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

Abstract

The activation of antigen receptors triggers two important signalling pathways originating from phosphatidylinositol(4,5)-bisphosphate [PtdIns(4,5)P(2)]. The first is phospholipase Cgamma (PLCgamma)-mediated hydrolysis of PtdIns(4,5)P(2), resulting in the activation of Ras, protein kinase C and Ca(2+) 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)P(2) to yield phosphatidylinositol(3,4,5)-trisphosphate, activating signalling pathways that overlap with PLCgamma 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 p110delta 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 p110delta in regulating antigen-dependent responses in these cell types.

Figure 1

Metabolism of PtdIns(4,5)P₂ by PLCγ and PI3K. PLCγ hydrolyses PtdIns(4,5)P₂ to yield Ins(1,4,5)P₃ and DAG, both of which function as signalling molecules. Ins(1,4,5)P₃ stimulates the release of Ca²⁺ from the ER into the cytosol, which triggers the nuclear translocation of NFAT. DAG binds to and activates RasGRP, which stimulates Ras and the Erk pathway, leading to AP-1-dependent transcription. Ras also binds to p110 and contributes to optimal PI3K activation. DAG binds to and activates PKC, which activates NF-κB through CARMA1, BCL10 and MALT1. By contrast, PI3K phosphorylates PtdIns(4,5)P₂ at position 3 to produce the membrane phosphoinositol lipid PtdIns(3,4,5)P₃. PtdIns(3,4,5)P₃ functions as an anchor and cofactor for proteins with PtdIns(3,4,5)P₃-binding PH domains such as Akt, Tec family kinases, and various GEFs and GAPs. Pdk1 is required to co-activate Akt. Akt phosphorylates and inactivates Foxo and GSK3. GSK3 can phosphorylate and inactivate NFAT. Akt stimulates mTOR through Tsc1 and Tsc2. Tec kinases can phosphorylate PLCγ and contribute to its optimal activity. PI3K signalling is antagonised by the Pten phosphoinositide phosphatase, which removes the 3-phosphate, and the SHIP phosphatase, which removes the 5-phosphate. The role of PI(3,4)P₂-binding proteins is still unknown. Although PLCγ and PI3K generate mutually exclusive second messenger signalling molecules, several of the pathways activated by these second messengers interact, and the signals are further integrated by the cell to promote gene transcription, cell growth and differentiation. p110δ seems to be the principal PI3K isoform in the context of antigen receptor signaling; however, p110α and p110β are also expressed in lymphocytes but their roles in antigen receptor signalling are unknown. Abbreviations: BCL10, B cell lymphoma 10; CARMA1, caspase recruitment domain (CARD)-containing membrane-associated guanylate kinase (MAGUK) protein 1; IP₃, inositol(1,4,5)-trisphosphate; MALT1, mucosa-associated lymphoid tissue lymphoma translocation protein 1; PI, phosphatidylinositol.

Figure 2

Antigen receptor complexes and p110δ antigen receptor signalling in different cell types has key commonalities, the most crucial of which is the phosphorylation of ITAM motifs found in proteins that are noncovalently associated with the polypeptides that bind to the antigen or antibody–antigen complexes. Src-family kinases phosphorylate these ITAM motifs, thus providing docking sites for Syk kinases. Syk kinases phosphorylate PLCγ, resulting in its activation, in addition to the recruitment of various cellular and membrane-bound adaptor proteins, such as LAT, Gab2, SLP-76 (in T cells and mast cells) and SLP-65 (in B cells) that nucleate larger signalling complexes . The upstream activators of PI3K in the context of antigen receptor signalling have not been definitively defined, and possible links with the SH2 domains of p85 are indicated by dashed arrows. Phosphorylation-independent interactions of PI3K with upstream signalling molecules are not shown. Note that the antigens are shown as monomers for illustrative purposes. In reality, dimers or oligomers of the ligands and receptors are required to trigger the signalling cascades shown. (a) BCR signalling. Lyn-dependent phosphorylation of Igα and Igβ ITAM motifs results in the recruitment of Syk and the phosphorylation of SLP-65. Several proteins in the BCR receptor complex have been implicated in binding to p85. BCAP is a transmembrane adaptor protein with YxxM motifs which becomes phosphorylated by Syk upon BCR activation. BCAP has been shown to regulate PI3K signalling in DT40 chicken cells but was found to be not required for PI3K signalling in primary mouse B cells . Similarly, Vav had been proposed to lie upstream of PI3K signalling ; however, BCR crosslinking of primary Vav-deficient B cells results in normal Akt phosphorylation (although Akt phosphorylation in response to BCR and CD19 coligation was Vav-dependent) . Other candidates that link PI3K to the activated BCR include Gab1, non T-cell activation linker (NTAL) and LAT. However, many of these interactions have only been identified in cell lines, and in several cases, their roles in PI3K signalling downstream of the BCR have not been confirmed in mice . (b) TCR signalling. Lck and Fyn phosphorylate ITAM motifs resulting in the recruitment of ZAP-70, which phosphorylates LAT. TRIM is a transmembrane adaptor protein that associates with the TCR and becomes phosphorylated on YxxM motifs. However, TRIM knockout mice show enhanced instead of impaired Akt phosphorylation . Vav has an important role in regulating Akt and Foxo phosphorylation. Although the exact biochemical link between Vav and PI3K is not clearly defined, it may reflect a more general role for Vav in assembling LAT complexes . CD28 can bind to the SH2 domains of p85 directly; however, CD28 can provide potent costimulatory signals independently of its association with PI3K . (c) FcɛRI signalling. Lyn phosphorylates ITAM motifs in the β and γ chains of the FcɛRI resulting in the recruitment of Syk, which phosphorylates GAB2 and LAT, inducing two parallel pathways through GAB2–PI3K and LAT–PLCγ. The link shown between NTAL and GAB2 is hypothetical. c-Kit can bind to p85 directly and can potentiate FcɛRI-stimulated degranulation. (d) Chemokine receptor signalling. The p110γ heterodimer binds to the Gβγ subunit released from Gα following GPCR stimulation with agonists such as chemokines. Despite the potent activation of p110γ by chemokines, p110γ seems to have a minor role in promoting lymphocyte chemotaxis . Instead, p110γ might promote the survival of developing thymocytes and memory T cells .