Involvement of the Na,K-ATPase in the induction of ion channels by palytoxin

Mitochondria possess a large, non-selective ionic current that is enhanced during cardiac injury

Mitochondrial ion channels are essential for energy production and cell survival. To avoid depleting the electrochemical gradient used for ATP synthesis, channels so far described in the mitochondrial inner membrane open only briefly, are highly ion-selective, have restricted tissue distributions, or have small currents. Here, we identify a mitochondrial inner membrane conductance that has strikingly different behavior from previously described channels. It is expressed ubiquitously, and transports cations non-selectively, producing a large, up to nanoampere-level, current. The channel does not lead to inner membrane uncoupling during normal physiology because it only becomes active at depolarized voltages. It is inhibited by external Ca2+, corresponding to the intermembrane space, as well as amiloride. This large, ubiquitous, non-selective, amiloride-sensitive (LUNA) current appears most active when expression of the mitochondrial calcium uniporter is minimal, such as in the heart. In this organ, we find that LUNA current magnitude increases two- to threefold in multiple mouse models of injury, an effect also seen in cardiac mitochondria from human patients with heart failure with reduced ejection fraction. Taken together, these features lead us to speculate that LUNA current may arise from an essential protein that acts as a transporter under physiological conditions, but becomes a channel under conditions of mitochondrial stress and depolarization.

Natural Products as Potential Antiviral Drugs: The Specific Case of Marine Biotoxins

To fight against various viral infections researchers turned to new chemical structures resulting from natural medicinal plants and more recently from "marine origin" as sources of active molecules against viral infections. The present manuscript describes complex marine origin drugs, their chemical complex structure, their therapeutic use, and their antiviral properties. Emphasis is placed more particularly on the properties of ionic channels (Na+, K+, Ca2+) blockers compounds from marine origin, named Dinotoxins, derived from "dinoflagellates microalgae". These compounds are of particular pharmaceutical interest since ionic channels blockers could be used to fight against a wide diversity of viruses, including SARS-CoV2 virus.

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.

NO Metabolites Levels in Human Red Blood Cells are Affected by Palytoxin, an Inhibitor of Na(+)/K(+)-ATPase Pump

Palytoxin (PTX), a marine toxin, represents an increasing hazard for human health. Despite its high toxicity for biological systems, the mechanisms triggered by PTX, are not well understood. The high affinity of PTX for erythrocyte Na(+)/K(+)-ATPase pump is largely known, and it indicates PTX as a sensitive tool to characterize the signal transducer role for Na(+)/K(+)-ATPase pump. Previously, it has been reported that in red blood cells (RBC), probably via a signal transduction generated by the formation of a PTX-Na(+)/K(+)-ATPase complex, PTX alters band 3 functions and glucose metabolism. The present study addresses the question of which other signaling pathways are regulated by Na(+)/K(+)-ATPase in RBC. Here it has been evidenced that PTX following its interaction with Na(+)/K(+)-ATPase pump, alters RBC morphology and this event is correlated to decreases by 30% in nitrites and nitrates levels, known as markers of plasma membrane eNOS activity. Orthovanadate (OV), an antagonist of PTX binding to Na(+)/K(+)-ATPase pump, was able to reverse the effects elicited by PTX. Finally, current investigation firstly suggests that Na(+)/K(+)-ATPase pump, following its interaction with PTX, triggers a signal transduction involved in NO metabolism regulation.

In vivo and in vitro effects of 42-hydroxy-palytoxin on mouse skeletal muscle: structural and functional impairment

Palytoxins (PLTXs) are known seafood contaminants and their entrance into the food chain raises concern about possible effects on human health. The increasing number of analogs being identified in edible marine organisms complicates the estimation of the real hazard associated with the presence of PLTX-like compounds. So far, 42-OH-PLTX is one of the few congeners available, and the study of its toxicity represents an important step toward a better comprehension of the mechanism of action of this family of compounds. From this perspective, the aim of this work was to investigate the in vivo and in vitro effect of 42-OH-PLTX on skeletal muscle, one of the most sensitive targets for PLTXs. Our results demonstrate that 42-OH-PLTX causes damage at the skeletal muscle level with a cytotoxic potency similar to that of PLTX. 42-OH-PLTX induces cytotoxicity and cell swelling in a Na(+)-dependent manner similar to the parent compound. However, the limited Ca(2+)-dependence of the toxic insult induced by 42-OH-PLTX suggests a specific mechanism of action for this analog. Our results also suggest an impaired response to the physiological agonist acetylcholine and altered cell elasticity.

Toxicity of palytoxin after repeated oral exposure in mice and in vitro effects on cardiomyocytes

Palytoxin (PLTX) is a highly toxic hydrophilic polyether detected in several edible marine organisms from intra-tropical areas, where seafood poisoning were reported. Symptoms usually start with gastro-intestinal malaise, often accompanied by myalgia, muscular cramps, dyspnea and, sometimes, arrhythmias. Monitoring programs in the Mediterranean Sea have detected PLTX-like molecules in edible mollusks and echinoderms. Despite the potential exposure of the human population and its high toxic potential, the toxicological profile of the molecule is still an issue. Thus, the effects of repeated oral administration of PLTX in mice were investigated. Seven days of PLTX administration caused lethality and toxic effects at doses ≥ 30 μg/kg/day. A NOAEL was estimated equal to 3 μg/kg/day, indicating a quite steep dose-response curve. This value, due to the limited number of animal tested, is provisional, although represents a sound basis for further testing. Macroscopic alterations at gastrointestinal level (gastric ulcers and intestinal fluid accumulation) were observed in mice dead during the treatment period. Histological analysis highlighted severe inflammation, locally associated with necrosis, at pulmonary level, as well as hyper-eosinophilia and fiber separation in myocardium. A cardiac damage was supported by the in vitro effect of the toxin on cardiomyocytes, indicating a severe and irreversible impairment of their electrical properties: electrophysiological recordings detected a progressive cell depolarization, arrest of action potentials and beating.

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.

Maitotoxin converts the plasmalemmal Ca(2+) pump into a Ca(2+)-permeable nonselective cation channel

Maitotoxin (MTX) activates Ca(2+)-permeable nonselective cation channels and causes a dramatic increase in cytosolic free Ca(2+) concentration ([Ca(2+)](i)) in every cell examined to date, but the molecular identity of the channels involved remains unknown. A clue came from studies of a structurally related marine toxin called palytoxin (PTX). PTX binds to the plasmalemmal Na(+)-K(+)-ATPase (NKA) and converts the Na(+) pump into a nonselective cation channel. Given the high permeability of the MTX channel for Ca(2+), we considered the possibility that MTX may bind to the plasmalemmal Ca(2+)-ATPase (PMCA) pump, and like PTX, convert the pump into a channel. To test this hypothesis, the PMCA was overexpressed in Spodoptera frugiperda (Sf9) insect cells and in human embryonic kidneys (HEK) 293 cells. In both cell types, enhanced expression of the PMCA was associated with a significant increase in MTX-induced whole cell membrane currents. The effect of MTX on whole cell currents in both wild-type and PMCA overexpressing HEK cells was sensitive to pump ligands including Ca(2+) and ATP. MTX-induced currents were significantly reduced by knockdown of PMCA1 in HEK cells using small interfering RNA or in mouse embryonic fibroblasts from genetically modified mice with the PMCA1(+/-) PMCA4(-/-) genotype. Finally, PMCA catalytic activity (i.e., Ca(2+)-ATPase) in isolated membranes, or in purified PMCA preparations, was inhibited by MTX. Together, these results suggest that MTX binds to and converts the PMCA pump into a Ca(2+)-permeable nonselective cation channel.

Palytoxin: membrane mechanisms of action

Palytoxin is a marine toxin originally isolated from the zoantharians of the genus Palythoa, but now is found in marine organisms ranging from dinoflagellates to fishes. With a MW of 2680, it is one of the largest nonpolymeric natural products ever found. Its complex structure has been elucidated and total synthesis has been achieved. With an LD(50) of 25 ng/kg for rabbits (the most sensitive species), it is one of the most lethal marine toxins. It binds to the Na,K-ATPase specifically with a K(D) of 20 pM. It has a unique action on the Na,K-ATPase, converting the pump into an ion channel and resulting in K(+) efflux, Na(+) influx and membrane depolarization. As a result palytoxin causes a wide spectrum of secondary pharmacological actions. By acting like a key to unlock the internal structure of the Na,K-ATPase, palytoxin holds promise as a useful tool for investigation of the pump molecule.

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.

Study of the neuronal effects of ouabain and palytoxin and their binding to Na,K-ATPases using an optical biosensor

The phycotoxin palytoxin (PTX) binds to Na,K-ATPase, inhibiting its activity and converting the pump into a channel. These mechanisms are poorly understood. We examined the effect of PTX on membrane potential (E(m)), intracellular calcium concentration ([Ca2+]i) and intracellular pH (pH(i)) in primary cultures of cerebellar granule cells (CGC) and compared PTX and ouabain actions in the same cellular parameters. In this system, PTX caused depolarization, intracellular calcium increase and acidification. This is similar to the effect of ouabain. Preincubation of the cells with ouabain, before addition of PTX, altered E(m), [Ca2+]i, and pH(i) in a fashion similar to that of ouabain alone. This suggest a direct interaction of PTX with the Na,K-ATPase. Therefore, we used a resonant mirror biosensor to evaluate the binding of PTX and ouabain to immobilized Na,K-ATPase. Ouabain binding to immobilized Na,K-ATPase was concentration-dependent. No binding of PTX to Na,K-ATPase was observed with up to 10 microM, or with PTX addition in the presence of ATP. The fact that ouabain binds to the pump in an immobilized conformation whereas not binding of PTX was observed indicates that PTX and ouabain do not share the same binding site, and PTX binding may require the tridimensional pump structure.

Palytoxin-induced cell death cascade in bovine aortic endothelial cells

The plasmalemmal Na(+)-K(+)-ATPase (NKA) pump is the receptor for the potent marine toxin palytoxin (PTX). PTX binds to the NKA and converts the pump into a monovalent cation channel that exhibits a slight permeability to Ca(2+). However, the ability of PTX to directly increase cytosolic free Ca(2+) concentration ([Ca(2+)](i)) via Na(+) pump channels and to initiate Ca(2+) overload-induced oncotic cell death has not been examined. Thus the purpose of this study was to determine the effect of PTX on [Ca(2+)](i) and the downstream events associated with cell death in bovine aortic endothelial cells. PTX (3-100 nM) produced a graded increase in [Ca(2+)](i) that was dependent on extracellular Ca(2+). The increase in [Ca(2+)](i) initiated by 100 nM PTX was blocked by pretreatment with ouabain with an IC(50) < 1 microM. The elevation in [Ca(2+)](i) could be reversed by addition of ouabain at various times after PTX, but this required much higher concentrations of ouabain (0.5 mM). These results suggest that the PTX-induced rise in [Ca(2+)](i) occurs via the Na(+) pump. Subsequent to the rise in [Ca(2+)](i), PTX also caused a concentration-dependent increase in uptake of the vital dye ethidium bromide (EB) but not YO-PRO-1. EB uptake was also blocked by ouabain added either before or after PTX. Time-lapse video microscopy showed that PTX ultimately caused cell lysis as indicated by release of transiently expressed green fluorescent protein (molecular mass 27 kDa) and rapid uptake of propidium iodide. Cell lysis was 1) greatly delayed by removing extracellular Ca(2+) or by adding ouabain after PTX, 2) blocked by the cytoprotective amino acid glycine, and 3) accompanied by dramatic membrane blebbing. These results demonstrate that PTX initiates a cell death cascade characteristic of Ca(2+) overload.

Quaternary organic amines inhibit Na,K pump current in a voltage-dependent manner: direct evidence of an extracellular access channel in the Na,K-ATPase

The effects of organic quaternary amines, tetraethylammonium (TEA) chloride and benzyltriethylammonium (BTEA) chloride, on Na,K pump current were examined in rat cardiac myocytes superfused in extracellular Na(+)-free solutions and whole-cell voltage-clamped with patch electrodes containing a high Na(+)-salt solution. Extracellular application of these quaternary amines competitively inhibited extracellular K(+) (K(+)(o)) activation of Na,K pump current; however, the concentration for half maximal inhibition of Na,K pump current at 0 mV (K(0)(Q)) by BTEA, 4.0 +/- 0.3 mM, was much lower than the K(0)(Q) for TEA, 26.6 +/- 0.7 mM. Even so, the fraction of the membrane electric field dissipated during K(+)(o) activation of Na,K pump current (lambda(K)), 39 +/- 1%, was similar to lambda(K) determined in the presence of TEA (37 +/- 2%) and BTEA (35 +/- 2%), an indication that the membrane potential (V(M)) dependence for K(+)(o) activation of the Na,K pump current was unaffected by TEA and BTEA. TEA was found to inhibit the Na,K pump current in a V(M)-independent manner, i.e., inhibition of current dissipated 4 +/- 2% of the membrane electric field. In contrast, BTEA dissipated 40 +/- 5% of the membrane electric field during inhibition of Na,K pump current. Thus, BTEA inhibition of the Na,K-ATPase is V(M)-dependent. The competitive nature of inhibition as well as the similar fractions of the membrane electric field dissipated during K(+)(o)-dependent activation and BTEA-dependent inhibition of Na,K pump current suggest that BTEA inhibits the Na,K-ATPase at or very near the enzyme's K(+)(o) binding site(s) located in the membrane electric field. Given previous findings that organic quaternary amines are not occluded by the Na,K-ATPase, these data clearly demonstrate that an ion channel-like structure provides access to K(+)(o) binding sites in the enzyme.

Palytoxin disrupts cardiac excitation-contraction coupling through interactions with P-type ion pumps

Palytoxin is a coral toxin that seriously impairs heart function, but its effects on excitation-contraction (E-C) coupling have remained elusive. Therefore, we studied the effects of palytoxin on mechanisms involved in atrial E-C coupling. In field-stimulated cat atrial myocytes, palytoxin caused elevation of diastolic intracellular Ca2+ concentration ([Ca2+]i), a decrease in [Ca2+]i transient amplitude, Ca2+ alternans followed by [Ca2+]i waves, and failures of Ca2+ release. The decrease in [Ca2+]i transient amplitude occurred despite high sarcoplasmic reticulum (SR) Ca2+ load. In voltage-clamped myocytes, palytoxin induced a current with a linear current-voltage relationship (reversal potential ∼5 mV) that was blocked by ouabain. Whole cell Ca2+ current and ryanodine receptor Ca2+ release channel function remained unaffected by the toxin. However, palytoxin significantly reduced Ca2+ pumping of isolated SR vesicles. In current-clamped myocytes stimulated at 1 Hz, palytoxin induced a depolarization of the resting membrane potential that was accompanied by delayed afterdepolarizations. No major changes of action potential configuration were observed. The results demonstrate that palytoxin interferes with the function of the sarcolemmal Na+-K+ pump and the SR Ca2+ pump. The suggested mode of palytoxin toxicity in the atrium involves the conversion of Na+-K+ pumps into nonselective cation channels as a primary event followed by depolarization, Na+ accumulation, and Ca2+ overload, which, in turn, causes arrhythmogenic [Ca2+]i waves and delayed afterdepolarizations.

Large diameter of palytoxin-induced Na/K pump channels and modulation of palytoxin interaction by Na/K pump ligands

Palytoxin binds to Na/K pumps to generate nonselective cation channels whose pore likely comprises at least part of the pump's ion translocation pathway. We systematically analyzed palytoxin's interactions with native human Na/K pumps in outside-out patches from HEK293 cells over a broad range of ionic and nucleotide conditions, and with or without cardiotonic steroids. With 5 mM internal (pipette) [MgATP], palytoxin activated the conductance with an apparent affinity that was highest for Na⁺-containing (K⁺-free) external and internal solutions, lowest for K⁺-containing (Na⁺-free) external and internal solutions, and intermediate for the mixed external Na⁺/internal K⁺, and external K⁺/internal Na⁺ conditions; with Na⁺ solutions and MgATP, the mean dwell time of palytoxin on the Na/K pump was about one day. With Na⁺ solutions, the apparent affinity for palytoxin action was low after equilibration of patches with nucleotide-free pipette solution. That apparent affinity was increased in two phases as the equilibrating [MgATP] was raised over the submicromolar, and submillimolar, ranges, but was increased by pipette MgAMPPNP in a single phase, over the submillimolar range; the apparent affinity at saturating [MgAMPPNP] remained ∼30-fold lower than at saturating [MgATP]. After palytoxin washout, the conductance decay that reflects palytoxin unbinding was accelerated by cardiotonic steroid. When Na/K pumps were preincubated with cardiotonic steroid, subsequent activation of palytoxin-induced conductance was greatly slowed, even after washout of the cardiotonic steroid, but activation could still be accelerated by increasing palytoxin concentration. These results indicate that palytoxin and a cardiotonic steroid can simultaneously occupy the same Na/K pump, each destabilizing the other. The palytoxin-induced channels were permeable to several large organic cations, including N-methyl-d-glucamine⁺, suggesting that the narrowest section of the pore must be ∼7.5 Å wide. Enhanced understanding of palytoxin action now allows its use for examining the structures and mechanisms of the gates that occlude/deocclude transported ions during the normal Na/K pump cycle.

Channel induction by palytoxin in yeast cells expressing Na+,K+-ATPase or its chimera with sarco/endoplasmic reticulum Ca2+-ATPase

Palytoxin (PTX) induces a cation channel through interaction with Na(+),K(+)-ATPase. It is unclear how this action relates to the enzyme catalytic activity. We examined whether the action of PTX depends on the catalytic domain specific for Na(+),K(+)-ATPase. Wild-type Na(+),K(+)-ATPase alpha-subunit (NNN) or its chimera (NCN), in which the catalytic domain was replaced with that of sarcoplasmic/endoplasmic reticulum Ca(2+)-ATPase, was co-expressed with beta-subunit in the yeast Saccharomyces cerevisiae. PTX (0.1-100 nM) increased K(+) efflux in NNN- or NCN-transfected cells to a similar degree but not in non-transfected cells. When ouabain-resistant NNN and NCN were expressed, PTX also increased K(+) efflux. Ouabain inhibited the effect of PTX in NNN or NCN cells but not in ouabain-resistant cells. These data suggest that the channel-forming action of PTX does not depend on the catalytic domain species.

Ion occlusion/deocclusion partial reactions in individual palytoxin-modified Na/K pumps

In P-type ion-motive ATPases, transported ions approach their binding sites from one membrane surface, become buried deep within "occluded" conformations in which the sites are inaccessible from either membrane side, and are then deoccluded and released to the opposite membrane surface. This describes an alternating-gate transport mechanism, in which the pump acts like an ion channel with two gates that open and close alternately. The occluded states ensure that one gate closes before the other can open, thus preventing the large electrodiffusive ion fluxes that would otherwise quickly undo the pump's electrochemical work. High-resolution crystal structures of two conformations of the SERCA (sarcoplasmic and endoplasmic reticulum Ca(2+)) P-type ATPase, together with mutagenesis results and analyses of structural models based on homology, have begun to provide a picture of the ion coordination sites in related P-type ATPases, including the Na/K pump. However, in no P-type ATPase are the structures and mechanisms of the gates known. The marine toxin, palytoxin (PTX), is known to bind to the Na/K pump and elicit a nonselective cation leak pathway, possibly by disrupting the strict coupling between the pump's inner and outer gates, allowing them to both be open. We recently found that ion flow through PTX-modified Na/K pump-channels appears to be modulated by two gates that can be regulated by the pump's physiological ligands in a manner suggesting that gating reflects underlying ion occlusion/deocclusion partial reactions. We review that work here and provide evidence that the pore of the PTX-induced pump-channel has a diameter > 6 A.

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.

Functional role of the N-terminus of Na(+),K(+)-ATPase alpha-subunit as an inactivation gate of palytoxin-induced pump channel

The N-terminus of the Na(+),K(+)-ATPase alpha-subunit shows some homology to that of Shaker-B K(+) channels; the latter has been shown to mediate the N-type channel inactivation in a ball-and-chain mechanism. When the Torpedo Na(+),K(+)-ATPase is expressed in Xenopus oocytes and the pump is transformed into an ion channel with palytoxin (PTX), the channel exhibits a time-dependent inactivation gating at positive potentials. The inactivation gating is eliminated when the N-terminus is truncated by deleting the first 35 amino acids after the initial methionine. The inactivation gating is restored when a synthetic N-terminal peptide is applied to the truncated pumps at the intracellular surface. Truncated pumps generate no electrogenic current and exhibit an altered stoichiometry for active transport. Thus, the N-terminus of the alpha-subunit appears to act like an inactivation gate and performs a critical step in the Na(+),K(+)-ATPase pumping function.

A hybrid between Na+,K+-ATPase and H+,K+-ATPase is sensitive to palytoxin, ouabain, and SCH 28080

Na(+),K(+)-ATPase is inhibited by cardiac glycosides such as ouabain, and palytoxin, which do not inhibit gastric H(+),K(+)-ATPase. Gastric H(+),K(+)-ATPase is inhibited by SCH28080, which has no effect on Na(+),K(+)-ATPase. The goal of the current study was to identify amino acid sequences of the gastric proton-potassium pump that are involved in recognition of the pump-specific inhibitor SCH 28080. A chimeric polypeptide consisting of the rat sodium pump alpha3 subunit with the peptide Gln(905)-Val(930) of the gastric proton pump alpha subunit substituted in place of the original Asn(886)-Ala(911) sequence was expressed together with the gastric beta subunit in the yeast Saccharomyces cerevisiae. Yeast cells that express this subunit combination are sensitive to palytoxin, which interacts specifically with the sodium pump, and lose intracellular K(+) ions. The palytoxin-induced K(+) efflux is inhibited by the sodium pump-specific inhibitor ouabain and also by the gastric proton pump-specific inhibitor SCH 28080. The IC(50) for SCH 28080 inhibition of palytoxin-induced K(+) efflux is 14.3 +/- 2.4 microm, which is similar to the K(i) for SCH 28080 inhibition of ATP hydrolysis by the gastric H(+),K(+)-ATPase. In contrast, palytoxin-induced K(+) efflux from cells expressing either the native alpha3 and beta1 subunits of the sodium pump or the alpha3 subunit of the sodium pump together with the beta subunit of the gastric proton pump is inhibited by ouabain but not by SCH 28080. The acquisition of SCH 28080 sensitivity by the chimera indicates that the Gln(905)-Val(930) peptide of the gastric proton pump is likely to be involved in the interactions of the gastric proton-potassium pump with SCH 28080.

Mechanisms underlying the hemolytic and ichthyotoxic activities of maitotoxin

Maitotoxin (MTX), a putative Ca(2+) channel activator produced by the dinoflagellate Gambierdiscus toxicus showed extremely potent hemolytic and ichthyotoxic activities. Hemolysis of 1% mouse blood cell suspension in saline occurred at 15 nM of MTX. The activity was enhanced six-fold in the presence of 10 microM of Ca(2+) and completely blocked by EDTA2Na, indicating its dependency on external Ca(2+). The MTX-induced hemolysis was little affected by L-type Ca(2+) channel blockers (diltiazem, nifedipine, verapamil) but was strongly inhibited by calmodulin blockers (prenylamine and chlorpromazine) or a phospholipase A2 inhibitor (quinacrine). MTX was mimicked by a calcium ionophore, calcimycin. Based on these results, a series of cellular events triggered by MTX were presumed to occur in the following sequence: increased Ca(2+) entry in cells, activation of calmodulin, promotion of phospholipase A2 activity, and finally destruction of cell membrane resulting from hydrolysis of membrane lipids. The sensitivity of blood cells to MTX varied significantly, dependent on the animal sources. Nucleated blood cells of carps and chickens were 100 times more resistant than those of mammals. LC(50) of MTX to freshwater fish Tanichthys albonubes in Ca(2+) free media (pH 8) was 5 nM but was markedly lowered to 3 pM by raising pH to 8 and increasing Ca(2+) concentration to 2 mM. In a marine environment MTX was 2000 times more toxic to fish than 42-di-hydrobrevetoxin-B (PbTx-3), one of the best known ichthyotoxins of red-tide origins.

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.

Palytoxin effects through interaction with the Na,K-ATPase in Xenopus oocyte

Palytoxin (PTX) is known to bind to Na,K-ATPase, to inhibit its activity, and to induce cation conductance, but the mechanism of these effects is still poorly understood. In Xenopus oocytes, PTX induced a large cation conductance, an effect that could be prevented or reversed by ouabain for oocytes expressing Xenopus Na,K-pumps but not with those expressing Bufo Na,K-pumps. In both cases patch-clamp experiments demonstrated a 7-8 pS channel in the presence of PTX. A large PTX-induced conductance could be observed with minimal Na,K-pump inhibition. From the single PTX-induced channel and macroscopic whole oocyte conductance, and the number of Na,K-pumps, we can conclude that PTX-induced conductance occurs through a direct interaction of PTX with a small number of Na,K-pumps.

Amino acids Val115-Ile126 of rat gastric H(+)-K(+)-ATPase confer high affinity for Sch-28080 to Na(+)-K(+)-ATPase

Na(+)-K(+)-ATPase is inhibited by cardiac glycosides and is insensitive to Sch-28080, an inhibitor of gastric H(+)-K(+)-ATPase. Gastric H(+)-K(+)-ATPase is not inhibited by cardiac glycosides. Both ouabain and, Sch-28080 binding are inhibited by K+, and it has been suggested that the inhibitors bind to corresponding regions on the alpha-subunit of each ion pump. For identification of regions of each pump that interact with the specific inhibitors, chimeric alpha-subunits consisting of selected regions from Na(+)-K(+)-ATPase and gastric H(+)-K(+)-ATPase have been prepared. One chimera (gM1/2) has been constructed from cDNA of the sheep alpha1-subunit of Na(+)-K(+)-ATPase by replacement of the last 12 amino acids of the first predicted transmembrane region (Ile99-Ile110) with corresponding amino acids from rat gastric H(+)-K(+)-ATPase. gM1/2 was expressed in yeast cells together with either the rat Na(+)-K(+)-ATPase beta 1-subunit (NK beta 1) or rat gastric H(+)-K(+)-ATPase beta-subunit (HK beta). Western blots show that the expression level of the chimeric alpha-subunit was comparable to the Na(+)-K(+)-ATPase alpha 1. Ouabain binds with high affinity to gM1/2+NK beta 1 [ouabain binding affinity (Kd) = 9.5 nM] but not to gM1/2+HK beta. The Kd for ouabain binding to Na(+)-K(+)-ATPase was 7.8 nM. Na(+)-K(+)-ATPase activity of gM1/2+NK beta 1 was inhibited both by ouabain and Sch-28080. The 50% inhibition concentration for Sch-28080 was 20-60 nM. Sch-28080 at 10 microM did not inhibit Mg(2+)- and Pi-dependent ouabain binding to gM1/2+NK beta 1. Ouabain (0.75 mM) inhibited palytoxin-induced K+ efflux from yeast cells expressing either gM1/2+NK beta 1 or gM1/2+NK beta, and Sch-28080 increased the palytoxin-induced K+ efflux from yeast cells expressing gM1/2+NK beta 1 or gM1/2+HK beta. These results implicate a small number of amino acids in the first transmembrane part of gastric H(+)-K(+)-ATPase as partial determinants of the sensitivity to Sch-28080. The data also suggest that ouabain and Sch-28080 do not bind to the same site on the chimera.

Palytoxin-induced single-channel currents from the sodium pump synthesized by in vitro expression

Palytoxin, the most potent animal toxin, is proposed to convert Na+/K(+)-ATPase into a cation-selective ion channel. Because of the ubiquity of pumps and channels in the living tissues used to study its mechanism of action, it is difficult to rule out that another site may be involved. In order to show that palytoxin selectively acts on Na+/K(+)-ATPase, two entirely in vitro methods were employed: (1) a cell-free expression system to synthesize the rat alpha 3 and beta 1 subunit proteins, and (2) single-channel recording of the synthetic Na+/K(+)-ATPase reconstituted in a planar lipid bilayer. Upon addition of palytoxin, single-channel currents were induced which had a conductance of 10 pS, in agreement with previous studies. In control experiments, when the cDNAs for Na+/K(+)-ATPase subunits were omitted, no single-channel currents were induced with palytoxin. Thus, the results show unambiguously that the Na+/ K(+)-ATPase is the site of action for palytoxin. Because palytoxin turns the Na+/K(+)-ATPase into a channel which can be detected by the exquisitely sensitive single-channel recording technique, the present results are the first to demonstrate the activity of in vitro synthesized Na+/K(+)-ATPase.