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

Background and Purpose Voltage-gated sodium channels (VGSCs) are assembled from two classes of subunits, a pore-bearing -subunit (NaV1) and one or two accessory -subunits (NaVs). Neurons in mammals can express one or more of seven isoforms of NaV1 and one or more of four isoforms of NaV. The peptide -conotoxins, like the guanidinium alkaloids tetrodotoxin (TTX) and saxitoxin (STX), inhibit VGSCs by blocking the pore in NaV1. Hitherto, the effects of NaV-subunit co-expression on the activity of these toxins have not been comprehensively assessed. Experimental Approach Four -conotoxins (-TIIIA, -PIIIA, -SmIIIA and -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 NaV1, 2, 3 or 4 and, for NaV1.7, binary combinations of thereof. Key Results Co-expression of NaV-subunits modifies the block by -conotoxins: in general, NaV1 or 3 co-expression tended to increase kon (in the most extreme instance by ninefold), whereas NaV2 or 4 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-subunit co-expression. Tests of NaV1:2 chimeras co-expressed with NaV1.7 suggest that the extracellular portion of the NaV subunit is largely responsible for altering -conotoxin kinetics. Conclusions and Implications These results are the first indication that NaV subunit co-expression can markedly influence -conotoxin binding and, by extension, the outer vestibule of the pore of VGSCs. -Conotoxins could, in principle, be used to pharmacologically probe the NaV subunit composition of endogenously expressed VGSCs.