Effects of Hyperkinetic, a β Subunit of Shaker Voltage-Dependent K+ Channels, on the Oxidation State of Presynaptic Nerve Terminals

Abstract

The Drosophila Hyperkinetic (Hk) gene encodes a β subunit of Shaker (Sh) K+ channels and shows high sequence homology to aldoketoreductase. Hk mutations are known to modify the voltage dependence and kinetics of Sh currents, which are also influenced by the oxidative state of the N-terminus region of the Sh channel, as demonstrated in heterologous expression experiments in frog oocytes. However, an in vivo role of Hk in cellular reduction/oxidation (redox) has not been demonstrated. By using a fluorescent indicator of reactive oxygen species (ROS), dihydrorhodamine-123 (DHR), we show that the presynaptic nerve terminal of larval motor axons is metabolically active, with more rapid accumulation of ROS in comparison with muscle cells. In Hk terminals, DHR fluorescence was greatly enhanced, indicating increased ROS levels. This observation implicates a role of the Hk β subunit in redox regulation in presynaptic terminals. This phenomenon was paralleled by the expected effects of the mutations affecting glutathione S-transferase S1 as well as applying H2O2 to wild-type synaptic terminals. Thus, our results also establish DHR as a useful tool for detecting ROS levels in the Drosophila neuromuscular junction.