Activation of the NLRP1 inflammasome and its ligand recognition in the antibacterial immune response of common carp (Cyprinus carpio)

The characterization of BCL-xL displays a non-apoptotic role in suppression of NLRP1 inflammasome assembly in common carp (Cyprinus carpio L.)

The NLRP1 inflammasome is a crucial muti-protein complex in the host anti-pathogen immune response. The previous studies have revealed that the anti-apoptotic protein BCL-xL played a non-apoptotic role by impeding the activation of NLRP1 inflammasome in mammals. However, the potential role of BCL-xL in regulating the inflammasome in fish remains unclear. In the present study, the BCL-xL (CcBCL-xL) was cloned from the head kidney of common carp (Cyprinus carpio L.), and its regulatory effect on the NLRP1 inflammasome was explored. It was found that CcBCL-xL predominantly localized in the brain, spleen and head kidney of common carp, and upon stimulation with Aeromonas hydrophila (A. hydrophila), Edwardsiella tarda (E. tarda), or spring viremia of carp virus (SVCV), the expression of CcBCL-xL significantly increased in multiple immune organs. The interaction between CcBCL-xL and CcNLRP1 was confirmed by co-immunoprecipitation and immunofluorescence. Meanwhile, we also found that CcBCL-xL significantly inhibited the assembly of the CcNLRP1 inflammasome, through ASC oligomerization, ASC specks formation and cytotoxicity experiments. Furthermore, our results revealed that CcBCL-xL interacted with the NACHT, LRR, FIIND, and CARD domains of CcNLRP1. Taken together, the results provide a theoretical foundation for further exploring the regulatory mechanism of NLRP1, and for the prevention and treatment of infectious diseases in fish.

The function of NLRP3 in anti-infection immunity and inflammasome assembly of common carp (Cyprinus carpio L.)

NLRP3 inflammasome can be activated by a variety of stimuli and plays an important role in protecting host from pathogen invasion and maintaining homeostasis. However, the activation mechanism of NLRP3 inflammasome in fish is still unclear. In the present study, the NLRP3 gene (CcNLRP3) was identified from common carp, which was 3069 bp in length and encoded a protein with five domains. Sequence analysis showed that NLRP3 was evolutionarily conserved, and CcNLRP3 was closely related to that in grass carp and zebrafish. Real-time PCR showed that CcNLRP3 was widely expressed in various immune-related tissues of healthy common carp, and significantly increased after stimulation with E. tarda, A. hydrophila and Cyprinus spring viremia virus (SVCV), suggesting that CcNLRP3 might be involved in the immune defense of common carp. The results of co-IP, spot formation, oligomerization and fluorescence localization showed that CcNLRP3 could interact with CcASC and assemble into inflammasome. The cytotoxicity assays showed that CcNLRP3 inflammasome was involved in the pyroptosis induced by CcGSDME. At the same time, CcNLRP3 could directly interact with CcCaspase-A/B and result in increased Caspase-B enzyme activity and LDH release, indicating that CcNLRP3 could also form inflammasome through ASC-independent pathway. Taken together, the results provide targets and theoretical basis for the prevention and control of infectious diseases in aquaculture.

The functions of two GSDMEs in pyroptosis of common carp (Cyprinus carpio L.) in canonical and non-canonical inflammasome pathways

Gasdermin family proteins are important effector proteins mediating pyroptosis and play an important role in innate immune response. GSDME can be cleaved by inflammatory Caspases at specific sites, releasing an active form of N-terminal fragment that binds to the plasma membrane to form pores and release cellular contents. Here, two GSDME genes, CcGSDME-like (CcGSDME-L) and CcGSDMEa, were cloned from common carp. The sequence similarity of the two genes were very high and more similar to DrGSDMEa of zebrafish in evolution. The expression levels of CcGSDME-L and CcGSDMEa can respond to the stimulation of Edwardsiella tarda. The results of cytotoxicity assay showed that CcGSDMEs were cleaved by the activation of canonical CcNLRP1 inflammasome, leading to obvious pyroptosis characteristics and increased cytotoxicity. In EPC cells, three CcCaspases responded to intracellular LPS stimulation and induced significantly cytotoxicity. In order to clarify the molecular mechanism of CcGSDME-induced pyroptosis, the N-terminal of CcGSDME-L (CcGSDME-L-NT) was expressed in 293T cells, which showed strong cytotoxicity and obvious pyroptosis characteristics. Fluorescence localization assay showed that the CcGSDME-L-NT was expressed on cell membrane, and CcGSDMEa-NT was located on the cell membrane or some organelle membranes. These findings can enrich the knowledge of CcNLRP1 inflammasome and GSDMEs mediated pyroptosis in common carp, and provide basic data for the prevention and treatment of fish infectious diseases.

Stimulator of interferon genes from Asian swamp eel (MaSTING) is involved in host defense against bacterial infection

Stimulator of interferon genes (STING) is an endoplasmic reticulum (ER)-associated protein that plays critical roles in innate immunity and pathogenesis of various diseases. To date, teleost STING against viral stimulation has been identified, whereas STING signaling events in fish against bacteria are not well understood. In the present study, the open reading frame (ORF) of STING from Asian swamp eel (Monopterus albus) was cloned (named MaSTING) and its roles in bacterial infection were investigated. Amino acid sequence alignment and phylogenetic analysis revealed that MaSTING had conserved structures with mammalian STING and shared the closest relationship with mandarin fish STING. Subcellular localization analysis showed that MaSTING distributed in the whole cytoplasm and mainly co-localized with ER. Expression pattern analysis found that MaSTING was constitutively expressed in all the examined tissues with the highest expression in the liver and spleen. Post stimulation with bacteria and various PAMPs, the expression of MaSTING was induced at indicated time points in the immune-related organs and isolated peripheral blood leucocytes. Furthermore, the mechanism underlying MaSTING against bacterial infection was further studied. The qPCR analysis showed that MaSTING overexpression promoted 2'3'-cGAMP induced the expression of IFN-1, ISG15, Viperin, Mx, IL-1β and TNF-α. Western blotting assay suggested that MaSTING significantly enhanced the phosphorylation of TANK-binding kinase 1 (TBK1) and p65. MaSTING also significantly increased the luciferase activity of IFN-1 and NF-κB promoters. Taken together, MaSTING is involved in host defense against bacterial infection by inducing the inflammatory response.

Emerging mechanisms and functions of inflammasome complexes in teleost fish

Inflammasomes are multiprotein complexes, which are assembled in response to a diverse range of exogenous pathogens and endogenous danger signals, leading to produce pro-inflammatory cytokines and induce pyroptotic cell death. Inflammasome components have been identified in teleost fish. Previous reviews have highlighted the conservation of inflammasome components in evolution, inflammasome function in zebrafish infectious and non-infectious models, and the mechanism that induce pyroptosis in fish. The activation of inflammasome involves the canonical and noncanonical pathways, which can play critical roles in the control of various inflammatory and metabolic diseases. The canonical inflammasomes activate caspase-1, and their signaling is initiated by cytosolic pattern recognition receptors. However the noncanonical inflammasomes activate inflammatory caspase upon sensing of cytosolic lipopolysaccharide from Gram-negative bacteria. In this review, we summarize the mechanisms of activation of canonical and noncanonical inflammasomes in teleost fish, with a particular focus on inflammasome complexes in response to bacterial infection. Furthermore, the functions of inflammasome-associated effectors, specific regulatory mechanisms of teleost inflammasomes and functional roles of inflammasomes in innate immune responses are also reviewed. The knowledge of inflammasome activation and pathogen clearance in teleost fish will shed new light on new molecular targets for treatment of inflammatory and infectious diseases.

Identification of three inflammatory Caspases in common carp (Cyprinus carpio L.) and its role in immune response against bacterial infection

Inflammatory Caspases are key effectors of the inflammasomes and play an important role in innate immune response. However, there are few studies on the homologs of inflammatory Caspases in bony fish. In the present study, three inflammatory Caspase genes were cloned from common carp and named CcCaspase-A1, CcCaspase-A2 and CcCaspase-B. Nucleotide sequences alignment revealed that the three Caspases were very similar in structure, which contained a PYD domain in the N-terminal, and a CASc domain in the C-terminal. In the phylogenetic tree, CcCaspase-A1 and CcCaspase-A2 were close to the Caspase-A of grass carp, and CcCaspase-B was close to the DrCaspase-B of zebrafish. In healthy common carp, the expression levels of CcCaspase-A1 and CcCaspase-A2 were the highest in the gills, and CcCaspase-B was the highest in the spleen. After immune stimulation with Edwardsiella tarda or Aeromonas hydrophila, the expression levels of all CcCaspases increased significantly. The fluorescence localization assays showed that all these CcCaspases were expressed in the cytoplasm, and were involved in the assembly of CcNLRP1 inflammasome. These results suggest that the inflammatory CcCaspases play a key role in immune response of common carp against bacterial infection, which may enrich the knowledge of inflammasome in fish, and provide basic data for the prevention and treatment of fish infectious diseases.