Abstract
Gain-of-function pathogenic missense KCNT1 variants are associated with several developmental and epileptic encephalopathies (DEE). With few exceptions, patients are heterozygous and there is a paucity of mechanistic information about how pathogenic variants increase KNa1.1 channel activity and the behaviour of heterotetrameric channels comprising both wild-type (WT) and variant subunits. To better understand these, we selected a range of variants across the DEE spectrum, involving mutations in different protein domains and studied their functional properties. Whole-cell electrophysiology was used to characterise homomeric and heteromeric KNa1.1 channel assemblies carrying DEE-causing variants in the presence and absence of 10 mM intracellular sodium. Voltage-dependent activation of homomeric variant KNa1.1 assemblies were more hyperpolarised than WT KNa1.1 and, unlike WT KNa1.1, exhibited voltage-dependent activation in the absence of intracellular sodium. Heteromeric channels formed by co-expression of WT and variant KNa1.1 had activation kinetics intermediate of homomeric WT and variant KNa1.1 channels, with residual sodium-independent activity. In general, WT and variant KNa1.1 activation followed a single exponential, with time constants unaffected by voltage or sodium. Mutating the threonine in the KNa1.1 selectivity filter disrupted voltage-dependent activation, but sodium-dependence remained intact. Our findings suggest that KNa1.1 gating involves a sodium-dependent activation gate that modulates a voltage-dependent selectivity filter gate. Collectively, all DEE-associated KNa1.1 mutations lowered the energetic barrier for sodium-dependent activation, but some also had direct effects on selectivity filter gating. Destabilisation of the inactivated unliganded channel conformation can explain how DEE-causing amino acid substitutions in diverse regions of the channel structure all cause gain-of-function.
Competing Interest Statement
The authors have declared no competing interest.
Footnotes
Conflicts of Interest statement The authors have no conflicts of interest to declare.
Main text edited for typos, font errors, and clarity. Summary scheme added as Figure 7.
Abbreviations
- (AD)SHE
- autosomal-dominant or sporadic sleep-related hypermotor epilepsy
- DEE
- developmental and epileptic encephalopathy
- EIMFS
- epilepsy of infancy with migrating focal seizures
- GOF
- gain-of-function
- K2P
- two pore domain-containing potassium channel subunit
- KNa1.1
- sodium-activated potassium channel subunit
- PO
- channel open probability
- WT
- wild-type