Dynamic recruitment of PAK1 to the immunological synapse is mediated by PIX independently of SLP-76 and Vav1

The opposing effects of two gene defects in STX11 and SLP76 on the disease in a patient with an inborn error of immunity

BACKGROUND

Inborn errors of immunity are mostly monogenic. However, disease phenotype and outcome may be modified by the coexistence of a second gene defect.

OBJECTIVE

We sought to identify the genetic basis of the disease in a patient who experienced bleeding episodes, pancytopenia, hepatosplenomegaly, and recurrent pneumonia that resulted in death.

METHODS

Genetic analysis was done using next-generation sequencing. Protein expression and phosphorylation were determined by immunoblotting. T-cell proliferation and F-actin levels were studied by flow cytometry.

RESULTS

The patient harbored 2 homozygous deletions in STX11 (c.369_370del, c.374_376del; p.V124fs60∗) previously associated with familial hemophagocytic lymphohistiocytosis and a novel homozygous missense variant in SLP76 (c.767C>T; p.T256I) that resulted in an approximately 85% decrease in SLP76 levels and absent T-cell proliferation. The patient's heterozygous family members showed an approximately 50% decrease in SLP76 levels but normal immune function. SLP76-deficient J14 Jurkat cells did not express SLP76 and had decreased extracellular signal-regulated kinase signaling, basal F-actin levels, and polymerization following T-cell receptor stimulation. Reconstitution of J14 cells with T256I mutant SLP76 resulted in low protein expression and abnormal extracellular signal-regulated kinase phosphorylation and F-actin polymerization after T-cell receptor activation compared with normal expression and J14 function when wild-type SLP76 was introduced.

CONCLUSIONS

The hypomorphic mutation in SLP76 tones down the hyperinflammation due to STX11 deletion, resulting in a combined immunodeficiency that overshadows the hemophagocytic lymphohistiocytosis phenotype. To our knowledge, this study represents the first report of the opposing effects of 2 gene defects on the disease in a patient with an inborn error of immunity.

Rapid increase in transferrin receptor recycling promotes adhesion during T cell activation

Background

T cell activation leads to increased expression of the receptor for the iron transporter transferrin (TfR) to provide iron required for the cell differentiation and clonal expansion that takes place during the days after encounter with a cognate antigen. However, T cells mobilise TfR to their surface within minutes after activation, although the reason and mechanism driving this process remain unclear.

Results

Here we show that T cells transiently increase endocytic uptake and recycling of TfR upon activation, thereby boosting their capacity to import iron. We demonstrate that increased TfR recycling is powered by a fast endocytic sorting pathway relying on the membrane proteins flotillins, Rab5- and Rab11a-positive endosomes. Our data further reveal that iron import is required for a non-canonical signalling pathway involving the kinases Zap70 and PAK, which controls adhesion of the integrin LFA-1 and eventually leads to conjugation with antigen-presenting cells.

Conclusions

Altogether, our data suggest that T cells boost their iron importing capacity immediately upon activation to promote adhesion to antigen-presenting cells.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-022-01386-0.

Liquid-Liquid Phase Separation at the Plasma Membrane-Cytosol Interface: Common Players in Adhesion, Motility, and Synaptic Function

Networks of scaffold proteins and enzymes assemble at the interface between the cytosol and specific sites of the plasma membrane, where these networks guide distinct cellular functions. Some of these plasma membrane-associated platforms (PMAPs) include shared core components that are able to establish specific protein-protein interactions, to produce distinct supramolecular assemblies regulating dynamic processes as diverse as cell adhesion and motility, or the formation and function of neuronal synapses. How cells organize such dynamic networks is still an open question. In this review we introduce molecular networks assembling at the edge of migrating cells, and at pre- and postsynaptic sites, which share molecular players that can drive the assembly of biomolecular condensates. Very recent experimental evidence has highlighted the emerging role of some of these multidomain/scaffold proteins belonging to the GIT, liprin-α and ELKS/ERC families as drivers of liquid-liquid phase separation (LLPS). The data point to an important role of LLPS: (i) in the formation of PMAPs at the edge of migrating cells, where LLPS appears to be involved in promoting protrusion and the turnover of integrin-mediated adhesions, to allow forward cell translocation; (ii) in the assembly of the presynaptic active zone and of the postsynaptic density deputed to the release and reception of neurotransmitter signals, respectively. The recent results indicate that LLPS at cytosol-membrane interfaces is suitable not only for the regulation of active cellular processes, but also for the continuous spatial rearrangements of the molecular interactions involved in these dynamic processes.

Arhgef6 (alpha-PIX) cytoskeletal regulator signals to GTPases and Cofilin to couple T cell migration speed and persistence

Immunity is governed by successful T cell migration, optimized to enable a T cell to fully scan its environment without wasted movement by balancing speed and turning. Here we report that the Arhgef6 RhoGEF (aka alpha-PIX; αPIX; Cool-2), an activator of small GTPases, is required to restrain cell migration speed and cell turning during spontaneous migration on 2D surfaces. In Arhgef6^-/- T cells, expression of Arhgef7 (beta-PIX; βPIX; Cool-1), a homolog of Arhgef6, was increased and correlated with defective activation and localization of Rac1 and CDC42 GTPases, respectively. Downstream of Arhgef6, PAK2 (p21-activated kinase 2) and LIMK1 phosphorylation was reduced, leading to increased activation of Cofilin, the actin-severing factor. Consistent with defects in these signaling pathways, Arhgef6^-/- T cells displayed abnormal bilobed lamellipodia and migrated faster, turned more, and arrested less than wild-type (WT) T cells. Using pharmacologic inhibition of LIMK1 (LIM domain kinase 1) to induce Cofilin activation in WT T cells, we observed increased migration speed but not increased cell turning. In contrast, inhibition of Cdc42 increased cell turning but not speed. These results suggested that the increased speed of the Arhgef6^-/- T cells is due to hyperactive Cofilin while the increased turning may be due to abnormal GTPase activation and recruitment. Together, these findings reveal that Arhgef6 acts as a repressor of T cell speed and turning by limiting actin polymerization and lamellipodia formation.

Escape of tumor cells from the NK cell cytotoxic activity

In recent years, NK cells, initially identified as potent cytotoxic effector cells, have revealed an unexpected complexity, both at phenotypic and functional levels. The discovery of different NK cell subsets, characterized by distinct gene expression and phenotypes, was combined with the characterization of the diverse functions NK cells can exert, not only as circulating cells, but also as cells localized or recruited in lymphoid organs and in multiple tissues. Besides the elimination of tumor and virus-infected cells, these functions include the production of cytokines and chemokines, the regulation of innate and adaptive immune cells, the influence on tissue homeostasis. In addition, NK cells display a remarkable functional plasticity, being able to adapt to the environment and to develop a kind of memory. Nevertheless, the powerful cytotoxic activity of NK cells remains one of their most relevant properties, particularly in the antitumor response. In this review, the process of tumor cell recognition and killing mediated by NK cells, starting from the generation of cytolytic granules and recognition of target cell, to the establishment of the NK cell immunological synapse, the release of cytotoxic molecules, and consequent tumor cell death is described. Next, the review focuses on the heterogeneous mechanisms, either intrinsic to tumors or induced by the tumor microenvironment, by which cancer cells can escape the NK cell-mediated attack.

Vav2 lacks Ca2+ entry-promoting scaffolding functions unique to Vav1 and inhibits T cell activation via Cdc42

Vav family guanine nucleotide exchange factors (GEFs) are essential regulators of immune function. Despite their structural similarity, Vav1 promotes and Vav2 opposes T cell receptor (TCR)-induced Ca2+ entry. By using a Vav1-deficient Jurkat T cell line, we find that Vav1 facilitates Ca2+ entry via non-catalytic scaffolding functions that are encoded by the catalytic core of Vav1 and flanking linker regions. We implicate, in this scaffolding function, a previously undescribed polybasic motif that is strictly conserved in Vav1 and absent from Vav2 in tetrapods. Conversely, the catalytic activity of Vav2 contributes to the suppression of TCR-mediated Ca2+ entry. By performing an in vivo 'GEF trapping' assay in intact cells, we demonstrate that Cdc42 interacts with the catalytic surface of Vav2 but not Vav1, and that Vav1 discriminates Cdc42 from Rac1 via F56 (W56 in Rac1). Finally, the Cdc42-specific inhibitor ZCL278 and the shRNA-mediated suppression of Cdc42 each prevent the inhibition of TCR-induced Ca2+ entry by Vav2. These findings define stark differences in the functions of Vav1 and Vav2, and provide an explanation for the differential usage of these Vav isoforms by immune subpopulations.

Unique-region phosphorylation targets LynA for rapid degradation, tuning its expression and signaling in myeloid cells

The activity of Src-family kinases (SFKs), which phosphorylate immunoreceptor tyrosine-based activation motifs (ITAMs), is a critical factor regulating myeloid-cell activation. We reported previously that the SFK LynA is uniquely susceptible to rapid ubiquitin-mediated degradation in macrophages, functioning as a rheostat regulating signaling (Freedman et al., 2015). We now report the mechanism by which LynA is preferentially targeted for degradation and how cell specificity is built into the LynA rheostat. Using genetic, biochemical, and quantitative phosphopeptide analyses, we found that the E3 ubiquitin ligase c-Cbl preferentially targets LynA via a phosphorylated tyrosine (Y32) in its unique region. This distinct mode of c-Cbl recognition depresses steady-state expression of LynA in macrophages derived from mice. Mast cells, however, express little c-Cbl and have correspondingly high LynA. Upon activation, mast-cell LynA is not rapidly degraded, and SFK-mediated signaling is amplified relative to macrophages. Cell-specific c-Cbl expression thus builds cell specificity into the LynA checkpoint.

Pak1 Kinase Promotes Activated T Cell Trafficking by Regulating the Expression of L-Selectin and CCR7

Normal function of the adaptive immune system requires trafficking of T cells between the blood and lymphoid organs. Lymphocyte homing to lymph nodes requires that they cross endothelial barriers present in blood vessels and lymphatics. This multi-step process requires a remodeling of the lymphocyte plasma membrane, which is mediated by the dynamic re-arrangement of the actin cytoskeleton. Pak1 plays a central role in cell morphology, adhesion and migration in various cell types. Here we demonstrate that Pak1 is required for activated CD4⁺ T cell trafficking to lymph nodes. Pak1 deficiency in T cells causes a defect in the transcription of CCR7 and L-selectin, thereby altering lymphocyte trafficking. Additionally, we report an increase in L-selectin shedding in Pak1-deficient T cells, which correlates with a decrease in the recruitment of calmodulin to the cytoplasmic tail of L-selectin during T cell activation. Overall, our findings demonstrate that by regulating the expression of two major lymph node homing molecules, L-selectin and CCR7, Pak1 mediates activated CD4⁺ T cell trafficking.

Filamin A Phosphorylation at Serine 2152 by the Serine/Threonine Kinase Ndr2 Controls TCR-Induced LFA-1 Activation in T Cells

The integrin LFA-1 (CD11a/CD18) plays a critical role in the interaction of T cells with antigen presenting cells (APCs) to promote lymphocyte differentiation and proliferation. This integrin can be present either in a closed or in an open active conformation and its activation upon T-cell receptor (TCR) stimulation is a critical step to allow interaction with APCs. In this study we demonstrate that the serine/threonine kinase Ndr2 is critically involved in the initiation of TCR-mediated LFA-1 activation (open conformation) in T cells. Ndr2 itself becomes activated upon TCR stimulation and phosphorylates the intracellular integrin binding partner Filamin A (FLNa) at serine 2152. This phosphorylation promotes the dissociation of FLNa from LFA-1, allowing for a subsequent association of Talin and Kindlin-3 which both stabilize the open conformation of LFA-1. Our data suggest that Ndr2 activation is a crucial step to initiate TCR-mediated LFA-1 activation in T cells.

Molecular Mechanisms of T Cells Activation by Dendritic Cells in Autoimmune Diseases

The interaction between T cell and dendritic cells (DCs) that leads to T cell activation affects the progression of the immune response including autoimmune diseases. Antigen presentation on immune cell surface, formation of an immunological synapse (IS), and specific identification of complex by T cells including two activating signals are necessary steps that lead to T cell activation. The formation of stimulatory IS involves the inclusion of costimulatory molecules, such as ICAM-1/LFA-1 and CD28/B7-1, and so on. Some fusion proteins and monoclonal antibodies targeting costimulatory molecules have been developed and approved to treat autoimmune diseases, including rheumatoid arthritis (RA), systemic lupus erythematosus (SLE), multiple sclerosis (MS), type I diabetes (T1D), inflammatory bowel disease (IBD), and psoriasis. These biological agents, including CTLA-4- and LFA-3-Ig, anti-CD3 monoclonal antibody, could prevent the successful engagement of DCs by T cell with significant efficacy and safety profile. In this article, we reviewed the molecular mechanisms of T cell activation during the interaction between T cells and DCs, and summarized some biological agents that target costimulatory molecules involved in the regulation of T cell activation.

GIT2-A keystone in ageing and age-related disease

Since its discovery, G protein-coupled receptor kinase-interacting protein 2, GIT2, and its family member, GIT1, have received considerable interest concerning their potential key roles in regulating multiple inter-connected physiological and pathophysiological processes. GIT2 was first identified as a multifunctional protein that is recruited to G protein-coupled receptors (GPCRs) during the process of receptor internalization. Recent findings have demonstrated that perhaps one of the most important effects of GIT2 in physiology concerns its role in controlling multiple aspects of the complex ageing process. Ageing can be considered the most prevalent pathophysiological condition in humans, affecting all tissue systems and acting as a driving force for many common and intractable disorders. The ageing process involves a complex interplay among various deleterious activities that profoundly disrupt the body's ability to cope with damage, thus increasing susceptibility to pathophysiologies such as neurodegeneration, central obesity, osteoporosis, type 2 diabetes mellitus and atherosclerosis. The biological systems that control ageing appear to function as a series of interconnected complex networks. The inter-communication among multiple lower-complexity signaling systems within the global ageing networks is likely coordinated internally by keystones or hubs, which regulate responses to dynamic molecular events through protein-protein interactions with multiple distinct partners. Multiple lines of research have suggested that GIT2 may act as one of these network coordinators in the ageing process. Identifying and targeting keystones, such as GIT2, is thus an important approach in our understanding of, and eventual ability to, medically ameliorate or interdict age-related progressive cellular and tissue damage.

HIV-1 Activates T Cell Signaling Independently of Antigen to Drive Viral Spread

HIV-1 spreads between CD4 T cells most efficiently through virus-induced cell-cell contacts. To test whether this process potentiates viral spread by activating signaling pathways, we developed an approach to analyze the phosphoproteome in infected and uninfected mixed-population T cells using differential metabolic labeling and mass spectrometry. We discovered HIV-1-induced activation of signaling networks during viral spread encompassing over 200 cellular proteins. Strikingly, pathways downstream of the T cell receptor were the most significantly activated, despite the absence of canonical antigen-dependent stimulation. The importance of this pathway was demonstrated by the depletion of proteins, and we show that HIV-1 Env-mediated cell-cell contact, the T cell receptor, and the Src kinase Lck were essential for signaling-dependent enhancement of viral dissemination. This study demonstrates that manipulation of signaling at immune cell contacts by HIV-1 is essential for promoting virus replication and defines a paradigm for antigen-independent T cell signaling.

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Unbiased global analysis of T cell signaling changes during HIV-1 cell-cell spread

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Experimental system to map dynamic signaling changes in mixed cell populations over time

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More than 200 host cell proteins are modified as HIV-1 disseminates between T cells

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HIV-1 activates antigen-independent TCR signaling to drive viral spread

Requirement of Treg-intrinsic CTLA4/PKCη signaling pathway for suppressing tumor immunity

The ability of Tregs to control the development of immune responses is essential for maintaining immune system homeostasis. However, Tregs also inhibit the development of efficient antitumor responses. Here, we explored the characteristics and mechanistic basis of the Treg-intrinsic CTLA4/PKCη signaling pathway that we recently found to be required for contact-dependent Treg-mediated suppression. We show that PKCη is required for the Treg-mediated suppression of tumor immunity in vivo. The presence of PKCη-deficient (Prkch-/-) Tregs in the tumor microenvironment was associated with a significantly increased expression of the costimulatory molecule CD86 on intratumoral CD103+ DCs, enhanced priming of antigen-specific CD8+ T cells, and greater levels of effector cytokines produced by these cells. Similar to mouse Tregs, the GIT/PAK/PIX complex also operated downstream of CTLA4 and PKCη in human Tregs, and GIT2 knockdown in Tregs promoted antitumor immunity. Collectively, our data suggest that targeting the CTLA4/PKCη/GIT/PAK/PIX signaling pathway in Tregs could represent a novel immunotherapeutic strategy to alleviate the negative impact of Tregs on antitumor immune responses.

Expanding functions of GIT Arf GTPase-activating proteins, PIX Rho guanine nucleotide exchange factors and GIT-PIX complexes

The GIT proteins, GIT1 and GIT2, are GTPase-activating proteins (inactivators) for the ADP-ribosylation factor (Arf) small GTP-binding proteins, and function to limit the activity of Arf proteins. The PIX proteins, α-PIX and β-PIX (also known as ARHGEF6 and ARHGEF7, respectively), are guanine nucleotide exchange factors (activators) for the Rho family small GTP-binding protein family members Rac1 and Cdc42. Through their multi-domain structures, GIT and PIX proteins can also function as signaling scaffolds by binding to numerous protein partners. Importantly, the constitutive association of GIT and PIX proteins into oligomeric GIT-PIX complexes allows these two proteins to function together as subunits of a larger structure that coordinates two distinct small GTP-binding protein pathways and serves as multivalent scaffold for the partners of both constituent subunits. Studies have revealed the involvement of GIT and PIX proteins, and of the GIT-PIX complex, in numerous fundamental cellular processes through a wide variety of mechanisms, pathways and signaling partners. In this Commentary, we discuss recent findings in key physiological systems that exemplify current understanding of the function of this important regulatory complex. Further, we draw attention to gaps in crucial information that remain to be filled to allow a better understanding of the many roles of the GIT-PIX complex in health and disease.

Sorting nexin 27 interactome in T-lymphocytes identifies zona occludens-2 dynamic redistribution at the immune synapse

T Lymphocyte recognition of antigens leads to the formation of a highly organized structure termed immune synapse (IS) by analogy with the neuronals synapse. Sorting nexin 27 (SNX27) controls the endosomal traffic of PSD95, Dlg1, ZO-1 (PDZ) domain-interacting proteins, and its alteration is associated with impaired synaptic function and neurological diseases. In T-lymphocytes, SNX27-positive vesicles polarize to the IS, the identity of SNX27 interactors in these conditions nonetheless remains unknown. Here we used proteomics to analyze the SNX27 interactome purified from IS-forming T cells, and confirmed the conserved nature of the SNX27/WASH/retromer association in hematopoietic cells. Furthermore, our comparative interactome analysis of SNX27 wild-type and a mutant-deficient for PDZ cargo recognition identified the epithelial cell-cell junction protein zona occludens-2 (ZO-2) as an IS component. Biochemistry and microscopy approaches in T cells confirmed SNX27/ZO-2 PDZ-dependent interaction, and demonstrated its role controlling the dynamic localization of ZO-2 at the IS. This study broadens our knowledge of SNX27 function in T lymphocytes, and suggests that pathways that delimit polarized structures in nervous and epithelial systems also participate in IS regulation.

GIT1 promotes lung cancer cell metastasis through modulating Rac1/Cdc42 activity and is associated with poor prognosis

G-protein-coupled receptor kinase interacting protein 1 (GIT1) is participated in cell movement activation, which is a fundamental process during tissue development and cancer progression. GIT1/PIX forming a functional protein complex that contributes to Rac1/Cdc42 activation, resulting in increasing cell mobility. Although the importance of Rac1/Cdc42 activation is well documented in cancer aggressiveness, the clinical importance of GIT1 remains largely unknown. Here, we investigated the clinical significance of GIT1 expression in non-small-cell lung cancer (NSCLC) and also verified the importance of GIT1-Rac1/Cdc42 axis in stimulating NSCLC cell mobility. The result indicated higher GIT1 expression patients had significantly poorer prognoses in disease-free survival (DFS) and overall survival (OS) compared with lower GIT1 expression patients. Higher GIT1 expression was an independent prognostic factor by multivariate analysis and associated with migration/invasion of NSCLC cells in transwell assay. In vivo studies indicated that GIT1 promotes metastasis of NSCLC cells. Finally, GIT1 was found to stimulate migration/invasion by altering the activity of Rac1/Cdc42 in NSCLC cells. Together, the GIT1 expression is associated with poor prognosis in patients with NSCLC. GIT1 is critical for the invasiveness of NSCLC cells through stimulating the activity of Rac1/Cdc42.

Cell cycle arrest caused by MEK/ERK signaling is a mechanism for suppressing growth of antigen-hyperstimulated effector T cells

Suppression of T-cell growth is an important mechanism for establishment of self-tolerance and prevention of unwanted prolonged immune responses that may cause tissue damage. Although negative selection of potentially self-reactive T cells in the thymus as well as in peripheral tissues has been extensively investigated and well documented, regulatory mechanisms to dampen proliferation of antigen-specific effector T cells in response to antigen stimulation remain largely unknown. Thus, in this work, we focus on the identification of growth suppression mechanisms of antigen-specific effector T cells. In order to address this issue, we investigated the cellular and molecular events in growth suppression of an ovalbumin (OVA)-specific T-cell clone after stimulation with a wide range of OVA-peptide concentrations. We observed that while an optimal dose of peptide leads to cell cycle progression and proliferation, higher doses of peptide reduced cell growth, a phenomenon that was previously termed high-dose suppression. Our analysis of this phenomenon indicated that high-dose suppression is a consequence of cell cycle arrest, but not Fas-Fas ligand-dependent apoptosis or T-cell anergy, and that this growth arrest occurs in S phase, accompanied by reduced expression of CDK2 and cyclin A. Importantly, inhibition of MEK/ERK activation eliminated this growth suppression and cell cycle arrest, while it reduced the proliferative response to optimal antigenic stimulation. These results suggest that cell cycle arrest is the major mechanism regulating antigen-specific effector T-cell expansion, and that the MEK/ERK signaling pathway has both positive and negative effects, depending on the strength of antigenic stimulation.

Differential Expression of the ARF GAP Genes GIT1 and GIT2 in Mouse Tissues

GIT1 and GIT2 belong to the family of ADP-ribosylation factor GTPase-activating proteins (ARF-GAP) and have been implicated in the regulation of G protein-coupled receptor sequestration, cell migration, T-cell activation, neuronal spine formation, and aggregate formation in Huntington's disease. Examination of endogenous GIT protein expression in tissues, however, has been hampered by the lack of GIT2-specific antibodies. To visualize GIT1 and GIT2 gene expression in mouse tissues, we created mice with β-galactosidase (β-Gal) reporters inserted into the two GIT genes. β-Gal staining confirmed the broad tissue distribution of GIT1 and GIT2 in the mouse but also revealed striking differences. GIT2 is expressed in most cells of the body, whereas GIT1 is restricted to only a subset of cells. For example, GIT2 is uniformly expressed throughout lung and liver, whereas GIT1 is restricted to cells lining blood vessels, bronchi, and bile ducts. Expression of GIT1 and GIT2 is mutually exclusive in the testes, where a developmental expression shift occurs, with GIT2 present in spermatogonia but GIT1 in mature spermatids. In conclusion, analysis of endogenous GIT expression revealed a nearly ubiquitous distribution of GIT2, whereas GIT1 is restricted to specific cell types even in tissues with apparently high GIT1 expression and is entirely absent from some tissues. (J Histochem Cytochem 55: 1039–1048, 2007)

GIT2 deficiency attenuates concanavalin A-induced hepatitis in mice

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GIT2 depletion attenuates Con A-induced immunological hepatic injuries.

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GIT2 depletion suppressed the activation and function of mouse CD4⁺ T cells.

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GIT2 depletion suppressed liver infiltration by lymphoid cells after Con A treatment.

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There were lower levels of proinflammatory cytokines in Git2^−/− mice after Con A injection.

G protein-coupled receptor kinase interactor 2 (GIT2) is a signaling scaffold protein involved in regulation of cytoskeletal dynamics and the internalization of G protein-coupled receptors (GPCRs). The short-splice form of GIT2 is expressed in peripheral T cells and thymocytes. However, the functions of GIT2 in T cells have not yet been determined. We show that treatment with Con A in a model of polyclonal T-lymphocyte activation resulted in marked inhibitions in the intrahepatic infiltration of inflammatory cells, cytokine response and acute liver failure in Git2^−/− mice. CD4⁺ T cells from Git2^−/− mice showed significant impairment in proliferation, cytokine production and signal transduction upon TCR-stimulated activation. Our results suggested that GIT2 plays an important role in T-cell function in vivo and in vitro.

Positive and negative regulation by SLP-76/ADAP and Pyk2 of chemokine-stimulated T-lymphocyte adhesion mediated by integrin α4β1

Stimulation by chemokines of integrin α4β1-dependent T-lymphocyte adhesion is a crucial step for lymphocyte trafficking. The adaptor Vav1 is required for chemokine-activated T-cell adhesion mediated by α4β1. Conceivably, proteins associating with Vav1 could potentially modulate this adhesion. Correlating with activation by the chemokine CXCL12 of T-lymphocyte attachment to α4β1 ligands, a transient stimulation in the association of Vav1 with SLP-76, Pyk2, and ADAP was observed. Using T-cells depleted for SLP-76, ADAP, or Pyk2, or expressing Pyk2 kinase-inactive forms, we show that SLP-76 and ADAP stimulate chemokine-activated, α4β1-mediated adhesion, whereas Pyk2 opposes T-cell attachment. While CXCL12-promoted generation of high-affinity α4β1 is independent of SLP-76, ADAP, and Pyk2, the strength of α4β1-VCAM-1 interaction and cell spreading on VCAM-1 are targets of regulation by these three proteins. GTPase assays, expression of activated or dominant-negative Rac1, or combined ADAP and Pyk2 silencing indicated that Rac1 activation by CXCL12 is a common mediator response in SLP-76-, ADAP-, and Pyk2-regulated cell adhesion involving α4β1. Our data strongly suggest that chemokine-stimulated associations between Vav1, SLP-76, and ADAP facilitate Rac1 activation and α4β1-mediated adhesion, whereas Pyk2 opposes this adhesion by limiting Rac1 activation.

αPIX Is a Trafficking Regulator that Balances Recycling and Degradation of the Epidermal Growth Factor Receptor

Endosomal sorting is an essential control mechanism for signaling through the epidermal growth factor receptor (EGFR). We report here that the guanine nucleotide exchange factor αPIX, which modulates the activity of Rho-GTPases, is a potent bimodal regulator of EGFR trafficking. αPIX interacts with the E3 ubiquitin ligase c-Cbl, an enzyme that attaches ubiquitin to EGFR, thereby labelling this tyrosine kinase receptor for lysosomal degradation. We show that EGF stimulation induces αPIX::c-Cbl complex formation. Simultaneously, αPIX and c-Cbl protein levels decrease, which depends on both αPIX binding to c-Cbl and c-Cbl ubiquitin ligase activity. Through interaction αPIX sequesters c-Cbl from EGFR and this results in reduced EGFR ubiquitination and decreased EGFR degradation upon EGF treatment. However, quantitatively more decisive for cellular EGFR distribution than impaired EGFR degradation is a strong stimulating effect of αPIX on EGFR recycling to the cell surface. This function depends on the GIT binding domain of αPIX but not on interaction with c-Cbl or αPIX exchange activity. In summary, our data demonstrate a previously unappreciated function of αPIX as a strong promoter of EGFR recycling. We suggest that the novel recycling regulator αPIX and the degradation factor c-Cbl closely cooperate in the regulation of EGFR trafficking: uncomplexed αPIX and c-Cbl mediate a positive and a negative feedback on EGFR signaling, respectively; αPIX::c-Cbl complex formation, however, results in mutual inhibition, which may reflect a stable condition in the homeostasis of EGF-induced signal flow.

The disease-linked Glu-26-Lys mutant version of Coronin 1A exhibits pleiotropic and pathway-specific signaling defects

Coronin 1A is involved in cell shape dynamics and Rac1 GTPase signaling. Loss-of-function mutations in the Coro1A gene promote severe immunodeficiency. An immunodeficiency-linked Coro1A point mutant is described that becomes dysfunctional due to changes in actin-binding, actin-remodeling, and signaling activities.

Coronin 1A (Coro1A) is involved in cytoskeletal and signaling events, including the regulation of Rac1 GTPase– and myosin II–dependent pathways. Mutations that generate truncated or unstable Coro1A proteins cause immunodeficiencies in both humans and rodents. However, in the case of the peripheral T-cell–deficient (Ptcd) mouse strain, the immunodeficiency is caused by a Glu-26-Lys mutation that targets a surface-exposed residue unlikely to affect the intramolecular architecture and stability of the protein. Here we report that this mutation induces pleiotropic effects in Coro1A protein, including the exacerbation of Coro1A-dependent actin-binding and -bundling activities; the formation of large meshworks of Coro1A^E26K-decorated filaments endowed with unusual organizational, functional, and staining properties; and the elimination of Coro1A functions associated with both Rac1 and myosin II signaling. By contrast, it does not affect the ability of Coro1A to stimulate the nuclear factor of activated T-cells (NF-AT). Coro1A^E26K is not a dominant-negative mutant, indicating that its pathological effects are derived from the inability to rescue the complete loss of the wild-type counterpart in cells. These results indicate that Coro1A^E26K behaves as either a recessive gain-of-function or loss-of-function mutant protein, depending on signaling context and presence of the wild-type counterpart in cells.

Pak2 is required for actin cytoskeleton remodeling, TCR signaling, and normal thymocyte development and maturation

The molecular mechanisms that govern thymocyte development and maturation are incompletely understood. The P21-activated kinase 2 (Pak2) is an effector for the Rho family GTPases Rac and Cdc42 that regulate actin cytoskeletal remodeling, but its role in the immune system remains poorly understood. In this study, we show that T-cell specific deletion of Pak2 gene in mice resulted in severe T cell lymphopenia accompanied by marked defects in development, maturation, and egress of thymocytes. Pak2 was required for pre-TCR β-selection and positive selection. Surprisingly, Pak2 deficiency in CD4 single positive thymocytes prevented functional maturation and reduced expression of S1P1 and KLF2. Mechanistically, Pak2 is required for actin cytoskeletal remodeling triggered by TCR. Failure to induce proper actin cytoskeletal remodeling impaired PLCγ1 and Erk1/2 signaling in the absence of Pak2, uncovering the critical function of Pak2 as an essential regulator that governs the actin cytoskeleton-dependent signaling to ensure normal thymocyte development and maturation.

DOI:http://dx.doi.org/10.7554/eLife.02270.001

T cells are a key element of the immune system. There are many different types of T cells, and they all have their origins in hematopoietic stem cells that are found in the bone marrow. These stem cells leave the bone marrow and circulate in the body until they reach an organ called the thymus, where they become early thymic progenitor cells. These progenitor cells then undergo a process called differentiation to become specific types of T cells, which mature in the thymus before moving to the blood. Although various molecules and mechanisms are known to be involved in the development of T cells, many details of this process are not understood.

One group of molecules that has been implicated in the differentiation of T cells is the p21-activated kinases. Kinases are proteins that activate or deactivate other proteins by adding phosphate groups to specific amino acids. Pak2 adds phosphorylate groups to various proteins that are involved in the reorganization of an important structure inside the cell called the cytoskeleton.

A kinase called Pak2 has an important role in the reorganization of the cytoskeleton, and since this reorganization is involved in almost all aspects of T cell biology, it seems plausible that Pak2 is also involved in the development of T cells. However, it has not been possible to test this idea because deleting the gene for Pak2 in mice results in their death.

Now, Phee et al. have overcome this problem by performing experiments in which the gene for Pak2 was only deleted in T cells. These mice had significantly fewer mature T cells than healthy mice. In particular, the absence of Pak2 in thymocytes (the cells that become T cells) prevented them from maturing into T cells, and also prevented them from producing a receptor protein that is needed for mature T cells to leave the thymus. This work implies that disruption of the Pak2-mediated signaling pathway that regulates the cytoskeleton may weaken the immune system in humans.

DOI:http://dx.doi.org/10.7554/eLife.02270.002

Protein kinase C-η controls CTLA-4-mediated regulatory T cell function

Regulatory T cells (T_reg cells), which maintain immune homeostasis and self-tolerance, form an immunological synapse (IS) with antigen-presenting cells (APCs). However, signaling events at the T_reg IS remain unknown. Here we show that protein kinase C-η (PKC-η) associated with CTLA-4 and was recruited to the T_reg IS. PKC-η-deficient T_reg cells displayed defective suppressive activity, including suppression of tumor immunity but not autoimmune colitis. Phosphoproteomic analysis revealed an association between CTLA-4-PKC-η and the GIT-PIX-PAK complex, an IS-localized focal adhesion complex. Defective activation of this complex in PKC-η-deficient T_reg cells was associated with reduced CD86 depletion from APCs by T_reg cells. These results reveal a novel CTLA-4-PKC-η signaling axis required for contact-dependent suppression, implicating this pathway as a potential cancer immunotherapy target.

Quantitative phosphoproteome analysis unveils LAT as a modulator of CD3ζ and ZAP-70 tyrosine phosphorylation

Signaling through the T cell receptor (TCR) initiates adaptive immunity and its perturbation may results in autoimmunity. The plasma membrane scaffolding protein LAT acts as a central organizer of the TCR signaling machinery to activate many functional pathways. LAT-deficient mice develop an autoimmune syndrome but the mechanism of this pathology is unknown. In this work we have compared global dynamics of TCR signaling by MS-based quantitative phosphoproteomics in LAT-sufficient and LAT-defective Jurkat T cells. Surprisingly, we found that many TCR-induced phosphorylation events persist in the absence of LAT, despite ERK and PLCγ1 phosphorylation being repressed. Most importantly, the absence of LAT resulted in augmented and persistent tyrosine phosphorylation of CD3ζ and ZAP70. This indicates that LAT signaling hub is also implicated in negative feedback signals to modulate upstream phosphorylation events. Phosphorylation kinetics data resulting from this investigation is documented in a database (phosphoTCR) accessible online. The MS data have been deposited to the ProteomeXchange with identifier PXD000341.

Regulation of ras exchange factors and cellular localization of ras activation by lipid messengers in T cells

The Ras-MAPK signaling pathway is highly conserved throughout evolution and is activated downstream of a wide range of receptor stimuli. Ras guanine nucleotide exchange factors (RasGEFs) catalyze GTP loading of Ras and play a pivotal role in regulating receptor-ligand induced Ras activity. In T cells, three families of functionally important RasGEFs are expressed: RasGRF, RasGRP, and Son of Sevenless (SOS)-family GEFs. Early on it was recognized that Ras activation is critical for T cell development and that the RasGEFs play an important role herein. More recent work has revealed that nuances in Ras activation appear to significantly impact T cell development and selection. These nuances include distinct biochemical patterns of analog versus digital Ras activation, differences in cellular localization of Ras activation, and intricate interplays between the RasGEFs during distinct T cell developmental stages as revealed by various new mouse models. In many instances, the exact nature of these nuances in Ras activation or how these may result from fine-tuning of the RasGEFs is not understood. One large group of biomolecules critically involved in the control of RasGEFs functions are lipid second messengers. Multiple, yet distinct lipid products are generated following T cell receptor (TCR) stimulation and bind to different domains in the RasGRP and SOS RasGEFs to facilitate the activation of the membrane-anchored Ras GTPases. In this review we highlight how different lipid-based elements are generated by various enzymes downstream of the TCR and other receptors and how these dynamic and interrelated lipid products may fine-tune Ras activation by RasGEFs in developing T cells.

Rac1 recruitment to the archipelago structure of the focal adhesion through the fluid membrane as revealed by single-molecule analysis

The focal adhesion (FA) is an integrin-based structure built in/on the plasma membrane (PM), linking the extracellular matrix to the actin stress-fibers, working as cell migration scaffolds. Previously, we proposed the archipelago architecture of the FA, in which FA largely consists of fluid membrane, dotted with small islands accumulating FA proteins: membrane molecules enter the inter-island channels in the FA zone rather freely, and the integrins in the FA-protein islands rapidly exchanges with those in the bulk membrane. Here, we examined how Rac1, a small G-protein regulating FA formation, and its activators αPIX and βPIX, are recruited to the FA zones. PIX molecules are recruited from the cytoplasm to the FA zones directly. In contrast, majorities of Rac1 molecules first arrive from the cytoplasm on the general inner PM surface, and then enter the FA zones via lateral diffusion on the PM, which is possible due to rapid Rac1 diffusion even within the FA zones, slowed only by a factor of two to four compared with that outside. The constitutively-active Rac1 mutant exhibited temporary and all-time immobilizations in the FA zone, suggesting that upon PIX-induced Rac1 activation at the FA-protein islands, Rac1 tends to be immobilized at the FA-protein islands.

LAT-independent Erk activation via Bam32-PLC-γ1-Pak1 complexes: GTPase-independent Pak1 activation

In T cells, the adaptor Bam32 is coupled to Erk activation downstream of the TCR by an unknown mechanism. We characterized in Jurkat cells and primary T lymphocytes a pathway dependent on Bam32-PLC-γ1-Pak1 complexes, in which Pak1 kinase activates Raf-1 and Mek-1, both upstream of Erk. In the Bam32-PLC-γ1-Pak1 complex, catalytically inactive PLC-γ1 is used as a scaffold linking Bam32 to Pak1. PLC-γ1(C-SH2) directly binds S141 of Bam32, preventing LAT-mediated activation of Ras by PLC-γ1. The Bam32-PLC-γ1 interaction enhances the binding of the SH3 domain of the phospholipase with Pak1. The PLC-γ1(SH3)-Pak1 interaction activates Pak1 independently of the small GTPases Rac1/Cdc42, previously described as being the only activators of Pak1 in T cells. Direct binding of the SH3 domain of PLC-γ1 to Pak1 dissociates inactive Pak1 homodimers, a mechanism required for Pak1 activation. We have thus uncovered a LAT/Ras-independent, Bam32-nucleated pathway that activates Erk signaling in T cells.

The adaptor protein and Arf GTPase-activating protein Cat-1/Git-1 is required for cellular transformation

Cat-1/Git-1 is a multifunctional protein that acts as a GTPase-activating protein (GAP) for Arf GTPases, as well as serves as a scaffold for a number of different signaling proteins. Cat-1 is best known for its role in regulating cell shape and promoting cell migration. However, whether Cat-1 might also contribute to cellular transformation is currently unknown. Here we show that ∼95% of cervical tumor samples examined overexpress Cat-1, suggesting that the up-regulation of Cat-1 expression is a frequent occurrence in this type of cancer. We demonstrate further that knocking down Cat-1 from NIH3T3 fibroblasts expressing an activated form of Cdc42 (Cdc42 F28L), or from the human cervical carcinoma (HeLa) cell line, inhibits the ability of these cells to form colonies in soft agar, an in vitro measure of tumorgenicity. The requirement for Cat-1 when assaying the anchorage-independent growth of transformed fibroblasts and HeLa cells is dependent on its ability to bind paxillin, while being negatively impacted by its Arf-GAP activity. Moreover, the co-expression of Cat-1 and an activated form of Arf6 in fibroblasts was sufficient to induce their transformation. These findings highlight novel roles for Cat-1 and its interactions with the Arf GTPases and paxillin in oncogenic transformation.

Feedback circuits monitor and adjust basal Lck-dependent events in T cell receptor signaling

The Src family kinase Lck is crucial for the initiation of TCR signaling. The activity of Lck is tightly controlled to prevent erroneous immune activation, yet it enables rapid cellular responses over a range of sensitivities to antigens. Here, in experiments with an analog-sensitive variant of the tyrosine kinase Csk, we report that Lck in T cells is dynamically controlled by an equilibrium between Csk and the tyrosine phosphatase CD45. By rapidly inhibiting Csk, we showed that changes in this equilibrium were sufficient to activate canonical TCR signaling pathways independently of ligand binding to the TCR. The activated signaling pathways showed sustained and enhanced phosphorylation compared to that in TCR-stimulated cells, revealing a feedback circuit that was sensitive to the basal signaling machinery. We identified the inhibitory adaptor molecule Dok-1 (downstream of kinase 1) as a candidate that may respond to alterations in basal signaling activity. Our results also suggest a role for Csk in the termination or dampening of TCR signals.

Coronin 1A promotes a cytoskeletal-based feedback loop that facilitates Rac1 translocation and activation

The activation of the Rac1 GTPase during cell signalling entails its translocation from the cytosol to membranes, release from sequestering Rho GDP dissociation inhibitors (RhoGDI), and GDP/GTP exchange. In addition to those steps, we show here that optimal Rac1 activation during cell signalling requires the engagement of a downstream, cytoskeletal-based feedback loop nucleated around the cytoskeletal protein coronin 1A and the Rac1 exchange factor ArhGEF7. These two proteins form a cytosolic complex that, upon Rac1-driven F-actin polymerization, translocates to juxtamembrane areas where it expands the pool of activated, membrane-bound Rac1. Such activity requires the formation of an F-actin/ArhGEF7-dependent physical complex of coronin 1A with Pak1 and RhoGDIα that, once assembled, promotes the Pak1-dependent dissociation of Rac1 from the Rac1/RhoGDIα complex and subsequent Rac1 activation. Genetic evidence demonstrates that this relay circuit is essential for generating sustained Rac1 activation levels during cell signalling.

Stromal cell-derived factor-1 signaling via the CXCR4-TCR heterodimer requires phospholipase C-β3 and phospholipase C-γ1 for distinct cellular responses

The CXCR4 chemokine receptor is a G protein-coupled receptor that signals in T lymphocytes by forming a heterodimer with the TCR. CXCR4 and TCR functions are consequently highly cross regulated, affecting T cell immune activation, cytokine secretion, and T cell migration. The CXCR4-TCR heterodimer stimulates T cell migration and activation of the ERK MAPK and downstream AP-1-dependent cytokine transcription in response to stromal cell-derived factor-1 (SDF-1), the sole chemokine ligand of CXCR4. These responses require Gi-type G proteins as well as TCR ITAM domains and the ZAP70 tyrosine kinase, thus indicating that the CXCR4-TCR heterodimer signals to integrate G protein-coupled receptor-associated and TCR-associated signaling molecules in response to SDF-1. Yet, the phospholipase C (PLC) isozymes responsible for coupling the CXCR4-TCR heterodimer to distinct downstream cellular responses are incompletely characterized. In this study, we demonstrate that PLC activity is required for SDF-1 to induce ERK activation, migration, and CXCR4 endocytosis in human T cells. SDF-1 signaling via the CXCR4-TCR heterodimer uses PLC-β3 to activate the Ras-ERK pathway and increase intracellular calcium ion concentrations, whereas PLC-γ1 is dispensable for these outcomes. In contrast, PLC-γ1, but not PLC-β3, is required for SDF-1-mediated migration via a mechanism independent of LAT. These results increase understanding of the signaling mechanisms employed by the CXCR4-TCR heterodimer, characterize new roles for PLC-β3 and PLC-γ1 in T cells, and suggest that multiple PLCs may also be activated downstream of other chemokine receptors to distinctly regulate migration versus other signaling functions.

T cell receptor (TCR)-induced tyrosine phosphorylation dynamics identifies THEMIS as a new TCR signalosome component

Stimulation of the T cell antigen receptor (TCR) induces formation of a phosphorylation-dependent signaling network via multiprotein complexes, whose compositions and dynamics are incompletely understood. Using stable isotope labeling by amino acids in cell culture (SILAC)-based quantitative proteomics, we investigated the kinetics of signal propagation after TCR-induced protein tyrosine phosphorylation. We confidently assigned 77 proteins (of 758 identified) as a direct or indirect consequence of tyrosine phosphorylation that proceeds in successive "signaling waves" revealing the temporal pace at which tyrosine kinases activate cellular functions. The first wave includes thymocyte-expressed molecule involved in selection (THEMIS), a protein recently implicated in thymocyte development but whose signaling role is unclear. We found that tyrosine phosphorylation of THEMIS depends on the presence of the scaffold proteins Linker for activation of T cells (LAT) and SH2 domain-containing lymphocyte protein of 76 kDa (SLP-76). THEMIS associates with LAT, presumably via the adapter growth factor receptor-bound protein 2 (Grb2) and with phospholipase Cγ1 (PLC-γ1). RNAi-mediated THEMIS knock-down inhibited TCR-induced IL-2 gene expression due to reduced ERK and nuclear factor of activated T cells (NFAT)/activator protein 1 (AP-1) signaling, whereas JNK, p38, or nuclear factor κB (NF-κB) activation were unaffected. Our study reveals the dynamics of TCR-dependent signaling networks and suggests a specific role for THEMIS in early TCR signalosome function.

Coordination of IL-7 receptor and T-cell receptor signaling by cell-division cycle 42 in T-cell homeostasis

T-cell homeostasis is essential for normal functioning of the immune system. IL-7 receptor (IL-7R) and T-cell receptor (TCR) signaling are pivotal for T-cell homeostatic regulation. The detailed mechanisms regulating T-cell homeostasis and how IL-7R and TCR signaling are coordinated are largely unknown. Here we demonstrate that T cell-specific deletion of cell-division cycle 42 (Cdc42) GTPase causes a profound loss of mature T cells. Deletion of Cdc42 leads to a markedly increased expression of growth factor independence-1 (Gfi-1) and represses expression of IL-7Rα. In the absence of Cdc42, aberrant ERK1/2 MAP kinase activity results in enhanced, TCR-mediated T-cell proliferation. In vivo reconstitution of effector-binding-defective Cdc42 mutants and the effector p21 protein-activated kinase 1 (PAK1) into Cdc42-deficient T cells showed that PAK1 is both necessary and sufficient for Cdc42-regulated T-cell homeostasis. Thus, T-cell homeostasis is maintained through a concerted regulation of Gfi-1-IL-7R-controlled cytokine responsiveness and ERK-mediated TCR signaling strength by the Cdc42-PAK1 signaling axis.

The adapter protein Nck: role of individual SH3 and SH2 binding modules for protein interactions in T lymphocytes

Nck is a ubiquitously expressed, primarily cytosolic adapter protein consisting of one SH2 domain and three SH3 domains. It links receptor and nonreceptor tyrosine kinases to actin cytoskeleton reorganizing proteins. In T lymphocytes, Nck is a crucial component of signaling pathways for T cell activation and effector function. It recruits actin remodeling proteins to T cell receptor (TCR)-associated activation clusters and thereby initiates changes in cell polarity and morphology. Moreover, Nck is crucial for the TCR-induced mobilization of secretory vesicles to the cytotoxic immunological synapse. To identify the interactome of Nck in human T cells, we performed a systematic screen for interaction partners in untreated or pervanadate-treated cells. We used GST fusion proteins containing full length Nck, the combined SH3 domains or the individual SH3 and SH2 domains to precipitate putative Nck interactors from cellular lysates. Protein bands were excised from gels, processed by tryptic in-gel digestion and analyzed by mass spectrometry. Using this approach, we confirmed previously established interactions (e.g., with Slp76, CD3 epsilon, WASP, and WIPF1) and identified several novel putative Nck-binding proteins. We subsequently verified the SH2 domain binding to the actin-binding protein HIP55 and to FYB/ADAP, and the SH3-mediated binding to the nuclear proteins SFPQ/NONO. Using laser scanning microscopy, we provide new evidence for a nuclear localization of Nck in human T cells. Our data highlight the fundamental role of Nck in the TCR-to-cytoskeleton crosstalk and point to yet unknown nuclear functions of Nck also in T lymphocytes.

TCR-induced activation of Ras proceeds at the plasma membrane and requires palmitoylation of N-Ras

Ras transmits manifold signals from the TCR at various crossroads in the life of a T cell. For example, selection programs in the thymus or the acquisition of a state of hypo-responsiveness known as anergy are just some of the T cell features known to be controlled by TCR-sparked signals that are intracellularly propagated by Ras. These findings raise the question of how Ras can transmit such a variety of signals leading to the shaping of equally many T cell traits. Because Ras proteins transit through endomembrane compartments on their way to the plasma membrane (PM), compartmentalized Ras activation at distinct subcellular sites represents a potential mechanism for signal diversification in TCR signaling. This hypothesis has been nurtured by studies in T cells engineered to overexpress Ras that reported distinct activation of Ras at the PM and Golgi. Contrary to this scenario, we report in this study that activation of endogenous Ras, imaged in live Jurkat T cells using novel affinity probes for Ras-GTP, proceeds only at the PM even upon enforced signal flux through the diacylglycerol/RasGRP1 pathway. Physiological engagement of the TCR at the immunological synapse in primary T cells caused focalized Ras-GTP accumulation also only at the PM. Analysis of palmitoylation-deficient Ras mutants, which are confined to endomembranes, confirmed that the TCR does not activate Ras in that compartment and revealed a critical function for palmitoylation in N-Ras/H-Ras activation. These findings identify the PM as the only site of TCR-driven Ras activation and document that endomembranes are not a signaling platform for Ras in T cells.

Mediation of T-cell activation by actin meshworks

Although the actin cytoskeleton and T-cell receptor (TCR) signaling complexes are seemingly distinct molecular structures, they are tightly integrated in T cells. The signaling pathways initiated by TCRs binding to peptide MHC complexes are extensively influenced by the actin cytoskeletal activities of the motile phase before TCR signaling, the signalosome scaffolding function of the cytoskeleton, and the translocation of signaling clusters that precedes the termination of signaling at these complexes. As these three successive phases constitute essentially all the steps consequent to immune synapse formation, it has become clear that the substantial physical forces and signaling interactions generated by the actin cytoskeleton dominate the signaling life cycle of TCR signalosomes. We discuss the contributions of the actin cytoskeleton to TCR signaling phases and model some remaining questions about how specific cytoskeletal factors regulate TCR signaling outcomes.

Regulation of thymocyte positive selection and motility by GIT2

Thymocytes are highly motile cells that migrate under the influence of chemokines in distinct thymic compartments as they mature. The motility of thymocytes is tightly regulated; however, the molecular mechanisms that control thymocyte motility are not well understood. Here we report that G protein-coupled receptor kinase-interactor 2 (GIT2) was required for efficient positive selection. Notably, Git2(-/-) double-positive thymocytes showed greater activation of the small GTPase Rac, actin polymerization and migration toward the chemokines CXCL12 (SDF-1) and CCL25 in vitro. By two-photon laser-scanning microscopy, we found that the scanning activity of Git2(-/-) thymocytes was compromised in the thymic cortex, which suggests GIT2 has a key role in regulating the chemokine-mediated motility of double-positive thymocytes.

Rho family GTPases and their regulators in lymphocytes

Rho family GTPases, and the proteins that regulate them, have important roles in many cellular processes, including cell division, survival, migration and adhesion. Although most of our understanding of these proteins has come from studies using cell lines, more recent gene targeting studies in mice are providing insights into the in vivo function of these proteins. Here we review recent progress revealing crucial roles for these proteins in lymphocyte development, activation, differentiation and migration. The emerging picture shows that Rho family GTPases transduce signals from receptors for antigens, chemokines and cytokines, as well as adhesion molecules and pattern recognition receptors, and that they function as focal points for crosstalk between different signalling pathways.

Vav links the T cell antigen receptor to the actin cytoskeleton and T cell activation independently of intrinsic Guanine nucleotide exchange activity

Background

T cell receptor (TCR) engagement leads to formation of signaling microclusters and induction of rapid and dynamic changes in the actin cytoskeleton, although the exact mechanism by which the TCR initiates actin polymerization is incompletely understood. The Vav family of guanine nucleotide exchange factors (GEF) has been implicated in generation of TCR signals and immune synapse formation, however, it is currently not known if Vav's GEF activity is required in T cell activation by the TCR in general, and in actin polymerization downstream of the TCR in particular.

Methodology/Principal Findings

Here, we report that Vav1 assembles into signaling microclusters at TCR contact sites and is critical for TCR-initiated actin polymerization. Surprisingly, Vav1 functions in TCR signaling and Ca⁺⁺ mobilization via a mechanism that does not appear to strictly depend on the intrinsic GEF activity.

Conclusions/Significance

We propose here a model in which Vav functions primarily as a tyrosine phosphorylated linker-protein for TCR activation of T cells. Our results indicate that, contrary to expectations based on previously published studies including from our own laboratory, pharmacological inhibition of Vav1's intrinsic GEF activity may not be an effective strategy for T cell-directed immunosuppressive therapy.

Integrating receptor signal inputs that influence small Rho GTPase activation dynamics at the immunological synapse

The Rho GTPase Cdc42 regulates cytoskeletal changes at the immunological synapse (IS) that are critical to T-cell activation. By imaging fluorescent activity biosensors (Raichu) using fluorescence lifetime imaging microscopy, Cdc42 activation was shown to display kinetics that are conditional on the specific receptor input (through two IS-associated receptors, CD3 and beta1 integrin). CD3-triggered Cdc42 activity is dependent on the cyto-2 (NPIY) motif of the beta1 integrin cytoplasmic domain. Perturbations of the ezrin-radixin-moesin (ERM) function blocked CD3- and beta1-dependent increases in Cdc42 activity. Both IS-associated receptors probably lie on a serial molecular pathway and transduce signals through the ERM-dependent machinery that is responsible for the remodeling and stabilization of the synapse. Cdc42 activity is impaired in beta1 integrin-deficient T cells that form conjugates with antigen-presenting cells but is partially restored in the context of an antigen-specific synapse. This restoration of Cdc42 activity is due, at least in part, to the recruitment and activation of beta2 integrin.

EphA2 engages Git1 to suppress Arf6 activity modulating epithelial cell-cell contacts

ADP-ribosylation factor (Arf) 6 activity is crucially involved in the regulation of E-cadherin-based cell-cell adhesions. Erythropoietin-producing hepatocellular carcinoma (Eph)-family receptors recognize ligands, namely, ephrins, anchored to the membrane of apposing cells, and they mediate cell-cell contact-dependent events. Here, we found that Arf6 activity is down-regulated in Madin-Darby canine kidney cells, which is dependent on cell density and calcium ion concentration, and we provide evidence of a novel signaling pathway by which ligand-activated EphA2 suppresses Arf6 activity. This EphA2-mediated suppression of Arf6 activity was linked to the induction of cell compaction and polarization, but it was independent of the down-regulation of extracellular signal-regulated kinase 1/2 kinase activity. We show that G protein-coupled receptor kinase-interacting protein (Git) 1 and noncatalytic region of tyrosine kinase (Nck) 1 are involved in this pathway, in which ligand-activated EphA2, via its phosphorylated Tyr594, binds to the Src homology 2 domain of Nck1, and then via its Src homology 3 domain binds to the synaptic localizing domain of Git1 to suppress Arf6 activity. We propose a positive feedback loop in which E-cadherin-based cell-cell contacts enhance EphA-ephrinA signaling, which in turn down-regulates Arf6 activity to enhance E-cadherin-based cell-cell contacts as well as the apical-basal polarization of epithelial cells.

Nck adapter proteins: functional versatility in T cells

Nck is a ubiquitously expressed adapter protein that is almost exclusively built of one SH2 domain and three SH3 domains. The two isoproteins of Nck are functionally redundant in many aspects and differ in only few amino acids that are mostly located in the linker regions between the interaction modules. Nck proteins connect receptor and non-receptor tyrosine kinases to the machinery of actin reorganisation. Thereby, Nck regulates activation-dependent processes during cell polarisation and migration and plays a crucial role in the signal transduction of a variety of receptors including for instance PDGF-, HGF-, VEGF- and Ephrin receptors. In most cases, the SH2 domain mediates binding to the phosphorylated receptor or associated phosphoproteins, while SH3 domain interactions lead to the formation of larger protein complexes. In T lymphocytes, Nck plays a pivotal role in the T cell receptor (TCR)-induced reorganisation of the actin cytoskeleton and the formation of the immunological synapse. However, in this context, two different mechanisms and adapter complexes are discussed. In the first scenario, dependent on an activation-induced conformational change in the CD3epsilon subunits, a direct binding of Nck to components of the TCR/CD3 complex was shown. In the second scenario, Nck is recruited to the TCR complex via phosphorylated Slp76, another central constituent of the membrane proximal activation complex. Over the past years, a large number of putative Nck interactors have been identified in different cellular systems that point to diverse additional functions of the adapter protein, e.g. in the control of gene expression and proliferation.

Structures of dimeric GIT1 and trimeric beta-PIX and implications for GIT-PIX complex assembly

GIT (G protein-coupled receptor kinase-interacting protein) and PIX (p21-activated kinase-interacting exchange factor) family proteins integrate signaling pathways involving Arf and Rho family GTPases. GIT1 and beta-PIX form a constitutively associated complex that acts as a scaffold to allow the formation of large multiprotein assemblies that regulate synaptogenesis, cell polarity and cell migration among other physiological processes. Complex formation is mediated by the GIT binding domain (GBD) in beta-PIX, which recognizes the Spa homology domain of GIT1. Both binding domains are adjacent to predicted coiled-coil segments that allow homo-oligomerization of GIT1 and beta-PIX, respectively. Oligomerization of GIT and PIX proteins is important for their physiological functions, and deletion of the coiled-coil domains interferes with correct subcellular localization and the GEF (guanine nucleotide exchange factor) activity of PIX. We have solved the crystal structures of the CC domains of GIT1 and beta-PIX and determined the stoichiometry of complex formation between the two proteins in order to understand the molecular architecture of the GIT1-beta-PIX complex. The crystal structure of the CC domain of GIT1 solved at 1.4 A resolution shows a dimeric, parallel CC that spans 67 A in length. Unexpectedly, and in contrast to prevalent dimeric models, the structure of the CC region of beta-PIX determined at 2.8 A resolution, combined with hydrodynamic studies, reveals that this protein forms a parallel trimer. Furthermore, we demonstrate that dimeric GIT and trimeric PIX form an unusual high-affinity heteropentameric complex in which each Spa homology domain of the GIT1 dimer recognizes one GBD of the beta-PIX trimer, leaving one GBD unoccupied. These results can serve as a basis to better understand oligomerization-dependent GIT1-beta-PIX-regulated signaling events and provide an insight into the architecture of large signaling complexes involving GIT1 and beta-PIX.

A RAC/CDC-42-independent GIT/PIX/PAK signaling pathway mediates cell migration in C. elegans

P21 activated kinase (PAK), PAK interacting exchange factor (PIX), and G protein coupled receptor kinase interactor (GIT) compose a highly conserved signaling module controlling cell migrations, immune system signaling, and the formation of the mammalian nervous system. Traditionally, this signaling module is thought to facilitate the function of RAC and CDC-42 GTPases by allowing for the recruitment of a GTPase effector (PAK), a GTPase activator (PIX), and a scaffolding protein (GIT) as a regulated signaling unit to specific subcellular locations. Instead, we report here that this signaling module functions independently of RAC/CDC-42 GTPases in vivo to control the cell shape and migration of the distal tip cells (DTCs) during morphogenesis of the Caenorhabditis elegans gonad. In addition, this RAC/CDC-42–independent PAK pathway functions in parallel to a classical GTPase/PAK pathway to control the guidance aspect of DTC migration. Among the C. elegans PAKs, only PAK-1 functions in the GIT/PIX/PAK pathway independently of RAC/CDC42 GTPases, while both PAK-1 and MAX-2 are redundantly utilized in the GTPase/PAK pathway. Both RAC/CDC42–dependent and –independent PAK pathways function with the integrin receptors, suggesting that signaling through integrins can control the morphology, movement, and guidance of DTC through discrete pathways. Collectively, our results define a new signaling capacity for the GIT/PIX/PAK module that is likely to be conserved in vertebrates and demonstrate that PAK family members, which are redundantly utilized as GTPase effectors, can act non-redundantly in pathways independent of these GTPases.

Cell migration is essential for the development and maintenance of metazoan tissue. A migrating cell must navigate through complex environments and properly interpret the signals present in its path. This cellular movement is accomplished through transduction of the signals into directed reorganization of the cellular structure. Among the most important molecules that orchestrate signals from the exterior of the cells into cellular movement are the small GTPases, which function in intracellular signal transduction cascades. We have studied the interactions between GTPases, their effectors, and the environmental signals during cellular migrations in C. elegans. We have found that while some GTPases do control the guidance of these migrating cells, a certain highly conserved complex of proteins thought to be involved in mediating GTPase signaling during cellular migrations in fact functions independently of these GTPases to specifically control the structure and movement of the migrating cells. These results have revealed an unexpected role of a well-known and highly conserved signaling complex, which is particularly important since members of this complex are associated with human mental retardation. Our results may imply that the disease phenotype is likely more complex than previously thought and may in fact occur from disruption of this novel pathway.

Differently phosphorylated forms of the cortactin homolog HS1 mediate distinct functions in natural killer cells

Here we investigated the involvement of HS1, the hematopoietic cell-specific homolog of cortactin, in the actin-based functions of natural killer cells. Involvement of HS1 in T cell regulation has been established, as HS1 is required for the formation of immune synapses. 'Knockdown' of HS1 in natural killer cells resulted in defective lysis of target cells, cell adhesion, chemotaxis and actin assembly at the lytic synapse. Phosphorylation of the tyrosine residue at position 397 (Tyr397) was required for adhesion to the integrin ligand ICAM-1 and for cytolysis, whereas phosphorylation of Tyr378 was required for chemotaxis. Phosphorylation of Tyr397 was also required for integrin signaling and recruitment of integrins, adaptors and actin to the lytic synapse. Thus, HS1 is essential for signaling and actin assembly in natural killer cells, and the functions of the two phosphorylated tyrosine residues are distinct and separable.