Stichodactyla toxin

Stichodactyla toxin (ShK) is a peptide toxin that blocks the voltage dependent potassium channels Kv1.1, Kv1.3 and Kv3.2.

Source

The stichodactyla toxin stems from the sea anemone "Stichodactyla helianthus". The "Stichodactyla helianthus" is a large, green, sessile, carpet-like sea anemone, from the Caribbean. It lives in shallow areas with mild to strong currents, and associates with clown fish. It is believed that it excretes toxins mainly to protect itself from the spiny lobster.Norton, R.S., Pennington, M.W., Wulff, H. (2004). Potassium channel blockade by the sea anemone toxin ShK for the treatment of multiple sclerosis and other autoimmune diseases. Current Medicinal Chemistry, 11, 3041-3052.]

"Helianthus" stems from the Greek words ἡλιος (meaning sun), and ἀνθος, meaning flower. Therefore, "S. helianthus" is also named the sun anemone.

Chemistry

ShK is a 35-residue basic peptide. It is cross-linked by three disulfide bridges: Cys3-Cys35, Cys12-Cys28, and Cys17-Cys32.Pennington, M.W., Lanigan, M.D.,Kalman, K., Mahnir, V.M., Rauer, H., McVaugh, S.T., Behm, D., Donaldson, D., Chandy, K.G., Kem, W.R., Norton, R.S. (1999) Role of Disulfide Bonds in the Structure and Potassium Channel Blocking Activity of ShK Toxin. Biochemistry, 38, 14549-14558] The amino acid sequence of the ShK toxin is Arg-Ser-Cys-Ile-Asp-Thr-Ile-Pro-Lys-Ser-Arg-Cys-Thr-Ala-Phe-Gln-Cys-Lys-His-Ser-Met-Lys-Tyr-Arg-Leu-Ser-Phe-Cys-Arg-Lys-Thr-Cys-Gly-Thr-Cys.Castaneda, O., Sotolongo, V., Amor, A.M., Stocklin, R., Anderson, A.M., Harvey, A.L., Engstrom, A., Wernstedt, C., Karlsson, E. (1995) Characterization of a potassium channel toxin from the caribbean sea anemone stichodactyla helianthus. Toxicon, 33, 603-613]

Target

ShK toxin blocks the K⁺ channels Kv1.1, Kv1.3 and Kv3.2Middleton, R.E., Sanchez, M., Linde, A., Bugianesi, R.M., Dai, G., Felix, J.P., Kporak, S.L., Staruch, M.J., Bruguera, M., Cox, R., Ghosh, A., Hwang, J., Jones, S., Kohler, M., Slaugter, R.S., McManus, O.B., Kaczorowski, G,J., Garcia M.L. (2003). Substitutions of a single residue in stichodactyla helianthus peptide, ShK-Dap22 reveals a novel pharmacological profile. Biochemistry, 42, 13698-13707] Yan, L., Herrington, J., Goldberg, E., Dulski, P.M., Bugianesi, R.M., Slaughter, R.S., Banerjee, P., Brochu, R.M., Priest, B.T., Kaczorowski, G..J., Rudy, B., Garcia, M.L. (2005). Stichodactyla helianthus peptide, a pharmacological tool for studying Kv3.2 channels. Molecular Pharmacology, 67, 1513–1521] . It binds to a shallow vestibule at the outer entrance of the ion conduction pathway. This blocks the entrance to the pore. As ShK toxin binds to the synaptosomal membranes, it facilitates an acetylcholine release at avian neuromuscular junctions.. The Kv1.1 and Kv1.3 channels are blocked by ShK at IC50 values of ~1 pM, which is a very low concentration, as compared to the concentration required to inhibit the Kv3.2 (which needs a concentration of about a 1000 times higher). The Kv1.3 channels set the resting potential of human T-lymphocytes. Inhibition of these channels causes the cell to depolarize, which results in an increase of intracellular calcium levels. These act to stimulate T cell receptors, which is important for T cell proliferation. Furthermore, Kv1.3 channels are upregulated in activated human effector memory cells while the Kv3.2 channels are expressed in neurons that fire at a high frequency (such as cortical GABAergic interneurons), due to their fast activation and deactivation rates. By blocking Kv3.2, ShK toxin depolarises the cortical GABAergic interneurons. Kv3.2 is also expressed in pancreatic beta cells. These cells are thought to play a role in their delayed-rectifier current, which regulates glucose-dependent firing. Therefore, ShK, as a Kv3.2 blocker, might be useful in the treatment of type-2 diabetes, although inhibition of the delayed-rectifier current has not yet been observed in human cells even when very high ShK concentrations were used.

Toxicity

Toxicity of ShK toxin in mice is quite low. The median paralytic dose is about 25mg/kg bodyweight (which translates to 0.5 mg per 20g mouse). ShK-Dap22 is less toxic, even a dose of 1.0 mg dose did not cause hyperactivity, seizures or mortality. The median paralytic dose was 200 mg/kg body weight.Kalman, K., Pennington, M.W., Lanigan, M.D., Nguyen, A., Rauer, H., Mahniri, V., Paschetto, K., Kemi, W.R., Grissmer, S., Gutman, G.A., Christian, E.P., Cahalan, M.D., Norton, R.S., Chandy, K.G. (1998) ShK-Dap22, a Potent Kv1.3-specific Immunosuppressive Polypeptide. The Journal of Biological Bhemistry, 49, 32697–32707]

Use

Because ShK toxin inhibits very specific potassium channels, it may serve as a useful pharmacological tool for studying these channels. Furthermore, patients with multiple sclerosis (MS) have higher levels of Kv1.3 channels in activated, myelin-reactive T cells, compared to naive or central memory T cells. Since Kv1.3 inhibitors, like the ShK toxin, suppress proliferation of T cells, this toxin might serve as a useful tool for studying MS. Shk-Dap22, in which Lys22 is replaced with a positively charged, non natural amino acid diaminopropionic acid, works as a highly selective and very potent blocker of T-lymphocyte Kv1.3 channels. This means that ShK-Dap22 might serve as an immunosuppressant in the treatment of autoimmune diseases.. Furthermore, because pancreatic beta cells, which have Kv3.2 channels, are thought to play a role in glucose-dependent firing, ShK, as a Kv3.2 blocker, might be useful in the treatment of type-2 diabetes, although inhibition of the delayed-rectifier current has not yet been observed in human cells even when very high ShK concentrations were used.

References