Requirement of phospholipase C-gamma 2 activation in surface immunoglobulin M-induced B cell apoptosis

The inositol 5-phosphatase INPP5B regulates B cell receptor clustering and signaling

Upon antigen binding, the B cell receptor (BCR) undergoes clustering to form a signalosome that propagates downstream signaling required for normal B cell development and physiology. BCR clustering is dependent on remodeling of the cortical actin network, but the mechanisms that regulate actin remodeling in this context remain poorly defined. In this study, we identify the inositol 5-phosphatase INPP5B as a key regulator of actin remodeling, BCR clustering, and downstream signaling in antigen-stimulated B cells. INPP5B acts via dephosphorylation of the inositol lipid PI(4,5)P2 that in turn is necessary for actin disassembly, BCR mobilization, and cell spreading on immobilized surface antigen. These effects can be explained by increased actin severing by cofilin and loss of actin linking to the plasma membrane by ezrin, both of which are sensitive to INPP5B-dependent PI(4,5)P2 hydrolysis. INPP5B is therefore a new player in BCR signaling and may represent an attractive target for treatment of B cell malignancies caused by aberrant BCR signaling.

Kinase inhibition in autoimmunity and inflammation

Despite recent advances in the treatment of autoimmune and inflammatory diseases, unmet medical needs in some areas still exist. One of the main therapeutic approaches to alleviate dysregulated inflammation has been to target the activity of kinases that regulate production of inflammatory mediators. Small-molecule kinase inhibitors have the potential for broad efficacy, convenience and tissue penetrance, and thus often offer important advantages over biologics. However, designing kinase inhibitors with target selectivity and minimal off-target effects can be challenging. Nevertheless, immense progress has been made in advancing kinase inhibitors with desirable drug-like properties into the clinic, including inhibitors of JAKs, IRAK4, RIPKs, BTK, SYK and TPL2. This Review will address the latest discoveries around kinase inhibitors with an emphasis on clinically validated autoimmunity and inflammatory pathways.

Syk facilitates phagosome-lysosome fusion by regulating actin-remodeling in complement-mediated phagocytosis

Effective phagocytosis is crucial for host defense against pathogens. Macrophages entrap pathogens into a phagosome and subsequently acidic lysosomes fuse to the phagosome. Previous studies showed the pivotal role of actin-remodeling mediated by phosphoinositide-related signaling in phagosome formation, but the mechanisms of phagosome-lysosome fusion remain unexplored. Here we show that in complement-mediated phagocytosis, phagosome-lysosome fusion requires the disappearance of F-actin structure surrounding the phagosome and a tyrosine kinase Syk plays a key role in this process. Using macrophage-like differentiated HL60 and Syk-knockout (Syk-KO) HL60 cells, we found that Syk-KO cells showed insufficient phagosome acidification caused by impaired fusion with lysosomes and permitted the survival of Candida albicans in complement-mediated phagocytosis. Phagosome tracking analysis showed that during phagosome internalization process, F-actin surrounding phagosomes disappeared in both parental and Syk-KO cells but this structure was reconstructed immediately only in Syk-KO cells. In addition, F-actin-stabilizing agent induced a similar impairment of phagosome-lysosome fusion. Collectively, Syk-derived signaling facilitates phagosome-lysosome fusion by regulating actin-remodeling.

The kinase IRAK4 promotes endosomal TLR and immune complex signaling in B cells and plasmacytoid dendritic cells

The dysregulation of multiple signaling pathways, including those through endosomal Toll-like receptors (TLRs), Fc gamma receptors (FcγR), and antigen receptors in B cells (BCR), promote an autoinflammatory loop in systemic lupus erythematosus (SLE). Here, we used selective small-molecule inhibitors to assess the regulatory roles of interleukin-1 receptor (IL-1R)-associated kinase 4 (IRAK4) and Bruton's tyrosine kinase (BTK) in these pathways. The inhibition of IRAK4 repressed SLE immune complex- and TLR7-mediated activation of human plasmacytoid dendritic cells (pDCs). Correspondingly, the expression of interferon (IFN)-responsive genes (IRGs) in cells and in mice was positively regulated by the kinase activity of IRAK4. Both IRAK4 and BTK inhibition reduced the TLR7-mediated differentiation of human memory B cells into plasmablasts. TLR7-dependent inflammatory responses were differentially regulated by IRAK4 and BTK by cell type: In pDCs, IRAK4 positively regulated NF-κB and MAPK signaling, whereas in B cells, NF-κB and MAPK pathways were regulated by both BTK and IRAK4. In the pristane-induced lupus mouse model, inhibition of IRAK4 reduced the expression of IRGs during disease onset. Mice engineered to express kinase-deficient IRAK4 were protected from both chemical (pristane-induced) and genetic (NZB/W_F1 hybrid) models of lupus development. Our findings suggest that kinase inhibitors of IRAK4 might be a therapeutic in patients with SLE.

Inhibition of Btk by Btk-specific concentrations of ibrutinib and acalabrutinib delays but does not block platelet aggregation mediated by glycoprotein VI

Ibrutinib and acalabrutinib are irreversible inhibitors of Bruton tyrosine kinase used in the treatment of B-cell malignancies. They bind irreversibly to cysteine 481 of Bruton tyrosine kinase, blocking autophosphorylation on tyrosine 223 and phosphorylation of downstream substrates including phospholipase C-γ2. In the present study, we demonstrate that concentrations of ibrutinib and acalabrutinib that block Bruton tyrosine kinase activity, as shown by loss of phosphorylation at tyrosine 223 and phospholipase C-γ2, delay but do not block aggregation in response to a maximally-effective concentration of collagen-related peptide or collagen. In contrast, 10- to 20-fold higher concentrations of ibrutinib or acalabrutinib block platelet aggregation in response to glycoprotein VI agonists. Ex vivo studies on patients treated with ibrutinib, but not acalabrutinib, showed a reduction of platelet aggregation in response to collagen-related peptide indicating that the clinical dose of ibrutinib but not acalabrutinib is supramaximal for Bruton tyrosine kinase blockade. Unexpectedly, low concentrations of ibrutinib inhibited aggregation in response to collagen-related peptide in patients deficient in Bruton tyrosine kinase. The increased bleeding seen with ibrutinib over acalabrutinib is due to off-target actions of ibrutinib that occur because of unfavorable pharmacodynamics.

A PIP2-derived amplification loop fuels the sustained initiation of B cell activation

Lymphocytes have evolved sophisticated signaling amplification mechanisms to efficiently activate downstream signaling after detection of rare ligands in their microenvironment. B cell receptor microscopic clusters (BCR microclusters) are assembled on the plasma membrane and recruit signaling molecules for the initiation of lymphocyte signaling after antigen binding. We identified a signaling amplification loop derived from phosphatidylinositol 4,5-biphosphate (PIP₂) for the sustained B cell activation. Upon antigen recognition, PIP₂ was depleted by phospholipase C-γ2 (PLC-γ2) within the BCR microclusters and was regenerated by phosphatidic acid-dependent type I phosphatidylinositol 4-phosphate 5-kinase outside the BCR microclusters. The hydrolysis of PIP₂ inside the BCR microclusters induced a positive feedback mechanism for its synthesis outside the BCR microclusters. The falling gradient of PIP₂ across the boundary of BCR microclusters was important for the efficient formation of BCR microclusters. Our results identified a PIP₂-derived amplification loop that fuels the sustained initiation of B cell activation.

PI(4,5)P2 determines the threshold of mechanical force-induced B cell activation

B lymphocytes use B cell receptors (BCRs) to sense the chemical and physical features of antigens. The activation of isotype-switched IgG-BCR by mechanical force exhibits a distinct sensitivity and threshold in comparison with IgM-BCR. However, molecular mechanisms governing these differences remain to be identified. In this study, we report that the low threshold of IgG-BCR activation by mechanical force is highly dependent on tethering of the cytoplasmic tail of the IgG-BCR heavy chain (IgG-tail) to the plasma membrane. Mechanistically, we show that the positively charged residues in the IgG-tail play a crucial role by highly enriching phosphatidylinositol (4,5)-biphosphate (PI(4,5)P2) into the membrane microdomains of IgG-BCRs. Indeed, manipulating the amounts of PI(4,5)P2 within IgG-BCR membrane microdomains significantly altered the threshold and sensitivity of IgG-BCR activation. Our results reveal a lipid-dependent mechanism for determining the threshold of IgG-BCR activation by mechanical force.

PLCγ2 promotes apoptosis while inhibits proliferation in rat hepatocytes through PKCD/JNK MAPK and PKCD/p38 MAPK signalling

OBJECTIVES

The PLCG2 (PLCγ2) gene is a member of PLC gene family encoding transmembrane signalling enzymes involved in various biological processes including cell proliferation and apoptosis. Our earlier study indicated that PLCγ2 may be involved in the termination of regeneration of the liver which is mainly composed of hepatocytes, but its exact biological function and molecular mechanism in liver regeneration termination remains unclear. This study aims to examine the role of PLCγ2 in the growth of hepatocytes.

MATERIALS AND METHODS

A recombinant adenovirus expressing PLCγ2 was used to infect primary rat hepatocytes. PLCγ2 mRNA and protein levels were detected by qRT-PCR and Western blot. The subcellular location of PLCγ2 protein was tested by an immunofluorescence assay. The proliferation of hepatocytes was measured by MTT assay. The cell cycle and apoptosis were analysed by flow cytometry. Caspase-3, -8 and -9 activities were measured by a spectrophotometry method. Phosphorylation levels of PKCD, JNK and p38 in the infected cells were detected by Western blot. The possible mechanism underlying the role of PLCγ2 in hepatocyte growth was also explored by adding a signalling pathway inhibitor.

RESULTS

Hepatocyte proliferation was dramatically reduced, while cell apoptosis was remarkably increased. The results demonstrated that PLCγ2 increased the phosphorylation of PKCD, p38 and JNK in rat hepatocytes. After PKCD activity was inhibited by the inhibitor Go 6983, the levels of both p-p38 and p-JNK MAPKs significantly decreased, and PLCγ2-induced cell proliferation inhibition and cell apoptosis were obviously reversed.

CONCLUSIONS

This study showed that PLCγ2 regulates hepatocyte growth through PKCD-dependently activating p38 MAPK and JNK MAPK pathways; this result was experimentally based on the further exploration of the effect of PLCγ2 on hepatocyte growth in vivo.

Silencing of phospholipase C gamma 2 promotes proliferation of rat hepatocytes in vitro

The management of hepatic failure is undoubtedly difficult, and poor results have led to the search for novel therapeutic approaches. Nowadays, anti-apoptotic gene therapy is considered as an ideal approach. It has been proved that phospholipase Cγ2 (PLCγ2) is involved in the apoptosis of immune cells and tumor cells; however, whether this gene is related to hepatocyte death is still unclear. This study examined the role of PLCγ2 by inhibiting its expression in rat hepatocytes with siRNA. We also further analyzed the cellular mechanism by which the expression inhibition of PLCγ2 induces cell death. Silencing PLCγ2 gene by adenovirus vector expressing PLCγ2-targeted siRNA caused the great decline in the number of G1- and G2/M phase cells, the significant increase in the number of S phase cells, and the obvious reduction in apoptosis index. In addition, silencing PLCγ2 gene relieved the rat hepatocyte damage, such as the cell shrinkage and chromatin condensation, nuclear fragmentation. Further analysis of Ad-PLCγ2 siRNA-transfected hepatocytes demonstrated that suppression of PLCγ2 gene expression could cause the caspase dependent cell death by inhibiting the signal pathway MEKK1/MKK4/JNK1/2/c-Jun. In conclusion, these findings suggest that interference with PLCγ2 expression could relieve the inhibitory effect of PLCγ2 on hepaocyte apoptosis, thus, promote proliferation through inactivating PKCδ-mediated JNK1/2 signaling pathway.

Enhancement or inhibition of PLCγ2 expression in rat hepatocytes by recombinant adenoviral vectors that contain full-length gene or siRNA

We investigated the effects of recombinant adenovirus vectors that overexpress or silence PLCγ2 on the expression of this gene during hepatocyte proliferation. Hepatocytes were isolated, identified by immunofluorescent cytochemical staining and infected by previously constructed Ad-PLCγ2 and Ad-PLCγ2 siRNA1, siRNA2 and siRNA3. Green fluorescent protein (GFP) expression was observed by fluorescence microscopy. Infection percentage was calculated by flow cytometry. mRNA and protein levels of PLCγ2 were detected by quantitative reverse transcription-PCR (qRT-PCR) and western blotting, respectively. The viability of the infected hepatocytes was measured by 3-(4,5-dimethylthiazol-2-yl)-2, 5-diphenyltetrazolium bromide (MTT) assay. We found that nearly 97% of cells were positive for the hepatocyte marker, CK18. After infection of Ad-PLCγ2 and Ad-PLCγ2 siRNA, more than 99% of hepatocytes expressed GFP significantly, and mRNA and protein expression of PLCγ2 was up-regulated significantly in Ad-PLCγ2 infected hepatocytes, but down-regulated in Ad-PLCγ2 siRNA2 infected cells. The cell proliferation rate decreased in PLCγ2-overexpressing cells, while the rate increased in PLCγ2-silencing cells. We verified that recombinant Ad-PLCγ2 and Ad-PLCγ2 siRNA2 were constructed successfully. These two recombinant vectors promoted or decreased the expression of PLCγ2 in rat hepatocytes and affected the cell proliferation rate, which provides a useful tool for further investigation of the role of PLCγ2 in hepatocyte apoptosis.

Characterization and expression analysis of Calmodulin (CaM) in orange-spotted grouper (Epinephelus coioides) in response to Vibrio alginolyticus challenge

Vibrio alginolyticus containing the highly toxic extracellular product is one of the most serious threats to grouper survival and its minimum lethal dose is approximately 500 CFU/g fish body weight in grouper. To study the toxic effects of V. alginolyticus on the immune system in teleost, Calmodulin (CaM), an important molecular indicator gene, was cloned from the orange-spotted grouper (Epinephelus coioides). The full-length Ec-CaM consisted of a 5'-UTR of 103 bp, an ORF of 450 bp and a 3'-UTR of 104 bp. The Ec-CaM gene encoded a protein of 149 amino acids with an estimated molecular mass of 16.4 kDa and a predicted isoelectric point of 3.93. The deduced amino acid sequence showed that Ec-CaM contained four highly conserved EF-hand domains known to be critical for the function of CaM. Ec-CaM was widely expressed and the highest expression level was observed in liver. Following V. alginolyticus challenge, a sharp increase level of respiratory burst activity and apoptosis ratio were observed. Further analyses of CaM expression and p53 expression in liver, kidney and spleen by qRT-PCR demonstrated that the up-regulated expression of CaM and p53 were observed in the vibrio challenge group. Western blotting analysis confirmed that the Ec-CaM protein was strongly induced in liver at 12 h post-injection, while a sharp increase of p53 protein expression was observed at 24 h post-injection. These results showed CaM expression serving as a potential molecular indicator may help to assess the toxicological effects of V. alginolyticus on the ROS generation and apoptotic process in grouper.

Rac-mediated Stimulation of Phospholipase Cγ2 Amplifies B Cell Receptor-induced Calcium Signaling

The Rho GTPase Rac is crucially involved in controlling multiple B cell functions, including those regulated by the B cell receptor (BCR) through increased cytosolic Ca(2+). The underlying molecular mechanisms and their relevance to the functions of intact B cells have thus far remained unknown. We have previously shown that the activity of phospholipase Cγ2 (PLCγ2), a key constituent of the BCR signalosome, is stimulated by activated Rac through direct protein-protein interaction. Here, we use a Rac-resistant mutant of PLCγ2 to functionally reconstitute cultured PLCγ2-deficient DT40 B cells and to examine the effects of the Rac-PLCγ2 interaction on BCR-mediated changes of intracellular Ca(2+) and regulation of Ca(2+)-regulated and nuclear-factor-of-activated-T-cell-regulated gene transcription at the level of single, intact B cells. The results show that the functional Rac-PLCγ2 interaction causes marked increases in the following: (i) sensitivity of B cells to BCR ligation; (ii) BCR-mediated Ca(2+) release from intracellular stores; (iii) Ca(2+) entry from the extracellular compartment; and (iv) nuclear translocation of the Ca(2+)-regulated nuclear factor of activated T cells. Hence, Rac-mediated stimulation of PLCγ2 activity serves to amplify B cell receptor-induced Ca(2+) signaling.

Role of Calcium Signaling in B Cell Activation and Biology

Increase in intracellular levels of calcium ions (Ca2+) is one of the key triggering signals for the development of B cell response to the antigen. The diverse Ca2+ signals finely controlled by multiple factors participate in the regulation of gene expression, B cell development, and effector functions. B cell receptor (BCR)-initiated Ca2+ mobilization is sourced from two pathways: one is the release of Ca2+ from the intracellular stores, endoplasmic reticulum (ER), and other is the prolonged influx of extracellular Ca2+ induced by depleting the stores via store-operated calcium entry (SOCE) and calcium release-activated calcium (CRAC) channels. The identification of stromal interaction molecule 1(STIM1), the ER Ca2+ sensor, and Orai1, a key subunit of the CRAC channel pore, has now provided the tools to understand the mode of Ca2+ influx regulation and physiological relevance. Herein, we discuss our current understanding of the molecular mechanisms underlying BCR-triggered Ca2+ signaling as well as its contribution to the B cell biological processes and diseases.

New insights into the DT40 B cell receptor cluster using a proteomic proximity labeling assay

In the vertebrate immune system, each B-lymphocyte expresses a surface IgM-class B cell receptor (BCR). When cross-linked by antigen or anti-IgM antibody, the BCR accumulates with other proteins into distinct surface clusters that activate cell signaling, division, or apoptosis. However, the molecular composition of these clusters is not well defined. Here we describe a quantitative assay we call selective proteomic proximity labeling using tyramide (SPPLAT). It allows proteins in the immediate vicinity of a target to be selectively biotinylated, and hence isolated for mass spectrometry analysis. Using the chicken B cell line DT40 as a model, we use SPPLAT to provide the first proteomic analysis of any BCR cluster using proximity labeling. We detect known components of the BCR cluster, including integrins, together with proteins not previously thought to be BCR-associated. In particular, we identify the chicken B-lymphocyte allotypic marker chB6. We show that chB6 moves to within about 30–40 nm of the BCR following BCR cross-linking, and we show that cross-linking chB6 activates cell binding to integrin substrates laminin and gelatin. Our work provides new insights into the nature and composition of the BCR cluster, and confirms SPPLAT as a useful research tool in molecular and cellular proteomics.

Negative selection of self-reactive chicken B cells requires B cell receptor signaling and is independent of the bursal microenvironment

Although the negative selection of self-reactive B cells in the bone marrow of mammals has been clearly demonstrated, it remains unclear in models of gut-associated B cell lymphopoiesis, such as that of the chicken (Gallus gallus). We have generated chicken surface IgM-related receptors in which the diversity region of the lamprey variable lymphocyte receptor (VLR) has been fused to the C region of chicken surface IgM (Tμ). Expression of a VLR:Tμ receptor with specificity for PE supported normal development of B cells, whereas a VLR:Tμ receptor specific to hen egg lysozyme (a self-antigen with respect to chicken B cells) induced, in vivo, complete deletion of VLR(HEL)Tμ-expressing B cells. In ovo i.v. injection of PE resulted in deletion of VLR(PE)Tμ-expressing Β cells in the embryo spleen, demonstrating that negative selection was independent of the bursal microenvironment. Although chickens transduced with a murine CD8α:chicken Igα fusion protein contained B cells expressing mCD8α:chIgα, cotransfection of the mCD8α:chIgα construct, together with thymus leukemia Ag (a natural ligand for mCD8α), resulted in reduced levels of mCD8α:chIgα-expressing B cells in inverse proportion to the levels of thymus leukemia Ag-expressing cells. Deletion of mCD8α:chIgα-expressing cells was specific for B cells and required active signaling downstream of the mCD8α:chIgα receptor. Ag-mediated negative selection of developing chicken B cells can therefore occur independently of the bursal microenvironment and is dependent on signaling downstream of the BCR.

PDK1 controls upstream PI3K expression and PIP3 generation

The PI3K/PDK1/Akt signaling axis is centrally involved in cellular homeostasis and controls cell growth and proliferation. Due to its key function as regulator of cell survival and metabolism, the dysregulation of this pathway is manifested in several human pathologies including cancers and immunological diseases. Thus, current therapeutic strategies target the components of this signaling cascade. In recent years, numerous feedback loops have been identified that attenuate PI3K/PDK1/Akt-dependent signaling. Here, we report the identification of an additional level of feedback regulation that depends on the negative transcriptional control of phosphatidylinositol 3-kinase (PI3K) class IA subunits. Genetic deletion of 3-phosphoinositide-dependent protein kinase 1 (PDK1) or the pharmacological inhibition of its downstream effectors, that is, Akt and mammalian target of rapamycin (mTOR), relieves this suppression and leads to the upregulation of PI3K subunits, resulting in enhanced generation of phosphatidylinositol-3,4,5-trisphosphate (PIP3). Apparently, this transcriptional induction is mediated by the concerted action of different transcription factor families, including the transcription factors cAMP-responsive element-binding protein and forkhead box O. Collectively, we propose that PDK1 functions as a cellular sensor that balances basal PIP3 generation at levels sufficient for survival but below a threshold being harmful to the cell. Our study suggests that the efficiency of therapies targeting the aberrantly activated PI3K/PDK1/Akt pathway might be increased by the parallel blockade of feedback circuits.

Diverse phosphorylation patterns of B cell receptor-associated signaling in naïve and memory human B cells revealed by phosphoflow, a powerful technique to study signaling at the single cell level

Following interaction with cognate antigens, B cells undergo cell activation, proliferation, and differentiation. Ligation of the B cell receptor (BCR) leads to the phosphorylation of BCR-associated signaling proteins within minutes of antigen binding, a process with profound consequences for the fate of the cells and development of effector immunity. Phosphoflow allows a rapid evaluation of various signaling pathways in complex heterogenous cell subsets. This novel technique was used in combination with multi-chromatic flow cytometry (FC) and fluorescent-cell barcoding (FCB) to study phosphorylation of BCR-associated signaling pathways in naïve and memory human B cell subsets. Proteins of the initiation (Syk), propagation (Btk, Akt), and integration (p38MAPK and Erk1/2) signaling units were studied. Switched memory (Sm) CD27+ and Sm CD27- phosphorylation patterns were similar when stimulated with anti-IgA or -IgG. In contrast, naïve and unswitched memory (Um) cells showed significant differences following IgM stimulation. Enhanced phosphorylation of Syk was observed in Um cells, suggesting a lower activation threshold. This is likely the result of higher amounts of IgM on the cell surface, higher pan-Syk levels, and enhanced susceptibility to phosphatase inhibition. All other signaling proteins evaluated also showed some degree of enhanced phosphorylation in Um cells. Furthermore, both the phospholipase C-γ2 (PLC-γ2) and phosphatidylinositol 3-kinase (PI3K) pathways were activated in Um cells, while only the PI3K pathway was activated on naïve cells. Um cells were the only ones that activated signaling pathways when stimulated with fluorescently labeled S. Typhi and S. pneumoniae. Finally, simultaneous evaluation of signaling proteins at the single cell level (multiphosphorylated cells) revealed that interaction with gram positive and negative bacteria resulted in complex and diverse signaling patterns. Phosphoflow holds great potential to accelerate vaccine development by identifying signaling profiles in good/poor responders.

Defense activation triggers differential expression of phospholipase-C (PLC) genes and elevated temperature induces phosphatidic acid (PA) accumulation in tomato

Recently, we provided the first genetic evidence for the requirement of tomato PLC4 and PLC6 genes in defense activation and disease resistance. The encoded enzymes were catalytically active as they were able to degrade phosphatidylinositol (PI), thereby producing diacylglycerol (DG). Here we report differential PLC gene expression following the initiation of defense signaling by the interaction between Cladosporium fulvum resistance (R) protein Cf-4 and its matching effector Avr4 in tomato hybrid seedlings that express both Cf-4 and Avr4. Furthermore, we observed that PLC3 and PLC6 gene expression is upregulated by elevated temperature in the control seedlings. This upregulation coincides with an increase in the levels of phosphatidic acid (PA) and a decrease in the levels of PI and phosphatidylinositol phosphate (PIP). The decrease in PI and PIP levels matches with the activation of PLC. In addition, the levels of the structural phospholipids phosphatidylcholine (PC), phosphatidylethanolamine (PE) and phosphatidylglycerol (PG) declined transiently during recovery after the exposure to elevated temperature., Further studies will be required to explain the mechanism causing the sustained accumulation of PA during recovery, combined with a reduction in the levels of structural phospholipids.

Identification of Ser/Thr phosphorylation sites in the C2-domain of phospholipase C γ2 (PLCγ2) using TRPM7-kinase

PLC-isozymes are central elements of cellular signaling downstream of numerous receptors. PLCγ2 is a pivotal component of B cell receptor (BCR) signaling. The regulation of PLCγ2-dependent signaling functions by Tyr-phosphorylation is well characterized, however, the potential role of Ser/Thr phosphorylation events remains undefined. TRPM7 is the fusion of a Ser/Thr kinase with an ion channel, and an essential component of Mg(2+)-homeostasis regulation. Although the interaction between the C2 domain of several PLC-isozymes and TRPM7 is well established, previous studies have focused on the effect of PLC-activity on TRPM7. Here, we investigated whether Ser/Thr phosphorylation sites in the C2 domain of PLCγ2 could be identified using TRPM7-kinase. We show that TRPM7-kinase phosphorylates PLCγ2 in its C2-domain at position Ser1164 and in the linker region preceding the C2-domain at position Thr1045. Using a complementation approach in PLCγ2(-/-) DT40 cells, we found that the PLCγ2-S1164A mutant fully restores BCR mediated Ca(2+)-responses under standard growth conditions. However, under hypomagnesic conditions, PLCγ2-S1164A fails to reach Ca(2+)-levels seen in cells expressing PLCγ2 wildtype. These results suggest that Mg(2+)-sensitivity of the BCR signaling pathway may be regulated by Ser/Thr phosphorylation of PLCγ2.

S-glutathionylation activates STIM1 and alters mitochondrial homeostasis

Oxidant stress induces constitutive calcium entry by tacking glutathiones onto the Orai CRAC channel activator STIM1.

Oxidant stress influences many cellular processes, including cell growth, differentiation, and cell death. A well-recognized link between these processes and oxidant stress is via alterations in Ca²⁺ signaling. However, precisely how oxidants influence Ca²⁺ signaling remains unclear. Oxidant stress led to a phenotypic shift in Ca²⁺ mobilization from an oscillatory to a sustained elevated pattern via calcium release–activated calcium (CRAC)–mediated capacitive Ca²⁺ entry, and stromal interaction molecule 1 (STIM1)– and Orai1-deficient cells are resistant to oxidant stress. Functionally, oxidant-induced Ca²⁺ entry alters mitochondrial Ca²⁺ handling and bioenergetics and triggers cell death. STIM1 is S-glutathionylated at cysteine 56 in response to oxidant stress and evokes constitutive Ca²⁺ entry independent of intracellular Ca²⁺ stores. These experiments reveal that cysteine 56 is a sensor for oxidant-dependent activation of STIM1 and demonstrate a molecular link between oxidant stress and Ca²⁺ signaling via the CRAC channel.

BLNK binds active H-Ras to promote B cell receptor-mediated capping and ERK activation

Cross-linked B cell receptor (BCR) aggregates on the cell surface, then assembles into the "cap" where Ras is co-localized, and transduces various intracellular signals including Ras-ERK activation. BCR signals induce proliferation, differentiation, or apoptosis of B cells depending on their maturational stage. The adaptor protein BLNK binds various signaling proteins and Igalpha, a signaling subunit of the BCR complex, and plays an important role in the BCR signal transduction. BLNK was shown to be required for activation of ERK, but not of Ras, after BCR cross-linking, raising a question how BLNK facilitates ERK activation. Here we demonstrate that BLNK binds the active form of H-Ras, and their binding is facilitated by BCR cross-linking. We have identified a 10-amino acid Ras-binding domain within BLNK that is necessary for restoration of BCR-mediated ERK activation in BLNK-deficient B cells and for anti-apoptotic signaling. The Ras-binding domain fused with a CD8alpha-Igalpha chimeric receptor could induce prolonged ERK phosphorylation, transcriptional activation of Elk1, as well as the capping of the receptor in BLNK-deficient B cells. These results indicate that BLNK recruits active H-Ras to the BCR complex, which is essential for sustained surface expression of BCR in the form of the cap and for the signal leading to functional ERK activation.

TREM-2 mediated signaling induces antigen uptake and retention in mature myeloid dendritic cells

Myeloid dendritic cells (mDC) activated with a B7-DC-specific cross-linking IgM Ab (B7-DC XAb) take up and retain Ag and interact with T cell compartments to affect a number of biologic changes that together cause strong antitumor responses and blockade of inflammatory airway disease in animal models. The molecular events mediating the initial responses in mDC remain unclear. In this study we show that B7-DC XAb caused rapid phosphorylation of the adaptor protein DAP12 and intracellular kinases Syk and phospholipase C-gamma1. Pretreatment of mDC with the Syk inhibitor piceatannol blocked B7-DC XAb-induced Ag uptake with a concomitant loss of tumor protection in mice. Vaccination with tumor lysate-pulsed wild-type B7-DC XAb-activated mDC, but not TREM-2 knockout XAb-activated mDC, protected mice from lethal melanoma challenge. Multimolecular caps appeared within minutes of B7-DC XAb binding to either human or mouse mDC, and FRET analysis showed that class II, CD80, CD86, and TREM-2 are recruited in tight association on the cell surface. When TREM-2 expression was reduced in wild-type mDC using short hairpin RNA or by using mDC from TREM-2 knockout mice, in vitro DC failed to take up Ag after B7-DC XAb stimulation. These results directly link TREM-2 signaling with one change in the mDC phenotype that occurs in response to this unique Ab. The parallel signaling events observed in both human and mouse mDC support the hypothesis that B7-DC cross-linking may be useful as a therapeutic immune modulator in human patients.

ATP modulation of Ca2+ release by type-2 and type-3 inositol (1, 4, 5)-triphosphate receptors. Differing ATP sensitivities and molecular determinants of action

ATP enhances Ca(2+) release from inositol (1,4,5)-trisphosphate receptors (InsP(3)R). However, the three isoforms of InsP(3)R are reported to respond to ATP with differing sensitivities. Ca(2+) release through InsP(3)R1 is positively regulated at lower ATP concentrations than InsP(3)R3, and InsP(3)R2 has been reported to be insensitive to ATP modulation. We have reexamined these differences by studying the effects of ATP on InsP(3)R2 and InsP(3)R3 expressed in isolation on a null background in DT40 InsP(3)R knockout cells. We report that the Ca(2+)-releasing activity as well as the single channel open probability of InsP(3)R2 was enhanced by ATP, but only at submaximal InsP(3) levels. Further, InsP(3)R2 was more sensitive to ATP modulation than InsP(3)R3 under similar experimental conditions. Mutations in the ATPB sites of InsP(3)R2 and InsP(3)R3 were generated, and the functional consequences of these mutations were tested. Surprisingly, mutation of the ATPB site in InsP(3)R3 had no effect on ATP modulation, suggesting an additional locus for the effects of ATP on this isoform. In contrast, ablation of the ATPB site of InsP(3)R2 eliminated the enhancing effects of ATP. Furthermore, this mutation had profound effects on the patterns of intracellular calcium signals, providing evidence for the physiological significance of ATP binding to InsP(3)R2.

Signaling molecules in sulfur mustard-induced cutaneous injury

OBJECTIVE

Sulfur mustard (SM) is a potent alkylating agent that can induce severe cutaneous injury. Though much is known regarding the gross pathology of SM injury, the molecular and cellular basis for this pathology is not well understood. General cellular processes such as inflammation, DNA damage response, and apoptosis have been hypothesized to be involved in SM injury. However, the specific molecules, signaling pathways, and gene products involved in the pathogenesis of SM injury have not been elucidated. This review discusses the molecular mechanisms observed in in vivo and in vitro models of cutaneous SM injury.

METHODS

The historical literature on the clinical pathology of SM-induced cutaneous injury is summarized, and recent work elucidating molecular signaling pathways involved in SM toxicity is extensively reviewed. In addition, this review focuses the discussion of SM-induced molecular mechanisms on those that have been experimentally validated in models of SM injury.

RESULTS

Recent work has uncovered potential roles for a number of signaling molecules. In particular, molecules in inflammatory signaling, DNA damage response, apoptosis signaling, and calcium signaling have been implicated in SM injury. These include signaling molecules involved in inflammation (e.g. p38 MAP kinase), apoptosis (e.g. p53, NF-kappa B, caspases, Fas), and cell stress responses (e.g. calcium, calmodulin).

CONCLUSIONS

Many of the molecules and mechanisms implicated in SM injury are now being experimentally validated. Critical questions are proposed that remain to be answered to increase our understanding of SM toxicity and accelerate the development of vesicant therapeutics.

Collagen promotes sustained glycoprotein VI signaling in platelets and cell lines

BACKGROUND

Glycoprotein (GP)VI is the major signaling receptor for collagen on platelets and signals via the associated FcRgamma-chain, which has an immunoreceptor tyrosine-containing activation motif (ITAM).

OBJECTIVE

To determine why GPVI-FcRgamma signals poorly, or not at all, in response to collagen in hematopoietic cell lines, despite robust responses to the GPVI-reactive snake venom toxin convulxin.

METHODS AND RESULTS

Using a nuclear factor of activated T-cells (NFAT) transcriptional reporter assay, a sensitive readout for sustained ITAM signaling, we demonstrate collagen-induced GPVI-FcRgamma signaling in hematopoietic cell lines. This is accompanied by relatively weak but sustained protein tyrosine phosphorylation, in contrast to the stronger but transient response to convulxin. Sustained signaling by collagen is also observed in platelets and is necessary for the maintenance of spreading on collagen. Finally, in cell lines, the inhibitory collagen receptor leukocyte-associated immunoglobulin-like receptor-1 (LAIR-1), which is not expressed on platelets but is present on most hematopoietic cells, inhibits GPVI responses to collagen but not convulxin.

CONCLUSION

The inability of previous studies to readily detect GPVI collagen signaling in cell lines is probably because of the weak but sustained nature of the signal and the presence of the inhibitory collagen receptor LAIR-1. In platelets, we propose that GPVI-FcRgamma has evolved to transmit sustained signals in order to maintain spreading over several hours, as well as facilitating rapid activation through release of feedback agonists and integrin activation. The establishment of a cell line NFAT assay will facilitate the molecular dissection of GPVI signaling and the identification of GPVI antagonists in drug discovery.

Calcium signalling and cell-fate choice in B cells

Alterations in the cytosolic concentration of calcium ions (Ca2+) transmit information that is crucial for the development and function of B cells. Cytosolic Ca2+ concentration is determined by a balance of active transport and gradient-driven Ca2+ fluxes, both of which are subject to the influence of multiple receptors and environmental sensing pathways. Recent advances in genomics have allowed for the compilation of an increasingly comprehensive list of Ca2+ transporters and channels expressed by B cells. The increasing understanding of the function and regulation of these proteins has begun to shift the frontier of Ca2+ physiology in B cells from molecular analysis to determining how diverse inputs to cytosolic Ca2+ concentration are integrated in specific immunological contexts.

The C-type lectin receptors CLEC-2 and Dectin-1, but not DC-SIGN, signal via a novel YXXL-dependent signaling cascade

The two lectin receptors, CLEC-2 and Dectin-1, have been shown to signal through a Syk-dependent pathway, despite the presence of only a single YXXL in their cytosolic tails. In this study, we show that stimulation of CLEC-2 in platelets and in two mutant cell lines is dependent on the YXXL motif and on proteins that participate in signaling by immunoreceptor tyrosine-based activation motif receptors, including Src, Syk, and Tec family kinases, and on phospholipase Cgamma. Strikingly, mutation of either Src homology (SH) 2 domain of Syk blocks signaling by CLEC-2 despite the fact that it has only a single YXXL motif. Furthermore, signaling by CLEC-2 is only partially dependent on the BLNK/SLP-76 family of adapter proteins in contrast to that of immunoreceptor tyrosine-based activation motif receptors. The C-type lectin receptor, Dectin-1, which contains a YXXL motif preceded by the same four amino acids as for CLEC-2 (DEDG), signals like CLEC-2 and also requires the two SH2 domains of Syk and is only partially dependent on the BLNK/SLP-76 family of adapters. In marked contrast, the C-type lectin receptor, DC-SIGN, which has a distinct series of amino acids preceding a single YXXL, signals independent of this motif. A mutational analysis of the DEDG sequence of CLEC-2 revealed that the glycine residue directly upstream of the YXXL tyrosine is important for CLEC-2 signaling. These results demonstrate that CLEC-2 and Dectin-1 signal through a single YXXL motif that requires the tandem SH2 domains of Syk but is only partially dependent on the SLP-76/BLNK family of adapters.

Intramolecular regulation of phospholipase C-gamma1 by its C-terminal Src homology 2 domain

Phosphoinositide-specific phospholipase C-gamma1 (PLC-gamma1) is a key enzyme that governs cellular functions such as gene transcription, secretion, proliferation, motility, and development. Here, we show that PLC-gamma1 is regulated via a novel autoinhibitory mechanism involving its carboxy-terminal Src homology (SH2C) domain. Mutation of the SH2C domain tyrosine binding site led to constitutive PLC-gamma1 activation. The amino-terminal split pleckstrin homology (sPHN) domain was found to regulate the accessibility of the SH2C domain. PLC-gamma1 constructs with mutations in tyrosine 509 and phenylalanine 510 in the sPHN domain no longer required an intact amino-terminal Src homology (SH2N) domain or phosphorylation of tyrosine 775 or 783 for activation. These data are consistent with a model in which the SH2C domain is blocked by an intramolecular interaction(s) that is released upon cellular activation by occupancy of the SH2N domain.

Human SLP-65 isoforms contribute differently to activation and apoptosis of B lymphocytes

The SH2 domain-containing leukocyte adaptor protein of 65 kDa (SLP-65) is the key effector for signaling downstream of the B-cell antigen receptor (BCR). SLP-65 controls not only B lymphopoiesis and humoral immunity but also possesses a yet poorly defined tumor suppressor activity that is lost in many cases of acute lymphoblastic leukemia. We found that the 2 isoforms of human SLP-65 are differentially involved in positive and negative B-cell signaling. Reconstitution experiments revealed that an atypical SH3 domain-binding motif, which is present in the long but not in the short SLP-65 isoform, mediates association to Grb2 and suppresses activation of mitogen-activated protein kinases p38 and JNK as well as up-regulation of c-Fos expression. In turn, the short isoform activates not only AP1-driven but also NF-kappaB-driven gene transcription more potently than the long isoform. Conversely, the long rather than the short SLP-65 isoform promotes BCR-induced B-cell apoptosis. Our data further delineate the structural requirements of positive and negative SLP-65 signal transduction in normal and neoplastic cells.

Nucleocytoplasmic trafficking of the Syk protein tyrosine kinase

The protein tyrosine kinase Syk couples the B-cell receptor (BCR) for antigen to multiple intracellular signaling pathways and also modulates cellular responses to inducers of oxidative stress in a receptor-independent fashion. In B cells, Syk is found in both the nuclear and cytoplasmic compartments but contains no recognizable nuclear localization or export signals. Through the analysis of a series of deletion mutants, we identified the presence of an unconventional shuttling sequence near the junction of the catalytic domain and the linker B region that accounts for Syk's subcellular localization. This localization is altered following prolonged engagement of the BCR, which causes Syk to be excluded from the nucleus. Nuclear exclusion requires the receptor-mediated activation of protein kinase C and new protein synthesis. Both of these processes also potentiate the activation of caspase 3 in cells in response to oxidative stress in a manner that is dependent on the localization of Syk outside of the nucleus. In contrast, restriction of Syk to the nucleus greatly diminishes the stress-induced activation of caspase 3.

Gap junctions and the propagation of cell survival and cell death signals

Gap junctions are a unique type of intercellular channels that connect the cytoplasm of adjoining cells. Each gap junction channel is comprised of two hemichannels or connexons and each connexon is formed by the aggregation of six protein subunits known as connexins. Gap junction channels allow the intercellular passage of small (< 1.5 kDa) molecules and regulate essential processes during development and differentiation. However, their role in cell survival and cell death is poorly understood. We review experimental data that support the hypothesis that gap junction channels may propagate cell death and survival modulating signals. In addition, we explore the hypothesis that hemichannels (or unapposed connexons) might be used as a paracrine conduit to spread factors that modulate the fate of the surrounding cells. Finally, direct signal transduction activity of connexins in cell death and survival pathways is addressed.

Selective role for superoxide in InsP3 receptor-mediated mitochondrial dysfunction and endothelial apoptosis

Reactive oxygen species (ROS) play a divergent role in both cell survival and cell death during ischemia/reperfusion (I/R) injury and associated inflammation. In this study, ROS generation by activated macrophages evoked an intracellular Ca2+ ([Ca2+]i) transient in endothelial cells that was ablated by a combination of superoxide dismutase and an anion channel blocker. [Ca2+]i store depletion, but not extracellular Ca2+ chelation, prevented [Ca2+]i elevation in response to O2*- that was inositol 1,4,5-trisphosphate (InsP3) dependent, and cells lacking the three InsP3 receptor (InsP3R) isoforms failed to display the [Ca2+]i transient. Importantly, the O2*--triggered Ca2+ mobilization preceded a loss in mitochondrial membrane potential that was independent of other oxidants and mitochondrially derived ROS. Activation of apoptosis occurred selectively in response to O2*- and could be prevented by [Ca2+]i buffering. This study provides evidence that O2*- facilitates an InsP3R-linked apoptotic cascade and may serve a critical function in I/R injury and inflammation.

The role of canonical transient receptor potential 7 in B-cell receptor-activated channels

Phospholipase C signaling stimulates Ca2+ entry across the plasma membrane through multiple mechanisms. Ca2+ store depletion stimulates store-operated Ca2+-selective channels, or alternatively, other phospholipase C-dependent events activate Ca2+-permeable non-selective cation channels. Transient receptor potential 7 (TRPC7) is a non-selective cation channel that can be activated by both mechanisms when ectopically expressed, but the regulation of native TRPC7 channels is not known. We knocked out TRPC7 in DT40 B-cells, which expresses both forms of Ca2+ entry. No difference in the store-operated current I(crac) was detected between TRPC7-/- and wild-type cells. Wild-type cells demonstrated nonstore-operated cation entry and currents in response to activation of the B-cell receptor or protease-activated receptor 2, intracellular dialysis with GTPgammaS, or application of the synthetic diacylglycerol oleyl-acetyl-glycerol. These responses were absent in TRPC7-/- cells but could be restored by transfection with human TRPC7. In conclusion, in B-lymphocytes, TRPC7 appeared to participate in the formation of ion channels that could be activated by phospholipase C-linked receptors. This represents the first demonstration of a physiological function for endogenous TRPC7 channels.

Genes, pathways and checkpoints in lymphocyte development and homeostasis

A plethora of genes involved in murine B and T cell development have been identified, and developmental pathways within the primary lymphoid tissues have been well delineated. The generation of a functional, but non-self reacting lymphocyte repertoire results from the completion of several checkpoints during lymphocyte development and competition for survival factors in the periphery. Improved knowledge of these developmental checkpoints and homeostatic mechanisms is critical for understanding human immunodeficiency, leukaemia/lymphoma and autoimmunity, which are conditions where checkpoints and homeostasis are likely to be deregulated.

Targeting Syk as a treatment for allergic and autoimmune disorders

Recent advances in our understanding of allergic and autoimmune disorders have begun to translate into novel, effective and safe medicines for these common maladies. Examples include an anti-IgE monoclonal antibody recently approved for severe asthmatics and the TNF-alpha antagonists that have demonstrated their ability to suppress rheumatoid arthritis, Crohn's disease and other chronic inflammatory processes. However, protein therapies are difficult and expensive to develop, manufacture and administer. Clearly, there is also a need for small-molecule inhibitors of novel targets that have safe and effective characteristics. Syk is an intracellular protein tyrosine kinase that was discovered 15 years ago as a key mediator of immunoreceptor signalling in a host of inflammatory cells including B cells, mast cells, macrophages and neutrophils. These immunoreceptors, including Fc receptors and the B-cell receptor, are important for both allergic diseases and antibody-mediated autoimmune diseases and thus pharmacologically interfering with Syk could conceivably treat these disorders. In addition, as Syk is positioned upstream in the cell signalling pathway, therapies targeting Syk may be more advantageous relative to drugs that inhibit a single downstream event. Syk inhibition during an allergic or asthmatic response will block three mast cell functions: the release of preformed mediators such as histamine, the production of lipid mediators such as leukotrienes and prostaglandins and the secretion of cytokines. In contrast, commonly used antihistamines or leukotriene receptor antagonists target only a single mediator of this complex cascade. Despite its expression in platelets and other non-haematopoietic cells, the role of Syk in regulating vascular homeostasis and other housekeeping functions is minimal or masked by redundant Syk-independent pathways. This suggests that targeting Syk would be an optimal approach to effectively treat a multitude of chronic inflammatory diseases without undue toxicity.

A New Tyrosine Phosphorylation Site in PLCγ1: The Role of Tyrosine 775 in Immune Receptor Signaling

Phospholipase Cgamma (PLCgamma) is a ubiquitous gatekeeper of calcium mobilization and diacylglycerol-mediated events induced by the activation of Ag and growth factor receptors. The activity of PLCgamma is regulated through its controlled membrane translocation and tyrosine (Y) phosphorylation. Four activation-induced tyrosine phosphorylation sites have been previously described (Y472, Y771, Y783, and Y1254), but their specific roles in Ag receptor-induced PLCgamma1 activation are not fully elucidated. Unexpectedly, we found that the phosphorylation of a PLCgamma1 construct with all four sites mutated to phenylalanine was comparable with that observed with wild-type PLCgamma1, suggesting the existence of an unidentified site(s). Sequence alignment with known phosphorylation sites in PLCgamma2 indicated homology of PLCgamma1 tyrosine residue 775 (Y775) with PLCgamma2 Y753, a characterized phosphorylation site. Tyrosine 775 was characterized as a phosphorylation site using phospho-specific anti-Y775 antiserum, and by mutational analysis. Phosphorylation of Y775 did not depend on the other tyrosines, and point mutation of PLCgamma1 Y775, or the previously described Y783, substantially reduced AgR-induced calcium, NF-AT, and AP-1 activation. Mutation of Y472, Y771, and Y1254 had no effect on overall PLCgamma1 phosphorylation or activation. Although the concomitant mutation of Y775 and Y783 abolished downstream PLCgamma1 signaling, these two tyrosines were sufficient to reconstitute the wild-type response in the absence of functional Y472, Y771, and Y1254. These data establish Y775 as a critical phosphorylation site for PLCgamma1 activation and confirm the functional importance of Y783.

RhoA GTPase regulates B cell receptor signaling

The RhoA GTPase controls many cellular functions, including gene transcription and actin polymerization. Several lines of evidence suggest that Rho GTPases are required for B cell receptor (BCR) signaling, but whether RhoA is necessary has not been investigated. Here, we show that RhoA is activated, downstream of PI3K, in response to BCR stimulation and is important for BCR-dependent calcium flux and cell proliferation. A RhoA dominant-negative mutant strongly inhibited BCR-dependent calcium mobilization. The RhoA-specific inhibitor, C3 toxin, inhibited both BCR-dependent calcium flux and cell proliferation. RhoA is important for BCR-dependent synthesis of IP(3) by PLCgamma2, but is not required for tyrosine phosphorylation of PLCgamma2. BCR-dependent synthesis of phosphatidylinositol-4,5-bisphosphate (PtdIns-4,5-P(2)) is inhibited in the absence of RhoA function. Providing exogenous PtdIns-4,5-P(2) restores BCR-dependent calcium flux in cells lacking functional RhoA. Our findings support a function for RhoA in BCR-dependent PtdIns-4,5-P(2) synthesis, PLCgamma2 activation, calcium mobilization, and cell proliferation.

Functional analysis of the green fluorescent protein-tagged inositol 1,4,5-trisphosphate receptor type 3 in Ca(2+) release and entry in DT40 B lymphocytes

We examined the function of GFP-IP(3)R3 (green fluorescent protein-tagged inositol 1,4,5-trisphosphate receptor type 3) in Ca(2+) release and entry using a mutant DT40 cell line (IP(3)R-KO) in which all three IP(3)R genes had been disrupted. GFP-IP(3)R3 fluorescence largely overlapped with the distribution of endoplasmic reticulum, whereas a portion of GFP-IP(3)R3 apparently co-localized with the plasma membrane. The application of IP(3) to permeabilized WT (wild-type) DT40 cells induced Ca(2+) release from internal stores. Although this did not occur in IP(3)R-KO cells it was restored by expression of GFP-IP(3)R3. In intact cells, application of anti-IgM, an activator of the BCR (B-cell receptor), or trypsin, a protease-activated receptor 2 agonist, did not cause any Ca(2+) response in IP(3)R-KO cells, whereas these treatments induced oscillatory or transient Ca(2+) responses in GFP-IP(3)R3-expressing IP(3)R-KO cells, as well as in WT cells. In addition, BCR activation elicited Ca(2+) entry in WT and GFP-IP(3)R3-expressing IP(3)R-KO cells but not in IP(3)R-KO cells. This BCR-mediated Ca(2+) entry was observed in the presence of La(3+), which blocks capacitative Ca(2+) entry. Thapsigargin depleted Ca(2+) stores and led to Ca(2+) entry in IP(3)R-KO cells irrespective of GFP-IP(3)R3 expression. In contrast with BCR stimulation, thapsigargin-induced Ca(2+) entry was completely blocked by La(3+), suggesting that the BCR-mediated Ca(2+) entry pathway is distinct from the capacitative Ca(2+) entry pathway. The present study demonstrates that GFP-IP(3)R3 could compensate for native IP(3)R in both IP(3)-induced Ca(2+) release and BCR-mediated Ca(2+) entry.

Dual Requirement for the Igα Immunoreceptor Tyrosine-Based Activation Motif (ITAM) and a Conserved Non-Igα ITAM Tyrosine in Supporting Igαβ-Mediated B Cell Development

Surface Ig (sIg) expression is a critical checkpoint during avian B cell development. Only cells that express sIg colonize bursal follicles, clonally expand, and undergo Ig diversification by gene conversion. Expression of a heterodimer, in which the extracellular and transmembrane domains of murine CD8alpha or CD8beta are fused to the cytoplasmic domains of chicken Igalpha (chIgalpha) or Igbeta, respectively (murine CD8alpha (mCD8alpha):chIgalpha + mCD8beta:chIgbeta), or an mCD8alpha:chIgalpha homodimer supported bursal B cell development as efficiently as endogenous sIg. In this study we demonstrate that B cell development, in the absence of chIgbeta, requires both the Igalpha ITAM and a conserved non-ITAM Igalpha tyrosine (Y3) that has been associated with binding to B cell linker protein (BLNK). When associated with the cytoplasmic domain of Igbeta, the Igalpha ITAM is not required for the induction of strong calcium mobilization or BLNK phosphorylation, but is still necessary to support B cell development. In contrast, mutation of the Igalpha Y3 severely compromised calcium mobilization when expressed as either a homodimer or a heterodimer with the cytoplasmic domain of Igbeta. However, coexpression of the cytoplasmic domain of Igbeta partially complemented the Igalpha Y3 mutation, rescuing higher levels of BLNK phosphorylation and, more strikingly, supporting B cell development.

Dual Phospholipase C/Diacylglycerol Requirement for Protein Kinase D1 Activation in Lymphocytes

The serine/threonine kinase protein kinase D1 (PKD1) is a protein kinase C (PKC) substrate that mediates antigen receptor signal transduction in lymphocytes. PKC phosphorylates serines 744/748 within the PKD1 catalytic domain, and this is proposed to be necessary and sufficient for enzyme activation. Hence, a PKD1 mutant with alanine substituted at positions 744 and 748 (PKD-S744A/S748A) is catalytically inactive. Conversely, a PKD1 mutant with glutamic residues substituted at positions 744 and 748 as phospho-mimics (PKD-S744E/S748E) is constitutively active when expressed in Cos7 or HeLa cells. The present study reveals that Ser-744/Ser-748 phosphorylation is required for PKD1 activation in lymphocytes. However, PKD-S744E/S748E is not constitutively active but, like the wild type enzyme, requires antigen receptor triggering or phorbol ester stimulation. Antigen receptor activation of wild type PKD is dependent on phospholipase C (PLC)/diacylglycerol (DAG) and PKC, whereas PKD-S744E/S748E is only dependent on PLC/DAG but no longer requires PKC. Hence, substitution of serines 744 and 748 with glutamic residues as phospho-mimics bypasses the PKC requirement for PKD1 activation but does not bypass the need for antigen receptors, PLC, or DAG. In lymphocytes, PKD1 is, thus, not regulated by PLC and PKC in a linear pathway; rather, PKD1 activation has more stringent requirements for integration of dual PLC signals, one mediated by PKCs and one that is PKC-independent.

Regulation of phospholipase C-gamma2 networks in B lymphocytes

The modulation of inositol-1,4,5-trisphosphate (IP3), a product of phospholipase C (PLC) activity, is one of a common signaling mechanism used in many biological systems. B lymphocytes also rely on IP3 and subsequent calcium signaling to ensure appropriate developmental outcomes, as well as antigen-specific responses. In establishing the optimal intensity and duration of the PLC-gamma activity, an important role has emerged for adaptor molecules, which direct the appropriate subcellular localization of PLC-gamma and induce its conformational changes. Generated IP3 binds to IP3 receptors located on the endoplasmic reticulum (ER), which in turn is essential for triggering calcium release from the ER and subsequent entry of extracellular calcium by so-called Ca2+ entry channels. Recent data has begun to shed new light on the connection between the calcium release and the influx of extracellular calcium, and the molecular identity of the Ca2+ entry channels.

Conservation of biological properties of the CD40 ligand, CD154 in a non-mammalian vertebrate

Signals delivered by the CD40 ligand, CD154, have crucial roles in immune responses in mammals, being required for development of germinal centres, maturation of T-dependent antibody responses, and generation of B-cell memory. To determine whether these functions were conserved in a non-mammalian species, a putative chicken CD 154 cDNA was used to make an oligomeric fusion protein, and to raise monoclonal antibodies. The antibodies detected surface expression on activated T-cells. The fusion protein detected expression of a receptor on B-cells, thrombocytes and macrophages. Biological effects of the fusion protein included induction of NO synthesis in a macrophage cell line, enhancement of splenic B-cell survival, and induction of apoptosis in a bursal lymphoma cell line. These observations demonstrated substantial functional equivalence with mammalian CD 154 and thus provided evidence for the early evolutionary emergence of the set of functions associated with this molecule, and its central role in the vertebrate immune system.

Lysosome is a primary organelle in B cell receptor-mediated apoptosis: an indispensable role of Syk in lysosomal function

To investigate the mechanism of B cell receptor (BCR)-mediated apoptosis, we utilized immature B cell lines, DT40 and WEHI-231. In both cell lines, BCR-crosslinking caused the increase in lysosomal pH with early apoptotic changes characterized by chromatin condensation and phosphatidylserine exposure. This increase was detected in c-Abl-deficient DT40 cells but not in Syk-deficient cells, which corresponded to the fact that the former cells but not the latter revealed BCR-induced apoptosis. In contrast, BCR-crosslinking caused no apparent change in mitochondrial transmembrane potential. Therefore, the lysosomal change might be a primary event in BCR-induced apoptosis in DT40 cells. The increased activity of cathepsin B and apoptosis-preventing effect of a cathepsin inhibitor suggested a significant role of lysosomal enzymes in this apoptosis. By microscopic studies, lysosomes of wild-type DT40 cells fused to BCR-carrying endosomes became enlarged and accumulated one another. In contrast, these changes of lysosomal dynamics did not occur in Syk-deficient cells but transfer of wild-type Syk restored the lysosomal changes and apoptosis. These results demonstrated that the lysosomal change accompanied with the activation of lysosomal enzymes is a primary step in BCR-crosslinking-mediated apoptosis and Syk is responsible for this step through the fusion of BCR-carrying endosomes to lysosomes.

Progressive loss of Syk and abnormal proliferation in breast cancer cells

The tumor suppressor gene Syk tyrosine kinase is absent or reduced in invasive breast cancer tissues and cell lines; its loss in breast tissues is linked to poor prognosis and metastasis. Also, evidence shows that in vitro Syk is involved in regulating proliferation. Here, we show by in situ hybridization on breast tissue sections that the loss of Syk expression is progressive during tumor development. Strikingly, Syk is already partially lost in normal epithelial tissue adjacent to the cancer lesion. In vivo, cell proliferation (as measured by the proliferative index Ki67) increased from normal to ductal carcinoma in situ to invasive, whereas Syk in situ staining in the same tissues decreased. In vitro, the presence of Syk was associated with reduced cell proliferation in an epidermal growth factor receptor-overexpressing breast cancer cell line, BT549, whereas changes in apoptosis were undetected. Concomitantly, the kinase activity of the proto-oncogene Src was reduced by approximately 30%. A 5-fold increase in abnormal mitoses was observed in the Syk-transfected cells compared with vector control. We propose that Syk is involved in the regulation of cell proliferation, possibly by controlling mechanisms of mitosis and cytokinesis via Src signal transduction pathway(s). Because of its progressive and early loss during tumor onset and development, monitoring of Syk loss in breast epithelial cells by noninvasive techniques such as ductal lavage may be a powerful tool for screening purposes.

The B lymphocyte adaptor molecule of 32 kilodaltons (Bam32) regulates B cell antigen receptor internalization

The B lymphocyte adaptor molecule of 32 kDa (Bam32) is an adaptor that plays an indispensable role in BCR signaling. In this study, we found that upon BCR ligation, Bam32 is recruited to the plasma membrane where it associates with BCR complexes and redistributes and internalizes with BCRs. BCR ligation induced colocalization of Bam32 with lipid rafts, clathrin, and actin filaments. An inhibitor of Src family protein tyrosine kinases (PTKs) blocked both BCR-induced tyrosine phosphorylation of Bam32 and BCR internalization. Moreover, BCR internalization is impaired in Bam32-/- and Lyn-/- cells, and expression of Bam32 with a mutation of its tyrosine phosphorylation site (Y139F) inhibited BCR internalization. These data suggest that Bam32 functions downstream of Src family PTKs to regulate BCR internalization. Bam32 deficiency does not affect tyrosine phosphorylation of clathrin or the association of clathrin with lipid rafts upon BCR cross-linking. However, BCR-induced actin polymerization is impaired in Bam32-/- cells. Collectively, these findings indicate a novel role of Bam32 in connecting Src family PTKs to BCR internalization by an actin-dependent mechanism.

An important role of phospholipase Cgamma1 in pre-B-cell development and allelic exclusion

Phospholipase Cgamma1 (PLCgamma1) has been reported to be expressed predominantly in T cells and to play an important role in T-cell receptor signaling. Here we show that PLCgamma1 is expressed throughout B-cell development, with high expression in B-cell progenitors, and is involved in pre-B-cell receptor (pre-BCR) signaling. Reduced expression of PLCgamma1, in the absence of PLCgamma2 (PLCgamma1+/-PLCgamma2-/-), impedes early B-cell development at the pro-B- to pre-B-cell transition and impairs immunoglobulin heavy chain allelic exclusion, hallmarks of defective pre-BCR signaling. In contrast, early B-cell development is largely normal, whereas late B-cell maturation is impaired in the absence of PLCgamma2 alone (PLCgamma2-/-) and overexpression of PLCgamma1 in PLCgamma2-/- mice fails to restore BCR-mediated B-cell proliferation and maturation. These studies reveal an essential role of PLCgamma1, distinct from that of PLCgamma2, in B-cell development.

Distinct mechanisms determine the patterns of differential activation of H-Ras, N-Ras, K-Ras 4B, and M-Ras by receptors for growth factors or antigen

Although GTPases of the Ras family have been implicated in many aspects of the regulation of cells, little is known about the roles of individual family members. Here, we analyzed the mechanisms of activation of H-Ras, N-Ras, K-Ras 4B, and M-Ras by two types of external stimuli, growth factors and ligation of the antigen receptors of B or T lymphocytes (BCRs and TCRs). The growth factors interleukin-3, colony-stimulating factor 1, and epidermal growth factor all preferentially activated M-Ras and K-Ras 4B over H-Ras or N-Ras. Preferential activation of M-Ras and K-Ras 4B depended on the presence of their polybasic carboxy termini, which directed them into high-buoyant-density membrane domains where the activated receptors, adapters, and mSos were also present. In contrast, ligation of the BCR or TCR resulted in activation of H-Ras, N-Ras, and K-Ras 4B, but not M-Ras. This pattern of activation was not influenced by localization of the Ras proteins to membrane domains. Activation of H-Ras, N-Ras, and K-Ras 4B instead depended on the presence of phospholipase C-gamma and RasGRP. Thus, the molecular mechanisms leading to activation of Ras proteins vary with the stimulus and can be influenced by either colocalization with activated receptors or differential sensitivity to the exchange factors activated by a stimulus.

Reverse genetic studies of the DNA damage response in the chicken B lymphocyte line DT40

In the 'post-genome' era, reverse genetics is one of the most informative and powerful means to investigate protein function. The chicken B lymphocyte line DT40 is widely used for reverse genetics because the cells have a number of advantages, including efficient gene targeting as well as a remarkably stable phenotype. Furthermore, the absence of functional p53 in DT40 cells enables identification of DNA damage using chromosome analysis by suppressing damage-induced apoptosis during interphase. This review summarizes the contribution of DT40 cells to reverse genetic studies of DNA damage response pathways in higher eukaryotic cells.

Obligatory role of Src kinase in the signaling mechanism for TRPC3 cation channels

Members of the canonical transient receptor potential (TRPC) subfamily of cation channels are candidates for capacitative and non-capacitative Ca2+ entry channels. When ectopically expressed in cell lines, TRPC3 can be activated by phospholipase C-mediated generation of diacylglycerol or by addition of synthetic diacylglycerols, independently of Ca2+ store depletion. Apart from this mode of regulation, little is known about other receptor-dependent signaling events that modulate TRPC3 activity. In the present study the role of tyrosine kinases in receptor- and diacylglycerol-dependent activation of TRPC3 was investigated. In HEK293 cells stably expressing TRPC3, pharmacological inhibition of tyrosine kinases, and specifically of Src kinases, abolished activation of TRPC3 by muscarinic receptor stimulation and by diacylglycerol. Channel regulation was lost following expression of a dominant-negative mutant of Src, or when TRPC3 was expressed in an Src-deficient cell line. In both instances, wild-type Src restored TRPC3 regulation. We conclude that Src plays an obligatory role in the mechanism for receptor and diacylglycerol activation of TRPC3.