Mapping microscopic order in plant and mammalian cells and tissues: novel differential polarization attachment for new generation confocal microscopes (DP-LSM)

Elucidation of the molecular architecture of complex, highly organized molecular macro-assemblies is an important, basic task for biology. Differential polarization (DP) measurements, such as linear (LD) and circular dichroism (CD) or the anisotropy of the fluorescence emission (r), which can be carried out in a dichrograph or spectrofluorimeter, respectively, carry unique, spatially averaged information about the molecular organization of the sample. For inhomogeneous samples-e.g. cells and tissues-measurements on macroscopic scale are not satisfactory, and in some cases not feasible, thus microscopic techniques must be applied. The microscopic DP-imaging technique, when based on confocal laser scanning microscope (LSM), allows the pixel by pixel mapping of anisotropy of a sample in 2D and 3D. The first DP-LSM configuration, which, in fluorescence mode, allowed confocal imaging of different DP quantities in real-time, without interfering with the 'conventional' imaging, was built on a Zeiss LSM410. It was demonstrated to be capable of determining non-confocally the linear birefringence (LB) or LD of a sample and, confocally, its FDLD (fluorescence detected LD), the degree of polarization (P) and the anisotropy of the fluorescence emission (r), following polarized and non-polarized excitation, respectively (Steinbach et al 2009 Acta Histochem.111 316-25). This DP-LSM configuration, however, cannot simply be adopted to new generation microscopes with considerably more compact structures. As shown here, for an Olympus FV500, we designed an easy-to-install DP attachment to determine LB, LD, FDLD and r, in new-generation confocal microscopes, which, in principle, can be complemented with a P-imaging unit, but specifically to the brand and type of LSM.

Analysis of cell surface molecular distributions and cellular signaling by flow cytometry

Flow cytometry is a fast analysis and separation method for large cell populations, based on collection and processing of optical signals gained on a cell-by-cell basis. These optical signals are scattered light and fluorescence. Owing to its unique potential ofStatistical data analysis and sensitive monitoring of (micro)heterogeneities in large cell populations, flow cytometry-in combination with microscopic imaging techniques-is a powerful tool to study molecular details of cellular signal transduction processes as well. The method also has a widespread clinical application, mostly in analysis of lymphocyte subpopulations for diagnostic (or research) purposes in diseases related to the immune system. A special application of flow cytometry is the mapping of molecular interactions (proximity relationships between membrane proteins) at the cell surface, on a cell-by-cell basis. We developed two approaches to study such questions; both are based ondistance-dependent quenching of excited state fluorophores (donors) by fluorescent or dark (nitroxide radical) acceptors via Förstertype dipole-dipole resonance energy transfer (FRET) and long-range electron transfer (LRET) mechanisms, respectively. A critical evaluation of these methods using donor- or acceptor-conjugated monoclonal antibodies (or their Fab fragments) to select the appropriate cell surface receptor or antigen will be presented in comparison with other approaches for similar purposes. The applicability of FRET and LRET for two-dimensional antigen mapping as well as for detection of conformational changes in extracellular domains of membrane-bound proteins is discussed and illustrated by examples of several lymphoma cell lines. Another special application area of flow cytometry is the analysis of different aspects of cellular signal transduction, e.g., changes of intracellular ion (Ca(2+), H(+), Na(+)) concentrations, regulation of ion channel activities, or more complex physiological responses of cell to external stimuli via correlated fluorescence and scatter signal analysis, on a cell-by-cell basis. This way different signaling events such as changes in membrane permeability, membrane potential, cell size and shape, ion distribution, cell density, chromatin structure, etc., can be easily and quickly monitored over large cell populations with the advantage of revealing microheterogeneities in the cellular responses. Flow cytometry also offers the possibility to follow the kinetics of slow (minute- and hour-scale) biological processes in cell populations. These applications are illustrated by the example of complex flow cytometric analysis of signaling in extracellular ATP-triggered apoptosis (programmed cell death) of murine thymic lymphocytes.

Two-dimensional receptor patterns in the plasma membrane of cells. A critical evaluation of their identification, origin and information content

A concise review is presented on the nature, possible origin and functional significance of cell surface receptor patterns in the plasma membrane of lymphoid cells. A special emphasize has been laid on the available methodological approaches, their individual virtues and sources of errors. Fluorescence energy transfer is one of the oldest available means for studying non-randomized co-distribution patterns of cell surface receptors. A detailed and critical description is given on the generation of two-dimensional cell surface receptor patterns based on pair-wise energy transfer measurements. A second hierarchical-level of receptor clusters have been described by electron and scanning force microscopies after immuno-gold-labeling of distinct receptor kinds. The origin of these receptor islands at a nanometer scale and island groups at a higher hierarchical (mum) level, has been explained mostly by detergent insoluble glycolipid-enriched complexes known as rafts, or detergent insoluble glycolipids (DIGs). These rafts are the most-likely organizational forces behind at least some kind of receptor clustering [K. Simons et al., Nature 387 (1997) 569]. These models, which have great significance in trans-membrane signaling and intra-membrane and intracellular trafficking, are accentuating the necessity to revisit the Singer-Nicolson fluid mosaic membrane model and substitute the free protein diffusion with a restricted diffusion concept [S.J. Singer et al., Science 175 (1972) 720].

Dynamic, yet structured: The cell membrane three decades after the Singer-Nicolson model

The fluid mosaic membrane model proved to be a very useful hypothesis in explaining many, but certainly not all, phenomena taking place in biological membranes. New experimental data show that the compartmentalization of membrane components can be as important for effective signal transduction as is the fluidity of the membrane. In this work, we pay tribute to the Singer-Nicolson model, which is near its 30th anniversary, honoring its basic features, "mosaicism" and "diffusion," which predict the interspersion of proteins and lipids and their ability to undergo dynamic rearrangement via Brownian motion. At the same time, modifications based on quantitative data are proposed, highlighting the often genetically predestined, yet flexible, multilevel structure implementing a vast complexity of cellular functions. This new "dynamically structured mosaic model" bears the following characteristics: emphasis is shifted from fluidity to mosaicism, which, in our interpretation, means nonrandom codistribution patterns of specific kinds of membrane proteins forming small-scale clusters at the molecular level and large-scale clusters (groups of clusters, islands) at the submicrometer level. The cohesive forces, which maintain these assemblies as principal elements of the membranes, originate from within a microdomain structure, where lipid-lipid, protein-protein, and protein-lipid interactions, as well as sub- and supramembrane (cytoskeletal, extracellular matrix, other cell) effectors, many of them genetically predestined, play equally important roles. The concept of fluidity in the original model now is interpreted as permissiveness of the architecture to continuous, dynamic restructuring of the molecular- and higher-level clusters according to the needs of the cell and as evoked by the environment.

Cell fusion experiments reveal distinctly different association characteristics of cell-surface receptors

The existence of small- and large-scale membrane protein clusters, containing dimers, oligomers and hundreds of proteins, respectively, has become widely accepted. However, it is largely unknown whether the internal structure of these formations is dynamic or static. Cell fusion was used to perturb the distribution of existing membrane protein clusters, and to investigate their mobility and associations. Scanning near-field optical microscopy, confocal and electron microscopy were applied to detect the exchange of proteins between large-scale protein clusters, whereas photobleaching fluorescence energy transfer was used to image the redistribution of existing small-scale membrane protein clusters. Large-scale clusters of major histocompatibility complex (MHC)-I exchanged proteins with each other and with MHC-II clusters. Similarly to MHC-I, large-scale MHC-II clusters were also dynamic. Exchange of components between small-scale protein clusters was not universal: intermixing did not take place in the case of MHC-II homoclusters; however, it was observed for homoclusters of MHC-I and for heteroclusters of MHC-I and MHC-II. These processes required a fluid state of the plasma membrane, and did not depend on endocytosis-mediated recycling of proteins. The redistribution of large-scale MHC-I clusters precedes the intermixing of small-scale clusters of MHC-I indicating a hierarchy in protein association. Investigation of a set of other proteins (α subunit of the interleukin 2 receptor, CD48 and transferrin receptor) suggested that a large-scale protein cluster usually exchanges components with the same type of clusters. These results offer new insight into processes requiring time-dependent changes in membrane protein interactions.

N-Alkane uptake and utilisation by Streptomyces strains

Streptomyces strains isolated from the Kuwait Burgan oil field were defined as S. griseoflavus, S. parvus, and S. plicatus utilised n-hexadecane, n-octadecane (purified fractions of mineral oil), kerosene, and crude oil as sole carbon and energy sources. The strains were incubated with n-alkanes and increase of the fatty acid content with chain length equivalent to the employed n-alkanes was observed. Signal transducing GTP-binding proteins (GBPs) play an important role in n-alkane uptake in streptomycetes. Specific activators of GBPs increased the uptake of hydrocarbons. Using the hydrophobic fluorescent dye diphenylhexatrien (DPH) as a probe, it was found that the microviscosity of the hydrophobic inner region of the cellular membrane is significantly lower in hydrocarbon utilisers than in non-utilisers. This difference probably reflects differences in the fatty acid composition of the strains. When cultures were grown in n-alkane containing media, electron microscopy revealed that the hydrocarbon utilisers showed less-electron dense areas as inclusions in the cytoplasm. Soil samples inoculated with Streptomyces strains eliminated hydrocarbons much faster than those not containing these strains, serving as control. When inorganic medium was supplied with n-hexadecane-1-14C as sole carbon and energy source, radioactive CO2 was detected. Since streptomycetes have not been used until now for oil elimination, though they are known as abundant soil bacteria tolerating extreme conditions, their possible use for bioremediation of hydrocarbon contaminated soils is discussed.

Clustering of class I HLA oligomers with CD8 and TCR: three-dimensional models based on fluorescence resonance energy transfer and crystallographic data

Fluorescence resonance energy transfer (FRET) data, in accordance with lateral mobility measurements, suggested the existence of class I HLA dimers and oligomers at the surface of live human cells, including the B lymphoblast cell line (JY) used in the present study. Intra- and intermolecular class I HLA epitope distances were measured on JY B cells by FRET using fluorophore-conjugated Ag-binding fragments of mAbs W6/32 and L368 directed against structurally well-characterized heavy and light chain epitopes, respectively. Out-of-plane location of these epitopes relative to the membrane-bound BODIPY-PC (2-(4,4-difluoro-5-(4-phenyl-1,3-butadienyl)-4-bora-3a,4a-diaza-s-indacene-3-pentanoyl)-1-hexadecanoyl-sn-glycero-3-phosphocholine) was also determined by FRET. Computer-simulated docking of crystallographic structures of class I HLA and epitope-specific Ag-binding fragments, with experimentally determined interepitope and epitope to cell surface distances as constraints, revealed several sterically allowed and FRET-compatible class I HLA dimeric and tetrameric arrangements. Extension of the tetrameric class I HLA model with interacting TCR and CD8 resulted in a model of a supramolecular cluster that may exist physiologically and serve as a functionally significant unit for a network of CD8-HLA-I complexes providing enhanced signaling efficiency even at low MHC-peptide concentrations at the interface of effector and APCs.

Cholesterol-dependent clustering of IL-2Ralpha and its colocalization with HLA and CD48 on T lymphoma cells suggest their functional association with lipid rafts

Immunogold staining and electron microscopy show that IL-2 receptor alpha-subunits exhibit nonrandom surface distribution on human T lymphoma cells. Analysis of interparticle distances reveals that this clustering on the scale of a few hundred nanometers is independent of the presence of IL-2 and of the expression of the IL-2R beta-subunit. Clustering of IL-2Ralpha is confirmed by confocal microscopy, yielding the same average cluster size, approximately 600-800 nm, as electron microscopy. HLA class I and II and CD48 molecules also form clusters of the same size. Disruption of cholesterol-rich lipid rafts with filipin or depletion of membrane cholesterol with methyl-beta-cyclodextrin results in the blurring of cluster boundaries and an apparent dispersion of clusters for all four proteins. Interestingly, the transferrin receptor, which is thought to be located outside lipid rafts, exhibits clusters that are only 300 nm in size and are less affected by modifying the membrane cholesterol content. Furthermore, transferrin receptor clusters hardly colocalize with IL-2Ralpha, HLA, and CD48 molecules (crosscorrelation coefficient is 0.05), whereas IL-2Ralpha colocalizes with both HLA and CD48 (crosscorrelation coefficient is between 0.37 and 0.46). This coclustering is confirmed by electron microscopy. The submicron clusters of IL-2Ralpha chains and their coclustering with HLA and CD48, presumably associated with lipid rafts, could underlie the efficiency of signaling in lymphoid cells.

Apoptosis of murine thymocytes induced by extracellular ATP is dose- and cytosolic pH-dependent

Thymocytes from young Balb/C mice responded to low extracellular ATP (ATPec) doses (< or = 0.3 mM) with a rapid intracellular acidification (mean pH: ca. 0.3 pH unit) that was inhibited by the Ca2+ channel blocker verapamil, or by suramin (50 microM) and TNP-ATP (40 microM), potent P2x (and P2y) purinoreceptor antagonists. ATPec also triggered a remarkable DNA fragmentation and cell shrinkage detectable only at these low doses. DNA fragmentation gradually disappears with increasing [ATPec] above 0.5 mM, with a concomitant dominance of cytosolic alkalinization of the cells. Suramin and TNP-ATP also blocked the ATPec-triggered DNA fragmentation efficiently. oATP, inhibitor of P2z nonspecific ATP-gated membrane pores, and 2 mM extracellular Mg2+ did not influence either the cytosolic acidification or the DNA fragmentation, but almost completely abolished the intracellular alkalinization characteristic of P2z receptor activation at high ATPec doses. Antagonist-sensitivity of the ATPec-induced membrane potential responses indicates that hyperpolarization is associated with intracellular acidification, while rapid depolarization is linked to alkalinization. These data together indicate that the Ca2+-dependent hyperpolarization and cytosolic acidification triggered by low ATPec doses are essential early signals in apoptosis of murine thymocytes and are likely mediated by P2x1 type ATP-gated ion channels. Subset specificity of the early purinergic signals suggests that the double positive thymocytes are most sensitive to ATPec showing both P2z and P2x receptor activation characteristics, the double negative thymocytes preferentially show P2z-type, while single positive (CD4- CD8+ or CD4+ CD8-) thymocytes respond mostly by weaker P2x-type changes, indicating that ATPec, similarly to adenosine may serve as a potential regulator of cell death and differentiation in the thymus.

Inhibition of IgE-mediated triggering of mast cells by complement-derived peptides interacting with the Fc epsilon RI

Mucosal type mast cells, in contrast to the serosal type ones, do not respond to cationic agents, or to the complement-derived peptides C3a and C5a. Earlier we have found that while C3a does not activate the rat mucosal type mast cells (line RBL-2H3), it strongly inhibits the IgE-mediated triggering of these cells, by interfering with the Fc epsilon RI-initiated signaling pathway. In the present study we further investigated the mechanism of this process. It is shown, that C3a interacts with the beta-chain of the Fc epsilon RI complex. Binding of the complement peptide to the cells apparently causes a decrease in the proximity of the IgE-binding Fc epsilon RI. Investigating certain sequences of C3a we found that the inhibition is caused by the C-terminal sequences of the complement-peptide, ranging from positions 56 to 77 and also by a shorter sequence, ranging from positions 56 to 64. The inhibitory effect of these peptides was observed both in the case of RBL-2H3 cells and mouse bone marrow derived mast cells.

Hydrocarbon uptake by Streptomyces

Oligocarbophilic Streptomyces strains capable of hydrocarbon uptake and utilization were isolated from the polluted desert of Kuwait and used in this study. Transmission electron-microscopy of hyphae revealed that they become enriched with large less electron dense areas in the cytoplasm, when biomass samples were incubated with alkanes. The Streptomyces isolate could utilize n-hexadecane as sole carbon and energy source and their fatty acid content showed an increase in the fatty acids with chain length equivalent to those of the alkane substrates. Fluorescence measurements of diphenylhexatriene dissolved in the representative alkane, n-hexadecane, showed that the kinetics of hydrocarbon uptake are quite different in hydrocarbon-utilizer compared with non-utilizer Streptomyces strain. Microviscosity of the cellular membrane of the utilizer strain was also different from that of the non-utilizer control strain Streptomyces griseus after incubation in the presence of n-hexadecane. Very likely the hydrocarbon utilizer transported these compounds more efficiently across their membranes and accumulated them as inclusions in the cytoplasm.

Supramolecular receptor structures in the plasma membrane of lymphocytes revealed by flow cytometric energy transfer, scanning force- and transmission electron-microscopic analyses

Receptors in the plasma membrane of blood cells in general and in that of lymphocytes in particular are supposed to move around in a random walk fashion relatively freely driven by thermal diffusion, as described by the Singer-Nicolson fluid mosaic membrane model. In this article we summarized data and techniques that indicated nonrandom codistribution patterns of receptor superstructures under conditions, where the generation of such molecular colocalizations by the methods themselves were excluded. Application of fluorescence energy transfer in a flow cytometer helped to analyze such codistribution patterns in cell populations. After normalizing energy transfer values for possible differences between labeling ratios of the targeting monoclonal antibodies and using the mean values of energy transfer distribution curves, two-dimensional receptor maps were generated from data obtained in a pair-wise fashion between receptors. Major histocompatibility complex (MHC) class I and II, intercellular adhesion molecule-1 (ICAM-1), TcR-CD3-CD4, tetraspan molecules (CD81, CD82, CD53), and the subunits of the multisubunit IL-2 receptor displayed nonrandom codistribution patterns sometimes with, but very frequently without induction by their ligand. Immunogold-bead "sandwich" labeling analyzed by atomic force microscopy has shown that such receptor "islands" existed also in "receptor-island-groups". This indicated the existence of nonrandom receptor distribution of MHC class I and II molecules also at an elevated hierarchical level. An analysis is given herein concerning a standardized approach. The apparent incompatibility of these supramolecular patterns with the Singer-Nicolson type "free-protein and lipid-mobility paradigm" was resolved by recommending an additional emphasis on the mosaicism of the membrane besides receptor mobility.

Preassembly of interleukin 2 (IL-2) receptor subunits on resting Kit 225 K6 T cells and their modulation by IL-2, IL-7, and IL-15: a fluorescence resonance energy transfer study

Assembly and mutual proximities of alpha, beta, and gamma(c) subunits of the interleukin 2 receptors (IL-2R) in plasma membranes of Kit 225 K6 T lymphoma cells were investigated by fluorescence resonance energy transfer (FRET) using fluorescein isothiocyanate- and Cy3-conjugated monoclonal antibodies (mAbs) that were directed against the IL-2R alpha, IL-2R beta, and gamma(c) subunits of IL-2R. The cell-surface distribution of subunits was analyzed at the nanometer scale (2-10 nm) by FRET on a cell-by-cell basis. The cells were probed in resting phase and after coculture with saturating concentrations of IL-2, IL-7, and IL-15. FRET data from donor- and acceptor-labeled IL-2R beta-alpha, gamma-alpha, and gamma-beta pairs demonstrated close proximity of all subunits to each other in the plasma membrane of resting T cells. These mutual proximities do not appear to represent mAb-induced microaggregation, because FRET measurements with Fab fragments of the mAbs gave similar results. The relative proximities were meaningfully modulated by binding of IL-2, IL-7, and IL-15. Based on FRET analysis the topology of the three subunits at the surface of resting cells can be best described by a "triangular model" in the absence of added interleukins. IL-2 strengthens the bridges between the subunits, making the triangle more compact. IL-7 and IL-15 act in the opposite direction by opening the triangle possibly because they associate their private specific alpha receptors with the beta and/or gamma(c) subunits of the IL-2R complex. These data suggest that IL-2R subunits are already colocalized in resting T cells and do not require cytokine-induced redistribution. This colocalization is significantly modulated by binding of relevant interleukins in a cytokine-specific manner.

Fluorescent lipid probes 12-AS and TMA-DPH report on selective, purinergically induced fluidity changes in plasma membranes of lymphoid cells

The effect of extracellular ATP (ATPex) on the anisotropy of 1-[4-(trimethylamino) phenyl]-6-phenyl-hexa-3,5 triene (TMA-DPH) and 12-anthroyloxi-stearic acid (12-AS) fluorescence was investigated in Balb/C mouse thymocytes and in JY human lymphoblasts. These cells have been shown recently to be sensitive and resistant to ATPex, respectively, in terms of cellular responses. Extracellular ATP (1 mM) induced a time-dependent elevation in the emission anisotropy of both probes (indicating an increased lipid packing density) in the plasma membrane of thymocytes. The maximal effect, at 37 degrees C, was observed between 20 and 60 min after ATPex administration, and followed by a gradual decrease of fluorescence anisotropy at longer times (60-180 min). ATPex did not change membrane fluidity of thymocytes below the phase transition temperature (at 18 degrees C). Oxidized ATP (oATP), a selective antagonist of P2z purinoreceptors, blocked the ATPex-induced decrease in membrane fluidity. Low ATPex concentrations (100-300 microM)--which are known to induce distinct signals (changes in membrane potential and intracellular Ph)--slightly fluidized the plasma membrane of thymocytes. This effect was partially blocked by quinine, a blocker of Ca(2+)-activated K+ channels. Neither 12-AS nor TMA-DPH showed any change in their emission anisotropy upon ATPex-treatment in the plasma membrane of the resistant human JY lymphoblast cells. No other signalling event (membrane potential change, Ca2+ response) is elicited by ATPex in this cell line. These data suggest that the changes in the membrane fluidity are likely consequences of specific, purinoreceptor-mediated signalling events, such as hyper-or depolarization of the plasma membrane or Ca2+ influx. These signals may induce changes in the conformation or lateral organization of membrane proteins, perturbing protein-lipid interactions, as well.

Effect of TSH and anti-TSH receptor antibodies on the plasma membrane potential of polymorphonuclear granulocytes

Effects of thyrotropin hormone (TSH) and anti-TSH receptor antibodies on the plasma membrane potential of polymorphonuclear granulocytes (PMN) were analyzed by means of flow cytometry. Both TSH and the autoantibody caused a rapid, dose-dependent hyperpolarization of the plasma membrane of PMNs. TSH was also able to mask (revert) the depolarizing effect of a chemotactic peptide, fMLP, on PMNs. No detectable rise in the cytosolic free calcium level accompanied the observed hyperpolarization. Quinine, a blocker of Ca(2+)-activated and voltage-gated K+ channels did not affect the hyperpolarization by TSH and antibodies. Decreasing the [K+] gradient across the plasma membrane by valinomycin, however, blocked the hyperpolarizing effect. Peptide362-376 (derived from the extracellular domain of TSH receptor) also blocked the hyperpolarization induced by both TSH and anti-TSHR antibodies. These data suggest that the observed hyperpolarization is a specific, receptor-mediated early signal during interaction of PMNs with TSH or anti-TSHR antibodies.

A photobleaching energy transfer analysis of CD8/MHC-I and LFA-1/ICAM-1 interactions in CTL-target cell conjugates

The photobleaching energy transfer (pbFRET) technique is a fluorescence method to measure proximity relationships between molecules, especially cell surface proteins, labeled with fluorophore-conjugated monoclonal antibodies, on a pixel-by-pixel base using digital imaging microscopy. This technique enables analysis of inter- and intramolecular proximities at cell surfaces at physiological conditions. We have developed a pbFRET approach to measure intercellular proximities in order to access spatial organization of interacting proteins in the contact region of two 'communicating' cells. Two examples, as possible application areas of this approach, are presented here: interaction between CD8 and MHC-I molecules in point contacts and interaction between LFA-1 and ICAM-1 molecules in focal contacts of CTL-target conjugates. The geometry of these protein contacts based on our resonance energy transfer (RET) data is consistent with the observed blocking effects of monoclonal antibodies (directed against the interacting proteins) on the cytolytic activity of CTLs and suggest a critical role for CD8beta-subunit in signal transmission in peripheral T-lymphocytes.

Modification of membrane cholesterol level affects expression and clustering of class I HLA molecules at the surface of JY human lymphoblasts

Recently we have found that class I HLA molecules, key elements of the antigen presentation system for CD8 + effector cells, show a clustered lateral distribution (homoassociation) at the surface of activated human T- and B-lymphocytes as well as virus-transformed T- and B-lymphoblasts, in contrast to a disperse distribution on resting human PBLs (Matk6 et al. (1994) J. Immunol. 152, 3353; Bene et al. (1994) Eur. J. Immunol. 24, 2115). Expression of beta2m-free HLA heavy chains and exogenous beta2m have been shown as potential regulation factors of HLA-I clustering, which in turn may affect cytotoxic activity of CD8+ effector cells. Here we report a study on the effect of plasma membrane-modification (by exogenous cholesterol and phosphatidylcholine) on the expression of free HLA heavy chains and beta2m-bound HLA-I molecules on JY human B-lymphoblasts. The modulating effect of these two treatments on the lipid fluidity of cells was demonstrated by fluorescence anisotropy of DPH lipid probe. The lateral clustering (association) of HLA-I molecules was detected by flow cytometric fluorescence resonance energy transfer (FCET) and digital imaging microscopic photobleaching energy transfer (pbFRET) methods, using flourescein-isothiocyanate (FITC) (donor)- and tetramethyl-rhodamine-isothiocyanate (TRITC) (acceptor)-labeled W6/32 or KE2 antibodies directed against intact HLA-I molecules. Cholesterol enrichment of the plasma membrane increased membrane fluidity and reduced the expression of heavy- and light-chain determinants of HLA-I molecules and free heavy chains (FHCs). This was accompanied with a higher degree of HLA-I clustering as shown by the enhanced intermolecular energy transfer efficiency. In contrast, cholesterol depletion resulted in membrane fluidization and increased expression of HLA-I epitopes. Our results suggest that both cholesterol level and lipid structure/fluidity of the plasma membrane in lymphoblastoid cells may also potentially regulate lateral organization and consequently the presentation efficiency of HLA-I molecules.

Changes in membrane potential of target cells promotes cytotoxic activity of effector T lymphocytes

The effector function of CD8+ lymphocytes depends on recognition by the TcR-CD3 complex of an oligopeptide presented by an MHC class I molecule on target cells. Recently it has been shown that MHC class I molecules change their conformation upon depolarization of human B lymphoblastoid JY cells. We studied here the effects of changes in membrane potential of target cells on the function of cytotoxic T lymphocytes (CTL). Selective alterations of plasma membrane potential of JY target cells were achieved by treatments with specific ionophore molecules as well as with Na(+)-K(+)-ATPase inhibitor, while the cytotoxic lymphocytes were not influenced. The plasma membrane was depolarized by gramicidin D and ouabain, while hyperpolarization was induced by valinomycin treatment. Alterations of the resting membrane potential of target cells in both direction resulted in an enhanced cytotoxic activity. The observed changes in cytolytic activities of cytotoxic T effectors may have a more general biological significance, namely apoptotic cells become depolarized after a given time, moreover neoplastic and virus infected cells also frequently show decreased membrane potential. A more efficient recognition of these cells by CTL is supposed to enhance the efficiency of their elimination, as well.

Plasma-membrane-bound macromolecules are dynamically aggregated to form non-random codistribution patterns of selected functional elements. Do pattern recognition processes govern antigen presentation and intercellular interactions?

Molecular recognition processes between cell surface elements are discussed with special reference to cell surface pattern formation of membrane-bound integral proteins. The existence, as detected by flow cytometric resonance energy transfer (Appendix), and significance of cell surface patterns involving the interleukin-2 receptor, the T-cell receptor-CD3 system, the intercellular adhesion molecule ICAM-1, and the major histocompatibility complex class I and class II molecules in the plasma membrane of lymphocytes are described. The modulation of antigen presentation by transmembrane potential changes is discussed, and a general role of transmembrane potential changes, and therefore of ion channel activities, adduced as one of the major regulatory mechanisms of cell-cell communication. A general role in the mediation and regulation of intercellular interactions is suggested for cell-surface macromolecular patterns. The dynamic pattern of protein and lipid molecules in the plasma membrane is generated by the genetic code, but has a remarkable flexibility and may be one of the major instruments of accommodation and recognition processes at the cellular level.

Structural hierarchy in the clustering of HLA class I molecules in the plasma membrane of human lymphoblastoid cells

Major histocompatibility complex (MHC) class I antigens in the plasma membranes of human T (HUT-102B2) and B (JY) lymphoma cells were probed by immunochemical reagents using fluorescence, transmission electron, and scanning force microscopies. Fluorescent labels were attached to monoclonal antibodies W6/32 or KE-2 directed against the heavy chain of HLA class I (A, B, C) and L368 or HB28 against the beta 2-microglobulin light chain. The topological distribution in the nanometer range was studied by photobleaching fluorescence resonance energy transfer (pbFRET) on single cells. A nonrandom codistribution pattern of MHC class I molecules was observed over distances of 2-10 nm. A second, nonrandom, and larger-scale topological organization of the MHC class I antigens was detected by indirect immunogold labeling and imaging by transmission electron microscopy (TEM) and scanning force microscopy (SFM). Although some differences in antigen distribution between the B- and T-cell lines were detected by pbFRET, both cell lines exhibited similar clustering patterns by TEM and SFM. Such defined molecular distributions on the surfaces of cells of the immune system may reflect an underlying specialization of membrane lipid domains and fulfill important functional roles in cell-cell contacts and signal transduction.

Ion-channel activities regulate transmembrane signaling in thymocyte apoptosis and T-cell activation

Several examples have shown that plasma membrane ion channels (e.g., Ca2+ and K+ channels) make an important contribution to lymphocyte activation or thymocyte apoptosis. Here we report on the importance of these ion channels in the sensitivity or resistance of lymphoid cells to extracellular ATP-induced apoptosis. Thymocytes of Balb/c mice responded to extracellular ATP (ATPex) sensitively, with an immediate increase in the intracellular calcium level and later with an increased membrane permeability to low MW markers. Mature (medullary) thymocytes showed a higher sensitivity than did cortical thymocytes. Three human lymphoma cell lines, including SUPT13, a cell line reported to be sensitive to TcR/CD3 activation-induced apoptosis, showed a high resistance to ATPex action. These observations suggest that maturation/differentiation state-dependent activity or disappearance of early ATP-receptor operated signaling systems (including ion channels) are critical for the cells in developing towards apoptosis. Using the patch-clamp technique we demonstrated that bretylium tosylate (a particular K(+)-channel blocker) known as inhibitor of T-lymphocyte proliferation also influences the single-channel properties of voltage-gated K+ channels through depressing whole-cell K+ currents. This finding is yet another example underlying the importance of K+ channel activity in T-lymphocyte proliferation.

Lateral organization of the ICAM-1 molecule at the surface of human lymphoblasts: a possible model for its co-distribution with the IL-2 receptor, class I and class II HLA molecules

Lateral distribution of the ICAM-1 molecule and its topological relationship (mutual proximity) to the heavy and light chains of class I HLA molecules, HLA-DR and interleukin-2 receptor alpha-chain (IL-2R alpha) were studied in the plasma membrane of HUT-102B2 T and JY B lymphoblastoid cell lines by the technique of flow cytometric energy transfer (FCET). Effects of adherency and treatments with recombinant interferon-gamma or tumor necrosis factor-alpha on the relative expression level of ICAM-1 to the above cell surface proteins were also investigated. While the cytokines did not significantly affect the ICAM-1 level of either cell line, an increased ICAM-1 expression was found on adherent JY cells. The ICAM-1 expression varied significantly with the cell cycle and culture conditions, as well. The statistical analysis of the differences observed in the energy transfer efficiency histograms resulted in a possible model of lateral co-distribution of these proteins in the plasma membrane. These two-dimensional patterns proved to be different for T and B lymphoma lines. ICAM-1 molecules showed a high degree of self-association on HUT-102B2 (T) cells, while they were mainly expressed as monomers on the surface of JY (B) cells. Both cells showed a significant (ca. 30%) difference between densities of the heavy and light chains of class I HLA antigen, suggesting a substantial amount of beta 2-microglobulin free heavy chains on these cell lines. The class I HLA molecules also showed partial self-association, but on both cell lines. The beta 2-microglobulin and the heavy chain of the class I HLA showed strongly different proximities to the IL-2R alpha, HLA-DR and ICAM-1 molecules, indicating that their orientations relative to the other proteins are dissimilar. IL-2R alpha molecules of the HUT-102B2 (T) cells are located mostly in the vicinity of the beta 2-microglobulin. In contrast, the local density of HLA-DR antigens is higher in the proximity of the heavy chain than in the vicinity of the beta 2-microglobulin. The possible functional significance of these protein patterns is also discussed herein.

Biphasic effect of extracellular ATP on the membrane potential of mouse thymocytes

Extracellular ATP induced changes in the membrane potential of thymocytes from BALB/c mice were analyzed. At concentrations below 0.1 mM, ATP hyperpolarizes the cell membrane on the time scale of development of the Ca(2+)-signal. After a longer time hyperpolarization turns to depolarization. ATP concentrations higher than 0.5 mM caused rapid depolarization without previous hyperpolarization. Verapamil, quinine or the absence of extracellular Ca2+ blocked the hyperpolarization by ATP. In Na(+)-free medium the magnitude of depolarization decreased. Our data suggest a contribution of Ca(2+)-activated K+ channels to the hyperpolarizing effect of ATP at lower concentrations. The direction of membrane potential changes is determined presumably by a sensitive balance of ATP-receptor mediated Ca(2+)- and Na(+)-influx and the Ca(2+)-activated K(+)-channel activity.

Effect of cyclosporin A on the membrane potential and Ca2+ level of human lymphoid cell lines and mouse thymocytes

The effect of the immunosuppressive cyclosporin A (CsA) on the cytosolic free Ca2+ concentration ([Ca2+]i) and membrane potential of human B and T lymphoblastoid cells and mouse thymocytes was studied in order to reveal some features of the early stage of drug-cell interaction. Cytosolic free Ca2+ concentration of the cells was measured by spectrofluorimetry using indo-1 and quin2 fluorescent calcium indicators. Membrane potential was monitored in a flow cytometer with oxonol dye. CsA applied at 2-20 micrograms/ml final concentrations caused a dose-dependent, rapid, transient rise of [Ca2+]i in all cell types. This effect could be blocked by chelating the extracellular Ca2+ with EGTA but was not sensitive to Ca2+ channel blockers verapamil and nifedipine or K+ channel blocker 4-aminopyridine. A possible explanation for the calcium mobilizing effect of CsA is an ionophore-like mode of action at the cell membrane level. Besides directly interfering with mitogenic signals, the elevation of [Ca2+]i could be responsible for an initial hyperpolarization observed in CsA-treated T lymphocytes. This hyperpolarization, however, was not detectable in B lymphoblastoid cells. A further difference between B and T cells was the diverse pattern of depolarization following CsA treatment. This variance in the behaviour of T and B lymphocytes and the diversity of membrane transport systems in its background could account for the different final outcome of the drug-cell interaction.

Interaction of the catalytic subunits of protein phosphatase-1 and 2A with inhibitor-1 and 2: a fluorescent study with sulfhydryl-specific pyrene maleimide

The catalytic subunits of protein phosphatase-1 and 2A were covalently modified in their reactive sulfhydryl groups with N-(3-Pyrene) maleimide resulting in fluorescent labeling of the proteins to an extent of 0.85 and 0.9 mole dye/mole enzyme, respectively. The reaction of the sulfhydryl group led to the partial inactivation of both phosphatase-1 and 2A. Inhibitor-1 and inhibitor-2 increased markedly the fluorescence intensity of the dye-phosphatase-1 conjugate implying that the labeled enzyme retained its ability to bind these proteins. In contrast, inhibitor-1 or inhibitor-2 had no influence on the fluorescence of the dye-phosphatase-2A conjugate. No change in either the fluorescence intensity or polarization of labeled phosphatase-1 and 2A was observed in the presence of thiophosphorylase a, suggesting a lack of interaction of these enzyme forms with the substrate after modification of the reactive sulfhydryl group.

The effect of transmembrane potential on the dynamic behavior of cell membranes

The relationship between transmembrane potential and lipid dynamics in the cytoplasmic membrane of mouse thymus cells has been investigated. Changes of transmembrane potential was followed by measuring the fluorescence emission of the anionic dye, bis-(1,3-dibutylbarbiturate)trimethine oxonol (diBa-C4-(3)). Assessment of lipid fluidity was carried out applying three fluorescent lipid probes, 1-[4-(trimethylammonium)phenyl]-6-phenyl-1,3,5-hexatriene (TMA-DPH), 12-(9-anthroyloxy)stearic acid (12-AS) and 1,6-diphenyl-1,3,5-hexatriene (DPH) used to monitor different structural regions of the bilayer. The fluorescence anisotropy of these probes was measured as a function of temperature at two values of transmembrane potential. In the case of DPH it proved to depend on the membrane potential in the higher temperature range (above 28 degrees C), while no such a dependence could be observed for DPH below this temperature range and for TMA-DPH and 12-AS in between 20 and 37 degrees C. These data suggest that changes in transmembrane potential are accompanied with some local alteration in membrane lipid dynamics and/or structure.

Intracellular free calcium concentration in lymphocytes of patients with muscular dystrophies

Intracellular free calcium concentration [( Ca2+]i) of human peripheral blood lymphocytes was determined by fluorescence spectroscopic measurements with quin2 in patients with different types of muscular dystrophy and in controls. The [Ca2+]i level in lymphocytes showed a significant increase in adult type (facioscapulohumeral and limb-girdle) muscular dystrophies, while it showed a decrease in Duchenne dystrophy as compared to the values of age- and sex-matched controls. The data obtained suggest an alteration in the effectiveness of the calcium pump in lymphocytes and may represent a sign of generalized membrane damage in these hereditary muscle diseases.

Luminescence spectroscopic approaches in studying cell surface dynamics

The major elements of membranes, such as proteins, lipids and polysaccharides, are in dynamic interaction with each other (Albertset al.1983). Protein diffusion in the lipid matrix of the membrane, the lipid diffusion and dynamic domain formation below and above their transition temperature from gel to fluid state, have many functional implications. This type of behaviour of membranes is often summarized in one frequently used word membrane fluidity (coined by Shinitzky & Henkart, 1979). The dynamic behaviour of the cell membrane includes rotational, translational and segmental movements of membrane elements (or their domain-like associations) in the plane of, and perpendicular to the membrane. The ever changing proximity relationships form a dynamic pattern of lipids, proteins and saccharide moieties and are usually described as ‘cell-surface dynamics’ (Damjanovichet al.1981). The knowledge about the above defined behaviour originates from experiments performed mostly on cytoplasmic membranes of eukaryotic cells. Nevertheless numerous data are available also on the mitochondrial and nuclear membranes, as well as endo (sarco-)plasmic reticulum (Martonosi, 1982; Slater, 1981; Siekevitz, 1981).

Accessibility of cell surface thiols in human lymphocytes is altered by ionophores or OKT-3 antibody

The accessibility of cell surface sulfhydryl groups in human peripheral lymphocytes was investigated with 5,5'-dithiobis-(2-nitrobenzoic acid) in the presence and absence of ionophore antibiotics and the monoclonal antibody, OKT-3. Only a few accessible protein thiols have been found on the cells as demonstrated by labeling with a fluorescent non-penetrating thiol-marker, monobromotrimethyl-ammoniobimane and the subsequent gel electrophoretic analysis of the protein pattern. Difference spectrophotometric measurement of thiol-DTNB reaction revealed that ionophores altering the transmembrane potential induced an enhanced cell surface thiol-exposure on the minute time scale. The effect showed a dependence on the external concentration of the cations. The binding of monoclonal antibody, OKT-3, directed against T3 complexes, resulted in a similar, concentration-dependent increase of thiol-accessibility. These data are interpreted as early membrane-effects of ionophores and the specific antibody including changes in the conformational equilibrium or vertical displacements of certain membrane proteins. These events are likely to be coupled to the changes in the transmembrane potential of the lymphocytes.

Structural and functional properties of Drosophila melanogaster phosphorylase: comparison with the rabbit skeletal muscle enzyme

Glycogen phosphorylase isolated from Drosophila melanogaster contains one pyridoxal 5'-phosphate per subunit; the coenzyme is in a hydrophobic environment. Fruit-fly phosphorylase a has lower KM for glucose-1-phosphate and is less sensitive to allosteric inhibitors than the b form of the enzyme. The amino acid composition of Drosophila phosphorylase differs from that of rabbit skeletal muscle phosphorylase. These two enzymes give distinct one dimensional peptide maps. The distribution of reactive SH-groups is markedly different in the insect and vertebrate phosphorylase. Fruit-fly phosphorylase a is dephosphorylated by either rabbit or Drosophila protein phosphatase-1 at a slower rate than rabbit muscle phosphorylase a.

A double-quenching method for studying protein dynamics: separation of the fluorescence quenching parameters characteristic of solvent-exposed and solvent-masked fluorophors

A novel experimental method, suitable for separate analysis of the quenching parameters characteristic of solvent-exposed and solvent-masked fluorophors of macromolecules, is described. The method is based on the modified Stern-Volmer analysis and requires simultaneous application of two kinds of quencher: one that can selectively quench the emission of exposed fluorophors (e.g., ionic quenchers) and another that is nonselective (e.g., oxygen or, in many cases, acrylamide), capable of quenching the fluorescence of both exposed and masked groups. In order to examine the accuracy of the model, a computer simulation was performed. The results showed that the errors are comparable to those arising from the conventional quenching experiments. The method is applicable to phosphorescence quenching as well and is extendable to time-resolved measurements (by replacing fluorescence intensities with lifetimes). The method was applied to resolve the quenching parameters of lysozyme fluorescence by the use of iodide as selective and acrylamide as nonselective quenchers. The determination of the acrylamide quenching constant associated with the internal fluorophor, Trp-108 (Kq = 3.5 M-1), permits specific studies on the dynamics of internal regions of the protein. The quenching constant determined for the more exposed residue Trp-62 (Kq = 1.6 M-1) provides local information about the surface independent of the electrostatic effects observed when an ionic quencher is used.

Dynamic interaction between functional groups in the active site of glycogen phosphorylase b

The quenching of coenzyme fluorescence in glycogen phosphorylase b is reinvestigated. Data with anionic quenchers show deviations from the original Stern-Volmer kinetics. A kinetic analysis based on measured lifetime data indicates a collisional quenching process, which is, however, not diffusion-controlled. It is proposed, that the quenching takes place primarily by enzyme-bound quencher species. The observed inhibition of the enzyme reaction by I- and IO-3 is consistent with this hypothesis. The inhibition pattern and spectral investigation refer to a true competition with the substrate, glucose-1-phosphate. So, this dynamic quenching can be regarded as an indicator of rapid conformational fluctuations which bring the two important active-site groups in contact. Effect of ligand binding on the quenching of coenzyme fluorescence should also be revaluated according to these results.

Förster-type energy transfer as a probe for changes in local fluctuations of the protein matrix

Much evidence, on both theoretical and experimental sides, indicates the importance of local fluctuations (in energy levels, conformational substates, etc.) of the macromolecular matrix in the biological activity of proteins. We describe here a novel application of the Förster-type energy-transfer process capable of monitoring changes both in local fluctuations and in conformational states of macromolecules. A new energy-transfer parameter, f, is defined as an average transfer efficiency, [E], normalized by the actual average quantum efficiency of the donor fluorescence, [phi D]. A simple oscillator model (for a one donor-one acceptor system) is presented to show the sensitivity of this parameter to changes in amplitudes of local fluctuations. The different modes of averaging (static, dynamic, and intermediate cases) occurring for a given value of the average transfer rate, [kt], and the experimental requirements as well as limitations of the method are also discussed. The experimental tests were performed on the ribonuclease T1-pyridoxamine 5'-phosphate conjugate (a one donor-one acceptor system) by studying the change of the f parameter with temperature, an environmental parameter expectedly perturbing local fluctuations of proteins. The parameter f increased with increasing temperature as expected on the basis of the oscillator model, suggesting that it really reflects changes of fluctuation amplitudes (significant changes in the orientation factor, k2, as well as in the spectral properties of the fluorophores can be excluded by anisotropy measurements and spectral investigations). Possibilities of the general applicability of the method are also discussed.

Segmental mobility in glycogen phosphorylase b

The dynamics and structuredness of the pyridoxal 5'-phosphate-binding region in glycogen phosphorylase b (EC 2.4.1.1) has been investigated with different techniques of fluorescence spectroscopy. Fluorescence polarization data of the thermal Perrin plot indicate some mobility in the cofactor binding site, while the isothermic measurements (at 20 degrees C, in high-viscosity solvents) demonstrate that the mobile unit carrying the emission oscillator is practically insensitive to the external viscosity. Characteristics of the thermal Perrin plots obtained for both native and reduced phosphorylase b can be interpreted either as a freely moving cofactor in a medium of high viscosity (0.3 P) or as the motion of a unit larger than a lysine-bonded pyridoxal 5'-phosphate in a medium with the viscosity of water. Data for acrylamide quenching and time-resolved fluorescence measurements suggest that the latter interpretation should valid. These data also suggest a tightly packed microenvironment around the pyridoxal moiety.

Heterotropic interactions of AMP and glucose binding sites in phosphorylase a are destroyed by limited proteolysis

Subtilisin BPN' hydrolyses a single peptide bond in phosphorylase a. The two proteolytic fragments are attached to each other by noncovalent bonds in solution as shown by gel filtration and ultracentrifugation studies. The subtilisin nicked phosphorylase a is inactive, however, still binds AMP and glucose as judged by equilibrium dialysis and fluorescence experiments. The modified enzyme can be dephosphorylated by protein phosphatase and AMP is an effective inhibitor of the dephosphorylation reaction. Glucose cannot cancel the AMP inhibition as well as cannot expel AMP from the nucleotide binding site. Thus a single nick in the polypeptide chain breaks the "communication" between the two ligand binding domains.

Effect of viscosity on enzyme-ligand dissociation. II. Role of the microenvironment

A theoretical treatment, describing a novel viscosity effect on decomposition of enzyme-ligand complexes, recently appeared (Somogyi et al., 1978). From this approach emerged a mechanistic picture of the manner in which increased viscosity lowers the value of the decomposition rate constant. A refined version of this model is presented herein. The analysis is extended to the molecular microenvironment ultimately responsible for mediating the "viscosity effect." Consideration is given to two major factors: (1) the role of viscosity in attenuating the excess chemical energy and (2) the statistical features of the microviscosity. In view of spatiotemporal inhomogeneity in the liquid structure, the concept of averaged microviscosity is introduced to parametrize the enzyme-ligand recombination probability. Quantitative predictions are consistent with models of liquid structure and with results from enzyme studies. The "viscosity effect" may contribute to substrate compartmentation in organized multi-enzyme systems in vivo.

A continuous assay method for glycogen synthesis from UDPglucose

A light scattering assay was developed for continuous monitoring of glycogen synthesis from UDPglucose. A close linear correlation between light-scattering signal and the amount of radioactively labelled glucose incorporated into glycogen suggests that the scattering signal is determined by the average molecular weight of the glycogen molecules. The method has the advantage of continuity, and it is suitable to follow the time course of the reaction at different conditions and to determine the initial rate of glycogen synthesis in a relatively simple and fast way. The possibility of measurement of glycogen synthase activity in muscle extracts was also tested.

Correlation between activity and dynamics of the protein matrix of phosphorylase b

Quenching of the tryptophan fluorescence of phosphorylase b was studied by using iodide and acrylamide. Steady-state measurements indicated that all indole side chains were accessible to the nonionic quencher, although only 3 out of the total of 12 residues could be quenched by I-. From Stern--Volmer plots and the fluorescence lifetime data, it was concluded that the quenching was mainly of dynamic character. The value of the collisional quenching rate constant was found to be an order of magnitude less than that obtained in the case of fully exposed tryptophans. The relatively high activation energy, 30.9 kJ/mol, of the diffusion-controlled process and the value of the activation entropy suggest that the diffusion takes place in a fluctuating, structured medium. In spite of the application of sensitive fluorescent techniques, no gross conformational changes were found in the presence of acrylamide. However, the catalytic rate of the glycogen synthesis was decreased with the residual activity of the enzyme, proportional to the concentration of the probe. Binding of activator (AMP) and substrates (glucose 1-phosphate and glycogen) was found to be unaffected by acrylamide in concentrations applied (0--0.8 M). In a similar manner, activation enthalpy did not change in the presence of the quencher either. The complete reversibility of both activity inhibition and fluorescence quenching ruled out the irreversible denaturation of the enzyme or the covalent modification of any of the functional groups. We concluded that a model, suggesting the cross-correlation of activity and fluctuation, was consistent with the experimental findings.