Fairwell Thomas

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 μm). 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 μm versus 1.5 ± 0.27%, 18 h, 18 μm (p < 0.0001); 37pA, 7.4 ± 0.85%, 18 h, 18μm versus 5.8 ± 0.20%, 18 h, 18μm (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.

Targeting of scavenger receptor class B type I by synthetic amphipathic α-helical-containing peptides blocks lipopolysaccharide (LPS) uptake and LPS-induced pro-inflammatory cytokine responses in THP-1 monocyte cells

Human scavenger receptor class B type I, CLA-1, mediates lipopolysaccharide (LPS) binding and internalization (Vishnyakova, T. G., Bocharov, A. V., Baranova, I. N., Chen, Z., Remaley, A. T., Csako, G., Eggerman, T. L., and Patterson, A. P. (2003) J. Biol. Chem. 278, 22771–22780). Because one of the recognition motifs in SR-B1 ligands is the anionic amphipathic α-helix, we analyzed the effects of model amphipathic α-helical-containing peptides on LPS uptake and LPS-stimulated cytokine production. The L-37pA model peptide, containing two class A amphipathic helices, bound with high affinity (Kd = 0.94 μg/ml) to CLA-1-expressing HeLa cells with a 10-fold increased capacity when compared with mock transfected HeLa cells. Both LPS and L-37pA colocalized with anti-CLA-1 antibody and directly bound CLA-1 as determined by cross-linking. SR-BI/CLA-1 ligands such as HDL, apoA-I, and L-37pA efficiently competed against iodinated L-37pA. Bacterial LPS, lipoteichoic acid, and hsp60 also competed against iodinated L-37pA. Model peptides blocked uptake of iodinated LPS in both mock transfected and CLA-1-overexpressing HeLa cells. Bound and internalized Alexa-L-37pA and BODIPY-LPS colocalized at the cell surface and perinuclear compartment. Both ligands were predominantly transported to the Golgi complex, colocalizing with the Golgi markers bovine serum albumin-ceramide, anti-Golgin97 antibody, and cholera toxin subunit B. A 100-fold excess of L-37pA nearly eliminated BODIPY-LPS binding and internalization. L-37pA and its d-amino acid analogue, D-37pA peptide were similarly effective in blocking LPS, Gram-positive bacterial wall component lipoteichoic acid and bacterial heat shock protein Gro-EL-stimulated cytokine secretion in THP-1 cells. In the same culture media used for the cytokine stimulation study, neither L-37pA nor D-37pA affected the Limulus amebocyte lysate activity of LPS, indicating that LPS uptake and cytokine stimulation were blocked independently of LPS neutralization. These results demonstrate that amphipathic helices of exchangeable apolipoproteins may represent a general host defense mechanism against inflammation.

Hepatic ABCG5/G8 overexpression reduces apoB-lipoproteins and atherosclerosis when cholesterol absorption is inhibited

We previously reported that liver-specific overexpression of ABCG5/G8 in mice is not atheroprotective, suggesting that increased biliary cholesterol secretion must be coupled with decreased intestinal cholesterol absorption to increase net sterol loss from the body and reduce atherosclerosis. To evaluate this hypothesis, we fed low density lipoprotein receptor-knockout (LDLr-KO) control and ABCG5/G8-transgenic (ABCG5/G8-Tg)×LDLr-KO mice, which overexpress ABCG5/G8 only in liver, a Western diet containing ezetimibe to reduce intestinal cholesterol absorption. On this dietary regimen, liver-specific ABCG5/G8 overexpression increased hepatobiliary cholesterol concentration and secretion rates (1.5-fold and 1.9-fold, respectively), resulting in 1.6-fold increased fecal cholesterol excretion, decreased hepatic cholesterol, and increased (4.4-fold) de novo hepatic cholesterol synthesis versus LDLr-KO mice. Plasma lipids decreased (total cholesterol, 32%; cholesteryl ester, 32%; free cholesterol, 30%), mostly as a result of reduced non-high density lipoprotein-cholesterol and apolipoprotein B (apoB; 36% and 25%, respectively). ApoB-containing lipoproteins were smaller and lipid-depleted in ABCG5/G8-Tg×LDLr-KO mice. Kinetic studies revealed similar 125I-apoB intermediate density lipoprotein/LDL fractional catabolic rates, but apoB production rates were decreased 37% in ABCG5/G8-Tg×LDLr-KO mice. Proximal aortic atherosclerosis decreased by 52% (male) and 59% (female) in ABCG5/G8-Tg×LDLr-KO versus LDLr-KO mice fed the Western/ezetimibe diet. Thus, increased biliary secretion, resulting from hepatic ABCG5/G8 overexpression, reduces atherogenic risk in LDLr-KO mice fed a Western diet containing ezetimibe. These findings identify distinct roles for liver and intestinal ABCG5/G8 in modulating sterol metabolism and atherosclerosis.

Neuronal calcium sensor-1 facilitates neuronal exocytosis through phosphatidylinositol 4-kinase

This work tested the theory that neuronal calcium sensor‐1 (NCS‐1) has effects on neurotransmitter release beyond its actions on membrane channels. We used nerve‐ending preparations where membrane channels are bypassed through membrane permeabilization made by mechanical disruption or streptolysin‐O. Nerve ending NCS‐1 and phosphatidylinositol 4‐kinase (PI4K) are largely or entirely particulate, so their concentrations in nerve endings remain constant after breaching the membrane. Exogenous, myristoylated NCS‐1 stimulated nerve ending phosphatidylinositol 4‐phosphate [PI(4)P] synthesis, but non‐myristoylated‐NCS‐1 did not. The N‐terminal peptide of NCS‐1 interfered with PI(4)P synthesis, and with spontaneous and Ca2+‐evoked release of both [3H]‐norepinephrine (NA) and [14C]‐glutamate (glu) in a concentration‐dependent manner. An antibody raised against the N‐terminal of NCS‐1 inhibited perforated nerve ending PI(4)P synthesis, but the C‐terminal antibody had no effects. Antibodies against the N‐ and C‐termini of NCS‐1 caused significant increases in mini/spontaneous/stimulation‐independent release of [3H]‐NA from perforated nerve endings, but had no effect on [14C]‐glu release. These results support the idea that NCS‐1 facilitates nerve ending neurotransmitter release and phosphoinositide production via PI4K and localizes these effects to the N‐terminal of NCS‐1. Combined with previous work on the regulation of channels by NCS‐1, the data are consistent with the hypothesis that a NCS‐1–PI4K (NP, neuropotentiator) complex may serve as an essential linker between lipid and protein metabolism to regulate membrane traffic and co‐ordinate it with ion fluxes and plasticity in the nerve ending.

Calcium and chlorpromazine binding to the EF-hand peptides of neuronal calcium sensor-1

Neuronal calcium sensor-1, a protein of calcium sensor family, is known to have four structural EF-hands. We have synthesised peptides corresponding to all the four EF-hands and studied their conformation and calcium-binding. Our data confirm that the first putative site, a non-canonical one (EF1), does not bind calcium. We have investigated if this lack of binding is due to the presence of non-favoured residues (particularly at +x and -z co-ordinating positions) of the loop. We have mutated these residues and found that after modification the peptides bound calcium. However, these mutated peptides (EF1 and its functional mutants) do not show any Ca(2+) induced changes in far-UV CD. EF2, EF3, and EF4 peptides bind Ca(2+), EF3 being the strongest binder, followed by EF4. Our data of Ca(2+)-binding to individual EF peptides show that there are three active Ca(2+)-binding sites in NCS-1. We have also studied the binding of a neuroleptic drug, chlorpromazine, with the protein as well as with its EF-hands. CPZ binds myristoylated as well as non-myristoylated NCS-1 in Ca(2+)-dependent manner, with dynamic interaction to myristoylated protein. CPZ does not bind to EF1, but binds to functional EF-hand peptides and induces changes in far-UV CD. Our results suggest that NCS-1 could be a target of such antipsychotic and neuroleptic drugs.