Impact of self-association on function of apolipoprotein A-I

The role of C-terminal ionic residues in self-association of apolipoprotein A-I

Apolipoprotein A-I (apoA-I) is the main protein of high-density lipoprotein and is comprised of a helical bundle domain and a C-terminal (CT) domain encompassing the last ~65 amino acid residues of the 243-residue protein. The CT domain contains three putative helices (helix 8, 9, and 10) and is critical for initiating lipid binding and harbors sites that mediate self-association of the lipid-free protein. Three lysine residues reside in helix-8 (K195, 206, 208), and three in helix-10 (K226, 238, 239). To determine the role of each CT lysine residue in apoA-I self-association, single, double and triple lysine to glutamine mutants were engineered via site-directed mutagenesis. Circular dichroism and chemical denaturation analysis revealed all mutants retained their structural integrity. Chemical crosslinking and size-exclusion chromatography showed a small effect on self-association when helix-8 lysine residues were changed into glutamine. In contrast, mutation of the three helix-10 lysine residues resulted in a predominantly monomeric protein and K226 was identified as a critical residue. When helix-10 glutamate residues 223, 234, or 235 were substituted with glutamine, reduced self-association was observed similar to that of the helix-10 lysine variants, suggesting ionic interactions between these residues. Thus, helix-10 is a critical part of apoA-I mediating self-association, and disruption of ionic interactions changes apoA-I from an oligomeric state into a monomer. Since the helix-10 triple mutant solubilized phospholipid vesicles at higher rates compared to wild-type apoA-I, this indicates monomeric apoA-I is more potent in lipid binding, presumably because helix-10 is fully accessible to interact with lipids.

Melittin Tryptophan Substitution with a Fluorescent Amino Acid Reveals the Structural Basis of Selective Antitumor Effect and Subcellular Localization in Tumor Cells

Melittin is a membrane-active peptide with strong anticancer activity against various cancers. Despite decades of research, the role of the singular Trp in the anticancer activity and selectivity of melittin remains poorly understood. Here, we propose a theranostic solution based on the substitution of Trp19 with a noncanonical fluorescent amino acid (DapAMCA). The introduction of DapAMCA residue in melittin stabilized the helical structure of the peptide, as evaluated by circular dichroism spectra and molecular dynamics simulations. In vitro hemolytic and anticancer activity assays revealed that introducing DapAMCA residue in melittin changed its mode of action with the cell membrane, resulting in reduced hemolytic toxicity and an improved the selectivity index (SI), with up to a five-fold increase compared to melittin. In vitro fluorescence imaging of DapAMCA-labeled melittin (MELFL) in cancer cells demonstrated high membrane-penetrating activity, with strong nuclear and nucleolar localization ability. These findings provide implications for novel anticancer therapies based on Trp-substituted designs and nuclear/nucleolar targeted therapy.

Interaction of lipid-free apolipoprotein A-I with cholesterol revealed by molecular modeling

We report the modeling of the interaction of differently self-associated lipid-free apoA-I with cholesterol monomer and tail-to-tail (TT) or face-to-face (FF) cholesterol dimer. Cholesterol dimerization is exploited to reconcile the existing experimental data on cholesterol binding to apoA-I with extremely low critical micelle concentration of cholesterol. Two crystal structures of 1-43 N-truncated apolipoprotein Δ(1-43)A-I tetramer (PDB ID: 1AV1, structure B), 185-243 C-truncated apolipoprotein Δ(185-243)A-I dimer (PDB ID: 3R2P, structure M) were analyzed. Cholesterol monomers bind to multiple binding sites in apoA-I monomer, dimer and tetramer with low, moderate and high energy (-10 to -28 kJ/mol with Schrödinger package), still insufficient to overcome the thermodynamic restriction by cholesterol micellization (-52.8 kJ/mol). The binding sites partially coincide with the putative cholesterol-binding motifs. However, apoA-I monomer and dimer existing in structure B, that contain nonoverlapping and non-interacting pairs of binding sites with high affinity for TT and FF cholesterol dimers, can bind in common 14 cholesterol molecules that correspond to existing values. ApoA-I monomer and dimer in structure M can bind in common 6 cholesterol molecules. The values of respective total energy of cholesterol binding up to 64.5 and 67.0 kJ/mol for both B and M structures exceed the free energy of cholesterol micellization. We hypothesize that cholesterol dimers may simultaneously interact with extracellular monomer and dimer of lipid-free apoA-I, that accumulate at acid pH in atheroma. The thermodynamically allowed apolipoprotein-cholesterol interaction outside the macrophage may represent a new mechanism of cholesterol transport by apoA-I from atheroma, in addition to ABCA1-mediated cholesterol efflux.

Analysis of pyrene-labelled apolipoprotein A-I oligomerization in solution: Spectra deconvolution and changes in P-value and excimer formation

ApoA-I is the main protein of HDL which has anti-atherogenic properties attributed to reverse cholesterol transport. It shares with other exchangeable apolipoproteins a high level of structural plasticity. In the lipid-free state, the apolipoprotein amphipathic α-helices interact intra- and inter-molecularly, providing structural stabilization by a complex self-association mechanism. In this study, we employed a multi-parametric fluorescent probe to study the self-association of apoA-I. We constructed six single cysteine mutants spanning positions along three helices: F104C, K107C (H4), K133C, L137C (H5), F225C and K226C (H10); and labelled them with N-Maleimide Pyrene. Taking advantage of its spectral properties, namely formation of an excited dimer (excimer) and polarity-dependent changes in its fluorescence fine structure (P-value), we monitored the apoA-I self-association in its lipid-free form as a function of its concentration. Interactions in helices H5 (K133C) and H10 (F225C and K226C) were highlighted by excimer emission; while polarity changes were reported in helix H4 (K107C), as well as in helices H5 and H10. Mathematical models were developed to enrich data analysis and estimate association constants (K_A) and oligomeric species distribution. Furthermore, we briefly discuss the usefulness of the multi-parametric fluorescent probe to monitor different equilibria, even at a single labelling position. Results suggest that apoA-I self-association must be considered to fully understand its physiological roles. Particularly, some contacts that stabilize discoidal HDL particles seem to be already present in the lipid-free apoA-I oligomers.

Structural and functional basis for increased HDL-cholesterol levels due to the naturally occurring V19L mutation in human apolipoprotein A-I

Several hereditary point mutations in human apolipoprotein A-I (apoA-I) have been associated with low HDL-cholesterol levels and/or increased coronary artery disease (CAD) risk. However, one apoA-I mutation, the V19L, recently identified in Icelanders, has been associated with increased HDL-cholesterol levels and decreased CAD risk. In an effort to gain mechanistic insight linking the presence of this mutation in apoA-I with the increase of HDL-cholesterol levels we evaluated the effect of V19L mutation on the conformational integrity and functional properties of apoA-I in lipid-free and lipidated form. ApoA-I[V19L] was found to be thermodynamically destabilized in lipid-free form and displays an increased capacity to associate with phospholipids compared to WT apoA-I. When associated to reconstituted HDL (rHDL), apoA-I[V19L] was more thermodynamically stabilized than WT apoA-I. ApoA-I[V19L] displayed normal capacity to promote ABCA1-mediated cholesterol efflux and to activate the enzyme LCAT, in lipid-free and rHDL-associated forms, respectively. Additionally, rHDL-associated apoA-I[V19L] showed normal capacity to promote ABCG1-mediated cholesterol efflux, but 45% increased capacity to promote SR-BI-mediated cholesterol efflux, while the SR-BI-mediated HDL-lipid uptake was normal. Overall, our findings show that the apoA-I V19L mutation does not affect the first steps of HDL biogenesis pathway. However, the increased capacity of apoA-I[V19L] to associate with phospholipids, in combination with the enhanced thermodynamic stability of lipoprotein-associated apoA-I[V19L] and increased capacity of apoA-I[V19L]-containing lipoprotein particles to accept additional cholesterol by SR-BI could account for the increased HDL-cholesterol levels observed in human carriers of the mutation.

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.

From a Highly Disordered to a Metastable State: Uncovering Insights of α-Synuclein

α-Synuclein (αS) is a major constituent of Lewy bodies, the insoluble aggregates that are the hallmark of one of the most prevalent neurodegenerative disorders, Parkinson's disease (PD). The vast majority of experiments in vitro and in vivo provide extensive evidence that a disordered monomeric form is the predominant state of αS in water solution, and it undergoes a large-scale disorder-to-helix transition upon binding to vesicles of different types. Recently, another form, tetrameric, of αS with a stable helical structure was identified experimentally. It has been shown that a dynamic intracellular population of metastable αS tetramers and monomers coexists normally; and the tetramer plays an essential role in maintaining αS homeostasis. Therefore, it is of interest to know whether the tetramer can serve as a means of preventing or delaying the start of PD. Before answering this very important question, it is, first, necessary to find out, on an atomistic level, a correlation between tetramers and monomers; what mediates tetramer formation and what makes a tetramer stable. We address these questions here by investigating both monomeric and tetrameric forms of αS. In particular, by examining correlations between the motions of the side chains and the main chain, steric parameters along the amino-acid sequence, and one- and two-dimensional free-energy landscapes along the coarse-grained dihedral angles γ and δ and principal components, respectively, in monomeric and tetrameric αS, we were able to shed light on a fundamental relationship between monomers and tetramers, and the key residues involved in mediating formation of a tetramer. Also, the reasons for the stability of tetrameric αS and inability of monomeric αS to fold are elucidated here.

Charged Residues in the C-Terminal Domain of Apolipoprotein A-I Modulate Oligomerization

Charged residues of the C-terminal domain of human apolipoprotein A-I (apoA-I) were targeted by site-directed mutagenesis. A series of mutant proteins was engineered in which lysine residues (Lys 195, 206, 208, 226, 238, and 239) or glutamate residues (Glu 234 and 235) were replaced by glutamine. The amino acid substitutions did not result in changes in secondary structure content or protein stability. Cross-linking and size-exclusion chromatography showed that the mutations resulted in reduced self-association, generating a predominantly monomeric apoA-I when five or six lysine residues were substituted. The rate of phosphatidylcholine vesicle solubilization was enhanced for all variants, with approximately a threefold rate enhancement for apoA-I lacking Lys 206, 208, 238, and 239, or Glu 234 and 235. Single or double mutations did not change the ability to protect lipolyzed low density lipoprotein from aggregation, but variants lacking >4 lysine residues were less effective in preventing lipoprotein aggregation. ApoA-I mediated cellular lipid efflux from wild-type mice macrophage foam cells was decreased for the variant with five lysine mutations. However, this protein was more effective in releasing cellular phosphatidylcholine and sphingomyelin from Abca1-null mice macrophage foam cells. This suggests that the mutations caused changes in the interaction with ABCA1 transporters and that membrane microsolubilization was primarily responsible for lipid efflux in cells lacking ABCA1. Taken together, this study indicates that ionic interactions in the C-terminal domain of apoA-I favor self-association and that monomeric apoA-I is more active in solubilizing phospholipid bilayers.

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.

Is ABCA1 a lipid transfer protein?

ABCA1 functions as a lipid transporter because it mediates the transfer of cellular phospholipid (PL) and free (unesterified) cholesterol (FC) to apoA-I and related proteins present in the extracellular medium. ABCA1 is a membrane PL translocase and its enzymatic activity leads to transfer of PL molecules from the cytoplasmic leaflet to the exofacial leaflet of a cell plasma membrane (PM). The presence of active ABCA1 in the PM promotes binding of apoA-I to the cell surface. About 10% of this bound apoA-I interacts directly with ABCA1 and stabilizes the transporter. Most of the pool of cell surface-associated apoA-I is bound to lipid domains in the PM that are created by the activity of ABCA1. The amphipathic α-helices in apoA-I confer detergent-like properties on the protein enabling it to solubilize PL and FC in these membrane domains to create a heterogeneous population of discoidal nascent HDL particles. This review focuses on current understanding of the structure-function relationships of human ABCA1 and the molecular mechanisms underlying HDL particle production.

Transfer of C-terminal residues of human apolipoprotein A-I to insect apolipophorin III creates a two-domain chimeric protein with enhanced lipid binding activity

Apolipophorin III (apoLp-III) is an insect apolipoprotein (18kDa) that comprises a single five-helix bundle domain. In contrast, human apolipoprotein A-I (apoA-I) is a 28kDa two-domain protein: an α-helical N-terminal domain (residues 1-189) and a less structured C-terminal domain (residues 190-243). To better understand the apolipoprotein domain organization, a novel chimeric protein was engineered by attaching residues 179 to 243 of apoA-I to the C-terminal end of apoLp-III. The apoLp-III/apoA-I chimera was successfully expressed and purified in E. coli. Western blot analysis and mass spectrometry confirmed the presence of the C-terminal domain of apoA-I within the chimera. While parent apoLp-III did not self-associate, the chimera formed oligomers similar to apoA-I. The chimera displayed a lower α-helical content, but the stability remained similar compared to apoLp-III, consistent with the addition of a less structured domain. The chimera was able to solubilize phospholipid vesicles at a significantly higher rate compared to apoLp-III, approaching that of apoA-I. The chimera was more effective in protecting phospholipase C-treated low density lipoprotein from aggregation compared to apoLp-III. In addition, binding interaction of the chimera with phosphatidylglycerol vesicles and lipopolysaccharides was considerably improved compared to apoLp-III. Thus, addition of the C-terminal domain of apoA-I to apoLp-III created a two-domain protein, with self-association, lipid and lipopolysaccharide binding properties similar to apoA-I. The apoA-I like behavior of the chimera indicate that these properties are independent from residues residing in the N-terminal domain of apoA-I, and that they can be transferred from apoA-I to apoLp-III.

PI(4,5)P2 Is Translocated by ABCA1 to the Cell Surface Where It Mediates Apolipoprotein A1 Binding and Nascent HDL Assembly

RATIONALE

The molecular mechanism by which ATP-binding cassette transporter A1 (ABCA1) mediates cellular binding of apolipoprotein A-I (apoA1) and nascent high-density lipoprotein (HDL) assembly is not well understood.

OBJECTIVE

To determine the cell surface lipid that mediates apoA1 binding to ABCA1-expressing cells and the role it plays in nascent HDL assembly.

METHODS AND RESULTS

Using multiple biochemical and biophysical methods, we found that apoA1 binds specifically to phosphatidylinositol (4,5) bis-phosphate (PIP2). Flow cytometry and PIP2 reporter-binding assays demonstrated that ABCA1 led to PIP2 redistribution from the inner to the outer leaflet of the plasma membrane. Enzymatic cleavage of cell surface PIP2 or decreased cellular PIP2 by knockdown of phosphatidylinositol-5-phosphate 4-kinase impaired apoA1 binding and cholesterol efflux to apoA1. PIP2 also increased the spontaneous solubilization of phospholipid liposomes by apoA1. Using site-directed mutagenesis, we found that ABCA1's PIP2 and phosphatidylserine translocase activities are independent from each other. Furthermore, we discovered that PIP2 is effluxed from cells to apoA1, where it is associated with HDL in plasma, and that PIP2 on HDL is taken up by target cells in a scavenger receptor-BI-dependent manner. Mouse plasma PIP2 levels are apoA1 gene dosage-dependent and are >1 μM in apoA1 transgenic mice.

CONCLUSIONS

ABCA1 has PIP2 floppase activity, which increases cell surface PIP2 levels that mediate apoA1 binding and lipid efflux during nascent HDL assembly. We found that PIP2 itself is effluxed to apoA1 and it circulates on plasma HDL, where it can be taken up via the HDL receptor scavenger receptor-BI.

Structural Insights into High Density Lipoprotein: Old Models and New Facts

The physiological link between circulating high density lipoprotein (HDL) levels and cardiovascular disease is well-documented, albeit its intricacies are not well-understood. An improved appreciation of HDL function and overall role in vascular health and disease requires at its foundation a better understanding of the lipoprotein's molecular structure, its formation, and its process of maturation through interactions with various plasma enzymes and cell receptors that intervene along the pathway of reverse cholesterol transport. This review focuses on summarizing recent developments in the field of lipid free apoA-I and HDL structure, with emphasis on new insights revealed by newly published nascent and spherical HDL models constructed by combining low resolution structures obtained from small angle neutron scattering (SANS) with contrast variation and geometrical constraints derived from hydrogen-deuterium exchange (HDX), crosslinking mass spectrometry, electron microscopy, Förster resonance energy transfer, and electron spin resonance. Recently published low resolution structures of nascent and spherical HDL obtained from SANS with contrast variation and isotopic labeling of apolipoprotein A-I (apoA-I) will be critically reviewed and discussed in terms of how they accommodate existing biophysical structural data from alternative approaches. The new low resolution structures revealed and also provided some answers to long standing questions concerning lipid organization and particle maturation of lipoproteins. The review will discuss the merits of newly proposed SANS based all atom models for nascent and spherical HDL, and compare them with accepted models. Finally, naturally occurring and bioengineered mutations in apoA-I, and their impact on HDL phenotype, are reviewed and discuss together with new therapeutics employed for restoring HDL function.

The complex interplay between ligand binding and conformational structure of the folate binding protein (folate receptor): Biological perspectives

This review analyzes how interplay between folate binding and changes in folate binding protein (FBP) conformation/self-association affects the biological function of FBP. Concentration-dependent, reversible self-association of hydrophobic apo-FBP at pI=7.4 is associated with decreased affinity for folate, probably due to shielding of binding sites between interacting hydrophobic patches. Titration with folate removes apo-monomers, favoring dissociation of self-associated apo-FBP into apo-monomers. Folate anchors to FBP through a network of hydrogen bonds and hydrophobic interactions, and the binding induces a conformational change with formation of hydrophilic and stable holo-FBP. Holo-FBP exhibits a ligand-mediated concentration-dependent self-association into multimers of great thermal and chemical stability due to strong intermolecular forces. Both ligand and FBP are thus protected against biological/physicochemical decomposition. In biological fluids with low FBP concentrations, e.g., saliva, semen and plasma, hydrophobic apo-monomers and hydrophilic holo-monomers associate into stable asymmetrical complexes with aberrant binding kinetics unless detergents, e.g., cholesterol or phospholipids are present.

Parkinson-causing α-synuclein missense mutations shift native tetramers to monomers as a mechanism for disease initiation

β-Sheet-rich α-synuclein (αS) aggregates characterize Parkinson's disease (PD). αS was long believed to be a natively unfolded monomer, but recent work suggests it also occurs in α-helix-rich tetramers. Crosslinking traps principally tetrameric αS in intact normal neurons, but not after cell lysis, suggesting a dynamic equilibrium. Here we show that freshly biopsied normal human brain contains abundant αS tetramers. The PD-causing mutation A53T decreases tetramers in mouse brain. Neurons derived from an A53T patient have decreased tetramers. Neurons expressing E46K do also, and adding 1-2 E46K-like mutations into the canonical αS repeat motifs (KTKEGV) further reduces tetramers, decreases αS solubility and induces neurotoxicity and round inclusions. The other three fPD missense mutations likewise decrease tetramer:monomer ratios. The destabilization of physiological tetramers by PD-causing missense mutations and the neurotoxicity and inclusions induced by markedly decreasing tetramers suggest that decreased α-helical tetramers and increased unfolded monomers initiate pathogenesis. Tetramer-stabilizing compounds should prevent this.

Myeloperoxidase-mediated Methionine Oxidation Promotes an Amyloidogenic Outcome for Apolipoprotein A-I

High plasma levels of apolipoprotein A-I (apoA-I) correlate with cardiovascular health, whereas dysfunctional apoA-I is a cause of atherosclerosis. In the atherosclerotic plaques, amyloid deposition increases with aging. Notably, apoA-I is the main component of these amyloids. Recent studies identified high levels of oxidized lipid-free apoA-I in atherosclerotic plaques. Likely, myeloperoxidase (MPO) secreted by activated macrophages in atherosclerotic lesions is the promoter of such apoA-I oxidation. We hypothesized that apoA-I oxidation by MPO levels similar to those present in the artery walls in atherosclerosis can promote apoA-I structural changes and amyloid fibril formation. ApoA-I was exposed to exhaustive chemical (H2O2) oxidation or physiological levels of enzymatic (MPO) oxidation and incubated at 37 °C and pH 6.0 to induce fibril formation. Both chemically and enzymatically oxidized apoA-I produced fibrillar amyloids after a few hours of incubation. The amyloid fibrils were composed of full-length apoA-I with differential oxidation of the three methionines. Met to Leu apoA-I variants were used to establish the predominant role of oxidation of Met-86 and Met-148 in the fibril formation process. Importantly, a small amount of preformed apoA-I fibrils was able to seed amyloid formation in oxidized apoA-I at pH 7.0. In contrast to hereditary amyloidosis, wherein specific mutations of apoA-I cause protein destabilization and amyloid deposition, oxidative conditions similar to those promoted by local inflammation in atherosclerosis are sufficient to transform full-length wild-type apoA-I into an amyloidogenic protein. Thus, MPO-mediated oxidation may be implicated in the mechanism that leads to amyloid deposition in the atherosclerotic plaques in vivo.

Preferential incorporation of shorter and less unsaturated acyl phospholipids into high density lipoprotein-like particles in the ABCA1- and ABCA7-mediated biogenesis with apoA-I

Molecular species of phosphatidylcholine (PC) and sphingomyelin (SPM) were globally analyzed for lipidomics in the nascent high-density lipoprotein (HDL)-like particles generated with human apolipoprotein A-I (apoA-I) form HEK293 cells where either human ATP binding cassette transporter (ABC) A1 or ABCA7 was transfected and overexpressed. SPM/PC ratio was higher in the ABCA1-mediated HDL than ABCA7-mediated HDL likely being related to their cholesterol content, while it was less than the ratio in the cell membrane in either case. Molecular species composition of hydrocarbon chain moiety in each phospholipid in the HDL largely reflected that in the cells the lipoprotein originated in, without remarkable difference between ABCA1 and ABCA7. Further analysis, however, revealed apparent preference for the molecules with shorter hydrocarbon chain length for both PC and SPM in their relative incorporation into HDL by ABCA1 and ABCA7. Likewise, it was in favor for less-unsaturated hydrocarbon chains of PC while this preference was not apparent for SPM. The results are consistent with the view that assembly of HDL particles with extracellular apoA-I is primarily with the cellular phospholipid molecules being regulated in part by their physicochemical nature.

Kinetic and thermodynamic analyses of spontaneous exchange between high-density lipoprotein-bound and lipid-free apolipoprotein A-I

It is thought that apolipoprotein A-I (apoA-I) spontaneously exchanges between high-density lipoprotein (HDL)-bound and lipid-free states, which is relevant to the occurrence of preβ-HDL particles in plasma. To improve our understanding of the mechanistic basis for this phenomenon, we performed kinetic and thermodynamic analyses for apoA-I exchange between discoidal HDL-bound and lipid-free forms using fluorescence-labeled apoA-I variants. Gel filtration experiments demonstrated that addition of excess lipid-free apoA-I to discoidal HDL particles promotes exchange of apoA-I between HDL-associated and lipid-free pools without alteration of the steady-state HDL particle size. Kinetic analysis of time-dependent changes in NBD fluorescence upon the transition of NBD-labeled apoA-I from HDL-bound to lipid-free state indicates that the exchange kinetics are independent of the collision frequency between HDL-bound and lipid-free apoA-I, in which the lipid binding ability of apoA-I affects the rate of association of lipid-free apoA-I with the HDL particles and not the rate of dissociation of HDL-bound apoA-I. Thus, C-terminal truncations or mutations that reduce the lipid binding affinity of apoA-I strongly impair the transition of lipid-free apoA-I to the HDL-bound state. Thermodynamic analysis of the exchange kinetics demonstrated that the apoA-I exchange process is enthalpically unfavorable but entropically favorable. These results explain the thermodynamic basis of the spontaneous exchange reaction of apoA-I associated with HDL particles. The altered exchangeability of dysfunctional apoA-I would affect HDL particle rearrangement, leading to perturbed HDL metabolism.

Direct detection of ABCA1-dependent HDL formation based on lipidation-induced hydrophobicity change in apoA-I

ABCA1 mediates the efflux of cholesterol and phospholipids into apoA-I to form HDL, which is important in the prevention of atherosclerosis. To develop a novel method for the evaluation of HDL formation, we prepared an apoA-I-POLARIC by labeling the specific residue of an apoA-I variant with a hydrophobicity-sensitive fluorescence probe that detects the environmental change around apoA-I during HDL formation. apoA-I-POLARIC possesses the intact ABCA1-dependent HDL formation activity and shows 4.0-fold higher fluorescence intensity in HDL particles than in the lipid-free state. Incubation of apoA-I-POLARIC with ABCA1-expressing cells, but not ABCA1-non-expressing cells, caused a 1.7-fold increase in fluorescence intensity. Gel filtration analysis demonstrated that the increase in fluorescence intensity of apoA-I-POLARIC represents the amount of apoA-I incorporated into the discoidal HDL particles rather than the amount of secreted cholesterol. THP-1 macrophage-mediated HDL formation and inhibition of HDL formation by cyclosporine A could also be measured using apoA-I-POLARIC. Furthermore, HDL formation-independent lipid release induced by microparticle formation or cell death was not detected by apoA-I-POLARIC. These results demonstrate that HDL formation by ABCA1-expressing cells can be specifically detected by sensing hydrophobicity change in apoA-I, thus providing a novel method for assessing HDL formation and screening of the HDL formation modulator.

Proteolysis of apolipoprotein A-I by secretory phospholipase A₂: a new link between inflammation and atherosclerosis

In the acute phase of the inflammatory response, secretory phospholipase A2 (sPLA2) reaches its maximum levels in plasma, where it is mostly associated with high density lipoproteins (HDL). Overexpression of human sPLA2 in transgenic mice reduces both HDL cholesterol and apolipoprotein A-I (apoA-I) plasma levels through increased HDL catabolism by an unknown mechanism. To identify unknown PLA2-mediated activities on the molecular components of HDL, we characterized the protein and lipid products of the PLA2 reaction with HDL. Consistent with previous studies, hydrolysis of HDL phospholipids by PLA2 reduced the particle size without changing its protein composition. However, when HDL was destabilized in the presence of PLA2 by the action of cholesteryl ester transfer protein or by guanidine hydrochloride treatment, a fraction of apoA-I, but no other proteins, dissociated from the particle and was rapidly cleaved. Incubation of PLA2 with lipid-free apoA-I produced similar protein fragments in the range of 6-15 kDa, suggesting specific and direct reaction of PLA2 with apoA-I. Mass spectrometry analysis of isolated proteolytic fragments indicated at least two major cleavage sites at the C-terminal and the central domain of apoA-I. ApoA-I proteolysis by PLA2 was Ca(2+)-independent, implicating a different mechanism from the Ca(2+)-dependent PLA2-mediated phospholipid hydrolysis. Inhibition of proteolysis by benzamidine suggests that the proteolytic and lipolytic activities of PLA2 proceed through different mechanisms. Our study identifies a previously unknown proteolytic activity of PLA2 that is specific to apoA-I and may contribute to the enhanced catabolism of apoA-I in inflammation and atherosclerosis.

The conformation of lipid-free human apolipoprotein A-I in solution

Apolipoprotein AI (apoA-I) is the principal acceptor of lipids from ATP-binding cassette transporter A1, a process that yields nascent high density lipoproteins. Analysis of lipidated apoA-I conformation yields a belt or twisted belt in which two strands of apoA-I lie antiparallel to one another. In contrast, biophysical studies have suggested that a part of lipid-free apoA-I was arranged in a four-helix bundle. To understand how lipid-free apoA-I opens from a bundle to a belt while accepting lipid it was necessary to have a more refined model for the conformation of lipid-free apoA-I. This study reports the conformation of lipid-free human apoA-I using lysine-to-lysine chemical cross-linking in conjunction with disulfide cross-linking achieved using selective cysteine mutations. After proteolysis, cross-linked peptides were verified by sequencing using tandem mass spectrometry. The resulting structure is compact with roughly four helical regions, amino acids 44-186, bundled together. C- and N-terminal ends, amino acids 1-43 and 187-243, respectively, are folded such that they lie close to one another. An unusual feature of the molecule is the high degree of connectivity of lysine40 with six other lysines, lysines that are close, for example, lysine59, to distant lysines, for example, lysine239, that are at the opposite end of the primary sequence. These results are compared and contrasted with other reported conformations for lipid-free human apoA-I and an NMR study of mouse apoA-I.

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.

The association of a distinct plasma proteomic profile with the cervical high-grade squamous intraepithelial lesion of Uyghur women: a 2D liquid-phase chromatography/mass spectrometry study

OBJECTIVE

To identify plasma protein biomarkers of cervical high-grade squamous intraepithelial lesion (HSIL) of Uyghur women by proteomics approach.

METHODS

Plasma protein samples of Uyghur women with HSIL and chronic cervicitis were analyzed with 2D HPLC followed by detection of target proteins with Linear Trap Quadrupole Mass Spectrometer (LTQ MS/MS).

RESULTS

We detected three upregulated and one downregulated protein peaks representing protein constituents distinguishing HSIL from controls by 2D HPLC, identified 31 target proteins by LTQ MS/MS. Further confirmed analysis with online software IPA® 8.7 and ELISA assay showed APOA1 and mTOR as potential biomarkers.

CONCLUSIONS

A distinct plasma proteomic profile may be associated with HSIL of Uyghur women.

The fibrillogenic L178H variant of apolipoprotein A-I forms helical fibrils

A number of amyloidogenic variants of apoA-I have been discovered but most have not been analyzed. Previously, we showed that the G26R mutation of apoA-I leads to increased β-strand structure, increased N-terminal protease susceptibility, and increased fibril formation after several days of incubation. In vivo, this and other variants mutated in the N-terminal domain (residues 26 to ∼90) lead to renal and hepatic accumulation. In contrast, several mutations identified within residues 170 to 178 lead to cardiac, laryngeal, and cutaneous protein deposition. Here, we describe the structural changes in the fibrillogenic variant L178H. Like G26R, the initial structure of the protein exhibits altered tertiary conformation relative to wild-type protein along with decreased stability and an altered lipid binding profile. However, in contrast to G26R, L178H undergoes an increase in helical structure upon incubation at 37°C with a half time (t_1/2) of about 12 days. Upon prolonged incubation, the L178H mutant forms fibrils of a diameter of 10 nm that ranges in length from 30 to 120 nm. These results show that apoA-I, known for its dynamic properties, has the ability to form multiple fibrillar conformations, which may play a role in the tissue-specific deposition of the individual variants.