PSD-95 Serine 73 phosphorylation is not required for induction of NMDA-LTD

Abstract

is a major scaffolding protein of the post-synaptic density (PSD) of a glutamatergic synapse. PSD-95, via interactions with stargazin, anchors AMPA receptors at the synapse and regulates AMPAR currents. The expression of PSD-95 is regulated during synaptic plasticity. It is, however, unknown whether this regulation is required for induction of functional plasticity of glutamatergic synapses. Here, we show that NMDA-induced long-term depression of synaptic transmission (NMDA-LTD) is accompanied by downregulation of PSD-95 protein levels. Using pharmacologic and molecular manipulations, we further demonstrate that the NMDA-induced downregulation of PSD-95 depends on the activation of CaMKII and CaMKII-driven phosphorylation of PSD-95 serine 73. Surprisingly, neither CaMKII activity nor CaMKII-dependent phosphorylation of PSD-95 serine 73 are required for the expression of NMDA-LTD. These results support the hypothesis that synaptic plasticity of AMPARs may occur without dynamic regulation of PSD-95 protein levels. PSD-95 is the major scaffolding protein of a glutamatergic synapse 1 affecting its stability, activity-dependent modifications 2-5 and functional plasticity 6-9. PSD-95 interacts directly with NMDA receptors and with AMPA receptors through an auxiliary protein stargazin 10,11. Interaction of PSD-95 with stargazin regulates synaptic content of AMPARs 10,12,13. In agreement with these findings mice lacking functional PSD-95 protein have greatly enhanced hippocampal, NMDAR-dependent long-term potentiation (LTP), whereas NMDAR-dependent long-term depression (LTD) is absent 6. Conversely, overexpression of PSD-95 occludes LTP 7,8 and decreases the threshold for LTD induction 9. Importantly, PSD-95 is a highly dynamic protein. Upon stimulation, PSD-95 is phosphorylated at serine 73 and transiently removed from the dendritic spine in a CaMKII-dependent manner 2. However, inhibition of this process by mutation of serine 73 to alanine does not affect the propensity to induce LTP 2. Sturgill et al. have also shown that NMDA-LTD provokes rapid destabilization of PSD-95 in the spine head 4. It is yet unknown whether LTD-induced elimination of PSD-95 protein from the dendritic spine is necessary for the expression of LTD. Here, using organotypic hippocampal cultures (OHC) we confirmed that PSD-95 protein levels were downregulated in the stratum radiatum of CA1 hippocampal field after induction of NMDA-LTD 4. Since it has been shown that CaMKII-dependent phosphorylation of PSD-95 on serine 73 (PSD-95:Ser73) regulates the binding of PSD-95 to NMDAR 14 and translocation of PSD-95 from activated spines 2 , we checked if CaMKII contributes to LTD-induced downregulation of PSD-95. Using pharmacological manipulations and AAV transfection approach we found that NMDA-LTD-induced downregulation of PSD-95 levels is regulated by CaMKII activity and CaMKII-driven phosphorylation of PSD-95:Ser73. Surprisingly, we also observed that neither CaMKII activity nor CaMKII-dependent phosphorylation of PSD-95:Ser73 are necessary for the ex...

Synaptic plasticity regulated by phosphorylation of PSD-95 Serine 73 in dorsal CA1 is required for contextual fear extinction

“…Here, we show that both aspects of the fear extinction-induced synaptic plasticity (spine elimination and growth of the remaining synapses) are coordinated by αCaMKII-dependent phosphorylation of PSD-95 at serine 73 (Gardoni et al, 2006). This is a new function of PSD-95 serine 73 as previously it was shown to be required for: PSD-95 dissociation from the NMDAR subunit NR2A after NMDAR stimulation (Gardoni et al, 2006), PSD-95 protein downregulation during LTD (Nowacka et al, 2020) and termination of synaptic growth after glutamate uncaging (Steiner et al, 2008). Thus none of these synaptic models explains synaptic processes observed during fear extinction as they predict excessive growth of the synapses and accumulation of PSD-95(S73A).…”

“…It was first described as a target of αCaMKII that promotes PSD-95 dissociation from the NMDA receptor subunit NR2A (Gardoni et al, 2006). Further studies showed that S73 phosphorylation induces PSD-95 activity-dependent trafficking that is necessary for termination of synaptic growth after NMDAR stimulation, as well as PSD-95 downregulation during NMDAR-dependent LTD (Nowacka et al, 2020;Steiner et al, 2008). Interestingly, the loss-of-function PSD-95 mutant mice lacking the guanylate kinase domain of PSD-95 (Migaud et al, 1998) show normal contextual fear memory but impaired extinction of contextual fear (Fitzgerald et al, 2015), indicating that PSD-95-dependent synaptic plasticity contributes to the updating rather than the formation of contextual fear memory.…”

PSD-95 in CA1 Area Regulates Spatial Choice Depending on Age

“…PSD-95 is a key protein at mature glutamatergic synapses (Aoki et al, 2001;Chen et al, 2011). Its expression is regulated during synaptic plasticity (Steiner et al, 2008;Nowacka et al, 2020) and predicts stability of dendritic spines (El-Husseini et al, 2000;Ehrlich et al, 2007;Murmu et al, 2013;Meyer et al, 2014). Knockdown of PSD-95 has been shown to impair NMDAR-dependent LTD (Ehrlich et al, 2007), as well as spatial learning, conditioned taste aversion and simple operant associative learning (Migaud et al, 1998;Elkobi et al, 2008;Fitzgerald et al, 2015).…”

Ketamine alleviates fear memory and spatial cognition deficits in PTSD rat model via BDNF signaling pathway of hippocampus and amygdala

“…Endocytosis of AMPA receptor was potentially necessary for memory remodeling after retrieval [55], while NMDAR activation was controlled by AMPA receptor endocytosis during hippocampal long-term depression(LTD) [56]. When PSD-95 is recruited into the synapse, NMDA receptors may migrate and aggregate, affecting the morphology, function, plasticity of synapses, and endocytosis of AMPA receptors [57]. Moreover, the BDNF and PSD-95 results showed a positive correlation in expression levels in the HIP and AMG, indicating that ketamine upregulated the PSD-95 protein and triggered the synaptic delivery of PSD-95 by promoting BDNF signaling to alleviate the PTSD-like effects.…”