Cl6a

{{Short description|Toxin}} '''μ-THTX-Cl6a''', also known as '''Cl6a''', is a 33-residue peptide toxin extracted from the venom of the spider ''Cyriopagopus longipes''. The toxin acts as an inhibitor of the tetrodotoxin-sensitive (TTX-S) voltage-gated sodium channel (Na_V1.7), thereby causing sustained reduction of Na_V1.7 currents.

{| class="wikitable "wikitable"" |+ '''Cl6a''' |'''Organism''' |''Cyriopagopus longipepes'' |- |'''Super family''' |Spider peptide toxins |- |'''Family''' |NaSpTx family 1 |- |'''Amino acid sequence''' |ACKGVFDPCTPGKNECCPNRVCSDKHKWCKWKI |- |'''Molecular weight''' |3775.6 Da<ref name="Zhang2020">Zhang, Q., Si, Y., Yang, L., Wang, L., Peng, S., Chen, Y., Chen, M., Zhou, X., & Liu, Z. (2020). Two Novel Peptide Toxins from the Spider Cyriopagopus longipes Inhibit Tetrodotoxin-Sensitive Sodium Channels. Toxins, 12(9), 529. DOI:[https://doi.org/10.3390/toxins12090529].</ref> |- |}

== Etymology and Source ==

Cl6a is a peptide extracted from the venom of the ''Cyriopagopus longipes'',<ref name="Zhang2020" /> which was first described by von Wirth and Striffler in 2005.<ref name="VonWirth2005"> Wirth, V. von & Striffler, B. F. (2005). Neue Erkenntnisse zur Vogelspinnen - Unterfamilie Ornithoctoninae, mit Beschreibung von Ornithoctonus aureotibialis sp. n. und Haplopelma longipes sp. n. (Araneae, Theraphosidae). Arthropoda 13(2): 2-27.</ref> This spider lives in multiple Southeast Asian countries such as Thailand, Cambodia and Laos. ''Cyriopagopus longipes'' originates from the ''Cyriopagopus'', which is a genus of the southeast Asian tarantula.<ref name="NaturalHistory2019">Natural History Museum Bern. (2019). NMBE - World Spider Catalog. Nmbe.ch.[https://wsc.nmbe.ch/].</ref>

== Chemistry ==

=== Structure ===

Cl6a is a 33 amino acid residue peptide toxin with a molecular weight of 3775.6 Dalton. Its molecular structure encompasses six cysteine residues, which demonstrate three disulfide bonds that assemble an inhibitor cystine knot (ICK) scaffold.<ref name="Zhang2020" /> Generally, ICK peptides are most prominently present in the venom of snails and spiders.<ref name="Zhu2003"> Zhu, S., Darbon, H., Dyason, K., Verdonck, F., & Tytgat, J. (2003). Evolutionary origin of inhibitor cystine knot peptides. The FASEB Journal, 17(12), 1765–1767.DOI:[https://doi.org/10.1096/fj.02-1044fje].</ref> These peptides usually function as gating modifier toxins affecting the gating and kinetic properties of voltage-gated ion channels.<ref name="Zhang2020" /> The ICK fold is characterized by two β-strands composing the polypeptide backbone from which two disulfide bonds originate.<ref name="Saez2010"> Saez, N. J., Senff, S., Jensen, J. E., Er, S. Y., Herzig, V., Rash, L. D., & King, G. F. (2010). Spider-Venom Peptides as Therapeutics. Toxins, 2(12), 2851–2871.DOI:[https://doi.org/10.3390/toxins2122851].</ref> These disulfide bonds and the polypeptide backbone form a ring structure, which is innervated by a third sulfide bond creating a pseudoknot.<ref name="Craik2001"> Craik, D. J., Daly, N. L., & Waine, C. (2001). The cystine knot motif in toxins and implications for drug design. Toxicon, 39(1), 43–60. DOI:[https://doi.org/10.1016/s0041-0101(00)00160-4].</ref><ref name="Saez2010" /> This comprises an extraordinarily stable protein structure, which is resistant to heat denaturation, extreme pH environments and proteolysis.<ref name="Saez2010" /><ref name="Zhang2018">Zhang, Y., Yang, Q., Zhang, Q., Peng, D., Chen, M., Liang, S., Zhou, X., & Liu, Z. (2018). Engineering Gain-of-Function Analogues of the Spider Venom Peptide HNTX-I, A Potent Blocker of the hNaV1.7 Sodium Channel. Toxins, 10(9), 358. DOI:[https://doi.org/10.3390/toxins10090358].</ref>

=== Family and homology ===

Cl6a belongs to the voltage-gated sodium channel (Na_V)-targeting spider toxin (NaSpTx) family 1,<ref name="Zhang2018" /> because it accommodates the same cysteine structure and ICK scaffold.<ref name="Zhang2020" /> Toxins of NaSpTx family 1 are characterized by containing the following motif present in the amino acid sequence of the toxins: (R/K)X(R/K)WCK. The amino acid sequence of Cl6a contains the NaSpTx family 1 motif KHKWCK. Cl6a highly resembles a similar sequence with other spider peptide toxins. For instance, the amino acid sequence of Cl6a shows 67% sequence similarity with Hainantoxin (HNTX) III and 97% with huwentoxin (HWTX) I.<ref name="Zhang2020" />

== Target ==

Cl6a is a selective antagonist of voltage-gated sodium channels. It targets TTX-S channels (Na_V1.2, Na_V1.3, Na_V1.4, Na_V1.6 and Na_V1.7) with the highest affinity against Na_V1.7 channels.<ref name="Zhang2020" /> Na_V1.7 channels are expressed in nociceptive dorsal root ganglion (DRG), sympathetic and olfactory sensory neurons. Na_V1.7 channels are located in epidermal free nerve endings and more centrally in the superficial lamina of the spinal cord.

== Mode of action ==

Cl6a induces an irreversible inhibition of Na_V1.7 peak currents with a slow onset of action. Cl6a alters the current-voltage relationship of Na_V1.7 channels by binding to site 4 of domain II (DII S3-S4), which contains acidic residues.<ref name="Zhang2020" /> Cl6a acquires a positively charged surface that interacts with the acidic residues of site 4.<ref name="Craik2001" /> Subsequently, the DII S3-S4 voltage sensor becomes trapped. Hereby, the channel is less sensitive to changes in the membrane voltage. As a consequence, the inward sodium current is not initiated. <ref name="Zhang2020" /> Consequently, there will be a decrease in neuronal excitability.<ref name="Shields2018">Shields, S. D., Deng, L., Reese, R. M., Dourado, M., Tao, J., Foreman, O., Chang, J. H., & Hackos, D. H. (2018). Insensitivity to Pain upon Adult-Onset Deletion of Nav1.7 or Its Blockade with Selective Inhibitors. The Journal of Neuroscience, 38(47), 10180–10201. DOI:[https://doi.org/10.1523/jneurosci.1049-18.2018].</ref>

== Toxicity ==

Cl6a selectively blocks Na_V1.7 channels, which are involved in peripheral pain relief.<ref name="DibHajj2013"> Dib-Hajj, S. D., Yang, Y., Black, J. A., & Waxman, S. G. (2013). The Na V 1.7 sodium channel: from molecule to man. Nature Reviews Neuroscience, 14(1), 49–62. DOI:[https://doi.org/10.1038/nrn3404].</ref> The half-maximal inhibitory concentration (IC₅₀) of Cl6a is 11.00 ± 2.5 nM.<ref name="Zhang2020" /><ref name="Berrouet2020">Berrouet, C., Dorilas, N., Rejniak, K. A., & Tuncer, N. (2020). Comparison of Drug Inhibitory Effects (IC50) in Monolayer and Spheroid Cultures. Bulletin of Mathematical Biology, 82(6). DOI:[https://doi.org/10.1007/s11538-020-00746-7].</ref> Generally, spider peptide toxins can incapacitate prey and hereby promote successful predation.<ref name="Neff2021">Neff, R. A., & Wickenden, A. D. (2021). Selective Targeting of Nav1.7 with Engineered Spider Venom-Based Peptides. Channels, 15(1), 179–193. DOI:[https://doi.org/10.1080/19336950.2020.1860382].</ref> In combination with the irreversible properties of Cl6a,<ref name="Zhang2020" /> this spider peptide toxin could indirectly be lethal to its prey.

== Therapeutic use ==

Cl6a exhibits a high affinity against Na_V1.7 channels, which are known to be critically involved in peripheral pain regulation. Other spider peptide toxins like HNTX-III and GpTx1 cause reversible inhibition of Na_V1.7 channels. However, Cl6a induces irreversible inhibition of Na_V1.7 channel activity. Therefore, this spider peptide toxin is a potential therapeutic target by obtaining prolonged blockage of Na_V1.7 channels. Nonetheless, Cl6a also inhibits Na_V1.4 and Na_V1.5 currents, which are involved in skeletal and cardiac muscle functioning.<ref name="Zhang2020" />

Since Cl6a contains the ICK motif, which is interestingly resistant to proteases, this peptide toxin could have further therapeutic implications. ICK peptides are stable in the human body for multiple days and demonstrate half-lives larger than 12 hours in a simulated gastric environment. Altogether, this implies that the ICK motif of Cl6a could contribute to the delivery of certain therapeutic agents to a specific location in the human body. <ref name="Saez2010" />

== References == <references />

Category:Ion channel toxins Category:Neurotoxins Category:Spider toxins