Formation of 3D Cholesterol Crystals from 2D Nucleation Sites in Lipid Bilayer Membranes: Implications for Atherosclerosis

Cholesterol crystals and atherosclerotic plaque instability: Therapeutic potential of Eicosapentaenoic acid

Atherosclerotic plaques associated with acute coronary syndromes (ACS), i.e. culprit lesions, frequently feature a ruptured fibrous cap with thrombotic complications. On imaging, these plaques exhibit a low attenuation, lipid-rich, necrotic core containing cholesterol crystals and are inherently unstable. Indeed, cholesterol crystals are causally associated with plaque vulnerability in vivo; their formation results from spontaneous self-assembly of cholesterol molecules. Cholesterol homeostasis is a central determinant of the physicochemical conditions leading to crystal formation, which are favored by elevated membrane free cholesterol content in plaque endothelial cells, smooth muscle cells, monocyte-derived macrophages, and foam cells, and equally by lipid oxidation. Emerging evidence from imaging trials in patients with coronary heart disease has highlighted the impact of intervention involving the omega-3 fatty acid, eicosapentaenoic acid (EPA), on vulnerable, low attenuation atherosclerotic plaques. Thus, EPA decreased features associated with unstable plaque by increasing fibrous cap thickness in statin-treated patients, by reducing lipid volume and equally attenuating intraplaque inflammation. Importantly, atherosclerotic plaques rapidly incorporate EPA; indeed, a high content of EPA in plaque tissue is associated with decreased plaque inflammation and increased stability. These findings are entirely consistent with the major reduction seen in cardiovascular events in the REDUCE-IT trial, in which high dose EPA was administered as its esterified precursor, icosapent ethyl (IPE); moreover, clinical benefit was proportional to circulating EPA levels. Eicosapentaenoic acid is efficiently incorporated into phospholipids, where it modulates cholesterol-enriched domains in cell membranes through physicochemical lipid interactions and changes in rates of lipid oxidation. Indeed, biophysical analyses indicate that EPA exists in an extended conformation in membranes, thereby enhancing normal cholesterol distribution while reducing propagation of free radicals. Such effects mitigate cholesterol aggregation and crystal formation. In addition to its favorable effect on cholesterol domain structure, EPA/IPE exerts pleiotropic actions, including antithrombotic, antiplatelet, anti-inflammatory, and proresolving effects, whose plaque-stabilizing potential cannot be excluded. Docosahexaenoic acid is distinguished from EPA by a higher degree of unsaturation and longer carbon chain length; DHA is thus predisposed to changes in its conformation with ensuing increase in membrane lipid fluidity and promotion of cholesterol aggregation into discrete domains. Such distinct molecular effects between EPA and DHA are pronounced under conditions of high cellular cholesterol content and oxidative stress. This review will focus on the formation and role of cholesterol monohydrate crystals in destabilizing atherosclerotic plaques, and on the potential of EPA as a therapeutic agent to attenuate the formation of deleterious cholesterol membrane domains and of cholesterol crystals. Such a therapeutic approach may translate to enhanced plaque stability and ultimately to reduction in cardiovascular risk.

Structure of a cholinergic cell membrane

While it has long been established that cell membranes are complex assemblies of proteins and bilayer-forming lipids, the inherent mobility and wide-ranging heterogeneity of the lipids have limited our ability to understand cell-membrane structure at a molecular level. Consequently, little is yet known about the protein-lipid and lipid-lipid interplay that exists in situ. The present study exploits the regular architecture of a cholinergic cell membrane to determine how the phospholipid and cholesterol organization is influenced by the protein surfaces and by differences in cholesterol concentration between the two leaflets. Lipids in the leaflet containing the most cholesterol form an ordered sterol-hydrocarbon “skin.” This hitherto unobserved hydrophobic-core structure has far-reaching implications in terms of how cholesterol-rich membranes are constructed and function.

Cell membranes are complex assemblies of proteins and lipids making transient or long-term associations that have yet to be characterized at a molecular level. Here, cryo-electron microscopy is applied to determine how phospholipids and cholesterol arrange between neighboring proteins (nicotinic acetylcholine receptors) of Torpedo cholinergic membrane. The lipids exhibit distinct properties in the two leaflets of the bilayer, influenced by the protein surfaces and by differences in cholesterol concentration. In the outer leaflet, the lipids show no consistent motif away from the protein surfaces, in keeping with their assumed fluidity. In the inner leaflet, where the cholesterol concentration is higher, the lipids organize into extensive close-packed linear arrays. These arrays are built from the sterol groups of cholesterol and the initial saturated portions of the phospholipid hydrocarbon chains. Together, they create an ordered ∼7 Å-thick “skin” within the hydrophobic core of the bilayer. The packing of lipids in the arrays appears to bear a close relationship to the linear cholesterol arrays that form crystalline monolayers at the air-water interface.

Examining atherosclerotic lesions in three dimensions at the nanometer scale with cryo-FIB-SEM

We employed in a correlative manner an unconventional combination of methods, comprising cathodoluminescence, cryo-scanning electron microscopy (SEM), and cryo-focused ion beam (FIB)-SEM, to examine the volumes of thousands of cubed micrometers from rabbit atherosclerotic tissues, maintained in close-to-native conditions, with a resolution of tens of nanometers. Data from three different intralesional regions, at the media-lesion interface, in the core, and toward the lumen, were analyzed following segmentation and volume or surface representation. The media-lesion interface region is rich in cells and lipid droplets, whereas the core region is markedly richer in crystals and has lower cell density. In the three regions, thin crystals appear to be associated with intracellular or extracellular lipid droplets and multilamellar bodies. Large crystals are independently positioned in the tissue, not associated with specific cellular components. This extensive evidence strongly supports the idea that the lipid droplet surfaces and the outer membranes of multilamellar bodies play a role in cholesterol crystal nucleation and growth and that crystal formation occurs, in part, inside cells. The correlative combination of methods that allowed the direct examination of cholesterol crystals and lipid deposits in the atherosclerotic lesions may be similarly used for high-resolution examination of other tissues containing pathological or physiological cholesterol deposits.

Killing the Culprit: Pharmacological Solutions to Get Rid of Cholesterol Crystals

Cholesterol crystals (CCs) play a key role in the pathophysiology of cardiovascular diseases (CVD) via triggering inflammation, plaque formation and subsequently plaque rupture. Although statins can stabilize plaques via calcification and alteration of the lipid composition within plaques, there is still a high residual risk of CVD events among statins users. Several studies have tried to blunt the detrimental effects of cholesterol crystals by pharmacological interventions. Cyclodexterins (CDs) and other nanoformulations, including polymers of CDs and liposomes, have the ability to dissolve CCs in vitro and in vivo. CDs were the first in their class that entered clinical trials and showed promising results, though their ototoxicity outweighed their benefits. Moreover, small molecules with structural similarity to cholesterol may also perturb cholesterol-cholesterol interactions and prevent from expansion of 2D crystalline domains to large 3D CCs. The results from ethyl eicosapentaenoic acid and ursodeoxycholic acid were encouraging and worth further consideration. In this review, the significance of CCs in pathogenesis of CVD is discussed and pharmacological agents with the ability to dissolve CCs or prevent from CCs formation are introduced.

Cholesterol Microdomain Enhances the Biofilm Eradication of Antibiotic Liposomes

Resistance and tolerance of biofilms to antibiotics is the greatest challenge in the treatment of bacterial infections. Therefore, developing an effective strategy against biofilms is a top priority. Liposomes are widely used as antibiotic drug carriers; however, common liposomes lack affinity for biofilms. Herein, biofilm-targeted antibiotic liposomes are created by simply adjusting their cholesterol content. The tailored liposomes exhibit significantly enhanced bacterial inhibition and biofilm eradication effects that are positively correlated with the cholesterol content of liposomes. The experiments further demonstrate that this enhanced effect can be ascribed to the effective drug release through the pores, which are formed by the combination of cholesterol microdomains in liposomal lipid bilayers with membrane-damaged toxins in biofilms. Consequently, liposome encapsulation with a high cholesterol concentration improves noticeably the pharmacodynamics and biocompatibility of antibiotics after pulmonary administration. This work may provide a new direction for the development of antibiofilm formulations that can be widely used for the treatment of infections caused by bacterial biofilms.

Polymorphism, Structure, and Nucleation of Cholesterol·H2O at Aqueous Interfaces and in Pathological Media: Revisited from a Computational Perspective

We revisit the important issues of polymorphism, structure, and nucleation of cholesterol·H₂O using first-principles calculations based on dispersion-augmented density functional theory. For the lesser known monoclinic polymorph, we obtain a fully extended H-bonded network in a structure akin to that of hexagonal ice. We show that the energy of the monoclinic and triclinic polymorphs is similar, strongly suggesting that kinetic and environmental effects play a significant role in determining polymorph nucleation. Furthermore, we find evidence in support of various O–H···O bonding motifs in both polymorphs that may result in hydroxyl disorder. We have been able to explain, via computation, why a single cholesterol bilayer in hydrated membranes always crystallizes in the monoclinic polymorph. We rationalize what we believe is a single-crystal to single-crystal transformation of the monoclinic form on increased interlayer growth beyond that of a single cholesterol bilayer, interleaved by a water bilayer. We show that the ice-like structure is also relevant to the related cholestanol·2H₂O and stigmasterol·H₂O crystals. The structure of stigmasterol hydrate both as a trilayer film at the air–water interface and as a macroscopic crystal further assists us in understanding the polymorphic and thermal behavior of cholesterol·H₂O. Finally, we posit a possible role for one of the sterol esters in the crystallization of cholesterol·H₂O in pathological environments, based on a composite of a crystalline bilayer of cholesteryl palmitate bound epitaxially as a nucleating agent to the monoclinic cholesterol·H₂O form.

Formation and Cellular Impact of Cholesterol Crystals in Health and Disease

Cholesterol crystals (CCs) were first discovered in atherosclerotic plaque tissue in the early 1900 and have since been observed and implicated in many diseases and conditions, including myocardial infarction, abdominal aortic aneurism, kidney disease, ocular diseases, and even central nervous system anomalies. Despite the widespread involvement of CCs in many pathologies, the mechanisms involved in their formation and their role in various diseases are still not fully understood. Current knowledge concerning the formation of CCs, as well as the molecular pathways activated upon cellular exposure to CCs, will be explored in this review. As CC formation is tightly associated with lipid metabolism, the role of cellular lipid homeostasis in the formation of CCs is highlighted, including the role of lysosomes. In addition, cellular pathways and processes known to be affected by CCs are described. In particular, CC-induced activation of the inflammasome and production of reactive oxygen species, along with the role of CCs in complement-mediated inflammation is discussed. Moreover, the clinical manifestation of embolized CCs is described with a focus on renal and skin diseases associated with CC embolism. Lastly, potential therapeutic measures that target either the formation of CCs or their impact on different cell types and tissues are highlighted.

Chirality affects cholesterol-oxysterol association in water, a computational study

Cholesterol (Chol) is the most prevalent sterol in the animal kingdom and an indispensable component of mammalian cell membranes. Chol content in the membrane is strictly controlled, although the oxidation of phospholipids may change the relative content of membrane Chol. An excess of it results in the formation of pure Chol microdomains in the membrane. It is likely that some Chol molecules detach from the domains and self-assemble in the aqueous environment. This may promote Chol microcrystallisation, which initiates the development of gallstones and atherosclerotic plaque. In this study, the molecular dynamics, free energy perturbation, umbrella sampling and Voronoi diagram methods are used to reveal the details of self-association of Chol and its oxidised forms (oxChol), namely 7α,β-hydroxycholesterol and 7α,β-hydroperoxycholesterol, in water. In the first part of the study the interactions between a sterol monomer and water over a short and longer timescale as well as the energy of hydration of each sterol are analysed. This helps one to understand Chol-Chol and Chol-OxChol with different chirality self-association in water better, which is analysed in the second part of the study. The Voronoi diagram approach is used to determine the relative arrangement of molecules in the dimer and, most importantly, to analyse the dehydration of the contacting surfaces of the assembling molecules. Free energy calculations indicate that Chol and 7β-hydroxycholesterol associate into the most stable dimer and that Chol-Chol is the next most stable of the five dimers studied. Employing different computational methods enables us to obtain an adequate picture of Chol-sterol self-association in water, which includes dynamic, energetic and temporal aspects of the process.

Mechanistic Aspects of the Evolution of 3D Cholesterol Crystallites in a Supported Lipid Membrane via a Quartz Crystal Microbalance with Dissipation Monitoring

The irreversible formation of cholesterol monohydrate crystals within biological membranes is the leading cause of various diseases, including atherosclerosis. Understanding the process of cholesterol crystallization is fundamentally important and could also lead to the development of improved therapeutic strategies. This has driven several studies investigating the effect of the environmental parameters on the induction of cholesterol crystallite growth and the structure of the cholesterol crystallites, while the kinetics and mechanistic aspects of the crystallite formation process within lipid membranes remain poorly understood. Herein, we fabricated cholesterol crystallites within a supported lipid bilayer (SLB) by adsorbing a cholesterol-rich bicellar mixture onto a glass and silica surface and investigated the real-time kinetics of cholesterol crystallite nucleation and growth using epifluorescence microscopy and quartz crystal microbalance with dissipation (QCM-D) monitoring. Microscopic imaging showed the evolution of the morphology of cholesterol crystallites from nanorod- and plate-shaped habits during the initial stage to mostly large, micron-sized three-dimensional (3D) plate-shaped crystallites in the end, which was likened to Ostwald ripening. QCM-D kinetics revealed unique signal responses during the later stage of the growth process, characterized by simultaneous positive frequency shifts, nonmonotonous energy dissipation shifts, and significant overtone dependence. Based on the optically observed changes in crystallite morphology, we discussed the physical background of these unique QCM-D signal responses and the mechanistic aspects of Ostwald ripening in this system. Together, our findings revealed mechanistic details of the cholesterol crystallite growth kinetics, which may be useful in biointerfacial sensing and bioanalytical applications.

Ultra-Stable Freestanding Lipid Membrane Array: Direct Visualization of Dynamic Membrane Remodeling with Cholesterol Transport and Enzymatic Reactions

Cell membranes actively change their local compositions, serving essential biological processes such as cellular signaling and endocytosis. Although membrane dynamics is vital in the cellular functions, the complexity of natural membranes has made its fundamental understanding and systematic assessment difficult. Here, a powerful artificial membrane system is developed for real-time visualization of the spatiotemporal dynamics of membrane remodeling. Through well-defined air/oil/water interfaces on grid holes, tens of planar lipid bilayer membranes are easily created, and their reproducibility, controllability, and generality are highlighted. The freestanding membranes are large but also highly stable, facilitating direct long-term monitoring of dynamic membrane reconstitution caused by external stimuli. As an example to demonstrate the superiority of this membrane system, the effect of cholesterol trafficking, which significantly affects biophysical properties of cell membranes, is investigated at different membrane compositions. Cholesterol transport into and out of the membranes at different rates causes anomalous lipid arrangements through cholesterol-mediated phase transitions and decomposition, which have never been witnessed before. Furthermore, enzyme-induced membrane dynamics is successfully shown in this platform; sphingomyelinases locally generate asymmetry between two membrane leaflets. This technique is broadly applicable for exploring the membrane heterogeneity under various membrane-based reactions, providing valuable insight into the membrane dynamics.

Hypothetical Pathway for Formation of Cholesterol Microcrystals Initiating the Atherosclerotic Process

Major factors leading to the development of atherosclerosis are a high cholesterol (Chol) level in the blood and oxidative stress. Both promote the formation of Chol microcrystals in blood vessel walls. Deposition of Chol microcrystals in arterial intima causes inflammation, which initiates and accompanies the atherosclerotic process in all its phases. One of the possible sources of Chol in the blood vessel walls is oxidized low-density lipoproteins-this atherosclerotic plaque formation pathway has already been described in the literature. Here, we hypothesize that initiation of the atherosclerotic process may involve Chol domains in the plasma membranes of arterial cells. Increased Chol content and the presence of polyunsaturated phospholipids in these membranes together with oxidative stress (phospholipid peroxidation) may lead to the formation of pure Chol bilayer domains that, with further peroxidation and increased Chol content, may collapse in the form of Chol seed crystals. Independent of their origin, Chol microcrystals activate inflammasomes, thereby stimulate immune responses, and initiate inflammation that may lead to the development of atherosclerosis. This new, hypothetical pathway has not yet been investigated in depth; however, data from the literature and our own results support its feasibility.

Two-Dimensional Immiscible Domain of Cholesterol in the Lipid Bilayer Membrane Promotes Early Stage Calcification by Inducing Oriented Nucleation of Hydroxyapatite

Biomineralization is characterized by the fact that the crystallization of inorganic minerals is guided by an in vivo biological interface. However, the interfaces that direct calcification are widely debated up to date. In this paper, it was found that the two-dimensional (2D) immiscible domain of cholesterol in the lipid bilayer can induce the deposition of calcium phosphate by rapidly promoting the nucleation of the hydroxyapatite (001) plane. This promotion effect is related to the high lattice matching degree between the 2D cholesterol immiscible domain and the (001) plane of hydroxyapatite (HAP), which leads to the heteroepitaxy of HAP. The lipid bilayer derived from cells or vesicles is the largest biological interface in the body, thus revealing whether the lipid bilayer can induce the deposition of calcium phosphate will facilitate the understanding of the important role of cells or vesicles in calcification.

Why Is Very High Cholesterol Content Beneficial for the Eye Lens but Negative for Other Organs?

The plasma membranes of the human lens fiber cell are overloaded with cholesterol that not only saturates the phospholipid bilayer of these membranes but also leads to the formation of pure cholesterol bilayer domains. Cholesterol level increases with age, and for older persons, it exceeds the cholesterol solubility threshold, leading to the formation of cholesterol crystals. All these changes occur in the normal lens without too much compromise to lens transparency. If the cholesterol content in the cell membranes of other organs increases to extent where cholesterol crystals forma, a pathological condition begins. In arterial cells, minute cholesterol crystals activate inflammasomes, induce inflammation, and cause atherosclerosis development. In this review, we will indicate possible factors that distinguish between beneficial and negative cholesterol action, limiting cholesterol actions to those performed through cholesterol in cell membranes and by cholesterol crystals.

Mechanical and Crystallographic Analysis of Cholesterol Crystals Puncturing Biological Membranes

Ischemic heart disease often leads to myocardial infarction and remains the most common cause for death in humans. Although the exact impetus for the infarction remains elusive, a mechanism has been proposed that relates the disease to the observed high cholesterol levels in the body. The mechanism claims that cholesterol crystallizes inside the arterial plaque into needle-shaped crystals. The crystals puncture the fibrous cap of the plaque, whereby the necrotic contents of the plaque are spilled, subsequently clotting the blood vessels. This hypothesis has not been given sufficient attention partly due to the purported softness of the organic crystals and the common platy habit of the known crystal forms of cholesterol. In this work it is shown that, from hydrophobic solutions that attempt to emulate the plaque contents, a new solid form of cholesterol crystallizes as prisms with mucronate tips, and they are sufficiently strong to puncture a lamb pericardium, which mimics the plaque cap. The properties of the crystals were assessed by mechanical, structural, and crystallographic analyses. The results support the hypothesis that the cholesterol crystals can be considered, at least within the framework of the proposed mechanism, a possible cause of myocardial infarction.

Two polymorphic cholesterol monohydrate crystal structures form in macrophage culture models of atherosclerosis

The formation of atherosclerotic plaques in the blood vessel walls is the result of LDL particle uptake, and consequently of cholesterol accumulation in macrophage cells. Excess cholesterol accumulation eventually results in cholesterol crystal deposition, the hallmark of mature atheromas. We followed the formation of cholesterol crystals in J774A.1 macrophage cells with time, during accumulation of LDL particles, using a previously developed correlative cryosoft X-ray tomography (cryo-SXT) and stochastic optical reconstruction microscopy (STORM) technique. We show, in the initial accumulation stages, formation of small quadrilateral crystal plates associated with the cell plasma membrane, which may subsequently assemble into large aggregates. These plates match crystals of the commonly observed cholesterol monohydrate triclinic structure. Large rod-like cholesterol crystals form at a later stage in intracellular locations. Using cryotransmission electron microscopy (cryo-TEM) and cryoelectron diffraction (cryo-ED), we show that the structure of the large elongated rods corresponds to that of monoclinic cholesterol monohydrate, a recently determined polymorph of the triclinic crystal structure. These monoclinic crystals form with an unusual hollow cylinder or helical architecture, which is preserved in the mature rod-like crystals. The rod-like morphology is akin to that observed in crystals isolated from atheromas. We suggest that the crystals in the atherosclerotic plaques preserve in their morphology the memory of the structure in which they were formed. The identification of the polymorph structure, besides explaining the different crystal morphologies, may serve to elucidate mechanisms of cholesterol segregation and precipitation in atherosclerotic plaques.

Studying biomineralization pathways in a 3D culture model of breast cancer microcalcifications

Microcalcifications serve as diagnostic markers for breast cancer, yet their formation pathway(s) and role in cancer progression are debated due in part to a lack of relevant 3D culture models that allow studying the extent of cellular regulation over mineralization. Previous studies have suggested processes ranging from dystrophic mineralization associated with cell death to bone-like mineral deposition. Here, we evaluated microcalcification formation in 3D multicellular spheroids, generated from non-malignant, pre-cancer, and invasive cell lines from the MCF10A human breast tumor progression series. The spheroids with greater malignancy potential developed necrotic cores, thus recapitulating spatially distinct viable and non-viable areas known to regulate cellular behavior in tumors in vivo. The spatial distribution of the microcalcifications, as well as their compositions, were characterized using nanoCT, electron-microscopy, and X-ray spectroscopy. Apatite microcalcifications were primarily detected within the viable cell regions and their number and size increased with malignancy potential of the spheroids. Levels of alkaline phosphatase decreased with malignancy potential, whereas levels of osteopontin increased. These findings support a mineralization pathway in which cancer cells induce mineralization in a manner that is linked to their malignancy potential, but that is distinct from physiological osteogenic mineralization.

Macrophages Shed Excess Cholesterol in Unique Extracellular Structures Containing Cholesterol Microdomains

Supplemental Digital Content is available in the text.

Objective—

Cells use various mechanisms to maintain cellular cholesterol homeostasis including efflux of cholesterol from the cellular plasma membrane to cholesterol acceptors such as HDLs (high-density lipoproteins). Little is known about the transfer of cholesterol from cells into the extracellular matrix. Using a unique monoclonal antibody that detects ordered cholesterol arrays (ie, cholesterol micro[or nano]-domains), we previously identified that particles containing these cholesterol domains accumulate in the extracellular matrix during cholesterol enrichment of human monocyte-derived macrophages and are found in atherosclerotic lesions. In this study, we further investigate these deposited particles containing cholesterol microdomains and discover their unexpected morphology.

Approach and Results—

Although appearing spherical at the resolution of the conventional fluorescence microscope, super-resolution immunofluorescence and atomic force microscopy of in situ cholesterol microdomains, and immunoelectron microscopy of isolated cholesterol microdomains revealed that the microdomains are not vesicles or 3-dimensional crystals but rather appear as branching irregularly shaped deposits of varying size. These cholesterol microdomain-containing deposits are shed from the plasma membrane into the extracellular matrix.

Conclusions—

To date, research on cellular excretion of excess cholesterol has demonstrated cellular cholesterol efflux in the form of membranous vesicles and discoidal HDL particles released into the fluid-phase medium. Shedding of plasma membrane cholesterol microdomains provides an additional mechanism for cells such as macrophages to maintain plasma membrane cholesterol homeostasis. Furthermore, recognition that macrophages shed cholesterol microdomains into the extracellular matrix is important to our understanding of extracellular buildup of cholesterol in atherosclerosis.

Successful reprogramming of cellular protein production through mRNA delivered by functionalized lipid nanoparticles

mRNA treatments represent an exciting approach to cure diseases that cannot be tackled with current therapeutics. However, the delivery of mRNA to target cells remains a challenge, but among the existing alternatives, lipid nanoparticles (LNPs) offer a promising answer to this. Here we determine the structure of LNPs encapsulating mRNA, consisting of a lipid mixture already evaluated in clinical trials. We show that the lipids are not homogeneously distributed across the LNP, and one of the lipids is localized mainly at its surface. The structural information enabled us to design LNPs that successfully modify intracellular protein production in two clinically relevant cell types. Our findings and approach provide a framework for understanding and optimizing vehicles for mRNA delivery.

The development of safe and efficacious gene vectors has limited greatly the potential for therapeutic treatments based on messenger RNA (mRNA). Lipid nanoparticles (LNPs) formed by an ionizable cationic lipid (here DLin-MC3-DMA), helper lipids (distearoylphosphatidylcholine, DSPC, and cholesterol), and a poly(ethylene glycol) (PEG) lipid have been identified as very promising delivery vectors of short interfering RNA (siRNA) in different clinical phases; however, delivery of high-molecular weight RNA has been proven much more demanding. Herein we elucidate the structure of hEPO modified mRNA-containing LNPs of different sizes and show how structural differences affect transfection of human adipocytes and hepatocytes, two clinically relevant cell types. Employing small-angle scattering, we demonstrate that LNPs have a disordered inverse hexagonal internal structure with a characteristic distance around 6 nm in presence of mRNA, whereas LNPs containing no mRNA do not display this structure. Furthermore, using contrast variation small-angle neutron scattering, we show that one of the lipid components, DSPC, is localized mainly at the surface of mRNA-containing LNPs. By varying LNP size and surface composition we demonstrate that both size and structure have significant influence on intracellular protein production. As an example, in both human adipocytes and hepatocytes, protein expression levels for 130 nm LNPs can differ as much as 50-fold depending on their surface characteristics, likely due to a difference in the ability of LNP fusion with the early endosome membrane. We consider these discoveries to be fundamental and opening up new possibilities for rational design of synthetic nanoscopic vehicles for mRNA delivery.

Inflammation in human carotid atheroma plaques

Inflammation in carotid atherosclerotic plaque is linked to plaque rupture and cerebrovascular accidents. The balance between pro- and anti-inflammatory mediators governs development of the plaque, and may mediate enhancement of lesion broadening or, on the contrary, delay progression. In addition to macrophages and endothelial cells, smooth muscle cells (SMCs), which are the dominant cell subset in advanced plaques, are crucial players in carotid atherosclerosis development given their ability to differentiate into distinct phenotypes in reponse to specific signals received from the environment of the lesion. Carotid atheroma SMCs actively contribute to the inflammation in the lesion because of their acquired capacity to produce inflammatory mediators. We review the successive stages of carotid atheroma plaque formation via fatty streak early-stage toward more advanced rupture-prone lesions and document involvement of cytokines and chemokines and their cellular sources and targets in plaque progression and rupture.

Methionine oxidized apolipoprotein A-I at the crossroads of HDL biogenesis and amyloid formation

Apolipoprotein A-I (apoA-I) shares with other exchangeable apolipoproteins a high level of structural plasticity. In the lipid-free state, the apolipoprotein amphipathic α-helices interact intra- and intermolecularly, providing structural stabilization by self-association. We have reported that lipid-free apoA-I becomes amyloidogenic upon physiologically relevant (myeloperoxidase-mediated) Met oxidation. In this study, we established that Met oxidation promotes amyloidogenesis by reducing the stability of apoA-I monomers and irreversibly disrupting self-association. The oxidized apoA-I monomers also exhibited increased cellular cholesterol release capacity and stronger association with macrophages, compared to nonoxidized apoA-I. Of physiologic relevance, preformed oxidized apoA-I amyloid fibrils induced amyloid formation in nonoxidized apoA-I. This process was enhanced when self-association of nonoxidized apoA-I was disrupted by thermal treatment. Solid state NMR analysis revealed that aggregates formed by seeded nonoxidized apoA-I were structurally similar to those formed by the oxidized protein, featuring a β-structure-rich amyloid fold alongside α-helices retained from the native state. In atherosclerotic lesions, the conditions that promote apoA-I amyloid formation are readily available: myeloperoxidase, active oxygen species, low pH, and high concentration of lipid-free apoA-I. Our results suggest that even partial Met oxidation of apoA-I can nucleate amyloidogenesis, thus sequestering and inactivating otherwise antiatherogenic and HDL-forming apoA-I.-Witkowski, A., Chan, G. K. L., Boatz, J. C., Li, N. J., Inoue, A. P., Wong, J. C., van der Wel, P. C. A., Cavigiolio, G. Methionine oxidized apolipoprotein A-I at the crossroads of HDL biogenesis and amyloid formation.

Shape Transformations of Lipid Bilayers Following Rapid Cholesterol Uptake

High cholesterol levels in the blood increase the risk of atherosclerosis. A common explanation is that the cholesterol increase in the plasma membrane perturbs the shape and functions of cells by disrupting the cell signaling pathways and the formation of membrane rafts. In this work, we show that after enhanced transient uptake of cholesterol, mono-component lipid bilayers change their shape similarly to cell membranes in vivo. The bilayers either expel lipid protrusions or spread laterally as a result of the ensuing changes in their lipid density, the mechanical constraints imposed on them, and the properties of cyclodextrin used as a cholesterol donor. In light of the increasingly recognized link between membrane tension and cell behavior, we propose that the physical adaptation of the plasma membrane to cholesterol uptake may play a substantial role in the biological response.

Nanoparticles as agents targeting cholesterol crystallization in atherosclerosis

This article introduces a hypothesis on nanoparticle-mediated modulation of cholesterol crystal behaviour in the atherosclerotic plaques. The role of cholesterol crystals in progression of atherosclerosis is emphasized. Proposed mechanism of spontaneous cholesterol crystal formation in the organism is discussed. Mechanisms and factors associated with the nanoparticle-mediated modulation of cholesterol crystal behaviour are proposed. Authors hypothesize that specially designed nanoparticles may therapeutically modulate cholesterol crystal behaviour in atherosclerosis. Nano-sized agents used in stent coatings and imaging techniques can possibly prevent cholesterol crystallization in the diseased vessels. On the other hand, new nanotechnologies should be implemented with caution as certain types of nanoparticles could become crystal seeds for cholesterol deposited in the atherosclerotically damaged vascular walls causing destabilization of the plaques. Studying nanoparticle-induced alterations of cholesterol crystal formation requires multidisciplinary approach involving biomedical researchers, computer scientists, and physical chemists specializing in crystal growth. The proposed hypothesis on nanoparticle-mediated modulation of cholesterol crystal behaviour may be relevant to other medical conditions including gallbladder stones, arthritis, and ophthalmological diseases such as synchysis scintillans.

Cholesterol crystallization within hepatocyte lipid droplets and its role in murine NASH

We recently reported that cholesterol crystals form in hepatocyte lipid droplets (LDs) in human and experimental nonalcoholic steatohepatitis. Herein, we assigned WT C57BL/6J mice to a high-fat (15%) diet for 6 months, supplemented with 0%, 0.25%, 0.5%, 0.75%, or 1% dietary cholesterol. Increasing dietary cholesterol led to cholesterol loading of the liver, but not of adipose tissue, resulting in fibrosing steatohepatitis at a dietary cholesterol concentration of ≥0.5%, whereas mice on lower-cholesterol diets developed only simple steatosis. Hepatic cholesterol crystals and crown-like structures also developed at a dietary cholesterol concentration ≥0.5%. Crown-like structures consisted of activated Kupffer cells (KCs) staining positive for NLRP3 and activated caspase 1, which surrounded and processed cholesterol crystal-containing remnant LDs of dead hepatocytes. The KCs processed LDs at the center of crown-like structures in the extracellular space by lysosomal enzymes, ultimately transforming into lipid-laden foam cells. When HepG2 cells were exposed to LDL cholesterol, they developed cholesterol crystals in LD membranes, which caused activation of THP1 cells (macrophages) grown in coculture; upregulation of TNF-alpha, NLRP3, and interleukin 1beta (IL1β) mRNA; and secretion of IL-1beta. In conclusion, cholesterol crystals form on the LD membrane of hepatocytes and cause activation and cholesterol loading of KCs that surround and process these LDs by lysosomal enzymes.

Depletion with Cyclodextrin Reveals Two Populations of Cholesterol in Model Lipid Membranes

Recent results provide evidence that cholesterol is highly accessible for removal from both cell and model membranes above a threshold concentration that varies with membrane composition. Here we measured the rate at which methyl-β-cyclodextrin depletes cholesterol from a supported lipid bilayer as a function of cholesterol mole fraction. We formed supported bilayers from two-component mixtures of cholesterol and a PC (phosphatidylcholine) lipid, and we directly visualized the rate of decrease in area of the bilayers with fluorescence microscopy. Our technique yields the accessibility of cholesterol over a wide range of concentrations (30-66 mol %) for many individual bilayers, enabling fast acquisition of replicate data. We found that the bilayers contain two populations of cholesterol, one with low surface accessibility and the other with high accessibility. A larger fraction of the total membrane cholesterol appears in the more accessible population when the acyl chains of the PC-lipid tails are more unsaturated. Our findings are most consistent with the predictions of the condensed-complex and cholesterol bilayer domain models of cholesterol-phospholipid interactions in lipid membranes.

Development of Correlative Cryo-soft X-ray Tomography and Stochastic Reconstruction Microscopy. A Study of Cholesterol Crystal Early Formation in Cells

We have developed a high resolution correlative method involving cryo-soft X-ray tomography (cryo-SXT) and stochastic optical reconstruction microscopy (STORM), which provides information in three dimensions on large cellular volumes at 70 nm resolution. Cryo-SXT morphologically identified and localized aggregations of carbon-rich materials. STORM identified specific markers on the desired epitopes, enabling colocalization between the identified objects, in this case cholesterol crystals, and the cellular environment. The samples were studied under ambient and cryogenic conditions without dehydration or heavy metal staining. The early events of cholesterol crystal development were investigated in relation to atherosclerosis, using as model macrophage cell cultures enriched with LDL particles. Atherosclerotic plaques build up in arteries in a slow process involving cholesterol crystal accumulation. Cholesterol crystal deposition is a crucial stage in the pathological cascade. Our results show that cholesterol crystals can be identified and imaged at a very early stage on the cell plasma membrane and in intracellular locations. This technique can in principle be applied to other biological samples where specific molecular identification is required in conjunction with high resolution 3D-imaging.

The Role of Cholesterol in the Pathogenesis of NASH

Lipotoxicity drives the development of progressive hepatic inflammation and fibrosis in a subgroup of patients with nonalcoholic fatty liver disease (NAFLD), causing nonalcoholic steatohepatitis (NASH) and even progression to cirrhosis and hepatocellular carcinoma (HCC). While the underlying molecular mechanisms responsible for the development of inflammation and fibrosis that characterize progressive NASH remain unclear, emerging evidence now suggests that hepatic free cholesterol (FC) is a major lipotoxic molecule critical in the development of experimental and human NASH. In this review, we examine the effects of excess FC in hepatocytes, Kupffer cells (KCs), and hepatic stellate cells (HSCs), and the subcellular mechanisms by which excess FC can induce cellular toxicity or proinflammatory and profibrotic effects in these cells.

Cholesterol crystal induced arterial inflammation and destabilization of atherosclerotic plaque

Evolution of plaque that is prone to rupture is characterized by inflammation and physical changes. Accumulation of low-density lipoprotein in the sub-intima provides esterified cholesterol (ESC) to macrophages and smooth muscle cells that convert it into free cholesterol (FRC) by cholesteryl ester hydrolases (CEHs). Membrane-bound cholesterol carriers transport FRC to high-density lipoprotein (HDL). Impaired HDL transport function and altered composition can lead to extracellular accumulation of FRC, whereas impaired membrane carrier activity can lead to intracellular FRC accumulation. Saturation of FRC can result in cholesterol crystallization with cell death and intimal injury. Disequilibrium between ESC and FRC can impact foam cell and cholesterol crystal (CC) formation. Cholesterol crystals initiate inflammation via NLRP3 inflammasome leading to interleukin-1β (IL-1β) production inducing C-reactive protein. Eventually, crystals growing from within the plaque and associated inflammation destabilize the plaque. Thus, inhibition of inflammation by antagonists to IL-1β or agents that dissolve or prevent CC formation may stabilize vulnerable plaques.