Usage of sphingosine kinase isoforms in mast cells is species and/or cell type determined

FcεRI engagement in mast cells (MCs) induces the activation of two distinct sphingosine kinase isoforms (SphK1 and SphK2) to produce sphingosine-1-phosphate, a mediator essential for MC responses. Whereas embryonic-derived SphK2-null MCs showed impaired responses to Ag, RNA silencing studies on other MC types indicated a dominant role for SphK1. Given the known functional heterogeneity of MCs, we explored whether the reported differences in SphK1 or SphK2 usage could be reflective of phenotypic differences between MC populations. Using lentiviral-based short hairpin RNA to silence SphK1 or SphK2, we found that SphK2 is required for murine MC degranulation, calcium mobilization, and cytokine and leukotriene production, irrespective of the tissue from which the MC progenitors were derived, the stage of MC granule maturity, or the conditions used for differentiation. This finding was consistent with the lack of a full allergic response in SphK2-null mice challenged to undergo passive cutaneous anaphylaxis. A redundant role for both SphKs was uncovered, however, in chemotaxis toward Ag in all MC types tested and in TNF-α production in certain MC types. In contrast, human MC responses were dependent only on SphK1, associating with a more robust expression of this isoform and a more varied representation of SphK variants relative to murine MCs. The findings show that the function of SphK1 and SphK2 can be interchangeable in MCs; however, an important determinant of SphK isoform usage is the species of origin and an influencing factor, the tissue from which MCs may be derived and/or their differentiation state.

The nascent truncation of FcϵRIβ confers calmodulin binding which is essential for microtubule formation and degranulation in human mast cells (177.17)

The gene locus containing the MS4A2 gene (encoding FcϵRIβ) has strong linkage to allergy susceptibility. MS4A2 was considered as a candidate gene, but studies on the functional consequence of mutations in FcϵRIβ have been disappointing. We have identified a truncation of FcϵRIβ (t-FcϵRIβ) in humans. In this study we sought to identify the function of this variant. We determined that t-FcϵRIβ forms a complex with Fyn kinase and Gab2 in the cytosol and that the truncation of FcϵRIβ exposes a calmodulin (CaM) binding domain. Using co-IPs we show that in the presence of Ca2+, CaM binds to the t-FcϵRIβ complex, allowing phosphorylation of Fyn and Gab2, which are involved in microtubule formation. Confocal microscopy revealed translocation of the t-FcϵRIβ complex to a tubulin-rich region after cell activation. Deconvolution and 3D modeling of the imaging data demonstrated that the translocated complex surrounds the centrosome, forming an intricate network with microtubules. Utilizing lentiviral shRNA to silence the FcϵRIβ variants, we identified that knockdown of t-FcϵRIβ attenuated microtubule formation, degranulation and cytokine production in response to thapsigargin. Ca2+ influx and PGD2 production were unaffected. These data suggest that t-FcϵRIβ propagates Ca2+ signals by binding CaM and initiating microtubule formation which is essential for mast cell degranulation and cytokine release. Thus, targeting t-FcϵRIβ may offer a novel approach to treatment of allergic diseases.

Lantrunculin B-induced filamentous actin disassembly enables human mast cell degranulation by SCF alone (177.7)

The release of inflammatory mediators from mast cells following antigen/IgE-mediated FcϵRI aggregation induces allergic reactions including anaphylaxis. Stem cell factor (SCF), the ligand for the mast cell growth factor receptor KIT, potentiates such release. However, despite inducing similar early signaling events as antigen, SCF alone induces minimal, if any, degranulation in the absence of other stimuli. We therefore investigated whether the process underlying these differences may lie in receptor-induced distal events involving the reorganization of the actin cytoskeleton. To explore this issue, we utilized latrunculin B, an inhibitor of actin polymerization which is known to potentiate FcϵRI-mediated mast cell degranulation. As expected, latrunculin B moderately potentiated IgE/antigen-induced human mast cell degranulation, This response was, however, markedly enhanced in the presence of SCF (100 ng/ml), especially at lower antigen concentrations, suggesting that the SCF dependent component was more sensitive to latrunculin B than the antigen component. Furthermore, although SCF alone failed to induce degranulation, in the presence of latrunculin B, it induced degranulation in a concentration-dependent manner to a similar extent as that produced by IgE/antigen. These data thus suggest that SCF provides inadequate signals for the necessary remodeling of the actin filament network for degranulation which is rectified by latrunculin B.

Chronic IL-33 exposure induces a hypo-responsive mast cell phenotype (177.13)

Antigen-mediated mast cell (MC) activation, and the ensuing release of inflammatory mediators, is a pivotal event in the initiation of such conditions as asthma and anaphylaxis. IL-33 is elevated in asthmatic lungs and thus has been hypothesized to play a role in the pathogenesis of atopic diseases. However, we observed that IL-33 by itself did not induce anaphylaxis in a mouse model. Indeed, peritoneal mast cells taken from chronically IL-33-treated mice, appeared less sensitive to antigen challenge. We therefore examined the direct effects of IL-33 on mouse and human MCs in culture. Cells exposed to IL-33 for >72 h had dramatically reduced antigen-mediated degranulation and generation of specific cytokines. This was not a consequence of reduced expression of the high affinity IgE receptor, FcϵRI, or KIT but reflected altered signaling in the IL-33-treated cells. In this respect, the required antigen-mediated calcium signal and cytoskeletal reorganization were diminished in IL-33-treated mast cells, and this was respectively associated with IL-33-dependent down-regulation of PLCγ1 and Hck expression. These data thus reveal that MCs can be reprogrammed to produce a hypo-responsive phenotype and furthermore suggest that IL-33 may have a protective, rather than a causative role, in allergic disorders. The ability to manipulate MC activation in this manner may provide novel opportunities for therapeutic intervention of MC-driven disease.

MTORC1 and mTORC2 control normal and neoplastic mast cell homoeostasis through regulation of cell division rather than maintenance of survival (151.20)

Mature mast cells are terminally differentiated and non-dividing. However, a pathologic increase in mast cells is associated with the myeloproliferative disorder mastocytosis which has been linked to expression of an activating mutation (D816V) in the mast cell growth factor receptor, KIT. We found that in primary human mast cells developed from peripheral blood CD34+ progenitor cells, KIT activation led to activation of mTORC1 and mTORC2 pathways through phosphorylation of mTOR (mammalian target of rapamycin) at S2481 and that these pathways were constitutively activated in both D816V positive (HMC-1.2) and negative (HMC-1.1, LAD2) neoplastic human mast cells. A dual acting inhibitor of the mTORC1 and mTORC2 pathways, Torin1, and a more selective mTORC1 inhibitor, Rapamycin, abolished proliferation of the malignant mast cells regardless of the presence of the D816V mutation. However, little reduction in numbers of the non-dividing primary mast cells was observed. The effects of the inhibitors on cell proliferation correlated to downregulation of mTORC1 and mTORC2 pathways which was verified by shRNA. Furthermore, no evidence of apoptosis in response to the inhibitors was observed in either neoplastic or primary mast cells. Collectively, our data demonstrate that by targeting the mTORC1 and mTORC2 pathways, it may be possible to inhibit the proliferation of rapidly dividing mast cell populations associated with mastocytosis while leaving normal mast cells largely unaffected.

Prostaglandin E2 induces activation of mTORC1 and mTORC2 in mast cells: selective utilization of mTORC2 for the regulation of chemotaxis and mediator release (151.6)

PGE2 is a potent modifier/regulator of multiple mast cell responses including FcϵRI-mediated degraulation, chemokine production, adhesion to basement membrane proteins, and chemotaxis. Nevertheless, how PGE2 regulates these diverse responses remains unclear. In mouse bone marrow-derived mast cells, we observed that PGE2 induced activation of mTORC1 (mTOR-raptor complex), as indicated by increased p70S6K and 4E-BP1 phosphorylation; and mTORC2 (mTOR-rictor complex), as indicated by increased phosphorylation of AKT at position Ser473. We thus investigated the roles of these signaling complexes in PGE2-mediated mast cell responses. Inhibition of mTORC1, through the use of the selective mTORC1 inhibitor rapamycin or by raptor-targeted shRNA, had minimal impact on PGE2-mediated chemotaxis or chemokine production. In contrast Torin, which inhibited both mTORC1 and mTORC2, and rictor-targeted shRNA, markedly reduced PGE2-mediated chemotaxis and this was associated with down-regulation of PGE2-induced cytoskeletal reorganization. Downregulation of mTORC2 furthermore attenuated PGE2-induced production of the chemokine CCL2, which was linked to a significant reduction in ROS production. Mast cell degranulation and adhesion were unaffected by down-regulation of either mTOR complex. Taken together, these data illustrate that mTOR2, is an important signal for chemotaxis and chemokine production from PGE2-activated mast cells.

Regulation of RGS13 expression by the tumor suppressor p53 in mast cells (86.22)

Regulators of G-protein Signaling (RGS) proteins negatively regulate GPCRs by accelerating the GTPase activity of Gα. In recent years, non-GPCR-related functions have also been described for these proteins. We found previously that RGS13 was enriched in mast cells and inhibited anaphylaxis in mice by suppressing IgE-mediated mast cell degranulation. We therefore examined the transcriptional regulation of RGS13 in mast cells. We confirmed the transcriptional start site (TSS) by 5’ RACE in cDNA from the human mast cell line LAD2 and demonstrated promoter activity in a ~ 1 kb element upstream of exon 1 using luciferase reporter assays. We also identified 2 potential p53 response elements (RE) in the region 2 kb upstream of the TSS by bioinformatic analysis. Overexpression and shRNA-mediated knockdown of endogenous p53 in LAD2 cells revealed an inverse relationship between p53 levels and RGS13 expression. Endogenous p53 in these cells bound exclusively to oligonucleotides containing the p53 RE most upstream of the TSS. Cells expressing double-stranded oligonucleotides containing this site or a p53 consensus site had enhanced promoter activity. These studies indicate that p53 suppresses RGS13 transcription in mast cells. Thus, upregulation of p53 activity in mast cells exposed to stress could reduce the threshold for degranulation as a result of decreased RGS13 expression.

Contrasting requirements for DAP12 in receptor-mediated mast cell activation (86.4)

By inducing FcϵRI aggregation, antigen/IgE promotes mast cell cytokine production; a response markedly enhanced by specific TLR agonists. Critical to the ability of FcϵRI to activate mast cells is the recruitment of the tyrosine kinase Syk via the FcϵRIγ chains. Recent reports in other cell types have indicated that TLR’s may alternatively signal via the adaptor molecule DAP12 which also has the ability to bind Syk following tyrosine phosphorylation. 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 (BMMCs) and, following its phosphorylation by pervanadate treatment, DAP12 recruited Syk. Although the TLR agonists LPS, Pam3Cys, and MALP2 induced cytokine production and synergistically enhanced this response mediated by FcϵRI, they failed to increase basal DAP12 phosphorylation. Furthermore, no up-regulation or down-regulation of TLR-mediated responses, nor FcϵRI-mediated responses, was observed in DAP12-/- BMMCs. In contrast, DAP12 phosphorylation and Syk recruitment were observed in mast cells following concanavalin A (Con A)-induced aggregation of mannose-glycosylated receptors, and these responses, together with ConA-induced degranulation, were substantially reduced in the DAP12-/- BMMCs. These data show that in mast cells DAP12 does not regulate TLR- or FcϵRI-mediated responses, but is required for responses elicited by mannose-glycosylated receptors.

P67L: a cystic fibrosis allele with mild effects found at high frequency in the Scottish population

Only three mutant cystic fibrosis (CF) alleles have to date been established as conferring a dominant mild effect on affected subjects who are compound heterozygotes. We now add a fourth, P67L, which occurs on about 1.4% of Scottish CF chromosomes. Among 13 patients (12 unrelated) with this allele, the average age at diagnosis was 22.5 +/- 11.3 years. None of the cases had consistently raised sweat chloride concentrations, the average value being 57 +/- 9 mmol/l; 77% of the patients were pancreatic sufficient. When compared to three other established mild CF alleles, R117H, A455E, and 3849 + 10kb C-T, a compound heterozygote for P67L has minimal disease and clinical suspicions are unlikely to be confirmed other than by DNA typing.

The incidence of cystic fibrosis in Scotland calculated from heterozygote frequencies

The incidence of cystic fibrosis (CF) has previously been calculated from epidemiological surveys and from neonatal screening. With the cloning of the CF gene it has become possible to derive incidence figures from heterozygote frequencies, provided that the distribution of mutant alleles among healthy carriers is the same as among affected people. We have estimated the allele frequencies for four CF mutations, AF508, G551D, G542X and R117H, in 14360 unselected women undergoing antenatal heterozygote screening. The proportion of R117H, an allele of known mild effect, was much greater for heterozygotes than for homozygotes. The incidence of CF was therefore calculated from the heterozygote frequencies of AF508, G551D and G542X in a larger cohort of 27 161 successively screened women. The point estimate for the incidence of CF in the Scottish population was 1 in 1984, with 95% confidence intervals of 1 in 1692 to 1 in 2336.

Antenatal screening for cystic fibrosis: a trial of the couple model

OBJECTIVE

To assess the delivery and acceptability of antenatal couple screening for cystic fibrosis. Carrier status was notified only when both members of a partnership had cystic fibrosis alleles and therefore a one in four risk of having an affected child.

DESIGN

Mouthwash samples were tested when both partners participated. Results were returned only to positive couples.

SETTING

Two large maternity hospitals in Edinburgh.

SUBJECTS

Screening was offered to all couples who booked at one of the two hospitals.

MAIN OUTCOME MEASURES

(a) The take up of screening, carriers and carrier couples identified, take up of prenatal diagnosis, and numbers of affected fetuses detected; (b) questionnaire measures of patient satisfaction and stress.

RESULTS

Screening was offered to 8536 couples. 714 (8.4%) were regarded as ineligible, usually because of late booking or absence of a partner. 1900 (24.3%) of the remainder declined screening. Among the 5922 screened couples, four tested positive--that is, both partners were cystic fibrosis heterozygotes. All four elected to have prenatal diagnosis. There were three terminations of pregnancy because of an affected fetus, one couple having two successive pregnancies with affected fetuses. The participation rate was 76% for eligible couples (5922/7822) and 69% for all couples (5922/8536). Only 89 screened couples (1.5%) requested information on individual carrier status. No anxiety was detected among a cohort of the screened population, and 99% of questioned participants expressed satisfaction with the concept of couple screening.

CONCLUSIONS

Antenatal couple screening is a satisfactory and acceptable way of screening for cystic fibrosis and has been adopted as routine in the two trial hospitals.

Mass screening for cystic fibrosis heterozygotes: two assay systems compared

Two different assay systems were used to detect mutant cystic fibrosis alleles in mouthwash samples from pregnant women attending antenatal clinics. In the first phase of the study, comprising 3110 consecutive samples, we compared the performance of an in-house system and a commercial amplification refractory mutation system (ARMS). Successful analyses were completed at first attempt in approximately 90% of samples, and the patient resampling rate was 1.1%. There were no false positives and only one detected false negative. In the second phase, comprising 3333 consecutive samples, only ARMS was used. Again, there were no false positives and the patient resampling rate dropped to 0.5%. On the basis of convenience and reduced operator time, we have elected to use ARMS for further screening for cystic fibrosis heterozygotes.

Mass screening for cystic fibrosis heterozygotes: two assay systems compared

Two different assay systems were used to detect mutant cystic fibrosis alleles in mouthwash samples from pregnant women attending antenatal clinics. In the first phase of the study, comprising 3110 consecutive samples, we compared the performance of an in-house system and a commercial amplification refractory mutation system (ARMS). Successful analyses were completed at first attempt in approximately 90% of samples, and the patient resampling rate was 1.1%. There were no false positives and only one detected false negative. In the second phase, comprising 3333 consecutive samples, only ARMS was used. Again, there were no false positives and the patient resampling rate dropped to 0.5%. On the basis of convenience and reduced operator time, we have elected to use ARMS for further screening for cystic fibrosis heterozygotes.

Prenatal cystic fibrosis carrier screening: factors in a woman's decision to decline testing

Among 2207 women eligible to be screened for cystic fibrosis (CF) carrier status during pregnancy, 325 (15 per cent) declined to be tested. Of these, 260 (80 per cent) answered a questionnaire soliciting their reasons for not participating. The main factor was opposition to termination of pregnancy, with 43 per cent being against termination for any reason and another 11 per cent against termination of a CF fetus. Other reasons given were partner's disapproval or non-participation (10 per cent), perceived risk of a CF child being low (7 per cent), the error rate of the test (6 per cent), and the generation of unacceptable levels of anxiety (5 per cent). Eleven women (4 per cent) said that they did not wish to be tested during pregnancy, but only six of these would have accepted screening at another time.

Prenatal screening for cystic fibrosis: attitudes and responses of participants

A screening programme to detect cystic fibrosis heterozygotes has been running in the antenatal clinics of a major Edinburgh maternity hospital for more than 2 years. A questionnaire was used to assess participants' knowledge of the genetics of the disorder and their attitudes to being screened. The respondents were 64 female heterozygotes and 63 of their non-heterozygous male partners, 101 female controls and 100 male controls. Although the two groups of controls received far less direct information than the carriers and their partners, all four groups were well informed about the genetics of cystic fibrosis and the significance of being a gene carrier. A majority of each group felt that adequate information had been given in the information leaflet, that they understood the purpose of screening and that they were glad to have participated. There was a consensus that CF carrier testing should be routinely offered to pregnant women, and also that it should be available in family planning clinics and GP health centres, but not in schools.

Prenatal screening for cystic fibrosis: psychological effects on carriers and their partners

This study aimed to assess the psychological impact of screening for cystic fibrosis (CF) carrier status in a population of pregnant women. A cohort of 1798 women, who accepted the offer of testing before 18 weeks of pregnancy, filled in a self administered questionnaire seeking information on their perceived risk of carrier status and their emotional response, as well as a general health questionnaire (GHQ). Sixty-four women identified as CF carriers had partners who received a negative test result. This group and their partners were assessed, together with selected controls, on four further occasions: (1) on receiving the carrier's positive test result; (2) on receiving the partner's negative test result; (3) six weeks later; (4) six weeks after delivery. The instruments used were the GHQ and the Symptom Rating Test (SRT). When compared to control subjects, carriers showed a significant increase in generalised psychological disturbance which could be attributed specifically to symptoms of anxiety and depression during the period (average four days) that they awaited their partner's test result. On receiving a partner's negative test result, the carriers returned to control levels and maintained this equilibrium. Although there was no significant difference in generalised psychological disturbance between partners and their selected controls, partners did become significantly more anxious and manifested feelings of inadequacy while awaiting their own test result. Both male partners and male control subjects were more likely to become anxious if their partner was distressed.

Purification and characterization of human recombinant IgE-Fc fragments that bind to the human high affinity IgE receptor

The Fc-region of immunoglobulin E (IgE) comprising C epsilon 2, C epsilon 3, and C epsilon 4 domains is sufficient for binding to the alpha chain of the high affinity IgE-Fc receptor (Fc epsilon RI alpha). In order to identify the smallest Fc fragment capable of binding to the Fc epsilon RI alpha with high affinity, various regions of the IgE-Fc molecule were expressed in COS cells and investigated for their ability to bind Fc epsilon RI alpha. The smallest fragment that showed Fc epsilon RI alpha binding activity spans amino acids 329-547 and lacks the entire C epsilon 2 domain. Two active fragments, viz. Fc epsilon(315-547) (containing Cys328 which is responsible for interchain S-S bonding) and Fc epsilon(329-547), have been overexpressed in CHO cells and purified to homogeneity. The purified proteins bind to the Fc epsilon RI alpha with high affinity, similar to native IgE. SDS-polyacrylamide gel electrophoresis analyses indicate that Fc epsilon(315-547) is an S-S-linked dimer of apparent molecular mass of 68 kDa. Fc epsilon(329-547) appears on SDS-gel as three distinct bands at approximately 32 kDa, both under reducing and nonreducing conditions. However, size exclusion chromatography and analytical ultracentrifugation studies suggest that Fc epsilon(329-547) also remains associated as a dimer. The presence of N-linked glycosylation was detected in both proteins. The deglycosylated form of Fc epsilon(315-547) was isolated after Endo F/N-glycosidase F digestion and demonstrated to have binding activity comparable to that of the mock-digested protein. These results suggest that the presence of N-linked sugars is not necessary for Fc epsilon RI alpha binding. Both proteins blocked the release of histamine from RBL cells expressing human Fc epsilon RI alpha in a dose-dependent manner. The availability of these recombinant IgE-Fc proteins will be helpful in elucidating the key epitopes essential for the binding of IgE to its high affinity receptor.

A preliminary trial of couple screening for cystic fibrosis: designing an appropriate information leaflet

An information leaflet, inviting participation in an antenatal screening trial for cystic fibrosis, was sent to 388 couples together with the pregnant woman's first clinic appointment. The leaflet pointed out that couples would be treated as a unit and that further action would be taken only if both partners were found to carry mutant alleles. Participants and non-participants were also asked to fill in a questionnaire eliciting their views on the leaflet. Three hundred and twelve (80%) questionnaires were returned and 253 (65%) couples elected to be screened. More than 90% of respondents found the leaflet easy to understand, although about 10% wanted more information on cystic fibrosis. The main reason for entering the trial was to avoid the birth of an affected child, and the main reason for non-entry was opposition to termination of pregnancy. There was little anxiety about the prospect of being screened. However, more than a third of couples mis-identified their risk of both carrying a CF gene, despite the figure of 1 in 600 being explicitly stated in the leaflet.

Prenatal screening for cystic fibrosis

Screening for carriers of CF (cystic fibrosis) is now possible but the best way of delivering such a service is unknown. In one model 4348 women attending antenatal clinics in an Edinburgh maternity hospital were invited to participate in a trial of prenatal screening. Mouthwash samples were tested for six CF alleles (85% of mutant genes) and when a woman was found to be a CF carrier her partner was also tested. Heterozygous couples were offered prenatal diagnosis. 609 (14%) women declined to enter the trial and another 574 (13%) were not screened, usually because of late booking. Among the remaining 3165 women there were 111 carriers of a CF gene (1 in 29). 4 of these 111 had carrier partners and these couples opted for prenatal diagnosis, the 1 pregnancy with an affected fetus being terminated. The psychological impact of screening was assessed by the general health questionnaire. There was a significant increase in stress at the time of the test result among women identified as carriers. However, this disappeared when their male partners tested normal and did not reappear later in the pregnancy. By providing time for couples to discuss the possibility of screening and by offering the test at a point (the antenatal booking clinic) at which most pregnant women are seen, this approach has advantages, provided that counselling is readily available.