Developing thymocytes discriminate positively from negatively selecting self ligands through a sharply switch-like signaling threshold: positive selection responds over a broad graded range of weak T-cell-receptor ligand affinities, whereas negative selection turns on abruptly. Prasad et al. (PNAS 2009) proposed that this asymmetry originates in how the two Ras activators are engaged - RasGRP1 produces graded Ras activation for weak ligands, while SOS's allosteric positive feedback ignites a threshold-like, bimodal Ras/ERK response for strong ligands - and explicitly flagged unresolved issues: unestablished SOS-Ras kinetics, simulated bimodality timing not matching experiments, possible downstream-feedback (MAPK/cPLA2) and Ras-compartmentalization (Golgi versus plasma membrane) alternatives, and untested in vivo consequences of SOS1/SOS2 deficiency or SOS-feedback blockade during thymic selection. The subsequent literature shows: in vivo deletion genetics (2011-2013) showed Sos1/Sos2 are dispensable for both selection outcomes with digital ERK persisting and RasGEF failsafe redundancy, contradicting the model's central prediction; allosteric-pocket-mutant reconstitution nonetheless requires SOS feedback for digital ERK; single-molecule reconstitution (2016-2024) challenges deterministic bistability as the source of bimodality; and a digital RSK phosphorylation module was identified downstream of ERK in thymocytes (2023). The mechanistic origin of the sharp selection boundary, and the in vivo role of SOS feedback within it, remains open.

Background

During T-cell development in the thymus, immature T cells (thymocytes) carrying randomly generated T-cell antigen receptors (TCRs) are screened for reactivity against self peptides bound to major histocompatibility complex proteins (pMHC). Thymocytes whose TCR binds self pMHC too weakly die by neglect, those binding with moderate affinity survive and mature (positive selection), and those binding too strongly are deleted (negative selection) - the basis of self-tolerance. A long-standing puzzle is the asymmetric character of these thresholds: positive selection operates over a broad, graded range of weak ligand affinities, whereas negative selection switches on abruptly, so that a very small increase in TCR-pMHC affinity across the selection threshold changes the outcome qualitatively (Daniels et al. 2006). Both selection modes require signaling from the TCR to the small GTPase Ras, which cycles between an inactive GDP-bound and an active GTP-bound state. Guanine nucleotide exchange factors (GEFs) catalyze GDP-to-GTP exchange (Ras activation) and GTPase-activating proteins (GAPs) accelerate GTP hydrolysis (Ras deactivation). Active Ras (RasGTP) switches on the three-tiered RAF-MEK-ERK kinase cascade, a mitogen-activated protein kinase (MAPK) pathway in which ERK (extracellular signal-regulated kinase) is the terminal kinase.

Lymphocytes express two families of RasGEFs with different regulatory logic. RasGRP1 is recruited to cellular membranes by diacylglycerol (DAG), a lipid second messenger produced when phospholipase C-γ1 (PLCγ1), attached to the transmembrane adaptor protein LAT (linker for activation of T cells), is phosphorylated downstream of TCR engagement. Son of Sevenless 1 and 2 (SOS1/SOS2) are recruited to phosphorylated LAT through the adaptor Grb2. SOS is distinctive because, besides its catalytic Ras-binding site, it contains an allosteric Ras-binding pocket: occupancy of this pocket by RasGTP markedly increases SOS's catalytic exchange activity, creating a positive feedback loop in which RasGTP promotes production of further RasGTP (Roose et al. 2007). In a signaling network, such product feedback can generate bistability - two stable steady states separated by an unstable one - so that individual cells settle into either a low- or a high-RasGTP state; a population then shows a bimodal ('digital', on/off) response instead of a graded ('analog') one, and can exhibit hysteresis, a memory of prior stimulation (Das et al. 2009).

Prasad et al. (2009) combined stochastic simulations of the TCR-LAT-PLCγ1-DAG-RasGRP/SOS network with flow-cytometric measurement of phosphorylated ERK in primary thymocytes. Their account: weak, positively selecting ligands produce only partial LAT phosphorylation and engage RasGRP, whose activity is not feedback-regulated, giving a graded Ras response that supports positive selection over a broad ligand range; strong, negatively selecting ligands cooperatively assemble fully phosphorylated LAT signalosomes that recruit enough SOS for its positive feedback to ignite, producing a sharp, threshold-like bimodal Ras/ERK response. Bimodal ERK distributions did appear in single thymocytes under strong TCR stimulation, whereas direct RasGRP activation (by the DAG analog PMA) gave graded responses. The paper itself, however, flagged unresolved issues that define this problem: the SOS-Ras kinetic parameters were not established (simulated bimodality emerged minutes later than experimentally observed unless the SOS GEF rate was raised and basal Ras activity assumed); the SOS-driven Ras increase might serve only to ignite downstream feedback modules, for example MAPK feedback through cytosolic phospholipase A2 (cPLA2, a phospholipid-hydrolyzing enzyme that generates arachidonic-acid-derived lipid mediators); compartmentalized Ras signaling - RasGRP activating Ras at the Golgi apparatus for positive selection versus SOS activating Ras at the plasma membrane for negative selection, as proposed by Daniels et al. (2006) - was treated only in supplemental modeling; and thymic selection in SOS1- or SOS2-deficient mice had not been examined, with the explicit prediction that SOS deficiency would profoundly impair negative selection while sparing positive selection.

The subsequent literature transformed this picture. Targeted deletion of Sos1 showed it is required at the earlier pre-TCR (beta-selection) checkpoint but dispensable for TCR-driven positive and negative selection (Kortum et al. 2011); systematic analysis of Sos1/Sos2/RasGRP1 combinations found that Sos1/Sos2 deletion shifts neither the negative-selection threshold nor its outcome in four experimental models, that ERK activation remaining via RasGRP1 alone stays digital in double-positive (CD4+CD8+) thymocytes, and that only combined Sos1/RasGRP1 deletion blocks negative selection - a failsafe redundancy among RasGEFs (Kortum et al. 2012). Conversely, reconstitution with the W729E allosteric-pocket SOS1 mutant in cell lines and thymocytes restores only analog ERK signaling, supporting a biochemical requirement for SOS allosteric feedback in the digital response (Jun et al. 2013). Primary human T-cell knockdown experiments likewise found TCR-mediated ERK activation independent of Grb2/SOS (Warnecke et al. 2012). Single-molecule reconstitution revealed that one membrane-resident SOS molecule can processively activate thousands of Ras molecules (Christensen et al. 2016), that bimodal Ras activity arises from this processivity as stochastic bursts without deterministic bistability (Lee et al. 2024), and that positive feedback in full-length SOS stems from release of its intramolecular autoinhibition rather than nucleotide-state-dependent allostery alone (Ren et al. 2024). Downstream of ERK, a stepwise, digital RSK (p90 ribosomal S6 kinase) phosphorylation module specifically induced by high-affinity ligands was shown to drive apoptosis of double-positive thymocytes (Funasaki et al. 2023). cPLA2α proved dispensable for thymocyte maturation (Rousseau et al. 2015). Reviews of this literature conclude that the RasGEF models of thymic selection require revision and that the mechanistic basis of the selection boundary remains unsettled (Kortum et al. 2013; Jun, Rubio and Roose 2013; Teixeiro and Daniels 2010).

Problem Statement

Determine the mechanistic origin, in vivo, of the sharp (digital) signaling boundary that separates positively from negatively selecting TCR ligands in double-positive thymocytes, by settling the status of the SOS-feedback hypothesis that first framed it (Prasad et al. 2009) and its named alternatives. Concretely, the task has three components: (1) whether feedback-mediated digital Ras activation by SOS is necessary and/or sufficient for the abrupt onset of negative selection at the threshold ligand potency, given that whole-molecule deletion leaves both selection outcomes intact while allosteric-mutant reconstitution abolishes digital ERK signaling; (2) which molecular module actually generates the observed single-cell digital response - deterministic bistability of the SOS-Ras loop, processivity-driven stochastic bimodality, RasGEF dosage and failsafe redundancy among Sos1/Sos2/RasGRP1, or downstream digital modules such as the RSK phosphorylation switch - including whether the required kinetic parameters (in vivo SOS GEF rate, basal RasGTP) permit the experimentally observed emergence of bimodality within 2-3 minutes of stimulation; and (3) whether compartmentalized Ras/ERK signaling (Golgi versus plasma membrane) or downstream feedback (MAPK/cPLA2) contributes to setting the boundary. These sub-questions jointly serve one objective: a causal, quantitatively supported account of how a graded increase in TCR-ligand affinity is converted into an all-or-none change of thymocyte fate, of which (1), (2) and (3) are the components and which yields quantitative predictions that discriminate among the candidate modules; targeted in vivo perturbations of SOS feedback during thymic selection and single-cell signaling readouts identifying at which level (Ras, ERK, or downstream) the digital character arises are the illustrative tests those predictions must ultimately face, not a separate deliverable the answer must design or perform.

References

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