Stonefish (Synanceia spp.) Ichthyocrinotoxins: An ecological review and prospectus for future research and biodiscovery

Marine organisms possess a diverse array of unique substances, many with wide ranging potential for applications in medicine, industry, and other sectors. Stonefish (Synanceia spp.), a bottom-dwelling fish that inhabit shallow and intertidal waters throughout the Indo-Pacific, harbour two distinct substances, a venom, and an ichthyocrinotoxin. Stonefish are well-known for the potent venom associated with their dorsal spines as it poses a significant risk to public health. Consequently, much of the research on stonefish focusses on the venom, with the aim of improving outcomes in cases of envenomation. However, there has been a notable lack of research on stonefish ichthyocrinotoxins, a class of toxin that is synthesised within specialised epithelial cells (i.e., tubercles) and exuded onto the skin. This has resulted in a substantial knowledge gap in our understanding of these animals. This review aims to bridge this gap by consolidating literature on the ecological functions and biochemical attributes of ichthyocrinotoxins present in various fish species and juxtaposing it with the current state of knowledge of stonefish ecology. We highlight the roles of ichthyocrinotoxins in predator defence, bolstering innate immunity, and mitigating integumentary interactions with parasites and detrimental fouling organisms. The objective of this review is to identify promising research avenues that could shed light on the ecological functions of stonefish ichthyocrinotoxins and their potential practical applications as therapeutics and/or industrial products.

Screening Snake Venoms for Toxicity to Tetrahymena Pyriformis Revealed Anti-Protozoan Activity of Cobra Cytotoxins

Snake venoms possess lethal activities against different organisms, ranging from bacteria to higher vertebrates. Several venoms were shown to be active against protozoa, however, data about the anti-protozoan activity of cobra and viper venoms are very scarce. We tested the effects of venoms from several snake species on the ciliate Tetrahymena pyriformis. The venoms tested induced T. pyriformis immobilization, followed by death, the most pronounced effect being observed for cobra Naja sumatrana venom. The active polypeptides were isolated from this venom by a combination of gel-filtration, ion exchange and reversed-phase HPLC and analyzed by mass spectrometry. It was found that these were cytotoxins of the three-finger toxin family. The cytotoxins from several cobra species were tested and manifested toxicity for infusorians. Light microscopy revealed that, because of the cytotoxin action, the infusorians' morphology was changed greatly, from teardrop-like to an almost spherical shape, this alteration being accompanied by a leakage of cell contents. Fluorescence microscopy showed that the fluorescently labelled cytotoxin 2 from cobra N. oxiana was localized mainly at the membrane of killed infusorians, indicating that cytotoxins may kill T. pyriformis by causing membrane rupture. This work is the first evidence of the antiprotozoal activity of cobra venom cytotoxins, as demonstrated by the example of the ciliate T. pyriformis.

Pardaxin-induced apoptosis enhances antitumor activity in HeLa cells

Pardaxin, a pore-forming antimicrobial peptide that encodes 33 amino acids was isolated from the Red Sea Moses sole, Pardachirus mamoratus. In this study, we investigated its antitumor activity in human fibrosarcoma (HT-1080) cells and epithelial carcinoma (HeLa) cells. In vitro results showed that the synthetic pardaxin peptide had antitumor activity in these two types of cancer cells and that 15μg/ml pardaxin did not lyse human red blood cells. Moreover, this synthetic pardaxin inhibited the proliferation of HT1080 cells in a dose-dependent manner and induced programmed cell death in HeLa cells. DNA fragmentation and increases in the subG1 phase and caspase 8 activities suggest that pardaxin caused HeLa cell death by inducing apoptosis, but had a different mechanism in HT1080 cells.

Membrane composition determines pardaxin's mechanism of lipid bilayer disruption

Pardaxin is a membrane-lysing peptide originally isolated from the fish Pardachirus marmoratus. The effect of the carboxy-amide of pardaxin (P1a) on bilayers of varying composition was studied using (15)N and (31)P solid-state NMR of mechanically aligned samples and differential scanning calorimetry (DSC). (15)N NMR spectroscopy of [(15)N-Leu(19)]P1a found that the orientation of the peptide's C-terminal helix depends on membrane composition. It is located on the surface of lipid bilayers composed of 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC) and is inserted in lipid bilayers composed of 1,2-dimyristoyl-phosphatidylcholine (DMPC). The former suggests a carpet mechanism for bilayer disruption whereas the latter is consistent with a barrel-stave mechanism. The (31)P chemical shift NMR spectra showed that the peptide significantly disrupts lipid bilayers composed solely of zwitterionic lipids, particularly bilayers composed of POPC, in agreement with a carpet mechanism. P1a caused the formation of an isotropic phase in 1-palmitoyl-2-oleoyl-phosphatidylethanolamine (POPE) lipid bilayers. This, combined with DSC data that found P1a reduced the fluid lamellar-to-inverted hexagonal phase transition temperature at very low concentrations (1:50,000), is interpreted as the formation of a cubic phase and not micellization of the membrane. Experiments exploring the effect of P1a on lipid bilayers composed of 4:1 POPC:cholesterol, 4:1 POPE:cholesterol, 3:1 POPC:1-palmitoyl-2-oleoyl-phosphatidylglycerol (POPG), and 3:1 POPE:POPG were also conducted, and the presence of anionic lipids or cholesterol was found to reduce the peptide's ability to disrupt bilayers. Considered together, these data demonstrate that the mechanism of P1a is dependent on membrane composition.

Purification of a toxic factor from Arabian Gulf catfish epidermal secretions

The Arabian Gulf catfish, Arius bilineatus (Valenciennes) secretes a proteinaceous epidermal secretion when threatened or injured. A toxic factor has been isolated and purified from the crude extract (crude skin toxin) of these secretions by a combination of gel filtration on Sephacryl S-300 and preparative discontinuous polyacrylamide gel electrophoresis. The purified skin toxin has a molecular weight of 39,000 Da and an isoelectric point (pI) of 5.45. Injection of the purified skin toxin into rabbits i.v. and determination of the LD50 indicated that the protein had been purified approximately 30 fold by these procedures. Injection of the purified skin toxin into rabbits caused agitation, convulsions and death within 5 min. Analysis of plasma levels of lactate dehydrogenase, glutamate-oxaloacetate transaminase and glutamate pyruvate transaminase in injected rabbits indicated that the skin toxin caused cardiac and liver damage to the animals.

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.

Presynaptic effects of the pardaxins, polypeptides isolated from the gland secretion of the flatfish Pardachirus marmoratus

The effects of the two toxic proteins Pardaxin I and II isolated from the gland secretion of the flatfish Pardachirus marmoratus on frog neuromuscular transmission have been investigated and compared to those of the gland secretion. Pardaxin I and II showed pre- but not postsynaptic neurotoxic effects. They increased the frequency of the spontaneous release of transmitter quanta in a dose-dependent and temperature-influenced way up to more than 100 times control values. At the same time the quantal content of the evoked end-plate potentials was greatly elevated. Pardaxin I was about 5 times more effective than Pardaxin II, and both were roughly in the same range of efficacy as the original gland secretion (w/v). The glycosteroids isolated from the same gland secretion were relatively ineffective in promoting neurotransmitter release; however, at high doses they had postsynaptic effects, as shown by a diminution of the amplitude of the evoked end-plate potentials. They did not reinforce the effect of the Pardaxins. At higher doses both the Pardaxins and the gland secretion induced depolarization of postsynaptic membranes, muscle cell contractions which could not be blocked by (+)-tubocurarine or by tetrodotoxin, and eventually also physical disruption of muscle cells. No effects on nerve conductance were observed. Pore-forming activity of the Pardaxins has already been demonstrated. It is suggested that their presynaptic effects are a result of a possible affinity to the nerve terminals, of their hydrophobicity and mainly of this pore-forming activity. These toxins might be valuable tools in neuroscience research.

Blood volume and red cell space in tissues of the rainbow trout, Salmo gairdneri

1. Whole body blood volume and red cell space of 22 tissues were measured in unanesthetized rainbow trout at 4, 12, 30, 60, 150 and 240 min after dorsal aortic injection of 51Cr-labeled red blood cells. 2. Apparent blood volume decreased during the initial 30 min after injection and increased thereafter. At 240 min the blood volume was 33.5 +/- 3.1 ml/kg body wt. 3. Tissue red cell space varied as a function of the interval between labeled red cell injection and tissue collection. Red cell space was highest in spleen followed by heart, kidney and liver. Lowest red cell spaces were found in stomach and red and white skeletal muscle. 4. Variability in blood volume and tissue red cell space over time suggests that caution should be exercised in the design of experiments that employ indicator dilution measurements to measure vascular volumes.

Increased 5-hydroxytryptamine and norepinephrine release from rat brain slices by the Red Sea flatfish toxin pardaxin

The effect of the Red Sea flatfish toxin pardaxin was examined on K+-evoked and on basal release of either [3H]norepinephrine or [3H]5-hydroxytryptamine from preloaded rat cortical slices. The K+-induced release of the neurotransmitters was stimulated in a dose-related manner at concentrations ranging from 0.5 to 4 micrograms/ml. Basal release of the two transmitters was elevated to a lesser extent. Although the stimulation of evoked release was approximately equivalent for the two neurotransmitters, the response to 5-hydroxytryptamine was reversible whereas that of norepinephrine was not washed by 20 min of superfusion. The mechanisms involved in producing these actions of pardaxin are not known; however, they may be mediated by changes in electrolyte fluxes across the neuronal membranes.

Synthetic surfactants: a new approach to the development of shark repellents

The toxic and shark repellent secretion of Pardachirus marmoratus (PMC) and a series of 15 surfactants and industrial detergents were assayed for their lethality to fish, termination of shark's tonic immobility and feeding inhibition of aggressive, hungry lemon sharks. Sodium and lithium lauryl sulfate salts were more potent than PMC in all three bioassays. Graduated activity, from mildly repellent to inactive, was demonstrated by several other surfactants. The abundance, versatility, chemical stability, modifiability, and low price of detergents justify their use in further development of effective shark repellents.

Pardaxin produces postjunctional muscle contraction in guinea-pig intestinal smooth muscle

The action of pardaxin (PX), a toxin isolated from the secretion of the Red Sea flatfish, Pardachirus marmoratus, was studied on longitudinal muscle of guinea-pig ileum. Pardaxin contracted the ileum and subsequently abolished muscle contraction to 5-hydroxytryptamine (5-HT), but did not affect the responses to acetylcholine (ACh) and substance P(SP). Pardaxin-induced contraction was only partially suppressed by atropine and not affected by tetrodotoxin or morphine. Preparations desensitized to 5-HT or SP responded normally to pardaxin. Pardaxin-induced contractions were normal in K+-depolarizing Krebs Ringer solution and not affected by black widow spider venom. It is concluded that the pardaxin-induced muscle contractions are not mediated through the release of neurotransmitters and do not involve 5-HT, SP or ACh receptors, but are due to a direct action on the muscle contractile mechanism.

Pardaxin increases solute permeability of gills and rectal gland in the dogfish shark (Squalus acanthias)

The action of the ichthyotoxic secretion of the Red Sea flatfish Pardachirus marmoratus and its derived toxin, pardaxin, was examined in the dogfish shark (Squalus acanthias). Pardaxin was more toxic when administered to the bathing medium than when injected into a dorsal artery and it transiently diminished the spiracular rate and caused a severe struggling response in the adult shark only when administered to the head region of the shark. Pardaxin caused a transient leakage to urea and sodium between the shark and the seawater. In the isolated perfused rectal gland pardaxin irreversibly reduced the rate of chloride secretion and concentration gradient of urea between perfusate and rectal gland fluid. In addition, ultrastructural studies on the rectal gland showed that ionic lanthanum penetrated the tight junctions and foci of cell necrosis were observed. These studies indicate that in shark the gills are the most probable target of the toxicity of pardaxin.

Conformation of pardaxin, the toxin of the flatfish Pardachirus marmoratus

Pardaxin is the toxic component isolated from the secretion of the Red Sea flatfish Pardachirus marmoratus. Pardaxin has attracted considerable attention recently because of its use as shark repellent. Conformational information regarding the peptide backbone, disulfide bonds, and tyrosine chromophores was obtained using Raman and circular dichroism (CD) spectroscopy. Analysis of the Raman spectra indicates that the toxin consists of 39% alpha-helix, 23% beta-structure, and 39% random coil. The occurrence of a band at 510--512 cm-1 indicates the conformational geometry of the disulfide C-C-S-S-C-C linkages to be gauche-gauche-gauche. From the lack of a peak in the 2500--2700 cm-1 region we concluded that all half-cystines are involved in disulfide linkages. The far-ultraviolet CD spectrum of pardaxin showed positive 196 and negative 208 and 222 nm bands. The best fit secondary structure based on the CD ellipticities was found to be: 23% alpha-helix, 21% beta-structure, and 56% random coil. Under highly acidic and alkaline conditions the far-ultraviolet pardaxin spectra showed extensive, reversible loss of its helical structure. Due to certain biological characteristics of pardaxin the possible influence of salt on pardaxin's conformation was examined. No evidence of conformation perturbation was detected in either the CD or Raman spectra of pardaxin.