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. Levels of extracellular calcium can increase at sites of infection and inflammation; however, the physiological significance of this has been unclear. This work shows that extracellular calcium acts as a danger signal, triggering the NLRP3 inflammasome via two G protein-coupled receptors.