Microelectrophoresis studies of the binding of glycosaminoglycans to phosphatidylcholine liposomes

Interaction of chondroitin sulfate with zwitterionic lipid membranes

Chondroitin sulfates (CSs) are important components of the extracellular matrix and side chains of membrane proteoglycans. These polysaccharides are, therefore, likely to interact with plasma membranes and play a significant role in modulating cellular functions. So far, the details of the processes occurring at the interface between the extracellular matrix and cellular membranes are not fully understood. In this study, we used experimental methods and atomic-scale molecular dynamics (MD) simulations to reveal the molecular picture of the interactions between CS and phosphocholine (PC) membranes, used as a simplified model of cell membranes. MD simulations reveal that the polysaccharide associates to the PC bilayer as a result of electrostatic interactions between the positively charged quaternary ammonium groups of choline and the negatively charged sulfate groups of CS. Compared to an aqueous medium, the adsorbed polysaccharide chains adopt more elongated conformations, which facilitates the electrostatic interactions with the membrane, and have a high degree of freedom to change their conformations and to adhere to and detach from the membrane surface. Penetrating slightly between the polar groups of the bilayer, they form a loosely anchored layer, but do not intrude into the hydrophobic region of the PC bilayer. The CS adsorption spread the PC headgroups apart, which is manifested by an increase in the value of the area pre lipid. The expansion of the lipid polar groups weakens the dispersion interactions between the lipid acyl chains. As a result, the lipid membrane in the membrane-polysaccharide contact areas becomes more fluid. Our outcomes may help to understand in detail the interaction of chondroitin sulfate with zwitterionic membranes at the molecular level, which is of biological interest since many biological processes depend on lipid-CS interactions.

Analytical challenges of glycosaminoglycans at biological interfaces

The analysis of glycosaminoglycans (GAGs) is a challenging task due to their high structural heterogeneity, which results in diverse GAG chains with similar chemical properties. Simultaneously, it is of high importance to understand their role and behavior in biological systems. It has been known for decades now that GAGs can interact with lipid molecules and thus contribute to the onset of atherosclerosis, but their interactions at and with biological interfaces, such as the cell membrane, are yet to be revealed. Here, analytical approaches that could yield important knowledge on the GAG-cell membrane interactions as well as the synthetic and analytical advances that make their study possible are discussed. Due to recent developments in laser technology, we particularly focus on nonlinear spectroscopic methods, especially vibrational sum-frequency generation spectroscopy, which has the potential to unravel the structural complexity of heterogeneous biological interfaces in contact with GAGs, in situ and in real time.

Chondroitin sulfate interacts mainly with headgroups in phospholipid monolayers

Sulfated glycosaminoglycans are precursors of the extracellular matrix used to treat diseases related to blood clotting and degenerative joint diseases. These medical applications have been well established, but the mode of action at the molecular level, which depends on the interaction with cell membranes, is not known in detail. In this study, we investigated the interaction between chondroitin sulfate (CS) and phospholipid monolayers that mimic cell membranes. From surface pressure isotherms and polarization-modulated infrared reflection absorption spectroscopy (PM-IRRAS), CS was found to interact mainly with the polar groups of dipalmitoyl phosphatidylcholine (DPPC) and dipalmitoyl phosphatidylglycerol (DPPG), with negligible penetration into the hydrophobic tails and only small changes in monolayer elasticity for the packing corresponding to a real cell membrane. The changes in surface pressure and surface potential isotherms depended on CS concentration and on the time allowed for its adsorption onto the monolayer, which points to a dynamic adsorption-desorption process. The charge of the phospholipid was also relevant, since CS induced order into DPPC monolayers while the opposite occurred for DPPG, according to the PM-IRRAS spectra. In summary, interaction with polar groups is responsible for the CS effects on model cell membranes.

Glycosaminoglycan-mediated selective changes in the aggregation states, zeta potentials, and intrinsic stability of liposomes

Though the aggregation of glycosaminoglycans (GAGs) in the presence of liposomes and divalent cations has been previously reported, the effects of different GAG species and minor changes in GAG composition on the aggregates that are formed are yet unknown. If minor changes in GAG composition produce observable changes in the liposome aggregate diameter or zeta potential, such a phenomenon may be used to detect potentially dangerous oversulfated contaminants in heparin. We studied the mechanism of the interactions between heparin and its oversulfated glycosaminoglycan contaminants with liposomes. Herein, we demonstrate that Mg(2+) acts to shield the incoming glycosaminoglycans from the negatively charged phosphate groups of the phospholipids and that changes in the aggregate diameter and zeta potential are a function of the glycosaminoglycan species and concentration as well as the liposome bilayer composition. These observations are supported by TEM studies. We have shown that the organizational states of the liposome bilayers are influenced by the presence of GAG and excess Mg(2+), resulting in a stabilizing effect that increases the T(m) value of DSPC liposomes; the magnitude of this effect is also dependent on the GAG species and concentration present. There is an inverse relationship between the percent change in aggregate diameter and the percent change in aggregate zeta potential as a function of GAG concentration in solution. Finally, we demonstrate that the diameter and zeta potential changes in POPC liposome aggregates in the presence of different oversulfated heparin contaminants at low concentrations allow for an accurate detection of oversulfated chondroitin sulfate at concentrations of as low as 1 mol %.

Fluorescent liposomes for differential interactions with glycosaminoglycans

We have successfully synthesized a lipid containing the pyranine dye as the hydrophilic headgroup. This lipid was incorporated into liposomes with 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine as the major component. The resultant liposomes displayed differential modulations in fluorescence emission intensity in the presence of nanomolar concentrations of different glycosaminoglycans. Linear discriminant analysis of the fluorescence response data demonstrate that the liposomes are able to distinguish between different GAGs. In addition, we also demonstrate that the liposomes incorporating the pyranine lipid are able to distinguish between dilute serum from healthy individuals and serum containing elevated chondroitin sulfate (simulated serum from an Alzheimer's disease patient).

Chondroitin 6-sulfate and dextran sulfate promote hypochlorite-induced peroxidation of phosphatidylcholine liposomes

In this work, we studied whether chondroitin sulfates and dextran sulfates (DXSs) can influence hypochlorite-induced peroxidation of phosphatidylcholine (PC) liposomes. Multilamellar liposomes (2 mg lipid/ml) were prepared in phosphate buffer, pH 7.4, with NaCl or not and exposed to reagent HOCl/ClO- (1mM) at 37 degrees C in the presence of different concentrations of chondroitin 6-sulfate (C6S), chondroitin 4-sulfate (C4S), DXS 8000, DXS 40,000, and DXS 500,000. Lipid peroxidation was assessed by thiobarbituric acid-reactive substance (TBARS) production. DXSs and C6S enhanced TBARS production in a dose-dependent manner. The decline in TBARS production at the relatively high C6S concentrations may be attributed to C4S present in C6S, since in contrast to C6S, C4S is known to react with hypochlorite. Dextrans, nonsulfated analogues of DXS, failed to modulate TBARS production. This fact indicates the important role of negatively charged sulfate groups for DXS to facilitate hypochlorite-induced peroxidation of PC liposomes. The electrostatic nature of the mechanism providing for the pro-oxidative effect of DXS was also supported by the influence of liposome surface charge and solution ionic strength on the extent of liposome peroxidation. The addition of calcium ions to the incubation mixture did not prevent the pro-oxidative action of DXS. The relevance of the results to atherogenesis is discussed.

Interfacial interaction between dextran sulfate and lipid monolayers: an electrochemical study

The interaction between dextran sulfate (DS) with zwitterionic dipalmitoylphosphatidylcholine (DPPC) and negatively charged dipalmitoylphosphatidic acid monolayers at different surface pressures at air-liquid and liquid-liquid interfaces was studied using Langmuir-Blodgett (LB) and electrochemical techniques. The negatively charged DS can bind to phospholipids via calcium ions. To investigate the mechanism of the adsorption of DS on lipid monolayers, compression isotherms (pi-A) and capacitance-potential curves were measured, and a theoretical model was developed to interpret the capacitance data. The compression of lipid monolayers in the presence of DS led to a more condensed hybrid layer, removing the LE-LC phase transition of DPPC. Lower surface pressures improved the binding of DS on the lipid monolayers via calcium bridges due to the electrostatic attraction. Alternating current voltammetry and cyclic voltammetry were used to monitor the transfer of a cationic beta-blocker (metoprolol) across lipid monolayers in the absence and presence of the polyelectrolyte and to compare with the transfer of the standard probe, tetraethylammonium cation. Results showed a strong dependence on (i) the surface pressure, (ii) the applied potential, and, (iii) in the case of the hybrid layer, the charge of the phospholipid headgroup. Finally, results were also confirmed by attenuated total reflection Fourier transform infrared spectroscopy, performed after transferring lipid multilayers onto a solid substrate by the LB method.

Relationship between conformation of polysaccharides -in the dilute regime and their interaction with a phospholipid bilayer

Interactions between polysaccharides and phospholipid bilayers have already been demonstrated in the literature but little is known about the influence of macromolecule conformations related to the solvent characteristics (pH, ions, ionic strength). In this study we have investigated the conformation of iono- and thermo-sensitive polysaccharides, iota- and kappa-carrageenans, and their interaction with a dimyristoylphosphatidylcholine (DMPC) model bilayer. The study was performed in two different media (NaCl 150 mmol/L, pH 6.5, and NaCl 300 mmol/L, pH 6.5). In the first part, the iota- and kappa-carrageenan samples have been characterized by size exclusion chromatography (SEC) coupled with a multi-angle laser light-scattering detector (MALLS). The SEC-MALLS results clearly show polysaccharide chain association at high ionic strength. In the second part, the polysaccharide-membrane interaction has been studied, using fluorescent probes embedded in the membrane. The thermotropic properties of the membrane were investigated by fluorescence depolarization of 1-(4-trimethylammonium-phenyl)-6-phenyl-1,3,5-hexatriene (TMA-DPH). The membrane surface accessibility was evaluated by fluorescence quenching of 2-(9-anthroyloxy) stearic acid (2-AS). Whatever the ionic strength tested, the polysaccharide presence notably enhances the membrane fluidity below the T(m). This sign of an interaction in the polar level of the membrane is more marked at low NaCl concentration. In contrast, the liposomes bilayer accessibility is drastically lowered when increasing the ionic strength. This is induced by macromolecular chain adsorption on the liposome surface, enhanced by the polysaccharide chain association. An ionic strength enhancement induces a conformational modification of the polysaccharide chains which modifies their ability to interact with the bilayer.

New role of glycosaminoglycans on the plasma membrane proposed by their interaction with phosphatidylcholine

Glycosaminoglycan side chains of membrane proteoglycans have been claimed to be located at the outermost layer of the glycocalyx surrounding the cell. In this study measurements by surface plasmon resonance and solid-phase assay have shown that both chondroitin sulfate and keratan sulfate but not heparin associate with phosphatidylcholine under physiological conditions. Spectrophotometric measurements also showed that chondroitin sulfate restricts the lateral diffusion of phosphatidylcholine in liposomes. These findings indicate that chondroitin sulfate and/or keratan sulfate chains of membrane proteoglycans crouch on the surface of the membrane while heparan sulfate chains stretch outward from the membrane surface as postulated traditionally.

Investigation of phospholipid area compression induced by calcium-mediated dextran sulfate interaction

The association of anionic polyelectrolytes such as dextran sulfate (DS) to zwitterionic phospholipid surfaces via Ca(2+) bridges results in a perturbation of lipid packing at physiologically relevant Ca(2+) concentrations. Lipid area compression was investigated in 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) multilamellar bilayer dispersions by (2)H-NMR and in monolayer studies. Binding of DS to DMPC surfaces via Ca(2+) results in denser lipid packing, as indicated by higher lipid chain order. DMPC order parameters are homogeneously increased throughout the lipid bilayer. Higher order translates into more extended hydrocarbon chains and decreased average lipid area per molecule. Area compression is reported as a function of DS concentration and molecular weight. Altering the NaCl and Ca(2+) concentrations modified electrostatic interactions between DS and phospholipid. A maximal area reduction of DeltaA = 2.7 A(2) per DMPC molecule is observed. The lipid main-phase transition temperature increases upon formation of DMPC/Ca(2+)/DS-complexes. Lipid area compression after addition of DS and Ca(2+) to the subphase was also observed in monolayer experiments. A decrease in surface tension of up to 3.5 mN/m at constant molecular area was observed. DS binds to the lipid headgroups by formation of Ca(2+) bridges without penetrating the hydrophobic region. We suggest that area compression is the result of an attractive electrostatic interaction between neighboring lipid molecules induced by high local Ca(2+) concentration due to the presence of DS. X-ray diffraction experiments demonstrate that DS binding to apposing bilayers reduces bilayer separation. We speculate that DS binding alters the phase state of low-density lipoproteins that associate with polyelectrolytes of the arterial connective tissue in the early stages of arteriosclerosis.

Heparin influence on alpha-staphylotoxin formed channel

The effects of heparin on ion channels formed by Staphylococcus aureus alpha-toxin (ST channel) in lipid bilayers were studied under voltage clamp conditions. Heparin concentrations as small as 100 pM induced a sharp dose-dependent increase in channel voltage sensitivity. This was only observed when heparin was added to the negative-potential side of lipid bilayers in the presence of divalent cations. Divalent cations differ in their efficiency: Zn2+>Ca2+>Mg2+. The apparent positive gating charge increased 2-3-fold with heparin addition as well as with acidification of the bathing solution. 'Free' carboxyl groups and carboxyl groups in ion pairs of the protein moiety are hypothesized to interact with sulfated groups of heparin through divalent cation bridges. The cis mouth of the channel (that protrudes beyond the membrane plane on the side of ST addition and to which voltage was applied) is less sensitive to heparin than the trans-mouth. It is suggested that charged residues which interact with heparin at the cis mouth of ST channels and which contribute to the effective gating charge at negative voltage may be physically different from those at the trans mouth and at positive voltage.

Ca2+-mediated interaction between dextran sulfate and dimyristoyl-sn-glycero-3-phosphocholine surfaces studied by 2H nuclear magnetic resonance

The binding of dextran sulfates (DSs) with varying chain lengths to phosphatidylcholine multilamellar vesicles was investigated as a function of polyelectrolyte, NaCl, and Ca2+ concentration. Attractive forces between negatively charged polyelectrolytes and zwitterionic phospholipids arise from the assembly of calcium bridges. The formation of calcium bridges between the sulfate groups on the dextran sulfate and the phosphate group of the lipid results in increased calcium binding in mixtures of DS and 1, 2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC). At high NaCl concentration, the plateau adsorption of DS 500 is increased. The strength of dextran sulfate binding to DMPC is reflected in the changes of the 2H NMR quadrupolar splittings of the headgroup methylenes. Association forces increase with the number of calcium bridges formed. Low-molecular-weight DS does not bind to DMPC surfaces whereas longer-chain DSs strongly influence headgroup structure as a result of strong association. DS binding increases with increasing concentration; however, further association of the polyelectrolyte can be promoted only if negative charges are sufficiently screened. DS binding to lipid bilayers is a complicated balance of calcium bridging and charge screening. From our data we postulate that the structure of the adsorbed layer resembles a lattice of DS strands sandwiched between the bilayer lamellae.

Lipids in predentine and dentine

Using two histochemical methods, malachite green-aldehyde and iodoplatinate, phospholipids were visualized in the predentine of rat incisors in the spaces located between collagen fibers and in dentine as needle-like structures located along individual or groups of mineralizing collagen fibers. The same staining pattern was seen with phospholipase A2-gold. Autoradiographic investigation using 3H choline as labelled precursor, visualized the incorporation of membrane-associated and extracellular choline-containing phosphatidyl choline and sphingomyelin. The cell and membrane-associated labelling decreased gradually between 24 and 4 days, whereas incorporation of the labelled precursor as stable extracellular matrix component was seen in dentine. In addition to these investigations, pharmacologically induced (suramine) and genetically (Krabbe's disease) lysosomal storage pathology was investigated. Defects due to lipid metabolism alterations were seen in predentine and/or in dentine. The major differences visualized here between the non-mineralized and mineralized compartments and interactions between phospholipids and proteoglycans, support the view that phospholipids as matrix components play an important role in the mechanisms of dentine formation and mineralization.

Characterization of dimyristoylphosphatidylcholine liposome aggregates induced by dextran sulfate and La(3+) by fluorescence spectroscopy

The addition of dextran sulfate (DS) to DMPC vesicles in the presence of di- and trivalent cations leads to a strong aggregation, resulting in a stack-like arrangement of the opposing membrane surfaces as shown by freeze-fracture electron microscopy. The strong aggregation is connected with a lipid mixing process, especially in the presence of La(3+) (measured by the NBD/Rh assay). The extent of lipid mixing depends on the molecular weight of DS and size of the DMPC vesicles. Additionally, a decrease in the surface dielectric constant of DMPC vesicles [measured by the emission shift of the fluorescent probe, dansylphosphatidyl-ethanolamine (DPE)] was observed. A direct dependence on the molecular weight (MW) of DS exists: the higher their MW, the higher the blue emission shift of the DPE probe. The results are discussed in terms of the theory proposed by Ohki and Arnold, which connects the decrease of the surface dielectric constant with the interaction parameters of phospholipid membranes.

Divalent cation-dependent interaction of sulfated polysaccharides with phosphatidylcholine and mixed phosphatidylcholine/phosphatidylglycerol liposomes

The Ca(2+)-dependent interaction of various polyanionic polysaccharides (chondroitin sulfate, heparin, dextran sulfate, beta-cyclodextrin sulfate, hyaluronic acid and carboxymethyldextran) with multilamellar dimyristoyl phosphatidylcholine (DMPC) liposomes was investigated by calorimetric and fluorescence spectroscopic measurements. It was found that an observed polysaccharide-induced phospholipid phase separation depends on the density of the sulfate groups along the polysaccharide chain independent of the presence of additional carboxyl groups. The phase separation resulting from the drastic dehydration of the covered membrane regions is monitored by the upward shift of the lipid phase transition and by the blue shift of the emission spectrum of a headgroup-dansylated phosphatidylethanolamine (DPE). This shift is only observable if the required polysaccharide chain length contains at least three glycosyl units. The Ca(2+)-mediated interaction of dextran sulfate with various phosphatidylcholines, differing in their compressibility, showed the maximal difference between the phase transition temperatures of the lipid phase covered by the polysaccharide and the uneffected lipid domains for dielaidinoyl phosphatidylcholine (DEPC), the most compressible phospholipid investigated here. Mixed negatively charged DMPC/dimyristoyl phosphatidylglycerol (DMPG) liposomes were found to compete with the likewise negatively charged dextran sulfate for the binding of Ca2+. At excess Ca2+ concentrations, the binding of the polysaccharide was strengthened, compared to pure DMPC liposomes. The monovalent cation sodium, was able to inhibit the interaction between the membrane surface and dextran sulfate. Various divalent cations were found to mediate the interaction, depending on their ionic radii and electron configuration. Within the second group of the periodic system Ca2+ is the most effective ion. However, within the horizontal forth period the ability to bind sulfated dextran to membrane surfaces decreases from Ca2+ to Ni2+, but then increases again if Cu2+ or Zn2+ was used as the mediating ion.

Interaction of dextran sulfate with phospholipid surfaces and liposome aggregation and fusion

The binding of dextran sulfate to phospholipid liposomes was investigated by microelectrophoresis experiments. The polyanion binds to neutral phospholipid liposomes (DMPC and PE) only in the presence of Ca2+. If positively charged stearylamine is incorporated in the vesicles dextran sulfate is bound without Ca2+. Negatively charged phospholipids as PS do not bind dextran sulfate, even in the presence of millimolar concentrations of Ca2+. The adsorption of dextran sulfate results in an aggregation of vesicles due to a bridging mechanism. In all cases the aggregation is followed by a disaggregation toward higher dextran sulfate concentrations. The disaggregation process starts at polymer concentrations smaller than the concentration of the onset of saturation of the adsorption. By use of the probe dilution method a fusion of small DMPC and DMPC/PE vesicles in the presence of Ca2+ and dextran sulfate was found.