Effect of microcystin-LR on protein phosphatase activity in fed and fasted juvenile goldfish Carassius auratus L

A Review of Common Cyanotoxins and Their Effects on Fish

Global warming and human-induced eutrophication drive the occurrence of various cyanotoxins in aquatic environments. These metabolites reveal diversified mechanisms of action, encompassing cyto-, neuro-, hepato-, nephro-, and neurotoxicity, and pose a threat to aquatic biota and human health. In the present paper, we review data on the occurrence of the most studied cyanotoxins, microcystins, nodularins, cylindrospermopsin, anatoxins, and saxitoxins, in the aquatic environment, as well as their potential bioaccumulation and toxicity in fish. Microcystins are the most studied among all known cyanotoxins, although other toxic cyanobacterial metabolites are also commonly identified in aquatic environments and can reveal high toxicity in fish. Except for primary toxicity signs, cyanotoxins adversely affect the antioxidant system and anti-/pro-oxidant balance. Cyanotoxins also negatively impact the mitochondrial and endoplasmic reticulum by increasing intracellular reactive oxygen species. Furthermore, fish exposed to microcystins and cylindrospermopsin exhibit various immunomodulatory, inflammatory, and endocrine responses. Even though cyanotoxins exert a complex pressure on fish, numerous aspects are yet to be the subject of in-depth investigation. Metabolites other than microcystins should be studied more thoroughly to understand the long-term effects in fish and provide a robust background for monitoring and management actions.

Osmoregulatory disturbance in Neotropical fish exposed to the crude extracts of the cyanobacterium, Radiocystis fernandoi

Blooms of cyanobacteria, a common event in eutrophic environments, result in the release of potentially toxic substances into the water. The cyanobacterium Radiocystis fernandoi produces microcystin (MC) and other peptides that may disturb homeostasis. This study evaluated the effect of intraperitoneal injections containing the crude extract (CE) of R. fernandoi strain R28 on the gills and kidneys of neotropical fish, Piaractus mesopotamicus, 3, 6 and 24 h post-injection. CE contained MC-RR, MC-YR and minor other oligopeptides. Plasma ions and the activities of the enzymes PP1 and PP2A, Na⁺/K⁺-ATPase (NKA), H⁺-ATPase (HA) and carbonic anhydrase (CA) were determined and morphological changes in both the gills and kidneys were characterized. Compared to controls, the concentration of Na⁺ within the plasma of P. mesopotamicus decreased after treatment with CE 3 h post treatment and increased after 24 h; the concentration of K⁺ decreased after 6 h. The activity of the endogenous PP1 and PP2A was unchanged in the gills and was inhibited in the kidneys 6 h after i.p. injection. In the gills, NKA activity increased after 3 h and decreased 6 h post i.p. exposure. Further, NKA activity did not differ from the controls 24-h post injection. In the kidneys, NKA, HA and CA activities were unaffected by treatment. The mitochondria-rich cell (MRC) density in the gills decreased after 3 h in the filament and 3 and 6 h in the lamellae and was restored to the control levels 24 h post-exposure. Filament epithelial hyperplasia and hypertrophy, lamellar atrophy and rupture of the lamellar epithelium were the most common effects of treatment in the gills. No histopathological changes occurred in the kidneys. This study demonstrates that a single dose of toxic CE from R. fernandoi can cause a transitory ion imbalance in P. mesopotamicus which is related to the changes in MRC levels and NKA activity. Ionic balance was recovered 24 h post i.p. injection, however, morphological changes that occurred in the gills took a longer amount of time to return to normal. To conclude, the effects of components contained within the CE of R. fernandoi may be harmful to P. mesopotamicus. In particular, the recovery of ionic regulation depends on MRC responses and histopathological changes produced by CE may affect gas exchange and other gill functions.

Molecular effects of Microcystin-LA in tilapia (Oreochromis niloticus)

Nile tilapia (Oreochromis niloticus) is a freshwater phytoplanktivorous fish species reported to accumulate and tolerate large amounts of cyanotoxins such as microcystins (MCs). The present study aimed to investigate molecular responses to the acute exposure of Nile tilapia to the Microcystin-LA analogue (MC-LA). Thus, the specimens were sublethally exposed to 1000 μg kg^-1 of MC-LA for 12, 24, 48, and 96 h. Gene expression of PP1, PP2A, GST, GPX and actin was analyzed by quantitative PCR. The protein abundance profile of PP2A was determined by immunoblotting, while the integrity of its biological function was assessed by a phosphatase enzymatic assay. PP2A activity was significantly and strongly reduced by MC-LA. A resulting feedback mechanism significantly increased PP2A gene expression and protein abundance in all assessed times. However, a recovery of that phosphatase activity was not observed. In this study, the observed increase in GPX gene expression was the only response that could be directly related to the unknown factors associated to the fish survival to such high dose exposure.

Microcystin-LR leads to oxidative damage and alterations in antioxidant defense system in liver and gills of Brycon amazonicus (SPIX & AGASSIZ, 1829)

Microcystin's (MCs) are toxins produced by several groups of cyanobacteria, in water bodies throughout the world, in a process which is being intensified by human action. Among the variants of MCs, MC-LR stands out for its distribution and toxicity. MCs are potent inhibitors of protein phosphatases 1 and 2 A, which causes disruption of the cytoskeleton and consequent cell death. They can also alter the antioxidant system and induce oxidative stress in various organs of many species. There is, however, a lack of information about the effects of MCs on the antioxidant system and oxidative damage in Brazilian fishes. This study evaluated the effect of microcystin-LR on the antioxidant system in liver and gills of the Brazilian fish Brycon amazonicus, after 48 h of i.p injection of 100 μg MC-LR.kg^-1 body mass. The liver exhibited increases in the activity of GST (74%) and GPx (217%), and a 47% decrease in SOD activity, with no changes in CAT values. In the gills of fish exposed to MC-LR, CAT and GPx activities did not show significant changes, while SOD and GST activity decreased by 66% and 37%, respectively. The GSH content did not change significantly in the liver, however, a decrease of 43% was observed in the gills. Oxidative damage measured by protein oxidation (PC) and lipoperoxidation (LPO) showed significant effects in both tissues. In hepatic tissue, there was no change in PC levels but LPO increased by 116%. Conversely, in the gills LPO levels did not change but PC increased by 317%. In conclusion, these data show that MC-LR induces oxidative damage in both tissues but in different ways, with being liver most sensitive to LPO and gills to PC. This also suggests that the gills are most sensitive to oxidative stress than liver, due to the inhibition of its antioxidant responses following MC-LR exposure.

Microcystin Bioaccumulation in Freshwater Fish at Different Trophic Levels from the Eutrophic Lake Chaohu, China

The spatial and temporal variations of microcystins (MCs) in fishes with different trophic levels were studied monthly in Lake Chaohu in 2014. MCs content in muscle was highest in phytoplanktivorous Hypophthalmichthys molitrix (H. molitrix), followed by omnivorous Carassius auratus (C. auratus), and was lowest in herbivorous Parabramis pekinensis (P. pekinensis) and carnivorous Coilia ectenes (C. ectenes). MCs concentration in liver was highest in C. auratus, followed by H. molitrix, and was lowest in P. pekinensis and C. ectene. The main uptake routes of MCs for C. auratus and H. molitrix were via the diet. The mechanism to counteract MCs had not been well developed in C. ectenes. H. molitrix and C. auratus from the western region as they had higher mean concentrations of MCs than fishes from the eastern region. The estimated daily intakes of MCs in 45.4% of muscle samples were higher than the provisional tolerable daily intake set by WHO.

Hepatotoxicity and metabolic effects of cellular extract of cyanobacterium Radiocystis fernandoi containing microcystins RR and YR on neotropical fish (Hoplias malabaricus)

The toxicological effect of cellular extract of cyanobacterium Radiocystis fernandoi strain R28 containing RR and YR microcystins was analyzed in the fish Hoplias malabaricus with emphasis on the liver structure and energetic metabolism, after short-term exposure. Fish were intraperitoneally (i.p.) injected with 100 μg of equivalent MC-LR kg^-1 body mass containing in the cellular extract of R. fernandoi strain R28. Twelve and 96 h post-injection, the plasma, liver and white muscle were sampled for biochemical analyses and liver was also sampled for morphological analyses. After i.p. injection, the activity of acid phosphatase (ACP), alanine aminotransferase (ALT) and direct bilirubin increased in the plasma, while ALT and aspartate aminotransferase (AST) decreased in the liver. Glucose, lactate and pyruvate increased while protein decreased in the plasma; glycogen, pyruvate and lactate decreased in the liver; and glycogen and glucose increased in the muscle. Ammonia increased in the plasma, liver and muscle. The hepatocyte cell shape changed from polyhedral to round after cellular extract injection; there was loss of biliary canaliculus organization, but the biliary duct morphology was conserved in the liver parenchyma. In conclusion, microcystins present in the cellular extract of R. fernandoi strain R28 affect the liver structure of H. malabaricus, but the liver was able to continuously produce energy by adjusting its intermediate metabolism; glycogenolysis and gluconeogenesis maintained glucose homeostasis and energy supply.

Crude extract of cyanobacteria (Radiocystis fernandoi, strain R28) induces liver impairments in fish

Radiocystis fernandoi R28 strain is a cyanobacterium which produces mostly the RR and YR microcystin variants (MC-RR and MC-YR, respectively). The effects of crude extract of the R. fernandoi strain R28 were evaluated on the protein phosphatases and on the structure and ultrastructure of the liver of the Neotropical fish, Hoplias malabaricus, after acute and subchronic exposure. Concomitantly, the accumulation of the majority of MCs was determined in the liver and muscle. The fish were exposed to 120.60 MC-RR+MC-LR kg-fish^-1 (=100μg MC-LReq kg-fish^-1) for 12 and 96h (one single dose, acute exposure) and 30days (one similar dose every 72h, subchronic exposure). MCs did not accumulate in the muscle but, in the liver, MC-YR accumulated after acute exposure and MC-RR and MC-YR accumulation occurred after subchronic exposure. Protein phosphatase 2A (PP2A) activity was inhibited only after subchronic exposure. Acute exposure induced liver hyperemia, hemorrhage, changes in hepatocytes and cord-like disorganization. At the ultrastructural level, the decreasing of glycogen and lipid levels, the swelling of mitochondria and whirling of endoplasmic reticulum suggested hepatocyte necrosis. Subchronic exposure resulted in a complete disarrangement of cord-like hepatocytes, some recovery of mitochondria and whirling endoplasmic reticulum and extensive connective tissues containing fibrous materials in the liver parenchyma. Despite microcystin toxicity and liver alterations, no tumor was induced by MCs. In conclusion, the increased algal mass of R. fernandoi in tropical freshwater, producing mainly MC-RR and MC-YR variants, results in fish liver impairments.

Quantitatively evaluating detoxification of the hepatotoxic microcystin-LR through the glutathione (GSH) pathway in SD rats

Glutathione (GSH) plays crucial roles in antioxidant defense and detoxification metabolism of microcystin-LR (MC-LR). However, the detoxification process of MC-LR in mammals remains largely unknown. This paper, for the first time, quantitatively analyzes MC-LR and its GSH pathway metabolites (MC-LR-GSH and MC-LR-Cys) in the liver of Sprague-Dawley (SD) rat after MC-LR exposure. Rats received intraperitoneal (i.p.) injection of 0.25 and 0.5 lethal dose 50 (LD50) of MC-LR with or without pretreatment of buthionine-(S,R)-sulfoximine (BSO), an inhibitor of GSH synthesis. The contents of MC-LR-GSH were relatively low during the experiment; however, the ratio of MC-LR-Cys to MC-LR reached as high as 6.65 in 0.5 LD50 group. These results demonstrated that MC-LR-GSH could be converted to MC-LR-Cys efficiently, and this metabolic rule was in agreement with the data of aquatic animals previously reported. MC-LR contents were much higher in BSO + MC-LR-treated groups than in the single MC-LR-treated groups. Moreover, the ratio of MC-LR-Cys to MC-LR decreased significantly after BSO pretreatment, suggesting that the depletion of GSH induced by BSO reduced the detoxification of MCs. Moreover, MC-LR remarkably induced liver damage, and the effects were more pronounced in BSO pretreatment groups. In conclusion, this study verifies the role of GSH in the detoxification of MC-LR and furthers our understanding of the biochemical mechanism for SD rats to counteract toxic cyanobacteria.

Damage and recovery of the ovary in female zebrafish i.p.-injected with MC-LR

Up to now, in vivo studies on toxic effects of microcystins (MCs) on the reproductive system are limited and the underlying molecular mechanisms of MCs-induced reproductive toxicity remain to be elucidated. In an acute toxic experiment, female zebrafish (Danio rerio) were injected intraperitoneally (i.p.) at doses of 50 and 200 μg MC-LR/kg body weight (BW) respectively, and histopathological lesions and antioxidant enzymatic activities and gene expression in the ovary were studied at 1, 3, 12, 24, 48 and 168 h post injection (hpi). Pathological lesions of zebrafish ovary progressed in severity and extent with the increasing exposure time and dose within 12 hpi. Concurrently, the increases in malondialdehyde (MDA) contents as well as the enzymatic activities and transcriptional levels of antioxidant enzymes catalase (CAT), superoxide dismutase (SOD) and glutathione peroxidase (GPx) showed the occurrence of oxidative stress, indicating that MC-LR induced adverse effects on the structure and functional activity of zebrafish ovary. Oxidative stress plays a significant role in the reproductive toxicity of MC-LR. The significant decrease of glutathione (GSH) content in zebrafish ovary suggested the importance of MC-LR detoxification by glutathione S-transferases (GST) via GSH. The final recovery of histostructure and antioxidative indices indicated that ovarian efficient antioxidant defense system might be an important mechanism of zebrafish to counteract MC-LR. Although the negative effects of MC-LR can be overcome by ovarian antioxidant system in this study, the potential reproductive risks of MC-LR should not be neglected because of its wide occurrence.

Induction of liver GST transcriptions by tert-butylhydroquinone reduced microcystin-LR accumulation in Nile tilapia (Oreochromis niloticus)

The cyanobacterial toxin, MC-LR, is predominantly presented during toxic cyanobacterial blooms and is consumed by phytoplanktivorous fish and zooplanktivorous fish directly. Detoxification of MC-LR in liver was believed to begin with conjugate formation with GSH, catalyzed by GSTs. MC-LR GSH conjugates display increased solubility and are subjected to accelerated biliary excretion. In this study, we showed that the mRNA transcriptions of GSTA, GPX and UCP2 were increased within 8h following MC-LR exposure in isolated hepatocytes of Nile tilapia, confirming the roles of phase II enzymes, especially GSTs, in MC-LR detoxification in tilapia. The widely used food-additive, synthetic antioxidant, tert-butylhydroquinone (tBHQ) has been shown to induce phase II enzymes including GSTs, via the antioxidant responsive elements (ARE) locate in the regulatory regions of these genes. Our results also showed that the transcription of various GSTs, including GSTA, GSTR2 and GSTT were significantly induced by tBHQ in Nile tilapia. In consistence, fish fed on tBHQ-containing diet (0.01 percent tBHQ) showed significantly reduced MC-LR accumulation in liver tissues 48 h after an oral administration of a single dose of 250 μg MC-LR/kg body weight (bwt). The findings in this research suggested that tBHQ could reduce MC-LR accumulations in liver, likely through the induction of phase II metabolizing enzymes such as GSTs. Subacute effects of tBHQ and its potential applications in fishery need to be further investigated.

Influence of the exposure way and the time of sacrifice on the effects induced by a single dose of pure Cylindrospermopsin on the activity and transcription of glutathione peroxidase and glutathione-S-transferase enzymes in Tilapia (Oreochromis niloticus)

Cylindrospermopsin is a cyanobacterial toxin frequently implicated in cyanobacterial blooms that is approaching an almost cosmopolitan distribution pattern. Moreover, the predominant extracellular availability of this cyanotoxin makes it particularly likely to be taken up by a variety of aquatic organisms including fish. Recently, Cylindrospermopsin has shown to alter the activity and gene expression of some of the glutathione related enzymes in tilapias (Oreochromis niloticus), but little is known about the influence of the route of exposure and the time of sacrifice after a single exposure to Cylindrospermopsin on these biomarkers. With this aim, tilapias were exposed by gavage or by intraperitoneal injection to a single dose of 200 μg kg(-1) bw of pure Cylindrospermopsin and after 24h or 5d they were sacrificed. The activity and relative mRNA expression by real-time PCR of antioxidant enzymes glutathione peroxidase and soluble glutathione-S-transferases (sGST) and the sGST protein abundance by Western blot analysis were evaluated in liver and kidney. Results showed differential responses in dependence on the variables considered with a higher toxicity with the intraperitoneal exposure and with 5d as time of sacrifice.

Oxidative stress and detoxification biomarker responses in aquatic freshwater vertebrates exposed to microcystins and cyanobacterial biomass

Cyanobacterial blooms represent a serious threat to the aquatic environment. Among other effects, biochemical markers have been studied in aquatic vertebrates after exposures to toxic cyanobacteria. Some parameters such as protein phosphatases may serve as selective markers of exposure to microcystins, but under natural conditions, fish are exposed to complex mixtures, which affect the overall biomarker response. This review aims to provide a critical summary of biomarker responses in aquatic vertebrates (mostly fish) to toxic cyanobacteria with a special focus on detoxification and oxidative stress. Detoxification biomarkers such as glutathione (GSH) and glutathione-S-transferase (GST) showed very high variability with poor general trends. Often, stimulations and/or inhibitions and/or no effects at GSH or GST have been reported, even within a single study, depending on many variables, including time, dose, tissue, species, etc. Most of the oxidative stress biomarkers (e.g., superoxide dismutase, catalase, glutathione peroxidase, and glutathione reductase) provided more consistent responses, but only lipid peroxidation (LPO) seemed to fulfill the criteria needed for biomarkers, i.e., a sufficiently long half-life and systematic response. Indeed, reviewed papers demonstrated that toxic cyanobacteria systematically elevate levels of LPO, which indicates the important role of oxidative damage in cyanobacterial toxicity. In summary, the measurement of biochemical changes under laboratory conditions may provide information on the mode of toxic action. However, comparison of different studies is very difficult, and the practical use of detoxification or oxidative stress biomarkers as diagnostic tools or early warnings of cyanobacterial toxicity is questionable.

The role of cysteine conjugation in the detoxification of microcystin-LR in liver of bighead carp (Aristichthys nobilis): a field and laboratory study

The role of glutathione (GSH) and cysteine (Cys) conjugates in the detoxification of microcystin-LR (MC-LR) in bighead carp (Aristichthys nobilis) was examined under laboratory and field conditions. Wild individuals of bighead carp were collected from 5 eutrophic lakes along the Yangtze River, while in laboratory experiment, bighead carp were injected intraperitoneally with 500 μg purified MC-LR/kg body weight (bw). Contents of MC-LR and its glutathione (MC-LR-GSH) and cysteine conjugates (MC-LR-Cys) in the liver of bighead carp were determined by liquid chromatography electrospray ionization mass spectrum (LC-ESI-MS). In laboratory experiment, low concentrations of MC-LR-GSH (mean: 0.042 μg/g dry weight (DW)) were always detectable, and the mean ratio of MC-LR-Cys to MC-LR-GSH was 6.55. While, in field study, relatively high MC-LR-Cys concentration (mean: 0.22 μg/g DW) was detected, whereas MC-LR-GSH was occasionally detectable, and the average ratio of MC-LR-Cys to MC-LR-GSH was as high as 71.49. A positive correlation was found between MC-LR-Cys concentration in the liver of bighead carp and MC-LR content in seston from the five lakes (r = 0.85). These results suggest that MC-LR-Cys might be much more important than MC-LR-GSH in the detoxification of MC-LR in fish liver, and that cysteine conjugation of MC-LR might be a physiological mechanism for the phytoplanktivorous bighead carp to counteract toxic cyanobacteria.

Dynamics of protein phosphatase gene expression in Corbicula fluminea exposed to microcystin-LR and to toxic Microcystis aeruginosa cells

This study investigated the in vivo effects of microcystins on gene expression of several phosphoprotein phosphatases (PPP) in the freshwater clam Corbicula fluminea with two different exposure scenarios. Clams were exposed for 96 h to 5 μg L⁻¹ of dissolved microcystin-LR and the relative changes of gene expression of three different types of PPP (PPP1, 2 and 4) were analyzed by quantitative real-time PCR. The results showed a significant induction of PPP2 gene expression in the visceral mass. In contrast, the cyanotoxin did not cause any significant changes on PPP1 and PPP4 gene expression. Based on these results, we studied alterations in transcriptional patterns in parallel with enzymatic activity of C. fluminea for PPP2, induced by a Microcystis aeruginosa toxic strain (1 × 10⁵ cells cm⁻³) during 96 h. The relative changes of gene expression and enzyme activity in visceral mass were analyzed by quantitative real-time PCR and colorimetric assays respectively. The clams exhibited a significant reduction of PPP2 activity with a concomitant enhancement of gene expression. Considering all the results we can conclude that the exposure to an ecologically relevant concentration of pure or intracellular microcystins (-LR) promoted an in vivo effect on PPP2 gene expression in C. fluminea.

Pathological modifications following sub-chronic exposure of medaka fish (Oryzias latipes) to microcystin-LR

Microcystins (MCs) are toxic monocyclic heptapeptides produced by many cyanobacteria. MCs, especially MC-LR, cause toxic effects in animals and are a recognized potent cause of environmental stress and health hazard in aquatic ecosystems when heavy blooms of cyanobacteria appear. Consequently, one of the major problems is the chronic exposure of fish to cyanotoxins in their natural environment. The present experiment involving chronic exposure confirmed initial findings on acute exposure to MC contamination: exacerbated physiological stress and tissue damage in several tissues of exposed medaka fish. The gonads were affected specifically. In female gonads the modifications included reduction of the vitellus storage, lysis of the gonadosomatic tissue and disruption of the relationships between the follicular cells and the oocytes. In the males, spermatogenesis appeared to be disrupted. This is the first report showing that a cyanotoxin can affect reproductive function, and so can impact on fish reproduction and thus fish stocks.

Oral toxicity of extracts of the microcystin-containing cyanobacterium Planktothrix agardhii to the medaka fish (Oryzias latipes)

As previously demonstrated the medaka fish appears to offer a good model for studies of microcystins (MCs) effects. Since cyanobacterial toxins are released with other molecules in the aquatic environment when the producers are dying, in this study, we performed additional experiments in order to compare the described effects obtained with the pure toxin microcystin-LR (MC-LR), among the most toxic MCs, to those induced by complex extracts of an MCs-producer Planktothrix agardhii, strain PMC 75.02 and a natural bloom containing the MCs-producer P. agardhii. The toxicity of these extracts containing several variants of MC was determined in adult medaka treated by gavage. Extracts of an MCs-free strain of P. agardhii (PMC 87.02) were assayed for comparison. Extracts effects were analysed on two tissues, liver and intestine by means of photon and transmission electron microscopy. MC was localized in these tissues by immunocytochemistry. No effect was detectable with extracts of the MCs-free P. agardhii strain. The two MCs-P. agardhii extracts (strain and natural bloom) were able to induce harmful effects in the liver and intestine of the medaka fish in acute intoxication by gavage. In these target organs as shown by toxin immunolocalization, reactions leading to cell disjunction and lysis were observed apparently associated with an immune reaction implying MC containing macrophages. These effects are similar to those previously described with photonic microscopy in medaka treated with pure MC-LR with additional results obtained under the electron microscope. Since no significant effect was detected with the MCs-free (PMC 87.02) extract, we then conclude that MCs, even in complex association with other cyanobacterial components, should be responsible for the toxic effects observed in treated fish.

Antioxidant response in liver of the phytoplanktivorous bighead carp (Aristichthys nobilis) intraperitoneally-injected with extracted microcystins

The worldwide occurrence of cyanobacterial blooms makes it necessary to perform environmental risk assessment procedures to monitor the effects of microcystins (MCs) on fish. Oxidative stress biomarkers are valuable tools in this regard. In the present study, phytoplanktivorous bighead carp (Aristichthys nobilis) were injected intraperitoneally (i.p.) with extracted MCs (mainly MC-RR and -LR) at two doses, 400 and 1,000 microg kg(-1) bw, and antioxidant responses of the liver as biomarkers of oxygen-mediated toxicity were studied at 1, 3, 12, 24 and 48 h after injection. Contents of reactive oxygen species (ROS) and activities of antioxidant enzymes [catalase (CAT), superoxide dismutase (SOD), glutathione peroxide (GPX), and glutathione reductase (GR)] as well as glutathione S-transferase (GST) in the liver in both dose groups showed a biphasic change with an increase at initial 3 h followed by a decrease after injection, owing to the roles of the antioxidant system in eliminating excessive ROS and regenerating glutathione (GSH). The increased GST was probably due to the high transcription of cytosolic GST alpha and rho, suggesting the importance of MCs detoxification by GSH pathway. The stable GSH levels in liver may be explained by the high basic GSH concentration in liver, and/or an increased GSH synthesis, suggesting a high ability to detoxify MCs and to release associated high oxidative pressure in phytoplantivorous fish.

Microcystin dynamics in aquatic organisms

Eutrophication of surface water has increased significantly during the past decade, resulting in increased occurrences of toxic blooms. Cyanotoxins have become a global health threat to humans, wild animals, or domestic livestock. Hepatotoxic microcystins (MC) are the predominant cyanotoxins, which accumulate in aquatic organisms and are transferred to higher trophic levels. This is an issue of major concern in aquatic toxicology, as it involves the risk for human exposure through the consumption of contaminated fish and other aquatic organisms. The persistence and detoxification of MC in aquatic organisms are important issues for public health and fishery economics. Bioaccumulation of MC depends on the toxicity of the strains, mode of feeding, and detoxication mechanisms. Although mussels, as sessile filter feeders, seem to be organisms that ingest more MC, other molluscs like gastropods, as well as zooplankton and fish, may also retain average similar levels of toxins. Edible animals such as some species of molluscs, crustaceans, and fish present different risk because toxins accumulate in muscle at low levels. Carnivorous fish seem to accumulate high MC concentrations compared to phytophagous or omnivorous fish. This review summarizes the existing data on the distribution and dynamics of MC in contaminated aquatic organisms.

Dose-dependent antioxidant responses and pathological changes in tenca (Tinca tinca) after acute oral exposure to Microcystis under laboratory conditions

The effects of cyanobacterial cells containing microcystins (MCs), toxins from cyanobacteria, on oxidative stress biomarkers from liver and kidney of Tenca fish (Tinca tinca) were investigated under laboratory conditions. Moreover, a histopathological study of liver, kidney, heart and intestine tissues was performed. Fish were orally exposed to cyanobacterial cells dosing 0, 5, 11, 25 and 55 microg MC-LR/fish mixed with the food. Results showed a dose-dependent decrease of superoxide dismutase (SOD) activity, and also of catalase (CAT) in the liver. Glutathione levels and protein oxidation, however, were not altered by the exposure to the cyanobacterial material. The microscopic study revealed tissue alterations even at the lower cyanobacterial cells doses. Onion-like hepatocytes in the liver, glomerulopathy in the kidney, loss of myofibrils in the heart and vacuolated enterocytes in the gastrointestinal tract were the main changes observed. These findings suggest that this fresh water fish can be adversely affected by cyanobacterial blooms in their natural habitats.

Detoxification and oxidative stress responses along with microcystins accumulation in Japanese quail exposed to cyanobacterial biomass

The cyanobacterial exposure has been implicated in mass mortalities of wild birds, but information on the actual effects of cyanobacteria on birds in controlled studies is missing. Effects on detoxification and antioxidant parameters as well as bioaccumulation of microcystins (MCs) were studied in birds after sub-lethal exposure to natural cyanobacterial biomass. Four treatment groups of model species Japanese quail (Coturnix coturnix japonica) were exposed to controlled doses of cyanobacterial bloom during acute (10 days) and sub-chronic (30 days) experiment. The daily doses of cyanobacterial biomass corresponded to 0.2-224.6 ng MCs/g body weight. Significant accumulation of MCs was observed in the liver for both test durations and slight accumulation also in the muscles of the highest treatment group from acute test. The greatest accumulation was observed in the liver of the highest treatment group in the acute test reaching average concentration of 43.7 ng MCs/g fresh weight. The parameters of detoxification metabolism and oxidative stress were studied in the liver, heart and brain. The cyanobacterial exposure caused an increase of activity of cytochrome P-450-dependent 7-ethoxyresorufin O-deethylase representing the activation phase of detoxification metabolism. Also the conjugation phase of detoxification, namely the activity of glutathione-S-transferase, was altered. Cyanobacterial exposure also modulated oxidative stress responses including the level of glutathione and activities of glutathione-related enzymes and caused increase in lipid peroxidation. The overall pattern of detoxification parameters and oxidative stress responses clearly separated the control and the lowest exposure group from all the higher exposed groups. This is the first controlled study documenting the induction of oxidative stress along with MCs accumulation in birds exposed to natural cyanobacterial biomass. The data also suggest that increased activities of detoxification enzymes could lead to greater biotransformation and elimination of the MCs at the longer exposure time.

Acute effects of microcystins on the transcription of antioxidant enzyme genes in crucian carp Carassius auratus

Recent evidences suggested that oxidative stress may play a significant role in the pathogenesis of MCs toxicity. In the present study, the acute effects of microcystins on the transcription of antioxidant enzyme genes were investigated in liver of crucian carp i.p.-injected with 50 mug MC-LReq per kg body weight (BW). We reported the cDNA sequences for four kinds of antioxidant enzyme (GSH-PX, CAT, Cu/Zn SOD, and GR) genes, and evaluated the oxidant stress induced by MCs through analyzing the transcription abundance of antioxidant enzyme genes using real-time PCR method. The time-dependent change of relative transcription abundance and expression of the antioxidant enzyme genes were determined at 1, 3, 12, 24, and 48 h. The transcription abundance varied among antioxidant enzymes, with GSH-PX and GR down-regulation, and CAT and SOD significantly upregulation. Based on these data, we tentatively concluded that the oxidant stress was induced by MCs, and caused the different response of the antioxidant enzyme genes.

Time-dependent oxidative stress responses after acute exposure to toxic cyanobacterial cells containing microcystins in tilapia fish (Oreochromis niloticus) under laboratory conditions

Microcystins (MCs) have been reported to induce oxidative stress in aquatic organisms including fish. The effect of acute exposure to toxic cyanobacterial material containing MCs on antioxidant enzymes and lipid peroxidation has been studied in liver, kidney and gills of tilapia fish (Oreochromis niloticus). Fish were orally exposed to a single dose of cyanobacterial cells containing 120 microg/fish MC-LR and sacrificed at 24 and 72 h. The activities of glutathione peroxidase (GPx), glutathione reductase (GR), superoxide dismutase (SOD) and catalase (CAT) enzymes in the studied organs decreased in general 24 and 72 h after the dose application, although elevation of CAT and GR was found in liver at 72 h post exposure in comparison to 24h values. In contrast, the lipid peroxidation level increased significantly in all the studied organs with the liver (3.6-fold) proving to be the most affected. Protein oxidation was also increased 1.5-fold in the liver. However, recovery in these parameters was observed in liver 72 h after exposure. The results show that an acute dose of MCs does not induce an adaptative response of the antioxidant enzymes, as a sub-chronic exposure to MCs in tilapia fish does, but a general decrease in them with an initial recovery of the oxidative damage after 72 h, expressed as enhancement of CAT and GR activities and a reduction of LPO and protein oxidation in comparison to 24h values.

Biochemical and ultrastructural changes of the liver and kidney of the phytoplanktivorous silver carp feeding naturally on toxic Microcystis blooms in Taihu Lake, China

Many experimental studies have documented the impact of microcystins (MC) on fish based on either intraperitoneal injection, or oral gavaging via the diet, but few experiments were conducted by MC exposure through natural food uptake in lakes. In this study, the phytoplanktivorous silver carp were stocked in a large pen set in Meiliang Bay of Taihu Lake where toxic Microcystis blooms occurred in the warm seasons. Fish samples were collected monthly and MC concentrations in liver and kidney of the fish were determined by LC-MS. The maximum MC concentrations in liver and kidney were present in July when damages in ultrastructures of the liver and kidney were revealed by electron microscope. In comparison with previous studies on common carp, silver carp showed less damage and presence of lysosome proliferation in liver and kidney. Silver carp might eliminate or lessen cell damage caused by MC through lysosome activation. Recovery in the ultrastructures of liver and kidney after Microcystis blooms was companied with a significant decrease or even disappearance of MC. Catalase and glutathione S-transferase in liver and kidney of silver carp during Microcystis blooms were significantly higher than before and after Microcystis blooms. The high glutathione pool in liver and kidney of silver carp suggests their high resistance to MC exposure. The efficient antioxidant defence may be an important mechanism of phytoplanktivorous fish like silver carp to counteract toxic Microcystis blooms.

Ecosystem consequences of cyanobacteria in the northern Baltic Sea

Cyanobacteria of the Baltic Sea have multiple effects on organisms that influence the food chain dynamics on several trophic levels. Cyanobacteria contain several bioactive compounds, such as alkaloids, peptides, and lipopolysaccharides. A group of nonribosomally produced oligopeptides, namely microcystins and nodularin, are tumor promoters and cause oxidative stress in the affected cells. Zooplankton graze on cyanobacteria, and when ingested, the hepatotoxins (nodularin) decrease the egg production of, for example, copepods. However, the observed effects are very variable, because many crustaceans are tolerant to nodularin and because cyanobacteria may complement the diet of grazers in small amounts. Cyanobacterial toxins are transferred through the food web from one trophic level to another. The transfer rate is relatively low in the pelagic food web, but reduced feeding and growth rates of fish larvae have been observed. In the benthic food web, especially in blue mussels, nodularin concentrations are high, and benthic feeding juvenile flounders have been observed to disappear from bloom areas. In the littoral ecosystem, gammarids have shown increased mortality and weakening of reproductive success under cyanobacterial exposure. In contrast, mysid shrimps seem to be tolerant to cyanobacterial exposure. In fish larvae, detoxication of nodularin poses a metabolic cost that is reflected as decreased growth and condition, which may increase their susceptibility to predation. Cyanobacterial filaments and aggregates also interfere with both hydromechanical and visual feeding of planktivores. The feeding appendages of mysid shrimps may clog, and the filaments interfere with prey detection of pike larvae. On the other hand, a cyanobacterial bloom may provide a refuge for both zooplankton and small fish. As the decaying bloom also provides an ample source of organic carbon and nutrients for the organisms of the microbial loop, the zooplankton species capable of selective feeding may thrive in bloom conditions. Cyanobacteria also compete for nutrients with other primary producers and change the nitrogen (N): phosphorus (P) balance of their environment by their N-fixation. Further, the bioactive compounds of cyanobacteria directly influence other primary producers, favoring cyanobacteria, chlorophytes, dinoflagellates, and nanoflagellates and inhibiting cryptophytes. As the selective grazers also shift the grazing pressure on other species than cyanobacteria, changes in the structure and functioning of the Baltic Sea communities and ecosystems are likely to occur during the cyanobacterial bloom season.

Foodweb transfer, accumulation, and depuration of microcystins, a cyanobacterial toxin, in pumpkinseed sunfish (Lepomis gibbosus)

Zooplankton accumulate microcystins (MC), a potent cyanobacteria toxin, and therefore may act as vectors of the toxin up the aquatic food web; however this transfer has not yet been quantified. In addition there is a lack of information regarding fish's ability to metabolize MC when administered a low dose over a longer period of time. We monitored MC concentrations in three levels of an aquatic food web: phytoplankton, zooplankton, and sunfish (Lepomis gibbosus). Bosmina appeared to be both a major accumulator of MC in zooplankton and the major vector of MC to sunfish. In an accumulation experiment, sunfish were brought into the laboratory and fed MC-rich zooplankton pellets (50 ng MC kg(-1)d(-1)) for 9 days. Zooplankton directly transferred MC to sunfish, resulting in liver and muscle tissue accumulation. However, after 6 days of accumulation fish significantly decreased concentrations in their liver and muscle tissue, indicating the induction of a detoxification and excretion pathway. Sunfish retained MC in their liver and muscle tissue, showing no significant changes in toxin concentration over 2 weeks of fasted depuration.

The presence of microcystins and other cyanobacterial bioactive peptides in aquatic fauna collected from Greek freshwaters

Toxic bloom-forming cyanobacteria can cause animal death and adversely affect human health. Blooms may contain microcystins (MCs), cyanobacterial heptapeptide hepatotoxins and other peptides such as anabaenopeptins and anabaenopeptilides. MCs have been shown to occur in various aquatic organisms including mussels, water snails, crustaceans and fish. Muscle and viscera samples from eight species of fish (Acipenser gueldenstaedtii, Carassius auratus, Carassius gibelio, Cyprinus carpio, Perca fluviatilis, Rutilus rubilio, Silurus aristotelis and Silurus glanis), a frog (Rana eperotica), a mussel (Anodonta sp.) and a water snail (Viviparus contectus) were analyzed by high-performance liquid chromatography (HPLC), protein phosphatase 1 (PP1) inhibition assay (PP1IA) and ELISA. MC(s) was detected in all fish, frog, mussel and water snail samples tested by PP1IA and ELISA, including the frog R. eperotica and the freshwater snail V. contectus, in which the occurrence of MCs was not previously known. MC concentration ranged from 20 to 1500 ng g(-1)dw and from 25 to 5400 ng g(-1)dw in muscle and visceral tissue of fishes and frogs, respectively. In mussel and water snail tissue MC concentration ranged from 1650 to 3495 ng g(-1)dw. HPLC analysis revealed peaks having the same UV spectrum as anabaenopeptin- or anabaenopeptilide-like compounds, not previously known to occur in aquatic fauna tissue. The concentrations of the compounds detected ranged from 1.5 to 230 microg g(-1)dw. Comparison of the PP1IA and ELISA showed that values obtained with PP1IA where higher than those obtained with ELISA. Anabaenopeptins and/or anabaenopeptilides occurring in faunal tissue may account for the higher PP1IA values as we found that PP1 activity was inhibited by the purified anabaenopeptins A (45-60% inhibition) and B (5-75% inhibition). Purified anabaenopeptilides 90A and 90B exhibited weaker PP1 inhibition activity (5-35 and 5-23% inhibition, respectively). This is the first report of MC occurrence in aquatic animals collected from freshwaters of southern Europe.

Recovery of MC-LR in fish liver tissue

Cyanotoxins, particularly microcystins (MCs), have been shown to be a hazard to human health. MCs accumulate in aquatic organisms probably as a result of irreversible binding to liver protein phosphatases. The aim of this study was to describe the recovery of MC from fish liver using various detection methods, with MC-LR as the representative congener. These findings are discussed in conjunction with the current procedures and limit values used for human risk assessment. Following incubation of liver homogenates with various MC-LR concentrations, the homogenates were extracted by a water/methanol/butanol mixture via different treatments and subsequently analyzed via the colorimetric protein phosphatase inhibition assay (cPPA), HPLC, and anti-Adda ELISA. Detection via cPPA appeared to yield the highest recovery of MC-LR, although the presence of unspecific background may have resulted in overestimation of the true recovery. The recoveries determined via HPLC and anti-Adda ELISA were comparable to each other. The limits of detection were 0.01-2.4 microg MC-LR/g liver tissue, depending on the method used. Maximum MC-LR recovery from samples incubated with 10 and 100 microg MC-LR/g ranged between 44% and 101%. Recovery from samples incubated with 1 microg MC-LR/g liver tissue was below 3%. Lower recovery is assumed to result from irreversible, covalent MC protein binding, as confirmed by Western blotting of liver homogenates with anti-Adda immunoprobing. The results demonstrate that further investigation of and improvement in routinely applied MC methods for fish tissue and/or food analyses are needed for a reliable risk assessment.

Chronic toxicity and responses of several important enzymes in Daphnia magna on exposure to sublethal microcystin-LR

In the current study, the toxicological mechanisms of microcystin-LR and its disadvantageous effects on Daphnia magna were examined. Survival rate, number of newborn, activity of several important enzymes [glutathione S-transferase (GST), lactate dehydrogenase (LDH), phosphatases, and glutathione], accumulated microcystins, and ultrastructural changes in different organs of Daphnia were monitored over the course of 21-day chronic tests. The results indicated that low concentrations of dissolved microcystin had no harmful effect on Daphnia. On the contrary, stimulatory effects were detected. In the presence of toxin at high dosage and for long-term exposure, GST and glutathione levels decreased significantly. The decreased enzyme activity in the antioxidant system probably was caused by detoxification reactions with toxins. And these processes of detoxification at the beginning of chronic tests may enable phosphatases in Daphnia magna to withstand inhibition by the toxins. At the same time, we also found that the LDH activity in test animals increased with exposure to microcystin-LR, indicating that adverse effects occurred in Daphnia. With microcystin given at a higher dosage or for a longer exposure, the effect on Daphnia magna was fatal. In the meantime, microcystin began to accumulate in Daphnia magna, and phosphatase activity started to be inhibited. From the ultrastructure results of cells in D. magna, we obtained new information: the alimentary canal may be the target organ affected by exposure of microcystins to D. magna. The results of the current study also suggested that the oxidative damage and PPI (protein phosphatase inhibition) mechanisms of vertebrates also are adapted to Daphnia.

Effect of microcystin-LR on protein phosphatase activity and glycogen content in isolated hepatocytes of fed and fasted juvenile goldfish Carassius auratus L

Toxic effects of microcystin-LR were studied in hepatocytes isolated from fed and fasted juvenile goldfish Carassius auratus (30 g body weight). The hepatocytes were incubated with 10 microgMC-LR l(-1) during 4 h. MC-LR induces no effect in terms of cell number and viability. The toxin accumulation pattern was different in fed and fasted treatments. MC-LR accumulated more rapidly in 'fasted' cells where the highest concentration was observed by 1 h of exposure. It was delayed to 4 h in the 'fed' cells. MC-LR accumulation induced a severe decrease in hepatic protein phosphatase activity in both treatments. It was almost totally inhibited in both treatments during the first hour of exposure. The glycogen content was significantly reduced after 2 h of exposure in the fasting treatments, but not in the feeding one.