Abstract

Synapses are highly dynamic structures that are formed, remodelled and eliminated throughout life. Following the initial peak of postnatal synaptogenesis these events determine and sculpt functional neural circuits. Whilst emerging evidence is unravelling many of the key molecules involved mechanistic details are still missing. We have previously demonstrated that secreted Wnts act as target-derived signals that regulate terminal arborisation of axons and presynaptic assembly. Loss of function studies using Wnt deficient mice combined with gain of function approaches showed that Wnt signalling regulates synapse formation by inducing the recruitment of synaptic components to future synaptic sites. Moreover, the use of the secreted Wnt antagonists like Frizzled related proteins (Sfrps) has allowed us to demonstrate by cellular and electrophysiological approaches that Wnt7a signalling regulates the formation of excitatory synapses in the hippocampus. The pattern of expression of Wnts, their receptors and many of the components of the canonical Wnt signalling pathway suggest that Wnts do not only regulate synapse formation but also synaptic maintenance. To further examine the mechanism of Wnt action at synapses, we have tested the contribution of the canonical Wnt signalling pathway that requires the co-receptor LRP6 , Dishevelled and inhibition Gsk3ß, a serine/threonine kinase. We used the Wnt antagonist Dickopf-1 (Dkk1), which directly binds to the LRP6 receptor, exclusively blocks canonical signalling. In inducible transgenic mice, expression of Dkk1 in the adult hippocampus decreases the level of presynaptic markers suggesting Wnt signaling is required for synaptic maintenance. In cultured hippocampal neurons, Dkk1 blocks Wnt-mediated presynaptic differentiation in hippocampal neurons. Dkk1 can also reduce the number of functional synaptic sites in the absence of exogenous Wnt. Intriguingly Dkk1 rapidly reduces (within 30 minutes) the number of presynaptic sites and the co-localisation of presynaptic proteins in a transcriptional independent manner. We also found ultrastructural correlates for synaptic disassembly and shrinking. Our data strongly suggest that canonical-Wnts regulate both the formation and maintenance of synapses in the central nervous system.