Liquid crystalline order in mucus

Mucus plays an exceptionally wide range of important biological roles. It operates as a protective, exchange, and transport medium in the digestive, respiratory, and reproductive systems of humans and other vertebrates. Mucus is a polymer hydrogel. It is secreted as discrete packages (secretory granules) by specialized secretory cells. Mucus hydrogel is stored in a condensed state inside the secretory granules. Depending upon the architecture of their constituent macromolecules and on the composition of the solvent, polymer gels can form liquid crystalline microstructures, with orientational order being exhibited over optically resolvable distances. Individual mucin molecules consist of alternating rigid segments (heavily glycosylated; hydrophilic) and flexible segments (nonglycosylated; hydrophobic). Polymer molecules consisting of rigid units linked by flexible spacers are frequently associated with liquid crystalline behavior, which again raises the possibility that mucus could form anisotropic fluid phases. Suggestions that mucins may be self-associating in dilute solution have previously been challenged on the basis of sedimentation-equilibrium studies performed on mucus in which potential sites of association were competitively blocked with inhibitors. However, the formation of stable liquid crystalline phases does not depend on the existence of inter- or intramolecular associations; these phases can form on the basis of steric considerations alone.     

Mucin macromolecules in normal, adenomatous, and carcinomatous colon: evidence for the neotransformation

Mucins are high molecular-weight glycoproteins having oligosaccharides attached to serine or threonine residues of the mucin core protein backbone by O-glycosidic linkages. They are major components of mucus, covering the luminal surfaces of epithelial respiratory, gastrointestinal and reproductive tracts, and responsible for its viscoelastic properties. The core proteins of mucins are encoded by different mucin genes. Aberrations in the cell surface carbohydrates including mucins have been regarded as a universal characteristic of the malignant transformation of cells. These alterations are considered to be relevant to the abnormal behaviour of cancer cells, such as altered cell adhesion or metastasis, and to the avoidance of immunological defence.     

Several Sterilization Strategies Maintain the Functionality of Mucin Glycoproteins

Mucin glycoproteins, the macromolecular components of mucus, combine a broad range of biomedically important properties. Among those is the ability of mucin solutions to act as excellent lubricants. However, to be able to use purified, endogenous mucin glycoproteins as components of a biomedical product, the mucins need to be sterile; this, in turn, makes it necessary to subject the mucins to quite harsh physical treatments, such as heat exposure, autoclaving, UV‐, or γ‐irradiation, which might compromise the functionality of the glycoproteins. Here, it is shown that mucins are indeed able to withstand most of those treatments without suffering significant lubrication impairment or structural degradation. Among those treatments, which left the mucins unharmed, γ‐irradiation is identified to be the most powerful one in terms of inactivating microbial contaminations. The obtained results demonstrate a remarkable sturdiness of mucins, which opens up broad possibilities for them to be further processed into materials, e.g., as parts of biomedical products.     

Slimy partners: the mucus barrier and gut microbiome in ulcerative colitis

Ulcerative colitis (UC) is a chronic recurrent intestinal inflammatory disease characterized by high incidence and young onset age. Recently, there have been some interesting findings in the pathogenesis of UC. The mucus barrier, which is composed of a mucin complex rich in O-glycosylation, not only provides nutrients and habitat for intestinal microbes but also orchestrates the taming of germs. In turn, the gut microbiota modulates the production and secretion of mucins and stratification of the mucus layers. Active bidirectional communication between the microbiota and its ‘slimy’ partner, the mucus barrier, seems to be a continually performed concerto, maintaining homeostasis of the gut ecological microenvironment. Any abnormalities may induce a disorder in the gut community, thereby causing inflammatory damage. Our review mainly focuses on the complicated communication between the mucus barrier and gut microbiome to explore a promising new avenue for UC therapy.Subject terms: Glycobiology, Ulcerative colitis     

泥鳅体表黏液的流变行为

研究了无鳞鱼———泥鳅的体表黏液流变行为,发现黏液的稳态流动存在着3个不同区域:第一区域内,黏度随剪切速率(γ)变化不显著,呈现牛顿流动行为;第二区域内,随γ增大,黏度下降,呈现非牛顿行为;第三区域内,随γ继续增大,黏度又基本保持不变.黏液表观黏度(ηa)与γ的关系可用Carreau模型很好地拟合,其增比黏度(ηsp)与浓度(c)的关系为ηsp∝c1.5,表明黏液处于亚浓缠结区域.在测试频率(ω)范围内,黏液的动态储能模量(G′)大于动态损耗模量(G″),表明与黏性相比较,弹性占优,且G′及G″随ω变化不显著.存在一临界温度(35℃),当低于35℃时,黏液黏度随温度变化不显著,当高于35℃时,黏液变性,表现出不同的流变行为.     

Transport properties and aggregation phenomena of polyoxyethylene sorbitane monooleate (polysorbate 80) in pig gastrointestinal mucin and mucus

The aqueous environment in the gastrointestinal tract frequently requires solubilization of hydrophobic drug molecules in appropriate drug delivery vehicles. An effective uptake/absorption and systemic exposure of a drug molecule entails many processes, one being transport properties of the vehicles through the mucus layer. The mucus layer is a complex mixture of biological molecules. Among them, mucin is responsible of the gel properties of this layer. In this study, we have investigated the diffusion of polyoxyethylene sorbitane monooleate (polysorbate 80), a commonly used nonionic surfactant, in aqueous solution, in mucin solutions at 0.25 and 5 wt %, and in mucus. These measurements were done by using the pulsed field gradient spin echo nuclear magnetic resonance (PGSE-NMR) technique. We conclude that polysorbate 80 is a mixture of non-surface-active molecules that can diffuse freely through all the systems investigated and of surface-active molecules that form micellar structures with transport properties strongly dependent on the environment. Polysorbate 80 micelles do not interact with mucin even though their diffusion is hindered by obstruction of the large mucin molecules. On the other hand, the transport is slowed down in mucus due to interactions with other components such as lipids depots. In the last part of this study, a hydrophobic NMR probe molecule has been included in the systems to mimic a hydrophobic drug molecule. The measurements done in aqueous solution revealed that the probe molecules were transported in a closely similar way as the polysorbate 80 micelles, indicating that they were dissolved in the micellar core. The situation was more complex in mucus. The probe molecules seem to dissolve in the lipid depots at low concentrations of polysorbate 80, which slows down their transport. At increasing concentration of polysorbate 80, the diffusion of the probe molecules increases indicating a continuous dissolution of hexamethyldisilane in the core of polysorbate 80 micelles.     

On the Structure of Poly-(Ethylene Oxide) When Immersed in Water

It can be shown that poly-(ethylene oxide) in its monomeric form, at molecular weights greater than about 1,000, cannot be soluble in water. Nonetheless, in actual practice poly-(ethylene oxide) is widely used for its hydrophilicity and for its unlimited aqueous solubility. The explanation for this apparent contradiction lies in the fact that poly-(ethylene oxide) molecules form a nonionic surfactant of a novel category, with its hydrophilic and hydrophobic chains attached to each other over their entire length so that its hydrophilic ethylene oxide side is exposed to the water interface while the hydrophobic poly-(ethylene) side remains hidden thanks to the formation of micelles.     

A Convergent Strategy for the Synthesis of Type‐1 Elongated Mucin Cores 1–3 and the Corresponding Glycopeptides

Mucins are a class of highly O‐glycosylated proteins found on the surface of cells in epithelial tissues. O‐Glycosylation is crucial for the functionality of mucins and changes therein can have severe consequences for an organism. With that in mind, the elucidation of interactions of carbohydrate binding proteins with mucins, whether in morbidly altered or unaltered conditions, continue to shed light on mechanisms involved in diseases like chronic inflammations and cancer. Despite the known importance of type‐1 and type‐2 elongated mucin cores 1–4 in glycobiology, the corresponding type‐1 structures are much less well studied. Here, the first chemical synthesis of extended mucin type‐1 O‐glycan core 1–3 amino acid structures based on a convergent approach is presented. By utilizing differentiation in acceptor reactivity, shared early stage Tn‐ and T‐acceptor intermediates were elongated with a common type‐1 [β‐D ‐Gal‐1,3‐β‐D ‐GlcNAc] disaccharide, which allows for straightforward preparation of diverse glycosylated amino acids carrying the type‐1 mucin core 1–3 saccharides. The obtained glycosylated 9‐fluorenylmethoxycarbonyl (Fmoc)‐protected amino acid building blocks were employed in synthesis of type‐1 mucin glycopeptides, which are useful in biological applications.     

Surface hydrophilization for polypropylene microporous membranes: A facile interfacial crosslinking approach

A hydrophilic surface is very important for hydrophobic separation membranes such as polypropylene microporous membranes (PPMMs). In this work a facile and effective method, interfacial crosslinking combined with pretreatment by dielectric barrier discharge plasma at atmospheric pressure, was developed for endowing PPMMs with a hydrophilic and charged surface. Poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA) and p-xylylene dichloride were selected for quaternization crosslinking to form a positively charged coating layer, which was characterized with FT-IR/ATR, XPS, and FESEM. Water contact angle and pure water flux measurements were conducted to evaluate the surface hydrophilicity. The influence of surface charges on protein filtration was also investigated. It is found that the mass gain of interfacial crosslinking increases almost linearly with increasing the PDMAEMA concentration from 0.5 to 10 g/L. The crosslinking degree is larger than 80% according to the XPS results, ensuring the stability of the crosslinking layer. The surface hydrophilicity is demonstrated by the sharp decrease of water contact angle from 145° to 20°. The pure water flux also increases 3 times under the optimized conditions. Furthermore, the results of protein filtration suggest that these highly hydrophilic and charged surfaces can effectively resist the fouling of proteins.     

Barrier Properties of Gastrointestinal Mucus to Nanoparticle Transport

Gastrointestinal mucus, a complex network of highly branched glycoproteins and macromolecules, is the first barrier through which orally delivered drug and gene vectors must traverse. The diffusion of such vectors can be restricted by the high adhesivity and viscoelasticity of mucus. In this investigation, the barrier properties of gastrointestinal mucus to particle transport were explored using real‐time multiple particle tracking. The influence of surface chemistry on particle transport rates was examined using amine‐, carboxylate‐, and sulfate‐modified polystyrene nanoparticles. A strong dependence of particle mobility in gastrointestinal mucus on surface charge was observed, with anionic particles diffusing 20–30 times faster than cationic particles. Comparison of diffusion coefficients calculated for gastrointestinal mucus with significantly varying values previously reported in the literature for other mucus sources, including cervicovaginal mucus and cystic fibrosis sputum, highlight the dependence of mucus barrier properties on the anatomical source. A significant degree of transport rate heterogeneity was also observed in native gastrointestinal mucus, suggesting a highly heterogeneous distribution of pore sizes. Furthermore, the suitability of purified mucin as a model system for transport studies was assessed by comparing particle transport rates between native intestinal mucus and purified porcine gastric mucin. Particle transport rates were approximately threefold lower in native mucus compared to purified mucin for anionic particles, yet comparable for cationic particles. Differences between barrier properties of the purified mucin preparation and native mucus depended on specific carrier properties, indicating that the purified mucin preparation does not provide an accurate model system for native mucus.

     

Description of the turbidity measurements near the phase transition temperature of poly(N-isopropyl acrylamide) copolymers: the effect of pH, concentration, hydrophilic and hydrophobic content on the turbidity

Absorbance values between 300 and 800 nm of aqueous solutions of poly(N-isopropyl acrylamide-co-itaconic acid-9.80), poly(N-isopropyl acrylamide-co-itaconic acid-52.05) and poly(N-isopropyl acrylamide)s containing Tegomer H-Si 2111 end groups and/or blocks were measured using a Shimadzu 160-A UV-visible spectrometer. Turbidities obtained from these absorbance values were used to interpret the macromolecular phase transition from a hydrophilic to a hydrophobic structure of the polymers. The effects of comonomer type and content, concentration of the solutions, pH and temperature on the coil-globule transition were discussed in terms of turbidity form factor, β related to size and shapes of particles and calculated by using the simplified form of Debye equation.The results presented in this work show that the presence of Tegomer H-Si 2111 (Si containing end groups and/or blocks) or high amount of itaconic acid (IA) in the chains prevent a collapse transition from hydrated extended coils to hydrophobic globules, which aggregate and form a separate phase (β<2). Furthermore, it was observed that in the case of concentrated solutions intermolecular hydrophobic interactions between isopropyl groups overcame the repulsive forces resulting from the ionized carboxylic acid groups of IA or surface active nature of Si containing hydrophobic groups (β>2). This stage of the transition corresponds to macroscopic phase separation after an intramolecular process.     

ACID-BASE INTERACTIONS IN THE CORNEA-TEAR FILM SYSTEM: SURFACE CHEMISTRY OF CORNEAL WETTING,CLEANING, LUBRICATION,HYDRATION AND DEFENSE

Various surface-chemical interactions among the corneal epithelium, ocular mucous gel, tear film and tear film contaminants (e.g., cellular debris, lipids, bacteria) are characterized based on their apolar and polar (acid-base) surface properties. Based on this approach, the surface-chemical pathways of the tear film breakup and of the corneal epithelial lubrication, cleansing, wetting and defense are proposed. A strong monopolar repulsion keeps mucus in the form of a highly hydrated gel, which cannot adhere to the normal glycocalix carrying epithelial surface, but forms an effective surface-chemical trap for the apolar and weakly polar hydrophobic contaminants. However, mucus-deficiency and/or a host of epithelial surface abnormalities (e.g., increased cell loss, chemical or morphologic changes, damage) can initiate a vicious cycle comprising of factors such as: increased mucus contamination, loss of mucus and epithelial hydrophilicity, abnormal adhesion and aggregation of mucus, reduced mucus mobility and faulty surface cleansing. All of these factors can conspire to produce a chronically unstable tear film secondary to the loss of corneal surface wettability.     

Mucin Interactions with Functionalized Polystyrene Latexes

The interactions of pig gastric mucin and bovine submaxillary mucin with carboxylate (PCM) and amino (PAM) polystyrene latexes with 750 and 1000 nm diameters have been studied in vitro. The mucin interaction increased when the pH decreased from 7.4 to 3.0 and when the electrolyte concentration increased from 86 to 205 mM. The driving force of the interaction was very probably nonionic. Under certain conditions, electrostatic attraction also was important for PAM. Under all experimental conditions tested, the mucins interacted less with PAM than with PCM. The functional groups of the latexes directed the conformation of the adsorbed mucins at the interface. At low pH, the mucins probably were adsorbed in multilayers.     

Water vapour sorption by the pedal mucus trail of a land snail

The hydrophilicity of pedal mucus trails deposited by snails influences the settlement of marine organisms and can potentially influence the trailing and homing mechanisms of terrestrial snails. The composition of pedal mucus deposited as a trail on a solid substrate by the giant African land snail (Achatina marginata) has been probed non-invasively using infrared ellipsometry. The primary chemical groups in the mucus (in its native state) have been identified through their characteristic infrared absorption frequencies. Water vapour sorption in the mucus trails in equilibrium with the atmosphere was measured as a function of the relative humidity (RH). When RH=84%, the mucus contains 53 volume percent water. The water sorption isotherm of the mucus trail can be described through a Flory–Huggins polymer/solvent interaction parameter of χ=0.54±0.1, which is comparable to the value for some synthetic hydrophilic polymers, such as poly(vinyl pyrrolidone).     

Molecular mechanisms of aqueous boundary lubrication by mucinous glycoproteins

Mucins have long been recognized as instrumental to biolubrication but the molecular details of their lubrication mechanisms have only been explored relatively recently. The glycoprotein PRG4, also known as lubricin, shares many features with mucins and appears to lubricate through similar mechanisms. A number of studies have contributed to a more in-depth understanding of mucin adsorption and layer formation on surfaces and the mechanisms by which these layers lubricate. Although mucinous glycoproteins differ in their aggregation properties, their adsorption behaviors on surfaces, and in their ability to reduce friction, they share important similarities favorable for lubrication. They are highly hydrated, they adsorb strongly to a broad range of surfaces, and the layers they form are both sterically and electrostatically repulsive, all attributes thought to contribute to boundary lubrication. They also hydrophilize hydrophobic surfaces, promoting the formation of aqueous fluid films that can lower friction at already relatively low sliding speeds. In this paper we briefly review current knowledge of mucin adsorption and lubrication, with a focus on recent advances.     

Synthesis and characterization of hydrophobically modified polyacrylamides and some observations on rheological properties

Hydrophobically modified polyacrylamide (PAM) polymers were synthesized by means of an aqueous micellar copolymerization, the di-alkyl substituted acrylamides di-n-propylacrylamide (DPAM) and di-n-octylacrylamide (DOAM) being used as hydrophobic comonomers. The number of hydrophobic blocks was varied using three different comonomer contents (f=1, 3 and 5 wt.%). The length of hydrophobic blocks, i.e. the number of hydrophobes per micelle, N_H, was controlled by the sodium dodecyl sulphate surfactant concentration. The effect of the type of hydrophobic comonomer and the number and length of hydrophobic units on the composition of the copolymer and its solubility and rheological properties were studied. The results indicate that the average copolymer composition is independent of the degree of conversion and surfactant concentration. Solubility and rheological measurements lead to a number of conclusions. First, DPAM is a weak hydrophobic monomer, all DPAM/AM (AM=acrylamide) copolymers being water soluble. Second, there is no strong hydrophobic interaction between DPAM units, in particular for low polymer concentration and N_H, and thus no strong associative thickening behaviour. Third, DOAM is a strong hydrophobic comonomer. Incorporation of a small amount of DOAM into PAM causes a dramatic enhancement in viscosity due to hydrophobic interactions. The properties of the copolymers are strongly dependent on the N_H values, most DOAM/AM copolymers being insoluble in water.     

Waterborne RAFT polymers

A recently invented novel family of RAFT (Reversible Addition-Fragmentation chain Transfer) agents having a common formula Z-C(S)-S-CR₂COOR¹ where Z = -SR, -NR₂, or -OR, and R¹ represents H or a variety of functional groups allows for tailoring their hydrophilicity-hydrophobicity balance. A limited hydrophilicity of the RAFT agents can be achieved which is sufficient for their diffusion through water, yet the agents are hydrophobic enough to phase-separate out of water. Thus, the limited hydrophilicity of otherwise hydrophobic agents allows them to be at the loci of polymerization making them suitable for the emulsion polymerization mechanism. With several RAFT agents, good control over molecular weight was demonstrated for a broad variety of ab initio acrylic emulsion polymers. For methyl methacrylate, a portion of RAFT did not engage, resulting in less than the theoretical number of polymer chains. It was found, however, that as little as ∼10 wt% of an acrylic monomer slowed down polymerization enough to engage all RAFT agent molecules and yield predicted molecular weights. A broad variety of colorless and odorless telechelic acrylic and methacrylic emulsion polymers were synthesized.Microemulsion and solution-dispersion techniques produced clean colloidally stable RAFT dispersions. These two techniques did not require RAFT agents with tailored hydrophilicity-hydrophobicity.The UV spectra and photooxidative stability of the RAFT polymers were studied. The RAFT fragment in polymers appeared to have no impact on their photooxidative stability.     

Synthesis of amphiphilic polyacetal by polycondensation of aldehyde and polyethylene glycol as an acid‐labile polymer for controlled release of aldehyde

A new acid‐labile polymer having acetal moieties in the main chain was synthesized by polycondensation of poly (ethylene glycol) (PEG) and lilial, an aldehyde widely used in fragrance applications. The hydrophobicity of the resulting acetal moiety and the hydrophilicity of the PEG chain allowed the polyacetal to exhibit amphiphilicity. The polyacetal derived from PEG having weight average molecular weight (M_w) of 1000 (PEG1000) was soluble in water and self‐associated to form associates in water. The polyacetals were hydrolyzed in acidic aqueous solutions to release hydrophobic lilial from the systems. The release rate of aldehyde from the polyacetal derived from PEG1000 was higher than that from the polyacetal derived from PEG having M_w of 400 (PEG400). These results supported that the release rate of lilial can be controlled by the chain length of PEG, on which hydrophilicity of polyacetal depends. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Interactions of β-Lactoglobulin with Bovine Submaxillary Mucin vs. Porcine Gastric Mucin: The Role of Hydrophobic and Hydrophilic Residues as Studied by Fluorescence Spectroscopy

The aim of this study was to investigate binding interactions between β-lactoglobulin (BLG) and two different mucins, bovine submaxillary mucins (BSM) and porcine gastric mucin (PGM), using intrinsic and extrinsic fluorescence spectroscopies. Intrinsic fluorescence spectra showed an enhanced decrease of fluorescence intensity of BLG at all pH conditions when BLG was mixed with PGM rather than with BSM. We propose that, unlike BSM, the tertiary structure of PGM changes and the hydrophobic regions are exposed at pH 3 due to protonation of negatively charged residues. Results suggest that PGM also facilitated the structural unfolding of BLG and its binding with PGM by a hydrophobic interaction, especially at acidic pH, which was further supported by extrinsic fluorescence spectroscopy. Hydrophobic interaction is suggested as the dominant interaction mechanism between BLG and PGM at pH 3, whereas electrostatic interaction is the dominant one between BLG and BSM.     

Cell surface photoengineering enables modeling of glycocalyx shedding dynamics

The cellular glycocalyx, composed of membrane associated glycoproteins and glycolipids, is a complex and dynamic interface that facilitates interactions between cells and their environment. The glycocalyx composition is continuously changing through biosynthesis of new glycoconjugates and membrane turnover. Various glycocalyx components, such as mucins, can also be rapidly shed from the cell surface in response to acute events, such as pathogenic threat. Mucins, which are large extended glycoproteins, deliver important protective functions against infection by creating a physical barrier at the cell surface and by capturing and clearing pathogens through shedding. Evaluating these mucin functions may provide better understanding of early stages of pathogenesis; however, tools to tailor the composition and dynamics of the glycocalyx with precision are still limited. Here, we report a chemical cell surface engineering strategy to model the shedding behavior of mucins with spatial and temporal control. We generated synthetic mucin mimetic glycopolymers terminated with a photolabile membrane anchor, which could be introduced into the membranes of living cells and, subsequently, released upon exposure to UV light. By tuning the molecular density of the artificial glycocalyx we evaluated lectin crosslinking and its effect on shedding, showing that lectins can stabilize the glycocalyx and limit release of the mucin mimetics from the cell surface. Our findings indicate that endogenous and pathogen-associated lectins, which are known to interact with the host-cell glycocalyx, may alter mucin shedding dynamics and influence the protective properties of the mucosal barrier. More broadly, we present a method which enables photoengineering of the glycocalyx and can be used to facilitate the study of glycocalyx dynamics in other biological contexts.

Engineering cell surfaces with light-responsive mucin mimetic glycopolymers enables modeling of mucosal glycocalyx shedding and its possible roles in mucosal epithelium protection.