Calcium-dependent and -independent acetylcholine release from electric organ synaptosomes by pardaxin: evidence of a biphasic action of an excitatory neurotoxin

The Molecular Basis of Toxins' Interactions with Intracellular Signaling via Discrete Portals

An understanding of the molecular mechanisms by which microbial, plant or animal-secreted toxins exert their action provides the most important element for assessment of human health risks and opens new insights into therapies addressing a plethora of pathologies, ranging from neurological disorders to cancer, using toxinomimetic agents. Recently, molecular and cellular biology dissecting tools have provided a wealth of information on the action of these diverse toxins, yet, an integrated framework to explain their selective toxicity is still lacking. In this review, specific examples of different toxins are emphasized to illustrate the fundamental mechanisms of toxicity at different biochemical, molecular and cellular- levels with particular consideration for the nervous system. The target of primary action has been highlighted and operationally classified into 13 sub-categories. Selected examples of toxins were assigned to each target category, denominated as portal, and the modulation of the different portal’s signaling was featured. The first portal encompasses the plasma membrane lipid domains, which give rise to pores when challenged for example with pardaxin, a fish toxin, or is subject to degradation when enzymes of lipid metabolism such as phospholipases A₂ (PLA₂) or phospholipase C (PLC) act upon it. Several major portals consist of ion channels, pumps, transporters and ligand gated ionotropic receptors which many toxins act on, disturbing the intracellular ion homeostasis. Another group of portals consists of G-protein-coupled and tyrosine kinase receptors that, upon interaction with discrete toxins, alter second messengers towards pathological levels. Lastly, subcellular organelles such as mitochondria, nucleus, protein- and RNA-synthesis machineries, cytoskeletal networks and exocytic vesicles are also portals targeted and deregulated by other diverse group of toxins. A fundamental concept can be drawn from these seemingly different toxins with respect to the site of action and the secondary messengers and signaling cascades they trigger in the host. While the interaction with the initial portal is largely determined by the chemical nature of the toxin, once inside the cell, several ubiquitous second messengers and protein kinases/ phosphatases pathways are impaired, to attain toxicity. Therefore, toxins represent one of the most promising natural molecules for developing novel therapeutics that selectively target the major cellular portals involved in human physiology and diseases.

Structure and orientation of pardaxin determined by NMR experiments in model membranes

Pardaxins are a class of ichthyotoxic peptides isolated from fish mucous glands. Pardaxins physically interact with cell membranes by forming pores or voltage-gated ion channels that disrupt cellular functions. Here we report the high-resolution structure of synthetic pardaxin Pa4 in sodium dodecylphosphocholine micelles, as determined by (1)H solution NMR spectroscopy. The peptide adopts a bend-helix-bend-helix motif with an angle between the two structure helices of 122 +/- 9 degrees , making this structure substantially different from the one previously determined in organic solvents. In addition, paramagnetic solution NMR experiments on Pa4 in micelles reveal that except for the C terminus, the peptide is not solvent-exposed. These results are complemented by solid-state NMR experiments on Pa4 in lipid bilayers. In particular, (13)C-(15)N rotational echo double-resonance experiments in multilamellar vesicles support the helical conformation of the C-terminal segment, whereas (2)H NMR experiments show that the peptide induces considerable disorder in both the head-groups and the hydrophobic core of the bilayers. These solid-state NMR studies indicate that the C-terminal helix has a transmembrane orientation in DMPC bilayers, whereas in POPC bilayers, this domain is heterogeneously oriented on the lipid surface and undergoes slow motion on the NMR time scale. These new data help explain how the non-covalent interactions of Pa4 with lipid membranes induce a stable secondary structure and provide an atomic view of the membrane insertion process of Pa4.

Pardaxin stimulation of phospholipases A2 and their involvement in exocytosis in PC-12 cells

Pardaxin (PX) is a voltage-dependent ionophore that stimulates catecholamine exocytosis from PC-12 pheochromocytoma cells both in the presence and absence of extracellular calcium. Using a battery of phospholipase A(2) inhibitors we show that PX stimulation of phospholipase A(2) (PLA(2)) enzymes is coupled with induction of exocytosis. We investigated the relationship between PX-induced PLA(2) activity and neurotransmitter release by measuring the levels of arachidonic acid (AA), prostaglandin E(2) (PGE(2)), and dopamine release. In the presence of extracellular calcium, the cytosolic PLA(2) inhibitor arachidonyl trifluoromethyl ketone (AACOCF(3)) inhibited by 100, 70, and 73%, respectively, the release of AA, PGE(2), and dopamine induced by PX. The mitogen-activated protein kinase/extracellular signal-regulated kinase inhibitor 2'-amino-3'-methoxyflavone (PD98059) reduced by 100 and 82%, respectively, the release of AA and PGE(2) induced by PX. In the absence of extracellular calcium, the calcium-independent PLA(2) (iPLA(2)) inhibitors methyl arachidonyl fluorophosphonate, AACOCF(3), and bromoenol lactone (BEL) inhibited by 80 to 90% PX stimulation of AA release, by 65 to 85% PX stimulation of PGE(2) release, and by 80 to 90% PX-induced dopamine release. Using vesicle fusion-based enzyme-linked immunosorbent assay we found similar levels of inhibition of PX-induced exocytosis by these inhibitors. Also, PX induced the formation of soluble N-ethylmaleimide-sensitive factor attachment protein receptor complexes, an effect that was augmented by N-methylmaleimide. This complex formation was completely inhibited by BEL. Botulinum toxins type C1 and F significantly inhibited the release of AA, PGE(2), and dopamine induced by PX. Our data suggest that PX stimulates exocytosis by activating cystolic PLA(2) and iPLA(2), leading to the generation of AA and eicosanoids, which, in turn, stimulate vesicle competence for fusion and neurotransmitter release.

Selective depletion of clear synaptic vesicles and enhanced quantal transmitter release at frog motor nerve endings produced by trachynilysin, a protein toxin isolated from stonefish (Synanceia trachynis) venom

Our previous observation that low concentrations of stonefish (Synanceia trachynis) venom elicit spontaneous quantal acetylcholine release from vertebrate motor nerve terminals prompted our present study to purify the quantal transmitter-releasing toxin present in the venom and to characterize the toxin's ability to alter the ultrastructure and immunoreactivity of frog motor nerve terminals. Fractionation of S. trachynis venom by sequential anion exchange fast protein-liquid chromatography (FPLC) and size-exclusion FPLC yielded a highly purified preparation of a membrane-perturbing (haemolytic) protein toxin, named trachynilysin. Trachynilysin (2-20 micrograms/ml) significantly increased spontaneous quantal acetylcholine release from motor endings, as detected by recording miniature endplate potentials from isolated frog cutaneous pectoris neuromuscular preparations. Ultrastructural analysis of nerve terminals in which quantal acetylcholine release was stimulated to exhaustion by 3 h exposure to trachynilysin revealed swelling of nerve terminals and marked depletion of small clear synaptic vesicles. However, trachynilysin did not induce a parallel depletion of large dense-core vesicles. Large dense core vesicles contained calcitonin gene-related peptide (CGRP), as revealed by colloidal gold immunostaining, and trachynilysin-treated nerve endings exhibited CGRP-like immunofluorescence similar to that of untreated terminals. Our results indicate that the ability of stonefish venom to elicit spontaneous quantal acetylcholine release from vertebrate motor nerve terminals is a function of trachynilysin, which selectively stimulates the release of small clear synaptic vesicles and impairs the recycling of small clear synaptic vesicles but does not affect the release of large dense-core vesicles. Trachynilysin may be a valuable tool for use in other secretory terminals to discriminate between neurotransmitter and neuropeptide release.

Ion selectivity of the channels formed by pardaxin, an ionophore, in bilayer membranes

Using the method of bilayer membranes at the tip of a patch pipette, the properties of the ionic channels produced by the ionophore toxin pardaxin were investigated. At low toxin concentrations, voltage-dependent, single-channel events were measured. The current-voltage curves were non-linear when determined in Tris-Cl solution, but were linear in K(+)-HEPES solution. Using asymmetric ion solutions, the ionic selectivity of pardaxin channels was estimated from the reversal potentials obtained. The sequence of the relative permeabilities for monovalent cations was Tl+ > Rb+ > Cs+ > K+,NH4+ > methylamine+ > Li+ > dimethylamine+ > Na+. Except for Li+, the selectivity sequence fitted the cations relative hydrated size. For bivalent ions the permeability of Ba2+, Sr2+, and Mn2+ relative to Mg2+ changed according to the relative hydrated size. For anions the selectivity sequence was I- > NO3- > Br- > Cl- > ClO4- > SCN- > BF- > HCOO- > F- > CH3COO-. The selectivity sequence for the small anions (I-, NO3-, Br-, Cl-) was different from their hydrated size. Pardaxin channel showed a modest ion selectivity between small anions and cations (PK:PCl:PNa = 1.28:1.00:0.56). Pardaxin is proposed as a biophysical model to study ionic channel selectivity.

Cell-lytic and antibacterial peptides that act by perturbing the barrier function of membranes: facets of their conformational features, structure-function correlations and membrane-perturbing abilities

Almost all hemolytic and antimicrobial peptides form part of the defense mechanism of species widely distributed across the evolutionary scale. Although these peptides are of varying lengths and composition, they form amphiphilic structures in a hydrophobic environment. They also have the ability to form channels in natural and model membranes. Hemolytic peptides have proven to be very useful in studying the mechanism of hemolysis and the permeability properties of red blood cells. Preliminary investigations indicate that these peptides may also be useful in the investigation of complex cellular phenomena like exocytosis and neurotransmission. Although molecules like vancomycin, bacitracin and penicillins have been extensively used as antibiotics for therapeutic purposes, most species throughout the evolutionary scale use peptides as antimicrobial agents. These peptides exert their activity by altering the permeability properties of the bacterial plasma membrane and do not interfere with macro molecular synthesis like the other antibiotics that are presently used in therapies. Hence it is likely that resistance to peptide antibacterial agents may not develop easily. Since the problem of antibiotic resistance is presently a particularly severe one, peptide antibiotics may be the drugs of choice in the future.