Regulation of Intestinal Barrier Function by Microbial Metabolites

The human gastrointestinal tract (GI) harbors a diverse population of microbial life that continually shapes host pathophysiological responses. Despite readily available abundant metagenomic data, the functional dynamics of gut microbiota remain to be explored in various health and disease conditions. Microbiota generate a variety of metabolites from dietary products that influence host health and pathophysiological functions. Since gut microbial metabolites are produced in close proximity to gut epithelium, presumably they have significant impact on gut barrier function and immune responses. The goal of this review is to discuss recent advances on gut microbial metabolites in the regulation of intestinal barrier function. While the mechanisms of action of these metabolites are only beginning to emerge, they mainly point to a small group of shared pathways that control gut barrier functions. Amidst expanding technology and broadening knowledge, exploitation of beneficial microbiota and their metabolites to restore pathophysiological balance will likely prove to be an extremely useful remedial tool. The human gastrointestinal tract (GI) harbors a diverse population of microbial life that continually shapes host pathophysiological responses. Despite readily available abundant metagenomic data, the functional dynamics of gut microbiota remain to be explored in various health and disease conditions. Microbiota generate a variety of metabolites from dietary products that influence host health and pathophysiological functions. Since gut microbial metabolites are produced in close proximity to gut epithelium, presumably they have significant impact on gut barrier function and immune responses. The goal of this review is to discuss recent advances on gut microbial metabolites in the regulation of intestinal barrier function. While the mechanisms of action of these metabolites are only beginning to emerge, they mainly point to a small group of shared pathways that control gut barrier functions. Amidst expanding technology and broadening knowledge, exploitation of beneficial microbiota and their metabolites to restore pathophysiological balance will likely prove to be an extremely useful remedial tool. SummaryGut microbial metabolites produced proximal to intestinal barrier regulate numerous host responsive activities. Current review article highlights recent advances in the area of gut metabolites and their impact on gut barrier function as well as their potential translational applications in regulating various disorders. Gut microbial metabolites produced proximal to intestinal barrier regulate numerous host responsive activities. Current review article highlights recent advances in the area of gut metabolites and their impact on gut barrier function as well as their potential translational applications in regulating various disorders. Humans and microbes have coevolved over millions of years, thereby contributing to the interdependency of host and microbiota physiological activities.1Li M. Wang B. Zhang M. Rantalainen M. Wang S. Zhou H. Zhang Y. Shen J. Pang X. Zhang M. Wei H. Chen Y. Lu H. Zuo J. Su M. Qiu Y. Jia W. Xiao C. Smith L.M. Yang S. Holmes E. Tang H. Zhao G. Nicholson J.K. Li L. Zhao L. Symbiotic gut microbes modulate human metabolic phenotypes.Proc Natl Acad Sci U S A. 2008; 105: 2117-2122Crossref PubMed Scopus (755) Google Scholar Diverse interactive associations of host cells and microbes lead to mild to severe cellular and molecular responses depending on the status of host pathophysiological conditions. 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Microbiota significantly influence the functions of the intestinal barrier that separates internal organs from harmful entities including microorganisms, luminal antigens, and luminal proinflammatory factors. Intestinal barrier function is compromised (barrier dysfunction) in several disease conditions leading to an increased translocation of bacteria, endotoxins, and other inflammatory mediators. Gut barrier dysfunction is associated with systemic inflammatory response resulting in aggravation of the pathophysiological status of underlying diseases. Recent studies have shown a significant correlation between gut barrier dysfunction with various gastrointestinal disease conditions such as inflammatory bowel disease (IBD), irritable bowel syndrome (IBS), and celiac disease. 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Lactobacillus plantarum MB452 enhances the function of the intestinal barrier by increasing the expression levels of genes involved in tight junction formation.BMC Microbiol. 2010; 10: 316Crossref PubMed Scopus (210) Google Scholar Detailed mechanisms are yet to be elucidated, but knowledge concerning the functional activities of metabolites on gut barrier function and the immune system is expanding gradually. Exploiting microbial metabolites to restore gut barrier integrity will prove to be an extremely useful remedial tool against several pathophysiological conditions. In this review, we discuss how the intestinal microbiota and their metabolites reinforce gut barrier function via bi-directional interactions with gut epithelial cells. Further, we will discuss potential treatment strategies to mitigate gut barrier dysfunction in various disorders. The gut barrier is composed of 3 main interlinked/interdependent layers that provides a physical barrier against bacterial intrusion from the gut lumen. These include the luminal mucus layer, the gut epithelial layer formed by a continuous sheet of epithelial cells, and a third, internal layer that forms the mucosal immune system. The gut barrier acts as a physical and immunological defense against luminal microorganisms, viruses, food antigens, and environmental toxins. The barrier is selectively permeable to allow for translocation of essential dietary nutrients, electrolytes, amino acids, short-chain fatty acids (SCFAs), sugars, water, and select microbial metabolites from the intestinal lumen into the circulation. The gut epithelial barrier is composed of a single cell layer epithelial cells interspersed with functionally specialized differentiated epithelial cells. These include enterocytes, Paneth cells, goblet cells, tuft cells, enteroendocrine cells, and microfold cells, which together form a continuous and polarized monolayer leading to the separation of the lumen from lamina propria (Table 1). Among these, Paneth cells and microfold cells are only present in the small intestine, whereas enterocytes, goblet cells, and enteroendocrine and tuft cells are present in both the small intestine and colon, reviewed previously.25Peterson L.W. Artis D. Intestinal epithelial cells: regulators of barrier function and immune homeostasis.Nat Rev Immunol. 2014; 14: 141-153Crossref PubMed Scopus (1145) Google Scholar, 26Okumura R. Takeda K. Roles of intestinal epithelial cells in the maintenance of gut homeostasis.Exp Mol Med. 2017; 49: e338Crossref PubMed Google Scholar, 27Allaire J.M. Crowley S.M. Law H.T. Chang S.-Y. Ko H.-J. 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Mucins in the gastrointestinal tract in health and disease.Front Biosci. 2001; 6: D1321-D1357Crossref PubMed Google Scholar and different types of intestinal cells play important and distinct roles in maintaining gut barrier function and homeostasis.Table 1Cell Types in Intestinal Barrier and Their Role in Barrier FunctionCell typesRole in gut barrier functionEnterocytes (small intestine, colon)•Responsible for physical barrier through junctional protein complexes.•Nutrient absorption and metabolization.•Balance of epithelial shedding.•Secretion of antimicrobial agents.•Changes in expression/localization of junction proteins regulate gut barrier permeability.Paneth cells (small intestine)•Source of AMPs.•Directly can sense microbes and critical for gut homeostasis.•Paneth cell dysfunction triggers inflammation and gut barrier dysfunction.•Lack of Paneth cells leads to necrotizing enterocolitis both in humans and mice.Goblet cells (small intestine)•Produce and release MUCs, the mucus forming glycoprotein to maintain mucosal barrier.•Lack of MUC2 or O-glycan or N-glycosylation lead to severe gut barrier dysfunction and generate susceptibility to colitis.Tuft cells (small intestine, colon)•Secrete IL-25 leading to release of IL-13 from type 2 innate lymphoid cells (ILC2) to promote goblet cell hyperplasia and mucin production.•Detect helminth infection and expulse.Enteroendocrine cells (small intestine, colon)•Secret hormones such as GLP-2.•GLP-2 induces of TJ proteins such as ZO-1 and occludin and attenuates TNF-α–induced changes in TJ proteins in colon epithelial cells.•GLP-2 enhances epithelial cell wound healing in TGF-β–dependent manner.M cells (small intestine)•Antigen uptake.•M cell damage, during chronic inflammatory conditions elevates uptake of microorganisms amplifying the inflammatory condition leading to increased gut barrier dysfunction.AMP, antimicrobial peptide; GLP-2, glucagon-like peptide-2; M cell, microfold cell; MUC, mucin; TGF-β, transforming growth factor beta; TJ, tight junction; TNF-α, tumor necrosis factor alpha. Open table in a new tab AMP, antimicrobial peptide; GLP-2, glucagon-like peptide-2; M cell, microfold cell; MUC, mucin; TGF-β, transforming growth factor beta; TJ, tight junction; TNF-α, tumor necrosis factor alpha. Intestinal epithelial cells (IECs) allow for selective penetration of nutrients, water, and electrolytes while simultaneously excluding microbial pathogen-associated molecular pattern, toxins, and foreign antigens.30Kunzelmann K. Mall M. Electrolyte transport in the mammalian colon: mechanisms and implications for disease.Physiol Rev. 2002; 82: 245-289Crossref PubMed Google Scholar Enterocytes connect with each other in a continuous epithelial cell layer via adhesive junctional proteins that make up tight junction (TJ) proteins, adherens junction (AJ) proteins, gap junction proteins, and desmosomes (Table 2).31Niessen C.M. Tight junctions/adherens junctions: basic structure and function.J Invest Dermatol. 2007; 127: 2525-2532Abstract Full Text Full Text PDF PubMed Scopus (410) Google Scholar,32Groschwitz K.R. Hogan S.P. Intestinal barrier function: molecular regulation and disease pathogenesis.J Allergy Clin Immunol. 2009; 124 (quiz 21–22): 3-20Abstract Full Text Full Text PDF PubMed Scopus (766) Google Scholar These protein complexes not only mechanically secure extracellular cell to cell interactions, but also regulate intracellular adaptor protein–mediated interactions within gut epithelial cells. Epithelial cells tightly regulate paracellular (space between cells) and transcellular (through cell) permeability by posttranslational modifications of TJ proteins.33Van Itallie C.M. Anderson J.M. Claudins and epithelial paracellular transport.Annu Rev Physiol. 2006; 68: 403-429Crossref PubMed Scopus (837) Google Scholar, 34Fung K.Y.Y. Fairn G.D. Lee W.L. 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The scaffolding protein ZO-1 coordinates actomyosin and epithelial apical specializations in vitro and in vivo.J Biol Chem. 2018; 293: 17317-17335Abstract Full Text Full Text PDF PubMed Scopus (20) Google Scholar TJ seals are generally formed when the extracellular domains of transmembrane TJ proteins anastomose with adjacent cells. TJ proteins regulate transport molecules based on their size/charge through the paracellular space. Adherence junctions and desmosomes primarily contribute mechanical contacts between adjacent cells. The physiological structures, properties, and functions of the gut epithelial junctional protein complexes have been extensively reviewed elsewhere.43Garcia-Hernandez V. Quiros M. Nusrat A. Intestinal epithelial claudins: expression and regulation in homeostasis and inflammation.Ann N Y Acad Sci. 2017; 1397: 66-79Crossref PubMed Scopus (104) Google Scholar, 44Laukoetter M.G. Bruewer M. Nusrat A. 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Tight junctions as targets and effectors of mucosal immune homeostasis.Cell Mol Gastroenterol Hepatol. 2020; 10: 327-340Abstract Full Text Full Text PDF PubMed Scopus (0) Google ScholarTable 2Structural Components of Intestinal Epithelial Cells and Gut Barrier FunctionStructural componentsJunctional proteinsExamples of junctional protein–mediated intestinal barrier dysfunctionTight junction proteinsZO, occludin, claudins, tricellulin, JAM•IFN-γ and TNF-α mediated organization of several TJ proteins such as ZO-1, claudin-1, claudin-4, occluding, and JAM-A downregulate intestinal epithelial barrier function.•Downregulation of claudin-3, claudin-4, claudin-5, and claudin-8 and increased claudin-2 expression and MLCK phosphorylation are reported to be associated with gut barrier function.Adherens junction proteinsCadherins, catenins•Downregulated E-cadherin–catenin complex mediates the impairment of the integrity of mucosal barrier.DesmosomeDesmoglein, desmocollins•Desmoglein 2 (Dsg2) deficiency leads to the loss of intestinal epithelial barrier integrity.Gap junctionsConnexin•Connexin-43 plays a role in intercellular communication mediated by extracellular vesicles, tunneling nanotubes and gap junctions.IFN-γ, interferon gamma; JAM, junctional adhesion molecules; MLCK, myosin light-chain kinase; TNF-α, tumor necrosis factor alpha; ZO, zonula occludens. Open table in a new tab IFN-γ, interferon gamma; JAM, junctional adhesion molecules; MLCK, myosin light-chain kinase; TNF-α, tumor necrosis factor alpha; ZO, zonula occludens. The functional or physical disruption of TJ, AJ, and gap junction proteins and desmosomes leads to increased gut permeability causing dysregulated translocation/transportation of inflammatory mediators, which potentially manifests as chronic gut inflammation. Increased inflammation further perpetuates the disruption of TJ proteins leading to increased permeability. The following sections will describe gut barrier dysfunction and impact of gut microbiota and their metabolites in the regulation of the gut barrier and its altered state in various inflammatory disorders. Defective gut epithelial barrier function in combination with immune dysregulation is associated with several gastrointestinal tract–related disorders including but not limited to IBD, IBS, drug-induced toxicity, and colon cancer. Increased gut barrier permeability occurs through dysregulation of epithelial apoptosis/enterocyte death, mucus degradation, and increased paracellular permeability due to disruption of TJs. These pathways are independently regulated and functional consequences are distinct from each other. The following section describes the mechanisms and potential factors responsible for barrier dysfunction. As described in previous sections, TJ proteins are responsible for paracellular transport and selective gut barrier permeability. It was reported that solutes and water cross TJs through 2 distinct pathways based on their size and charge selectivity: the pore pathway48Zuo L. Kuo W.T. Turner J.R. Tight junctions as targets and effectors of mucosal immune homeostasis.Cell Mol Gastroenterol Hepatol. 2020; 10: 327-340Abstract Full Text Full Text PDF PubMed Scopus (0) Google Scholar, 49Turner J.R. 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Occludin OCEL-domain interactions are required for maintenance and regulation of the tight junction barrier to macromolecular flux.Mol Biol Cell. 2013; 24: 3056-3068Crossref PubMed Scopus (94) Google Scholar The pore pathway is exclusively size and charge selective and excludes molecules with a diameter ≤8 Å and is a high-conductance route. CLDN-2 as well as CLDNs 10a, 10b, 15, 16, and 17 were shown to play critical role in the pore pathway. In contrast to the pore pathway, the leak pathway allows macromolecule flux with an exclusion limit of ∼100 Å with lower conductance.51Buschmann M.M. Shen L. Rajapakse H. Raleigh D.R. Wang Y. Wang Y. Lingaraju A. Zha J. Abbott E. McAuley E.M. Breskin L.A. Wu L. Anderson K. Turner J.R. Weber C.R. Occludin OCEL-domain interactions are required for maintenance and regulation of the tight junction barrier to macromolecular flux.Mol Biol Cell. 2013; 24: 3056-3068Crossref PubMed Scopus (94) Google Scholar The leak pathway is believed to be regulated by myosin light-chain kinase (MLCK), where constitutively active MLCK is sufficient to increase the leak pathway–dependent permeability both in vitro and in vivo.52Shen L. Black E.D. Witkowski E.D. Lencer W.I. Guerriero V. Schneeberger E.E. Turner J.R. Myosin light chain phosphorylation regulates barrier function by remodeling tight junction structure.J Cell Sci. 2006; 119: 2095-2106Crossref PubMed Scopus (308) Google Scholar,53Su L. Shen L. Clayburgh D.R. Nalle S.C. Sullivan E.A. Meddings J.B. Abraham C. Turner J.R. Targeted epithelial tight junction dysfunction causes immune activation and contributes to development of experimental colitis.Gastroenterology. 2009; 136: 551-563Abstract Full Text Full Text PDF PubMed Scopus (273) Google Scholar It was reported that MLCK, OCLN, and ZO-1 like TJ proteins play an important role in regulation of the leak pathway.54Yu A.S. McCarthy K.M. Francis S.A. McCormack J.M. Lai J. Rogers R.A. Lynch R.D. Schneeberger E.E. Knockdown of occludin expression leads to diverse phenotypic alterations in epithelial cells.Am J Physiol Cell Physiol. 2005; 288: C1231-C1241Crossref PubMed Scopus (239) Google Scholar Detailed discussions of paracellular permeability and potential mechanisms have been published.48Zuo L. Kuo W.T. Turner J.R. Tight junctions as targets and effectors of mucosal immune homeostasis.Cell Mol Gastroenterol Hepatol. 2020; 10: 327-340Abstract Full Text Full Text PDF PubMed Scopus (0) Google Scholar,49Turner J.R. Intestinal mucosal barrier function in health and disease.Nat Rev Immunol. 2009; 9