Immune responses of phenoloxidase and superoxide dismutase in the manila clam Venerupis philippinarum challenged with Vibrio tapetis--Part I: Spatio-temporal evolution of enzymes' activities post-infection

Probiotics from kefir: Evaluating their immunostimulant and antioxidant potential in the carpet shell clam (Ruditapesdecussatus)

This study aimed to investigate the impact of incorporating kefir into the diet on biometric parameters, as well as the immune and antioxidant responses of the carpet shell clam (Ruditapes decussatus) after an experimental infection by Vibrio alginolyticus. Clams were divided into a control group and a treated group. The control group was fed on spirulina (Arthrospira platensis) alone. While, the treated group was fed on spirulina supplemented with 10% dried kefir. After 21 days, clams were immersed in a suspension of V. alginolyticus 5 × 105 CFU mL -1 for 30 min. Seven days after experimental infection, survival was 100% in both groups. The obtained results showed a slight increase in weight and condition index in clams fed with kefir-supplemented diet for 21 days compared to control clams. Regarding antioxidant responses, the treated group showed higher superoxide dismutase activity compared to the control group. However, the malondialdehyde level was lower in the treated clams than in the control. In terms of immune parameters, the treated group showed slightly elevated activities of phenoloxidase, lysozyme and alkaline phosphatase, whereas a decreased lectin activity was observed compared to the control group. The obtained results suggest that kefir enhanced both the antioxidant and immune response of infected clams.

Implication of the Type IV Secretion System in the Pathogenicity of Vibrio tapetis, the Etiological Agent of Brown Ring Disease Affecting the Manila Clam Ruditapes philippinarum

Vibrio tapetis is a Gram-negative bacterium that causes infections of mollusk bivalves and fish. The Brown Ring Disease (BRD) is an infection caused by V. tapetis that primarily affects the Manila clam Ruditapes philippinarum. Recent studies have shown that a type IV secretion system (T4SS) gene cluster is exclusively found in strains of V. tapetis pathogenic to clams. However, whether the T4SS is implicated or not during the infection process remains unknown. The aim of this study was to create and characterize a V. tapetis T4SS null mutant, obtained by a near-complete deletion of the virB4 gene, in order to determine the role of T4SS in the development of BRD. This study demonstrated that the T4SS is neither responsible for the loss of hemocyte adhesion capacities, nor for the decrease of the lysosomal activity during BRD. Nevertheless, we observed a 50% decrease of the BRD prevalence and a decrease of mortality dynamics with the ΔvirB4 mutant. This work demonstrates that the T4SS of V. tapetis plays an important role in the development of BRD in the Manila clam.

External pH modulation during the growth of Vibrio tapetis, the aetiological agent of brown ring disease

AIMS

Brown ring disease (BRD) is an infection of the Manila clam Ruditapes philippinarum due to the pathogen Vibrio tapetis. During BRD, clams are facing immunodepression and shell biomineralization alteration. In this paper, we studied the role of pH on the growth of the pathogen and formulated hypothesis on the establishment of BRD by V. tapetis.

METHODS AND RESULTS

In this study, we monitored the evolution of pH during the growth of V. tapetis in a range of pH and temperatures. We also measured the pH of Manila clam haemolymph and extrapallial fluids (EPFs) during infection by V. tapetis. We highlighted that V. tapetis modulates the external pH during its growth, to a value of 7·70. During the development of BRD, V. tapetis also influences EPFs and haemolymph pH in vitro in the first hours of exposure and in vivo after 3 days of infection.

CONCLUSIONS

Our experiments have shown a close interaction between V. tapetis CECT4600, a pathogen of Manila clam that induces BRD, and the pH of different compartments of the animals during infection. These results indicate that the bacterium, through a direct mechanism or as a consequence of physiological changes encountered in the animal during infection, is able to interfere with the pH of Manila clam fluids. This pH modification might promote the infection process or at least create an imbalance within the animal that would favour its persistence. This last hypothesis should be tested in future experiment.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study is the first observation of pH modifications in the context of BRD and might orient future research on the fine mechanisms of pH modulation associated with BRD.

Differences in proteomic profile between two haemocyte types, granulocytes and hyalinocytes, of the flat oyster Ostrea edulis

Haemocytes play a dominant role in shellfish immunity, being considered the main defence effector cells in molluscs. These cells are known to be responsible for many functions, including chemotaxis, cellular recognition, attachment, aggregation, shell repair and nutrient transport and digestion. There are two basic cell types of bivalve haemocytes morphologically distinguishable, hyalinocytes and granulocytes; however, functional differences and specific abilities are poorly understood: granulocytes are believed to be more efficient in killing microorganisms, while hyalinocytes are thought to be more specialised in clotting and wound healing. A proteomic approach was implemented to find qualitative differences in the protein profile between granulocytes and hyalinocytes of the European flat oyster, Ostrea edulis, as a way to evaluate functional differences. Oyster haemolymph cells were differentially separated by Percoll® density gradient centrifugation. Granulocyte and hyalinocyte proteins were separated by 2D-PAGE and their protein profiles were analysed and compared with PD Quest software; the protein spots exclusive for each haemocyte type were excised from gels and analysed by MALDI-TOF/TOF with a combination of mass spectrometry (MS) and MS/MS for sequencing and protein identification. A total of 34 proteins were identified, 20 unique to granulocytes and 14 to hyalinocytes. The results suggested differences between the haemocyte types in signal transduction, apoptosis, oxidation reduction processes, cytoskeleton, phagocytosis and pathogen recognition. These results contribute to identify differential roles of each haemocyte type and to better understand the oyster immunity mechanisms, which should help to fight oyster diseases.

Comparative immune response during the juvenile and adult stages of two abalones under Vibrio harveyi challenge

Mass mortality of juvenile hybrid (Haliotis discus hannai ♀× H. fulgens ♂, DF) and adult H. discus hannai (DD) occurs in south China during the summer. This study showed that the juvenile DF and adult DD exhibited significantly lower survival rates than juvenile DD and adult DF under 72 h pathogenic bacteria (Vibrio harveyi) challenge at different temperatures (20 °C and 28 °C). Phenoloxidase (PO) and superoxide dismutase (SOD) activities were significantly higher in juvenile DD compared to juvenile DF, whereas that in adult abalone was the opposite. Juvenile DD and adult DF also exhibited advantages in terms of immune-related gene expression (TRAF, TLR, MIF, Lys, Spi, Cat, TNF, and SOD) compared to juvenile DF and adult DD. The data reveals immunocompetence differences in DD and DF at the juvenile and adult stages.

The effects of glyphosate and AMPA on the mediterranean mussel Mytilus galloprovincialis and its microbiota

Glyphosate, the most widely used herbicide worldwide, targets the 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) enzyme in the shikimate pathway found in plants and some microorganisms. While the potential for glyphosate to induce a broad range of biological effects in exposed organisms has been demonstrated, the global molecular mechanisms of toxicity and potential effects in bacterial symbionts remain unclear, in particular for ecologically important marine species such as bivalve molluscs. Here, the effects of glyphosate (GLY), its degradation product aminomethylphosphonic acid (AMPA), and a mixture of both (MIX) on the mussel M. galloprovincialis were assessed in a controlled experiment. For the first time, next generation sequencing (RNA-seq and 16S rRNA amplicon sequencing) was used to evaluate such effects at the molecular level in both the host and its respective microbiota. The results suggest that the variable capacity of bacterial species to proliferate in the presence of these compounds and the impairment of host physiological homeostasis due to AMPA and GLY toxicity may cause significant perturbations to the digestive gland microbiota, as well as elicit the spread of potential opportunistic pathogens such as Vibrio spp.. The consequent host-immune system activation identified at the molecular and cellular level could be aimed at controlling changes occurring in the composition of symbiotic microbial communities. Overall, our data raise further concerns about the potential adverse effects of glyphosate and AMPA in marine species, suggesting that both the effects of direct toxicity and the ensuing changes occurring in the host-microbial community must be taken into consideration to determine the overall ecotoxicological hazard of these compounds.

Transcriptomic analysis of clam extrapallial fluids reveals immunity and cytoskeleton alterations in the first week of Brown Ring Disease development

The Brown Ring Disease is an infection caused by the bacterium Vibrio tapetis on the Manila clam Ruditapes philippinarum. The process of infection, in the extrapallial fluids (EPFs) of clams, involves alteration of immune functions, in particular on hemocytes which are the cells responsible of phagocytosis. Disorganization of the actin-cytoskeleton in infected clams is a part of what leads to this alteration. This study is the first transcriptomic approach based on collection of extrapallial fluids on living animals experimentally infected by V. tapetis. We performed differential gene expression analysis of EPFs in two experimental treatments (healthy-against infected-clams by V. tapetis), and showed the deregulation of 135 genes. In infected clams, a downregulation of transcripts implied in immune functions (lysosomal activity and complement- and lectin-dependent PRR pathways) was observed during infection. We also showed a deregulation of transcripts encoding proteins involved in the actin cytoskeleton organization such as an overexpression of β12-Thymosin (which is an actin sequestration protein) or a downregulation of proteins that closely interact with capping proteins such as Coactosin, that counteract action of capping proteins, or Profilin. We validated these transcriptomic results by cellular physiological analyses that showed a decrease of the lysosome amounts and the disorganization of actin cytoskeleton in infected hemocytes.

MITF Regulates Downstream Genes in Response to Vibrio parahaemolyticus Infection in the Clam Meretrix Petechialis

The microphthalmia-associated transcription factor (MITF) is a basic helix-loop-helix-leucine zipper protein that plays a key role in cell proliferation, survival and immune defense through the direct transcriptional control of downstream genes. We have found that MITF participates in the immune response to Vibrio parahaemolyticus infection in the clam Meretrix petechialis. In this study, we focused on how MITF functions in immunity. First, PO, CTSK, and BCL-2 were identified as the target genes of MpMITF in the clam by RNAi. EMSAs showed direct binding between the MpMITF protein and the E-box of the MpPO, MpCTSK, and MpBCL-2 promoters. Yeast one-hybrid assays also suggested that MpMITF could activate the expression of these three downstream genes. These results demonstrated that the transcriptional expression of MpPO, MpCTSK, and MpBCL-2 is directly regulated by MpMITF. Second, we analyzed the roles of MpPO, MpCTSK, and MpBCL-2 in clam immunity. The mRNA expression of MpPO, MpCTSK, and MpBCL-2 increased significantly after V. parahaemolyticus challenge, which implied that these genes might take part in the immune defense against V. parahaemolyticus challenge in clams. The purified recombinant proteins, MpPO and MpCTSK, inhibited the growth of V. parahaemolyticus. Additionally, the apoptosis rate of clam haemocytes rose significantly when the activity of MpBCL-2 was suppressed. These results revealed that MpPO, MpCTSK, and MpBCL-2 are involved in the immune defense against V. parahaemolyticus. This study supports the idea that the MpMITF pathway plays a key role in immune defense through the direct regulation of the downstream genes MpPO, MpCTSK, and MpBCL-2 in the clam, M. petechialis.

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.

Exposure to the toxic dinoflagellate Alexandrium catenella modulates juvenile oyster Crassostrea gigas hemocyte variables subjected to different biotic conditions

The Pacific oyster Crassostrea gigas is an important commercial species cultured throughout the world. Oyster production practices often include transfers of animals into new environments that can be stressful, especially at young ages. This study was undertaken to determine if a toxic Alexandrium bloom, occurring repeatedly in French oyster beds, could modulate juvenile oyster cellular immune responses (i.e. hemocyte variables). We simulated planting on commercial beds by conducting a cohabitation exposure of juvenile, "specific pathogen-free" (SPF) oysters (naïve from the environment) with previously field-exposed oysters to induce interactions with new microorganisms. Indeed, toxic Alexandrium spp. exposures have been reported to modulate bivalve interaction with specific pathogens, as well as physiological and immunological variables in bivalves. In summary, SPF oysters were subjected to an artificial bloom of Alexandrium catenella, simultaneously with a cohabitation challenge. Exposure to A. catenella, and thus to the paralytic shellfish toxins (PSTs) and extracellular bioactive compounds produced by this alga, induced higher concentration, size, complexity and reactive oxygen species (ROS) production of circulating hemocytes. Challenge by cohabitation with field-exposed oysters also activated these hemocyte responses, suggesting a defense response to new microorganism exposure. These hemocyte responses to cohabitation challenge, however, were partially inhibited by A. catenella exposure, which enhanced hemocyte mortality, suggesting either detrimental effects of the interaction of both stressors on immune capacity, or the implementation of an alternative immune strategy through apoptosis. Indeed, no infection with specific pathogens (herpesvirus OsHV-1 or Vibrio aesturianus) was detected. Additionally, lower PST accumulation in challenged oysters suggests a physiological impairment through alteration of feeding-related processes. Overall, results of this study show that a short-term exposure to A. catenella combined with an exposure to a modified microbial community inhibited some hemocyte responses, and likely compromised physiological condition of the juvenile oysters.

Dietary alpha-linolenic acid/linoleic acid ratios modulate intestinal immunity, tight junctions, anti-oxidant status and mRNA levels of NF-κB p65, MLCK and Nrf2 in juvenile grass carp (Ctenopharyngodon idella)

This study was conducted to investigate the effects of dietary alpha-linolenic acid/linoleic acid (ALA/LNA) ratios on the immune response, tight junctions, antioxidant status and immune-related signaling molecules mRNA levels in the intestine of juvenile grass carp (Ctenopharyngodon idella). A total of 1260 juvenile grass carp with an average initial weight of 8.78 ± 0.03 g were fed diets with different ALA/LNA ratios (0.01, 0.34, 0.68, 1.03, 1.41, 1.76 and 2.15) for 60 days. Results indicated that ALA/LNA ratio of 1.03 significantly increased acid phosphatase, lysozyme activities and complement C3 contents, promoted interleukin 10, transforming growth factor β1 and κB inhibitor α mRNA abundance, whereas suppressed pro-inflammatory cytokines (interleukin 1β, interleukin 8, tumor necrosis factor α and interferon γ2) and signal molecules (IκB kinase β, IκB kinase γ and nuclear factor κB p65) mRNA levels in the intestine (P < 0.05), suggesting that optimal dietary ALA/LNA ratio improved intestinal immune response of juvenile fish. Additionally, ALA/LNA ratio of 1.03 significantly promoted Claudin-3, Claudin-b, Claudin-c, Occludin and ZO-1 gene transcription, whereas reduced Claudin-15a and myosin light-chain kinase mRNA levels in the intestine, suggesting that appropriate dietary ALA/LNA ratio strengthened tight junctions in the intestine of juvenile fish. Meanwhile, ALA/LNA ratio of 1.03 noticeably elevated glutathione contents, copper/zinc superoxide dismutase, glutathione peroxidase, glutathione S-transferase and glutathione reductase activities and mRNA levels, as well as signaling molecule nuclear factor erythoid 2-related factor 2 gene transcriptional abundance in the intestine, suggesting that proper ratio of dietary ALA/LNA ameliorate the intestinal antioxidant status of juvenile fish. Based on the quadratic regression analysis of the complement C3 content in the distal intestine and malondialdehyde content in the whole intestine, optimal ALA/LNA ratio for maximum growth of juvenile grass carp (8.78-72.00 g) were estimated to be 1.13 and 1.12, respectively.

Effect of nutritive status on Mytilus galloprovincialis pollution biomarkers: Implications for large-scale monitoring programs

Biomarkers have been extensively used in monitoring programs with the aim of assessing the biological effects of pollutants on marine organisms and determining environmental status. Data obtained from these programs are sometimes difficult to interpret due to the large amount of natural variables affecting biological processes, which could act as confounding factors on biomarker responses. The main aim of this work was to identify the effect of one of these variables, the food availability, and consequently, the mussel nutritive status, on biomarker responses. For that purpose, mussels (Mytilus galloprovincialis) were conditioned to three different food rations for 2 months in order to create three mussel nutritive statuses and afterwards, each status was exposed to three nominal concentrations of fluoranthene (FLU) for 3 weeks. A battery of biomarkers was considered in this study to cover a wide range of organism responses, both physiological (scope for growth - SFG) and biochemical (superoxide dismutase - SOD, catalase - CAT, glutathione reductase - GR, glutathione peroxidase - GPx, glutathione-S-transferase - GST and phenoloxidase - PO activities, and lipid membrane peroxidation - LPO). The results obtained, evidenced that most of the studied biomarkers (SFG, SOD, CAT, GPx, and PO) were strongly affected by mussel nutritive status, showing higher values at lower status, whereas the effect of toxicant was not always evident, masked by the nutritive status effect. This paper demonstrates that toxicants are not the only source of variability modulating pollution biomarkers, and confirms nutritive status as a major factor altering biochemical and physiological biomarkers.

DNA methylation is associated with expression level changes of galectin gene in mantle wound healing process of pearl oyster, Pinctada fucata

Galectin is one important member of pattern recognition proteins that plays a pivotal role in regulating innate immune response of invertebrates. In this study, we cloned the promoter sequence of a tandem-repeat galectin from the pearl oyster Pinctada fucata (P. fucata). The quantitative real-time PCR analysis revealed that galectin mRNA expression in mantle tissues were firstly up-regulated from time points of 2 h-24 h, and then down-regulated from time points of 24 h-168 h after mantle injury. The genome methylation level of mantle tissue was inversely related to galectin mRNA expression (Pearson correlation: -0.554, P: 0.154). The "CpG4-6" methylation level in promoter region of galectin was significant positive correlated with the mRNA expression (Pearson correlation: 0.313, P: 0.049). The results indicated that galectin gene may be involved in immune response in mantle wound healing process of P. fucata, and DNA methylation may be a regulation factor of gene expression.

Immune responses of phenoloxidase and superoxide dismutase in the manila clam Venerupis philippinarum challenged with Vibrio tapetis--part II: combined effect of temperature and two V. tapetis strains

Manila clams, Venerupis philippinarum (Adams and Reeve, 1850), were experimentally infected with two different bacterial strains and challenged with two different temperatures. Bacterial strains used in this study were Vibrio tapetis strain CECT4600(T), the causative agent of Brown Ring Disease (BRD) and V. tapetis strain LP2, supposed less virulent to V. philippinarum. V. tapetis is considered to proliferate at low temperatures, i.e. under 21 °C. In a global warming context we could hypothesize a decrease of mass mortalities caused by V. tapetis but these thermal changes could also directly impact the immune system of the host V. philippinarum. Thus, the aim of this study was to investigate the effects of the extrapallial injection with V. tapetis combined with temperature challenge on two enzymes activities in V. philippinarum. More precisely, after infection, phenoloxidase (PO) and superoxide dismutase (SOD), two major enzymes involved in immune response, were studied for 30 days in two compartments: the mantle and the hemolymph. Conchyolin Deposit Stages (CDS) and Shell Repair Stages (SRS) were also determined 30 days post-injection as a proxy of the virulence of the tested strains. In this study, we highlighted that host-pathogen interaction in a varying environment affects the enzymatic response of the host. The coupled effect of V. tapetis injection and temperature challenge was detected 30 days post injection and resulted in virulence differences. These findings were supported by CDS and SRS determination in clams and lead to the conclusion that clam's immunity could be enhanced at 22 °C while V. tapetis virulence is lowered at this temperature. Another result of our study was the increase of PO and SOD basal activities as clams are exposed to warmer temperature.

Development of a Taqman real-time PCR assay for rapid detection and quantification of Vibrio tapetis in extrapallial fluids of clams

The Gram-negative bacterium Vibrio tapetis is known as the causative agent of Brown Ring Disease (BRD) in the Manila clam Venerupis (=Ruditapes) philippinarum. This bivalve is the second most important species produced in aquaculture and has a high commercial value. In spite of the development of several molecular methods, no survey has been yet achieved to rapidly quantify the bacterium in the clam. In this study, we developed a Taqman real-time PCR assay targeting virB4 gene for accurate and quantitative identification of V. tapetis strains pathogenic to clams. Sensitivity and reproducibility of the method were assessed using either filtered sea water or extrapallial fluids of clam injected with the CECT4600(T) V. tapetis strain. Quantification curves of V. tapetis strain seeded in filtered seawater (FSW) or extrapallial fluids (EF) samples were equivalent showing reliable qPCR efficacies. With this protocol, we were able to specifically detect V. tapetis strains down to 1.125 10(1) bacteria per mL of EF or FSW, taking into account the dilution factor used for appropriate template DNA preparation. This qPCR assay allowed us to monitor V. tapetis load both experimentally or naturally infected Manila clams. This technique will be particularly useful for monitoring the kinetics of massive infections by V. tapetis and for designing appropriate control measures for aquaculture purposes.