Kalkitoxin, a lipopeptide derived from the cyanobacterium Lyngbya majuscula, induces NMDA receptor mediated neuronal necrosis, blocks voltage-dependent sodium channels, and induces cellular hypoxia by inhibiting the electron transport chain (ETC) complex 1.
Contents
Chemistry
Kalkitoxine is a lipopeptide toxin with a molecular weight of 366.604Da. Its chemical formula is C21H38N2OS. The kalkitoxin carbon chain contains two double bonds, a ring system, which is in the natural form a 2,4-disubstituted thiazoline, and an additional carbonyl-group. These four different groups are unsaturated, which causes kalkitoxin to have four degrees of unsaturation: double bonds can be broken and new functional groups can be added to the molecule. In kalkitoxin, there is one thiazoline group and four methylgroups. These groups are carbon atoms, attached to four different types of atoms. This causes kalkitoxin to have a total of five asymmetric carbon centers in kalkitoxin. The four methylgroups, the different stereochemistry, and a N-methylgroup all contribute to the toxicity of kalkitoxin.
Targets
Kalkitoxin may activate the NMDA receptor. It also blocks the voltage-gated sodium channel and the electron transport chain (ETC) complex 1. It remains unknown how exactly kalkitoxin binds to the voltage-gated sodium channel. Neurotoxin sit 1 and 2 have been ruled out as possible binding sites, whereas neurotoxin site 7 is suggested as binding site for kalkitoxin. This is probable, because there is inhibition of the channel by kalkitoxin when deltamethrin, which has positive allosteric effects, is present. This could be because molecular determinants for binding are similar in kalkitoxin and deltamethrin.
Mode of Action
Kalkitoxin induces delayed neuronal necrosis in cerebellar granule cells of the rat. This neuronal necrosis proved to be NMDA-receptor mediated. These receptors are normally activated by glutamate and other excitotoxic compounds and can induce neuronal necrosis. It is not yet known if the toxin induces necrosis directly or via the release of excitotoxic compounds.
Secondly, Kalkitoxin blocks voltage-gated sodium channels, thereby inhibiting Ca2+ release that normally occurs when the voltage-gated sodium channel is activated, in a concentration dependent matter. Calcium release has been shown to induce lactate dehydrogenase (LDH) production. The amount of LDH is a measure for neuronal cell death. In the presence of kalkitoxin there is also a concentration-dependent inhibition of neuronal cell death and LDH production (9). The mechanism behind this inhibition is still unknown.
Thirdly, kalkitoxin blocks the electron transport chain (ETC) complex 1, one of the protein complexes involved in mitochondrial respiration. By blocking the ETC complex 1, kalkitoxin potently inhibits hypoxia-inducible factor-1 (HIF-1) activation. HIF-1 is a transcription factor, which enhances the expression of genes that increase oxygen availability, as well as genes that decrease oxygen consumption. Inhibition of HIF-1, which is one of the main effects of kalkitoxin, thus induces cellular hypoxia.
Toxicity
Kalkitoxin is ichthyotoxic to goldfish (Carassius auratus, LC50: 700nM) and to aquatic crustacean brine shrimp (Artemia salina, LC50: 150-180nM ). Kalkitoxin also has been shown to have delayed neurotoxic effects on cerebellar granule cells of the rat (LC50: 3,86nM).