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  • What is the mechanism by which Wnt

    2024-07-09

    What is the mechanism by which Wnt7a-Fz7 signaling enhances AMPAR localization and synaptic strength? Following induction of LTP, the number of AMPARs rapidly increases at extrasynaptic sites (Makino and Malinow, 2009, Yang et al., 2008a) through PKA-dependent phosphorylation of GluA1 at S845 (He et al., 2009, Man et al., 2007, Oh et al., 2006, Yang et al., 2008a). Although this phosphorylation site is not required for direct synaptic AMPAR incorporation (Esteban et al., 2003), extrasynaptic receptors can be recruited to the synapse through lateral diffusion in the surface membrane (Bassani et al., 2013, Opazo and Choquet, 2011). A study showed that the rapid recruitment of AMPARs to synapses from an extrasynaptic pool through lateral diffusion and gpr44 inhibitor activity-dependent trapping of these receptors at the PSD is imperative for LTP induction (Penn et al., 2017). Moreover, the replenishment of the extracellular pool of AMPARs is through exocytosis and is required to maintain LTP (Penn et al., 2017). Here, we demonstrate that Wnt7a through Fz7 increased PKA-mediated phosphorylation of the GluA1 subunit at S845 and promoted the localization of AMPARs at extrasynaptic sites within 10–20 min. These findings, together with our time-lapse recordings and single-particle tracking data, are consistent with the model proposed by Penn et al. (2017). Thus, our results suggest that Wnts could play an important role in AMPAR synaptic recruitment at early and later stages of LTP. Activation of CaMKII is also required for the localization and confinement of AMPARs at synaptic sites (Esteban et al., 2003, Herring and Nicoll, 2016, Hayashi et al., 2000, Opazo et al., 2010). Although CaMKII phosphorylates AMPARs at S831, this post-translational modification is not required for AMPAR synaptic localization (Hayashi et al., 2000, Opazo et al., 2010). In contrast, CaMKII phosphorylates SynGAP, a Ras-GTPase that inhibits ERK signaling (Rumbaugh et al., 2006), resulting in the loss of this protein from dendritic spines with the concomitant increase in AMPARs (Araki et al., 2015). Consistent with this model, and in gpr44 inhibitor to results obtained with Wnt5a (Codocedo et al., 2015), we found that Wnt7a through Fz7 rapidly promotes the loss of SynGAP from spines in a CaMKII-dependent manner, resulting in the activation of the Ras-ERK pathway. Thus, our findings suggest that Wnt7a-CaMKII signaling regulates the synaptic localization of AMPARs through rapid changes in SynGAP localization.
    Experimental Procedures
    Introduction Epilepsy is one of the most common and disabling neurologic diseases and is characterized by recurrent seizure activity. Approximately 65 million people worldwide suffer from epilepsy (Thurman et al., 2011), and 30% of people with newly diagnosed epilepsy will become pharmacoresistant (Keller et al., 2017), eventually progressing to intractable epilepsy (IE) (Kwan and Brodie, 2000). Temporal lobe epilepsy (TLE) is a common form of epilepsy that is often pharmacoresistant, which results in poor prognoses for patients with TLE (Keller et al., 2017). The development of epilepsy refers to a process in which an initial brain-damaging insult triggers a cascade of molecular and neuronal network changes including neuronal death, neurogenesis, gliosis, and network reorganization that eventually lead to recurrent seizures (Rakhade and Jensen, 2009). The underlying mechanism of TLE has been investigated in previous studies (Drexel et al., 2017; Lu et al., 2016; Wang et al., 2017), but researchers still have an incomplete understanding of the underlying mechanism of TLE, which is worthy of further study. Endophilin A1, a member of the endophilin A family, is encoded by the gene SH3GL2 and primarily expressed in the nervous system (Yang et al., 2015). As a membrane-binding protein, endophilin A1 colocalizes with the postsynaptic density (PSD) (Yang et al., 2015), shows an association with glutamate receptors internalization (Kaneko et al., 2005; Masuda et al., 2006), and regulates the excitatory synaptic transmission (Yang et al., 2015). These characteristics of endophilin A1 indicate that it has multiple functions in synaptic plasticity and, furthermore, regulates neuronal hyperexcitability, which may contribute to epileptogenesis and seizure generation. On the basis of the fundamental characteristics of endophilin A1, we propose that it might play a critical role in epilepsy.