Induction of apoptosis by UV in the flat oyster, Ostrea edulis

Effects of ambient UVB light on Pacific oyster Crassostrea gigas mantle tissue based on multivariate data

Ambient ultraviolet radiation (UVB) from solar and artificial light presents serious environmental risks to aquatic ecosystems. The Pacific oyster, Crassostrea gigas, perceives changes in the external environment primarily through its mantle tissue, which contains many nerve fibers and tentacles. Changes within the mantles can typically illustrate the injury of ambient UVB. In this study, a comprehensive analysis of phenotypic, behavioral, and physiological changes demonstrated that extreme UVB radiation (10 W/m²) directly suppressed the behavioral activities of C. gigas. Conversely, under ambient UVB radiation (5 W/m²), various physiological processes exhibited significant alterations in C. gigas, despite the behavior remaining relatively unaffected. Using mathematical model analysis, the integrated analysis of the full-length transcriptome, proteome, and metabolome showed that ambient UVB significantly affected the metabolic processes (saccharide, lipid, and protein metabolism) and cellular biology processes (autophagy, apoptosis, oxidative stress) of the C. gigas mantle. Subsequently, using Procrustes analysis and Pearson correlation analysis, the association between multi-omics data and physiological changes, as well as their biomarkers, revealed the effect of UVB on three crucial biological processes: activation of autophagy signaling (key factors: Ca2+, LC3B, BECN1, caspase-7), response to oxidative stress (reactive oxygen species, heat shock 70, cytochrome c oxidase), and recalibration of energy metabolism (saccharide, succinic acid, translation initiation factor IF-2). These findings offer a fresh perspective on the integration of multi-data from non-model animals in ambient UVB risk assessment.

Benzotriazole UV stabilizer-induced genotoxicity in freshwater benthic clams: A survey on apoptosis, oxidative stress, histopathology and transcriptomics

Benzotriazole UV stabilizer-329 (UV-329) is frequently detected in various environmental and biological matrices. However, the toxicity effect on freshwater benthos induced by UV-329 has rarely been described. In this study, genotoxicity, apoptosis, oxidative stress, histopathological alterations, siphoning behavior, and bioaccumulation in the gill and digestive gland of Corbicula fluminea exposed to UV-329 at 10, 100, and 1000 μg/L for 21 days were investigated. Toxicity screening using transcriptomics confirmed that UV-329 preferentially stimulated cellular process-related pathways including gap junctions, apoptosis, phagosomes and necroptosis. The transcript levels of a large number of apoptosis genes were significantly upregulated. This apoptosis mechanism was further confirmed by the fact that UV-329 exposure significantly increased the percentage of apoptotic cells, activated caspase-3, -8, and -9, and affected the antioxidant enzyme activities. Following exposure to 1000 μg/L UV-329, significant histological alterations were reflected in the corrosion of cilia, cellular swelling of epithelial cells in the gills, degeneration of digestive tubules, and necrosis of epithelial cells in the digestive glands. These results may aid in elucidating the toxicity mechanism of UV329 in bivalves and evaluating the hazards of UV-329 in benthic ecosystems.

Effects of elevated temperature on 8-OHdG expression in the American oyster (Crassostrea virginica): Induction of oxidative stress biomarkers, cellular apoptosis, DNA damage and γH2AX signaling pathways

Global temperature is increasing due to anthropogenic activities and the effects of elevated temperature on DNA lesions are not well documented in marine organisms. The American oyster (Crassostrea virginica, an edible and commercially important marine mollusk) is an ideal shellfish species to study oxidative DNA lesions during heat stress. In this study, we examined the effects of elevated temperatures (24, 28, and 32 °C for one-week exposure) on heat shock protein-70 (HSP70, a biomarker of heat stress), 8‑hydroxy-2'-deoxyguanosine (8-OHdG, a biomarker of pro-mutagenic DNA lesion), double-stranded DNA (dsDNA), γ-histone family member X (γH2AX, a molecular biomarker of DNA damage), caspase-3 (CAS-3, a key enzyme of apoptotic pathway) and Bcl-2-associated X (BAX, an apoptosis regulator) protein and/or mRNA expressions in the gills of American oysters. Immunohistochemical and qRT-PCR results showed that HSP70, 8-OHdG, dsDNA, and γH2AX expressions in gills were significantly increased at high temperatures (28 and 32 °C) compared with control (24°C). In situ TUNEL analysis showed that the apoptotic cells in gill tissues were increased in heat-exposed oysters. Interestingly, the enhanced apoptotic cells were associated with increased CAS-3 and BAX mRNA and/or protein expressions, along with 8-OHdG levels in gills after heat exposure. Moreover, the extrapallial (EP) fluid (i.e., extracellular body fluid) protein concentrations were lower; however, the EP glucose levels were higher in heat-exposed oysters. Taken together, these results suggest that heat shock-driven oxidative stress alters extracellular body fluid conditions and induces cellular apoptosis and DNA damage, which may lead to increased 8-OHdG levels in cells/tissues in oysters.

Bivalve Haemocyte Subpopulations: A Review

Bivalve molluscs stand out for their ecological success and their key role in the functioning of aquatic ecosystems, while also constituting a very valuable commercial resource. Both ecological success and production of bivalves depend on their effective immune defence function, in which haemocytes play a central role acting as both the undertaker of the cellular immunity and supplier of the humoral immunity. Bivalves have different types of haemocytes, which perform different functions. Hence, identification of cell subpopulations and their functional characterisation in immune responses is essential to fully understand the immune system in bivalves. Nowadays, there is not a unified nomenclature that applies to all bivalves. Characterisation of bivalve haemocyte subpopulations is often combined with 1) other multiple parameter assays to determine differences between cell types in immune-related physiological activities, such as phagocytosis, oxidative stress and apoptosis; and 2) immune response to different stressors such as pathogens, temperature, acidification and pollution. This review summarises the major and most recent findings in classification and functional characterisation of the main haemocyte types of bivalve molluscs.

Inhibition of ultraviolet-induced sea cucumber (Stichopus japonicus) autolysis by maintaining coelomocyte intracellular calcium homeostasis

Apoptosis plays a critical role in sea cucumber autolysis. To investigate the ultraviolet (UV)-induced apoptosis, sea cucumbers with and without injection of BAPTA-AM (cytosolic calcium chelator) were exposed to UV (15 W/m²) for 30 min. The results showed that UV irradiation caused several changes in sea cucumber coelomocytes, including calcium imbalance, abnormal morphology of endoplasmic reticulum, upregulation of pro-apoptotic proteins CRT, CHOP, and caspases 9 and 3, and downregulation of anti-apoptotic protein Bcl-2. A comparison between the two groups showed that injection of the calcium chelator into sea cucumbers helped maintain coelomocyte intracellular calcium homeostasis and suppressed other abnormal changes caused by ER stress, indicating apoptosis in sea cucumbers is mediated by calcium imbalance and follows the activation of the ER stress pathway. Therefore, this study broadens understanding of the apoptotic mechanism involved in sea cucumber autolysis, which is helpful in developing preservative agents for sea cucumbers.

Elucidating multilevel toxicity response differences between tris(1,3-dichloro-2-propyl) phosphate and its primary metabolite in Corbicula fluminea

Tris(1,3-dichloro-2-propyl) phosphate (TDCIPP) and its primary metabolite, bis(1,3-dichloro-2-propyl) phosphate (BDCIPP) are frequently detected in aquatic environments. However, information regarding the biotoxicity of these compounds to bivalves is limited. We explored the multilevel physiological responses of Corbicula fluminea to TDCIPP and BDCIPP. The results indicated that TDCIPP/BDCIPP bioaccumulation in bivalves was positively correlated with their hydrophobicity. Furthermore, the higher body burden of TDCIPP in digestive glands led to significantly higher levels of ethoxyresorufin-O-deethylase (EROD), glutathione S-transferase (GST), and P-glycoprotein (p < 0.05). Owing to different molecular structures of inducers, upregulations of cyp4, gstm1, and abcb1 mRNA exhibited different sensitivities to TDCIPP and BDCIPP. Although Phase-I and Phase-II biotransformation and the multixenobiotic resistance (MXR) system were activated to protect bivalves from TDCIPP or BDCIPP, digestive glands produced large amounts of reactive oxygen species (ROS). Moreover, oxidative stress, the percentage of apoptotic cells in digestive glands, and inhibition of siphoning behaviour in TDCIPP treatments were higher than those in BDCIPP treatments (p < 0.05), indicating that TDCIPP was more toxic to bivalves than BDCIPP. Lower bioaccumulation and rapid metabolism of BDCIPP in vivo may contribute to alleviating its toxicity. This research establishes a foundation for further understanding the differences between the toxic mechanisms of TDCIPP and its metabolites.

Cellular and molecular complementary immune stress markers for the model species Dreissena polymorpha

This study aimed to combine cellular and molecular analyses for better detail the effects of various stresses on a sentinel species of freshwater invertebrate. For this purpose, the hemocytes of the zebra mussel, Dreissena polymorpha, were exposed to different stresses at two different intensities, high or low: chemical (cadmium and ionomycin), physical (ultraviolet B), or biological ones (Cryptosporidium parvum and Toxoplasma gondii). After exposure, flow cytometry and droplet digital PCR analyses were performed on the same pools of hemocytes. Several responses related to necrosis, apoptosis, phagocytosis, production of nitric oxide and expression level of several genes related to the antioxidant, detoxification and immune systems were evaluated. Results showed that hemocyte integrity was compromised by both chemical and physical stress, and cellular markers of phagocytosis reacted to ionomycin and protozoa. While cadmium induced oxidative stress and necrosis, ionomycin tends to modulate the immune response of hemocytes. Although both biological stresses led to a similar immune response, C. parvum oocysts induced more effects than T. gondii, notably through the expression of effector caspases gene and an increase in hemocyte necrosis. This suggests different management of the two protozoa by the cell. This work provides new knowledge of biomarkers in the zebra mussel, at both cellular and molecular levels, and contributes to elucidate the mechanisms of action of different kinds of stress in this species.

Applications of flow cytometry in molluscan immunology: Current status and trends

Flow cytometry (FCM) is routinely used in fundamental and applied research, clinical practice, and clinical trials. In the last three decades, this technique has also become a routine tool used in immunological studies of molluscs to analyse physical and chemical characteristics of haemocytes. Here, we briefly review the current implementation of FCM in the field of molluscan immunology. These applications cover a diverse range of practices from straightforward total cell counts and cell viability to characterize cell subpopulations, and further extend to analyses of DNA content, phagocytosis, oxidative stress and apoptosis. The challenges and prospects of FCM applications in immunological studies of molluscs are also discussed.

Involvement of apoptosis in the dialogue between the parasite Bonamia ostreae and the flat oyster Ostrea edulis

The protozoan parasite Bonamia ostreae has been associated with the decline of flat oyster Ostrea edulis populations in some European countries. Control of shellfish diseases mostly relies on prevention measures including transfer restrictions and stock management measures such as breeding programmes. These prevention and mitigation measures require a better understanding of interactions between host and pathogens. Previous in vitro studies allowed identifying apoptosis as a mechanism activated by the flat oyster in response to B. ostreae. However, these experiments also suggested that the parasite is able to regulate apoptosis in order to survive and multiply within hemocytes. By simplifying the conditions of infection, in vitro studies allow identifying most distinct features of the response of the host. In order to appreciate the relative importance of apoptosis in this response at the oyster scale, in vivo trials were carried out by injecting with parasites oysters from two French locations, Quiberon Bay (Brittany) and Diana Lagoon (Corsica). Apoptosis was investigated on pools of hemolymph from oysters collected at early and later times after injection using previously developed tools. Apoptotic cellular activities including intracytoplasmic calcium concentration, mitochondrial membrane potential and phosphatidyl serine externalization were analysed using flow cytometry. Moreover, the expression of flat oyster genes involved in both extrinsic and intrinsic pathways was measured using real time quantitative PCR.

Elevated temperature attenuates ovarian functions and induces apoptosis and oxidative stress in the American oyster, Crassostrea virginica: potential mechanisms and signaling pathways

Global climate change is predicted to intensify thermal stress in marine and coastal organisms, affecting their development, growth, and reproductive functions. In this study, we performed histological observations on ovarian development, immunohistochemical analyses of ovarian heat shock protein-70 (HSP70), nitrotyrosine protein (NTP, an indicator of reactive nitrogen species (RNS)), and dinitrophenyl protein (DNP, an indicator of protein oxidation) expressions, in situ TUNEL assay for cellular apoptosis, biochemical analyses of ovarian caspase-3/7 activity and protein carbonyl (PC, a measure of reactive oxygen species (ROS)) contents, nitrate/nitrite (NOx) levels, and extrapallial fluid (EPF, an important body fluid) pH in the American oyster, Crassostrea virginica. Oysters were exposed to medium (28 °C) and high (32 °C) temperatures under controlled laboratory conditions for 1 week. Oysters exposed to higher temperatures significantly decreased the number and diameter of eggs, and EPF protein concentrations compared with controls (24 °C). In contrast, EPF pH, ovarian HSP70 mRNA levels, and protein expression were increased after heat exposure, consistent with increased ovarian apoptosis. The enhanced apoptosis in ovaries was associated with increased ovarian caspase-3/7 activity, PC contents, NOx levels, and NTP and DNP expressions in heat-exposed oysters. Collectively, these results suggest that higher temperatures drastically increase RNS and ROS levels, increasing incidence of apoptosis and subsequently reducing ovarian functions in oysters.

Copper-induced immunomodulation in mussel (Perna canaliculus) haemocytes

Copper is a common contaminant in aquatic environments, which may cause physiological dysfunction in marine organisms. However, the toxicity mechanisms of copper in marine bivalves is not fully understood. In this study, we applied an integrated approach that combines flow cytometry and Gas Chromatography-Mass Spectrometry (GC-MS)-based metabolomics to characterize cellular and molecular mechanisms of copper immunotoxicity in New Zealand Greenshell™ mussel (Perna canaliculus) haemolymph. Flow cytometric results showed significant increases in haemocyte mortality, production of reactive oxygen species and apoptosis (via alteration of caspase 3/7 and mitochondrial membrane potential) of haemocytes exposed to increasing total concentrations of Cu2+ (62.5, 125.0 and 187.5 μM) compared to a low Cu2+ concentration (25.0 μM) and control (0.0 μM). In addition to flow cytometric data, our metabolomics results showed alterations of 25 metabolites within the metabolite profile of Cu2+-exposed haemolymph (125 μM) compared to those of control samples. Changes in levels of these metabolites may be considered important signatures of oxidative stress (e.g., glutathione) and apoptosis processes (e.g., alanine, glutamic acid). This study provides insights into the cellular and molecular mechanisms of oxidative stress and apoptosis in marine bivalves and highlights the applicability and reliability of metabolomic techniques for immunotoxicological studies in marine organisms.

Molecular and cellular characterization of apoptosis in flat oyster a key mechanisms at the heart of host-parasite interactions

Bonamia ostreae has been associated with the decline of flat oyster Ostrea edulis populations in some European countries. This obligatory intracellular parasite persists and multiplies into hemocytes. Previous in vitro experiments showed that apoptosis is activated in hemocytes between 1 h and 4 h of contact with the parasite. The flat oyster uses the apoptosis pathway to defend against B. ostreae. However, the parasite might be also able to modulate this response in order to survive in its host. In order to investigate this hypothesis the apoptotic response of the host was evaluated using flow cytometry, transmission electron microscopy and by measuring the response of genes involved in the apoptotic pathway after 4 h. In parallel, the parasite response was investigated by measuring the expression of B. ostreae genes involved in different biological functions including cell cycle and cell death. Obtained results allow describing molecular apoptotic pathways in O. edulis and confirm that apoptosis is early activated in hemocytes after a contact with B. ostreae. Interestingly, at cellular and molecular levels this process appeared downregulated after 44 h of contact. Concurrently, parasite gene expression appeared reduced suggesting that the parasite could inhibit its own metabolism to escape the immune response.

Effect of UVB radiation exposure in the expression of genes and proteins related to apoptosis in freshwater prawn embryos

Our previous studies showed that embryos of the freshwater prawn Macrobrachium olfersii exposed to ultraviolet B (UVB) radiation exhibited DNA damage, excessive ROS production, mitochondrial dysfunction and increased hsp70 expression, which are able, independently or together, to induce apoptosis. Thus, we attempted to elucidate some key apoptosis-related genes (ARG) and apoptosis-related proteins (ARP) and their expression during different stages of embryonic development, as well as to characterize the chronology of ARG expression and ARP contents after UVB radiation insult. We demonstrate that p53, Bax and Caspase3 genes are active in the embryonic cells at early embryonic developmental stages, and that the Bcl2 gene is active from the mid-embryonic stage. After UVB radiation exposure, we found an increase in ARP such as p53 and Bak after 3h of exposure. Moreover, an increase in ARG transcript levels for p53, Bax, Bcl2 and Caspase3 was observed at 6h after UVB exposure. Then, after 12h of UVB radiation exposure, an increase in Caspase3 gene expression and protein was observed, concomitantly with an increased number of apoptotic cells. Our data reveal that ARG and ARP are developmentally regulated in embryonic cells of M. olfersii and that UVB radiation causes apoptosis after 12h of exposure. Overall, we demonstrate that embryonic cells of M. olfersii are able to active the cell machinery against environmental changes, such as increased incidence of UVB radiation in aquatic ecosystems.

The paralytic shellfish toxin, saxitoxin, enters the cytoplasm and induces apoptosis of oyster immune cells through a caspase-dependent pathway

Exposure of the toxin-producing dinoflagellate Alexandrium catenella (A. catenella) was previously demonstrated to cause apoptosis of hemocytes in the oyster species Crassostrea gigas. In this work, a coumarin-labeled saxitoxin appeared to spread throughout the cytoplasm of the hemocytes. PSTs, including saxitoxin, were also shown to be directly responsible for inducing apoptosis in hemocytes, a process dependent on caspase activation and independent of reactive oxygen species (ROS) production. A series of in vitro labelling and microscopy experiments revealed that STX and analogs there of induced nuclear condensation, phosphatidylserine exposure, membrane permeability, and DNA fragmentation of hemocytes. Unlike in vertebrates, gonyautoxin-5 (GTX5), which is present in high concentrations in A. catenella, was found to be more toxic than saxitoxin (STX) to oyster immune cells. Altogether, results show that PSTs produced by toxic dinoflagellates enter the cytoplasm and induce apoptosis of oyster immune cells through a caspase-dependent pathway. Because of the central role of hemocytes in mollusc immune defense, PST-induced death of hemocytes could negatively affect resistance of bivalve molluscs to microbial infection.

Conservation and divergence of mitochondrial apoptosis pathway in the Pacific oyster, Crassostrea gigas

Apoptosis is considered a crucial part of the host defense system in oysters according to previous reports; however, the exact process by which this occurs remains unclear. Besides, mitochondrial apoptosis is the primary method of apoptosis in vertebrate cells, but has been poorly studied in invertebrates and is quite controversial. In this study, we investigated the molecular mechanism of mitochondrial apoptosis in the Pacific oyster Crassostrea gigas. Notably, we show that most key elements involved in the vertebrate mitochondrial apoptosis pathway – including mitochondrial outer membrane permeabilization, cytochrome c release, and caspase activation – are also present in C. gigas. In contrast, the lack of Bcl-2 homology 3-only subfamily members and apoptotic protease activating factor-1 (APAF-1) protein revealed evolutionary diversity from other phyla. Our results support that mitochondrial apoptosis in animals predates the emergence of vertebrates, but suggest that an unexpectedly diverse mitochondrial apoptosis pathway may exist in invertebrates. In addition, our work provided new clues for an improved understanding of how bivalve acclimate themselves to an inconstant environment.

Haemocytes collected from experimentally infected Pacific oysters, Crassostrea gigas: Detection of ostreid herpesvirus 1 DNA, RNA, and proteins in relation with inhibition of apoptosis

Recent transcriptomic approaches focused on anti-viral immunity in molluscs lead to the assumption that the innate immune system, such as apoptosis, plays a crucial role against ostreid herpesvirus type 1 (OsHV-1), infecting Pacific cupped oyster, Crassostrea gigas. Apoptosis constitutes a major mechanism of anti-viral response by limiting viral spread and eliminating infected cells. In this way, an OsHV-1 challenge was performed and oysters were monitored at three times post injection to investigate viral infection and host response: 2h (early after viral injection in the adductor muscle), 24h (intermediate time), and 48h (just before first oyster mortality record). Virus infection, associated with high cumulative mortality rates (74% and 100%), was demonstrated in haemocytes by combining several detection techniques such as real-time PCR, real-time RT PCR, immunofluorescence assay, and transmission electron microscopy examination. High viral DNA amounts ranged from 5.46×104 to 3.68×105 DNA copies ng-1 of total DNA, were detected in dead oysters and an increase of viral transcripts was observed from 2, 24, and 48hpi for the five targeted OsHV-1 genes encoding three putative membrane proteins (ORFs 25, 41, and 72), a putative dUTPase (ORF 75), and a putative apoptosis inhibitor (ORF 87). Apoptosis was studied at molecular and cellular levels with an early marker (phosphatidyl-serine externalisation measured by flow cytometry and epifluorescence microscopy) and a later parameter (DNA fragmentation by terminal deoxynucleotidyltransferase-mediated dUTP nick end labeling assay (TUNEL)). The down-regulation of genes encoding proteins involved in the activation of the apoptotic pathway (TNF and caspase 3) and the up-regulation of genes encoding anti-apoptotic proteins (IAP-2, and Bcl-2) suggested an important anti-apoptosis phenomenon in haemocytes from OsHV-1 infected oysters at 24 and 48hpi. Additionally, more phosphatidyl-serines were externalized and more cells with DNA fragmentation were observed in haemocytes collected from artificial seawater injected oysters than in haemocytes collected from OsHV-1 infected oysters at 24 and 48hpi, suggesting an inhibition of the apoptotic process in presence of the virus. In conclusion, this study is the first to focus on C. gigas haemocytes, cells involved in the host immune defense, during an OsHV-1 challenge in controlled conditions by combining various and original approaches to investigate apoptosis at molecular and cellular levels.

Flat oyster follows the apoptosis pathway to defend against the protozoan parasite Bonamia ostreae

The in vitro model Ostrea edulis hemocyte - Bonamia ostreae is interesting to investigate host-parasite interactions at the cellular level. Indeed, this unicellular parasite infects the flat oyster Ostrea edulis and multiplies within hemocytes, the central effectors of oyster defenses. Apoptosis is a mechanism used by many organisms to eliminate infected cells. In order to study the potential involvement of this mechanism in the oyster response to B. ostreae, in vitro experiments were carried out by exposing hemocytes from the naturally susceptible oyster O. edulis and a resistant oyster species Crassostrea gigas to live and heat-inactivated parasites. Hemocyte apoptotic response was measured using a combination of flow cytometry and microscopy analyses. Whatever the host species was, the parasite was engulfed in hemocytes and induced an increase of apoptotic parameters including intracytoplasmic calcium concentration, mitochondrial membrane potential or phosphatidyl-serine externalization as well as ultrastructural modifications. However, the parasite appears more able to infect flat oyster than cupped oyster hemocytes and the apoptotic response was more important against live than dead parasites in the natural host than in C. gigas. Our results suggest that O. edulis specifically responds to B. ostreae by inducing apoptosis of hemocytes.