Alternative splicing of KCNQ2 potassium channel transcripts contributes to the functional diversity of M-currents

The region of alternative splicing in the KCNQ2 potassium channel gene was determined by RNase protection analysis of KCNQ2 mRNA transcripts. Systematic analysis of KCNQ2 alternative splice variant expression in rat superior cervical ganglia revealed multiple variant isoforms. One class of KCNQ2 splice variants, those that contained exon 15a, was found to have significantly different kinetics to those of the other isoforms. These transcripts encoded channel subunits that, when co-expressed with the KCNQ3 subunit, activated and deactivated approximately 2.5 times more slowly than other isoforms. Deletion of exon 15a in these isoforms produced a reversion to the faster kinetics. Comparison of the kinetic properties of the cloned channel splice variants with those of the native M-current suggests that alternative splicing of the KCNQ2 gene may contribute to the variation in M-current kinetics seen in vivo. The KCNQ2 and KCNQ3 potassium channel genes encode subunits that co-assemble to form heteromeric channels that underlie the M-current in sympathetic neurons and most probably also contribute to M-currents in central neurons (Wang et al. 1998). The M-current is a slowly activating and deactivating current that plays a critical role in controlling neuronal excitability (Brown, 1988; Yamada et al. 1989; Wang & McKinnon, 1995). In accordance with this property, mutations in either gene can produce benign familial neonatal epilepsy (Singh et al. 1998; Biervert et al. 1998; Charlier et al. 1998). The KCNQ2 gene has a complex genomic structure (Nakamura et al. 1998; Biervert & Steinlein, 1999) and can produce multiple differentially spliced transcripts (Nakamura et al. 1998; Tinel et al. 1998). To date, the functional properties of channels encoded by alternatively spliced transcripts produced from the KCNQ2 gene have not been studied systematically. For this reason, we have attempted to determine the complete diversity of KCNQ2 mRNA transcripts expressed in rat sympathetic ganglia and have examined the functional properties of the channels encoded by these alternative splice variants. We chose the rat superior cervical ganglia (SCG) because neurons in these ganglia are most commonly used for studies of M-channel function and the properties of the M-channel are best described in this tissue.