Targeted Sos1 deletion reveals its critical role in early T-cell development

Kortum, Robert L.; Sommers, Connie L.; Alexander, Clayton P.; Pinski, John M.; Li, Wenmei; Grinberg, Alex; Lee, Jan; Love, Paul E.; Samelson, Lawrence E.

doi:10.1073/pnas.1104295108

Cited by 57 publications

(114 citation statements)

References 53 publications

(87 reference statements)

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“…Agonist-driven negative selection (that elicits strong and transient ERK activation) (28) might instead favor SOS1-GRB2/LAT-type signalosomes during the late phases of (or post) positive selection, when THEMIS levels decrease, thus favoring cell death. The observation that positive section requires THEMIS (1-3), but not SOS proteins (27), and recent data that SOS1 is instead required for negative selection (29) agree with this model. A functional role of constitutive THEMIS-GRB2 association in thymocyte positive selection agrees with data on conditional ablation of GRB2 in thymocytes (30), which showed that GRB2-SH2 and -SH3C (required for association to THEMIS) were found to be indispensable for thymocyte development (30).…”

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confidence: 72%

GRB2-Mediated Recruitment of THEMIS to LAT Is Essential for Thymocyte Development

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Brockmeyer

et al. 2013

The Journal of Immunology

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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.

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supporting

confidence: 72%

GRB2-Mediated Recruitment of THEMIS to LAT Is Essential for Thymocyte Development

Paster

Brockmeyer

et al. 2013

The Journal of Immunology

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“…In agreement with this model, RasGRP1 without SOS sends ana-log ERK signals (10,12,25), ERK phosphorylation and positive selection are impaired in Rasgrp1 Ϫ/Ϫ mice (18), ERK1 and ERK2 doubly deficient thymocytes demonstrate impaired positive selection (27), and Grb2 haploinsufficiency (28) or conditional inactivation of Sos1 (29) in thymocytes leaves positive selection intact. However, recent studies have demonstrated that this model is incomplete; negative selection is intact in ERK1 Ϫ/Ϫ ERK2 Ϫ/Ϫ thymocytes (30) as well as in thymocytes with conditional inactivation of Sos1 (29), even in the context of SOS2 deletion (31).…”

Section: Tcr-induced Erk Phosphorylation Is Impaired In Rasgrp1mentioning

confidence: 66%

“…However, recent studies have demonstrated that this model is incomplete; negative selection is intact in ERK1 Ϫ/Ϫ ERK2 Ϫ/Ϫ thymocytes (30) as well as in thymocytes with conditional inactivation of Sos1 (29), even in the context of SOS2 deletion (31). Thus, TCR-induced digital ERK activation alone cannot be sufficient for the negative selection signal.…”

Section: Tcr-induced Erk Phosphorylation Is Impaired In Rasgrp1mentioning

confidence: 99%

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

Jun

Yang

Chen

et al. 2013

Molecular and Cellular Biology

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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 C␥ (PLC␥)-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.

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“…These data, in conjunction with the marked downregulation of Sos1 and upregulation of RasGRP1 protein levels observed between DN and DP thymocytes, have suggested an alternative model in which the RasGEF expression profile is the major determinant of Ras activation at these critical intrathymic checkpoints (12). A caveat to this second model is that genetic studies to date have not described a role for either Sos1 or RasGRP1 in negative selection (12,20). Therefore, whether Ras is involved in negative selection at the TCR checkpoint remains a critical, unanswered question.…”

mentioning

confidence: 93%

Deconstructing Ras Signaling in the Thymus

Kortum

Sommers

Pinski

et al. 2012

Molecular and Cellular Biology

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Thymocytes must transit at least two distinct developmental checkpoints, governed by signals that emanate from either the pre-T cell receptor (pre-TCR) or the TCR to the small G protein Ras before emerging as functional T lymphocytes. Recent studies have shown a role for the Ras guanine exchange factor (RasGEF) Sos1 at the pre-TCR checkpoint. At the second checkpoint, the quality of signaling through the TCR is interrogated to ensure the production of an appropriate T cell repertoire. Although Ras-GRP1 is the only confirmed RasGEF required at the TCR checkpoint, current models suggest that the intensity and character of Ras activation, facilitated by both Sos and RasGRP1, will govern the boundary between survival (positive selection) and death (negative selection) at this stage. Using mouse models, we have assessed the independent and combined roles for the RasGEFs Sos1, Sos2, and RasGRP1 during thymocyte development. Although Sos1 was the dominant RasGEF at the pre-TCR checkpoint, combined Sos1/RasGRP1 deletion was required to effectively block development at this stage. Conversely, while RasGRP1 deletion efficiently blocked positive selection, combined RasGRP1/Sos1 deletion was required to block negative selection. This functional redundancy in RasGEFs during negative selection may act as a failsafe mechanism ensuring appropriate central tolerance. Tcell development is initiated when immature precursor cells emigrate from the fetal liver or adult bone marrow to the thymus. In the thymus, these cells undergo a receptor-driven differentiation program, passing through at least two distinct developmental checkpoints before emerging as functional T lymphocytes (2). At the first checkpoint, a properly rearranged T cell receptor ␤ (TCR␤) chain pairs with a pre-TCR␣ chain to form a pre-T cell receptor (pre-TCR). The pre-TCR signals in a ligand-independent manner to promote proliferation and drive differentiation from the CD4 Ϫ CD8 Ϫ double-negative (DN) to the CD4 ϩ CD8 ϩ double-positive (DP) stage of thymocyte development. At the second checkpoint, the strength and quality of signaling through the mature TCR is interrogated. Cells that fail to signal through the TCR die by neglect, cells expressing a TCR that binds self-antigen in the context of the major histocompatibility complex (MHC) with strong affinity generate strong TCR signals and die via TCR-dependent apoptotic pathways (negative selection), whereas cells expressing a TCR that has weak affinity for self antigen-MHC complexes signal weakly and selectively survive (positive selection) (8). Genetic studies have shown that signaling from either the pre-TCR or the TCR, through the adaptors LAT and Slp-76, to the small G protein Ras and the downstream extracellular signal-regulated kinase (ERK) cascade is required for thymocyte development at both checkpoints (1,7,14,17,22,(24)(25)(26). However, an understanding of how Ras signals are generated to drive thymocyte development has remained enigmatic, despite intense study over the past 20 years.Ras is thought to be...

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Targeted Sos1 deletion reveals its critical role in early T-cell development

Cited by 57 publications

References 53 publications

GRB2-Mediated Recruitment of THEMIS to LAT Is Essential for Thymocyte Development

GRB2-Mediated Recruitment of THEMIS to LAT Is Essential for Thymocyte Development

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

Deconstructing Ras Signaling in the Thymus

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