Affinity for phosphatidylinositol 4,5-bisphosphate determines muscarinic agonist sensitivity of Kv7 K+ channels

摘要

Kv7 K+-channel subunits differ in their apparent affinity for PIP2 and are differentially expressed in nerve, muscle, and epithelia in accord with their physiological roles in those tissues. To investigate how PIP2 affinity affects the response to physiological stimuli such as receptor stimulation, we exposed homomeric and heteromeric Kv7.2, 7.3, and 7.4 channels to a range of concentrations of the muscarinic receptor agonist oxotremorine-M (oxo-M) in a heterologous expression system. Activation of M1 receptors by oxo-M leads to PIP2 depletion through Gq and phospholipase C (PLC). Chinese hamster ovary cells were transiently transfected with Kv7 subunits and M1 receptors and studied under perforated-patch voltage clamp. For Kv7.2/7.3 heteromers, the EC50 for current suppression was 0.44 ± 0.08 μM, and the maximal inhibition (Inhibmax) was 74 ± 3% ( n = 5–7). When tonic PIP2 abundance was increased by overexpression of PIP 5-kinase, the EC50 was shifted threefold to the right (1.2 ± 0.1 μM), but without a significant change in Inhibmax (73 ± 4%, n = 5). To investigate the muscarinic sensitivity of Kv7.3 homomers, we used the A315T pore mutant (Kv7.3T) that increases whole-cell currents by 30-fold without any change in apparent PIP2 affinity. Kv7.3T currents had a slightly right-shifted EC50 as compared with Kv7.2/7.3 heteromers (1.0 ± 0.8 μM) and a strongly reduced Inhibmax (39 ± 3%). In contrast, the dose–response curve of homomeric Kv7.4 channels was shifted considerably to the left (66 ± 8 nM), and Inhibmax was slightly increased (81 ± 6%, n = 3–4). We then studied several Kv7.2 mutants with altered apparent affinities for PIP2 by coexpressing them with Kv7.3T subunits to boost current amplitudes. For the lower affinity (Kv7.2 (R463Q)/Kv7.3T) or higher affinity (Kv7.2 (R463E)/Kv7.3T) channels, the EC50 and Inhibmax were similar to Kv7.4 or Kv7.3T homomers (0.12 ± 0.08 μM and 79 ± 6% [ n = 3–4] and 0.58 ± 0.07 μM and 27 ± 3% [ n = 3–4], respectively). The very low-affinity Kv7.2 (R452E, R459E, and R461E) triple mutant was also coexpressed with Kv7.3T. The resulting heteromer displayed a very low EC50 for inhibition (32 ± 8 nM) and a slightly increased Inhibmax (83 ± 3%, n = 3–4). We then constructed a cellular model that incorporates PLC activation by oxo-M, PIP2 hydrolysis, PIP2 binding to Kv7-channel subunits, and K+ current through Kv7 tetramers. We were able to fully reproduce our data and extract a consistent set of PIP2 affinities.