Gene name - NMDA receptor 1 and NMDA receptor 2
Cytological map position- 83A6-83A7 and 2B1-2B1
Function - glutamate-gated ion channel
Symbol - Nmdar1 and Nmdar2
Genetic map position - 3R and X
Classification - N-methyl-D-aspartate selective glutamate receptor activity
Cellular location - surface transmembrane
N-methyl-D-aspartate (NMDA) receptors are one of three pharmacologically distinct subtypes of ionotropic receptors that mediate a majority of excitatory neurotransmission in the brain via the endogenous amino acid, L-glutamate. NMDARs form heteromeric complexes usually comprised of the essential NR1 subunit and various NR2 subunits. Molecular and electrophysiological properties of NMDARs suggest that they may be the Hebbian 'coincidence detectors' hypothesized to underlie associative learning. Because of the nonspecificity of drugs that modulate NMDAR function or the relatively chronic genetic manipulations of various NMDAR subunits from mammalian studies, conclusive evidence for such an acute role for NMDARs in adult behavioral plasticity, however, is lacking. Moreover, a role for NMDARs in memory consolidation remains controversial (Xia, 2005; full text of article).
The Drosophila genome encodes two NMDAR homologs, dNR1 and dNR2. When coexpressed in Xenopus oocytes or Drosophila S2 cells, dNR1 and dNR2 form functional NMDARs with several of the distinguishing molecular properties observed for vertebrate NMDARs, including voltage/Mg2+-dependent activation by glutamate. Both proteins are weakly expressed throughout the entire brain but show preferential expression in several neurons surrounding the dendritic region of the mushroom bodies. Hypomorphic mutations of the essential dNR1 gene disrupt olfactory learning, and this learning defect is rescued with wild-type transgenes. Importantly, Pavlovian learning is disrupted in adults within 15 hr after transient induction of a dNR1 antisense RNA transgene. Extended training is sufficient to overcome this initial learning defect, but long-term memory (LTM) specifically is abolished under these training conditions. In conclusion, this study uses a combination of molecular-genetic tools to (1) generate genomic mutations of the dNR1 gene, (2) rescue the accompanying learning deficit with a dNR1+ transgene, and (3) rapidly and transiently knockdown dNR1+ expression in adults, thereby demonstrating an evolutionarily conserved role for the acute involvement of NMDARs in associative learning and memory (Xia, 2005).
The NMDAR channel is highly permeable to Ca2+ and Na+, and its opening requires simultaneous binding of glutamate and postsynaptic membrane depolarization. Once activated, the NMDAR channel allows calcium influx into the postsynaptic cell where calcium triggers a cascade of biochemical events resulting in synaptic changes (Xia, 2005).
Cellular studies have suggested that NMDAR is involved in several forms of synaptic plasticity, including long-term potentiation and long-term depression. The NMDAR possesses an interesting molecular property, namely, a voltage-dependent blockade of glutamate-induced calcium flux. This suggests NMDAR's role as the above mentioned 'Hebbian coincidence detector' underlying associative learning. Additional, non-Hebbian cellular mechanisms appear necessary, however, to model associative learning adequately. To that end, behavioral studies attempting to demonstrate an acute role for mammalian NMDARs in associative learning and/or memory have been limited by (1) the nonspecificity of drugs that modulate NMDAR function or (2) the relatively chronic genetic manipulations of various NMDAR subunits. Whether NMDARs also are involved with memory consolidation is even more controversial (Xia, 2005).
In invertebrates, pharmacological manipulations have suggested that NMDA-like receptors mediate associative learning in Aplysia (Roberts, 2003) and memory recall in honeybee (Si, 2004), and the function of an NR1 homolog, NMR-1, has been characterized in C. elegans (Brockie, 2001). These studies did not determine which potential NMDAR homologs form functional NMDARs, (Dingledine, 1999), however, direct demonstrations of roles for specific NMDAR genes in behavioral plasticity still are lacking in these model systems. Therefore molecular, genetic, electrophysiological, and behavioral experiments were persued on the Drosophila NMDAR subunit genes, dNR1 (Ultsch, 1993) and dNR2, which together establish an acute role for NMDAR in associative learning and in long-term memory consolidation (Xia, 2005).
Homology searches of the Drosophila genome database and cloning suggest dNR1 is the only gene bearing high amino acid sequence similarity to the mammalian NMDA receptor subunit NR1. Compared with its vertebrate counterpart, dNR1 shows high homology with respect to its entire size, domain structures, and active physiological sites. dNR2 appears to be the sole gene encoding the Drosophila homolog of mammalian NR2, although there are four NR2 family members in vertebrates (Yamakura, 1999). dNR2 undergoes alternative splicing, however, to generate eight different transcripts and three protein variants. The domain structures of dNR2 show high homology to vertebrate NR2, but its entire size, active physiological sites, and molecular function are only moderately conserved from its mammalian counterparts (Xia, 2005).
The dNR1 transcript is highly regulated during development and is expressed at high levels in late embryos when the larval nervous system is formed, in late pupae when the adult central nervous system develops, and in adult head. Western blots confirmed that both proteins are expressed at a high level in adult head but not in the body. Immunostaining also indicates that they may be expressed throughout the whole brain and at especially high levels in several neurons surrounding the calyx of the MBs. The interpretation of generally weak expression of dNR1 and dNR2 is further supported by Western blots showing a detectable band from single-head preparations. Thus, dNR1 and dNR2 likely function together in most places, which is in agreement with functional analyses. In contrast, dNR1 appears to have a broader pattern of preferential expression than dNR2 in adult brain, suggesting alternative associations with other endogenous glutamate receptors. Alternatively, dNR1 alone may form functional NMDAR channels in vivo, given its weak but significant NMDA-selective response in Xenopus oocytes. It might be noted, however, that functional NMDA receptors can be formed by expression of NR1 alone in Xenopus oocytes but not in mammalian cell lines. Finally, dNR1 has an RSS (Retention Signal Sequence) motif at its C terminus, similar to its mammalian homolog, suggesting that dNR1, when not associated with dNR2 or other glutamate receptors, may be retained in the ER rather than inserted in the cell membrane (Xia, 2005).
Coexpression of dNR1 and dNR2-2 in Xenopus oocytes generated NMDA-selective responses. Similarly, functional homomeric receptors can be formed within the AMPA and kainate subunit families but probably not for NMDA receptors in vertebrates, and highly active NMDAR channels are only formed when the NR1 subunit is expressed in combination with one of the four NR2 subunits (Dingledine, 1999; Mori, 1995). Pharmacological, anatomical, biochemical, and immunological studies also have established heteromeric, but not homomeric, assembly of NMDAR channel subunits in vivo (Mori, 1995). The physiological features that distinguish NMDAR from other ionotropic glutamate receptors are (1) high permeability to Ca2+, (2) selective activation by NMDA and L-asparate, (3) modulation by glycine as the coagonist for glutamate, and (4) voltage-dependent blockade by Mg2+ (Dingledine, 1999). The electrophysiological profile of dNR1 and dNR2 coexpressed in Xenopus oocytes or Drosophila S2 cells reveals that the functional NMDARs produce most of these distinguishing characteristics including selective activation by NMDA and L-asparate, modulation by glycine as the coagonist for glutamate, and voltage- and Mg2+-dependent conductance. Thus, Drosophila likely has functional NMDARs consisting of two subunits, dNR1 and dNR2 (Xia, 2005).
The NMDA-selective conductance was sensitive to Mg2+ blockade only in Drosophila S2 cells but not in Xenopus oocytes up to 10 mM, which is highly reminiscent of NMDA receptors in C. elegans (Brockie, 2001). Proper external ionic conditions for oocytes and insect cells are remarkably different. The endogenous Mg2+ concentration for fly muscle, for instance, is about ten times higher than that for oocytes, suggesting that invertebrate NMDA receptors have evolved to be less sensitive to Mg2+. Molecular evidence exists in support of this conclusion. Replacement of the asparagine residue in the pore-forming TM2 domain reduces but does not abolish Mg2+ block for mammalian NR receptors (Dingledine, 1999). This crucial asparagine residue in dNR2 subunits is replaced by glutamine. In addition, TM1, TM4, and the short linker between TM2 and TM3 domains also are critical determinants for Mg2+ block (Kuner, 1996). Although the linker appears conserved in dNR2, TM1 and TM4 are not (Xia, 2005).
Fly NMDA receptors have been shown to regulate the larval locomotor rhythm (Cattaert, 2001). This effect can be blocked completely by MK801, requiring binding to the same asparagine residue to execute its antagonist effect (Ferrer-Montiel, 1995). MK801 also suppresses NMDAR-mediated juvenile hormone biosynethesis in cockroach (Chiang, 2002) (Xia, 2005).
This study provided the first demonstration that NMDARs are required acutely for associative learning in Drosophila. Pavlovian task is a form of fear conditioning which uses well-defined odors as conditioned stimuli (CSs) and footshock as an unconditioned stimulus (US). When tested immediately after Pavlovian conditioning (one training session), flies homozygous for either of two different hypomorphic mutations performed poorly in this task, although they seem to grow normally, do not show any obvious behavioral abnormalities, and most importantly, show normal sensorimotor responses to the stimuli used for this task. The learning deficit in dNR1 mutants can be rescued fully in transgenic flies carrying either of two different genomic constructs containing the dNR transcription unit, which constitutes definitive proof that this transcription unit is responsible for the phenotypic defect observed in these mutants (Xia, 2005).
dNR1 is acutely required for associative learning. Disruption of dNR1, with an hs-GAL4 driver to induce expression of a dNR1 antisense message, yielded a learning deficit specifically and transiently. These results rule out any potential developmental explanation for the adult learning defect. The data extend to insects similar findings from pharmacological and genetic studies in mammals and provide the strongest argument to date that adult learning and memory depend on proper NMDA receptor function (Xia, 2005).
Acute disruption of dNR1 also disrupts 1-day memory after spaced training, without affecting 1-day memory after massed training. The specific abolition of LTM, without affecting 1-day memory after massed training, is similar to that produced by induced expression of a CREB-repressor transgene and indicates a specific disruption of cycloheximde-sensitive LTM with no effect on cycloheximide-insensitive ARM. Hence, CREB-dependent LTM formation appears to depend on normal NMDA receptor function. The cAMP/PKA/CREB signaling pathway has been shown to be involved in diverse processes ranging from hippocampal LTP and barrel formation to learning and memory in mammals. In most of these experimental contexts, activation of NMDARs is required for LTM formation (Riedel, 2003). Recent experiments in mammals also have revealed NMDAR-dependent activation of CREB during LTP and LTM in both amygdala and hippocampus (Schulz, 1999; Cammarota, 2000). Interestingly, two functionally distinct NMDA receptor signaling complexes have been identified: synaptic and extrasynaptic (Hardingham, 2002). Synaptic NMDARs can cause sustained CREB phosphorylation and CRE-mediated gene expression, whereas extrasynaptic NMDARs actively suppress CREB activity via an as yet unknown mechanism. Hence, it seems likely that synaptic NMDAR complexes regulate memory formation by controlling nuclear signaling to CREB (Xia, 2005).
This characterization of a role for NMDA receptors in behavioral plasticity of Drosophila again reinforces the notion that the functional homologies among various model systems is appreciable. Many intracellular signaling proteins are known to be physically associated with vertebrate NMDA receptors (Husi, 2000). The newly identified NMDAR complex consist of more than 80 different proteins, organized into receptor, adaptor, signaling, cytoskeletal, cell adhesion, and novel proteins (Husi, 2000). Genetic and pharmacological disruptions of several components of the NMDAR complex produce learning impairments in rodents. Obvious Drosophila homologs can be identified for a majority of these 80 proteins. Among of them are NR1, PKA subunits, PKC isoforms, and NF1. Disruptions of these genes yield associative learning deficits in flies (Xia, 2005).
The conservation of NMDA-dependent behavioral plasticity in invertebrates further demonstrates that a unified mechanism underlies associative learning and memory. Because behavioral plasticity is tightly associated with synaptic plasticity, it is speculated that similar cellular mechanisms of NMDAR-mediated long-term changes, such as LTP and LTD, may also exist in the adult insect brain. Drosophila genetics now can be applied to discover additional genes and signaling pathways important for NMDAR-dependent plasticity (Xia, 2005).
In conclusion, this study has established that Drosophila likely has functional NMDARs consisting of two subunits, dNR1 and dNR2. Combined expression of both dNR1 and dNR2 generated NMDA-selective responses, whereas expression of either of them individually no significant NMDA-dependent responses in oocytes. The eletrophysiological profile of dNR1 and dNR2 coexpressed in Xenopus oocytes or Drosophila S2 cells reveals that the functional NMDARs produce most of these distinguishing properties specific to mammalian counterparts including selective activation by NMDA and L-asparate, modulation by glycine as the coagonist for glutamate, and voltage- and Mg2+-dependent conductance (Xia, 2005).
This study also demonstrates that NMDARs not only are involved acutely for associative learning but also are required for LTM consolidation. Genomic mutations of the essential dNR1 gene yield defects in a Pavlovian olfactory learning task, and these learning defects are fully rescued by two different genomic transgenes containing the dNR1+ coding sequence. Importantly, it was shown that Pavlovian learning is disrupted within 15 hr via transient induction in adults of a dNR1 antisense RNA transgene. Finally, the transient knockdown of dNR1 also specifically abolishes the consolidation of protein synthesis- and CREB-dependent LTM (Xia, 2005).
Naive Drosophila larvae show vigorous chemotaxis toward many odorants including ethyl acetate (EA). Chemotaxis toward EA is substantially reduced after a 5-min pre-exposure to the odorant and recovers with a half-time of ~20 min. An analogous behavioral decrement can be induced without odorant-receptor activation through channelrhodopsin-based, direct photoexcitation of odorant sensory neurons (OSNs). The neural mechanism of short-term habituation (STH) requires the (1) Rutabaga adenylate cyclase; (2) transmitter release from predominantly GABAergic local interneurons (LNs); (3) GABA-A receptor function in projection neurons (PNs) that receive excitatory inputs from OSNs; and (4) NMDA-receptor function in PNs. These features of STH cannot be explained by simple sensory adaptation and, instead, point to plasticity of olfactory synapses in the antennal lobe as the underlying mechanism. These observations suggest a model in which NMDAR-dependent depression of the OSN-PN synapse and/or NMDAR-dependent facilitation of inhibitory transmission from LNs to PNs contributes substantially to short-term habituation (Larkin, 2010).
Experience-induced plasticity of synapses is believed to be a fundamental mechanism of learning and memory. However, central synaptic changes that underlie memory have not been clearly defined, even for relatively simple nonassociative learning processes such as habituation (Larkin, 2010).
During habituation, unreinforced exposure to a repeated or prolonged stimulus results in a reversible decrease in response to that stimulus. Habituation probably serves as an important building block for more complex cognitive function. By allowing unchanging or irrelevant stimuli to be ignored, it allows cognitive resources to be focused on more salient stimuli (Larkin, 2010 and references therein).
The neural basis of short-term habituation (STH) is best studied in the marine snail, Aplysia californica. Here STH (lasting ~30 min) of the defensive gill-withdrawal reflex in response to tactile stimulation of the siphon is thought to arise from presynaptic depression of transmitter release at sensorimotor synapses. However, even here, presynaptic plasticity may not be cell-autonomous, potentially requiring, for instance, activity of yet-to-be-identified interneurons (Larkin, 2010).
Several forems of habituation have been described in Drosophila and are often shown to require the function of genes that regulate cAMP-dependent forms of associative memory. For instance, habituation of proboscis extension reflex as well as odor-evoked startle reflex in adult Drosophila requires rutabaga (rut)-encoded Ca2+/calmodulin-sensitive adenylyl cyclase. In addition, habituation of the ethanol-induced startle response requires the shaggy/GSK-3 signaling pathway. Despite such pioneering observations, the mechanisms of these various forms of habituation, even whether the primary neuronal changes are purely sensory or involve plasticity of central synapses (involving centrally located interneurons that may integrate various different kinds of modulatory, inhibitory, and excitatory inputs), remain poorly understood (Larkin, 2010).
Recent advances in understanding the circuitry that underlies Drosophila olfactory behavior, as well as the development of new tools to perturb identified neurons in vivo, has opened the opportunity for understanding mechanisms of olfactory habituation at the level of the underlying neural circuitry (Larkin, 2010).
In the larval olfactory system, 21 olfactory sensory neurons (OSNs), each expressing a single odorant receptor (together with the broadly expressed Or83b co-receptor), synapse, respectively, onto 21 cognate projection neurons (PNs) within 21 glomeruli in the larval antennal lobe (AL). Local, predominantly GABAergic interneurons (LNs) synapse widely within the antennal lobe, interlinking different glomeruli. Various neuromodulatory synapses also form on the larval antennal lobe and mushroom body. Thus, odorant-stimulated signals in sensory neurons are processed in the antennal lobe, modulated by motivational or emotional states, and relayed through projection neurons to higher brain centers (Larkin, 2010).
Previous work has shown that in Drosophila larvae, olfactory chemotaxis decreases after odorant pre-exposure. This study shows that this behavioral habituation, alternatively referred to as 'adaptation' by some previous investigators, arises from mechanisms of synaptic plasticity. This study demonstrates that odorant receptor activation is not necessary for olfactory habituation; however, local interneuron activity and projection neuron signaling is necessary. These observations suggest a model in which habituation occurs by a pathway in which NMDA receptors in projection neurons signal depression of OSN-PN synapses and/or facilitation of LN-PN synapses (Larkin, 2010).
Previous studies have not clearly discriminated between peripheral and central mechanisms. Indeed, the term 'adaptation,' better applied to sensory neuron changes such as receptor desensitization, has often been used interchangeably with the term 'habituation', which is usually restricted to behavioral changes arising from central synaptic mechanisms (Larkin, 2010). .
The form of larval olfactory STH characterized in this study displays at least some of the defining behavioral characteristics of habituation. First, there is a behavioral decrement in response to repeated or sustained application of a particular stimulus. Second, STH shows spontaneous recovery with time in the absence of the habituating stimulus. And third, STH is susceptible to dishabituation when habituated larvae are presented with of a strong or noxious stimulus. The property of dishabituation is particularly significant, as an important way of distinguishing between habituation and either fatigue or sensory adaptation. Dishabituation shows that the habituated animal retains the capability to respond and suggests that the attenuated behavioral response arises from some form of active suppression. Thus, the behavioral data suggest (1) that the term 'habituation' may be better used in place of 'adaptation,' while referring to the behavioral phenomenon that was studied; and (2) that STH probably arises from central synaptic mechanisms, rather than sensory neuron adaptation (Larkin, 2010).
Three main lines of data support the conclusion that STH arises from a central synaptic mechanism that resides in the antennal lobe, rather than from adaptation of olfactory receptor signaling in the OSN. First, behavioral decrements similar to STH can be induced by direct depolarization of OSNs, indicating that STH may potentially be induced by processes stimulated by activation action-potential firing in OSNs, independently of olfactory receptor activation. Second, and more striking, STH requires synaptic-vesicle exocytosis from local interneurons during the process of odorant exposure, when STH is being established. This requirement is incompatible with an exclusively sensory mechanism. Third, STH requires the function of NMDA receptors on postsynaptic projection neurons. This last observation also provides a particularly strong argument for a synaptic mechanism, indicating a need for plasticity of OSN and/or LN synapses made onto dendrites of projection neurons in the antennal lobe. Given that OSNs are excitatory and LNs are primarily inhibitory, it appears most likely that NMDAR functions in PNs to depress excitatory OSN-PN synapses and/or to potentiate inhibition by strengthening the LN-PN synapse. It is suggestd that the LN-PN mechanism may be involved because (1) LN transmission seems necessary for both induction and expression of habituation; and (2) the process of dishabituation could be attractively explained as arising from the inhibition of local inhibitory synapses through descending neuromodulation. A requirement for facilitation of the LN-PN synapse would be consistent with previous studies (Sachse, 2007) showing that adult-long-term olfactory habituation is associated with an increase in odor-evoked calcium fluxes in GABAergic processes within the Drosophila antennal lobe (Larkin, 2010).
Based both on experimental and theoretical arguments, a simple model is suggested for short-term olfactory habituation. Since this is a model, no claim is being made to to having ruled out additional major contributing mechanisms, It is suggested that during initial odorant pre-exposure, dendritic NMDA receptors on projection neurons detect and respond to membrane depolarization occurs coincident with transmitter release from LNs. Calcium entry through dendritic NMDA receptors may trigger a local retrograde signal required for facilitation of transmitter release from the LNs. Although existing data do not rule out functions for rutabaga in higher larval brain centers, it is suggested that either the generation of a retrograde signal in PN dendrites or the presynaptic response of LNs to this signal could be dependent on the rut adenylate cyclase. In habituated animals, facilitation of GABA release would reduce odor-evoked projection neuron outputs to higher brain centers, thereby reducing olfactory behavior. As NMDAR signaling would only occur at active glomeruli, this mechanism can account not only for the observed odor selectivity of habituation, but also the instances of cross-habituation (Larkin, 2010).
Such a model also naturally suggests a hypothesis for the mechanism of dishabituation: namely, that dishabituating stimuli cause release of neuromodulators that act to reduce GABA release from local inhibitory synapses (Larkin, 2010).
Given the remarkable similarities in the anatomical organization of insect and mammalian olfactory systems, a significant conservation of olfactory mechanisms would be expected. In rodents, at least two forms of habituation have been described, lasting 2-3 and 30-60 min, respectively: the latter equivalent in timescale to larval STH described in this study. Consistent with a similar underlying mechanism, the more persistent form of olfactory habituation can be blocked by an N-methyl-D-aspartate (NMDA) receptor antagonist in the olfactory bulb, a structure homologous to the insect antennal lobe. Thus, larval STH described in this study has some similarities to a previously characterized form of mammalian olfactory habituation. Analysis of the underlying mechanisms is therefore likely to provide directly transferable insights in mammalian olfaction. The data make the prediction that the activity of mammalian olfactory interneurons, either periglomerular or granule cells, is critical for the establishment and display of at least one timescale of olfactory habituation (Larkin, 2010).
In addition to providing some insight into mechanisms of olfactory habituation in mammals, it possible that circuit mechanisms of larval olfactory habituation are relevant to other forms of behavioral habituation. In at least three previous instances, increased inhibition has been associated with attenuated behavior. For example, habituation of an escape reflex mediated by the lateral giant fibers in the crayfish has been associated with enhanced GABAergic transmission onto giant fibers. Similarly, LTP of inhibitory synapses controlling excitability of the Mauthner cell has been associated with reduced escape behavior in goldfish. Furthermore, ethanol, a potentiator of GABA synapses, has been shown to enhance habituation of a motor pathway in the frog spinal cord. Could these different instances of habituation all involve circuit mechanisms similar to those used in Drosophila larval olfactory behavior (Larkin, 2010)?
In all brain regions, principal/projection neurons are subject to inhibitory feedback modulation and a pathway that has been appreciated as potentially essential for neuronal homeostasis. Potentiation of inhibitory feedback triggered by the pattern of principle cell activation would be predicted to preferentially dampen this particular output pattern. Thus, the circuit mechanism suggest in this study is theoretically generalizable to other and more complex forms of habituation. Further experiments will be required to determine the validity of this very testable hypothesis (Larkin, 2010).
The importance of habituation has been underlined by the fact that deficits in sensory gating and pre-pulse inhibition (PPI), processes with similarities to habituation, have been linked with various neurological problems, including autism and schizophrenia. Indeed, a circuit model for understanding schizophrenia has specifically proposed that altered negative feedback in the hippocampus may underlie both positive and negative symptoms of schizophrenia (Larkin, 2010).
In addition, defects in habituation or habituation-like processes have been described in Fragile X syndrome and migraines. It has also been shown to have important effects relating to learning disabilities, age-related changes in learning, and substance abuse. If mechanisms of olfactory habituation prove to be general, then studies of olfactory plasticity may prove relevant for other forms of cognition as well as for human neurological disease (Larkin, 2010).
The dNR1 gene is a large gene, containing 15 exons (Ultsch, 1993). Exon 1 (noncoding) undergoes alternative splicing, giving rise to two different transcripts, which contain the same coding sequence but which differ in the 5′ untranslated region. The putative dNR1 protein from these splice forms faithfully maintains all the major structural features of NR1 receptor. The protein contains one hydrophobic region at the amino terminus supposedly as the signal peptide, three hydrophobic transmembrane regions (TM1), a hydrophobic pore-forming segment in the carboxyl terminal half, and two ligand binding domains (S1–S2) with high homology to bacterial amino acid binding proteins. dNR1 also has a potential type II PDZ domain binding motif at its C terminus (X-Ψ-X-Ψ, where Ψ is a hydrophobic amino acid), suggesting interactions with other PDZ domain-containing proteins (Sheng, 2001). Most of the important amino acid residues for ligand binding are conserved in dNR1. A key asparagine residue (N631) is present in the TM2 domain and presumably controls the Ca2+ permeability and voltage-dependent Mg2+ blockade (Burnashev, 1992) (Xia, 2005).
dNR2, as confirmed by complete cloning, appears to be the only gene encoding the fly NR2 homolog, whereas there are four mammalian members in the NR2 subfamily (Dingledine, 1999; Yamakura, 1999). dNR2 undergoes alternative splicing, mostly at the 5′ untranslated region, generating eight different transcripts that may encode three different proteins. Full-length cDNAs have been isolated for all eight variants. Six of them contain the same coding sequence but differ from each other at the 5′ untranslated region, with five of them containing a separate noncoding exon 1. All three deduced NR2 proteins bear highest homology to NMR-2 in C. elegans, rat NR2D and NR2B, with respect to their overall sequence or their ligand binding and pore-forming transmembrane domains. Several anti-peptide monoclonal or polyclonal anti-dNR2 antibodies have been generated that specifically recognized two different bands on Westerns. Because two of the putative dNR2 peptides were predicted to have similar molecular weight, it is still unclear whether the two bands in fact contained all three protein variants (Xia, 2005).
The domain structures of NR2 receptors are largely conserved in dNR2, but its general sequence homology and the active physiological sites only moderately mimic its mammalian counterparts. The protein contains four hydrophobic regions (TM1–TM4) in the carboxyl terminal half that align perfectly with the three hydrophobic transmembrane regions and a hydrophobic pore-forming segment (TM2) in other ionotropic glutamate receptors (Dingledine, 1999). Like its rat counterpart, dNR2 has conserved major determinants of glutamate binding in the N-terminal ligand binding domain (S1) preceding transmembrane segment TM1 and the loop (S2) between TM3 and TM4 (Dingledine, 1999). The two asparagine residues, which are present in the TM2 domain of NMDA receptors and control the Ca2+ permeability and voltage-dependent Mg2+ blockade (Dingledine, 1999), however, are not conserved in dNR2. Finally, the type I PDZ binding motif (X-S/T-X-V) is not present in dNR2, whereas it is well conserved in all vertebrate NR2 homologs (Sheng, 2001). Thus, Drosophila NMDA receptors may physically interact with PDZ domain-containing proteins through dNR1 but not dNR2, which is usually the case in vertebrates (Xia, 2005).
date revised: 5 February 2007
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