Actin cage for the synaptic vesicle liquid phase

Synaptic vesicles in nerve terminals are organized into a cluster at the active zone, where they fuse with the presynaptic membrane during synaptic activity. This cluster is proposed to be organized as a liquid phase dependent on synapsin. Using the lamprey giant reticulospinal synapse, we demonstrate that synaptic activity triggers the formation of an actin filament cage that encapsulates the synaptic vesicle cluster. These filaments grow from the periactive zone toward and over the vesicle cluster. Acute disruption of this F-actin cage by cytochalasin D, latrunculin A, or C2 toxin from Clostridium botulinum results in the detachment of the cluster from the release site during stimulation. Thus, the F-actin cage functions to maintain the synaptic vesicle liquid phase at the active zone during synaptic activity.