title: Specificity, affinity and efficacy of iota-conotoxin RXIA, an agonist of voltage-gated sodium channels Na(V)1.2, 1.6 and 1.7.
abstract: The excitotoxic conopeptide iota-RXIA induces repetitive action potentials in frog motor axons and seizures upon intracranial injection into mice. We recently discovered that iota-RXIA shifts the voltage-dependence of activation of voltage-gated sodium channel Na(V)1.6 to a more hyperpolarized level. Here, we performed voltage-clamp experiments to examine its activity against rodent Na(V)1.1 through Na(V)1.7 co-expressed with the beta1 subunit in Xenopus oocytes and Na(V)1.8 in dissociated mouse DRG neurons. The order of sensitivity to iota-RXIA was Na(V)1.6 > 1.2 > 1.7, and the remaining subtypes were insensitive. The time course of iota-RXIA-activity on Na(V)1.6 during exposure to different peptide concentrations were well fit by single-exponential curves that provided k(obs). The plot of k(obs)versus [iota-RXIA] was linear, consistent with a bimolecular reaction with a K(d) of approximately 3 microM, close to the steady-state EC(50) of approximately 2 microM. iota-RXIA has an unusual residue, D-Phe, and the analog with an L-Phe instead, iota-RXIA[L-Phe44], had a two-fold lower affinity and two-fold faster off-rate than iota-RXIA on Na(V)1.6 and furthermore was inactive on Na(V)1.2. iota-RXIA induced repetitive action potentials in mouse sciatic nerve with conduction velocities of both A- and C-fibers, consistent with the presence of Na(V)1.6 at nodes of Ranvier as well as in unmyelinated axons. Sixteen peptides homologous to iota-RXIA have been identified from a single species of Conus, so these peptides represent a rich family of novel sodium channel-targeting ligands.
authors.affiliation: Department of Biology, University of Utah, Salt Lake City, Utah 84112, USA.
authors.forename: Brian
authors.initials: B
authors.lastname: Fiedler
authors.forename: Min-Min
authors.initials: MM
authors.lastname: Zhang
authors.forename: Oga
authors.initials: O
authors.lastname: Buczek
authors.forename: Layla
authors.initials: L
authors.lastname: Azam
authors.forename: Grzegorz
authors.initials: G
authors.lastname: Bulaj
authors.forename: Raymond S
authors.initials: RS
authors.lastname: Norton
authors.forename: Baldomero M
authors.initials: BM
authors.lastname: Olivera
authors.forename: Doju
authors.initials: D
authors.lastname: Yoshikami
mesh_terms: D000200:Action Potentials / Q000187:drug effects; D000595:Amino Acid Sequence; D000818:Animals; D002478:Cells, Cultured; D003001:Cloning, Molecular; D020916:Conotoxins / Q000737:chemistry / Q000494:pharmacology*; D004305:Dose-Response Relationship, Drug; D004558:Electric Stimulation; D051379:Mice; D008958:Models, Molecular; D008969:Molecular Sequence Data; D009474:Neurons / Q000187:drug effects / Q000378:metabolism; D009865:Oocytes / Q000378:metabolism; D018408:Patch-Clamp Techniques; D011487:Protein Conformation; D021122:Protein Subunits; D051381:Rats; D012584:Sciatic Nerve / Q000378:metabolism / Q000502:physiology; D062687:Sodium Channel Agonists; D014981:Xenopus
chemical_list: D020916:Conotoxins; D021122:Protein Subunits; D062687:Sodium Channel Agonists; C524340:conotoxin iota-RXIA, Conus radiatus