A molecular dissection of lymphocyte unresponsiveness induced by sustained calcium signalling

In lymphocytes, integration of Ca²⁺ and other signalling pathways results in productive activation, while unopposed Ca²⁺ signalling leads to decreased responsiveness to subsequent stimulation (anergy). The Ca²⁺-regulated transcription factor NFAT has an integral role in both aspects of lymphocyte function. NFAT cooperates with the transcription factor AP-1 (Fos/Jun) to up-regulate genes involved in productive activation of lymphocytes. However, in the absence of AP-1, NFAT imposes an opposing genetic programme that leads to lymphocyte anergy. Anergy is implemented at least partly through proteolytic degradation of the key signalling proteins PKCθ and PLCγ1. Sustained Ca²⁺-calcineurin signalling increases mRNA and protein levels of the E3 ubiquitin ligases Itch, CblB and Grail and induces expression of Tsg101, the ubiquitin-binding component of the ESCRT1 endosomal sorting complex. Subsequent stimulation or homotypic cell adhesion promotes membrane translocation of Itch and the related protein Nedd4, resulting in PKCθ and PLCγ1 degradation. T cells from Itch- and CblB-deficient mice are resistant to anergy induction. Anergic T cells show impaired calcium mobilization after TCR triggering and are unable to maintain a mature immunological synapse. Thus Ca²⁺-calcineurin-NFAT signalling links gene transcription to a multi-step programme that leads to impaired signal transduction in anergic T cells.