Combined effects of cadmium, temperature and hypoxia-reoxygenation on mitochondrial function in rainbow trout (Oncorhynchus mykiss)

Osmo-respiratory compromise in the mosshead sculpin (Clinocottus globiceps): effects of temperature, hypoxia, and re-oxygenation on rates of diffusive water flux and oxygen uptake

In nature, mosshead sculpins (Clinocottus globiceps) are challenged by fluctuations in temperature and oxygen levels in their environment. However, it is unclear how mosshead sculpins modulate the permeability of their branchial epithelia to water and O2 in response to temperature or hypoxia stress. Acute decrease in temperature from 13 to 6 oC reduced diffusive water flux rate by 22% and ṀO2 by 51%, whereas acute increase in temperature from 13 to 25 oC increased diffusive water flux rate by 217% and ṀO2 by 140%, yielding overall Q10 values of 2.08 and 2.47 respectively. Acute reductions in oxygen tension from >95% to 20% or 10% air saturation did not impact diffusive water flux rates, however, ṀO2 was reduced significantly by 36% and 65% respectively. During 1-h or 3-h recovery periods diffusive water flux rates were depressed while ṀO2 exhibited overshoots beyond the normoxic control level. Many responses differed from those seen in our parallel earlier study on the tidepool sculpin, a cottid with similar hypoxia tolerance but much smaller gill area that occupies a similar environment. Overall, our data suggest that during temperature stress, diffusive water flux rates and ṀO2 follow the traditional osmo-respiratory compromise pattern, but during hypoxia and re-oxygenation stress, diffusive water flux rates are decoupled from ṀO2.

Combined effects of salinity and intermittent hypoxia on mitochondrial capacity and reactive oxygen species efflux in the Pacific oyster, Crassostrea gigas

Coastal environments commonly experience fluctuations in salinity and hypoxia-reoxygenation (H/R) stress that can negatively affect mitochondrial functions of marine organisms. Although intertidal bivalves are adapted to these conditions, the mechanisms that sustain mitochondrial integrity and function are not well understood. We determined the rates of respiration and reactive oxygen species (ROS) efflux in the mitochondria of oysters, Crassostrea gigas, acclimated to high (33 psu) or low (15 psu) salinity, and exposed to either normoxic conditions (control; 21% O2) or short-term hypoxia (24 h at <0.01% O2) and subsequent reoxygenation (1.5 h at 21% O2). Further, we exposed isolated mitochondria to anoxia in vitro to assess their ability to recover from acute (∼10 min) oxygen deficiency (<0.01% O2). Our results showed that mitochondria of oysters acclimated to high or low salinity did not show severe damage and dysfunction during H/R stress, consistent with the hypoxia tolerance of C. gigas. However, acclimation to low salinity led to improved mitochondrial performance and plasticity, indicating that 15 psu might be closer to the metabolic optimum of C. gigas than 33 psu. Thus, acclimation to low salinity increased mitochondrial oxidative phosphorylation rate and coupling efficiency and stimulated mitochondrial respiration after acute H/R stress. However, elevated ROS efflux in the mitochondria of low-salinity-acclimated oysters after acute H/R stress indicates a possible trade-off of higher respiration. The high plasticity and stress tolerance of C. gigas mitochondria may contribute to the success of this invasive species and facilitate its further expansion into brackish regions such as the Baltic Sea.

Cardiac Hypoxia Tolerance in Fish: From Functional Responses to Cell Signals

Aquatic animals are increasingly challenged by O₂ fluctuations as a result of global warming, as well as eutrophication processes. Teleost fish show important species-specific adaptability to O₂ deprivation, moving from intolerance to a full tolerance of hypoxia and even anoxia. An example is provided by members of Cyprinidae which includes species that are amongst the most tolerant hypoxia/anoxia teleosts. Living at low water O₂ requires the mandatory preservation of the cardiac function to support the metabolic and hemodynamic requirements of organ and tissues which sustain whole organism performance. A number of orchestrated events, from metabolism to behavior, converge to shape the heart response to the restricted availability of the gas, also limiting the potential damages for cells and tissues. In cyprinids, the heart is extraordinarily able to activate peculiar strategies of functional preservation. Accordingly, by using these teleosts as models of tolerance to low O₂, we will synthesize and discuss literature data to describe the functional changes, and the major molecular events that allow the heart of these fish to sustain adaptability to O₂ deprivation. By crossing the boundaries of basic research and environmental physiology, this information may be of interest also in a translational perspective, and in the context of conservative physiology, in which the output of the research is applicable to environmental management and decision making.

Iron Dysregulation in Mitochondrial Dysfunction and Alzheimer’s Disease

Alzheimer’s disease (AD) is a devastating progressive neurodegenerative disease characterized by neuronal dysfunction, and decreased memory and cognitive function. Iron is critical for neuronal activity, neurotransmitter biosynthesis, and energy homeostasis. Iron accumulation occurs in AD and results in neuronal dysfunction through activation of multifactorial mechanisms. Mitochondria generate energy and iron is a key co-factor required for: (1) ATP production by the electron transport chain, (2) heme protein biosynthesis and (3) iron-sulfur cluster formation. Disruptions in iron homeostasis result in mitochondrial dysfunction and energetic failure. Ferroptosis, a non-apoptotic iron-dependent form of cell death mediated by uncontrolled accumulation of reactive oxygen species and lipid peroxidation, is associated with AD and other neurodegenerative diseases. AD pathogenesis is complex with multiple diverse interacting players including Aβ-plaque formation, phosphorylated tau, and redox stress. Unfortunately, clinical trials in AD based on targeting these canonical hallmarks have been largely unsuccessful. Here, we review evidence linking iron dysregulation to AD and the potential for targeting ferroptosis as a therapeutic intervention for AD.

Anoxia-reoxygenation modulates cadmium-induced liver mitochondrial reactive oxygen species emission during oxidation of glycerol 3-phosphate

Aquatic organisms are frequently exposed to multiple stressors including low dissolved oxygen (O₂) and metals such as cadmium (Cd). Reduced O₂ concentration and Cd exposure alter cellular function in part by impairing energy metabolism and dysregulating reactive oxygen species (ROS) homeostasis. However, little is known about the role of mitochondrial glycerol 3-phosphate dehydrogenase (mGPDH) in ROS homeostasis in fish and its response to environmental stress. In this study, mGPDH activity and the effects of anoxia-reoxygenation (A-RO) and Cd on ROS (as hydrogen peroxide, H₂O₂) emission in rainbow trout liver mitochondria during oxidation of glycerol 3-phosphate (G3P) were probed. Trout liver mitochondria exhibited low mGPDH activity that supported a low respiratory rate but substantial H₂O₂ emission rate. Cd evoked a low concentration stimulatory-high concentration inhibitory H₂O₂ emission pattern that was blunted by A-RO. At specific redox centers, Cd suppressed H₂O₂ emission from site I_Q, but stimulated emission from sites III_Qo and G_Q. In contrast, A-RO stimulated H₂O₂ emission from site I_Q following 15 min exposure and augmented Cd-stimulated emission from site II_F after 30 min exposure but did not alter the rate of H₂O₂ emission from sites III_Qo and G_Q. Additionally, Cd neither altered the activities of catalase, glutathione peroxidase, or thioredoxin reductase nor the concentrations of total glutathione, reduced glutathione, or oxidized glutathione. Overall, this study indicates that oxidation of G3P drives ROS production from mGPDH and complexes I, II and III, whereas Cd directly modulates redox sites but not antioxidant defense systems to alter mitochondrial H₂O₂ emission.

Effects of long-term cadmium exposure on growth, antioxidant defense and DNA methylation in juvenile Nile tilapia (Oreochromis niloticus)

Cadmium (Cd) is a ubiquitous environmental contaminant, posing serious threats to aquatic organisms. The aims of the present study were to investigate the effects of long-term Cd exposure on the growth, GH/IGF axis, antioxidant defense and DNA methylation in juvenile Nile tilapia (Oreochromis niloticus). To this end, juvenile Nile tilapia were exposed to 0, 10 and 50 µg∙L^-1 Cd for 45 and 90 days. The obtained results revealed that exposure to high concentrations of Cd significantly decreased body mass and body length, and down-regulated mRNA levels of GHRs, IGF-I and IGF-II in the liver of Nile tilapia. Cd exposure induced oxidative stress including the reduction of antioxidant activities and increases of malondialdehyde (MDA) and 8-hydroxydeoxyguanosine (8-OHdG) contents. Beside, the global DNA methylation levels significantly decreased with increasing Cd concentration and exposure time, which might result from increased oxidative DNA damage, the down-regulated expression of DNMT3a and DNMT3b and up-regulated expression of TET1 and TET2. In conclusion, long-term Cd exposure could inhibit growth, reduce antioxidant capacity and lead to oxidative damages to lipid and DNA, and decrease global DNA methylation level in juvenile Nile tilapia.

Effects of waterborne cadmium exposure on Spinibarbus sinensis hepatopancreas and kidney: Mitochondrial cadmium accumulation and respiratory metabolism

To examine the relationship between heavy metal accumulation in mitochondria and their respiration function in fish during in vivo exposure, juvenile Spinibarbus sinensis were exposed to different waterborne cadmium (Cd) concentrations for up to 28 days. We measured the state III respiration rate and cytochrome c oxidase (CCO) activity of mitochondria in hepatopancreas and kidney and the accumulated Cd concentrations in mitochondria and heat-stable protein (HSP) fractions. Dose- and time-dependent Cd accumulation occurred at different levels in both organs, but was lower in hepatopancreas. When hepatopancreas mitochondrial Cd concentrations in Cd-exposed groups were > 5.5 μg/g dwt, their state III respiration rates were significantly lower than the control. CCO activity of hepatopancreas mitochondria exhibited decreasing dose- and time-dependent trends. However, kidney mitochondria respiratory activities were not affected significantly by Cd exposure. Cd concentrations in kidney HSP fraction were 2-5 times higher than in hepatopancreas under all exposure conditions, and were mainly present as non-deleterious metallothionein (MT)-Cd complexes. These results suggest that Cd accumulation occurred in hepatopancreas and kidney mitochondria of S. sinensis following waterborne Cd exposure, which significantly inhibited the respiration function of hepatopancreas mitochondria but did not have a deleterious effect on kidney mitochondria. The inhibitory pattern of hepatopancreas mitochondrial Cd concentrations related to function exhibited threshold and saturation effects, suggesting the capacity of S. sinensis to manage Cd toxicity. The difference in the relative proportion of Cd occurring as MT-Cd complexes in organs likely causes the organ-specific effects of Cd on hepatopancreas and kidney mitochondrial function.

Carbonyl cyanide 3-chlorophenylhydrazone induced the imbalance of mitochondrial homeostasis in the liver of Megalobrama amblycephala: A dynamic study

Carbonylcyanide-3-chlorophenylhydrazone (CCCP) is a protonophore, which causes uncoupling of proton gradient in the inner mitochondrial membrane, thus inhibiting the rate of ATP synthesis. However, this information is manly derived from mammals, while its effects on the mitochondrial homeostasis of aquatic animals are largely unknown. In this study, the mitochondrial homeostasis of a carp fish Megalobrama amblycephala was investigated systematically in a time-course manner by using CCCP. Fish was injected intraperitoneally with CCCP (1.8 mg/kg per body weight) and DMSO (control), respectively. The results showed that CCCP treatment induced hepatic mitochondrial oxidative stress, as was evidenced by the significantly increased MDA and PC contents coupled with the decreased SOD and MnSOD activities. Meanwhile, mitochondrial fission was up-regulated remarkably characterized by the increased transcriptions of Drp-1, Fis-1 and Mff. However, the opposite was true for mitochondrial fusion, as was indicative of the decreased transcriptions of Mfn-1, Mfn-2 and Opa-1. This consequently triggered mitophagy, as was supported by the accumulated mitochondrial autophagosomes and the increased protein levels of PINK1, Parkin, LC3-II and P62 accompanied by the increased LC3-II/LC3-I ratio. Mitochondrial biogenesis and function both decreased significantly addressed by the decreased activities of CS, SDH and complex I, IV and V, as well as the protein levels of PGC-1β coupled with the decreased transcriptions of TFAM, COX-1, COX-2 and ATP-6. Unlikely, DMSO treatment exerted little influence. Overall, CCCP treatment resulted in the imbalance of mitochondrial homeostasis in Megalobrama amblycephala by promoting mitochondrial oxidative stress, fission and mitophagy, but depressing mitochondrial fusion, biogenesis and function.

Comparative Phosphoproteomics Reveals a Role for AMPK in Hypoxia Signaling in Testes of Oriental River Prawn (Macrobrachium nipponense)

Hypoxia is one of the major stresses in aquaculture animals. Recently, we reported that hypoxia disrupts the endocrine system and inhibits testicular function of oriental river prawns (Macrobrachium nipponense), but the molecular mechanism of testes responded to hypoxia remains largely unknown. In the present study, we aimed to integrate whole phosphoproteomic profiles of hypoxia-treated testes of the oriental river prawn (Macrobrachium nipponense). We successfully isolated sperm cells and evaluated the mitochondrial morphology and function using laser confocal microscopy, flow cytometry, and biochemical analyses. Quantitative proteomics identified 117 differentially abundant phosphorylated proteins, and these proteins are mainly involved in the pathways related to cellular processes, including autophagy, apoptosis, and the FoxO signaling pathway. Protein-protein interaction analysis clustered these phosphoproteins into three groups, many of which have been suggested to impact carbohydrate metabolism, autophagy, and signal regulation in testes. Western blotting confirmed that phosphorylated proteins including AMPK, ULK1, and TP53 (of the AMPK pathway) may contribute to testicular dysfunction caused by hypoxia. Further, we investigated the potential roles of AMP-activated protein kinase (AMPK)'s in testes mitochondrial autophagy and apoptosis in M. nipponense as induced by hypoxia. Simultaneous knockdown of AMPKα in sperm cells led to a decrease in FOXO3a phosphorylation at Ser413, upregulation of caspase-3 and caspase-9 activities, and an increased apoptosis rate. These results improve our understanding of hypoxia-induced energy metabolism disorders in the testes of M. nipponense.

Modulation of mitochondrial site-specific hydrogen peroxide efflux by exogenous stressors

Oxygen (O₂) deprivation and metals are common environmental stressors and their exposure to aquatic organisms can induce oxidative stress by disrupting cellular reactive oxygen species (ROS) homeostasis. Mitochondria are a major source of ROS in the cell wherein a dozen sites located on enzymes of the electron transport system (ETS) and substrate oxidation produce superoxide anion radicals (O₂˙‾) or hydrogen peroxide (H₂O₂). Sites located on ETS enzymes can generate ROS by forward electron transfer (FET) and reverse electron transfer (RET) reactions; however, knowledge of how exogenous stressors modulate site-specific ROS production is limited. We investigated the effects of anoxia-reoxygenation and cadmium (Cd) on H₂O₂ emission in fish liver mitochondria oxidizing glutamate-malate, succinate or palmitoylcarnitine-malate. We find that anoxia-reoxygenation attenuates H₂O₂ emission while the effect of Cd depends on the substrate, with monotonic responses for glutamate-malate and palmitoylcarnitine-malate, and a biphasic response for succinate. Anoxia-reoxygenation exerts a substrate-dependent inhibition of mitochondrial respiration which is more severe with palmitoylcarnitine-malate compared with succinate or glutamate-malate. Additionally, specific mitochondrial ROS-emitting sites were sequestered using blockers of electron transfer and the effects of anoxia-reoxygenation and Cd on H₂O₂ emission were evaluated. Here, we find that site-specific H₂O₂ emission capacities depend on the substrate and the direction of electron flow. Moreover, anoxia-reoxygenation alters site-specific H₂O₂ emission rates during succinate and glutamate-malate oxidation whereas Cd imposes monotonic or biphasic H₂O₂ emission responses depending on the substrate and site. Contrary to our expectation, anoxia-reoxygenation blunts the effect of Cd. These results suggest that the effect of exogenous stressors on mitochondrial oxidant production is governed by their impact on energy conversion reactions and mitochondrial redox poise. Moreover, direct increased ROS production seemingly does not explain the increased adverse effects associated with combined exposure of aquatic organisms to Cd and low dissolved oxygen levels.

Transcriptome sequencing reveals the effects of cadmium toxicity on the cold tolerance of the wolf spider Pirata subpiraticus

As the predominant predator of pests in rice fields, spiders have been exposed to cadmium (Cd) pollution for a long time. The livability of spiders during the overwintering period is closely related to population growth in spring, but the effects of Cd on spider's survival of cold hardness and the underlining mechanism remain unclear. In the present study, we found that some growth parameters (body length, width, mass and livability) in the wolf spider Pirata subpiraticus were altered distinctively under Cd stress. To investigate the effects of Cd toxicity on the spider at molecular levels, RNA-sequencing was performed on the spiderlings undergoing ambient temperature alterations. Transcriptome data showed that a total of 807 differentially expressed genes (DEGs) were yielded in the comparison. The obtained DEGs were mainly linked with metabolism-related process, including oxidoreductase activity and lipid transport, and 25 DEGs were associated with the reported cryoprotectants, including glycerol, arginine, cysteine, heat shock protein, glucose and mannose. Growth factors (insulin growth factor, platelet-derived growth factor and transforming growth factor) and cytochrome P450 encoding genes were dramatically expressed in the spider. Furthermore, transcriptional factors (TFs) family were characterized according to the transcriptomic profile, and ZBTB TFs were represented the most distinctive alterations in the characterized genes. Collectively, our study illustrated that Cd poses disadvantageous effects on the growth of P. subpiraticus at cold ambient temperature, and the spiders are capable of responding to the adverse Cd stress by expressing the genes involved in the metabolism of energy substances, cryoprotectants and immune-related components.

Reverse translation: effects of acclimation temperature and acute temperature challenges on oxygen consumption, diffusive water flux, net sodium loss rates, Q10 values and mass scaling coefficients in the rainbow trout (Oncorhynchus mykiss)

Our understanding is limited on how fish adjust the effective permeability of their branchial epithelium to ions and water while altering O₂ uptake rate (MO₂) with acute and chronic changes in temperature. We investigated the effects of acclimation temperature (8 °C, 13 °C and 18 °C) and acute temperature challenges [acute rise (acclimated at 8 °C, measured at 13 °C and 18 °C), acute drop (acclimated at 18 °C, measured at 8 °C and 13 °C) and intermediate (acclimated at 13 °C, measured at 8 °C and 18 °C)] on routine MO₂, diffusive water flux, and net sodium loss rates in 24-h fasted rainbow trout (Oncorhynchus mykiss). In the temperature challenge tests, measurements were made during the first hour. In acclimated trout at all temperatures, allometric mass scaling coefficients were much higher for diffusive water flux than for MO₂. Furthermore, the diffusive water flux rate was more responsive (overall Q₁₀ = 2.75) compared to MO₂ (Q₁₀ = 1.80) over the 8-18 °C range, and for both, Q₁₀ values were greater at 8-13 °C than at 13-18 °C. The net Na⁺ flux rates were highly sensitive to acclimation temperature with an overall Q₁₀ of 3.01 for 8-18 °C. In contrast, very different patterns occurred in trout subjected to acute temperature challenges. The net Na⁺ flux rate was temperature-insensitive with a Q₁₀ around 1.0. Both MO₂ and diffusive water flux rates exhibited lower Q₁₀ values than for the acclimated rates in response to either acute increases or decreases in temperature. These results fit Pattern 5 of Precht (undercompensation, reverse effect) and more precisely Pattern IIB of Prosser (reverse translation). These inverse compensatory patterns suggest that trout do not alter their rates very much when undergoing acute thermal challenges (diurnal fluctuations, migration through the thermocline). The greater changes seen with acclimation may be adaptive to long-term seasonal changes in temperature. We discuss the roles of aquaporins, spontaneous activity, and recent feeding in these responses.

Cumulative effects of cadmium and natural stressors (temperature and parasite infection) on molecular and biochemical responses of juvenile rainbow trout

The simultaneous presence of natural and anthropogenic stressors in aquatic ecosystems can challenge the identification of factors causing decline in fish populations. These stressors include chemical mixtures and natural abiotic and biotic factors such as water temperature and parasitism. Effects of cumulative stressors may vary from antagonism to synergism at the organismal or population levels and may not be predicted from exposure to individual stressors. This study aimed to evaluate the combined effects of chronic exposure to cadmium (Cd) and elevated water temperature (23 °C) or parasite infection in juvenile rainbow trout (Oncorhynchus mykiss) using a multi-level biological approach, including RNA-sequencing. Fish were exposed to diet-borne Cd (6 μg Cd/g wet feed), individually and in combination with thermal (23 °C) or parasitic stressors, for 28 days. The parasite challenge consisted of a single exposure to glochidia (larvae) of the freshwater mussel (Strophitus undulatus), which encysts in fish gills, fins and skin. Results indicated lower fish length, weight, and relative growth rate in fish exposed to a higher water temperature (23 °C). Body condition and hepatosomatic index of trout were, however, higher in the 23 °C temperature treatment compared to the control fish kept at 15 °C. Exposure to thermal stress or parasitism did not influence tissue Cd bioaccumulation. More than 700 genes were differentially transcribed in fish exposed to the individual thermal stress treatment. However, neither Cd exposure nor parasite infection affected the number of differentially transcribed genes, compared to controls. The highest number of differentially transcribed genes (969 genes) was observed in trout exposed to combined Cd and high temperature stressors; these genes were mainly related to stress response, protein folding, calcium metabolism, bone growth, energy metabolism, and immune system; functions overlapped with responses found in fish solely exposed to higher water temperature. Only 40 genes were differentially transcribed when fish were exposed to Cd and glochidia and were related to the immune system, apoptosis process, energy metabolism and malignant tumor. These results suggest that dietary Cd may exacerbate the temperature stress and, to a lesser extent, parasitic infection stress on trout transcriptomic responses. Changes in the concentrations of liver ethoxyresorufin-o-deethylase, heat shock protein 70 and thiobarbituric acid reactive substances coupled to changes in the activities of cellular glutathione S-transferase and glucose-6-phosphate dehydrogenase were also observed at the cellular level. This study may help understand effects of freshwater fish exposure to cumulative stressors in a changing environment.

Cardiac mitochondrial function, nitric oxide sensitivity and lipid composition following hypoxia acclimation in sablefish

In fishes, the effect of O₂ limitation on cardiac mitochondrial function remains largely unexplored. The sablefish (Anoplopoma fimbria) encounters considerable variations in environmental oxygen availability, and is an interesting model for studying the effects of hypoxia on fish cardiorespiratory function. We investigated how in vivo hypoxia acclimation (6 months at 40% then 3 weeks at 20% air saturation) and in vitro anoxia-reoxygenation affected sablefish cardiac mitochondrial respiration and reactive oxygen species (ROS) release rates using high-resolution fluorespirometry. Further, we investigated how hypoxia acclimation affected the sensitivity of mitochondrial respiration to nitric oxide (NO), and compared mitochondrial lipid and fatty acid (FA) composition between groups. Hypoxia acclimation did not alter mitochondrial coupled or uncoupled respiration, or respiratory control ratio, ROS release rates, P ₅₀ or superoxide dismutase activity. However, it increased citrate synthase activity (by ∼20%), increased the sensitivity of mitochondrial respiration to NO inhibition (i.e., the NO IC₅₀ was 25% lower), and enhanced the recovery of respiration (by 21%) and reduced ROS release rates (by 25-30%) post-anoxia. In addition, hypoxia acclimation altered mitochondrial FA composition [increasing arachidonic acid (20:4ω6) and eicosapentaenoic acid (20:5ω3) proportions by 11 and 14%, respectively], and SIMPER analysis revealed that the phospholipid:sterol ratio was the largest contributor (24%) to the dissimilarity between treatments. Overall, these results suggest that hypoxia acclimation may protect sablefish cardiac bioenergetic function during or after periods of O₂ limitation, and that this may be related to alterations in mitochondrial sensitivity to NO and to adaptive changes in membrane composition (fluidity).

Effects of bioenergetics, temperature and cadmium on liver mitochondria reactive oxygen species production and consumption

A by-product of mitochondrial substrate oxidation and electron transfer to generate cellular energy (ATP) is reactive oxygen species (ROS). Superoxide anion radical and hydrogen peroxide (H₂O₂) are the proximal ROS produced by the mitochondria. Because low levels of ROS serve critical regulatory roles in cell physiology while excessive levels or inappropriately localized ROS result in aberrant physiological states, mitochondrial ROS need to be tightly regulated. While it is known that regulation of mitochondrial ROS involves balancing the rates of production and removal, the effects of stressors on these processes remain largely unknown. To illuminate how stressors modulate mitochondrial ROS homeostasis, we investigated the effects of temperature and cadmium (Cd) on H₂O₂ emission and consumption in rainbow trout liver mitochondria. We show that H₂O₂ emission rates increase with temperature and Cd exposure. Energizing mitochondria with malate-glutamate or succinate increased the rate of H₂O₂ emission; however, Cd exposure imposed different patterns of H₂O₂ emission depending on the concentration and substrate. Specifically, mitochondria respiring on malate-glutamate exhibited a saturable graded concentration-response curve that plateaued at 5 μM while mitochondria respiring on succinate had a biphasic concentration-response curve characterized by a spike in the emission rate at 1 μM Cd followed by gradual diminution at higher Cd concentrations. To explain the observed substrate- and concentration-dependent effects of Cd, we sequestered specific mitochondrial ROS-emitting sites using blockers of electron transfer and then tested the effect of the metal. The results indicate that the biphasic H₂O₂ emission response imposed by succinate is due to site II_F but is further modified at sites I_Q and III_Qo. Moreover, the saturable graded H₂O₂ emission response in mitochondria energized with malate-glutamate is consistent with effect of Cd on site I_F. Additionally, Cd and temperature acted cooperatively to increase mitochondrial H₂O₂ emission suggesting that increased toxicity of Cd at high temperature may be due to increased oxidative insult. Surprisingly, despite their clear stimulatory effect on H₂O₂ emission, Cd, temperature and bioenergetic status did not affect the kinetics of mitochondrial H₂O₂ consumption; the rate constants and half-lives for all the conditions tested were similar. Overall, our study indicates that the production processes of rainbow trout liver mitochondrial H₂O₂ metabolism are highly responsive to stressors and bioenergetics while the consumption processes are recalcitrant. The latter denotes the presence of a robust H₂O₂ scavenging system in liver mitochondria that would maintain H₂O₂ homeostasis in the face of increased production and reduced scavenging capacity.

Acute and sub-chronic toxicity bioassays of Olive Mill Wastewater on the Eastern mosquitofish Gambusia holbrooki

Olive oil production generates large volumes of wastewaters mostly in peri-Mediterranean countries with adverse impacts on the biota of the receiving aquatic systems. Few studies have however documented its toxicity on aquatic species, with an almost total lack of relative studies on fish. We assessed the acute and sub-chronic OMW toxicity, as well as the acute and sub-chronic behavioural, morphological and biochemical effects of OMW exposure on the mosquitofish Gambusia holbrooki. LC₅₀ values of the acute bioassays ranged from 7.31% (24 h) to 6.38% (96 h). Behavioural symptoms of toxicity included hypoactivity and a shift away from the water surface, coupled with a range of morphological alterations, such as skin damage, excessive mucus secretion, hemorrhages, fin rot and exophhalmia, with indications also of gill swelling and anemia. Biochemical assays showed that OMW toxicity resulted in induction of catalase (CAT) and inhibition of acetylcholinesterase (AChE) activities. The implications of our results at the level of environmental policy for the sustainable management of the olive mill industry, i.e. the effective restriction of untreated OMW disposal of in adjacent waterways, as well as the implementation of new technologies that reduce their impact (detoxification and/or revalorization of its residues) are discussed.

Respiratory complex II in mitochondrial dysfunction-mediated cytotoxicity: Insight from cadmium

In the present work we studied action of several inhibitors of respiratory complex II (CII) of mitochondrial electron transport chain, namely malonate and thenoyltrifluoroacetone (TTFA) on Cd²⁺-induced toxicity and cell mortality, using two rat cell lines, pheochromocytoma PC12 and ascites hepatoma AS-30D and isolated rat liver mitochondria (RLM). It was shown that malonate, an endogenous competitive inhibitor of dicarboxylate-binding site of CII, restored in part RLM respiratory function disturbed by Cd²⁺. In particular, malonate increased both phosphorylating and maximally uncoupled respiration rates in KCl medium in the presence of CI substrates as well as palliated changes in basal and resting state respiration rates produced by the heavy metal on the mitochondria energized by CI or CII substrates. Notably, malonate enhanced Cd²⁺-induced swelling of the mitochondria energized by CI substrates in KCl and, in a much lesser extent and at higher [Cd²⁺], in sucrose media but did not influence on the Cd²⁺ effects in NaCl medium. Besides, malonate did not affect swelling in sucrose media of RLM energized by CIV substrates under using of Cd²⁺ or Ca²⁺ whereas it strongly increased the mitochondrial swelling produced by selenite. In addition, malonate produced some protection against Cd²⁺-promoted necrotic death of AS-30D and PC12 cells and reduced intracellular reactive oxygen species (ROS) formation evoked by Cd²⁺ in PC12 cells. Importantly, TTFA, an irreversible competitive inhibitor of Q-binding site of CII, per se induced apoptosis of AS-30D cells which was inhibited by co-treatment with Cd²⁺ as well as decreased the Cd²⁺-enhanced intracellular ROS formation. In turn, decylubiquinone (dUb) at low μM concentrations did not protect AS-30D cells against the Cd²⁺-induced necrosis and enhanced the Cd²⁺-induced apoptosis of the cells. High μM concentrations of dUb were highly toxic for the cells. As consequence, the findings give new evidence indicative of critical involvement of CII in mechanism(s) of Cd²⁺-produced cytotoxicity and support the notion on CII as a perspective pharmacological target in mitochondria dysfunction-mediated conditions and diseases.

Interactive effects of chronic dietary selenomethionine and cadmium exposure in rainbow trout (Oncorhynchus mykiss): A preliminary study

The present study investigated the interactive effects of dietary cadmium (Cd) and selenium (Se) on the tissue-specific (liver, kidney, and muscle) accumulation of these two elements, hepatic oxidative stress response, and morphometrics in rainbow trout (Oncorhynchus mykiss) during chronic exposure. Fish were exposed to elevated dietary Cd (45 μg g^-1 dry wt.), and medium (10 μg g^-1 dry wt.) or high (45 μg g^-1 dry wt.) dietary selenium (added as selenomethionine), both alone and in combination, for 30 days. Exposure to dietary Cd alone caused oxidative stress in fish as reflected by reduced thiol redox (GSH:GSSG), increased lipid peroxidation, and induction of anti-oxidative enzymes (catalase, superoxide dismutase, and glutathione peroxidase) in the liver. Also, an increase in tissue-specific Cd burden and impaired morphometrics (hepato-somatic index and condition factor) were also recorded in fish following exposure to dietary Cd. In contrast, the dietary co-exposure to Cd and Se (at both medium and high doses) resulted in a decrease in Cd burden in the liver and kidney of fish. However, co-exposure to medium, but not high, dose of dietary Se completely alleviated Cd-induced oxidative stress and impaired morphometrics in fish, indicating that the reduced Cd tissue burden might not have been the primary factor behind the amelioration of Cd toxicity by Se. Overall, our study demonstrated that the protective effect of Se against the chronic Cd toxicity in fish is mainly mediated by the anti-oxidative properties of Se, but this protective effect is dose-specific and occurs only at a moderate exposure dose.

Effects of heat and cadmium exposure on stress-related responses in the liver of female zebrafish: Heat increases cadmium toxicity

In this study, female zebrafish (Danio rerio) were exposed to 26°C or 34°C, 0 or 197μg/L cadmium (Cd), singly or in combination for 7days. Multiple stress-related indicators were evaluated in the liver. Mortality, lipid peroxidation (LPO) and ultrastructural damage increased significantly by Cd exposure alone, and were not affected by heat alone. Interestingly, the combined exposure increased LPO, ultrastructural damage, and mortality compared with Cd exposure alone. The results indicated that elevated temperature increased Cd toxicity, which could be explained by several reasons. Firstly, Cd-exposed fish failed to activate the antioxidant defense system under heat stress. Secondly, expression levels of heat shock protein 70 (HSP70) were not significantly up-regulated by heat in Cd-exposed fish but increased by 117 times in Cd-free fish. Besides, hypermethylation of heat shock factor (HSF) binding motif in HSP70 promoter was observed during the combined exposure, indicating that simultaneous exposure may have partially suppressed the cytoprotective up-regulation of HSP70. Thirdly, heat induced an immunosuppressive effect in Cd-exposed fish, as reflected by the reduced mRNA and activity levels of nitric oxide synthase (iNOS) and interleukin-1β (IL-1β) expression levels. Finally, heat down-regulated Zir-, Irt-like protein 8 (ZIP8) and copper transporter 1 (CTR1) and up-regulated metallothioneins (MTs) in Cd-exposed fish, possibly suggesting Cu and Zn depletion and Cd accumulation. Hence, our data provide evidences that warmer temperatures can potentiate Cd toxicity, involved in the regulation of gene transcription, enzymatic activity, and DNA methylation. We found that heat indicators showed varied sensitivity between normal and Cd-exposed fish, emphasizing that the field metal pollution should be carefully considered when evaluating effects of climate change.

Heat-induced oxidative stress and inflammation involve in cadmium pollution history in the spleen of zebrafish

Zebrafish were exposed to 0, 2.5 and 5 μg/L cadmium (Cd) for 10 weeks, and then each group was exposed to 26 °C(control) and 32 °C (high temperature) for 7 days. 22 indicators were compared between 26 °C and 32 °C in the spleen, including body weight, LPO and NO levels, activity levels of Cu/Zn-SOD, CAT and iNOS, MTs protein levels, and mRNA levels of Nrf2, Cu/Zn-SOD, CAT, HSF1, HSF2, HSP70, MTF-1, MTs, IL-6, IL-10, IL-1β, TNF-α, iNOS and NF-κB. Most indicators were not significantly affected by heat in fish from no Cd pollution. However, almost all of indicators were responsive to heat in fish pre-exposed to Cd. Several indicators were sensitive to heat in fish pre-exposed to 2.5 μg/L Cd such as iNOS activities, and mRNA levels of iNOS and IL-10. Most other indicators were sensitive to heat in fish pre-exposed to 5 μg/L. The mRNA levels of HSP70 and MTF-1 were up-regulated by heat in fish pre-exposed to 0, 2.5 and 5 μg/L Cd. However, the magnitude of increase was the greatest in fish pre-exposed to 5 μg/L Cd. These differences between control and high temperature would serve as biomarkers to distinguish healthy from Cd-polluted group. The findings imply that metal pollution history should be carefully considered when screening heat biomarkers in fish.

Heat indicators of oxidative stress, inflammation and metal transport show dependence of cadmium pollution history in the liver of female zebrafish

Environmental stressors such as high temperature and metal exposure may occur sequentially, simultaneously, previously in aquatic ecosystems. However, information about whether responses to high temperature depend on Cd exposure history is still unknown in fish. Zebrafish were exposed to 0 (group 1), 2.5 (group 2) and 5μg/L (group 3) cadmium (Cd) for 10 weeks, and then each group was subjected to Cd-free water maintained at 26°C and 32°C for 7days respectively. 26 indicators were used to compare differences between 26°C and 32°C in the liver of female zebrafish, including 5 biochemical indicators (activity of Cu/Zn-SOD, CAT and iNOS; LPO; MT protein), 8 molecular indicators of oxidative stress (mRNA levels of Nrf2, Cu/Zn-SOD, CAT, HSF1, HSF2, HSP70, MTF-1 and MT), 5 molecular indicators of inflammation (mRNA levels of IL-6, IL-1β, TNF-α, iNOS and NF-κB), 8 molecular indicators of metal transport (mRNA levels of, ZnT1, ZnT5, ZIP8, ZIP10, ATP7A, ATP7B and CTR1). All biochemical indicators were unchanged in group 1 and changed in group 2 and 3. Contrarily, differences were observed in almost all of molecular indicators of inflammation and metal transport in group 1, about half in group 2, and few in group 3. We also found that all molecular indicators of oxidative stress in group 2 and fewer in group 1 and 3 were significantly affected by heat. Our data indicated that heat indicators of oxidative stress, inflammation and metal transport showed dependence of previous cadmium exposure in the liver of zebrafish, emphasizing metal pollution history should be carefully considered when evaluating heat stress in fish.

Preheating mitigates cadmium toxicity in zebrafish livers: Evidence from promoter demethylation, gene transcription to biochemical levels

The working hypothesis for this study was that moderate heat stress would alleviate the deleterious effects of subsequent cadmium (Cd) exposure on fish. Thus, zebrafish (Danio rerio) were subjected to water maintained at 26°C and 34°C for 4days, and then exposed to 0 or 200μg/L Cd for 1 week at 26°C. Multiple indicators were measured from livers of zebrafish at different levels, including DNA, RNA, protein and enzymatic activity associated with oxidative stress, inflammation and metal transport. The ameliorative effect of preheatinging on Cd toxicity was demonstrated. In the Cd-exposed groups, preheating decreased mortality and lipid peroxidation, increased activity levels of catalase (CAT) and copper/zinc-superoxide dismutase (Cu/Zn-SOD), and up-regulated mRNA levels of heat shock protein 70 (HSP70) and heat shock factor 2 (HSF2). Preheating also mitigated Cd-induced increases in protein and mRNA levels of metallothioneins (MTs), and mRNA levels of several inflammation-related genes. Furthermore, preheating alone dramatically up-regulated mRNA levels of genes related to antioxidant and immune defenses, zinc and copper transporters, protein folding, and reduced methylation levels in the HSF binding motif of the HSP70 promoter. Overall, preheating-induced accumulation of transcripts via demethylation might support the rapid defense responses at post-transcriptional levels caused by subsequent Cd exposure, indicating an adaptive mechanism for organisms exposed to one mild stressor followed by another.

Immunosuppressive effects and associated compensatory responses in zebrafish after full life-cycle exposure to environmentally relevant concentrations of cadmium

In natural environments, fish survive in polluted water by cadmium (Cd) throughout their whole life cycle. However, little information is available on Cd toxicity considering a life cycle assessment. The present study investigated effects of environmental levels of cadmium (0, 2.5, and 5μg/L) on immune responses in liver and spleen of zebrafish for 15 weeks, from embryos to sexually maturity. Nitric oxide (NO) levels and iNOS activity declined in liver and spleen of zebrafish exposed to 5μg/L Cd, suggesting an immunosuppressive effect. The result was further supported by the decreased transcriptional levels of proinflammatory cytokines by Cd, such as interleukin-6 (IL-6), interleukin-10 (IL-10), interleukin-1β (IL-1β), and tumour necrosis factor-α (TNF-α) in liver. However, a sharp increase in the mRNA levels of these cytokines was observed in spleen of zebrafish exposed to Cd. The increased mRNA expression of these proinflammatory cytokines may be the secondary effect following immunosuppression and just reflect a compensatory mechanism for coping with the decreased immunity, which may explain an increase in mRNA levels and a decrease in iNOS activity in spleen of zebrafish exposed to Cd. In liver, the down-regulated mRNA levels of iNOS paralleled with the decreased iNOS activity, suggesting a synchronous response from a molecular level to a biochemical level. Positive correlations between mRNA expression levels of nuclear transcription factor κB (NF-κB) and proinflammatory cytokines were also observed, suggesting that NF-κB might be required for the protracted induction of inflammatory genes. The corresponding changes in the mRNA levels of the inhibitor of κBα (IκBαa and IκBαb) may form a feedback loop to restore transcriptional activity of NF-κB. Furthermore, splenic ROS levels were increased by 5μg/L Cd, possibly activating NF-κB pathway. Taken together, immunosuppressive effects and tissue-dependent compensatory responses were demonstrated in zebrafish after full life-cycle exposure to environmental levels of Cd, indicating a compromise between survival and immunity.