Elucidating cylindrospermopsin toxicity via synthetic analogues: An in vitro approach

Cytotoxic Effects and Oxidative Stress Produced by a Cyanobacterial Cylindrospermopsin Producer Extract versus a Cylindrospermopsin Non-Producing Extract on the Neuroblastoma SH-SY5Y Cell Line

The incidence and interest of cyanobacteria are increasing nowadays because they are able to produce some toxic secondary metabolites known as cyanotoxins. Among them, the presence of cylindrospermopsin (CYN) is especially relevant, as it seems to cause damage at different levels in the organisms: the nervous system being the one most recently reported. Usually, the effects of the cyanotoxins are studied, but not those exerted by cyanobacterial biomass. The aim of the present study was to assess the cytotoxicity and oxidative stress generation of one cyanobacterial extract of R. raciborskii non-containing CYN (CYN-), and compare its effects with those exerted by a cyanobacterial extract of C. ovalisporum containing CYN (CYN+) in the human neuroblastoma SH-SY5Y cell line. Moreover, the analytical characterization of potential cyanotoxins and their metabolites that are present in both extracts of these cultures was also carried out using Ultrahigh Performance Liquid Chromatography-Mass Spectrometry, in tandem (UHPLC-MS/MS). The results show a reduction of cell viability concentration- and time-dependently after 24 and 48 h of exposure with CYN+ being five times more toxic than CYN-. Furthermore, the reactive oxygen species (ROS) increased with time (0-24 h) and CYN concentration (0-1.11 µg/mL). However, this rise was only obtained after the highest concentrations and times of exposure to CYN-, while this extract also caused a decrease in reduced glutathione (GSH) levels, which might be an indication of the compensation of the oxidative stress response. This study is the first one performed in vitro comparing the effects of CYN+ and CYN-, which highlights the importance of studying toxic features in their natural scenario.

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.

Response of Oreochromis niloticus (Teleostei: Cichlidae) exposed to a guanitoxin-producing cyanobacterial strain using multiple biomarkers

Changes in environmental conditions in aquatic ecosystems caused by anthropic actions can modify the composition of primary producers, promoting the excessive proliferation of cyanobacteria. These organisms can form cyanobacterial blooms, which directly affect aquatic life. The present study investigated the mutagenicity of the cyanobacterium Sphaerospermopsis torques-reginae (strain ITEP-024), guanitoxin-producing (natural organophosphate), and sublethal effects on fish in relevant environment concentrations. For this, the Ames test (Salmonella/microsome) was performed as a mutagenic assay for extracts of the ITEP-024 strain. Specimens of Oreochromis niloticus (Teleostei: Cichlidae) were subjected to acute 96 h exposure to different concentrations of aqueous extract of the strain: C = control group; T1 = 31.25 mg/L; T2 = 62.5 mg/L; T3 = 125 mg/L; and T4 = 250 mg/L. Genotoxic, biochemical, osmoregulatory, and physiologic biomarkers were analyzed. Our results showed that the cyanobacterium had a weak mutagenic response for the TA102 strain of Salmonella with and without metabolic activation by S9. Strains TA98 and TA100 were not affected. Fish from treatments T3 and T4 showed changes in oxidative stress (CAT, SOD, and GST enzymes), inhibition of the enzyme acetylcholinesterase activity, micronucleus formation, and osmoregulatory disorders. No guanitoxin accumulation was detected in the different tissues of O. niloticus by LC-MS/MS. Our results showed unprecedented mutagenicity data of the guanitoxin-producing cyanobacteria by the Ames test and biochemical, osmoregulatory, and genotoxic disorders in fish, providing efficient aquatic contamination biomarkers. Despite the great concern related to the presence of guanitoxin in blooms in freshwater ecosystems, its concentration is not yet regulated, and thus there is no monitoring agenda in current legislation.

7-epi-cylindrospermopsin and microcystin producers among diverse Anabaena/Dolichospermum/Aphanizomenon CyanoHABs in Oregon, USA

Several genomes of Nostocales ADA clade members from the US Pacific Northwest were recently sequenced. Biosynthetic genes for microcystin, cylindrospermopsin or anatoxin-a were present in 7 of the 15 Dolichospermum/Anabaena strains and none of the 5 Aphanizomenon flos-aquae (AFA) strains. Toxin analyses (ELISA and LC-MS/MS) were conducted to quantitate and identify microcystin (MC) and cylindrospermopsin (CYN) congeners/analogs in samples dominated by Dolichospermum spp. of known genome sequence. MC-LR was the main congener produced by Dolichospermum spp. from Junipers Reservoir, Lake Billy Chinook and Odell Lake, while a congener provisionally identified as [Dha⁷]MC-HtyR was produced by a Dolichospermum sp. in Detroit Reservoir. A second Dolichospermum sp. from Detroit Reservoir was found to produce 7-epi-CYN, with 7-deoxy-CYN also present, but no CYN. The monitoring history of each of these lakes indicates the capacity for high levels of cyanotoxins during periods when Dolichospermum spp. are the dominant cyanobacteria. The diversity of ADA strains found in the US Pacific NW emphasizes the importance of these cyanobacteria as potentially toxic HAB formers in this temperate climatic region. Our results linking congener and genetic identity add data points that will help guide development of improved tools for predicting congener specificity from cyanotoxin gene sequences.

Albicanol modulates oxidative stress and the p53 axis to suppress profenofos induced genotoxicity in grass carp hepatocytes

The organophosphorus pesticide profenofos (PFF) is widely used as an environmental contaminant, and it can remain in water bodies causing serious harm to aquatic organisms. Albicanol is a sesquiterpenoid with potent antioxidant and antagonistic activities against heavy metal toxicity. However, the mechanism of PFF induced genotoxicity in fish hepatocytes and the role Albicanol can play in this process are unknown. In this study, the model was established by treating grass carp hepatocytes with PFF (150 μM) and/or Albicanol (5 × 10^-5 μg mL^-1) for 24 h. The results showed that PFF exposure arrested L8824 cells in the G1-S phase. PFF caused the increase of MDA level in L8824 cells, while the decrease of SOD, CAT and T-AOC levels caused oxidative stress. Elevated levels of γH2AX, tail moment, tail length, % DNA and 8-OHdG indicated that PFF caused DNA damage in L8824 cells. PFF inhibited the expression levels of cell cycle related regulatory genes (cyclin A, cyclin D, cyclin E, CDK2 and CDK4) by upregulating p53/p21 genes and activating the p53 signaling pathway. Albicanol was used to significantly reduce the above effects caused by PFF exposure on hepatocytes in grass carp. Albicanol could reduce the increase in the proportion of cells in the G1-S phase caused by PFF. In summary, Albicanol could inhibit the genotoxicity of L8824 cells resulted from PFF exposure by decreasing oxidative stress and the p53 pathway.

Synthesis of (+)-(R)-Tiruchanduramine

The absolute stereochemistry of the marine alkaloid (+)-(R)-tiruchanduramine was established via a convergent total synthesis in six steps and 15.5% overall yield from Fmoc-D-Dab(Boc)-OH.

Treatment of cylindrospermopsin by hydroxyl and sulfate radicals: Does degradation equal detoxification?

Drinking water treatment ultimately aims to provide safe and harmless drinking water. Therefore, the suitability of a treatment process should not only be assessed based on reducing the concentration os a pollutant concentration but, more importantly, on reducing its toxicity. Hence, the main objective of this study was to answer whether the degradation of a highly toxic compound of global concern for drinking water equals its detoxification. We, therefore, investigated the treatment of cylindrospermopsin (CYN) by •OH and SO₄^-• produced in Fenton and Fenton-like reactions. Although SO₄^-• radicals removed the toxin more effectively, both radical species substantially degraded CYN. The underlying degradation mechanisms were similar for both radical species and involved hydroxylation, dehydrogenation, decarboxylation, sulfate group removal, ring cleavage, and further fragmentation. The hydroxymethyl uracil and tricyclic guanidine moieties were the primary targets. Furthermore, the residual toxicity, assessed by a 3-dimensional human in vitro liver model, was substantially reduced during the treatment by both radical species. Although the results indicated that some of the formed degradation products might still be toxic, the overall reduction of the toxicity together with the proposed degradation pathways allowed us to conclude: "Yes, degradation of CYN equals its detoxification!".

Phosphorus deficiency stimulates dominance of Cylindrospermopsis through facilitating cylindrospermopsin-induced alkaline phosphatase secretion: Integrating field and laboratory-based evidences

Potentially toxic Cylindrospermopsis raciborskii blooms are of emerging concerns, as its scale is spreading from tropical regions to high latitudes, increasing the risk of aquatic biota being exposed to cylindrospermopsin (CYN). So far, CYN-producing C. raciborskii strains have only been reported in tropical waters which are commonly phosphorus (P)-deficient, where they can dominate phytoplankton communities. However, the influence of CYN on phytoplankton communities under different P status remains unclear. In this study, we first analyzed the summer observations of 120 tropical reservoirs in Guangdong Province. The proportion of potential CYN-producers was significantly higher in P-deficient and CYN-present reservoirs than that in P-sufficient or CYN-absent ones. This suggested that in P-deficient condition, the potential CYN producers might gain more advantages by the help of CYN. Then, in laboratory experiments we found that upon P deprivation, CYN did not inhibit the cell growth of other algal cells, but significantly stimulates them to secret more alkaline phosphatase (ALP) than in P-sufficient condition. Through transcriptomics, we further revealed that under such P-deficient condition, CYN remarkably induced intracellular nitrogen allocation and protein export system by activating the PIK3/Akt-cGMP/PKG signaling pathways in Scenedesmus bijugatus, thus enhancing its ALP secretion. Our study implies that CYN-induced ALP secretion is facilitated upon P deficiency, thus supporting the dominance of its producers C. raciborskii.

Effects of cylindrospermopsin, its decomposition products, and anatoxin-a on human keratinocytes

Toxins produced by cyanobacteria (cyanotoxins) are among the most dangerous natural compounds. In recent years, there have been many published papers related to the toxic alkaloids cylindrospermopsin (CYN) and anatoxin-a (ANTX-a), which are synthesized by several freshwater species of cyanobacteria (i.e. Raphidiopsis raciborskii and Anabaena flos-aquae) and are some of the most common cyanotoxins in aquatic reservoirs. The harmful properties of CYN are wide and primarily include cytotoxicity. To date, several analogs and decomposition products of CYN have been described, which can potentially increase its toxic effects in living organisms. The mode of action of ANTX-a is different than that observed after CYN exposure and involves structures in the nervous system. One of the most frequent situations in which cyanotoxins are introduced into the human body is by skin contact with contaminated water, i.e., during water sports, fishing or agriculture. Unfortunately, to date, knowledge on the influence of CYN, its decomposition products, and ANTX-a on human skin is limited. In this paper, we investigated the impact of CYN, its decomposition products, and ANTX-a on the proliferation of human keratinocytes, which provide a protective barrier on the skin. Moreover, we described the cytotoxic effects developed in the selected cell type and estimated the ability of the keratinocytes to migrate under the influence of the studied cyanotoxins. The obtained results suggest that CYN and its decomposition products at concentrations corresponding to that determined for CYN in nature (1 μg·mL^-1) are strong inhibitors of keratinocyte proliferation (70% inhibition within 24 h for pure CYN). The cytotoxic effects of CYN and the CYN decomposition products on keratinocytes was also significant, and the pure toxin (1 μg·mL^-1) was estimated to be 35% after 24 h of exposure. Similarly, harmful effects caused by CYN and its byproducts were observed during keratinocyte migration, and the initial form of the toxin (1 μg·mL^-1) showed 40% inhibition within 16 h. Different results were obtained for ANTX-a. The toxic effects of this compound on human keratinocytes estimated by the applied tests was observed only at the highest tested concentration (10 μg·mL^-1) and after a long period of exposure. The results presented in this paper are, to the best of our knowledge, the first description of the influence of CYN, CYN decomposition products, and ANTX-a on human epidermal cells. Clearly, CYN and its decomposition products are serious threats not only when acting on internal organs but also during the skin contact stage. Further studies on cyanotoxins should focus on the determination of their decomposition products and ecotoxicology in natural aquatic environments.

The chemistry and biology of guanidine secondary metabolites

Covering: 2017-2019Guanidine natural products isolated from microorganisms, marine invertebrates and terrestrial plants, amphibians and spiders, represented by non-ribosomal peptides, guanidine-bearing polyketides, alkaloids, terpenoids and shikimic acid derived, are the subject of this review. The topics include the discovery of new metabolites, total synthesis of natural guanidine compounds, biological activity and mechanism-of-action, biosynthesis and ecological functions.

Application of advanced HepG2 3D cell model for studying genotoxic activity of cyanobacterial toxin cylindrospermopsin

Cylindrospermopsin (CYN) is an emerging cyanotoxin increasingly being found in freshwater cyanobacterial blooms worldwide. Humans and animals are exposed to CYN through the consumption of contaminated water and food as well as occupational and recreational water activities; therefore, it represents a potential health threat. It exhibits genotoxic effects in metabolically active test systems, thus it is considered as pro-genotoxic. In the present study, the advanced 3D cell model developed from human hepatocellular carcinoma (HepG2) cells was used for the evaluation of CYN cyto-/genotoxic activity. Spheroids were formed by forced floating method and were cultured for three days under static conditions prior to exposure to CYN (0.125, 0.25 and 0.5 μg/mL) for 72 h. CYN influence on spheroid growth was measured daily and cell survival was determined by MTS assay and live/dead staining. The influence on cell proliferation, cell cycle alterations and induction of DNA damage (γH2AX) was determined using flow cytometry. Further, the expression of selected genes (qPCR) involved in the metabolism of xenobiotics, proliferation, DNA damage response, apoptosis and oxidative stress was studied. Results revealed that CYN dose-dependently reduced the size of spheroids and affected cell division by arresting HepG2 cells in G1 phase of the cell cycle. No induction of DNA double strand breaks compared to control was determined at applied conditions. The analysis of gene expression revealed that CYN significantly deregulated genes encoding phase I (CYP1A1, CYP1A2, CYP3A4, ALDH3A) and II (NAT1, NAT2, SULT1B1, SULT1C2, UGT1A1, UGT2B7) enzymes as well as genes involved in cell proliferation (PCNA, TOP2α), apoptosis (BBC3) and DNA damage response (GADD45a, CDKN1A, ERCC4). The advanced 3D HepG2 cell model due to its more complex structure and improved cellular interactions provides more physiologically relevant information and more predictive data for human exposure, and can thus contribute to more reliable genotoxicity assessment of chemicals including cyanotoxins.

Alternative Isolation Protocol for Desulfo and Zwitterionic Cylindrospermopsin Alkaloids and Comparison of Their Toxicity in HepG2 Cells

The term cylindrospermopsins (CYNs) refers to a structurally related class of cyanobacterial metabolites comprised of a tricyclic guanidine group and a hydroxymethyluracil moiety. Most reports in environmental aquatic samples refer to cylindrospermopsin (CYN), and reports on other CYN alkaloids are scarce, due, in part, to a lack of versatile isolation protocols. Thus, using commercially available solid phase extraction (SPE) cartridges, we optimized an isolation protocol for the complete recovery of CYN, 7-deoxy-cylindrospermopsin (7D-CYN) and 7-deoxy-desulfo-cylindrospermopsin (7D-desulfo-CYN) from the same aliquot. The isolation protocol was adaptable depending on the nature of the sample (solid biomass, culture broth or environmental water sample) and tolerates up to 4 L of dense culture broth or 400 mg of lyophilized biomass. To quantitate the CYN alkaloids, we validated an LC-DAD-MS² method, which takes advantage of the UV absorption of the uracil group (λ 262 nm). Using electrospray ionization (ESI) in a positive ion mode, the high-resolution MS¹ data confirms the presence of the protonated alkaloids, and the MS² fragment assignment is reported as complementary proof of the molecular structure of the CYNs. We isolated three CYN alkaloids with different water solubility using the same lyophilized sample, with a purity that ranged from 95% to 99%. The biological activity of the purified CYNs, along with a synthetic degradation product of CYN (desulfo-cylindrospermopsin), was evaluated by assessing necrosis and apoptosis in vitro using flow cytometry. CYN’s lethal potency in HepG2 cells was greater than the other analogs, due to the presence of all four functional groups: guanidine, uracil, C-7 hydroxyl and the sulfate residue.

In Vitro Toxicological Screening of Stable and Senescing Cultures of Aphanizomenon, Planktothrix, and Raphidiopsis

Toxicity of cyanobacteria is the subject of ongoing research, and a number of toxic metabolites have been described, their biosynthesis pathways have been elucidated, and the mechanism of their action has been established. However, several knowledge gaps still exist, e.g., some strains produce hitherto unknown toxic compounds, while the exact dynamics of exerted toxicity during cyanobacterial growth still requires further exploration. Therefore, the present study investigated the toxicity of extracts of nine freshwater strains of Aphanizomenon gracile, an Aphanizomenon sp. strain isolated from the Baltic Sea, a freshwater strain of Planktothrix agardhii, and two strains of Raphidiopsis raciborskii obtained from 25- and 70-day-old cultures. An in vitro experimental model based on Cyprinus carpio hepatocytes (oxidative stress markers, DNA fragmentation, and serine/threonine protein activity) and brain homogenate (cholinesterase activity) was employed. The studied extracts demonstrated toxicity to fish cells, and in general, all examined extracts altered at least one or more of considered parameters, indicating that they possess, to some degree, toxic potency. Although the time from which the extracts were obtained had a significant importance for the response of fish cells, we observed strong variability between the different strains and species. In some strains, extracts that originated from 25-day-old cultures triggered more harmful effects on fish cells compared to those obtained from 70-day-old cultures, whereas in other strains, we observed the opposite effect or a lack of a significant change. Our study revealed that there was no clear or common pattern regarding the degree of cyanobacterial bloom toxicity at a given stage of development. This means that young cyanobacterial blooms that are just forming can pose an equally toxic threat to aquatic vertebrates and ecosystem functioning as those that are stable or old with a tendency to collapse. This might be largely due to a high variability of strains in the bloom.

Plastics in Cyanobacterial Blooms-Genotoxic Effects of Binary Mixtures of Cylindrospermopsin and Bisphenols in HepG2 Cells

Ever-expanding environmental pollution is causing a rise in cyanobacterial blooms and the accumulation of plastics in water bodies. Consequently, exposure to mixtures of cyanotoxins and plastic-related contaminants such as bisphenols (BPs) is of increasing concern. The present study describes genotoxic effects induced by co-exposure to one of the emerging cyanotoxins-cylindrospermopsin (CYN)-(0.5 µg/mL) and BPs (bisphenol A (BPA), S (BPS), and F (BPF); (10 µg/mL)) in HepG2 cells after 24 and 72 h of exposure. The cytotoxicity was evaluated with an MTS assay and genotoxicity was assessed through the measurement of the induction of DNA double strand breaks (DSB) with the γH2AX assay. The deregulation of selected genes (xenobiotic metabolic enzyme genes, DNA damage, and oxidative response genes) was assessed using qPCR. The results showed a moderate reduction of cell viability and induction of DSBs after 72 h of exposure to the CYN/BPs mixtures and CYN alone. None of the BPs alone reduced cell viability or induced DSBs. No significant difference was observed between CYN and CYN/BPs exposed cells, except with CYN/BPA, where the antagonistic activity of BPA against CYN was indicated. The deregulation of some of the tested genes (CYP1A1, CDKN1A, GADD45A, and GCLC) was more pronounced after exposure to the CYN/BPs mixtures compared to single compounds, suggesting additive or synergistic action. The present study confirms the importance of co-exposure studies, as our results show pollutant mixtures to induce effects different from those confirmed for single compounds.

A Review on the Study of Cyanotoxins in Paleolimnological Research: Current Knowledge and Future Needs

Cyanobacterial metabolites are increasingly studied, in regards to their biosynthesis, ecological role, toxicity, and potential biomedical applications. However, the history of cyanotoxins prior to the last few decades is virtually unknown. Only a few paleolimnological studies have been undertaken to date, and these have focused exclusively on microcystins and cylindrospermopsins, both successfully identified in lake sediments up to 200 and 4700 years old, respectively. In this paper, we review direct extraction, quantification, and application of cyanotoxins in sediment cores, and put forward future research prospects in this field. Cyanobacterial toxin research is also compared to other paleo-cyanobacteria tools, such as sedimentary pigments, akinetes, and ancient DNA isolation, to identify the role of each tool in reproducing the history of cyanobacteria. Such investigations may also be beneficial for further elucidation of the biological role of cyanotoxins, particularly if coupled with analyses of other abiotic and biotic sedimentary features. In addition, we identify current limitations as well as future directions for applications in the field of paleolimnological studies on cyanotoxins.