Abstracts

Neuronal Kv7 (KCNQ) channels composed of Kv7.2 and Kv7.3 subunits generate voltage-dependent slow-activating, non-inactivating outward potassium currents (I_M) which hyperpolarize the resting membrane potential (RMP), potently suppressing repetitive and burst firing of action potentials (APs). Consistent with their critical role in neuronal excitability, pathogenic mutations in Kv7.2 and Kv7.3 cause neonatal epilepsies, including benign familial neonatal epilepsy (BFNE) and epileptic encephalopathy (EE), characterized by neurodevelopmental delay and intellectual disability. Although these channels are enriched at the neuronal axons in hippocampus and cortex, recent immuno-electron microscopy (EM) studies in primate prefrontal cortex have detected neuronal Kv7 subunits within dendritic spines. In this study, we tested the hypothesis that Kv7 channels are present at excitatory synapses of rodent hippocampal and cortical pyramidal neurons, where their K⁺ current suppresses excitatory synaptic transmission.

Using a biochemical approach and structured illumination microscopy, we detected Kv7.2 and Kv7.3 in dendritic spines as well as the postsynaptic density (PSD) of rodent hippocampal neurons, where they colocalize with PSD95. Calcium imaging revealed decreased calcium influx through NMDA receptors in dendritic spines upon pharmacological activation of Kv7 channels, suggesting that postsynaptic membrane hyperpolarization induced by Kv7 opening suppressed NMDA receptor activity.

We found that HA-tagged Kv7.3 alone localized to dendritic spines via its Ankyrin-G (AnkG) binding motif, whereas spine localization of HA-Kv7.2 required co-expression with Kv7.3. Both Kv7.2 and Kv7.3 bind to the short 190 kDa isoform of AnkG, the only AnkG isoform present in dendritic spines. Additionally, their spine localization was even increased by AnkG190 co-expression. Lastly, BFNE and EE mutations variably disrupted their postsynaptic targeting.

These findings reveal the novel postsynaptic localization and function of Kv7 channels, shifting the field to consider the direct impact of postsynaptic Kv7 dysfunction on synaptic transmission and the pathogenic mechanism underlying Kv7-associated neonatal epilepsy.