SIT is a transmembrane adapter protein that modulates signals emanating from the T-cell receptor (TCR

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 guanylreleasing 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.Keywords: Activation kinetics r Ras-Erk activation r RasGRP1 r Sos r Sustained signaling Additional supporting information may be found in the online version of this article at the publisher's web-site IntroductionSignaling via the Ras-Erk (where Ras is rat sarcoma) cascade presides over the regulation of a variety of cellular processes such as proliferation, differentiation, migration, and cell death. One of the puzzling aspects of Ras-Erk activation is that it can elicit distinct outcomes even in the same cell type. Recent progress has shown that variations in the duration, the magnitude, and the compartmentalization of Erk activation determine the specificity of the signaling output and the consequent cellular outcome [1].Upon ligation of the TCR, the activation of Ras is thought to be mediated via the action of two guanine nucleotide exchange Correspondence: Dr. Luca Simeoni e-mail: luca.simeoni@med.ovgu.de factors, Ras guanyl-releasing protein 1 (RasGRP1) and Son of sevenless 1 (Sos1). A major step forward in our understanding of the molecular mechanism underlying the regulation of the Ras-Erk cascade in T cells has been recently made [2,3]. Studies based on lymphoid cell lines and in silico simulations have, indeed, shown that Ras activation depends on the coordinated action of Ras-GRP1 and Sos. According to the proposed model, TCR ligation results in the generation of active Ras (RasGTP) exclusively via RasGRP1. However, this initial pool of active Ras is not sufficient to trigger full T-cell activation. Therefore, in order to prime T cells, a prolonged stimulation with the same antigen is required. This will lead to a progressive increase in the amount of intracellular RasGTP until a certain threshold is reached. At this point, Sos comes into play. The pool of RasGTP generated by RasGRP1 primes Sos by binding an allosteric regulatory site thereby activating a positive feedback loop and allowing full T-cell activation. Despite the fact that it is well-characterized how RasGRP1 and Sos orch...

Upon TCR stimulation by peptide-MHC complexes, CD4+ T cells undergo activation and proliferation. This process will ultimately culminate in T-cell differentiation and the acquisition of effector functions. The production of specific cytokines by differentiated CD4+ T cells is crucial for the generation of the appropriate immune response. Altered CD4+ T-cell activation and cytokine production result in chronic inflammatory conditions and autoimmune disorders. miRNAs have been shown to be important regulators of T-cell biology. In this study, we have focused our investigation on miR-20a, a member of the miR-17-92 cluster, whose expression is decreased in patients suffering from multiple sclerosis. We have found that miR-20a is rapidly induced upon TCR-triggering in primary human naïve CD4+ T cells and that its transcription is regulated in a Erk-, NF-κB-, and Ca++-dependent manner. We have further shown that overexpression of miR-20a inhibits TCR-mediated signaling but not the proliferation of primary human naïve CD4+ T cells. However, miR-20a overexpression strongly suppresses IL-10 secretion and moderately decreases IL-2, IL-6 and IL8 production, which are crucial regulators of inflammatory responses. Our study suggests that miR-20a is a new player in the regulation of TCR signaling strength and cytokine production.