ADP-heptose: a bacterial PAMP detected by the host sensor ALPK1

The β-d-manno-heptoses are immune agonists across kingdoms

Bacterial small molecule metabolites such as adenosine-diphosphate-d-glycero-β-d-manno-heptose (ADP-heptose) and their derivatives act as effective innate immune agonists in mammals. We show that functional nucleotide-diphosphate-heptose biosynthetic enzymes (HBEs) are distributed widely in bacteria, archaea, eukaryotes, and viruses. We identified a conserved STTR5 motif as a hallmark of heptose nucleotidyltransferases that can synthesize not only ADP-heptose but also cytidine-diphosphate (CDP)- and uridine-diphosphate (UDP)-heptose. Both CDP- and UDP-heptoses are agonists that trigger stronger alpha-protein kinase 1 (ALPK1)-dependent immune responses than ADP-heptose in human and mouse cells and mice. We also produced ADP-heptose in archaea and verified its innate immune agonist functions. Hence, the β-d-manno-heptoses are cross-kingdom, small-molecule, pathogen-associated molecular patterns that activate the ALPK1-dependent innate immune signaling cascade.

Characterization of the ADP-β-D-manno-heptose biosynthetic enzymes from two pathogenic Vibrio strains

ADP-activated β-D-manno-heptoses (ADP-β-D-manno-heptoses) are precursors for the biosynthesis of the inner core of lipopolysaccharide in Gram-negative bacteria. Recently, ADP-D-glycero-β-D-manno-heptose (ADP-D,D-manno-heptose) and its C-6'' epimer, ADP-L-glycero-β-D-manno-heptose (ADP-L,D-manno-heptose), were identified as potent pathogen-associated molecular patterns (PAMPs) that can trigger robust innate immune responses. Although the production of ADP-D,D-manno-heptose has been studied in several different pathogenic Gram-negative bacteria, current knowledge of ADP-β-D-manno-heptose biosynthesis in Vibrio strains remains limited. Here, we characterized the biosynthetic enzymes of ADP-D,D-manno-heptose and the epimerase that converts it to ADP-L,D-manno-heptose from Vibrio cholerae (the causative agent of pandemic cholera) and Vibrio parahaemolyticus (non-cholera pathogen causing vibriosis with clinical manifestations of gastroenteritis and wound infections) in comparison with their isozymes from Escherichia coli. Moreover, we discovered that β-D-mannose 1-phosphate, but not α-D-mannose 1-phosphate, could be activated to its ADP form by the nucleotidyltransferase domains of bifunctional kinase/nucleotidyltransferases HldEVC (from V. cholerae) and HldEVP (from V. parahaemolyticus). Kinetic analyses of the nucleotidyltransferase domains of HldEVC and HldEVP together with the E. coli-derived HldEEC were thus carried out using β-D-mannose 1-phosphate as a mimic sugar substrate. Overall, our works suggest that V. cholerae and V. parahaemolyticus are capable of synthesizing ADP-β-D-manno-heptoses and lay a foundation for further physiological function explorations on manno-heptose metabolism in Vibrio strains. KEY POINTS: • Vibrio strains adopt the same biosynthetic pathway as E. coli in synthesizing ADP-β-D-manno-heptoses. • HldEs from two Vibrio strains and E. coli could activate β-D-mannose 1-phosphate to ADP-β-D-mannose. • Comparable nucleotidyltransfer efficiencies were observed in the kinetic studies of HldEs.

The Therapeutic Role and Mechanism of Glabridin Under Aspergillus fumigatus Infection

Purpose: To characterize the efficiency of glabridin alone and in combination with clinical antifungals in Aspergillus fumigatus keratitis. Methods: The broth microdilution method was performed to investigate whether glabridin exerted an antifungal role on planktonic cells and immature and mature biofilm. Antifungal mechanism was evaluated by Sorbitol and Ergosterol Assays. The synergistic effect of glabridin and antifungals was assessed through the checkerboard microdilution method and time-killing test. Regarding anti-inflammatory role, inflammatory substances induced by A. fumigatus were assessed by real-time quantitative polymerase chain reaction, western blot, and enzyme-linked immunosorbent assay. Drug toxicity was assessed by Draize test in vivo. Macrophage phenotypes were examined by flow cytometry. Results: Regarding antifungal activity, glabridin destroyed fungal cell wall and membrane on planktonic cells and suppressed immature and mature biofilm formation. After combining with natamycin or amphotericin B, glabridin possessed a potent synergistic effect against A. fumigatus. Regarding anti-inflammatory aspects, Dectin-1, toll‑like receptor (TLR)-2 and TLR-4 expression of human corneal epithelial cells were significantly elevated after A. fumigatus challenge and reduced by glabridin. The elevated expression of interleukin-1β and tumor necrosis factor-alpha induced by A. fumigatus or corresponding agonists were reversed by glabridin, equivalent to the effect of corresponding inhibitors. Glabridin could also contribute to anti-inflammation by downregulating inflammatory mediator expression to suppress macrophage infiltration. Conclusions: Glabridin contributed to fungal clearance by destroying fungal cell wall and membrane, and disrupting biofilm. Combining glabridin with clinical antifungals was superior in reducing A. fumigatus growth. Glabridin exerted an anti-inflammatory effect by downregulating proinflammatory substance expression and inhibiting macrophage infiltration, which provide a potential agent and treatment strategies for fungal keratitis.

Pathogenicity and virulence of Francisella tularensis

Tularaemia is a zoonotic disease caused by the Gram-negative bacterium, Francisella tularensis. Depending on its entry route into the organism, F. tularensis causes different diseases, ranging from life-threatening pneumonia to less severe ulceroglandular tularaemia. Various strains with different geographical distributions exhibit different levels of virulence. F. tularensis is an intracellular bacterium that replicates primarily in the cytosol of the phagocytes. The main virulence attribute of F. tularensis is the type 6 secretion system (T6SS) and its effectors that promote escape from the phagosome. In addition, F. tularensis has evolved a peculiar envelope that allows it to escape detection by the immune system. In this review, we cover tularaemia, different Francisella strains, and their pathogenicity. We particularly emphasize the intracellular life cycle, associated virulence factors, and metabolic adaptations. Finally, we present how F. tularensis largely escapes immune detection to be one of the most infectious and lethal bacterial pathogens.

Masking of typical TLR4 and TLR5 ligands modulates inflammation and resolution by Helicobacter pylori

Helicobacter pylori is a paradigm of chronic bacterial infection and is associated with peptic ulceration and malignancies. H. pylori uses specific masking mechanisms to avoid canonical ligands from activating Toll-like receptors (TLRs), such as lipopolysaccharide (LPS) modification and specific flagellin sequences that are not detected by TLR4 and TLR5, respectively. Thus, it was believed for a long time that H. pylori evades TLR recognition as a crucial strategy for immune escape and bacterial persistence. However, recent data indicate that multiple TLRs are activated by H. pylori and play a role in the pathology. Remarkably, H. pylori LPS, modified through changes in acylation and phosphorylation, is mainly sensed by other TLRs (TLR2 and TLR10) and induces both pro- and anti-inflammatory responses. In addition, two structural components of the cag pathogenicity island-encoded type IV secretion system (T4SS), CagL and CagY, were shown to contain TLR5-activating domains. These domains stimulate TLR5 and enhance immunity, while LPS-driven TLR10 signaling predominantly activates anti-inflammatory reactions. Here, we discuss the specific roles of these TLRs and masking mechanisms during infection. Masking of typical TLR ligands combined with evolutionary shifting to other TLRs is unique for H. pylori and has not yet been described for any other species in the bacterial kingdom. Finally, we highlight the unmasked T4SS-driven activation of TLR9 by H. pylori, which mainly triggers anti-inflammatory responses.

Innate activation of human neutrophils and neutrophil-like cells by the pro-inflammatory bacterial metabolite ADP-heptose and Helicobacter pylori

Lipopolysaccharide inner core heptose metabolites, including ADP-heptose, play a substantial role in the activation of cell-autonomous innate immune responses in eukaryotic cells, via the ALPK1-TIFA signaling pathway, as demonstrated for various pathogenic bacteria. The important role of LPS heptose metabolites during Helicobacter pylori infection of the human gastric niche has been demonstrated for gastric epithelial cells and macrophages, while the role of heptose metabolites on human neutrophils has not been investigated. In this study, we aimed to gain a better understanding of the activation potential of bacterial heptose metabolites for human neutrophil cells. To do so, we used pure ADP-heptose and, as a bacterial model, H. pylori, which can transport heptose metabolites into the human host cell via the Cag Type 4 Secretion System (CagT4SS). Main questions were how bacterial heptose metabolites impact on the pro-inflammatory activation, alone and in the bacterial context, and how they influence maturation of human neutrophils. Results of the present study demonstrated that neutrophils respond with high sensitivity to pure heptose metabolites, and that global regulation networks and neutrophil maturation are influenced by heptose exposure. Furthermore, activation of human neutrophils by live H. pylori is strongly impacted by the presence of LPS heptose metabolites and the functionality of its CagT4SS. Similar activities were determined in cell culture neutrophils of different maturation states and in human primary neutrophils. In conclusion, we demonstrated that specific heptose metabolites or bacteria producing heptoses exhibit a strong activity on cell-autonomous innate responses of human neutrophils.

Helicobacter pylori Modulates Heptose Metabolite Biosynthesis and Heptose-Dependent Innate Immune Host Cell Activation by Multiple Mechanisms

Heptose metabolites including ADP-d-glycero-β-d-manno-heptose (ADP-heptose) are involved in bacterial lipopolysaccharide and cell envelope biosynthesis. Recently, heptoses were also identified to have potent proinflammatory activity on human cells as novel microbe-associated molecular patterns. The gastric pathogenic bacterium Helicobacter pylori produces heptose metabolites, which it transports into human cells through its Cag type 4 secretion system. Using H. pylori as a model, we have addressed the question of how proinflammatory ADP-heptose biosynthesis can be regulated by bacteria. We have characterized the interstrain variability and regulation of heptose biosynthesis genes and the modulation of heptose metabolite production by H. pylori, which impact cell-autonomous proinflammatory human cell activation. HldE, a central enzyme of heptose metabolite biosynthesis, showed strong sequence variability between strains and was also variably expressed between strains. Amounts of gene transcripts in the hldE gene cluster displayed intrastrain and interstrain differences, were modulated by host cell contact and the presence of the cag pathogenicity island, and were affected by carbon starvation regulator A (CsrA). We reconstituted four steps of the H. pylori lipopolysaccharide (LPS) heptose biosynthetic pathway in vitro using recombinant purified GmhA, HldE, and GmhB proteins. On the basis of one- and two-dimensional nuclear magnetic resonance (NMR) spectroscopy and mass spectrometry, the structures of major reaction products were identified as β-d-ADP-heptose and β-heptose-1-monophosphate. A proinflammatory heptose-monophosphate variant was also identified for the first time as a novel cell-active product in H. pylori bacteria. Separate purified HldE subdomains and variant HldE allowed us to uncover additional strain variation in generating heptose metabolites. IMPORTANCE Bacterial heptose metabolites, intermediates of lipopolysaccharide (LPS) biosynthesis, are novel microbe-associated molecular patterns (MAMPs) that activate proinflammatory signaling. In the gastric pathogen Helicobacter pylori, heptoses are transferred into host cells by the Cag type IV secretion system, which is also involved in carcinogenesis. Little is known about how H. pylori, which is highly strain variable, regulates heptose biosynthesis and downstream host cell activation. We report here that the regulation of proinflammatory heptose production by H. pylori is strain specific. Heptose gene cluster activity is modulated by the presence of an active cag pathogenicity island (cagPAI), contact with human cells, and the carbon starvation regulator A. Reconstitution with purified biosynthesis enzymes and purified bacterial lysates allowed us to biochemically characterize heptose pathway products, identifying a heptose-monophosphate variant as a novel proinflammatory metabolite. These findings emphasize that the bacteria use heptose biosynthesis to fine-tune inflammation and also highlight opportunities to mine the heptose biosynthesis pathway as a potential therapeutic target against infection, inflammation, and cancer.

MvfR Controls Tolerance to Polymyxin B by Regulating rfaD in Pseudomonas aeruginosa

Polymyxins are currently the last-resort antibiotics for the treatment of multidrug-resistant Gram-negative bacterial infections. To expand the understanding of the intrinsic resistance mechanism against polymyxins, a laboratory strain of Pseudomonas aeruginosa PAO1 was subjected to serial passage in the presence of sublethal doses of polymyxin B over a period of 30 days. By whole-genome sequencing of successively isolated polymyxin B-resistant isolates, we identified a frameshift mutation (L183fs) in the mvfR gene that further increased polymyxin resistance in the pmrB mutant background. A ΔmvfR mutation alone showed higher tolerance to polymyxin B due to altered lipopolysaccharide (LPS) on the surface of bacterial cells, which decreases its outer membrane permeability. In the ΔmvfR mutant, polymyxin B treatment caused the upregulation of rfaD, the gene involved in LPS core oligosaccharide synthesis, which is responsible for polymyxin tolerance. To the best of our knowledge, this is the first report of mvfR mutation conferring polymyxin resistance in P. aeruginosa via increased integrity of bacterial outer membrane. IMPORTANCE Antibiotic resistance imposes a considerable challenge for the treatment of P. aeruginosa infections. Polymyxins are the last-resort antibiotics for the treatment of multidrug-resistant P. aeruginosa infections. Understanding the development and mechanisms of bacterial resistance to polymyxins may provide clues for the development of new or improved therapeutic strategies effective against P. aeruginosa. In this study, using an in vitro evolution assay in combination with whole-genome sequencing, we demonstrated that MvfR controls tolerance to polymyxin B by regulating the rfaD gene in P. aeruginosa. Our results reveal a novel mechanism employed by P. aeruginosa in the defense against polymyxin antibiotics.

A Bittersweet Kiss of Gram-Negative Bacteria: The Role of ADP-Heptose in the Pathogenesis of Infection

Due to the global crisis caused by the dramatic rise of drug resistance among Gram-negative bacteria, there is an urgent need for a thorough understanding of the pathogenesis of infections of such an etiology. In light of the limited availability of new antibiotics, therapies aimed at host-pathogen interactions emerge as potential treatment modalities. Thus, understanding the mechanism of pathogen recognition by the host and immune evasion appear to be the key scientific issues. Until recently, lipopolysaccharide (LPS) was recognized as a major pathogen-associated molecular pattern (PAMP) of Gram-negative bacteria. However, recently, ADP-L-glycero-β-D-manno-heptose (ADP-heptose), an intermediate carbohydrate metabolite of the LPS biosynthesis pathway, was discovered to activate the hosts' innate immunity. Therefore, ADP-heptose is regarded as a novel PAMP of Gram-negative bacteria that is recognized by the cytosolic alpha kinase-1 (ALPK1) protein. The conservative nature of this molecule makes it an intriguing player in host-pathogen interactions, especially in the context of changes in LPS structure or even in its loss by certain resistant pathogens. Here, we present the ADP-heptose metabolism, outline the mechanisms of its recognition and the activation of its immunity, and summarize the role of ADP-heptose in the pathogenesis of infection. Finally, we hypothesize about the routes of the entry of this sugar into cytosol and point to emerging questions that require further research.

The Role of Microbiota-Derived Metabolites in Colorectal Cancer

The impact of bacterial members of the microbiota on the development of colorectal cancer (CRC) has become clear in recent years. However, exactly how bacteria contribute to the development of cancer is often still up for debate. The impact of bacteria-derived metabolites, which can influence the development of CRC either in a promoting or inhibiting manner, is undeniable. Here, we discuss the effects of the most well-studied bacteria-derived metabolites associated with CRC, including secondary bile acids, short-chain fatty acids, trimethylamine-N-oxide and indoles. We show that the effects of individual metabolites on CRC development are often nuanced and dose- and location-dependent. In the coming years, the array of metabolites involved in CRC development will undoubtedly increase further, which will emphasize the need to focus on causation and mechanisms and the clearly defined roles of bacterial species within the microbiota.

Alpha-kinase1 promotes tubular injury and interstitial inflammation in diabetic nephropathy by canonical pyroptosis pathway

BACKGROUND

Alpha-kinase 1 (ALPK1) is a master regulator in inflammation and has been proved to promote renal fibrosis by promoting the production of IL-1β in diabetic nephropathy (DN) mice. Pyroptosis is involved in high glucose (HG)-induced tubular cells injury, characterized by activation of Gasdermin D (GSDMD) and the release of IL-1β and IL-18, resulting in inflammatory injury in DN. It is reasonable to assume that ALPK1 is involved in pyroptosis-related tubular injury in DN. However, the mechanism remains poorly defined.

METHODS

Immunohistochemistry (IHC) staining was performed to detect the expression of pyroptosis- and fibrosis-related proteins in renal sections of DN patients and DN mice. DN models were induced through injection of streptozotocin combined with a high-fat diet. Protein levels of ALPK1, NF-κB, Caspase-1, GSDMD, IL-1β, IL-18 and α-SMA were detected by Western blot. HK-2 cells treated with high-glucose (HG) served as an in vitro model. ALPK1 small interfering RNA (siRNA) was transfected into HK-2 cells to down-regulate ALPK1. The pyroptosis rates were determined by flow cytometry. The concentrations of IL-1β and IL-18 were evaluated by ELISA kits. Immunofluorescence staining was used to observe translocation of NF-κB and GSDMD.

RESULTS

The heat map of differentially expressed genes showed that ALPK1, Caspase-1 and GSDMD were upregulated in the DN group. The expression levels of ALPK1, Caspase-1, GSDMD and CD68 were increased in renal biopsy tissues of DN patients by IHC. ALPK1expression and CD68⁺ macrophages were positively correlated with tubular injury in DN patients. Western blot analysis showed increased expressions of ALPK1, phospho-NF-κB P65, GSDMD-NT, and IL-1β in renal tissues of DN mice and HK-2 cells, accompanied with increased renal fibrosis-related proteins (FN, α-SMA) and macrophages infiltration in interstitial areas. Inhibition of ALPK1 attenuated HG-induced upregulation expressions of NF-κB, pyroptosis-related proteins Caspase-1, GSDMD-NT, IL-1β, IL-18, α-SMA, and pyroptosis level in HK-2 cells. Also, the intensity and nuclear translocation of NF-κB and membranous translocation of GSDMD were ameliorated in HG-treated HK-2 cells after treatment with ALPK1 siRNA.

CONCLUSIONS

Our data suggest that ALPK1/NF-κB pathway initiated canonical caspase-1-GSDMD pyroptosis pathway, resulting in tubular injury and interstitial inflammation of DN.

ALPK1 Accelerates the Pathogenesis of Osteoarthritis by Activating NLRP3 Signaling

Alpha-kinase 1 (ALPK1), a member of the alpha-kinase family, has been shown to be involved in mediating inflammatory responses and is strongly associated with gout; however, its modulatory role in osteoarthritis (OA) remains unclear. Here, we uncovered elevation of ALPK1 in degraded cartilage of destabilized medial meniscus (DMM) and collagenase-induced osteoarthritis (CIOA), two different mouse OA models induced by mechanical stress or synovitis. Intraarticular administration of recombinant human ALPK1 (rhALPK1) in vivo exacerbated OA pathogenesis in both DMM and CIOA mice, whereas ALPK1 knockout reversed this process. In vitro study demonstrated that ALPK1 aggravates metabolic disturbances in chondrocytes by enhancing the production of NOD-like receptor protein 3 (NLRP3), an inflammasome sensors driving interlukin-1β (IL-1β)-mediated inflammatory conditions. Furthermore, the selective inhibition of nuclear factor-κB (NF-κB) or NLRP3 indicates that NLRP3 is a downstream signaling governed by NF-κB in ALPK1-activated chondrocytes. Collectively, these results establish ALPK1 as a novel catabolic regulator of OA pathogenesis, and targeting this signaling may be a promising treatment strategy for OA. © 2022 American Society for Bone and Mineral Research (ASBMR).

The ADP-Heptose Biosynthesis Enzyme GmhB is a Conserved Gram-Negative Bacteremia Fitness Factor

Klebsiella pneumoniae is a leading cause of Gram-negative bacteremia, which is a major source of morbidity and mortality worldwide. Gram-negative bacteremia requires three major steps: primary site infection, dissemination to the blood, and bloodstream survival. Because K. pneumoniae is a leading cause of health care-associated pneumonia, the lung is a common primary infection site leading to secondary bacteremia. K. pneumoniae factors essential for lung fitness have been characterized, but those required for subsequent bloodstream infection are unclear. To identify K. pneumoniae genes associated with dissemination and bloodstream survival, we combined previously and newly analyzed insertion site sequencing (InSeq) data from a murine model of bacteremic pneumonia. This analysis revealed the gene gmhB as important for either dissemination from the lung or bloodstream survival. In Escherichia coli, GmhB is a partially redundant enzyme in the synthesis of ADP-heptose for the lipopolysaccharide (LPS) core. To characterize its function in K. pneumoniae, an isogenic knockout strain (ΔgmhB) and complemented mutant were generated. During pneumonia, GmhB did not contribute to lung fitness and did not alter normal immune responses. However, GmhB enhanced bloodstream survival in a manner independent of serum susceptibility, specifically conveying resistance to spleen-mediated killing. In a tail-vein injection of murine bacteremia, GmhB was also required by K. pneumoniae, E. coli, and Citrobacter freundii for optimal fitness in the spleen and liver. Together, this study identifies GmhB as a conserved Gram-negative bacteremia fitness factor that acts through LPS-mediated mechanisms to enhance fitness in blood-filtering organs.

Helicobacter pylori-induced NF-κB: trailblazer for gastric pathophysiology

NF-κB signaling pathways, induced by a variety of triggers, play a key role in regulating the expression of genes involved in the immune response and cellular responses to stress. The human pathogen Helicobacter pylori induces classical and alternative NF-κB signaling pathways via its effector ADP-L-glycero-β-D-manno-heptose (ADP-heptose). We review H. pylori- and NF-κB-dependent alterations in cellular processes and associated maladaptation leading to deleterious gastric pathophysiology that have implications for the diagnosis and treatment of gastric diseases. Therapeutic options for gastric cancer (GC) include clinically relevant small molecule inhibitors of NF-κB and epigenetic therapy approaches. In this context, gastric organoid biobanks originated from patient material, represent a valuable platform for translational applications to predict patient responses to chemotherapy, with a view to personalized medicine.

Pathogen-Associated Molecular Patterns: The Synthesis of Heptose Phosphates and Derivatives

Lipopolysaccharide biosynthesis metabolites, such as d-glycero-β-d-manno-heptopyranosyl 1,7-diphosphate, d-glycero-β-d-manno-heptopyranosyl phosphate, and adenosine 5′-(l-glycero-β-d-manno-heptopyranosyl)diphosphate, have been found to activate NF-κB via alpha-kinase 1 and TRAF-interacting protein with forkhead associated domain. This axis has been determined as a novel pathway of innate immunity yet to be targeted for immunomodulatory treatment approaches. Key in understanding this new axis has been the ability to synthesize these metabolites. The design of synthetic analogues and probes have also been published not only to design new drugs, but also to gain insight into the mechanism of action for these compounds. The focus of the present review is the synthesis of heptose phosphate metabolites­ as well as synthetic analogues and probes.

1 Introduction

2 Synthesis of d-glycero-d-manno-Heptose

2.1 Using d-Mannose as Starting Material

2.2 Using d-Ribose as Starting Material

2.3 Using 2,2-Dimethyl-1,3-dioxan-5-one as Starting Material

3 Synthesis of l-glycero-d-manno-Heptose

3.1 Using d-Mannose as Starting Material

3.2 Using 2,2-Dimethyl-1,3-dioxan-5-one as Starting Material

3.3 Using l-Lyxose as Starting Material

4 Synthesis of Heptose Phosphates

4.1 Synthesis of d-glycero-β-d-manno-Heptose 1,7-Diphosphate

4.2 Synthesis of Heptose Phosphate Derivatives

4.2.1 Development of Scaffolds for Conjugation

4.2.2 Development of Heptose Phosphates Derivatives for Cell Intake and Metabolic Stability

5 Conclusion and Outlook

ADP-heptose enables Helicobacter pylori to exploit macrophages as a survival niche by suppressing antigen-presenting HLA-II expression

The persistence of Helicobacter pylori in the human gastric mucosa implies that the immune response fails to clear the infection. We found that H. pylori compromises the antigen presentation ability of macrophages, because of the decline of the presenting molecules HLA-II. Here, we reveal that the main bacterial factor responsible for this effect is ADP-heptose, an intermediate metabolite in the biosynthetic pathway of lipopolysaccharide (LPS) that elicits a pro-inflammatory response in gastric epithelial cells. In macrophages, it upregulates the expression of miR146b which, in turn, would downmodulate CIITA, the master regulator for HLA-II genes. Hence, H. pylori, utilizing ADP-heptose, exploits a specific arm of macrophage response to establish its survival niche in the face of the immune defense elicited in the gastric mucosa.

A Journey from Structure to Function of Bacterial Lipopolysaccharides

Lipopolysaccharide (LPS) is a crucial constituent of the outer membrane of most Gram-negative bacteria, playing a fundamental role in the protection of bacteria from environmental stress factors, in drug resistance, in pathogenesis, and in symbiosis. During the last decades, LPS has been thoroughly dissected, and massive information on this fascinating biomolecule is now available. In this Review, we will give the reader a third millennium update of the current knowledge of LPS with key information on the inherent peculiar carbohydrate chemistry due to often puzzling sugar residues that are uniquely found on it. Then, we will drive the reader through the complex and multifarious immunological outcomes that any given LPS can raise, which is strictly dependent on its chemical structure. Further, we will argue about issues that still remain unresolved and that would represent the immediate future of LPS research. It is critical to address these points to complete our notions on LPS chemistry, functions, and roles, in turn leading to innovative ways to manipulate the processes involving such a still controversial and intriguing biomolecule.

Comparative Analysis of Colon Cancer-Derived Fusobacterium nucleatum Subspecies: Inflammation and Colon Tumorigenesis in Murine Models

Fusobacteria are commonly associated with human colorectal cancer (CRC), but investigations are hampered by the absence of a stably colonized murine model. Further, Fusobacterium nucleatum subspecies isolated from human CRC have not been investigated. While F. nucleatum subspecies are commonly associated with CRC, their ability to induce tumorigenesis and contributions to human CRC pathogenesis are uncertain. We sought to establish a stably colonized murine model and to understand the inflammatory potential and virulence genes of human CRC F. nucleatum, representing the 4 subspecies, animalis, nucleatum, polymorphum, and vincentii. Five human CRC-derived and two non-CRC derived F. nucleatum strains were tested for colonization, tumorigenesis, and cytokine induction in specific-pathogen-free (SPF) and/or germfree (GF) wild-type and ApcMin/+ mice, as well as in vitro assays and whole-genome sequencing (WGS). SPF wild-type and ApcMin/+ mice did not achieve stable colonization with F. nucleatum, whereas certain subspecies stably colonized some GF mice but without inducing colon tumorigenesis. F. nucleatum subspecies did not form in vivo biofilms or associate with the mucosa in mice. In vivo inflammation was inconsistent across subspecies, whereas F. nucleatum induced greater cytokine responses in a human colorectal cell line, HCT116. While F. nucleatum subspecies displayed genomic variability, no distinct virulence genes associated with human CRC strains were identified that could reliably distinguish these strains from non-CRC clinical isolates. We hypothesize that the lack of F. nucleatum-induced tumorigenesis in our model reflects differences in human and murine biology and/or a synergistic role for F. nucleatum in concert with other bacteria to promote carcinogenesis. IMPORTANCE Colon cancer is a leading cause of cancer morbidity and mortality, and it is hypothesized that dysbiosis in the gut microbiota contributes to colon tumorigenesis. Fusobacterium nucleatum, a member of the oropharyngeal microbiome, is enriched in a subset of human colon tumors. However, it is unclear whether this genetically varied species directly promotes tumor formation, modulates mucosal immune responses, or merely colonizes the tumor microenvironment. Mechanistic studies to address these questions have been stymied by the lack of an animal model that does not rely on daily orogastric gavage. Using multiple murine models, in vitro assays with a human colon cancer cell line, and whole-genome sequencing analysis, we investigated the proinflammatory and tumorigenic potential of several F. nucleatum clinical isolates. The significance of this research is development of a stable colonization model of F. nucleatum that does not require daily oral gavages in which we demonstrate that a diverse library of clinical isolates do not promote tumorigenesis.