Increasing complexity of NLRP3 inflammasome regulation

Abstract

Inflammasomes are multiprotein complexes that assemble upon detection of danger signals to activate the inflammatory enzyme caspase-1, trigger secretion of the highly proinflammatory cytokine IL-1β, and induce an inflammatory cell death called pyroptosis. Distinctiveness of the nucleotide-binding oligomerization (NOD), Leucine-rich repeat (LRR)-containing protein (NLRP3) inflammasome resides in the diversity of molecules that induce its activation, indicating a certain intricacy. Furthermore, besides the canonical activation of NLRP3 in response to various stimuli, caspase-11-dependent detection of intracellular LPS activates NLRP3 through a noncanonical pathway. Several aspects of the NLRP3 inflammasome are not characterized or remain unclear. In this review, we summarize the different modes of NLRP3 activation. We describe recent insights into post-translational and cellular regulation that confer further complexity to NLRP3 inflammasomes.

Keywords: ASC; IL-1; NLRP3; caspase-1; caspase-11; inflammasome; phosphorylation; ubiquitination.

Figure 1.. Activation of the canonical NLRP3 inflammasome

Activation of the NLRP3 inflammasome first requires the priming of NLRP3. Activation of membrane TLR4 by LPS from Gram-negative bacteria, for instance, or detection of DAMPs result in downstream activation of NF-kB, which promotes the transcription of NLRP3. Specific activation of NLRP3 occurs mostly downstream of K⁺ efflux, which can be generated by bacterial pore-forming toxins or ATP-induced activation of P2X7 purinergic receptor P2X7R, although how NLRP3 is activated by reduction of intracellular K⁺ concentration remains unknown. Lysosomal or mitochondrial damage can also lead to activation of NLRP3 via uncharacterized pathways. After sensing of these specific stimuli, the sensor NLRP3 assembles together with the adaptor ASC and the effector pro-caspase-1, via homotypic interactions between the N-terminal PYD domain of NLRP3 and the PYD domain of ASC, as well as between the respective CARD domains of ASC and pro-caspase-1. The kinase NEK7 is required for assembly of the NLRP3 inflammasome as well. Assembly of the NLRP3 inflammasome leads to activation of caspase-1, which then cleaves the pro-forms of IL-1β and IL-18, resulting in the secretion of biologically active cytokines, as well as Gasdermin D, resulting in pyroptosis via the formation of pores at the plasma membrane.

Figure 2.. Activation of the non-canonical NLRP3 inflammasome

Activation of the non-canonical NLRP3 inflammasome involves interferon-inducible caspase-11. Induction of IFN-β expression by Gram-negative bacteria relies on activation of endosomal/phagosomal TLR4 and the signaling pathway downstream of the adaptor TRIF. Secreted IFN-β then activates IFNAR, notably in an autocrine manner, which promotes expression of pro-caspase-11 and additional IFN-inducible proteins such as GBPs. Caspase-11 is activated by direct binding of intracellular LPS, released from the phagosomes, to the CARD domain of caspase-11. GBPs are recruited to the phagosome and participate in phagosomal lysis and release of LPS in the cytoplasm. Active caspase-11 directly cleaves Gasdermin D to induce pyroptosis, and also leads to activation of the NLRP3 inflammasome via uncharacterized mechanisms.

Figure 3.. Regulation of the NLRP3 inflammasome by post-translational modifications and sub-cellular localization

A. Ubiquitination and deubiquitination, as well as phosphorylation and dephosphorylation, regulate NLRP3 inflammasome activation. Post-translational events targeting the sensor NLRP3 and the adaptor ASC are summarized, including the name of the enzymes involved (when identified), the type of modification and the consequence for NLRP3 inflammasome activity. PolyUb = polyubiquitination, DUB = deubiquitination, P- = phosphorylation, De-P- = dephosphorylation. B. The sub-cellular localization of NLRP3 may determine NLRP3 inflammasome activation. Different possible localizations of NLRP3 are indicated, including cytosolic or Endoplasmic Reticulum (ER) localizations in the absence of activation. When NLRP3 is activated, several relocalization have been described, including translocation to mitochondria where NLRP3 functionally interacts with MAVS, the Golgi apparatus where NLRP3 physically interacts with SREBP2 and SCAP, or the Trans-Golgi network (TGN) where NLRP3 binds to phosphatidylinositol-4-phosphate (PtdIns4P). How these localizations enable the activation of the NLRP3 inflammasome remain unclear.