Interaction of the neuropeptide met-enkephalin with zwitterionic and negatively charged bicelles as viewed by 31P and 2H solid-state NMR

Arrangement of lipid vesicles and bicelle-like structures formed in the presence of Aβ(25-35) peptide

Our complementary experimental data and molecular dynamics (MD) simulations results reveal the structure of previously observed lipid bicelle-like structures (BLSs) formed in the presence of amyloid-beta peptide Aβ(25-35) below the main phase transition temperature (Tm) of saturated phosphatidylcholine lipids and small unilamellar vesicles (SUVs) above this temperature. First, we show by using solid-state 31P nuclear magnetic resonance (NMR) spectroscopy that our BLSs being in the lipid gel phase demonstrate magnetic alignment along the magnetic field of NMR spectrometer and undergo a transition to SUVs in the lipid fluid phase when heated through the Tm. Secondly, thanks to the BLS alignment we present their lipid structure. Lipids are found located not only in the flat bilayered part but also around its perimeter, which is corroborated by the results of coarse-grained (CG) MD simulations. Finally, peptides appear to mix randomly with lipids in SUVs while assuming predominantly unordered secondary structures revealed by circular dichroism (CD), Raman spectroscopy, and all-atom MD simulations. Importantly, the former is changing little when the system undergoes morphological transitions between BLSs and SUVs. Our structural results then offer a platform for studying and understanding mechanisms of morphological transformations caused by the disruptive effect of amyloid-beta peptides on the lipid bilayer.

Ultrafast vibrational dynamics of the tyrosine ring mode and its application to enkephalin insertion into phospholipid membranes as probed by two-dimensional infrared spectroscopy

Enkephalins are small opioid peptides whose binding conformations are catalyzed by phospholipid membranes. Binding to opioid receptors is determined by the orientation of tyrosine and phenylalanine side chains. In this work, we investigate the effects of different charged phospholipid headgroups on the insertion of the tyrosine side chain into a lipid bilayer using a combination of 2D IR spectroscopy, anharmonic DFT calculations, and third order response function modeling. The insertion is probed by using the ∼1515 cm^-1 tyrosine ring breathing mode, which we found exhibits rich vibrational dynamics on the picosecond timescale. These dynamics include rapid intramolecular vibrational energy redistribution (IVR), where some of the energy ends up in a dark state that shows up as an anharmonically shifted combination band. The waiting-time dependent 2D IR spectra also show an unusual line shape distortion that affects the extraction of the frequency-frequency correlation function (FFCF), which is the dynamic observable of interest that reflects the tyrosine side chain's insertion into the lipid bilayer. We proposed three models to account for this distortion: a hot-state exchange model, a local environment dependent IVR model, and a coherence transfer model. A qualitative analysis of these models suggests that the local environment dependent IVR rate best explains the line shape distortion, while the coherence transfer model best reproduced the effects on the FFCF. Even with these complex dynamics, we found that the tyrosine ring mode's FFCF is qualitatively correlated with the degree of insertion expected from the different phospholipid headgroups.

Bicelles and nanodiscs for biophysical chemistry

Membrane nanoobjects are very important tools to study biomembrane properties. Two types are described herein: Bicelles and Nanodiscs. Bicelles are obtained by thorough water mixing of long chain and short chain lipids and may take the form of membranous discs of 10-50 nm. Temperature-composition-hydration diagrams have been established for Phosphatidylcholines and show limited domains of existence. Bicelles can be doped with charged lipids, surfactants or with cholesterol and offer a wide variety of membranous platforms for structural biology. Internal dynamics as measured by solid-state NMR is very similar to that of liposomes in their fluid phase. Because of the magnetic susceptibility anisotropy of the lipid chains, discs may be aligned along or perpendicular to the magnetic field. They may serve as weak orienting media to provide distance information in determining the 3D structure of soluble proteins. In different conditions they show strong orienting properties which may be used to study the 3D structure, topology and dynamics of membrane proteins. Lipid Bicelles with biphenyl chains or doped with lanthanides show long lasting remnant orientation after removing the magnetic field due to smectic-like properties. An alternative to pure lipid Bicelles is provided by nanodiscs where the half torus composed by short chain lipids is replaced by proteins. This renders the nano-objects less fragile as they can be used to stabilize membrane protein assemblies to be studied by electron microscopy. Internal dynamics is again similar to liposomes except that the phase transition is abolished, possibly due to lateral constrain imposed by the toroidal proteins limiting the disc size. Advantages and drawbacks of both nanoplatforms are discussed.

The influence of the stereochemistry and C-end chemical modification of dermorphin derivatives on the peptide-phospholipid interactions

In this work the conformation of dermorphin, Tyr-D-Ala-Phe-Gly-Tyr-Pro-Ser-NH₂, an opioid peptide and its analogues with different stereochemistry of alanine and different C-terminus is studied in aqueous and membrane environments. Using two-dimensional NMR techniques we demonstrate that in D₂O/H₂O peptides with D-alanine have extended conformation, while for the L-isomers more compact conformation is preferred. The analysis of ROESY HR MAS spectra of the peptides interacting with the DMPC bilayer indicates that both stereoisomers have still more extended conformation compared to aqueous phase, as shown by much weaker intermolecular interactions. The influence of Ala residue stereochemistry is also reflected in the interactions of the studied peptides with model membranes, as shown by the ³¹P NMR static spectra, in which the shapes of the phosphorus NMR signals originating from D-isomers correspond to spherically shaped vesicles in the presence of external magnetic field, in comparison to a more elongated ones observed for L-isomers, while TEM photographs shows that upon addition of D-isomers larger lipid vesicles are formed, in contrast to smaller ones for L-isomers. The location of aromatic fragments of dermorphins in the membrane is determined based on static ²H NMR and ¹H¹H RFDR MAS experiments. All aromatic rings were found to be inserted in the hydrophobic part of the bilayer, with the exception of the Tyr5 rings of D-Ala dermorphins. The influence of the C-terminal modification was found to be almost imperceptible.

Effects of Amphipathic Polypeptides on Membrane Organization Inferred from Studies Using Bicellar Lipid Mixtures

SP-B_63-78, a lung surfactant protein fragment, and magainin 2, an antimicrobial peptide, are amphipathic peptides with the same overall charge but different biological functions. Deuterium nuclear magnetic resonance has been used to compare the interactions of these peptides with dispersions of 1,2-dimyristoyl- sn-glycero-3-phophocholine (DMPC)/1,2-dihexanoyl- sn-glycero-3-phophocholine (DHPC) (4:1) and DMPC/1,2-dimyristoyl- sn-glycero-3-phopho-(1'-rac-glycerol) (DMPG)/DHPC (3:1:1), two mixtures of long-chain and short-chain lipids that display bicellar behavior. This study exploited the sensitivity of a bicellar system structural organization to factors that modify partitioning of their lipid components between different environments. In small bicelle particles formed at low temperatures, short-chain components preferentially occupy curved rim environments around bilayer disks of the long-chain components. Changes in chain order and lipid mixing, on heating, can drive transitions to more extended assemblies including a magnetically orientable phase at intermediate temperature. In this work, neither peptide had a substantial effect on the behavior of the zwitterionic DMPC/DHPC mixture. For bicellar mixtures containing the anionic lipid DMPG, the peptide SP-B_63-78 lowered the temperature at which magnetically orientable particles coalesced into more extended lamellar structures. SP-B_63-78 did not promote partitioning of the zwitterionic and anionic long-chain lipid components into different environments. Magainin 2, on the other hand, was found to promote separation of the anionic lipid, DMPG, and the zwitterionic lipid, DMPC, into different environments for temperatures above 34 °C. The contrast between the effects of these two peptides on the lipid mixtures studied appears to be consistent with their functional roles in biological systems.

Behavior of Most Widely Spread Lipids in Isotropic Bicelles

Isotropic bicelles are a widely used membrane mimetic for structural studies of membrane proteins and their transmembrane domains. Simple and cheap in preparation, they contain a patch of lipid bilayer that reproduces the native environment of membrane proteins. Despite the obvious power of bicelles in reproducing the various kinds of environments, the vast majority of structural studies employ the single lipid/detergent system. On the other hand, even if the alternative bicelle composition is used, the properties of mixtures are not characterized, and the mere presence of lipid bilayer and discoidal shape of bicelle particles is not confirmed. Here we present an extensive investigation of various bicellar mixtures and describe the behavior of bicelles with lipids other than classical DMPC, namely sphingomyelins (SM), phosphatidylethanolamines (PE), phosphatidylglycerols (PG), phosphatidylserines (PS), and cholesterol. These lipids are rarely used in modern structural biology, but can help a lot in understanding the influence of the membrane composition on the properties of both integral and peripheral membrane proteins. Additionally, the ability of diheptanoylphosphatidylcholine (DH7PC) to serve as a rim-forming agent was investigated. We followed the phase transitions as revealed by ³¹P NMR and size of particles measured by ¹H NMR diffusion as the criteria of the proper morphology and structure of bicelles. As an outcome, we state that SM exclusively, and PG/PS in mixtures with zwitterionic lipids can form small isotropic bicelles, which reproduce the key features of lipid behavior in bilayers. Mixtures, containing exclusively the anionic lipids, fail to reveal the lipid phase transition and do not follow the size predicted for the ideal bicelle particles. PE and DH7PC are the unwanted components of bicellar mixtures, and cholesterol can be added to bicelles, however, with certain precautions. In combination with our several most recent works, this study provides a practical guide for the preparation of small isotropic bicelles.

Phase Transitions in Small Isotropic Bicelles

Isotropic phospholipid bicelles are one of the most prospective membrane mimetics for the structural studies of membrane proteins in solution. Recent works provided an almost full set of data regarding the properties of isotropic bicelles; however, one major aspect of their behavior is still under consideration: the possible mixing between the lipid and detergent in the bilayer area. This problem may be resolved by studying the lipid phase transitions in bicelle particles. In the present work, we investigate two effects: phase transitions of bilayer lipids and temperature-induced growth of isotropic bicelles using the NMR spectroscopy. We propose an approach to study the phase transitions in isotropic bicelles based on the properties of ³¹P NMR spectra of bilayer-forming lipids. We show that phase transitions in small bicelles are "fractional", particles with the liquid-crystalline and gel bilayers coexist in solution at certain temperatures. We study the effects of lipid fatty chain type and demonstrate that the behavior of various lipids in bilayers is reproduced in the isotropic bicelles. We show that the temperature-induced growth of isotropic bicelles is not related directly to the phase transition but is the result of the reversible fusion of bicelle particles. In accordance with our data, rim detergents also have an impact on phase transitions: detergents that resist the temperature-induced growth provide the narrowest and most expressed transitions at higher temperatures. We demonstrate clearly that phase transitions take place even in the smallest bicelles that are applicable for structural studies of membrane proteins by solution NMR spectroscopy. This last finding, together with other data draws a thick line under the long-lasting argument about the relevance of small isotropic bicelles. We show with certainty that the small bicelles can reproduce the most fundamental property of lipid membranes: the ability to undergo phase transition.

Membrane solid-state NMR in Canada: A historical perspective

This manuscript presents an overview of more than 40years of membrane solid-state nuclear magnetic resonance (NMR) research in Canada. This technique is a method of choice for the study of the structure and dynamics of lipid bilayers; bilayer interactions with a variety of molecules such as membrane peptides, membrane proteins and drugs; and to investigate membrane peptide and protein structure, dynamics, and topology. Canada has a long tradition in this field of research, starting with pioneering work on natural and model membranes in the 1970s in a context of emergence of biophysics in the country. The 1980s and 1990s saw an emphasis on studying lipid structures and dynamics, and peptide-lipid and protein-lipid interactions. The study of bicelles began in the 1990s, and in the 2000s there was a rise in the study of membrane protein structures. Novel perspectives include using dynamic nuclear polarization (DNP) for membrane studies and using NMR in live cells. This article is part of a Special Issue entitled: Biophysics in Canada, edited by Lewis Kay, John Baenziger, Albert Berghuis and Peter Tieleman.

Comparative study of the structure and interaction of the pore helices of the hERG and Kv1.5 potassium channels in model membranes

The hERG channel is a voltage-gated potassium channel found in cardiomyocytes that contributes to the repolarization of the cell membrane following the cardiac action potential, an important step in the regulation of the cardiac cycle. The lipids surrounding K⁺ channels have been shown to play a key role in their regulation, with anionic lipids shown to alter gating properties. In this study, we investigate how anionic lipids interact with the pore helix of hERG and compare the results with those from Kv1.5, which possesses a pore helix more typical of K⁺ channels. Circular dichroism studies of the pore helix secondary structure reveal that the presence of the anionic lipid DMPS within the bilayer results in a slight unfolding of the pore helices from both hERG and Kv1.5, albeit to a lesser extent for Kv1.5. In the presence of anionic lipids, the two pore helices exhibit significantly different interactions with the lipid bilayer. We demonstrate that the pore helix from hERG causes significant perturbation to the order in lipid bicelles, which contrasts with only small changes observed for Kv1.5. These observations suggest that the atypical sequence of the pore helix of hERG may play a key role in determining how anionic lipids influence its gating.

Characterization of Small Isotropic Bicelles with Various Compositions

Structural studies of membrane proteins are of great importance and interest, with solution and solid state NMR spectroscopy being very promising tools for that task. However, such investigations are hindered by a number of obstacles, and in the first place by the fact that membrane proteins need an adequate environment that models the cell membrane. One of the most widely used and prospective membrane mimetics is isotropic bicelles. While large anisotropic bicelles are well-studied, the field of small bicelles contains a lot of "white spots". The present work reports the radii of particles and concentration of the detergents in the monomeric state in solutions of isotropic bicelles, formed by 1,2-dihexanoyl-sn-glycero-3-phosphocholine (DHPC), 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate (CHAPS), 3-[(3-cholamidopropyl)dimethylammonio]-2-hydroxy-1-propanesulfonate (CHAPSO), and sodium cholate, as a function of lipid/detergent ratio and temperature. These parameters were measured using (1)H NMR diffusion spectroscopy for the bicelles composed of lipids with saturated fatty chains of different length and lipids, containing unsaturated fatty acid residue. The influence of a model transmembrane protein (membrane domain of rat TrkA) on the properties of bicelles and the effect of the bicelle size and composition on the properties of the transmembrane protein were investigated with heteronuclear NMR and nuclear Overhauser effect spectroscopy. We show that isotropic bicelles that are applicable for solution NMR spectroscopy behave as predicted by the theoretical models and are likely to be bicelles rather than mixed micelles. Using the obtained data, we propose a simple approach to control the size of bicelles at low concentrations. On the basis of our results, we compared different rim-forming agents and selected CHAPS as a detergent of choice for structural studies in bicelles, if the deuteration of the detergent is not required.

Functional stability of rhodopsin in a bicelle system: evaluating G protein activation by rhodopsin in bicelles

Rhodopsin is a prototypical member of the G protein-coupled receptors (GPCRs). This photoreceptor is responsible for initiating the visual signaling transduction cascade upon interaction with its heterotrimeric G protein, transducin (Gt), after light activation. Like all transmembrane proteins, rhodopsin is embedded within a phospholipid bilayer. Many studies have proposed that the membrane composition of this bilayer is an important factor for receptor function during the activation process. Here we describe the methods and assays used to evaluate the function of purified and reconstituted rhodopsin in bicelles.

Characterization of the morphology of fast-tumbling bicelles with varying composition

Small, fast-tumbling bicelles are frequently used in solution NMR studies of protein-lipid interactions. For this purpose it is critical to have information about the organization of the lipids within the bicelle structure. We have studied the morphology of small, fast-tumbling bicelles containing DMPC and DHPC as a function of temperature, lipid concentration, and the relative ratio (q value) of lipid (DMPC) to detergent (DHPC) amounts. Dynamic light scattering and cryo-transmission electron microscopy techniques were used to measure the size of the bicelles and to monitor the shape and dispersity of the particles in the samples. The stability and size of DMPC-containing bicelle mixtures were found to be highly dependent on temperature and the total lipid concentration for mixtures with q = 1 and q = 1.5. Stable DMPC/DHPC bicelles are only formed at low q values (0.5). Bicelle mixtures with q > 0.5 appear to be multidisperse containing more than one component, one with r(H) around 2.5 nm and one with r(H) of 6-8 nm. This is interpreted as a coexistence of small (possibly mixed micelles) bicelles and much larger bicelles. Incubating the sample at 37 °C increases the phase separation. Moreover, low total amphiphile concentrations and low q values lead to the formation of a temperature-independent morphology, interpreted as the formation of small particles in which the DHPC and DMPC are more mixed. On the basis of these results, we propose the existence of a critical bicelle concentration, a parameter that determines the existence of bilayered bicelles, which varies with q value. This polymorphism was not observed at any concentrations for q = 0.5 bicelles, for which a small but detectable temperature dependence was observed at high concentrations. The results demonstrate that q = 0.5 mixtures predominantly form "classical" bicelles, but that caution is needed when using fast-tumbling mixtures with q values higher than 0.5.

Investigation of the mechanism of action of novel amphipathic peptides: insights from solid-state NMR studies of oriented lipid bilayers

We have investigated in the present study the effect of both non-selective and selective cationic 14-mer peptides on the lipid orientation of DMPC bilayers by (31)P solid-state nuclear magnetic resonance (NMR) spectroscopy. Depending on the position of substitution, these peptides adopt mainly either an α-helical structure able to permeabilize DMPC and DMPG vesicles (non-selective peptides) or an intermolecular β-sheet structure only able to permeabilize DMPG vesicles (selective peptides). Several systems have been investigated, namely bilayers mechanically oriented between glass plates as well as bicelles oriented with their normal perpendicular or parallel to the external magnetic field. The results have been compared with spectral simulations with the goal of elucidating the difference in the interaction of these two types of peptides with zwitterionic lipid bilayers. The results indicate that the perturbation induced by selective peptides is much greater than that induced by non-selective peptides in all the lipid systems investigated, and this perturbation has been associated to the aggregation of the selective β-sheet peptides in these systems. On the other hand, the oriented lipid spectra obtained in the presence of non-selective peptides suggest the presence of toroidal pores. This article is part of a Special Issue entitled: Interfacially Active Peptides and Proteins. Guest Editors: William C. Wimley and Kalina Hristova.

Interaction of methionine-enkephalins with raft-forming lipids: monolayers and BAM experiments

Enkephalins (Tyr-Gly-Gly-Phe-Met/Leu) are opioid peptides with proven antinociceptive action in organism. They interact with opioid receptors belonging to G-protein coupled receptor superfamily. It is known that these receptors are located preferably in membrane rafts composed mainly of sphingomyelin (Sm), cholesterol (Cho), and phosphatidylcholine. In the present work, using Langmuir's monolayer technique in combination with Wilhelmy's method for measuring the surface pressure, the interaction of synthetic methionine-enkephalin and its amidated derivative with 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), Sm, and Cho, as well as with their double and triple mixtures, was studied. From the pressure/area isotherms measured, the compressional moduli of the lipids and lipid-peptide monolayers were determined. Our results showed that the addition of the synthetic enkephalins to the monolayers studied led to change in the lipid monolayers characteristics, which was more evident in enkephalinamide case. In addition, using Brewster angle microscopy (BAM), the surface morphology of the lipid monolayers, before and after the injection of both enkephalins, was determined. The BAM images showed an increase in surface density of the mixed surface lipids/enkephalins films, especially with double and triple component lipid mixtures. This effect was more pronounced for the enkephalinamide as well. These observations showed that there was an interaction between the peptides and the raft-forming lipids, which was stronger for the amidated peptide, suggesting a difference in folding of both enkephalins. Our research demonstrates the potential of lipid monolayers for elegant and simple membrane models to study lipid-peptide interactions at the plane of biomembranes.

Response to hydrostatic pressure of bicellar dispersions containing an anionic lipid: pressure-induced interdigitation

Bicellar dispersions of chain perdeuterated 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC-d54), 1,2-dimyristoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (DMPG), and 1,2-dihexanoyl-sn-glycero-3-phosphocholine (DHPC), with molar ratios of 3:1:1, were studied using variable-pressure (2)H NMR spectroscopy at hydrostatic pressures up to 125 MPa. Upon warming of the dispersions, spectra at ambient pressure indicated a progressive coalescence from small bilayered disks undergoing isotropic reorientation to more extended micellar structures in which spectra indicated anisotropic reorientation and, under some conditions, magnetic orientation and finally to randomly oriented lamellae or multilamellar vesicles. Temperatures for the onsets of anisotropic reorientation and random lamellar orientation increased with pressure at rates of 0.22 and 0.15 °C/MPa, respectively. In the 3.5-T magnetic field used for this work, magnetic orientation within the intermediate phase was not observed at 83 MPa or higher pressures. Comparison of spectra obtained at fixed pressure showed significant asymmetry between behaviors upon warming and cooling. For samples of DMPC-d54/DMPG/DHPC (3:1:1), but not DMPC-d54/DHPC (4:1), a persistent interdigitated phase was formed after repeated cooling from high temperature at 83 MPa. This is likely a metastable phase and might reflect kinetic trapping of the short-chain lipid component, DHPC, in a nonequilibrium spatial distribution as temperature is lowered at high pressure. Bicellar dispersions typically behave differently upon warming and cooling, and these observations could provide some insight into the observed behaviors in such systems. This work also suggests the possibility of trapping bicellar dispersions in persistent nonequilibrium morphologies.

Interaction of the C-terminal peptide of pulmonary surfactant protein B (SP-B) with a bicellar lipid mixture containing anionic lipid

The hydrophobic lung surfactant SP-B is essential for respiration. SP-B promotes spreading and adsorption of surfactant at the alveolar air-water interface and may facilitate connections between the surface layer and underlying lamellar reservoirs of surfactant material. SP-B_63–78 is a cationic and amphipathic helical peptide containing the C-terminal helix of SP-B. ²H NMR has been used to examine the effect of SP-B_63–78 on the phase behavior and dynamics of bicellar lipid dispersions containing the longer chain phospholipids DMPC-d₅₄ and DMPG and the shorter chain lipid DHPC mixed with a 3∶1∶1 molar ratio. Below the gel-to-liquid crystal phase transition temperature of the longer chain components, bicellar mixtures form small, rapidly reorienting disk-like particles with shorter chain lipid components predominantly found around the highly curved particle edges. With increasing temperature, the particles coalesce into larger magnetically-oriented structures and then into more extended lamellar phases. The susceptibility of bicellar particles to coalescence and large scale reorganization makes them an interesting platform in which to study peptide-induced interactions between lipid assemblies. SP-B_63–78 is found to lower the temperature at which the orientable phase transforms to the more extended lamellar phase. The peptide also changes the spectrum of motions contributing to quadrupole echo decay in the lamellar phase. The way in which the peptide alters interactions between bilayered micelle structures may provide some insight into some aspects of the role of full-length SP-B in maintaining a functional surfactant layer in lungs.

Induction on differentiation and modulation of bone marrow progenitor of dendritic cell by methionine enkephalin (MENK)

Methionine enkephalin (MENK), the endogenous neuropeptide, is known to exert direct effects on the neuroendocrine and the immune systems and participates in regulation of various functions of cells related to both the innate and adaptive immune systems. Dendritic cells (DCs) play important role in initiating and regulating T cell responses. The aim of this work is to investigate the effects of MENK on differentiation, maturation, and function of DCs derived from murine bone marrow progenitors (BM-derived DCs). Our result showed that MENK could induce BM-derived DCs to polarize predominantly to mDC subtype, rather than pDC both in vivo and in vitro, and this was in favor of Th1 response. BM-derived DCs, after treatment with MENK, up-regulated the expressions of MHC class II and key costimulatory molecules. Result by RT-PCR showed MENK could increase expressions of delta and kappa receptors on BM-derived DCs. Also MENK promoted BM-derived DCs to secret higher levels of proinflammatory cytokines of IL-12p70, TNF-α. Furthermore, differentiated BM-derived DCs treated with MENK displayed higher activity to induce allogeneic T cell proliferation and MENK also inhibited tumor growth in vivo and induced apoptosis of tumor cells in vitro. Thus, it is concluded that MENK could be an effective inducer of BM-derived DCs and might be a new therapeutic agent for cancer, as well as other immune handicapped disease. Also we may consider MENK as a potential adjuvant in vaccine preparation.

Methionine enkephalin (MENK) improved the functions of bone marrow-derived dendritic cells (BMDCs) loaded with antigen

The aim of this investigation is to look at whether MENK could improve antitumor effect of CD8+T cell elicited by BMDCs. We investigated the effects of MENK on the differentiation, maturation, and functions of murine BMDC loaded with Rac-1 antigens (RG) and CTL of tumor specific immune response elicited by the BMDC in vitro and in vivo. The production of cytokine IL-12 and TNF-α secreted by BMDCs in the presence of MENK was assayed with ELISA and key surface markers of CD40, CD86, CD83 and MHC-II on the BMDCs were analyzed with use of flow cytometry (FCM). In addition, the activities to induce CD8+ T cell proliferation, along with displayed cytotoxicity of the CD8+T cells(CTL) by the BMDCs after treatment with MENK were determined with use of FCM as well as MTS. Our results indicated that MENK induced phenotypic and functional maturation of BMDC loaded with RG antigen, as evidenced by higher level of expression of key surface markers and more production of cytokines. Subsequently, the BMDC activated by MENK intensified immune responses mounted by CTL, resulting in stronger antitumor activity. Our results suggest that MENK could be working as an effective immune adjuvant in vaccine preparation for cancer fight and other immune related diseases. We concluded that MENK could be a positive immune modulator in the improved functions of BMDCs loaded with antigen as well as in CD8+T cell mediated anti-tumor responses.

Bicelles: lipid nanostructured platforms with potential dermal applications

Bicelles emerge as promising membrane models, and because of their attractive combination of lipid composition, small size and morphological versatility, they become new targets in skin research. Bicelles are able to modify skin biophysical parameters and modulate the skin's barrier function, acting to enhance drug penetration. Because of their nanostructured assemblies, bicelles have the ability to penetrate through the narrow intercellular spaces of the stratum corneum of the skin to reinforce its lipid lamellae. The bicelle structure also allows for the incorporation of different molecules that can be carried through the skin layers. All of these characteristics can be modulated by varying the lipid composition and experimental conditions. The remarkable versatility of bicelles is their most important characteristic, which makes their use possible in various fields. This system represents a platform for dermal applications. In this review, an overview of the main properties of bicelles and their effects on the skin are presented.

Formation of kinetically trapped nanoscopic unilamellar vesicles from metastable nanodiscs

Zwitterionic long-chain lipids (e.g., dimyristoyl phosphatidylcholine, DMPC) spontaneously form onion-like, thermodynamically stable structures in aqueous solutions (commonly known as multilamellar vesicles, or MLVs). It has also been reported that the addition of zwitterionic short-chain (i.e., dihexanoyl phosphatidylcholine, DHPC) and charged long-chain (i.e., dimyristoyl phosphatidylglycerol, DMPG) lipids to zwitterionic long-chain lipid solutions results in the formation of unilamellar vesicles (ULVs). Here, we report a kinetic study on lipid mixtures composed of DMPC, DHPC, and DMPG. Two membrane charge densities (i.e., [DMPG]/[DMPC] = 0.01 and 0.001) and two solution salinities (i.e., [NaCl] = 0 and 0.2 M) are investigated. Upon dilution of the high-concentration samples at 50 °C, thermodynamically stable MLVs are formed, in the case of both weakly charged and high salinity solution mixtures, implying that the electrostatic interactions between bilayers are insufficient to cause MLVs to unbind. Importantly, in the case of these samples small angle neutron scattering (SANS) data show that, initially, nanodiscs (also known as bicelles) or bilayered ribbons form at low temperatures (i.e., 10 °C), but transform into uniform size, nanoscopic ULVs after incubation at 10 °C for 20 h, indicating that the nanodisc is a metastable structure. The instability of nanodiscs may be attributed to low membrane rigidity due to a reduced charge density and high salinity. Moreover, the uniform-sized ULVs persist even after being heated to 50 °C, where thermodynamically stable MLVs are observed. This result clearly demonstrates that these ULVs are kinetically trapped, and that the mechanical properties (e.g., bending rigidity) of 10 °C nanodiscs favor the formation of nanoscopic ULVs over that of MLVs. From a practical point of view, this method of forming uniform-sized ULVs may lend itself to their mass production, thus making them economically feasible for medical applications that depend on monodisperse lipid-based systems for therapeutic and diagnostic purposes.

Structural Versatility of Bicellar Systems and Their Possibilities as Colloidal Carriers

Bicellar systems are lipid nanostructures formed by long- and short-chained phospholipids dispersed in aqueous solution. The morphological transitions of bicellar aggregates due to temperature, composition and time variations have been revised in this work. To this end, two bicellar systems have been considered; one formed by dimyristoyl-phosphatidylcholine (DMPC) and dihexanoyl- phosphatidylcholine (DHPC) and another formed by dipalmitoyl-phosphatidylcholine (DPPC) and DHPC. The relationship between the magnetic alignment, the morphology of the aggregates and the phase transition temperature (T_m) of lipids is discussed. In general terms, the non-alignable samples present rounded objects at temperature below the T_m. Above this temperature, an increase of viscosity is followed by the formation of large elongated aggregates. Alignable samples presented discoidal objects below the T_m. The best alignment was achieved above this temperature with large areas of lamellar stacked bilayers and some multilamellar vesicles. The effect of the inclusion of ceramides with different chain lengths in the structure of bicelles is also revised in the present article. A number of physical techniques show that the bicellar structures are affected by both the concentration and the type of ceramide. Systems are able to incorporate 10% mol of ceramides that probably are organized forming domains. The addition of 20% mol of ceramides promotes destabilization of bicelles, promoting the formation of mixed systems that include large structures. Bicellar systems have demonstrated to be morphologically stable with time, able to encapsulate different actives and to induce specific effects on the skin. These facts make bicellar systems good candidates as colloidal carriers for dermal delivery. However, water dilution induces structural changes and formation of vesicular structures in the systems; stabilization strategies have been been explored in recent works and are also updated here.

Diffusion NMR and bicelle morphology

This minireview focuses on diffusion NMR studies in bicelles. Following a discourse on diffusion fundamentals, and a comparative overview of fluorescence and NMR-based techniques for measuring diffusion, the pulsed field gradient (PFG) NMR diffusion method is introduced, emphasizing its specific advantages and limitations when applied to diffusion measurements in macroscopically oriented lamellar systems such as magnetically aligned bicelles. The utility of PFG NMR diffusion measurements in bicellar model membrane systems for examining lateral diffusion of membrane-bound molecular species is demonstrated, along with certain features of lateral diffusion that such studies illuminate. Further, those aspects of bicelle morphology that have been resolved using PFG NMR diffusion studies of various molecular weight soluble polymeric species are reviewed. The discussion concludes with an outline of future prospects for diffusion NMR studies in bicelles.

Temperature driven annealing of perforations in bicellar model membranes

Bicellar model membranes composed of 1,2-dimyristoylphosphatidylcholine (DMPC) and 1,2-dihexanoylphosphatidylcholine (DHPC), with a DMPC/DHPC molar ratio of 5, and doped with the negatively charged lipid 1,2-dimyristoylphosphatidylglycerol (DMPG), at DMPG/DMPC molar ratios of 0.02 or 0.1, were examined using small angle neutron scattering (SANS), (31)P NMR, and (1)H pulsed field gradient (PFG) diffusion NMR with the goal of understanding temperature effects on the DHPC-dependent perforations in these self-assembled membrane mimetics. Over the temperature range studied via SANS (300-330 K), these bicellar lipid mixtures exhibited a well-ordered lamellar phase. The interlamellar spacing d increased with increasing temperature, in direct contrast to the decrease in d observed upon increasing temperature with otherwise identical lipid mixtures lacking DHPC. (31)P NMR measurements on magnetically aligned bicellar mixtures of identical composition indicated a progressive migration of DHPC from regions of high curvature into planar regions with increasing temperature, and in accord with the "mixed bicelle model" (Triba, M. N.; Warschawski, D. E.; Devaux, P. E. Biophys. J.2005, 88, 1887-1901). Parallel PFG diffusion NMR measurements of transbilayer water diffusion, where the observed diffusion is dependent on the fractional surface area of lamellar perforations, showed that transbilayer water diffusion decreased with increasing temperature. A model is proposed consistent with the SANS, (31)P NMR, and PFG diffusion NMR data, wherein increasing temperature drives the progressive migration of DHPC out of high-curvature regions, consequently decreasing the fractional volume of lamellar perforations, so that water occupying these perforations redistributes into the interlamellar volume, thereby increasing the interlamellar spacing.

Interactions of tyrosine in Leu-enkephalin at a membrane-water interface: an ultrafast two-dimensional infrared study combined with density functional calculations and molecular dynamics simulations

The interactions of neuropeptides and membranes play an important role in peptide hormone function. Our current understanding of peptide-membrane interactions remains limited due to the paucity of experimental techniques capable of probing such interactions. In this work, we study the nature of opioid peptide-membrane interactions using ultrafast two-dimensional infrared (2D IR) spectroscopy. The high temporal resolution of 2D IR is particularly suited for studying highly flexible opioid peptides. We investigate the location of the tyrosine (Tyr) side chain of leucine-enkephalin (Lenk) in lipid bilayer membranes by measuring spectral diffusion of the phenolic ring vibrational mode in three different systems: Lenk in lipid bilayer membranes (bicelles), Lenk in deuterated water, and p-cresol in deuterated water. Frequency-frequency correlation functions obtained from waiting-time-dependent 2D IR spectra reveal an ultrafast decaying component with an approximately 1 ps time constant that is common for all three systems. On the basis of density functional theory calculations and molecular dynamics simulations, this spectral diffusion component is attributed to hydrogen-bond dynamics of the phenolic hydroxyl group interacting with bulk water. Unlike p-cresol in water, both Lenk systems exhibit static spectral inhomogeneity, which can be attributed to conformational distributions of Lenk that do not interconvert within 4 ps. Our results suggest that the Tyr side chain of Lenk in bicelles is located at the water-abundant region at the membrane-water interface and not embedded into the hydrophobic core.

Membrane interactions and dynamics of a 21-mer cytotoxic peptide: a solid-state NMR study

We have investigated the membrane interactions and dynamics of a 21-mer cytotoxic model peptide that acts as an ion channel by solid-state NMR spectroscopy. To shed light on its mechanism of membrane perturbation, (31)P and (2)H NMR experiments were performed on 21-mer peptide-containing bicelles. (31)P NMR results indicate that the 21-mer peptide stabilizes the bicelle structure and orientation in the magnetic field and perturbs the lipid polar head group conformation. On the other hand, (2)H NMR spectra reveal that the 21-mer peptide orders the lipid acyl chains upon binding. (15)N NMR experiments performed in DMPC bilayers stacked between glass plates also reveal that the 21-mer peptide remains at the bilayer surface. (15)N NMR experiments in perpendicular DMPC bicelles indicate that the 21-mer peptide does not show a circular orientational distribution in the bicelle planar region. Finally, (13)C NMR experiments were used to study the 21-mer peptide dynamics in DMPC multilamellar vesicles. By analyzing the (13)CO spinning sidebands, the results show that the 21-mer peptide is immobilized upon membrane binding. In light of these results, we propose a model of membrane interaction for the 21-mer peptide where it lies at the bilayer surface and perturbs the lipid head group conformation.

Bicelles: A natural 'molecular goniometer' for structural, dynamical and topological studies of molecules in membranes

Major biological processes occur at the biological membrane. One of the great challenges is to understand the function of chemical or biological molecules inside the membrane; as well of those involved in membrane trafficking. This requires obtaining a complete picture of the in situ structure and dynamics as well as the topology and orientation of these molecules in the membrane lipid bilayer. These led to the creation of several innovative models of biological membranes in order to investigate the structure and dynamics of amphiphilic molecules, as well as integral membrane proteins having single or multiple transmembrane segments. Because the determination of the structure, dynamics and topology of molecules in membranes requires a macroscopic alignment of the system, a new membrane model called 'bicelles' that represents a crossover between lipid vesicles and classical micelles has become very popular due to its property of spontaneous self-orientation in magnetic fields. In addition, crucial factors involved in mimicking natural membranes, such as sample hydration, pH and salinity limits, are easy to control in bicelle systems. Bicelles are composed of mixtures of long chain (14-18 carbons) and short chain phospholipids (6-8 carbons) hydrated up to 98% with buffers and may adopt various morphologies depending on lipid composition, temperature and hydration. We have been developing bicelle systems under the form of nano-discs made of lipids with saturated or biphenyl-containing fatty acyl chains. Depending on the lipid nature, these membranous nano-discs may be macroscopically oriented with their normal perpendicular or parallel to the magnetic field, providing a natural 'molecular goniometer' for structural and topological studies, especially in the field of NMR. Bicelles can also be spun at the magic angle and lead to the 3D structural determination of molecules in membranes.

Evidence for effect of GM1 on opioid peptide conformation: NMR study on leucine enkephalin in ganglioside-containing isotropic phospholipid bicelles

Enkephalins are endogenous neuropeptides that have opioid-like activities and compete with morphines for the receptor binding. The binding of these neuropeptides to membrane appears crucial since enkephalins interact with the nerve cell membranes to achieve bioactive conformations that fit onto multiple receptor sites (micro, delta, and kappa). Using NMR spectroscopy, we have determined the solution structure of the small opiate pentapeptide leucine enkephalin in the presence of isotropic phospholipid bicelles: phosphocholine bicelles (DMPC:CHAPS 1:4) and phosphocholine bicelles doped with ganglioside GM1 (DMPC:CHAPS:GM1 1:4:0.3). Bicelles containing GM1 were found to interact strongly with leucine enkephalin, whereas a somewhat weaker interaction was observed in the case of bicelles without GM1. Structure calculation from torsion angles, chemical shifts, and NOE-based distance constraints explored that the peptide could flexibly switch between several mu- and delta-selective conformations in both the bicelles though micro-selective conformations turned out to be geometrically preferred in each bicellar system. A detailed analysis of the structures presented supports the variance over the singly associated conformation of enkephalin in nerve cell membranes.

Solid-state NMR analysis of the orientation and dynamics of epigallocatechin gallate, a green tea polyphenol, incorporated into lipid bilayers

Catechins are the principle polyphenolic compounds in green tea; the four major compounds identified are epicatechin (EC), epigallocatechin (EGC), epicatechin gallate (ECg) and epigallocatechin gallate (EGCg). Tea catechins tend to attach externally to their targets, such as viral envelopes, cell membranes, or the surface of low-density lipoproteins. In order to further our understanding of the molecular mobility of these compounds in cells, we examined the interaction of tea catechins with lipid membranes using solid-state NMR techniques. Our previous work indicated that the EGCg molecule is incorporated into lipid bilayers in a unique orientation. However, the detailed configuration, orientation, and dynamics of EGCg in lipid bilayers have not been well-characterized. Here, we investigated the orientation and dynamics of EGCg incorporated into multi-lamellar vesicles (MLVs) and bicelles using solid-state NMR spectroscopy.

Structural divergence among cannabinoids influences membrane dynamics: A 2H Solid-State NMR analysis

Cannabinoids are compounds that can modulate neuronal functions and immune responses via their activity at the CB(1) receptor. We used (2)H NMR order parameters and relaxation rate determination to delineate the behavior of magnetically aligned phospholipid bilayers in the presence of several structurally distinct cannabinoid ligands. THC (Delta(9)-Tetrahydrocannabinol) and WIN-55,212-2 were found to lower the phase transition temperature of the DMPC and to destabilize their acyl chains leading to a lower average S(CD) ( approximately 0.13), while methanandamide and CP-55,940 exhibited unusual properties within the lipid bilayer resulting in a greater average S(CD) ( approximately 0.14) at the top of the phospholipid upper chain. The CB(1) antagonist AM281 had average S(CD) values that were higher than the pure DMPC lipids, indicating a stabilization of the lipid bilayer. R(1Z) versus |S(CD)|(2) plots indicated that the membrane fluidity is increased in the presence of THC and WIN-55,212-2. The interaction of CP-55,940 with a variety of zwitterionic and charged membranes was also assessed. The unusual effect of CP-55,940 was present only in bicelles composed of DMPC. These studies strongly suggest that cannabinoid action on the membrane depends upon membrane composition as well as the structure of the cannabinoid ligands.

The role of hydrophobic interactions in positioning of peripheral proteins in membranes

BACKGROUND

Three-dimensional (3D) structures of numerous peripheral membrane proteins have been determined. Biological activity, stability, and conformations of these proteins depend on their spatial positions with respect to the lipid bilayer. However, these positions are usually undetermined.

RESULTS

We report the first large-scale computational study of monotopic/peripheral proteins with known 3D structures. The optimal translational and rotational positions of 476 proteins are determined by minimizing energy of protein transfer from water to the lipid bilayer, which is approximated by a hydrocarbon slab with a decadiene-like polarity and interfacial regions characterized by water-permeation profiles. Predicted membrane-binding sites, protein tilt angles and membrane penetration depths are consistent with spin-labeling, chemical modification, fluorescence, NMR, mutagenesis, and other experimental studies of 53 peripheral proteins and peptides. Experimental membrane binding affinities of peripheral proteins were reproduced in cases that did not involve a helix-coil transition, specific binding of lipids, or a predominantly electrostatic association. Coordinates of all examined peripheral proteins and peptides with the calculated hydrophobic membrane boundaries, subcellular localization, topology, structural classification, and experimental references are available through the Orientations of Proteins in Membranes (OPM) database.

CONCLUSION

Positions of diverse peripheral proteins and peptides in the lipid bilayer can be accurately predicted using their 3D structures that represent a proper membrane-bound conformation and oligomeric state, and have membrane binding elements present. The success of the implicit solvation model suggests that hydrophobic interactions are usually sufficient to determine the spatial position of a protein in the membrane, even when electrostatic interactions or specific binding of lipids are substantial. Our results demonstrate that most peripheral proteins not only interact with the membrane surface, but penetrate through the interfacial region and reach the hydrocarbon interior, which is consistent with published experimental studies.

Human Opioid Peptide Met-Enkephalin Binds to Anionic Phosphatidylserine in High Preference to Zwitterionic Phosphatidylcholine: Natural-Abundance 13C NMR Study on the Binding State in Large Unilamellar Vesicles

A human opioid neuropeptide, Met-enkephalin (M-Enk: Tyr1-Gly2-Gly3-Phe4-Met5), having no net charge binds to anionic phosphatidylserine (PS) in high preference to zwitterionic phosphatidylcholine (PC). The binding mechanism in the PS and PC bilayers was studied on the basis of the inter- and intramolecular interaction data obtained by natural-abundance 13C nuclear magnetic resonance (NMR) of the peptide. Prominent upfield changes of the 13C resonance were observed in the C-terminal residue upon binding to PS, whereas no such marked change was observed upon binding to PC. The upfield chemical shift changes with their characteristic carbon site dependence are ascribed to the electrostatic binding between the peptide C-terminal CO2- and the PS headgroup NH3+. Despite the net negative charge of the PS bilayer surface, M-Enk thus anchors the negatively charged C-terminus. In the N-terminal residue, on the other hand, marked downfield chemical shift changes are observed upon binding to both the PS and PC bilayers, the magnitude of the changes being much larger in the PS system. The downfield changes with their characteristic carbon site dependence are ascribed to the electrostatic binding between the peptide N-terminal NH3+ and the lipid headgroup negative charge(s) (CO2- or PO4- in PS, PO4- in PC). Perturbation on the signal half-widths due to membrane binding also indicates the preferential and deeper binding of M-Enk on the PS membrane surface than on the PC membrane surface. Local charge cancellation takes place efficiently between M-Enk termini and the PS headgroups and compensates for the strong electrostatic hydration of the ionic groups. Distribution of the charged (positive and negative) and uncharged sites in the headgroups along the bilayer normal is responsible for the marked difference between PS and PC headgroups in controlling the binding state of the zwitterionic M-Enk.

Phospholamban and its phosphorylated form interact differently with lipid bilayers: a 31P, 2H, and 13C solid-state NMR spectroscopic study

Phospholamban (PLB) is a 52-amino acid integral membrane protein that helps to regulate the flow of Ca(2+) ions in cardiac muscle cells. Recent structural studies on the PLB pentamer and the functionally active monomer (AFA-PLB) debate whether its cytoplasmic domain, in either the phosphorylated or dephosphorylated states, is alpha-helical in structure as well as whether it associates with the lipid head groups (Oxenoid, K. (2005) Proc. Natl. Acad. Sci. U.S.A. 102, 10870-10875; Karim, C. B. (2004) Proc. Natl. Acad. Sci. U.S.A. 101, 14437-14442; Andronesi, C.A. (2005) J. Am. Chem. Soc. 127, 12965-12974; Li, J. (2003) Biochemistry 42, 10674-10682; Metcalfe, E. E. (2005) Biochemistry 44, 4386-4396: Clayton, J. C. (2005) Biochemistry 44, 17016-17026). Comparing the secondary structure of the PLB pentamer and its phosphorylated form (P-PLB) as well as their interaction with the lipid bilayer is crucial in order to understand its regulatory function. Therefore, in this study, the full-length wild-type (WT) PLB and P-PLB were incorporated into 1-palmitoyl-2-oleoyl-sn-glycero-phosphocholine (POPC) phospholipid bilayers and studied utilizing solid-state NMR spectroscopy. The analysis of the (2)H and (31)P solid-state NMR data of PLB and P-PLB in POPC multilamellar vesicles (MLVs) indicates that a direct interaction takes place between both proteins and the phospholipid head groups. However, the interaction of P-PLB with POPC bilayers was less significant compared that with PLB. Moreover, the secondary structure using (13)C=O site-specific isotopically labeled Ala15-PLB and Ala15-P-PLB in POPC bilayers suggests that this residue, located in the cytoplasmic domain, is a part of an alpha-helical structure for both PLB and P-PLB.