Functional genomic studies on an immune- and antiviral-related gene of MyD88 in orange-spotted grouper, Epinephelus coioides

Ctenopharyngodon idella Tollip regulates MyD88-induced NF-κB activation

Toll-interacting protein (Tollip) and MyD88 are key components of the TLR/IL-1R signaling pathway in mammals. MyD88 is known as a universal adaptor protein involving in TLR/IL-1R-induced NF-κB activation. Tollip is a crucial negative regulator of TLR-mediated innate immune responses. Previous studies have demonstrated that teleost Tollip served as a negative regulator of MyD88-dependent TLR signaling pathway. However, the mechanism is still unclear. In particular, the effect of TBD, C2, and CUE domains of Tollip on MyD88-NF-κB signaling pathway remains to be elucidated. In this study, we found that the response of grass carp Tollip (CiTollip) to LPS stimulation was faster and stronger than that of poly I:C treatment, and CiTollip diminished the expression of tnf-α induced by LPS. Further assays indicated that except for the truncated mutant of △CUE2 (1-173 aa), wild type CiTollip and other truncated mutants (△N-(52-276 aa), △C2-(173-276 aa) and △CUE1-(1-231 aa)) could associate with MyD88 and negatively regulate MyD88-induced NF-κB activation. It suggested that the C-terminal (173-276 aa), in particular the connection section between C2 and CUE domains (173-231 aa), played a pivotal role in suppressing MyD88-induced activation of NF-κB.

Spotted knifejaw (Oplegnathus punctatus) MyD88: Intracellular localization, signal transduction function and immune responses to bacterial infection

Myeloid differentiation factor 88 (MyD88) links members of the toll-like receptor (TLR) and interleukin-1 receptor (IL-1R) superfamily to the downstream activation of NF-κB as a "bridge" molecular in response to exogenous pathogen, but the function in spotted knifejaw (Oplegnathus. punctatus), a commercial fish in China, is still unknown. We present a functional analysis of spotted knifejaw MyD88 (OppMyD88) with a typical death domain (DD) at the N-terminus and a conservative Toll/IL-1R (TIR) domain at the C-terminus and suggest that MyD88 is important for the activation of TLR-mediated NF-κB with the synergy between domains. Subcellular localization showed that OppMyD88 was distributed in the cytoplasm in a condensed form. Tissues expression profiling analysis showed that OppMyD88 ubiquitously expressed in all tested tissues with the highest expression in the liver, as determined by real-time PCR. The expression of OppMyD88 significantly upregulated in the liver, spleen, kidney and gills within 120 h post Vibrio anguillarum infection. Moreover, we further confirmed that over-expressed OppMyD88 could also induce apoptosis. These results indicate that OppMyD88 might possess important roles in defense against microbial infection and other biological processes in spotted knifejaw similar to those in mammals, which will deepen our understandings in innate immunity of spotted knifejaw.

Comparison of polymeric immunoglobulin receptor between fish and mammals

Polymeric immunoglobulin receptor (pIgR) functions in transporting polymeric immunoglobulin across epithelial cells into external secretion in animals. During animal evolution, fish was situated at a transition point on the phylogenetic spectrum between species possessing only innate immunity (i.e., invertebrates) and species depending heavily on adaptive immunity (i.e., mammals). Previous studies reported that fish and mammals significantly differ in pIgR. This review summarized the differences in pIgR structure, function, and transcriptional regulation between fish and mammals. A model of the transcriptional regulation of the pIgR gene was suggested. In this model, microbes could activate Toll-like receptor, trigger the cascade reactions in the signaling pathway, and then activate transcription factors that regulate pIgR expression through combining with the pIgR promoter. This review provides some suggestions for further studies on the function and regulatory mechanism of pIgR in fish and other animals.

Molecular cloning, structural modeling, and expression analysis of MyD88 and IRAK4 of golden pompano (Trachinotus ovatus)

MyD88 and IRAK4 are important components of TLR signaling pathways. However, information about MyD88 and IRAK4 is vacant in golden pompano (Trachinotus ovatus), a marine teleost with great commercial value. Thus, in this study the full lengths of trMyD88 and trIRAK4 were cloned from golden pompano using RT-PCR and RACE-PCR methods. trMyD88 was 1213 bp in length, encoding a putative protein of 288 amino acids (aa), consisting of a 99 aa of death domain at its N-terminal and a 137 aa of the TIR domain at its C-terminal. trIRAK4 was 1606 bp in length, encoding a putative protein of 469 aa, including an N-terminal death domain and a central kinase domain, connected by a ProST domain. Other domains or aa residues needed for their functions were also identified in trMyD88 and trIRAK4. Physicochemical features and 3-D structures of trMyD88 and trIRAK4 were also analyzed. Quantitative real-time PCR revealed that the 2 genes were ubiquitously expressed in tissues from healthy pompano, especially highly in the spleen and head kidney, indicating their roles in the immune response. Further, trMyD88 and trIRAK4 were up-regulated at 12 h after the Vibrio alginilyticus and polyI:C challenge and continued to 48 h post challenge. Our results demonstrated that MyD88 and IRAK4 played important roles in the golden pompano innate immune system, providing the basis for further study of the signaling pathways that these 2 genes are involved in.

Function analysis of fish Tollip gene in response to virus infection

Toll-interacting protein (Tollip) is one of the important regulatory proteins of Toll-like receptor (TLR) signaling pathways. In previous studies, a Tollip sequence of grouper (Epinephelus coioides) was identified and the signal transduction functions of Tollip were studied. However, the response of Tollip to virus infection has not been characterized from grouper. In the present paper, the Tollip homolog (EtTollip) from grouper (Epinephelus tauvina) was cloned and its immune response to Singapore grouper iridovirus (SGIV) was investigated. EtTollip shares significant similarities to other mammalian Tollips, which contain a centrally localized protein kinase C conserved region 2 (C2) domain and a C-terminal CUE domain. After challenging with SGIV, the expression levels of EtTollip were altered in the spleen and head kidney of grouper. EtTollip mainly aggregated in the cytoplasm in a condensed state and was also distributed on the membranes of GS cells. EtTollip significantly inhibited the activities of NF-κB and IFN-β luciferase reporter when transfected into grouper spleen (GS) cells. SGIV can increase the activities of NF-κB and IFN-β luciferase reporter, especially to IFN-β. When transfected EtTollip with EcMyd88, the activity of NF-κB was increased, while transfected EtTollip with EcIRF3, the activity of IFN-β was significantly increased. Over-expressed EtTollip inhibited the transcription of SGIV genes significantly in GS cells, and silencing of EtTollip with siRNA led to increase of SGIV genes loads. Taken together, the results provide new insights in to the importance of Tollip as evolutionarily conserved molecule for grouper innate immunity against virus infection.

Molecular cloning and expression studies of the adapter molecule myeloid differentiation factor 88 (MyD88) in turbot (Scophthalmus maximus)

Myeloid differentiation factor 88 (MyD88) is an adapter protein involved in the interleukin-1 receptor (IL-1R) and Toll-like receptor (TLR)-mediated activation of nuclear factor-kappaB (NF-κB). In this study, a full length cDNA of MyD88 was cloned from turbot, Scophthalmus maximus. It is 1619 bp in length and contains an 858-bp open reading frame that encodes a peptide of 285 amino acid residues. The putative turbot (Sm)MyD88 protein possesses a N-terminal death domain and a C-terminal Toll/IL-1 receptor (TIR) domain known to be important for the functions of MyD88 in mammals. Phylogenetic analysis grouped SmMyD88 with other fish MyD88s. SmMyD88 mRNA was ubiquitously expressed in all examined tissues of healthy turbots, with higher levels observed in immune-relevant organs. To explore the role of SmMyD88, its gene expression profile in response to stimulation of lipopolysaccharide (LPS), CpG oligodeoxynucleotide (CpG-ODN) or turbot reddish body iridovirus (TRBIV) was studied in the head kidney, spleen, gills and muscle over a 7-day time course. The results showed an up-regulation of SmMyD88 transcript levels by the three immunostimulants in all four examined tissues, with the induction by CpG-ODN strongest and initiated earliest and inducibility in the muscle very weak. Additionally, TRBIV challenge resulted in a quite high level of SmMyD88 expression in the spleen, whereas the two synthetic immunostimulants induced the higher levels in the head kidney. These data provide insights into the roles of SmMyD88 in the TLR/IL-1R signaling pathway of the innate immune system in turbot.

Blunt Snout Bream (Megalobrama amblycephala) MyD88 and TRAF6: characterisation, comparative homology modelling and expression

MyD88 and TRAF6 play an essential role in the innate immune response in most animals. This study reports the full-length MaMyD88 and MaTRAF6 genes identified from the blunt snout bream (Megalobrama amblycephala) transcriptome profile. MaMyD88 is 2501 base pairs (bp) long, encoding a putative protein of 284 amino acids (aa), including the N-terminal DEATH domain of 78 aa and the C-terminal TIR domain of 138 aa. MaTRAF6 is 2252 bp long, encoding a putative protein of 542 aa, including the N-terminal low-complexity region, RING domain (40 aa), a coiled-coil region (64 aa) and C-terminal MATH domain (147 aa). Coding regions of MaMyD88 and MaTRAF6 genomic sequences consisted of five and six exons, respectively. Physicochemical and functional characteristics of the proteins were analysed. Alpha helices were dominant in the secondary structure of the proteins. Homology models of the MaMyD88 and MaTRAF6 domains were constructed applying the comparative modelling method. RT-qPCR was used to analyse the expression of MaMyD88 and MaTRAF6 mRNA transcripts in response to Aeromonas hydrophila challenge. Both genes were highly upregulated in the liver, spleen and kidney during the first 24 h after the challenge. While MyD88 and TRAF6 have been reported in various aquatic species, this is the first report and characterisation of these genes in blunt snout bream. This research also provides evidence of the important roles of these two genes in the blunt snout bream innate immune system.

Grouper (Epinephelus coioides) MyD88 and Tollip: intracellular localization and signal transduction function

Myeloid differentiation factor 88 (MyD88) and Toll-interacting protein (Tollip) are two important regulatory proteins of the Toll-like receptor (TLR) signaling pathways. In this paper, a Tollip sequence of grouper (Epinephelus coioides) was identified and the signal transduction functions of Tollip and MyD88 were studied. The full length of E. coioides Tollip (EcTollip) cDNA with an open reading frame (ORF) of 1734 nucleotides encoded a putative protein of 274 amino acid residues. The EcTollip protein had conservative domains with mammalian homologous proteins, and high identity (78%-95%) with other vertebrates. MyD88 and Tollip were distributed in the HeLa cytoplasm in a highly condensed form. Over-expression of MyD88 could activate nuclear factor-κB (NF-κB) and its function was dependent on the death domain and ID domain on the N-terminal. Some important functional sites of mammalian MyD88 also affected fish MyD88 signal transduction. Tollip impaired NF-κB signals activated by MyD88, and its activity was dependent on the coupling of ubiquitin to the endoplasmic reticulum degradation (CUE) domain on the C-terminal. These results suggest that MyD88 and Tollip of fish and mammals are conservative on function during evolution.

Toll-pathway in tiger shrimp (Penaeus monodon) responds to white spot syndrome virus infection: evidence through molecular characterisation and expression profiles of MyD88, TRAF6 and TLR genes

The Toll-pathway plays key roles in regulating the innate immune response in invertebrates. Myeloid differentiation factor 88 (MyD88) and Tumour necrosis factor receptor (TNFR)-associated factor 6 (TRAF6) are key molecules in this signalling pathway. To investigate the role of Toll-pathway in innate immune response of shrimp, Penaeus monodon, MyD88 (PmMyD88) and TRAF6 (PmTRAF6) were identified and characterised. PmMyD88 cDNA is 1716 bp long with an open reading frame (ORF) of 1449 bp encoding a putative protein of 482 amino acids, with a death domain, a TIR domain and C-terminal extension domain. PmTRAF6 cDNA is 2563 bp long with an ORF of 1785 bp (594 amino acids) with an N-terminal RING-type zinc finger domain, two TRAF-type zinc finger domains, a coiled region and a MATH domain. In healthy shrimp, PmMyD88, PmTRAF6 and PmToll were detected in 15 tissues with the highest expression in midgut, eyestalk and lymphoid organ, respectively. Responses of these genes to WSSV in experimentally-infected P. monodon as well as in cultured haemocytes and also effect of poly I:C on the gene expression in vitro was investigated at six time-points in seven tissues. PmToll showed significant up-regulation at all time-points of infection in six tissues and until 24 h post-infection in vitro. However, poly I:C-induced haemocytes showed up-regulation of the gene until 48 h post-exposure. WSSV caused significant up-regulation of PmMyD88 in most of the tissues tested. The virus challenge as well as poly I:C induction in vitro also resulted in significant up-regulation of the gene. Up-regulated expression of PmTRAF6 was detected in haemocytes and lymphoid organ at late stage of infection. In vitro virus challenge showed significant up-regulation of PmTRAF6 at almost all time-points whereas no significant change in the expression was observed on poly I:C induction. The responses of these key genes, observed in the present study, suggest that Toll-pathway as a whole may play a crucial role in the immune response against viruses in shrimp.