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

Long acyl chain ceramides govern cholesterol and cytoskeleton dependence of membrane outer leaflet dynamics

The spatiotemporal dynamics of the plasma membrane is a consequence of fine-tuned interactions between membrane components. However, the precise identity of molecular factors that maintain this delicate balance, which is lost even in cell membrane derived mimics, remains elusive. Here, we use two cell lines, CHO-K1 and RBL-2H3, which show differences in outer membrane organization, dynamics, and cytoskeleton coupling, to investigate the underlying factors. To our surprise, knock-down of the cytoskeleton-interacting Immunoglobulin E receptor, which is abundant in RBL-2H3 but not in CHO-K1 cells, is not responsible for lipid confinement or cytoskeleton coupling. A subsequent lipidomic analysis of the two cell membranes revealed differences in total membrane ceramide content (C16 to C24). Analysis of the dynamics and organization of ceramide treated live cell membranes by imaging fluorescence correlation spectroscopy demonstrates that C24 and C16 saturated ceramides uniquely alter membrane dynamics by promoting the formation of cholesterol-independent domains and by elevating the inter-leaflet coupling.

Video-Rate Bioluminescence Imaging of Degranulation of Mast Cells Attached to the Extracellular Matrix

Degranulation refers to the secretion of inflammatory mediators, such as histamine, serotonin, and proteases, that are stored within the granules of mast cells and that trigger allergic reactions. The amount of these released mediators has been measured biochemically using cell mass. To investigate degranulation in living single cells, fluorescence microscopy has traditionally been used to observe the disappearance of granules and the appearance of these discharged granules within the plasma membrane by membrane fusion and the movement of granules inside the cells. Here, we developed a method of video-rate bioluminescence imaging to directly detect degranulation from a single mast cell by measuring luminescence activity derived from the enzymatic reaction between Gaussia luciferase (GLase) and its substrate coelenterazine. The neuropeptide Y (NPY), which was reported to colocalize with serotonin in the secretory granules, fused to GLase (NPY-GLase) was efficiently expressed in rat basophilic leukemia (RBL-2H3) cells, a mast-cell line, using a preferred human codon-optimized gene. Bioluminescence imaging analysis of RBL-2H3 cells expressing NPY-GLase and adhered on a glass-bottomed dish showed that the luminescence signals from the resting cells were negligible, while the luminescence signals of the secreted NPY-GLase were repeatedly detected after the addition of an antigen. In addition, this imaging method was applicable for observing degranulation in RBL-2H3 cells that adhered to the extracellular matrix (ECM). These results indicated that video-rate bioluminescence imaging using GLase will be a useful tool for detecting degranulation in single mast cells adhered to a variety of ECM proteins.

Mast Cell Mediators Inhibit Osteoblastic Differentiation and Extracellular Matrix Mineralization

Mast cells are multifunctional immune cells that participate in many important processes such as defense against pathogens, allergic reactions, and tissue repair. These cells perform their functions through the release of a wide variety of mediators. This release occurs mainly through cross-linking IgE (immunoglobulin E) bound to high affinity IgE receptors by multivalent antigens. The abundance of mast cells in connective tissue, surrounding blood vessels, and their involvement in the early stages of bone repair support the possibility of physiological and pathological interactions between mast cells and osteoblasts. However, the participation of mast cell mediators in osteogenesis is not fully understood. Therefore, the objective of this work was to investigate the role of mast cell mediators in the acquisition of the osteogenic phenotype in vitro. The results show that pooled mast cell mediators can affect proliferation, morphology, and cytoskeleton of osteoblastic cells, and impair the activity and expression of alkaline phosphatase as well as the expression of bone sialoprotein. Also, mast cell mediators inhibit the expression of mRNA for those proteins and inhibit the formation and maturation of calcium nodules and consequently inhibit mineralization. Therefore, mast cell mediators can modulate osteogenesis and are potential therapeutic targets for treatments of bone disorders.

The FcεRI Signaling Cascade and Integrin Trafficking Converge at Patterned Ligand Surfaces

We examined the spatial targeting of early and downstream signaling mediated by the IgE receptor (FcεRI) in RBL mast cells utilizing surface-patterned 2,4 dinitrophenyl (DNP) ligands. Micron-sized features of DNP are presented as densely immobilized conjugates of bovine serum albumin (DNP-BSA) or mobile in a supported lipid bilayer (DNP-SLB). Although soluble anti-DNP IgE binds uniformly across features for both pattern types, IgE bound to FcεRI on cells shows distinctive distributions: uniform for DNP-SLB and edge-concentrated for DNP-BSA. These distributions of IgE-FcεRI propagate to the spatial recruitment of early signaling proteins, including spleen tyrosine kinase (Syk), linker for activation of T cells (LAT), and activated phospholipase C gamma 1 (PLCγ1), which all localize with engaged receptors. We found stimulated polymerization of F-actin is not required for Syk recruitment but is progressively involved in the recruitment of LAT and PLCγ1. We further found β1- and β3-integrins colocalize with IgE-FcεRI at patterned ligand surfaces as cells spread. This recruitment corresponds to directed exocytosis of recycling endosomes (REs) containing these integrins and their fibronectin ligand. Together, our results show targeting of signaling components, including integrins, to regions of clustered IgE-FcεRI in processes that depend on stimulated actin polymerization and outward trafficking of REs.

Sphingolipid Organization in the Plasma Membrane and the Mechanisms That Influence It

Sphingolipids are structural components in the plasma membranes of eukaryotic cells. Their metabolism produces bioactive signaling molecules that modulate fundamental cellular processes. The segregation of sphingolipids into distinct membrane domains is likely essential for cellular function. This review presents the early studies of sphingolipid distribution in the plasma membranes of mammalian cells that shaped the most popular current model of plasma membrane organization. The results of traditional imaging studies of sphingolipid distribution in stimulated and resting cells are described. These data are compared with recent results obtained with advanced imaging techniques, including super-resolution fluorescence detection and high-resolution secondary ion mass spectrometry (SIMS). Emphasis is placed on the new insight into the sphingolipid organization within the plasma membrane that has resulted from the direct imaging of stable isotope-labeled lipids in actual cell membranes with high-resolution SIMS. Super-resolution fluorescence techniques have recently revealed the biophysical behaviors of sphingolipids and the unhindered diffusion of cholesterol analogs in the membranes of living cells are ultimately in contrast to the prevailing hypothetical model of plasma membrane organization. High-resolution SIMS studies also conflicted with the prevailing hypothesis, showing sphingolipids are concentrated in micrometer-scale membrane domains, but cholesterol is evenly distributed within the plasma membrane. Reductions in cellular cholesterol decreased the number of sphingolipid domains in the plasma membrane, whereas disruption of the cytoskeleton eliminated them. In addition, hemagglutinin, a transmembrane protein that is thought to be a putative raft marker, did not cluster within sphingolipid-enriched regions in the plasma membrane. Thus, sphingolipid distribution in the plasma membrane is dependent on the cytoskeleton, but not on favorable interactions with cholesterol or hemagglutinin. The alternate views of plasma membrane organization suggested by these findings are discussed.

Plasma membrane organization and dynamics is probe and cell line dependent

The action and interaction of membrane receptor proteins take place within the plasma membrane. The plasma membrane, however, is not a passive matrix. It rather takes an active role and regulates receptor distribution and function by its composition and the interaction of its lipid components with embedded and surrounding proteins. Furthermore, it is not a homogenous fluid but contains lipid and protein domains of various sizes and characteristic lifetimes which are important in regulating receptor function and signaling. The precise lateral organization of the plasma membrane, the differences between the inner and outer leaflet, and the influence of the cytoskeleton are still debated. Furthermore, there is a lack of comparisons of the organization and dynamics of the plasma membrane of different cell types. Therefore, we used four different specific membrane markers to test the lateral organization, the differences between the inner and outer membrane leaflet, and the influence of the cytoskeleton of up to five different cell lines, including Chinese hamster ovary (CHO-K1), Human cervical carcinoma (HeLa), neuroblastoma (SH-SY5Y), fibroblast (WI-38) and rat basophilic leukemia (RBL-2H3) cells by Imaging Total Internal Reflection (ITIR)-Fluorescence Correlation Spectroscopy (FCS). We measure diffusion in the temperature range of 298-310K to measure the Arrhenius activation energy (E_Arr) of diffusion and apply the FCS diffusion law to obtain information on the spatial organization of the probe molecules on the various cell membranes. Our results show clear differences of the FCS diffusion law and E_Arr for the different probes in dependence of their localization. These differences are similar in the outer and inner leaflet of the membrane. However, these values can differ significantly between different cell lines raising the question how molecular plasma membrane events measured in different cell lines can be compared. This article is part of a Special Issue entitled: Interactions between membrane receptors in cellular membranes edited by Kalina Hristova.

Graphene Oxide Nanosheets Stimulate Ruffling and Shedding of Mammalian Cell Plasma Membranes

Graphene oxide (GO) has attracted intense interest for use in living systems and environmental applications. GO's compatibility with mammalian cells is sometimes inferred from its low cytotoxicity, but such conclusions ignore non-lethal effects that will influence GO's utility. Here we demonstrate, with rat basophilic leukemia (RBL) cells, profound plasma membrane (PM) ruffling and shedding induced by GO using confocal and live cell fluorescence microscopy, as well as scanning electron microscopy. These membrane structures contain immunoglobulin E receptors, are resistant to detergents, and lack detectable fluorescence labeling of F-actin and fibronectin. The formation of these membrane structures correlates with a loss of contact inhibition between RBL cells. We observe similar cellular responses towards GO for NIH-3T3 fibroblast cells and MDA-MB-231 human breast cancer cells. These findings reveal a previously unreported cellular response towards foreign nanomaterials. Membrane ruffling and shedding raise fundamental questions about how GO interacts with the PM, as well as its potential to modulate cellular mechanosensing for tissue engineering, stem cell differentiation, and other biomedical applications.

Partitioning, diffusion, and ligand binding of raft lipid analogs in model and cellular plasma membranes

Several simplified membrane models featuring coexisting liquid disordered (Ld) and ordered (Lo) lipid phases have been developed to mimic the heterogeneous organization of cellular membranes, and thus, aid our understanding of the nature and functional role of ordered lipid-protein nanodomains, termed "rafts". In spite of their greatly reduced complexity, quantitative characterization of local lipid environments using model membranes is not trivial, and the parallels that can be drawn to cellular membranes are not always evident. Similarly, various fluorescently labeled lipid analogs have been used to study membrane organization and function in vitro, although the biological activity of these probes in relation to their native counterparts often remains uncharacterized. This is particularly true for raft-preferring lipids ("raft lipids", e.g. sphingolipids and sterols), whose domain preference is a strict function of their molecular architecture, and is thus susceptible to disruption by fluorescence labeling. Here, we analyze the phase partitioning of a multitude of fluorescent raft lipid analogs in synthetic Giant Unilamellar Vesicles (GUVs) and cell-derived Giant Plasma Membrane Vesicles (GPMVs). We observe complex partitioning behavior dependent on label size, polarity, charge and position, lipid headgroup, and membrane composition. Several of the raft lipid analogs partitioned into the ordered phase in GPMVs, in contrast to fully synthetic GUVs, in which most raft lipid analogs mis-partitioned to the disordered phase. This behavior correlates with the greatly enhanced order difference between coexisting phases in the synthetic system. In addition, not only partitioning, but also ligand binding of the lipids is perturbed upon labeling: while cholera toxin B binds unlabeled GM1 in the Lo phase, it binds fluorescently labeled GMI exclusively in the Ld phase. Fluorescence correlation spectroscopy (FCS) by stimulated emission depletion (STED) nanoscopy on intact cellular plasma membranes consistently reveals a constant level of confined diffusion for raft lipid analogs that vary greatly in their partitioning behavior, suggesting different physicochemical bases for these phenomena.

Orthogonal patterning of multiple biomolecules using an organic fluorinated resist and imprint lithography

The ability to spatially deposit multiple biomolecules onto a single surface with high-resolution while retaining biomolecule stability and integrity is critical to the development of micro- and nanoscale biodevices. While conventional lithographic patterning methods are attractive for this application, they typically require the use of UV exposure and/or harsh solvents and imaging materials, which may be damaging to fragile biomolecules. Here, we report the development of a new patterning process based on a fluorinated patterning material that is soluble in hydrofluoroether solvents, which we show to be benign to biomolecules, including proteins and DNA. We demonstrate the implementation of these materials into an orthogonal processing system for patterning multibiomolecule arrays by imprint lithography at room temperature. We further showcase this method's capacity for fabricating patterns of receptor-specific ligands for fundamental cell studies.

Roles for ca(2+) mobilization and its regulation in mast cell functions

Mobilization of Ca²⁺ in response to IgE receptor-mediated signaling is a key process in many aspects of mast cell function. Here we summarize our current understanding of the molecular bases for this process and the roles that it plays in physiologically relevant mast cell biology. Activation of IgE receptor signaling by antigen that crosslinks these complexes initiates Ca²⁺ mobilization as a fast wave that is frequently followed by a series of Ca²⁺ oscillations which are dependent on Ca²⁺ influx-mediated by coupling of the endoplasmic reticulum luminal Ca²⁺ sensor STIM1 to the calcium release activated calcium channel protein Orai1. Granule exocytosis depends on this process, together with the activation of protein kinase C isoforms, and specific roles for these signaling steps are beginning to be understood. Ca²⁺ mobilization also plays important roles in stimulated exocytosis of recycling endosomes and newly synthesized cytokines, as well as in antigen-mediated chemotaxis of rat mucosal mast cells. Phosphoinositide metabolism plays key roles in all of these processes, and we highlight these roles in several cases.

Ca2+ waves initiate antigen-stimulated Ca2+ responses in mast cells

Ca(2+) mobilization is central to many cellular processes, including stimulated exocytosis and cytokine production in mast cells. Using single cell stimulation by IgE-specific Ag and high-speed imaging of conventional or genetically encoded Ca(2+) sensors in rat basophilic leukemia and bone marrow-derived rat mast cells, we observe Ca(2+) waves that originate most frequently from the tips of extended cell protrusions, as well as Ca(2+) oscillations throughout the cell that usually follow the initiating Ca(2+) wave. In contrast, Ag conjugated to the tip of a micropipette stimulates local, repetitive Ca(2+) puffs at the region of cell contact. Initiating Ca(2+) waves are observed in most rat basophilic leukemia cells stimulated with soluble Ag and are sensitive to inhibitors of Ca(2+) release from endoplasmic reticulum stores and to extracellular Ca(2+), but they do not depend on store-operated Ca(2+) entry. Knockdown of transient receptor potential channel (TRPC)1 and TRPC3 channel proteins by short hairpin RNA reduces the sensitivity of these cells to Ag and shifts the wave initiation site from protrusions to the cell body. Our results reveal spatially encoded Ca(2+) signaling in response to immunoreceptor activation that utilizes TRPC channels to specify the initiation site of the Ca(2+) response.

Focal adhesion proteins connect IgE receptors to the cytoskeleton as revealed by micropatterned ligand arrays

Patterned surfaces that present specific ligands in spatially defined arrays are used to examine structural linkages between clustered IgE receptors (IgE-Fc epsilonRI) and the cytoskeleton in rat basophilic leukemia (RBL) mast cells. We showed with fluorescence microscopy that cytoskeletal F-actin concentrates in the same regions as cell surface IgE-Fc epsilonRI that bind to the micrometer-size patterned ligands. However, the proteins mediating these cytoskeletal connections and their functional relevance were not known. We now show that whereas the adaptor proteins ezrin and moesin do not detectably concentrate with the array of clustered IgE-Fc epsilonRI, focal adhesion proteins vinculin, paxillin, and talin, which are known to link F-actin with integrins, accumulate in these regions on the same time scale as F-actin. Moreover, colocalization of these focal adhesion proteins with clustered IgE-Fc epsilonRI is enhanced after addition of fibronectin-RGD peptides. Significantly, the most prominent rat basophilic leukemia cell integrin (alpha5) avoids the patterned regions occupied by the ligands and associates preferentially with exposed regions of the silicon substrate. Thus, spatial separation provided by the patterned surface reveals that particular focal adhesion proteins, which connect to the actin cytoskeleton, associate with ligand-cross-linked IgE-Fc epsilonRI, independently of integrins. We investigated the functional role of one of these proteins, paxillin, in IgE-Fc epsilonRI-mediated signaling by using small interfering RNA. From these results, we determine that paxillin reduces stimulated phosphorylation of the Fc epsilonRI beta subunit but enhances stimulated Ca(2+) release from intracellular stores. The results suggest that paxillin associated with clustered IgE-Fc epsilonRI has a net positive effect on Fc epsilonRI signaling.

Cholesterol-dependent separation of the beta2-adrenergic receptor from its partners determines signaling efficacy: insight into nanoscale organization of signal transduction

Determining the role of lipid raft nanodomains in G protein-coupled receptor signaling remains fraught by the lack of assays directly monitoring rafts in native membranes. We thus combined extensive biochemical and pharmacological approaches to a nanoscale strategy based on bioluminescence resonance energy transfer (BRET) to assess the spatial and functional influence of cholesterol-rich liquid-ordered lipid nanodomains on beta2 adrenergic receptor (beta2AR) signaling. The data revealed that whereas beta2AR did not partition within liquid-ordered lipid phase, a pool of G protein and adenylyl cyclase (AC) were sequestered in these domains. Destabilization of the liquid-ordered phase by cholesterol depletion led to a lateral redistribution of Galphas and AC that favored interactions between the receptor and its signaling partners as assessed by BRET. This resulted in an increased basal and agonist-promoted beta2AR-stimulated cAMP production that was partially dampened as a result of constitutive protein kinase A-dependent phosphorylation and desensitization of the receptor. This restraining influence of nanodomains on beta2AR signaling was further substantiated by showing that liquid-ordered lipid phase stabilization using caveolin overexpression or increasing membrane cholesterol amount led to an inhibition of beta2AR-associated signaling. Given the emerging concept that clustering of receptors and effectors into signaling platforms contributes to the efficacy and selectivity of signal transduction, our results support a model whereby cholesterol-promoted liquid-ordered lipid phase-embedding Gs and AC allows their lateral separation from the receptor, thus restraining the basal activity and controlling responsiveness of beta2AR signaling machinery within larger signaling platforms.

Real-time cross-correlation image analysis of early events in IgE receptor signaling

Signaling in mast cells and basophils is mediated through IgE and its high affinity cell surface receptor, FcepsilonRI. Crosslinking of the receptors by a cognate multivalent antigen leads to degranulation and release of mediators of the allergic immune response. Using multicolor fluorescence confocal microscopy, we probed the spatio-temporal dynamics of early events in the IgE receptor signal cascade. We monitored the recruitment of GFP-/CFP-labeled signaling proteins by acquiring sequential images with time resolution of 3 s during stimulation of RBL-2H3 mast cells with multivalent antigen. A fluorescent tag on the antigen allowed us to visualize the plasma membrane localization of crosslinked receptors, and fluorescent cholera toxin B served as a plasma membrane marker. We developed an automated image analysis scheme to quantify the recruitment of fluorescent intracellular proteins to the plasma membrane and to assess the time-dependent colocalization of these and other membrane-associated proteins with crosslinked receptors as measured by cross-correlation between the plasma membrane distributions of the two fluorophores. This automated method permits analysis of thousands of individual images from multiple experiments for each cross-correlation pair. We systematically applied this analysis to characterize stimulated interactions of IgE receptors with several signaling proteins, including the tyrosine kinases Lyn and Syk, and the adaptor protein LAT. Notably, for Syk-CFP we observed a rapid stimulated translocation to the plasma membrane but very little colocalization with aggregated receptors. Our results demonstrate the utility of this simple, automated method to monitor protein interactions quantitatively during cell signaling.

Structural determinants for partitioning of lipids and proteins between coexisting fluid phases in giant plasma membrane vesicles

The structural basis for organizational heterogeneity of lipids and proteins underlies fundamental questions about the plasma membrane of eukaryotic cells. A current hypothesis is the participation of liquid ordered (Lo) membrane domains (lipid rafts) in dynamic compartmentalization of membrane function, but it has been difficult to demonstrate the existence of these domains in live cells. Recently, giant plasma membrane vesicles (GPMVs) obtained by chemically induced blebbing of cultured cells were found to phase separate into optically resolvable, coexisting fluid domains containing Lo-like and liquid disordered (Ld)-like phases as identified by fluorescent probes. In the present study, we used these GPMVs to investigate the structural bases for partitioning of selected lipids and proteins between coexisting Lo-like/Ld-like fluid phases in compositionally complex membranes. Our results with lipid probes show that the structure of the polar headgroups, in addition to acyl chain saturation, can significantly affect partitioning. We find that the membrane anchor of proteins and the aggregation state of proteins both significantly influence their distributions between coexisting fluid phases in these biological membranes. Our results demonstrate the value of GPMVs for characterizing the phase preference of proteins and lipid probes in the absence of detergents and other perturbations of membrane structure.

Fluorescence resonance energy transfer between lipid probes detects nanoscopic heterogeneity in the plasma membrane of live cells

Fluorescence resonance energy transfer (FRET) between matched carbocyanine lipid analogs in the plasma membrane outer leaflet of RBL mast cells was used to investigate lateral distributions of lipids and to develop a general method for quantitative measurements of lipid heterogeneity in live cell membranes. FRET measured as fluorescence quenching of long-chain donor probes such as DiO-C18 is greater with long-chain, saturated acceptor probes such as DiI-C16 than with unsaturated or shorter-chain acceptors with the same chromophoric headgroup compared at identical concentrations. FRET measurements between these lipid probes in model membranes support the conclusion that differential donor quenching is not caused by nonideal mixing or spectroscopic differences. Sucrose gradient analysis of plasma membrane-labeled, Triton X-100-lysed cells shows that proximity measured by FRET correlates with the extent of lipid probe partitioning into detergent-resistant membranes. FRET between DiO-C16 and DiI-C16 is sensitive to cholesterol depletion and disruption of liquid order (Lo) by short-chain ceramides, and it is enhanced by cross linking of Lo-associated proteins. Consistent results are obtained when homo-FRET is measured by decreased fluorescence anisotropy of DiI-C16. These results support the existence of nanometer-scale Lo/liquid disorder heterogeneity of lipids in the outer leaflet of the plasma membrane in live cells.

Large-scale fluid/fluid phase separation of proteins and lipids in giant plasma membrane vesicles

The membrane raft hypothesis postulates the existence of lipid bilayer membrane heterogeneities, or domains, supposed to be important for cellular function, including lateral sorting, signaling, and trafficking. Characterization of membrane lipid heterogeneities in live cells has been challenging in part because inhomogeneity has not usually been definable by optical microscopy. Model membrane systems, including giant unilamellar vesicles, allow optical fluorescence discrimination of coexisting lipid phase types, but thus far have focused on coexisting optically resolvable fluid phases in simple lipid mixtures. Here we demonstrate that giant plasma membrane vesicles (GPMVs) or blebs formed from the plasma membranes of cultured mammalian cells can also segregate into micrometer-scale fluid phase domains. Phase segregation temperatures are widely spread, with the vast majority of GPMVs found to form optically resolvable domains only at temperatures below approximately 25 degrees C. At 37 degrees C, these GPMV membranes are almost exclusively optically homogenous. At room temperature, we find diagnostic lipid phase fluorophore partitioning preferences in GPMVs analogous to the partitioning behavior now established in model membrane systems with liquid-ordered and liquid-disordered fluid phase coexistence. We image these GPMVs for direct visual characterization of protein partitioning between coexisting liquid-ordered-like and liquid-disordered-like membrane phases in the absence of detergent perturbation. For example, we find that the transmembrane IgE receptor FcepsilonRI preferentially segregates into liquid-disordered-like phases, and we report the partitioning of additional well known membrane associated proteins. Thus, GPMVs now provide an effective approach to characterize biological membrane heterogeneities.

Membrane order and molecular dynamics associated with IgE receptor cross-linking in mast cells

Cholesterol-rich microdomains (or "lipid rafts") within the plasma membrane have been hypothesized to exist in a liquid-ordered phase and play functionally important roles in cell signaling; however, these microdomains defy detection using conventional imaging. To visualize domains and relate their nanostructure and dynamics to mast cell signaling, we use two-photon (760 nm and 960 nm) fluorescence lifetime imaging microscopy and fluorescence polarization anisotropy imaging, with comparative one-photon anisotropy imaging and single-point lifetime and anisotropy decay measurements. The inherent sensitivity of ultrafast excited-state dynamics and rotational diffusion to the immediate surroundings of a fluorophore allows for real-time monitoring of membrane structure and organization. When the high affinity receptor for IgE (FcepsilonRI) is extensively cross-linked with anti-IgE, molecules associated with cholesterol-rich microdomains (e.g., saturated lipids (the lipid analog diI-C(18) or glycosphingolipids)) and lipid-anchored proteins coredistribute with cross-linked IgE-FcepsilonRI. We find an enhancement in fluorescence lifetime and anisotropy of diI-C(18) and Alexa 488-labeled IgE-FcepsilonRI in the domains where these molecules colocalize. Our results suggest that fluorescence lifetime and, particularly, anisotropy permit us to correlate the recruitment of lipid molecules into more ordered domains that serve as platforms for IgE-mediated signaling.

Selective membrane exclusion in phagocytic and macropinocytic cups

Specialized eukaryotic cells can ingest large particles and sequester them within membrane-delimited phagosomes. Many studies have described the delivery of lysosomal proteins to the phagosome, but little is known about membrane sorting during the early stages of phagosome formation. Here we used Dictyostelium discoideum amoebae to analyze the membrane composition of newly formed phagosomes. The membrane delimiting the closing phagocytic cup was essentially derived from the plasma membrane, but a subgroup of proteins was specifically excluded. Interestingly the same phenomenon was observed during the formation of macropinosomes, suggesting that the same sorting mechanisms are at play during phagocytosis and macropinocytosis. Analysis of mutant strains revealed that clathrin-associated adaptor complexes AP-1, -2 and -3 were not necessary for this selective exclusion and, accordingly, ultrastructural analysis revealed no evidence for vesicular transport around phagocytic cups. Our results suggest the existence of a new, as yet uncharacterized, sorting mechanism in phagocytic and macropinocytic cups.

Dynamic and reversible restructuring of the ER induced by PDMP in cultured cells

In many cells, the endoplasmic reticulum (ER) contains segregated smooth and rough domains, but the mechanism of this segregation is unclear. Here, we used a HeLa cell line, inducibly expressing a GFP fusion protein [GFP-b(5)tail] anchored to the ER membrane, as a tool to investigate factors influencing ER organisation. Induction of GFP-b(5)tail expression caused proliferation of the ER, but its normal branching polygonal meshwork architecture was maintained. Experiments designed to test the effects of drugs that alter ceramide levels revealed that treatment of these cells with Phenyl-2-decanoyl-amino-3-morpholino-1-propanol-hydrocholride (PDMP) generated patches of segregated smooth ER, organised as a random tubular network, which rapidly dispersed after removal of the drug. The effect of PDMP was independent of its activity as sphingolipid synthesis inhibitor, but could be partially reversed by a membrane-permeant Ca(2+) chelator. Although the smooth ER patches maintained connectivity with the remaining ER, they appeared to represent distinct domains differing in protein and lipid composition from the remaining ER. PDMP did not cause detachment of membrane-bound ribosomes, indicating that smooth ER patch generation was due to a reorganisation of pre-existing ribosome-free areas. Our results demonstrate a dynamic relationship between smooth and rough ER and have implications for the mechanisms regulating ER architecture.

Endoplasmic reticulum architecture: structures in flux

The endoplasmic reticulum (ER) is a dynamic pleiomorphic organelle containing continuous but distinct subdomains. The diversity of ER structures parallels its many functions, including secretory protein biogenesis, lipid synthesis, drug metabolism and Ca2+ signaling. Recent studies are revealing how elaborate ER structures arise in response to subtle changes in protein levels, dynamics, and interactions as well as in response to alterations in cytosolic ion concentrations. Subdomain formation appears to be governed by principles of self-organization. Once formed, ER subdomains remain malleable and can be rapidly transformed into alternative structures in response to altered conditions. The mechanisms that modulate ER structure are likely to be important for the generation of the characteristic shapes of other organelles.

Coexisting domains in the plasma membranes of live cells characterized by spin-label ESR spectroscopy

The importance of membrane-based compartmentalization in eukaryotic cell function has become broadly appreciated, and a number of studies indicate that these eukaryotic cell membranes contain coexisting liquid-ordered (L(o)) and liquid-disordered (L(d)) lipid domains. However, the current evidence for such phase separation is indirect, and so far there has been no direct demonstration of differences in the ordering and dynamics for the lipids in these two types of regions or their relative amounts in the plasma membranes of live cells. In this study, we provide direct evidence for the presence of two different types of lipid populations in the plasma membranes of live cells from four different cell lines by electron spin resonance. Analysis of the electron spin resonance spectra recorded over a range of temperatures, from 5 to 37 degrees C, shows that the spin-labeled phospholipids incorporated experience two types of environments, L(o) and L(d), with distinct order parameters and rotational diffusion coefficients but with some differences among the four cell lines. These results suggest that coexistence of lipid domains that differ significantly in their dynamic order in the plasma membrane is a general phenomenon. The L(o) region is found to be a major component in contrast to a model in which small liquid-ordered lipid rafts exist in a 'sea' of disordered lipids. The results on ordering and dynamics for the live cells are also compared with those from model membranes exhibiting coexisting L(o) and L(d) phases.

Lipid segregation and IgE receptor signaling: A decade of progress

Recent work to characterize the roles of lipid segregation in IgE receptor signaling has revealed a mechanism by which segregation of liquid ordered regions from disordered regions of the plasma membrane results in protection of the Src family kinase Lyn from inactivating dephosphorylation by a transmembrane tyrosine phosphatase. Antigen-mediated crosslinking of IgE receptors drives their association with the liquid ordered regions, commonly called lipid rafts, and this facilitates receptor phosphorylation by active Lyn in the raft environment. Previous work showed that the membrane skeleton coupled to F-actin regulates stimulated receptor phosphorylation and downstream signaling processes, and more recent work implicates cytoskeletal interactions with ordered lipid rafts in this regulation. These and other results provide an emerging view of the complex role of membrane structure in orchestrating signal transduction mediated by immune and other cell surface receptors.

Molecular partitioning during host cell penetration by Toxoplasma gondii

During invasion by Toxoplasma gondii, host cell transmembrane proteins are excluded from the forming parasitophorous vacuole membrane (PVM) by the tight apposition of host and parasite cellular membranes. Previous studies suggested that the basis for the selective partitioning of membrane constituents may be a preference for membrane microdomains, and this hypothesis was herein tested. The partitioning of a diverse group of molecular reporters for raft and nonraft membrane subdomains was monitored during parasite invasion by time-lapse video or confocal microscopy. Unexpectedly, both raft and nonraft lipid probes, as well as both raft and nonraft cytosolic leaflet proteins, flowed unhindered past the host-parasite junction into the PVM. Moreover, neither a raft-associated type 1 transmembrane protein nor its raft-dissociated counterpart accessed the PVM, while a multispanning membrane raft protein readily did so. Considered together with previous data, these studies demonstrate that selective partitioning at the host-parasite interface is a highly complex process, in which raft association favors, but is neither necessary nor sufficient for, inclusion into the T. gondii PVM.

Membrane domains

Considerable evidence shows that lateral inhomogeneities in lipid composition and physical properties exist in biological membranes. These membrane lipid domains are proposed to play important roles in processes such as signal transduction and membrane traffic. However, there is not at present an adequate description of the nature of these lipid domains in terms of their size, abundance, composition, or dynamics. We discuss the current analyses of the properties and function of membrane domains in cells and compare their properties with chemically simpler model membrane systems that can be understood in greater detail.

Visualization of plasma membrane compartmentalization with patterned lipid bilayers

Micrometer-size patterned lipid bilayers containing liganded lipids are used to control the location and size of receptor clusters and enable direct visualization of structural reorganization of cellular components. Subsequent to concentration of Fcepsilon receptor I, the mast cell receptor for IgE, and colocalized tyrosine phosphorylation activity, Lyn kinase and other proteins anchored to the inner leaflet of the plasma membrane redistribute selectively with the receptor clusters in a process that depends on actin polymerization. Surprisingly, outer leaflet components characteristically associated with lipid rafts do not detectably coredistribute with these inner leaflet components. Cell activation using patterned surfaces provides unique insights into cell membrane structural organization, revealing dynamic, large-scale uncoupling of inner and outer leaflet components of lipid rafts.

Quantitative analysis of the fluorescence properties of intrinsically fluorescent proteins in living cells

The main potential of intrinsically fluorescent proteins (IFPs), as noninvasive and site-specific markers, lies in biological applications such as intracellular visualization and molecular genetics. However, photophysical studies of IFPs have been carried out mainly in aqueous solution. Here, we provide a comprehensive analysis of the intracellular environmental effects on the steady-state spectroscopy and excited-state dynamics of green (EGFP) and red (DsRed) fluorescent proteins, using both one- and two-photon excitation. EGFP and DsRed are expressed either in the cytoplasm of rat basophilic leukemia (RBL-2H3) mucosal mast cells or anchored (via LynB protein) to the inner leaflet of the plasma membrane. The fluorescence lifetimes (within approximately 10%) and spectra in live cells are basically the same as in aqueous solution, which indicate the absence of both IFP aggregation and cellular environmental effects on the protein folding under our experimental conditions. However, comparative time-resolved anisotropy measurements of EGFP reveal a cytoplasmic viscosity 2.5 +/- 0.3 times larger than that of aqueous solution at room temperature, and also provide some insights into the LynB-EGFP structure and the heterogeneity of the cytoplasmic viscosity. Further, the oligomer configuration and internal depolarization of DsRed, previously observed in solution, persists upon expression in these cells. DsRed also undergoes an instantaneous three-photon induced color change under 740-nm excitation, with efficiently nonradiative green species. These results confirm the implicit assumption that in vitro fluorescence properties of IFPs are essentially valid for in vivo applications, presumably due to the beta-barrel protection of the embodied chromophore. We also discuss the relevance of LynB-EGFP anisotropy for specialized domains studies in plasma membranes.

Involvement of the AP-1 adaptor complex in early steps of phagocytosis and macropinocytosis

The best described function of the adaptor complex-1 (AP-1) is to participate in the budding of clathrin-coated vesicles from the trans-Golgi network and endosomes. Here, we show that AP-1 is also localized to phagocytic cups in murine macrophages as well as in Dictyostelium amoebae. AP-1 is recruited to phagosomal membranes at this early stage of phagosome formation and rapidly dissociates from maturing phagosomes. To establish the role of AP-1 in phagocytosis, we made used of Dictyostelium mutant cells (apm1(-) cells) disrupted for AP-1 medium chain. In this mutant, phagocytosis drops by 60%, indicating that AP-1 is necessary for efficient phagocytosis. Furthermore, phagocytosis in apm1(-) cells is more affected for large rather than small particles, and cells exhibiting incomplete engulfment are then often observed. This suggests that AP-1 could participate in the extension of the phagocytic cup. Interestingly, macropinocytosis, a process dedicated to fluid-phase endocytosis and related to phagocytosis, is also impaired in apm1(-) cells. In summary, our data suggest a new role of AP-1 at an early stage of phagosome and macropinosome formation.

Antigen-stimulated trafficking from the recycling compartment to the plasma membrane in RBL mast cells

Binding of fluorescein isothiocyanate (FITC)-conjugated cholera toxin B subunit to ganglioside GM1 on RBL-2H3 cells at 37 degrees C results in labeling of the plasma membrane as well as a pool of perinuclear intracellular membranes identified as the endosomal recycling compartment. Antigen-mediated activation of IgE receptor signaling causes rapid, sustained outward trafficking of these labeled endosomes, that is monitored as an increase in FITC fluorescence due to relief of quenching in the acidic endosomes upon delivery to the plasma membrane. Stimulation of this process depends on the integrity of cholesterol-dependent lipid rafts and occurs in response to Ca2+-mobilizing thapsigargin as well as antigen. Inhibitors of some early signaling enzymes stimulated by FcepsilonRI, including Syk tyrosine kinase and phosphoinositide 3-kinase, have little or no effect on this trafficking response. Other signaling pathways, including activation of phospholipase C and Ca2+ influx, do not appear to be necessary for the initiation of the outward trafficking response, but they contribute to maintaining the sustained phase of this process. Consistent with this, antigen-stimulated ruffles are labeled with FITC-cholera toxin B in a Ca2+-dependent manner. Thus, this trafficking response provides a mechanism by which an internal membrane pool can contribute to plasma membrane remodeling during stimulated membrane ruffling, cell motility, and phagocytosis.

Lipid rafts orchestrate signaling by the platelet receptor glycoprotein VI

The platelet collagen receptor glycoprotein VI (GPVI) couples to the immune receptor adaptor Fc receptor gamma-chain (FcRgamma) and signals using many of the same intracellular signaling molecules as immune receptors. Studies of immune receptor signaling have revealed a critical role for specialized areas of the cell membrane known as lipid rafts, which are enriched in essential signaling molecules. However, the role of lipid rafts in signaling in nonimmune cells such as platelets remains poorly defined. This study shows that GPVI-FcRgamma does not constitutively associate with rafts, but is recruited to lipid rafts following receptor stimulation in both GPVI-expressing RBL-2H3 cells and human platelets. FcRgamma is required for GPVI association with lipid rafts, as mutant GPVI receptors that do not couple to FcRgamma were unable to associate with lipid rafts after receptor clustering. Following GPVI stimulation in platelets, virtually all phosphorylated FcRgamma was found in lipid rafts, but inhibition of FcRgamma phosphorylation did not block receptor association with lipid rafts. This work demonstrates that lipid rafts orchestrate GPVI receptor signaling in platelets in a manner analogous to immune cell receptors and supports a model of GPVI signaling in which FcRgamma phosphorylation is controlled by ligand-dependent association with lipid rafts.

Membrane sorting in the endocytic and phagocytic pathway of Dictyostelium discoideum

To study sorting in the endocytic pathway of a phagocytic and macropinocytic cell, monoclonal antibodies to membrane proteins of Dictyostelium discoideum were generated. Whereas the p25 protein was localized to the cell surface, p80 was mostly present in intracellular endocytic compartments as observed by immunofluorescence as well as immunoelectron microscopy analysis. The p80 gene was identified and encodes a membrane protein presumably involved in copper transport. Expression of chimeric proteins revealed that the cytoplasmic domain of p80 was sufficient to cause constitutive endocytosis and localization of the protein to endocytic compartments. Dileucine- and tyrosine-based endocytic signals described previously in mammalian systems were also capable of targeting chimera to endocytic compartments. In phagocytosing cells no membrane sorting was observed during formation of the phagosome. Both p25 and p80 were incorporated non-selectively in nascent phagosomes, and then retrieved shortly after phagosome closure. Our results emphasize the fact that very active membrane traffic takes place in phagocytic and macropinocytic cells. This is coupled with precise membrane sorting to maintain the specific composition of endocytic compartments.

Cholesterol depletion induces large scale domain segregation in living cell membranes

Local inhomogeneities in lipid composition play a crucial role in regulation of signal transduction and membrane traffic. Nevertheless, most evidence for microdomains in cells remains indirect, and the nature of membrane inhomogeneities has been difficult to characterize. We used lipid analogs and lipid-anchored proteins with varying fluidity preferences to examine the effect of modulating cellular cholesterol on domain formation. We show that lowering cholesterol levels induces formation of visible micrometer-scale domains in the plasma membrane of several mammalian cell types with complementary distributions of fluorescent lipid analogs with preferences for fluid or ordered domains. A uniform distribution is restored by cholesterol repletion. Unexpectedly, cholesterol depletion does not visibly alter the distribution of a crosslinked or uncrosslinked glycosylphosphatidylinositol-anchored protein (the folate receptor). We also examined the effect of varying cholesterol content on the cold Triton X-100 solubility of several membrane constituents. Although a cholesterol analog, dehydroergosterol, and a glycosylphosphatidylinositol-anchored protein are largely retained after extraction, a lipid analog with saturated 16-carbon acyl chains is largely removed when the cellular cholesterol level is lowered. This result indicates that after cholesterol depletion molecules in the more ordered domains can be extracted differentially by cold nonionic detergents.

Cytoskeleton-dependent membrane domain segregation during neutrophil polarization

On treatment with chemoattractant, the neutrophil plasma membrane becomes organized into detergent-resistant membrane domains (DRMs), the distribution of which is intimately correlated with cell polarization. Plasma membrane at the front of polarized cells is susceptible to extraction by cold Triton X-100, whereas membrane at the rear is resistant to extraction. After cold Triton X-100 extraction, DRM components, including the transmembrane proteins CD44 and CD43, the GPI-linked CD16, and the lipid analog, DiIC(16), are retained within uropods and cell bodies. Furthermore, CD44 and CD43 interact concomitantly with DRMs and with the F-actin cytoskeleton, suggesting a mechanism for the formation and stabilization of DRMs. By tracking the distribution of DRMs during polarization, we demonstrate that DRMs progress from a uniform distribution in unstimulated cells to small, discrete patches immediately after activation. Within 1 min, DRMs form a large cap comprising the cell body and uropod. This process is dependent on myosin in that an inhibitor of myosin light chain kinase can arrest DRM reorganization and cell polarization. Colabeling DRMs and F-actin revealed a correlation between DRM distribution and F-actin remodeling, suggesting that plasma membrane organization may orient signaling events that control cytoskeletal rearrangements and, consequently, cell polarity.

Phospholipases stimulate secretion in RBL mast cells

Roles for glycerophospholipids in exocytosis have been proposed, but remain controversial. Phospholipases are stimulated following the activation of the high-affinity receptor for immunoglobulin E (IgE) in mast cells. To study the biochemical sequelae that lead to degranulation, broken cell systems were employed. We demonstrate that the addition of three distinct types of exogenous phospholipases (i.e., bcPLC, scPLD, and tfPLA(2)), all of which hydrolyze phosphatidylcholine (PC), trigger degranulation in permeabilized RBL-2H3 cells, a mucosal mast cell line. Production of bioactive lipids by these phospholipases promotes release of granule contents through the plasma membrane and acts downstream of PKC, PIP(2), and Rho subfamily GTPases in regulated secretion. These exogenous phospholipase-induced degranulation pathways circumvent specific factors activated following stimulation of the IgE receptor as well as in ATP- and GTP-dependent intracellular pathways. Taken together, these results suggest that regulated secretion may be achieved in vitro in the absence of cytosolic factors via phospholipase activation and that products of PC hydrolysis can promote exocytosis in mast cells.

Cross-correlation analysis of inner-leaflet-anchored green fluorescent protein co-redistributed with IgE receptors and outer leaflet lipid raft components

To investigate the structural basis for membrane interactions that occur between Lyn tyrosine kinase and IgE-Fc(epsilon)RI or other components of lipid rafts, we prepared a green fluorescent protein analog of Lyn (PM-EGFP) and used cross-correlation analysis to quantify co-redistributions of aggregates that occur after IgE-Fc(epsilon)RI is cross-linked on the cell surface. PM-EGFP, which contains minimally the palmitoylation and myristoylation sites on Lyn, was compared with another inner leaflet probe, EGFP-GG, which contains a prenylation site and a polybasic sequence similar to K-ras. Confocal fluorescence microscopy was used to examine co-redistributions of these inner leaflet components with IgE-Fc(epsilon)RI and outer leaflet raft components, ganglioside GD1b and glycosylphosphotidylinositol-linked Thy-1, under conditions where the latter were cross-linked externally to form large patches at the cell surface. The cross-correlation analysis was developed and characterized with simulations representing cell surface distributions, and parameters from the cross-correlation curves, rho(o) (peak height) and A (peak area), were shown to be reliable measures of the extent of co-redistributed aggregates and their size. Cross-correlation analysis was then applied to quantify co-redistributions of the fluorescently labeled inner and outer leaflet components on RBL-2H3 cells. As visually observed and parameterized in this manner, PM-EGFP was found to co-redistribute with lipid rafts significantly more than EGFP-GG or an endogenous prenylated protein, Cdc42. These quantitative results are consistent with previous analyses of Lyn co-redistributions and support the hypothesis that the functionally important interaction of Lyn with cross-linked IgE- Fc(epsilon)RI is due to their mutual co-association with lipid rafts.

Mutant RBL mast cells defective in Fc epsilon RI signaling and lipid raft biosynthesis are reconstituted by activated Rho-family GTPases

Characterization of defects in a variant subline of RBL mast cells has revealed a biochemical event proximal to IgE receptor (Fc epsilon RI)-stimulated tyrosine phosphorylation that is required for multiple functional responses. This cell line, designated B6A4C1, is deficient in both Fc epsilon RI-mediated degranulation and biosynthesis of several lipid raft components. Agents that bypass receptor-mediated Ca(2+) influx stimulate strong degranulation responses in these variant cells. Cross-linking of IgE-Fc epsilon RI on these cells stimulates robust tyrosine phosphorylation but fails to mobilize a sustained Ca(2+) response. Fc epsilon RI-mediated inositol phosphate production is not detectable in these cells, and failure of adenosine receptors to mobilize Ca(2+) suggests a general deficiency in stimulated phospholipase C activity. Antigen stimulation of phospholipases A(2) and D is also defective. Infection of B6A4C1 cells with vaccinia virus constructs expressing constitutively active Rho family members Cdc42 and Rac restores antigen-stimulated degranulation, and active Cdc42 (but not active Rac) restores ganglioside and GPI expression. The results support the hypothesis that activation of Cdc42 and/or Rac is critical for Fc epsilon RI-mediated signaling that leads to Ca(2+) mobilization and degranulation. Furthermore, they suggest that Cdc42 plays an important role in the biosynthesis and expression of certain components of lipid rafts.

Interactions between Fc(epsilon)RI and lipid raft components are regulated by the actin cytoskeleton

Previous studies showed that crosslinking of IgE-Fc(epsilon)RI complexes on RBL-2H3 mast cells causes their association with isolated detergent-resistant membranes, also known as lipid rafts, in a cholesterol-dependent process that precedes initiation of signaling by these receptors. To investigate these interactions on intact cells, we examined the co-redistribution of raft components with crosslinked IgE-Fc(epsilon)RI using confocal microscopy. After several hours of crosslinking at 4 degrees C, the glycosylphosphatidylinositol-linked protein Thy-1 and the Src-family tyrosine kinase Lyn co-redistribute with IgE-Fc(epsilon)RI in large patches at the plasma membrane. Under these conditions, F-actin also undergoes dramatic co-segregation with Fc(epsilon)RI and raft components but is dispersed following a brief warm-up to 37 degrees C. When crosslinking of IgE-Fc(epsilon)RI is initiated at higher temperatures, co-redistribution of raft components with patched Fc(epsilon)RI is not readily detected unless stimulated F-actin polymerization is inhibited by cytochalasin D. In parallel, cytochalasin D converts transient antigen-stimulated tyrosine phosphorylation to a more sustained response. Sucrose gradient analysis of lysed cells reveals that crosslinked IgE-Fc(epsilon)RI remains associated with lipid rafts throughout the time course of the transient phosphorylation response but undergoes a time-dependent shift to higher density that is prevented by cytochalasin D. Our results indicate that interactions between Lyn and crosslinked IgE-Fc(epsilon)RI are regulated by stimulated F-actin polymerization, and this is best explained by a segregation of anchored raft components from more mobile ones.

Role of membrane organization and membrane domains in endocytic lipid trafficking

Lipid compositions vary greatly among organelles, and specific sorting mechanisms are required to establish and maintain these distinct compositions. In this review, we discuss how the biophysical properties of the membrane bilayer and the chemistry of individual lipid molecules play a role in the intracellular trafficking of the lipids themselves, as well as influencing the trafficking of transmembrane proteins. The large diversity of lipid head groups and acyl chains lead to a variety of weak interactions, such as ionic and hydrogen bonding at the lipid/water interfacial region, hydrophobic interactions, and van-der-Waals interactions based on packing density. In simple model bilayers, these weak interactions can lead to large-scale phase separations, but in more complex mixtures, which mimic cell membranes, such phase separations are not observed. Nevertheless, there is growing evidence that domains (i.e., localized regions with non-random lipid compositions) exist in biological membranes, and it is likely that the formation of these domains are based on interactions similar to those that lead to phase separations in model systems. Sorting of lipids appears to be based in part on the inclusion or exclusion of certain types of lipids in vesicles or tubules as they bud from membrane organelles.

Induction of caveolae in the apical plasma membrane of Madin-Darby canine kidney cells

In this paper, we have analyzed the behavior of antibody cross-linked raft-associated proteins on the surface of MDCK cells. We observed that cross-linking of membrane proteins gave different results depending on whether cross-linking occurred on the apical or basolateral plasma membrane. Whereas antibody cross-linking induced the formation of large clusters on the basolateral membrane, resembling those observed on the surface of fibroblasts (Harder, T., P. Scheiffele, P. Verkade, and K. Simons. 1998. J. Cell Biol. 929-942), only small ( approximately 100 nm) clusters formed on the apical plasma membrane. Cross-linked apical raft proteins e.g., GPI-anchored placental alkaline phosphatase (PLAP), influenza hemagglutinin, and gp114 coclustered and were internalized slowly ( approximately 10% after 60 min). Endocytosis occurred through surface invaginations that corresponded in size to caveolae and were labeled with caveolin-1 antibodies. Upon cholesterol depletion the internalization of PLAP was completely inhibited. In contrast, when a non-raft protein, the mutant LDL receptor LDLR-CT22, was cross-linked, it was excluded from the clusters of raft proteins and was rapidly internalized via clathrin-coated pits. Since caveolae are normally present on the basolateral membrane but lacking from the apical side, our data demonstrate that antibody cross-linking induced the formation of caveolae, which slowly internalized cross-linked clusters of raft-associated proteins.

Cdc42 and Rac stimulate exocytosis of secretory granules by activating the IP(3)/calcium pathway in RBL-2H3 mast cells

We have expressed dominant-active and dominant-negative forms of the Rho GTPases, Cdc42 and Rac, using vaccinia virus to evaluate the effects of these mutants on the signaling pathway leading to the degranulation of secretory granules in RBL-2H3 cells. Dominant-active Cdc42 and Rac enhance antigen-stimulated secretion by about twofold, whereas the dominant-negative mutants significantly inhibit secretion. Interestingly, treatment with the calcium ionophore, A23187, and the PKC activator, PMA, rescues the inhibited levels of secretion in cells expressing the dominant-negative mutants, implying that Cdc42 and Rac act upstream of the calcium influx pathway. Furthermore, cells expressing the dominant-active mutants exhibit elevated levels of antigen-stimulated IP(3) production, an amplified antigen-stimulated calcium response consisting of both calcium release from internal stores and influx from the extracellular medium, and an increase in aggregate formation of the IP(3) receptor. In contrast, cells expressing the dominant-negative mutants display the opposite phenotypes. Finally, we are able to detect an in vitro interaction between Cdc42 and PLCgamma1, the enzyme immediately upstream of IP(3) formation. Taken together, these findings implicate Cdc42 and Rac in regulating the exocytosis of secretory granules by stimulation of IP(3) formation and calcium mobilization upon antigen stimulation.

How does the plasma membrane participate in cellular signaling by receptors for immunoglobulin E?

Accumulating evidence strongly supports the view that the plasma membrane participates in transmembrane signaling by IgE-receptors (IgE-Fc epsilon RI) through the formation of lipid-based domains, also known as rafts. Ongoing biochemical and biophysical experiments investigate the composition, structure, and dynamics of the corresponding membrane components and how these are related to functional coupling between Fc epsilon RI and Lyn tyrosine kinase to initiate signaling in mast cells.

Electron spin resonance characterization of liquid ordered phase of detergent-resistant membranes from RBL-2H3 cells

The dynamic structure of detergent-resistant membranes (DRMs) isolated from RBL-2H3 cells was characterized using two different acyl chain spin-labeled phospholipids (5PC and 16PC), a headgroup labeled sphingomyelin (SM) analog (SD-Tempo) and a spin-labeled cholestane (CSL). It was shown, by comparison to dispersions of SM, dipalmitoylphosphatidylcholine (DPPC), and DPPC/cholesterol of molar ratio 1, that DRM contains a substantial amount of liquid ordered phase: 1) The rotational diffusion rates (R( perpendicular)) of 16PC in DRM between -5 degrees C and 45 degrees C are nearly the same as those in molar ratio DPPC/Chol = 1 dispersions, and they are substantially greater than R( perpendicular) in pure DPPC dispersions in the gel phase studied above 20 degrees C; 2) The order parameters (S) of 16PC in DRM at temperatures above 4 degrees C are comparable to those in DPPC/Chol = 1 dispersions, but are greater than those in DPPC dispersions in both the gel and liquid crystalline phases. 3) Similarly, R( perpendicular) for 5PC and CSL in DRM is greater than in pure SM dispersions in the gel phase, and S for these labels in DRM is greater than in the SM dispersions in both the gel and liquid crystalline phases. 4) R( perpendicular) of SD-Tempo in DRM is greater than in dispersions of SM in both gel and liquid phases, consistent with the liquid-like mobility in the acyl chain region in DRM. However, S of SD-Tempo in DRM is substantially less than that of this spin label in SM in gel and liquid crystalline phases (in absolute values), indicating that the headgroup region in DRMs is less ordered than in pure SM. These results support the hypothesis that plasma membranes contain DRM domains with a liquid ordered phase that may coexist with a liquid crystalline phase. There also appears to be a coexisting region in DRMs in which the chain labels 16PC and 5PC are found to cluster. We suggest that other biological membranes containing high concentrations of cholesterol also contain a liquid ordered phase.