InsP3R-SEC5 interaction on phagosomes modulates innate immunity to Candida albicans by promoting cytosolic Ca2+ elevation and TBK1 activity

Long Yang,Wenwen Gu,Lan Yan,Benjamin Chun-Kit Tong,King-Ho Cheung,Yuanying Jiang,Jun Yang

doi:10.1186/s12915-018-0507-6

Abstract

BackgroundCandida albicans (C. albicans) invasion triggers antifungal innate immunity, and the elevation of cytoplasmic Ca2+ levels via the inositol 1,4,5-trisphosphate receptor (InsP3R) plays a critical role in this process. However, the molecular pathways linking the InsP3R-mediated increase in Ca2+ and immune responses remain elusive.ResultsIn the present study, we find that during C. albicans phagocytosis in macrophages, exocyst complex component 2 (SEC5) promotes InsP3R channel activity by binding to its C-terminal α-helix (H1), increasing cytosolic Ca2+ concentrations ([Ca2+]c). Immunofluorescence reveals enriched InsP3R-SEC5 complex formation on phagosomes, while disruption of the InsP3R-SEC5 interaction by recombinant H1 peptides attenuates the InsP3R-mediated Ca2+ elevation, leading to impaired phagocytosis. Furthermore, we show that C. albicans infection promotes the recruitment of Tank-binding kinase 1 (TBK1) by the InsP3R-SEC5 interacting complex, leading to the activation of TBK1. Subsequently, activated TBK1 phosphorylates interferon regulatory factor 3 (IRF-3) and mediates type I interferon responses, suggesting that the InsP3R-SEC5 interaction may regulate antifungal innate immune responses not only by elevating cytoplasmic Ca2+ but also by activating the TBK1-IRF-3 pathway.ConclusionsOur data have revealed an important role of the InsP3R-SEC5 interaction in innate immune responses against C. albicans.

Highlights

Candida albicans (C. albicans) invasion triggers antifungal innate immunity, and the elevation of cytoplasmic Ca2+ levels via the inositol 1,4,5-trisphosphate receptor (InsP3R) plays a critical role in this process
C. albicans were added to bone marrow-derived macrophages (BMDMs), and [cytosolic Ca2+ concentrations (Ca2+]c) levels were recorded in individual macrophages for 10–20 min during the course of phagocytosis
We examined BMDMs internalizing C. albicans for 15 min through immunofluorescence microscopy and found that InsP3R was localized to phagosomes engulfing C. albicans (Fig. 1b)

Summary

Introduction

Candida albicans (C. albicans) invasion triggers antifungal innate immunity, and the elevation of cytoplasmic Ca2+ levels via the inositol 1,4,5-trisphosphate receptor (InsP3R) plays a critical role in this process. Systemic fungal infection by C. albicans may be life-threatening in immunocompromised patients who have undergone surgery, chemotherapy, or organ or bone marrow transplantation [1, 2]. Yang et al BMC Biology (2018) 16:46 generate inositol 1,4,5-trisphosphate (InsP3), which triggers the activation of the inositol 1,4,5-trisphosphate receptor (InsP3R) signaling pathway [6]. InsP3R is a major endoplasmic reticulum (ER) Ca2+ channel, and its activation promotes the release of Ca2+ from intracellular Ca2+ stores, resulting in an elevation of cytosolic Ca2+ concentrations ([Ca2+]c) [7, 8]. Vaeth et al reported that such intracellular Ca2+ elevation in macrophages and dendritic cells is required for the activation of key downstream antifungal functions such as phagocytosis, cytokine production, and inflammasome activation [9]

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Conclusion