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ScienceDirect NMDA receptor dysfunction in autism spectrum disorders Eun-Jae Lee1, Su Yeon Choi2 and Eunjoon Kim2,3 Abnormalities and imbalances in neuronal excitatory and inhibitory synapses have been implicated in diverse neuropsychiatric disorders including autism spectrum disorders (ASDs). Increasing evidence indicates that dysfunction of NMDA receptors (NMDARs) at excitatory synapses is associated with ASDs. In support of this, human ASD-associated genetic variations are found in genes encoding NMDAR subunits. Pharmacological enhancement or suppression of NMDAR function ameliorates ASD symptoms in humans. Animal models of ASD display bidirectional NMDAR dysfunction, and correcting this deficit rescues ASD-like behaviors. These findings suggest that deviation of NMDAR function in either direction contributes to the development of ASDs, and that correcting NMDAR dysfunction has therapeutic potential for ASDs. Addresses 1 Graduate School of Medical Science and Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, Republic of Korea 2 Department of Biological Sciences, KAIST, Daejeon 305-701, Republic of Korea 3 Center for Synaptic Brain Dysfunctions, Institute for Basic Science (IBS), Daejeon 305-701, Republic of Korea Corresponding author: Kim, Eunjoon ([email protected])
Among known synaptic proteins implicated in ASD are metabotropic glutamate receptors (mGluRs). Functional enhancement and suppression of mGluR5 are associated with fragile X syndrome and tuberous sclerosis, respectively, which share autism as a common phenotype [5,6]. More recently, ionotropic glutamate receptors, namely NMDA receptors (NMDARs) and AMPA receptors (AMPARs), have also been implicated in ASDs [7]. In this review, we will focus on NMDA receptors and summarize evidence supporting the hypothesis that NMDAR dysfunction contributes to ASDs, and, by extension, that correcting NMDAR dysfunction has therapeutic potential for ASDs.
ASD-related human NMDAR genetic variants Clinical studies on ASDs have identified genetic variants of NMDAR subunit genes. Specifically, de novo mutations have been identified in the GRIN2B gene, encoding the GluN2B subunit [8,9,10–12]. In addition, SNP analyses have linked both GRIN2A (GluN2A subunit) and GRIN2B with ASDs [13]. Because assembled NMDARs contain four subunits, each with distinct properties [14], ASD-related GRIN2A/GRIN2B variants likely alter the functional properties of NMDARs and/or NMDAR-dependent plasticity.
Current Opinion in Pharmacology 2015, 20:8–13 This review comes from a themed issue on Neurosciences Edited by Pierre Paoletti and Jean-Philippe Pin For a complete overview see the Issue and the Editorial http://dx.doi.org/10.1016/j.coph.2014.10.007 1471-4892/# 2014 Elsevier Ltd. All rights reserved.
Introduction Autism spectrum disorders (ASDs) represent neurodevelopmental disorders characterized by two core symptoms; (1) impaired social interaction and communication, and (2) restricted and repetitive behaviors, interests, and activities. ASDs affect 1% of the population, and are considered to be highly genetic in nature [1,2]. A large number (600) of ASD-related genetic variations have been identified (sfari.org), and target gene functions are apparently quite diverse. However, some fall onto common pathways, including synaptic function and chromosome remodeling [3,4], suggesting that core mechanisms may exist. Current Opinion in Pharmacology 2015, 20:8–13
NMDAR modulators in ASD therapy Pharmacological modulation of NMDAR function can improve ASD symptoms. D-cycloserine (DCS), an NMDAR agonist, significantly ameliorates social withdrawal [15] and repetitive behavior [16] in individuals with ASD. These results suggest that reduced NMDAR function may contribute to the development of ASDs in humans. Elevated NMDAR function is also implicated in ASDs. Memantine, an NMDAR antagonist, and its analogue amantadine improve ASD-related symptoms including social deficits, inappropriate language, stereotypy, cognitive impairments, lethargy, irritability, inattention, and hyperactivity [17,18]. These results, together with the DCS results, highlight the importance of a normal range of NMDAR function, and suggest that deviation of NMDAR function in either direction leads to ASD. This concept is in line with the emerging view that synaptic function within a normal range is important and its deviation causes ASDs and intellectual disability [6,19].
Animal models of ASD with NMDAR dysfunction Animal studies have provided further support for the contribution of NMDAR dysfunction to ASDs. As is www.sciencedirect.com
NMDAR dysfunction and ASDs Lee, Choi and Kim 9
the case in humans, positive or negative modulators of NMDARs can normalize ASD-like phenotypes in animals. We can divide these animal studies into two groups. The first group consists of animals in which NMDAR modulators were shown to normalize both NMDAR dysfunction and ASD-like behaviors, establishing strong association between NMDARs and ASD phenotypes (Fig. 1). In the second group, NMDAR modulators were shown to rescue ASD-like behaviors, but NMDAR dysfunction and its correction have not been demonstrated. Specific examples of each group are described below and summarized in Table 1, sorted based on NMDAR function.
Group 1: ASD models with data showing rescue of both NMDAR dysfunction and ASDlike behaviors Neuroligin-1
Mice lacking neuroligin-1, an excitatory postsynaptic adhesion molecule, show reduced NMDAR function in the hippocampus and striatum, as evidenced by a decrease in NMDA/AMPA ratio and long-term potentiation (LTP) [20–22]. Neuroligin-1 is thought to enhance synaptic NMDAR function, by directly interacting with and promoting synaptic localization of NMDARs [22]. Figure 1 Brain function
Neuroligin-1/ mice show markedly increased selfgrooming, which is rapidly (30 min) attenuated by systemic injection of DCS [20]. Shank2
Shank2 is an excitatory postsynaptic scaffold important for synaptic protein assembly and signaling. Shank2/ mice lacking exons 6 and 7, a mutation found in humans [23], show reduced hippocampal NMDAR function and social deficits that are rapidly normalized by systemic DCS treatment [24]. CDPPB, a positive allosteric modulator of mGluR5 that potentiates NMDARs [25], similarly normalizes NMDAR dysfunction and behavioral deficits, consistent with the idea that indirectly modulating NMDARs through mGluR5 is a viable approach for treating ASDs [26]. Notably, mGluR5-dependent longterm depression (LTD) is unchanged in Shank2/ synapses, indicating that mGluR5 function is normal and suggesting that CDPPB-dependent rescue does not involve normalization of mGluR5 activity per se. Another line of Shank2/ mice that lacks only exon 7, a mutation also found in humans [23], exhibits enhanced rather than reduced NMDAR function, but shows social deficits similar to those observed in exon 6 + 7-deleted mice [27]. Although the mechanistic details are not clear, these results argue that deviation of NMDAR function in either direction can lead to ASD-like phenotypes. IRSp53
Neuroligin-1–/– Shank2 –/– (exon 6+7)
IRSp53 –/–
Tbr1+/– GluD1–/– BTBR Balb/c Selectively bred rats
Normal NMDAR function
Reduced
Group 2
Group 1 Phenotype
Excessive
Rescue
Phenotype
Rescue
ASD-like behaviors
Group 2: ASD models with data showing rescue of ASD-like behaviors but no demonstrated NMDAR dysfunction
NMDAR dysfunction
Current Opinion in Pharmacology
Bidirectional NMDAR dysfunction in animal models of ASD. Animal models of ASD with bidirectional NMDAR dysfunction can be positioned on either side of an NMDAR function curve. Model animals were divided into two groups. Group 1: NMDAR modulators normalize both NMDAR dysfunction and ASD-like behaviors (green). Group 2: NMDAR modulators rescue ASD-like behaviors, but NMDAR dysfunction and its rescue have not been demonstrated (orange). Note that Group 2 animals are tentatively placed on the left-hand side of the slope based on the observed DCS rescue of their ASD-like phenotypes, but the directions of their NMDAR dysfunctions remain to be experimentally determined. www.sciencedirect.com
IRSp53 is a postsynaptic scaffold that regulates F-actin in dendritic spines. IRSp53/ mice show an increased NMDA/AMPA ratio and impaired social interaction [28]. Memantine normalizes both NMDAR dysfunction and social deficits. MPEP, a negative allosteric modulator of mGluR5, similarly rescues the deficits in these animals, which exhibit normal mGluR-dependent LTD. These results suggest that NMDAR hyperfunction is associated with ASD-like phenotypes in IRSp53/ mice. Taken together with the results from neuroligin-1/ and Shank2/ mice, this associates bidirectional NMDAR dysfunction with ASD-like phenotypes.
Tbr1
Tbr1 is a transcriptional regulator, one of whose targets is the gene encoding the GluN2B subunit of NMDARs. Mice haploinsufficient for Tbr1 (Tbr1+/) show structural abnormalities in the amygdala and limited GluN2B induction upon behavioral stimulation [29]. Both systemic injection and local amygdalar infusion of DCS rescue social deficits and impaired associative memory in Tbr1+/ mice [29]. However, reduced NMDAR function and its DCS-dependent correction have not been demonstrated. Current Opinion in Pharmacology 2015, 20:8–13
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Table 1 NMDAR dysfunctions in animal models of ASD. Animal models of ASD are summarized with specific details of NMDAR dysfunction, ASDlike behavior, and pharmacological rescue. Hp, hippocampus; Str, striatum; NA, not available; DCS, D-cycloserine NMDAR function #
Region of NMDAR dysfunction
Animals
Neuroligin-1
/
mice
Hp, Str
ASD-like behaviors
Ref.
Sociability
Repetitive behavior
Pharmacological rescue
Grooming"
Grooming by DCS
[20–22]
Grooming and jumping"
Sociability by DCS and CDPPB
[24]
Shank2/ mice (exons 6+7)
Hp
Mildly impaired Impaired
"
IRSp53/mice
Hp
Impaired
Not increased (grooming and marble burying)
Sociability by memantine and MPEP
[28]
N/A
Tbr1+/ mice
N/A
Impaired
Not increased (grooming) N/A Grooming"
Sociability by DCS
[29]
Sociability by DCS Sociability and grooming by DCS, MPEP, and GRN-529 Sociability and stereotypy by DCS Sociability by NMDAR agonist, GLYX-13
[30,52] [32,33,34]
GluD1/ mice BTBR inbred mice
Impaired Impaired
Balb/c inbred mice
Impaired
Selectively bred rats
Impaired
GluD1
Mice lacking GluD1, a member of the delta family of ionotropic glutamate receptors, exhibit diminished social interaction and impaired contextual fear conditioning, both of which are rescued by DCS [30]. BTBR mice
BTBR is an inbred mouse model of ASD that shows social interaction deficits, impaired ultrasonic vocalization (USV), and repetitive behavior [31]. DCS improves social interaction and self-grooming in BTBR mice [32], suggesting that NMDAR function is reduced. In addition, negative allosteric modulators of mGluR5 such as MPEP and GRN-529 attenuate repetitive behavior and social deficits in BTBR mice [33,34], although whether this involves NMDAR modulation has not been determined. Balb/c mice
Balb/c is another inbred mouse strain with reduced sociability. DCS improves social interaction and stereotypy in these animals [35,36]. Notably, Balb/c mice show heightened behavioral sensitivity to the NMDAR antagonist, MK801 [37], suggesting that NMDAR function is reduced in these animals. Selectively bred rats
Rats selectively bred for low rates of play-related prosocial USVs show reduced social interaction and increased monotonous (non-frequency modulated) USVs. These Current Opinion in Pharmacology 2015, 20:8–13
Spontaneous stereotypy N/A
[35,36] [38]
USV phenotypes are rescued by GLYX-13, an NMDAR glycine site partial agonist [38].
NMDAR dysfunction in the development of ASDs The results from animal studies mentioned above lead to several fundamental questions that need to be discussed. First, how might mutations in diverse ASD-related genes converge onto NMDAR dysfunctions? Second, how does NMDAR dysfunction alter neuron and circuit functions to cause ASD-like phenotypes? Third, how does NMDAR dysfunction in either direction lead to similar ASD-like phenotypes? NMDARs have been shown to form large synaptic protein complexes together with other receptors, scaffolding proteins, signaling molecules, and cytoskeletal components [39], where a delicate balance between all the components would be required to fine-tune the function of ‘the NMDA receptosome’ in a homeostatic manner. Alterations in the levels or functions in any of these components may disturb the equilibrium and leads to NMDAR dysfunction. Regarding the second question, one straightforward possibility is that NMDAR dysfunction may impact neuronal or synaptic development, or NMDAR-dependent synaptic transmission or plasticity in mature neurons [14]. At molecular and cellular levels, NMDAR dysfunction may alter diverse downstream processes in a neuron, including synaptic signaling, AMPAR trafficking, www.sciencedirect.com
NMDAR dysfunction and ASDs Lee, Choi and Kim 11
neuronal excitability, excitation/inhibition balance (E/I balance), gene expression, and action potential firing. For the last question, one possibility is that NMDAR dysfunction may occur at excitatory synapses on interneuronal dendrites. For instance, reduced NMDAR function in interneurons likely decrease inhibitory input onto target neurons, increasing their excitability, a result that can also be caused by enhanced NMDAR function in the target neurons. In line with this, the impaired social interaction caused by elevated E/I balance in pyramidal neurons of the medial prefrontal cortex by optogenetic stimulation has been shown to be partially rescued by increasing inhibitory input onto these neurons by optogenetic excitation of adjacent parvalbumin-positive interneurons [40]. In addition, NMDAR dysfunction may also occur in modulatory neurons including dopaminergic, adrenergic, and cholinergic neurons, which have diverse influences on excitatory and inhibitory neurons.
Perspectives It should be noted that the hypothesized link between NMDAR dysfunction and ASDs currently relies on a small number of clinical and animal studies. Thus, a larger dataset will clearly be needed to verify the hypothesis. Notably, altered NMDAR functions have been reported in a number of known mouse or rat models of ASD, including Shank3DC/DC mice [41], Neuroligin3R451C knock-in mice [42–45,46], Fmr1/ mice [47], and mice and rats prenatally exposed to valproic acid [48,49], a teratogen known to induce ASDs in humans. However, whether correcting the NMDAR function among the diverse electrophysiological alterations observed in these animals can rescue some of the ASD-like behavioral deficits remain to be determined. An important question regarding the NMDAR dysfunction hypothesis would be to determine how NMDAR dysfunction leads to defects in neurons, circuits, and behaviors. Molecular and cellular mechanisms downstream of NMDAR activation should be carefully analyzed and dissected for specific circuit and behavioral defects. Additionally, experimental approaches and interpretations will require further refinement. First, many behavioral rescue experiments on animal models of ASD have used adult animals (i.e., 8–12-week-old mice), whereas electrophysiological measurements are often made in younger animals (i.e., 3–4 weeks). Though experimentally unavoidable, this approach creates a time gap between the two datasets that should be minimized and carefully interpreted. www.sciencedirect.com
Pharmacological rescue of both NMDAR dysfunction and ASD-like behaviors in experimental animals strongly links the two factors, but does not necessarily establish that NMDAR dysfunction causes the ASD-like behaviors. One way to address this question is to carefully monitor the changes occurring along the developmental time axis and, if possible, attempt conditional gene ablation or pharmacological rescue over specific developmental time windows, as previously reported [50,51]. ASDs involve diverse core and comorbid symptoms. Consistent with this, a single autism-related mutation, neuroligin-3 R451C, causes diverse synaptic phenotypes in different brain regions and circuits. Therefore, synaptic changes should be analyzed in greater detail, ideally using brain region-specific and cell type-specific conditional gene ablation, as recently reported. The NMDAR dysfunction hypothesis likely explains only a part of the behavioral deficits, as already discovered in many rescue studies. Modulators of mGluR5, in addition to NMDARs and AMPARs, have been considered to be a new means of regulating glutamatergic transmission [4,26]. Therefore, pharmacological rescue of animal models of ASD should ideally involve modulation of both NMDARs and mGluR5, or even other NMDA-modulatory approaches, to better facilitate translation to clinical therapy. Lastly, because our hypothesis associates bidirectional NMDAR dysfunction with ASDs, there may be clinical cases, such as where individuals with reduced NMDAR function are treated with NMDAR antagonists, which might aggravate the situation and affect the interpretation.
Conflict of interest Nothing declared.
Acknowledgement This work was supported by the Institute for Basic Science (IBS-R002-D1 to E.K.).
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Current Opinion in Pharmacology 2015, 20:8–13
ScienceDirect NMDA receptor dysfunction in autism spectrum disorders Eun-Jae Lee1, Su Yeon Choi2 and Eunjoon Kim2,3 Abnormalities and imbalances in neuronal excitatory and inhibitory synapses have been implicated in diverse neuropsychiatric disorders including autism spectrum disorders (ASDs). Increasing evidence indicates that dysfunction of NMDA receptors (NMDARs) at excitatory synapses is associated with ASDs. In support of this, human ASD-associated genetic variations are found in genes encoding NMDAR subunits. Pharmacological enhancement or suppression of NMDAR function ameliorates ASD symptoms in humans. Animal models of ASD display bidirectional NMDAR dysfunction, and correcting this deficit rescues ASD-like behaviors. These findings suggest that deviation of NMDAR function in either direction contributes to the development of ASDs, and that correcting NMDAR dysfunction has therapeutic potential for ASDs. Addresses 1 Graduate School of Medical Science and Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, Republic of Korea 2 Department of Biological Sciences, KAIST, Daejeon 305-701, Republic of Korea 3 Center for Synaptic Brain Dysfunctions, Institute for Basic Science (IBS), Daejeon 305-701, Republic of Korea Corresponding author: Kim, Eunjoon ([email protected])
Among known synaptic proteins implicated in ASD are metabotropic glutamate receptors (mGluRs). Functional enhancement and suppression of mGluR5 are associated with fragile X syndrome and tuberous sclerosis, respectively, which share autism as a common phenotype [5,6]. More recently, ionotropic glutamate receptors, namely NMDA receptors (NMDARs) and AMPA receptors (AMPARs), have also been implicated in ASDs [7]. In this review, we will focus on NMDA receptors and summarize evidence supporting the hypothesis that NMDAR dysfunction contributes to ASDs, and, by extension, that correcting NMDAR dysfunction has therapeutic potential for ASDs.
ASD-related human NMDAR genetic variants Clinical studies on ASDs have identified genetic variants of NMDAR subunit genes. Specifically, de novo mutations have been identified in the GRIN2B gene, encoding the GluN2B subunit [8,9,10–12]. In addition, SNP analyses have linked both GRIN2A (GluN2A subunit) and GRIN2B with ASDs [13]. Because assembled NMDARs contain four subunits, each with distinct properties [14], ASD-related GRIN2A/GRIN2B variants likely alter the functional properties of NMDARs and/or NMDAR-dependent plasticity.
Current Opinion in Pharmacology 2015, 20:8–13 This review comes from a themed issue on Neurosciences Edited by Pierre Paoletti and Jean-Philippe Pin For a complete overview see the Issue and the Editorial http://dx.doi.org/10.1016/j.coph.2014.10.007 1471-4892/# 2014 Elsevier Ltd. All rights reserved.
Introduction Autism spectrum disorders (ASDs) represent neurodevelopmental disorders characterized by two core symptoms; (1) impaired social interaction and communication, and (2) restricted and repetitive behaviors, interests, and activities. ASDs affect 1% of the population, and are considered to be highly genetic in nature [1,2]. A large number (600) of ASD-related genetic variations have been identified (sfari.org), and target gene functions are apparently quite diverse. However, some fall onto common pathways, including synaptic function and chromosome remodeling [3,4], suggesting that core mechanisms may exist. Current Opinion in Pharmacology 2015, 20:8–13
NMDAR modulators in ASD therapy Pharmacological modulation of NMDAR function can improve ASD symptoms. D-cycloserine (DCS), an NMDAR agonist, significantly ameliorates social withdrawal [15] and repetitive behavior [16] in individuals with ASD. These results suggest that reduced NMDAR function may contribute to the development of ASDs in humans. Elevated NMDAR function is also implicated in ASDs. Memantine, an NMDAR antagonist, and its analogue amantadine improve ASD-related symptoms including social deficits, inappropriate language, stereotypy, cognitive impairments, lethargy, irritability, inattention, and hyperactivity [17,18]. These results, together with the DCS results, highlight the importance of a normal range of NMDAR function, and suggest that deviation of NMDAR function in either direction leads to ASD. This concept is in line with the emerging view that synaptic function within a normal range is important and its deviation causes ASDs and intellectual disability [6,19].
Animal models of ASD with NMDAR dysfunction Animal studies have provided further support for the contribution of NMDAR dysfunction to ASDs. As is www.sciencedirect.com
NMDAR dysfunction and ASDs Lee, Choi and Kim 9
the case in humans, positive or negative modulators of NMDARs can normalize ASD-like phenotypes in animals. We can divide these animal studies into two groups. The first group consists of animals in which NMDAR modulators were shown to normalize both NMDAR dysfunction and ASD-like behaviors, establishing strong association between NMDARs and ASD phenotypes (Fig. 1). In the second group, NMDAR modulators were shown to rescue ASD-like behaviors, but NMDAR dysfunction and its correction have not been demonstrated. Specific examples of each group are described below and summarized in Table 1, sorted based on NMDAR function.
Group 1: ASD models with data showing rescue of both NMDAR dysfunction and ASDlike behaviors Neuroligin-1
Mice lacking neuroligin-1, an excitatory postsynaptic adhesion molecule, show reduced NMDAR function in the hippocampus and striatum, as evidenced by a decrease in NMDA/AMPA ratio and long-term potentiation (LTP) [20–22]. Neuroligin-1 is thought to enhance synaptic NMDAR function, by directly interacting with and promoting synaptic localization of NMDARs [22]. Figure 1 Brain function
Neuroligin-1/ mice show markedly increased selfgrooming, which is rapidly (30 min) attenuated by systemic injection of DCS [20]. Shank2
Shank2 is an excitatory postsynaptic scaffold important for synaptic protein assembly and signaling. Shank2/ mice lacking exons 6 and 7, a mutation found in humans [23], show reduced hippocampal NMDAR function and social deficits that are rapidly normalized by systemic DCS treatment [24]. CDPPB, a positive allosteric modulator of mGluR5 that potentiates NMDARs [25], similarly normalizes NMDAR dysfunction and behavioral deficits, consistent with the idea that indirectly modulating NMDARs through mGluR5 is a viable approach for treating ASDs [26]. Notably, mGluR5-dependent longterm depression (LTD) is unchanged in Shank2/ synapses, indicating that mGluR5 function is normal and suggesting that CDPPB-dependent rescue does not involve normalization of mGluR5 activity per se. Another line of Shank2/ mice that lacks only exon 7, a mutation also found in humans [23], exhibits enhanced rather than reduced NMDAR function, but shows social deficits similar to those observed in exon 6 + 7-deleted mice [27]. Although the mechanistic details are not clear, these results argue that deviation of NMDAR function in either direction can lead to ASD-like phenotypes. IRSp53
Neuroligin-1–/– Shank2 –/– (exon 6+7)
IRSp53 –/–
Tbr1+/– GluD1–/– BTBR Balb/c Selectively bred rats
Normal NMDAR function
Reduced
Group 2
Group 1 Phenotype
Excessive
Rescue
Phenotype
Rescue
ASD-like behaviors
Group 2: ASD models with data showing rescue of ASD-like behaviors but no demonstrated NMDAR dysfunction
NMDAR dysfunction
Current Opinion in Pharmacology
Bidirectional NMDAR dysfunction in animal models of ASD. Animal models of ASD with bidirectional NMDAR dysfunction can be positioned on either side of an NMDAR function curve. Model animals were divided into two groups. Group 1: NMDAR modulators normalize both NMDAR dysfunction and ASD-like behaviors (green). Group 2: NMDAR modulators rescue ASD-like behaviors, but NMDAR dysfunction and its rescue have not been demonstrated (orange). Note that Group 2 animals are tentatively placed on the left-hand side of the slope based on the observed DCS rescue of their ASD-like phenotypes, but the directions of their NMDAR dysfunctions remain to be experimentally determined. www.sciencedirect.com
IRSp53 is a postsynaptic scaffold that regulates F-actin in dendritic spines. IRSp53/ mice show an increased NMDA/AMPA ratio and impaired social interaction [28]. Memantine normalizes both NMDAR dysfunction and social deficits. MPEP, a negative allosteric modulator of mGluR5, similarly rescues the deficits in these animals, which exhibit normal mGluR-dependent LTD. These results suggest that NMDAR hyperfunction is associated with ASD-like phenotypes in IRSp53/ mice. Taken together with the results from neuroligin-1/ and Shank2/ mice, this associates bidirectional NMDAR dysfunction with ASD-like phenotypes.
Tbr1
Tbr1 is a transcriptional regulator, one of whose targets is the gene encoding the GluN2B subunit of NMDARs. Mice haploinsufficient for Tbr1 (Tbr1+/) show structural abnormalities in the amygdala and limited GluN2B induction upon behavioral stimulation [29]. Both systemic injection and local amygdalar infusion of DCS rescue social deficits and impaired associative memory in Tbr1+/ mice [29]. However, reduced NMDAR function and its DCS-dependent correction have not been demonstrated. Current Opinion in Pharmacology 2015, 20:8–13
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Table 1 NMDAR dysfunctions in animal models of ASD. Animal models of ASD are summarized with specific details of NMDAR dysfunction, ASDlike behavior, and pharmacological rescue. Hp, hippocampus; Str, striatum; NA, not available; DCS, D-cycloserine NMDAR function #
Region of NMDAR dysfunction
Animals
Neuroligin-1
/
mice
Hp, Str
ASD-like behaviors
Ref.
Sociability
Repetitive behavior
Pharmacological rescue
Grooming"
Grooming by DCS
[20–22]
Grooming and jumping"
Sociability by DCS and CDPPB
[24]
Shank2/ mice (exons 6+7)
Hp
Mildly impaired Impaired
"
IRSp53/mice
Hp
Impaired
Not increased (grooming and marble burying)
Sociability by memantine and MPEP
[28]
N/A
Tbr1+/ mice
N/A
Impaired
Not increased (grooming) N/A Grooming"
Sociability by DCS
[29]
Sociability by DCS Sociability and grooming by DCS, MPEP, and GRN-529 Sociability and stereotypy by DCS Sociability by NMDAR agonist, GLYX-13
[30,52] [32,33,34]
GluD1/ mice BTBR inbred mice
Impaired Impaired
Balb/c inbred mice
Impaired
Selectively bred rats
Impaired
GluD1
Mice lacking GluD1, a member of the delta family of ionotropic glutamate receptors, exhibit diminished social interaction and impaired contextual fear conditioning, both of which are rescued by DCS [30]. BTBR mice
BTBR is an inbred mouse model of ASD that shows social interaction deficits, impaired ultrasonic vocalization (USV), and repetitive behavior [31]. DCS improves social interaction and self-grooming in BTBR mice [32], suggesting that NMDAR function is reduced. In addition, negative allosteric modulators of mGluR5 such as MPEP and GRN-529 attenuate repetitive behavior and social deficits in BTBR mice [33,34], although whether this involves NMDAR modulation has not been determined. Balb/c mice
Balb/c is another inbred mouse strain with reduced sociability. DCS improves social interaction and stereotypy in these animals [35,36]. Notably, Balb/c mice show heightened behavioral sensitivity to the NMDAR antagonist, MK801 [37], suggesting that NMDAR function is reduced in these animals. Selectively bred rats
Rats selectively bred for low rates of play-related prosocial USVs show reduced social interaction and increased monotonous (non-frequency modulated) USVs. These Current Opinion in Pharmacology 2015, 20:8–13
Spontaneous stereotypy N/A
[35,36] [38]
USV phenotypes are rescued by GLYX-13, an NMDAR glycine site partial agonist [38].
NMDAR dysfunction in the development of ASDs The results from animal studies mentioned above lead to several fundamental questions that need to be discussed. First, how might mutations in diverse ASD-related genes converge onto NMDAR dysfunctions? Second, how does NMDAR dysfunction alter neuron and circuit functions to cause ASD-like phenotypes? Third, how does NMDAR dysfunction in either direction lead to similar ASD-like phenotypes? NMDARs have been shown to form large synaptic protein complexes together with other receptors, scaffolding proteins, signaling molecules, and cytoskeletal components [39], where a delicate balance between all the components would be required to fine-tune the function of ‘the NMDA receptosome’ in a homeostatic manner. Alterations in the levels or functions in any of these components may disturb the equilibrium and leads to NMDAR dysfunction. Regarding the second question, one straightforward possibility is that NMDAR dysfunction may impact neuronal or synaptic development, or NMDAR-dependent synaptic transmission or plasticity in mature neurons [14]. At molecular and cellular levels, NMDAR dysfunction may alter diverse downstream processes in a neuron, including synaptic signaling, AMPAR trafficking, www.sciencedirect.com
NMDAR dysfunction and ASDs Lee, Choi and Kim 11
neuronal excitability, excitation/inhibition balance (E/I balance), gene expression, and action potential firing. For the last question, one possibility is that NMDAR dysfunction may occur at excitatory synapses on interneuronal dendrites. For instance, reduced NMDAR function in interneurons likely decrease inhibitory input onto target neurons, increasing their excitability, a result that can also be caused by enhanced NMDAR function in the target neurons. In line with this, the impaired social interaction caused by elevated E/I balance in pyramidal neurons of the medial prefrontal cortex by optogenetic stimulation has been shown to be partially rescued by increasing inhibitory input onto these neurons by optogenetic excitation of adjacent parvalbumin-positive interneurons [40]. In addition, NMDAR dysfunction may also occur in modulatory neurons including dopaminergic, adrenergic, and cholinergic neurons, which have diverse influences on excitatory and inhibitory neurons.
Perspectives It should be noted that the hypothesized link between NMDAR dysfunction and ASDs currently relies on a small number of clinical and animal studies. Thus, a larger dataset will clearly be needed to verify the hypothesis. Notably, altered NMDAR functions have been reported in a number of known mouse or rat models of ASD, including Shank3DC/DC mice [41], Neuroligin3R451C knock-in mice [42–45,46], Fmr1/ mice [47], and mice and rats prenatally exposed to valproic acid [48,49], a teratogen known to induce ASDs in humans. However, whether correcting the NMDAR function among the diverse electrophysiological alterations observed in these animals can rescue some of the ASD-like behavioral deficits remain to be determined. An important question regarding the NMDAR dysfunction hypothesis would be to determine how NMDAR dysfunction leads to defects in neurons, circuits, and behaviors. Molecular and cellular mechanisms downstream of NMDAR activation should be carefully analyzed and dissected for specific circuit and behavioral defects. Additionally, experimental approaches and interpretations will require further refinement. First, many behavioral rescue experiments on animal models of ASD have used adult animals (i.e., 8–12-week-old mice), whereas electrophysiological measurements are often made in younger animals (i.e., 3–4 weeks). Though experimentally unavoidable, this approach creates a time gap between the two datasets that should be minimized and carefully interpreted. www.sciencedirect.com
Pharmacological rescue of both NMDAR dysfunction and ASD-like behaviors in experimental animals strongly links the two factors, but does not necessarily establish that NMDAR dysfunction causes the ASD-like behaviors. One way to address this question is to carefully monitor the changes occurring along the developmental time axis and, if possible, attempt conditional gene ablation or pharmacological rescue over specific developmental time windows, as previously reported [50,51]. ASDs involve diverse core and comorbid symptoms. Consistent with this, a single autism-related mutation, neuroligin-3 R451C, causes diverse synaptic phenotypes in different brain regions and circuits. Therefore, synaptic changes should be analyzed in greater detail, ideally using brain region-specific and cell type-specific conditional gene ablation, as recently reported. The NMDAR dysfunction hypothesis likely explains only a part of the behavioral deficits, as already discovered in many rescue studies. Modulators of mGluR5, in addition to NMDARs and AMPARs, have been considered to be a new means of regulating glutamatergic transmission [4,26]. Therefore, pharmacological rescue of animal models of ASD should ideally involve modulation of both NMDARs and mGluR5, or even other NMDA-modulatory approaches, to better facilitate translation to clinical therapy. Lastly, because our hypothesis associates bidirectional NMDAR dysfunction with ASDs, there may be clinical cases, such as where individuals with reduced NMDAR function are treated with NMDAR antagonists, which might aggravate the situation and affect the interpretation.
Conflict of interest Nothing declared.
Acknowledgement This work was supported by the Institute for Basic Science (IBS-R002-D1 to E.K.).
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![[Autism spectrum disorders].](/assets/img/hold.png)
