finally, we also propose that the effects of specific microbial taxa, especially those that exert their effects through metabolites, are highly dependent on the host dietary intake. What emerges is a complex multifaceted interaction that has been challenging to disentangle in human studies, contributing to the divergence of findings across heterogeneous cohorts with differing geography, dietary preferences, and genetics. Nonetheless, this provides a complex and individualized, yet tractable, model of how the gut microbiota regulate susceptibility to MS, and potentially progression of this disease. Thus, we conclude that prophylactic or therapeutic modulation of the gut microbiome to prevent or treat MS will require a careful and personalized consideration of host genetics, baseline gut microbiota composition, and dietary inputs.

The influence of aging and the microbiome in multiple sclerosis and other neurologic diseases

The human gut microbiome is well-recognized as a key player in maintaining health. However, it is a dynamic entity that changes across the lifespan. How the microbial changes that occur in later decades of life shape host health or impact age-associated inflammatory neurological diseases such as multiple sclerosis (MS) is still unclear. Current understanding of the aging gut microbiome is largely limited to cross-sectional observational studies. Moreover, studies in humans are limited by confounding host-intrinsic and extrinsic factors that are not easily disentangled from aging. This review provides a comprehensive summary of existing literature on the aging gut microbiome and its known relationships with neurological diseases, with a specific focus on MS. We will also discuss preclinical animal models and human studies that shed light on the complex microbiota-host interactions that have the potential to influence disease pathology and progression in aging individuals. Lastly, we propose potential avenues of investigation to deconvolute features of an aging microbiota that contribute to disease, or alternatively promote health in advanced age.

The Role of Gut-derived Short-Chain Fatty Acids in Multiple Sclerosis

Multiple sclerosis (MS) is a chronic condition affecting the central nervous system (CNS), where the interplay of genetic and environmental factors influences its pathophysiology, triggering immune responses and instigating inflammation. Contemporary research has been notably dedicated to investigating the contributions of gut microbiota and their metabolites in modulating inflammatory reactions within the CNS. Recent recognition of the gut microbiome and dietary patterns as environmental elements impacting MS development emphasizes the potential influence of small, ubiquitous molecules from microbiota, such as short-chain fatty acids (SCFAs). These molecules may serve as vital molecular signals or metabolic substances regulating host cellular metabolism in the intricate interplay between microbiota and the host. A current emphasis lies on optimizing the health-promoting attributes of colonic bacteria to mitigate urinary tract issues through dietary management. This review aims to spotlight recent investigations on the impact of SCFAs on immune cells pivotal in MS, the involvement of gut microbiota and SCFAs in MS development, and the considerable influence of probiotics on gastrointestinal disruptions in MS. Comprehending the gut-CNS connection holds promise for the development of innovative therapeutic approaches, particularly probiotic-based supplements, for managing MS.

Gut-Brain Interactions and Their Impact on Astrocytes in the Context of Multiple Sclerosis and Beyond

Multiple Sclerosis (MS) is a chronic autoimmune inflammatory disease of the central nervous system (CNS) that leads to physical and cognitive impairment in young adults. The increasing prevalence of MS underscores the critical need for innovative therapeutic approaches. Recent advances in neuroimmunology have highlighted the significant role of the gut microbiome in MS pathology, unveiling distinct alterations in patients' gut microbiota. Dysbiosis not only impacts gut-intrinsic processes but also influences the production of bacterial metabolites and hormones, which can regulate processes in remote tissues, such as the CNS. Central to this paradigm is the gut-brain axis, a bidirectional communication network linking the gastrointestinal tract to the brain and spinal cord. Via specific routes, bacterial metabolites and hormones can influence CNS-resident cells and processes both directly and indirectly. Exploiting this axis, novel therapeutic interventions, including pro- and prebiotic treatments, have emerged as promising avenues with the aim of mitigating the severity of MS. This review delves into the complex interplay between the gut microbiome and the brain in the context of MS, summarizing current knowledge on the key signals of cross-organ crosstalk, routes of communication, and potential therapeutic relevance of the gut microbiome. Moreover, this review places particular emphasis on elucidating the influence of these interactions on astrocyte functions within the CNS, offering insights into their role in MS pathophysiology and potential therapeutic interventions.

Game of microbes: the battle within - gut microbiota and multiple sclerosis

Multiple sclerosis (MS) is a chronic and progressive autoimmune disease of the central nervous system (CNS), with both genetic and environmental factors contributing to the pathobiology of the disease. While human leukocyte antigen (HLA) genes have emerged as the strongest genetic factor, consensus on environmental risk factors are lacking. Recently, trillions of microbes residing in our gut (microbiome) have emerged as a potential environmental factor linked with the pathobiology of MS as PwMS show gut microbial dysbiosis (altered gut microbiome). Thus, there has been a strong emphasis on understanding the factors (host and environmental) regulating the composition of the gut microbiota and the mechanism(s) through which gut microbes contribute to MS disease, especially through immune system modulation. A better understanding of these interactions will help harness the enormous potential of the gut microbiota as a therapeutic approach to treating MS.

Impact of Microbiome-Brain Communication on Neuroinflammation and Neurodegeneration

The gut microbiome plays a pivotal role in maintaining human health, with numerous studies demonstrating that alterations in microbial compositions can significantly affect the development and progression of various immune-mediated diseases affecting both the digestive tract and the central nervous system (CNS). This complex interplay between the microbiota, the gut, and the CNS is referred to as the gut-brain axis. The role of the gut microbiota in the pathogenesis of neurodegenerative diseases has gained increasing attention in recent years, and evidence suggests that gut dysbiosis may contribute to disease development and progression. Clinical studies have shown alterations in the composition of the gut microbiota in multiple sclerosis patients, with a decrease in beneficial bacteria and an increase in pro-inflammatory bacteria. Furthermore, changes within the microbial community have been linked to the pathogenesis of Parkinson's disease and Alzheimer's disease. Microbiota-gut-brain communication can impact neurodegenerative diseases through various mechanisms, including the regulation of immune function, the production of microbial metabolites, as well as modulation of host-derived soluble factors. This review describes the current literature on the gut-brain axis and highlights novel communication systems that allow cross-talk between the gut microbiota and the host that might influence the pathogenesis of neuroinflammation and neurodegeneration.

Feeding the gut microbiome: impact on multiple sclerosis

Multiple sclerosis (MS) is a multifactorial neurological disease characterized by chronic inflammation and immune-driven demyelination of the central nervous system (CNS). The rising number of MS cases in the last decade could be partially attributed to environmental changes, among which the alteration of the gut microbiome driven by novel dietary habits is now of particular interest. The intent of this review is to describe how diet can impact the development and course of MS by feeding the gut microbiome. We discuss the role of nutrition and the gut microbiota in MS disease, describing preclinical studies on experimental autoimmune encephalomyelitis (EAE) and clinical studies on dietary interventions in MS, with particular attention to gut metabolites-immune system interactions. Possible tools that target the gut microbiome in MS, such as the use of probiotics, prebiotics and postbiotics, are analyzed as well. Finally, we discuss the open questions and the prospects of these microbiome-targeted therapies for people with MS and for future research.

The immunomodulatory roles of the gut microbiome in autoimmune diseases of the central nervous system: Multiple sclerosis as a model

The gut-associated lymphoid tissue is a primary activation site for immune responses to infection and immunomodulation. Experimental evidence using animal disease models suggests that specific gut microbes significantly regulate inflammation and immunoregulatory pathways. Furthermore, recent clinical findings indicate that gut microbes' composition, collectively named gut microbiota, is altered under disease state. This review focuses on the functional mechanisms by which gut microbes promote immunomodulatory responses that could be relevant in balancing inflammation associated with autoimmunity in the central nervous system. We also propose therapeutic interventions that target the composition of the gut microbiota as immunomodulatory mechanisms to control neuroinflammation.

Possible role of the NLRP3 inflammasome and the gut-brain axis in multiple sclerosis-related depression

Multiple sclerosis (MS) is an autoimmune and demyelinating disease of the central nervous system that results from complex interactions between genetic and environmental determinants. Patients with MS exhibit a high risk of depression, however, the exact pathomechanisms remain largely unknown. It is becoming widely accepted that the gut-brain axis (GBA) disorders may exert an influence on neuroinflammation and psychiatric symptoms, including so-called MS-related depression. The element suggested as a bridge between intestinal disorders, depression, and MS is an inflammatory response with the central role of the NLR family pyrin domain containing 3 (NLRP3) inflammasome. The pro-inflammatory activity of effector cytokines of the NLRP3 inflammasome forms the hypothesis that it is actively involved in the development of inflammatory and autoimmune diseases. Despite extensive reviews considering the possible origins of MS-related depression, its complex pathophysiology prevents any easy determination of its underlying mechanisms. This paper aims to discuss molecular mechanisms related to the GBA axis that can mediate dysbiosis, intestinal barrier dysfunction, disruption of blood-brain barrier integrity, neuroinflammation, and subsequent manifestation of MS-related major depressive disorder.

Inhibition of Th1 activation and differentiation by dietary guar gum ameliorates experimental autoimmune encephalomyelitis

Dietary fibers are potent modulators of immune responses that can restrain inflammation in multiple disease contexts. However, dietary fibers encompass a biochemically diverse family of carbohydrates, and it remains unknown how individual fiber sources influence immunity. In a direct comparison of four different high-fiber diets, we demonstrate a potent ability of guar gum to delay disease and neuroinflammation in experimental autoimmune encephalomyelitis, a T cell-mediated mouse model of multiple sclerosis. Guar gum-specific alterations to the microbiota are limited, and disease protection appears to be independent of fiber-induced increases in short-chain fatty acid levels or regulatory CD4⁺ T cells. Instead, CD4⁺ T cells of guar gum-supplemented mice are less encephalitogenic due to reduced activation, proliferation, Th1 differentiation, and altered migratory potential. These findings reveal specificity in the host response to fiber sources and define a pathway of fiber-induced immunomodulation that protects against pathologic neuroinflammation.

Moving beyond descriptive studies: harnessing metabolomics to elucidate the molecular mechanisms underpinning host-microbiome phenotypes

Advances in technology and software have radically expanded the scope of metabolomics studies and allow us to monitor a broad transect of central carbon metabolism in routine studies. These increasingly sophisticated tools have shown that many human diseases are modulated by microbial metabolism. Despite this, it remains surprisingly difficult to move beyond these statistical associations and identify the specific molecular mechanisms that link dysbiosis to the progression of human disease. This difficulty stems from both the biological intricacies of host-microbiome dynamics as well as the analytical complexities inherent to microbiome metabolism research. The primary objective of this review is to examine the experimental and computational tools that can provide insights into the molecular mechanisms at work in host-microbiome interactions and to highlight the undeveloped frontiers that are currently holding back microbiome research from fully leveraging the benefits of modern metabolomics.

Microbial Metabolites in Multiple Sclerosis: Implications for Pathogenesis and Treatment

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

Identification of Novel Key Genes and Pathways in Multiple Sclerosis Based on Weighted Gene Coexpression Network Analysis and Long Noncoding RNA-Associated Competing Endogenous RNA Network

Objective

Multiple sclerosis (MS) is an autoimmune disease of the central nervous system characterized by chronic inflammation and demyelination. This study is aimed at identifying crucial genes and molecular pathways involved in MS pathogenesis.

Methods

Raw data in GSE52139 were collected from the Gene Expression Omnibus. The top 50% expression variants were subjected to weighted gene coexpression network analysis (WGCNA), and the key module associated with MS occurrence was identified. A long noncoding RNA- (lncRNA-) associated competing endogenous RNA (ceRNA) network was constructed in the key module. The hub gene candidates were subsequently verified in an individual database.

Results

Of the 18 modules obtained, the cyan module was designated as the key module. The established ceRNA network was composed of seven lncRNAs, 45 mRNAs, and 21 microRNAs (miRNAs), and the FAM13A-AS1 was the lncRNA with the highest centrality. Functional assessments indicated that the genes in the cyan module primarily gathered in ribosome-related functional terms. Interestingly, the targeted mRNAs of the ceRNA network enriched in diverse categories. Moreover, highly expressed CYBRD1, GNG12, and SMAD1, which were identified as hub genes, may be associated with “valine leucine and isoleucine degradation,” “base excision repair,” and “fatty acid metabolism,” respectively, according to the results of single gene-based genomes and gene set enrichment analysis (GSEA).

Conclusions

Combined with the WGCNA and ceRNA network, our findings provide novel insights into the pathogenesis of MS. The hub genes discovered herein might also serve as novel biomarkers that correlate with the development and management of MS.

Remote regulation of type 2 immunity by intestinal parasites

The intestinal tract is the target organ of most parasitic infections, including those by helminths and protozoa. These parasites elicit prototypical type 2 immune activation in the host's immune system with striking impact on the local tissue microenvironment. Despite local containment of these parasites within the intestinal tract, parasitic infections also mediate immune adaptation in peripheral organs. In this review, we summarize the current knowledge on how such gut-tissue axes influence important immune-mediated resistance and disease tolerance in the context of coinfections, and elaborate on the implications of parasite-regulated gut-lung and gut-brain axes on the development and severity of airway inflammation and central nervous system diseases.

The intestinal microbiota: from health to disease, and back

Our understanding of the composition and the function of the intestinal microbiota has significantly increased over the past few years. In a series of reviews focusing on the role of the intestinal microbiota in health and disease, we explore recent conceptual and technological advances in this rapidly evolving research arena.