SNX‐482: A Novel Class E Calcium Channel Antagonist from Tarantula Venom

Abstract

Calcium channels are represented by at least 9 distinct genes (calcium channel classes A-I), corresponding to at least 5 functional and pharmacological "types" (L, N, P/Q, R and T). Selective L-, N-, and T-type channel antagonists are either in clinical use or in late stage clinical trials, while antagonists of P/Q channels are known to be toxic. No selective ligand has been identified for the R-type (class E), and its function and pharmacology are consequently, poorly understood. We review recent work on the discovery and initial characterization of SNX-482, the first known selective antagonist of R-type calcium channels. SNX-482 is a 41 residue acidic peptide with three disulfide bonds that has been isolated from the venom of the African tarantula, Hysterocrates gigas. In cell-based assays, it is a potent and selective inhibitor of the class E or R-type calcium channel. SNX-482 blocks some but not all native R-type currents: it blocks an R-type current in vertebrate neurohypophysis, but it does not block an R-type current in cerebellar granule cells. The peptide blocks oxytocin but not vasopressin release, suggesting a possible utility for SNX-482 as a neuroendocrine modulator. The peptide possesses antiseizure activity in several animal models of epilepsy, suggesting that class E antagonists may have pharmacological use in seizure disorders.

µ‐Opioid Receptor Preferentially Inhibits Oxytocin Release from Neurohypophysial Terminals by Blocking R‐type Ca2+ Channels

“…Non-additive effects of DAMGO and SNX-482 on Ca 2+ -currents, [Ca 2+ ] i and oxytocin release (32)(33)(34)(35). Experiments in which the order of application of these drugs was reversed produced similar results (n ¼ 3; data not shown).…”

“…activation and inactivation) that are consistent with those of the R ‐type Ca 2+ ‐channel (32). Low concentrations of the polypeptide SNX‐482 (34, 35) can block a similar component of the neurohypophyseal barium current (I Ba ), as does DAMGO. Those terminals that were insensitive to low concentrations of DAMGO were also insensitive to SNX 482 (n = 4; data not shown).…”

“…R ‐type Ca 2+ channels in neurophypophysial terminals show similar biophysical properties to those of recombinant channels expressed by the α 1E (Cav2.3) gene in HEK cells (32, 33). By contrast to other isoforms present in the CNS, the R ‐type in neurophypophysial terminals, as well as its dependent oxytocin release, have been shown to be highly sensitive (IC 50 = 1.4 n M ) to the polypeptide toxin SNX 482 (32, 34, 35). In the present study, we make use of this specific block by SNX 482 to identify the R ‐type channel and, in conjunction with other known specific Ca 2+ channel blockers, expose any opioid effect on this channel subtype.…”

Modulation/physiology of calcium channel sub-types in neurosecretory terminals

“…These properties are most consistent with the “R”-type Ca 2+ -channel. At maximally effective concentrations (20 nM), SNX 482, a polypeptide toxin able to specifically block α 1E (R) sub-type Ca 2+ currents expressed in HEK cells [76,78], abolished the resistant component of OT release and the resistant component of the neurohypophysial Ca 2+ current from these terminals (see Fig. 1B).…”

Age-Dependent Contribution of P/Q- and R-Type Ca2+Channels to Neuromuscular Transmission inLethargicMice

“…Nimodipine was purchased from Sigma-Aldrich (St. Louis, MO), v-agatoxin IVA (v-Aga-IVA) from Alomone Labs (Jerusalem, Israel), v-conotoxin GVIA (v-CTx-GVIA) from Bachem (Torrance, CA), and SNX-482 (C 192 H 274 N 52 O 60 S 7 ; structure in Newcomb et al, 1998Newcomb et al, , 2000 from Ascent Scientific (Princeton, NJ). HEPES, EGTA, paraformaldehyde, Triton X-100, and tetramethylrhodamine a-bungarotoxin were all purchased from Sigma-Aldrich.…”