Role of Inflammasome in Autoinflammatory Diseases

Mutations in the gene encoding NLRP3 cause a spectrum of autoinflammatory diseases known as the cryopyrin-associated periodic syndromes (CAPS).  NLRP3 is a key component of one of several distinct cytoplasmic multiprotein complexes (inflammasomes) that mediate the maturation of the proinflammatory cytokine interleukin-1 (IL-1) by activating caspase-1.  Although several models for inflammasome activation, such as K+ efflux, generation of reactive oxygen species (ROS), and lysosomal destabilization 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 calcium sensing receptor (CaSR) activates the NLRP3 inflammasome, mediated by increased intracellular calcium and decreased cellular cyclic AMP (cAMP).  Calcium 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.  The CaSR activates the NLRP3 inflammasome through phospholipase C (PLC), which catalyzes inositol trisphosphate (IP3) production and thereby induces release of calcium from endoplasmic reticulum (ER) stores. The increased cytoplasmic calcium promotes the assembly of inflammasome components, and intracellular calcium is required for spontaneous inflammasome activity in cells from CAPS patients.  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-1 production from CAPS patients’ peripheral blood mononuclear cells (PBMCs) is attenuated by increasing cAMP.  Taken together, these findings suggest that calcium and cAMP are two key molecular regulators of the NLRP3 inflammasome that have critical roles in the molecular pathogenesis of CAPS.