The immunomodulation of inducible nitric oxide in scallop Chlamys farreri

NO production of granulocytes associated with antibacterial immune response in Tegillarca granosa

Nitric oxide (NO) is a signaling molecule and immune effector produced by the nitric oxide synthases (NOS), which involved to various physiological processes of animals. In marine bivalves, hemocytes play important roles in antimicrobial innate immune response. Although hemocyte-derived NO has been detected in several bivalves, the immune function of hemocyte-derived NO is not well understood. Here, we investigated the antibacterial response of hemocyte-derived NO in the blood clam Tegillarca granosa. Two types of hemocytes including erythrocytes and granulocytes were isolated by Percoll density gradient centrifugation, their NO production and TgNOS expression level were analyzed. The results showed that NO was mainly produced in granulocytes and almost no detected in erythrocytes. The granulocytes showed significantly higher NO level and TgNOS expression level than the erythrocytes. And the TgNOS expression level was significantly increased in granulocytes after Vibro parahemolyticus challenge. In addition, the NO donor sodium nitroprusside (SNP) significantly increased the NO production of hemocytes to kill pathogenic bacteria. In summary, the results revealed that granulocytes-derived NO play vital roles in the antimicrobial immune response of the blood clam.

Nitric oxide synthase regulates coelomocytes apoptosis through the NF-κB signaling pathway in the sea cucumber Apostichopus japonicus

Nitric oxide synthase (NOS) was initially discovered to participate in the generation of nitric oxide as a defense mechanism against pathogenic infections. In recent years, it has been found that NOS plays a pivotal role in regulating apoptosis and inflammation in mammals. However, the mechanisms underlying NOS-mediated apoptosis in invertebrates remain largely unclear. In this study, we found that the Apostichopus japonicus NOS (AjNOS) expression levels were upregulated by 2.20-fold and 3.46-fold after being challenged with Vibrio splendidus at concentrations of 107 CFU mL-1 and 108 CFU mL-1 for 12 h compared to the control group, respectively. Under these conditions, the rates of coelomocytes apoptosis were increased from 14.7% to 32.7% and 45.4%, respectively. Treatment with NOS inhibitor (l-NAME) resulted in a reduction of coelomocytes apoptosis rates from 32.6% to 26.5% in V. splendidus (107 CFU mL-1) groups and from 42.3% to 33.3% in V. splendidus (108 CFU mL-1) groups, respectively. NOS has been reported to regulate apoptosis through IκBα phosphorylation. Simultaneously, exposure to V. splendidus in conjunction with l-NAME resulted in down-regulation of AjIκBα phosphorylation levels compared to the group infected solely with V. splendidus. Furthermore, immunofluorescence analysis revealed that treatment with l-NAME or interference of AjNOS using siRNA inhibited translocation of AjNF-κB/p65 (RelA) into the nucleus. Previous studies have shown that NF-κB can down-regulate expression levels of Bcl-2 family members, which is an important pathway for regulating apoptosis. In the present study, treatment with l-NAME was found to promote anti-apoptotic AjBcl-2 mRNA increase to 1.41-fold and protein expression increase to 1.86-fold at 12 h post V. splendidus challenge. However, these effects were suppressed by PMA (an NF-κB activator). Overall, our findings demonstrate that AjNOS regulates coelomocytes apoptosis induced by V. splendidus through activation of the AjNF-κB signaling pathway and down-regulation of AjBcl-2 in A. japonicus.

Investigating the internal system of defense of Gastropoda Aplysia depilans (Gmelin, 1791): Focus on hemocytes

The internal defense system of mollusks represents an efficient protection against pathogens and parasites, involving several biological immune processes, such as phagocytosis, encapsulation, cytotoxicity, and antigenic recognition of self/non-self. Mollusks possess professional, migratory, and circulating cells that play a key role in the defense of the organism, the hemocytes. Several studies have been performed on hemocytes from different mollusks, but, to date, these cells are still scarcely explored. Different hemocyte populations have been found, according to the presence or absence of granules, size, and the species of mollusks studied. Our study aims to deepen the knowledge of the hemocytes of the gastropod Aplysia depilans using morphological techniques and light and confocal microscopy, testing Toll-like receptor 2, inducible nitric oxide synthetase, and nicotinic acetylcholine receptor alpha 7 subunit. Our results show two hemocyte populations distinguishable by size, and presence/absence of granules in the cytoplasm, strongly positive for the antibodies tested, suggesting for the first time the presence of these receptors on the surface of sea hare hemocytes by immunohistochemistry. These data help in the understanding of the immune system of this gastropod, providing additional data for comprehending the evolution of the defense response in metazoan phylogenesis.

Immune diversity in lophotrochozoans, with a focus on recognition and effector systems

Lophotrochozoa is one of the most species-rich but immunologically poorly explored phyla. Although lack of acquired response in a narrow sense, lophotrochozoans possess various genetic mechanisms that enhance the diversity and specificity of innate immune system. Here, we review the recent advances of comparative immunology studies in lophotrochozoans with focus on immune recognition and effector systems. Haemocytes and coelomocytes are general important yet understudied player. Comparative genomics studies suggest expansion and functional divergence of lophotrochozoan immune reorganization systems is not as "homogeneous and simple" as we thought including the large-scale expansion and molecular divergence of pattern recognition receptors (PRRs) (TLRs, RLRs, lectins, etc.) and signaling adapters (MyD88s etc.), significant domain recombination of immune receptors (RLR, NLRs, lectins, etc.), extensive somatic recombination of fibrinogenrelated proteins (FREPs) in snails. Furthermore, there are repeatedly identified molecular mechanisms that generate immune effector diversity, including high polymorphism of antimicrobial peptides and proteins (AMPs), reactive oxygen and nitrogen species (RONS) and cytokines. Finally, we argue that the next generation omics tools and the recently emerged genome editing technicism will revolutionize our understanding of innate immune system in a comparative immunology perspective.

Toxicity response to benzo[α]pyrene exposure: Modulation of immune parameters of the bay scallop, Argopectenirradians

Bay scallops were exposed to four BaP concentrations (0.5, 1.0, 10 and 50 μg/L) for 72 h to elucidate their immune response. Immune parameters were evaluated by measuring nitric oxide (NO) levels in hemolymph. Additionally, we measured peptidoglycan recognition proteins (PGRP), fibrinogen-domain-containing protein (FReDC1), metallothionein (MT), and heat shock protein (HSP) 70 mRNA expression in digestive diverticula. NO as well as FReDC1 and MT expression in each BaP group increased significantly over time except for the BaP 0.5 group. The PGRP and HSP70 mRNA expression in the BaP 50 group increased in the range 6-24 h and then decreased. In situ hybridization also confirmed that there was higher MT mRNA expression in the BaP 50 group than in the control group at 72 h. Our results suggest that higher levels of BaP dampened scallop immune responses, while simultaneously reducing their ability to cope with oxidative stress and DNA damage. BaP exposure can be considered a potential immune inducer in bay scallop.

Toxicity assessment of organophosphorus in Ruditapes decussatus via physiological, chemical and biochemical determination: A case study with the compounds γ-oximo- and γ-amino-phosphonates and phosphine oxides

Organophosphorus derivatives are widely used in human health care and have been detected in aquatic ecosystems. These compounds may pose significant risks to non-target exposed organisms and only limited studies are available on bioconcentration and the effects of organophosphorus derivatives on marine organisms. The aim of this work was to evaluate the possible toxic effects of two concentrations (20 and 40 μg/L) of γ-oximo- and γ-amino-phosphonates and phosphine oxides in mediterranean clams Ruditapes decussatus exposed for 14 days using different biomarkers and the changes of filtration and respiration rate. The use of clams in ecotoxicity evaluation is thus mandatory to assess the feasibility of assessing oxidative stress on R. decussatus after being exposed to γ-oximo- and γ-amino-phosphonates and phosphine oxides. The oxidative status was analyzed by measuring oxidative stress biomarkers RNS and ROS production in mitochondria, superoxide dismutase (SOD), catalase (CAT), glutathione-S-transferases (GSTs), lipid peroxidation (LPO) and acetylcholinesterase (AChE), whose alteration was indicative of organophosphorus exposure, in both gills and digestive gland of the clams. No significant alterations in RNS, ROS production, SOD, CAT and AChE activities and MDA content were observed in both organs of clams treated with γ-oximophosphine oxides. It was possible then to hypothesize that γ-oximophosphine oxides may have probably exerted an incomplete alteration of antioxidant defenses and damage, which was changed by the activation of defense mechanisms. On the contrary, oxidative stress parameters were changed after exposure to γ-amino-phosphonates and phosphine oxides. In addition, metals accumulation, filtration and respiration rates were altered following exposure to all the studied organophosphorus compounds.

Expression Pattern of Nitric Oxide Synthase during Development of the Marine Gastropod Mollusc, Crepidula fornicata

Nitric Oxide (NO) plays a key role in the induction of larval metamorphosis in several invertebrate phyla. The inhibition of the NO synthase in Crepidula fornicata, a molluscan model for evolutionary, developmental, and ecological research, has been demonstrated to block the initiation of metamorphosis highlighting that endogenous NO is crucial in the control of this developmental and morphological process. Nitric Oxide Synthase contributes to the development of shell gland, digestive gland and kidney, being expressed in cells that presumably correspond to FMRF-amide, serotoninergic and catecolaminergic neurons. Here we identified a single Nos gene in embryonic and larval transcriptomes of C. fornicata and studied its localization during development, through whole-mount in situ hybridization, in order to compare its expression pattern with that of other marine invertebrate animal models.

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

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

A novel tumor necrosis factor in the Pacific oyster Crassostrea gigas mediates the antibacterial response by triggering the synthesis of lysozyme and nitric oxide

Tumor necrosis factors (TNFs) are a group of multifunctional inflammatory cytokines involved in various pathological and immune processes. Recently, a few primitive TNFs have been characterized from molluscs, which play important roles in modulating cell apoptosis, phagocytosis and production of immune-related enzymes. In the present study, a novel TNF (named as CgTNF-2) with the activity to mediate antibacterial response was identified from the Pacific oyster Crassostrea gigas. The open reading frame of CgTNF-2 was of 783 bp encoding a putative polypeptide of 261 amino acids with a typical TNF domain. The deduced amino acid sequence of CgTNF-2 shared high identity with that of TNFs previously identified from other molluscs, such as 96.1% identity with that in oyster C. hongkongensis, 33.7% identity with that in scallop Mizuhopecten yessoensis and 33.0% identity with CgTNF-1 in oyster C. gigas. There were two distinct TNF branches of vertebrate and invertebrate in the phylogenetic tree, and CgTNF-2 was firstly clustered with TNF-14 from C. hongkongensis, and then clustered with other molluscan TNFs. The mRNA transcripts of CgTNF-2 were widely expressed in various oyster tissues, with the highest expression level in hemocytes. The expression level of CgTNF-2 increased significantly at 6 h (2.45-fold and 6.20-fold, respectively, p < 0.05) after peptidoglycan and lipopolysaccharides treatments, and peaked at 12 h (31.86-fold and 7.90-fold, respectively, p < 0.05). The recombinant protein of CgTNF-2 (rCgTNF-2) inhibited the growth of human alveolar basal epithelial (A549) cells at a concentration of 800 ng/mL. After the oysters received an injection of rCgTNF-2, the serum from those oysters exhibited significantly higher antibacterial activity compared to that from control group, evidenced by inhibiting the growth of Vibrio splendidus. Moreover, the lysozyme activity as well as the contents of nitric oxide in the oyster serum also increased significantly. The above results collectively suggested that CgTNF-2 was a novel member of bivalve TNF-α family, which could prompt the antibacterial activity by inducing the lysozyme activity and the production of nitric oxide in the innate immune response of oyster.

Effects of Marine Toxin Domoic Acid on Innate Immune Responses in Bay Scallop Argopecten irradians

Domoic acid (DA) is an amnesic shellfish poisoning toxin produced by some species of the genera Pseudo-nitzschia and Nitzschia. This toxin has harmful effects on various species, especially scallops. This study aimed to investigate the effects of DA exposure on the immune and physical responses of bay scallop, Argopecten irradians. Various immunological and physical parameters were assessed (acid phosphatase (ACP), alkaline phosphatase (ALP), lactate dehydrogenase (LDH), lipid peroxide (LPO), nitric oxide (NO), and the total protein content) in the haemolymph of scallops at 3, 6, 12, 24, and 48 h post-exposure to DA at different concentrations (10, 50, and 100 ng/mL). Moreover, the expression of immune-related genes (CLT-6, FREP, HSP90, MT, PGRP, and PrxV) was assessed. The activities of ACP, ALP, and LDH and the total protein content and LPO increased upon exposure to DA at different concentrations, while NO levels were decreased. Furthermore, immune-related genes were assessed upon DA exposure. Our results showed that exposure to DA negatively impacts immune function and disrupts physiological activities in bay scallops.

The immunomodulatory function of invertebrate specific neuropeptide FMRFamide in oyster Crassostrea gigas

As one of the most important neuropeptides identified only in invertebrates of Mollusca, Annelida and Arthropoda, FMRFamide (Phe-Met-Arg-Phe-NH₂) involves in multiple physiological processes, such as mediating cardiac frequency and contraction of somatic and visceral muscles. However, its modulatory role in the immune defense has not been well understood. In the present study, an FMRFamide precursor (designed as CgFMRFamide) was identified in oyster Crassostrea gigas, which could be processed into nineteen FMRFamide peptides. Phylogenetic analysis revealed that CgFMRFamide shared high similarity with other identified FMRFamides in mollusks. The mRNA of CgFMRFamide was mainly concentrated in the tissues of visceral ganglia, hepatopancreas and hemocytes, and a consistent distribution of FMRFamide peptide was confirmed by immunohistochemistry and immunocytochemistry assays. The mRNA expression level of CgFMRFamide in hemocytes was significantly up-regulated after immune stimulation with lipopolysaccharide (LPS). After the concentration of FMRFamide was increased by exogenous injection, the in vivo expressions of pro-inflammatory cytokine CgIL17-5, as well as the apoptosis-related CgCaspase-1 and CgCaspase-3 in hemocytes were promptly increased (p < 0.05), but the concentration of signal molecule nitric oxide (NO) was significantly down-regulated (p < 0.05). Meanwhile, an increased phosphorylation of p38 MAP kinase in hemocytes was also detected after the FMRFamide injection. These results collectively demonstrated that the conserved FMRFamide could not only respond to immune stimulation, but also regulate the expression of immune effectors and apoptosis-related genes, which might be mediated by p38 MAP kinase pathway, thereby effectively involved in clearing pathogens and maintaining homeostasis in oysters.

Metal accumulation, biochemical and behavioral responses on the Mediterranean clams Ruditapes decussatus exposed to two photocatalyst nanocomposites (TiO2 NPs and AuTiO2NPs)

Nanoparticle decoration with noble metal represents a promising alternative to improve their photocatalytic and photovoltaic properties. However, toxicity can be influenced by such modification, as the bioavailability of these substances may be influenced. To understand how decoration influences the NP impacts in marine ecosystems, we exposed suspension-feeding clams, Ruditapes decussatus, to two photocatalyst nanocomposites, TiO₂ NPs and AuTiO₂ NPs, over 2 concentrations, 50 μg L^-1and 100 μg L^-1, in a laboratory experiment. Accumulation of Au and Ti in gills and digestive gland was noted in clams after exposure to TiO₂ NPs and AuTiO₂ NPs using inductively coupled plasma optic emission spectroscopy (ICP-OES). TiO₂ and AuTiO₂ NPs alter the behavior of the clams Ruditapes decussatus by reducing filtration and respiration rates. Furthermore, the highest concentration of TiO₂NPs induces an overproduction of H₂O₂ in gills and digestive gland and NO production only in gills. Superoxide dismutase (SOD), Catalase (CAT), Glutathione-S-transferase (GST) and acetylcholinesterase (AChE) activities were induced in gills and digestives gland in concentration and nanocomposite type dependent manner. Decorated form presented higher Malondialdehyde (MDA) levels in gills and digestive gland than the undecorated form, suggesting different mechanisms of action that may be mediated through oxidative stress. In conclusion, the considered parameters could represent reliable biomarkers for the assessment of NP toxicity on R. decussatus as biological biomonitoring model. In addition, based on the obtained results, nanoparticle decoration influences the toxicity of metal nanoparticles in marine organism.

Involvement of inducible nitric oxide synthase (iNOS) in immune-functioning of Paphia malabarica (Chemnitz, 1782)

In recent years, the role of inducible nitric oxide synthase (iNOS) isoform has been widely studied because of its immunological relevance in higher organisms as well as invertebrates including bivalves. However, little is known about the immunological role of iNOS in Paphia malabarica defense mechanism. In this study, we immunodetected the presence of iNOS in P. malabarica hemocytes using antibody N9657 monoclonal anti-nitric oxide synthase. In addition, increased iNOS activity was evident in response to a higher bacterial dosage (Vibrio parahaemolyticus and V. cholerae), highlighting the dose-dependent iNOS activity induction. Also, higher bacterial survivability was observed in the presence of iNOS inhibitor, i.e., S-methylisothiourea hemisulphate (SMIS) thus, validating the bactericidal role of iNOS. These findings implicate the involvement of iNOS in immune-functioning of P. malabarica. Future work should focus on elucidating the expression and regulation of pathogenesis in P. malabarica, involving iNOS.

The Neuroendocrine-Immune Regulation in Response to Environmental Stress in Marine Bivalves

Marine bivalves, which include many species worldwide, from intertidal zones to hydrothermal vents and cold seeps, are important components of the ecosystem and biodiversity. In their living habitats, marine bivalves need to cope with a series of harsh environmental stressors, including biotic threats (bacterium, virus, and protozoan) and abiotic threats (temperature, salinity, and pollutants). In order to adapt to these surroundings, marine bivalves have evolved sophisticated stress response mechanisms, in which neuroendocrine regulation plays an important role. The nervous system and hemocyte are pillars of the neuroendocrine system. Various neurotransmitters, hormones, neuropeptides, and cytokines have been also characterized as signal messengers or effectors to regulate humoral and cellular immunity, energy metabolism, shell formation, and larval development in response to a vast array of environmental stressors. In this review substantial consideration will be devoted to outline the vital components of the neuroendocrine system identified in bivalves, as well as its modulation repertoire in response to environmental stressors, thereby illustrating the dramatic adaptation mechanisms of molluscs.

The ethanol extract of honeysuckle stem modulates the innate immunity of Chinese mitten crab Eriocheir sinensis against Aeromonas hydrophila

Honeysuckle stem had been used as feed additives to modulate immunity in breeding industry, which was limited in the aquaculture field. In this study, the immunomodulation of honeysuckle stem ethanol extract (designed as HSE) on Chinese mitten crab Eriocheir sinensis was detected. The crabs fed with HSE diets for 30 days had higher level of the total haemocyte count (HTC), lysozyme activity and PO activity (P < 0.05), and had no obvious affect on the phagocytic activity, NO and TNF-α level. When challenged with Aeromonas hydrophila (1.0 × 10⁷ colony-forming units), HSE exhibited weak antibacterial activity against A. hydrophila and increased survival rate of crabs. The decreasing of THC and the increasing of TNF-α concentration, EsCaspase and EsLITAF mRNA expression level were all inhibited significantly by HSE treatment (P < 0.05), when the crabs were challenged by A. hydrophila. Moreover, the following immune parameters of crabs were enhanced by HSE treatment after A. hydrophila infection, including the rising of phagocytosis index and phagocytic rate of haemocyte, the rising of lysozyme, PO, NOS activities and nitric oxide concentration (P < 0.05). Therefore, it was concluded that HSE had great potential to develop into feed additive of crabs, which could enhance the innate immunity of Chinese mitten crabs E. sinensis effectively after A. hydrophila infection.

Annual assessment of the sea urchin (Paracentrotus lividus) humoral innate immune status: Tales from the north Portuguese coast

Innate immune status of the sea urchin Paracentrotus lividus population from two different rocky shore beaches in the northern Portuguese coast was evaluated for a period of one year. Although some ecological studies regarding the effect of toxics on the immune parameters of the sea urchin were made in Portuguese waters, there is a current lack of knowledge concerning their immune status all over the year. In perspective of a changing ecosystem in these waters due to global warming and colonization of new species, it is important to assess the status of the major species living in the area. In this way, immune parameters such as total protein content, nitric oxide concentration, haemolytic activity, protease activity, lysozyme concentration and bactericidal activity were evaluated in the perivisceral coelomic fluid, and were correlated with the gonadal index of the population and water parameters. Also, the spawning period can upset some immune status parameters, and others such as haemolytic activity and bactericidal activity against Vibrio anguillarum, showed a clear correlation with the gonad maturation status. The knowledge of the basal immune status of the species could serve as ecological indicator of some stress agent or contaminant into the field; also, coelomic fluid is suggested as good quality marker to assess the immune status of sea urchins.

The oyster immunity

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

The neuroendocrine immunomodulatory axis-like pathway mediated by circulating haemocytes in pacific oyster Crassostrea gigas

The neuroendocrine-immune (NEI) regulatory network is a complex system, which plays an indispensable role in the immunity of host. In this study, a neuroendocrine immunomodulatory axis (NIA)-like pathway mediated by the nervous system and haemocytes was characterized in the oyster Crassostrea gigas. Once invaded pathogen was recognized by the host, the nervous system would temporally release neurotransmitters to modulate the immune response. Instead of acting passively, oyster haemocytes were able to mediate neuronal immunomodulation promptly by controlling the expression of specific neurotransmitter receptors on cell surface and modulating their binding sensitivities, thus regulating intracellular concentration of Ca²⁺. This neural immunomodulation mediated by the nervous system and haemocytes could influence cellular immunity in oyster by affecting mRNA expression level of TNF genes, and humoral immunity by affecting the activities of key immune-related enzymes. In summary, though simple in structure, the ‘nervous-haemocyte’ NIA-like pathway regulates both cellular and humoral immunity in oyster, meaning a world to the effective immune regulation of the NEI network.

Toxicological perturbations of zinc oxide nanoparticles in the Coelatura aegyptiaca mussel

More research is needed to understand the interactions of nanoparticles (NPs) with aquatic organisms and their mechanism of toxic action. Zinc oxide nanoparticles (ZnONPs) are the most used engineered metal oxide NPs in consumer products. The present study was designed to evaluate the cytotoxicity, genotoxicity and digestive gland (DG) as well as gill histopathology of the freshwater molluscan bivalve Coelatura aegyptiaca following exposure to ZnONPs (2, 10 and 50 mg/L) for 6 consecutive days. Exposure to ZnONPs (10 and 50 mg/L) induced a significant increase in malondialdehyde, superoxide dismutase and nitric oxide with a concomitant decrease in reduced glutathione, glutathione-S-transferase and catalase levels in the haemolymph, DG and gills of the treated mussels. Following exposure to ZnONPs (50 mg/L), the DG exhibited gradual changes in glandular activity showing hypertrophy and hyperplasia in the glandular cells and irregularity of lamellae and swelling of filaments in the gills. The present investigation revealed that oxidative stress induction, genotoxicity in the haemocytes and histological alterations in the DG and gills of C. aegyptiaca could be the main mechanisms involved in ZnONPs toxicity in aquatic organisms. Thereby, it is suggested that ZnONPs should be applied with more precautions in relevant industries, and occupational health surveillance should be necessarily considered.

A glutamic acid decarboxylase (CgGAD) highly expressed in hemocytes of Pacific oyster Crassostrea gigas

Glutamic acid decarboxylase (GAD), a rate-limiting enzyme to catalyze the reaction converting the excitatory neurotransmitter glutamate to inhibitory neurotransmitter γ-aminobutyric acid (GABA), not only functions in nervous system, but also plays important roles in immunomodulation in vertebrates. However, GAD has rarely been reported in invertebrates, and never in molluscs. In the present study, one GAD homologue (designed as CgGAD) was identified from Pacific oyster Crassostrea gigas. The full length cDNA of CgGAD was 1689 bp encoding a polypeptide of 562 amino acids containing a conserved pyridoxal-dependent decarboxylase domain. CgGAD mRNA and protein could be detected in ganglion and hemocytes of oysters, and their abundance in hemocytes was unexpectedly much higher than those in ganglion. More importantly, CgGAD was mostly located in those granulocytes without phagocytic capacity in oysters, and could dynamically respond to LPS stimulation. Further, after being transfected into HEK293 cells, CgGAD could promote the production of GABA. Collectively, these findings suggested that CgGAD, as a GABA synthase and molecular marker of GABAergic system, was mainly distributed in hemocytes and ganglion and involved in neuroendocrine-immune regulation network in oysters, which also provided a novel insight to the co-evolution between nervous system and immune system.

Immune response of the bay scallop, Argopecten irradians, after exposure to the algicide palmitoleic acid

Palmitoleic acid (PA) is an effective algicide against the toxin-producing dinoflagellate Alexandrium tamarense; however, its effects on the immune system of the edible bay scallop Argopecten irradians are unclear. Therefore, we investigated the effects of PA on the immune response in A. irradians by assessing total haemocyte counts (THC), alkaline phosphatase activity (ALP), nitrite oxide (NO), glutathione (GSH), and lactate dehydrogenase (LDH) levels, as well as the expression of immune-related genes (FREP, PGRP, HSP90, MnSOD, and Cu/ZnSOD) at various hours post-exposure (hpe) to the compound. THC decreased in PA-treated groups, whereas ALP increased significantly in all of the PA treatment groups at 3 hpe, after which it significantly decreased. The LDH and NO levels were significantly enhanced in the high and medium concentration group. Notably, the GSH level increased in all PA treatment groups at each time interval. Our study revealed that after treatment with different concentrations of PA, variable effects on the expression of genes involved in the immune system response were observed. The results of our study demonstrate that immersing scallops in PA at effective concentrations could result in differential effects on immune system responses and expression of immune-related genes. Specifically, PA may disrupt the endocrine system or affect signal transduction pathways in the scallops. Therefore, the present study highlights the potential risk of using the PA as an algicide to control algal bloom outbreaks in the marine environment.

Marine Toxin Okadaic Acid Affects the Immune Function of Bay Scallop (Argopecten irradians)

Okadaic acid (OA) is produced by dinoflagellates during harmful algal blooms and is a diarrhetic shellfish poisoning toxin. This toxin is particularly problematic for bivalves that are cultured for human consumption. This study aimed to reveal the effects of exposure to OA on the immune responses of bay scallop, Argopecten irradians. Various immunological parameters were assessed (total hemocyte counts (THC), reactive oxygen species (ROS), malondialdehyde (MDA), glutathione (GSH), lactate dehydrogenase (LDH), and nitric oxide (NO) in the hemolymph of scallops at 3, 6, 12, 24, and 48 h post-exposure (hpe) to different concentrations of OA (50, 100, and 500 nM). Moreover, the expression of immune-system-related genes (CLT-6, FREP, HSP90, MT, and Cu/ZnSOD) was also measured. Results showed that ROS, MDA, and NO levels and LDH activity were enhanced after exposure to different concentrations of OA; however, both THC and GSH decreased between 24-48 hpe. The expression of immune-system-related genes was also assessed at different time points during the exposure period. Overall, our results suggest that exposure to OA had negative effects on immune system function, increased oxygenic stress, and disrupted metabolism of bay scallops.

Effective activation of antioxidant system by immune-relevant factors reversely correlates with apoptosis of Eisenia andrei coelomocytes

Oxidative stress is harmful to the microbes but also to the host, and may result in bystander damage or death. Because of this, respiratory burst triggered in phagocytes by pathogens is counteracted by production of antioxidative factors. The aim of this work was to examine effectiveness of the latter system in earthworms Eisenia andrei by induction of reactive oxygen species, lipofuscin and phenoloxidase by natural (LPS, zymosan, Micrococus luteus) and synthetic (phorbol ester, PMA) stimulants. The compounds impaired numbers, viability (increased apoptosis) and composition of coelomocytes, and triggered the antioxidant activity of catalase and selenium-dependent glutathione peroxidase. The natural pathogenic compounds, unlike PMA, strongly activated antioxidative responses that diminished cell apoptosis. Moreover, repeated exposure to the same or different pathogenic compounds did not induce respiratory burst exhausted phenotype showing that coelomocytes are constantly at bay to withstand numerous infections. The current study reveals importance and efficiency of the oxidative-antioxidative systems in annelids but also confirms its evolutionary conservatism and complexity even in lower taxa of the animal kingdom.

The first description of complete invertebrate arginine metabolism pathways implies dose-dependent pathogen regulation in Apostichopus japonicus

In this study, three typical members representative of different arginine metabolic pathways were firstly identified from Apostichopus japonicus, including nitric oxide synthase (NOS), arginase, and agmatinase. Spatial expression analysis revealed that the AjNOS transcript presented negative expression patterns relative to those of Ajarginase or Ajagmatinase in most detected tissues. Furthermore, Vibrio splendidus-challenged coelomocytes and intestine, and LPS-exposed primary coelomocytes could significantly induce AjNOS expression, followed by obviously inhibited Arginase and AjAgmatinase transcripts at the most detected time points. Silencing the three members with two specific siRNAs in vivo and in vitro collectively indicated that AjNOS not only compete with Ajarginase but also with Ajagmatinase in arginine metabolism. Interestingly, Ajarginase and Ajagmatinase displayed cooperative expression profiles in arginine utilization. More importantly, live pathogens of V. splendidus and Vibrio parahaemolyticus co-incubated with primary cells also induced NO production and suppressed arginase activity in a time-dependent at an appropriate multiplicity of infection (MOI) of 10, without non-pathogen Escherichia coli. When increasing the pathogen dose (MOI = 100), arginase activity was significantly elevated, and NO production was depressed, with a larger magnitude in V. splendidus co-incubation. The present study expands our understanding of the connection between arginine's metabolic and immune responses in non-model invertebrates.

The immunomodulation of nicotinic acetylcholine receptor subunits in Zhikong scallop Chlamys farreri

Nicotinic acetylcholine receptor (nAChR), the best-studied ionotropic neuron receptor protein, is a key player in neuronal communication, and it has been reported to play an important role in immunomodulation of vertebrates. Although nAChRs have also been identified in most invertebrates, the knowledge about their immunomodulation is still limited. In the present study, two scallop nAChR genes were identified from Chlamys farreri (designed as CfnAChR1 and CfnAChR2), which encoded 384 and 443 amino acids, respectively. The conserved disulfide-linked cystines, ion selectivity residues and the hydrophobic gating residues (L251, V255 and V259) were identified in CfnAChR1 and CfnAChR2. The immunoreactivities of CfnAChR1 and CfnAChR2 were observed in all the tested scallop tissues, including adductor muscle, mantle, gill, hepatopancreas, kidney and gonad. After LPS (0.5 mg mL(-1)) stimulation, the expression of CfnAChR1 mRNA in haemocytes increased significantly by 9.83-fold (P < 0.05) and 12.93-fold (P < 0.05) at 3 h and 24 h, respectively. While the expression level of CfnAChR2 mRNA increased 43.94% at 12 h after LPS stimulation (P < 0.05). After TNF-α (50 ng mL(-1)) stimulation, the expression levels of CfnAChR1 and CfnAChR2 both increased significantly at 1 h, which were 21.33-fold (P < 0.05) and 2.44-fold (P < 0.05) of that in the PBS group, respectively. The results collectively indicated that the cholinergic nervous system in scallops could be activated by immune stimulations through CfnAChR1 and CfnAChR2, which function as the links between the cholinergic nervous system and immune system.

The immune system and its modulation mechanism in scallop

Scallops are a cosmopolitan family of bivalves, and some of them are highly prized as dominant aquaculture species. In the past decades, there have been increasing studies on the basic biology and immunology of scallops, and this review summarizes the research progresses of immune system and its modulation mechanism in scallop. As invertebrate, scallops lack adaptive immunity and they have evolved an array of sophisticated strategies to recognize and eliminate various invaders by employing a set of molecules and cells. It is evident that basic immune reactions such as immune recognition, signal transduction, and effector synthesis involved in immune response are accomplished in a variety of ways. They rely upon an extensive repertoire of phagocytosis, apoptosis and encapsulation of the circulating hemocytes for eliminating invasive pathogens, as well as the production of immune effectors that are active against a large range of pathogens or sensitive for the environmental stress. Furthermore, the molecular constitutions, metabolic pathways and immunomodulation mechanisms of the primitive catecholaminergic, cholinergic, enkephalinergic system and NO system in scallop are also discussed, which can be taken as an entrance to better understand the origin and evolution of the neuroendocrine-immune regulatory network in lower invertebrates.

Mutual modulation between norepinephrine and nitric oxide in haemocytes during the mollusc immune response

Nitric oxide (NO) is one of the most important immune molecules in innate immunity of invertebrates, and it can be regulated by norepinephrine in ascidian haemocytes. In the present study, the mutual modulation and underlying mechanism between norepinephrine and NO were explored in haemocytes of the scallop Chlamys farreri. After lipopolysaccharide stimulation, NO production increased to a significant level at 24 h, and norepinephrine concentration rose to remarkable levels at 3 h and 12~48 h. A significant decrease of NO production was observed in the haemocytes concomitantly stimulated with lipopolysaccharide and α-adrenoceptor agonist, while a dramatic increase of NO production was observed in the haemocytes incubated with lipopolysaccharide and β-adrenoceptor agonist. Meanwhile, the concentration of cyclic adenosine monophosphate (cAMP) decreased significantly in the haemocytes treated by lipopolysaccharide and α/β-adrenoceptor agonist, while the content of Ca(2+) was elevated in those triggered by lipopolysaccharide and β-adrenoceptor agonist. When the haemocytes was incubated with NO donor, norepinephrine concentration was significantly enhanced during 1~24 h. Collectively, these results suggested that norepinephrine exerted varied effects on NO production at different immune stages via a novel α/β-adrenoceptor-cAMP/Ca(2+) regulatory pattern, and NO might have a feedback effect on the synthesis of norepinephrine in the scallop haemocytes.

The immunomodulation of inducible hydrogen sulfide in Pacific oyster Crassostrea gigas

Hydrogen sulfide (H2S) is an important gasotransmitter, which plays indispensable roles in cardiovascular, nervous and immune systems of vertebrates. However, the information about the immunomodulation of H2S in invertebrates is still very limited. In the present study, the temporal expression profile of cystathionine γ lyase in oyster Crassostrea gigas (CgCSE) was investigated after the oysters were stimulated by lipopolysaccharide. The expression levels of CgCSE mRNA transcripts in hemocytes increased significantly at 12h (1.31-fold of the PBS group, P<0.05) after LPS stimulation. The immunomodulation of inducible H2S in oyster was examined by monitoring the alterations of both cellular and humoral immune parameters in response to the stimulations of LPS, LPS+Na2S and LPS+propargylglycine (PAG). The total hemocyte counts (THC) and hemolymph PO activity increased significantly after LPS stimulation, and the increase could be further enhanced by adding PAG, while inhibited by appending Na2S. The phagocytosis activity of hemocytes was also increased firstly after LPS treatment, and the increase was enhanced by adding Na2S but inhibited after appending PAG. The anti-bacterial activity in hemolymph increased at 3h post LPS treatment, and then decreased after adding PAG. The total SOD activity of hemolymph was also elevated at 6h post LPS treatment, and the elevated activity was depressed by adding Na2S. These results collectively indicated that H2S might play crucial roles in the immune response of oyster via modulating the turnover and phagocytosis of hemocytes, and regulating the anti-bacterial activity and proPO activation in the hemolymph.

The comparative proteomics analysis revealed the modulation of inducible nitric oxide on the immune response of scallop Chlamys farreri

Nitric oxide (NO) is an important gasotransmitter which plays a key role on the modulation of immune response in all vertebrates and invertebrates. In the present study, the modulation of inducible NO on immune response of scallop Chlamys farreri was investigated via proteomic analysis. Total proteins from hepatopancreas of scallops treated with lipopolysaccharide (LPS) and/or the inhibitor of vertebrate inducible NO synthase (S-methylisothiourea sulfate, SMT) for 12 h were analyzed via 2-D PAGE and ImageMaster 2D Platinum. There were 890, 1189 and 1046 protein spots detected in the groups treated by phosphate buffered saline (PBS), LPS and LPS+SMT, respectively, and 26 differentially expressed protein spots were identified among them. These proteins were annotated with binding or catalytic activity, and most of them were involved in metabolic or cellular processes. Some immune-related or antioxidant-related molecules such as single Ig IL-1-related receptor, guanine nucleotide-binding protein subunit beta-like protein and peroxiredoxin were identified, and the changes of their expression levels in LPS group were intensified significantly after adding SMT. The decreased expression level of tyrosinase and increased level of glutathione S-transferase 4 in LPS group were diametrically reversed by appending SMT. Moreover, interferon stimulated exonuclease gene 20-like protein and copper chaperone for superoxide dismutase were only induced by LPS+SMT stimulation but not by LPS stimulation. These data indicated that NO could modulate many immunity processes in scallop, such as NF-κB transactivation, cytoskeleton reorganization and other pivotal processes, and it was also involved in the energy metabolism, posttranslational modification, detoxification and redox balance during the immune response.

The modulation of catecholamines on immune response of scallop Chlamys farreri under heat stress

Catecholamines (CAs) play key roles in mediating the physiological responses to various stresses. In the present study, the expression of CA-related genes were examined in the hemocytes of scallop Chlamys farreri under heat stress, and several immune or metabolism-related parameters were investigated after heat stress and adrenoceptor antagonist stimulation. After the scallops were cultured at 28°C, the mRNA expression level of dopa decarboxylase (CfDDC) and α-adrenoceptor (CfαAR) increased significantly (P<0.01), whereas that of monoamine oxidase (CfMAO) was down-regulated in the first 6h (P<0.05), and then up-regulated to the maximum level at 24h (P<0.01). In the hemocytes of scallops injected with adrenoceptor antagonist, the expression levels of peptidoglycan-recognition protein (CfPGRP-S1) and C-type lectin (CfLec-1) began to increase significantly at 2 and 3h post propranolol and high temperature treatment, respectively (P<0.01). While the up-regulation of heat shock protein 70 (CfHSP70) post heat stress was significantly inhibited by prazosin injection (P<0.01), and that of hexokinase (CfHK) was inhibited by both prazosin and propranolol injection (P<0.01). Moreover, the remarkable increase of relative specific activity of SOD in the hemolymph post heat stress (P<0.01) was further up-regulated early after prazosin or propranolol injection (P<0.01), while that of the relative anti-bacterial ability was down-regulated by prazosin or propranolol treatment (P<0.01). These results collectively indicated that the catecholaminergic neuroendocrine system in scallop could be activated by heat stress to release CAs, which subsequently modulated the immune response and energy metabolism via α- and β-adrenoceptors.

A scallop nitric oxide synthase (NOS) with structure similar to neuronal NOS and its involvement in the immune defense

BACKGROUND

Nitric oxide synthase (NOS) is responsible for synthesizing nitric oxide (NO) from L-arginine, and involved in multiple physiological functions. However, its immunological role in mollusc was seldom reported.

METHODOLOGY

In the present study, an NOS (CfNOS) gene was identified from the scallop Chlamys farreri encoding a polypeptide of 1486 amino acids. Its amino acid sequence shared 50.0~54.7, 40.7~47.0 and 42.5~44.5% similarities with vertebrate neuronal (n), endothelial (e) and inducible (i) NOSs, respectively. CfNOS contained PDZ, oxygenase and reductase domains, which resembled those in nNOS. The CfNOS mRNA transcripts expressed in all embryos and larvae after the 2-cell embryo stage, and were detectable in all tested tissues with the highest level in the gonad, and with the immune tissues hepatopancreas and haemocytes included. Moreover, the immunoreactive area of CfNOS distributed over the haemocyte cytoplasm and cell membrane. After LPS, β-glucan and PGN stimulation, the expression level of CfNOS mRNA in haemocytes increased significantly at 3 h (4.0-, 4.8- and 2.7-fold, respectively, P < 0.01), and reached the peak at 12 h (15.3- and 27.6-fold for LPS and β-glucan respectively, P < 0.01) and 24 h (17.3-fold for PGN, P < 0.01). In addition, TNF-α also induced the expression of CfNOS, which started to increase at 1 h (5.2-fold, P < 0.05) and peaked at 6 h (19.9-fold, P < 0.01). The catalytic activity of the native CfNOS protein was 30.3 ± 0.3 U mgprot(-1), and it decreased significantly after the addition of the selective inhibitors of nNOS and iNOS (26.9 ± 0.4 and 29.3 ± 0.1 U mgprot(-1), respectively, P < 0.01).

CONCLUSIONS

These results suggested that CfNOS, with identical structure with nNOS and similar enzymatic characteristics to nNOS and iNOS, played the immunological role of iNOS to be involved in the scallop immune defense against PAMPs and TNF-α.