Mini-reviewRegulation of NMDA receptors by phosphorylation
Introduction
Ionotropic glutamate receptors mediate most excitatory neuronal transmission in the brain and play essential roles in the regulation of synaptic activity. Dysfunction of these receptors contributes to many neurological and psychiatric disorders, including Alzheimer's disease, Parkinson's disease, and schizophrenia (Cull-Candy et al., 2001, Waxman and Lynch, 2005). Depending on their specific response to different pharmacological agents, ionotropic glutamate receptors can be subdivided into N-methyl-d-aspartate (NMDA), α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), and kainate receptors (Dingledine et al., 1999, Hollmann and Heinemann, 1994). Among the ionotropic glutamate receptors, NMDA receptor channels have several unique features, including voltage-sensitive block by extracellular Mg2+, high permeability to Ca2+, and unusually slow activation/deactivation kinetics (Cull-Candy et al., 2001). The Mg2+ block acts as a molecular coincidence switch, with Mg2+ being removed from the pore of the channel when postsynaptic cells are depolarized. The relief of the Mg2+ block, together with agonist binding, leads to Ca2+ influx through the NMDA receptor channel that in turn regulates synaptic strength through Ca2+-activated signaling cascades.
Three families of genes (NR1, NR2 and NR3) have been identified that encode the NMDA receptor subunits (Cull-Candy et al., 2001). Functional NMDA receptors are tetramers composed of two essential NR1 subunits assembling with two NR2 subunits or in some cases, an NR2 and an NR3 subunit (Cull-Candy and Leszkiewicz, 2004). Crystallographic analysis, in combination with biochemical and electrophysiological studies, indicates that the NR1–NR2 heterodimer is the functional unit in tetrameric NMDA receptors (Furukawa et al., 2005). A unique feature of NMDA receptors is that receptor activation requires the binding of the co-agonist glycine in addition to glutamate (Erreger et al., 2004). Therefore, functional NMDA receptors require both an NR1 subunit, which contains the glycine binding site, and an NR2 subunit, which binds to glutamate. In addition to the formation of diheteromeric receptors (e.g. NR1/NR2B), there is compelling evidence for the existence of triheteromeric NMDA receptors (e.g. NR1/NR2A/NR2B) (Cull-Candy and Leszkiewicz, 2004). Many studies have demonstrated that NR2 and NR3 subunits confer distinct electrophysiological properties to the NMDA receptors (Cull-Candy and Leszkiewicz, 2004). Therefore, variability in NMDA receptor subunit composition is an important source of diversity for functional regulation of NMDA receptors.
The specific subunit composition of NMDA receptors varies at distinct synapses in different developmental stages. The NR1 subunit is a single gene product and, as an essential subunit, is found ubiquitously throughout the brain. In contrast, NR2 subunits (NR2A-D) are encoded by four distinct genes and are differentially expressed throughout the brain and during development. Among NR2 subunits, the expression patterns of NR2A and NR2B are relatively broad and both are developmentally regulated, with a concurrent decrease in NR2B and increase in NR2A expression as neurons mature. NR2C is restricted primarily to the cerebellum and is expressed later in development. In contrast, NR2D is predominantly expressed early in development and is localized mainly in thalamic and hypothalamic nuclei and in the brainstem (Monyer et al., 1994). The NR3A subunit is widely distributed early in development (Ciabarra et al., 1995, Sucher et al., 1995), whereas NR3B is restricted primarily to motor neurons (Chatterton et al., 2002). Endogenous NMDA receptors typically contain NR1 and NR2 subunits, with NR3 subunits only incorporated in a subpopulation of NMDA receptors playing a modulatory role (Cull-Candy and Leszkiewicz, 2004).
NMDA receptor subunits contain a long extracellular N-terminal domain, three true transmembrane segments, a re-entrant pore loop, and an intracellular C-terminal domain of variable length. The C-terminal domain is the most divergent region of the protein when comparing NMDA receptor subunits, consistent with it playing a critical role in the diversity conferred on NMDA receptors by different subunit compositions. Whereas the N-terminal domain and extracellular loop form the ligand-binding pocket (Furukawa and Gouaux, 2003), the C-terminal tail regulates receptor interactions with a variety of cytosolic proteins. These protein-protein interactions dictate the precise intracellular trafficking and localization of NMDA receptors. In addition, different NMDA receptor subunits can couple receptors to distinct intracellular signaling complexes. For example, NR2B specifically interacts with the protein SynGAP, which is a Ras GTPase activating protein demonstrated to selectively inhibit NMDA-stimulated ERK signaling (Kim et al., 2005). Also, NR2A and NR2B bind to active calcium/calmodulin-dependent protein kinase II (CaMKII) with different affinities (Strack and Colbran, 1998), which results in different forms of synaptic plasticity (Barria and Malinow, 2005). Finally, the C-termini of NMDA receptor subunits are substrates for post-translational modifications such as phosphorylation. Phosphorylation regulates many cellular processes including protein activity, localization and mobility. In addition, phosphorylation is an important regulator of many protein-protein interactions. Direct phosphorylation of ionotropic glutamate receptors is a key mechanism regulating channel function and receptor localization at synapses (Lee, 2006).
Section snippets
Functional regulation of NMDA receptors by serine/threonine phosphorylation
Many serine/threonine phosphorylation sites have been identified in NMDA receptor subunits, which are substrates for cAMP-dependent protein kinase A (PKA), protein kinase C (PKC), protein kinase B (PKB), CaMKII, cyclin-dependent kinase-5 (Cdk5), and casein kinase II (CKII) (Fig. 1). These kinases can regulate intracellular trafficking or channel properties of NMDA receptors, resulting in changes in synaptic strength underlying many forms of synaptic plasticity (Lee, 2006). Although in some
Tyrosine phosphorylation of NMDA receptors
In addition to serine/threonine phosphorylation, NMDA receptor function is also regulated by protein tyrosine kinases (PTKs). For example, NMDA receptor currents are potentiated by increasing PTK activity and reduced by decreasing PTK activity (Wang and Salter, 1994, Wang et al., 1996). PTKs, especially Src and Fyn, are important modulators of NMDA receptors. Early studies characterizing the molecular components of the PSD revealed that several proteins were highly phosphorylated on tyrosine
Conclusion
Protein phosphorylation is an important mechanism modulating the function of NMDA receptors. Although there has been considerable progress in studying NMDA receptor regulation by phosphorylation, many aspects of NMDA receptor phosphorylation remain to be explored. For example, phosphorylation of NR3A and NR3B has not been reported. However, based on the studies in other NMDA receptor subunits and given the structural similarity of these subunits with other NMDA family members, it is expected
Acknowledgements
We thank John T.R. Isaac for valuable comments on the manuscript. This work was supported by the NINDS Intramural Research Program, National Institutes of Health and a NINDS Career Transition Award (to B.C.).
References (87)
- et al.
NMDA receptor subunit composition controls synaptic plasticity by regulating binding to CaMKII
Neuron
(2005) - et al.
Regulation of NR1/NR2C N-methyl-D-aspartate (NMDA) receptors by phosphorylation
The Journal of biological chemistry
(2006) - et al.
Binding of the inward rectifier K+ channel Kir 2.3 to PSD-95 is regulated by protein kinase A phosphorylation
Neuron
(1996) - et al.
NMDA receptor subunits: diversity, development and disease
Current Opinion in Neurobiology
(2001) - et al.
Inactivation of NMDA receptors by direct interaction of calmodulin with the NR1 subunit
Cell
(1996) - et al.
Protein kinase C activation modulates alpha-calmodulin kinase II binding to NR2A subunit of N-methyl-d-aspartate receptor complex
The Journal of Biological Chemistry
(2001) - et al.
A region of the rat N-methyl-d-aspartate receptor 2A subunit that is sufficient for potentiation by phorbol esters
Neuroscience Letters
(2001) - et al.
Differential roles of NR2A- and NR2B-containing NMDA receptors in Ras-ERK signaling and AMPA receptor trafficking
Neuron
(2005) - et al.
Calcineurin acts via the C-terminus of NR2A to modulate desensitization of NMDA receptors
Neuropharmacology
(2002) - et al.
Differential tyrosine phosphorylation of N-methyl-d-aspartate receptor subunits
The Journal of Biological Chemistry
(1995)
Differential binding of the AP-2 adaptor complex and PSD-95 to the C-terminus of the NMDA receptor subunit NR2B regulates surface expression
Neuropharmacology
Synaptic plasticity and phosphorylation
Pharmacology & Therapeutics
Modulation of D2R-NR2B interactions in response to cocaine
Neuron
Protein phosphorylation of ligand-gated ion channels
Methods in Enzymology
Developmental and regional expression in the rat brain and functional properties of four NMDA receptors
Neuron
Activity-dependent mRNA splicing controls ER export and synaptic delivery of NMDA receptors
Neuron
Characterization of Fyn-mediated tyrosine phosphorylation sites on GluR epsilon 2 (NR2B) subunit of the N-methyl-d-aspartate receptor
The Journal of Biological Chemistry
Forward transport. 14-3-3 binding overcomes retention in endoplasmic reticulum by dibasic signals
Cell
Identification of a phosphorylation site for calcium/calmodulin-dependent protein kinase II in the NR2B subunit of the N-methyl-d-aspartate receptor
The Journal of Biological Chemistry
The synaptic localization of NR2B-containing NMDA receptors is controlled by interactions with PDZ proteins and AP-2
Neuron
Beta-adrenergic regulation of synaptic NMDA receptors by cAMP-dependent protein kinase
Neuron
Glutamate receptor phosphorylation and synaptic plasticity
Current Opinion in Neurobiology
Serines 890 and 896 of the NMDA receptor subunit NR1 are differentially phosphorylated by protein kinase C isoforms
Neurochemistry International
Coordinated PKA and PKC phosphorylation suppresses RXR-mediated ER retention and regulates the surface delivery of NMDA receptors
Neuropharmacology
Protein kinase C transiently activated heteromeric N-methyl-d-aspartate receptor channels independent of the phosphorylatable C-terminal splice domain and of consensus phosphorylation sites
The Journal of Biological Chemistry
PDZ domain suppression of an ER retention signal in NMDA receptor NR1 splice variants
Neuron
Autophosphorylation-dependent targeting of calcium/ calmodulin-dependent protein kinase II by the NR2B subunit of the N-methyl-d-aspartate receptor
The Journal of Biological Chemistry
Mechanism and regulation of calcium/calmodulin-dependent protein kinase II targeting to the NR2B subunit of the N-methyl-d-aspartate receptor
The Journal of Biological Chemistry
Characterization of protein kinase A and protein kinase C phosphorylation of the N-methyl-d-aspartate receptor NR1 subunit using phosphorylation site-specific antibodies
The Journal of Biological Chemistry
14-3-3 dimers probe the assembly status of multimeric membrane proteins
Current Biology
NMDA receptors in layer 4 spiny stellate cells of the mouse barrel cortex contain the NR2C subunit
The Journal of Neuroscience
Excitatory glycine receptors containing the NR3 family of NMDA receptor subunits
Nature
Protein kinase C reduces Mg2+ block of NMDA-receptor channels as a mechanism of modulation
Nature
Regulation of the NMDA receptor complex and trafficking by activity-dependent phosphorylation of the NR2B subunit PDZ ligand
The Journal of Neuroscience
Cloning and characterization of chi-1: a developmentally regulated member of a novel class of the ionotropic glutamate receptor family
The Journal of Neuroscience
cAMP-dependent protein kinase mediates activity-regulated synaptic targeting of NMDA receptors
The Journal of Neuroscience
The glutamate receptor ion channels
Pharmacological Reviews
Developmental regulation of tyrosine phosphorylation of the NR2D NMDA glutamate receptor subunit in rat central nervous system
Journal of Neurochemistry
Splice variant-specific interaction of the NMDA receptor subunit NR1 with neuronal intermediate filaments
The Journal of Neuroscience
Synaptic protein tyrosine kinase: partial characterization and identification of endogenous substrates
Journal of Neurochemistry
Glutamate receptor gating
Critical Reviews in Neurobiology
Cited by (327)
Calcium- and calmodulin-dependent inhibition of NMDA receptor currents
2024, Biophysical JournalActivation of mGlu<inf>2/3</inf> receptors in the striatum alleviates L-DOPA-induced dyskinesia and inhibits abnormal postsynaptic molecular expression
2023, Pharmacology Biochemistry and BehaviorGluN1 antibody causes behavioral deficits in prepulse inhibition and memory through CaMKIIβ signaling
2022, Journal of NeuroimmunologyLong-term cyclosporine A treatment promotes anxiety-like behavior: Possible relation with glutamate signaling in rat hippocampus
2022, Journal of Affective Disorders Reports