Inhibition of embryonic development by microcystin-LR in zebrafish, Danio rerio

Unveiling the molecular mechanisms and developmental consequences of mercury (Hg) toxicity in zebrafish embryo-larvae: A comprehensive approach

Mercury (Hg) is a global contaminant affecting aquatic ecosystems' health. Chronic exposure to Hg has shown that the normal development of zebrafish embryo-larvae is affected. However, the molecular mechanisms behind the toxicity of Hg on fish embryonic development are still poorly understood. This work aimed to investigate the effects of Hg exposure on zebrafish embryo-larvae using a combined approach at individual (mortality, embryo development and locomotor behavior) and biochemical (neurotoxicity and oxidative stress enzymatic activities and protein phosphatase expression) levels. The Fish Embryo Toxicity assay followed the Organization for Economic Cooperation and Development Guideline 236 and used a concentration range between 13 and 401 μg Hg/L. Lethal and developmental endpoints were examined at 24, 48, 72 and 96 hpf. Biochemical markers, including Acetylcholinesterase (AChE), Catalase (CAT), Glutathione Reductase (GR), and Glutathione-S-Transferase (GST) activities and, for the first time, the expression of the protein phosphatase 1 gamma (PP1γ) was assessed after 24, 48, 72 and 96 h of exposure to 10 and 100 μg Hg/L. The behavioral effects of a sublethal range of Hg (from 0.8 to 13 μg Hg/L) were assessed using an automated video tracking system at 120 hpf. Several developmental abnormalities on zebrafish embryos and larvae, including pericardial edema, spin and tail deformities and reduced rate of consumption of the yolk sac, were found after exposure to Hg (LC50 at 96 hpf of 139 μg Hg/L) with EC50 values for total malformations ranging from 22 to 264 μg Hg/L. After 96 hpf, no significant effects were observed in the CAT and GR activities. However, an increase in the GST activity in a concentration and time-dependent manner was found, denoting possible stress-related adaptation of zebrafish embryos to deleterious effects of Hg exposure. The AchE activity showed a response pattern in line with the behavioral responses. At the lowest concentration tested, no significant effects were found for the AChE activity, whereas a decrease in AChE activity was observed at 100 μg Hg/L, suggesting that exposure to Hg induced neurotoxic effects in zebrafish embryos which in turn may explain the lack of equilibrium found in this study (EC50 at 96 hpf of 83 μg Hg/L). Moreover, a decrease in the PP1γ expression was found after 96 h of exposure to 10 and 100 μg Hg/L. Thus, we suggest that Hg may be an inhibitor of PP1γ in zebrafish embryos-larvae and thus, along with the alterations in the enzymatic activity of GST, explain some of the developmental malformations observed, as well as the lack of equilibrium. Hence, in this study, we propose the use of PP1 expression, in combination with apical and biochemical endpoints, as a precursor for assessing Hg's toxic mechanism on embryonic development.

Visualization Tools for Detecting Microcystin-LR in the Biological System via Near-Infrared Fluorescent Probes

Numerous toxicological and epidemiological studies have shown that microcystin-LR (MC-LR) could cause a variety of toxicity to humans and animals. However, the absence of effective methods to trace MC-LR in biological systems has hindered the in-depth understanding of the mechanism of MC-LR toxicity. Near-infrared (NIR) fluorescent probes are crucial tools for accurate visualization and in-depth study of specific molecules in biological systems. Due to the lack of effective design strategies, NIR fluorescent probes for imaging MC-LR specifically in biological systems have not been reported yet. In order to address this pressing issue, herein, we have introduced a new and facile strategy to improve MC-LR detection and imaging in biological systems, and based on this design strategy, three NIR fluorescence probes (MC-RdTPA1, MC-RdTPA2, and MC-RdTPE1) have been constructed. These probes have several advantages: (i) have long emission wavelength and large Stokes shifts, which have great potential in vivo imaging applications; (ii) could selectively visualize MC-LR in cells; and (iii) showed stable fluorescence intensity in the pH range of 5.0-7.0. This work may provide a new avenue for the detection of MC-LR in biological systems and new tool to advance our knowledge of the mechanism of MC-LR toxicity.

Biotests in Cyanobacterial Toxicity Assessment-Efficient Enough or Not?

Cyanobacteria are a diverse group of organisms known for producing highly potent cyanotoxins that pose a threat to human, animal, and environmental health. These toxins have varying chemical structures and toxicity mechanisms and several toxin classes can be present simultaneously, making it difficult to assess their toxic effects using physico-chemical methods, even when the producing organism and its abundance are identified. To address these challenges, alternative organisms among aquatic vertebrates and invertebrates are being explored as more assays evolve and diverge from the initially established and routinely used mouse bioassay. However, detecting cyanotoxins in complex environmental samples and characterizing their toxic modes of action remain major challenges. This review provides a systematic overview of the use of some of these alternative models and their responses to harmful cyanobacterial metabolites. It also assesses the general usefulness, sensitivity, and efficiency of these models in investigating the mechanisms of cyanotoxicity expressed at different levels of biological organization. From the reported findings, it is clear that cyanotoxin testing requires a multi-level approach. While studying changes at the whole-organism level is essential, as the complexities of whole organisms are still beyond the reach of in vitro methodologies, understanding cyanotoxicity at the molecular and biochemical levels is necessary for meaningful toxicity evaluations. Further research is needed to refine and optimize bioassays for cyanotoxicity testing, which includes developing standardized protocols and identifying novel model organisms for improved understanding of the mechanisms with fewer ethical concerns. In vitro models and computational modeling can complement vertebrate bioassays and reduce animal use, leading to better risk assessment and characterization of cyanotoxins.

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.

Microcystin-LR aerosol induces inflammatory responses in healthy human primary airway epithelium

Harmful algal blooms plague bodies of freshwater globally. These blooms are often composed of outgrowths of cyanobacteria capable of producing the heptapeptide Microcystin-LR (MC-LR) which is a well-known hepatotoxin. Recently, MC-LR has been detected in aerosols generated from lake water. However, the risk for human health effects due to MC-LR inhalation exposure have not been extensively investigated. In this study, we exposed a fully differentiated 3D human airway epithelium derived from 14 healthy donors to MC-LR-containing aerosol once a day for 3 days. Concentrations of MC-LR ranged from 100 pM to 1 µM. Although there were little to no detrimental alterations in measures of the airway epithelial function (i.e. cell survival, tissue integrity, mucociliary clearance, or cilia beating frequency), a distinct shift in the transcriptional activity was found. Genes related to inflammation were found to be upregulated such as C-C motif chemokine 5 (CCL5; log2FC = 0.57, p = 0.03) and C-C chemokine receptor type 7 (CCR7; log2FC = 0.84, p = 0.03). Functionally, conditioned media from MC-LR exposed airway epithelium was also found to have significant chemo-attractive properties for primary human neutrophils. Additionally, increases were found in the concentration of secreted chemokine proteins in the conditioned media such as CCL1 (log2FC = 5.07, p = 0.0001) and CCL5 (log2FC = 1.02, p = 0.046). These results suggest that MC-LR exposure to the human airway epithelium is capable of inducing an inflammatory response that may potentiate acute or chronic disease.

Cylindrospermopsin impairs zebrafish (Danio rerio) embryo development

Cyanotoxins are among common contaminants that can impair human, animal, and environmental health. Cylindrospermopsin (CYN) is an abundant form of cyanotoxins elevated following algal bloom in the water worldwide. Previous studies have described CYN effects on several organs in mammals. However, little is known about its toxicity mechanisms in other vertebrates. This study aims to characterize the developmental effects of CYN using zebrafish larvae as an aquatic model organism. A wide range of CYN concentrations (0-2000 μg/L) was tested using a morphometric approach for survival, hatching, various growth and developmental abnormalities. We also investigated the expression of genes related to oxidative stress, osmoregulation, and thyroid function. Exposure to CYN resulted in decreased growth, increased developmental abnormalities such as pericardial and yolk sac edema as well as swim bladder absence. In addition, CYN increased tr1a, and decreased dio1 and dio3 transcript levels which are involved in thyroid-mediated function. It also increased transcript levels related to oxidative stress, including hsp70, ahr1a, cyp1a, gpx and cat. Lastly, CYN exposure increased aqp3a and decreased dab2, which are involved in osmoregulation with a threshold of 10 μg/L. The present study demonstrates multiple effects of exposure to environmentally relevant CYN concentrations in zebrafish embryos.

Sublethal exposure to Microcystis aeruginosa extracts during embryonic development reduces aerobic swimming capacity in juvenile zebrafish

In the last decades, cyanobacterial harmful algal blooms (CyanoHABs) pose an intensifying ecological threat. Microcystis aeruginosa is a common CyanoHAB species in freshwater ecosystems, with severe toxic effects in a wide range of organisms. In the present paper we examined whether transient and short (48 h) exposure of fish embryos to sublethal levels of M. aeruginosa crude extract (200 mg biomass dw L^-1) affects swimming performance at later life stages (end of metamorphosis, ca 12 mm TL, 22,23 days post-fertilization). Pre-exposed metamorphosing larvae presented a significant decrease in swimming performance (9.7 ± 1.6 vs 11.4 ± 1.7 TL s^-1 in the control group, p < 0.01), and a significant decrease in the ventricle length-to-depth ratio (1.23 ± 0.15 vs 1.42 ± 0.15 in control fish, p < 0.05). In addition, extract-exposed fish presented significantly elevated rates of vertebral abnormalities (82 ± 13% vs 7 ± 4% in the control group), mainly consisting of the presence of extra neural and haemal processes. No significant differences between groups were detected in survival and growth rates. Results are discussed in respect to the mechanisms that might mediate the detected cyanobacterial effects. This is the first evidence of a direct link between sublethal exposure to M. aeruginosa during the embryonic period and swimming performance at later life-stages. Decreased swimming performance, altered cardiac shape, and elevated vertebral abnormalities in response to early exposure to M. aeruginosa could have significant effects on fish populations in the wild.

The latest advances in the reproductive toxicity of microcystin-LR

Microcystin-LR (MC-LR) is an emerging environmental pollutant produced by cyanobacteria that poses a threat to wild life and human health. In recent years, the reproductive toxicity of MC-LR has gained widespread attention, a large number of toxicological studies have filled the gaps in past research and more molecular mechanisms have been elucidated. Hence, this paper reviews the latest research advances on MC-LR-induced reproductive toxicity. MC-LR can damage the structure and function of the testis, ovary, prostate, placenta and other organs of animals and then reduce their fertility. Meanwhile, MC-LR can also be transmitted through the placenta to the offspring causing trans-generational and developmental toxicity including death, malformation, growth retardation, and organ dysfunction in embryos and juveniles. The mechanisms of MC-LR-induced reproductive toxicity mainly include the inhibition of protein phosphatase 1/2 A (PP1/2 A) activity and the induction of oxidative stress. On the one hand, MC-LR triggers the hyperphosphorylation of certain proteins by inhibiting intracellular PP1/2 A activity, thereby activating multiple signaling pathways that cause inflammation and blood-testis barrier destruction, etc. On the other hand, MC-LR-induced oxidative stress can result in cell programmed death via the mitochondrial and endoplasmic reticulum pathways. It is worth noting that epigenetic modifications are also involved in reproductive cell apoptosis, which may be an important direction for future research. Furthermore, this paper proposes for the first time that MC-LR can produce estrogenic effects in animals as an environmental estrogen. New findings and suggestions in this review could be areas of interest for future research.

Microcystin-LR Does Not Alter Cell Survival and Intracellular Signaling in Human Bronchial Epithelial Cells

Changes in ecological and environmental factors lead to an increased occurrence of cyanobacterial water blooms, while secondary metabolites-producing cyanobacteria pose a threat to both environmental and human health. Apart from oral and dermal exposure, humans may be exposed via inhalation and/or swallowing of contaminated water and aerosols. Although many studies deal with liver toxicity, less information about the effects in the respiratory system is available. We investigated the effects of a prevalent cyanotoxin, microcystin-LR (MC-LR), using respiratory system-relevant human bronchial epithelial (HBE) cells. The expression of specific organic-anion-transporting polypeptides was evaluated, and the western blot analysis revealed the formation and accumulation of MC-LR protein adducts in exposed cells. However, MC-LR up to 20 μM neither caused significant cytotoxic effects according to multiple viability endpoints after 48-h exposure, nor reduced impedance (cell layer integrity) over 96 h. Time-dependent increase of putative MC-LR adducts with protein phosphatases was not associated with activation of mitogen-activated protein kinases ERK1/2 and p38 during 48-h exposure in HBE cells. Future studies addressing human health risks associated with inhalation of toxic cyanobacteria and cyanotoxins should focus on complex environmental samples of cyanobacterial blooms and alterations of additional non-cytotoxic endpoints while adopting more advanced in vitro models.

Combined Danio rerio embryo morbidity, mortality and photomotor response assay: A tool for developmental risk assessment from chronic cyanoHAB exposure

Freshwater harmful algal blooms produce a broad array of bioactive compounds, with variable polarity. Acute exposure to cyanotoxins can impact the liver, nervous system, gastrointestinal tract, skin, and immune function. Increasing evidence suggests chronic effects from low-level exposures of cyanotoxins and other associated bioactive metabolites of cyanobacterial origin. These sundry compounds persist in drinking and recreational waters and challenge resource managers in detection and removal. A systematic approach to assess the developmental toxicity of cyanobacterial metabolite standards was employed utilizing a robust and high throughput developmental Danio rerio embryo platform that incorporated a neurobehavioral endpoint, photomotor response. Subsequently, we applied the platform to cyanobacterial bloom surface water samples taken from temperate recreational beaches and tropical lake subsistence drinking water sources as a model approach. Dechorionated Danio rerio embryos were statically immersed beginning at four to six hours post fertilization at environmentally relevant concentrations, and then assessed at 24 h and 5 days for morbidity, morphological changes, and photomotor response. At least one assessed endpoint deviated significantly for exposed embryos for 22 out of 25 metabolites examined. Notably, the alkaloid lyngbyatoxin-a resulted in profound, dose-dependent morbidity and mortality beginning at 5 μg/L. In addition, hydrophobic components of extracts from beach monitoring resulted in potent morbidity and mortality despite only trace cyanotoxins detected. The hydrophilic extracts with several order of magnitude higher concentrations of microcystins resulted in no morbidity or mortality. Developmental photomotor response was consistently altered in environmental bloom samples, independent of the presence or concentration of toxins detected in extracts. While limited with respect to more polar compounds, this novel screening approach complements specific fingerprinting of acutely toxic metabolites with robust assessment of developmental toxicity, critical for chronic exposure scenarios.

Microcystin and pyriproxyfen are toxic to early stages of development in Rhamdia quelen: An experimental and modelling study

The recent increase of freshwater eutrophication has favored cyanobacteria blooms and consequently the increase of toxins such as microcystin-LR in aquatic environments, but few is know about the associated effect of toxin and other compounds. Pyriproxyfen is an insecticide indicated by WHO (World Health Organization) to control Aedes aegypti mosquito (vector of Dengue, Chikungunya and Zika diseases), however, the effects are not well described to non-target species, such as fish. The early life stages (ELS) of fish are more sensitive to chemical stress due to higher metabolic rate, immature immune system and high superficial area/volume ratio. In the current study, ELS of R. quelen a Neotropical fish were exposed to environmentally realistic concentrations of microcystin (1, 10 and 100 µg L^-1_; M1, M2 and M3 groups, respectively) from an algal extract, pyriproxyfen (1 and 10 µg L^-1, P1 and P2) and their association (co-exposure). The hatching, survival and larvae deformities were analyzed, and applied a mathematical model to evaluate the effects on the population size along further generations. Both compounds were toxic to embryos/larvae of fish, but the effects were more pronounced in M2, P1M2 and P2M1 for hatching and M2, P1M2, P2M1 and P1 for survival. Deformities prevailed in groups exposed to the chemicals at 48 hpf (hours post-fertilization) were suggestions of toxicological interaction in P1M2, P2M1 and P2M2 at 48 and 72 hpf. In 96 hpf, the levels of deformities were lower than in previous times. Model predicted population density over 100 years decreased to lower than 0.5 (50%) in all groups, except for P1M1, indicating risk of extinction. P1M2 had the worse results, followed by M2, P1M3 and P2M1. Cyanobacterial blooms can lead to microcystin-LR levels higher than M2 (10 µg L^-1), and the suggestion of toxicological interaction with pyriproxyfen is relevant because both compounds may potentially coexist in aquatic environments. Finally, mathematical models may provide an ecological interpretation of the risk of exposure of fish.

Single and combined exposure of microcystin-LR and nitrite results in reproductive endocrine disruption via hypothalamic-pituitary-gonadal-liver axis

Microcystin-LR (MC-LR) released by Microcystis blooms degradation usually co-exists with a chemical called nitrite, posing a serious harm to aquatic organisms. To assess the single and combined effects of MC-LR and nitrite on the reproductive endocrine system, a fully factorial experiment was designed and adult male zebrafish (Danio rerio) were exposed to 9 treatment combinations of MC-LR (0, 3, 30 μg/L) and nitrite (0, 2, 20 mg/L) for 30 d. The results showed that both MC-LR and nitrite caused concentration-dependent effects including the growth inhibition, decreased gonad index as well as testicular injuries with widen intercellular spaces and seminiferous epithelium deteriorations. And testicular pathological changes in the co-exposure groups of MC-LR and nitrite were similar but more serious than those in single-factor exposure groups. Concurrently, exposure to MC-LR or nitrite alone could significantly decrease T levels by downregulating gene expressions (gnrh2, lhβ, ar, lhr) in the hypothalamic-pituitary-gonadal-liver-axis (HPGL-axis), and there were significant interactions between MC-LR and nitrite on them. In contrast, E2 levels as well as transcriptional levels of cyp19a1b, cyp19a1a and vtg1 showed significant inductions with increasing MC-LR concentrations, indicating an estrogen-like effect of MC-LR. Our findings illustrated that co-exposure of MC-LR and nitrite synergistically cause reproductive dysfunction by interfering with the HPGL axis in male fish, which prompt us to focus more on the potential risks in fish reproduction and even population dynamics due to the wide occurrence of toxic cyanobacterial blooms.

Differential Toxicity of Cyanobacteria Isolated from Marine Sponges towards Echinoderms and Crustaceans

Marine sponges and cyanobacteria have a long history of co-evolution, with documented genome adaptations in cyanobionts. Both organisms are known to produce a wide variety of natural compounds, with only scarce information about novel natural compounds produced by cyanobionts. In the present study, we aimed to address their toxicological potential, isolating cyanobacteria (n = 12) from different sponge species from the coast of Portugal (mainland, Azores, and Madeira Islands). After large-scale growth, we obtained both organic and aqueous extracts to perform a series of ecologically-relevant bioassays. In the acute toxicity assay, using nauplii of Artemia salina, only organic extracts showed lethality, especially in picocyanobacterial strains. In the bioassay with Paracentrotus lividus, both organic and aqueous extracts produced embryogenic toxicity (respectively 58% and 36%), pointing to the presence of compounds that interfere with growth factors on cells. No development of pluteus larvae was observed for the organic extract of the strain Chroococcales 6MA13ti, indicating the presence of compounds that affect skeleton formation. In the hemolytic assay, none of the extracts induced red blood cells lysis. Organic extracts, especially from picoplanktonic strains, proved to be the most promising for future bioassay-guided fractionation and compounds isolation. This approach allows us to classify the compounds extracted from the cyanobacteria into effect categories and bioactivity profiles.

Effects of Microcystis on development of early life stage Japanese medaka (Oryzias latipes): Comparative toxicity of natural blooms, cultured Microcystis and microcystin-LR

Freshwater cyanobacterial harmful algal blooms (CyanoHABs) caused by algae in the genus Microcystis have been increasing in frequency and severity in recent decades. Microcystis blooms threaten aquatic organisms through effects associated with the rapid increase of biomass and the production of the hepatotoxin microcystin (MC) by toxic strains. Among fish, effects of blooms are likely to be more severe for early life stages, and physiological impacts on this life stage could significantly impact recruitment and fish populations. This study explores the effects of Microcystis blooms on the development of fish using the model organism, the Japanese medaka (Oryzias latipes), under realistic exposure conditions. Medaka embryos were exposed to natural blooms collected from New York City (USA) lakes, lab cultures of Microcystis, and MC-LR solutions. Field collected samples were more toxic than lab cultures (even when compared at the same algal density or MC concentration), causing decreased survival, premature time to hatch, reduced body length, yolk sac edema, and decreased heart rate, while lab culture exposures only resulted in bradycardia. Heart rate was the most sensitive endpoint measured, being depressed in embryos exposed to both lab cultures and field collected blooms. Generalized linear model analysis indicated bradycardia was statistically associated with both cell densities of blooms and MC concentrations, while single factor analysis indicated that MC concentrations had a stronger correlation compared to cell densities. However, MC exposure could not fully explain the effects observed, as exposures to MC-LR solutions alone were not able to reduce heart rate as severely as algal exposures. Collectively, these experiments indicate that factors beyond exposure to MC or even isolated Microcystis strains influence heart rate of fish exposed to Microcystis blooms. Enhanced mortality, depressed heart rate, and abnormal development observed in response to environmentally realistic exposures of Microcystis blooms could affect success of fish at both individual or population levels.

Toxicological and biochemical responses of the earthworm Eisenia fetida to cyanobacteria toxins

Irrigation with eutrophic water containing cyanobacteria toxins poses a potential risk to soil animals. To evaluate ecotoxicological effect of microcystins (MCs) on earthworms, filter paper acute toxicity test, avoidance test and a 14-d artificial soil test were carried out. No acute toxicity was found in the filter paper test, and earthworms showed no avoidance response to MCs exposure. In the artificial soil test, Eisenia fetida were allowed to grow in presence or absence of MCs (0, 1, 10, 100, 1000 μg kg⁻¹ of soil) for 1, 7, and 14 d. Results showed that MCs could bioaccumulated in earthworm. A stimulatory effect on catalase and glutathione oxidase activities induced by MCs was found on day 1, and both of them were significantly inhibited at 100 and 1000 μg kg⁻¹ on days 14. The superoxide dismutase activity was relatively insensitive. Significant increase of malondialdehyde content and decrease of neutral red retention time were observed at 100 and 1000 μg kg⁻¹ on days 7 and 14. Our results suggest that MCs induces oxidative stress on earthworms, which leads to disruption of the antioxidant system and lipid peroxidation, as well as alterations in lysosomal membrane stability.

Preliminary evidence for snail deformation from a Eutrophic lake

The incidence of deformities in snails Bellamya aeruginosa was investigated in a typical eutrophicated lake - Taihu Lake. A total of 15 105 specimens were collected, and 0.18-0.93% of the snails exhibited abnormal tentacle bifurcations. Abnormally developed snails were all female and were found in regions with relatively high Chlorophyll a levels (12.40±7.23μg/L). As tentacles are sexually dimorphic in B. aeruginosa, we postulated that factors associated with eutrophication might be responsible for the partial masculinization of tentacles in females. Differential gene expression analyses revealed that a number of unigenes were significantly up-regulated or down-regulated in snails sampled from three locations having high Chlorophyll a levels compared with snails sampled from the region with lower Chlorophyll a level (2.95μg/L). Thus, transcriptomic profiling revealed potential molecular signal of eutrophication that can lead to developmental abnormalities in this species.

Transcriptional and Behavioral Responses of Zebrafish Larvae to Microcystin-LR Exposure

Microcystins are cyclic heptapeptides that constitute a diverse group of toxins produced by cyanobacteria. One of the most toxic variants of this family is microcystin-LR (MCLR) which is a potent inhibitor of protein phosphatase 2A (PP2A) and induces cytoskeleton alterations. In this study, zebrafish larvae exposed to 500 μg/L of MCLR for four days exhibited a 40% reduction of PP2A activity compared to the controls, indicating early effects of the toxin. Gene expression profiling of the MCLR-exposed larvae using microarray analysis revealed that keratin 96 (krt96) was the most downregulated gene, consistent with the well-documented effects of MCLR on cytoskeleton structure. In addition, our analysis revealed upregulation in all genes encoding for the enzymes of the retinal visual cycle, including rpe65a (retinal pigment epithelium-specific protein 65a), which is critical for the larval vision. Quantitative real-time PCR (qPCR) analysis confirmed the microarray data, showing that rpe65a was significantly upregulated at 50 μg/L and 500 μg/L MCLR in a dose-dependent manner. Consistent with the microarray data, MCLR-treated larvae displayed behavioral alterations such as weakening response to the sudden darkness and hypoactivity in the dark. Our work reveals new molecular targets for MCLR and provides further insights into the molecular mechanisms of MCLR toxicity during early development.

Microcystin-LR induced developmental toxicity and apoptosis in zebrafish (Danio rerio) larvae by activation of ER stress response

Recent studies have demonstrated that cyanobacteria-derived Microcystin-LR (MC-LR) can cause developmental toxicity and trigger apoptosis in zebrafish (Danio rerio) larvae, but the underlying mechanisms remain largely unknown. In this study, we tested the hypothesis that the mechanism by which MC-LR induces developmental toxicity is through activation of endoplasmic reticulum (ER) stress. MC-LR (4.0 μM) exposure through submersion caused serious developmental toxicity, such as malformation, growth delay and decreased heart rates in zebrafish larvae, which could be inhibited by ER stress blocker, tauroursodeoxycholic acid (TUDCA, 20 μM). Meanwhile, acridine orange (AO) staining showed TUDCA could rescue cell apoptosis in heart area in zebrafish larvae resulted by MC-LR exposure. Real-time polymerase chain reaction (real-time PCR) analysis demonstrated that MC-LR induced activation of ER stress which consequently triggered apoptosis in zebrafish larvae. Protein expression examined by western blot indicated that MC-LR could activate MAPK8/Bcl-2/Bax pathway and caspase-dependent apoptotic pathway in zebrafish larva and the effects were mitigated by inhibition of ER stress. Taken together, the results observed in this study suggested that ER stress plays a critical role in developmental toxicity and apoptosis in zebrafish embryos exposed to MC-LR.

The toxic effects of microcystin-LR on mouse lungs and alveolar type II epithelial cells

OBJECTIVES

Microcystin-leucine arginine (MC-LR) is produced by cyanobacteria and can accumulate in lungs through blood circulation. However, the effect of MC-LR on lung remains unclear. In this study, we investigated the chronic, low-dose effect of MC-LR on mouse lung tissues and the influence of MC-LR on mouse alveolar type II epithelial cells (ATII cells).

METHODS

MC-LR was orally administered to mice at 0, 1, 10, and 40 μg/L for 6 consecutive months and mouse lungs were obtained for histopathological and immunoblot analysis. ATII cells were cultured in various concentrations of MC-LR (0, 0.5, 5, 50, 500 nmol/L) for indicated time and the cell viability and proteins change were tested.

RESULTS

Our study revealed that the chronic, low-dose MC-LR exposure induced alveolar collapse and lung cell apoptosis as well as the breach of cell junction integrity. Furthermore, following treatment with MC-LR, ATII cells could uptake MC-LR, resulting in apoptosis and disruption of cell junction integrity.

CONCLUSIONS

These data support the toxic potential of low-dose MC-LR in rendering chronic injury to lung tissues.

Comparative effects of nodularin and microcystin-LR in zebrafish: 2. Uptake and molecular effects in eleuthero-embryos and adult liver with focus on endoplasmic reticulum stress

Microcystin (MC) and nodularin are structurally similar cyanobacterial toxins that inhibit protein phosphatases. Additional modes of action are poorly known, in particular for nodularin. In our associated work, we showed that active cellular uptake is mediated by the organic anion transporting polypeptide drOatp1d1 in zebrafish (Faltermann et al., 2016). Here, we assessed the transcriptional expression of three genes encoding three uptake transporters during embryonic development from 24h post fertilization (hpf) to 168 hpf. Transcripts of drOatp1d1 and drOatp2b1 are present at 24 hpf. The abundance increased after hatching and remained about constant up to 168 hpf. Transcripts of drOatp2b1 were most abundant, while drOapt1f transcripts showed very low relative abundance compared to drOatp1d1 and drOatp2b1. We further demonstrated the uptake of fluorescent labeled MC-LR in eleuthero-embryos and its accumulation in the glomerulus of the pronephros. An important molecular effect of MC-LR in human liver cells is the induction of endoplasmic reticulum (ER)-stress. Here, we investigated, whether MC-LR and nodularin similarly lead to induction of ER-stress in zebrafish by analyzing changes of mRNA levels of genes indicative of ER-stress. In zebrafish liver organ cultures short- and long-term exposures to 0.15 and 0.3 μmol L(-1) MC-LR, and 0.5 and 1 μM L(-1) nodularin led to significant transcriptional induction of several ER-stress marker genes, including the chaperone glucose regulated protein 78 (bip), the spliced form of x-box binding protein (xbp-1s), the CCAAT-enhancer-binding protein homologous protein (chop) and activating transcription factor 4 (atf4). Furthermore, strong transcriptional changes occurred for tumor necrosis factor alpha (tnfa) and dual specificity phosphatase 5 (dusp5), associated with mitogen activated protein kinase (MAPK) pathway. However, no alterations in transcript levels of pro-apoptotic genes Bcl-2 like protein 4 (bax) and p53 occurred. In contrast to adult liver, MC-LR and nodularin did not result in detectable changes of mRNA levels of selected target genes involved in ER-stress in zebrafish eleuthero-embryos, nor was the abundance of transcripts belonging to the MAPK and pro-apoptosis pathways altered. In conclusion, our data indicate that MC-LR and nodularin have similar transcriptional effects. They lead to changes in mRNA levels of genes that suggest induction of ER-stress, and furthermore, lead to increased level of tnfα mRNA in the adult liver, which suggests a novel (transcriptional) mode of action in fish. However, although taken up by eleuthero-embryos, no transcriptional changes induced by these cyanobacterial toxins were detected. This is probably due to action to specific organs such as liver and kidneys that could not be identified by whole-embryo sampling.

Reproduction impairment and endocrine disruption in female zebrafish after long-term exposure to MC-LR: A life cycle assessment

Microcystin-LR (MC-LR) has been found to cause reproductive and developmental impairments as well as to disrupt sex hormone homeostasis of fish during acute and sub-chronic toxic experiments. However, fish in natural environments are continuously exposed to MC-LR throughout their entire life cycle as opposed to short-term exposure. Here, we tested the hypothesis that the mechanism by which MC-LR harms female fish reproduction and development within natural water bodies is through interference of the reproductive endocrine system. In the present study, zebrafish hatchlings (5 d post-fertilization) were exposed to 0, 0.3, 3 and 30 μg/L MC-LR for 90 d until reaching sexual maturity. Female zebrafish were selected, and the changes in growth and developmental indicators, ovarian ultrastructure as well as the levels of gonadal steroid hormones and vitellogenin (VTG) were examined along with the transcription of related genes in the hypothalamic-pituitary-gonadal-liver axis (HPGL-axis). The results showed for the first time, a life cycle exposure to MC-LR caused growth inhibition, decreased ovary weight and ovarian ultra-pathological lesions. Decreased ovarian testosterone levels indicated that MC-LR disrupted sex steroid hormone balance. Significantly up-regulated transcription of brain FSHβ and LHβ along with ovarian ERα, FSHR and LHR suggested positive feedback regulation in the HPGL-axis was induced as a compensatory mechanism for MC-LR damage. It was also noted that ovarian VTG content and hepatic ERα and VTG1 expression were all down-regulated, which might be responsible for reduced vitellus storage noted in our histological observations. Our findings indicate that a life cycle exposure to MC-LR impairs the development and reproduction of female zebrafish by disrupting the transcription of related HPGL-axis genes, suggesting that MC-LR has potential adverse effects on fish reproduction and thus population dynamics in MCs-contaminated aquatic environment.

Endocrine, teratogenic and neurotoxic effects of cyanobacteria detected by cellular in vitro and zebrafish embryos assays

Cyanobacteria contain various types of bioactive compounds, which could cause adverse effects on organisms. They are released into surface waters during cyanobacterial blooms, but there is little information on their potential relevance for effects in vivo. In this study presence of bioactive compounds was characterized in cyanobacteria Microcystis aeruginosa (Chroococcales), Planktothrix agardhii (Oscillatoriales) and Aphanizomenon gracile (Nostocales) with selected in vitro assays. The in vivo relevance of detected bioactivities was analysed using transgenic zebrafish embryos tg(cyp19a1b-GFP). Teratogenic potency was assessed by analysis of developmental disorders and effects on functions of the neuromuscular system by video tracking of locomotion. Estrogenicity in vitro corresponded to 0.95-54.6 ng estradiol equivalent(g dry weight (dw))(-1). In zebrafish embryos, estrogenic effects could not be detected potentially because they were masked by high toxicity. There was no detectable (anti)androgenic/glucocorticoid activity in any sample. Retinoid-like activity was determined at 1-1.3 μg all-trans-retinoic acid equivalent(g dw)(-1). Corresponding to the retinoid-like activity A. gracile extract also caused teratogenic effects in zebrafish embryos. Furthermore, exposure to biomass extracts at 0.3 gd wL(-1) caused increase of body length in embryos. There were minor effects on locomotion caused by 0.3 gd wL(-1)M. aeruginosa and P. agardhii extracts. The traditionally measured cyanotoxins microcystins did not seem to play significant role in observed effects. This indicates importance of other cyanobacterial compounds at least towards some species or their developmental phases. More attention should be paid to activity of retinoids, estrogens and other bioactive substances in phytoplankton using in vitro and in vivo bioassays.

Retinoid-like activity and teratogenic effects of cyanobacterial exudates

Retinoic acids and their derivatives have been recently identified by chemical analyses in cyanobacteria and algae. Given the essential role of retinoids for vertebrate development this has raised concerns about a potential risk for vertebrates exposed to retinoids during cyanobacterial blooms. Our study focuses on extracellular compounds produced by phytoplankton cells (exudates). In order to address the capacity for the production of retinoids or compounds with retinoid-like activity we compared the exudates of ten cyanobacteria and algae using in vitro reporter gene assay. Exudates of three cyanobacterial species showed retinoid-like activity in the range of 269-2,265 ng retinoid equivalents (REQ)/L, while there was no detectable activity in exudates of the investigated algal species. The exudates of one green alga (Desmodesmus quadricaudus) and the two cyanobacterial species with greatest REQ levels, Microcystis aeruginosa and Cylindrospermopsis raciborskii, were selected for testing of the potential relation of retinoid-like activity to developmental toxicity in zebrafish embryos. The exudates of both cyanobacteria were indeed provoking diverse teratogenic effects (e.g. tail, spine and mouth deformation) and interference with growth in zebrafish embryos, while such effects were not observed for the alga. Fish embryos were also exposed to all-trans retinoic acid (ATRA) in a range equivalent to the REQ concentrations detected in exudates by in vitro bioassays. Both the phenotypes and effective concentrations of exudates corresponded to ATRA equivalents, supporting the hypothesis that the teratogenic effects of cyanobacterial exudates are likely to be associated with retinoid-like activity. The study documents that some cyanobacteria are able to produce and release retinoid-like compounds into the environment at concentrations equivalent to those causing teratogenicity in zebrafish. Hence, the characterization of retinoid-like and teratogenic potency should be included in the assessment of the potential adverse effects caused by the release of toxic and bioactive compounds during cyanobacterial blooms.

The role of apoptosis in MCLR-induced developmental toxicity in zebrafish embryos

We previously demonstrated that cyanobacteria-derived microcystin-leucine-arginine (MCLR) is able to induce developing toxicity, such as malformation, growth delay and also decreased heart rates in zebrafish embryos. However, the molecular mechanisms by which MCLR induces its toxicity during the development of zebrafish remain largely unknown. Here, we evaluate the role of apoptosis in MCLR-induced developmental toxicity. Zebrafish embryos were exposed to various concentrations of MCLR (0, 0.2, 0.5, 2, and 5.0 mg L(-1)) for 96 h, at which time reactive oxygen species (ROS) was significantly induced in the 2 and 5.0 mg L(-1) MCLR exposure groups. Acridine orange (AO) staining and terminal deoxynucleotide transferase-mediated deoxy-UTP nick end labelling (TUNEL) assay showed that MCLR exposure resulted in cell apoptosis. To test the apoptotic pathway, the expression pattern of several apoptotic-related genes was examined for the level of enzyme activity, gene and protein expression, respectively. The overall results demonstrate that MCLR induced ROS which consequently triggered apoptosis in the heart of developing zebrafish embryos. Our results also indicate that the p53-Bax-Bcl-2 pathway and the caspase-dependent apoptotic pathway play major roles in MCLR-induced apoptosis in the developing embryos.

Toxicological implications of microcystins for zebrafish embryos in the presence of other environmental pollutants

Microcystins (MCs) interact with environmental contaminants as well as various other congeners of the MC family in the natural environment and with antioxidants in the exposed organisms. These interactions are likely to modify the toxicological behavior of MCs at the cellular level. The present study was conducted to determine the toxicological response of extracellular MCs in aquatic systems under environmentally relevant conditions. Microcystin-leucine-arginine (MCLR) and microcystin-arginine-arginine (MCRR) were introduced at different concentrations in a single-component (MCLR or MCRR) or dual-component (MCLR and MCRR) system to zebrafish embryos in the presence of inorganic elements (Hg, As, Pb, and Cd) and nutrient species (NO3 (-) , PO4 (3-) , and Cl(-1) ). Hatchability, heart rate, and mortality of zerbrafish embryos were monitored together with changes in the activity of glutathione-S-transferase (GST) to evaluate their response on exposure to MCLR and MCRR. There was a significant reduction in all these parameters at higher doses of MCLR and MCRR (>100 ng/mL), implying bioaccumulation of these MCs in embryos and adverse effects on early development stages of the fish. It was further observed that PO4 (3-) and Cl(-) enhanced the toxic effects of MCLR and MCRR while NO3 (-) attenuated their toxic effects. In contrast, all 4 toxic elements together increased the toxicity of MCLR and MCRR to embryos compared with their single-component counterparts. Thus, the toxic effects of MCs depend not only on their relative environmental concentrations, but also on those of other environmental pollutants and the levels of antioxidants in exposed organisms.

The ecotoxicological evaluation of Cylindrospermopsis raciborskii from Lake Balaton (Hungary) employing a battery of bioassays and chemical screening

Ecotoxicity of four Cylindrospermopsis raciborskii strains (ACT 9502, ACT 9503, ACT 9504, ACT 9505) isolated from Lake Balaton (Hungary) was evaluated in four aquatic bioassays including the Thamnocephalus platyurus acute lethality test; Daphnia magna acute immobilization assay; D. magna feeding inhibition assay and Danio rerio embryo developmental toxicity assay, assisted by chemical screening for known toxins by HPLC-MS. For reference, we analyzed in parallel the toxin content and toxic effects of two previously characterized toxin-producing strains: the Australian cylindrospermopsin producer AQS C. raciborskii and the anatoxins producer Oscillatoria sp. PCC 6506. Bioassays were used to evaluate the overall toxicity of the hydrophilic bioactive metabolites pool synthesized by the selected cyanobacteria. Chemical screening has proven that the ACT C. raciborskii extracts investigated did not contained cylindrospermopsins and anatoxins. The relative toxicity of the ACT C. raciborskii aqueous extracts observed in each bioassay was comparable to the effects recorded for the anatoxins producer PCC 6506 strain while toxicity values (EC50/LC50) calculated for the AQS extract were in general one order of magnitude lower. Concerning sublethal effects of ACT C. raciborskii extracts to the D. rerio embryogenesis, the general morphological abnormality observed was a significant retardation of development. Overall, our results suggest that C. raciborskii populating Lake Balaton produce metabolites with significant bioactive potencies. Therefore, continued investigation of these unknown compounds is required.

Polymethoxy-1-alkenes from Aphanizomenon ovalisporum inhibit vertebrate development in the zebrafish (Danio rerio) embryo model

Cyanobacteria are recognized producers of a wide array of toxic or otherwise bioactive secondary metabolites. The present study utilized the zebrafish (Danio rerio) embryo as an aquatic animal model of vertebrate development to identify, purify and characterize lipophilic inhibitors of development (i.e., developmental toxins) from an isolate of the freshwater cyanobacterial species, Aphanizomenon ovalisporum.Bioassay-guided fractionation led to the purification, and subsequent chemical characterization, of an apparent homologous series of isotactic polymethoxy-1-alkenes (1–6), including three congeners (4–6) previously identified from the strain, and two variants previously identified from other species (2 and 3), as well as one apparently novel member of the series (1). Five of the PMAs in the series (1–5) were purified in sufficient quantity for comparative toxicological characterization, and toxicity in the zebrafish embryo model was found to generally correlate with relative chain length and/or methoxylation. Moreover, exposure of embryos to a combination of variants indicates an apparent synergistic interaction between the congeners. Although PMAs have been identified previously in cyanobacteria, this is the first report of their apparent toxicity. These results, along with the previously reported presence of the PMAs from several cyanobacterial species, suggest a possibly widespread distribution of the PMAs as toxic secondary metabolites and warrants further chemical and toxicological investigation.

Waterborne exposure to microcystin-LR alters thyroid hormone levels and gene transcription in the hypothalamic-pituitary-thyroid axis in zebrafish larvae

Microcystin-leucine-arginine (MCLR) is the most toxic and the most commonly encountered variant of microcystins (MCs) in aquatic environment, and it has the potential for disrupting thyroid hormone homeostasis, but the molecular mechanisms underlying this process have not yet been clarified. In the present study, we observed body growth retardation associated with decreased levels of thyroid hormones (THs) in zebrafish larvae, highlighting the interferences of MCLR with the growth of fish larvae. To further our understanding of mechanisms of MCLR-induced endocrine toxicity, quantitative real-time PCR analysis was performed on hypothalamic-pituitary-thyroid (HPT) axis related genes of developing zebrafish embryos exposed to 100, 300 and 500 μg L(-1) MCLR until 96 h post-fertilization. The expression of several genes in the HPT system, i.e., corticotropin-releasing factor (CRF), thyroid-stimulating hormone (TSH), sodium/iodide symporter (NIS), thyroglobulin (TG), thyroid receptors (TRα and TRβ) and iodothyronine deiodinases (Dio1 and Dio2) was examined using quantitatively real-time PCR. The gene expression levels of CRF, TSH, NIS and TG were significantly induced after exposure to 500 μg L(-1) MCLR. The transcription of TRs gene was down-regulated in a concentration-dependent manner. Up-regulation and down-regulation of Deio1 and Deio2 gene expression, respectively, were observed upon exposure to MCLR. The above results indicated that MCLR could alter gene expression in the HPT axis which might subsequently contribute to MCLR-induced thyroid disruption.

Analysis of microRNA expression in embryonic developmental toxicity induced by MC-RR

As cynobacterial blooms frequently occur in fresh waters throughout the world, microcystins (MCs) have caused serious damage to both wildlife and human health. MCs are known to have developmental toxicity, however, the possible molecular mechanism is largely unknown. This is the first toxicological study to integrate post-transcriptomic, proteomic and bioinformatics analysis to explore molecular mechanisms for developmental toxicity of MCs in zebrafish. After being microinjected directly into embryos, MC-RR dose-dependently decreased survival rates and increased malformation rates of embryos, causing various embryo abnormalities including loss of vascular integrity and hemorrhage. Expressions of 31 microRNAs (miRNAs) and 78 proteins were significantly affected at 72 hours post-fertilisation (hpf). Expressions of miR-430 and miR-125 families were also significantly changed. The altered expressions of miR-31 and miR-126 were likely responsible for the loss of vascular integrity. MC-RR significantly reduced the expressions of a number of proteins involved in energy metabolism, cell division, protein synthesis, cytoskeleton maintenance, response to stress and DNA replication. Bioinformatics analysis shows that several aberrantly expressed miRNAs and proteins (involved in various molecular pathways) were predicted to be potential MC-responsive miRNA-target pairs, and that their aberrant expressions should be the possible molecular mechanisms for the various developmental defects caused by MC-RR.

Transmembrane transport of microcystin to Danio rerio zygotes: insights into the developmental toxicity of environmental contaminants

Microcystins (MCs) produced by cyanobacteria and their continuing "blooms" are a worldwide problem owing to the toxicity of microcystin-LR (MC-LR) to plants and animals. In the present study, we investigated membrane transport of MC-LR and its toxic effects on zebrafish embryos using fragmentation of embryos, scanning electron microscope (SEM), fluorescence microscopy, and toxic exposure tests. At a concentration < 0.04 mmol/l, MC-LR was predominantly adsorbed on outer membrane surface of embryos according to Langmuir isotherm. The absorption characteristics of MC-LR within the range from 0.05 to 0.4 mmol/l conformed to Freundlich isotherm model. At concentrations > 0.50 mmol/l MC-LR directly entered the cytoplasm via partition. Thinning and disruption of membranes was confirmed using SEM and fluorescence morphological observations. Exposure to different concentrations of MC-LR resulted in differences in membrane transport and toxicity characteristics. At low concentrations, more than 75% of the adsorbed MC-LR accumulated on the outer membrane surface and resulted in axial malformation, tail curving, and tail twisting. Increasing the concentration of MC-LR to between 0.05 and 0.4 mmol/l improved membrane transport and it was evident in cytoplasm of embryos, resulting in serious pericardial edema, hatching gland edema, hemagglutination, hemorrhage, and vacuolization. At > 0.50 mmol/l, more than 70% of the adsorbed MC-LR entered the cytoplasm and this was lethal to the embryos. The current research outlines a new method and mechanism for the transmembrane transport of large molecular weight organic compounds and could be important for studies concerning molecular toxicology.

Cytotoxic and antimitotic activities in aqueous extracts of eight cyanobacterial strains isolated from the marine sponge Petrosia ficiformis

Marine cyanobacteria are photosynthetic prokaryotes of significant ecological interest, living free or in association with invertebrates. They are also considered as excellent sources of antineoplastic, antibacterial, antiviral and antifungal compounds. In this work, aqueous extracts from eight cyanobacterial strains isolated from the Mediterranean sponge Petrosia ficiformis have been investigated for their bioactive properties. Bioassays with human erythrocytes, Artemia salina nauplii, and Paracentrotus lividus gametes and embryos were performed. Some aqueous extracts exhibited citolytic effect on human erythrocytes and toxic activity against A. salina nauplii. Furthermore antimitotic activity was evidenced during sea urchin embryos development and disorganization of blastomeres with altered cell-cell contact was also induced. Some of the isolated cyanobacterial strains, belonging to Leptolyngbya and Synechococcus genera with an high citotoxic activity, should be further investigated to better characterize their bioactive molecules. Our data confirm cyanobacteria as an interesting source of novel bioactive compounds with potential applications in pharmaceutics.

p53 Plays an important role in cell fate determination after exposure to microcystin-LR

BACKGROUND

Microcystin-LR, a cyclic heptapeptide, possesses the ability to inhibit the serine/threonine protein phosphatases PP1 and PP2A and, consequently, exhibits acute hepatocytotoxicity. Moreover, microcystin-LR induces cellular proliferation, resulting in tumor-promoting activity in hepatocytes. However, mechanisms that regulate the balance between cell death and proliferation after microcystin-LR treatment remain unclear.

OBJECTIVE

We examined the contribution of the transcription factor p53, as well as that of the hepatic uptake transporter for microcystin-LR, organic anion transporting polypeptide 1B3 (OATP1B3), to the cellular response to microcystin-LR exposure.

METHODS

We analyzed intracellular signaling responses to microcystin-LR by immunoblotting and real-time reverse-transcriptase polymerase chain reaction techniques using HEK293 human embryonic kidney cells stably transfected with SLCO1B3 (HEK293-OATP1B3). In addition, we analyzed the effect of attenuation of p53 function, via the p53 inhibitor pifithrin-alpha, and knockdown of p53 mRNA on the cytotoxicity of microcystin-LR using a 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay.

RESULTS

Microcystin-LR induced the phosphorylation and accumulation of p53 in HEK293-OATP1B3 cells, which resulted in up-regulation of the expression of p53 transcript targets, including p21 and seven in absentia homolog 1 (siah-1). In addition, microcystin-LR activated Akt signaling through the phosphorylation of Akt and glycogen synthase kinase 3beta. Although Akt signaling was activated, the accumulation of p53 led cells to apoptosis after treatment with 50 nM microcystin-LR for 24 hr. Both pharmacological inhibition of transcription factor activity of p53 by pifithrin-alpha and knockdown of p53 with small hairpin RNA attenuated the susceptibility of HEK293-OATP1B3 cells to microcystin-LR.

CONCLUSIONS

This study demonstrates the importance of p53 in the regulation of cell fate after exposure to microcystin-LR. Our results suggest that, under conditions of p53 inactivation (including p53 mutation), chronic exposure to low doses of microcystin-LR may lead to cell proliferation through activation of Akt signaling. Results of this study may contribute to the development of chemoprevention and chemotherapeutic approaches to microcystin-LR poisoning.

Changes in the relative expression pattern of multiple vitellogenin genes in adult male and larval zebrafish exposed to exogenous estrogens

Production of the lipoprotein vitellogenin (Vg) is induced in fish upon exposure to estrogens and is a biomarker of endocrine disruption in fish. In some fish, three types of Vg (VgA, VgB, and VgC) are recognized and transcribed from at least three distinct Vg genes (vtg). We investigated expression of vtg coding for Vg1A/B, Vg2A/B, and VgC in adult male and larval zebrafish exposed to various estrogenic substances. Quantitative PCR was conducted for transcripts of each vtg and a control gene (beta-actin). Male fish were exposed to 17beta-estradiol (E2) and 17alpha-ethinylestradiol, total RNA was extracted from excised liver, and histopathology of liver, trunk kidney, and gonads was conducted. Larval fish were exposed to 10 different estrogenic substances and total RNA was extracted from groups of whole larvae. In adult male fish, the relative fold change varied, but pattern of expression change (i.e., Vg1A/B > Vg2A/B > VgC) was consistent. Larger males exposed to E2 had significantly higher induction of each vtg. In larval zebrafish, the relative fold change in vtg expression varied according to specific estrogenic substance tested, but the pattern of change (i.e., Vg2A/B > Vg1A/B > VgC) was consistent for each substance that induced vtg.

Compensatory growth induced in zebrafish larvae after pre-exposure to a Microcystis aeruginosa natural bloom extract containing microcystins

Early life stage tests with zebrafish (Danio rerio) were used to detect toxic effects of compounds from a Microcystis aeruginosa natural bloom extract on their embryolarval development. We carried out the exposure of developing stages of fish to complex cyanobacterial blooms containing hepatotoxic molecules - microcystins. Fish embryo tests performed with the bloom extract containing 3 mg·L⁻¹ Eq microcystin-LR showed that after 24 h of exposure all fish embryos died. The same tests performed with other diluted extracts (containing 0.3, 0.1 and 0.03 mg·L⁻¹ Eq microcystin-LR) were shown to have an influence on zebrafish development and a large number of embryos showed malformation signs (edema, bent and curving tail). After hatching the larvae were transferred to a medium without toxins to follow the larval development under the new conditions. The specific growth of the pre-exposed larvae was significantly more important than that of the control larvae. This may represent a compensatory growth used to reduce the difference in size with the control fish noted after hatching.

Toxicity of cylindrospermopsin, and other apparent metabolites from Cylindrospermopsis raciborskii and Aphanizomenon ovalisporum, to the zebrafish (Danio rerio) embryo

Cyanobacteria produce a diverse array of toxic or otherwise bioactive compounds that pose growing threats to human and environmental health. We utilized the zebrafish (Danio rerio) embryo, as a model of vertebrate development, to investigate the inhibition of development pathways (i.e. developmental toxicity) by the cyanobacterial toxin, cylindrospermopsin (CYN), as well as extracts from various isolates of Cylindrospermopsis raciborskii and Aphanizomenon ovalisporum. CYN was toxic only when injected directly into embryos, but not by direct immersion at doses up to 50mug/ml. Despite the dose dependency of toxicity observed following injection of CYN, no consistent patterns of developmental defects were observed, suggesting that toxic effects of CYN may not target specific developmental pathways. In contrast, direct immersion of embryos in all of the extracts resulted in both increased mortality and reproducible, consistent, developmental dysfunctions. Interestingly, there was no correlation of developmental toxicity observed for these extracts with the presence of CYN or with previously reported toxicity for these strains. These results suggest that CYN is lethal to zebrafish embryos, but apparently inhibits no specific developmental pathways, whereas other apparent metabolites from C. raciborskii and A. ovalisporum seem to reproducibly inhibit development in the zebrafish model. Continued investigation of these apparent, unknown metabolites is needed.

Cyanobacterial toxins as allelochemicals with potential applications as algaecides, herbicides and insecticides

Cyanobacteria ("blue-green algae") from marine and freshwater habitats are known to produce a diverse array of toxic or otherwise bioactive metabolites. However, the functional role of the vast majority of these compounds, particularly in terms of the physiology and ecology of the cyanobacteria that produce them, remains largely unknown. A limited number of studies have suggested that some of the compounds may have ecological roles as allelochemicals, specifically including compounds that may inhibit competing sympatric macrophytes, algae and microbes. These allelochemicals may also play a role in defense against potential predators and grazers, particularly aquatic invertebrates and their larvae. This review will discuss the existing evidence for the allelochemical roles of cyanobacterial toxins, as well as the potential for development and application of these compounds as algaecides, herbicides and insecticides, and specifically present relevant results from investigations into toxins of cyanobacteria from the Florida Everglades and associated waterways.

Protein phosphatase PP1 gamma 2 in sperm morphogenesis and epididymal initiation of sperm motility

The serine/threonine phosphatase (PP1) isoform PP1 gamma 2, predominantly expressed in the testis, is a key enzyme in spermatozoa. High PP1 gamma 2 catalytic activity holds motility in check in immature spermatozoa. Inhibition of PP1 gamma 2 causes motility initiation in immature spermatozoa and motility stimulation and changes in flagellar beat parameters in mature spermatozoa. The PP1 gamma 2 isoform is present in all mammalian spermatozoa studied: mouse, rat, hamster, bovine, non-human primate and man. We have now identified at least four of its regulatory proteins that regulate distinct pools of PP1 gamma 2 within spermatozoa. Our studies provide new insights into biochemical mechanisms underlying development and regulation of sperm motility. We hypothesize that changes in sperm PP1 gamma 2 activity as a result of phosphorylation and reversible binding of the regulatory proteins to the catalytic subunit are critical in the development and regulation of motility and the ability of sperm to fertilize eggs. Targeted disruption of the Ppp1cc gene, which encodes the PP1 gamma 1 or PP1 gamma 2 isoforms, causes male infertility in mice as a result of impaired spermiogenesis. Our observations suggest that, in addition to motility, the protein phosphatase PP1 gamma 2 might play an isoform-specific function in the development of specialized flagellar structures of mammalian spermatozoa.

Analysis of Ppp1cc-Null Mice Suggests a Role for PP1gamma2 in Sperm Morphogenesis1

Serine/threonine protein phosphatase 1 (PP1) consists of four ubiquitously expressed major isoforms, two of which, PP1gamma1 and PP1gamma2, are derived by alternative splicing of a single gene, Ppp1cc. PP1gamma2 is the most abundant isoform in the testis, and is a key regulator of sperm motility. Targeted disruption of the Ppp1cc gene causes male infertility in mice due to impaired spermiogenesis. This study was undertaken to determine the expression patterns of specific PP1 isoforms in testes of wild-type mice and to establish how the defects produced in Ppp1cc-null developing sperm are related to the loss of PP1gamma isoform expression. We observed that PP1gamma2 was prominently expressed in the cytoplasm of secondary spermatocytes and round spermatids as well as in elongating spermatids and testicular and epididymal spermatozoa, whereas its expression was weak or absent in spermatogonia, pachytene spermatocytes, and interstitial cells. In contrast, a high level of PP1gamma1 expression was observed in interstitial cells, whereas much weaker expression was observed in all stages of spermatogenesis. Another PP1 isoform, PP1alpha, was predominant in spermatogonia, pachytene spermatocytes, and interstitial cells. Examining the temporal expression of PP1 enzymes in testes revealed a striking postnatal increase in PP1gamma2 levels compared with other isoforms. Testicular sperm tails from Ppp1cc-null mice showed malformed mitochondrial sheaths and extra outer dense fibers in both the middle and principal pieces. These data suggest that in addition to its previously documented role in motility, PP1gamma2 is involved in sperm tail morphogenesis.

Effect of different cyanobacterial biomasses and their fractions with variable microcystin content on embryonal development of carp (Cyprinus carpio L.)

While numerous studies focused on the effects of microcystins, the role of other components of complex cyanobacterial water blooms in toxicity is poorly understood. In this study we have evaluated effects of various fractions of cyanobacterial biomass with different composition and microcystin content on embryolarval development of carp (Cyprinus carpio). The following samples (fractions) of four natural water blooms were prepared and tested: complex cyanobacterial biomass, crude aqueous extract of biomass, cellular pellet remaining from aqueous extract, permeate (i.e. microcystin-free fraction prepared during C-18 solid-phase extraction; SPE), and eluate (i.e. fraction prepared by SPE containing mostly microcystins). Complex biomass and the crude aqueous extract (regardless of microcystin content and/or microcystin variants present) in the sample were the most toxic. On the other hand, eluate fractions of all samples containing microcystins in concentrations 8-255 microgL(-1) induced no or only weak toxic effects. Exposures of fish to permeate fractions (with removed microcystins) of two samples dominated by Aphanizomenon sp. and Planktothrix sp. resulted in significant mortality, while other two samples dominated by Microcystis spp. induced minor effects. We have also observed significant inhibition of glutathione S-transferases (GST) at most fractions of the Aphanizomenon sp. and Planktothrix sp. dominated samples. Our data indicate that cyanobacterial water blooms as well complex biomass extracts induce significant embryolarval toxicity in common carp. However, these effects were independent of microcystin content, and the most pronounced effects were observed with the non-Microcystis dominated samples. Therefore, a critical examination of microcystin role in overall ecotoxicology of complex cyanobacterial blooms is needed.

Toxicity of complex cyanobacterial samples and their fractions in Xenopus laevis embryos and the role of microcystins

This work evaluated the effects of various cyanobacterial fractions in Frog Embryo Teratogenesis Assay Xenopus (FETAX) with African clawed frog embryos. Fractions were prepared from five biomasses with different dominant genera (Microcystis, Aphanizomenon, Anabaena, Planktothrix) and different microcystin content. Effects of following fractions were investigated: (I) homogenate of complex cyanobacterial biomass, (II) cell debris (pellet) after centrifugation of complex biomass, (III) supernatant after centrifugation of complex biomass (= crude aqueous extract), (IV) permeate after passing of crude extract through C-18 column (fraction devoid of microcystins), and (V) eluate from C-18 column (containing microcystins, if present). Besides classical parameters evaluated in 96 h FETAX (mortality, growth inhibition, malformations), we have also assessed the effects on biochemical markers of oxidative stress and detoxification (glutathione pool, GSH; activity of glutathione peroxidase, GPx; glutathione reductase, GR; activity of glutathione-S-transferase, GST). Complex biomass (I) and aqueous extract (III) were generally the most toxic fractions in terms of mortality and growth inhibition, whereas eluates containing microcystins (V) were generally less toxic. On the other hand, the same fraction (eluates) induced significant malformations in low concentrations but the effects were not related to the content of microcystins. Biomarkers were affected in variable manner but no significant effect or clear relation to microcystin content was observed. Our data support the hypothesis that microcystins are not the only or major toxic compounds in the complex cyanobacterial samples (at least for some species) and that more attention should be paid to other components of complex cyanobacterial biomass including non-specific parameters such as oxygen content or toxic ammonia released during bacterial decay of organic material.

Microcystins cause embryonic toxicity in mice

Microcystins produced by freshwater cyanobacteria are potent hepatotoxins and can cause animal intoxications and human illnesses. In the present study, the effects of microcystins on the embryonic development of Kunming mice were determined using cell extracts of Microcystis aeruginosa from the Nanwan reservoir, China. Forty-eight pregnant mice were divided into four groups of 12 mice. Pregnant mice in three experimental groups were injected intraperitoneally with cell extracts at doses equivalent to 3, 6, or 12 microg microcystins/kg body weight daily from gestational days 6-15, while the mice in the control group were injected on the same schedule with sterilized saline. Mice were killed on the 18th day of gestation and embryonic and fetal developmental indexes checked. The fetal mice were also examined for anomalies of external, skeletal, and internal organs. The results demonstrated a significant decrease in body weight gain of pregnant mice in the 12 microg/kg dose group when compared to the control group (p<0.05). Differences in mean body weight, body length, and tail length of the fetuses were also found in these two groups (p<0.05). However, no significant difference in these characteristics was detected in the 6 or 3 microg/kg dose groups when compared to the control group (p>0.05). Four fetuses in the 6 microg/kg body dose group were found to have a curving tail. Additionally, petechial hemorrhage and hydropic degeneration were observed in the livers of fetuses in the 6 and 12 microg/kg experimental groups. These results suggested that microcystins had both maternal and embryonic toxicity in mice.