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Calitoxin, also known as CLX, is a sea anemone toxin which is likely to affect voltage-gated sodium channels in invertebrates. Two different toxins, namely CLX-1 and CLX-2, can be distinguished. It targets invertebrates, among other crabs and octopuses.

Etymology

‘’Calitoxin’’(CLX), derives its name from the sea anemone Calliactis parasitica.

Sources

Calitoxin is a highly potent neurotoxin, which can be found in the nematocysts, organelles in stinging cells, of the Calliactis parasitica. This sea anemone is a species from the Hormathiidae family and is present along the European coasts of the Atlantic Ocean and in the Mediterranean Sea.

Structure and chemistry

The formula for calitoxin is C₂₀₃H₃₀₅N₅₅O₇₂S₇. The amino acid sequence is markedly dissimilar from other known sea anemones toxins. Two genes coding for two highly homologous calitoxins are discovered and analyzed, namely CLX-1 and CLX-2, both consisting of 46 amino acids and originating from a precursor peptide of 79 amino acids where the C-terminus determines whether it will be the mature CLX-1 or CLX-2. In the mature CLX, one base-pair substitution is responsible for a single Glu to Lys replacement in the coding region of CLX-2, leading to the difference between the two peptides. The structural organization of these two genes show a high degree of homology. This might imply that the two different peptides have the same biological function. This cannot yet be confirmed because only CLX-1 has been isolated from Calliactis parasitca.

Target and mode of action

The precise action of calitoxin has not been clarified yet. Calitoxin may influence the activity of voltage-gated sodium channels in motor neurons by inhibiting their inactivation. This leads to prolongation of presynaptic action potentials and thus massive neurotransmitter release in the nerve terminals, which in turn causes a strong muscle contraction. Experimental evidence comes from an experiment with tetrodotoxin (TTX), a blocker of voltage-gated sodium channels. In the presence of TTX in the extracellular medium, no action potentials are recorded in the postsynaptic area of the muscular cells. It seems that in the presence of TTX, calitoxin is not able to exert a postsynaptic effect. Because it is known that TTX engages on the presynaptic voltage-gated sodium channels, it is likely that calitoxin influences the presynaptic voltage-gated sodium channels as well by inhibiting their inactivation. In short, calitoxin seems to influence the presynaptic motorneurons resulting in a strong postsynaptic motor respons. These muscle contractions create a constant maximal contraction in the muscles, which causes paralysis.

Function in nature

Sea anemones produce toxins, such as Calitoxin, in their stinging cells (cnidocytes). These cells contain organnelles called nematocysts. When triggered, an envenomation response occurs. This can result in injury to target organisms, including capture of prey, defense against predatory organisms, or against agressors from within their own species. In a natural setting, Calliactis parasitica establishes a mutualistic relationship with the hermit crab Pagurus bernhardus. The sea anemone identifies shells inhabited by the hermit crab and attaches. Through stings against potential predators, C. parasitica provides protection for the hermit crab. In return, the sea anemone gains an advantage in accessing a broader distribution of food sources, as the crab moves across the ocean floor. Octopuses will avoid shells bearing C. parasitica.

Toxicity

Calitoxin has been tested for activity on the crab Carcinus mediterraneus, by injecting a 0.1 mL solution with 0.2 µg lyophilized CLX of into the hemocoel. This revealed that the toxic unit corresponds to this minimum amount of the toxin inducing muscle contractions in the crab causing paralysis within 1 minute from injection. The LD₅₀ is unknown.

References

1. ^ Cariello, L; de Santis, A; Fiore, F; Piccoli, R; Spagnuolo, A; Zanetti, L; Parente, A (21 Mar 1989). "Calitoxin, a neurotoxic peptide from the sea anemone Calliactis parasitica: amino acid sequence and electrophysiological properties". Biochemistry. 28 (6): 2484–9. PMID 2567180. {{cite journal}}: |access-date= requires |url= (help)
2. ^ Spagnuolo, Antonietta; Zanetti, Laura; Cariello, Lucio; Piccoli, Renata. "Isolation and characterization of two genes encoding calitoxins, neurotoxic peptides from Calliactis parasitica (Cnidaria)". Gene. 138 (1–2): 187–191. doi:10.1016/0378-1119(94)90805-2.
3. Kastin, edited by Abba J. (2006). Handbook of biologically active peptides. Amsterdam: Academic Press. pp. 363–364. ISBN 0123694426. Retrieved 13 October 2014. {{cite book}}: |first1= has generic name (help)
4. John Fish & Susan Fish (2011). "Calliactis parasitica (Couch)". A Student's Guide to the Seashore (3rd ed.). Cambridge University Press. p. 96. ISBN 978-0-521-72059-5.
5. Roger T. Hanlon & John B. Messenger (1998). "Learning and the development of behaviour". Cephalopod Behaviour. Cambridge University Press. pp. 132–148. ISBN 978-0-521-64583-6.

• Invertebrate toxins
• Ion channel toxins