title: Co-expression of Na(V)β subunits alters the kinetics of inhibition of voltage-gated sodium channels by pore-blocking μ-conotoxins.
abstract: OBJECTIVE
  Voltage-gated sodium channels (VGSCs) are assembled from two classes of subunits, a pore-bearing α-subunit (NaV 1) and one or two accessory β-subunits (NaV βs). Neurons in mammals can express one or more of seven isoforms of NaV 1 and one or more of four isoforms of NaV β. The peptide μ-conotoxins, like the guanidinium alkaloids tetrodotoxin (TTX) and saxitoxin (STX), inhibit VGSCs by blocking the pore in NaV 1. Hitherto, the effects of NaV β-subunit co-expression on the activity of these toxins have not been comprehensively assessed.
  
  
  METHODS
  Four μ-conotoxins (μ-TIIIA, μ-PIIIA, μ-SmIIIA and μ-KIIIA), TTX and STX were tested against NaV 1.1, 1.2, 1.6 or 1.7, each co-expressed in Xenopus laevis oocytes with one of NaV β1, β2, β3 or β4 and, for NaV 1.7, binary combinations of thereof.
  
  
  RESULTS
  Co-expression of NaV β-subunits modifies the block by μ-conotoxins: in general, NaV β1 or β3 co-expression tended to increase kon (in the most extreme instance by ninefold), whereas NaV β2 or β4 co-expression decreased kon (in the most extreme instance by 240-fold). In contrast, the block by TTX and STX was only minimally, if at all, affected by NaV β-subunit co-expression. Tests of NaV β1 : β2 chimeras co-expressed with NaV 1.7 suggest that the extracellular portion of the NaV β subunit is largely responsible for altering μ-conotoxin kinetics.
  
  
  CONCLUSIONS
  These results are the first indication that NaV β subunit co-expression can markedly influence μ-conotoxin binding and, by extension, the outer vestibule of the pore of VGSCs. μ-Conotoxins could, in principle, be used to pharmacologically probe the NaV β subunit composition of endogenously expressed VGSCs.
authors.affiliation: Department of Biology, University of Utah, Salt Lake City, UT, USA.
authors.forename: Min-Min
authors.initials: MM
authors.lastname: Zhang
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Wilson
authors.forename: Layla
authors.initials: L
authors.lastname: Azam
authors.forename: Joanna
authors.initials: J
authors.lastname: Gajewiak
authors.forename: Jean E
authors.initials: JE
authors.lastname: Rivier
authors.forename: Grzegorz
authors.initials: G
authors.lastname: Bulaj
authors.forename: Baldomero M
authors.initials: BM
authors.lastname: Olivera
authors.forename: Doju
authors.initials: D
authors.lastname: Yoshikami
mesh_terms: D000818:Animals; D020916:Conotoxins / Q000494:pharmacology*; D005260:Female; D007700:Kinetics; D009865:Oocytes / Q000378:metabolism; D020033:Protein Isoforms / Q000378:metabolism; D051381:Rats; D026941:Sodium Channel Blockers / Q000494:pharmacology*; D062546:Voltage-Gated Sodium Channel beta Subunits / Q000378:metabolism; D061566:Voltage-Gated Sodium Channels / Q000378:metabolism*; D014982:Xenopus laevis
chemical_list: D020916:Conotoxins; D020033:Protein Isoforms; D026941:Sodium Channel Blockers; D062546:Voltage-Gated Sodium Channel beta Subunits; D061566:Voltage-Gated Sodium Channels