SPAK and OSR1 Sensitive Kir2.1 K⁺ Channels

BACKGROUND/AIMS: Kir2.1 (KCNJ2) channels are expressed in neurons, skeletal muscle and cardiac tissue and maintain the resting membrane potential. The activity of those channels is regulated by diverse signalling molecules. The present study explored whether Kir2.1 channels are sensitive to the transporter and channels regulating kinases SPAK (SPS1-related proline/alanine-rich kinase) and OSR1 (oxidative stress-responsive kinase 1), which are in turn regulated by WNK (with-no-K[Lys]) kinases. METHODS: cRNA encoding Kir2.1 was injected into Xenopus laevis oocytes with or without additional injection of cRNA encoding wild-type SPAK, constitutively active T233E SPAK, WNK insensitive T233A SPAK, catalytically inactive D212A SPAK, wild-type OSR1, constitutively active T185E OSR1, WNK insensitive T185A OSR1 and catalytically inactive D164A OSR1. Inwardly rectifying K+ channel activity was quantified utilizing dual electrode voltage clamp and Kir2.1 channel protein abundance in the cell membrane was measured utilizing chemiluminescence of Kir2.1 containing an extracellular HA-tag epitope. RESULTS: Kir2.1 activity was significantly enhanced by wild-type SPAK and T233E SPAK, but not by T233A SPAK and D212A SPAK, as well as by wild-type OSR1 and T185E OSR1, but not by T185A OSR1 and D164A OSR1. As shown for SPAK, the kinases enhanced Kir2.1 protein abundance in the cell membrane. The difference of current and conductance between oocytes expressing Kir2.1 together with SPAK or OSR1 and oocytes expressing Kir2.1 alone was dissipated following a 24 hours inhibition of channel insertion into the cell membrane by brefeldin A (5 µM). CONCLUSIONS: SPAK and OSR1 are both stimulators of Kir2.1 activity. They are presumably effective by enhancing channel insertion into the cell membrane.