Specific expression of antimicrobial peptide and HSP70 genes in response to heat-shock and several bacterial challenges in mussels

Investigation of the highly endangered Pinna nobilis' mass mortalities: Seasonal and temperature patterns of health status, antioxidant and heat stress responses

Recently, P. nobilis populations have suffered a tremendous reduction, with pathogens potentially playing a crucial role. Considering its highly endangered status, mechanisms leading to mass mortalities were examined in one or multiple pathogens infected populations. Thus, seasonal antioxidant enzymatic activities, hsp70 and catalase mRNA levels, were investigated in two different Greek populations, during mass mortality events in summer of 2020. Samples were collected from Fthiotis and Lesvos during February (T^oC 14 ± 1.2 and 15 ± 1 respectively), April (T^oC 18 ± 1.2 and 17 ± 1.3 respectively), and June (T^oC 24.5 ± 1.5 and 21.5 ± 1.5 respectively) 2020. In July of the same year (T^oC 26.5 ± 1.7 in Fthiotis and 24.5 ± 1.7 in Lesvos), no live specimens were found. All biochemical parameters and phylogenetic analysis suggest that pathogen infection increases P. nobilis sensitivity to water temperature, subsequently leading to mass mortality. The latter was obvious in Fthiotis individuals, in which Haplosporidium pinnae was also observed with Mycobacterium spp., compared to Lesvos individuals.

Molecular Mechanisms Underlying Vibrio Tolerance in Ruditapes philippinarum Revealed by Comparative Transcriptome Profiling

The clam Ruditapes philippinarum is an important species in the marine aquaculture industry in China. However, in recent years, the aquaculture of R. philippinarum has been negatively impacted by various bacterial pathogens. In this study, the transcriptome libraries of R. philippinarum showing different levels of resistance to challenge with Vibrio anguillarum were constructed and RNA-seq was performed using the Illumina sequencing platform. Host immune factors were identified that responded to V. anguillarum infection, including C-type lectin domain, glutathione S-transferase 9, lysozyme, methyltransferase FkbM domain, heat shock 70 kDa protein, Ras-like GTP-binding protein RHO, C1q, F-box and BTB/POZ domain protein zf-C2H2. Ten genes were selected and verified by RT-qPCR, and nine of the gene expression results were consistent with those of RNA-seq. The lectin gene in the phagosome pathway was expressed at a significantly higher level after V. anguillarum infection, which might indicate the role of lectin in the immune response to V. anguillarum. Comparing the results from R. philippinarum resistant and nonresistant to V. anguillarum increases our understanding of the resistant genes and key pathways related to Vibrio challenge in this species. The results obtained here provide a reference for future immunological research focusing on the response of R. philippinarum to V. anguillarum infection.

Does the environmental history of mussels have an effect on the physiological response to additional stress under experimental conditions?

Expected effects on marine biota of the ongoing elevation of water temperature and high latitudes is of major concern when considering the reliability of coastal ecosystem production. To compare the capacity of coastal organisms to cope with a temperature increase depending on their environmental history, responses of adult blue mussels (Mytilus spp.) taken from two sites differentially exposed to chemical pollution were investigated during an experimental exposure to a thermal stress. Immune parameters were notably altered by extreme warming and transcriptional changes for a broad selection of genes were associated to the temperature increase following a two-step response pattern. Site-specific responses suggested an influence of environmental history and support the possibility of a genetic basis in the physiological response. However no meaningful difference was detected between the response of hybrids and M galloprovincialis. This study brings new information about the capacity of mussels to cope with the ongoing elevation of water temperature in these coastal ecosystems.

Gene Expression Profiles in Two Razor Clam Populations: Discerning Drivers of Population Status

With rapidly changing marine ecosystems, shifts in abundance and distribution are being documented for a variety of intertidal species. We examined two adjacent populations of Pacific razor clams (Siliqua patula) in lower Cook Inlet, Alaska. One population (east) supported a sport and personal use fishery, but this has been closed since 2015 due to declines in abundance, and the second population (west) continues to support commercial and sport fisheries. We used gene expression to investigate potential causes of the east side decline, comparing razor clam physiological responses between east and west Cook Inlet. The target gene profile used was developed for razor clam populations in Alaska based on physiological responses to environmental stressors. In this study, we identified no differences of gene expression between east and west populations, leading to two potential conclusions: (1) differences in factors capable of influencing physiology exist between the east and west and are sufficient to influence razor clam populations but are not detected by the genes in our panel, or (2) physiological processes do not account for the differences in abundance, and other factors such as predation or changes in habitat may be impacting the east Cook Inlet population.

NMR Metabolite Profiles of the Bivalve Mollusc Mytilus galloprovincialis Before and After Immune Stimulation With Vibrio splendidus

The hemolymph metabolome of Mytilus galloprovincialis injected with live Vibrio splendidus bacteria was analyzed by ¹H-NMR spectrometry. Changes in spectral hemolymph profiles were already detected after mussel acclimation (3 days at 18 or 25 °C). A significant decrease of succinic acid was accompanied by an increase of most free amino acids, mytilitol, and, to a smaller degree, osmolytes. These metabolic changes are consistent with effective osmoregulation, and the restart of aerobic respiration after the functional anaerobiosis occurred during transport. The injection of Vibrio splendidus in mussels acclimated at 18°C caused a significant decrease of several amino acids, sugars, and unassigned chemical species, more pronounced at 24 than at 12 h postinjection. Correlation heatmaps indicated dynamic metabolic adjustments and the relevance of protein turnover in maintaining the homeostasis during the response to stressful stimuli. This study confirms NMR-based metabolomics as a feasible analytical approach complementary to other omics techniques in the investigation of the functional mussel responses to environmental challenges.

Transcriptomic Response of Mussel Gills After a Vibrio splendidus Infection Demonstrates Their Role in the Immune Response

Mussels (Mytilus galloprovincialis) are filter feeder bivalves that are constantly in contact with a wide range of microorganisms, some of which are potentially pathogenic. How mussels recognize and respond to pathogens has not been fully elucidated to date; therefore, we investigated the immune mechanisms that these animals employ in response to a bacterial bath infection from the surrounding water, mimicking the response that mussels mount under natural conditions. After the bath infection, mussels were able to remove the bacteria from their bodies and from the water tank. Accordingly, antibacterial activity was detected in gill extracts, demonstrating that this tissue plays a central role in removing and clearing potential pathogens. A transcriptomic study performed after a bath infection with Vibrio splendidus identified a total of 1,156 differentially expressed genes. The expression levels of genes contributing to a number of biological processes, such as immune response activation pathways and their regulation with cytokines, cell recognition, adhesion and apoptosis, were significantly modulated after infection, suggesting that the gills play important roles in pathogen recognition, as well as being activators and regulators of the mussel innate immune response. In addition to RNA-seq analysis, long non-coding RNAs and their neighboring genes were also analyzed and exhibited modulation after the bacterial challenge. The response of gills against bath infection was compared with the findings of a previous transcriptomic study on hemocytes responding to systemic infection, demonstrating the different and specific functions of gills. The results of this study indicate that recognition processes occur in the gill, thereby activating the effector agents of the immune response to overcome bacterial infection.

Immune priming: the secret weapon of the insect world

Insects are a highly successful group of animals that inhabit almost every habitat and environment on Earth. Part of their success is due to a rapid and highly effective immune response that identifies, inactivates, and eliminates pathogens. Insects possess an immune system that shows many similarities to the innate immune system of vertebrates, but they do not possess an equivalent system to the antibody-mediated adaptive immune response of vertebrates. However, some insect do display a process known as immune priming in which prior exposure to a sublethal dose of a pathogen, or pathogen-derived material, leads to an elevation in the immune response rendering the insect resistant to a subsequent lethal infection a short time later. This process is mediated by an increase in the density of circulating hemocytes and increased production of antimicrobial peptides. Immune priming is an important survival strategy for certain insects while other insects that do not show this response may have colony-level behaviors that may serve to limit the success of pathogens. Insects are now widely used as in vivo models for studying microbial pathogens of humans and for assessing the in vivo efficacy of antimicrobial agents. Knowledge of the process of immune priming in insects is essential in these applications as it may operate and augment the perceived in vivo antimicrobial activity of novel compounds.Abbreviations: 1,3-dibenzyl-4,5-diphenyl-imidazol-2-ylidene silver(I) acetate; SBC3: antimicrobial peptides; AMPs: dorsal-related immunity factor; DIF: Down syndrome cell adhesion molecule; Dscam: Lipopolysaccharide; LPS: Pathogen-associated molecular patterns; PAMPS: Patterns recognition receptors; PRR: Prophenoloxidase; PO: Toll-like receptors; TLRs: Toll/IL-1R; TIR, Transgenerational Immune Priming; TgIP: Tumor necrosis factor-α; TNF-α.

Hemocytes released in seawater act as Trojan horses for spreading of bacterial infections in mussels

Global warming has been associated with increased episodes of mass mortality events in invertebrates, most notably in bivalves. Although the spread of pathogens is one of multiple factors that contribute to such mass mortality events, we don’t fully understand the pathophysiological consequences of sea warming on invertebrates. In this work, we show that in temperature stress conditions, circulating hemocytes in mussels leave the hemolymph to gain access to the intervalvar fluid before being released in seawater. External hemocytes can survive for several hours in seawater before entering other mussels. When infected by bacteria, externally-infected hemocytes can enter naive mussels and promote bacterial dissemination in the host. These results reveal the existence of a new opportunistic mechanism used by pathogens to disseminate in marine ecosystems. Such mechanisms may explain how thermal anomalies triggered by global warming can favor episodic mass mortality observed in recent years in marine ecosystem.

In-Depth In Silico Search for Cuttlefish (Sepia officinalis) Antimicrobial Peptides Following Bacterial Challenge of Haemocytes

Cuttlefish (Sepia officinalis) haemocytes are potential sources of antimicrobial peptides (AMPs). To study the immune response to Vibrio splendidus and identify new AMPs, an original approach was developed based on a differential transcriptomic study and an in-depth in silico analysis using multiple tools. Two de novo transcriptomes were retrieved from cuttlefish haemocytes following challenge by V. splendidus or not. A first analysis of the annotated transcripts revealed the presence of Toll/NF-κB pathway members, including newly identified factors such as So-TLR-h, So-IKK-h and So-Rel/NF-κB-h. Out of the eight Toll/NF-κB pathway members, seven were found up-regulated following V. splendidus challenge. Besides, immune factors involved in the immune response were also identified and up-regulated. However, no AMP was identified based on annotation or conserved pattern searches. We therefore performed an in-depth in silico analysis of unannotated transcripts based on differential expression and sequence characteristics, using several tools available like PepTraq, a homemade software program. Finally, five AMP candidates were synthesized. Among them, NF19, AV19 and GK28 displayed antibacterial activity against Gram-negative bacteria. Each peptide had a different spectrum of activity, notably against Vibrio species. GK28—the most active peptide—was not haemolytic, whereas NF19 and AV19 were haemolytic at concentrations between 50 and 100 µM, 5 to 10 times higher than their minimum inhibitory concentration.

Monitoring nearshore ecosystem health using Pacific razor clams (Siliqua patula) as an indicator species

An emerging approach to ecosystem monitoring involves the use of physiological biomarker analyses in combination with gene transcription assays. For the first time, we employed these tools to evaluate the Pacific razor clam (Siliqua patula), which is important both economically and ecologically, as a bioindicator species in the northeast Pacific. Our objectives were to (1) develop biomarker and gene transcription assays with which to monitor the health of the Pacific razor clam, (2) acquire baseline biomarker and gene transcription reference ranges for razor clams, (3) assess the relationship between physiological and gene transcription assays and (4) determine if site-level differences were present. Pacific razor clams were collected in July 2015 and 2016 at three sites within each of two national parks in southcentral Alaska. In addition to determining reference ranges, we found differences in biomarker assay and gene transcription results between parks and sites which indicate variation in both large-scale and local environmental conditions. Our intent is to employ these methods to evaluate Pacific razor clams as a bioindicator of nearshore ecosystem health. Links between the results of the biomarker and gene transcription assays were observed that support the applicability of both assays in ecosystem monitoring. However, we recognize the need for controlled studies to examine the range of responses in physiology and gene transcripts to different stressors.

Physiological and gene transcription assays to assess responses of mussels to environmental changes

Coastal regions worldwide face increasing management concerns due to natural and anthropogenic forces that have the potential to significantly degrade nearshore marine resources. The goal of our study was to develop and test a monitoring strategy for nearshore marine ecosystems in remote areas that are not readily accessible for sampling. Mussel species have been used extensively to assess ecosystem vulnerability to multiple, interacting stressors. We sampled bay mussels (Mytilus trossulus) in 2015 and 2016 from six intertidal sites in Lake Clark and Katmai National Parks and Preserves, in south-central Alaska. Reference ranges for physiological assays and gene transcription were determined for use in future assessment efforts. Both techniques identified differences among sites, suggesting influences of both large-scale and local environmental factors and underscoring the value of this combined approach to ecosystem health monitoring.

Ralstonia eutropha, containing high poly-β-hydroxybutyrate levels, regulates the immune response in mussel larvae challenged with Vibrio coralliilyticus

Marine invertebrates rely mainly on innate immune mechanisms that include both humoral and cellular responses. Antimicrobial peptides (AMPs), lysozyme and phenoloxidase activity, are important components of the innate immune defense system in marine invertebrates. They provide an immediate and rapid response to invading microorganisms. The impact of amorphous poly-β-hydroxybutyrate (PHB-A) (1 mg PHB-A L^-1) on gene expression of the AMPs mytimycin, mytilinB, defensin and the hydrolytic enzyme lysozyme in infected blue mussel larvae was investigated during "in vivo" challenge tests with Vibrio coralliilyticus (10⁵ CFU mL^-1). RNAs were isolated from mussel larvae tissue, and AMPs were quantified by q-PCR using the 18srRNA gene as a housekeeping gene. Our data demonstrated that AMPs genes had a tendency to be upregulated in challenged mussel larvae, and the strongest expression was observed from 24 h post-exposure onwards. The presence of both PHB-A and the pathogen stimulated the APMs gene expression, however no significant differences were noticed between treatments or between exposure time to the pathogen V. coralliilyticus. Looking at the phenoloxidase activity in the infected mussels, it was observed that the addition of PHB-A significantly increased the activity.

Evaluation of the immune responses of the brown mussel Perna perna as indicators of fecal pollution

The mussel Perna perna is an intertidal bivalve that is widely distributed, cultivated and consumed in South Africa, Brazil and Venezuela. Among marine resources, bivalve mollusks are one of the most impacted by anthropogenic pollution, as they can accumulate pathogenic bacteria and water pollutants. Hemocytes are molluscan defense cells, and their abundance and functions can be affected in response to contaminants, such as bacterial load. However, no previous study has investigated the immune response of P. perna hemocytes. The aim of this study was to evaluate several immune parameters in P. perna as indicators of fecal pollution in mussel hemolymph and in seawater. We collected mussels and adjacent seawater from beaches with different levels of fecal contamination in Rio de Janeiro state (Brazil): Vermelha Beach (VB); Icaraí Beach (IB); Urca Beach (UB); and Jurujuba Beach (JB). Hemocyte parameters (density, morphology, phagocytic activity and production of Reactive Oxygen Species - ROS) were evaluated using flow cytometry. We quantified Fecal Indicator Bacteria (FIB) in seawater by the multiple tubes technique for each beach and for hemolymph by the spread-plate technique. In agreement with historical evaluation of fecal contamination levels, UB presented the highest FIB abundance in seawater (thermotolerant coliforms, TEC = 1600 NMP 100 mL^-1), whereas VB exhibited the lowest (TEC = 17 NMP 100 mL^-1). UB mussels had six and eight times higher hemocyte density and phagocytic activity, respectively, than mussels from VB. Mussels from VB and IB presented a significantly lower number of total coliforms in hemolymph and a significantly higher relative internal complexity of hemocytes than those from UB and JB (p ≤ 0.01, PERMANOVA). ROS production by hemocytes was significantly lower in mussels from VB compared to those from JB (p = 0.04, ANOVA). Our results indicate a significant relationship between the level of fecal contamination in aquatic environments and the immune response of mussel hemocytes. Immune-related parameters may therefore be useful as indicators of bivalve health and environmental quality. Our flow cytometric analysis of P. perna hemocytes represents a new approach for studying Perna perna biology and might represent a novel tool for measuring organic pollution and water quality.

The immune response of Mytilus edulis hemocytes exposed to Vibrio splendidus LGP32 strain: A transcriptomic attempt at identifying molecular actors

The marine mussel Mytilus edulis, tolerant to a wide range of environmental changes, combines a key role as a sentinel species for environmental monitoring programs and a significant economic importance. Mortality events caused by infective agents and parasites have not been described in mussels, which suggests an efficient immune system. This study aims at identifying the molecular mechanisms involved in the early immune responses M. edulis' hemocytes challenged with Vibrio splendidus LGP32 strain during 2, 4 and 6 h. A total of 149,296 assembled sequences has been annotated and compared to KEGG reference pathways. Several immune related sequences were identified such as Toll-Like receptors (TLRs), transcription factors, cytokines, protease inhibitors, stress proteins and sequences encoding for proteins involved in cell adhesion, phagocytosis, oxidative stress, apoptosis and autophagy. Differential gene expression clustered 10 different groups of transcripts according to kinetics of transcript occurrence. Sequences were assigned to biological process gene ontology categories. Sequences encoding for galectins, fibrinogen-related proteins, TLRs, MyD88, some antimicrobial peptides, lysosomal hydrolases, heat shock proteins and protease inhibitors, as well as proteins of oxidative stress and apoptosis were identified as differently regulated during the exposure to V. splendidus LGP32. The levels of candidate transcripts were quantified in M. edulis' hemocytes exposed to V. splendidus LGP32 and 7SHRW by using branched DNA technology. Transcripts encoding for inhibitor kappa B, inhibitor of apoptosis proteins, tumor protein D54, serine/threonine-proteine kinase SIK2 were identified as up-regulated in hemocytes exposed to both strains.

Bacicyclin, a new antibacterial cyclic hexapeptide from Bacillus sp. strain BC028 isolated from Mytilus edulis

A new cyclic hexapeptide, cyclo-(Gly-Leu-Val-IIe-Ala-Phe), named bacicyclin (1), was isolated from a marine Bacillus sp. strain associated with Mytilus edulis. The sequences of the amino acid building blocks of the cyclic peptide and its structure were determined by 1D- and 2D-NMR techniques. Marfey's analysis showed that the amino acid building blocks had L-configuration in all cases except for alanine and phenylalanine, which had D-configuration. Bacicyclin (1) exhibited antibacterial activity against the clinically relevant strains Enterococcus faecalis and Staphylococcus aureus with minimal inhibitory concentration values of 8 and 12 µM, respectively. These results demonstrate the potential of marine bacteria as a promising source for the discovery of new antibiotics.

Responses of Mytilus galloprovincialis hemocytes to environmental strains of Vibrio parahaemolyticus, Vibrio alginolyticus, Vibrio vulnificus

Marine bivalves are exposed to different types of bacteria in the surrounding waters, in particular of the Vibrio genus. In the hemocytes of the mussel Mytilus spp. immune responses to different vibrios have been largely characterized. However, little information is available on the hemocyte responses to human pathogenic vibrios commonly detected in coastal waters and bivalve tissues that are involved in seafood-borne diseases. In this work, functional parameters of the hemocytes from the Mediterranean mussel M. galloprovincialis were evaluated in response to in vitro challenge with different vibrios isolated from environmental samples of the Adriatic sea (Italy): V. parahaemolyticus Conero, V. alginolyticus 1513 and V. vulnificus 509. V. parahaemolyticus ATCC 43996 was used for comparison. At the 50:1 bacteria hemocyte ratio, only V. parahaemolyticus strains induced significant lysosomal membrane destabilisation. Stimulation of extracellular lysozyme release, total ROS, O₂^- and NO production were observed, although to different extents and with distinct time courses for different vibrios, V. vulnificus 509 in particular. Further comparisons between V. parahaemolyticus Conero and V. vulnificus 509 showed that only the latter induced dysregulation of the phosphorylation state of p38 MAP Kinase and apoptotic processes. The results indicate that mussel hemocytes can mount an efficient immune response towards V. parahaemolyticus and V. alginolyticus strains, whereas V. vulnificus 509 may affect the hemocyte function. This is the first report on immune responses of mussels to local environmental isolates of human pathogenic vibrios. These data reinforce the hypothesis that Mytilus hemocytes show specific responses to different vibrio species and strains.

Potential Response to Selection of HSP70 as a Component of Innate Immunity in the Abalone Haliotis rufescens

Assessing components of the immune system may reflect disease resistance. In some invertebrates, heat shock proteins (HSPs) are immune effectors and have been described as potent activators of the innate immune response. Several diseases have become a threat to abalone farming worldwide; therefore, increasing disease resistance is considered to be a long-term goal for breeding programs. A trait will respond to selection only if it is determined partially by additive genetic variation. The aim of this study was to estimate the heritability (h²) and the additive genetic coefficient of variation (CV_A) of HSP70 as a component of innate immunity of the abalone Haliotis rufescens, in order to assess its potential response to selection. These genetic components were estimated for the variations in the intracellular (in haemocytes) and extracellular (serum) protein levels of HSP70 in response to an immunostimulant agent in 60 full-sib families of H. rufescens. Levels of HSP70 were measured twice in the same individuals, first when they were young and again when they were pre-harvest adults, to estimate the repeatability (R), the h² and the potential response to selection of these traits at these life stages. High HSP70 levels were observed in abalones subjected to immunostimulation in both the intracellular and extracellular haemolymph fractions. This is the first time that changes in serum levels of HSP70 have been reported in response to an immune challenge in molluscs. HSP70 levels in both fractions and at both ages showed low h² and R, with values that were not significantly different from zero. However, HSP70 induced levels had a CV_A of 13.3–16.2% in young adults and of 2.7–8.1% in pre-harvest adults. Thus, despite its low h², HSP70 synthesis in response to an immune challenge in red abalone has the potential to evolve through selection because of its large phenotypic variation and the presence of additive genetic variance, especially in young animals.

Rearing effect of biofloc on antioxidant and antimicrobial transcriptional response in Litopenaeus stylirostris shrimp facing an experimental sub-lethal hydrogen peroxide stress

This study compares the antioxidant and antimicrobial transcriptional expression of blue shrimps reared according to two different systems, BioFloc Technology (BFT) and Clear sea Water (CW) and their differential responses when facing an experimental sublethal hydrogen peroxide stress. After 30 days of rearing, juvenile shrimps were exposed to H2O2 stress at a concentration of 30 ppm during 6 h. The oxidative stress caused by H2O2 was examined in the digestive glands of the shrimp, in which antioxidant enzyme (AOE) and antimicrobial peptide (AMP) gene expression were analysed by quantitative real-time PCR. Results showed that rearing conditions did not affect the expression of genes encoding AOEs or AMPs. However, H2O2 stress induced a differential response in expression between shrimps from the two rearing treatments (BFT and CW). Comparative analysis of the expression profiles indicates that catalase transcripts were significantly upregulated by H2O2 stress for BFT shrimps while no change was observed for CW shrimps. In contrast, H2O2 caused down-regulation of superoxide dismutase and glutathione transferase transcripts and of the three AMP transcripts studied (penaeidin 2 and 3, and crustin) for CW shrimps, while no effect was observed on BFT shrimp transcript levels. These results suggested that BFT shrimps maintained antioxidant and AMP responses after stress and therefore can effectively protect their cells against oxidative stress, while CW shrimp immune competence seems to decrease after stress.

Methods for quantifying gene expression in ecoimmunology: from qPCR to RNA-Seq

Historically, the use of cutting-edge molecular techniques to study immunological gene expression and related cellular pathways has been largely limited to model organisms. Few studies have been performed that quantify the molecular immunological responses of non-model species, especially in response to environmental factors, life-history events, or exposure to parasites. This dearth of information has largely occurred due to the lack of available non-model species-specific gene sequences and immunological reagents and also due to prohibitively expensive technology. However, with the rapid development of various sequencing and transcriptomic technologies, profiling the gene expression of non-model organisms has become possible. Technologies and concepts explored here include an overview of current technologies for quantifying gene expression, including: qPCR, multiplex branched DNA assays, microarrays, and profiling gene expression (RNA sequencing [RNA-Seq]) based on next-generation sequencing. Examples of the advancement of these technologies in non-model systems are discussed. Additionally, applications, limitations, and feasibility of the use of these methodologies in non-model systems to address questions in ecological immunology and disease-ecology are specifically addressed.

Interactions between Mytilus galloprovincialis hemocytes and the bivalve pathogens Vibrio aestuarianus 01/032 and Vibrio splendidus LGP32

Marine bivalves can accumulate large numbers of bacteria, in particular Vibrio species, whose persistence in bivalve tissues largely depends on their sensitivity to the bactericidal activity of circulating hemocytes and hemolymph soluble factors. The interactions between vibrios and hemolymph have been investigated, in particular in bivalve species susceptible to infection by certain Vibrio spp. and strains. In this work, the effects of two bivalve pathogens, Vibrio splendidus LGP32 (V.s.) and Vibrio aestuarianus 01/032 (V.a.), isolated from oyster mortality outbreaks, on the hemocytes of Mytilus galloprovincialis were investigated. In vitro, V.s., but not V.a., induced a dramatic decrease in lysosomal membrane stability-LMS in the hemocytes; both vibrios induced a moderate lysozyme release, with V.s. > V.a.. The V.s.-induced decrease in LMS was mediated by activation of PI-3Kinase, as shown by use of different kinase inhibitors. TEM analysis showed rapid internalization of both vibrios; however, V.s. lead to cellular and lysosomal damage and was able to survive within the hemocytes, whereas significant killing of V.a. was observed. In vivo, in mussels challenged with either vibrio and sampled at 6, 24 and 96 h post-injection, transient decreases in hemocyte LMS and progressive increases in serum lysozyme activity were observed, with V.s. > V.a.. Moreover, whereas V.a. was efficiently cleared from hemolymph, V.s. showed significant growth, that was maximal at 24 h p.i. when lowest LMS values were recorded in the hemocytes. Both vibrios also induced significant decreases in LMS in the digestive gland, again with V.s. > V.a.. The results indicate distinct interactions between mussel hemocytes and the two vibrio strains tested. The effects of V.s. may be due to the capacity of this strain to interfere with the signaling pathways involved in hemocyte function, thus escaping the bactericidal activity of the host cell, as observed for certain mammalian pathogens. Although V.s. is considered not pathogenic to Mytilus, this vibrio strain can affect the lysosomal function at the cellular and tissue level, thus leading to stressful conditions.

Toll-like receptors and MyD88 adaptors in Mytilus: complete cds and gene expression levels

TLR- and MyD88-related sequences have been previously investigated in Mytibase and then in new transcript reads obtained by Illumina technology from the mussel, Mytilus galloprovincialis. Based on full cds and domain organizations of virtual translations, we identified 23 Toll-like receptors (TLRs) and 3 MyD88 adaptors. MgTLRs can be arranged in 4 clusters according to extra-cellular LRR domain content. MgTLR-b, -i and -k were the only ones containing a multiple cysteine cluster (mccTLR), a domain composition also found in Drosophila Toll-1 and 18-wheeler. The 3 MyD88 we identified in M. galloprovincialis were also retrieved from Mytilus edulis, as well as MgTLR-b and -i. All MgTLRs were constitutively expressed in digestive gland whereas only 4 of them were also present in hemocytes. On the opposite, the 3 MgMyD88s were constitutively expressed in all the tissues. In vivo challenge of M. galloprovincialis with bacteria caused the up regulation of only MgTLR-i, but of all the 3 MgMyD88s. Highest response was induced by Gram-negative Vibrio anguillarum at 9h p.i. Injection of filamentous fungus, Fusarium oxysporum, resulted in up regulation of MgTLR-i and MgMyD88-c at 9h p.i. Such similar pattern of responses suggested MgMyD88-c represents the intra cytoplasm partner of MgTLR-i. Their interaction constituted the first cellular event revealing the existence of a Toll-signaling pathway in Lophotrochozoa.

In vivo effects of n-TiO2 on digestive gland and immune function of the marine bivalve Mytilus galloprovincialis

Due to the increasing production of nanoparticles (NPs) and their potential release in the aquatic environment, evaluation of their biological impact on aquatic organisms represents a major concern. Suspension feeding invertebrates, in particular bivalve mollusks, may play a role in NP biotransformation and transfer through food webs and may represent a significant target for NP toxicity. In this work, the in vivo effects of titanium dioxide (n-TiO2), one of the most widespread NPs in use, were investigated in the bivalve Mytilus galloprovincialis, largely utilised as a sentinel for marine contamination. Mussels were exposed for 96h to different concentrations of n-TiO2 suspensions (1, 10 and 100μgL(-1)) and multiple responses were evaluated in the digestive gland and immune cells, the haemocytes. In the digestive gland, n-TiO2 affected lysosomal and oxidative stress biomarkers and decreased transcription of antioxidant and immune-related genes. In the haemocytes, n-TiO2 decreased lysosomal membrane stability-LMS and phagocytosis, increased oxyradical production and transcription of antimicrobial peptides; moreover, pre-apoptotic processes were observed. The effects of n-TiO2 on digestive gland and haemocytes were distinct, also depending on the endpoint and on nominal NP concentrations, with many significant responses elicited by the lowest concentrations tested. The results show that n-TiO2, at concentrations close to predicted environmental levels, significantly affected different functional and molecular parameters of mussel digestive gland and immune cells. In particular, the observed changes in immune parameters that represent significant biomarkers of exposure at the organism level suggest that exposure to n-TiO2 may pose a serious risk to mussel health.

Molecular cloning and expression analysis of a cytosolic heat shock protein 70 gene from mud crab Scylla serrata

Heat shock protein 70s (Hsp70s) play important roles in resisting environmental stresses and stimulating innate immune system. To understand the immune defense mechanisms of Scylla serrata, a full-length cytosolic Hsp70 cDNA of S. serrata (designated as SSHsp70) was obtained by reverse transcriptase-polymerase chain reaction (RT-PCR) coupled with rapid amplification of cDNA ends (RACE). The full-length of SSHsp70 cDNA was 2235 bp, with a 5' untranslated region of 105 bp, a 3' untranslated region of 174 bp, and an open reading frame of 1956 bp encoding a polypeptide of 651 amino acids with an estimated molecular mass of 71.3 kDa and an estimated isoelectric point of 5.55. The cloned SSHsp70 belonged to a cytosolic Hsp70 family. Three typical Hsp70 signature motifs were detected in SSHsp70 by InterPro analysis. Quantitative PCR (qPCR) was used to detect tissue distribution and mRNA expression levels of SSHsp70 under different stress conditions. The obviously high levels of SSHsp70 transcript were in hemocyte, heart, hepatopancreas and gill, whereas low levels were detected in muscle, eyestalk, stomach, and gut. In different temperature treatments, the expression levels of SSHsp70 in low or high temperatures were higher than those in temperate temperature. In pathogen challenge treatments, the mRNA expression level of SSHsp70 reached a maximum level after 18 h and then dropped progressively. In different salt concentration treatments, the mRNA expression level of SSHsp70 had a minimum level at 25‰ salt concentration and high expression levels at high or low salt concentration. In different nitrite concentration treatments, the mRNA expression level of SSHsp70 increased progressively with the increase of nitrite concentration. The results confirmed Hsp70 could be used as a tool for evolution and phylogenetic analysis, a kind of potential biomarker, and a disease resistance factor used in application.

Sequence analysis of a normalized cDNA library of Mytilus edulis hemocytes exposed to Vibrio splendidus LGP32 strain

In the past decades, reports on bivalves' pathogens and associated mortalities have steadily increased. To face pathogenic micro-organisms, bivalves rely on innate defenses established in hemocytes which are essentially based on phagocytosis and cytotoxic reactions. As a step towards a better understanding of the molecular mechanisms involved in the mussel Mytilus edulis innate immune system, we constructed and sequenced a normalized cDNA library specific to M. edulis hemocytes unchallenged (control) and challenged with Vibrio splendidus LGP32 strain for 2, 4 and 6 h. A total of 1,024,708 nucleotide reads have been generated using 454 pyrosequencing. These reads have been assembled and annotated into 19,622 sequences which we believe cover most of the M. edulis hemocytes transcriptome. These sequences were successfully assigned to biological process, cellular component, and molecular function Gene Ontology (GO) categories. Several transcripts related to immunity and stress such as some fibrinogen related proteins and Toll-like receptors, the complement C1qDC, some antioxidant enzymes and antimicrobial peptides have already been identified. In addition, Toll-like receptors signaling pathways and the lysosome and apoptosis mechanisms were compared to KEGG reference pathways. As an attempt for large scale RNA sequencing, this study focuses on identifying and annotating transcripts from M. edulis hemocytes regulated during an in vitro experimental challenge with V. splendidus. The bioinformatic analysis provided a reference transcriptome, which could be used in studies aiming to quantify the level of transcripts using high-throughput analysis such as RNA-Seq.

Functional and molecular responses in Mytilus edulis hemocytes exposed to bacteria, Vibrio splendidus

This study aims at examining the morphological, functional and molecular responses of Mytilus edulis hemocytes exposed to different strains of Gram-negative bacteria Vibrio splendidus (a virulent strain V. splendidus LGP32, V. splendidus LGP32 Δvsm without metalloprotease and an environmental type strain V. splendidus 7SHRW) at a 1:3 ratio for 2, 4, and 6 h. Our data showed that hemocytes could have a discriminative capacity towards microorganisms. Both V. splendidus LGP32 strains had an effect on hemocyte adhesion, phagocytosis abilities and oxidative burst, whereas the environmental strain 7SHRW induced weak and delayed hemocyte responses. At a molecular level, differential levels of candidate transcripts were measured in M. edulis hemocytes exposed to V. splendidus LGP32-GFP and 7SHRW. Mainly, a down-regulation of defensin was recorded in hemocytes exposed to V. splendidus LGP32. A significant up-regulation of lysozyme and proteasome 26S was observed at 2 h followed by a down-regulation at 4 and 6 h of exposure to the LGP32 strain. Similarly, SOD and GPx genes were up-regulated 2 h post-exposure to LGP32 strain and their expressions decreased after 4 and 6 h post-exposure. Further analysis is however needed in a near future to relate the transcript level variations with the physiological process.

Fragments of β-thymosin from the sea urchin Paracentrotus lividus as potential antimicrobial peptides against staphylococcal biofilms

The immune mediators in echinoderms can be a potential source of novel antimicrobial peptides (AMPs) applied toward controlling pathogenic staphylococcal biofilms that are intrinsically resistant to conventional antibiotics. The peptide fraction <5 kDa from the cytosol of coelomocytes of the sea urchin Paracentrotus lividus (5-CC) was tested against a group of Gram-positive and Gram-negative pathogen reference strains. The 5-CC of P. lividus was active against all planktonic-tested strains but also showed antibiofilm properties against staphylococcal strains. Additionally, we demonstrated the presence of three small peptides in the 5-CC belonging to segment 9-41 of a P. lividusβ-thymosin. The smallest of these peptides in particular, showed the common chemical-physical characteristics of AMPs. This novel AMP from β-thymosin has high potential activity as an antibiofilm agent, acting on slow-growing bacterial cells that exhibit a reduced susceptibility to conventional antibiotics and represent a reservoir for recurrent biofilm-associated infections.

Thermal tolerance of the crab Pachygrapsus marmoratus: intraspecific differences at a physiological (CTMax) and molecular level (Hsp70)

Temperature is one of the most important variables influencing organisms, especially in the intertidal zone. This work aimed to test physiological and molecular intraspecific differences in thermal tolerance of the crab Pachygrapsus marmoratus (Fabricius, 1787). The comparisons made focused on sex, size, and habitat (estuary and coast) differences. The physiological parameter was upper thermal limit, tested via the critical thermal maximum (CTMax) and the molecular parameter was total heat shock protein 70 (Hsp70 and Hsp70 plus Hsc70) production, quantified via an enzyme-linked imunosorbent assay. Results showed that CTMax values and Hsp70 production are higher in females probably due to different microhabitat use and potentially due to different hormonal regulation in males and females. Among females, non-reproducing ones showed a higher CTMax value, but no differences were found in Hsp70, even though reproducing females showed higher variability in Hsp70 amounts. As reproduction takes up a lot of energy, its allocation for other activities, including stress responses, is lower. Juveniles also showed higher CTMax and Hsp70 expression because they occur in greater shore heights and ageing leads to alterations in protein synthesis. Comparing estuarine and coastal crabs, no differences were found in CTMax but coastal crabs produce more Hsp70 than estuarine crabs because they occur in drier and hotter areas than estuarine ones, which occur in moister environments. This work shows the importance of addressing intraspecific differences in the stress response at different organizational levels. This study shows that these differences are key factors in stress research, climate research, and environmental monitoring.

Individual variability of mytimycin gene expression in mussel

The antifungal peptide mytimycin (MytM) is synthesized by hemocytes of the Mediterranean mussel, Mytilus galloprovincialis. In addition to sequence and gene structure diversities previously reported from pooled hemocytes, the present report focused on the expression of mytm gene in individual M. galloprovincialis, before and after challenge. Within untreated mussel, MytM mRNA was observed by ISH in about 42% of circulating hemocytes, characterized by large, diffuse nucleus. Injection with Fusarium oxysporum increased such percentage, but in only some of the mussels. Similarly, MytM gene expression increased after injection in only some of the mussels, as measured by qPCR. Responders and not responders are common evidence in any given population of organisms. Nevertheless, even if the use of proper pool size selection has been practised to find out and evaluate the most common response trends, individual analyses must be regarded as optimal.

MIF from mussel: coding sequence, phylogeny, polymorphism, 3D model and regulation of expression

Three macrophage migration inhibitory factor (MIF)-related sequences were identified from a Mytilus galloprovincialis EST library. The consensus sequence included a 5'-UTR of 32 nucleotides, the complete ORF of 345 nucleotides, and a 3'-UTR of 349 nucleotides. As for other MIFs, M. galloprovincialis ORF does not include any signal or C-terminus extensions. The translated sequence of 115 amino acids possesses a molecular mass of 12,681.4, a pI of 6.27 and a stability index of 21.48. Its 3D structure resembles human MIF except for one shorter α-helix. Although evolutionary separated from ticks and vertebrates, Mg-MIF appeared to be closely related to Pinctada fucata and Haliotis, but not to Chlamys farreri and Biomphalaria glabrata. Numerous mutation points were observed within the Mg-MIF ORF, defining 11 amino acid variants within the mussels from Palavas-France and 14 amino acid variants within the mussels from Palermo-Italy. The 2 major variants from Palavas were identical to 2 of the 4 major variants from Palermo. In all the 18 Mg-MIF variants, residues involved in tautomerase and in oxidoreductase activities were conserved. Generally, one mussel expressed 2 Mg-MIF amino acid sequences but with different frequencies of occurrence. Mg-MIF is constitutively expressed principally in hemocytes and in the mantle. In contrast to other animal models, Mg-MIF expression was always down regulated following challenge by bacteria and fungi, confirming previous data obtained with microarray. Down regulation started as soon as 1 h and Mg-MIF expression returned to background 9-48 h after the challenge. Exception was regarding the yeast, Candidaalbicans, down-regulation between 9 and 72 h, suggesting yeast and bacteria-filamentous fungi trigger different mechanisms of elimination.

Cellular and transcriptional responses of Crassostrea gigas hemocytes exposed in vitro to brevetoxin (PbTx-2)

Hemocytes mediate a series of immune reactions essential for bivalve survival in the environment, however, the impact of harmful algal species and their associated phycotoxins upon bivalve immune system is under debate. To better understand the possible toxic effects of these toxins, Crassostrea gigas hemocytes were exposed to brevetoxin (PbTx-2). Hemocyte viability, monitored through the neutral red retention and MTT reduction assays, and apoptosis (Hoechst staining) remained unchanged during 12 h of exposure to PbTx-2 in concentrations up to 1000 µg/L. Despite cell viability and apoptosis remained stable, hemocytes incubated for 4 h with 1000 µg/L of PbTx-2 revealed higher expression levels of Hsp70 (p < 0.01) and CYP356A1 (p < 0.05) transcripts and a tendency to increase FABP expression, as evaluated by Real-Time quantitative PCR. The expression of other studied genes (BPI, IL-17, GSTO, EcSOD, Prx6, SOD and GPx) remained unchanged. The results suggest that the absence of cytotoxic effects of PbTx-2 in Crassostrea gigas hemocytes, even at high concentrations, allow early defense responses to be produced by activating protective mechanisms associated to detoxification (CYP356A1 and possibly FABP) and stress (Hsp70), but not to immune or to antioxidant (BPI, IL-17, EcSOD, Prx6, GPx and SOD) related genes.

Molecular profiling of marine fauna: integration of omics with environmental assessment of the world's oceans

Many species that contribute to the commercial and ecological richness of our marine ecosystems are harbingers of environmental change. The ability of organisms to rapidly detect and respond to changes in the surrounding environment represents the foundation for application of molecular profiling technologies towards marine sentinel species in an attempt to identify signature profiles that may reside within the transcriptome, proteome, or metabolome and that are indicative of a particular environmental exposure event. The current review highlights recent examples of the biological information obtained for marine sentinel teleosts, mammals, and invertebrates. While in its infancy, such basal information can provide a systems biology framework in the detection and evaluation of environmental chemical contaminant effects on marine fauna. Repeated evaluation across different seasons and local marine environs will lead to discrimination between signature profiles representing normal variation within the complex milieu of environmental factors that trigger biological response in a given sentinel species and permit a greater understanding of normal versus anthropogenic-associated modulation of biological pathways, which prove detrimental to marine fauna. It is anticipated that incorporation of contaminant-specific molecular signatures into current risk assessment paradigms will lead to enhanced wildlife management strategies that minimize the impacts of our industrialized society on marine ecosystems.

Gene expression specificity of the mussel antifungal mytimycin (MytM)

We previously reported the nucleotide sequences and diversity of mytimycin (MytM) from the Mediterranean mussel, Mytilus galloprovincialis. Using real-time PCR (q-PCR), we observed that the MytM gene was mainly expressed in circulating hemocytes and to a less extent in the mantle. In vivo challenge with bacteria or with the yeast, Candida albicans, did not increase the expression as measured by q-PCR in hemocytes. By contrast, injection of the filamentous fungus, Fusarium oxysporum, induced a sudden and strong increase of expression at 9h p.i. (stimulation index of 25.7 ± 2.1). Optimum stimulating dose was 10(4) spores of F. oxysporum per mussel. In the same samples, AMP mytilin and myticin showed no stimulation. Consequently, we hypothesized the existence of 2 different signal transduction pathways, one activated by bacteria and yeast, the other triggered by filamentous fungi. A second challenge performed with F. oxysporum 24 h after the first challenge induced an increase of MytM gene expression (stimulation index of 3.5 ± 1.7). However, this second increase was significantly lower than the first, suggesting less efficient response rather than significant protection.

Immunomodulation in Mytilus galloprovincialis by non-toxic doses of hexavalent chromium

In aquatic organisms, the immune function can be affected by exposure to environmental pollutants, including heavy metals. In vertebrate systems, different forms of Cr have been shown induce either immunostimulatory or immunosuppressive processes. Hexavalent Cr, Cr(VI), is an important contaminant released from both domestic and industrial effluents, and the predominant chemical form of the metal in aquatic ecosystems. In this work, the in vitro and in vivo effects of Cr(VI) on immune parameters of the marine bivalve Mytilus galloprovincialis were evaluated. Hemocyte incubation with different concentrations of Cr(VI) (0.1-1-10-100 μM) induced a dose-dependent decrease in lysosomal membrane stability (LMS). Decreases in extracellular lysozyme release and phagocytic activity were also observed, with stronger effects at lower metal concentrations. On the other hand, in these conditions, Cr(VI) stimulated extracellular superoxide production and nitrite accumulation. The effects of Cr(VI) were also evaluated in mussels exposed to the metal (0.1-1-10 μg L(-1), corresponding to nanomolar concentrations) for 96 h. Decreases in hemocyte LMS values and in serum lysozyme activity were observed with increasing metal concentrations. Decreased phagocytic activity and increased NO production were recorded, with stronger effects at lower concentrations. In these conditions, decreased Total Hemocyte Counts (THC), but no necrotic/apoptotic processes were observed. Moreover, Cr(VI) at both 0.1 and 1 μg L(-1) seemed to induce significant changes in transcription of immune genes (lysozyme, Mytilin C, Myticin B, defensin, MgC1q), of the serotonin receptor (5-HTR) and of the stress protein HSP70, whereas that of the anti-apoptotic gene p53 was unaffected. Overall, the results indicate that exposure to non-toxic, environmentally relevant concentrations of Cr(VI) can modulate functional and molecular immune parameters in M. galloprovincialis.

Massively parallel amplicon sequencing reveals isotype-specific variability of antimicrobial peptide transcripts in Mytilus galloprovincialis

Background

Effective innate responses against potential pathogens are essential in the living world and possibly contributed to the evolutionary success of invertebrates. Taken together, antimicrobial peptide (AMP) precursors of defensin, mytilin, myticin and mytimycin can represent about 40% of the hemocyte transcriptome in mussels injected with viral-like and bacterial preparations, and unique profiles of myticin C variants are expressed in single mussels. Based on amplicon pyrosequencing, we have ascertained and compared the natural and Vibrio-induced diversity of AMP transcripts in mussel hemocytes from three European regions.

Methodology/Principal Findings

Hemolymph was collected from mussels farmed in the coastal regions of Palavas (France), Vigo (Spain) and Venice (Italy). To represent the AMP families known in M. galloprovincialis, nine transcript sequences have been selected, amplified from hemocyte RNA and subjected to pyrosequencing. Hemolymph from farmed (offshore) and wild (lagoon) Venice mussels, both injected with 10⁷Vibrio cells, were similarly processed. Amplicon pyrosequencing emphasized the AMP transcript diversity, with Single Nucleotide Changes (SNC) minimal for mytilin B/C and maximal for arthropod-like defensin and myticin C. Ratio of non-synonymous vs. synonymous changes also greatly differed between AMP isotypes. Overall, each amplicon revealed similar levels of nucleotidic variation across geographical regions, with two main sequence patterns confirmed for mytimycin and no substantial changes after immunostimulation.

Conclusions/Significance

Barcoding and bidirectional pyrosequencing allowed us to map and compare the transcript diversity of known mussel AMPs. Though most of the genuine cds variation was common to the analyzed samples we could estimate from 9 to 106 peptide variants in hemolymph pools representing 100 mussels, depending on the AMP isoform and sampling site. In this study, no prevailing SNC patterns related to geographical origin or Vibrio injection emerged. Whether or not the contact with potential pathogens can increase the amount of AMP transcript variants in mussels requires additional study.

Relationship between mRNA biomarker candidates and location near a marine municipal wastewater outfall in the benthic indicator species Modiolus modiolus (L.)

The deep-sea horse mussel Modiolus modiolus (L.) is a sentinel bivalve species used for the assessment of potential biological exposure to anthropogenic contaminants in benthic environments. Using a combination of endpoints that included gross biological metrics, reproductive status, tissue contaminant load, and mRNA abundance profiles, we characterized variation in the local M. modiolus population situated in different spatial zones relative to a municipal wastewater outfall. Significant differences were observed in reproductive indicators, growth parameters, and abundance of four specific mRNA transcripts representative of stress response or membrane transport (CAT, NET/SCF6, ABCA4 and HSP70) in adductor muscle tissue of animals adjacent to the wastewater outfall. Concentrations of metals and organic chemicals in M. modiolus tissue were generally highest directly at the outfall site with much lower levels at 100-800 m from the outfall. This general pattern did not match the mRNA profiles. HSP70 and ABCA4 mRNA showed increased abundance in all regions adjacent to the municipal outfall compared to the reference site. One site group located within 100-200 m south/south-easterly of the outfall had increased levels of all four transcripts. Some mRNAs showed significant correlations with nickel, arsenic, lead, selenium, copper, and one of thirteen polycyclic aromatic hydrocarbons measured (dibenzo(a,h)-anthracene). Three mRNAs (CAT, NET/SCF6, and ABCA4) were negatively correlated with bis(2-ethylhexyl)phthalate. The data suggest that these benthic organisms are exhibiting biological responses to the outfall and support an alternate interpretation regarding dispersal of contaminants. The potential effects of emerging chemicals of concern entering the receiving environment merits further assessment.

Immunological function in marine invertebrates: responses to environmental perturbation

The inception of ecological immunology has led to an increase in the number of studies investigating the impact of environmental stressors on host immune defence mechanisms. This in turn has led to an increased understanding of the importance of invertebrate groups for immunological research. This review discusses the advances made within marine invertebrate ecological immunology over the past decade. By demonstrating the environmental stressors tested, the immune parameters typically investigated, and the species that have received the greatest level of investigation, this review provides a critical assessment of the field of marine invertebrate ecological immunology. In highlighting the methodologies employed within this field, our current inability to understand the true ecological significance of any immune dysfunction caused by environmental stressors is outlined. Additionally, a number of examples are provided in which studies successfully demonstrate a measure of immunocompetence through alterations in disease resistance and organism survival to a realized pathogenic threat. Consequently, this review highlights the potential to advance our current understanding of the ecological and evolutionary significance of environmental stressor related immune dysfunction. Furthermore, the potential for the advancement of our understanding of the immune system of marine invertebrates, through the incorporation of newly emerging and novel molecular techniques, is emphasized.

Molluscan biological and chemical diversity: secondary metabolites and medicinal resources produced by marine molluscs

The phylum Mollusca represents an enormous diversity of species with eight distinct classes. This review provides a taxonomic breakdown of the published research on marine molluscan natural products and the medicinal products currently derived from molluscs, in order to identify priority targets and strategies for future research. Some marine gastropods and bivalves have been of great interest to natural products chemists, yielding a diversity of chemical classes and several drug leads currently in clinical trials. Molluscs also feature prominently in a broad range of traditional natural medicines, although the active ingredients in the taxa involved are typically unknown. Overall secondary metabolites have only been investigated from a tiny proportion (<1%) of molluscan species. At the class level, the number of species subject to chemical studies mirrors species richness and our relative knowledge of the biology of different taxa. The majority of molluscan natural products research is focused within one of the major groups of gastropods, the opisthobranchs (a subgroup of Heterobranchia), which are primarily comprised of soft-bodied marine molluscs. Conversely, most molluscan medicines are derived from shelled gastropods and bivalves. The complete disregard for several minor classes of molluscs is unjustified based on their evolutionary history and unique life styles, which may have led to novel pathways for secondary metabolism. The Polyplacophora, in particular, have been identified as worthy of future investigation given their use in traditional South African medicines and their abundance in littoral ecosystems. As bioactive compounds are not always constitutively expressed in molluscs, future research should be targeted towards biosynthetic organs and inducible defence reactions for specific medicinal applications. Given the lack of an acquired immune system, the use of bioactive secondary metabolites is likely to be ubiquitous throughout the Mollusca and broadening the search field may uncover interesting novel chemistry.

Insights into the innate immunity of the Mediterranean mussel Mytilus galloprovincialis

Background

Sessile bivalves of the genus Mytilus are suspension feeders relatively tolerant to a wide range of environmental changes, used as sentinels in ecotoxicological investigations and marketed worldwide as seafood. Mortality events caused by infective agents and parasites apparently occur less in mussels than in other bivalves but the molecular basis of such evidence is unknown. The arrangement of Mytibase, interactive catalogue of 7,112 transcripts of M. galloprovincialis, offered us the opportunity to look for gene sequences relevant to the host defences, in particular the innate immunity related genes.

Results

We have explored and described the Mytibase sequence clusters and singletons having a putative role in recognition, intracellular signalling, and neutralization of potential pathogens in M. galloprovincialis. Automatically assisted searches of protein signatures and manually cured sequence analysis confirmed the molecular diversity of recognition/effector molecules such as the antimicrobial peptides and many carbohydrate binding proteins. Molecular motifs identifying complement C1q, C-type lectins and fibrinogen-like transcripts emerged as the most abundant in the Mytibase collection whereas, conversely, sequence motifs denoting the regulatory cytokine MIF and cytokine-related transcripts represent singular and unexpected findings. Using a cross-search strategy, 1,820 putatively immune-related sequences were selected to design oligonucleotide probes and define a species-specific Immunochip (DNA microarray). The Immunochip performance was tested with hemolymph RNAs from mussels injected with Vibrio splendidus at 3 and 48 hours post-treatment. A total of 143 and 262 differentially expressed genes exemplify the early and late hemocyte response of the Vibrio-challenged mussels, respectively, with AMP trends confirmed by qPCR and clear modulation of interrelated signalling pathways.

Conclusions

The Mytibase collection is rich in gene transcripts modulated in response to antigenic stimuli and represents an interesting window for looking at the mussel immunome (transcriptomes mediating the mussel response to non-self or abnormal antigens). On this basis, we have defined a new microarray platform, a mussel Immunochip, as a flexible tool for the experimental validation of immune-candidate sequences, and tested its performance on Vibrio-activated mussel hemocytes. The microarray platform and related expression data can be regarded as a step forward in the study of the adaptive response of the Mytilus species to an evolving microbial world.

Effects of vibrio challenge on digestive gland biomarkers and antioxidant gene expression in Mytilus galloprovincialis

In bivalve molluscs, responses to bacterial infection have been largely characterized in terms of both functional responses and gene expression in the immune cells, the hemocytes. The effects of bacterial challenge at the tissue level, where bacterial infection may cause stressful conditions, have not been so far specifically investigated. Biomarkers are widely utilised to evaluate the health status of bivalves, from the molecular to the organism level, in response to both natural and anthropogenic stressors. In this work, the effects of in vivo challenge with heat-killed vibrio species, Vibrio splendidus LGP32 and Vibrio anguillarum (ATCC19264), on different biomarkers in the digestive gland of the marine bivalve Mytilus galloprovincialis were investigated. Mussels were injected with either vibrio and tissues sampled at 3, 6 and 24 h post injection (p.i.). Lysosomal biomarkers, such as lysosomal membrane stability (LMS) and lipofuscin accumulation, as well as specific activities of antioxidant enzymes (catalase and glutathione transferase-GST) were evaluated. Moreover, the expression of antioxidant molecules (catalase, GST-pi and metallothioneins MT10 and MT20) was determined by quantitative RT-PCR. Both V. splendidus and V. anguillarum significantly affected all parameters measured, to a different extent and at different times p.i. Interestingly, whereas both vibrios induced lysosomal membrane destabilisation and increases in the activities of antioxidant enzymes, distinct responses were observed in terms of lysosomal lipofuscin accumulation and expression of antioxidant molecules. In particular, V. splendidus induced a general increase in the transcription of antioxidant genes, indicating that Mytilus digestive gland can mount an efficient antioxidant response towards this vibrio species. On the other hand, a general down-regulation or no effect was observed with V. anguillarum. The lack of this response was reflected in stronger oxidative stress conditions in the digestive gland of mussels challenged with V. anguillarum, as indicated by higher levels of lysosomal lipofuscin observed at longer times p.i. Overall, these data indicate that lysosomal and oxidative stress biomarkers could be usefully applied in order to monitor early changes in the health status of bivalves induced by bacteria. Moreover, the results support the hypothesis that host responses to bacteria may be taken into account when interpreting biomarker data in ecotoxicological studies.

Differential gene expression of gamma-actin, Toll-like receptor 2 (TLR-2) and interleukin-1 receptor-associated kinase 4 (IRAK-4) in Mya arenaria haemocytes induced by in vivo infections with two Vibrio splendidus strains

Immune function gene expression in Mya arenaria haemocytes was evaluated following in vivo infection with Vibrio splendidus LGP32-GFP and 7SHRW. Elongation factor 1alpha (EF-1alpha) with 2 (EF-2), after challenge with LGP32-GFP, and EF-1alpha with the ribosomal protein S-18, after challenge with 7SHRW, were found to be the most stable housekeeping genes. Using these internal controls and comparing the regulation induced by both strains, up-regulation of gamma-actin, down-regulation of TLR-2 and up-regulation of IRAK-4 was significantly higher after challenge with LGP32-GFP (p<0.001, p=0.001 and p<0.05, respectively). These results suggest specific responses at a molecular level modulated by the bacterial strains. LGP32-GFP induced marked responses which coincide with a similar trend previously found on phenotypic responses under our experimental model.

Expression of Mytilus immune genes in response to experimental challenges varied according to the site of collection

Mussels live in diverse coastal environments experience various physical, chemical and biological conditions, which they counteract with functional adjustments and heritable adaptive changes. In order to investigate possible differences in immune system capabilities, we analyzed by qPCR the expression levels of 4 immune genes (defensin, mytilin B, myticin B, lysozyme) and HSP70 in the Mediterranean mussel, Mytilus galloprovincialis collected in 3 European farming areas {Atlantic Ocean-Ría de Vigo-Spain (RV), French Mediterranean Gulf of Lion-Palavas-Prévost lagoon (PP) and Northern Adriatic Sea-Venice-Italy (VI)} in response to one injection of one of the 3 bacterial species (Vibrio splendidus LGP32, Vibrio anguillarum, Micrococcus lysodeikticus), and to heat shock or cold stress. We confirmed that the 5 genes are constitutively expressed in hemocytes, defensin being the less expressed, myticin B the highest. As suspected, the same gene resulted differently expressed according to mussel group, with the biggest difference being for HSP70 and lysozyme and lowest expression of all the 5 genes in mussels from RV. In addition, gene expression levels varied according to the challenge. Most frequent effect of bacterial injections was down-regulation, especially for mytilin B and myticin B. Heat shock enhanced transcript levels, particularly in mussels from RV, whereas cold stress had no effect. In situ hybridization of labelled probes on mussel hemocytes indicated that bacterial injections did not change the mRNA patterns of defensin and myticin B whereas mytilin B mRNA almost disappeared. In conclusion, these results demonstrated that constitutive level, nature and intensity of immune gene expression regulations strongly depended from mussel group, and support the concept of gene-environment interactions.

Influence of temperature, salinity and E. coli tissue content on immune gene expression in mussel: results from a 2005-2008 survey

Several bivalves, including mussels, suffered from mortalities particularly in summer. To look for the possible effect of environmental parameters on immune capacities, Mytilus galloprovincialis were collected monthly from August 2005 to July 2008 from the Palavas Laguna, French Mediterranean coast. Q-PCR was used to quantify the expression of three antimicrobial peptide genes (defensin, mytilin B and myticin B), in addition to lysozyme and HSP70. House keeping gene was 28S rRNA. Defensin, myticin B and lysozyme appeared more expressed in spring-summer than in winter. In contrast, HSP70 expression was higher in winter. Statistical studies using principal component analysis (PCA) and multiple regression models revealed positive influence of temperature on 28S rRNA, defensin, myticin B and lysozyme expressions, but not on mytilin B and HSP70. The positive influence was significant for defensin and lysozyme expression, but relationships cannot be quantified. Similarly, salinity appeared to influence defensin expression, but this relationship cannot be quantified neither. E. coli tissue content appeared without influence. Consequently, there was no clear relationship between environmental parameters and immune-related gene expressions, demonstrating anti-infectious capabilities cannot be evaluated using only the expression of such genes as markers.

Molecular cloning and expression analysis of the liver-expressed antimicrobial peptide 2 (LEAP-2) gene in grass carp

Liver-expressed antimicrobial peptide 2 (LEAP-2) is a novel antimicrobial peptide (AMP) recently found in vertebrates, and exhibits distinct amino acid sequence, secondary structure and expression pattern from other peptides. In this study, the LEAP-2 gene and its full-length cDNA were cloned from grass carp. Grass carp LEAP-2 gene consists of two introns and three exons. The translated product contains 92 amino acids, including a 26 amino acids signal peptide and a mature peptide of 41 amino acids. Grass carp LEAP-2 gene was expressed in a wide range of tissues except blood, with the highest level of transcripts found in liver. Upon induction by Aeromonas hydrophila, its expression was significantly up-regulated in liver, gill, skin, muscle, spleen, blood, head kidney, heart and intestine, but down-regulated in trunk kidney and brain. The transcript level was high in embryos at the 16-cell stage but declined gradually afterwards, suggesting that LEAP-2 transcripts in early embryos might be maternal. Mature peptides obtained by in vitro expression displayed selective antimicrobial activities. These results together further our understanding of the physiological function of LEAP-2 in vertebrates.

Schistosoma mansoni infection of juvenile Biomphalaria glabrata induces a differential stress response between resistant and susceptible snails

Schistosomes develop successfully in susceptible snails but are encapsulated and killed in resistant ones. Mechanism(s) shaping these outcomes involves the parasites ability to evade the snail's defenses. RNA analysis from resistant (BS-90), non-susceptible (LAC2) and susceptible (NMRI) juvenile Biomphalaria glabrata to Schistosoma mansoni revealed that stress-related genes, heat shock protein 70 (Hsp 70) and reverse transcriptase (RT), were dramatically co-induced early in susceptible snails, but not in resistant/non-susceptible ones. These transcripts were, however, down regulated upon exposure to irradiated parasites although penetration behavior of irradiated vs. normal parasites were the same, indicating that Hsp 70 and RT regulation was elicited by infection and not injury. Understanding molecular events involved in stress response transcriptional regulation of Hsp 70 in juvenile snails could pave a way towards the identification of genes involved in schistosome/snail interactions.