The pharmacological actions on guinea-pig ileum of crude venoms from the marine gastropods Conus striatus and Conus magus

Conus regius-Derived Conotoxins: Novel Therapeutic Opportunities from a Marine Organism

Conus regius is a marine venomous mollusk of the Conus genus that captures its prey by injecting a rich cocktail of bioactive disulfide bond rich peptides called conotoxins. These peptides selectively target a broad range of ion channels, membrane receptors, transporters, and enzymes, making them valuable pharmacological tools and potential drug leads. C. regius-derived conotoxins are particularly attractive due to their marked potency and selectivity against specific nicotinic acetylcholine receptor subtypes, whose signalling is involved in pain, cognitive disorders, drug addiction, and cancer. However, the species-specific differences in sensitivity and the low stability and bioavailability of these conotoxins limit their clinical development as novel therapeutic agents for these disorders. Here, we give an overview of the main pharmacological features of the C. regius-derived conotoxins described so far, focusing on the molecular mechanisms underlying their potential therapeutic effects. Additionally, we describe adoptable chemical engineering solutions to improve their pharmacological properties for future potential clinical translation.

A perspective on toxicology of Conus venom peptides

The evolutionarily unique and ecologically diverse family Conidae presents fundamental opportunities for marine pharmacology research and drug discovery. The focus of this investigation is to summarize the worldwide distribution of Conus and their species diversity with special reference to the Indian coast. In addition, this study will contribute to understanding the structural properties of conotoxin and therapeutic application of Conus venom peptides. Cone snails can inject a mix of various conotoxins and these venoms are their major weapon for prey capture, and may also have other biological purposes, and some of these conotoxins fatal to humans. Conus venoms contain a remarkable diversity of pharmacologically active small peptides; their targets are an iron channel and receptors in the neuromuscular system. Interspecific divergence is pronounced in venom peptide genes, which is generally attributed to their species specific biotic interactions. There is a notable interspecific divergence observed in venom peptide genes, which can be justified as of biotic interactions that stipulate species peculiar habitat and ecology of cone snails. There are several conopeptides used in clinical trials and one peptide (Ziconotide) has received FDA approval for treatment of pain. This perspective provides a comprehensive overview of the distribution of cone shells and focus on the molecular approach in documenting their taxonomy and diversity with special reference to geographic distribution of Indian cone snails, structure and properties of conopeptide and their pharmacological targets and future directions.

Two toxins from Conus striatus that individually induce tetanic paralysis

We describe structural properties and biological activities of two related O-glycosylated peptide toxins isolated from injected (milked) venom of Conus striatus, a piscivorous snail that captures prey by injecting a venom that induces a violent, spastic paralysis. One 30 amino acid toxin is identified as kappaA-SIVA (termed s4a here), and another 37 amino acid toxin, s4b, corresponds to a putative peptide encoded by a previously reported cDNA. We confirm the amino acid sequences and carry out structural analyses of both mature toxins using multiple mass spectrometric techniques. These include electrospray ionization ion-trap mass spectrometry and nanoelectrospray techniques for small volume samples, as well as matrix-assisted laser desorption/ionization time of flight mass spectrometric analysis as a complementary method to assist in the determination of posttranslational modifications, including O-linked glycosylation. Physiological experiments indicate that both s4a and s4b induce intense repetitive firing of the frog neuromuscular junction, leading to a tetanic contracture in muscle fiber. These effects apparently involve modification of voltage-gated sodium channels in motor axons. Notably, application of either s4a or s4b alone mimics the biological effects of the whole injected venom on fish prey.

The marine biologist—Bob Endean

Bob Endean was a dedicated marine biologist with an extensive knowledge of coral reef communities in the Great Barrier Reef and fauna in subtropical Queensland waters. He commenced a study of venomous and poisonous marine animals dangerous to man at a time when the field was new, employing a variety of techniques to investigate the venom apparatus, mode of delivery of venom or toxin, mode of toxic action on excitable tissues, and biochemistry of venom or toxin. Determination of the pharmacological properties of crude venom from Conus marine snails advanced characterization of conotoxins by later workers. A study of four types of nematocysts from the box-jellyfish Chironex fleckeri provided information as to their structure, function, and mechanism of discharge; myotoxins T1 and T2 were isolated from microbasic mastigophores. Endean studied poisonous stonefish (Synanceia trachynis) and, with Ann Cameron, scorpionfish (Notesthes robusta); investigations of ciguatera and of paralytic shellfish poisoning were initiated. He organized the collection of Australian frogs which led to the isolation of caerulein by Erspamer in Italy. Endean highlighted the ecological danger of the population explosion of the crown-of-thorns starfish (Acanthaster planci) and provided the impetus for the creation of the Great Barrier Reef Marine Park.

Multiple vasoactive factors in epidermal secretions of the Arabian Gulf catfish, Arius bilineatus (Valenciennes)

1. The Arabian Gulf catfish produces proteinaceous epidermal secretions when threatened or injured. 2. The soluble fraction of the catfish epidermal secretions (SES) has vasoconstrictor activities on sheep renal arteries, which can be inhibited by several antagonists, including atropine, indomethacin, prazosin, and verapamil. 3. Mepyramine, yohimbine, and ketanserin have negligible effects on SES-induced contraction. 4. SES exhibits a significant tachyphylaxis upon addition of a second (8.4% reduction) and third (47% reduction) dose of SES to the organ bath, which can be partially prevented by addition of a fresh arterial section prior to each addition. 5. A vasoconstricting activity has been partially purified from SES by gel filtration on Fractogel HW-65(F) and appears to be a protein with a pI near 7.3. This activity is affected only by verapamil and prazosin.

The mechanism of the inotropic action of striatoxin, a novel polypeptide toxin from a marine snail, in isolated cardiac muscle

1. Striatoxin (StTX), a novel polypeptide from a marine snail, caused a dose-dependent increase in contractility in the isolated atria of guinea-pig and rat in the concentration-range of 2 x 10(-9) to 3 x 10(-8)M and 3 x 10(-6)M, respectively. 2. In guinea-pig atria, the StTX-induced inotropic effect was inhibited by tetrodotoxin but not by cimetidine or chlorpheniramine. Practolol, propranolol or reserpine caused only partial block of this inotropic action. 3. In isolated single cells from rat hearts, StTX caused an increase in the degree and the rate of contraction. 4. In guinea-pig atria, StTX provoked action potentials with a plateau phase of long duration without affecting the maximum rate of rise, the amplitude of action potential and the resting membrane potential. This prolongation was also reversed by tetrodotoxin. 5. In guinea-pig cardiac myocytes, whole-cell patch-clamp experiments showed that StTX slowed Na channel inactivation without affecting the time course of channel activation. The voltage dependence of Na currents was not altered by StTX. 6. The residual currents, but not peak currents were markedly enhanced by StTX. 7. These results suggest that StTX causes prolongation of the action potential duration probably due to slowed inactivation of Na inward currents and enhanced residual currents and that this may result in an increase in Ca2+ availability in cardiac muscle cells. This could explain the cardiotonic action of StTX.

Effects of venom from Conus striatus on the delayed rectifier potassium current of molluscan neurons

The effect of crude venom extracted from venom ducts of Conus striatus on the delayed rectifier potassium current (IK) of the marine mollusc Aplysia californica was studied using voltage clamp techniques. Initial experiments indicated that the venom had phospholipase activity which destroyed the cells. The use of phospholipase inhibitors prevented destruction of the cell and permitted long-term electrophysiological measurements to be made. Application of the venom to unclamped cells caused a dramatic increase in the frequency of action potentials associated with a depolarization of the membrane potential. A broadening of the action potential was also observed. Three separate effects of the venom were observed on IK in voltage clamped cells: an increase in peak current (effect I), a slowing of both the activation and inactivation kinetics (effect K) and a decrease in the peak current (effect D). All three effects were dose dependent and both effects on peak current were greater at more depolarized membrane potentials. The data suggest that the three effects on IK are caused by different components of the venom. Effect D appears to be caused by a heat-labile compound of molecular weight greater than 50,000, effect I by a heat-stable compound of less than 50,000 and effect K by a heat-stable compound of intermediate molecular size.

Peptide neurotoxins from fish-hunting cone snails

To paralyze their more agile prey, the venomous fish-hunting cone snails (Conus) have developed a potent biochemical strategy. They produce several classes of toxic peptides (conotoxins) that attack a series of successive physiological targets in the neuromuscular system of the fish. The peptides include presynaptic omega-conotoxins that prevent the voltage-activated entry of calcium into the nerve terminal and release of acetylcholine, postsynaptic alpha-conotoxins that inhibit the acetylcholine receptor, and muscle sodium channel inhibitors, the mu-conotoxins, which directly abolish muscle action potentials. These distinct peptide toxins share several common features: they are relatively small (13 to 29 amino acids), are highly cross-linked by disulfide bonds, and strongly basic. The fact that they inhibit sequential steps in neuromuscular transmission suggests that their action is synergistic rather than additive. Five new omega-conotoxins that block presynaptic calcium channels are described. They vary in their activity against different vertebrate classes, and also in their actions against different synapses from the same animal. There are susceptible forms of the target molecule in peripheral synapses of fish and amphibians, but those of mice are resistant. However, the mammalian central nervous system is clearly affected, and these toxins are thus of potential significance for investigating the presynaptic calcium channels.

Structure-activity relations of conotoxins at the neuromuscular junction

The anticholinergic actions of synthetic conotoxin GI analogues and their structure-activity relationships were studied. Conotoxins competitively blocked the nicotinic acetylcholine receptors in neuromuscular junctions of preparations of rat sciatic nerve, M. gastrocnemius and frog abdominal muscles. They did not have a ganglion-blocking action, at least in the parasympathetic ganglion, or an anti-muscarinic action. The pA2 values of the synthetic conotoxin analogues indicated that the major factors determining the activity of conotoxin are the structural conformation of the peptide defined by two disulfide bridges, and the presence of a proline residue and C-terminal amide.

Potent excitatory effects of a cardiotonic protein from the venom of the marine snail Conus magus on the guinea-pig left atria, vas deferens and ileum

An active component (MAC) of Conus magus venom has been purified by gel filtration and ion exchange chromatographies, monitored by cardiotonic activity in the guinea-pig isolated left atria. MAC was estimated to be a protein of mol. wt between 45,000 and 65,000 by gel filtration and protease experiments. The purified protein at concentrations above 10(-7) g/ml elicited powerful rhythmic contractions of the guinea-pig vas deferens, rhythmic transient contractions of the ileum followed by relaxations, and a marked increase in the contractile force of the left atria. These responses to the toxin were abolished by tetrodotoxin. These results suggest that the action of MAC on the ileum and vas deferens may be mediated via nerves. It is also suggested that the major mechanism of action is due to activation of Na channels on the cell membrane.

Excitatory and inhibitory effects of a myotoxin from Conus magus venom on the mouse diaphragm, the guinea-pig atria, taenia caeci, ileum and vas deferens

A myotoxin purified from Conus magus venom elicited complete loss of contractile response of the mouse diaphragm to electrical stimulation followed by a gradual rise in the baseline, an increase in the contractile force of the guinea-pig left atria, a tonic contraction of the guniea-pig taenia caeci and powerful rhythmic contractions of the guinea-pig ileum and vas deferens. These excitatory effects of the toxin were blocked by treatment with tetrodotoxin, suggesting that these effects were due to an increase in Na+ permeability of the cell membrane.

Biphasic mechanical responses of the guinea-pig isolated ileum to the venom of the marine snail Conus striatus

Venom extract of Conus striatus elicited a rhythmic, transient contraction of the guinea-pig isolated ileum followed by a relaxation at concentrations greater than 1 microgram/ml, which was abolished by tetrodotoxin and a low-Na medium. The contraction induced by the venom was inhibited by atropine but not mecamylamine, whereas the relaxation was not affected by bretylium, guanethidine or phentolamine. These results suggest that the contraction of the ileum induced by the venom is due to the excitation of cholinergic nerves, while the relaxation is mediated through non-adrenergic inhibitory nerves.