Differential signalling during B-cell maturation

Naturally occurring autoimmune disease in (NZB X NZW) F1 mice is correlated with suppression of MZ B cell development due to aberrant B Cell Receptor (BCR) signaling, which is exacerbated by exposure to inorganic mercury

Autoimmune diseases are multifactorial and include environmental as well as genetic drivers. Although much progress has been made in understanding the nature of genetic underpinnings of autoimmune disease, by comparison much less is understood regarding how environmental factors interact with genetics in the development of autoimmunity and autoimmune disease. In this report, we utilize the (NZB X NZW) F1 mouse model of Systemic Lupus Erythematosus (SLE). Mercury is a xenobiotic that is environmentally ubiquitous and is epidemiologically linked with the development of autoimmunity. Among other attributes of human SLE, (NZB X NZW) F1 mice spontaneously develop autoimmune-mediated kidney disease. It has been previously shown that if (NZB X NZW) F1 mice are exposed to inorganic mercury (Hg2+), the development of autoimmunity, including autoimmune kidney pathology, is accelerated. We now show that in these mice the development of kidney disease is correlated with a decreased percentage of marginal zone (MZ) B cells in the spleen. In Hg2+-intoxicated mice, kidney disease is significantly augmented, and matched by a greater decrease in MZ B cell splenic percentages than found in control mice. In Hg2+- intoxicated mice, the decrease in MZ B cells appears to be linked to aberrant B Cell Receptor (BCR) signal strength in transitory 2 (T2) B cells, developmental precursors of MZ B cells.

Mercury intoxication disrupts tonic signaling in B cells, and may promote autoimmunity due to abnormal phosphorylation of STIM-1 and other autoimmunity risk associated phosphoproteins involved in BCR signaling

Epidemiological studies link exposure to mercury with autoimmune disease. Unfortunately, in spite of considerable effort, no generally accepted mechanistic understanding of how mercury functions with respect to the etiology of autoimmune disease is currently available. Nevertheless, autoimmune disease often arises because of defective B cell signaling. Because B cell signaling is dependent on phosphorylation cascades, in this report, we have focused on how mercury intoxication alters phosphorylation of B cell proteins in antigen-non stimulated (tonic) mouse (BALB/c) splenic B cells. Specifically, we utilized mass spectrometric techniques to conduct a comprehensive unbiased global analysis of the effect of inorganic mercury (Hg2+) on the entire B cell phosphoproteome. We found that the effects were pleotropic in the sense that large numbers of pathways were impacted. However, confirming our earlier work, we found that the B cell signaling pathway stood out from the rest, in that phosphoproteins which had sites which were affected by Hg2+, exhibited a much higher degree of connectivity, than components of other pathways. Further analysis showed that many of these BCR pathway proteins had been previously linked to autoimmune disease. Finally, dose response analysis of these BCR pathway proteins showed STIM1_S575, and NFAT2_S259 are the two most Hg2+ sensitive of these sites. Because STIM1_S575 controls the ability of STIM1 to regulate internal Ca2+, we speculate that STIM1 may be the initial point of disruption, where Hg2+ interferes with B cell signaling leading to systemic autoimmunity, with the molecular effects pleiotropically propagated throughout the cell by virtue of Ca2+ dysregulation.

Defective Bcl-2 expression in memory B cells from common variable immunodeficiency patients

Common variable immunodeficiency (CVID) is a primary immunodeficiency characterized by hypogammaglobulinemia and different degrees of B cell compartment alteration. Memory B cell differentiation requires the orchestrated activation of several intracellular signaling pathways that lead to the activation of a number of factors, such as nuclear factor kappa B (NF-κB) which, in turn, promote transcriptional programs required for long-term survival. The aim of this study was to determine if disrupted B cell differentiation, survival and activation in B cells in CVID patients could be related to defects in intracellular signaling pathways. For this purpose, we selected intracellular readouts that reflected the strength of homeostatic signaling pathways in resting cells, as the protein expression levels of the Bcl-2 family which transcription is promoted by NF-κB. We found reduced Bcl-2 protein levels in memory B cells from CVID patients. We further explored the possible alteration of this crucial prosurvival signaling pathway in CVID patients by analysing the expression levels of mRNAs from anti-apoptotic proteins in naive B cells, mimicking T cell-dependent activation in vitro with CD40L and interleukin (IL)-21. BCL-XL mRNA levels were decreased, together with reduced levels of AICDA, after naive B-cell activation in CVID patients. The data suggested a molecular mechanism for this tendency towards apoptosis in B cells from CVID patients. Lower Bcl-2 protein levels in memory B cells could compromise their long-term survival, and a possible less activity of NF-κB in naive B cells, may condition an inabilityto increase BCL-XL mRNA levels, thus not promoting survival in the germinal centers.

Simulation of the Dynamics of Primary Immunodeficiencies in B Cells

Primary immunodeficiencies (PIDs) are a group of over 300 hereditary, heterogeneous, and mainly rare disorders that affect the immune system. Various aspects of immune system and PID proteins and genes have been investigated and facilitate systems biological studies of effects of PIDs on B cell physiology and response. We reconstructed a B cell network model based on data for the core B cell receptor activation and response processes and performed semi-quantitative dynamic simulations for normal and B cell PID failure modes. The results for several knockout simulations correspond to previously reported molecular studies and reveal novel mechanisms for PIDs. The simulations for CD21, CD40, LYN, MS4A1, ORAI1, PLCG2, PTPRC, and STIM1 indicated profound changes to major transcription factor signaling and to the network. Significant effects were observed also in the BCL10, BLNK, BTK, loss-of-function CARD11, IKKB, MALT1, and NEMO, simulations whereas only minor effects were detected for PIDs that are caused by constitutively active proteins (PI3K, gain-of-function CARD11, KRAS, and NFKBIA). This study revealed the underlying dynamics of PID diseases, confirms previous observations, and identifies novel candidates for PID diagnostics and therapy.

Genome-wide association study of cell-mediated immune response in chicken

Cell-mediated immunity (CMI) causes the intracellular destruction of the antigen or elimination of the host cell to make animals resistant against exogenous antigens and cancers. In this study, a genome-wide association study (GWAS) was carried out to identify genomic regions associated with CMI in chicken using chicken 60k high-density single nucleotide polymorphism (SNP) array. Genomic relationships were taken into account to adjust for population structure. In order to account for multiple testing, chromosome-wise false discovery rate was controlled at 5% and 10% levels. Moreover, a comparison of the power of fixed and mixed linear models based on genomic inflation factor was carried out. Mixed linear model (MLM) had better inflation rate, and therefore the results from MLM were used for subsequent analysis. Three significantly associated SNPs (FDR < 0.05) on chromosome 24 and linkage group E22C19W28_E50C23, and three suggestively associated SNPs (FDR < 0.1) on chromosome 1, 5 and 16 were identified. Pathway analysis showed that two biological pathways, which are related to immune response, were strongly associated with the candidate genes surrounding identified SNPs, and their influences were mostly on antigen processing and presentation, and cellular structure.

Requirement for PLCγ2 in IL-3 and GM-CSF-stimulated MEK/ERK phosphorylation in murine and human hematopoietic stem/progenitor cells

Even though the involvement of intracellular Ca(2+) Ca(i)(2+) in hematopoiesis has been previously demonstrated, the relationship between Ca(i)(2+) signaling and cytokine-induced intracellular pathways remains poorly understood. Herein, the molecular mechanisms integrating Ca(2+) signaling with the extracellular signal-regulated kinase 1/2 (ERK1/2) pathway in primary murine and human hematopoietic stem/progenitor cells stimulated by IL-3 and GM-CSF were studied. Our results demonstrated that IL-3 and GM-CSF stimulation induced increased inositol 1,4,5-trisphosphate (IP(3) ) levels and Ca(i)(2+) release in murine and human hematopoietic stem/progenitor cells. In addition, Ca(i)(2+) signaling inhibitors, such as inositol 1,4,5-trisphosphate receptor antagonist (2-APB), PKC inhibitor (GF109203), and CaMKII inhibitor (KN-62), blocked phosphorylation of MEK activated by IL-3 and GM-CSF, suggesting the participation of Ca(2+) -dependent kinases in MEK activation. In addition, we identify phospholipase Cγ2 (PLCγ2) as a PLCγ responsible for the induction of Ca(2+) release by IL-3 and GM-CSF in hematopoietic stem/progenitor cells. Furthermore, the PLCγ inhibitor U73122 significantly reduced the numbers of granulocyte-macrophage colony-forming units after cytokine stimulation. Similar results were obtained in both murine and human hematopoietic stem/progenitor cells. Taken together, these data indicate a role for PLCγ2 and Ca(2+) signaling through the modulation of MEK in both murine and human hematopoietic stem/progenitor cells.

Ca2+ signaling induced by sphingosine 1-phosphate and lysophosphatidic acid in mouse B cells

Lysophospholipids (LPLs) such as lysophosphatidic acid (LPA) and sphingosine 1-phosphate (S1P) are chemotactic for lymphocytes, and increases of in cytosolic [Ca(2+)] signal the regulation of lymphocyte activation and migration. Here, the authors investigated the effects of LPA and S1P on [Ca(2+)](c) in mouse B cell lines (WEHI-231 and Bal-17) and primary B cells isolated from mouse spleen and bone marrow, and focused on the modulation of store-operated Ca(2+) entry (SOCE) by LPLs. In Bal-17 (a mature B cell line) both LPA and S1P induced a transient [Ca(2+)](c) increase via a phospholipase C pathway. In addition, pretreatment with LPLs was found to augment thapsigargin-induced SOCE in Bal-17 cells. However, in WEHI-231 (an immature B cell line) LPLs had no significant effect on [Ca(2+)](c) or SOCE. Furthermore, in freshly isolated splenic B cells (SBCs) and bone marrow B cells (BMBCs), LPLs induced only a small increase in [Ca(2+)](c). Interestingly, however, pretreatment with LPLs markedly increased SOCE in primary B cells, and this augmentation was more prominent in BMBCs than SBCs. The unidirectional influx of Ca(2+) was measured using Ba(2+) as a surrogate ion. Similarly, Ba(2+) influx was also found to be markedly increased by LPLs in SBCs and BMBCs. Summarizing, LPLs were found to strongly augment SOCE-mediated Ca(2+)-signaling in mouse B cells. However, unlike the mature Bal-17 cell line, PLC-dependent Ca(2+) release was insignificant in primary B cells and inWEHI-231.

CD83 regulates splenic B cell maturation and peripheral B cell homeostasis

The central function of murine CD83 that is expressed on thymic epithelial cells is to induce the progression of double-positive thymocytes to single CD4-positive T cells. Several lines of evidence suggest an additional role for CD83 in the regulation of peripheral T and B cell responses. Here we show that CD83 is expressed by immature B cells and regulates their further maturation and survival in the periphery. Employing mixed bone marrow chimeras, we compare wild-type, CD83 over-expressing and CD83-deficient B cells within the same host. CD83 over-expression on the immature B cells themselves led to an accumulation of transitional B cells and a reciprocally reduced maturation of follicular B cells that was strictly correlated to the intensity of CD83 over-expression. The absence of CD83 on B cells resulted in a decreased maturation of marginal zone B cells and conferred a mild selection advantage for B cell survival in the periphery. Consenting with these findings, the over-expression of CD83 specifically and dose dependently interfered with homeostasis of B cells while T cell survival was not affected by CD83 over-expression over a period of 30 weeks. Taken together, our data suggest that CD83 negatively regulates B cell maturation and survival.

Role of protein tyrosine kinase and Erk1/2 activities in the Toll-like receptor 2-induced cellular activation of murine B cells by neisserial porin

Neisserial porins are potent immune adjuvants and have been demonstrated to stimulate and induce the activation of human and murine B lymphocytes. Their immunopotentiating ability is due largely to the upregulation of the surface expression of the costimulatory ligand CD86 (B7-2) on B cells and other antigen-presenting cells. Porin-induced activation is dependent on the innate immune pattern recognition receptor Toll-like receptor 2 (TLR2). These data have led us to investigate the signal transduction events induced by PorB from Neisseria meningitidis and then, using inhibitors of these pathways, to establish the mechanism by which this bacterial major outer membrane protein induces CD86 upregulation and the proliferation of murine B cells. PorB was able to induce (i) protein tyrosine kinase (PTK) activity, (ii) the phosphorylation of Erk1 and Erk2, and (iii) IkappaB-alpha phosphorylation, leading to NF-kappaB nuclear translocation in B cells in a TLR2-dependent manner. PorB-induced NF-kappaB nuclear translocation was not dependent on either PTK or Erk1/2 activities. However, B-cell proliferation and the induction of increased surface expression of CD86 by PorB were dependent on PTK activity and not Erk1/2 activation. In conclusion, PorB acts through TLR2 as a B-cell mitogen, triggering tyrosine phosphorylation of various cellular proteins that are involved in proliferation and CD86 expression, as well as the phosphorylation of Erk1/2, which is not necessary for CD86 upregulation or the proliferation of B cells.

Differentiation of gastric surface mucous cells (GSM06) induced by air-liquid interface is regulated partly through mitogen-activated protein kinase pathway

BACKGROUND AND AIM

The aim of the present study was to examine the role of mitogen-activated protein (MAP) kinase pathway on gastric surface epithelium using an established cell culture model in which differentiation is promoted in GSM06 cells by air-liquid interface.

METHODS

A double-dish culture system of mouse gastric surface mucous cell line GSM06 in Ham's F12 medium supplemented with 10% fetal calf serum and 50 microg/mL gentamicin at 37 degrees C in a humidified atmosphere of 5% CO(2) in air was used for an air-liquid interface. Culture cells were examined on histology, cell proliferation was evaluated by bromodeoxy-uridine (BrdU) uptake, and western blot analysis of extracellular signal-regulated kinase (ERK)1/2 and phosphate ERK1/2. On day 3, U0126, an inhibitor of MAP kinase kinase (MEK), was added to medium of incubated cells.

RESULTS

GSM06 cells were differentiated with an air-liquid interface for 3 weeks. Compared to immersion control culture, phosphorylated ERK 1/2 expression increased significantly. This increase was completely suppressed with U0126, and tall columnar cells developed by air-liquid interface in GSM06 were not observed in U0126-treated cells. Increase in BrdU uptake with air-liquid interface was suppressed by U0126.

CONCLUSION

These results suggested that MAP kinase signaling, activated by air-liquid interface, was, at least in part, related to cell differentiation in GSM06 cells induced by air-liquid interface.

Inhibitors of protein kinase C sensitise multiple myeloma cells to common genotoxic drugs

OBJECTIVES

Despite high dose treatment regimes multiple myeloma (MM) disease is still not curable. Patients become resistant to cytotoxic drugs and die of disease progression. Therefore, besides new cytotoxic compounds drug sensitisers are urgently needed.

METHODS

The MM cell lines U266, OPM-2, RPMI-8226 and NCI-H929 were incubated with the common anti-myeloma drugs like melphalan together with protein kinase C (PKC) inhibitors. Growth inhibition was measured using the water-soluble tetrazolium salt 4-[3-(4-iodophenyl)-2-(4-nitrophenyl)-2H-5-tetrazolio]-1,3-benzene disulfonate (WST-1 assay), and apoptosis was determined by flow cytometry after staining with fluorescein isothiocyanate-labeled annexin V (annexin-V-FITC) and propidium iodide. Intracellular signalling was shown by western blotting.

RESULTS

In this study we show that the combination of melphalan or doxorubicin with a PKC inhibitor, Gö6976 or enzastaurin, strongly increases cell toxicity. Increase of cytotoxicity is shown to be due to increased induction of apoptosis. Furthermore, we show that the protective effect of human bone marrow stromal cells (hBMSC) is abrogated by the PKC inhibitors. Finally, western blotting experiments revealed that incubation of myeloma cells with cytotoxic drugs like melphalan or doxorubicin leads to increased phosphorylation and therefore degradation of inhibitor of nuclear factor kappa B (IkappaB) and release of nuclear factor kappa B (NFkappaB). In contrast, enzastaurin inhibits phosphorylation of IkappaB.

CONCLUSIONS

We conclude that the combination of conventional drugs and PKC inhibitors might be very effective and represents a new strategy in the treatment of MM.

Filarial nematode secreted product ES-62 is an anti-inflammatory agent: therapeutic potential of small molecule derivatives and ES-62 peptide mimetics

1. The 'hygiene hypothesis' postulates that the recent increased incidence of allergic or autoimmune diseases (e.g. asthma, type I diabetes) in the West reflects an absence of appropriate priming of the immune response by infectious agents, such as parasitic worms, during childhood. 2. Consistent with this, it has long been recognized that several autoimmune disorders, such as rheumatoid arthritis (RA), a T helper (Th) 1-mediated autoimmune disease characterized by excess production of pro-inflammatory cytokines, such as tumour necrosis factor-alpha, exhibit reduced incidence and severity in geographical regions with high parasite load, suggesting that environmental factors may subtly alter disease progression. 3. Infection with worms also appears to suppress Th2-biased inflammatory disorders, such as asthma, because there also appears to be an inverse correlation between parasite load and atopy. This is perhaps more surprising, given that helminths often induce strong Th2-type immune responses characterized by release of specific cytokines, such as interleukin (IL)-4, IL-5 and IL-13. 4. Therefore, these findings suggest that the co-evolution of helminths with hosts, which has resulted in the ability of worms to modulate inflammatory responses in order to promote parasite survival, may also have generated a predisposition for the host to develop autoimmunity and allergy in the absence of infection. 5. The mechanisms underlying such immunomodulation are not clear, but appear to involve the release of parasite-derived molecules that allow the worms to modulate or evade the host immune response by a number of mechanisms, including skewing of cytokine responses and the induction of T regulatory cells. 6. In the present review we discuss the properties of one such filarial nematode-derived immunomodulatory molecule, namely ES-62, its anti-inflammatory action and the therapeutic potential of small molecule derivatives and peptides that mimic its action.

Transcriptional regulation of the bovine leukemia virus promoter by the cyclic AMP-response element modulator tau isoform

Bovine leukemia virus (BLV) expression is controlled at the transcriptional level through three Tax(BLV)-responsive elements (TxREs) responsive to the viral transactivator Tax(BLV). The cAMP-responsive element (CRE)-binding protein (CREB) has been shown to interact with CRE-like sequences present in the middle of each of these TxREs and to play critical transcriptional roles in both basal and Tax(BLV)-transactivated BLV promoter activity. In this study, we have investigated the potential involvement of the cAMP-response element modulator (CREM) in BLV transcriptional regulation, and we have demonstrated that CREM proteins were expressed in BLV-infected cells and bound to the three BLV TxREs in vitro. Chromatin immunoprecipitation assays using BLV-infected cell lines demonstrated in the context of chromatin that CREM proteins were recruited to the BLV promoter TxRE region in vivo. Functional studies, in the absence of Tax(BLV), indicated that ectopic CREMtau protein had a CRE-dependent stimulatory effect on BLV promoter transcriptional activity. Cross-link of the B-cell receptor potentiated CREMtau transactivation of the viral promoter. Further experiments supported the notion that this potentiation involved CREMtau Ser-117 phosphorylation and recruitment of CBP/p300 to the BLV promoter. Although CREB and Tax(BLV) synergistically transactivated the BLV promoter, CREMtau repressed this Tax(BLV)/CREB synergism, suggesting that a modulation of the level of Tax(BLV) transactivation through opposite actions of CREB and CREMtau could facilitate immune escape and allow tumor development.

Production of Ins(1,3,4,5)P4 mediated by the kinase Itpkb inhibits store-operated calcium channels and regulates B cell selection and activation

Antigen receptor-mediated production of inositol-1,4,5-trisphosphate (Ins(1,4,5)P3) in lymphocytes triggers the release of Ca2+ from intracellular stores; this release of Ca2+ results in the opening of store-operated Ca2+ channels in the plasma membrane. Here we report that mice lacking Ins(1,4,5)P3 3-kinase B (Itpkb), which converts Ins(1,4,5)P3 to inositol-1,3,4,5-tetrakisphosphate (Ins(1,3,4,5)P4), had impaired B lymphocyte development and defective immunoglobulin G3 antibody responses to a T lymphocyte-independent antigen. Itpkb-deficient B lymphocytes had the phenotypic and functional features of tolerant B lymphocytes and showed enhanced activity of store-operated Ca2+ channels after B lymphocyte receptor stimulation, which was reversed by the provision of exogenous Ins(1,3,4,5)P4. Our data identify Itpkb and its product Ins(1,3,4,5)P4 as inhibitors of store-operated Ca2+ channels and crucial regulators of B cell selection and activation.

Neutrophils are a key component of the antitumor efficacy of topical chemotherapy with ingenol-3-angelate

Harnessing neutrophils for the eradication of cancer cells remains an attractive but still controversial notion. In this study, we provide evidence that neutrophils are required to prevent relapse of skin tumors following topical treatment with a new anticancer agent, ingenol-3-angelate (PEP005). Topical PEP005 treatment induces primary necrosis of tumor cells, potently activates protein kinase C, and was associated with an acute T cell-independent inflammatory response characterized by a pronounced neutrophil infiltrate. In Foxn1(nu) mice depleted of neutrophils and in CD18-deficient mice (in which neutrophil extravasation is severely impaired) PEP005 treatment was associated with a >70% increase in tumor relapse rates. NK cell or monocyte/macrophage deficiency had no effect on relapse rates. Both in vitro and in mice, PEP005 induced MIP-2/IL-8, TNF-alpha, and IL-1beta, all mediators of neutrophil recruitment and activation. In vitro, PEP005 activated human endothelial cells resulting in neutrophil adhesion and also induced human neutrophils to generate tumoricidal-reactive oxygen intermediates. Treatment of tumors with PEP005 significantly elevated the level of anticancer Abs, which were able to promote neutrophil-mediated Ab-dependent cellular cytotoxicity (ADCC) in vitro. PEP005 treatment of tumors grown in SCID mice was also associated with >70% increase in tumor relapse rates. Taken together, these data suggest a central role for neutrophil-mediated ADCC in preventing relapse. PEP005-mediated cure of tumors therefore appears to involve initial chemoablation followed by a neutrophil-dependent ADCC-mediated eradication of residual disease, illustrating that neutrophils can be induced to mediate important anticancer activity with specific chemotherapeutic agents.

Proteome analysis of B-cell maturation

Proteins affected by anti-mIgM stimulation during B-cell maturation were identified using 2-DE-based proteomics. We investigated the proteome profiles of stimulated and nonstimulated Ramos B-cells at eight time points during 5 d and compared the obtained proteomic data to the corresponding data from DNA-microarray studies. Anti-mIgM stimulation of the cells resulted in significant differences (> or =twofold) in the protein abundance close to 100 proteins and differences in post-translational protein modifications. Forty-eight up- or down-regulated proteins were identified by mass spectrometric methods and database searches. The identities of a further nine proteins were revealed by comparing their positions to the known proteins in other lymphocyte 2-DE databases. Several of the proteins are directly related to the functional and morphological characteristics of B-cells, such as cytoskeleton rearrangement and intracellular signalling triggered by the crosslinking of B-cell receptors. In addition to proteins known to be involved in human B-cell maturation, we identified several proteins that were not previously linked to lymphocyte differentiation. The results provide deeper insights into the process of B-cell maturation and may lead to novel therapeutic strategies for immunodeficiencies. An interactive 2-DE reference map is available at http://bioinf.uta.fi/BcellProteome.

Transcription factors that regulate memory in humoral responses

At least three types of B lymphocytes are important for providing memory in a humoral immune response: 'classical' memory cells that do not secrete immunoglobulin (Ig), long-lived plasma cells (LLPCs) in the bone marrow, and 'innate-like' B-1 cells. In this review, our work on B-lymphocyte-induced maturation protein-1 (Blimp-1), a critical regulator of terminal B-cell differentiation, is discussed in the context of current knowledge of all transcriptional controls that regulate these three types of B cells. Blimp-1 is not required for formation of memory cells, but it is required for them to progress toward becoming plasma cells. Blimp-1 is required for Ig secretion in plasma cells and in B-1 cells. Induction of the activator X-box-binding protein-1 and formation of mu-secreted mRNA depend on Blimp-1 in both cell types. Finally, even after their formation, LLPCs in the bone marrow continue to require Blimp-1 for their maintenance.

Expression profile of RhoGTPases and RhoGEFs during RANKL-stimulated osteoclastogenesis: identification of essential genes in osteoclasts

RhoGTPases regulate actin cytoskeleton dynamics, a key element in osteoclast biology. We identified three novel genes induced during RANKL-stimulated osteoclastogenesis among RhoGTPases and their exchange factors that are essential in osteoclast biology.

INTRODUCTION

During the process of differentiation, adhesion to the bone matrix or osteolysis, the actin cytoskeleton of osteoclasts undergoes profound reorganization. RhoGTPases are key regulators of actin dynamics. They control cell adhesion, migration, and morphology through their action on actin cytoskeleton. In mice, there are 18 low molecular weight RhoGTPases. They are activated by guanine nucleotide exchange factors: the RhoGEFs. There are 76 RhoGEFs in mice: 65 belong to the Dbl family and 11 to the CZH family. To identify novel genes among RhoGTPases and RhoGEFs important in osteoclasts, we established the expression profiles of the complete families of RhoGTPases and RhoGEFs during RANKL-stimulated osteoclastogenesis.

MATERIALS AND METHODS

The RAW264.7 cell line, mouse bone marrow macrophages, and hematopoietic stem cells were used as precursors for RANKL-induced osteoclastogenesis. Gene arrays and real-time quantitative PCR analyses were performed to establish the transcription profiles of RhoGTPase and RhoGEF genes during differentiation. Small hairpin RNA was used to knock down genes of interest.

RESULTS

Of the 18 RhoGTPases and 76 RhoGEFs, the expression of three genes was upregulated by RANKL: the RhoGTPase RhoU/Wrch1, the Dbl family exchange factor Arhgef8/Net1, and the CZH family exchange factor Dock5. The inductions were observed in gene array and real-time quantitative PCR experiments performed in RAW264.7 cells. They were further confirmed in bone marrow macrophages and hematopoietic stem cells. Silencing of Wrch1 and Arhgef8 expression severely inhibited differentiation and affected osteoclast morphology. Dock5 suppression was lethal in osteoclast precursors while having no effect in fibroblasts.

CONCLUSIONS

We identified three genes among RhoGTPase signaling pathways that are upregulated during RANKL-induced osteoclastogenesis. These genes are novel essential actors in osteoclasts, most likely through the control of actin cytoskeleton dynamics.

Protein tyrosine phosphatase receptor-type O truncated (PTPROt) regulates SYK phosphorylation, proximal B-cell-receptor signaling, and cellular proliferation

The strength and duration of B-cell-receptor (BCR) signaling depends upon the balance between protein tyrosine kinase (PTK) activation and protein tyrosine phosphatase (PTP) inhibition. BCR-dependent activation of the SYK PTK initiates downstream signaling events and amplifies the original BCR signal. Although BCR-associated SYK phosphorylation is clearly regulated by PTPs, SYK has not been identified as a direct PTP substrate. Herein, we demonstrate that SYK is a major substrate of a tissue-specific and developmentally regulated PTP, PTP receptor-type O truncated (PTPROt). PTPROt is a member of the PTPRO family (also designated GLEPP, PTP-Ø, PTP-oc, and PTPu2), a group of highly conserved receptor-type PTPs that are thought to function as tumor suppressor genes. The overexpression of PTPROt inhibited BCR-triggered SYK tyrosyl phosphorylation, activation of the associated adaptor proteins SHC and BLNK, and downstream signaling events, including calcium mobilization and mitogen-activated protein kinase/extracellular signal-regulated kinase (MAPK/ERK) activation. PTPROt overexpression also inhibited lymphoma cell proliferation and induced apoptosis in the absence of BCR cross-linking, suggesting that the phosphatase modulates tonic BCR signaling.

CD72 down-modulates BCR-induced signal transduction and diminishes survival in primary mature B lymphocytes

CD72, a 45-kDa type II transmembrane glycoprotein carrying an ITIM motif, is believed to be an inhibitory coreceptor of the BCR. Mature B cells lacking CD72 show enhanced Ca(2+) mobilization and are hyperproliferative in response to BCR ligation. However, the signal transduction pathways downstream of BCR signaling that transmit the inhibitory effect of CD72 in mature B cells remain unknown. To address this question, we used hen egg lysozyme-specific BCR transgenic mice to elucidate the differential cell signaling between wild-type and CD72-deficient B cells in response to hen egg lysozyme Ag stimulation. Our results demonstrate that CD72 predominantly down-regulates the major signal transduction pathways downstream of the BCR, including NF-AT, NF-kappaB, ERK, JNK, p38-MAPK, and PI3K/Akt in mature B cells. CD72 ligation with anti-CD72 Ab (K10.6), which mimics the binding of CD100 (a natural ligand for CD72) to release the inhibitory function of CD72, augments cell proliferation, Ca(2+) flux, IkappaBalpha activation, and ERK MAPK activity upon Ag stimulation in wild-type B cells. In addition, we show direct evidence that CD72 promotes cell cycle arrest and apoptosis after Ag stimulation in mature B cells. Taken together, our findings conclude that CD72 plays a dominant role as a negative regulator of BCR signaling in primary mature B lymphocytes.

Introduction to immune cell signalling

The dynamic interaction of cells of the immune system with other cells, antigens and secreted factors determines the nature of an immune response. The response of individual cells is governed by the sequence of intracellular signalling events triggered following the association of cell surface molecules during cell-cell contact or the detection of soluble molecules of host or pathogen origin. In this review we will first outline the general principles of intracellular signal transduction. We will then describe the signalling pathways triggered following the recognition of antigen, as well as the detection of cytokines, and discuss how the signalling pathways activated regulate the effector response.