Micron-scale phase segregation in lipid monolayers induced by myelin basic protein in the presence of a cholesterol analog

In Vitro Application of Langmuir Monolayer Model: The interfacial Behavior of Myelin Basic Protein with a Plasma Membrane Model

The stability and compactness of myelin structure and brain homeostasis depend on MBP and glial cell plasma membrane interactions. In order to get more detailed mechanisms of this interaction, the MBP of different concentrations interacting with plasma membrane (POPC/POPE/POPS/Cholesterol (Chol)) model to form bionic membrane was studied by atomic force microscopy (AFM) and Langmuir monolayer technology. The surface pressure(π)-area(A) curve is analyzed by the elastic modulus ([Formula: see text]) and two-dimensional virial equation of state (2D-VES), and the second virial coefficient of the interaction between MBP and plasma membrane molecules was calculated. (i) According to two-dimensional virial equation, it could be analyzed that with the increase of MBP concentration in the subphase, the value of the second virial coefficient increases also, which indicates that MBP is absorbed into lipid membrane of the plasma membrane model at low surface pressure and that the interaction between the molecules is spatial repulsive force, and (ii) in the monolayers with MBP, resulting in an increasing mean molecular area and monolayer stability due to hydrophobic and electrostatic interactions between the positively charged MBP with hydrophobic residues and negatively charged POPS and neutral lipid (POPC, POPE). AFM surface topographic results correspond to the results of the curve analysis, indicating that MBP of different concentrations has significant influences on alignment and conformation of plasma membrane, which is of great medical value and biological significance to the application of interaction between MBP and myelin lipid membrane in treatment of central nervous diseases. Adsorption model of interaction between MBP and plasma membrane model.

Cardiolipin remodeling enables protein crowding in the inner mitochondrial membrane

Mitochondrial cristae are extraordinarily crowded with proteins, which puts stress on the bilayer organization of lipids. We tested the hypothesis that the high concentration of proteins drives the tafazzin-catalyzed remodeling of fatty acids in cardiolipin, thereby reducing bilayer stress in the membrane. Specifically, we tested whether protein crowding induces cardiolipin remodeling and whether the lack of cardiolipin remodeling prevents the membrane from accumulating proteins. In vitro, the incorporation of large amounts of proteins into liposomes altered the outcome of the remodeling reaction. In yeast, the concentration of proteins involved in oxidative phosphorylation (OXPHOS) correlated with the cardiolipin composition. Genetic ablation of either remodeling or biosynthesis of cardiolipin caused a substantial drop in the surface density of OXPHOS proteins in the inner membrane of the mouse heart and Drosophila flight muscle mitochondria. Our data suggest that OXPHOS protein crowding induces cardiolipin remodelling and that remodeled cardiolipin supports the high concentration of these proteins in the inner mitochondrial membrane.

Regulation of phase boundaries and phase-segregated patterns in model membranes

Demixing of components has long been described in model membranes. It is a consequence of non-ideal lateral interactions between membrane components, and it causes the presence of segregated phases, forming patches (domains) of different properties, thus introducing heterogeneity into the membrane. In the present review we first describe the processes through which domains are generated, how they grow, and why they are rounded, striped or fractal-like, as well as why they get distributed forming defined patterns. Next, we focus on the effect of an additive on a lipid mixture, which usually induces shifts in demixing points, thus stabilizing or destabilizing the phase-segregated state. Results found for different model membranes are summarized, detailing the ways in which phase segregation and the generated patterns may be modulated. We focus on which are, from our viewpoint, the most relevant regulating factors affecting the surface texture observed in model membranes. This article is part of a Special Issue entitled: Emergence of Complex Behavior in Biomembranes edited by Marjorie Longo.

Effects of Concentration and Surface Pressure on MBP Interaction with Cholesterol in Langmuir Films

Predicting the mechanism of MBP binding to cholesterol is meaningful in understanding how MBP participate in lateral membrane organization. The interaction of MBP with cholesterol monolayer was investigated at three surface pressures on 10 mM Tris-HCl buffer with the different concentrations of MBP. The results show that π-A isotherms shift to larger molecular area at all pressures. By means of analyzing π-T curves, a surface pressure increase was obtained. Results indicated that the greater the protein concentration in the subphase, the larger the increase of surface pressure. In addition, changes in monolayer surface morphology and domain formation were performed by AFM. These results provide more direct and convincing evidence for the MBP interaction with cholesterol. The MBP-cholesterol interaction suggests a significant concentrations and surface pressure dependence and is probably governed by hydrogen bonds. The date presented could help to understand at least one of the molecular mechanisms through which MBP affects lateral organization of the cholesterol membrane.

Ratiometric Fluorescence Live Imaging Analysis of Membrane Lipid Order in Arabidopsis Mitotic Cells Using a Lipid Order-Sensitive Probe

Eukaryotic cells contain membranes exhibiting different levels of lipid order mostly related to their relative amount of sterol-rich domains, thought to mediate temporal and spatial organization of cellular processes. We previously provided evidence in Arabidopsis thaliana that sterols are crucial for execution of cytokinesis, the last stage of cell division. Recently, we used di-4-ANEPPDHQ, a fluorescent probe sensitive to order of lipid phases, to quantify the level of membrane order of the cell plate, the membrane structure separating daughter cells during somatic cytokinesis of higher plant cells. By employing quantitative, ratiometric fluorescence microscopy for mapping localized lipid order levels, we revealed that the Arabidopsis cell plate represents a high-lipid-order domain of the plasma membrane. Here, we describe step-by-step protocols and troubleshooting for ratiometric live imaging procedures employing the di-4-ANEPPDHQ fluorescent probe for quantification of membrane lipid order during plant cell division in suspension cell cultures and roots of Arabidopsis thaliana.

Searching for line active molecules on biphasic lipid monolayers

In membranes with phase coexistence, line tension appears as an important parameter for the determination of the amount of domains, as well as their size and their shape, thus defining the membrane texture. Different molecules have been proposed as "linactants" (i.e. molecules that reduce the line tension, thereby modulating the membrane texture). In this work, we explore the efficiency of different molecules as linactants in monolayers with two coexisting phases of different thicknesses. We tested the linactant ability of a molecule with chains of different saturation degrees, another molecule with different chain lengths and a bulky molecule. In this way, we show in the same system the effect of molecules with chains of different rigidities, with an intrinsic thickness mismatch and with a bulky moiety, thereby analyzing different hypotheses of how a molecule may change the line tension in a monolayer system. Both lipids with different hydrocarbon chains did not act as linactants, while only one of the bulky molecules tested decreased the line tension in the monolayer studied. We conclude that there are no universal rules for the structure of a molecule that enable us to predict that it will behave as a linactant and thus, designing linactants appears to be a difficult task and a challenge for future studies. Furthermore, in regard to the membrane texture, there was no direct influence of the line tension in the distribution of domain sizes.

Zn(2+)-dependent surface behavior of diacylglycerol pyrophosphate and its mixtures with phosphatidic acid at different pHs

Diacylglycerol pyrophosphate (DGPP) is a minor lipid that attenuates the phosphatidic acid (PA) signal, and also DGPP itself would be a signaling lipid. Diacylglycerol pyrophosphate is an anionic phospholipid with a pyrophosphate group attached to diacylglycerol that was shown to respond to changes of pH, thus affecting the surface organization of DGPP and their interaction with PA. In this work, we have investigated how the presence of Zn(2+) modulates the surface organization of DGPP and its interaction with PA at acidic and basic pHs. Both lipids formed expanded monolayers at pHs 5 and 8. At pH 5, monolayers formed by DGPP became stiffer when Zn(2+)was added to the subphase, while the surface potential decreased. At this pH, Zn(2+) induced a phase transition from an expanded to a condensed-phase state in monolayers formed by PA. Conversely, at pH 8 the effects induced by the presence of Zn(2+) on the surface behaviors of the pure lipids were smaller. Thus, the interaction of the bivalent cation with both lipids was modulated by pH and by the ionization state of the polar head groups. Mixed monolayers of PA and DGPP showed a non-ideal behavior and were not affected by the presence of Zn(2+) at pH 8. This could be explained considering that when mixed, the lipids formed a closely packed monolayer that could not be further modified by the cation. Our results indicate that DGPP and PA exhibit expanded- and condensed-phase states depending on pH, on the proportion of each lipid in the film and on the presence of Zn(2+). This may have implications for a possible role of DGPP as a signaling lipid molecule.

Myelin architecture: zippering membranes tightly together

Rapid nerve conduction requires the coating of axons by a tightly packed multilayered myelin membrane. In the central nervous system, myelin is formed from cellular processes that extend from oligodendrocytes and wrap in a spiral fashion around an axon, resulting in the close apposition of adjacent myelin membrane bilayers. In this review, we discuss the physical principles underlying the zippering of the plasma membrane of oligodendrocytes at the cytoplasmic and extracellular leaflet. We propose that the interaction of the myelin basic protein with the cytoplasmic leaflet of the myelin bilayer triggers its polymerization into a fibrous network that drives membrane zippering and protein extrusion. In contrast, the adhesion of the extracellular surfaces of myelin requires the down-regulation of repulsive components of the glycocalyx, in order to uncover weak and unspecific attractive forces that bring the extracellular surfaces into close contact. Unveiling the mechanisms of myelin membrane assembly at the cytoplasmic and extracelluar sites may help to understand how the myelin bilayers are disrupted and destabilized in the different demyelinating diseases.

Modification of plasma membrane organization in tobacco cells elicited by cryptogein

Lipid mixtures within artificial membranes undergo a separation into liquid-disordered and liquid-ordered phases. However, the existence of this segregation into microscopic liquid-ordered phases has been difficult to prove in living cells, and the precise organization of the plasma membrane into such phases has not been elucidated in plant cells. We developed a multispectral confocal microscopy approach to generate ratiometric images of the plasma membrane surface of Bright Yellow 2 tobacco (Nicotiana tabacum) suspension cells labeled with an environment sensitive fluorescent probe. This allowed the in vivo characterization of the global level of order of this membrane, by which we could demonstrate that an increase in its proportion of ordered phases transiently occurred in the early steps of the signaling triggered by cryptogein and flagellin, two elicitors of plant defense reactions. The use of fluorescence recovery after photobleaching revealed an increase in plasma membrane fluidity induced by cryptogein, but not by flagellin. Moreover, we characterized the spatial distribution of liquid-ordered phases on the membrane of living plant cells and monitored their variations induced by cryptogein elicitation. We analyze these results in the context of plant defense signaling, discuss their meaning within the framework of the "membrane raft" hypothesis, and propose a new mechanism of signaling platform formation in response to elicitor treatment.

Axonal selection and myelin sheath generation in the central nervous system

The formation of myelin in the central nervous system is a multi-step process that involves coordinated cell-cell interactions and dramatic changes in plasma membrane architecture. First, oligodendrocytes send our numerous highly ramified processes to sample the axonal environment and decide which axon(s) to select for myelination. After this decision is made and individual axon to oligodendrocyte contact has been established, the exploratory process of the oligodendrocyte is converted into a flat sheath that spreads and winds along and around its associated axon to generate a multilayered membrane stack. By compaction of the opposing extracellular layers of membrane and extrusion of almost all cytoplasm from the intracellular domain of the sheath, the characteristic membrane-rich multi-lamellar structure of myelin is formed. Here we highlight recent advances in identifying biophysical and signalling based mechanisms that are involved in axonal selection and myelin sheath generation by oligodendrocytes. A thorough understanding of the mechanisms underlying these events is a prerequisite for the design of novel myelin repair strategies in demyelinating and dysmyelinating diseases.

Molecular determinants for the line tension of coexisting liquid phases in monolayers

The line tension (λ) in biphasic membranes has been determined in monolayers and bilayers using a variety of techniques. In this work we present a novel approach to the determination of λ in monolayers with liquid/liquid phase coexistence, overcoming several of the drawbacks of current techniques. Using our method, we determined the line tension of liquid/liquid phases in binary mixtures of different lipids and a molecule similar to cholesterol but less oxidizable. We analyzed the effect of the hydrocarbon chain length and the polar head-group of the non-sterol lipid and found the latter to exert much more influence than the former. The presence of PE led to high λ values, PG to low values and PS and PC to intermediate values. The line tension showed a strong correlation with the critical packing parameter of the phospholipid. The spontaneous curvature displayed by the phases constituted by a particular lipid appears to be an important parameter for determining the line tension in mixed films.

Self-segregation of myelin membrane lipids in model membranes

Rapid conduction of nerve impulses requires coating of axons by myelin sheaths, which are multilamellar, lipid-rich membranes produced by oligodendrocytes in the central nervous system. To act as an insulator, myelin has to form a stable and firm membrane structure. In this study, we have analyzed the biophysical properties of myelin membranes prepared from wild-type mice and from mouse mutants that are unable to form stable myelin. Using C-Laurdan and fluorescence correlation spectroscopy, we find that lipids are tightly organized and highly ordered in myelin isolated from wild-type mice, but not from shiverer and ceramide synthase 2 null mice. Furthermore, only myelin lipids from wild-type mice laterally segregate into physically distinct lipid phases in giant unilamellar vesicles in a process that requires very long chain glycosphingolipids. Taken together, our findings suggest that oligodendrocytes exploit the potential of lipids to self-segregate to generate a highly ordered membrane for electrical insulation of axons.

Electrostatic field effects on membrane domain segregation and on lateral diffusion

Natural membranes are organized structures of neutral and charged molecules bearing dipole moments which generate local non-homogeneous electric fields. When subjected to such fields, the molecules experience net forces that can modify the lipid and protein organization, thus modulating cell activities and influencing (or even dominating) the biological functions. The energetics of electrostatic interactions in membranes is a long-range effect which can vary over distance within r^-1 to r^-3. In the case of a dipole interacting with a plane of dipoles, e.g. a protein interacting with a lipid domain, the interaction is stronger than two punctual dipoles and depends on the size of the domain. In this article, we review several contributions on how electrostatic interactions in the membrane plane can modulate the phase behavior, surface topography and mechanical properties in monolayers and bilayers.

Equivalent aqueous phase modulation of domain segregation in myelin monolayers and bilayer vesicles

Purified myelin can be spread as monomolecular films at the air/aqueous interface. These films were visualized by fluorescence and Brewster angle microscopy, showing phase coexistence at low and medium surface pressures (<20-30 mN/m). Beyond this threshold, the film becomes homogeneous or not, depending on the aqueous subphase composition. Pure water as well as sucrose, glycerol, dimethylsulfoxide, and dimethylformamide solutions (20% in water) produced monolayers that become homogeneous at high surface pressures; on the other hand, the presence of salts (NaCl, CaCl(2)) in Ringer's and physiological solution leads to phase domain microheterogeneity over the whole compression isotherm. These results show that surface heterogeneity is favored by the ionic milieu. The modulation of the phase-mixing behavior in monolayers is paralleled by the behavior of multilamellar vesicles as determined by small-angle and wide-angle x-ray scattering. The correspondence of the behavior of monolayers and multilayers is achieved only at high surface pressures near the equilibrium adsorption surface pressure; at lower surface pressures, the correspondence breaks down. The equilibrium surface tension on all subphases corresponds to that of the air/alkane interface (27 mN/m), independently on the surface tension of the clean subphase.

Interaction of myelin basic protein with actin in the presence of dodecylphosphocholine micelles

The 18.5 kDa myelin basic protein (MBP), the most abundant splice isoform in human adult myelin, is a multifunctional, intrinsically disordered protein that maintains compact assembly of the myelin sheath in the central nervous system. Protein deimination and phosphorylation are two key posttranslational modifications whose balance determines local myelin microdomain stability and function. It has previously been shown that MBP in solution causes both polymerization of G-actin to F-actin and bundling of the microfilaments, and binds them to a negatively charged membrane. However, the binding parameters, and the roles of different possible interacting domains of membrane-associated MBP, have not yet been investigated. Here, we compared the interaction of unmodified (rmC1) and pseudodeiminated (rmC8) recombinant murine MBP (full-length charge variants), and of two terminal deletion variants (rmDeltaC and rmDeltaN), with actin in the presence of DPC (dodecylphosphocholine) to mimic a membrane environment. Our results show that although both charge variants polymerized and bundled actin, the maximal polymerization/bundling due to rmC1 occurred at a lower molar ratio compared to rmC8. In the presence of DPC, rmC1 appeared to be more active than rmC8 in its ability to polymerize and bundle actin, and the binding affinity of both charge variants to G-actin became higher. Moreover, of the two deletion variants studied in the presence of DPC, the one lacking the C-terminal domain (rmDeltaC) was more active compared to the variant lacking the N-terminal domain (rmDeltaN) but exhibited weaker binding to actin. Thus, whereas the N-terminal domain of MBP can be more important for the MBP's actin polymerization activity and membrane-association, the C-terminal domain can regulate its interaction with actin.