Phosphoinositides Control the Activity of Slick and Slack K+ Channels

Abstract

Phosphoinositides are acidic membrane phospholipids involved in cell signaling in eukaryotes. The most common isoform in the plasma membrane, phosphatidylinositol 4,5-biphosphate, usually named PIP2, has proven to be necessary for the activity of a number of ion channels and transporters. PIP2 regulates all members of the Kir and KCNQ channel families; furthermore it also directly modulates Slo1 (BKCa) and Slo3 channels. Slick (Slo2.1) and Slack (Slo2.2) channels are high-conductance K+ channels widely distributed in the CNS. The present work evaluates the PIP2 sensitivity of Slo2 channels, namely Slick and Slack K+ channels. To address this issue, both channels were expressed in Xenopus laevis oocytes and currents were measured by two-electrode voltage clamp before and after pre-incubation of oocytes in 10μM PIP2. Two isoforms of PIP2: PI(3,4)P2 and PI(4,5)P2 activated K+ currents through Slick and Slack channels. Manipulation of endogenous PIP2 levels using nanomolar concentrations of wortmannin, which is known to block PI3 kinase leading to PIP2 build-up in the membrane, resulted in increased currents through Slick and Slack channels. In contrast, application of micromolar concentrations of wortmannin, which block both PI3 and PI4 kinases leading to PIP2 depletion from the membrane, resulted in decreased activity of both channels. Treatment with neomycin, a polycation which acts as PIP2 scavenger and prevents it from interacting with the channels, produced a stepwise reduction in the activity of Slick and Slack channels induced by exogenous PIP2. In conclusion, the present study demonstrates for the first time that Slick and Slack K+ channels are regulated by the availability of PIP2 in the membrane. Therefore, phosphatidylinositol 4,5-biphosphate acts as common activator of the Slo K+ channel family despite their different structures and physiological roles.