Subthreshold A-type K(+) currents (ISA s) have been recorded from the cell bodies of hippocampal and neocortical interneurons as well as neocortical pyramidal neurons. Kv4 channels are responsible for the somatodendritic ISA s. It has been proposed that neuronal Kv4 channels are ternary complexes including pore-forming Kv4 subunits, K(+) channel-interacting proteins (KChIPs), and dipeptidyl peptidase-like proteins (DPPLs). However, colocalization evidence was still lacking. The distribution of DPP10 mRNA in rodent brain has been reported but its protein localization remains unknown. In this study, we generated a DPP10 antibody to label DPP10 protein in adult rat brain by immunohistochemistry. Absent from glia, DPP10 proteins appear mainly in the cell bodies of DPP10(+) neurons, not only at the plasma membrane but also in the cytoplasm. At least 6.4% of inhibitory interneurons in the hippocampus coexpressed Kv4.3, KChIP1, and DPP10, with the highest density in the CA1 strata alveus/oriens/pyramidale and the dentate hilus. Colocalization of Kv4.3/KChIP1/DPP10 was also detected in at least 6.9% of inhibitory interneurons scattered throughout the neocortex. Both hippocampal and neocortical Kv4.3/KChIP1/DPP10(+) inhibitory interneurons expressed parvalbumin or somatostatin, but not calbindin or calretinin. Furthermore, we found colocalization of Kv4.2/Kv4.3/KChIP3/DPP10 in neocortical layer 5 pyramidal neurons and olfactory bulb mitral cells. Together, although DPP10 is also expressed in some brain neurons lacking Kv4 (such as parvalbumin- and somatostatin-positive Golgi cells in the cerebellum), colocalization of DPP10 with Kv4 and KChIP at the plasma membrane of ISA -expressing neuron somata supports the existence of Kv4/KChIP/DPPL ternary complex in vivo.

Membrane excitability in the axonal growth cones of embryonic neurons influences axon growth. Voltage-gated K (Kv) channels are key factors in controlling membrane excitability, but whether they regulate axon growth remains unclear. Here, we report that Kv3.4 is expressed in the axonal growth cones of embryonic spinal commissural neurons, motoneurons, dorsal root ganglion neurons, retinal ganglion cells, and callosal projection neurons during axon growth. Our (cultured dorsal spinal neurons of chick embryos) and (developing chick spinal commissural axons and rat callosal axons) findings demonstrate that knockdown of Kv3.4 by a specific shRNA impedes axon initiation, elongation, pathfinding, and fasciculation. In cultured dorsal spinal neurons, blockade of Kv3.4 by blood depressing substance II suppresses axon growth via an increase in the amplitude and frequency of Ca influx through T-type and L-type Ca channels. Electrophysiological results show that Kv3.4, the major Kv channel in the axonal growth cones of embryonic dorsal spinal neurons, is activated at more hyperpolarized potentials and inactivated more slowly than it is in postnatal and adult neurons. The opening of Kv3.4 channels effectively reduces growth cone membrane excitability, thereby limiting excessive Ca influx at subthreshold potentials or during Ca-dependent action potentials. Furthermore, excessive Ca influx induced by an optogenetic approach also inhibits axon growth. Our findings suggest that Kv3.4 reduces growth cone membrane excitability and maintains [Ca] at an optimal concentration for normal axon growth. Accumulating evidence supports the idea that impairments in axon growth contribute to many clinical disorders, such as autism spectrum disorders, corpus callosum agenesis, Joubert syndrome, Kallmann syndrome, and horizontal gaze palsy with progressive scoliosis. Membrane excitability in the growth cone, which is mainly controlled by voltage-gated Ca (Cav) and K (Kv) channels, modulates axon growth. The role of Cav channels during axon growth is well understood, but it is unclear whether Kv channels control axon outgrowth by regulating Ca influx. This report shows that Kv3.4, which is transiently expressed in the axonal growth cones of many types of embryonic neurons, acts to reduce excessive Ca influx through Cav channels and thus permits normal axon outgrowth.