Effect of the arrangement of tandem repeating units of class A amphipathic alpha-helixes on lipid interaction

Artificial High Density Lipoprotein Nanoparticles in Cardiovascular Research

Lipoproteins are endogenous nanoparticles which are the major transporter of fats and cholesterol in the human body. They play a key role in the regulatory mechanisms of cardiovascular events. Lipoproteins can be modified and manipulated to act as drug delivery systems or nanocarriers for contrast agents. In particular, high density lipoproteins (HDL), which are the smallest class of lipoproteins, can be synthetically engineered either as nascent HDL nanodiscs or spherical HDL nanoparticles. Reconstituted HDL (rHDL) particles are formed by self-assembly of various lipids and apolipoprotein AI (apo-AI). A variety of substances including drugs, nucleic acids, signal emitting molecules, or dyes can be loaded, making them efficient nanocarriers for therapeutic applications or medical diagnostics. This review provides an overview about synthesis techniques, physicochemical properties of rHDL nanoparticles, and structural determinants for rHDL function. We discuss recent developments utilizing either apo-AI or apo-AI mimetic peptides for the design of pharmaceutical rHDL formulations. Advantages, limitations, challenges, and prospects for clinical translation are evaluated with a special focus on promising strategies for the treatment and diagnosis of atherosclerosis and cardiovascular diseases.

Preparation of Lipid Nanodisks Containing Apolipoprotein E-Derived Synthetic Peptides for Biocompatible Delivery Vehicles Targeting Low-Density Lipoprotein Receptor

High-density lipoprotein (HDL) particles that are formed in vivo adopt a disk-shaped structure, in which the periphery of the discoidal phospholipid bilayer is surrounded by apolipoprotein. Such discoidal nanoparticles can be reconstituted with certain apolipoproteins and phospholipids and are commonly called lipid nanodisks. Apolipoprotein E (apoE), one of the HDL constituent proteins, serves as a ligand for the low-density lipoprotein (LDL) receptor. Thus, it is considered that biocompatible delivery vehicles targeting LDL receptors could be prepared by incorporating apoE as the protein component of lipid nanodisks. To enhance targeting efficiency, we designed lipid nanodisks with a large number of ligands using a peptide with the LDL receptor-binding region of apoE combined with a high lipid affinity sequence (LpA peptide). In our study, the LpA peptide spontaneously formed discoidal complexes (LpA nanodisks) of approximately 10 nm in size, equivalent to native HDL. LpA peptides on nanodisks adopted highly α-helical structures, a competent conformation capable of interacting with LDL receptors. As anticipated, the uptake of LpA nanodisks into LDL receptor-expressing cells (HepG2) was higher than that of apoE nanodisks, suggesting an enhanced targeting efficiency via the enrichment of LDL receptor-binding regions on the particle. Biodistribution studies using ¹¹¹In-labeled LpA nanodisks showed little splenic accumulation and prolonged retention in blood circulation, reflecting the biocompatibility of LpA nanodisks. High accumulation of ¹¹¹In-labeled LpA nanodisks was observed in the liver as well as in implanted tumors, which abundantly express LDL receptors. Thus, LpA nanodisks are potential biocompatible delivery vehicles targeting LDL receptors.

Dimeric peptides with three different linkers self-assemble with phospholipids to form peptide nanodiscs that stabilize membrane proteins

Three dimers of the amphipathic α-helical peptide 18A have been synthesized with different interhelical linkers inserted between the two copies of 18A. The dimeric peptides were denoted 'beltides' where Beltide-1 refers to the 18A-dimer without a linker, Beltide-2 is the 18A-dimer with proline (Pro) as a linker and Beltide-3 is the 18A-dimer linked by two glycines (Gly-Gly). The self-assembly of the beltides with the phospholipid DMPC was studied with and without the incorporated membrane protein bacteriorhodopsin (bR) through a combination of coarse-grained MD simulations, size-exclusion chromatography (SEC), circular dichroism (CD) spectroscopy, small-angle scattering (SAS), static light scattering (SLS) and UV-Vis spectroscopy. For all three beltides, MD and combined small-angle X-ray and -neutron scattering were consistent with a disc structure composed by a phospholipid bilayer surrounded by a belt of peptides and with a total disc diameter of approximately 10 nm. CD confirmed that all three beltides were α-helical in the free form and with DMPC. However, as shown by SEC the different interhelical linkers clearly led to different properties of the beltides. Beltide-3, with the Gly-Gly linker, was very adaptable such that peptide nanodiscs could be formed for a broad range of different peptide to lipid stoichiometries and therefore also possible disc-sizes. On the other hand, both Beltide-2 with the Pro linker and Beltide-1 without a linker were less adaptable and would only form discs of certain peptide to lipid stoichiometries. SLS revealed that the structural stability of the formed peptide nanodiscs was also highly affected by the linkers and it was found that Beltide-1 gave more stable discs than the other two beltides. With respect to membrane protein stabilization, each of the three beltides in combination with DMPC stabilizes the seven-helix transmembrane protein bacteriorhodopsin significantly better than the detergent octyl glucoside, but no significant difference was observed between the three beltides. We conclude that adaptability, size, and structural stability can be tuned by changing the interhelical linker while maintaining the properties of the discs with respect to membrane protein stabilization.

High-density lipoprotein mimetics: promises and challenges

The concept of lipoprotein mimetics was developed and extensively tested in the last three decades. Most lipoprotein mimetics were designed to recreate one or several functions of high-density lipoprotein (HDL) in the context of cardiovascular disease; however, the application of this approach is much broader. Lipoprotein mimetics should not just be seen as a set of compounds aimed at replenishing a deficiency or dysfunctionality of individual elements of lipoprotein metabolism but rather as a designer concept with remarkable flexibility and numerous applications in medicine and biology. In the present review, we discuss the fundamental design principles used to create lipoprotein mimetics, mechanisms of their action, medical indications and efficacy in animal models and human studies.

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

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

Anti-inflammatory and cholesterol-reducing properties of apolipoprotein mimetics: a review

Reduced levels of HDL cholesterol (HDL-C) are a strong independent predictor of coronary artery disease (CAD) risk. The major anti-atherogenic function of HDL is to mediate reverse cholesterol transport. This response is highly dependent on apoA-I and apoE, protein components of HDL. Randomized clinical trials have assessed effects of several classes of drugs on plasma cholesterol levels in CAD patients. Agents including cholestyramine, fibrates, niacin, and statins significantly lower LDL cholesterol (LDL-C) and induce modest increases in HDL-C, but tolerance issues and undesirable side effects are common. Additionally, residual risk may be present in patients with persistently low HDL-C and other complications despite a reduction in LDL-C. These observations have fueled interest in the development of new pharmacotherapies that positively impact circulating lipoproteins. The goal of this review is to discuss the therapeutic potential of synthetic apolipoprotein mimetic peptides. These include apoA-I mimetic peptides that have undergone initial clinical assessment. We also discuss newer apoE mimetics that mediate the clearance of atherogenic lipids from the circulation and possess anti-inflammatory properties. One of these (AEM-28) has recently been given orphan drug status and is undergoing clinical trials.

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

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

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.

α-Synuclein oligomers with broken helical conformation form lipoprotein nanoparticles

α-Synuclein (αS) is a membrane-binding protein with sequence similarity to apolipoproteins and other lipid-carrying proteins, which are capable of forming lipid-containing nanoparticles, sometimes referred to as "discs." Previously, it has been unclear whether αS also possesses this property. Using cryo-electron microscopy and light scattering, we found that αS can remodel phosphatidylglycerol vesicles into nanoparticles whose shape (ellipsoidal) and dimensions (in the 7-10-nm range) resemble those formed by apolipoproteins. The molar ratio of αS to lipid in nanoparticles is ∼1:20, and αS is oligomeric (including trimers and tetramers). Similar nanoparticles form when αS is added to vesicles of mitochondrial lipids. This observation suggests a mechanism for the previously reported disruption of mitochondrial membranes by αS. Circular dichroism and four-pulse double electron electron resonance experiments revealed that in nanoparticles αS assumes a broken helical conformation distinct from the extended helical conformation adopted when αS is bound to intact vesicles or membrane tubules. We also observed αS-dependent tubule and nanoparticle formation in the presence of oleic acid, implying that αS can interact with fatty acids and lipids in a similar manner. αS-related nanoparticles might play a role in lipid and fatty acid transport functions previously attributed to this protein.

Acute-phase serum amyloid A: perspectives on its physiological and pathological roles

Serum amyloid A (SAA), a protein originally of interest primarily to investigators focusing on AA amyloidogenesis, has become a subject of interest to a very broad research community. SAA is still a major amyloid research topic because AA amyloid, for which SAA is the precursor, is the prototypic model of in vivo amyloidogenesis and much that has been learned with this model has been applicable to much more common clinical types of amyloid. However, SAA has also become a subject of considerable interest to those studying (i) the synthesis and regulation of acute phase proteins, of which SAA is a prime example, (ii) the role that SAA plays in tissue injury and inflammation, a situation in which the plasma concentration of SAA may increase a 1000-fold, (iii) the influence that SAA has on HDL structure and function, because during inflammation the majority of SAA is an apolipoprotein of HDL, (iv) the influence that SAA may have on HDL's role in reverse cholesterol transport, and therefore, (v) SAA's potential role in atherogenesis. However, no physiological role for SAA, among many proposed, has been widely accepted. None the less from an evolutionary perspective SAA must have a critical physiological function conferring survival-value because SAA genes have existed for at least 500 million years and SAA's amino acid sequence has been substantially conserved. An examination of the published literature over the last 40 years reveals a great deal of conflicting data and interpretation. Using SAA's conserved amino acid sequence and the physiological effects it has while in its native structure, namely an HDL apolipoprotein, we argue that much of the confounding data and interpretation relates to experimental pitfalls not appreciated when working with SAA, a failure to appreciate the value of physiologic studies done in the 1970-1990 and a current major focus on putative roles of SAA in atherogenesis and chronic disease. When viewed from an evolutionary perspective, published data suggest that acute-phase SAA is part of a systemic response to injury to recycle and reuse cholesterol from destroyed and damaged cells. This is accomplished through SAA's targeted delivery of HDL to macrophages, and its suppression of ACAT, the enhancement of neutral cholesterol esterase and ABC transporters in macrophages. The recycling of cholesterol during serious injury, when dietary intake is restricted and there is an immediate and critical requirement of cholesterol in the generation of myriads of cells involved in inflammation and repair responses, is likely SAA's important survival role. Data implicating SAA in atherogenesis are not relevant to its evolutionary role. Furthermore, in apoE(-/-) mice, domains near the N- and C- termini of SAA inhibit the initiation and progression of aortic lipid lesions illustrating the conflicting nature of these two sets of data.

Apolipoprotein A-I and A-I mimetic peptides: a role in atherosclerosis

Cardiovascular disease remains a major cause of morbidity and mortality in the westernized world. Atherosclerosis is the underlying cause of most cardiovascular diseases. Atherosclerosis is a slowly evolving chronic inflammatory disorder involving the intima of large and medium sized arteries that is initiated in response to high plasma lipid levels, especially LDL. Cells of both the innate and adaptive immunity are involved in this chronic inflammation. Although high plasma LDL levels are a major contributor to most stages of the evolution of atherosclerosis, HDL and its major protein apoA-I possess properties that attenuate and may even reverse atherosclerosis. Two major functions are the ability to induce the efflux of cholesterol from cells, particularly lipid-loaded macrophages, in the artery wall for transfer to the liver, a process referred to as reverse cholesterol transport, and the ability to attenuate the pro-inflammatory properties of LDL. The removal of cellular cholesterol from lipid-loaded macrophages may also be anti-inflammatory. One of the most promising therapies to enhance the anti-atherogenic, anti-inflammatory properties of HDL is apoA-I mimetic peptides. Several of these peptides have been shown to promote cellular cholesterol efflux, attenuate the production of pro-inflammatory cytokines by macrophages, and to attenuate the pro-inflammatory properties of LDL. This latter effect may be related to their high affinity for oxidized lipids present in LDL. This review discusses the functional properties of the peptides and their effect on experimental atherosclerosis and the results of initial clinical studies in humans.

Membrane curvature induction and tubulation are common features of synucleins and apolipoproteins

Synucleins and apolipoproteins have been implicated in a number of membrane and lipid trafficking events. Lipid interaction for both types of proteins is mediated by 11 amino acid repeats that form amphipathic helices. This similarity suggests that synucleins and apolipoproteins might have comparable effects on lipid membranes, but this has not been shown directly. Here, we find that α-synuclein, β-synuclein, and apolipoprotein A-1 have the conserved functional ability to induce membrane curvature and to convert large vesicles into highly curved membrane tubules and vesicles. The resulting structures are morphologically similar to those generated by amphiphysin, a curvature-inducing protein involved in endocytosis. Unlike amphiphysin, however, synucleins and apolipoproteins do not require any scaffolding domains and curvature induction is mediated by the membrane insertion and wedging of amphipathic helices alone. Moreover, we frequently observed that α-synuclein caused membrane structures that had the appearance of nascent budding vesicles. The ability to function as a minimal machinery for vesicle budding agrees well with recent findings that α-synuclein plays a role in vesicle trafficking and enhances endocytosis. Induction of membrane curvature must be under strict regulation in vivo; however, as we find it can also cause disruption of membrane integrity. Because the degree of membrane curvature induction depends on the concerted action of multiple proteins, controlling the local protein density of tubulating proteins may be important. How cellular safeguarding mechanisms prevent such potentially toxic events and whether they go awry in disease remains to be determined.

Asymmetry in the lipid affinity of bihelical amphipathic peptides. A structural determinant for the specificity of ABCA1-dependent cholesterol efflux by peptides

ApoA-I contains a tandem array of amphipathic helices with varying lipid affinity, which are critical in its ability to bind and remove lipids from cells by the ABCA1 transporter. In this study, the effect of asymmetry in the lipid affinity of amphipathic helices in a bihelical apoA-I mimetic peptide, 37pA, on lipid efflux by the ABCA1 transporter was examined. Seven peptide variants of 37pA were produced by substituting a varying number of hydrophobic amino acids for alanine on either one or both helices. The 5A peptide with five alanine substitutions in the second helix had decreased helical content compared with 37pA (5A, 12+/-1% helicity; 37pA, 28+/-2% helicity) and showed less self-association but, similar to the parent peptide, was able to readily solubilize phospholipid vesicles. Furthermore, 5A, unlike the parent peptide 37pA, was not hemolytic (37pA, 27+/-2% RBC lysis, 2 h, 18 microm). Finally, the 5A peptide stimulated cholesterol and phospholipid efflux by the ABCA1 transporter with higher specificity (ABCA1-transfected versus untransfected cells) than 37pA (5A, 9.7+/-0.77%, 18 h, 18 microm versus 1.5+/-0.27%, 18 h, 18 microm (p<0.0001); 37pA, 7.4+/-0.85%, 18 h, 18 microm versus 5.8+/-0.20%, 18 h, 18 microm (p=0.03)). In summary, we describe a novel bihelical peptide with asymmetry in the lipid affinity of its helices and properties similar to apoA-I in terms of specificity for cholesterol efflux by the ABCA1 transporter and low cytotoxicity.

Structural requirements for antioxidative and anti-inflammatory properties of apolipoprotein A-I mimetic peptides

Recently, attention has been focused on pharmacological treatments that increase HDL cholesterol to prevent coronary artery disease. Despite three decades of extensive research of human apolipoprotein A-I (apoA-I), the major protein component of HDL, the molecular basis for its antiatherogenic and anti-inflammatory functions remain elusive. Another protein component of HDL, apoA-II, has structural features similar to those of apoA-I but does not possess atheroprotective properties. To understand the molecular basis for the effectiveness of apoA-I, we used model synthetic peptides. We designed analogs of the class A amphipathic helical motif in apoA-I that is responsible for solubilizing phospholipids. None of these analogs has sequence homology to apoA-I, but all are similar in their lipid-associating structural motifs. Although all of these peptide analogs interact with phospholipids to form peptide:lipid complexes, the biological properties of these analogs are different. Physical-chemical and NMR studies of these peptides have enabled the delineation of structural requirements for atheroprotective and anti-inflammatory properties in these peptides. It has been shown that peptides that interact strongly with lipid acyl chains do not have antiatherogenic and anti-inflammatory properties. In contrast, peptides that associate close to the lipid head group (and hence do not interact strongly with the lipid acyl chain) are antiatherogenic and anti-inflammatory. Understanding the structure and function of apoA-I and HDL through studies of the amphipathic helix motif may lead to peptide-based therapies for inhibiting atherosclerosis and other related inflammatory lipid disorders.

Apolipoprotein A-I-mimetic peptides with antioxidant actions

To augment antioxidant action of apolipoprotein A-I (Apo A-I)-mimetic peptide, the peptide F3,6,14,18 18A (DWFKAFYDKVAEKFKEAF) was modified by incorporating antioxidant amino acid residues. Introduction of His residue at position 2 or 3 at N-terminal of the peptide remarkably enhanced antioxidant action against Cu2+ oxidation of LDL and the capability of sequestering Cu2+. Likewise, the substitution of Ala for Cys residue at position 12 increased antioxidant action against Cu2+ oxidation of LDL. Additionally, the Cys substitution contributed to enhanced capabilities in the removal of hypochlorous acid (HOCl) and 13-hydroperoxyoctadecadienoic acid. Furthermore, the combined incorporation of His and Cys residues enhanced antioxidant actions in preventing Cu2+ oxidation and reducing HOCl and hydroperoxide levels. Separately, in solubilizing phosphatidylcholine, either peptides with His residue at N-terminal position 2 or 3, or those containing Cys residue at position 11 or 12 were equipotent to peptide F3,6,14,18 18A. Further, the lipid-solubilizing ability of those containing both His and Cys residues was comparable to that of peptide F3,6,14,18 18A. In support of this, a similar structural importance was observed with Trp fluorescence study illustrating the penetration of peptides in phosphatidylcholine liposome. Besides, the modified peptides were also comparable to peptide F3,6,14,18 18A in restoring phosphatidylserine-induced loss of PON1 activity. These results indicate that the insertion of His or Cys residue into peptide F3,6,14,18 18A at appropriate positions could lead to enhanced antioxidant action with no significant change of lipid-solubilizing action.

Helix-Turn-Helix Peptides That Form α-Helical Fibrils: Turn Sequences Drive Fibril Structure

Models of apolipoprotein A-I (apo A-I), the main protein of high-density lipoprotein, predict that it contains 10 amphiphilic, alpha-helical segments connected by turns. We synthesized four peptides with two identical 18-residue, amphiphilic, alpha-helical segments (Anantharamaiah, G. M., et al. (1985) J. Biol. Chem. 260, 10248-10255) connected by putative turn sequences from apo A-I: (1) Ac-DWLKAFYDKVAEKLKEAFKVEPLRADWLKAFYDKVAEKLKEAF-NH2, (2) Ac-DWLKAFYDKVAEKLKEAFGLLPVLEDWLKAFYDKVAEKLKEAF-NH2, (3) Ac-DWLKAFYDKVAEKLKEAFKVQPYLDDWLKAFYDKVAEKLKEAF-NH2, and (4) Ac-DWLKAFYDKVAEKLKEAFNGGARLADWLKAFYDKVAEKLKEAF-NH2. Surprisingly, peptides 1-3 formed fibrils after incubation (37 degrees C, 10 mM sodium phosphate, pH 7.60), but in contrast to beta-sheet amyloid fibrils, these did not bind thioflavin T and they induced a blue shift in the spectrum of Congo red. CD (peptides 1-3) and FTIR (peptides 1 and 2) of the fibrils showed significant alpha-helical character. Synchrotron X-ray fiber diffraction on a magnetically aligned sample of 1 confirmed the alpha-helical character in the fibrils and indicated that the helical axes are oriented perpendicular to the fibril axis. In contrast, peptide 4, containing two Gly residues but no Pro in the turn, formed only a small amount of nonfibrillar precipitate after prolonged incubation. Peptide 4P (peptide 4 with a Pro in place of the central Ala) and peptide 5, containing a PEG block in lieu of the central turn, were similar to peptide 4 in not forming fibrils, possibly because the region linking the helices was unstructured. These studies indicate that varying turn sequences between longer amphiphilic alpha-helical segments can drive the structure of fibrils.

Effect of an arginine-rich amphipathic helical peptide on plasma cholesterol in dyslipidemic mice

We have shown that the dual domain peptide Ac-hE18A-NH(2), in which LRKLRKRLLR, (141-150 region of human apo E) covalently linked to a class A lipid-associating domain, is able to associate with apo B-containing lipoproteins and enhance their clearance both in vitro and in vivo. We present here the differential effects of this peptide on the plasma cholesterol levels in different mouse models. The peptide intravenously administered (100 microg) into C57BL/6J mice on atherogenic diet, apo E null, and apo E null/LDL-receptor (LDL-R) null double knock out mouse models, was able to rapidly reduce plasma cholesterol levels within 2 min, and the effect persisted for more than 6 h. The reduction was limited to the VLDL and IDL/LDL fractions; HDL was not reduced in any mouse model studied. However, the peptide had no effect on the plasma cholesterol levels in C57BL/6J mice on normal diet, LDL-R null mice on normal chow, and LDL-R null mice on Western diet. Administration to LDL-R null mice of 125I-labelled human lipoproteins incubated with peptide resulted in accelerated human VLDL and LDL clearance with associated increase of radioactivity in the liver. These results, coupled with our earlier in vitro observations, indicate that the Arg-rich peptide-assisted rapid clearance of plasma cholesterol in dyslipidemic mice is due to the peptide targeting apo B-48-containing atherogenic lipoproteins to the liver for increased uptake and degradation.

Studies of synthetic peptides of human apolipoprotein A-I containing tandem amphipathic alpha-helixes

In mature human apolipoprotein A-I (apo A-I), the amino acid residues 1-43 are encoded by exon 3, whereas residues 44-243 are encoded by exon 4 of the apo A-I gene. The region encoded by exon 4 of the apo A-I gene contains 10 tandem amphipathic alpha-helixes; their location and the class to which they belong are as follows: helix 1 (44-65, class A1), helix 2 (66-87, class A1), helix 3 (88-98, class Y), helix 4 (99-120, class Y), helix 5 (121-142, class A1), helix 6 (143-164, class A1), helix 7 (165-186, class A1), helix 8 (187-208, class A1), helix 9 (209-219, class Y), and helix 10 (220-241, class Y). To examine the effects of multiple tandem amphipathic helixes compared to individual helixes of apo A-I on lipid association, we have studied lipid-associating properties of the following peptides: Ac-44-87-NH2 (peptide 1-2), Ac-66-98-NH2 (peptide 2-3), Ac-66-120-NH2 (peptide 2-3-4), Ac-88-120-NH2 (peptide 3-4), Ac-99-142-NH2 (peptide 4-5), Ac-121-164-NH2 (peptide 5-6), Ac-143-186-NH2 (peptide 6-7), Ac-165-208-NH2 (peptide 7-8), Ac-187-219-NH2 (peptide 8-9), and Ac-209-241-NH2 (peptide 9-10). To study lipid-associating properties of the region encoded by exon 3 of the apo A-I gene, 1-33-NH2 (peptide G) has also been studied. The results of the present study indicate that, among the peptides studied, peptides 1-2 and 9-10 possess significantly higher lipid affinity than the other peptides, with peptide 9-10 having higher lipid affinity than peptide 1-2, as evidenced by (i) higher helical content in the presence of 1, 2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), (ii) faster rate of association with DMPC multilamellar vesicles (MLV), (iii) greater reduction in the enthalpy of gel to liquid-crystalline phase transition of DMPC MLV, (iv) higher exclusion pressure from an egg yolk phosphatidylcholine monolayer, and (v) higher partitioning into 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine MLV. A comparison of the free energies of lipid association (DeltaG) of the peptides studied here with those studied previously by us [Palgunachari, M. N. , et al. (1996) Arterioscler. Thromb. Vasc. Biol. 16, 328-338] indicates that, except for the peptides 4-5 and 5-6, other peptides possess higher lipid affinities compared to constituent helixes. However, the lipid affinities of the peptides studied here are neither higher than nor equal to the sum of the lipid affinities of the constituent helixes. This indicates the absence of cooperativity among the adjacent amphipathic helical domains of apo A-I for lipid association. As indicated by DeltaG, the lipid affinity of peptide 4-5 is higher than peptide 5 but lower than peptide 4; the lipid affinity of peptide 5-6 is lower than both peptides 5 and 6. Implications of these results for the structure and function of apo A-I are discussed.

Deletion of central alpha-helices in human apolipoprotein A-I: effect on phospholipid association

In order to better understand the structure-function properties of apolipoprotein (apo) A-I, we have constructed and expressed three apoA-I mutants using a system previously described for the expression of human apolipoprotein A-I (Rec.-apoA-I). These mutants (corresponding to deletion of apoA-I residues 100-143, 122-165, 144-186) have been studied for their ability to form reconstituted apoA-I-containing lipoproteins (LpA-I) with POPC and DMPC, and for their structural and physical properties. Rec.- and native apoA-I can form homogeneous discoidal Lp2A-I over a wide range of POPC/apoA-I ratios [(20-130)/1] and exhibit sizes ranging from 9.5 to 10.5 nm. When recombined with varying POPC content [(20-130)/1, POPC/A-I)], the three mutants produce homogeneous discoidal Lp2A-I that contain a low POPC/A-I molar ratio [(20-40)/l for all mutants] and exhibit a nearly constant size [7.5-7.6 nm for delta (100-143) and 7.9-8.0 nm for the other two mutants]. Kinetics of association of these proteins with DMPC are similar for delta (100-143) and Rec.-apoA-I (t 1/2 of 4.0 and 4.4 min, respectively) but appear significantly reduced for delta (122-165) and delta (144-186) (t 1/2 of 7.5 and 6.9 min, respectively). While in the lipid-free form, all proteins have a similar thermodynamic stability with a very comparable free energy of unfolding (delta GD degree) for the alpha-helical structure, as determined by isothermal denaturation studies. delta-(100-143) has a significantly lower alpha-helical content (33%) as compared to the other proteins [40, 41, and 45% for Rec.-apoA-I. delta (122-165), and delta (144-186), respectively]. When associated to POPC, delta (122-165) and delta (144-186) have a higher alpha-helicity (63 and 63%) and an enhanced stability (2.5 and 2.3 kcal/mol, respectively) as compared to delta (100-143) (49% and 1.8 kcal/mol) and Rec.-apoA-I (52% and 1.9 kcal/mol). These results suggest that the amphipathic alpha-helices within residues 100-186 are directly involved in interactions with phospholipids. The helical region 100-121 appears to be more important to the stabilization of the lipid-apoprotein complex formed whereas helices within residues 122-186 appear to be critical to the initial rates of association of the apoprotein with DMPC. These data suggest that an important role of the central domain 100-186 may be to maintain the plasticity of apoA-I and its ability to form different classes of HDL particles. Therefore, it is likely that this region may also play an important role in the functional properties of this apoprotein.

Interaction of model class A1, class A2, and class Y amphipathic helical peptides with membranes

To test the hypothesis that differences in the lipid affinity of exchangeable apolipoproteins are due to the presence of different classes of amphipathic alpha-helical motifs which differ primarily in the distribution of charged amino acid residues, we designed and synthesized model peptides mimicking class A1, class A2, and class Y amphipathic helices present in these apolipoproteins. Both class A1 and class A2 helices have positive residues at the polar-nonpolar interface and negative residues at the center of the polar face. However, clustering of positive and negative residues is less exact in class A1 compared to class A2 helices. The class Y helices have two negative residue clusters on the polar face separating the two arms and the base of the Y motif formed by three positive residue clusters. The lipid affinities of three 18 residue model peptides representing these classes, Ac-18A1-NH2 (Ac-ELLEKWAEKLAALKEALK-NH2), Ac-18A2-NH2 (Ac-ELLEKWKEALAALAEKLK-NH2), and Ac-18Y-NH2 (Ac-ELLKAWKEALEALKEKLA-NH2), were determined by right-angle light scattering, circular dichroism spectroscopy, differential scanning calorimetry, and fluorescence spectroscopy. The observed rank order of lipid affinity of these three peptides is: Ac-18A2-NH2 > Ac-18Y-NH2 > Ac-18A1-NH2. This order is consistent with the known lipid affinity of exchangeable apolipoproteins containing class A1, class A2, and class Y helices (class A2 > class Y > class A1). Results of this study illustrate the important role of interfacial lysine residues in modulating the lipid affinity of amphipathic helices and suggest that the effect of interfacial lysine residues in increasing lipid affinity is additive. We propose that interfacial lysine residues, in addition to widening the hydrophobic face because of snorkeling, also help anchor the amphipathic helix in the lipid bilayer.

Only the two end helixes of eight tandem amphipathic helical domains of human apo A-I have significant lipid affinity. Implications for HDL assembly

Human apolipoprotein A-I (apo A-I) possesses multiple tandem repeating 22-mer amphipathic alpha-helixes. Computer analysis and studies of model synthetic peptides and recombinant protein-lipid complexes of phospholipids have suggested that apo A-I interacts with HDL surface lipids through cooperation among its individual amphipathic helical domains. To delineate the overall lipid-associating properties of apo A-I, the first step is to understand the lipid-associating properties of individual amphipathic helical domains. To this end, we synthesized and studied each of the eight tandem repeating 22-mer domains of apo A-I: residues 44-65, 66-87, 99-120, 121-142, 143-164, 165-186, 187-208, and 220-241. Among the 22-mers, only the N- and C-terminal peptides (44-65 and 220-241) were effective in clarifying multilamellar vesicles (MLVs) of dimyristoylphosphatidylcholine (DMPC). These two peptides also exhibited the highest partition coefficient into 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylcholine liposomes, the highest exclusion pressure for penetration into an egg yolk phosphatidylcholine monolayer, and the greatest reduction in the enthalpy of the gel-to-liquid crystalline phase transition of DMPC MLVs. These results suggest that the strong, lipid-associating properties of apo A-I are localized to the N- and C-terminal amphipathic domains. Although each of the eight peptides studied has an amphipathic structure, models based on changes in residual effective amino acid hydrophobicity resulting from differing depths of helix penetration into the lipid are best able to explain the high lipid affinity possessed by the two terminal domains. Differential scanning calorimetry (DSC) studies showed that on a molar basis, apo A-I is about 10 times more effective than the most effective peptide analyzed in reducing the enthalpy of the gel-to-liquid crystalline phase transition of DMPC MLVs. Because previous proteolysis experiments coupled with the present DSC results suggest that the lipid-associating domains of apo A-I are distributed throughout the length of the 243 amino acid residues, we propose that the terminal amphipathic helical domains are involved in the initial binding of apo A-I to the lipid surface to form HDL particles, followed by cooperative binding of the middle six amphipathic helical domains, perhaps aided by salt-bridge formation between adjacent helixes arranged in an antiparallel orientation.

Mechanisms for the modulation of membrane bilayer properties by amphipathic helical peptides

The amphipathic helix, in which hydrophobic and hydrophilic residues are grouped on opposing faces, is a structural motif found in many peptides and proteins that bind to membranes. One of the physical properties of membranes that can be altered by the binding of amphipathic helices is membrane monolayer curvature strain. Class A amphipathic helices, which are present in exchangeable plasma lipoproteins, can stabilize membranes by reducing negative monolayer curvature strain; proline-punctuated class A amphipathic helical segments are particularly effective in this regard. This property is suggested to be associated with some of the beneficial biological effects of this protein. On the other hand, lytic amphipathic helical peptides can act by increasing negative curvature strain or by forming pores composed of helical clusters. Thus, different amphipathic helical peptides can be membrane stabilizing or be lytic to membranes, depending on the structural motif of the helix, which in turn determines the nature of its association with membranes. Features of these peptides that are responsible for their specific properties are discussed.