Extracellular Ca2+ is a danger signal activating the NLRP3 inflammasome through G protein-coupled calcium sensing receptors

Abstract

Activation of the NLRP3 inflammasome enables monocytes and macrophages to release high levels of interleukin-1β during inflammatory responses. Concentrations of extracellular calcium can increase at sites of infection, inflammation or cell activation. Here we show that increased extracellular calcium activates the NLRP3 inflammasome via stimulation of G protein-coupled calcium sensing receptors. Activation is mediated by signalling through the calcium-sensing receptor and GPRC6A via the phosphatidyl inositol/Ca2+ pathway. The resulting increase in the intracellular calcium concentration triggers inflammasome assembly and Caspase-1 activation. We identified necrotic cells as one source for excess extracellular calcium triggering this activation. In vivo, increased calcium concentrations can amplify the inflammatory response in the mouse model of carrageenan-induced footpad swelling, and this effect was inhibited in GPRC6A−/− mice. Our results demonstrate that G-protein-coupled receptors can activate the inflammasome, and indicate that increased extracellular calcium has a role as a danger signal and amplifier of inflammation.

Inflammasomes and its importance in viral infections

“…pathway. In addition, in the mouse model of carrageenan-induced footpad swelling, increased calcium concentration was found to amplify the inflammatory response and this effect was inhibited in Gprc6a -/-mice [41]. This work was further supported by the finding that extracellular Ca 2?…”

Interleukin-10 Modulates Antigen Presentation by Dendritic Cells through Regulation of NLRP3 Inflammasome Assembly during Chlamydia Infection

“…They have been shown to regulate the processing and release of IL-1␤ through pannexin-1-mediated pore formation, which activates NLRP3 (25). Calcium is known to increase the assembly and activation of the NLRP3 inflammasome (26), while high levels of cAMP have been shown to reduce the activity of the NLRP3 inflammasome (27). We investigated the expression of the ATP-gated P2X7 ion channels and intracellular cAMP and Ca 2ϩ concentrations in Chlamydia-pulsed WT and IL-10 Ϫ/Ϫ DCs using Western blotting, ELISA, and fluorescent Ca 2ϩ imaging analyses.…”

NOD-Like Receptors: Versatile Cytosolic Sentinels

“…Murakami et al (207) demonstrated that multiple known NLRP3 stimuli induce calcium signaling and provided a mechanism where increased cytosolic calcium released from the ER causes mitochondria damage (increased ROS) and mitochondrial DNA release to activate the inflammasome. Follow-up studies by two independent groups identify calcium receptors CASR (calcium-sensing receptor) and GPCR6A as likely upstream channels that activate the NLRP3 inflammasome through PLC, a signaling molecule that releases calcium from the ER (157,242). The ability of specific ions to regulate PRR signaling is an intriguing finding and could represent a potential host-defense mechanism that serves as an early warning signal against foreign agents.…”

“…Altogether, the NLRP3 inflammasome integrates multiple signals to protect the host against different forms of cellular stress, and a common integrator of these pathways remains to be determined. Importantly, perturbation in cytosolic ion fluxes (calcium levels) has recently been proposed as a common upstream signal that all NLRP3 stimuli modulate to activate the inflammasome (described in a different section below) (157,207,242). Future studies will need to investigate if other ionic molecules such as chloride ions are also actively engaged in NLRP3 inflammasome activation, as a balance of ion levels is tightly regulated to maintain cellular osmolarity and membrane potential.…”