Preparation and incorporation of probe-labeled apoA-I for fluorescence resonance energy transfer studies of rHDL

Apolipoprotein A-I carboxy-terminal domain residues 187-243 are required for adiponectin-induced cholesterol efflux

Adiponectin exerts its atheroprotection by stimulating adenosine triphosphate binding cassette transporter A1 (ABCA1)-mediated cholesterol efflux to apolipoprotein A-I (apoA-I). However, involvement of the apoA-I residues in this process have not been studied. In Tamm-Horsfall 1 (THP-1) macrophages and baby hamster kidney (BHK) cells we assessed adiponectin's potential to restore cholesterol efflux in the presence of apoA-I and ABCA1 mutants, respectively. Adiponectin was unable to restore efflux from THP-1 macrophages in the presence of apoA-I carboxy-terminal domain (CTD) successive mutants from residues 187-243 versus apoA-I mutants alone. Furthermore, adiponectin did not significantly influence cholesterol efflux to apoA-I from BHK-ABCA1 mutant cells. Adiponectin appears to require functional apoA-I CTD residues 187-243 and wild-type ABCA1 to mediate efficient cholesterol efflux from THP-1 macrophages and BHK cells, respectively. Therefore, adiponectin cannot rescue defective cholesterol efflux in apoA-I- or ABCA1-mutant conditions, but rather increases cholesterol efflux in wild-type apoA-I conditions compared to apoA-I exposure alone.

Purification and HDL-like particle formation of apolipoprotein A-I after co-expression with the EDDIE mutant of Npro autoprotease

Apolipoprotein A-I (ApoA-I) is the major protein constituent of high-density lipoprotein particles, and as such is involved in cholesterol transport and activation of LCAT (the lecithin:cholesterol acyltransferase). It may also form amyloidal deposits in the body, showing the multifaceted interactions of ApoA-I. In order to facilitate the study of ApoA-I in various systems, we have developed a protocol based on recombinant expression in E. coli. ApoA-I is protected from degradation by driving its expression to inclusion bodies using a tag: the EDDIE mutant of Npro autoprotease from classical swine fever virus. Upon refolding, EDDIE will cleave itself off from the target protein. The result is a tag-free ApoA-I, with its N-terminus intact. ApoA-I was then purified using a five-step procedure composed of anion exchange chromatography, immobilized metal ion affinity chromatography, hydrophobic interaction chromatography, boiling and size exclusion chromatography. This led to protein of high purity as confirmed with SDS-PAGE and mass spectrometry. The purified ApoA-I formed discoidal objects in the presence of zwitterionic phospholipid DMPC, showing its retained function of interacting with lipids. The protocol was also tested by expression and purification of two ApoA-I mutants, both of which could be purified in the same manner as the wildtype, showing the robustness of the protocol.

Interleukin-17 Drives Interstitial Entrapment of Tissue Lipoproteins in Experimental Psoriasis

Lipoproteins trapped in arteries drive atherosclerosis. Extravascular low-density lipoprotein undergoes receptor uptake, whereas high-density lipoprotein (HDL) interacts with cells to acquire cholesterol and then recirculates to plasma. We developed photoactivatable apoA-I to understand how HDL passage through tissue is regulated. We focused on skin and arteries of healthy mice versus those with psoriasis, which carries cardiovascular risk in man. Our findings suggest that psoriasis-affected skin lesions program interleukin-17-producing T cells in draining lymph nodes to home to distal skin and later to arteries. There, these cells mediate thickening of the collagenous matrix, such that larger molecules including lipoproteins become entrapped. HDL transit was rescued by depleting CD4⁺ T cells, neutralizing interleukin-17, or inhibiting lysyl oxidase that crosslinks collagen. Experimental psoriasis also increased vascular stiffness and atherosclerosis via this common pathway. Thus, interleukin-17 can reduce lipoprotein trafficking and increase vascular stiffness by, at least in part, remodeling collagen.

High-Density Lipoprotein Biogenesis: Defining the Domains Involved in Human Apolipoprotein A-I Lipidation

The first step in removing cholesterol from a cell is the ATP-binding cassette transporter 1 (ABCA1)-driven transfer of cholesterol to lipid-free or lipid-poor apolipoprotein A-I (apoA-I), which yields cholesterol-rich nascent high-density lipoprotein (nHDL) that then matures in plasma to spherical, cholesteryl ester-rich HDL. However, lipid-free apoA-I has a three-dimensional (3D) conformation that is significantly different from that of lipidated apoA-I on nHDL. By comparing the lipid-free apoA-I 3D conformation of apoA-I to that of 9-14 nm diameter nHDL, we formulated the hypothetical helical domain transitions that might drive particle formation. To test the hypothesis, ten apoA-I mutants were prepared that contained two strategically placed cysteines several of which could form intramolecular disulfide bonds and others that could not form these bonds. Mass spectrometry was used to identify amino acid sequence and intramolecular disulfide bond formation. Recombinant HDL (rHDL) formation was assessed with this group of apoA-I mutants. ABCA1-driven nHDL formation was measured in four mutants and wild-type apoA-I. The mutants contained cysteine substitutions in one of three regions: the N-terminus, amino acids 34 and 55 (E34C to S55C), central domain amino acids 104 and 162 (F104C to H162C), and the C-terminus, amino acids 200 and 233 (L200C to L233C). Mutants were studied in the locked form, with an intramolecular disulfide bond present, or unlocked form, with the cysteine thiol blocked by alkylation. Only small amounts of rHDL or nHDL were formed upon locking the central domain. We conclude that both the N- and C-terminal ends assist in the initial steps in lipid acquisition, but that opening of the central domain was essential for particle formation.

Acrolein impairs the cholesterol transport functions of high density lipoproteins

High density lipoproteins (HDL) are considered athero-protective, primarily due to their role in reverse cholesterol transport, where they transport cholesterol from peripheral tissues to the liver for excretion. The current study was designed to determine the impact of HDL modification by acrolein, a highly reactive aldehyde found in high abundance in cigarette smoke, on the cholesterol transport functions of HDL. HDL was chemically-modified with acrolein and immunoblot and mass spectrometry analyses confirmed apolipoprotein crosslinking, as well as acrolein adducts on apolipoproteins A-I and A-II. The ability of acrolein-modified HDL (acro-HDL) to serve as an acceptor of free cholesterol (FC) from COS-7 cells transiently expressing SR-BI was significantly decreased. Further, in contrast to native HDL, acro-HDL promotes higher neutral lipid accumulation in murine macrophages as judged by Oil Red O staining. The ability of acro-HDL to mediate efficient selective uptake of HDL-cholesteryl esters (CE) into SR-BI-expressing cells was reduced compared to native HDL. Together, the findings from our studies suggest that acrolein modification of HDL produces a dysfunctional particle that may ultimately promote atherogenesis by impairing functions that are critical in the reverse cholesterol transport pathway.

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.

Mimicry of high-density lipoprotein: functional peptide-lipid nanoparticles based on multivalent peptide constructs

We describe an approach for engineering peptide-lipid nanoparticles that function similarly to high-density lipoprotein (HDL). Branched, multivalent constructs, bearing multiple 23- or 16-amino-acid peptides, were designed, synthesized, and combined with phospholipids to produce nanometer-scale discoidal HDL-like particles. A variety of biophysical techniques were employed to characterize the constructs, including size exclusion chromatography, analytical ultracentrifuge sedimentation, circular dichroism, transmission electron microscopy, and fluorescence spectroscopy. The nanoparticles functioned in vitro (human and mouse plasma) and in vivo (mice) to rapidly remodel large native HDLs into small lipid-poor HDL particles, which are key acceptors of cholesterol in reverse cholesterol transport. Fluorescent labeling studies showed that the constituents of the nanoparticles readily distributed into native HDLs, such that the peptide constructs coexisted with apolipoprotein A-I (apoA-I), the main structural protein in HDLs. Importantly, nanolipid particles containing multivalent peptides promoted efficient cellular cholesterol efflux and were functionally superior to those derived from monomeric apoA-I mimetic peptides. The multivalent peptide-lipid nanoparticles were also remarkably stable toward enzymatic digestion in vitro and displayed long half-lives and desirable pharmacokinetic profiles in mice, providing a real practical advantage over previously studied linear or tandem helical peptides. Encouragingly, a two-week exploratory efficacy study in a widely used animal model for atherosclerosis research (LDLr-null mice) using nanoparticles constructed from a trimeric peptide demonstrated an exceptional 50% reduction in the plasma total cholesterol levels compared to the control group. Altogether, the studies reported here point to an attractive avenue for designing synthetic, HDL-like nanoparticles, with potential for treating atherosclerosis.

Conformation of dimeric apolipoprotein A-I milano on recombinant lipoprotein particles

Apolipoprotein A-I Milano (apoA-I(Milano)) is a naturally occurring human mutation of wild-type apolipoprotein A-I (apoA-I(WT)) having cystine substituted for arginine(173). Two molecules of apo-I(WT) form disks with phospholipid having a defined relationship between the apoA-I(WT) molecules. ApoA-I(Milano) forms cystine homodimers that would not allow the protein to adopt the conformation reported for apoA-I(WT). The conformational constraints for dimeric apoA-I(Milano) recombinant high-density lipoprotein (rHDL) disks made with phospholipid were deduced from a combination of chemical cross-linking and mass spectrometry. Lysine-selective homobifunctional cross-linkers were reacted with homogeneous rHDL having diameters of 78 and 125 A. After reduction, cross-linked apoA-I(Milano) was separated from monomeric apoprotein by gel electrophoresis and then subjected to in-gel trypsin digest. Cross-linked peptides were confirmed by MS/MS sequencing. The cross-links provided distance constraints that were used to refine models of lipid-bound dimeric apoA-I(Milano). These studies suggest that a single dimeric apoA-I(Milano) on 78 A diameter rHDL girdles the edge of a phospholipid disk assuming a "belt" conformation similar to the "belt" region of apoA-I(WT) on rHDL. However, the C-terminal end of dimeric apoA-I(Milano) wraps around the periphery of the particle to shield the fatty acid chains from water rather than folding back onto the "belt" as does apoA-I(WT). The two apoA-I(Milano) dimers on a 125 A diameter rHDL do not encircle the periphery of a phospholipid disk but appear to reside on the surface of a laminar micelle.

Double superhelix model of high density lipoprotein

High density lipoprotein (HDL), the carrier of so-called "good" cholesterol, serves as the major athero-protective lipoprotein and has emerged as a key therapeutic target for cardiovascular disease. We applied small angle neutron scattering (SANS) with contrast variation and selective isotopic deuteration to the study of nascent HDL to obtain the low resolution structure in solution of the overall time-averaged conformation of apolipoprotein AI (apoA-I) versus the lipid (acyl chain) core of the particle. Remarkably, apoA-I is observed to possess an open helical shape that wraps around a central ellipsoidal lipid phase. Using the low resolution SANS shapes of the protein and lipid core as scaffolding, an all-atom computational model for the protein and lipid components of nascent HDL was developed by integrating complementary structural data from hydrogen/deuterium exchange mass spectrometry and previously published constraints from multiple biophysical techniques. Both SANS data and the new computational model, the double superhelix model, suggest an unexpected structural arrangement of protein and lipids of nascent HDL, an anti-parallel double superhelix wrapped around an ellipsoidal lipid phase. The protein and lipid organization in nascent HDL envisages a potential generalized mechanism for lipoprotein biogenesis and remodeling, biological processes critical to sterol and lipid transport, organismal energy metabolism, and innate immunity.

Three-dimensional models of HDL apoA-I: implications for its assembly and function

The purpose of this review is to highlight recent advances toward the refinement of a three-dimensional structure for lipid-bound apolipoprotein A-I (apoA-I) on recombinant HDL. Recently, X-ray crystallography has yielded a new structure for full-length, lipid-free apoA-I. Although this approach has not yet been successful in solving the three-dimensional structure of lipid-bound apoA-I, analysis of the X-ray structures has been of immense help in the interpretation of structural data obtained from other methods that yield structural information. Recent studies emphasize the use of mass spectrometry to unambiguously identify cross-linked peptides or to quantify solvent accessibility using hydrogen-deuterium exchange. The combination of mass spectrometry, molecular modeling, molecular dynamic analysis, and small-angle X-ray diffraction has provided additional structural information on apoA-I folding that complements previous approaches.

Ratio determination of plasma wild-type and L159R apoA-I using mass spectrometry: tools for studying apoA-IFin

In this report, methods are described to isolate milligram quantities of a mutant apolipoprotein A-I (apoA-I) protein for use in structure-function studies. Expression of the L159R apoA-I mutation in humans reduces the concentration of plasma wild-type apoA-I, thus displaying a dominant negative phenotype in vivo. Earlier attempts to express and isolate this mutant protein resulted in extensive degradation and protein misfolding. Using an Escherichia coli expression system used predominantly for the isolation of soluble apoA-I mutant proteins, we describe the expression and purification of L159R apoA-I (apoA-I(Fin)) from inclusion bodies. In addition, we describe a mass spectrometric method for measuring the L159R-to-wild-type apoA-I ratio in a 1 microl plasma sample. These new methods will facilitate further studies into the mechanism behind the dominant negative phenotype associated with the expression of the L159R apoA-I protein in humans.

Intermolecular contact between globular N-terminal fold and C-terminal domain of ApoA-I stabilizes its lipid-bound conformation: studies employing chemical cross-linking and mass spectrometry

The structure of apoA-I on discoidal high density lipoprotein (HDL) was studied using a combination of chemical cross-linking and mass spectrometry. Recombinant HDL particles containing 145 molecules of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine and two molecules of apoA-I with a 96-A diameter were treated with the lysine-specific cross-linker, dithiobis(succinimidylpropionate) at varying molar ratios from 2:1 to 200:1. At low molar ratios of dithiobis(succinimidylpropionate) to apoA-I, two products were obtained corresponding to approximately 53 and approximately 80 kDa. At high molar ratios, these two products merged, yielding a product of approximately 59 kDa, close to the theoretical molecular mass of dimeric apoA-I. To identify the intermolecular cross-links giving rise to the two different sized products, bands were excised from the gel, digested with trypsin, and then analyzed by liquid chromatography-electrospray-tandem mass spectrometry. In addition, tandem mass spectrometry of unique cross-links found in the 53- and 80-kDa products suggested that a distinct conformation exists for lipid-bound apoA-I on 96-A recombinant HDL, emphasizing the inherent flexibility and malleability of the N termini and its interaction with its C-terminal domain.

Quality control in the apoA-I secretory pathway

From a total of 47 known apolipoprotein A-I (apoA-I) mutations, only 18 are linked to low plasma HDL apoA-I concentrations, and 78% of these map to apoA-I helices 6 and 7 (residues 143-186). Gene transfer and transgenic mouse studies have shown that several helix 6 apoA-I mutations have reduced hepatic HDL production. Our objective was to examine the impact of helix 6 modifications on intracellular biosynthetic processing and secretion of apoA-I. Cells were transfected with wild-type or mutant apoA-I, radiolabeled with [(35)S]Met/Cys, and then placed in unlabeled medium for up to 4 h. Results show that >90% of newly synthesized wild-type apoA-I was secreted by 60 min. Over the same length of time, only 20% of helix 6 deletion mutant (Delta 6 apoA-I) was secreted, whereas 80% remained cell associated. Microscopic and biochemical studies revealed that cell-associated Delta 6 apoA-I was located predominantly within the cytoplasm as lipid-protein inclusions, whereas wild-type apoA-I was localized in the endoplasmic reticulum/Golgi. Results using other helix deletions or helix 6 substitution mutations indicated that only complete removal of helix 6 resulted in massive cytoplasmic accumulation. These data suggest that alterations in native apoA-I conformation can lead to aberrant trafficking and accumulation of apolipoprotein-phospholipid structures. Thus, conformation-dependent alterations in intracellular trafficking and turnover may underlie the reduced plasma HDL concentrations observed in individuals harboring deletion mutations within helix 6.

Structure-function relationships of apolipoprotein A-I: a flexible protein with dynamic lipid associations

PURPOSE OF REVIEW

Apolipoprotein A-I is the major structural protein of HDL. Its physicochemical properties maintain a delicate balance between maintenance of stable lipoproteins and the ability to associate with and dissociate from the lipid transported. Here we review the progress made in the last 2-3 years on the structure-function relationships of apolipoprotein A-I, including elements related to the ATP binding cassette transporter A1.

RECENT FINDINGS

Current evidence now supports the so-called 'belt' or 'hairpin' models for apolipoprotein A-I conformation when bound to discoidal lipoproteins. In-vivo expression of apolipoprotein A-I mutant proteins has shown that both the N- and C-terminal domains are important for lipid association as well as for the esterification reaction, particularly binding of cholesteryl esters and formation of mature alpha-migrating lipoproteins. This property is apparently quite distinct from the activation of the enzyme lecithin cholesterol acyl transferase, which requires interaction with the central helix 6. The interaction of apolipoprotein A-I with the ATP binding cassette transporter A1 has been shown to require the C-terminal domain, which is proposed to mediate the opening of the helix bundle formed by lipid-free or lipid-poor apolipoprotein A-I and allow its association with hydrophobic binding sites.

SUMMARY

Significant progress has been made in the understanding of the molecular mechanisms controlling the folding of apolipoprotein A-I and its interaction with lipids and various other protein factors involved in HDL metabolism.

Optimized bacterial expression of human apolipoprotein A-I

Apolipoprotein A-I (apoA-I) serves critical functions in plasma lipoprotein metabolism as a structural component of high density lipoprotein, activator of lecithin:cholesterol acyltransferase, and acceptor of cellular cholesterol as part of the reverse cholesterol transport pathway. In an effort to facilitate structure:function studies of human apoA-I, we have optimized a plasmid vector for production of recombinant wild type (WT) and mutant apoA-I in bacteria. To facilitate mutagenesis studies, subcloning, and DNA manipulation, numerous silent mutations have been introduced into the apoA-I cDNA, generating 13 unique restriction endonuclease sites. The coding sequence for human apoA-I has been modified by the introduction of additional silent mutations that eliminate 18 separate codons that employ tRNAs that are of low or moderate abundance in Escherichia coli. Yields of recombinant apoA-I achieved using the optimized cDNA were 100+/-20 mg/L bacterial culture, more than fivefold greater than yields routinely obtained with the original cDNA. Site-directed mutagenesis of the apoA-I cDNA was performed to generate a Glu2Asp mutation in the N-terminal sequence of apoA-I. This modification, which creates an acid labile Asp-Pro peptide bond between amino acids 2 and 3, permits specific chemical cleavage of an N-terminal His-Tag fusion peptide used for rapid protein purification. The product protein's primary structure is identical to WT apoA-I in all other respects. Together, these changes in apoA-I cDNA and bacterial expression protocol significantly improve the yield of apoA-I protein without compromising the relative ease of purification.

ApoA-I structure on discs and spheres. Variable helix registry and conformational states

Apolipoprotein A-I (apoA-I) readily forms discoidal high density lipoprotein (HDL) particles with phospholipids serving as an ideal transporter of plasma cholesterol. In the lipid-bound conformation, apoA-I activates the enzyme lecithin:cholesterol acyltransferase stimulating the formation of cholesterol esters from free cholesterol. As esterification proceeds cholesterol esters accumulate within the hydrophobic core of the discoidal phospholipid bilayer transforming it into a spherical HDL particle. To investigate the change in apoA-I conformation as it adapts to a spherical surface, fluorescence resonance energy transfer studies were performed. Discoidal rHDL particles containing two lipid-bound apoA-I molecules were prepared with acceptor and donor fluorescent probes attached to cysteine residues located at specific positions. Fluorescence quenching was measured for probe combinations located within repeats 5 and 5 (residue 132), repeats 5 and 6 (residues 132 and 154), and repeats 6 and 6 (residue 154). Results from these experiments indicated that each of the 2 molecules of discoidal bound apoA-I exists in multiple conformations and support the concept of a "variable registry" rather than a "fixed helix-helix registry." Additionally, discoidal rHDL were transformed in vitro to core-containing particles by incubation with lecithin:cholesterol acyltransferase. Compositional analysis showed that core-containing particles contained 11% less phospholipid and 633% more cholesterol ester and a total of 3 apoA-I molecules per particle. Spherical particles showed a lowering of acceptor to donor probe quenching when compared with starting rHDL. Therefore, we conclude that as lipid-bound apoA-I adjusts from a discoidal to a spherical surface its intermolecular interactions are significantly reduced presumably to cover the increased surface area of the particle.