The Pyrin Inflammasome in Health and Disease

Abstract

The pyrin inflammasome has evolved as an innate immune sensor to detect bacterial toxin-induced Rho guanosine triphosphatase (Rho GTPase)-inactivation, a process that is similar to the "guard" mechanism in plants. Rho GTPases act as molecular switches to regulate a variety of signal transduction pathways including cytoskeletal organization. Pathogens can modulate Rho GTPase activity to suppress host immune responses such as phagocytosis. Pyrin is encoded by MEFV, the gene that is mutated in patients with familial Mediterranean fever (FMF). FMF is the prototypic autoinflammatory disease characterized by recurring short episodes of systemic inflammation and is a common disorder in many populations in the Mediterranean basin. Pyrin specifically senses modifications in the activity of the small GTPase RhoA, which binds to many effector proteins including the serine/threonine-protein kinases PKN1 and PKN2 and actin-binding proteins. RhoA activation leads to PKN-mediated phosphorylation-dependent pyrin inhibition. Conversely, pathogen virulence factors downregulate RhoA activity in a variety of ways, and these changes are detected by the pyrin inflammasome irrespective of the type of modifications. MEFV pathogenic variants favor the active state of pyrin and elicit proinflammatory cytokine release and pyroptosis. They can be inherited either as a dominant or recessive trait depending on the variant's location and effect on the protein function. Mutations in the C-terminal B30.2 domain are usually considered recessive, although heterozygotes may manifest a biochemical or even a clinical phenotype. These variants are hypomorphic in regard to their effect on intramolecular interactions, but ultimately accentuate pyrin activity. Heterozygous mutations in other domains of pyrin affect residues critical for inhibition or protein oligomerization, and lead to constitutively active inflammasome. In healthy carriers of FMF mutations who have the subclinical inflammatory phenotype, the increased activity of pyrin might have been protective against endemic infections over human history. This finding is supported by the observation of high carrier frequencies of FMF-mutations in multiple populations. The pyrin inflammasome also plays a role in mediating inflammation in other autoinflammatory diseases linked to dysregulation in the actin polymerization pathway. Therefore, the assembly of the pyrin inflammasome is initiated in response to fluctuations in cytoplasmic homeostasis and perturbations in cytoskeletal dynamics.

Keywords: RhoA GTPases; Yersinia toxins; autoinflammatory diseases; familial Mediterranean fever; pyrin inflammasome; serine-threonine kinase.

Figure 1

Schematic representation of the structure and function of NLRP1, NLRP3, NLRC4/IPAF, AIM2, and Pyrin Inflammasomes. Shown are the minimal NLRP1, NLRP3, NLRC4/IPAF, AIM2, and pyrin inflammasomes, their corresponding stimulation signals, described resulting downstream effects and associated disorders. Formation of the inflammasome initiates autocatalytic activation of caspase-1, resulting in cleavage of the pro-enzyme into the active p20 and p10 subunits. The active enzyme is assembled in the form of heterodimers and cause release of mature IL-1β/IL-18 and gasdermin D cleavage.

Figure 2

Schematic representation of the MEFV gene and the encoded pyrin protein. Of the more than 300 nucleotide variants described in the MEFV gene, only shown are the ones that are clearly associated to disease phenotypes. The most common FMF-associated mutations reside in exon 10, that encodes the B30.2 domain (B30.2). Mutations in the B30.2 domain tend to be transmitted in an autosomal-recessive fashion, while the mutations in exons 2, 3 and 5 more often exhibit an autosomal-dominant pattern of inheritance. Known pyrin interaction partners are depicted next to their corresponding or putative interaction domain of the pyrin protein: ASC (apoptosis-associated speck-like protein containing a CARD), 14-3-3 (14-3-3 protein), PKN1/2 (serine-threonine kinases PKN1 and PKN2), p65 (transcription factor p65), IκB (NF-κB inhibitor), PSTPIP1 (proline serine threonine phosphatase-interacting protein).

Figure 3

Proposed interaction model of Yersinia pestis effectors, Yops, with the pyrin inflammasome. Yersinia pestis effectors YopE (by promoting GTP hydrolysis) and YopT (by catalyzing cleavage and detachment of RhoA from the plasma membrane) inactivate RhoA. This results in reduced PKN1/2 activity, which in turn promotes the assembly of a pyrin inflammasome. Other bacterial toxins such as Clostridium difficile toxin B/TcdB (by glycosylation) or deleterious mutations in key enzymes of the mevalonate kinase pathway (Mut-MVK) also cause RhoA-inactivation. By delivering the additional effector YopM to the host cell, Yersinia pestis maintains virulence. YopM stimulates the PKN-1/2-mediated phosphorylation of pyrin and thereby the inhibition of pyrin inflammasome by hijacking host kinases PKN-1/2.

Figure 4

Proposed molecular mechanism of regulation of the pyrin inflammasome. Geranylgeranylation (mevalonate kinase pathway) and release of GEF-H1 (i.e., through the microtubule polymerization inhibitor colchicine) induce RhoA activity. RhoA effector kinases, PKN1 and PKN2, cause subsequent phosphorylation of pyrin and binding to inhibitory proteins 14-3-3. This mechanism is enhanced by PKN1/2 activating substances and release of GEF-H1, while low GEF-H1 or defective MVK-pathway function leads to inactivation of PKN1/2. Decreased phosphorylation of pyrin is associated with pyrin inflammasome activation and release of mature IL-1β and IL-18. The release of IL-1β and IL-18 is further facilitated by the production of the plasma membrane pore-forming N-terminal fragment of gasdermin D.

Figure 5

Current summary of autoinflammatory diseases mediated by the pyrin inflammasome. Pyrin interacts with ASC and caspase-1 to form the canonical inflammasome that process pro-IL-1β and pro-IL-18 into its mature forms, IL-1β and IL-18. Mutations in pyrin increase activity of the inflammasome and cause familial Mediterranean fever (FMF) and pyrin-associated autoinflammation with neutrophilic dermatosis (PAAND). While FMF mutations only enhance the activity of pyrin, PAAND-associated mutations lead to constitutive activation of pyrin, which is reflected by a higher production of IL-1 and IL-18 in PAAND than FMF. The molecular consequence of other dominantly inherited pyrin mutations (PADD) is still unknown. Mutations in MVK associated with MKD/HIDS act by downregulating RhoA activity that is important for pyrin inhibition. Mutations in PSTPIP1 (PAPA syndrome), WDR1 (PFIT), and ARPC1B affect actin polymerization, thus it is likely that these perturbations in the cytoskeleton are detected by the pyrin inflammasome.