Abstract
In the central nervous system, the M-current plays a critical role in regulating subthreshold electrical excitability of neurons, determining their firing properties and responsiveness to synaptic input. The M-channel is mainly formed by subunits Kv7.2 and Kv7.3 that co-assemble to form a heterotetrametric channel. Mutations in Kv7.2 and Kv7.3 are associated with hyperexcitability phenotypes including benign familial neonatal epilepsy (BFNE) and neonatal epileptic encephalopathy (NEE). SGK1.1, the neuronal isoform of the serum and glucocorticoids-regulated kinase 1 (SGK1), increases M-current density in neurons, leading to reduced excitability and protection against seizures. Herein, using two-electrode voltage clamp on Xenopus laevis oocytes, we demonstrate that SGK1.1 selectively activates heteromeric Kv7 subunit combinations underlying the M-current. Importantly, activated SGK1.1 increases M-channel activity in the presence of two different epilepsy mutations found in Kv7.2, R207W and A306T. In addition, proximity ligation assays in the N2a cell line allowed us to address the effect of these mutations on Kv7-SGK1.1-Nedd4 molecular associations, a proposed pathway underlying augmentation of M-channel activity by SGK1.1
Highlights
Mutations affecting ion channel subunit-encoding genes underly several forms of epilepsy
In the first set of experiments, we compared the effect of wild type (WT) serum and glucocorticoids-regulated kinase 1 (SGK1).1 with that of constitutively active SGK1.1 mutant S515D on Kv7.2/3-mediated M-currents
We found that coexpression of WT SGK1.1 with Kv7.2/3 resulted in significantly increased peak and tail currents (Figures 1A–C) without altering normalized conductance (Figure 1D), as we have previously described (Miranda et al, 2013)
Summary
Mutations affecting ion channel subunit-encoding genes underly several forms of epilepsy (for a recent review see Oyrer et al, 2018). In the CNS, KCNQ2, and KCNQ3 genes encode, respectively, the Kv7.2 and Kv7.3 K+ channel subunits, which co-assemble and form a heterotetrametric channel underlying the M-current (Brown and Adams, 1980; Wang et al, 1998). M-channels are active at subthreshold membrane potentials (near −60 mV). Their activation is slow and they do not contribute to the repolarization of individual action potentials. The M-current plays a critical role in dynamically regulating subthreshold electrical excitability of neurons, determining their firing properties and responsiveness to synaptic input
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