Hippocampal NMDA receptors are important for behavioural inhibition but not for encoding associative spatial memories

The idea that an NMDA receptor (NMDAR)-dependent long-term potentiation-like process in the hippocampus is the neural substrate for associative spatial learning and memory has proved to be extremely popular and influential. However, we recently reported that mice lacking NMDARs in dentate gyrus and CA1 hippocampal subfields (GluN1^ΔDGCA1 mice) acquired the open field, spatial reference memory watermaze task as well as controls, a result that directly challenges this view. Here, we show that GluN1^ΔDGCA1 mice were not impaired during acquisition of a spatial discrimination watermaze task, during which mice had to choose between two visually identical beacons, based on extramaze spatial cues, when all trials started at locations equidistant between the two beacons. They were subsequently impaired on test trials starting from close to the decoy beacon, conducted post-acquisition. GluN1^ΔDGCA1 mice were also impaired during reversal of this spatial discrimination. Thus, contrary to the widely held belief, hippocampal NMDARs are not required for encoding associative, long-term spatial memories. Instead, hippocampal NMDARs, particularly in CA1, act as part of a comparator system to detect and resolve conflicts arising when two competing, behavioural response options are evoked concurrently, through activation of a behavioural inhibition system. These results have important implications for current theories of hippocampal function.

Hypertrophy and altered activity of the adrenal cortex in Homer 1 knockout mice

Homer 1 gene products are involved in synaptic transmission and plasticity, and hence, distinct behavioral abnormalities, including anxiety- and depression-like behaviors, have been observed in Homer 1 knockout (KO) mice. Here we report that Homer 1 KO mice additionally exhibit a pronounced endocrine phenotype, displaying a profoundly increased adrenal gland weight and increased adrenal/body weight ratio. Histological examinations of Homer 1 deficient adrenal glands revealed an increased size of the adrenal cortex, especially the sizes of the zona fasciculata and zona glomerulosa. Moreover, the plasma corticosterone and aldosterone were higher in Homer 1 KO than wild-type (WT) mice while the plasma ACTH levels were not different between the genotypes. The in vivo ACTH test revealed that corticosterone and aldosterone plasma levels were higher in saline injected Homer 1 KO mice than in WT mice (saline injected mice served as controls for the respective groups of ACTH-injected animals), but the magnitude of steroid responses to ACTH was similar in both genotypes. In contrast, an in vitro experiment performed on isolated cells of adrenal cortex clearly showed increased production of both steroids in response to ACTH in Homer 1 KO cells, which is in line with an ~8-fold increase in the expression of ACTH receptor mRNA in the adrenal cortex of these mutants. These results, together with the detection of Homer 1 mRNA and protein in the adrenal cortex of WT mice, indicate that Homer 1 directly affects the steroidogenic function of the adrenal glands.

Deletion of glutamate receptor-A (GluR-A) AMPA receptor subunits impairs one-trial spatial memory

Genetically modified mice lacking the glutamate receptor A (GluR-A) subunit of the AMPA receptor (GluR-A-/- mice) display normal spatial reference memory but impaired spatial working memory (SWM). This study tested whether the SWM impairment in these mice could be explained by a greater sensitivity to within-session proactive interference. The SWM performance of GluR-A-/- and wild-type mice was assessed during nonmatching-to-place testing under conditions in which potential proactive interference from previous trials was reduced or eliminated. SWM was impaired in GluR-A-/- mice, both during testing with pseudotrial-unique arm presentations on the radial maze and when conducting each trial on a different 3-arm maze, each in a novel testing room. Experimentally naive GluR-A-/- mice also exhibited chance performance during a single trial of spontaneous alternation. This 1-trial spatial memory deficit was present irrespective of the delay between the sample information and the response choice (0 or 45 min) and the length of the sample phase (0.5 or 5 min). These results imply that the SWM deficit in GluR-A-/- mice is not due to increased susceptibility to proactive interference.

Impaired spatial working memory but spared spatial reference memory following functional loss of NMDA receptors in the dentate gyrus

Novel spatially restricted genetic manipulations can be used to assess contributions made by synaptic plasticity to learning and memory, not just selectively within the hippocampus, but even within specific hippocampal subfields. Here we generated genetically modified mice (NR1^ΔDG mice) exhibiting complete loss of the NR1 subunit of the N-methyl-d-aspartate receptor specifically in the granule cells of the dentate gyrus. There was no evidence of any reduction in NR1 subunit levels in any of the other hippocampal subfields, or elsewhere in the brain. NR1^ΔDG mice displayed severely impaired long-term potentiation (LTP) in both medial and lateral perforant path inputs to the dentate gyrus, whereas LTP was unchanged in CA3-to-CA1 cell synapses in hippocampal slices. Behavioural assessment of NR1^ΔDG mice revealed a spatial working memory impairment on a three-from-six radial arm maze task despite normal hippocampus-dependent spatial reference memory acquisition and performance of the same task. This behavioural phenotype resembles that of NR1^ΔCA3 mice but differs from that of NR1^ΔCA1 mice which do show a spatial reference memory deficit, consistent with the idea of subfield-specific contributions to hippocampal information processing. Furthermore, this pattern of selective functional loss and sparing is the same as previously observed with the global GluR-A l-α-amino-3-hydroxy-5-methyl-4-isoxazelopropionate receptor subunit knockout, a mutation which blocks the expression of hippocampal LTP. The present results show that dissociations between spatial working memory and spatial reference memory can be induced by disrupting synaptic plasticity specifically and exclusively within the dentate gyrus subfield of the hippocampal formation.

Complex, multimodal behavioral profile of the Homer1 knockout mouse

Proteins of the Homer1 immediate early gene family have been associated with synaptogenesis and synaptic plasticity suggesting broad behavioral consequences of loss of function. This study examined the behavior of male Homer1 knockout (KO) mice compared with wild-type (WT) and heterozygous mice using a battery of 10 behavioral tests probing sensory, motor, social, emotional and learning/memory functions. KO mice showed mild somatic growth retardation, poor motor coordination, enhanced sensory reactivity and learning deficits. Heterozygous mice showed increased aggression in social interactions with conspecifics. The distribution of mGluR5 and N-methyl-D-aspartate receptors (NMDA) receptors appeared to be unaltered in the hippocampus (HIP) of Homer1 KO mice. The results indicate an extensive range of disrupted behaviors that should contribute to the understanding of the Homer1 gene in brain development and behavior.

Behavioral and neurochemical phenotyping of Homer1 mutant mice: possible relevance to schizophrenia

Homer proteins are involved in the functional assembly of postsynaptic density proteins at glutamatergic synapses and are implicated in learning, memory and drug addiction. Here, we report that Homer1-knockout (Homer1-KO) mice exhibit behavioral and neurochemical abnormalities that are consistent with the animal models of schizophrenia. Relative to wild-type mice, Homer1-KO mice exhibited deficits in radial arm maze performance, impaired prepulse inhibition, enhanced 'behavioral despair', increased anxiety in a novel objects test, enhanced reactivity to novel environments, decreased instrumental responding for sucrose and enhanced MK-801- and methamphetamine-stimulated motor behavior. No-net-flux in vivo microdialysis revealed a decrease in extracellular glutamate content in the nucleus accumbens and an increase in the prefrontal cortex. Moreover, in Homer1-KO mice, cocaine did not stimulate a rise in frontal cortex extracellular glutamate levels, suggesting hypofrontality. These behavioral and neurochemical data derived from Homer1 mutant mice are consistent with the recent association of schizophrenia with a single-nucleotide polymorphism in the Homer1 gene and suggest that the regulation of extracellular levels of glutamate within limbo-corticostriatal structures by Homer1 gene products may be involved in the pathogenesis of this neuropsychiatric disorder.

SVC: structured visualization of evolutionary sequence conservation

We have developed a web application for the detailed analysis and visualization of evolutionary sequence conservation in complex vertebrate genes. Given a pair of orthologous genes, the protein-coding sequences are aligned. When these sequences are mapped back onto their encoding exons in the genomes, a scaffold of the conserved gene structure naturally emerges. Sequence similarity between exons and introns is analysed and embedded into the gene structure scaffold. The visualization on the SVC server provides detailed information about evolutionarily conserved features of these genes. It further allows concise representation of complex splice patterns in the context of evolutionary conservation. A particular application of our tool arises from the fact that around mRNA editing sites both exonic and intronic sequences are highly conserved. This aids in delineation of these sites. SVC is available at .

Restoration of spatial working memory by genetic rescue of GluR-A–deficient mice

Gene-targeted mice lacking the AMPA receptor subunit GluR-A (also called GluR1 encoded by the gene Gria1,) have deficits in hippocampal CA3-CA1 long-term potentiation (LTP) and have profoundly impaired hippocampus-dependent spatial working memory (SWM) tasks, although their spatial reference memory remains normal. Here we show that forebrain-localized expression of GFP-tagged GluR-A subunits in GluR-A-deficient mice rescues SWM, paralleling its rescue of CA3-CA1 LTP. This provides powerful new evidence linking hippocampal GluR-A-dependent synaptic plasticity to rapid, flexible memory processing.

GluR-A-dependent synaptic plasticity is required for the temporal encoding of nonspatial information

Four related experiments studied operant performance of mice on differential reinforcement of low rates of responding (DRL) paradigms. Experiment 1 showed that excitotoxic hippocampal lesions impaired performance of a 10-s DRL schedule (DRL-10). Experiments 2 and 3 showed that GluR-A AMPA receptor subunit knockout mice, which are deficient in CA3-CA1 long-term potentiation (LTP), were markedly impaired at 15 s (DRL-15), but less impaired at DRL-10. Experiment 4 compared DRL-15 performance in mice from the 2 strains from which the GluR-A colony was derived and showed that they did not differ. The results show that GluR-A-containing AMPA receptors are required for normal performance on hippocampus-dependent, nonspatial working memory tasks, consistent with a role for GluR-A in the temporal encoding (what happened when) of nonspatial information.

Spatial reference memory in GluR-A-deficient mice using a novel hippocampal-dependent paddling pool escape task

Genetically modified mice lacking the L-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptor subunit, GluR-A (GluR1), and deficient in hippocampal CA3-CA1 long-term potentiation (LTP), were assessed on a novel, hippocampal-dependent spatial reference memory, paddling pool escape task. The mice were required to use the extramaze cues around the laboratory to find a hidden escape tube that was in a constant location at one of 12 possible positions around the perimeter of the paddling pool, in order to escape from shallow water. The knockout mice performed well on this task. They displayed a small initial impairment (in terms of both escape latencies and choice errors), but they were soon as efficient as the wild-type mice in escaping from the water. This was further demonstrated by performance during a 20-s probe trial in which the exit tube was blocked. Both groups of mice spent most of the time searching in the quadrant of the pool in which the exit tube had previously been located. In a subsequent experiment, entirely normal spatial acquisition was observed in the knockout mice when the paddling pool was moved to a novel spatial environment. The GluR-A -/- mice were also unimpaired in a further reversal phase in which the correct exit location was moved by 180 degrees around the perimeter wall. These results are consistent with previous watermaze studies, providing further demonstration of intact hippocampus-dependent spatial reference memory in GluR-A knockout mice. They contrast strikingly with the profound deficits in hippocampus-dependent, short-term, flexible spatial working memory observed in these knockout mice. This study also demonstrates a novel behavioral task for assessing spatial memory in genetically modified mice. This task shares the behavioral profile of the well-established watermaze paradigm, but may have advantages for the study of genetically modified mice.

A Comparison of GluR-A-Deficient and Wild-Type Mice on a Test Battery Assessing Sensorimotor, Affective, and Cognitive Behaviors

Previous studies have demonstrated a spatial working memory deficit in glutamate receptor (GluR)-A (GluR1) AMPA receptor subunit knockout mice. The present study evaluated male and female wild-type and GluR-A-/- mice on a test battery that assessed sensorimotor, affective, and cognitive behaviors. Results revealed a behavioral phenotype more extensive than previously described. GluR-A-/- mice were hyperactive, displayed a subtle lack of motor coordination, and were generally more anxious than wild-type controls. In addition, they showed a deficit in spontaneous alternation, consistent with previous reports of a role for GluR-A-dependent plasticity in hippocampus-dependent, spatial working memory. Although changes in motor coordination or anxiety cannot explain the dissociations already reported within the spatial memory domain, it is clear that they could significantly affect interpretation of results obtained in other kinds of behavioral tasks.

GluR-A-Deficient mice display normal acquisition of a hippocampus-dependent spatial reference memory task but are impaired during spatial reversal

Acquisition and reversal of a spatial discrimination were assessed in an appetitive, elevated plus-maze task in 4 groups of mice: knockout mice lacking the AMPA receptor subunit GluR-A (GluR1), wild-type controls, mice with cytotoxic hippocampal lesions, and controls that had undergone sham surgery. In agreement with previous studies using tasks such as the water maze, GluR-A(-/-) mice were unimpaired during acquisition of the spatial discrimination task, whereas performance in the hippocampalgroup remained at chance levels. In contrast to their performance during acquisition, the GluR-A(-/-) mice displayed a mild deficit during reversal of the spatial discrimination and were profoundly impaired during discrete trial, rewarded-alternation testing on the elevated T maze. The latter result suggests a short-term, flexible spatial working memory impairment in GluR-A(-/-) mice, which might also underlie their mild deficit during spatial reversal.

Spatial memory dissociations in mice lacking GluR1

Gene-targeted mice lacking the AMPA receptor subunit GluR1 (GluR-A) have deficits in hippocampal CA3-CA1 long-term potentiation. We now report that they showed normal spatial reference learning and memory, both on the hidden platform watermaze task and on an appetitively motivated Y-maze task. In contrast, they showed a specific spatial working memory impairment during tests of non-matching to place on both the Y-maze and an elevated T-maze. In addition, successful watermaze and Y-maze reference memory performance depended on hippocampal function in both wild-type and mutant mice; bilateral hippocampal lesions profoundly impaired performance on both tasks, to a similar extent in both groups. These results suggest that different forms of hippocampus-dependent spatial memory involve different aspects of neural processing within the hippocampus.

The Adhesion Molecule on Glia (AMOG) Is Widely Expressed by Astrocytes in Developing and Adult Mouse Brain

Adhesion molecule on glia (AMOG) is a 45 - 50 kD cell surface glycoprotein structurally similar to the Na, K-ATPase beta-subunit and associated with the catalytic subunit of this enzyme. Previous immunofluorescence results had suggested that AMOG is transiently expressed on Bergmann glia during mouse cerebellar development, and antibody-inhibition results have implicated it in the migration of granule neurons. We report that, while AMOG mRNA is detected in Bergmann glia during the migratory period, this astrocyte derivative continues to express AMOG mRNA at similar levels in adult mice suggesting a functional role for AMOG in adulthood. Evidence from RNA and protein blot analyses that AMOG is present before birth, increasing about ten fold in adult mouse brain and cerebellum is also provided. RNA blot analysis of astrocyte-enriched cell populations and in situ hybridization results show that astrocytes synthesize AMOG mRNA in all regions of the developing and adult brain. In the adult, AMOG mRNA is more abundant in grey than white matter and, among grey matter regions, highest in cerebellar cortex. These results indicate a relationship between density of neuronal elements and AMOG expression. It is further speculated that AMOG is part of a Na,K-ATPase complex expressed preferentially by astrocytes in mouse brain.

Codon-improved Cre recombinase (iCre) expression in the mouse

By applying the mammalian codon usage to Cre recombinase, we improved Cre expression, as determined by immunoblot and functional analysis, in three different mammalian cell lines. The improved Cre (iCre) gene was also designed to reduce the high CpG content of the prokaryotic coding sequence, thereby reducing the chances of epigenetic silencing in mammals. Transgenic iCre expressing mice were obtained with good frequency, and in these mice loxP-mediated DNA recombination was observed in all cells expressing iCre. Moreover, iCre fused to two estrogen receptor hormone binding domains for temporal control of Cre activity could also be expressed in transgenic mice. However, Cre induction after administration of tamoxifen yielded only low Cre activity. Thus, whereas efficient activation of Cre fusion proteins in the brain needs further improvements, our studies indicate that iCre should facilitate genetic experiments in the mouse.

Impaired NMDA receptor function in mouse olfactory bulb neurons by tetracycline-sensitive NR1 (N598R) expression

High Ca(2+) permeability and its control by voltage-dependent Mg(2+) block are defining features of NMDA receptors. These features are lost if the principal NR1 subunit carries an asparagine (N) to arginine (R) substitution in a critical channel site at NR1 position 598. NR1(R) expression from a single allele in gene-targeted NR1(+/R) mice is lethal soon after birth, precluding analysis of altered synaptic functions later in life. We therefore employed the forebrain specific alphaCaMKII promoter to drive tTA-mediated tetracycline sensitive transcription of transgenes for NR1(R) and for lacZ as reporter. Transgene expression was observed in cortex, striatum, hippocampus, amygdala and olfactory bulb and was mosaic in all these forebrain regions. It was highest in olfactory bulb granule cells, in most of which Ca(2+) permeability and voltage-dependent Mg(2+) block of NMDA receptors were reduced to different extents. This indicates significant impairment of NMDA receptor function by NR1(R) in presence of the wild-type NR1 complement. Indeed, even though NR1(R) mRNA constituted only 18% of the entire NR1 mRNA population in forebrain, the transgenic mice died during adolescence unless transgene expression was suppressed by doxycycline. Thus, glutamate receptor function can be altered in the mouse by regulated NR1(R) transgene expression.

Dominance of the lurcher mutation in heteromeric kainate and AMPA receptor channels

Homomeric glutamate receptor (GluR) channels become spontaneously active when the last alanine residue within the invariant SYTANLAAF-motif in the third membrane segment is substituted by threonine. The same mutation in the orphan GluRdelta2 channel is responsible for neurodegeneration in "Lurcher" (Lc) mice. Since most native GluRs are composed of different subunits, we investigated the effect of an Lc-mutated subunit in heteromeric kainate and AMPA receptors expressed in HEK293 cells. Kainate receptor KA2 subunits, either wild type or carrying the Lc mutation (KA2(Lc)), are retained inside the cell but are surface-expressed when assembled with GluR6 subunits. Importantly, KA2(Lc) dominates the gating of KA2(Lc)/GluR6(WT) channels, as revealed by spontaneous activation and by slowed desensitization and deactivation kinetics of ligand-activated whole-cell currents. Moreover, the AMPA receptor subunit GluR-B(Lc)(Q) which forms spontaneously active homomeric channels with rectifying current-voltage relationships, dominates the gating of heteromeric GluR-B(Lc)(Q)/GluR-A(R) channels. The spontaneous currents of these heteromeric AMPAR channels show linear current-voltage relationships, and the ligand-activated whole-cell currents display slower deactivation and desensitization kinetics than the respective wild-type channels. For heteromeric Lc-mutated kainate and AMPA receptors, the effects on kinetics were reduced relative to the homomeric Lc-mutated forms. Thus, an Lc-mutated subunit can potentially influence heteromeric channel function in vivo, and the severity of the phenotype will critically depend on the levels of homomeric GluR(Lc) and heteromeric GluR(Lc)/GluR(WT) channels.

Genetic manipulation of key determinants of ion flow in glutamate receptor channels in the mouse

Glutamate receptor channels are built around an ancient pore loop structure which defines the inner channel environment and which is connected to structures for channel gating. This pore loop, which corresponds to the M2 region of the receptor subunits, enters the lipid bilayer from the intracellular side in an alpha-helical configuration, then kinks to form a random coil and exits the lipid bilayer at the intracellular side. The narrow constriction of the channel is formed by amino acid residues that occupy a position shortly after the end of the alpha-helical part of M2. These residues determine ion selectivity and conductance properties of the glutamate-gated channel. The critical residues are asparagines for NMDA receptor subunits and glutamine or arginine for AMPA and kainate receptor subunits. Presence of arginine in the critical channel position of AMPA and kainate receptors is controlled by site-selective RNA editing. To study the importance of these critical channel residues in the mouse, we introduced codon changes in the endogenous genes for NMDA and AMPA receptor subunits. Our results show that changes in the critical channel position are not tolerated, but lead to early death. Therefore, the impact on adult synaptic function and plasticity by glutamate receptor channels with changed ion selectivity and conductance needs to be addressed by conditional expression of the mutant receptors.

Conditional restoration of hippocampal synaptic potentiation in Glur-A-deficient mice

Plasticity of mature hippocampal CA1 synapses is dependent on l-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptors containing the glutamate receptor A (GluR-A) subunit. In GluR-A-deficient mice, plasticity could be restored by controlled expression of green fluorescent protein (GFP)-tagged GluR-A, which contributes to channel formation and displayed the developmental redistribution of AMPA receptors in CA1 pyramidal neurons. Long-term potentiation (LTP) induced by pairing or tetanic stimulation was rescued in adult GluR-A(-/-) mice when (GFP)GluR-A expression was constitutive or induced in already fully developed pyramidal cells. This shows that GluR-A-independent forms of synaptic plasticity can mediate the establishment of mature hippocampal circuits that are prebuilt to express GluR-A-dependent LTP.

Morphine-induced dependence and sensitization are altered in mice deficient in AMPA-type glutamate receptor-A subunits

AMPA-type glutamate receptors have been suggested to be involved in the neurobiological mechanisms of drug addiction. We have made use of two mouse lines, which both have modulated AMPA receptor responses. The first line is entirely deficient in glutamate receptor-A (GluR-A) subunits (A-/- knock-out line) and, in the second one, the Q582 residue of GluR-A subunits is replaced by an arginine residue (R/R mutants), which reduces the calcium permeability and channel conductance of the receptors containing this mutated subunit. Mice of both lines are healthy, but they show slightly increased locomotor activity. Acute morphine administration enhanced locomotor activity of the GluR-A-/- and GluR-A(R/R) mice, at least as much as that of their wild-type littermates. Only in the GluR-A-/- mice did we observe reduced tolerance development in tail-flick antinociception and less severe naloxone-precipitated withdrawal symptoms after treatment with increasing morphine doses, without differences in plasma and brain morphine levels when compared with wild type. Repeated daily morphine administration sensitized the locomotor activity responses in the GluR-A-/- and GluR-A(R/R) mice only when given in the measuring cages, whereas the wild-type mice showed slightly increased responses also when the repeated treatment was given in their home cages. Normal or even enhanced context-dependent sensitization was observed also with repeated amphetamine administration in the GluR-A subunit-deficient mice. The results indicate that AMPA receptors are involved in the acute and chronic effects of morphine, including context-independent sensitization, and that the GluR-A subunit itself is important for morphine tolerance and dependence.

Channel-lining residues of the AMPA receptor M2 segment: structural environment of the Q/R site and identification of the selectivity filter

In AMPA receptor channels, a single amino acid residue (Q/R site) of the M2 segment controls permeation of calcium ions, single-channel conductance, blockade by intracellular polyamines, and permeation of anions. The structural environment of the Q/R site and its positioning with regard to a narrow constriction were probed with the accessibility of substituted cysteines to positively and negatively charged methanethiosulfonate reagents, applied from the extracellular and cytoplasmic sides of the channel. The accessibility patterns confirm that the M2 segment forms a pore loop with the Q/R site positioned at the tip of the loop (position 0) facing the extracellular vestibule. Cytoplasmically accessible residues on the N- and C-terminal sides of position 0 form the ascending alpha-helical (-8 to -1) and descending random coil (+1 to +6) components of the loop, respectively. Substitution of a glycine residue at position +2 with alanine strongly decreased the permeability of organic cations, indicating that position +2 contributes to the narrow constriction. The anionic 2-sulfonatoethyl-methanethiosufonate reacted with a cysteine at position 0 only from the external side and with cysteines at positions +1 to +4 only from the cytoplasmic side. These results suggest that charge selectivity occurs external to the constriction (+2) and possibly involves interactions of ions with the negative electrostatic potential created by the dipole of the alpha-helix formed by the ascending limb of the loop.

A GFP-equipped bidirectional expression module well suited for monitoring tetracycline-regulated gene expression in mouse

Doxycycline (Dox)-sensitive co-regulation of two transcriptionally coupled transgenes was investigated in the mouse. For this, we generated four independent mouse lines carrying coding regions for green fluorescent protein (GFP) and beta-galactosidase in a bicistronic, bidirectional module. In all four lines the expression module was silent but was activated when transcription factor tTA was provided by the alpha-CaMKII-tTA transgene. In vivo analysis of GFP fluorescence, beta-galactosidase and immunochemical stainings revealed differences in GFP and beta-galactosidase levels between the lines, but comparable patterns of expression. Strong signals were found in neurons of the olfactory system, neocortical, limbic lobe and basal ganglia structures. Weaker expression was limited to thalamic, pontine and medullary structures, the spinal cord, the eye and to some Purkinje cells in the cerebellum. Strong GFP signals were always accompanied by intense beta-galactosidase activity, both of which could be co-regulated by Dox. We conclude that the tTA-sensitive bidirectional expression module is well suited to express genes of interest in a regulated manner and that GFP can be used to track transcriptional activity of the module in the living mouse.

Using reporter genes to label selected neuronal populations in transgenic mice for gene promoter, anatomical, and physiological studies

This review summarizes recent work on the use of reporter genes to label selected neuronal populations in transgenic mice, with particular emphasis on gonadotropin-releasing hormone (GnRH) neurons. Reporter genes discussed are the lacZ, green fluorescent protein (GFP), luc, and bla genes, which encode the reporter proteins beta-galactosidase, GFP, luciferase, and beta-lactamase, respectively. Targeted transgenic expression of these reporter proteins is obtained by fusing the corresponding reporter gene, with or without a subcellular localization signal, to a cell type- or brain region-specific gene promoter. Mice carrying GnRH promoter-driven reporter genes have proven useful for revealing the promoter elements required for cell type-specific expression of GnRH, the full anatomical profile of the GnRH neuronal network, and its electrophysiological activity, suggesting that similar approaches will assist in elucidating the properties of other neuronal populations as well.

Respiration and parturition affected by conditional overexpression of the Ca2+-activated K+ channel subunit, SK3

In excitable cells, small-conductance Ca2+-activated potassium channels (SK channels) are responsible for the slow after-hyperpolarization that often follows an action potential. Three SK channel subunits have been molecularly characterized. The SK3 gene was targeted by homologous recombination for the insertion of a gene switch that permitted experimental regulation of SK3 expression while retaining normal SK3 promoter function. An absence of SK3 did not present overt phenotypic consequences. However, SK3 overexpression induced abnormal respiratory responses to hypoxia and compromised parturition. Both conditions were corrected by silencing the gene. The results implicate SK3 channels as potential therapeutic targets for disorders such as sleep apnea or sudden infant death syndrome and for regulating uterine contractions during labor.

Developmental profile of kainate receptor subunit KA1 revealed by Cre expression in YAC transgenic mice

To determine the spatio-temporal expression in brain of the high-affinity kainate receptor subunit KA1, we generated transgenic mice expressing Cre recombinase from the KA1 gene on a chromosomally integrated 550 kb yeast artificial chromosome (YAC). Activity of the KA1 gene promoter during brain development was visualized by Cre immunohistochemistry, and by X-gal staining of beta-galactosidase induced by Cre recombinase in double transgenic KA1-Cre/lacZ indicator mice. During early brain development, expression from the YAC-carried KA1-Cre transgene was observed in all major brain areas, predicting a function for KA1 in the developing central nervous system. In the adult brain, KA1-Cre transgene expression was restricted mainly to hippocampal CA3 pyramidal and dentate gyrus granule cells, an adult expression pattern characteristic for the endogenous KA1 alleles. KA1-Cre transgenic mice may help in elucidating the role of floxed genes ablated in vivo in KA1 expressing neurons.

The Lurcher mutation identifies delta 2 as an AMPA/kainate receptor-like channel that is potentiated by Ca(2+)

Neurodegeneration in Lurcher (Lc) mice results from constitutive activation of delta 2, a subunit of ionotropic glutamate receptors (GluRs) with unknown natural ligands and channel properties. Homo-oligomeric channels of GluR-delta2 with the Lurcher mutation (GluR-delta 2(Lc)) expressed in human embryonic kidney 293 cells showed a doubly rectifying current-voltage relation reminiscent of the block by intracellular polyamines in AMPA/kainate channels. Similarly, the fraction of the total current carried by Ca(2+) was approximately 2-3%, comparable with that found in Ca(2+)-permeable AMPA/kainate channels. Currents through GluR-delta 2(Lc) channels were also potentiated by extracellular Ca(2+) in a biphasic manner, with maximal potentiation occurring at physiological concentrations of Ca(2+). We examined the functional role of the Q/R site in GluR-delta 2(Lc) by replacing glutamine with arginine. Analogous to AMPA/kainate receptors, GluR-delta 2(Lc)(R) channels showed no voltage-dependent block by intracellular polyamines and were nominally impermeable to Ca(2+). The potentiation by Ca(2+), however, remained intact. Hence, GluR-delta 2(Lc) channels are functionally similar to the AMPA/kainate receptor channels, consistent with the high-sequence identity shared by these subunits within the channel-lining M2 and M3 segments. Furthermore, potentiation by Ca(2+) and a permeability to Ca(2+) comparable with that of AMPA/kainate receptors provide a possible cause for cell death in Lurcher mice and may contribute to cerebellar long-term depression under physiological conditions.

Point mutation in an AMPA receptor gene rescues lethality in mice deficient in the RNA-editing enzyme ADAR2

RNA editing by site-selective deamination of adenosine to inosine alters codons and splicing in nuclear transcripts, and therefore protein function. ADAR2 (refs 7, 8) is a candidate mammalian editing enzyme that is widely expressed in brain and other tissues, but its RNA substrates are unknown. Here we have studied ADAR2-mediated RNA editing by generating mice that are homozygous for a targeted functional null allele. Editing in ADAR2-/- mice was substantially reduced at most of 25 positions in diverse transcripts; the mutant mice became prone to seizures and died young. The impaired phenotype appeared to result entirely from a single underedited position, as it reverted to normal when both alleles for the underedited transcript were substituted with alleles encoding the edited version exonically. The critical position specifies an ion channel determinant, the Q/R site, in AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate) receptor GluR-B pre-messenger RNA. We conclude that this transcript is the physiologically most important substrate of ADAR2.

C-Terminal truncation of NR2A subunits impairs synaptic but not extrasynaptic localization of NMDA receptors

NMDA receptors interact via the extended intracellular C-terminal domain of the NR2 subunits with constituents of the postsynaptic density for purposes of retention, clustering, and functional regulation at central excitatory synapses. To examine the role of the C-terminal domain of NR2A in the synaptic localization and function of NR2A-containing NMDA receptors in hippocampal Schaffer collateral-CA1 pyramidal cell synapses, we analyzed mice which express NR2A only in its C-terminally truncated form. In CA1 cell somata, the levels, activation, and deactivation kinetics of extrasynaptic NMDA receptor channels were comparable in wild-type and mutant NR2A(Delta)(C/)(Delta)(C) mice. At CA1 cell synapses, however, the truncated receptors were less concentrated than their full-length counterparts, as indicated by immunodetection in cultured neurons, synaptosomes, and postsynaptic densities. In the mutant, the NMDA component of evoked EPSCs was reduced in a developmentally progressing manner and was even more reduced in miniature EPSCs (mEPSCs) elicited by spontaneous glutamate release. Moreover, pharmacologically isolated NMDA currents evoked by synaptic stimulation had longer latencies and displayed slower rise and decay times, even in the presence of an NR2B-specific antagonist. These data strongly suggest that the C-terminal domain of NR2A subunits is important for the precise synaptic arrangement of NMDA receptors.

Dysfunctions in mice by NMDA receptor point mutations NR1(N598Q) and NR1(N598R)

NMDA receptors in mice were mutated by gene targeting to substitute asparagine (N) in position 598 of the NR1 subunit to glutamine (Q) or arginine (R). Animals expressing exclusively the mutated NR1 alleles, NR1(Q/Q) and NR1(-/R) mice, developed a perinatally lethal phenotype mainly characterized by respiratory failure. The dysfunctions were partially rescued in heterozygous mice by the presence of pure wild-type receptors. Thus, NR1(+/Q) mice exhibited reduced life expectancy, with females being impaired in nurturing; NR1(+/R) mice displayed signs of underdevelopment such as growth retardation and impaired righting reflex, and died before weaning. We analyzed the key properties of NMDA receptors, high Ca(2+) permeability, and voltage-dependent Mg(2+) block, in the mutant mice. Comparison of the complex physiological and phenotypical changes observed in the different mutants indicates that properties controlled by NR1 subunit residue N598 are important for autonomic brain functions at birth and during postnatal development. We conclude that disturbed NMDA receptor signaling mediates a variety of neurological phenotypes.

Importance of AMPA receptors for hippocampal synaptic plasticity but not for spatial learning

Gene-targeted mice lacking the L-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptor subunit GluR-A exhibited normal development, life expectancy, and fine structure of neuronal dendrites and synapses. In hippocampal CA1 pyramidal neurons, GluR-A-/- mice showed a reduction in functional AMPA receptors, with the remaining receptors preferentially targeted to synapses. Thus, the CA1 soma-patch currents were strongly reduced, but glutamatergic synaptic currents were unaltered; and evoked dendritic and spinous Ca2+ transients, Ca2+-dependent gene activation, and hippocampal field potentials were as in the wild type. In adult GluR-A-/- mice, associative long-term potentiation (LTP) was absent in CA3 to CA1 synapses, but spatial learning in the water maze was not impaired. The results suggest that CA1 hippocampal LTP is controlled by the number or subunit composition of AMPA receptors and show a dichotomy between LTP in CA1 and acquisition of spatial memory.

Studies on conditional gene expression in the brain

This manuscript summarizes our recent attempts to regulate in vitro and in vivo the expression of genes encoding components and regulators of the postsynaptic machinery along with marker genes such as lacZ and GFP. In particular, we studied tTA-dependent regulation and utilized Cre in combination with reversible silencing by intron engineering of dominant negative alleles. We further present a "knockin" approach for on-site artificial regulation of chromosomal genes.

GABA- and glutamate-activated channels in green fluorescent protein-tagged gonadotropin-releasing hormone neurons in transgenic mice

Mice were generated expressing green fluorescent protein (GFP) under the control of the gonadotropin-releasing hormone (GnRH) promoter. Green fluorescence was observed in, and restricted to, GnRH-immunopositive neuronal somata in the olfactory bulb, ganglion terminale, septal nuclei, diagonal band of Broca (DBB), preoptic area (POA), and caudal hypothalamus, as well as GnRH neuronal dendrites and axons, including axon terminals in the median eminence and organum vasculosum of the lamina terminalis (OVLT). Whole-cell recordings from GFP-expressing GnRH neurons in the OVLT-POA-DBB region revealed a firing pattern among GFP-expressing GnRH neurons distinct from that of nonfluorescent neurons. Nucleated patches of GFP-expressing GnRH neurons exhibited pronounced responses to fast application of GABA and smaller responses to L-glutamate and AMPA. One-fifth of the nucleated patches responded to NMDA. The GABA-A, AMPA, and NMDA receptor channels on GnRH neurons mediating these responses may play a role in the modulation of GnRH secretory oscillations.

EphrinB ligands recruit GRIP family PDZ adaptor proteins into raft membrane microdomains

Transmembrane ephrinB proteins have important functions during embryonic patterning as ligands for Eph receptor tyrosine kinases and presumably as signal-transducing receptor-like molecules. Consistent with "reverse" signaling, ephrinB1 is localized in sphingo-lipid/cholesterol-enriched raft microdomains, platforms for the localized concentration and activation of signaling molecules. Glutamate receptor-interacting protein (GRIP) and a highly related protein, which we have termed GRIP2, are recruited into these rafts through association with the C-terminal PDZ target site of ephrinB1. Stimulation of ephrinB1 with soluble EphB2 receptor ectodomain causes the formation of large raft patches that also contain GRIP proteins. Moreover, a GRIP-associated serine/threonine kinase activity is recruited into ephrinB1-GRIP complexes. Our findings suggest that GRIP proteins provide a scaffold for the assembly of a multiprotein signaling complex downstream of ephrinB ligands.

NMDAR channel segments forming the extracellular vestibule inferred from the accessibility of substituted cysteines

In NMDA receptor channels, the M2 loop forms the narrow constriction and the cytoplasmic vestibule. The identity of an extracellular vestibule leading toward the constriction remained unresolved. Using the substituted cysteine accessibility method (SCAM), we identified channel-lining residues of the NR1 subunit in the region preceding M1 (preM1), the C-terminal part of M3 (M3C), and the N-terminal part of M4 (M4N). These residues are located on the extracellular side of the constriction and, with one exception, are exposed to the pore independently of channel activation, suggesting that the gate is at the constriction or further cytoplasmic to it. Permeation of Ca2+ ions was decreased by mutations in M3C and M4N, but not by mutations in preM1, suggesting a functionally distinct contribution of the segments to the extracellular vestibule of the NMDA receptor channel.

Neurological dysfunctions in mice expressing different levels of the Q/R site-unedited AMPAR subunit GluR-B

We generated mouse mutants with targeted AMPA receptor (AMPAR) GluR-B subunit alleles, functionally expressed at different levels and deficient in Q/R-site editing. All mutant lines had increased AMPAR calcium permeabilities in pyramidal neurons, and one showed elevated macroscopic conductances of these channels. The AMPAR-mediated calcium influx induced NMDA-receptor-independent long-term potentiation (LTP) in hippocampal pyramidal cell connections. Calcium-triggered neuronal death was not observed, but mutants had mild to severe neurological dysfunctions, including epilepsy and deficits in dendritic architecture. The seizure-prone phenotype correlated with an increase in the macroscopic conductance, as independently revealed by the effect of a transgene for a Q/R-site-altered GluR-B subunit. Thus, changes in GluR-B gene expression and Q/R site editing can affect critical architectural and functional aspects of excitatory principal neurons.

The AMPA receptor subunit GluR-B in its Q/R site-unedited form is not essential for brain development and function

Calcium permeability of L-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptors (AMPARs) in excitatory neurons of the mammalian brain is prevented by coassembly of the GluR-B subunit, which carries an arginine (R) residue at a critical site of the channel pore. The codon for this arginine is created by site-selective adenosine deamination of an exonic glutamine (Q) codon at the pre-mRNA level. Thus, central neurons can potentially control the calcium permeability of AMPARs by the level of GluR-B gene expression as well as by the extent of Q/R-site editing, which in postnatal brain, positions the R codon into >99% of GluR-B mRNA. To study whether the small amount of unedited GluR-B is of functional relevance, we have generated mice carrying GluR-B alleles with an exonic arginine codon. We report that these mutants manifest no obvious deficiencies, indicating that AMPAR-mediated calcium influx into central neurons can be solely regulated by the levels of Q/R site-edited GluR-B relative to other AMPAR subunits. Notably, a targeted GluR-B gene mutant with 30% reduced GluR-B levels had 2-fold higher AMPAR-mediated calcium permeability in hippocampal pyramidal cells with no sign of cytotoxicity. This constitutes proof in vivo that elevated calcium influx through AMPARs need not generate pathophysiological consequences.

Candidate editases for GluR channels in single neurons of rat hippocampus and cerebellum

RNA editing by site selective adenosine deamination changes codons in several nuclear transcripts in the mammalian brain and affects critical properties of the encoded proteins, as exemplified by the calcium permeability of AMPA receptor channels. The recently cloned RNA dependent adenosine deaminases ADAR1, ADAR2 and ADAR3 form a small family of sequence-related candidate editases which are expressed in brain and other tissues at distinct levels and patterns. We have employed single-cell polymerase chain reaction of hippocampal CA1 and CA3 pyramidal neurons and cerebellar Purkinje and Bergmann glial cells in an attempt to evaluate the expression of these enzymes at a cellular level. We found ADAR2 expressed in all cells analyzed; approximately 50% of the cells co-expressed ADAR1 or ADAR3. The differential ADAR expression revealed by our study might underlie the distinct editing efficiencies and selectivities in different GluR subunit transcripts.

Differentiation of glycine antagonist sites of N-methyl-D-aspartate receptor subtypes. Preferential interaction of CGP 61594 with NR1/2B receptors

The binding site for the co-agonist glycine on N-methyl-D-aspartate (NMDA) receptors has been mapped to the NR1 subunit whereas binding of the principal agonist glutamate is mediated by the NR2 subunits. Using the novel glycine site antagonist and photoaffinity label CGP 61594, distinct contributions of the NR2 subunit variants to the glycine antagonist binding domains of NMDA receptor subtypes are demonstrated. High affinity sites for CGP 61594 were exclusively displayed by NR1/2B receptors, as shown by their co-distribution with the NR2B subunit, by subunit-selective immunoprecipitation and by functional analysis of NR1/2B receptors expressed in Xenopus oocytes (inhibitory potency, IC50 = 45 +/- 11 nM). Other NMDA receptor subtypes are clearly distinguished by reduced inhibitory potencies for CGP 61594, being low for NR1/2A and NR1/2D receptors (IC50 = 430 +/- 105 nM and 340 +/- 61 nM, respectively) and intermediate for NR1/2C receptors (IC50 = 164 +/- 27 nM). Glycine antagonist sites with low and intermediate affinity for [3H]CGP 61594 were detected also in situ by radioligand binding in brain areas predominantly expressing the NR2A and NR2C subunits, respectively. Thus, [3H]CGP 61594 is the first antagonist radioligand that reliably distinguishes the glycine site of NMDA receptor subtypes. [3H]CGP 61594 is a promising tool to identify the NR2 subunit domains that contribute to differential glycine antagonist sites of NMDA receptor subtypes.

RNA editing of brain glutamate receptor channels: mechanism and physiology

Glutamate-gated cation selective channels mediate fast excitatory neurotransmission in the mammalian brain. Functionally critical channel positions contain amino acid residues not predicted from the exonic sequence for the channel subunits. The codons for these residues are created in the respective primary gene transcripts by the site selective deamination of adenosine to inosine. This type of RNA editing requires a short double-stranded RNA structure formed by the exonic sequence around the adenosine targeted for deamination with a complementary sequence in the downstream intron and hence, it precedes splicing. Candidate enzymes for nuclear transcript editing currently comprise three molecularly cloned mammalian RNA-dependent adenosine deaminases. Two of these are expressed in most body tissues, perhaps indicating that adenosine deamination in transcripts is more global than has been recognized. Indeed, numerous mRNAs in different tissues may contain inosine residues and encode proteins with amino acid substitutions and different properties relative to the exonically encoded forms. If so, RNA editing by adenosine deamination may significantly enlarge the functional repertoire of the mammalian genome.

Importance of the intracellular domain of NR2 subunits for NMDA receptor function in vivo

NMDA receptors, a class of glutamate-gated cation channels with high Ca2+ conductance, mediate fast transmission and plasticity of central excitatory synapses. We show here that gene-targeted mice expressing NMDA receptors without the large intracellular C-terminal domain of any one of three NR2 subunits phenotypically resemble mice made deficient in that particular subunit. Mice expressing the NR2B subunit in a C-terminally truncated form (NR2B(deltaC/deltaC) mice) die perinatally. NR2A(deltaC/deltaC) mice are viable but exhibit impaired synaptic plasticity and contextual memory. These and NR2C(deltaC/deltaC) mice display deficits in motor coordination. C-terminal truncation of NR2 subunits does not interfere with the formation of gateable receptor channels that can be synaptically activated. Thus, the phenotypes of our mutants appear to reflect defective intracellular signaling.

Regional, developmental and interspecies expression of the four NMDAR2 subunits, examined using monoclonal antibodies

Mouse monoclonal antibodies were raised against bacterially expressed protein sequences of the NR2A, NR2B, NR2C and NR2D subunits of the rat NMDA receptor. From immunoblots of rat brain proteins, the apparent molecular weights of these subunits were 165, 170, 135 and 145 kDa, respectively. Proteins of similar masses were observed on immunoblots of specifically transfected HEK293 cells. Deglycosylation with endoglycosidase F reduced the mass of each endogenous NR2 subunit by approximately 10 kDa. In distribution studies, NR2A-immunoreactive protein (IRP) was located throughout the adult rat brain, NR2B-IRP was primarily in the forebrain, NR2C-IRP was predominantly in the cerebellum and NR2D-IRP was mainly found in the thalamus, midbrain and brainstem. Whereas NR2A- and NR2C-IRPs increased during rat brain post-natal development, NR2B- and NR2D-IRPs were abundant at birth and declined with age, especially in cerebellum. NR2-IRPs of mouse, rabbit, frog and human brain were of sizes similar to those of the corresponding rat subunits and were similarly distributed. In summary, NR2 subunits are large glycoproteins whose specific expression profiles in the brain are developmentally and regionally regulated and which are similarly expressed in a variety of species.

Tissue specific control regions of the N-methyl-D-aspartate receptor subunit NR2C promoter

In the mouse brain, the N-methyl-D-aspartate receptor subunit NR2C (epsilon-3) is mainly detected in the cerebellar granule cells starting from the second week of postnatal life. In order to improve our understanding of molecular mechanisms of this neuron-specific, spatial and temporal gene expression, different promoter fragments were used to control indicator genes in nondifferentiated rat pheochromocytoma (PC12) cells, in human embryonal kidney (HEK293) cells and in transgenic mice. A 400 bp NR2C promoter region upstream of the transcriptional start site was identified as a basal promoter that was negatively regulated possibly by a neuron restrictive silencer element (NRSE) that is localized 664 base pairs downstream from the transcriptional start sites.

Mammalian RNA-dependent deaminases and edited mRNAs

The past year has witnessed major progress in the field of mammalian nuclear RNA editing. Two new sequence-related RNA-dependent adenosine deaminases, distantly related to the previously characterized double-stranded RNA adenosine deaminase DRADA/dsRAD, have been molecularly characterized. One of these deaminases edits in vitro with precision for the molecular determinant that controls the Ca2+ permeability of fast synaptic glutamate-gated cation channels. This deaminase, like DRADA, is expressed in many tissues and the search is now on for more substrates of these RNA-editing enzymes. Moreover, the physiological role of the apolipoprotein B RNA editing enzyme APOBEC-1 has been investigated in genetically manipulated mice.