Mucins and Their Role in Shaping the Functions of Mucus Barriers

The Relationship between Mucins and Ulcerative Colitis: A Systematic Review

Mucins are a family of glycosylated proteins which are the primary constituents of mucus and play a dynamic role in the regulation of the protective mucosal barriers throughout the human body. Ulcerative colitis (UC) is an Inflammatory Bowel Disease (IBD) characterised by continuous inflammation of the inner layer of the large intestine, and in this systematic review we analyse currently available data to determine whether alterations exist in mucin activity in the colonic mucosa of UC patients. Database searches were conducted to identify studies published between 1990 and 2020 that assess the role of mucins in cohorts of UC patients, where biopsy specimens were resected for analysis and control groups were included for comparison. 5497 articles were initially identified and of these 14 studies were systematically selected for analysis, a further 2 articles were identified through citation chaining. Therefore, 16 studies were critically reviewed. 13 of these studies assessed the role of MUC2 in UC and the majority of articles indicated that alterations in MUC2 structure or synthesis had an impact on the colonic mucosa, although conflicting results were presented regarding MUC2 expression. This review highlights the importance of further research to enhance our understanding of mucin regulation in UC and summarises data that may inform future studies.

Long-term evolution of the epithelial cell secretome in preclinical 3D models of the human bronchial epithelium

The human bronchial epithelium is the first line of defense against atmospheric particles, pollutants, and respiratory pathogens such as the novel SARS-CoV-2. The epithelial cells form a tight barrier and secrete proteins that are major components of the mucosal immune response. Functional in vitro models of the human lung are essential for screening the epithelial response and assessing the toxicity and barrier crossing of drugs, inhaled particles, and pollutants. However, there is a lack of models to investigate the effect of chronic exposure without resorting to animal testing. Here, we developed a 3D model of the human bronchial epithelium using Calu-3 cell line and demonstrated its viability and functionality for 21 days without subculturing. We investigated the effect of reduced Fetal Bovine Serum supplementation in the basal medium and defined the minimal supplementation needed to maintain a functional epithelium, so that the amount of exogenous serum proteins could be reduced during drug testing. The long-term evolution of the epithelial cell secretome was fully characterized by quantitative mass spectrometry in two preclinical models using Calu-3 or primary NHBE cells. 408 common secreted proteins were identified while significant differences in protein abundance were observed with time, suggesting that 7-10 days are necessary to establish a mature secretome in the Calu-3 model. The associated Reactome pathways highlight the role of the secreted proteins in the immune response of the bronchial epithelium. We suggest this preclinical 3D model can be used to evaluate the long-term toxicity of drugs or particles on the human bronchial epithelium, and subsequently to investigate their effect on the epithelial cell secretions.

Exploring the interplay of mucin with biologically-relevant amorphous magnesium-calcium phosphate nanoparticles

HYPOTHESIS

It has been recently shown that, in our organism, the secretions of Ca²⁺, Mg²⁺ and phosphate ions lead to the precipitation of amorphous magnesium-calcium phosphate nanoparticles (AMCPs) in the small intestine, where the glycoprotein mucin is one of the most abundant proteins, being the main component of the mucus hydrogel layer covering gut epithelium. Since AMCPs precipitate in vivo in a mucin-rich environment, we aim at studying the effect of this glycoprotein on the formation and features of endogenous-like AMCPs.

EXPERIMENTS

AMCPs were synthesized from aqueous solution in the presence of different concentrations of mucin, and the obtained particles were characterised in terms of crystallinity, composition and morphology. Solid State NMR investigation was also performed in order to assess the interplay between mucin and AMCPs at a sub-nanometric level.

FINDING

Results show that AMCPs form in the presence of mucin and the glycoprotein is efficiently incorporated in the amorphous particles. NMR indicates the existence of interactions between AMCPs and mucin, revealing how AMCPs in mucin-hybrid nanoparticles affect the features of both proteic and oligosaccharidic portions of the glycoprotein. Considering that the primary function of mucin is the protection of the intestine from pathogens, we speculate that the nature of the interaction between AMCPs and mucin described in the present work might be relevant to the immune system, suggesting a novel type of scenario which could be investigated by combining physico-chemical and biomedical approaches.

Photoinactivation of Staphylococci with 405 nm Light in a Trachea Model with Saliva Substitute at 37 °C

The globally observed rise in bacterial resistance against antibiotics has increased the need for alternatives to antibiotic treatments. The most prominent and important pathogen bacteria are the ESKAPE pathogens, which include among others Staphylococcus aureus, Klebsiella pneumoniae and Acinetobacter baumannii. These species cause ventilator-associated pneumonia (VAP), which accounts for 24% of all nosocomial infections. In this study we tested the efficacy of photoinactivation with 405 nm violet light under conditions comparable to an intubated patient with artificial saliva for bacterial suspension at 37 °C. A technical trachea model was developed to investigate the visible light photoinactivation of Staphylococcus carnosus as a non-pathogen surrogate of the ESKAPE pathogen S. aureus (MRSA). The violet light was coupled into the tube with a fiber optic setup. The performed tests proved, that photoinactivation at 37 °C is more effective with a reduction of almost 3 log levels (99.8%) compared to 25 °C with a reduction of 1.2 log levels. The substitution of phosphate buffered saline (PBS) by artificial saliva solution slightly increased the efficiency during the experimental course. The increased efficiency might be caused by a less favorable environment for bacteria due to for example the ionic composition.

Home, sweet home: how mucus accommodates our microbiota

As a natural environment for human-microbiota interactions, healthy mucus houses a remarkably stable and diverse microbial community. Maintaining this microbiota is essential to human health, both to support the commensal bacteria that perform a wide array of beneficial functions and to prevent the outgrowth of pathogens. However, how the host selects and maintains a specialized microbiota remains largely unknown. In this viewpoint, we propose several strategies by which mucus may regulate the composition and function of the human microbiota and discuss how compromised mucus barriers in disease can give rise to microbial dysbiosis.

Nanoparticle Delivery Systems with Cell-Specific Targeting for Pulmonary Diseases

Respiratory disorders are among the most important medical problems threatening human life. The conventional therapeutics for respiratory disorders are hindered by insufficient drug concentrations at pathological lesions, lack of cell-specific targeting, and various biobarriers in the conducting airways and alveoli. To address these critical issues, various nanoparticle delivery systems have been developed to serve as carriers of specific drugs, DNA expression vectors, and RNAs. The unique properties of nanoparticles, including controlled size and distribution, surface functional groups, high payload capacity, and drug release triggering capabilities, are tailored to specific requirements in drug/gene delivery to overcome major delivery barriers in pulmonary diseases. To avoid off-target effects and improve therapeutic efficacy, nanoparticles with high cell-targeting specificity are essential for successful nanoparticle therapies. Furthermore, low toxicity and high degradability of the nanoparticles are among the most important requirements in the nanoparticle designs. In this review, we provide the most up-to-date research and clinical outcomes in nanoparticle therapies for pulmonary diseases. We also address the current critical issues in key areas of pulmonary cell targeting, biosafety and compatibility, and molecular mechanisms for selective cellular uptake.

Sex-dependent modulation of immune development in mice by secretory IgA-coated Lactobacillus reuteri isolated from breast milk

Lactobacilli, commonly present in human breast milk, appear to colonize the neonatal gut and provide protection to infants against various infections, thereby promoting immune development. This study examined the potential probiotic role of breast milk-derived Lactobacillus reuteri FN041 in immune development in mice. The FN041 were gavaged either to BALB/c dams (n = 6/group) during the lactation period or to their offspring (n = 6/sex per intervention) after weaning separately (cointervention). All interventions induced increased intestinal barriers in 5-wk-old offspring, especially in the females. Immunoglobulin A plasmocytes in ileal tissue and secretory IgA (sIgA) in ileal contents increased in all 5-wk-old offspring of cointervention. The activation of mRNA expression of 17 genes was sex-dependent, especially in 5-wk-old offspring. Broader genes were regulated in female mice. The effect of cointervention on the Shannon index of total microbiota is sex-related. The Shannon index of sIgA-coated microbiota increased in both sexes. The sIgA-coated microbiota showed intergroup differences according to β diversity, especially in female mice that showed an increase in Bifidobacterium of Actinobacteria. The sIgA-coated Bifidobacterium was positively correlated with mRNA expression of Tlr9. The sIgA-coated Lactobacillus in male offspring was negatively correlated with mRNA expression of Cldn2. In conclusion, L. reuteri FN041 promoted the production of intestinal sIgA and the expression of genes related to antimicrobial peptides in the offspring and enhanced the function of the mucosal barrier, depending on sex and treatment manner.

Particle Diffusivity and Free-Energy Profiles in Hydrogels from Time-Resolved Penetration Data

A combined experimental and theoretical method to simultaneously determine diffusivity and free-energy profiles of particles that penetrate into inhomogeneous hydrogel systems is presented. As the only input, arbitrarily normalized concentration profiles from fluorescence intensity data of labeled tracer particles for different penetration times are needed. The method is applied to dextran molecules of varying size that penetrate into hydrogels of polyethylene-glycol chains with different lengths that are covalently cross-linked by hyperbranched polyglycerol hubs. Extracted dextran bulk diffusivities agree well with fluorescence correlation spectroscopy data obtained separately. Empirical scaling laws for dextran diffusivities and free energies inside the hydrogel are identified as a function of the dextran mass. An elastic free-volume model that includes dextran as well as polyethylene-glycol linker flexibility quantitively describes the repulsive dextran-hydrogel interaction free energy, which is of steric origin, and furthermore suggests that the hydrogel mesh-size distribution is rather broad and particle penetration is dominated by large hydrogel pores. Particle penetration into hydrogels for steric particle-hydrogel interactions is thus suggested to be governed by an elastic size-filtering mechanism that involves the tail of the hydrogel pore-size distribution.

Design, synthesis and biological applications of glycopolypeptides

Carbohydrates play essential structural and biochemical roles in all living organisms. Glycopolymers are attractive as well-defined biomimetic analogs to study carbohydrate-dependent processes, and are widely applicable biocompatible materials in their own right. Glycopolypeptides have shown great promise in this area since they are closer structural mimics of natural glycoproteins than other synthetic glycopolymers and can serve as carriers for biologically active carbohydrates. This review highlights advances in the area of design and synthesis of such materials, and their biomedical applications in therapeutic delivery, tissue engineering, and beyond.

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.

Unraveling mucin domains in cancer and metastasis: when protectors become predators

A dynamic mucosal layer shields the epithelial cells lining the body cavities and is made up of high molecular weight, heavily glycosylated, multidomain proteins called mucins. Mucins, broadly grouped into transmembrane and secreted mucins, are the first responders to any mechanical or chemical insult to the epithelia and help maintain tissue homeostasis. However, their intrinsic properties to protect and repair the epithelia are exploited during oncogenic processes, where mucins are metamorphosed to aid the tumor cells in their malignant journey. Diverse domains, like the variable number tandem repeats (VNTR), sea urchin sperm protein enterokinase and agrin (SEA), adhesion-associated domain (AMOP), nidogen-like domain (NIDO), epidermal growth factor-like domain (EGF), and von Willebrand factor type D domain (vWD) on mucins, including MUC1, MUC4, MUC5AC, MUC5B, and MUC16, have been shown to facilitate cell-to-cell and cell-to-matrix interactions, and cell-autonomous signaling to promote tumorigenesis and distant dissemination of tumor cells. Several obstacles have limited the study of mucins, including technical difficulties in working with these huge glycoproteins, the dearth of scientific tools, and lack of animal models; thus, the tissue-dependent and domain-specific roles of mucins during mucosal protection, chronic inflammation, tumorigenesis, and hematological dissemination of malignant cells are still unclear. Future studies should try to integrate information on the rheological, molecular, and biological characteristics of mucins to comprehensively delineate their pathophysiological role and evaluate their suitability as targets in future diagnostic and therapeutic strategies.

Tunnels for Protein Export from the Endoplasmic Reticulum

The functions of coat protein complex II (COPII) coats in cargo packaging and the creation of vesicles at the endoplasmic reticulum are conserved in eukaryotic protein secretion. Standard COPII vesicles, however, cannot handle the secretion of metazoan-specific cargoes such as procollagens, apolipoproteins, and mucins. Metazoans have thus evolved modules centered on proteins like TANGO1 (transport and Golgi organization 1) to engage COPII coats and early secretory pathway membranes to engineer a novel mode of cargo export at the endoplasmic reticulum.

MUC Glycoproteins: Potential Biomarkers and Molecular Targets for Cancer Therapy

MUC proteins have great significance as prognostic and diagnostic markers as well as a potential target for therapeutic interventions in most cancers of glandular epithelial origin. These are high molecular weight glycosylated proteins located in the epithelial lining of several tissues and ducts. Mucins belong to a heterogeneous group of large O-glycoproteins that can be either secreted or membrane-bound. Glycosylation, a post-translational modification affects the biophysical, functional and biochemical properties and provides structural complexity for these proteins. Aberrant expression and glycosylation of mucins contribute to tumour survival and proliferation in many cancers, which in turn activates numerous signalling pathways such as NF-kB, ERα, HIF, MAPK, p53, c-Src, Wnt and JAK-STAT, etc. This subsequently induces cancer cell growth, proliferation and metastasis. The present review mainly demonstrates the functional aspects of MUC glycoproteins along with its unique signalling mechanism and role of aberrant glycosylation in cancer progression and therapeutics. The importance of MUC proteins and its subtypes in a wide spectrum of cancers including but not limited to breast cancer, colorectal cancer, endometrial and cervical cancer, lung cancer, primary liver cancer, pancreatic cancer, prostate cancer and ovarian cancer has been exemplified with significance in targeting the same. Several patents associated with the MUC proteins in the field of cancer therapy are also emphasized in the current review.

An ovary-specific mucin is associated with choriogenesis mediated by prostaglandin signaling in Spodoptera exigua

Polytrophic ovarioles of Spodoptera exigua, a lepidopteran insect, begins with the development of oocytes and differentiation of nurse cells followed by vitellogenesis and choriogenesis. Compared with previtellogenic and vitellogenic developments, choriogenesis has not been clearly understood yet in endocrine control. This study investigated the expression and function of a mucin-like structural protein of S. exigua called Se-Mucin1 in choriogenesis. It was highly expressed in ovarioles containing chorionated oocytes. The expression level of Se-Mucin1 was increased during adult stage as early as 18 h after adult emergence, reaching the maximal level at 24 h and later. Interestingly, DNA amount of Se-Mucin1 was increased by almost four folds during early adult stage while other genes (hexokinase and glyceraldehyde-3-phosphate dehydrogenase) not directly associated with chorion formation did not show genomic DNA increase, suggesting specific gene amplification of Se-Mucin1. RNA interference (RNAi) suppressed Se-Mucin1 expression by injecting 1 μg of double-strand RNA to teneral females (<5 h after emergence), which exhibited significantly impaired fecundity and egg hatching rate. Eggs laid by RNAi-treated females were malformed in eggshell structures with loss of mesh-like fibers. Treatment with aspirin, a prostaglandin (PG) biosynthesis inhibitor, suppressed the induction of Se-Mucin1 expression during early adult stage and impaired egg development. An addition of PGE₂ significantly rescued such impairment in Se-Mucin1 expression and subsequent egg development. These results suggest that PGs mediate choriogenesis of S. exigua by activating the expression of chorion-associated genes including Se-Mucin1.

Tear-film breakup: The role of membrane-associated mucin polymers

Mucin polymers in the tear film protect the corneal surface from pathogens and modulate the tear-film flow characteristics. Recent studies have suggested a relationship between the loss of membrane-associated mucins and premature rupture of the tear film in various eye diseases. This work aims to elucidate the hydrodynamic mechanisms by which loss of membrane-associated mucins causes premature tear-film rupture. We model the bulk of the tear film as a Newtonian fluid in a two-dimensional periodic domain, and the lipid layer at the air-tear interface as insoluble surfactants. Gradual loss of membrane-associated mucins produces growing areas of exposed cornea in direct contact with the tear fluid. We represent the hydrodynamic consequences of this morphological change through two mechanisms: an increased van der Waals attraction due to loss of wettability on the exposed area, and a change of boundary condition from an effective negative slip on the mucin-covered areas to the no-slip condition on exposed cornea. Finite-element computations, with an arbitrary Lagrangian-Eulerian scheme to handle the moving interface, demonstrate a strong effect of the elevated van der Waals attraction on precipitating tear-film breakup. The change in boundary condition on the cornea has a relatively minor role. Using realistic parameters, our heterogeneous mucin model is able to predict quantitatively the shortening of tear-film breakup time observed in diseased eyes.

An overview of gastrointestinal mucus rheology under different pH conditions and introduction to pH-dependent rheological interactions with PLGA and chitosan nanoparticles

This review discusses the physicochemical and mechanical properties of porcine gastrointestinal mucus from a rheological point of view. Considering mucus as a viscoelastic gel that functions as a biological barrier by limiting particles passage, lubricating the gastrointestinal tract, and protecting the stomach from gastric acids. The viscoelastic and protective properties of mucus are mainly produced by its mucin network, which is stabilized through electrostatic, hydrophobic and hydrogen bonding interactions. Otherwise, mucus rheology is determined by its polyanionic nature at physiological pH. At neutral pH, mucus presents a viscous behavior produced by chains crosslinking. While, at acidic pH, mucus exhibits an elastic behavior related with the extended conformation that produces mucus gelation at the stomach. Additionally, rheology studies the degree of adhesion between a polymer-mucus mixture through rheological synergism, and how it varies at different pH conditions. Finally, mucoadhesion phenomenon is exemplified with chitosan (cationic) and poly (lactic-co-glycolic) acid (anionic) polymers.

Antibacterial Activity of N,O-Acylated Chitosan Derivative

The antibacterial activity of N,O-acylated chitosan derivative with linoleic acid (CH_LA) was tested by disc and well diffusion, agar impregnation and microdilution methods against Staphylococcus aureus, Escherichia coli and Helicobacter pylori strains. Hydrophobically modified chitosan (HMC) was expected to exhibit enhanced antibacterial activity and specific mucin interactions. Although diffusion tests have not indicated the antibacterial potential of chitosan (CH) or CH_LA, the results of the microdilution method demonstrated that tested polymers significantly reduced the amount of living bacteria cells in different concentrations depending on the microorganism. Additionally, CH_LA was characterized by enhanced antibacterial activity compared to CH, which may suggest a different mechanism of interaction with S. aureus and H. pylori. Furthermore, the UV-VIS analysis revealed that the amphiphilic character of derivative led to strong CH_LA–mucin interactions. The study proved the high potential of CH_LA in antibacterial applications, especially for the gastrointestinal tract.

Design and Synthesis of Mucin-Inspired Glycopolymers

Mucins are bottlebrush biopolymers that are glycoproteins on the surfaces of cells and as hydrogels secreted inside and outside the body. Mucin function in biology includes cell-cell recognition, signaling, protection, adhesion, and lubrication. Because of their attractive and diverse properties, mucins have recently become the focus of synthetic efforts by researchers who hope to understand and emulate these biomaterials. This review is focused on the development of methodologies for preparing mucin-inspired synthetic oligomers and glycopolymers, including solid-phase synthesis, polymerization of glycosylated monomers, and post-polymerization grafting of glycans to polymer chains. How these synthetic mucins have been used in health applications is discussed. Natural mucins are formed from a conserved set of monomers that are combined into chains of different sequences and lengths to achieve materials with widely diverse properties. Adopting this design paradigm from natural mucins could lead to next-generation bioinspired synthetic materials.

Mucin Biopolymers and Their Barrier Function at Airway Surfaces

In the lung, the airway epithelium produces secreted and tethered mucin biopolymers to form a mucus hydrogel layer and a surface-attached polymer brush layer. These layers work in concert to facilitate the cilia-mediated transport of mucus for the capture and clearance of inhaled materials to prevent lung damage. The mechanisms by which mucin biopolymers protect the lung from injury have been an intense area of study in airway biology for the past several decades. In this feature article, we will discuss how airway mucins achieve these protective barrier functions. We will present the key findings, rooted in polymer and surface science, that have aided in understanding mucin barrier function. In addition, we will describe how this work may influence the design of nanoparticles to overcome the mucus barrier to effective drug delivery.

The Landscape of CAR-T Cell Clinical Trials against Solid Tumors-A Comprehensive Overview

Simple Summary

Certain immune cells, namely T cells, of cancer patients can be genetically manipulated to express so-called chimeric antigen receptors (CARs), which enables these cells to kill the tumor cells after recognition by the receptor. This therapy is very successful in the treatment of hematologic tumors such as lymphoma or leukemia. However, tumors growing as a solid mass are less susceptible to this kind of treatment. This review summarizes known data of all clinical trials using this therapy against solid tumors that are registered at clinicaltrials.gov.

Abstract

CAR-T cells showed great potential in the treatment of patients with hematologic tumors. However, the clinical efficacy of CAR-T cells against solid tumors lags behind. To obtain a comprehensive overview of the landscape of CAR-T cell clinical trials against this type of cancer, this review summarizes all the 196 studies registered at clinicaltrials.gov. Special focus is on: (1) geographical distribution; (2) targeted organs, tumor entities, and antigens; (3) CAR transfer methods, CAR formats, and extra features introduced into the T cells; and (4) patient pretreatments, injection sites, and safety measurements. Finally, the few data on clinical outcome are reported. The last assessment of clinicaltrials.gov for the data summarized in this paper was on 4 August 2020.

Interactions of microorganisms with host mucins: a focus on Candida albicans

Mucus is an important host innate defense factor that lines most epithelial cell layers of the body and provides crucial physical and biological protection against pathogenic microorganisms. Mucins are the main glycoproteins of mucus that are responsible for interacting with microorganisms and are critical for the antimicrobial properties of mucus. The mechanisms by which microorganisms interact with mucins are poorly understood, especially in terms of fungi, and these interactions are continually evolving. Work in bacterial pathogens has shown that mucins inhibit bacterial virulence traits, including quorum sensing, toxin secretion and biofilm formation. Among the fungal clade, the common opportunistic human fungal pathogen and commensal Candida albicans engages in constant battle with the host innate immune system. This battle creates strong selective pressures for C. albicans to evolve in response to the host. Recent work in C. albicans found that mucins inhibit specific virulence traits, such as surface adherence, filamentation, biofilm formation and the production of secreted proteases. Here we review the current knowledge of microbial interactions with mucins, with a special emphasis on the interactions between C. albicans and mucins.

Muc5ac Expression Protects the Colonic Barrier in Experimental Colitis

BACKGROUND

The mucus gel layer (MGL) lining the colon is integral to exclusion of bacteria and maintaining intestinal homeostasis in health and disease. Some MGL defects allowing bacteria to directly contact the colonic surface are commonly observed in ulcerative colitis (UC). The major macromolecular component of the colonic MGL is the secreted gel-forming mucin MUC2, whose expression is essential for homeostasis in health. In UC, another gel-forming mucin, MUC5AC, is induced. In mice, Muc5ac is protective during intestinal helminth infection. Here we tested the expression and functional role of MUC5AC/Muc5ac in UC biopsies and murine colitis.

METHODS

We measured MUC5AC/Muc5ac expression in UC biopsies and in dextran sulfate sodium (DSS) colitis. We performed DSS colitis in mice deficient in Muc5ac (Muc5ac-/-) to model the potential functional role of Muc5ac in colitis. To assess MGL integrity, we quantified bacterial-epithelial interaction and translocation to mesenteric lymph nodes. Antibiotic treatment and 16S rRNA gene sequencing were performed to directly investigate the role of bacteria in murine colitis.

RESULTS

Colonic MUC5AC/Muc5ac mRNA expression increased significantly in active UC and murine colitis. Muc5ac-/- mice experienced worsened injury and inflammation in DSS colitis compared with control mice. This result was associated with increased bacterial-epithelial contact and translocation to the mesenteric lymph nodes. However, no change in microbial abundance or community composition was noted. Antibiotic treatment normalized colitis severity in Muc5ac-/- mice to that of antibiotic-treated control mice.

CONCLUSIONS

MUC5AC/Muc5ac induction in the acutely inflamed colon controls injury by reducing bacterial breach of the MGL.

An enzymatic toolkit for selective proteolysis, detection, and visualization of mucin-domain glycoproteins

Densely O-glycosylated mucin domains are found in a broad range of cell surface and secreted proteins, where they play key physiological roles. In addition, alterations in mucin expression and glycosylation are common in a variety of human diseases, such as cancer, cystic fibrosis, and inflammatory bowel diseases. These correlations have been challenging to uncover and establish because tools that specifically probe mucin domains are lacking. Here, we present a panel of bacterial proteases that cleave mucin domains via distinct peptide- and glycan-based motifs, generating a diverse enzymatic toolkit for mucin-selective proteolysis. By mutating catalytic residues of two such enzymes, we engineered mucin-selective binding agents with retained glycoform preferences. StcE^E447D is a pan-mucin stain derived from enterohemorrhagic Escherichia coli that is tolerant to a wide range of glycoforms. BT4244^E575A derived from Bacteroides thetaiotaomicron is selective for truncated, asialylated core 1 structures commonly associated with malignant and premalignant tissues. We demonstrated that these catalytically inactive point mutants enable robust detection and visualization of mucin-domain glycoproteins by flow cytometry, Western blot, and immunohistochemistry. Application of our enzymatic toolkit to ascites fluid and tissue slices from patients with ovarian cancer facilitated characterization of patients based on differences in mucin cleavage and expression patterns.

Tusks, the extra-oral teeth

OBJECTIVE

The present review aims to: a) describe the features that support tusks in extra-oral position, and b) represent distinctive features of tusks, which provide insights into tusks adaptation to ambient conditions.

DESIGN

A comprehensive review of scientific literature relevant to tusks and comparable dental tissues was conducted.

RESULTS

The oral cavity provides a desirable condition which is conducive to tooth health. Therefore, it remains questionable how the bare (exposed) tusks resist the extra-oral conditions. The common features among tusked mammals indicate that the structural (e.g. the peculiar dentinal alignment), cellular (e.g. low or lack of cell populations in the tusk), hormonal (e.g. androgens), and behavioral traits have impact on a tusk's preservation and occurrence.

CONCLUSIONS

Understanding of bare mineralized structures, such as tusks and antlers, and their compatibility with different environments, can provide important insight into oral biology.

Tools for probing host-bacteria interactions in the gut microenvironment: From molecular to cellular levels

Healthy function of the gut microenvironment is dependent on complex interactions between the bacteria of the microbiome, epithelial and immune (host) cells, and the surrounding tissue. Misregulation of these interactions is implicated in disease. A range of tools have been developed to study these interactions, from mechanistic studies to therapeutic evaluation. In this Digest, we highlight select tools at the cellular and molecular level for probing specific cell-microenvironment interactions. Approaches are overviewed for controlling and probing cell-cell interactions, from transwell and microfluidic devices to engineered bacterial peptidoglycan fragments, and cell-matrix interactions, from three-dimensional scaffolds to chemical handles for in situ modifications.

Supercharged Proteins and Polypeptides

Electrostatic interactions play a vital role in nature. Biomacromolecules such as proteins are orchestrated by electrostatics, among other intermolecular forces, to assemble and organize biochemistry. Natural proteins with a high net charge exist in a folded state or are unstructured and can be an inspiration for scientists to artificially supercharge other protein entities. Recent findings show that supercharging proteins allows for control of their properties such as temperature resistance and catalytic activity. One elegant method to transfer the favorable properties of supercharged proteins to other proteins is the fabrication of fusions. Genetically engineered, supercharged unstructured polypeptides (SUPs) are just one promising fusion tool. SUPs can also be complexed with artificial entities to yield thermotropic and lyotropic liquid crystals and liquids. These architectures represent novel bulk materials that are sensitive to external stimuli. Interestingly, SUPs undergo fluid-fluid phase separation to form coacervates. These coacervates can even be directly generated in living cells or can be combined with dissipative fiber assemblies that induce life-like features. Supercharged proteins and SUPs are developed into exciting classes of materials. Their synthesis, structures, and properties are summarized. Moreover, potential applications are highlighted and challenges are discussed.

Intestinal Mucosal Barrier Function Restoration in Mice by Maize Diet Containing Enriched Flavan-4-Ols

Inflammatory bowel disease (IBD), a chronic intestinal inflammatory condition, awaits safe and effective preventive strategies. Naturally occurring flavonoid compounds are promising therapeutic candidates against IBD due to their great antioxidant potential and ability to reduce inflammation and improve immune signaling mediators in the gut. In this study, we utilized two maize near-isogenic lines flavan-4-ols-containing P1-rr (F+) and flavan-4-ols-lacking p1-ww (F-) to investigate the anti-inflammatory property of flavan-4-ols against carboxymethylcellulose (CMC)-induced low-grade colonic inflammation. C57BL/6 mice were exposed to either 1% CMC (w/v) or water for a total of 15 weeks. After week six, mice on CMC treatment were divided into four groups. One group continued on the control diet. The second and third groups were supplemented with F+ at 15% or 25% (w/w). The fourth group received diet supplemented with F- at 15%. Here we report that mice consuming F+(15) and F+(25) alleviated CMC-induced increase in epididymal fat-pad, colon histology score, pro-inflammatory cytokine interleukin 6 expression and intestinal permeability compared to mice fed with control diet and F-(15). F+(15) and F+(25) significantly enhanced mucus thickness in CMC exposed mice (p < 0.05). These data collectively demonstrated the protective effect of flavan-4-ol against colonic inflammation by restoring intestinal barrier function and provide a rationale to breed for flavan-4-ols enriched cultivars for better dietary benefits.

Quantifying and Engineering Mucus Adhesion of Probiotics

Mucus in the gastrointestinal (GI) tract is the primary point-of-interaction between humans and their gut microbiota. This intimates that mucus not only ensures protection against endogenous and exogenous opportunists but also provisions for the human microbiota to reside and flourish. With the emergence of living therapeutics, engineered microbes can deliver and produce increasingly complex medicine, and controlling the mucoadhesive properties of different microbial chassis can dictate dose-response in a patient. Here we present a redesigned, in vitro, plate-based assay to measure the mucus adhesion of various probiotics. Cell-mucus interactions were isolated by immobilizing mucus to the plate surface. Binding parameters were derived for each probiotic strain by measuring cell adhesion over a wide range of cell concentrations, providing dose-dependent adhesion metrics. Surface proteins and cell components known to influence mucoadhesion were then heterologously expressed or altered in Lactococcus lactis MG1363 and Escherichia coli Nissle 1917 to control mucus-binding capacity, avidity, and cooperativity.

Potential Interaction between Galectin-2 and MUC5AC in Mouse Gastric Mucus

Galectins are a group of animal lectins characterized by their specificity for β-galactosides. Of these, galectin-2 (Gal-2) is predominantly expressed in the gastrointestinal tract. In the current study, we used a mouse gastric mucous fraction to investigate whether Gal-2 is secreted from epithelial cells and identify its potential ligands in gastric mucus. Gal-2 was detected in the mouse gastric mucous fraction and could be eluted from it by the addition of lactose. Affinity chromatography using recombinant mouse galectin-2 (mGal-2)-immobilized adsorbent and subsequent LC-MS/MS identified MUC5AC, one of the major gastric mucin glycoproteins, as a potential ligand of mGal-2. Furthermore, MUC5AC was detected in the mouse gastric mucous fraction by Western blotting, and recombinant mGal-2 was adsorbed to this fraction in a carbohydrate-dependent manner. These results suggested that Gal-2 and MUC5AC in mouse gastric mucus interact in a β-galactoside-dependent manner, resulting in a stronger barrier structure protecting the mucosal surface.

Biolubrication synergy: Hyaluronan - Phospholipid interactions at interfaces

The manner in which nature has solved lubrication issues has fascinated scientists for centuries, in particular when considering that lubrication is achieved in aqueous media. The most outstanding system in this respect is likely the synovial joint, where close to frictionless motion is realized under different loads and shear rates. This review article focuses on two components present in the synovial area, hyaluronan and phospholipids. We recapitulate what has been learned about their interactions at interfaces from recent experiments, with focus on results obtained using reflectivity techniques at large scale facilities. In parallel, modelling experiments have been carried out and from these efforts new detailed knowledge about how hyaluronan and phospholipids interact has been gained. In this review we combine findings from modelling and experiments to gain deeper insight. Finally, we summarize what has been learned of the lubrication performance of mixtures of phospholipids and hyaluronan.

Mucins and Pathogenic Mucin-Like Molecules Are Immunomodulators During Infection and Targets for Diagnostics and Vaccines

Mucins and mucin-like molecules are highly O-glycosylated proteins present on the cell surface of mammals and other organisms. These glycoproteins are highly diverse in the apoprotein and glycan cores and play a central role in many biological processes and diseases. Mucins are the most abundant macromolecules in mucus and are responsible for its biochemical and biophysical properties. Mucin-like molecules cover various protozoan parasites, fungi and viruses. In humans, modifications in mucin glycosylation are associated with tumors in epithelial tissue. These modifications allow the distinction between normal and abnormal cell conditions and represent important targets for vaccine development against some cancers. Mucins and mucin-like molecules derived from pathogens are potential diagnostic markers and targets for therapeutic agents. In this review, we summarize the distribution, structure, role as immunomodulators, and the correlation of human mucins with diseases and perform a comparative analysis of mucins with mucin-like molecules present in human pathogens. Furthermore, we review the methods to produce pathogenic and human mucins using chemical synthesis and expression systems. Finally, we present applications of mucin-like molecules in diagnosis and prevention of relevant human diseases.

Cosmc overexpression enhances malignancies in human colon cancer

Cosmc is known as a T‐synthase‐specific molecular chaperone that plays a crucial role in the process of O‐glycosylation. Cosmc dysfunction leads to inactive T‐synthase and results in aberrant O‐glycosylation, which is associated with various tumour malignancies. However, it is unclear whether Cosmc has some other functions beyond its involvement in O‐glycosylation. In this study, we aimed to investigate the functional role of Cosmc in human colorectal cancer (CRC). We first assessed the expression levels of Cosmc in human CRC specimens and then forcedly expressed Cosmc in human CRC cell lines (HCT116, SW480) to examine its impact on cellular behaviours. The mechanisms for aberrant expression of Cosmc in CRC tissues and the altered behaviours of tumour cells were explored. It showed that the mRNA and protein levels of Cosmc were markedly elevated in human CRC specimens relative to normal colorectal tissues. The occurrence of endoplasmic reticulum (ER) stress may largely contribute to the increased Cosmc expression in cancer tissue and cells. Cosmc overexpression in CRC cells significantly promoted cell migration and invasion, which could be attributed to the activation of the epithelial‐mesenchymal transition (EMT) pathway rather than aberrant O‐glycosylation. These data indicate that Cosmc expression was elevated in human CRC possibly caused by ER stress, which further enhanced malignancies through the activation of EMT but independently of aberrant O‐glycosylation.

Mucin glycans attenuate the virulence of Pseudomonas aeruginosa in infection

A slimy, hydrated mucus gel lines all wet epithelia in the human body, including the eyes, lungs, and gastrointestinal and urogenital tracts. Mucus forms the first line of defence while housing trillions of microorganisms that constitute the microbiota¹. Rarely do these microorganisms cause infections in healthy mucus¹, suggesting that mechanisms exist in the mucus layer that regulate virulence. Using the bacterium Pseudomonas aeruginosa and a three-dimensional (3D) laboratory model of native mucus, we determined that exposure to mucus triggers downregulation of virulence genes that are involved in quorum sensing, siderophore biosynthesis and toxin secretion, and rapidly disintegrates biofilms-a hallmark of mucosal infections. This phenotypic switch is triggered by mucins, which are polymers that are densely grafted with O-linked glycans that form the 3D scaffold inside mucus. Here, we show that isolated mucins act at various scales, suppressing distinct virulence pathways, promoting a planktonic lifestyle, reducing cytotoxicity to human epithelia in vitro and attenuating infection in a porcine burn model. Other viscous polymer solutions lack the same effect, indicating that the regulatory function of mucin does not result from its polymeric structure alone. We identify that interactions with P. aeruginosa are mediated by mucin-associated glycans (mucin glycans). By isolating glycans from the mucin backbone, we assessed the collective activity of hundreds of complex structures in solution. Similar to their grafted counterparts, free mucin glycans potently regulate bacterial phenotypes even at relatively low concentrations. This regulatory function is likely dependent on glycan complexity, as monosaccharides do not attenuate virulence. Thus, mucin glycans are potent host signals that 'tame' microorganisms, rendering them less harmful to the host.

Effect of pH on the viscoelastic properties of pig gastric mucus

Mucus is a biomaterial with peculiar, gel-like viscoelastic properties, and bearing different functionalities, depending on the different mucosae it covers. It is clear that these functionalities have to stay effective throughout the in vivo broad range of physiological pH values at which the mucus is exposed. We sought here to determine the effect of pH on the rheological properties of ex vivo mucus. We demonstrate that viscoelastic properties of gastric mucus are quite "stable" to pH changes, in marked contrast with the pH sensitivity of purified mucin gels. We also find that the rheological features of porcine gastric mucus are reversible when the system is first alkalized up to solubilization (pH > 8.5) and then re-acidified to its initial pH value.

IgA-Targeted Lactobacillus jensenii Modulated Gut Barrier and Microbiota in High-Fat Diet-Fed Mice

IgA-coated Lactobacillus live in the mucous layer of the human or mammalian intestine in close proximity to epithelial cells. They act as potential probiotics for functional food development, but their physiological regulation has not yet been studied. We isolated IgA-targeted (Lactobacillus jensenii IgA21) and lumen lactic acid bacterial strains (Pediococcus acidilactici FS1) from the fecal microbiota of a healthy woman. C57BL/6 mice were fed a normal (CON) or high fat diet (HFD) for 6 weeks and then treated with IgA21 or FS1 for 4 weeks. HFD caused dyslipidemia, mucosal barrier damage, and intestinal microbiota abnormalities. Only IgA21 significantly inhibited dyslipidemia and gut barrier damage. This was related to significant up-regulation of mucin-2, PIgR mRNA expression, and colonic butyrate production (P < 0.05 vs. HFD). Unlike IgA21, FS1 caused a more pronounced gut dybiosis than did HFD, and, in particular, it induced a significant decrease in the Bacteroidales S24-7 group and an increase in Desulfovibrionaceae (P < 0.05 vs. CON). In conclusion, IgA-coated and non-coated lactic acid bacteria of gut have been demonstrated to differentially affect the intestinal barrier and serum lipids. This indicates that IgA-bound bacteria possess the potential to more easily interact with the host gut to regulate homeostasis.

A Mucin-Like Protein Is Essential for Oviposition in Nilaparvata lugens

Mucins play a variety of roles; for example, in vertebrates, mucins lubricate epithelial surfaces and protect tissue from physical and biological damage, however, knowledge of insect mucins is limited. Here, we identified an eggshell-related mucin-like protein, NlESMuc, in the brown planthopper (BPH), Nilaparvata lugens. NlESMuc was specifically expressed in the follicular cells from the egg chambers of the ovarioles. RNA interference (RNAi) was used to perform functional analysis of NlESMuc. Adult female BPH with NlESMuc knockdown had significantly reduced fecundity, including more difficult oviposition, lower egg production, and eggs that could not hatch. Scanning electron microscopy showed that, in NlESMuc knocked-down BPH, the ultrastructure of the eggshells of fully developed oocytes was loose, and the cross-section showed many small droplets of about 0.1-μm diameter. Based on the results, it is concluded that NlESMuc is an eggshell-related protein and essential for normal oviposition. Our findings help to provide new targets for pesticide design and RNAi-based BPH control and will also provide new insights into insect eggshells and insect mucins.

Non-Gaussian, non-ergodic, and non-Fickian diffusion of tracers in mucin hydrogels

Native mucus is polymer-based soft-matter material of paramount biological importance. How non-Gaussian and non-ergodic is the diffusive spreading of pathogens in mucus? We study the passive, thermally driven motion of micron-sized tracers in hydrogels of mucins, the main polymeric component of mucus. We report the results of the Bayesian analysis for ranking several diffusion models for a set of tracer trajectories [C. E. Wagner et al., Biomacromolecules, 2017, 18, 3654]. The models with "diffusing diffusivity", fractional and standard Brownian motion are used. The likelihood functions and evidences of each model are computed, ranking the significance of each model for individual traces. We find that viscoelastic anomalous diffusion is often most probable, followed by Brownian motion, while the model with a diffusing diffusion coefficient is only realised rarely. Our analysis also clarifies the distribution of time-averaged displacements, correlations of scaling exponents and diffusion coefficients, and the degree of non-Gaussianity of displacements at varying pH levels. Weak ergodicity breaking is also quantified. We conclude that-consistent with the original study-diffusion of tracers in the mucin gels is most non-Gaussian and non-ergodic at low pH that corresponds to the most heterogeneous networks. Using the Bayesian approach with the nested-sampling algorithm, together with the quantitative analysis of multiple statistical measures, we report new insights into possible physical mechanisms of diffusion in mucin gels.