Probing the Role of NMDA Receptor NR2B in Development and Schizophrenia
5 January 2012. The NR2B subunit of the N-methyl-D-aspartate (NMDA) receptor plays a crucial role in the development and function of the cortex, according to a new report in the December 8 issue of Neuron. Led by Benjamin Hall of Tulane University in New Orleans, Louisiana, a team of researchers created a mouse model in which the NR2B subunit of the NMDA receptor was genetically replaced by the NR2A subunit. They observed altered α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor signaling in these animals, as well as a behavioral phenotype reminiscent of schizophrenia.
The glutamate hypothesis of schizophrenia, which posits dysfunction of the NMDA receptor (see SRF Current Hypothesis), is based, in part, on the observation that NMDA receptor antagonists can reproduce symptoms of the illness, and the fact that many genes reported to confer risk for schizophrenia are glutamatergic. Additionally, mice with reduced NMDA function have been utilized as models of schizophrenia (Mohn et al., 1999; see also SRF related news story).
The heteromultimeric NMDA receptor is composed of two NR1 and two NR2 subunits. NR1 is encoded by a single gene, while four genes encode NR2, producing subunits NR2A-D. During embryogenesis and early postnatal life, NMDA receptors exclusively contain NR1 and NR2B; NR2A incorporation occurs later in development (Monyer et al., 1994; Sheng et al., 1994). The study of the role of the NR2B subtype during development has been particularly difficult due to the lethality of NR2B knockout mice, as well as the fact that, in traditional models, loss of NR2B early in development also results in loss of all NMDA signaling. In the current study, Hall and colleagues have circumvented these issues by creating the "NR2B --> NR2A" mouse, which exhibits functional NMDA receptor current despite complete loss of the NR2B subunit.
Role of NR2B in AMPA receptor-mediated signaling
NMDA receptors primarily regulate synaptic strength through control of AMPA receptors (Esteban, 2008). To examine the role of NR2B loss on AMPA signaling, first author Chih-Chieh Wang and colleagues measured AMPA receptor-mediated current in cortical cultures from NR2B --> NR2A mice. They found that AMPA-mediated current was increased, consistent with prior data showing that NMDA receptor activation suppresses AMPA receptor function (Hall et al., 2007). In these animals, Wang and colleagues also observed a deficit in a protein translation-dependent form of homeostatic synaptic plasticity, the scaling of synaptic strength to maintain stability mediated in part by changes in AMPA receptor signaling (Turrigiano, 2008; Gainey et al., 2009). A similar deficit was observed in conditional NR2B knockout mice that lack NR2B only in the cortex and hippocampus, but have normal levels in subcortical regions, suggesting that the impaired homeostatic synaptic plasticity is due to a loss of NR2B rather than premature expression of NR2A. After further probing the mechanisms underlying NR2B-mediated regulation of AMPA receptor current, the researchers found that the regulation of protein translation was dependent on the mammalian/mechanistic target of rapamycin (mTOR) pathway, and also required an association between NR2B and α calcium-calmodulin kinase II (CaMKII).
Role of NR2B in social functioning
Behavioral testing of the NR2B --> NR2A animals revealed hyperlocomotion and reduced social exploratory behavior, as measured by a social approach assay. The researchers also examined conditional NR2B conditional knockout mice and observed similar locomotive and social deficits, suggesting (similar to the homeostatic synaptic plasticity data) that the behavioral effects observed are due to a loss of NR2B rather than early expression of NR2A. However, since NR2B --> NR2A mice performed worse on the social interaction task than the conditional NR2B knockouts, premature NR2A expression may also enhance the social deficit.
This study demonstrates that signaling through NR2A is unable to rescue the deficits produced by loss of NR2B, and points to the NR2B subunit as a crucial determinant of signaling through AMPA receptors, synaptic function, and social behavior.—Allison A. Curley.
Wang CC, Held RG, Chang SC, Yang L, Delpire E, Ghosh A, Hall BJ. A Critical Role for GluN2B-Containing NMDA Receptors in Cortical Development and Function. Neuron . 2011 Dec 8 ; 72(5):789-805. Abstract
Comments on News and Primary Papers
Comment by: Stefan Kolata
, Kazu Nakazawa
Submitted 21 February 2012
Posted 21 February 2012
The original NMDA receptor (NMDAR) hypofunction theory of schizophrenia was predicated on the discovery that, in adulthood, NMDAR antagonists mimicked disease symptomatology and exacerbated symptoms in schizophrenic patients (Javitt and Zukin, 1991). Recent advances have since shown that, in addition to this effect in adulthood, there may be a postnatal developmental sensitive period necessary for NMDAR hypofunction to later manifest as schizophrenia phenotypes. For instance, in mice, schizophrenia-like phenotypes were observed when NR1 (GluN1) was ablated selectively in corticolimbic interneurons after postnatal day 7, but not when the knockout occurred after adolescence (Belforte et al., 2010). Similarly, transient antagonism of NMDA during development later resulted in schizophrenia-like phenotypes in adult rats (Stefani and Moghaddam, 2005; Baier et al., 2009). In the present work by Wang et al. (2011), using an elegant molecular genetic technique, Benjamin Hall and his colleagues were able to show that it is perhaps the NR2B (GluN2B) subunit during this developmental period that is most critical for the later development of the symptomatology. NR2B is highly expressed during this postnatal sensitive period, and is only later replaced by NR2A (GluN2A) in most NMDA receptors. The present paper showed that an early replacement of NR2B with NR2A recapitulated some of the NMDA hypomorph phenotypes. While these results are very intriguing and dovetail nicely with the emerging thinking about the neurodevelopmental role of NMDARs, the possible involvement of NR2A itself in schizophrenia should not be lost. Impairment of NR2A results in several schizophrenia-like phenotypes, including a reduction in parvalbumin immunoreactivity, impaired fast-spiking interneuron maturation, altered dopamine metabolism, and a hyperlocomotion response in the open field that is rescued by antipsychotic treatment (Zhang and Sun, 2011; Miyamoto et al., 2001). Further studies of synapses, neurons, and neuronal networks regulated by NR2A and NR2B may lead to a better understanding of the mechanisms underlying the NMDAR hypofunction theory of schizophrenia.
Baier PC, Blume A, Koch J, Marx A, Fritzer G, Aldenhoff JB, Schiffelholz T. Early postnatal depletion of NMDA receptor development affects behaviour and NMDA receptor expression until later adulthood in rats--a possible model for schizophrenia. Behav Brain Res. 2009 Dec 14;205(1):96-101. Abstract
Belforte JE, Zsiros V, Sklar ER, Jiang Z, Yu G, Li Y, Quinlan EM, Nakazawa K. Postnatal NMDA receptor ablation in corticolimbic interneurons confers schizophrenia-like phenotypes. Nat Neurosci. 2010 Jan;13(1):76-83. Abstract
Javitt DC, Zukin SR. Recent advances in the phencyclidine model of schizophrenia. Am J Psychiatry. 1991 Oct;148(10):1301-8. Abstract
Miyamoto Y, Yamada K, Noda Y, Mori H, Mishina M, Nabeshima T. Hyperfunction of dopaminergic and serotonergic neuronal systems in mice lacking the NMDA receptor epsilon1 subunit. J Neurosci. 2001 Jan 15;21(2):750-7. Abstract
Stefani MR, Moghaddam B. Transient N-methyl-D-aspartate receptor blockade in early development causes lasting cognitive deficits relevant to schizophrenia. Biol Psychiatry. 2005 Feb 15;57(4):433-6. Abstract
Zhang Z, Sun QQ. Development of NMDA NR2 subunits and their roles in critical period maturation of neocortical GABAergic interneurons. Dev Neurobiol. 2011 Mar;71(3):221-45. Abstract
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Related News: Getting Specific: Conditional Knockouts Address Glutamate HypothesisComment by: Margarita Behrens
Submitted 17 November 2009
Posted 17 November 2009
Since the discovery that phencyclidine and its analog ketamine exert their pro-psychotic effects through antagonism of NMDA receptors (Javitt and Zukin, 1991), the mechanisms by which these drugs exert these effects have been the subject of intensive research. These studies led to the hypo-NMDA theory of schizophrenia by Olney and collaborators that proposed that “blockade of NMDA receptors triggers a complex network disturbance featuring inactivation of inhibitory neurons and consequent disinhibition of excitatory pathways…” (Olney et al., 1999). Based on the effects of prolonged exposure of primary cultured neurons to selective and non-selective NMDAR antagonists, it was proposed that NMDARs expressed by the subpopulation of parvalbumin-positive (PV) fast spiking interneurons were the target of the antagonists, and that these glutamate receptors played a fundamental role in the maintenance of the GABAergic phenotype of the interneurons (Kinney et al., 2006). Using the Cre-LoxP system to produce the selective ablation of NMDARs in mouse corticolimbic interneurons, Kazu Nakasawa and colleagues now elegantly support this hypothesis in the latest issue of Nature Neuroscience (Belforte et al., 2009). Furthermore, they demonstrate the neurodevelopmental origin of schizophrenia-like behaviors by showing that it is the dysfunction of NMDARs during the period of active maturation of PV-interneurons that increases the chance of behavioral disruptions in late adolescence/early adulthood. These results give strong support to the hypothesis that disruption of the normal maturation of PV-interneurons will produce permanent changes of the inhibitory circuitry in cortex, thus profoundly affecting cortical network function (Behrens and Sejnowski, 2009).
An interesting outcome of Belforte’s results is that, per se, the diminished activity of NMDARs in PV-interneurons does not lead to behavioral disruption, but when these animals undergo the stress of being reared in isolation they manifest the schizophrenia-like behavior. The effects of isolation rearing on PV-interneurons and behavior were recently related to the activation of the superoxide producing enzyme NADPH-oxidase (Nox2) in brain (Schiavone et al., 2009). Treatment of these animals with the Nox2 inhibitor apocynin prevented the loss of GABAergic phenotype of PV-interneurons as well as the behavioral derangements produced by the isolation rearing.
These results have bearing on the effects of NMDAR antagonist exposure, where it was shown that activation of this same enzyme (Nox2) is responsible for the effects of the antagonists on the GABAergic phenotype of PV-interneurons (Behrens et al., 2007; Behrens et al., 2008). Therefore, we can speculate that the pro-psychotic effects of NMDAR-antagonists occur by a double-hit mechanism: first, blocking NMDAR activity in PV-interneurons leads to the loss of their GABAergic phenotype; and, second, inducing the activation of the IL-6/Nox2 pathway further promotes this loss even in the absence of the antagonist. However, it is still not clear why diminished activity of NMDARs in PV-interneurons is only consequential during the period of active maturation of PV-interneuronal circuits, and renders the cortical circuitry vulnerable to the sustained activation of the IL-6/Nox2 pathway. One possible answer is that inactivation of NMDARs in PV-interneurons during early postnatal development disrupts the development of PV-interneuronal synaptic contacts. This could lead to cortical networks that have all neurons in place but with a subset dysfunctional. In turn, this faulty network may be more vulnerable to the effects of activation of the IL-6/Nox2 pathway, such that when this pathway is activated, i.e., by social isolation, it leads to aberrant oscillatory activity in brain and cognitive disruption as observed in schizophrenia.
Javitt DC, Zukin SR. Recent advances in the phencyclidine model of schizophrenia. Am J Psychiatry. 1991 Oct 1;148(10):1301-8. Abstract
Olney JW, Newcomer JW, Farber NB. NMDA receptor hypofunction model of schizophrenia. J Psychiatr Res. 1999 Nov-Dec ;33(6):523-33. Abstract
Kinney JW, Davis CN, Tabarean I, Conti B, Bartfai T, Behrens MM. A specific role for NR2A-containing NMDA receptors in the maintenance of parvalbumin and GAD67 immunoreactivity in cultured interneurons. J Neurosci . 2006 Feb 1 ; 26(5):1604-15. Abstract
Belforte JE, Zsiros V, Sklar ER, Jiang Z, Yu G, Li Y, Quinlan EM, Nakazawa K. Postnatal NMDA receptor ablation in corticolimbic interneurons confers schizophrenia-like phenotypes. Nat Neurosci. 2009 Nov 15. Abstract
Behrens MM, Sejnowski TJ. Does schizophrenia arise from oxidative dysregulation of parvalbumin-interneurons in the developing cortex? Neuropharmacology. 2009 Sep 1;57(3):193-200. Abstract
Schiavone S, Sorce S, Dubois-Dauphin M, Jaquet V, Colaianna M, Zotti M, Cuomo V, Trabace L, Krause KH. Involvement of NOX2 in the development of behavioral and pathologic alterations in isolated rats. Biol Psychiatry. 2009 Aug 15;66(4):384-92. Abstract
Behrens MM, Ali SS, Dao DN, Lucero J, Shekhtman G, Quick KL, Dugan LL. Ketamine-induced loss of phenotype of fast-spiking interneurons is mediated by NADPH-oxidase. Science. 2007 Dec 7;318(5856):1645-7. Abstract
Behrens MM, Ali SS, Dugan LL. Interleukin-6 mediates the increase in NADPH-oxidase in the ketamine model of schizophrenia. J Neurosci. 2008 Dec 17;28(51):13957-66. Abstract
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