Epileptic encephalopathy-related Kv2.1 mutants impair channel clustering and membrane distribution but not neuronal excitability

The voltage-gated potassium channel Kv2.1, which is encoded by the epileptic encephalopathy-associated gene KCNB1, is a primary driver of delayed-rectifier K+ currents in neurons. These currents contribute to high frequency firing by preventing depolarization block due to Na+ channel inactivation. Wild-type (WT) channels are localized at the soma, proximal dendrites and axons, forming large aggregates (clusters) via their C-terminal proximal restriction and clustering domain (PRC). This study investigates the biophysical and functional consequences of two C-terminal truncation mutations (Y529* and R579*), identified in patients with epileptic encephalopathy, which disrupt this critical clustering domain. Cluster formation was impaired, though not abolished, in neurons expressing mutated subunits together with endogenous WT subunits. Consistent with this clustering deficit, single-molecule imaging revealed altered channel dynamics, with WT channels remaining largely immobile, while mutated forms exhibited intermittent diffusion punctuated by transient immobilization events. This behavior was reproduced by diffusion-capture simulations considering channels with different number of PRC domains (i.e. with mutant subunits) and labile scaffolding interactions. Patch-clamp recordings revealed no significant difference in excitability between WT and mutant-expressing neurons. However, we observed increased firing in both conditions compared to non-transfected neurons. This suggests that Kv2.1 overexpression (for both WT and mutant channels) counterintuitively enhances neuronal excitability. Our data challenge a canonical role of this voltage-gated potassium channel in dampening neuronal firing, indicating that its overall expression level can be a critical and paradoxical driver of hyperexcitability.