Potassium channels in neuronal development

ABSTRACT Early in the development of the brain, the Kv3.4 voltage-dependent potassium channel is highly expressed in all major fiber tracts as they elongate towards their final destination. In most of these tracts, expression of Kv3.4 disappears as synapses form. The canonical function of Kv channels is to generate K+ flux during action potential firing in order to repolarize the plasma membrane, modulating the influx of Na+ and Ca2+ ions. In contrast, recent findings indicate that Kv3.4 triggers the formation of filopodia and promotes neurite outgrowth even in the absence of ion flux through the channel. Kv3.4 also differs from other closely related channels in that it directly binds the cell adhesion molecule Protocadherin 9 (PCDH9). Binding to PCDH9 is absolutely required for Kv3.4 to be retained and function as a K+ channel in the plasma membrane. The experiments in this proposal will test the hypothesis that the binding of Kv3.4 to PCDH9 allows the latter protein to activate the WAVE complex, triggering the extension of filopodia and neurite outgrowth by nucleation of actin filaments. Biochemical, imaging and patch clamp studies will determine how the ability of Kv3.4 to trigger the formation of filopodia in mammalian cells and the extension of neurites in cerebellar granule neurons is affected by membrane potential and by voltage-dependent inactivation of the channel. Experiments will determine the regions of the cytoplasmic C-termini of Kv3.4 and PCDH9 that are required for the protection of the channel from degradation and for coupling the Kv3.4/PCDH9 complex to the actin-nucleating WAVE complex. Finally, experiments in the intact nervous system will determine how loss of Kv3.4 affects the cellular organization of the cerebellum, and whether this can be rescued by early re-expression of wild-type Kv3.4 or of channel mutants that do not conduct K+ ions or interact with PCDH9. Because marked increases in Kv3.4 levels occur in adults during several disease states, including spinal cord injury, Alzheimer’s Disease and Huntington’s Disease, our findings will provide novel insights into plastic changes in neurite growth associated with these conditions and provide potential therapeutics targeted at the Kv3.4-PCDH9 complex.