title: Mechanism and molecular basis for the sodium channel subtype specificity of µ-conopeptide CnIIIC.
abstract: OBJECTIVE
  Voltage-gated sodium channels (Na(V) channels) are key players in the generation and propagation of action potentials, and selective blockade of these channels is a promising strategy for clinically useful suppression of electrical activity. The conotoxin µ-CnIIIC from the cone snail Conus consors exhibits myorelaxing activity in rodents through specific blockade of skeletal muscle (Na(V) 1.4) Na(V) channels.
  
  
  METHODS
  We investigated the activity of µ-CnIIIC on human Na(V) channels and characterized its inhibitory mechanism, as well as the molecular basis, for its channel specificity.
  
  
  RESULTS
  Similar to rat paralogs, human Na(V) 1.4 and Na(V) 1.2 were potently blocked by µ-CnIIIC, the sensitivity of Na(V) 1.7 was intermediate, and Na(V) 1.5 and Na(V) 1.8 were insensitive. Half-channel chimeras revealed that determinants for the insensitivity of Na(V) 1.8 must reside in both the first and second halves of the channel, while those for Na(V) 1.5 are restricted to domains I and II. Furthermore, domain I pore loop affected the total block and therefore harbours the major determinants for the subtype specificity. Domain II pore loop only affected the kinetics of toxin binding and dissociation. Blockade by µ-CnIIIC of Na(V) 1.4 was virtually irreversible but left a residual current of about 5%, reflecting a 'leaky' block; therefore, Na(+) ions still passed through µ-CnIIIC-occupied Na(V) 1.4 to some extent. TTX was excluded from this binding site but was trapped inside the pore by µ-CnIIIC.
  
  
  CONCLUSIONS
  Of clinical significance, µ-CnIIIC is a potent and persistent blocker of human skeletal muscle Na(V) 1.4 that does not affect activity of cardiac Na(V) 1.5.
authors.affiliation: Center for Molecular Biomedicine, Department of Biophysics, Friedrich Schiller University of Jena & Jena University Hospital, Germany.
authors.forename: René
authors.initials: R
authors.lastname: Markgraf
authors.forename: Enrico
authors.initials: E
authors.lastname: Leipold
authors.forename: Jana
authors.initials: J
authors.lastname: Schirmeyer
authors.forename: Marianne
authors.initials: M
authors.lastname: Paolini-Bertrand
authors.forename: Oliver
authors.initials: O
authors.lastname: Hartley
authors.forename: Stefan H
authors.initials: SH
authors.lastname: Heinemann
mesh_terms: D000818:Animals; D001665:Binding Sites; D020916:Conotoxins / Q000494:pharmacology*; D052078:Conus Snail; D057809:HEK293 Cells; D006801:Humans; D062553:NAV1.4 Voltage-Gated Sodium Channel / Q000187:drug effects* / Q000378:metabolism; D062554:NAV1.5 Voltage-Gated Sodium Channel / Q000187:drug effects / Q000378:metabolism; D012964:Sodium / Q000378:metabolism; D026941:Sodium Channel Blockers / Q000494:pharmacology*; D015222:Sodium Channels / Q000187:drug effects* / Q000378:metabolism
chemical_list: D020916:Conotoxins; D062553:NAV1.4 Voltage-Gated Sodium Channel; D062554:NAV1.5 Voltage-Gated Sodium Channel; D026941:Sodium Channel Blockers; D015222:Sodium Channels; D012964:Sodium