Co-expression of NaVβ subunits alters the kinetics of inhibition of voltage-gated sodium channels by pore-blocking μ-conotoxins

Abstract

Background and Purpose Voltage-gated sodium channels (VGSCs) are assembled from two classes of subunits, a pore-bearing α-subunit (NaV1) and one or two accessory β-subunits (NaVβs). Neurons in mammals can express one or more of seven isoforms of NaV1 and one or more of four isoforms of NaVβ. The peptide μ-conotoxins, like the guanidinium alkaloids tetrodotoxin (TTX) and saxitoxin (STX), inhibit VGSCs by blocking the pore in NaV1. Hitherto, the effects of Na Vβ-subunit co-expression on the activity of these toxins have not been comprehensively assessed. Experimental Approach Four μ-conotoxins (μ-TIIIA, μ-PIIIA, μ-SmIIIA and μ-KIIIA), TTX and STX were tested against NaV1.1, 1.2, 1.6 or 1.7, each co-expressed in Xenopus laevis oocytes with one of NaVβ1, β2, β3 or β4 and, for NaV1.7, binary combinations of thereof. Key Results Co-expression of NaVβ-subunits modifies the block by μ-conotoxins: in general, NaVβ1 or β3 co-expression tended to increase k on (in the most extreme instance by ninefold), whereas Na Vβ2 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 NaVβ-subunit co-expression. Tests of NaVβ1: β2 chimeras co-expressed with NaV1.7 suggest that the extracellular portion of the Na Vβ subunit is largely responsible for altering μ-conotoxin kinetics. Conclusions and Implications These results are the first indication that NaVβ 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 NaVβ subunit composition of endogenously expressed VGSCs. © 2012 The British Pharmacological Society.