Lecithin cholesterol acyltransferase (LCAT) activity in the presence of Apo-AI-derived peptides exposed to disorder–order conformational transitions

Although the association of Apo AI with HDLs has been proposed to activate LCAT activity, the detailed molecular mechanisms involved in the process are not known. Therefore, in this study we have investigated how conformational changes in several exposed regions of Apo-AI might cause LCAT activation and for this purpose, designed a strategy to investigate three Apo AI-derived peptides. Since these peptides present the ability to adopt several secondary structure conformations, they were used to determine whether LCAT activity could be modulated in the presence of a particular conformation. Circular dichroism experiments showed that Apo AI-derived peptides in PBS displayed a disordered arrangement, with a strong tendency to adopt β-sheet and random conformational structures as a function of concentration. However, in the presence of Lyso-C₁₂PC, maximal percentages of α-helical structures were observed. Performed in human plasma, time-course experiments of LCAT activity under control conditions reached the highest level of ³H-cholesteryl esters after 2.5 h incubation. In the presence of Apo AI-derived peptides, a significant increase in the production of ³H-cholesteryl esters was observed. The present study provides an important insight into the potential interactions between LCAT and lipoproteins and also suggests that peptides, initially present in a disordered conformation, are able to sense the lipid environment provided by lipoproteins of plasma and following a disorder-to-order transition, change their conformation to an ordered α-helix.     

Lecithin:cholesterol acyltransferase deficiency protects against cholesterol-induced hepatic endoplasmic reticulum stress in mice

We recently reported that lecithin:cholesterol acyltransferase (LCAT) knock-out mice, particularly in the LDL receptor knock-out background, are hypersensitive to insulin and resistant to high fat diet-induced insulin resistance (IR) and obesity. We demonstrated that chow-fed Ldlr-/-xLcat+/+ mice have elevated hepatic endoplasmic reticulum (ER) stress, which promotes IR, compared with wild-type controls, and this effect is normalized in Ldlr-/-xLcat-/- mice. In the present study, we tested the hypothesis that hepatic ER cholesterol metabolism differentially regulates ER stress using these models. We observed that the Ldlr-/-xLcat+/+ mice accumulate excess hepatic total and ER cholesterol primarily attributed to increased reuptake of biliary cholesterol as we observed reduced biliary cholesterol in conjunction with decreased hepatic Abcg5/g8 mRNA, increased Npc1l1 mRNA, and decreased Hmgr mRNA and nuclear SREBP2 protein. Intestinal NPC1L1 protein was induced. Expression of these genes was reversed in the Ldlr-/-xLcat-/- mice, accounting for the normalization of total and ER cholesterol and ER stress. Upon feeding a 2% high cholesterol diet (HCD), Ldlr-/-xLcat-/- mice accumulated a similar amount of total hepatic cholesterol compared with the Ldlr-/-xLcat+/+ mice, but the hepatic ER cholesterol levels remained low in conjunction with being protected from HCD-induced ER stress and IR. Hepatic ER stress correlates strongly with hepatic ER free cholesterol but poorly with hepatic tissue free cholesterol. The unexpectedly low ER cholesterol seen in HCD-fed Ldlr-/-xLcat-/- mice was attributable to a coordinated marked up-regulation of ACAT2 and suppressed SREBP2 processing. Thus, factors influencing the accumulation of ER cholesterol may be important for the development of hepatic insulin resistance.     

Detoxification of oxidized LDL by transferring its oxidation product(s) to lecithin:cholesterol acyltransferase

In the present study, we isolated modified LCAT (m-LCAT) by hydroxyapatite column chromatography after incubation of crude LCAT (after DEAE SephadexA-50 column chromatography, penultimate step of LCAT purification) with oxidized LDL (oxLDL) at 37 degrees C for 1 h. The activity was found to be about 30% lower than that of native LCAT (n-LCAT). When activity was determined in the presence of oxLDL, m-LCAT was less inhibited than n-LCAT by oxLDL. Treatments of purified LCAT either at 56 degrees C for 30 min, at 100 degrees C for 10 min, or with 6 mM 5-5' -dithiobis-2-nitrobenzoic acid or 9 mM diisopropyl fluorophosphates (each at 37 degrees C for 30 min) resulted in the loss of its cholesterol-esterifying activity. When examined for their ability to detoxify oxLDL, native LCAT and LCAT treated at 56 degrees C for 30 min were found to detoxify oxLDL. These results indicate that oxidation product(s) of LDL is transferred and bound to LCAT in a way that does not depend on its cholesterol-esterifying activity, but rather on the availability of the sulfhydryl group of cysteine residue and the hydroxyl group of serine residue.     

Design and synthesis of biologically active peptides: a 'tail' of amino acids can modulate activity of synthetic cyclic peptides

In earlier work, we synthesized a cyclic 9-amino acid peptide (AFPep, cyclo[EKTOVNOGN]) and showed it to be useful for prevention and therapy of breast cancer. In an effort to explore the structure–function relationships of AFPep, we have designed analogs that bear a short ‘tail’ (one or two amino acids) attached to the cyclic peptide distal to its pharmacophore. Analogs that bore a tail of either one or two amino acids, either of which had a hydrophilic moiety in the side chain (e.g., cyclo[EKTOVNOGN]FS) exhibited greatly diminished biological activity (inhibition of estrogen-stimulated uterine growth) relative to AFPep. Analogs that bore a tail of either one or two amino acids which had hydrophobic (aliphatic or aromatic) side chains (e.g., cyclo[EKTOVNOGN]FI) retained (or had enhanced) growth inhibition activity. Combining in the same biological assay a hydrophilic-tailed analog with either AFPep or a hydrophobic-tailed analog resulted in decreased activity relative to that for AFPep or for the hydrophobic-tailed analog alone, suggesting that hydrophilic-tailed analogs are binding to a biologically active receptor. An analog with a disrupted pharmacophore (cyclo[EKTOVGOGN]) exhibited little or no growth inhibition activity. An analog with a hydrophilic tail and a disrupted pharmacophore (cyclo[EKTOVGOGN]FS) exhibited no growth inhibition activity of its own and did not affect the activity of a hydrophobic-tailed analog, but enhanced the growth inhibition activity of AFPep. These results are discussed in the context of a two-receptor model for binding of AFPep and ring-and-tail analogs. We suggest that tails on cyclic peptides may comprise a useful method to enhance diversity of peptide design and specificity of ligand–receptor interactions.     

Hydrophobicity-tuned anion responsiveness underlies endosomolytic cargo delivery mediated by amphipathic vehicle peptides

Peptide conformation can change subject to environment cues. This concept also applies to many cationic amphipathic peptides (CAPs) known to have cell membrane lytic or penetrative activities. Well-conditioned CAPs can match the properties of the target membrane to support their intended biological functions, e.g., intracellular cargo delivery; however, the intricacy in such conditioning surpasses our current understanding. Here we focused on hydrophobicity, a key biophysical property that dictates the membrane activity of CAPs, and applied a structure–function strategy to evolve a template peptide for endosomolytic cargo delivery. The template was subjected to iterative adjustment to balance hydrophobicity between its N-terminal linear and C-terminal helical domains. We demonstrate that the obtained peptide, LP6, could dramatically promote cargo cell entry and facilitate cytosolic delivery of biomacromolecules such as FITC-dextran, saporin, and human IgG. Among the evolved peptide series, LP6 has low cytotoxicity and moderate hydrophobicity, exhibits maximum change in helical conformation in response to negatively charged phospholipids, and also shows an apparent aggregational behavior in response to sialic acid enrichment. These attributes of LP6 collectively indicate that its anion-responsive conformational change is a critical underlining of its endosomolytic cargo delivery capability. Our results also suggest that modulation of hydrophobicity serves as a key to the precise tuning of CAP''s membrane activity for future biomedical applications.     

Use of synthetic peptides for the detection and quantification of autoantibodies

Summary and conclusions The rapid progress made over the last 10 years in the identification of individual autoantigens and in the localization of the epitopes involved, has resulted in a parallel reduction in the complexity of the antigen required for the detection of autoantibodies. The ability to use synthetic peptides as antigens is a remarkable culmination of this process considering that many antigenic particles contain multiple proteins (eg. Sm consist of 8 or more individual proteins).Despite the fact that patients with SLE have a polyclonal hypergammaglobulinemia, excellent correlations between ELISAs utilizing the P2 or SmB/B synthetic peptides, ELISAs utilizing r proteins and immunoblotting were obtained [28, 38, 50]. However, false positive/non-specific binding to a P2-BSA-glutaraldehyde conjugate has been observed with serum from old MRL/lpr mice (unpublished observations). In addition, some of the results obtained in human autoimmune diseases suggest that non-specific binding may be problematic in some instances. It is difficult, at present, to know whether the higher frequencies of detection of autoantibodies to certain synthetic peptide antigens reflect increased sensitivity or decreased specificity.Synthetic peptide antigens have beeen used to detect autoantibodies in both organ specific and multisystem autoimmune diseases. In only a small number of cases have these reagents been rigorously tested for sensitivity and specificity. Despite this, synthetic peptides have been shown to be valuable for detection and quantification of autoantibodies in certain clinical situations. Undoubtedly, further progress in epitope mapping of autoantigens coupled with technological advances in protein synthesis and improved prediction of protein structure will lead to a large number of synthetic peptide antigens for research and clinical applications. It is unlikely that short synthetic peptides will substitute for native proteins in all instances since some autoantibodies show a striking preference for conformational epitopes.Abbreviations r recombinant - SLE systemic lupus erythematosus     

Biological activity of synthetic peptides analogous to heat-stable enterotoxin produced by Yersinia enterocolitica

Abstract 3 peptides were synthesized chemically by following the primary structure of heat-stable enterotoxin (ST) produced by Yersinia enterocolitica . A peptide 1–30, having the whole sequence of 30 amino-acid residues, showed a ST activity similar to that of analogue peptide 15–30 composed of the C-terminal 16 amino acid residues. The c-GMP levels of L cells increased through an interaction with peptide 1–30 but not with peptide 15–30, while membranes isolated from broken L cells responded to both. Peptide 1–11, composed of the N-terminal 11 amino-acid residues, showed no biological activity.     

Biophysical studies of the interactions between 14-mer and 21-mer model amphipathic peptides and membranes: Insights on their modes of action

We have investigated the interactions between synthetic amphipathic peptides and zwitterionic model membranes. Peptides with 14 and 21 amino acids composed of leucines and phenylalanines modified by the addition of crown ethers have been synthesized. The 14-mer and 21-mer peptides both possess a helical amphipathic structure as revealed by circular dichroism. To shed light on their mechanism of membrane interaction, different complementary biophysical techniques have been used such as circular dichroism, fluorescence, membrane conductivity measurement and NMR spectroscopy. Results obtained by these different techniques show that the 14-mer peptide is a membrane perturbator that facilitate the leakage of species such as calcein and Na ions, while the 21-mer peptide acts as an ion channel. ³¹P solid-state NMR experiments on multilamellar vesicles reveal that the dynamics and/or orientation of the polar headgroups are greatly affected by the presence of the peptides. Similar results have also been obtained in mechanically oriented DLPC and DMPC bilayers where different acyl chain lengths seem to play a role in the interaction. On the other hand, ²H NMR experiments on multilamellar vesicles demonstrate that the acyl chain order is affected differently by the two peptides. Based on these studies, mechanisms of action are proposed for the 14-mer and 21-mer peptides with zwitterionic membranes.     

Helix stabilization of amphipathic peptides by hydrocarbon stapling increases cholesterol efflux by the ABCA1 transporter

Apolipoprotein mimetic peptides are short amphipathic peptides that efflux cholesterol from cells by the ABCA1 transporter and are being investigated as therapeutic agents for cardiovascular disease. We examined the role of helix stabilization of these peptides in cholesterol efflux. A 23-amino acid long peptide (Ac-VLEDSFKVSFLSALEEYTKKLNTQ-NH2) based on the last helix of apoA-I (A10) was synthesized, as well as two variants, S1A10 and S2A10, in which the third and fourth and third and fifth turn of each peptide, respectively, were covalently joined by hydrocarbon staples. By CD spectroscopy, the stapled variants at 24 °C were more helical in aqueous buffer than A10 (A10 17%, S1A10 62%, S2A10 97%). S1A10 and S2A10 unlike A10 were resistant to proteolysis by pepsin and chymotrypsin. S1A10 and S2A10 showed more than a 10-fold increase in cholesterol efflux by the ABCA1 transporter compared to A10. In summary, hydrocarbon stapling of amphipathic peptides increases their helicity, makes them resistant to proteolysis and enhances their ability to promote cholesterol efflux by the ABCA1 transporter, indicating that this peptide modification may be useful in the development of apolipoprotein mimetic peptides.     

Lecithin cholesterol acyltransferase null mice are protected from diet-induced obesity and insulin resistance in a gender-specific manner through multiple pathways

Complete lecithin cholesterol acyltransferase (LCAT) deficiency uniformly results in a profound HDL deficiency. We recently reported unexpected enhanced insulin sensitivity in LCAT knock-out mice in the LDL receptor knock-out background (Ldlr(-/-)×Lcat(-/-); double knock-out (DKO)), when compared with their Ldlr(-/-)×Lcat(+/+) (single knock-out (SKO)) controls. Here, we report that LCAT-deficient mice (DKO and Lcat(-/-)) are protected against high fat high sucrose (HFHS) diet-induced obesity without hypophagia in a gender-specific manner compared with their respective (SKO and WT) controls. The metabolic phenotypes are more pronounced in the females. Changes in endoplasmic reticulum stress were examined as a possible mechanism for the metabolic protection. The female DKO mice developed attenuated HFHS-induced endoplasmic reticulum stress as evidenced by a lack of increase in mRNA levels of the hepatic unfolded protein response (UPR) markers Grp78 and CHOP compared with SKO controls. The DKO female mice were also protected against diet-induced insulin resistance. In white adipose tissue of chow-fed DKO mice, we also observed a reduction in UPR, gene markers for adipogenesis, and markers for activation of Wnt signaling. In skeletal muscles of female DKO mice, we observed an unexpected increase in UCP1 in association with increase in phospho-AMPKα, PGC1α, and UCP3 expressions. This increase in UCP1 was associated with ectopic islands of brown adipocytes between skeletal muscle fibers. Our findings suggest that LCAT deficiency confers gender-specific protection against diet-induced obesity and insulin resistance at least in part through regulation in UPR, white adipose tissue adipogenesis, and brown adipocyte partitioning.     

Generation of species-specific antihemoglobin antibodies by immunization with synthetic peptides of human hemoglobin

Four peptides (7–16 residues) representing nonconserved regions of human hemoglobin (Hb) were selected for synthesis by comparison of the amino acid sequence of human Hb with those of the most common domesticated animals. Mouse antisera resulting from immunization with the synthetic peptides were investigated for binding to a panel of animal Hbs using solid-phase radioimmunoassay (RIA). One of the peptides elicited antibodies which bound specifically to human Hb, but not to any Hb of the nonprimate animals tested. The results show that the peptide immunogen chosen on the basis of dissimilarity between regions of different species is useful for the generation of species-specific antibodies. Such antibodies could serve as valuable tools for clinical screening of fecal occult blood trait and for forensic identification of bloodstains of human origin.     

Ion channels formed by amphipathic helical peptides

Channel forming peptides (CFPs) are amphipathic peptides, of length ca. 20 residues, which adopt an -helical conformation in the presence of lipid bilayers and form ion channels with electrophysiological properties comparable to those of ion channel proteins. We have modelled CFP channels as bundles of parallel trans-bilayer helices surrounding a central ion-permeable pore. Ion-channel interactions have been explored via accessible surface area calculations, and via evaluation of changes in van der Waals and electrostatic energies as a K⁺ ion is translated along the length of the pore. Two CFPs have been modelled: (a) zervamicin-A1-16, a synthetic apolar peptaibol related to alamethicin, and (b) -toxin from Staphylococcus aureus. Both of these CFPs have previously been shown to form ion channels in planar lipid bilayers, and have been shown to have predominantly helical conformations. Zervamicin-A1-16 channels were modelled as bundles of 4 to 8 parallel helices. Two related helix bundle geometries were explored. K⁺channel interactions have been shown to involve exposed backbone carbonyl oxygen atoms. -Toxin channels were modelled as bundles of 6 parallel helices. Residues Q3, D11 and D18 generate favourable K⁺-channel interactions. Rotation of W15 about its C-C bond has been shown to be capable of occluding the central pore, and is discussed as a possible model for sidechain conformational changes in relation to ion channel gating.     

Binding of synthetic peptides by a human monoclonal IgM with an unusual combining site structure

Using X-ray crystallography, a human monoclonal IgM cryoglobulin (Mez) was found to have an unusual combining site topography. Analysis of the unliganded Fv at 2.6 A resolution revealed that the HCDR3 had partitioned the active site into two compartments [Ramsland PA et al. 2000. Mol. Immunol. 37: 295-310]. The two cavities had dimensions and chemical properties that were compatible with the binding of peptides. In this study, libraries of peptides were prepared using solid-phase synthesis. Binding of the intact Mez IgM to these peptides was tested by enzyme-linked immunoassays. Screening of 400 dipeptides revealed that binding was markedly skewed toward amino acids with aromatic side-chains (Phe and Trp), especially when located in the second position. Preferential recognition of aromatic side-chains by Mez IgM was confirmed with larger peptides of three to five residues, but C-terminal positioning was not favored in these peptides. Mez IgM also showed binding propensities for acidic residues (Asp and Glu) as well as several other side-chains with different chemical properties, including His, Pro, Asn and Gln. Mez IgM recognized sets of overlapping octapeptides representing the sequences of the constant domains of human IgG1 heavy chains. These peptides represented similar stretches of polypeptide on the three-dimensional structures of all three constant domains (CH1, CH2 and CH3). Thus, Mez IgM may recognize structurally homologous regions of immunoglobulin domains, which were conserved during the evolution of the immune system.     

Studies of synthetic peptide analogs of the amphipathic helix. Effect of charge distribution, hydrophobicity, and secondary structure on lipid association and lecithin:cholesterol acyltransferase activation

Four peptides capable of forming an amphipathic alpha-helix have been synthesized and their conformational and lipid-binding properties studied. These peptides have been designed to vary the alpha-helix-forming potential as well as the charge distribution of the model peptide. The resulting peptide analogs and their complexes with dimyristoyl phosphatidylcholine were studied by using right angle light scattering, negative stain electron microscopy, nondenaturing gradient gel electrophoresis, circular dichroism, intrinsic tryptophan fluorescence, and differential scanning calorimetry techniques. The four analogs, [Glu4,9, Leu11,17] (reverse-18A, [Glu4,9, Leu5,11,17] reverse-18A, [Glu1,8, Leu11,17] 18A, and [Glu1,8, Leu5,11,17] 18A were derived from a model amphipathic peptide Asp-Trp-Leu-Lys-Ala-Phe-Tyr-Asp-Lys-Val-Ala-Glu-Lys-Leu-Lys-Glu-Ala-Phe (18A) whose lipid-associating properties strongly mimic apolipoprotein A-I or derived from Lys-Trp-Leu-Asp-Ala-Phe-Tyr-Lys-Asp-Val-Ala-Lys-Glu-Leu-Glu-Lys-Ala-Phe (reverse-18A), a peptide with little affinity for lipid and having a reversed charge distribution compared to the 18A peptide. We have shown that by substituting glutamic acid and leucine for aspartic acid and alanine, respectively, in a weak lipid-associating amphipathic helix peptide, the lipid-associating ability can be increased. Thus, peptides with both kinds of charge distribution can associate with the lipid. The ability of the peptide to disrupt phospholipid bilayers, however, is higher for 18A analogs compared to the reverse-18A analogs even after increasing the helix-forming potential and hydrophobicity. In addition to forming smaller lipoprotein particles, the modified 18A analogs were much superior to the modified reverse-18A analogs in their ability to activate the enzyme lecithin:cholesterol acyltransferase. This demonstrates that the positions of charged residues in the amphipathic helix play an important role in lecithin:cholesterol acyltransferase activation.     

Conformational studies of the N-terminal lipid-associating domain of human apolipoprotein C-I by CD and 1H NMR spectroscopy.

A peptide comprising the N-terminal 38 residues of human apolipoprotein C-I (apoC-I(1-38)) was synthesized using solid-phase methods and its solution conformation studied by CD and 1H NMR spectroscopy. The CD data indicate that apoC-I(1-38) has a similar helical content (55%) in the presence of saturating amounts of SDS or egg yolk lysophosphatidylcholine. A structural ensemble of SDS-bound apoC-I(1-38) was calculated from 464 NOE-based distance restraints using distance geometry methods. ApoC-I(1-38) adopts a helical structure between residues V4 and K30 and an extended C-terminus from Q31 when associated with SDS. The region K12-G15 undergoes slow conformational exchange as indicated by above-average amide resonance linewidths, large temperature coefficients, and fast exchange (< 2 h) of backbone amide protons with deuterium. The mobility of K12-G15 is reflected in the poorly defined dihedral angles of K12 and E13 in the calculated ensemble of structures. The average structure of apoC-I(1-38) is curved toward its hydrophobic face with bends of 125 degrees, centered at K12/E13, and 150 degrees, centered at K21. This curvature appears to be driven by the interaction of two hydrophobic clusters, one formed by residues L8, L11, F14, and L18, and the other by L25, I26, and I29, with the amphiphile SDS. Based on our present structural definition of apoC-I(1-38) and the previously obtained structure of the fragment apoC-I(35-53), we propose the secondary structure of intact apolipoprotein C-I.     

Proteolysis of synthetic peptides by type A botulinum neurotoxin

Type A botulinum neurotoxin catalyzed the hydrolysis of synthetic peptides based on the sequence of the 25-kD synaptosomal protein SNAP-25. In each peptide, the toxin cleaved at a single glutaminyl-arginine bond corresponding to residues 197 and 198 of SNAP-25, confirming earlier reports on the enzymatic specificity of the toxin in synaptosomal preparations. Metal chelators inhibited catalysis, consistent with a metalloprotease activity. In contrast to tetanus toxin and other botulinum toxin serotypes, type A toxin hydrolyzed relatively short, 17-to 20-residue peptides. In the substrates, SNAP-25 residue 202 and one or more of residues 187–191 were required for efficient hydrolysis, but residues 167–186 and 203–206 were not. The highest rates of hydrolysis were found when the C-terminal residues of the peptides were amidated.     

Efficient synthetic signal peptides for Streptomyces

A short synthetic signal peptide (SSSP) of 26 amino acid and a long one of 35 amino acids (LSSP), having an additional ribosome binding site (RBS), were synthesized. The SSSP sequence was based on the comparison of known efficient Streptomycessignal sequences. The SSSP and the LSSP were connected to the Streptomycessp. TO1 amylase gene (amyTO1) without its signal peptide. These constructions, when cloned into Streptomycessp. TO1 and placed under the control of the ermE-up promoter of Saccharopolyspora erythrea, increased the secretion of the amylase up to six-fold when compared to the natural amyTO1 signal peptide.     

Effect of introducing a short amyloidogenic sequence from the Aβ peptide at the N‐terminus of 18‐residue amphipathic helical peptides

Fibril formation is the hallmark of pathogenesis in Alzheimer's disease and other amyloid disorders caused by conformational alterations leading to the aggregation of soluble monomers. Aβ40 self‐associates to form amyloid fibrils. Its central seven‐residue segment KLVFFAE (Aβ16–22), which is thought to be crucial for fibril formation of the full‐length peptide, forms fibrils even in isolation. Context‐dependent induction of amyloid formation by such sequences in peptides, which otherwise do not have that propensity, is of considerable interest. We have examined the effect of introducing the Aβ16–22 sequence at the N‐terminus of two amphipathic helical 18‐residue peptides Ac‐WYSEMKRNVQRLERAIEE‐am and Ac‐KQLIRFLKRLDRNLWGLA‐am, which have high average hydrophobic moment <μH> values but have net charges of 0 and +4, respectively, at neutral pH. Upon incubation in aqueous buffer, fibril‐like aggregates were discernible by transmission electron microscopy for the peptide with only 0 net charge, which also displayed ThT binding and β‐structure. Although both the sequences have been derived from amphipathic helical segments in globular proteins and possess high average hydrophobic moments, the +4 charge peptide lacks the ability to form fibrils, while the peptide with 0 charge has the tendency to form fibrillar structures. Variation in the net charge and the presence of several glutamic acids in the sequence of the peptide with net charge 0 appear to favor the formation of fibrils when the Aβ16–22 sequence is attached at the N‐terminus. Copyright © 2011 European Peptide Society and John Wiley & Sons, Ltd.