Elsevier

Neuropharmacology

Volume 53, Issue 3, September 2007, Pages 362-368
Neuropharmacology

Mini-review
Regulation of NMDA receptors by phosphorylation

https://doi.org/10.1016/j.neuropharm.2007.05.018Get rights and content

Abstract

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.

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.).

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