[Comparative study of Wuhan-like and omicron-like variants of SARS-CoV-2 in experimental animal models]

INTRODUCTION

The variability of SARS-CoV-2 appeared to be higher than expected, the emergence of new variants raises concerns. The aim of the work was to compare the pathogenicity of the Wuhan and BA.1.1/Omicron variants in BALB/c mice and Syrian hamsters.

MATERIALS AND METHODS

The study used strains of SARS-CoV-2: Dubrovka phylogenetically close to Wuhan-Hu-1, and LIA phylogenetically close to Omicron, BALB/c mice, transgenic mice B6.Cg-Tg(K18-ACE2)2Prlmn/HEMI Hemizygous for Tg(K18-ACE2)2Prlmn, Syrian golden hamsters. Animals were infected intranasally, pathogenicity was estimated by a complex of clinical, pathomorphological and virological methods.

RESULTS

Comparative studies of SARS-CoV-2 Dubrovka and LIA strains on animal models demonstrated their heterogeneous pathogenicity. In parallel infection of BALB/c mice with Dubrovka and LIA variants, the infection proceeded without serious clinical signs and lung damage. Infection with the LIA strain resulted to a systemic disease with a high concentration of viral RNA in the lungs and brain tissues of animals. The presence of viral RNA in mice infected with the Dubrovka strain was transient and undetectable in the lungs by day 7 post-infection. Unlike the mouse model, in hamsters, the Dubrovka strain had a greater pathogenicity than the LIA strain. In hamsters infected with the Dubrovka strain lung lesions were more significant, and the virus spread through organs, in particular in brain tissue, was observed. In hamsters infected with the LIA strain virus was not detected in brain tissue.

CONCLUSION

The study of various variants of SARS-CoV-2 in species initially unsusceptible to SARS-CoV-2 infection is important for monitoring zoonotic reservoirs that increase the risk of spread of new variants in humans.

Physiological Variation in Response to Vibrio and Hypoxia by Aquacultured Eastern Oysters in the Southeastern United States

Eastern oysters (Crassostrea virginica) have long been recognized as model organisms of extreme environmental tolerance, showing resilience to variation in temperature, salinity, hypoxia and microbial pathogens. These phenotypic responses, however, show variability between geographic locations or habitats (e.g., tidal). Physiological, morphological and genetic differences occur in populations throughout a species' geographical range, which may have been shaped by regional abiotic and biotic variations. Few studies of C. virginica have explored the combined factors of physiological mechanisms of divergent phenotypes between locations and the genetic relationships of individuals between these locations. To characterize genetic relationships of four locations with aquacultured oysters along the North Carolina and Virginia coast, we sequenced a portion of cytochrome oxidase subunit I (COI) that revealed significant variation in haplotype distribution between locations. We then measured mitochondrial physiology and expression of the innate immunity response of hemocytes to lab acclimation and combined stress conditions to compare basal expression and stress response in oysters between these locations. For stress sensing genes, toll-like receptors had the strongest location-specific response to hypoxia and Vibrio, whereas mannose receptor and a stress-receptor were specific to hypoxia and bacteria, respectively. The expression of stress response genes also showed location-specific and stressor-specific changes in expression, particularly for big defensin and the complement gene Cq3. Our results further suggested that genetic similarity of oysters from different locations was not clearly related to physiological and molecular responses. These results are informative for understanding the range of physiological plasticity for stress responses in this commercially important oyster species. They also have implications in the oyster farming industry as well as conservation efforts to restore endangered native oyster beds.

Sex and sex-related differences in gamete development progression impinge on biomarker responsiveness in sentinel mussels

In marine pollution monitoring, the biomarkers recorded in sentinel organisms are influenced by natural confounding factors that may jeopardise their interpretation. Among these confounding factors, little is known about the influence of sex along the annual reproductive cycle. The present investigation aims at contributing to understand how sex and sex-related differences in gamete development progression impinge on biomarker baseline values and on biomarker responsiveness to pollution in sentinel mussels. Mussels (Mytilus galloprovincialis) were collected from a relatively clean locality and from a chronically polluted site in the Basque Coast (Bay of Biscay) in January, April, August and November. Sex and gametogenesis stages were determined for each mussel. Tissue concentration of metals and PAHs was analysed. A battery of biomarkers was investigated: cytochrome c oxidase, pyruvate kinase and phosphoenolpyruvate carboxykinase enzyme activities; levels of protein carbonyls, malondialdehyde and 4-hydroxy-2-nonenal; lysosomal enlargement and membrane stability; intracellular neutral lipid accumulation; cell type composition and thinning of the digestive gland epithelium; and survival-in-air. Sex- and reproductive stage-related differences were found in bioaccumulation and in the values and responsiveness of most of the biomarkers. However, the patterns of sex-related differences were not consistent across all biomarkers. The differences in the biomarker responses between females and males also depended on the season, reflecting the progression of the gametogenesis cycle. Thus, selecting mussels of one specific sex does not seem to be a crucial requisite to carry out biomarker-based monitoring; yet, it is highly recommended to identify sex condition and gamete developmental stage of each mussel to test for the potentially confounding effects of sex, reproductive status and sex-related variability along the reproductive cycle.

Effects of seawater salinity and pH on cellular metabolism and enzyme activities in biomineralizing tissues of marine bivalves

Molluscan shell formation is a complex energy demanding process sensitive to the shifts in seawater CaCO₃ saturation due to changes in salinity and pH. We studied the effects of salinity and pH on energy demand and enzyme activities of biomineralizing cells of the Pacific oyster (Crassostrea gigas) and the hard-shell clam (Mercenaria mercenaria). Adult animals were exposed for 14 days to high (30), intermediate (18), or low (10) salinity at either high (8.0-8.2) or low (7.8) pH. Basal metabolic cost as well as the energy cost of the biomineralization-related cellular processes were determined in isolated mantle edge cells and hemocytes. The total metabolic rates were similar in the hemocytes of the two studied species, but considerably higher in the mantle cells of C. gigas compared with those of M. mercenaria. Cellular respiration was unaffected by salinity in the clams' cells, while in oysters' cells the highest respiration rate was observed at intermediate salinity (18). In both studied species, low pH suppressed cellular respiration. Low pH led to an upregulation of Na⁺/K⁺ ATPase activity in biomineralizing cells of oysters and clams. Activities of Ca²⁺ ATPase and H⁺ ATPase, as well as the cellular energy costs of Ca²⁺ and H⁺ transport in the biomineralizing cells were insensitive to the variation in salinity and pH in the two studied species. Variability in cellular response to low salinity and pH indicates that the disturbance of shell formation under these conditions has different underlying mechanisms in the two studied species.

Food-type may jeopardize biomarker interpretation in mussels used in aquatic toxicological experimentation

To assess the influence of food type on biomarkers, mussels (Mytilus galloprovincialis) were maintained under laboratory conditions and fed using 4 different microalgae diets ad libitum for 1 week: (a) Isochrysis galbana; (b) Tetraselmis chuii; (c) a mixture of I. galbana and T. chuii; and (d) a commercial food (Microalgae Composed Diet, Acuinuga). Different microalgae were shown to present different distribution and fate in the midgut. I. galbana (≈4 μm Ø) readily reached digestive cells to be intracellularly digested. T. chuii (≈10 μm Ø and hardly digestible) was retained in stomach and digestive ducts for long times and extracellularly digested. Based on these findings, it appeared likely that the presence of large amounts of microalgal enzymes and metabolites might interfere with biochemical determinations of mussel's biomarkers and/or that the diet-induced alterations of mussels' digestion could modulate lysosomal and tissue-level biomarkers. To test these hypotheses, a battery of common biochemical, cytological and tissue-level biomarkers were determined in the gills (including activities of pyruvate kinase, phosphoenolpyruvate carboxykinase and cytochrome c oxidase) and the digestive gland of the mussels (including protein, lipid, free glucose and glycogen total content, lysosomal structural changes and membrane stability, intracellular accumulation of neutral lipids and lipofuscins, changes in cell type composition and epithelial thinning, as well as altered tissue integrity). The type of food was concluded to be a major factor influencing biomarkers in short-term experiments though not all the microalgae affected biomarkers and their responsiveness in the same way. T. chuii seemed to alter the nutritional status, oxidative stress and digestion processes, thus interfering with a variety of biomarkers. On the other hand, the massive presence of I. galbana within digestive cells hampered the measurement of cytochemical biomarkers and rendered less reliable the results of biochemical biomarkers (as these could be attributed to both the mussel and the microalgae). Research to optimize dietary food type, composition, regime and rations for toxicological experimentation is urgently needed. Meanwhile, a detailed description of the food type and feeding conditions should be always provided when reporting aquatic toxicological experiments with mussels, as a necessary prerequisite to compare and interpret the biological responses elicited by pollutants.

Bioenergetic responses of freshwater mussels Unio tumidus to the combined effects of nano-ZnO and temperature regime

Bivalves from the cooling reservoirs of electrical power plants (PP) are exposed to the chronic heating and chemical pollution making them a suitable model to study the combined effects of these stressors. We investigated the effect of in situ exposures to chemical and thermal pollution in the PP cooling ponds on the metabolic responses of unionid bivalves (Unio tumidus) to a novel widespread pollutant, ZnO nanoparticles (nZnO). Male U. tumidus from the reservoirs of Dobrotvir and Burshtyn PPs (DPP and BPP) were maintained in clean water at 18 °C, or exposed for 14 days to one of the following conditions: nZnO (3.1 μM) or Zn²⁺ (3.1 μM, a positive control for Zn impacts) at 18 °C, elevated temperature (T, 25 °C), or nZnO at 25 °C (nZnO + T). Baseline levels of glycogen, lipids and ATP were similar in the two studied populations, whereas the levels of proteins, lactate/pyruvate ratio (L/P) and extralysosomal cathepsin D level were higher in the tissues of BPP mussels. The levels of glycogen and glucose declined in most experimental exposures indicating elevated energy demand except for a slight increase in the digestive gland of warming-exposed BPP mussels and in the gills of the nZnO + T-exposed DPP-mussels. Experimental exposures stimulated cathepsin D activity likely reflecting onset of autophagic processes to compensate for stress-induced energy demand. No depletion of ATP in Zn-containing exposures was observed indicating that the cellular metabolic adjustments were sufficient for such compensation. Unexpectedly, experimental warming mitigated most metabolic responses to nZnO in co-exposures. Our data thus indicate that metabolic effects of nZnO strongly depend on the environmental context of the mussels (such as temperature and acclimation history) which must be taken into account for the molecular and cellular biomarker-based assessment of the nanoparticle effects in the field.

The influence of short-term experimental fasting on biomarker responsiveness in oil WAF exposed mussels

Mussels are widely used in toxicological experimentation; however, experimental setups are not standardized yet. Although there is evidence of changes in biomarker values during food digestion and depending on the mussel nutritive status, the mode of feeding differs among toxicological experiments. Typically, mussels are fed with different diets in different long-term experiments, while fasting is the most common approach for short-term studies. Consequently, comparisons among experiments and reliable interpretations of biomarker results are often unfeasible. The present investigation aimed at determining the influence of fasting (against feeding with Isochrysis galbana) on biomarkers and their responsiveness in mussels exposed for 96 h to the water accommodated fraction (WAF) of a heavy fuel oil (0%, 6.25%, 12.5% and 25% WAF in sea water). PAH tissue levels in digestive gland and a battery of biomarkers were compared. WAF exposure led to decrease of cytochrome-C-oxidase activity, modulated glutathione-S-transferase activity, augmented lipid peroxidation, inhibited acetyl cholinesterase (AChE) activity, and led to lysosomal enlargement (Vv_LYS and S/V_LYS) and membrane destabilisation, lipofuscin accumulation, and histopathological alterations (Vv_BAS, MLR/MET and CTD ratio) in the digestive gland epithelium; and were integrated as IBR/n (biological response index). Overall, no significant changes were recorded in AChE activity, S/V_LYS and CTD ratio in any experimental treatment, while all the other biomarkers showed significant changes depending on the fasting/feeding condition, the exposure to WAF and/or their interaction. As a result, the integrated biomarker index IBR/n was higher at increasing WAF exposure levels both in fasted and fed mussels albeit the response was more marked in the latter. The response profiles were qualitatively similar between fasted and fed mussels but quantitatively more pronounced in fed mussels, especially upon exposure to the highest concentration (25% WAF). Therefore, it is highly recommended that mussels are also supplied with food during short-term, like during long-term toxicological experiments. This practice would avoid the interference of fasting with biological responses elicited by the tested chemicals and allow for reliable comparison with data obtained in long-term experiments and monitoring programmes.

Potential trade-offs between biomineralization and immunity revealed by shell properties and gene expression profiles of two closely related Crassostrea species

Species of the Ostreidae family are key ecosystem engineers and many of them - including Crassostrea gigas and Crassostreavirginica - are commercially important aquaculture species. Despite similarities in their morphology and ecology, these two species differ in their ability to defend against pathogens, potentially reflecting species-specific differential specialization of hemocytes on immune defense versus biomineralization. To test this hypothesis, we investigated the expression levels of immune- and biomineralization-related genes as well as mineralogical and mechanical properties of the shells and the calcium sequestration ability of the hemocytes of C. gigas and C. virginica The expression of biomineralization-related genes was higher in C. virginica than in C. gigas in multiple tissues including the mantle edge and hemocytes, while the expression of immune genes was higher in the hemocytes of C. gigas Hemocytes of C. virginica contained more calcium (stored intracellularly as calcium carbonate mineral) compared with those of C. gigas Analysis of the adult shells showed that the crystallinity of calcite was higher and the laths of the foliated layer of the shell were thicker in C. virginica than in C. gigas Mechanically, the shells of C. virginica were stiffer, harder and stronger than those of C. gigas Taken together, our results show that the species-specific differences in physiology (such as disease resistance and exoskeleton properties) are reflected at the cellular and molecular levels in the differential specialization of hemocytes on potentially competing functions (immunity and biomineralization) as well as different expression profiles of other tissues involved in biomineralization (such as the mantle edge).

Detoxification and cellular stress responses of unionid mussels Unio tumidus from two cooling ponds to combined nano-ZnO and temperature stress

Bivalve mollusks from the cooling reservoirs of fuel power plants (PP) are acclimated to the chronic heating and chemical pollution. We investigated stress responses of the mussels from these ponds to determine their tolerance to novel environmental pollutant, zinc oxide nanoparticles (nZnO). Male Unio tumidus from the reservoirs of Dobrotvir and Burschtyn PPs (DPP and BPP), Ukraine were exposed for 14 days to nZnO (3.1 μM), Zn²⁺ (3.1 μM) at 18 °C, elevated temperature (T, 25 °C), or nZnO at 25 °C (nZnO + T). Control groups were held at 18 °C. Zn-containing exposures resulted in the elevated concentrations of total and Zn-bound metallothionein (MT and Zn-MT) in the digestive gland, an increase in the levels of non-metalated MT (up to 5 times) and alkali-labile phosphates and lysosomal membrane destabilization in hemocytes. A common signature of nZnO exposures was modulation of the multixenobiotic-resistance protein activity (a decrease in the digestive gland and increase in the gills). The origin of population strongly affected the cellular stress responses of mussels. DPP-mussels showed depletion of caspase-3 in the digestive gland and up-regulation of HSP70, HSP72 and HSP60 levels in the gill during most exposures, whereas in the BPP-mussels caspase-3 was up-regulated and HSPs either downregulated or maintained stable. BPP-mussels were less adapted to heating shown by a glutathione depletion at elevated temperature (25 °C). Comparison with the earlier studies on mussels from pristine habitats show that an integrative 'eco-exposome'-based approach is useful for the forecast of the biological responses to novel adverse effects on aquatic organisms.

Intermittent hypoxia leads to functional reorganization of mitochondria and affects cellular bioenergetics in marine molluscs

Fluctuations in oxygen (O2) concentrations represent a major challenge to aerobic organisms and can be extremely damaging to their mitochondria. Marine intertidal molluscs are well-adapted to frequent O2 fluctuations, yet it remains unknown how their mitochondrial functions are regulated to sustain energy metabolism and prevent cellular damage during hypoxia and reoxygenation (H/R). We used metabolic control analysis to investigate the mechanisms of mitochondrial responses to H/R stress (18 h at <0.1% O2 followed by 1 h of reoxygenation) using hypoxia-tolerant intertidal clams Mercenaria mercenaria and hypoxia-sensitive subtidal scallops Argopecten irradians as models. We also assessed H/R-induced changes in cellular energy balance, oxidative damage and unfolded protein response to determine the potential links between mitochondrial dysfunction and cellular injury. Mitochondrial responses to H/R in scallops strongly resembled those in other hypoxia-sensitive organisms. Exposure to hypoxia followed by reoxygenation led to a strong decrease in the substrate oxidation (SOX) and phosphorylation (PHOS) capacities as well as partial depolarization of mitochondria of scallops. Elevated mRNA expression of a reactive oxygen species-sensitive enzyme aconitase and Lon protease (responsible for degradation of oxidized mitochondrial proteins) during H/R stress was consistent with elevated levels of oxidative stress in mitochondria of scallops. In hypoxia-tolerant clams, mitochondrial SOX capacity was enhanced during hypoxia and continued rising during the first hour of reoxygenation. In both species, the mitochondrial PHOS capacity was suppressed during hypoxia, likely to prevent ATP wastage by the reverse action of FO,F1-ATPase. The PHOS capacity recovered after 1 h of reoxygenation in clams but not in scallops. Compared with scallops, clams showed a greater suppression of energy-consuming processes (such as protein turnover and ion transport) during hypoxia, indicated by inactivation of the translation initiation factor EIF-2α, suppression of 26S proteasome activity and a dramatic decrease in the activity of Na(+)/K(+)-ATPase. The steady-state levels of adenylates were preserved during H/R exposure and AMP-dependent protein kinase was not activated in either species, indicating that the H/R exposure did not lead to severe energy deficiency. Taken together, our findings suggest that mitochondrial reorganizations sustaining high oxidative phosphorylation flux during recovery, combined with the ability to suppress ATP-demanding cellular functions during hypoxia, may contribute to high resilience of clams to H/R stress and help maintain energy homeostasis during frequent H/R cycles in the intertidal zone.

Biomineralization-related specialization of hemocytes and mantle tissues of the Pacific oyster Crassostrea gigas

The molluscan exoskeleton (shell) plays multiple important roles including structural support, protection from predators and stressors, and physiological homeostasis. Shell formation is a tightly regulated biological process that allows molluscs to build their shells even in environments unfavorable for mineral precipitation. Outer mantle edge epithelial cells (OME) and hemocytes were implicated in this process; however, the exact functions of these cell types in biomineralization are not clear. Pacific oysters (Crassostrea gigas) were used to study differences in the expression profiles of selected biomineralization-related genes in hemocytes and mantle cells, and the functional characteristics of hemocytes such as adhesion, motility and phagocytosis. The specialized role of OME in shell formation was supported by high expression levels of the extracellular matrix (ECM) related and cell-cell interaction genes. Density gradient separation of hemocytes revealed distinct phenotypes based on the cell morphology, gene expression patterns, motility and adhesion characteristics. These hemocyte fractions can be categorized into two functional groups, i.e. biomineralization and immune response cells. Gene expression profiles of the putative biomineralizing hemocytes indicate that in addition to their proposed role in mineral transport, hemocytes also contribute to the formation of the ECM, thus challenging the current paradigm of the mantle as the sole source of the ECM for shell formation. Our findings corroborate the specialized roles of hemocytes and the OME in biomineralization and emphasize complexity of the biological controls over shell formation in bivalves.

Interactive effects of copper exposure and environmental hypercapnia on immune functions of marine bivalves Crassostrea virginica and Mercenaria mercenaria

Estuarine organisms such as bivalves are commonly exposed to trace metals such as copper (Cu) and hypercapnia (elevated CO2 levels) in their habitats, which may affect their physiology and immune function. This study investigated the combined effects of elevated CO2 levels (∼800-2000 μatm PCO2, such as predicted by the near-future scenarios of global climate change) and Cu (50 μg l(-1)) on immune functions of the sediment dwelling hard clams Mercenaria mercenaria and an epifaunal bivalve, the eastern oyster Crassostrea virginica. Clams and oysters were exposed for 4 weeks to different CO2 and Cu levels, and tissue Cu burdens and immune parameters were assessed to test the hypothesis that hypercapnia will enhance Cu uptake due to the higher bioavailability of free Cu(2+) and increase the immunomodulatory effects of Cu. Exposure to Cu stimulated key immune parameters of clams and oysters leading to increased number of circulating hemocytes, higher phagocytosis and adhesion ability of hemocytes, as well as enhanced antiparasitic and antibacterial properties of the hemolymph reflected in higher activities of lysozyme and inhibitors of cysteine proteases. Lysozyme activation by Cu exposure was most prominent in normocapnia (∼400 μatm PCO2) and an increase in the levels of the protease inhibitors was strongest in hypercapnia (∼800-2000 μatm PCO2), but other immunostimulatory effects of Cu were evident in all PCO2 exposures. Metabolic activity of hemocytes of clams and oysters (measured as routine and mitochondrial oxygen consumption rates) was suppressed by Cu exposure likely reflecting lower rates of ATP synthesis and/or turnover. However, this metabolic suppression had no negative effects of the studied immune functions of hemocytes such as phagocytosis or adhesion capacity. Hypercapnia (∼800-2000 μatm PCO2) slightly but significantly enhanced accumulation of Cu in hemocytes, consistent with higher Cu(2+) bioavailability in CO2-acidified water, but had little effect on cellular and humoral immune traits of clams and oysters. These findings indicate that low levels of Cu contamination may enhance immunity of estuarine bivalves while moderate hypercapnia (such as predicted by the near future scenarios of the global climate change) does not strongly affect their immune parameters.

Effects of intermittent hypoxia on oxidative stress and protein degradation in molluscan mitochondria

Oxygen fluctuations represent a common stressor in estuarine and intertidal environments and can compromise the mitochondrial integrity and function in marine organisms. We assessed the role of mitochondrial protection mechanisms (ATP-dependent and ATP-independent mitochondrial proteases, and antioxidants) in tolerance to intermittent hypoxia or anoxia in three species of marine bivalves: the hypoxia tolerant hard clams (Mercenaria mercenaria) and oysters (Crassostrea virginica), and a hypoxia-sensitive subtidal scallop (Argopecten irradians). In clams and oysters, mitochondrial tolerance to hypoxia (18 h at 5% O2), anoxia (18 h at 0.1% O2) and subsequent reoxygenation was associated with the ability to maintain the steady-state activity of ATP-dependent and ATP-independent mitochondrial proteases and an anticipatory upregulation of the total antioxidant capacity (TAOC) under the low oxygen conditions. No accumulation of end-products of lipid or protein peroxidation was found during intermittent hypoxia or anoxia in clams and oysters (except for an increase in protein carbonyl concentration after hypoxia-reoxygenation in oysters). In contrast, hypoxia/anoxia and reoxygenation strongly suppressed activity of the ATP-dependent mitochondrial proteases in hypoxia-sensitive scallops. This suppression was associated with accumulation of oxidatively damaged mitochondrial proteins (including carbonylated proteins and proteins conjugated with a lipid peroxidation product malondialdehyde) despite high TAOC levels in scallop mitochondria. These findings highlight a key role of mitochondrial proteases in protection against hypoxia-reoxygenation stress and adaptations to frequent oxygen fluctuations in intertidal mollusks.

Effects of environmental hypercapnia and metal (Cd and Cu) exposure on acid-base and metal homeostasis of marine bivalves

Elevated CO2 levels reduce seawater pH and may affect bioavailability of trace metals in estuaries. We studied the interactive effects of common metal pollutants (50 μg l(-1) Cd or Cu) and PCO2 (~395, 800 and 2000 μatm) on metal levels, intracellular pH, expression of metal binding proteins and stress biomarkers in estuarine bivalves Crassostrea virginica (oysters) and Mercenaria mercenaria (hard clams). Cd (but not Cu or hypercapnia) exposure affected the acid-base balance of hemocytes resulting in elevated intracellular pH. Cd and Cu exposure led to the increase in the tissue metal burdens, and metal accumulation was reduced by elevated PCO2 in the mantle but not hemocytes. No change was found in the intracellular free Cd(2+), Cu(2+) or Fe(2+) during Cu or Cd exposure indicating that these metals are bound to intracellular ligands. Free Zn(2+) content in oyster hemocytes was suppressed by Cd and Cu exposure and below the detection limits in clam hemocytes, which went hand-in-hand with the elevated mRNA expression of metallothioneins and ferritin in Cd- and Cu-exposed bivalves, enhanced by hypercapnia. The metal-binding and antioxidant mechanisms of oysters and clams were sufficient to effectively maintain intracellular redox status, even though metal exposure combined with moderate hypercapnia (~800 μatm PCO2) led to the elevated production of reactive oxygen species in hemocytes. Overall, while hypercapnia modulates metal accumulation, binding capacity and oxidative stress in estuarine bivalves, the physiological effects of elevated CO2 are mild compared to the effects of other common stressors.

Interactive effects of metal pollution and ocean acidification on physiology of marine organisms

Changes in the global environment such as ocean acidification (OA) may interact with anthropogenic pollutants including trace metals threatening the integrity of marine ecosystems. We analyze recent studies on the interactive effects of OA and trace metals on marine organisms with a focus on the physiological basis of these interactions. Our analysis shows that the responses to elevated CO2 and metals are strongly dependent on the species, developmental stage, metal biochemistry and the degree of environmental hypercapnia, and cannot be directly predicted from the CO2-induced changes in metal solubility and speciation. The key physiological functions affected by both the OA and trace metal exposures involve acid-base regulation, protein turnover and mitochondrial bioenergetics, reflecting the sensitivity of the underlying molecular and cellular pathways to CO2and metals. Physiological interactions between elevated CO2 and metals may impact the organisms’ capacity to maintain acid-base homeostasis and reduce the amount of energy available for fitness-related functions such as growth, development and reproduction thereby affecting survival and performance of estuarine populations. Environmental hypercapnia may also affect the marine food webs by altering predator-prey interactions and the trophic transfer of metals in the food chain. However, our understanding of the degree to which these effects can impact the function and integrity of marine ecosystems is limited due the scarcity of the published research and its bias towards certain taxonomic groups. Future research priorities should include studies of metal x PCO2 interactions focusing on critical physiological functions (including acid-base, protein and energy homeostasis) in a greater range of ecologically and economically important marine species, as well as including the field populations naturally exposed (and potentially adapted) to different levels of metals and CO2 in their environments.

Safety and efficacy of high-dose adeno-associated virus 9 encoding sarcoplasmic reticulum Ca(2+) adenosine triphosphatase delivered by molecular cardiac surgery with recirculating delivery in ovine ischemic cardiomyopathy

OBJECTIVE

Therapeutic safety and efficacy are the basic prerequisites for clinical gene therapy. We investigated the effect of high-dose molecular cardiac surgery with recirculating delivery (MCARD)-mediated adeno-associated virus 9 (AAV9)/sarcoplasmic reticulum Ca(2+) adenosine triphosphatase (SERCA2a) gene delivery on clinical parameters, oxidative stress, humoral and cellular immune responses, and cardiac remodeling.

METHODS

Ischemic cardiomyopathy was generated in a sheep model. The sheep were assigned to 1 of 2 groups: control (n = 10) and study (MCARD, n = 6). The control group underwent no intervention and the study group received 10(14) genome copies of AAV9/SERCA2a 4 weeks after infarction.

RESULTS

Our ischemic model produced reliable infarcts leading to heart failure. The baseline ejection fraction in the MCARD group was 57.6% ± 1.6% versus 61.2% ± 1.9% in the control group (P > .05). At 12 weeks after infarction, the MCARD group had superior left ventricular function compared with the control group: stroke volume index, 46.6 ± 1.8 versus 35.8 ± 2.5 mL/m(2) (P < .05); ejection fraction, 46.2% ± 1.9% versus 38.7% ± 2.5% (P < .05); and left ventricular end-systolic and end-diastolic dimensions, 41.3 ± 1.7 versus 48.2 ± 1.4 mm and 51.2 ± 1.5 versus 57.6 ± 1.7 mm, respectively (P < .05). The markers of oxidative stress were significantly reduced in the infarct zone in the MCARD group. No positive T-cell-mediated immune response was seen in the MCARD group at any point. Myocyte hypertrophy was also significantly attenuated in the MCARD group compared with the control group.

CONCLUSIONS

Cardiac overexpression of the SERCA2a gene by way of MCARD is a safe therapeutic intervention. It significantly improves left ventricular function, decreases markers of oxidative stress, abrogates myocyte hypertrophy, arrests remodeling, and does not induce a T-cell-mediated immune response.

Immunomodulation by the interactive effects of cadmium and hypercapnia in marine bivalves Crassostrea virginica and Mercenaria mercenaria

Estuarine organisms are exposed to multiple stressors including large fluctuations in partial pressure of carbon dioxide (P2CO) and concentrations of trace metals such as cadmium (Cd) that can affect their survival and fitness. Ocean acidification due to the increasing atmospheric (P2CO) leads to a decrease in pH and shifts in the carbonate chemistry of seawater which can change bioavailability and toxicity of metals. We studied the interactive effects of (P2CO) and Cd exposure on metal levels, metabolism and immune-related functions in hemocytes of two ecologically and economically important bivalve species, Mercenaria mercenaria (hard shell clam) and Crassostrea virginica (Eastern oyster). Clams and oysters were exposed to combinations of three (P2CO) levels (∼400, 800 and 2000 μatm (P2CO), corresponding to the present day conditions and the projections for the years 2100 and 2250, respectively) and two Cd concentrations (0 and 50 μg l(-1)) in seawater. Following four weeks of exposure to Cd, hemolymph of both species contained similar Cd levels (50-70 μg l(-1)), whereas hemocytes accumulated intracellular Cd burdens up to 15-42 mg l(-1), regardless of the exposure P2CO. Clam hemocytes had considerably lower Cd burdens than those of oysters (0.7-1 ng 10(-6) cells vs. 4-6 ng 10(-6) cells, respectively). Cd exposure suppressed hemocyte metabolism and increased the rates of mitochondrial proton leak in normocapnia indicating partial mitochondrial uncoupling. This Cd-induced mitochondrial uncoupling was alleviated in hypercapnia. Cd exposure suppressed immune-related functions in hemocytes of clams and oysters, and these effects were exacerbated at elevated (P2CO). Thus, elevated (P2CO) combined with Cd exposure resulted in decrease in phagocytic activity and adhesion capacity as well as lower expression of mRNA for lectin and heat shock protein (HSP70) in clam and oyster hemocytes. In oysters, combined exposure to elevated (P2CO) and Cd also led to reduced activity of lysozyme in hemocytes and hemolymph. Overall, our study shows that moderately elevated (P2CO) (∼800-2000 μatm P2CO) potentiates the negative effects of Cd on immunity and thus may sensitize clams and oysters to pathogens and diseases during seasonal hypercapnia and/or ocean acidification in polluted estuaries.

Interactive effects of pH and metals on mitochondrial functions of intertidal bivalves Crassostrea virginica and Mercenaria mercenaria

Intertidal bivalves experience broad fluctuations of environmental temperature, pH and oxygen content which could change their intracellular pH. They are also exposed to trace metals such as cadmium (Cd) and copper (Cu) that accumulate in their tissues and may negatively affect mitochondrial functions and bioenergetics. We determined the interactive effects of pH and trace metals (25 μM Cd or Cu) on mitochondrial functions (including respiration and membrane potentials in both ADP-stimulated (state 3) and resting (state 4) states) of two common marine bivalves, the hard clams (Mercenaria mercenaria) and eastern oysters (Crassostrea virginica). In the absence of the trace metals, mitochondrial functions of C. virginica and M. mercenaria were insensitive to pH in a broad physiologically relevant range (6.6-7.8). Mitochondrial respiration was generally suppressed by 25 μM Cd or Cu (with the stronger effects observed for ADP-stimulated compared to the resting respiration) while the mitochondrial membrane potential was unaffected. pH modulated the effects of Cu and Cd on mitochondrial respiration of the bivalves. In oysters, Cu suppressed ADP-stimulated mitochondrial respiration at high and low pH values (6.6 and 7.8, respectively), but had no effect in the intermediate pH range (7.0-7.4). In clams, the negative effect of Cu on ADP-stimulated respiration was only observed at extremely high pH (7.8). A decrease in pH was also protective against Cd in mitochondria of clams and oysters. In clams, 25 μM Cd suppressed ADP-stimulated respiration at all pH; however, at low pH (6.6-7.0) this suppression was paralleled by a decrease in the rates of proton leak thereby effectively restoring mitochondrial coupling. In oysters, the inhibitory effects of Cd on ADP-stimulated respiration were fully abolished at low pH (6.6-7.0). This indicates that moderate acidosis (such as occurs during exposure to air, extreme salinities or elevated CO2 levels in the intertidal zone) may have a beneficial side-effect of protecting mitochondria of clams and oysters against the toxic effects of trace metals in polluted estuaries.

Short-term acute hypercapnia affects cellular responses to trace metals in the hard clams Mercenaria mercenaria

Estuarine and coastal habitats experience large fluctuations of environmental factors such as temperature, salinity, partial pressure of CO2 ( [Formula: see text] ) and pH; they also serve as the natural sinks for trace metals. Benthic filter-feeding organisms such as bivalves are exposed to the elevated concentrations of metals in estuarine water and sediments that can strongly affect their physiology. The effects of metals on estuarine organisms may be exacerbated by other environmental factors. Thus, a decrease in pH caused by high [Formula: see text] (hypercapnia) can modulate the effects of trace metals by affecting metal bioavailability, accumulation or binding. To better understand the cellular mechanisms of interactions between [Formula: see text] and trace metals in marine bivalves, we exposed isolated mantle cells of the hard clams (Mercenaria mercenaria) to different levels of [Formula: see text] (0.05, 1.52 and 3.01 kPa) and two major trace metal pollutants - cadmium (Cd) and copper (Cu). Elevated [Formula: see text] resulted in a decrease in intracellular pH (pHi) of the isolated mantle cells from 7.8 to 7.4. Elevated [Formula: see text] significantly but differently affected the trace metal accumulation by the cells. Cd uptake was suppressed at elevated [Formula: see text] levels while Cu accumulation has greatly accelerated under hypercapnic conditions. Interestingly, at higher extracellular Cd levels, labile intracellular Cd(2+) concentration remained the same, while intracellular levels of free Zn(2+) increased suggesting that Cd(2+) substitutes bound Zn(2+) in these cells. In contrast, Cu exposure did not affect intracellular Zn(2+) but led to a profound increase in the intracellular levels of labile Cu(2+) and Fe(2+). An increase in the extracellular concentrations of Cd and Cu led to the elevated production of reactive oxygen species under the normocapnic conditions (0.05 kPa [Formula: see text] ); surprisingly, this effect was mitigated in hypercapnia (1.52 and 3.01 kPa). Overall, our data reveal complex and metal-specific interactions between the cellular effects of trace metals and [Formula: see text] in clams and indicate that variations in environmental [Formula: see text] may modulate the biological effects of trace metals in marine organisms.

Effects of temperature and cadmium exposure on the mitochondria of oysters (Crassostrea virginica) exposed to hypoxia and subsequent reoxygenation

Intertidal bivalves are commonly exposed to multiple stressors including periodic hypoxia, temperature fluctuations and pollution, which can strongly affect energy metabolism. We used top-down control and elasticity analyses to determine the interactive effects of intermittent hypoxia, cadmium (Cd) exposure and acute temperature stress on mitochondria of the eastern oyster Crassostrea virginica. Oysters were acclimated at 20°C for 30 days in the absence or presence of 50 μg l(-1) Cd and then subjected to a long-term hypoxia (6 days at <0.5% O(2) in seawater) followed by normoxic recovery. Mitochondrial function was assessed at the acclimation temperature (20°C), or at elevated temperature (30°C) mimicking acute temperature stress in the intertidal zone. In the absence of Cd or temperature stress, mitochondria of oysters showed high resilience to transient hypoxia. In control oysters at 20°C, hypoxia/reoxygenation induced elevated flux capacity of all three studied mitochondrial subsystems (substrate oxidation, phosphorylation and proton leak) and resulted in a mild depolarization of resting mitochondria. Elevated proton conductance and enhanced capacity of phosphorylation and substrate oxidation subsystems may confer resistance to hypoxia/reoxygenation stress in oyster mitochondria by alleviating production of reactive oxygen species and maintaining high aerobic capacity and ATP synthesis rates during recovery. Exposure to environmental stressors such as Cd and elevated temperatures abolished the putative adaptive responses of the substrate oxidation and phosphorylation subsystems, and strongly enhanced proton leak in mitochondria of oysters subjected to hypoxia/reoxygenation stress. Our findings suggest that Cd exposure and acute temperature stress may lead to the loss of mitochondrial resistance to hypoxia and reoxygenation and thus potentially affect the ability of oysters to survive periodic oxygen deprivation in coastal and estuarine habitats.

Interactive effects of salinity and elevated CO2 levels on juvenile eastern oysters, Crassostrea virginica

Rising levels of atmospheric CO2 lead to acidification of the ocean and alter seawater carbonate chemistry, which can negatively impact calcifying organisms, including mollusks. In estuaries, exposure to elevated CO2 levels often co-occurs with other stressors, such as reduced salinity, which enhances the acidification trend, affects ion and acid–base regulation of estuarine calcifiers and modifies their response to ocean acidification. We studied the interactive effects of salinity and partial pressure of CO2 (PCO2) on biomineralization and energy homeostasis in juveniles of the eastern oyster, Crassostrea virginica, a common estuarine bivalve. Juveniles were exposed for 11 weeks to one of two environmentally relevant salinities (30 or 15 PSU) either at current atmospheric PCO2 (∼400 μatm, normocapnia) or PCO2 projected by moderate IPCC scenarios for the year 2100 (∼700–800 μatm, hypercapnia). Exposure of the juvenile oysters to elevated PCO2 and/or low salinity led to a significant increase in mortality, reduction of tissue energy stores (glycogen and lipid) and negative soft tissue growth, indicating energy deficiency. Interestingly, tissue ATP levels were not affected by exposure to changing salinity and PCO2, suggesting that juvenile oysters maintain their cellular energy status at the expense of lipid and glycogen stores. At the same time, no compensatory upregulation of carbonic anhydrase activity was found under the conditions of low salinity and high PCO2. Metabolic profiling using magnetic resonance spectroscopy revealed altered metabolite status following low salinity exposure; specifically, acetate levels were lower in hypercapnic than in normocapnic individuals at low salinity. Combined exposure to hypercapnia and low salinity negatively affected mechanical properties of shells of the juveniles, resulting in reduced hardness and fracture resistance. Thus, our data suggest that the combined effects of elevated PCO2 and fluctuating salinity may jeopardize the survival of eastern oysters because of weakening of their shells and increased energy consumption.

Proteomic response to elevated PCO2 level in eastern oysters, Crassostrea virginica: evidence for oxidative stress

Estuaries are characterized by extreme fluctuations in CO2 levels due to bouts of CO2 production by the resident biota that exceed its capacity of CO2 consumption and/or the rates of gas exchange with the atmosphere and open ocean waters. Elevated partial pressures of CO2 (PCO2; i.e. environmental hypercapnia) decrease the pH of estuarine waters and, ultimately, extracellular and intracellular pH levels of estuarine organisms such as mollusks that have limited capacity for pH regulation. We analyzed proteomic changes associated with exposure to elevated PCO2 in the mantle tissue of eastern oysters (Crassostrea virginica) after 2 weeks of exposure to control (∼39 Pa PCO2) and hypercapnic (∼357 Pa PCO2) conditions using two-dimensional gel electrophoresis and tandem mass spectrometry. Exposure to high PCO2 resulted in a significant proteome shift in the mantle tissue, with 12% of proteins (54 out of 456) differentially expressed under the high PCO2 compared with control conditions. Of the 54 differentially expressed proteins, we were able to identify 17. Among the identified proteins, two main functional categories were upregulated in response to hypercapnia: those associated with the cytoskeleton (e.g. several actin isoforms) and those associated with oxidative stress (e.g. superoxide dismutase and several peroxiredoxins as well as the thioredoxin-related nucleoredoxin). This indicates that exposure to high PCO2 (∼357 Pa) induces oxidative stress and suggests that the cytoskeleton is a major target of oxidative stress. We discuss how elevated CO2 levels may cause oxidative stress by increasing the production of reactive oxygen species (ROS) either indirectly by lowering organismal pH, which may enhance the Fenton reaction, and/or directly by CO2 interacting with other ROS to form more free radicals. Although estuarine species are already exposed to higher and more variable levels of CO2 than other marine species, climate change may further increase the extremes and thereby cause greater levels of oxidative stress.

Interactive effects of cadmium and hypoxia on metabolic responses and bacterial loads of eastern oysters Crassostrea virginica Gmelin

Pollution by toxic metals including cadmium (Cd) and hypoxia are important stressors in estuaries and coastal waters which may interactively affect sessile benthic organisms, such as oysters. We studied metabolic responses to prolonged hypoxic acclimation (2 weeks at 5% O2) in control and Cd-exposed (30 d at 50 μg L(-1) Cd) oysters Crassostrea virginica, and analyzed the effects of these stressors on abundance of Vibrio spp. in oysters. Hypoxia-acclimated oysters retained normal standard metabolic rates (SMR) at 5% O2, in contrast to a decline of SMR observed during acute hypoxia. However, oysters spent more time actively ventilating in hypoxia than normoxia resulting in enhanced Cd uptake and 2.7-fold higher tissue Cd burdens in hypoxia. Cd exposure led to a significant decrease in tissue glycogen stores, increase in free glucose levels and elevated activity of glycolytic enzymes (hexokinase and aldolase) indicating a greater dependence on carbohydrate catabolism. A compensatory increase in activities of two key mitochondrial enzymes (citrate synthase and cytochrome c oxidase) was found during prolonged hypoxia in control oysters but suppressed in Cd-exposed ones. Cd exposure also resulted in a significant increase in abundance of Vibrio parahaemolyticus and Vibrio vulnificus levels during normoxia and hypoxia, respectively. Overall, Cd- and hypoxia-induced changes in metabolic profile, Cd accumulation and bacterial flora of oysters indicate that these stressors can synergistically impact energy homeostasis, performance and survival of oysters in polluted estuaries and have significant consequences for transfer of Cd and bacterial pathogens to the higher levels of the food chain.

Molecular characterization and mRNA expression of two key enzymes of hypoxia-sensing pathways in eastern oysters Crassostrea virginica (Gmelin): hypoxia-inducible factor α (HIF-α) and HIF-prolyl hydroxylase (PHD)

Oxygen homeostasis is crucial for development, survival and normal function of all metazoans. A family of transcription factors called hypoxia-inducible factors (HIF) is critical in mediating the adaptive responses to reduced oxygen availability. The HIF transcription factor consists of a constitutively expressed β subunit and an oxygen-dependent α subunit; the abundance of the latter determines the activity of HIF and is regulated by a family of O(2)- and Fe(2+)-dependent enzymes prolyl hydroxylases (PHDs). Currently very little is known about the function of this important pathway and the molecular structure of its key players in hypoxia-tolerant intertidal mollusks including oysters, which are among the animal champions of anoxic and hypoxic tolerance and thus can serve as excellent models to study the role of HIF cascade in adaptations to oxygen deficiency. We have isolated transcripts of two key components of the oxygen sensing pathway - the oxygen-regulated HIF-α subunit and PHD - from an intertidal mollusk, the eastern oyster Crassostrea virginica, and determined the transcriptional responses of these two genes to anoxia, hypoxia and cadmium (Cd) stress. HIF-α and PHD homologs from eastern oysters C. virginica show significant sequence similarity and share key functional domains with the earlier described isoforms from vertebrates and invertebrates. Phylogenetic analysis shows that genetic diversification of HIF and PHD isoforms occurred within the vertebrate lineage indicating functional diversification and specialization of the oxygen-sensing pathways in this group, which parallels situation observed for many other important genes. HIF-α and PHD homologs are broadly expressed at the mRNA level in different oyster tissues and show transcriptional responses to prolonged hypoxia in the gills consistent with their putative role in oxygen sensing and the adaptive response to hypoxia. Similarity in amino acid sequence, domain structure and transcriptional responses between HIF-α and PHD homologs from oysters and other invertebrate and vertebrate species implies the highly conserved functions of these genes throughout the evolutionary history of animals, in accordance with their critical role in oxygen sensing and homeostasis.

Effects of cadmium on anaerobic energy metabolism and mRNA expression during air exposure and recovery of an intertidal mollusk Crassostrea virginica

Marine organisms are exposed to periodical oxygen deficiency and pollution stress in estuarine and coastal zones which may strongly affect their performance and survival. We studied the combined effects of exposure to a common pollutant, cadmium (Cd), and intermittent anoxia on anaerobic metabolism, energy status and mRNA expression of 13 genes involved in and/or controlled by the hypoxia inducible factor-1 (HIF-1) pathway in hepatopancreas of an intertidal bivalve, the eastern oyster Crassostrea virginica. In control oysters, prolonged anoxia resulted in a selective suppression of nitric oxide synthase (NOS) and upregulation of cytochrome c oxidase subunit IV (COX4) while the levels of other transcripts remained unchanged. During post-anoxic recovery, mRNA expression of hypoxia inducible factor-1alpha (HIF-1alpha) was elevated, phosphoenolpyruvate carboxykinase (PEPCK), NOS and LON protease suppressed, and mRNA expression of other studied genes not changed. Notably, most of the key glycolytic genes that are stimulated by HIF-1 in mammals, either remained unchanged or were downregulated in anoxic oysters suggesting a different mechanism of molecular response to oxygen deficiency. Patterns of transcriptional response during anoxia and reoxygenation were significantly altered by Cd exposure in a gene-specific manner. Anaerobic metabolism (indicated by accumulation of l-alanine, succinate and acetate during anoxia) was also suppressed in Cd-exposed oysters. In control oysters, ATP turnover rate (M(ATP)) during anoxia was mostly sustained by anaerobic glycolysis with negligible contributions from ATP and PLA breakdown. In contrast, in Cd-exposed oysters ATP breakdown contributed significantly to anaerobic M(ATP). Thus, while control oysters could efficiently defend the ATP levels and tissue energy status during prolonged anoxia, Cd-exposed oysters experienced a disturbance in tissue energy balance indicated by the depletion of ATP, a rapid decline in adenylate energy charge and increase in ADP/ATP ratios. This energy deficiency combined with suppression of anaerobic metabolism may strongly affect performance and survival of oysters in polluted estuaries where metal pollution may co-occur with "dead zones".

Effects of elevated temperature and cadmium exposure on stress protein response in eastern oysters Crassostrea virginica (Gmelin)

Stress proteins such as heat shock proteins (HSPs) and metallothioneins (MTs) play a key role in cellular protection against environmental stress. Marine ectotherms such as eastern oysters Crassostrea virginica are commonly exposed to multiple stressors including temperature and pollution by metals such as cadmium (Cd) in estuaries and coastal zones; however, the combined effects of these stressors on their cellular protection mechanisms are poorly understood. We acclimated C. virginica from populations adapted to different thermal regimes (Washington, North Carolina and Texas) at a common temperature of 12 degrees C, and analyzed their expression of MTs and HSPs (cytosolic HSP69, HSC72-77, HSP90 and mitochondrial HSP60) in response to the combined acute temperature stress and long-term Cd exposure. Overall, HSP and MT induction patterns were similar in oysters from the three studied geographically distant populations. HSP69 and MTs were significantly up-regulated by Cd and temperature stress implying their important role in cellular stress protection. In contrast, HSC72-77, HSP60 and HSP90 were not consistently induced by either acute heat or Cd exposure. The induction temperature for MTs was higher than for HSP69 (>28 degrees C vs. 20 degrees C, respectively), and MTs were more strongly induced by Cd than by temperature stress (to up to 38-94-fold compared by 3.5-7.5-fold, respectively) consistent with their predominant role in metal detoxification. Notably, heat stress did not result in an additional increase in metallothionein expression in Cd-exposed oysters suggesting a capacity limitation during the combined exposure to Cd and temperature stress. Levels of HSP69 and in some cases, HSC72-77 and HSP90 were lower in Cd-exposed oysters as compared to their control counterparts during heat stress indicating that simultaneous exposure to these two stressors may have partially suppressed the cytoprotective upregulation of molecular chaperones. These limitations of stress protein response may contribute to the reduced thermotolerance of oysters from metal-polluted environments.

Molecular characterization and expression of a novel homolog of uncoupling protein 5 (UCP5) from the eastern oyster Crassostrea virginica (Bivalvia: Ostreidae)

Uncoupling proteins (UCPs) belong to the mitochondrial anion carrier gene family which has been implicated in diverse physiological functions ranging from thermoregulation to antioxidant defense. In mammals, the UCP family is well characterized and contains five members (UCP1-5). In contrast, invertebrate homologues of uncoupling proteins are much less studied both from the viewpoints of structure and function. In this study we report nucleotide and predicted protein structure of an important member of UCP family, UCP5 from eastern oysters Crassostrea virginica. UCP5 from oysters appears to be a close homolog of the mammalian brain mitochondrial carrier protein (BMCP1, or UCP5) and is the first full-length UCP described from a Lophotrochozoan invertebrate. Evolutionary analysis of UCP sequences indicates at least three monophyletic UCP branches (UCP1-3, UCP4 and UCP5) that have diverged early in the evolution, prior to the divergence of vertebrates and invertebrates. In oysters, two forms of UCP5 transcript are found (UCP5S and UCP5L) that differ by 152 bp in length due to the presence of an intron in UCP5L. UCP5 was expressed in all studied oyster tissues, unlike mammals, where UCP5 is predominantly expressed in brains and male gonads. Hypoxia-reoxygenation stress, sublethal Cd exposure (50 ?g L(?1) Cd for 56 days) and acclimation to different temperatures (12 and 20 °C) had no significant effect on UCP5 mRNA expression in oysters indicative of its relative unimportance in antioxidant defense and temperature adaptation of oyster mitochondria. These data suggest that despite the relatively high degree of evolutionary conservation of the UCP5 amino acid sequence, its functional significance in mitochondria changed in the course of evolution of mollusks and vertebrates.

Differential sensitivity to cadmium of key mitochondrial enzymes in the eastern oyster, Crassostrea virginica Gmelin (Bivalvia: Ostreidae)

Combined effects of cadmium (Cd) and temperature on key mitochondrial enzymes [including Complexes I-IV of electron transport chain and Krebs cycle enzymes citrate synthase (CS), and NAD- and NADP-dependent isocitrate dehydrogenases (NAD-IDH and NADP-IDH)] were studied in a marine ectotherm, Crassostrea virginica in order to better understand the mechanisms of Cd-induced impairment of mitochondrial function. Matrix enzymes including CS and isocitrate dehydrogenases were the most sensitive to Cd making Krebs cycle a likely candidate to explain Cd-induced impairment of mitochondrial substrate oxidation. CS and NAD-IDH had IC(50) of 26 and 65 microM at the acclimation temperature (15 degrees C) and 65 (CS) and 1.5 (NAD-IDH) microM at elevated temperature (25 degrees C), respectively. Mitochondrial NADP-IDH was the most sensitive to Cd with IC(50) of 14 and 3.4 microM at 15 degrees and 25 degrees C, respectively. Electron transport chain (ETC) complexes were significantly less sensitive to the direct effects of Cd with IC(50) ranging from 260 to >>400 microM. Temperature increase led to a higher sensitivity of mitochondrial enzymes to the inhibitory effects of Cd as indicated by a decline in IC(50) with the exception of Complex III from gills and CS from gills and hepatopancreas. Cd exposure also resulted in a decrease in activation energy of mitochondrial enzymes suggesting that mitochondria from Cd-exposed oysters could exhibit reduced capacity to respond to temperature rise with an adequate increase in the substrate flux. These interactive effects of Cd and temperature on mitochondrial enzymes could negatively affect metabolic performance of oysters and possibly other ectotherms in polluted environments during temperature increase such as expected during the global climate change and/or tidal or seasonal warming in estuarine and coastal waters.

Effects of cadmium exposure on expression and activity of P-glycoprotein in eastern oysters, Crassostrea virginica Gmelin

Heavy metal pollution is a worldwide problem, and cadmium (Cd) is one of the most noxious pollutants in aquatic environments. We studied P-glycoprotein (P-gp) expression and function in control and Cd exposed (50microgL(-1) Cd, 30-40 days) oysters Crassostrea virginica as a possible mechanism of cell protection against Cd. Our data show that P-gp is expressed on cell membrane and in mitochondria of oyster gills and hepatopancreas. Inhibitor studies with verapamil, cyclosporine A and JS-2190 suggest that in the gills, mitochondrial P-gp pumps substrates from cytosol into the mitochondria, while cell membrane P-gp pumps substrates from cytosol out of the cell. Cd exposure resulted in a 2-2.5-fold increase in P-gp protein expression in cell membranes and a 3.5-7-fold increase in transport activity measured as the inhibitor-sensitive rhodamine B extrusion rate. In contrast, p-gp mRNA levels were similar in control and Cd-exposed oysters. No difference in P-gp protein expression was observed between mitochondria of control and Cd-exposed oysters but the apparent transport activity was higher in mitochondria from Cd-exposed oysters. Overall, a stronger increase in substrate transport activity in Cd-exposed oysters compared to a relatively weaker change in P-gp protein levels suggests that P-gp activity is post-translationally regulated. Our data show that direct determination of P-gp transport activity may be the best measure of the xenobiotic-resistant phenotype, whereas p-gp mRNA levels are not a good marker due to the likely involvement of multiple post-transcriptional regulatory steps. Cd exposure resulted in a significantly elevated rate of oxygen consumption of isolated oyster gills by 46%. Specific inhibitors of ATPase function of P-gp (cyclosporine A and JS-2190) had no significant effect on tissue oxygen consumption indicating that P-gp contribution to energy budget is negligible and supporting indirect estimates based on the ATP stoichiometry of substrate transport that also suggest low energy demand for P-gp function.

Effects of cadmium on cellular protein and glutathione synthesis and expression of stress proteins in eastern oysters, Crassostrea virginica Gmelin

Cadmium (Cd) is an important toxicant in estuarine and coastal environments that can strongly affect energy balance of aquatic organisms by increasing the organism's basal energy demand and reducing its aerobic capacity. Mechanisms of cadmium-induced increase in basal metabolic costs are not well understood and may involve elevated detoxification costs due to the synthesis of cellular protective proteins and glutathione. We studied the short-term effects of cadmium exposure (4 h) on protein and glutathione (GSH) synthesis and expression of stress proteins (heat shock proteins HSP60, HSP70 and HSP90) and metallothioneins in isolated gill and hepatopancreas cells of the eastern oyster, Crassostrea virginica. Our study showed that exposure to cadmium resulted in a dose-dependent increase in the rate of protein synthesis in oyster cells, which reached 150% of the control at the highest tested Cd level (2000 micromol l(-1)). GSH synthesis was significantly inhibited by the highest Cd concentrations, especially in hepatopancreas, which resulted in a slight but significant decrease in the total GSH concentrations. Elevated protein synthesis was associated with the increased expression of metallothioneins and heat shock proteins. Interestingly, stress protein response differed considerably between gill and hepatopancreas cells. In hepatopancreas, expression of metallothionein mRNA (measured by real-time PCR) increased 2-8-fold in response to Cd exposure, whereas no significant increase in metallothionein expression was found in Cd-exposed gill cells. By contrast, HSP60 and HSP70 protein levels increased significantly in Cd-exposed gill cells (by 1.5-2-fold) but not in hepatopancreas. No change in HSP90 expression was detected in response to Cd exposure in oyster cells. These data indicate that metallothionein expression may provide sufficient protection against Cd-induced damage to intracellular proteins in hepatopancreas, alleviating the need for overexpression of molecular chaperones. By contrast, Cd detoxification mechanisms such as inducible metallothioneins and GSH appear to be insufficient to fully prevent protein damage in gill cells, thus necessitating induction of HSPs as a secondary line of cellular defense. Therefore, gills are likely to be among the most Cd-sensitive tissues in oysters, which may have important implications for impaired oxygen uptake contributing to energy misbalance and reduced aerobic scope in Cd-exposed oysters.