Analysis of the diffusion of bile salt/phospholipid micelles in rat intestinal mucin

Changes in Mechanical Properties of Vesicles by Mucin in Aqueous Solution

The mechanical properties of vesicles were investigated as they were prepared, according to the ratio of mucin to dipalmitoylphosphatidylcholine (DPPC), using an atomic force microscope (AFM). After the confirmation of the vesicle adsorption on a mica surface, an AFM-tip deflection, caused by the interaction between the tip and the vesicle, was measured. The deflection showed that the tip broke through into the vesicle twice. Each break meant a tip-penetration into the upper and lower portion of the vesicle. Only the first penetration allowed the Hertzian model available to estimate the vesicle mechanical moduli. Two moduli reduced as the ratio of mucin to DPPC increased to 0.5, but the moduli were little changed above the 0.5 ratio. These results seem to be a platform for the effect of the mucin on the plasma-membrane anchoring and cellular signaling.

Purified mucins in drug delivery research

One of the main challenges in the field of drug delivery remains the development of strategies to efficiently transport pharmaceuticals across mucus barriers, which regulate the passage and retention of molecules and particles in all luminal spaces of the body. A thorough understanding of the molecular mechanisms, which govern such selective permeability, is key for achieving efficient translocation of drugs and drug carriers. For this purpose, model systems based on purified mucins can contribute valuable information. In this review, we summarize advances that were made in the field of drug delivery research with such mucin-based model systems: First, we give an overview of mucin purification procedures and discuss the suitability of model systems reconstituted from purified mucins to mimic native mucus. Then, we summarize techniques to study mucin binding. Finally, we highlight approaches that made use of mucins as building blocks for drug delivery platforms or employ mucins as active compounds.

A simulated mucus layer protects Lactobacillus reuteri from the inhibitory effects of linoleic acid

Lactobacillus reuteri is a commensal, beneficial gut microbe that colonises the intestinal mucus layer, where it makes close contact with the human host and may significantly affect human health. Here, we investigated the capacity of linoleic acid (LA), the most common polyunsaturated fatty acid (PUFA) in a Western-style diet, to affect L. reuteri ATCC PTA 6475 prevalence and survival in a simulated mucus layer. Short-term (1 h) survival and mucin-agar adhesion assays of a log-phase L. reuteri suspension in intestinal water demonstrated that the simulated mucus layer protected L. reuteri against the inhibitory effects of LA by lowering its contact with the bacterial cell membrane. The protective effect of the simulated mucus layer was further evaluated using a more complex and dynamic model of the colon microbiota (SHIME®), in which L. reuteri survival was monitored during 6 days of daily exposure to LA in the absence (L-SHIME) and presence (M-SHIME) of a simulated mucus layer. After 6 days, luminal L- and M-SHIME L. reuteri plate counts had decreased by 3.1±0.5 and 2.6±0.9 log cfu/ml, respectively. Upon supplementation of 1.0 g/l LA, the decline in the luminal L. reuteri population started earlier than was observed for the control. In contrast, mucin-agar levels of L. reuteri (in the M-SHIME) remained unaffected throughout the experiment even in the presence of high concentrations of LA. Overall, the results of this study indicate the importance of the mucus layer as a protective environment for beneficial gut microbes to escape from stress by high loads of the antimicrobial PUFA LA to the colon, i.e. due to a Western-style diet.

Concentration dependent aggregation properties of chlorhexidine salts

PURPOSE

Chlorhexidine (CHX), a chemical antiseptic, is known to bind to dentin and has been shown to be effective in treating bacterial infections caused by microbes. The solubility and aggregation properties of CHX salts were determined to guide the development of a sustained release formulation for long-term disinfection.

METHODS

The amount of CHX in solution was determined as a function of counterion concentration (chloride, acetate (Ac) or gluconate (G)) by UV spectrophotometry at 255nm. The weight average molecular weight was determined from the angular dependence of the scattered light. Proton NMR spectroscopy was used to investigate the dependence of the peak intensity and chemical shift on solution concentration and diffusion measurements were performed by Fourier-transform pulsed-field gradient spin-echo (PFG-SE) (1)H NMR.

RESULTS

The observed CHX concentration was highly dependent on the type and concentration of salt present in solution with the greatest CHX concentration achieved with gluconate, moderate to low with diacetate, and very low with dichloride solutions. Addition of sodium gluconate enhanced the amount of CHX-Ac(2) in solution; however, only low concentrations of chlorhexidine can be achieved in the presence of chloride ions. For solutions of CHX-G(2), the aggregate number appeared to range from a dimer at 40mM to perhaps a pentamer at 150mM. In contrast, no aggregation of CHX-Cl(2) or CHX-Ac(2) was detected, which was corroborated by diffusion NMR results. The change in chemical shift of protons is consistent with association of the phenyl group of one CHX with the hexamethylene chain of a second CHX. Based on the analysis of NMR peak intensities of CHX, gluconate, and acetate in saturated solutions, it appears that solubilization of the diacetate species occurs within digluconate aggregates, since the solubility product of chlorhexidine diacetate is such that the concentration of CHX will exceed the critical micelle concentration (CMC). However, no solubilization of CHX-Cl(2) occurs because the solubility product falls below the CMC.

CONCLUSIONS

The low concentration of CHX that can be achieved in physiological concentrations of chloride in the oral cavity may be problematic for dental and slow release formulations. Achieving a high concentration of CHX appears to require that the monomer be present at a concentration greater than that required to produce self-association.

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.

Diffusion of Nutrients Molecules and Model Drug Carriers Through Mucin Layer Investigated by Magnetic Resonance Imaging with Chemical Shift Resolution

Magnetic resonance imaging (MRI) with chemical shift resolution is a recent extension of MRI and it provides information about species resolved molecular transport on the macroscopic scale in complex systems. In this contribution, we show that by using this novel method, one can predict the behavior of drug and food molecules when they are in contact with the mucosal layer in the gastrointestinal tract. For the first time, the transport properties of a mixture of nutrients (i.e., a solution of ethanol and glucose) and of a model drug carrier (i.e., an equimolar solution of cationic and nonionic surfactants) through a mucin gel have been investigated. This study shows that transport properties of the diffusing molecules through a mucin gel are dependent on their size and physicochemical properties. In addition, we show that mucin gel acts as an efficient selective barrier. It favors the disintegration of mixed micelles of nonionic and cationic surfactants by stopping the diffusion of cationic surfactants with slightly affecting the diffusion of the nonionic surfactants.

Interaction of bile salts with gastrointestinal mucins

The properties of three mucins were examined to identify the structural features responsible for their functional difterences. Bovine submaxillary mucin (BSM), porcine gastric mucin (PGM), and rat intestinal mucin (RIM) were each characterized, and high carbohydrate contents were found for RIM and PGM. The amino acid compositions were typical of mucin glycoproteins, with over half comprising small, neutral amino acids. Thereafter, each mucin was equilibrated with three different series of concentrations of the bile salts sodium taurocholate, sodium taurodeoxycholate, and sodium taurochenodeoxycholate. Following multiple centrifugations, the supernatant and mucin pellet concentrations of the bile salts were measured. The bile salt pellet concentration was plotted as a function of supernatant concentration, and from the slopes, the excluded volumes were calculated as 25, 29-44, and 28 55 mL/g for BSM, RIM, and PGM, respectively. The intercepts were 8-10, 2-3, and 1-3 mM for BSM, RIM, and PGM, respectively, which represents an estimate of the bound concentration of bile salt. Differences among the bile salts were observed in the excluded volume and amount bound, but no trends were evident. The bile salts may interact as aggregates with the hydrophobic areas and carbohydrate side chains of the mucins, providing favorable sites for association. The binding at low concentrations with exclusion at high concentrations is significant for modulating the absorption of lipid aggregates from the intestine. Finally, the differences among the mucins reflect the unique structure function relationship of these gastrointestinal mucins.

Excluded volume effect of rat intestinal mucin on taurocholate/phosphatidylcholine mixed micelles

The interaction of bile salt/phospholipid mixed micelles with an intestinal mucin has been investigated to provide the foundation for the transport of ingested fat and poorly water-soluble drugs through the intestinal mucous layer. Egg phosphatidylcholine (PC) was equilibrated with sodium taurocholate (TC) to generate several series of solutions, which had different intermicellar concentrations of TC. Within each series, each solution had the same IMC and thereby micelle sizes, but varied with respect to micelle concentration. These solutions were combined with isolated rat intestinal mucin, equilibrated, and then separated by centrifugation. The supernatant and mucin pellet were assayed for PC and TC, and the diffusion coefficient of PC was measured in the supernatant by PFG-SE NMR spectroscopy. For each series, four linear relationships were found; TC supernatant concentration plotted as a function of PC supernatant concentration; TC pellet concentration plotted as a function of PC pellet concentration; TC pellet concentration plotted as a function of TC supernatant concentration; and PC pellet concentration plotted as a function of PC supernatant concentration. Theoretical analysis of these results indicated that mucin excludes from 25 to 80% of the bile salt/phospholipid mixed micelles with greater exclusion observed with larger micelle size. There is preferential association of the taurocholate with intestinal mucin, when present in the mixed micelle region of the phase diagram. The association coupled with exclusion would allow mucin to modulate the concentration of bile salt at the epithelial surface.

Distribution and diffusion of sodium taurocholate and egg phosphatidylcholine aggregates in rat intestinal mucin

PURPOSE

The permeability of rat intestinal mucin (RIM) to sodium taurocholate/egg phosphatidylcholine (TC/PC)-mixed micelles has been investigated.

METHODS

The time dependence for the equilibration of TC/PC-mixed lipid micelles with isolated RIM was determined. Thereafter the distribution of TC/PC-mixed lipid micelles was assessed at low and high PC and intermicellar concentrations (IMC) and with different RIM concentrations. The equilibrium distribution of PC and TC was determined by analysis for phosphorus and by high-performance liquid chromatography, respectively, as well as by nuclear magnetic resonance spectroscopy. In addition, the diffusion coefficients of water, PC, and TC were measured by pulsed field gradient nuclear magnetic resonance spectroscopy. Two model solutes, phenylmethyltrimethylsilane (PTMS) and tetramethylsilyl-tetradeutero-proprionic acid (TSP), were added to the high PC, low IMC samples, and the diffusion coefficients were determined.

RESULTS

The time to reach equilibrium was 2 days for a system with a high intermicellar concentration of sodium taurocholate. At low PC concentrations, RIM had slightly higher PC concentrations relative to the control. In contrast, at high PC concentrations, RIM samples had lower PC concentrations. The concentration of TC was largely independent of mucin concentration. The water diffusivity was reduced proportionately to the concentration of RIM, and analysis indicated that about 150 g of water moved as a kinetic unit with each gram of mucin. The diffusion coefficients of PC were also reduced with increasing RIM concentration. The magnetization decay of TC did not always follow a monoexponential decay, reflecting the simultaneous diffusion and exchange among sites imparting different relaxation behavior on the TC. Magnetization decay curves were simulated and the diffusivity of TC in mucin was estimated. The diffusion coefficient of TSP was 10 times larger than that of PTMS in the presence of micelles and mucin.

CONCLUSIONS

RIM is highly hydrated, and dilute solutions have a minor exclusive effect on the high concentration of PC/TC micelles. At low concentrations of PC, there appears to be preferential association of the PC with the RIM. The permeability of mucin to solutes in the presence of bile salt mixed micelles critically depends on the degree of association of the solute with the micelle.