Identification of a gene encoding a membrane-anchored toll-like receptor 5 (TLR5M) in Oplegnathus fasciatus that responds to flagellin challenge and activates NF-κB

UNC93B1 facilitates the localization and signaling of TLR5M in Epinephelus coioides

Toll-like receptor 5 (TLR5), serving as a sensor of bacterial flagellin, mediates the innate immune response to actively engage in the host's immune processes against pathogen invasion. However, the mechanism underlying TLR5-mediated immune response in fish remains unclear. Despite the presumed cell surface expression of TLR5 member form (TLR5M), its trafficking dynamics remain elusive. Here, we have identified Epinephelus coioides TLR5M as a crucial mediator of Vibrio flagellin-induced cytokine expression in grouper cells. EcTLR5M facilitated the activation of NF-κB signaling pathway in response to flagellin stimulation and exerted a modest influence on the mitogen-activated protein kinase (MAPK)-extracellular regulated kinase (ERK) signaling. The trafficking chaperone Unc-93 homolog B1 (EcUNC93B1) participated in EcTLR5M-mediated NF-κB signaling activation and downstream cytokine expression. In addition, EcUNC93B1 combined with EcTLR5M to mediate its exit from the endoplasmic reticulum, and also affected its post-translational maturation. Collectively, these findings first discovered that EcTLR5M mediated the flagellin-induced cytokine expression primarily by regulating the NF-κB signaling pathway, and EcUNC93B1 mediated EcTLR5M function through regulating its trafficking and post-translational maturation. This research expanded the understanding of fish innate immunity and provided a novel concept for the advancement of anti-vibrio immunity technology.

Identification and characterization of toll-like receptor genes in silver pomfret (Pampus argenteus) and their involvement in the host immune response to Photobacterium damselae subsp. Damselae and Nocardia seriolae infection

Toll-like receptors (TLRs) are vital pattern recognition receptors that play a critical role in the innate immune response against pathogenic attack. Among the bacteria commonly found in the culture process of silver pomfret, Photobacterium damselae subsp. Damselae (PDD, gram-negative) and Nocardia seriolae (NS, gram-positive), can cause large-scale mortality in this fish species. However, there is currently no research on the role of TLRs in mediating the immune response of silver pomfret to these two bacterial infections. Therefore, in this study, we identified nine PaTLRs family members, including several fish-specific TLRs (TLR14 and TLR21). Phylogenetic analysis revealed that these PaTLRs genes could be classified into five subfamilies, namely TLR1, TLR3, TLR5, TLR7, and TLR11, indicating their evolutionary conservation. To further explore the interactions of TLR genes with immune-related mediators, protein and protein interaction network (PPI) results were generated to explain the association of TLR genes with TNF receptor-associated factor 6 (TRAF6) and other relevant genes in the MyD88-dependent pathway and NF-κb signaling pathway. Subsequently, RT-qPCR was conducted to verify the expression patterns of the nine TLR genes in the gills, skin, kidney, liver, and spleen of healthy fish, with most of the TLRs showing high expression levels in the spleen. Following infection with PDD and NS, these PaTLRs exhibited different expression patterns in the spleen, with PaTLR2, PaTLR3, PaTLR5, PaTLR7, PaTLR9, and PaTLR14 being significantly up-regulated. Furthermore, when spleen cells were treated with bacterial compositions, the majority of PaTLRs expression was up-regulated in response to Lipopolysaccharide (LPS) and lipophosphorylcholic acid (LTA) treatment, except for PaTLR21. Finally, changes in the expression levels of TLR-interacting genes were also observed under the stimulation of bacteria and bacterial compositions. The results of this study provide a preliminary reference for further understanding the mechanism of the innate immune response of the TLR gene family in silver pomfret and offer theoretical support for addressing the disease problems encountered during large-scale fish breeding.

Identification and functional characteristics of two TLR5 subtypes in S. grahami

TLR5, as a member of Toll-like receptors (TLRs) family in mammals, is responsible for recognizing bacterial flagellin and initiating innate immunity, but its function is still unclear in fish species. In this study, two family members of TLR5 were cloned and identified from Sinocyclocheilus grahami (S. grahami), named sgTLR5a and sgTLR5b. The length of coding sequence of sgTLR5a and sgTLR5b is 2,622 bp and 2,658 bp, encoding 873 and 885 amino acids, respectively. Molecular phylogenetic analysis indicates that sgTLR5a and sgTLR5b have the closest genetic relationship with TLR5M (membrane-type) of Cyprinus carpio and Schizothorax prenanti, respectively. sgTLR5a and sgTLR5b were widely expressed in various tested tissues, of which the expression levels were the highest in skin tissue. After stimulations of Aeromonas hydrophila (A. hydrophila) and flagellin, the expression levels of sgTLR5a and sgTLR5b in liver, spleen and head kidney tissues were strongly up-regulated, but LPS stimulation only increased the expression of sgTLR5b in these tissues. The luciferase reporter assay displayed that sgTLR5a and sgTLR5b could specifically recognize bacterial flagellin and A. hydrophila and activate the downstream NF-κB signaling pathway in HEK293T cells. Moreover, the overexpression of sgTLR5a and sgTLR5b in EPC cells up-regulated the expression levels of IL-8 and TNF. sgTLR5a and sgTLR5b were observed to locate in the intracellular region by confocal microscope. Interestingly, we found that the NF-κB signaling pathway was positively regulated by co-transfecting sgTLR5a or sgTLR5b with TLR trafficking chaperone sgUNC93B1. In conclusion, our results reveal sgTLR5a and sgTLR5b may play an important role in antibacterial response by activating the NF-κB signaling pathway.

TLR5 recognizes Aeromonas hydrophila flagellin and interacts with MyD88 in Nile tilapia

Toll-like receptor 5 (TLR5) is responsible for bacterial flagellin recognition in vertebrates. In the present study, TLR5M was identified in the Nile tilapia Oreochromis niloticus (OnTLR5), containing a conserved LRR domain, a transmembrane region and a C-terminal TIR domain, similar to that of other fishes and mammals. OnTLR5 was broadly expressed in all the tissues examined, presenting the highest expression levels in the blood and the lowest in the kidney. OnTLR5 was detected from 2 d postfertilization (dpf) to 8 dpf during embryonic development. Moreover, expression levels of OnTLR5 were clearly altered in all five tissues examined in response to Streptococcus agalactiae infection in vivo. Overexpression of OnTLR5 in HEK293T cells revealed that OnTLR5 was distributed in the cytoplasm and significantly increased NF-κB activation. In response to cotransfection with OnMyd88, OnTLR5 significantly upregulated OnMyd88-induced NF-κB activation. Pulldown assays showed that OnTLR5 interacts with OnMyd88 and revealed an interaction between TLR5 and Aeromonas hydrophila flagellin. Taken together, these findings suggest that OnTLR5 plays important roles in TLR/IL-1R signalling pathways and the immune response to pathogen invasion.

Membrane orthologs of TLR5 of tongue sole Cynoglossus semilaevis: Expression patterns, signaling pathway and antibacterial property

Mammalian toll-like receptor 5 (TLR5) is crucial for recognizing bacterial flagellin and initiating the inflammatory signaling cascades via myeloid differentiation factor 88 (MyD88) signaling pathway, which plays vital roles in innate immune against pathogenic bacteria. Herein, we reported the signaling pathway and antibacterial property of tongue sole (Cynoglossus semilaevis) membrane forms of TLR5 (i.e. CsTLR5M1and CsTLR5M2). CsTLR5M1/M2 contain 936 and 885 amino acid residues respectively. CsTLR5M1 shares 86.7% overall sequence identities with CsTLR5M2. CsTLR5M1/M2 possess the same extracellular domain (ECD) and transmembrane domain (TMD), but the different toll-interleukin-1 receptor (TIR) domain. CsTLR5M1/M2 expression occurred constitutively in multiple tissues and regulated by bacterial stimulation. Recombinant CsTLR5M1/M2 (rCsTLR5M) could bind to flagellin and Gram-negative/positive bacteria, which could suppress bacterial growth. Stimulation of the CsTLR5M pathway by flagellin resulted in increased expression of MyD88-dependent signaling molecules and inflammatory cytokines. Blocking rCsTLR5M by antibody markedly reduced the phagocytosis and ROS production of peripheral blood leukocytes (PBLs), which in turn in vivo promoted the dissemination of bacteria. Overall, these observations add new insights into the signaling pathway and immune function of teleost TLR5M.

Comparative Characterization of Two cxcl8 Homologs in Oplegnathus fasciatus: Genomic, Transcriptional and Functional Analyses

CXCL8 (interleukin-8, IL-8) is a CXC family chemokine that recruits specific target cells and mediates inflammation and wound healing. This study reports the identification and characterization of two cxcl8 homologs from rock bream, Oplegnathus fasciatus. Investigation of molecular signature, homology, phylogeny, and gene structure suggested that they belonged to lineages 1 (L1) and 3 (L3), and designated Ofcxcl8-L1 and Ofcxcl8-L3. While Ofcxcl8-L1 and Ofcxcl8-L3 revealed quadripartite and tripartite organization, in place of the mammalian ELR (Glu-Leu-Arg) motif, their peptides harbored EMH (Glu-Met-His) and NSH (Asn-Ser-His) motifs, respectively. Transcripts of Ofcxcl8s were constitutively detected by Quantitative Real-Time PCR (qPCR) in 11 tissues examined, however, at different levels. Ofcxcl8-L1 transcript robustly responded to treatments with stimulants, such as flagellin, concanavalin A, lipopolysaccharide, and poly(I:C), and pathogens, including Edwardsiella tarda, Streptococcus iniae, and rock bream iridovirus, when compared with Ofcxcl8-L3 mRNA. The differences in the putative promoter features may partly explain the differential transcriptional modulation of Ofcxcl8s. Purified recombinant OfCXCL8 (rOfCXCL8) proteins were used in in vitro chemotaxis and proliferation assays. Despite the lack of ELR motif, both rOfCXCL8s exhibited leukocyte chemotactic and proliferative functions, where the potency of rOfCXCL8-L1 was robust and significant compared to that of rOfCXCL8-L3. The results, taken together, are indicative of the crucial importance of Ofcxcl8s in inflammatory responses and immunoregulatory roles in rock bream immunity.

Toll-Like Receptor 5 of Golden Pompano Trachinotus ovatus (Linnaeus 1758): Characterization, Promoter Activity and Functional Analysis

Toll-like receptors (TLRs), as important pattern recognition receptors, represent a significant component of fish immune systems and play an important role in resisting the invasion of pathogenic microorganisms. The TLR5 subfamily contains two types of TLR5, the membrane form of TLR5 (TLR5M) and the soluble form of TLR5 (TLR5S), whose detailed functions have not been completely elucidated. In the present study, we first identified two genes, TLR5M (ToTLR5M) and TLR5S (ToTLR5S), from golden pompano (Trachinotus ovatus). The full-length ToTLR5M and ToTLR5S cDNA are 3644 bp and 2329 bp, respectively, comprising an open reading frame (ORF) of 2673 bp, encoding 890 amino acids, and an ORF of 1935 bp, encoding 644 amino acids. Both the ToTLR5s possess representative TLR domains; however, only ToTLR5M has transmembrane and intracellular TIR domains. Moreover, the transcription of two ToTLR5s was significantly upregulated after stimulation by polyinosinic:polycytidylic acid (poly (I:C)), lipopolysaccharide (LPS), and flagellin in both immune-related tissues (liver, intestine, blood, kidney, and skin) and nonimmune-related tissue (muscle). Furthermore, the results of bioinformatic and promoter analysis show that the transcription factors GATA-1 (GATA Binding Protein 1), C/EBPalpha (CCAAT Enhancer Binding Protein Alpha), and ICSBP (Interferon (IFN) consensus sequence binding protein) may play a positive role in moderating the expression of two ToTLR5s. Overexpression of ToTLR5M and ToTLR5S notably increases NF-κB (nuclear factor kappa-B) activity. Additionally, the binding assay revealed that two rToTLR5s can bind specifically to bacteria and pathogen-associated molecular patterns (PAMPs) containing Vibrio harveyi, Vibrio anguillarum, Vibrio vulnificus, Escherichia coli, Photobacterium damselae, Staphylococcus aureus, Aeromonas hydrophila, LPS, poly(I:C), flagellin, and peptidoglycan (PGN). In conclusion, the present study may help to elucidate the function of ToTLR5M/S and clarify their possible roles in the fish immune response to bacterial infection.

Vibrio parahaemolyticus flagellin induces cytokines expression via toll-like receptor 5 pathway in orange-spotted grouper, Epinephelus coioides

Vibrio parahaemolyticus is the major pathogen of vibriosis in aquatic animals and causes inflammation that may be related to tissue damage. Here, we have established a V. parahaemolyticus flagellin stimulation model using grouper spleen (GS) cell line. Purified V. parahaemolyticus flagellin was used to stimulate GS cells. Our results showed that the mRNA levels of orange-spotted grouper (Epinephelus coioides) toll-like receptor 5M (EcTLR5M), EcTLR5S and downstream cytokines, such as interferon-γ2 (IFN-γ2), interleukin-6 (IL-6) and tumor necrosis factor-α (TNF-α), were all significantly increased after stimulation with V. parahaemolyticus flagellin in GS cells. Gene silencing of the EcTLR5M and EcTLR5S in GS cells by using small interfering RNA resulted in suppression of the V. parahaemolyticus flagellin-induced cytokines expression. We further demonstrated that activation of both mitogen-activated protein kinases (MAPKs) and nuclear factor-kappa B (NF-κB) were required for cytokines expression. We observed that the phosphorylation of NF-κB inhibitor-α (IκBα), extracellular signal-regulated kinase (ERK) and p38 were induced following treatment with flagellin. Additionally, most of p65, a NF-κB subunit, was found to translocate to the nucleus after 60 min stimulation. Overall, our results suggest that V. parahaemolyticus flagellin influences cytokines expression, such as IFN-γ2, IL-6 and TNF-α, via EcTLR5s recognition and MAPKs/NF-κB signaling pathway activation in GS cells.

Transcriptome-wide identification, functional characterization, and expression analysis of two novel invertebrate-type Toll-like receptors from disk abalone (Haliotis discus discus)

Toll-like receptors (TLRs) are well-known pattern recognition receptors that play key immunological roles in a diverse range of organisms. In this study, two novel invertebrate TLRs from disk abalone (designated as AbTLR-A and AbTLR-B) were identified and functionally characterized for the first time. AbTLR-A and AbTLR-B comprised the typical TLR domain architecture containing an extracellular leucine-rich repeat domain, transmembrane domain, and Toll/interleukin-1 receptor domain. Expressional analysis revealed that both TLRs were constitutively expressed at all the early embryonic stages of disk abalone analyzed, with the highest level of AbTLR-A found at the 16-cell stage and AbTLR-B at the trochophore stage. According to tissue distribution analysis, prominent mRNA expression of AbTLR-A and AbTLR-B was detected in the hemocytes and gills, respectively. AbTLR-A and AbTLR-B mRNAs were significantly up-regulated in response to Gram-negative Vibrio parahemolyticus, Gram-positive Listeria monocytogenes, and viral hemorrhagic septicemia virus injections in abalone hemocytes and gills. Overexpression of AbTLR-A and AbTLR-B in HEK293T cells directly activated nuclear factor-κB (NF-κB) and activator protein-1 (AP-1) responsive reporters. Neither TLRs showed a high response to pathogen-associated molecular patterns in vitro. Co-expression of AbTLR-A and AbTLR-B with AbMyD88-2 and AbMyD88-X activated NF-κB-responsive reporters in a synergetic manner. These findings demonstrate the involvement of AbTLR-A and AbTLR-B in abalone innate immunity.

Genomic structure and molecular characterization of Toll-like receptors 1 and 2 from golden pompano Trachinotus ovatus (Linnaeus, 1758) and their expression response to three types of pathogen-associated molecular patterns

Toll-like receptors (TLRs) play an essential role in the immune response. Here two Toll-like receptors from golden pompano (Trachinotus ovatus), ToTLR1 and ToTLR2, were characterized, the full-length cDNAs were 3126 bp and 7430 bp, and the deduced proteins consisted of 801 and 825 amino acids, respectively. ToTLR1 and ToTLR2 both contained the typical TLR domain architecture including signal peptide, leucine rich repeat (LRR), C-terminal LRR domain at the extracellular region and Toll/interleukin (IL)-1 receptor (TIR) domain in the cytoplasmic region. ToTLR1 only had one intron and two exons, but ToTLR2 consisted of twelve introns and thirteen exons. The promoters of ToTLR1 and ToTLR2 contained several putative transcription factor binding sites. Phylogenetic analysis showed that ToTLR1 and ToTLR2 were clustered into the clade of TLR1 and TLR2, respectively. Tissues distribution analysis indicated that both genes were ubiquitously expressed in all examined tissues, with higher expression levels observed in blood, head-kidney and spleen. After injection with poly inosinic:cytidylic [poly(I:C)], flagellin and lipopolysaccharides (LPS), ToTLR1 and ToTLR2 mRNAs were significantly up-regulated in the immune related tissues, indicating the possible the role of ToTLR1 and ToTLR2 in defense against pathogenic microbes. Further research should be carried out to identify ligands of fish TLR1 and TLR2 in order to understand the function of these receptors.

Molecular cloning and expression study on Toll-like receptor 5M in turbot, Scophthalmus maximus

Toll-like receptor 5 (TLR5) is responsible for the recognition of bacterial flagellin in mammals and play an important role in innate immunity. In the present study, a TLR5M gene was cloned from turbot, Scophthalmus maximus, and its immune responsive expression was subsequently studied in vivo. The Scophthalmus maximus (Sm)TLR5M gene is 4268 bp in length, consists of four exons and three introns and encodes a peptide of 892 amino acids (aa). The deduced protein possesses a signal peptide sequence, a leucine-rich repeat (LRR) domain composed of 23 LRR motifs, a transmembrane (TM) domain and a Toll/interleukin-1 receptor (TIR) domain. Phylogenetic analysis grouped SmTLR5M with other teleost TLR5Ms. A number of binding sites for transcription factors involved in immune response regulation were predicted in the 5'-flanking region of SmTLR5M. Quantitative real-time PCR (qPCR) analysis demonstrated that SmTLR5M mRNA was expressed ubiquitously with higher levels in head kidney and spleen. Its expression following stimulation with flagellin and lipopolysaccharide (LPS) was further tested in gills, spleen, head kidney and muscle. The maximum increases of SmTLR5M transcript levels ranged from 1.3 to 6.8-fold and appeared at 3 h to 5 day post-injection depending on different organs and stimuli. These findings suggest that SmTLR5M may play an important role in immune responses to infections with bacterial pathogens.

MicroRNA regulation of Toll-like receptor signaling pathways in teleost fish

The innate immune system is the first line defense mechanism that recognizes, responds to, controls or eliminates invading pathogens. Toll-like receptors (TLRs) are a critical family of pattern recognition receptors (PRRs) tightly regulated by complex mechanisms involving many molecules to ensure a beneficial outcome in response to foreign invaders. MicroRNAs (miRNAs), a transcriptional and posttranscriptional regulator family in a wide range of biological processes, have been identified as new molecules related to the regulation of TLR-signaling pathways in immune responses. To date, at least 22 TLR types have been identified in more than a dozen different fish species. However, the functions and underlying mechanisms of miRNAs in the regulation of inflammatory responses related to the TLR-signaling pathway in fish is lacking. In this review, we summarize the regulation of miRNA expression profiles in the presence of TLR ligands or pathogen infections in teleost fish. We focus on the effects of miRNAs in regulating TLR-signaling pathways by targeting multiple molecules, including TLRs themselves, TLR-associated signaling proteins, and TLR-induced cytokines. An understanding of the relationship between the TLR-signaling pathways and miRNAs may provide new insights for drug intervention to manipulate immune responses in fish.

Molecular characterization and expression of toll-like receptor 5 genes from Pelteobagrus vachellii

Toll-like receptor 5 (TLR5) is an important pathogen recognition receptor (PRR) that recognizes the flagellin protein of pathogenic bacteria and plays a fundamental role in activating the innate immune response. In this study, full-length pvTLR5m (membrane) and pvTLR5s (soluble) genes were cloned from darkbarbel catfish Pelteobagrus vachellii, and their expression and that of downstream genes were analyzed following exposure to the Aeromonas hydrophila pathogen. The 3009 bp pvTLR5m cDNA includes a 2652 bp open reading frame (ORF) encoding 884 amino acids. The 2422 bp pvTLR5s cDNA includes a 1944 bp ORF encoding a predicted protein of 648 amino acids. The genes are most closely related to TLR5m (75%) and TLR5s (69%) from Ictalurus punctatus, respectively, and both have a typical TLR structure. Both genes were constitutively expressed in all examined tissues, and most abundantly in the head kidney and spleen. Following pathogen challenge, pvTLR5m and pvTLR5s expression was increased significantly (P <0.05) and peaked at 24 and 12 h post-exposure in the liver, 24 and 12 h in the head kidney, and 48 and 24 h in the spleen, respectively. The downstream genes interleukin-1β (IL-1β), IL-12 and tumor necrosis factor-alpha (TNF-α) were significantly up-regulated following pathogen exposure in spleen, and the NF-kB inhibitor (IκB) was down-regulated. These findings indicated that pvTLR5 may play an important role in the immune responses to A. hydrophila. These results provide new insight to elucidate the immune signalling pathways of fish TLR.

Characterization, expression, and evolutionary analysis of new TLR3 and TLR5M genes cloned from the spiny eel Mastacembelus armatus

Toll-like receptors (TLRs) play an important role in innate and adaptive immunity. Here, we identify two new TLRs from the spiny eel Mastacembelus armatus (TLR3 and membrane TLR5M). Both MaTLR3 and MaTLR5M were expressed in all tested tissues; expression was highest in liver and spleen, respectively. After infection with Vibrio parahaemolyticus, expression of both TLRs fluctuated and differed significantly from controls at several time points. The predicted three-dimensional model of the MaTLR3 and MaTLR5M proteins indicates that most sites under positive selection were located in the extracellular domains of TLRs. Evolutionary analysis detected positively selected sites in the ancestral lineages of vertebrates, amphibians and reptiles. Multiple ML methods recovered 10 positively selected sites in teleost TLR3 and 24 in TLR5M, and most sites were located in leucine-rich repeat domain, possibly related to an "arms-race" co-evolution with pathogens.