NaVβ Subunits Modulate the Inhibition of NaV1.8 by the Analgesic Gating Modifier μO-Conotoxin MrVIB

Voltage-gated sodium channels (VGSCs) consist of a pore-forming alpha-subunit and regulatory beta-subunits. Several families of neuroactive peptides of Conus snails target VGSCs, including mu O-conotoxins and mu-conotoxins. Unlike mu-conotoxins and the guanidinium alkaloid saxitoxin (STX), which are pore blockers, mu O-conotoxins MrVIA and MrVIB inhibit VGSCs by modifying channel gating. mu O-MrVIA/B can block Na(V)1.8 (a tetrodotoxin-resistant isoform of VGSCs) and have analgesic properties. The effect of Na-V beta-subunit coexpression on susceptibility to block by mu O-MrVIA/B and STX has, until now, not been reported. Here, we show that beta 1-, beta 2-, beta 3-, and beta 4-subunits, when individually co-expressed with Na(V)1.8 in Xenopus laevis oocytes, increased the k(on) of the block produced by mu O-MrVIB (by 3-, 32-, 2-, and 7-fold, respectively) and modestly decreased the apparent k(off). Strong depolarizing prepulses markedly accelerated MrVIB washout with rates dependent on beta-subunit coexpression. Thus, coexpression of beta-subunits with Na(V)1.8 can strongly influence the affinity of the conopeptide for the channel. This observation is of particular interest because beta-subunit expression can be dynamic, e. g., beta 2-expression is up-regulated after nerve injury (J Neurosci, 25: 10970-10980, 2005); therefore, the effectiveness of a mu O-conotoxin as a channel blocker could be enhanced by the conditions that may call for its use therapeutically. In contrast to MrVIB's action, the STX-induced block of Na(V)1.8 was only marginally, if at all, affected by coexpression of any of the beta-subunits. Our results raise the possibility that mu O-conotoxins and perhaps other gating modifiers may provide a means to functionally assess the beta-subunit composition of VGSC complexes in neurons.