Large-scale co-aggregation of fluorescent lipid probes with cell surface proteins

Lipid domain structure of the plasma membrane revealed by patching of membrane components

Lateral assemblies of glycolipids and cholesterol, "rafts," have been implicated to play a role in cellular processes like membrane sorting, signal transduction, and cell adhesion. We studied the structure of raft domains in the plasma membrane of non-polarized cells. Overexpressed plasma membrane markers were evenly distributed in the plasma membrane. We compared the patching behavior of pairs of raft markers (defined by insolubility in Triton X-100) with pairs of raft/non-raft markers. For this purpose we cross-linked glycosyl-phosphatidylinositol (GPI)-anchored proteins placental alkaline phosphatase (PLAP), Thy-1, influenza virus hemagglutinin (HA), and the raft lipid ganglioside GM1 using antibodies and/or cholera toxin. The patches of these raft markers overlapped extensively in BHK cells as well as in Jurkat T-lymphoma cells. Importantly, patches of GPI-anchored PLAP accumulated src-like protein tyrosine kinase fyn, which is thought to be anchored in the cytoplasmic leaflet of raft domains. In contrast patched raft components and patches of transferrin receptor as a non-raft marker were sharply separated. Taken together, our data strongly suggest that coalescence of cross-linked raft elements is mediated by their common lipid environments, whereas separation of raft and non-raft patches is caused by the immiscibility of different lipid phases. This view is supported by the finding that cholesterol depletion abrogated segregation. Our results are consistent with the view that raft domains in the plasma membrane of non-polarized cells are normally small and highly dispersed but that raft size can be modulated by oligomerization of raft components.

Transient confinement of a glycosylphosphatidylinositol-anchored protein in the plasma membrane

Glycosylphosphatidylinositol (GPI)-anchored proteins participate in many cell surface functions; however, the molecular associations of these lipid-linked proteins within the plasma membrane are not well understood. Recent biochemical analyses of detergent insoluble membrane fractions have suggested that GPI-anchored proteins may be associated with glycosphingolipid (GSL)-enriched domains that also contain cholesterol and signaling molecules such as Src family kinases and, in some cases, caveolae. The movements of two components of the putative GSL-enriched domains, Thy-1, a GPI-anchored protein, and GM1, a GSL, were followed with single particle tracking on C3H 10T1/2 cell surfaces and categorized into four modes of lateral transport, fast diffusion, slow anomalous diffusion, diffusion confined to 325-370 nm diameter regions, and a fraction of molecules that was essentially stationary on the 6.6 s time scale. Longer observations (60 s) showed that Thy-1 and GM1 are transiently confined for 7-9 s to regions averaging 260-330 nm in diameter. Approximately 35-37% of both Thy-1 and GM1 undergo confined diffusion, whereas only 16% of fluorescein phosphatidylethanolamine, a phospholipid analog which is not expected to be found in the GSL domains, experience confined diffusion to regions averaging approximately 230 nm in diameter. Further, when glycosphingolipid expression was reduced approximately 40% with the glucosylceramide synthase inhibitor, d-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol, the percentage of trajectories exhibiting confinement and the size of the confining domain for Thy-1 were reduced approximately 1.5-fold. In contrast, extraction of cells with Triton X-100 leaves the fraction of molecules confined and the domain sizes of Thy-1 and GM1 unchanged. Our results are consistent with the preferential association of GPI-anchored proteins with glycosphingolipid-enriched domains and suggest that the confining domains may be the in vivo equivalent of the detergent insoluble membrane fractions.

On the origin of sphingolipid/cholesterol-rich detergent-insoluble cell membranes: physiological concentrations of cholesterol and sphingolipid induce formation of a detergent-insoluble, liquid-ordered lipid phase in model membranes

Detergent-insoluble membrane fragments that are rich in sphingolipid and cholesterol can be isolated from both cell lysates and model membranes. We have proposed that these arise from membranes that are in the liquid-ordered phase both in vivo and in vitro [Schroeder et al. (1994) Proc. Natl. Acad. Sci. U.S.A. 91, 12130-12134]. In order to detect formation of the liquid-ordered phase while avoiding possible detergent artifacts, we have now used fluorescence quenching to examine the phase behavior of mixtures of phosphatidylcholines, sphingolipids, and cholesterol. Phase separation was found in binary mixtures of either dipalmitoylphosphatidylcholine (DPPC) or sphingomyelin (SM) and a nitroxide-labeled phosphatidylcholine (12SLPC). A DPPC- or SM-enriched solidlike gel phase coexisted with a 12SLPC-enriched liquid-disordered fluid phase at 23 degrees C. As expected, phase separation was not seen at low concentrations of DPPC or SM. Instead, only a uniform fluid phase was present. Including 33 mol % cholesterol in model membranes greatly promoted phase separation. Phase separation was seen at higher temperatures and/or at lower concentrations of DPPC or SM in the presence of cholesterol than in its absence. Mixtures of DPPC or SM and cholesterol are known to form the liquid-ordered phase. Therefore, the fact that phase separation was observed in the cholesterol-containing membranes shows that liquid-ordered and liquid-disordered phase domains coexist. At 37 degrees C, the SM-enriched liquid-ordered phase was first seen at a SM/PC ratio of close to 0.25, when SM made up 17% of the total lipid including cholesterol. (This is similar to or less than the SM concentration of the plasma membranes of mammalian cells.) Furthermore, the detergent insolubility of cholesterol-containing model membranes correlated well with the amount of liquid-ordered phase as detected by fluorescence quenching. Thus, the detergent-insoluble membranes isolated from cells are likely to exist in the liquid-ordered phase prior to detergent extraction. The promotion of liquid-ordered phase formation may be an important function of cholesterol and sphingolipids in cells and may be a major distinction between the cholesterol- and sphingolipid-rich plasma membrane and most other cellular membranes.

Caveolae, DIGs, and the dynamics of sphingolipid-cholesterol microdomains

There is accumulating evidence that lateral assemblies (rafts) of sphingolipids and cholesterol form platforms that serve to support numerous cellular events in membrane traffic and signal transduction. Raft membrane microdomains are thought to function by preferentially associating with specific proteins while excluding others. The basic forces driving raft formation are lipid interactions which are, per se, weak and transient. Sphingolipid rafts should therefore be considered to be dynamic structures in which cholesterol plays an important role as a linker. Caveolins influence these dynamics by forming stabilized raft domains in intracellular membranes as well as at the plasma membrane. Recent data suggest that clustering of raft components could regulate raft dynamics and therefore represents an important feature in the function of these membrane microdomains.

Lipid microdomains in cell surface membranes

Lipid microdomains within cell membranes are detected by a variety of experimental techniques, each of which characterizes microdomains on a different time and spatial scale. The sum of the data on lipid microdomains has yet to be integrated into a single model of cell membrane structure. Indeed, one highlight of the past year is a new analysis of experimental results which yields a model of a cell membrane which need not contain any microdomains. Other highlights are an estimate of the number of phospholipid molecules in a membrane microdomain and the detection of domain formation in cell membranes in real time. Some progress has also been made in visualizing lipid microdomains within cell membranes. We still await, however, a new model of membrane structure that integrates all experimental results.

Evidence supporting a role for microfilaments in regulating the coupling between poorly dissociable IgE-Fc epsilonRI aggregates downstream signaling pathways

Aggregation of Fc epsilonRI, the high-affinity receptor for IgE, on RBL-2H3 mast cells caused by reversible ligands such as multivalent antigen causes cellular responses that can be halted by subsequent addition of excess monovalent ligand. In contrast, Ca2+ and degranulation responses elicited by effectively irreversible streptavidin cross-linking of biotinylated IgE-Fc epsilonRI are not stopped by addition of excess biotin after stimulation is initiated. These results support previous conclusions based on studies with covalent oligomers of IgE that stable cross-links can continue to deliver stimulatory signals for extended periods of time. Dissociation measured in the presence of monovalent hapten reveals two populations of IgE-Fc epsilonRI cross-linked by multivalent antigen that differ in functional effectiveness. Aggregates with readily dissociable cross-links are normally responsible for triggering essentially all of the degranulation response, whereas aggregates with poorly dissociable cross-links apparently do not trigger this response. Treatment of RBL-2H3 cells with cytochalasin D, an inhibitor of actin polymerization, enhances downstream signaling and enables the less readily dissociable aggregates to stimulate Ca2+ and degranulation responses. Under these conditions, cytochalasin D does not affect hapten-mediated dissociation of multivalent antigen, nor does it prevent hapten from reversing tyrosine phosphorylation of Syk. Cytochalasin D alone causes tyrosine phosphorylation of a protein at approximately 75 kDa, and it reduces hapten-induced reversal of antigen-stimulated tyrosine phosphorylation of several other proteins. Taken together, these results indicate that stimulated actin polymerization normally regulates the coupling of aggregated Fc epsilonRI to downstream signaling pathways, and they provide an explanation for seeming discrepancies between responses to stable and reversible cross-links.

Fatty acid alteration and the lateral diffusion of lipids in the plasma membrane of keratinocytes

The fluorescent probe diI was used to study the lateral mobility of lipids in in vitro strains of living adult human keratinocytes grown in four different media. One medium was essential fatty acid deficient (EFAD) and low in calcium ion, a medium known to yield cells that proliferate rapidly and contain lipid with extremely low levels of essential fatty acids. Two other media were supplemented with essential fatty acids (FAS), media that are known to result in cells that grow more slowly and have normalized fatty acid proportions. A fourth medium consisted of 1 microM all-trans-retinoic acid added to the fatty acid-supplemented medium (FAS-RA), a medium known to produce cells that are highly proliferative, with a growth rate greater than that of the FAS strains and similar to that of the EFAD strains. The keratinocytes grown in these four media were studied using the fluorescence recovery after photobleaching (FRAP) technique to determine the lateral diffusion rate of diI in the plasma membranes. Our results showed a positive correlation between growth rate and diffusion coefficient (D): the diffusion coefficient of diI was higher in the EFAD or FAS-RA cells than in the FAS cells. The measurement of D among the FAS cells fell into two groups. One group was similar to the single group seen in the EFAD cells, but the other group was composed of much lower D values. The other FRAP parameters (mobile fraction and bleach depth) were larger in the "slow" group than in the "fast" group. This trend of negative correlation between these parameters and D was also found within the fast group. These results are interpreted in terms of possible changes in membrane structure or morphology that might be indirectly associated with the fatty acid alterations, including the possible presence of areas in senescing keratinocytes where plasma membranes collapse to form an interacting system of lipid bilayers.

Assembly and intracellular targeting of the betagamma subunits of heterotrimeric G proteins

The assembly in living cells of heterotrimeric guanine nucleotide binding proteins from their constituent alpha, beta, and gamma subunits is a complex process, compounded by the multiplicity of the genes that encode them, and the diversity of receptors and effectors with which they interact. Monoclonal anti-beta antibodies (ARC5 and ARC9), raised against immunoaffinity purified beta gamma complexes, recognize beta subunits when not associated with gamma and can thus be used to monitor assembly of beta gamma complexes. Complex formation starts immediately after synthesis and is complete within 30 min. Assembly occurs predominantly in the cytosol, and association of beta gamma complexes with the plasma membrane fraction starts between 15-30 min of chase. Three pools of beta subunits can be distinguished based on their association with gamma subunits, their localization, and their detergent solubility. Association of beta and alpha subunits with detergent-insoluble domains occurs within 1 min of chase, and increases to reach a plateau of near complete detergent resistance within 30 min of chase. Brefeldin A treatment does not interfere with delivery of beta gamma subunits to detergent-insoluble domains, suggesting that assembly of G protein subunits with their receptors occurs distally from the BFA-imposed block of intracellular membrane trafficking and may occur directly at the plasma membrane.

Compartmentalized activation of the high affinity immunoglobulin E receptor within membrane domains

The earliest known step in the activation of the high affinity IgE receptor, FcepsilonRI, is the tyrosine phosphorylation of its beta and gamma subunits by the Src family tyrosine kinase, Lyn. We report here that aggregation-dependent association of FcepsilonRI with specialized regions of the plasma membrane precedes its tyrosine phosphorylation and appears necessary for this event. Tyrosine phosphorylation of beta and gamma occurs in intact cells only for FcepsilonRI that associate with these detergent-resistant membrane domains, which are enriched in active Lyn. Furthermore, efficient in vitro tyrosine phosphorylation of FcepsilonRI subunits occurs only for those associated with isolated domains. This association and in vitro phosphorylation are highly sensitive to low concentrations of detergent, suggesting that lipid-mediated interactions with Lyn are important in FcepsilonRI activation. Participation of membrane domains accounts for previously unexplained aspects of FcepsilonRI-mediated signaling and may be relevant to signaling by other multichain immune receptors.

Noncovalent associations of T lymphocyte surface proteins

A number of T cell surface transmembrane molecules such as CD2, CD4, CD8, lymphocyte functional antigen (LFA)-1 and CD45 are known to interact functionally with the T cell receptor (TCR) complex during T cell activation. Several previous communications have also reported physical associations between some of these molecules. On the other hand, there are indications that signaling through T cell surface molecules anchored via glycosylphosphatidylinositol (GPI), such as Thy-1, Ly-6 or CD59, is dependent on the TCR. Therefore, it was of interest to determine in a systematic way which T cell surface molecules are noncovalently associated with the TCR/CD3 complex and with the major intracellular signaling molecules, the protein tyrosine kinases of the Src family. To this aim, membrane proteins of human thymoma HPB-ALL cells were solubilized in a solution of the mild detergent Brij-58 and subjected to immunoprecipitation followed by in vitro kinase assays. Two types of large complexes containing protein tyrosine kinases were observed: the first one contained CD3 and the transmembrane proteins CD2, CD4, CD5, CD6, CD7, CD8, CD11a, CD38, CD43, CD45, CD71, CD98 and CD99 and the other contained mainly the GPI-anchored proteins CD48, CD55, CD59 and CDw108 as well as a fraction of CD4 and CD8. The GPI-anchored protein complexes were of larger size and lower buoyant density than the CD3 complexes. In agreement with these biochemical data, co-cross-linking of CD3 with most of the relevant transmembrane proteins on the surface of another T cell line, Jurkat, markedly enhanced tyrosine phosphorylation of several intracellular proteins. These data indicate the existence of at least two types of membrane microdomains of very different composition in the membranes of T cells which may play a role in signaling through different types of receptors and in functional cooperation between TCR/CD3 and various accessory molecules.

Submicrosecond phospholipid dynamics using a long-lived fluorescence emission anisotropy probe

The use of the long-lived fluorescence probe coronene (mean value of tau(FL) approximately 200 ns) is described for investigating submicrosecond lipid dynamics in DPPC model bilayer systems occurring below the lipid phase transition. Time-resolved fluorescence emission anisotropy decay profiles, measures as a function of increasing temperature toward the lipid-phase transition temperature (T(C)), for coronene-labeled DPPC small unilamellar vesicles (SUVs), are best described in most cases by three rotational decay components (phi(i = 3)). We have interpreted these data using two dynamic lipid bilayer models. In the first, a compartmental model, the long correlation time (phi(N)) is assigned to immobilized coronene molecules located in "gel-like" or highly ordered lipid phases (S-->1) of the bilayer, whereas a second fast rotational time (phi(F) approximately 2-5 ns) is associated with probes residing in more "fluid-like" regions (with corresponding lower ordering, S-->0). Interests here have focused on the origins of an intermediate correlation time (50-100 ns), the associated amplitude (beta(G)) of which increases with increasing temperature. Such behavior suggests a changing rotational environment surrounding the coronene molecules, arising from fluidization of gel lipid. The observed effective correlation time (phi(EFF)) thus reflects a discrete gel-fluid lipid exchange rate (k(FG)). A refinement of the compartmental model invokes a distribution of gel-fluid exchange rates (d(S,T)) corresponding to a distribution of lipid order parameters and is based on an adapted Landau expression for describing "gated" packing fluctuations. A total of seven parameters (five thermodynamic quantities, defined by the free energy versus temperature expansion; one gating parameter (gamma) defining a cooperative "melting" requirement; one limiting diffusion rate (or frequency factor: d(infinity))) suffice to predict complete anisotropy decay curves measured for coronene at several temperatures below the phospholipid T(C). The thermodynamic quantities are associated with the particular lipid of interest (in this case DPPC) and have been determined previously from ultrasound studies, thus representing fixed constants. Hence resolved variables are r(O), temperature-dependent gate parameters (gamma), and limiting diffusion rates (d(infinity)). This alternative distribution model is attractive because it provides a general probe-independent expression for distributed lipid fluctuation-induced probe rotational rates occurring within bilayer membranes below the phospholipid phase transition on the submicrosecond time scale.

Fc epsilon RI-mediated association of 6-micron beads with RBL-2H3 mast cells results in exclusion of signaling proteins from the forming phagosome and abrogation of normal downstream signaling

Cells of the mucosal mast cell line, RBL-2H3, are normally stimulated to degranulate after aggregation of high affinity receptors for IgE (Fc epsilon RI) by soluble cross-linking ligands. This cellular degranulation process requires sustained elevation of cytoplasmic Ca2+. In this study, we investigated the response of RBL-2H3 cells to 6-micron beads coated with IgE-specific ligands. These ligand-coated beads cause only small, transient Ca2+ responses, even though the same ligands added in soluble form cause larger, more sustained Ca2+ responses. The ligand-coated 6-micron beads also fail to stimulate significant degranulation of RBL-2H3 cells, whereas much larger ligand-coated Sepharose beads stimulate ample degranulation. Confocal fluorescence microscopy shows that the 6-micron beads (but not the Sepharose beads) are phagocytosed by RBL-2H3 cells and that, beginning with the initial stages of bead engulfment, there is exclusion of many plasma membrane components from the 6-micron bead/cell interface, including p53/56lyn and several other markers for detergent-resistant membrane domains, as well as an integrin and unliganded IgE-Fc epsilon RI. The fluorescent lipid probe DiIC16 is a marker for the membrane domains that is excluded from the cell/bead interface, whereas a structural analogue, fast DiI, which differs from DiIC16 by the presence of unsaturated acyl chains, is not substantially excluded from the interface. None of these components are excluded from the interface of RBL-2H3 cells and the large Sepharose beads. Additional confocal microscopy analysis indicates that microfilaments are involved in the exclusion of plasma membrane components from the cell/bead interface. These results suggest that initiation of phagocytosis diverts normal signaling pathways in a cytoskeleton-driven membrane clearance process that alters the physiological response of the cells.

Fc epsilon RI-mediated recruitment of p53/56lyn to detergent-resistant membrane domains accompanies cellular signaling

Detergent-resistant plasma membrane structures, such as caveolae, have been implicated in signalling, transport, and vesicle trafficking functions. Using sucrose gradient ultracentrifugation, we have isolated low-density, Triton X-100-insoluble membrane domains from RBL-2H3 mucosal mast cells that contain several markers common to caveolae, including a src-family tyrosine kinase, p53/56lyn. Aggregation of Fc epsilon RI, the high-affinity IgE receptor, causes a significant increase in the amount of p53/56lyn associated with these low-density membrane domains. Under our standard conditions for lysis, IgE-Fc epsilon RI fractionates with the majority of the solubilized proteins, whereas aggregated receptor complexes are found at a higher density in the gradient. Stimulated translocation of p53/56lyn is accompanied by increased tyrosine phosphorylation of several proteins in the low-density membrane domains as well as enhanced in vitro tyrosine kinase activity toward these proteins and an exogenous substrate. With a lower detergent-to-cell ratio during lysis, significant Fc epsilon RI remains associated with these membrane domains, consistent with the ability to coimmunoprecipitate tyrosine kinase activity with Fc epsilon RI under similar lysis conditions [Pribluda, V. S., Pribluda, C. & Metzger, H. (1994) Proc. Natl. Acad. Sci. USA 91, 11246-11250]. These results indicate that specialized membrane domains may be directly involved in the coupling of receptor aggregation to the activation of signaling events.

Influence of obstacles on lipid lateral diffusion: computer simulation of FRAP experiments and application to proteoliposomes and biomembranes

Fluorescence Recovery After Photobleaching experiments were simulated using a computer approach in which a membrane lipid leaflet was mimicked using a triangular lattice obstructed with randomly distributed immobile and non-overlapping circular obstacles. Influence of the radius r and area fraction c of these obstacles and of the radius R of the observation area on the relative diffusion coefficient D* (Eq. (1)) and mobile fraction M was analyzed. A phenomenological equation relating D* to r and c was established. Fitting this equation to the FRAP data we obtained with the probe NBD-PC embedded in bacteriorhodopsin/egg-PC multilayers suggests that this transmembrane protein rigidifies the surrounding lipid phase over a distance of about 18 A (approximately equal to two lipid layers) from the protein surface. In contrast, analysis of published diffusion constants obtained for lipids in the presence of gramicidin suggests that in terms of lateral diffusion, this relatively small polypeptide does not significantly affect the surrounding lipid phase. With respect to the mobile fraction M, and for point obstacles above the percolation threshold, an increase in R led to a decrease in M which can be associated with the existence of closed domains whose average size and diffusion properties can be determined. Adaptation of this model to the re-interpretation of the FRAP data obtained by Yechiel and Edidin (J Cell Biol (1987) 115:755-760) for the plasma membrane of human fibroblasts consistently leads to the suggestion that the lateral organization of this membrane would be of the confined type, with closed lipid domains of approximately equal to 0.5 microns 2 in area.