Piccolo Regulates the Dynamic Assembly of Presynaptic F-actin

Filamentous (F)-actin is a known regulator of the synaptic vesicle (SV) cycle, with roles in SV mobilization, fusion, and endocytosis. However, the molecular pathways that regulate its dynamic assembly within presynaptic boutons remain unclear. In this study, we have used shRNA-mediated knockdown to demonstrate that Piccolo, a multidomain protein of the active zone cytomatrix, is a key regulator of presynaptic F-actin assembly. Boutons lacking Piccolo exhibit enhanced activity-dependent Synapsin1a dispersion and SV exocytosis, and reduced F-actin polymerization and CaMKII recruitment. These phenotypes are rescued by stabilizing F-actin filaments and mimicked by knocking down Profilin2, another regulator of presynaptic F-actin assembly. Importantly, we find that mice with a targeted deletion of exon 14 from the Pclo gene, reported to lack >95% of Piccolo, continue to express multiple Piccolo isoforms. Furthermore, neurons cultured from these mice exhibit no defects in presynaptic F-actin assembly due to the expression of these isoforms at presynaptic boutons. These data reveal that Piccolo regulates neurotransmitter release by facilitating activity-dependent F-actin assembly and the dynamic recruitment of key signaling molecules into presynaptic boutons, and highlight the need for new genetic models with which to study Piccolo loss of function.