The influence of actin microfilaments on signaling by the receptor with high-affinity for IgE

Cytoskeletal Transport, Reorganization, and Fusion Regulation in Mast Cell-Stimulus Secretion Coupling

Mast cells are well known for their role in allergies and many chronic inflammatory diseases. They release upon stimulation, e.g., via the IgE receptor, numerous bioactive compounds from cytoplasmic secretory granules. The regulation of granule secretion and its interaction with the cytoskeleton and transport mechanisms has only recently begun to be understood. These studies have provided new insight into the interaction between the secretory machinery and cytoskeletal elements in the regulation of the degranulation process. They suggest a tight coupling of these two systems, implying a series of specific signaling effectors and adaptor molecules. Here we review recent knowledge describing the signaling events regulating cytoskeletal reorganization and secretory granule transport machinery in conjunction with the membrane fusion machinery that occur during mast cell degranulation. The new insight into MC biology offers novel strategies to treat human allergic and inflammatory diseases targeting the late steps that affect harmful release from granular stores leaving regulatory cytokine secretion intact.

Functional nanoscale coupling of Lyn kinase with IgE-FcεRI is restricted by the actin cytoskeleton in early antigen-stimulated signaling

Spatial targeting of signaling components to activated receptors on the plasma membrane is key for initiating signal transduction. The actin cytoskeleton restricts antigen-stimulated colocalization of IgE-FcεRI with membrane-anchored signaling partner Lyn kinase, and this regulation is mediated by organization of plasma membrane lipids.

The allergic response is initiated on the plasma membrane of mast cells by phosphorylation of the receptor for immunoglobulin E (IgE), FcεRI, by Lyn kinase after IgE-FcεRI complexes are cross-linked by multivalent antigen. Signal transduction requires reorganization of receptors and membrane signaling proteins, but this spatial regulation is not well defined. We used fluorescence localization microscopy (FLM) and pair-correlation analysis to measure the codistribution of IgE-FcεRI and Lyn on the plasma membrane of fixed cells with 20- to 25-nm resolution. We directly visualized Lyn recruitment to IgE-FcεRI within 1 min of antigen stimulation. Parallel FLM experiments captured stimulation-induced FcεRI phosphorylation and colocalization of a saturated lipid-anchor probe derived from Lyn’s membrane anchorage. We used cytochalasin and latrunculin to investigate participation of the actin cytoskeleton in regulating functional interactions of FcεRI. Inhibition of actin polymerization by these agents enhanced colocalization of IgE-FcεRI with Lyn and its saturated lipid anchor at early stimulation times, accompanied by augmented phosphorylation within FcεRI clusters. Ising model simulations provide a simplified model consistent with our results. These findings extend previous evidence that IgE-FcεRI signaling is initiated by colocalization with Lyn in ordered lipid regions and that the actin cytoskeleton regulates this functional interaction by influencing the organization of membrane lipids.

Loss of TRPC1-mediated Ca2+ influx contributes to impaired degranulation in Fyn-deficient mouse bone marrow-derived mast cells

MC degranulation requires the influx of calcium from the extracellular environment. Orai1/STIM1 is essential to MC SOCE, as shown in rat peritoneal MCs, the rat MC lines (RBL-2H3), or in Orai1 null embryo liver-derived, cultured MCs. However, minimal information exists about the role of other calcium channels expressed on these cells. Here, we demonstrate that the nonselective TRPC1 participates in FcεRI-mediated calcium entry in mouse BMMCs. We found that Fyn null MCs, which have an impaired degranulation response, expressed reduced levels of TRPC1, had normal depletion of intracellular calcium stores but an impaired calcium influx, and failed to depolymerize cortical F-actin (a key step for granule-plasma membrane fusion). Partial RNAi silencing of TRPC1 expression in WT MCs (to the level of Fyn null MCs) mimicked the Fyn null defect in calcium influx, cortical F-actin depolymerization, and MC degranulation. Ectopic expression of Fyn or TRPC1 in Fyn null MCs restored calcium responses and cortical F-actin depolymerization and increased MC degranulation. Together with our findings that expression of Orai1 is not altered in Fyn null MCs, our findings suggest that TRPC1 participates in calcium influx and other key events required for MC degranulation. This demonstrates that in addition to a role described previously for Orai1 in promoting MC degranulation, nonselective cation channels participate in promoting the exocytotic response.

Insights into function of the immunological synapse from studies with supported planar bilayers

Innate and adaptive immunity is dependent upon reliable cell-cell communication mediated by direct interactions of cell surface receptors with ligands integrated into the surface of apposing cells or bound directly to the surface as in complement deposition or antibody mediated recognition through Fc receptors. Supported lipid bilayers formed on glass surfaces offer a useful model system in which to explore some basic features of molecular interactions in immunological relevant contacts, which include signal integration and effector functions through immunological synapses and kinapses. We have exploited that lateral mobility of molecules in the supported planar bilayers and fluorescence microscopy to develop a system for measurement of two-dimensional affinities and kinetic rates in the contact area, which is of immunological interest. Affinity measurements are based on a modified Scatchard analysis. Measurements of kinetic rates are based on fluorescence photo bleaching after recovery at the level of the entire contact area. This has been coupled to a reaction-diffusion equation that allows calculation of on- and off-rates. We have found that mixtures of ligands in supported planar bilayers can effectively activate T lymphocytes and simultaneously allow monitoring of the immunological synapse. Recent studies in planar bilayers have provided additional insights into organization principles of cell-cell interfaces. Perennial problems in understanding cell-cell communication are yielding quantitative measurements based on planar bilayers in areas of ligand-driven receptor clustering and the role of the actin cytoskeleton in immune cell activation. A major goal for the field is determining quantitative rules involved in signaling complex formation by innate and adaptive receptor systems.

Supported bilayers at the vanguard of immune cell activation studies

Biological adhesion between cells is critical for development of multicellular organisms and for the function of the adaptive immune system of vertebrates. A gap in understanding of adhesion systems arises from the difficulty of collecting quantitative data on the molecular interactions underlying adhesion, which is typically studied by population statistics such as percent adhesion in the presence of empirically defined forces to separate less adherent cells. Supported lipid bilayers formed on glass surfaces offer a useful model system in which to explore some basic features of molecular interactions in adhesive contacts. We have exploited the lateral mobility of molecules in the supported planar bilayers and fluorescence microscopy to develop a system for measurement of two-dimensional affinities and kinetic rates in contact areas. Affinity measurements are based on a modified Scatchard analysis. Measurements of kinetic rates are based on fluorescence photobleaching after recovery at the level of the entire contact area. This has been coupled to a reaction-diffusion equation that allows calculation of on- and off-rates. We have found that mixtures of ligands in supported planar bilayers can effectively activate T lymphocytes and simultaneously allow monitoring of the immunological synapse. Recent studies in planar bilayers have provided additional insights into organization principles of cell-cell interfaces. Perennial problems in understanding cell-cell communication are yielding to quantitative measurements based on planar bilayers in areas of ligand driven receptor clustering and the role of the actin cytoskeleton in immune cell activation. A major goal for the field is determining quantitative rules involved in signaling complex formation.

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.

Kinetic proofreading model

Kinetic proofreading is an intrinsic property of the cell signaling process. It arises as a consequence of the multiple interactions that occur after a ligand triggers a receptor to initiate a ignaling cascade and it ensures that false signals do not propagate to completion. In order for an active signaling complex to form after a ligand binds to a cell surface receptor, a sequence of binding and phosphorylation events must occur that are rapidly reversed if the ligand dissociates from the receptor. This gives rise to a mechanism by which cells can discriminate among ligands that bind to the same receptor but form ligand-receptor complexes with different lifetimes. We review experiments designed to test for kinetic proofreading and models that exhibit kinetic proofreading.

Kinetic proofreading of ligand-FcepsilonRI interactions may persist beyond LAT phosphorylation

Cells may discriminate among ligands with different dwell times for receptor binding through a mechanism called kinetic proofreading in which the formation of an activated receptor complex requires a progression of events that is aborted if the ligand dissociates before completion. This mechanism explains how, at equivalent levels of receptor occupancy, a rapidly dissociating ligand can be less effective than a more slowly dissociating analog at generating distal cellular responses. Simple mathematical models predict that kinetic proofreading is limited to the initial complex; once the signal passes to second messengers, the dwell time no longer regulates the signal. This suggests that an assay for kinetic proofreading might be used to determine which activation events occur within the initial signaling complex. In signaling through the high affinity IgE receptor FcepsilonRI, the transmembrane adaptor called linker for activation of T cells (LAT) is thought to nucleate a distinct secondary complex. Experiments in which the concentrations of two ligands with different dwell times are adjusted to equalize the level of LAT phosphorylation in rat basophilic leukemia 2H3 cells show that Erk2 phosphorylation, intracellular Ca(2+), and degranulation exhibit kinetic proofreading downstream of LAT phosphorylation. These results suggest that ligand-bound FcepsilonRI and LAT form a complex that is required for effective signal transmission.

Regulation of allergy by Fc receptors

The aggregation of high-affinity IgE receptors (FcepsilonRI) on mast cells and basophils has long been known as the critical event that initiates allergic reactions. Monomeric IgE was recently found to induce a variety of effects when binding to FcepsilonRI. Upregulation of FcepsilonRI only requires binding, whereas other responses require FcepsilonRI aggregation. Interestingly, FcepsilonRI aggregation has recently been understood to generate a mixture of positive and negative intracellular signals. Mast cells and basophils also express low-affinity and, under specific conditions, high-affinity IgG receptors. When co-engaging these receptors with FcepsilonRI, IgG antibodies can amplify or dampen IgE-induced mast cell activation. On the basis of these findings, it has been proposed that FcRs can be used as targets and/or tools for new therapeutic approaches to allergies.