Molecular mechanisms and functions of pyroptosis, inflammatory caspases and inflammasomes in infectious diseases

Abstract

Cell death is a fundamental biological phenomenon that is essential for the survival and development of an organism. Emerging evidence also indicates that cell death contributes to immune defense against infectious diseases. Pyroptosis is a form of inflammatory programmed cell death pathway activated by human and mouse caspase-1, human caspase-4 and caspase-5, or mouse caspase-11. These inflammatory caspases are used by the host to control bacterial, viral, fungal, or protozoan pathogens. Pyroptosis requires cleavage and activation of the pore-forming effector protein gasdermin D by inflammatory caspases. Physical rupture of the cell causes release of the pro-inflammatory cytokines IL-1β and IL-18, alarmins and endogenous danger-associated molecular patterns, signifying the inflammatory potential of pyroptosis. Here, we describe the central role of inflammatory caspases and pyroptosis in mediating immunity to infection and clearance of pathogens.

Keywords: bacteria; caspase-1; caspase-11; caspase-4; caspase-5; cell death; gasdermin D; infection; inflammasomes; inflammation; inflammatory caspases; interferons; lysis; lytic; necroptosis; necrosis; pores; pyroptosis; viruses.

Figure 1. Programed cell death pathways are regulated by different molecular components

(a and b) Apoptosis can be activated via intrinsic and extrinsic pathways. (a) Intrinsic apoptosis requires BCL-2 homology domain 3 (BH3)-only proteins, which engages BAX and BAK activation. This leads to apoptosome assembly via APAF1 and caspase-9, resulting in the activation of the effectors caspase-3 and caspase-7, and apoptosis. (b) Extrinsic apoptosis requires death receptors to induce dimerization of caspase-8 or caspase-10 through the adaptor protein FADD. Active caspase-8 and caspase-10 cleave and activate caspase-3 and caspase-7, leading to apoptosis. (c and d) Pyroptosis can be induced via caspase-1, human caspase-4 and caspase-5, or mouse caspase-11. (c) The inflammasome sensors NLRP3, AIM2 and Pyrin require the inflammasome adaptor protein ASC in order to form a caspase-1-containing inflammasome complex. The inflammasome sensors NLRC4 and NLRP1b can directly bind to caspase-1 without ASC. Caspase-1 mediates cleavage of the substrate gasdermin D, generating an N-terminal fragment of gasdermin D that induces pyroptosis. (d) Human caspase-4 and caspase-5 or mouse caspase-11 directly cleave gasdermin D to induce pyroptosis. (e and f) Necroptosis can be induced via RIPK1 or independently of RIPK1. (e) Death receptors induce activation of RIPK1, RIPK3 and MLKL, leading to necroptosis. FADD, caspase-8, and the caspase-8 paralogue FLIP form a complex to inhibit RIPK1-dependent necroptosis. (f) TLR3 and TLR4 can directly recruit and activate RIPK3 via the adaptor protein TRIF, independently of RIPK1. In addition, ZBP1 (also known as DLM-1 and DAI) can bind and activate RIPK3. Interferons (IFNs) can also activate RIPK3. In this context, RIPK1, along with FADD, caspase-8, and FLIP inhibit RIPK3-dependent necroptosis.

Figure 2. Molecular basis of caspase-1-dependent pyroptosis and caspase-1-independent pyroptosis

Caspase-1-dependent pyroptosis requires activation of the canonical inflammasomes. In this pathway, pathogen-associated molecular patterns or danger-associated molecular patterns activate their respective inflammasome sensors, including NLRP1b, NLRP3, NLRC4, AIM2 or Pyrin. Activation of the NLRP3 and NLRC4 inflammasomes requires the kinase NEK7 and ligand-binding NAIP proteins, respectively. Inflammasome sensors trigger recruitment of the inflammasome adaptor ASC and the cysteine protease caspase-1 into the same macromolecular complex. Caspase-1 directly cleaves gasdermin D and the precursor cytokines pro-IL-1β and pro-IL-18, initiating pyroptosis and maturation of IL-1β and IL-18, respectively. The 31-kDa cleaved N-terminal portion of gasdermin D forms pores on the host cell membrane to mediate the release of cytoplasmic contents. Caspase-1-independent pyroptosis requires activation of the non-canonical inflammasome. In this pathway, cytosolic LPS from Gram-negative bacteria is recognized by either caspase-4 or caspase-5 in human cells or by caspase-11 in mouse cells. These inflammatory caspases directly cleave gasdermin D and initiate pyroptosis. The N-terminal fragment also activates the NLRP3 inflammasome and caspase-1-dependent maturation of IL-1β and IL-18.

Figure 3. Type I IFN signaling regulates pathogen-induced inflammasome activation and pyroptosis

LPS from Gram-negative bacteria is recognized by TLR4, inducing TRIF-dependent type I IFN production. Francisella novicida is a cytosolic bacterium which is recognized by the DNA sensor cGAS, inducing STING-dependent type I IFN production. RNA from influenza A virus triggers the RNA sensors TLR7 via the adaptor MyD88 and RIG-I via the adaptor MAVS, both inducing production of type I IFNs. The type I IFN signaling pathway is activated via the transcription factors STAT1, STAT2 and IRF9, leading to upregulation of caspase-11, IFN-inducible GTPases, including guanylate-binding proteins (GBPs) and Immunity-related GTPases (IRGs), and other IFN-inducible proteins including the sensor ZBP1. GBP2 ruptures the vacuole containing Gram-negative bacteria, mediating the liberation of LPS into the cytoplasm for recognition by caspase-11. IRGB10 further disrupts Gram-negative bacteria to increase LPS accessibility for detection by caspase-11. GBP2, GBP5 and IRGB10 directly target the bacterial membrane of cytosolic-dwelling F. novicida, exposing its DNA for sensing by AIM2. Activation of the caspase-11-NLRP3 inflammasome in response to Gram-negative bacteria and activation of the AIM2 inflammasome in response to F. novicida lead to pyroptosis via gasdermin D. ZBP1 recognizes proteins from the influenza A virus and induce pyroptosis, necroptosis, and apoptosis via RIPK3, FADD and caspase-8.