The calcium-sensing receptor regulates the NLRP3 inflammasome through [Ca.sup.2+] and cAMP

Mutations in the gene encoding NLRP3 cause a spectrum of auto-inflammatory diseases known as cryopyrin-associated periodic syndromes (CAPS) (1). NLRP3 is a key component of one of several distinct cytoplasmic multiprotein complexes (inflammasomes) that mediate the maturation of the proinflammatory cytokine interleukin-1β (IL-1β) by activating caspase-1. Although several models for inflammasome activation, such as [K.sup.+] efflux (2), generation of reactive oxygen species (3) and lysosomal destabilization (4), have been proposed, the precise molecular mechanism of NLRP3 inflammasome activation, as well as the mechanism by which CAPS-associated mutations activate NLRP3, remain to be elucidated. Here we show that the murine calcium-sensing receptor (CASR) activates the NLRP3 inflammasome, mediated by increased intracellular [Ca.sup.2+] and decreased cellular cyclic AMP (cAMP). [Ca.sup.2+] or other CASR agonists activate the NLRP3 inflammasome in the absence of exogenous ATP, whereas knockdown of CASR reduces inflammasome activation in response to known NLRP3 activators. CASR activates the NLRP3 inflammasome through phospholipase C, which catalyses inositol-1,4,5-trisphosphate production and thereby induces release of [Ca.sup.2+] from endoplasmic reticulum stores. The increased cytoplasmic [Ca.sup.2+] promotes the assembly of inflammasome components, and intracellular [Ca.sup.2+] is required for spontaneous inflammasome activity in cells from patients with CAPS. CASR stimulation also results in reduced intracellular cAMP, which independently activates the NLRP3 inflammasome. cAMP binds to NLRP3 directly to inhibit inflammasome assembly, and downregulation of cAMP relieves this inhibition. The binding affinity of cAMP for CAPS-associated mutant NLRP3 is substantially lower than for wild-type NLRP3, and the uncontrolled mature IL-1β production from CAPS patients' peripheral blood mononuclear cells is attenuated by increasing cAMP. Taken together, these findings indicate that [Ca.sup.2+] and cAMP are two key molecular regulators of the NLRP3 inflammasome that have critical roles in the molecular pathogenesis of CAPS.
NLRP3 inflammasome activation by extracellular ATP is mediated through the purinergic receptor, P2X7R, and is inhibited by high concentrations of extracellular [K.sup.+], which may block [K.sup.+] efflux (5). However, ATP-driven [...]