Deconstructing Ras signaling in the thymus

Exploring the multifaceted role of RASGRP1 in disease: immune, neural, metabolic, and oncogenic perspectives

RAS guanyl releasing protein 1 (RASGRP1) is a guanine nucleotide exchange factor (GEF) characterized by the presence of a RAS superfamily GEF domain. It functions as a diacylglycerol (DAG)-regulated nucleotide exchange factor, specifically activating RAS through the exchange of bound GDP for GTP. Activation of RAS by RASGRP1 has a wide range of downstream effects at the cellular level. Thus, it is not surprising that many diseases are associated with RASGRP1 disorders. Here, we present an overview of the structure and function of RASGRP1, its crucial role in the development, expression, and regulation of immune cells, and its involvement in various signaling pathways. This review comprehensively explores the relationship between RASGRP1 and various diseases, elucidates the underlying molecular mechanisms of RASGRP1 in each disease, and identifies potential therapeutic targets. This study provides novel insights into the role of RASGRP1 in insulin secretion and highlights its potential as a therapeutic target for diabetes. The limitations and challenges associated with studying RASGRP1 in disease are also discussed.

Cutting Edge: Induced Loss of Rasgrp1 in Peripheral CD4+ T Cells of Conditional Rasgrp1-Deficient Mice Reveals an Essential Role for Rasgrp1 in TCR/CD28-Induced Ras-MAPK Signaling

Ras guanine nucleotide-releasing protein 1 (Rasgrp1) is a Ras guanine nucleotide exchange factor that participates in the activation of the Ras-ERK signaling pathway in developing T cells and is required for efficient thymic T cell positive selection. However, the role of Rasgrp1 in mature peripheral T cells has not been definitively addressed, in part because peripheral T cells from constitutive Rasgrp1-deficient mice show an abnormal activated phenotype. In this study, we generated an inducible Rasgrp1-deficient mouse model to allow acute disruption of Rasgrp1 in peripheral CD4+ T cells in the context of normal T cell development. TCR/CD28-mediated activation of Ras-ERK signaling was blocked in Rasgrp1-deficient peripheral CD4+ T cells. Furthermore, Rasgrp1-deficient CD4+ T cells were unable to synthesize IL-2 and the high-affinity IL-2R and were unable to proliferate in response to TCR/CD28 stimulation. These findings highlight an essential function for Rasgrp1 for TCR/CD28-induced Ras-ERK activation in peripheral CD4+ T cells.

Isoform-specific functions of Ras in T-cell development and differentiation

Ras GTPases, well characterized for their role in oncogenesis, are the cells' molecular switches that signal to maintain immune homeostasis through cellular development, proliferation, differentiation, survival, and apoptosis. In the immune system, T cells are the central players that cause autoimmunity if dysregulated. Antigen-specific T-cell receptor (TCR) stimulation activates Ras-isoforms, which exhibit isoform-specific activator and effector requirements, functional specificities, and a selective role in T-cell development and differentiation. Recent studies show the role of Ras in T-cell-mediated autoimmune diseases; however, there is a scarcity of knowledge about the role of Ras in T-cell development and differentiation. To date, limited studies have demonstrated Ras activation in response to positive and negative selection signals and Ras isoform-specific signaling, including subcellular signaling, in immune cells. The knowledge of isoform-specific functions of Ras in T cells is essential, but still inadequate to develop the T-cell-targeted Ras isoform-specific treatment strategies for the diseases caused by altered Ras-isoform expression and activation in T cells. In this review, we discuss the role of Ras in T-cell development and differentiation, critically analyzing the isoform-specific functions.

Development of Son of Sevenless Homologue 1 (SOS1) Modulators To Treat Cancers by Regulating RAS Signaling

Son of sevenless homologue 1 (SOS1) protein is universally expressed in cells and plays an important role in the RAS signaling pathway. Specifically, this protein interacts with RAS in response to upstream stimuli to promote guanine nucleotide exchange in RAS and activates the downstream signaling pathways. Thus, targeting SOS1 is a new approach for treating RAS-driven cancers. In this Perspective, we briefly summarize the structural and functional aspects of SOS1 and focus on recent advances in the discovery of activators, inhibitors, and PROTACs that target SOS1. This review aims to provide a timely and updated overview on the strategies for targeting SOS1 in cancer therapy.

A Focused Review of Ras Guanine Nucleotide-Releasing Protein 1 in Immune Cells and Cancer

Four Ras guanine nucleotide-releasing proteins (RasGRP1 through 4) belong to the family of guanine nucleotide exchange factors (GEFs). RasGRPs catalyze the release of GDP from small GTPases Ras and Rap and facilitate their transition from an inactive GDP-bound to an active GTP-bound state. Thus, they regulate critical cellular responses via many downstream GTPase effectors. Similar to other RasGRPs, the catalytic module of RasGRP1 is composed of the Ras exchange motif (REM) and Cdc25 domain, and the EF hands and C1 domain contribute to its cellular localization and regulation. RasGRP1 can be activated by a diacylglycerol (DAG)-mediated membrane recruitment and protein kinase C (PKC)-mediated phosphorylation. RasGRP1 acts downstream of the T cell receptor (TCR), B cell receptors (BCR), and pre-TCR, and plays an important role in the thymocyte maturation and function of peripheral T cells, B cells, NK cells, mast cells, and neutrophils. The dysregulation of RasGRP1 is known to contribute to numerous disorders that range from autoimmune and inflammatory diseases and schizophrenia to neoplasia. Given its position at the crossroad of cell development, inflammation, and cancer, RASGRP1 has garnered interest from numerous disciplines. In this review, we outline the structure, function, and regulation of RasGRP1 and focus on the existing knowledge of the role of RasGRP1 in leukemia and other cancers.

Case report: The cardio-facio-cutaneous syndrome due to a novel germline mutation in MAP2K1: A multifaceted disease with immunodeficiency and short stature

Cardio-facio-cutaneous syndrome (CFCS) belongs to the group of RASopathies, clinical disorders defined by disruptions in the RAS/MAPK signaling pathway. It is caused by heterozygous gain-of-function germline mutations in genes encoding protein kinases: BRAF, MAP2K1 (MEK1), MAP2K2 (MEK2), and in the GTPase-encoding gene KRAS. CFCS is characterized by craniofacial dysmorphic features, congenital heart defects, severe malnutrition, proportionate short stature, anomalies within the structure of skin and hair, and psychomotor disability. The pathophysiology of growth impairment is multifactorial with feeding difficulties, growth hormone deficiency, and insensitivity. Immunodeficiency has not been hitherto reported as an integral part of CFCS yet an increased activation of the RAS/MAPK signaling pathway may contribute to explaining the causal relationship between RASopathy and the dysfunctions within the B and T lymph cell compartments resulting in a deficiency in T cell costimulation and B cell maturation with impaired class switch recombination, somatic hypermutation, and high-affinity antibody production. We report on a boy born prematurely at 32 WGA, with the perinatal period complicated by pneumonia, respiratory distress syndrome, and valvular pulmonary stenosis. The boy suffered from recurrent pneumonia, obstructive bronchitis, sepsis, urinary tract infection, and recurrent fevers. He presented with severe hypotrophy, psychomotor disability, short stature, craniofacial dysmorphism, dental hypoplasia, sparse hair, and cryptorchidism. Whole genome sequencing showed a novel heterozygous pathogenic germline missense variant: c.364A > G; p.Asn122Asp in the MAP2K1 gene, supporting the diagnosis of CFCS. The immunological workup revealed hypogammaglobulinemia, IgG subclass, and specific antibody deficiency accompanied by decreased numbers of T helper cells and naive and memory B cells. Replacement immunoglobulin therapy with timely antibiotic prophylaxis were instituted. At the age of six years, growth hormone deficiency was diagnosed and the rGH therapy was started. The ever-increasing progress in genetic studies contributes to establishing the definitive CFCS diagnosis and sheds the light on the interrelated genotype-phenotype heterogeneity of RASopathies. Herein, we add new phenotypic features of predominating humoral immunodeficiency to the symptomatology of CFCS with a novel mutation in MAP2K1. While CFCS is a multifaceted disease, increased pediatricians' awareness is needed to prevent the delay in diagnostics and therapeutic interventions.

The Role of Wild-Type RAS in Oncogenic RAS Transformation

The RAS family of oncogenes (HRAS, NRAS, and KRAS) are among the most frequently mutated protein families in cancers. RAS-mutated tumors were originally thought to proliferate independently of upstream signaling inputs, but we now know that non-mutated wild-type (WT) RAS proteins play an important role in modulating downstream effector signaling and driving therapeutic resistance in RAS-mutated cancers. This modulation is complex as different WT RAS family members have opposing functions. The protein product of the WT RAS allele of the same isoform as mutated RAS is often tumor-suppressive and lost during tumor progression. In contrast, RTK-dependent activation of the WT RAS proteins from the two non-mutated WT RAS family members is tumor-promoting. Further, rebound activation of RTK–WT RAS signaling underlies therapeutic resistance to targeted therapeutics in RAS-mutated cancers. The contributions of WT RAS to proliferation and transformation in RAS-mutated cancer cells places renewed interest in upstream signaling molecules, including the phosphatase/adaptor SHP2 and the RasGEFs SOS1 and SOS2, as potential therapeutic targets in RAS-mutated cancers.

Nras Q61R/+ and Kras-/- cooperate to downregulate Rasgrp1 and promote lympho-myeloid leukemia in early T-cell precursors

Kras^−/−; Nras^Q61R/+ mice develop early onset of T-cell malignancy that recapitulates many biological and molecular features of human ETP-ALL.

We identify Rasgrp1 as a negative regulator of Ras/ERK signaling in oncogenic Nras-driven ETP-like leukemia.

Early T-cell precursor acute lymphoblastic leukemia (ETP-ALL) is an aggressive subtype of T-cell ALL. Although genetic mutations hyperactivating cytokine receptor/Ras signaling are prevalent in ETP-ALL, it remains unknown how activated Ras signaling contributes to ETP-ALL. Here, we find that in addition to the frequent oncogenic RAS mutations, wild-type (WT) KRAS transcript level was significantly downregulated in human ETP-ALL cells. Similarly, loss of WT Kras in Nras^Q61R/+ mice promoted hyperactivation of extracellular signal-regulated kinase (ERK) signaling, thymocyte hyperproliferation, and expansion of the ETP compartment. Kras^−/−; Nras^Q61R/+ mice developed early onset of T-cell malignancy that recapitulates many biological and molecular features of human ETP-ALL. Mechanistically, RNA-sequencing analysis and quantitative proteomics study identified that Rasgrp1, a Ras guanine nucleotide exchange factor, was greatly downregulated in mouse and human ETP-ALL. Unexpectedly, hyperactivated Nras/ERK signaling suppressed Rasgrp1 expression and reduced Rasgrp1 level led to increased ERK signaling, thereby establishing a positive feedback loop to augment Nras/ERK signaling and promote cell proliferation. Corroborating our cell line data, Rasgrp1 haploinsufficiency induced Rasgrp1 downregulation and increased phosphorylated ERK level and ETP expansion in Nras^Q61R/+ mice. Our study identifies Rasgrp1 as a negative regulator of Ras/ERK signaling in oncogenic Nras-driven ETP-like leukemia.

SOS GEFs in health and disease

SOS1 and SOS2 are the most universal and widely expressed family of guanine exchange factors (GEFs) capable or activating RAS or RAC1 proteins in metazoan cells. SOS proteins contain a sequence of modular domains that are responsible for different intramolecular and intermolecular interactions modulating mechanisms of self-inhibition, allosteric activation and intracellular homeostasis. Despite their homology, analyses of SOS1/2-KO mice demonstrate functional prevalence of SOS1 over SOS2 in cellular processes including proliferation, migration, inflammation or maintenance of intracellular redox homeostasis, although some functional redundancy cannot be excluded, particularly at the organismal level. Specific SOS1 gain-of-function mutations have been identified in inherited RASopathies and various sporadic human cancers. SOS1 depletion reduces tumorigenesis mediated by RAS or RAC1 in mouse models and is associated with increased intracellular oxidative stress and mitochondrial dysfunction. Since WT RAS is essential for development of RAS-mutant tumors, the SOS GEFs may be considered as relevant biomarkers or therapy targets in RAS-dependent cancers. Inhibitors blocking SOS expression, intrinsic GEF activity, or productive SOS protein-protein interactions with cellular regulators and/or RAS/RAC targets have been recently developed and shown preclinical and clinical effectiveness blocking aberrant RAS signaling in RAS-driven and RTK-driven tumors.

Increased baseline RASGRP1 signals enhance stem cell fitness during native hematopoiesis

Oncogenic mutations in RAS genes, like KRAS^G12D or NRAS^G12D, trap Ras in the active state and cause myeloproliferative disorder and T cell leukemia (T-ALL) when induced in the bone marrow via Mx1CRE. The RAS exchange factor RASGRP1 is frequently overexpressed in T-ALL patients. In T-ALL cell lines overexpression of RASGRP1 increases flux through the RASGTP/RasGDP cycle. Here we expanded RASGRP1 expression surveys in pediatric T-ALL and generated a RoLoRiG mouse model crossed to Mx1CRE to determine the consequences of induced RASGRP1 overexpression in primary hematopoietic cells. RASGRP1-overexpressing, GFP-positive cells outcompeted wild type cells and dominated the peripheral blood compartment over time. RASGRP1 overexpression bestows gain-of-function colony formation properties to bone marrow progenitors in medium containing limited growth factors. RASGRP1 overexpression enhances baseline mTOR-S6 signaling in the bone marrow, but not in vitro cytokine-induced signals. In agreement with these mechanistic findings, hRASGRP1-ires-EGFP enhances fitness of stem- and progenitor- cells, but only in the context of native hematopoiesis. RASGRP1 overexpression is distinct from KRAS^G12D or NRAS^G12D, does not cause acute leukemia on its own, and leukemia virus insertion frequencies predict that RASGRP1 overexpression can effectively cooperate with lesions in many other genes to cause acute T cell leukemia.

Graded diacylglycerol kinases α and ζ activities ensure mucosal-associated invariant T-cell development in mice

Mucosal-associated invariant T (MAIT) cells participate in both protective immunity and pathogenesis of diseases. Most murine MAIT cells express an invariant TCRVα19-Jα33 (iVα19) TCR, which triggers signals crucial for their development. However, signal pathways downstream of the iVα19TCR and their regulation in MAIT cells are unknown. Diacylglycerol (DAG) is a critical second messenger that relays the TCR signal to multiple downstream signaling cascades. DAG is terminated by DAG kinase (DGK)-mediated phosphorylation and conversion to phosphatidic acid. We have demonstrated here that downregulation of DAG caused by enhanced DGK activity impairs late-stage MAIT cell maturation in both thymus and spleen. Moreover, deficiency of DGKζ but not DGKα by itself causes modest decreases in MAIT cells, and deficiency of both DGKα and ζ results in severe reductions of MAIT cells in an autonomous manner. Our studies have revealed that DAG signaling is not only critical but also must be tightly regulated by DGKs for MAIT cell development and that both DGKα and, more prominently, DGKζ contribute to the overall DGK activity for MAIT cell development.

Efficient CD4Cre-Mediated Conditional KRas Expression in Alveolar Macrophages and Alveolar Epithelial Cells Causes Fatal Hyperproliferative Pneumonitis

The CD4Cre transgenic model has been widely used for T cell-specific gene manipulation. We report unexpected highly efficient Cre-mediated recombination in alveolar macrophages (AMFs), bronchial epithelial cells (BECs), and alveolar epithelial cells (AECs) in this strain of mice. Different from CD4 T cells, AMFs, AECs, and BECs do not express detectable Cre protein, suggesting that Cre protein is either very transiently expressed in these cells or only expressed in their precursors. Mice carrying a conditional constitutively active KRas (caKRas) allele and the CD4Cre transgene contain not only hyperactivated T cells but also develop severe AMF accumulation, AEC and BEC hyperplasia, and adenomas in the lung, leading to early lethality correlated with caKRas expression in these cells. We propose that caKRas-CD4Cre mice represent, to our knowledge, a novel model of proliferative pneumonitis involving macrophages and epithelial cells and that the CD4Cre model may offer unique usefulness for studying gene functions simultaneously in multilineages in the lung. Our observations, additionally, suggest that caution in data interpretation is warranted when using the CD4Cre transgenic model for T cell-specific gene manipulation, particularly when lung pathophysiological status is being examined.

Identification of rare coding variants in TYK2 protective for rheumatoid arthritis in the Japanese population and their effects on cytokine signalling

OBJECTIVE

Although genome-wide association studies (GWAS) have identified approximately 100 loci for rheumatoid arthritis (RA), the disease mechanisms are not completely understood. We evaluated the pathogenesis of RA by focusing on rare coding variants.

METHODS

The coding regions of 98 candidate genes identified by GWAS were sequenced in 2294 patients with RA and 4461 controls in Japan. An association analysis was performed using cases and controls for variants, genes and domains of TYK2. Cytokine responses for two associated variants (R231W, rs201917359; and R703W, rs55882956) in TYK2 as well as a previously reported risk variant (P1004A, rs34536443) for multiple autoimmune diseases were evaluated by reporter assays.

RESULTS

A variant in TYK2 (R703W) showed a suggestive association (p=5.47×10^-8, OR=0.48). We observed more accumulation of rare coding variants in controls in TYK2 (p=3.94×10^-12, OR=0.56). The four-point-one, ezrin, radixin, moesin (FERM; 2.14×10^-3, OR=0.66) and pseudokinase domains (1.63×10^-8, OR=0.52) of TYK2 also showed enrichment of variants in controls. R231W in FERM domain especially reduced interleukin (IL)-6 and interferon (IFN)-γ signalling, whereas P1104A in kinase domain reduced IL-12, IL-23 and IFN-α signalling. R703W in pseudokinase domain reduced cytokine signals similarly to P1104A, but the effects were weaker than those of P1104A.

CONCLUSIONS

The FERM and pseudokinase domains in TYK2 were associated with the risk of RA in the Japanese population. Variants in TYK2 had different effects on cytokine signalling, suggesting that the regulation of selective cytokine signalling is a target for RA treatment.

Oncogenic RAS isoforms show a hierarchical requirement for the guanine nucleotide exchange factor SOS2 to mediate cell transformation

About a third of tumors have activating mutations in HRAS, NRAS, or KRAS, genes encoding guanosine triphosphatases (GTPases) of the RAS family. In these tumors, wild-type RAS cooperates with mutant RAS to promote downstream effector activation and cell proliferation and transformation, suggesting that upstream activators of wild-type RAS are important modulators of mutant RAS-driven oncogenesis. The guanine nucleotide exchange factor (GEF) SOS1 mediates KRAS-driven proliferation, but little is understood about the role of SOS2. We found that RAS family members have a hierarchical requirement for the expression and activity of SOS2 to drive cellular transformation. In mouse embryonic fibroblasts (MEFs), SOS2 critically mediated mutant KRAS-driven, but not HRAS-driven, transformation. Sos2 deletion reduced epidermal growth factor (EGF)-dependent activation of wild-type HRAS and phosphorylation of the kinase AKT in cells expressing mutant RAS isoforms. Assays using pharmacological inhibitors revealed a hierarchical requirement for signaling by phosphoinositide 3-kinase (PI3K) in promoting RAS-driven cellular transformation that mirrored the requirement for SOS2. KRAS-driven transformation required the GEF activity of SOS2 and was restored in Sos2^-/- MEFs by expression of constitutively activated PI3K. Finally, CRISPR/Cas9-mediated deletion of SOS2 reduced EGF-stimulated AKT phosphorylation and synergized with MEK inhibition to revert the transformed phenotype of human KRAS mutant pancreatic and lung tumor cells. These results indicate that SOS2-dependent PI3K signaling mediates mutant KRAS-driven transformation, revealing therapeutic targets in KRAS-driven cancers. Our data also reveal the importance of three-dimensional culture systems in investigating the mediators of mutant KRAS.

Thymic progenitors of TCRαβ+ CD8αα intestinal intraepithelial lymphocytes require RasGRP1 for development

Golec et al. show that RasGRP1, a critical Ras activator in thymocytes, is required for TCRαβ⁺CD8αα IEL development by regulating the survival of a heterogeneous population of thymic progenitors that receive a strong TCR signal. Therefore, RasGRP1 is necessary for thymic selection events stemming from strong or weak TCR signals.

Strong T cell receptor (TCR) signaling largely induces cell death during thymocyte development, whereas weak TCR signals induce positive selection. However, some T cell lineages require strong TCR signals for differentiation through a process termed agonist selection. The signaling relationships that underlie these three fates are unknown. RasGRP1 is a Ras activator required to transmit weak TCR signals leading to positive selection. Here, we report that, despite being dispensable for thymocyte clonal deletion, RasGRP1 is critical for agonist selection of TCRαβ⁺CD8αα intraepithelial lymphocyte (IEL) progenitors (IELps), even though both outcomes require strong TCR signaling. Bim deficiency rescued IELp development in RasGRP1^−/− mice, suggesting that RasGRP1 functions to promote survival during IELp generation. Additionally, expression of CD122 and the adhesion molecules α₄β₇ and CD103 define distinct IELp subsets with differing abilities to generate TCRαβ⁺CD8αα IEL in vivo. These findings demonstrate that RasGRP1-dependent signaling underpins thymic selection processes induced by both weak and strong TCR signals and is differentially required for fate decisions derived from a strong TCR stimulus.

Age and sex differences in microRNAs expression during the process of thymus aging

The gender-biased thymus involution and the importance of microRNAs (miRNAs, miRs) expression in modulating the thymus development have been reported in many studies. However, how males and females differ in so many ways in thymus involution remains unclear. To address this question, we investigated the miRNA expression profiles in both untreated 3- and 12-month-old female and male mice thymuses. The results showed that 7 and 18 miRNAs were defined as the sex- and age-specific miRNAs, respectively. The expression of miR-181c-5p, miR-20b-5p, miR-98b-5p, miR-329-3p, miR-341-5p, and miR-2137 showed significant age-difference in mice thymus by quantitative polymerase chain reaction. High expression levels of miR-2137 were detected in mice thymic epithelial cells and gradually increased during the process of thymus aging. MiR-27b-3p and miR-378a-3p of the female-biased miRNAs were confirmed as the sex- and estrogen-responsive miRNAs in mice thymus in vivo. Their potential target genes and the pathway were identified by the online software. Possible regulation roles of sex- and age-specific miRNA expression during the process of thymus aging were discussed. Our results suggested that these miRNAs may be potential biomarkers for the study of sex- and age-specific thymus aging and involution.

Suppression of Th17 cell differentiation by misshapen/NIK-related kinase MINK1

T helper type 17 cells (Th17 cells) are major contributors to many autoimmune diseases. In this study, we demonstrate that the germinal center kinase family member MINK1 (misshapen/NIK-related kinase 1) negatively regulates Th17 cell differentiation. The suppressive effect of MINK1 on induction of Th17 cells is mediated by the inhibition of SMAD2 activation through direct phosphorylation of SMAD2 at the T324 residue. The importance of MINK1 to Th17 cell differentiation was strengthened in the animal model of experimental autoimmune encephalomyelitis (EAE). Moreover, we show that the reactive oxygen species (ROS) scavenger N-acetyl cysteine boosts Th17 cell differentiation in a MINK1-dependent manner and exacerbates the severity of EAE. Thus, we have not only established MINK1 as a critical regulator of Th17 cell differentiation, but also clarified that accumulation of ROS may limit the generation of Th17 cells. The contribution of MINK1 to ROS-regulated Th17 cell differentiation may suggest an important mechanism for the development of autoimmune diseases influenced by antioxidant dietary supplements.

Sos1 disruption impairs cellular proliferation and viability through an increase in mitochondrial oxidative stress in primary MEFs

Using a 4-hydroxytamoxifen (4OHT)-inducible, conditional Sos1-null mutation, we analyzed wild-type (WT), single Sos1-KO, Sos2-KO and double Sos1/2 KO primary mouse embryonic fibroblasts (MEF) with an aim at evaluating the functional specificity or redundancy of the Sos1 and Sos2 alleles at the cellular level. The 4OHT-induced Sos1-KO and Sos1/2-DKO MEFs exhibited distinct flat morphology, enlarged cell perimeter and altered cytoskeletal organization that were not observed in the WT and Sos2-KO counterparts. The Sos1-KO and Sos1/2-DKO MEFs also displayed significant accumulation, in comparison with WT and Sos2-KO MEFs, of cytoplasmic vesicular bodies identified as autophagosomes containing degraded mitochondria by means of electron microscopy and specific markers. Cellular proliferation and migration were impaired in Sos1-KO and Sos1/2-DKO MEFs in comparison with WT and Sos2-KO MEFs, whereas cell adhesion was only impaired upon depletion of both Sos isoforms. RasGTP formation was practically absent in Sos1/2-DKO MEFs as compared with the other genotypes and extracellular signal-regulated kinase phosphorylation showed only significant reduction after combined Sos1/2 depletion. Consistent with a mitophagic phenotype, in vivo labeling with specific fluorophores uncovered increased levels of oxidative stress (elevated intracellular reactive oxygen species and mitochondrial superoxide and loss of mitochondrial membrane potential) in the Sos1-KO and the Sos1/2-DKO cells as compared with Sos2-KO and WT MEFs. Interestingly, treatment of the MEF cultures with antioxidants corrected the altered phenotypes of Sos1-KO and Sos1/2-DKO MEFs by restoring their altered perimeter size and proliferative rate to levels similar to those of WT and Sos2-KO MEFs. Our data uncover a direct mechanistic link between Sos1 and control of intracellular oxidative stress, and demonstrate functional prevalence of Sos1 over Sos2 with regards to cellular proliferation and viability.

The linker for activation of T cells (LAT) signaling hub: from signaling complexes to microclusters

Since the cloning of the critical adapter, LAT (linker for activation of T cells), more than 15 years ago, a combination of multiple scientific approaches and techniques continues to provide valuable insights into the formation, composition, regulation, dynamics, and function of LAT-based signaling complexes. In this review, we will summarize current views on the assembly of signaling complexes nucleated by LAT. LAT forms numerous interactions with other signaling molecules, leading to cooperativity in the system. Furthermore, oligomerization of LAT by adapter complexes enhances intracellular signaling and is physiologically relevant. These results will be related to data from super-resolution microscopy studies that have revealed the smallest LAT-based signaling units and nanostructure.

Enhanced T-cell activation and differentiation in lymphocytes from transgenic mice expressing ubiquitination-resistant 2KR LAT molecules

Linker for activation of T cells (LAT) is critical for the propagation of T-cell signals upon T-cell receptor (TCR) activation. Previous studies demonstrated that substitution of LAT lysines with arginines (2KR LAT) resulted in decreased LAT ubiquitination and elevated T-cell signaling, indicating that LAT ubiquitination is a molecular checkpoint for attenuation of T-cell signaling. To investigate the role of LAT ubiquitination in vivo, we have generated transgenic mice expressing WT and ubiquitin-defective 2KR LAT. On TCR stimulation of T cells from these mice, proximal signaling and cytokine production was elevated in 2KR versus wild-type (WT) LAT mice. Enhanced cytolytic activity as well as T-helper responses were observed on LAT expression, which were further elevated by 2KR LAT expression. Despite greater T-effector function, WT or 2KR LAT expression did not have any effect on clearance of certain pathogens or tumors. Our data support the model that lack of tumor clearance is due to increased differentiation and acquisition of effector phenotype that is associated with suboptimal immunity in an immunotherapy model. Thus, our data further reinforce the role of LAT ubiquitination in TCR signaling and uncovers a novel role for LAT in driving T-cell differentiation.

Absence of both Sos-1 and Sos-2 in peripheral CD4(+) T cells leads to PI3K pathway activation and defects in migration

Sos-1 and Sos-2 are ubiquitously expressed Ras-guanine exchange factors involved in Erk-MAP kinase pathway activation. Using mice lacking genes encoding Sos-1 and Sos-2, we evaluated the role of these proteins in peripheral T-cell signaling and function. Our results confirmed that TCR-mediated Erk activation in peripheral CD4(+) T cells does not depend on Sos-1 and Sos-2, although IL-2-mediated Erk activation does. Unexpectedly, however, we show an increase in AKT phosphorylation in Sos-1/2dKO CD4(+) T cells upon TCR and IL-2 stimulation. Activation of AKT was likely a consequence of increased recruitment of PI3K to Grb2 upon TCR and/or IL-2 stimulation in Sos-1/2dKO CD4(+) T cells. The increased activity of the PI3K/AKT pathway led to downregulation of the surface receptor CD62L in Sos-1/2dKO T cells and a subsequent impairment in T-cell migration.

The Ras GTPase-activating protein Rasal3 supports survival of naive T cells

The Ras-mitogen-activated protein kinase (MAPK) pathway is crucial for T cell receptor (TCR) signaling in the development and function of T cells. The significance of various modulators of the Ras-MAPK pathway in T cells, however, remains to be fully understood. Ras-activating protein-like 3 (Rasal3) is an uncharacterized member of the SynGAP family that contains a conserved Ras GTPase-activating protein (GAP) domain, and is predominantly expressed in the T cell lineage. In the current study, we investigated the function and physiological roles of Rasal3. Our results showed that Rasal3 possesses RasGAP activity, but not Rap1GAP activity, and represses TCR-stimulated ERK phosphorylation in a T cell line. In systemic Rasal3-deficient mice, T cell development in the thymus including positive selection, negative selection, and β-selection was unaffected. However, the number of naive, but not effector memory CD4 and CD8 T cell in the periphery was significantly reduced in Rasal3-deficient mice, and associated with a marked increase in apoptosis of these cells. Indeed, survival of Rasal3 deficient naive CD4 T cells in vivo by adoptive transfer was significantly impaired, whereas IL-7-dependent survival of naive CD4 T cells in vitro was unaltered. Collectively, Rasal3 is required for in vivo survival of peripheral naive T cells, contributing to the maintenance of optimal T cell numbers.

CBAP promotes thymocyte negative selection by facilitating T-cell receptor proximal signaling

T-cell receptor (TCR)-transduced signaling is critical to thymocyte development at the CD4/CD8 double-positive stage, but the molecules involved in this process are not yet fully characterized. We previously demonstrated that GM-CSF/IL-3/IL-5 receptor common β-chain-associated protein (CBAP) modulates ZAP70-mediated T-cell migration and adhesion. On the basis of the high expression of CBAP during thymocyte development, we investigated the function of CBAP in thymocyte development using a CBAP knockout mouse. CBAP-deficient mice showed normal early thymocyte development and positive selection. In contrast, several negative selection models (including TCR transgene, superantigen staphylococcal enterotoxin B, and anti-CD3 antibody treatment) revealed an attenuation of TCR-induced thymocyte deletion in CBAP knockout mice. This phenotype correlated with a reduced accumulation of BIM upon TCR crosslinking in CBAP-deficient thymocytes. Loss of CBAP led to reduced TCR-induced phosphorylation of proteins involved in both proximal and distal signaling events, including ZAP70, LAT, PLCγ1, and JNK1/2. Moreover, TCR-induced association of LAT signalosome components was reduced in CBAP-deficient thymocytes. Our data demonstrate that CBAP is a novel component in the TCR signaling pathway and modulates thymocyte apoptosis during negative selection.

Differential function of Themis CABIT domains during T cell development

Themis (also named Gasp) is a newly identified Grb2-binding protein that is essential for thymocyte positive selection. Despite the possible involvement of Themis in TCR-mediated signal transduction, its function remains unresolved and controversial. Themis contains two functionally uncharacterized regions called CABIT (cysteine-containing, all-β in Themis) domains, a nuclear localization signal (NLS), and a proline-rich sequence (PRS). To elucidate the role of these motifs in Themis’s function in vivo, we established a series of mutant Themis transgenic mice on a Themis^−/− background. Deletion of the highly conserved Core motif of CABIT1 or CABIT2 (Core1 or Core2, respectively), the NLS, or the PRS abolished Grb2-association, as well as TCR-dependent tyrosine-phosphorylation and the ability to induce positive selection in the thymus. The NLS and Core1 motifs were required for the nuclear localization of Themis, whereas Core2 and PRS were not. Furthermore, expression of ΔCore1- but not ΔCore2-Themis conferred dominant negative-type inhibition on T cell development. Collectively, our current results indicate that PRS, NLS, CABIT1, and CABIT2 are all required for positive selection, and that each of the CABIT domains exerts distinct functions during positive selection.

Sos1 regulates sustained TCR-mediated Erk activation

The duration and/or the magnitude of Ras-Erk activation are known to be crucial for cell-fate decisions. In T cells, sustained Erk activation correlates with differentiation/proliferation, whereas transient Erk activation parallels with unresponsiveness/apoptosis. The mechanism by which Son of sevenless (Sos) proteins and Ras guanyl-releasing protein 1 (RasGRP1) contribute to dynamics of Erk activation in mature T cells is not yet known. Here, we have assessed this issue using stimuli inducing either transient or sustained TCR signaling and RNA interference mediated suppression of Sos1, Sos2, and RasGRP1 expression in primary human T cells. We found that transient Erk activation depends on RasGRP1 but not on Sos. Conversely, sustained Erk signaling and T-cell activation depend on both Sos1 and RasGRP1. In summary, our data show for the first time that the two guanine nucleotide exchange factors expressed in T cells are differentially involved in the regulation of the duration of Erk phosphorylation and T-cell activation.

The ability of Sos1 to oligomerize the adaptor protein LAT is separable from its guanine nucleotide exchange activity in vivo

The activation of the small guanosine triphosphatase Ras by the guanine nucleotide exchange factor (GEF) Sos1 (Son of Sevenless 1) is a central feature of many receptor-stimulated signaling pathways. In developing T cells (thymocytes), Sos1-dependent activation of extracellular signal-regulated kinase (ERK) is required to stimulate cellular proliferation and differentiation. We showed that in addition to its GEF activity, Sos1 acted as a scaffold to nucleate oligomerization of the T cell adaptor protein LAT (linker for activation of T cells) in vivo. The scaffold function of Sos1 depended on its ability to bind to the adaptor protein Grb2. Furthermore, the GEF activity of Sos1 and the Sos1-dependent oligomerization of LAT were separable functions in vivo. Whereas the GEF activity of Sos1 was required for optimal ERK phosphorylation in response to T cell receptor (TCR) stimulation, the Sos1-dependent oligomerization of LAT was required for maximal TCR-dependent phosphorylation and activation of phospholipase C-γ1 and Ca(2+) signaling. Finally, both of these Sos1 functions were required for early thymocyte proliferation. Whereas transgenic restoration of either the GEF activity or the LAT oligomerization functions of Sos1 alone failed to rescue thymocyte development in Sos1-deficient mice, simultaneous reconstitution of these two signals in the same cell restored normal T cell development. This ability of Sos1 to act both as a RasGEF and as a scaffold to nucleate Grb2-dependent adaptor oligomerization may also occur in other Grb2-dependent pathways, such as those activated by growth factor receptors.

The Ras GTPase-activating protein neurofibromin 1 promotes the positive selection of thymocytes

TCR-mediated activation of the Ras signaling pathway is critical for T cell development in the thymus and function in the periphery. However, which members of a family of Ras GTPase-activating proteins (RasGAPs) negatively regulate Ras activation in T cells is unknown. In this study we examined a potential function for the neurofibromin 1 (NF1) RasGAP in the T cell lineage with the use of T cell-specific NF1-deficient mice. Surprisingly, on an MHC class I-restricted TCR transgenic background, NF1 was found to promote thymocyte positive selection. By contrast, NF1 neither promoted nor inhibited the negative selection of thymocytes. In the periphery, NF1 was found to be necessary for the maintenance of normal numbers of naïve CD4⁺ and CD8⁺ T cells but was dispensable as a regulator of TCR-induced Ras activation, cytokine synthesis, proliferation and differentiation and death. These findings point to a novel unexpected role for NF1 in T cell development as well as a regulator of T cell homeostasis.

RasGRP Ras guanine nucleotide exchange factors in cancer

RasGRP proteins are activators of Ras and other related small GTPases by the virtue of functioning as guanine nucleotide exchange factors (GEFs). In vertebrates, four RasGRP family members have been described. RasGRP-1 through -4 share many structural domains but there are also subtle differences between each of the different family members. Whereas SOS RasGEFs are ubiquitously expressed, RasGRP proteins are expressed in distinct patterns, such as in different cells of the hematopoietic system and in the brain. Most studies have concentrated on the role of RasGRP proteins in the development and function of immune cell types because of the predominant RasGRP expression profiles in these cells and the immune phenotypes of mice deficient for Rasgrp genes. However, more recent studies demonstrate that RasGRPs also play an important role in tumorigenesis. Examples are skin- and hematological-cancers but also solid malignancies such as melanoma or prostate cancer. These novel studies bring up many new and unanswered questions related to the molecular mechanism of RasGRP-driven oncogenesis, such as new receptor systems that RasGRP appears to respond to as well as regulatory mechanism for RasGRP expression that appear to be perturbed in these cancers. Here we will review some of the known aspects of RasGRP biology in lymphocytes and will discuss the exciting new notion that RasGRP Ras exchange factors play a role in oncogenesis downstream of various growth factor receptors.

Functional redundancy of Sos1 and Sos2 for lymphopoiesis and organismal homeostasis and survival

Sos1 and Sos2 are ubiquitously expressed, universal Ras guanine nucleotide exchange factors (Ras-GEFs) acting in multiple signal transduction pathways activated by upstream cellular kinases. The embryonic lethality of Sos1 null mutants has hampered ascertaining the specific in vivo contributions of Sos1 and Sos2 to processes controlling adult organism survival or development of hematopoietic and nonhematopoietic organs, tissues, and cell lineages. Here, we generated a tamoxifen-inducible Sos1-null mouse strain allowing analysis of the combined disruption of Sos1 and Sos2 (Sos1/2) during adulthood. Sos1/2 double-knockout (DKO) animals died precipitously, whereas individual Sos1 and Sos2 knockout (KO) mice were perfectly viable. A reduced percentage of total bone marrow precursors occurred in single-KO animals, but a dramatic depletion of B-cell progenitors was specifically detected in Sos1/2 DKO mice. We also confirmed a dominant role of Sos1 over Sos2 in early thymocyte maturation, with almost complete thymus disappearance and dramatically higher reduction of absolute thymocyte counts in Sos1/2 DKO animals. Absolute counts of mature B and T cells in spleen and peripheral blood were unchanged in single-KO mutants, while significantly reduced in Sos1/2 DKO mice. Our data demonstrate functional redundancy between Sos1 and Sos2 for homeostasis and survival of the full organism and for development and maturation of T and B lymphocytes.

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.

Structural analysis of autoinhibition in the Ras-specific exchange factor RasGRP1

RasGRP1 and SOS are Ras-specific nucleotide exchange factors that have distinct roles in lymphocyte development. RasGRP1 is important in some cancers and autoimmune diseases but, in contrast to SOS, its regulatory mechanisms are poorly understood. Activating signals lead to the membrane recruitment of RasGRP1 and Ras engagement, but it is unclear how interactions between RasGRP1 and Ras are suppressed in the absence of such signals. We present a crystal structure of a fragment of RasGRP1 in which the Ras-binding site is blocked by an interdomain linker and the membrane-interaction surface of RasGRP1 is hidden within a dimerization interface that may be stabilized by the C-terminal oligomerization domain. NMR data demonstrate that calcium binding to the regulatory module generates substantial conformational changes that are incompatible with the inactive assembly. These features allow RasGRP1 to be maintained in an inactive state that is poised for activation by calcium and membrane-localization signals.

DOI:http://dx.doi.org/10.7554/eLife.00813.001

Individual cells within the human body must grow, divide or specialize to perform the tasks required of them. The fates of these cells are often directed by proteins in the Ras family, which detect signals from elsewhere in the body and orchestrate responses within each cell. The activities of these proteins must be tightly controlled, because cancers and developmental diseases can result if Ras proteins are not properly regulated.

Binding to the small molecule GTP activates Ras and causes conformational changes that allow it to interact with other proteins in various signaling pathways in the cell. GTP is loaded into Ras by proteins called nucleotide exchange factors, which can replace ‘used’ nucleotides with ‘fresh’ ones to activate Ras.

These nucleotide exchange factors are also tightly regulated. For example, the genes for many exchange factors are only switched on after particular signals are received, which can restrict their presence to defined times and locations (e.g., cells or tissues). Also, when activating signals are absent, nucleotide exchange factors commonly reside in the cytoplasm, whereas the Ras proteins remain bound to lipid membranes inside the cell.

RasGRP1 is a nucleotide exchange factor that controls the development of immune cells, and leukemia and lupus can result if it is not regulated correctly. However, many questions about RasGRP1 remain unanswered, including how it is able to remain inactive, and how it is activated by various different signals.

Iwig et al. have now revealed the mechanisms through which RasGRP1 suppresses Ras signaling in immune cells by solving the structures of two fragments of RasGRP1 and then using a combination of structural, biochemical and cell-based methods to explore how it is activated. These analyses revealed that inactive RasGRP1 adopts a conformation in which one of its regulatory elements blocks access to the Ras binding site. Surprisingly, RasGRP1 can form dimers; this hides the portions of the protein that associate with the membrane and thereby keeps RasGRP1 away from Ras. Iwig et al. also found that two signals, calcium ions and a lipid called diacylglycerol, overcome these inhibitory mechanisms by changing the conformation of RasGRP1 and recruiting it to the membrane.

These studies provide a framework for understanding how disease-associated mutations in RasGRP1 bypass the regulatory mechanisms that insure proper immune cell development, and will be critical for developing therapeutic agents that inhibit RasGRP1 activity.

DOI:http://dx.doi.org/10.7554/eLife.00813.002

Activation of Extracellular Signal-Regulated Kinase but Not of p38 Mitogen-Activated Protein Kinase Pathways in Lymphocytes Requires Allosteric Activation of SOS

Thymocytes convert graded T cell receptor (TCR) signals into positive selection or deletion, and activation of extracellular signal-related kinase (ERK), p38, and Jun N-terminal protein kinase (JNK) mitogen-activated protein kinases (MAPKs) has been postulated to play a discriminatory role. Two families of Ras guanine nucleotide exchange factors (RasGEFs), SOS and RasGRP, activate Ras and the downstream RAF-MEK-ERK pathway. The pathways leading to lymphocyte p38 and JNK activation are less well defined. We previously described how RasGRP alone induces analog Ras-ERK activation while SOS and RasGRP cooperate to establish bimodal ERK activation. Here we employed computational modeling and biochemical experiments with model cell lines and thymocytes to show that TCR-induced ERK activation grows exponentially in thymocytes and that a W729E allosteric pocket mutant, SOS1, can only reconstitute analog ERK signaling. In agreement with RasGRP allosterically priming SOS, exponential ERK activation is severely decreased by pharmacological or genetic perturbation of the phospholipase Cγ (PLCγ)-diacylglycerol-RasGRP1 pathway. In contrast, p38 activation is not sharply thresholded and requires high-level TCR signal input. Rac and p38 activation depends on SOS1 expression but not allosteric activation. Based on computational predictions and experiments exploring whether SOS functions as a RacGEF or adaptor in Rac-p38 activation, we established that the presence of SOS1, but not its enzymatic activity, is critical for p38 activation.

Ras and extracellular signal-regulated kinase signaling in thymocytes and T cells

Extracellular signal-regulated kinase (ERK) activation is important for both thymocyte development and T cell function. Classically, signal transduction from the T cell antigen receptor (TCR) to ERK is thought to be regulated by signaling from Ras guanine nucleotide exchange factors (GEFs), through the small G protein Ras, to the three-tiered Raf-MAPK/ERK kinase (MEK)-ERK kinase cascade. Developing and mature T cells express four members of two RasGEF families, RasGRP1, RasGRP4, son of sevenless 1 (Sos1), and Sos2, and several models describing combined signaling from these RasGEFs have been proposed. However, recent studies suggest that existing models need revision to include both distinct and overlapping roles of multiple RasGEFs during thymocyte development and novel, Ras-independent signals to ERK that have been identified in peripheral T cells.

GRB2-mediated recruitment of THEMIS to LAT is essential for thymocyte development

Thymocyte-expressed molecule involved in selection (THEMIS) is a recently identified regulator of thymocyte positive selection. THEMIS's mechanism of action is unknown, and whether it has a role in TCR-proximal signaling is controversial. In this article, we show that THEMIS and the adapter molecule growth factor receptor-bound protein 2 (GRB2) associate constitutively through binding of a conserved PxRPxK motif within the proline-rich region 1 of THEMIS to the C-terminal SH3-domain of GRB2. This association is indispensable for THEMIS recruitment to the immunological synapse via the transmembrane adapter linker for activation of T cells (LAT) and for THEMIS phosphorylation by Lck and ZAP-70. Two major sites of tyrosine phosphorylation were mapped to a YY-motif close to proline-rich region 1. The YY-motif was crucial for GRB2 binding, suggesting that this region of THEMIS might control local phosphorylation-dependent conformational changes important for THEMIS function. Finally, THEMIS binding to GRB2 was required for thymocyte development. Our data firmly assign THEMIS to the TCR-proximal signaling cascade as a participant in the LAT signalosome and suggest that the THEMIS-GRB2 complex might be involved in shaping the nature of Ras signaling, thereby governing thymic selection.

RasGRP1, but not RasGRP3, is required for efficient thymic β-selection and ERK activation downstream of CXCR4

T cell development is a highly dynamic process that is driven by interactions between developing thymocytes and the thymic microenvironment. Upon entering the thymus, the earliest thymic progenitors, called CD4⁻CD8⁻ ‘double negative’ (DN) thymocytes, pass through a checkpoint termed “β-selection” before maturing into CD4⁺CD8⁺ ‘double positive’ (DP) thymocytes. β-selection is an important developmental checkpoint during thymopoiesis where developing DN thymocytes that successfully express the pre-T cell receptor (TCR) undergo extensive proliferation and differentiation towards the DP stage. Signals transduced through the pre-TCR, chemokine receptor CXCR4 and Notch are thought to drive β-selection. Additionally, it has long been known that ERK is activated during β-selection; however the pathways regulating ERK activation remain unknown. Here, we performed a detailed analysis of the β-selection events in mice lacking RasGRP1, RasGRP3 and RasGRP1 and 3. We report that RasGRP1 KO and RasGRP1/3 DKO deficient thymi show a partial developmental block at the early DN3 stage of development. Furthermore, DN3 thymocytes from RasGRP1 and RasGRP1/3 double knock-out thymi show significantly reduced proliferation, despite expression of the TCRβ chain. As a result of impaired β-selection, the pool of TCRβ⁺ DN4 is significantly diminished, resulting in inefficient DN to DP development. Also, we report that RasGRP1 is required for ERK activation downstream of CXCR4 signaling, which we hypothesize represents a potential mechanism of RasGRP1 regulation of β-selection. Our results demonstrate that RasGRP1 is an important regulator of proliferation and differentiation at the β-selection checkpoint and functions downstream of CXCR4 to activate the Ras/MAPK pathway.

A phospholipase C-γ1-independent, RasGRP1-ERK-dependent pathway drives lymphoproliferative disease in linker for activation of T cells-Y136F mutant mice

Mice expressing a germline mutation in the phospholipase C-γ1-binding site of linker for activation of T cells (LAT) show progressive lymphoproliferation and ultimately die at 4-6 mo age. The hyperactivated T cells in these mice show defective TCR-induced calcium flux but enhanced Ras/ERK activation, which is critical for disease progression. Despite the loss of LAT-dependent phospholipase C-γ1 binding and activation, genetic analysis revealed RasGRP1, and not Sos1 or Sos2, to be the major Ras guanine exchange factor responsible for ERK activation and the lymphoproliferative phenotype in these mice. Analysis of isolated CD4(+) T cells from LAT-Y136F mice showed altered proximal TCR-dependent kinase signaling, which activated a Zap70- and LAT-independent pathway. Moreover, LAT-Y136F T cells showed ERK activation that was dependent on Lck and/or Fyn, protein kinase C-θ, and RasGRP1. These data demonstrate a novel route to Ras activation in vivo in a pathological setting.

The adaptor protein LAT serves as an integration node for signaling pathways that drive T cell activation

T cells are essential for the adaptive immune response to pathogens. However, dysfunctional T cell activity has been implicated in numerous diseases, including the failure of organ transplants, allergic reactions, asthma, autoimmune disorders, and coronary artery disease. T cell responses to pathogens require the induction of the primary activating receptor, the T cell receptor (TCR), along with other costimulatory and adhesion receptors. Signal transduction pathways activated downstream of these receptors drive T cell responses required for the immune response and disease progression. A key question in our understanding of the mechanism of T cell activation is how signaling pathways emanating from multiple receptors integrate together to alter T cell effector functions. One integration node for intracellular signaling is the membrane-associated adaptor protein linker for the activation of T cells or LAT. Upon stimulation of the TCR and other receptors, LAT is phosphorylated at several tyrosines residues on its cytoplasmic tail. This leads to the binding of SH2 domain-containing proteins and their associated molecules and the formation of large multiprotein complexes. These dynamic and highly regulated signaling complexes facilitate the production of second messengers, activate downstream pathways, induce actin cytoskeleton polymerization, and stimulate the activity of multiple transcription factors. Thus, signaling pathways from several receptors feed into LAT, which then integrates this information and selectively induces pathways critical for T cell activation and the adaptive immune response.

T cell Receptor Signal Transduction in T lymphocytes

The T cell receptor (TCR) recognizes self or foreign antigens presented by major histocompatibility complex (MHC) molecules. Engagement of the TCR triggers the formation of multi-molecular signalosomes that lead to the generation of second messengers and subsequent activation of multiple distal signaling cascades, such as the Ca+2-calcineurin-NFAT, RasGRP1-Ras-Erk1/2, PKCθ-IKK-NFκB, and TSC1/2-mTOR pathways. These signaling cascades control many aspects of T cell biology. Mechanisms have been evolved to fine-tune TCR signaling to maintain T cell homeostasis and self-tolerance, and to properly mount effective responses to microbial infection. Defects or deregulation of TCR signaling has been implicated in the pathogenesis of multiple human diseases.