RasGRP1 transduces low-grade TCR signals which are critical for T cell development, homeostasis, and differentiation

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.

A novel homozygous mutation in RASGRP1 that predisposes to immune dysregulation and immunodeficiency associated with uncontrolled Epstein-Barr virus-induced B cell proliferation

BACKGROUND

RASGRP1-deficiency results in an immune dysregulation and immunodeficiency that manifest as autoimmunity, lymphoproliferation, lymphopenia, defective T cell function, and increased incidence of Epstein-Bar Virus infections and lymphomas.

OBJECTIVE

To investigate the mechanism of autoimmune hemolytic anemia and infections in a male patient of consanguineous parents from Lebanon.

METHODS

Genetic diagnosis was obtained using next generation and Sanger sequencing. Protein expression and phosphorylation were determined by immunoblotting. T and B cell development and function were studied by flow cytometry. Cytokine and immunoglobulin secretions were quantified by enzyme-linked immunosorbent assay.

RESULTS

The patient suffered from severe lymphopenia especially affecting the T cell compartment. Genetic analysis revealed a homozygous insertion of adenine at position 1396_1397 in RASGRP1 that abolished protein expression and downstream Ras signaling. T cells from the patient showed severe activation defects resulting in uncontrolled Epstein-Bar Virus-induced B cell proliferation. B cells from the patient were normal.

CONCLUSION

This report expands the spectrum of mutations in patients with RasGRP1 deficiency, and provides evidence for the important role RasGRP1 plays in the ability of T cells to control Epstein-Bar Virus-induced B cell proliferation.

CLINICAL IMPLICATIONS

Following diagnosis, the patient will be maintained on oral valganciclovir and monitored regularly for Epstein-Bar Virus infections to avoid the development of Epstein-Bar Virus- induced B cell lymphoma. He is also candidate for hematopoietic stem cell transplantation.

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.

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.

RASGRP2 is a potential immune-related biomarker and regulates mitochondrial-dependent apoptosis in lung adenocarcinoma

Background

Ras guanine nucleotide-releasing protein 2 (RASGRP2), one of the guanine nucleotide exchange factors (GEFs), has attracted much attention in recent years. However, the correlation between RASGRP2 and immune infiltration and malignant features in lung adenocarcinoma (LUAD) has rarely been mentioned.

Methods

The Limma package and the LASSO regression model were performed to screen for differentially expressed genes. Data from the TCGA and 5 GEO databases were used to explore the expression level of RASGRP2 in LUAD patients. A weighted co-expression network and LinkFinder module were established to find the related genes of RASGRP2. The ESTIMATE algorithm was used to analyze the correlation between RASGRP2 and immune infiltration in LUAD. Tumor-infiltrating immune cells were sorted and sequenced at the single-cell level to analyze differences in RASGRP2. Real-time PCR and immunohistochemistry were performed in the real-world cohort to verify the expression of RASGRP2 and its correlation with immune-related genes. Clone formation and EdU assays were used to verify the proliferation ability. The proportion of apoptotic cells was analyzed by flow cytometry. Observation of mitochondrial membrane potential (MMP) changes by fluorescence microscopy.

Results

Our results suggested that decreased RASGRP2 was associated with worse clinical parameters and prognosis in LUAD patients. And we constructed a FLI1-HSA-miR-1976-RASGRP2 transcriptional network to support the role of RASGRP2. Enrichment analysis revealed that RASGRP2 was involved in lymphocyte activation and leukocyte adhesion. RASGRP2 was found to be positively correlated with the infiltration of most immune cells, immunoregulators, and chemokines in a subsequent study. Meanwhile, the real-world cohort confirmed that the expression levels of PDCD1, CTLA4, CD40LG, CCL14, CXCR5, and CCR7 were higher in the high-RASGRP2 expression group. Cytological experiments proved that RASGRP2 inhibited cell proliferation in LUAD by regulating mitochondrial-dependent apoptosis.

Conclusion

RASGRP2 was a potential immune-related biomarker of LUAD. In addition, RASGRP2 was involved in the malignant progression of LUAD through the regulation of mitochondrial-dependent apoptosis.

RasGRP1 promotes the acute inflammatory response and restricts inflammation-associated cancer cell growth

An acute inflammatory response needs to be properly regulated to promote the elimination of pathogens and prevent the risk of tumorigenesis, but the relevant regulatory mechanism has not been fully elucidated. Here, we report that Ras guanine nucleotide-releasing protein 1 (RasGRP1) is a bifunctional regulator that promotes acute inflammation and inhibits inflammation-associated cancer. At the mRNA level, Rasgrp1 activates the inflammatory response by functioning as a competing endogenous RNA to specifically promote IL-6 expression by sponging let-7a. In vivo overexpression of the Rasgrp1 3' untranslated region enhances lipopolysaccharide-induced systemic inflammation and dextran sulphate sodium-induced colitis in Il6^+/+ mice but not in Il6^-/- mice. At the protein level, RasGRP1 overexpression significantly inhibits the tumour-promoting effect of IL-6 in hepatocellular carcinoma progenitor cell-like spheroids. Examination of the EGFR signalling pathway shows that RasGRP1 inhibits inflammation-associated cancer cell growth by disrupting the EGFR-SOS1-Ras-AKT signalling pathway. Tumour patients with high RasGRP1 expression have better clinical outcomes than those with low RasGRP1 expression. Considering that acute inflammation rarely leads to tumorigenesis, this study suggests that RasGRP1 may be an important bifunctional regulator of the acute inflammatory response and tumour growth.

RASA2 ablation in T cells boosts antigen sensitivity and long-term function

The efficacy of adoptive T cell therapies for cancer treatment can be limited by suppressive signals from both extrinsic factors and intrinsic inhibitory checkpoints1,2. Targeted gene editing has the potential to overcome these limitations and enhance T cell therapeutic function3-10. Here we performed multiple genome-wide CRISPR knock-out screens under different immunosuppressive conditions to identify genes that can be targeted to prevent T cell dysfunction. These screens converged on RASA2, a RAS GTPase-activating protein (RasGAP) that we identify as a signalling checkpoint in human T cells, which is downregulated upon acute T cell receptor stimulation and can increase gradually with chronic antigen exposure. RASA2 ablation enhanced MAPK signalling and chimeric antigen receptor (CAR) T cell cytolytic activity in response to target antigen. Repeated tumour antigen stimulations in vitro revealed that RASA2-deficient T cells show increased activation, cytokine production and metabolic activity compared with control cells, and show a marked advantage in persistent cancer cell killing. RASA2-knockout CAR T cells had a competitive fitness advantage over control cells in the bone marrow in a mouse model of leukaemia. Ablation of RASA2 in multiple preclinical models of T cell receptor and CAR T cell therapies prolonged survival in mice xenografted with either liquid or solid tumours. Together, our findings highlight RASA2 as a promising target to enhance both persistence and effector function in T cell therapies for cancer treatment.

Comparative transcriptome analysis reveals molecular adaptations underlying distinct immunity and inverted resting posture in bats

Understanding how natural selection shapes unique traits in mammals is a central topic in evolutionary biology. The mammalian order Chiroptera (bats) is attractive for biologists as well as the general public due to their specific traits of extraordinary immunity and inverted resting posture. However, genomic resources for bats that occupy key phylogenetic positions are not sufficient, which hinders comprehensive investigation of the molecular mechanisms underpinning the origin of specific traits in bats. Here, we sequenced the transcriptomes of five bats that are phylogenetically divergent and occupy key positions in the phylogenetic tree of bats. In combination with the available genomes of 19 bats and 21 other mammals, we built a database consisting of 10,918 one-to-one ortholog genes and reconstructed phylogenetic relationships of these mammals. We found that genes related to immunity, bone remodeling and cardiovascular system are targets of natural selection along the ancestral branch of bats. Further analyses revealed that the T cell receptor signaling pathway involved in immune adaptation is specifically enriched in bats. Moreover, molecular adaptations of bone remodeling, cardiovascular system, and balance sensing may help to explain the reverted resting posture in bats. Our study provides valuable transcriptome resources, enabling us to tentatively identify genetic changes associated with bat-specific traits. This work is among the first to advance our understanding of molecular underpinnings of inverted resting posture in bats, which could provide insight into healthcare applications such as hypertension in humans. This article is protected by copyright. All rights reserved.

Prioritization of autoimmune disease-associated genetic variants that perturb regulatory element activity in T cells

Genome-wide association studies (GWASs) have uncovered hundreds of autoimmune disease-associated loci; however, the causal genetic variants within each locus are mostly unknown. Here, we perform high-throughput allele-specific reporter assays to prioritize disease-associated variants for five autoimmune diseases. By examining variants that both promote allele-specific reporter expression and are located in accessible chromatin, we identify 60 putatively causal variants that enrich for statistically fine-mapped variants by up to 57.8-fold. We introduced the risk allele of a prioritized variant (rs72928038) into a human T cell line and deleted the orthologous sequence in mice, both resulting in reduced BACH2 expression. Naive CD8 T cells from mice containing the deletion had reduced expression of genes that suppress activation and maintain stemness and, upon acute viral infection, displayed greater propensity to become effector T cells. Our results represent an example of an effective approach for prioritizing variants and studying their physiologically relevant effects.

Overarching roles of diacylglycerol signaling in cancer development and antitumor immunity

Diacylglycerol (DAG) is a lipid second messenger that is generated in response to extracellular stimuli and channels intracellular signals that affect mammalian cell proliferation, survival, and motility. DAG exerts a myriad of biological functions through protein kinase C (PKC) and other effectors, such as protein kinase D (PKD) isozymes and small GTPase-regulating proteins (such as RasGRPs). Imbalances in the fine-tuned homeostasis between DAG generation by phospholipase C (PLC) enzymes and termination by DAG kinases (DGKs), as well as dysregulation in the activity or abundance of DAG effectors, have been widely associated with tumor initiation, progression, and metastasis. DAG is also a key orchestrator of T cell function and thus plays a major role in tumor immunosurveillance. In addition, DAG pathways shape the tumor ecosystem by arbitrating the complex, dynamic interaction between cancer cells and the immune landscape, hence representing powerful modifiers of immune checkpoint and adoptive T cell-directed immunotherapy. Exploiting the wide spectrum of DAG signals from an integrated perspective could underscore meaningful advances in targeted cancer therapy.

Stripenn detects architectural stripes from chromatin conformation data using computer vision

Architectural stripes tend to form at genomic regions harboring genes with salient roles in cell identity and function. Therefore, the accurate identification and quantification of these features are essential for understanding lineage-specific gene regulation. Here, we present Stripenn, an algorithm rooted in computer vision to systematically detect and quantitate architectural stripes from chromatin conformation measurements using various technologies. We demonstrate that Stripenn outperforms existing methods and highlight its biological applications in the context of B and T lymphocytes. By comparing stripes across distinct cell types and different species, we find that these chromatin features are highly conserved and form at genes with prominent roles in cell-type-specific processes. In summary, Stripenn is a computational method that borrows concepts from widely used image processing techniques to demarcate and quantify architectural stripes.

Ras GEF Mouse Models for the Analysis of Ras Biology and Signaling

Animal models have become in recent years a crucial tool to understand the physiological and pathological roles of many cellular proteins. They allow analysis of the functional consequences of [1] complete or partial (time- or organ-limited) removal of specific proteins (knockout animals), [2] the exchange of a wild-type allele for a mutant or truncated version found in human illnesses (knock-in), or [3] the effect of overexpression of a given protein in the whole body or in specific organs (transgenic mice). In this regard, the study of phenotypes in Ras GEF animal models has allowed researchers to find specific functions for otherwise very similar proteins, uncovering their role in physiological contexts such as memory formation, lymphopoiesis, photoreception, or body homeostasis. In addition, mouse models have been used to unveil the functional role of Ras GEFs under pathological conditions, including Noonan syndrome, skin tumorigenesis, inflammatory diseases, diabetes, or ischemia among others. In the following sections, we will describe the methodological approaches employed for Ras GEF animal model analyses, as well as the main discoveries made.

Dysregulated RASGRP1 expression through RUNX1 mediated transcription promotes autoimmunity

RasGRP1 is a Ras guanine nucleotide exchange factor, and an essential regulator of lymphocyte receptor signaling. In mice, Rasgrp1 deletion results in defective T lymphocyte development. RASGRP1-deficient patients suffer from immune deficiency, and the RASGRP1 gene has been linked to autoimmunity. However, how RasGRP1 levels are regulated, and if RasGRP1 dosage alterations contribute to autoimmunity remains unknown. We demonstrate that diminished Rasgrp1 expression caused defective T lymphocyte selection in C57BL/6 mice, and that the severity of inflammatory disease inversely correlates with Rasgrp1 expression levels. In patients with autoimmunity, active inflammation correlated with decreased RASGRP1 levels in CD4+ T cells. By analyzing H3K27 acetylation profiles in human T cells, we identified a RASGRP1 enhancer that harbors autoimmunity-associated SNPs. CRISPR-Cas9 disruption of this enhancer caused lower RasGRP1 expression, and decreased binding of RUNX1 and CBFB transcription factors. Analyzing patients with autoimmunity, we detected reduced RUNX1 expression in CD4+ T cells. Lastly, we mechanistically link RUNX1 to transcriptional regulation of RASGRP1 to reveal a key circuit regulating RasGRP1 expression, which is vital to prevent inflammatory disease.

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.

The RNA binding protein SRSF1 is a master switch of gene expression and regulation in the immune system

Serine/Arginine splicing factor 1 (SRSF1) is an RNA binding protein abundantly expressed in most tissues. The pleiotropic functions of SRSF1 exert multiple roles in gene expression by regulating major steps in transcription, processing, export through the nuclear pores and translation of nascent RNA transcripts. The aim of this review is to highlight recent findings in the functions of this protein and to describe its role in immune system development, functions and regulation.

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.

Mechanistic Characterization of RASGRP1 Variants Identifies an hnRNP-K-Regulated Transcriptional Enhancer Contributing to SLE Susceptibility

Systemic lupus erythematosus (SLE) is an autoimmune disease with a strong genetic component. We recently identified a novel SLE susceptibility locus near RASGRP1, which governs the ERK/MAPK kinase cascade and B-/T-cell differentiation and development. However, precise causal RASGRP1 functional variant(s) and their mechanisms of action in SLE pathogenesis remain undefined. Our goal was to fine-map this locus, prioritize genetic variants likely to be functional, experimentally validate their biochemical mechanisms, and determine the contribution of these SNPs to SLE risk. We performed a meta-analysis across six Asian and European cohorts (9,529 cases; 22,462 controls), followed by in silico bioinformatic and epigenetic analyses to prioritize potentially functional SNPs. We experimentally validated the functional significance and mechanism of action of three SNPs in cultured T-cells. Meta-analysis identified 18 genome-wide significant (p < 5 × 10⁻⁸) SNPs, mostly concentrated in two haplotype blocks, one intronic and the other intergenic. Epigenetic fine-mapping, allelic, eQTL, and imbalance analyses predicted three transcriptional regulatory regions with four SNPs (rs7170151, rs11631591-rs7173565, and rs9920715) prioritized for functional validation. Luciferase reporter assays indicated significant allele-specific enhancer activity for intronic rs7170151 and rs11631591-rs7173565 in T-lymphoid (Jurkat) cells, but not in HEK293 cells. Following up with EMSA, mass spectrometry, and ChIP-qPCR, we detected allele-dependent interactions between heterogeneous nuclear ribonucleoprotein K (hnRNP-K) and rs11631591. Furthermore, inhibition of hnRNP-K in Jurkat and primary T-cells downregulated RASGRP1 and ERK/MAPK signaling. Comprehensive association, bioinformatics, and epigenetic analyses yielded putative functional variants of RASGRP1, which were experimentally validated. Notably, intronic variant (rs11631591) is located in a cell type-specific enhancer sequence, where its risk allele binds to the hnRNP-K protein and modulates RASGRP1 expression in Jurkat and primary T-cells. As risk allele dosage of rs11631591 correlates with increased RASGRP1 expression and ERK activity, we suggest that this SNP may underlie SLE risk at this locus.

Genome-wide CRISPR Screens in T Helper Cells Reveal Pervasive Crosstalk between Activation and Differentiation

T helper type 2 (Th2) cells are important regulators of mammalian adaptive immunity and have relevance for infection, autoimmunity, and tumor immunology. Using a newly developed, genome-wide retroviral CRISPR knockout (KO) library, combined with RNA-seq, ATAC-seq, and ChIP-seq, we have dissected the regulatory circuitry governing activation and differentiation of these cells. Our experiments distinguish cell activation versus differentiation in a quantitative framework. We demonstrate that these two processes are tightly coupled and are jointly controlled by many transcription factors, metabolic genes, and cytokine/receptor pairs. There are only a small number of genes regulating differentiation without any role in activation. By combining biochemical and genetic data, we provide an atlas for Th2 differentiation, validating known regulators and identifying factors, such as Pparg and Bhlhe40, as part of the core regulatory network governing Th2 helper cell fates.

Decreased Expression of Serine/Arginine-Rich Splicing Factor 1 in T Cells From Patients With Active Systemic Lupus Erythematosus Accounts for Reduced Expression of RasGRP1 and DNA Methyltransferase 1

OBJECTIVE

T cells from systemic lupus erythematosus (SLE) patients have reduced protein levels of RasGRP1, a guanine nucleotide exchange factor for Ras, and increased transcript of alternatively spliced (AS) forms lacking exon 11. Serine/arginine-rich splicing factor 1 (SRSF1) binds pre-messenger RNA (pre-mRNA) to regulate AS forms of several genes, including CD3ζ in SLE T cells. This study was undertaken to assess whether SRSF1 controls the expression of RasGRP1 in T cells from patients with SLE.

METHODS

We studied T cells from 45 SLE patients and 18 healthy subjects. Expression levels of SRSF1, wild-type (WT) RasGRP1, and DNA methyltransferase 1 (DNMT1) were assessed by quantitative polymerase chain reaction. Direct binding of SRSF1 to exon 11 of RasGRP1 mRNA was evaluated with an oligonucleotide-protein pulldown assay. Healthy T cells and SLE T cells were treated with SRSF1-specific small interfering RNA or SRSF1 expression vector, respectively, and then evaluated for mRNA/protein expression.

RESULTS

SRSF1 expression levels were significantly lower in T cells from SLE patients compared to those from healthy subjects, and correlated inversely with disease activity and positively with levels of RasGRP1-WT and DNMT1. SRSF1 bound directly to exon 11 of RasGRP1 mRNA. Silencing of SRSF1 in human T cells led to increased ratios of RasGRP1-AS to RasGRP1-WT and decreased levels of RasGRP1 protein, whereas overexpression of SRSF1 in SLE T cells caused recovery of RasGRP1, which in turn induced DNMT1/interleukin-2 expression.

CONCLUSION

SRSF1 controls the alternative splicing of RasGRP1 and subsequent protein expression. Our findings extend evidence that alternative splicing plays a central role in the aberrant T cell function in patients with SLE by controlling the expression of multiple genes.

Loss of RASGRP1 in humans impairs T-cell expansion leading to Epstein-Barr virus susceptibility

Inherited CTPS1, CD27, and CD70 deficiencies in humans have revealed key factors of T-lymphocyte expansion, a critical prerequisite for an efficient immunity to Epstein-Barr virus (EBV) infection. RASGRP1 is a T-lymphocyte-specific nucleotide exchange factor known to activate the pathway of MAP kinases (MAPK). A deleterious homozygous mutation in RASGRP1 leading to the loss RASGRP1 expression was identified in two siblings who both developed a persistent EBV infection leading to Hodgkin lymphoma. RASGRP1-deficient T cells exhibited defective MAPK activation and impaired proliferation that was restored by expression of wild-type RASGRP1. Similar defects were observed in T cells from healthy individuals when RASGRP1 was downregulated. RASGRP1-deficient T cells also exhibited decreased CD27-dependent proliferation toward CD70-expressing EBV-transformed B cells, a crucial pathway required for expansion of antigen-specific T cells during anti-EBV immunity. Furthermore, RASGRP1-deficient T cells failed to upregulate CTPS1, an important enzyme involved in DNA synthesis. These results show that RASGRP1 deficiency leads to susceptibility to EBV infection and demonstrate the key role of RASGRP1 at the crossroad of pathways required for the expansion of activated T lymphocytes.

Unexpected positive control of NFκB and miR-155 by DGKα and ζ ensures effector and memory CD8+ T cell differentiation

Signals from the T-cell receptor (TCR) and γ-chain cytokine receptors play crucial roles in initiating activation and effector/memory differentiation of CD8 T-cells. We report here that simultaneous deletion of both diacylglycerol kinase (DGK) α and ζ (DKO) severely impaired expansion of CD8 effector T cells and formation of memory CD8 T-cells after Listeria monocytogenes infection. Moreover, ablation of both DGKα and ζ in preformed memory CD8 T-cells triggered death and impaired homeostatic proliferation of these cells. DKO CD8 T-cells were impaired in priming due to decreased expression of chemokine receptors and migration to the draining lymph nodes. Moreover, DKO CD8 T-cells were unexpectedly defective in NFκB-mediated miR-155 transcript, leading to excessive SOCS1 expression and impaired γ-chain cytokine signaling. Our data identified a DGK-NFκB-miR-155-SOCS1 axis that bridges TCR and γ-chain cytokine signaling for robust CD8 T-cell primary and memory responses to bacterial infection.

A Histidine pH sensor regulates activation of the Ras-specific guanine nucleotide exchange factor RasGRP1

RasGRPs are guanine nucleotide exchange factors that are specific for Ras or Rap, and are important regulators of cellular signaling. Aberrant expression or mutation of RasGRPs results in disease. An analysis of RasGRP1 SNP variants led to the conclusion that the charge of His 212 in RasGRP1 alters signaling activity and plasma membrane recruitment, indicating that His 212 is a pH sensor that alters the balance between the inactive and active forms of RasGRP1. To understand the structural basis for this effect we compared the structure of autoinhibited RasGRP1, determined previously, to those of active RasGRP4:H-Ras and RasGRP2:Rap1b complexes. The transition from the autoinhibited to the active form of RasGRP1 involves the rearrangement of an inter-domain linker that displaces inhibitory inter-domain interactions. His 212 is located at the fulcrum of these conformational changes, and structural features in its vicinity are consistent with its function as a pH-dependent switch.

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.

2B4-SAP signaling is required for the priming of naive CD8+ T cells by antigen-expressing B cells and B lymphoma cells

Mutations in SH2D1A gene that encodes SAP (SLAM-associated protein) result in X-linked lymphoproliferative disease (XLP), a rare primary immunodeficiency disease defined by exquisite sensitivity to the B-lymphotropic Epstein-Barr virus (EBV) and B cell lymphomas. However, the precise mechanism of how the loss of SAP function contributes to extreme vulnerability to EBV and the development of B cell lymphomas remains unclear. Here, we investigate the hypothesis that SAP is critical for CD8⁺ T cell immune surveillance of antigen (Ag)-expressing B cells or B lymphoma cells under conditions of defined T cell receptor (TCR) signaling. Sh2d1a^-/- CD8⁺ T cells exhibited greatly diminished proliferation relative to wild type when Ag-presenting-B cells or -B lymphoma cells served as the primary Ag-presenting cell (APC). By contrast, Sh2d1a^-/- CD8⁺ T cells responded equivalently to wild-type CD8⁺ T cells when B cell-depleted splenocytes, melanoma cells or breast carcinoma cells performed Ag presentation. Through application of signaling lymphocyte activation molecule (SLAM) family receptor blocking antibodies or SLAM family receptor-deficient CD8⁺ T cells and APCs, we found that CD48 engagement on the B cell surface by 2B4 is crucial for initiating SAP-dependent signaling required for the Ag-driven CD8⁺ T cell proliferation and differentiation. Altogether, a pivotal role for SAP in promoting the expansion and differentiation of B cell-primed viral-specific naive CD8⁺ T cells may explain the selective immune deficiency of XLP patients to EBV and B cell lymphomas.

Diacylglycerol Kinases in T Cell Tolerance and Effector Function

Diacylglycerol kinases (DGKs) are a family of enzymes that regulate the relative levels of diacylglycerol (DAG) and phosphatidic acid (PA) in cells by phosphorylating DAG to produce PA. Both DAG and PA are important second messengers cascading T cell receptor (TCR) signal by recruiting multiple effector molecules, such as RasGRP1, PKCθ, and mTOR. Studies have revealed important physiological functions of DGKs in the regulation of receptor signaling and the development and activation of immune cells. In this review, we will focus on recent progresses in our understanding of two DGK isoforms, α and ζ, in CD8 T effector and memory cell differentiation, regulatory T cell development and function, and invariant NKT cell development and effector lineage differentiation.

Dok-1 and Dok-2 Regulate the Formation of Memory CD8+ T Cells

Diverse signals received by CD8⁺ T cells are integrated to achieve the required magnitude of cell expansion and the appropriate balance of effector/memory CD8⁺ T cell generation. Notably, the strength and nature of TCR signaling influence the differentiation and functional capacity of effector and memory CD8⁺ T cells. Dok-1 and Dok-2, the two members of the Dok family expressed in T cells, negatively regulate TCR signaling in vitro. However, the role of Dok proteins in modulating T cell function in vivo has not yet studied. We studied the function of Dok-1 and Dok-2 proteins in the regulation of the CD8⁺ T cell response to vaccinia virus infection. Comparison of responses to vaccinia virus expressing OVA peptide SIINFEKL by wild-type and Dok-1/2^-/- CD8⁺ OT-I cells showed that the absence of Dok-1 and Dok-2 slightly reduced the magnitude of virus-specific effector CD8⁺ T cell expansion. This was not due to reduced proliferation or enhanced apoptosis of effector CD8⁺ T cells. Dok-1/2-deficient effector CD8⁺ T cells showed increased cell surface TCR expression following virus infection in vivo and increased expression of granzyme B and TNF upon stimulation with peptide Ag ex vivo. Finally, Dok-1/2-deficient effector CD8⁺ T had a severe defect in survival that resulted in impaired generation of memory CD8⁺ T cells. These results reveal the critical involvement of Dok-1 and Dok-2 in a negative-feedback loop that prevents overactivation of CD8⁺ T cells and promotes memory formation.

RasGRP1 and RasGRP3 Are Required for Efficient Generation of Early Thymic Progenitors

T cell development is dependent on the migration of progenitor cells from the bone marrow to the thymus. Upon reaching the thymus, progenitors undergo a complex developmental program that requires inputs from various highly conserved signaling pathways including the Notch and Wnt pathways. To date, Ras signaling has not been implicated in the very earliest stages of T cell differentiation, but members of a family of Ras activators called RasGRPs have been shown to be involved at multiple stages of T cell development. We examined early T cell development in mice lacking RasGRP1, RasGRP3, and RasGRPs 1 and 3. We report that RasGRP1- and RasGRP3-deficient thymi show significantly reduced numbers of early thymic progenitors (ETPs) relative to wild type thymi. Furthermore, RasGRP1/3 double-deficient thymi show significant reductions in ETP numbers compared with either RasGRP1 or RasGRP3 single-deficient thymi, suggesting that both RasGRP1 and RasGRP3 regulate the generation of ETPs. In addition, competitive bone marrow chimera experiments reveal that RasGRP1/3 double-deficient progenitors intrinsically generate ETPs less efficiently than wild type progenitors. Finally, RasGRP1/3-deficient progenitors show impaired migration toward the CCR9 ligand, CCL25, suggesting that RasGRP1 and RasGRP3 may regulate progenitor entry into the thymus through a CCR9-dependent mechanism. These data demonstrate that, in addition to Notch and Wnt, the highly conserved Ras pathway is critical for the earliest stages of T cell development and further highlight the importance of Ras signaling during thymocyte maturation.

Autoantibody-Mediated Pulmonary Alveolar Proteinosis in Rasgrp1-Deficient Mice

Pulmonary alveolar proteinosis (PAP) is a rare lung syndrome caused by the accumulation of surfactants in the alveoli. The most prevalent clinical form of PAP is autoimmune PAP (aPAP) whereby IgG autoantibodies neutralize GM-CSF. GM-CSF is a pleiotropic cytokine that promotes the differentiation, survival, and activation of alveolar macrophages, the cells responsible for surfactant degradation. IgG-mediated neutralization of GM-CSF thereby inhibits alveolar macrophage homeostasis and function, leading to surfactant accumulation and innate immunodeficiency. Importantly, there are no rodent models for this disease; therefore, underlying immune mechanisms regulating GM-CSF-specific IgG in aPAP are not well understood. In this article, we identify that autoimmune-prone Rasgrp1-deficient mice develop aPAP: 1) Rasgrp1-deficient mice exhibit reduced pulmonary compliance and lung histopathology characteristic of PAP; 2) alveolar macrophages from Rasgrp1-deficient mice are enlarged and exhibit reduced surfactant degradation; 3) the concentration of GM-CSF-specific IgG is elevated in both serum and bronchoalveolar lavage fluid from Rasgrp1-deficient mice; 4) GM-CSF-specific IgG is capable of neutralizing GM-CSF bioactivity; and 5) Rasgrp1-deficient mice also lacking CD275/ICOSL, a molecule necessary for conventional T cell-dependent Ab production, have reduced GM-CSF-specific autoantibody and do not develop PAP. Collectively, these studies reveal that Rasgrp1-deficient mice, to our knowledge, represent the first rodent model for aPAP.

CD275-Independent IL-17-Producing T Follicular Helper-like Cells in Lymphopenic Autoimmune-Prone Mice

T cells undergo homeostatic expansion and acquire an activated phenotype in lymphopenic microenvironments. Restoration of normal lymphocyte numbers typically re-establishes normal homeostasis, and proinflammatory cytokine production returns to baseline. Mice deficient in guanine nucleotide exchange factor RasGRP1 exhibit dysregulated homeostatic expansion, which manifests as lymphoproliferative disease with autoantibody production. Our previous work revealed that autoreactive B cells lacking RasGRP1 break tolerance early during development, as well as during germinal center responses, suggesting that T cell-independent and T cell-dependent mechanisms are responsible. Examination of whether a particular T cell subset is involved in the breach of B cell tolerance revealed increased Th17 cells in Rasgrp1-deficient mice relative to control mice. Rasgrp1-deficient mice lacking IL-17R had fewer germinal centers, and germinal centers that formed contained fewer autoreactive B cells, suggesting that IL-17 signaling is required for a break in B cell tolerance in germinal centers. Interestingly, a fraction of Th17 cells from Rasgrp1-deficient mice were CXCR5(+) and upregulated levels of CD278 coordinate with their appearance in germinal centers, all attributes of T follicular helper cells (Tfh17). To determine whether CD278-CD275 interactions were required for the development of Tfh17 cells and for autoantibody, Rasgrp1-deficient mice were crossed with CD275-deficient mice. Surprisingly, mice deficient in RasGRP1 and CD275 formed Tfh17 cells and germinal centers and produced similar titers of autoantibodies as mice deficient in only RasGRP1. Therefore, these studies suggest that requirements for Tfh cell development change in lymphopenia-associated autoimmune settings.

RasGRP1 Is an Essential Signaling Molecule for Development of B1a Cells with Autoantigen Receptors

B1a cells, particularly the PD-L2(+) B1a cell subset, are enriched with autoantigen-specific receptors. However, the underlying molecular mechanism responsible for the skewed selection of autoreactive B1a cells remains unclear. In this study, we find that B1 cells express only Ras guanyl nucleotide-releasing protein (RasGRP) 1, whereas B2 cells express mostly RasGRP3 and little RasGRP1. RasGRP1 is indispensable for transduction of weak signals. RasGRP1 deficiency markedly impairs B1a cell development and reduces serum natural IgM production; in particular, B1a cells that express autoantigen receptors, such as anti-phosphatidylcholine B1a cells, are virtually eliminated. Thus, unlike Btk and other signalosome components, RasGRP1 deficiency selectively affects only the B1a cell population with autoantigen receptors rather than the entire pool of B1a cells.

Common RASGRP1 Gene Variants That Confer Risk of Type 2 Diabetes

BACKGROUND

Genome-wide association studies have validated the RAS guanyl nucleotide-releasing protein 1 (RASGRP1) gene as a type 2 diabetes (T2D) susceptibility locus.

AIMS

This study aimed to replicate and verify the association of RASGRP1 tag single-nucleotide polymorphisms with T2D in a Chinese Han population.

METHODS

Eleven common variants of RASGRP1 were detected using TaqMan technology in 1272 healthy controls and 1234 T2D patients. All study participants were unrelated members of the Han ethnic group in China. In this study, the rs7403531 genotype frequency differed significantly between T2D patients and controls (allele: adjusted p=8.30×10(-6), genotype: adjusted p=2.50×10(-5), OR=1.366 [95% CI=1.206-1.546]). The rs4465567-rs4567661 C-A and C-C haplotypes were also significant risk factors for T2D (adjusted p=0.0002 and p=0.0006, respectively) with a global p-value of 6.46×10(-5). These results indicate that in a Chinese Han population, RASGRP1 variants, particularly rs7403531, confer a risk for developing T2D.

RasGRP3 limits Toll-like receptor-triggered inflammatory response in macrophages by activating Rap1 small GTPase

Host immune cells can detect and destruct invading pathogens via pattern-recognition receptors. Small Rap GTPases act as conserved molecular switches coupling extracellular signals to various cellular responses, but their roles as regulators in Toll-like receptor (TLR) signalling have not been fully elucidated. Here we report that Ras guanine nucleotide-releasing protein 3 (RasGRP3), a guanine nucleotide-exchange factor activating Ras and Rap1, limits production of proinflammatory cytokines (especially IL-6) in macrophages by activating Rap1 on activation by low levels of TLR agonists. We demonstrate that RasGRP3, a dominant member of RasGRPs in macrophages, impairs TLR3/4/9-induced IL-6 production and relieves dextrane sulphate sodium-induced colitis and collagen-induced arthritis. In RasGRP3-deficient RAW264.7 cells obtained by CRISPR-Cas9 genome editing, TLR3/4/9-induced activation of Rap1 was inhibited while ERK1/2 activation was enhanced. Our study suggests that RasGRP3 limits inflammatory response by activating Rap1 on low-intensity pathogen infection, setting a threshold for preventing excessive inflammatory response.

Toll like receptors (TLRs) couple microbial sensing to initiation of immune responses, which are essential for defense against pathogens but may cause immunopathology when activated excessively. Here the authors show that RasGRP3 sets a threshold of TLR activation to prevent immunopathology.

T-cells null for the MED23 subunit of mediator express decreased levels of KLF2 and inefficiently populate the peripheral lymphoid organs

MED23, a subunit of the Mediator coactivator complex, is important for the expression of a subset of MAPK/ERK pathway-responsive genes, the constituents of which vary between cell types for reasons that are not completely clear. MAPK/ERK pathway-dependent processes are essential for T-cell development and function, but whether MED23 has a role in this context is unknown. We generated Med23 conditional knockout mice and induced Med23 deletion in early T-cell development using the lineage specific Lck-Cre transgene. While the total cell number and distribution of cell populations in the thymuses of Med23^flox/flox;Lck-Cre mice were essentially normal, MED23 null T-cells failed to efficiently populate the peripheral lymphoid organs. MED23 null thymocytes displayed decreased expression of the MAPK/ERK-responsive genes Egr1, Egr2, as well as of the membrane glycoprotein Cd52 (CAMPATH-1). MED23 null CD4 single-positive thymocytes also showed decreased expression of KLF2 (LKLF), a T-cell master regulatory transcription factor. Indeed, similarities between the phenotypes of mice lacking MED23 or KLF2 in T-cells suggest that KLF2 deficiency in MED23 null T-cells is one of their key defects. Mechanistic experiments using MED23 null MEFs further suggest that MED23 is required for full activity of the MAPK-responsive transcription factor MEF2, which has previously been shown to mediate Klf2 expression. In summary, our data indicate that MED23 has critical roles in enabling T-cells to populate the peripheral lymphoid organs, possibly by potentiating MEF2-dependent expression of the T-cell transcription factor KLF2.

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.

Targeted deletion of RasGRP1 impairs skin tumorigenesis

Ras is frequently activated in cutaneous squamous cell carcinoma, a prevalent form of skin cancer. However, the pathways that contribute to Ras-induced transformation have not been entirely elucidated. We have previously demonstrated that in transgenic mice, overexpression of the Ras activator RasGRP1 promotes the formation of spontaneous skin tumors and enhances malignant progression in the multistage carcinogenesis skin model that relies on the oncogenic activation of H-Ras. Utilizing a RasGRP1 knockout mouse model (RasGRP1 KO), we now show that lack of RasGRP1 reduced the susceptibility to skin tumorigenesis. The dependency on RasGRP1 was associated with a diminished response to the phorbol ester tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA). Specifically, we found impairment of epidermal hyperplasia induced by TPA through keratinocyte proliferation. Using a keratinocyte cell line that carries a ras oncogenic mutation, we also demonstrated that RasGRP1 could further activate Ras in response to TPA. Thus, we propose that RasGRP1 upregulates signaling from Ras and contributes to epidermal tumorigenesis by increasing the total dosage of active Ras.

Rasgrp1 mutation increases naive T-cell CD44 expression and drives mTOR-dependent accumulation of Helios⁺ T cells and autoantibodies

Missense variants are a major source of human genetic variation. Here we analyze a new mouse missense variant, Rasgrp1^Anaef, with an ENU-mutated EF hand in the Rasgrp1 Ras guanine nucleotide exchange factor. Rasgrp1^Anaef mice exhibit anti-nuclear autoantibodies and gradually accumulate a CD44^hi Helios⁺ PD-1⁺ CD4⁺ T cell population that is dependent on B cells. Despite reduced Rasgrp1-Ras-ERK activation in vitro, thymocyte selection in Rasgrp1^Anaef is mostly normal in vivo, although CD44 is overexpressed on naïve thymocytes and T cells in a T-cell-autonomous manner. We identify CD44 expression as a sensitive reporter of tonic mTOR-S6 kinase signaling through a novel mouse strain, chino, with a reduction-of-function mutation in Mtor. Elevated tonic mTOR-S6 signaling occurs in Rasgrp1^Anaef naïve CD4⁺ T cells. CD44 expression, CD4⁺ T cell subset ratios and serum autoantibodies all returned to normal in Rasgrp1^AnaefMtor^chino double-mutant mice, demonstrating that increased mTOR activity is essential for the Rasgrp1^Anaef T cell dysregulation.

DOI:http://dx.doi.org/10.7554/eLife.01020.001

Our DNA contains more than three billion nucleotides. Each of these nucleotides can be an A, C, G or T, and groups of three neighboring nucleotides within our DNA are used to represent the 20 amino acids that are used to make proteins. This means that changing just one nucleotide can cause one amino acid to be replaced by a different amino acid in the protein encoded by that stretch of DNA: AAA and AAG code for the amino acid lysine, for example, but AAC and AAT code for asparagine. Known as missense gene variants, these changes can also increase or decrease the expression of the gene.

Every person has thousands of missense gene variants, including about 12,000 inherited from their parents. Sometimes these variants have no consequence, but they can be harmful if replacing the correct amino acid with a different amino acid prevents the protein from performing an important task. In particular, missense gene variants in genes that encode immune system proteins are likely to play a role in diseases of the immune system. For example, variants near a gene called Rasgrp1 have been linked to two autoimmune diseases – type 1 diabetes and Graves’ disease—in which the immune system mistakenly attacks the body’s own cells and tissues.

Now Daley et al. have shed new light on the mechanism by which a missense gene variant in Rasgrp1 can cause autoimmune diseases. Mice with this mutation show signs of autoimmune disease, but their T cells—white blood cells that have a central role in the immune system – develop normally despite this mutation. Instead, Daley et al. found that a specific type of T cell, called T helper cells, accumulated in large numbers in the mutant mice and stimulated cells of a third type—immune cells called B cells—to produce autoantibodies. The production of autoantibodies is a common feature of autoimmune diseases.

Daley et al. traced the origins of the T helper cells to excessive activity on a signaling pathway that involves a protein called mTOR, and went on to show that treatment with the drug rapamycin counteracted the accumulation of the T helper cells and reduced the level of autoimmune activity. In addition to exemplifying how changing just one amino acid change can have a profound effect, the work of Daley et al. is an attractive model for exploring how missense gene variants in people can contribute to autoimmune diseases.

DOI:http://dx.doi.org/10.7554/eLife.01020.002

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.

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.

Multiple checkpoint breach of B cell tolerance in Rasgrp1-deficient mice

Lymphopenic hosts offer propitious microenvironments for expansion of autoreactive B and T cells. Despite this, many lymphopenic hosts do not develop autoimmune disease, suggesting that additional factors are required for breaching self-tolerance in the setting of lymphopenia. Mice deficient in guanine nucleotide exchange factor Rasgrp1 develop a lymphoproliferative disorder with features of human systemic lupus erythematosus. Early in life, Rasgrp1-deficient mice have normal B cell numbers but are T lymphopenic, leading to defective homeostatic expansion of CD4 T cells. To investigate whether B cell-intrinsic mechanisms also contribute to autoimmunity, Rasgrp1-deficient mice were bred to mice containing a knockin autoreactive BCR transgene (564Igi), thereby allowing the fate of autoreactive B cells to be assessed. During B cell development, the frequency of receptor-edited 564Igi B cells was reduced in Rasrp1-deficient mice compared with Rasgrp1-sufficient littermate control mice, suggesting that tolerance was impaired. In addition, the number of 564Igi transitional B cells was increased in Rasgrp1-deficient mice compared with control mice. Immature 564Igi B cells in bone marrow and spleen lacking RasGRP1 expressed lower levels of Bim mRNA and protein, suggesting that autoreactive B cells elude clonal deletion during development. Concomitant with increased serum autoantibodies, Rasgrp1-deficient mice developed spontaneous germinal centers at 8-10 wk of age. The frequency and number of 564Igi B cells within these germinal centers were significantly increased in Rasgrp1-deficient mice relative to control mice. Taken together, these studies suggest that autoreactive B cells lacking Rasgrp1 break central and peripheral tolerance through both T cell-independent and -dependent mechanisms.

Regulation of Lipid Signaling by Diacylglycerol Kinases during T Cell Development and Function

Diacylglycerol (DAG) and phosphatidic acid (PA) are bioactive lipids synthesized when the T cell receptor binds to a cognate peptide-MHC complex. DAG triggers signaling by recruiting Ras guanyl-releasing protein 1, PKCθ, and other effectors, whereas PA binds to effector molecules that include mechanistic target of rapamycin, Src homology region 2 domain-containing phosphatase 1, and Raf1. While DAG-mediated pathways have been shown to play vital roles in T cell development and function, the importance of PA-mediated signals remains less clear. The diacylglycerol kinase (DGK) family of enzymes phosphorylates DAG to produce PA, serving as a molecular switch that regulates the relative levels of these critical second messengers. Two DGK isoforms, α and ζ, are predominantly expressed in T lineage cells and play an important role in conventional αβ T cell development. In mature T cells, the activity of these DGK isoforms aids in the maintenance of self-tolerance by preventing T cell hyper-activation and promoting T cell anergy. In this review, we discuss the roles of DAG-mediated pathways, PA-effectors, and DGKs in T cell development and function. We also highlight recent work that has uncovered previously unappreciated roles for DGK activity, for instance in invariant NKT cell development, anti-tumor and anti-viral CD8 responses, and the directional secretion of soluble effectors.

Dysregulated RasGRP1 responds to cytokine receptor input in T cell leukemogenesis

Enhanced signaling by the small guanosine triphosphatase Ras is common in T cell acute lymphoblastic leukemia/lymphoma (T-ALL), but the underlying mechanisms are unclear. We identified the guanine nucleotide exchange factor RasGRP1 (Rasgrp1 in mice) as a Ras activator that contributes to leukemogenesis. We found increased RasGRP1 expression in many pediatric T-ALL patients, which is not observed in rare early T cell precursor T-ALL patients with KRAS and NRAS mutations, such as K-Ras(G12D). Leukemia screens in wild-type mice, but not in mice expressing the mutant K-Ras(G12D) that encodes a constitutively active Ras, yielded frequent retroviral insertions that led to increased Rasgrp1 expression. Rasgrp1 and oncogenic K-Ras(G12D) promoted T-ALL through distinct mechanisms. In K-Ras(G12D) T-ALLs, enhanced Ras activation had to be uncoupled from cell cycle arrest to promote cell proliferation. In mouse T-ALL cells with increased Rasgrp1 expression, we found that Rasgrp1 contributed to a previously uncharacterized cytokine receptor-activated Ras pathway that stimulated the proliferation of T-ALL cells in vivo, which was accompanied by dynamic patterns of activation of effector kinases downstream of Ras in individual T-ALLs. Reduction of Rasgrp1 abundance reduced cytokine-stimulated Ras signaling and decreased the proliferation of T-ALL in vivo. The position of RasGRP1 downstream of cytokine receptors as well as the different clinical outcomes that we observed as a function of RasGRP1 abundance make RasGRP1 an attractive future stratification marker for T-ALL.

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.

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 genome-wide association study identifies GRK5 and RASGRP1 as type 2 diabetes loci in Chinese Hans

Substantial progress has been made in identification of type 2 diabetes (T2D) risk loci in the past few years, but our understanding of the genetic basis of T2D in ethnically diverse populations remains limited. We performed a genome-wide association study and a replication study in Chinese Hans comprising 8,569 T2D case subjects and 8,923 control subjects in total, from which 10 single nucleotide polymorphisms were selected for further follow-up in a de novo replication sample of 3,410 T2D case and 3,412 control subjects and an in silico replication sample of 6,952 T2D case and 11,865 control subjects. Besides confirming seven established T2D loci (CDKAL1, CDKN2A/B, KCNQ1, CDC123, GLIS3, HNF1B, and DUSP9) at genome-wide significance, we identified two novel T2D loci, including G-protein-coupled receptor kinase 5 (GRK5) (rs10886471: P = 7.1 × 10(-9)) and RASGRP1 (rs7403531: P = 3.9 × 10(-9)), of which the association signal at GRK5 seems to be specific to East Asians. In nondiabetic individuals, the T2D risk-increasing allele of RASGRP1-rs7403531 was also associated with higher HbA(1c) and lower homeostasis model assessment of β-cell function (P = 0.03 and 0.0209, respectively), whereas the T2D risk-increasing allele of GRK5-rs10886471 was also associated with higher fasting insulin (P = 0.0169) but not with fasting glucose. Our findings not only provide new insights into the pathophysiology of T2D, but may also shed light on the ethnic differences in T2D susceptibility.

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.