An old probe sheds new light on BK channel pore structure

499 C o m m e n t a r y In many scientific areas, including drug discovery, the greatest challenge is picking the best targets and questions to pursue. To develop improved treatments using target-based therapeutics, a better understanding of the roles of specific channel types in normal and patho-physiological function is needed. Development of pharmacological probes provides a proven approach for target validation in which selective channel modulators can be developed as needed and then tested in animal models and human tissues. This approach is limited by availability of suitable ion channel modulators, which must display appropriate potency, selectivity, and phys-iochemical and pharmacokinetic properties to enable meaningful pharmacological experiments. An understanding of the mechanism of action of a probe molecule is also vital for interpreting its effects at the cellular, organ, and in vivo levels. A new paper by Zhou and Lingle (2014) provides a detailed mechanistic study of paxilline block of high conductance calcium-activated potassium channels (referred to as BK, maxi-K, or slo1 channels). Zhou and Lingle find that paxilline, a secondary fungal metabolite produced by Penicillium paxilli, blocks BK channels by an unusual closed-state mechanism. This information provides a framework to interpret the effects of this compound at the cellular and tissue levels, as well as insights into the gating mechanism of these channels. A brief history of paxilline Paxilline is a member of a group of indole diterpene natural products that are potent and selective blockers of BK channels (Knaus et al., 1994). These BK channel inhibitors were identified in a biochemical screen using iodinated charybdotoxin (125 I-ChTx), a scorpion toxin peptide that blocks BK channels by binding at the extra-cellular pore entrance and occluding potassium flux (MacKinnon and Miller, 1988; Giangiacomo et al., 2008). At the time the indole diterpene inhibitors were identified , binding assays afforded one of the few viable higher throughput methods for screening ion channels. Knaus et al. (1994) described eight structurally related indole diterpenes, which all inhibited BK channels in vascular smooth muscle at low nanomolar or sub-nanomolar Correspondence to Owen B. McManus: o w e n b m c m a n u s @ g m a i l. c o m concentrations. Initial experiments with these indole diterpenes showed high selectivity for inhibition of BK channels compared with other ion channels, and these agents remain among the most potent and selective BK channel inhibitors described. Paxilline …