Ether-à-go-go-related gene K+ channels contribute to threshold excitability of mouse auditory brainstem neurons.

The ionic basis of excitability requires identification and characterisation of expressed channels and their specific roles in native neurons. We have exploited principal neurons of the medial nucleus of the trapezoid body (MNTB) as a model system for examining voltage-gated K(+) channels, because of their known function and simple morphology. Here we show that channels of the ether-à-go-go-related gene family (ERG, Kv11; encoded by kcnh) complement Kv1 channels in regulating neuronal excitability around threshold voltages. Using whole-cell patch clamp from brainstem slices, the selective ERG antagonist E-4031 reduced action potential (AP) threshold and increased firing on depolarisation. In P12 mice, under voltage-clamp with elevated [K(+)](o) (20 mm), a slowly deactivating current was blocked by E-4031 or terfenadine (V(0.5,act) = -58.4 +/- 0.9 mV, V(0.5,inact) = -76.1 +/- 3.6 mV). Deactivation followed a double exponential time course (tau(slow) = 113.8 +/- 6.9 ms, tau(fast) = 33.2 +/- 3.8 ms at -110 mV, tau(fast) 46% peak amplitude). In P25 mice, deactivation was best fitted by a single exponential (tau(fast) = 46.8 +/- 5.8 ms at -110 mV). Quantitative RT-PCR showed that ERG1 and ERG3 were the predominant mRNAs and immunohistochemistry showed expression as somatic plasma membrane puncta on principal neurons. We conclude that ERG currents complement Kv1 currents in limiting AP firing at around threshold; ERG may have a particular role during periods of high activity when [K(+)](o) is elevated. These ERG currents suggest a potential link between auditory hyperexcitability and acoustic startle triggering of cardiac events in familial LQT2.

Activation gating of hERG potassium channels: S6 glycines are not required as gating hinges.

The opening of ion channels is proposed to arise from bending of the pore inner helices that enables them to pivot away from the central axis creating a cytosolic opening for ion diffusion. The flexibility of the inner helices is suggested to occur either at a conserved glycine located adjacent to the selectivity filter (glycine gating hinge) and/or at a second site occupied by glycine or proline containing motifs. Sequence alignment with other K+ channels shows that hERG possesses glycine residues (Gly648 and Gly657) at each of these putative hinge sites. In apparent contrast to the hinge hypotheses, substitution of both glycine residues for alanine causes little effect on either the voltage-dependence or kinetics of channel activation, and open state block by intracellular blockers. Substitution of the glycines with larger hydrophobic residues causes a greater propensity for the channel to open. We propose that in contrast to Shaker the pore of hERG is intrinsically more stable in the open than the closed conformation and that substitution at Gly648 or Gly657 further shifts the gating equilibrium to favor the open state. Molecular dynamics simulations indicate the S6 helices of hERG are inherently flexible, even in the absence of the glycine residues. Thus hERG activation gating exhibits important differences to other Kv channels. Our findings indicate that the hERG inner helix glycine residues are required for the tight packing of the channel helices and that the flexibility afforded by glycine or proline residues is not universally required for activation gating.