Abstract
µ-Conotoxins are small, potent, peptide voltage-gated sodium (Na(V)) channel inhibitors characterised by a conserved cysteine framework. Despite promising in vivo studies indicating analgesic potential of these compounds, selectivity towards the therapeutically relevant subtype Na(V)1.7 has so far been limited. We recently identified a novel µ-conotoxin, SxIIIC, which potently inhibits human Na(V)1.7 (hNa(V)1.7). SxIIIC has high sequence homology with other µ-conotoxins, including SmIIIA and KIIIA, yet shows different Na(V) channel selectivity for mammalian subtypes. Here, we evaluated and compared the inhibitory potency of µ-conotoxins SxIIIC, SmIIIA and KIIIA at hNa(V) channels by whole-cell patch-clamp electrophysiology and discovered that these three closely related µ-conotoxins display unique selectivity profiles with significant variations in inhibitory potency at hNa(V)1.7. Analysis of other µ-conotoxins at hNa(V)1.7 shows that only a limited number are capable of inhibition at this subtype and that differences between the number of residues in loop 3 appear to influence the ability of µ-conotoxins to inhibit hNa(V)1.7. Through mutagenesis studies, we confirmed that charged residues in this region also affect the selectivity for hNa(V)1.4. Comparison of µ-conotoxin NMR solution structures identified differences that may contribute to the variance in hNa(V)1.7 inhibition and validated the role of the loop 1 extension in SxIIIC for improving potency at hNa(V)1.7, when compared to KIIIA. This work could assist in designing µ-conotoxin derivatives specific for hNa(V)1.7.