Monitoring cholesterol crystallization from lithogenic model bile by time-lapse density gradient ultracentrifugation

Fast Morphological Gallbladder Changes Triggered by a Hypercholesterolemic Diet

INTRODUCTION AND AIM

Obesity is a worldwide epidemic problem, described as a risk factor for hepatic diseases, such as non-alcoholic fatty liver disease and other pathologies related to development of cholesterol crystals and cholesterol gallbladder stones. It has been reported that cholesterol overload may cause hepatic damage; however, little is known about the effects of an acute hypercholesterolemic diet on the gallbladder. The aim of this manuscript was to evaluate the impact of a cholesterol-rich diet on the gallbladder.

MATERIAL AND METHODS

The study included ten eight-week-old C57BL6 male mice, which were divided into two study groups and fed different diets for 48 h: a hypercholesterolemic diet and a balanced Chow diet. After 48 h, the mice were analyzed by US with a Siemens Acuson Antares equipment. Mice were subsequently sacrificed to carry out a cholesterol analysis with a Refloton System (Roche), a crystal analysis with a Carl Zeiss microscope with polarized light, and a histological analysis with Hematoxylin-eosin staining.

RESULTS

The hypercholesterolemic diet induced an increase in gallbladder size and total cholesterol content in the bile, along with important histological changes.

CONCLUSION

Cholesterol overloads not only trigger hepatic damage, but also affect the gallbladder significantly.

Biliary cholesterol crystallization characterized by single-crystal cryogenic electron diffraction

Cholesterol crystals are the building blocks of cholesterol gallstones. The exact structure of early-forming crystals is still controversial. We combined cryogenic-temperature transmission electron microscopy with cryogenic-temperature electron diffraction to sequentially study crystal development and structure in nucleating model and native gallbladder biles. The growth and long-term stability of classic cholesterol monohydrate (ChM) crystals in native and model biles was determined. In solutions of model bile with low phospholipid-to-cholesterol ratio, electron diffraction provided direct proof of a novel transient polymorph that had an elongated habit and unit cell parameters differing from those of classic triclinic ChM. This crystal is exactly the monoclinic ChM phase described by Solomonov and coworkers (Biophysical J., In press) in cholesterol monolayers compressed on the air-water interface. We observed no evidence of anhydrous cholesterol crystallization in any of the biles studied. In conclusion, classic ChM is the predominant and stable form in native and model biles. However, under certain (low phospholipid) conditions, transient intermediate polymorphs may form. These findings, documenting single-crystal analysis in bulk solution, provide an experimental approach to investigating factors influencing biliary cholesterol crystal nucleation and growth as well as other processes of nucleation and crystallization in liquid systems.

Spontaneous cholecysto- and hepatolithiasis in Mdr2-/- mice: a model for low phospholipid-associated cholelithiasis

Previously, we identified needle-like and filamentous, putatively "anhydrous" cholesterol crystallization in vitro at very low phospholipid concentrations in model and native biles. Our aim now was to address whether spontaneous gallstone formation occurs in Mdr2 (Abcb4) knockout mice that are characterized by phospholipid-deficient bile. Biliary phenotypes and cholesterol crystallization sequences in fresh gallbladder biles and non-fixed liver sections were determined by direct and polarizing light microscopy. The physical chemical nature and composition of crystals and stones were determined by sucrose density centrifugation and before mass and infrared spectroscopy. Gallbladder biles of Mdr2(-/-) mice precipitate needle-like cholesterol crystals at 12 weeks of age on chow. After 15 weeks, more than 50% of Mdr2(-/-) mice develop gallbladder stones, with female mice displaying a markedly higher gallstone-susceptibility. Although gallbladder biles of Mdr2(-/-) mice contain only traces (</= 1.1 mM) of phospholipid and cholesterol, they become supersaturated with cholesterol and plot in the left 2-phase zone of the ternary phase diagram, consistent with "anhydrous" cholesterol crystallization. Furthermore, more than 40% of adult female Mdr2(-/-) mice show intra- and extrahepatic bile duct stones. In conclusion, spontaneous gallstone formation is a new consistent feature of the Mdr2(-/-) phenotype. The Mdr2(-/-) mouse is therefore a model for low phospholipid-associated cholelithiasis recently described in humans with a dysfunctional mutation in the orthologous ABCB4 gene. The mouse model supports the concept that this gene is a monogenic risk factor for cholesterol gallstones and a target for novel therapeutic strategies.

A technique for isolation of rubella virus-like particles by sucrose gradient ultracentrifugation using Coomassie brilliant blue G crystals

An improved method for the isolation of rubella virus-like particles (RVLP) from cell culture supernatant of transfected Chinese hamster ovary (CHO24S) cells is described. It employs a combination of membrane filtration with sucrose gradient ultracentrifugation. It was found that staining the RVLP band with Coomassie brilliant blue G (CBB) resulted in the CBB crystals adsorbing RVLP. After ultracentrifugation (25,000 rpm, 3h, 4 degrees C) a sharp blue band with crystals (diameter 30-40 microm) was observed (at a density of 1.250 g/ml at 25 degrees C) in a 30-60% sucrose gradient. Using a combination of SDS-PAGE and Western blotting techniques, E1 rubella virus structural protein was detected only in the solutions derived from the sharp blue band. A decrease in crystal concentration a few millimeters above or below the main band was associated with a decrease in protein concentration. By dilution with a saturated ice-cold 30% sucrose solution it was possible to pellet the crystals by centrifugation (15,000 rpm, 10 min). SDS-PAGE showed a much higher concentration of RVLP structural protein in the pellet than in the supernatant. This RVLP-containing material is especially suitable for the preparation of rubella virus immunoblot stripes.

Cholesterol gallstones

Cholesterol cholelithiasis is common in Western populations and represents a consequence of altered cholesterol homeostasis. Gallstones form because of a complex and incompletely understood series of metabolic and physicochemical events that promote cholesterol crystallization in bile. In the context of current paradigms, this article reviews recent progress in research on biliary lipid metabolism and the pathogenesis of cholesterol gallstones.

Cholesterol monohydrate nucleation in ultrathin films on water

The growth of a cholesterol crystalline phase, three molecular layers thick at the air-water interface, was monitored by grazing incidence x-ray diffraction and x-ray reflectivity. Upon compression, a cholesterol film transforms from a monolayer of trigonal symmetry and low crystallinity to a trilayer, composed of a highly crystalline bilayer in a rectangular lattice and a disordered top cholesterol layer. This system undergoes a phase transition into a crystalline trilayer incorporating ordered water between the hydroxyl groups of the top and middle sterol layers in an arrangement akin to the triclinic 3-D crystal structure of cholesterol x H(2)O. By comparison, the cholesterol derivative stigmasterol transforms, upon compression, directly into a crystalline trilayer in the rectangular lattice. These results may contribute to an understanding of the onset of cholesterol crystallization in pathological lipid deposits.

Arachidyl amido cholanoic acid (Aramchol) is a cholesterol solubilizer and prevents the formation of cholesterol gallstones in inbred mice

We have recently synthesized fatty acid bile acid conjugates (FABAC) that were able to reduce and retard cholesterol crystallization in model and human biles. When given orally, they prevented the formation of cholesterol crystals in the bile of hamsters. The aim of the present study was to determine whether the FABAC are cholesterol solubilizers, whether they can dissolve pre-existing crystals, whether they can prevent the formation of cholesterol gallstones, and to investigate the optimal type of bond between the fatty acid and bile acid. The presence of cholesterol crystals was determined by light microscopy, and the total crystal mass of precipitated crystals was measured by chemical means. Inbred (C57J/L) mice on a lithogenic diet were used to evaluate cholesterol crystal formation, dissolution, and gallstone formation in vivo. Arachidyl amido cholanoic acid (Aramchol) was the FABAC used in the present experiments. At equimolar amounts, the cholesterol-solubilizing capacity of Aramchol was higher than that of taurocholate and similar to that of phosphatidylcholine. The addition of Aramchol dissolved approximately 50% of pre-existing crystals in model bile solutions. The same phenomenon was demonstrated in human bile ex vivo, with a dose-response effect. All inbred mice developed cholesterol crystals in bile after 10-14 d on the lithogenic diet. Thereafter, supplementation of the diet with Aramchol progressively reduced the proportion of mice with crystals to 25% after 28 d. On the lithogenic diet, 100% of inbred mice developed cholesterol gallstones in the gallbladder by day 21. None of the mice whose diet was supplemented with 0.5 mg or 1.0 mg of Aramchol/d developed stones or crystals. FABAC are a new class of molecules that are cholesterol solubilizers and which are able to dissolve cholesterol crystals in bile. Upon oral administration, they dissolve pre-existing cholesterol crystals and prevent the formation of gallstones in gallstone-susceptible mice.

Microstructural evolution of lipid aggregates in nucleating model and human biles visualized by cryogenic transmission electron microscopy

Obtaining reliable information on the physical state and ultrastructure of bile is difficult because of its mixed aqueous-lipid composition and thermodynamic metastability. We have used time-lapse cryogenic transmission electron microscopy (cryo-TEM) combined with video-enhanced light microscopy (VELM) to study microstructural evolution in nucleating bile. A well-characterized model bile and gallbladder biles from cholesterol and pigment gallstone patients were studied sequentially during cholesterol nucleation and precipitation. In model bile, cholesterol crystallization was preceded by the appearance of the following distinct microstructures: spheroidal micelles (3-5 nm), discoidal membrane patches (50-150 nm) often in multiple layers (2-10), discs (50-100 nm), and unilamellar (50-200 nm) and larger multilamellar vesicles (MLVs). The membrane patches and discs appeared to be short-lived intermediates in a micelle-to-vesicle transition. Vesicular structures formed by growth and closure of patches as well as by budding off from vesicles with fewer bilayers. MLVs became more abundant, uniform, and concentric as a function of time. In native bile, all the above microstructures, except discoidal membrane patches, were observed. However, native MLVs were more uniform and concentric from the beginning. When cholesterol crystals appeared by light microscopy, MLVs were always detected by cryo-TEM. Edges of early cholesterol crystals were lined up with micelles and MLVs in a way suggesting an active role in feeding crystal growth from these microstructures. These findings, for the first time documented by cryo-TEM in human bile, provide a microstructural framework that can serve as a basis for investigation of specific factors that influence biliary cholesterol nucleation and crystal formation.

Role of bile salt hydrophobicity in distribution of phospholipid species to carriers in supersaturated model bile solutions

BACKGROUND

Phospholipid species modulate cholesterol-holding capacity and, therefore, regulate bile metastability.

METHODS

In this study, we investigated the effect of bile salt hydrophobicity on the distribution of phospholipids among lipid particles in supersaturated model bile solutions (total lipid concentration, 9 g/dL; taurocholate/phospholipid ratio 3.0, cholesterol saturation index 1.3), by using gel permeation chromatography.

RESULTS

With an increase of bile salt hydrophobicity in the elution buffer, the uptake of cholesterol and phospholipids into bile salt micelles was increased, associated with an increased cholesterol/phospholipid molar ratio of the vesicles. In contrast, there was an inverse correlation between the hydrophobicity of the phospholipid species in the vesicles and that of bile salts in the elution buffer, suggesting that hydrophobic bile salts induced preferential uptake of hydrophobic phospholipids into bile salt micelles, while less hydrophobic phospholipids, with a relatively low cholesterol-holding capacity, remained in the vesicles.

CONCLUSIONS

These data indicate that bile salt hydrophobicity regulates vesicular cholesterol metastability by modulating the hydrophobicity of phospholipids in vesicles, as well as the lipid distribution among various biliary lipid particles.

The effects of phospholipid molecular species on cholesterol crystallization in model biles: the influence of phospholipid head groups

BACKGROUND/AIMS

Variations in the molecular species of biliary phospholipids have been shown to exert major effects on cholesterol solubility and carriers in model and human biles. The aim of this study was to explore systematically the effects of various phospholipid head groups on the cholesterol crystallization process in model biles.

METHODS

Three different control model biles were prepared using varying proportions of egg lecithin, cholesterol and Na taurocholate. In the test biles, 20% of the egg lecithin was replaced with synthetic phosphatidylserine, phosphatidylethanolamine, phosphatidylglycerol or phosphatidylcholine, keeping the phospholipid acyl chains and other biliary lipids constant in each experiment.

RESULTS

Phosphatidylserine and phosphatidylglycerol significantly prolonged the crystal observation time, from 2 days to 10 and 6 days, respectively (p<0.02), while phosphatidylethanolamine had little and phosphatidylcholine no effect. The crystal growth rate was significantly slowed down with 20% phospholipid replacement in the following order: phosphatidylglycerol >phosphatidylserine >phosphatidylethanolamine. The total crystal mass after 14 days, as measured by chemical analysis, was reduced by 59% with phosphatidylserine (p<0.05), and by 73% with phosphatidylglycerol (p<0.05); while phosphatidylethanolamine had little effect. The precipitable cholesterol crystal fractions after 14 days were significantly reduced with phosphatidylserine (54%) and phosphatidylglycerol (37%), but not with phosphatidylethanolamine or phosphatidylcholine.

CONCLUSIONS

Variations in the head groups of biliary phospholipids may markedly slow down the cholesterol crystallization process in model biles.

Increased saturation of the fatty acids in the sn-2 position of phospholipids reduces cholesterol crystallization in model biles

Changes in the molecular structure of biliary phospholipids were shown to have major effects on cholesterol solubility, carriers and crystallization in human and model biles. This study investigated systematically the effects of varying saturation of the phosphatidylcholine (PC) sn-2 fatty acid on the cholesterol crystallization process in 3 different model biles. Twenty % of the egg PC (EPC) in these biles were replaced by synthetic PC's with 16:0-18:0, 16:0-18:1, or 16:0-18:2 fatty acyl chains. With 18:0 in the sn-2 position, the crystal observation time (COT) was prolonged from 2 days in the control EPC solution to 14 days (p<0.05). The crystal growth rate (CGR) was reduced from 0.1 OD/day to unmeasurable levels, and the total crystal mass on day 14 decreased by 86%. The introduction of one (18:1), and two (18:2) double bonds in the sn-2 fatty acid rapidly reversed these effects. Ultracentrifugal analysis showed precipitable cholesterol as monohydrate crystals. In the 16:0-18:0 test solution, most of the precipitable cholesterol remained in the supersaturated multilamellar vesicles. Saturation of the biliary PC sn-2 fatty acyl chain prolongs the COT, slows the CGR, reduces the crystal mass, and extends cholesterol solubility in multilamellar vesicles. Desaturation of the sn-2 fatty acid reverses these effects.

Calcium and the anionic polypeptide fraction (APF) have opposing effects on cholesterol crystallization in model bile

BACKGROUND/AIMS

Cholesterol gallstones contain both calcium and biliary proteins, but their respective roles in gallstone pathogenesis are unknown. We have studied the effects of calcium and a major biliary protein, anionic polypeptide fraction, on the process of cholesterol crystallization in bile.

METHODS

Anionic polypeptide fraction was purified from human bile. Model bile composed of cholesterol, egg yolk lecithin and sodium taurocholate was prepared in a lipid concentration (18 mM, 37 mM, and 120 mM, respectively) simulating lithogenic human gallbladder bile. The crystallization process was observed by phase contrast light microscopy, and sequential separation of precipitable cholesterol structures by sucrose density gradient ultracentrifugation.

RESULTS

Addition of calcium, or anionic polypeptide fraction alone, or both together did not influence the crystal observation time of bile (the time which elapsed from initiation of supersaturation to the first appearance of crystals). However, the rate and quantity of cholesterol precipitation and crystal formation were affected by both. Calcium increased in a dose-dependent manner the cholesterol monohydrate crystal mass before apparent equilibrium was reached. This effect was inhibited by anionic polypeptide fraction, which increased the amount of cholesterol within precipitable phospholipid vesicles, and decreased the rate of crystal formation. Fluorescence-labeled anionic polypeptide fraction revealed that anionic polypeptide fraction (with and without calcium) was primarily associated with vesicle aggregates.

CONCLUSIONS

Our data demonstrate that calcium and anionic polypeptide fraction have opposing effects on the process of cholesterol crystallization and the resultant crystal mass without influencing the crystal observation time of bile. These findings suggest that biliary proteins, in addition to being crystallization effectors by themselves, may further influence cholesterol crystallization and gallstone formation by interacting with calcium and possibly other elements that coexist in bile.