User:Bri-Sch/Cll1

Toxin Cll1 is present in the venom of the Mexican scorpion (Centruroides limpidus limpidus) and by binding to neurotoxin binding site 4 of specific isoforms of sodium channels, it drives the threshold for voltage dependant activation of the sodium channel toward more negative membrane potential. This results in the sodium channels being easier to activate, to such an extent that this may even induce spontaneous activation.

Etymology and Source
The toxin Cll1 is named after its producing species, the Centruroides Limpidus Limpidus. Despite the fact that many different species of scorpions are endemic in Mexico, only the eight subspecies - all belonging to the Centruroides familie - are dangerous to humans. One of these subspecies is the Centruroides Limpidus Limpidus. Along with Cll1, multiple toxins are excreted in the venom by this scorpion.

Chemistry
Cll1 is a long chain neuropeptide belonging to the scorpion toxin superfamily. Within this superfamily, it is a member of the sodium channel inhibitors. More specifically, Cll1 is classified as a member of the beta-toxin subfamily based on its way of influencing the sodium channels. The global secondary structure of Cll1 is very similar to that of other scorpion beta-toxins. Important similar components of the protein are the alpha-helix and triple stranded antiparallel beta-sheet. Additionally, a third important similar feature in structure determination is the four disulfide bridges between the amino acid residues. The latter in particular is typical for beta-scorpion toxins. For these structural features are well preserved throughout the beta-scorpion toxins, it is suggested they are important in the interaction with the targeted sodium channels. Despite all similarities, some unusual amino acid residues are present on the Cll1 peptide, one of which being Trp18 (Tryptophan, at 18th amino acid location). Trp18 is a hydrophobic residue and it can be found at the surface of the molecule. This location is quite unusual for a hydrophobic residue. In mammalian directed toxins, it is more common for a hydrophilic residue to be present at this location, whereas in crustacean or insect directed peptides the residues on the membrane are mainly hydrophobic. The fact that Cll1 has hydrophobic residues present on its surface explains why Cll1 is rather a crustacean directed toxin than a mammalian one.

Target
Cll1 targets, like the classical scorpion beta-toxin, the voltages gated sodium channels (Nav). Voltage gated sodium channels are build up of 4 subunits, each consisting of 6 transmembrane segments (S1-S6). The beta-toxins bind to the extracellular end of the S4 at the loop between the 3th and 4th segment of the second domain. S4 functions as a voltage sensor and by binding this segment it alters the voltage dependent opening of the channel.

Mode of action
Cll1 roughly influences three intrinsic properties of the targeted sodium channel. Each of these are discussed below separately.

Voltage dependent activation
First of all, Cll1 has an effect on the voltage dependent activation. As previously mentioned, Cll1 binds to S4 of the voltage gated sodium channels. Binding to this segment causes a shift in the threshold of activation of the sodium channel towards a more negative membrane potential. This means that the amount of depolarisation which is required to open the sodium channel is less in presence of Cll1 compared to when Cll1 is absent. Therefore the channel opens faster when Cll1 is bound. Seven different isoforms of the voltage gated sodium channels (Nav1.1-Nav1.7) have been studied in the presence of Cll1. In almost all of these seven isoforms, Cll1 was found to have an effect on the voltage dependent activation. It has only a minor effect on the Nav1.1-1.4 and Nav1.7 channels, but a much larger effect was shown on the isoform Nav1.6 of this sodium channel.

Peak current
Secondly, Cll1 causes a reduction of the peak current when bound to the voltage activated sodium channels. This effect was present in all but one of the seven tested isoforms (Nav1.1-Nav1.6). The only isoform that showed no reduction in peak current was Nav1.7.

Resurgent current
Thirdly, it has been shown that Cll1 has the ability to induce resurgent current. This effect has also been demonstrated for other beta-scorpion toxins. The resurgent current is the strongest in Nav1.6, but it is also present in other isoforms of the voltage activated sodium channels. The effect on the other isoform of the Nav channels, however, is very limited.

Toxicity and Treatment
The LD50 of the Cll1 toxin in mice is 85 μg/kg.

A possible treatment of an intoxication with Cll1 toxin is the use of single chain variable fragments (scFv). It was found that scFv can lower toxicity of one LD50 dose of Cll1 in mice.

Other possible treatments find their origin in traditional Mexican medicine. Several herbs used in traditional Mexican medicine have been proven to be effective in treating an intoxication from the whole venom from the C. limpidus limpidus in mice. Research has shown that, when administering Bouvardia ternifolia dissolved in haxane extract or in methanol extract simultaneously with the venom of the C. limpidus limpidus, the LD50 significantly increased. The antitoxic effect of Aristolochia elegans was lower than for B. ternifolia and it only resulted in a significant increase in LD50 when dissolved in methanol. Vitex mollis was also tested, but it did not have a significant antitoxic effect. Off course these herbs have all been tested for the whole venom of the C. Limpidus limpidus, including the other toxic components, and therefore the results cannot directly be extrapolated to the effect it would have on an intoxication with Cll1 as a single component.