Sequence determinants of subtype‐specific actions of KCNQ channel openers

Abstract

Retigabine (RTG) is the first approved anti-epileptic drug that acts via activation of voltage-gated potassium channels, targeting KCNQ channels that underlie the neuronal M-current. RTG exhibits little specificity between KCNQ2-5 as a result of conservation of a Trp residue in the pore domain that binds to the drug. The RTG analogue ICA-069673 ('ICA73') exhibits much stronger effects on KCNQ2 channels, including a large hyperpolarizing shift of the voltage-dependence of activation, an ∼2-fold enhancement of peak current and pronounced subtype specificity for KCNQ2 over KCNQ3. Based on ICA73 sensitivity of chimeric constructs of the transmembrane segments of KCNQ2 and KCNQ3, this drug appears to interact with the KCNQ2 voltage sensor (S1-S4) rather than the pore region targeted by RTG. KCNQ2 point mutants in the voltage sensor were generated based on KCNQ2/KCNQ3 sequence differences, and screened for ICA73 sensitivity. These experiments reveal that KCNQ2 residues F168 and A181 in the S3 segment are essential determinants of ICA73 subtype specificity. Mutations at either position in KCNQ2 abolish the ICA73-mediated gating shift, but preserve RTG sensitivity. Interestingly, A181P mutant channels show little ICA73-mediated gating shift but retain current potentiation by the drug. Mutations (L198F and P211A), which introduce these critical KCNQ2 residues at corresponding positions in KCNQ3, transplant partial ICA73 sensitivity. These findings demonstrate that RTG and ICA73 act via distinct mechanisms, and also reveal specific residues that underlie subtype specificity of KCNQ channel openers.

M‐type K+ channels in peripheral nociceptive pathways

“…By introducing a CF3‐group at the 4‐position of the benzylamine moiety and a fluorine at the 3‐position of the aniline ring (Figure E), they generated a compound (RL648_81) which is over 15 times more potent than retigabine at K v 7.2/K v 7.3 channels (EC 50 0.2 μM vs. ~3 μM) and does not affect K v 7.4 and K v 7.5 (Kumar et al, ). Another recent compound, ICA‐069673 (and related derivatives such as ICA‐27243 and ztz series compounds), displays remarkable selectivity for K v 7.2 over K v 7.3 (Figure F) (Wickenden et al, ; Gao et al, ; Wang et al, ).…”

“…Here, we review the current literature on the expression in pain pathways, functional role and a potential for therapeutic targeting of a family of K + channels called ‘M channels’ (KCNQ, K v 7), which are increasingly recognized as one of the important mechanisms controlling nociceptive fibre activity. Several earlier reviews on this or related topics are available (Munro and Dalby‐Brown, ; Gribkoff, ; Rivera‐Arconada et al, ; Wickenden and McNaughton‐Smith, ; Du and Gamper, ; Wang and Li, ), so here, we will focus on recent developments in the field as well as on the emerging understanding of the sites of analgesic efficacy of M channel potentiating drugs (‘openers’) within the mammalian nervous system. Since most of the available evidence concerns the M channels expressed within the peripheral somatosensory system, we will mainly focus on the role of M channels in peripheral pain pathways.…”

Functional up‐regulation of the M‐current by retigabine contrasts hyperexcitability and excitotoxicity on rat hypoglossal motoneurons

“…; Wang et al . ).Thus, the results of the present study provide further evidence of possible therapeutic outcomes with respect to positive modulation of the I M in the context of hyperexcitability and excitotoxicity.…”

Specificity of M‐channel activators: binding or effect?

“…The study published in this issue of The Journal of Physiology by Wang and coworkers (Wang et al . ) brings us one important step closer to understanding how subtype specificity may be achieved. Wang and coworkers compared the effects of the retigabine analogue ICA069673 (called ICA73) on homomeric KCNQ2 and KCNQ3 channels and found that ICA73, in contrast to retigabine, is a potent activator of KCNQ2, but has basically no effect on KCNQ3.…”