Shal-gene-related voltage-dependent potassium (Kv4) channels mediate rapidly inactivating A-type currents. Kv4.2 channels are the moelcular correlate of the neuronal subthreshold-activating A-type current (I_SA), which controls dendritic excitation. Certain forms of synaptic long-term potentiation (LTP) in the hippocampus are accompanied by a local down-regulation of Kv4.2 channels, and in animal models of temporal lobe epilepsy (TLE) and cortical malformations a general reduced availability of functional Kv4.2 channels can be observed. However, the molecular mechanisms underlying plasticity-related and pathophysiological Kv4.2 channel remodelling have remained obscure. Native Kv4.2 channel complexes are associated with Kv Channel Interacting Proteins (KChIPs), belonging to the Neuronal Calcium Sensor (NCS) family of Ca²⁺-binding proteins, and with Dipeptidyl aminopeptidase (DPP)-like proteins. Both accessory subunits have been shown to enhance functional surface expression and to modulate gating parameters of Kv4.2 channels in heterologous expression systems.

We asked if the effects observed in heterologous expression systems reflect a role for accessory subunits in the normal trafficking, targeting and functioning, and/or during pathophysiological remodelling of Kv4.2 channels. To answer this question, we transfected cultured rat hippocampal neurons with cDNA coding for Kv4.2 wild-type or a KChIP binding-deficient mutant (Kv4.2A14K). The channel constructs were decorated with an external HA- and a C-terminal EGFP-tag. Combined analysis of EGFP self-fluorescence and HA immunostaining showed that Kv4.2A14K-HA-EGFP is transported into dendrites and gets to the surface membrane. Co-transfection with KChIP2 cDNA further increased dendritic surface expression of wild-type but not KChIP binding-deficient Kv4.2 mutants.

Electrophysiological recordings in the whole-cell and nucleated-patch configuration confirmed our immunocytochemistry results, with Kv4.2 + KChIP2 co-transfected neurons yielding the largest A-type current amplitudes. Based on these results we studied glutamatergic postsynaptic signals in control (EGFP-transfected) and Kv4.2 overexpressing (Kv4.2 + KChIP2 co-transfected) neurons. Notably, miniature excitatory postsynaptic currents (mEPSCs) and potentials (mEPSPs) exhibited no difference between the two groups.

Our results indicate that KChIP binding is not an absolute requirement for dendritic targeting and surface expression of Kv4.2 channels in hippocampal neurons. Furthermore, our results show that, quantal postsynaptic events (mEPSCs and mEPSPs) are not under direct control of Kv4.2 channels. It is likely, that the subthreshold activity of Kv4.2 channels and NCS properties of Kv4.2-associated KChIPs come into play only during events with massive Ca²⁺ influx like the backpropagation of action potentials and evoked synaptic responses.

Supported by grants BA 2055/4 and SFB 444 from the Deutsche Forschungsgemeinschaft to RB.