Calcineurin/NFAT signaling and innate host defence: a role for NOD1-mediated phagocytic functions

Abstract

The calcineurin/nuclear factor of activated T cells (NFATs) signaling pathway plays a central role in T cell mediated adaptive immune responses, but a number of recent studies demonstrated that calcineurin/NFAT signaling also plays a key role in the control of the innate immune response by myeloid cells. Calcineurin inhibitors, such as cyclosporine A (CsA) and tacrolimus (FK506), are commonly used in organ transplantation to prevent graft rejection and in a variety of immune diseases. These immunosuppressive drugs have adverse effects and significantly increase host's susceptibility towards bacterial or fungal infections. Recent studies highlighted the role of NFAT signaling in fungal infection and in the control of the pattern recognition receptor nucleotide-binding oligomerization domain-containing protein 1 (NOD1), which predominantly senses invasive Gram-negative bacteria and mediates neutrophil phagocytic functions. This review summarises some of the current knowledge concerning the role of NFAT signaling in the innate immune response and the recent advances on NFAT-dependent inhibition of NOD1-mediated innate immune response caused by CsA, which may contribute to sensitizing transplant recipients to bacterial infection.

Figure 1

Differential activation of NFATc in myeloid cells. Schematic representation of the activation and inhibition of NFATc by calcineurin inhibitors in myeloid cells. (A) Inhibition of calcineurin/NFAT signaling by cyclosporine A (CsA) or the NFAT peptide inhibitor, VIVIT, antagonizes Flt3-L-induced development of bone marrow myeloid cells and increases the number and proliferation of myeloid progenitors [12](B) Antigens- and anti-IgE-dependent activation of NFAT2/c1 and up-regulation of COX2 expression and release of PGE2. CsA and the peptide inhibitor of NFAT, VIVIT, both inhibited antigens- and anti-IgE-mediated activation of COX2 and PGE2 in human neutrophils [45]. (C) LPS activates NFAT3/c4 and NFAT4/c3 in mouse macrophages, and CsA and VIVIT both induced significant inhibition of the LPS-induced TNF production [44]. (D) Zymosan and curdlan failed to activate IL-10, COX2, Egr1 and Egr2 expressions regulated by NFAT2/c1 and NFAT4/c3 in CsA-treated and CnB-deficient neutrophils, indicating that the dectin-1 receptor is the upstream activator of calcineurin. C. albicans killing was not affected in NFAT-deficient neutrophils, suggesting that the CnB regulation of antifungal response may occur through an NFAT-independent anti-microbial mechanism [46]. (E) Downexpression of the NFAT2/c1 by silencing RNA (siRNA) impaired the activation of NOD1 induced by UPEC. CsA or the cell permeable 11R-VIVIT inhibited the UPEC-induced NOD1 expression and NOD1-mediated neutrophil functions (migration capacity, phagocytosis, bacterial killing) [47]. DC: Dendritic cell; MΦ: Macrophage; PMN: Polymorphonuclear neutrophil.

Figure 2

NOD1 signaling and interactions with Rho GTPases and NFATc. (Left panel) Schematic representation of the activation of NOD1 interacting with Caspase recruitment domain (CARD)-containing kinase RIPK2, which leads to activation of RIPK2 and subsequent recruitment and activation of TAK1. The TAK1 complex then induces the polyubiquitination of the IKK-β kinase, degradation of the NF-κB repressor IκB, nuclear translocation of NF-Kb, and transcription of pro-inflammatory mediators. NOD1 stimulating agonists also activate JNK. The guanine exchanger SopE from salmonella activates Rho GTPase Rac1 and Cdc42, which form a protein complex with NOD1 and the heat shock protein 90 (not shown). Activation of Rac1 and Cdc42 induces the activation of NOD1/RIPK2 signaling and induction of NF-κB-mediated pro-inflammatory mediators. Downregulation of NOD1 expression by calcineurin inhibitors or silencing NFATc1 mRNA markedly impairs bacterial-mediated activation of PMN functions (e.g. neutrophil migration and E. coli phagocytic killing capacities). (Right panel) Schematic view of the NFATc activation pathway by cell surface receptors coupled to Ca²⁺ mobilization. Ca²⁺-dependent activation of calmodulin (CaM) and calcineurin B (CnB) enables CaM binding to CnA regulatory region, and CnA activating conformational change. Activated calcineurin then dephosphorylates the phosphorylation motifs of NFATc, allowing NFATc to translocate to the nucleus. Nuclear NFATc, in collaboration with other transcription factors (such as AP-1), then induce gene transcription. Calcineurin activity is inhibited by CsA or FK506, which binds to their intracellular immunophillin receptors. The high-affinity calcineurin-binding VIVIT peptides are potent blockers of NFATc. IKK: I kappa B kinase; JNK: c-Jun N-terminal kinases; NEMO: NF-κB essential modulator; NF-κB: nuclear factor κB; RIPK2: Receptor-interacting serine/threonine-protein kinase 2; TAK1: Transforming growth factor-beta-activated kinase 1; TCR: T cell receptor.