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

Cellular effects of microplastics are influenced by their dimension: Mechanistic relationships and integrated criteria for particles definition

The definition of microplastics (MPs) is nowadays too generic from a biological perspective, since different characteristics of these particles might influence their effects. To provide experimental evidence that size is an important factor to be considered, Mediterranean mussels Mytilus galloprovincialis were exposed to five size classes of polyethylene fragments (PE-MPs, 20-50 μm, 50-100 μm, 100-250 μm, 250-500 μm, 500-1000 μm). After 10 days of exposure, MPs ingestion and mechanistic relationships between particles size and cellular effects were analysed through a wide panel of biological alterations, including immune system responses, cholinergic function, antioxidant system, lipid metabolism and peroxidation. Results were further elaborated through a Weight of Evidence approach, summarizing the overall biological significance of obtained results in a hazard index based on the number and magnitude of variations and their toxicological relevance. PE-MPs 500-1000 μm were identified as the less biologically reactive size class due to the limited ingestion of particles coupled with the lack of biological effects, followed by PE-MPs 250-500 μm, which slightly altered the cholinergic function and lysosomal membranes. Conversely, PE-MPs smaller than 250 μm provoked a more consistent onset of biological alterations in terms of immune system composition and functioning, redox homeostasis, and lipid metabolism. The overall findings of this study highlight the importance of considering the size of particles for monitoring and risk assessment of MPs, introducing a more integrated evaluation of plastic pollution that, beside particles concentration, should adequately weigh those characteristics triggering the onset of biological effects.

A C-type lectin with a single carbohydrate-recognition domain (CRD) containing unique QPN/WDD motifs from Tegillarca granosa is involved in the innate immune defense

C-type lectins (CTLs), a superfamily of Ca2+-dependent carbohydrate-recognition proteins, serve as pattern recognition receptors (PRRs) in the immune response of many species. However, little is currently known about the CTLs of the commercially and ecologically important bivalve species, blood clam (Tegillarca granosa). In this study, a CTL (designated as TgCTL-1) with a single carbohydrate-recognition domain (CRD) containing unique QPN/WDD motifs was identified in the blood clam through transcriptome and whole-genome searching. Multiple alignment and phylogenetic analysis strongly suggested that TgCTL-1 was a new member of the CTL superfamily. Expression analysis demonstrated that TgCTL-1 was highly expressed in the hemocytes and visceral mass of the clam under normal condition. In addition, the expression of TgCTL-1 was shown to be significantly up-regulated upon pathogen challenge. Moreover, the recombinant TgCTL-1 (rTgCTL-1) displayed agglutinating and binding activities against both the gram-positive and gram-negative bacteria tested in a Ca2+-dependent manner. Furthermore, it was found that the in vitro phagocytic activity of hemocytes was significantly enhanced by rTgCTL-1. In general, our results showed that TgCTL-1 was an inducible acute-phase secretory protein, playing crucial roles in recognizing, agglutinating, and binding to pathogenic bacteria as well as modulating phagocytic activity of hemocytes in the innate immune defense of blood clam.

Potential pathway and mechanisms underlining the immunotoxicity of benzo[a]pyrene to Chlamys farreri

The long-distance migration of polycyclic aromatic hydrocarbons (PAHs) promotes their release into the marine environment, posing a serious threat to marine life. Studies have shown that PAHs have significant immunotoxicity effects on bivalves, but the exact mechanism of immunotoxicity remains unclear. This paper aims to investigate the effects of exposure to 0.4, 2, and 10 μg/L of benzo(a)pyrene (B[a]P) on the immunity of Chlamys farreri under environmental conditions, as well as the potential molecular mechanism. Multiple biomarkers, including phagocytosis rate, metabolites, neurotoxicity, oxidative stress, DNA damage, and apoptosis, were adopted to assess these effects. After exposure to 0.4, 2, and 10 μg/L B[a]P, obvious concentration-dependent immunotoxicity was observed, indicated by a decrease in the hemocyte index (total hemocyte count, phagocytosis rate, antibacterial and bacteriolytic activity). Analysis of the detoxification metabolic system in C. farreri revealed that B[a]P produced B[a]P-7,8-diol-9,10-epoxide (BPDE) through metabolism, which led to an increase in the expression of protein tyrosine kinase (PTK). In addition, the increased content of neurotransmitters (including acetylcholine, γ -aminobutyric acid, enkephalin, norepinephrine, dopamine, and serotonin) and related receptors implied that B[a]P might affect immunity through neuroendocrine system. The changes in signal pathway factors involved in immune regulation indicated that B[a]P interfered with Ca2+ and cAMP signal transduction via the BPDE-PTK pathway or neuroendocrine pathway, resulting in immunosuppression. Additionally, B[a]P induced the increase in reactive oxygen species (ROS) content and DNA damage, as well as an upregulation of key genes in the mitochondrial pathway and death receptor pathway, leading to the increase of apoptosis rate. Taken together, this study comprehensively investigated the detoxification metabolic system, neuroendocrine system, and cell apoptosis to explore the toxic mechanism of bivalves under B[a]P stress.

Hologenome analysis reveals independent evolution to chemosymbiosis by deep-sea bivalves

BACKGROUND

Bivalves have independently evolved a variety of symbiotic relationships with chemosynthetic bacteria. These relationships range from endo- to extracellular interactions, making them ideal for studies on symbiosis-related evolution. It is still unclear whether there are universal patterns to symbiosis across bivalves. Here, we investigate the hologenome of an extracellular symbiotic thyasirid clam that represents the early stages of symbiosis evolution.

RESULTS

We present a hologenome of Conchocele bisecta (Bivalvia: Thyasiridae) collected from deep-sea hydrothermal vents with extracellular symbionts, along with related ultrastructural evidence and expression data. Based on ultrastructural and sequencing evidence, only one dominant Thioglobaceae bacteria was densely aggregated in the large bacterial chambers of C. bisecta, and the bacterial genome shows nutritional complementarity and immune interactions with the host. Overall, gene family expansions may contribute to the symbiosis-related phenotypic variations in different bivalves. For instance, convergent expansions of gaseous substrate transport families in the endosymbiotic bivalves are absent in C. bisecta. Compared to endosymbiotic relatives, the thyasirid genome exhibits large-scale expansion in phagocytosis, which may facilitate symbiont digestion and account for extracellular symbiotic phenotypes. We also reveal that distinct immune system evolution, including expansion in lipopolysaccharide scavenging and contraction of IAP (inhibitor of apoptosis protein), may contribute to the different manners of bacterial virulence resistance in C. bisecta.

CONCLUSIONS

Thus, bivalves employ different pathways to adapt to the long-term co-existence with their bacterial symbionts, further highlighting the contribution of stochastic evolution to the independent gain of a symbiotic lifestyle in the lineage.

Gamma-aminobutyric acid (GABA) suppresses hemocyte phagocytosis by binding to GABA receptors and modulating corresponding downstream pathways in blood clam, Tegillarca granosa

Although accumulating data demonstrated that gamma-aminobutyric acid (GABA), an inhibitory neurotransmitter, plays an important regulatory role in immunity of vertebrates, its immunomodulatory function and mechanisms of action remain poorly understood in invertebrates such as bivalve mollusks. In this study, the effect of GABA on phagocytic activity of hemocytes was evaluated in a commercial bivalve species, Tegillarca granosa. Furthermore, the potential regulatory mechanism underpinning was investigated by assessing potential downstream targets. Data obtained demonstrated that in vitro GABA incubation significantly constrained the phagocytic activity of hemocytes. In addition, the GABA-induced suppression of phagocytosis was markedly relieved by blocking of GABA_A and GABA_B receptors using corresponding antagonists. Hemocytes incubated with lipopolysaccharides (LPS) and GABA had significant higher K⁺-Cl^- cotransporter 2 (KCC2) content compared to the control. In addition, GABA treatment led to an elevation in intracellular Cl^-, which was shown to be leveled down to normal by blocking the ionotropic GABA_A receptor. Treatment with GABA_A receptor antagonist also rescued the suppression of GABA_A receptor-associated protein (GABARAP), KCC, TNF receptor associated factor 6 (TRAF6), inhibitor of nuclear factor kappa-B kinase subunit alpha (IKKα), and nuclear factor kappa B subunit 1 (NFκB) caused by GABA incubation. Furthermore, incubation of hemocytes with GABA resulted in a decrease in cAMP content, an increase in intracellular Ca²⁺, and downregulation of cAMP-dependent protein kinase (PKA), calmodulin kinase II (CAMK2), calmodulin (CaM), calcineurin (CaN), TRAF6, IKKα, and NFκB. All these above-mentioned changes were found to be evidently relieved by blocking the metabotropic G-protein-coupled GABA_B receptor. Our results suggest GABA may play an inhibitory role on phagocytosis through binding to both GABA_A and GABA_B receptors, and subsequently regulating corresponding downstream pathways in bivalve invertebrates.

Marine heatwaves hamper neuro-immune and oxidative tolerance toward carbamazepine in Mytilus galloprovincialis

The increased frequency and intensity of short-term extreme warming phenomena have been associated to harsh biological and ecosystem outcomes (i.e., mass mortalities in marine organisms). Marine heatwaves (MHWs), occurring when seasonal temperature threshold is exceeded for at least 5 consecutive days, may reduce the tolerance of coastal species toward additional pressures, but interactions between such multiple stressors are virtually unexplored. The present study aimed to characterize in Mytilus galloprovincialis the influence of a simulated MHW scenario on the toxicological effects of the pharmaceutical carbamazepine (CBZ), ubiquitously detected in the marine environment and chosen as model compound for this relevant class of emerging contaminants. The bioaccumulation of CBZ and responsiveness of various biological parameters, including immune system, antioxidant status, lipid metabolism and cellular integrity, were analyzed in exposed mussels both during and after the end of the heatwave. MHW appeared to strongly modulate accumulation of CBZ, paralleled by weakened immunocompetence and onset of oxidative disturbance that finally evolved to cellular damages and lipid metabolism disorders. Elaboration of the overall results through a quantitative Weight of Evidence model, revealed the highest hazard in organisms exposed to both the stressors 10 days after the end of the heatwave, suggesting that MHWs could leave a footprint on the capability of mussels to counteract CBZ toxicity, thus affecting their vulnerability and predisposition to adverse effects toward multiple stressors.

Immunotoxicity pathway and mechanism of benzo[a]pyrene on hemocytes of Chlamys farreri in vitro

Benzo[a]pyrene (B[a]P), a typical PAHs widely existing in the marine environment, has been extensively studied for its immunotoxicity due to its persistence and high toxicity. Nevertheless, the immunotoxicity mechanism remain incompletely understood. In this study, isolated hemocytes of Chlamys farreri were exposed at three concentrations of B[a]P (5, 10 and 15 μg/mL), and the effects of B[a]P on detoxification metabolism, signal transduction, humoral immune factors, exocytosis and phagocytosis relevant proteins and immune function at 0, 6, 12, 24 h were studied. Results illustrated the AhR, ARNT and CYP1A1 were significantly induced by B[a]P at 12 h. Additionally, the content of B[a]P metabolite BPDE increased in a dose-dependent manner with pollutants. Under B[a]P stimulation, the expressions of PTK (Src, Fyn) and PLC-Ca²⁺-PKC pathway gene increased significantly, while the transcription level of AC-cAMP-PKA pathway gene decreased remarkably. Additionally, the expressions of nuclear transcription factors (CREB, NF-κB), complement system genes and C-type lectin genes up-regulated obviously. The gene expressions of phagocytosis and exocytosis related proteins were also notably affected. 5 μg/mL B[a]P could promote phagocytosis in a transitory time, but with the increase of exposure time and concentration of B[a]P, the phagocytosis, antibacterial and bacteriolytic activities gradually decreased. These results indicated that similar to vertebrates, BPDE, the metabolite of B[a]P, mediated downstream signal transduction via PTK in bivalves. The declined of the immune defense ability of hemocytes might be closely related to the inhibition of AC-cAMP-PKA pathway and the imbalance of intracellular Ca²⁺ pathway. In addition, the results manifested that complement and lectin systems play a significant role in regulating immune response. In this study, the direct relationship between detoxification metabolism and immune signal transduction in bivalves under B[a]P stress was demonstrated for the first time, which provided important information for the potential molecular mechanism of B[a]P-induced immune system disorder in bivalves.

B-type allatostatin regulates immune response of hemocytes in mud crab Scylla paramamosain

B-type allatostatins (AST-B) are neuropeptides that have important physiological roles in arthropods, they have also been identified in a number of crustacean species. Recent research on neuroendocrine-immune (NEI) regulatory system in invertebrates has exploded, it reveals that the NEI network plays an indispensable role in optimizing the immune response and maintaining homeostasis. Herein, mud crab Scylla paramamosain provides a primitive and ancient model to study crosstalk between the neuroendocrine and immune systems. In this study, qRT-PCR analysis showed that the nervous system was the main production site for Sp-AST-B mRNA in S. paramamosain, while its receptor gene (Sp-AST-BR) mRNA could be detected in all the analyzed tissues including hemocytes. This reveals that AST-B might act as a pleiotropic neuropeptide. In situ hybridization further confirmed that granular cells of hemocyte subpopulations express Sp-AST-BR. Time-course analysis revealed that bacteria-analog LPS or virus-analog Poly (I:C) challenge significantly induced Sp-AST-B expression in the thoracic ganglion, and the expression of Sp-AST-BR in hemocytes were also positively changed. Furthermore, mud crabs treated with a synthetic AST-B peptide significantly increased the mRNA levels of AST-BR, nuclear factor-κB (NF-κB) transcription factor (Dorsal and Relish), pro-inflammatory cytokine (IL-16) and immune-effector molecules, and also dramatically enhanced the nitric oxide (NO) production and phagocytic activity in hemocytes. Meanwhile dsRNA-mediated knockdown of Sp-AST-B remarkably suppressed the NO concentrations, phagocytic activity and the expression of immune related genes, resulting in markedly impaired ability of crabs to inhibit bacterial proliferation in vivo. Combined, these data demonstrate that AST-B induced innate immune in the mud crab.

Microplastics aggravate the bioaccumulation of three veterinary antibiotics in the thick shell mussel Mytilus coruscus and induce synergistic immunotoxic effects

Marine bivalves living in pollution-prone coastal areas may be simultaneously coexposed to veterinary antibiotic residuals and microplastics (MPs). However, the synergistic effects of these two types of emergent pollutants have not been fully elucidated in mussel species. Therefore, the immunotoxic effects of the three representative antibiotics, oxytetracycline (OTC, 270 ng/L), florfenicol (FLO, 42 ng/L), and sulfamethoxazole (SMX, 140 ng/L), with and without the copresence of polystyrene MPs (0.26 mg/L, dimeter: 500 nm), were investigated in the thick shell mussel. Our data showed that the immunity was significantly hampered by exposure to the pollutants and MP-antibiotic coexposure induced synergistic immunotoxicity. For instance, compared to those treated with antibiotics (OTC, FLO, and SMX) alone, mussels coexposed to antibiotic and MPs had significantly lower phagocytic rate (further decline by approximately 28.80%, 34.21%, and 11.22%, respectively) and total hemocyte count (further reduced by approximately 37.45%, 61.67%, and 46.32%, respectively). Exposure to the pollutants tested also led to inductions in intracellular reactive oxygen species (ROS), decreases in the F-actin cytoskeleton, declines in the cell viability of hemocytes, and downregulation of cytoskeleton- and immune-related genes. In addition, mussels coexposed to antibiotic-MP accumulated significantly greater amounts of antibiotics, which may partially explain the synergic immunotoxic effect detected. Exposure to pollutants tested also led to suppression in the activity of glutathione-S-transferase (GST) and downregulation of detoxification-related genes whereas induction in the level of lipid peroxidation (indicated by MDA content) in gills, which may facilitate the entry whereas constrain the exclusion of antibiotics and therefore result in an elevation in accumulation of antibiotics.

Fine polystyrene microplastics render immune responses more vulnerable to two veterinary antibiotics in a bivalve species

Living in close proximity to the sediment of coastal areas, bivalves may be exposed to veterinary antibiotic residuals and microplastics (MPs) simultaneously. However, the immunotoxic impacts of veterinary antibiotics remain unknown in bivalves, let alone their interactions with MPs. Therefore, the immune responses of two representative veterinary antibiotics, oxytetracycline and florfenicol, was investigated in a bivalve species, the blood clam (Tegillarca granosa). The effects of the copresence of MPs on the immune responses triggered by these antibiotics were also analyzed. Results showed that exposure to antibiotics investigated led to significant alteration in hematic parameters and reduction in lectin content in serum. In addition to inducing ROS production, aggravating lipid peroxidation and DNA damage, and suppressing the hemocyte viability, antibiotic treatments also downregulated the expression of immune- and detoxification-related genes but upregulated apoptosis-related Caspase-3. Furthermore, the toxic impacts of antibiotics were found to be significantly increased by the copresence of MPs.

Modulatory function of calmodulin on phagocytosis and potential regulation mechanisms in the blood clam Tegillarca granosa

Unlike vertebrate species, invertebrates lack antigen-antibody mediated immune response and mainly rely on haemocyte phagocytosis to fight against pathogen infection. Recently, studies conducted in model vertebrates demonstrated that the multifunctional protein calmodulin (CaM) plays an important role in regulating immune responses. However, the intrinsic relation between CaM and phagocytosis process remains poorly understood in invertebrate species such as bivalve mollusks. Therefore, in the present study, the immunomodulatory function of CaM on haemocyte phagocytosis was verified in the blood clam, Tegillarca granosa, using the CaM-specific inhibitor N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide hydrochloride (W-7). Results obtained show that CaM inhibition significantly suppressed the phagocytic activity of haemocytes. In addition, CaM inhibition constrained intracellular Ca²⁺ elevation, hampered actin cytoskeleton assembly, suppressed calcineurin (CaN) activity, and disrupted NF-κB activation in haemocytes upon LPS induction. Furthermore, expression of seven selected genes from the actin cytoskeleton regulation- and immune-related pathways were significantly downregulated whereas those of CaM and CaN from the Ca²⁺-signaling pathway were significantly upregulated by in vitro incubation of haemocytes with W-7. For the first time, the present study demonstrated that CaM play an important role in phagocytosis modulation in bivalve species. In addition, the intracellular Ca²⁺ and downstream Ca²⁺-signaling-, actin cytoskeleton regulation-, and immune-related pathways offer candidate routes through which CaM modulates phagocytosis.