Stichodactyla toxin

Structure

ShK is a 35-residue basic peptide first discovered in the sea anemone Stichodactyla helianthus by Professor Olga Castaneda from the University of Havana, Cuba, and her collaborators in Sweden. The formula is C₁₆₉H₂₇₄N₅₄O₄₈S₇. It is cross-linked by three disulfide bridges: Cys3-Cys35, Cys12-Cys28, and Cys17-Cys32 (see figure below). 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. ShK is stabilized by three disulfide bridges and consists of two short α-helices comprising residues 14-19 and 21-24. The N-terminal eight residues of ShK adopt an extended conformation, followed by a pair of interlocking turns that resemble a 3₁₀ helix, while its C-terminal Cys35 residue forms a nearly head-to-tail cyclic structure through a disulfide bond with Cys3. Protein domains with structural resemblance to ShK have been described in 688 proteins, most of them from C. elegans (InterPro: IPR003582). The SMART database at the EMBL has a list of 688 proteins containing 1315 ShK-like sequences (http://smart.embl-heidelberg.de). Other proteins containing domains with similar structures include the cysteine-rich secretory protein snake toxins natrin, triflin, and stecrisp, the Toxocara canis mucins, secreted peptides from the dog hookworm Ancylostoma caninum, and the human proteins Tpx-1 and matrix metalloprotease 23 (MMP23).

Target

ShK toxin blocks the K⁺ channels K_v1.1, K_v1.3, K_v1.6, K_v3.2 and K_Ca3.1, The peptide binds to all four subunits in the K_v1.3 tetramer through its interaction with the shallow vestibule at the outer entrance of the ion conduction pathway. The peptide's Lysine²² residue occludes the channel pore like a "cork in a bottle". This blocks the entrance to the pore.

ShK blocks the K_v1.3 channel in T cells with a K_d of about 11 pM. It blocks the neuronal K_v1.1 and K_v1.6 channels with K_ds of 16 pM and 200 pM respectively. The K_v3.2 and K_Ca3.1 channels are more than 1000 times less sensitive to the peptide.

Several ShK analogs have been generated to enhance specificity for the K_v1.3 channel over the K_v1.1, K_v1.6 and K_v3.2 channels. The first analog that showed some degree of specificity was ShK-Dap22. Attaching a fluorescein to the N-terminus of the peptide via a hydrophilic AEEA linker (2-aminoethoxy-2-ethoxy acetic acid; mini-PEG) resulted in a peptide, ShK-F6CA, with 100-fold specificity for K_v1.3 over K_v1.1 and related channels. Based on this surprising finding additional analogs were made. ShK-170 [a.k.a. ShK(L5)],contains a L-phosphotyrosine in place of the fluorescein in ShK-F6CA. It blocks K_v1.3 with a K_d of 69 pM and shows exquisite specificity for K_v1.3. However, it is chemically unstable. To improve stability a new analog, ShK-186 [a.k.a. SL5], was made with the C-terminal carboxyl of ShK-170 replaced by an amide; ShK-186 is otherwise identical to ShK-170. In rats and squirrel monkeys, an indium-labeled ShK-186 analog called ShK-221, was slowly released from the injection site and maintained blood levels above the channel blocking dose for 3–5 days ShK-192 is a new analog with increased stability. It contains norleucine²¹ in place of methionine²¹ to avoid methionine oxidation, and the terminal phosphotyrosine is replaced by a non-hydrolyzable para-phosphonophenylalanine (Ppa) group. ShK-192 is effective in ameliorating disease in rat models of multiple sclerosis. The D-diastereomer of ShK is also stable but blocks K_v1.3 with 2800-fold potency than the L-form (K_d = 36 nM) and it only exhibits 2-fold specificity for K_v1.3 over K_v1.1. ShK-K-amide is a new analog with a C-terminal lysine. It blocks Kv1.3 with roughly 50-fold greater potency (IC50 of 26 ± 3 pM) than Kv1.1 ( IC50 of 942 ± 120 pM), and suppresses proliferation of human T cells (IC50 ≈ 3 nM).

K_v1.3 and K_Ca3.1 regulate membrane potential and calcium signaling of T cells. Calcium entry through the CRAC channel is promoted by potassium efflux through the K_v1.3 and K_Ca3.1 potassium channels. Blockade of K_v1.3 channels in effector-memory T cells by ShK-186 suppresses calcium signaling, cytokine production (interferon-gamma, interleukin 2) and cell proliferation. In vivo, ShK-186 paralyzes effector-memory T cells at the sites of inflammation and prevent their reactivation in inflamed tissues. In contrast, ShK-186 does not affect the homing to and motility within lymph nodes of naive and central memory T cells, most likely because these cells express the K_Ca3.1 channel and are therefore protected from the effect of K_v1.3 blockade. In proof-of-concept studies, ShK and its analogs have prevented and treated disease in rat models of multiple sclerosis, rheumatoid arthritis, and delayed type hypersensitivity. ShK-186, due to its durable pharmacological action, is effective in ameliorating disease in rat models of delayed type hypersensitivity, multiple sclerosis (experimental autoimmune encephalomyelitis) and rheumatoid arthritis (pristane induced arthritis) when administered once every 2–5 days. ShK-186 has completed non-clinical safety studies. ShK-186 is the subject of an open Investigational New Drug (IND) application in the USA, and has completed human phase 1A and 1B trials in healthy volunteers.

As ShK toxin binds to the synaptosomal membranes, it facilitates an acetylcholine release at avian neuromuscular junctions while the K_v3.2 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 K_v3.2, ShK toxin depolarises the cortical GABAergic interneurons. K_v3.2 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 K_v3.2 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 25 mg/kg bodyweight (which translates to 0.5 mg per 20 g mouse). In rats the therapeutic safety index was greater than 75-fold.

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.

ShK-170 [a.k.a. ShK(L5)] does not cause significant toxicity in vitro. The peptide was not toxic to human and rat lymphoid cells incubated for 48 h with 100 nM of ShK-170 (>1200 times greater than the K_v1.3 half-blocking dose). The same high concentration of ShK-170 was negative in the Ames test on tester strain TA97A, suggesting that it is not a mutagen. ShK-170 had no effect on heart rate or heart rate variability parameters in either the time or the frequency domain in rats. It does not block the hERG (K_v11.1) channel that is associated with drug-associated cardiac arrhythmias. Repeated daily administration of the peptide by subcutaneous injection (10 µg/kg/day) for 2 weeks to rats does not cause any changes in blood counts, blood chemistry or in the proportion of thymocyte or lymphocyte subsets. Furthermore, the rats administered the peptide gain weight normally.

ShK-186 [a.k.a. SL5] is also safe. Repeated daily administration by subcutaneous injection of ShK-186 (100 µg/kg/day) for 4 weeks to rats does not cause any changes in blood counts, blood chemistry or histopathology. Furthermore, ShK-186 did not compromise the protective immune response to acute influenza viral infection or acute bacterial (Chlamydia) infection in rats at concentrations that were effective in ameliorating autoimmune diseases in rat models. Interestingly, rats repeatedly administered ShK-186 for a month by subcutaneous injection (500 µg/kg/day) developed low titer anti-ShK antibodies. The reason for the low immunogenicity of the peptide is not well understood. ShK-186 has completed GLP (Good Laboratory Practice) non-clinical safety studies in rodents and non-human primates. ShK-186 (aka Dalazatide) which was licensed to Kineta Bio is the subject of an open Investigational New Drug (IND) application in the United States of America, and has recently completed human phase 1A and 1b trials in healthy volunteers. A second human phase 1b was recently completed in 2015 in psoriasis patients. Dalazatide was shown to significantly ameliorate symptoms in 90% patients with active plaque psoriasis with a 60 mcg weekly dose.

Many groups are developing K_v1.3 blockers for the treatment of autoimmune diseases.

Use

Because ShK toxin is a specific inhibitor of K_v1.1, K_v1.3, K_v1.6, K_v3.2 and K_Ca3.1, it may serve as a useful pharmacological tool for studying these channels. The K_v1.3 specific ShK analogs, ShK-170, ShK-186 and ShK-192, have been demonstrated to be effective in rat models of autoimmune diseases, and these or related analogs might have use as therapeutics for human autoimmune diseases.

K_v1.3 is also considered a therapeutic target for the treatment of obesity, for enhancing peripheral insulin sensitivity in patients with type-2 diabetes mellitus, and for preventing bone resorption in periodontal disease. Furthermore, because pancreatic beta cells, which have K_v3.2 channels, are thought to play a role in glucose-dependent firing, ShK, as a K_v3.2 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.