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

Regorafenib loaded self-assembled lipid-based nanocarrier for colorectal cancer treatment via lymphatic absorption

Oral chemotherapy can improve the life quality of patients; however, the therapeutic effects are limited by low bioavailability and rapid in vivo elimination of anticancer drugs. Here, we developed a regorafenib (REG)-loaded self-assembled lipid-based nanocarrier (SALN) to improve oral absorption and anti-colorectal cancer efficacy of REG through lymphatic absorption. SALN was prepared with lipid-based excipients to utilize lipid transport in the enterocytes and enhance lymphatic absorption of the drug in the gastrointestinal tract. The particle size of SALN was 106 ± 10 nm. SALNs were internalized by the intestinal epithelium via the clathrin-mediated endocytosis, and then transported across the epithelium via the chylomicron secretion pathway, resulting in a 3.76-fold increase in drug epithelial permeability (P_app) compared to the solid dispersion (SD). After oral administration to rats, SALNs were transported by the endoplasmic reticulum, Golgi apparatus, and secretory vesicles of enterocytes and were found in the lamina propria of intestinal villi, abdominal mesenteric lymph, and plasma. The oral bioavailability of SALN was 65.9-fold and 1.70-fold greater than that of the coarse powder suspension and SD, respectively, and was highly dependent on the lymphatic route of absorption. Notably, SALN prolonged the elimination half-life of the drug (9.34 ± 2.51 h) compared to the solid dispersion (3.51 ± 0.46 h), increased the biodistribution of REG in the tumor and gastrointestinal (GI) tract, decreased biodistribution in the liver, and showed better therapeutic efficacy than the solid dispersion in colorectal tumor-bearing mice. These results demonstrated that SALN is promising for the treatment of colorectal cancer via lymphatic transport and has potential for clinical translation.

Effects of Biliary Phospholipids on Cholesterol Crystallization and Growth in Gallstone Formation

The prevalence of cholesterol gallstone disease is increasing, primarily due to the global epidemic of obesity associated with insulin resistance, and this trend leads to a considerable healthcare, financial, and social burden worldwide. Although phospholipids play an essential role in maintaining cholesterol solubility in bile through both mixed micelles and vesicles, little attention has been paid to the impact of biliary phospholipids on the pathogenesis of cholesterol gallstone formation. A reduction or deficiency of biliary phospholipids results in a distinctly abnormal metastable physical-chemical state of bile predisposing to supersaturation with cholesterol. Changes in biliary phospholipid concentrations influence cholesterol crystallization by yielding both liquid crystalline and "anhydrous" crystalline metastable intermediates, evolving into classical parallelogram-shaped cholesterol monohydrate crystals in supersaturated bile. As a result, five distinct crystallization pathways, A-E, have been defined, mainly based on the prime habits of liquid and solid crystals in the physiological or pathophysiological cholesterol saturation of gallbladder and hepatic bile. This review concisely summarizes the chemical structures and physical-chemical properties of biliary phospholipids and their physiological functions in bile formation and cholesterol solubility in bile, as well as comprehensively discusses the latest advances in the role of biliary phospholipids in cholesterol crystallization and growth in gallstone formation, largely based on the findings from clinical and animal studies and in vitro experiments. The insights gleaned from uncovering the cholelithogenic mechanisms are expected to form a fundamental framework for investigating the hitherto elusive events in the earliest stage of cholesterol nucleation and crystallization. This may help to identify better measures for early diagnosis and prevention in susceptible subjects and effective treatment of patients with gallstones.

Genetic Analysis of ABCB4 Mutations and Variants Related to the Pathogenesis and Pathophysiology of Low Phospholipid-Associated Cholelithiasis

Clinical studies have revealed that the ABCB4 gene encodes the phospholipid transporter on the canalicular membrane of hepatocytes, and its mutations and variants are the genetic basis of low phospholipid-associated cholelithiasis (LPAC), a rare type of gallstone disease caused by a single-gene mutation or variation. The main features of LPAC include a reduction or deficiency of phospholipids in bile, symptomatic cholelithiasis at <40 years of age, intrahepatic sludge and microlithiasis, mild chronic cholestasis, a high cholesterol/phospholipid ratio in bile, and recurrence of biliary symptoms after cholecystectomy. Needle-like cholesterol crystals, putatively “anhydrous” cholesterol crystallization at low phospholipid concentrations in model and native bile, are characterized in ABCB4 knockout mice, a unique animal model for LPAC. Gallbladder bile with only trace amounts of phospholipids in these mice is supersaturated with cholesterol, with lipid composition plotting in the left two-phase zone of the ternary phase diagram, consistent with “anhydrous” cholesterol crystallization. In this review, we summarize the molecular biology and physiological functions of ABCB4 and comprehensively discuss the latest advances in the genetic analysis of ABCB4 mutations and variations and their roles in the pathogenesis and pathophysiology of LPAC in humans, based on the results from clinical studies and mouse experiments. To date, approximately 158 distinct LPAC-causing ABCB4 mutations and variants in humans have been reported in the literature, indicating that it is a monogenic risk factor for LPAC. The elucidation of the ABCB4 function in the liver, the identification of ABCB4 mutations and variants in LPAC patients, and the exploration of gene therapy for ABCB4 deficiency in animal models can help us to better understand the cellular, molecular, and genetic mechanisms underlying the onset of the disease, and will pave the way for early diagnosis and prevention of susceptible subjects and effective intervention for LPAC in patients.

Cholesterol-phospholipid interactions resist the detergent effect of bovine bile

Sphingomyelin (SM) and cholesterol complexation gives rise to detergent-resistant liquid-ordered domains. The persistence of these domains and subsequent mixed micelle formation was examined in the presence of bile under physiological digestive in vitro conditions for vesicles comprising either SM/cholesterol, porcine brain phosphatidylcholine (BPC)/cholesterol, or soy phosphatidylcholine (SPC)/cholesterol bilayers, the latter two systems having no liquid-ordered domains. Micellization of these digested phospholipid multilamellar vesicle systems was confirmed by transmission electron microscopy. Bovine bile was found to consist of large multilamellar sheets which subsumed phospholipid vesicles to form aggregated superstructures. Budding off from these superstructures were vesicle-to-micelle transition intermediates: unilamellar vesicles and cylindrical micelles. The presence of cholesterol (60/40 phospholipid/cholesterol mol/mol) delayed the initial rapid onset of digestion, but not for BPC and SPC vesicle systems. Acyl chain order/disorder before and after vesicle-to-micelle transition of all three phospholipid/cholesterol systems was examined using Raman spectroscopy. The addition of bovine bile to both PC/cholesterol vesicle systems reduced the overall ratio of acyl chain disorder to order. In SM/cholesterol vesicles with ≤ 20% mol cholesterol, only the lateral inter-acyl chain packing was reduced, whereas for SM/cholesterol vesicles with ≥ 30% mol cholesterol, a higher proportion of gauche-to-trans isomerization was apparent, demonstrating that SM/cholesterol complexes modify the acyl chain structure of micelles.

Mapping the intermediate digestion phases of human healthy intestinal contents from distal ileum and caecum at fasted and fed state conditions

OBJECTIVES

To investigate at the ultrastructural level, the colloidal phases formed in the lumen of the distal ileum and caecum of healthy adults.

METHODS

Cryogenic transmission electron microscopy (Cryo-TEM) was employed to image the intermediate colloidal phases of human intestinal contents collected from distal ileum and caecum of two healthy volunteers under fasted and fed state conditions.

KEY FINDINGS

In samples collected both in the fasted and fed states, Cryo-TEM study revealed the presence of large spherical unilamellar and occasionally bi-lamellar and oligolamellar vesicles with diameters ranging from 50 to 200 nm for both volunteers in distal ileum and caecum. Bilayer fragments were frequently observed in caecal samples. Plate-like structures resembling the morphology of cholesterol plates were visualised in all samples. Elongated structures were observed in the fed state in distal ileum and caecum for both volunteers, whereas no micellar structures could be detected for all samples.

CONCLUSIONS

This study provides a framework for understanding the structure of colloidal phases, and it may assist in elucidating the role of dosing conditions on drug absorption from the distal ileum and caecum.

Oral delivery system prolongs blood circulation of docetaxel nanocapsules via lymphatic absorption

An original oral formulation of docetaxel nanocapsules (NCs) embedded in microparticles elicited in rats a higher bioavailability compared with the i.v. administration of the commercial docetaxel solution, Taxotere. In the present study, various animal studies were designed to elucidate the absorption process of docetaxel from such a delivery system. Again, the docetaxel NC formulation elicited a marked enhanced absorption compared with oral Taxotere in minipigs, resulting in relative bioavailability and Cmax values 10- and 8.4-fold higher, respectively, confirming the previous rat study results. It was revealed that orally absorbed NCs altered the elimination and distribution of docetaxel, as shown in the organ biodistribution rat study, due to their reinforced coating, while transiting through the enterocytes by surface adsorption of apoproteins and phospholipids. These findings were demonstrated by the cryogenic-temperature transmission electron microscopy results and confirmed by the use of a chylomicron flow blocker, cycloheximide, that prevented the oral absorption of docetaxel from the NC formulation in an independent pharmacokinetic study. The lipoproteinated NCs reduced the docetaxel release in plasma and its distribution among the organs. The improved anticancer activity compared with i.v. Taxotere, observed in the metastatic lung cancer model in Severe Combined Immune Deficiency-beige (SCID-bg) mice, should be attributed to the extravasation effect, leading to the lipoproteinated NC accumulation in lung tumors, where they exert a significant therapeutic action. To the best of our knowledge, no study has reported that the absorption of NCs was mediated by a lymphatic process and reinforced during their transit.

Curvature instability in a chiral amphiphile self-assembly

We present the first experimental evidence for the morphological transition from twisted to helical ribbons in amphiphile aggregates. This transition, from structures possessing negative Gaussian curvature to helically curved structures, is shown to be directly linked to ribbon width. Time-resolved cryotransmission electron microscopy images of a peptidomimetic amphiphile further capture the dynamic transformation between the two geometries along a single ribbon unit. Quantitative analysis indicates that both ribbon width and pitch grow with ribbon maturation, maintaining a constant ratio.

Crystalline lipid domains: characterization by X-ray diffraction and their relation to biology

Biological membranes comprise thousands of different lipids, differing in their alkyl chains, headgroups, and degree of saturation. It is estimated that 5% of the genes in the human genome are responsible for regulating the lipid composition of cell membranes. Conceivably, the functional explanation for this diversity is found, at least in part, in the propensity of lipids to segregate into distinct domains, which are important for cell function. X-ray diffraction has been used increasingly to characterize the packing and phase behavior of lipids in membranes. Crystalline domains have been studied in synthetic membranes using wide- and small-angle X-ray scattering, and grazing incidence X-ray diffraction. Herein we summarize recent results obtained using the various X-ray methods, discuss the correlation between crystalline domains and liquid ordered domains studied with other techniques, and the relevance of crystalline domains to functional lipid domains in biological membranes.

Unraveling the mechanism of nanotube formation by chiral self-assembly of amphiphiles

The self-assembly of nanotubes from chiral amphiphiles and peptide mimics is still poorly understood. Here, we present the first complete path to nanotubes by chiral self-assembly studied with C(12)-β(12) (N-α-lauryl-lysyl-aminolauryl-lysyl-amide), a molecule designed to have unique hybrid architecture. Using the technique of direct-imaging cryo-transmission electron microscopy (cryo-TEM), we show the time-evolution from micelles of C(12)-β(12) to closed nanotubes, passing through several types of one-dimensional (1-D) intermediates such as elongated fibrils, twisted ribbons, and coiled helical ribbons. Scattering and diffraction techniques confirm that the fundamental unit is a monolayer lamella of C(12)-β(12), with the hydrophobic tails in the gel state and β-sheet arrangement. The lamellae are held together by a combination of hydrophobic interactions, and two sets of hydrogen-bonding networks, supporting C(12)-β(12) monomers assembly into fibrils and associating fibrils into ribbons. We further show that neither the "growing width" model nor the "closing pitch" model accurately describe the process of nanotube formation, and both ribbon width and pitch grow with maturation. Additionally, our data exclusively indicate that twisted ribbons are the precursors for coiled ribbons, and the latter structures give rise to nanotubes, and we show chirality is a key requirement for nanotube formation.

On the origin of primitive cells: from nutrient intake to elongation of encapsulated nucleotides

Recent major discoveries in membrane biophysics hold the key to a modern understanding of the origin of life on Earth. Membrane bilayer vesicles have been shown to provide a multifaceted microenvironment in which protometabolic reactions could have developed. Cell-membrane-like aggregates of amphiphilic molecules capable of retaining encapsulated oligonucleotides have been successfully created in the laboratory. Sophisticated laboratory studies on the origin of life now show that elongation of the DNA primer takes place inside fatty acid vesicles when activated nucleotide nutrients are added to the external medium. These studies demonstrate that cell-like vesicles can be sufficiently permeable to allow for the intake of charged molecules such as activated nucleotides, which can then take part in copying templates in the protocell interior. In this Review we summarize recent experiments in this area and describe a possible scenario for the origin of primitive cells, with an emphasis on the elongation of encapsulated nucleotides.

Live-cell imaging in Caenorhabditis elegans reveals the distinct roles of dynamin self-assembly and guanosine triphosphate hydrolysis in the removal of apoptotic cells

Dynamins are large GTPases that oligomerize along membranes. Dynamin's membrane fission activity is believed to underlie many of its physiological functions in membrane trafficking. Previously, we reported that DYN-1 (Caenorhabditis elegans dynamin) drove the engulfment and degradation of apoptotic cells through promoting the recruitment and fusion of intracellular vesicles to phagocytic cups and phagosomes, an activity distinct from dynamin's well-known membrane fission activity. Here, we have detected the oligomerization of DYN-1 in living C. elegans embryos and identified DYN-1 mutations that abolish DYN-1's oligomerization or GTPase activities. Specifically, abolishing self-assembly destroys DYN-1's association with the surfaces of extending pseudopods and maturing phagosomes, whereas inactivating guanosine triphosphate (GTP) binding blocks the dissociation of DYN-1 from these membranes. Abolishing the self-assembly or GTPase activities of DYN-1 leads to common as well as differential phagosomal maturation defects. Whereas both types of mutations cause delays in the transient enrichment of the RAB-5 GTPase to phagosomal surfaces, only the self-assembly mutation but not GTP binding mutation causes failure in recruiting the RAB-7 GTPase to phagosomal surfaces. We propose that during cell corpse removal, dynamin's self-assembly and GTP hydrolysis activities establish a precise dynamic control of DYN-1's transient association to its target membranes and that this control mechanism underlies the dynamic recruitment of downstream effectors to target membranes.

Imperfect dissolution in nonionic block copolymer and surfactant mixtures

Self-assembled copolymer micelles have been widely explored for numerous applications including cosmetic formulations and detergency, drug delivery, and agriculture. In many of these technologies at least trace amounts of surfactants and detergents are present, yet little is known regarding their effect on the copolymer micelle structure. In this paper we examine the influence of a nonionic micelle-forming surfactant, Triton X-100, on spherical, nonionic polymeric micelles composed of poly(butadiene)-co-poly(polyethylene oxide). Using cryo-TEM we find that relatively small surfactant concentrations (less than 1:1 molar ratio) are sufficient to disrupt the copolymer assemblies, and to yield, via dimerization, mixed polymer-surfactant micelles with characteristic diameters. Saturation of the polymeric micelles is reached with approximately 3 mM surfactant (1:8 mol ratio). Upon saturation, and in high surfactant excess, coexistence of two homogeneous micellar populations is found: saturated polymer-surfactant micelles, and much smaller micelles of pure surfactant. The lack of complete demicellization of the polymeric micelles is explained by packing constraints of the polymer hydrophobic chains by the added surfactant. This behavior is found to be characteristic of polymeric molecules with hydrophobic-to-hydrophilic molecular weight ratio close to, or exceeding, 0.75. We further found that structural transitions in polymer-surfactant mixtures are fast, and the systems reach equilibrium at time scales characteristic to the small molecule, in contrast with the slow equilibration in polymer-polymer mixtures.

A comparative study of microstructural development in paired human hepatic and gallbladder biles

Cholesterol gallstones usually develop in the gallbladder and rarely form in bile ducts even in patients with highly lithogenic bile. Bile concentration and proteins (e.g. mucin) may affect crystallization, but the exact nature of this effect, especially in relation to crystallization pathways and microstructural evolution remains unclear. We examined lipid microstructures in paired hepatic and gallbladder biles to reveal ones that are essential for crystallization. Combining digital light microscopy with cryogenic-temperature transmission electron microscopy we are able to directly visualize and compare the time evolution of lipid microstructures in paired hepatic, gallbladder and diluted gallbladder biles of gallstone patients and controls, without drying or separating. Gallbladder bile exhibited several multilamellar vesicles and spheroidal micelles preceding and throughout crystallization. Vesicle morphology changed before crystallization was observed. In contrast, hepatic bile revealed almost no crystallization and while a variety of unilamellar vesicles and spheroidal micelles existed throughout the examination, multilamellar vesicles were rare. Diluted gallbladder bile was different from native gallbladder bile, as well as the paired hepatic bile, yielding occasional crystallization. Our findings suggest that maturing multilamellar vesicles precede (and at least partially initiate) crystallization in gallbladder bile. Although microstructural development seems to be concentration dependent, dilution of gallbladder bile to hepatic bile concentrations neither makes it identical to hepatic bile, nor prevents crystallization.

Microemulsions as transdermal drug delivery vehicles

Microemulsions are clear, stable, isotropic mixtures of oil, water, and surfactant, frequently in combination with a cosurfactant. Microemulsions have been intensively studied during the last decades by many scientists and technologists because of their great potential in many food and pharmaceutical applications. The use of microemulsions is advantageous not only due to the facile and low cost preparation, but also because of the improved bioavailability. The increased absorption of drugs in topical applications is attributed to enhancement of penetration through the skin by the carrier. Saturated and unsaturated fatty acids serving as an oil phase are frequently used as penetration enhancers. The most popular enhancer is oleic acid. Other permeation enhancers commonly used in transdermal formulations are isopropyl myristate, isopropyl palmitate, triacetin, isostearylic isostearate, R(+)-limonene and medium chain triglycerides. The most popular among the enhancing permeability surfactants are phospholipids that have been shown to enhance drug permeation in a different mode. l-alpha-phosphatidylcholine from egg yolk, l-alpha-phosphatidylcholine 60%, from soybean and dioleylphosphatidyl ethanolamine which are in a fluid state may diffuse into the stratum corneum and enhance dermal and transdermal drug penetration, while distearoylphosphatidyl choline which is in a gel-state has no such capability. Other very commonly used surfactants are Tween 20, Tween 80, Span 20, Azone, Plurol Isostearique and Plurol Oleique. As cosurfactants commonly serve short-chain alkanols such as ethanol and propylene glycol. Long-chain alcohols, especially 1-butanol, are known for their enhancing activity as well. Decanol was found to be an optimum enhancer among other saturated fatty alcohols that were examined (from octanol to myristyl alcohol). Many enhancers are concentration-dependent; therefore, optimal concentration for effective promotion should be determined. The delivery rate is dependent on the type of the drug, the structure and ingredients of the carrier, and on the character of the membrane in use. Each formulation should be examined very carefully, because every membrane alters the mechanism of penetration and can turn an enhancer to a retarder. Various potential mechanisms to enhance drug penetration through the skin include directly affecting the skin and modifying the formulation so the partition, diffusion, or solubility is altered. The combination of several enhancement techniques such as the use of iontophoresis with fatty acids leads to synergetic drug penetration and to decrease in skin toxicity. Selected studies of various microemulsions containing certain drugs including retinoic acid, 5-fluorouracil, triptolide, ascorbic acid, diclofenac, lidocaine, and prilocaine hydrochloride in transdermal formulations are presented in this review. In conclusion, microemulsions were found as an effective vehicle of the solubilization of certain drugs and as protecting medium for the entrapped of drugs from degradation, hydrolysis, and oxidation. It can also provide prolonged release of the drug and prevent irritation despite the toxicity of the drug. Yet, in spite of all the advantages the present formulations lack several key important characteristics such as cosmetic-permitted surfactants, free dilution in water capabilities, stability in the digestive tracts and sufficient solubilization capacity.

Cholesterol gallstone disease

With a prevalence of 10-15% in adults in Europe and the USA, gallstones are the most common digestive disease needing admission to hospital in the West. The interplay between interprandial and postprandial physiological responses to endogenous and dietary lipids underscores the importance of coordinated hepatobiliary and gastrointestinal functions to prevent crystallisation and precipitation of excess biliary cholesterol. Indeed, identifying the metabolic and transcriptional pathways that drive the regulation of biliary lipid secretion has been a major achievement in the field. We highlight scientific advances in protein and gene regulation of cholesterol absorption, synthesis, and catabolism, and biliary lipid secretion with respect to the pathogenesis of cholesterol gallstone disease. We discuss the physical-chemical mechanisms of gallstone formation in bile and the active role of the gallbladder and the intestine. We also discuss gaps in our knowledge of the pathogenesis of gallstone formation and the potential for gene targeting in therapy.

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.

A kinetic study of the growth of fatty acid vesicles

Membrane vesicles composed of fatty acids can be made to grow and divide under laboratory conditions, and thus provide a model system relevant to the emergence of cellular life. Fatty acid vesicles grow spontaneously when alkaline micelles are added to buffered vesicles. To investigate the mechanism of this process, we used stopped-flow kinetics to analyze the dilution of non-exchanging FRET probes incorporated into preformed vesicles during growth. Oleate vesicle growth occurs in two phases (fast and slow), indicating two pathways for the incorporation of fatty acid into preformed vesicles. We propose that the fast phase, which is stoichiometrically limited by the preformed vesicles, results from the formation of a "shell" of fatty acid around a vesicle, followed by rapid transfer of this fatty acid into the preformed vesicle. The slower phase may result from incorporation of fatty acid which had been trapped in an intermediate state. We provide independent evidence for the rapid transformation of micelles into an aggregated intermediate form after transfer from high to low pH. Our results show that the most efficient incorporation of added oleate into oleic acid/oleate vesicles occurs under conditions that avoid a large transient increase in the micelle/vesicle ratio.

Trapping crystal nucleation of cholesterol monohydrate: relevance to pathological crystallization

Crystalline nucleation of cholesterol at the air-water interface has been studied via grazing incidence x-ray diffraction using synchrotron radiation. The various stages of cholesterol molecular assembly from monolayer to three bilayers incorporating interleaving hydrogen-bonded water layers in a monoclinic cholesterol.H(2)O phase, has been monitored and their structures characterized to near atomic resolution. Crystallographic evidence is presented that this multilayer phase is similar to that of a reported metastable cholesterol phase of undetermined structure obtained from bile before transformation to the triclinic phase of cholesterol.H(2)O, the thermodynamically stable macroscopic form. According to grazing incidence x-ray diffraction measurements and crystallographic data, a transformation from the monoclinic film structure to a multilayer of the stable monohydrate phase involves, at least initially, an intralayer cholesterol rearrangement in a single-crystal-to-single-crystal transition. The preferred nucleation of the monoclinic phase of cholesterol.H(2)O followed by transformation to the stable monohydrate phase may be associated with an energetically more stable cholesterol bilayer arrangement of the former and a more favorable hydrogen-bonding arrangement of the latter. The relevance of this nucleation process of cholesterol monohydrate to pathological crystallization of cholesterol from cell biomembranes is discussed.

Rapid constriction of lipid bilayers by the mechanochemical enzyme dynamin

Dynamin, a large GTPase, is located at the necks of clathrin-coated pits where it facilitates the release of coated vesicles from the plasma membrane upon GTP binding, and hydrolysis. Previously, we have shown by negative stain electron microscopy that wild-type dynamin and a dynamin mutant lacking the C-terminal proline-rich domain, DeltaPRD, form protein-lipid tubes that constrict and vesiculate upon addition of GTP. Here, we show by time-resolved cryo-electron microscopy (cryo-EM) that DeltaPRD dynamin in the presence of GTP rapidly constricts the underlying lipid bilayer, and then gradually disassembles from the lipid. In agreement with the negative stain results, the dynamin tubes constrict from 50 to 40 nm, and their helical pitch decreases from approximately 13 to 9.4 nm. However, in contrast to the previous results, examination by cryo-EM shows that the lipid bilayer remains intact and small vesicles or fragments do not form upon GTP binding and hydrolysis. Therefore, the vesicle formation seen by negative stain may be due to the lack of mobility of the dynamin tubes on the grid during the GTP-induced conformational changes. Our results confirm that dynamin is a mechanochemical enzyme and suggest that during endocytosis dynamin is directly responsible for membrane constriction. In the cell, other proteins may enhance the activity of dynamin or the constraints induced by the surrounding coated pit and plasma membrane during constriction may cause the final membrane fission event.

The Epitaxial Growth of Cholesterol Crystals from Bile Solutions on Calcite Substrates

Epitaxial relationships between the surfaces of inorganic and bioorganic crystals can be an important factor in crystal nucleation and growth processes in a variety of biological environments. Crystalline cholesterol monohydrate (ChM), a constituent of both gallstone and atherosclerotic plaques, is often found in association with assorted mineral phases. Using in situ atomic force microscopy (AFM) and well-characterized model bile solutions, the nucleation and epitaxial growth of ChM on calcite (104) surfaces in real-time is demonstrated. The growth rates of individual cholesterol islands formed on calcite substrates were determined at physiological temperatures. Evidence of Ostwald's ripening was also observed under these experimental conditions. The energetics of various (104) calcite/(001) ChM interfaces were calculated to determine the most stable interfacial structure. These simulations suggest that the interface is fully hydrated and that cholesterol hydroxyl groups are preferentially positioned above carbonate ions in the calcite surface. This combination of experimental and theoretical work provides a clearer picture of how preexisting mineral seeds might provide a viable growth template that can reduce the energetic barrier to cholesterol nucleation under some physiological conditions.

Cholesterol crystal morphology in acalculous gallbladder disease

BACKGROUND

Biliary crystal morphology is best described in patients with gallbladder stones, but most patients undergoing bile collection for microscopy have a clinical diagnosis of acalculous gallbladder disease. We investigated the morphology of biliary crystals in such patients.

STUDY

Bile was obtained for polarizing microscopy from fresh cholecystectomy specimens of patients with a clinical diagnosis of acalculous or calculous gallbladder disease. Slides for microscopy were prepared by touch contact with bile in freshly opened gallbladder specimens, and following aspiration of gallbladder bile through a 5-French cannula.

RESULTS

Bile was examined from five patients with a clinical diagnosis of acalculous gallbladder disease and five patients with known gallstones. Needle-like cholesterol crystals predominated in most patients without gallstones, whereas plate-like and dot-like crystals were more common in patients with gallstones. All three crystal types were seen in most patients. Crystal morphology was not affected by aspiration of bile through a 5-French cannula.

CONCLUSIONS

Birefringent needles and dots should be recognized as cholesterol crystals during bile microscopy. These crystal morphologies may predominate in some patients with a clinical diagnosis of acalculous gallbladder disease.

Incorporation of cholesterol in sphingomyelin- phosphatidylcholine vesicles has profound effects on detergent-induced phase transitions

Vesicle <--> micelle transitions are important phenomena during bile formation and intestinal lipid processing. The hepatocyte canalicular membrane outer leaflet contains appreciable amounts of phosphatidylcholine (PC) and sphingomyelin (SM), and both phospholipids are found in the human diet. Dietary SM enrichment inhibits intestinal cholesterol absorption. We therefore studied detergent-induced vesicle --> micelle transitions in SM-PC vesicles. Phase transitions were evaluated by spectrophotometry and cryotransmission electron microscopy (cryo-TEM) after addition of taurocholate (3-7 mM) to SM-PC vesicles (4 mM phospholipid, SM/PC 40%/60%, without or with 1.6 mM cholesterol). After addition of excess (5-7 mM) taurocholate, SM-PC vesicles were more sensitive to micellization than PC vesicles. As shown by sequential cryo-TEM, addition of equimolar (4 mM) taurocholate to SM-PC vesicles induced formation of open vesicles, then (at the absorbance peak) fusion of bilayer fragments into large open structures (around 200 nm diameter) coexisting with some multilamellar or fused vesicles and thread-like micelles and, finally, transformation into an uniform picture with long thread-like micelles. Incorporation of cholesterol in the SM/PC bilayer changed initial vesicular shape from spherical into ellipsoid and profoundly increased detergent resistance. Disk-like micelles and multilamellar vesicles, and then extremely large vesicular structures, were observed by sequential cryo-TEM under these circumstances, with persistently increased absorbance values by spectrophotometry. These findings may be relevant for bile formation and intestinal lipid processing. Inhibition of intestinal cholesterol absorption by dietary SM enrichment may relate to high resistance against bile salt-induced micellization of intestinal lipids in presence of the sphingolipid.

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.

Gallstones

At the turn of the millennium, significant advances continue to be made into the epidemiology and pathophysiology of gallstone disease. The NHANES III study, the largest American population-based study of gallstone disease, estimated that more than 20 million Americans have undergone gallbladder surgery or currently have gallstones. Insulin resistance may be an independent risk factor for gallstone disease.Cholecystokinin receptors may be responsible for the altered motility of the gallbladder smooth muscle, and mucin may play an underestimated role as a pronucleating factor. For the first time, researchers have been able to directly observe cholesterol crystallization in human bile. Improved understanding of the multiple factors involved in the pathogenesis of gallstone disease should lead to new therapeutic and preventive strategies.