Developmentally regulated changes in cellular compartmentation and synaptic distribution of actin in hippocampal...

Actin dynamics and actin-based motility are important for neurite outgrowth and synapse plasticity. Recent work implicates actin in synapse assembly, but the morphological relationship between actin and synapses during development is unclear. Here we used developing hippocampal neurons grown in culture to examine the relationship between F- and G-actin and clusters of synaptic proteins. Both F- and G-actin are most enriched in dendritic and axonal growth cones, but only G-actin is present within the distal tips of filopodia. Outside of growth cones, F-actin levels are greater in dendrites than in axons, whereas G-actin levels are slightly greater in axons than in dendrites. The distribution of both F- and G-actin is consistent with their presence at synapses, but only F-actin levels become detectably enhanced at synaptic sites. Quantitative analyses suggest that first-forming synapses are associated with enhanced levels of pre- and postsynaptic F-actin that do not necessarily remain elevated during synapse maturation. However, nearly all mature excitatory synapses become associated with high, mostly postsynaptic concentrations of F-actin contained principally within dendritic spines. Mature shaft and GABAergic synapses are also associated with enhanced levels of F-actin, but to a lesser degree. Thus, although F-actin is essential for function and maintenance of young synapses, it need not be highly concentrated at every site. The large increase in postsynaptic F-actin concentration observed in mature neurons is likely to reflect actin's role in dendritic spine morphology and in synapse plasticity.