Phosphatidylinositol 4,5-bisphosphate alters pharmacological selectivity for epilepsy-causing KCNQ potassium channels

Abstract

Pharmacological augmentation of neuronal KCNQ muscarinic (M) currents by drugs such as retigabine (RTG) represents a first-in-class therapeutic to treat certain hyperexcitatory diseases by dampening neuronal firing. Whereas all five potassium channel subtypes (KCNQ1-KCNQ5) are found in the nervous system, KCNQ2 and KCNQ3 are the primary players that mediate M currents. We investigated the plasticity of subtype selectivity by two M current effective drugs, retigabine and zinc pyrithione (ZnPy). Retigabine is more effective on KCNQ3 than KCNQ2, whereas ZnPy is more effective on KCNQ2 with no detectable effect on KCNQ3. In neurons, activation of muscarinic receptor signaling desensitizes effects by retigabine but not ZnPy. Importantly, reduction of phosphatidylinositol 4,5-bisphosphate (PIP 2 ) causes KCNQ3 to become sensitive to ZnPy but lose sensitivity to retigabine. The dynamic shift of pharmacological selectivity caused by PIP 2 may be induced orthogonally by voltagesensitive phosphatase, or conversely, abolished by mutating a PIP 2 site within the S4-S5 linker of KCNQ3. Therefore, whereas drugchannel binding is a prerequisite, the drug selectivity on M current is dynamic and may be regulated by receptor signaling pathways via PIP 2 . . They open at subthreshold voltages and the channel activity is functionally down-regulated by G protein coupled receptor (GPCR) signaling pathways through intracellular secondary messengers, primarily phosphatidylinositol 4,5-bisphosphate (PIP 2 ) (1, 2). In the nervous system, KCNQ2 and KCNQ3 subtypes are major determinants for potassium current sensitive to muscarinic (M) receptor signaling, hence also known as M current (3). The combination of sensitivity to subthreshold voltages and down-regulation by receptor signaling makes M current a critical component for controlling membrane potential in the nervous system, and consequently neuronal firing (4-6). KCNQ2-5 channels, with few exceptions (7), are primarily expressed in the brain and peripheral nervous system. KCNQ1, although it is found in brain (8), is more abundantly expressed in the heart to mediate slow delayed rectifier K + current (also known as I Ks ) that contributes to the termination of action potential, hence regulating QT duration (9).Retigabine (RTG) or ethyl N-[2-amino-4-[(4-fluorophenyl) methylamino] phenyl]carbamate, a positive allosteric modulator (or an opener) for neuronal KCNQ channel, is a first-in-class therapeutic for antiepileptic treatment (10-12). Its clinical use to dampen the hyperexcitatory activity has greatly intensified the interest in a more detailed understanding of chemical augmentation of voltage-sensitive channel activity. Retigabine has selectivity for KCNQ2, -3, -4, and -5 potassium channels but not KCNQ1 potassium channels (13). Tryptophan (W) 236 in KCNQ2 is conserved among the sensitive channels. Interestingly, KCNQ2 channels with W236L mutation (or KCNQ2 W236L ) are functional but no longer sensitive to retigabine (14,15). This and other evidence supports the idea that W236 is a ...

Dynamic PIP 2 interactions with voltage sensor elements contribute to KCNQ2 channel gating

“…1B). The closing of the KCNQ2 channel in response to decreased PIP 2 concentration is consistent with previous observations (6). The M1 and Dr-VSP experiments support the positive effects of PIP 2 on current amplitude.…”

“…We first cotransfected KCNQ2 and Musicrinic receptor 1 (M1) in the cells, where PIP 2 is reduced through the phospholipase C (PLC)-mediated lipid hydrolysis (3,14). Consistent with previous reports (6,11,19), we observed that activation of the M1 receptor by Oxo-M (5 μM) induces a dramatic suppression of the current amplitude (Fig. 1A).…”

“…Depletion of PIP 2 suppresses the current of the KCNQ2 channel (6,11,19). However, the effects of PIP 2 on the voltage sensitivity of the KCNQ2 channel remain unclear.…”

“…Structural studies on Kir channels (1,2,5) demonstrated that PIP 2 directly interacts with the channels. Subsequent studies supported that PIP 2 also interacts directly with voltage-gated potassium (Kv) channels (6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19). Several positive residues that may be critical for PIP 2 activity have been identified (7,11,18,(20)(21)(22)(23)(24).…”

Ion channel regulation by β-secretase BACE1 – enzymatic and non-enzymatic effects beyond Alzheimer's disease

“…1B1), similar to neurons in which Mcurrent was suppressed by drugs or genetic manipulations. 94,98,99 Furthermore, the M-current blocker XE991, which increased input resistance and firing frequency, and reduced latency to first AP in control neurons, failed to do so in mutant neurons. In voltage-clamp recordings, M-currents of acutely isolated CA1 pyramidal neurons from BACE1-deficient mice were reduced by 2/3 and displayed faster decay kinetics when compared to their wild type counterparts (Fig.…”