Stamatakou, E; (2015) The role of Wnt signalling in actin dynamics during synapse formation. Doctoral thesis , UCL (University College London).

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Abstract

Wnt secreted proteins play key roles in the formation of neuronal circuits by regulating axon guidance, axonal remodelling, dendritogenesis and synaptogenesis. We have previously shown that Wnts promote axonal remodelling through the regulation of actin cytoskeleton and the formation of dendritic spines. However, the downstream events modulated by Wnts to regulate actin dynamics during these processes remain elusive. Here, we identified the actin-capping protein Epidermal growth factor receptor kinase substrate 8 (Eps8) as a direct interactor of Dishevelled-1 (Dvl1), a key scaffold protein and integrator of Wnt signalling. Expression of Eps8 mimics Dvl1-induced axonal remodelling by promoting enlargement and F-actin accumulation in growth cones from dorsal root ganglia (DRG) neurons. Importantly, we show that Eps8 is required for Wnt3a-mediated axonal remodelling. Our findings demonstrate that Eps8 is a downstream effector of Wnt signalling during axonal remodelling. Dendritic spine morphogenesis critically depends on actin dynamics, a process that is modulated by signalling molecules and neuronal activity through poorly described mechanisms. Here we report that Eps8 is required for spine morphogenesis in hippocampal neurons. Our gain- and loss-of-function studies demonstrate that Eps8 promotes the formation of dendritic spines but inhibits filopodium formation. However, Eps8 does not affect spine growth nor modulates Dvl1-mediated spine enlargement, indicating that Eps8 regulates spine formation through a Wnt-independent pathway. Loss of function of Eps8 results in increased actin polymerization, but also actin turnover within dendritic spines, as revealed by free-barbed end and FRAP assays, consistent with a role for Eps8 as an actin-capping protein. Interestingly, Eps8 promotes the localisation of excitatory synapses on spines, without affecting the total number of synapses or basal synaptic transmission. Importantly, Eps8 silencing impairs the structural and functional plasticity of synapses induced by long-term potentiation. These results demonstrate a novel role for Eps8 in spine formation and in activity-mediated synaptic plasticity.

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