Sodium channel gene defects in the periodic paralyses

A peptide segment critical for sodium channel inactivation functions as an inactivation gate in a potassium channel

The short cytoplasmic peptide segment connecting domains III and IV of voltage-gated sodium channels (III-IV linker) is essential for fast inactivation. To test the functional similarity between the III-IV linker and the potassium channel inactivation particle, we attached the III-IV linker to the amino terminus of a noninactivating potassium channel. This chimeric channel inactivated rapidly and displayed biophysical properties similar to Shaker A-type potassium channels. Recovery from inactivation in the chimeric channels was accelerated by high external potassium, consistent with the idea that potassium ions passing through the channel displaced the III-IV linker inactivation particle. A mutation that completely abolishes fast inactivation in rat brain sodium channels also completely abolished inactivation in the chimera. These results demonstrate that the sodium channel III-IV linker can function as a fast inactivation gate and suggest a functional relationship between the fast inactivation processes of sodium and potassium channels.

Transporters, channels and human disease

The past year has seen significant advances in our understanding of the molecular biology of ion channels and transporters and their role in human disease. The star of the show has to be the cystic fibrosis chloride channel about which an enormous amount of information has been accumulated, and the functional effects of some of the mutations found in cystic fibrosis patients have been characterized.

Accessory subunits and sodium channel inactivation

Recent studies have shown that the accessory subunits of the voltage-gated sodium channel can modify its inactivation properties. Other studies have demonstrated that the cytoplasmic linker between domains III and IV is critical for fast inactivation. Future work should help to define the mechanisms by which these processes occur, and how mutations affecting sodium channel inactivation result in human neurological diseases.

Periodic paralysis, myotonia congenita and sarcolemmal ion channels: a success of the candidate gene approach

The classification of periodic paralyses and myotonic syndromes has been a subject of debates for the last 40 yr. Recent advances in molecular biology have led geneticists to reconsider this old question, using a candidate gene approach. Two groups of disorders have now emerged: (1) muscle sodium channel-associated diseases which include hyperkalemic periodic paralysis and its clinical variants, as well as paramyotonia congenita; (2) muscle chloride channel-associated disorders which comprise both the dominant and recessive form of myotonia congenita. This review is focussed on these recent discoveries.