Phosphorylation of SNAP-23 by IkappaB kinase 2 regulates mast cell degranulation

Cross-talk between zinc and calcium regulates ion transport: A role for the zinc receptor, ZnR/GPR39

Zinc is essential for many physiological functions, with a major role in digestive system, skin health, and learning and memory. On the cellular level, zinc is involved in cell proliferation and cell death. A selective zinc sensing receptor, ZnR/GPR39 is a Gq-coupled receptor that acts via the inositol trisphosphate pathway to release intracellular Ca2+. The ZnR/GPR39 serves as a mediator between extracellular changes in Zn2+ concentration and cellular Ca2+ signalling. This signalling pathway regulates ion transporters activity and thereby controls the formation of transepithelial gradients or neuronal membrane potential, which play a fundamental role in the physiological function of these tissues. This review focuses on the role of Ca2+ signalling, and specifically ZnR/GPR39, with respect to the regulation of the Na+/H+ exchanger, NHE1, and of the K+/Cl- cotransporters, KCC1-3, and also describes the physiological implications of this regulation.

Methiothepin downregulates SNAP-23 and inhibits degranulation of rat basophilic leukemia cells and mouse bone marrow-derived mast cells

In the present study, we found that methiothepin (a nonselective 5-hydroxytryptamine [5-HT] receptor antagonist) inhibited antigen-induced degranulation in rat basophilic leukemia cells and mouse bone marrow-derived mast cells. Although antigen stimulation induces release of histamine and serotonin (5-HT) by exocytosis and mast cells express several types of 5-HT receptor, the detailed role of these receptors remains unclear. Here, pretreatment of cells with methiothepin attenuated increased intracellular Ca2+ concentration, phosphorylated critical upstream signaling components (Src family tyrosine kinases, Syk, and PLCγ1), and suppressed TNF-α secretion via inhibition of Akt (a Ser/Thr kinase activated by PI3K)and ERK phosphorylation. Furthermore, it inhibited PMA/ionomycin-induced degranulation; this finding suggested that methiothepin affected downstream signaling. IκB kinase β phosphorylates synaptosomal associated protein 23, which regulates the fusion events of the secretory granule/plasma membrane after mast cell activation, resulting in degranulation. We showed that methiothepin blocked PMA/ionomycin-induced phosphorylation of synaptosomal associated protein 23 by inhibiting its interaction with IκB kinase β. Together with the results of selective 5-HT antagonists, it is suggested that methiothepin inhibits mast cell degranulation by downregulating upstream signaling pathways and exocytotic fusion machinery through mainly 5-HT1A receptor. Our findings provide that 5-HT antagonists may be used to relieve allergic reactions.

Potential Role of Moesin in Regulating Mast Cell Secretion

Mast cells have existed for millions of years in species that never suffer from allergic reactions. Hence, in addition to allergies, mast cells can play a critical role in homeostasis and inflammation via secretion of numerous vasoactive, pro-inflammatory and neuro-sensitizing mediators. Secretion may utilize different modes that involve the cytoskeleton, but our understanding of the molecular mechanisms regulating secretion is still not well understood. The Ezrin/Radixin/Moesin (ERM) family of proteins is involved in linking cell surface-initiated signaling to the actin cytoskeleton. However, how ERMs may regulate secretion from mast cells is still poorly understood. ERMs contain two functional domains connected through a long α-helix region, the N-terminal FERM (band 4.1 protein-ERM) domain and the C-terminal ERM association domain (C-ERMAD). The FERM domain and the C-ERMAD can bind to each other in a head-to-tail manner, leading to a closed/inactive conformation. Typically, phosphorylation on the C-terminus Thr has been associated with the activation of ERMs, including secretion from macrophages and platelets. It has previously been shown that the ability of the so-called mast cell "stabilizer" disodium cromoglycate (cromolyn) to inhibit secretion from rat mast cells closely paralleled the phosphorylation of a 78 kDa protein, which was subsequently shown to be moesin, a member of ERMs. Interestingly, the phosphorylation of moesin during the inhibition of mast cell secretion was on the N-terminal Ser56/74 and Thr66 residues. This phosphorylation pattern could lock moesin in its inactive state and render it inaccessible to binding to the Soluble NSF attachment protein receptors (SNAREs) and synaptosomal-associated proteins (SNAPs) critical for exocytosis. Using confocal microscopic imaging, we showed moesin was found to colocalize with actin and cluster around secretory granules during inhibition of secretion. In conclusion, the phosphorylation pattern and localization of moesin may be important in the regulation of mast cell secretion and could be targeted for the development of effective inhibitors of secretion of allergic and inflammatory mediators from mast cells.

Endocannabinoid modulation of allergic responses: Focus on the control of FcεRI-mediated mast cell activation

Allergic reactions are highly prevalent pathologies initiated by the production of IgE antibodies against harmless antigens (allergens) and the activation of the high-affinity IgE receptor (FcεRI) expressed in the surface of basophils and mast cells (MCs). Research on the mechanisms of negative control of those exacerbated inflammatory reactions has been intense in recent years. Endocannabinoids (eCBs) show important regulatory effects on MC-mediated immune responses, mainly inhibiting the production of pro-inflammatory mediators. However, the description of the molecular mechanisms involved in eCB control of MC activation is far from complete. In this review, we aim to summarize the available information regarding the role of eCBs in the modulation of FcεRI-dependent activation of that cell type, emphasizing the description of the eCB system and the existence of some of its elements in MCs. Unique characteristics of the eCB system and cannabinoid receptors (CBRs) localization and signaling in MCs are mentioned. The described and putative points of cross-talk between CBRs and FcεRI signaling cascades are also presented. Finally, we discuss some important considerations in the study of the effects of eCBs in MCs and the perspectives in the field.

Exocytic machineries differentially control mediator release from allergen-triggered RBL-2H3 cells

BACKGROUND

Mast cells utilize SNAREs (soluble-N-ethyl-maleimide sensitive factor attachment protein receptors) and SM (Sec1/Munc18) proteins to secrete/exocytose a variety of proinflammatory mediators. However, whether a common SNARE-SM machinery is responsible remains unclear.

METHODS

Four vesicle/granule-anchored SNAREs (VAMP2, VAMP3, VAMP7, and VAMP8) and two Munc18 homologs (Munc18a and Munc18b) were systematically knocked down or knocked out in RBL-2H3 mast cells and antigen-induced release of β-hexosaminidase, histamine, serotonin, and TNF was examined. Phenotypes were validated by rescue experiments. Immunofluorescence studies were performed to determine the subcellular distribution of key players.

RESULTS

The reduction of VAMP8 expression inhibited the exocytosis of β-hexosaminidase, histamine, and serotonin but not TNF. Unexpectedly, however, confocal microscopy revealed substantial co-localization between VAMP8 and TNF, and between TNF and serotonin. Meanwhile, the depletion of other VAMPs, including knockout of VAMP3, had no impact on the release of any of the mediators examined. On the other hand, TNF exocytosis was diminished specifically in stable Munc18bknockdown cells, in a fashion that was rescued by exogenous, RNAi-resistant Munc18b. In line with this, TNF was co-localized with Munc18b (47%) to a much greater extent than with Munc18a (13%).

CONCLUSION

Distinct exocytic pathways exist in mast cells for the release of different mediators.

B Cell Activation Results in IKK-Dependent, but Not c-Rel- or RelA-Dependent, Decreases in Transcription of the B Cell Tolerance-Inducing Gene Ets1

Ets1 is a key transcription factor in B cells that is required to prevent premature differentiation into Ab-secreting cells. Previously, we showed that BCR and TLR signaling downregulate Ets1 levels and that the kinases PI3K, Btk, IKK, and JNK are required for this process. PI3K is important in activating Btk by generating the membrane lipid phosphatidylinositol (3,4,5)-trisphosphate, to which Btk binds via its PH domain. Btk in turn is important in activating the IKK kinase pathway, which it does by activating phospholipase Cγ2→protein kinase Cβ signaling. In this study, we have further investigated the pathways regulating Ets1 in mouse B cells. Although IKK is well known for its role in activating the canonical NF-κB pathway, IKK-mediated downregulation of Ets1 does not require either RelA or c-Rel. We also examined the potential roles of two other IKK targets that are not part of the NF-κB signaling pathway, Foxo3a and mTORC2, in regulating Ets1. We find that loss of Foxo3a or inhibition of mTORC2 does not block BCR-induced Ets1 downregulation. Therefore, these two pathways are not key IKK targets, implicating other as yet undefined IKK targets to play a role in this process.

Rerouting trafficking circuits through posttranslational SNARE modifications

Soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) are membrane-associated trafficking proteins that confer identity to lipid membranes and facilitate membrane fusion. These functions are achieved through the complexing of Q-SNAREs with a specific cognate target R-SNARE, leading to the fusion of their associated membranes. These SNARE complexes then dissociate so that the Q-SNAREs and R-SNAREs can repeat this cycle. Whilst the basic function of SNAREs has been long appreciated, it is becoming increasingly clear that the cell can control the localisation and function of SNARE proteins through posttranslational modifications (PTMs), such as phosphorylation and ubiquitylation. Whilst numerous proteomic methods have shown that SNARE proteins are subject to these modifications, little is known about how these modifications regulate SNARE function. However, it is clear that these PTMs provide cells with an incredible functional plasticity; SNARE PTMs enable cells to respond to an ever-changing extracellular environment through the rerouting of membrane traffic. In this Review, we summarise key findings regarding SNARE regulation by PTMs and discuss how these modifications reprogramme membrane trafficking pathways.

Investigation of the Anti-Inflammatory Activity of Fusaproliferin Analogues Guided by Transcriptome Analysis

Background: Excessive inflammation results in severe tissue damage as well as serious acute or chronic disorders, and extensive research has focused on finding new anti-inflammatory hit compounds with safety and efficacy profiles from natural products. As promising therapeutic entities for the treatment of inflammation-related diseases, fusaproliferin and its analogs have attracted great interest. However, the underlying anti-inflammatory mechanism is still poorly understood and deserves to be further investigated.

Methods: For the estimation of the anti-inflammatory activity of fusaproliferin (1) and its analogs (2-4)in vitro and in vivo, lipopolysaccharide (LPS)-induced RAW264.7 macrophages and zebrafish embryos were employed. Then, transcriptome analysis was applied to guide subsequent western blot analysis of critical proteins in related signaling pathways. Surface plasmon resonance assays (SPR) combined with molecular docking analyses were finally applied to evaluate the affinity interactions between 1-4 and TLR4 and provide a possible interpretation of the downregulation of related signaling pathways.

Results: 1-4 significantly attenuated the production of inflammatory messengers, including nitric oxide (NO), reactive oxygen species (ROS), interleukin-6 (IL-6), tumor necrosis factor-α (TNF-α), and interleukin-1β (IL-1β), as well as nitric oxide synthase (iNOS) and cyclooxygenase-2 (COX-2), in LPS-induced RAW264.7 macrophages. Transcriptome analyses based on RNA-seq indicated the ability of compound 1 to reverse LPS stimulation and the nuclear factor kappa-B (NF-κB) and mitogen-activated protein kinase (MAPKs) signaling pathways contribute to the anti-inflammatory process. Experimental verification at the protein level revealed that 1 can inhibit the activation of inhibitor of NF-κB kinase (IKK), degradation of inhibitor of NF-κB (IκB), and phosphorylation of NF-κB and reduce nuclear translocation of NF-κB. 1 also decreased the phosphorylation of MAPKs, including p38, extracellular regulated protein kinases (ERK), and c-Jun N-terminal kinase (JNK). SPR assays and molecular docking results indicated that 1-4 exhibited affinity for the TLR4 protein with KD values of 23.5–29.3 μM.

Conclusion: Fusaproliferin and its analogs can be hit compounds for the treatment of inflammation-associated diseases.

SNAP23 regulates KCC2 membrane insertion and activity following mZnR/GPR39 activation in hippocampal neurons

Modulation of the neuronal K⁺/Cl⁻ cotransporter 2 (KCC2) activity, which mediates Cl⁻ export, is critical to neuronal function. Here, we demonstrate that KCC2 interacts with the SNARE protein synaptosome-associated protein 23, SNAP23, an essential component of membrane insertion machinery. Using KCC2 truncated mutants, we show that KCC2 C-terminal domain is essential for membrane targeting and SNAP23-dependent upregulation of KCC2 activity triggered by activation of the Zn²⁺-sensitive receptor mZnR/GPR39 in HEK293 cells. Expression of SNAP23 phosphorylation-insensitive mutants or inhibition of its upstream activator IκB kinase (IKK) prevents mZnR/GPR39 upregulation of KCC2 activity in mouse hippocampal neurons. We further find that SNAP23 interacts with Syntaxin 1A and KCC2, and that all three proteins exhibit increased membrane insertion following mZnR/GPR39 activation in neurons. Our results elucidate a G-protein-coupled receptor-dependent pathway for regulation of KCC activity, mediated via interaction with SNARE proteins.

•

Neuronal K⁺/Cl⁻ cotransporter 2 (KCC2) is regulated via interaction with SNAP23

•

Zn²⁺ enhances interaction and membrane insertion of SNAP23, Syntaxin 1A, and KCC2

•

Zn²⁺-dependent mZnR/GPR39 regulation of KCC2 requires SNAP23 phosphorylation

•

Epithelial KCC3 regulation by ZnR/GPR39 also requires SNAP23

Calcineurin Aα Contributes to IgE-Dependent Mast-Cell Mediator Secretion in Allergic Inflammation

Mast cells (MCs) are key mediators of allergic inflammation through the activation of cross-linked immunoglobulin E (IgE) bound to the high-affinity IgE receptor (FcϵRI) on the cell surface, leading to the release of biologically potent mediators, either from preformed granules or newly synthesized. Pharmacological inhibitors have been developed to target a key signaling protein phosphatase in this pathway, calcineurin, yet there is a lack of genetic and definitive evidence for the various isoforms of calcineurin subunits in FcϵRI-mediated responses. In this study, we hypothesized that deficiency in the calcineurin Aα isoform will result in a decreased allergic immune response by the MCs. In a model of passive cutaneous anaphylaxis, there was a reduction in vascular permeability in MC-deficient mouse tissues reconstituted with calcineurin subunit A (CnAα) gene-knockout (CnAα^−/−) MCs, and in vitro experiments identified a significant reduction in release of preformed mediators from granules. Furthermore, released levels of de novo synthesized cytokines were reduced upon FcϵRI activation of CnAα^−/− MCs in vitro. Characterizing the mechanisms associated with this deficit response, we found a significant impairment of nuclear factor of kappa light polypeptide gene enhancer in B cell phosphorylation and impaired nuclear factor kappa-light-chain-enhancer of activated B-cell inhibitor alpha (NF-κB) activation. Thus, we concluded that CnAα contributes to the release of preformed mediators and newly synthesized mediators from FcϵRI-mediated activation of MCs, and this regulation includes NF-κB signaling.

Diverse innate stimuli activate basophils through pathways involving Syk and IκB kinases

Mature basophils play critical inflammatory roles during helminthic, autoimmune, and allergic diseases through their secretion of histamine and the type 2 cytokines interleukin 4 (IL-4) and IL-13. Basophils are activated typically by allergen-mediated IgE cross-linking but also by endogenous "innate" factors. The aim of this study was to identify the innate stimuli (cytokines, chemokines, growth factors, hormones, neuropeptides, metabolites, and bacterial products) and signaling pathways inducing primary basophil activation. Basophils from naïve mice or helminth-infected mice were cultured with up to 96 distinct stimuli and their influence on basophil survival, activation, degranulation, and IL-4 or IL-13 expression were investigated. Activated basophils show a heterogeneous phenotype and segregate into distinct subsets expressing IL-4, IL-13, activation, or degranulation markers. We find that several innate stimuli including epithelial derived inflammatory cytokines (IL-33, IL-18, TSLP, and GM-CSF), growth factors (IL-3, IL-7, TGFβ, and VEGF), eicosanoids, metabolites, TLR ligands, and type I IFN exert significant direct effects on basophils. Basophil activation mediated by distinct upstream signaling pathways is always sensitive to Syk and IκB kinases-specific inhibitors but not necessarily to NFAT, STAT5, adenylate cyclase, or c-fos/AP-1 inhibitors. Thus, basophils are activated by very diverse mediators, but their activation seem controlled by a core checkpoint involving Syk and IκB kinases.

STAP-2 Adaptor Protein Regulates Multiple Steps of Immune and Inflammatory Responses

Signal-transducing adaptor protein (STAP)-2 is an adaptor molecule involved in regulation of several intracellular signaling events in immune cells. STAP-2 contains a pleckstrin homology domain at the N-terminus, an src homology domain in the central portion and a proline-rich region at the C-terminus. STAP-2 also has a YXXQ motif, which is a potential signal transducer and activator of transcription (STAT)3-binding site. STAP-2 influences the STAT3 and STAT5 activity, integrin-mediated T cell adhesion, chemokine-induced T cell migration, Fas-mediated T cell apoptosis, Toll-like receptor-mediated macrophage functions, macrophage colony-stimulating factor-induced macrophage activation, and the high-affinity immunoglobulin E receptor-mediated mast cell activation. This article reviews the current understanding of roles of the STAP-2 during immune and/or inflammatory responses, and discusses possible therapeutic applications of targeting STAP-2 proteins in immune-related disorders.

PKCε controls the fusion of secretory vesicles in mast cells in a phosphatidic acid-dependent mode

PKCε is highly expressed in mast cells and plays a fundamental role in the antigen-triggered activation of the allergic reaction. Although its regulation by diacylglycerols has been described, its regulation by acidic phospholipids and how this regulation leads to the control of downstream vesicle secretion is barely known. Here, we used structural and evolutionary studies to find the molecular mechanism that explains the selectivity of the C1B domain of PKCε by Phosphatidic Acid (PA). This resided in a collection of Arg residues that form a specific rim on the outer surface of the C1B domain, around the diacylglycerol binding cleft. In RBL-2H3 cells, this basic rim allowed the kinase to respond specifically to phosphatidic acid signals that induced its translocation to the plasma membrane and subsequent activation. Further experiments in cells that overexpress PKCε and a mutant of the PA binding site, showed that PA-dependent PKCε activation increased vesicle degranulation in RBL-2H3 cells, and this correlated with increased SNAP23 phosphorylation. Over-expression of PKCε in these cells also induced an increase in the number of docked vesicles containing SNAP23, when stimulated with PA. This accumulation could be attributed to the stabilizing effect of phosphorylation on the formation of the SNARE complex, which ultimately led to increased release of content in the presence of Ca²⁺ during the fusion process. Therefore, these findings reinforce the importance of PA signaling in the activation of PKCε, which could be an important target to inhibit the exacerbated responses of these cells in the allergic reaction.

Inflammation rapidly recruits mammalian GMP and MDP from bone marrow into regional lymphatics

Innate immune cellular effectors are actively consumed during systemic inflammation, but the systemic traffic and the mechanisms that support their replenishment remain unknown. Here, we demonstrate that acute systemic inflammation induces the emergent activation of a previously unrecognized system of rapid migration of granulocyte-macrophage progenitors and committed macrophage-dendritic progenitors, but not other progenitors or stem cells, from bone marrow (BM) to regional lymphatic capillaries. The progenitor traffic to the systemic lymphatic circulation is mediated by Ccl19/Ccr7 and is NF-κB independent, Traf6/IκB-kinase/SNAP23 activation dependent, and is responsible for the secretion of pre-stored Ccl19 by a subpopulation of CD205+/CD172a+ conventional dendritic cells type 2 and upregulation of BM myeloid progenitor Ccr7 signaling. Mature myeloid Traf6 signaling is anti-inflammatory and necessary for lymph node myeloid cell development. This report unveils the existence and the mechanistic basis of a very early direct traffic of myeloid progenitors from BM to lymphatics during inflammation.

The high affinity IgE receptor: a signaling update

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

Cytoskeletal Transport, Reorganization, and Fusion Regulation in Mast Cell-Stimulus Secretion Coupling

Mast cells are well known for their role in allergies and many chronic inflammatory diseases. They release upon stimulation, e.g., via the IgE receptor, numerous bioactive compounds from cytoplasmic secretory granules. The regulation of granule secretion and its interaction with the cytoskeleton and transport mechanisms has only recently begun to be understood. These studies have provided new insight into the interaction between the secretory machinery and cytoskeletal elements in the regulation of the degranulation process. They suggest a tight coupling of these two systems, implying a series of specific signaling effectors and adaptor molecules. Here we review recent knowledge describing the signaling events regulating cytoskeletal reorganization and secretory granule transport machinery in conjunction with the membrane fusion machinery that occur during mast cell degranulation. The new insight into MC biology offers novel strategies to treat human allergic and inflammatory diseases targeting the late steps that affect harmful release from granular stores leaving regulatory cytokine secretion intact.

Authentic and Ectopically Expressed MRGPRX2 Elicit Similar Mechanisms to Stimulate Degranulation of Mast Cells

The identification of the Mas-related G-protein-coupled receptors (Mrgpr) as targets of diverse stimuli of mast cells (MCs), including neuropeptides and pseudo-allergy causing drugs, has placed these receptors at a prime position in MC research. However, the species-dependent diversity of these receptors raises the need for an adequate model for investigating the human MRGPRX2 receptor. RBL-2H3 cells, stably transfected with MRGPRX2 (RBL-MRGPRX2), are increasingly used for this purpose. Therefore, we investigated whether ectopically expressed MRGPRX2, in rat MCs, recapitulates its authentic signaling. To this purpose, we performed a broad comparative study of the responses of human LAD-2 MCs that express MRGPRX2 endogenously, and RBL-MRGPRX2 cells to compound 48/80, substance P and vancomycin, three proto-type ligands of MRGPRX2. We demonstrate that both models share similar dose-response relationships, kinetics and sensitivities to a wide range of signaling targeting drugs. Therefore, our results indicate that ectopically expressed MRGPRX2 preserves the signaling pathways employed to evoke human MC degranulation, which we show to rely on ERK1/2 MAP kinases, phospholipase C (PLC) and autophagy-related signaling. Importantly, we also show that the underlying mechanisms of MRGPRX2-triggered MC degranulation in either LAD-2 or RBL-MRGPRX2 cells are different from those elicited by its rodent orthologs.

TNF Production in Activated RBL-2H3 Cells Requires Munc13-4

Mast cell activation triggers intricate signaling pathways that promote the expression and/or release of a wide range of mediators including tumor necrosis factor (TNF; also known as TNFα). In this study, we investigated the connection between TNF secretion and TNF production, exploiting RBL-2H3 cells (a tumor analog of mucosal mast cells) that are depleted of Munc13-4, a crucial component of the mast cell exocytic machinery. We showed that antigen/IgE elicited robust TNF production in RBL-2H3 cells, but not in Munc13-4 knockout cells. The production defect was corrected when Munc13-4 was reintroduced into the knockout cell line, suggesting that the phenotype was not caused by any secondary effect derived from the knockout approach. Furthermore, pre-incubation of RBL-2H3 cells with R-7050, an antagonist of TNF receptor-dependent signaling, was shown to block TNF production without inhibiting TNF release. These observations provide fresh evidence for a robust feed-back loop to boost TNF production in activated mast cells.

Pancreas-specific SNAP23 depletion prevents pancreatitis by attenuating pathological basolateral exocytosis and formation of trypsin-activating autolysosomes

Intrapancreatic trypsin activation by dysregulated macroautophagy/autophagy and pathological exocytosis of zymogen granules (ZGs), along with activation of inhibitor of NFKB/NF-κB kinase (IKK) are necessary early cellular events in pancreatitis. How these three pancreatitis events are linked is unclear. We investigated how SNAP23 orchestrates these events leading to pancreatic acinar injury. SNAP23 depletion was by knockdown (SNAP23-KD) effected by adenovirus-shRNA (Ad-SNAP23-shRNA/mCherry) treatment of rodent and human pancreatic slices and in vivo by infusion into rat pancreatic duct. In vitro pancreatitis induction by supraphysiological cholecystokinin (CCK) or ethanol plus low-dose CCK were used to assess SNAP23-KD effects on exocytosis and autophagy. Pancreatitis stimuli resulted in SNAP23 translocation from its native location at the plasma membrane to autophagosomes, where SNAP23 would bind and regulate STX17 (syntaxin17) SNARE complex-mediated autophagosome-lysosome fusion. This SNAP23 relocation was attributed to IKBKB/IKKβ-mediated SNAP23 phosphorylation at Ser95 Ser120 in rat and Ser120 in human, which was blocked by IKBKB/IKKβ inhibitors, and confirmed by the inability of IKBKB/IKKβ phosphorylation-disabled SNAP23 mutant (Ser95A Ser120A) to bind STX17 SNARE complex. SNAP23-KD impaired the assembly of STX4-driven basolateral exocytotic SNARE complex and STX17-driven SNARE complex, causing respective reduction of basolateral exocytosis of ZGs and autolysosome formation, with consequent reduction in trypsinogen activation in both compartments. Consequently, pancreatic SNAP23-KD rats were protected from caerulein and alcoholic pancreatitis. This study revealed the roles of SNAP23 in mediating pathological basolateral exocytosis and IKBKB/IKKβ's involvement in autolysosome formation, both where trypsinogen activation would occur to cause pancreatitis. SNAP23 is a strong candidate to target for pancreatitis therapy.

Regulatory Mechanism of SNAP23 in Phagosome Formation and Maturation

Synaptosomal associated protein of 23 kDa (SNAP23), a plasma membrane-localized soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE), is a ubiquitously expressed protein that is generally involved in fusion of the plasma membrane and secretory or endosomal recycling vesicles during several types of exocytosis. SNAP23 is expressed in phagocytes, such as neutrophils, macrophages, and dendritic cells, and functions in both exocytosis and phagocytosis. This review focuses on the function of SNAP23 in immunoglobulin G Fc receptor-mediated phagocytosis by macrophages. SNAP23 and its partner SNAREs mediate fusion of the plasma membrane with intracellular organelles or vesicles to form phagosomes as well as the fusion of phagosomes with endosomes or lysosomes to induce phagosome maturation, characterized by reactive oxygen species production and acidification. During these processes, SNAP23 function is regulated by phosphorylation. In addition, microtubule-associated protein 1A/1B light chain 3 (LC3)-associated phagocytosis, which tightly promotes or suppresses phagosome maturation depending on the foreign target, requires SNAP23 function. SNAP23 that is enriched on the phagosome membrane during LC3-associated phagocytosis may be phosphorylated or dephosphorylated, thereby enhancing or inhibiting subsequent phagosome maturation, respectively. These findings have increased our understanding of the SNAP23-associated membrane trafficking mechanism in phagocytes, which has important implications for microbial pathogenesis and innate and adaptive immune responses.

ELKS1 controls mast cell degranulation by regulating the transcription of Stxbp2 and Syntaxin 4 via Kdm2b stabilization

ELKS1 is a protein with proposed roles in regulated exocytosis in neurons and nuclear factor κB (NF-κB) signaling in cancer cells. However, how these two potential roles come together under physiological settings remain unknown. Since both regulated exocytosis and NF-κB signaling are determinants of mast cell (MC) functions, we generated mice lacking ELKS1 in connective tissue MCs (Elks1^f/f Mcpt5-Cre) and found that while ELKS1 is dispensable for NF-κB-mediated cytokine production, it is essential for MC degranulation both in vivo and in vitro. Impaired degranulation was caused by reduced transcription of Syntaxin 4 (STX4) and Syntaxin binding protein 2 (Stxpb2), resulting from a lack of ELKS1-mediated stabilization of lysine-specific demethylase 2B (Kdm2b), which is an essential regulator of STX4 and Stxbp2 transcription. These results suggest a transcriptional role for active-zone proteins like ELKS1 and suggest that they may regulate exocytosis through a novel mechanism involving transcription of key exocytosis proteins.

Paclitaxel Induces Immunogenic Cell Death in Ovarian Cancer via TLR4/IKK2/SNARE-Dependent Exocytosis

Emerging evidence shows that the efficacy of chemotherapeutic drugs is reliant on their capability to induce immunogenic cell death (ICD), thus transforming dying tumor cells into antitumor vaccines. We wanted to uncover potential therapeutic strategies that target ovarian cancer by having a better understanding of the standard-of-care chemotherapy treatment. Here, we showed in ovarian cancer that paclitaxel induced ICD-associated damage-associated molecular patterns (DAMP, such as CALR exposure, ATP secretion, and HMGB1 release) in vitro and elicited significant antitumor responses in tumor vaccination assays in vivo Paclitaxel-induced TLR4 signaling was essential to the release of DAMPs, which led to the activation of NF-κB-mediated CCL2 transcription and IkappaB kinase 2-mediated SNARE-dependent vesicle exocytosis, thus exposing CALR on the cell surface. Paclitaxel induced endoplasmic reticulum stress, which triggered protein kinase R-like ER kinase activation and eukaryotic translation initiation factor 2α phosphorylation independent of TLR4. Paclitaxel chemotherapy induced T-cell infiltration in ovarian tumors of the responsive patients; CALR expression in primary ovarian tumors also correlated with patients' survival and patient response to chemotherapy. These findings suggest that the effectiveness of paclitaxel relied upon the activation of antitumor immunity through ICD via TLR4 and highlighted the importance of CALR expression in cancer cells as an indicator of response to paclitaxel chemotherapy in ovarian cancer.

IκB kinase 2 is not essential for platelet activation

Platelets are small anucleate cells that release a plethora of molecules to ensure functional hemostasis. It has been reported that IκB kinase 2 (IKK2), the central enzyme of the inflammatory NF-κB pathway, is involved in platelet activation, because megakaryocyte/platelet-specific deletion of exons 6 and 7 of IKK2 resulted in platelet degranulation defects and prolonged bleeding. We aimed to investigate the role of IKK2 in platelet physiology in more detail, using a platelet-specific IKK2 knockout via excision of exon 3, which makes up the active site of the enzyme. We verified the deletion on genomic and transcriptional levels in megakaryocytes and were not able to detect any residual IKK2 protein; however, platelets from these mice did not show any functional impairment in vivo or in vitro. Bleeding time and thrombus formation were not affected in platelet-specific IKK2-knockout mice. Moreover, platelet aggregation, glycoprotein GPIIb/IIIa activation, and degranulation were unaltered. These observations were confirmed by pharmacological inhibition of IKK2 with TPCA-1 and BMS-345541, which did not affect activation of murine or human platelets over a wide concentration range. Altogether, our results imply that IKK2 is not essential for platelet function.

Thiazolidine-2,4-dione-based irreversible allosteric IKK-β kinase inhibitors: Optimization into in vivo active anti-inflammatory agents

Selective kinase inhibitors development is a cumbersome task because of ATP binding sites similarities across kinases. On contrast, irreversible allosteric covalent inhibition offers opportunity to develop novel selective kinase inhibitors. Previously, we reported thiazolidine-2,4-dione lead compounds eliciting in vitro irreversible allosteric inhibition of IKK-β. Herein, we address optimization into in vivo active anti-inflammatory agents. We successfully developed potent IKK-β inhibitors with the most potent compound eliciting IC₅₀ = 0.20 μM. Cellular assay of a set of active compounds using bacterial endotoxin lipopolysaccharide (LPS)-stimulated macrophages elucidated significant in vitro anti-inflammatory activity. In vitro evaluation of microsomal and plasma stabilities showed that the promising compound 7a is more stable than compound 7p. Finally, in vivo evaluation of 7a, which has been conducted in a model of LPS-induced septic shock in mice, showed its ability to protect mice against septic shock induced mortality. Accordingly, this study presents compound 7a as a novel potential irreversible allosteric covalent inhibitor of IKK-β with verified in vitro and in vivo anti-inflammatory activity.

The cysteine-rich domain of synaptosomal-associated protein of 23 kDa (SNAP-23) regulates its membrane association and regulated exocytosis from mast cells

Synaptosomal-associated protein of 23 kDa (SNAP-23) plays an important role during regulated exocytosis of various inflammatory mediators, stored in secretory granules, from mast cells in response to physiological triggers. It is however synthesized as a soluble protein, and the mechanisms by which free SNAP-23 gets peripherally associated with membrane for the regulation of exocytosis, are poorly defined. SNAP-23 contains a hydrophobic domain with five closely spaced cysteines which get palmitoylated, and we show that SNAP-23 cysteine mutants show differential membrane association when transfected in rat basophilic leukemia (RBL) mast cells. SNAP-23 Cys^- mutant, devoid of all five cysteines, and SNAP-23 P119A (proline to alanine) mutant, that likely interferes with palmitoylation of SNAP-23 by palmitoyl transferases are completely cytosolic. Mutating specific cysteines (Cys; C) to leucine or phenylalanine (L or F; retains hydrophobicity but lacks palmitoylation) partially decreases the membrane association of SNAP-23 which is further hampered by alanine (A; has lesser hydrophobicity, and lacks palmitoylation) mutation at C79, C80 or C83 position. Cloning a transmembrane domain MDR3_1-145 from multidrug resistance protein into SNAP-23 Cys^- mutant is able to partially restore its membrane association. Regulated exocytosis studies using co-transfected human growth hormone (hGH) secretion reporter plasmid revealed that overexpression of SNAP-23 Cys^- and P119A mutants significantly inhibits the overall extent of exocytosis from RBL mast cells, whereas expression of SNAP-23 Cys^--MDR3_1-145 fusion protein is able to restore exocytosis. These results establish that the cysteine-rich domain of SNAP-23 regulates its membrane association and thereby also regulates exocytosis from mast cells.

Anaphylactic Degranulation of Mast Cells: Focus on Compound Exocytosis

Anaphylaxis is a notorious type 2 immune response which may result in a systemic response and lead to death. A precondition for the unfolding of the anaphylactic shock is the secretion of inflammatory mediators from mast cells in response to an allergen, mostly through activation of the cells via the IgE-dependent pathway. While mast cells are specialized secretory cells that can secrete through a variety of exocytic modes, the most predominant mode exerted by the mast cell during anaphylaxis is compound exocytosis-a specialized form of regulated exocytosis where secretory granules fuse to one another. Here, we review the modes of regulated exocytosis in the mast cell and focus on compound exocytosis. We review historical landmarks in the research of compound exocytosis in mast cells and the methods available for investigating compound exocytosis. We also review the molecular mechanisms reported to underlie compound exocytosis in mast cells and expand further with reviewing key findings from other cell types. Finally, we discuss the possible reasons for the mast cell to utilize compound exocytosis during anaphylaxis, the conflicting evidence in different mast cell models, and the open questions in the field which remain to be answered.

Syntaxin 3, but not syntaxin 4, is required for mast cell-regulated exocytosis, where it plays a primary role mediating compound exocytosis

Mast cells (MCs) participate in allergy, inflammation, and defense against pathogens. They release multiple immune mediators via exocytosis, a process that requires SNARE proteins, including syntaxins (Stxs). The identity of the Stxs involved in MC exocytosis remains controversial. Here, we studied the roles of Stx3 and -4 in fully developed MCs from conditional knockout mice by electrophysiology and EM, and found that Stx3, and not Stx4, is crucial for MC exocytosis. The main defect seen in Stx3-deficient MCs was their inability to engage multigranular compound exocytosis, while leaving most single-vesicle fusion events intact. We used this defect to show that this form of exocytosis is not only required to accelerate MC degranulation but also essential to achieve full degranulation. The exocytic defect was severe but not absolute, indicating that an Stx other than Stx3 and -4 is also required for exocytosis in MCs. The removal of Stx3 affected only regulated exocytosis, leaving other MC effector responses intact, including the secretion of cytokines via constitutive exocytosis. Our in vivo model of passive systemic anaphylaxis showed that the residual exocytic function of Stx3-deficient MCs was sufficient to drive a full anaphylactic response in mice.

Botulinum toxin blocks mast cells and prevents rosacea like inflammation

BACKGROUND

Rosacea is a chronic inflammatory skin condition whose etiology has been linked to mast cells and the antimicrobial peptide cathelicidin LL-37. Individuals with refractory disease have demonstrated clinical benefit with periodic injections of onabotulinum toxin, but the mechanism of action is unknown.

OBJECTIVES

To investigate the molecular mechanism by which botulinum toxin improves rosacea lesions.

METHODS

Primary human and murine mast cells were pretreated with onabotulinum toxin A or B or control. Mast cell degranulation was evaluated by β-hexosaminidase activity. Expression of botulinum toxin receptor Sv2 was measured by qPCR. The presence of SNAP-25 and VAMP2 was established by immunofluorescence. In vivo rosacea model was established by intradermally injecting LL-37 with or without onabotulinum toxin A pretreatment. Mast cell degranulation was assessed in vivo by histologic counts. Rosacea biomarkers were analyzed by qPCR of mouse skin sections.

RESULTS

Onabotulinum toxin A and B inhibited compound 48/80-induced degranulation of both human and murine mast cells. Expression of Sv2 was established in mouse mast cells. Onabotulinum toxin A and B increased cleaved SNAP-25 and decreased VAMP2 staining in mast cells respectively. In mice, injection of onabotulinum toxin A significantly reduced LL-37-induced skin erythema, mast cell degranulation, and mRNA expression of rosacea biomarkers.

CONCLUSIONS

These findings suggest that onabotulinum toxin reduces rosacea-associated skin inflammation by directly inhibiting mast cell degranulation. Periodic applications of onabotulinum toxin may be an effective therapy for refractory rosacea and deserves further study.

Cryptotanshinone inhibits IgE‑mediated degranulation through inhibition of spleen tyrosine kinase and tyrosine‑protein kinase phosphorylation in mast cells

Atopic dermatitis (AD) is a type of chronic skin inflammation and one of the most common relapsing allergic diseases, which presents with a severe rash and itchy skin lesions. The pathogenesis of AD is primarily associated with hyper‑activated mast cells, which makes them an effective treatment target. After cross‑linking the antigen/immunoglobulin (Ig) E complex binds to its high affinity receptor FcεRl on the surface of mast cells. The cells subsequently secrete excessive pro‑inflammatory mediators, including histamine and cytokines, which lead to pruritus and immune cell infiltration in the skin lesions. The present study screened natural compounds that have an inhibitory effect on IgE/antigen‑mediated secretory activity. It was revealed that cryptotanshinone (CRT), a natural compound extracted from Salvia miltiorrhiza Bunge, had inhibitory effects on the IgE/antigen complex. The underlying mechanism by which CRT exerted an anti‑allergy/inflammatory function was investigated using rat basophilic leukaemia (RBL) cells for degranulation assays and a 1‑chloro‑2,4‑dinitrobenzene (DNCB)‑induced AD Balb/c mouse model for in vivo study. CRT effectively mitigated the secretion of pro‑inflammatory cytokines, including tumor necrosis factor‑α and interleukin 1β, as well as immune cell infiltration into skin lesions in a mouse model of AD‑like skin disease induced by dinitrochlorobenzene. The inhibitory effect of CRT on IgE‑mediated mast cell degranulation was mediated by the inhibition of tyrosine kinase‑dependent degranulation signalling pathways involving spleen tyrosine kinase and Lyn. The present study revealed CRT as an inhibitor of mast cell degranulation. Therefore, CRT may be considered for development as a therapeutic drug to treat IgE‑mediated skin diseases.

Phosphorylation of SNAP-23 at Ser95 causes a structural alteration and negatively regulates Fc receptor-mediated phagosome formation and maturation in macrophages

SNAP-23 is a plasma membrane-localized soluble N-ethylmaleimide–sensitive factor attachment protein receptors (SNARE) involved in Fc receptor (FcR)-mediated phagocytosis. However, the regulatory mechanism underlying its function remains elusive. Using phosphorylation-specific antibodies, SNAP-23 was found to be phosphorylated at Ser95 in macrophages. To understand the role of this phosphorylation, we established macrophage lines overexpressing the nonphosphorylatable S95A or the phosphomimicking S95D mutation. The efficiency of phagosome formation and maturation was severely reduced in SNAP-23-S95D–overexpressing cells. To examine whether phosphorylation at Ser95 affected SNAP-23 structure, we constructed intramolecular Förster resonance energy transfer (FRET) probes of SNAP-23 designed to evaluate the approximation of the N termini of the two SNARE motifs. Interestingly, a high FRET efficiency was detected on the membrane when the S95D probe was used, indicating that phosphorylation at Ser95 caused a dynamic structural shift to the closed form. Coexpression of IκB kinase (IKK) 2 enhanced the FRET efficiency of the wild-type probe on the phagosome membrane. Furthermore, the enhanced phagosomal FRET signal in interferon-γ–activated macrophages was largely dependent on IKK2, and this kinase mediated a delay in phagosome-lysosome fusion. These results suggested that SNAP-23 phosphorylation at Ser95 played an important role in the regulation of SNARE-dependent membrane fusion during FcR-mediated phagocytosis.

Dynamic cycling of t-SNARE acylation regulates platelet exocytosis

Platelets regulate vascular integrity by secreting a host of molecules that promote hemostasis and its sequelae. Given the importance of platelet exocytosis, it is critical to understand how it is controlled. The t-SNAREs, SNAP-23 and syntaxin-11, lack classical transmembrane domains (TMDs), yet both are associated with platelet membranes and redistributed into cholesterol-dependent lipid rafts when platelets are activated. Using metabolic labeling and hydroxylamine (HA)/HCl treatment, we showed that both contain thioester-linked acyl groups. Mass spectrometry mapping further showed that syntaxin-11 was modified on cysteine 275, 279, 280, 282, 283, and 285, and SNAP-23 was modified on cysteine 79, 80, 83, 85, and 87. Interestingly, metabolic labeling studies showed incorporation of [3H]palmitate into the t-SNAREs increased although the protein levels were unchanged, suggesting that acylation turns over on the two t-SNAREs in resting platelets. Exogenously added fatty acids did compete with [3H]palmitate for t-SNARE labeling. To determine the effects of acylation, we measured aggregation, ADP/ATP release, as well as P-selectin exposure in platelets treated with the acyltransferase inhibitor cerulenin or the thioesterase inhibitor palmostatin B. We found that cerulenin pretreatment inhibited t-SNARE acylation and platelet function in a dose- and time-dependent manner whereas palmostatin B had no detectable effect. Interestingly, pretreatment with palmostatin B blocked the inhibitory effects of cerulenin, suggesting that maintaining the acylation state is important for platelet function. Thus, our work shows that t-SNARE acylation is actively cycling in platelets and suggests that the enzymes regulating protein acylation could be potential targets to control platelet exocytosis in vivo.

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

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

Diverse exocytic pathways for mast cell mediators

Mast cells play pivotal roles in innate and adaptive immunities but are also culprits in allergy, autoimmunity, and cardiovascular diseases. Mast cells respond to environmental changes by initiating regulated exocytosis/secretion of various biologically active compounds called mediators (e.g. proteases, amines, and cytokines). Many of these mediators are stored in granules/lysosomes and rely on intricate degranulation processes for release. Mast cell stabilizers (e.g. sodium cromoglicate), which prevent such degranulation processes, have therefore been clinically employed to treat asthma and allergic rhinitis. However, it has become increasingly clear that different mast cell diseases often involve multiple mediators that rely on overlapping but distinct mechanisms for release. This review illustrates existing evidence that highlights the diverse exocytic pathways in mast cells. We also discuss strategies to delineate these pathways so as to identify unique molecular components which could serve as new drug targets for more effective and specific treatments against mast cell-related diseases.

Effects of opioid and nonopioid analgesics on canine wheal formation and cultured human mast cell degranulation

Mast cell (MC) degranulation has been implicated in the side effect profile of a variety of clinically useful agents. Thus, after intrathecal delivery, formation of space-occupying, meningeally-derived masses may be related to local MC degranulation. We systematically characterized degranulating effects of opioid and nonopioid analgesics on cutaneous flares in the dog and in primary human MC (hMC) cultures.

METHODS

Dogs were anesthetized with IV propofol and received intradermal (ID) injections (50μL). Flare diameters were measured at 30min. Drugs showing flare responses were tested after intramuscular (IM) cromolyn (10mg/kg), a MC stabilizer. Human primary MCs (human cord blood CD34⁺/CD45⁺ cells) were employed and β-hexosaminidase in cell-free supernatants were measured to assess degranulation.

RESULTS

A significant skin flare for several classes of agents was observed including opioids, ziconotide, ketamine, ST-91, neostigmine, adenosine, bupivacaine, lidocaine, MK-801 and 48/80. Tizanidine, fentanyl, alfentanil, gabapentin and baclofen produced no flare. Flare produced by all ID agents, except adenosine, bupivacaine and lidocaine, was reduced by cromolyn. Naloxone had no effect upon opiate or 48/80 evoked flares. In hMC studies, 48/80 resulted in a concentration-dependent release of β-hexosaminidase. The rank order of drug-induced hMC β-hexosaminidase release was similar to that for flares.

CONCLUSIONS

A variety of therapeutically useful drugs degranulate MCs. This action may account for side effects such as the intrathecal granuloma resulting from spinally-delivered opioids. This degranulating effect may be useful in predicting potential intrathecal toxicity in the development of novel agents.

Rab5 is critical for SNAP23 regulated granule-granule fusion during compound exocytosis

Compound exocytosis is considered the most massive mode of exocytosis, during which the membranes of secretory granules (SGs) fuse with each other to form a channel through which the entire contents of their granules is released. The underlying mechanisms of compound exocytosis remain largely unresolved. Here we show that the small GTPase Rab5, a known regulator of endocytosis, is pivotal for compound exocytosis in mast cells. Silencing of Rab5 shifts receptor-triggered secretion from a compound to a full exocytosis mode, in which SGs individually fuse with the plasma membrane. Moreover, we show that Rab5 is essential for FcεRI-triggered association of the SNARE protein SNAP23 with the SGs. Direct evidence is provided for SNAP23 involvement in homotypic SG fusion that occurs in the activated cells. Finally, we show that this fusion event is prevented by inhibition of the IKKβ2 kinase, however, neither a phosphorylation-deficient nor a phosphomimetic mutant of SNAP23 can mediate homotypic SG fusion in triggered cells. Taken together our findings identify Rab5 as a heretofore-unrecognized regulator of compound exocytosis that is essential for SNAP23-mediated granule-granule fusion. Our results also implicate phosphorylation cycles in controlling SNAP23 SNARE function in homotypic SG fusion.

Mechanisms of the action of adenine on anti-allergic effects in mast cells

Introduction

Mast cells play an important role in allergic responses.

Methods

We herein demonstrated the mechanisms of inhibitory effect of adenine on IgE/antigen‐induced degranulation and TNF‐α release in mast cells.

Results

We found that these effects were dependent on the amino group of adenine because purine only weakly inhibited degranulation. Adenine also inhibited Ca²⁺ ionophore‐ and thapsigargin‐induced degranulation, however, this inhibitory effect was weaker than that of the antigen. Therefore, the inhibitory effects of adenine on degranulation may be mediated before as well as after the Ca²⁺ raise under the antigen stimulus. Adenine inhibited antigen‐induced Syk and the subsequent induction of AKT and ERK activation under FcϵRI‐mediated signal. Adenine also attenuated antigen‐induced increase in Ca²⁺. Furthermore, adenine inhibited IgE/antigen‐induced IKKα/β activation, which is involved in degranulation. Finally, adenine protected mice against anaphylactic allergic responses in vivo.

Conclusions

The present study revealed a key role of adenine in the attenuation of allergic responses through the inhibition of Syk‐mediated signal transduction and IKK‐mediated degranulation.

NF-κB and the Transcriptional Control of Inflammation

The NF-κB transcription factor was discovered 30 years ago and has since emerged as the master regulator of inflammation and immune homeostasis. It achieves this status by means of the large number of important pro- and antiinflammatory factors under its transcriptional control. NF-κB has a central role in inflammatory diseases such as rheumatoid arthritis, inflammatory bowel disease, and autoimmunity, as well as diseases comprising a significant inflammatory component such as cancer and atherosclerosis. Here, we provide an overview of the studies that form the basis of our understanding of the role of NF-κB subunits and their regulators in controlling inflammation. We also describe the emerging importance of posttranslational modifications of NF-κB in the regulation of inflammation, and highlight the future challenges faced by researchers who aim to target NF-κB transcriptional activity for therapeutic benefit in treating chronic inflammatory diseases.

PKC-dependent phosphorylation of Munc18a at Ser313 in activated RBL-2H3 cells

Protein Kinase C (PKC) regulates the release of pro-inflammatory compounds from IgE/antigen-activated mast cells by unknown mechanisms. In this study, we show for the first time that PKC inhibitor Ro-03-0432, which inhibits RBL-2H3 exocytosis/degranulation in a concentration-dependent fashion, prevents the phosphorylation of membrane fusion factor Munc18a at Ser 313. Our study provides fresh evidence that PKC-dependent protein phosphorylation may contribute to the intricate regulation of mast cell degranulation by directly targeting the fusion factors.

Stimulation of nAchRα7 Receptor Inhibits TNF Synthesis and Secretion in Response to LPS Treatment of Mast Cells by Targeting ERK1/2 and TACE Activation

The cholinergic anti-inflammatory pathway is recognized as one of the main mechanisms of neuromodulation of the immune system. Activation of the α7 nicotinic acetylcholine receptor (nAchRα7) suppresses cytokine synthesis in distinct immune cells but the molecular mechanisms behind this effect remain to be fully described. Mast cells (MCs) are essential players of allergic reactions and innate immunity responses related to chronic inflammation. Activation of TLR4 receptor in MCs leads to the rapid secretion of pre-synthesized TNF from intracellular pools and to the activation of NFκB, necessary for de novo synthesis of TNF and other cytokines. Here we report that the nAchRα7 receptor specific agonist GTS-21 inhibits TLR4-induced secretion of preformed TNF from MCs in vivo and in vitro. Utilizing bone marrow-derived mast cells (BMMCs) it was found that GTS-21 also diminished secretion of de novo synthesized TNF, TNF mRNA accumulation and IKK-dependent p65-NFκB phosphorylation in response to LPS. nAchRα7 triggering prevented TLR4-induced ERK1/2 phosphorylation, which resulted an essential step for TNF secretion due to the phosphorylation of the metallopeptidase responsible for TNF maturation (TACE). Main inhibitory actions of GTS-21 were prevented by AG490, an inhibitor of JAK-2 kinase. Our results show for the first time, that besides the prevention of NFκB-dependent transcription, inhibitory actions of nAchRα7 triggering include the blockade of pathways leading to exocytosis of granule-stored cytokines in MCs.

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

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

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

The comings and goings of MHC class I molecules herald a new dawn in cross-presentation

MHC class I (MHC-I) molecules are the centerpieces of cross-presentation. They are loaded with peptides derived from exogenous sources and displayed on the plasma membrane to communicate with CD8 T cells, relaying a message of tolerance or attack. The study of cross-presentation has been focused on the relative contributions of the vacuolar versus cytosolic pathways of antigen processing and the location where MHC-I molecules are loaded. While vacuolar processing generates peptides loaded onto vacuolar MHC-I molecules, how and where exogenous peptides generated by the proteasome and transported by TAP meet MHC-I molecules for loading has been a matter of debate. The source and trafficking of MHC-I molecules in dendritic cells have largely been ignored under the expectation that these molecules came from the Endoplasmic reticulum (ER) or the plasma membrane. New studies reveal a concentrated pool of MHC-I molecules in the endocytic recycling compartment (ERC). These pools are rapidly mobilized to phagosomes carrying microbial antigens, and in a signal-dependent manner under the control of Toll-like receptors. The phagosome becomes a dynamic hub receiving traffic from multiple sources, the ER-Golgi intermediate compartment for delivering the peptide-loading machinery and the ERC for deploying MHC-I molecules that alert CD8 T cells of infection.

Interaction of primary mast cells with Borrelia burgdorferi (sensu stricto): role in transmission and dissemination in C57BL/6 mice

Background

Borrelia burgdorferi (sensulato), the causative agent of Lyme borreliosis is a bacterium transmitted by hard ticks, Ixodes spp. Bacteria are injected into the host skin during the tick blood meal with tick saliva. There, Borrelia and saliva interact together with skin cells such as keratinocytes, fibroblasts, mast cells and other specific immune cells before disseminating to target organs.

Methods

To study the role of mast cells in the transmission of Lyme borreliosis, we isolated mouse primary mast cells from bone marrow and incubated them in the presence of Borrelia burgdorferi (sensu stricto) and tick salivary gland extract. We further analyzed their potential role in vivo, in a mouse model of deficient in mast cells (Kit^wsh−/− mice).

Results

To our knowledge, we report here for the first time the bacteria ability to induce the inflammatory response of mouse primary mast cells. We show that OspC, a major surface lipoprotein involved in the early transmission of Borrelia, induces the degranulation of primary mast cells but has a limited effect on the overall inflammatory response of these cells. In contrast, whole bacteria have an opposite effect. We also show that mast cell activation is significantly inhibited by tick salivary gland extract. Finally, we demonstrate that mast cells are likely not the only host cells involved in the early transmission and dissemination of Borrelia since the use of mast cell deficient Kit^wsh−/− mice shows a limited impact on these two processes in the context of this mouse genetic background.

Conclusions

The absence of mast cells did not change the replication rate of Borrelia in the skin. However, in the absence of mast cells, Borrelia dissemination to the joints was faster. Mast cells do not control skin bacterial proliferation during primary infection and the establishment of the primary infection, as shown in the C57BL/6 mouse model studied. Nevertheless, the Borrelia induced cytotokine modulation on mast cells might be involved in long term and/or repeated infections and protect from Lyme borreliosis due to the development of a hypersensitivity to tick saliva.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-017-2243-0) contains supplementary material, which is available to authorized users.

Peroxisome proliferator-activated receptor γ agonist suppresses mast cell maturation and induces apoptosis

Peroxisome proliferator-activated receptor gamma (PPAR γ), is important in the immunoregulation of the allergic response. Mast cells are the most important inflammatory cells in immediate hypersensitivity and allergic diseases. However, there is limited information regarding the effects of PPAR γ on mast cell maturation. In the present study, mouse bone marrow-derived mast cells (BMMCs) were cultured in interleukin (IL)-3 and stem cell factor (SCF), in the presence or absence of the PPAR γ agonist, pioglitazone (PIO). The expression levels of the tyrosine kinase receptor CD117 and the high affinity IgE receptor FcεRI α, were assessed by flow cytometry, cell viability was assessed by Alamar-Blue assay and histamine release was determined by measuring the activity of β-hexosaminidase. IL-3 and SCF are required for the development of mast cells in vitro. PIO dose-dependently inhibited the expression of CD117 and FcεRI α, and the maturation of BMMCs. Treatment with PIO additionally inhibited the formation of granules and reduced the expression of β-hexosaminidase. In addition, reverse transcription-polymerase chain reaction analysis revealed that BMMCs treated with PIO expressed a lower level of mast cell protease (MCP)-6 mRNA and PIO treatment enhanced the level of PPAR γ mRNA. Furthermore, PIO induced mast cell progenitor apoptosis. PPAR γ agonists may maintain mast cell homeostasis by inhibiting maturation of their precursors. The inhibitory effects of PPAR γ agonists include suppression of the activation of mast cells and a decrease in mast cell function in the inflammatory response. Therefore, PPAR γ agonists may serve as effective anti-inflammatory reagents in the treatment of allergic reactions.

Na+ influx via Orai1 inhibits intracellular ATP-induced mTORC2 signaling to disrupt CD4 T cell gene expression and differentiation

T cell effector functions require sustained calcium influx. However, the signaling and phenotypic consequences of non-specific sodium permeation via calcium channels remain unknown. α-SNAP is a crucial component of Orai1 channels, and its depletion disrupts the functional assembly of Orai1 multimers. Here we show that α-SNAP hypomorph, hydrocephalus with hopping gait, Napa^hyh/hyh mice harbor significant defects in CD4 T cell gene expression and Foxp3 regulatory T cell (Treg) differentiation. Mechanistically, TCR stimulation induced rapid sodium influx in Napa^hyh/hyh CD4 T cells, which reduced intracellular ATP, [ATP]_i. Depletion of [ATP]_i inhibited mTORC2 dependent NFκB activation in Napa^hyh/hyh cells but ablation of Orai1 restored it. Remarkably, TCR stimulation in the presence of monensin phenocopied the defects in Napa^hyh/hyh signaling and Treg differentiation, but not IL-2 expression. Thus, non-specific sodium influx via bonafide calcium channels disrupts unexpected signaling nodes and may provide mechanistic insights into some divergent phenotypes associated with Orai1 function.

DOI:http://dx.doi.org/10.7554/eLife.25155.001

Peroxisome proliferator-activated receptor-β/δ modulates mast cell phenotype

The peroxisome proliferator-activated receptor-β/δ (PPARβ/δ) is known to have multiple anti-inflammatory effects, typically observed in endothelial cells, macrophages, T cells and B cells. Despite the fact that mast cells are important mediators of inflammation, to date, the role of PPARβ/δ in mast cells has not been examined. Hence, the present study examined the hypothesis that PPARβ/δ modulates mast cell phenotype. Bone-marrow-derived mast cells (BMMCs) and peritoneal mast cells from Pparβ/δ^+/+ mice expressed higher levels of high-affinity IgE receptor (FcεRI) compared with Pparβ/δ^-/- mice. BMMCs from Pparβ/δ^+/+ mice also exhibited dense granules, associated with higher expression of enzymes and proteases compared with Pparβ/δ^-/- mice. Resting BMMCs from Pparβ/δ^+/+ mice secreted lower levels of inflammatory cytokines, associated with the altered activation of phospholipase Cγ1 and extracellular signal-regulated kinases compared with Pparβ/δ^-/- mice. Moreover, the production of cytokines by mast cells induced by various stimuli was highly dependent on PPARβ/δ expression. This study demonstrates that PPARβ/δ is an important regulator of mast cell phenotype.