The Pacific oyster Crassostrea gigas is a sessile bivalve mollusc whose homeostasis relies, at least partially, upon cells circulating in hemolymph and referred to as hemocytes. Oyster’s hemocytes have been reported to produce reactive oxygen species (ROS), even in absence of stimulation. Although ROS production in bivalve molluscs is mostly studied for its defence involvement, ROS may also be involved in cellular and tissue homeostasis. ROS sources have not yet been described in oyster hemocytes. The objective of the present work was to characterize the ROS sources in unstimulated hemocytes. We studied the effects of chemical inhibitors on the ROS production and the mitochondrial membrane potential (Δψm) of hemocytes. First, this work confirmed the specificity of JC-10 probe to measure Δψm in oyster hemocytes, without being affected by ΔpH, as reported in mammalian cells. Second, results show that ROS production in unstimulated hemocytes does not originate from cytoplasmic NADPH-oxidase, nitric oxide synthase or myeloperoxidase, but from mitochondria. In contrast to mammalian cells, incubation of hemocytes with rotenone (complex I inhibitor) had no effect on ROS production. Incubation with antimycin A (complex III inhibitor) resulted in a dose-dependent ROS production decrease while an over-production is usually reported in vertebrates. In hemocytes of C. gigas, the production of ROS seems similarly dependent on both Δψm and ΔpH. These findings point out differences between mammalian models and bivalve cells, which warrant further investigation about the fine characterization of the electron transfer chain and the respective involvement of mitochondrial complexes in ROS production in hemocytes of bivalve molluscs.
Citation: Donaghy L, Kraffe E, Le Goïc N, Lambert C, Volety AK, Soudant P (2012) Reactive Oxygen Species in Unstimulated Hemocytes of the Pacific Oyster Crassostrea gigas: A Mitochondrial Involvement. PLoS ONE7(10): e46594. https://doi.org/10.1371/journal.pone.0046594
Editor: Jo Anne Powell-Coffman, Iowa State University, United States of America
Received: July 16, 2012; Accepted: September 3, 2012; Published: October 3, 2012
Copyright: © Donaghy et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: The present work has been supported by a post-doctoral grant from CNRS INEE to L. Donaghy. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Competing interests: The authors have declared that no competing interests exist.
The Pacific oyster Crassostrea gigas is a sessile bivalve mollusc which inhabits intertidal zones. This invertebrate species is subjected not only to natural environmental variations but also to pathogenic infections and anthropogenic stressors. Due to their sessile nature, oysters face changing environment while maintaining their homeostasis to survive. Homeostasis in C. gigas relies, at least partially, upon cells circulating in hemolymph and infiltrating in tissues, referred to as hemocytes –. Hemocytes are involved in various and numerous physiological functions including nutrient digestion, transportation and distribution , , as well as shell and tissue repair , . Hemocytes also mediate cellular internal defence through accumulation and detoxification of toxicants –, phagocytosis and encapsulation of invading, foreign, biological material –.
Reactive oxygen species (ROS) are small molecules considered, at least in vertebrates, as involved in internal defence through the elimination of internalized non-self particles such as pathogens –. ROS can, however, interact with host cells’ proteins, lipids, carbohydrates and nucleic acids, irreversibly altering the spatial conformation and function of the impacted molecule , . Consequently, the concept of ROS as agents of cellular damage has been widely accepted for a long time.
The production of ROS was previously reported in hemocytes of various bivalve species, including oysters –, mussels , , scallops ,  and clams , . Due to the involvement of hemocytes in host defence, ROS production was mostly investigated in the immunological context (Reviewed in ). The production of ROS in chemically or biologically stimulated hemocytes was then suggested as NADPH-oxidase related –. In 2003, Lambert et al.  first developed flow cytometric methods to investigate the ROS production in hemocytes of bivalves and showed that Pacific oyster hemocytes produce ROS in absence of any kind of stimulation. We recently summarized the ROS pathways commonly accepted but not yet characterized in stimulated bivalves’ hemocytes  (Fig. 1).
Molecular targets of inhibitors (grey boxes) are indicated. NO, nitric oxide; ONOO−, peroxynitrite; O2−, superoxide; H2O2, hydrogen peroxide; HOCl, hypochloride; NOS, nitric oxide synthase; SOD, superoxide dismutase; MPO, myeloperoxidase. Adapted from Donaghy et al., 2009 .
Mitochondria are organelles that generate most of the cell's supply of energy during respiration, through the production of Adenosine TriPhosphate (ATP) by the F0F1-ATP synthase (Fig. 2). During mitochondrial electron transport, a concomitant transfer of protons (H+) across the inner mitochondrial membrane out of the mitochondrial matrix creates and maintains an electrochemical transmembrane potential. The H+ gradient is then used by F0F1-ATP synthase for the phosphorylation of Adenosine DiPhosphate (ADP) to ATP. Such H+ motive force is a physico-chemical parameter consisting of two components: a voltage gradient, Δψm, the mitochondrial membrane potential, and a pH gradient, ΔpH . Mitochondria were described as the main sources of ROS production in unstimulated vertebrate cells ,  and there is evidence that mitochondrial production of ROS plays a role in the maintenance of cellular and tissue homeostasis, at least in vertebrates. Indeed, mitochondrial ROS can activate cell proliferation as well as immune and inflammatory responses. Mitochondrial ROS were also reported to be involved in the regulation of glycogen storage, ion-channel function, oxygen sensing, regulation of cellular pH, intracellular trafficking and cell adhesion capacities , .
Molecular targets of inhibitors (grey boxes) are indicated.
The objective of the present work was to characterize the intracellular sources (NADPH-oxidase, nitric oxide synthase, myeloperoxidase, mitochondrial electron transport chain) involved in the production of ROS in unstimulated hemocytes from the marine bivalve species C. gigas, through the use of chemical inhibitors. In order to investigate whether ROS production in unstimulated hemocytes is dependent on the mitochondrial activity, we also studied the mitochondrial membrane potential (Δψm).
Materials and Methods
Adult Pacific oysters were collected from the Bay of Brest (04°34′ W 48°21′ N; Brittany, France) from October 2009 to April 2010. No specific permits were required for the described field studies. The location is not privately-owned or protected in any way, and Pacific oyster is not endangered or protected species. Animals were maintained in a flow-through seawater system for a maximum of two weeks before experiments.
Hemolymph was withdrawn from the adductor muscle of the oysters using a 2 mL plastic syringe fitted with a 21-gauge needle, through a notch extemporaneously made in the anterior part of the shell. Collected hemolymph was held on ice to reduce cell clumping. Each individual hemolymph sample was microscopically examined to eliminate samples with contaminating particles, such as gametes or tissue debris, and then filtered through an 80-µm mesh. Each experiment was performed on three randomly constituted pools with at least 4 individual oysters for each pool. Pooling of hemolymph was necessitated to obtain enough volume of hemolymph to test the effects of all concentrations of inhibitors and controls simultaneously. In addition, this enabled us to minimize high individual variability.
Modulators of Intracellular ROS Production
Working solutions of chemicals were prepared by serial dilutions of stock solutions. Diluents (DMSO and ethanol) were assessed as non-toxic for hemocytes and concentrations were kept constant between each dilution (Table 2). Final concentrations of chemicals are mentioned in Table 2, text and figures. Briefly, diluted hemolymph (1∶4 in filtered sterile seawater) was incubated with chemical inhibitors and specific fluorescent probes for 30 minutes at 18°C in the dark. After incubation, samples were kept on ice until flow cytometric analyses. Cellular parameters were evaluated using flow cytometer FACSCalibur (Becton-Dickinson) equipped with argon laser 488 nm. Collected data were analysed using WinMDI 2.9 software.
Toxicity of Inhibitors: Assessment of Morphology and Mortality of Hemocytes
Toxicity of inhibitors was determined through the evaluation of morphology and mortality of hemocytes using a double staining procedure including SYBR Green I (1/1000 of stock solution) (Invitrogen) and propidium iodide (10 µg mL−1) (PI, Sigma), adapted from Delaporte et al. . Hemocytes of C. gigas are composed of two sub-populations, hyalinocytes and granulocytes, whose morphology was based upon relative flow-cytometric parameters, Forward Scatter (FSC) and Side Scatter (SSC) . FSC and SSC commonly measure particle size and internal complexity, respectively. SYBR Green I is a DNA-binding probe which spontaneously penetrates both viable and dead cells. Contrastingly, membranes of viable cells do not allow PI to penetrate; whereas, altered membranes are permeable to PI. Dead cells are characterized by loss of membrane integrity and are, therefore, double stained by SYBR Green I and PI.
Intracellular ROS Production
Determination of ROS production was performed using 2′7′-dichlorofluorescein diacetate (final concentration 10 µM) (DCFH-DA; Molecular Probes, Invitrogen), a membrane permeable, non-fluorescent probe. Inside hemocytes, the -DA radical is first hydrolyzed by esterase enzymes. Intracellular hydrogen peroxide (H2O2), as well as superoxide ion (O2•−; , ), then oxidizes DCFH to the fluorescent DCF molecule. Oxidation of DCFH can also be mediated by nitrite radicals (NO2 or N2O3) . DCF green fluorescence, detected on the FL1 detector (530/30 nm band pass) of the flow cytometer (FACSCalibur, Becton Dickinson), is proportional to the ROS production. Relative ROS production is expressed as percentage of control condition (i.e., without inhibitor).
Mitochondrial Membrane Potential
Estimation of mitochondrial membrane potential (Δψm) was performed using JC-10 (Interchim; final concentration 5 µM), a membrane permeable fluorescent probe. JC-10 enters selectively into mitochondria and exists as two forms, monomeric or aggregate, depending upon Δψm and not ΔpH , . The JC-10 monomer form predominates in mitochondria with low Δψm and emits in the green wavelength (525–530 nm). The JC-10 aggregate form accumulates in mitochondria with high Δψm and emits in the orange wavelength (590 nm). JC-10 forms (monomer/aggregate) can change reversibly. Fluorescence intensities of JC-10 monomers and aggregates were quantified, respectively, by FL1 (530/30 nm) and FL2 (585/42 nm) detectors of the flow cytometer. The JC-10 aggregate/monomer ratio is assumed to be proportional to mitochondrial membrane potential intensity , .
Although data are presented as percentage of control (i.e., without inhibitor) and expressed as mean (n = 3) ± confidence interval (α = 0.05), statistical analyses were performed on untransformed flow cytometry values. Similarity of variances was first assessed using F-test. In all conditions, no significant difference was observed between variances. The significance of differences between means of control vs. each treatment was therefore analysed with T-tests (p<0.05), using Statgraphics Plus 5 software (Manugistics, Inc., Rockville, MD, USA).
As previously stated, hemocytes of the Pacific oyster, C. gigas, are comprised of two sub-populations . Unstimulated hyalinocytes show lower basal levels of ROS and Δψm than granulocytes. Both sub-populations reacted similarly to tested inhibitors. The following results reflect the responses of the whole hemocyte population.
Diphenyleneiodonium Chloride (DPI)
No toxic effect could be detected on hemocytes incubated with DPI in the range of concentrations tested. The detected intracellular ROS formation rate of hemocytes was not altered by DPI from 0.01 to 1 µM (Fig. 3). Incubation of hemocytes with concentrations of DPI higher than 5 µM highly decreased the ROS production, reaching 40% of control at 10 µM. The Δψm of hemocytes displayed no modification from 0.01 to 0.1 µM (Fig. 3). Application of 0.5 µM of DPI however resulted in a statistically significant major decrease of Δψm (28% of control condition). The Δψm was abolished with 5 µM of DPI.
Apocynin, L-N5-(1-iminoethyl) Ornithine Hydrochloride (L-NIO), 4′-Aminobenzoic Acid Hydrazide (ABAH)
In the range of concentrations tested, from 0.01 up to 50 µM, apocynin, L-NIO and ABAH displayed no toxicity toward hemocytes. Incubation of hemocytes with apocynin, L-NIO and ABAH resulted in no modification of detected intracellular ROS, whatever the concentration. Apocynin, L-NIO and ABAH induced no modification of Δψm in hemocytes (data not shown).
Carbonyl Cyanide 4-(Trifluoromethoxy) Phenylhydrazone (FCCP)
As expected, the Δψm of hemocytes decreased very quickly with low concentration of FCCP (0.05 µM) until being abolished at 0.5 µM (Fig. 4A). An inhibition of the ROS production occurred from 0.5 to 10 µM of FCCP (Fig. 4A). Concentrations of FCCP higher than 10 µM were toxic for the hemocytes, as determined by SYBR Green/PI double staining. While mortality of hemocytes in controls was 3.3%, mortality reached ∼20% with 25 and 50 µM of FCCP. Furthermore, morphology of hemocytes incubated with 25 and 50 µM of FCCP was also altered: internal complexity (SSC) decreased to 60% of control (data not shown).
As expected, the Δψm of hemocytes incubated with valinomycin at concentrations higher than 0.05 µM was reduced, and abolished at 1 µM (Fig. 4B). In contrast, the rate of intracellular ROS formation in hemocytes remained constant until 1 µM of valinomycin and then slightly decreased at 5 and 10 µM (Fig. 4B).
The Δψm of hemocytes incubated with nigericin (0.01 to 0.5 µM) remained stable and similar to the control condition while incubation with 1 µM of nigericin resulted in a small but significant increase of the Δψm, up to 125% of the control condition (Fig. 4C). Higher concentrations of nigericin then induced a decrease of Δψm, up to 55% inhibition. The detected intracellular ROS formation rate in hemocytes decreased with concentrations of nigericin ranging from 0.1 to 1 µM, reaching a maximum of 30% inhibition (Fig. 4C). Incubation of hemocytes with 5 and 10 µM of nigericin resulted in an increase of ROS production rate.
Rotenone had no toxicity on hemocytes in the range of concentrations tested. Intracellular ROS and Δψm of hemocytes were not modified by rotenone and remained similar to the control, whatever the concentrations tested (data not shown).
The Δψm significantly decreased until being abolished from concentrations ranging from 0.1 to 1 µM of antimycin A (Fig. 5). The rate of ROS production in hemocytes was inhibited by antimycin A, reaching 50% inhibition at 10 µM (Fig. 5).
Salicylhydroxamic Acid (SHAM)
SHAM induced no toxicity toward hemocytes in the range of concentrations used. The Δψm was not altered when hemocytes were incubated with SHAM, except for the two highest concentrations (500 and 1000 µM), which resulted in a significant major inhibition of Δψm (20% of control condition) (Fig. 6A). The detected intracellular ROS were not altered in hemocytes incubated with SHAM.
The Δψm of hemocytes incubated with oligomycin was not altered from 0.01 to 0.1 µM, and then decreased up to 20% of control at 10 µM of oligomycin (Fig. 6B). Incubation of hemocytes with oligomycin reduced the intracellular ROS rates to 50% of control from 0.5 to 10 µM of oligomycin (Fig. 6B). As observed with antimycin A, the inhibition of ROS by oligomycin also statistically occurred prior to the inhibition of Δψm.
The production of ROS was previously observed in unstimulated oysters’ hemocytes  but sources were not yet fully characterized. In the present work, the absence of effect of apocynin, L-NIO and ABAH on ROS production rates suggested that the ROS production in unstimulated hemocytes does not originate from NADPH-oxidase, nitric oxide synthase and myeloperoxidase, contrastingly with Lambert et al.  results. Using different chemical inhibitors to investigate intracellular sources of ROS in unstimulated hemocytes, Lambert et al.  concluded that about 80–85% of the intracellular ROS production originated from NADPH-oxidase complexes, with the remaining 15–20% supposed to originate from mitochondrial activity. The main reason for misinterpretation by Lambert et al.  could have been due to the use of DPI as a specific inhibitor of NADPH-oxidase complexes. DPI is a broad inhibitor of flavoprotein oxidoreductases, including NADPH-oxidase , but has also been demonstrated to interfere with nitric oxide synthase , mitochondrial respiratory chain NADH reductase (complex I) and succinyl dehydrogenase (complex II) –. In vertebrates, the influence of DPI on ROS production has been described as mostly related to the inhibition of mitochondrial complex I rather than complex II , resulting in an increased production of mitochondrial superoxide. In the present work, incubation of hemocytes with DPI inhibited ROS production, as previously reported in human unstimulated monocytes/macrophages  and peripheral blood mononuclear cells . Interestingly, incubation of oyster hemocytes with rotenone, an inhibitor of mitochondrial complex I, did not alter Δψm and ROS production rates as observed with DPI. Rotenone was previously demonstrated to efficiently inhibit complex I in oyster hemocytes , but does not seem to alter ROS production rates, as reported here. For its part, DPI may have disturbed markedly mitochondrial activity in hemocytes, as shown by the observed major decrease of Δψm. Reasons for differences in ROS production patterns observed with rotenone and DPI between bivalves and mammalian cells still remain unknown and require further investigation.
Antimycin A inhibits the respiratory chain at complex III between cytochrome b and cytochrome c1 , . As expected, inhibition of electron transport chain by antimycin A inhibited Δψm –. Incubation of hemocytes with antimycin A also highly inhibited ROS production. Contrastingly, an increase of ROS production has usually been reported with antimycin A in mammalian cells , . In these cells, flow of electrons is inhibited by antimycin A, resulting in a backflow of electrons through complex I which leads to an increase of ROS production . Contrastingly with vertebrates, the electron transport chain of invertebrates is branched with additional terminal oxidases such as the alternative oxidase (AOX) . Studies on marine invertebrates demonstrated the presence of the AOX ,  and the genetic basis for an alternative pathway has been documented in C. gigas , . AOX accepts electrons directly from ubiquinol and reduces O2 to H2O, thus representing both a branch in the respiratory chain and an additional terminal oxidase (Fig. 2). By preventing the over-reduction of the respiratory chain, AOX could act to limit ROS generation , . AOX could then be hypothesised to being involved in limiting ROS generation due to the backflow of electrons through complex I during incubation of hemocytes with antimycin A. Application of SHAM (inhibitor of AOX) to hemocytes simultaneously incubated with antimycin A did not, however, result in an increase of ROS production (data not shown).
The inhibition of AOX by SHAM alone resulted in no modification of ROS production, suggesting that this oxidase is not spontaneously involved in the ROS production in unstimulated hemocytes. The Δψm was also not altered by SHAM, except with the two highest concentrations tested. As AOX does not participate in the establishment of the electrochemical gradient, the observed inhibition of Δψm might be due to a non-specific effect of SHAM at high concentrations. Indeed, it has been suggested that SHAM might inhibit the cytochrome c oxidase (complex IV) , the terminal oxidase of the respiratory chain. Inhibition of mitochondrial complex IV could be responsible for the collapse of Δψm without affecting ROS production.
The final complex of the oxidative phosphorylation machinery is the F0F1-ATP synthase (complex V) which allows protons to return to the matrix, and uses their potential energy to synthesize ATP  (Fig. 2). It has been previously reported in isolated mitochondria that inhibition of complex V may induce an increase of ROS production , . Electron transfer driven by complexes I to IV slows down, increasing the “dwell time” for electrons on the complexes and the chance to form superoxide radicals . In the present work, inhibition of complex V resulted in a decrease of ROS production, as reported by Karawajew et al.  in a human leukemia cell line. In oyster hemocytes, maintenance of the Δψm during complex V inhibition might rely on the leak of H+ through the inner mitochondrial membrane to the matrix , . The two highest concentrations of oligomycin resulted in a drastic decrease of Δψm. An identical pattern of response has been reported in human neutrophils for concentrations higher than 1 µM , and might be non-specific and related to additional effects of oligomycin on cells. Oligomycin has been reported to induce hyperpolarization of the cell membranes, resulting in cytoplasmic membrane permeability . Variations in cellular content in ions (e.g., Na+, K+) might alter the osmotic equilibrium between cytoplasm and mitochondria, leading to the observed decrease of Δψm and ROS production.
The ionophores nigericin, valinomycin and FCCP were used to study the relative involvement of Δψm and ΔpH in ROS production from mitochondria. Nigericin inhibits ΔpH without affecting Δψm, while valinomycin decreases Δψm. Lastly, FCCP inhibits both Δψm and ΔpH by increasing proton leak , . Our results confirmed the specificity of JC-10 probe to measure Δψm in unstimulated hemocytes, without being affected by ΔpH, as reported in mammalian cells , . Incubation of hemocytes with valinomycin, which inhibits Δψm, resulted in a 30% reduction of ROS production. Similarly, inhibition of ΔpH by nigericin also decreased ROS production by 30%. In oyster hemocytes, Δψm and ΔpH then play a role in the production of ROS, and it appears that none seems to dominate. In contrast, Lambert and Brand  previously demonstrated that the ROS production in isolated mitochondria from skeletal muscle of rats was more dependent on the ΔpH across the mitochondrial inner membrane than on the Δψm. More recently, Selivanov et al.  reported that the pH itself, not the gradient, was an essential factor defining ROS production by the respiratory chain of rat brain mitochondria. Marine bivalve molluscs and terrestrial mammalians such as rats have evolved and exist in totally different environments, in terms of osmolytes and oxygen availability. Oysters are intertidal animals daily exposed to the air for hours. During emersion phases, depletion of available oxygen occurs within the shell . Simultaneously, accumulation of CO2 causes a decrease in tissue and hemolymph pH , . To date, although no data have been provided to support the fact that such tissue pH decrease affects mitochondrial pH, it is obvious that terrestrial mammalians do not naturally experience such pH variations. This may provide first clues about the observed evolutionary divergence regarding the relative importance of Δψm and ΔpH for the ROS production.
In summary, the present work evaluated the sources of ROS in unstimulated hemocytes of the Pacific oyster, C. gigas. Our results show that ROS in unstimulated hemocytes originate from mitochondria and not from NADPH-oxidase, nitric oxide synthase or myeloperoxidase. Contrastingly with mammalian cells, incubation of hemocytes with rotenone (complex I inhibitor) had no effect on ROS production. Incubation with antimycin A (complex III inhibitor) resulted in a dose-dependent ROS production decrease while an over-production is usually reported in vertebrates. The existence of an AOX in the mitochondrial inner membrane of C. gigas might limit over-production of ROS when complex III is inhibited. These findings warrant further investigations about the fine characterization of the electron transfer chain and ROS formation in hemocytes mitochondria of bivalve molluscs, compared with vertebrate models.
Conceived and designed the experiments: LD EK NLG PS. Performed the experiments: LD NLG. Analyzed the data: LD NLG. Contributed reagents/materials/analysis tools: CL PS. Wrote the paper: LD EK AKV PS.
- 1. Auffret M (1988) Bivalve hemocyte morphology. Am Fish Soc Spec Pub 18: 169–177.
- 2. Donaghy L, Lambert C, Choi KS, Soudant P (2009) Hemocytes of the carpet shell clam (Ruditapes decussatus) and the Manila clam (Ruditapes philippinarum): Current knowledge and future prospects. Aquaculture 297: 10–24.
- 3. Hine PM (1999) The inter-relationships of bivalve haemocytes. Fish Shellfish Immunol 9: 367–385.
- 4. Cheng TC (1996) Hemocytes: forms and functions. In: Kennedy VS, Newell RIE, Eble AF, editor. The eastern oyster Crassostrea virginica. College Park, MD, USA: Maryland Sea Grant Book. 299–333.
- 5. Beninger PG, Le Pennec G, Le Pennec M (2003) Demonstration of nutrient pathway from the digestive system to oocytes in the gonad intestinal loop of the scallop Pecten maximus L. Biol Bull. 205: 83–92.
- 6. Sparks AK, Morado JF (1988) Inflammation and wound repair in bivalve molluscs. In: Fisher WS, editor. Disease processes in marine bivalve molluscs. Bethesda, MD, USA: American Fisheries Society. 139–152.
- 7. Mount AS, Wheeler AP, Paradkar RP, Snider D (2004) Hemocyte-mediated shell mineralization in the eastern oyster. Science 304: 297–300.
- 8. Fisher WS (2004) Antimicrobial activity of copper and zinc accumulated in eastern oyster amebocytes. J Shellfish Res 23: 321–351.
- 9. Fisher WS (2004) Relationship of amebocytes and terrestrial elements to adult shell deposition in eastern oysters. J Shellfish Res 23: 353–367.
- 10. Matozzo V, Ballarin L, Pampanin DM, Marin MG (2001) Effects of copper and cadmium exposure on functional responses of hemocytes in the clam, Tapes philippinarum. Arch Environ Contam Toxicol 41: 163–170.
- 11. Canesi L, Gallo G, Gavioli M, Pruzzo C (2002) Bacteria-hemocyte interactions and phagocytosis in marine bivalves. Microsc Res Tech 57: 469–476.
- 12. Chagot D, Comps M, Boulo V, Ruano F, Grizel H (1987) Histological study of a cellular reaction in Ruditapes decussatus infected by a protozoan. Aquaculture 67: 260–261.
- 13. Cheng TC (2000) Cellular defense mechanisms in oysters. In: Fingerman N, Nagabhushanam R, editor. Recent advances in marine biotechnology Immunobiology and pathology. Enfield, NH, USA: Science Publishers. 43–83.
- 14. Chu FLE (2000) Defense mechanisms of marine bivalves. In: Fingerman N, Nagabhushanam R, editor. Recent advances in marine biotechnology Immunobiology and pathology. Enfield, NH, USA: Science Publishers. 1–42.
- 15. Bedard K, Krause KH (2007) The NOX family of ROS-generating NADPH oxidases: physiology and pathophysiology. Physiol Rev 87: 245–313.
- 16. Lam GY, Huang J, Brumell JH (2010) The many roles of NOX2 NADPH oxidase-derived ROS in immunity. Semin Immunopathol 32: 415–430.
- 17. Nahrevanian H (2009) Involvement of Nitric Oxide and Its Up/Down Stream Molecules in the Immunity Against Parasitic Infections. Braz J Infect Dis 13: 440–448.
- 18. Bartosz G (2009) Reactive oxygen species: destroyers or messengers? Biochem Pharmacol 77: 1303–1315.
- 19. Larson KG, Roberson BS, Hetrick FM (1989) Effect of environmental pollutants on the chemiluminescence of hemocytes from the American oyster Crassostrea virginica. Dis Aquat Organ 6: 131–136.
- 20. Hégaret H, Wikfors GH, Soudant P (2003) Flow cytometric analysis of haemocytes from eastern oysters, Crassostrea virginica, subjected to a sudden temperature elevation: II. Haemocyte functions: aggregation, viability, phagocytosis, and respiratory burst. J Exp Mar Bio Ecol 293: 249–265.
- 21. Lambert C, Soudant P, Choquet G, Paillard C (2003) Measurement of Crassostrea gigas hemocyte oxidative metabolism by flow cytometry and the inhibiting capacity of pathogenic vibrios. Fish Shellfish Immunol 15: 225–240.
- 22. Goedken M, De Guise S (2004) Flow cytometry as a tool to quantify oyster defence mechanisms. Fish Shellfish Immunol 16: 539–552.
- 23. Labreuche Y, Lambert C, Soudant P, Boulo V, Huvet A, et al. (2006) Cellular and molecular hemocyte responses of the Pacific oyster, Crassostrea gigas, following bacterial infection with Vibrio aestuarianus strain 01/32. Microbes Infect 8: 2715–2724.
- 24. Donaghy L, Kim BK, Hong HK, Park HS, Choi KS (2009) Flow cytometry studies on the populations and immune parameters of the hemocytes of the Suminoe oyster, Crassostrea ariakensis. Fish Shellfish Immunol 27: 296–301.
- 25. Pipe RK (1992) Generation of reactive oxygen metabolites by the haemocytes of the mussel Mytilus edulis. Dev Comp Immunol 16: 111–122.
- 26. Ordas MC, Novoa B, Figueras A (2000) Modulation of the chemiluminescence response of Mediterranean mussel (Mytilus galloprovincialis) haemocytes. Fish Shellfish Immunol 10: 611–622.
- 27. Chen MY, Yang HS, Delaporte M, Zhao SJ, Xing K (2007) Immune responses of the scallop Chlamys farreri after air exposure to different temperatures. J Exp Mar Bio Ecol 345: 52–60.
- 28. Chen MY, Yang HS, Delaporte M, Zhao SJ (2007) Immune condition of Chlamys farreri in response to acute temperature challenge. Aquaculture 271: 479–487.
- 29. Cima F, Matozzo V, Marin MG, Ballarin L (2000) Haemocytes of the clam Tapes philippinarum (Adams & Reeve, 1850): morphofunctional characterisation. Fish Shellfish Immunol 10: 677–693.
- 30. Buggé DA, Hégaret H, Wikfors GH, Allam B (2007) Oxidative burst in hard clam (Mercenaria mercenaria) haemocytes. Fish Shellfish Immunol 23: 188–196.
- 31. Lambert C, Soudant P, Jegaden M, Delaporte M, Labreuche Y, et al. (2007) In vitro modulation of reactive oxygen and nitrogen intermediate (ROI/RNI) production in Crassostrea gigas hemocytes. Aquaculture 270: 413–421.
- 32. Takahashi KG, Mori K (2000) Functional Profiles of Hemocytes in the bio-defense Process of the Pacific Oyster, Crassostrea gigas. Tohoku J Agric Res 51: 15–27.
- 33. Manduzio H, Rocher B, Durand F, Galap C, Leboulenger F (2005) The point about oxidative stress in molluscs. Invertebrate Surviv J 2: 91–104.
- 34. Solaini G, Sgarbi G, Lenaz G, Baracca A (2007) Evaluating mitochondrial membrane potential in cells. Biosci Rep 27: 11–21.
- 35. Murphy MP (2009) How mitochondria produce reactive oxygen species. Biochem J 417: 1–13.
- 36. Stowe DF, Camara AK (2009) Mitochondrial reactive oxygen species production in excitable cells: modulators of mitochondrial and cell function. Antioxid Redox Signal 11: 1373–1414.
- 37. Delaporte M, Soudant P, Moal J, Lambert C, Quéré C, et al. (2003) Effect of a mono-specific algal diet on immune functions in two bivalve species - Crassostrea gigas and Ruditapes philippinarum. J Exp Biol 206: 3053–3064.
- 38. Le Grand F, Kraffe E, Marty Y, Donaghy L, Soudant P (2011) Membrane phospholipid composition of hemocytes in the Pacific oyster Crassostrea gigas and the Manila clam Ruditapes philippinarum. Comp Biochem Physiol A Mol Integr Physiol 159: 383–391.
- 39. Curtin JF, Donovan M, Cotter TG (2002) Regulation and measurement of oxidative stress in apoptosis. J Immunol Methods 265: 49–72.
- 40. Rao KM, Padmanabhan J, Kilby DL, Cohen HJ, Currie MS, et al. (1992) Flow cytometric analysis of nitric oxide production in human neutrophils using dichlorofluorescein diacetate in the presence of a calmodulin inhibitor. J Leukoc Biol 51: 496–500.
- 41. Wardman P (2007) Fluorescent and luminescent probes for measurement of oxidative and nitrosative species in cells and tissues: progress, pitfalls, and prospects. Free Radic Biol Med 43: 995–1022.
- 42. Reers M, Smith TW, Chen LB (1991) J-Aggregate Formation Of A Carbocyanine As A Quantitative Fluorescent Indicator Of Membrane-Potential. Biochemistry 30: 4480–4486.
- 43. Smiley ST, Reers M, Mottola-Hartshorn C, Lin M, Chen A, et al. (1991) Intracellular heterogeneity in mitochondrial membrane potentials revealed by a J-aggregate-forming lipophilic cation JC-1. Proc Natl Acad Sci USA 88: 3671–3675.
- 44. Cossarizza A, Ceccarelli D, Masini A (1996) Functional heterogeneity of an isolated mitochondrial population revealed by cytofluorometric analysis at the single organelle level. Exp Cell Res 222: 84–94.
- 45. Cross AR, Jones OT (1986) The effect of the inhibitor diphenylene iodonium on the superoxide-generating system of neutrophils. Specific labelling of a component polypeptide of the oxidase. Biochem J 237: 111–116.
- 46. Stuehr DJ, Fasehun OA, Kwon NS, Gross SS, Gonzalez JA, et al. (1991) Inhibition of macrophage and endothelial cell nitric oxide synthase by diphenyleneiodonium and its analogs. Faseb J 5: 98–103.
- 47. Luetjens CM, Bui NT, Sengpiel B, Munstermann G, Poppe M, et al. (2000) Delayed mitochondrial dysfunction in excitotoxic neuron death: cytochrome c release and a secondary increase in superoxide production. J Neurosci 20: 5715–5723.
- 48. Li N, Ragheb K, Lawler G, Sturgis J, Rajwa B, et al. (2003) DPI induces mitochondrial superoxide-mediated apoptosis. Free Radic Biol Med 34: 465–477.
- 49. Aldieri E, Riganti C, Polimeni M, Gazzano E, Lussiana C, et al. (2008) Classical inhibitors of NOX NAD(P)H oxidases are not specific. Curr Drug Metab 9: 686–696.
- 50. Bulua AC, Simon A, Maddipati R, Pelletier M, Park H, et al. (2011) Mitochondrial reactive oxygen species promote production of proinflammatory cytokines and are elevated in TNFR1-associated periodic syndrome (TRAPS). J Exp Med 208: 519–533.
- 51. Li Y, Trush MA (1998) Diphenyleneiodonium, an NAD(P)H oxidase inhibitor, also potently inhibits mitochondrial reactive oxygen species production. Biochem Biophys Res Commun 253: 295–299.
- 52. Kurochkin IO, Ivanina AV, Eilers S, Downs CA, May LA, et al. (2009) Cadmium affects metabolic responses to prolonged anoxia and reoxygenation in eastern oysters (Crassostrea virginica). Am J Physiol Regul Integr Comp Physiol 297: 1262–1272.
- 53. Alexandre A, Lehninger AL (1984) Bypasses of the antimycin a block of mitochondrial electron transport in relation to ubisemiquinone function. Biochim Biophys Acta 767: 120–129.
- 54. Maguire JJ, Kagan VE, Packer L (1992) Electron-Transport Between Cytochrome-C And Alpha-Tocopherol. Biochem Biophys Res Commun 188: 190–197.
- 55. Kalbacova M, Vrbacky M, Drahota Z, Melkova Z (2003) Comparison of the effect of mitochondrial inhibitors on mitochondrial membrane potential in two different cell lines using flow cytometry and spectrofluorometry. Cytometry A 52: 110–116.
- 56. Balaban RS, Nemoto S, Finkel T (2005) Mitochondria, oxidants, and aging. Cell 120: 483–495.
- 57. Campo ML, Kinnally KW, Tedeschi H (1992) The effect of antimycin A on mouse liver inner mitochondrial membrane channel activity. J Biol Chem 267: 8123–8127.
- 58. Han YH, Park WH (2010) The effects of MAPK inhibitors on antimycin A-treated Calu-6 lung cancer cells in relation to cell growth, reactive oxygen species, and glutathione. Mol Cell Biochem 333: 211–219.
- 59. Zmijewski JW, Lorne E, Banerjee S, Abraham E (2009) Participation of mitochondrial respiratory complex III in neutrophil activation and lung injury. Am J Physiol Lung Cell Mol Physiol 296: 624–634.
- 60. Hoffman DL, Brookes PS (2009) Oxygen sensitivity of mitochondrial reactive oxygen species generation depends on metabolic conditions. J Biol Chem 284: 16236–16245.
- 61. McDonald A, Vanlerberghe G (2004) Branched mitochondrial electron transport in the Animalia: presence of alternative oxidase in several animal phyla. IUBMB Life 56: 333–341.
- 62. Buchner T, Abele D, Portner HO (2001) Oxyconformity in the intertidal worm Sipunculus nudus: the mitochondrial background and energetic consequences. Comp Biochem Physiol B Biochem Mol Biol 129: 109–120.
- 63. Tschischka K, Abele D, Portner HO (2000) Mitochondrial oxyconformity and cold adaptation in the polychaete Nereis pelagica and the bivalve Arctica islandica from the Baltic and White Seas. J Exp Biol 203: 3355–3368.
- 64. McDonald AE (2009) Alternative oxidase: what information can protein sequence comparisons give us? Physiol Plant 137: 328–341.
- 65. Keller M, Sommer AM, Portner HO, Abele D (2004) Seasonality of energetic functioning and production of reactive oxygen species by lugworm (Arenicola marina) mitochondria exposed to acute temperature changes. J Exp Biol 207: 2529–2538.
- 66. St-Pierre J, Buckingham JA, Roebuck SJ, Brand MD (2002) Topology of superoxide production from different sites in the mitochondrial electron transport chain. J Biol Chem 277: 44784–44790.
- 67. Poyton RO, Ball KA, Castello PR (2009) Mitochondrial generation of free radicals and hypoxic signaling. Trends Endocrinol Metab 20: 332–340.
- 68. Karawajew L, Rhein P, Czerwony G, Ludwig WD (2005) Stress-induced activation of the p53 tumor suppressor in leukemia cells and normal lymphocytes requires mitochondrial activity and reactive oxygen species. Blood 105: 4767–4775.
- 69. Brand MD, Nicholls DG (2011) Assessing mitochondrial dysfunction in cells. Biochem J 435: 297–312.
- 70. van Raam BJ, Sluiter W, de Wit E, Roos D, Verhoeven AJ, et al. (2008) Mitochondrial membrane potential in human neutrophils is maintained by complex III activity in the absence of supercomplex organisation. PLoS One 3: e2013.
- 71. Otero MJ, Carrasco L (1984) Action of oligomycin on cultured mammalian cells. Permeabilization to translation inhibitors. Mol Cell Biochem 61: 183–191.
- 72. Garlid KD, Paucek P (2003) Mitochondrial potassium transport: the K(+) cycle. Biochim Biophys Acta 1606: 23–41.
- 73. Selivanov VA, Zeak JA, Roca J, Cascante M, Trucco M, et al. (2008) The role of external and matrix pH in mitochondrial reactive oxygen species generation. J Biol Chem 283: 29292–29300.
- 74. Lambert AJ, Brand MD (2004) Superoxide production by NADH:ubiquinone oxidoreductase (complex I) depends on the pH gradient across the mitochondrial inner membrane. Biochem J 382: 511–517.
- 75. Allen SM, Burnett LE (2008) The effects of intertidal air exposure on the respiratory physiology and the killing activity of hemocytes in the pacific oyster, Crassostrea gigas (Thunberg). J Exp Mar Biol Ecol 357: 165–171.
- 76. Burnett LE, Stickle WB (2001) Physiological responses to hypoxia; Rabalais NN, Turner RE, editors. Washington, D.C. 101–114 p.