br Results br Discussion Here we uncovered a role for

Results
Discussion Here, we uncovered a role for Wnt-Fz7 signaling in LTP-dependent spine plasticity and synaptic localization of AMPARs. We demonstrated that LTP induction increased Wnt7a/b protein at EZLink Sulfo-NHS-LC-Biotin Kit and spines. Direct blockade of endogenous Wnts impaired LTP-mediated structural spine plasticity and the synaptic targeting of GluA1-containing AMPARs. We identified Fz7 as the postsynaptic receptor for Wnt7a required for spine plasticity and LTP induction. We also demonstrated that Wnt signaling regulates the membrane localization of AMPARs through PKA-dependent phosphorylation of GluA1 and facilitated the synaptic localization of these receptors by CaMKII-mediated loss of SynGAP at dendritic spines (see model in Figure 7E). Our findings reveal a critical role for Wnts in LTP induction, because these secreted factors trigger downstream signaling events that are required for NMDAR-mediated structural and functional plasticity. The molecular mechanisms that contribute to the structural and functional plasticity of synapses have been extensively studied. The central role of the neurotransmitter glutamate as the main initiator of LTP-mediated synaptic plasticity is widely recognized; however, the identity and function of secreted molecules that support this role of glutamate is not fully understood. While extracellular signals such as BDNF are recognized as LTP mediators (Zagrebelsky and Korte, 2014), the importance of other extracellular signaling proteins, including Wnts, in LTP-mediated structural and functional plasticity is unclear. Wnt5a regulates NMDAR-mediated synaptic transmission and LTP expression, but the effect of this Wnt protein is slow, taking approximately 20 min (Cerpa et al., 2011). Similarly, Wnt5a requires 1–2 hr to enhance dendritic spine size and synapse formation in cultured hippocampal neurons and 30 min to regulate spine number (Ramírez et al., 2016). Here, we demonstrate that Wnt5a does not induce fast changes in AMPAR localization or dendritic spine size. Collectively, these findings suggest that Wnt5a may function during later stages of LTP. Here we discover a unique role for Wnt7a/b at early stages of LTP. HFS of SCs, which induces LTP at CA3-CA1 synapses, specifically increased the levels of endogenous Wnt7a/b protein in the stratum radiatum and is rapidly elevated (5 min) at dendritic spines following cLTP. Given the expression of Wnt7a and Wnt7b at principal neurons in the hippocampus, Wnt7a/b protein may come from presynaptic and/or postsynaptic neurons in the stratum radiatum. Although the mechanisms controlling the rapid accumulation and/or release of Wnt7a/b remain to be elucidated, Wnts could be stored and then released from exosomes at the synapse, as observed at the Drosophila NMJ (Budnik et al., 2016, Padamsey et al., 2017). Further studies are required to demonstrate the mechanism by which activity regulates the synaptic accumulation and secretion of Wnts. Wnt7a, through Fz7 receptors, modulated LTP-associated structural and functional plasticity. Acute blockade of endogenous Wnts with Sfrps suppresses the structural plasticity of dendritic spines and LTP induction and maintenance. This result is consistent with our previous work showing that long-term in vivo blockade of endogenous Wnts with Dkk1, another secreted Wnt antagonist, affects LTP (Marzo et al., 2016). Early LTP events depend on the precise regulation of AMPAR content (Henley and Wilkinson, 2016, Huganir and Nicoll, 2013, Temkin et al., 2017). Consistently, Wnt7a rapidly increased the accumulation of AMPARs at dendritic spines, thereby elevating AMPAR-mediated excitatory postsynaptic current (EPSC) amplitude. Using live imaging of SEP-GluA1-expressing neurons and single-particle tracking, we showed that Wnt7a rapidly increased spine size and synaptic AMPAR levels while increasing the number of immobile synaptic AMPARs. The timing of these events is similar to those observed after LTP induction (Kopec et al., 2006, Makino and Malinow, 2009, Tanaka and Hirano, 2012, Yang et al., 2008b). We demonstrated that Wnt7a specifically regulated spine plasticity and AMPAR localization through Fz7 receptors, which were located postsynaptically. Fz7 loss of function prevented cLTP-induced AMPAR accumulation at the surface and inhibits synaptic potentiation following a whole-cell LTP pairing protocol. Thus, our studies delineate a pathway modulating structural and functional plasticity through the Wnt7a-Fz7 signaling cascade.