Regulation of RasGRP1 Function in T Cell Development and Activation by Its Unique Tail Domain

Abstract

The Ras-guanyl nucleotide exchange factor RasGRP1 plays a critical role in T cell receptor-mediated Erk activation. Previous studies have emphasized the importance of RasGRP1 in the positive selection of thymocytes, activation of T cells, and control of autoimmunity. RasGRP1 consists of a number of well-characterized domains, which it shares with its other family members; however, RasGRP1 also contains an ∼200 residue-long tail domain, the function of which is unknown. To elucidate the physiological role of this domain, we generated knock-in mice expressing RasGRP1 without the tail domain. Further analysis of these knock-in mice showed that thymocytes lacking the tail domain of RasGRP1 underwent aberrant thymic selection and, following TCR stimulation, were unable to activate Erk. Furthermore, the deletion of the tail domain led to enhanced CD4 + T cell expansion in aged mice, as well as the production of autoantibodies. Mechanistically, the tail-deleted form of RasGRP1 was not able to traffic to the cell membrane following stimulation, indicating a potential reason for its inability to activate Erk. While the DAG-binding C1 domain of RasGRP1 has long been recognized as an important factor mediating Erk activation, we have revealed the physiological relevance of the tail domain in RasGRP1 function and control of Erk signaling.

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

“…These phenotypes are recapitulated in RasGRP1 d/d and RasGRP1 Anaef mice (41, 42), which express RasGRP1 with a deleted tail domain or a mutated EF-hand, respectively. In humans, a high incidence of aberrant RasGRP1 splice variants exclusively occurs in SLE patients (43, 44).…”

“…Young RasGRP1-deficient mice exhibit T cell lymphopenia. However, the adult mice exhibit a substantial expansion of CD4 + T cells (41, 45). Thus, autoimmune phenotypes have been attributed to T cell lymphopenia in these animals.…”

“…Development of autoimmune disease in a lymphopenic host needs a key “second hit” (46). Secondly, RasGRP1 d/d (41) and RasGRP1 Anaef (42) mice produce substantial antinuclear autoantibody, while T cell development in these mice is mildly impaired or normal, respectively. Thus, the mechanism responsible for the autoimmune disorder in RasGRP1-deficient mice remains unclear.…”

Importance of the REM (Ras exchange) domain for membrane interactions by RasGRP3

“…The EF hands control plasma membrane targeting by counteracting the SuPT domain which negatively regulates the PT domain [46]. The plasma membrane targeting domain is directly responsible for targeting RasGRP1 to the plasma membrane in some cell types [47]. In the PT domain, a small segment enriched in basic and hydrophobic residues, the BHC motif has been identified [48].…”

Diacylglycerol Kinases in T Cell Tolerance and Effector Function

“…Additionally, RasGRP1-Ras-Erk1/2 pathway functions upstream for TCR-induced mTORC1, mTORC2, and PI3K activation in T cells (Gorentla et al, 2011). RasGRP1 plays an essential role in conventional αβ T cell development (Dower et al, 2000; Fuller et al, 2012), particularly for the selection of thymocytes that express weak TCR signals (Priatel et al, 2002) and for early iNKT cell development (Shen et al, 2011a). While RasGRP1 appears dispensable for overall γδT cell development, it ensures IL-17 expressing γδT17 lineage differentiation and TCR-induced γδT cell activation (Chen et al, 2012).…”