Innate immune inflammatory cell death: PANoptosis and PANoptosomes in host defense and disease

Abstract

Regulated cell death (RCD) triggered by innate immune activation is an important strategy for host survival during pathogen invasion and perturbations of cellular homeostasis. There are two main categories of RCD, including nonlytic and lytic pathways. Apoptosis is the most well-characterized nonlytic RCD, and the inflammatory pyroptosis and necroptosis pathways are among the best known lytic forms. While these were historically viewed as independent RCD pathways, extensive evidence of cross-talk among their molecular components created a knowledge gap in our mechanistic understanding of RCD and innate immune pathway components, which led to the identification of PANoptosis. PANoptosis is a unique innate immune inflammatory RCD pathway that is regulated by PANoptosome complexes upon sensing pathogens, pathogen-associated molecular patterns (PAMPs), damage-associated molecular patterns (DAMPs) or the cytokines produced downstream. Cytosolic innate immune sensors and regulators, such as ZBP1, AIM2 and RIPK1, promote the assembly of PANoptosomes to drive PANoptosis. In this review, we discuss the molecular components of the known PANoptosomes and highlight the mechanisms of PANoptosome assembly, activation and regulation identified to date. We also discuss how PANoptosomes and mutations in PANoptosome components are linked to diseases. Given the impact of RCD, and PANoptosis specifically, across the disease spectrum, improved understanding of PANoptosomes and their regulation will be critical for identifying new therapeutic targets and strategies.

Keywords: Apoptosis; Caspases; Inflammasome; Necroptosis; PANoptosis; PANoptosome; Pyroptosis.

Figure 1.. Examples of RCD signaling pathways in response to cellular insults.

Cellular insults such as pathogenic infection or alterations in cellular homeostasis can act as ligands to activate the innate immune system. The activation of sensor molecules results in an inflammatory response and RCD such as apoptosis, pyroptosis, necroptosis and PANoptosis. Apoptosis occurs through either the intrinsic or extrinsic route, depending on the stimulus. In intrinsic apoptosis, permeabilization of the mitochondrial outer membrane results in cytochrome c release to activate APAF1 and form the apoptosome with pro–CASP9. Activation of CASP9 by this complex allows it to cleave and activate CASP3 and CASP7 to execute apoptosis. Extrinsic stimuli can activate death receptors resulting in a FADD/CASP8/RIPK1 complex that leads to CASP8 activation, which then activates CASP3 and CASP7 to execute cell death. CASP8 can also initiate intrinsic apoptosis (not shown). Pyroptosis involves the formation of the inflammasome. The prototypical NLRP3 inflammasome is shown, which contains the sensor NLRP3, the adaptor ASC and pro–CASP1. Formation of the inflammasome facilitates the activation of CASP1 to cleave its downstream substrates, including the inflammatory cytokines IL-1β and IL-18 and the pore-formingmolecule GSDMD.Necroptosis occurs when the apoptotic CASP8 is absent or inhibited. The TNF-mediated necroptotic pathway is illustrated here, where binding of TNF to its receptor (TNFR) results in the formation of the necrosome, containing RIPK1 and RIPK3. This interaction leads to phosphorylation of RIPK1/RIPK3 and the recruitment of MLKL for its phosphorylation. Phosphorylated MLKL translocates into the inner leaflet of the plasma membrane to disrupt the plasma membrane integrity, leading to cell death and the release of DAMPs. PANoptosis is a unique innate immune inflammatory RCD pathway that is regulated by multifaceted PANoptosome complexes, which integrate components from other RCD pathways. The canonical ZBP1-PANoptosome is illustrated here, where IAV activates ZBP1 to form a complex containing inflammasome components, such as ASC; complex II components, such as CASP8; necrosome components, such as RIPK3; along with other PANoptosome components. PANoptosome formation induces activation of downstream molecules including CASP1 to activate GSDMD, IL-1β and IL-18; CASP3 and CASP7 to cleave and activate GSDME and other apoptotic caspase targets; andMLKL. APAF1, apoptotic peptidase activating factor 1; CASP1, caspase-1; CASP3, caspase-3; CASP7, caspase-7; CASP8, caspase-8; CASP9, caspase-9; DAMPs, damage-associated molecular patterns; DISC, death-inducing signaling complex; FADD, FAS-associated death domain protein; GSDMD, gasdermin D; GSDME, gasdermin E; IAV, influenza A virus; IL-18, interleukin-18; IL-1β, interleukin-1 beta; MLKL, mixed lineage kinase domain-like pseudokinase; NLRP3, NOD-like receptor (NLR) protein 3; RCD, regulated cell death; RHIM, RIP homotypic interaction motif; RIPK1, receptor-interacting protein kinase 1; RIPK3, receptorinteracting protein kinase 3; TNF, tumor necrosis factor; TNFR, tumor necrosis factor receptor; ZBP1, Z-DNA binding protein 1. Figure created with BioRender.

Figure 2.. Molecular interactions among pyroptotic, apoptotic and necroptotic molecules.

The interactome of key molecules from PANoptosis, including pyroptotic, apoptotic and necroptotic molecules, is shown. Colored dotted connecting lines denote the individual pathways originally associated with each molecule. Gray solid connecting lines indicate interactions that were compiled from STRING PPI; interactions with an interaction score ≥ 0.9 are shown.