Atomic Force Microscopy Reveals Aggregation of Gastric Mucin at Low pH

Double-globular structure of porcine stomach mucin: a small-angle X-ray scattering study

We present evidence from small-angle X-ray scattering synchrotron experiments that porcine stomach mucin (MUC6) contains a double-globular comb structure. Analysis of the amino acid sequence of the peptide comb backbone indicates that the globular structure is determined by both the charge and hydrophobicity of the amino acids and the placement of the short hydrophilic carbohydrate side chains (approximately 2.5 nm). The double-globular structure is, thus, due to a block copolymer type hydrophobic polyampholyte charge instability in contrast to the random copolymer instabilities observed previously with synthetic polyelectrolytes (particularly polystyrene sulfonates). Careful filtering was required to exclude multimonomer aggregates from the X-ray measurements. A double Guinier analysis ( R g approximately 26 nm) and a double power law fit are consistent with two globules per chain in low salt conditions. The average radius of the globules is approximately 10 nm in salt- free condition (double Guinier fit) and the average distance of intrachain separation of the globules is 48 nm. The addition of salt causes a significant decrease in the radius of gyration (14 nm 100 mM NaCl) of the chains and is attributed to the contraction of the glycosylated peptide spacer between the two globules (the globular size continues to be approximately 10 nm and the globule separation is then 18 nm). Without salt, the scaling of the semidilute mesh size (xi) as a function of the mucin concentration (c) is xi approximately c (-0.45)compared with xi approximately c (-0.28) in high salt conditions, highlighting the globular nature of the chains. In contrast, hydrophilic flexible polyelectrolytes have a stronger concentration dependence of xi when excess salt is added.

Boronate-containing polymers form affinity complexes with mucin and enable tight and reversible occlusion of mucosal lumen by poly(vinyl alcohol) gel

Copolymers of N-acryloyl-m-aminophenylboronic acid (NAAPBA) with N,N-dimethylacrylamide (DMAA) formed insoluble interpolymer complexes with mucin from porcine stomach at pH 9.0. The complex formation based on boronate-sugar interactions took place between the similarly charged macromolecules and resulted in coacervate particles formation, which depended both on pH and ionic strength of the solution. The coacervation rate displayed a maximum at the intermediate DMAA-NAAPBA copolymer: mucin weight ratio, that is a pattern typical of interpolymer complex formation. The effective hydrodynamic particle diameter of the coacervates monotonously grew from 155+/-20 nm up to 730+/-120 nm in 2 days in 0.1M sodium bicarbonate buffer solution, pH 9.0. Electrophoretic mobility of the resultant nanoparticles was intermediate between those of individual polymers, whereas the particles zeta-potential was -7.5+/-0.4 mV in the above buffer solution. Pre-treatment of the inner mucosal epithelium of excised male pig urethras with 5% (w/v) solutions of acrylamide-NAAPBA or DMAA-NAAPBA copolymers at pH 8.8 allowed for tight occlusion of the lumen by poly(vinyl alcohol)-borax gel injected via a two-way catheter. Leakage of 0.15M NaCl solution through the thus occluded organs could be prevented, while the leakage through the organs occluded by the gel without the pre-treatment was unavoidable. The gel plug could be quickly dissolved on demand after injection of 5% (w/v) aqueous fructose solution into the lumen. The described technique may be useful for temporal occlusion of mucosal lumens in living organisms. In contrast to the conventional mucoadhesive polymers like polyacrylic acid or chitosan, the boronate-containing copolymers display their mucoadhesivity at weakly alkaline pH of 8-9 and physiological ionic strength.

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.

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.

Effect of pH on the association behavior in aqueous solutions of pig gastric mucin

In this study, dynamic light scattering (DLS), turbidity, and rheo-small angle light scattering (rheo-SALS) methods have been utilized to examine the impact of pH (1 < or = pH < or = 7) on aqueous solutions of noncommercial purified pig gastric mucin. The asymmetric flow field-flow fractionation (AFFFF) measurements established that the mucin sample has a high molecular weight and is polydisperse. DLS measurements on dilute solutions of mucin disclosed large interchain aggregates at pH 2, where the polymer has a low charge density or is uncharged. At lower or higher values of pH, mucin is charged and the tendency of forming interpolymer complexes is affected. In the semidilute concentration regime, pronounced junction zones ('lumps' of polymer) are evolved and a heterogeneous connected network is formed at pH 2, whereas the association structures are disintegrated (smaller 'lumps') at lower or higher pH values due to electrostatic repulsive interactions, and a more homogeneous network is evolved. The DLS and viscosity results at pH 1 indicate the development of a fragmented network, composed of contracted chains that are decorated by some positive charges. The effect of shear flow on the structure of semidilute solutions of mucin was investigated with the aid of rheo-SALS methods. The scattered intensity revealed a strong upturn at low values of the wave vector (q) for mucin solutions at pH 2 and pH 4, which suggests the evolution of large association domains. At these pH values, a flow-induced anisotropy in the 2D SALS patterns in the form of elliptical shapes was observed at high shear rates.

Globular structure of human ovulatory cervical mucus

Human cervical mucus is a heterogeneous mixture of mucin glycoproteins whose relative concentration changes during the ovulatory phases, thereby producing different mucus aggregation structures that can periodically permit the transit of spermatozoa for fertilization. In preovulatory phase, mucus is arranged in compact fiber-like structures where sperm transit is hindered. Previously, through observations made of fixed and dehydrated samples, a permissive structure in the ovulatory phase was attributed to the larger diameters of pores in the mucus network. Instead, by means of atomic force microscopy, we can show, for the first time, that unfixed ovulatory mucus is composed by floating globules of mucin aggregates. This finding sheds new light on the mechanism that governs spermatozoa transit toward the uterine cavity. In addition, we demonstrate that the switch from globular ovulatory to fibrous preovulatory mucus largely depends on a pH-driven mechanism. Analysis of mucin 5B primary sequence, the main mucin in ovulatory mucus, highlights pH-sensitive domains that are associated to flexible regions prone to drive aggregation. We suggest an involvement of these domains in the fiber-to-globule switch in cervical mucus.

Rheology of gastric mucin exhibits a pH-dependent sol-gel transition

Gastric mucin, a high molecular weight glycoprotein, is responsible for providing the gel-forming properties and protective function of the gastric mucus layer. Bulk rheology measurements in the linear viscoelastic regime show that gastric mucin undergoes a pH-dependent sol-gel transition from a viscoelastic solution at neutral pH to a soft viscoelastic gel in acidic conditions, with the transition occurring near pH 4. In addition to pH-dependent gelation behavior in this system, further rheological studies under nonlinear deformations reveal shear thinning and an apparent yield stress in this material which are also highly influenced by pH.

PFG-NMR diffusometry: A tool for investigating the structure and dynamics of noncommercial purified pig gastric mucin in a wide range of concentrations

For the first time, Pulsed Field Gradient-Nuclear Magnetic Resonance, a powerful noninvasive tool for studying the dynamics and structure of complex gels, has been used to measure diffusion of probe molecules in aqueous solutions/gels of noncommercial purified pig gastric mucin (PGM), in a concentration range up to 5 wt %. Complementary data were obtained from rheology measurements. The combination of techniques revealed a strong pH dependency of the structure of the PGM samples while changes in concentration, ionic strength, and temperature appeared to induce less pronounced alterations. Viscosity was found to vary in a nonmonotonous way with pH, with the more viscous solutions found at intermediate pH. We propose that this finding is due to a reduced charge density at lower pH, which is expected to continuously increase the relative importance of hydrophobic associations. The results suggest a loose network of expanded fully charged PGM molecules with considerable mobility at neutral pH (pH 7.4). At intermediate pH (pH 4), a three-dimensional expanded network is favored. At pH 1, the charge density is low and microphase separation occurs since hydrophobic associations prevail. This leads to the formation of clusters concentrated in PGM molecules separated by regions depleted in PGM. The results obtained increase our knowledge about the gastric mucosal layer, which in vivo contains mucin in the same concentration range as that of the samples investigated here.

In vitro evaluation of the mucoadhesive properties of polysaccharide-based nanoparticulate oral drug delivery systems

Impedance quartz crystal microbalance (QCM) and surface plasmon resonance (SPR) measurements were performed in order to assess the mucoadhesive properties of hydrophobically modified (HM) derivatives of dextran (DEX), with an average molecular weight of 10,000 Da, and of hydroxypropylcellulose (HPC), with an average molecular weight of 80,000 Da. The measurements involved (1) treatment of a hydrophobic surface with bovine submaxillary gland mucin (BSM) under various pH conditions (2.0-8.0) and (2) treatment of the BSM layer with buffer solutions of the amphiphilic polysaccharides (pH 3.0 and 7.0). Control measurements were carried out with DEX, HPC, and chitosan (CH) used as a model mucoadhesive polymer. All HM-polysaccharides were shown to adsorb onto a BSM layer, the extent of adsorption increasing with increasing hydrophobicity of the samples. Under the same conditions, HPC and CH interacted with the BSM layer, but DEX showed no affinity to BSM. All the results suggest that HM-polysaccharide micellar systems have the potential of enhancing the bioavailability of poorly adsorbed drugs in peroral delivery.