Most, if not all, animals coexist with a complement of prokaryotic symbionts that confer a variety of physiologic benefits. In humans, the interaction between animal and bacterial cells is especially ...important in the gastrointestinal tract. Technical and conceptual advances have enabled rapid progress in characterizing the taxonomic composition, metabolic capacity, and immunomodulatory activity of the human gut microbiota, allowing us to establish its role in human health and disease. The human host coevolved with a normal microbiota over millennia and developed, deployed, and optimized complex immune mechanisms that monitor and control this microbial ecosystem. These cellular mechanisms have homeostatic roles beyond the traditional concept of defense against potential pathogens, suggesting these pathways contribute directly to the well-being of the gut. During their coevolution, the bacterial microbiota has established multiple mechanisms to influence the eukaryotic host, generally in a beneficial fashion, and maintain their stable niche. The prokaryotic genomes of the human microbiota encode a spectrum of metabolic capabilities beyond that of the host genome, making the microbiota an integral component of human physiology. Gaining a fuller understanding of both partners in the normal gut-microbiota interaction may shed light on how the relationship can go awry and contribute to a spectrum of immune, inflammatory, and metabolic disorders and may reveal mechanisms by which this relationship could be manipulated toward therapeutic ends.
Reactive oxygen species (ROS) are a chemical class of molecules that have generally been conceptualized as deleterious entities, albeit ones whose destructive properties could be harnessed as ...antimicrobial effector functions to benefit the whole organism. This appealingly simplistic notion has been turned on its head in recent years with the discovery of the NADPH oxidases, or Noxes, a family of enzymes dedicated to the production of ROS in a variety of cells and tissues. The Nox-dependent, physiological generation of ROS is highly conserved across virtually all multicellular life, often as a generalized response to microbes and/or other exogenous stressors. This review discusses the current knowledge of the role of physiologically generated ROS and the enzymes that form them in both normal biology and disease.
Enzymes with intracellular activity have significant potential to treat diseases. Protein nanoparticles (NPs) considerably enhance intracellular delivery of enzymes. We have previously shown that a ...Salmonella effector enzyme, AvrA, delivered by NPs is capable of modulating inflammatory signals in a murine dextran sulfate sodium (DSS) colitis model. The NPs were instilled intrarectally, limiting delivery to the distal colon. Localized intestinal delivery of protein therapeutics via the oral route is a highly attractive alternative approach. However, the harsh conditions in the gastrointestinal tract can severely reduce protein function. The approach described here is to deliver therapeutic protein NPs encapsulated within gastro-protective microparticles (MPs) made from alginate and chitosan that subsequently release NPs in the small intestine and colon. A flow focusing microfluidic device was used to form alginate droplets encapsulating protein NPs. Droplets were then simultaneously crosslinked with calcium and coated with chitosan. Protein NPs encapsulated within crosslinked alginate/chitosan MPs were protected and retained their activity after incubation in simulated gastric fluid (SGF). Subsequent incubation in simulated intestinal fluid (SIF) induced release of bioactive protein NPs. Oral administration of AvrA NPs encapsulated in alginate/chitosan MPs delivered protein to intestinal epithelia and reduced clinical and histological scores of inflammation in a murine DSS-induced colitis model. Altogether, NPs in alginate/chitosan MPs are a potential oral delivery vehicle for protein therapeutics.
Inflammatory bowel disease (IBD) affects 6.8 million people globally. A variety of factors have been implicated in IBD pathogenesis, including host genetics, immune dysregulation and gut microbiota ...alterations. Emerging evidence implicates intestinal epithelial glycosylation as an underappreciated process that interfaces with these three factors. IBD is associated with increased expression of truncated O-glycans as well as altered expression of terminal glycan structures. IBD genes, glycosyltransferase mislocalization, altered glycosyltransferase and glycosidase expression and dysbiosis drive changes in the glycome. These glycan changes disrupt the mucus layer, glycan-lectin interactions, host-microorganism interactions and mucosal immunity, and ultimately contribute to IBD pathogenesis. Epithelial glycans are especially critical in regulating the gut microbiota through providing bacterial ligands and nutrients and ultimately determining the spatial organization of the gut microbiota. In this Review, we discuss the regulation of intestinal epithelial glycosylation, altered epithelial glycosylation in IBD and mechanisms for how these alterations contribute to disease pathobiology. We hope that this Review provides a foundation for future studies on IBD glycosylation and the emergence of glycan-inspired therapies for IBD.
The microbiota that inhabits the mammalian intestine can influence a range of physiological functions, including the modulation of immune responses, enhancement epithelial barrier function, and the ...stimulation of cell proliferation. While the mechanisms by which commensal prokaryotes stimulate immune signaling networks are well-characterized, less is known about the mechanistic control over homeostatic pathways within tissues. Recent reports by our research group have demonstrated that contact between the gut epithelia and some groups of enteric commensal bacteria prompts the rapid generation of reactive oxygen species (ROS) within host cells. Whereas the bacterial-induced production of ROS in phagocytes in response to ligand binding to Formyl Peptide Receptors (FPRs) and ensuing activation of NADPH oxidase 2 (Nox2) is a well-defined mechanism, ROS generated by other cell types such as intestinal epithelia in response to microbial signals via FPRs and the NADPH oxidase 1 (Nox1) is less appreciated. Importantly, enzymatically generated ROS have been shown to function as second messengers in many signal transduction pathways via the transient oxidative activity on sensor proteins bearing oxidant-sensitive thiol groups. Examples of redox sensitive proteins include tyrosine phosphatases that serve as regulators of MAPK pathways, focal adhesion kinase, as well as components involved NF-kB activation. Here, we review the leading edge discoveries gleaned from investigations that focus on microbial-induced generation of ROS and their functional effects on host physiology. These studies identify the functional molecular elements and mechanistic events that mediate the established effects of the normal microbiota on intestinal physiology.
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•Bacterial contact with host epithelial cells can result in redox signaling.•ROS is produced by mechanisms involving formyl peptide receptors and NAPDH oxidases.•Distinct bacterial taxa have different abilities to stimulate redox signaling.•Redox signaling is involved in cytoprotection, cellular motility and proliferation.
Abstract
The microbiota that occupies the mammalian intestine can modulate a range of physiological functions, including control over immune responses, epithelial barrier function, and cellular ...proliferation. While commensal prokaryotic organisms are well known to stimulate inflammatory signaling networks, less is known about control over homeostatic pathways. Recent work has shown that gut epithelia contacted by enteric commensal bacteria rapidly generate reactive oxygen species (ROS). While the induced production of ROS in professional phagocytes via stimulation of formyl peptide receptors (FPRs) and activation of NADPH oxidase 2 (Nox2) is a well-studied process, ROS are also similarly elicited in other cell types, including intestinal epithelia, in response to microbial signals via FPRs and the epithelial NADPH oxidase 1 (Nox1). ROS generated by Nox enzymes have been shown to function as critical second messengers in multiple signal transduction pathways via the rapid and transient oxidative inactivation of a distinct class of sensor proteins bearing oxidant-sensitive thiol groups. These redox-sensitive proteins include tyrosine phosphatases that serve as regulators of MAP kinase pathways, focal adhesion kinase, as well as components involved in NF-κB activation. As microbe-elicited ROS has been shown to stimulate cellular proliferation and motility, and to modulate innate immune signaling, we hypothesize that many of the established effects of the normal microbiota on intestinal physiology may be at least partially mediated by this ROS-dependent mechanism.
It is known that the gut microbiota, the numerically vast and taxonomically diverse microbial communities that thrive in a symbiotic fashion within our alimentary tract, can affect the normal ...physiology of the gastrointestinal tract and liver. Further, disturbances of the microbiota community structure from both endogenous and exogenous influences as well as the failure of host responsive mechanisms have been implicated in a variety of disease processes. Mechanistically, alterations in intestinal permeability and dysbiosis of the microbiota can result in inflammation, immune activation, and exposure to xenobiotic influences. Additionally, the gut and liver are continually exposed to small molecule products of the microbiota with proinflammatory, gene regulatory, and oxidative properties. Long-term coevolution has led to tolerance and incorporation of these influences into normal physiology and homeostasis; conversely, changes in this equilibrium from either the host or the microbial side can result in a wide variety of immune, inflammatory, metabolic, and neoplastic intestinal and hepatic disorders.
Many studies have suggested a role for gut-resident microbes (the “gut microbiome”) in modulating host health; however, the mechanisms by which they impact systemic physiology remain largely unknown. ...In this study, metabolomic and transcriptional profiling of germ-free and conventionalized mouse liver revealed an upregulation of the Nrf2 antioxidant and xenobiotic response in microbiome-replete animals. Using a Drosophila-based screening assay, we identified members of the genus Lactobacillus capable of stimulating Nrf2. Indeed, the human commensal Lactobacillus rhamnosus GG (LGG) potently activated Nrf2 in the Drosophila liver analog and the murine liver. This activation was sufficient to protect against two models of oxidative liver injury, acetaminophen overdose and acute ethanol toxicity. Characterization of the portal circulation of LGG-treated mice by tandem mass spectrometry identified a small molecule activator of Nrf2, 5-methoxyindoleacetic acid, produced by LGG. Taken together, these data demonstrate a mechanism by which intestinal microbes modulate hepatic susceptibility to oxidative injury.
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•The gut microbiome induces the Nrf2 antioxidant response pathway in the liver•Gut-resident Lactobacilli induce hepatic Nrf2 in both Drosophila and mice•Oral delivery of Lactobacillus rhamnosus GG protects against oxidative liver injury•Lactobacilli-derived 5-methoxyindoleacetic acid activates Nrf2
Saeedi et al. demonstrate that the gut microbiome acts at a distance to activate host antioxidant responses in the liver. These responses can be amplified by exogenous administration of Lactobacilli and protect against oxidative liver injury. Finally, they identify a Lactobacilli-derived small molecule that mediates, in part, these effects.
It is now well understood that the eukaryotic host has evolved multiple mechanisms to monitor and respond to the diverse and biochemically active microbiota that thrives in a symbiotic fashion in the ...gut and other tissues. Generally, these mechanisms are based on traditional notions of innate and adaptive immune processes, which are mediated by recognition of, and response to, microbially derived macromolecules. Microbes themselves are metabolically active and contribute a vast array of small molecules, not present in germ-free model systems, with diverse putative and unknown biological function, and intensive work is ongoing to unravel their roles in physiological systems. Metazoans have evolved and maintain distinct gene regulatory networks to detect and respond to environmental, non-self-molecules (xenobiotics), and interestingly, recent investigation has shown that these pathways are operational in the detection and response to microbiota-derived small metabolites. These processes likely represent a general mechanism of host-microbe crosstalk, and they have clinical implications in drug and xenobiotic metabolism.
An optimal gut microbiota influences many beneficial processes in the metazoan host. However, the molecular mechanisms that mediate and function in symbiont-induced host responses have not yet been ...fully characterized. Here, we report that cellular ROS enzymatically generated in response to contact with lactobacilli in both mice and Drosophila has salutary effects against exogenous insults to the intestinal epithelium via the activation of Nrf2 responsive cytoprotective genes. These data show that the xenobiotic-inducible Nrf2 pathway participates as a signaling conduit between the prokaryotic symbiont and the eukaryotic host. Indeed, our data imply that the capacity of lactobacilli to induce redox signaling in epithelial cells is a highly conserved hormetic adaptation to impel cellular conditioning to exogenous biotic stimuli. These data also highlight the role the microbiota plays in eukaryotic cytoprotective pathways and may have significant implications in the characterization of a eubiotic microbiota.
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•Commensal lactobacilli activate the xenobiotic responsive Nrf2 pathway•Nrf2 pathway activation upregulates cytoprotective genes in both Drosophila and mice•Nrf2 target genes protect against environmental stresses•Lactobacilli transduce cytoprotective effects via Nrf2 pathway activation
Jones et al. report that the commensal gut bacterial taxa lactobacilli are able to mediate beneficial cytoprotective effects in the gut of both flies and mice. These highly conserved events are mediated by ROS-dependent activation of the Nrf2 xenobiotic pathway and conserved effector genes.
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