Pharmacological properties of the interaction of a sea anemone polypeptide toxin with cardiac cells in culture

Three approaches have been used to analyze the mechanism of action of a sea anemone neurotoxin on cultured chick embryonic cardiac cells: 1) electrophysiological measurements; 2) simultaneous recordings of contraction properties; and 3) measurements of cationic influx of 22Na+ and 45Ca++ The chick embryo cell cultures consisted of 3-day aggregates and monolayer cultures which have electrophysiological properties of the early embryonic type and 16-day aggregates which have electrophysiological properties of the adult type. All types of cardiac cell cultures responded similarly to exposure to the 47 amino acid long sea anemone toxin extracted from Anemonia sulcata. The polypeptide toxin provoked action potentials with a plateau phase of long duration, a slowing down of the beating rate and simultaneously with the prolonged action potential an increase in amplitude and duration of cardiac contractions. Our results indicate: 1) that the site of action of the sea anemone toxin on cardiac cell is the Na+ channel as in other excitable system; 2) that the sea anemone toxin can reveal unexpressed ("silent") fast Na+ channels in cardiac cells of the early embryonic type; and 3) that the increase in amplitude and duration of cardiac contractions caused by the polypeptide toxin is most probably due to an indirect activation of the Na+-Ca++ exchange system.

Properties of the interaction of the sodium channel with permeant monovalent cations

The use of sea anemone toxin, veratridine and scorpion toxin which specifically interact with the gating system of the sodium channel and maintain the channel in an open conformation has permitted a study of the mechanism of transport of monovalent cations through the selectivity filter of this channel. The initial rate of 22Na+ influx through the tetrodotoxin-sensitive Na+ channels of excitable cells is dependent upon the external concentrations of Na+ and Na+-substitutes with the following properties. (a) It is saturable at high Na+ concentrations and increases with the external Na+ concentration in a cooperative manner (nH = 1.6). (b) At low external Na+ concentrations (1 mM), it is activated and then inhibited by increasing external concentrations of monovalent cations such as Li+, guanidinium, hydrazinium, hydroxylamine and K+. The activating effect of these cations disappears at higher external Na+ concentrations (10 mM). The experimental data are consistent with a model involving at least two allosteric cation-binding sites per Na+ channel. The binding of monovalent cations to Na+ sites is characterized by a high positive homotropic cooperativity. Most of the work describes the properties of the Na+ channel in neuroblastoma cells. The mechanism has also been shown to be valid for excitable cells of other types and origins.

Synthesis of new, highly radioactive tetrodotoxin derivatives and their binding properties to the sodium channel

The Pfitzner-Moffatt oxidation procedure has been used to prepare five derivatives of tetrodotoxin by covalent attachment of the oxidized toxin to lysine, glycine, beta-alanine or ethylenediamine. These derivates reach specific radioactivities between 5 and 45 Ci/mmol. The equilibrium properties of binding of these tetrodotoxin derivatives to membrane-embedded or solubilized sodium channels from crustacean nerves have been analysed and compared to the binding properties of tritiated tetrodotoxin and saxitoxin to the same biological systems. All these highly radiolabelled derivatives associate to the tetrodotoxin binding component with properties similar to those of tetrodotoxin itself. The synthetic route described in this paper may be used to prepare other types of tetrodotoxin derivatives such as fluorescent derivatives for example.

Binding of sea anemone toxin to receptor sites associated with gating system of sodium channel in synaptic nerve endings in vitro

Iodination of toxin II from the sea anemone Anemonia sulcata gives a labeled monoiododerivative that retains 80% of the original neurotoxicity. This derivative binds specifically to rat brain synaptosomes at 20 degrees C and pH 7.4 with a second-order rate constant of association ka = 4.6 x 10(4) M-1 sec-1 and a first-order rate constant of dissociation kd = 1.1 x 10(-2) sec-1. The binding occurs on the Na+ channel at a binding site distinct from that of other gating system toxins like batrachotoxin, veratridine, grayanotoxin, aconitine, and pyrethroids. The maximal binding capacity Bmax is 3.2 pmol/mg of protein (i.e., about two sea anemone toxin binding sites per tetrodotoxin binding site) and the Kd is 240 nM for the monoiododerivative and 150 nM for the native toxin. Corresponding binding parameters for the association of a 125I-labeled derivative of toxin II from the scorpion Androctonus australis Hector are Bmax = 0.3 pmol/mg of protein and Kd = 1 nM, whereas the Kd of the unmodified scorpion toxin is 0.6 nM. Competition experiments involving scorpion toxins, sea anemone toxins, and synaptosomes demonstrate that, although the sea anemone toxin is able to displace the scorpion toxin bound to synaptosomes, the scorpion toxin does not displace the sea anemone toxin. The sea anemone toxin but not the scorpion toxin binds to depolarized synaptosomes. Differences between binding properties of the two polypeptide toxins are analyzed in the discussion.

The sodium channel in non-impulsive cells. Interaction with specific neurotoxins

The cell line C9 used in this paper has a resting potential of --50 mV (+/- 10 mV) but is unable to generate an action potential upon electrical stimulation. The cell membrane has receptors for the selectivity filter toxin tetrodotoxin as well as for the gating system toxins, veratridine, scorpion toxin and sea anemone toxin. The Na+ channel which remains silent to electrical stimulation in the absence of toxins can be chemically activated by the gating system toxins. This has been demonstarted by electrophysiological techniques and by 22Na+ flux studies. The electrophysiological approach has shown that the sea anemone toxin is able to induce a spontaneous slow-wave activity inhibited by tetrodotoxin. 22Na+ influx analyses have shown that veratridine and the sea anemone toxin produce an important increase of the initial rate of 22Na+ influx into the C9 cell. The stimulation of 22Na+ entry by these gating system toxins is similar to that found using spiking neuroblastoma cells. Veratridine and the sea anemone toxin on one hand as well as veratridine and the scorpion toxin on the other hand are synergistic in their action to stabilize an open and highly permeable form of the sodium channel. Stimulation of 22Na+ entry into the cell through the sodium channel maintained open by the gating system neurotoxins is completely suppressed by tetrodotoxin.

Interaction of phencyclidine ("angel dust") with a specific receptor in rat brain membranes

[3H]Phencyclidine binds to synaptic membranes from rat brain in a saturable, reversible, and selective fashion, with a dissociation constant Kd of 0.25 microM and a maximal binding capacity of 2.4 pmol/mg of membrane protein--i.e., 250 pmol/g of brain. The binding activity is concentrated in synaptosomal fractions, is higher in cerebral cortex and corpus striatum than in other parts of the rat brain, and is not detectable in the spinal cord. Only molecules of the phencyclidine series and ketamine are able to bind to the phencyclidine receptor. [3H]Phencyclidine bound to its receptor is not displaced by the classical neurotransmitters or neuromodulators. There is a good correlation between the apparent affinities of a series of phencyclidine analogs for the phencyclidine receptor and the pharacological activities of these analogs as measured by the rotarod assay.

Synthesis and mode of action on axonal membranes of photoactivable derivatives of tetrodotoxin

Two photoactivable derivatives of tetrodotoxin have been synthesized. Electrophysiological experiments on crab giant axons and competitive binding with [3H]-tetrodotoxin for the tetrodotoxin receptor indicate that they are only 4.5 to 7.5 times less active than tetrodotoxin itself. These compounds give a reversible block of the sodium channel in the dark but after ultraviolet irradiation they provoke an irreversible blockade of the channel.

The 1H nuclear-magnetic-resonance spectra of Neurotoxin I and cardiotoxin Vii4 from Naja mossambica mossambica

Two toxins from the venom of Naja mossambica mossambica, neurotoxin I and cardiotoxin VII4, were investigated in aqueous solution by high-resolution 1H nuclear magnetic resonance (NMR) techniques at 360 MHz. The spectral characterization of the proteins included determination of the number of slowly exchanging amide protons which can be observed in 2H2O solution, measurement of the amide proton chemical shifts and exchange rates, characterization of the aromatic spin systems and the internal mobilities of aromatic rings, and studies of the pH dependence of the NMR spectra. For numerous resonances of labile and non-labile protons quite outstanding pH titration shifts were observed. It is suggested that these NMR parameters provide a useful basis for comparative structural studies of different proteins in the large group of homologous snake toxins. As a first application the NMR data presently available in the literature on neurotoxin II from Naja naja oxiana, toxin alpha from Naja nigricollis and erabutoxin a and b from Laticauda semifasciata have been used to compare these three proteins with neurotoxin I from Naja mossambica mossambica. This preliminary comparative study provides evidence that the same type of spatial structure prevails for these four homologous neurotoxins and that the folding of the backbone corresponds quite closely to that observed in the crystal structure of erabutoxin b. A second application is the comparison of cardiotoxin VII4 from Naja mossambica mossambica with the neurotoxins. The experimental data indicate that the folding of the polypeptide backbone is closely similar, but that the cardiotoxin molecule is markedly more flexible than the neurotoxins.

Isolation and partial characterization of rat CNS axolemma enriched fractions

Axolemma-enriched fractions were prepared from rat brain by osmotic shock of a purified preparation of myelinated axons and subsequent separation of myelin, two axolemma-enriched fractions and myelin-free axons by density gradient centrifugation. Compared with the starting whole homogenate, the fractions were enriched in specific activity of Na+K+ ATPase, acetylcholinesterase, 5'nucleotidase as well as 2',3'-cyclic nucleotide 3'phosphohydrolase. Compared with myelin, the axolemmal fractions are greatly enriched in high molecular weight proteins. The 1.0/1.2 fraction has a predominant peak of fucose-labeled glycoprotein with a molecular weight between that of the myelin associated glycoprotein and the Wolfgram protein which is absent from the myelin glycoprotein profile. Polyacrylamide gel electrophoresis showed that the protein profile of myelin isolated by this procedure was similar to that of myelin isolated by other procedures and that the myelin specific basic and proteolipid proteins were virtually absent in the axolemma-enriched fractions. Both axolemma fractions were enriched in higher MW proteins, some of which resembled proteins in the myelin protein profile. Both axolemma-enriched fractions specifically bind between 2 and 3 pmoles of [3H]tetrodotoxin per mg protein. The axolemma-enriched fractions incorporated [3H]leucine and [14C]fucose exclusively into high molecular weight proteins and glycoproteins. In contrast myelin concomitantly isolated with the axolemma-enriched fractions had a significant amount of [3H]leucine labeled protein in myelin proteolipid and basic proteins. In addition to the myelin associated g-ycoprotein the [14C]fucose labeled a glycoprotein of slightly larger apparent molecular weight than proteolipid protein was found in the myelin fraction while the comparable labeled glycoprotein was absent in the axolemma-enriched fractions. The possible extent of contamination of these fractions by myelin or myelin subfractions and relationship of these axolemma-enriched fractions to other axolemma preparations are discussed.