Pyroptosis is a unique, pro‐inflammatory form of lytic cell death that is initiated by the activation of inflammatory caspases. The caspase substrate gasdermin D (GSDMD) plays a critical function in ...pyroptosis, yet the precise mode of action of this molecule in cell death execution remained unclear. Several recent reports, including a The EMBO Journal article, show that the caspase‐matured N‐terminal fragment of GSDMD is recruited to lipid membranes to form pore‐like structures, which constitutes the key effector mechanism of pyroptotic cell death.
During pyroptosis, an N‐terminal cleavage fragment of gasdermin D relocates to the plasma membrane, forming pores that trigger pro‐inflammatory cell death.
NLRP3 is the most studied inflammasome sensor due to its crucial involvement in sterile and infection-triggered inflammation. Although its molecular mode of activation remains to be defined, it is ...well established that low intracellular potassium concentrations result in its activation. This functionality allows the classical NLRP3 pathway to serve as a highly sensitive, but non-specific surveillance mechanism responding to any type of perturbation that breaches plasma membrane integrity and the associated potassium gradient across the membrane. Here, we review our current knowledge on potassium efflux-dependent NLRP3 activation, with a special focus on how major cell death programs are rendered pro-inflammatory by secondary NLRP3 activation. Apart from the “alternative inflammasome” as the major exception to the rule, this connection explains the fundamental importance of NLRP3 in cell death-associated inflammation and firmly establishes NLRP3 as a principal surveillance mechanism of cellular integrity.
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•NLRP3 is activated by low intracellular potassium levels.•This pathway serves as a surveillance mechanism of plasma membrane integrity.•Due to this functionality, major forms of programmed cell death result in secondary NLRP3 activation.
Highlights • Alternative NLRP3 inflammasome activation comprises a distinct signaling entity. • Genetic and phenotypic criteria distinguish alternative and classical NLRP3 signaling. • This includes ...absence of pyroptosis and GSDMD-independent IL-1β secretion.
The Integrator is a specialized 3′ end-processing complex involved in cleavage and transcription termination of a subset of nascent RNA polymerase II transcripts, including small nuclear RNAs ...(snRNAs). We provide evidence of the modular nature of the Integrator complex by biochemically characterizing its two subcomplexes, INTS5/8 and INTS10/13/14. Using cryoelectron microscopy (cryo-EM), we determined a 3.5-Å-resolution structure of the INTS4/9/11 ternary complex, which constitutes Integrator’s catalytic core. Our structure reveals the spatial organization of the catalytic nuclease INTS11, bound to its catalytically impaired homolog INTS9 via several interdependent interfaces. INTS4, a helical repeat protein, plays a key role in stabilizing nuclease domains and other components. In this assembly, all three proteins form a composite electropositive groove, suggesting a putative RNA binding path within the complex. Comparison with other 3′ end-processing machineries points to distinct features and a unique architecture of the Integrator’s catalytic module.
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•Native Integrator complex is highly modular•INTS5/8, INTS10/13/14, and INTS4/9/11 exist as stable sub-complexes•Cryo-EM structure of the INTS4/9/11 reveals a putative substrate binding grove•CTD1 of INTS9/11 is required for recruitment of INTS4
Pfleiderer and Galej performed biochemical and structural analysis of the Integrator complex, one of the three main 3′ end-processing machineries in higher eukaryotes. The study reveals the modular nature of the Integrator complex, and the cryo-EM structure of its catalytic core provides detailed insights into its architecture and assembly mechanism.
TLR8 is among the highest-expressed pattern-recognition receptors in the human myeloid compartment, yet its mode of action is poorly understood. TLR8 engages two distinct ligand binding sites to ...sense RNA degradation products, although it remains unclear how these ligands are formed in cellulo in the context of complex RNA molecule sensing. Here, we identified the lysosomal endoribonuclease RNase T2 as a non-redundant upstream component of TLR8-dependent RNA recognition. RNase T2 activity is required for rendering complex single-stranded, exogenous RNA molecules detectable for TLR8. This is due to RNase T2’s preferential cleavage of single-stranded RNA molecules between purine and uridine residues, which critically contributes to the supply of catabolic uridine and the generation of purine-2′,3′-cyclophosphate-terminated oligoribonucleotides. Thus-generated molecules constitute agonistic ligands for the first and second binding pocket of TLR8. Together, these results establish the identity and origin of the RNA-derived molecular pattern sensed by TLR8.
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•Lysosomal RNase T2 activity functions upstream of TLR8•RNase T2 cleaves ssRNA between purines and uridine (U)•This releases U and purine-2′,3′-cyclophosphate-terminated RNAs activating TLR8•Staphylococcus aureus is sensed in a manner dependent on RNase T2 and TLR8
Human TLR8 activation involves recognition of specific RNA products generated by the enzyme RNase T2.
Detection of cytosolic DNA constitutes a central event in the context of numerous infectious and sterile inflammatory conditions. Recent studies have uncovered a bipartite mode of cytosolic DNA ...recognition, in which the cGAS-STING axis triggers antiviral immunity, whereas AIM2 triggers inflammasome activation. Here, we show that AIM2 is dispensable for DNA-mediated inflammasome activation in human myeloid cells. Instead, detection of cytosolic DNA by the cGAS-STING axis induces a cell death program initiating potassium efflux upstream of NLRP3. Forward genetics identified regulators of lysosomal trafficking to modulate this cell death program, and subsequent studies revealed that activated STING traffics to the lysosome, where it triggers membrane permeabilization and thus lysosomal cell death (LCD). Importantly, the cGAS-STING-NLRP3 pathway constitutes the default inflammasome response during viral and bacterial infections in human myeloid cells. We conclude that targeting the cGAS-STING-LCD-NLRP3 pathway will ameliorate pathology in inflammatory conditions that are associated with cytosolic DNA sensing.
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•DNA inflammasome activation in human monocytes depends on NLRP3, but not AIM2•cGAS-STING signals independently of its type I IFN response upstream of NLRP3•STING activation orchestrates a lysosomal cell death program that engages NLRP3•The cGAS-STING-NLRP3 axis is the default DNA inflammasome in human myeloid cells
In humans, a cGAS-STING-lysosomal cell death-NLRP3 pathway is responsible for the inflammasome response to bacterial and viral DNA with AIM2 being dispensable.
Interleukin-1β (IL-1β) is a cytokine whose bioactivity is controlled by activation of the inflammasome. However, in response to lipopolysaccharide, human monocytes secrete IL-1β independently of ...classical inflammasome stimuli. Here, we report that this constituted a species-specific response that is not observed in the murine system. Indeed, in human monocytes, lipopolysaccharide triggered an “alternative inflammasome” that relied on NLRP3-ASC-caspase-1 signaling, yet was devoid of any classical inflammasome characteristics including pyroptosome formation, pyroptosis induction, and K+ efflux dependency. Genetic dissection of the underlying signaling pathway in a monocyte transdifferentiation system revealed that alternative inflammasome activation was propagated by TLR4-TRIF-RIPK1-FADD-CASP8 signaling upstream of NLRP3. Importantly, involvement of this signaling cascade was limited to alternative inflammasome activation and did not extend to classical NLRP3 activation. Because alternative inflammasome activation embraces both sensitivity and promiscuity of TLR4, we propose a pivotal role for this signaling cascade in TLR4-driven, IL-1β-mediated immune responses and immunopathology in humans.
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•LPS by itself triggers IL-1β secretion in human, but not in murine monocytes•Human monocytes activate an alternative inflammasome in response to LPS•This proceeds independently of K+ efflux, pyroptosome formation, and pyroptosis•Alternative but not classical inflammasome signals via TLR4-TRIF-RIPK1-FADD-CASP8
How human monocytes secrete processed IL-1β upon LPS challenge is unknown. Hornung and colleagues report that LPS triggers an alternative NLRP3 inflammasome pathway in human monocytes. Unlike classical NLRP3 inflammasome signaling, alternative inflammasome activation proceeds independently of potassium efflux, pyroptosome formation, and pyroptosis, while it engages TLR4-TRIF-RIPK1-FADD-CASP8 upstream of NLRP3.
We study a one-dimensional topological superconductor, the Kitaev chain, under the influence of a non-Hermitian but PT -symmetric potential. This potential introduces gain and loss in the system in ...equal parts. We show that the stability of the topological phase is influenced by the gain/loss strength and explicitly derive the bulk topological invariant in a bipartite lattice as well as compute the corresponding phase diagram using analytical and numerical methods. Furthermore, we find that the edge state is exponentially localized near the ends of the wire despite the presence of gain and loss of probability amplitude in that region.
Inflammasome activation culminates in activation of caspase‐1, which leads to the maturation and subsequent release of cytokines of the interleukin 1 (IL‐1) family and results in a particular form of ...cell death known as pyroptosis. In addition, in the murine system, a so‐called non‐canonical inflammasome involving caspase‐11 has been described that directly responds to cytosolic LPS. Here, we show that the human monocytic cell line THP1 activates the inflammasome in response to cytosolic LPS in a TLR4‐independent fashion. This response is mediated by caspase‐4 and accompanied by caspase‐1 activation, pyroptosis, and IL‐1β maturation. In addition to caspase‐4, efficient IL‐1β conversion upon intracellular LPS delivery relies on potassium efflux, NLRP3, ASC, and caspase‐1, indicating that although caspase‐4 activation alone is sufficient to induce pyroptosis, this process depends on the NLRP3 inflammasome activation to drive IL‐1β maturation. Altogether, this study provides evidence for the presence of a non‐canonical inflammasome in humans and its dependence on caspase‐4.
Effector-Triggered Immunity Remick, Brenna C; Gaidt, Moritz M; Vance, Russell E
Annual review of immunology,
04/2023, Letnik:
41, Številka:
1
Journal Article
Recenzirano
Odprti dostop
The innate immune system detects pathogens via germline-encoded receptors that bind to conserved pathogen ligands called pathogen-associated molecular patterns (PAMPs). Here we consider an additional ...strategy of pathogen sensing called effector-triggered immunity (ETI). ETI involves detection of pathogen-encoded virulence factors, also called effectors. Pathogens produce effectors to manipulate hosts to create a replicative niche and or block host immunity. Unlike PAMPs, effectors are often diverse and rapidly evolving and can thus be unsuitable targets for direct detection by germline-encoded receptors. Effectors are instead often sensed indirectly via detection of their virulence activities. ETI is a viable strategy for pathogen sensing and is used across diverse phyla, including plants, but the molecular mechanisms of ETI are complex compared to simple receptor ligand-based PAMP detection. Here we survey the mechanisms and functions of ETI, with a particular focus on emerging insights from animal studies. We suggest that many examples of ETI may remain to be discovered, hiding in plain sight throughout immunology.
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