Stages of synapse development defined by dependence on F-actin.
It has been widely speculated that actin plays a central role in CNS synapse assembly, but such a requirement for actin filaments (F-actin) has not yet been demonstrated experimentally. We used hippocampal neurons grown in culture and the actin depolymerizing agent, latrunculin A, to examine directly the relationship between F-actin and synapse formation and maturation. During the first week in culture, actin depolymerization results in a near complete loss of synapses defined by synaptophysin-labeled vesicle clusters, synaptic vesicle recycling, and ultrastructure. Over the second week in culture, F-actin becomes increasingly stable, but actin depolymerization no longer disrupts basic synaptic structure. There is, however, a reduction in the number and size of synaptophysin-labeled clusters and in the size of vesicle clusters undergoing FM4-64 recycling, suggesting that synaptic vesicle anchoring remains partially dependent on F-actin. By 18 d in culture, synaptophysin clusters and synaptic vesicle recycling are largely resistant to F-actin depolymerization. The decrease in synapse dependence on F-actin correlates well with the acquisition and retention of presynaptic scaffolding proteins such as Bassoon and postsynaptic scaffolding proteins such as those of the postsynaptic density-95 family. Increased activity stabilizes F-actin and its associated proteins at synaptic sites, suggesting a correlation between active synapses, actin stability, and synapse stability. Our findings demonstrate that F-actin is essential for the development and maintenance of young synapses. Because F-actin is also highly regulatable, we propose that F-actin may be a principal target for stabilizing or destabilizing signals that ultimately result in synapse maintenance or elimination.
Zhang
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Fishberg Research Center for Neurobiology and Program in Cell Adhesion, The Mount Sinai School of Medicine, New York, New York 10029, USA.
Benson
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D000199:Actins / Q000187:drug effects / Q000378:metabolism*; D000818:Animals; D019086:Bridged Bicyclo Compounds, Heterocyclic / Q000494:pharmacology; D015820:Cadherins / Q000378:metabolism; D002478:Cells, Cultured; D005109:Extracellular Matrix / Q000378:metabolism; D005456:Fluorescent Dyes; D006624:Hippocampus; D009419:Nerve Tissue Proteins / Q000378:metabolism; D009474:Neurons / Q000187:drug effects / Q000378:metabolism* / Q000648:ultrastructure; D051381:Rats; D013569:Synapses / Q000187:drug effects / Q000378:metabolism* / Q000648:ultrastructure; D013572:Synaptic Vesicles / Q000378:metabolism / Q000648:ultrastructure; D016708:Synaptophysin / Q000378:metabolism; D013844:Thiazoles / Q000494:pharmacology; D053778:Thiazolidines; D013997:Time Factors
D000199:Actins; D019086:Bridged Bicyclo Compounds, Heterocyclic; C113688:Bsn protein, rat; D015820:Cadherins; D005456:Fluorescent Dyes; D009419:Nerve Tissue Proteins; D016708:Synaptophysin; D013844:Thiazoles; D053778:Thiazolidines; C033652:postsynaptic density proteins; C037067:latrunculin A