Cloning and characterization of a novel caspase-8-like gene in Crassostrea gigas

Species-specific CgCaspase-Cg-5 in the pacific oyster induces haemocyte apoptosis by regulating the mRNA expression of apoptosis-related genes in the early stage of immune response

Caspases are cysteinyl aspartate-specific proteinases, playing critical roles in apoptotic pathway to induce apoptosis and inflammatory response. In this study, the expanded repertoire of Caspases was revealed in the Pacific oyster Crassostrea gigas, and a total of 30 Caspases were identified from the genomic and stress-induced transcriptomic databases of the Pacific oyster. They were clustered into CgCaspase-2/9, CgCaspase-8/10, CgCaspase-3/6/7, CgCaspase-Cg, and CgCaspase-L. CgCaspase-Cg subgroup was found to be specifically expanded after a positive selection in oyster with average Ka/Ks of 0.50. The mRNA expression of CgCaspase-Cg-5 was found to be obviously induced against various bacterial and viral stimulations or environmental stresses. The relative expression level of CgCaspase-Cg-5 in haemocytes increased and reached the peak at 6 h after Vibrio splendidus stimulation, which was 5.57-fold of that in the control group (p < 0.01). In the oysters whose CgCaspase-Cg-5 expression was knocked down, the mRNA expression of apoptosis-related genes including CgBcl2, CgBax, CgCaspase3 and CgCaspase9 changed significantly at 12 h after V. splendidus stimulation. The expression of CgBax, CgCaspase3 and CgCaspase9 decreased, which was 0.64-fold (p < 0.05), 0.53-fold (p < 0.05) and 0.62-fold (p < 0.01), while the expression of CgBcl2 increased, which was 2.81-fold (p < 0.01) of that in the EGFP-dsRNA group, respectively. Meanwhile, the apoptotic rate of haemocytes (1.90 ± 0.71%) significantly decreased compared to that in the EGFP-dsRNA group (5.40 ± 0.72%) (p < 0.05), and the histological damages of widened cell spacing, gill filament swelling and loose cytoplasm were observed in the CgCaspase-Cg-5-knockdown oysters after V. splendidus stimulation. Collectively, CgCaspase-Cg subgroup was specifically expanded in oyster and some bivalve species, and species-specific CgCaspase-Cg-5 regulated the mRNA expression of the apoptosis-related genes to induce haemocyte apoptosis in the early stage of immune response. This provided insight into the evolutionary and functional characteristics of Caspase repertoire in the Pacific oyster and highlighted the important role of CgCaspase-Cg-5 in the response to pathogen infection and environmental stresses.

The involvement of CgCaspase-8-2 in regulating the expressions of cytokines, antibacterial peptide and autophagy-related genes in oysters

Caspase-8 has been reported to be involved not only in apoptosis, but also in many other important immune response processes, such as inflammation and autophagy. In the present study, the open reading frame of CgCaspase-8-2 was cloned from the Pacific oyster Crassostrea gigas, which was of 2160 bp encoding 737 amino acids. There were two death effector domains (DEDs) and a cysteine aspartase cysteine structural (CASc) domain in the deduced amino acid sequences of CgCaspase-8-2. The mRNA expressions of CgCaspase-8-2 in haemocytes and gills all increased significantly after Vibrio splendidus stimulation at 3 h, 6 h, and 24 h. The cleaved CgCaspase-8-2 protein was observed in haemocytes at 3 h after V. splendidus stimulation and the expression of CgCaspase-8-2 protein was relatively higher in granulocytes, compared with that in agranulocytes. In CgCaspase-8-2-RNAi oysters, the mRNA expressions of CgIL17s (CgIL17-1, -2, -3, -4, -6), CgTNF, CgIFNLP and CgBigDef1 all decreased significantly at 12 h after V. splendidus stimulation. Meanwhile, the mRNA expressions of CgATG5 and CgBeclin1 decreased significantly at 12 h after V. splendidus stimulation, while CgBcl2 increased significantly. These results indicated that CgCaspase-8-2 was involved in not only the regulation of cytokine and antibacterial peptide production, but also autophagy-related gene expressions.

In vitro Evaluation of Programmed Cell Death in the Immune System of Pacific Oyster Crassostrea gigas by the Effect of Marine Toxins

Programmed cell death (PCD) is an essential process for the immune system's development and homeostasis, enabling the remotion of infected or unnecessary cells. There are several PCD's types, depending on the molecular mechanisms, such as non-inflammatory or pro-inflammatory. Hemocytes are the main component of cellular immunity in bivalve mollusks. Numerous infectious microorganisms produce toxins that impair hemocytes functions, but there is little knowledge on the role of PCD in these cells. This study aims to evaluate in vitro whether marine toxins induce a particular type of PCD in hemocytes of the bivalve mollusk Crassostrea gigas during 4 h at 25°C. Hemocytes were incubated with two types of marine toxins: non-proteinaceous toxins from microalgae (saxitoxin, STX; gonyautoxins 2 and 3, GTX2/3; okadaic acid/dynophysistoxin-1, OA/DTX-1; brevetoxins 2 and 3, PbTx-2,-3; brevetoxin 2, PbTx-2), and proteinaceous extracts from bacteria (Vibrio parahaemolyticus, Vp; V. campbellii, Vc). Also, we used the apoptosis inducers, staurosporine (STP), and camptothecin (CPT). STP, CPT, STX, and GTX 2/3, provoked high hemocyte mortality characterized by apoptosis hallmarks such as phosphatidylserine translocation into the outer leaflet of the cell membrane, exacerbated chromatin condensation, DNA oligonucleosomal fragments, and variation in gene expression levels of apoptotic caspases 2, 3, 7, and 8. The mixture of PbTx-2,-3 also showed many apoptosis features; however, they did not show apoptotic DNA oligonucleosomal fragments. Likewise, PbTx-2, OA/DTX-1, and proteinaceous extracts from bacteria Vp, and Vc, induced a minor degree of cell death with high gene expression of the pro-inflammatory initiator caspase-1, which could indicate a process of pyroptosis-like PCD. Hemocytes could carry out both PCD types simultaneously. Therefore, marine toxins trigger PCD's signaling pathways in C. gigas hemocytes, depending on the toxin's nature, which appears to be highly conserved both structurally and functionally.

Phylogenetic analysis of the caspase family in bivalves: implications for programmed cell death, immune response and development

BACKGROUND

Apoptosis is an important process for an organism's innate immune system to respond to pathogens, while also allowing for cell differentiation and other essential life functions. Caspases are one of the key protease enzymes involved in the apoptotic process, however there is currently a very limited understanding of bivalve caspase diversity and function.

RESULTS

In this work, we investigated the presence of caspase homologues using a combination of bioinformatics and phylogenetic analyses. We blasted the Crassostrea gigas genome for caspase homologues and identified 35 potential homologues in the addition to the already cloned 23 bivalve caspases. As such, we present information about the phylogenetic relationship of all identified bivalve caspases in relation to their homology to well-established vertebrate and invertebrate caspases. Our results reveal unexpected novelty and complexity in the bivalve caspase family. Notably, we were unable to identify direct homologues to the initiator caspase-9, a key-caspase in the vertebrate apoptotic pathway, inflammatory caspases (caspase-1, - 4 or - 5) or executioner caspases-3, - 6, - 7. We also explored the fact that bivalves appear to possess several unique homologues to the initiator caspase groups - 2 and - 8. Large expansions of caspase-3 like homologues (caspase-3A-C), caspase-3/7 group and caspase-3/7-like homologues were also identified, suggesting unusual roles of caspases with direct implications for our understanding of immune response in relation to common bivalve diseases. Furthermore, we assessed the gene expression of two initiator (Cg2A, Cg8B) and four executioner caspases (Cg3A, Cg3B, Cg3C, Cg3/7) in C. gigas late-larval development and during metamorphosis, indicating that caspase expression varies across the different developmental stages.

CONCLUSION

Our analysis provides the first overview of caspases across different bivalve species with essential new insights into caspase diversity, knowledge that can be used for further investigations into immune response to pathogens or regulation of developmental processes.

The involvement of a regucalcin in suppressing hemocyte apoptosis in Pacific oyster Crassostrea gigas

Regucalcin (RGN), also known as senescence marker protein-30 (SMP30), plays a vital role in the regulation of Ca²⁺ homeostasis. In the present study, a regucalcin (designated as CgRGN) was identified from Pacific oyster Crassostrea gigas. The complete cDNA sequence of CgRGN was of 1059 bp, containing an open reading frame of 933 bp which encoded a protein of 310 amino acids. The deduced amino acid sequence of CgRGN shared similarity with other RGNs from the genome of C. gigas as well as other species. The mRNA transcripts of CgRGN were universally detected in all tested tissues, with higher level in hepatopancreas, labial palp, and gills. The relative expression level of CgRGN in hemocytes was significantly up-regulated (p < 0.05) at 3, 12, 72, and 96 h after the stimulation of lipopolysaccharide (LPS). After CgRGN expression was interfered by specific CgRGN-dsRNA, the hemocytes apoptosis rate increased dramatically at 12 h post LPS stimulation (1.56 fold, p < 0.01), compared to the control group. The caspase-3 activity in hemocytes and NO concentration in hemolymph increased significantly (p < 0.05) in dsCgRGN injection oysters. These results collectively indicated that CgRGN could suppress LPS-induced apoptosis and be involved in the immune response of oysters.

New insights into the Manila clam and PAMPs interaction based on RNA-seq analysis of clam through in vitro challenges with LPS, PGN, and poly(I:C)

Background

Manila clam (Ruditapes philippinarum) is a worldwide commercially important marine bivalve species. In recent years, however, microbial diseases caused high economic losses and have received increasing attention. To understand the molecular basis of the immune response to pathogen-associated molecular patterns (PAMPs) in R. philippinarum, transcriptome libraries of clam hepatopancreas were constructed at 24 h post-injection with Lipopolysaccharide (LPS), peptidoglycan (PGN), and polyinosinic-polycytidylic acid (poly(I:C)) and phosphate-buffered saline (PBS) control by using RNA sequencing technology (RNA-seq).

Results

A total of 832, 839, and 188 differentially expressed genes (DEGs) were found in LPS, PGN, and poly(I:C) challenge group compared with PBS control, respectively. Several immune-related genes and pathways were activated in response to the different PAMPs, suggesting these genes and pathways might specifically participate in the immune response to pathogens. Besides, the analyses provided useful complementary data to compare different PAMPs challenges in vivo. Functional enrichment analysis of DEGs demonstrated that PAMPs responsive signal pathways were related to apoptosis, signal transduction, immune system, and signaling molecules and interaction. Several shared or specific DEGs response to different PAMPs were revealed in R. philippinarum, including pattern recognition receptors (PRRs), antimicrobial peptides (AMPs), interferon-induced proteins (IFI), and some other immune-related genes were found in the present work.

Conclusions

This is the first study employing high throughput transcriptomic sequencing to provide valuable genomic resources and investigate Manila clam response to different PAMPs through in vivo challenges with LPS, PGN, and poly(I:C). The results obtained here provide new insights to understanding the immune characteristics of R. philippinarum response to different PAMPs. This information is critical to elucidate the molecular basis of R. philippinarum response to different pathogens invasion, which potentially can be used to develop effective control strategies for different pathogens.

Molecular characterization of caspase-8-like and its expression induced by microcystin-LR in grass carp (Ctenopharygodon idella)

Caspase-8, an initiator caspase, plays a vital role in apoptosis. In this study, caspase-8-like (named as Cicaspase-8-like), a homologue of caspase-8, was identified in grass carp (Ctenopharygodon idella). The full-length cDNA sequence of CiCaspase-8-like was 1409 bp and contained a 162 bp 5'-UTR, a 239 bp 3'-UTR and a 1008 bp coding sequence. The putative amino acids sequence was 335 residues long, including a large subunit (P20) and a small subunit (P10), but lacking conserved death effector domains. A histidine active site DHSQMDAFVCCVLSHG and a cysteine active-site motif KPKLFFIQACQG were found in P20. Phylogenetic analysis showed that Cicaspase-8-like clustered with the caspase-8 and caspase-8-like of other fish and grouped closely with Carassius auratus caspase-8-like. Quantitative real-time PCR revealed that the Cicaspase-8-like mRNA were expressed constitutively in all tested tissues from healthy grass carp, with high expression level in the blood, spleen, liver and gill, indicating its role in immune reaction. The expression of Cicaspase-8-like mRNA was decreased significantly in the liver because of the stress caused by microcystin-LR (MC-LR) (75 and 100 μg MC-LR/kg BW) at 24 h and 96 h post injection (P < 0.05), but it was increased significantly in grass carp treated with 25 μg MC-LR/kg BW at 24 h (P < 0.05) post injection. Cleaved fragments of Cicaspase-8-like were observed using western blot analysis, and the expression of Cicaspase-8-like protein was increased after MC-LR treatments. Moreover, the expression of both caspase-9 and caspase-3 mRNA increased significantly after treatment with the three doses of MC-LR. TUNEL assay results showed remarkable changes in apoptosis after the MC-LR treatment. These results suggest that Cicaspase-8-like is an important caspase and plays an essential role in MC-LR-induced apoptosis.

Gene expression profiles at different stages for formation of pearl sac and pearl in the pearl oyster Pinctada fucata

Background

The most critical step in the pearl formation during aquaculture is issued to the proliferation and differentiation of outer epithelial cells of mantle graft into pearl sac. This pearl sac secretes various matrix proteins to produce pearls by a complex physiological process which has not been well-understood yet. Here, we aimed to unravel the genes involved in the development of pearl sac and pearl, and the sequential expression patterns of different shell matrix proteins secreted from the pearl sac during pearl formation by pearl oyster Pinctada fucata using high-throughput transcriptome profiling.

Results

Principal component analysis (PCA) showed clearly different gene expression profiles between earlier (before 1 week) and later stages (1 week to 3 months) of grafting. Immune-related genes were highly expressed between 0 h – 24 h (donor dependent) and 48 h – 1 w (host dependent), and in the course of wound healing process pearl sac was developed by two weeks of graft transplantation. Moreover, for the first time, we identified some stem cell marker genes including ABCG2, SOX2, MEF2A, HES1, MET, NRP1, ESR1, STAT6, PAX2, FZD1 and PROM1 that were expressed differentially during the formation of pearl sac. The expression profiling of 192 biomineralization-related genes demonstrated that most of the shell matrix proteins (SMPs) involved in prismatic layer formation were first up-regulated and then gradually down-regulated indicating their involvement in the development of pearl sac and the onset of pearl mineralization. Most of the nacreous layer forming SMPs were up-regulated at 2 weeks after the maturation of pearl sac. Nacrein, MSI7 and shematrin involved in both layer formation were highly expressed during 0 h – 24 h, down-regulated up to 1 week and then up-regulated again after accomplishment of pearl sac formation.

Conclusions

Using an RNA-seq approach we unraveled the expression pattern of the key genes involved in the development of pearl sac and pearl as a result of host immune response after grafting. These findings provide valuable information in understanding the molecular mechanism of pearl formation and immune response in P. fucata.

Electronic supplementary material

The online version of this article (10.1186/s12864-019-5579-3) contains supplementary material, which is available to authorized users.

The modulation of Smac/DIABLO on mitochondrial apoptosis induced by LPS in Crassostrea gigas

The mitochondrial pathway of apoptosis is well studied as the major mechanism of physiological cell death in vertebrates. In the present study, a second mitochondria-derived activator of caspases (Smac)/direct inhibitor of apoptosis-binding protein (IAP) with low pI protein (DIABLO) (designated as CgSmac) was identified from oyster Crassostrea gigas. The open reading frame of CgSmac was of 966 bp nucleotides encoding a predicted polypeptide of 321 amino acids with a conserved Smac/DIABLO domain containing a potential IAP-binding motif of VMPV. CgSmac proteins were distributed in hemocytes and co-localized with mitochondria. Western blotting analysis revealed that CgSmac proteins mainly existed in the dimer form in hemocytes, and the monomeric precursors and mature monomers were also detected. After lipopolysaccharide (LPS) stimulation, the mRNA expression of CgSmac in hemocytes was significantly up-regulated and peaked at 6 h (12.26-fold, p < 0.05), and the protein level of its dimers was significantly up-regulated at 6 h, 12 h, 24 h, and 48 h, while that of CgSmac monomers was up-regulated at 6 h, 12 h and down-regulated at 24 h, 48 h. The decrease of mitochondrial membrane potential indicated that the occurrence of early stage of apoptosis in primary cultured hemocytes was induced by LPS, and RNA interference (RNAi) of CgSmac could not rescue this decrease. The caspase-3 activity in primary cultured hemocytes was significantly suppressed after RNAi of CgSmac. Correspondingly, the total apoptotic rate of primary cultured hemocytes was also significantly suppressed in dsCgSmac + LPS group (31.57%) compared to dsEGFP + LPS group (40.27%, p < 0.05), which in turn demonstrated the conserved pro-apoptotic function of CgSmac. Furthermore, the early apoptotic rate (10.4% vs. 8.5%, p < 0.05) was significantly higher in dsCgSmac + LPS group than that of dsEGFP + LPS group, while the necrosis (7.7% vs. 10.0%, p < 0.05) and late apoptotic rates (13.4% vs. 21.9%, p < 0.05) were lower in dsCgSmac + LPS group than those of dsEGFP + LPS group. Collectively, CgSmac could activate mitochondrial apoptosis pathway by promoting caspase-3 activity in oyster hemocytes against exogenous LPS invasion. These results provided new insights on oyster apoptosis and the immune defense mechanisms in invertebrates.

A novel caspase-6 from sea cucumber Holothuria leucospilota: Molecular characterization, expression analysis and apoptosis detection

In this study, a novel caspase-6 named HLcaspase-6 was identified from sea cucumber Holothuria leucospilota. The full-length cDNA of HLcaspase-6 is 2195 bp in size, containing a 126 bp 5'-untranslated region (UTR), a 1043 bp 3'-UTR and a 1026 bp open reading frame (ORF) encoding a protein of 341 amino acids with a deduced molecular weight of 38.57 kDa. HLcaspase-6 contains the common signatures of the caspase family, including the conserved pentapeptide motif QACRG, as well as the P20 and P10 domains. In addition, HLcaspase-6 contains a short pro-domain. HLcaspase-6 mRNA is ubiquitously expressed in all tissues examined, with the highest transcript level in the intestine, followed by coelomocytes. In in vitro experiments, the expression of HLcaspase-6 mRNA in coelomocytes was significantly up-regulated by lipopolysaccharides (LPS) or polyriboinosinic-polyribocytidylic acid [poly (I:C)] challenge, suggesting that HLcaspase-6 might play important roles in the innate immune defense of sea cucumber against bacterial and viral infections. Moreover, we further confirmed that overexpression of HLcaspase-6 could induce apoptosis and activate the p53 signal pathway.

A novel caspase-associated recruitment domain (CARD) containing protein (CgCARDCP-1) involved in LPS recognition and NF-κB activation in oyster (Crassostrea gigas)

Caspase-associated recruitment domain (CARD) containing proteins play critical roles in molecular interaction and regulation of various signaling pathways, such as the activation of caspase and NF-κB singling pathway in the process of apoptosis or inflammation. In the present study, a novel CARD containing protein (designed CgCARDCP-1) was identified and characterized from oyster Crassostrea gigas. Molecular feature analysis revealed that, the open reading frame (ORF) of CgCARDCP-1 gene was 759 bp encoding a polypeptide of 253 amino acids with a conserved N-terminal CARD domain and two transcriptional coactivator p15 (PC4) domains in C-terminus. Homologous alignment showed that the amino acid sequence of CgCARDCP-1 shared 30%-46% identity with that of caspase-2. By RT-PCR detection, the mRNA transcripts of CgCARDCP-1 were found to be widely distributed in various tissues of oyster with the highest expression level in hemocytes and mantle. And CgCARDCP-1 protein was mostly distributed in the cytoplasm of oyster hemocytes as shown by immunohistochemistry. Moreover, the CgCARDCP-1 mRNA expression level in hemocytes was significantly up-regulated after lipopolysaccharide (LPS) and Vibrio splendidus stimulations. The recombinant CgCARDCP-1 displayed strong binding activity with LPS in vitro. In addition, after transfected into the HEK-293T cell with luciferase reporter system, CgCARDCP-1 could significantly promote the NF-κB activation (1.29-fold, p < 0.05) compared to that in the control group. These results collectively demonstrated that the CgCARDCP-1 might serve as a recognition molecule for LPS and a regulator of NF-κB activation in the immune response of oyster.

The oyster immunity

Oysters, the common name for a number of different bivalve molluscs, are the worldwide aquaculture species and also play vital roles in the function of ecosystem. As invertebrate, oysters have evolved an integrated, highly complex innate immune system to recognize and eliminate various invaders via an array of orchestrated immune reactions, such as immune recognition, signal transduction, synthesis of antimicrobial peptides, as well as encapsulation and phagocytosis of the circulating haemocytes. The hematopoietic tissue, hematopoiesis, and the circulating haemocytes have been preliminary characterized, and the detailed annotation of the Pacific oyster Crassostrea gigas genome has revealed massive expansion and functional divergence of innate immune genes in this animal. Moreover, immune priming and maternal immune transfer are reported in oysters, suggesting the adaptability of invertebrate immunity. Apoptosis and autophagy are proved to be important immune mechanisms in oysters. This review will summarize the research progresses of immune system and the immunomodulation mechanisms of the primitive catecholaminergic, cholinergic, neuropeptides, GABAergic and nitric oxidase system, which possibly make oysters ideal model for studying the origin and evolution of immune system and the neuroendocrine-immune regulatory network in lower invertebrates.

The first tropical sea cucumber caspase-8 from Holothuria leucospilota: Molecular characterization, involvement of apoptosis and inducible expression by immune challenge

In this study, the first tropical sea cucumber caspase-8 named HLcaspase-8 was identified from Holothuria leucospilota. The full-length cDNA of HLcaspase-8 is 2293 bp in size, containing a 245 bp 5'-untranslated region (UTR), a 521 bp 3'-UTR and a 1527 bp open reading frame (ORF) encoding a protein of 508 amino acids with a deduced molecular weight of 57.47 kDa. Besides the common signatures of caspase family including conserved cysteine active site pentapeptide motif QACQG, P20 domain and P10 domain, HLcaspase-8 also contains a characteristic DED domain. The over-expression of HLcaspase-8 in HEK293T cells showed that HLcaspase-8 protein could induce apoptosis and the apoptosis could be promoted by TNF-α, indicating that the apoptosis induced by HLcaspase-8 might also be triggered via a receptor-mediated pathway. Moreover, the expression of HLcaspase-8 in in vitro experiments performed in coelomocytes was significantly up-regulated by lipopolysaccharides (LPS) or polyriboinosinic-polyribocytidylic Acid [poly (I:C)] challenge, suggesting that the sea cucumber caspase-8 might play some important roles in the innate immune defense against bacterial and viral infections.

The self-activation and LPS binding activity of executioner caspase-1 in oyster Crassostrea gigas

Executioner caspases play important roles in apoptotic pathway and immune defense, which is considered to coordinate the execution phase of apoptosis by cleaving multiple structural and repair proteins. However, the knowledge about the activation mechanism and function of executioner caspases in mollusks, especially marine bivalves is limited. In the present study, the full-length cDNA sequence of caspase-1 was cloned from oyster Crassostrea gigas, which encoded a predicted protein containing a small subunit (p10) and large subunit (p20) with a conserved caspase active site QACRG similar to that of human executioner caspase-3/7. SDS-polyacrylamide gel electrophoresis and western blot results demonstrated that the CgCaspase-1 zymogen could be cleaved into p20p10, p20 and p10 in prokaryotic expression systems, and the C-terminus of CgCaspase-1 was also cleaved into p20 and p10. Both of the recombinant CgCaspase-1 (rCgCaspase-1) and the C-terminus of CgCaspase-1 (rCgCaspase-1-C) exhibited similar caspase activity towards proteolytic substrate Ac-DMQD-pNA and Ac-DEVD-pNA. However, the recombinant N-terminus of CgCaspase-1 (rCgCaspase-1-N) did not display any caspase activity. Moreover, the inhibitor of both caspase-3/7 and pan-caspase could significantly inhibit the proteolytic activity of rCgCaspase-1. The strong binding activities towards lipopolysaccharide (LPS) of both rCgCaspase-1 and rCgCaspase-1-C were revealed by ELISA techniques and western blotting. A high level of CgCaspase-1 mRNA transcripts was detected in the gills and hemocytes by quantitative real-time PCR, and the CgCaspase-1 protein was mainly located in the cytoplasm of oyster hemocytes by immunofluorescence assay. These results collectively suggested that CgCaspase-1 was a homolog of executioner caspase-3/7, which could be self-activated through proteolytic cleavage in prokaryotic expression systems, and performed caspase and LPS binding activities in the innate immune response of oyster.

Expression of a novel complement C3 gene in the razor clam Sinonovacula constricta and its role in innate immune response and hemolysis

Complement component 3 (C3) is a core component of the complement system, and directly participates in immune regulation and immune defense. Isoforms of C3 have been reported in several species of vertebrate, but invertebrates, and more specifically clams, have been less well studied. An isoform of C3, named ScC3-2, was identified in Sinonovacula constricta (Chinese razor clam). ScC3-2 included eight conserved regions, a thioester bond and two predicted junction sites (α-β and α-γ). The gene was expressed in the liver, gill, foot, hemolymph, mantle, gonad and siphon tissues. The gene was significantly upregulated in umbo larvae, suggesting that initial larval immunity may develop in umbo larvae. Moreover, the ScC3-2 mRNA expression patterns after challenge with Vibrio parahemolyticus and Micrococcus lysodeikticus exhibited an obvious upregulation at 8 h in the hemolymph and at 4 h in the liver, respectively. Furthermore, ScC3-2 showed effective membrane rupture of heterologous rabbit erythrocytes. The ScC3-2 protein was located on the surface of the cells during the process of hemolysis. After a comparative analysis, we suggest that the major structure and function of ScC3 and ScC3-2 are analogous. Our findings suggest that ScC3-2 plays an important immune function, and an intricate complement response may exist in S. constricta.

Conservation and divergence of mitochondrial apoptosis pathway in the Pacific oyster, Crassostrea gigas

Apoptosis is considered a crucial part of the host defense system in oysters according to previous reports; however, the exact process by which this occurs remains unclear. Besides, mitochondrial apoptosis is the primary method of apoptosis in vertebrate cells, but has been poorly studied in invertebrates and is quite controversial. In this study, we investigated the molecular mechanism of mitochondrial apoptosis in the Pacific oyster Crassostrea gigas. Notably, we show that most key elements involved in the vertebrate mitochondrial apoptosis pathway – including mitochondrial outer membrane permeabilization, cytochrome c release, and caspase activation – are also present in C. gigas. In contrast, the lack of Bcl-2 homology 3-only subfamily members and apoptotic protease activating factor-1 (APAF-1) protein revealed evolutionary diversity from other phyla. Our results support that mitochondrial apoptosis in animals predates the emergence of vertebrates, but suggest that an unexpectedly diverse mitochondrial apoptosis pathway may exist in invertebrates. In addition, our work provided new clues for an improved understanding of how bivalve acclimate themselves to an inconstant environment.

Molecular and functional characterization of caspase-8 from the big-belly seahorse (Hippocampus abdominalis)

Apoptosis is a physiological process that can also participate in host immune defense mechanisms, including tumor growth suppression along with homeostasis and maturation of immune cells. Caspases are known to be involved in cellular apoptotic signaling; among them, caspase-8 plays an important role in the initiation phase of the apoptotic death cascade. In the current study, we molecularly characterized a caspase-8 homolog (designated as HaCasp-8) from Hippocampus abdominalis. The HaCasp-8 gene harbors a 1476 bp open reading frame (ORF) that codes for a protein of 492 amino acids (aa) with a predicted molecular mass of 55 kDa. HaCasp-8 houses the typical domain architecture of known initiator caspases, including the death effector domain and the carboxyl-terminal catalytic domain. As expected, phylogenetic analysis reflected a closer evolutionary relationship of HaCasp-8 with its teleostean similitudes. The results of our qPCR assays confirmed the ubiquitous expression of HaCasp-8 in physiologically important tissues examined, with pronounced expression levels in ovary tissues, followed by blood cells. HaCasp-8 expression at the mRNA level was found to be significantly modulated by lipopolysaccharide, polyinosinic:polycytidylic acid, Streptococcus iniae, and Edwardsiella tarda injection. Overexpression of HaCasp-8 could trigger a significant level of cell death in HEK293T cells, suggesting its putative role in cell death. Taken together, our findings suggest that HaCasp-8 is an important component in the caspase cascade, and its expression can be significantly modulated under pathogen stress conditions in the big-belly seahorse.

Complement C3 gene: Expression characterization and innate immune response in razor clam Sinonovacula constricta

Complement component 3 (C3) is central to the complement system, playing an important role in immune defense, immune regulation and immune pathology. Several C3 genes have been characterized in invertebrates but very few in shellfish. The C3 gene was identified from the razor clam Sinonovacula constricta, referred to here as Sc-C3. It was found to be highly homologous with the C3 gene of Ruditapes decussatus. All eight model motifs of the C3 gene were found to be included in the thiolester bond and the C345C region. Sc-C3 was widely expressed in all healthy tissues with expression being highest in hemolymph. A significant difference in expression was revealed at the umbo larvae development stage. The expression of Sc-C3 was highly regulated in the hemolymph and liver, with a distinct response pattern being noted after a challenge with Micrococcus lysodeikticus and Vibrio parahemolyticus. It is therefore suggested that a complicated and unique response pathway may be present in S. constricta. Further, serum of S. constricta containing Sc-C3 was extracted. This was activated by LPS or bacterium for verification for function. The more obvious immune function of Sc-C3 was described as an effective membrane rupture in hemocyte cells of rabbit, V. parahemolyticus and Vibrio anguillarum. Thus, Sc-C3 plays an essential role in the immune defense of S. constricta.

Characterization and expression of a novel caspase gene: Evidence of the expansion of caspases in Crassostrea gigas

Caspases are a group of cysteine-aspartate proteases involved in apoptosis and a variety of non-apoptotic processes. In this study, a novel caspase gene was cloned and its potential role in apoptosis was investigated. The caspase gene (CgCasp 3/7) has an open reading frame of 1626bp encoding 541 amino acids containing the conserved functional domains and motifs of effector caspases. Its amino acid sequence shows low identity with the other effector caspases of Crassostrea gigas and contains a unique long intersubunit linker (IL). The CgCasp 3/7 mRNA was expressed highly in oocytes and then decreased gradually after fertilization, indicating CgCasp 3/7 could function in oocyte apoptosis. In adult tissues, it is located primarily in the gills and hepatopancreas. We examined the mRNA expression of CgCasp 3/7 in gills of oysters immersed in ambient (17°C) or heated (27°C) seawater. The thermal stress stimulated mRNA expression of CgCasp 3/7 by 2.5- and 4.1-fold at 2h and 6h post-treatment, respectively, indicating CgCasp3/7 was involved in the early response to thermal stress. To examine the function of the IL, CgCasp 3/7 and CgCasp 3/7-T (with a truncated IL) were expressed using an in vitro translation system and their DEVDase activity was measured. Both proteins showed a significantly higher level of DEVDase activity than control, but CgCasp3/7-T had lower DEVDase activity than CgCasp3/7, indicating CgCasp3/7 had DEVDase activity and the IL was required for maximal DEVDase activity. Our study adds to the complexity of caspases in C. gigas.