| Scorpion short toxin | |||||||||
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| Identifiers | |||||||||
| Symbol | Toxin_2 | ||||||||
| Pfam | PF00451 | ||||||||
| Pfam clan | CL0054 | ||||||||
| InterPro | IPR001947 | ||||||||
| PROSITE | PDOC00875 | ||||||||
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Margatoxin (MgTX) is a peptide that selectively inhibits Kv1.3 voltage-dependent potassium channels. It is found in the venom of Centruroides margaritatus , also known as the Central American Bark Scorpion. Margatoxin was first discovered in 1993. It was purified from scorpion venom and its amino acid sequence was determined.
Margatoxin is a peptide of 39 amino acids with a molecular weight of 4185 Dalton. The primary amino acid sequence of margatoxin is as follows:
Or, when translated to one-letter sequence,
There are disulfide bridges between Cys7-Cys29, Cys13-Cys34 and Cys17-Cys36.
Margatoxin is classified as a "scorpion short toxin" by Pfam, showing sequence homology with other potassium channel blockers, such as charybdotoxin (44%), kaliotoxin (54%), iberiotoxin (41%) and noxiustoxin (79%), which are also derived from scorpion venom. [2]
Margatoxin is a peptide originally purified from the venom of the scorpion Centrutoides margaritatus (Central American Bark Scorpion). Scorpion toxins are specific and have a high affinity for their targets, and this makes them good tools to characterize various receptor proteins involved in ion channel functioning. Because only low amounts of natural toxins can be isolated from scorpion venoms, a chemical synthesis approach has been utilised to produce sufficient protein for research. This approach is not only produces enough material to study the effects on potassium channels but ensures purity as toxin isolated from the scorpion venom risks contamination by other active compounds. [3]
Margatoxin can be chemically synthesized using the solid phase synthesis technique. The compound gained by this technique was compared with the natural, purified margatoxin. Both compounds had the same physical and biological properties. The chemically synthesized margatoxin is now used to study the role of Kv1.3 channels. [2]
Margatoxin blocks potassium channels Kv1.1 Kv1.2 en Kv1.3. Kv1.2 channel regulates neurotransmitter release associated with heart rate, insulin secretion, neuronal excitability, epithelial electrolyte transport, smooth muscle contraction, immunological response and cell volume. Kv1.3 channels are expressed in T and B lymphocytes. [4] Margatoxin irreversibly inhibits the proliferation of human T-cells in a concentration of 20 μM. At lower concentrations, this inhibition is reversible.
Margatoxin significantly reduces outward currents of Kv1.3 channels and depolarized resting membrane potential. It increases the time necessary to conduct action potentials in the cell in response to a stimulus. Acetylcholine (ACh) plays a key role in activation of nicotinic and muscarinic ACh-receptors. Margatoxin influences nicotinic ACh-receptor agonist-induced norepinephrine release. Upon activation of muscarinic ACh receptors with bethanechol, margatoxin-sensitive current was suppressed. Therefore, it was concluded that Kv1.3 affects the function of postganglionic sympathetic neurons, so one could suggest that Kv1.3 influences sympathetic control of cardiovascular function. [5]
Kv1.3-channels can be found in various cells, including T-lymphocytes and macrophages. To activate an immune response a T-lymphocyte has to come into contact with a macrophage. The macrophage can then produce cytokines, such as IL-1, IL-6, and TNF-α. Cytokines are cell signaling molecules that can enhance the immune response. Kv1.3-channels are important for the activation of T-lymphocytes, and thus for the activation of macrophages. The disturbance of the function of Kv1.3-channels, for example due to inhibition of these channels, will lower the cytokines production and lymphocyte proliferation in vitro. This would lead to immune response suppression in vivo.
Kv channels are regulated during proliferation and regulation of macrophages and their activity is important during cell responses. In contrast to leukocytes which have monomeric Kv1.3 channels, macrophages have heterotetrameric Kv1.3/Kv1.5 channels. These heterotetramers plays a role in regulating the membrane potential of macrophages on different stages of macrophage activation by lymphocytes. Potassium channels are involved in leukocyte activation by calcium. The possible different conformations of these Kv1.3 and 1.5 complexes can affect the immune response. Margatoxin inhibits Kv1.3 channels, so no heterodimers can be formed. The effect of margatoxin is similar to the effect of DEX. DEX diminishes amount of K1.3 channels by binding to GC receptor, which leads to downregulating of expression of K1.3 channels. Both margatoxin and DEX lead to immune suppression. [6]
Ion channels play a key role in lymphocyte signal transduction. Potassium channels are required for the activation of T-cells. Pharmacological inhibition of Potassium channels can be useful in the treatment of immune diseases. The membrane potential exerts powerful effects on the lymphocyte activation. The resting potential results primarily from a potassium-diffusion potential contributed by potassium channels. Margatoxin depolarizes resting human T cells. Pharmacological studies suggest that functional potassium channels are required in the activation of T- and B-cells. KV channel blockers inhibit activation, gene expression, killing by cytotoxic T cells and NK cells, lymphokine secretion and proliferation. Margatoxin blocks mitogen-induced proliferation, the mixed lymphocyte response and the secretion of Interleukin-2 and interferon-gamma (IFN-γ). This provides the strongest available evidence for a role of KV channels in mitogenesis. [7]
Margatoxin can have several different effects on the body: [8]
The chronic effects target the heart, nerves, lungs, skeleton and muscles.
The median lethal dose (LD50) of margatoxin is 59.9 mg/kg, so Centruroides margaritatus stings are not dangerous to humans except as a result of possible anaphylactic responses. They do cause pain, local swelling, and tingling for 3–4 hours, but no intervention beyond symptomatic relief should be necessary.
Margatoxin leads to the depolarization of human and pig cells in vitro. [9] By blocking 99% of the KV1.3-channels, margatoxin inhibits the proliferation response of T-cells in mini-swine. Furthermore, it suppresses a B-cell response to allogenic immunization and inhibits the delayed-type hypersensitivity reaction to tuberculin. [9] In pigs, the protein's half-life is two hours. When the peptide is continuously infused, it leads to diarrhea and hypersalivation. [10] However, no major toxic effects are observed in animals. In contrast to when the plasma concentration of margatoxin is higher than 10nM, the transient hyperactivity occurs in pigs. It might be an effect of Kv1.1 and Kv1.2 channels in the brain.
Kv1.3 is already linked with proliferation of lymphocytes, vascular smooth cells, oligodendrocytes and cancer cells. Recent studies[ when? ] have shown that there is therapeutical potential for Kv1.3-blockers such as Margatoxin.
In a minipig treatment a study with margatoxin has been conducted. An eight-day treatment led to a prolonged immune suppression that lasted three to four weeks after termination of dosing. Thymic atrophy (reduced thymus) was observed. Especially the cells in the cortical region had decreased in number [9]
Neointimal Hyperplasia is the movement and proliferation of smooth muscle cells into the luminal area of a blood vessel. This generates a new inner structure that can block blood flow. This is commonly seen to cause failure of interventional clinical procedures that include placement of stents and bypass grafts.
Due to changes in potassium channel type the vascular smooth muscle cells switch from the contractile to proliferating phenotype. It is suggested that Kv1.3 is important in proliferating vascular smooth muscle cells. Inhibitors of such channels suppress vascular smooth muscle proliferation, stenosis following injury, and neointimal hyperplasia. Studies shows that margatoxin is a high potency inhibitor of vascular cell migration, with an IC50 (half maximal inhibitory concentration) of 85 pM. In this study, a negative effect was also found. There have been vasoconstrictor effects observed in some arteries, but elevated blood pressure has not appeared as a significant concern. [5]
Slotoxin is a peptide from Centruroides noxius Hoffmann scorpion venom. It belongs to the short scorpion toxin superfamily.
Maurotoxin is a peptide toxin from the venom of the Tunisian chactoid scorpion Scorpio maurus palmatus, from which it was first isolated and from which the chemical gets its name. It acts by blocking several types of voltage-gated potassium channel.
Potassium voltage-gated channel, shaker-related subfamily, member 3, also known as KCNA3 or Kv1.3, is a protein that in humans is encoded by the KCNA3 gene.
Cobatoxin is a toxin present in the venom of the scorpion Centruroides noxius. It blocks two potassium channel subtypes; voltage-gated and calcium-activated channels.
Stichodactyla toxin is a 35-residue basic peptide from the sea anemone Stichodactyla helianthus that blocks a number of potassium channels. Related peptides form a conserved family of protein domains known as the ShkT domain. Another well-studied toxin of the family is BgK from Bunodosoma granulifera.
Kaliotoxin (KTX) inhibits potassium flux through the Kv1.3 voltage-gated potassium channel and calcium-activated potassium channels by physically blocking the channel-entrance and inducing a conformational change in the K+-selectivity filter of the channel.
BmTx3 is a neurotoxin, which is a component of the venom of the scorpion Buthus Martensi Karsch. It blocks A-type potassium channels in the central nervous system and hERG-channels in the heart.
Discrepin (α-KTx15.6) is a peptide from the venom of the Venezuelan scorpion Tityus discrepans. It acts as a neurotoxin by irreversibly blocking A-type voltage-dependent K+-channels.
BeKm-1 is a toxin from the Central Asian scorpion Buthus eupeus. BeKm-1 acts by selectively inhibiting the human Ether-à-go-go Related Gene (hERG) channels, which are voltage gated potassium ion channels.
Guangxitoxin, also known as GxTX, is a peptide toxin found in the venom of the tarantula Plesiophrictus guangxiensis. It primarily inhibits outward voltage-gated Kv2.1 potassium channel currents, which are prominently expressed in pancreatic β-cells, thus increasing insulin secretion.
Hanatoxin is a toxin found in the venom of the Grammostola spatulata tarantula. The toxin is mostly known for inhibiting the activation of voltage-gated potassium channels, most specifically Kv4.2 and Kv2.1, by raising its activation threshold.
Anuroctoxin is a peptide from the venom of the Mexican scorpion Anuroctonus phaiodactylus. This neurotoxin belongs to the alpha family of potassium channel acting peptides. It is a high-affinity blocker of Kv1.3 channels.
Butantoxin (BuTX) is a compound of the venom of three Brazilian and an Argentinean scorpion species of the genus Tityus. Butantoxin reversibly blocks the voltage-gated K+ channels Shaker B and Kv1.2, and the Ca2+-activated K+ channelsKCa 1.1 and KCa 3.1.
Ergtoxin is a family of toxins that can be isolated from the venom of several members of the Mexican scorpion genus of Centruroides. These toxins target hERG potassium channels.
Pi3 toxin is a purified peptide derivative of the Pandinus imperator scorpion venom. It is a potent blocker of voltage-gated potassium channel, Kv1.3 and is closely related to another peptide found in the venom, Pi2.
Spinoxin is a 34-residue peptide neurotoxin isolated from the venom of the Malaysian black scorpion Heterometrus spinifer. It is part of the α-KTx6 subfamily and exerts its effects by inhibiting voltage-gated potassium channels, specifically Kv1.2 and Kv1.3.
HgeTx1 (systematic name: α-KTx 6.14) is a toxin produced by the Mexican scorpion Hoffmanihadrurus gertschi that is a reversible blocker of the Shaker B K+-channel, a type of voltage-gated potassium channels.
ImKTx88 is a selective inhibitor of the Kv1 ion channel family that can be isolated from the venom of the Isometrus maculatus. This peptide belongs to the α-KTx subfamily and is classified as a pore-blocking toxin.
BmP02, also known as α-KTx 9.1 or Bmkk(6), is a toxin from the Buthus Martensi Karsch (BmK) scorpion. The toxin acts on potassium channels, blocking Kv1.3 and slowing the deactivation of Kv4.2. BmP02 is not toxic to humans or mice.
OdK2 is a toxin found in the venom of the Iranian scorpion Odonthobuthus doriae. It belongs to the α-KTx family, and selectively blocks the voltage-gated potassium channel Kv1.3 (KCNA3).
