Van der Waals interactions between cell surfaces

Physicochemical Principles of Adhesion Mechanisms in the Brain

The brain functions through neuronal circuits and networks that are synaptically connected. This type of connection can exist due to physical forces that interact to stabilize local contacts in the brain. Adhesion is a fundamental physical phenomenon that allows different layers, phases, and tissues to connect. Similarly, synaptic connections are stabilized by specialized adhesion proteins. This review discusses the basic physical and chemical properties of adhesion. Cell adhesion molecules (CAMs) such as cadherins, integrins, selectins, and immunoglobulin family of cell adhesion molecules (IgSF) will be discussed, and their role in physiological and pathological brain function. Finally, the role of CAMs at the synapse will be described. In addition, methods for studying adhesion in the brain will be presented.

A van der Waals force-based adhesion study of stem cells exposed to cold atmospheric plasma jets

Cold atmospheric plasma has established its effect on cell adhesion. Given the importance of cell adhesion in stem cells, the current study investigates the effect of plasma treatment on Human Bone Marrow Mesenchymal Stem Cells (HBMMSCs) adhesion by which the differentiation and fate of cells are determined. In this paper, adhesion modification is considered not only for cell- ECM (Extra cellular Matrix), but also between suspended cells, and enhanced adhesions were found in both circumstances. Regarding the previous works, the increase of the cell–ECM adhesion during the plasma therapy was mostly attributed to the enhancement of the production and activity of integrin proteins. Nevertheless, considering the importance of van der Waals forces at the cellular level, the effect of cold plasma on VDWFs and so its effect on adhesion is investigated in this work for the first time, to the best of our knowledge. For this purpose, employing the semi-empirical methods, the role of the plasma therapy on the VDWF between the cells has been studied at three levels; (a) plasma-induced dipole formation, (b) Hammaker coefficient modification of culture medium, and c) cell roughness modification. For suspended cell condition, we conclude and support that van der Waals forces (VDWFs) enhancement has a key role in cell adhesion processes. We believe that, the present work gives a new physical insight in studying the plasma therapy method at the cellular level.

Lipid Status of A2780 Ovarian Cancer Cells after Treatment with Ruthenium Complex Modified with Carbon Dot Nanocarriers: A Multimodal SR-FTIR Spectroscopy and MALDI TOF Mass Spectrometry Study

Simple Summary

Developing new anticancer medicaments is focused on inducing controlled elimination of tumor tissue without severe side effects. It is essential to enable the medicament to reach the target molecule without provoking the immune response too early. The first cellular changes might occur already at the level of the cell membrane, composed mainly of lipids. Therefore, we used spectroscopic techniques to study the interaction of potential metallodrug [Ru(η⁵-C₅H₅)(PPh₃)₂CN] (RuCN) with lipids of A2780 ovarian cancer cells and investigated if these changes are affected by the presence of drug carriers (carbon dots (CDs) and nitrogen-doped carbon dots (N-CDs)). Our results showed that CDs and N-CDs prevent lysis and moderate oxidative stress of lipids caused by metallodrug, still keeping the antitumor activity and potential to penetrate through the lipid bilayer. Therefore, Ru drug loading to carriers balances the anticancer efficiency and leads to better anticancer outcomes by reducing the oxidative stress that has been linked to cancer progression.

Abstract

In the last decade, targeting membrane lipids in cancer cells has been a promising approach that deserves attention in the field of anticancer drug development. To get a comprehensive understanding of the effect of the drug [Ru(η⁵-Cp)(PPh₃)₂CN] (RuCN) on cell lipidic components, we combine complementary analytical approaches, matrix-assisted laser desorption and ionization time-of-flight mass spectrometry (MALDI TOF MS) and synchrotron radiation-based Fourier transform infrared (SR-FTIR) spectroscopy. Techniques are used for screening the effect of potential metallodrug, RuCN, without and with drug carriers (carbon dots (CDs) and nitrogen-doped carbon dots (N-CDs)) on the lipids of the human ovarian cancer cell line A2780. MALDI TOF MS results revealed that the lysis of ovarian cancer membrane lipids is promoted by RuCN and not by drug carriers (CDs and N-CDs). Furthermore, SR-FTIR results strongly suggested that the phospholipids of cancer cells undergo oxidative stress after the treatment with RuCN that was accompanied by the disordering of the fatty acid chains. On the other hand, using (N-)CDs as RuCN nanocarriers prevented the oxidative stress caused by RuCN but did not prevent the disordering of the fatty acid chain packing. Finally, we demonstrated that RuCN and RuCN/(N-)CDs alter the hydration of the membrane surface in the membrane–water interface region.

Colloidal transport and deposition through dense vegetation

The effectiveness of submerged synthetic aquatic vegetation on removal of colloids from flowing water was investigated to explore retention of particulate nonpoint source pollutants in aquatic systems. In colloid transport experiments, the deposition rate coefficient of colloids in dense vegetation is often taken as spatially constant. This assumption was tested by experiments and modeling aimed at quantifying changes in colloid retention with travel distance in submerged synthetic aquatic vegetation. Experiments were performed in a 10-m long, 0.6-m wide flume with a 5-cm water depth under different fluid velocities, initial colloid concentrations, and solution pH values. A model accounting for advection, dispersion and first-order kinetic deposition described the experimental data. The colloid deposition rate coefficient showed a power-law decrease with travel distance, and reached a steady state value before the end of the flume. Measured changes in colloid properties with transport distance (ζ potential and size) could not explain the observed decrease. While gravity was shown to contribute to the decrease, its impact was too weak to explain the decreasing power law trend, suggesting that processes operating in granular media to produce similar outcomes may also apply to submerged vegetation.

A coarse-grained xDLVO model for colloidal protein-protein interactions

Colloidal protein-protein interactions (PPIs) of attractive and repulsive nature modulate the solubility of proteins, their aggregation, precipitation and crystallization. Such interactions are very important for many biotechnological processes, but are complex and hard to control, therefore, difficult to be understood in terms of measurements alone. In diluted protein solutions, PPIs can be estimated from the osmotic second virial coefficient, B22, which has been calculated using different methods and levels of theory. The most popular approach is based on the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory and its extended versions, i.e. xDLVO. Despite much efforts, these models are not fully quantitative and must be fitted to experiments, which limits their predictive value. Here, we report an extended xDLVO-CG model, which extends existing models by a coarse-grained representation of proteins and the inclusion of an additional ion-protein dispersion interaction term. We demonstrate for four proteins, i.e. lysozyme (LYZ), subtilisin (Subs), bovine serum albumin (BSA) and immunoglobulin (IgG1), that semi-quantitative agreement with experimental values without the need to fit to experimental B22 values. While most likely not the final step in the nearly hundred years of research in PPIs, xDLVO-CG is a step towards predictive PPIs calculations that are transferable to different proteins.

3D analysis of human islet amyloid polypeptide crystalline structures in Drosophila melanogaster

Expression of the Alzheimer’s disease associated polypeptide Aβ42 and the human polypeptide hormon islet amyloid polypeptide (hIAPP) and the prohormone precursor (hproIAPP) in neurons of Drosophila melanogaster leads to the formation of protein aggregates in the fat body tissue surrounding the brain. We determined the structure of these membrane-encircled protein aggregates using transmission electron microscopy (TEM) and observed the dissolution of protein aggregates after starvation. Electron tomography (ET) as an extension of transmission electron microscopy revealed that these aggregates were comprised of granular subunits having a diameter of 20 nm aligned into highly ordered structures in all three dimensions. The three dimensional (3D) lattice of hIAPP granules were constructed of two unit cells, a body centered tetragonal (BCT) and a triclinic unit cell. A 5-fold twinned structure was observed consisting of the cyclic twinning of the BCT and triclinic unit cells. The interaction between the two nearest hIAPP granules in both unit cells is not only governed by the van der Waals forces and the dipole-dipole interaction but potentially also by filament-like structures that can connect the nearest neighbors. Hence, our 3D structural analysis provides novel insight into the aggregation process of hIAPP in the fat body tissue of Drosophila melanogaster.

Hydrodynamic versus Surface Interaction Impacts of Roughness in Closing the Gap between Favorable and Unfavorable Colloid Transport Conditions

Recent experiments revealed that roughness decreases the gap in colloid attachment between favorable (repulsion absent) and unfavorable (repulsion present) conditions through a combination of hydrodynamic slip and surface interactions with asperities. Hydrodynamic slip was calibrated to experimentally observed tangential colloid velocities, demonstrating that slip length was equal to maximum asperity relief, thereby providing a functional relationship between slip and roughness metrics. Incorporation of the slip length in mechanistic particle trajectory simulations yielded the observed modest decrease in attachment over rough surfaces under favorable conditions, with the observed decreased attachment being due to reduced colloid delivery rather than decreased attraction. Cumulative interactions with multiple asperities acting within the zone of colloid-surface interaction were unable to produce the observed dramatic increased attachment and decreased reversibility with increased roughness under unfavorable conditions, necessitating inclusion of nanoscale attractive heterogeneity that was inferred to have codeveloped with roughness. Simulated attachment matched experimental observations when the spatial frequency of larger heterodomains (nanoscale zones of attraction) increased disproportionately relative to smaller heterodomains as roughness increased, whereas attachment was insensitive to asperity properties, including the number of interactions per asperity and asperity height; colloid detachment simulations were highly sensitive to these parameters. These cumulative findings reveal that hydrodynamic slip moderately decreases colloid bulk delivery, nanoscale heterogeneity dramatically enhances colloid attachment, and multiple interactions among asperities decrease detachment from rough surfaces.

Membrane flexibility, free fatty acids, and the onset of vascular and neurological lesions in type 2 diabetes

Free fatty acids released from human adipose tissue contain a limited amount of non-esterified poly-cis-unsaturated fatty acids. In cases of elevated plasma free fatty acids, this condition ultimately leads to a shift from unsaturated to saturated fatty-acyl chains in membrane phospholipids. Because this shift promotes the physical attractive van der Waals interactions between phospholipid acyl chains, it increases stiffness of both erythrocyte and endothelial membranes, which causes a reduction in both insulin-independent and insulin-dependent Class 1 glucose transporters, a reduction in cell membrane functionality, and a decreased microcirculatory blood flow which results in tissue hypoxia. Against the background of these processes, we review recently published experimental phospholipid data obtained from Drosophila melanogaster and from human erythrocytes of controls and patients with type 2 diabetes, with and without retinopathy, along the way free fatty acids interfere with eye and kidney function in patients with type 2 diabetes and give rise to endoplasmic reticulum stress, reduced insulin sensitivity, and ischemia.

The effect of the physical properties of the substrate on the kinetics of cell adhesion and crawling studied by an axisymmetric diffusion-energy balance coupled model

In this paper an analytical approach to study the effect of the substrate physical properties on the kinetics of adhesion and motility behavior of cells is presented. Cell adhesion is mediated by the binding of cell wall receptors and substrate's complementary ligands, and tight adhesion is accomplished by the recruitment of the cell wall binders to the adhesion zone. The binders' movement is modeled as their axisymmetric diffusion in the fluid-like cell membrane. In order to preserve the thermodynamic consistency, the energy balance for the cell-substrate interaction is imposed on the diffusion equation. Solving the axisymmetric diffusion-energy balance coupled equations, it turns out that the physical properties of the substrate (substrate's ligand spacing and stiffness) have considerable effects on the cell adhesion and motility kinetics. For a rigid substrate with uniform distribution of immobile ligands, the maximum ligand spacing which does not interrupt adhesion growth is found to be about 57 nm. It is also found that as a consequence of the reduction in the energy dissipation in the isolated adhesion system, cell adhesion is facilitated by increasing substrate's stiffness. Moreover, the directional movement of cells on a substrate with gradients in mechanical compliance is explored with an extension of the adhesion formulation. It is shown that cells tend to move from soft to stiff regions of the substrate, but their movement is decelerated as the stiffness of the substrate increases. These findings based on the proposed theoretical model are in excellent agreement with the previous experimental observations.

Cell substratum adhesion during early development of Dictyostelium discoideum

Vegetative and developed amoebae of Dictyostelium discoideum gain traction and move rapidly on a wide range of substrata without forming focal adhesions. We used two independent assays to quantify cell-substrate adhesion in mutants and in wild-type cells as a function of development. Using a microfluidic device that generates a range of hydrodynamic shear stress, we found that substratum adhesion decreases at least 10 fold during the first 6 hr of development of wild type cells. This result was confirmed using a single-cell assay in which cells were attached to the cantilever of an atomic force probe and allowed to adhere to untreated glass surfaces before being retracted. Both of these assays showed that the decrease in substratum adhesion was dependent on the cAMP receptor CAR1 which triggers development. Vegetative cells missing talin as the result of a mutation in talA exhibited slightly reduced adhesive properties compared to vegetative wild-type cells. In sharp contrast to wild-type cells, however, these talA mutant cells did not show further reduction of adhesion during development such that after 5 hr of development they were significantly more adhesive than developed wild type cells. In addition, both assays showed that substrate adhesion was reduced in 0 hr cells when the actin cytoskeleton was disrupted by latrunculin. Consistent with previous observations, substrate adhesion was also reduced in 0 hr cells lacking the membrane proteins SadA or SibA as the result of mutations in sadA or sibA. However, there was no difference in the adhesion properties between wild type AX3 cells and these mutant cells after 6 hr of development, suggesting that neither SibA nor SadA play an essential role in substratum adhesion during aggregation. Our results provide a quantitative framework for further studies of cell substratum adhesion in Dictyostelium.

Mechanical model of vertical nanowire cell penetration

Direct access into cells' interiors is essential for biomolecular delivery, gene transfection, and electrical recordings yet is challenging due to the cell membrane barrier. Recently, molecular delivery using vertical nanowires (NWs) has been demonstrated for introducing biomolecules into a large number of cells in parallel. However, the microscopic understanding of how and when the nanowires penetrate cell membranes is still lacking, and the degree to which actual membrane penetration occurs is controversial. Here we present results from a mechanical continuum model of elastic cell membrane penetration through two mechanisms, namely through "impaling" as cells land onto a bed of nanowires, and through "adhesion-mediated" penetration, which occurs as cells spread on the substrate and generate adhesion force. Our results reveal that penetration is much more effective through the adhesion mechanism, with NW geometry and cell stiffness being critically important. Stiffer cells have higher penetration efficiency, but are more sensitive to NW geometry. These results provide a guide to designing nanowires for applications in cell membrane penetration.

Is adhesion superficial? Silicon wafers as a model system to study van der Waals interactions

Adhesion is a key issue for researchers of various fields, it is therefore of uppermost importance to understand the parameters that are involved. Commonly, only surface parameters are employed to determine the adhesive forces between materials. Yet, van der Waals forces act not only between atoms in the vicinity of the surface, but also between atoms in the bulk material. In this review, we describe the principles of van der Waals interactions and outline experimental and theoretical studies investigating the influence of the subsurface material on adhesion. In addition, we present a collection of data indicating that silicon wafers with native oxide layers are a good model substrate to study van der Waals interactions with coated materials.

Lipid composition of cell membranes and its relevance in type 2 diabetes mellitus

Identifying the causative relationship between the fatty acid composition of cell membranes and type 2 diabetes mellitus fundamentally contributes to the understanding of the basic pathophysiological mechanisms of the disease. Important outcomes of the reviewed studies appear to support the hypotheses that the flexibility of a membrane determined by the ratio of (poly)unsaturated to saturated fatty acyl chains of its phospholipids influences the effectiveness of glucose transport by insulin-independent glucose transporters (GLUTs) and the insulin-dependent GLUT4, and from the prediabetic stage on a shift from unsaturated towards saturated fatty acyl chains of membrane phospholipids directly induces a decrease in glucose effectiveness and insulin sensitivity. In addition, it has become evident that a concomitant increase in stiffness of both plasma and erythrocyte membranes may decrease the microcirculatory flow, leading ultimately to tissue hypoxia, insufficient tissue nutrition, and diabetes-specific microvascular pathology. As to the etiology of type 2 diabetes mellitus, a revised hypothesis that attempts to accommodate the reviewed findings is presented.

Innate non-specific cell substratum adhesion

Adhesion of motile cells to solid surfaces is necessary to transmit forces required for propulsion. Unlike mammalian cells, Dictyostelium cells do not make integrin mediated focal adhesions. Nevertheless, they can move rapidly on both hydrophobic and hydrophilic surfaces. We have found that adhesion to such surfaces can be inhibited by addition of sugars or amino acids to the buffer. Treating whole cells with αlpha-mannosidase to cleave surface oligosaccharides also reduces adhesion. The results indicate that adhesion of these cells is mediated by van der Waals attraction of their surface glycoproteins to the underlying substratum. Since glycoproteins are prevalent components of the surface of most cells, innate adhesion may be a common cellular property that has been overlooked.

Influence of the subsurface composition of a material on the adhesion of staphylococci

Controlling the interface between bacteria and solid materials has become an important task in biomedical science. For a fundamental and comprehensive understanding of adhesion it is necessary to seek quantitative information about the involved interactions. Most studies concentrate on the modification of the surface (chemical composition, hydrophobicity, or topography) neglecting, however, the influence of the bulk material, which always contributes to the overall interaction via van der Waals forces. In this study, we applied AFM force spectroscopy and flow chamber experiments to probe the adhesion of Staphylococcus carnosus to a set of tailored Si wafers, allowing for a separation of short- and long-range forces. We provide experimental evidence that the subsurface composition of a substrate influences bacterial adhesion. A coarse estimation of the strength of the van der Waals forces via the involved Hamaker constants substantiates the experimental results. The results demonstrate that the uppermost layer is not solely responsible for the strength of adhesion. Rather, for all kinds of adhesion studies, it is equally important to consider the contribution of the subsurface.

Agglutination of Histoplasma capsulatum by IgG monoclonal antibodies against Hsp60 impacts macrophage effector functions

Histoplasma capsulatum can efficiently survive within macrophages, facilitating H. capsulatum translocation from the lung into the lymphatics and bloodstream. We have recently generated monoclonal antibodies (MAbs) to an H. capsulatum surface-expressed heat shock protein of 60 kDa (Hsp60) that modify disease in a murine histoplasmosis model. Interestingly, the MAbs induced different degrees of yeast cell agglutination in vitro. In the present study, we characterized the agglutination effects of the antibodies to Hsp60 on H. capsulatum yeast cells by light microscopy, flow cytometry, dynamic light scattering, measuring zeta potential, and using optical tweezers. We found that immunoglobulin Gs (IgGs) to Hsp60 cause H. capsulatum aggregation dependent on the (i) concentration of MAbs, (ii) MAb binding constant, and (iii) IgG subclass. Furthermore, infection of macrophages using agglutinates of various sizes after incubation with different Hsp60-binding MAbs induced association to macrophages through distinct cellular receptors and differentially affected macrophage antifungal functions. Hence, the capacity of IgG MAbs to agglutinate H. capsulatum significantly impacted pathogenic mechanisms of H. capsulatum during macrophage infection, and the effect was dependent on the antibody subclass and antigen epitope.

Using the atomic force microscope to study the interaction between two solid supported lipid bilayers and the influence of synapsin I

To measure the interaction between two lipid bilayers with an atomic force microscope one solid supported bilayer was formed on a planar surface by spontaneous vesicle fusion. To spontaneously adsorb lipid bilayers also on the atomic force microscope tip, the tips were first coated with gold and a monolayer of mercapto undecanol. Calculations indicate that long-chain hydroxyl terminated alkyl thiols tend to enhance spontaneous vesicle fusion because of an increased van der Waals attraction as compared to short-chain thiols. Interactions measured between dioleoylphosphatidylcholine, dioleoylphosphatidylserine, and dioleoyloxypropyl trimethylammonium chloride showed the electrostatic double-layer force plus a shorter-range repulsion which decayed exponentially with a decay length of 0.7 nm for dioleoylphosphatidylcholine, 1.2 nm for dioleoylphosphatidylserine, and 0.8 nm for dioleoyloxypropyl trimethylammonium chloride. The salt concentration drastically changed the interaction between dioleoyloxypropyl trimethylammonium chloride bilayers. As an example for the influence of proteins on bilayer-bilayer interaction, the influence of the synaptic vesicle-associated, phospholipid binding protein synapsin I was studied. Synapsin I increased membrane stability so that the bilayers could not be penetrated with the tip.

Lysis of Large Unilamellar Vesicles Induced by Analogs of the Fusion Peptide of Influenza Virus Hemagglutinin

We have developed a micropipette aspiration assay to observe the lysis of large (20-30 &mgr;m diameter) vesicles aspirated using micropipettes. Single membrane lysis events can be seen with the light microscope and are followed using fluorescence assays and video microscopy. In this study, we have examined the ability of two analogs of the fusion peptide from influenza virus hemagglutinin to induce the lysis of large unilamellar egg phosphatidylcholine vesicles, as a function of peptide concentration and pH. X31 is a wild-type peptide from one strain of Influenza A, and E5 is an analogue which has several residues replaced by glutamate residues. Both peptides were found to induce lysis of large vesicles in a pH-dependent manner. Both peptides exhibited maximal activity at pH 5, measured in terms of both rate and extent of lysis. E5 was active at much lower concentrations than X31. Our results with both peptides are compared to results published from other laboratories.

Effects of dextran on hippocampal brain slice water, extracellular space, calcium kinetics and histology

Hippocampal brain slices are valuable models for studying brain function but are compromised by several artifacts, including significant water gain and histologic injury, which occur under certain incubation conditions. Addition of colloid to Krebs-Ringer buffer (K-R) has been shown to eliminate water gain but has not achieved widespread acceptance. We confirm prior observations that dextran and PEG lessen the increase in slice mass during incubation in a dose-dependent manner with no water gain occurring at 4% concentrations. However, we also observe that addition of colloid to standard K-R induces severe neuronal pyknosis. Fortunately, the pyknosis can be eliminated by reduction in buffer osmolarity through adjustment of NaCl, producing markedly improved slice histology in dextran buffer, especially in the CA3 and CA4 regions of the hippocampus which are severely injured when incubated submerged in K-R at 37 degrees C. Extracellular space markers are not affected by either colloid. The volume of distribution for 45Ca is much larger in dextran buffers than in K-R and variability of 45Ca kinetics is also reduced. In the presence of dextran, hypoxia induces significant slice water gain, a relatively selective histologic injury and an alteration of tissue Ca2+ kinetics. Use of dextran buffers may eliminate many troubling brain slice artifacts.

Micropipette manipulation technique for the monitoring of pH-dependent membrane lysis as induced by the fusion peptide of influenza virus

We have assembled a micropipette aspiration assay to measure membrane destabilization events in which large (20-30 microns diameter) unilamellar vesicles are manipulated and exposed to membrane destabilizing agents. Single events can be seen with a light microscope and are recorded using both a video camera and a photomultiplier tube. We have performed experiments with a wild-type fusion peptide from influenza virus (X31) and found that it induces pH-dependent, stochastic lysis of large unilamellar vesicles. The rate and extent of lysis are both maximum at pH 5; the maximum rate of lysis is 0.018 s-1 at pH 5. An analysis of our data indicates that the lysis is not correlated either to the size of the vesicles or to the tension created in the vesicle membranes by aspiration.

Kinetics of specific and nonspecific adhesion of red blood cells on glass

Fixed spherical human red blood cells suspended in 17% sucrose were allowed to adhere on either clean glass surfaces or glass surfaces preincubated with antibodies specific to a certain blood group antigen. The adhesion experiments were performed in an impinging jet apparatus, in which the cells are subjected to stagnation point flow. The objective of this study was to compare the efficiencies of nonspecific and specific (antigen-antibody mediated) adhesion of red blood cells on glass surfaces. The efficiency was defined as the ratio of the experimental adhesion rate to that calculated based on numerical solutions of the mass transfer equation, taking into account hydrodynamic interactions as well as colloidal forces. The efficiency for nonspecific adhesion was nearly unity at flow rates lower than 85 microliter/s (corresponding to a wall shear rate, Gw, of 30 s-1 at a radial distance of 110 microns from the stagnation point). The values of efficiency dropped at higher flow rates, due to an increase in the tangential force. The critical deposition concentration is found to occur at 120-150 mM NaCl, which is consistent with the theoretically predicted values. At low salt concentrations, the experimental values are higher than the theoretical ones. Similar discrepancies have been found in many colloidal systems. Introducing steric repulsion by adsorbing a layer of albumin molecules on the glass completely prevents nonspecific adhesion at flow rates below 60 microliter/s (Gw congruent to 15 s-1). The efficiency of specific adhesion depends both on the concentration of antibody molecules on the surface and the flow rate. Normal red cells adhere more readily through antigen-antibody bonds than fixed cells. Fixed spherical cells have a higher adhesion efficiency than fixed biconcave ones.

Membrane interactions in nerve myelin. I. Determination of surface charge from effects of pH and ionic strength on period

We have used x-ray diffraction to study the interactions between myelin membranes in the sciatic nerve (PNS) and optic nerve (CNS) as a function of pH (2-10) and ionic strength (0-0.18). The period of myelin was found to change in a systematic manner with pH and ionic strength. PNS periods ranged from 165 to 250 A or more, while CNS periods ranged from 150 to 230 A. The native periods were observed only near physiological ionic strength at neutral or alkaline pH. The smallest periods were observed in the pH range 2.5-4 for PNS myelin and pH 2.5-5 for CNS myelin. The minimum period was also observed for PNS myelin after prolonged incubation in distilled water. At pH 4, within these acidic pH ranges, myelin period increased slightly with ionic strength; however, above these ranges, the period increased with pH and decreased with ionic strength. Electron density profiles calculated at different pH and ionic strength showed that the major structural alteration underlying the changes in period was in the width of the aqueous space at the extracellular apposition of membranes; the width of the cytoplasmic space was virtually constant. Assuming that the equilibrium myelin periods are determined by a balance of nonspecific forces/i.e., the electrostatic repulsion force and the van der Walls attractive force, as well as the short-range repulsion force (hydration force, or steric stabilization), then values in the period-dependency curve can be used to define the isoelectric pH and exclusion length of the membrane. The exclusion length, which is related to the minimum period at isoelectric pH, was used to calculate the electrostatic repulsion force given the other forces. The electrostatic repulsion was then used to calculate the surface potential, which in turn was used to calculate the surface charge density (at different pH and ionic strength). We found the negative surface charge increases with pH at constant ionic strength and with ionic strength at constant pH. We suggest that the former is due to deprotonation of the ionizable groups on the surface while the latter is due to ion binding. Interpretation of our data in terms of the chemical composition of myelin is given in the accompanying paper (Inouye and Kirschner, 1988). We also calculated the total potential energy functions for the different equilibrium periods and found that the energy minima became shallower and broader with increasing membrane separation. Finally, it was difficult to account directly for certain structural transitions from a balance of nonspecific forces. Such transitions included the abrupt appearance of the native period at alkaline pH and physiological ionic strength and the discontinuous compaction after prolonged treatment in distilled water. Possibly, in PNS myelin conformational modification of PO glycoprotein occurs under these conditions. The invariance of the cytoplasmic space suggests the presence of specific short-range interactions between surfaces at this apposition.

Ca2+-induced fusion of large unilamellar phosphatidylserine/cholesterol vesicles

The effect of cholesterol on the Ca2+-induced aggregation and fusion of large unilamellar phosphatidylserine (PS) vesicles has been investigated. Mixing of aqueous vesicle contents was followed continuously with the Tb/dipicolinate assay, while the dissociation of pre-encapsulated Tb/dipicolinate complex was taken as a measure of the release of vesicle contents. Vesicles consisting of pure PS or PS/cholesterol mixtures at molar ratios of 4:1, 2:1 and 1:1 were employed at three different lipid concentrations, each at four different Ca2+ concentrations. The results could be well simulated in terms of a mass-action kinetic model, providing separately the rate constants of vesicle aggregation, c11, and of the fusion reaction itself, f11. In the analyses the possibility of deaggregation of aggregated vesicles was considered explicitly. Values of both c11 and f11 increase steeply with the Ca2+ concentration increasing from 2 to 5 mM. With increasing cholesterol content of the vesicles the value of c11 decreases, while the rate of the actual fusion reaction, f11, increases. Remarkably, the effect of cholesterol on both aggregation and fusion is quite moderate. The presence of cholesterol in the vesicle bilayer does not affect the leakage of vesicle contents during fusion.

Free energy potential for aggregation of giant, neutral lipid bilayer vesicles by Van der Waals attraction

Here, we report the first direct observation of Van der Waals' attraction between biomembrane capsules using measurements of the free energy reduction per unit area of membrane-membrane contact formation. In these studies, the membrane capsules were reconstituted neutral (egg phosphatidylcholine) lipid bilayers of giant (greater than 10(-3) cm diam) vesicles. Micromanipulation methods were used to select and maneuver two vesicles into proximity for contact; after adhesion was allowed to occur, the extent of contact formation was regulated through the vesicle membrane tensions that were controlled by micropipette suction. The free energy reduction per unit area of contact formation was proportional to the membrane tension multiplied by a simple function of the pipette and vesicle dimensions. The free energy potential for Van der Waals attraction between the neutral bilayers in 120 mM NaCl solutions was 1.5 X 10(-2) ergs/cm2. Also, when human serum albumin was added to the medium in the range of 0-1 mg/ml, the free energy potential for bilayer-bilayer adhesion was not affected. Using published values for equilibrium spacing between lipid bilayers in multilamellar lipid-water dispersions and the theoretical equation for van der Waals attraction between continuous dielectric layers, we calculated the value for the Hamaker coefficient of the Van der Waals attraction to be 5.8 X 10(-14) ergs.

Cell adhesion. Competition between nonspecific repulsion and specific bonding

We develop a thermodynamic calculus for the modeling of cell adhesion. By means of this approach, we are able to compute the end results of competition between the formation of specific macromolecular bridges and nonspecific repulsion arising from electrostatic forces and osmotic (steric stabilization) forces. Using this calculus also allows us to derive in a straightforward manner the effects of cell deformability, the Young's modulus for stretching of bridges, diffusional mobility of receptors, heterogeneity of receptors, variation in receptor number, and the strength of receptor-receptor binding. The major insight that results from our analysis concerns the existence and characteristics of two phase transitions corresponding, respectively, to the onset of stable cell adhesion and to the onset of maximum cell-cell or cell-substrate contact. We are also able to make detailed predictions of the equilibrium contact area, equilibrium number of bridges, and the cell-cell or cell-substrate separation distance. We illustrate how our approach can be used to improve the analysis of experimental data, by means of two concrete examples.

Modulatory effects of different temperatures and Ca2+ concentrations on gangliosides and phospholipids in monolayers at air/water interfaces and their possible functional role

Gangliosides are neuraminic acid-containing glycolipids preferently localized in nervous membranes and showing physicochemical peculiarities, e.g., drastically changing amphiphilic properties by Ca2+ binding. On account of this they are favorite compounds to act as modulators of membraneous organization and functions during synaptic transmission. Lipid monolayers are suitable experimental systems for the study of the surface behavior of amphipatic molecules and therefore are useful to interpret membraneous organization. The surface pressure/area isotherms of monolayers of different individual gangliosides (GM1, GD1a, GD1b, GT1b) of an artificial reconstituted and a natural ganglioside mixture from bovine brain and of ganglioside mixtures from different brain parts of summer- and winter-adapted dsungarian hamsters were compared at three temperatures (11, 20, and 37 degrees C) with egg phosphatidylcholine (PC) and phosphatidylserine (PS) monolayers. The monolayers were formed in a Teflon trough on a triethanolamine/HCl-buffered (pH 7.4) subphase, in some cases containing different amounts of CaCl2. The surface pressure/area isotherms of ganglioside monolayers, in contrast to phospholipids, generally showed slowly rising slopes, with transitions from the liquid-expanded to the liquid-condensed state at a surface pressure of 20-30 mN/m. Ganglioside monolayers, in particular from GD1a or GT1b versus GD1b or from mixtures from summer- versus winter-adapted hamster brain, were differently affected by temperature and/or by Ca2+. PS monolayers were slightly condensed only by Ca2+. PC monolayers, however, were influenced neither by temperature nor by Ca2+. In mixed monolayers of the unpolar natural lipid cholesterol (Ch) and the disialoganglioside GD1a, intermolecular interactions were indicated. Ganglioside monolayers, in contrast to phospholipids, were shown to be easily modulated by temperature and/or Ca2+ ions, thus enabling gangliosides to act as possible membrane modulators, e.g., during synaptic transmission. In particular, the differences concerning the influences of temperature and/or Ca2+ on the surface behavior of ganglioside mixtures from the brain of summer- compared with winter-adapted hamsters are correlated with other physiologically relevant data.

Cell-cell adhesion in the immune system

Interactions between cells are essential for function and regulation of the immune system. Some of these interactions are mediated by soluble factors such as immunoglobulins or interleukins, which can serve as chemical messengers that transmit information between widely separated cells. In other cases interaction requires physical contact between the cells and cell-cell adhesion. In this review George Bell emphasizes adhesion mediated by specific cell-surface receptors and/or ligands, including instances in which the receptors have not been identified and are inferred from specificity of interaction rather than biochemical characterization.

The physical principles of energy transduction in chloroplast thylakoid membranes

Photosynthesis in green plants or algae may be represented by an overall equation:

The energy necessary to promote this overall reaction is derived from light through absorption by pigment molecules — chiefly the chlorophylls.

Photosynthesis occurs in chloroplasts - subcellular organelles in which all the chlorophyll pigments are located. The chloroplasts comprise membranous, structures, and can be classified into two types. To the first type belong chloroplasts with appressed stacks of lamellar membranes, termed grana. These chloroplasts occur in mesophyll cells (C3plants). The second type of chloroplasts are those with lamellar membranes that do not form the grana structures; they occur in bundle sheath cells of maize and other monocotyledons (C4plants, Hatch & Slack, 1970). In algae a greater diversity of structure occurs (Kirk & Tilney-Basset, 1978). Fluorescence microscopy indicates that chlorophyll molecules are localized mainly in the grana membrane regions of mesophyll-type chloroplasts and uniformly throughout the bundle sheath cells (Spencer & Wildman, 1962). Mesophyll chloroplasts are flattened saucer-shaped organelles (20 or more in each cell) of between 5000 and 10000 nm in diameter, and of thickness 1000–2000 nm, whilst the individual grana are each of the order of 300–500 nm in diameter. The available evidence suggests that individual lamellar membranes are arranged to form vesicles, or sacks where the internal space is completely delimited from the external space. These individual closed membrane structures were termed thylakoids (Menke, 1962).

Effects of ovarian hormones on cell membranes in the rat uterus. III. The surface carbohydrates at the apex of the luminal epithelium

Histochemical techniques, including radioisotope histochemistry, have been used to investigate the nature of the surface carbohydrates at the apex of cells of the luminal epithelium of the rat uterus under various hormonal conditions. Binding of ruthenium red was quantitatively similar in ovariectomized rats without further treatment and in those given three daily injections of progesterone. Ruthenium red binding was significantly lower after 3 days treatment with estradiol, and also after 3 days treatment with progesterone with an additional dose of estradiol on day 3, a regime known to produce an epithelium receptive to the implanting blastocyst. Binding of concanavalin A (con A), whether studied by electron microscope histochemistry after incubation of tissue with con A-horseradish peroxidase, or by light microscope autoradiography after incubation with 3H-con A, was not statistically different in any of the four groups of rats. The results with ruthenium red show a reduction in net negative charge of the carbohydrates on the apical cell membrane in conditions permitting implantation: this change is not due to variations in the amounts of the neutral carbohydrates, mannose and glucose, as demonstrated by con A.

Mass action kinetics of phosphatidylserine vesicle fusion as monitored by coalescence of internal vesicle volumes

The kinetics of Ca2+-induced fusion of sonicated phosphatidylserine vesicles is analyzed by means of the mass action model. The results of calculations are shown to simulate the experimental results for the mixing of aqueous vesicle volumes, release of vesicle contents and for the observed increase in light scattering [Wilschut, J., Düzgünes, N., Fraley, R., & Papahadjopoulos, D. (1980) Biochemistry (first of three papers in this issue)]. The calculations give the distribution of vesicle sizes during the initial stages of the fusion process and an estimate for the occurrence of multiple fusion events. It is estimated that during the first few seconds from the beginning of the fusion process in the above systems only a small fraction of the material trapped will leak during each fusion event. The fraction of material which leaks per fusion event is further reduced with increased Ca2+ concentrations. The values of the rates of fusion which describe the above experiments suggest that the rate limiting step of the overall fusion reaction is the aggregation and close approach of vesicles to each other rather than the fusion event per se.

Models for the specific adhesion of cells to cells

A theoretical framework is proposed for the analysis of adhesion between cells or of cells to surfaces when the adhesion is mediated by reversible bonds between specific molecules such as antigen and antibody, lectin and carbohydrate, or enzyme and substrate. From a knowledge of the reaction rates for reactants in solution and of their diffusion constants both in solution and on membranes, it is possible to estimate reaction rates for membrane-bound reactants. Two models are developed for predicting the rate of bond formation between cells and are compared with experiments. The force required to separate two cells is shown to be greater than the expected electrical forces between cells, and of the same order of magnitude as the forces required to pull gangliosides and perhaps some integral membrane proteins out of the cell membrane.