Cytoskeleton in mast cell signaling

Anaphylactic degranulation by mast cells requires the mobilization of inflammasome components

The inflammasome components NLRP3 and ASC are cytosolic proteins, which upon sensing endotoxins or danger cues, form multimeric complexes to process interleukin (IL)-1β for secretion. Here we found that antigen (Ag)-triggered degranulation of IgE-sensitized mast cells (MCs) was mediated by NLRP3 and ASC. IgE-Ag stimulated NEK7 and Pyk2 kinases in MCs to induce the deposition of NLRP3 and ASC on granules and form a distinct protein complex (granulosome) that chaperoned the granules to the cell surface. MCs deficient in NLRP3 or ASC did not form granulosomes, degranulated poorly in vitro and did not evoke systemic anaphylaxis in mice. IgE-Ag-triggered anaphylaxis was prevented by an NLRP3 inhibitor. In endotoxin-primed MCs, pro-IL-1β was rapidly packaged into granules after IgE-Ag stimulation and processed within granule remnants by proteases after degranulation, causing lethal anaphylaxis in mice. During IgE-Ag-mediated degranulation of endotoxin-primed MCs, granulosomes promoted degranulation, combined with exteriorization and processing of IL-1β, resulting in severe inflammation.

Regulation of microtubule nucleation in mouse bone marrow-derived mast cells by ARF GTPase-activating protein GIT2

Aggregation of high-affinity IgE receptors (FcϵRIs) on granulated mast cells triggers signaling pathways leading to a calcium response and release of inflammatory mediators from secretory granules. While microtubules play a role in the degranulation process, the complex molecular mechanisms regulating microtubule remodeling in activated mast cells are only partially understood. Here, we demonstrate that the activation of bone marrow mast cells induced by FcϵRI aggregation increases centrosomal microtubule nucleation, with G protein-coupled receptor kinase-interacting protein 2 (GIT2) playing a vital role in this process. Both endogenous and exogenous GIT2 were associated with centrosomes and γ-tubulin complex proteins. Depletion of GIT2 enhanced centrosomal microtubule nucleation, and phenotypic rescue experiments revealed that GIT2, unlike GIT1, acts as a negative regulator of microtubule nucleation in mast cells. GIT2 also participated in the regulation of antigen-induced degranulation and chemotaxis. Further experiments showed that phosphorylation affected the centrosomal localization of GIT2 and that during antigen-induced activation, GIT2 was phosphorylated by conventional protein kinase C, which promoted microtubule nucleation. We propose that GIT2 is a novel regulator of microtubule organization in activated mast cells by modulating centrosomal microtubule nucleation.

Hapten-Decorated DNA Nanostructures Decipher the Antigen-Mediated Spatial Organization of Antibodies Involved in Mast Cell Activation

The immunological response of mast cells is controlled by the multivalent binding of antigens to immunoglobulin E (IgE) antibodies bound to the high-affinity receptor FcεRI on the cell membrane surface. However, the spatial organization of antigen-antibody-receptor complexes at the nanometer scale and the structural constraints involved in the initial events at the cell surface are not yet fully understood. For example, it is unclear what influence the affinity and nanoscale distance between the binding partners involved have on the activation of mast cells to degranulate inflammatory mediators from storage granules. We report the use of DNA origami nanostructures (DON) functionalized with different arrangements of the haptenic 2,4-dinitrophenyl (DNP) ligand to generate multivalent artificial antigens with full control over valency and nanoscale ligand architecture. To investigate the spatial requirements for mast cell activation, the DNP-DON complexes were initially used in surface plasmon resonance (SPR) analysis to study the binding kinetics of isolated IgE under physiological conditions. The most stable binding was observed in a narrow window of approximately 16 nm spacing between haptens. In contrast, affinity studies with FcεRI-linked IgE antibodies on the surface of rat basophilic leukemia cells (RBL-2H3) indicated virtually no distance-dependent variations in the binding of the differently structured DNP-DON complexes but suggested a supramolecular oligovalent nature of the interaction. Finally, the use of DNP-DON complexes for mast cell activation revealed that antigen-directed tight assembly of antibody-receptor complexes is the critical factor for triggering degranulation, even more critical than ligand valence. Our study emphasizes the significance of DNA nanostructures for the study of fundamental biological processes.

Infant Skin Friendly Adhesive Hydrogel Patch Activated at Body Temperature for Bioelectronics Securing and Diabetic Wound Healing

Adhesive-caused injury is a great threat for infants with premature skin or diabetic patients with fragile skin because extra-strong adhesion might incur pain, inflammation, and exacerbate trauma upon removal. Herein, we present a skin-friendly adhesive hydrogel patch based on protein-polyphenol complexation strategy, which leads to a thermoresponsive network sensitive to body temperature. The adhesion of the hydrogel is smartly activated after contacting with warm skin, whereas the painless detachment is easily realized by placing an ice bag on the surface of the hydrogel. The hydrogel exhibits an immunomodulatory performance that prevents irritation and allergic reactions during long-period contact with the skin. Thus, the hydrogel patch works as a conformable and nonirritating interface to guarantee nondestructively securing bioelectronics on infant skin for healthcare. Furthermore, the hydrogel patch provides gentle adhesion to wounded skin and provides a favorable environment to speed up the healing process for managing diabetic wounds.

Inhibition of negative feedback for persistent epithelial cell-cell junction contraction by p21-activated kinase 3

Actin-mediated mechanical forces are central drivers of cellular dynamics. They generate protrusive and contractile dynamics, the latter of which are induced in concert with myosin II bundled at the site of contraction. These dynamics emerge concomitantly in tissues and even each cell; thus, the tight regulation of such bidirectional forces is important for proper cellular deformation. Here, we show that contractile dynamics can eventually disturb cell–cell junction contraction in the absence of p21-activated kinase 3 (Pak3). Upon Pak3 depletion, contractility induces the formation of abnormal actin protrusions at the shortening junctions, which causes decrease in E-cadherin levels at the adherens junctions and mislocalization of myosin II at the junctions before they enough shorten, compromising completion of junction shortening. Overexpressing E-cadherin restores myosin II distribution closely placed at the junctions and junction contraction. Our results suggest that contractility both induces and perturbs junction contraction and that the attenuation of such perturbations by Pak3 facilitates persistent junction shortening.

Actin and myosin operate at cell–cell junctions during junctional shortening. Here the authors show that prolonged actomyosin contractility can compromise junctional shortening, and that Pak3 is required for attenuation of abnormal active protrusive structure and thus keeps junction contraction, appropriate E-cadherin distribution, and junction shortening in Drosophila.

Mechanobiology in the Comorbidities of Ehlers Danlos Syndrome

Ehlers-Danlos Syndromes (EDSs) are a group of connective tissue disorders, characterized by skin stretchability, joint hypermobility and instability. Mechanically, various tissues from EDS patients exhibit lowered elastic modulus and lowered ultimate strength. This change in mechanics has been associated with EDS symptoms. However, recent evidence points toward a possibility that the comorbidities of EDS could be also associated with reduced tissue stiffness. In this review, we focus on mast cell activation syndrome and impaired wound healing, comorbidities associated with the classical type (cEDS) and the hypermobile type (hEDS), respectively, and discuss potential mechanobiological pathways involved in the comorbidities.

Rab44 regulates murine mast cell-driven anaphylaxis through kinesin-1-dependent secretory granule translocation

BACKGROUND

Mast cells (MCs) are key effectors of the allergic response. Following the cross-linking of IgE receptors (FcεRIs), they release crucial inflammatory mediators through degranulation. Although degranulation depends critically on secretory granule (SG) trafficking towards the plasma membrane, the molecular machinery underlying this transport has not been fully characterized.

OBJECTIVE

Here, we analyzed the function of Rab44, a large atypical Rab GTPase highly expressed in MC, in MC degranulation process.

METHODS

Murine KO mouse models (KO^Rab44 and DKO^Kif5b/Rab44) were used to perform passive cutaneous anaphylaxis (PCA) experiments and analyze granule translocation in derived bone-marrow-derived MCs (BMMCs) during degranulation.

RESULTS

We demonstrate that mice lacking Rab44 (KO^Rab44) in their BMMCs are impaired in their ability to translocate and degranulate SGs at the plasma membrane upon FcεRI stimulation. Accordingly, KO^Rab44 mice were less sensitive to IgE-mediated passive cutaneous anaphylaxis in vivo. A lack of Rab44 did not impair early FcεRI-stimulated signaling pathways, microtubule reorganization, lipid mediator or cytokine secretion. Mechanistically, Rab44 appears to interact with and function as part of the previously described kinesin-1-dependent transport pathway.

CONCLUSIONS

Our results highlight a novel role of Rab44 as a regulator of SG transport during degranulation and anaphylaxis acting through the kinesin-1-dependent microtubule transport machinery. Rab44 can thus be considered as a potential target for modulating MC degranulation and inhibiting IgE-mediated allergic reactions.

Mast cell granule motility and exocytosis is driven by dynamic microtubule formation and kinesin-1 motor function

Mast cells are tissue-resident immune cells that have numerous cytoplasmic granules which contain preformed pro-inflammatory mediators. Upon antigen stimulation, sensitized mast cells undergo profound changes to their morphology and rapidly release granule mediators by regulated exocytosis, also known as degranulation. We have previously shown that Rho GTPases regulate exocytosis, which suggests that cytoskeleton remodeling is involved in granule transport. Here, we used live-cell imaging to analyze cytoskeleton remodeling and granule transport in real-time as mast cells were antigen stimulated. We found that granule transport to the cell periphery was coordinated by de novo microtubule formation and not F-actin. Kinesore, a drug that activates the microtubule motor kinesin-1 in the absence of cargo, inhibited microtubule-granule association and significantly reduced exocytosis. Likewise, shRNA knock-down of Kif5b, the kinesin-1 heavy chain, also reduced exocytosis. Imaging showed granules accumulated in the perinuclear region after kinesore treatment or Kif5b knock-down. Complete microtubule depolymerization with nocodazole or colchicine resulted in the same effect. A biochemically enriched granule fraction showed kinesin-1 levels increase in antigen-stimulated cells, but are reduced by pre-treatment with kinesore. Kinesore had no effect on the levels of Slp3, a mast cell granule cargo adaptor, in the granule-enriched fraction which suggests that cargo adaptor recruitment to granules is independent of motor association. Taken together, these results show that granules associate with microtubules and are driven by kinesin-1 to facilitate exocytosis.

Mast Cells and Acupuncture Analgesia

Mast cells are widely distributed in various parts of the human body and play a vital role in the progression of many diseases. Recently, the close relationship between mast cells and acupoints was elucidated, and the role of mast cells in acupuncture analgesia has attracted the attention of researchers worldwide. Using mast cells, acupuncture analgesia and acupoint as key words to search CNKI, PubMed, Web of Science and other databases, combining the representative articles in these databases with the published research papers of our group, we summarized: The enrichment of mast cells and the dense arrangement of collagen fibers, microvessels, and nerves form the basis for acupoints as the reaction sites of acupuncture; acupuncture can cause the deformation of collagen fibers and activate TRPV channels on mast cells membrane, so as to stimulate mast cells to release bioactive substances and activate nerve receptors to generate analgesic effect; system biology models are set up to explain the quantitative process of information initiation and transmission at acupuncture points, and indicate that the acupuncture effect depends on the local mast cells density. In a conclusion, this review will give a scientific explanation of acupuncture analgesia from the material basis of acupoints, the local initiation, and afferent biological mechanism.

Multimodal fluorescently labeled polymer-coated GdF3 nanoparticles inhibit degranulation in mast cells

Multimodal gadolinium fluoride nanoparticles belong to potential contrast agents useful for bimodal optical fluorescence and magnetic resonance imaging. However, the metallic nature of the nanoparticles, similarly to some paramagnetic iron oxides, might induce allergic and anaphylactic reactions in patients after administration. A reduction of these adverse side effects is a priority for the safe application of the nanoparticles. Herein, we prepared paramagnetic poly(4-styrenesulfonic acid-co-maleic acid) (PSSMA)-stabilized GdF₃ nanoparticles with surface modified by Atto 488-labeled poly(styrene-grad-2-dimethylaminoethyl acrylate)-block-poly(2-dimethylaminoethyl acrylate) (PSDA-A488) with reactive amino groups for introduction of an additional imaging (luminescence) modality and possible targeting of anticancer drugs. The saturation magnetization of GdF₃@PSSMA particles according to SQUID magnetometry reached 157 Am² kg^-1 at 2 K and magnetic field of 7 T. GdF₃@PSSMA-PSDA-A488 nanoparticles were well tolerated by human cervical adenocarcinoma (HeLa), mouse bone marrow-derived mast cells (BMMC), and rat basophilic mast cells (RBL-2H3); the particles also affected cell morphology and protein tyrosine phosphorylation in mast cells. Moreover, the nanoparticles interfered with the activation of mast cells by multivalent antigens and inhibited calcium mobilization and cell degranulation. These findings show that the new multimodal GdF₃-based nanoparticles possess properties useful for various imaging methods and might minimize mast cell degranulation incurred after future nanoparticle diagnostic administration.

The role of Sp140 revealed in IgE and mast cell responses in Collaborative Cross mice

Mouse IgE and mast cell (MC) functions have been studied primarily using inbred strains. Here, we (a) identified effects of genetic background on mouse IgE and MC phenotypes, (b) defined the suitability of various strains for studying IgE and MC functions, and (c) began to study potentially novel genes involved in such functions. We screened 47 Collaborative Cross (CC) strains, as well as C57BL/6J and BALB/cJ mice, for strength of passive cutaneous anaphylaxis (PCA) and responses to the intestinal parasite Strongyloides venezuelensis (S.v.). CC mice exhibited a diversity in PCA strength and S.v. responses. Among strains tested, C57BL/6J and CC027 mice showed, respectively, moderate and uniquely potent MC activity. Quantitative trait locus analysis and RNA sequencing of BM-derived cultured MCs (BMCMCs) from CC027 mice suggested Sp140 as a candidate gene for MC activation. siRNA-mediated knock-down of Sp140 in BMCMCs decreased IgE-dependent histamine release and cytokine production. Our results demonstrated marked variations in IgE and MC activity in vivo, and in responses to S.v., across CC strains. C57BL/6J and CC027 represent useful models for studying MC functions. Additionally, we identified Sp140 as a gene that contributes to IgE-dependent MC activation.

MYO1F Regulates IgE and MRGPRX2-Dependent Mast Cell Exocytosis

The activation and degranulation of mast cells is critical in the pathogenesis of allergic inflammation and modulation of inflammation. Recently, we demonstrated that the unconventional long-tailed myosin, MYO1F, localizes with cortical F-actin and mediates adhesion and migration of mast cells. In this study, we show that knockdown of MYO1F by short hairpin RNA reduces human mast cell degranulation induced by both IgE crosslinking and by stimulation of the Mas-related G protein-coupled receptor X2 (MRGPRX2), which has been associated with allergic and pseudoallergic drug reactions, respectively. Defective degranulation was accompanied by a reduced reassembly of the cortical actin ring after activation but reversed by inhibition of actin polymerization. Our data show that MYO1F is required for full Cdc42 GTPase activation, a critical step in exocytosis. Furthermore, MYO1F knockdown resulted in less granule localization in the cell membrane and fewer fissioned mitochondria along with deficient mitochondria translocation to exocytic sites. Consistent with that, AKT and DRP1 phosphorylation are diminished in MYO1F knockdown cells. Altogether, our data point to MYO1F as an important regulator of mast cell degranulation by contributing to the dynamics of the cortical actin ring and the distribution of both the secretory granules and mitochondria.

RACK1 plays a critical role in mast cell secretion and Ca2+ mobilization by modulating F-actin dynamics

Although RACK1 is known to act as a signaling hub in immune cells, its presence and role in mast cells (MCs) is undetermined. MC activation via antigen stimulation results in mediator release and is preceded by cytoskeleton reorganization and Ca2+ mobilization. In this study, we found that RACK1 was distributed throughout the MC cytoplasm both in vivo and in vitro. After RACK1 knockdown (KD), MCs were rounded, and the cortical F-actin was fragmented. Following antigen stimulation, in RACK1 KD MCs, there was a reduction in cortical F-actin, an increase in monomeric G-actin and a failure to organize F-actin. RACK1 KD also increased and accelerated degranulation. CD63+ secretory granules were localized in F-actin-free cortical regions in non-stimulated RACK1 KD MCs. Additionally, RACK1 KD increased antigen-stimulated Ca2+ mobilization, but attenuated antigen-stimulated depletion of ER Ca2+ stores and thapsigargin-induced Ca2+ entry. Following MC activation there was also an increase in interaction of RACK1 with Orai1 Ca2+-channels, β-actin and the actin-binding proteins vinculin and MyoVa. These results show that RACK1 is a critical regulator of actin dynamics, affecting mediator secretion and Ca2+ signaling in MCs. This article has an associated First Person interview with the first author of the paper.

The high affinity IgE receptor: a signaling update

Here we update receptor proximal and distant signaling events of the mast cell high affinity IgE receptor (FcεRI) launching immediate type I hypersensitivity and an inflammatory cytokine-chemokine cascade. Different physiologic antigen concentrations, their affinity, and valency for the IgE ligand produce distinct intracellular signaling events with different outcomes. Investigating mast cell degranulation has revealed a complex molecular machinery that relays proximal signaling to cytoskeletal reorganization, granule transport and membrane fusion. Several new phosphorylation- and calcium-responsive effectors have been described. FcεRI signaling also promotes de novo gene transcription. Recent progress has identified enhancers at genes that are upregulated in mast cells after stimulation through FcεRI using next generation sequencing methods. Enhancers at genes that respond to antigenic stimulation in human mast cells revealed Ca²⁺-dependency. Stimulation-responsive super enhancers in mouse mast cells have also been identified. Mast cell lineage-determining transcription factor GATA2 primes these enhancers to respond to antigenic stimulation.

The actin cytoskeleton and mast cell function

The application of high and super-resolution microscopy techniques has extended the possibilities of studying actin dynamics in mast cells (MCs). These studies demonstrated the close correlation between actin-driven changes in cell morphology and the functions that MC perform during their life cycle. Dynamic conversions between actin polymerization and depolymerization support MC degranulation and leading to the release of the preformed, secretory granule (SG)-contained, inflammatory mediators. Cell flattening inflicting an actin porous geometry and clearing of cortical actin, characterize the secretory actin phenotype. In contrast, pericentral actin clusters, that entrap the SGs, characterize the migratory actin phenotype, which supports MC migration, but restricts MC degranulation. Multiple actin binding and actin interacting proteins regulate these actin rearrangements, in compliance with the signals elicited by the respective activating receptors. Here, we review recent findings on the interplay between the actin cytoskeleton and MC migration and degranulation.

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.

Two Japanese pepper (Zanthoxylum piperitum) fruit-derived compounds attenuate IgE-mediated allergic response in vitro and in vivo via inhibition of mast cell degranulation

Zanthoxylum piperitum (ZP, 'Japanese pepper') is a traditional medicine and pepper used in Asian countries such as Japan. Hydroxy-α-sanshool, a pungent-tasting substance contained within ZP, has been reported to slightly suppress immunoglobulin E (IgE)-mediated mast cell degranulation. The current study aims to newly identify anti-allergic compounds derived from ZP. We examine the inhibitory mechanisms behind IgE-mediated mast cell degranulation. By inhibitory effect-guided isolation, we identified degranulation inhibitory compounds derived from ZP fruit: 1-acetoxy-7-hydroxy-3, 7-dimethylocta-2E, 5E-diene (ZP1) and 8-hydroxygeranyl acetate (ZP2). ZP1 and ZP2 inhibited IgE-mediated degranulation and A23187-mediated degranulation in RBL-2H3 mast cells. Our findings suggest the inhibition of degranulation by ZP1 and ZP2 was by inhibition of Lyn phosphorylation, followed by inhibition of intracellular Ca²⁺ mobilization, protein kinase C alpha phosphorylation, membrane ruffling, and granule-to-plasma membrane fusion. Oral administration of ZP1 or ZP2 attenuated an IgE-mediated passive cutaneous anaphylactic reaction in mice. Histological observation suggests that this effect occurred via inhibition of mast cell degranulation. These findings indicate that ZP1 and ZP2 attenuate allergic reaction via inhibition of IgE-mediated mast cell degranulation.

Mast Cell Biology at Molecular Level: a Comprehensive Review

Mast cells (MCs) are portions of the innate and adaptive immune system derived from bone marrow (BM) progenitors that are rich in cytoplasmic granules. MC maturation, phenotype, and function are determined by their microenvironment. MCs accumulate at inflammatory sites associated with atopy, wound healing, and malignancies. They interact with the external environment and are predominantly located in close proximity of blood vessels and sensory nerves. MCs are key initiators and modulators of allergic, anaphylactic, and other inflammatory reactions, by induction of vasodilation, promoting of vascular permeability, recruitment of inflammatory cells, facilitation of adaptive immune responses, and modulation of angiogenesis, and fibrosis. They express a wide range of receptors, e.g., for IgE (FcεRI), IgG (FcγR), stem cell factor (SCF) (KIT receptor or CD117), complement (including C5aR), and cytokines, that upon activation trigger various signaling pathways. The final consequence of such ligand receptor-based activation of MCs is the release of a broad array of mediators which are classified in three categories. While some mediators are preformed and remain stored in granules such as heparin, histamine, and enzymes mainly chymase and tryptase, others are de novo synthesized only after activation including LTB4, LTD4, PDG2, and PAF, and the cytokines IL-10, IL-8, IL-5, IL-3, IL-1, GM-CSF, TGF-β, VEGF, and TNF-α. Depending on the stimulus, MCs calibrate their pattern of mediator release, modulate the amplification of allergic inflammation, and are involved in the resolution of the immune responses. Here, we review recent findings and reports that help to understand the MC biology, pathology, and physiology of diseases with MC involvement.

The Role of Mast Cells in IgE-Independent Lung Diseases

Mast cells (MCs) are granular cells of the innate immune system which develop from CD34+/CD117+ progenitors and play a role in orchestrating adaptive immune responses. They have a well-known role in allergic reactions following immunoglobulin (Ig)E-mediated activation of the cell-surface expressed IgE high-affinity receptor (FcεRI). MCs can also respond to various other stimuli due to the expression of a variety of receptors including toll-like receptors (TLRs), immunoglobulin (IgG) receptors (FcγR), complement receptors such as C5a (CD88) expressed by skin MCs, neuropeptides receptors including nerve growth factor receptor, (NGFR), cytokines receptors such as (IL)-1R and IL-3R, and chemokines receptors including CCR-1 and CCR-3. MCs release three groups of mediators upon degranulation differentiated according to their chemical composition, storage, and time to release. These include preformed mediators (mainly histamine, tryptase, and chymase), de novo synthesized mediators such as prostaglandin (PG)D2, leukotriene (LT)B4 and LTD4, and cytokines including IL-1β, IL-3, tumor necrosis factor (TNF)α, and transforming growth factor(TGF)-β. Emerging evidence indicates a role for IgE-independent MC activation in the late-stage asthmatic response as well as in non-allergic airway diseases including chronic obstructive pulmonary disease (COPD), idiopathic pulmonary fibrosis (IPF), and lung cancer. MC infiltration/activation has been reported in some, but not all, studies of lung cancer. MC-derived TNF-α possesses tumor-suppressive activity while IL-1β supports tumor progression and metastasis. In IPF lungs, an increase in density of tryptase- and chymase-positive MCs (MCTC) and overexpression of TGF-β support the fibrosis progression. MC-derived chymase activates latent TGF-β that induces the differentiation of fibroblasts to matrix-producing myofibroblasts. In summary, increasing evidence highlights a critical role of MCs in non-allergic diseases that may indicate new approaches for therapy.

The antipsychotic drug pimozide inhibits IgE-mediated mast cell degranulation and migration

BACKGROUND

Mast cells (MCs) mediate a key role in allergic diseases. Detailed studies of how the neuroleptic drug pimozide affects MC activity are lacking. The aim of this study was to investigate pimozide inhibition of immunoglobulin E (IgE)-mediated MC activation and MC-mediated allergic responses.

METHOD

MCs were stimulated with anti-dinitrophenyl (DNP) IgE antibodies and DNP-horse serum albumin (HSA) antigen (Ag), and anti-allergic pimozide effects were detected by measuring β-hexosaminidase levels. Morphological changes were observed histologically. In vivo pimozide effects were assessed in passive cutaneous anaphylaxis (PCA) and ovalbumin (OVA)-sensitized active systemic anaphylaxis mouse (ASA) model experiments. Levels of phosphorylated (p-) SYK (spleen tyrosine kinase) and MAPKs (mitogen-activated protein kinases) were detected in western blots.

RESULTS

We found that pimozide inhibited MC degranulation, reduced MC release of β-hexosaminidase dose-dependently in activated RBL-2H3 (IC₅₀: 13.52 μM) and bone marrow derived MC (BMMC) (IC₅₀: 42.42 μM), and reduced MC morphological changes. The IgE/Ag-induced migration effect was suppressed by pimozide treatment dose-dependently. Pimozide down-regulated IgE/Ag-induced phosphorylation of SYK and MAPKs in activated MCs. Moreover, pimozide attenuated allergic reactions in PCA and ASA model mice, and decreased MC populations among splenic cells.

CONCLUSIONS

The antipsychotic drug pimozide can suppress IgE-mediated MC activation in vitro and in vivo and should be considered for repurposing to suppress MC-mediated diseases.

Aurora kinase inhibitor tozasertib suppresses mast cell activation in vitro and in vivo

BACKGROUND AND PURPOSE

Mast cells are important in allergic reactions. Here, we assessed the anti-allergic effects of the anti-cancer drug tozasertib specifically regarding regulatory effects on mast cell activation.

EXPERIMENTAL APPROACH

Tozasertib effects on mast cell degranulation were determined by measuring β-hexosaminidase and histamine release and by assessing morphological changes in RBL-2H3 and mouse bone marrow-derived mast cells (BMMCs) stimulated with mouse anti-dinitrophenyl (DNP)-IgE/DNP-human serum albumin or human LAD2 cells activated with phorbol-12-myristate 13-acetate plus calcium ionophore (PMACI). Western blots were performed to detect the expression of molecules involved in NF-κB, MAPK, and Aurora kinase signalling. in vivo anti-allergic effects of tozasertib were determined in the murine IgE-mediated passive cutaneous anaphylaxis (PCA) and ovalbumin (OVA)-induced active systemic anaphylaxis (ASA) models.

KEY RESULTS

Tozasertib treatment decreased high-affinity IgE receptor (FcεRI) or PMACI-mediated degranulation in RBL-2H3 cells and in BMMCs or LAD2 cells as shown by β-hexosaminidase or histamine levels. Similarly, tozasertib prevented morphological changes in mast cells, such as particle release and F-actin reorganization. In addition, tozasertib markedly decreased expression of phosphorylated (p)-NF-κB p65, p-Erk1/2, p-p38, and p-Aurora A/B, indicating that tozasertib can inhibit the signalling pathway mediating mast cell activation. Tozasertib attenuated IgE/Ag-induced PCA dose-dependently, as shown by reduced Evans blue staining. Similarly, tozasertib reduced body temperature levels and serum histamine levels in OVA-challenged ASA mice.

CONCLUSION AND IMPLICATIONS

The Aurora kinase inhibitor tozasertib suppressed mast cell activation in vitro and in vivo. Tozasertib may be a potential drug, targeting mast cell activation, to treat allergic diseases or mastocytosis.

Interplay between Convective and Viscoelastic Forces Controls the Morphology of In Vitro Paclitaxel-Stabilized Microtubules

Microtubules (MTs) are self-assembling, high-aspect-ratio tubular nanostructures formed from the polymerization of tubulin protein. MTs are capable of globally assembling into optically birefringent morphologies, but there is disagreement on the mechanisms driving this behavior. We investigated the temporal evolution of paclitaxel (PTX)-stabilized MT solutions under a range of in vitro conditions. Significant morphological differences were observed in the polymerized PTX-MT solutions as a consequence of varying the orientation of the reaction vessel (vertical vs. horizontal), the type of heating source (hot plate vs. incubator), the incubation time, and the concentration of PTX (high vs. low). The most robust birefringent patterns were found only in vertically oriented cuvettes that were heated asymmetrically on a hot plate, suggesting dependence upon a convective flow, which we confirmed with a combination of optical and thermal imaging. Higher concentrations of PTX led to denser PTX-MT domain formation and brighter birefringence, due to more complete polymerization. Combining our experimental observations, we conclude that birefringent patterns arise principally through a combination of convective and viscoelastic forces, and we identify the sequence of dynamical stages through which they evolve.

Low-dose ionizing radiation attenuates mast cell migration through suppression of monocyte chemoattractant protein-1 (MCP-1) expression by Nr4a2

Purpose: The aim of this study was to investigate whether low-dose ionizing radiation attenuates mast cell migration by modulating migration-associated signaling pathways and the expression of chemotactic cytokines.Materials and methods: IgE-sensitized RBL-2H3 mast cells were exposed with ionizing radiation at 0.01, 0.05, 0.1, or 0.5 Gy using a ¹³⁷Cs γ-irradiator and stimulated with 2,4-dinitrophenol-human serum albumin. Cell migration was determined using a transwell assay system, F-actin distribution using Alex Fluor 488-conjugated phalloidin, expression of various signaling proteins by Western blotting, mRNA expression by RT-PCR.Results: Low-dose ionizing radiation significantly suppressed mast cell migration induced by IgE-mediated mast cell activation. Furthermore, low-dose ionizing radiation altered cell morphology, as reflected by changes in F-actin distribution, and inhibited the activation of PI3K, Btk, Rac1, and Cdc42. These effects were mediated by Nr4a2, an immune-modulating factor. Knockdown of Nr4a2 reduced mast cell migration, inhibited the PI3K and Btk signaling pathways, and reduced expression of the chemotactic cytokine monocyte chemoattractant protein-1 (MCP-1). We further demonstrated that direct blockade of MCP-1 using neutralizing antibodies inhibits mast cell migration.Conclusion: Low-dose ionizing radiation inhibits mast cell migration through the regulation production of MCP-1 by Nr4a2 in the activated mast cell system.

Proteomic analysis of male rat nucleus accumbens, dorsal hippocampus and amygdala on conditioned place aversion induced by morphine withdrawal

Addiction is characterized by compulsive drug seeking and taking behavior, which is thought to result from persistent neuroadaptations, encoded by changes of gene expression. We previously demonstrated that the changes in synaptic plasticity were required for the formation of aversive memories associated with morphine withdrawal. However, the proteins involved in synaptic plasticity and aversive memory formation have not been well explored. In the present study, we employed a two-dimensional gel electrophoresis (2-DE)-based proteomic technique to detect the changes of protein expression in the nucleus accumbens, amygdala and dorsal hippocampus of the rats that had developed conditioned morphine withdrawal. We found that twenty-three proteins were significantly altered in the amygdala and dorsal hippocampus after conditioned morphine withdrawal. These proteins can be classified into multiple categories, such as energy metabolism, signal transduction, synaptic transmission, cytoskeletal proteins, chaperones, and protein metabolism according to their biological functions. Eight proteins related to synaptic plasticity were further confirmed by western blot analysis. It is very likely that these identified proteins may contribute to conditioned morphine withdrawal-induced neural plasticity and aversive memory formation. Thus, our work will help understand the potential mechanism associated with generation of drug withdrawal memories.

SLAP Is a Negative Regulator of FcεRI Receptor-Mediated Signaling and Allergic Response

Binding of antigen to IgE-high affinity FcεRI complexes on mast cells and basophils results in the release of preformed mediators such as histamine and de novo synthesis of cytokines causing allergic reactions. Src-like adapter protein (SLAP) functions co-operatively with c-Cbl to negatively regulate signaling downstream of the T cell receptor, B cell receptor, and receptor tyrosine kinases (RTK). Here, we investigated the role of SLAP in FcεRI-mediated mast cell signaling, using bone marrow derived mast cells (BMMCs) from SLAP knock out (SLAP KO) mice. Mature SLAP-KO BMMCs displayed significantly enhanced antigen induced degranulation and synthesis of IL-6, TNFα, and MCP-1 compared to wild type (WT) BMMCs. In addition, SLAP KO mice displayed an enhanced passive cutaneous anaphylaxis response. In agreement with a negative regulatory role, SLAP KO BMMCs showed enhanced FcεRI-mediated signaling to downstream effector kinases, Syk, Erk, and Akt. Recombinant GST-SLAP protein binds to the FcεRIβ chain and to the Cbl-b in mast cell lysates, suggesting a role in FcεRI down regulation. In addition, the ubiquitination of FcεRIγ chain and antigen mediated down regulation of FcεRI is impaired in SLAP KO BMMCs compared to the wild type. In line with these findings, stimulation of peripheral blood human basophils with FcεRIα antibody, or a clinically relevant allergen, resulted in increased SLAP expression. Together, these results indicate that SLAP is a dynamic regulator of IgE-FcεRI signaling, limiting allergic responses.

Regulation of the pleiotropic effects of tissue-resident mast cells

Mast cells (MCs), which are best known for their detrimental role in patients with allergic diseases, act in a diverse array of physiologic and pathologic functions made possible by the plurality of MC types. Their various developmental avenues and distinct sensitivity to (micro-) environmental conditions convey extensive heterogeneity, resulting in diverse functions. We briefly summarize this heterogeneity, elaborate on molecular determinants that allow MCs to communicate with their environment to fulfill their tasks, discuss the protease repertoire stored in secretory lysosomes, and consider different aspects of MC signaling. Furthermore, we describe key MC governance mechanisms (ie, the high-affinity receptor for IgE [FcεRI]), the stem cell factor receptor KIT, the IL-4 system, and both Ca²⁺- and phosphatase-dependent mechanisms. Finally, we focus on distinct physiologic functions, such as chemotaxis, phagocytosis, host defense, and the regulation of MC functions at the mucosal barriers of the lung, gastrointestinal tract, and skin. A deeper knowledge of the pleiotropic functions of MC mediators, as well as the molecular processes of MC regulation and communication, should enable us to promote beneficial MC traits in physiology and suppress detrimental MC functions in patients with disease.

Kit-independent mast cell adhesion mediated by Notch

Kit/CD117 plays a crucial role in the cell-cell and cell-matrix adhesion of mammalian mast cells (MCs); however, it is unclear whether other adhesion molecule(s) perform important roles in the adhesion of MCs. In the present study, we show a novel Kit-independent adhesion mechanism of mouse cultured MCs mediated by Notch family members. On stromal cells transduced with each Notch ligand gene, Kit and its signaling become dispensable for the entire adhesion process of MCs from tethering to spreading. The Notch-mediated spreading of adherent MCs involves the activation of signaling via phosphatidylinositol 3-kinases and mitogen-activated protein kinases, similar to Kit-mediated spreading. Despite the activation of the same signaling pathways, while Kit supports the adhesion and survival of MCs, Notch only supports adhesion. Thus, Notch family members are specialized adhesion molecules for MCs that effectively replace the adhesion function of Kit in order to support the interaction of MCs with the surrounding cellular microenvironments.

Blocking dephosphorylation at Serine 120 residue in t-SNARE SNAP-23 leads to massive inhibition in exocytosis from mast cells

Mast cells (MCs) respond to allergen challenge by release of pre-stored inflammatory mediators from their secretory granules, on cross-linking of Fc(epsilon) receptor I (Fc(epsilon)RI) receptors. The target-SNARE (t-SNARE) SNAP-23 has been shown to play an important role in MC exocytosis and undergoes transient phosphorylation at Serine 95 (S95) and Serine 120 (S120), concomitant with mediator release. During current study we explored the importance of transient nature of phosphorylation at S120 in MC exocytosis. A phosphomimetic SNAP-23-S120D mutant of rodent SNAP-23 was cloned into EGFP vector and its effect on the exocytosis and the mechanisms involved was studied in RBL-2H3 MC line. Secretion reporter assay with SNAP-23-S120D transfected MCs revealed a very significant inhibition of exocytosis, and reduced ruffling in response to Fc(epsilon)RI cross-linking. Further, the effect of this mutation on localization of SNAP-23 in MCs was studied. Immunofluorescence microscopy studies and membrane-cytosol fractionation of green fluorescent protein-tagged SNAP- 23-S120D (GFP-SNAP-23-S120D) transfected MCs showed that a large proportion of GFP-SNAP-23-S120D was residing in cytosol unlike wild-type SNAP-23, in resting and activated MCs and even the membrane associated portion was on internal lysosomal membranes than plasma membrane. These studies imply that dephosphorylation of S120 is important for SNAP-23 membrane association dynamics and subsequently MC degranulation.

Computational modeling of single-cell mechanics and cytoskeletal mechanobiology

Cellular cytoskeletal mechanics plays a major role in many aspects of human health from organ development to wound healing, tissue homeostasis and cancer metastasis. We summarize the state-of-the-art techniques for mathematically modeling cellular stiffness and mechanics and the cytoskeletal components and factors that regulate them. We highlight key experiments that have assisted model parameterization and compare the advantages of different models that have been used to recapitulate these experiments. An overview of feed-forward mechanisms from signaling to cytoskeleton remodeling is provided, followed by a discussion of the rapidly growing niche of encapsulating feedback mechanisms from cytoskeletal and cell mechanics to signaling. We discuss broad areas of advancement that could accelerate research and understanding of cellular mechanobiology. A precise understanding of the molecular mechanisms that affect cell and tissue mechanics and function will underpin innovations in medical device technologies of the future. WIREs Syst Biol Med 2018, 10:e1407. doi: 10.1002/wsbm.1407 This article is categorized under: Models of Systems Properties and Processes > Mechanistic Models Physiology > Mammalian Physiology in Health and Disease Models of Systems Properties and Processes > Cellular Models.

Self-Assembling Peptide Nanoscaffold That Activates Human Mast Cells

Engineering biomaterials to manipulate the immune response to elicit specific therapeutic outcomes is a burgeoning field of research. Mast cells play a distinct and central role in the innate immune response, and are characterized by their rapid release of a myriad of proinflammatory mediators in response to stimulation. These mediators are central to protective actions such as wound healing, angiogenesis, and host defense against pathogens and animal venoms. Considering that mast cells are widely distributed in tissues that interface with the external environment, and are loaded with large amounts of preformed protective compounds, they are ideal targets for novel immunotherapies. Here we report that, by using an engineered nanoscaffold, human mast cells can be contact activated in cell and primary human skin tissue culture using a specific receptor-ligand mechanism. The IgE independent PAMP-12 peptide activates human mast cells through the recently identified Mas-related G-protein coupled receptor member X2 (MRGPRX₂) receptor. The PAMP-12 motif was conjugated, via a glycine spacer, with the self-assembling peptide (RADA)₄ and mixed with unmodified (RADA)₄ to form a nanofiber matrix; mast cell activation was influenced directly by this ratio. Moreover, conjugating the PAMP-12 motif within the matrix was shown to only activate local, tissue-resident mast cells. The result of ex vivo human skin tissue tests confirmed that the engineered nanoscaffold successfully activated skin-resident mast cells by contact. Thus, this nanoscaffold design may provide a new platform to modulate localized mast cell functions thereby facilitating their protective role in the skin.

The transcriptional program, functional heterogeneity, and clinical targeting of mast cells

Cildir et al. discuss the recent findings in transcriptional regulation of mast cell development and activation and provide insights into the plasticity and clinical targeting of mast cell functions.

Mast cells are unique tissue-resident immune cells that express an array of receptors that can be activated by several extracellular cues, including antigen–immunoglobulin E (IgE) complexes, bacteria, viruses, cytokines, hormones, peptides, and drugs. Mast cells constitute a small population in tissues, but their extraordinary ability to respond rapidly by releasing granule-stored and newly made mediators underpins their importance in health and disease. In this review, we document the biology of mast cells and introduce new concepts and opinions regarding their role in human diseases beyond IgE-mediated allergic responses and antiparasitic functions. We bring to light recent discoveries and developments in mast cell research, including regulation of mast cell functions, differentiation, survival, and novel mouse models. Finally, we highlight the current and future opportunities for therapeutic intervention of mast cell functions in inflammatory diseases.

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.

Anti-allergic activity of 2,4,6-trihydroxy-3-geranylacetophenone (tHGA) via attenuation of IgE-mediated mast cell activation and inhibition of passive systemic anaphylaxis

tHGA, a geranyl acetophenone compound originally isolated from a local shrub called Melicope ptelefolia, has been previously reported to prevent ovalbumin-induced allergic airway inflammation in a murine model of allergic asthma by targeting cysteinyl leukotriene synthesis. Mast cells are immune effector cells involved in the pathogenesis of allergic diseases including asthma by releasing cysteinyl leukotrienes. The anti-asthmatic properties of tHGA could be attributed to its inhibitory effect on mast cell degranulation. As mast cell degranulation is an important event in allergic responses, this study aimed to investigate the anti-allergic effects of tHGA in cellular and animal models of IgE-mediated mast cell degranulation. For in vitro model of IgE-mediated mast cell degranulation, DNP-IgE-sensitized RBL-2H3 cells were pre-treated with tHGA before challenged with DNP-BSA to induce degranulation. For IgE-mediated passive systemic anaphylaxis, Sprague Dawley rats were sensitized by intraperitoneal injection of DNP-IgE before challenged with DNP-BSA. Both in vitro and in vivo models showed that tHGA significantly inhibited the release of preformed mediators (β-hexosaminidase and histamine) as well as de novo mediators (interleukin-4, tumour necrosis factor-α, prostaglandin D₂ and leukotriene C₄). Pre-treatment of tHGA also prevented IgE-challenged RBL-2H3 cells and peritoneal mast cells from undergoing morphological changes associated with mast cell degranulation. These findings indicate that tHGA possesses potent anti-allergic activity via attenuation of IgE-mediated mast cell degranulation and inhibition of IgE-mediated passive systemic anaphylaxis. Thus, tHGA may have the potential to be developed as a mast cell stabilizer for the treatment of allergic diseases in the future.

Cytoskeletal Regulation of Inflammation and Its Impact on Skin Blistering Disease Epidermolysis Bullosa Acquisita

Actin remodelling proteins regulate cytoskeletal cell responses and are important in both innate and adaptive immunity. These responses play a major role in providing a fine balance in a cascade of biological events that results in either protective acute inflammation or chronic inflammation that leads to a host of diseases including autoimmune inflammation mediated epidermolysis bullosa acquisita (EBA). This review describes the role of the actin cytoskeleton and in particular the actin remodelling protein called Flightless I (Flii) in regulating cellular inflammatory responses and its subsequent effect on the autoimmune skin blistering disease EBA. It also outlines the potential of an antibody based therapy for decreasing Flii expression in vivo to ameliorate the symptoms associated with EBA.

Alpha-tubulin enhanced renal tubular cell proliferation and tissue repair but reduced cell death and cell-crystal adhesion

Adhesion of calcium oxalate (CaOx) crystals on renal tubular epithelial cells is a critical event for kidney stone disease that triggers many cascades of cellular response. Our previous expression proteomics study identified several altered proteins in MDCK renal tubular cells induced by CaOx crystals. However, functional significance of those changes had not been investigated. The present study thus aimed to define functional roles of such proteome data. Global protein network analysis using STRING software revealed α-tubulin, which was decreased, as one of central nodes of protein-protein interactions. Overexpression of α-tubulin (pcDNA6.2-TUBA1A) was then performed and its efficacy was confirmed. pcDNA6.2-TUBA1A could maintain levels of α-tubulin and its direct interacting partner, vimentin, after crystal exposure. Also, pcDNA6.2-TUBA1A successfully reduced cell death to almost the basal level and increased cell proliferation after crystal exposure. Additionally, tissue repair capacity was improved in pcDNA6.2-TUBA1A cells. Moreover, cell-crystal adhesion was reduced by pcDNA6.2-TUBA1A. Finally, levels of potential crystal receptors (HSP90, HSP70, and α-enolase) on apical membrane were dramatically reduced to basal levels by pcDNA6.2-TUBA1A. These findings implicate that α-tubulin has protective roles in kidney stone disease by preventing cell death and cell-crystal adhesion, but on the other hand, enhancing cell proliferation and tissue repair function.

Inhibitory effects of enzyme-treated dried sardine extract on IgE-mediated degranulation of RBL-2H3 cells and a murine model of Japanese cedar pollinosis

Protease-digested small dried sardine extract relieved allergic symptoms in mice through inhibiting microtubule dynamics.

Interaction between Flavivirus and Cytoskeleton during Virus Replication

Flaviviruses are potentially human pathogens that cause major epidemics worldwide. Flavivirus interacts with host cell factors to form a favourable virus replication site. Cell cytoskeletons have been observed to have close contact with flaviviruses, which expands the understanding of cytoskeleton functions during virus replication, although many detailed mechanisms are still unclear. The interactions between the virus and host cytoskeletons such as actin filaments, microtubules, and intermediate filaments have provided insight into molecular alterations during the virus infection, such as viral entry, in-cell transport, scaffold assembly, and egress. This review article focuses on the utilization of cytoskeleton by Flavivirus and the respective functions during virus replication.

Engineered Nanostructures of Haptens Lead to Unexpected Formation of Membrane Nanotubes Connecting Rat Basophilic Leukemia Cells

A recent finding reports that co-stimulation of the high-affinity immunoglobulin E (IgE) receptor (FcεRI) and the chemokine receptor 1 (CCR1) triggered formation of membrane nanotubes among bone-marrow-derived mast cells. The co-stimulation was attained using corresponding ligands: IgE binding antigen and macrophage inflammatory protein 1α (MIP1 α), respectively. However, this approach failed to trigger formation of nanotubes among rat basophilic leukemia (RBL) cells due to the lack of CCR1 on the cell surface (Int. Immunol. 2010, 22 (2), 113-128). RBL cells are frequently used as a model for mast cells and are best known for antibody-mediated activation via FcεRI. This work reports the successful formation of membrane nanotubes among RBLs using only one stimulus, a hapten of 2,4-dinitrophenyl (DNP) molecules, which are presented as nanostructures with our designed spatial arrangements. This observation underlines the significance of the local presentation of ligands in the context of impacting the cellular signaling cascades. In the case of RBL, certain DNP nanostructures suppress antigen-induced degranulation and facilitate the rearrangement of the cytoskeleton to form nanotubes. These results demonstrate an important scientific concept; engineered nanostructures enable cellular signaling cascades, where current technologies encounter great difficulties. More importantly, nanotechnology offers a new platform to selectively activate and/or inhibit desired cellular signaling cascades.

Mast cells in airway diseases and interstitial lung disease

Mast cells are major effector cells of inflammation and there is strong evidence that mast cells play a significant role in asthma pathophysiology. There is also a growing body of evidence that mast cells contribute to other inflammatory and fibrotic lung diseases such as chronic obstructive pulmonary disease and idiopathic pulmonary fibrosis. This review discusses the role that mast cells play in airway diseases and highlights how mast cell microlocalisation within specific lung compartments and their cellular interactions are likely to be critical for their effector function in disease.

DOCK5 functions as a key signaling adaptor that links FcεRI signals to microtubule dynamics during mast cell degranulation

Mast cells play a key role in the induction of anaphylaxis, a life-threatening IgE-dependent allergic reaction, by secreting chemical mediators that are stored in secretory granules. Degranulation of mast cells is triggered by aggregation of the high-affinity IgE receptor, FcεRI, and involves dynamic rearrangement of microtubules. Although much is known about proximal signals downstream of FcεRI, the distal signaling events controlling microtubule dynamics remain elusive. Here we report that DOCK5, an atypical guanine nucleotide exchange factor (GEF) for Rac, is essential for mast cell degranulation. As such, we found that DOCK5-deficient mice exhibit resistance to systemic and cutaneous anaphylaxis. The Rac GEF activity of DOCK5 is surprisingly not required for mast cell degranulation. Instead, DOCK5 associated with Nck2 and Akt to regulate microtubule dynamics through phosphorylation and inactivation of GSK3β. When DOCK5-Nck2-Akt interactions were disrupted, microtubule formation and degranulation response were severely impaired. Our results thus identify DOCK5 as a key signaling adaptor that orchestrates remodeling of the microtubule network essential for mast cell degranulation.

Prediction of substrate sites for protein phosphatases 1B, SHP-1, and SHP-2 based on sequence features

Tyrosine phosphorylation plays crucial roles in numerous physiological processes. The level of phosphorylation state depends on the combined action of protein tyrosine kinases and protein tyrosine phosphatases. Detection of possible phosphorylation and dephosphorylation sites can provide useful information to the functional studies of relevant proteins. Several studies have focused on the identification of protein tyrosine kinase substrates. However, compared with protein tyrosine kinases, the prediction of protein tyrosine phosphatase substrates involved in the balance of protein phosphorylation level falls behind. This paper described a method that utilized the k-nearest neighbor algorithm to identity the substrate sites of three protein tyrosine phosphatases based on the sequence features of manually collected dephosphorylation sites. In the performance evaluation, both sensitivities and specificities could reach above 75% for all three protein tyrosine phosphatases. Finally, the method was applied on a set of known tyrosine phosphorylation sites to search for candidate substrates.

Complex chemoattractive and chemorepellent Kit signals revealed by direct imaging of murine mast cells in microfluidic gradient chambers

Besides its cooperating effects on stem cell proliferation and survival, Kit ligand (KL) is a potent chemotactic protein. While transwell assays permit studies of the frequency of migrating cells, the lack of direct visualization precludes dynamic chemotaxis studies. In response, we utilize microfluidic chambers that enable direct observation of murine bone marrow-derived mast cells (BMMC) within stable KL gradients. Using this system, individual Kit+ BMMC were quantitatively analyzed for migration speed and directionality during KL-induced chemotaxis. Our results indicated a minimum activating threshold of ~3 ng ml(-1) for chemoattraction. Analysis of cells at KL concentrations below 3 ng ml(-1) revealed a paradoxical chemorepulsion, which has not been described previously. Unlike chemoattraction, which occurred continuously after an initial time lag, chemorepulsion occurred only during the first 90 minutes of observation. Both chemoattraction and chemorepulsion required the action of G-protein coupled receptors (GPCR), as treatment with pertussis toxin abrogated directed migration. These results differ from previous studies of GPCR-mediated chemotaxis, where chemorepulsion occurred at high ligand concentrations. These data indicate that Kit-mediated chemotaxis is more complex than previously understood, with the involvement of GPCRs in addition to the Kit receptor tyrosine kinase and the presence of both chemoattractive and chemorepellent phases.