Arp2/3 Is Required for Axonal Arbor Terminal Retraction in Cerebellar Granule Neurons

Although selective pruning of subsets of branches is a key step for axonal remodeling, the underlying mechanisms have not fully elucidated. We have reported that the retraction rate of axonal arbor is higher in the terminals, where less kinesin is sorted, and inhibiting actin turnover by treatment with latrunculin A prevented retraction in such terminals. In the present study, we investigated the effects of pharmacological inhibitors for the regulators of actin polymerization/destabilization on the elongation and retraction of arbor terminals in cerebellar granule neurons. Inhibition of an actin nucleator Arp2/3 with CK-666 or CK-869 inhibited the retraction of arbor terminals. However, inhibitors for PAK/LIMK, upstream signaling of ADF/cofilin, reduced the elongation rate but not the retraction rate. A photoconversion analysis revealed that turnover of F-actin at the tip of the axonal terminal is faster in the presence of the Arp2/3 inhibitor compared with the control and neurons treated with the LIMK inhibitor. Our results indicate the important role of Arp2/3 for the retraction of axonal arbor terminals in cerebellar granule neurons.