Regulation of lymphocyte function by ORAI and STIM proteins in infection and autoimmunity

Abstract

Store-operated Ca(2+) entry (SOCE) in cells of the immune system is mediated by Ca(2+) release-activated Ca(2+) (CRAC) channels that are formed by ORAI1 and its homologues ORAI2 and ORAI3. They are activated by stromal interaction molecules (STIM) 1 and 2 in response to depletion of endoplasmic reticulum Ca(2+) stores. Loss-of-function mutations in the human ORAI1 and STIM1 genes abolish CRAC channel function and SOCE in a variety of non-excitable cells including lymphocytes and other immune cells, resulting in a unique clinical syndrome termed CRAC channelopathy. It is dominated by severe immunodeficiency and autoimmunity due to impaired SOCE and defects in the function of several lymphocyte subsets. These include CD8(+) T cells, CD4(+) effector and regulatory T cells, natural killer (NK) cells and B cells. This review provides a concise discussion of the role of CRAC channels in these lymphocyte populations and the regulation of adaptive immune responses to infection, in autoimmunity and inflammation.

Figure 1. ORAI1, STIM1 and STIM2 mediate SOCE and control gene expression in distinct T cell subsets

A, SOCE is activated following T cell receptor (TCR) or B cell receptor (BCR, not shown) ligation in T and B cells. In T cells, the protein tyrosine kinases Lck and ZAP-70 initiate a signalling cascade that results in the activation of PLCγ1 and production of InsP₃, a second messenger whose binding to the InsP₃ receptor leads to the release of Ca²⁺ from the ER. The subsequent reduction of [Ca²⁺]_ER causes the dissociation of Ca²⁺ from EF hand (EFh) domains in the N terminus of STIM1 and STIM2, unfolding of the adjacent EFh-SAM domains and oligomerization of STIM molecules. Oligomerized STIM assembles in puncta localized at junctions formed by the ER and plasma membrane to which ORAI channels are recruited. STIM1 binds to and activates ORAI1 via a CRAC activation domain (CAD or CCb9) in its C terminus. Opening of ORAI CRAC channels in the plasma membrane results in sustained Ca²⁺ influx and activation of several Ca²⁺ regulated enzymes and transcription factors. Of particular importance in lymphocytes is the serine/threonine phosphatase calcineurin, which dephosphorylates NFAT (nuclear factor of activated T cells) and thereby enables it to translocate to the nucleus and bind to promoters and conserved non-coding DNA sequences (CNS) of many target genes. The expression of several cytokines (IL-17A, IL-22, IL-21) in Th17 cells and the lineage-specific transcription factor Foxp3 in Treg cells are regulated by NFAT. (Note: only NFAT and other Ca²⁺ dependent transcription factors regulating cytokine and Foxp3 expression are shown in this figure.)

Figure 2. ORAI1, STIM1 and STIM2 control SOCE and cytokine production in CD4⁺ T cells

A, T cell-specific deletion of STIM2 in CD4⁺ T cells from Stim2^fl/fl Cd4Cre mice does not interfere with the peak of Ca²⁺ influx (*) but impairs sustained SOCE (**) compared to wild-type control (Ctrl) T cells. Gradually more severe defects in SOCE are observed (in this order) in ORAI1-, STIM1- and STIM1/2-deficient CD4⁺ T cells (from Oh-Hora et al. 2008; McCarl et al. 2010 and authors unpublished observations). T cells were stimulated with PMA (10 nm), ionomycin (0.5 μm) and thapsigargin (TG, 1 μm) in Ringer solution containing 0 or 2 mm Ca²⁺ as indicated. In Orai1^R93W knock-in mice, the endogenous Orai1 gene was replaced with a mutant (Arg → Trp) allele that encodes a non-functional ORAI1 protein. B, summary of SOCE levels in ORAI/STIM-deficient murine CD4⁺ T cells. C, cytokine production by CD4⁺ T cells from the indicated mice that were stimulated for 6 h with PMA and ionomycin and analysed by intracellular cytokine staining in flow cytometry (from Oh-Hora et al. 2008; McCarl et al. 2010; Copyright 2010. The American Association of Immunologists, Inc.). Note that >99% of wild-type Ctrl T cells express IFNγ or IFNγ and IL-2, whereas this percentage is gradually reduced in STIM2-, ORAI1- and STIM1-deficient CD4⁺ T cells.

Figure 3. Role of SOCE in the differentiation and function of CD4⁺ T cells

Naive CD4⁺ T cells differentiate into distinct T helper (Th) cell types and regulatory T (Treg) cells whose role in immunity is indicated on the right. Differentiation into Th1, Th2, Th17 or Treg cells is influenced by the cytokine milieu in which TCR stimulation occurs. Indicated are the cytokines and STAT (signal transducer and activator of transcription) molecules that initiate CD4⁺ T cell differentiation (left column) and promote expression of lineage specific transcription factors (indicated above arrows). In Treg cells, SOCE is required for Foxp3 expression and Treg differentiation. In Th17 cells, SOCE controls expression of IL-21 and the receptor for IL-23 (IL-23R), cytokines which have been implicated in the terminal differentiation and survival of Th17 cells. In addition to influencing CD4⁺ T cell differentiation, SOCE is essential for effector functions of Th1, Th2, Th17 and Treg cells. The levels of SOCE required for the differentiation and function of distinct CD4⁺ T cell types are indicated by blue boxes (light blue: low levels of SOCE sufficient; dark blue: robust, sustained SOCE required).