Homeostatic Neuronal Plasticity Alters Axon Initial Segment Actin Membrane Skeleton Periodicity

Homeostatic plasticity allows neurons to maintain their activity around a set point, contributing to neuronal network stability over time. Increasing or decreasing neuronal activity not only regulates the levels of ion channels in neurons, but also leads to structural plasticity of the axon initial segment (AIS), the site of action potential initiation. Although previous studies have shown that activity could change the length and position of the AIS along the axon to compensate for changes in neuronal activity, it is currently unknown whether homeostatic plasticity also regulates the periodicity of the AIS actin cytoskeleton. Actin filaments, which are components of the axon plasma membrane skeleton, form periodic rings with a 190-nm spacing. Here, we examined whether the integrity of the actin ring periodicity at the AIS is plastic, depending on prolonged changes in neuronal activity. To induce AIS homeostatic plasticity, we either increased neuronal activity chronically by blocking the AIS enriched Kv7 voltage-gated potassium channels with XE991 or chronically decreased it by using the pan-Kv7 channel opener retigabine (RTG), or alternatively, the voltage-gated sodium channel blocker TTX at different neuronal maturation stages (DIV10 and DIV16). We found that prolonged exposure to XE991 relocated AIS away from soma and disrupted the periodicity of actin rings at DIV10. However, the actin ring periodicity remained unaffected at DIV16. In comparison, long-term treatment with RTG or TTX induced relocation of AIS towards the soma and reduction in AIS length, respectively. In parallel, RTG and TTX disrupted the actin ring periodicity at both developmental stages. These observations suggest that homeostatic plasticity not only changes the AIS position and length, but it could also modulate the periodicity of the AIS actin membrane skeleton, raising the possibility that the mechanical properties of the AIS are also plastic.