Detecting, visualising, and quantifying mucins

An ex vivo rat trachea model reveals abnormal airway physiology and a gland secretion defect in cystic fibrosis

Cystic fibrosis (CF) is a genetic disease hallmarked by aberrant ion transport that results in delayed mucus clearance, chronic infection, and progressive lung function decline. Several animal models have been developed to study the airway anatomy and mucus physiology in CF, but they are costly and difficult to maintain, making them less accessible for many applications. A more available CFTR-/- rat model has been developed and characterized to develop CF airway abnormalities, but consistent dosing of pharmacologic agents and longitudinal evaluation remain a challenge. In this study, we report the development and characterization of a novel ex vivo trachea model that utilizes both wild type (WT) and CFTR-/- rat tracheae cultured on a porcine gelatin matrix. Here we show that the ex vivo tracheae remain viable for weeks, maintain a CF disease phenotype that can be readily quantified, and respond to stimulation of mucus and fluid secretion by cholinergic stimulation. Furthermore, we show that ex vivo tracheae may be used for well-controlled pharmacological treatments, which are difficult to perform on freshly excised trachea or in vivo models with this degree of scrutiny. With improved interrogation possible with a durable trachea, we also established firm evidence of a gland secretion defect in CFTR-/- rat tracheae compared to WT controls. Finally, we demonstrate that the ex vivo tracheae can be used to generate high mucus protein yields for subsequent studies, which are currently limited by in vivo mucus collection techniques. Overall, this study suggests that the ex vivo trachea model is an effective, easy to set up culture model to study airway and mucus physiology.

Human ocular mucins: The endowed guardians of sight

Mucins are an ancient group of glycoproteins that provide viscoelastic, lubricating and hydration properties to fluids bathing wet surfaced epithelia. They are involved in the protection of underlying tissues by forming a barrier with selective permeability properties. The expression, processing and spatial distribution of mucins are often determined by organ-specific requirements that in the eye involve protecting against environmental insult while allowing the passage of light. The human ocular surface epithelia have evolved to produce an extremely thin and watery tear film containing a distinct soluble mucin product secreted by goblet cells outside the visual axis. The adaptation to the ocular environment is notably evidenced by the significant contribution of transmembrane mucins to the tear film, where they can occupy up to one-quarter of its total thickness. This article reviews the tissue-specific properties of human ocular mucins, methods of isolation and detection, and current approaches to model mucin systems recapitulating the human ocular surface mucosa. This knowledge forms the fundamental basis to develop applications with a promising biological and clinical impact.

The effect of ingredients commonly used in nasal and inhaled solutions on the secretion of mucus in vitro

Hypersecretion of mucus is associated with impaired mucociliary clearance that can influence the retention of active pharmaceutical ingredients in the airway but is also linked with recurrent airway disease. Therefore, the effect on mucin secretion of a range of ingredients used in solutions delivered to the nose and lung was studied. Mucin secretion from explants of ovine epithelium was quantified using an enzyme-linked lectin assay (ELLA) or sandwich ELLA depending on the compatibility of the ingredients with the assay. Benzalkonium chloride (0.015% w/w), Methocel™ E50 premium LV (1.0% w/w), propylene glycol (1.5% w/w), potassium sorbate + propylene glycol (0.3% w/w + 1.5% w/w) and polysorbate 80 (0.025% w/w), used at common working concentrations, all increased the secretion of mucin from the explants (P < 0.05). Ethylenediamine tetraacetic acid-disodium salt (EDTA) (0.015% w/w), Avicel® RC591 (1.5% w/w), fluticasone furoate (0.0004% w/w, concentration in solution) and dimethyl sulfoxide (DMSO) (0.2% w/w) did not affect mucin secretion. Compounds increasing mucin secretion could alter the rate of mucociliary clearance and the mucus could provide a barrier to drug absorption. This could predispose patients to disease and affect the activity of delivered drugs, decreasing or increasing their clinical efficacy.

Time Course of Salivary Protein Responses to Cranberry-Derived Polyphenol Exposure as a Function of PROP Taster Status

Astringency is a complex oral sensation, commonly experienced when dietary polyphenols interact with salivary proteins. Most astringent stimuli alter protein levels, which then require time to be replenished. Although it is standard practice in astringency research to provide breaks in between stimuli, there is limited consensus over the amount of time needed to restore the oral environment to baseline levels. Here we examined salivary protein levels after exposure to 20 mL of a model stimulus (cranberry polyphenol extract, 0.75 g/L CPE) or unsweetened cranberry juice (CJ), over a 10 min period. Whole saliva from healthy subjects (n = 60) was collected at baseline and after 5 and 10 min following either stimulus. Five families of proteins: basic proline-rich proteins (bPRPs); acidic proline-rich proteins (aPRPs); histatins; statherin; and S-type cystatins, were analyzed in whole saliva via HPLC-low resolution-ESI-IT-MS, using the area of the extracted ion current (XIC) peaks. Amylase was quantified via immunoblotting. In comparison to baseline (resting), both stimuli led to a rise in levels of aPRPs (p < 0.000) at 5 min which remained elevated at 10 min after stimulation. Additionally, an interaction of PROP taster status and time was observed, wherein super-tasters had higher levels of amylase in comparison to non-tasters after stimulation with CJ at both timepoints (p = 0.014–0.000). Further, male super-tasters had higher levels of bPRPs at 5 min after stimulation with both CJ and CPE (p = 0.015–0.007) in comparison to baseline. These data provide novel findings of interindividual differences in the salivary proteome that may influence the development of astringency and that help inform the design of sensory experiments of astringency.

Changes in mucociliary clearance over time in children with cystic fibrosis

OBJECTIVES

(a) To quantify changes in mucociliary clearance (MCC) over time in children with cystic fibrosis (CF) and the relationship between MCC and rate of infection with Pseudomonas aeruginosa (PA); (b) to determine the impact of MCC on the evolution of CF lung disease; and (c) to explore the role of mucus composition as a determinant of MCC.

METHODS

Children with CF, who had previously undergone an MCC measurement (visit 1), underwent the following tests 3 to 10 years later: (a) second MCC measurement (visit 2); (b) multiple breath washout to assess ventilation inhomogeneity, expressed as lung clearance index (LCI); (c) high resolution computed tomography lung scan (HRCT); and (d) induced sputum test. Number of PA + cultures/year between visits was documented and mucus dry weight was quantified in the children and adult controls.

RESULTS

Nineteen children completed both visits. Median time between visits was 4.6 years. Clearance declined 30% between visits. Lower MCC on visit 2 was associated with more PA+ cultures/year between visits. Lower MCC values on visit 1 were associated with higher LCI values and higher HRCT scores on visit 2. Mucus dry weight was significantly higher in children with CF compared with controls. Higher dry weights were associated with lower MCC.

CONCLUSIONS

Mucociliary clearance declines significantly over time in children with CF. The decline is associated with PA infection rate and is affected by mucus composition. Children with early slowing of MCC appear to be at risk for developing ventilation inhomogeneity and parenchymal lung damage when they are older.

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.

Silver nanoclusters-based fluorescent biosensing strategy for determination of mucin 1: Combination of exonuclease I-assisted target recycling and graphene oxide-assisted hybridization chain reaction

A novel label-free fluorescent biosensing strategy was described for the sensitive detection of mucin 1 (MUC1). It consisted of an M-shaped aptamer probe for exonuclease I (Exo I)-assisted target recycling (EATR) amplification, and two AgNCs-hairpin probes for graphene oxide (GO)-assisted hybridization chain reaction (HCR) amplification. Based on the specificity of aptamer-target recognition, the addition of MUC1 caused a conformational change in the M-shaped aptamer probe, which was split into a MUC1-P3 complex and a P1-P2 duplex. Exo I then catalyzed the cleavage of aptamer sequence P3 from the MUC1-P3 complex and released the target MUC1. The released target MUC1 was free to bind with a new M-shaped probe to perform EATR amplification. Furthermore, the P1-P2 duplex with three single-stranded arms can act as a primer to initiate HCR between hairpin probes AgNCs-H1 and AgNCs-H2. In the process of HCR, two AgNCs-hairpins were autonomously cross-opened, generating long linear double-stranded nanowires containing large numbers of AgNCs. These nanowires cannot be quenched by GO due to the weak affinity between the long double-stranded DNA and GO, thereby retaining a strong fluorescent signal indicative of the concentration of MUC1. With these designs, in addition to an extremely low detection limit of 0.36 fg mL^-1, the method exhibited an acceptable linear response to detect MUC1 from 1 fg mL^-1 to 1 ng mL^-1. Additionally, this method could be exerted with a high degree of success to detect MUC1 in diluted human serum with satisfactory results.

Technical note: Evaluation of 3 methods to determine mucin protein concentration in ileal digesta of young preweaning calves

The use of alternative sources of protein to substitute for milk proteins in milk replacers (MR) can increase the synthesis of endogenous proteins and therefore alter ileal or total-tract digestibility calculations. Mucin is the main component of gastrointestinal mucus and represents the greatest contribution to total endogenous protein. Mucin is difficult to isolate and has not been extensively studied in dairy calves. We explored 3 different procedures to analyze and estimate mucin protein (MUP) in ileal digesta of young dairy calves. Ileal digesta samples were collected from nine 30-d-old ileal-cannulated calves that were enrolled in a 3 × 3 replicated Latin square with 5-d periods. The 3 diets were a control whey protein-based MR (WPC), an isonitrogenous MR in which 50% of the protein was from enzyme-treated soybean meal (ESBM), and an N-free MR (NFREE). Mucin protein concentration and flow were analyzed by fractionation of the digesta and ethanol precipitation; this process served as the reference method. Alternative methods to estimate MUP consisted of using commercial enzymatic kits to analyze glucosamine (N-acetylglucosamine, GlcNAc) and galactosamine (N-acetylgalactosamine, GalNAc), 2 amino-sugars that are highly enriched in mucin. Before GlcNAc determination, samples were processed using 3 different procedures: sample clarification (GLCL), clarification plus hydrolysis (GLCH), and hydrolysis alone (GLHL). The MUP was estimated by regression of the GlcNAc and GalNAc values using previously validated equations. According with the bias and agreement analysis, none of the methods yielded MUP values similar to the reference method. However, GLHL showed a strong association with the reference method (ρ = 0.73). It allowed identifying the smaller MUP flows with NFREE compared with the other 2 diets and detecting the greater flow of ESBM than WPC, as observed with the reference method. Using the GlcNAc values from GLHL and the MUP measured with the reference method, we were able to establish a linear relationship between both methods (adjusted R2 = 0.75). We found that the GLHL method enabled detecting differences in MUP ileal flows between diets differing in protein level and source. Inferences about MUP secretions must be done cautiously because many dietary and physiological factors are involved. The adoption of practical techniques to determine MUP can help to increase our knowledge about gastrointestinal tract function and to improve the accuracy of MR digestibility calculations.

Mucosubstances in the porcine gastrointestinal tract: Fixation, staining and quantification

Mucins are of great interest in intestinal research and histochemical methods are often employed to identify them. Since it is in the nature of mucins that they are "hard to hold onto" once they come into contact with water, a frequently used medium in histochemistry, there are a number of challenges that may decrease diagnostic accuracy. As the outcome of methods published for microscopic detection of mucosubstances proved to be unsatisfactory in our hands, the aim was the establishment of a reliable and reproducible protocol. Tissue samples were available from pig feeding experiments. In the present study, we focus on a fixation / staining procedure without making comparisons between differently fed pigs. Several fixation and staining procedures were evaluated for their use in semiautomatic quantification and quality assessment of different mucus fractions simultaneous on one tissue section. Cryostat sectioning, subsequent fixation steps with heat, ethanol and modified Bouin's solution, followed by triple staining with high iron diamine, alcian blue and periodic acid-Schiff turned out to be the best method to identify sulfomucin, sialomucin and neutral mucin simultaneous on one tissue section. This methodology resulted in very good morphology of goblet cells with intact mucin containing vesicles within the cells, which was comparable to ultrastructural electron microscopical observations. Semiautomatic quantification of different mucins was possible. In conclusion, reliable mucus quantification and assessment of mucus quality requires strictly tested procedures. According to our experience, the most important aim after cryosectioning is fast fixation of the mucosubstances, which requires a combination of different fixation steps.

Development of a Novel ex vivo Nasal Epithelial Cell Model Supporting Colonization With Human Nasal Microbiota

The nasal mucosa provides first line defense against inhaled pathogens while creating a unique microenvironment for bacterial communities. Studying the impact of microbiota in the nasal cavity has been difficult due to limitations with current models including explant cultures, primary cells, or neoplastic cell lines. Most notably, none have been shown to support reproducible colonization by bacterial communities from human donors. Therefore, to conduct controlled studies of the human nasal ecosystem, we have developed a novel ex vivo mucosal model that supports bacterial colonization of a cultured host mucosa created by immortalized human nasal epithelial cells (NEC). For this model, immortalized NEC established from 5 male and 5 female donors were cultured with an air-interfaced, apical surface on a porous transwell membrane. NEC were grown from nasal turbinate tissues harvested from willed bodies or from discarded tissue collected during sinonasal procedures. Immortalized cells were evaluated through molecular verification of cell type, histological confirmation of tissue differentiation including formation of tight junctions, NEC multilayer viability, metabolism, physiology and imaging of the luminal surface by scanning electron microscopy. Results showed proper differentiation and multilayer formation at seven to 10 days after air interface that was maintained for up to 3 weeks. The optimized mucosal cultures created an environment necessary to sustain colonization by nasal microbiomes (NMBs) that were collected from healthy volunteers, cryogenically preserved and characterized with customized quantitative polymerase chain reaction (qPCR) arrays. Polymicrobial communities of nasal bacteria associated with healthy and inflamed states were consistently reproduced in matured NEC co-cultures by transplant of NMBs from multiple community types. The cultured NMBs were stable after an initial period of bacterial replication and equilibration. This novel ex vivo culture system is the first model that supports controlled cultivation of NMBs, allowing for lab-based causation studies and further experimentation to explore the complexities of host-microbe and microbe-microbe interactions.

Mucin gel assembly is controlled by a collective action of non-mucin proteins, disulfide bridges, Ca2+-mediated links, and hydrogen bonding

Mucus is characterized by multiple levels of assembly at different length scales which result in a unique set of rheological (flow) and mechanical properties. These physical properties determine its biological function as a highly selective barrier for transport of water and nutrients, while blocking penetration of pathogens and foreign particles. Altered integrity of the mucus layer in the small intestine has been associated with a number of gastrointestinal tract pathologies such as Crohn’s disease and cystic fibrosis. In this work, we uncover an intricate hierarchy of intestinal mucin (Muc2) assembly and show how complex rheological properties emerge from synergistic interactions between mucin glycoproteins, non-mucin proteins, and Ca²⁺. Using a novel method of mucus purification, we demonstrate the mechanism of assembly of Muc2 oligomers into viscoelastic microscale domains formed via hydrogen bonding and Ca²⁺-mediated links, which require the joint presence of Ca²⁺ ions and non-mucin proteins. These microscale domains aggregate to form a heterogeneous yield stress gel-like fluid, the macroscopic rheological properties of which are virtually identical to that of native intestinal mucus. Through proteomic analysis, we short-list potential protein candidates implicated in mucin assembly, thus paving the way for identifying the molecules responsible for the physiologically critical biophysical properties of mucus.

A detection and quantification label-free tool to speed up downstream processing of model mucins

Mucins are high-molecular weight glycoproteins (0.25-20 MDa) containing one or more domains that are heavily O-glycosylated. Their implications as targets for cancer treatment have increased the interest in these glycoproteins, mainly in the fields of vaccines and antibodies. However, mucins present high heterogeneity, posing challenges that affect purification processes and quality control analysis. In that sense, it is necessary to develop and improve downstream processes and analytical methods to characterize these products. Here a tool based on biolayer interferometry analysis to improve mucin's detection and quantification in a fast, simple and label free-way is presented. Taking advantage of lectin recognition of mucins' carbohydrate structures, several lectins were evaluated and immobilized on streptavidin biosensors. Different assay conditions were optimized and the most suitable lectin, Aleuria aurantia lectin (AAL), was selected. Bovine Submaxillary Gland and human MUC5B mucins were used as proof of concept and were successfully detected and quantified at different stages of purification. High sensitivity levels were achieved with LOD and LOQ of 3.8 μg mL-1 and 11.7 μg mL-1 for BSM, and 0.2 μg mL-1 and 0.6 μg mL-1 for MUC5B. AAL binding specificity was also confirmed with fucose competition assays. Our method represents an advance on mucins detection and quantification since the existing methods present several disadvantages for process development. Hereafter, it can be applied to the optimization of new or already established downstream processes for mucins' purification.

The cell surface mucin MUC1 limits the severity of influenza A virus infection

Cell surface mucin (cs-mucin) glycoproteins are constitutively expressed at the surface of respiratory epithelia where pathogens such as influenza A virus (IAV) gain entry into cells. Different members of the cs-mucin family each express a large and heavily glycosylated extracellular domain that towers above other receptors on the epithelial cell surface, a transmembrane domain that enables shedding of the extracellular domain, and a cytoplasmic tail capable of triggering signaling cascades. We hypothesized that IAV can interact with the terminal sialic acids presented on the extracellular domain of cs-mucins, resulting in modulation of infection efficiency. Utilizing human lung epithelial cells, we found that IAV associates with the cs-mucin MUC1 but not MUC13 or MUC16. Overexpression of MUC1 by epithelial cells or the addition of sialylated synthetic MUC1 constructs, reduced IAV infection in vitro. In addition, Muc1^-/- mice infected with IAV exhibited enhanced morbidity and mortality, as well as greater inflammatory mediator responses compared to wild type mice. This study implicates the cs-mucin MUC1 as a critical and dynamic component of the innate host response that limits the severity of influenza and provides the foundation for exploration of MUC1 in resolving inflammatory disease.

Mucins in Gastric Cancer - An Update

Mucins are high-molecular-weight glycoproteins expressed throughout the gastrointestinal tract, with a key role in mucosal protection and function. In gastric cancer expression of MUC5AC and MUC1 is reduced and denovo expression of MUC2 occurs. With progressive loss of tumor differentiation and increased tumor stage, expression of MUC5AC and MUC1 is further reduced, and MUC2 decreases. Isolated MUC2 expression (the intestinal phenotype) correlates with metastatic spread and poor survival. There is emerging evidence that MUC1 acts as an oncoprotein when overexpressed. The cytoplasmic tail of MUC1 interacts with the H. pylori virulence factor cagA and is a major effector of the wnt-β catenin intracellular signalling cascade. Polymorphism in the MUC1 gene has been identified in gastric cancer patients and may have a prospective role in the stratification of high-risk subjects. The MUC1 gene also mediates resistance to the recombinant HER2/neu antibody trastuzumab. Future research efforts will examine targeting MUC1 for therapeutic purposes.

IL-10 promotes production of intestinal mucus by suppressing protein misfolding and endoplasmic reticulum stress in goblet cells

BACKGROUND & AIMS

Protein misfolding and endoplasmic reticulum (ER) stress have been observed in intestinal secretory cells from patients with inflammatory bowel diseases and induce intestinal inflammation in mice. However, it is not clear how immune factors affect ER stress and therefore disease symptoms.

METHODS

We analyzed the effects of interleukin (IL)-10 on ER stress in intestinal tissues in wild-type C57BL/6, Winnie, IL-10(-/-), and Winnie × IL-10(+/-) mice. In Winnie mice, misfolding of the intestinal mucin Muc2 initiates ER stress and inflammation. We also analyzed the effects of different inhibitors of IL-10 signaling and the N-glycosylation inhibitor tunicamycin in cultured human LS174T goblet cells.

RESULTS

Administration of neutralizing antibodies against IL-10 or its receptor (IL-10R1) to Winnie mice rapidly exacerbated ER stress and intestinal inflammation compared with mice given vehicle (controls). Antibodies against IL-10 also increased accumulation of misfolded Muc2 in the ER of goblet cells of Winnie mice and increased T-cell production of inflammatory cytokines. Winnie × IL-10(+/-) mice and IL-10(-/-) mice with a single Winnie allele each developed more severe inflammation than Winnie mice or IL-10(-/-) mice. Administration of tunicamycin to wild-type mice caused intestinal ER stress, which increased when IL-10R1 was blocked. In LS174T cells, induction of ER stress with tunicamycin and misfolding of MUC2 were reduced by administration of IL-10; this reduction required STAT1 and STAT3. In LS174T cells incubated with tunicamycin, IL-10 up-regulated genes involved in MUC2 folding and in ER-associated degradation and maintained correct folding of MUC2, its transport from the ER, and its O-glycosylation and secretion.

CONCLUSIONS

IL-10 prevents protein misfolding and ER stress by maintaining mucin production in goblet cells and helps the intestine preserve the mucus barrier.

Molecular beacon-based quantitiation of epithelial tumor marker mucin 1

Mucin 1 (Muc1) is a glycoprotein expressed on most epithelial cell surfaces, which has been confirmed as a useful biomarker for the diagnosis of early cancers. In this study, we demonstrate that a quantum dot (QD)-aptamer beacon acts by folding-induced dissociation of a DNA intercalating dye, BOBO-3, in the presence of the target molecules, Muc1. Release of intercalated BOBO-3s from the QD-conjugated aptamers results in a decrease in QD fluorescence resonance energy transfer (FRET)-mediated BOBO-3 emission, allowing for label-free Muc1 detection and quantitation. We attain highly specific and wide-range detection (from 50nM to 20μM) of Muc1, suggesting that our QD-aptamer beacon can be a potential alternative to immuno-based assays for Muc1 detection. The detection methodology is expected to be improved for the early diagnosis of different types of epithelial cancers of large populations.