Characterization of the maitotoxin-induced calcium influx pathway from human skin fibroblasts

Synergistic Effect of Transient Receptor Potential Antagonist and Amiloride against Maitotoxin Induced Calcium Increase and Cytotoxicity in Human Neuronal Stem Cells

Maitotoxins (MTX) are among the most potent marine toxins identified to date causing cell death trough massive calcium influx. However, the exact mechanism for the MTX-induced calcium entry and cytotoxicity is still unknown. In this work, the effect of MTX-1 on the cytosolic free calcium concentration and cellular viability of human neuronal stem cells was evaluated. MTX elicited a concentration-dependent decrease in cell viability which was already evident after 1 h of treatment with 0.25 nM MTX; however, at a concentration of 0.1 nM, the toxin did not cause cell death even after 14 days of exposure. Moreover, the toxin caused a concentration dependent rise in the cytosolic calcium concentration which was maximal at toxin concentrations of 1 nM and dependent on the presence of extracellular calcium on the bathing solution. Several pharmacological approaches were employed to evaluate the role of canonical transient potential receptor channels (TRPC) on the MTX effects. The results presented here lead to the identification of the TRPC4 channels as contributors to the MTX effects in human neuronal cells. Both, the calcium increase and the cytotoxicity of MTX were either fully (for the calcium increase) or partially (in the case of cytotoxicity) reverted by the blockade of canonical TRPC4 receptors with the selective antagonist ML204. Furthermore, the sodium proton exchanger blocker amiloride also partially inhibited the calcium rise and the cell death elicited by MTX while the combination of amiloride and ML204 fully prevented both the cytotoxicity and the calcium rise elicited by the toxin.

Maitotoxin-4, a Novel MTX Analog Produced by Gambierdiscus excentricus

Maitotoxins (MTXs) are among the most potent toxins known. These toxins are produced by epi-benthic dinoflagellates of the genera Gambierdiscus and Fukuyoa and may play a role in causing the symptoms associated with Ciguatera Fish Poisoning. A recent survey revealed that, of the species tested, the newly described species from the Canary Islands, G. excentricus, is one of the most maitotoxic. The goal of the present study was to characterize MTX-related compounds produced by this species. Initially, lysates of cells from two Canary Island G. excentricus strains VGO791 and VGO792 were partially purified by (i) liquid-liquid partitioning between dichloromethane and aqueous methanol followed by (ii) size-exclusion chromatography. Fractions from chromatographic separation were screened for MTX toxicity using both the neuroblastoma neuro-2a (N2a) cytotoxicity and Ca2+ flux functional assays. Fractions containing MTX activity were analyzed using liquid chromatography coupled to high-resolution mass spectrometry (LC-HRMS) to pinpoint potential MTX analogs. Subsequent non-targeted HRMS analysis permitted the identification of a novel MTX analog, maitotoxin-4 (MTX4, accurate mono-isotopic mass of 3292.4860 Da, as free acid form) in the most toxic fractions. HRMS/MS spectra of MTX4 as well as of MTX are presented. In addition, crude methanolic extracts of five other strains of G. excentricus and 37 other strains representing one Fukuyoa species and ten species, one ribotype and one undetermined strain/species of Gambierdiscus were screened for the presence of MTXs using low resolution tandem mass spectrometry (LRMS/MS). This targeted analysis indicated the original maitotoxin (MTX) was only present in one strain (G. australes S080911_1). Putative maitotoxin-2 (p-MTX2) and maitotoxin-3 (p-MTX3) were identified in several other species, but confirmation was not possible because of the lack of reference material. Maitotoxin-4 was detected in all seven strains of G. excentricus examined, independently of their origin (Brazil, Canary Islands and Caribbean), and not detected in any other species. MTX4 may therefore serve as a biomarker for the highly toxic G. excentricus in the Atlantic area.

The opening of maitotoxin-sensitive calcium channels induces the acrosome reaction in human spermatozoa: differences from the zona pellucida

The acrosome reaction (AR), an absolute requirement for spermatozoa and egg fusion, requires the influx of Ca²(+) into the spermatozoa through voltage-dependent Ca²(+) channels and store-operated channels. Maitotoxin (MTx), a Ca²(+)-mobilizing agent, has been shown to be a potent inducer of the mouse sperm AR, with a pharmacology similar to that of the zona pellucida (ZP), possibly suggesting a common pathway for both inducers. Using recombinant human ZP3 (rhZP3), mouse ZP and two MTx channel blockers (U73122 and U73343), we investigated and compared the MTx- and ZP-induced ARs in human and mouse spermatozoa. Herein, we report that MTx induced AR and elevated intracellular Ca²(+) ([Ca²(+)](i)) in human spermatozoa, both of which were blocked by U73122 and U73343. These two compounds also inhibited the MTx-induced AR in mouse spermatozoa. In disagreement with our previous proposal, the AR triggered by rhZP3 or mouse ZP was not blocked by U73343, indicating that in human and mouse spermatozoa, the AR induction by the physiological ligands or by MTx occurred through distinct pathways. U73122, but not U73343 (inactive analogue), can block phospholipase C (PLC). Another PLC inhibitor, edelfosine, also blocked the rhZP3- and ZP-induced ARs. These findings confirmed the participation of a PLC-dependent signalling pathway in human and mouse zona protein-induced AR. Notably, edelfosine also inhibited the MTx-induced mouse sperm AR but not that of the human, suggesting that toxin-induced AR is PLC-dependent in mice and PLC-independent in humans.

Attenuation of maitotoxin-induced cytotoxicity in rat aortic smooth muscle cells by inhibitors of Na+/Ca2+ exchange, and calpain activation

The highly potent marine toxin maitotoxin (MTX) evoked an increase in cytosolic Ca(2+) levels in fura-2 loaded rat aortic smooth muscle cells, which was dependent on extracellular Ca(2+). This increase was almost fully inhibited by KB-R7943, a potent selective inhibitor of the reverse mode of the Na(+)/Ca(2+) exchanger (NCX). Cell viability was assessed using ethidium bromide uptake and the alamarBlue cytotoxicity assay. In both assays MTX-induced toxicity was attenuated by KB-R7943, as well as by MDL 28170, a membrane permeable calpain inhibitor. Maitotoxin-evoked contractions of rat aortic strip preparations in vitro, which persist following washout of the toxin, were relaxed by subsequent addition of KB-R7943 or MDL 28170, either in the presence of, or following washout of MTX. These results suggest that MTX targets the Na(+)/Ca(2+) exchanger and causes it to operate in reverse mode (Na(+) efflux/Ca(2+) influx), thus leading to calpain activation, NCX cleavage, secondary Ca(2+) overload and cell death.

Maitotoxin potently promotes Ca2+ influx in mouse spermatogenic cells and sperm, and induces the acrosome reaction

Maitotoxin (MTX), a potent marine toxin, activates Ca2+ entry via nonselective cation channels in a wide variety of cells. The identity of the channels involved in MTX action remains unknown. In mammalian sperm, Ca2+ entry through store-operated channels regulates a number of physiological events including the acrosome reaction (AR). Here we report that MTX produced an increase in the intracellular concentration of Ca2+ ([Ca2+]i) in spermatogenic cells that depended on extracellular Ca2+. Ni2+ and SKF96365 diminished the MTX-activated Ca2+ uptake, at concentrations they inhibit store-operated channels, and in a similar manner as they inhibit the Ca2+ influx activated following depletion of intracellular stores by thapsigargin (Tpg). In addition, MTX significantly increased [Ca2+]i in single mature sperm and effectively induced the AR with a half-maximal concentration (ED50) of approximately 1.1 nM. Notably, SKF96365 similarly inhibited the MTX-induced increase in sperm [Ca2+]i and the AR triggered by the toxin, Tpg and zona pellucida. These results suggest that putative MTX-activated channels may be involved in the Ca2+ influx required for mouse sperm AR.

Therapy and prophylaxis of inhaled biological toxins

This review highlights the current lack of therapeutic and prophylactic treatments for use against inhaled biological toxins, especially those considered as potential biological warfare (BW) or terrorist threats. Although vaccine development remains a priority, the use of rapidly deployable adjunctive therapeutic or prophylactic drugs could be life-saving in severe cases of intoxication or where vaccination has not been possible or immunity not established. The current lack of such drugs is due to many factors. Thus, methods involving molecular modelling are limited by the extent to which the cellular receptor sites and mode of action and structure of a toxin need to be known. There is also our general lack of knowledge of what effect individual toxins will have when inhaled into the lungs - whether and to what extent the action will be cell specific and cytotoxic or rather an acute inflammatory response requiring the use of immunomodulators. Possible sources of specific high-affinity toxin antagonists being investigated include monoclonal antibodies, selected oligonucleotides (aptamers) and derivatized dendritic polymers (dendrimers). The initial selection of suitable agents of these kinds can be made using cytotoxicity assays involving cultured normal human lung cells and a range of suitable indicators. The possibility that a mixture of selected antibody, aptamer or dendrimer-based materials for one or more toxins could be delivered simultaneously as injections or as inhaled aerosol sprays should be investigated.

Evidence that Ca2+-release-activated Ca2+ channels in rat hepatocytes are required for the maintenance of hormone-induced Ca2+ oscillations

Store-operated Ca(2+) channels in liver cells have been shown previously to exhibit a high selectivity for Ca(2+) and to have properties indistinguishable from those of Ca(2+)-release-activated Ca(2+) (CRAC) channels in mast cells and lymphocytes [Rychkov, Brereton, Harland and Barritt (2001) Hepatology 33, 938-947]. The role of CRAC channels in the maintenance of hormone-induced oscillations in the cytoplasmic free Ca(2+) concentration ([Ca(2+)](cyt)) in isolated rat hepatocytes was investigated using several inhibitors of CRAC channels. 2-Aminoethyl diphenylborate (2-APB; 75 microM), Gd(3+) (1 microM) and 1-[beta-[3-(4-methoxyphenyl)propoxy]-4-methoxyphenethyl]-1H-imidazole hydrochloride (SK&F 96365; 50 microM) each inhibited vasopressin- and adrenaline (epinephrine)-induced Ca(2+) oscillations (measured using fura-2). The characteristics of this inhibition were similar to those of inhibition caused by decreasing the extracellular Ca(2+) concentration to zero by addition of EGTA. The effect of 2-APB was reversible. In contrast, LOE-908 [( R, S )-(3,4-dihydro-6,7-dimethoxy-isochinolin-1-yl)-2-phenyl- N, N -di[2-(2,3,4-trimethoxyphenyl)ethyl]acetamide mesylate] (30 microM), used commonly to block Ca(2+) inflow through intracellular-messenger-activated, non-selective cation channels, did not inhibit the Ca(2+) oscillations. In the absence of added extracellular Ca(2+), 2-APB, Gd(3+) and SK&F 96365 did not alter the kinetics of the increase in [Ca(2+)](cyt) induced by a concentration of adrenaline or vasopressin that induces continuous Ca(2+) oscillations at the physiological extracellular Ca(2+) concentration. Ca(2+) inflow through non-selective cation channels activated by maitotoxin could not restore Ca(2+) oscillations in cells treated with 2-APB to block Ca(2+) inflow through CRAC channels. Evidence for the specificity of the pharmacological agents for inhibition of CRAC channels under the conditions of the present experiments with hepatocytes is discussed. It is concluded that Ca(2+) inflow through CRAC channels is required for the maintenance of hormone-induced Ca(2+) oscillations in isolated hepatocytes.

Modulation of the maitotoxin response by intracellular and extracellular cations

The aim of the present study was to characterize the role played by intracellular and extracellular calcium and sodium on the maitotoxin (MTX) response in Chinese hamster ovary (CHO) cells. The results presented here indicated that: (1) MTX activates calcium and sodium influx in a concentration-dependent manner; (2) extracellular calcium is required for the sodium influx; (3) removal of the extracellular sodium did not prevent the MTX-induced calcium influx; (4) elevation in the intracellular calcium concentration potentiates the MTX response; and (5) MTX, at the concentrations tested, did not compromise cell viability.

Characterization of the maitotoxin-activated cationic current from human skin fibroblasts

The maitotoxin (MTX)-induced cationic current (I(mtx)) from human skin fibroblasts was characterized using the patch-clamp technique in whole-cell configuration. Under resting conditions (absence of MTX), the main current observed is produced by an outwardly rectifying K(+) channel which is inhibited by 1 mM TEA. The current reversal potential was -86 mV (n = 12). MTX (500 pM) activated a current with a linear current-voltage relationship and a reversal potential of -10 mV (n = 10). Replacing the extracellular Na(+) and K(+) with N-methyl-D-glucamine (NMDG) caused a shift of the reversal potential to a value below -100 mV, indicating that Na(+) and K(+), but not NMDG, carry I(mtx). Further ion selectivity experiments showed that Ca(2+) carries I(mtx) also. The resulting permeability sequence obtained with the Goldman-Hodgkin-Katz equation yielded Na(+) (1) approximately equal to K(+) (1) > Ca(2+) (0.87). The I(mtx) activation time course reflected the changes in intracellular Ca(2+) and Na(+) measured with the fluorescent indicators fura-2 and SBFI, respectively, suggesting that the activation of I(mtx) brings about an increment in intracellular Ca(2+) and Na(+). Reducing the extracellular Ca(2+) concentration below 1.8 mM prevented the activation of I(mtx) and the increment in intracellular Na(+) induced by MTX. Mn(2+) and Mg(2+) could not replace Ca(2+), but Ba(2+) could replace Ca(2+). MTX activation of current in 10 mM Ba(2+) was approximately 50 % of that induced in the presence of 1.8 mM Ca(2+). When 5 mM of the Ca(2+) chelator BAPTA was included in the patch pipette, MTX either failed to activate the current or induced a small current (less than 15 % of the control), indicating that intracellular Ca(2+) is also required for the activation of I(mtx). Intracellular Ba(2+) can replace Ca(2+) as an activator of I(mtx). However, in the presence of 10 mM Ba(2+) the activation by MTX of the current was 50 % less than the activation with nM concentrations of free intracellular Ca(2+).

Maitotoxin activates an endogenous non-selective cation channel and is an effective initiator of the activation of the heterologously expressed hTRPC-1 (transient receptor potential) non-selective cation channel in H4-IIE liver cells

The structures and mechanisms of activation of non-selective cation channels (NSCCs) are not well understood although NSCCs play important roles in the regulation of metabolism, ion transport, cell volume and cell shape. It has been proposed that TRP (transient receptor potential) proteins are the molecular correlates of some NSCCs. Using fura-2 and patch-clamp recording, it was shown that the maitotoxin-activated cation channels in the H4-IIE rat liver cell line admit Ca(2+), Mn(2+) and Na(+), have a high selectivity for Na(+) compared with Ca(2+), and are inhibited by Gd(3+) (half-maximal inhibition at 1 microM). Activation of the channels by maitotoxin was inhibited by increasing the extracellular Ca(2+) concentration or by inclusion of 10 mM EGTA in the patch pipette. mRNA encoding TRP proteins 1, 2 and 3 at levels comparable with those in brain was detected using reverse transcriptase-polymerase chain reaction in poly(A)(+) RNA prepared from H4-IIE cells and freshly-isolated rat hepatocytes. In H4-IIE cells transiently transfected with cDNA encoding hTRPC-1, the expressed hTRPC-1 protein was chiefly located at intracellular sites and at the plasma membrane. Cells expressing hTRPC-1 exhibited a substantial enhancement of maitotoxin-initiated Ca(2+) inflow and a modest enhancement of thapsigargin-initiated Ca(2+) inflow (measured using fura-2) and no enhancement of the highly Ca(2+)-selective store-operated Ca(2+) current (measured using patch-clamp recording). In cells expressing hTRPC-1, maitotoxin activated channels which were not found in untransfected cells, have an approximately equal selectivity for Na(+) and Ca(2+), and are inhibited by Gd(3+) (half-maximal inhibition at 3 microM). It is concluded that in liver cells (i) maitotoxin initiates the activation of endogenous NSCCs with a high selectivity for Na(+) compared with Ca(2+); (ii) TRP proteins 1, 2 and 3 are expressed; (iii) maitotoxin is an effective initiator of activation of heterologously expressed hTRPC-1 channels; and (iv) the endogenous TRP-1 protein is unlikely to be the molecular counterpart of the maitotoxin-activated NSCCs nor the highly Ca(2+)-selective store-operated Ca(2+) channels.

Maitotoxin-induced free Ca changes in single rat hepatocytes

We report the results using bioluminescent and fluorescent indicators to investigate maitotoxin-induced free Ca changes in single rat hepatocytes. Maitotoxin generated a steadily rising free Ca increase after a long lag period. The free Ca increase was dependent on extracellular calcium and could be antagonised by chelation of extracellular calcium or the inclusion of nickel in the superfusate. Manganese-induced quench of cytoplasmic Fura2 dextran revealed an accelerated rate of calcium entry during the final period of the lag phase, immediately prior to the free Ca increase. Imaging experiments demonstrated a markedly different part of free Ca mobilisation compared with glycogenolytic stimuli. Moreover, the use of a combination of hormonal stimuli and maitotoxin revealed that some cells could exhibit free Ca oscillations despite steadily rising intracellular free Ca level. The significance of these observations in terms of the mechanism of action of maitotoxin and the mechanism of free Ca transient generation is discussed.

Maitotoxin and P2Z/P2X(7) purinergic receptor stimulation activate a common cytolytic pore

The effects of maitotoxin (MTX) on plasmalemma permeability are similar to those caused by stimulation of P2Z/P2X(7) ionotropic receptors, suggesting that 1) MTX directly activates P2Z/P2X(7) receptors or 2) MTX and P2Z/P2X(7) receptor stimulation activate a common cytolytic pore. To distinguish between these two possibilities, the effect of MTX was examined in 1) THP-1 monocytic cells before and after treatment with lipopolysaccharide and interferon-gamma, a maneuver known to upregulate P2Z/P2X(7) receptor, 2) wild-type HEK cells and HEK cells stably expressing the P2Z/P2X(7) receptor, and 3) BW5147.3 lymphoma cells, a cell line that expresses functional P2Z/P2X(7) channels that are poorly linked to pore formation. In control THP-1 monocytes, addition of MTX produced a biphasic increase in the cytosolic free Ca(2+) concentration ([Ca(2+)](i)); the initial increase reflects MTX-induced Ca(2+) influx, whereas the second phase correlates in time with the appearance of large pores and the uptake of ethidium. MTX produced comparable increases in [Ca(2+)](i) and ethidium uptake in THP-1 monocytes overexpressing the P2Z/P2X(7) receptor. In both wild-type HEK and HEK cells stably expressing the P2Z/P2X(7) receptor, MTX-induced increases in [Ca(2+)](i) and ethidium uptake were virtually identical. The response of BW5147.3 cells to concentrations of MTX that produced large increases in [Ca(2+)](i) had no effect on ethidium uptake. In both THP-1 and HEK cells, MTX- and Bz-ATP-induced pores activate with similar kinetics and exhibit similar size exclusion. Last, MTX-induced pore formation, but not channel activation, is greatly attenuated by reducing the temperature to 22 degrees C, a characteristic shared by the P2Z/P2X(7)-induced pore. Together, the results demonstrate that, although MTX activates channels that are distinct from those activated by P2Z/P2X(7) receptor stimulation, the cytolytic/oncotic pores activated by MTX- and Bz-ATP are indistinguishable.

Modulation of a calcium-activated chloride current by Maitotoxin

The effect of Maitotoxin (MTX) on the calcium-activated chloride current (ICl-Ca) from Xenopus oocytes was studied, applying the two-electrode voltage clamp technique. MTX increased the current amplitude at all the voltages explored and reduced the time to reach the maximum current level (time to peak). At low toxin concentrations (15 pM), both effects were fully reversible. Activation of ICl-Ca by MTX was secondary to the increment in the intracellular Ca2+ concentration induced by this toxin, since incubation of the oocytes with the cell-permeant Ca2+ chelator BAPTA-AM, greatly reduced the effect of MTX on ICl-Ca. Furthermore, external chloride ions removal also diminished the MTX effect on the current, strongly suggesting that the main current activated by MTX is ICl-Ca. Subsequent applications of a fixed toxin concentration after toxin washout resulted in enhanced ICl-Ca, suggesting that the toxin effect potentiates.