Self-association of mucin

Pinching dynamics, extensional rheology, and stringiness of saliva substitutes

Saliva substitutes are human-made formulations extensively used in medicine, food, and pharmaceutical research to emulate human saliva's biochemical, tribological, and rheological properties. Even though extensional flows involving saliva are commonly encountered in situations such as swallowing, coughing, sneezing, licking, drooling, gleeking, and blowing spit bubbles, rheological evaluations of saliva and its substitutes in most studies rely on measured values of shear viscosity. Natural saliva possesses stringiness or spinnbarkeit, governed by extensional rheology response, which cannot be evaluated or anticipated from the knowledge of shear rheology response. In this contribution, we comprehensively examine the rheology of twelve commercially available saliva substitutes using torsional rheometry for rate-dependent shear viscosity and dripping-onto-substrate (DoS) protocols for extensional rheology characterization. Even though most formulations are marketed as having suitable rheology, only three displayed measurable viscoelasticity and strain-hardening. Still, these too, failed to emulate the viscosity reduction with the shear rate observed for saliva or match perceived stringiness. Finally, we explore the challenges in creating saliva-like formulations for dysphagia patients and opportunities for using DoS rheometry for diagnostics and designing biomimetic fluids.

Understanding mucus modulation behavior of chitosan oligomers and dextran sulfate combining light scattering and calorimetric observations

This study reports the fundamental understanding of mucus-modulatory strategies combining charged biopolymers with distinct molecular weights and surface charges. Here, key biophysical evidence supports that low-molecular-weight (Mw) polycation chitosan oligosaccharides (COSs) and high-Mw polyanion dextran sulfate (DS) exhibit distinct thermodynamic signatures upon interaction with mucin (MUC), the main protein of mucus. While the COS → MUC microcalorimetric titrations released ~14 kcal/mol and ~60 kcal/mol, the DS → MUC titrations released ~1200 and ~1450 kcal/mol at pH of 4.5 and 6.8, respectively. The MPT-2 titrations of COS → MUC and DS → MUC indicated a greater zeta potential variation at pH = 4.5 (relative variation = 815 % and 351 %, respectively) than at pH = 6.8 (relative variation = 282 % and 136 %, respectively). Further, the resultant binary (COS-MUC) and ternary (COS-DS-MUC) complexes showed opposite behavior (aggregation and charge inversion events) according to the pH environment. Most importantly, the results indicate that electrostatics could not be the driving force that governs COS-MUC interactions. To account for this finding, we proposed a two-level abstraction model. Macro features emerge collectively from individual interactions occurring at the molecular level. Therefore, to understand the outcomes of mucus modulatory strategy based on charged biopolymers it is necessary to integrate both visions into the same picture.

Anatomical site-specific carbohydrate availability impacts Streptococcus pneumoniae virulence and fitness during colonization and disease

Streptococcus pneumoniae (Spn) colonizes the nasopharynx asymptomatically but can also cause severe life-threatening disease. Importantly, stark differences in carbohydrate availability exist between the nasopharynx and invasive disease sites, such as the bloodstream, which most likely impact Spn's behavior. Herein, using chemically-defined media (CDM) supplemented with physiological levels of carbohydrates, we examined how anatomical-site specific carbohydrate availability impacted Spn physiology and virulence. Spn grown in CDM modeling the nasopharynx (CDM-N) had reduced metabolic activity, slower growth rate, demonstrated mixed acid fermentation with marked H₂O₂ production, and were in a carbon-catabolite repression (CCR)-derepressed state versus Spn grown in CDM modeling blood (CDM-B). Using RNA-seq, we determined the transcriptome for Spn WT and its isogenic CCR deficient mutant in CDM-N and CDM-B. Genes with altered expression as a result of changes in carbohydrate availability or catabolite control protein deficiency, respectively, were primarily involved in carbohydrate metabolism, but also encoded for established virulence determinants such polysaccharide capsule and surface adhesins. We confirmed that anatomical site-specific carbohydrate availability directly influenced established Spn virulence traits. Spn grown in CDM-B formed shorter chains, produced more capsule, were less adhesive, and were more resistant to macrophage killing in an opsonophagocytosis assay. Moreover, growth of Spn in CDM-N or CDM-B prior to the challenge of mice impacted relative fitness in a colonization and invasive disease model, respectively. Thus, anatomical site-specific carbohydrate availability alters Spn physiology and virulence, in turn promoting anatomical-site specific fitness.

Alkaline pH Increases Swimming Speed and Facilitates Mucus Penetration for Vibrio cholerae

The diarrheal disease cholera is still a burden for populations in developing countries with poor sanitation. To develop effective vaccines and prevention strategies against Vibrio cholerae, we must understand the initial steps of infection leading to the colonization of the small intestine.

Intestinal mucus is the first line of defense against intestinal pathogens. It acts as a physical barrier between epithelial tissues and the lumen that enteropathogens must overcome to establish a successful infection. We investigated the motile behavior of two Vibrio cholerae strains (El Tor C6706 and Classical O395) in mucus using single-cell tracking in unprocessed porcine intestinal mucus. We determined that V. cholerae can penetrate mucus using flagellar motility and that alkaline pH increases swimming speed and, consequently, improves mucus penetration. Microrheological measurements indicate that changes in pH between 6 and 8 (the physiological range for the human small intestine) had little effect on the viscoelastic properties of mucus. Finally, we determined that acidic pH promotes surface attachment by activating the mannose-sensitive hemagglutinin (MshA) pilus in V. cholerae El Tor C6706 without a measurable change in the total cellular concentration of the secondary messenger cyclic dimeric GMP (c-di-GMP). Overall, our results support the hypothesis that pH is an important factor affecting the motile behavior of V. cholerae and its ability to penetrate mucus. Therefore, changes in pH along the human small intestine may play a role in determining the preferred site for V. cholerae during infection.

IMPORTANCE The diarrheal disease cholera is still a burden for populations in developing countries with poor sanitation. To develop effective vaccines and prevention strategies against Vibrio cholerae, we must understand the initial steps of infection leading to the colonization of the small intestine. To infect the host and deliver the cholera toxin, V. cholerae has to penetrate the mucus layer protecting the intestinal tissues. However, the interaction of V. cholerae with intestinal mucus has not been extensively investigated. In this report, we demonstrated using single-cell tracking that V. cholerae can penetrate intestinal mucus using flagellar motility. In addition, we observed that alkaline pH improves the ability of V. cholerae to penetrate mucus. This finding has important implications for understanding the dynamics of infection, because pH varies significantly along the small intestine, between individuals, and between species. Blocking mucus penetration by interfering with flagellar motility in V. cholerae, reinforcing the mucosa, controlling intestinal pH, or manipulating the intestinal microbiome will offer new strategies to fight cholera.

Clinically adaptable polymer enables simultaneous spatial analysis of colonic tissues and biofilms

Microbial influences on host cells depend upon the identities of the microbes, their spatial localization, and the responses they invoke on specific host cell populations. Multimodal analyses of both microbes and host cells in a spatially resolved fashion would enable studies into these complex interactions in native tissue environments, potentially in clinical specimens. While techniques to preserve each of the microbial and host cell compartments have been used to examine tissues and microbes separately, we endeavored to develop approaches to simultaneously analyze both compartments. Herein, we established an original method for mucus preservation using Poloxamer 407 (also known as Pluronic F-127), a thermoreversible polymer with mucus-adhesive characteristics. We demonstrate that this approach can preserve spatially-defined compartments of the mucus bi-layer in the colon and the bacterial communities within, compared with their marked absence when tissues were processed with traditional formalin-fixed paraffin-embedded (FFPE) pipelines. Additionally, antigens for antibody staining of host cells were preserved and signal intensity for 16S rRNA fluorescence in situ hybridization (FISH) was enhanced in poloxamer-fixed samples. This in turn enabled us to integrate multimodal analysis using a modified multiplex immunofluorescence (MxIF) protocol. Importantly, we have formulated Poloxamer 407 to polymerize and cross-link at room temperature for use in clinical workflows. These results suggest that the fixative formulation of Poloxamer 407 can be integrated into biospecimen collection pipelines for simultaneous analysis of microbes and host cells.

Acidic Submucosal Gland pH and Elevated Protein Concentration Produce Abnormal Cystic Fibrosis Mucus

In response to respiratory insults, airway submucosal glands secrete copious mucus strands to increase mucociliary clearance and protect the lung. However, in cystic fibrosis, stimulating submucosal glands has the opposite effect, disrupting mucociliary transport. In cystic fibrosis (CF) pigs, loss of cystic fibrosis transmembrane conductance regulator (CFTR) anion channels produced submucosal gland mucus that was abnormally acidic with an increased protein concentration. To test whether these variables alter mucus, we produced a microfluidic model of submucosal glands using mucus vesicles from banana slugs. Acidic pH and increased protein concentration decreased mucus gel volume and increased mucus strand elasticity and tensile strength. However, once mucus strands were formed, changing pH or protein concentration largely failed to alter the biophysical properties. Likewise, raising pH or apical perfusion did not improve clearance of mucus strands from CF airways. These findings reveal mechanisms responsible for impaired mucociliary transport in CF and have important implications for potential treatments.

Chemical and Microstructural Characterization of pH and [Ca2+] Dependent Sol-Gel Transitions in Mucin Biopolymer

Mucus is responsible for controlling transport and barrier function in biological systems, and its properties can be significantly affected by compositional and environmental changes. In this study, the impacts of pH and CaCl₂ were examined on the solution-to-gel transition of mucin, the primary structural component of mucus. Microscale structural changes were correlated with macroscale viscoelastic behavior as a function of pH and calcium addition using rheology, dynamic light scattering, zeta potential, surface tension, and FTIR spectroscopic characterization. Mucin solutions transitioned from solution to gel behavior between pH 4–5 and correspondingly displayed a more than ten-fold increase in viscoelastic moduli. Addition of CaCl₂ increased the sol-gel transition pH value to ca. 6, with a twofold increase in loss moduli at low frequencies and ten-fold increase in storage modulus. Changing the ionic conditions—specifically [H⁺] and [Ca²⁺] —modulated the sol-gel transition pH, isoelectric point, and viscoelastic properties due to reversible conformational changes with mucin forming a network structure via  non-covalent cross-links between mucin chains.

Non-C-mannosylable mucin CYS domains hindered proper folding and secretion of mucin

The CYS domain occurs in multiple copies in many gel-forming mucins. It is believed that CYS domains can interact with each other in a reversible manner, suggesting a key role of the domain in gel formation. This domain always contains in its amino-terminal sequence the C-mannosylation motif WXXW, but whether the CYS domain is C-mannosylated is debated, and the putative role of C-mannosylation of the domain is unclear. We prepared recombinant CYS domains of the human mucin MUC5B with (WXXW→AXXW) and without a single amino acid mutation and mini-5B mucins made of a large Ser/Thr/Pro region flanked by two CYS domains with the WXXW motif or with the mutated AXXW motif on the first, second or both CYS domains. We found that the single CYS domain and the two CYS domains of mini-5B mucin must be C-mannosylable for the efficient maturation and secretion of the recombinant molecules; otherwise, they are retained in the cell and co-localized with a resident enzyme of the endoplasmic reticulum.

The effect of saliva on the fate of nanoparticles

OBJECTIVES

The design of nanocarriers for local drug administration to the lining mucosa requires a sound knowledge of how nanoparticles (NPs) interact with saliva. This contact determines whether NPs agglomerate and become immobile due to size- and interaction-filtering effects or adsorb on the cell surface and are internalized by epithelial cells. The aim of this study was to examine the behavior of NPs in saliva considering physicochemical NP properties.

MATERIALS AND METHODS

The salivary pore-size distribution was determined, and the viscosity of the fluid inside of the pores was studied with optical tweezers. Distinct functionalized NPs (20 and 200 nm) were dispersed in saliva and salivary buffers and characterized, and surface-bound MUC5B and MUC7 were analyzed by 1D electrophoresis and immunoblotting. NP mobility was recorded, and cellular uptake studies were performed with TR146 cells.

RESULTS

The mode diameter of the salivary mesh pores is 0.7 μm with a peak width of 1.9 μm, and pores are filled with a low-viscosity fluid. The physicochemical properties of the NPs affected the colloidal stability and mobility: compared with non-functionalized particles, which did not agglomerate and showed a cellular uptake rate of 2.8%, functionalized particles were immobilized, which was correlated with agglomeration and increased binding to mucins.

CONCLUSION

The present study showed that the salivary microstructure facilitates NP adsorption. However, NP size and surface functionalization determine the colloidal stability and cellular interactions.

CLINICAL RELEVANCE

The sound knowledge of NP interactions with saliva enables the improvement of current treatment strategies for inflammatory oral diseases.

Generation of mucin gel particles with self-degradable and -releasable properties

Preparation of nano-sized particles via assembly of mucins and incorporation of lysozymes made it possible to gradually degrade mucin particles upon enzymatic cleavage of mucin molecules, facilitating the release of incorporated substances including lysozyme.

The innate immune properties of airway mucosal surfaces are regulated by dynamic interactions between mucins and interacting proteins: the mucin interactome

Chronic lung diseases such as cystic fibrosis, chronic bronchitis, and asthma are characterized by hypersecretion and poor clearance of mucus, which are associated with poor prognosis and mortality. Little is known about the relationship between the biophysical properties of mucus and its molecular composition. The mucins MUC5B and MUC5AC are traditionally believed to generate the characteristic biophysical properties of airway mucus. However, the contribution of hundreds of globular proteins to the biophysical properties of mucus is not clear. Approximately one-third of the total mucus proteome comprises distinct, multi-protein complexes centered around airway mucins. These complexes constitute a discrete entity we call the "mucin interactome". The data suggest that while the majority of these proteins interact with mucins via electrostatic and weak interactions, some interact through very strong hydrophobic and/or covalent interactions. Using reagents that interfere with protein-protein interactions, the complexes can be disassembled, and mucus rheology can be dramatically altered. Using MUC5B-glutathione S-transferase (GST) and MUC5B-galectin-3 as a representative of these interactions, we provide evidence that individual mucin protein interactions can alter the biophysical properties of mucus and modulate the biological function of the protein. We propose that the key mechano- and bio-active functions of mucus depend on the dynamic interactions between mucins and globular proteins. These observations challenge the paradigm that mucins are the only molecules that confer biophysical properties of mucus. These observations may ultimately lead to a greater understanding of the system and guide the development of strategies for more effective interventions using better therapeutic agents.

Hagfish slime and mucin flow properties and their implications for defense

When hagfish (Myxinidae) are attacked by predators, they form a dilute, elastic, and cohesive defensive slime made of mucins and protein threads. In this study we propose a link between flow behavior and defense mechanism of hagfish slime. Oscillatory rheological measurements reveal that hagfish slime forms viscoelastic networks at low concentrations. Mucins alone did not contribute viscoelasticity, however in shear flow, viscosity was observed. The unidirectional flow, experienced by hagfish slime during suction feeding by predators, was mimicked with extensional rheology. Elongational stresses were found to increase mucin viscosity. The resulting higher resistance to flow could support clogging of the attacker's gills. Shear flow in contrast decreases the slime viscosity by mucin aggregation and leads to a collapse of the slime network. Hagfish may benefit from this collapse when trapped in their own slime and facing suffocation by tying a sliding knot with their body to shear off the slime. This removal could be facilitated by the apparent shear thinning behavior of the slime. Therefore hagfish slime, thickening in elongation and thinning in shear, presents a sophisticated natural high water content gel with flow properties that may be beneficial for both, defense and escape.

Particle tracking microrheology of purified gastrointestinal mucins

The rheological characteristics of gastric and duodenal mucin solutions, the building blocks of the mucus layer that covers the epithelia of the two organs, were investigated using particle tracking microrheology. We used biochemically well characterized purified porcine mucins (MUC5AC and MUC2) as models for human mucins, to probe their viscoelasticity as a function of mucin concentration and pH. Furthermore, we used both reducing (dithiothreitol, DTT) and chaotropic agents (guanidinium chloride and urea) to probe the mesoscopic forces that mediate the integrity of the polymer network. At neutral pH both gastric and duodenal mucins formed self-assembled semi-dilute networks above a certain critical mucin concentration (c*) with the viscosity (η) scaling as η∼c(0.53±0.08) for MUC5AC and η∼c(0.53±0.06) for MUC2, where c is the mucin concentration. Above an even higher mucin concentration threshold (ce , the entanglement concentration) reptation occurs and there is a dramatic increase in the viscosity scaling, η∼c(3.92±0.38) for MUC5AC and η∼c(5.1±0.8) for MUC2. The dynamics of the self-assembled comb polymers is examined in terms of a scaling model for flexible polyelectrolyte combs. Both duodenum and gastric mucin are found to be pH switchable gels, gelation occurring at low pHs. There is a hundred-fold increase in the elastic shear modulus once the pH is decreased. The addition of DTT, guanidinium chloride and urea disassembles both the semi-dilute and gel structures causing a large increase in the compliance (decrease in their shear moduli). Addition of the polyphenol EGCG has a reverse effect on mucin viscoelasticity, that is, it triggers a sol-gel transition in semi-dilute mucin solutions at neutral pH.

Biological modeling of mucus to modulate mucus barriers

A recent study using a transgenic mouse, whose intestinal mucus contains a molecule made of 12 copies of a domain found in many gelling mucins, demonstrates that it is possible to strengthen mucus properties in situ, leading to promising new treatment strategies in diseases in which the mucosal barrier is impaired.

Salivary mucins in host defense and disease prevention

Mucus forms a protective coating on wet epithelial surfaces throughout the body that houses the microbiota and plays a key role in host defense. Mucins, the primary structural components of mucus that creates its viscoelastic properties, are critical components of the gel layer that protect against invading pathogens. Altered mucin production has been implicated in diseases such as ulcerative colitis, asthma, and cystic fibrosis, which highlights the importance of mucins in maintaining homeostasis. Different types of mucins exist throughout the body in various locations such as the gastrointestinal tract, lungs, and female genital tract, but this review will focus on mucins in the oral cavity. Salivary mucin structure, localization within the oral cavity, and defense mechanisms will be discussed. These concepts will then be applied to present what is known about the protective function of mucins in oral diseases such as HIV/AIDS, oral candidiasis, and dental caries.

Threonine Requirement of the Enterally Fed Term Infant in the First Month of Life

OBJECTIVE

Threonine is one of the essential amino acids. Its major fate is incorporation into intestinal mucosal proteins and synthesis of secretory glycoproteins. Therefore, it has an important function in the neonatal gut barrier integrity. The objective was to quantify the threonine requirement in fully enterally fed term neonates by means of the indicator amino acid oxidation (IAAO) method, using L-[1-C]phenylalanine as indicator.

METHODS

After a 24-hour test diet adaptation, containing randomly assigned amounts of threonine (range 5-182 mg · kg · day), the participating neonates received a primed continuous infusion of [C]bicarbonate and L-[1-C]phenylalanine. At baseline and during the plateau phase of both infusions, breath samples were obtained for CO2. The fractional L-[1-C]phenylalanine oxidation (FCO2) was estimated and plotted against the threonine intakes. Biphasic linear regression crossover analysis was used to calculate the breakpoint of the FCO2, representing the mean threonine requirement. Data are presented as mean ± SD.

RESULTS

Thirty-two term neonates (gestational age 39 ± 1 weeks, birth weight 3.3 ± 0.3 kg, mean postnatal age 10 ± 4 days) were studied. The mean threonine requirement was estimated to be 68 mg · kg · day with an upper and lower 95% confidence interval of 104 and 32 mg · kg · day, respectively (r = 0.37).

CONCLUSIONS

The determined threonine requirement is extremely close to the existing requirement recommendations (∼90% of the present World Health Organization requirement guidelines). Infant formula preparations presently on the market, however, contain up to twice as much threonine as recommended. The threonine intake in formula-fed infants may therefore be reduced considerably.

Boundary lubrication by associative mucin

Mucus lubricants are widely distributed in living organisms. Such lubricants consist of a gel structure constructed by associative mucin. However, limited tribological studies exist on associative mucin fluids. The present research is the first to investigate the frictional behavior of a typical intact vertebrate mucin (loach skin mucin), which can recover the gel structure of mucus via hydrophobic association under physiological conditions (5-10 mg/mL loach skin mucin dissolved in water). Both rough hydrophobic and hydrophilic polydimethylsiloxane (PDMS) rubber plates were used as friction substrates. Up to 10 mg/mL loach skin mucin dissolved in water led to a 10-fold reduction in boundary friction of the two substrates. The boundary-lubricating ability for hydrophilic PDMS decreased with rubbing time, whereas that for hydrophobic PDMS remained constant. The boundary-lubricating abilities of the mucin on hydrophobic PDMS and hydrophilic PDMS showed almost similar responses toward changing concentration or sodium dodecyl sulfate (SDS). The mucin fluids reduced boundary friction coefficients (μ) only at concentrations (c) in which intermucin associations were formed, with a relationship shown as μ ∼ c(-0.7). Destroying intermucin associations by SDS largely impaired the boundary-lubricating ability. Results reveal for the first time that intermolecular association of intact mucin in bulk solution largely enhances boundary lubrication, whereas tightly adsorbed layer plays a minor role in the lubrication. This study indicates that associated mucin should contribute considerably to the lubricating ability of biological mucus in vivo.

Rheological behavior of hydrophobically modified polysulfobetaine methacrylate aqueous solution

Polysulfobetaine methacrylate (PSBMA) that was hydrophobically modified with methacrylic acid 2,3-epoxypropyl ester (GMA) was synthesized via micellar copolymerization method. Viscosity of the hydrophobically modified PSBMA solution was sensitive to added salt concentration.

Development and application of an inhalation bioaccessibility method (IBM) for lead in the PM10 size fraction of soil

An approach for assessing the inhalation bioaccessibility of Pb in the PM10 size fraction is presented, using an in vitro simulated epithelial lung fluid to represent the extracellular environment of the lung. The developed inhalation bioaccessibility method (IBM) is applied to a range of urban surface soils and mining wastes obtained from Mitrovica, Kosovo, a site where impacts upon human health following exposure to Pb have been internationally publicised. All Pb determinations were undertaken by inductively coupled plasma mass spectrometry (ICP-MS). The pseudo-total concentration of Pb (microwave acid digestion using aqua-regia) varied between matrices: smelter (20,900-72,800mgkg(-1)), topsoil (274-13,700mgkg(-1)), and tailings (2990mgkg(-1)-25,300mgkg(-1)). The in vitro inhalation bioaccessibility was typically several orders of magnitude lower: smelter (7.0-965mgkg(-1)), topsoil (9.8-1060mgkg(-1)), and tailings (0.7mgkg(-1)-49.2mgkg(-1)). The % inhalation bioaccessibility ranged from 0.02 to 11.0%, with the higher inhalation bioaccessible Pb concentrations being observed for samples from the Bosniak Mahalla area of Mitrovica (an area proposed for the relocation of internally displaced peoples). The estimated inhalation dose (for adults) calculated from the PM10 pseudo-total Pb concentration ranged from 0.369 to 1.284μgkg(-1)BWday(-1) (smelter), 0.005-0.242μgkg(-1)BWday(-1) (topsoil), and 0.053-0.446μgkg(-1)BWday(-1) (tailings). When daily inhalation doses were calculated using the bioaccessible Pb concentration the modelled exposure doses were much lower: smelter (0.0001-0.0170μgkg(-1)BWday(-1)), topsoil (0.0002-0.0187μgkg(-1)BWday(-1)) and tailings (0.0001-0.0009μgkg(-1)BWday(-1)). Modelled for the neutral pH conditions of the interstitial lung environment, the results indicate a low potential inhalation bioaccessibility for Pb in these samples. Given the already elevated environmental Pb burden experienced by the local population, where significant prolonged dust or particulate generating activities are taking place, or where the inhaled particles are phagocytized, then inhalation exposure has the potential to significantly add to the overall Pb burden. Such data are important for local policy makers to better enable them to assess risk, especially in areas where soils/dusts have elevated levels of contamination.

Nicotine alters mucin rheological properties

Tobacco smoke exposure, the major cause of chronic obstructive pulmonary disease (COPD), instigates a dysfunctional clearance of thick obstructive mucus. However, the mechanism underlying the formation of abnormally viscous mucus remains elusive. We investigated whether nicotine can directly alter the rheological properties of mucin by examining its physicochemical interactions with human airway mucin gels secreted from A549 lung epithelial cells. Swelling kinetics and multiple particle tracking were utilized to assess mucin gel viscosity change when exposed to nicotine. Herein we show that nicotine (≤50 nM) significantly hindered postexocytotic swelling and hydration of released mucins, leading to higher viscosity, possibly by electrostatic and hydrophobic interactions. Moreover, the close association of nicotine and mucins allows airway mucus to function as a reservoir for prolonged nicotine release, leading to correlated pathogenic effects. Our results provide a novel explanation for the maltransport of poorly hydrated mucus in smokers. More importantly, this study further indicates that even low-concentration nicotine can profoundly increase mucus viscosity and thus highlights the health risks of secondhand smoke exposure.

The microstructure and bulk rheology of human cervicovaginal mucus are remarkably resistant to changes in pH

The protective barrier, lubricant, and clearance functions of mucus are intimately coupled to its microstructure and bulk rheology. Mucus gels consist of a network of mucin biopolymers along with lipids, salts, and other proteins and exhibit similar biochemical and physical properties across diverse mucosal surfaces. Nevertheless, mucus is exposed to a broad range of pH values throughout the human body. Protein functions are typically sensitive to small changes in pH, and prior investigations using reconstituted, purified mucin gels suggested mucus undergoes a transition from a low-viscosity liquid at neutral pH to a highly viscoelastic solid at low pH. We sought to determine whether those observations hold for fresh, minimally perturbed human mucus ex vivo by using different-sized muco-inert nanoparticles to probe microstructure and cone-and-plate rheometry to measure bulk rheology. We demonstrate that both the microstructure and bulk rheology of fresh, undiluted, and minimally perturbed cervicovaginal mucus exhibit relatively minor changes from pH 1-2 to 8-9, in marked contrast with the pH sensitivity of purified mucin gels. Our work also suggests additional components in mucus secretions, typically eliminated during mucin purification and reconstitution, may play an important role in maintaining the protective properties of mucus.

Understanding glycoprotein behaviours using Raman and Raman optical activity spectroscopies: characterising the entanglement induced conformational changes in oligosaccharide chains of mucin

We illustrate the great potential of Raman and ROA spectroscopies for investigating the structure and organisation of glycoproteins and the complex matrices they can form. In combination these spectroscopic techniques are sensitive to changes in conformation revealing details of secondary and tertiary structures, probing hydrogen bonding interactions, as well as resolving side chain orientation and the absolute configuration of chiral substructures. To demonstrate this potential we have characterised the structural changes in a complex glycoprotein, mucin. Spectral changes were observed during the entanglement transition as the mucin concentration was increased. By applying two-dimensional correlation analysis (2DCos) to the ROA and Raman concentration-dependent spectral sets delicate transitions in mucin conformation could also be determined. From ~20-40 mg/ml conformational transitions assigned mainly to the sugar N-acetyl-d-galactosamine (GalNAc), which is the linking saccharide unit to the protein backbone, were monitored. Further changes in local oligosaccharide conformation above 40 mg/ml were also monitored, together with other structural transitions observed in the protein core, particularly β-structure formation. Consequently, these spectral techniques were shown to monitor the formation of transient entanglements formed by brush-brush interactions between oligosaccharide combs of mucin molecules identifying changes in both carbohydrate and protein moieties. This work clearly shows how these methods can be used to elucidate fresh insights into the complex behaviour of these large complex molecules.

Continuum-kinetic-microscopic model of lung clearance due to core-annular fluid entrainment

The human lung is protected against aspirated infectious and toxic agents by a thin liquid layer lining the interior of the airways. This airway surface liquid is a bilayer composed of a viscoelastic mucus layer supported by a fluid film known as the periciliary liquid. The viscoelastic behavior of the mucus layer is principally due to long-chain polymers known as mucins. The airway surface liquid is cleared from the lung by ciliary transport, surface tension gradients, and airflow shear forces. This work presents a multiscale model of the effect of airflow shear forces, as exerted by tidal breathing and cough, upon clearance. The composition of the mucus layer is complex and variable in time. To avoid the restrictions imposed by adopting a viscoelastic flow model of limited validity, a multiscale computational model is introduced in which the continuum-level properties of the airway surface liquid are determined by microscopic simulation of long-chain polymers. A bridge between microscopic and continuum levels is constructed through a kinetic-level probability density function describing polymer chain configurations. The overall multiscale framework is especially suited to biological problems due to the flexibility afforded in specifying microscopic constituents, and examining the effects of various constituents upon overall mucus transport at the continuum scale.

Mucus as a barrier to lipophilic drugs

Mucus is a complex hydrogel, comprising glycoproteins, lipids, salts, DNA, enzymes and cellular debris, covering many epithelial surfaces in the human body. Once secreted, mucin forms a barrier to protect the underlying tissues against the extracellular environment. Mucus can therefore adversely affect the absorption or action of drugs administered by the oral, pulmonary, vaginal, nasal or other routes. Solubility and lipophilicity are key factors determining drug absorption, as a drug has to be soluble in the body fluids at the site of absorption and must also possess enough lipophilicity to permeate the biological membrane. Evidence has accumulated over the past 40 years indicating that poorly soluble drugs will interact with mucus glycoprotein. Studies of the permeability of native or purified mucous gels are important when it comes to understanding the relative importance of hindered diffusion versus drug binding in mucous layers. This review highlights the current understanding of the drug-mucin interaction and also examines briefly the interaction of polymers and particles with the mucus matrix. While the concept of mucoadhesion was thought to provide an intensified and prolonged contact to mucosal absorption sites, mucopenetrating properties are nowadays being discussed for (nano)particulate carriers to overcome the mucus as a barrier and enhance drug delivery through mucus.

Surface induced self-organization of comb-like macromolecules

We present a review of the theoretical and experimental evidence for the peculiar properties of comb copolymers, demonstrating the uniqueness of these materials among other polymer architectures. These special properties include an increase in stiffness upon increasing side-chain length, the spontaneous curvature of adsorbed combs, rod-globule transition, and specific intramolecular self-assembly. We also propose a theory of chemically heterogeneous surface nanopattern formation in ultrathin films of comblike macromolecules containing two different types (A and B) of incompatible side chains (so-called binary combs). Side chains of the binary combs are strongly adsorbed on a surface and segregated with respect to the backbone. The thickness of surface domains formed by the B side chains is controlled by the interaction with the substrate. We predict the stability of direct and inverse disc-, torus- and stripelike nanostructures. Phase diagrams of the film are constructed.

Interactions between human neutrophils and mucin-coated surfaces

Recently, we showed microscopically that bovine (BSM), porcine (PGM) and human (MG1) mucin coatings could suppress the adhesion of neutrophils to a polyethylene terephthalate-based model biomaterial (Thermanox). Here, using the release of reactive oxygen species (ROS) as a marker of material-induced neutrophil activation, the strong surface-passivating effects of these mucin coatings were corroborated. Under optimal adsorption conditions, all mucin species performed equally well, thus indicating a high degree of functional homology between the mucins. Cell adhesion and morphology correlated well with the release of ROS. Quartz crystal microbalance (QCM-D) analysis linked low neutrophil activation to efficient mucin surface-shielding. Interestingly, the shielding power appeared equal for thick expanded and thin compact mucin coatings. Combined mucin-serum coatings were found to be highly surface-passivating. Particularly, since our data suggested partly synergistic mucin-serum action, we highlight the possibility that pre-adsorbed mucins could provide favorable support for adsorbing host components.

Saliva as research material: Biochemical, physicochemical and practical aspects

Whole saliva is a complex mixture of proteins and other molecules which originate from several sources. The biochemical and physicochemical properties of saliva contribute to the numerous functions of saliva in, e.g., speech, maintaining oral and general health, and food processing. Interest in saliva has increased in the last few years for its potential to diagnose viral, bacterial and systemic diseases. The use of saliva as research material may pose particular problems due to its inherent variability and instability. This review describes practical aspects of salivary as research material with emphasis on protein biochemistry and physical chemistry.

Charge and interfacial behavior of short side-chain heavily glycosylated porcine stomach mucin

The current accepted model for high-molecular-weight gastric mucins of the MUC family is that they adopt a polydisperse coil conformation in bulk solutions. We develop this model using well-characterized highly purified porcine gastric mucin and examine the molecules' charge and interfacial adsorption. "Orthana" mucin has short side-chains, low levels of sialic acid residues, and includes minute amounts of cystine residues that can be responsible for the self-polymerization of mucin. Atomic force microscopy and transmission electron microscopy are used to examine the interfacial behavior of the mucin and clearly demonstrate the existence of discrete spherical subunits within the mucin molecules, with sizes in agreement with static light scattering, dynamic light scattering, and zeta potential measurements. Furthermore images indicate the combs are assembled with a beads on a string conformation; the daisy chain model. Zeta potential measurements establish the polyampholyte nature of the mucin molecules, which is used to explain their adsorption behavior on similarly charged surfaces.

Metabolism of threonine in newborn infants

Threonine kinetics, threonine oxidative pathway, and the relationship between threonine and whole body protein turnover were quantified in 10 healthy term infants during the first 48 h after birth. The kinetic data were obtained 6 h after the last feed (fasting) and in response to formula feeding, using [U-(13)C(4),(15)N]threonine, [(2)H(5)]phenylalanine, and [(15)N]glycine tracers. The rate of carbon dioxide production (Vco(2)) and (13)C enrichment of the expired CO(2) were measured to quantify the rate of oxidation of threonine. The rate of appearance (R(a)) of threonine (136 +/- 37 micromol.kg(-1).h(-1)) was higher in newborn infants than that reported in adults. Formula feeding resulted in a significant decrease in threonine R(a) (P < 0.05). A significant positive correlation was seen between phenylalanine R(a) and threonine R(a), both during fasting and after formula feeding (r(2) = 0.65). In contrast to a 1:1 ratio of threonine and phenylalanine in mixed muscle protein, threonine R(a) relative to phenylalanine R(a) was 2.2 +/- 0.4. The fractional rate of threonine flux oxidized was 20% during fasting and 26% (P < 0.05) in response to nutrient administration. There was a significant correlation between plasma threonine concentration and threonine oxidation (r(2) = 0.75). No measurable incorporation of threonine in plasma glycine was seen. These data suggest that threonine is exclusively degraded by the glycine-independent serine/threonine dehydratase pathway. A higher flux of threonine relative to phenylalanine indicates higher turnover of threonine enriched proteins.

Mixtures of mucin and oppositely charged surfactant aggregates with varying charge density. Phase behavior, association, and dynamics

The nonionic surfactant Tween80 is a commonly used excipient in drug formulations containing an active substance with low aqueous solubility. Model drug vehicles with varying charge density were obtained by mixing Tween80 (PS-80) with the cationic surfactant Tetradecyltrimethylammonium chloride (TTAC), thus forming mixed micelles. The micelles were mixed with the negatively charged polyelectrolyte mucin, which is a component in the protective mucus layer covering epithelial cell linings. Depending on the composition of the mixture, complex-formation could be followed by precipitation. Using X-ray diffraction, it was found that the precipitate contained a lamellar phase with properties sensitive to the proportion of PS-80. Higher amounts of PS-80 were found to oppose phase separation. Further analysis in the one-phase region, or alternatively of the supernatant of two-phase samples, by (1)H NMR, HPLC, and diffusion measurements with PGSE-NMR led to the conclusions that at low proportion of PS-80 aggregates composed of mixed (PS-80 and TTAC) micelles and mucin were formed, whereas increased concentrations of PS-80 favored the dissolution of the precipitate and limited the interactions between mixed micelles and the polymer.

Mucous Systems Show a Novel Mechanical Response to Applied Deformation

Mucous secretions have a wide range of biological functions that are intimately linked with their rheological properties. In addition, many mucous secretions are exposed to significant stress and deformation during physiological function. This study has examined the rheological response of three mucous systems, native pig gastric mucus, purified mucin gels, and mucin alginate gels, to increasing applied stress to a level sufficient to induce flow behavior. A novel, frequency-dependent stress hardening was observed in all three systems. This hardening behavior may play a significant role in the ability of mucous systems to resist mechanical disruption in the physiological state.

Dynamic molecular resolution imaging of preocular fluid impressions

AIM

The preocular fluid is renewed with molecules secreted by the underlying cells and with lacrimal gland secretions, while maintaining a stable surface topography. The authors tested the hypothesis that interactions between gelled and newly inserted mucins are the key to this stability.

METHODS

Using atomic force microscopy, the authors studied the topography of the freshly isolated preocular fluid obtained by impression cytology. The effects of adding mucins to this impression were compared with adding mucins to a pure mucin macromolecular assembly as a single component control to the more complex preocular fluid. The control structure was built up by repeated addition of pure ocular mucin to a tethering surface.

RESULTS

Imaging at molecular resolution showed a thin layer of superficial preocular fluid with an appearance consistent with a gel that was very flat, with surface roughness of approximately 0.1 nm. Mucin molecules adhering to a clean flat surface maintained their individual character when overlapping, whereas molecules integrating in the impression could not be followed individually. Both the preocular impression and the pure mucin assembly were stable under imaging for at least 90 minutes. The roughness of the pure mucin network decreased as more mucin was added. In contrast, there was a small increase in the roughness of the 2.25 microm2 area of impression over the 60 minutes of continuous imaging, although locally there appeared to be infill of low height features. Disulphide bond breaking resulted in the collapse of the imaged structure in both the pure mucin control and the more complex ex vivo preocular impression.

CONCLUSIONS

Polymeric mucins linked by disulphide bonds prevent or lessen loss of ocular surface material into the surrounding aqueous tears.

Human preocular mucins reflect changes in surface physiology

BACKGROUND/AIMS

Mucin function is associated with both peptide core and glycosylation characteristics. The authors assessed whether structural alterations occurring during mucin residence in the tear film reflect changes in ocular surface physiology.

METHODS

Ocular surface mucus was collected from normal volunteers as N-acetyl cysteine (NAcCys) washes or directly from the speculum after cataract surgery. To assess the influence of surface health on mucins, NAcCys washings were also obtained from patients with symptoms, but no clinical signs of dry eye (symptomatics). Mucins were extracted in guanidine hydrochloride (GuHCl) with protease inhibitors. Buoyant density of mucin species, a correlate of glycosylation density, was followed by reactivity with anti-peptide core antibodies. Mucin hydrodynamic volume was assessed by gel filtration on Sepharose CL2B.

RESULTS

Surface fluid and mucus contained soluble forms of MUC1, MUC2, MUC4, and MUC5AC and also the same species requiring DTT solubilisation. Reactivity with antibodies to MUC2 and MUC5AC peaked at 1.3-1.5 g/ml in normals, while dominated by underglycosylated forms in symptomatics. Surface mucins were predominantly smaller than intracellular species. MUC2 size distributions were different in symptomatics and normals, while those of MUC5AC were similar in these two groups.

CONCLUSIONS

A reduction in surface mucin size indicates post-secretory cleavage. Dissimilarities in surface mucin glycosylation and individual MUC size distributions in symptomatics suggest changes in preocular mucin that might precede dry eye signs.

Calcium-dependent protein interactions in MUC5B provide reversible cross-links in salivary mucus

The macromolecular organization within saliva was investigated by tracer diffusion measurements of fluorescent polystyrene microspheres by fluorescence recovery after photobleaching using a confocal microscope (confocal-FRAP). There was a concentration-dependent reduction in microsphere diffusion; this was much greater in the presence of calcium (10 mm) and was reduced by the addition of EGTA (10 mm). These effects on tracer diffusion showed that native saliva contained a macromolecular organization that was sensitive to free calcium concentrations. This was supported by a major increase in the weight average molecular weight of the high molecular weight mucin fraction in saliva (10-62 x 106) and an increase in intrinsic viscosity of saliva (733 to 1203 ml/g) both caused by calcium. Analysis of the change in tracer diffusion in saliva showed a 20-fold increase in the apparent pore size (from 130 nm in 10 mm CaCl2 to 2600 nm in 10 mm EGTA at physiological concentration). The effect was specific for calcium and was unaffected by up to 2 m NaCl. The calcium binding activity was contained in a high buoyant density fraction of saliva excluded from Sepharose CL-2B. Calcium binding to this fraction gave an approximate Kd of 7 x 10-6 m, and the binding was irreversibly destroyed by treatment with 6 m guanidinium chloride and by mild reduction, suggesting it to be to a protein site. This fraction of saliva was shown to contain MUC5B as the single major protein species by positive ion electrospray ionization-tandem mass spectrometry analysis. The results suggested that oligomeric MUC5B in saliva is assembled into much larger linear or branched assemblies through calcium-mediated protein cross-links.

The gel matrix of gastric mucus is maintained by a complex interplay of transient and nontransient associations

The gel nature of mucus is fundamental to its physiological functions; however, the structure of the mucus gel matrix is unclear. Here, small and large deformation rheology has been used to investigate the physical nature of the gel matrix and the forces that maintain this matrix in pig gastric mucus. The gelation process in mucus has been shown to be comparable with that of other polymer gel systems. Nongelling portions of mucin have been identified within the gel network, and the importance of transient, relaxable interactions to the maintenance of the mucus gel matrix has been demonstrated. The structure of the mucus gel matrix is considered in relation to the functional properties of mucus gels.