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Deficiency of mevalonate kinase (MVK) causes systemic inflammation. However, the molecular mechanisms linking the mevalonate pathway to inflammation remain obscure. Geranylgeranyl pyrophosphate (GGPP), a non-sterol intermediate of the mevalonate pathway, is the substrate for protein geranylgeranylation, protein post-translational modification catalyzed by protein geranylgeranyl transferase I (GGTase I). Pyrin is an innate immune sensor that forms an active inflammasome in response to bacterial toxins. Mutations in MEFV (encoding human PYRIN) cause autoinflammatory Familial Mediterranean Fever (FMF) syndrome. Here, we show that protein geranylgeranylation enables Toll-like receptor (TLR)-induced phosphatidylinositol-3-OH kinase PI(3)K) activation by promoting the interaction between the small GTPase Kras and the PI(3)K catalytic subunit p110δ. Macrophages deficient for GGTase I or p110δ exhibited constitutive interleukin-1β release that was MEFV-dependent, but NLRP3-, AIM2- and NLRC4- inflammasome independent. In the absence of protein geranylgeranylation, compromised PI(3)K activity allows for an unchecked TLR-induced inflammatory responses and constitutive activation of the Pyrin inflammasome.

Summary Inflammasome activation is associated with numerous diseases. However, in vivo detection of the activated inflammasome complex has been limited by a dearth of tools. We developed transgenic mice that ectopically express the fluorescent adaptor protein, ASC (apoptosis-associated speck-like protein containing a caspase recruitment domain), and characterized the formation of assembled inflammasome complexes (“specks”) in primary cells and tissues. In addition to hematopoietic cells, we found that a stromal population in the lung tissues forms specks during the early phase of influenza infection whereas myeloid cells showed speck formation after two days. In a peritonitis and Group B streptococcus infection models, a higher percentage of neutrophils formed specks at early phases of infection, while dendritic cells formed specks at later time points. Furthermore, speck-forming cells underwent pyroptosis, and extensive release of specks to the extracellular milieu in vivo. These data underscore the importance of free specks during inflammatory processes in vivo.

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