Neuraminidase 1-mediated desialylation of the mucin 1 ectodomain releases a decoy receptor that protects against Pseudomonas aeruginosa lung infection

Stenotrophomonas maltophilia provokes NEU1-mediated release of a flagellin-binding decoy receptor that protects against lethal infection

Stenotrophomonas maltophilia (Sm), a multidrug-resistant pathogen often isolated from immunocompromised individuals, presents its flagellin to multimeric tandem repeats within the ectodomain of mucin-1 (MUC1-ED), expressed on airway epithelia. Flagellated Sm increases neuraminidase-1 (NEU1) sialidase association with and desialylation of MUC1-ED. This NEU1-mediated MUC1-ED desialylation unmasks cryptic binding sites for Sm flagellin, increasing flagellin and Sm binding to airway epithelia. MUC1 overexpression increases receptor number whereas NEU1 overexpression elevates receptor binding affinity. Silencing of either MUC1 or NEU1 reduces the flagellin-MUC1 interaction. Sm/flagellin provokes MUC1-ED autoproteolysis at a juxtamembranous glycine-serine peptide bond. MUC1-ED shedding from the epithelium not only occurs in vitro, but in the bronchoalveolar compartments of Sm/flagellin-challenged mice and patients with ventilator-associated Sm pneumonia. Finally, the soluble flagellin-targeting, MUC1-ED decoy receptor dose-dependently inhibits multiple Sm flagellin-driven pathogenic processes, in vitro, including motility, biofilm formation, adhesion, and proinflammatory cytokine production, and protects against lethal Sm lung infection, in vivo.

Infection and the Glycome─New Insights into Host Response

Glycans play critical roles in the host-pathogen interactions leading to infection. However, we still understand very little about the dynamic nature of glycosylation in response to infection and its function in modulating host immunity. Many of the host proteins involved in immune defense are glycoproteins. Furthermore, the innate immune system recognizes glycans. The glycoform of a protein can impact proteolytic stability, receptor interactions, serum half-life, and other aspects. New, cutting-edge chemical biology tools are shedding light on the interplay between infection and the host glycome. In this review, we highlight new work on the importance of dynamic glycosylation of host proteins in the innate and adaptive immune pathways in response to infection. These include recent findings on altered glycoprofiles of mucins, complement components, and antibodies.

Bulky glycocalyx drives cancer invasiveness by modulating substrate-specific adhesion

The majority of the eukaryotic cell surface is decorated with a layer of membrane-attached polysaccharides and glycoproteins collectively referred to as the glycocalyx. While the formation of a bulky glycocalyx has been associated with the cancer progression, the mechanisms by which the glycocalyx regulates cancer invasiveness are incompletely understood. We address this question by first documenting subtype-specific expression of the major glycocalyx glycoprotein Mucin-1 (MUC1) in breast cancer patient samples and breast cancer cell lines. Strikingly, glycocalyx disruption led to inhibition of 2D motility, loss of 3D invasion, and reduction of clonal scattering in breast cancer cells at the population level. Tracking of 2D cell motility and 3D invasiveness of MUC1-based sorted subpopulations revealed the fastest motility and invasiveness in intermediate MUC1-expressing cells, with glycocalyx disruption abolishing these effects. While differential sensitivity in 2D motility is attributed to a nonmonotonic dependence of focal adhesion size on MUC1 levels, higher MUC1 levels enhance 3D invasiveness via increased traction generation. In contrast to inducing cell rounding on collagen-coated substrates, high MUC1 level promotes cell adhesion and confers resistance to shear flow on substrates coated with the endothelial surface protein E-selectin. Collectively, our findings illustrate how MUC1 drives cancer invasiveness by differentially regulating cell-substrate adhesion in a substrate-dependent manner.

The role of sialidase Neu1 in respiratory diseases

Neu1 is a sialidase enzyme that plays a crucial role in the regulation of glycosylation in a variety of cellular processes, including cellular signaling and inflammation. In recent years, numerous evidence has suggested that human NEU1 is also involved in the pathogenesis of various respiratory diseases, including lung infection, chronic obstructive pulmonary disease (COPD), asthma, and pulmonary fibrosis. This review paper aims to provide an overview of the current research on human NEU1 and respiratory diseases.

Morphological and metabolic changes in microglia exposed to cadmium: Cues on neurotoxic mechanisms

Microglial cells play a key role in protecting the central nervous system from pathogens and toxic compounds and are involved in the pathogenesis of different neurodegenerative diseases. Cadmium is a widespread toxic heavy metal, released into the environment at a rate of 30,000 tons/year by anthropogenic activities; it is easily uptaken by the human body through diet and cigarette smoke, as well as by occupational exposure. Once inside the body, cadmium enters the cells and substitutes to zinc and other divalent cations altering many biological functions. Its extremely long half-life makes it a serious health threat. Recent data suggest a role for heavy metals in many neurodegenerative diseases; however, the role of cadmium is still to be elucidated. In this work we report the investigation of cadmium toxicity towards murine BV2 microglial cells, a widely used model for the study of neurodegeneration. Results show that increasing cadmium concentrations increase oxidative stress, a proposed mechanism of neurodegeneration, but also that BV2 cells can keep oxidative stress under control by increasing glutathione reduction. Moreover, cadmium induces alterations of cell morphology and metabolism leading to mitochondrial impairment, without switching the cells to Warburg effect. Finally cadmium induces the release of proinflammatory cytokines, but does not markedly switch BV2 cells to M1 phenotype.

Immunomodulatory Properties of Vitamin D in the Intestinal and Respiratory Systems

Vitamin D plays a crucial role in modulating the innate immune response by interacting with its intracellular receptor, VDR. In this review, we address vitamin D/VDR signaling and how it contributes to the regulation of intestinal and respiratory microbiota. We additionally review some components of the innate immune system, such as the barrier function of the pulmonary and intestinal epithelial membranes and secretion of mucus, with their respective modulation by vitamin D. We also explore the mechanisms by which this vitamin D/VDR signaling mounts an antimicrobial response through the transduction of microbial signals and the production of antimicrobial peptides that constitute one of the body’s first lines of defense against pathogens. Additionally, we highlight the role of vitamin D in clinical diseases, namely inflammatory bowel disease and acute respiratory distress syndrome, where excessive inflammatory responses and dysbiosis are hallmarks. Increasing evidence suggests that vitamin D supplementation may have potentially beneficial effects on those diseases.

Sialidase Inhibitors with Different Mechanisms

Sialidases, or neuraminidases, are enzymes that catalyze the hydrolysis of sialic acid (Sia)-containing molecules, mostly removal of the terminal Sia (desialylation). By desialylation, sialidase can modulate the functionality of the target compound and is thus often involved in biological pathways. Inhibition of sialidases with inhibitors is an important approach for understanding sialidase function and the underlying mechanisms and could serve as a therapeutic approach as well. Transition-state analogues, such as anti-influenza drugs oseltamivir and zanamivir, are major sialidase inhibitors. In addition, difluoro-sialic acids were developed as mechanism-based sialidase inhibitors. Further, fluorinated quinone methide-based suicide substrates were reported. Sialidase product analogue inhibitors were also explored. Finally, natural products have shown competitive inhibiton against viral, bacterial, and human sialidases. This Perspective describes sialidase inhibitors with different mechanisms and their activities and future potential, which include transition-state analogue inhibitors, mechanism-based inhibitors, suicide substrate inhibitors, product analogue inhibitors, and natural product inhibitors.

Mammalian Neuraminidases in Immune-Mediated Diseases: Mucins and Beyond

Mammalian neuraminidases (NEUs), also known as sialidases, are enzymes that cleave off the terminal neuraminic, or sialic, acid resides from the carbohydrate moieties of glycolipids and glycoproteins. A rapidly growing body of literature indicates that in addition to their metabolic functions, NEUs also regulate the activity of their glycoprotein targets. The simple post-translational modification of NEU protein targets-removal of the highly electronegative sialic acid-affects protein folding, alters protein interactions with their ligands, and exposes or covers proteolytic sites. Through such effects, NEUs regulate the downstream processes in which their glycoprotein targets participate. A major target of desialylation by NEUs are mucins (MUCs), and such post-translational modification contributes to regulation of disease processes. In this review, we focus on the regulatory roles of NEU-modified MUCs as coordinators of disease pathogenesis in fibrotic, inflammatory, infectious, and autoimmune diseases. Special attention is placed on the most abundant and best studied NEU1, and its recently discovered important target, mucin-1 (MUC1). The role of the NEU1 - MUC1 axis in disease pathogenesis is discussed, along with regulatory contributions from other MUCs and other pathophysiologically important NEU targets.

MUC1 ectodomain is a flagellin-targeting decoy receptor and biomarker operative during Pseudomonas aeruginosa lung infection

We previously reported that flagellin-expressing Pseudomonas aeruginosa (Pa) provokes NEU1 sialidase-mediated MUC1 ectodomain (MUC1-ED) desialylation and MUC1-ED shedding from murine lungs in vivo. Here, we asked whether Pa in the lungs of patients with ventilator-associated pneumonia might also increase MUC1-ED shedding. The levels of MUC1-ED and Pa-expressed flagellin were dramatically elevated in bronchoalveolar lavage fluid (BALF) harvested from Pa-infected patients, and each flagellin level, in turn, predicted MUC1-ED shedding in the same patient. Desialylated MUC1-ED was only detected in BALF of Pa-infected patients. Clinical Pa strains increased MUC1-ED shedding from cultured human alveolar epithelia, and FlaA and FlaB flagellin-expressing strains provoked comparable levels of MUC1-ED shedding. A flagellin-deficient isogenic mutant generated dramatically reduced MUC1-ED shedding compared with the flagellin-expressing wild-type strain, and purified FlaA and FlaB recapitulated the effect of intact bacteria. Pa:MUC1-ED complexes were detected in the supernatants of alveolar epithelia exposed to wild-type Pa, but not to the flagellin-deficient Pa strain. Finally, human recombinant MUC1-ED dose-dependently disrupted multiple flagellin-driven processes, including Pa motility, Pa biofilm formation, and Pa adhesion to human alveolar epithelia, while enhancing human neutrophil-mediated Pa phagocytosis. Therefore, shed desialylated MUC1-ED functions as a novel flagellin-targeting, Pa-responsive decoy receptor that participates in the host response to Pa at the airway epithelial surface.

The sialidase NEU1 directly interacts with the juxtamembranous segment of the cytoplasmic domain of mucin-1 to inhibit downstream PI3K-Akt signaling

The extracellular domain (ED) of the membrane-spanning sialoglycoprotein, mucin-1 (MUC1), is an in vivo substrate for the lysosomal sialidase, neuraminidase-1 (NEU1). Engagement of the MUC1-ED by its cognate ligand, Pseudomonas aeruginosa-expressed flagellin, increases NEU1-MUC1 association and NEU1-mediated MUC1-ED desialylation to unmask cryptic binding sites for its ligand. However, the mechanism(s) through which intracellular NEU1 might physically interact with its surface-expressed MUC1-ED substrate are unclear. Using reciprocal coimmunoprecipitation and in vitro binding assays in a human airway epithelial cell system, we show here that NEU1 associates with the MUC1-cytoplasmic domain (CD) but not with the MUC1-ED. Prior pharmacologic inhibition of the NEU1 catalytic activity using the NEU1-selective sialidase inhibitor, C9-butyl amide-2-deoxy-2,3-dehydro-N-acetylneuraminic acid, did not diminish NEU1-MUC1-CD association. In addition, glutathione-S-transferase (GST) pull-down assays using the deletion mutants of the MUC1-CD mapped the NEU1-binding site to the membrane-proximal 36 aa of the MUC1-CD. In a cell-free system, we found that the purified NEU1 interacted with the immobilized GST-MUC1-CD and the purified MUC1-CD associated with the immobilized 6XHis-NEU1, indicating that the NEU1-MUC1-CD interaction was direct and independent of its chaperone protein, protective protein/cathepsin A. However, the NEU1-MUC1-CD interaction was not required for the NEU1-mediated MUC1-ED desialylation. Finally, we demonstrated that overexpression of either WT NEU1 or a catalytically dead NEU1 G68V mutant diminished the association of the established MUC1-CD binding partner, PI3K, to MUC1-CD and reduced downstream Akt kinase phosphorylation. These results indicate that NEU1 associates with the juxtamembranous region of the MUC1-CD to inhibit PI3K-Akt signaling independent of NEU1 catalytic activity.

[Biofilm Eradication Four-Step Strategy: Study of Using Self-Assembled Azithromycin/Rhamnolipid Nanoparticles for Removing Pseudomonas aeruginosa Biofilm]

OBJECTIVE

To investigate the in vitro eradicative effect of self-assembled azithromycin/rhamnolipid nanoparticles (AZI-RHL NPs) on P seudomonas aeruginosa ( P. aeruginosa) biofilm.

METHODS

AZI-RHL NPs were prepared and characterized. The minimum inhibitory concentration (MIC) of AZI-RHL NPs on planktonic P. aeruginosa was measured by the broth microdilution method. The eradicative effect of AZI-RHL NPs on P. aeruginosa biofilm was evaluated via crystal violet staining and SYTO 9/PI live/dead staining. Fluorescence labeling was used to measure the eradicative effect of NPs on extracellular polymeric substances (EPS). In addition, crystal violet staining was performed to evaluate the inhibitory effect of AZI-RHL NPs on the adhesion of P. aeruginosa on human bronchial epithelial BEAS-2B cells. To investigate the ability of AZI-RHL NPs to penetrate mucus, the interaction between NPs and mucin was measured via particle size changes after co-incubation with mucin solution.

RESULTS

The AZI-RHL NPs had a particle size of about 121 nm and were negatively charged on the surface, displaying a high encapsulation efficiency and a high drug loading capacity of 96.72% and 45.08% for AZI, respectively and 99.38% and 53.07% for RHL, respectively. The MIC of AZI-RHL NPs on planktonic P. aeruginosa was half of that of using AZI alone. AZI-RHL NPs displayed the capacity to effectively destroy the biofilm structure and remove the proteins and polysaccharides in EPS, eradicating biofilms in addition to reducing the survival rate of bacteria in the biofilm. AZI-RHL NPs were shown to have inhibited P. aeruginosa adhesion on BEAS-2B cells and prevented the residual bacteria from forming a new biofilm. There was no significant change in the particle size of NPs after co-incubation with mucin solution, indicating a weak interaction between NPs and mucin, and suggesting that NPs could penetrate the mucus and reach the P. aeruginosa infection sites.

CONCLUSION

AZI-RHL NPs were able to effectively enhance the removal of P. aeruginosa biofilm through a four-step strategy of biofilm eradication, including penetrating the mucus, disintegrating the biofilm structure, killing the bacteria dispersed from biofilm, and preventing the adhesion of residual bacteria. We hope that this study will provide a replicable common strategy for the treatment of refractory infections caused by P. aeruginosa and other types of biofilms.

Decoy Technology as a Promising Therapeutic Tool for Atherosclerosis

Cardiovascular diseases (CVDs) have been classified into several types of disease, of which atherosclerosis is the most prevalent. Atherosclerosis is characterized as an inflammatory chronic disease which is caused by the formation of lesions in the arterial wall. Subsequently, lesion progression and disruption ultimately lead to heart disease and stroke. The development of atherosclerosis is the underlying cause of approximately 50% of all deaths in westernized societies. Countless studies have aimed to improve therapeutic approaches for atherosclerosis treatment; however, it remains high on the global list of challenges toward healthy and long lives. Some patients with familial hypercholesterolemia could not get intended LDL-C goals even with high doses of traditional therapies such as statins, with many of them being unable to tolerate statins because of the harsh side effects. Furthermore, even in patients achieving target LDL-C levels, the residual risk of traditional therapies is still significant thus highlighting the necessity of ongoing research for more effective therapeutic approaches with minimal side effects. Decoy-based drug candidates represent an opportunity to inhibit regulatory pathways that promote atherosclerosis. In this review, the potential roles of decoys in the treatment of atherosclerosis were described based on the in vitro and in vivo findings.

The host glycomic response to pathogens

Glycans play important roles in the biology of infectious diseases. Although glycans are expressed on both the pathogens and the host, the functions and dynamics of the host glycome during infection are not well understood. Recent years have witnessed new discoveries on the host glycome respsonse to infection, as well as related mechanisms and their implications. Herein, we present a brief review on the latest findings in this field and put them in the context of host immunity.

Therapeutic Effect of Neuraminidase-1-Selective Inhibition in Mouse Models of Bleomycin-Induced Pulmonary Inflammation and Fibrosis

Pulmonary fibrosis remains a serious biomedical problem with no cure and an urgent need for better therapies. Neuraminidases (NEUs), including NEU1, have been recently implicated in the mechanism of pulmonary fibrosis by us and others. We now have tested the ability of a broad-spectrum neuraminidase inhibitor, 2,3-dehydro-2-deoxy-N-acetylneuraminic acid (DANA), to modulate the in vivo response to acute intratracheal bleomycin challenge as an experimental model of pulmonary fibrosis. A marked alleviation of bleomycin-induced body weight loss and notable declines in accumulation of pulmonary lymphocytes and collagen deposition were observed. Real-time polymerase chain reaction analyses of human and mouse lung tissues and primary human lung fibroblast cultures were also performed. A predominant expression and pronounced elevation in the levels of NEU1 mRNA were observed in patients with idiopathic pulmonary fibrosis and bleomycin-challenged mice compared with their corresponding controls, whereas NEU2, NEU3, and NEU4 were expressed at far lower levels. The levels of mRNA for the NEU1 chaperone, protective protein/cathepsin A (PPCA), were also elevated by bleomycin. Western blotting analyses demonstrated bleomycin-induced elevations in protein expression of both NEU1 and PPCA in mouse lungs. Two known selective NEU1 inhibitors, C9-pentyl-amide-DANA (C9-BA-DANA) and C5-hexanamido-C9-acetamido-DANA, dramatically reduced bleomycin-induced loss of body weight, accumulation of pulmonary lymphocytes, and deposition of collagen. Importantly, C9-BA-DANA was therapeutic in the chronic bleomycin exposure model with no toxic effects observed within the experimental timeframe. Moreover, in the acute bleomycin model, C9-BA-DANA attenuated NEU1-mediated desialylation and shedding of the mucin-1 ectodomain. These data indicate that NEU1-selective inhibition offers a potential therapeutic intervention for pulmonary fibrotic diseases. SIGNIFICANCE STATEMENT: Neuraminidase-1-selective therapeutic targeting in the acute and chronic bleomycin models of pulmonary fibrosis reverses pulmonary collagen deposition, accumulation of lymphocytes in the lungs, and the disease-associated loss of body weight-all without observable toxic effects. Such therapy is as efficacious as nonspecific inhibition of all neuraminidases in these models, thus indicating the central role of neuraminidase-1 as well as offering a potential innovative, specifically targeted, and safe approach to treating human patients with a severe malady: pulmonary fibrosis.

Defensive Properties of Mucin Glycoproteins during Respiratory Infections-Relevance for SARS-CoV-2

Mucus plays a pivotal role in protecting the respiratory tract against microbial infections. It acts as a primary contact site to entrap microbes and facilitates their removal from the respiratory tract via the coordinated beating of motile cilia. The major components of airway mucus are heavily O-glycosylated mucin glycoproteins, divided into gel-forming mucins and transmembrane mucins. The gel-forming mucins MUC5AC and MUC5B are the primary structural components of airway mucus, and they enable efficient clearance of pathogens by mucociliary clearance. MUC5B is constitutively expressed in the healthy airway, whereas MUC5AC is upregulated in response to inflammatory challenge. MUC1, MUC4, and MUC16 are the three major transmembrane mucins of the respiratory tracts which prevent microbial invasion, can act as releasable decoy receptors, and activate intracellular signal transduction pathways. Pathogens have evolved virulence factors such as adhesins that facilitate interaction with specific mucins and mucin glycans, for example, terminal sialic acids. Mucin expression and glycosylation are dependent on the inflammatory state of the respiratory tract and are directly regulated by proinflammatory cytokines and microbial ligands. Gender and age also impact mucin glycosylation and expression through the female sex hormone estradiol and age-related downregulation of mucin production. Here, we discuss what is currently known about the role of respiratory mucins and their glycans during bacterial and viral infections of the airways and their relevance for the novel coronavirus severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Understanding the impact of microbe-mucin interaction in the respiratory tract could inspire the development of novel therapies to boost mucosal defense and combat respiratory infections.

Burkholderia cenocepacia-host cell contact controls the transcription activity of the trimeric autotransporter adhesin BCAM2418 gene

Cell-to-cell early contact between pathogens and their host cells is required for the establishment of many infections. Among various surface factors produced by bacteria that allow an organism to become established in a host, the class of adhesins is a primary determinant. Burkholderia cenocepacia adheres to the respiratory epithelium of cystic fibrosis patients and causes chronic inflammation and disease. Cell-to-cell contacts are promoted by various kinds of adhesins, including trimeric autotransporter adhesins (TAAs). We observed that among the 7 TAA genes found in the B. cenocepacia K56-2 genome, two of them (BCAM2418 and BCAS0236) express higher levels of mRNA following physical contact with host cells. Further analysis revealed that the B. cenocepacia K56-2 BCAM2418 gene shows an on-off switch after an initial colonization period, exhibits a strong expression dependent on the host cell type, and enhances its function on cell adhesion. Furthermore, our analysis revealed that adhesion to mucin-coated surfaces dramatically increases the expression levels of BCAM2418. Abrogation of mucin O-glycans turns BCAM2418 gene expression off and impairs bacterial adherence. Overall, our findings suggest that glycosylated extracellular components of host membrane might be a binding site for B. cenocepacia and a signal for the differential expression of the TAA gene BCAM2418.

Role of the mucins in pathogenesis of COPD: implications for therapy

Introduction: Evidence accumulated in the last decade has started to reveal the enormous complexity in the expression, interactions and functions of the large number of different mucins present in the different compartments of the human lower airways. This occurs both in normal subjects and in COPD patients in different clinical phases and stages of severity.Areas covered: We review the known physiological mechanisms that regulate mucin production in human lower airways of normal subjects, the changes in mucin synthesis/secretion in COPD patients and the clinical efficacy of drugs that modulate mucin synthesis/secretion.Expert opinion: It is evident that the old simplistic concept that mucus hypersecretion in COPD patients is associated with negative clinical outcomes is not valid and that the therapeutic potential of 'mucolytic drugs' is under-appreciated due to the complexity of the associated molecular network(s). Likewise, our current knowledge of the effects of the drugs already available on the market that target mucin synthesis/secretion/structure in the lower airways is extremely limited and often indirect and more well-controlled clinical trials are needed in this area.