Orai1-NFAT signalling pathway triggered by T cell receptor stimulation

T cell receptor (TCR) stimulation plays a crucial role in development, homeostasis, proliferation, cell death, cytokine production, and differentiation of T cells. Thus, in depth understanding of TCR signalling is crucial for development of therapy targeting inflammatory diseases, improvement of vaccination efficiency, and cancer therapy utilizing T cell-based strategies. TCR activation turns on various signalling pathways, one of the important one being the Ca2+-calcineurin-nuclear factor of activated T cells (NFAT) signalling pathway. Stimulation of TCRs triggers depletion of intracellular Ca2+ store and in turn, initiates store-operated Ca2+ entry (SOCE), one of the major mechanisms to raise the intracellular Ca2+ concentrations in T cells. Ca2+-release-activated-Ca2+ (CRAC) channels are a prototype of store-operated Ca2+ (SOC) channels in immune cells that are very well characterized. Recent identification of STIM1 as the endoplasmic reticulum (ER) Ca2+ sensor and Orai1 as the pore subunit has dramatically advanced the understanding of CRAC channels and provides a molecular tool to investigate the physiological outcomes of Ca2+ signalling during immune responses. In this review, we focus on our current understanding of CRAC channel activation, regulation, and downstream calcineurin-NFAT signaling pathway.