Transmembrane region of N-methyl-D-aspartate receptor (NMDAR) subunit is required for receptor subunit assembly

N-Methyl-D-aspartate receptors (NMDARs), one of three main classes of ionotropic glutamate receptors, play major roles in synaptic plasticity, synaptogenesis, and excitotoxicity. Unlike non-NMDA receptors, NMDARs are thought to comprise obligatory heterotetrameric complexes mainly composed of GluN1 and GluN2 subunits. When expressed alone in heterogenous cells, such as HEK293 cells, most of the NMDAR subunits can neither leave the endoplasmic reticulum (ER) nor be expressed in the cell membrane because of the ER retention signals. Only when NMDARs are heteromerically assembled can the ER retention signals be masked and NMDARs be expressed in the surface membrane. However, the mechanisms underlying NMDAR assembly remain poorly understood. To identify regions in subunits that mediate this assembly, we made a series of truncated or chimeric cDNA constructs. Using FRET measurement in living cells combined with immunostaining and coimmunoprecipitation analysis, we examined the assembly-determining domains of NMDAR subunits. Our results indicate that the transmembrane region of subunits is necessary for the assembly of NMDAR subunits, both for the homodimer and the heteromer.     

NR2A but not NR2B N-methyl-D-aspartate receptor subunit is altered in the visual cortex of BDNF-knock-out mice

1. Aim of the present paper is to study the expression of N-Methyl-D-Aspartate receptor (NMDAR) subunits NR2A and NR2B within mouse visual cortex.2. To investigate the influence of neurotrophic factor of NGF family (neurotrophins) on NMDAR expression we used mutant mice carrying a deletion in the gene for brain-derived neurotrophic factor (BDNF), a well-known neurotrophin expressed in visual cortex.3. Western blot and immunohistochemistry were performed at postnatal day P12–14, P21–23, and adulthood showing that both subunits change during postnatal development.4. Absence of BDNF induced a reduction of NR2A level. This effect was specific since the other subunit investigated, NR2B, was not affected in mutant mice.5. We conclude that endogenous BDNF modulates NMDAR expression in the developing visual cortex.     

Trafficking of the NMDAR2B receptor subunit distal cytoplasmic tail from endoplasmic reticulum to the synapse

NMDA receptor NR2A/B subunits have PDZ-binding domains on their extreme C-termini that are known to interact with the PSD-95 family and other PDZ proteins. We explore the interactions between PSD-95 family proteins and the NR2A/B cytoplasmic tails, and the consequences of these interactions, from the endoplasmic reticulum (ER) through delivery to the synapse in primary rat hippocampal and cortical cultured neurons. We find that the NR2A/B cytoplasmic tails cluster very early in the secretory pathway and interact serially with SAP102 beginning at the intermediate compartment, and then PSD-95. We further establish that colocalization of the distal C-terminus of NR2B and PSD-95 begins at the trans-Golgi Network (TGN). Formation of NR2B/PSD-95/SAP102 complexes is dependent on the PDZ binding domain of NR2B subunits, but association with SAP102 and PSD-95 plays no distinguishable role in cluster pre-formation or initial targeting to the vicinity of the synapse. Instead the PDZ binding domain plays a role in restricting cell-surface clusters to postsynaptic targets.     

Key amino acid residues within the third membrane domains of NR1 and NR2 subunits contribute to the regulation of the surface delivery of N-methyl-D-aspartate receptors

N-methyl-d-aspartate (NMDA) receptors are glutamate ionotropic receptors that play critical roles in synaptic transmission, plasticity, and excitotoxicity. The functional NMDA receptors, heterotetramers composed mainly of two NR1 and two NR2 subunits, likely pass endoplasmic reticulum quality control before they are released from the endoplasmic reticulum and trafficked to the cell surface. However, the mechanism underlying this process is not clear. Using truncated and mutated NMDA receptor subunits expressed in heterologous cells, we found that the M3 domains of both NR1 and NR2 subunits contain key amino acid residues that contribute to the regulation of the number of surface functional NMDA receptors. These key residues are critical neither for the interaction between the NR1 and NR2 subunits nor for the formation of the functional receptors, but rather they regulate the early trafficking of the receptors. We also found that the identified key amino acid residues within both NR1 and NR2 M3 domains contribute to the regulation of the surface expression of unassembled NR1 and NR2 subunits. Thus, our data identify the unique role of the membrane domains in the regulation of the number of surface NMDA receptors.     

Characterization of Fyn-mediated tyrosine phosphorylation sites on GluR epsilon 2 (NR2B) subunit of the N-methyl-D-aspartate receptor

The N-methyl-d-aspartate (NMDA) receptors play critical roles in synaptic plasticity, neuronal development, and excitotoxicity. Tyrosine phosphorylation of NMDA receptors by Src-family tyrosine kinases such as Fyn is implicated in synaptic plasticity. To precisely address the roles of NMDA receptor tyrosine phosphorylation, we identified Fyn-mediated phosphorylation sites on the GluR epsilon 2 (NR2B) subunit of NMDA receptors. Seven out of 25 tyrosine residues in the C-terminal cytoplasmic region of GluR epsilon 2 were phosphorylated by Fyn in vitro. Of these 7 residues, Tyr-1252, Tyr-1336, and Tyr-1472 in GluR epsilon 2 were phosphorylated in human embryonic kidney fibroblasts when co-expressed with active Fyn, and Tyr-1472 was the major phosphorylation site in this system. We then generated rabbit polyclonal antibodies specific to Tyr-1472-phosphorylated GluR epsilon 2 and showed that Tyr-1472 of GluR epsilon 2 was indeed phosphorylated in murine brain using the antibodies. Importantly, Tyr-1472 phosphorylation was greatly reduced in fyn mutant mice. Moreover, Tyr-1472 phosphorylation became evident when hippocampal long term potentiation started to be observed, and its magnitude became larger in murine brain. Finally, Tyr-1472 phosphorylation was significantly enhanced after induction of long term potentiation in the hippocampal CA1 region. These data suggest that Tyr-1472 phosphorylation of GluR epsilon 2 is important for synaptic plasticity.     

Export from the endoplasmic reticulum of assembled N-methyl-d-aspartic acid receptors is controlled by a motif in the c terminus of the NR2 subunit

Functional N-methyl-d-aspartic acid (NMDA) receptors are formed from the assembly of NR1 and NR2 subunits. When expressed alone, the major NR1 splice variant and the NR2 subunits are retained in the endoplasmic reticulum (ER), reflecting a quality control mechanism found in many complex multisubunit proteins to ensure that only fully assembled and properly folded complexes reach the cell surface. Recent studies have identified an RRR motif in the C terminus of the NR1 subunit, which controls the ER retention of the unassembled subunit. Here we investigated the mechanisms controlling the ER retention of the NR2 subunit and the export of the assembled complex from the ER. We found that Tac chimeras of the C terminus of the NR2B subunit show that an ER retention signal is also present in the NR2B subunit. In assembled complexes, ER retention signals on the individual subunits must be overcome to allow the complex to leave the ER. One common mechanism involves mutual masking of the signals on the individual subunits. Our data do not support such a mechanism for regulating the release of assembled NMDA receptors from the ER. We found that the motif, HLFY, immediately following transmembrane domain 4 of the NR2 subunit, is required for the assembled complex to exit from the ER. Mutation of this motif allowed the assembly of NR1 and NR2 subunits into a complex that was functional, based on MK-801 binding, but it is retained in the ER. These results are consistent with HLFY functioning as a signal that is necessary for the release of the assembled functional NMDA receptor complex from the ER.     

Transcription of the NR1 subunit of the N-methyl-D-aspartate receptor is down-regulated by excitotoxic stimulation and cerebral ischemia

The N-methyl-D-aspartate (NMDA) type of glutamate receptor (NMDAR) plays central roles in normal and pathological neuronal functioning. We have examined the regulation of the NR1 subunit of the NMDAR in response to excessive activation of this receptor in in vitro and in vivo models of excitotoxicity. NR1 protein expression in cultured cortical neurons was specifically reduced by stimulation with 100 microM NMDA or glutamate. NMDA decreased NR1 protein amounts by 71% after 8 h. Low NMDA concentrations (< or = 10 microM) had no effect. NR1 down-regulation was inhibited by the general NMDAR antagonist DL-AP5 and also by ifenprodil, which specifically antagonizes NMDARs containing NR2B subunits. Arrest of NMDAR signaling with DL-AP5 after brief exposure to NMDA did not prevent subsequent NR1 decrease. Down-regulation of NR1 did not involve calpain cleavage but resulted from a decrease in de novo synthesis consequence of reduced mRNA amounts. In contrast, NMDA did not alter the expression of NR2A mRNA or newly synthesized protein. In neurons transiently transfected with an NR1 promoter/luciferase reporter construct, promoter activity was reduced by 68% after 2 h of stimulation with NMDA, and its inhibition required extracellular calcium. A similar mechanism of autoregulation of the receptor probably operates during cerebral ischemia, because NR1 mRNA and protein were strongly decreased at early stages of blood reperfusion in the infarcted brains of rats subjected to occlusion of the middle cerebral artery. Because NR1 is the obligatory subunit of NMDARs, this regulatory mechanism will be fundamental to NMDAR functioning.     

Comparative analysis of different competitive antagonists interaction with NR2A and NR2B subunits of N-methyl-D-aspartate (NMDA) ionotropic glutamate receptor

The antagonist-bound conformation of the NR2A and NR2B subunits of N-methyl-D-aspartate (NMDA) ionotropic glutamate receptor are modeled using the crystal structure of the DCKA (5,7-dichloro-kynurenic acid)-bound form of the NR1 subunit ligand-binding core (S1S2). Five different competitive NMDA receptor antagonists [(1) DL-AP5; (2) DL-AP7; (3) CGP-37847; (4) CGP 39551; (5) (RS)-CPP] have been docked into both NR2A and NR2B subunits. Experimental studies report NR2A and NR2B subunits having dissimilar interactions and affinities towards the antagonists. However, the molecular mechanism of this difference remains unexplored. The distinctive features in the antagonist's interaction with these two different but closely related (approximately 80% sequence identity at this region) subunits are analyzed from the patterns of their hydrogen bonding. The regions directly involved in the antagonist binding have been classified into seven different interaction sites. Two conserved hydrophilic pockets located at both the S1 and S2 domains are found to be crucial for antagonist binding. The positively charged (Lys) residues present at the second interaction site and the invariant residue (Arg) located at the fourth interaction site are seen to influence ligand binding. The geometry of the binding pockets of NR2A and NR2B subunits have been determined from the distance between the C-alpha atoms in the residues interacting with the ligands. The binding pockets are found to be different for NR2A and NR2B. There are gross dissimilarities in competitive antagonist binding between these two subunits. The binding pocket geometry identified in this study may have the potential for future development of selective antagonists for the NR2A or NR2B subunit.     

Mind bomb-2 is an E3 ligase that ubiquitinates the N-methyl-D-aspartate receptor NR2B subunit in a phosphorylation-dependent manner

The N-methyl-D-aspartate receptor (NMDAR) plays a critical role in synaptic plasticity. Post-translational modifications of NMDARs, such as phosphorylation, alter both the activity and trafficking properties of NMDARs. Ubiquitination is increasingly being recognized as another post-translational modification that can alter synaptic protein composition and function. We identified Mind bomb-2 as an E3 ubiquitin ligase that interacts with and ubiquitinates the NR2B subunit of the NMDAR in mammalian cells. The protein-protein interaction and the ubiquitination of the NR2B subunit were found to be enhanced in a Fyn phosphorylation-dependent manner. Immunocytochemical studies reveal that Mind bomb-2 is localized to postsynaptic sites and colocalizes with the NMDAR in apical dendrites of hippocampal neurons. Furthermore, we show that NMDAR activity is down-regulated by Mind bomb-2. These results identify a specific E3 ubiquitin ligase as a novel interactant with the NR2B subunit and suggest a possible mechanism for the regulation of NMDAR function involving both phosphorylation and ubiquitination.     

The I-II loop of the Ca2+ channel alpha1 subunit contains an endoplasmic reticulum retention signal antagonized by the beta subunit

The auxiliary beta subunit is essential for functional expression of high voltage-activated Ca2+ channels. This effect is partly mediated by a facilitation of the intracellular trafficking of alpha1 subunit toward the plasma membrane. Here, we demonstrate that the I-II loop of the alpha1 subunit contains an endoplasmic reticulum (ER) retention signal that severely restricts the plasma membrane incorporation of alpha1 subunit. Coimmunolabeling reveals that the I-II loop restricts expression of a chimera CD8-I-II protein to the ER. The beta subunit reverses the inhibition imposed by the retention signal. Extensive deletion of this retention signal in full-length alpha1 subunit facilitates the cell surface expression of the channel in the absence of beta subunit. Our data suggest that the beta subunit favors Ca2+ channel plasma membrane expression by inhibiting an expression brake contained in beta-binding alpha1 sequences.     

Arginine 260 of the amino-terminal domain of NR1 subunit is critical for tissue-type plasminogen activator-mediated enhancement of N-methyl-D-aspartate receptor signaling

Tissue-type plasminogen activator (tPA) has been involved in both physiological and pathological glutamatergic-dependent processes, such as synaptic plasticity, seizure, trauma, and stroke. In a previous study, we have shown that the proteolytic activity of tPA enhances the N-methyl-D-aspartate (NMDA) receptor-mediated signaling in neurons (Nicole, O., Docagne, F., Ali, C., Margaill, I., Carmeliet, P., MacKenzie, E. T., Vivien, D., and Buisson, A. (2001) Nat. Med. 7, 59-64). Here, we show that tPA forms a direct complex with the amino-terminal domain (ATD) of the NR1 subunit of the NMDA receptor and cleaves this subunit at the arginine 260. Furthermore, point mutation analyses show that arginine 260 is necessary for both tPA-induced cleavage of the ATD of NR1 and tPA-induced potentiation of NMDA receptor signaling. Thus, tPA is the first binding protein described so far to interact with the ATD of NR1 and to modulate the NMDA receptor function.     

Discovery of (R)-1-[2-hydroxy-3-(4-hydroxy-phenyl)-propyl]-4-(4-methyl-benzyl)-piperidin-4-ol: a novel NR1/2B subtype selective NMDA receptor antagonist

Starting from Ro-25-6981 as a lead compound, highly potent and selective NR1/2B subtype selective NMDA receptor antagonists, with low activity at alpha(1) adrenergic receptors were developed.     

N-Methyl-D-aspartate (NMDA) receptor subunit NR1 forms the substrate for oligomeric assembly of the NMDA receptor

The time course of the assembly of the N-methyl-D-aspartate receptor was examined in a cell line expressing it under the control of the dexamethasone promoter. These studies suggested a delay between the appearance of the NR1 and NR2A subunits and their stable association as examined by co-immunoprecipitation of NR1 and NR2A. This prompted us to examine the stability and folding of the individual subunits using nonreduced polyacrylamide gels and the sulfhydryl cross-linker BMH. Both studies showed that the NR1 subunit was expressed in a monomer and dimer form, whereas both NR2 and NR3 showed substantial aggregation on both nonreduced gels and after cross-linking. Protein degradation experiments showed that NR1 was relatively stable, whereas NR2 and NR3 were more rapidly degraded. When co-expressed with NR1, NR2 was more stable. Fluorescence recovery after photobleaching experiments showed that, under conditions of reduced ATP, the diffusion rate of NR2 and NR3 in the endoplasmic reticulum was reduced, whereas that of NR1 was unaffected. Together these data show that NR1 folds stably when expressed alone, unlike NR2 and NR3, and provides the substrate for assembly of the N-methyl-D-aspartate receptor.     

The C-terminal HDEL sequence is sufficient for retention of secretory proteins in the endoplasmic reticulum (ER) but promotes vacuolar targeting of proteins that escape the ER

Proteins are co-translationally transferred into the endo-plasmic reticulum (ER) and then either retained or transported to different intracellular compartments or to the extracellular space. Various molecular signals necessary for retention in the ER or targeting to different compartments have been identified. In particular, the HDEL and KDEL signals used for retention of proteins in yeast and animal ER have also been described at the C-terminal end of soluble ER processing enzymes in plants. The fusion of a KDEL extension to vacuolar proteins is sufficient for their retention in the ER of transgenic plant cells. However, recent results obtained using the same strategy indicate that HDEL does not contain sufficient information for full retention of phaseolin expressed in tobacco. In the present study, an HDEL C-terminal extension was fused to the vacuolar or extracellular (Δpro) forms of sporamin. The resulting SpoHDEL or ΔproHDEL, as well as Spo and Δpro, were expressed at high levels in transgenic tobacco cells ( Nicotiana tabacum cv BY2). The intracellular location of these different forms of recombinant sporamin was studied by subcellular fractionation. The results clearly indicate that addition of an HDEL extension to either Spo or Δpro induces accumulation of these sporamin forms in a compartment that co-purifies with the ER markers NADH cytochrome C reductase, binding protein (BiP) and calnexin. In addition, a significant SpoHDEL or ΔproHDEL fraction that escapes the ER retention machinery is transported to the vacuole. From these results, it may be proposed that, in addition to its function as an ER retention signal, HDEL could also act in quality control by targeting chaperones or chaperone-bound proteins that escape the ER to the plant lysosomal compartment for degradation.     

Cleavage of the NR2B subunit amino terminus of N-methyl-D-aspartate (NMDA) receptor by tissue plasminogen activator: identification of the cleavage site and characterization of ifenprodil and glycine affinities on truncated NMDA receptor

Thrombolysis using tissue plasminogen activator (tPA) has been the key treatment for patients with acute ischemic stroke for the past decade. Recent studies, however, suggest that this clot-busting protease also plays various roles in brain physiological and pathophysiological glutamatergic-dependent processes, such as synaptic plasticity and neurodegeneration. In addition, increasing evidence implicates tPA as an important neuromodulator of the N-methyl-d-aspartate (NMDA) receptors. Here, we demonstrate that recombinant human tPA cleaves the NR2B subunit of NMDA receptor. Analysis of NR2B in rat brain lysates and cortical neurons treated with tPA revealed concentration- and time-dependent degradation of NR2B proteins. Peptide sequencing studies performed on the cleaved-off products obtained from the tPA treatment on a recombinant fusion protein of the amino-terminal domain of NR2B revealed that tPA-mediated cleavage occurred at arginine 67 (Arg(67)). This cleavage is tPA-specific, plasmin-independent, and removes a predicted ~4-kDa fragment (Arg(27)-Arg(67)) from the amino-terminal domain of the NR2B protein. Site-directed mutagenesis of putative cleavage site Arg(67) to Ala(67) impeded tPA-mediated degradation of recombinant protein. This analysis revealed that NR2B is a novel substrate of tPA and suggested that an Arg(27)-Arg(67)-truncated NR2B-containing NMDA receptor could be formed. Heterologous expression of NR2B with Gln(29)-Arg(67) deleted is functional but exhibits reduced ifenprodil inhibition and increased glycine EC(50) with no change in glutamate EC(50). Our results confirmed NR2B as a novel proteolytic substrate of tPA, where tPA may directly interact with NR2B subunits leading to a change in pharmacological properties of NR2B-containing NMDA receptors.     

4-(3,4-dihydro-1H-isoquinolin-2yl)-pyridines and 4-(3,4-dihydro-1H-isoquinolin-2-yl)-quinolines as potent NR1/2B subtype selective NMDA receptor antagonists

A series of 4-(3,4-dihydro-1H-isoquinolin-2yl)-pyridines and analogous quinolines was prepared and evaluated as NR1/2B subtype selective NMDA receptor antagonists. 2-Hydroxyalkylamino substitution combines high affinity with selectivity (vs alpha1 and M1 receptors) and activity in vivo.     

Identification of an amino acid sequence within GABA(A) receptor beta3 subunits that is important for receptor assembly

GABA(A) receptors are chloride ion channels that can be opened by GABA, the most important inhibitory transmitter in the CNS. In the mammalian brain the majority of these pentameric receptors is composed of two alpha, two beta and one gamma subunit. To achieve the correct order of subunits around the pore, each subunit must form specific contacts via its plus (+) and minus (-) side. To identify a sequence on the beta3 subunit important for assembly, we generated various full-length or truncated chimeric beta3 constructs and investigated their ability to assemble with alpha1 and gamma2 subunits. It was demonstrated that replacement of the sequence beta3(76-89) by the homologous alpha1 sequence impaired assembly with alpha1 but not with gamma2 subunits in alpha1beta3gamma2-GABA(A) receptors. Other experiments indicated that assembly was impaired via the beta3(-) side of the chimeric subunit. Within the sequence beta3(76-89) the sequence beta3(85-89) seemed to be of primary importance for assembly with alpha1 subunits. A comparison with the structure of the acetylcholine-binding protein supports the conclusion that the sequence beta3(85-89) is located at the beta3(-) side and indicates that it contains amino acid residues that might directly interact with the (+) side of the neighbouring alpha1 subunit.     

Novel N1-(benzyl)cinnamamidine derived NR2B subtype-selective NMDA receptor antagonists

Novel (E)-N(1)-(benzyl)cinnamamidines were prepared and evaluated as NR2B subtype NMDA receptor ligands. Excellent affinity was achieved by appropriate substitution of either phenyl ring. The 2-methoxybenzyl compound 1h had approximately 1,000-fold lower IC(50) in NR2B than NR2A-containing cells. Replacement of the styryl unit by 2-naphthyl was well tolerated.     

Antagonists of the human CCR5 receptor as anti-HIV-1 agents. Part 4: synthesis and structure-activity relationships for 1-[N-(methyl)-N-(phenylsulfonyl)amino]-2-(phenyl)-4-(4-(N-(alkyl)-N-(benzyloxycarbonyl)amino)piperidin-1-yl)butanes

(2S)-2-(3-Chlorophenyl)-1-[N-(methyl)-N-(phenylsulfonyl)amino]-4-[spiro(2,3-dihydrobenzthiophene-3,4'-piperidin-1'-yl)]butane S-oxide (1b) has been identified as a potent CCR5 antagonist having an IC50=10 nM. Herein, structure-activity relationship studies of non-spiro piperidines are described, which led to the discovery of 4-(N-(alkyl)-N-(benzyloxycarbonyl)amino)piperidine derivatives (3-5) as potent CCR5 antagonists.