Regulation of phospholipase C-gamma2 networks in B lymphocytes

Identification of Molecular Subtypes and a Prognostic Signature Based on Inflammation-Related Genes in Colon Adenocarcinoma

Both tumour-infiltrating immune cells and inflammation-related genes that can mediate immune infiltration contribute to the initiation and prognosis of patients with colon cancer. In this study, we developed a method to predict the survival outcomes among colon cancer patients and direct immunotherapy and chemotherapy. We obtained patient data from The Cancer Genome Atlas (TCGA) and captured inflammation-related genes from the GeneCards database. The package “ConsensusClusterPlus” was used to generate molecular subtypes based on inflammation-related genes obtained by differential expression analysis and univariate Cox analysis. A prognostic signature including four genes (PLCG2, TIMP1, BDNF and IL13) was also constructed and was an independent prognostic factor. Cluster 2 and higher risk scores meant worse overall survival and higher expression of human leukocyte antigen and immune checkpoints. Immune cell infiltration calculated by the estimate, CIBERSORT, TIMER, ssGSEA algorithms, tumour immune dysfunction and exclusion (TIDE), and tumour stemness indices (TSIs) were also compared on the basis of inflammation-related molecular subtypes and the risk signature. In addition, analyses of stratification, somatic mutation, nomogram construction, chemotherapeutic response prediction and small-molecule drug prediction were performed based on the risk signature. We finally used qRT–PCR to detect the expression levels of four genes in colon cancer cell lines and obtained results consistent with the prediction. Our findings demonstrated a four-gene prognostic signature that could be useful for prognostication in colon cancer patients and designing personalized treatments, which could provide new versions of personalized management for these patients.

The Phospholipase Cγ2 Mutants R665W and L845F Identified in Ibrutinib-resistant Chronic Lymphocytic Leukemia Patients Are Hypersensitive to the Rho GTPase Rac2 Protein

Mutations in the gene encoding phospholipase C-γ2 (PLCγ2) have been shown to be associated with resistance to targeted therapy of chronic lymphocytic leukemia (CLL) with the Bruton's tyrosine kinase inhibitor ibrutinib. The fact that two of these mutations, R665W and L845F, imparted upon PLCγ2 an ∼2-3-fold ibrutinib-insensitive increase in the concentration of cytosolic Ca2+ following ligation of the B cell antigen receptor (BCR) led to the assumption that the two mutants exhibit constitutively enhanced intrinsic activity. Here, we show that the two PLCγ2 mutants are strikingly hypersensitive to activation by Rac2 such that even wild-type Rac2 suffices to activate the mutant enzymes upon its introduction into intact cells. Enhanced "basal" activity of PLCγ2 in intact cells is shown using the pharmacologic Rac inhibitor EHT 1864 and the PLCγ2F897Q mutation mediating Rac resistance to be caused by Rac-stimulated rather than by constitutively enhanced PLCγ2 activity. We suggest that R665W and L845F be referred to as allomorphic rather than hypermorphic mutations of PLCG2 Rerouting of the transmembrane signals emanating from BCR and converging on PLCγ2 through Rac in ibrutinib-resistant CLL cells may provide novel drug treatment strategies to overcome ibrutinib resistance mediated by PLCG2 mutations or to prevent its development in ibrutinib-treated CLL patients.

The Inositol Trisphosphate/Calcium Signaling Pathway in Health and Disease

Many cellular functions are regulated by calcium (Ca2+) signals that are generated by different signaling pathways. One of these is the inositol 1,4,5-trisphosphate/calcium (InsP3/Ca2+) signaling pathway that operates through either primary or modulatory mechanisms. In its primary role, it generates the Ca2+ that acts directly to control processes such as metabolism, secretion, fertilization, proliferation, and smooth muscle contraction. Its modulatory role occurs in excitable cells where it modulates the primary Ca2+ signal generated by the entry of Ca2+ through voltage-operated channels that releases Ca2+ from ryanodine receptors (RYRs) on the internal stores. In carrying out this modulatory role, the InsP3/Ca2+ signaling pathway induces subtle changes in the generation and function of the voltage-dependent primary Ca2+ signal. Changes in the nature of both the primary and modulatory roles of InsP3/Ca2+ signaling are a contributory factor responsible for the onset of a large number human diseases.

Reversing Tolerance in Isotype Switch-Competent Anti-Insulin B Lymphocytes

Autoreactive B lymphocytes that escape central tolerance and mature in the periphery are a liability for developing autoimmunity. IgG insulin autoantibodies that predict type 1 diabetes and complicate insulin therapies indicate that mechanisms for tolerance to insulin are flawed. To examine peripheral tolerance in anti-insulin B cells, we generated C57BL/6 mice that harbor anti-insulin VDJH-125 site directed to the native IgH locus (VH125(SD)). Class switch-competent anti-insulin B cells fail to produce IgG Abs following T cell-dependent immunization of VH125(SD) mice with heterologous insulin, and they exhibit markedly impaired proliferation to anti-CD40 plus insulin in vitro. In contrast, costimulation with LPS plus insulin drives robust anti-insulin B cell proliferation. Furthermore, VH125(SD) mice produce both IgM and IgG2a anti-insulin Abs following immunization with insulin conjugated to type 1 T cell-independent Brucella abortus ring test Ag (BRT). Anti-insulin B cells undergo clonal expansion in vivo and emerge as IgM(+) and IgM(-) GL7(+)Fas(+) germinal center (GC) B cells following immunization with insulin-BRT, but not BRT alone. Analysis of Igκ genes in VH125(SD) mice immunized with insulin-BRT reveals that anti-insulin Vκ from the preimmune repertoire is selected into GCs. These data demonstrate that class switch-competent anti-insulin B cells remain functionally silent in T cell-dependent immune responses, yet these B cells are vulnerable to reversal of anergy following combined BCR/TLR engagement that promotes Ag-specific GC responses and Ab production. Environmental factors that lead to infection and inflammation could play a critical yet underappreciated role in driving loss of tolerance and promoting autoimmune disease.

TLR7 and TLR9 in SLE: when sensing self goes wrong

Autoreactive B and T cells are present in healthy, autoimmunity-free individuals, but they are kept in check by various regulatory mechanisms. In systemic lupus erythematosus (SLE) patients, however, autoreactive cells are expanded, activated, and produce large quantities of autoantibodies, directed especially against nuclear antigens. These antibodies form immune complexes with self-nucleic acids present in SLE serum. Since self-DNA and self-RNA in the form of protein complexes can act as TLR9 and TLR7 ligands, respectively, TLR stimulation is suggested as an additional signal contributing to activation and/or modulation of the aberrant adaptive immune response. Data from mouse models suggest a pathogenic role for TLR7 and a protective role for TLR9 in the pathogenesis of SLE. Future investigations are needed to elucidate the underlying modulatory mechanisms and the role of TLR7 and TLR9 in the complex pathogenesis of human SLE.

Mammalian phospholipase C

Phospholipase C (PLC) converts phosphatidylinositol 4,5-bisphosphate (PIP(2)) to inositol 1,4,5-trisphosphate (IP(3)) and diacylglycerol (DAG). DAG and IP(3) each control diverse cellular processes and are also substrates for synthesis of other important signaling molecules. PLC is thus central to many important interlocking regulatory networks. Mammals express six families of PLCs, each with both unique and overlapping controls over expression and subcellular distribution. Each PLC also responds acutely to its own spectrum of activators that includes heterotrimeric G protein subunits, protein tyrosine kinases, small G proteins, Ca(2+), and phospholipids. Mammalian PLCs are autoinhibited by a region in the catalytic TIM barrel domain that is the target of much of their acute regulation. In combination, the PLCs act as a signaling nexus that integrates numerous signaling inputs, critically governs PIP(2) levels, and regulates production of important second messengers to determine cell behavior over the millisecond to hour timescale.

Immune modulation by Fas ligand reverse signaling: lymphocyte proliferation is attenuated by the intracellular Fas ligand domain

Fas ligand (FasL) not only induces apoptosis in Fas receptor-bearing target cells, it is also able to transmit signals into the FasL-expressing cell via its intracellular domain (ICD). Recently, we described a Notch-like proteolytic processing of FasL that leads to the release of the FasL ICD into the cytoplasm and subsequent translocation into the nucleus where it may influence gene transcription. To study the molecular mechanism underlying such reverse FasL signaling in detail and to analyze its physiological importance in vivo, we established a knockout/knockin mouse model, in which wild-type FasL was replaced with a deletion mutant lacking the ICD. Our results demonstrate that FasL ICD signaling impairs activation-induced proliferation in B and T cells by diminishing phosphorylation of phospholipase C γ, protein kinase C, and extracellular signal-regulated kinase 1/2. We also demonstrate that the FasL ICD interacts with the transcription factor lymphoid-enhancer binding factor-1 and inhibits lymphoid-enhancer binding factor-1–dependent transcription. In vivo, plasma cell numbers, generation of germinal center B cells, and, consequently, production of antigen-specific immunoglobulin M antibodies in response to immunization with T cell–dependent or T cell–independent antigen are negatively affected in presence of the FasL ICD, suggesting that FasL reverse signaling participates in negative fine-tuning of certain immune responses.

Molecular underpinning of B-cell anergy

A byproduct of the largely stochastic generation of a diverse B-cell specificity repertoire is production of cells that recognize autoantigens. Indeed, recent studies indicate that more than half of the primary repertoire consists of autoreactive B cells that must be silenced to prevent autoimmunity. While this silencing can occur by multiple mechanisms, it appears that most autoreactive B cells are silenced by anergy, wherein they populate peripheral lymphoid organs and continue to express unoccupied antigen receptors yet are unresponsive to antigen stimulation. Here we review molecular mechanisms that appear operative in maintaining the antigen unresponsiveness of anergic B cells. In addition, we present new data indicating that the failure of anergic B cells to mobilize calcium in response to antigen stimulation is not mediated by inactivation of stromal interacting molecule 1, a critical intermediary in intracellular store depletion-induced calcium influx.

Unique properties of memory B cells of different isotypes

Memory antibody responses are typically seen to T-cell-dependent antigens and are characterized by the rapid production of high titers of high-affinity antigen-specific antibody. The hallmark of T-cell-dependent memory B cells is their expression of a somatically mutated, isotype-switched B-cell antigen receptor, features that are mainly generated in germinal centers. Classical studies have focused on isotype-switched memory B cells (mainly IgG isotype) and demonstrated their unique intrinsic properties in terms of localization and responsiveness to antigen re-exposure. However, recent advances in monitoring antigen-experienced B cells have revealed the considerable heterogeneity of memory B cells, which include unswitched IgM(+) and/or unmutated memory B cells. The IgM and IgG type memory B cells reside in distinct locations and appear to possess distinct origins and effector functions, together orchestrating humoral memory responses.

Echinacea-induced cytosolic Ca2+ elevation in HEK293

Background

With a traditional medical use for treatment of various ailments, herbal preparations of Echinacea are now popularly used to improve immune responses. One likely mode of action is that alkamides from Echinacea bind to cannabinoid type 2 (CB2) receptors and induce a transient increase in intracellular Ca²⁺. Here, we show that unidentified compounds from Echinacea purpurea induce cytosolic Ca²⁺ elevation in non-immune-related cells, which lack CB2 receptors and that the Ca²⁺ elevation is not influenced by alkamides.

Methods

A non-immune human cell line, HEK293, was chosen to evaluate E. purpurea root extracts and constituents as potential regulators of intracellular Ca²⁺ levels. Changes in cytosolic Ca²⁺ levels were monitored and visualized by intracellular calcium imaging. U73122, a phospholipase C inhibitor, and 2-aminoethoxydiphenyl borate (2-APB), an antagonist of inositol-1,4,5-trisphosphate (IP₃) receptor, were tested to determine the mechanism of this Ca²⁺ signaling pathway. E. purpurea root ethanol extracts were fractionated by preparative HPLC, screened for bioactivity on HEK293 cells and by GC-MS for potential constituent(s) responsible for this bioactivity.

Results

A rapid transient increase in cytosolic Ca²⁺ levels occurs when E. purpurea extracts are applied to HEK293 cells. These stimulatory effects are phospholipase C and IP₃ receptor dependent. Echinacea-evoked responses could not be blocked by SR 144528, a specific CB2 receptor antagonist, indicating that CB2 is not involved. Ca²⁺ elevation is sustained after the Echinacea-induced Ca²⁺ release from intracellular Ca²⁺ stores; this longer-term effect is abolished by 2-APB, indicating a possible store operated calcium entry involvement. Of 28 HPLC fractions from E. purpurea root extracts, six induce cytosolic Ca²⁺ increase. Interestingly, GC-MS analysis of these fractions, as well as treatment of HEK293 cells with known individual and combined chemicals, indicates the components thought to be responsible for the major immunomodulatory bioactivity of Echinacea do not explain the observed Ca²⁺ response. Rather, lipophilic constituents of unknown structures are associated with this bioactivity.

Conclusions

Our data indicate that as yet unidentified constituents from Echinacea stimulate an IP₃ receptor and phospholipase C mediation of cytosolic Ca²⁺ levels in non-immune mammalian cells. This pathway is distinct from that induced in immune associated cells via the CB2 receptor.

B cell signaling and fate decision

Antigen receptors on the surface of B lymphocytes trigger adaptive immune responses after encountering their cognate antigens but also control a series of antigen-independent checkpoints during B cell development. These physiological processes are regulated by the expression and function of cell surface receptors, intracellular signaling molecules, and transcription factors. The function of these proteins can be altered by a dynamic array of post-translational modifications, using two interconnected mechanisms. These modifications can directly induce an altered conformational state in the protein target of the modification itself. In addition, they can create new binding sites for other protein partners, thereby contributing to where and when such multiple protein assemblies are activated within cells. As a new type of post-transcriptional regulator, microRNAs have emerged to influence the development and function of B cells by affecting the expression of target mRNAs.

Regulation of the Src homology 2 domain-containing inositol 5'-phosphatase (SHIP1) by the cyclic AMP-dependent protein kinase

Many agents that activate hematopoietic cells use phos pha tidyl ino si tol 3,4,5-trisphosphate (PtdIns 3,4,5-P(3)) to initiate signaling cascades. The SH2 domain-containing inositol 5' phosphatase, SHIP1, regulates hematopoietic cell function by opposing the action of phos pha tidyl ino si tol 3-kinase and reducing the levels of PtdIns 3,4,5-P(3). Activation of the cyclic AMP-de pend ent protein kinase (PKA) also opposes many of the pro-inflammatory responses of hematopoietic cells. We tested to see whether the activity of SHIP1 was regulated via phos pho ryl a tion with PKA. We prepared pure recombinant SHIP1 from HEK-293 cells and found it can be rapidly phos pho ryl a ted by PKA to a stoichiometry of 0.6 mol of PO(4)/mol of SHIP1. In (32)P-labeled HEK-293 cells transfected with SHIP1, stimulation with Sp-adenosine 3',5'-cyclic monophosphorothioate triethylammonium salt hydrate (Sp-cAMPS) or activation of the beta-adrenergic receptor increased the phos pho ryl a tion state of SHIP1. Inhibition of protein phosphatase activity with okadaic acid also increased the phos pho ryl a tion of SHIP1. Phosphorylation of SHIP1 in vitro or in cells by PKA increased the 5' phosphatase activity of SHIP1 by 2-3-fold. Elevation of Ca(2+) in DT40 cells in response to B cell receptor cross-linking, an indicator of PtdIns 3,4,5-P(3) levels, was markedly blunted by pretreatment with Sp-cAMPS. This effect was absent in SHIP(-/-) DT40 cells showing that the effect of Sp-cAMPS in DT40 cells is SHIP1-de pend ent. Sp-cAMPS also blunted the ability of the B cell receptor to increase the phos pho ryl a tion of Akt in DT40 and A20 cells. Overall, activation of G protein-coupled receptors that raise cyclic AMP cause SHIP1 to be phosphorylated and stimulate its inositol phosphatase activity. These results outline a novel mechanism of SHIP1 regulation.

PLC-gamma2 is essential for formation and maintenance of memory B cells

Resting antigen-experienced memory B cells are thought to be responsible for the more rapid and robust antibody responses after antigen reencounter, which are the hallmark of memory humoral responses. The molecular basis for the development and survival of memory B cells remains largely unknown. We report that phospholipase C (PLC) γ2 is required for efficient formation of germinal center (GC) and memory B cells. Moreover, memory B cell homeostasis is severely hampered by inducible loss of PLC-γ2. Accordingly, mice with a conditional deletion of PLC-γ2 in post-GC B cells had an almost complete abrogation of the secondary antibody response. Collectively, our data suggest that PLC-γ2 conveys a survival signal to GC and memory B cells and that this signal is required for a productive secondary immune response.