K⁺Channel Kv3.4 Is Essential for Axon Growth by Limiting the Influx of Ca²⁺into Growth Cones

Abstract

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.

Distribution of Acid Sensing Ion Channels in Axonal Growth Cones and Presynaptic Membrane of Cultured Hippocampal Neurons

“…To investigate the localization of ASICs at growth cones, we performed immunocytochemistry for endogenous ASIC1a and ASIC2a, which are the primary ASIC subunits expressed in the central nervous systems in DIV3-DIV6 hippocampal neurons, using anti-ASIC1a and anti-ASIC2a antibodies. To facilitate the identification of growth cones, the neurons were coimmunostained with phalloidin (Huang et al, 2017). Immunocytochemistry was performed at DIV3 and DIV6 due to easy morphological recognition.…”

Kir6.2-deficient mice develop somatosensory dysfunction and axonal loss in the peripheral nerves

“…These results suggest that the inhibition of the K ATP channel may exacerbate neural degeneration, but the activation prevents degeneration. It has been reported that membrane excitability in the axonal growth cones of iScience Article embryonic neurons influences axonal growth (Huang et al, 2017). According to the report, increased membrane excitability inhibited axonal growth.…”

Potassium channels and autism spectrum disorder: An overview

“…Neuronal excitability is partly mediated by voltage‐gated ion channels that could transform the initial neuronal signaling to an action potential and give a rise to subsequent activities affected by biophysical properties and localization of these channels within neurons (Lewis et al., 2011). In excitable cells, Kv channels are key factors regulating membrane excitability by governing K + current changes (Huang et al., 2017). Kv channels are tetramers composed of four α subunits arranged around a central pore which controls the K + current efflux.…”