Pannexin-1-Mediated Recognition of Bacterial Molecules Activates the Cryopyrin Inflammasome Independent of Toll-like Receptor Signaling

Abstract

Cryopyrin is essential for caspase-1 activation triggered by Toll-like receptor (TLR) ligands in the presence of adenosine triphosphate (ATP). However, the events linking bacterial products and ATP to cryopyrin remain unclear. Here we demonstrate that cryopyrin-mediated caspase-1 activation proceeds independently of TLR signaling, thus dissociating caspase-1 activation and IL-1beta secretion. Instead, caspase-1 activation required pannexin-1, a hemichannel protein that interacts with the P2X(7) receptor. Direct cytosolic delivery of multiple bacterial products including lipopolysaccharide, but not flagellin, induced caspase-1 activation via cryopyrin in the absence of pannexin-1 activity or ATP stimulation. However, unlike Ipaf-dependent caspase-1 activation, stimulation of the pannexin-1-cryopyrin pathway by several intracellular bacteria was independent of a functional bacterial type III secretion system. These results provide evidence for cytosolic delivery and sensing of bacterial molecules as a unifying model for caspase-1 activation and position pannexin-1 as a mechanistic link between bacterial stimuli and the cryopyrin inflammasome.

The inflammasomes in health and disease: from genetics to molecular mechanisms of autoinflammation and beyond

“…33,51 ATP binds to the purinergic receptor P2X7, resulting in the opening of the protein channel pannexin-1. 52 This allows potassium efflux, inflammasome activation and ATP release. 53 However, it has been proposed that the pore formation following pannexin-1 recruitment could allow cytosolic access to microbial products, such as lipopolysaccharide (LPS), which can directly activate the inflammasome.…”

“…53 However, it has been proposed that the pore formation following pannexin-1 recruitment could allow cytosolic access to microbial products, such as lipopolysaccharide (LPS), which can directly activate the inflammasome. 52 Although it is unclear whether the pore width would be sufficient to allow entrance of such microbial-derived molecules.…”

Les inflammasomes

“…Chez l'homme, la famille des ALR comporte quatre membres : AIM2, IFI16 (interferon -inducible protein 16), MNDA (myeloid cell nuclear differentiation antigen), et IFIX (interferon inducible protein X) [9]. L'extrémité carboxy-terminale des ALR contient un domaine de liaison à l'ADN double-brin appelé HIN200.…”

“…L'activation de ce récepteur membranaire entraîne l'ouverture d'un canal protéique (la pannexine) et un efflux d'ions potassium hors de la cellule. La diminution de la concentration intracellulaire en potassium induit un changement de conformation de NLRP3 et l'activation de l'inflammasome NLRP3 [15][16][17]. De la même manière, des variations de l'homéostasie calcique pourraient participer à la détection des dommages membranaires via l'activation de NLRP3 [18].…”

P2X7 receptor and macrophage function

“…Pelegrin and Surprenant have shown that inhibition of pannexin-1 can block caspase-1 activation even in the presence of potassium efflux suggesting that it functions downstream of potassium's efflux [34]. Interestingly, pannexin-1 may also function to bring PAMPs into the cytosol for pathogen sensing [35].…”