A method for probing the affinity of peptides for amphiphilic surfaces

Exploring Molecular-Biomembrane Interactions with Surface Plasmon Resonance and Dual Polarization Interferometry Technology: Expanding the Spotlight onto Biomembrane Structure

The molecular analysis of biomolecular-membrane interactions is central to understanding most cellular systems but has emerged as a complex technical challenge given the complexities of membrane structure and composition across all living cells. We present a review of the application of surface plasmon resonance and dual polarization interferometry-based biosensors to the study of biomembrane-based systems using both planar mono- or bilayers or liposomes. We first describe the optical principals and instrumentation of surface plasmon resonance, including both linear and extraordinary transmission modes and dual polarization interferometry. We then describe the wide range of model membrane systems that have been developed for deposition on the chips surfaces that include planar, polymer cushioned, tethered bilayers, and liposomes. This is followed by a description of the different chemical immobilization or physisorption techniques. The application of this broad range of engineered membrane surfaces to biomolecular-membrane interactions is then overviewed and how the information obtained using these techniques enhance our molecular understanding of membrane-mediated peptide and protein function. We first discuss experiments where SPR alone has been used to characterize membrane binding and describe how these studies yielded novel insight into the molecular events associated with membrane interactions and how they provided a significant impetus to more recent studies that focus on coincident membrane structure changes during binding of peptides and proteins. We then discuss the emerging limitations of not monitoring the effects on membrane structure and how SPR data can be combined with DPI to provide significant new information on how a membrane responds to the binding of peptides and proteins.

Extracellular transport of cell-size particles and tumor cells by dendritic cells in culture

Many particulate materials of sizes approximating that of a cell disseminate after being introduced into the body. While some move about within phagocytic inflammatory cells, others appear to move about outside of, but in contact with, such cells. In this report, we provide unequivocal photomicroscopic evidence that cultured, mature, human dendritic cells can transport in extracellular fashion over significant distances both polymeric beads and tumor cells. At least in the case of polymeric beads, both fibrinogen and the β2-integrin subunit, CD18, appear to play important roles in the transport process. These discoveries may yield insight into a host of disease-related phenomena, including and especially tumor cell invasion and metastasis.

Analysis of membrane binding equilibria of peripheral proteins: allowance for excluded area of bound protein

When peripheral proteins bind to phospholipid membranes lacking discrete binding sites, steric repulsion between bound protein molecules may result in a reduction of the surface area available to additional bound protein by an amount significantly greater than the actual area occupied by bound protein. An approximate treatment of this effect demonstrates that neglect of area exclusion by bound protein may lead to significant errors in the evaluation of equilibrium association constants and the fractional coverage of membrane surface area.

Cooperative adsorption of proteins onto lipid membranes

The adsorption of proteins onto a lipid membrane depends on and thus reflects the energetics of the underlying substrate. This is particularly relevant for mixed membranes that contain lipid species with different affinities for the adsorbed proteins. In this case, there is an intricate interplay between lateral membrane organization and the number of adsorbed proteins. Most importantly, proteins often tend to enhance the propensity of the lipid mixture to form clusters, domains, or to macroscopically phase separate. Sigmoidal binding isotherms are the typical signature of the corresponding cooperativity in protein adsorption. We discuss the underlying thermodynamic basis, and compare various theoretical binding models for protein adsorption onto mixed membranes. We also present experimental data for the adsorption of the C2A protein motif and analyse to what extent these data reflect cooperative binding.

Structure and mobility of lipid membranes on a thermoplastic substrate

Supported lipid membranes constitute one of the most important model systems for cell membranes. The properties of lipid membranes supported by the hydrophobic solid polymer cyclic olefin copolymer (COC) were investigated. Lipid layers consisting of varying amounts of 1,2-dioleoyl-3-trimethylammonium propane (DOTAP, cationic) and 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC, neutral) prepared by vesicle fusion and solvent exchange were compared. All lipid mixtures coated the COC surface homogeneously forming a fluid membrane as verified by fluorescence microscopy and fluorescence recovery after photobleaching (FRAP). The exact structure of the supported membranes was determined by synchrotron reflectivity experiments using a microfluidic chamber. The X-ray data are in agreement with a compressed (head-to-head distance = 29 angstroms) and less densely packed bilayer.

Mitogenicity of a spread film of monophosphoryl lipid A

When spread at the air-water interface, monophosphoryl lipid A (MPLA) forms stable insoluble monolayers that collapse at approximately 55 dyn/cm. At collapse, the exclusion area of each molecule is approximately 119 Angstrom(2), consistent with the cross-sectional area of the lipid's 6 acyl chains. The nominal thickness of such films is approximately 22 Angstrom, determined, presumably, by the length of the acyl chains. For biological modeling of MPLA films, a system was developed in which monolayers of the lipid are supported by monodisperse hydrophobic beads of microscopic dimensions. Beads coated with MPLA monolayers within which the nominal area of each molecule is approximately equivalent to the "take-off" area of the lipid at the air-water interface, 280 Angstrom(2), are mitogenic for spleen cells. Given the natural occurrence of lipid A in the bacterial cell wall as well as the inherent stability of lipid A films, it seems reasonable to assume that at least some of the biological activities attributed to the lipid derive from its presentation/operation at an interface, i.e., on a surface. We propose beads coated with adsorbed films of lipid A will prove useful tools for modeling the activities of the lipid both in vitro and in vivo, and for elucidating the surface dependency and structural requirements of those activities.

Phospholipid barrier to fibrinolysis: role for the anionic polar head charge and the gel phase crystalline structure

The massive presence of phospholipids is demonstrated in frozen sections of human arterial thrombi. Purified platelet phospholipids and synthetic phospholipids retard in vitro tissue-type plasminogen activator (tPA)-induced fibrinolysis through effects on plasminogen activation and plasmin function. The inhibition of plasminogen activation on the surface of fibrin correlates with the fraction of anionic phospholipid. The phospholipids decrease the amount of tPA penetrating into the clot by 75% and the depth of the reactive surface layer occupied by the activator by up to 30%, whereas for plasmin both of these parameters decrease by approximately 50%. The phospholipids are not only a diffusion barrier, they also bind the components of the fibrinolytic system. Isothermal titration calorimetry shows binding characterized with dissociation constants in the range 0.35-7.64 microm for plasmin and tPA (lower values with more negative phospholipids). The interactions are endothermic and thermodynamically driven by an increase in entropy, probably caused by the rearrangements in the ordered gel structure of the phospholipids (in line with the stronger inhibition at gel phase temperatures compared with liquid crystalline phase temperatures). These findings show a phospholipid barrier, which should be overcome during lysis of arterial thrombi.

Reactivity of human platelets with immobilized fibrinogen is dictated by the chemical character of the surface

Quiescent platelets readily adhere to surface-immobilized fibrinogen. In contrast, platelets exposed to soluble fibrinogen do not demonstrate such activity. As part of an effort to characterize this phenomenon, a solid-phase reagent was prepared by adsorbing human fibrinogen to polystyrene-divinylbenzene microparticles. Using a suspension of human platelets, phase-contrast microscopy was used to quantitate directly platelets bound to fibrinogen-coated beads. This method is fast, straightforward, and requires minimal amounts of reagents and sample. An existing turbidimetric assay was modified to monitor optically the rate and extent of platelet-fibrinogen binding. When platelet-rich plasma was added to a stirred suspension of fibrinogen-coated beads, the rate of aggregation was related directly to the concentration of fibrinogen on the bead surface. This response could not be mitigated by the thrombin inhibitor, hirudin, indicating that any thrombin generated in the reaction has no role in bead aggregation. Conversely, the alpha(IIb)beta(3) antagonist, abciximab (ReoPro), completely prevented aggregation, implicating specific fibrinogen-alpha(IIb)beta(3) interactions as responsible for the observed effect. Beads coated with either albumin or a densely packed, pure film of the neutral phospholipid, phosphatidylcholine (lecithin), do not aggregate under identical conditions, nor do fibrinogen-coated beads aggregate when platelet-depleted plasma is added. When fibrinogen was coated to beads as a mixed film with lecithin, a striking increase in reactivity toward platelets was demonstrated, compared to unmodified beads. These studies indicate that the observed adhesion of platelets to beads is a direct result of platelet-fibrinogen interactions and platelets respond differently to fibrinogen when presented as a mixed film with lipid, compared to the protein alone at an interface.

Identification of synapsin I peptides that insert into lipid membranes

The synapsins constitute a family of synaptic vesicle-associated phosphoproteins essential for regulating neurotransmitter release and synaptogenesis. The molecular mechanisms underlying the selective targeting of synapsin I to synaptic vesicles are thought to involve specific protein-protein interactions, while the high-affinity binding to the synaptic vesicle membrane may involve both protein-protein and protein-lipid interactions. The highly hydrophobic N-terminal region of the protein has been shown to bind with high affinity to the acidic phospholipids phosphatidylserine and phosphatidylinositol and to penetrate the hydrophobic core of the lipid bilayer. To precisely identify the domains of synapsin I which mediate the interaction with lipids, synapsin I was bound to liposomes containing the membrane-directed carbene-generating reagent 3-(trifluoromethyl)-3-(m-[125I]iodophenyl)diazirine and subjected to photolysis. Isolation and N-terminal amino acid sequencing of 125I-labelled synapsin I peptides derived from CNBr cleavage indicated that three distinct regions in the highly conserved domain C of synapsin I insert into the hydrophobic core of the phospholipid bilayer. The boundaries of the regions encompass residues 166-192, 233-258 and 278-327 of bovine synapsin I. These regions are surface-exposed in the crystal structure of domain C of bovine synapsin I and are evolutionarily conserved among isoforms across species. The present data offer a molecular explanation for the high-affinity binding of synapsin I to phospholipid bilayers and synaptic vesicles.

Identification of synapsin I peptides that insert into lipid membranes

The synapsins constitute a family of synaptic vesicle-associated phosphoproteins essential for regulating neurotransmitter release and synaptogenesis. The molecular mechanisms underlying the selective targeting of synapsin I to synaptic vesicles are thought to involve specific protein-protein interactions, while the high-affinity binding to the synaptic vesicle membrane may involve both protein-protein and protein-lipid interactions. The highly hydrophobic N-terminal region of the protein has been shown to bind with high affinity to the acidic phospholipids phosphatidylserine and phosphatidylinositol and to penetrate the hydrophobic core of the lipid bilayer. To precisely identify the domains of synapsin I which mediate the interaction with lipids, synapsin I was bound to liposomes containing the membrane-directed carbene-generating reagent 3-(trifluoromethyl)-3-(m-[125I]iodophenyl)diazirine and subjected to photolysis. Isolation and N-terminal amino acid sequencing of 125I-labelled synapsin I peptides derived from CNBr cleavage indicated that three distinct regions in the highly conserved domain C of synapsin I insert into the hydrophobic core of the phospholipid bilayer. The boundaries of the regions encompass residues 166-192, 233-258 and 278-327 of bovine synapsin I. These regions are surface-exposed in the crystal structure of domain C of bovine synapsin I and are evolutionarily conserved among isoforms across species. The present data offer a molecular explanation for the high-affinity binding of synapsin I to phospholipid bilayers and synaptic vesicles.

Noncovalent immobilized artificial membrane chromatography, an improved method for describing peptide-lipid bilayer interactions

A promising approach in assessing hydrophobic peptide-membrane interactions is the use of reversed-phase high-performance liquid chromatography. The present study describes the preparation and properties of a noncovalent immobilized artificial membrane (noncovalent IAM) stationary phase. The noncovalent IAM phase was prepared by coating the C18 chains of a reversed-phase HPLC column with the phospholipid ditetradecanoyl-sn-glycero-3-phosphocholine. Lipid coating was achieved by pumping a lipid solution in water-2-propanol through the column. The formation of a bilayer-like structure on the chromatographic surface was confirmed by calculating the phospholipid surface density of the stationary phase. The surface density was determined to be approximately 1.95 mumol m-2, which is close to that of lipid vesicles. The coating was found to be stable in chromatographic elution systems containing less than 35% of acetonitrile. Employing this new technique, we determined interaction parameters of a set of helical antibacterial magainin-2-amide peptides with pairwise substitutions of adjacent amino acids by their D-enatiomers. The results demonstrate that the chromatographic retention behavior of peptides on noncovalent IAM stationary phase shows an excellent correlation with lipid affinities to phospholipid vesicles.

A turbidimetric assay for quantitating functional fibrin(ogen) using polystyrene-divinylbenzene microparticles

A sensitive assay has been developed to quantitate fibrinogen in plasma or in other aqueous solutions. Microscopic latex particles, modified with a mixed monomolecular film of lecithin and fibrinogen, are used as a solid-phase reagent. These lecithin/fibrinogen-coated beads aggregate when stirred in the presence of thrombin and, when solution-phase fibrinogen is added, the increased rate of aggregation is proportional to the concentration of soluble fibrinogen. Using a sample volume of 200 microl, as little as 15 nM ( approximately 5 microg ml-1) fibrinogen can be measured. Fibrinogen determinations using the bead assay compared favorably with those derived from a standard clinical assay, with a correlation coefficient (r2) of 0.9710 over a range of 2.5 to 28.0 microM. Analytic precision was comparable to available assays, with typical coefficients of variation of 12.7 and 7.1% for fibrinogen concentrations of 30 nM and 15.0 microM, respectively. The method has a dynamic range of 15 nM to over 3.0 microM and offers the advantage of being sensitive to 20-fold lower concentrations of fibrinogen compared to routine clot-based methods. Unlike immunological assays, e.g., ELISA, it measures only the functional protein. This bead method should prove to be of greatest use to investigators measuring low levels of functional fibrin(ogen).

A phospholipase A2 kinetic and binding assay using phospholipid-coated hydrophobic beads

A novel kinetic and membrane-binding assay for phospholipase A2 (PLA2) has been developed utilizing phospholipid-coated hydrophobic styrene-divinylbenzene beads (5.2 +/- 0.3 microm diameter). Phospholipids formed a stable monolayer film on styrene-divinylbenzene beads with average surface packing density of (1.3 +/- 0.2) x 10(-2) molecule/A2. Secretory PLA2 readily hydrolyzed 1-palmitoyl-2-[3H]-oleoyl-sn-glycero-3-phosphoglycerol coated on styrene-divinylbenzene beads which could be easily monitored by measuring the radioactivity of fatty acid released to solution in the presence of bovine serum albumin. For human cytosolic PLA2 with high specificity for sn-2 arachidonyl group, styrene-divinylbenzene beads coated with 1-stearoyl-2-[14C]-arachidonyl-sn-glycero-3-phosphocholine and dioleoylglycerol (7:3, mol/mol) were used as substrate. PLA2 activity was linearly proportional to the enzyme concentration in the range from 1 to 150 nM for human class II secretory PLA2 and from 1 to 20 nM for cytosolic PLA2; the specific activity was 1.6 and 1.7 micromol/min/mg, respectively. Finally, styrene-divinylbenzene beads coated with polymerized 1,2-bis[12-(lipoyloxy) dodecanoyl]-sn-glycero-3-phosphoglycerol were used to measure the membrane binding affinity of PLA2, which in conjunction with kinetic data provides important insights into how PLA2 interacts with membranes.

Evidence for prothrombin production and thrombin expression by phorbol ester-treated THP-1 cells

With the addition of fibrinogen, fibrin clots form in serum-free culture medium recovered from phorbol ester-treated THP-1 cells. We attribute this coagulant activity to thrombin generated as a consequence of cell stimulation because the coagulant activity exists in serum-free culture medium from treated cells only, and it is inhibited by hirudin. The thrombin does not derive from a prothrombin/thrombin contaminant since no detectable prothrombin/thrombin preexists in either the serum-free culture medium or the fibrinogen preparations used for our experiments. We hypothesized that the thrombin is synthesized by the cells themselves. In support of this hypothesis, we found that prothrombin mRNA is expressed in THP-1 cells following their treatment with phorbol ester. Accompanying expression of this mRNA, prothrombin antigen becomes detectable in lysates of PMA-treated THP-1 cells, and thrombin antigen and activity become detectable in both lysates and culture medium of treated cells. These results are consistent with the notion that certain cells of myelomonocytic lineage are capable of synthesizing proteins relevant to coagulation.

Adsorption and coagulability of fibrinogen on atheromatous lipid surfaces

Fibrinogen, the precursor of the blood clot matrix and a major constituent of atherosclerotic lesions, is shown to adsorb with high affinity to hydrophobic beads coated with cholesteryl oleate, cholesterol, or loosely packed lecithin. The quantity of fibrinogen that binds to cholesterol- or lecithin-coated beads decreases as the surface concentration of the lipid increases; densely packed films lecithin bind little,if any, if the protein. In sharp contrast, the appreciable quantity of fibrinogen that binds to cholesteryl oleate-coated beads is indifferent to the surface concentration of that lipid. Not unexpectedly, the quantity of fibrinogen that binds to beads coated with mixtures of cholesteryl oleate and lecithin increases with increasing concentration of the cholesteryl ester. When bound, fibrinogen can be converted by thrombin to fibrin and nucleate clot formation as manifested by the aggregation of stirred beads. These results indicate that hydrophobic, atheromatous lipid surfaces, particularly those rich in cholesteryl esters, may be predisposed to thrombosis by virtue of their inherent capacity to bind functional fibrinogen.

Determination of the affinity of vitamin D metabolites to serum vitamin D binding protein using assay employing lipid-coated polystyrene beads

We have developed an assay to measure the affinity of serum vitamin D binding protein for 25-hydroxyvitamin D3, 1,25-dihydroxyvitamin D3, and vitamin D3, using uniform diameter (6.4 microns) polystyrene beads coated with phosphatidylcholine and vitamin D metabolites as the vitamin D donor. The lipid metabolite coated beads have a solid core, and thus all of the vitamin D metabolites are on the bead surface from which transfer to protein occurs. After incubating these beads in neutral buffer for 3 h, essentially no 3H-labeled vitamin D metabolites desorb from this surface. Phosphatidylcholine/vitamin D metabolite-coated beads (1 microM vitamin D metabolite) were incubated with varying concentrations of serum vitamin D binding protein under conditions in which the bead surfaces were saturated with protein, but most of the protein was free in solution. After incubation, beads were rapidly centrifuged without disturbing the equilibrium of binding and vitamin D metabolite bound to sDBP in solution was assayed in the supernatant. All three vitamin D metabolites became bound to serum vitamin D binding protein, and after 10 min of incubation the transfer of the metabolites to serum vitamin D binding protein was time independent. The transfer followed a Langmuir isotherm, and the Kd for each metabolite binding to serum vitamin D binding protein was derived by nonlinear least-squares fit analysis. From this analysis the following values for the Kd were obtained: 5.59 x 10(-6) M, 25-hydroxyvitamin D; 9.45 x 10(-6) M, 1,25-dihydroxyvitamin D; and 9.17 x 10(-5) M, vitamin D. In conclusion, we have developed a method which avoids problems encountered in previous assays and allows the precise and convenient determination of binding affinities of vitamin D metabolites and serum vitamin D binding protein.

Quantitation of plasma factor XIIIa activity using fibrin-coated microscopic latex beads

Following exposure to thrombin, monodisperse microscopic polystyrene-divinylbenzene beads coated with a mixed film of lecithin and fibrinogen aggregate as a consequence of interbead fibrin polymerization. These bead aggregates rapidly dissociate in 5 M urea. Treatment of aggregates with factor XIIIa results in a dose-dependent decrease of the rate of aggregate dissociation in urea. The rate of disaggregation is readily quantitated by turbidimetry. We have exploited this phenomenon to develop a rapid and sensitive method for quantitating factor XIIIa activity in plasma. Using 20 microliter of human plasma the method measures the activity of factor XIIIa to a level of 0.03 U ml-1 with a precision of +/-5%.

A turbidimetric method for measuring fibrin formation and fibrinolysis at solid-liquid interfaces

We have developed a rapid and sensitive method by which to quantitate proteolysis of fibrin(ogen) at interfaces. Microscopic polystyrene-divinylbenzene beads coated with a mixed monomolecular film of lecithin and fibrinogen aggregate in aqueous media following exposure to thrombin or enzymes of thrombin-like activity. This aggregation is a consequence of interbead association of fibrin. As an indirect measure of the rate of fibrin formation, the rate of aggregation of beads can be used advantageously to assay enzymes and enzyme regulators pertinent to coagulation. Since the apparent absorbance of monodisperse beads is greater than that of bead aggregates, determination of the rate of change of apparent absorbance of a stirred dispersion of beads following addition of enzyme or enzyme-regulator mixture is a convenient and simple means by which to quantitate the rate of bead aggregation. Using a simple spectrophotometer or aggregometer, the method can be used to quantitate as little as 0.0005 NIH unit of thrombin. Aggregates of fibrin-coated beads can be disaggregated by several proteinases, most notably plasmin. Thus, just as bead aggregation can be used to quantitate effectors of fibrin formation, dissociation of aggregates of fibrin-coated beads can be used to quantitate effectors of fibrinolysis. Using disaggregation as a measure of fibrinolysis, the method is sensitive to as little as 0.005 unit of plasmin. Fibrin(ogen)-coated beads should prove a useful tool for studying proteolysis of fibrin(ogen) in general, and adsorbed fibrin(ogen) in particular.

Ionization and surface properties of verapamil and several verapamil analogues

We have investigated the ionization and surface properties of verapamil (5-[(3,4-dimethoxyphenethyl)methylamino]-2-(3, 4-dimethoxyphenyl)-2-isopropylvaleronitrile, 1) and several verapamil analogues since these properties appear to be involved in the biologic activities of these compounds. Our results show that verapamil and its analogues are surface-active and bind to amphiphilic surfaces. The affinity toward, as well as the capacity of, an amphiphilic surface for verapamil and its ionizable analogues is pH dependent, with the surface having both higher affinity and capacity for the neutral form of the molecules. Thus, verapamil exists as protonated and neutral forms, both of which are free in solution and adsorbed to the interface, and the ionization of verapamil at an interface changes with respect to its ionization in solution. From analyses of the pH dependency of surface binding and of solution and interfacial ionizations, we determined the values of the four equilibrium constants. These equilibrium constants permit correlative studies between the pH-dependent abundance of each species and biologic activity. We discuss preliminary studies which indicate that the negative inotropic effect of verapamil is mediated by the membrane-bound neutral form of the drug.