Mechanism and molecular basis for the sodium channel subtype specificity of μ-conopeptide CnIIIC

BACKGROUND AND PURPOSE Voltage-gated sodium channels (NaV channels) are key players in the generation and propagation of action potentials, and selective blockade of these channels is a promising strategy for clinically useful suppression of electrical activity. The conotoxin mu-CnIIIC from the cone snail Conus consors exhibits myorelaxing activity in rodents through specific blockade of skeletal muscle (NaV1.4) NaV channels. EXPERIMENTAL APPROACH We investigated the activity of mu-CnIIIC on human NaV channels and characterized its inhibitory mechanism, as well as the molecular basis, for its channel specificity. KEY RESULTS Similar to rat paralogs, human NaV1.4 and NaV1.2 were potently blocked by mu-CnIIIC, the sensitivity of NaV1.7 was intermediate, and NaV1.5 and NaV1.8 were insensitive. Half-channel chimeras revealed that determinants for the insensitivity of NaV1.8 must reside in both the first and second halves of the channel, while those for NaV1.5 are restricted to domains I and II. Furthermore, domain I pore loop affected the total block and therefore harbours the major determinants for the subtype specificity. Domain II pore loop only affected the kinetics of toxin binding and dissociation. Blockade by mu-CnIIIC of NaV1.4 was virtually irreversible but left a residual current of about 5%, reflecting a leaky block; therefore, Na+ ions still passed through mu-CnIIIC-occupied NaV1.4 to some extent. TTX was excluded from this binding site but was trapped inside the pore by mu-CnIIIC. CONCLUSION AND IMPLICATIONS Of clinical significance, mu-CnIIIC is a potent and persistent blocker of human skeletal muscle NaV1.4 that does not affect activity of cardiac NaV1.5.