Paramyotonia congenita mutations reveal different roles for segments S3 and S4 of domain D4 in hSkM1 sodium channel gating.

Abstract

Mutations in the gene encoding the voltage-gated sodium channel of skeletal muscle (SkM1) have been identified in a group of autosomal dominant diseases, characterized by abnormalities of the sarcolemmal excitability, that include paramyotonia congenita (PC) and hyperkalemic periodic paralysis (HYPP). We previously reported that PC mutations cause in common a slowing of inactivation in the human SkM1 sodium channel. In this investigation, we examined the molecular mechanisms responsible for the effects of L1433R, located in D4/$3, on channel gating by creating a series of additional mutations at the 1433 site. Unlike the R1448C mutation, found in D4/$4, which produces its effects largely due to the loss of the positive charge, change of the hydropathy of the side chain rather than charge is the primary factor mediating the effects of L1433R. These two mutations also differ in their effects on recovery from inactivation, conditioned inactivation, and steady state inactivation of the hSkM1 channels. We constructed a double mutation containing both L1433R and R1448C. The double mutation closely resembled R1448C with respect to alterations in the kinetics of inactivation during depolarization and voltage dependence, but was indistinguishable from L1433R in the kinetics of recovery from inactivation and steady state inactivation. No additive effects were seen, suggesting that these two segments interact during gating. In addition, we found that these mutations have different effects on the delay of recovery from inactivation and the kinetics of the tail currents, raising a question whether this delay is a reflection of the deactivation process. These results suggest that the $3 and $4 segments play distinct roles in different processes of hSkM1 channel gating: D4/$4 is critical for the deactivation and inactivation of the open channel while D4/$3 has a dominant role in the recovery of inactivated channels. However, these two segments interact during the entry to, and exit from, inactivation states.

A revised view of cardiac sodium channel “blockade” in the long-QT syndrome

“…A slowing of open-state inactivation with coexistent enhancement of closed-state inactivation for this hH1 mutant is consistent with gating phenotypes reported for mutations in the domain IV, S4 segment of the human skeletal muscle Na channel (hSkM1) linked to paramyotonia congenita (24,33).…”

Local anesthetics as effectors of allosteric gating. Lidocaine effects on inactivation-deficient rat skeletal muscle Na channels.

“…Such an effect on inactivation can indirectly influence local anesthetic action; hence, the reduction in use-dependent block by this mutation may not specifically define the location of the local anesthetic receptor. The profound influence of externally positioned mutations on fast inactivation (30,52,72) suggests that the structural characteristics of the inactivation-gate docking site are sensitive to remote allosteric effects. Recent work suggests that the externally bound ␣-scorpion and sea anemone toxins (44) disrupt fast inactivation through a conformational change transmitted through the S3 and S4 transmembrane repeats (73).…”

Slow Closed-State Inactivation: A Novel Mechanism Underlying Ramp Currents in Cells Expressing the hNE/PN1 Sodium Channel

“…Several studies (Ji et al, 1996;Dib-Hajj et al, 1997;Chen et al, 1998) have implicated the S3-S4 linker of domain 4 as being important in repriming kinetics. Interestingly, there is a threonine (T1590) in the domain 4 S3-S4 linker of hNE (and NaS) at a position where most of the other sodium channel isoforms, including hSkM1 and rBII, have a lysine.…”