Disease-causing Slack potassium channel mutations produce opposite effects on excitability of excitatory and inhibitory neurons

SUMMARY

KCNT1encodes the sodium-activated potassium channel Slack (KCNT1, K _Na1.1), an important mediator of neuronal membrane excitability. Gain-of-function (GOF) mutations in humans lead cortical network hyperexcitability and seizures, as well as very severe intellectual disability. Using a mouse model of Slack GOF-associated epilepsy, we found that both excitatory and inhibitory neurons of the cerebral cortex have increased Na ⁺-dependent K ⁺(K _Na) currents and voltage-dependent sodium (Na _V) currents. The characteristics of the increased K _Nacurrents were, however, different in the two cell types such that the intrinsic excitability of excitatory neurons was enhanced but that of inhibitory neurons was suppressed. We further showed that the expression of Na _Vchannel subunits, particularly that of Na _V1.6, is upregulated and that the length of the axon initial segment (AIS) and of axonal Na _Vimmunostaining is increased in both neuron types. We found that the proximity of the AIS to the soma is shorter in excitatory neurons than in inhibitory neurons of the mutant animals, potentially contributing to the different effects on membrane excitability. Our study on the coordinate regulation of K _Nacurrents and the expression of Na _Vchannels may provide a new avenue for understanding and treating epilepsies and other neurological disorders.

In brief

In a genetic mouse model of Na ⁺-activated K ⁺potassium channel gene Slack-related childhood epilepsy, Wu et al. show that a disease-causing gain-of-function (GOF) mutation R455Hin Slack channel causes opposite effects on excitability of cortical excitatory and inhibitory neurons. In contrast to heterologous expression systems, they find that the increase in potassium current substantially alters the expression of sodium channel subunits, resulting in increased lengths of axonal initial segments.

Highlights

GOF mutations in Slack potassium channel cause elevated outward K ⁺currents and inward voltage-dependent Na ⁺(Na _V) currents in cortical neurons

Slack GOF does not alter the expression of Slack channel but upregulates the expression of Na _Vchannel

Slack GOF enhances the excitability of excitatory neurons but suppresses the firing of inhibitory interneurons

Slack GOF alters the length of AIS in both excitatory and inhibitory neurons

Proximity of AIS to the soma is different between excitatory neuron and inhibitory neuron