Keeping hyperactive voltage-gated sodium channels in silent mode

Skeletal muscle voltage-gated sodium channels (Na_v1.4) are membrane proteins that dictate the action potential threshold and underlie the rapid depolarizing phase, or upstroke, of the action potential in this tissue. Alterations in sodium channel activity not only impact the threshold and the upstroke of the action potential, but can also affect its duration and the initiation of subsequent action potentials. The primary functional unit of the voltage-gated sodium channel is a 220 kDa polypeptide α-subunit and a single 36 kDa β-subunit. Mutations in the SCN4A gene encoding the Na_v1.4 channel α-subunit cause five skeletal muscle disorders: potassium aggravated myotonia, paramyotonia congenita, hyperkalaemic periodic paralysis, hypokalaemic periodic paralysis, and a form of congenital myasthenic syndrome (Simkin & Bendahhou, 2011). Depending on the disease, the triggering factors could be rest following exercise, cold, alcohol, stress, potassium ingestion and carbohydrates.

When open, a single Na_v1.4 pore allows up to 10 million sodium ions per second to pass across the cell membrane. The ability of sodium ions to flow through the pore is controlled by a relatively complex set of gating mechanisms that were first put forth by Hodgkin & Huxley (1952). At negative membrane potentials (e.g. −80 mV), voltage-gated sodium channels are in a resting or closed state and sodium ions cannot flow through the channel pore. In response to rapid depolarizing changes in transmembrane voltage, these closed channels can rapidly open in less than a millisecond, allowing sodium ions to flow along the electrochemical driving force into cells. Activation of sodium channels involves conformational changes that open the pore of the channel. If the membrane is rapidly repolarized, the activation gate can quickly return to the closed position by a process that is termed deactivation. Activation and deactivation are driven by the movement of gating charges. It is the transfer of these gating charges across the membrane that dictates the voltage dependence of sodium channel activation. Sodium channels also exhibit a distinct process termed inactivation that involves the blocking of the pore by a part of the sodium channel complex that is distinct from the activation gate. Inactivation can occur within a millisecond, playing an important role in modulating the duration of action potentials. Sodium channels must be hyperpolarized for several milliseconds in order to recover from inactivation and therefore differences in inactivation properties can also play a role in regulating the firing frequency of excitable cells. Rather, it is believed that binding of the inactivation gate depends on movement of one or more of the voltage sensors that control activation. Sodium channels can inactivate without complete activation. This so-called closed-state inactivation is slower than open-state inactivation. Changes in the proportion of channels that are in the closed versus inactivated states can play a role in modulating the relative excitability of neurons and muscle cells.

The situation is further complicated because skeletal muscle sodium channels can also undergo much slower inactivation (with time constants on the order of seconds to minutes) and it has been proposed that this form of inactivation may also be caused by some of the disease-associated Na_v1.4 mutations (Ruff, 1994; Cummins & Sigworth, 1996). Slow inactivation is thought to be kinetically and functionally distinct from the fast-inactivation process. It appears to be a separate voltage-dependent process that provides long-term modulation of sodium channel availability.

Over 40 disease-associated mutations have been identified in the α-subunit of the Na_v1.4 channel. Although the mutations may appear to be randomly scattered throughout the channel sequence, some patterns seem to exist. All the aforementioned gating steps of Na_v1.4 channels are affected by these mutations: activation, fast and slow inactivation, deactivation, and recovery from inactivation.

In this issue of The Journal of Physiology, Zhao et al. (2012) report on the functional properties of the M1476I missense mutation located in the linker S4–S5 of domain IV of the Na_v1.4. Interestingly, this mutation was found in the largest cohort of sodium channel myotonia associated with a single Na_v1.4 mutation (Rossignol et al. 2007). Several defects, produced by disruption of fast inactivation and leading to a gain of function, were identified upon expression in a mammalian cell background: increase in the persistent current, slow rate of inactivation from the open state, acceleration of the recovery from fast inactivation, and a depolarized shift in the voltage dependence. Cold-induced myotonia was prominent in this cohort and present in 41% of the cases. The authors elegantly addressed this clinical phenotype by studying the channels at low temperatures (23°C, 18°C and 10°C). They clearly show that the persistent current is increased in the M1476I compared to wild-type channels. An increase in the persistent current can cause enhanced depolarization leading to repetitive muscle activity as seen in these families.

Treatment for myotonia has been focused on reducing the involuntary action potential bursts without blocking the voluntary high-frequency muscle stimulation. Agents such as mexiletine and flecainide (orally administered methylated lidocaine derivatives) prevent repetitive action potential firing and myotonia by reversibly blocking sodium channels (Wang et al. 2004). Like most local anaesthetics, mexiletine produces both tonic and use-dependent block of sodium channels. Several studies have shown that the efficacy of mexiletine is mainly due to the open-channel block of the persistent sodium current.

Mexiletine was efficient in vivo by reducing myotonic activity in the present case as well as in other sodium channel myotonia mutations. In an extreme case that has recently been reported with severe myotonia in newborn babies, treatment with mexiletine has prevented death in two patients (Lion-François et al. 2010). Like in the actual M1476I mutation, in vitro studies of the related mutation revealed a large persistent current in the A799S life-threatening mutated Na_v1.4 channels (Simkin et al. 2011).

Additional investigations on mexiletine action are needed in order to better understand how this anti-arrhythmic drug preferentially binds to the open state of the sodium channels and to shed light on the heterogeneity of the clinical symptoms observed with mexiletine.