Abstract
Missense variants are a major source of human genetic variation. Here we analyze a new mouse missense variant, Rasgrp1(Anaef), with an ENU-mutated EF hand in the Rasgrp1 Ras guanine nucleotide exchange factor. Rasgrp1(Anaef) mice exhibit anti-nuclear autoantibodies and gradually accumulate a CD44(hi) Helios(+) PD-1(+) CD4(+) T cell population that is dependent on B cells. Despite reduced Rasgrp1-Ras-ERK activation in vitro, thymocyte selection in Rasgrp1(Anaef) is mostly normal in vivo, although CD44 is overexpressed on naïve thymocytes and T cells in a T-cell-autonomous manner. We identify CD44 expression as a sensitive reporter of tonic mTOR-S6 kinase signaling through a novel mouse strain, chino, with a reduction-of-function mutation in Mtor. Elevated tonic mTOR-S6 signaling occurs in Rasgrp1(Anaef) naïve CD4(+) T cells. CD44 expression, CD4(+) T cell subset ratios and serum autoantibodies all returned to normal in Rasgrp1(Anaef)Mtor(chino) double-mutant mice, demonstrating that increased mTOR activity is essential for the Rasgrp1(Anaef) T cell dysregulation. DOI: http://dx.doi.org/10.7554/eLife.01020.001.
Highlights
Positive and negative selection of thymocytes generates a population of T lymphocytes with a broad spectrum of antigen-specific T cell receptors (TCR) (Starr et al, 2003; Kortum et al, 2013)
We found that pharmacological inhibition of mTOR with rapamycin in mice treated with low doses that avoid toxicity (Coenen et al, 2007; Araki et al, 2009), resulted in a dose-dependent decrease in CD44 expression on wildtype and Rasgrp1Anaef thymocytes (Figure 8H)
By analyzing a missense mutation in the Ras guanine nucleotide releasing protein 1 (Rasgrp1) EF-hand, the results dissociate this new function of Rasgrp1 from its well-known role in thymic positive selection
Summary
Positive and negative selection of thymocytes generates a population of T lymphocytes with a broad spectrum of antigen-specific T cell receptors (TCR) (Starr et al, 2003; Kortum et al, 2013). It was recognized early on that the small GTPase Ras plays a role (Swan et al, 1995). Three Ras guanine exchange factor (RasGEF) families can activate Ras: SOS, RasGRP, and RasGRF (Stone, 2011). Following TCR engagement, Son of Sevenless (SOS)-1 and -2 are recruited to the plasma membrane via a Grb2phospho-LAT interaction. The second messenger diacylglycerol (DAG), generated via PLCγ, directly recruits Ras guanine nucleotide releasing protein 1 (Rasgrp1) to the plasma membrane
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