The ins and outs of IgE-dependent mast-cell exocytosis

C-kit mutations determine dasatinib mechanism of action in HMC-1 neoplastic mast cells: dasatinib differently regulates PKCδ translocation in HMC-1(560) and HMC-1(560,816) cell lines

PURPOSE

The second generation of tyrosine kinase inhibitors is a group of compounds that inhibit c-kit receptor activity and therefore widely used in the treatment of mastocytosis. In this research, the relationship between the mechanism of action of tyrosine kinase inhibitors and protein kinase C is investigated in HMC-1(560) or HMC-1(560,816) cell lines.

RESULTS

From all the tyrosine kinase inhibitors tested, nilotinib is the compound that has the highest cytotoxic effect against HMC-1(560) mast cell line, while midostaurin is the most potent in HMC-1(560,816). Moreover, an increase on histamine release is observed after protein kinase C activation either in HMC-1(560) or HMC-1(560,816) cells. Furthermore, dasatinib increases histamine release in both mast cell lines, which could be related with the secondary reactions previously described in dasatinib-treated patients. Dasatinib also induces Ca(2+)-dependent protein kinase C isoforms translocation from the cytosol to the membrane, whereas protein kinase Cδ is translocated from the cytosol to the nucleus in the HMC-1(560,816) cell line, but not in HMC-1(560) cells.

CONCLUSION

Results obtained demonstrate that dasatinib induces an important cytotoxic effect in both HMC-1 cell lines and differently regulates protein kinase Cδ in HMC-1(560) and HMC-1(560,816) cells. Finally, our results confirm that PKCδ is an essential target for dasatinib.

Brain "fog," inflammation and obesity: key aspects of neuropsychiatric disorders improved by luteolin

Brain "fog" is a constellation of symptoms that include reduced cognition, inability to concentrate and multitask, as well as loss of short and long term memory. Brain "fog" characterizes patients with autism spectrum disorders (ASDs), celiac disease, chronic fatigue syndrome, fibromyalgia, mastocytosis, and postural tachycardia syndrome (POTS), as well as "minimal cognitive impairment," an early clinical presentation of Alzheimer's disease (AD), and other neuropsychiatric disorders. Brain "fog" may be due to inflammatory molecules, including adipocytokines and histamine released from mast cells (MCs) further stimulating microglia activation, and causing focal brain inflammation. Recent reviews have described the potential use of natural flavonoids for the treatment of neuropsychiatric and neurodegenerative diseases. The flavone luteolin has numerous useful actions that include: anti-oxidant, anti-inflammatory, microglia inhibition, neuroprotection, and memory increase. A liposomal luteolin formulation in olive fruit extract improved attention in children with ASDs and brain "fog" in mastocytosis patients. Methylated luteolin analogs with increased activity and better bioavailability could be developed into effective treatments for neuropsychiatric disorders and brain "fog."

Mast cells, brain inflammation and autism

Increasing evidence indicates that brain inflammation is involved in the pathogenesis of neuropsychiatric diseases. Mast cells (MCs) are located perivascularly close to neurons and microglia, primarily in the leptomeninges, thalamus, hypothalamus and especially the median eminence. Corticotropin-releasing factor (CRF) is secreted from the hypothalamus under stress and, together with neurotensin (NT), can stimulate brain MCs to release inflammatory and neurotoxic mediators that disrupt the blood-brain barrier (BBB), stimulate microglia and cause focal inflammation. CRF and NT synergistically stimulate MCs and increase vascular permeability; these peptides can also induce each other׳s surface receptors on MCs leading to autocrine and paracrine effects. As a result, brain MCs may be involved in the pathogenesis of "brain fog," headaches, and autism spectrum disorders (ASDs), which worsen with stress. CRF and NT are significantly increased in serum of ASD children compared to normotypic controls further strengthening their role in the pathogenesis of autism. There are no clinically affective treatments for the core symptoms of ASDs, but pilot clinical trials using natural-antioxidant and anti-inflammatory molecules reported statistically significant benefit.

Targeting IL-33 in autoimmunity and inflammation

Interleukin-33 (IL-33) belongs to the IL-1 family of cytokines. Whereas IL-1 is processed and released by live immune cells in response to infection or other triggers, IL-33 is mostly released as a danger signal ("alarmin") from damaged cells. IL-33 may also be processed and released from activated mast cells (MCs) with subsequent autocrine and paracrine actions. IL-33 augments the stimulatory effects of IgE and substance P on MCs but can also trigger release of cytokines from MCs on its own. Blood IL-33 levels are increased in asthma, atopic dermatitis, multiple sclerosis, rheumatoid arthritis, and Sjögren's syndrome. However, prolonged elevation of IL-33 downregulates FcεRI and may be protective in atherosclerosis, suggesting different roles in immune-regulated diseases. Even though neutralizing IL-33, knocking-down its receptor, or using its soluble "decoy" receptor has resulted in anti-inflammatory effects, there appear to be different outcomes in different tissues. Hence, selective regulation of IL-33 synthesis, release, and signaling may be required to provide effective treatment options.

Mast cell and autoimmune diseases

Mast cells are important in innate immune system. They have been appreciated as potent contributors to allergic reaction. However, increasing evidence implicates the important role of mast cells in autoimmune disease like rheumatoid arthritis and multiple sclerosis. Here we review the current stage of knowledge about mast cells in autoimmune diseases.

Chaperoning of closed syntaxin-3 through Lys46 and Glu59 in domain 1 of Munc18 proteins is indispensable for mast cell exocytosis

Understanding how Munc18 proteins govern exocytosis is crucial because mutations of this protein cause severe secretion deficits in neuronal and immune cells. Munc18-2 has indispensable roles in the degranulation of mast cell, partly by binding and chaperoning a subset of syntaxin isoforms. However, the key syntaxin that, crucially, participates in the degranulation – whose levels and intracellular localization are regulated by Munc18-2 – remains unknown. Here, we demonstrate that double knockdown of Munc18-1 and Munc-2 in mast cells results in greatly reduced degranulation accompanied with strikingly compromised expression levels and localization of syntaxin-3. This phenotype is fully rescued by wild-type Munc18 proteins but not by the K46E, E59K and K46E/E59K mutants of Munc-18 domain 1, each of which exhibits completely abolished binding to 'closed' syntaxin-3. Furthermore, knockdown of syntaxin-3 strongly impairs degranulation. Collectively, our data argue that residues Lys46 and Glu59 of Munc18 proteins are indispensable for mediating the interaction between Munc18 and closed syntaxin-3, which is essential for degranulation by chaperoning syntaxin-3. Our results also indicate that the functional contribution of these residues differs between immune cell degranulation and neuronal secretion.

Neuroinflammatory Mechanisms of Connective Tissue Fibrosis: Targeting Neurogenic and Mast Cell Contributions

Significance: The pathogenesis of fibrogenic wound and connective tissue healing is complex and incompletely understood. Common observations across a vast array of human and animal models of fibroproliferative conditions suggest neuroinflammatory mechanisms are important upstream fibrogenic events. Recent Advances: As detailed in this review, mast cell hyperplasia is a common observation in fibrotic tissue. Recent investigations in human and preclinical models of hypertrophic wound healing and post-traumatic joint fibrosis provides evidence that fibrogenesis is governed by a maladaptive neuropeptide-mast cell-myofibroblast signaling pathway. Critical Issues: The blockade and manipulation of these factors is providing promising evidence that if timed correctly, the fibrogenic process can be appropriately regulated. Clinically, abnormal fibrogenic healing responses are not ubiquitous to all patients and the identification of those at-risk remains an area of priority. Future Directions: Ultimately, an integrated appreciation of the common pathobiology shared by many fibrogenic connective tissue conditions may provide a scientific framework to facilitate the development of novel antifibrotic prevention and treatment strategies.

Approaches for analyzing the roles of mast cells and their proteases in vivo

The roles of mast cells in health and disease remain incompletely understood. While the evidence that mast cells are critical effector cells in IgE-dependent anaphylaxis and other acute IgE-mediated allergic reactions seems unassailable, studies employing various mice deficient in mast cells or mast cell-associated proteases have yielded divergent conclusions about the roles of mast cells or their proteases in certain other immunological responses. Such "controversial" results call into question the relative utility of various older versus newer approaches to ascertain the roles of mast cells and mast cell proteases in vivo. This review discusses how both older and more recent mouse models have been used to investigate the functions of mast cells and their proteases in health and disease. We particularly focus on settings in which divergent conclusions about the importance of mast cells and their proteases have been supported by studies that employed different models of mast cell or mast cell protease deficiency. We think that two major conclusions can be drawn from such findings: (1) no matter which models of mast cell or mast cell protease deficiency one employs, the conclusions drawn from the experiments always should take into account the potential limitations of the models (particularly abnormalities affecting cell types other than mast cells) and (2) even when analyzing a biological response using a single model of mast cell or mast cell protease deficiency, details of experimental design are critical in efforts to define those conditions under which important contributions of mast cells or their proteases can be identified.

Treatment of murine mast cells with IgEκ and protein L enhances apoptotic cell death induced by IL-3 withdrawal

Engagement of the high-affinity IgE receptor (FcεRI) can be either protective or non-protective against apoptotic cell death (ACD) in bone marrow-derived murine mast cells (BMMCs) after IL-3 withdrawal, depending on the avidity between IgE and its antigen. We recently reported that protein L (PpL), a bacterial Igκ-binding soluble protein, is able to stimulate intracellular signaling to induce activation of BMMCs by interacting with the IgEκ-FcεRI complex. However, it is unclear if cross-linking of FcεRI with IgEκ and PpL prevents or enhances IL-3-dependent ACD in BMMCs. In the present study, we found that IL-3-dependent ACD of BMMCs is accelerated by loading soluble PpL in the presence of IgEκ-occupied FcεRIα. For this purpose, soluble PpL was incorporated into the BMMCs. Unlike soluble PpL, immobilized PpL failed to enhance ACD, although both forms of PpL induced IL-6 production equally in BMMCs. In addition, we observed that DNS5-BSA protected anti-DNS IgE-sensitized BMMCs from IL-3 depletion-mediated ACD by inducing the production of autocrine IL-3. In contrast, DNS5-PpL enhanced IL-3 withdrawal-induced ACD of anti-DNS IgE-sensitized BMMCs and reduced the production of autocrine IL-3. These findings suggest that PpL increases IL-3 withdrawal-induced ACD of IgEκ-sensitized BMMCs by incorporating PpL into the BMMCs and that this internalized PpL may interfere with survival signals via FcεRI.

Iopromide in combination with IFN-γ induces the activation of HMC-1 cells via IL-4 and MCP-1 expression

In this study, we investigated whether IFN-γ has a role in contrast-medium-induced adverse reactions. Iopromide, a nonionic iodinated contrast agent, slightly induced mast cell proliferation and significantly increased the expression of IL-4 and MCP-1 at low doses. The pretreatment of cells with IFN-γ dramatically increased the expression of iopromide-induced IL-4 and MCP-1. An evaluation of mast cell activator secretion revealed that IFN-γ- or IL-4-pretreated HMC-1 cells released dramatically increased levels of β-hexosaminidase and histamine when stimulated with iopromide. We also found that the migration of EoL-1 and THP-1 cells was significantly increased in culture conditions with iopromide-stimulated IL-4-pretreated HMC-1 cells. Taken together, our findings suggest that measuring IFN-γ or IL-4 levels in serum would be helpful as a potential biomarker of adverse patient reactions and that blocking IFN-γ or IL-4 may be crucial in preventing the delayed allergy-like reaction induced by contrast medium in patients with various diseases.

The role of mast cells in cancers

Mast cells are immune cells that accumulate in the tumors and their microenvironment during disease progression. Mast cells are armed with a wide array of receptors that sense environment modifications and, upon stimulation, they are able to secrete several biologically active factors involved in the modulation of tumor growth. For example, mast cells are able to secrete pro-angiogenic and growth factors but also pro- and anti-inflammatory mediators. Recent studies have allowed substantial progress in understanding the role of mast cells in tumorigenesis/disease progression but further studies are necessary to completely elucidate their impact in the pathophysiology of cancer. Here we review observations suggesting that mast cells could modulate tumor growth in humans. We also discuss the drawbacks related to observations from mast cell-deficient mouse models, which could have consequences in the determination of a potential causative relationship between mast cells and cancer. We believe that the understanding of the precise role of mast cells in tumor development and progression will be of critical importance for the development of new targeted therapies in human cancers.

The phylogenetic profile of mast cells

Mast cells (MCs) are tissue-based immune cells that participate to both innate and adaptive immunities as well as to tissue-remodelling processes. Their evolutionary history appears as a fascinating process, whose outline we can only partly reconstruct according to current remnant evidence. MCs have been identified in all vertebrate classes, and a cell population with the overall characteristics of higher vertebrate MCs is identifiable even in the most evolutionarily advanced fish species. In invertebrates, cells related to vertebrate MCs have been recognized in ascidians, a class of urochordates which appeared approximately 500 million years ago. These comprise the granular hemocyte with intermediate characteristics of basophils and MCs and the "test cell" (see below). Both types of cells contain histamine and heparin, and provide defensive functions. The test cell releases tryptase after stimulation with compound 48/80. A leukocyte ancestor operating in the context of a primitive local innate immunity probably represents the MC phylogenetic progenitor. This cell was likely involved in phagocytic and killing activity against pathogens and operated as a general inducer of inflammation. This early type of defensive cell possibly expressed concomitant tissue-reparative functions. With the advent of recombinase activating gene (RAG)-mediated adaptive immunity in the Cambrian era, some 550 million years ago, and the emergence of early vertebrates, MC progenitors differentiated towards a more complex cellular entity. Early MCs probably appeared in the last common ancestor we shared with hagfish, lamprey, and sharks about 450-500 million years ago.

Mast cell plasticity and sphingosine-1-phosphate in immunity, inflammation and cancer

Mast cells (MC) are found in all vascularized tissues at homeostasis and, until recently, were viewed only as effector cells of allergic reactions via degranulation, the canonical process through which MC release mediators, including histamine and pre-formed proteases and cytokines such as TNF. Cross-linking of IgE bound to surface high affinity receptors for IgE (FcɛRI) by a specific antigen (Ag) triggers signaling events leading to degranulation. We and others have reported the concomitant production and export of an influential multifaceted sphingolipid mediator, sphingosine-1-phosphate (S1P) transported outside of MC by ATP-binding cassettes (ABC) transporters, i.e., independently of degranulation. Indeed, the MC horizon expanded by the discovery of their unique ability to selectively release mediators depending upon the stimulus and receptors involved. Aside from degranulation and transporter usage, MC are also endowed with piecemeal degranulation, a slower process during which mediator release occurs with minor morphological changes. The broad spectrum of pro- and anti-inflammatory bioactive substances MC produce and release, their amounts and delivery pace render these cells bona fide fine-tuners of the immune response. In this viewpoint article, MC developmental, phenotypic and functional plasticity, its modulation by microRNAs and its relevance to immunity, inflammation and cancer will be discussed.

Regulation of the tyrosine phosphorylation of Phospholipid Scramblase 1 in mast cells that are stimulated through the high-affinity IgE receptor

Engagement of high-affinity immunoglobulin E receptors (FcεRI) activates two signaling pathways in mast cells. The Lyn pathway leads to recruitment of Syk and to calcium mobilization whereas the Fyn pathway leads to phosphatidylinositol 3-kinase recruitment. Mapping the connections between both pathways remains an important task to be completed. We previously reported that Phospholipid Scramblase 1 (PLSCR1) is phosphorylated on tyrosine after cross-linking FcεRI on RBL-2H3 rat mast cells, amplifies mast cell degranulation, and is associated with both Lyn and Syk tyrosine kinases. Here, analysis of the pathway leading to PLSCR1 tyrosine phosphorylation reveals that it depends on the FcRγ chain. FcεRI aggregation in Fyn-deficient mouse bone marrow-derived mast cells (BMMC) induced a more robust increase in FcεRI-dependent tyrosine phosphorylation of PLSCR1 compared to wild-type cells, whereas PLSCR1 phosphorylation was abolished in Lyn-deficient BMMC. Lyn association with PLSCR1 was not altered in Fyn-deficient BMMC. PLSCR1 phosphorylation was also dependent on the kinase Syk and significantly, but partially, dependent on detectable calcium mobilization. Thus, the Lyn/Syk/calcium axis promotes PLSCR1 phosphorylation in multiple ways. Conversely, the Fyn-dependent pathway negatively regulates it. This study reveals a complex regulation for PLSCR1 tyrosine phosphorylation in FcεRI-activated mast cells and that PLSCR1 sits at a crossroads between Lyn and Fyn pathways.

The role of peptides derived from Spirulina maxima in downregulation of FcεRI-mediated allergic responses

SCOPE

Spirulina has been found suitable for use as a bioactive additive. It is an excellent source of protein that can be hydrolyzed into bioactive peptides. Two peptides LDAVNR (P1) and MMLDF (P2) purified from enzymatic hydrolysate of Spirulina maxima have been reported to be effective against early atherosclerotic responses. In this study, the intracellular mechanism involved in the downregulation of these peptides on high-affinity IgE receptor-mediated allergic reaction was further investigated.

METHODS AND RESULTS

RBL-2H3 mast cells were pretreated with P1 or P2 and sensitized with dinitrophenyl-specific IgE antibody before stimulation of antigen dinitrophenyl-BSA. It was revealed that P1 and P2 exhibited significant inhibition on mast-cell degranulation via decreasing histamine release and intracellular Ca(2+) elevation. The inhibitory activity of P1 was found due to blockade of calcium- and microtubule-dependent signaling pathways. Meanwhile, the inhibition of P2 was involved in suppression of phospholipase Cγ activation and reactive oxygen species production. Moreover, the suppressive effects of P1 and P2 on generation of IL-4 were evidenced via depression of nuclear factor-κB translocation.

CONCLUSION

These findings indicate that peptides P1 and P2 from S. maxima may be promising candidates of antiallergic therapeutics, contributing to development of bioactive food ingredients for amelioration of allergic diseases.

Vesicular trafficking and signaling for cytokine and chemokine secretion in mast cells

Upon activation mast cells (MCs) secrete numerous inflammatory compounds stored in their cytoplasmic secretory granules by a process called anaphylactic degranulation, which is responsible for type I hypersensitivity responses. Prestored mediators include histamine and MC proteases but also some cytokines and growth factors making them available within minutes for a maximal biological effect. Degranulation is followed by the de novo synthesis of lipid mediators such as prostaglandins and leukotrienes as well as a vast array of cytokines, chemokines, and growth factors, which are responsible for late phase inflammatory responses. While lipid mediators diffuse freely out of the cell through lipid bilayers, both anaphylactic degranulation and secretion of cytokines, chemokines, and growth factors depends on highly regulated vesicular trafficking steps that occur along the secretory pathway starting with the translocation of proteins to the endoplasmic reticulum. Vesicular trafficking in MCs also intersects with endocytic routes, notably to form specialized cytoplasmic granules called secretory lysosomes. Some of the mediators like histamine reach granules via specific vesicular monoamine transporters directly from the cytoplasm. In this review, we try to summarize the available data on granule biogenesis and signaling events that coordinate the complex steps that lead to the release of the inflammatory mediators from the various vesicular carriers in MCs.

3-Dimensional Tracking of Non-blinking 'Giant' Quantum Dots in Live Cells

While semiconductor quantum dots (QDs) have been used successfully in numerous single particle tracking (SPT) studies due to their high photoluminescence efficiency, photostability, and broad palette of emission colors, conventional QDs exhibit fluorescence intermittency or 'blinking,' which causes ambiguity in particle trajectory analysis and limits tracking duration. Here, non-blinking 'giant' quantum dots (gQDs) are exploited to study IgE-FcεRI receptor dynamics in live cells using a confocal-based 3D SPT microscope. There is a 7-fold increase in the probability of observing IgE-FcεRI for longer than 1 min using the gQDs compared to commercially available QDs. A time-gated photon-pair correlation analysis is implemented to verify that selected SPT trajectories are definitively from individual gQDs and not aggregates. The increase in tracking duration for the gQDs allows the observation of multiple changes in diffusion rates of individual IgE-FcεRI receptors occurring on long (>1 min) time scales, which are quantified using a time-dependent diffusion coefficient and hidden Markov modeling. Non-blinking gQDs should become an important tool in future live cell 2D and 3D SPT studies, especially in cases where changes in cellular dynamics are occurring on the time scale of several minutes.

Unraveling the significance of IgE autoantibodies in organ-specific autoimmunity: lessons learned from bullous pemphigoid

Bullous pemphigoid (BP), a cutaneous autoimmune blistering disease, has provided a useful model to elucidate a role for IgE in autoimmunity. IgE antibodies specific for the BP180 autoantigen are detected in sera and biopsy samples from the majority of BP patients. In BP biopsies, both IgE and BP180 antigen localize to the surface of mast cells, and incubation of circulating basophils from these patients with BP180 protein triggered degranulation. The in vivo pathogenicity of BP180-specific IgE was confirmed in mouse models, where injection of purified BP IgE into human skin grafted onto nu/nu mice replicated the early phase of lesion development, including mast cell degranulation, eosinophil infiltration and development of urticarial plaques. In addition, IgE antibodies from patient sera bind to BP180 on basal keratinocytes, resulting in internalization of BP180, production of inflammatory cytokines, IL-6 and IL-8, and a decrease in the number of hemidesmosomes at the basement membrane zone. These findings have led to therapeutic trials of the anti-IgE monoclonal antibody omalizumab in BP, resulting in substantial improvement in the patients' disease. Overall, the work in BP provides the first evidence for a pathogenic role for IgE in autoimmunity.

Mast cell tryptase induces eosinophil recruitment in the pleural cavity of mice via proteinase-activated receptor 2

Proteinase-activated receptor (PAR) 2 has been implicated in eosinophil migration. Mast cell (MC) tryptase has been similarly implicated in allergic diseases through the activation of PAR-2, but the role of this receptor in MC tryptase-induced inflammation is not well elucidated. This study aims to investigate the ability of MC tryptase or PAR-2 activating peptide (SLIGRL-NH2) to induce eosinophil recruitment to the pleural cavity of mice. Mast cell tryptase-injected mice were pretreated with PAR-2 antagonist ENMD-1068. Mice injected with SLIGRL-NH2 were pretreated with mast cell tryptase inhibitor APC 366, and eosinophil migration into the pleural cavity and PAR-2 expression was analyzed after 24 or 48 h. SLIGRL-NH2-induced eosinophil recruitment was inhibited by APC 366, and MC tryptase-induced eosinophil recruitment was abolished by ENMD-1068. MC tryptase induced PAR-2 expression on pleural eosinophils. Our results demonstrate a key role for PAR-2 in mediating eosinophil recruitment in MC tryptase-induced pleurisy in mice. The ability of MC tryptase to inducing PAR-2 expression on eosinophils corroborates the relevance of MC tryptase and PAR-2 on modulating eosinophil migration.

Distinct stages of stimulated FcεRI receptor clustering and immobilization are identified through superresolution imaging

Recent advances in fluorescence localization microscopy have made it possible to image chemically fixed and living cells at 20 nm lateral resolution. We apply this methodology to simultaneously record receptor organization and dynamics on the ventral surface of live RBL-2H3 mast cells undergoing antigen-mediated signaling. Cross-linking of IgE bound to FcεRI by multivalent antigen initiates mast cell activation, which leads to inflammatory responses physiologically. We quantify receptor organization and dynamics as cells are stimulated at room temperature (22°C). Within 2 min of antigen addition, receptor diffusion coefficients decrease by an order of magnitude, and single-particle trajectories are confined. Within 5 min of antigen addition, receptors organize into clusters containing ∼100 receptors with average radii of ∼70 nm. By comparing simultaneous measurements of clustering and mobility, we determine that there are two distinct stages of receptor clustering. In the first stage, which precedes stimulated Ca(2+) mobilization, receptors slow dramatically but are not tightly clustered. In the second stage, receptors are tightly packed and confined. We find that stimulation-dependent changes in both receptor clustering and mobility can be reversed by displacing multivalent antigen with monovalent ligands, and that these changes can be modulated through enrichment or reduction in cellular cholesterol levels.

Proteinase-activated receptor 2 blockade impairs CCL11- or allergen-induced eosinophil recruitment in experimental pleurisy

Although proteinase-activated receptor (PAR)-2 has been implicated in inflammatory diseases, its role in regulating eosinophil recruitment in response to chemoattractants remains unclear. Here, we investigated the role of PAR-2 and PAR-2-activating Mast Cell (MC) tryptase on chemokine C-C motif ligand (CCL)11- and antigen-induced eosinophil recruitment to the pleural cavity of BALB/c mice. The PAR-2-activating peptide H-Ser-Leu-Ile-Gly-Arg-Leu-NH2 (SLIGRL-NH2) induced eosinophil recruitment whereas PAR-2 blockade inhibited ovalbumin (OVA)- or CCL11-induced eosinophil recruitment. Moreover, OVA and CCL11 induced PAR-2 expression in pleural leukocytes, and the MC tryptase inhibitor APC 366 ([N-(1-hydroxy-2-napthoyl)-l-arginyl-l-prolinamide hydrochloride]) abolished CCL11-induced eosinophil recruitment. These results suggest a pro inflammatory effect of PAR-2 and support a role for MC tryptase mediating eosinophil migration via PAR-2 signaling. Taken together, our results suggest that PAR-2 activation through endogenous MC tryptase activity could be required, at least partially, to mediate CCL11-induced eosinophil migration.

Rac1 and Rac2 control distinct events during antigen-stimulated mast cell exocytosis

The release of preformed mediators from immune cells is through a process described as exocytosis. In mast cells, exocytosis is regulated by several coordinated intracellular signaling pathways. Here, we investigated the role of the hematopoietic-specific Rho GTPase, Rac2, and the ubiquitously expressed Rac1, in controlling mast cell exocytosis. These two isoforms showed equivalent levels of expression in mouse BMMCs. Although Rac1 and Rac2 share 92% sequence identity, they were not functionally redundant, as Rac2-/- BMMCs were defective in exocytosis, even though Rac1 levels were unaffected. Antigen-stimulated WT mast cells underwent a series of morphological transitions: initial flattening, followed by actin-mediated peripheral membrane ruffling and calcium influx, which preceded exocytosis. Whereas membrane ruffling was unaffected in Rac2-/- BMMCs, calcium influx was decreased significantly. Calcium influx was studied further by examining SOCE. In Rac2-/- BMMCs, the activation of PLCγ1 and calcium release from intracellular stores occurred normally; however, activation of plasma membrane calcium channels was defective, shown by the lack of extracellular calcium influx and a reduction of YFP-STIM1 puncta at the plasma membrane. Additionally, we used the small molecule Rac inhibitor, EHT 1864, to target Rac signaling acutely in WT BMMCs. EHT 1864 blocked exocytosis and membrane ruffling completely in conjunction with exocytosis. Our findings suggest that antigen-stimulated membrane ruffling in mast cells is a Rac1-mediated process, as this persisted in the absence of Rac2. Therefore, we define distinct modes of Rac-regulated mast cell exocytosis: Rac2-mediated calcium influx and Rac1-mediated membrane ruffling.

Escherichia coli exposure inhibits exocytic SNARE-mediated membrane fusion in mast cells

Mast cells orchestrate the allergic response through the release of proinflammatory mediators, which is driven by the fusion of cytoplasmic secretory granules with the plasma membrane. During this process, SNARE proteins including Syntaxin4, SNAP23 and VAMP8 play a key role. Following stimulation, the kinase IKKβ interacts with and phosphorylates the t-SNARE SNAP23. Phosphorylated SNAP23 then associates with Syntaxin4 and the v-SNARE VAMP8 to form a ternary SNARE complex, which drives membrane fusion and mediator release. Interestingly, mast cell degranulation is impaired following exposure to bacteria such as Escherichia coli. However, the molecular mechanism(s) by which this occurs is unknown. Here, we show that E. coli exposure rapidly and additively inhibits degranulation in the RBL-2H3 rat mast cell line. Following co-culture with E. coli, the interaction between IKKβ and SNAP23 is disrupted, resulting in the hypophosphorylation of SNAP23. Subsequent formation of the ternary SNARE complex between SNAP23, Syntaxin4 and VAMP8 is strongly reduced. Collectively, these results demonstrate that E. coli exposure inhibits the formation of VAMP8-containing exocytic SNARE complexes and thus the release of VAMP8-dependent granules by interfering with SNAP23 phosphorylation.

Actin cytoskeleton reorganization correlates with polarization of secretory vesicle and cell morphology in the degranulation of mast cell subtypes in human colon tissues

Mast cells play a central role in the intestinal immune response. To investigate the relationship between degranulation, cell polarization and the reorganization of actin cytoskeleton of mast cells, we used fluorescence or gold labeling methods to identify different mast cell subtypes in human colon. The reorganization of filamentous actin was visualized and then the polarization of secretory vesicles, as well as cell surfaces, was analyzed by fluorescence microscopy and electron microscopy. Our results first showed a diversity of filamentous actin assembly or disassembly within the contacting cell membrane of different mast cell subtypes. The polarization and degranulation of secretory vesicles was not only accompanied with the assembly and disassembly of filamentous actin at the cell periphery, but also with changes of cell surface polarization. Our study provides an insight into the local membranous structures and suggested correlations of cytoskeleton arrangement with the polarization of secretory vesicles and cell surface configuration during mast cell degranulation.

Autoreactive IgE is prevalent in systemic lupus erythematosus and is associated with increased disease activity and nephritis

The presence of autoantibodies in systemic lupus erythematosus, particularly those of the IgG subclass, have long been associated with disease onset and activity. Here we explored the prevalence of autoreactive IgE in SLE and its relevance to disease in French (n = 79) and United States (US) (n = 117) cohorts with a mean age of 41.5 ± 12.7 and 43.6 ± 15.3 years and disease duration of 13.5 ± 8.5 and 16.6 ± 11.9 years, respectively. Our findings show that approximately 65% of all SLE subjects studied produced IgE antibodies to the seven autoantigens tested. This positivity was increased to almost 83% when only those subjects with active disease were considered. SLE subjects who were positive for anti-dsDNA, -Sm, and -SSB/La -specific IgE showed a highly significant association in the levels of these antibodies with disease activity similar to that of the corresponding IgG's. A strong association of IgE autoantibodies with active nephritis was also found in the combined cohort analysis. A test of the predictive value of autoreactive IgE's and IgGs for disease activity (SLE Disease Activity Index (SLEDAI) ≥ 4) revealed that the best predictors were dsDNA-specific IgE and IgG, and that the age of an SLE subject influenced this predictive model. The finding argue that the overall levels of IgE autoantibodies, independently or in combination with IgG autoantibodies, may serve as indicators of active disease.

Molecular editing of cellular responses by the high-affinity receptor for IgE

Cellular responses elicited by cell surface receptors differ according to stimulus strength. We investigated how the high-affinity receptor for immunoglobulin E (IgE) modulates the response of mast cells to a high- or low-affinity stimulus. Both high- and low-affinity stimuli elicited similar receptor phosphorylation; however, differences were observed in receptor cluster size, mobility, distribution, and the cells' effector responses. Low-affinity stimulation increased receptor association with the Src family kinase Fgr and shifted signals from the adapter LAT1 to the related adapter LAT2. LAT1-dependent calcium signals required for mast cell degranulation were dampened, but the role of LAT2 in chemokine production was enhanced, altering immune cell recruitment at the site of inflammation. These findings uncover how receptor discrimination of stimulus strength can be interpreted as distinct in vivo outcomes.

[Role of adaptor molecule Gab2 in mast cell-mediated allergy response]

Mast cells are major players in allergic responses. IgE-dependent activation through Fc epsilon RI leads to degranulation and cytokine production, both of which require Gab2. To clarify how the signals diverge at Gab2, we established Gab2 knock-in mice that express Gab2 mutated at either the PI-3K- or SHP-2-binding sites. Examination of these mutants showed that both binding sites were required for the degranulation and anaphylaxis response, but not for cytokine production or contact hypersensitivity. Furthermore, the PI-3K- but not the SHP-2-binding site was important for granule translocation during degranulation. We also identified a small GTPase, ARF1, as the downstream target of PI-3K that regulates granule translocation. Fc epsilon RI-stimulation induced ARF1 activation, and this response was dependent on Fyn and the PI-3K-binding site of Gab2. ARF1 activity was required for the Fc epsilon RI-mediated granule translocation. These results indicate that Fyn/Gab2/PI-3K/ARF1-mediated signaling is specifically involved in granule translocation and the anaphylaxis response. In this review, I discussed how Gab2 controls biological events especially for mast cell degranulation and allergy response.

Chronic Inflammation: Synergistic Interactions of Recruiting Macrophages (TAMs) and Eosinophils (Eos) with Host Mast Cells (MCs) and Tumorigenesis in CALTs. M-CSF, Suitable Biomarker for Cancer Diagnosis!

Ongoing debates, misunderstandings and controversies on the role of inflammation in cancer have been extremely costly for taxpayers and cancer patients for over four decades. A reason for repeated failed clinical trials (90% ± 5 failure rates) is heavy investment on numerous genetic mutations (molecular false-flags) in the chaotic molecular landscape of site-specific cancers which are used for "targeted" therapies or "personalized" medicine. Recently, unresolved/chronic inflammation was defined as loss of balance between two tightly regulated and biologically opposing arms of acute inflammation ("Yin"-"Yang" or immune surveillance). Chronic inflammation could differentially erode architectural integrities in host immune-privileged or immune-responsive tissues as a common denominator in initiation and progression of nearly all age-associated neurodegenerative and autoimmune diseases and/or cancer. Analyses of data on our "accidental" discoveries in 1980s on models of acute and chronic inflammatory diseases in conjunctival-associated lymphoid tissues (CALTs) demonstrated at least three stages of interactions between resident (host) and recruited immune cells: (a), acute phase; activation of mast cells (MCs), IgE Abs, histamine and prostaglandin synthesis; (b), intermediate phase; down-regulation phenomenon, exhausted/degranulated MCs, heavy eosinophils (Eos) infiltrations into epithelia and goblet cells (GCs), tissue hypertrophy and neovascularization; and (c), chronic phase; induction of lymphoid hyperplasia, activated macrophages (Mfs), increased (irregular size) B and plasma cells, loss of integrity of lymphoid tissue capsular membrane, presence of histiocytes, follicular and germinal center formation, increased ratios of local IgG1/IgG2, epithelial thickening (growth) and/or thinning (necrosis) and angiogenesis. Results are suggestive of first evidence for direct association between inflammation and identifiable phases of immune dysfunction in the direction of tumorigenesis. Activated MFs (TAMs or M2) and Eos that are recruited by tissues (e.g., conjunctiva or perhaps lung airways) whose principal resident immune cells are MCs and lymphocytes are suggested to play crucial synergistic roles in enhancing growth promoting capacities of host toward tumorigenesis. Under oxidative stress, M-CSF may produce signals that are cumulative/synergistic with host mediators (e.g., low levels of histamine), facilitating tumor-directed expression of decoy receptors and immune suppressive factors (e.g., dTNFR, IL-5, IL-10, TGF-b, PGE2). M-CSF, possessing superior sensitivity and specificity, compared with conventional markers (e.g., CA-125, CA-19-9) is potentially a suitable biomarker for cancer diagnosis and technology development. Systematic monitoring of interactions between resident and recruited cells should provide key information not only about early events in loss of immune surveillance, but it would help making informed decisions for balancing the inherent tumoricidal (Yin) and tumorigenic (Yang) properties of immune system and effective preventive and therapeutic approaches and accurate risk assessment toward improvement of public health.

Application of optical imaging technology on the in vitro assessment of mast cell degranulation

A new approach for anaphylactoid reaction assessment is proposed and it could be applied in allergen screening in drug safety and allergen detection in clinic.

Munc18-2 and syntaxin 3 control distinct essential steps in mast cell degranulation

Mast cell degranulation requires N-ethylmaleimide-sensitive factor attachment protein receptors (SNARE) and mammalian uncoordinated18 (Munc18) fusion accessory proteins for membrane fusion. However, it is still unknown how their interaction supports fusion. In this study, we found that small interfering RNA-mediated silencing of the isoform Munc18-2 in mast cells inhibits cytoplasmic secretory granule (SG) release but not CCL2 chemokine secretion. Silencing of its SNARE-binding partner syntaxin 3 (STX3) also markedly inhibited degranulation, whereas combined knockdown produced an additive inhibitory effect. Strikingly, while Munc18-2 silencing impaired SG translocation, silencing of STX3 inhibited fusion, demonstrating unique roles of each protein. Immunogold studies showed that both Munc18-2 and STX3 are located on the granule surface, but also within the granule matrix and in small nocodazole-sensitive clusters of the cytoskeletal meshwork surrounding SG. After stimulation, clusters containing both effectors were detected at fusion sites. In resting cells, Munc18-2, but not STX3, interacted with tubulin. This interaction was sensitive to nocodazole treatment and decreased after stimulation. Our results indicate that Munc18-2 dynamically couples the membrane fusion machinery to the microtubule cytoskeleton and demonstrate that Munc18-2 and STX3 perform distinct, but complementary, functions to support, respectively, SG translocation and membrane fusion in mast cells.

Inhibitory effects of a cationic liposome on allergic reaction mediated by mast cell activation

Several studies have shown that cationic liposomes exert immunomodulatory effects with low immunogenicity and toxicity, and offer advantages such as easy preparation and targeting. Cationic liposomes not only transport DNA to immune cells but also enhance the function of antigen presenting cells such as dendritic cells and macrophages. Here, we investigated the effect of a particular cationic liposome on mast cell function during allergic reaction. We found that the cationic liposomes bound to the mast cell surface suppressed degranulation induced by cross-linking of high affinity immunoglobulin E receptors in a time- and dose-dependent manner. The suppression of degranulation was mediated by impairment of the sustained level of intracellular Ca(2+) concentration ([Ca(2+)]i) derived from the inhibition of store-operated Ca(2+) entry. The decrease in sustained elevation of [Ca(2+)]i led to the suppression of phosphorylation of soluble N-ethylmaleimide-sensitive factor attachment protein receptor proteins such as SNAP-23, syntaxin-4, which are necessary for membrane fusion between secretory granules and the plasma membrane during degranulation. Furthermore, the cationic liposomes suppressed vascular permeability elevation induced by mast cell activation in mice. These results showed that cationic liposomes possess the novel property of inhibiting mast cell activation, suggesting the possibility of developing cationic liposomes as anti-allergic effectors.

Food allergen--induced mast cell degranulation is dependent on PI3K-mediated reactive oxygen species production and upregulation of store-operated calcium channel subunits

The importance of Ca(2+) influx via store-operated calcium channels (SOCs) leading to mast cell degranulation is well known in allergic disease. However, the underlying mechanisms are not fully understood. With food-allergic rat model, the morphology of degranulated mast cell was analysed by toluidine blue stain and electron microscope. Ca(2+) influx via SOCs was checked by Ca(2+) imaging confocal microscope. Furthermore, the mRNA and protein expression of SOCs subunits were investigated using qPCR and Western blot. We found that ovalbumin (OVA) challenge significantly increased the levels of Th2 cytokines and OVA-specific IgE in allergic animals. Parallel to mast cell activation, the levels of histamine in serum and supernatant of rat peritoneal lavage solution were remarkably increased after OVA treatment. Moreover, the Ca(2+) entry through SOCs evoked by thapsigargin was increased in OVA-challenged group. The mRNA and protein expressions of SOC subunits, stromal interaction molecule 1 (STIM1) and Orail (calcium-release-activated calcium channel protein 1), were dramatically elevated under food-allergic condition. Administration of Ebselen, a scavenger of reactive oxygen species (ROS), significantly attenuated OVA sensitization-induced intracellular Ca(2+) rise and upregulation of SOCs subunit expressions. Intriguingly, pretreatment with PI3K-specific inhibitor (Wortmannin) partially abolished the production of ROS and subsequent elevation of SOCs activity and their subunit expressions. Taken together, these results imply that enhancement of SOC-mediated Ca(2+) influx induces mast cell activation, contributing to the pathogenesis of OVA-stimulated food allergy. PI3K-dependent ROS generation involves in modulating the activity of SOCs by increasing the expressions of their subunit.

Morphine prevents lipopolysaccharide-induced TNF secretion in mast cells blocking IκB kinase activation and SNAP-23 phosphorylation: correlation with the formation of a β-arrestin/TRAF6 complex

We have previously shown that morphine pretreatment inhibits mast cell-dependent TNF production after LPS injection in the murine peritoneal cavity. In this study, we used bone marrow-derived mast cells (BMMCs) to investigate the molecular mechanisms of that inhibition. We found that morphine prevented LPS-induced TNF secretion in these cells. The observed inhibition was not due to morphine-induced TLR4 internalization and it was related to the blockage of preformed TNF secretion. LPS-induced TNF exocytosis in BMMCs was dependent on tetanus toxin-insensitive vesicle-associated membrane proteins and calcium mobilization, as well as PI3K, MAPK, and IκB kinase (IKK) activation. TNF secretion was also associated to the phosphorylation of synaptosomal-associated protein 23 (SNAP-23), which was found forming a complex with IKK in LPS-activated BMMCs. Morphine pretreatment prevented TLR4-dependent ERK and IKK phosphorylation. Analyzing the signaling events upstream of IKK activation, we found diminished TGF-β-activated kinase 1 (TAK1) phosphorylation and TNFR-associated factor (TRAF) 6 ubiquitination in BMMCs pretreated with morphine and stimulated with LPS. Morphine pretreatment provoked a marked increase in the formation of a molecular complex composed of TRAF6 and β-arrestin-2. Naloxone and a combination of μ and δ opioid receptor antagonists prevented morphine inhibitory actions. In conclusion, our results show that activation of μ and δ opioid receptors with morphine suppresses TLR4-induced TNF release in mast cells, preventing the IKK-dependent phosphorylation of SNAP-23, which is necessary for TNF exocytosis, and this inhibition correlates with the formation of a β-arrestin-2/TRAF6 complex. To our knowledge, these findings constitute the first evidence of molecular crosstalk between opioid receptors and the TLR4 signal transduction system in mast cells.

Polydatin attenuated food allergy via store-operated calcium channels in mast cell

AIM: To investigate the effect of polydatin (PD), a resveratrol glucoside, on mast cell degranulation and anti-allergic activity.

METHODS: After the rats were orally sensitized with ovalbumin (OVA) for 48 d and underwent PD treatment for 4 d, all the rats were stimulated by 100 mg/mL OVA for 24 h and then sacrificed for the following experiments. The small intestines from all the groups were prepared for morphology examination by hematoxylin and eosin staining. We also used a smooth muscle organ bath to evaluate the motility of the small intestines. The OVA-specific immunoglobulin E (IgE) production and interleukin-4 (IL-4) levels in serum or supernatant of intestinal mucosa homogenates were analyzed by enzyme-linked immunosorbent assay (ELISA). Using toluidine blue stain, the activation and degranulation of isolated rat peritoneal mast cells (RPMCs) were analyzed. Release of histamine from RPMCs was measured by ELISA, and regulation of PD on intracellular Ca²⁺ mobilization was investigated by probing intracellular Ca²⁺ with fluo-4 fluorescent dye, with the signal recorded and analyzed.

RESULTS: We found that intragastric treatment with PD significantly reduced loss of mucosal barrier integrity in the small intestine. However, OVA-sensitization caused significant hyperactivity in the small intestine of allergic rats, which was attenuated by PD administration by 42% (1.26 ± 0.13 g vs OVA 2.18 ± 0.21 g, P < 0.01). PD therapy also inhibited IgE production (3.95 ± 0.53 ng/mL vs OVA 4.53 ± 0.52 ng/mL, P < 0.05) by suppressing the secretion of Th2-type cytokine, IL-4, by 34% (38.58 ± 4.41 pg/mL vs OVA 58.15 ± 6.24 pg/mL, P < 0.01). The ratio of degranulated mast cells, as indicated by vehicles (at least five) around the cells, dramatically increased in the OVA group by 5.5 fold (63.50% ± 15.51% vs phosphate-buffered saline 11.15% ± 8.26%, P < 0.001) and fell by 65% after PD treatment (21.95% ± 4.37% vs OVA 63.50% ± 15.51%, P < 0.001). PD mediated attenuation of mast cell degranulation was further confirmed by decreased histamine levels in both serum (5.98 ± 0.17 vs OVA 6.67 ± 0.12, P < 0.05) and intestinal mucosa homogenates (5.83 ± 0.91 vs OVA 7.35 ± 0.97, P < 0.05). Furthermore, we demonstrated that administration with PD significantly decreased mast cell degranulation due to reduced Ca²⁺ influx through store-operated calcium channels (SOCs) (2.35 ± 0.39 vs OVA 3.51 ± 0.38, P < 0.01).

CONCLUSION: Taken together, our data indicate that PD stabilizes mast cells by suppressing intracellular Ca²⁺ mobilization, mainly through inhibiting Ca²⁺ entry via SOCs, thus exerting a protective role against OVA-sensitized food allergy.

Toxoplasma gondii inhibits mast cell degranulation by suppressing phospholipase Cγ-mediated Ca(2+) mobilization

Toxoplasma gondii is well-known to subvert normal immune responses, however, mechanisms are incompletely understood. In particular, its capacity to alter receptor-activated Ca²⁺-mediated signaling processes has not been well-characterized. In initial experiments, we found evidence that T. gondii infection inhibits Ca²⁺ responses to fMetLeuPhe in murine macrophages. To further characterize the mechanism of inhibition of Ca²⁺ mobilization by T. gondii, we used the well-studied RBL mast cell model to probe the capacity of T. gondii to modulate IgE receptor-activated signaling within the first hour of infection. Ca²⁺ mobilization that occurs via IgE/FcεRI signaling leads to granule exocytosis in mast cells. We found that T. gondii inhibits antigen-stimulated degranulation in infected cells in a strain-independent manner. Under these conditions, we found that cytoplasmic Ca²⁺ mobilization, particularly antigen-mediated Ca²⁺ release from intracellular stores, is significantly reduced. Furthermore, stimulation-dependent activation of Syk kinase leading to tyrosine phosphorylation and activation of phospholipase Cγ is inhibited by infection. Therefore, we conclude that inhibitory effects of infection are likely due to parasite-mediated inhibition of the tyrosine kinase signaling cascade that results in reduced hydrolysis of phosphatidylinositol 4,5-bisphosphate. Interestingly, inhibition of IgE/FcεRI signaling persists when tachyzoite invasion is arrested via cytochalasin D treatment, suggesting inhibition is mediated by a parasite-derived factor secreted into the cells during the invasion process. Our study provides direct evidence that immune subversion by T. gondii is initiated concurrently with invasion.

Pathogenic role of mast cells in experimental eosinophilic esophagitis

Eosinophilic esophagitis (EoE) is a chronic allergic disease characterized by esophageal intraepithelial eosinophils, extracellular eosinophil granule deposition, induced mast cell accumulation, and epithelial cell hyperplasia. However, the processes involved in the development of a number of these characteristics are largely unknown. Herein, we tested the hypothesis whether induced mast cell accumulation in the esophagus has a role in promoting EoE pathogenesis. Accordingly, we induced experimental EoE in wild-type mice, mast cell-deficient WWv mice, and mast cell-reconstituted WWv mice. We report that esophageal mast cell numbers increase in parallel with eosinophils in a dose- and time-dependent manner following the induction of allergen-induced EoE. The induced mast cells are localized in the esophageal lamina propria and muscular mucosa but have no influence on promoting esophageal eosinophilia. The 5'-bromodeoxyuridine (BrdU) incorporation analysis indicated that mast cells have a significant role in muscle cell hyperplasia and hypertrophy. In addition, the wild-type and mast cell-reconstituted WWv mice showed a comparable number of BrdU⁺ cells in the esophageal muscular mucosa following allergen-induced EoE. In conclusion, we provide for the first time direct evidence that mast cell promotes muscle cell hyperplasia and hypertrophy and may have a significant role in promoting esophageal functional abnormalities in EoE.

A truncated splice-variant of the FcεRIβ receptor subunit is critical for microtubule formation and degranulation in mast cells

Human linkage analyses have implicated the MS4A2-containing gene locus (encoding FcεRIβ) as a candidate for allergy susceptibility. We have identified a truncation of FcεRIβ (t-FcεRIβ) in humans that contains a putative calmodulin-binding domain and thus, we sought to identify the role of this variant in mast cell function. We determined that t-FcεRIβ is critical for microtubule formation and degranulation and that it may perform this function by trafficking adaptor molecules and kinases to the pericentrosomal and Golgi region in response to Ca2+ signals. Mutagenesis studies suggest that calmodulin binding to t-FcεRIβ in the presence of Ca2+ could be critical for t-FcεRIβ function. In addition, gene targeting of t-FcεRIβ attenuated microtubule formation, degranulation, and IL-8 production downstream of Ca2+ signals. Therefore, t-FcεRIβ mediates Ca2+ -dependent microtubule formation, which promotes degranulation and cytokine release. Because t-FcεRIβ has this critical function, it represents a therapeutic target for the downregulation of allergic inflammation.

Negative control of mast cell degranulation and the anaphylactic response by the phosphatase lipin1

Mast cells play a critical role in the pathogenesis of allergic diseases; however, how mast cell function is regulated is still not well understood. Both phosphatidic acid (PA) and diacylglycerol (DAG) are important secondary messengers involved in mast cell activ-ation. Lipin1 is a phosphatidate phosphatase that hydrolyzes PA to produce DAG, but the role of lipin1 in mast cell function has been thus far unknown. Here we show that lipin1 is an important and selective inhibitor of mast cell degranulation. Lipin1 deficiency enhanced FcεRI-mediated β-hexosaminidase and prostaglandin D2 release from mast cells in vitro and exacerbated the passive systemic anaphylaxis reaction in vivo. Lipin1 deficiency, however, did not exert obvious effects on IL-6 or TNF-α production following FcεRI engagement. FcεRI-induced PKC and SNAP-23 phosphorylation were augmented in the lipin1-deficient mast cells. Moreover, inhibition of PKC activity reduced SNAP-23 phosphorylation and mast cell degranulation in lipin1-deficient mast cells. Together, our findings suggest that lipin1 may negatively control mast cell degranulation and the anaphylactic response through inhibiting the PKC-SNAP-23 pathway.

SNARE complex-mediated degranulation in mast cells

Mast cell function and dysregulation is important in the development and progression of allergic and autoimmune disease. Identifying novel proteins involved in mast cell function and disease progression is the first step in the design of new therapeutic strategies. Soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) are a family of proteins demonstrated to mediate the transport and fusion of secretory vesicles to the membrane in mast cells, leading to the subsequent release of the vesicle cargo through an exocytotic mechanism. The functional role[s] of specific SNARE family member complexes in mast cell degranulation has not been fully elucidated. Here, we review recent and historical data on the expression, formation and localization of various SNARE proteins and their complexes in murine and human mast cells. We summarize the functional data identifying the key SNARE family members that appear to participate in mast cell degranulation. Furthermore, we discuss the utilization of RNA interference (RNAi) methods to validate SNARE function and the use of siRNA as a therapeutic approach to the treatment of inflammatory disease. These studies provide an overview of the specific SNARE proteins and complexes that serve as novel targets for the development of new therapies to treat allergic and autoimmune disease.

Serum- and glucocorticoid-inducible kinase SGK1 regulates reorganization of actin cytoskeleton in mast cells upon degranulation

Aggregation of the high-affinity IgE receptor (FcεRI) on mast cells (MCs) causes MC degranulation, a process that involves cortical F-actin disassembly. Actin depolymerization may be triggered by increase of cytosolic Ca(2+). Entry of Ca(2+) through the Ca(2+) release-activated Ca(2+) (CRAC) channels is under powerful regulation by the serum- and glucocorticoid-inducible kinase SGK1. Moreover, FcεRI-dependent degranulation is decreased in SGK1-deficient (sgk1(-/-)) MCs. The present study addressed whether SGK1 is required for actin cytoskeleton rearrangement in MCs and whether modulation of actin architecture could underlie decreased degranulation of sgk1(-/-) MCs. Confirming previous results, release of β-hexosaminidase reflecting FcεRI-dependent degranulation was impaired in sgk1(-/-) MCs compared with sgk1(+/+) MCs. When CRAC channels were inhibited by 2-aminoethoxydiphenyl borate (2-APB; 50 μM), MC degranulation was strongly decreased in both sgk1(+/+) and sgk1(-/-) MCs and the difference between genotypes was abolished. Moreover, degranulation was impaired by actin-stabilizing (phallacidin) and enhanced by actin-disrupting (cytochalasin B) agents to a similar extent in sgk1(+/+) MCs and sgk1(-/-) MCs, implying a regulatory role of actin reorganization in this event. In line with this, measurements of monomeric (G) and filamentous (F) actin content by FACS analysis and Western blotting of detergent-soluble and -insoluble cell fractions indicated an increase of the G/F-actin ratio in sgk1(+/+) MCs but not in sgk1(-/-) MCs upon FcεRI ligation, an observation reflecting actin depolymerization. In sgk1(+/+) MCs, FcεRI-induced actin depolymerization was abolished by 2-APB. The observed actin reorganization was confirmed by confocal laser microscopic analysis. Our observations uncover SGK1-dependent Ca(2+) entry in mast cells as a novel mechanism regulating actin cytoskeleton.

Novel PKCα-mediated phosphorylation site(s) on cofilin and their potential role in terminating histamine release

PKCα phosphorylates cofilin at Ser-23 and/or Ser-24 during degranulation, and the novel phosphorylation contributes to F-actin remodeling by regulating the ability of cofilin to bind to 14-3-3ζ and to depolymerize and/or sever F-actin. This highly regulated mechanism is necessary for the proper termination of degranulation.

Using specific inhibitors, kinase-negative mutants, and small interfering RNA against protein kinase Cα (PKCα) or PKCβI, we find that PKCβI positively regulates degranulation in rat basophilic leukemia–2H3 cells, whereas PKCα negatively regulates degranulation. Mass spectrometric and mutagenic analyses reveal that PKCα phosphorylates cofilin at Ser-23 and/or Ser-24 during degranulation. Overexpression of a nonphosphorylatable form (S23,24A), but not that of a mutant-mimicking phosphorylated form (S23,24E), increases degranulation. Furthermore, the S23,24A mutant binds to F-actin and retains its depolymerizing and/or cleavage activity; conversely, the S23,24E mutant is unable to sever actin filaments, resulting in F-actin polymerization. In addition, the S23,24E mutant preferentially binds to the 14-3-3ζ protein. Fluorescence-activated cell sorting analysis with fluorescein isothiocyanate–phalloidin and simultaneous observation of degranulation, PKC translocation, and actin polymerization reveals that during degranulation, actin polymerization is dependent on PKCα activity. These results indicate that a novel PKCα-mediated phosphorylation event regulates cofilin by inhibiting its ability to depolymerize F-actin and bind to 14-3-3ζ, thereby promoting F-actin polymerization, which is necessary for cessation of degranulation.