Molecular determinants for the subtype specificity of μ-conotoxin SIIIA targeting neuronal voltage-gated sodium channels

Abstract

Voltage-gated sodium channels (Na V channels) play a pivotal role in neuronal excitability; they are specifically targeted by μ-conotoxins from the venom of marine cone snails. These peptide toxins bind to the outer vestibule of the channel pore thereby blocking ion conduction through Na V channels. μ-Conotoxin SIIIA from Conus striatus was shown to be a potent inhibitor of neuronal sodium channels and to display analgesic effects in mice, albeit the molecular targets are not unambiguously known. We therefore studied recombinant Na V channels expressed in mammalian cells using the whole-cell patch-clamp method. Synthetic μSIIIA slowly and partially blocked rat Na V1.4 channels with an apparent IC 50 of 0.56 ± 0.29 μM; the block was not complete, leaving at high concentration a residual current component of about 10% with a correspondingly reduced single-channel conductance. At 10 μM, μSIIIA potently blocked rat Na V1.2, rat and human Na V1.4, and mouse Na V1.6 channels; human Na V1.7 channels were only inhibited by 58.1 ± 4.9%, whereas human Na V1.5 as well as rat and human Na V1.8 were insensitive. Employing domain chimeras between rNa V1.4 and hNa V1.5, we located the determinants for μSIIIA specificity in the first half of the channel protein with a major contribution of domain-2 and a minor contribution of domain-1. The latter was largely accounted for by the alteration in the TTX-binding site (Tyr401 in rNa V1.4, Cys for Na V1.5, and Ser for Na V1.8). Introduction of domain-2 pore loops of all tested channel isoforms into rNa V1.4 conferred the μSIIIA phenotype of the respective donor channels highlighting the importance of the domain-2 pore loop as the major determinant for μSIIIA’s subtype specificity. Single-site substitutions identified residue Ala728 in rNa V1.4 as crucial for its high sensitivity toward μSIIIA. Likewise, Asn889 at the homologous position in hNa V1.7 is responsible for the channel’s reduced μSIIIA sensitivity. These results will pave the way for the rational design of selective Na V-channel antagonists for research and medical applications.