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Published on: August 8, 2014
Deconstructing Ras signaling in the thymus
Robert L Kortum1, Connie L Sommers, John M Pinski
1Laboratory of Cellular and Molecular Biology, National Cancer Institute, National Institutes of Health, Bethesda, Maryland, USA.
Abstract:
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 RasGRP1 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.
Insights
This study reveals functional redundancy between Ras guanine exchange factors (RasGEFs) Sos1 and RasGRP1 during T cell development. Their combined action ensures proper T cell selection, preventing errors in central tolerance.
Area of Science:
- Immunology
- Cell Biology
- Molecular Biology
Background:
- T cell development requires precise signaling through the pre-T cell receptor (pre-TCR) and T cell receptor (TCR).
- Ras guanine exchange factors (RasGEFs) like Sos1 and RasGRP1 mediate signals from these receptors to the small G protein Ras.
- Sos1 is implicated in the pre-TCR checkpoint, while RasGRP1 is essential for the TCR checkpoint.
Purpose of the Study:
- To investigate the independent and combined roles of Sos1, Sos2, and RasGRP1 in thymocyte development.
- To elucidate the mechanisms by which RasGEFs regulate T cell selection at distinct developmental checkpoints.
Main Methods:
- Utilized mouse models to assess thymocyte development.
- Examined the impact of deleting specific RasGEFs (Sos1, Sos2, RasGRP1) individually and in combination.
Main Results:
- Sos1 is the dominant RasGEF at the pre-TCR checkpoint, but combined Sos1/RasGRP1 deletion is needed to fully block development.
- RasGRP1 deletion blocks positive selection, while combined RasGRP1/Sos1 deletion is necessary to inhibit negative selection.
- Demonstrated functional redundancy between Sos1 and RasGRP1, particularly during negative selection.
Conclusions:
- Sos1 and RasGRP1 exhibit functional redundancy during thymocyte development, especially at the negative selection stage.
- This redundancy may serve as a failsafe mechanism to ensure appropriate central tolerance.
- Understanding RasGEF function is critical for comprehending T cell repertoire formation and preventing autoimmunity.
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