The interplay between size, morphology, stability, and functionality of high-density lipoprotein subclasses

Flipped C-Terminal Ends of APOA1 Promote ABCA1-Dependent Cholesterol Efflux by Small HDLs

BACKGROUND

Cholesterol efflux capacity (CEC) predicts cardiovascular disease independently of high-density lipoprotein (HDL) cholesterol levels. Isolated small HDL particles are potent promoters of macrophage CEC by the ABCA1 (ATP-binding cassette transporter A1) pathway, but the underlying mechanisms are unclear.

METHODS

We used model system studies of reconstituted HDL and plasma from control and lecithin-cholesterol acyltransferase (LCAT)-deficient subjects to investigate the relationships among the sizes of HDL particles, the structure of APOA1 (apolipoprotein A1) in the different particles, and the CECs of plasma and isolated HDLs.

RESULTS

We quantified macrophage and ABCA1 CEC of 4 distinct sizes of reconstituted HDL. CEC increased as particle size decreased. Tandem mass spectrometric analysis of chemically cross-linked peptides and molecular dynamics simulations of APOA1, the major protein of HDL, indicated that the mobility of C-terminus of that protein was markedly higher and flipped off the surface in the smallest particles. To explore the physiological relevance of the model system studies, we isolated HDL from LCAT-deficient subjects, whose small HDLs (like reconstituted HDLs) are discoidal and composed of APOA1, cholesterol, and phospholipid. Despite their very low plasma levels of HDL particles, these subjects had normal CEC. In both the LCAT-deficient subjects and control subjects, the CEC of isolated extra-small HDL (a mixture of extra-small and small HDL by calibrated ion mobility analysis) was 3- to 5-fold greater than that of the larger sizes of isolated HDL. Incubating LCAT-deficient plasma and control plasma with human LCAT converted extra-small and small HDL particles into larger particles, and it markedly inhibited CEC.

CONCLUSIONS

We present a mechanism for the enhanced CEC of small HDLs. In smaller particles, the C-termini of the 2 antiparallel molecules of APOA1 are "flipped" off the lipid surface of HDL. This extended conformation allows them to engage with ABCA1. In contrast, the C-termini of larger HDLs are unable to interact productively with ABCA1 because they form a helical bundle that strongly adheres to the lipid on the particle. Enhanced CEC, as seen with the smaller particles, predicts decreased cardiovascular disease risk. Thus, extra-small and small HDLs may be key mediators and indicators of the cardioprotective effects of HDL.

Flipped C-Terminal Ends of APOA1 Promote ABCA1-dependent Cholesterol Efflux by Small HDLs

Background

Cholesterol efflux capacity (CEC) predicts cardiovascular disease (CVD) independently of HDL cholesterol (HDL-C) levels. Isolated small HDL particles are potent promoters of macrophage CEC by the ABCA1 pathway, but the underlying mechanisms are unclear.

Methods

We used model system studies of reconstituted HDL and plasma from control and lecithin-cholesterol acyltransferase (LCAT)-deficient subjects to investigate the relationships among the sizes of HDL particles, the structure of APOA1 in the different particles, and the CECs of plasma and isolated HDLs.

Results

We quantified macrophage and ABCA1 CEC of four distinct sizes of reconstituted HDL (r-HDL). CEC increased as particle size decreased. MS/MS analysis of chemically crosslinked peptides and molecular dynamics simulations of APOA1 (HDL's major protein) indicated that the mobility of that protein's C-terminus was markedly higher and flipped off the surface in the smallest particles. To explore the physiological relevance of the model system studies, we isolated HDL from LCAT-deficient subjects, whose small HDLs-like r-HDLs-are discoidal and composed of APOA1, cholesterol, and phospholipid. Despite their very low plasma levels of HDL particles, these subjects had normal CEC. In both the LCAT-deficient subjects and control subjects, the CEC of isolated extra-small HDL (a mixture of extra-small and small HDL by calibrated ion mobility analysis) was 3-5-fold greater than that of the larger sizes of isolated HDL. Incubating LCAT-deficient plasma and control plasma with human LCAT converted extra-small and small HDL particles into larger particles, and it markedly inhibited CEC.

Conclusions

We present a mechanism for the enhanced CEC of small HDLs. In smaller particles, the C-termini of the two antiparallel molecules of APOA1 are flipped off the lipid surface of HDL. This extended conformation allows them to engage with ABCA1. In contrast, the C-termini of larger HDLs are unable to interact productively with ABCA1 because they form a helical bundle that strongly adheres to the lipid on the particle. Enhanced CEC, as seen with the smaller particles, predicts decreased CVD risk. Thus, extra-small and small HDLs may be key mediators and indicators of HDL's cardioprotective effects.

Boosting Cholesterol Efflux from Foam Cells by Sequential Administration of rHDL to Deliver MicroRNA and to Remove Cholesterol in a Triple-Cell 2D Atherosclerosis Model

Cardiovascular disease, the leading cause of mortality worldwide, is primarily caused by atherosclerosis, which is characterized by lipid and inflammatory cell accumulation in blood vessels and carotid intima thickening. Although disease management has improved significantly, new therapeutic strategies focused on accelerating atherosclerosis regression must be developed. Atherosclerosis models mimicking in vivo-like conditions provide essential information for research and new advances toward clinical application. New nanotechnology-based therapeutic opportunities have emerged with apoA-I nanoparticles (recombinant/reconstituted high-density lipoproteins, rHDL) as ideal carriers to deliver molecules and the discovery that microRNAs participate in atherosclerosis establishment and progression. Here, a therapeutic strategy to improve cholesterol efflux is developed based on a two-step administration of rHDL consisting of a first dose of antagomiR-33a-loaded rHDLs to induce adenosine triphosphate-binding cassette transporters A1 overexpression, followed by a second dose of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine rHDLs, which efficiently remove cholesterol from foam cells. A triple-cell 2D-atheroma plaque model reflecting the cellular complexity of atherosclerosis is used to improve efficiency of the nanoparticles in promoting cholesterol efflux. The results show that sequential administration of rHDL potentiates cholesterol efflux indicating that this approach may be used in vivo to more efficiently target atherosclerotic lesions and improve prognosis of the disease.

HDL maturation and remodelling

Cholesterol in the circulation is mostly transported in an esterified form as a component of lipoproteins. The majority of these cholesteryl esters are produced in nascent, discoidal high density lipoproteins (HDLs) by the enzyme, lecithin:cholesterol acyltransferase (LCAT). Discoidal HDLs are discrete populations of particles that consist of a phospholipid bilayer, the hydrophobic acyl chains of which are shielded from the aqueous environment by apolipoproteins that also confer water solubility on the particles. The progressive LCAT-mediated accumulation of cholesteryl esters in discoidal HDLs generates the spherical HDLs that predominate in normal human plasma. Spherical HDLs contain a core of water insoluble, neutral lipids (cholesteryl esters and triglycerides) that is surrounded by a surface monolayer of phospholipids with which apolipoproteins associate. Although spherical HDLs all have the same basic structure, they are extremely diverse in size, composition, and function. This review is concerned with how the biogenesis of discoidal and spherical HDLs is regulated and the mechanistic basis of their size and compositional heterogeneity. Current understanding of the impact of this heterogeneity on the therapeutic potential of HDLs of varying size and composition is also addressed in the context of several disease states.

Effects of Elaidic Acid on HDL Cholesterol Uptake Capacity

Recently we established a cell-free assay to evaluate “cholesterol uptake capacity (CUC)” as a novel concept for high-density lipoprotein (HDL) functionality and demonstrated the feasibility of CUC for coronary risk stratification, although its regulatory mechanism remains unclear. HDL fluidity affects cholesterol efflux, and trans fatty acids (TFA) reduce lipid membrane fluidity when incorporated into phospholipids (PL). This study aimed to clarify the effect of TFA in HDL-PL on CUC. Serum was collected from 264 patients after coronary angiography or percutaneous coronary intervention to measure CUC and elaidic acid levels in HDL-PL, and in vitro analysis using reconstituted HDL (rHDL) was used to determine the HDL-PL mechanism affecting CUC. CUC was positively associated with HDL-PL levels but negatively associated with the proportion of elaidic acid in HDL-PL (elaidic acid in HDL-PL/HDL-PL ratio). Increased elaidic acid-phosphatidylcholine (PC) content in rHDL exhibited no change in particle size or CUC compared to rHDL containing oleic acid in PC. Recombinant human lecithin-cholesterol acyltransferase (LCAT) enhanced CUC, and LCAT-dependent enhancement of CUC and LCAT-dependent cholesterol esterification were suppressed in rHDL containing elaidic acid in PC. Therefore, CUC is affected by HDL-PL concentration, HDL-PL acyl group composition, and LCAT-dependent cholesterol esterification. Elaidic acid precipitated an inhibition of cholesterol uptake and maturation of HDL; therefore, modulation of HDL-PL acyl groups could improve CUC.

(r)HDL in theranostics: how do we apply HDL's biology for precision medicine in atherosclerosis management?

High-density lipoproteins (HDL) are key players in cholesterol metabolism homeostasis since they are responsible for transporting excess cholesterol from peripheral tissues to the liver. Imbalance in this process, due to either excessive accumulation or impaired clearance, results in net cholesterol accumulation and increases the risk of cardiovascular disease (CVD). Therefore, significant effort has been focused on the development of therapeutic tools capable of either directly or indirectly enhancing HDL-guided reverse cholesterol transport (RCT). More recently, in light of the emergence of precision nanomedicine, there has been renewed research interest in attempting to take advantage of the development of advanced recombinant HDL (rHDL) for both therapeutic and diagnostic purposes. In this review, we provide an update on the different approaches that have been developed using rHDL, focusing on the rHDL production methodology and rHDL applications in theranostics. We also compile a series of examples highlighting potential future perspectives in the field.

High-Density Lipoproteins as Homeostatic Nanoparticles of Blood Plasma

It is well known that blood lipoproteins (LPs) are multimolecular complexes of lipids and proteins that play a crucial role in lipid transport. High-density lipoproteins (HDL) are a class of blood plasma LPs that mediate reverse cholesterol transport (RCT)-cholesterol transport from the peripheral tissues to the liver. Due to this ability to promote cholesterol uptake from cell membranes, HDL possess antiatherogenic properties. This function was first observed at the end of the 1970s to the beginning of the 1980s, resulting in high interest in this class of LPs. It was shown that HDL are the prevalent class of LPs in several types of living organisms (from fishes to monkeys) with high resistance to atherosclerosis and cardiovascular disorders. Lately, understanding of the mechanisms of the antiatherogenic properties of HDL has significantly expanded. Besides the contribution to RCT, HDL have been shown to modulate inflammatory processes, blood clotting, and vasomotor responses. These particles also possess antioxidant properties and contribute to immune reactions and intercellular signaling. Herein, we review data on the structure and mechanisms of the pleiotropic biological functions of HDL from the point of view of their evolutionary role and complex dynamic nature.

Sensitive Morphological Characterization of Oriented High-Density Lipoprotein Nanoparticles Using 31 P NMR Spectroscopy

The biological function of high-density lipoprotein (HDL) nanoparticles, the so-called good cholesterol that is associated with a low risk of heart disease, depends on their composition, morphology, and size. The morphology of HDL particles composed of apolipoproteins, lipids and cholesterol is routinely visualised by transmission electron microscopy (TEM), but higher-resolution tools are needed to observe more subtle structural differences between particles of different composition. Here, reconstituted HDL formulations are oriented on glass substrates and solid-state 31 P NMR spectroscopy is shown to be highly sensitive to the surface curvature of the lipid headgroups. The spectra report potentially functionally important differences in the morphology of different HDL preparations that are not detected by TEM. This method provides new morphological insights into HDL comprising a naturally occurring apolipoprotein A-I mutant, which may be linked to its atheroprotective properties, and holds promise as a future research tool in the clinical analysis of plasma HDL.

Cholesterol Efflux Efficiency of Reconstituted HDL Is Affected by Nanoparticle Lipid Composition

Cardiovascular disease (CVD), the leading cause of mortality worldwide is primarily caused by atherosclerosis, which is promoted by the accumulation of low-density lipoproteins into the intima of large arteries. Multiple nanoparticles mimicking natural HDL (rHDL) have been designed to remove cholesterol excess in CVD therapy. The goal of this investigation was to assess the cholesterol efflux efficiency of rHDLs with different lipid compositions, mimicking different maturation stages of high-density lipoproteins (HDLs) occurring in vivo. Methods: the cholesterol efflux activity of soybean PC (Soy-PC), 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), DPPC:Chol:1-palmitoyl-2-hydroxy-sn-glycero-3-phosphocholine (LysoPC) and DPPC:18:2 cholesteryl ester (CE):LysoPC rHDLs was determined in several cell models to investigate the contribution of lipid composition to the effectiveness of cholesterol removal. Results: DPPC rHDLs are the most efficient particles, inducing cholesterol efflux in all cellular models and in all conditions the effect was potentiated when the ABCA1 transporter was upregulated. Conclusions: DPPC rHDLs, which resemble nascent HDL, are the most effective particles in inducing cholesterol efflux due to the higher physical binding affinity of cholesterol to the saturated long-chain-length phospholipids and the favored cholesterol transfer from a highly positively curved bilayer, to an accepting planar bilayer such as DPPC rHDLs. The physicochemical characteristics of rHDLs should be taken into consideration to design more efficient nanoparticles to promote cholesterol efflux.

Head-to-Head Comparison of the Penetration Efficiency of Lipid-Based Nanoparticles into Tumor Spheroids

Most tumor-targeted drug delivery systems must overcome a large variety of physiological barriers before reaching the tumor site and diffuse through the tight network of tumor cells. Many studies focus on optimizing the first part, the accumulation of drug carriers at the tumor site, ignoring the penetration efficiency, i.e., a measure of the ability of a drug delivery system to overcome tumor surface adherence and uptake. We used three-dimensional (3D) tumor spheroids in combination with light-sheet fluorescence microscopy in a head-to-head comparison of a variety of commonly used lipid-based nanoparticles, including liposomes, PEGylated liposomes, lipoplexes, and reconstituted high-density lipoproteins (rHDL). Whilst PEGylation of liposomes only had minor effects on the penetration efficiency, we show that lipoplexes are mainly associated with the periphery of tumor spheroids, possibly due to their positive surface charge, leading to fusion with the cells at the spheroid surface or aggregation. Surprisingly, the rHDL showed significantly higher penetration efficiency and high accumulation inside the spheroid. While these findings indeed could be relevant when designing novel drug delivery systems based on lipid-based nanoparticles, we stress that the used platform and the detailed image analysis are a versatile tool for in vitro studies of the penetration efficiency of nanoparticles in tumors.

Diabetes Impairs Cellular Cholesterol Efflux From ABCA1 to Small HDL Particles

RATIONALE

HDL (high-density lipoprotein) may be cardioprotective because it accepts cholesterol from macrophages via the cholesterol transport proteins ABCA1 (ATP-binding cassette transporter A1) and ABCG1 (ATP-binding cassette transporter G1). The ABCA1-specific cellular cholesterol efflux capacity (ABCA1 CEC) of HDL strongly and negatively associates with cardiovascular disease risk, but how diabetes mellitus impacts that step is unclear.

OBJECTIVE

To test the hypothesis that HDL's cholesterol efflux capacity is impaired in subjects with type 2 diabetes mellitus.

METHODS AND RESULTS

We performed a case-control study with 19 subjects with type 2 diabetes mellitus and 20 control subjects. Three sizes of HDL particles, small HDL, medium HDL, and large HDL, were isolated by high-resolution size exclusion chromatography from study subjects. Then we assessed the ABCA1 CEC of equimolar concentrations of particles. Small HDL accounted for almost all of ABCA1 CEC activity of HDL. ABCA1 CEC-but not ABCG1 CEC-of small HDL was lower in the subjects with type 2 diabetes mellitus than the control subjects. Isotope dilution tandem mass spectrometry demonstrated that the concentration of SERPINA1 (serpin family A member 1) in small HDL was also lower in subjects with diabetes mellitus. Enriching small HDL with SERPINA1 enhanced ABCA1 CEC. Structural analysis of SERPINA1 identified 3 amphipathic α-helices clustered in the N-terminal domain of the protein; biochemical analyses demonstrated that SERPINA1 binds phospholipid vesicles.

CONCLUSIONS

The ABCA1 CEC of small HDL is selectively impaired in type 2 diabetes mellitus, likely because of lower levels of SERPINA1. SERPINA1 contains a cluster of amphipathic α-helices that enable apolipoproteins to bind phospholipid and promote ABCA1 activity. Thus, impaired ABCA1 activity of small HDL particles deficient in SERPINA1 could increase cardiovascular disease risk in subjects with diabetes mellitus.

Single-Molecule 3D Images of "Hole-Hole" IgG1 Homodimers by Individual-Particle Electron Tomography

The engineering of immunoglobulin-G molecules (IgGs) is of wide interest for improving therapeutics, for example by modulating the activity or multiplexing the specificity of IgGs to recognize more than one antigen. Optimization of engineered IgG requires knowledge of three-dimensional (3D) structure of synthetic IgG. However, due to flexible nature of the molecules, their structural characterization is challenging. Here, we use our reported individual-particle electron tomography (IPET) method with optimized negative-staining (OpNS) for direct 3D reconstruction of individual IgG hole-hole homodimer molecules. The hole-hole homodimer is an undesired variant generated during the production of a bispecific antibody using the knob-into-hole heterodimer technology. A total of 64 IPET 3D density maps at ~15 Å resolutions were reconstructed from 64 individual molecules, revealing 64 unique conformations. In addition to the known Y-shaped conformation, we also observed an unusual X-shaped conformation. The 3D structure of the X-shaped conformation contributes to our understanding of the structural details of the interaction between two heavy chains in the Fc domain. The IPET approach, as an orthogonal technique to characterize the 3D structure of therapeutic antibodies, provides insight into the 3D structural variety and dynamics of heterogeneous IgG molecules.

Apolipoprotein A1 Forms 5/5 and 5/4 Antiparallel Dimers in Human High-density Lipoprotein

Apolipoprotein A1 (APOA1), the major protein of high-density lipoprotein (HDL), contains 10 helical repeats that play key roles in protein-protein and protein-lipid interactions. The current structural model for HDL proposes that APOA1 forms an antiparallel dimer in which helix 5 in monomer 1 associates with helix 5 in monomer 2 along a left-left (LL5/5) interface, forming a protein complex with a 2-fold axis of symmetry centered on helix 5. However, computational studies suggest that other orientations are possible. To test this idea, we used a zero-length chemical cross-linking reagent that forms covalent bonds between closely apposed basic and acidic residues. Using proteolytic digestion and tandem mass spectrometry, we identified amino acids in the central region of the antiparallel APOA1 dimer of HDL that were in close contact. As predicted by the current model, we found six intermolecular cross-links that were consistent with the antiparallel LL5/5 registry. However, we also identified three intermolecular cross-links that were consistent with the antiparallel LL5/4 registry. The LL5/5 is the major structural conformation of the two complexes in both reconstituted discoidal HDL particles and in spherical HDL from human plasma. Molecular dynamic simulations suggest that that LL5/5 and LL5/4 APOA1 dimers possess similar free energies of dimerization, with LL5/5 having the lowest free energy. Our observations indicate that phospholipidated APOA1 in HDL forms different antiparallel dimers that could play distinct roles in enzyme regulation, assembly of specific protein complexes, and the functional properties of HDL in humans.

Oxidation of methionine residues in human apolipoprotein A-I generates a potent pro-inflammatory molecule

Amyloid deposits of apolipoprotein A-I (apoA-I) and inflammation are common in atherosclerotic arteries. In this study, we investigated the interplay between oxidation of apoA-I methionine residues (Met(O)-ApoA-I), a known amyloidogenic modification of apoA-I, and the inflammatory response of immune cells. Soluble pre-fibrillar Met(O)-ApoA-I, but not apoA-I, induced intracellular accumulation of pro-interleukin (IL)-1β and secretion of the pro-inflammatory cytokines tumor necrosis factor α (TNFα) and IL-6 in mouse bone marrow-derived macrophages (BMDMs) and human primary monocytes. Additionally, secretion of mature IL-1β was also activated in human monocytes. The pro-inflammatory activity of Met(O)-ApoA-I was Toll-like receptor 4 (TLR4)-dependent and CD36-independent and was solely determined by oxidation of apoA-I methionine residues, in particular Met-86 and Met-148. In contrast, amyloid fibrils or reconstituted high-density lipoproteins (HDLs) generated from Met(O)-ApoA-I did not induce cytokine production in BMDMs. Although lipid-free Met(O)-ApoA-I remained functional in extracting lipids from cells and generating HDL, it gained strong pro-inflammatory properties that may aggravate local inflammation in the arteries and atherosclerosis. Our study indicates that oxidation of apoA-I methionine residues produces a potent danger-associated molecular pattern capable of stimulating pro-inflammatory cytokine secretion at levels similar to those induced by known pathogen-associated molecular patterns, such as lipopolysaccharide.

Tertiary structure of apolipoprotein A-I in nascent high-density lipoproteins

Understanding the function of high-density lipoprotein (HDL) requires detailed knowledge of the structure of its primary protein, apolipoprotein A-I (APOA1). However, APOA1 flexibility and HDL heterogeneity have confounded decades of efforts to determine high-resolution structures and consistent models. Here, molecular dynamics simulations totaling 30 μs on two nascent HDLs, each with 2 APOA1 and either 160 phospholipids and 24 cholesterols or 200 phospholipids and 20 cholesterols, show that residues 1-21 of the N-terminal domains of APOA1 interact via strong salt bridges. Residues 26-43 of one APOA1 in the smaller particle form a hinge on the disc edge, which displaces the C-terminal domain of the other APOA1 to the phospholipid surface. The proposed structures are supported by chemical cross-linking, Rosetta modeling of the N-terminal domain, and analysis of the lipid-free ∆185APOA1 crystal structure. These structures provide a framework for understanding HDL maturation and revise all previous models of nascent HDL.

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.

Assessing the mechanisms of cholesteryl ester transfer protein inhibitors

Cholesteryl ester transfer protein (CETP) inhibitors are a new class of therapeutics for dyslipidemia that simultaneously improve two major cardiovascular disease (CVD) risk factors: elevated low-density lipoprotein (LDL) cholesterol and decreased high-density lipoprotein (HDL) cholesterol. However, the detailed molecular mechanisms underlying their efficacy are poorly understood, as are any potential mechanistic differences among the drugs in this class. Herein, we used electron microscopy (EM) to investigate the effects of three of these agents (Torcetrapib, Dalcetrapib and Anacetrapib) on CETP structure, CETP-lipoprotein complex formation and CETP-mediated cholesteryl ester (CE) transfer. We found that although none of these inhibitors altered the structure of CETP or the conformation of CETP-lipoprotein binary complexes, all inhibitors, especially Torcetrapib and Anacetrapib, increased the binding ratios of the binary complexes (e.g., HDL-CETP and LDLCETP) and decreased the binding ratios of the HDL-CETP-LDL ternary complexes. The findings of more binary complexes and fewer ternary complexes reflect a new mechanism of inhibition: one distal end of CETP bound to the first lipoprotein would trigger a conformational change at the other distal end, thus resulting in a decreased binding ratio to the second lipoprotein and a degraded CE transfer rate among lipoproteins. Thus, we suggest a new inhibitor design that should decrease the formation of both binary and ternary complexes. Decreased concentrations of the binary complex may prevent the inhibitor was induced into cell by the tight binding of binary complexes during lipoprotein metabolism in the treatment of CVD.

Eicosapentaenoic Acid-Enriched High-Density Lipoproteins Exhibit Anti-Atherogenic Properties

BACKGROUND

It has previously been reported that oral administration of purified eicosapentaenoic acid (EPA) generates EPA-rich high-density lipoprotein (HDL) particles with a variety of anti-inflammatory properties. In this study, the mechanism underlying the anti-atherogenic effects of EPA-rich HDL using reconstituted HDL (rHDL) was investigated.Methods and Results:rHDL was generated by the sodium cholate dialysis method, using apolipoprotein A-1 protein, cholesterol, and various concentrations of EPA-phosphatidylcholine (PC) or egg-PC. Increased EPA-PC contents in rHDL resulted in decreased particle size. Next, the effects of rHDL containing various amounts (0-100% of total PC) of EPA-PC on vascular cell adhesion molecule-1 (VCAM-1) expression in human umbilical vein endothelial cells (HUVECs) was examined. Cytokine-stimulated VCAM-1 expression was inhibited in a dose-dependent manner based on the amount of EPA-PC in rHDL. Surprisingly, the incubation of HUVECs with EPA-rich rHDL resulted in the production of resolvin E3 (RvE3), an anti-inflammatory metabolite derived from EPA. Incubation with EPA-PC alone did not adequately induce RvE3 production, suggesting that RvE3 production requires an endothelial cell-HDL interaction. The increased anti-inflammatory effects of EPA-rich HDL may be explained by EPA itself and RvE3 production. Furthermore, the increase in EPA-PC content enhanced cholesterol efflux.

CONCLUSIONS

The EPA-enriched HDL particles exhibit cardioprotective properties via the production of anti-inflammatory lipid metabolites and the increase in cholesterol efflux.

Serum amyloid A self-assembles with phospholipids to form stable protein-rich nanoparticles with a distinct structure: A hypothetical function of SAA as a "molecular mop" in immune response

Serum amyloid A (SAA) is an acute-phase protein whose action in innate immunity and lipid homeostasis is unclear. Most circulating SAA binds plasma high-density lipoproteins (HDL) and reroutes lipid transport. In vivo SAA binds existing lipoproteins or generates them de novo upon lipid uptake from cells. We explored the products of SAA-lipid interactions and lipoprotein remodeling in vitro. SAA complexes with palmitoyl-oleoyl phosphocholine (POPC) were analyzed for structure and stability using circular dichroism and fluorescence spectroscopy, electron microscopy, gel electrophoresis and gel filtration. The results revealed the formation of 8-11nm lipoproteins that were∼50% α-helical and stable at near-physiological conditions but were irreversibly remodeled at T_m∼52°C. Similar HDL-size nanoparticles formed spontaneously at ambient conditions or upon thermal remodeling of parent lipoproteins containing various amounts of proteins and lipids, including POPC and cholesterol. Therefore, such HDL-size particles formed stable kinetically accessible structures in a wide range of conditions. Based on their size and stoichiometry, each particle contained about 12 SAA and 72 POPC molecules, with a protein:lipid weight ratio circa 2.5:1, suggesting a structure distinct from HDL. High stability of these nanoparticles and their HDL-like size suggest that similar lipoproteins may form in vivo during inflammation or injury when SAA concentration is high and membranes from dead cells require rapid removal. We speculate that solubilization of membranes by SAA to generate lipoproteins in a spontaneous energy-independent process constitutes the primordial function of this ancient protein, providing the first line of defense in clearing cell debris from the injured sites.

Nanodiscs in Membrane Biochemistry and Biophysics

Membrane proteins play a most important part in metabolism, signaling, cell motility, transport, development, and many other biochemical and biophysical processes which constitute fundamentals of life on the molecular level. Detailed understanding of these processes is necessary for the progress of life sciences and biomedical applications. Nanodiscs provide a new and powerful tool for a broad spectrum of biochemical and biophysical studies of membrane proteins and are commonly acknowledged as an optimal membrane mimetic system that provides control over size, composition, and specific functional modifications on the nanometer scale. In this review we attempted to combine a comprehensive list of various applications of nanodisc technology with systematic analysis of the most attractive features of this system and advantages provided by nanodiscs for structural and mechanistic studies of membrane proteins.

Cell lipid metabolism modulators 2-bromopalmitate, D609, monensin, U18666A and probucol shift discoidal HDL formation to the smaller-sized particles: implications for the mechanism of HDL assembly

ATP-binding cassette transporter A1 (ABCA1) mediates formation of disc-shaped high-density lipoprotein (HDL) from cell lipid and lipid-free apolipoprotein A-I (apo A-I). Discoidal HDL particles are heterogeneous in physicochemical characteristics for reasons that are understood incompletely. Discoidal lipoprotein particles similar in characteristics and heterogeneity to cell-formed discoidal HDL can be reconstituted from purified lipids and apo A-I by cell-free, physicochemical methods. The heterogeneity of reconstituted HDL (rHDL) is sensitive to the lipid composition of the starting lipid/apo A-I mixture. To determine whether the heterogeneity of cell-formed HDL is similarly sensitive to changes in cell lipids, we investigated four compounds that have well-established effects on cell lipid metabolism and ABCA1-mediated cell cholesterol efflux. 2-Bromopalmitate, D609, monensin and U18666A decreased formation of the larger-sized, but dramatically increased formation of the smaller-sized HDL. 2-Bromopalmitate did not appear to affect ABCA1 activity, subcellular localization or oligomerization, but induced dissolution of the cholesterol-phospholipid complexes in the plasma membrane. Arachidonic and linoleic acids shifted HDL formation to the smaller-sized species. Tangier disease mutations and inhibitors of ABCA1 activity wheat germ agglutinin and AG 490 reduced formation of both larger-sized and smaller-sized HDL. The effect of probucol was similar to the effect of 2-bromopalmitate. Taking rHDL formation as a paradigm, we propose that ABCA1 mutations and activity inhibitors reduce the amount of cell lipid available for HDL formation, and the compounds in the 2-bromopalmitate group and the polyunsaturated fatty acids change cell lipid composition from one that favors formation of the larger-sized HDL particles to one that favors formation of the smaller-sized species.

Short-term isocaloric fructose restriction lowers apoC-III levels and yields less atherogenic lipoprotein profiles in children with obesity and metabolic syndrome

BACKGROUND AND AIMS

Dietary fructose may play a role in the pathogenesis of metabolic syndrome (MetS). In a recently published study of obese children with MetS, we showed that isocaloric fructose restriction reduced fasting triglyceride (TG) and LDL-cholesterol (LDL-C). In these ancillary analyses, we tested the hypothesis that these effects were also accompanied by improved quantitative and qualitative changes in LDL and HDL subclasses and their apolipoproteins; as well as change in VLDL, particularly apoC-III.

METHODS

Obese children with MetS (n = 37) consumed a diet that matched self-reported macronutrient composition for nine days, with the exception that dietary fructose was reduced from 11.7 ± 4.0% to 3.8 ± 0.5% of daily calories and substituted with glucose (in starch). Participants underwent fasting biochemical analyses on Days 0 and 10. HDL and LDL subclasses were analyzed using the Lipoprint HDL and LDL subfraction analysis systems from Quantimetrix.

RESULTS

Significant reductions in apoB (78 ± 24 vs. 66 ± 24 mg/dl) apoC-III (8.7 ± 3.5 vs. 6.5 ± 2.6 mg/dl) and apoE (4.6 ± 2.3 vs. 3.6 ± 1.1 mg/dl), all p < 0.001) were observed. LDL size increased by 0.87 Å (p = 0.008). Small dense LDL was present in 25% of our cohort and decreased by 68% (p = 0.04). Small HDL decreased by 2.7% (p < 0.001) and large HDL increased by 2.4% (p = 0.04). The TG/HDL-C ratio decreased from 3.1 ± 2.5 to 2.4 ± 1.4 (p = 0.02). These changes in fasting lipid profiles correlated with changes in insulin sensitivity.

CONCLUSIONS

Isocaloric fructose restriction for 9 days improved lipoprotein markers of CVD risk in children with obesity and MetS. The most dramatic reduction was seen for apoC-III, which has been associated with atherogenic hypertriglyceridemia.

Small-angle X-ray scattering of the cholesterol incorporation into human ApoA1-POPC discoidal particles

Structural and functional aspects of high-density lipoproteins have been studied for over half a century. Due to the plasticity of this highly complex system, new aspects continue to be discovered. Here, we present a structural study of the human Apolipoprotein A1 (ApoA1) and investigate the role of its N-terminal domain, the so-called globular domain of ApoA1, in discoidal complexes with phospholipids and increasing amounts of cholesterol. Using a combination of solution-based small-angle x-ray scattering (SAXS) and molecular constrained data modeling, we show that the ApoA1-1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC)-based particles are disk shaped with an elliptical cross section and composed by a central lipid bilayer surrounded by two stabilizing ApoA1 proteins. This structure is very similar to the particles formed in the so-called nanodisc system, which is based on N-terminal truncated ApoA1 protein. Although it is commonly agreed that the nanodisc is plain disk shaped, several more advanced structures have been proposed for the full-length ApoA1 in combination with POPC and cholesterol. This prompted us to make a detailed comparative study of the ApoA1 and nanodisc systems upon cholesterol uptake. Based on the presented SAXS analysis it is found that the N-terminal domains of ApoA1-POPC-cholesterol particles are not globular but instead an integrated part of the protein belt stabilizing the particles. Upon incorporation of increasing amounts of cholesterol, the presence of the N-terminal domain allows the bilayer thickness to increase while maintaining an overall flat bilayer structure. This is contrasted by the energetically more strained and less favorable lens shape required to fit the SAXS data from the N-terminal truncated nanodisc system upon cholesterol incorporation. This suggests that the N-terminal domain of ApoA1 actively participates in the stabilization of the ApoA1-POPC-cholesterol discoidal particle and allows for a more optimal lipid packing upon cholesterol uptake.

Formation of stable nanodiscs by bihelical apolipoprotein A-I mimetic peptide

Nanodiscs are composed of scaffold protein or peptide such as apolipoprotein A-I (apoA-I) and phospholipids. Although peptide-based nanodiscs have an advantage to modulate the size of nanodiscs by changing phospholipid/peptide ratios, they are usually less stable than apoA-I-based nanodiscs. In this study, we designed a novel nanodisc scaffold peptide (NSP) that has proline-punctuated bihelical amphipathic structure based on apoA-I mimetic peptides. NSP formed α-helical structure on 1-palmitoyl-2-oleoyl phosphatidylcholine (POPC) nanodiscs prepared by cholate dialysis method. Dynamic light scattering measurements demonstrated that diameters of NSP nanodiscs vary depending upon POPC/NSP ratios. Comparison of thermal unfolding of nanodiscs monitored by circular dichroism measurements demonstrated that NSP forms much more stable nanodiscs with POPC than monohelical peptide, 4F, exhibiting comparable stability to apoA-I-POPC nanodiscs. Intrinsic Trp fluorescence measurements showed that Trp residues of NSP exhibit more hydrophobic environment than that of 4 F on nanodiscs, suggesting the stronger interaction of NSP with phospholipids. Thus, the bihelical structure of NSP appears to increase the stability of nanodiscs because of the enhanced interaction of peptides with phospholipids. In addition, NSP as well as 4F spontaneously solubilized POPC vesicles into nanodiscs without using detergent. These results indicate that bihelical NSP forms nanodiscs with comparable stability to apoA-I and has an ability to control the size of nanodiscs simply by changing phospholipid/peptide ratios.

HDL-apolipoprotein A-I exchange is independently associated with cholesterol efflux capacity

HDL is the primary mediator of cholesterol mobilization from the periphery to the liver via reverse cholesterol transport (RCT). A critical first step in this process is the uptake of cholesterol from lipid-loaded macrophages by HDL, a function of HDL inversely associated with prevalent and incident cardiovascular disease. We hypothesized that the dynamic ability of HDL to undergo remodeling and exchange of apoA-I is an important and potentially rate-limiting aspect of RCT. In this study, we investigated the relationship between HDL-apoA-I exchange (HAE) and serum HDL cholesterol (HDL-C) efflux capacity. We compared HAE to the total and ABCA1-specific cholesterol efflux capacity of 77 subjects. We found that HAE was highly correlated with both total (r = 0.69, P < 0.0001) and ABCA1-specific (r = 0.47, P < 0.0001) efflux, and this relationship remained significant after adjustment for HDL-C or apoA-I. Multivariate models of sterol efflux capacity indicated that HAE accounted for approximately 25% of the model variance for both total and ABCA1-specific efflux. We conclude that the ability of HDL to exchange apoA-I and remodel, as measured by HAE, is a significant contributor to serum HDL efflux capacity, independent of HDL-C and apoA-I, indicating that HDL dynamics are an important factor in cholesterol efflux capacity and likely RCT.

Quantification of HDL particle concentration by calibrated ion mobility analysis

BACKGROUND

It is critical to develop new metrics to determine whether HDL is cardioprotective in humans. One promising approach is HDL particle concentration (HDL-P), the size and concentration of HDL in plasma. However, the 2 methods currently used to determine HDL-P yield concentrations that differ >5-fold. We therefore developed and validated an improved approach to quantify HDL-P, termed calibrated ion mobility analysis (calibrated IMA).

METHODS

HDL was isolated from plasma by ultracentrifugation, introduced into the gas phase with electrospray ionization, separated by size, and quantified by particle counting. We used a calibration curve constructed with purified proteins to correct for the ionization efficiency of HDL particles.

RESULTS

The concentrations of gold nanoparticles and reconstituted HDLs measured by calibrated IMA were indistinguishable from concentrations determined by orthogonal methods. In plasma of control (n = 40) and cerebrovascular disease (n = 40) participants, 3 subspecies of HDL were reproducibility measured, with an estimated total HDL-P of 13.4 (2.4) μmol/L. HDL-C accounted for 48% of the variance in HDL-P. HDL-P was significantly lower in participants with cerebrovascular disease (P = 0.002), and this difference remained significant after adjustment for HDL cholesterol concentrations (P = 0.02).

CONCLUSIONS

Calibrated IMA accurately determined the concentration of gold nanoparticles and synthetic HDL, strongly suggesting that the method could accurately quantify HDL particle concentration. The estimated stoichiometry of apolipoprotein A-I determined by calibrated IMA was 3-4 per HDL particle, in agreement with current structural models. Furthermore, HDL-P was associated with cardiovascular disease status in a clinical population independently of HDL cholesterol.

Arginine-directed glycation and decreased HDL plasma concentration and functionality

BACKGROUND/OBJECTIVES

Decreased plasma concentration of high-density lipoprotein cholesterol (HDL-C) is a risk factor linked to increased risk of cardiovascular disease (CVD). Decreased anti-atherogenic properties of HDL are also implicated in increased CVD risk. The cause is unknown but has been linked to impaired glucose tolerance. The aim of this study was to quantify the modification of HDL by methylglyoxal and related dicarbonyls in healthy people and patients with type 2 diabetes characterise structural, functional and physiological consequences of the modification and predict the importance in high CVD risk groups.

SUBJECTS/METHODS

Major fractions of HDL, HDL2 and HDL3 were isolated from healthy human subjects and patients with type 2 diabetes and fractions modified by methylglyoxal and related dicarbonyl metabolites quantified. HDL2 and HDL3 were glycated by methylglyoxal to minimum extent in vitro and molecular, functional and physiological characteristics were determined. A one-compartment model of HDL plasma clearance was produced including formation and clearance of dicarbonyl-modified HDL.

RESULTS

HDL modified by methylglyoxal and related dicarbonyl metabolites accounted for 2.6% HDL and increased to 4.5% in patients with type 2 diabetes mellitus (T2DM). HDL2 and HDL3 were modified by methylglyoxal to similar extents in vitro. Methylglyoxal modification induced re-structuring of the HDL particles, decreasing stability and plasma half-life in vivo. It occurred at sites of apolipoprotein A-1 in HDL linked to membrane fusion, intramolecular bonding and ligand binding. Kinetic modelling of methylglyoxal modification of HDL predicted a negative correlation of plasma HDL-C with methylglyoxal-modified HDL. This was validated clinically. It also predicted that dicarbonyl modification produces 2-6% decrease in total plasma HDL and 5-13% decrease in functional HDL clinically.

CONCLUSIONS

These results suggest that methylglyoxal modification of HDL accelerates its degradation and impairs its functionality in vivo, likely contributing to increased risk of CVD-particularly in high CVD risk groups.

Optimized negative staining: a high-throughput protocol for examining small and asymmetric protein structure by electron microscopy

Structural determination of proteins is rather challenging for proteins with molecular masses between 40 - 200 kDa. Considering that more than half of natural proteins have a molecular mass between 40 - 200 kDa^1,2, a robust and high-throughput method with a nanometer resolution capability is needed. Negative staining (NS) electron microscopy (EM) is an easy, rapid, and qualitative approach which has frequently been used in research laboratories to examine protein structure and protein-protein interactions. Unfortunately, conventional NS protocols often generate structural artifacts on proteins, especially with lipoproteins that usually form presenting rouleaux artifacts. By using images of lipoproteins from cryo-electron microscopy (cryo-EM) as a standard, the key parameters in NS specimen preparation conditions were recently screened and reported as the optimized NS protocol (OpNS), a modified conventional NS protocol ³ . Artifacts like rouleaux can be greatly limited by OpNS, additionally providing high contrast along with reasonably high‐resolution (near 1 nm) images of small and asymmetric proteins. These high-resolution and high contrast images are even favorable for an individual protein (a single object, no average) 3D reconstruction, such as a 160 kDa antibody, through the method of electron tomography^4,5. Moreover, OpNS can be a high‐throughput tool to examine hundreds of samples of small proteins. For example, the previously published mechanism of 53 kDa cholesteryl ester transfer protein (CETP) involved the screening and imaging of hundreds of samples ⁶. Considering cryo-EM rarely successfully images proteins less than 200 kDa has yet to publish any study involving screening over one hundred sample conditions, it is fair to call OpNS a high-throughput method for studying small proteins. Hopefully the OpNS protocol presented here can be a useful tool to push the boundaries of EM and accelerate EM studies into small protein structure, dynamics and mechanisms.

Surface behavior of apolipoprotein A-I and its deletion mutants at model lipoprotein interfaces

Apolipoprotein A-I (apoA-I) has a great conformational flexibility to exist in lipid-free, lipid-poor, and lipid-bound states during lipid metabolism. To address the lipid binding and the dynamic desorption behavior of apoA-I at lipoprotein surfaces, apoA-I, Δ(185-243)apoA-I, and Δ(1-59)(185-243)apoA-I were studied at triolein/water and phosphatidylcholine/triolein/water interfaces with special attention to surface pressure. All three proteins are surface active to both interfaces lowering the interfacial tension and thus increasing the surface pressure to modify the interfaces. Δ(185-243)apoA-I adsorbs much more slowly and lowers the interfacial tension less than full-length apoA-I, confirming that the C-terminal domain (residues 185-243) initiates the lipid binding. Δ(1-59)(185-243)apoA-I binds more rapidly and lowers the interfacial tension more than Δ(185-243)apoA-I, suggesting that destabilizing the N-terminal α-helical bundle (residues 1-185) restores lipid binding. The three proteins desorb from both interfaces at different surface pressures revealing that different domains of apoA-I possess different lipid affinity. Δ(1-59)(185-243)apoA-I desorbs at lower pressures compared with apoA-I and Δ(185-243)apoA-I indicating that it is missing a strong lipid association motif. We propose that during lipoprotein remodeling, surface pressure mediates the adsorption and partial or full desorption of apoA-I allowing it to exchange among different lipoproteins and adopt various conformations to facilitate its multiple functions.

Activation of paraoxonase 1 is associated with HDL remodeling ex vivo

BACKGROUND

We hypothesize that during high density lipoprotein (HDL) remodeling PON1 reaches an optimal distribution in HDL subclasses by which it achieves maximum activity. We conducted this study to gain insight on PON1 fate and activation during short-term HDL remodeling ex vivo.

METHODS

Serum from 8 healthy volunteers was either frozen at -80°C (time 0) or incubated under sterile conditions for up to 48h at 37°C or at 4°C. Aliquots were taken at 3, 6, 9, 24 and 48 h and immediately frozen at -80°C. PON1 activities were measured, as well as PON1 and apolipoprotein distributions in HDL subclasses by gradient gel electrophoresis.

RESULTS

The first novel finding in our study is the evidence provided for a significant activation of both lactonase and arylesterase activities of PON1 that ensues in a very short time frame of incubation of serum ex vivo at 37°C. All subjects studied displayed these changes, the activation was apparent in <3h, peaked at 6h and amounted to >20%. This is associated with a temperature and time-dependent redistribution of PON1 activity in HDL subclasses, with an increase in activity in both very large HDL2 and small HDL3 in the first phase (3-9h), followed by a progressive transfer of PON1 to very large HDL2 as the particles mature. These changes are paralleled by the appearance of weak, but apparent PON1 activity at subspecies that correspond to sdLDL. During the first phase of PON1 activation and shifts, a parallel shift of apoE can be evidenced: at 3-9h, apoE increases in sdLDL, after that time it is lost from HDL and also from sdLDL and stays in VLDL at the origin of the run. ApoA-I shifts towards larger particles, which parallels the change in PON1. As HDL matures there is a progressive shift of apoA-II towards larger HDL. Low levels of apoA-IV at the initiation of the incubation are followed by time dependent quick disappearance of apoA-IV in HDL which parallels the changes in PON1, apoE and A-II.

CONCLUSION

Short, ex vivo incubation of serum leads to quick activation of PON1 associated with transfers to HDL3c, large HDL and sdLDL. The process is blocked by CETP and LCAT inhibitors. The data suggest that HDL maturation optimizes PON1 activity. These findings may be of interest for future studies aimed at modulating PON-1 activity for its cardioprotective effects and suggest a new mechanism whereby CETP inhibitors failed in clinical trials.

Molecules that mimic apolipoprotein A-I: potential agents for treating atherosclerosis

Certain amphipathic α-helical peptides can functionally mimic many of the properties of full-length apolipoproteins, thereby offering an approach to modulate high-density lipoprotein (HDL) for combating atherosclerosis. In this Perspective, we summarize the key findings and advances over the past 25 years in the development of peptides that mimic apolipoproteins, especially apolipoprotein A-I (apoA-I). This assemblage of information provides a reasonably clear picture of the state of the art in the apolipoprotein mimetic field, an appreciation of the potential for such agents in pharmacotherapy, and a sense of the opportunities for optimizing the functional properties of HDL.

Conservation of apolipoprotein A-I's central domain structural elements upon lipid association on different high-density lipoprotein subclasses

The antiatherogenic properties of apolipoprotein A-I (apoA-I) are derived, in part, from lipidation-state-dependent structural elements that manifest at different stages of apoA-I's progression from lipid-free protein to spherical high-density lipoprotein (HDL). Previously, we reported the structure of apoA-I's N-terminus on reconstituted HDLs (rHDLs) of different sizes. We have now investigated at the single-residue level the conformational adaptations of three regions in the central domain of apoA-I (residues 119-124, 139-144, and 164-170) upon apoA-I lipid binding and HDL formation. An important function associated with these residues of apoA-I is the activation of lecithin:cholesterol acyltransferase (LCAT), the enzyme responsible for catalyzing HDL maturation. Structural examination was performed by site-directed tryptophan fluorescence and spin-label electron paramagnetic resonance spectroscopies for both the lipid-free protein and rHDL particles 7.8, 8.4, and 9.6 nm in diameter. The two methods provide complementary information about residue side chain mobility and molecular accessibility, as well as the polarity of the local environment at the targeted positions. The modulation of these biophysical parameters yielded new insight into the importance of structural elements in the central domain of apoA-I. In particular, we determined that the loosely lipid-associated structure of residues 134-145 is conserved in all rHDL particles. Truncation of this region completely abolished LCAT activation but did not significantly affect rHDL size, reaffirming the important role of this structural element in HDL function.

HDL-apoA-I exchange: rapid detection and association with atherosclerosis

High density lipoprotein (HDL) cholesterol levels are associated with decreased risk of cardiovascular disease, but not all HDL are functionally equivalent. A primary determinant of HDL functional status is the conformational adaptability of its main protein component, apoA-I, an exchangeable apolipoprotein. Chemical modification of apoA-I, as may occur under conditions of inflammation or diabetes, can severely impair HDL function and is associated with the presence of cardiovascular disease. Chemical modification of apoA-I also impairs its ability to exchange on and off HDL, a critical process in reverse cholesterol transport. In this study, we developed a method using electron paramagnetic resonance spectroscopy (EPR) to quantify HDL-apoA-I exchange. Using this approach, we measured the degree of HDL-apoA-I exchange for HDL isolated from rabbits fed a high fat, high cholesterol diet, as well as human subjects with acute coronary syndrome and metabolic syndrome. We observed that HDL-apoA-I exchange was markedly reduced when atherosclerosis was present, or when the subject carries at least one risk factor of cardiovascular disease. These results show that HDL-apoA-I exchange is a clinically relevant measure of HDL function pertinent to cardiovascular disease.

Active plasma membrane P-type H+-ATPase reconstituted into nanodiscs is a monomer

Plasma membrane H(+)-ATPases form a subfamily of P-type ATPases responsible for pumping protons out of cells and are essential for establishing and maintaining the crucial transmembrane proton gradient in plants and fungi. Here, we report the reconstitution of the Arabidopsis thaliana plasma membrane H(+)-ATPase isoform 2 into soluble nanoscale lipid bilayers, also termed nanodiscs. Based on native gel analysis and cross-linking studies, the pump inserts into nanodiscs as a functional monomer. Insertion of the H(+)-ATPase into nanodiscs has the potential to enable structural and functional characterization using techniques normally applicable only for soluble proteins.

The secondary structure of apolipoprotein A-I on 9.6-nm reconstituted high-density lipoprotein determined by EPR spectroscopy

Apolipoprotein A-I (ApoA-I) is the major protein component of high-density lipoprotein (HDL), and is critical for maintenance of cholesterol homeostasis. During reverse cholesterol transport, HDL transitions between an array of subclasses, differing in size and composition. This process requires ApoA-I to adapt to changes in the shape of the HDL particle, transiting from an apolipoprotein to a myriad of HDL subclass-specific conformations. Changes in ApoA-I structure cause alterations in HDL-specific enzyme and receptor-binding properties, and thereby direct the HDL particle through the reverse cholesterol transport pathway. In this study, we used site-directed spin label spectroscopy to examine the conformational details of the ApoA-I central domain on HDL. The motional dynamics and accessibility to hydrophobic/hydrophilic relaxation agents of ApoA-I residues 99–163 on 9.6-nm reconstituted HDL was analyzed by EPR. In previous analyses, we examined residues 6–98 and 164–238 (of ApoA-I's 243 residues), and combining these findings with the current results, we have generated a full-length map of the backbone structure of reconstituted HDL-associated ApoA-I. Remarkably, given that the majority of ApoA-I's length is composed of amphipathic helices, we have identified nonhelical residues, specifically the presence of a β-strand (residues 149–157). The significance of these nonhelical residues is discussed, along with the other features, in the context of ApoA-I function in contrast to recent models derived by other methods.

The maltose ABC transporter: action of membrane lipids on the transporter stability, coupling and ATPase activity

The coupling between ATP hydrolysis and substrate transport remains a key question in the understanding of ABC-mediated transport. We show using the MalFGK2 complex reconstituted into nanodiscs, that membrane lipids participate directly to the coupling reaction by stabilizing the transporter in a low energy conformation. When surrounded by short acyl chain phospholipids, the transporter is unstable and hydrolyzes large amounts of ATP without inducing maltose. The presence of long acyl chain phospholipids stabilizes the conformational dynamics of the transporter, reduces its ATPase activity and restores dependence on maltose. Membrane lipids therefore play an essential allosteric function, they restrict the transporter ATPase activity to increase coupling to the substrate. In support to the notion, we show that increasing the conformational dynamics of MalFGK2 with mutations in MalF increases the transporter ATPase activity but decreases the maltose transport efficiency.

Crystal structure of Δ(185-243)ApoA-I suggests a mechanistic framework for the protein adaptation to the changing lipid load in good cholesterol: from flatland to sphereland via double belt, belt buckle, double hairpin and trefoil/tetrafoil

Apolipoprotein A-I (apoA-I) is the major protein of plasma high-density lipoproteins (HDLs), macromolecular assemblies of proteins and lipids that remove cell cholesterol and protect against atherosclerosis. HDL heterogeneity, large size (7.7-12 nm), and ability to exchange proteins have prevented high-resolution structural analysis. Low-resolution studies showed that two apoA-I molecules form an antiparallel α-helical "double belt" around an HDL particle. The atomic-resolution structure of the C-terminal truncated lipid-free Δ(185-243)apoA-I, determined recently by Mei and Atkinson, provides unprecedented new insights into HDL structure-function. It allows us to propose a molecular mechanism for the adaptation of the full-length protein to increasing lipid load during cholesterol transport. ApoA-I conformations on small, midsize, and large HDLs are proposed based on the tandem α-helical repeats and the crystal structure of Δ(185-243)apoA-I and are validated by comparison with extensive biophysical data reported by many groups. In our models, the central half of the double belt ("constant" segment 66-184) is structurally conserved while the N- and C-terminal half ("variable" segments 1-65 and 185-243) rearranges upon HDL growth. This includes incremental unhinging of the N-terminal bundle around two flexible regions containing G39 and G65 to elongate the belt, along with concerted swing motion of the double belt around G65-P66 and G185-G186 hinges that are aligned on various-size particles, to confer two-dimensional surface curvature to spherical HDLs. The proposed conformational ensemble integrates and improves several existing HDL models. It helps provide a structural framework necessary to understand functional interactions with over 60 other HDL-associated proteins and, ultimately, improve the cardioprotective function of HDL.

Optimized negative-staining electron microscopy for lipoprotein studies

BACKGROUND

Negative-staining (NS), a rapid, simple and conventional technique of electron microscopy (EM), has been commonly used to initially study the morphology and structure of proteins for half a century. Certain NS protocols however can cause artifacts, especially for structurally flexible or lipid-related proteins, such as lipoproteins. Lipoproteins were often observed in the form of rouleau as lipoprotein particles appeared to be stacked together by conventional NS protocols. The flexible components of lipoproteins, i.e. lipids and amphipathic apolipoproteins, resulted in the lipoprotein structure being sensitive to the NS sample preparation parameters, such as operational procedures, salt concentrations, and the staining reagents.

SCOPE OF REVIEW

The most popular NS protocols that have been used to examine lipoprotein morphology and structure were reviewed.

MAJOR CONCLUSIONS

The comparisons show that an optimized NS (OpNS) protocol can eliminate the rouleau artifacts of lipoproteins, and that the lipoproteins are similar in size and shape as statistically measured from two EM methods, OpNS and cryo-electron microscopy (cryo-EM). OpNS is a high-throughput, high-contrast and high-resolution (near 1nm, but rarely better than 1nm) method which has been used to discover the mechanics of a small protein, 53kDa cholesterol ester transfer protein (CETP), and the structure of an individual particle of a single protein by individual-particle electron tomography (IPET), i.e. a 14Å-resolution IgG antibody three-dimensional map.

GENERAL SIGNIFICANCE

It is suggested that OpNS can be used as a general protocol to study the structure of proteins, especially highly dynamic proteins with equilibrium-fluctuating structures.

Reconstitution of the Escherichia coli cell division ZipA-FtsZ complexes in nanodiscs as revealed by electron microscopy

ZipA is an element of the bacterial division ring complex that provides an anchor to the membrane to FtsZ, a GTPase ancestor of tubulin. In vitro reconstitution and characterization of these interactions is challenged by the difficulty to integrate a physiological membrane environment. Here a single copy of the full-length ZipA protein from Escherichia coli incorporated into phospholipid bilayer nanodiscs (Nd-ZipA) has been visualized using negative-staining electron microscopy (EM). The EM images reveal the presence of discs, mostly organized in two distinct populations of 11 and 13nm in diameter. The globular FtsZ-binding C-terminal domain of ZipA (ZBD) was not visible in 3D reconstructions of Nd-ZipA or 2D averages, suggesting that this domain is separated from the membrane by the large flexible domain connecting the N-terminal trans-membrane region to the ZBD. We tested if Nd-ZipA were appropriate models for the in vitro reconstitution of ZipA-FtsZ interactions. First we observed that the ZBD region of ZipA was accessible for the interaction with other proteins in the context of the nanodisc, as revealed by its recognition by specific antibodies. In addition, Nd-ZipA attached to carbon coated EM grids, but not empty nanodiscs, were able to capture FtsZ filaments without inducing significant filament bundling, consistent with a model in which FtsZ filaments are loosely attached to the cell-membrane. These observations are compatible with the plastic nature of the ZipA-FtsZ complexes formed at the membrane, evidenced in the moderate binding affinity of Nd-ZipA to FtsZ oligomers and polymers recently measured.

Nascent high density lipoproteins formed by ABCA1 resemble lipid rafts and are structurally organized by three apoA-I monomers

This report details the lipid composition of nascent HDL (nHDL) particles formed by the action of the ATP binding cassette transporter A1 (ABCA1) on apolipoprotein A-I (apoA-I). nHDL particles of different size (average diameters of ∼ 12, 10, 7.5, and <6 nm) and composition were purified by size-exclusion chromatography. Electron microscopy suggested that the nHDL were mostly spheroidal. The proportions of the principal nHDL lipids, free cholesterol, glycerophosphocholine, and sphingomyelin were similar to that of lipid rafts, suggesting that the lipid originated from a raft-like region of the cell. Smaller amounts of glucosylceramides, cholesteryl esters, and other glycerophospholipid classes were also present. The largest particles, ∼ 12 nm and 10 nm diameter, contained ∼ 43% free cholesterol, 2-3% cholesteryl ester, and three apoA-I molecules. Using chemical cross-linking chemistry combined with mass spectrometry, we found that three molecules of apoA-I in the ∼ 9-14 nm nHDL adopted a belt-like conformation. The smaller (7.5 nm diameter) spheroidal nHDL particles carried 30% free cholesterol and two molecules of apoA-I in a twisted, antiparallel, double-belt conformation. Overall, these new data offer fresh insights into the biogenesis and structural constraints involved in forming nascent HDL from ABCA1.

Apolipoprotein A-I helical structure and stability in discoidal high-density lipoprotein (HDL) particles by hydrogen exchange and mass spectrometry

To understand high-density lipoprotein (HDL) structure at the molecular level, the location and stability of α-helical segments in human apolipoprotein (apo) A-I in large (9.6 nm) and small (7.8 nm) discoidal HDL particles were determined by hydrogen-deuterium exchange (HX) and mass spectrometry methods. The measured HX kinetics of some 100 apoA-I peptides specify, at close to amino acid resolution, the structural condition of segments throughout the protein sequence and changes in structure and stability that occur on incorporation into lipoprotein particles. When incorporated into the large HDL particle, the nonhelical regions in lipid-free apoA-I (residues 45-53, 66-69, 116-146, and 179-236) change conformation from random coil to α-helix so that nearly the entire apoA-I molecule adopts helical structure (except for the terminal residues 1-6 and 237-243). The amphipathic α-helices have relatively low stability, in the range 3-5 kcal/mol, indicating high flexibility and dynamic unfolding and refolding in seconds or less. A segment encompassed by residues 125-158 exhibits bimodal HX labeling indicating co-existing helical and disordered loop conformations that interchange on a time scale of minutes. When incorporated around the edge of the smaller HDL particle, the increase in packing density of the two apoA-I molecules forces about 20% more residues out of direct contact with the phospholipid molecules to form disordered loops, and these are the same segments that form loops in the lipid-free state. The region of disc-associated apoA-I that binds the lecithin-cholesterol acyltransferase enzyme is well structured and not a protruding unstructured loop as reported by others.

IPET and FETR: experimental approach for studying molecular structure dynamics by cryo-electron tomography of a single-molecule structure

The dynamic personalities and structural heterogeneity of proteins are essential for proper functioning. Structural determination of dynamic/heterogeneous proteins is limited by conventional approaches of X-ray and electron microscopy (EM) of single-particle reconstruction that require an average from thousands to millions different molecules. Cryo-electron tomography (cryoET) is an approach to determine three-dimensional (3D) reconstruction of a single and unique biological object such as bacteria and cells, by imaging the object from a series of tilting angles. However, cconventional reconstruction methods use large-size whole-micrographs that are limited by reconstruction resolution (lower than 20 Å), especially for small and low-symmetric molecule (<400 kDa). In this study, we demonstrated the adverse effects from image distortion and the measuring tilt-errors (including tilt-axis and tilt-angle errors) both play a major role in limiting the reconstruction resolution. Therefore, we developed a “focused electron tomography reconstruction” (FETR) algorithm to improve the resolution by decreasing the reconstructing image size so that it contains only a single-instance protein. FETR can tolerate certain levels of image-distortion and measuring tilt-errors, and can also precisely determine the translational parameters via an iterative refinement process that contains a series of automatically generated dynamic filters and masks. To describe this method, a set of simulated cryoET images was employed; to validate this approach, the real experimental images from negative-staining and cryoET were used. Since this approach can obtain the structure of a single-instance molecule/particle, we named it individual-particle electron tomography (IPET) as a new robust strategy/approach that does not require a pre-given initial model, class averaging of multiple molecules or an extended ordered lattice, but can tolerate small tilt-errors for high-resolution single “snapshot” molecule structure determination. Thus, FETR/IPET provides a completely new opportunity for a single-molecule structure determination, and could be used to study the dynamic character and equilibrium fluctuation of macromolecules.

Crystal structure of C-terminal truncated apolipoprotein A-I reveals the assembly of high density lipoprotein (HDL) by dimerization

Apolipoprotein A-I (apoA-I) plays important structural and functional roles in plasma high density lipoprotein (HDL) that is responsible for reverse cholesterol transport. However, a molecular understanding of HDL assembly and function remains enigmatic. The 2.2-Å crystal structure of Δ(185-243)apoA-I reported here shows that it forms a half-circle dimer. The backbone of the dimer consists of two elongated antiparallel proline-kinked helices (five AB tandem repeats). The N-terminal domain of each molecule forms a four-helix bundle with the helical C-terminal region of the symmetry-related partner. The central region forms a flexible domain with two antiparallel helices connecting the bundles at each end. The two-domain dimer structure based on helical repeats suggests the role of apoA-I in the formation of discoidal HDL particles. Furthermore, the structure suggests the possible interaction with lecithin-cholesterol acyltransferase and may shed light on the molecular details of the effect of the Milano, Paris, and Fin mutations.