Background/Purpose: Mutations in the genes encoding pyrin and mevalonate kinase (MVK) cause the autoinflammatory diseases familial Mediterranean fever (FMF) and hyperimmunoglobulinemia D syndrome (HIDS), respectively.  The inflammation of both diseases is mediated by interleukin-1β (IL-1β).  Recently it has been reported that pyrin forms an inflammasome, a multiprotein complex that mediates the maturation of IL-1β by activating caspase-1, in response to bacterial modifications of RhoA.  However, the precise molecular mechanism of pyrin inflammasome activation, as well as the mechanism by which FMF-associated mutations activate pyrin, remain to be elucidated.  Thus, here we investigated the molecular mechanism of pyrin inflammasome activation and the molecular pathogenesis of FMF and HIDS.

Methods: We studied IL-1β production in immune cells from wild type mice and from several knockout and knockin mouse strains, as well as from FMF and HIDS patients and healthy controls, in response to LPS and/or various other bacterial toxins, and in the presence of pharmacologic agents targeting the Rho GTPase or adenylate cyclase pathways.  Protein interactions were studied by immunoprecipitation.

Results: The Clostridial TcdB and C3 toxins, which inactivate RhoA, activate IL-1β maturation by a pathway that is Mefv-, Asc-, and Caspase-1-dependent, but Nlrp3-, Nlrc4-, and Aim2-independent.  IL-1β secretion induced by the Clostridial toxins or from bone marrow-derived macrophages (BMDMs) of FMF knockin (KI) mice is inhibited both by the bacterial CNF toxin, which activates RhoA, and by colchicine.  Colchicine also inhibits the constitutive IL-1β secretion from peripheral blood mononuclear cells (PBMCs) of FMF patients.  In addition, the constitutive IL-1b secretion from FMF patients’ PBMCs or BMDMs of FMF-KI mice is potentiated by cAMP, which has a role in suppressing RhoA through PKA-mediated direct phosphorylation.  RhoA inhibition-induced inflammasome activation is mediated by reduced activities of downstream RhoA-effector kinase, serine/threonine protein kinase C-related kinase (PKN/PRK).  The kinase domain of PKN binds to pyrin directly and phosphorylates two serine residues (S208 and S242 of human pyrin).  The phosphorylated pyrin is recognized by 14-3-3 proteins, which negatively regulate the pyrin inflammasome.  The binding affinity of 14-3-3 proteins as well as PKN for FMF-associated mutant pyrin is substantially lower than for wild-type pyrin, and the constitutive IL-1β secretion from FMF or HIDS patients’ PBMCs or macrophages of FMF-KI mice is attenuated by activating PKNs. Defects in prenylation, seen in HIDS, lead to RhoA inactivation and consequent pyrin inflammasome activation. Taken together, these data suggest that the inflammations in FMF or HIDS are caused by the constitutive activation of the pyrin inflammasome due to the decreased interaction of pyrin with 14-3-3 proteins.

Conclusion: These data directly implicate Rho GTPase in the regulation of the pyrin inflammasome, and suggest that this pathway is also important in HIDS.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/activation-of-the-pyrin-inflammasome-through-the-rhoa-signaling-pathway-in-familial-mediterranean-fever-fmf-and-hyperimmunoglobulinemia-d-syndrome-hids/