Gu, S.; Tzingounis, A. V.; Lykotrafitis, G.

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 ion channels levels in neurons but also leads to structural plasticity of the axon initial segment (AIS), the site of action potential initiation. Prior studies show that activity can alter AIS length and position along the axon, but it is unknown whether homeostatic plasticity also regulates the periodicity of the AIS actin cytoskeleton. Actin filaments in the axon plasma membrane skeleton form periodic rings with 190-nm spacing. Here, we examined whether the integrity of actin-ring periodicity at the AIS is plastic following prolonged changes in neuronal activity. We induced AIS homeostatic plasticity by chronically increasing activity with XE991, a blocker of AIS-enriched Kv7 channels, or decreasing activity with the pan-Kv7 opener retigabine (RTG) or the voltage-gated sodium channel blocker TTX, at two maturation stages (DIV10 and DIV16). We found that prolonged exposure to XE991 relocated the AIS away from the soma and disrupted actin-ring periodicity at DIV10 but periodicity was unaffected at DIV16. In comparison, long-term treatment with RTG or TTX induced AIS relocation towards the soma and reduced AIS length, respectively. Both RTG and TTX disrupted actin-ring periodicity at both developmental stages. These observations suggest that homeostatic plasticity not only alters 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.