A fluorimetric method based on changes in membrane potential for screening paralytic shellfish toxins in mussels

Current Trends and New Challenges in Marine Phycotoxins

Marine phycotoxins are a multiplicity of bioactive compounds which are produced by microalgae and bioaccumulate in the marine food web. Phycotoxins affect the ecosystem, pose a threat to human health, and have important economic effects on aquaculture and tourism worldwide. However, human health and food safety have been the primary concerns when considering the impacts of phycotoxins. Phycotoxins toxicity information, often used to set regulatory limits for these toxins in shellfish, lacks traceability of toxicity values highlighting the need for predefined toxicological criteria. Toxicity data together with adequate detection methods for monitoring procedures are crucial to protect human health. However, despite technological advances, there are still methodological uncertainties and high demand for universal phycotoxin detectors. This review focuses on these topics, including uncertainties of climate change, providing an overview of the current information as well as future perspectives.

Fish gill damage by harmful microalgae newly explored by microelectrode ion flux estimation techniques

Harmful algal blooms (HAB) are responsible for massive mortalities of wild and aquacultured fish due to noticeable gill damage, but the precise fish-killing mechanisms remain poorly understood. A non-invasive microelectrode ion flux estimation (MIFE) technique was successfully applied to assess changes in membrane-transport processes in a model fish gill cell line exposed to harmful microplankton. Net Ca²⁺, H⁺, K⁺ ion fluxes in the rainbow trout cell line RTgill-W1 were monitored before and after addition of lysed cells of this Paralytic Shellfish Toxins (PST) producer along with purified endocellular dinoflagellate PST. It was demonstrated that PST alone do not play a role in fish gill damage during A. catenella outbreaks as previously thought, but that other ichthyotoxic metabolites from lysed algal cells (i.e. lipid peroxidation products or other unknown metabolites) result in net K⁺ efflux from fish gill cells and thereby gill cell death.

Ultrasensitive sensing of Androctonus australis hector scorpion venom toxins in biological fluids using an electrochemical graphene quantum dots/nanobody-based platform

Rapid and sensitive detection of low levels of scorpion venom toxins in biological fluids is of tremendous importance for decision-taking in cases of envenomation by scorpions stings. In Tunisia, at least 1200 severe envenomation cases by Androctonus australis hector (Aah) scorpion stings were reported annually. In this work, we report on a novel electrochemical immuno-sandwich to detect the Aah50 toxic fraction within the Aah scorpion venom using the bispecific nanobody format specially designed to highly recognize and neutralize the two most toxic molecules in the AahG50 venom fraction (i.e. AahI and AahII toxins), graphene quantum dots (GQDs) constructed on the surface carbon screen-printed electrodes. Hydroquinone/H₂O₂/peroxidase system was used to amplify the current in order to achieve the detection of low levels of AahG50. Electrochemical studies revealed a high sensitivity toward the AahG50 with a sensitivity of 18.2 nA mL pg^-1 and a picomolar limit of detection as low as 0.55 pg mL^-1. The platform exhibits very good metrological performances such as repeatability, reproducibility, selectivity and long storage stability. Matrix effect was found to be insignificant as demonstrated by assays performed in human blood serum and urine.

Innovative detection methods for aquatic algal toxins and their presence in the food chain

Detection of aquatic algal toxins has become critical for the protection of human health. During the last 5 years, techniques such as optical, electrochemical, and piezoelectric biosensors or fluorescent-microsphere-based assays have been developed for the detection of aquatic algal toxins, in addition to optimization of existing techniques, to achieve higher sensitivities, specificity, and speed or multidetection. New toxins have also been incorporated in the array of analytical and biological methods. The impact of the former innovation on this field is highlighted by recent changes in legal regulations, with liquid chromatography-mass spectrometry becoming the official reference method for marine lipophilic toxins and replacing the mouse bioassay in many countries. This review summarizes the large international effort to provide routine testing laboratories with fast, sensitive, high-throughput, multitoxin, validated methods for the screening of seafood, algae, and water samples.

In vitro approaches to evaluate toxicity induced by organotin compounds tributyltin (TBT), dibutyltin (DBT), and monobutyltin (MBT) in neuroblastoma cells

The toxic effects of the organotin compounds (OTCs) monobutyltin (MBT), dibutyltin (DBT), and tributyltin (TBT) were evaluated in vitro in a neuroblastoma human cell line. Mechanisms of cell death, apoptosis versus necrosis, were studied by using several markers: inhibition of cell viability and proliferation, F-actin, and mitochondrial membrane potential changes as well as reactive oxygen species (ROS) production and DNA fragmentation. The most toxic effects were detected with DBT and TBT even at very low concentrations (0.1-1 μM). In contrast, MBT induced lighter cytotoxic changes at the higher doses tested. None of the studied compounds stimulated propidium iodide uptake, although the most toxic chemical, TBT, caused lactate dehydrogenase release at the higher concentrations tested. These findings suggest that in neuroblastoma, OTC-induced cytotoxicity involves different pathways depending on the compound, concentration, and incubation time. A screening method for DBT and TBT quantification based on cell viability loss was developed, allowing a fast detection alternative to complex methodology.

An overview on the marine neurotoxin, saxitoxin: genetics, molecular targets, methods of detection and ecological functions

Marine neurotoxins are natural products produced by phytoplankton and select species of invertebrates and fish. These compounds interact with voltage-gated sodium, potassium and calcium channels and modulate the flux of these ions into various cell types. This review provides a summary of marine neurotoxins, including their structures, molecular targets and pharmacologies. Saxitoxin and its derivatives, collectively referred to as paralytic shellfish toxins (PSTs), are unique among neurotoxins in that they are found in both marine and freshwater environments by organisms inhabiting two kingdoms of life. Prokaryotic cyanobacteria are responsible for PST production in freshwater systems, while eukaryotic dinoflagellates are the main producers in marine waters. Bioaccumulation by filter-feeding bivalves and fish and subsequent transfer through the food web results in the potentially fatal human illnesses, paralytic shellfish poisoning and saxitoxin pufferfish poisoning. These illnesses are a result of saxitoxin’s ability to bind to the voltage-gated sodium channel, blocking the passage of nerve impulses and leading to death via respiratory paralysis. Recent advances in saxitoxin research are discussed, including the molecular biology of toxin synthesis, new protein targets, association with metal-binding motifs and methods of detection. The eco-evolutionary role(s) PSTs may serve for phytoplankton species that produce them are also discussed.

First direct fluorescence polarization assay for the detection and quantification of spirolides in mussel samples

In 2009, we achieve the first inhibition FP assay to detect imine cyclic toxins. In the present paper we propose a new FP assay for direct quantify spirolides. This new method has resulted in significant improvement of sensitivity, rapidity and accessibility. In the method design, nicotinic acetylcholine receptor from Torpedo marmorata membranes labelled with a derivative of fluorescein was used. Spirolides, 13-desmethyl spirolide C (13-desMeC) and 13,19-didesmethyl spirolide C (13,19-didesMeC) were extracted and purified from cultures of the Alexandrium ostenfeldii dinoflagellate. Data showed the decrease of FP when toxin concentration was increased. Thus, a relationship between the FP units and the spirolides amount present in a sample was obtained. This direct assay is a reproducible, simple and very sensitive method with a detection limit about 25 nM for 13-desMeC and 150 nM for 13,19-didesMeC. The procedure was used to measure spirolides in mussel samples using an extraction and clean up protocol suitable for the FP assay. Results obtained show that this method is able to quantify 13-desMeC in the range of 50-350 μg kg(-1) meat. Other liposoluble toxins did not interfere with the assay, proving a specific method. Moreover, the matrix do not affect in the range of toxin concentrations that involving risk of spirolides intoxication.

Biological methods for marine toxin detection

The presence of marine toxins in seafood poses a health risk to human consumers which has prompted the regulation of the maximum content of marine toxins in seafood in the legislations of many countries. Most marine toxin groups are detected by animal bioassays worldwide. Although this method has well known ethical and technical drawbacks, it is the official detection method for all regulated phycotoxins except domoic acid. Much effort by the scientific and regulatory communities has been focused on the development of alternative techniques that enable the substitution or reduction of bioassays; some of these have recently been included in the official detection method list. During the last two decades several biological methods including use of biosensors have been adapted for detection of marine toxins. The main advances in marine toxin detection using this kind of technique are reviewed. Biological methods offer interesting possibilities for reduction of the number of biosassays and a very promising future of new developments.

Use of biosensors as alternatives to current regulatory methods for marine biotoxins

Marine toxins are currently monitored by means of a bioassay that requires the use of many mice, which poses a technical and ethical problem in many countries. With the exception of domoic acid, there is a legal requirement for the presence of other toxins (yessotoxin, saxitoxin and analogs, okadaic acid and analogs, pectenotoxins and azaspiracids) in seafood to be controlled by bioassay, but other toxins, such as palytoxin, cyclic imines, ciguatera and tetrodotoxin are potentially present in European food and there are no legal requirements or technical approaches available to identify their presence. The need for alternative methods to the bioassay is clearly important, and biosensors have become in recent years a feasible alternative to animal sacrifice. This review will discuss the advantages and disadvantages of using biosensors as alternatives to animal assays for marine toxins, with particular focus on surface plasmon resonance (SPR) technology.

Reporter gene assay for detection of shellfish toxins

OBJECTIVE

To explore the potential reporter gene assay for the detection of sodium channel-specific toxins in shellfish as an alternative for screening harmful algal bloom (HAB) toxins, considering the fact that the existing methods including HPLC and bioassay are inappropriate for identifying HAB toxins which poses a serious problem on human health and shellfish industry.

METHODS

A reporter plasmid pEGFP-c-fos containing c-fos promoter and EGFP was constructed and transfected into T24 cells using LipofectAMINE 2000. Positive transfectants were screened by G418 to produce a pEGFP-c-fos-T24 cell line. After addition of increasing neurotoxic shellfish poison (NSP) or GTX2,3, primary components of paralytic shellfish poison (PSP), changes in expression of EGFP in the cell line were observed under a laser scanning confocal microscope and quantified with Image-pro Plus software.

RESULTS

Dose-dependent changes in the intensity of green fluorescence were observed for NSP in a range from 0 to 10 ng/mL and for GTX2,3 from 0 to 16 ng/mL.

CONCLUSION

pEGFP-c-fos-T24 can be applied in detecting HAB toxins, and cell-based assay can be used as an alternative for screening sodium channel-specific HAB toxins.

Characterization of the Ag mediated surface-enhanced Raman spectroscopy of saxitoxin

The rapid detection and quantification of saxitoxin (STX) is reported using surface-enhanced Raman spectroscopy (SERS) with a colloidal hydrosol of silver nanoparticles. Under the conditions of our experiments, the limit of detection (LD) for STX using SERS is 3 nM, with a limit of quantification (LQ) of 20 nM. It is shown that the SERS method is rapid, with spectra being collected in as little as 5 seconds total integration time for a 40 nM STX sample. In order to improve the signal-to-noise ratio, SERS spectra were generally collected with a total integration time of 1 minute (6 accumulations of 10 seconds each), with no need for extensive sample work-up or substrate preparation. Based on these results, the SERS technique shows great promise for the future detection and quantification of STX molecules in aqueous solutions.

Flow cytometric detection of saxitoxins using fluorescent voltage-sensitive dyes

By virtue of their ability to block depolarization of nerve cells, the saxitoxins exert the toxic effects associated with paralytic shellfish poisoning and allow for their detection through various methodologies. When veratridine-induced depolarization is followed using voltage-sensitive fluorescent dyes, the presence of these toxic blocking agents can be observed as a decrease in fluorescence of dye-treated nerve cells. Detection using flow cytometry provides for selection of the most responsive population of cultured mouse neuroblastoma (Neuro 2a) cells thereby enhancing assay sensitivity and this approach can be accomplished in real time. The method is demonstrated in preliminary studies using saxitoxin and crude shellfish extracts.

The cytoskeleton, a structure that is susceptible to the toxic mechanism activated by palytoxins in human excitable cells

Palytoxin is a marine toxin responsible for a fatal type of poisoning in humans named clupeotoxism, with symptoms such as neurologic disturbances. It is believed that it binds to the Na(+)/K(+)-ATPase from the extracellular side and modifies cytosolic ions; nevertheless, its effects on internal cell structures, such as the cytoskeleton, which might be affected by these initial events, have not been fully elucidated. Likewise, ostreocin-D, an analog of palytoxin, has been only recently found, and its action on excitable cells is therefore unknown. Therefore, our aim was to investigate the modifications of ion fluxes associated with palytoxin and ostreocin-D activities, and their effects on an essential cytoskeletal component, the actin system. We used human neuroblastoma cells and fluorescent dyes to detect changes in membrane potential, intracellular Ca(2+) concentration, cell detachment, and actin filaments. Fluorescence values were obtained with spectrofluorymetry, laser-scanning cytometry, and confocal microscopy; the last of these was also used for recording images. Palytoxin and ostreocin-D modified membrane permeability as a first step, triggering depolarization and increasing Ca(2+) influx. The substantial loss of filamentous actin, and the morphologic alterations elicited by both toxins, are possibly secondary to their action on ion channels. The decrease in polymerized actin seemed to be Ca(2+)-independent; however, this ion could be related to actin cytoskeletal organization. Palytoxin and ostreocin-D alter the ion fluxes, targeting pathways that involve the cytoskeletal dynamics of human excitable cells.

Effects of a Synthetic Analog of Polycavernoside A on Human Neuroblastoma Cells

BACKGROUND

Polycavernoside A is a glycosidic marine toxin first extracted from the red alga Polycavernosa tsudai in 1991 when 3 people died after the ingestion of this food. Polycavernoside A is an interesting molecule because of its complex macrolide structure and strong bioactivity. However, the target site of this toxin has not been characterized.

METHODS

We studied the effects of a synthethic analog of polycavernoside A on human neuroblastoma cells by measuring changes in membrane potential with bis-oxonol and variations in intracellular calcium levels with fura-2. Fluorescent phalloidin was utilized for assaying activity on actin cytoskeleton.

RESULTS

Data showed that this polycavernoside A analog induced a membrane depolarization and an increase in cytosolic calcium levels.

CONCLUSION

These results provide the first insight into the mode of action of polycavernoside A, suggesting that: i) this toxin triggers an initial extracellular calcium entry neither produced across L-type voltage-gated calcium channels nor activation of muscarinic receptors ii) there is a depolarization induced by the toxin and due to the extracellular calcium entry.

The Sodium Channel of Human Excitable Cells is a Target for Gambierol

BACKGROUND

Gambierol is a polycyclic ether toxin with the same biogenetic origin as ciguatoxins. Gambierol has been associated with neurological symptoms in humans even though its mechanism of action has not been fully characterized.

METHODS

We studied the effect of gambierol in human neuroblastoma cells by using bis-oxonol to measure membrane potential and FURA-2 to monitor intracellular calcium.

RESULTS

We found that this toxin: i) produced a membrane depolarization, ii) potentiated the effect of veratridine on membrane potential iii) decreased ciguatoxin-induced depolarization and iv) increased cytosolic calcium in neuroblastoma cells.

CONCLUSION

These results indicate that gambierol modulate ion fluxes by acting as a partial agonist of sodium channels.

Spectroscopic detection of Saxitoxin: an alternative to mouse bioassay

Herein, we report the surface modification of quartz with a coumaryl-aza-crown-6 derivative to detect Saxitoxin using fluorescence enhancement through Photoinduced Electron Transfer and the sensitivity with this system approaches the limit of the mouse bioassay which is the current benchmark for Saxitoxin detection.

Functional assays in marine biotoxin detection

The contamination of seafood by algal toxins regularly affects animals living in several areas of the world, and the number of toxic phycotoxins which are being characterized is steadily increasing. The extreme dynamics characterizing the field of algal toxins has stimulated the development of tools to be implemented in the monitoring of contamination of seafood by individual toxin classes. Under these circumstances, functional assays which can encompass the analytical potential of chemical methods and the predictive features of biological tests are sought. A variety of functional assays for the detection of phycotoxins has been developed in the last 20 years, and the analysis of their features reveals that their specificity is related to the hierarchical level of the biological response to the toxin that has been exploited for its detection. Ideally, analytical methods which could allow accurate estimates of the overall toxicity of multiple classes of toxins in a single procedure would provide the best means for the highest standards in consumer protection and the most rational and economical tools in the management of risks posed by phycotoxins in a wider scale. The achievement of a "systemic functional assay for marine biotoxins" does not appear to be at hand, but its inclusion among the foreseeable events is fully justified by the new research tools and approaches which have become available for the high throughput analysis of entire molecular domains at the cellular level.

Fluorescent microplate cell assay to measure uptake and metabolism of glucose in normal human lung fibroblasts

This is the first report of a fluorimetric microplate assay to assess glucose uptake and metabolism in eukaryotic cells. The assay was carried out incubating normal human lung fibroblasts in the wells of microtiter trays with a fluorescent D-glucose derivative, 2-N-7-(nitrobenz-2-oxa-1,3-diazol-4-yl)amino-2-deoxy-D-glucose (2-NBDG). This dye could be incorporated by glucose transporting systems in living cells. Substrate uptake was determined by analysing the data obtained with a fluorescence microplate reader. Variables studied in the development of the assay included dye concentration and incubation period. We found that this cell assay is very sensitive, reproducible, provides fast results and graphical display of data. It requires small sample volumes and allows handling of a large number of samples simultaneously. Okadaic acid was used to assess this microplate assay in the field of cytotoxicity. This diarrhetic shellfish toxin is a tumour promoter and a specific inhibitor of protein phosphatases 1 and 2A. The exposition of cells to okadaic acid (0.1 nM-1 microM) at different time intervals causes a decrease in intracellular glucose (40-50% over controls). Results obtained with okadaic acid are the starting point to evaluate application of the method to routine toxicity probes.