How bradykinin alters the lipid membrane structure: a spin label comparative study with bradykinin fragments and other cations

Activity of Bradykinin B2 Receptor Is Regulated by Long-Chain Polyunsaturated Fatty Acids

The molecular and cellular mechanisms by which long-chain polyunsaturated fatty acids (LCPUFA) exert their beneficial effects on cardiovascular health remain obscure. While both LCPUFA and bradykinin (BK) signaling pathway play a role in the cardiovascular system, any direct link between the two is yet to be established. Using picosecond time-resolved fluorescence microscopy and a genetically engineered bradykinin B₂ receptor (B₂R) sensor (B2K-CC), we detected LCPUFA-induced conformational responses in the B₂R similar to those caused by its cognate ligand, BK. The selective B₂R antagonist (HOE-140) blocked the eicosapentaenoic acid (EPA, C20∶5, n-3) induced conformational response of the B2K-CC. Further analysis suggests that LCPUFA are capable of direct, B₂R-dependent activation of extracellular ligand-regulated kinases (ERK). From a wide range of fatty acids studied, varying in chain length, saturation, and position of double bonds, EPA, docosahexaenoic (DHA, C22∶6, n-3), docosadienoic (DDA, C22∶2, n-6), and dihomo-gamma linoleic (DGLA, C20∶3, n-6) fatty acids caused the highest ERK phosphorylation. EPA or DHA dependent ERK phosphorylation was inhibited by the selective B₂R antagonist. We show that LCPUFA stimulates downstream signaling by B₂R such as B₂R-dependent phosphorylation and expression regulation of endothelial nitric-oxide synthase (eNOS). Further analysis indicated that LCPUFA also alters levels of the eNOS transcription factor, kruppel-like factor 2 (KLF2). Moreover we show that EPA increases membrane fluidity on the same time scale as B₂R conformational response, suggesting that partitioning of LCPUFA into bilayer is a primary step required for receptor activation. In summary our data show that LCPUFA activate B₂R receptor at nanomolar concentrations suggesting a novel molecular mechanism by which fatty acids may affect the cardiovascular system.

Influence of applied potential on bradykinin adsorption onto Ag, Au, and Cu electrodes

Surface‐enhanced Raman scattering, electrochemistry, and generalized two‐dimensional correlation analysis (G2DCA) methods were used to characterize bradykinin (BK), a hormone which is known to be involved in small‐cell and non‐small‐cell lung carcinoma and prostate cancer. BK was deposited onto Ag, Au, and Cu electrode surfaces under different applied electrode potentials (−1.000 V to 0.200 V) in aqueous solutions. Based on the analysis of the enhancement, the broadening, and the shifts in the wavenumbers of individual bands, specific conclusions were drawn regarding the peptide geometry and changes in this geometry that occurred when the electrode type and applied electrode potential were varied. Briefly, BK deposited onto the Ag, Au, and Cu electrode surfaces showed bands that were due to the vibrations of moieties in contact with or in close proximity to the electrode surfaces and were thus located on the same side of the polypeptide backbone. These moieties included the Phe, Arg, and Pro residues. The findings for adsorbed BK were fully supported by G2DCA, which also allowed us to determine the order in which changes occurred when the electrode potential was changed. In addition, it was found that at negative electrode potentials, the Phe rings and methylene groups interact with Ag electrode surface. No such interaction was observed for Au and Cu electrodes. Copyright © 2013 John Wiley & Sons, Ltd.

Molecular dynamics simulations of the interactions of kinin peptides with an anionic POPG bilayer

We have performed molecular dynamics simulations of peptide hormone bradykinin (BK) and its fragment des-Arg9-BK in the presence of an anionic lipid bilayer, with an aim toward delineating the mechanism of action related to their bioactivity. Starting from the initial aqueous environment, both of the peptides are quickly adsorbed and stabilized on the cell surface. Whereas BK exhibits a stronger interaction with the membrane and prefers to stay on the interface, des-Arg9-BK, with the loss of C-terminal Arg, penetrates further. The heterogeneous lipid-water interface induces β-turn-like structure in the otherwise inherently flexible peptides. In the membrane-bound state, we observed C-terminal β-turn formation in BK, whereas for des-Arg9-BK, with the deletion of Arg9, turn formation occurred in the middle of the peptide. The basic Arg residues anchor the peptide to the bilayer by strong electrostatic interactions with charged lipid headgroups. Simulations with different starting orientations of the peptides with respect to the bilayer surface lead to the same observations, namely, the relative positioning of the peptides on the membrane surface, deeper penetration of the des-Arg9-BK, and the formation of turn structures. The lipid headgroups adjacent to the bound peptides become substantially tilted, causing bilayer thinning near the peptide contact region and increase the degree of disorder in nearby lipids. Again, because of hydrogen bonding with the peptide, the neighboring lipid's polar heads exhibit considerably reduced flexibility. Corroborating findings from earlier experiments, our results provide important information about how the lipid environment promotes peptide orientation/conformation and how the peptide adapts to the environment.

Structural characterization of photopolymerizable binary liposomes containing diacetylenic and saturated phospholipids

The use of liposomes to encapsulate materials has received widespread attention for drug delivery, transfection, diagnostic reagent, and as immunoadjuvants. Phospholipid polymers form a new class of biomaterials with many potential applications in medicine and research. Of interest are polymeric phospholipids containing a diacetylene moiety along their acyl chain since these kinds of lipids can be polymerized by Ultra-Violet (UV) irradiation to form chains of covalently linked lipids in the bilayer. In particular the diacetylenic phosphatidylcholine 1,2-bis(10,12-tricosadiynoyl)-sn-glycero-3-phosphocholine (DC8,9PC) can form intermolecular cross-linking through the diacetylenic group to produce a conjugated polymer within the hydrocarbon region of the bilayer. As knowledge of liposome structures is certainly fundamental for system design improvement for new and better applications, this work focuses on the structural properties of polymerized DC8,9PC:1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) liposomes. Liposomes containing mixtures of DC8,9PC and DMPC, at different molar ratios, and exposed to different polymerization cycles, were studied through the analysis of the electron spin resonance (ESR) spectra of a spin label incorporated into the bilayer, and the calorimetric data obtained from differential scanning calorimetry (DSC) studies. Upon irradiation, if all lipids had been polymerized, no gel-fluid transition would be expected. However, even samples that went through 20 cycles of UV irradiation presented a DSC band, showing that around 80% of the DC8,9PC molecules were not polymerized. Both DSC and ESR indicated that the two different lipids scarcely mix at low temperatures, however few molecules of DMPC are present in DC8,9PC rich domains and vice versa. UV irradiation was found to affect the gel-fluid transition of both DMPC and DC8,9PC rich regions, indicating the presence of polymeric units of DC8,9PC in both areas. A model explaining lipids rearrangement is proposed for this partially polymerized system.

DNA alters the bilayer structure of cationic lipid diC14-amidine: a spin label study

Cationic lipids-DNA complexes (lipoplexes) have been used for delivery of nucleic acids into cells in vitro and in vivo. Despite the fact that, over the last decade, significant progress in the understanding of the cellular pathways and mechanisms involved in lipoplexes-mediated gene transfection have been achieved, a convincing relationship between the structure of lipoplexes and their in vivo and in vitro transfection activity is still missing. How does DNA affect the lipid packing and what are the consequences for transfection efficiency is the point we want to address here. We investigated the bilayer organization in cationic liposomes by electron spin resonance (ESR). Phospholipids spin labeled at the 5th and 16th carbon atoms were incorporated into the DNA/diC14-amidine complex. Our data demonstrate that electrostatic interactions involved in the formation of DNA-cationic lipid complex modify the packing of the cationic lipid membrane. DNA rigidifies the amidine fluid bilayer and fluidizes the amidine rigid bilayer just below the gel-fluid transition temperature. These effects were not observed with single nucleotides and are clearly related to the repetitive charged motif present in the DNA chain and not to a charge-charge interaction. These modifications of the initial lipid packing of the cationic lipid may reorient its cellular pathway towards different routes. A better knowledge of the cationic lipid packing before and after interaction with DNA may therefore contribute to the design of lipoplexes capable to reach specific cellular targets.

Fusion Peptide−Phospholipid Noncovalent Interactions As Observed by Nanoelectrospray FTICR−MS

Viral infections are propagated by the fusing of the viral membrane with a host cell membrane. Initiation of the fusion process occurs upon perturbation of the membrane of the cell under attack by a subunit of the viral protein known as a fusion peptide. Fusion peptides must insert into the lipid-rich host cell membrane to initiate rupture and merging of the two entities, but much remains unknown about the details of the fusion process. We present detailed electrospray mass spectrometry studies of binding specificities of model fusion peptides P294 and P326 with cell membrane phospholipids, i.e., phosphatidylcholines (PCs, such as 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC)) and phosphatidylglycerols (PGs, such as 1,2-dimyristoyl-sn-glycero-3-[phospho-rac-(1-glycerol)] (DMPG)). The fusion peptides clearly bind more strongly to negatively charged DMPG than to zwitterionic DMPC. Detected binding between P294/P326 and PC/PG in 100% aqueous solution was disrupted by addition of methanol, which is known to weaken hydrophobic interactions; a higher percentage of methanol was needed to destroy a stronger initial binding. Further increases in the methanol volume fraction generally resulted in a reappearance of peptide-lipid binding, with binding strength quotients of 1,2-dilauroyl-sn-glycero-3-phosphocholine (DLPC)/1,2-dilauroyl-sn-glycero-3-[phospho-rac-(1-glycerol)] (DLPG)-peptide complexes rising more steeply than those of DMPC/DMPG-peptide complexes. Compared to fusion peptides P294 and P326, a hydrophilic peptide, fibrinopeptide B, showed much weaker affinity for zwitterionic DMPC, but had moderate binding affinity to negatively charged DMPG in 100% aqueous solutions. However, upon progressive addition of methanol, this hydrophilic peptide showed only a minor initial decrease in binding to DMPG before the detected binding eventually increased. These results contrast with those obtained for the hydrophobic peptides, and offer corroborative evidence that hydrophobic interactions play a key role in the mass spectrometrically observed binding between fusion peptides and phospholipids. Because the rate of viral infection has been found to be pH-dependent, the effect of initial solution pH on peptide-lipid binding was also studied. As the pH was lowered, P326-DMPC binding had a steep and immediate weakening, whereas the P294-DMPC binding was slightly strengthened at pH 3.7 and then gradually weakened with a further decrease in pH. Both P326 and P294 exhibited affinities toward unsaturated lipids; (18:1)PC bound slightly more strongly to P294 than (18:3)PC. These experiments offer further evidence of the ability of electrospray mass spectrometry to provide binding information concerning noncovalent interactions that were established principally by the hydrophobic effect in solution.

Interaction and structural study of kinin peptide bradykinin and ganglioside monosialylated 1 micelle

Partitioning of small proteins and peptides from the aqueous to membrane phase is often coupled with folding. In this work we examine the binding and folding of the kinin peptide, bradykinin (BK), in the presence of the ganglioside monosialylated 1 (GM1) micelle. Using two-dimensional NMR techniques, we have shown that at low concentration, GM1 micelle is able to induce a turn conformation to BK. A pulsed-field gradient diffusion NMR study indicated that the peptide partitions into the GM1 micelle with a DeltaG(part) of -3.14 +/- 0.03 kcal/mol. A saturation transfer difference (STD) NMR study indicated that the binding is mostly through hydrophobic residues.

Conformational alteration of bradykinin in presence of GM1 micelle

We report here the interaction of bradykinin with ganglioside GM1 by circular dichroism, steady-state fluorescence, and one-dimensional 1H NMR spectroscopy. Circular dichroism spectroscopy is indicative of a turn formation of bradykinin backbone in the presence of GM1 micelle. The fluorescence quenching efficiencies of iodide and acrylamide are substantially reduced, indicating a shielding of phenylalanine residue of bradykinin from aqueous environment. Significant line broadening of NMR resonances of bradykinin, suggestive of motional restriction, is observed.

DMPG gel-fluid thermal transition monitored by a phospholipid spin labeled at the acyl chain end

Low ionic strength aqueous dispersion of dimyristoyl phosphatidylglycerol (DMPG) presents a rather peculiar gel-fluid thermal transition behavior. The lipid main phase transition occurs over a large temperature interval (ca. 17 degrees C), along which several calorimetric peaks are observed. Using lipids spin labeled at the acyl chain end, a two-peak electron spin resonance (ESR) spectrum is observed along that temperature transition region (named intermediate phase), at three different microwave frequencies: L-, X- and Q-bands. The intermediate phase ESR spectra are analyzed, and shown to be most likely due to spin labels probing two distinct types of lipid organization in the DMPG bilayer. Based on the ESR spectra parameters, a model for the DMPG intermediate phase is proposed, where rather fluid and hydrated domains, possibly high curvature regions, coexist with patches that are more rigid and hydrophobic.

Ortho-aminobenzoic acid-labeled bradykinins in interaction with lipid vesicles: fluorescence study

The peptide hormone bradykinin (BK) (Arg(1)-Pro(2)-Pro(3)-Gly(4)-Phe(5)-Ser(6)-Pro(7)-Phe(8)-Arg(9)) and its shorter homolog BK(1-5) (Arg(1)-Pro(2)-Pro(3)-Gly(4)-Phe(5)) were labeled with the extrinsic fluorescent probe ortho-aminobenzoic acid (Abz) bound to the N-terminal and amidated in the C-terminal carboxyl group (Abz-BK-NH(2) and Abz-BK(1-5)-NH(2)). The fragment des-Arg(9)-BK was synthesized with the Abz fluorescent probe attached to the 3-amino group of 2,3-amino propionic acid (DAP), which positioned the Abz group at the C-terminal side of BK sequence, constituting the peptide des-Arg(9)-BK-DAP(Abz)-NH(2). The spectral characteristics of the probe were similar in the three peptides, and their fluorescent properties were monitored to study the interaction of the peptides with anionic vesicles of dimyristoylphosphatidylglycerol (DMPG). Time-resolved fluorescence experiments showed that the fluorescence decay of the peptides was best described by double-exponential kinetics, with mean lifetimes values around 8.0 ns in buffer pH 7.4 that increased about 10% in the presence of DMPG vesicles. About a 10-fold increase, compared with the values in aqueous solution, was observed in the steady-state anisotropy in the presence of vesicles. A similar increase was also observed for the rotational correlation times obtained from time-resolved anisotropy decay profiles, and related to the overall tumbling of the peptides. Equilibrium binding constants for the peptide-lipid interaction were examined monitoring anisotropy values in titration experiments and the electrostatic effects were evaluated through Gouy-Chapman potential calculations. Without corrections for electrostatic effects, the labeled fragment Abz-BK(1-5)-NH(2) presented the major affinity for DMPG vesicles. Corrections for the changes in peptide concentration due to electrostatic interactions suggested higher affinity of the BK fragments to the hydrophobic phase of the bilayer.

Structural and thermal characterization of dioctadecyldimethylammonium bromide dispersions by spin labels

Dioctadecyldimethylammonium bromide (DODAB) dispersions obtained by simply mixing the amphiphile in water, and by bath-sonication, were investigated by electron spin resonance (ESR) of stearic acids and their methyl ester derivatives, labeled at the 5th and 16th carbons of the acyl chain. The ESR spectra indicate that the non-sonicated dispersions are formed mainly by one population of DODAB vesicles, either in the gel (T<T(m)) or in the liquid-crystalline (T>T(m)) state. Around T(m) there is a co-existence of the two phases, with a thermal hysteresis of about 3.2 degrees C. In sonicated DODAB dispersions, spin labels indicate two different environments even for temperatures far below T(m): one similar to that obtained with non-sonicated samples, a gel phase, and another one in the liquid-crystalline state. The fluid phase domain present below T(m) could correspond to either the periphery of bilayer fragments, reported to be present in sonicated DODAB dispersions, or to high curvature vesicles.

Thermal transitions of DMPG bilayers in aqueous solution: SAXS structural studies

Dimyristoylphosphatidylglycerol (DMPG) has been extensively studied as a model for biological membranes, since phosphatidylglycerol is the most abundant anionic phospholipid in prokaryotic cells. At low ionic strengths, this lipid presents a peculiar thermal behavior, with two sharp changes in the light scattering profile, at temperatures named here T(on)(m) and T(off)(m). Structural changes involved in the DMPG thermal transitions are here investigated by small angle X-ray scattering (SAXS), and compared to the results yielded by differential scanning calorimetry (DSC) and electron spin resonance (ESR). The SAXS results show a broad peak, indicating that DMPG is organized in single bilayers, for the range of temperature studied (10-45 degrees C). SAXS intensity shows an unusual effect, starting to decrease at T(on)(m), and presenting a sharp increase at T(off)(m). The bilayer electron density profiles, obtained from modeling the SAXS curves, show a gradual decrease in electron density contrast (attributed to separation between charged head groups) and in bilayer thickness between T(on)(m) and T(off)(m). Results yielded by SAXS, DSC and ESR indicate that a chain melting process starts at T(on)(m), but a complete fluid phase exists only for temperatures above T(off)(m), with structural changes occurring at the bilayer level in the intermediate region.

Correlation between the effects of a cationic peptide on the hydration and fluidity of anionic lipid bilayers: a comparative study with sodium ions and cholesterol

The cationic tridecapeptide alpha-melanocyte stimulating hormone (alpha-MSH) is known to interact with anionic vesicles of 1,2-dimyristoyl-sn-glycero-3-phosphoglycerol (DMPG), partially penetrating the lipid membrane. In the lipid liquid crystal phase, phospholipid derivatives spin labeled at the different C-atoms along the acyl chain, show that the peptide increases the bilayer packing at all depths. Parallel to that, there is an increase in the probe's isotropic hyperfine splittings, indicating that the peptide significantly decreases the membrane hydrophobic barrier. Accordingly, it is suggested that the increase in membrane packing yielded by alpha-MSH is partly due to a greater level of interchain hydration. This result is compared to the increase in packing and decrease in polarity yielded by cholesterol, and the absence of structural or polar alterations with Na+. The latter result shows that the peptide effect is not related to an increase of positive charges at the anionic vesicle surface. Alterations on the lipid bilayer polar profile measured by the nitroxide hyperfine splitting z component in frozen samples are shown to be different from those obtained at room temperature. However, it is shown here that a certain correlation can be drawn between the increase in polarity measured in frozen samples and the packing effect caused by the different molecules in the lipid gel phase.