Atomic force microscopy to study the effects of ITIM-bearing FcgammaRIIB on the activation of RBL-2H3 cells

Receptor dynamics regulates actin polymerization state through phosphorylation of cofilin in mast cells

Aggregation of IgE bound to the high-affinity IgE receptor (FcεRI) by a multivalent antigen induces mast cell activation, while disaggregation of aggregated FcεRI by monomer hapten immediately terminates degranulation mediated by dephosphorylation of Syk and mediates a decrease in intracellular Ca²⁺ concentration ([Ca²⁺]_i). The actin polymerization state is intimately involved in mast cell activation mediated by FcεRI aggregation. However, the relation between aggregation-disaggregation of FcεRI and actin rearrangement in mast cells is not well understood. The addition of a multivalent antigen rapidly depolymerized actin filaments, while the subsequent addition of monomer hapten rapidly recovered actin polymerization. Whereas cofilin, an actin-severing protein, was temporally dephosphorylated several minutes after a multivalent antigen stimulation and the addition of monomer hapten rapidly increased cofilin phosphorylation level within 30 s. The removal of extracellular Ca²⁺ instead of monomer hapten addition did not restore cofilin phosphorylation, suggesting that the significant decrease in [Ca²⁺]_i by monovalent hapten was not a critical reason for the actin rearrangement. Additionally, monovalent hapten did not completely reduce [Ca²⁺]_i in mast cells pretreated with jasplakinolide, an inhibitor of actin depolymerization. These results suggest that the multivalent antigen-induced actin depolymerization mediated by cofilin dephosphorylation, and the subsequent addition of monovalent hapten in the F-actin severing state efficiently elicited actin re-polymerization by cofilin phosphorylation.

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.

Atomic force microscopy study of ionomycin-induced degranulation in RBL-2H3 cells

Mast cell degranulation is the typical anaphylaxis process of mast cells associated with the release of cytokines, eicosanoids and their secretory granules, which play very important roles in the allergic inflammatory response of the human body upon anaphylactogen stimulation. The calcium ionophore ionomycin is widely used as a degranulation induction agent for mast cell degranulation studies. In the present work, ionomycin-induced degranulation of RBL-2H3 basophilic leukemia cell line cells was investigated in vitro by high resolution atomic force microscopy (AFM). Ionomycin, which could increase the intracellular free Ca²⁺ level and β-Hexosaminidase release, was found to induce the formation of a kind of peculiar vesicles in the cytoplasm area of RBL-2H3 cells. Those vesicles induced by ionomycin would desintegrate to release a larger amount of granules surrounding RBL-2H3 cells by the controlling of F-actin. These results provide the precise morphological information of ionomycin-induced mast cell degranulation at nanoscale, which could benefit our understanding of ionomycin-induced mast cell anaphylaxis model and also validate the applicability of AFM for the detection of allergic inflammatory response in mast cells. SCANNING 38:525-534, 2016. © 2016 Wiley Periodicals, Inc.

High-resolution three-dimensional imaging of the rich membrane structures of bone marrow-derived mast cells

Atomic force microscopy (AFM) enables high-resolution three-dimensional (3D) imaging of cultured bone marrow-derived mast cells. Cells were immobilized by a quick centrifugation and fixation to preserve their transient cellular morphologies followed by AFM characterization in buffer. This "fix-and-look" approach preserves the structural integrity of individual cells. Well-known membrane morphologies, such as ridges and microvilli, are visualized, consistent with prior electron microscopy observations. Additional information including the 3D measurements of these characteristic features are attained from AFM topographs. Filopodia and lamellopodia, associated with cell spreading, were captured and visualized in three dimensions. New morphologies are also revealed, such as high-density ridges and micro-craters. This investigation demonstrates that the "fix-and-look" approach followed by AFM imaging provides an effective means to characterize the membrane structure of hydrated cells with high resolution. The quantitative imaging and measurements pave the way for systematic correlation of membrane structural features with the biological status of individual cells.

Tetraspanin-Fc receptor interactions

The tetraspanins are a superfamily of membrane glycoproteins which facilitate the interaction of membrane and intracellular signalling molecules (e.g., integrins, pro-growth factors and their receptors, protein kinase-C) in the formation of membrane signalling microdomains (sometimes referred to as the tetraspanin web). A proximal localisation/association of tetraspanins with Fc receptors (FcR) has been implied by the repeated rediscoveries of tetraspanins as the targets of antibodies which activate platelets and other blood cells through co-ligation of FcR. Direct evidence of tetraspanin-FcR interactions has come from immunoprecipitation and co-immunofluorescence studies. The functional effects of this interaction remain unclear, but tetraspanins have been identified as negative regulators of FcR signalling independently of co-ligation, indicating potential roles in modulating FcR function in co-ordination with the activity of other signalling/adhesion molecules in the tetraspanin web. Given their capacity to influence FcR signalling, tetraspanins could provide specific therapeutic targets for immune disorders including rheumatoid arthritis, asthma and allergies.

The effects of ITIM-bearing FcgammaRIIB on the nuclear shuttling of MAP kinase in RBL-2H3 cells

We have studied the effects of ITIM-bearing FcgammaRIIB2 on the FcepsilonRI-dependent nuclear shuttling of mitogen-activated protein (MAP) kinase (ERK2) in rat basophilic leukemia (RBL-2H3) cells. The cross-linking of FcepsilonRI elicited the sustained increase of the intracellular calcium ion concentration ([Ca(2+)](i)) and the translocation of ERK2 from the cytoplasm to the nucleus. The import of ERK2 to the nucleus reached the maximum at 6-7 min, thereafter ERK2 was exported within 30 min. The co-clustering of FcepsilonRI and FcgammaRIIB2 increased the [Ca(2+)](i) and induced the import of ERK2. However, the calcium increase was transient and ERK2 was rapidly exported to the cytoplasm. In addition, the phosphorylation of ERK2 and the production of TNF-alpha were decreased in case of co-clustering of FcepsilonRI and FcgammaRIIB2. This suggested that the co-clustering negatively control the production of pro-inflammatory cytokines through the suppression of nuclear shuttling of ERK2.