mu-Conotoxins (mu-CTXs) block skeletal muscle Na(+) channels with an affinity 1-2 orders of magnitude higher than cardiac and brain Na(+) channels. Although a number of conserved pore residues are recognized as critical determinants of mu-CTX block, the molecular basis of isoform-specific toxin sensitivity remains unresolved. Sequence comparison of the domain II (DII) S5-S6 loops of rat skeletal muscle (mu1, Na(v)1.4), human heart (hh1, Na(v)1.5), and rat brain (rb1, Na(v)1.1) Na(+) channels reveals substantial divergence in their N-terminal S5-P linkers even though the P-S6 and C-terminal P segments are almost identical. We used Na(v)1.4 as the backbone and systematically converted these DII S5-P isoform variants to the corresponding residues in Na(v)1.1 and Na(v)1.5. The Na(v)1.4–>Na(v)1.5 variant substitutions V724R, C725S, A728S, D730S, and C731S (Na(v)1.4 numbering) reduced block of Na(v)1.4 by 4-, 86-, 12-, 185-, and 55-fold respectively, rendering the skeletal muscle isoform more “cardiac-like.” Conversely, an Na(v)1.5–> Na(v)1.4 chimeric construct in which the Na(v)1.4 DII S5-P linker replaces the analogous segment in Na(v)1.5 showed enhanced mu-CTX block. However, these variant determinants are conserved between Na(v)1.1 and Na(v)1.4 and thus cannot explain their different sensitivities to mu-CTX. Comparison of their sequences reveals two variants at Na(v)1.4 positions 729 and 732: Ser and Asn in Na(v)1.4 compared with Thr and Lys in Na(v)1.1, respectively. The double mutation S729T/N732K rendered Na(v)1.4 more “brain-like” (30-fold downward arrow in block), and the converse mutation T925S/K928N in Na(v)1.1 reproduced the high affinity blocking phenotype of Na(v)1.4. We conclude that the DII S5-P linker, although lying outside the conventional ion-conducting pore, plays a prominent role in mu-CTX binding, thus shaping isoform-specific toxin sensitivity.

We describe the properties of a family of 22-amino acid peptides, the mu-conotoxins, which are useful probes for investigating voltage-dependent sodium channels of excitable tissues. The mu-conotoxins are present in the venom of the piscivorous marine snail, Conus geographus L. We have purified seven homologs of the mu-conotoxin set and determined their amino acid sequences, as follows, where Hyp = trans-4-hydroxyproline. GIIIA R.D.C.C.T.Hyp.Hyp.K.K.C.K.D.R.Q.C.K.Hyp.Q.R.C.C.A-NH2 [Pro6]GIIIA R.D.C.C. T.P.Hyp.K.K.C.K.D.R.Q.C.K.Hyp.Q.R.C.C.A-NH2 [Pro7]GIIIA R.D.C.C.T.Hyp.P.K.K.C.K.D.R.Q.C.R.Hyp.Q.R.C.C.A-NH2 GIIIB R.D.C.C.T.Hyp.Hyp.R.K.C.K.D.R.R.C.K.Hyp.M.K.C.C.A-NH2 [Pro6]GIIIB R.D.C.C.T.P.Hyp.R.K.C.K.D.R.R. C.K.Hyp.M.K.C.C.A-NH2 [Pro7]GIIIB R.D.C.C.T.Hyp.P.R.K.C.K.D.R.R.C.K.Hyp.M.K.C.C.A-NH2 GIIIC R.D.C.C.T.Hyp.Hyp.K.K.C.K.D.R.R.C.K.Hyp.L.K.C.C.A-NH2. Using the major peptide (GIIIA) in electrophysiological studies on nerve-muscle preparations and in single channel studies using planar lipid bilayers, we have established that the toxin blocks muscle sodium channels, while having no discernible effect on nerve or brain sodium channels. In bilayers the blocking kinetics of GIIIA were derived by statistical analysis of discrete transitions between blocked and unblocked states of batrachotoxin-activated sodium channels from rat muscle. The kinetics conform to a single-site, reversible binding equilibrium with a voltage-dependent binding constant. The measured value of the equilibrium KD for GIIIA is 100 nM at OmV, decreasing e-fold/34 mV of hyperpolarization. This voltage dependence of blocking is similar to that of tetrodotoxin and saxitoxin as measured by the same technique. The tissue specificity and kinetic characteristics suggest that the mu-conotoxins may serve as useful ligands to distinguish sodium channel subtypes in different tissues.

mu-Conotoxin (mu-CTX) specifically occludes the pore of voltage-dependent Na(+) channels. In the rat skeletal muscle Na(+) channel (mu1), we examined the contribution of charged residues between the P loops and S6 in all four domains to mu-CTX block. Conversion of the negatively charged domain II (DII) residues Asp-762 and Glu-765 to cysteine increased the IC(50) for mu-CTX block by approximately 100-fold (wild-type = 22.3 +/- 7.0 nm; D762C = 2558 +/- 250 nm; E765C = 2020 +/- 379 nm). Restoration or reversal of charge by external modification of the cysteine-substituted channels with methanethiosulfonate reagents (methanethiosulfonate ethylsulfonate (MTSES) and methanethiosulfonate ethylammonium (MTSEA)) did not affect mu-CTX block (D762C: IC(50, MTSEA+) = 2165.1 +/- 250 nm; IC(50, MTSES-) = 2753.5 +/- 456.9 nm; E765C: IC(50, MTSEA+) = 2200.1 +/- 550.3 nm; IC(50, MTSES-) = 3248.1 +/- 2011.9 nm) compared with their unmodified counterparts. In contrast, the charge-conserving mutations D762E (IC(50) = 21.9 +/- 4.3 nm) and E765D (IC(50) = 22.0 +/- 7.0 nm) preserved wild-type blocking behavior, whereas the charge reversal mutants D762K (IC(50) = 4139.9 +/- 687.9 nm) and E765K (IC(50) = 4202.7 +/- 1088.0 nm) destabilized mu-CTX block even further, suggesting a prominent electrostatic component of the interactions between these DII residues and mu-CTX. Kinetic analysis of mu-CTX block reveals that the changes in toxin sensitivity are largely due to accelerated toxin dissociation (k(off)) rates with little changes in association (k(on)) rates. We conclude that the acidic residues at positions 762 and 765 are key determinants of mu-CTX block, primarily by virtue of their negative charge. The inability of the bulky MTSES or MTSEA side chain to modify mu-CTX sensitivity places steric constraints on the sites of toxin interaction.

Critical illness myopathy is a disorder in which skeletal muscle becomes electrically inexcitable. We previously demonstrated that a shift in the voltage dependence of fast inactivation of sodium currents contributes to inexcitability of affected fibres in an animal model of critical illness myopathy in which denervated rat skeletal muscle is treated with corticosteroids (steroid-denervated; SD). In the current study we examined whether expression of Nav1.5 contributes to the altered voltage dependence of sodium channel inactivation in SD muscle. We used TTX and mu-conotoxin GIIIB to selectively block Nav1.4 in SD muscle and found that the level of Nav1.5 did not correlate closely with the shift in fast inactivation. Surprisingly, we found that the voltage dependence of inactivation of Nav1.4 was similar to that of Nav1.5 in skeletal muscle in vivo. In severely affected fibres, inactivation of both Nav1.4 and Nav1.5 was shifted towards hyperpolarized potentials. We examined the role of denervation and steroid treatment in the shift of the voltage dependence of inactivation and found that both denervation and steroid treatment contribute to the shift in inactivation. Our results suggest that modulation of the voltage dependence of inactivation of both Nav1.4 and Nav1.5 in vivo contributes to loss of electrical excitability in SD muscle.