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

Simulated ischemia in live cerebral slices is mimicked by opening the Na+/K+ pump: clues to the generation of spreading depolarization

The gray matter of the higher brain undergoes spreading depolarization (SD) in response to the increased metabolic demand of ischemia, promoting acute neuronal injury and death following stroke, traumatic brain injury, or sudden cardiac arrest. The mechanism linking ischemic failure of the Na+/K+ ATPase (NKA) pump to the immediate onset of a large inward current driving SD has remained a mystery because blockade of conventional ion channels does not prevent SD nor ischemic neuron death. The marine poison palytoxin (PLTX) specifically binds the NKA at picomolar concentrations, converting this transporter to an open cationic channel, causing sudden neuronal Na+ influx and K+ efflux. This pump failure, together with induction of a strong inward current, should evoke SD-like activity in gray matter. Indeed, 1-10 nM PLTX applied to live coronal brain slices of rodents induces a propagating depolarization remarkably like SD induced by oxygen/glucose deprivation (OGD). This PLTX depolarization (PD) mimicked other effects of OGD. In the neocortex, as an elevated light transmittance (LT) front passed by an extracellular pipette, a distinct negative DC shift indicated mass cell depolarization, whether induced by bath OGD or PLTX. Either treatment induced strong SD-like responses in the same higher or lower brain regions. Furthermore, we imaged identical real-time OGD-SD or PD effects upon live pyramidal neurons using 2-photon microscopy. Taken together, these findings support our proposal that an endogenous PLTX-like molecule may open the NKA to conduct Na+ influx/K+ efflux, thereby driving SD and, in its wake, ensuing neuronal damage.NEW & NOTEWORTHY With stroke, traumatic brain injury, or sudden cardiac arrest, there is no therapeutic drug to aid brain recovery. Within 2 min of severe ischemia, a wave of spreading depolarization (SD) propagates through affected gray matter. More SDs arise over hours, expanding the injury. This period represents a therapeutic window to inhibit recurring SD and reduce neuronal damage, but we do not understand the underlying molecular sequence. Here, we argue for a novel molecule to target.

The structure of the neurotoxin palytoxin determined by MicroED

Palytoxin (PTX) is a potent neurotoxin found in marine animals that can cause serious symptoms such as muscle contractions, haemolysis of red blood cells and potassium leakage. Despite years of research, very little is known about the mechanism of PTX. However, recent advances in the field of cryoEM, specifically the use of microcrystal electron diffraction (MicroED), have allowed us to determine the structure of PTX. It was discovered that PTX folds into a hairpin motif and is able to bind to the extracellular gate of Na,K-ATPase, which is responsible for maintaining the electrochemical gradient across the plasma membrane. These findings, along with molecular docking simulations, have provided important insights into the mechanism of PTX and can potentially aid in the development of molecular agents for treating cases of PTX exposure.

The Critical Role of Spreading Depolarizations in Early Brain Injury: Consensus and Contention

BACKGROUND

When a patient arrives in the emergency department following a stroke, a traumatic brain injury, or sudden cardiac arrest, there is no therapeutic drug available to help protect their jeopardized neurons. One crucial reason is that we have not identified the molecular mechanisms leading to electrical failure, neuronal swelling, and blood vessel constriction in newly injured gray matter. All three result from a process termed spreading depolarization (SD). Because we only partially understand SD, we lack molecular targets and biomarkers to help neurons survive after losing their blood flow and then undergoing recurrent SD.

METHODS

In this review, we introduce SD as a single or recurring event, generated in gray matter following lost blood flow, which compromises the Na⁺/K⁺ pump. Electrical recovery from each SD event requires so much energy that neurons often die over minutes and hours following initial injury, independent of extracellular glutamate.

RESULTS

We discuss how SD has been investigated with various pitfalls in numerous experimental preparations, how overtaxing the Na⁺/K⁺ ATPase elicits SD. Elevated K⁺ or glutamate are unlikely natural activators of SD. We then turn to the properties of SD itself, focusing on its initiation and propagation as well as on computer modeling.

CONCLUSIONS

Finally, we summarize points of consensus and contention among the authors as well as where SD research may be heading. In an accompanying review, we critique the role of the glutamate excitotoxicity theory, how it has shaped SD research, and its questionable importance to the study of early brain injury as compared with SD theory.

Endogenous transport systems in the Xenopus laevis oocyte plasma membrane

Oocytes of the South African clawed frog Xenopus laevis are widely used as a heterologous expression system for the characterization of transport systems such as passive and active membrane transporters, receptors and a whole plethora of other membrane proteins originally derived from animal or plant tissues. The large size of the oocytes and the high degree of expression of exogenous mRNA or cDNA makes them an optimal tool, when compared with other expression systems such as yeast, Escherichia coli or eukaryotic cell lines, for the expression and functional characterization of membrane proteins. This easy to handle expression system is becoming increasingly attractive for pharmacological research. Commercially available automated systems that microinject mRNA into the oocytes and perform electrophysiological measurements fully automatically allow for a mass screening of new computer designed drugs to target membrane transport proteins. Yet, the oocytes possess a large variety of endogenous membrane transporters and it is absolutely mandatory to distinguish the endogenous transporters from the heterologous, expressed transport systems. Here, we review briefly the endogenous membrane transport systems of the oocytes.

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.

Specific and dynamic detection of palytoxins by in vitro microplate assay with human neuroblastoma cells

Palytoxin is one of the most complex and biggest molecules known to show extreme acute toxicity. The dinoflagellate Ostreopsis spp., the producer organism of palytoxin, has been shown to be distributed worldwide, thus making palytoxin an emerging toxin. Rat-derived hepatocytes (Clone 9) and BE (2)-M17 human neuroblastoma cells were used to test palytoxin or palytoxin-like compounds by measuring the cell metabolic rate with Alamar Blue. The dose-dependent decrease in viability was specifically inhibited by ouabain in the case of BE (2)-M17 neuroblastoma cells. This is a functional, dynamic and simple test for palytoxins with high sensitivity (as low as 0.2 ng/ml). This method was useful for toxin detection in Ostreopsis extracts and naturally contaminated mussel samples. A comparative study testing toxic mussel extracts by LC (liquid chromatography)-MS/MS (tandem MS), MBA (mouse bioassay), haemolysis neutralization assay and a cytotoxicity test indicated that our method is suitable for the routine determination and monitoring of palytoxins and palytoxin-like compounds.

Model and simulation of Na+/K+ pump phosphorylation in the presence of palytoxin

The ATP hydrolysis reactions responsible for the Na(+)/K(+)-ATPase phosphorylation, according to recent experimental evidences, also occur for the PTX-Na(+)/K(+) pump complex. Moreover, it has been demonstrated that PTX interferes with the enzymes phosphorylation status. However, the reactions involved in the PTX-Na(+)/K(+) pump complex phosphorylation are not very well established yet. This work aims at proposing a reaction model for PTX-Na(+)/K(+) pump complex, with similar structure to the Albers-Post model, to contribute to elucidate the PTX effect over Na(+)/K(+)-ATPase phosphorylation and dephosphorylation. Computational simulations with the proposed model support several hypotheses and also suggest: (i) phosphorylation promotes an increase of the open probability of induced channels; (ii) PTX reduces the Na(+)/K(+) pump phosphorylation rate; (iii) PTX may cause conformational changes to substates where the Na(+)/K(+)-ATPase may not be phosphorylated; (iv) PTX can bind to substates of the two principal states E1 and E2, with highest affinity to phosphorylated enzymes and with ATP bound to its low-affinity sites. The proposed model also allows previewing the behavior of the PTX-pump complex substates for different levels of intracellular ATP concentrations.

Sodium flux ratio in Na/K pump-channels opened by palytoxin

Palytoxin binds to Na(+)/K(+) pumps in the plasma membrane of animal cells and opens an electrodiffusive cation pathway through the pumps. We investigated properties of the palytoxin-opened channels by recording macroscopic and microscopic currents in cell bodies of neurons from the giant fiber lobe, and by simultaneously measuring net current and (22)Na(+) efflux in voltage-clamped, internally dialyzed giant axons of the squid Loligo pealei. The conductance of single palytoxin-bound "pump-channels" in outside-out patches was approximately 7 pS in symmetrical 500 mM [Na(+)], comparable to findings in other cells. In these high-[Na(+)], K(+)-free solutions, with 5 mM cytoplasmic [ATP], the K(0.5) for palytoxin action was approximately 70 pM. The pump-channels were approximately 40-50 times less permeable to N-methyl-d-glucamine (NMG(+)) than to Na(+). The reversal potential of palytoxin-elicited current under biionic conditions, with the same concentration of a different permeant cation on each side of the membrane, was independent of the concentration of those ions over the range 55-550 mM. In giant axons, the Ussing flux ratio exponent (n') for Na(+) movements through palytoxin-bound pump-channels, over a 100-400 mM range of external [Na(+)] and 0 to -40 mV range of membrane potentials, averaged 1.05 +/- 0.02 (n = 28). These findings are consistent with occupancy of palytoxin-bound Na(+)/K(+) pump-channels either by a single Na(+) ion or by two Na(+) ions as might be anticipated from other work; idiosyncratic constraints are needed if the two Na(+) ions occupy a single-file pore, but not if they occupy side-by-side binding sites, as observed in related structures, and if only one of the sites is readily accessible from both sides of the membrane.

Palytoxin acts on Na(+),K (+)-ATPase but not nongastric H(+),K (+)-ATPase

Palytoxin (PTX) opens a pathway for ions to pass through Na,K-ATPase. We investigate here whether PTX also acts on nongastric H,K-ATPases. The following combinations of cRNA were expressed in Xenopus laevis oocytes: Bufo marinus bladder H,K-ATPase alpha(2)- and Na,K-ATPase beta(2)-subunits; Bufo Na,K-ATPase alpha(1)- and Na,K-ATPase beta(2)-subunits; and Bufo Na,K-ATPase beta(2)-subunit alone. The response to PTX was measured after blocking endogenous Xenopus Na,K-ATPase with 10 microM ouabain. Functional expression was confirmed by measuring (86)Rb uptake. PTX (5 nM: ) produced a large increase of membrane conductance in oocytes expressing Bufo Na,K-ATPase, but no significant increase occurred in oocytes expressing Bufo H,K-ATPase or in those injected with Bufo beta(2)-subunit alone. Expression of the following combinations of cDNA was investigated in HeLa cells: rat colonic H,K-ATPase alpha(1)-subunit and Na,K-ATPase beta(1)-subunit; rat Na,K-ATPase alpha(2)-subunit and Na,K-ATPase beta(2)-subunit; and rat Na,K-ATPase beta(1)- or Na,K-ATPase beta(2)-subunit alone. Measurement of increases in (86)Rb uptake confirmed that both rat Na,K and H,K pumps were functional in HeLa cells expressing rat colonic HKalpha(1)/NKbeta(1) and NKalpha(2)/NKbeta(2). Whole-cell patch-clamp measurements in HeLa cells expressing rat colonic HKalpha(1)/NKbeta(1) exposed to 100 nM PTX showed no significant increase of membrane current, and there was no membrane conductance increase in HeLa cells transfected with rat NKbeta(1)- or rat NKbeta(2)-subunit alone. However, in HeLa cells expressing rat NKalpha(2)/NKbeta(2), outward current was observed after pump activation by 20 mM K(+) and a large membrane conductance increase occurred after 100 nM PTX. We conclude that nongastric H,K-ATPases are not sensitive to PTX when expressed in these cells, whereas PTX does act on Na,K-ATPase.

Palytoxin-induced effects on partial reactions of the Na,K-ATPase

The interaction of palytoxin with the Na,K-ATPase was studied by the electrochromic styryl dye RH421, which monitors the amount of ions in the membrane domain of the pump. The toxin affected the pump function in the state P-E2, independently of the type of phosphorylation (ATP or inorganic phosphate). The palytoxin-induced modification of the protein consisted of two steps: toxin binding and a subsequent conformational change into a transmembrane ion channel. At 20 degrees C, the rate-limiting reaction had a forward rate constant of 10(5) M(-1)s(-1) and a backward rate constant of about 10(-3) s(-1). In the palytoxin-modified state, the binding affinity for Na+ and H+ was increased and reached values between those obtained in the E1 and P-E2 conformation under physiological conditions. Even under saturating palytoxin concentrations, the ATPase activity was not completely inhibited. In the Na/K mode, approximately 50% of the enzyme remained active in the average, and in the Na-only mode 25%. The experimental findings indicate that an additional exit from the inhibited state exists. An obvious reaction pathway is a slow dephosphorylation of the palytoxin-inhibited state with a time constant of approximately 100 s. Analysis of the effect of blockers of the extracellular and cytoplasmic access channels, TPA+ and Br2-Titu3+, respectively, showed that both access channels are part of the ion pathway in the palytoxin-modified protein. All experiments can be explained by an extension of the Post-Albers cycle, in which three additional states were added that branch off in the P-E2 state and lead to states in which the open-channel conformation is introduced and returns into the pump cycle in the occluded E2 state. The previously suggested molecular model for the channel state of the Na,K-ATPase as a conformation in which both gates between binding sites and aqueous phases are simultaneously in their open state is supported by this study.

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.

Actin cytoskeleton of rabbit intestinal cells is a target for potent marine phycotoxins

Biotoxins produced by harmful marine microalgae (phycotoxins) can be accumulated into seafood, representing a great risk for public health. Some of these phycotoxins are responsible for a variety of gastrointestinal disturbances; however, the relationship between their mechanism of action and toxicity in intestinal cells is still unknown. The actin cytoskeleton is an important and highly complicated structure in intestinal cells, and on that basis our aim has been to investigate the effect of representative phycotoxins on the enterocyte cytoskeleton. We have quantified for the first time the loss of enterocyte microfilament network induced by each toxin and recorded fluorescence images using a laser-scanning cytometer and confocal microscopy. Our data show that pectenotoxin-6, maitotoxin, palytoxin and ostreocin-D cause a significant reduction in the actin cytoskeleton. In addition, we found that the potency of maitotoxin, palytoxin and ostreocin-D to damage filamentous actin is related to Ca2+ influx in enterocytes. Those results identify the cytoskeleton as an early target for the toxic effect of those toxins.

Functional effects of Na+,K+-ATPase gene mutations linked to familial hemiplegic migraine

Familial hemiplegic migraine type 2, an autosomal dominant form of migraine with aura, has been associated with four distinct mutations in the alpha2-subunit of the Na+,K+-ATPase. We have introduced these mutations in the alpha2-subunit of the human Na+,K+-ATPase and the corresponding mutations in the Bufo marinus alpha1-subunit and studied these mutants by expression in Xenopus oocyte. Metabolic labeling studies showed that the mutants were synthesized and associated with the beta-subunit, except for the alpha2HW887R mutant, which was poorly synthesized, and the alpha1BW890R, which was partially retained in the endoplasmic reticulum. [3H]ouabain binding showed the presence of the alpha2HR689Q and alpha2HM731T at the membrane, whereas the alpha2HL764P and alpha2HW887R could not be detected. Functional studies with the mutants of the B. marinus Na+,K+-ATPase showed a reduced or abolished electrogenic activity and a low K+ affinity for the alpha1BW890R mutant. Through different mechanisms, all these mutations result in a strong decrease of the functional expression of the Na+,K+-pump. The decreased activity in alpha2 isoform of the Na+,K+-pump expressed in astrocytes seems an essential component of hemiplegic migraine pathogenesis and may be responsible for the cortical spreading depression, which is one of the first events in migraine attacks.

Recent Insights into the Structure and Mechanism of the Sodium Pump

The sodium pump (or Na-K-ATPase) is essential to the function of animal cells. Publication of the related calcium pump (SERCA) structure together with several recent results from a variety of approaches allow us to propose a mechanistic model to answer the question: “How does the sodium pump pump?”

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.

Cysteine-scanning mutagenesis study of the sixth transmembrane segment of the Na,K-ATPase alpha subunit

The accessibility of the residues of the sixth transmembrane segment (TM) of the Bufo marinus Na,K-ATPase alpha subunit was explored by cysteine scanning mutagenesis. Methanethiosulfonate reagents reached only the two most extracellular positions (T803, D804) in the native conformation of the Na,K-pump. Palytoxin induced a conductance in all mutants, including D811C, T814C and D815C which showed no active electrogenic transport. After palytoxin treatment, four additional positions (V805, L808, D811 and M816) became accessible to the sulfhydryl reagent. We conclude that one side of the sixth TM helix forms a wall of the palytoxin-induced channel pore and, probably, of the cation pathway from the extracellular side to one of their binding sites.

Electrophysiological and hemolytic activity elicited by the venom of the jellyfish Cassiopea xamachana

In this study, we determined hemolysis activity in human and sheep erythrocytes, and characterized the electrical responses in Xenopus oocyte membrane elicited by the venom of the jellyfish Cassiopea xamachana (Cx). The Cx venom produced hemolysis in both species, being more potent on human red cells. The electrophysiological study showed that the Cx venom elicited three different responses in the oocytes. One current was generated in all the oocytes tested and corresponded with a slow inward current (I(Cx)) associated with an increase in membrane conductance. I(Cx) was concentration-dependent and had a reversal potential of -10.3+/-0.4 mV. Ionic substitution studies indicated that the conductive pathway was mainly permeable to cations and non-selective. The oocyte membrane resistance was completely recovered after washout of the venom, this suggested that the effect was due to generation of a specific membrane conductance as opposed to a possible non-specific membrane breakdown. A comparative study with three distinct native cationic channels present in the oocyte membrane [i.e. (1) hemi-gap-junction channels, (2) mechanosensitive channels, and (3) the ouabain-sensitive channel activated by palytoxin], showed that I(Cx) might correspond to opening of mechanosensitive channels or to activation of an unknown cationic channel located in the oocyte membrane. The bioactive fraction eliciting I(Cx) were peptides and was separated from two other peptidic hemolytic fractions by chromatography.

The cytoplasmic C-terminal fragment of polycystin-1 regulates a Ca2+-permeable cation channel

The cytoplasmic C-terminal portion of the polycystin-1 polypeptide (PKD1(1-226)) regulates several important cell signaling pathways, and its deletion suffices to cause autosomal dominant polycystic kidney disease. However, a functional link between PKD1 and the ion transport processes required to drive renal cyst enlargement has remained elusive. We report here that expression at the Xenopus oocyte surface of a transmembrane fusion protein encoding the C-terminal portion of the PKD1 cytoplasmic tail, PKD1(115-226), but not the N-terminal portion, induced a large, Ca(2+)-permeable cation current, which shifted oocyte reversal potential (E(rev)) by +33 mV. Whole cell currents were sensitive to inhibition by La(3+), Gd(3+), and Zn(2+), and partially inhibited by SKF96365 and amiloride. Currents were not activated by bath hypertonicity, but were inhibited by acid pH. Outside-out patches pulled from PKD1(115-226)-expressing oocytes exhibited a 5.1-fold increased NP(o) of endogenous 20-picosiemens cation channels of linear conductance. PKD1(115-226)-injected oocytes also exhibited elevated NP(o) of unitary calcium currents in outside-out and cell-attached patches, and elevated calcium permeability documented by fluorescence ratio and (45)Ca(2+) flux experiments. Both Ca(2+) conductance and influx were inhibited by La(3+). Mutation of candidate phosphorylation sites within PKD1(115-226) abolished the cation current. We conclude that the C-terminal cytoplasmic tail of PKD1 up-regulates inward current that includes a major contribution from Ca(2+)-permeable nonspecific cation channels. Dysregulation of these or similar channels in autosomal dominant polycystic kidney disease may contribute to cyst formation or expansion.

Structure of the 5th transmembrane segment of the Na,K-ATPase alpha subunit: a cysteine-scanning mutagenesis study

To study the structure of the pathway of cations across the Na, K-ATPase, we applied the substituted cysteine accessibility method to the putative 5th transmembrane segment of the alpha subunit of the Na,K-ATPase of the toad Bufo marinus. Only the most extracellular amino acid position (A(796)) was accessible from the extracellular side in the native Na,K-pump. After treatment with palytoxin, six other positions (Y(778), L(780), S(782), P(785), E(786) and L(791)), distributed along the whole length of the segment, became readily accessible to a small-size methanethiosulfonate compound (2-aminoethyl methanethiosulfonate). The accessible residues are not located on the same side of an alpha-helical model but the pattern of reactivity would rather suggest a beta-sheet structure for the inner half of the putative transmembrane segment. These results demonstrate the contribution of the 5th transmembrane segment to the palytoxin-induced channel and indicate which amino acid positions are exposed to the pore of this channel.

Distribution and sequestration of palytoxin in coral reef animals

In the reefs off the Colombian coast (Caribbean Sea) and around Lizard Island, Australia (Pacific), palytoxin (PTX), which has been detected in zoanthid species of the genus Palythoa, also occurred in various other marine organisms living in close association with zoanthid colonies, e.g. sponges (Porifera), soft corals (Alcyonaria), gorgonians (Gorgonaria), mussels, and crustaceans. Predators, e.g. polychaete worms (Hermodice carunculata), a starfish (Acanthaster planci) and fish (Chaetodon species) feeding on Palythoa colonies, accumulate high toxin concentrations in their organs, where PTX is stored in its active form. The high level of toxin tolerance observed in marine animals may enable the wide distribution of PTX in marine biota and its transport and sequestration in food chains.