The Cholinergic and Adrenergic Autocrine Signaling Pathway Mediates Immunomodulation in Oyster Crassostrea gigas

Neuroendocrine functions of monoamines in invertebrates: Focus on bivalve molluscs

Monoamines (MA) such as serotonin, catecholamines (dopamine, norepinephrine, epinephrine), and trace amines (octopamine, tyramine), are neurotransmitters and neuroendocrine modulators in vertebrates, that contribute to adaptation to the environment. Although MA are conserved in evolution, information is still fragmentary in invertebrates, given the diversity of phyla and species. However, MA are crucial in homeostatic processes in these organisms, where the absence of canonical endocrine glands in many groups implies that the modulation of physiological functions is essentially neuroendocrine. In this review, we summarize available information on MA systems in invertebrates, with focus on bivalve molluscs, that are widespread in different aquatic environments, where they are subjected to a variety of environmental stimuli. Available data are reviewed on the presence of the different MA in bivalve tissues, their metabolism, target cells, signaling pathways, and the physiological functions modulated in larval and adult stages. Research gaps and perspectives are highlighted, in order to enrich the framework of knowledge on MA neuroendocrine functions, and on their role in adaptation to ongoing and future environmental changes.

Ecological benefits of artificial light at night (ALAN): Accelerating the development and metamorphosis of marine shellfish larvae

Urbanization has led to increasing use of artificial light at night (ALAN), which has rapidly become an important source of pollution in many cities. To identify the ALAN effects on the embryonic development of the Pacific abalone Haliotis discus hannai, we first exposed larvae to natural light with a light period of 12 L:12D (control, Group CTR). We then exposed larvae to three different light regimes. Larvae in Group NL were exposed to full spectrum artificial light from 18:00 to 00:00 to simulate the lighting condition at night, whereas Groups BL and YL were illuminated at the same time interval with 450 nm of short-wavelength blue light and 560 nm of long-wavelength orange light, respectively, to simulate billboard lighting at night. There were significantly higher hatching success and metamorphosis rates of larvae in Group BL than in Group YL or CTR (P < 0.05). The larvae in Group YL had the highest abnormality rate and took the longest time to complete metamorphosis. Transcriptomic studies revealed significantly higher expression levels of genes related to RNA transport, DNA replication, and protein processing in endoplasmic reticulum pathways in Group BL compared to the other groups. In the metabolomic analysis, we identified prostaglandin B1, tyramine, d-fructose 6-phosphate, L-adrenaline, leukotriene C4, and arachidonic acid as differential metabolic markers, as they play a vital part in helping larvae adapt to different ALAN conditions. Multi-omics correlation analysis of pairwise comparisons between all of the groups suggested that the biosynthesis of unsaturated fatty acids (FAs) and arachidonic acid metabolism pathways were significantly enriched (P < 0.05). Further quantitative analysis of the fatty acid (FA) contents revealed that 42 out of 50 FAs were down-regulated in Group BL and up-regulated in Group YL, which suggested that the synthesis, catabolism, and metabolism of FAs are crucial for the larval response to different spectral components of ALAN. For the first time, we report positive rather than negative effects of artificial blue light at night on the embryonic development of a benthic marine species. These results are significant for unbiased and full-scale assessment of the ecological effects of ALAN and for understanding the structural stability of the marine benthic community.

Whole-genome bisulfite sequencing reveals the function of DNA methylation in the allotransplantation immunity of pearl oysters

Introduction

In the pearl culture industry, a major challenge is the overactive immunological response in pearl oysters resulting from allotransplantation, leading to shell-bead rejection and death. To better understand the molecular mechanisms of postoperative recovery and the regulatory role of DNA methylation in gene expression, we analyzed the changes in DNA methylation levels after allotransplantation in pearl oyster Pinctada fucata martensii, and elucidated the regulatory function of DNA methylation in promoter activity of nicotinic acetylcholine receptor (nAChR) gene.

Methods

We constructed nine DNA methylomes at different time points after allotransplantation and used bisulfite genomic sequencing PCR technology (BSP) to verify the methylation status in the promoter of nAChR. We performed Dual luciferase assays to determine the effect of the dense methylation region in the promoter on transcriptional activity and used DNA pull-down and mass spectrometry analysis to assess the capability of transcription factor binding with the dense methylation region.

Result

The DNA methylomes reveal that CG-type methylation is predominant, with a trend opposite to non-CG-type methylation. Promoters, particularly CpG island-rich regions, were less frequently methylated than gene function elements. We identified 5,679 to 7,945 differentially methylated genes (DMGs) in the gene body, and 2,146 to 3,385 DMGs in the promoter at each time point compared to the pre-grafting group. Gene ontology and pathway enrichment analyses showed that these DMGs were mainly associated with "cellular process", "Membrane", "Epstein-Barr virus infection", "Notch signaling pathway", "Fanconi anemia pathway", and "Nucleotide excision repair". Our study also found that the DNA methylation patterns of the promoter region of nAChR gene were consistent with the DNA methylomics data. We further demonstrated that the dense methylation region in the promoter of nAChR affects transcriptional activity, and that the methylation status in the promoter modulates the binding of different transcription factors, particularly transcriptional repressors.

Conclusion

These findings enhance our understanding of the immune response and regulation mechanism induced by DNA methylation in pearl oysters after allotransplantation.

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.

Lipidomic insights into the immune response and pearl formation in transplanted pearl oyster Pinctada fucata martensii

During pearl culture, the excess immune responses may induce nucleus rejection and death of pearl oysters after transplantation. To better understand the immune response and pearl formation, lipidomic analysis was applied to investigate changes in the serum lipid profile of pearl oyster Pinctada fucata martensii following transplantation. In total, 296 lipid species were identified by absolute quantitation. During wound healing, the content of TG and DG initially increased and then decreased after 3 days of transplantation with no significant differences, while the level of C22:6 decreased significantly on days 1 and 3. In the early stages of transplantation, sphingosine was upregulated, whereas PC and PUFAs were downregulated in transplanted pearl oyster. PI was upregulated during pearl sac development stages. GP and LC-PUFA levels were upregulated during pearl formation stage. In order to identify enriched metabolic pathways, pathway enrichment analysis was conducted. Five metabolic pathways were found significantly enriched, namely glycosylphosphatidylinositol-anchor biosynthesis, glycerophospholipid metabolism, alpha-linolenic acid metabolism, linoleic acid metabolism and arachidonic acid metabolism. Herein, results suggested that the lipids involved in immune response, pearl sac maturation, and pearl formation in the host pearl oyster after transplantation, which might lead to an improvement in the survival rate and pearl quality of transplanted pearl oyster.

RGD-Labeled Hemocytes With High Migration Activity Display a Potential Immunomodulatory Role in the Pacific Oyster Crassostrea gigas

Immunocyte migration to infection sites is important for host cellular defense, but the main types of migrating hemocytes and their mechanisms against pathogen invasions are unclear in invertebrates. In the present study, a population of hemocytes in the Pacific oyster Crassostrea gigas labeled with a fluorescein isothiocyanate (FITC)-conjugated Arg-Gly-Asp (RGD)-containing peptide was sorted. RGD⁺ hemocytes were characterized by a smaller cell size and cytoplasmic-nucleo ratio, fewer cytoplasmic granules, and higher levels of myeloperoxidase, reactive oxygen species, and intracellular free calcium concentration. RGD⁺ hemocytes exhibited a high level of migration activity, which was further induced after V. splendidus infection. Transcriptome analysis revealed that RGD⁺ hemocytes highly expressed a series of migration-related genes, which together with migration-promoting genes were significantly upregulated after V. splendidus infection. The neuroendocrine system was also proven to regulate the migration activity of RGD⁺ hemocytes, especially with the excitatory neuroendocrine factor dopamine, which promoted migration activity as confirmed by receptor blocking assays. Meanwhile, RGD⁺ hemocytes could highly express immunomodulatory factor interleukin (IL)-17s and their receptor genes, which was positively related to the production of antimicrobial peptides in whole hemocytes after V. splendidus infection. Collectively, this study identified a specific hemocyte population, i.e., RGD⁺ hemocytes, that shows high migration activity in response to pathogen infection and exerts a potential immunomodulatory role by highly expressing IL-17s that might enhance the hemocytes’ antimicrobial peptide production in oysters.

The exploration of neuroendocrine regulation of crustacean hyperglycemic hormone (CHH) on innate immunity of Litopenaeus vannamei under ammonia-N stress

To unveil the neuroendocrine-immune (NEI) mechanism of crustaceans under high ambient ammonia-N, crustacean hyperglycemic hormone (CHH) in L. vannamei was knocked down under 20 mg/L ammonia-N exposure. The results showed that the expression of CHH in the eyestalks decreased significantly when CHH was silenced. After CHH was knocked down, the levels of CHH, ACh, DA, NE, and 5-HT in the haemolymph decreased significantly. Correspondingly, the expressions of GC, ACh₇R, DM1, DA₁R, and 5-HT₇R in haemocytes down-regulated significantly, while DA₄R and α₂AR up-regulated significantly. Besides, the expression of Toll3 reduced significantly. And significantly changes occurred in the levels of G protein effectors (AC and PLC), second messengers (cAMP, cGMP, CaM, and DAG), protein kinases (PKA, PKC and PKG), and nuclear transcription factors (CREB, Dorsal, Relish and NKRF). Furthermore, immune defense proteins (BGBP and PPO3, Crustin A, ALF, LYC, TNFα, and IL-16), phagocytosis-related proteins (Cubilin, Integrin, Peroxinectin, Mas-like protein, and Dynamin-1) and exocytosis-related proteins (SNAP-25, VAMP-2 and Syntaxin) changed significantly. Eventually, a significant decrease in the levels of THC, haemocytes phagocytosis rate, plasma PO, antibacterial and bacteriolytic activities was detected. Therefore, these results indicate that under ammonia-N stress, the combination of CHH and GC mainly affects exocytosis of shrimp through the cGMP-PKG-CREB pathway. Simultaneously, CHH stimulates the release of biogenic amines, and then activate G protein effectors after binding to their specific receptors, to regulate exocytosis mainly via the cAMP-PKA-CREB pathway and influence phagocytosis primarily by the cAMP-PKA-NF-κB pathway. CHH can enhance ACh, and then activate G protein effectors after binding to the receptors, and finally regulate exocytosis mainly through the cAMP-PKA-CREB pathway and regulate phagocytosis by the cAMP-PKA-NF-κB pathway. CHH can also promote Toll3-NF-κB pathway, thereby affecting the expressions of immune defense factors. This study contributes to a further understanding of the NEI mechanism of crustacean in response to environmental stress.

Characterization and expression analysis of tandemly duplicated nicotinic acetylcholine receptors in pearl oysters after stimulation of pathogen-related molecular patterns

Nicotinic acetylcholine receptors (nAChRs) are a class of ligand-gated ion channels that participate in signal transduction and are reported to play an important role in the immunomodulation of vertebrates and invertebrates. Previous studies have shown that the nAChRs in mollusks have undergone large-scale expansion after tandem repeats and retrotransposition, with the most expansion observed in bivalves. This study characterized the sequence of a tandem repeat nAChR unique to several bivalve mollusks and investigated its functions in Pinctada fucata martensii. Firstly, phylogenetic analysis revealed that the tandem arrays of nAChRs existed before bivalve differentiation and m ost tandem-replicated nAChR genes have a conserved genomic structure and domain combination. In present study, five tandemly duplicated nAChR genes were cloned from P. f. martensii and designated as PmnAChR-1 to PmnAChR-5. qRT-PCR analysis revealed that five PmnAChRs were specifically expressed in adult gills. In addition, after PAMP stimulation, the expression of PmnAChRs in hemocytes of P. f. martensii were strongly induced but exhibited different responses to different stimuli. PmnAChR-1, PmnAChR-4, and PmnAChR-5 exhibited strong and wide responsiveness to lipopolysaccharide (LPS) stimulation but had no response to peptidoglycan (PGN) stimulation. PmnAChR-2 expression was notably upregulated at 6 h after PGN challenge but had no response to LPS stimulation. Polyinosinic-polycytidylic acid challenge upregulated nearly all PmnAChRs, except for PmnAChR-5. Furthermore, Pm-miR-873-3p, Pm-miR-4577, Pm-miR-103a-3p, and Pm-miR-6753-3p were identified as the regulatory miRNA of PmnAChR-1, PmnAChR-3, PmnAChR-4, and PmnAChR-5, respectively. These findings suggested that these tandem arrays of nAChRs are unique to bivalves, and the tandem duplication of nAChR genes may be involved in the immune regulation process after pathogen stimulation.

Association of Salivary Cholinesterase With Arthropod Vectors of Disease

Acetylcholinesterase (AChE) was previously reported to be present in saliva of the southern cattle tick, Rhipicephalus (Boophilus) microplus (Canestrini), with proposed potential functions to 1) reduce acetylcholine toxicity during rapid engorgement, 2) modulate host immune responses, and 3) to influence pathogen transmission and establishment in the host. Potential modulation of host immune responses might include participation in salivary-assisted transmission and establishment of pathogens in the host as has been reported for a number of arthropod vector-borne diseases. If the hypothesis that tick salivary AChE may alter host immune responses is correct, we reasoned that similar cholinesterase activities might be present in saliva of additional arthropod vectors. Here, we report the presence of AChE-like activity in the saliva of southern cattle ticks, Rhipicephalus (Boophilus) microplus; the lone star tick, Amblyomma americanum (Linnaeus); Asian tiger mosquitoes, Aedes albopictus (Skuse); sand flies, Phlebotomus papatasi (Scopoli); and biting midges, Culicoides sonorensis Wirth and Jones. Salivary AChE-like activity was not detected for horn flies Haematobia irritans (L.), stable flies Stomoxys calcitrans (L.), and house flies Musca domestica L. Salivary cholinesterase (ChE) activities of arthropod vectors of disease-causing agents exhibited various Michaelis-Menten KM values that were each lower than the KM value of bovine serum AChE. A lower KM value is indicative of higher affinity for substrate and is consistent with a hypothesized role in localized depletion of host tissue acetylcholine potentially modulating host immune responses at the arthropod bite site that may favor ectoparasite blood-feeding and alter host defensive responses against pathogen transmission and establishment.

Acetylcholine suppresses phagocytosis via binding to muscarinic- and nicotinic-acetylcholine receptors and subsequently interfering Ca2+- and NFκB-signaling pathways in blood clam

Though immunomodulation via cholinergic neurotransmitter acetylcholine (ACh), an important part of neuroendocrine-immune (NEI) regulatory network, has been well established in vertebrate species, the mechanisms remain poorly understood in invertebrates. In the present study, the immunomodulatory effect of ACh on haemocyte phagocytosis was investigated in an invertebrate bivalve species, Tegillarca granosa. Data obtained showed that in vitro ACh incubation suppressed phagocytic activity of haemocytes along with a significant elevation in intracellular Ca²⁺. In addition, the expressions of genes from Ca²⁺ signaling pathway were significantly induced whereas those from NF-κB signaling pathway were significantly down-regulated by ACh incubation. Furthermore, these adverse impacts of ACh were significantly relieved by the blocking of muscarinic acetylcholine receptors (mAChRs) or nicotinic acetylcholine receptors (nAChRs) using corresponding antagonists. Our study suggests that ACh suppresses phagocytosis via binding to both mAChRs and nAChRs, which disrupts intracellular Ca²⁺ homeostasis and subsequently interferes with downstream Ca²⁺ and NF-κB signaling pathways.

A Signaling Pathway to Mediate the Combined Immunomodulation of Acetylcholine and Enkephalin in Oyster Crassostrea gigas

Molluscs have evolved a primitive but complete neuroendocrine-immune (NEI) system with a vast array of neurotransmitters to conduct both humoral and cellular immunomodulation. Previous studies have illustrated the immune functions of several key neurotransmitters. However, the combined effects of multiple neurotransmitters and the signaling pathway to mediate such immunomodulation have not been well-understood. In the present study, iTRAQ and LC-ESI-MS/MS approaches were employed to investigate the combined immunomodulation functions of two crucial neurotransmitters, acetylcholine (ACh), and [Met⁵]-enkephalin (ENK), in oyster Crassostrea gigas. A total number of 5,379 proteins were identified from hemocytes of oysters after the treatments with Ach and ENK separately or simultaneously, and 1,475 of them were found to be significantly up-regulated, while 1,115 of them were significantly down-regulated. The protein expression patterns in the groups treated by ACh and ENK separately were quite similar, which were dramatically different from that in the group treated by ACh+ENK. One hundred seventy-two proteins were found to be differentially expressed in all the three neurotransmitter treatment groups. Functional validation suggested that ACh and ENK possibly modulate the immune response in oyster hemocytes by enhancing pathogen recognition, cell apoptosis, and the enzyme activities of superoxide dismutase (SOD). Moreover, GO enrichment and co-expression network analyses implied that the combined immunomodulation of ACh and ENK might be mediated by p53, EGF-R–ErbB, and Fc gamma R (FcγR) signaling pathways. These results collectively indicated that multiple neurotransmitters executed a combined and ordered immune regulation through common signaling cascades in molluscs, which was under delicate control to maintain the homeostasis.

Oxidative stress: Noradrenaline as an integrator of responses in the neuroendocrine and immune systems of the ascidian Phallusia nigra

Neurotransmitters play key roles in regulating the homeostasis of organisms in stressful environments. Noradrenaline (NA) is the main neurotransmitter known to modulate immunological parameters, and is important in the crosstalk between the neuroendocrine and immune systems. In this study, using the ascidian Phallusia nigra, we analyzed the level of catecholamines (CA) in the plasma after mechanical stress, and the effect of NA on the oxidative stress (OS) displayed by immune cells. We measured the concentration of reactive oxygen species (ROS), and analyzed whether α- and/or β-adrenoreceptors (ARs) are involved in ROS modulation, lipid peroxidation (LPO), antioxidant capacity against peroxyl radicals (ACAP), and activity of the enzymes catalase (CAT) and glutathione S transferase (GST) in immune cells after incubation with different concentrations of NA, with or without zymosan (ZnA) challenge. The results showed that NA reduced ROS production, even in immune cells challenged with ZnA, and that this modulation occurred through α₁-and β₁-ARs. ACAP levels showed different responses, depending on whether immune cells were challenged or not with ZnA, and also depending on the NA concentration: 1.0 μM NA increased ACAP levels, but 10.0 μM reduced ACAP levels. NA enhanced the activity of CAT and GST in ZnA-challenged and non-challenged immune cells, while 1.0 and 10.0 μM NA effectively reduced LPO. Taken together, these results show that NA can protect cells from ROS damage, decreasing ROS production and LPO, and enhancing ACAP as well as the activity of CAT and GST. The approach used here with this model contributes to understanding the relationship between the neuroendocrine and immune systems, revealing new effects of NA on OS regulation in ascidians.

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.

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

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

A membrane-bound dopamine β-hydroxylase highly expressed in granulocyte of Pacific oyster Crassostrea gigas

Dopamine β-hydroxylase (DBH) is one of key rate-limiting enzymes converting dopamine to norepinephrine. It locates not only in catecholaminergic neuron system, but also in immunocytes and plays roles in the immune response of vertebrates. However, the knowledge about the function of DBH in immune system is still very limited in invertebrates. In the present study, the DBH gene family with seven members was screened from Crassostrea gigas genome, and their mRNA expressions in various tissues were recorded. Among them, one DBH (designated CgDBH-1) with high expression level in oyster hemocytes was further characterized. The deduced amino acid sequence of CgDBH-1 was predicted to contain a transmembrane domain and shared 30.1% and 30.9% similarity with that in Mus musculus and Homo sapiens, respectively. CgDBH-1 was closely clustered with DBH from Aplysia californica in the phylogenetic tree. The recombinant protein of CgDBH-1 (rCgDBH-1) exhibited significant enzymatic activity (0.54 ± 0.019 pmol L^-1 min^-1) to synthesize norepinephrine. Importantly, the mRNA transcript of CgDBH-1 was highly expressed in oyster hemocytes, and the highest expression level was observed in granulocytes among the three types of hemocytes, which was 8.18-fold (p < 0.01) of that in agranulocytes. Moreover, the expression of CgDBH-1 in hemocytes was significantly increased at the late stage of immune response. The CgDBH-1 protein was mainly co-localized with the granules and endoplasmic reticulum (ER) of granulocytes. These results collectively suggested that CgDBH-1, as a novel molluscan norepinephrine synthesizing enzyme highly expressed in granulocytes, involved in the late-stage immune response of oysters, which provided vital insight to understand the crosstalk between neuroendocrine and immune systems in invertebrates.

Enzymes and non-enzymatic antioxidants responses to sequential cold stress in polymorphic noble scallop Chlamys nobilis with different total carotenoids content

Noble scallop, an economically important edible marine bivalve displays polymorphism in shells (golden and brown) and flesh colors (orange and white). Mass mortality of noble scallops usually occurs during the winter months. Interestingly, carotenoid-rich golden scallops demonstrated much higher survival rates than brown scallops in winter. In order to understand the response of polymorphic noble scallops to sequential cold stress, the present study aimed to investigate the enzyme and non-enzymatic antioxidant responses of golden and brown scallops under sequential cold stress. Parameters evaluated included total carotenoid content (TCC), fatty acid composition, total antioxidant capacity (TAC), methylenedioxyamphetamine (MDA) content, catalase (CAT) activity, and superoxide dismutase (SOD) enzyme activity. The results of the present study revealed that golden scallops have higher cold tolerance than brown scallops. Golden and brown scallops are well adapted to low water temperature of above 12 °C, but in areas where winter water temperatures are below 12 °C, golden scallops are more suitable for aquaculture than brown scallops. The findings of this study are crucial to understanding the physiological responses of polymorphic scallops to cold stress and identify suitable candidates for winter aquaculture.

Massive expansion and diversity of nicotinic acetylcholine receptors in lophotrochozoans

Background

Nicotinic acetylcholine receptors (nAChRs) are among the oldest and most conserved transmembrane receptors involved in signal transduction. Despite the prevalence and significance of cholinergic signaling, the diversity and evolution of nAChRs are not fully understood.

Result

By comparative genomic analysis, we found massive expansions of nAChR genes in molluscs and some other lophotrochozoans. The expansion is particularly pronounced in stationary bivalve molluscs with simple nervous systems, with the number of nAChR genes ranging from 99 to 217 in five bivalves, compared with 10 to 29 in five ecdysozoans and vertebrates. The expanded molluscan nAChR genes tend to be intronless and in tandem arrays due to retroposition followed by tandem duplication. Phylogenetic analysis revealed diverse nAChR families in the common ancestor of bilaterians, which subsequently experienced lineage-specific expansions or contractions. The expanded molluscan nAChR genes are highly diverse in sequence, domain structure, temporal and spatial expression profiles, implying diversified functions. Some molluscan nAChR genes are expressed in early development before the development of the nervous system, while others are involved in immune and stress responses.

Conclusion

The massive expansion and diversification of nAChR genes in bivalve molluscs may be a compensation for reduced nervous systems as part of adaptation to stationary life under dynamic environments, while in vertebrates a subset of specialized nAChRs are retained to work with advanced nervous systems. The unprecedented diversity identified in molluscs broadens our view on the evolution and function of nAChRs that are critical to animal physiology and human health.

Oxidative stress responses of golden and brown noble scallops Chlamys nobilis to acute cold stress

The noble scallop Chlamys nobilis is an important edible marine bivalve that is widely cultivated in the sea of southern China. Unfortunately, the mass mortality of noble scallops frequently occurs during the winter months. The present study investigated the effects of acute cold stress (8 °C) to the physiological responses of polymorphic noble scallops, by assessing the HSP70 gene expression, total carotenoid content (TCC), total antioxidant capacity (TAC), malondialdehyde (MDA) content, catalase (CAT) activity and superoxide dismutase (SOD) enzymatic activity in different tissues of golden and brown scallops. The results of the present study revealed that MDA, TCC and CAT increased drastically in most tissues in the early stage of acute cold stress (0-3 h), but TCC, SOD and CAT generally showed a downward trend. Within 3-6 h of acute cold stress, MDA content decreased in most tissues and the SOD content increased significantly in most tissues, while TCC and CAT remained at peak. After 6 h of acute cold stress, MDA content continued to increase in most tissues, while TCC, CAT, SOD and TAC decreased or remained at a lower level. For HSP70 expression, up-regulation of the HSP70 gene was observed only in mantle of brown scallops and hemolymph of golden scallops at 3 h and 24 h, respectively. The findings of the present study can better understand the physiological response of noble scallops to acute cold stress.

Transcriptomic analysis of α-synuclein knockdown after T3 spinal cord injury in rats

Background

Endogenous α-synuclein (α-Syn) is involved in many pathophysiological processes in the secondary injury stage after acute spinal cord injury (SCI), and the mechanism governing these functions has not been thoroughly elucidated to date. This research aims to characterize the effect of α-Syn knockdown on transcriptional levels after SCI and to determine the mechanisms underlying α-Syn activity based on RNA-seq.

Result

The establishment of a rat model of lentiviral vector-mediated knockdown of α-Syn in Sprague-Dawley rats with T3 spinal cord contusion (LV_SCI group). The results of the RNA-seq analysis showed that there were 337 differentially expressed genes (DEGs) between the SCI group and the LV_SCI group, and 153 DEGs specific to LV_SCI between the (SCI vs LV_SCI) and (SCI vs CON) comparisons. The top 20 biological transition terms were identified by Gene ontology (GO) analysis. The Kyoto Gene and Genomic Encyclopedia (KEGG) analysis showed that the LV_SCI group significantly upregulated the cholinergic synaptic & nicotine addiction and the neuroactive ligand receptor interaction signaling pathway. Enriched chord analysis analyzes key genes. Further cluster analysis, gene and protein interaction network analysis and RT-qPCR results showed that Chrm2 and Chrnb2 together significantly in both pathways. The proliferation of muscarinic cholinergic receptor subtype 2 (Chrm2) and nicotinic cholinergic receptor subtype β2 (Chrnb2), and the neurogenesis were elevated in the injury site of LV_SCI group by immunofluorescence. Further by subcellular localization, the LV_SCI group enhanced the expression of Chrnb2 at the cell membrane.

Conclusion

Knockdown of α-Syn after SCI enhance motor function and promote neurogenesis probably through enhancing cholinergic signaling pathways and neuroreceptor interactions. This study not only further clarifies the understanding of the mechanism of knockdown of α-Syn on SCI but also helps to guide the treatment strategy for SCI.

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.

Transcriptome analysis reveals the activation of neuroendocrine-immune system in shrimp hemocytes at the early stage of WSSV infection

Background

Functional communications between nervous, endocrine and immune systems are well established in both vertebrates and invertebrates. Circulating hemocytes act as fundamental players in this crosstalk, whose functions are conserved during the evolution of the main groups of metazoans. However, the roles of the neuroendocrine-immune (NEI) system in shrimp hemocytes during pathogen infection remain largely unknown.

Results

In this study, we sequenced six cDNA libraries prepared with hemocytes from Litopenaeus vannamei which were injected by WSSV (white spot syndrome virus) or PBS for 6 h using Illumina Hiseq 4000 platform. As a result, 3444 differentially expressed genes (DEGs), including 3240 up-regulated genes and 204 down-regulated genes, were identified from hemocytes after WSSV infection. Among these genes, 349 DEGs were correlated with innate immunity and categorized into seven groups based on their predictive function. Interestingly, 18 genes encoded putative neuropeptide precursors were induced significantly by WSSV infection. Furthermore, some genes were mapped to several typical processes in the NEI system, including proteolytic processing of prohormones, amino acid neurotransmitter pathways, biogenic amine biosynthesis and acetylcholine signaling pathway.

Conclusions

The data suggested that WSSV infection triggers the activation of NEI in shrimp, which throws a light on the pivotal roles of NEI system mediated by hemocytes in shrimp antiviral immunity.

Electronic supplementary material

The online version of this article (10.1186/s12864-019-5614-4) contains supplementary material, which is available to authorized users.

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

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

Transcriptional changes in the Japanese scallop (Mizuhopecten yessoensis) shellinfested by Polydora provide insights into the molecular mechanism of shell formation and immunomodulation

The Japanese scallop (Mizuhopecten yessoensis) is one of the most important aquaculture species in Asian countries; however, it has suffered severe infection by Polydora in northern China in recent years, causing great economic losses. The Polydora parasitizes the shell of scallops, badly destroying the shell's structure. To investigate the molecular response mechanism of M. yessoensis to Polydora infestion, a comprehensive and niche-targeted cDNA sequence database for diseased scallops was constructed. Additionally, the transcriptional changes in the edge mantle, central mantle and hemocytes, tissues directly related to the disease, were first described in this study. The results showed that genes involved in shell formation and immunomodulation were significantly differentially expressed due to the infestation. Different transcriptional changes existed between the two mantle regions, indicating the different molecular functions likely responsible for the formation of different shell layers. The differential expression of genes for immune recognition, signal transduction and pathogen elimination presented an integrated immune response process in scallops. Moreover, neuromodulation and glycometabolism involved in the regulation process with relevant function significantly enriched. The study provides valuable information for mechanism study of shell formation and immunomodulation in scallops.

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.