Distinct roles of phosphoinositide-3 kinase and phospholipase Cgamma2 in B-cell receptor-mediated signal transduction

The role of PLCγ2 in immunological disorders, cancer, and neurodegeneration

Phosphatidylinositol-specific phospholipase Cγ2 (PLCγ2) is a critical signaling molecule activated downstream from a variety of cell surface receptors that contain an intracellular immunoreceptor tyrosine-based activation motif. These receptors recruit kinases such as Syk, BTK, and BLNK to phosphorylate and activate PLCγ2, which then generates 1D-myo-inositol 1,4,5-trisphosphate and diacylglycerol. These well-known second messengers are required for diverse membrane functionality including cellular proliferation, endocytosis, and calcium flux. As a result, PLCγ2 dysfunction is associated with a variety of diseases including cancer, neurodegeneration, and immune disorders. The diverse pathologies associated with PLCγ2 are exemplified by distinct genetic variants. Inherited mutations at this locus cause PLCγ2-associated antibody deficiency and immune dysregulation, in some cases with autoinflammation. Acquired mutations at this locus, which often arise as a result of BTK inhibition to treat chronic lymphocytic leukemia, result in constitutive downstream signaling and lymphocyte proliferation. Finally, a third group of PLCγ2 variants actually has a protective effect in a variety of neurodegenerative disorders, presumably by increased uptake and degradation of deleterious neurological aggregates. Therefore, manipulating PLCγ2 activity either up or down could have therapeutic benefit; however, we require a better understanding of the signaling pathways propagated by these variants before such clinical utility can be realized. Here, we review the signaling roles of PLCγ2 in hematopoietic cells to help understand the effect of mutations driving immune disorders and cancer and extrapolate from this to roles which may relate to protection against neurodegeneration.

PLCγ-dependent mTOR signalling controls IL-7-mediated early B cell development

The precise molecular mechanism underlying the regulation of early B cell lymphopoiesis is unclear. The PLCγ signaling pathway is critical for antigen receptor-mediated lymphocyte activation, but its function in cytokine signaling is unknown. Here we show that PLCγ1/PLCγ2 double deficiency in mice blocks early B cell development at the pre-pro-B cell stage and renders B cell progenitors unresponsive to IL-7. PLCγ pathway inhibition blocks IL-7-induced activation of mTOR, but not Stat5. The PLCγ pathway activates mTOR through the DAG/PKC signaling branch, independent of the conventional Akt/TSC/Rheb signaling axis. Inhibition of PLCγ/PKC-induced mTOR activation impairs IL-7-mediated B cell development. PLCγ1/PLCγ2 double-deficient B cell progenitors have reduced expression of genes related to B cell lineage, IL-7 signaling, and cell cycle. Thus, IL-7 receptor controls early B lymphopoiesis through activation of mTOR via PLCγ/DAG/PKC signaling, not via Akt/Rheb signaling.

Novel G6B gene variant causes familial autosomal recessive thrombocytopenia and anemia

OBJECTIVE

To characterize the underlying genetic and molecular defects in a consanguineous family with lifelong blood disorder manifested with thrombocytopenia (low platelets count) and anemia.

METHODS

Genetic linkage analysis, exome sequencing, and functional genomics were carried out to identify and characterize the defective gene.

RESULTS

We identified a novel truncation mutation (p.C108*) in chromosome 6 open reading frame 25 (C6orf25) gene in this family. We also showed the p.C108* mutation was responsible for destabilizing the encoded truncated G6B protein. Unlike the truncated form, wild-type G6B expression resulted in enhanced K562 differentiation into megakaryocytes and erythrocytes. C6orf25, also known as G6B, is an effector protein for the key hematopoiesis regulators, Src homology region 2 domain-containing phosphatases SHP-1 and SHP-2.

CONCLUSION

G6B seems to act through an autosomal recessive mode of disease transmission in this family and regarded as the gene responsible for the observed hematological disorder. This inference is well supported further by in vivo evidence where similar outcomes were reported from G6b^-/- and SHP1/2 DKO mouse models.

p85-RhoGDI2, a novel complex, is required for PSGL-1-induced β1 integrin-mediated lymphocyte adhesion to VCAM-1

P-selectin glycoprotein ligand-1 and β1 integrin play essential roles in T cell trafficking during inflammation. E-selectin and vascular cell adhesion molecule-1 are their ligands expressed on inflammation-activated endothelium. During the tethering and rolling of lymphocytes on endothelium, P-selectin glycoprotein ligand-1 binds E-selectin and induces signals. Subsequently, β1 integrin is activated and mediates stable adhesion. However, the intracellular signal pathways from PSGL-1 to β1 integrin have not yet been fully understood. Here, we find that p85, a regulatory subunit of phosphoinositide 3-kinase, forms a novel complex with Rho-GDP dissociation inhibitor-2, a lymphocyte-specific RhoGTPases dissociation inhibitor. Phosporylations of the p85-bound Rho-GDP dissociation inhibitor-2 on 130 and 153 tyrosine residues by c-Abl and Src were required for the complex to be recruited to P-selectin glycoprotein ligand-1 and thereby regulate β1 integrin-mediated T cell adhesion to vascular cell adhesion molecule-1. Both shRNAs to Rho-GDP dissociation inhibitor-2 and p85 and over-expression of Rho-GDP dissociation inhibitor-2 Y130F and Y153F significantly reduced the above-mentioned adhesion. Although Rho-GDP dissociation inhibitor-2 in the p85-Rho-GDP dissociation inhibitor-2 complex was also phosphorylated on 24 tyrosine residue by Syk, the phosphorylation is not required for the adhesion. Taken together, we find that specific phosphorylations on 130 and 153 tyrosine residues of p85-bound Rho-GDP dissociation inhibitor-2 are pivotal for P-selectin glycoprotein ligand-1-induced β1 integrin-mediated lymphocyte adhesion to vascular cell adhesion molecule-1. This will shed new light on the mechanisms that connect leukocyte initial rolling with subsequent adhesion.

The viral latency-associated nuclear antigen augments the B-cell response to antigen in vivo

Gammaherpesviruses, including Kaposi sarcoma-associated herpesvirus (KSHV), establish latency in B cells. We hypothesized that the KSHV latency-associated nuclear antigen (LANA/orf73) provides a selective advantage to infected B cells by driving proliferation in response to antigen. To test this, we used LANA B-cell transgenic mice. Eight days after immunization with antigen without adjuvant, LANA mice had significantly more activated germinal center (GC) B cells (CD19(+) PNA(+) CD71(+)) than controls. This was dependent upon B-cell receptor since LANA did not restore the GC defect of CD19 knockout mice. However, LANA was able to restore the marginal zone defect in CD19 knockout mice.

The follicular versus marginal zone B lymphocyte cell fate decision

Bone marrow-derived B cells make an important cell fate choice to develop into either follicular B cells or marginal zone B cells in the spleen, which depends on signalling through the B cell receptor, Notch2, the receptor for B cell-activating factor and the canonical nuclear factor-kappaB pathway, as well as signals involved in the migration and anatomical retention of marginal zone B cells. Recent information discussed in this Review reconciles some of the controversies regarding the role of the B cell receptor in this cell fate decision and a clearer picture has also emerged regarding the anatomical location of ligands for Notch2 in the spleen. This cell fate decision could provide mechanistic insights that are relevant to other commitment events in lymphocytes.

Deletion of PI3K-p85alpha gene impairs lineage commitment, terminal maturation, cytokine generation and cytotoxicity of NK cells

Class IA phosphotidylinositol-3-kinases (PI3Ks) are a family of p85/p110 heterodimeric lipid kinases that are important in regulating signaling events in B and T cells. However, their role in natural killer (NK) cells is not understood. Here, using mice that lack the regulatory p85alpha subunit and its alternatively spliced variants p55alpha/p50alpha (collectively termed as p85alpha(-/-)), we defined the role of PI3K in NK cell development and function. p85alpha(-/-) mice had impaired lineage commitment leading to reduced NK cellularity in the bone marrow and liver. p85alpha(-/-) NK cells showed a defective Ly49 subset specification and a decreased expression of CD43. Lack of p85alpha severely reduced the NK-mediated cytotoxicity against tumor cells representing 'induced-self' and 'missing-self'. More importantly, NKG2D and NK1.1 receptor-mediated cytokine and chemokine generation was significantly compromised in p85alpha(-/-) NK cells. These results reveal a previously unrecognized role of p85alpha in the development, terminal maturation, cytokine/chemokine generation and tumor clearance of NK cells.

The recirculating B cell pool contains two functionally distinct, long-lived, posttransitional, follicular B cell populations

Disparate models for the development of peripheral B cells may reflect significant heterogeneity in recirculating long-lived B cells that have not been previously accounted for. We show in this study that the murine recirculating B cell pool contains two distinct, long-lived, posttransitional, follicular B cell populations. Follicular Type I IgM(low) B cells require Ag-derived and Btk-dependent signals for their development and make up the majority of cells in the recirculating follicular B cell pool. Follicular type II B cells do not require Btk- or Notch-2-derived signals, make up about a third of the long-lived recirculating B cell pool, and can develop in the absence of Ag. These two follicular populations exhibit differences in basal tyrosine phosphorylation and in BCR-induced proliferation, suggesting that they may represent functionally distinct populations of long-lived recirculating B cells.

Transitional B cell fate is associated with developmental stage-specific regulation of diacylglycerol and calcium signaling upon B cell receptor engagement

Functional peripheral mature follicular B (FoB) lymphocytes are thought to develop from immature transitional cells in a BCR-dependent manner. We have previously shown that BCR cross-linking in vitro results in death of early transitional (T1) B cells, whereas late transitional (T2) B cells survive and display phenotypic characteristics of mature FoB cells. We now demonstrate that diacylglycerol (DAG), a lipid second messenger implicated in cell survival and differentiation, is produced preferentially in T2 compared with T1 B cells upon BCR cross-linking. Consistently, inositol 1,4,5-triphosphate is also produced preferentially in T2 compared with T1 B cells. Unexpectedly, the initial calcium peak appears similar in both T1 and T2 B cells, whereas sustained calcium levels are higher in T1 B cells. Pretreatment with 2-aminoethoxydiphenylborate, an inhibitor of inositol 1,4,5-triphosphate receptor-mediated calcium release, and verapamil, an inhibitor of L-type calcium channels, preferentially affects T1 B cells, suggesting that distinct mechanisms regulate calcium mobilization in each of the two transitional B cell subsets. Finally, BCR-mediated DAG production is dependent upon Bruton's tyrosine kinase and phospholipase C-gamma2, enzymes required for the development of FoB from T2 B cells. These results suggest that calcium signaling in the absence of DAG-mediated signals may lead to T1 B cell tolerance, whereas the combined action of DAG and calcium signaling is necessary for survival and differentiation of T2 into mature FoB lymphocytes.

Antigen receptor signalling: a distinctive role for the p110delta isoform of PI3K

The activation of antigen receptors triggers two important signalling pathways originating from phosphatidylinositol(4,5)-bisphosphate [PtdIns(4,5)P₂]. The first is phospholipase Cγ (PLCγ)-mediated hydrolysis of PtdIns(4,5)P₂, resulting in the activation of Ras, protein kinase C and Ca²⁺ flux. This culminates in profound alterations in gene expression and effector-cell responses, including secretory granule exocytosis and cytokine production. By contrast, phosphoinositide 3-kinases (PI3Ks) phosphorylate PtdIns(4,5)P₂ to yield phosphatidylinositol(3,4,5)-trisphosphate, activating signalling pathways that overlap with PLCγ or are PI3K-specific. Pathways that are PI3K-specific include Akt-mediated inactivation of Foxo transcription factors and transcription-independent regulation of glucose uptake and metabolism. The p110δ isoform of PI3K is the main source of PI3K activity following antigen recognition by B cells, T cells and mast cells. Here, we review the roles of p110δ in regulating antigen-dependent responses in these cell types.