G protein-coupled receptors and the modification of FcepsilonRI-mediated mast cell activation

Inhibitory effects of benzodiazepines on the adenosine A(2B) receptor mediated secretion of interleukin-8 in human mast cells

The activation of adenosine A(2B) receptors in human mast cells causes pro-inflammatory responses such as the secretion of interleukin-8. There is evidence for an inhibitory effect of benzodiazepines on mast cell mediated symptoms in patients with systemic mast cell activation disease. Therefore, we investigated the effects of benzodiazepines on adenosine A(2B) receptor mediated interleukin-8 production in human mast cell leukaemia (HMC1) cells by an enzyme linked immunosorbent assay. The adenosine analogue N-ethylcarboxamidoadenosine (NECA, 0.3-3 μM) increased interleukin-8 production about 5-fold above baseline. This effect was attenuated by the adenosine A(2B) receptor antagonist MRS1754 (N-(4-cyanophenyl)-2-{4-(2,3,6,7-tetrahydro-2,6-dioxo-1,3-dipropyl-1H-purin-8-yl)phenoxy}-acetamide) 1 μM. In addition, diazepam, 4'-chlorodiazepam and flunitrazepam (1-30 μM) markedly reduced NECA-induced interleukin-8 production in that order of potency, whereas clonazepam showed only a modest inhibition. The inhibitory effect of diazepam was not altered by flumazenil 10 μM or PK11195 (1-(2-chlorophenyl)-N-methyl-N-(1-methylpropyl)-3-isoquinolinecarboxamide) 10 μM. Diazepam attenuated the NECA-induced expression of mRNA encoding for interleukin-8. Moreover, diazepam and flunitrazepam reduced the increasing effects of NECA on cAMP-response element- and nuclear factor of activated t-cells-driven luciferase reporter gene activities in HMC1 cells. Neither diazepam nor flunitrazepam affected NECA-induced increases in cellular cAMP levels in CHO Flp-In cells stably expressing recombinant human adenosine A(2B) receptors, excluding a direct action of benzodiazepines on human adenosine A(2B) receptors. In conclusion, this is the first study showing an inhibitory action of benzodiazepines on adenosine A(2B) receptor mediated interleukin-8 production in human mast (HMC1) cells. The rank order of potency indicates the involvement of an atypical benzodiazepine binding site.

The role of P2Y(14) and other P2Y receptors in degranulation of human LAD2 mast cells

Mast cell degranulation affects many conditions, e.g., asthma and urticaria. We explored the potential role of the P2Y(14) receptor (P2Y(14)R) and other P2Y subtypes in degranulation of human LAD2 mast cells. All eight P2YRs were expressed at variable levels in LAD2 cells (quantitative real-time RT-PCR). Gene expression levels of ADP receptors, P2Y(1)R, P2Y(12)R, and P2Y(13)R, were similar, and P2Y(11)R and P2Y(4)R were highly expressed at 5.8- and 3.8-fold of P2Y(1)R, respectively. Least expressed P2Y(2)R was 40-fold lower than P2Y(1)R, and P2Y(6)R and P2Y(14)R were ≤50 % of P2Y(1)R. None of the native P2YR agonists alone induced β-hexosaminidase (β-Hex) release, but some nucleotides significantly enhanced β-Hex release induced by C3a or antigen, with a rank efficacy order of ATP > UDPG ≥ ADP >> UDP, UTP. Although P2Y(11)R and P2Y(4)R are highly expressed, they did not seem to play a major role in degranulation as neither P2Y(4)R agonist UTP nor P2Y(11)R agonists ATPγS and NF546 had a substantial effect. P2Y(1)R-selective agonist MRS2365 enhanced degranulation, but ~1,000-fold weaker compared to its P2Y(1)R potency, and the effect of P2Y(6)R agonist 3-phenacyl-UDP was negligible. The enhancement by ADP and ATP appears mediated via multiple receptors. Both UDPG and a synthetic agonist of the P2Y(14)R, MRS2690, enhanced C3a-induced β-Hex release, which was inhibited by a P2Y(14)R antagonist, specific P2Y(14)R siRNA and pertussis toxin, suggesting a role of P2Y(14)R activation in promoting human mast cell degranulation.

Mast cell chemotaxis - chemoattractants and signaling pathways

Migration of mast cells is essential for their recruitment within target tissues where they play an important role in innate and adaptive immune responses. These processes rely on the ability of mast cells to recognize appropriate chemotactic stimuli and react to them by a chemotactic response. Another level of intercellular communication is attained by production of chemoattractants by activated mast cells, which results in accumulation of mast cells and other hematopoietic cells at the sites of inflammation. Mast cells express numerous surface receptors for various ligands with properties of potent chemoattractants. They include the stem cell factor (SCF) recognized by c-Kit, antigen, which binds to immunoglobulin E (IgE) anchored to the high affinity IgE receptor (FcεRI), highly cytokinergic (HC) IgE recognized by FcεRI, lipid mediator sphingosine-1-phosphate (S1P), which binds to G protein-coupled receptors (GPCRs). Other large groups of chemoattractants are eicosanoids [prostaglandin E₂ and D₂, leukotriene (LT) B₄, LTD₄, and LTC₄, and others] and chemokines (CC, CXC, C, and CX3C), which also bind to various GPCRs. Further noteworthy chemoattractants are isoforms of transforming growth factor (TGF) β1–3, which are sensitively recognized by TGF-β serine/threonine type I and II β receptors, adenosine, C1q, C3a, and C5a components of the complement, 5-hydroxytryptamine, neuroendocrine peptide catestatin, tumor necrosis factor-α, and others. Here we discuss the major types of chemoattractants recognized by mast cells, their target receptors, as well as signaling pathways they utilize. We also briefly deal with methods used for studies of mast cell chemotaxis and with ways of how these studies profited from the results obtained in other cellular systems.

G Protein βγ-subunit signaling mediates airway hyperresponsiveness and inflammation in allergic asthma

Since the Gβγ subunit of Gi protein has been importantly implicated in regulating immune and inflammatory responses, this study investigated the potential role and mechanism of action of Gβγ signaling in regulating the induction of airway hyperresponsiveness (AHR) in a rabbit model of allergic asthma. Relative to non-sensitized animals, OVA-sensitized rabbits challenged with inhaled OVA exhibited AHR, lung inflammation, elevated BAL levels of IL-13, and increased airway phosphodiesterase-4 (PDE4) activity. These proasthmatic responses were suppressed by pretreatment with an inhaled membrane-permeable anti-Gβγ blocking peptide, similar to the suppressive effect of glucocorticoid pretreatment. Extended mechanistic studies demonstrated that: 1) corresponding proasthmatic changes in contractility exhibited in isolated airway smooth muscle (ASM) sensitized with serum from OVA-sensitized+challenged rabbits or IL-13 were also Gβγ-dependent and mediated by MAPK-upregulated PDE4 activity; and 2) the latter was attributed to Gβγ-induced direct stimulation of the non-receptor tyrosine kinase, c-Src, resulting in downstream activation of ERK1/2 and its consequent transcriptional upregulation of PDE4. Collectively, these data are the first to identify that a mechanism involving Gβγ-induced direct activation of c-Src, leading to ERK1/2-mediated upregulation of PDE4 activity, plays a decisive role in regulating the induction of AHR and inflammation in a rabbit model of allergic airway disease.

CCL2 promotes P2X4 receptor trafficking to the cell surface of microglia

P2X4 receptors (P2X4Rs), a subtype of the purinergic P2X family, play important roles in regulating neuronal and glial functions in the nervous system. We have previously shown that the expression of P2X4Rs is upregulated in activated microglia after peripheral nerve injury and that activation of the receptors by extracellular ATP is crucial for maintaining nerve injury-induced pain hypersensitivity. However, the regulation of P2X4R expression on the cell surface of microglia is poorly understood. Here, we identify the CC chemokine receptor CCR2 as a regulator of P2X4R trafficking to the cell surface of microglia. In a quantitative cell surface biotinylation assay, we found that applying CCL2 or CCL12, endogenous ligands for CCR2, to primary cultured microglial cells, increased the levels of P2X4R protein on the cell surface without changing total cellular expression. This effect of CCL2 was prevented by an antagonist of CCR2. Time-lapse imaging of green fluorescent protein (GFP)-tagged P2X4R in living microglial cells showed that CCL2 stimulation increased the movement of P2X4R-GFP particles. The subcellular localization of P2X4R immunofluorescence was restricted to lysosomes around the perinuclear region. Notably, CCL2 changed the distribution of lysosomes with P2X4R immunofluorescence within microglial cells and induced release of the lysosomal enzyme β-hexosaminidase, indicating lysosomal exocytosis. Moreover, CCL2-stimulated microglia enhanced Akt phosphorylation by ATP applied extracellularly, a P2X4R-mediated response. These results indicate that CCL2 promotes expression of P2X4R protein on the cell surface of microglia through exocytosis of P2X4R-containing lysosomes, which may be a possible mechanism for pain hypersensitivity after nerve injury.

Regulators of Ca(2+) signaling in mast cells: potential targets for treatment of mast cell-related diseases?

A calcium signal is essential for degranulation, generation of eicosanoids and optimal production of cytokines in mast cells in response to antigen and other stimulants. The signal is initiated by phospholipase C-mediated production of inositol1,4,5-trisphosphate resulting in release of stored Ca(2+) from the endoplasmic reticulum (ER) and Golgi. Depletion of these stores activates influx of extracellular Ca(2+), usually referred to as store-operated calcium entry (SOCE), through the interaction of the Ca(2+)-sensor, stromal interacting molecule-1 (STIM1 ), in ER with Orai1(CRACM1) and transient receptor potential canonical (TRPC) channel proteins in the plasma membrane (PM). This interaction is enabled by microtubular-directed reorganization of ER to form ER/PM contact points or "punctae" in which STIM1 and channel proteins colocalize. The ensuing influx of Ca(2+) replenishes Ca(2+) stores and sustains elevated levels of cytosolic Ca(2+) ions-the obligatory signal for mast-cell activation. In addition, the signal can acquire spatial and dynamic characteristics (e.g., calcium puffs, waves, oscillations) that encode signals for specific functional outputs. This is achieved by coordinated regulation of Ca(2+) fluxes through ATP-dependent Ca(2+)-pumps and ion exchangers in mitochondria, ER and PM. As discussed in this chapter, studies in mast cells revealed much about the mechanisms described above but little about allergic and autoimmune diseases although studies in other types of cells have exposed genetic defects that lead to aberrant calcium signaling in immune diseases. Pharmacologic agents that inhibit or activate the regulatory components of calcium signaling in mast cells are also discussed along with the prospects for development of novel SOCE inhibitors that may prove beneficial in the treatment inflammatory mast-cell related diseases.

Distinct PGE2-responder and non-responder phenotypes in human mast cell populations: "all or nothing" enhancement of antigen-dependent mediator release

Reports indicate that prostaglandin (PG)E(2) markedly enhances antigen-mediated degranulation in mouse bone marrow-derived mast cells (BMMCs) but not in human mast cells (HuMCs). We have examined the underlying mechanism(s) for this disparity in HuMCs derived from the peripheral blood of multiple donors in addition to mouse BMMCs. HuMCs from half of these donors failed to respond to PGE(2) and the PGE(2) EP3 receptor agonist, sulprostone. However, HuMCs from the remaining donors and the LAD2 human MC line responded to PGE(2) and sulprostone with marked enhancement of antigen-mediated degranulation and IL-8 production in a similar manner to that observed in mouse BMMCs. The EP2 agonist, butaprost, failed to modulate antigen-mediated responses in any type of MCs. These distinct phenotypes could not be explained by differences in EP2 or EP3 expression nor by differences in the ability of PGE(2) to elevate levels of cAMP, a signal recognized to down-regulate mast cell activation. Moreover, both responder and non-responder HuMC populations exhibited similar activation of phosphatidylinositol 3-kinase, and MAP kinases. However, translocation of PLCγ(1) to the cell membrane and the associated calcium signal were enhanced only in the responder HuMC population indicating that the link between EP3 and PLCγ is impaired in the non-responder HuMCs.

CONCLUSIONS

These data provide a cautionary note for the translating of observations in the mouse to human mast cell-dependent disorders, but may also provide a basis for examining the effects of co-activating receptors in patients susceptible to allergic conditions.

Tyrosine kinase inhibitors in the treatment of systemic mastocytosis

Systemic mastocytosis (SM) is a heterogeneous disease, vast majority of these patients have a gain of function mutation in the gene encoding the tyrosine kinase KIT (KIT(D816V)). A small subset of SM patients with KIT(D816V) mutation require cytoreductive therapy. In these patients, tyrosine kinase inhibitors (TKIs) have been actively investigated over the last decade because of codon 816 KIT mutations causing constitutive activation of tyrosine kinase activity of the molecule. The main question has been whether the success story with imatinib in chronic myeloid leukemia (CML), another disease associated with a constitutively active tyrosine kinase, could be mimicked in mastocytosis. However, the results from various TKIs in SM with KIT(D816V) mutation have been disappointing to date. Only a few of the TKIs sufficiently block KIT(D816V) activity and have shown promising clinical results. The data from these studies indicate that, apart from KIT(D816V), other kinase targets and target pathways may play a role in disease evolution and progression, especially in patients with SM with an associated clonal hematological non-mast cell lineage disease (SM-AHNMD). Imatinib is effective in patients with increased mast cells and eosinophils associated with FIP1L1/PDGFRA+ (e.g., myeloid neoplasm with eosinophilia and rearrangement of PDGFRA) or rare patients with SM associated with KIT mutations outside of exon 17. This review will focus on the KIT receptor, KIT mutations, and the effects of the mutations in SM. The preclinical and clinical activities of FDA approved TKIs (for CML) as well as novel TKIs in SM will be evaluated.

Distinct and shared roles of β-arrestin-1 and β-arrestin-2 on the regulation of C3a receptor signaling in human mast cells

Background

The complement component C3a induces degranulation in human mast cells via the activation of cell surface G protein coupled receptors (GPCR; C3aR). For most GPCRs, agonist-induced receptor phosphorylation leads to the recruitment of β-arrestin-1/β-arrestin-2; resulting in receptor desensitization and internalization. Activation of GPCRs also leads to ERK1/2 phosphorylation via two temporally distinct pathways; an early response that reflects G protein activation and a delayed response that is G protein independent but requires β-arrestins. The role of β-arrestins on C3aR activation/regulation in human mast cells, however, remains unknown.

Methodology/Principal Findings

We utilized lentivirus short hairpin (sh)RNA to stably knockdown the expression of β-arrestin-1 and β-arrrestin-2 in human mast cell lines, HMC-1 and LAD2 that endogenously expresses C3aR. Silencing β-arrestin-2 attenuated C3aR desensitization, blocked agonist-induced receptor internalization and rendered the cells responsive to C3a for enhanced NF-κB activity as well as chemokine generation. By contrast, silencing β-arrestin-1 had no effect on these responses but resulted in a significant decrease in C3a-induced mast cell degranulation. In shRNA control cells, C3a caused a transient ERK1/2 phosphorylation, which peaked at 5 min but disappeared by 10 min. Knockdown of β-arrestin-1, β-arrestin-2 or both enhanced the early response to C3a and rendered the cells responsive for ERK1/2 phosphorylation at later time points (10–30 min). Treatment of cells with pertussis toxin almost completely blocked both early and delayed C3a-induced ERK1/2 phosphorylation in β-arrestin1/2 knockdown cells.

Conclusion/Significance

This study demonstrates distinct roles for β-arrestins-1 and β-arrestins-2 on C3aR desensitization, internalization, degranulation, NF-κB activation and chemokine generation in human mast cells. It also shows that both β-arrestin-1 and β-arrestin-2 play a novel and shared role in inhibiting G protein-dependent ERK1/2 phosphorylation. These findings reveal a new level of complexity for C3aR regulation by β-arrestins in human mast cells.

Generation, isolation, and maintenance of human mast cells and mast cell lines derived from peripheral blood or cord blood

Antigen-mediated mast cell activation is a pivotal step in the initiation of allergic disorders including anaphylaxis and atopy. To date, studies aimed at investigating the mechanisms regulating these responses, and studies designed to identify potential ways to prevent them, have primarily been conducted in rodent mast cells. However, to understand how these responses pertain to human disease, and to investigate and develop novel therapies for the treatment of human mast cell-driven disease, human mast cell models may have greater relevance. Recently, a number of systems have been developed to allow investigators to readily obtain sufficient quantities of human mast cells to conduct these studies. These mast cells release the appropriate suite of inflammatory mediators in response to known mast cell activators including antigen. These systems have also been employed to examine the signaling events regulating these responses. Proof of principle studies has also demonstrated utility of these systems for the identification of potential inhibitors of mast cell activation and growth. In this unit, techniques for the development and culture of human mast cells from their progenitors and the culture of human mast cell lines are described. The relative merits and drawbacks of each model are also described.

Measuring mast cell mediator release

Mediators released from activated mast cells are responsible for the allergic inflammatory reactions associated with disease states such as anaphylaxis and atopy. These mediators are released as a consequence of immediate degranulation and phospholipid metabolism upon mast cell activation, followed by delayed cytokine gene expression. Thus, techniques that monitor indices of these events in mast cell culture systems, in association with biochemical analysis of parameters of cell signaling, are critical to our understanding of the molecular mechanisms regulating mast cell-mediated disease. Furthermore, these systems can be adapted for high-throughput screens to identify potential inhibitors of mast cell activation that may provide potential leads for novel therapies for these diseases. In this unit, we describe approaches that can be readily used or adapted to a variety of rodent and human mast cell culture systems for the determination of degranulation, phospholipid-derived inflammatory mediator production, and cytokine generation.

Amplification of cytokine production through synergistic activation of NFAT and AP-1 following stimulation of mast cells with antigen and IL-33

IL-33 is associated with atopic and autoimmune diseases and, as reported here, it interacts synergistically with Ag to markedly enhance production of inflammatory cytokines in rodent mast cells even in the absence of degranulation. Investigation of the underlying mechanisms revealed that synergy in signaling occurred at the level of TGF-β-activated kinase 1, which was then transmitted downstream through JNK, p38 MAP kinase, and AP-1. Stimulation of the Ca(2+) /calcineurin/NFAT pathway by Ag, which IL-33 did not, was critical for the synergy between Ag and IL-33. For example, selective stimulation of the NFAT pathway by thapsigargin also markedly enhanced responses to IL-33 in a calcineurin-dependent manner. As indicated by luciferase-reporter assays, IL-33 failed to stimulate the transcriptional activities of NFAT and AP-1 but augmented the activation of these transcription factors by Ag or thapsigargin. Robust stimulation of NF-κB transcriptional activity by IL-33 was also essential for the synergy. These and pharmacologic data suggested that the enhanced production of cytokines resulted in part from amplification of the activation of AP-1 and NFAT as well as co-operative interactions among transcription factors. IL-33 may retune mast cell responses to Ag toward enhanced cytokine production and thus determine the symptoms and severity of Ag-dependent allergic and autoimmune diseases.

Regulation of mast cell responses in health and disease

Mast cells are multifunctional cells that initiate not only IgE-dependent allergic diseases but also play a fundamental role in innate and adaptive immune responses to microbial infection. They are also thought to play a role in angiogenesis, tissue remodeling, wound healing, and tumor repression or growth. The broad scope of these physiologic and pathologic roles illustrates the flexible nature of mast cells, which is enabled in part by their phenotypic adaptability to different tissue microenvironments and their ability to generate and release a diverse array of bioactive mediators in response to multiple types of cell-surface and cytosolic receptors. There is increasing evidence from studies in cell cultures that release of these mediators can be selectively modulated depending on the types or groups of receptors activated. The intent of this review is to foster interest in the interplay among mast cell receptors to help understand the underlying mechanisms for each of the immunological and non-immunological functions attributed to mast cells. The second intent of this review is to assess the pathophysiologic roles of mast cells and their products in health and disease. Although mast cells have a sufficient repertoire of bioactive mediators to mount effective innate and adaptive defense mechanisms against invading microorganisms, these same mediators can adversely affect surrounding tissues in the host, resulting in autoimmune disease as well as allergic disorders.

Mast cell biology: introduction and overview

In recent years, the field of mast cell biology has expanded well beyond the boundaries of atopic disorders and anaphy laxis, on which it has been historically focused. The biochemical and signaling events responsible for the development and regulation of mast cells has been increasingly studied, aided in large part by novel breakthroughs in laboratory techniques used to study these cells. The result of these studies has been a more comprehensive definition of mast cells that includes added insights to their overall biology as well as the various disease states that can now be traced to defects in mast cells. This introductory chapter outlines and highlights the various topics of mast cell biology that will be discussed in further detail in subsequent chapters.

The role of mast cells in atrial natriuretic peptide-induced cutaneous inflammation

Atrial natriuretic peptide (ANP) is widely distributed throughout the heart, skin, gastrointestinal and genital tracts, and nervous and immune systems. ANP acts to mediate vasodilation and induces mast cell activation in both human and rats in vitro. However, the mechanisms of ANP-induced mast cell activation, the extent to which ANP can induce tissue swelling, mast cell degranulation, and granulocyte infiltration in mouse skin are not fully understood. This issue was investigated by treatment with ANP in rat peritoneal mast cells (RPMCs) and mouse peritoneal mast cells (MPMCs) in vitro and by injection of ANP into the skin of congenic normal WBB6F1/J-Kit+/Kit+ +/+, genetically mast cell-deficient WBB6F1/J-Kit(W)/Kit(W-v) (W/W(v)) and mast cell-engrafted W/W(v) (BMCMC→W/W(v)) mice in vivo. ANP induced the release of histamine and TNF-α from RPMCs and enhanced serotonin release from MPMCs, in a dose-dependent fashion, as well as reduced cAMP level of RPMCs in vitro. In +/+ mice, ANP induced significant tissue swelling, mast cell degranulation, and granulocyte infiltration in a dose-dependent manner, whereas not in genetically mast cell-deficient W/W(v) mice. However, ANP-induced cutaneous inflammation has been restored in BMCMC→W/W(v) mice. These data indicate that mast cells play a key role in the ANP-induced cutaneous inflammation.

Prostaglandin E2 activates and utilizes mTORC2 as a central signaling locus for the regulation of mast cell chemotaxis and mediator release

Prostaglandin (PG) E(2), a potent mediator produced in inflamed tissues, can substantially influence mast cell responses including adhesion to basement membrane proteins, chemotaxis, and chemokine production. However, the signaling pathways by which PGE(2) induces mast cell chemotaxis and chemokine production remains undefined. In this study, we identified the downstream target of phosphatidylinositol 3-kinase, mammalian target of rapamycin (mTOR), as a key regulator of these responses. In mouse bone marrow-derived mast cells, PGE(2) was found to induce activation of mTORC1 (mTOR complexed to raptor) as indicated by increased p70S6K and 4E-BP1 phosphorylation, and activation of mTORC2 (mTOR complexed to rictor), as indicated by increased phosphorylation of AKT at position Ser(473). Selective inhibition of the mTORC1 cascade by rapamycin or by the use of raptor-targeted shRNA failed to decrease PGE(2)-mediated chemotaxis or chemokine generation. However, inhibition of the mTORC2 cascade through the dual mTORC1/mTORC2 inhibitor Torin, or through rictor-targeted shRNA, resulted in a significant attenuation in PGE(2)-mediated chemotaxis, which was associated with a comparable decrease in actin polymerization. Furthermore, mTORC2 down-regulation decreased PGE(2)-induced production of the chemokine monocyte chemoattractant protein-1 (CCL2), which was linked to a significant reduction in ROS production. These findings are consistent with the conclusion that activation of mTORC2, downstream of PI3K, represents a critical signaling locus for chemotaxis and chemokine release from PGE(2)-activated mast cells.

Mast cells contribute to the mucosal adjuvant effect of CTA1-DD after IgG-complex formation

Mast cell activation is one of the most dramatic immune-mediated responses the body can encounter. In the worst scenario (i.e., anaphylaxis), this response is fatal. However, the importance of mast cells as initiators and effectors of both innate and adaptive immunity in healthy individuals has recently been appreciated. It was reported that mast cell activation can be used as an adjuvant to promote Ag-specific humoral immune responses upon vaccination. In this study, we have used a clinically relevant mucosal adjuvant, cholera toxin A1 subunit (CTA1)-DD, which is a fusion protein composed of CTA1, the ADP-ribosylating part of cholera toxin, and DD, two Ig-binding domains derived from Staphylococcus aureus protein A. CTA1-DD in combination with polyclonal IgG induced degranulation and production of TNF-alpha from mouse mast cells. Furthermore, CTA1-DD and polyclonal IgG complex induced mast cell degranulation in mouse skin tissue and nasal mucosa. We also found that intranasal immunization with hapten (4-hydroxy-3-nitrophenyl) acetyl (NP) coupled to chicken gammaglobulin admixed with CTA1-DD complexed with polyclonal IgG greatly enhanced serum IgG anti-NP Ab responses and stimulated higher numbers of NP-specific plasma cells in the bone marrow as compared with that observed in mice immunized with NP-chicken gammaglobulin with CTA1-DD alone. This CTA1-DD/IgG complex-mediated enhancement was mast cell dependent because it was absent in mast cell-deficient Kit(W-sh/W-sh) mice. In conclusion, our data suggest that a clinically relevant adjuvant, CTA1-DD, exerts additional augmenting effects through activation of mucosal mast cells, clearly demonstrating that mast cells could be further exploited for improving the efficacy of mucosal vaccines.

Btk-dependent Rac activation and actin rearrangement following FcepsilonRI aggregation promotes enhanced chemotactic responses of mast cells

Mast cells infiltrate the sites of inflammation associated with chronic atopic disease and during helminth and bacterial infection. This process requires receptor-mediated cell chemotaxis across a concentration gradient of their chemotactic ligands. In vivo, mast cells are likely to be exposed to several such agents, which can cooperate in a synergistic manner to regulate mast cell homing. Here, we report that chemotaxis of mouse bone-marrow-derived mast cells (BMMCs) in response to the chemoattractants stem-cell factor (SCF) and prostaglandin (PG)E(2), is substantially enhanced following antigen-dependent ligation of the high-affinity receptor for IgE (FcεRI). These responses were associated with enhanced activation of phosphoinositide 3-kinase (PI3K), and downstream activation of the tyrosine protein kinase Btk, with subsequent enhanced phospholipase (PL)Cγ-mediated Ca(2+) mobilization, Rac activation and F-actin rearrangement. Antigen-induced chemotaxis, and the ability of antigen to amplify responses mediated by SCF, adenosine and PGE(2) were suppressed following inhibition of PI3K, and were impaired in BMMCs derived from Btk(-/-) mice. There were corresponding decreases in the PLCγ-mediated Ca(2+) signal, Rac activation and F-actin rearrangement, which, as they are essential for BMMC chemotaxis, accounts for the impaired migration of Btk-deficient cells. Taken together, these data demonstrate that, by regulating signaling pathways that control F-actin rearrangement, Btk is crucial for the ability of antigen to amplify mast-cell chemotactic responses.

Cooperation of adapter molecules in proximal signaling cascades during allergic inflammation

Activation of mast cells through their high-affinity immunoglobulin E receptor (FcepsilonRI) plays an important role in allergic disorders. Other mast cell-activating stimuli, such as Toll-like receptor (TLR) ligands, synergize with FcepsilonRI to enhance allergic inflammation. Thus, there is much interest in understanding how signaling occurs downstream of these receptors. One key event for FcepsilonRI-mediated mast cell activation is the inducible formation of multimolecular proximal signaling complexes. These complexes are nucleated by adapter proteins, scaffolds that localize various signaling molecules through their multiple molecule-binding domains. Here we review recent findings in proximal signaling cascades with an emphasis on how adapter molecules cooperate with each other to generate an optimal signal in mast cells, and we discuss how signals crosstalk between FcepsilonRI and TLRs in enhancing mast cell activation. Deciphering the molecular mechanisms leading to mast cell activation will hopefully bring new ideas for the development of novel therapeutics to control allergic diseases.

Activation of human mast cells through the platelet-activating factor receptor

BACKGROUND

In human subjects platelet-activating factor (PAF) concentrations are markedly increased in the plasma after anaphylactic reactions, and these correlate strongly with the severity of the response. The mechanism for the systemic spread of mast cell (MC) activation in anaphylaxis is often assumed to relate to the hematogenous spread of allergen, but this is implausible, and amplification mechanisms need to be considered.

OBJECTIVE

We have investigated the ability of PAF to induce human MC degranulation using skin, lung, and peripheral blood (PB)-derived cultured MCs and the signaling pathways activated in PB-derived MCs in response to PAF.

METHODS

The expression of PAF receptor was investigated by means of RT-PCR and Western blot analysis. Cell-signaling pathways in PB-derived MCs in response to PAF were investigated by analyzing the effect of various inhibitors and the silencing of phospholipase C (PLC) mRNA on PAF-mediated histamine release.

RESULTS

We show for the first time that PAF induces histamine release from human lung MCs and PB-derived MCs but not skin MCs. Activation of PAF receptor-coupled G(alphai) leads to degranulation through PLCgamma1 and PLCbeta2 activation in human MCs. PAF-induced degranulation was rapid, being maximal at 5 seconds, and was partially dependent on extracellular Ca(2+).

CONCLUSION

Our findings provide a mechanism whereby PAF mediates an amplification loop for MC activation in the generation of anaphylaxis.

Substance P downregulates expression of the high affinity IgE receptor (FcepsilonRI) by human mast cells

The effect of the neuropeptide substance P (SP) on human mast cell (MC) phenotype is poorly understood. In this study, SP effects on human MC expression of the high affinity IgE receptor (FcepsilonRI) were characterized. SP downregulated expression of FcepsilonRI mRNA and protein by approximately 50% and in a concentration dependent manner, the effect was partially mediated by engagement of the neurokinin-1 receptor (NK1R) and resulted in reduced mast cell activation. Sensitization of MC with IgE prior to SP exposure protected MC from SP-mediated FcepsilonRI downregulation. SP release may inhibit MC responses to allergens and these results may have implications in neuroinflammatiion and stress.

Mechanisms of mast cell signaling in anaphylaxis

The recent development of a consensus definition and proposed diagnostic criteria for anaphylaxis offers promise for research efforts and a better understanding of the epidemiology and pathogenesis of this enigmatic and life-threatening disease. This review examines basic principles and recent research advances in the mechanisms of mast cell signaling believed to underlie anaphylaxis. The unfolding complexity of mast cell signaling suggests that the system is sensitive to regulation by any of several individual signaling pathways and intermediates and that complementary pathways regulate mast cell activation by amplified signals. The signaling events underlying anaphylactic reactions have largely been identified through experiments in genetically modified mice and supported by biochemical studies of mast cells derived from these mice. These studies have revealed that signaling pathways exist to both upregulate and downregulate mast cell responses. In this review we will thus describe the key molecular players in these pathways in the context of anaphylaxis.

Regulation of Ca2+ signaling with particular focus on mast cells

Calcium signals mediate diverse cellular functions in immunological cells. Early studies with mast cells, then a preeminent model for studying Ca2+-dependent exocytosis, revealed several basic features of calcium signaling in non-electrically excitable cells. Subsequent studies in these and other cells further defined the basic processes such as inositol 1,4,5-trisphosphate-mediated release of Ca2+ from Ca2+ stores in the endoplasmic reticulum (ER); coupling of ER store depletion to influx of external Ca2+ through a calcium-release activated calcium (CRAC) channel now attributed to the interaction of the ER Ca2+ sensor, stromal interacting molecule-1 (STIM1), with a unique Ca2+-channel protein, Orai1/CRACM1, and subsequent uptake of excess Ca2+ into ER and mitochondria through ATP-dependent Ca2+ pumps. In addition, transient receptor potential channels and ion exchangers also contribute to the generation of calcium signals that may be global or have dynamic (e.g., waves and oscillations) and spatial resolution for specific functional readouts. This review discusses past and recent developments in this field of research, the pharmacologic agents that have assisted in these endeavors, and the mast cell as an exemplar for sorting out how calcium signals may regulate multiple outputs in a single cell.

Molecular regulation of mast cell development and maturation

Mast cells play a crucial role in the pathogenesis of allergic diseases. In recent years, tremendous progresses have been made in studies of mast cell origination, migration, proliferation, maturation and survival, and the cytokines regulating these activities. These advances have significantly improved our understandings to mast cell biology and to the molecular mechanisms of mast cells in the pathogenesis of allergic diseases.

Functional evidence for the expression of P2X1, P2X4 and P2X7 receptors in human lung mast cells

BACKGROUND AND PURPOSE

P2X receptors are widely expressed in cells of the immune system with varying functions. This study sought to characterize P2X receptor expression in the LAD2 human mast cell line and human lung mast cells (HLMCs).

EXPERIMENTAL APPROACH

Reverse transcriptase polymerase chain reaction (RT-PCR) and patch clamp studies were used to characterize P2X expression in mast cells using a range of pharmacological tools.

KEY RESULTS

RT-PCR revealed P2X1, P2X4 and P2X7 transcripts in both cell types; mRNA for P2X6 was also detected in LAD2 cells. Under whole-cell patch clamp conditions, rapid application of ATP (1-1000 microM) to cells clamped at -60 mV consistently evoked inward currents in both types of cells. Brief application of ATP (1 s) evoked a rapidly desensitizing P2X1-like current in both cell types. This current was also elicited by alphabetamethylene ATP (10 microM, 94% cells, n= 31) and was antagonized in LAD2 cells by NF 449 (1 microM) and pyridoxal phosphate-6-azo(benzene-2,4-disulphonic acid) (1-10 microM). A P2X7-like non-desensitizing current in response to high concentrations of ATP (1-5 mM) was also seen in both cell types (96% LAD2, n= 24; 54% HLMCs, n= 24) which was antagonized by AZ11645373 (1 microM). P2X7-like responses were also evoked in LAD2 cells by 2'(3')-0-(4-benzoylbenzoyl)ATP (300 microM). A P2X4-like current was evoked by 100 microM ATP (80% LAD2, n= 10; 21% HLMCs, n= 29), the amplitude and duration of which was potentiated by ivermectin (3 microM).

CONCLUSION AND IMPLICATIONS

Our data confirmed the presence of functional P2X1, P2X4 and P2X7 receptors in LAD2 cells and HLMCs.

Regulation of G-protein-coupled signaling pathways in allergic inflammation

Allergic diseases such as asthma are elicited by maladaptive activation of immune cells such as mast cells and lymphocytes by otherwise innocuous allergens. The numerous mediators secreted by such cells promote both acute inflammation and, in many instances, chronic tissue remodeling. Most of these compounds exert their effects on end-organ targets such as epithelial and endothelial cells and airway smooth muscle by activating G-protein-coupled receptors (GPCRs), which are by far the most abundant type of cell surface receptor. Since GPCRs are also the most common target of allergy therapeutics, a better understanding of their intracellular signaling mechanisms is vital to improve the efficacy of such drugs or to develop new targets. In this review, we focus on some of the new regulatory elements that control the duration and amplitude of GPCR signal transduction pathways in immune effector cells and end-organ structural cells affected by allergic inflammation.

Amplification mechanisms for the enhancement of antigen-mediated mast cell activation

Activation of mast cells in the allergic inflammatory response occurs via the high affinity receptor for IgE (FcepsilonRI) following receptor aggregation induced by antigen-mediated cross-linking of IgE-occupied FcepsilonRI. Recent observations suggest this response is profoundly influenced by other factors that reduce the threshold for, and increase the extent of, mast cell activation. For example, under experimental conditions, cell surface receptors such as KIT and specific G protein-coupled receptors synergistically enhance FcepsilonRI-mediated mast cell degranulation and cytokine production. Activating mutations in critical signaling molecules may also contribute to such responses. In this review, we describe our research exploring the mechanisms regulating these synergistic interactions and, furthermore, discuss the relevance of our observations in the context of clinical considerations.

The tyrosine kinase network regulating mast cell activation

Mast cell mediator release represents a pivotal event in the initiation of inflammatory reactions associated with allergic disorders. These responses follow antigen-mediated aggregation of immunoglobulin E (IgE)-occupied high-affinity receptors for IgE (Fc epsilon RI) on the mast cell surface, a response which can be further enhanced following stem cell factor-induced ligation of the mast cell growth factor receptor KIT (CD117). Activation of tyrosine kinases is central to the ability of both Fc epsilon RI and KIT to transmit downstream signaling events required for the regulation of mast cell activation. Whereas KIT possesses inherent tyrosine kinase activity, Fc epsilon RI requires the recruitment of Src family tyrosine kinases and Syk to control the early receptor-proximal signaling events. The signaling pathways propagated by these tyrosine kinases can be further upregulated by the Tec kinase Bruton's tyrosine kinase and downregulated by the actions of the tyrosine Src homology 2 domain-containing phosphatase 1 (SHP-1) and SHP-2. In this review, we discuss the regulation and role of specific members of this tyrosine kinase network in KIT and Fc epsilon RI-mediated mast cell activation.

Our perception of the mast cell from Paul Ehrlich to now

Just over a century ago Paul Ehrlich received the Nobel Prize for his studies of immunity. This review describes one of his legacies, the histochemical description of the mast cell, and the research that has ensued since then. After a long period of largely descriptive studies, which revealed little about the biological role of the mast cell, the field was galvanized in the 1950s by the recognition that the mast cell was the main repository of histamine and a key participant in anaphylactic reactions. Although the mast cell was long-viewed in these terms, recent research has now shown that the mast cell also plays a key role in innate and adaptive immune responses, autoimmune disease, and possibly tissue homeostasis by virtue of its expression of a diverse array of receptors and biologically active products. In addition, the responsiveness of mast cells to immunological and pathological stimulants is highly modulated by the tissue cytokine environment and by synergistic, or inhibitory, interactions among the various mast cell receptor systems. This once enigmatic cell of Paul Ehrlich has proved to be both adaptable and multifunctional.

Sustained activation of the tyrosine kinase Syk by antigen in mast cells requires local Ca2+ influx through Ca2+ release-activated Ca2+ channels

Mast cell activation involves cross-linking of IgE receptors followed by phosphorylation of the non-receptor tyrosine kinase Syk. This results in activation of the plasma membrane-bound enzyme phospholipase Cgamma1, which hydrolyzes the minor membrane phospholipid phosphatidylinositol 4,5-bisphosphate to generate diacylglycerol and inositol trisphosphate. Inositol trisphosphate raises cytoplasmic Ca2+ concentration by releasing Ca2+ from intracellular stores. This Ca2+ release phase is accompanied by sustained Ca2+ influx through store-operated Ca2+ release-activated Ca2+ (CRAC) channels. Here, we find that engagement of IgE receptors activates Syk, and this leads to Ca2+ release from stores followed by Ca2+ influx. The Ca2+ influx phase then sustains Syk activity. The Ca2+ influx pathway activated by these receptors was identified as the CRAC channel, because pharmacological block of the channels with either a low concentration of Gd3+ or exposure to the novel CRAC channel blocker 3-fluoropyridine-4-carboxylic acid (2',5'-dimethoxybiphenyl-4-yl)amide or RNA interference knockdown of Orai1, which encodes the CRAC channel pore, all prevented the increase in Syk activity triggered by Ca2+ entry. CRAC channels and Syk are spatially close together, because increasing cytoplasmic Ca2+ buffering with the fast Ca2+ chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetrakis failed to prevent activation of Syk by Ca2+ entry. Our results reveal a positive feedback step in mast cell activation where receptor-triggered Syk activation and subsequent Ca2+ release opens CRAC channels, and the ensuing local Ca2+ entry then maintains Syk activity. Ca2+ entry through CRAC channels therefore provides a means whereby the Ca2+ and tyrosine kinase signaling pathways can interact with one another.

The multiple roles of phosphoinositide 3-kinase in mast cell biology

Mast cells play a central role in the initiation of inflammatory responses associated with asthma and other allergic disorders. Receptor-mediated mast cell growth, differentiation, homing to their target tissues, survival and activation are all controlled, to varying degrees, by phosphoinositide-3-kinase (PI3K)-driven pathways. It is not fully understood how such diverse responses can be differentially regulated by PI3K. However, recent studies have provided greater insight into the mechanisms that control, and those that are controlled by, different PI3K subunit isoforms in mast cells. In this review, we discuss how PI3K influences the mast cell processes described above. Furthermore, we describe how different mast cell receptors use alternative isoforms of PI3K for these functions and discuss potential downstream targets of these isoforms.

Targeting human mast cells expressing g-protein-coupled receptors in allergic diseases

The G-protein-coupled receptors (GPCRs) are the largest known group of integral membrane receptor proteins and are the most common targets of pharmacotherapy. Mast cells (MCs) have been reported to play an important role in allergic diseases, such as urticaria and bronchial asthma. There is an increasing body of clinical evidence that MCs are recruited into allergic reactions by non-IgE-dependent mechanisms. Human MCs are activated and secrete histamine in response to neuropeptides, such as substance P and somatostatin, mediated by a GPCR, MRGX2. The microenvironment surrounding MCs in their resident tissues is likely to contain multiple factors that modify antigen-dependent MC activation. MCs express various GPCRs, and since the function of human MCs is modulated by various GPCR ligands, such as adenosine and sphingosine-1-phosphate, which are present in high levels in the bronchial alveolar lavage fluid of asthmatic patients, the GPCRs expressed on MCs may play an important role in human allergic diseases. The GPCRs expressed on MCs may serve as drug targets for the treatment of allergic diseases.

Pharmacological targeting of the KIT growth factor receptor: a therapeutic consideration for mast cell disorders

KIT is a member of the tyrosine kinase family of growth factor receptors which is expressed on a variety of haematopoietic cells including mast cells. Stem cell factor (SCF)-dependent activation of KIT is critical for mast cell homeostasis and function. However, when KIT is inappropriately activated, accumulation of mast cells in tissues results in mastocytosis. Such dysregulated KIT activation is a manifestation of specific activating point mutations within KIT, with the human D816V mutation considered as a hallmark of human systemic mastocytosis. A number of other activating mutations in KIT have recently been identified and these mutations may also contribute to aberrant mast cell growth. In addition to its role in mast cell growth, differentiation and survival, localized concentration gradients of SCF may control the targeting of mast cells to specific tissues and, once resident within these tissues, mast cell activation by antigen may also be amplified by SCF. Thus, KIT inhibitors may have potential application in multiple conditions linked to mast cells including systemic mastocytosis, anaphylaxis, and asthma. In this review, we discuss the role of KIT in the context of mast cells in these disease states and how recent advances in the development of inhibitors of KIT activity and function may offer novel therapies for the treatment of these disorders.

Unraveling the complexities of sphingosine-1-phosphate function: the mast cell model

Sphingosine-1-phosphate (S1P) is a lipid mediator involved in diverse biological processes, from vascular and neural development to the regulation of lymphocyte trafficking. Many of its functions are regulated by five widely expressed S1P G-protein-coupled receptors (S1P(1-5)). S1P is produced mostly intracellularly, thus, much of its potential as an autocrine and paracrine mediator depends on how, when, and where it is generated or secreted out of the cells. However, S1P can also have intracellular activity independent of its receptors, adding to the complexity of S1P function. The mast cell, a major effector cell during an allergic response, has proven instrumental towards understanding the complex regulation and function of S1P. Antigen (Ag) engagement of the IgE receptor in mast cells stimulates sphingosine kinases, which generate S1P and are involved in the activation of calcium fluxes critical for mast cell responses. In addition, mast cells secrete considerable amounts of S1P upon activation, thus affecting the surrounding tissues and recruiting inflammatory cells. Export of S1P is also involved in the autocrine transactivation of S1P receptors present in mast cells. The in vivo response of mast cells, however, is not strictly dependent on their ability to generate S1P, but they are also affected by changes in S1P in the environment previous to Ag challenge. This review will discuss the recent advances towards understanding the intricacies of S1P generation, secretion and regulation in mast cells. In addition, how S1P receptors are activated and their involvement in mast cell functions will also be covered, including new insights on the role of S1P in the mast cell-mediated allergic response of systemic anaphylaxis.

Synergistic activation of phospholipases Cgamma and Cbeta: a novel mechanism for PI3K-independent enhancement of FcepsilonRI-induced mast cell mediator release

Antigen/IgE-mediated mast cell activation via FcvarepsilonRI can be markedly enhanced by the activation of other receptors expressed on mast cells and these receptors may thus contribute to the allergic response in vivo. One such receptor family is the G protein-coupled receptors (GPCRs). Although the signaling cascade linking FcvarepsilonRI aggregation to mast cell activation has been extensively investigated, the mechanisms by which GPCRs amplify this response are relatively unknown. To investigate this, we utilized prostaglandin (PG)E2 based on initial studies demonstrating its greater ability to augment antigen-mediated degranulation in mouse mast cells than other GPCR agonists examined. This enhancement, and the ability of PGE2 to amplify antigen-induced calcium mobilization, was independent of phosphoinositide 3-kinase but was linked to a pertussis toxin-sensitive synergistic translocation to the membrane of phospholipase (PL)Cgamma and PLCbeta and to an enhancement of PLCgamma phosphorylation. This "trans-synergistic" activation of PLCbeta and gamma, in turn, enhanced production of inositol 1,4,5-trisphosphate, store-operated calcium entry, and activation of protein kinase C (PKC) (alpha and beta). These responses were critical for the promotion of degranulation. This is the first report of synergistic activation between PLCgamma and PLCbeta that permits reinforcement of signals for degranulation in mast cells.