Reduced axonal surface expression and phosphoinositide sensitivity in Kv7 channels disrupts their function to inhibit neuronal excitability in Kcnq2 epileptic encephalopathy

Neuronal K(v)7/KCNQ channels are voltage-gated potassium channels composed of K(v)7.2/KCNQ2 and K(v)7.3/KCNQ3 subunits. Enriched at the axonal membrane, they potently suppress neuronal excitability. De novo and inherited dominant mutations in K(v)7.2 cause early onset epileptic encephalopathy characterized by drug resistant seizures and profound psychomotor delay. However, their precise pathogenic mechanisms remain elusive. Here, we investigated selected epileptic encephalopathy causing mutations in calmodulin (CaM)-binding helices A and B of K(v)7.2. We discovered that R333W, K526N, and R532W mutations located peripheral to CaM contact sites decreased axonal surface expression of heteromeric channels although only R333W mutation reduced CaM binding to K(v)7.2. These mutations also altered gating modulation by phosphatidylinositol 4,5-bisphosphate (PIP2), revealing novel PIP2 binding residues. While these mutations disrupted K(v)7 function to suppress excitability, hyperexcitability was observed in neurons expressing K(v)7.2-R532W that displayed severe impairment in voltage-dependent activation. The M518 V mutation at the CaM contact site in helix B caused most defects in K(v)7 channels by severely reducing their CaM binding, K+ currents, and axonal surface expression. Interestingly, the M518 V mutation induced ubiquitination and accelerated proteasome-dependent degradation of K(v)7.2, whereas the presence of K(v)7.3 blocked this degradation. Furthermore, expression of K(v)7.2M518V increased neuronal death. Together, our results demonstrate that epileptic encephalopathy mutations in helices A and B of K(v)7.2 cause abnormal K(v)7 expression and function by disrupting K(v)7.2 binding to CaM and/or modulation by PIP2. We propose that such multiple K(v)7 channel defects could exert more severe impacts on neuronal excitability and health, and thus serve as pathogenic mechanisms underlying Kcnq2 epileptic encephalopathy.