Janet M. Oliver

Actin restricts Fc|[epsiv]|RI diffusion and facilitates antigen-induced receptor immobilization

The actin cytoskeleton has been implicated in restricting diffusion of plasma membrane components. Here, simultaneous observations of quantum dot-labelled FcɛRI motion and GFP-tagged actin dynamics provide direct evidence that actin filament bundles define micron-sized domains that confine mobile receptors. Dynamic reorganization of actin structures occurs over seconds, making the location and dimensions of actin-defined domains time-dependent. Multiple FcɛRI often maintain extended close proximity without detectable correlated motion, suggesting that they are co-confined within membrane domains. FcɛRI signalling is activated by crosslinking with multivalent antigen. We show that receptors become immobilized within seconds of crosslinking. Disruption of the actin cytoskeleton results in delayed immobilization kinetics and increased diffusion of crosslinked clusters. These results implicate actin in membrane partitioning that not only restricts diffusion of membrane proteins, but also dynamically influences their long-range mobility, sequestration and response to ligand binding.

High resolution mapping of mast cell membranes reveals primary and secondary domains of Fc(epsilon)RI and LAT.

In mast cells, cross-linking the high-affinity IgE receptor (FcεRI) initiates the Lyn-mediated phosphorylation of receptor ITAMs, forming phospho-ITAM binding sites for Syk. Previous immunogold labeling of membrane sheets showed that resting FcεRI colocalize loosely with Lyn, whereas cross-linked FcεRI redistribute into specialized domains (osmiophilic patches) that exclude Lyn, accumulate Syk, and are often bordered by coated pits. Here, the distribution of FcεRI β is mapped relative to linker for activation of T cells (LAT), Grb2-binding protein 2 (Gab2), two PLCγ isoforms, and the p85 subunit of phosphatidylinositol 3-kinase (PI3-kinase), all implicated in the remodeling of membrane inositol phospholipids. Before activation, PLCγ1 and Gab2 are not strongly membrane associated, LAT occurs in small membrane clusters separate from receptor, and PLCγ2, that coprecipitates with LAT, occurs in clusters and along cytoskeletal cables. After activation, PLCγ2, Gab2, and a portion of p85 colocalize with FcεRI β in osmiophilic patches. LAT clusters enlarge within 30 s of receptor activation, forming elongated complexes that can intersect osmiophilic patches without mixing. PLCγ1 and another portion of p85 associate preferentially with activated LAT. Supporting multiple distributions of PI3-kinase, FcεRI cross-linking increases PI3-kinase activity in anti-LAT, anti-FcεRIβ, and anti-Gab2 immune complexes. We propose that activated mast cells propagate signals from primary domains organized around FcεRIβ and from secondary domains, including one organized around LAT.

Dysregulated FcεRI Signaling and Altered Fyn and SHIP Activities in Lyn-Deficient Mast Cells

Studies in B cells from Lyn-deficient mice have identified Lyn as both a kinetic accelerator and negative regulator of signaling through the BCR. The signaling properties of bone marrow-derived mast cells from Lyn−/− mice (Lyn−/− BMMCs) have also been explored, but their signaling phenotype remains controversial. We confirm that Lyn−/− BMMCs release more β-hexosaminidase than wild-type BMMCs following FcεRI cross-linking and show that multiple mast cell responses to FcεRI cross-linking (the phosphorylation of receptor subunits and other proteins, the activation of phospholipase Cγ isoforms, the mobilization of Ca2+, the synthesis of phosphatidylinositol 3,4,5-trisphosphate, the activation of the α4β1 integrin, VLA-4) are slow to initiate in Lyn−/− BMMCs, but persist far longer than in wild-type cells. Mechanistic studies revealed increased basal as well as stimulated phosphorylation of the Src kinase, Fyn, in Lyn−/− BMMCs. Conversely, there was very little basal or stimulated tyrosine phosphorylation or activity of the inositol phosphatase, SHIP, in Lyn−/− BMMCs. We speculate that Fyn may substitute (inefficiently) for Lyn in signal initiation in Lyn−/− BMMCs. The loss of SHIP phosphorylation and activity very likely contributes to the increased levels of phosphatidylinositol 3,4,5-trisphosphate and the excess FcεRI signaling in Lyn−/− BMMCs. The unexpected absence of the transient receptor potential channel, Trpc4, from Lyn−/− BMMCs may additionally contribute to their altered signaling properties.

Multiple Defects in FcεRI Signaling in Syk-Deficient Nonreleaser Basophils and IL-3-Induced Recovery of Syk Expression and Secretion

Human basophils respond to Ag-induced cross-linking of their high affinity IgE receptor, FcεRI, by releasing histamine and other mediators from granules, producing IL-4 and other cytokines and, as shown in this study, by forming membrane ruffles and showing increased very late Ag-4 (VLA-4)-mediated adhesion to VCAM-1-expressing target cells. We have identified five blood donors whose basophils lack detectable levels of the FcεRI-associated protein tyrosine kinase, Syk. Despite showing no obvious ultrastructural differences from normal basophils, nonreleaser basophils fail to form membrane ruffles, to show increased VLA-4-mediated adhesive activity, or to produce IL-4 in response to FcεRI cross-linking. Although Syk protein levels are suppressed in basophils from all five donors, Syk mRNA is consistently present. Furthermore, culturing nonreleaser basophils for 4 days with IL-3 restores Syk protein expression and FcεRI-mediated histamine release. Understanding the reversible suppression of Syk protein expression in nonreleaser basophils, and learning to replicate this property in patients with allergic inflammation could be a powerful and specific way to limit symptomatic disease.

Mapping ErbB receptors on breast cancer cell membranes during signal transduction.

Distributions of ErbB receptors on membranes of SKBR3 breast cancer cells were mapped by immunoelectron microscopy. The most abundant receptor, ErbB2, is phosphorylated, clustered and active. Kinase inhibitors ablate ErbB2 phosphorylation without dispersing clusters. Modest co-clustering of ErbB2 and EGFR, even after EGF treatment, suggests that both are predominantly involved in homointeractions. Heregulin leads to dramatic clusters of ErbB3 that contain some ErbB2 and EGFR and abundant PI 3-kinase. Other docking proteins, such as Shc and STAT5, respond differently to receptor activation. Levels of Shc at the membrane increase two- to five-fold with EGF, whereas pre-associated STAT5 becomes strongly phosphorylated. These data suggest that the distinct topography of receptors and their docking partners modulates signaling activities.

Small, Mobile FcɛRI Receptor Aggregates Are Signaling Competent

Crosslinking of IgE-bound FcepsilonRI triggers mast cell degranulation. Previous fluorescence recovery after photobleaching (FRAP) and phosphorescent anisotropy studies suggested that FcepsilonRI must immobilize to signal. Here, single quantum dot (QD) tracking and hyperspectral microscopy methods were used for defining the relationship between receptor mobility and signaling. QD-IgE-FcepsilonRI aggregates of at least three receptors remained highly mobile over extended times at low concentrations of antigen that induced Syk kinase activation and near-maximal secretion. Multivalent antigen, presented as DNP-QD, also remained mobile at low doses that supported secretion. FcepsilonRI immobilization was marked at intermediate and high antigen concentrations, correlating with increases in cluster size and rates of receptor internalization. The kinase inhibitor PP2 blocked secretion without affecting immobilization or internalization. We propose that immobility is a feature of highly crosslinked immunoreceptor aggregates and a trigger for receptor internalization, but is not required for tyrosine kinase activation leading to secretion.

Identification of the FcϵRI-activated tyrosine kinases Lyn, Syk, and Zap-70 in human basophils

Abstract Background: In human blood basophils, cross-linking the high-affinity IgE receptor FcϵRI with multivalent antigen activates a signaling pathway leading to Ca 2+ mobilization, actin polymerization, shape changes, secretion, and cytokine production. Methods and Results: The role of tyrosine kinases in human FcϵRI signaling was explored by using human basophils isolated by Percoll gradient centrifugation followed by negative and/or positive selection with antibody-coated magnetic beads. FcϵRI cross-linking of more than 95% pure basophil preparations activates the protein-tyrosine kinases Lyn and Syk, previously linked to FcϵRI-coupled rodent mast cell activation, as well as Zap-70, previously implicated in T-cell receptor signaling, and causes the tyrosine phosphorylation of multiple proteins. The presence of Lyn, Syk, and Zap-70 in basophils was confirmed by Western blotting in lysates of highly purified basophils and independently by confocal fluorescence microscopy in cells labeled simultaneously with kinase-specific antibodies and with the basophil-specific antibody 2D7. Comparable amounts of Lyn and Syk were found in basophils and B cells, whereas T cells appear to have greater amounts of Zap-70 than basophils. The tyrosine kinase inhibitor piceatannol spares IgE-mediated Lyn activation but inhibits IgE-induced Syk and Zap-70 activation as well as overall protein tyrosine phosphorylation and secretion. Overall protein-tyrosine phosphorylation increases steadily over a range of anti-IgE concentrations that are low to optimal for secretion. However, tyrosine phosphorylation continues to increase at high anti-IgE concentrations that elicit very little secretion (the characteristic high-dose inhibition of secretion). Conclusions: Our data demonstrate the association of anti-IgE–stimulated, protein-tyrosine phosphorylation by a cascade of tyrosine kinases, including Zap-70 as well as Lyn and Syk, with the initiation of FcϵRI-mediated signaling in human basophils. (J Allergy Clin Immunol 1998;102:304-15.)

FcεRI signaling observed from the inside of the mast cell membrane

Abstract Crosslinking the high affinity IgE receptor, FceRI, on basophils and mast cells initiates cascades of biochemical events leading to degranulation, membrane ruffling and other physiological responses. Downstream of FceRI and its coupled tyrosine kinases, Lyn and Syk, scores of different proteins and lipids are implicated in these signaling cascades and new players are being identified continuously. Here, we use immunogold probes to label receptors and signaling proteins on the cytoplasmic face of membrane sheets prepared from RBL-2H3 mast cells and transmission electron microscopy to examine their distributions in relationship to each other and to features of the membrane. New topographical data are integrated with existing knowledge of the biochemistry of FceRI signaling and of cell shape during signaling to implicate at least two distinct membrane domains in FceRI signaling. “Primary signaling domains”, also called osmiophilic patches, are recognized by their dark staining with osmium, adjacency to coated pits (previously mapped to planar membrane between lamellae) and by the characteristic presence of receptor, Syk and PLCγ2, but not Lyn. “Secondary signaling domains” are characterized by the presence of large elliptical linker for activation of T cells (LAT) rafts and of PLCγ1 (previously mapped to lamellae) but not receptor. The signaling proteins, Vav, Grb2, Cbl and Gab2, and the endocytic proteins, AP2 and clathrin, all map to the primary domains, while the p85 regulatory subunit of phosphatidylinositol 3 (PI 3)-kinase maps to both domains. Recognition that FceRI signaling is controlled not only by which chemical species are available for interaction, but also by where the interactions occur, may provide new opportunities for the modeling of signaling cascades and new targets for the development of drugs to treat allergies and asthma.

Immunologically mediated signaling in basophils and mast cells: finding therapeutic targets for allergic diseases in the human FcεR1 signaling pathway

The high affinity IgE receptor, FceRI, plays key roles in an array of acute and chronic human allergic reactions including asthma, allergic rhinitis, atopic dermatitis, urticaria and anaphylaxis. In humans and rodents, this receptor is found at high levels on basophils and mast cells where its activation by IgE and multivalent antigen produces mediators and cytokines responsible for FceRI-dependent acute inflammation. Mast cells can additionally contribute to sustained inflammatory responses by internalizing antigen bound to IgE-FceRI complexes for processing to peptides and presentation to T cells. In humans, the FceRI is also expressed, at lower density, on monocytes, macrophages and dendritic cells (DC) where its likely functions again include both signaling to mediator and cytokine production and antigen presentation. Our laboratories have focused on defining the earliest steps in the FceRI signaling cascade in basophils and mast cells and on developing new routes to control allergic inflammation based on inhibiting these events. Here, we describe novel strategies to limit antigen-stimulated FceRI signaling by: (1) sequestering the FceRI-associated protein-tyrosine kinase, Lyn, that initiates FceRI signaling; (2) eliminating; or (3) inactivating the protein-tyrosine kinase, Syk, that propagates FceRI signaling; and (4) establishing inhibitory crosstalk between FceRI and a co-expressed receptor, FcγRII, that again limits FceRI-mediated Syk activation. These strategies may form the basis for new therapies for allergic inflammation.

The Src kinase Lyn is a negative regulator of mast cell proliferation

Previous investigators have reported that deletion of the protein tyrosine kinase Lyn alters mast cell (MC) signaling responses but does not affect or reduces the cytokine‐mediated proliferation of mouse bonemarrow‐derived MC (BMMC) precursors and of mature MC. We observed that Lyn‐deficient mice have more peritoneal MC than wild‐type (WT) mice. Studies to explore this unexpected result showed that Lyn−/− BM cells expand faster than WT cells in response to interleukin (IL)‐3 and stem‐cell factor over the 4–5 weeks required to produce a >95% pure population of granular, receptor with high affinity for immunoglobulin E‐positive BMMC. Furthermore, differentiated Lyn−/− BMMC continue to proliferate more rapidly than WT BMMC and undergo less apoptosis in response to cytokine withdrawal. Additionally, Lyn−/− BMMC support greater IL‐3‐mediated phosphorylation of the prosurvival kinase, Akt, and the proliferative kinase, extracellular‐regulated kinase 1/2. These results identify Lyn as a negative regulator of murine MC survival and proliferation.