The low density lipoprotein receptor active conformation of apolipoprotein E. Helix organization in n-terminal domain-phospholipid disc particles

Apolipoprotein E3 Containing Nanodiscs as Vehicles for Transport and Targeted Delivery of Flavonoid Luteolin

Luteolin is a flavonoid that possesses multiple beneficial biological properties, such as anticancer, antioxidant, and anti-inflammatory effects. The objective of this study is to test the hypothesis that luteolin can be transported across a cell via a nanodisc delivery system and delivered to intracellular sites. Luteolin was incorporated into reconstituted high-density lipoprotein complexes made up of apolipoprotein E3 (apoE3) N-terminal domain (apoE3NT) and 1,2-dimystrioyl-sn-glycero-3-phosphocholine. ApoE3NT confers the ability on nanodiscs to traverse the plasma membrane via low-density lipoprotein receptor or scavenger receptor-B1. Physicochemical characterization revealed that the nanodiscs were 17-22 nm in diameter as demonstrated by native polyacrylamide gel electrophoresis and dynamic lightering analysis and ∼660 kDa in size, with a luteolin content of ∼4 luteolin molecules/nanodisc. Luteolin appeared to be embedded in the nonpolar core of nanodiscs, as revealed by fluorescence quenching and polarization analysis and spectroscopic characterization. The presence of luteolin did not affect the ability of apoE3NT to mediate binding and cellular uptake of luteolin containing nanodiscs in macrophages, as inferred from immunofluorescence analysis that revealed apoE- and lipid-related fluorescence as punctate perinuclear vesicles and from flow cytometry studies. Lastly, luteolin appeared to be localized in the nucleus, having escaped the lysosomes following disassembly of the nanodiscs as suggested by fluorescence spectroscopy and microscopy analyses. Taken together, nanodiscs offer the potential to effectively transport luteolin and potentially therapeutic drugs into perinuclear sites in cells, where they can be available to enter the nucleus.

Molecular dynamics simulation of apolipoprotein E3 lipid nanodiscs

Nanodiscs are binary discoidal complexes of a phospholipid bilayer circumscribed by belt-like helical scaffold proteins. Using coarse-grained and all-atom molecular dynamics simulations, we explore the stability, size, and structure of nanodiscs formed between the N-terminal domain of apolipoprotein E3 (apoE3-NT) and variable number of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) molecules. We study both parallel and antiparallel double-belt configurations, consisting of four proteins per nanodisc. Our simulations predict nanodiscs containing between 240 and 420 DMPC molecules to be stable. The antiparallel configurations exhibit an average of 1.6 times more amino acid interactions between protein chains and 2 times more ionic contacts, compared to the parallel configuration. With one exception, DMPC order parameters are consistently larger in the antiparallel configuration than in the parallel one. In most cases, the root mean square deviation of the positions of the protein backbone atoms is smaller in the antiparallel configuration. We further report nanodisc size, thickness, radius of gyration, and solvent accessible surface area. Combining all investigated parameters, we hypothesize the antiparallel protein configuration leading to more stable and more rigid nanodiscs than the parallel one.

Insights into the C-terminal domain of apolipoprotein E from chimera studies with apolipophorin III

Apolipoprotein E3 (apoE) is a critical cholesterol transport protein in humans and is composed of two domains: a well characterized N-terminal (NT) domain that harbors the low-density lipoprotein LDL receptor, and a less understood C-terminal (CT) domain that is the site of protein oligomerization and initiation of lipid binding. To better understand the domain structure of apoE, the CT domain was fused to apolipophorin III (apoLp-III), a single-domain, monomeric apolipoprotein of insect origin, to yield a chimeric protein, apoLp-III/CT-apoE. Recombinant apoLp-III/CT-apoE maintained an overall helical content similar to that of the parent proteins, while chemical induced unfolding studies indicated that its structural integrity was not compromised. Analysis using 1-anilinonaphthalene-8-sulfonic acid (ANS), a sensitive fluorescent indicator of exposed hydrophobic sites and protein folding, demonstrated that whereas apoLp-III provided few ANS binding sites, apoLp-III/CT-apoE harbored an abundance of ANS binding sites. Thus, this indicated tertiary structure formation in CT-apoE when part of the chimera. Size-exclusion chromatography and chemical crosslinking analysis demonstrated that while apoLp-III is monomeric, the chimeric protein formed large oligomeric complexes, similar to native apoE3. Compared to apoLp-III, the chimera showed a two-fold enhancement in phospholipid vesicle solubilization rates and a significantly improved ability to bind to lipolyzed low-density lipoprotein, preventing the onset of lipoprotein aggregation at concentrations comparable to that of parent CT-apoE. These results confirm that high lipid binding and self-association sites are located in the CT domain of apoE, and that these properties can be transferred to an unrelated apolipoprotein, demonstrating that these properties operate independently from the NT domain.

The LDL receptor binding domain of apolipoprotein E directs the relative orientation of its C-terminal segment in reconstituted nascent HDL

Apolipoprotein E (apoE) (299 residues) is a highly helical protein that plays a critical role in cholesterol homeostasis. It comprises a four-helix bundle N-terminal (NT) and a C-terminal (CT) domain that can exist in lipid-free and lipid-associated states. In humans, there are two major apoE isoforms, apoE3 and apoE4, which differ in a single residue in the NT domain, with apoE4 strongly increasing risk of Alzheimer's disease (AD) and cardiovascular diseases (CVD). It has been proposed that the CT domain initiates rapid lipid binding, followed by a slower NT domain helix bundle opening and lipid binding to yield discoidal reconstituted high density lipoprotein (rHDL). However, the contribution of the NT domain on the CT domain organization in HDL remains poorly understood. To understand this, we employed Cys-specific cross-linking and spatially-sensitive fluorophores in the NT and CT domains of apoE3 and apoE4, and in isolated CT domain. We noted that the helices in isolated CT domain are oriented parallel to those in the neighboring molecule in rHDL, whereas full length apoE3 and apoE4 adopt either an anti-parallel or hairpin-like organization. It appears that the bulky NT domain determines the spatial organization of its CT domain in rHDL, a finding that has significance for apoE4, which is more susceptible to proteolytic cleavage in AD brains, showing increased accumulation of neurotoxic NT and CT fragments. We envisage that the structural organization of HDL apoE would have profound functional consequences in its ability to regulate cholesterol homeostasis in AD and CVD.

Conformational Reorganization of Apolipoprotein E Triggered by Phospholipid Assembly

Apolipoprotein E (apoE), a major determinant protein for lipid metabolism, actively participates in lipid transport in the central nervous system via high-affinity interaction with the low-density lipoprotein receptor (LDLR). Prior evidences indicate that the phospholipids first need to assemble around apoE before the protein can recognize its receptor. However, despite multiple attempts via spectroscopic and biochemical investigations, it is unclear what are the impacts of lipid assembly on the globular structure of apoE. Here, using a combination of all-atom and coarse-grained molecular dynamics simulations, we demonstrate that an otherwise compact tertiary fold of monomeric apoE3 spontaneously unwraps in an aqueous phospholipid solution in two distinct stages. Interestingly, these structural reorganizations are triggered by an initial localized binding of lipid molecules to the C-terminal domain of the protein, which induce a rapid separation of the C-terminal domain of apoE3 from the rest of its tertiary fold. This is followed by a slow lipid-induced interhelix separation event within the N-terminal domain of the protein, as seen in an extensively long coarse-grained simulation. Remarkably, the resultant complex takes the shape of an "open conformation" of the lipid-stabilized unwrapped protein, which intriguingly coincides with an earlier proposal by a small-angle X-ray scattering (SAXS) experiment. The lipid-binding activity and the lipid-induced protein conformation are found to be robust across a monomeric mutant and wild-type sequence of apoE3. The "open" complex derived in coarse-grained simulation retains its structural morphology after reverse-mapping to the all-atom representation. Collectively, the investigation puts forward a plausible structure of currently elusive conformationally activated state of apoE3, which is primed for recognition by the lipoprotein receptor and can be exploited for eventual lipid transport.

ApoE and ApoE Nascent-Like HDL Particles at Model Cellular Membranes: Effect of Protein Isoform and Membrane Composition

Apolipoprotein E (ApoE), an important mediator of lipid transportation in plasma and the nervous system, plays a large role in diseases such as atherosclerosis and Alzheimer's. The major allele variants ApoE3 and ApoE4 differ only by one amino acid. However, this difference has major consequences for the physiological behaviour of each variant. In this paper, we follow (i) the initial interaction of lipid-free ApoE variants with model membranes as a function of lipid saturation, (ii) the formation of reconstituted High-Density Lipoprotein-like particles (rHDL) and their structural characterisation, and (iii) the rHDL ability to exchange lipids with model membranes made of saturated lipids in the presence and absence of cholesterol [1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) or 1-palmitoyl-2-oleoyl-glycero-3-phosphocholine (POPC) with and without 20 mol% cholesterol]. Our neutron reflection results demonstrate that the protein variants interact differently with the model membranes, adopting different protein conformations. Moreover, the ApoE3 structure at the model membrane is sensitive to the level of lipid unsaturation. Small-angle neutron scattering shows that the ApoE containing lipid particles form elliptical disc-like structures, similar in shape but larger than nascent or discoidal HDL based on Apolipoprotein A1 (ApoA1). Neutron reflection shows that ApoE-rHDL do not remove cholesterol but rather exchange saturated lipids, as occurs in the brain. In contrast, ApoA1-containing particles remove and exchange lipids to a greater extent as occurs elsewhere in the body.

Apolipoprotein E: Structural Insights and Links to Alzheimer Disease Pathogenesis

Apolipoprotein E (ApoE) is of great interest due to its role as a cholesterol/lipid transporter in the central nervous system (CNS) and as the most influential genetic risk factor for Alzheimer disease (AD). Work over the last four decades has given us important insights into the structure of ApoE and how this might impact the neuropathology and pathogenesis of AD. In this review, we highlight the history and progress in the structural and molecular understanding of ApoE and discuss how these studies on ApoE have illuminated the physiology of ApoE, receptor binding, and interaction with amyloid-β (Aβ). We also identify future areas of study needed to advance our understanding of how ApoE influences neurodegeneration.

A convenient protein library for spectroscopic calibrations

While several Raman, CD or FTIR spectral libraries are available for well-characterized proteins of known structure, proteins themselves are usually very difficult to acquire, preventing a convenient calibration of new instruments and new recording methods. The problem is particularly critical in the field of FTIR spectroscopy where numerous new methods are becoming available on the market.

The present papers reports the construction of a protein library (cSP92) including commercially available products, that are well characterized experimentally for their purity and solubility in conditions compatible with the recording of FTIR spectra and whose high-resolution structure is available.

Overall, 92 proteins were selected. These proteins cover well the CATH space at the level of classes and architectures. In terms of secondary structure content, an analysis of their high-resolution structure by DSSP shows that the mean content in the different secondary structures present in cSP92 is very similar to the mean content found in the PDB.

The 92-protein set is analyzed in details for the distribution of helix length, number of strands in β- sheets, length of β-strands and amino acid content, all features that may be important for the interpretation of FTIR spectra.

Reconfiguring Nature's Cholesterol Accepting Lipoproteins as Nanoparticle Platforms for Transport and Delivery of Therapeutic and Imaging Agents

Apolipoproteins are critical structural and functional components of lipoproteins, which are large supramolecular assemblies composed predominantly of lipids and proteins, and other biomolecules such as nucleic acids. A signature feature of apolipoproteins is the preponderance of amphipathic α-helical motifs that dictate their ability to make extensive non-covalent inter- or intra-molecular helix-helix interactions in lipid-free states or helix-lipid interactions with hydrophobic biomolecules in lipid-associated states. This review focuses on the latter ability of apolipoproteins, which has been capitalized on to reconstitute synthetic nanoscale binary/ternary lipoprotein complexes composed of apolipoproteins/peptides and lipids that mimic native high-density lipoproteins (HDLs) with the goal to transport drugs. It traces the historical development of our understanding of these nanostructures and how the cholesterol accepting property of HDL has been reconfigured to develop them as drug-loading platforms. The review provides the structural perspective of these platforms with different types of apolipoproteins and an overview of their synthesis. It also examines the cargo that have been loaded into the core for therapeutic and imaging purposes. Finally, it lays out the merits and challenges associated with apolipoprotein-based nanostructures with a future perspective calling for a need to develop "zip-code"-based delivery for therapeutic and diagnostic applications.

Cellular Uptake and Clearance of Oxidatively-modified Apolipoprotein E3 by Cerebral Cortex Endothelial Cells

Apolipoprotein E3 (apoE3) plays a critical role in the metabolism of lipoproteins and lowers plasma lipid levels by serving as a ligand for the low-density lipoprotein receptor (LDLr) family of proteins and by promoting macrophage cholesterol efflux. The current study examines the effect of acrolein (an endogenously generated metabolite and an environmental pollutant) modification on the structure and function of apoE3. Acrolein modification was confirmed in Western blots by reactivity with acrolein–lysine-specific antibody and by the presence of oligomeric species due to cross-linking. LC-MS/MS analysis revealed modification of 10 out of 12 lysines in apoE3, with N^ε-(3-methylpyridinium)-lysine being the predominant form of modification, and Lys75 being a ‘hot spot’ in terms of susceptibility to oxidation. Circular dichroism spectroscopy showed no major change in overall secondary structure compared to unmodified apoE3. Reconstituted high density lipoprotein (HDL) bearing acrolein modified apoE3 showed loss of binding to soluble LDLr; however, incubation with mouse endothelioma bEnd.3 cells showed that it was internalized. Incubation with excess LDL did not abolish cellular uptake of acrolein modified apoE3, suggesting alternative mechanism(s) not involving LDLr. Incubation with anti-CD36 antibody did not show a decrease in internalization while incubation with anti- lectin-like oxidized LDL receptor 1 (LOX1) showed partial internalization. However, incubation with anti-scavenger receptor class B type I (SRB1) antibody abolished internalization of acrolein modified apoE3. Taken together, our studies suggest that acrolein modification of apoE3 at lysine residues leads to increase in net negative charge, and as a consequence, results in clearance by LOX1 and SRB1 on endothelial cells. Overall, oxidative modification of apoE3 likely impairs its role in regulating plasma cholesterol homeostasis, eventually leading to lipid disorders.

Ordered opening of LDL receptor binding domain of human apolipoprotein E3 revealed by hydrogen/deuterium exchange mass spectrometry and fluorescence spectroscopy

Apolipoprotein E3 (apoE3) is an exchangeable apolipoprotein that plays a critical role in cholesterol homeostasis. The N-terminal (NT) domain of apoE3 (residues 1-191) is folded into a helix bundle comprised of 4 amphipathic α-helices: H1, H2, H3 and H4, flanked by flexible helices N1 and N2, and Hinge Helix 1 (Hinge H1), at the N-and C-terminal sides of the helix bundle, respectively. The NT domain plays a critical role in binding to the low density lipoprotein receptor (LDLR), which eventually leads to lowering of plasma cholesterol levels. In order to be recognized by the LDLR, the helix bundle has to open and undergo a conformational change. The objective of the study was to understand the mechanism of opening of the helix bundle. Hydrogen/deuterium exchange mass spectrometry (HDX-MS) revealed that apoE3 NT domain adopts several disordered and unfolded regions, with H2 exhibiting relatively little protection against exchange-in compared to H1, H3, and H4. Site-directed fluorescence labeling indicated that H2 not only has the highest degree of solvent exposure but also the most flexibility in the helix bundle. It also indicated that the lipoprotein behavior of H1 was significnatly different from that of H2, H3 and H4. These results suggest that the opening of the helix bundle is likely initiated at the flexible end of H2 and the loop linking H2/H3, and involves movement of H2/H3 away from H1/H4. Together, these observations offer mechanistic insight suggesting a regulated helix bundle opening of apoE3 NT domain can be triggered by lipid binding.

Lipidated apolipoprotein E4 structure and its receptor binding mechanism determined by a combined cross-linking coupled to mass spectrometry and molecular dynamics approach

Apolipoprotein E (apoE) is a forefront actor in the transport of lipids and the maintenance of cholesterol homeostasis, and is also strongly implicated in Alzheimer’s disease. Upon lipid-binding apoE adopts a conformational state that mediates the receptor-induced internalization of lipoproteins. Due to its inherent structural dynamics and the presence of lipids, the structure of the biologically active apoE remains so far poorly described. To address this issue, we developed an innovative hybrid method combining experimental data with molecular modeling and dynamics to generate comprehensive models of the lipidated apoE4 isoform. Chemical cross-linking combined with mass spectrometry provided distance restraints, characterizing the three-dimensional organization of apoE4 molecules at the surface of lipidic nanoparticles. The ensemble of spatial restraints was then rationalized in an original molecular modeling approach to generate monomeric models of apoE4 that advocated the existence of two alternative conformations. These two models point towards an activation mechanism of apoE4 relying on a regulation of the accessibility of its receptor binding region. Further, molecular dynamics simulations of the dimerized and lipidated apoE4 monomeric conformations revealed an elongation of the apoE N-terminal domain, whereby helix 4 is rearranged, together with Arg172, into a proper orientation essential for lipoprotein receptor association. Overall, our results show how apoE4 adapts its conformation for the recognition of the low density lipoprotein receptor and we propose a novel mechanism of activation for apoE4 that is based on accessibility and remodeling of the receptor binding region.

Among the proteins involved in the transport of lipids and their distribution to the cells, apolipoprotein E (apoE) mediates the internalization of cholesterol rich lipoproteins by acting as a ligand for cell-surface receptors. In the central nervous system, while apoE is the major cholesterol transport protein, a dysfunction of apoE in the transport and metabolism of lipids is associated with Alzheimer’s disease. A molecular understanding of the mechanisms underlying the receptor binding abilities of apoE is crucial to address its biological functions, but is so far hindered by the dynamic and complex nature of these assemblies. We have designed an original hybrid approach combining experimental data and bioinformatics tools to generate high resolution models of lipidated apoE. Based on these models, we can propose how apoE adapts its conformation at the surface of lipid nanoparticles. Further, we propose a novel mechanism of regulation of the activation and receptor recognition of apoE that could prove valuable to interpret its role in Alzheimer and apoE-related cardiovascular diseases.

The serine protease HtrA1 contributes to the formation of an extracellular 25-kDa apolipoprotein E fragment that stimulates neuritogenesis

Apolipoprotein-E (apoE) is a glycoprotein highly expressed in the brain, where it appears to play a role in lipid transport, β-amyloid clearance, and neuronal signaling. ApoE proteolytic fragments are also present in the brain, but the enzymes responsible for apoE fragmentation are unknown, and the biological activity of specific apoE fragments remains to be determined. Here we utilized SK-N-SH neuroblastoma cells differentiated into neurons with all-trans-retinoic acid (ATRA) to study extracellular apoE proteolysis. ApoE fragments were detectable in culture supernatants after 3 days, and their levels were increased for up to 9 days in the presence of ATRA. The concentration of apoE fragments was positively correlated with levels of the neuronal maturation markers (PSD95 and SMI32). The most abundant apoE fragments were 25- and 28-kDa N-terminal fragments that both contained sialylated glycosylation and bound to heparin. We detected apoE fragments only in the extracellular milieu and not in cell lysates, suggesting that an extracellular protease contributes to apoE fragmentation. Of note, siRNA-mediated knockdown of high-temperature requirement serine peptidase A1 (HtrA1) and a specific HtrA1 inhibitor reduced apoE 25-kDa fragment formation by 41 and 86%, respectively. Recombinant 25-kDa fragment apoE and full-length apoE both stimulated neuritogenesis in vitro, increasing neuroblastoma neurite growth by more than 2-fold over a 6-day period. This study provides a cellular model for assessing apoE proteolysis, indicates that HtrA1 regulates apoE 25-kDa fragment formation under physiological conditions, and reveals a new neurotrophic function for the apoE 25-kDa fragment.

Interaction of a model apolipoprotein, apoLp-III, with an oil-phospholipid interface

Lipid droplets are "small" organelles that play an important role in de novo synthesis of new membrane, and steroid hormones, as well as in energy storage. The way proteins interact specifically with the oil-(phospho-)lipid monolayer interface of lipid droplets is a relatively unexplored but crucial question. Here, we use our home built liquid droplet tensiometer to mimic intracellular lipid droplets and study protein-lipid interactions at this interface. As model neutral lipid binding protein, we use apoLp-III, an amphipathic α-helix bundle protein. This domain is also found in proteins from the perilipin family and in apoE. Protein binding to the monolayer is studied by the decrease in the oil/water surface tension. Previous work used POPC (one of the major lipids found on lipid droplets) to form the phospholipid monolayer on the triolein surface. Here we expand this work by incorporating other lipids with different physico-chemical properties to study the effect of charge and lipid head-group size. This study sheds light on the affinity of this important protein domain to interact with lipids.

Amyloidogenicity at a Distance: How Distal Protein Regions Modulate Aggregation in Disease

The misfolding of proteins to form amyloid is a key pathological feature of several progressive, and currently incurable, diseases. A mechanistic understanding of the pathway from soluble, native protein to insoluble amyloid is crucial for therapeutic design, and recent efforts have helped to elucidate the key molecular events that trigger protein misfolding. Generally, either global or local structural perturbations occur early in amyloidogenesis to expose aggregation-prone regions of the protein that can then self-associate to form toxic oligomers. Surprisingly, these initiating structural changes are often caused or influenced by protein regions distal to the classically amyloidogenic sequences. Understanding the importance of these distal regions in the pathogenic process has highlighted many remaining knowledge gaps regarding the precise molecular events that occur in classic aggregation pathways. In this review, we discuss how these distal regions can influence aggregation in disease and the recent technical and conceptual advances that have allowed this insight.

Targeted intracellular delivery of resveratrol to glioblastoma cells using apolipoprotein E-containing reconstituted HDL as a nanovehicle

The objective of this study is to transport and deliver resveratrol to intracellular sites using apolipoprotein E3 (apoE3). Reconstituted high-density lipoprotein (rHDL) bearing resveratrol (rHDL/res) was prepared using phospholipids and the low-density lipoprotein receptor (LDLr)-binding domain of apoE3. Biophysical characterization revealed that resveratrol was partitioned into the phospholipid bilayer of discoidal rHDL/res particles (~19 nm diameter). Co-immunoprecipitation studies indicated that the LDLr-binding ability of apoE3 was retained. Cellular uptake of resveratrol to intracellular sites was evaluated in glioblastoma A-172 cells by direct fluorescence using chemically synthesized NBD-labeled resveratrol (res/NBD) embedded in rHDL/res. Competition and inhibition studies indicate that the uptake is by receptor mediated endocytosis via the LDLr, with co-localization of apoE3 and res/NBD in late endosomes/lysosomes. We propose that rHDL provides an ideal hydrophobic milieu to sequester resveratrol and that rHDL containing apoE3 serves as an effective “nanovehicle” to transport and deliver resveratrol to targeted intracellular sites.

Lipoprotein assembly and function in an evolutionary perspective

Circulatory fat transport in animals relies on members of the large lipid transfer protein (LLTP) superfamily, including mammalian apolipoprotein B (apoB) and insect apolipophorin II/I (apoLp-II/I). ApoB and apoLp-II/I, constituting the structural (non-exchangeable) basis for the assembly of various lipoproteins, acquire lipids through microsomal triglyceride-transfer protein, another LLTP family member, and bind them by means of amphipathic α-helical and β-sheet structural motifs. Comparative research reveals that LLTPs evolved from the earliest animals and highlights the structural adaptations in these lipid-binding proteins. Thus, in contrast to apoB, apoLp-II/I is cleaved post-translationally by a furin, resulting in the appearance of two non-exchangeable apolipoproteins in the single circulatory lipoprotein in insects, high-density lipophorin (HDLp). The remarkable structural similarities between mammalian and insect lipoproteins notwithstanding important functional differences relate to the mechanism of lipid delivery. Whereas in mammals, partial delipidation of apoB-containing lipoproteins eventually results in endocytic uptake of their remnants, mediated by members of the low-density lipoprotein receptor (LDLR) family, and degradation in lysosomes, insect HDLp functions as a reusable lipid shuttle capable of alternate unloading and reloading of lipid. Also, during muscular efforts (flight activity), an HDLp-based lipoprotein shuttle provides for the transport of lipid for energy generation. Although a lipophorin receptor - a homolog of LDLR - was identified that mediates endocytic uptake of HDLp during specific developmental periods, the endocytosed lipoprotein appears to be recycled in a transferrin-like manner. These data highlight that the functional adaptations in the lipoprotein lipid carriers in mammals and insects also emerge with regard to the functioning of their cognate receptors.

A pyrene based fluorescence approach to study conformation of apolipoprotein E3 in macrophage-generated nascent high density lipoprotein

Apolipoprotein E3 (apoE3) is an anti-atherogenic apolipoprotein with the ability to exist in lipid-free and lipoprotein-associated states. During atherosclerosis, its function in promoting cholesterol efflux from macrophages via the ATP-binding cassette transporter A1 (ABCA1) takes a prominent role, leading to generation of nascent high density lipoprotein (nHDL) particles. The objective of this study is to understand the conformation adopted by apoE3 in macrophage-generated nHDL using a fluorescence spectroscopic approach involving pyrene. Pyrene-labeled recombinant human apoE3 displayed a robust ability to stimulate ABCA1-mediated cholesterol efflux from cholesterol-loaded J774 macrophages (which do not express apoE), comparable to that elicited by unlabeled apoE3. The nHDL recovered from the conditioned medium revealed the presence of apoE3 by immunoblot analysis. A heterogeneous population of nHDL bearing exogenously added apoE3 was generated with particle size varying from ∼12 to ∼19 nm in diameter, corresponding to molecular mass of ∼450 to ∼700 kDa. The lipid: apoE3 ratio varied from ∼60:1 to 10:1. A significant extent of pyrene excimer emission was noted in nHDL, indicative of spatial proximity between Cys112 on neighboring apoE3 molecules similar to that noted in reconstituted HDL. Cross-linking analysis using Cys-specific cross-linkers revealed the predominant presence of dimers. Taken together the data indicate a double belt arrangement of apoE molecules on nHDL. A similar organization of the C-terminal tail of apoE on nHDL was noted when pyrene-apoEA277C(201-299) was used as the cholesterol acceptor. These studies open up the possibility of using exogenously labeled apoE3 to generate nHDL for structural and conformational analysis.

Impact of apolipoprotein E on Alzheimer's disease

A key feature of Alzheimer's disease (AD) is deposition of extracellular amyloid plaque comprised chiefly of the amyloid β (Aβ) peptide. Studies of Aβ have shown that it may be catabolized by proteolysis or cleared from brain via members of the low-density lipoprotein receptor family. Alternatively, Aβ can undergo a conformational transition from α-helix to β-sheet, a conformer that displays a propensity to self-associate, oligomerize and form fibrils. Furthermore, β- sheet conformers catalyze conversion of other α-helical Aβ peptides to β-sheet, feeding the oligomer and fibril assembly process. A factor that influences the fate of Aβ in the extracellular space is apolipoprotein (apo) E. Polymorphism at position 112 or 158 in apoE give rise to three major isoforms. One isoform in particular, apoE4 (Arg at 112 and 158), has generated considerable interest since the discovery that it is the major genetic risk factor for development of late onset AD. Despite this striking correlation, the molecular mechanism underlying apoE4's association with AD remains unclear. A tertiary structural feature distinguishing apoE4 from apoE2 and apoE3, termed domain interaction, is postulated to affect the conformation and orientation of its' two independently folded domains. This feature has the potential to influence apoE4's interaction with Aβ, its sensitivity to proteolysis or its lipid accrual and receptor binding activities. Thus, domain interaction may constitute the principal molecular feature of apoE4 that predisposes carriers to late onset AD. By understanding the contribution of apoE4 to AD at the molecular level new therapeutic or prevention strategies will emerge.

An ER protein functionally couples neutral lipid metabolism on lipid droplets to membrane lipid synthesis in the ER

Eukaryotic cells store neutral lipids such as triacylglycerol (TAG) in lipid droplets (LDs). Here, we have addressed how LDs are functionally linked to the endoplasmic reticulum (ER). We show that, in S. cerevisiae, LD growth is sustained by LD-localized enzymes. When LDs grow in early stationary phase, the diacylglycerol acyl-transferase Dga1p moves from the ER to LDs and is responsible for all TAG synthesis from diacylglycerol (DAG). During LD breakdown in early exponential phase, an ER membrane protein (Ice2p) facilitates TAG utilization for membrane-lipid synthesis. Ice2p has a cytosolic domain with affinity for LDs and is required for the efficient utilization of LD-derived DAG in the ER. Ice2p breaks a futile cycle on LDs between TAG degradation and synthesis, promoting the rapid relocalization of Dga1p to the ER. Our results show that Ice2p functionally links LDs with the ER and explain how cells switch neutral lipid metabolism from storage to consumption.

Solution structure studies of monomeric human TIP47/perilipin-3 reveal a highly extended conformation

Tail-interacting protein of 47 kDa (TIP47) has two putative functions: lipid biogenesis and mannose 6-phosphate receptor recycling. Progress in understanding the molecular details of these two functions has been hampered by the lack of structural data on TIP47, with a crystal structure of the C-terminal domain of the mouse homolog constituting the only structural data in the literature so far. Our studies have first provided a strategy to obtain pure monodisperse preparations of the full-length TIP47/perilipin-3 protein, as well as a series of N-terminal truncation mutants with no exogenous sequences. These constructs have then enabled us to obtain the first structural characterization of the full-length protein in solution. Our work demonstrates that the N-terminal region of TIP47/perilipin-3, in contrast to the largely helical C-terminal region, is predominantly β-structure with turns and bends. Moreover, we show that full-length TIP47/perilipin-3 adopts an extended conformation in solution, with considerable spatial separation of the N- and C-termini that would likely translate into a separation of functional domains.

A two-step binding model of PCSK9 interaction with the low density lipoprotein receptor

PCSK9 (proprotein convertase subtilisin-like/kexin type 9) is an emerging target for pharmaceutical intervention. This multidomain protein interacts with the LDL receptor (LDLR), promoting receptor degradation. Insofar as PCSK9 inhibition induces a decrease in plasma cholesterol levels, understanding the nature of the binding interaction between PCSK9 and the LDLR is of critical importance. In this study, the ability of PCSK9 to compete with apoE3 N-terminal domain-containing reconstituted HDL for receptor binding was examined. Whereas full-length PCSK9 was an effective competitor, the N-terminal domain (composed of the prodomain and catalytic domain) was not. Surprisingly, the C-terminal domain (CT domain) of PCSK9 was able to compete. Using a direct binding interaction assay, we show that the PCSK9 CT domain bound to the LDLR in a calcium-dependent manner and that co-incubation with the prodomain and catalytic domain had no effect on this binding. To further characterize this interaction, two LDLR fragments, the classical ligand-binding domain (LBD) and the EGF precursor homology domain, were expressed in stably transfected HEK 293 cells and isolated. Binding assays showed that the PCSK9 CT domain bound to the LBD at pH 5.4. Thus, CT domain interaction with the LBD of the LDLR at endosomal pH constitutes a second step in the PCSK9-mediated LDLR binding that leads to receptor degradation.

Apolipoprotein E: from lipid transport to neurobiology

Apolipoprotein (apo) E has a storied history as a lipid transport protein. The integral association between cholesterol homeostasis and lipoprotein clearance from circulation are intimately related to apoE's function as a ligand for cell-surface receptors of the low-density lipoprotein receptor family. The receptor binding properties of apoE are strongly influenced by isoform specific amino acid differences as well as the lipidation state of the protein. As understanding of apoE as a structural component of circulating plasma lipoproteins has evolved, exciting developments in neurobiology have revitalized interest in apoE. The strong and enduring correlation between the apoE4 isoform and age of onset and increased risk of Alzheimer's disease has catapulted apoE to the forefront of neurobiology. Using genetic tools generated for study of apoE lipoprotein metabolism, transgenic "knock-in" and gene-disrupted mice are now favored models for study of its role in a variety of neurodegenerative diseases. Key structural knowledge of apoE and isoform-specific differences is driving research activity designed to elucidate how a single amino acid change can manifest such profoundly significant pathological consequences. This review describes apoE through a lens of structure-based knowledge that leads to hypotheses that attempt to explain the functions of apoE and isoform-specific effects relating to disease mechanism.

High density lipoprotein structure-function and role in reverse cholesterol transport

High density lipoprotein (HDL) possesses important anti-atherogenic properties and this review addresses the molecular mechanisms underlying these functions. The structures and cholesterol transport abilities of HDL particles are determined by the properties of their exchangeable apolipoprotein (apo) components. ApoA-I and apoE, which are the best characterized in structural terms, contain a series of amphipathic alpha-helical repeats. The helices located in the amino-terminal two-thirds of the molecule adopt a helix bundle structure while the carboxy-terminal segment forms a separately folded, relatively disorganized, domain. The latter domain initiates lipid binding and this interaction induces changes in conformation; the alpha-helix content increases and the amino-terminal helix bundle can open subsequently. These conformational changes alter the abilities of apoA-I and apoE to function as ligands for their receptors. The apoA-I and apoE molecules possess detergent-like properties and they can solubilize vesicular phospholipid to create discoidal HDL particles with hydrodynamic diameters of ~10 nm. In the case of apoA-I, such a particle is stabilized by two protein molecules arranged in an anti-parallel, double-belt, conformation around the edge of the disc. The abilities of apoA-I and apoE to solubilize phospholipid and stabilize HDL particles enable these proteins to be partners with ABCA1 in mediating efflux of cellular phospholipid and cholesterol, and the biogenesis of HDL particles. ApoA-I-containing nascent HDL particles play a critical role in cholesterol transport in the circulation whereas apoE-containing HDL particles mediate cholesterol transport in the brain. The mechanisms by which HDL particles are remodeled by lipases and lipid transfer proteins, and interact with SR-BI to deliver cholesterol to cells, are reviewed.

Apolipoprotein A-V N-terminal domain lipid interaction properties in vitro explain the hypertriglyceridemic phenotype associated with natural truncation mutants

The N-terminal 146 residues of apolipoprotein (apo) A-V adopt a helix bundle conformation in the absence of lipid. Because similarly sized truncation mutants in human subjects correlate with severe hypertriglyceridemia, the lipid binding properties of apoA-V(1-146) were studied. Upon incubation with phospholipid in vitro, apoA-V(1-146) forms reconstituted high density lipoproteins 15-17 nm in diameter. Far UV circular dichroism spectroscopy analyses of lipid-bound apoA-V(1-146) yielded an alpha-helix secondary structure content of 60%. Fourier transformed infrared spectroscopy analysis revealed that apoA-V(1-146) alpha-helix segments align perpendicular with respect to particle phospholipid fatty acyl chains. Fluorescence spectroscopy of single Trp variant apoA-V(1-146) indicates that lipid interaction is accompanied by a conformational change. The data are consistent with a model wherein apoA-V(1-146) alpha-helices circumscribe the perimeter of a disk-shaped bilayer. The ability of apoA-V(1-146) to solubilize dimyristoylphosphatidylcholine vesicles at a rate faster than full-length apoA-V suggests that N- and C-terminal interactions in the full-length protein modulate its lipid binding properties. Preferential association of apoA-V(1-146) with murine plasma HDL, but not with VLDL, suggests that particle size is a determinant of its lipoprotein binding specificity. It may be concluded that defective lipoprotein binding of truncated apoA-V contributes to the hypertriglyceridemia phenotype associated with truncation mutations in human subjects.

The helix bundle: a reversible lipid binding motif

Apolipoproteins are the protein components of lipoproteins that have the innate ability to inter convert between a lipid-free and a lipid-bound form in a facile manner, a remarkable property conferred by the helix bundle motif. Composed of a series of four or five amphipathic alpha-helices that fold to form a helix bundle, this motif allows the en face orientation of the hydrophobic faces of the alpha-helices in the protein interior in the lipid-free state. A conformational switch then permits helix-helix interactions to be substituted by helix-lipid interactions upon lipid binding interaction. This review compares the apolipoprotein high-resolution structures and the factors that trigger this switch in insect apolipophorin III and the mammalian apolipoproteins, apolipoprotein E and apolipoprotein A-I, pointing out the commonalities and key differences in the mode of lipid interaction. Further insights into the lipid-bound conformation of apolipoproteins are required to fully understand their functional role under physiological conditions.

Apolipophorin III interaction with model membranes composed of phosphatidylcholine and sphingomyelin using differential scanning calorimetry

Apolipophorin III (apoLp-III) from Locusta migratoria was employed as a model apolipoprotein to gain insight into binding interactions with lipid vesicles. Differential scanning calorimetry (DSC) was used to measure the binding interaction of apoLp-III with liposomes composed of mixtures of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and sphingomyelin (SM). Association of apoLp-III with multilamellar liposomes occurred over a temperature range around the liquid crystalline phase transition (L(alpha)). Qualitative and quantitative data were obtained from changes in the lipid phase transition upon addition of apoLp-III. Eleven ratios of DMPC and SM were tested from pure DMPC to pure SM. Broadness of the phase transition (T(1/2)), melting temperature of the phase transition (T(m)) and enthalpy were used to determine the relative binding affinity to the liposomes. Multilamellar vesicles composed of 40% DMPC and 60% SM showed the greatest interaction with apoLp-III, indicated by large T(1/2) values. Pure DMPC showed the weakest interaction and liposomes with lower percentage of DMPC retained domains of pure DMPC, even upon apoLp-III binding indicating demixing of liposome lipids. Addition of apoLp-III to rehydrated liposomes was compared to codissolved trials, in which lipids were rehydrated in the presence of protein, forcing the protein to interact with the lipid system. Similar trends between the codissolved and non-codissolved trials were observed, indicating a similar binding affinity except for pure DMPC. These results suggested that surface defects due to non-ideal packing that occur at the phase transition temperature of the lipid mixtures are responsible for apolipoprotein-lipid interaction in DMPC/SM liposomes.

TIP47 functions in the biogenesis of lipid droplets

TIP47 (tail-interacting protein of 47 kD) was characterized as a cargo selection device for mannose 6-phosphate receptors (MPRs), directing their transport from endosomes to the trans-Golgi network. In contrast, our current analysis shows that cytosolic TIP47 is not recruited to organelles of the biosynthetic and endocytic pathways. Knockdown of TIP47 expression had no effect on MPR distribution or trafficking and did not affect lysosomal enzyme sorting. Therefore, our data argue against a function of TIP47 as a sorting device. Instead, TIP47 is recruited to lipid droplets (LDs) by an amino-terminal sequence comprising 11-mer repeats. We show that TIP47 has apolipoprotein-like properties and reorganizes liposomes into small lipid discs. Suppression of TIP47 blocked LD maturation and decreased the incorporation of triacylglycerol into LDs. We conclude that TIP47 functions in the biogenesis of LDs.

The N-terminus of apolipoprotein A-V adopts a helix bundle molecular architecture

Previous studies of recombinant full-length human apolipoprotein A-V (apoA-V) provided evidence of the presence of two independently folded structural domains. Computer-assisted sequence analysis and limited proteolysis studies identified an N-terminal fragment as a candidate for one of the domains. C-Terminal truncation variants in this size range, apoA-V(1-146) and apoA-V(1-169), were expressed in Escherichia coli and isolated. Unlike full-length apoA-V or apoA-V(1-169), apoA-V(1-146) was soluble in neutral-pH buffer in the absence of lipid. Sedimentation equilibrium analysis yielded a weight-average molecular weight of 18811, indicating apoA-V(1-146) exists as a monomer in solution. Guanidine HCl denaturation experiments at pH 3.0 yielded a one-step native to unfolded transition that corresponds directly with the more stable component of the two-stage denaturation profile exhibited by full-length apoA-V. On the other hand, denaturation experiments conducted at pH 7.0 revealed a less stable structure. In a manner similar to that of known helix bundle apolipoproteins, apoA-V(1-146) induced a relatively small enhancement in 8-anilino-1-naphthalenesulfonic acid fluorescence intensity. Quenching studies with single-Trp apoA-V(1-146) variants revealed that a unique site predicted to reside on the nonpolar face of an amphipathic alpha-helix was protected from quenching by KI. Taken together, the data suggest the 146 N-terminal residues of human apoA-V adopt a helix bundle molecular architecture in the absence of lipid and, thus, likely exist as an independently folded structural domain within the context of the intact protein.

Analysis of the structural and dynamic properties of human N-terminal domain of apolipoprotein E by molecular dynamics simulations

Whereas the lipid-free N-terminal domain of apolipoprotein E (apoE-NT) adopts a four-helix bundle, the lipid-bound form is believed to undergo a large conformational change likely to be characterized by the opening of the bundle. ApoE-NT in a water/alcohol mixture was also shown to experience conformational changes exhibiting similarities with those induced upon lipid binding. The structure and dynamics of apoE-NT have been here investigated by analyzing 40 ns and 60 ns molecular dynamics simulations performed in water and in a water/propanol mixture, respectively. The overall structural properties show alterations of the tertiary structure of apoE-NT in the water/alcohol system in agreement with those observed experimentally. In contrast, in the water simulation, the sampled conformations remain closer to the crystal structure that served as a starting point for both simulations. Interestingly, several propanol molecules are seen to penetrate two hydrophobic regions of the bundle interior. One of these regions is enclosed in part by the short helix (H1') connecting helices 1 and 2 of the bundle which has been experimentally shown to be important for modulating lipid binding activity of apoE-NT. Principal component analysis of the water/propanol trajectory confirms that the region including H1' is the locus of the largest motion. Another region involves the loop connecting helix 2 and helix 3 which has been hypothesized to play the role of a hinge in the opening of the bundle.

Quantifying size distributions of nanolipoprotein particles with single-particle analysis and molecular dynamic simulations

Self-assembly of purified apolipoproteins and phospholipids results in the formation of nanometer-sized lipoprotein complexes, referred to as nanolipoprotein particles (NLPs). These bilayer constructs are fully soluble in aqueous environments and hold great promise as a model system to aid in solubilizing membrane proteins. Size variability in the self-assembly process has been recognized for some time, yet limited studies have been conducted to examine this phenomenon. Understanding the source of this heterogeneity may lead to methods to mitigate heterogeneity or to control NLP size, which may be important for tailoring NLPs for specific membrane proteins. Here, we have used atomic force microscopy, ion mobility spectrometry, and transmission electron microscopy to quantify NLP size distributions on the single-particle scale, specifically focusing on assemblies with 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and a recombinant apolipoprotein E variant containing the N-terminal 22 kDa fragment (E422k). Four discrete sizes of E422k/DMPC NLPs were identified by all three techniques, with diameters centered at approximately 14.5, 19, 23.5, and 28 nm. Computer simulations suggest that these sizes are related to the structure and number of E422k lipoproteins surrounding the NLPs and particles with an odd number of lipoproteins are consistent with the double-belt model, in which at least one lipoprotein adopts a hairpin structure.

Apolipoprotein E isoform-specific binding to the low-density lipoprotein receptor

Apolipoprotein E (apoE) is a ligand for members of the low-density lipoprotein receptor (LDLR) family and functions in plasma cholesterol homeostasis. A fluorescence-based assay has been employed in molecular studies of receptor-ligand interactions. Competition experiments revealed isoform-specific differences in binding of lipid-associated apoE N terminal (NT) domain to a recombinant soluble LDLR (sLDLR). In a similar manner, lipid--associated-but not lipid-free--full-length apoE3 showed binding activity to sLDLR. The molecular chaperone, receptor-associated protein, inhibited apoE3-NT-phospholipid complex binding to sLDLR. Kinetic studies of apoE3-NT-phospholipid complex interaction with sLDLR revealed time-dependent effects of apoE-NT isoform binding to sLDLR. The results reveal a discerning method for study of the molecular basis of ligand interactions that likely influence receptor function in maintenance of whole body cholesterol homeostasis.

How lipids influence the mode of action of membrane-active peptides

The human, multifunctional peptide LL-37 causes membrane disruption by distinctly different mechanisms strongly dependent on the nature of the membrane lipid composition, varying not only with lipid headgroup charge but also with hydrocarbon chain length. Specifically, LL-37 induces a peptide-associated quasi-interdigitated phase in negatively charged phosphatidylglycerol (PG) model membranes, where the hydrocarbon chains are shielded from water by the peptide. In turn, LL-37 leads to a disintegration of the lamellar organization of zwitterionic dipalmitoyl-phosphatidylcholine (DPPC) into disk-like micelles. Interestingly, interdigitation was also observed for the longer-chain C18 and C20 PCs. This dual behavior of LL-37 can be attributed to a balance between electrostatic interactions reflected in different penetration depths of the peptide and hydrocarbon chain length. Thus, our observations indicate that there is a tight coupling between the peptide properties and those of the lipid bilayer, which needs to be considered in studies of lipid/peptide interaction. Very similar effects were also observed for melittin and the frog skin peptide PGLa. Therefore, we propose a phase diagram showing different lipid/peptide arrangements as a function of hydrocarbon chain length and LL-37 concentration and suggest that this phase diagram is generally applicable to membrane-active peptides localized parallel to the membrane surface.

Structure-function properties of the apoE-dependent COX-2 pathway in vascular smooth muscle cells

Apolipoprotein (apoE) E is a multifunctional protein that plays a critical role in atherogenesis, in part by regulating the intimal proliferation of vascular smooth muscle cells. Recently, a novel cyclooxygenase (COX)-2 pathway was shown to contribute to the anti-proliferative action of human apoE3 in vascular smooth muscle cells (VSMC). Here, we provide insight into the structure-function properties by which apoE mediates these effects. ApoE3 is most effective in promoting COX-2 expression as a lipid-free protein and is less active after lipidation. Alterations in the stability of the helix bundle N-terminal domain of apoE that contains the binding site for the low density lipoprotein (LDL) receptor and heparin do not affect the up-regulation of the COX-2 pathway. In addition, the apoE2, 3, and 4 isoforms are all capable of up-regulating the COX-2 pathway. Finally, the effect of apoE on COX-2 was found to be independent of expression on the VSMC surface of the LDL receptor and heparan sulfate proteoglycans (HSPG). In summary, our data indicates that apoE, particularly in the lipid-free state, can up-regulate COX-2 in murine vascular smooth muscle cells apparently independently of binding to the LDLR, LRP or HSPG.

Anionic phospholipids inhibit apolipoprotein E--low-density lipoprotein receptor interactions

Apolipoprotein E (apoE) is a ligand for members of the low-density lipoprotein receptor (LDLR) family. Lipid-free apoE is not recognized by LDLR, yet interaction with lipid confers receptor recognition properties. Although lipid interaction is known to induce a conformational change in apoE, it is not known if the lipid composition of the resulting complex influences binding. Using reconstituted lipoprotein particles of apoE3 N-terminal (NT) domain and dimyristoylphosphatidylcholine (DMPC), maximal LDLR binding was observed at DMPC:apoE3-NT ratios >2.5:1 (w/w). ApoE3-NT lipid particles prepared with egg sphingomyelin were functional as LDLR ligands while complexes formed with the anionic phospholipids dimyristoylphosphatidylglycerol or dimyristoylphosphatidylserine (DMPS) were not. In the case of apoE3-NT, lipid particles comprised of a mixture of DMPC and DMPS, a DMPS concentration dependent inhibition of LDLR binding activity was observed. Thus, in addition to affecting apoE conformational status, the lipid composition of ligand particles can modulate LDLR binding activity.

Lipid-induced extension of apolipoprotein E helix 4 correlates with low density lipoprotein receptor binding ability

Apolipoprotein E (apoE) serves as a ligand for the low density lipoprotein receptor (LDLR) only when bound to lipid. The N-terminal domain of lipid-free apoE exists as globular 4-helix bundle that is conferred with LDLR recognition ability after undergoing a lipid binding-induced conformational change. To investigate the structural basis for this phenomenon, site-directed spin label electron paramagnetic resonance spectroscopy experiments were conducted, focusing on the region near the C-terminal end of helix 4 (Ala-164). Using C112S apoE-N-terminal as template, a series of single cysteine substitution variants (at sequence positions 161, 165, 169, 173, 176, and 181) were produced, isolated, and labeled with the nitroxide probe, methane thiosulfonate. Electron paramagnetic resonance analysis revealed that lipid association induced fixed secondary structure in a region of the molecule known to exist as random coil in the lipid-free state. In a complementary approach, site-directed fluorescence analysis using an environmentally sensitive probe indicated that the lipid-induced transition of this region of the protein to alpha helix was accompanied by relocation to a more hydrophobic environment. In studies with full-length apoE single Cys variants, a similar random coil to stable backbone transition was observed, consistent with the concept that lipid interaction induced an extension of helix 4 beyond the boundary defining its lipid-free conformation. This structural transition likely represents a key conformational change necessary for manifestation of the LDLR recognition properties of apoE.

Amino-terminal domain stability mediates apolipoprotein E aggregation into neurotoxic fibrils

The three isoforms of apolipoprotein (apo) E are strongly associated with different risks for Alzheimer's disease: apoE4>apoE3>apoE2. Here, we show at physiological salt concentrations and pH that native tetramers of apoE form soluble aggregates in vitro that bind the amyloid dyes thioflavin T and Congo red. However, unlike classic amyloid fibrils, the aggregates adopt an irregular protofilament-like morphology and are seemingly highly alpha-helical. The aggregates formed at substantially different rates (apoE4>apoE3>apoE2) and were significantly more toxic to cultured neuronal cells than the tetramer. Since the three isoforms have large differences in conformational stability that can influence aggregation and amyloid pathways, we tested the effects of mutations that increased or decreased stability. Decreasing the conformational stability of the amino-terminal domain of apoE increased aggregation rates and vice versa. Our findings provide a new perspective for an isoform-specific pathogenic role for apoE aggregation in which differences in the conformational stability of the amino-terminal domain mediate neurodegeneration.

Role of leucine zipper motif in apoE3 N-terminal domain lipid binding activity

The N terminal domain of human apolipoprotein E3 (apoE3-NT) functions as a ligand for members of the low-density lipoprotein receptor (LDLR) family. Whereas lipid-free apoE3-NT adopts a stable four-helix bundle conformation, a lipid binding induced conformational change is required for LDLR recognition. To investigate the role of a leucine zipper motif identified in the helix bundle on lipid binding activity, three leucine residues in helix 2 (Leu63, Leu71 and Leu78) were replaced by alanine. Recombinant "leucine to alanine" (LA) apoE3-NT was produced in E. coli, isolated and characterized. Stability studies revealed a transition midpoint of guanidine hydrochloride induced denaturation of 2.7 M and 2.1 M for wild type (WT) and LA apoE3-NT, respectively. Results from fluorescent dye binding assays revealed that, compared to WT apoE3-NT, LA apoE3-NT has an increased content of solvent exposed hydrophobic surfaces. In phospholipid vesicle solubilization assays, LA apoE3-NT was more effective than WT apoE3-NT at inducing a time-dependent decrease in dimyristoylphosphatidylglycerol vesicle light scattering intensity. Likewise, in lipoprotein binding assays, LA apoE3-NT protected human low-density lipoprotein from phospholipase C induced aggregation to a greater extent than WT apoE3-NT. On the other hand, LA apoE3-NT and WT apoE3-NT were equivalent in terms of their ability to bind a soluble LDLR fragment. The results suggest that the leucine zipper motif confers stability to the apoE3-NT helix bundle state and may serve to modulate lipid binding activity of this domain and, thereby, influence the conformational transition associated with manifestation of LDLR binding activity.

Apolipoprotein E structure: insights into function

Human apolipoprotein E (apoE) is a member of the family of soluble apolipoproteins. Through its interaction with members of the low-density lipoprotein receptor family, apoE has a key role in lipid transport both in the plasma and in the central nervous system. Its three common structural isoforms differentially affect the risk of developing atherosclerosis and neurodegenerative disorders, including Alzheimer's disease. Because the function of apoE is dictated by its structure, understanding the structural properties of apoE and its isoforms is required both to determine its role in disease and for the development of therapeutic strategies.

Nanodisk-associated amphotericin B clears Leishmania major cutaneous infection in susceptible BALB/c mice

Nanometer-scale, apolipoprotein-stabilized phospholipid bilayer disk complexes (nanodisks [ND]) harboring the toxic and poorly soluble antileishmanial agent amphotericin B (AMB) were examined for efficacy in treatment of Leishmania major-infected BALB/c mice (Mus musculus). L. major-infected mice were intraperitoneally (i.p.) treated with AMB-ND in 0-, 1-, and 5-mg/kg doses at 24 h, 48 h, and 4, 7, 14, and 21 days postinfection in two experiments. L. major-infected mice were i.p. treated with phosphate-buffered saline, 5 mg/kg AMB-ND, or 5 mg/kg lipid-associated amphotericin B (liposomal amphotericin B, AmBisome) at 24 h, 48 h, and 10, 20, 30, and 40 days postinfection in one experiment. Parasite numbers, footpad lesion size progression, and development of cytokine responses were assayed at days 7, 15, 30, 50, 140, and 250 or at days 14, 30, 50, 95, and 140 postinfection. Mice administered AMB-ND in 1- or 5-mg/kg doses were significantly protected from L. major, displaying decreases in lesion size and parasite burden, particularly at the 5-mg/kg dosage level. In contrast to the i.p. treated AmBisome group, BALB/c mice treated with i.p. AMB-ND completely cleared an L. major infection by 140 to 250 days postinfection, with no lesions remaining and no parasites isolated from infected animals. Restimulated mixed lymphocyte culture cytokine responses (interleukin-4 [IL-4], IL-12, IL-10, NO, and gamma interferon) were unchanged by AMB-ND administration compared to controls. The marked clearance of Leishmania parasites from a susceptible strain of mice without an appreciable change in the cytokine response suggests that AMB-ND represent a potentially useful formulation for treatment of intrahistiocytic organisms.

Replacement of helix 1' enhances the lipid binding activity of apoE3 N-terminal domain

The N-terminal domain of human apolipoprotein E (apoE-NT) harbors residues critical for interaction with members of the low-density lipoprotein receptor (LDLR) family. Whereas lipid free apoE-NT adopts a stable four-helix bundle conformation, a lipid binding induced conformational adaptation is required for manifestation of LDLR binding ability. To investigate the structural basis for this conformational change, the short helix connecting helix 1 and 2 in the four-helix bundle was replaced by the sequence NPNG, introducing a beta-turn. Recombinant helix-to-turn (HT) variant apoE3-NT was produced in Escherichia coli, isolated and characterized. Stability studies revealed a denaturation transition midpoint of 1.9 m guanidine hydrochloride for HT apoE3-NT vs. 2.5 M for wild-type apoE3-NT. Wild-type and HT apoE3-NT form dimers in solution via an intermolecular disulfide bond. Native PAGE showed that reconstituted high-density lipoprotein prepared with HT apoE3-NT have a diameter in the range of 9 nm and possess binding activity for the LDLR on cultured human skin fibroblasts. In phospholipid vesicle solubilization assays, HT apoE3-NT was more effective than wild-type apoE3-NT at inducing a time dependent decrease in dimyristoylphosphatidylglycerol vesicle light scattering intensity. In lipoprotein binding assays, HT apoE3-NT protected human low-density lipoprotein from phospholipase C induced aggregation to a greater extent that wild-type apoE3-NT. The results indicate that a mutation at one end of the apoE3-NT four-helix bundle markedly enhances the lipid binding activity of this protein. In the context of lipoprotein associated full-length apoE, increased lipid binding affinity of the N-terminal domain may alter the balance between receptor-active and -inactive conformational states.

The structure of human apolipoprotein E2, E3 and E4 in solution. 2. Multidomain organization correlates with the stability of apoE structure

The stabilities toward thermal and chemical denaturation of three recombinant isoforms of human apolipoprotein E (r-apoE2, r-apoE3 and r-apoE4), human plasma apoE3, the recombinant amino-terminal (NT) and the carboxyl-terminal (CT) domains of plasma apoE3 at pH 7 were studied using near and far ultraviolet circular dichroism (UV CD), fluorescence and size-exclusion chromatography. By far UV CD, thermal unfolding was irreversible for the intact apoE isoforms and consisted of a single transition. The r-apoE3 was found to be less stable as compared to the plasma protein and the stability of recombinant isoforms was r-apoE4<r-apoE3<r-apoE2. The thermal denaturation of the isolated NT- and CT-domains of apoE3 was largely reversible and included two transitions. The NT-domain was more resistant to heating than the CT-domain, both of which were more resistant than the intact protein. By near UV CD, the thermal unfolding was biphasic. When compared, thermal unfolding of the secondary and tertiary structures appeared to occur concurrently in r-apoE2 whereas heating affected the tertiary structure, initially, in r-apoE3 and r-apoE4. Denaturation with guanidine hydrochloride did not follow a two-state transition. A three-state treatment of the denaturation curves revealed the order of stability as r-apoE4<r-apoE3<r-apoE2 for the whole proteins as well as that for the NT-domains, as established by fluorescence and far UV CD spectroscopy, whereas the CT-domains had roughly similar stabilities. There are isoform-specific differences in the stability and in the state of association and the unfolding of both the NT- and CT-domains may be more complex than a two-state transition.

Rerouting lipoprotein nanoparticles to selected alternate receptors for the targeted delivery of cancer diagnostic and therapeutic agents

We report that a lipoprotein-based nanoplatform generated by conjugating tumor-homing molecules to the protein components of naturally occurring lipoproteins reroutes them from their normal lipoprotein receptors to other selected cancer-associated receptors. Multiple copies of these targeting moieties may be attached to the same nanoparticle, or a variety of different targeting moieties can be attached. Such a diverse set of tumor-homing molecules could be used to create a variety of conjugated lipoproteins as multifunctional, biocompatible nanoplatforms with a broad application to both cancer imaging and treatment. The same principle can be applied to imaging and treatment of other diseases and for monitoring specific tissues. To validate this concept, we prepared a low-density lipoprotein (LDL)-based folate receptor (FR)-targeted agent by conjugating folic acid to the Lys residues of the apolipoprotein B (apoB)-100 protein. To demonstrate the ability of the lipoprotein-based nanoplatform to deliver surface-loaded and core-loaded payloads, the particles were labeled either with the optical reporter 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine that was intercalated in the phospholipid monolayer or with the lipophilic photodynamic therapy agent, tetra-t-butyl-silicon phthalocyanine bisoleate, that was reconstituted into the lipid core. Cellular localization of the labeled LDL was monitored by confocal microscopy and flow cytometry in FR-overexpressing KB cells, in FR-nonexpressing CHO and HT-1080 cells, and in LDL receptor-overexpressing HepG2 cells. These studies demonstrate that the folic acid conjugation to the Lys side-chain amino groups blocks binding to the normal LDL receptor and reroutes the resulting conjugate to cancer cells through their FRs.

Structural analysis of lipoprotein E particles

Apolipoprotein E (apoE) is a key regulator of cholesterol homeostasis. Human apoE has three common isoforms, each with different risk implications for cardiovascular and neurodegenerative disease. Neither the structure of lipoprotein E particles nor the structural consequences of the isoform differences are known. In this investigation, synthetic lipoprotein particles were prepared by complexing phospholipids with full-length apoE isoforms, or with truncated N-terminal and C-terminal domains of apoE. These particles were examined with calorimetry, electron microscopy, circular dichroism spectroscopy, and internal reflection infrared spectroscopy. Results indicate that particles made with the three full-length apoE isoforms are discoidal in shape, and structurally indistinguishable. Thus, differences in their pathological consequences are not due to gross differences in particle structure. Although apoE is predominantly helical, and the axes of the helices are parallel to the flat surfaces of the particles, the orientational order of lipid acyl chains is low and inconsistent with the belt model of lipoprotein A-I structure. Instead, the data suggest that there are at least two different types of apoE-lipid interactions within lipoprotein E particles. One type occurs between apoE helices and the edge of the lipid bilayer as in the belt model, while a second type involves apoE helices that situate in the plane of the membrane and disturb acyl chain order. These interactions allow LpE particles to form with different protein/lipid ratios, and they account for the structure of LpE particles made with only the truncated domains.

Model of biologically active apolipoprotein E bound to dipalmitoylphosphatidylcholine

Apolipoprotein (apo)E plays a critical role in cholesterol transport, through high affinity binding to the low density lipoprotein receptor. This interaction requires apoE to be associated with a lipoprotein particle. To determine the structure of biologically active apoE on a lipoprotein particle, we crystallized dipalmitoylphosphatidylcholine particles containing two apoE molecules and determined the molecular envelope of apoE at 10 Angstroms resolution. On the basis of the molecular envelope and supporting biochemical evidence, we propose a model in which each apoE molecule is folded into a helical hairpin with the binding region for the low density lipoprotein receptor at its apex.

Structure and physiologic function of the low-density lipoprotein receptor

The low-density lipoprotein receptor (LDLR) is responsible for uptake of cholesterol-carrying lipoprotein particles into cells. The receptor binds lipoprotein particles at the cell surface and releases them in the low-pH environment of the endosome. The focus of the current review is on biochemical and structural studies of the LDLR and its ligands, emphasizing how structural features of the receptor dictate the binding of low-density lipoprotein (LDL) and beta-migrating forms of very low-density lipoprotein (beta-VLDL) particles, how the receptor releases bound ligands at low pH, and how the cytoplasmic tail of the LDLR interfaces with the endocytic machinery.

Modulation of apolipoprotein E structure by domain interaction: differences in lipid-bound and lipid-free forms

Interaction of the amino- and carboxyl-terminal domains in apolipoprotein (apo) E, referred to as domain interaction, is predicted to be more pronounced in apoE4 than in apoE3 and to underlie the association of apoE4 with Alzheimer and cardiovascular diseases. However, direct physical proof for the domain interaction concept is lacking. To address this issue, fluorescence resonance energy transfer and electron paramagnetic resonance spectroscopy were used to probe the spatial proximity of the two domains of apoE. Both methods demonstrated that the two domains are closer in both lipid-free and phospholipid-bound apoE4 than in apoE3 as a result of domain interaction. In addition, as shown by electron paramagnetic resonance, the domains of apoE4 move apart to resemble more closely the distance in apoE3 when the isoforms are bound to triglyceride-rich emulsion particles. These results demonstrate that domain interaction is a structural property of apoE4 and that apoE adopts different conformations when complexed to different lipids.

The apoE isoform binding properties of the VLDL receptor reveal marked differences from LRP and the LDL receptor

Apolipoprotein E (apoE) associates with lipoproteins and mediates their interaction with members of the LDL receptor family. ApoE exists as three common isoforms that have important distinct functional and biological properties. Two apoE isoforms, apoE3 and apoE4, are recognized by the LDL receptor, whereas apoE2 binds poorly to this receptor and is associated with type III hyperlipidemia. In addition, the apoE4 isoform is associated with the common late-onset familial and sporadic forms of Alzheimer's disease. Although the interaction of apoE with the LDL receptor is well characterized, the specificity of other members of this receptor family for apoE is poorly understood. In the current investigation, we have characterized the binding of apoE to the VLDL receptor and the LDL receptor-related protein (LRP). Our results indicate that like the LDL receptor, LRP prefers lipid-bound forms of apoE, but in contrast to the LDL receptor, both LRP and the VLDL receptor recognize all apoE isoforms. Interestingly, the VLDL receptor does not require the association of apoE with lipid for optimal recognition and avidly binds lipid-free apoE. It is likely that this receptor-dependent specificity for various apoE isoforms and for lipid-free versus lipid-bound forms of apoE is physiologically significant and is connected to distinct functions for these receptors.

Orientation and mode of lipid-binding interaction of human apolipoprotein E C-terminal domain

ApoE (apolipoprotein E) is an anti-atherogenic lipid transport protein that plays an integral role in lipoprotein metabolism and cholesterol homoeostasis. Lipid association educes critical functional features of apoE, mediating reduction in plasma and cellular cholesterol levels. The 10-kDa CT (C-terminal) domain of apoE facilitates helix-helix interactions in lipid-free state to promote apoE self-association and helix-lipid interactions during binding with lipoproteins, although the mode of lipid-binding interaction is not well understood. We investigated the mode of lipid-binding interaction and orientation of apoE CT domain on reconstituted lipoproteins. Isolated recombinant human apoE CT domain (residues 201-299) possesses a strong ability to interact with phospholipid vesicles, yielding lipoprotein particles with an apparent molecular mass of approximately 600 kDa, while retaining the overall alpha-helical content. Electron microscopy and non-denaturing PAGE analysis of DMPC (dimyristoylphosphatidylcholine)--apoE CT domain lipoprotein complexes revealed discoidal complexes with a diameter of approx. 17 nm. Cross-linking apoE CT domain on discoidal particles yielded dimeric species as the major product. Attenuated total reflectance Fourier transform IR spectroscopy of phospholipid-apoE CT domain complexes reveals that the helical axis is oriented perpendicular to fatty acyl chains of the phospholipid. Fluorescence quenching analysis of DMPC-apoE CT domain discoidal complexes by spin-labelled stearic acid indicated a relatively superficial location of the native tryptophan residues with respect to the plane of the phospholipid bilayer. Taken together, we propose that apoE CT domain interacts with phospholipid vesicles, forming a long extended helix that circumscribes the discoidal bilayer lipoprotein complex.