Dendritic spines are the post-synaptic sites of most excitatory synapses in the brain, and changes in their morphology are implicated in synaptic plasticity and long-term memory. F-actin dynamics are thought to be a basis for both the formation of dendritic spines during development and their structural plasticity (Ethell and Pasquale, 2005; Pontrello and Ethell, 2009). We have shown that the F-actin-severing protein cofilin, which is regulated by phosphorylation, can induce remodeling of mature dendritic spines in hippocampal neurons (Shi et al., 2009). We also demonstrate that β-Arrestins play an important role in spatial control over cofilin activity in dendritic spines, which underlies NMDA-mediated dendritic spine remodeling. Cofilin activity in dendritic spines is regulated through CaMKII-mediated suppression of cofilin activity by phosphorylation, and calcineurin-dependent cofilin activation through its dephosphorylation. In addition, while the EphB receptor promotes spine stabilization through cofilin inactivation, the NMDA receptor prevents EphB-mediated cofilin inactivation through calcineurin. NMDAR activation also promotes the translocation of cofilin to dendritic spines, an event that requires cofilin dephosphorylation and is also dependent on β-Arrestins, which have recently been shown to scaffold cofilin with its regulators, LIM kinase and slingshot phosphatase (Zoudilova et al., 2010). Our studies demonstrate that cofilin clustering in the spines is affected in both β-Arrestin1- and β-Arrestin2- deficient neurons under normal synaptic activity, and a constitutively-active cofilinS3A mutant fails to translocate to spines in response to NMDA in β-Arrestin2 KO neurons. Moreover, while wt neurons display dendritic spine remodeling in response to NMDA or with over-expression of cofilinS3A, β-Arrestin2-deficient neurons are resistant to both NMDA-induced and cofilinS3A-induced spine remodeling. In contrast, dominant-negative cofilinS3D prevents NMDA-induced dendritic spine remodeling in wt neurons, and also rescues a mature spine phenotype that is lost in β-Arrestin1 KO neurons. In addition, over-expression of β-Arrestin in the KO neurons rescues spine abnormalities. β-Arrestin1-deficient neurons also develop immature spines in vivo, whereas hippocampal neurons lacking β-Arrestin2 develop normal mature spines, but fail to remodel in response to NMDA. Our studies demonstrate novel functions of β-Arrestin1 in the development of mature dendritic spines, and β-Arrestin2 in NMDAR-mediated dendritic spine plasticity through spatial control over cofilin activity.
