The N‐methyl‐D‐aspartate receptors (NMDARs) are ionotropic glutamate receptors that mediate the flux of calcium (Ca2+) into the post‐synaptic compartment. Ca2+ influx subsequently triggers the ...activation of various intracellular signalling cascades that underpin multiple forms of synaptic plasticity. Functional NMDARs are assembled as heterotetramers composed of two obligatory GluN1 subunits and two GluN2 or GluN3 subunits. Four different GluN2 subunits (GluN2A‐D) are present throughout the central nervous system; however, they are differentially expressed, both developmentally and spatially, in a cell‐ and synapse‐specific manner. Each GluN2 subunit confers NMDARs with distinct ion channel properties and intracellular trafficking pathways. Regulated membrane trafficking of NMDARs is a dynamic process that ultimately determines the number of NMDARs at synapses, and is controlled by subunit‐specific interactions with various intracellular regulatory proteins. Here we review recent progress made towards understanding the molecular mechanisms that regulate the trafficking of GluN2‐containing NMDARs, focusing on the roles of several key synaptic proteins that interact with NMDARs via their carboxyl termini.
The N‐methyl‐D‐aspartate glutamate receptors (NMDARs) mediate calcium‐dependent signalling that underpins multiple forms of synaptic plasticity. Different GluN2 (GluN2A‐D) subunits confer NMDARs with distinct ion channel properties and intracellular trafficking pathways. This review article summarizes the current knowledge of the molecular mechanisms that regulate the trafficking of GluN2‐containing NMDARs, focusing on the roles of several key‐binding partners.
Highlights ► AMPA receptor phosphorylation regulates receptor trafficking and function. ► AMPA receptor palmitoylation plays differential roles in receptor trafficking. ► AMPA receptor ubiquitination ...functions in receptor endocytosis and degradation. ► Posttranslational modifications interact with each other to modulate AMPA receptors.
Phosphorylation regulates surface and synaptic expression of NMDA receptors (NMDARs). Both the tyrosine kinase Fyn and the tyrosine phosphatase striatal-enriched protein tyrosine phosphatase (STEP) ...are known to target the NMDA receptor subunit GluN2B on tyrosine 1472, which is a critical residue that mediates NMDAR endocytosis. STEP reduces the surface expression of NMDARs by promoting dephosphorylation of GluN2B Y1472, whereas the synaptic scaffolding protein postsynaptic density protein 95 (PSD-95) stabilizes the surface expression of NMDARs. However, nothing is known about a potential functional interaction between STEP and PSD-95. We now report that STEP61 binds to PSD-95 but not to other PSD-95 family members. We find that PSD-95 expression destabilizes STEP61 via ubiquitination and degradation by the proteasome. Using subcellular fractionation, we detect low amounts of STEP61 in the PSD fraction. However, STEP61 expression in the PSD is increased upon knockdown of PSD-95 or in vivo as detected in PSD-95–KO mice, demonstrating that PSD-95 excludes STEP61 from the PSD. Importantly, only extrasynaptic NMDAR expression and currents were increased upon STEP knockdown, as is consistent with low STEP61 localization in the PSD. Our findings support a dual role for PSD-95 in stabilizing synaptic NMDARs by binding directly to GluN2B but also by promoting synaptic exclusion and degradation of the negative regulator STEP61.
In cerebral cortex there is a developmental switch from NR2B- to NR2A-containing NMDA receptors (NMDARs) driven by activity and sensory experience. This subunit switch alters NMDAR function, ...influences synaptic plasticity, and its dysregulation is associated with neurological disorders. However, the mechanisms driving the subunit switch are not known. Here, we show in hippocampal CA1 pyramidal neurons that the NR2B to NR2A switch driven acutely by activity requires activation of NMDARs and mGluR5, involves PLC, Ca2+ release from IP3R-dependent stores, and PKC activity. In mGluR5 knockout mice the developmental NR2B-NR2A switch in CA1 is deficient. Moreover, in visual cortex of mGluR5 knockout mice, the NR2B-NR2A switch evoked in vivo by visual experience is absent. Thus, we establish that mGluR5 and NMDARs are required for the activity-dependent NR2B-NR2A switch and play a critical role in experience-dependent regulation of NMDAR subunit composition in vivo.
► The NR2B to 2A subunit switch at CA1 synapses requires NMDARs and mGluR5 ► The NR2 subunit switch requires PLC, IP3, and PKC signaling ► The acute NR2 subunit switch is absent in slices from mGluR5 KO mice ► mGluR5 KO mice exhibit a deficient experience-driven NR2 subunit switch
The metabotropic glutamate receptors (mGlu receptors) are G protein-coupled receptors that bind to the excitatory neurotransmitter glutamate and are important in the modulation of neuronal ...excitability, synaptic transmission, and plasticity in the central nervous system. Trafficking of mGlu receptors in and out of the synaptic plasma membrane is a fundamental mechanism modulating excitatory synaptic function through regulation of receptor abundance, desensitization, and signaling profiles. In this review, we cover the regulatory mechanisms determining surface expression and endocytosis of mGlu receptors, with particular focus on post-translational modifications and receptor-protein interactions. The literature we review broadens our insight into the precise events defining the expression of functional mGlu receptors at synapses, and will likely contribute to the successful development of novel therapeutic targets for a variety of developmental, neurological, and psychiatric disorders.
•Metabotropic glutamate receptors (mGlu receptors) undergo constitutive and agonist-induced endocytosis.•The endocytosis of mGlu receptors is regulated by receptor phosphorylation and protein-protein interactions.•Many proteins involved in mGlu receptor trafficking bind to the intracellular C-terminus or loop domains of mGlu receptors.•The intracellular trafficking pathways of mGlu receptors are remarkably diverse.
CaMKII is one of the most studied synaptic proteins, but many critical issues regarding its role in synaptic function remain unresolved. Using a CRISPR-based system to delete CaMKII and replace it ...with mutated forms in single neurons, we have rigorously addressed its various synaptic roles. In brief, basal AMPAR and NMDAR synaptic transmission both require CaMKIIα, but not CaMKIIβ, indicating that, even in the adult, synaptic transmission is determined by the ongoing action of CaMKIIα. While AMPAR transmission requires kinase activity, NMDAR transmission does not, implying a scaffolding role for the CaMKII protein instead. LTP is abolished in the absence of CaMKIIα and/or CaMKIIβ and with an autophosphorylation impaired CaMKIIα (T286A). With the exception of NMDAR synaptic currents, all aspects of CaMKIIα signaling examined require binding to the NMDAR, emphasizing the essential role of this receptor as a master synaptic signaling hub.
N-Methyl-
d-aspartate (NMDA) receptors are critical for neuronal development and synaptic plasticity. The molecular mechanisms underlying the synaptic localization and functional regulation of NMDA ...receptors have been the subject of extensive studies. In particular, phosphorylation has emerged as a fundamental mechanism that regulates NMDA receptor trafficking and can alter the channel properties of NMDA receptors. Here we summarize recent advances in the characterization of NMDA receptor phosphorylation, emphasizing subunit-specific phosphorylation, which differentially controls the trafficking and surface expression of NMDA receptors.
We recently identified an autism spectrum disorder/intellectual disability (ASD/ID)-related
mutation hotspot in the Rac1-activating GEF1 domain of the protein Trio. Trio is a Rho guanine nucleotide ...exchange factor (RhoGEF) that is essential for glutamatergic synapse function. An ASD/ID-related mutation identified in Trio's GEF1 domain, Trio D1368V, produces a pathologic increase in glutamatergic synaptogenesis, suggesting that Trio is coupled to synaptic regulatory mechanisms that govern glutamatergic synapse formation. However, the molecular mechanisms by which Trio regulates glutamatergic synapses are largely unexplored. Here, using biochemical methods, we identify an interaction between Trio and the synaptogenic protein Neuroligin 1 (NLGN1) in the brain. Molecular biological approaches were then combined with super-resolution dendritic spine imaging and whole-cell voltage-clamp electrophysiology in hippocampal slices from male and female rats to examine the impact ASD/ID-related Trio mutations have on NLGN1-mediated synaptogenesis. We find that an ASD/ID-related mutation in Trio's eighth spectrin repeat region, Trio N1080I, inhibits Trio's interaction with NLGN1 and prevents Trio D1368V-mediated synaptogenesis. Inhibiting Trio's interaction with NLGN1 via Trio N1080I blocked NLGN1-mediated synaptogenesis and increases in synaptic NMDA Receptor function but not NLGN1-mediated increases in synaptic AMPA Receptor function. Finally, we show that the aberrant synaptogenesis produced by Trio D1368V is dependent on NLGN signaling. Our findings demonstrate that ASD/ID-related mutations in Trio are able to pathologically increase as well as decrease NLGN-mediated effects on glutamatergic neurotransmission, and point to an NLGN1-Trio interaction as part of a key pathway involved in ASD/ID etiology.
A number of genes have been implicated in the development of autism spectrum disorder/intellectual disability (ASD/ID) in humans. It is now important to identify relationships between these genes to uncover specific cellular regulatory pathways that contribute to these disorders. In this study, we discover that two glutamatergic synapse regulatory proteins implicated in ASD/ID, Trio and Neuroligin 1, interact with one another to promote glutamatergic synaptogenesis. We also identify ASD/ID-related mutations in Trio that either inhibit or augment Neuroligin 1-mediated glutamatergic synapse formation. Together, our results identify a synaptic regulatory pathway that, when disrupted, likely contributes to the development of ASD/ID. Going forward, it will be important to determine whether this pathway represents a point of convergence of other proteins implicated in ASD/ID.
Traditionally, hippocampal long-term potentiation (LTP) of synaptic strength requires Ca2+/calmodulin (CaM)-dependent protein kinase II (CaMKII) and other kinases, whereas long-term depression (LTD) ...requires phosphatases. Here, we found that LTD also requires CaMKII and its phospho-T286-induced “autonomous” (Ca2+-independent) activity. However, whereas LTP is known to induce phosphorylation of the AMPA-type glutamate receptor (AMPAR) subunit GluA1 at S831, LTD instead induced CaMKII-mediated phosphorylation at S567, a site known to reduce synaptic GluA1 localization. GluA1 S831 phosphorylation by “autonomous” CaMKII was further stimulated by Ca2+/CaM, as expected for traditional substrates. By contrast, GluA1 S567 represents a distinct substrate class that is unaffected by such stimulation. This differential regulation caused GluA1 S831 to be favored by LTP-type stimuli (strong but brief), whereas GluA1 S567 was favored by LTD-type stimuli (weak but prolonged). Thus, requirement of autonomous CaMKII in opposing forms of plasticity involves distinct substrate classes that are differentially regulated to enable stimulus-dependent substrate-site preference.
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•Autonomous CaMKII activity is required not only for LTP, but also for LTD•LTP stimuli (strong but brief) favor traditional substrate site phosphorylation•LTD stimuli (weak but prolonged) instead favor a distinct substrate class•Deciding factor in substrate choice is further Ca2+/CaM stimulation of CaMKII
CaMKII and its “autonomous” activity, induced by T286-autophosphorylation, is a crucial mediator of long-term potentiation (LTP) of synaptic strength. In this study, Dell’Acqua, Bayer, and colleagues show that this CaMKII autonomy is also required for long-term depression (LTD), an opposing form of synaptic plasticity. These opposing functions involve stimulus-dependent differential substrate site selection on GluA1: S831 (a traditional substrate favored by LTP-type stimuli) versus S567 (a distinct substrate class instead favored by LTD-type stimuli).
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