Identification, expression patterns, evolutionary characteristics and recombinant protein activities analysis of CD209 gene from Megalobrama amblycephala

Characterization and Expression Analysis of Genes from Megalobrama amblycephala Encoding Hemoglobins with Extracellular Microbicidal Activity

Hemoglobin (Hb) usually comprises two α and two β subunits, forming a tetramer responsible for oxygen transportation and storage. Few studies have elucidated fish hemoglobin immune functions. Megalobrama amblycephala is a freshwater-cultured fish prevalent in China. We identified two M. amblycephala hemoglobin subunits and analyzed their expression patterns and antibacterial activities. The respective full-length cDNA sequences of the M. amblycephala Hb α (MaHbα) and β (MaHbβ) subunits were 588 and 603 bp, encoding 143 and 148 amino acids. MaHbα and MaHbβ were highly homologous to hemoglobins from other fish, displaying typical globin-like domains, most heme-binding sites, and tetramer interface regions highly conserved in teleosts. In phylogenetic analyses, the hemoglobin genes from M. amblycephala and other cypriniformes clustered into one branch, and those from other fishes and mammals clustered into other branches, revealing fish hemoglobin conservation. These M. amblycephala Hb subunits exhibit different expression patterns in various tissues and during development. MaHbα is mainly expressed in the blood and brain, while MaHbβ gene expression is highest in the muscle. MaHbα expression was detectable and abundant post-fertilization, with levels fluctuating during the developmental stages. MaHbβ expression began at 3 dph and gradually increased. Expression of both M. amblycephala Hb subunits was down-regulated in most examined tissues and time points post-Aeromonas hydrophila infection, which might be due to red blood cell (RBC) and hematopoietic organ damage. Synthetic MaHbα and MaHbβ peptides showed excellent antimicrobial activities, which could inhibit survival and growth in five aquatic pathogens. Two M. amblycephala hemoglobin subunits were identified, and their expression patterns and antibacterial activities were analyzed, thereby providing a basis for the understanding of evolution and functions of fish hemoglobins.

Molecular characterization and functional analysis of CD209E from Nile Tilapia (Oreochromis Niloticus) involved in immune response to bacterial infection

CD209 plays significant roles in pathogen recognition, innate and adaptive immunity, and cell-cell interactions. In the present study, a CD209 antigen-like protein E from Nile tilapia (Oreochromis niloticus) (designated as OnCD209E) was identified and characterized. OnCD209E contains an open reading frame (ORF) of 771 bp encoding a 257 amino acid protein, as well as the carbohydrate recognition domain (CRD). Multiple sequence analysis exhibits that the amino acid sequence of OnCD209E was relatively high homologous to that of partial fish, especially the highly conserved CRD, in which four conserved disulfide-bonded cysteine residues, WIGL conserved motif and two Ca²⁺/carbohydrate-binding sites (EPD and WFD motifs) were founded. Quantitative real-time PCR and Western Blot revealed that OnCD209E mRNA/protein is generally expressed in all tissues examined, but with wealth in head kidney and spleen tissues. The mRNA expression of OnCD209E was significantly increased in brain, head kidney, intestine, liver, and spleen tissues in response to the stimulation with polyinosinic-polycytidylic acid, Streptococcus agalactiae and Aeromonas hydrophila in vitro. Recombinant OnCD209E protein exhibited detectable bacterial binding and agglutination activity against different bacteria as well as inhibited the proliferation of tested bacteria. Subcellular localization analysis revealed that OnCD209E was mostly localized in the cell membrane. Moreover, overexpression of OnCD209E could activate nuclear factor-kappa B reporter genes in HEK-293T cells. Collectively, these results demonstrated that CD209E may potentially involve in immune response of Nile tilapia against bacterial infection.

Molecular characterization and expression patterns of CXCL8 gene from blunt snout bream (Megalobrama amblycephala) and its chemotactic effects on macrophages and neutrophils

CXCL8 is a typical CXC-type chemokine, which mediates the migration of immune cells from blood vessels to the site of inflammation or injury to clear pathogenic microorganisms and repair damaged tissues. In this study, Megalobrama amblycephala CXCL8 (MaCXCL8) gene was identified and characterized. Sequence analysis showed that the deduced MaCXCL8 protein possessed the typical structure of CXCL8 from other species, with the characteristic CXC cysteine residues in the N-terminal and accompanied by a DLR motif (Asp-Leu-Arg motif). Phylogenetic analysis revealed that MaCXCL8 was homologous to that of Ctenopharyngodon idella and other cyprinid fishes. MaCXCL8 gene was expressed in all detected healthy tissues, with the highest expression levels in the spleen, and its expression was significantly up-regulated upon the challenge of Aeromonas hydrophila and Lipopolysaccharide (LPS) both in juvenile M. amblycephala tissues and primary macrophages. The immunohistochemical assay showed that MaCXCL8 was mainly distributed in the nucleus and cytoplasm, and its expression levels increased observably with the prolongation of bacterial infection. In addition, recombinant MaCXCL8 protein exhibited significant chemotactic effects on neutrophils and macrophages. In conclusion, MaCXCL8 is involved in the immune response of M. amblycephala, and these findings will be helpful to understand the biological roles of MaCXCL8 and provide a theoretical basis for the prevention and control of fish bacterial diseases.

Immunogenicity and protective efficacy of OmpA subunit vaccine against Aeromonas hydrophila infection in Megalobrama amblycephala: An effective alternative to the inactivated vaccine

Aeromonas hydrophila is a kind of zoonotic pathogen, which can cause bacterial septicemia in fish and bring huge economic losses to global aquaculture. Outer membrane proteins (Omps) are conserved antigens of Aeromonas hydrophila, which can be developed as subunit vaccines. To evaluate the protective efficacy of inactivated vaccine and recombinant outer membrane protein A (OmpA) subunit vaccine against A. hydrophila in juvenile Megalobrama amblycephala, the present study investigated the immunogenicity and protective effects of both vaccines, as well as the non-specific and specific immune response of M. amblycephala. Compared with the non-vaccinated group, both inactivated and OmpA subunit vaccines improved the survival rate of M. amblycephala upon infection. The protective effects of OmpA vaccine groups were better than that of the inactivated vaccine groups, which should be attributed to the reduced bacterial load and enhanced host immunity in the vaccinated fish. ELISA assay showed that the titer of serum immunoglobulin M (IgM) specific to A. hydrophila up-regulated significantly in the OmpA subunit vaccine groups at 14 d post infection (dpi), which should contribute to better immune protective effects. In addition, vaccination enhanced host bactericidal abilities might also attribute to the regulation of the activities of hepatic and serum antimicrobial enzymes. Moreover, the expression of immune-related genes (SAA, iNOS, IL-1 β, IL-6, IL-10, TNF α, C3, MHC I, MHC II, CD4, CD8, TCR α, IgM, IgD and IgZ) increased in all groups post infection, which was more significant in the vaccinated groups. Furthermore, the number of immunopositive cells exhibiting different epitopes (CD8, IgM, IgD and IgZ) that were detected by immunohistochemical assay had increased in the vaccinated groups post infection. These results show that vaccination effectively stimulated host immune response (especially OmpA vaccine groups). In conclusion, these results indicated that both the inactivated vaccine and OmpA subunit vaccine could protect juvenile M. amblycephala against A. hydrophila infection, of which OmpA subunit vaccine provided more effective immune protection and can be used as an ideal candidate for the A. hydrophila vaccine.

Comparative Expression Profiling Reveals the Regulatory Effects of Dietary Mannan Oligosaccharides on the Intestinal Immune Response of Juvenile Megalobrama amblycephala against Aeromonas hydrophila Infection

Mannan oligosaccharides (MOS) are functional oligosaccharides with beneficial effects on the non-specific immunity of Megalobrama amblycephala, but systematic studies on the immunomodulatory mechanisms of MOS are still lacking. To investigate the protective mechanisms of three different levels of dietary MOS supplementation on the intestinal immunity of juvenile M. amblycephala, comparative digital gene expression (DGE) profiling was performed. In this study, 622 differentially expressed genes (DEGs) were identified, while the similar expression tendency of 34 genes by qRT-PCR validated the accuracy of the DGE analyses. Gene Ontology (GO) enrichment revealed that the DEGs were mainly enriched in two functional categories of biological process and molecular function. Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis revealed that the DEGs were mainly related to complement and coagulation cascades, coagulation cascades, platelet activation, natural killer cell mediated cytotoxicity, Fc gamma R-mediated phagocytosis and antigen processing and presentation. In addition, the pro-inflammatory, apoptosis and tight junction-related genes were more significantly up-regulated upon infection in the dietary MOS groups to enhance host immune functions and maintain the stability of the intestinal barrier. These results will be helpful to clarify the regulatory mechanism of MOS on the intestinal immunity of M. amblycephala and lay the theoretical foundation for the prevention and protection of fish bacterial diseases.

Alterations of the Mucosal Immune Response and Microbial Community of the Skin upon Viral Infection in Rainbow Trout (Oncorhynchus mykiss)

The skin is the largest organ on the surface of vertebrates, which not only acts as the first line of defense against pathogens but also harbors diverse symbiotic microorganisms. The complex interaction between skin immunity, pathogens, and commensal bacteria has been extensively studied in mammals. However, little is known regarding the effects of viral infection on the skin immune response and microbial composition in teleost fish. In this study, we exposed rainbow trout (Oncorhynchus mykiss) to infectious hematopoietic necrosis virus (IHNV) by immersion infection. Through pathogen load detection and pathological evaluation, we confirmed that IHNV successfully invaded the rainbow trout, causing severe damage to the epidermis of the skin. qPCR analyses revealed that IHNV invasion significantly upregulated antiviral genes and elicited strong innate immune responses. Transcriptome analyses indicated that IHNV challenge induced strong antiviral responses mediated by pattern recognition receptor (PRR) signaling pathways in the early stage of the infection (4 days post-infection (dpi)), and an extremely strong antibacterial immune response occurred at 14 dpi. Our 16S rRNA sequencing results indicated that the skin microbial community of IHNV-infected fish was significantly richer and more diverse. Particularly, the infected fish exhibited a decrease in Proteobacteria accompanied by an increase in Actinobacteria. Furthermore, IHNV invasion favored the colonization of opportunistic pathogens such as Rhodococcus and Vibrio on the skin, especially in the later stage of infection, leading to dysbiosis. Our findings suggest that IHNV invasion is associated with skin microbiota dysbiosis and could thus lead to secondary bacterial infection.

Molecular Characterization, Expression, Evolutionary Selection, and Biological Activity Analysis of CD68 Gene from Megalobrama amblycephala

CD68 is a highly glycosylated transmembrane glycoprotein that belongs to the lysosome-associated membrane glycoprotein family and is involved in various immune processes. In this study, Megalobrama amblycephala CD68 (MaCD68) was cloned and characterized, and its expression patterns and evolutionary characteristics were analyzed. The coding region of MaCD68 was 987 bp, encoding 328 amino acids, and the predicted protein molecular weight was 34.9 kDa. MaCD68 contained two transmembrane helical structures and 18 predicted N-glycosylation sites. Multiple sequence alignments showed that the MaCD68 protein had high homology with other fish, and their functional sites were also highly conserved. Phylogenetic analysis revealed that MaCD68 and other cypriniformes fish clustered into one branch. Adaptive evolution analysis identified several positively selected sites of teleost CD68 using site and branch-site models, indicating that it was under positive selection pressure during evolution. Quantitative real-time reverse transcription polymerase chain reaction analysis showed that MaCD68 was highly expressed in the head kidney, spleen, and heart. After Aeromonas hydrophila infection, MaCD68 was significantly upregulated in all tested tissues, peaking at 12 h post-infection (hpi) in the kidney and head kidney and at 120 hpi in the liver and spleen, suggesting that MaCD68 participated in the innate immune response of the host against bacterial infection. Immunohistochemical and immunofluorescence analyses also showed that positive signals derived from the MaCD68 protein were further enhanced after bacterial and lipopolysaccharide treatment, which suggested that MaCD68 is involved in the immune response and could be used as a macrophage marker. Biological activity analysis indicated that recombinant MaCD68 (rMaCD68) protein had no agglutination or bactericidal effects on A. hydrophila but did have these effects on Escherichia coli. In conclusion, these results suggest that MaCD68 plays a vital role in the immune response against pathogens, which is helpful in understanding the immune responses and mechanisms of M. amblycephala.