A novel stimulator of interferon gene (STING) from Larimichthys crocea and their involvement in immune response to ectoparasite Cryptocaryon irritans infection.

The marine white spot disease caused by protozoan ectoparasite Cryptocaryon irritans is a severe problem to the large yellow croaker farming industry. To understand the molecular immune mechanisms and improve its immune capacity are particular important. STING, one of the important second messengers in innate immune response process, plays pivotal roles in defensing against different pathogenic microorganisms. Many reports have pointed that STING could not only combine the uncovered dsDNA, ssDNA directly in the cytoplasm from the pathogens or biology itself, but it also could recognize cyclic diguanylate monophosphate (c-di-GMP), cyclic diadenylate monophosphate (c-di-AMP). Based on the STING sequence, a variant of the L. crocea STING (termed LcSTING2) was found by accident. RACE was used to clone the full-length cDNA of LcSTING2 which contained a 5'- UTR of 154 bp, a 3'-UTR of 592 bp and an ORF of 1227 bp encoding 408 amino acids. The predicted protein molecular weight (Mw) was 45.83 KDa and the estimated theoretical isoelectric point (pI) was 6.24. The deduced protein of LcSTING2 contains no signal peptide. One transmembrane motif (TM) in the N-terminal region, a TMEM173 domain and five putative motifs (RXR) found in resident endoplasmic reticulum proteins were also conserved. According to the partial genomic sequence, the unknown sequences were amplified, it contained 6 exons and 5 introns, and all the exon-intron boundaries were in accordance with classical GT-AG rule (GT/intron/AG). The similarity shared with fishes was higher than other high vertebrates. qRT-PCR results showed that LcSTING (two variants) distributed in all examined tissues, and it was the most abundant in gill. After challenged by C. irritans, LcSTING mRNA only appeared instantaneous up-regulation during the infective stage of theronts in the head kidney and was also up-regulated during the whole infectious cycle of C. irritans in the liver, which implied LcSTING gene was likely to be involved in the defensing against C. irritans infection, it is the first time to explore the STING taking part in the immune response to ectoparasite rather than bacterium or viruses.

Identification, expression and antibacterial activities of an antimicrobial peptide NK-lysin from a marine fish Larimichthys crocea.

As fundamental immunologic mechanism, the innate immunity system is more important than the specific immunity system in teleost fishes during pathogens infection. Antimicrobial peptides are integral parts of the innate immune system, and play significant roles against pathogens infection. NK-lysin, the compounds of the natural killer cells and cytotoxic T cells, are potent and effective antimicrobial peptides widely distributed in animals. In this study, we reported the sequence characteristics, expression profiles and antibacterial activities of a NK-lysin gene (Lc-NK-lysin) from a commercially important marine fish, the large yellow croaker (Larimichthys crocea). The open reading frame of Lc-NK-lysin cDNA sequence was 447 bp in length, coding 148 amino acids. The genomic DNA of Lc-NK-lysin has the common features of NK-lysin family, consisting of five exons and four introns, and in its deduced mature peptide, there are six well-conserved cysteine residues and a Saposin B domain. Lc-NK-lysin was expressed in all tested tissues (skin, muscle, gill, brain, head kidney, heart, liver, spleen, stomach and intestine) with different expression patterns. In pathogens infection the expression profiles of Lc-NK-lysin varied significantly in gill, head kidney, spleen and liver, indicating its role in immune response. Two peptides (Lc-NK-lysin-1 and Lc-NK-lysin-2) divided from the core region of the Lc-NK-lysin mature polypeptide were chemically synthesized and their antibacterial activities were examined; the potential function on the inhibition of bacteria propagation was revealed. Our results suggested that Lc-NK-lysin is a typical member of the NK-lysin family and as an immune-related gene it involves in the immune response when pathogens invasion.

Transcriptome analysis of the Larimichthys crocea liver in response to Cryptocaryon irritans.

The large yellow croaker (Larimichthys crocea) is an economically important marine fish cultured in China and East Asian countries and is facing a serious threat from Cryptocaryon irritans, which is a protozoan ectoparasite that infects most reared marine fish species. To understand the molecular immune mechanisms underlying the response to C. irritans, we first performed a comparative gene transcription analysis using livers from C. irritans-immunized L. croceas and from a control group through RNA-Seq technology. After the removal of low-quality sequences and assembly, 51360 contigs were obtained, with an average length of 1066.93 bp. Further, a blast analysis indicates that 30747 contigs can be annotated based on homology with matches in the NT, NR, gene, and string databases. A gene ontology analysis was used to classify 21598 genes according to three major functional categories: molecular function, cellular component, and biological process. Moreover, 14470 genes were found in 303 KEGG pathways. We used RSEM and EdgeR to determine that 3841 genes were significantly differentially expressed (FDR < 0.001), including 2129 up-regulated genes and 1712 down-regulated genes. A significant enrichment analysis of these differentially expressed genes and isogenes revealed major immune-related pathways, including the toll-like receptor, complement and coagulation cascades, and chemokine signaling pathways. In addition, 28748 potential simple sequence repeats (SSRs) were detected from 12776 transcripts, and 62992 candidate single nucleotide polymorphisms (SNPs) were identified in the L. croceas liver transcriptome. This study characterized a gene expression pattern for normal and C. irritans-immunized L. croceas for the first time and not only sheds new light on the molecular mechanisms underlying the host-C. irritans interaction but also facilitates future studies on L. croceas gene expression and functional genomics.

Molecular characterization and expression analysis of interferon-gamma in the large yellow croaker Larimichthys crocea.

The large yellow croaker Larimichthys crocea is an important mariculture fish species in China, and the bacterium Vibrio harveyi (V. harveyi) and the ciliate protozoan Cryptocaryon irritans (C. irritans) are the two major pathogens in its aquaculture sector. Interferon-gamma (IFN-γ) plays important roles in regulating both innate and cell mediated immune responses as an inflammatory cytokine. In this study, we obtained the nucleotide sequence of IFN-γ from the large yellow croaker (LcIFN-γ). The phylogenetic relationship tree of 18 available IFN-γ genes was constructed based on their sequences. Expression analyses in 10 various tissues were conducted after the croaker challenged with V. harveyi and C. irritans, respectively. Real time PCR analysis showed that the expression of LcIFN-γ was observed broadly in health individuals. After injected with V. harveyi, the 10 tissues had a higher expression of IFN-γ at the first day (1 d); only spleen, muscle, intestine, heart and skin had higher expressions after infected with C. irritans at 1 d. Major immune tissues (skin, gill, head kidney and spleen) and detoxification tissue (liver) were sampled at 0 h, 6 h, 1 d, 2 d, 3 d, 4 d, 5 d and 7 d to understand the expression trends of LcIFN-γ after challenged with C. irritans. The expressions of LcIFN-γ in skin and gill (the primary immune organs) showed a clear correlative relationship with the life cycle of C. irritans.