Phosphorylation status of the NR2B subunit of NMDA receptor regulates its interaction with calcium/calmodulin-dependent protein kinase II

Ca²⁺ influx through NMDA-type glutamate receptor at excitatory synapses causes activation of post-synaptic Ca²⁺/calmodulin-dependent protein kinase type II (CaMKII) and its translocation to the NR2B subunit of NMDA receptor. The major binding site for CaMKII on NR2B undergoes phosphorylation at Ser1303, in vivo . Even though some regulatory effects of this phosphorylation are known, the mode of dephosphorylation of NR2B-Ser1303 is still unclear. We show that phosphorylation status at Ser1303 enables NR2B to distinguish between the Ca²⁺/calmodulin activated form and the autonomously active Thr286-autophosphorylated form of CaMKII. Green fluorescent protein–α-CaMKII co-expressed with NR2B sequence in human embryonic kidney 293 cells was used to study intracellular binding between the two proteins. Binding in vitro was studied by glutathione- S -transferase pull-down assay. Thr286-autophosphorylated α-CaMKII or the autophosphorylation mimicking mutant, T286D-α-CaMKII, binds NR2B sequence independent of Ca²⁺/calmodulin unlike native wild-type α-CaMKII. We show enhancement of this binding by Ca²⁺/calmodulin. Phosphorylation or a phosphorylation mimicking mutation on NR2B (NR2B-S1303D) abolishes the Ca²⁺/calmodulin-independent binding whereas it allows the Ca²⁺/calmodulin-dependent binding of α-CaMKII in vitro . Similarly, the autonomously active mutants, T286D-α-CaMKII and F293E/N294D-α-CaMKII, exhibited Ca²⁺-independent binding to non-phosphorylatable mutant of NR2B under intracellular conditions. We also show for the first time that phosphatases in the brain such as protein phosphatase 1 and protein phosphatase 2A dephosphorylate phospho-Ser1303 on NR2B.     

Sequence determinants on the NR2A and NR2B subunits of NMDA receptor responsible for specificity of phosphorylation by CaMKII

Calcium/calmodulin-dependent protein kinase type II (CaMKII) and NMDA-type glutamate receptor (NMDAR) are neuronal proteins involved in learning and memory. CaMKII binds to the NR2B subunit of NMDAR in more than one mode, a stable association involving a noncatalytic site on CaMKII and an enzyme-substrate mode of interaction by its catalytic site. The latter binding results in phosphorylation of serine-1303 on NR2B. We have investigated this binding by studying the kinetics of phosphorylation of synthetic peptides harboring nested sequences of the phosphorylation site motif. We find that residues 1292-1297 of NR2B enhance the affinity of the catalytic site-mediated binding of CaMKII to the minimal phosphorylation site motif, 1298-1308 of NR2B, as evident from measurements of K(m) values for phosphorylation. However, CaMKII shows decreased affinity towards the closely related NR2A subunit due to an -Ile-Asn- motif present as a natural insertion in the analogous sequence on NR2A.     

Modulation of D2R-NR2B interactions in response to cocaine

Dopamine-glutamate interactions in the neostriatum determine psychostimulant action, but the underlying molecular mechanisms remain elusive. Here we found that dopamine stimulation by cocaine enhances a heteroreceptor complex formation between dopamine D2 receptors (D2R) and NMDA receptor NR2B subunits in the neostriatum in vivo. The D2R-NR2B interaction is direct and occurs in the confined postsynaptic density microdomain of excitatory synapses. The enhanced D2R-NR2B interaction disrupts the association of Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) with NR2B, reduces NR2B phosphorylation at a CaMKII-sensitive site (Ser1303), and inhibits NMDA receptor-mediated currents in medium-sized striatal neurons. Furthermore, the regulated D2R-NR2B interaction is critical for constructing behavioral responsiveness to cocaine. Our findings here uncover a direct and dynamic D2R-NR2B interaction in striatal neurons in vivo. This type of dopamine-glutamate integration at the receptor level may be responsible for synergistically inhibiting the D2R-mediated circuits in the basal ganglia and fulfilling the stimulative effect of psychostimulants.     

Displacement of alpha-actinin from the NMDA receptor NR1 C0 domain By Ca2+/calmodulin promotes CaMKII binding

Ca2+ influx through the N-methyl-d-aspartate (NMDA)-type glutamate receptor triggers activation and postsynaptic accumulation of Ca2+/calmodulin-dependent kinase II (CaMKII). CaMKII, calmodulin, and alpha-actinin directly bind to the short membrane proximal C0 domain of the C-terminal region of the NMDA receptor NR1 subunit. In a negative feedback loop, calmodulin mediates Ca2+-dependent inactivation of the NMDA receptor by displacing alpha-actinin from NR1 C0 upon Ca2+ influx. We show that Ca2+-depleted calmodulin and alpha-actinin simultaneously bind to NR1 C0. Upon addition of Ca2+, calmodulin dislodges alpha-actinin. Either the N- or C-terminal half of calmodulin is sufficient for Ca2+-induced displacement of alpha-actinin. Whereas alpha-actinin directly antagonizes CaMKII binding to NR1 C0, the addition of Ca2+/calmodulin shifts binding of NR1 C0 toward CaMKII by displacing alpha-actinin. Displacement of alpha-actinin results in the simultaneous binding of calmodulin and CaMKII to NR1 C0. Our results reveal an intricate mechanism whereby Ca2+ functions to govern the complex interactions between the two most prevalent signaling molecules in synaptic plasticity, the NMDA receptor and CaMKII.     

AlphaCaMKII binding to the C-terminal tail of NMDA receptor subunit NR2A and its modulation by autophosphorylation.

Ca2+/calmodulin-dependent protein kinase II (CaMKII), a multifunctional, widely distributed enzyme, is enriched in post-synaptic densities (PSDs). Here, we demonstrate that CaMKII binds to a discrete C-terminal region of the NR2A subunit of NMDA receptors and promotes the phosphorylation of a Ser residue of this NMDA receptor subunit. Glutathione S-transferase (GST)-NR2A(1349-1464) binds native CaMKII from solubilised hippocampal PSDs in 'pull-out' and overlay experiments and this binding is competed by recombinant alphaCaMKII(1-315). The longer GST-NR2A(1244-1464), although containing the CaMKII phosphosite Ser-1289, binds the kinase with a lower efficacy. CaMKII association to NR2A(1349-1464) is positively modulated by kinase autophosphorylation in the presence of Ca2+/calmodulin. These data provide direct evidence for a mechanism modulating the synaptic strength.     

Regulation of calcium/calmodulin-dependent protein kinase II docking to N-methyl-D-aspartate receptors by calcium/calmodulin and alpha-actinin

Ca(2+) influx through the N-methyl-d-aspartate (NMDA)-type glutamate receptor leads to activation and postsynaptic accumulation of Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) and ultimately to long term potentiation, which is thought to be the physiological correlate of learning and memory. The NMDA receptor also serves as a CaMKII docking site in dendritic spines with high affinity binding sites located on its NR1 and NR2B subunits. We demonstrate that high affinity binding of CaMKII to NR1 requires autophosphorylation of Thr(286). This autophosphorylation reduces the off rate to a level (t(12) = approximately 23 min) that is similar to that observed for dissociation of the T286D mutant CaMKII (t(12) = approximately 30 min) from spines after its glutamate-induced accumulation (Shen, K., Teruel, M. N., Connor, J. H., Shenolikar, S., and Meyer, T. (2000) Nat. Neurosci. 3, 881-886). CaMKII as well as the previously identified NR1 binding partners calmodulin and alpha-actinin bind to the short C-terminal portion of the C0 region of NR1. Like Ca(2+)/calmodulin, autophosphorylated CaMKII competes with alpha-actinin-2 for binding to NR1. We conclude that the NR1 C0 region is a key site for recruiting CaMKII to the postsynaptic site, where it may act in concert with calmodulin to modulate the stimulatory role of alpha-actinin interaction with the NMDA receptor.     

Nucleotides and Phosphorylation Bi-directionally Modulate Ca2+/Calmodulin-dependent Protein Kinase II (CaMKII) Binding to the N-Methyl-d-aspartate (NMDA) Receptor Subunit GluN2B

The Ca²⁺/calmodulin (CaM)-dependent protein kinase II (CaMKII) and the NMDA-type glutamate receptor are key regulators of synaptic plasticity underlying learning and memory. Direct binding of CaMKII to the NMDA receptor subunit GluN2B (formerly known as NR2B) (i) is induced by Ca²⁺/CaM but outlasts this initial Ca²⁺-stimulus, (ii) mediates CaMKII translocation to synapses, and (iii) regulates synaptic strength. CaMKII binds to GluN2B around S1303, the major CaMKII phosphorylation site on GluN2B. We show here that a phospho-mimetic S1303D mutation inhibited CaM-induced CaMKII binding to GluN2B in vitro, presenting a conundrum how binding can occur within cells, where high ATP concentration should promote S1303 phosphorylation. Surprisingly, addition of ATP actually enhanced the binding. Mutational analysis revealed that this positive net effect was caused by four modulatory effects of ATP, two positive (direct nucleotide binding and CaMKII T286 autophosphorylation) and two negative (GluN2B S1303 phosphorylation and CaMKII T305/6 autophosphorylation). Imaging showed positive regulation by nucleotide binding also within transfected HEK cells and neurons. In fact, nucleotide binding was a requirement for efficient CaMKII interaction with GluN2B in cells, while T286 autophosphorylation was not. Kinetic considerations support a model in which positive regulation by nucleotide binding and T286 autophosphorylation occurs faster than negative modulation by GluN2B S1303 and CaMKII T305/6 phosphorylation, allowing efficient CaMKII binding to GluN2B despite the inhibitory effects of the two slower reactions.     

Multivalent interactions of calcium/calmodulin-dependent protein kinase II with the postsynaptic density proteins NR2B, densin-180, and alpha-actinin-2

Dendritic calcium/calmodulin-dependent protein kinase II (CaMKII) is dynamically targeted to the synapse. We show that CaMKIIalpha is associated with the CaMKII-binding proteins densin-180, the N-methyl-D-aspartate receptor NR2B subunit, and alpha-actinin in postsynaptic density-enriched rat brain fractions. Residues 819-894 within the C-terminal domain of alpha-actinin-2 constitute the minimal CaMKII-binding domain. Similar amounts of Thr286-autophosphorylated CaMKIIalpha holoenzyme [P-T286]CaMKII bind to alpha-actinin-2 as bind to NR2B (residues 1260-1339) or to densin-180 (residues 1247-1495) in glutathione-agarose cosedimentation assays, even though the CaMKII-binding domains share no amino acid sequence similarity. Like NR2B, alpha-actinin-2 binds to representative splice variants of each CaMKII gene (alpha, beta, gamma, and delta), whereas densin-180 binds selectively to CaMKIIalpha. In addition, C-terminal truncated CaMKIIalpha monomers can interact with NR2B and alpha-actinin-2, but not with densin-180. Soluble alpha-actinin-2 does not compete for [P-T286]CaMKII binding to immobilized densin-180 or NR2B. However, soluble densin-180, but not soluble NR2B, increases CaMKII binding to immobilized alpha-actinin-2 by approximately 10-fold in a PDZ domain-dependent manner. A His6-tagged NR2B fragment associates with GST-densin or GST-actinin but only in the presence of [P-T286]CaMKII. Similarly, His6-tagged densin-180 or alpha-actinin fragments associate with GST-NR2B in a [P-T286]CaMKII-dependent manner. In addition, GST-NR2B and His6-tagged alpha-actinin can bind simultaneously to monomeric CaMKII subunits. In combination, these data support a model in which [P-T286]CaMKIIalpha can simultaneously interact with multiple dendritic spine proteins, possibly stabilizing the synaptic localization of CaMKII and/or nucleating a multiprotein synaptic signaling complex.     

Calcium/Calmodulin-Dependent Protein Kinase II Is Associated with NR2A/B Subunits of NMDA Receptor in Postsynaptic Densities

Abstract: NMDA receptors and Ca²⁺/calmodulin-dependent kinase II (CaMKII) have been reported to be highly concentrated in the postsynaptic density (PSD). Although the possibility that CaMKII in PSD might be associated with specific proteins has been put forward, the protein or proteins determining the targeting of the kinase in PSD have not yet been identified. Here we report that CaMKII binds to NR2A and NR2B subunits of NMDA receptors in PSD isolated from cortex and hippocampus. The association of NMDA receptor subunits and CaMKII was assessed by immunoprecipitating PSD proteins with antibodies specific for NR2A/B and CaMKII: CaMKII coprecipitated with NR2A/B and NR1 but not with other glutamate ionotropic receptor subunits, such as GluR1 and GluR2-3. A direct association between CaMKII and NR2A/B subunits was further confirmed by overlay experiments using either ³²P-autophosphorylated CaMKII or ³²P-NR2A/B and by evaluating the formation of a CaMKII-NR2A/B complex by means of the cross-linker disuccimidyl suberate. These data demonstrate an association between the NMDA receptor complex and CaMKII in the postsynaptic compartment, suggesting that this colocalization may be relevant for synaptic plasticity.     

Association of calcium/calmodulin-dependent kinase II with developmentally regulated splice variants of the postsynaptic density protein densin-180

In a continuing search for proteins that target calcium/calmodulin-dependent protein kinase II (CaMKII) to postsynaptic density (PSD) substrates important in synaptic plasticity, we showed that the PSD protein densin-180 binds CaMKII. Four putative splice variants (A-D) of the cytosolic tail of densin-180 are shown to be differentially expressed during brain development. Densin-180 splicing affects CaMKII phosphorylation of specific serine residues. Variants A, B, and D, but not C, bind CaMKII stoichiometrically and with high affinity, mediated by a differentially spliced domain. Densin-180 differs from the previously identified CaMKII-binding protein NR2B in that binding does not strictly require CaMKII autophosphorylation. Binding of densin-180 and NR2B to CaMKII is noncompetitive, indicating different interaction sites on CaMKII. Expression of the membrane-targeted CaMKII-binding domain of densin-180 confers membrane localization to coexpressed CaMKII without requiring calcium mobilization, suggesting that densin-180 plays a role in the constitutive association of CaMKII with PSDs.     

Protein kinase C promotes N-methyl-D-aspartate (NMDA) receptor trafficking by indirectly triggering calcium/calmodulin-dependent protein kinase II (CaMKII) autophosphorylation

Regulation of neuronal NMDA receptor (NMDAR) is critical in synaptic transmission and plasticity. Protein kinase C (PKC) promotes NMDAR trafficking to the cell surface via interaction with NMDAR-associated proteins (NAPs). Little is known, however, about the NAPs that are critical to PKC-induced NMDAR trafficking. Here, we showed that calcium/calmodulin-dependent protein kinase II (CaMKII) could be a NAP that mediates the potentiation of NMDAR trafficking by PKC. PKC activation promoted the level of autophosphorylated CaMKII and increased association with NMDARs, accompanied by functional NMDAR insertion, at postsynaptic sites. This potentiation, along with PKC-induced long term potentiation of the AMPA receptor-mediated response, was abolished by CaMKII antagonist or by disturbing the interaction between CaMKII and NR2A or NR2B. Further mutual occlusion experiments demonstrated that PKC and CaMKII share a common signaling pathway in the potentiation of NMDAR trafficking and long-term potentiation (LTP) induction. Our results revealed that PKC promotes NMDA receptor trafficking and induces synaptic plasticity through indirectly triggering CaMKII autophosphorylation and subsequent increased association with NMDARs.     

Differential modulation of Ca2+/calmodulin-dependent protein kinase II activity by regulated interactions with N-methyl-D-aspartate receptor NR2B subunits and alpha-actinin

Neuronal Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) interacts with several prominent dendritic spine proteins, which have been termed CaMKII-associated proteins. The NR2B subunit of N-methyl-d-aspartate (NMDA)-type glutamate receptor, densin-180, and alpha-actinin bind comparable, approximately stoichiometric amounts of Thr(286)-autophosphorylated CaMKIIalpha, forming a ternary complex (Robison, A. J., Bass, M. A., Jiao, Y., Macmillan, L. B., Carmody, L. C., Bartlett, R. K., and Colbran, R. J. (2005) J. Biol. Chem. 280, 35329-35336), but their impacts on CaMKII function are poorly understood. Here we show that these interactions are differentially regulated and exert distinct effects on CaMKII activity. Nonphosphorylated and Thr(286)-autophosphorylated CaMKII bind to alpha-actinin with similar efficacy, but autophosphorylation at Thr(305/306) or Ca(2+)/calmodulin binding significantly reduce this binding. Moreover, alpha-actinin antagonizes CaMKII activation by Ca(2+)/calmodulin, as assessed by autophosphorylation and phosphorylation of a peptide substrate. CaMKII binding to densin (1247-1542) is partially independent of Thr(286) autophosphorylation and is unaffected by Ca(2+)-independent autophosphorylation or Ca(2+)/calmodulin. In addition, the CaMKII binding domain of densin-180 has little effect on CaMKII activity. In contrast, the interaction of CaMKIIalpha with NR2B requires either Thr(286) autophosphorylation or the binding of both Ca(2+)/calmodulin and adenine nucleotides. NR2B inhibits both the Ca(2+)/calmodulin-dependent and autonomous activities of CaMKII by a mechanism that is competitive with autocamtide-2 substrate, non-competitive with syntide-2 substrate, and uncompetitive with respect to ATP. In combination, these data suggest that dynamically regulated interactions with CaMKII-associated proteins could play pleiotropic roles in finetuning CaMKII signaling in defined subcellular compartments.     

Phosphorylated CaMKII post-synaptic binding to NR2B subunits in the anterior cingulate cortex mediates visceral pain in visceral hypersensitive rats

The NR2B subunit of NMDA receptor in the anterior cingulate cortex (ACC) is up-regulated in viscerally hypersensitive (VH) rats induced by colonic anaphylaxis. It plays a critical role in modulation of ACC sensitization and visceral pain responses. Given the key role of calcium/calmodulin-dependent protein kinase II (CaMKII) in synaptic plasticity and behavior learning and memory, we hypothesize that phosphorylation of CaMKII binding to NR2B mediates visceral pain in VH states. We performed in vivo electroporation of CaMKII siRNA produced inhibition of colorectal distension-induced visceromotor response in the VH rats. The NR2B, CaMKII and P-CaMKII-Thr2?? protein levels were increased in 180%, 220% and 304% fold in the post-synaptic density (PSD) fraction in VH rats separately. Western blotting following co-immunoprecipitation showed that P-CaMKII-Thr2?? bound to NR2B in the PSD, which was increased to 267% of control in VH rats. Administration of CaMKII antagonist Antennapedia-CaMKIINtide suppressed visceromotor response in VH rats in parallel with decrease of NR2B levels and reduction of the NR2B-P-CaMKII-Thr2?? protein complex in PSD. In conclusion, CaMKII is a critical signaling molecule in the ACC glutamatergic synaptic transmission and phosphorylation of CaMKII at Thr286, which binds to NR2B subunit at post-synaptic site, modulates visceral pain in viscerally hypersensitive state.     

Protein kinase C activation modulates alpha-calmodulin kinase II binding to NR2A subunit of N-methyl-D-aspartate receptor complex

The N-methyl-d-aspartate (NMDA) receptor subunits NR2 possess extended intracellular C-terminal domains by which they can directly interact with a large number of postsynaptic density (PSD) proteins involved in synaptic clustering and signaling. We have previously shown that PSD-associated alpha-calmodulin kinase II (alphaCaMKII) binds with high affinity to the C-terminal domain of the NR2A subunit. Here, we show that residues 1412-1419 of the cytosolic tail of NR2A are critical for alphaCaMKII binding, and we identify, by site directed mutagenesis, PKC-dependent phosphorylation of NR2A(Ser(1416)) as a key mechanism in inhibiting alphaCaMKII-binding and promoting dissociation of alphaCaMKII.NR2A complex. In addition, we show that stimulation of PKC activity in hippocampal slices either with phorbol esters or with the mGluRs specific agonist trans-1-amino-1,3- cyclopentanedicarboxylic acid (t-ACPD) decreases alphaCaMKII binding to NMDA receptor complex. Thus, our data provide clues on understanding the molecular basis of a direct cross-talk between alphaCaMKII and PKC pathways in the postsynaptic compartment.     

Interaction of peptide substrate outside the active site influences catalysis by CaMKII

The interaction of calcium/calmodulin-dependent protein kinase II (CaMKII) with the NR2B subunit of N-methyl-D-aspartate-type glutamate receptor is thought to be one of the important events leading to synaptic plasticity. CaMKII binds NR2B by its catalytic site and by the autophosphorylation site binding pocket (APBP), a non-catalytic site. Mutagenesis of Glu-236, a residue in the APBP of CaMKII that is likely to be interacting with NR2B, influences phosphorylation of NR2B. The phosphorylation of syntide-2, a classical catalytic site substrate of CaMKII, is influenced to a much lesser extent by this mutation. Taken together these results indicate that interaction of NR2B at the non-catalytic site of CaMKII influences catalysis. Our data suggest that kinetic models of peptide substrate phosphorylation by CaMKII should incorporate the non-catalytic mode of binding of peptides that is dependent on the sequence of the peptide.     

Identification of molecular determinants that are important in the assembly of N-methyl-D-aspartate receptors

To determine which domains of the N-methyl-d-aspartate (NMDA) receptor are important for the assembly of functional receptors, a number of N- and C-terminal truncations of the NR1a subunit have been produced. Truncations containing a complete ligand binding domain bound glycine antagonist and gave binding constants similar to those of the native subunit, suggesting they were folding to form antagonist binding sites. Since NR2A is not transported to the cell surface unless it is associated with NR1 (McIlhinney, R. A. J., Le Bourdellès, B., Tricuad, N., Molnar, E., Streit, P., and Whiting, P. J. (1998) Neuropharmacology 37, 1355-1367), surface expression of NR2A can be used to monitor the association of the subunits. There was progressive loss of NR2A cell surface expression as the N terminus of NR1a was shortened, with complete loss when truncated beyond residue 380. Removal of the C terminus and/or the last transmembrane domain did not affect NR2A surface expression. Similar results were obtained in co-immunoprecipitation experiments. The oligomerization status of the co-expressed NR1a constructs and NR2A subunits was investigated using a non-denaturing gel electrophoresis system (blue native-polyacrylamide gel electrophoresis) and sucrose density gradient centrifugation. The blue native-polyacrylamide gel electrophoresis system also showed that the NR1a subunits could form a homodimer, which was confirmed using soluble constructs of the NR1a subunit. Together these results suggest the residues N-terminal of residue 380 are important for the association of NR2A with NR1a and that the complete N-terminal domain of the NR1a subunit is required for oligomerization with NR2A.     

Characterization of a central Ca2+/calmodulin-dependent protein kinase IIalpha/beta binding domain in densin that selectively modulates glutamate receptor subunit phosphorylation

The densin C-terminal domain can target Ca(2+)/calmodulin-dependent protein kinase IIα (CaMKIIα) in cells. Although the C-terminal domain selectively binds CaMKIIα in vitro, full-length densin associates with CaMKIIα or CaMKIIβ in brain extracts and in transfected HEK293 cells. This interaction requires a second central CaMKII binding site, the densin-IN domain, and an "open" activated CaMKII conformation caused by Ca(2+)/calmodulin binding, autophosphorylation at Thr-286/287, or mutation of Thr-286/287 to Asp. Mutations in the densin-IN domain (L815E) or in the CaMKIIα/β catalytic domain (I205/206K) disrupt the interaction. The amino acid sequence of the densin-IN domain is similar to the CaMKII inhibitor protein, CaMKIIN, and a CaMKIIN peptide competitively blocks CaMKII binding to densin. CaMKII is inhibited by both CaMKIIN and the densin-IN domain, but the inhibition by densin is substrate-selective. Phosphorylation of a model peptide substrate, syntide-2, or of Ser-831 in AMPA receptor GluA1 subunits is fully inhibited by densin. However, CaMKII phosphorylation of Ser-1303 in NMDA receptor GluN2B subunits is not effectively inhibited by densin in vitro or in intact cells. Thus, densin can target multiple CaMKII isoforms to differentially modulate phosphorylation of physiologically relevant downstream targets.     

Effects of streptozotocin-diabetes on the hippocampal NMDA receptor complex in rats

In animal models of diabetes mellitus, such as the streptozotocin-diabetic rat (STZ-rat), spatial learning impairments develop in parallel with a reduced expression of long-term potentiation (LTP) and enhanced expression of long-term depression (LTD) in the hippocampus. This study examined the time course of the effects of STZ-diabetes and insulin treatment on the hippocampal post-synaptic glutamate N-methyl-D-aspartate (NMDA) receptor complex and other key proteins regulating hippocampal synaptic transmission in the post-synaptic density (PSD) fraction. In addition, the functional properties of the NMDA-receptor complex were examined. One month of STZ-diabetes did not affect the NMDA receptor complex. In contrast, 4 months after induction of diabetes NR2B subunit immunoreactivity, CaMKII and Tyr-dependent phosphorylation of the NR2A/B subunits of the NMDA receptor were reduced and alphaCaMKII autophosphorylation and its association to the NMDA receptor complex were impaired in STZ-rats compared with age-matched controls. Likewise, NMDA currents in hippocampal pyramidal neurones measured by intracellular recording were reduced in STZ-rats. Insulin treatment prevented the reduction in kinase activities, NR2B expression levels, CaMKII-NMDA receptor association and NMDA currents. These findings strengthen the hypothesis that altered post-synaptic glutamatergic transmission is related to deficits in learning and plasticity in this animal model.     

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.     

Reduced ethanol inhibition of N-methyl-D-aspartate receptors by deletion of the NR1 C0 domain or overexpression of alpha-actinin-2 proteins

The depressant actions of ethanol on central nervous system activity appear to be mediated by its actions on a number of important membrane associated ion channels including the N-methyl-d-aspartate (NMDA) subtype of ionotropic glutamate receptor. Although no specific site of action for ethanol on the NMDA receptor has been found, previous studies suggest that the ethanol sensitivity of the receptor may be affected by intracellular C-terminal domains of the receptor that regulate the calcium-dependent inactivation of the receptor. In the present study, co-expression of the NR2A subunit and an NR1 subunit that lacks the alternatively spliced intracellular C1 cassette did not reduce the effects of ethanol on channel function as measured by patch-clamp electrophysiology. Full inhibition was also observed in cells expressing an NR1 subunit truncated at the end of the C0 domain (NR1(863stop)). However, the inhibitory effects of ethanol were reduced by expression of an NR1 C0 domain deletion mutant (NR1(Delta839-863)), truncation mutant (NR1(858stop)), or a triple-point mutant (Arg to Ala, Lys to Ala, and Asn to Ala at 859-861) previously shown to significantly reduce calcium-dependent inactivation. A similar reduction in the effects of ethanol on wild-type NR1/2A but not NR1/2B or NR1/2C receptors was observed after co-expression of full-length or truncated human skeletal muscle alpha-actinin-2 proteins that produce a functional knockout of the C0 domain. The effects of ethanol on hippocampal and cortical NMDA-induced currents were similarly attenuated in low calcium recording conditions, suggesting that a C0 domain-dependent process may confer additional ethanol sensitivity to NMDA receptors.