Mode of action of palytoxin on the release of acetylcholine from rat cerebrocortical synaptosomes

Phycotoxins in Marine Shellfish: Origin, Occurrence and Effects on Humans

Massive phytoplankton proliferation, and the consequent release of toxic metabolites, can be responsible for seafood poisoning outbreaks: filter-feeding mollusks, such as shellfish, mussels, oysters or clams, can accumulate these toxins throughout the food chain and present a threat for consumers' health. Particular environmental and climatic conditions favor this natural phenomenon, called harmful algal blooms (HABs); the phytoplankton species mostly involved in these toxic events are dinoflagellates or diatoms belonging to the genera Alexandrium, Gymnodinium, Dinophysis, and Pseudo-nitzschia. Substantial economic losses ensue after HABs occurrence: the sectors mainly affected include commercial fisheries, tourism, recreational activities, and public health monitoring and management. A wide range of symptoms, from digestive to nervous, are associated to human intoxication by biotoxins, characterizing different and specific syndromes, called paralytic shellfish poisoning, amnesic shellfish poisoning, diarrhetic shellfish poisoning, and neurotoxic shellfish poisoning. This review provides a complete and updated survey of phycotoxins usually found in marine invertebrate organisms and their relevant properties, gathering information about the origin, the species where they were found, as well as their mechanism of action and main effects on humans.

Microelectrode array (MEA) platform as a sensitive tool to detect and evaluate Ostreopsis cf. ovata toxicity

In the last decade, the occurrence of harmful dinoflagellate blooms of the genus Ostreopsis has increased both in frequency and in geographic distribution with adverse impacts on public health and the economy. Ostreopsis species are producers of palytoxin-like toxins (putative palytoxin and ovatoxins) which are among the most potent natural non-protein compounds known to date, exhibiting extreme toxicity in mammals, including humans. Most existing toxicological data are derived from in vivo mouse assay and are related to acute effects of pure palytoxin, without considering that the toxicity mechanism of dinoflagellates can be dependent on the varying composition of complex biotoxins mixture and on the presence of cellular components. In this study, in vitro neuronal networks coupled to microelectrode array (MEA)-based system are proposed, for the first time, as sensitive biosensors for the evaluation of marine alga toxicity on mammalian cells. Toxic effect was investigated by testing three different treatments of laboratory cultured Ostreopsis cf. ovata cells: filtered and re-suspended algal cells; filtered, re-suspended and sonicated algal cells; conditioned growth medium devoid of algal cells. The great sensitivity of this system revealed the mixture of PTLX-complex analogues naturally released in the growth medium and the different potency of the three treatments to inhibit the neuronal network spontaneous electrical activity. Moreover, by means of the multiparametric analysis of neuronal network activity, the approach revealed a different toxicity mechanism of the cellular component compared to the algal conditioned growth medium, highlighting the potential active role of the first treatment.

Palytoxin action on the Na(+),K(+)-ATPase and the disruption of ion equilibria in biological systems

Palytoxin-group toxins (PlTX) exert their potent biological activity by altering mechanisms of ion homeostasis in excitable and non-excitable tissues. This review will describe major aspects that led to the relatively early identification of the Na(+),K(+)-ATPase as the molecular target and receptor of the toxin in sensitive systems. The importance of this pump in the normal functioning of animal cells has driven extensive investigative efforts. The recognized molecular mechanism of action of PlTX involves its binding to the extracellular portion of alpha subunit of this plasma membrane protein, which converts an enzyme carrying ions against their concentration gradients at the expense of chemical energy (ATP) into a non-selective cation channel, allowing passive flow of ions following their concentration gradients. More recent findings have indicated that PlTX would interfere with the normal strict coupling between inner and outer gates of the pump controlling the ion access to the Na(+),K(+)-ATPase, allowing the gates to be simultaneously open. The ability of PlTX to make internal portions of the Na(+),K(+)-ATPase accessible to relatively large molecules has been exploited to characterize the structure-function relationship of the pump, leading to a better understanding of its ion translocation pathway. Thus, forty years from the isolation of this potent marine biotoxin, a considerable understanding of its mode of action and of its potential as a research tool have been achieved and are the basis for promising future advancement in the characterization of biological systems and their alteration by PlTX.

Neurotoxins from marine dinoflagellates: a brief review

Dinoflagellates are not only important marine primary producers and grazers, but also the major causative agents of harmful algal blooms. It has been reported that many dinoflagellate species can produce various natural toxins. These toxins can be extremely toxic and many of them are effective at far lower dosages than conventional chemical agents. Consumption of seafood contaminated by algal toxins results in various seafood poisoning syndromes: paralytic shellfish poisoning (PSP), neurotoxic shellfish poisoning (NSP), amnesic shellfish poisoning (ASP), diarrheic shellfish poisoning (DSP), ciguatera fish poisoning (CFP) and azaspiracid shellfish poisoning (ASP). Most of these poisonings are caused by neurotoxins which present themselves with highly specific effects on the nervous system of animals, including humans, by interfering with nerve impulse transmission. Neurotoxins are a varied group of compounds, both chemically and pharmacologically. They vary in both chemical structure and mechanism of action, and produce very distinct biological effects, which provides a potential application of these toxins in pharmacology and toxicology. This review summarizes the origin, structure and clinical symptoms of PSP, NSP, CFP, AZP, yessotoxin and palytoxin produced by marine dinoflagellates, as well as their molecular mechanisms of action on voltage-gated ion channels.

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.

Palytoxin-induced increase in cytosolic-free Ca(2+) in mouse spleen cells

The effect of palytoxin (C(129)H(223)N(3)O(54)) on Ca(2+) homeostasis in immune cells has not been studied. Therefore, we investigated the effect of palytoxin on the cytosolic-free Ca(2+) concentration ([Ca(2+)](i)) in mouse spleen cells using a fluorescence Ca(2+) indicator, fura-2. Palytoxin (0.1-100 nM) increased [Ca(2+)](i) in a concentration-dependent manner. The palytoxin-induced increase in [Ca(2+)](i) was abolished by the omission of extracellular Ca(2+) or 1-[beta-[3-(4-methoxyphenyl)propoxy]-4-methoxyphenethyl]-1H-imidazole hydrochloride (SKF-96365, 100 microM), and was greatly inhibited by Ni(2+) (2 mM). Ouabain (0.5-1 mM) partially inhibited the palytoxin-induced response. There was no effect of decreased extracellular Na(+) (6.2 mM), tetrodotoxin (1 microM), verapamil (10 microM), nifedipine (10 microM), omega-agatoxin IVA (200 nM), omega-conotoxin GVIA (1 microM), omega-conotoxin MVIIC (500 nM), or La(3+) (100 microM). These results suggest that palytoxin increases [Ca(2+)](i) in mouse spleen cells by stimulating Ca(2+) entry through an SKF-96365-, Ni(2+)-sensitive pathway.

Measurement of intracellular Na+ concentration by a Na+-sensitive fluorescent dye, sodium-binding benzofuran isophthalate, in porcine adrenal chromaffin cells--usage of palytoxin as a Na+ ionophore

Palytoxin was found to equilibrate sodium ions (Na+) across the cell membrane much faster than dose gramicidin, which has been frequently used to calibrate the intracellular Na+ concentration ([Na+]in), in cells treated with a Na+-sensitive fluorescent dye, sodium-binding benzofuran isophthalate (SBFI). Palytoxin was capable of equilibrating Na+ in cells treated with SBFI-acetoxymethyl ester (SBFI-AM) and in voltage-clamped cells loaded with SBFI through a patch pipette. Nicotine caused a dose-dependent increase in ([Na+]in) in porcine adrenal chromaffin cells treated with SBFI-AM and caused a simultaneous increase in [Na+]in and inward current in the voltage-clamped cells loaded with SBFI. Palytoxin has an advantage of calibrating ([Na+]in) in a shorter time than dose gramicidin because of its powerful ionophoretic activity.

Inhibition of sodium-dependent uptake processes in purified rat brain synaptosomes by Lophozozymus pictor toxin and palytoxin

To get an insight into the mechanism of neurotoxicity exhibited by Lophozozymus pictor toxin (LPTX) and the toxin isolated from P.caribaeorum (C-PTX) studies were carried out on the effect of these toxins on the uptake of selected substrates (neurotransmitters, amino acids and glucose) in isolated nerve endings. The toxins were found to inhibit the uptake of gamma-aminobutyric acid (GABA), noradrenaline, choline, L-leucine and 2-deoxy-D-glucose in rat brain synaptosomes. LPTX- or C-PTX-induced inhibition of synaptosomal uptake was reduced in the absence of Na+ in the assay medium. Synaptosomes exposed to LPTX and C-PTX release K+ in a dose-dependent manner. Ouabain, a selective inhibitor of the plasma membrane Na+, K(+)-ATPase could inhibit LPTX- and C-PTX-induced K+ efflux from synaptosomes and alleviate the toxin-induced inhibition of synaptosomal GABA uptake. It appears that the induction of ionic flux is the primary cause of toxicity by these toxins leading to the inhibition of Na(+)-dependent uptake processes in synaptosomes. The antagonistic action of ouabain suggests the involvement of the membrane sodium pump in the development of cytotoxicity.

Palytoxin. Recent electrophysiological and pharmacological evidence for several mechanisms of action

1. Palytoxin is one of the most potent toxins known so far. It acts as an haemolysin and alters the functioning of excitable cells. 2. A primary action of palytoxin in excitable cells is to induce the activity of a small conductance (9-25 pS), non-selective cationic channel which then triggers secondary activations of voltage dependent Ca2+ channels and of Na+/Ca2+ exchange. This results in neurotransmitter release by nerve terminals and contractions of striated and smooth muscle cells. 3. Palytoxin induced channels are blocked by amiloride derivatives such as 3,4 dichlorobenzamil. They are also blocked by ouabain but at concentrations higher than those required to inhibit the (Na+,K+)ATPase. 4. A second and independent action of palytoxin is to open a membrane conductive pathway for H+ that drives H+ inside the cells and secondarily activates Na+/H+ exchange activity. 5. A third action of PTX in chick cardiomyocytes is to raise [Ca2+]i in a manner independent of its depolarizing action or of its action on intracellular pH. 6. It is suggested that PTX probably has more than one site of action in excitable cells and that it may act as an agonist for a family of low conductance channels that conduct Na+/K+, H+ and Ca2+ions.

[Release of acetylcholine and its regulation]

The mechanism of acetylcholine (ACh) release and its regulation is a widely studied subject still underdebated. Although the vesicular hypothesis for ACh release is at present largely accepted, alternative theories have been proposed. ACh release is triggered by calcium influx through specific presynaptic Ca2+ channels. The modulation of this calcium influx appears as the main mechanism through which ACh release is regulated. This can be achieved by direct modification of the presynaptic Ca2+ channel opening or indirectly by a change in the polarization level of the presynaptic membrane due to the opening or closing of other presynaptic channels (usually K+ channels). The increase in the intracellular Ca2+ concentration that triggers ACh release is also under the control of Ca2+ membrane exchanges and intracellular Ca2+ buffers. ACh synthesis that takes place in the cytoplasm of the terminal, can itself be modulated leading to changes in the quantity of ACh available for release. All these regulatory mechanisms can be initiated by the activation of presynaptic receptors to either ACh itself (autoreceptors) or to other transmitters (heteroreceptors). Most often, these presynaptic receptors seem to require the transducing role of G proteins and the involvement of various second messengers. Some illnesses concerning the cholinergic system can be related to a disfunction of one of these presynaptic regulatory mechanisms.