| Notexin | |||||||
|---|---|---|---|---|---|---|---|
 | |||||||
| Identifiers | |||||||
| Organism | |||||||
| Symbol | N/A | ||||||
| CAS number | 37223-96-4 | ||||||
| PDB | 1AE7 | ||||||
| UniProt | P00608 | ||||||
| |||||||
Notexin is a toxin produced by the tiger snake (Notechis scutatus). It is a myotoxic and presynaptic, neurotoxic phospholipase A2 (PLA2s). [1] These are enzymes that hydrolyze the bond between a fatty acid tail and glycerol in fatty acids on the 2-position. [2]
The name notexin comes from the fact that this toxin was first found to be the major component in the venom of the tiger snake. [3] The name notexin is thus a combination of the genus name Notechis and the word toxin. The tiger snake was first described by Wilhelm Peters in 1861. [4] The toxin was first purified more than a hundred years later in 1972 by Karlsson et al. [3] [5] This prompted more research into notexin. [5] [6]
Notexin consists of a single molecule. This molecule is a single-peptide chain of 119 amino acid residues that are cross-linked with 7 disulfide-bridges. [7]
X-ray diffraction has been used to determine the crystal structure of notexin and led to the conclusion that notexin belongs to either the P3121 or P3221 space group with lattice parameters a = b = 74.6 Å, c = 49.0 Å with a β of 120⁰. This data was found with a resolution of 2.0 Å and had an R-factor of 16.5%. [8] For protein data, this R-factor is usually 20%, [9] indicating that the crystal structure of notexin is relatively well defined.
The supramolecular structure of notexin is very similar to that of other PLA2s. Both notexin and many PLA2s contain four characteristic main helices (the αA, αB, αC and αE helices) and a short carboxyl end helix in their secondary structure. Also the active site seems to be similar enough to that of other PLA2s in order to use their model building studies when discussing enzymatic properties. Notexin does deviate significantly from other PLA2s due to different main chain lengths and its conformation in the 69th amino acid residue. [8]
The active site of notexin contains His-48. This residue is in close contact with the carboxylate oxygens of an Asp-99 residue, which is also present in notexin. [8] For most PLA2s the wall of the active site is covered with hydrophobic residues. When a lone pair on the oxygen of water attacks the ester, the His-48 residue facilitates a proton transfer and the substrates's carbonyl oxygen is possibly fixated and stabilized by the positively charged NH-groups on the PLA2s. [10]
Notexin is generally lethal if it enters the bloodstream in rats. This lethal effect is the result of a presynaptic blockade of transmission across neuromuscular junctions of the breathing muscles, causing asphyxiation. It has also been shown to have myotoxic effects upon intravenous injection. [8] Myotoxic effects generally entail muscle necrosis. [11]
It was proposed (Dixon et al., 1996) [12] that this myotoxicity of notexin is the result of notexin binding to the sarcolemma, causing hypercontraction and thereby muscle necrosis as a result of the membrane between places of hypercontraction rupturing. [12] The presynaptic activity is, however, much more potent, at least in mice. [8]
Notexin causes an indirect reduction or complete end of the release of acetylcholine in the affected nerve terminals. This acetylcholine normally causes an action potential and thereby muscle contraction. It was found that this reduction of acetylcholine release was caused by an impaired recycling of synaptic vesicles as a reduction in the content of synaptic vesicles and abnormally large vesicles were observed in the affected tissues. This was followed by shrinking of the nerve terminals and the amount of vesicles in these terminals decreasing. [8]
The exact way of interaction with the cell is unknown, but it is suggested that notexin, like other PLA2s, interacts with high-affinity specific protein receptors or low-affinity lipid domains of muscle cells and motor neurons. Interaction of notexin with the plasma membrane results in the hydrolysis of the phospholipids in the cell membrane. [13] A study [14] showed that without the PLA2 activity, notexin also has membrane damaging effects, suggesting that notexin has multiple mechanisms to damage the cell membrane.
Cell membranes become permeable for ions and cause an influx of Ca2+ from the extracellular medium. In muscle cells the influx of Ca2+ causes hypercontraction of myofilaments, which can cause mechanical damage to the plasma membrane. [12] [13] The mitochondria will take up Ca2+, eventually leading to a reduced mitochondrial functionality. The high Ca2+ concentration in the cytosol activates Ca2+-dependent proteinases, calpains, [15] and the endogenous Ca2+-dependent phospholipase A2. The calpains degrade the cytoskeletal components of the cell and Ca2+-dependent phospholipase A2 hydrolyses the cell membrane, which leads to further cell degradation and bigger influx of Ca2+. At a certain point the damage is irreversible and necrosis of the cell occurs. [12] [13]
In neurons the influx of calcium causes the release of the ready-to-release synaptic vesicles and the reserve pool of synaptic vesicles. [13] [16] Research [16] showed that neurons, after treatment with notexin, had strongly reduced numbers of synaptic vesicles. These results seem to indicate that notexin inhibits the endocytosis of new synaptic vesicles, besides the exocytosis as a result of the Ca2+ influx. [13] [16] [17] Like in muscle cells, the Ca2+ influx in neurons also leads to reduced mitochondrial functionality and the activation of calpains and endogenous Ca2+-dependent PLA2s. This leads to the same structural damage as in muscle cells. [13]
Experimental research also showed that notexinhad nephrotoxic effects on mice. A study [18] showed that depending on the dose, there was renal tubular and glomerular damage within 24 hours.
Not much about the metabolism of notexin is known. However, studies have shown that the toxin can be made ineffective by specific antibodies. [19] [20] [21] In a certain study, mice become resistant to notexin, similar isoforms of the toxin and other venoms from the same origin. This was done by exposing the mice to the non-detoxified notexin. It was found that the C-terminal part of the notexin peptide chain is the binding site for these antibodies and thus it is known that this is the site for an antigenic domain. [19] Another study showed that at least some of the antigens that are able to block the effects of notexin do this by cross-neutralizations. [20] Certain antibodies have also been shown to have different affinities for different notexin isoforms. These different isoforms occur in snakes that have different geographical locations. [21] It is thus not the case that notexin antibodies necessarily bind to all notexin isoforms with the same affinity.
Notexin causes pain at the site of the bite followed by excessive salivation, weakness, drowsiness, difficulty breathing, decreased blood pressure and paralysis of lips, larynx, tongue and facial muscles. Possibly, blurring of vision, ptosis, headaches and convulsions may also occur. [22] [23]
There has been no research on the reaction of notexin in humans, however it is known that upon injection with the toxin, muscle damage and myoglobinuria will follow. [12] Data has shown that the tiger snake is one of the major causes of snake bites in Australia leading to being the second most common cause of death from snakebites. [24]
Several results on the toxic effects of notexin in rodents have been reported. [23] [25] When injecting 1 to 2 μg of pure toxin into a rat soleus muscle, it will destroy all of the muscle fibers. [25] In mice the LD50 is 0.214 mg/kg when applied subcutaneously and 0.04 mg/kg when applied intravenously. [23] [25] There have also been researches into functional and morphological properties of regrowing mouse extensor digitorum longus (EDL) muscles after a notexin injection. Three days after injection there was complete fiber breakdown and loss of functional capacity. After ten days the muscles were made up entirely of regrowing fibers. [26]
There is no notexin antivenom available on the market. There are two general tiger snake antivenoms available that potentially could work. [27] There are no studies found on the effectiveness of the tiger snake antivenoms on notexin.
A neuromuscular junction is a chemical synapse between a motor neuron and a muscle fiber.
In a neuron, synaptic vesicles store various neurotransmitters that are released at the synapse. The release is regulated by a voltage-dependent calcium channel. Vesicles are essential for propagating nerve impulses between neurons and are constantly recreated by the cell. The area in the axon that holds groups of vesicles is an axon terminal or "terminal bouton". Up to 130 vesicles can be released per bouton over a ten-minute period of stimulation at 0.2 Hz. In the visual cortex of the human brain, synaptic vesicles have an average diameter of 39.5 nanometers (nm) with a standard deviation of 5.1 nm.
Snake venom is a highly toxic saliva containing zootoxins that facilitates in the immobilization and digestion of prey. This also provides defense against threats. Snake venom is injected by unique fangs during a bite, whereas some species are also able to spit venom.
End plate potentials (EPPs) are the voltages which cause depolarization of skeletal muscle fibers caused by neurotransmitters binding to the postsynaptic membrane in the neuromuscular junction. They are called "end plates" because the postsynaptic terminals of muscle fibers have a large, saucer-like appearance. When an action potential reaches the axon terminal of a motor neuron, vesicles carrying neurotransmitters are exocytosed and the contents are released into the neuromuscular junction. These neurotransmitters bind to receptors on the postsynaptic membrane and lead to its depolarization. In the absence of an action potential, acetylcholine vesicles spontaneously leak into the neuromuscular junction and cause very small depolarizations in the postsynaptic membrane. This small response (~0.4mV) is called a miniature end plate potential (MEPP) and is generated by one acetylcholine-containing vesicle. It represents the smallest possible depolarization which can be induced in a muscle.
A latrotoxin is a high-molecular mass neurotoxin found in the venom of spiders of the genus Latrodectus as well as at least one species of another genus in the same family, Steatoda nobilis. Latrotoxins are the main active components of the venom and are responsible for the symptoms of latrodectism.
Poneratoxin is a paralyzing neurotoxic peptide made by the bullet ant Paraponera clavata. It prevents inactivation of voltage gated sodium channels and therefore blocks synaptic transmission in the central nervous system. Specifically, poneratoxin acts on voltage gated sodium channels in skeletal muscle fibers, causing paralysis, and nociceptive fibers, causing pain. It is rated as a 4 plus on the Schmidt sting pain index, the highest possible rating with that system, and its effects can cause waves of pain up to twelve hours after a single sting. It is additionally being studied for its uses in biological insecticides.
Gloydius blomhoffii, commonly known as the mamushi, Japanese moccasin, Japanese pit viper, Qichun snake, Salmusa or Japanese mamushi, is a venomous pit viper species found in Japan. It was once considered to have 4 subspecies, but it is now considered monotypic.
Venomous fish are species of fish which produce strong mixtures of toxins harmful to humans which they deliberately deliver by means of a bite, sting, or stab, resulting in an envenomation. As a contrast, poisonous fish also produce a strong toxin, but they do not bite, sting, or stab to deliver the toxin, instead being poisonous to eat because the human digestive system does not destroy the toxin they contain in their bodies. Venomous fish do not necessarily cause poisoning if they are eaten, as the digestive system often destroys the venom.
Taicatoxin (TCX) is a snake toxin that blocks voltage-dependent L-type calcium channels and small conductance Ca2+-activated K+ channels. The name taicatoxin (TAIpan + CAlcium + TOXIN) is derived from its natural source, the taipan snake, the site of its action, calcium channels, and from its function as a toxin. Taicatoxin was isolated from the venom of Australian taipan snake, Oxyuranus scutellatus scutellatus. TCX is a secreted protein, produced in the venom gland of the snake.
Calciseptine (CaS) is a natural neurotoxin isolated from the black mamba Dendroaspis p. polylepis venom. This toxin consists of 60 amino acids with four disulfide bonds. Calciseptine specifically blocks L-type calcium channels, but not other voltage-dependent Ca2+ channels such as N-type and T-type channels.
Taipoxin is a potent myo- and neurotoxin that was isolated from the venom of the coastal taipan Oxyuranus scutellatus or also known as the common taipan. Taipoxin like many other pre-synaptic neurotoxins are phospholipase A2 (PLA2) toxins, which inhibit/complete block the release of the motor transmitter acetylcholine and lead to death by paralysis of the respiratory muscles (asphyxia). It is the most lethal neurotoxin isolated from any snake venom to date.
β-Bungarotoxin is a form of bungarotoxin that is fairly common in Krait venoms. It is the prototypic class of snake β-neurotoxins. There are at least five isoforms, coded β1 to β5, assembled from different combinations of A and Bchains.
The high-affinity choline transporter (ChT) also known as solute carrier family 5 member 7 is a protein in humans that is encoded by the SLC5A7 gene. It is a cell membrane transporter and carries choline into acetylcholine-synthesizing neurons.
Varespladib is an inhibitor of the IIa, V, and X isoforms of secretory phospholipase A2 (sPLA2). The molecule acts as an anti-inflammatory agent by disrupting the first step of the arachidonic acid pathway of inflammation. From 2006 to 2012, varespladib was under active investigation by Anthera Pharmaceuticals as a potential therapy for several inflammatory diseases, including acute coronary syndrome and acute chest syndrome. The trial was halted in March 2012 due to inadequate efficacy. The selective sPLA2 inhibitor varespladib (IC50 value 0.009 μM in chromogenic assay, mole fraction 7.3X10-6) was studied in the VISTA-16 randomized clinical trial (clinicaltrials.gov Identifier: NCT01130246) and the results were published in 2014. The sPLA2 inhibition by varespladib in this setting seemed to be potentially harmful, and thus not a useful strategy for reducing adverse cardiovascular outcomes from acute coronary syndrome. Since 2016, scientific research has focused on the use of Varespladib as an inhibitor of snake venom toxins using various types of in vitro and in vivo models. Varespladib showed a significant inhibitory effect to snake venom PLA2 which makes it a potential first-line drug candidate in snakebite envenomation therapy. In 2019, the U.S. Food and Drug Administration (FDA) granted varespladib orphan drug status for its potential to treat snakebite.
Micrurus lemniscatus, commonly known as the South American coral snake, is a species of venomous snake in the family Elapidae. The species is endemic to South America.
α-Neurotoxins are a group of neurotoxic peptides found in the venom of snakes in the families Elapidae and Hydrophiidae. They can cause paralysis, respiratory failure, and death. Members of the three-finger toxin protein family, they are antagonists of post-synaptic nicotinic acetylcholine receptors (nAChRs) in the neuromuscular synapse that bind competitively and irreversibly, preventing synaptic acetylcholine (ACh) from opening the ion channel. Over 100 α-neurotoxins have been identified and sequenced.
Three-finger toxins are a protein superfamily of small toxin proteins found in the venom of snakes. Three-finger toxins are in turn members of a larger superfamily of three-finger protein domains which includes non-toxic proteins that share a similar protein fold. The group is named for its common structure consisting of three beta strand loops connected to a central core containing four conserved disulfide bonds. The 3FP protein domain has no enzymatic activity and is typically between 60-74 amino acid residues long. Despite their conserved structure, three-finger toxin proteins have a wide range of pharmacological effects. Most members of the family are neurotoxins that act on cholinergic intercellular signaling; the alpha-neurotoxin family interacts with muscle nicotinic acetylcholine receptors (nAChRs), the kappa-bungarotoxin family with neuronal nAChRs, and muscarinic toxins with muscarinic acetylcholine receptors (mAChRs).
Mipartoxin-I is a neurotoxin produced by Micrurus mipartitus, a venomous coral snake distributed in Central and South America. This toxin causes a neuromuscular blockade by blocking the nicotinic acetylcholine receptor. It is the most abundant component in the venom.
Crotoxin (CTX) is the main toxic compound in the snake venom of the South American rattlesnake, Crotalus durissus terrificus. Crotoxin is a heterodimeric beta-neurotoxin, composed of an acidic, non-toxic and non-enzymatic subunit (CA), and a basic, weakly toxic, phospholipase A2 protein (CB). This neurotoxin causes paralysis by both pre- and postsynaptic blocking of acetylcholine signalling.
MiDCA1, short for Micrurus dumerili carinicauda 1, is a β-neurotoxin primarily affecting presynaptic synapses, where it interferes with the release of neurotransmitters by inhibiting potassium (K+) channels. This toxin belongs to the phospholipase A2 (PLA2) family but distinguishes itself by existing as a monomer, unlike some other PLA2 toxins. It occurs naturally in the venom of the coral snake Micrurus dumerili carinicauda.
