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

Genetically encodable biosensors for Ras activity

Genetically encoded Ras biosensors have been instrumental in illuminating the spatiotemporal dynamics of Ras activity since the beginning of the imaging revolution of the early 21st century. In general, these sensors employ Ras sensing units coupled with fluorescent proteins. These biosensors have not only helped elucidate Ras signalling dynamics at the plasma membrane but also revealed novel roles for Ras signalling within subcellular compartments such as the Golgi apparatus. In this review, we discuss the different classes of biosensors used to measure Ras activity and discuss their importance in uncovering new roles for Ras activity in cellular signalling and behavior.

Stratification of patients with KRAS-mutated advanced non-small cell lung cancer: improving prognostics

INTRODUCTION

KRAS is the most frequently mutated oncogene in cancer and encodes a key signaling protein in tumors. Due to its high affinity for GTP and the lack of a large binding pocket that allosteric inhibitors can occupy, KRAS has long been considered 'non-druggable.' Finding effective treatment measures for patients with KRAS mutations is our top priority.

AREAS COVERED

In this article, we will provide an overview of the KRAS pathway and review the current state of therapeutic strategies for targeting oncogenic KRAS, as well as their potential to improve outcomes in patients with KRAS-mutant malignancies. We will also discuss the development of these strategies and gave an outlook on prospects.

EXPERT OPINION

KRAS mutations have posed a significant challenge in the treatment of advanced non-small cell lung cancer (NSCLC) over the past few decades. However, the emergence of immunotherapy and KRAS inhibitors, such as Sotorasib (AMG 510) and Adagrasib (MRTX849), has marked a new era in cancer therapy. As more research and clinical trials continue, we anticipate the development of more effective treatment strategies and better options for lung cancer patients.

Wiskott-Aldrich syndrome protein interacts and inhibits diacylglycerol kinase alpha promoting IL-2 induction

Background

Phosphorylation of diacylglycerol by diacylglycerol-kinases represents a major inhibitory event constraining T cell activation upon antigen engagement. Efficient TCR signalling requires the inhibition of the alpha isoform of diacylglycerol kinase, DGKα, by an unidentified signalling pathway triggered by the protein adaptor SAP. We previously demonstrated that, in SAP absence, excessive DGKα activity makes the T cells resistant to restimulation-induced cell death (RICD), an apoptotic program counteracting excessive T cell clonal expansion.

Results

Herein, we report that the Wiskott-Aldrich syndrome protein (WASp) inhibits DGKα through a specific interaction of the DGKα recoverin homology domain with the WH1 domain of WASp. Indeed, WASp is necessary and sufficient for DGKα inhibition, and this WASp function is independent of ARP2/3 activity. The adaptor protein NCK-1 and the small G protein CDC42 connect WASp-mediated DGKα inhibition to SAP and the TCR signalosome. In primary human T cells, this new signalling pathway is necessary for a full response in terms of IL-2 production, while minimally affecting TCR signalling and restimulation-induced cell death. Conversely, in T cells made resistant to RICD by SAP silencing, the enhanced DAG signalling due to DGKα inhibition is sufficient to restore apoptosis sensitivity.

Conclusion

We discover a novel signalling pathway where, upon strong TCR activation, the complex between WASp and DGKα blocks DGKα activity, allowing a full cytokine response.

CD6-mediated inhibition of T cell activation via modulation of Ras

BACKGROUND

CD6 is one of many cell surface receptors known to regulate signal transduction upon T cell activation. However, whether CD6 mediates costimulatory or inhibitory signals is controversial. When T cells engage with antigen presenting cells (APCs), CD6 interacts with its ligand CD166 at the cell-cell interface while the cytosolic tail assembles a complex signalosome composed of adaptors and effector enzymes, that may either trigger activating signaling cascades, or instead modulate the intensity of signaling. Except for a few cytosolic adaptors that connect different components of the CD6 signalosome, very little is known about the mechanistic effects of the cytosolic effectors that bind CD6.

METHODS

Jurkat model T cells were transfected to express wild-type (WT) CD6, or a cytoplasmic truncation, signaling-disabled mutant, CD6Δcyt. The two resulting cell lines were directly activated by superantigen (sAg)-loaded Raji cells, used as APCs, to assess the net signaling function of CD6. The Jurkat cell lines were further adapted to express a FRET-based unimolecular HRas biosensor that reported the activity of this crucial GTPase at the immunological synapse.

RESULTS

We show that deletion of the cytosolic tail of CD6 enhances T-cell responses, indicating that CD6 restrains T-cell activation. One component of the CD6-associated inhibitory apparatus was found to be the GTPase activating protein of Ras (RasGAP), that we show to associate with CD6 in a phosphorylation-dependent manner. The FRET HRas biosensor that we developed was demonstrated to be functional and reporting the activation of the T cell lines. This allowed to determine that the presence of the cytosolic tail of CD6 results in the down-regulation of HRas activity at the immunological synapse, implicating this fundamental GTPase as one of the targets inhibited by CD6.

CONCLUSIONS

This study provides the first description of a mechanistic sequence of events underlying the CD6-mediated inhibition of T-cell activation, involving the modulation of the MAPK pathway at several steps, starting with the coupling of RasGAP to the CD6 signalosome, the repression of the activity of Ras, and culminating in the reduction of ERK1/2 phosphorylation and of the expression of the T-cell activation markers CD69 and IL-2R α chain. Video abstract.

Regulation of T cell function by protein S-acylation

S-acylation, the reversible lipidation of free cysteine residues with long-chain fatty acids, is a highly dynamic post-translational protein modification that has recently emerged as an important regulator of the T cell function. The reversible nature of S-acylation sets this modification apart from other forms of protein lipidation and allows it to play a unique role in intracellular signal transduction. In recent years, a significant number of T cell proteins, including receptors, enzymes, ion channels, and adaptor proteins, were identified as S-acylated. It has been shown that S-acylation critically contributes to their function by regulating protein localization, stability and protein-protein interactions. Furthermore, it has been demonstrated that zDHHC protein acyltransferases, the family of enzymes mediating this modification, also play a prominent role in T cell activation and differentiation. In this review, we aim to highlight the diversity of proteins undergoing S-acylation in T cells, elucidate the mechanisms by which reversible lipidation can impact protein function, and introduce protein acyltransferases as a novel class of regulatory T cell proteins.

Fluorescent Biosensor for Measuring Ras Activity in Living Cells

The small GTPase Ras is a critical regulator of cell growth and proliferation. Its activity is frequently dysregulated in cancers, prompting decades of work to pharmacologically target Ras. Understanding Ras biology and developing effective Ras therapeutics both require probing Ras activity in its native context, yet tools to measure its activities in cellulo are limited. Here, we developed a ratiometric Ras activity reporter (RasAR) that provides quantitative measurement of Ras activity in living cells with high spatiotemporal resolution. We demonstrated that RasAR can probe live-cell activities of all the primary isoforms of Ras. Given that the functional roles of different isoforms of Ras are intimately linked to their subcellular distribution and regulation, we interrogated the spatiotemporal regulation of Ras utilizing subcellularly targeted RasAR and uncovered the role of Src kinase as an upstream regulator to inhibit HRas. Furthermore, we showed that RasAR enables capture of KRasG12C inhibition dynamics in living cells upon treatment with KRasG12C covalent inhibitors, including ARS1620, Sotorasib, and Adagrasib. We found in living cells a residual Ras activity lingers for hours in the presence of these inhibitors. Together, RasAR represents a powerful molecular tool to enable live-cell interrogation of Ras activity and facilitate the development of Ras inhibitors.

Immune Checkpoint Receptors Signaling in T Cells

The characterization of the receptors negatively modulating lymphocyte function is rapidly advancing, driven by success in tumor immunotherapy. As a result, the number of immune checkpoint receptors characterized from a functional perspective and targeted by innovative drugs continues to expand. This review focuses on the less explored area of the signaling mechanisms of these receptors, of those expressed in T cells. Studies conducted mainly on PD-1, CTLA-4, and BTLA have evidenced that the extracellular parts of some of the receptors act as decoy receptors for activating ligands, but in all instances, the tyrosine phosphorylation of their cytoplasmatic tail drives a crucial inhibitory signal. This negative signal is mediated by a few key signal transducers, such as tyrosine phosphatase, inositol phosphatase, and diacylglycerol kinase, which allows them to counteract TCR-mediated activation. The characterization of these signaling pathways is of great interest in the development of therapies for counteracting tumor-infiltrating lymphocyte exhaustion/anergy independently from the receptors involved.

Kinase-mediated RAS signaling via membraneless cytoplasmic protein granules

Receptor tyrosine kinase (RTK)-mediated activation of downstream effector pathways such as the RAS GTPase/MAP kinase (MAPK) signaling cascade is thought to occur exclusively from lipid membrane compartments in mammalian cells. Here, we uncover a membraneless, protein granule-based subcellular structure that can organize RTK/RAS/MAPK signaling in cancer. Chimeric (fusion) oncoproteins involving certain RTKs including ALK and RET undergo de novo higher-order assembly into membraneless cytoplasmic protein granules that actively signal. These pathogenic biomolecular condensates locally concentrate the RAS activating complex GRB2/SOS1 and activate RAS in a lipid membrane-independent manner. RTK protein granule formation is critical for oncogenic RAS/MAPK signaling output in these cells. We identify a set of protein granule components and establish structural rules that define the formation of membraneless protein granules by RTK oncoproteins. Our findings reveal membraneless, higher-order cytoplasmic protein assembly as a distinct subcellular platform for organizing oncogenic RTK and RAS signaling.

Tumor microenvironmental influences on dendritic cell and T cell function: A focus on clinically relevant immunologic and metabolic checkpoints

Cancer immunotherapy is fast becoming one of the most promising means of treating malignant disease. Cancer vaccines, adoptive cell transfer therapies, and immune checkpoint blockade have all shown varying levels of success in the clinical management of several cancer types in recent years. However, despite the clinical benefits often achieved by these regimens, an ongoing problem for many patients is the inherent or acquired resistance of their cancer to immunotherapy. It is now appreciated that dendritic cells and T lymphocytes both play key roles in antitumor immune responses and that the tumor microenvironment presents a number of barriers to the function of these cells that can ultimately limit the success of immunotherapy. In particular, the engagement of several immunologic and metabolic checkpoints within the hostile tumor microenvironment can severely compromise the antitumor functions of these important immune populations. This review highlights work from both preclinical and clinical studies that has shaped our understanding of the tumor microenvironment and its influence on dendritic cell and T cell function. It focuses on clinically relevant targeted and immunotherapeutic strategies that have emerged from these studies in an effort to prevent or overcome immune subversion within the tumor microenvironment. Emphasis is also placed on the potential of next-generation combinatorial regimens that target metabolic and immunologic impediments to dendritic cell and T lymphocyte function as strategies to improve antitumor immune reactivity and the clinical outcome of cancer immunotherapy going forward.

Multiple sources of signal amplification within the B-cell Ras/MAPK pathway

The Ras-Map kinase (MAPK) cascade underlies functional decisions in a wide range of cell types and organisms. In B-cells, positive feedback-driven Ras activation is the proposed source of the digital (all or none) MAPK responses following antigen stimulation. However, an inability to measure endogenous Ras activity in living cells has hampered our ability to test this model directly. Here we leverage biosensors of endogenous Ras and ERK activity to revisit this question. We find that B-cell receptor (BCR) ligation drives switch-like Ras activation and that lower BCR signaling output is required for the maintenance versus the initiation of Ras activation. Surprisingly, digital ERK responses persist in the absence of positive feedback-mediated Ras activation, and digital ERK is observed at a threshold level of Ras activation. These data suggest an independent analogue-to-digital switch downstream of Ras activation and reveal that multiple sources of signal amplification exist within the Ras-ERK module of the BCR pathway.

Ras signals principally via Erk in G1 but cooperates with PI3K/Akt for Cyclin D induction and S-phase entry

Numerous studies exploring oncogenic Ras or manipulating physiological Ras signalling have established an irrefutable role for Ras as driver of cell cycle progression. Despite this wealth of information the precise signalling timeline and effectors engaged by Ras, particularly during G1, remain obscure as approaches for Ras inhibition are slow-acting and ill-suited for charting discrete Ras signalling episodes along the cell cycle. We have developed an approach based on the inducible recruitment of a Ras-GAP that enforces endogenous Ras inhibition within minutes. Applying this strategy to inhibit Ras stepwise in synchronous cell populations revealed that Ras signaling was required well into G1 for Cyclin D induction, pocket protein phosphorylation and S-phase entry, irrespective of whether cells emerged from quiescence or G2/M. Unexpectedly, Erk, and not PI3K/Akt or Ral was activated by Ras at mid-G1, albeit PI3K/Akt signalling was a necessary companion of Ras/Erk for sustaining cyclin-D levels and G1/S transition. Our findings chart mitogenic signaling by endogenous Ras during G1 and identify limited effector engagement restricted to Raf/MEK/Erk as a cogent distinction from oncogenic Ras signalling.

Deep analysis of immune response and metabolic signature in children with food protein induced enterocolitis to cow's milk

Background

Food Protein-Induced Enterocolitis Syndrome (FPIES) is considered to be a non-IgE mediated food allergy. However, its pathogenesis remains poorly understood and biomarkers are lacking. We aimed to perform in-depth characterization of humoral and cellular immune responses in children with cow’s milk (CM)-FPIES and investigated whether there is a FPIES metabolomic signature.

Methods

Children with CM-FPIES and control subjects with an IgE-mediated CM allergy (IgE-CMA), both avoiding CM, were recruited on the day of an oral food challenge. Blood samples were collected before the challenge. Total and specific levels of IgE, IgG1-4, IgA, IgM and IgD to various whey and casein allergens and to their gastroduodenal digestion products were measured in plasma, using plasma from CM-tolerant peanut allergic patients (IgE-PA, not avoiding CM) as additional controls. Cytokine secretion and cellular proliferation were analyzed after stimulation of PBMC with different CM allergens. Metabolomic profiles were obtained for plasma samples using liquid chromatography coupled to high-resolution mass spectrometry.

Results

Nine children with CM-FPIES and 12 control subjects (6 IgE-CMA and 6 IgE-PA) were included. In children with CM-FPIES, total Ig concentrations were lower than in control subjects, specific Ig against CM components were weak to undetectable, and no specific IgE against CM digestion products were detected. Moreover, in CM-FPIES patients, we did not find any Th cell proliferation or associated cytokine secretion after allergen reactivation, whereas such responses were clearly found in children with IgE-CMA. Plasma metabolic profiles were different between CM allergic patients, with significantly lower concentrations of various fatty acids and higher concentrations of primary metabolites such as amino acids in CM-FPIES compared to IgE-CMA patients.

Conclusions

In CM-FPIES, both humoral and cellular specific immune responses are weak or absent, and this is not related to CM avoidance. A metabolomic signature was identified in patients with CM-FPIES that may be useful for the diagnosis and management of this disease.

Electronic supplementary material

The online version of this article (10.1186/s13601-018-0224-9) contains supplementary material, which is available to authorized users.

The spatiotemporal regulation of RAS signalling

Nearly 30% of human tumours harbour mutations in RAS family members. Post-translational modifications and the localisation of RAS within subcellular compartments affect RAS interactions with regulator, effector and scaffolding proteins. New insights into the control of spatiotemporal RAS signalling reveal that activation kinetics and subcellular compartmentalisation are tightly coupled to the generation of specific biological outcomes. Computational modelling can help utilising these insights for the identification of new targets and design of new therapeutic approaches.

Metabolic pathways in T cell activation and lineage differentiation

Recent advances in the field of immunometabolism support the concept that fundamental processes in T cell biology, such as TCR-mediated activation and T helper lineage differentiation, are closely linked to changes in the cellular metabolic programs. Although the major task of the intermediate metabolism is to provide the cell with a constant supply of energy and molecular precursors for the production of biomolecules, the dynamic regulation of metabolic pathways also plays an active role in shaping T cell responses. Key metabolic processes such as glycolysis, fatty acid and mitochondrial metabolism are now recognized as crucial players in T cell activation and differentiation, and their modulation can differentially affect the development of T helper cell lineages. In this review, we describe the diverse metabolic processes that T cells engage during their life cycle from naïve towards effector and memory T cells. We consider in particular how the cellular metabolism may actively support the function of T cells in their different states. Moreover, we discuss how molecular regulators such as mTOR or AMPK link environmental changes to adaptations in the cellular metabolism and elucidate the consequences on T cell differentiation and function.

Feedback activation of neurofibromin terminates growth factor-induced Ras activation

Background

Growth factors induce a characteristically short-lived Ras activation in cells emerging from quiescence. Extensive work has shown that transient as opposed to sustained Ras activation is critical for the induction of mitogenic programs. Mitogen-induced accumulation of active Ras-GTP results from increased nucleotide exchange driven by the nucleotide exchange factor Sos. In contrast, the mechanism accounting for signal termination and prompt restoration of basal Ras-GTP levels is unclear, but has been inferred to involve feedback inhibition of Sos. Remarkably, how GTP-hydrolase activating proteins (GAPs) participate in controlling the rise and fall of Ras-GTP levels is unknown.

Results

Monitoring nucleotide exchange of Ras in permeabilized cells we find, unexpectedly, that the decline of growth factor-induced Ras-GTP levels proceeds in the presence of unabated high nucleotide exchange, pointing to GAP activation as a major mechanism of signal termination. Experiments with non-hydrolysable GTP analogues and mathematical modeling confirmed and rationalized the presence of high GAP activity as Ras-GTP levels decline in a background of high nucleotide exchange. Using pharmacological and genetic approaches we document a raised activity of the neurofibromatosis type I tumor suppressor Ras-GAP neurofibromin and an involvement of Rsk1 and Rsk2 in the down-regulation of Ras-GTP levels.

Conclusions

Our findings show that, in addition to feedback inhibition of Sos, feedback stimulation of the RasGAP neurofibromin enforces termination of the Ras signal in the context of growth-factor signaling. These findings ascribe a precise role to neurofibromin in growth factor-dependent control of Ras activity and illustrate how, by engaging Ras-GAP activity, mitogen-challenged cells play safe to ensure a timely termination of the Ras signal irrespectively of the reigning rate of nucleotide exchange.

Electronic supplementary material

The online version of this article (doi:10.1186/s12964-016-0128-z) contains supplementary material, which is available to authorized users.

Mapping the functional versatility and fragility of Ras GTPase signaling circuits through in vitro network reconstitution

The Ras-superfamily GTPases are central controllers of cell proliferation and morphology. Ras signaling is mediated by a system of interacting molecules: upstream enzymes (GEF/GAP) regulate Ras's ability to recruit multiple competing downstream effectors. We developed a multiplexed, multi-turnover assay for measuring the dynamic signaling behavior of in vitro reconstituted H-Ras signaling systems. By including both upstream regulators and downstream effectors, we can systematically map how different network configurations shape the dynamic system response. The concentration and identity of both upstream and downstream signaling components strongly impacted the timing, duration, shape, and amplitude of effector outputs. The distorted output of oncogenic alleles of Ras was highly dependent on the balance of positive (GAP) and negative (GEF) regulators in the system. We found that different effectors interpreted the same inputs with distinct output dynamics, enabling a Ras system to encode multiple unique temporal outputs in response to a single input. We also found that different Ras-to-GEF positive feedback mechanisms could reshape output dynamics in distinct ways, such as signal amplification or overshoot minimization. Mapping of the space of output behaviors accessible to Ras provides a design manual for programming Ras circuits, and reveals how these systems are readily adapted to produce an array of dynamic signaling behaviors. Nonetheless, this versatility comes with a trade-off of fragility, as there exist numerous paths to altered signaling behaviors that could cause disease.

RasGRP1 overexpression in T-ALL increases basal nucleotide exchange on Ras rendering the Ras/PI3K/Akt pathway responsive to protumorigenic cytokines

Ras GTPases are activated by RasGEFs and inactivated by RasGAPs, which stimulate the hydrolysis of RasGTP to inactive RasGDP. GTPase-impairing somatic mutations in RAS genes, such as KRAS(G12D), are among the most common oncogenic events in metastatic cancer. A different type of cancer Ras signal, driven by overexpression of the RasGEF RasGRP1 (Ras guanine nucleotide-releasing protein 1), was recently implicated in pediatric T-cell acute lymphoblastic leukemia (T-ALL) patients and murine models, in which RasGRP1 T-ALLs expand in response to treatment with interleukins (ILs) 2, 7 and 9. Here, we demonstrate that IL-2/7/9 stimulation activates Erk and Akt pathways downstream of Ras in RasGRP1 T-ALL but not in normal thymocytes. In normal lymphocytes, RasGRP1 is recruited to the membrane by diacylglycerol (DAG) in a phospholipase C-γ (PLCγ)-dependent manner. Surprisingly, we find that leukemic RasGRP1-triggered Ras-Akt signals do not depend on acute activation of PLCγ to generate DAG but rely on baseline DAG levels instead. In agreement, using three distinct assays that measure different aspects of the RasGTP/GDP cycle, we established that overexpression of RasGRP1 in T-ALLs results in a constitutively high GTP-loading rate of Ras, which is constantly counterbalanced by hydrolysis of RasGTP. KRAS(G12D) T-ALLs do not show constitutive GTP loading of Ras. Thus, we reveal an entirely novel type of leukemogenic Ras signals that is based on a RasGRP1-driven increased in flux through the RasGTP/GDP cycle, which is mechanistically very different from KRAS(G12D) signals. Our studies highlight the dynamic balance between RasGEF and RasGAP in these T-ALLs and put forth a new model in which IL-2/7/9 decrease RasGAP activity.

Methods to study the Ras2 protein activation state and the subcellular localization of Ras-GTP in Saccharomyces cerevisiae

Ras proteins were highly conserved during evolution. They function as a point of convergence for different signalling pathways in eukaryotes and are involved in a wide range of cellular responses (shift from gluconeogenic to fermentative growth, breakdown of storage carbohydrates, stress resistance, growth control and determination of life span, morphogenesis and development, and others). These proteins are members of the small GTPase superfamily, which are active in the GTP-bound form and inactive in the GDP-bound form. Given the importance of studies on the Ras protein activation state to understand the detailed mechanism of Ras-mediated signal transduction, we provide here a simple, sensitive, and reliable method, based on the high affinity interaction of Ras-GTP with the Ras binding domain (RBD) of Raf1, to measure the level of Ras2-GTP on total Ras2 in Saccharomyces cerevisiae. Moreover, to study the localization of Ras-GTP in vivo in single S. cerevisiae cells, we expressed a probe consisting of a GFP fusion with a trimeric Ras Binding Domain of Raf1 (eGFP-RBD3), which was proven to be a useful live-cell biosensor for Ras-GTP in mammalian cells.

Real-time visualization and quantification of native Ras activation in single living cells

Members of the Ras family of small guanosine triphosphate phosphohydrolases are GDP/GTP-binding proteins that function as pivotal molecular switches in multiple cell biological processes. The prototypical Ras family members K-Ras, N-Ras, and H-Ras, in particular, have been the focus of intense research for the last 30 years owing to their critical function as signalling nodes in the control of cell growth and proliferation and as drivers of oncogenic transformation. One aspect that has attracted much attention in recent times is the spatial control of Ras activity, which is dictated largely by a series of posttranslational modifications that do effectively govern the subcellular distribution and trafficking of Ras. Accordingly, strong emphasis has been placed on developing methodological microscopy-based approaches for the visualization of active Ras-GTP complexes at subcellular resolution. Here we describe the use of a collection of fluorescent affinity probes for the real-time visualization of Ras-GTP in live cells. These probes are multivalent and thus feature high avidity/affinity to Ras-GTP, which obviates the over-expression of Ras and enables one to image endogenous Ras-GTP formation. In addition, this chapter details the use of automated segmentation strategies for the unbiased quantification of probe-derived fluorescence at individual subcellular sites like the plasma membrane and endomembranes.

Graded inhibition of oncogenic Ras-signaling by multivalent Ras-binding domains

BACKGROUND

Ras is a membrane-associated small G-protein that funnels growth and differentiation signals into downstream signal transduction pathways by cycling between an inactive, GDP-bound and an active, GTP-bound state. Aberrant Ras activity as a result of oncogenic mutations causes de novo cell transformation and promotes tumor growth and progression.

RESULTS

Here, we describe a novel strategy to block deregulated Ras activity by means of oligomerized cognate protein modules derived from the Ras-binding domain of c-Raf (RBD), which we named MSOR for multivalent scavengers of oncogenic Ras. The introduction of well-characterized mutations into RBD was used to adjust the affinity and hence the blocking potency of MSOR towards activated Ras. MSOR inhibited several oncogenic Ras-stimulated processes including downstream activation of Erk1/2, induction of matrix-degrading enzymes, cell motility and invasiveness in a graded fashion depending on the oligomerization grade and the nature of the individual RBD-modules. The amenability to accurate experimental regulation was further improved by engineering an inducible MSOR-expression system to render the reversal of oncogenic Ras effects controllable.

CONCLUSION

MSOR represent a new tool for the experimental and possibly therapeutic selective blockade of oncogenic Ras signals.

Ras palmitoylation is necessary for N-Ras activation and signal propagation in growth factor signalling

Ras GTPases undergo post-translational modifications that govern their subcellular trafficking and localization. In particular, palmitoylation of the Golgi tags N-Ras and H-Ras for exocytotic transport and residency at the PM (plasma membrane). Following depalmitoylation, PM-Ras redistributes to all subcellular membranes causing an accumulation of palmitate-free Ras at endomembranes, including the Golgi and endoplasmic reticulum. Palmitoylation is unanimously regarded as a critical modification at the crossroads of Ras activity and trafficking control, but its precise relevance to native wild-type Ras function in growth factor signalling is unknown. We show in the present study by use of palmitoylation-deficient N-Ras mutants and via the analysis of palmitate content of agonist-activated GTP-loaded N-Ras that only palmitoylated N-Ras becomes activated by agonists. In line with an essential role of palmitoylation in Ras activation, dominant-negative RasS17N loses its blocking potency if rendered devoid of palmitoylation. Live-cell Ras-GTP imaging shows that N-Ras activation proceeds only at the PM, consistent with activated N-Ras-GTP being palmitoylated. Finally, palmitoylation-deficient N-Ras does not sustain EGF (epidermal growth factor) or serum-elicited mitogenic signalling, confirming that palmitoylation is essential for signal transduction by N-Ras. These findings document that N-Ras activation proceeds at the PM and suggest that depalmitoylation, by removing Ras from the PM, may contribute to the shutdown of Ras signalling.

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.

Features of Ras activation by a mislocalized oncogenic tyrosine kinase: FLT3 ITD signals via K-Ras at the plasma membrane of Acute Myeloid Leukemia cells

FLT3 ITD (FMS-like tyrosine kinase 3 with internal tandem duplication) is an important oncoprotein in Acute Myeloid Leukemia (AML). Owing to its constitutive kinase activity FLT3 ITD accumulates partially at endomembranes, a feature shared with other disease-associated, mutated receptor tyrosine kinases. Since Ras proteins also transit through endomembranes we have investigated the possible existence of an intracellular FLT3 ITD/Ras signaling pathway by comparing Ras signaling of FLT3 ITD with that of wild-type FLT3. Ligand stimulation activated both K- and N-Ras in cells expressing wild-type FLT3. Life-cell Ras-GTP imaging revealed ligand-induced Ras activation at the plasma membrane (PM). FLT3 ITD dependent constitutive activation of K-Ras and N-Ras was also observed primarily at the PM, supporting the view that the PM-resident pool of FLT3 ITD engaged the Ras/Erk pathway in AML cells. Accordingly, specific interference with FLT3 ITD/Ras signaling at the PM using PM-restricted dominant negative K-RasS17N potently inhibited cell proliferation and promoted apoptosis, corroborating that Ras signalling is crucial for FLT3 ITD dependent cell transformation and confirming that FLT3 ITD addresses PM-bound Ras despite its pronounced mislocalization to endomembranes.

Regulation and Function of the RasGRP Family of Ras Activators in Blood Cells

Ras guanyl nucleotide releasing proteins (RasGRPs) are guanyl nucleotide exchange factors that activate Ras and related GTPases such as Rap. Like Sos proteins, RasGRPs have a catalytic region composed of a Ras exchange motif (REM) and a CDC25 domain. RasGRPs also possess a pair of atypical EF hands that may bind calcium in vivo and a C1 domain resembling the diacylglycerol (DAG)-binding domain of protein kinase C. DAG directly activates RasGRPs by a membrane recruitment mechanism as well as indirectly by PKC-mediated phosphorylation. RasGRPs are prominently expressed in blood cells. RasGRP1 acts downstream of TCR, while RasGRP1 and RasGRP3 both act downstream of BCR. Together, they regulate Ras in adaptive immune cells. RasGRP2, through Rap, plays a role in controlling platelet adhesion, while RasGRP4 controls Ras activation in mast cells. RasGRP malfunction likely contributes to autoimmunity and may contribute to blood malignancies. RasGRPs might prove to be viable drug targets. The intracellular site of RasGRP action and the relationship between RasGRPs and other Ras regulatory mechanisms are subjects of lively debate.

SAP-mediated inhibition of diacylglycerol kinase α regulates TCR-induced diacylglycerol signaling

Diacylglycerol kinases (DGKs) metabolize diacylglycerol to phosphatidic acid. In T lymphocytes, DGKα acts as a negative regulator of TCR signaling by decreasing diacylglycerol levels and inducing anergy. In this study, we show that upon costimulation of the TCR with CD28 or signaling lymphocyte activation molecule (SLAM), DGKα, but not DGKζ, exits from the nucleus and undergoes rapid negative regulation of its enzymatic activity. Inhibition of DGKα is dependent on the expression of SAP, an adaptor protein mutated in X-linked lymphoproliferative disease, which is essential for SLAM-mediated signaling and contributes to TCR/CD28-induced signaling and T cell activation. Accordingly, overexpression of SAP is sufficient to inhibit DGKα, whereas SAP mutants unable to bind either phospho-tyrosine residues or SH3 domain are ineffective. Moreover, phospholipase C activity and calcium, but not Src-family tyrosine kinases, are also required for negative regulation of DGKα. Finally, inhibition of DGKα in SAP-deficient cells partially rescues defective TCR/CD28 signaling, including Ras and ERK1/2 activation, protein kinase C membrane recruitment, induction of NF-AT transcriptional activity, and IL-2 production. Thus SAP-mediated inhibition of DGKα sustains diacylglycerol signaling, thereby regulating T cell activation, and it may represent a novel pharmacological strategy for X-linked lymphoproliferative disease treatment.

Ras trafficking, localization and compartmentalized signalling

Ras proteins are proto-oncogenes that are frequently mutated in human cancers. Three closely related isoforms, HRAS, KRAS and NRAS, are expressed in all cells and have overlapping but distinctive functions. Recent work has revealed how differences between the Ras isoforms in their trafficking, localization and protein-membrane orientation enable signalling specificity to be determined. We review the various strategies used to characterize compartmentalized Ras localization and signalling. Localization is an important contextual modifier of signalling networks and insights from the Ras system are of widespread relevance for researchers interested in signalling initiated from membranes.

Dynamic palmitoylation and the role of DHHC proteins in T cell activation and anergy

Although protein S-palmitoylation was first characterized >30 years ago, and is implicated in the function, trafficking, and localization of many proteins, little is known about the regulation and physiological implications of this posttranslational modification. Palmitoylation of various signaling proteins required for TCR-induced T cell activation is also necessary for their proper function. Linker for activation of T cells (LAT) is an essential scaffolding protein involved in T cell development and activation, and we found that its palmitoylation is selectively impaired in anergic T cells. The recent discovery of the DHHC family of palmitoyl acyl transferases and the establishment of sensitive and quantitative proteomics-based methods for global analysis of the palmitoyl proteome led to significant progress in studying the biology and underlying mechanisms of cellular protein palmitoylation. We are using these approaches to explore the palmitoyl proteome in T lymphocytes and, specifically, the mechanistic basis for the impaired palmitoylation of LAT in anergic T cells. This chapter reviews the history of protein palmitoylation and its role in T cell activation, the DHHC family and new methodologies for global analysis of the palmitoyl proteome, and summarizes our recent work in this area. The new methodologies will accelerate the pace of research and provide a greatly improved mechanistic and molecular understanding of the complex process of protein palmitoylation and its regulation, and the substrate specificity of the novel DHHC family. Reversible protein palmitoylation will likely prove to be an important posttranslational mechanism that regulates cellular responses, similar to protein phosphorylation and ubiquitination.