PTEN permits acute increases in D3-phosphoinositide levels following TCR stimulation but inhibits distal signaling events by reducing the basal activity of Akt

Viral Particle-Mediated SAMHD1 Depletion Sensitizes Refractory Glioblastoma to DNA-Damaging Therapeutics by Impairing Homologous Recombination

The current standard-of-care treatment for glioblastoma includes DNA damaging agents, γ-irradiation (IR) and temozolomide (TMZ). These treatments fail frequently and there is limited alternative strategy. Therefore, identifying a new therapeutic target is urgently needed to develop a strategy that improves the efficacy of the existing treatments. Here, we report that tumor samples from GBM patients express a high level of SAMHD1, emphasizing SAMHD1's importance. The depletion of SAMHD1 using virus-like particles containing Vpx, VLP(+Vpx), sensitized two independent GBM cell lines (LN-229 and U-87) to veliparib, a well-established PARP inhibitor, and slowed cell growth in a dose-dependent manner. In the mouse GBM xenograft model, Vpx-mediated SAMHD1 depletion reduced tumor growth and SAMHD1 knockout (KO) improved survival. In combination with IR or TMZ, SAMHD1 KO and exposure to 50% growth inhibitory dose (gID50) of VLP(+Vpx) displayed a synergistic effect, resulting in impaired HR, and improved LN-229 cells' sensitivity to TMZ and IR. In conclusion, our finding demonstrates that SAMHD1 promotes GBM resistance to treatment, and it is a plausible therapeutic target to improve the efficacy of TMZ and IR in GBM. Furthermore, we show that Vpx could be a potential therapeutic tool that can be utilized to deplete SAMHD1 in GBM.

MicroRNA-183 and microRNA-96 are associated with autoimmune responses by regulating T cell activation

MircoRNAs (miRs) are small molecules that regulate gene expression at the posttranscriptional level. They have been proposed to be involved in the regulation of several immune responses including autoimmunity. Here, we identified miR-183 and miR-96 to be highly expressed in CD4⁺ T cells from peripheral blood of Graves' orbitopathy (GO) patients as well as in human and murine T cells upon activation in vitro. By using Luciferase-based binding assays, we identified EGR-1 as target for miR-183 and miR-96. Overexpression of miR-183 and miR-96 in murine CD4⁺ T cells by retroviral gene transfer resulted in decreased EGR-1 and PTEN expression, elevated Akt phosphorylation and enhanced proliferation. In contrast, treatment of murine CD4⁺ T cells with specific antagomiRs increased EGR-1 and PTEN expression and interfered with the proliferative activity upon stimulation in vitro. Strikingly, adoptive transfer of miR-183 and miR-96 overexpressing antigen-specific T cells into INS-HA/Rag2KO mice accelerated the development of autoimmune diabetes, whereas transfer of antagomiR-treated cells delayed the disease onset. These results indicate that miR-183 and miR-96 have the ability to regulate the strength of T cell activation and thereby the development and severity of T cell-dependent autoimmune diseases.

TCR-mediated functions are enhanced in activated peripheral blood T cells isolated from leucocyte reduction systems

Buffy coats are the most common method for the acquisition of activated primary human T cells for research or clinical applications, but recently leukocyte reduction system (LRS) cones have emerged as a viable source for these cells. In this study, we determined if activated human T cells derived from buffy coats or LRS cones had different functionality. No changes in the expression of surface receptors were observed except for a significant increase in CD44 expression on T cells isolated from LRS cones. LRS cone-derived T cells trended towards higher receptor-mediated cytokine production and had significantly increased donor-to-donor variability in IFN-γ production. TCR-induced ERK1/ERK2 and AKT phosphorylation was also increased in T cells isolated from LRS cones. In conclusion, LRS cones are an excellent source of T cells for clinical and research applications, but these cells have subtle functional differences from T cells isolated using standard buffy coats.

The N terminus of SKAP55 enables T cell adhesion to TCR and integrin ligands via distinct mechanisms

The T cell receptor (TCR) triggers the assembly of "SLP-76 microclusters," which mediate signals required for T cell activation. In addition to regulating integrin activation, we show that Src kinase-associated phosphoprotein of 55 kD (SKAP55) is required for microcluster persistence and movement, junctional stabilization, and integrin-independent adhesion via the TCR. These functions require the dimerization of SKAP55 and its interaction with the adaptor adhesion and degranulation-promoting adaptor protein (ADAP). A "tandem dimer" containing two ADAP-binding SKAP55 Src homology 3 (SH3) domains stabilized SLP-76 microclusters and promoted T cell adhesion via the TCR, but could not support adhesion to integrin ligands. Finally, the SKAP55 dimerization motif (DM) enabled the coimmunoprecipitation of the Rap1-dependent integrin regulator Rap1-GTP-interacting adaptor molecule (RIAM), the recruitment of talin into TCR-induced adhesive junctions, and "inside-out" signaling to β1 integrins. Our data indicate that SKAP55 dimers stabilize SLP-76 microclusters, couple SLP-76 to the force-generating systems responsible for microcluster movement, and enable adhesion via the TCR by mechanisms independent of RIAM, talin, and β1 integrins.

Prior TLR5 induction in human T cells results in a transient potentiation of subsequent TCR-induced cytokine production

Activation of TLRs by components required for pathogen viability results in increased inflammation and an enhanced immune response to infection. Unlike their effects on other immune cells, TLR activation in the absence of T cell antigen receptor (TCR) induction has little effect on T cell activity. Instead, the simultaneous induction of TLR and TCR results in increased cytokine release compared to TCR treatment alone. Thus, the current model states that TLRs alter T cell function only if activated at the same time as the TCR. In this study, we tested the novel hypothesis that prior TLR induction can also alter TCR-mediated functions. We found that human T cells responded to ligands for TLR2 and TLR5. However, only prior TLR5 induction potentiated subsequent TCR-mediated cytokine production in human T cells. This response required at least 24h of TLR5 induction and lasted for approximately 24-36h after removal of a TLR5 ligand. Interestingly, prior TLR5 induction enhanced TCR-mediated activation of Akt without increasing Lck, LAT or ERK kinase phosphorylation. Together, our studies show that TLR5 induction leads to a transient increase in the sensitivity of T cells to TCR stimulation by selectively enhancing TCR-mediated Akt function, highlighting that timeframe when TLR5 can potentiate TCR-induced downstream functions are significantly longer that previously appreciated.

AKT/SGK-sensitive phosphorylation of GSK3 in the regulation of L-selectin and perforin expression as well as activation induced cell death of T-lymphocytes

Survival and function of T-lymphocytes critically depends on phosphoinositide (PI) 3 kinase. PI3 kinase signaling includes the PKB/Akt and SGK dependent phosphorylation and thus inhibition of glycogen synthase kinase GSK3α,β. Lithium, a known unspecific GSK3 inhibitor protects against experimental autoimmune encephalomyelitis. The present study explored, whether Akt/SGK-dependent regulation of GSK3 activity is a determinant of T cell survival and function. Experiments were performed in mutant mice in which Akt/SGK-dependent GSK3α,β inhibition was disrupted by replacement of the serine residue in the respective SGK/Akt-phosphorylation consensus sequence by alanine (gsk3(KI)). T cells from gsk3(KI) mice were compared to T cells from corresponding wild type mice (gsk3(WT)). As a result, in gsk3(KI) CD4(+) cells surface CD62L (L-selectin) was significantly less abundant than in gsk3(WT) CD4(+) cells. Upon activation in vitro T cells from gsk3(KI) mice reacted with enhanced perforin production and reduced activation induced cell death. Cytokine production was rather reduced in gsk3(KI) T cells, suggesting that GSK3 induces effector function in CD8(+) T cells. In conclusion, PKB/Akt and SGK sensitive phosphorylation of GSK3α,β is a potent regulator of perforin expression and activation induced cell death in T lymphocytes.

Rule-based modeling of signal transduction: a primer

Biological cells accomplish their physiological functions using interconnected networks of genes, proteins, and other biomolecules. Most interactions in biological signaling networks, such as bimolecular association or covalent modification, can be modeled in a physically realistic manner using elementary reaction kinetics. However, the size and combinatorial complexity of such reaction networks have hindered such a mechanistic approach, leading many to conclude that it is premature and to adopt alternative statistical or phenomenological approaches. The recent development of rule-based modeling languages, such as BioNetGen (BNG) and Kappa, enables the precise and succinct encoding of large reaction networks. Coupled with complementary advances in simulation methods, these languages circumvent the combinatorial barrier and allow mechanistic modeling on a much larger scale than previously possible. These languages are also intuitive to the biologist and accessible to the novice modeler. In this chapter, we provide a self-contained tutorial on modeling signal transduction networks using the BNG Language and related software tools. We review the basic syntax of the language and show how biochemical knowledge can be articulated using reaction rules, which can be used to capture a broad range of biochemical and biophysical phenomena in a concise and modular way. A model of ligand-activated receptor dimerization is examined, with a detailed treatment of each step of the modeling process. Sections discussing modeling theory, implicit and explicit model assumptions, and model parameterization are included, with special focus on retaining biophysical realism and avoiding common pitfalls. We also discuss the more advanced case of compartmental modeling using the compartmental extension to BioNetGen. In addition, we provide a comprehensive set of example reaction rules that cover the various aspects of signal transduction, from signaling at the membrane to gene regulation. The reader can modify these reaction rules to model their own systems of interest.

Control of fibroblast fibronectin expression and alternative splicing via the PI3K/Akt/mTOR pathway

Fibronectin (FN), a ubiquitous glycoprotein that plays critical roles in physiologic and pathologic conditions, undergoes alternative splicing which distinguishes plasma FN (pFN) from cellular FN (cFN). Although both pFN and cFN can be incorporated into the extracellular matrix, a distinguishing feature of cFN is the inclusion of an alternatively spliced exon termed EDA (for extra type III domain A). The molecular steps involved in EDA splicing are well-characterized, but pathways influencing EDA splicing are less clear. We have previously found an obligate role for inhibition of the tumor suppressor phosphatase and tensin homologue on chromosome 10 (PTEN), the primary regulator of the PI3K/Akt pathway, in fibroblast activation. Here we show TGF-beta, a potent inducer of both EDA splicing and fibroblast activation, inhibits PTEN expression and activity in mesenchymal cells, corresponding with enhanced PI3K/Akt signaling. In pten(-/-) fibroblasts, which resemble activated fibroblasts, inhibition of Akt attenuated FN production and decreased EDA alternative splicing. Moreover, inhibition of mammalian target of rapamycin (mTOR) in pten(-/-) cells also blocked FN production and EDA splicing. This effect was due to inhibition of Akt-mediated phosphorylation of the primary EDA splicing regulatory protein SF2/ASF. Importantly, FN silencing in pten(-/-) cells resulted in attenuated proliferation and migration. Thus, our results demonstrate that the PI3K/Akt/mTOR axis is instrumental in FN transcription and alternative splicing, which regulates cell behavior.

Comparison of T cell receptor-induced proximal signaling and downstream functions in immortalized and primary T cells

BACKGROUND

Human T cells play an important role in pathogen clearance, but their aberrant activation is also linked to numerous diseases. T cells are activated by the concurrent induction of the T cell receptor (TCR) and one or more costimulatory receptors. The characterization of signaling pathways induced by TCR and/or costimulatory receptor activation is critical, since these pathways are excellent targets for novel therapies for human disease. Although studies using human T cell lines have provided substantial insight into these signaling pathways, no comprehensive, direct comparison of these cell lines to activated peripheral blood T cells (APBTs) has been performed to validate their usefulness as a model of primary T cells.

METHODOLOGY/PRINCIPAL FINDINGS

We used quantitative biochemical techniques to compare the activation of two widely used human T cell lines, Jurkat E6.1 and HuT78 T cells, to APBTs. We found that HuT78 cells were similar to APBTs in proximal TCR-mediated signaling events. In contrast, Jurkat E6.1 cells had significantly increased site-specific phosphorylation of Pyk2, PLCgamma1, Vav1, and Erk1/Erk2 and substantially more Ca2+ flux compared to HuT78 cells and APBTs. In part, these effects appear to be due to an overexpression of Itk in Jurkat E6.1 cells compared to HuT78 cells and APBTs. Both cell lines differ from APBTs in the expression and function of costimulatory receptors and in the range of cytokines and chemokines released upon TCR and costimulatory receptor activation.

CONCLUSIONS/SIGNIFICANCE

Both Jurkat E6.1 and HuT78 T cells had distinct similarities and differences compared to APBTs. Both cell lines have advantages and disadvantages, which must be taken into account when choosing them as a model T cell line.

PIP3 pathway in regulatory T cells and autoimmunity

Regulatory T cells (Tregs) play an important role in preventing both autoimmune and inflammatory diseases. Many recent studies have focused on defining the signal transduction pathways essential for the development and the function of Tregs. Increasing evidence suggest that T-cell receptor (TCR), interleukin-2 (IL-2) receptor (IL-2R), and co-stimulatory receptor signaling are important in the early development, peripheral homeostasis, and function of Tregs. The phosphoinositide-3 kinase (PI3K)-regulated pathway (PIP3 pathway) is one of the major signaling pathways activated upon TCR, IL-2R, and CD28 stimulation, leading to T-cell activation, proliferation, and cell survival. Activation of the PIP3 pathway is also negatively regulated by two phosphatidylinositol phosphatases SHIP and PTEN. Several mouse models deficient for the molecules involved in PIP3 pathway suggest that impairment of PIP3 signaling leads to dysregulation of immune responses and, in some cases, autoimmunity. This review will summarize the current understanding of the importance of the PIP3 pathway in T-cell signaling and the possible roles this pathway performs in the development and the function of Tregs.

Phosphoinositide lipid phosphatases: natural regulators of phosphoinositide 3-kinase signaling in T lymphocytes

The phosphoinositide 3-kinase signaling pathway has been implicated in a range of T lymphocyte cellular functions, particularly growth, proliferation, cytokine secretion, and survival. Dysregulation of phosphoinositide 3-kinase-dependent signaling and function in leukocytes, including B and T lymphocytes, has been implicated in many inflammatory and autoimmune diseases. As befits a pivotal signaling cascade, several mechanisms exist to ensure that the pathway is tightly regulated. This minireview focuses on two lipid phosphatases, viz. the 3'-phosphatase PTEN (phosphatase and tensin homolog deleted on chromosome 10) and SHIP (Src homology 2 domain-containing inositol-5-phosphatase). We discuss their role in regulating T lymphocyte signaling as well their potential as future therapeutic targets.

Signalling to suit function: tailoring phosphoinositide 3-kinase during T-cell activation

Members of the CD28 family of co-receptors are crucial determinants of the outcome of T-cell activation. These receptors interact with ligands in the B7 family and either costimulate or co-inhibit signals through antigen-specific receptors. The T-cell-costimulatory molecules CD28 and inducible costimulator recruit and activate class 1A phosphoinositide 3-kinase (PI3K). Interestingly, the co-inhibitory molecules cytotoxic T lymphocyte antigen-4 and B and T lymphocyte attenuator also interact with class 1A PI3K. However, all co-inhibitory receptors share an ability to oppose activation of the key PI3K effector protein kinase B (also known as Akt). Recent evidence suggests that distinct mechanisms exist to limit Akt activation by different co-inhibitory receptors. This article examines how differential positive or negative regulation of the PI3K-Akt signalling pathway by CD28 family receptors enables functional differences between the receptors.

Persistence of cooperatively stabilized signaling clusters drives T-cell activation

Antigen recognition triggers the recruitment of the critical adaptor protein SLP-76 to small macromolecular clusters nucleated by the T-cell receptor (TCR). These structures develop rapidly, in parallel with TCR-induced increases in tyrosine phosphorylation and cytosolic calcium, and are likely to contribute to TCR-proximal signaling. Previously, we demonstrated that these SLP-76-containing clusters segregate from the TCR and move towards the center of the contact interface. Neither the function of these clusters nor the structural requirements governing their persistence have been examined extensively. Here we demonstrate that defects in cluster assembly and persistence are associated with defects in T-cell activation in the absence of Lck, ZAP-70, or LAT. Clusters persist normally in the absence of phospholipase C-gamma1, indicating that in the absence of a critical effector, these structures are insufficient to drive T-cell activation. Furthermore, we show that the critical adaptors LAT and Gads localize with SLP-76 in persistent clusters. Mutational analyses of LAT, Gads, and SLP-76 indicated that multiple domains within each of these proteins contribute to cluster persistence. These data indicate that multivalent cooperative interactions stabilize these persistent signaling clusters, which may correspond to the functional complexes predicted by kinetic proofreading models of T-cell activation.

Respiratory chain deficiency slows down cell-cycle progression via reduced ROS generation and is associated with a reduction of p21CIP1/WAF1

We have used HeLa cells without mitochondrial DNA (rho0-cells) and transient rho0-phenocopies, obtained from wild-type cells by short-term treatment with ethidium bromide, to analyze how the absence of a functional mitochondrial respiratory chain slows down proliferation. We ruled out an energetic problem (ATP/ADP content) as well as defective synthesis of pyrimidine, iron-sulfur clusters or heme as important causes for the proliferative defect. Flow cytometric analysis revealed that reactive oxygen species were reduced in rho0-cells and in rho0-phenocopies, and that, quite unusually, all stages of the cell cycle were slowed down. Specific quenching of mitochondrial ROS with the ubiquinone analog MitoQ also resulted in slower growth. Some important cell-cycle regulators were reduced in rho0-cells: cyclin D3, cdk6, p18INK4C, p27KIP1, and p21CIP1/WAF1. In the rho0-phenocopies, the expression pattern did not fully duplicate the complex response observed in rho0-cells, and mainly p21CIP1/WAF1 was downregulated. Activities of the growth regulatory PKB/Akt and MAPK/ERK-signaling pathways did not correlate with proliferation rates of rho0-cells and rho0-phenocopies. Our study demonstrates that loss of a functional mitochondrial electron transport chain inhibits cell-cycle progression, and we postulate that this occurs through the decreased concentration of reactive oxygen species, leading to downregulation of p21CIP1/WAF1.

T cell receptor for antigen induces linker for activation of T cell-dependent activation of a negative signaling complex involving Dok-2, SHIP-1, and Grb-2

Adaptor proteins positively or negatively regulate the T cell receptor for antigen (TCR) signaling cascade. We report that after TCR stimulation, the inhibitory adaptor downstream of kinase (Dok)-2 and its homologue Dok-1 are involved in a multimolecular complex including the lipid phosphatase Src homology 2 domain–containing inositol polyphosphate 5′-phosphatase (SHIP)-1 and Grb-2 which interacts with the membrane signaling scaffold linker for activation of T cells (LAT). Knockdown of LAT and SHIP-1 expression indicated that SHIP-1 favored recruitment of Dok-2 to LAT. Knockdown of Dok-2 and Dok-1 revealed their negative control on Akt and, unexpectedly, on Zap-70 activation. Our findings support the view that Dok-1 and -2 are critical elements of a LAT-dependent negative feedback loop that attenuates early TCR signal. Dok-1 and -2 may therefore exert a critical role in shaping the immune response and as gatekeepers for T cell tolerance.

Modulation of the PI 3-kinase-Akt signalling pathway by IGF-I and PTEN regulates the differentiation of neural stem/precursor cells

Neural stem cells depend on insulin-like growth factor I (IGF-I) for differentiation. We analysed how activation and inhibition of the PI 3-kinase-Akt signalling affects the number and differentiation of mouse olfactory bulb stem cells (OBSCs). Stimulation of the pathway with insulin and/or IGF-I, led to an increase in Akt phosphorylated on residues Ser473 and Thr308 (P-Akt(Ser473) and P-Akt(Thr308), respectively) in proliferating OBSCs, and in differentiating cells. Conversely, P-Akt(Ser473) levels decreased by 50% in the OB of embryonic day 16.5-18.5 IGF-I knockout mouse embryos. Overexpression of PTEN, a negative regulator of the PI 3-kinase pathway, caused a reduction in the basal levels of P-Akt(Ser473) and P-Akt(Thr308) and a minor reduction in IGF-I-stimulated P-Akt(Ser473). Although PTEN overexpression decreased the proportion of neurons and astrocytes in the absence of insulin/IGF-I, it did not alter the proliferation or survival of OBSCs. Accordingly, overexpression of a catalytically inactive PTEN mutant promoted OBSCs differentiation. Inhibition of PI 3-kinase by LY294002 produced strong and moderate reductions in IGF-I-stimulated P-Akt(Ser473) and P-Akt(Thr308), respectively. Consequently, LY294002 reduced the proliferation of OBSCs and the number of neurons and astrocytes, and also augmented cell death. These findings indicate that OBSC differentiation is more sensitive to lower basal levels of P-Akt than proliferation or death. By regulating P-Akt levels in opposite ways, IGF-I and PTEN contribute to the fine control of neurogenesis in the olfactory bulb.

Early phosphorylation kinetics of proteins involved in proximal TCR-mediated signaling pathways

Activation of T cells via the stimulation of the TCR plays a central role in the adaptive immunological response. Although much is known about TCR-stimulated signaling pathways, there are still gaps in our knowledge about the kinetics and sequence of events during early activation and about the in vivo specificity of kinases involved in these proximal signaling pathways. This information is important not only for understanding the activation of signaling pathways important for T cell function but also for the development of drug targets and computer-based molecular models. In this study, phospho-specific Abs directed toward individual sites on signaling proteins were used to investigate the early phosphorylation kinetics of proteins involved in proximal TCR-induced pathways. These studies indicate that linker for activation of T cells' tyrosines have substantially different phosphorylation kinetics and that Src homology 2 domain-containing leukocyte protein of 76 kDa has rapid, transient phosphorylation kinetics compared to other proteins. In additions, we provide evidence that ZAP-70 is the primary in vivo kinase for LAT tyrosine 191 and that Itk plays a role in the phosphorylation of tyrosine 783 on phospholipase C-gamma1. In total, these studies give new insight into the sequence, kinetics and specificity of early TCR-mediated signaling events that are vital for T cell activation.

Protein phosphatase 2A is a negative regulator of IL-2 production in patients with systemic lupus erythematosus

Decreased IL-2 production in systemic lupus erythematosus (SLE) represents a central component of the disease immunopathology. We report that the message, protein, and enzymatic activity of the catalytic subunit of protein phosphatase 2A (PP2Ac), but not PP1, are increased in patients with SLE regardless of disease activity and treatment and in a disease-specific manner. Treatment of SLE T cells with PP2Ac-siRNA decreased the protein levels and activity of PP2Ac in a specific manner and increased the levels of phosphorylated cAMP response element-binding protein and its binding to the IL2 and c-fos promoters, as well as increased activator protein 1 activity, causing normalization of IL-2 production. Our data document increased activity of PP2A as a novel SLE disease-specific abnormality and define a distinct mechanism whereby it represses IL-2 production. We propose the use of PP2Ac-siRNA as a novel tool to correct T cell IL-2 production in SLE patients.