The gene encoding the glutamate receptor subunit GluR5 is located on human chromosome 21q21.1-22.1 in the vicinity of the gene for familial amyotrophic lateral sclerosis

Structure, Function, and Regulation of the Kainate Receptor

Neural communication and modulation are complex processes. Ionotropic glutamate receptors (iGluRs) significantly contribute to mediating the fast-excitatory branch of neurotransmission in the mammalian brain. Kainate receptors (KARs), a subfamily of the iGluRs, act as modulators of the neuronal circuitry by playing important roles at both the post- and presynaptic sites of specific neurons. The functional tetrameric receptors are formed by two different gene families, low agonist affinity (GluK1-GluK3) and high agonist affinity (GluK4-GluK5) subunits. These receptors garnered attention in the past three decades, and since then, much work has been done to understand their localization, interactome, physiological functions, and regulation. Cloning of the receptor subunits (GluK1-GluK5) in the early 1990s led to recombinant expression of kainate receptors in heterologous systems. This facilitated understanding of the functional differences between subunit combinations, splice variants, trafficking, and drug discovery. Structural studies of individual domains and recent full-length homomeric and heteromeric kainate receptors have revealed unique functional mechanisms, which have answered several long-standing questions in the field of kainate receptor biology. In this chapter, we review the current understanding of kainate receptors and associated disorders.

Pathogenic Mechanisms in Sporadic Amyotrophic Lateral Sclerosis

In recognition of the 100th anniversary of Charcot’s death we have reviewed possible pathogenic mechanisms in amyotrophic lateral sclerosis (ALS). Advances in the last 5 years in molecular biology and genetics have identified mutations in the cytosolic dismutase (SODI) gene in some patients with familial ALS raising the possibility that oxidative stress may be involved in the pathogenesis. An excitotoxic pathogenesis has been implicated based on elevated plasma and CSF levels of amino acids and altered contents of amino acids in the nervous system of ALS patients and changes in the number of excitatory amino acid receptors. ALS sera containing antibodies to L-type calcium channels and the development of immune mediated lower and upper and lower motor neuron models have revitalized research efforts focusing on an immune basis for ALS. Other pathogenic mechanisms which have been the subject of recent research include elemental toxicity, apoptosis and programmed cell death and possibly a deficiency or abnormality in growth factors. Pathogenic processes for ALS must account for an increasing incidence of ALS, male preponderance, and the selective vulnerability of the corticomotoneuronal system.

Anti-Glutamate Therapy in Amyotrophic Lateral Sclerosis: A Trial Using Lamotrigine

Glutamate excitotoxicity is implicated in the pathogenesis of amyotrophic lateral sclerosis (ALS). We report the results of a double blind, placebo controlled, trial using 100 mg of oral daily lamotrigine (3,5-diamino-6-(2,3 dichlorophenyl)-l,2,4-triazine) which inhibits glutamate release. 67 patients were entered and at trial termination of 1.5 years 15 had withdrawn (9 active and 6 placebo) and 12 had died (6 active and 6 placebo). Mean age at entry was 57.5 years for the active and 58.6 years for the placebo groups. Patients were seen at 3 monthly intervals and scored according to neurological deficit based upon age of onset, bulbar and respiratory involvement, ambulation and functional disability. The mean change in clinical scores for the active versus placebo groups over the trial period was 7.1 ± 3.3 and 9.0 ± 3.3 respectively (0.05 < p < 0.10). Changes in cortical threshold and MEP/CMAP ratios to magnetic stimulation also did not differ significantly between the two groups. We conclude that lamotrigine in the doses administered does not alter the course of ALS.

Metabotropic actions of kainate receptors in the control of glutamate release in the hippocampus

Kainate-type glutamate receptors (KARs) structurally present the credentials of the other ionotropic glutamate receptor (iGluR) family members (NMDA and AMPA receptors), but functionally often purport examples of a metabotropic mode of operation. In the present chapter, we describe these metabotropic roles of KARs in the modulation of glutamate release in the hippocampus at CA3 Schaffer Collateral (SC)-CA1 Pyramidal Cell (PC) synapses and dentate gyrus granule cell Mossy Fiber (MF)-CA3 PC synapses. As autoreceptors on SC terminals, KARs inhibit the release of glutamate at SC-CA1 PC synapses through a mechanism dependent on a pertussis toxin-sensitive G(i/o) protein thought to couple via its Gβγ subunit to a decrease in Ca(2+) channel function. At MF-CA3 PC synapses, autoreceptors on MF terminals respond diametrically depending on the agonist concentration. At low KA concentrations (< 100 nM), a G-protein-independent process invokes the activation of proteins kinase A (PKA) to effect a facilitation of glutamate release. This facilitation possibly involves the Ca(2+)-dependent (rather than GPCR-dependent) activation of adenylate cyclase (AC). At high KA concentrations (<100 nM), a mechanism involving a pertussis toxin-sensitive G(i/o) protein is invoked to inhibit AC activity and thereby suppress PKA activity. Taken together with the heterosynaptic regulation of GABA release by KARs working with a metabotropic modus operandi, there is therefore compelling evidence that these ionotropic glutamate receptors are involved in a noncanonical modulation of glutamate release that does not rely on their typical ionotropic activity.

Senile myoclonic epilepsy: delineation of a common condition associated with Alzheimer's disease in Down syndrome

In Down syndrome (DS), epilepsy is frequent in all age classes and is recognized as a significant cause of additional handicap and morbidity. Longer life expectancy has led to the recognition of the high incidence of both Alzheimer's disease and seizures in elderly persons with DS. Neuropathological markers of AD are found in all DS brains and clinical symptoms of AD become apparent by the age of 60 years and above in over 50% of DS subjects. Following preliminary description of myoclonic seizures and/or myoclonic epilepsy in isolated cases or small series, we wish to report the diagnostic criteria, treatment and prognosis of a specific and recognizable form of epilepsy associated with AD in a larger group of middle-aged to elderly DS patients. This markedly under-recognized entity may indeed concern an already large and steadily increasing number of patients. We reviewed all medical records of patients with DS referred to our centers (Centre Saint Paul-Gastaut, Marseille; Epilepsy Unit, Montpellier University Hospital; Department of Neurology, Hospital General de Asturias, Oviedo) since 1995. DS had been diagnosed in all at birth, and all presented with the typical morphological changes associated with DS. We selected all cases (18) referred as adults with new onset of myoclonic jerks (MJ) and/or behavioral or cognitive deterioration (CD).

Gene expression profiling of human neural progenitor cells following the serum-induced astrocyte differentiation

Neural stem cells (NSC) with self-renewal and multipotent properties could provide an ideal cell source for transplantation to treat spinal cord injury, stroke, and neurodegenerative diseases. However, the majority of transplanted NSC and neural progenitor cells (NPC) differentiate into astrocytes in vivo under pathological environments in the central nervous system, which potentially cause reactive gliosis. Because the serum is a potent inducer of astrocyte differentiation of rodent NPC in culture, we studied the effect of the serum on gene expression profile of cultured human NPC to identify the gene signature of astrocyte differentiation of human NPC. Human NPC spheres maintained in the serum-free culture medium were exposed to 10% fetal bovine serum (FBS) for 72 h, and processed for analyzing on a Whole Human Genome Microarray of 41,000 genes, and the microarray data were validated by real-time RT-PCR. The serum elevated the levels of expression of 45 genes, including ID1, ID2, ID3, CTGF, TGFA, METRN, GFAP, CRYAB and CSPG3, whereas it reduced the expression of 23 genes, such as DLL1, DLL3, PDGFRA, SOX4, CSPG4, GAS1 and HES5. Thus, the serum-induced astrocyte differentiation of human NPC is characterized by a counteraction of ID family genes on Delta family genes. Coimmunoprecipitation analysis identified ID1 as a direct binding partner of a proneural basic helix-loop-helix (bHLH) transcription factor MASH1. Luciferase assay indicated that activation of the DLL1 promoter by MASH1 was counteracted by ID1. Bone morphogenetic protein 4 (BMP4) elevated the levels of ID1 and GFAP expression in NPC under the serum-free culture conditions. Because the serum contains BMP4, these results suggest that the serum factor(s), most probably BMP4, induces astrocyte differentiation by upregulating the expression of ID family genes that repress the proneural bHLH protein-mediated Delta expression in human NPC.

Metabotropic actions of kainate receptors in the CNS

Kainate receptors (KARs), together with NMDA and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate receptors (AMPA), are typically described as ionotropic glutamate receptors. Although ionotropic functions for KARs are beginning to be characterized in multiple brain regions, both, in the pre- and post-synaptic compartments of the synapse, there is accumulating evidence that KARs mediate some of their effects without invoking ion-fluxes. Thus, since 1998, when the first metabotropic action of KARs was described in the modulation of GABA release in hippocampal interneurons, there have been increasing reports that some of the functions of KARs involve the participation of intracellular signalling cascades and depend on G protein activation. These surprising observations, attesting metabotropic actions of KARs, akin to those usually attributed to seven transmembrane region G protein-coupled receptors, make the physiological classification and description of glutamate receptors more complex. In the present review, we describe the metabotropic roles of KARs in the CNS and discuss the intriguing properties of this receptor which, structurally shows all the facets of a typical ionotropic receptor, but appears to express a metabotropic remit at some key synapses.

Isolation and characterization of LCHN: a novel factor induced by transient global ischemia in the adult rat hippocampus

Using mRNA differential display to identify cerebral ischemia-responsive mRNAs, we isolated and cloned a cDNA derived from a novel gene, that has been designated LCHN. Antisense mRNA in situ hybridization and immunoblotting confirmed LCHN expression to be induced in the rat hippocampus following transient forebrain ischemia. The deduced amino acid sequence of the novel LCHN cDNA contains an open reading frame of 455 amino acids, encoding a protein with a predicted molecular mass of approximately 51 kDa. Although LCHN is highly conserved between rat, mouse, and human, the deduced amino acid sequence of LCHN does not possess significant homology to other known genes. LCHN immunoreactivity is detected within the somatodendritic compartment of neurons, is also present on dendritic growth cones, but is not detected on astrocytes. The induction of LCHN in the hippocampus following ischemic injury may have functional consequences, as the ectopic over-expression of LCHN generated neurons with longer and more branched axons and dendrites. Taken together, these data suggest that LCHN could play a role in neuritogenesis, as well as in neuronal recovery and/or restructuring in the hippocampus following transient cerebral ischemia.

Associations between human disease genes and overlapping gene groups and multiple amino acid runs

Overlapping gene groups (OGGs) arise when exons of one gene are contained within the introns of another. Typically, the two overlapping genes are encoded on opposite DNA strands. OGGs are often associated with specific disease phenotypes. In this report, we identify genes with OGG architecture and genes encoding multiple long amino acid runs and examine their relations to diseases. OGGs appear to be susceptible to genomic rearrangements as happens commonly with the loci of the DiGeorge syndrome on human chromosome 22. We also examine the degree of conservation of OGGs between human and mouse. Our analyses suggest that (i) a high proportion of genes in OGG regions are disease-associated, (ii) genomic rearrangements are likely to occur within OGGs, possibly as a consequence of anomalous sequence features prevalent in these regions, and (iii) multiple amino acid runs are also frequently associated with pathologies.

Glutamate receptor genes: susceptibility factors in schizophrenia and depressive disorders?

Schizophrenia, depression, and bipolar disorder are three major neuropsychiatric disorders that are among the leading causes of disability and have enormous economic impacts on our society. Although several neurotransmitter systems have been suggested to play a role in their etiology, we still have not identified any gene or molecular mechanism that might lead to genetic susceptibility for or protection against these neuropsychiatric disorders. The glutamatergic receptor system, and in particular the N-methyl-D-aspartate (NMDA) receptor complex, has long been implicated in their etiology. I review the current molecular evidence that supports a critical role for the glutamatergic receptor system in schizophrenia and the potential involvement of this receptor system in depression and bipolar disorder. It is likely that mutations in glutamate receptor genes might alter the risk of developing one of these disorders. Potential future research directions designed to identify these mutations and to elucidate their effect on mental health will be discussed.

Molecular physiology of kainate receptors

A decade ago, our understanding of the molecular properties of kainate receptors and their involvement in synaptic physiology was essentially null. A plethora of recent studies has altered this situation profoundly such that kainate receptors are now regarded as key players in the modulation of transmitter release, as important mediators of the postsynaptic actions of glutamate, and as possible targets for the development of antiepileptic and analgesic drugs. In this review, we summarize our current knowledge of the properties of kainate receptors focusing on four key issues: 1) their structural and biophysical features, 2) the important progress in their pharmacological characterization, 3) their pre- and postsynaptic mechanisms of action, and 4) their involvement in a series of physiological and pathological processes. Finally, although significant progress has been made toward the elucidation of their importance for brain function, kainate receptors remain largely an enigma and, therefore, we propose some new roads that should be explored to obtain a deeper understanding of this young, but intriguing, class of proteins.

Evidence for an interferon-related inflammatory reaction in the trisomy 16 mouse brain leading to caspase-1-mediated neuronal apoptosis

The trisomy of human chromosome 21 (Down syndrome) is the leading genetic cause of learning difficulties in children, and predisposes this population to the early onset of the neurodegeneration of Alzheimer's disease. Down syndrome is associated with increased interferon (IFN) sensitivity resulting in unexpectedly high levels of IFN inducible gene products including Fas, complement factor C3, and neuronal HLA I which could result in a damaging inflammatory reaction in the brain. Consistent with this possibility, we report here that the trisomy 16 mouse fetus has significantly increased whole brain IFN-gamma and Fas receptor immunoreactivity and that cultured whole brain trisomy 16 mouse neurons have increased basal levels of caspase 1 activity and altered homeostasis of intracellular calcium and pH. The trisomic neurons also showed a heightened sensitivity to the increase in both Fas receptor levels and caspase 1 activity we observed when IFN-gamma was added to the neuron culture media. Because of the autoregulatory nature of IFN activity, and the IFN inducing capability of caspase-1-activated cytokine activity, our data argue in favor of the possibility of an interferon-mediated, self-perpetuating, inflammatory response in the trisomy brain that could subserve the loss of neuron viability seen in this trisomy 16 mouse model for Down syndrome.

Excitotoxic profile of LY339434, a GluR5 agonist, in cultured murine cortical neurons

The neurotoxic profile of (2S,4R, 6E)-2-amino-4-carboxy-7-(2-naphthyl)hept-6-enoic acid (LY339434), a low-affinity kainate receptor subtype 5 (GluR5) agonist at recombinant human glutamate receptors, was evaluated to investigate the involvement of GluR5 in excitotoxic neuronal death. Murine cortical neurons were exposed to treatments for 24 h and assessed by a cell viability assay and phase-contrast microscopy. LY339434 (1-1000 microM) caused a concentration-dependent decrease in cell viability (EC(50)=11.4+/-1.2 microM) that was only attenuated by (5R, 10S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5, 10-imine (MK-801, 10 microM), but not by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 50 microM) or 1-(4-aminophenyl)-4-methyl-7,8-methylenedioxy-5H-2,3-benzodiazepine (GYKI 52466, 20 microM). Labeling with nucleic acid binding dyes revealed that LY339434 induced few apoptotic-like characteristics. These findings indicate that in cultured murine cortical neurons, LY339434 acts predominantly through N-methyl-D-aspartate (NMDA) receptors rather than GluR5 to effect neuronal death that is rapid and involves predominantly necrosis rather than morphological apoptosis.

Low-affinity kainate receptor agonists induce insult-dependent apoptosis and necrosis in cultured murine cortical neurons

Overstimulation of ionotropic glutamate receptors leads to excitotoxic neuronal death, which has been implicated in the neurodegeneration of neurological diseases. The present study examined the role of putative low-affinity kainate receptor subtype (GluR5-7) agonists in excitotoxicity in cultured murine cortical neurons. The concentration-dependent decrease in cell viability induced by the agonists kainate (1-1,000 microM) and (RS)-2-amino-3-(hydroxy-5-tert-butylisoxazol-4-yl) propanoic acid (ATPA; 1-1,000 microM) was only attenuated by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 10 microM) and 1-(4-aminophenyl)-4-methyl-7,8-methylenedioxy-5H-2,3-benzodiazepine (GYKI 52466; 20 microM). (S)-5-iodowillardiine (1-1,000 microM)-induced toxicity was attenuated by CNQX (20 microM), GYKI 52466 (20 microM) and MK-801 (10 microM); however, (2S, 4R)-4-methylglutamate (1-120 microM)-induced toxicity was not attenuated by the antagonists. None of the agonists possessed selective actions at GluR5-7. Morphological observations (phase-contrast and fluorescence microscopy) revealed that the agonists induced two distinct patterns of neuronal injury. After 24 hr of treatment, low concentrations of agonists (1-30 microM) produced cellular shrinkage and nuclear granulation consistent with slow, apoptotic-like neuronal death. Pyknotic labeling with the DNA binding dye Sytox green confirmed these apoptotic characteristics, which significantly decreased with increasing concentrations. After 4 hr, increasing concentrations of agonists (100-1,000 microM) induced cellular swelling, with subsequent extracellular debris; labeling with propidium iodide revealed isolated nuclei consistent with the increased involvement of rapid necrosis. Thus, all putative GluR5-7 agonists produced excitotoxicity across a necrotic-apoptotic continuum in murine cortical neuron cultures.

Recent advances in the genetics of epilepsy: insights from human and animal studies

Progress in understanding the genetics of epilepsy is proceeding at a dizzying pace. Due in large part to rapid progress in molecular genetics, gene defects underlying many of the inherited epilepsies have been mapped, and several more are likely to be added each year. In this review, we summarize the available information on the genetic basis of human epilepsies and epilepsy syndromes, and correlate these advances with rapidly expanding information about the mechanisms of epilepsy gained from both spontaneous and transgenic animal models. We also provide practical suggestions for clinicians confronted with families in which multiple members are afflicted with epilepsy.

Effects of overexpression of the cytoplasmic copper-zinc superoxide dismutase on the survival of neurons in vitro

The cytoplasmic copper-zinc superoxide dismutase (Cu, Zn SOD; SOD-1) is an abundant and well-conserved intracellular antioxidant enzyme which has been implicated in a number of oxidative stress mediated phenomena, especially Down Syndrome, in which SOD-1 activity is increased due to triplication of chromosome 21 containing the gene and, in hereditary amyotrophic lateral sclerosis, in which the gene is mutated. Overexpression of SOD-1 could theoretically, therefore, lead to increased vulnerability to oxidative stress in two distinct manners: increasing steady-state hydrogen peroxide levels or increasing toxic side reactions. We used two mouse neuronal culture systems--one in which the murine chromosome containing SOD-1 is triplicated and one in which human SOD-1 is a transgene--to test the effect of overexpression of this enzyme on antioxidant status in general and specifically on glutamate mediated oxidative stress. We found that SOD-1 overexpression increases antioxidant status at the same time it decreases vulnerability to glutamate.

Blood glutamate levels in patients with motor neuron disease

This study was undertaken to evaluate the role of excitatory amino acid glutamate (Glu) in the pathophysiology of motor neuron disease (MND). It was observed that blood Glu levels were significantly higher in MND patients with respect to healthy controls. The data indicate that Glu homeostasis is altered in the patients with MND.

Responses to NMDA in cultured hippocampal neurons from trisomy 16 embryonic mice

The trisomy 16 (Ts16) mouse is regarded as a model of human trisomy 21 (Ts21), or Down syndrome. The ionic current evoked by the glutamate receptor agonist N-methyl-D-aspartate (NMDA) was investigated in cultured hippocampal neurons from embryonic Ts16 and control diploid mice. In both Ts16 and control neurons, NMDA- (6-150 microM) evoked a similar inward current. The reversal potential, the minimum current, the dose response plot of the conductance, the effect of Mg2+ on the current-voltage plot and the inhibition by D-2-amino-5-phosphonovaleric acid (AP5; 50 microM) showed no significant difference between Ts16 and control neurons. These data suggest that, although voltage-dependent ion channels are reported to have altered active properties in Ts16 neurons, NMDA-evoked currents are not altered.

Excitatory amino acids in health and disease

PURPOSE

To review the role of excitatory neurotransmitters in normal mammalian brain function, the concept of excitotoxic neuronal death as an important final common path in a variety of diseases, and modification of excitatory synaptic transmission as an important new pharmacological principle. These principles are discussed, with special emphasis on diseases of importance to older adults.

DATA SOURCES

A MEDLINE search from 1966 to May 1995 was undertaken, as well as a manual search of current issues of clinical and basic neuroscience journals, for articles that addressed glutamate N-methyl-D-aspartate and/or excitotoxicity.

STUDY SELECTION

A total of 5398 original and 68 review articles were identified that addressed animal and human experimentation relevant to excitotoxic neuronal death. There were 364 articles with potential significance for clinical application identified; 132 of the most recent references are provided.

DATA EXTRACTION

All articles were classified into three categories: general receptor, biology pathogenesis of disease, and pharmacotherapy.

RESULTS

Glutamic and aspartic acids are the physiological mediators of most excitatory synaptic transmission. This is critical to several normal nervous system functions, including memory and long-term modification of synaptic transmission and nociception. Activation of the inotropic NMDA and non-NMDA receptors increases transmembrane calcium and sodium fluxes, and the metabotropic glutamate receptor activation results in generation of inositol triphosphate and inhibition of adenylate cyclase. Numerous modulatory sites exist, especially on the NMDA receptor. Nitric oxide, arachidonic acid, superoxide, and intracellular calcium overload are the ultimate mediators of neuronal death. Glutamate re-uptake transporters belong to a unique family of amino acid transport systems, the malfunction of which is intricately involved in disease pathogenesis. Ischemic stroke, hypoglycemia, Parkinson's disease, alcohol intoxication and withdrawal, Alzheimer's disease, epilepsy, and chronic pain syndromes are only some of the important clinical neurological disorders with a major pathogenic role for the excitatory amino acids.

CONCLUSIONS

Pharmacological manipulation of the excitatory amino acid receptors is likely to be of benefit in important and common diseases of the nervous system. Only a few of the currently available drugs that modify excitatory neurotransmission, such as remacemide, lamotrigine, and tizanidine, have an acceptable therapeutic index. The identification of numerous receptor subtypes, topographic variabilities of distribution, and multiple modulatory sites will provide a true challenge to the neuropharmacologist.

Amyotrophic lateral sclerosis is a multifactorial disease

Amyotrophic lateral sclerosis (ALS) is probably biphasic. An initial trigger(s) is followed by a terminal cascade coinciding with the onset of neurological deficits. The terminal cascade involves interactive multifactorial pathogenic mechanisms. Aging must play a crucial role leading to multiple defective or degraded gene products accumulating with progressing years. This in turn leads to failure of receptor integrity and resulting excitotoxicity, free radical accumulation, failure of neurotrophism, and possibly immunological disturbances. These events are predated by months or years by a trigger which is also likely to be multifactorial and cumulative. Evidence suggests that environmental factors may be important triggers. Failure of specific glutamate transporters and calcium binding proteins may account for selective vulnerability of the corticomotoneuronal system. It is postulated that in ALS the primary target cell is the corticomotoneuron or the local circuit interneurons which modulate its activity. Glia cells may play an important role in the demise of the corticomotoneuronal cell. The disordered corticomotoneuron induces excessive excitatory transmitter (glutamate?) release at the corticomotoneuronal-spinal-motoneuronal synapse resulting in the subsequent demise of this neuron.

Sporadic ALS and chromosome 22: evidence for a possible neurofilament gene defect

ALS is associated with the P2 blood group phenotype. Molecular evidence now shows the gene encoding this antigen to be on the long arm of human chromosome 22 near the newly discovered gene for heavy neurofilament (NF-H). Since an ALS-type condition can be generated in transgenic mice expressing the human NF-H gene, and since the gene for the CNTF-related cytokine leukemia inhibitory factor (LIF) is located adjacent to this gene, it is hypothesized that a defect on the chromosome 22 band region q12 is involved in the pathogenesis of sporadic ALS.

Expression and regulation of kainate and AMPA receptors in uncommitted and committed neural progenitors

Here we review experimental evidence of non-NMDA glutamate receptor expression in the embryonic central nervous system. AMPA- and kainate-preferring glutamate receptor subunit mRNA transcripts are detected in embryonic neurons, glia and neural progenitors. Functional assays demonstrate that in some cell subpopulations ionotropic glutamate receptors are expressed by progenitors before synapse formation and terminal differentiation, and may be present before lineage determination is specified. The activation of these receptors triggers induction of immediate early gene transcription in progenitor cells. The cloning and transcriptional analysis of upstream regulatory regions of glutamate receptor genes governing their temporal and tissue-specific expression are also discussed.

Genetic and physical mapping of the GLUR5 glutamate receptor gene on human chromosome 21

Glutamate receptors (GluRs) mediate excitatory neurotransmission and may have important roles in central nervous system disorders. To characterize the human GLUR5 gene, which is located on human chromosome 21q22.1, we isolated cDNAs, genomic phage lambda clones, and yeast artificial chromosomes (YACs) and developed sequence tagged sites (STSs) and simple sequence length polymorphisms (SSLPs) for GLUR5. Genetic mapping with a tetranucleotide AGAT repeat named GLUR5/AGAT (six alleles observed, 70% heterozygosity) placed GLUR5 5 cM telomeric to APP (D21S210) and 3 cM centromeric to SOD1 (D21S223). The human GLUR5 gene is located near the familial amyotrophic lateral sclerosis (FALS) locus; linkage analysis of GLUR5 SSLPs in FALS pedigrees yielded negative lod scores, consistent with the recent association of the FALS locus with the SOD1 gene. Physical mapping of GLUR5 using a YAC contig suggested that the GLUR5 gene spans approximately 400-500kb, and is within 280kb of D21S213. The large size of the GLUR5 gene raises questions regarding its functional significance. Our GLUR5 YAC contig includes clones found in the Genethon chromosome 21 YAC contig, and reference to the larger contig indicates the orientation centromere--D21S213-GLUR5 5' end-GLUR5/AGAT--GLUR5 3' end--SOD1. The development of GLUR5/AGAT should permit rapid determination of the status of the GLUR5 gene in individuals with partial trisomy or monosomy of chromosome 21. Such studies may provide insights concerning the possible role of GLUR5 in Down syndrome.

Infantile spasms in children with Down syndrome

The authors retrospectively identified 17 children with Down syndrome who developed infantile spasms, and analysed their etiology, EEG findings, response to medication, development and prognosis. Compared with symptomatic infantile spasms in the general population, which have a poor prognosis, these patients had a relatively benign outcome with regard to seizure control: only three of 16 survivors currently have seizures and seven of the 16 currently receive anticonvulsants. Developmental outcome was poorest in those with a superimposed hypoxic insult and in those who regressed developmentally at the onset of the spasms and did not regain developmental milestones. The overall neurological prognosis for children with Down syndrome and infantile spasms appears to be better than for children with infantile spasms in the general population.

Lethal congenital contracture syndrome: further delineation and genetic aspects

In a national morphology based study of lethal arthrogryposis between 1979 and 1992, 40 fetuses and infants with lethal congenital contracture syndrome (LCCS, McKusick 253310) were found in Finland. The incidence of LCCS in Finland was 1:19,000 births. There were 20 affected males and 20 affected females in 26 families. In 16 cases the pregnancy was terminated after the prenatal diagnosis of total akinesia and fetal hydrops on ultrasound. There were 19 stillborn infants and five were born showing signs of life, but died within one hour. The segregation analyses yielded 0.45 affected by the "singles" method and 0.34 by the "sib" method. The birthplaces of the grandparents were located in the sparsely populated north east of Finland. This finding supports the existence of an autosomal recessive LCCS gene in Finland, particularly in the north eastern part.

Cloning, functional coexpression, and pharmacological characterisation of human cDNAs encoding NMDA receptor NR1 and NR2A subunits

Using expression cloning, and more recently using polymerase chain reaction cloning approaches, a family of rat N-methyl-D-aspartate (NMDA) receptor subunit cDNAs has been described (NR1, NR2A, NR2B, NR2C, and NR2D). Here we report cloning and sequencing of cDNAs encoding isoforms of the human NR1 subunit (NR1a, NR1d, and NR1e) that differ at their C-terminal end as a result of alternative splicing and also of a cDNA encoding the human NR2A subunit. The deduced amino acid sequences of the human NR1 subunit isoforms differed from the published rat NR1 subunit sequences at only eight positions, all of which were N-terminal to the alternatively spliced domains. The human NR2A subunit deduced amino acid sequence differed from the published rat NR2A subunit sequence at 81 of the 1,464 amino acids, with most of the substitutions being located in the C-terminal half of the subunit. The gene for NR2A has been localised to human chromosome 16. We also report the expression and pharmacological characterisation of recombinant human NR1a/NR2A heteromeric receptors in Xenopus oocytes. These receptors had EC50 values of 2.14 and 2.05 microM for glutamate and glycine, respectively, and an IC50 of 46.8 microM for Mg2+. Responses were antagonised by D-2-amino-5-phosphonovalerate, L-689,560, pH 6.3, zinc, and MK-801. No modulatory effect was observed on application of ifenprodil, confirming previous observations with rat NR1 + NR2A recombinant receptors.

Excitatory amino acids, free radicals and the pathogenesis of motor neuron disease

The cause of motor neuron disease (MND) remains unknown, but the pathogenic involvement of excitatory amino acid (EAA) neurotransmitters and related exogenous compounds has been proposed. We discuss current concepts of the mechanisms of action of EAAs and the evidence for links between these neurotransmitters and free radical hypotheses of neuronal damage. These concepts are especially pertinent following reports of mutations in the gene encoding the free radical scavenging enzyme, copper-zinc superoxide dismutase, in familial MND. New approaches to treatment are suggested by advances in understanding of the disease.

Mechanism of epilepsy

Epilepsy is a collection of diverse disorders that together affect approximately 1% of the general population. Current therapies are largely symptomatic and are aimed at controlling seizures in affected individuals. This review focuses on emerging insights into mechanisms underlying the most common form of epilepsy--complex partial epilepsy--and also addresses progress in molecular genetic approaches. Such developments will hopefully lead to more effective therapies.

An inherited lower motor neuron disease of pigs: clinical signs in two litters and pathology of an affected pig

A chronic progressive neurodegeneration, called hereditary porcine neuronal system degeneration (HPNSD), was recognized in a swine herd in Devon, England. Adult pigs that were presumed carriers of the dominantly inherited trait for HPNSD were transferred from England, where a breeding colony was maintained for 9 years, to the Wyoming State Veterinary Laboratory (WSVL) for study. Two litters of affected piglets were born to 2 carrier sows at the WSVL. Clinical signs of muscular tremors, paresis, or ataxia developed at 12-59 days of age in 4 of 6 liveborn pigs. Three other pigs were stillborn. In the 4 affected liveborn pigs, clinical signs progressed and included symmetrical (3 pigs) or asymmetrical (1 pig) posterior paresis, bilateral knuckling of metatarsal-phalangeal or carpal joints, poor exercise tolerance, and in 1 pig, marked hind limb hypermetria. A 34-kg gilt exhibiting clinical signs of muscular tremors and posterior paresis and clinical signs for 22 days was euthanized and examined postmortem at 83 days of age. Apart from decubitus ulcers, gross lesions were absent. Microscopically, perikaryal vacuolation and osmiophilic lipid droplets were observed in atrophic alpha motor neurons in the spinal cord. There was axonal (Wallerian) degeneration in sulcomarginal and dorsal spinocerebellar tracts. Axonal degeneration also involved ventral but not dorsal spinal nerve roots, and was present in eight peripheral nerves sampled for histopathology. Changes in skeletal muscles were consistent with denervation atrophy and were most pronounced in M. tibialis cranialis of the 6 muscles sampled. Immunohistochemical staining of spinal cord for phosphorylated and nonphosphorylated neurofilaments did not reveal abnormal patterns, unlike some well-characterized inherited motor neuron diseases in other species.

Chromosomal localization of gene for human glutamate receptor subunit-7

We isolated a human glutamate receptor subunit 7 (GluR-7) cosmid after high stringency screening of a human genomic placental library using a rat GluR-7 cDNA as a probe. A 614-bp fragment of the GluR-7 cosmid was sequenced, and an exon that encodes 53 amino acids was found between two introns. The exon exhibited 89% and 96% identity at the nucleotide and amino acid levels, respectively, with the corresponding region of rat GluR-7. The human GluR-7 was classified as a kainate subtype glutamate receptor based on its homology to rat GluR-7. Using somatic cell hybrid analysis, human GluR-7 was localized to chromosome 1. Fine mapping analysis using FISH localized the gene to 1p33-34. Since glutamate receptors of the kainate subtype have been implicated in neurodegenerative disorders, establishing the precise map position of human GluR-7 subunit is an important step towards evaluating this locus as a candidate for mutations in neurodegenerative disorders.

Chromosomal localization of glutamate receptor genes: relationship to familial amyotrophic lateral sclerosis and other neurological disorders of mice and humans

Receptors for the major excitatory neurotransmitter glutamate may play key roles in neurodegeneration. The mouse Glur-5 gene maps to chromosome 16 between App and Sod-1. The homologous human GLUR5 gene maps to the corresponding region of human chromosome 21, which contains the locus for familial amyotrophic lateral sclerosis. This location, and other features, render GLUR5 a possible candidate gene for familial amyotrophic lateral sclerosis. In addition, dosage imbalance of GLUR5 may have a role in the trisomy 21 (Down syndrome). Further characterization of the murine glutamate receptor family includes mapping of Glur-1 to the same region as neurological mutants spasmodic, shaker-2, tipsy, and vibrator on chromosome 11; Glur-2 near spastic on chromosome 3; Glur-6 near waltzer and Jackson circler on chromosome 10; and Glur-7 near clasper on chromosome 4.