Cloning and molecular characterization of two complement Bf/C2 genes in large yellow croaker (Pseudosciaena crocea)

Structural comparison and expression function analysis of BF/C2 in Ctenopharyngodon idella and Squaliobarbus curriculus

Ctenopharyngodon idella and Squaliobarbus curriculus, members of the Cyprinidae family and Yaroideae subfamily, have shown different levels of resistance to grass carp reo virus (GCRV), with S. curriculus exhibiting higher resilience. In the pursuit to explore the distinctions in the structural and expression traits of BF/C2 (A,B) between the two species, we conducted an analysis involving the cloning and examination of various coding sequences (CDS). We successfully cloned the CDS of ci-BF/C2A and ci-BF/C2B from C. idella, which spanned 2259 bp and 2514 bp respectively, encoding 752 and 837 amino acids. Similarly, the CDS of sc-BF/C2A and sc-BF/C2B from S. curriculus were cloned, featuring lengths of 1353 bp and 2517 bp and encoding 450 and 838 amino acids, respectively. A chromosome collinearity assessment revealed that ci-BF/C2A demonstrated collinearity with sc-BF/C2A, a finding not replicated with ci-BF/C2B and sc-BF/C2B. Delving into gene structure, we discerned that ci-BF/C2A harbored a greater number of Tryp_SPc domains compared to sc-BF/C2A. Following this, we engineered and purified six prokaryotic recombinant proteins: CI-BF/C2A, CI-BF/C2A1 (a variant resulting from the deletion of the Tryp_SPc domain of CI-BF/C2A), CI-BF/C2A2 (representing the Tryp_SPc domain of CI-BF/C2A), CI-BF/C2B, SC-BF/C2A, and SC-BF/C2B. Through serum co-incubation tests with these recombinant proteins, we established the activation of the complement marker C3 in each case. Utilizing fluorescence quantitative expression analysis, we observed ubiquitous expression of ci-BF/C2A and ci-BF/C2B across all grass carp tissues, predominantly in the liver. This pattern mirrored in S. curriculus, where sc-BF/C2A was highly expressed in the gills, and sc-BF/C2B manifested notably in the liver. Kidney cell infection experiments on both species revealed enhanced resistance to GCRV post-incubation with the recombinant proteins. Notably, cells treated with SC-BF/C2 (A, B) exhibited pronounced resilience compared to those treated with CI-BF/C2 (A, B, A1, A2). However, cells incubated with CI-BF/C2A1 and CI-BF/C2A2 showed strengthen resistance relative to cells treated with CI-BF/C2A and CI-BF/C2B. In GCRV infection trials on grass carp, ci-BF/C2A and ci-BF/C2B expressions reached a zenith on the seventh day post-infection, highlighting a distinctive functional mode in immune defense against GCRV infection orchestrated by BF/C2. The empirical data underscores the pivotal role of the Tryp_SPc domain in immune responses to GCRV infection, pinpointing its influence on ci-BF/C2A expression. Conclusively, this investigation provides a foundational understanding of the unique immune function characteristics of BF/C2 in grass carp, paving the way for further scholarly exploration in this realm.

Complement proteins detected through iTRAQ-based proteomics analysis of serum from black carp Mylopharyngodon piceus in response to experimentally induced Aeromonas hydrophila infection

The black carp Mylopharyngodon piceus is one of the culturally important '4 famous domestic fishes' in China. Recently, infectious diseases caused by Aeromonas hydrophila have drastically altered the operation of the black carp farming industry. In the present study, isobaric tags for relative and absolute quantitation (iTRAQ) were combined with mass spectrometry analysis to screen for differentially abundant black carp serum proteins in response to experimentally induced A. hydrophila infection. A total of 86 differentially abundant proteins were quantified at 24 h post-infection, including 78 down-regulated proteins and 8 up-regulated proteins. The down-regulated proteins included complement C1q subcomponent subunit C, complement factor B/C2A, complement factor B/C2B, complement C3-Q1, complement C3, and complement C4-2. Bioinformatic analysis indicated that the differentially abundant proteins were mainly associated with complement and coagulation cascades (27.9%). Moreover, real-time PCR (qPCR) analysis revealed changes in the gene expression of both C3 and B/C2A in blood cells, liver, kidney, gills, and intestines of the black carp infected with A. hydrophila. However, mRNA expression levels did not consistently correlate with the corresponding protein levels. A polyclonal antibody was prepared using a synthetic C3 peptide. Immunofluorescence analysis showed that the expression of C3 in the kidney was increased with A. hydrophila infection. This work provides a useful characterization of the impact of A. hydrophila infection on the complement system of the black carp.

Membrane attack complex-associated molecules from redlip mullet (Liza haematocheila): Molecular characterization and transcriptional evidence of C6, C7, C8β, and C9 in innate immunity

The redlip mullet (Liza haematocheila) is one of the most economically important fish in Korea and other East Asian countries; it is susceptible to infections by pathogens such as Lactococcus garvieae, Argulus spp., Trichodina spp., and Vibrio spp. Learning about the mechanisms of the complement system of the innate immunity of redlip mullet is important for efforts towards eradicating pathogens. Here, we report a comprehensive study of the terminal complement complex (TCC) components that form the membrane attack complex (MAC) through in-silico characterization and comparative spatial and temporal expression profiling. Five conserved domains (TSP1, LDLa, MACPF, CCP, and FIMAC) were detected in the TCC components, but the CCP and FIMAC domains were absent in MuC8β and MuC9. Expression analysis of four TCC genes from healthy redlip mullets showed the highest expression levels in the liver, whereas limited expression was observed in other tissues; immune-induced expression in the head kidney and spleen revealed significant responses against Lactococcus garvieae and poly I:C injection, suggesting their involvement in MAC formation in response to harmful pathogenic infections. Furthermore, the response to poly I:C may suggest the role of TCC components in the breakdown of the membrane of enveloped viruses. These findings may help to elucidate the mechanisms behind the complement system of the teleosts innate immunity.

Natural Resistance Associated Macrophage Protein Is Involved in Immune Response of Blunt Snout Bream, Megalobrama amblycephala

The natural resistance-associated macrophage protein gene (Nramp), has been identified as one of the significant candidate genes responsible for modulating vertebrate natural resistance to intracellular pathogens. Here, we identified and characterized a new Nramp family member, named as maNramp, in the blunt snout bream. The full-length cDNA of maNramp consists of a 153 bp 5′UTR, a 1635 bp open reading frame encoding a protein with 544 amino acids, and a 1359 bp 3′UTR. The deduced protein (maNRAMP) possesses the typical structural features of NRAMP protein family, including 12 transmembrane domains, three N-linked glycosylation sites, and a conserved transport motif. Phylogenetic analysis revealed that maNRAMP shares the significant sequence consistency with other teleosts, and shows the higher sequence similarity to mammalian Nramp2 than Nramp1. It was found that maNramp expressed ubiquitously in all normal tissues tested, with the highest abundance in the spleen, followed by the head kidney and intestine, and less abundance in the muscle, gill, and kidney. After lipopolysaccharide (LPS) stimulation, the mRNA level of maNramp was rapidly up-regulated, which reached a peak level at 6 h. Altogether, these results indicated that maNramp might be related to fish innate immunity and similar to mammalian Nramp1 in function.

A teleost complement factor Ba possesses antimicrobial activity and inhibits bacterial infection in fish

Complement factor B (Bf) is a component of the complement system. Following activation of the alternative pathway of the complement system, factor B is cleaved into Ba and Bb fragments. In fish, the Bf of rainbow trout is known to act as a C3 convertase, but the function of the Ba fragment is essentially unknown. In this study, we examined the expression patterns of tongue sole Cynoglossus semilaevis Bf (named CsBf) and the biological activity of the Ba fragment of CsBf (named CsBa). CsBf possesses the conserved domains of Bf and shares 39.9%-56.4% sequence identities with other fish Bf. CsBf expression was high in liver, muscle, and heart, and low in intestine, blood, and kidney. Bacterial infection significantly induced CsBf expression in kidney, spleen, and liver in a time-dependent manner. Recombinant CsBa (rCsBa) exhibited apparent binding capacities to bacteria and tongue sole peripheral blood leukocytes, and binding of rCsBa to bacteria inhibited bacterial growth. When overexpressed in tongue sole, CsBa significantly reduced bacterial dissemination in fish tissues. Together these results indicate for the first time that a fish Ba possesses antibacterial effect as well as immune cell-binding capacity, and thus probably plays a role in host immune defense against bacterial infection.

Molecular characterization and expression analysis of complement component C9 gene in the whitespotted bambooshark, Chiloscyllium plagiosum

Complement system is known as highly sophisticated immune defense mechanism for antigen recognition as well as effector functions. Activation of the terminal pathway of the complement system leads to the assembly of terminal complement complexes (C5b-9), which induces the characteristic complement-mediated cytolysis. The lytic activity of shark complement involves functional analogues of mammalian C8 and C9. In this article, a full-length cDNA of C9 (CpC9) is identified from cartilaginous species, the whitespotted bambooshark, Chiloscyllium plagiosum by RACE. The CpC9 cDNA is 2263 bp in length, encoding a protein of 603 amino acids, which shares 42% and 43% identity with human and Xenopus C9 respectively. Through sequence alignment and comparative analysis, the CpC9 protein was found well conserved, with the typical modular architecture in TCCs and nearly unanimous cysteine composition from fish to mammal. Phylogenetic analysis places it in a clade with C9 orthologs in higher vertebrate and as a sister taxa to the Xenopus. Expression analysis revealed that CpC9 is constitutively highly expressed in shark liver, with much less or even undetectable expression in other tissues; demonstrating liver is the primary tissue for C9synthesis. To sum up, the structural conservation and distinctive phylogenetics might indicate the potentially vital role of CpC9 in shark immune response, though it remains to be confirmed by further study.

Advances in research of fish immune-relevant genes: a comparative overview of innate and adaptive immunity in teleosts

Fish is considered to be an important model in comparative immunology studies because it is a representative population of lower vertebrates serving as an essential link to early vertebrate evolution. Fish immune-relevant genes have received considerable attention due to its role in improving understanding of both fish immunology and the evolution of immune systems. In this review, we discuss the current understanding of teleost immune-relevant genes for both innate and adaptive immunity, including pattern recognition receptors, antimicrobial peptides, complement molecules, lectins, interferons and signaling factors, inflammatory cytokines, chemokines, adaptive immunity relevant cytokines and negative regulators, major histocompatibility complexes, immunoglobulins, and costimulatory molecules. The implications of these factors on the evolutionary history of immune systems were discussed and a perspective outline of innate and adaptive immunity of teleost fish was described. This review may provide clues on the evolution of the essential defense system in vertebrates.

Molecular cloning, characterization and expression of the complement component Bf/C2 gene in grass carp

The complement system is an integral part of the host immune system and plays an immunoregulatory role at the interface between the innate and acquired immune responses. Factor B (Bf) serves as the catalytic subunit of complement C3 convertase in the alternative pathway (AP), while in the classical pathway (CP), this function is subjected to C2. In this study, we cloned and characterized the two Bf/C2 genes of grass carp, gcBf/C2A and gcBf/C2B. The gcBf/C2A and gcBf/C2B cDNA sequences are 2259 and 3004 bp in length, and the open reading frames (ORFs) of gcBf/C2A and gcBf/C2B were found to encode peptides of 752 and 837 amino acids, respectively. The genes share 30.7% amino acid identity with each other and 32.4-38.3% and 31.4-33% with the Bf and C2 genes in humans and mice. GcBf/C2A and gcBf/C2B were expressed in a wide range of grass carp tissues, with the highest level of expression of both genes detected in the liver. After a challenge with Aeromonas hydrophila, gcBf/C2A was significantly upregulated, especially at 4 h after infection, and the significantly higher expression of gcBf/C2B (27.3-fold) was found in the head kidney at 24 h post-challenge. The expression of gcBf/C2A was quickly upregulated at 1 day post-hatching and peaked at 5 days post-hatching. The maximum expression of gcBf/C2B was found at 1 day post-hatching. In conclusion, our data enables a better understanding of the physiological function of the Bf/C2 complement genes in vertebrates.

Alternative complement pathway of channel catfish (Ictalurus punctatus): molecular characterization, mapping and expression analysis of factors Bf/C2 and Df

The complement system is important in both innate and adaptive host defense against microbial infection in vertebrates. It contains three pathways: the classical, alternative, and lectin pathways. Complement component factors B and D are two crucial proteases in the alternative pathway. In this study, the genes of complement factors Bf/C2 and Df from channel catfish, Ictalurus punctatus were identified and characterized. Two complement factor B-related genes, Bf/C2A and Bf/C2B, and factor D gene Df were identified. Phylogenetic analysis suggested that Bf/C2A and Bf/C2B is likely orthologous to factor B and factor C2, respectively. Southern blot results suggested that these three genes are all single-copy genes in the catfish genome. The catfish Bf/C2A, Bf/C2B and Df genes were genetically mapped on linkage group 3, 20 and 29, respectively. Bf/C2A and Bf/C2B are highly expressed in liver and kidney, while Df is highly expressed in gill and spleen. After infection with Edwardsiella ictaluri, the expression of Bf/C2A, Bf/C2B and Df genes were found to be remarkably induced in the gill, liver, spleen and kidney at some sampling times, indicating that these three complement factors play a pivotal role in immune responses after the bacterial infection in catfish.

The complement system in teleost fish: progress of post-homolog-hunting researches

Studies on the complement system of bony fish are now finishing a stage of homologue-hunting identification of the components, unveiling existence of almost all the orthologues of mammalian complement components in teleost. Genomic and transcriptomic data for several teleost species have contributed much for the homologue-hunting research progress. Only an exception is identification of orthologues of mammalian complement regulatory proteins and complement receptors. It is of particular interest that teleost complement components often exist as multiple isoforms with possible functional divergence. This review summarizes research progress of teleost complement system following the molecular identification and sequence analysis of the components. The findings of extensive expression analyses of the complement components with special emphasis of their prominent extrahepatic expression, acute-phase response to immunostimulation and various microbial infections, and ontogenic development including maternal transfer are discussed to infer teleost-specific functions of the complement system. Importance of the protein level characterization of the complement components is also emphasized, especially for understanding of the isotypic diversity of the components, a unique feature of teleost complement system.