Regulator of complement activation (RCA) group 2 gene cluster in zebrafish: identification, expression, and evolution

Ayu (Plecoglossus altivelis) CD46 isoforms protect the cells from autologous complement attack

CD46 is an important immune regulatory receptor with multiple functions. However, studies on the function of teleost CD46, especially the different CD46 isoforms are limited. In this study, we identified three membrane cofactor protein (MCP, CD46) gene isoforms from ayu (Plecoglossus altivelis) and tentatively named as PaCD46 isoforms. PaCD46 isoforms were generated by alternative splicing and all consisted of four conserved short consensus repeats (SCRs), and the variable serine-threonine-proline-rich domain, transmembrane hydrophobic domain, and cytoplasmic tail. Phylogenetic analysis showed that the isoforms clustered together with other fish CD46 and then with higher animal CD46. Western blotting analysis of peripheral blood mononuclear cells (PBMC) revealed three bands, all of which had much larger molecular weights than the theoretical values of the three PaCD46 isoforms. Moreover, three PaCD46 isoforms were individually expressed on HEK293 cells, and Western blotting showed the similar band profile to that of PBMC. The recombinant extracellular domain of the PaCD46 isoforms, obtained by expression in Pichia pastoris, significantly reduced hemolysis activity of ayu sera. Furthermore, each of the three PaCD46 isoforms respectively protected the HEK293 cells expressing the isoform. The isoforms were also identified for their protection of autologous PBMC from complement activation. These results provided the first evidence that PaCD46 isoforms may be complement regulatory proteins to prevent complement-induced damage to self-tissue.

Spatially conserved motifs in complement control protein domains determine functionality in regulators of complement activation-family proteins

Regulation of complement activation in the host cells is mediated primarily by the regulators of complement activation (RCA) family proteins that are formed by tandemly repeating complement control protein (CCP) domains. Functional annotation of these proteins, however, is challenging as contiguous CCP domains are found in proteins with varied functions. Here, by employing an in silico approach, we identify five motifs which are conserved spatially in a specific order in the regulatory CCP domains of known RCA proteins. We report that the presence of these motifs in a specific pattern is sufficient to annotate regulatory domains in RCA proteins. We show that incorporation of the lost motif in the fourth long-homologous repeat (LHR-D) in complement receptor 1 regains its regulatory activity. Additionally, the motif pattern also helped annotate human polydom as a complement regulator. Thus, we propose that the motifs identified here are the determinants of functionality in RCA proteins.

The complement factor H (CFH) and its related protein 2 (CFHR2) mediating immune response in large yellow croaker Larimichthys crocea

Complement is a complex innate immune surveillance system, playing a key role in host homeostasis, inflammation, and in the defense against pathogens. Complement regulators are crucial to prevent the injudicious production of these mediators and potential injury to self tissues. Here, we identified the complement factor H (CFH) and its related gene 2 (CFHR2) homologs from large yellow croaker (Larimichthys crocea), named LcCfh and LcCfhr2, respectively. The deduced LcCfh and LcCfhr2 proteins shared significant structural similarities and identified codes for a polypeptide consisting of various numbers of highly conserved SCR domains. LcCfh, LcCfhr1 and LcCfhr2 genes were detected in all examined tissues with predominantly expressions in liver, spleen and kidney, and their expressions all increased upon Vibrio alginolyticus challenge. In vitro assays showed that recombinant LcCfh was likely to act as a cofactor of CFI and played a negative regulation role in complement system, when recombinant LcCfhr2 seemed to play mechanisms independent of the activity of CFH. Both recombinant LcCfh and LcCfhr2 took participate in inflammatory reaction despite of the inequal ability to mediate pro-inflammation response. These data provide a new insight into the functional activities of teleost complement system.

Transcriptome profiling and expression analysis of immune responsive genes in the liver of Golden mahseer (Tor putitora) challenged with Aeromonas hydrophila

Transcriptome profiling has been used to decipher the novel mechanisms behind immune responses of the fishes. However, the molecular mechanism underlining immune response in mahseer is not studied so far. Fishes are greatly affected by bacterial pathogens such as Aeromonas hydrophila. In this study, transcriptome response of golden mahseer (Tor putitora) infected with A. hydrophila was examined using paired end Illumina sequencing of liver tissue to understand the immune response of the fish. The de novo assembly generated 61,042 unigenes ranging from 200 to 9322 bp in length and an average length of 463 bp. The gene ontology annotations resulted a total of 131,826 term assignments to the annotated transcriptome including 60,846 (46.16%) allocations from the biological process; 21,603 (16.39%) from molecular function and 49,377 (37.46%) from cellular components. Differential gene expression analysis of the transcriptome data from challenged and control group revealed 1104 upregulated and 1304 down-regulated unigenes. The differentially expressed genes were mainly involved in the pathways including cell surface receptor signaling, TH1 and TH2 cell differentiation, pathogen recognition, and immune system process/defense response especially complement cascade. Twelve unigenes including ankyrin, serum amyloid, hsp4b, STAT3, complement factor c3 and c7 were validated using qPCR and found differentially expressed in accordance with in silico expression analysis. The results obtained in this study will provide the first and crucial information on the molecular mechanism of mahseer fishes against bacterial infection.

Transcriptional response to heat shock in liver of snow trout (Schizothorax richardsonii)--a vulnerable Himalayan Cyprinid fish

The snow trout (Schizothorax richardsonii) belonging to family Cyprinidae, is an endemic fish of the Himalayan region. The species is tagged as vulnerable species in the IUCN red list of threatened species. The fish thrives well in snowmelt water of several streams and rivers in the region but are occasionally exposed to more than 20 °C during the summer season. Therefore, we have used deep RNA sequencing to decipher the transcriptome of snow trout and characterize the genes and molecular pathways involved in heat shock response. In this study 72,601,298 and 65,428,283 raw reads for heat-shocked and control, respectively, were obtained by Illumina paired-end sequencing technology. The de novo assembled transcriptome was tested for differential gene expression across the treatment groups. The quality of assembly was high with N75 and N50 lengths of 461 and 1274 bases, respectively. A total of 65 unique transcripts were differentially expressed in liver under heat shock and control. Annotated blast matches reveal that differentially expressed transcripts correspond to critical chaperones and molecular pathways, previously shown to be important for thermal stress in other fish species. Eight randomly selected heat-stressed responsive transcripts were also observed to be upregulated during qRT-PCR analysis. This study is the preliminary step to understanding the responses during sudden environmental changes like heat shock. The reference transcriptome database would also aid further studies on biological and physiological aspects of the snow trout under abiotic stresses.

Complement regulatory protein genes in channel catfish and their involvement in disease defense response

Complement system is one of the most important defense systems of innate immunity, which plays a crucial role in disease defense responses in channel catfish. However, inappropriate and excessive complement activation could lead to potential damage to the host cells. Therefore the complement system is controlled by a set of complement regulatory proteins to allow normal defensive functions, but prevent hazardous complement activation to host tissues. In this study, we identified nine complement regulatory protein genes from the channel catfish genome. Phylogenetic and syntenic analyses were conducted to determine their orthology relationships, supporting their correct annotation and potential functional inferences. The expression profiles of the complement regulatory protein genes were determined in channel catfish healthy tissues and after infection with the two main bacterial pathogens, Edwardsiella ictaluri and Flavobacterium columnare. The vast majority of complement regulatory protein genes were significantly regulated after bacterial infections, but interestingly were generally up-regulated after E. ictaluri infection while mostly down-regulated after F. columnare infection, suggesting a pathogen-specific pattern of regulation. Collectively, these findings suggested that complement regulatory protein genes may play complex roles in the host immune responses to bacterial pathogens in channel catfish.

A CD46-like molecule functional in teleost fish represents an ancestral form of membrane-bound regulators of complement activation

In the complement system, the regulators of complement activation (RCA) play crucial roles in controlling excessive complement activation and in protecting host cell from misdirected attack of complement. Several members of RCA family have been cloned from cyclostome and bony fish species and classified into soluble and membrane-bound type as in mammalian RCA factors. Complement-regulatory functions have been described only for soluble RCA of lamprey and barred sand bass; however, little is known on the biological function of the membrane-bound RCA proteins in the lower vertebrates. In this study, a membrane-bound RCA protein, designated teleost complement-regulatory membrane protein (Tecrem), was cloned and characterized for its complement-regulatory roles. Carp Tecrem, an ortholog of a zebrafish type 2 RCA, ZCR1, consists of four short consensus repeat modules, a serine/threonine/proline-rich domain, a transmembrane region, and a cytoplasmic domain, from the N terminus, as does mammalian CD46. Tecrem showed a ubiquitous mRNA expression in carp tissues, agreeing well with the putative regulatory role in complement activation. A recombinant Chinese hamster ovary cell line bearing carp Tecrem showed a significantly higher tolerance against lytic activity of carp complement and less deposition of C3-S, the major C3 isotypes acting on the target cell, than control Chinese hamster ovary (mock transfectant). Anti-Tecrem mAb enhanced the depositions of carp C3 and two C4 isotypes on autologous erythrocytes. Thus, the present findings provide the evidence of complement regulation by a membrane-bound group 2 RCA in bony fish, implying the host-cell protection is an evolutionarily conserved mechanism in regulation of the complement system.

Complement system in zebrafish

Zebrafish is recently emerging as a model species for the study of immunology and human diseases. Complement system is the humoral backbone of the innate immune defense, and our knowledge as such in zebrafish has dramatically increased in the recent years. This review summarizes the current research progress of zebrafish complement system. The global searching for complement components in genome database, together with published data, has unveiled the existence of all the orthologues of mammalian complement components identified thus far, including the complement regulatory proteins and complement receptors, in zebrafish. Interestingly, zebrafish complement components also display some distinctive features, such as prominent levels of extrahepatic expression and isotypic diversity of the complement components. Future studies should focus on the following issues that would be of special importance for understanding the physiological role of complement components in zebrafish: conclusive identification of complement genes, especially those with isotypic diversity; analysis and elucidation of function and mechanism of complement components; modulation of innate and adaptive immune response by complement system; and unconventional roles of complement-triggered pathways.

Complement protective epitopes and CD55-microtubule complexes facilitate the invasion and intracellular persistence of uropathogenic Escherichia coli

BACKGROUND

 Escherichia coli-bearing Dr-adhesins (Dr+ E. coli) cause chronic pyelonephritis in pregnant women and animal models. This chronic renal infection correlates with the capacity of bacteria to invade epithelial cells expressing CD55. The mechanism of infection remains unknown.

METHODS

 CD55 amino acids in the vicinity of binding pocket-Ser155 for Dr-adhesin were mutated to alanine and subjected to temporal gentamicin-invasion/gentamicin-survival assay in Chinese hamster ovary cells. CD55/microtubule (MT) responses were studied using confocal/electron microscopy, and 3-dimensional structure analysis.

RESULTS

 Mutant analysis revealed that complement-protective CD55-Ser165 and CD55-Phe154 epitopes control E. coli invasion by coregulating CD55-MT complex expression. Single-point CD55 mutations changed E. coli to either a minimally invasive (Ser165Ala) or a hypervirulent pathogen (Phe154Ala). Thus, single amino acid modifications with no impact on CD55 structure and bacterial attachment can have a profound impact on E. coli virulence. While CD55-Ser165Ala decreased E. coli invasion and led to dormant intracellular persistence, intracellular E. coli in CD55-Phe154Ala developed elongated forms (multiplying within vacuoles), upregulated CD55-MT complexes, acquired CD55 coat, and escaped phagolysosomal fusion.

CONCLUSIONS

 E. coli target complement-protective CD55 epitopes for invasion and exploit CD55-MT complexes to escape phagolysosomal fusion, leading to a nondestructive parasitism that allows bacteria to persist intracellularly.

Molecular characterization and expression analysis of three membrane-bound complement regulatory protein isoforms in the ginbuna crucian carp Carassius auratus langsdorfii

Regulators of complement activation (RCA) play a role in protecting cells from excessive complement activation in humans. cDNA corresponding to three isoforms of teleost membrane-bound RCA protein (gTecrem) have been identified in the ginbuna crucian carp. gTecrem-1 consists of seven short consensus repeats (SCRs), whereas gTecrem-2 and gTecrem-3 have four SCRs. While gTecrem-1 possesses a tyrosine phosphorylation site in its cytoplasmic region, gTecrem-2 and gTecrem-3 lack the site. Tissue distribution analysis showed that gTecrem-1 and gTecrem-2 mRNAs were expressed in almost all tissues examined, whereas gTecrem-2 expression was not significantly detected in gill, liver, or intestine. Furthermore, analysis showed that gTecrem-1 was expressed in both peripheral blood leukocytes (PBLs) and erythrocytes and was also expressed in T cell subsets such as CD4(+), CD8(+) T cells, and IgM(+) B cells. gTecrem-2 expression was not detected in either PBLs or erythrocytes, whereas gTecrem-3 was expressed only in erythrocytes. These results suggested that gTecrem isoforms may serve different functional roles; gTecrem-1, which is expressed in T cells and possesses a tyrosine phosphorylation site, may act as a complement regulator and a cellular receptor in adaptive immunity.

Expression and functional analysis of properdin in zebrafish Danio rerio

Properdin, an upregulator of the alternative complement pathway, has been thoroughly studied in the mammalian species, but its research in the lower vertebrates such as fish is rather limited. Additionally, information regarding the structure-activity relationship of properdin remains rather fragmentary. In this report, we showed that zebrafish properdin gene zfp was abundantly expressed in the liver of adult fish, while it was primarily expressed in the brain, neural plate, developing lens, and neutrophil in the early embryos/larvae. Recombinant TSR modules of zfP were demonstrated to be able to bind to C3b, LPS, LTA and both gram-negative and positive bacteria. Moreover, TSR5 of zfP was able to enhance the phagocytosis of microbes by macrophages. These results together support the notion that properdin is a pattern recognition molecule capable of identifying non-self antigens/structures, and indicate that TSR5 plays a central role in the capacity of properdin to promote phagocytosis. It is also suggested that properdin is associated with the pattern formation and immune defense of early developing embryos/larvae.