Unmasking of a Novel Potassium Current in Drosophilaby a Mutation and Drugs

Abstract

The delayed rectifier potassium current plays a critical role in cellular physiology. This current (I(K)) in Drosophila larvae is believed to be a single current. However, a likely null mutation in the Shab K(+) channel gene (Shab(3)) reduces I(K) but does not eliminate it. This raises a question as to whether or not the entire I(K) passes through channels encoded by one gene. Similarly, an incomplete blockade of I(K) by high concentrations of quinidine, a selective I(K) blocker, raises a question as to whether I(K) consists of two components that are differentially sensitive to quinidine. We have addressed these questions by a combined use of genetics, pharmacology, and physiology. The current component removed by the Shab(3) mutation differed from the remaining component in activation kinetics, inactivation kinetics, threshold of activation, and voltage dependence. The two components showed strong differences in sensitivity to quinidine. Physiological properties of the current component removed by the Shab(3) mutation were similar to those of the quinidine-sensitive fraction of I(K). Complementary to this, properties of the current component remaining in the Shab(3) mutant muscles were similar to those of the quinidine-resistant fraction of I(K). These observations strongly suggest that, in contrast to the current belief, I(K) consists of two components in Drosophila, which are genetically, pharmacologically, and physiologically distinct. These components are being called I(KS) and I(KF). I(KS) is carried via Shab-encoded channels. I(KF) defines a new voltage-activated K(+) current in Drosophila.

Distinct Frequency-Dependent Regulation of Nerve Terminal Excitability and Synaptic Transmission by IA and IK Potassium Channels Revealed by Drosophila Shaker and Shab Mutations

“…Heterologous expression of Shab produces sustained delayed rectifier K ϩ currents (Wei et al, 1990), consistent with the phenotypes of Shab mutants (i.e., reduced I K in neurons) (I.-F. Peng and C.-F. Wu, unpublished observation) and muscle cells (Singh and Singh, 1999;Chopra et al, 2000). Additionally, dominant-negative Kv2 expression reduces I K in vertebrate cultured neurons (Blaine and Ribera, 2001;Malin and Nerbonne, 2002).…”

“…5A). We found that giant ejps similar to those observed in Shab could be induced by repetitive nerve stimulation in WT larvae treated with quinidine, which is known to block specifically Shab currents at low concentrations in larval muscles (Singh and Singh, 1999) and in cultured neu- rons (I.-F. Peng and C.-F. Wu, unpublished observation). In contrast, WT larvae treated with 4-AP (200 M to 1 mM) displayed a significantly increased ejp (tens of millivolts) immediately after a single nerve stimulus (Fig.…”

“…Our results demonstrate a functional division between Sh I A and Shab I K in regulation of synaptic transmission within different temporal domains and frequency ranges. (Singh and Singh, 1999) and cultured neurons (I.-F. Peng and C.-F. Wu, unpublished observations). Inactivating K ϩ current I A is not detectable in muscle cells (Wu and Haugland, 1985) and decreased in amplitude in the soma of cultured neurons (Zhao et al, 1995) (I.-F. Peng and C.-F. Wu, unpublished observation) in both Sh alleles.…”

“…Because postsynaptic muscle cells also express Shab I K (Singh and Singh, 1999), electrical activity in muscle cells could be affected by Shab mutations. Under normal conditions, larval muscles do not produce all-or-none action potentials (Wu and Haugland, 1985).…”

Complementation of Physiological and Behavioral Defects by a Slowpoke Ca2+ ‐Activated K+ Channel Transgene

“…The Shaker gene produces the channel that conducts the fast-activating voltage-dependent current called I A (Baumann et al, 1987;Kamb et al, 1987;Papazian et al, 1987). In larval muscle, the Shab gene has been shown to encode a channel that contributes to I K (Hegde et al, 1999;Singh and Singh, 1999). I CS is a slowactivating Ca 2ϩ -dependent current (I Kc in Wei and Salkoff, 1986).…”