Phosphatidylinositol-3,4,5-trisphosphate (PtdIns-3,4,5-P3)/Tec kinase-dependent calcium signaling pathway: a target for SHIP-mediated inhibitory signals

INPP5D/SHIP1: Expression, Regulation and Roles in Alzheimer's Disease Pathophysiology

Alzheimer's disease (AD) is the most common form of dementia, accounting for approximately 38.5 million cases of all-cause dementia. Over 60% of these individuals live in low- and middle-income countries and are the worst affected, especially by its deleterious effects on the productivity of both patients and caregivers. Numerous risk factors for the disease have been identified and our understanding of gene-environment interactions have shed light on several gene variants that contribute to the most common, sporadic form of AD. Microglial cells, the innate immune cells of the central nervous system (CNS), have long been established as guardians of the brain by providing neuroprotection and maintaining cellular homeostasis. A protein with a myriad of effects on various important signaling pathways that is expressed in microglia is the Src Homology 2 (SH2) domain-containing Inositol 5' Phosphatase 1 (SHIP1) protein. Encoded by the INPP5D (Inositol Polyphosphate-5-Phosphatase D) gene, SHIP1 has diminutive effects on most microglia signaling processes. Polymorphisms of the INPP5D gene have been found to be associated with a significantly increased risk of AD. Several studies have elucidated mechanistic processes by which SHIP1 exerts its perturbations on signaling processes in peripheral immune cells. However, current knowledge of the controllers of INPP5D/SHIP1 expression and the idiosyncrasies of its influences on signaling processes in microglia and their relevance to AD pathophysiology is limited. In this review, we summarize these discoveries and discuss the potential of leveraging INPP5D/SHIP1 as a therapeutic target for Alzheimer's disease.

Activated Src kinases downstream of BCR-ABL and Flt3 induces proteasomal degradation of SHIP1 by phosphorylation of tyrosine 1021

Within the various subtypes of ALL, patients with a BCR-ABL-positive background as well as with a genetic change in the KMT2A gene have by far the worst survival probabilities. Interestingly, both subtypes are characterized by highly activated tyrosine kinases. SHIP1 serves as an important negative regulator of the PI3K/AKT signaling pathway, which is often constitutively activated in ALL. The protein expression of SHIP1 is decreased in most T-ALL and in some subgroups of B-ALL. In this study, we analyzed the expression of SHIP1 protein in detail in the context of groups with aberrant activated tyrosine kinases, namely BCR-ABL (Ph+) and Flt3 (KMT2A translocations). We demonstrate that constitutively activated Src kinases downstream of BCR-ABL and receptor tyrosine kinases reduce the SHIP1 expression in a SHIP1-Y1021 phosphorylated-dependent manner with subsequent ubiquitin marked proteasomal degradation. Inhibition of BCR-ABL (Imatinib), Flt3 (Quizartinib) or Src-Kinase-Family (Saracatinib) leads to significant reconstitution of SHIP1 protein expression. These results further support a functional role of SHIP1 as tumor suppressor protein and could be the basis for the establishment of a targeted therapy form.

Acute Csk inhibition hinders B cell activation by constraining the PI3 kinase pathway

B lymphocytes recognize pathogenic antigens and become activated via their B cell receptors (BCR). This BCR-dependent activation is controlled by Src-family kinases (SFKs). It is unclear how B cells tolerate the fluctuations of SFK activities and maintain unresponsiveness in the absence of foreign antigens. Using a chemical-genetic system, we acutely inhibited C-terminal Src kinase to enhance the SFK activity in mouse B cells. Surprisingly, we observed marked inhibition of BCR-downstream signaling due to associated impairment of the phosphatidylinositol-trisphosphate pathway. These results reveal the critical importance of maintaining a proper amount of SFK activity in quiescent B cells for appropriate BCR-dependent responses, which may be critical for naïve B cell unresponsiveness to self-antigens to maintain peripheral tolerance.

T cell antigen receptor (TCR) and B cell antigen receptor (BCR) signaling are initiated and tightly regulated by Src-family kinases (SFKs). SFKs positively regulate TCR signaling in naïve T cells but have both positive and negative regulatory roles in BCR signaling in naïve B cells. The proper regulation of their activities depends on the opposing actions of receptor tyrosine phosphatases CD45 and CD148 and the cytoplasmic tyrosine kinase C-terminal Src kinase Csk. Csk is a major negative regulator of SFKs. Using a PP1-analog-sensitive Csk (Csk^AS) system, we have previously shown that inhibition of Csk^AS increases SFK activity, leading to augmentation of responses to weak TCR stimuli in T cells. However, the effects of Csk inhibition in B cells were not known. In this study, we surprisingly found that inhibition of Csk^AS led to marked inhibition of BCR-stimulated cytoplasmic free calcium increase and Erk activation despite increased SFK activation in B cells, contrasting the effects observed in T cells. Further investigation revealed that acute Csk^AS inhibition suppressed BCR-mediated phosphatidylinositol 3,4,5-trisphosphate (PIP3) production in B cells. Restoring PIP3 levels in B cells by CD19 cross-linking or SHIP1 deficiency eliminated the negative regulatory effect of Csk^AS inhibition. This reveals the critical role of Csk in maintaining an appropriate level of SFK activity and regulating PIP3 amounts as a means of compensating for SFK fluctuations to prevent inappropriate B cell activation. This regulatory mechanism controlling PIP3 amounts may also contribute to B cell anergy and self-tolerance.

Implications of a 'Third Signal' in NK Cells

Innate and adaptive immune systems are evolutionarily divergent. Primary signaling in T and B cells depends on somatically rearranged clonotypic receptors. In contrast, NK cells use germline-encoded non-clonotypic receptors such as NCRs, NKG2D, and Ly49H. Proliferation and effector functions of T and B cells are dictated by unique peptide epitopes presented on MHC or soluble humoral antigens. However, in NK cells, the primary signals are mediated by self or viral proteins. Secondary signaling mediated by various cytokines is involved in metabolic reprogramming, proliferation, terminal maturation, or memory formation in both innate and adaptive lymphocytes. The family of common gamma (γc) cytokine receptors, including IL-2Rα/β/γ, IL-7Rα/γ, IL-15Rα/β/γ, and IL-21Rα/γ are the prime examples of these secondary signals. A distinct set of cytokine receptors mediate a ‘third’ set of signaling. These include IL-12Rβ1/β2, IL-18Rα/β, IL-23R, IL-27R (WSX-1/gp130), IL-35R (IL-12Rβ2/gp130), and IL-39R (IL-23Rα/gp130) that can prime, activate, and mediate effector functions in lymphocytes. The existence of the ‘third’ signal is known in both innate and adaptive lymphocytes. However, the necessity, context, and functional relevance of this ‘third signal’ in NK cells are elusive. Here, we define the current paradigm of the ‘third’ signal in NK cells and enumerate its clinical implications.

CD5 signalosome coordinates antagonist TCR signals to control the generation of Treg cells induced by foreign antigens

CD5 is characterized as an inhibitory coreceptor with an important regulatory role during T cell development. The molecular mechanism by which CD5 operates has been puzzling and its function in mature T cells suggests promoting rather than repressing effects on immune responses. Here, we combined quantitative mass spectrometry and genetic studies to analyze the components and the activity of the CD5 signaling machinery in primary T cells. We found that T cell receptor (TCR) engagement induces the selective phosphorylation of CD5 tyrosine 429, which serves as a docking site for proteins with adaptor functions (c-Cbl, CIN85, CRKL), connecting CD5 to positive (PI3K) and negative (UBASH3A, SHIP1) regulators of TCR signaling. c-CBL acts as a coordinator in this complex enabling CD5 to synchronize positive and negative feedbacks on TCR signaling through the other components. Disruption of CD5 signalosome in mutant mice reveals that it modulates TCR signal outputs to selectively repress the transactivation of Foxp3 and limit the inopportune induction of peripherally induced regulatory T cells during immune responses against foreign antigen. Our findings bring insights into the paradigm of coreceptor signaling, suggesting that, in addition to providing dualistic enhancing or dampening inputs, coreceptors can engage concomitant stimulatory and inhibitory signaling events, which act together to promote specific functional outcomes.

Role of MCC/Eisosome in Fungal Lipid Homeostasis

One of the best characterized fungal membrane microdomains is the MCC/eisosome. The MCC (membrane compartment of Can1) is an evolutionarily conserved ergosterol-rich plasma membrane domain. It is stabilized on its cytosolic face by the eisosome, a hemitubular protein complex composed of Bin/Amphiphysin/Rvs (BAR) domain-containing Pil1 and Lsp1. These two proteins bind directly to phosphatidylinositol 4,5-bisphosphate and promote the typical furrow-like shape of the microdomain, with highly curved edges and bottom. While some proteins display stable localization in the MCC/eisosome, others enter or leave it under particular conditions, such as misbalance in membrane lipid composition, changes in membrane tension, or availability of specific nutrients. These findings reveal that the MCC/eisosome, a plasma membrane microdomain with distinct morphology and lipid composition, acts as a multifaceted regulator of various cellular processes including metabolic pathways, cellular morphogenesis, signalling cascades, and mRNA decay. In this minireview, we focus on the MCC/eisosome's proposed role in the regulation of lipid metabolism. While the molecular mechanisms of the MCC/eisosome function are not completely understood, the idea of intracellular processes being regulated at the plasma membrane, the foremost barrier exposed to environmental challenges, is truly exciting.

The Coordination Between B Cell Receptor Signaling and the Actin Cytoskeleton During B Cell Activation

B-cell activation plays a crucial part in the immune system and is initiated via interaction between the B cell receptor (BCR) and specific antigens. In recent years with the help of modern imaging techniques, it was found that the cortical actin cytoskeleton changes dramatically during B-cell activation. In this review, we discuss how actin-cytoskeleton reorganization regulates BCR signaling in different stages of B-cell activation, specifically when stimulated by antigens, and also how this reorganization is mediated by BCR signaling molecules. Abnormal BCR signaling is associated with the progression of lymphoma and immunological diseases including autoimmune disorders, and recent studies have proved that impaired actin cytoskeleton can devastate the normal activation of B cells. Therefore, to figure out the coordination between the actin cytoskeleton and BCR signaling may reveal an underlying mechanism of B-cell activation, which has potential for new treatments for B-cell associated diseases.

Phospholipid subcellular localization and dynamics

Membrane biology seeks to understand how lipids and proteins within bilayers assemble into large structures such as organelles and the plasma membranes. Historically, lipids were thought to merely provide structural support for bilayer formation and membrane protein function. Research has now revealed that phospholipid metabolism regulates nearly all cellular processes. Sophisticated techniques helped identify >10,000 lipid species suggesting that lipids support many biological processes. Here, we highlight the synthesis of the most abundant glycerophospholipid classes and their distribution in organelles. We review vesicular and nonvesicular transport pathways shuttling lipids between organelles and discuss lipid regulators of membrane trafficking and second messengers in eukaryotic cells.

SHIP negatively regulates type II immune responses in mast cells and macrophages

SHIP is a hematopoietic-specific lipid phosphatase that dephosphorylates PI3K-generated PI(3,4,5)-trisphosphate. SHIP removes this second messenger from the cell membrane blunting PI3K activity in immune cells. Thus, SHIP negatively regulates mast cell activation downstream of multiple receptors. SHIP has been referred to as the "gatekeeper" of mast cell degranulation as loss of SHIP dramatically increases degranulation or permits degranulation in response to normally inert stimuli. SHIP also negatively regulates Mϕ activation, including both pro-inflammatory cytokine production downstream of pattern recognition receptors, and alternative Mϕ activation by the type II cytokines, IL-4, and IL-13. In the SHIP-deficient (SHIP-/- ) mouse, increased mast cell and Mϕ activation leads to spontaneous inflammatory pathology at mucosal sites, which is characterized by high levels of type II inflammatory cytokines. SHIP-/- mast cells and Mϕs have both been implicated in driving inflammation in the SHIP-/- mouse lung. SHIP-/- Mϕs drive Crohn's disease-like intestinal inflammation and fibrosis, which is dependent on heightened responses to innate immune stimuli generating IL-1, and IL-4 inducing abundant arginase I. Both lung and gut pathology translate to human disease as low SHIP levels and activity have been associated with allergy and with Crohn's disease in people. In this review, we summarize seminal literature and recent advances that provide insight into SHIP's role in mast cells and Mϕs, the contribution of these cell types to pathology in the SHIP-/- mouse, and describe how these findings translate to human disease and potential therapies.

Dual enhancement of T and NK cell function by pulsatile inhibition of SHIP1 improves antitumor immunity and survival

The success of immunotherapy in some cancer patients has revealed the profound capacity for cytotoxic lymphocytes to eradicate malignancies. Various immunotherapies work by blocking key checkpoint proteins that suppress immune cell activity. The phosphatase SHIP1 (SH2-containing inositol polyphosphate 5-phosphatase) limits signaling from receptors that activate natural killer (NK) cells and T cells. However, unexpectedly, genetic ablation studies have shown that the effector functions of SHIP1-deficient NK and T cells are compromised in vivo. Because chronic activation of immune cells renders them less responsive to activating signals (a host mechanism to avoid autoimmunity), we hypothesized that the failure of SHIP1 inhibition to induce antitumor immunity in those studies was caused by the permanence of genetic ablation. Accordingly, we found that reversible and pulsatile inhibition of SHIP1 with 3-α-aminocholestane (3AC; "SHIPi") increased the antitumor response of NK and CD8⁺ T cells in vitro and in vivo. Transient SHIP1 inhibition in mouse models of lymphoma and colon cancer improved the median and long-term tumor-free survival rates. Adoptive transfer assays showed evidence of immunological memory to the tumor in hematolymphoid cells from SHIPi-treated, long-term surviving mice. The findings suggest that a pulsatile regimen of SHIP1 inhibition might be an effective immunotherapy in some cancer patients.

Alzheimer's Disease: From Genetic Variants to the Distinct Pathological Mechanisms

Being the most common cause of dementia, AD is a polygenic and neurodegenerative disease. Complex and multiple factors have been shown to be involved in its pathogenesis, of which the genetics play an indispensable role. It is widely accepted that discovery of potential genes related to the pathogenesis of AD would be of great help for the understanding of neurodegeneration and thus further promote molecular diagnosis in clinic settings. Generally, AD could be clarified into two types according to the onset age, the early-onset AD (EOAD) and the late-onset AD (LOAD). Progresses made by genetic studies on both EOAD and LOAD are believed to be essential not only for the revolution of conventional ideas but also for the revelation of new pathological mechanisms underlying AD pathogenesis. Currently, albeit the genetics of LOAD is much less well-understood compared to EOAD due to its complicated and multifactorial essence, Genome-wide association studies (GWASs) and next generation sequencing (NGS) approaches have identified dozens of novel genes that may provide insight mechanism of LOAD. In this review, we analyze functions of the genes and summarize the distinct pathological mechanisms of how these genes would be involved in the pathogenesis of AD.

Regulation of immune cell signaling by SHIP1: A phosphatase, scaffold protein, and potential therapeutic target

The phosphoinositide phosphatase SHIP is a critical regulator of immune cell activation. Despite considerable study, the mechanisms controlling SHIP activity to ensure balanced cell activation remain incompletely understood. SHIP dampens BCR signaling in part through its association with the inhibitory coreceptor Fc gamma receptor IIB, and serves as an effector for other inhibitory receptors in various immune cell types. The established paradigm emphasizes SHIP's inhibitory receptor-dependent function in regulating phosphoinositide 3-kinase signaling by dephosphorylating the phosphoinositide PI(3,4,5)P₃ ; however, substantial evidence indicates that SHIP can be activated independently of inhibitory receptors and can function as an intrinsic brake on activation signaling. Here, we integrate historical and recent reports addressing the regulation and function of SHIP in immune cells, which together indicate that SHIP acts as a multifunctional protein controlled by multiple regulatory inputs, and influences downstream signaling via both phosphatase-dependent and -independent means. We further summarize accumulated evidence regarding the functions of SHIP in B cells, T cells, NK cells, dendritic cells, mast cells, and macrophages, and data suggesting defective expression or activity of SHIP in autoimmune and malignant disorders. Lastly, we discuss the biological activities, therapeutic promise, and limitations of small molecule modulators of SHIP enzymatic activity.

Trans-inhibition of activation and proliferation signals by Fc receptors in mast cells and basophils

Allergic and autoimmune inflammation are associated with the activation of mast cells and basophils by antibodies against allergens or auto-antigens, respectively. Both cell types express several receptors for the Fc portion of antibodies, the engagement of which by antigen-antibody complexes controls their responses. When aggregated on the plasma membrane, high-affinity immunoglobulin E (IgE) receptors (FcεRI) and low-affinity IgG receptors (FcγRIIIA in mice, FcγRIIA in humans) induce these cells to release and secrete proinflammatory mediators, chemokines, and cytokines that account for clinical symptoms. When coaggregated with activating receptors on the same cells, other low-affinity IgG receptors (FcγRIIB in both species) inhibit mast cell and basophil activation. We found that FcγRIIB inhibited not only signals triggered by activating receptors with which they were coengaged (cis-inhibition), but also signals triggered by receptors engaged independently (trans-inhibition). Trans-inhibition acted upon the FcεRI-dependent activation of mouse mast cells, mouse basophils, and human basophils, and upon growth factor receptor (Kit)-dependent normal mouse mast cell proliferation, as well as the constitutive in vitro proliferation and the in vivo growth of oncogene (v-Abl)-transformed mastocytoma cells. Trans-inhibition was induced by receptors, whether inhibitory (FcγRIIB) or activating (FcεRI), which recruited the lipid phosphatase SHIP1. By hydrolyzing PI(3,4,5)P₃, SHIP1 induced a global unresponsiveness that affected biological responses triggered by receptors that use phosphoinositide 3-kinase to signal. These data suggest that trans-inhibition controls numerous physiological and pathological processes, and that it may be used as a therapeutic tool in inflammation, especially but not exclusively, in allergy and autoimmunity.

The early activation of memory B cells from Wiskott-Aldrich syndrome patients is suppressed by CD19 downregulation

Wiskott-Aldrich syndrome (WAS) pediatric patients exhibit a deficiency in humoral immune memory. However, the mechanism by which Wiskott-Aldrich syndrome protein (WASP) regulates the differentiation and activation of memory B cells remains elusive. Here we examine the early activation events of memory B cells from the peripheral blood mononuclear cells of WAS patients and age-matched healthy controls (HCs) using total internal reflection fluorescence microscopy. In response to stimulation through the B-cell receptor (BCR), memory B cells from HCs showed significantly higher magnitudes of BCR clustering and cell spreading than naive B cells from the same individuals. This was associated with increases in CD19 recruitment to the BCR and the activation of its downstream signaling molecule Btk and decreases in FcγRIIB recruitment and the activation of its downstream molecule Src homology 2-containing inositol 5' phosphatase (SHIP). However, these enhanced signaling activities mediated by CD19 and Btk are blocked in memory B cells from WAS patients, whereas the activation of FcγRIIB and SHIP was increased. Although the expression levels of CD19, Btk, and FcγRIIB did not change between CD27(-) and CD27(+) B cells of HCs, the protein and mRNA levels of CD19 but not Btk and FcγRIIB were significantly reduced in both CD27(-) and CD27(+) B cells of WAS patients, compared with those of HCs. Overall, our study suggests that WASP is required for memory B-cell activation, promoting the activation by positive regulating CD19 transcription and CD19 recruitment to the BCR.

Protein tyrosine phosphatase 1B (PTP1B) is dispensable for IgE-mediated cutaneous reaction in vivo

Mast cells play a critical role in allergic reactions. The cross-linking of FcεRI-bound IgE with multivalent antigen initiates a cascade of signaling events leading to mast cell activation. It has been well-recognized that cross linking of FcεRI mediates tyrosine phosphorylation. However, the mechanism involved in tyrosine dephosphorylation in mast cells is less clear. Here we demonstrated that protein tyrosine phosphatase 1B (PTP1B)-deficient mast cells showed increased IgE-mediated phosphorylation of the signal transducer and activator of transcription 5 (STAT5) and enhanced production of CCL9 (MIP-1γ) and IL-6 in IgE-mediated mast cells activation in vitro. However, IgE-mediated calcium mobilization, β-hexaosaminidase release (degranulation), and phosphorylation of IκB and MAP kinases were not affected by PTP1B deficiency. Furthermore, PTP1B deficient mice showed normal IgE-dependent passive cutaneous anaphylaxis and late phase cutaneous reactions in vivo. Thus, PTP1B specifically regulates IgE-mediated STAT5 pathway, but is redundant in influencing mast cell function in vivo.

Phosphatidylinositol-3,4,5-triphosphate and cellular signaling: implications for obesity and diabetes

Phosphatidylinositol-3,4,5-triphosphate (PtdIns(3,4,5)P₃) is one of the most important phosphoinositides and is capable of activating a wide range of proteins through its interaction with their specific binding domains. Localization and activation of these effector proteins regulate a number of cellular functions, including cell survival, proliferation, cytoskeletal rearrangement, intracellular vesicle trafficking, and cell metabolism. Phosphoinositides have been investigated as an important agonist-dependent second messenger in the regulation of diverse physiological events depending upon the phosphorylation status of their inositol group. Dysregulation in formation as well as metabolism of phosphoinositides is associated with various pathophysiological disorders such as inflammation, allergy, cardiovascular diseases, cancer, and metabolic diseases. Recent studies have demonstrated that the impaired metabolism of PtdIns(3,4,5)P₃ is a prime mediator of insulin resistance associated with various metabolic diseases including obesity and diabetes. This review examines the current status of the role of PtdIns(3,4,5)P₃ signaling in the regulation of various cellular functions and the implications of dysregulated PtdIns(3,4,5)P₃ signaling in obesity, diabetes, and their associated complications.

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.

Molecular dynamic simulation to explore the molecular basis of Btk-PH domain interaction with Ins(1,3,4,5)P4

Bruton's tyrosine kinase contains a pleckstrin homology domain, and it specifically binds inositol 1,3,4,5-tetrakisphosphate (Ins(1,3,4,5)P4), which is involved in the maturation of B cells. In this paper, we studied 12 systems including the wild type and 11 mutants, K12R, S14F, K19E, R28C/H, E41K, L11P, F25S, Y40N, and K12R-R28C/H, to investigate any change in the ligand binding site of each mutant. Molecular dynamics simulations combined with the method of molecular mechanics/Poisson-Boltzmann solvent-accessible surface area have been applied to the twelve systems, and reasonable mutant structures and their binding free energies have been obtained as criteria in the final classification. As a result, five structures, K12R, K19E, R28C/H, and E41K mutants, were classified as “functional mutations,” whereas L11P, S14F, F25S, and Y40N were grouped into “folding mutations.” This rigorous study of the binding affinity of each of the mutants and their classification provides some new insights into the biological function of the Btk-PH domain and related mutation-causing diseases.

The Myc-miR-17-92 axis amplifies B-cell receptor signaling via inhibition of ITIM proteins: a novel lymphomagenic feed-forward loop

The c-Myc oncoprotein regulates >15% of the human transcriptome and a limited number of microRNAs (miRNAs). Here, we establish that in a human B-lymphoid cell line, Myc-repressed, but not Myc-stimulated, genes are significantly enriched for predicted binding sites of Myc-regulated miRNAs, primarily those comprising the Myc-activated miR-17~92 cluster. Notably, gene set enrichment analysis demonstrates that miR-17∼92 is a major regulator of B-cell receptor (BCR) pathway components. Many of them are immunoreceptor tyrosine inhibitory motif (ITIM)-containing proteins, and ITIM proteins CD22 and FCGR2B were found to be direct targets of miR-17∼92. Consistent with the propensity of ITIM proteins to recruit phosphatases, either MYC or miR-17~92 expression was necessary to sustain phosphorylation of spleen tyrosine kinase (SYK) and the B-cell linker protein (BLNK) upon ligation of the BCR. Further downstream, stimulation of the BCR response by miR-17-92 resulted in the enhanced calcium flux and elevated levels of Myc itself. Notably, inhibition of the miR-17~92 cluster in diffuse large B-cell lymphoma (DLBCL) cell lines diminished the BCR response as measured by SYK and BLNK phosphorylation. Conversely, human DLBCLs of the BCR subtype express higher Myc and mir17hg transcript levels than other subtypes. Hence, the Myc-miR-17-92-BCR axis, frequently affected by genomic rearrangements, constitutes a novel lymphomagenic feed-forward loop.

Role of inositol poly-phosphatases and their targets in T cell biology

T lymphocytes play a critical role in host defense in all anatomical sites including mucosal surfaces. This not only includes the effector arm of the immune system, but also regulation of immune responses in order to prevent autoimmunity. Genetic targeting of PI3K isoforms suggests that generation of PI(3,4,5)P3 by PI3K plays a critical role in promoting effector T cell responses. Consequently, the 5'- and 3'-inositol poly-phosphatases SHIP1, SHIP2, and phosphatase and tensin homolog capable of targeting PI(3,4,5)P3 are potential genetic determinants of T cell effector functions in vivo. In addition, the 5'-inositol poly-phosphatases SHIP1 and 2 can shunt PI(3,4,5)P3 to the rare but potent signaling phosphoinositide species PI(3,4)P2 and thus these SHIP1/2, and the INPP4A/B enzymes that deplete PI(3,4)P2 may have precise roles in T cell biology to amplify or inhibit effectors of PI3K signaling that are selectively recruited to and activated by PI(3,4)P2. Here we summarize recent genetic and chemical evidence that indicates the inositol poly-phosphatases have important roles in both the effector and regulatory functions of the T cell compartment. In addition, we will discuss future genetic studies that might be undertaken to further elaborate the role of these enzymes in T cell biology as well as potential pharmaceutical manipulation of these enzymes for therapeutic purposes in disease settings where T cell function is a key in vivo target.

Phospholipase C-β in immune cells

Great progress has recently been made in structural and functional research of phospholipase C (PLC)-β. We now understand how PLC-β isoforms (β1-β4) are activated by GTP-bound Gαq downstream of G protein-coupled receptors. Numerous studies indicate that PLC-βs participate in the differentiation and activation of immune cells that control both the innate and adaptive immune systems. The PLC-β3 isoform also interplays with tyrosine kinase-based signaling pathways, to inhibit Stat5 activation by recruiting the protein-tyrosine phosphatase SHP-1, with which PLC-β3 and Stat5 form a multi-molecular signaling platform, named SPS complex. The SPS complex has important regulatory roles in tumorigenesis and immune cell activation.

PKIS: computational identification of protein kinases for experimentally discovered protein phosphorylation sites

Background

Dynamic protein phosphorylation is an essential regulatory mechanism in various organisms. In this capacity, it is involved in a multitude of signal transduction pathways. Kinase-specific phosphorylation data lay the foundation for reconstruction of signal transduction networks. For this reason, precise annotation of phosphorylated proteins is the first step toward simulating cell signaling pathways. However, the vast majority of kinase-specific phosphorylation data remain undiscovered and existing experimental methods and computational phosphorylation site (P-site) prediction tools have various limitations with respect to addressing this problem.

Results

To address this issue, a novel protein kinase identification web server, PKIS, is here presented for the identification of the protein kinases responsible for experimentally verified P-sites at high specificity, which incorporates the composition of monomer spectrum (CMS) encoding strategy and support vector machines (SVMs). Compared to widely used P-site prediction tools including KinasePhos 2.0, Musite, and GPS2.1, PKIS largely outperformed these tools in identifying protein kinases associated with known P-sites. In addition, PKIS was used on all the P-sites in Phospho.ELM that currently lack kinase information. It successfully identified 14 potential SYK substrates with 36 known P-sites. Further literature search showed that 5 of them were indeed phosphorylated by SYK. Finally, an enrichment analysis was performed and 6 significant SYK-related signal pathways were identified.

Conclusions

In general, PKIS can identify protein kinases for experimental phosphorylation sites efficiently. It is a valuable bioinformatics tool suitable for the study of protein phosphorylation. The PKIS web server is freely available at http://bioinformatics.ustc.edu.cn/pkis.

Phosphoinositides: tiny lipids with giant impact on cell regulation

Phosphoinositides (PIs) make up only a small fraction of cellular phospholipids, yet they control almost all aspects of a cell's life and death. These lipids gained tremendous research interest as plasma membrane signaling molecules when discovered in the 1970s and 1980s. Research in the last 15 years has added a wide range of biological processes regulated by PIs, turning these lipids into one of the most universal signaling entities in eukaryotic cells. PIs control organelle biology by regulating vesicular trafficking, but they also modulate lipid distribution and metabolism via their close relationship with lipid transfer proteins. PIs regulate ion channels, pumps, and transporters and control both endocytic and exocytic processes. The nuclear phosphoinositides have grown from being an epiphenomenon to a research area of its own. As expected from such pleiotropic regulators, derangements of phosphoinositide metabolism are responsible for a number of human diseases ranging from rare genetic disorders to the most common ones such as cancer, obesity, and diabetes. Moreover, it is increasingly evident that a number of infectious agents hijack the PI regulatory systems of host cells for their intracellular movements, replication, and assembly. As a result, PI converting enzymes began to be noticed by pharmaceutical companies as potential therapeutic targets. This review is an attempt to give an overview of this enormous research field focusing on major developments in diverse areas of basic science linked to cellular physiology and disease.

New insights into the regulation of neutrophil NADPH oxidase activity in the phagosome: a focus on the role of lipid and Ca(2+) signaling

SIGNIFICANCE

Reactive oxygen species, produced by the phagosomal NADPH oxidase of neutrophils, play a significant physiological role during normal defense. Their role is not only to kill invading pathogens, but also to act as modulators of global physiological functions of phagosomes. Given the importance of NADPH oxidase in the immune system, its activity has to be decisively controlled by distinctive mechanisms to ensure appropriate regulation at the phagosome.

RECENT ADVANCES

Here, we describe the signal transduction pathways that regulate phagosomal NADPH oxidase in neutrophils, with an emphasis on the role of lipid metabolism and intracellular Ca(2+) mobilization.

CRITICAL ISSUES

The potential involvement of Ca(2+)-binding S100A8 and S100A9 proteins, known to interact with the plasma membrane NADPH oxidase, is also considered.

FUTURE DIRECTIONS

Recent technical progress in advanced live imaging microscopy will permit to focus more accurately on phagosomal rather than plasma membrane NADPH oxidase regulation during neutrophil phagocytosis.

Aberrant antibody affinity selection in SHIP-deficient B cells

The strength of the Ag receptor signal influences development and negative selection of B cells, and it might also affect B-cell survival and selection in the GC. Here, we have used mice with B-cell-specific deletion of the 5'-inositol phosphatase SHIP as a model to study affinity selection in cells that are hyperresponsive to Ag and cytokine receptor stimulation. In the absence of SHIP, B cells have lower thresholds for Ag- and interferon (IFN)-induced activation, resulting in augmented negative selection in the BM and enhanced B-cell maturation in the periphery. Despite a tendency to spontaneously downregulate surface IgM expression, SHIP deficiency does not alter anergy induction in response to soluble hen-egg lysozyme Ag in the MDA4 transgenic model. SHIP-deficient B cells spontaneously produce isotype-switched antibodies; however, they are poor responders in immunization and infection models. While SHIP-deficient B cells form GCs and undergo mutation, they are not properly selected for high-affinity antibodies. These results illustrate the importance of negative regulation of B-cell responses, as lower thresholds for B-cell activation promote survival of low affinity and deleterious receptors to the detriment of optimal Ab affinity maturation.

PI3K and STAT3: a new alliance

Recent proteomic data have uncovered an interdependence of PI3K and STAT3. In PI3K-tranformed murine cells, STAT3 is phosphorylated on Y705 and activated in a PI3K-dependent manner. Dominant negative STAT3 interferes with PI3K-induced oncogenic transformation. Phosphorylation of STAT3 in PI3K-transformed murine cells is mediated by the TEC kinase BMX. Observations on glioblastoma stem cells reveal similar critical roles for STAT3 and BMX. The new data document an important role of STAT3 in PI3K-driven oncogenic transformation and mark BMX as a promising therapeutic target that could enhance the effectiveness of PI3K inhibitors.

SIGNIFICANCE

The PI3K–TOR and STAT3 signaling pathways represent two distinct regulatory networks. The discovery of a functional link between these pathways is significant for our understanding of PI3K- and STAT3-driven oncogenic mechanisms and identifies the TEC kinase BMX as a new cancer target.

GPCR activation of Ras and PI3Kc in neutrophils depends on PLCb2/b3 and the RasGEF RasGRP4

The molecular mechanisms by which receptors regulate the Ras Binding Domains of the PIP3-generating, class I PI3Ks remain poorly understood, despite their importance in a range of biological settings, including tumorigenesis, activation of neutrophils by pro-inflammatory mediators, chemotaxis of Dictyostelium and cell growth in Drosophila. We provide evidence that G protein-coupled receptors (GPCRs) can stimulate PLCb2/b3 and diacylglycerol- dependent activation of the RasGEF, RasGRP4 in neutrophils. The genetic loss of RasGRP4 phenocopies knock-in of a Ras-insensitive version of PI3Kc in its effects on PI3Kc-dependent PIP3 accumulation, PKB activation, chemokinesis and reactive oxygen species (ROS) formation. These results establish a new mechanism by which GPCRs can stimulate Ras, and the broadly important principle that PLCs can control activation of class I PI3Ks.

The B-cell receptor signaling pathway as a therapeutic target in CLL

Targeted therapy with imatinib and other selective tyrosine kinase inhibitors has transformed the treatment of chronic myeloid leukemia. Unlike chronic myeloid leukemia, chronic lymphocytic leukemia (CLL) lacks a common genetic aberration amenable to therapeutic targeting. However, our understanding of normal B-cell versus CLL biology points to differences in properties of B-cell receptor (BCR) signaling that may be amenable to selective therapeutic targeting. The application of mouse models has further expanded this understanding and provides information about targets in the BCR signaling pathway that may have other important functions in cell development or long-term health. In addition, overexpression or knockout of selected targets offers the potential to validate targets genetically using new mouse models of CLL. The initial success of BCR-targeted therapies has promoted much excitement in the field of CLL. At the present time, GS-1101, which reversibly inhibits PI3Kδ, and ibrutinib (PCI-32765), an irreversible inhibitor of Bruton tyrosine kinase, have generated the most promising early results in clinical trials including predominately refractory CLL where durable disease control has been observed. This review provides a summary of BCR signaling, tools for studying this pathway relevant to drug development in CLL, and early progress made with therapeutics targeting BCR-related kinases.

The role of the tec kinase Bruton's tyrosine kinase (Btk) in leukocyte recruitment

Recruitment of leukocytes into inflamed tissue is a key component of the immune system. The activation of integrins on leukocytes is required for their recruitment into the inflamed tissue. Btk is a cytoplasmic nonreceptor tyrosine kinase belonging to the Tec-kinase family. It plays a key role in B-cell development and function, and recently published studies revealed important roles of Btk in myeloid cells. Btk might be activated through a variety of receptors leading to activation of integrins as the pivotal element in leukocyte recruitment. This review focuses on the role of Btk in B-lymphocyte homing and in neutrophil recruitment.

An involvement of neurokinin-1 receptor in FcεRΙ-mediated RBL-2H3 mast cell activation

OBJECTIVE AND DESIGN

To determine whether the neurokinin-1 receptor (NK1R) plays a role in the activation of RBL-2H3 mast cells after FcεRΙ aggregation.

MATERIALS AND METHODS

NK1R expression in RBL-2H3 cells was inhibited by small hairpin RNA (shRNA) against NK1R, and determined by western blotting. For activation, both NK1R knockdown and control RBL-2H3 cells were sensitized by dinitrophenol (DNP)-specific IgE and stimulated with the antigen DNP-bovine serum albumin (BSA). Following the activation of RBL-2H3 cells, monocyte chemoattractant protein (MCP-1) production and intracellular calcium flux were monitored by ELISA and confocal microscopy assay, respectively. For investigation of the signaling mechanism, phosphorylation of mitogen-activated protein kinases (MAPKs) after RBL-2H3 cell activation was assessed by western blotting.

RESULTS

shRNA-NK1R mediated an effective inhibition of NK1R expression in RBL-2H3 cells. Protein production of MCP-1 was reduced by more than 55 % in NK1R knockdown RBL-2H3 cells compared with control RBL-2H3 cells. In addition, both calcium mobilization and phosphorylation levels of MAPKs (Erk1/2, JNK, and p38) after DNP-BSA stimulation (via FcεRΙ) were decreased due to the inhibition of NK1R expression.

CONCLUSION

NK1R is required for the activation of RBL-2H3 cells following FcεRΙ engagement and involved in the regulation of MAPK signaling pathways.

Shp2 activates Fyn and Ras to regulate RBL-2H3 mast cell activation following FcεRI aggregation

The protein-tyrosine phosphatase (PTP) Shp2 has been implicated in many immunoreceptor signaling pathways, but its role in immunoreceptor FcεRI signaling, which leads to the activation of mast cells and blood basophils, is still largely undefined. Using Shp2 knockdown RBL-2H3 (RBL) mast cells, we here reported that Shp2 is required for the activation of RBL cells induced by FcεRI. FcεRΙ-evoked degranulation, calcium mobilization, and synthesis of cytokine transcripts (IL-1β, IL-10, and monocyte chemoattractant protein 1 (MCP-1)) were reduced in Shp2 knockdown RBL cells. Signaling regulatory mechanism investigation using immunoblotting, immunoprecipitation, and GST pull-down assay reveals that the down-regulation of Shp2 expression in RBL cells leads to decreased activities of Fyn, PLCγ, JNK, p38MAPK, and Ras/Erk1/2 after FcεRΙ aggregation. Further studies suggest that Paxillin phosphoryaltion was also impaired, but PAG phosphorylation was normal after FcεRΙ stimulation as a consequence of the inhibition of Shp2 expression in RBL cells. Collectively, our data strongly indicate that Shp2 is essential for the activation of RBL cells in response to FcεRΙ aggregation. Shp2 regulates this process through Fyn and Ras with no involvement of PAG. In addition, we identify Paxillin as an indirect substrate of Shp2 in FcεRΙ-initiated signaling of RBL cells.

The B-cell receptor signaling pathway as a therapeutic target in CLL

Targeted therapy with imatinib and other selective tyrosine kinase inhibitors has transformed the treatment of chronic myeloid leukemia. Unlike chronic myeloid leukemia, chronic lymphocytic leukemia (CLL) lacks a common genetic aberration amenable to therapeutic targeting. However, our understanding of normal B-cell versus CLL biology points to differences in properties of B-cell receptor (BCR) signaling that may be amenable to selective therapeutic targeting. The application of mouse models has further expanded this understanding and provides information about targets in the BCR signaling pathway that may have other important functions in cell development or long-term health. In addition, overexpression or knockout of selected targets offers the potential to validate targets genetically using new mouse models of CLL. The initial success of BCR-targeted therapies has promoted much excitement in the field of CLL. At the present time, GS-1101, which reversibly inhibits PI3Kδ, and ibrutinib (PCI-32765), an irreversible inhibitor of Bruton tyrosine kinase, have generated the most promising early results in clinical trials including predominately refractory CLL where durable disease control has been observed. This review provides a summary of BCR signaling, tools for studying this pathway relevant to drug development in CLL, and early progress made with therapeutics targeting BCR-related kinases.

Transmembrane adaptor proteins in the high-affinity IgE receptor signaling

Aggregation of the high-affinity IgE receptor (FcεRI) initiates a cascade of signaling events leading to release of preformed inflammatory and allergy mediators and de novo synthesis and secretion of cytokines and other compounds. The first biochemically well defined step of this signaling cascade is tyrosine phosphorylation of the FcεRI subunits by Src family kinase Lyn, followed by recruitment and activation of spleen tyrosine kinase (Syk). Activity of Syk is decisive for the formation of multicomponent signaling assemblies, the signalosomes, in the vicinity of the receptors. Formation of the signalosomes is dependent on the presence of transmembrane adaptor proteins (TRAPs). These proteins are characterized by a short extracellular domain, a single transmembrane domain, and a cytoplasmic tail with various motifs serving as anchors for cytoplasmic signaling molecules. In mast cells five TRAPs have been identified [linker for activation of T cells (LAT), non-T cell activation linker (NTAL), linker for activation of X cells (LAX), phosphoprotein associated with glycosphingolipid-enriched membrane microdomains (PAG), and growth factor receptor-bound protein 2 (Grb2)-binding adaptor protein, transmembrane (GAPT)]; engagement of four of them (LAT, NTAL, LAX, and PAG) in FcεRI signaling has been documented. Here we discuss recent progress in the understanding of how TRAPs affect FcεRI-mediated mast cell signaling. The combined data indicate that individual TRAPs have irreplaceable roles in important signaling events such as calcium response, degranulation, cytokines production, and chemotaxis.

PI3Kδ inhibitors in cancer: rationale and serendipity merge in the clinic

Several phosphoinositide 3-kinase (PI3K) inhibitors are in the clinic and many more are in preclinical development. CAL-101, a selective inhibitor of the PI3Kδ isoform, has shown remarkable success in certain hematologic malignancies. Although PI3Kδ signaling plays a central role in lymphocyte biology, the degree of single-agent therapeutic activity of CAL-101 during early-phase development has been somewhat unexpected. CAL-101 works in part by blocking signals from the microenvironment that normally sustain leukemia and lymphoma cells in a protective niche. As PI3Ks enter the arena of molecular-targeted therapies, CAL-101 provides proof of principle that isoform-selective compounds can be effective in selected cancer types and patient populations.

SIGNIFICANCE

A key question is whether compounds targeting a single PI3K catalytic isoform can provide meaningful single agent efficacy in cancer cells that express multiple isoforms. Clinical studies of the drug CAL-101 have provided a significant advance by showing that selective targeting of PI3Kδ achieves efficacy in chronic lymphocytic leukemia, in part through targeting the tumor microenvironment.

Inhibitor and activator: dual functions for SHIP in immunity and cancer

SHIP1 is at the nexus of intracellular signaling pathways in immune cells that mediate bone marrow (BM) graft rejection, production of inflammatory and immunosuppressive cytokines, immunoregulatory cell formation, the BM niche that supports development of the immune system, and immune cancers. This review summarizes how SHIP participates in normal immune physiology or the pathologies that result when SHIP is mutated. This review also proposes that SHIP can have either inhibitory or activating roles in cell signaling that are determined by whether signaling pathways distal to PI3K are promoted by SHIP's substrate (PI(3,4,5)P(3) ) or its product (PI(3,4)P(2) ). This review also proposes the "two PIP hypothesis" that postulates that both SHIP's product and its substrate are necessary for a cancer cell to achieve and sustain a malignant state. Finally, due to the recent discovery of small molecule antagonists and agonists for SHIP, this review discusses potential therapeutic settings where chemical modulation of SHIP might be of benefit.

Concerted action of Helios and Ikaros controls the expression of the inositol 5-phosphatase SHIP

Ikaros family transcription factors have a key role in lymphoid development, and their aberrant function contributes to a multitude of lymphoid malignancies. Ikaros and Helios bind to similar DNA sequences, and Helios associates with Ikaros-containing chromatin remodeling complexes. Previously, we have shown that loss of Ikaros leads to diminished BCR-signaling strength. In this study, we describe a Helios-deficient chicken DT40 B-cell line with a BCR signaling phenotype that is the opposite to that of Ikaros-deficient cells. In contrast to Ikaros-deficient cells, Helios(-/-) B cells exhibit increased calcium release to the cytoplasm after BCR crosslinking, but diminished BCR-induced phosphorylation of signaling molecules. The inositol 5-phosphatase SHIP, an important regulator in several signaling pathways, is differentially expressed in Ikaros- and Helios-deficient cells. In the absence of Ikaros, SHIP is upregulated, whereas Helios deficiency leads to the downregulation of SHIP expression. We also show with ChIP that Ikaros binds to the promoter of the INPP5D gene-encoding SHIP. Considering the critical role of SHIP in the BCR signaling pathway, our findings provide insight into the mechanism of how both Helios and Ikaros are involved in the regulation of BCR signaling.

The B-lymphoid Grb2 interaction code

The growth factor receptor-bound protein 2 (Grb2) is a ubiquitously expressed and evolutionary conserved adapter protein possessing a plethora of described interaction partners for the regulation of signal transduction. In B lymphocytes, the Grb2-mediated scaffolding function controls the assembly and subcellular targeting of activating as well as inhibitory signalosomes in response to ligation of the antigen receptor. Also, integration of simultaneous signals from B-cell coreceptors that amplify or attenuate antigen receptor signal output relies on Grb2. Hence, Grb2 is an essential signal integrator. The key question remains, however, of how pathway specificity can be maintained during signal homeostasis critically required for the balance between immune cell activation and tolerance induction. Here, we summarize the molecular network of Grb2 in B cells and introduce a proteomic approach to elucidate the interactome of Grb2 in vivo.

Phosphoinositide 3-kinase-regulated adapters in lymphocyte activation

Signaling via phosphoinositide 3-kinases (PI3Ks) has emerged as a central component of lymphocyte activation via immunoreceptors, costimulatory receptors, cytokine receptors, and chemokine receptors. The discovery of phosphoinositide-binding pleckstrin homology (PH) domains has substantially increased understanding of how PI3Ks activate cellular responses. Accumulating evidence indicates that PH-domain containing adapter molecules provide important links between PI3K and lymphocyte function. Here, we review data on PI3K-regulated adapter proteins of the Grb-associated binder (GAB), Src kinase-associated phosphoprotein (SKAP), and B-lymphocyte adapter molecule of 32 kDa (Bam32)/ dual-adapter for phosphotyrosine and 3-phosphoinositides (DAPP)/TAPP families, with a focus on the latter group. Current data support the model that recruitment of these adapters to the plasma membrane of activated lymphocytes is driven by the phosphoinositides phosphatidylinositol-3,4,5-tris-phosphate and phosphatidylinositol-3,4-bisphosphate, generated through the action of PI3Ks and under the regulatory control of lipid phosphatases Src homology 2 domain-containing inositol phosphatase (SHIP), phosphatase and tensin homolog, and inositol polyphosphate 4-phosphatase. At the plasma membrane, these adapters serve to assemble distinct protein complexes. Bam32/DAPP1 and SKAPs function to promote activation of monomeric guanosine triphosphatases, including Rac and Rap, and promote integrin activation, lymphocyte adhesion to matrix proteins, and cell:cell interactions between B and T lymphocytes. GABs can provide feedforward amplification or feedback inhibition of PI3K signaling. Current work is further defining the molecular interactions driven by these molecules and identifying the functions of TAPP adapters, which also appear to be involved in lymphocyte adhesion and are specific effectors downstream of the SHIP product phosphatidylinositol-3,4-bisphosphate.

FcepsilonRI-induced activation by low antigen concentrations results in nuclear signals in the absence of degranulation

High affinity IgE receptor (FcvarepsilonRI)-induced activation of mast cells results in degranulation and generation of leukotrienes and cytokines. FcvarepsilonRI-induced mast cell activation was analyzed at a single cell basis using a rat basophilic leukemia (RBL-2H3) cell line transfected with a reporter plasmid containing three tandem NFAT (nuclear factor of activated T cells) binding sites fused to enhanced green fluorescent protein (GFP). Surprisingly, with this sensitive detection system, there is activation of IgE sensitized cells at concentrations of antigen as low as 10pg/ml, which was 10-fold lower than was detected by degranulation. There were differences in signaling pathways leading to degranulation compared to NFAT-mediated gene activation. Both signaling to NFAT activation and degranulation required Syk and calcineurin. However inhibitors of the phosphatidylinositol 3-kinase pathway blocked degranulation but did not NFAT activation. The results also indicate that NFAT was activated at lower intracellular signals compared to degranulation. Therefore, FcvarepsilonRI activation can result in nuclear signals in the absence of the release of mediators.

Differential roles for the inositol phosphatase SHIP in the regulation of macrophages and lymphocytes

The SH2 domain-containing inositol 5'-phosphatase (SHIP) negatively regulates antigen, cytokine, and Fc receptor signaling pathways in immune cells. Our knowledge of the function of SHIP largely derives from in vitro studies that utilized SHIP-deficient cell lines and immune cells isolated from SHIP null mice. To avoid the pleiotropic effects observed in mice with germline deletion of SHIP, we have used the Cre-lox system to generate SHIP conditional knockout mice with deletion in specific immune cell populations. In this review we summarize our observations from mice with deletion of SHIP in lymphocyte and macrophage lineages and contrast them with earlier data gathered by the analysis of SHIP null mice.

Fine tuning the immune response with PI3K

The phosphoinositide 3-kinase (PI3K) family of lipid kinases regulates diverse aspects of lymphocyte behavior. This review discusses how genetic and pharmacological tools have yielded an increasingly detailed understanding of how PI3K enzymes function at different stages of lymphocyte development and activation. Following antigen receptor engagement, activated PI3K generates 3-phosphorylated inositol lipid products that serve as membrane targeting signals for numerous proteins involved in the assembly of multiprotein complexes, termed signalosomes, and immune synapse formation. In B cells, class IA PI3K is the dominant subgroup whose loss causes profound defects in development and antigen responsiveness. In T cells, both class IA and IB PI3K contribute to development and immune function. PI3K also regulates both chemokine responsiveness and antigen-driven changes in lymphocyte trafficking. PI3K modulates the function not only of effector T cells, but also regulatory T cells; these disparate functions culminate in unexpected autoimmune phenotypes in mice with PI3K-deficient T cells. Thus, PI3K signaling is not a simple switch to promote cellular activation, but rather an intricate web of interactions that must be properly balanced to ensure appropriate cellular responses and maintain immune homeostasis. Defining these complexities remains a challenge for pharmaceutical development of PI3K inhibitors to combat inflammation and autoimmunity.

Conformational snapshots of Tec kinases during signaling

The control of cellular signaling cascades is of utmost importance in regulating the immune response. Exquisitely precise protein-protein interactions and chemical modification of substrates by enzymatic catalysis are the fundamental components of the signals that alert immune cells to the presence of a foreign antigen. In particular, the phosphorylation events induced by protein kinase activity must be spatially and temporally regulated by specific interactions to maintain a normal and effective immune response. High resolution structures of many protein kinases along with supporting biochemical data are providing significant insight into the intricate regulatory mechanisms responsible for controlling cellular signaling. The Tec family kinases are immunologically important kinases for which regulatory details are beginning to emerge. This review focuses on bringing together structural insights gained over the years to develop an understanding of how domain interactions both within the Tec kinases and between the Tec kinases and other signaling molecules control immune cell function.

The role of the inositol polyphosphate 5-phosphatases in cellular function and human disease

Phosphoinositides are membrane-bound signalling molecules that regulate cell proliferation and survival, cytoskeletal reorganization and vesicular trafficking by recruiting effector proteins to cellular membranes. Growth factor or insulin stimulation induces a canonical cascade resulting in the transient phosphorylation of PtdIns(4,5)P(2) by PI3K (phosphoinositide 3-kinase) to form PtdIns(3,4,5)P(3), which is rapidly dephosphorylated either by PTEN (phosphatase and tensin homologue deleted on chromosome 10) back to PtdIns(4,5)P(2), or by the 5-ptases (inositol polyphosphate 5-phosphatases), generating PtdIns(3,4)P(2). The 5-ptases also hydrolyse PtdIns(4,5)P(2), forming PtdIns4P. Ten mammalian 5-ptases have been identified, which share a catalytic mechanism similar to that of the apurinic/apyrimidinic endonucleases. Gene-targeted deletion of 5-ptases in mice has revealed that these enzymes regulate haemopoietic cell proliferation, synaptic vesicle recycling, insulin signalling, endocytosis, vesicular trafficking and actin polymerization. Several studies have revealed that the molecular basis of Lowe's syndrome is due to mutations in the 5-ptase OCRL (oculocerebrorenal syndrome of Lowe). Futhermore, the 5-ptases SHIP [SH2 (Src homology 2)-domain-containing inositol phosphatase] 2, SKIP (skeletal muscle- and kidney-enriched inositol phosphatase) and 72-5ptase (72 kDa 5-ptase)/Type IV/Inpp5e (inositol polyphosphate 5-phosphatase E) are implicated in negatively regulating insulin signalling and glucose homoeostasis in specific tissues. SHIP2 polymorphisms are associated with a predisposition to insulin resistance. Gene profiling studies have identified changes in the expression of various 5-ptases in specific cancers. In addition, 5-ptases such as SHIP1, SHIP2 and 72-5ptase/Type IV/Inpp5e regulate macrophage phagocytosis, and SHIP1 also controls haemopoietic cell proliferation. Therefore the 5-ptases are a significant family of signal-modulating enzymes that govern a plethora of cellular functions by regulating the levels of specific phosphoinositides. Emerging studies have implicated their loss or gain of function in human disease.

Survival of monocytes and macrophages and their role in health and disease

Macrophages are versatile cells involved in health and disease. These cells act as scavengers to rid the body of apoptotic and senescent cells and debris through their phagocytic function. Although this is a primary function of these cells, macrophages play vital roles in inflammation and repair of damaged tissue. Macrophages secrete a large number of cytokines, chemokines and growth factors that recruit and activate a variety of cell types to inflamed tissue compartments. These cells are also critical in cell-mediated immunity and in the resolution of inflammation. Since macrophages, and their precursors, blood monocytes, are important in regulating and resolving inflammation, prolonged cellular survival in tissue compartments could be detrimental. Thus, factors that regulate the fate of monocyte and macrophage survival are important in cellular homeostasis. In this article, we will explore stimuli and the intracellular pathways important in regulating macrophage survival and implication in human disease.

Negative signaling by inhibitory receptors: the NK cell paradigm

Receptors carrying immunoreceptor tyrosine-based inhibition motifs (ITIMs) in their cytoplasmic tail control a vast array of cellular responses, ranging from autoimmunity, allergy, phagocytosis of red blood cells, graft versus host disease, to even neuronal plasticity in the brain. The inhibitory function of many receptors has been deduced on the basis of cytoplasmic ITIM sequences. Tight regulation of natural killer (NK) cell cytotoxicity and cytokine production by inhibitory receptors specific for major histocompatibility complex class I molecules has served as a model system to study the negative signaling pathway triggered by an ITIM-containing receptor in the physiological context of NK-target cell interactions. Advances in our understanding of the molecular details of inhibitory signaling in NK cells have provided a conceptual framework to address how ITIM-mediated regulation controls cellular reactivity in diverse cell types.

Btk regulates localization, in vivo activation, and class switching of anti-DNA B cells

The autoimmune disease systemic lupus erythematosus (SLE) is characterized by loss of tolerance to nuclear antigens such as chromatin, DNA, and RNA. This focused autoreactivity is thought to arise from the ability of DNA or RNA specific B cells to receive dual signals from the BCR and TLR9 or TLR7, respectively. The Tec kinase Btk is necessary for the production of anti-DNA antibodies in several murine models of SLE. To assess the role of Btk in the fate of DNA reactive B cells, we generated Btk-/- mice carrying the 56R anti-DNA Ig transgene on the C57BL/6 background. dsDNA specific B cells were present in 56R.Btk-/- mice, although they were not preferentially localized to the marginal zone. These cells were able to proliferate in response to large CpG DNA containing fragments that require BCR-induced internalization to access TLR9. However, anti-DNA antibodies were not observed in the serum of 56R.Btk-/- mice. A transgene expressing a low level of Btk in B cells (Btk(lo)) restored anti-DNA IgM in these mice. This correlated with partial rescue of proliferative response to BCR engagement and TLR9-induced IL-10 secretion in Btk(lo) B cells. anti-DNA IgG was not observed in 56R.Btk(lo) mice, however. This was likely due, at least in part, to a role for Btk in controlling the expression of T-bet and AID in cells stimulated with CpG DNA. Thus, Btk is required for the initial loss of tolerance to DNA and the subsequent production of pathogenic autoantibodies once tolerance is breached.

FcgammaRIIB signals inhibit BLyS signaling and BCR-mediated BLyS receptor up-regulation

These studies investigate how interactions between the BCR and FcgammaRIIB affect B lymphocyte stimulator (BLyS) recep-tor expression and signaling. Previous studies showed that BCR ligation up-regulates BLyS binding capacity in mature B cells, reflecting increased BLyS receptor levels. Here we show that FcgammaRIIB coaggregation dampens BCR-induced BLyS receptor up-regulation. This cross-regulation requires BCR and FcgammaRIIB coligation, and optimal action relies on the Src-homology-2 (SH2)-containing inositol 5 phosphase-1 (SHIP1). Subsequent to FcgammaRIIB/BCR coaggregation, the survival promoting actions of BLyS are attenuated, reflecting reduced BLyS receptor signaling capacity in terms of Pim 2 maintenance, noncanonical NF-kappaB activation, and Bcl-xL levels. These findings link the negative regulatory functions of FcgammaRIIB with BLyS-mediated B-cell survival.