Induction of phenoloxidase and other immunological activities in Sydney rock oysters challenged with microbial pathogen-associate molecular patterns

Transcriptomic Response to Perkinsus marinus in Two Crassostrea Oysters Reveals Evolutionary Dynamics of Host-Parasite Interactions

Infectious disease outbreaks are causing widespread declines of marine invertebrates including corals, sea stars, shrimps, and molluscs. Dermo is a lethal infectious disease of the eastern oyster Crassostrea virginica caused by the protist Perkinsus marinus. The Pacific oyster Crassostrea gigas is resistant to Dermo due to differences in the host-parasite interaction that is not well understood. We compared transcriptomic responses to P. marinus challenge in the two oysters at early and late infection stages. Dynamic and orchestrated regulation of large sets of innate immune response genes were observed in both species with remarkably similar patterns for most orthologs, although responses in C. virginica were stronger, suggesting strong or over-reacting immune response could be a cause of host mortality. Between the two species, several key immune response gene families differed in their expansion, sequence variation and/or transcriptional response to P. marinus, reflecting evolutionary divergence in host-parasite interaction. Of note, significant upregulation of inhibitors of apoptosis (IAPs) was observed in resistant C. gigas but not in susceptible C. virginica, suggesting upregulation of IAPs is an active defense mechanism, not a passive response orchestrated by P. marinus. Compared with C. gigas, C. virginica exhibited greater expansion of toll-like receptors (TLRs) and positive selection in P. marinus responsive TLRs. The C1q domain containing proteins (C1qDCs) with the galactose-binding lectin domain that is involved in P. marinus recognition, were only present and significantly upregulated in C. virginica. These results point to previously undescribed differences in host defense genes between the two oyster species that may account for the difference in susceptibility, providing an expanded portrait of the evolutionary dynamics of host-parasite interaction in lophotrochozoans that lack adaptive immunity. Our findings suggest that C. virginica and P. marinus have a history of coevolution and the recent outbreaks may be due to increased virulence of the parasite.

The insect, Galleria mellonella, is a compatible model for evaluating the toxicology of okadaic acid

The polyether toxin, okadaic acid, causes diarrhetic shellfish poisoning in humans. Despite extensive research into its cellular targets using rodent models, we know little about its putative effect(s) on innate immunity. We inoculated larvae of the greater wax moth, Galleria mellonella, with physiologically relevant doses of okadaic acid by direct injection into the haemocoel (body cavity) and/or gavage (force-feeding). We monitored larval survival and employed a range of cellular and biochemical assays to assess the potential harmful effects of okadaic acid. Okadaic acid at concentrations ≥ 75 ng/larva (≥ 242 μg/kg) led to significant reductions in larval survival (> 65%) and circulating haemocyte (blood cell) numbers (> 50%) within 24 h post-inoculation. In the haemolymph, okadaic acid reduced haemocyte viability and increased phenoloxidase activities. In the midgut, okadaic acid induced oxidative damage as determined by increases in superoxide dismutase activity and levels of malondialdehyde (i.e. lipid peroxidation). Our observations of insect larvae correspond broadly to data published using rodent models of shellfish-poisoning toxidrome, including complementary LD₅₀ values: 206–242 μg/kg in mice, ~ 239 μg/kg in G. mellonella. These data support the use of this insect as a surrogate model for the investigation of marine toxins, which offers distinct ethical and financial incentives.

Hybridization improved bacteria resistance in abalone: Evidence from physiological and molecular responses

Hybridization is an effective way of improving germplasm in abalone, as it often generates benign traits in the hybrids. The hybrids of Haliotis discus hannai and H. gigantea have shown heterosis in terms of disease resistance than one or both parental species. In the present study, to elucidate the physiological and molecular mechanism of this heterosis, we analyzed the dynamic changes of several immune indexes including survival rate, total circulating haemocyte count (THC), phagocytic activity, reactive oxygen species level (ROS) and phenoloxidase activity (PO) in two parental species, H. discus hannai (DD) and H. gigantea (GG), and their reciprocal hybrids H. discus hannai ♀ × H. gigantea ♂ (DG), H. gigantea ♀ × H. discus hannai ♂ (GD) challenged with a mixture of Vibrio harveyi, V. alginolyticus and V. parahaemolyticus (which have been demonstrated to be pathogenic to abalone). Besides, we cloned and analyzed three important immune genes: heat shock protein 70 (hsp70), ferritin and cold shock domain protein (csdp) in H. discus hannai and H. gigantea, then further investigated their mRNA level changes in the four abalone genotypes after bacterial challenge. Results showed that these physiological and molecular parameters were significantly induced by bacterial exposure, and their changing patterns were obviously different between the four genotypes: (1) Survival rates of the two hybrids were higher than both parental species after bacterial exposure; (2) DG had higher THC than the other three genotypes; (3) Phagocytosis responded slower in the hybrids than in the parental species; (4) DD's ROS level was lower than the other three genotypes at 48 h post infection; (5) Phenoloxidase activity was lower in DD during the infection compared to the other genotypes; (6) mRNA levels of hsp70 and csdp, were always lower in at least one parental species (DD) than in the hybrids after the bacterial exposure. Results from this study indicate that the hybrids are more active or efficient in immune system function, hence they could effectively defense against a bacterial invasion, leading to higher survival rates after challenge. This study provides physiological and molecular evidences for interpreting the disease resistant heterosis in this abalone hybrid system, which could help us in a better understanding and utilization of heterosis in abalone aquaculture.

Recent findings on phenoloxidases in bivalves

The production of melanin is a complex process involving biochemical cascades, such as the pro-phenoloxidase (proPO) system, and enzymes, such as phenoloxidases (POs). Different studies have shown a strong correlation between the decrease in PO activities and the occurrence of diseases in bivalve invertebrates, leading to mortalities in the host. Results of these studies suggest that POs could play a fundamental role in defense mechanisms in bivalves. This article reviews the fundamental knowledge on the proPO system in bivalves and the methods used to assess PO activities. Finally, this is the first report on the major findings of laboratory and field studies that indicate that a type of PO in bivalves, the laccase enzyme, is inducible and involved in the 1) immune 2) antioxidant and 3) detoxification roles in bivalves, and might be an ecological potential biomarker of environmental stress.

Metabolic responses of clam Ruditapes philippinarum exposed to its pathogen Vibrio tapetis in relation to diet

We investigated the effect of brown ring disease (BRD) development and algal diet on energy reserves and activity of enzymes related to energy metabolism, antioxidant system and immunity in Manila clam, Ruditapes philippinarum. We found that algal diet did not impact the metabolic response of clams exposed to Vibrio tapetis. At two days post-injection (dpi), activities of superoxide dismutase and glutathione peroxidase (GPx) decreased whereas activities of nitric oxide synthase (iNOS) and catalase increased in infected clams, although no clinical signs were visible (BRD-). At 7 dpi, activities of several antioxidant and immune-related enzymes were markedly increased in BRD-likely indicating an efficient reactive oxygen species (ROS) scavenging compared to animals which developed clinical signs of BRD (BRD+). Therefore, resistance to BRD clinical signs appearance was associated with higher detoxification of ROS and enhancement of immune response. This study provides new biochemical indicators of disease resistance and a more comprehensive view of the global antioxidant response of clam to BRD development.

A review of the immune molecules in the sea cucumber

It is very important to identify and characterize the immune-related genes that respond to pathogens. Until recently, only some of the immune-related genes in sea cucumbers had been characterized. Their expression patterns after pathogen challenges have been analyzed via expressed sequence tag libraries, microarray studies and proteomic approaches. These genes include lectins, antimicrobial peptides, lysozyme, enzymes, clotting protein, pattern recognition proteins, Toll receptors, complement C3 and other humoral factors that might participate in the innate immune system of sea cucumbers. Although the participation of some of these immune molecules in the sea cucumber's innate immune defense against invading pathogens has been demonstrated, the functions of many of the molecules remain unclear. This review focuses on the discovery and functional characterization of the immune-related molecules from the sea cucumber for the first time and provides new insights into the immune mechanisms of the sea cucumber, which opens new possibilities for developing drugs for novel anti-bacterial and antiviral applications in fisheries.

Immunotoxicity of washing soda in a freshwater sponge of India

The natural habitat of sponge, Eunapius carteri faces an ecotoxicological threat of contamination by washing soda, a common household cleaning agent of India. Washing soda is chemically known as sodium carbonate and is reported to be toxic to aquatic organisms. Domestic effluent, drain water and various human activities in ponds and lakes have been identified as the major routes of washing soda contamination of water. Phagocytosis and generation of cytotoxic molecules are important immunological responses offered by the cells of sponges against environmental toxins and pathogens. Present study involves estimation of phagocytic response and generation of cytotoxic molecules like superoxide anion, nitric oxide and phenoloxidase in E. carteri under the environmentally realistic concentrations of washing soda. Sodium carbonate exposure resulted in a significant decrease in the phagocytic response of sponge cells under 4, 8, 16 mg/l of the toxin for 96h and all experimental concentrations of the toxin for 192h. Washing soda exposure yielded an initial increase in the generation of the superoxide anion and nitric oxide followed by a significant decrease in generation of these cytotoxic agents. Sponge cell generated a high degree of phenoloxidase activity under the experimental exposure of 2, 4, 8, 16 mg/l of sodium carbonate for 96 and 192 h. Washing soda induced alteration of phagocytic and cytotoxic responses of E. carteri was indicative to an undesirable shift in their immune status leading to the possible crises of survival and propagation of sponges in their natural habitat.

In vitro antibacterial analysis of phenoloxidase reaction products from the sea cucumber Apostichopus japonicus

Three phenoloxidases (POs) of Apostichopus japonicus, AjPOs (AjPO1, AjPO2 and AjPO3), were partially purified from the coelomocytes with an electrophoretic method, and then employed for the in vitro antibacterial analysis. Using L-3,4-dihydroxyphenylalanine (L-DOPA) as a substrate, AjPO1 and AjPO2-derived compounds inhibited the growth of Vibrio splendidus and Staphylococcus aureus, while AjPO3-derived compounds only inhibited the growth of V. splendidus. When dopamine was used as a substrate, AjPO1 and AjPO3-derived compounds inhibited the growth of V. splendidus and Vibrio harveyi, while AjPO2-derived compounds only inhibited the growth of V. splendidus. Moreover, AjPO1-derived compounds showed stronger inhibition in V. harveyi than AjPO3-derived compounds did. However, all of the three AjPO reaction products showed no inhibitions on the growth of Pseudoalteromonas nigrifaciens, Shewanella baltica, Micrococcus lysodeikticus, Streptococcus dysgalactiae and Nocardiopsis sp. with L-DOPA or dopamine as a substrate. Scanning electron microscope (SEM) observation of V. harveyi treated by AjPOs and dopamine showed that AjPO1-derived compounds resulted in massive bacteriolysis, AjPO2-derived compounds caused no obvious alteration on bacterial morphology, and AjPO3-derived compounds increased the ratio of spheroidal bacteria. All these results suggested that AjPO reaction products derived by L-DOPA and dopamine had different but limited antibacterial spectrum, and the different antibacterial effects observed among three AjPOs resulted from the different reaction products generated by AjPOs with the same substrate.

Infectious microbial diseases and host defense responses in Sydney rock oysters

Aquaculture has long been seen as a sustainable solution to some of the world's growing food shortages. However, experience over the past 50 years indicates that infectious diseases caused by viruses, bacteria, and eukaryotes limit the productivity of aquaculture. In extreme cases, these types of infectious agents threaten the viability of entire aquaculture industries. This article describes the threats from infectious diseases in aquaculture and then focuses on one example (QX disease in Sydney rock oysters) as a case study. QX appears to be typical of many emerging diseases in aquaculture, particularly because environmental factors seem to play a crucial role in disease outbreaks. Evidence is presented that modulation of a generic subcellular stress response pathway in oysters is responsible for both resistance and susceptibility to infectious microbes. Understanding and being able to manipulate this pathway may be the key to sustainable aquaculture.

Phenoloxidase from the sea cucumber Apostichopus japonicus: cDNA cloning, expression and substrate specificity analysis

Phenoloxidase (PO) is a crucial component of the immune system of echinoderms. In the present study, the full-length cDNA of PO (AjPO) was cloned from coelomocytes of the sea cucumber Apostichopus japonicus using 3'- and 5'-rapid amplification of cDNA ends (RACE) PCR method, which is 2508 bp, with an open reading frame (ORF) of 2040 bp encoding 679 amino acids. AjPO contains a transmembrane domain, and three Cu-oxidase domains with copper binding centers formed by 10 histidines, one cysteine and one methionine respectively. Phylogenetic analysis revealed that AjPO was clustered with laccase-type POs of invertebrates. Using the isolated membrane proteins as crude AjPO, the enzyme could catalyze the substrates catechol, L-3,4-dihydroxyphenylalanine (L-DOPA), dopamine and hydroquinone, but failed to oxidize tyrosine. The results described above collectively proved that AjPO was a membrane-binding laccase-type PO. The quantitative real-time PCR (qRT-PCR) analysis revealed that AjPO mRNA was expressed in muscle, body wall, coelomocytes, tube feet, respiratory tree and intestine with the highest expression level in coelomocytes. AjPO could be significantly induced by lipopolysaccharide (LPS), peptidoglycan (PGN), Zymosan A and polyinosinic-polycytidylic acid (PolyI:C), suggesting AjPO is closely involved in the defense against the infection of bacteria, fungi and double-stranded RNA viruses.

Laccase-like activity in the hemolymph of Venerupis philippinarum: characterization and kinetic properties

The phenoloxidases (POs) include tyrosinases (EC 1.14.18.1), catecholases (EC 1.10.3.1) and laccases (EC 1.10.3.2) and are known to play a role in the immune defences of many invertebrates. For the Manila clam, Venerupis philippinarum, the exact role is not known, especially with regard to defences against Brown Ring Disease (BRD), which leads to high mortalities along European coasts. In order to understand the role and functioning of PO in V. philippinarum, the first step, and aim of this study, was to biochemically characterize the PO activity in the circulating hemolymph. Various substrates were tested and the common PO substrates L-DOPA, Catechol and dopamine exhibited good affinity with the enzyme and consequent low K(m) values (3.75, 1.97, 4.91 mM, respectively). A single tyrosinase-specific substrate, PHPPA, was oxidized, but the affinity for it was low (K(m) = 47.33 mM). Three tested laccase-specific substrates were oxidized by V. philippinarum PO (PPD, OPD and hydroquinone) and affinity was higher than for PHPPA. The results obtained with the substrate were confirmed by the use of different inhibitors: CTAB, a laccase-specific inhibitor inhibited PO activity greatly but not completely, whereas 4-Hr, specific to catecholases and tyrosinases, inhibited PO activity to a lesser extent. The results lead us to conclude that V. philippinarum PO activity in the circulating hemolymph, is mainly a laccase-like activity but there is also a smaller-scale tyrosinase-like activity. The inhibition mechanisms were also determined using dose-response substrate concentration for an inhibitor concentration equal or close to the IC50. Optimal conditions for the enzyme activity were also determined using L-DOPA as substrate, showing that its optimal temperature and pH are around 40 °C and 8.4 respectively. The enzyme is denatured for temperatures above 50 °C.

Shell colour polymorphism, injuries and immune defense in three helicid snail species, Cepaea hortensis, Theba pisana and Cornu aspersum maximum

Shell colour polymorphism is a widespread feature of various land snail species. In our study we aimed at elucidating the question whether there is a correlation between shell colouration and immune defense in three land snail species by comparing phenoloxidase (PO) activity levels of different morphs after immunostimulation via Zymosan A-injection. Since phenoloxidase is involved both in immune defense as well as in melanin production, the PO activity level is particularly interesting when trying to resolve this question. Even though Zymosan A failed to induce PO activity rendering a comparison of inducible PO activity impossible, an interesting difference between pale and dark morphs of all tested species could be observed: dark snails were less affected by hemolymph withdrawal and were able to maintain or regenerate a significantly higher PO activity level after hemolymph withdrawal than pale snails. Possible implications of this observation are discussed.

Hemocyanin-derived phenoloxidase activity: a contributing factor to hyperpigmentation in Nephrops norvegicus

The phenomenon of hyperpigmentation (melanosis) in shellfish has long been attributed to phenoloxidase enzymes. Over the last number of years, the oxygen carrier hemocyanin, has demonstrated several immune- and physiological functionalities, most notably, inducible phenoloxidase activity. In this study, hemocyanin purified from the hemolymph of Nephrops norvegicus displays diphenoloxidase activity in the presence of a number of elicitors and retains structural and functional integrity throughout the process of freeze-thawing (at -25 °C). Conversely, cellular phenoloxidase activity (present in cell-lysates), demonstrates >98% reduction in activity after freeze-thawing. We present evidence that hemocyanin may act as a causative agent of hyperpigmentation in N. norvegicus. The inhibition of hemocyanin-derived phenoloxidase activity is discussed, and for the first time, the biophysical interactions of shellfish hemocyanin with known phenoloxidase inhibitors are presented.

Induction of phenoloxidase and other immunological activities in the humoral fluids of amphioxus Branchiostoma belcheri challenged with Lipopolysaccharide (LPS)

The knowledge concerning the humoral immunity is scarce in amphioxus Branchiostoma belcheri. This study measured the humoral parameters including phenoloxidase (PO) activity, lysozyme activity, antimicrobial activity, microbial agglutinin, and hemagglutinin in amphioxus humoral fluids before and after lipopolysaccharide (LPS) challenge. Humoral fluids from unchallenged Branchiostoma belcheri (B. belcheri) had PO activity, lysozyme, antimicrobial, microbial agglutinating, and hemagglutinating activities, which may represent part of the baseline level of innate immunity in this organism. After challenge with LPS, many humoral parameters were all increased significantly including the PO activity, lysozyme activity, growth-inhibiting activities against Gram-negative bacteria Escherichia coli (E. coli) and Vibrio alginolyticus (V. alginolyticus), growth-inhibiting activities against Gram-positive bacteria Staphylococcus aureus (S. aureus) and Bacillus subtilis (B. subtilis), microbial agglutinating activities against Micrococcus lysodeikticus (M. lysodeikticus), B. subtilis, and S. aureus, and hemagglutinating activities against rabbit and human A and O erythrocytes. In contrast, the agglutinating activities against V. harveyi and E. coli and the hemagglutinating activity against human B erythrocytes in the humoral fluids were reduced in response to LPS challenge. It appears that the humoral fluids of B. belcheri contain components that are able to differentiate different microbes and different human blood cell types.

The phenoloxidase activity and antibacterial function of a tyrosinase from scallop Chlamys farreri

Tyrosinase (TYR), also known as monophenol monooxygenase, is a ubiquitous binuclear copper-containing enzyme which catalyzes the hydroxylation of phenols to catechols and the oxidation of catechols to quinones. In the present study, the cDNA of a tyrosinase (CfTYR) was identified from scallop Chlamys farreri, which encoded a polypeptide of 486 amino acids. The CfTYR mRNA transcripts were expressed in all the tested tissues, including haemocytes, adductor muscle, kidney, hepatopancreas, gill, gonad and mantle, with the highest level in mantle. The expression level of CfTYR mRNA in haemocytes decreased significantly during 3-6 h after LPS stimulation, and reached the lowest level at 6 h (0.05-fold, P < 0.05). Then, it began to increase at 12 h (0.32-fold, P > 0.05), and reached the highest level at 24 h (2.91-fold, P < 0.05). At 3 h after LPS stimulation, the phenoloxidase activity catalyzing L-dopa and dopamine in haemolymph increased significantly to 53.13 and 40.36 U mg(-1) respectively, but it decreased to 10.82 U mg(-1) and even undetectable level after CfTYR activity was inhibited. Furthermore, the antibacterial activity of haemolymph against Escherichia coli was also increased significantly at 3 h after LPS stimulation, but it decreased significantly when the haemolymph was treated by TYR inhibitor. The recombinant protein of the mature CfTYR peptide expressed in the in vitro Glycoprotein Expression Kit displayed phenoloxidase activity of 64.36 ± 5.51 U mg(-1) in the present of trypsinase and Cu(2+). These results collectively suggested that CfTYR was a homologue of tyrosinase in scallop C. farreri with the copper-dependence phenoloxidase activity, and it could be induced after immune stimulation and mediate immune response for the elimination of invasive pathogens in scallop.

Morphological characterization and functional immune response of the carpet shell clam (Ruditapes decussatus) haemocytes after bacterial stimulation

The morphology and functionality of Ruditapes decussatus haemocytes have been characterized by light microscopy and flow cytometry, leading to the identification of three different cellular subpopulations. Granulocytes were the largest cells, the hyalinocytes were smaller and contained fewer granules and the intermediate cells showed a size similar to hyalinocytes and a higher number of granules. The phagocytosis of different particles and the associated production of oxygen radicals were measured by flow cytometric methods. Granulocytes were the most active cells, followed by the intermediate cells and hyalinocytes. The effect of stimulation of haemocytes with lipopolysaccharide (LPS), with a heat inactivated bacterial mixture or with the infection of Vibrio splendidus on the cell viability and the expression of selected immune-related genes were studied. While significant low levels of damaged cells were registered in LPS-stimulated cells, the treatment with dead bacteria or V. splendidus reduced cell viability 1 h, 3 h and 6 h after treatment. The stimulation of haemocytes with LPS and dead bacteria induced changes in the expression of defender against cell death (DAD-1), thrombin, prosaposin, inhibitor of apoptosis (IAP), factor B and C3 complement component.

Suppression substractive hybridisation (SSH) and real time PCR reveal differential gene expression in the Pacific cupped oyster, Crassostrea gigas, challenged with Ostreid herpesvirus 1

Virus-induced genes were identified using suppression subtractive hybridisation (SSH) from Pacific cupped oyster, Crassostrea gigas, haemocytes challenged by OsHV-1. A total of 304 clones from SSH forward library were sequenced. Among these sequences, some homologues corresponded to (i) immune related genes (macrophage express protein, IK cytokine, interferon-induced protein 44 or multicopper oxidase), (ii) apoptosis related genes (Bcl-2) and (iii) cell signalling and virus receptor genes (glypican). Molecular characterization and phylogenic analysis of 3 immune-related genes (macrophage expressed protein, multicopper oxidase and immunoglobulin domain cell adhesion molecule) were performed. Finally, quantitative PCR revealed significant changes in the expression of immune related genes (multicopper oxidase, macrophage expressed protein, myeloid differentiation factor 88 and interferon-induced protein 44) in oysters experimentally challenged with OsHV-1. These findings provide a first basis for studying the role of innate immunity in response to viruses in bivalves and identified genes may serve as markers of interest in breeding programs in order to obtain selected oysters presenting OsHV-1 resistance.

Induction of phenoloxidases in the humoral fluids of amphioxus Branchiostoma belcheri by Vibrio alginolyticus and Escherichia coli

The humoral immune responses of amphioxus Branchiostoma belcheri to microbial challenge remain open to date. Here we examined the changes in PO activity in the humoral fluids in amphioxus before and after challenge with Escherichia coli and Vibrio alginolyticus. It was found that PO activity in the humoral fluids is markedly increased by challenge with E. coli and V. alginolyticus; and the microbial challenge results in a significant rise in subunit 2 of the three PO subunits, making PO subunit 2 a marker enzyme responsive to microbial challenge. This is the first report on microbial induction of the immune-related molecules like PO in B. belcheri.