SNAREs and associated regulators in the control of exocytosis in the RBL-2H3 mast cell line

Mast cells participate in inflammation and allergies by releasing biologically active mediators stored in numerous cytoplasmic granules. Degranulation is tightly controlled and requires activation of cell surface receptors, such as the high affinity IgE receptor (FcepsilonRI). Here, we discuss some of the key components of the molecular machinery that regulates the final steps of fusion between the granular and plasma membrane based on results obtained with the rat mast cell line RBL-2H3. We emphasize the role of soluble N-ethylmaleimide attachment protein receptors (SNAREs) proteins such as syntaxin 4 that can promote membrane fusion through formation of a stable complex with SNAP-23. We also highlight the role of a Ser/Thr kinase found to be associated with Rab3D, a negative regulator of degranulation. Associated kinase activity, which diminishes after stimulation as a consequence of intracellular calcium increases, specifically phosphorylates syntaxin 4 thereby affecting its capacity to bind to its t-SNARE partner SNAP-23. Our results suggest a new way of how Rab3 GTPases may intersect with the function of SNAREs thought to be universal mediators of membrane fusion.     

The RBL-2H3 cell line: its provenance and suitability as a model for the mast cell

The RBL-2H3 cell line is a commonly used histamine-releasing cell line used in inflammation, allergy and immunological research. Quite commonly, it is referred to in research papers as a mast cell line, despite the fact that it is derived from basophils. There is also a lack of consistency, both between different research groups using the same cell line and with both mast cell and basophil physiology. The review follows the development of the RBL-2H3 cell line from its inception and then goes on to assess the nature of the cell line in terms of its characteristics and its response to various stimuli. The relationship of this cell line to the various mast cell subtypes and basophils is discussed and it is concluded that while the RBL-2H3 cell line shares some characteristics with both mast cells and basophils, it is not fully representative of either.     

The expanding roles of ITAM adapters FcRγ and DAP12 in myeloid cells

Summary:  The adapter proteins DAP12 and FcRγ associate with a wide spectrum of receptors in a variety of innate immune cells to mediate intracellular signaling pathways when their cognate receptor is engaged. These adapter proteins are coupled to their receptors through charged residues within the transmembrane regions of the adapter and receptor. DAP12 and FcRγ contain specific protein domains (referred to as immunoreceptor tyrosine-based activation motifs) that serve as the substrates and docking sites for kinases, allowing amplification of intracellular signaling reactions. Recent research has broadened the repertoire of receptors that utilize these adapters for signaling to include not only novel immunoglobulin superfamily members but also cytokine receptors, integrins, and other adhesion molecules. There is abundant evidence that these multifunctional signaling adapters also mediate inhibitory activity, downmodulating signaling from Toll-like receptors and other heterologous receptors. In this review, we discuss the newly described receptors that utilize DAP12 and/or FcRγ adapters to modulate innate immune responses.     

Sympathetic nerve contact alters membrane resistance of cells of the RBL-2H3 mucosal mast cell line.

Indirect evidence links sensory nerves with mast cells (MC) in inflammatory reactions of airway, skin, and intestine. Isolated MC secrete histamine, serotonin, and other inflammatory mediators in response to neuropeptides such as substance P (SP) in vitro. To obtain direct evidence of nerve/MC interactions, we used a tissue culture model involving the co-culture of murine sympathetic neurons and rat basophilic leukemia (RBL) cells (homologous to mucosal MC). An electrophysiologic analysis of the consequences of neuron/RBL cell contacts showed that neurite contact with RBL cells reduced the control input resistance (Ro) of 61.8 +/- 3.2 (n = 110) M omega to 22.4 +/- 4.8 (n = 13) M omega (P less than 0.01) without change in the membrane potential. Time course studies showed that Ro of RBL cells with neurite contact was always lower by 30 to 54% than adjacent RBL cells lacking such contact. This effect was not seen in RBL cells cultured on rat fibroblasts. Direct application of SP, bradykinin, and somatostatin, but not acetylcholine, noradrenaline, or the putative neurotransmitter ATP, could partly mimic the effect of neurite contact. Therefore, neurotransmitter release from sympathetic neurons in contact with RBL cells may decrease RBL cell membrane resistance, possibly leading to activation.     

Variants of the mucosal mast cell line (RBL-2H3) deficient in a functional membrane glycoprotein.

We have isolated and characterized subpopulations of the rat mucosal mast cell line, RBL-2H3, carrying either high or low density of a glycoprotein, recently established as mast cell function-associated antigen (MAFA, Ortega et al., 1991), on their surface. These populations were investigated in order to better define the involvement of the MAFA in coupling the immunological stimulation of mast cells to mediator release. The MAFA density on the cell surface of the deficient subpopulation was less than or equal to 10-20% that of the parental population and this phenotype was found to be stably maintained for several months. In contrast, the MAFA-enriched cells had maximally twice the number of copies per cell surface than that of the parental population and this phenotype was less stable. Significantly, low copy number of MAFA on the cell's surface was accompanied by a markedly different secretory response, i.e. (i) a considerable decrease in the secretory response to the Fc epsilon RI-mediated stimulus (ii) a marked enhancement of the ionomycin induced secretion. In order to gain insight into the causes for this decrease in cellular response to the Fc epsilon RI-mediated stimulus, we measured the amplitudes of several biochemical processes which are assigned to the stimulus-secretion coupling cascade. The Fc epsilon RI-mediated uptake of 45Ca2+ by the MAFA-deficient cells was considerably lower than that of the parental and MAFA-enriched cells. Similarly, these cell's Fc epsilon RI-induced rise in [Ca2+]i (both the initial transient as well as the sustained elevation), was markedly lower than that of the parental line and the MAFA-enriched cells. Moreover, the low initial transient rise in [Ca2+]i was found to be correlated with the decrease in Fc epsilon RI-mediated IP3 levels. We therefore examined the cell's content of the phosphatidyl-inositides hydrolyzing enzyme, phospholipase C gamma 1. This was found to be similar in the parental line and in its derived subpopulations. However, PLC gamma 1 activation, as measured by the time course of phosphorylation of its tyrosines, showed a marked difference: while PLC gamma 1 tyrosine phosphorylation, in the parental cells, was only transient (detected already 1 min after antigen addition and declined afterwards to basal levels at ca. 10 min), in the MAFA-deficient cells, tyrosine phosphorylated PLC gamma 1 was also observed 1 min after antigen addition, yet showed no decrease with time in its phosphorylation intensity for up to 30 min.(ABSTRACT TRUNCATED AT 400 WORDS)     

丹参酮对RBL-2H3肥大细胞增殖凋亡以及活化脱颗粒的影响

研究丹参酮对肥大细胞增殖凋亡以及活化脱颗粒的影响,深入探讨丹参酮的抗炎药理机制。应用细胞培养、流式细胞术等方法,观察丹参酮等药物对体外培养的RBL-2H3肥大细胞增殖凋亡以及活化脱颗粒的影响。结果:丹参酮I、IIA对肥大细胞增殖有明显的抑制作用,其IC50约为20μmol/L,并能明显促进细胞凋亡,且对RBL-2H3细胞活化脱颗粒有较强的抑制作用。结论:丹参酮对肥大细胞功能具有明显的抑制作用,可能是其发挥抗炎作用的一个重要方面。     

青藤碱对RBL-2H3肥大细胞增殖凋亡以及活化脱颗粒的影响

目的 通过研究青藤碱对肥大细胞增殖凋亡以及活化脱颗粒的影响,深入探讨青藤碱的抗炎作用机理.方法 应用细胞培养,流式细胞术等方法,观察青藤碱等药物对体外培养的RBL-2H3肥大细胞增殖凋亡以及活化脱颗粒的影响.结果 青藤碱对肥大细胞增殖有明显的抑制作用,其IG50约为1 mmol/L,并能促进肥大细胞凋亡,且对RBL-2H3肥大细胞活化脱颗粒有较强的抑制作用.结论青藤碱可能部分通过下调肥大细胞功能发挥抗炎抗风湿作用.     

Small-interfering RNA-mediated identification and regulation of the ternary SNARE complex mediating RBL-2H3 mast cell degranulation

Dysregulation of mast cell function contributes to allergic and autoimmune disease that affects more than 70 million persons in the United States alone. Identifying novel mast cell targets that mediate disease or disease progression is required for the development of innovative therapeutics for the treatment of allergy/asthma and autoimmune disease. RNA interference technologies offer hope both as basic research tools for target identification and as potential, novel, specific therapeutics. Soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) are a family of evolutionarily conserved proteins that have been postulated to mediate the transport and fusion of inflammatory mediator-laden vesicles to the membrane in mast cells leading to their subsequent exocytosis. The functional role(s) of specific SNARE family member complexes in mast cell degranulation has not been fully elucidated. Here, we characterize the functional importance of SNARE complexes in FcεRI receptor-mediated degranulation of RBL-2H3 cells utilizing RNA interference. We demonstrate that ternary SNARE complexes of synaptosomal-associated protein-23, Syntaxin 4 and vesicle-associated membrane protein-7 (VAMP-7) or VAMP-8 are directly involved in mast cell degranulation. Additionally, we evaluate the siRNAs directed against these molecules as potential therapeutic agents for disease intervention. These studies have identified specific SNARE proteins and complexes that serve as novel targets for the development of siRNA therapies to treat allergic and autoimmune disease.     

羧胺三唑对RBL-2H3细胞活化脱颗粒的影响

目的研究羧胺三唑(CAI)对RBL-2H3肥大细胞增殖、凋亡及活化脱颗粒的影响,探索CAI的抗感染作用机制。方法以C48/80诱导RBL-2H3细胞活化脱颗粒模型,中性红染色法观察细胞脱颗粒的形态学,分别用ELISA法和底物显色法检测细胞培养上清中组胺和β-氨基己糖苷酶的释放水平,CCK-8法测定细胞活力,Hoechst 33342荧光染色法检测细胞凋亡。结果与对照组相比,10、20和40μmol/L CAI能够不同程度抑制C48/80诱导的RBL-2H3细胞脱颗粒反应,20和40μmol/L CAI能够降低C48/80诱导的组胺释放(P0.01),40μmol/L CAI能够降低β-氨基己糖苷酶的释放(P0.01)。另外,所用各浓度的CAI对细胞增殖和凋亡均无明显影响。结论 CAI能有效抑制RBL-2H3肥大细胞的活化脱颗粒,此作用并不是通过细胞毒发挥作用的。CAI可能部分通过下调肥大细胞的功能活化,发挥其抗感染作用。     

Serine/threonine protein kinases synergistically regulate phospholipase D1 and 2 and secretion in RBL-2H3 mast cells

The role of phospholipase (PL) D in secretion was examined in RBL-2H3 mast cells which contain both PLD1 and 2. The effects of pharmacologic stimulants and inhibitors of Ca(2+)/calmodulin-dependent kinase II, protein kinase C, and protein kinase A suggested that all three kinases synergistically stimulate PLD and, when associated with a calcium signal, secretion as well to indicate a possible linkage between these two events. Overexpression of either PLD1 or 2 markedly enhanced the activation of PLD by pharmacologic stimulants as well as antigen and both isoforms thus appear co-ordinately regulated. As the expressed PLD1 was associated with secretory granules and PLD2 with the plasma membrane, the two isoforms may serve distinct but complementary functions in secretion.     

HMBOX1 negatively regulates NK cell functions by suppressing the NKG2D/DAP10 signaling pathway

HMBOX1 is a new member of the homeobox family. Homeobox members have been reported to participate in embryonic development and systemic metabolism, but the function of HMBOX1 remains unclear, especially in the hematopoietic system. Here, we show that HMBOX1 is expressed at a high level in primary human NK cells but is expressed at much lower levels in NK cell lines. Overexpression of HMBOX1 significantly inhibited NK cell activities, including natural cytotoxicity against tumor cells, the level of CD107a (a marker protein for degranulation) and the production of cytolytic proteins (perforin and granzymes). More interestingly, HMBOX1 negatively regulated the expression of NKG2D and the activation of the NKG2D/DAP10 signaling pathway in NK cells. This effect was reversed by knocking down HMBOX1. Taken together, these findings demonstrate that HMBOX1 may act as a negative regulator of NK cell functions via suppressing the NKG2D/DAP10 signaling pathway.     

反义RNA抑制synaptotagminⅡ在RBL-2H3的表达及对胞吐的作用

目的：研究SynaptotagminⅡ（简称syt2）在RBL-2H3（简称RBL）中的表达及其在胞吐中的作用。方法：通过Westernblot检测Syt2在RBL中的表达情况。用高保真酶扩增Syt2，与pEGFP-N1构建全长反义基因表达载体，电穿孔转染RBL，G418筛选获得稳定转染细胞。通过钙离子载体和抗原刺激，应用Westernblot检测稳定转染细胞和对照细胞分泌的组织蛋白酶D，分析Syl2对胞吐的影响。结果：在RBL中检测到Syt2表达。构建了pEGFP-N1-Syt2-AS质粒，插入片段测序结果与GenBank登录号NM012665（ratsyt2）序列完全一致。经转染和G418筛选，获得了稳定转染细胞RBL-Syt2-AS，Westernblot结果显示，两株RBL-Syt2-AS表达的syt2均明显减少，分别只有对照的8％和10％。经刺激后，RBL-Syt2．AS分泌的组织蛋白酶D较对照明显增加。结论：Syt在RBL中表达，其对RBL溶酶体胞吐起负调控作用。     

The Distribution of Synaptotagmin Ⅱ in RBL-2H3 and Its Regulation on Exocytosis of Lysosomes in RBL-2H3

Synaptotagmin （Syt） constitutes a family of membrane-trafficking proteins, so far nearly 20 Syts have been discovered. Extensive work showed that synatotagmins were a potential Ca^2＋ sensor for regulated exocytosis. This study was to investigate the expression and location of synaptotagmin II （Syt2） in RBL-2H3 （RBL） and its role in regulating exocytosis of RBL. The expression of Syt2 in RBL was confirmed by Western blot. The recombinant expression vector pEGFP-N1-Syt2 was constructed and transfected into RBL by electroporation, the stable transfectant RBL-Syt2-S expressing fusion protein Syt2-EGFP were obtained and Syt2 was highly concentrated at plasma membrane with little detected in cytoplasm. To analyze the role of Syt2 during exocytosis of RBL, the release of cathepsin D was assayed by immunoblotting. Compared with control, the release of cathepsin D by RBL-Syt2-S was markedly decreased. The results indicated that Syt2 played a negative regulation in exocytosis of lysosomes in RBL. Cellular ＆ Molecular Immunology. 2005;2（3）：205-209.     

The Distribution of Synaptotagmin II in RBL-2H3 and Its Regulation on Exocytosis of Lysosomes in RBL-2H3

Synaptotagmin(Syt)constitutes a family of membrane-trafficking proteins,so far nearly 20 Syts have beendiscovered.Extensive work showed that synatotagmins were a potential Ca~(2+) sensor for regulated exocytosis.Thisstudy was to investigate the expression and location of synaptotagmin Ⅱ(Syt2)in RBL-2H3(RBL)and its role inregulating exocytosis of RBL.The expression of Syt2 in RBL was confirmed by Western blot.The recombinantexpression vector pEGFP-N1-Syt2 was constructed and transfected into RBL by electroporation,the stabletransfectant RBL-Syt2-S expressing fusion protein Syt2-EGFP were obtained and Syt2 was highly concentrated atplasma membrane with little detected in cytoplasm.To analyze the role of Syt2 during exocytosis of RBL,therelease of cathepsin D was assayed by immunoblotting.Compared with control,the release of cathepsin D byRBL-Syt2-S was markedly decreased.The results indicated that Syt2 played a negative regulation in exocytosis oflysosomes in RBL.Cellular & Molecular Immunology.2005;2(3):205-209.     

TLR-mediated signaling pathways circumvent the requirement for DAP12 in mast cells for the induction of inflammatory mediator release

TLR, expressed on the surface of mast cells, respond to a variety of bacterial and viral components to induce and enhance high-affinity IgE receptor-mediated cytokine production. Recent reports have indicated that specific TLR-dependent responses in macrophages and dendritic cells are regulated by the ITAM-containing molecule, DAP12. When phosphorylated, DAP12 recruits Syk, which is a critical molecule for mast cell activation. We therefore examined whether DAP12 similarly regulates TLR-mediated responses in mast cells. DAP12 was confirmed to be expressed in both human and mouse mast cells and, upon phosphorylation, to recruit Syk. However, although TLR agonists induced cytokine production, and synergistically enhanced high-affinity IgE receptor-mediated cytokine production, surprisingly, they failed to increase DAP12 phosphorylation in mouse bone marrow-derived mast cells (BMMC). Furthermore, normal TLR-mediated responses were observed in DAP12(-/-) BMMC. However, DAP12 phosphorylation and subsequent Syk recruitment were observed in BMMC following Con A-induced aggregation of mannose-glycosylated receptors, and these responses, together with Con A-induced degranulation, were substantially reduced in the DAP12(-/-) BMMC. These data demonstrate that TLR have differential requirements for DAP12 for their function in different cell types and that the inability of TLR to influence mast cell degranulation may be linked to their inability to utilize DAP12 to recruit Syk.     

Clustering the mast cell function-associated antigen (MAFA) leads to tyrosine phosphorylation of p62Dok and SHIP and affects RBL-2H3 cell cycle

The mast cell function-associated antigen (MAFA) is a type II membranal glycoprotein expressed by rat mast cells and basophils. MAFA clustering by its specific monoclonal antibody, (mAb) G63, efficiently inhibits the FcvarepsilonRI induced secretory response of mucosal-type mast cells of the RBL-2H3 line, as well as bone marrow-derived mast cells. Here we present results which suggest that MAFA has also a capacity of modulating the cell cycle of the RBL-2H3 line. We found that MAFA clustering, by mAb G63 or by its F(ab')2 fragments, reduces the cell proliferation rate. Cell cycle analysis by flow cytometry revealed that the number of cells in sub-G phase is considerably higher for cells on which MAFA was clustered. Results of biochemical experiments established that MAFA clustering leads to a marked increase in the transient tyrosine phosphorylation of the adaptor protein p62(Dok) and the inositol phosphatase SHIP. Concomitantly, their respective binding to RasGAP and Shc was increased. Furthermore, the GTP binding protein Sos1 was found to dissociate from Shc upon MAFA clustering, suggesting that SHIP and Sos1 compete for Shc binding. We therefore suggest that MAFA has also a role in regulating RBL-2H3 cell proliferation rate by inhibiting RasGTP formation in the Ras signaling pathway.