Differential Control of Small-conductance Calcium-activated Potassium Channel Diffusion by Actin in Different Neuronal Subcompartments

Abstract

Small-conductance calcium-activated potassium (SK) channels show a ubiquitous distribution on neurons, in both somatodendritic and axonal regions. SK channels are associated with neuronal activity regulating action potential frequency, dendritic excitability, and synaptic plasticity. Although the physiology of SK channels and the mechanisms that control their surface expression levels have been investigated extensively, little is known about what controls SK channel diffusion in the neuronal plasma membrane. This aspect is important, as the diffusion of SK channels at the surface may control their localization and proximity to calcium channels, hence increasing the likelihood of SK channel activation by calcium. In this study, we successfully investigated the diffusion of SK channels labeled with quantum dots on human embryonic kidney cells and dissociated hippocampal neurons by combining a single-particle tracking method with total internal reflection fluorescence microscopy. We observed that actin filaments interfere with SK mobility, decreasing their diffusion coefficient. We also found that during neuronal maturation, SK channel diffusion was gradually inhibited on somatodendritic compartments. Importantly, we observed that axon barriers formed at approximately days in vitro (DIV) 6 and restricted the diffusion of SK channels on the axon initial segment (AIS). However, after neuron maturation, SK channels on the AIS were strongly immobilized, even after disruption of the actin network, suggesting that crowding may cause this effect. Altogether, our work provides insight into how SK channels diffuse on the neuronal plasma membrane and how actin and membrane crowding impacts SK channel diffusion.

Ion Channel Lateral Diffusion Reveals the Maturation Process of the Neuronal Actin Cytoskeleton

“…SK channels can also be found in the soma and in axons, where they likely contribute to spike frequency adaptation. Gu and colleagues used single-particle tracking 2 of SK channels in different areas of pyramidal hippocampal neurons in cultures of different ages. The diffusion coefficient of SK channels was determined along the maturation process of the neuronal culture using biotinylated apamin and streptavidin-conjugated quantum dots to label the channels in combination with total internal reflection microscopy.…”