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Biomedical subjects

A Plaitakis

Publications and source records attributed to A Plaitakis.

At least 37 records · Page 2Linked to original sources
PubMed1994

Molecular cloning of human brain glutamate/aspartate transporter II.

Glutamate transporters are membrane-bound proteins which are localized in glial cells and/or pre-synaptic glutamatergic nerve endings and are essential for the removal and termination of action of synaptic glutamate. Several cDNAs encoding glutamate transporters have been isolated from mammalian tissues, including human cerebellum. Here, we screened cDNA libraries derived from human brain stem and cerebellum, and isolated a novel cDNA that encodes for a glutamate transporter. This cDNA predicts a protein which contains 565 amino acids and is homologous to a rat brain Na(+)-dependent glutamate/aspartate transporter. The new cDNA is expressed in brain and is structurally distinct from the previously reported human glutamate transporter cDNA.

Amino Acid Sequence↗
PubMed1993

Cloning and characterization of a glutamate transporter cDNA from human cerebellum.

The glutamate/aspartate transporters are essential for the elimination and recycling of synaptic glutamate released from nerve endings during neurotransmission. Evidence suggests that these processes are altered in ischemia and neuronal degenerations linked to excitotoxicity. We screened a cDNA library constructed from human cerebellar mRNA, and isolated a cDNA that shows an 88.5% and a 98.7% sequence similarity at the nucleotide and amino acid level, respectively, with a rat brain specific Na(+)-dependent glutamate/aspartate transporter. The human cDNA is expressed in brain and it may prove useful in the study of human neurodegenerations linked to glutamate dysfunction.

Amino Acid Sequence↗
PubMed1993

Altered metabolism of excitatory amino acids, N-acetyl-aspartate and N-acetyl-aspartyl-glutamate in amyotrophic lateral sclerosis.

Since recent studies provided evidence for abnormal glutamate metabolism in amyotrophic lateral sclerosis, we measured amino acid levels in the fasting plasma of 52 ALS patients and an equal number of controls of a similar age. In addition, the content of amino acids, N-acetyl-aspartate (NAA) and N-acetyl-aspartyl-glutamate (NAAG) were measured in spinal cord and brain tissue obtained at autopsy from patients dying of ALS. Results showed significant elevations (by about 70%) in the plasma levels of glutamate in the ALS patients as compared to controls. In contrast, glutamate levels were significantly decreased in all CNS regions studied of ALS patients (by 21-40%), with the greatest changes occurring in the spinal cord. The ratio of glutamine to glutamate was altered significantly in the spinal cord ALS tissue. In addition, reductions in the levels of aspartate (by 32-35%), NAA, and NAAG (by 40-48%) were found in the spinal cord of ALS patients. These results are consistent with a generalized defect in the metabolism of neuroexcitotoxic amino acids. An altered distribution of these compounds may occur between their intracellular and extracellular pools with resultant abnormal potentiation of excitatory transmission mediated by glutamate receptors and selective degeneration of motor neurons.

Adult↗
PubMed1993

MRI and CT features of cerebellar degeneration.

The many conventional classifications of cerebellar degeneration are usually based on information obtained in post-mortem examinations. On the other hand, neuroimagings, particularly with follow-up imaging studies, can demonstrate morphologic changes at various stages of disease evolution in living patients, thus providing a better understanding and evaluation of the disease processes, leading towards an earlier and more accurate diagnosis. Obviously, some patients require determination of a biochemical marker or markers in the final diagnosis. In Friedreich's ataxia, major changes are severe atrophy of the spinal cord with flattening of its posterior aspect. The medulla oblongata becomes smaller and the vermian and paravermian structures adjacent to the primary fissures become mildly atrophic. In hexosaminidase deficiency, there is pancerebellar atrophy with marked dilatation of the fourth ventricle. Cerebellar atrophy is more marked in the hemispheres than in the vermis, while the brain stem shows little change. The frontal and parietal sulci are usually slightly prominent. In cerebello-olivary atrophy (also called cortical cerebellar degeneration), there is atrophy of the superior vermis, especially the declive, folium and tuber. There is also atrophy of the lateral part of the cerebellar hemispheres, giving an appearance of the "fish-mouth deformity" on parasagittal sections. The fourth ventricle may be greatly enlarged. In dominant olivopontocerebellar atrophy (OPCA), the Menzel type is characterized by cerebellar atrophy of the "fine comb" type with the greatest involvement in the anterior lobe and in the upper part of the middle lobe. The hemispheres are more involved than the vermis. The brainstem, especially the pons, and the brachia pontis are also atrophic. In severe cases, the changes are very marked. Although the fourth ventricle is large, it lacks the ballooning characteristic of OPCA with slow saccades. In OPCA with slow saccades with or without retinal degeneration, the most pathognomonic features are "ballooning of the fourth ventricle" due to excavation of its floor and the "molar tooth deformity" secondary to severe atrophy of the pons, brachia pontis and conjunctiva. The cerebellum usually shows pancerebellar atrophy of the "fine comb" type. In recessive OPCA, cerebellar atrophy is most marked in the lateral part of the cerebellar hemispheres with "fish mouth deformity" secondary to drop-out of the tertiary and secondary folia from the primary folia. This feature is less marked in cases of atypical cerebello-olivary atrophy. In late-onset sporadic OPCA with autonomic failure, the cerebellum, especially its lateral portions and the brainstem, are variably involved in the atrophic processes, ranging from very mild to severe involvement.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1993

Glutamate dehydrogenase deficiency in cerebellar degenerations: clinical, biochemical and molecular genetic aspects.

Glutamate dehydrogenase (GDH), an enzyme central to glutamate metabolism, is significantly reduced in patients with heterogenous neurological disorders characterized by multiple system atrophy (MSA) and predominant involvement of the cerebellum and its connections. In human brain, GDH exists in multiple isoforms differing in their isoelectric point and molecular mass. These are differentially reduced in quantity and altered in catalytic activity in patients with clinically distinct forms of MSA, thus suggesting that these GDH isoproteins are under different genetic control. Dysregulation of glutamate metabolism occurs in patients with GDH deficiency and is thought to mediate the disease's neurodegeneration via neuroexcitotoxic mechanisms. This possibility is supported by additional data showing that glutamate binding sites are significantly decreased in cerebellar tissue obtained at autopsy from MSA patients. At the molecular biological level, several cDNAs specific for human GDH have been isolated recently and cloned. Northern blot analysis of various human tissues, including brain, has revealed the presence of multiple GDH-specific mRNAs. In addition, multiple GDH-specific genes are present in humans and these data are consistent with the possibility that the various GDH isoproteins are encoded by different genes. These advances have laid the groundwork for characterizing the human GDH genes and their products in health and disease.

Animals↗
PubMed1993

Modulation of monoaminergic and amino acid transmission as a means for therapeutic intervention in ataxia.

In recent years, substantial progress has been made in understanding the organization and function of the cerebellum at the neuronal, synaptic, and molecular level. More than any other region of the brain, the cerebellum utilizes amino acids as its main excitatory and inhibitory transmitters. Excitatory amino acids, such as glutamate and aspartate, in addition to serving as chemical messengers, may also mediate neurodegenerative processes in human ataxic disorders. Of the monoamines, serotonin has been proposed as a neuromodulator in the cerebellum and is thought to play a role in the pathophysiology of ataxia in animal models, and human cerebellar disorders. These considerations raise the possibility that pharmacologic modification of amino acid and serotonergic transmission may provide a means for therapeutic intervention in ataxia.

Amino Acids↗
PubMed1991

Quantitative autoradiographic study of L-glutamate binding sites in normal and atrophic human cerebellum.

In the present work the distribution of L-glutamate binding sites in the different layers of human cerebellum of normal individuals and of seven patients who died with olivopontocerebellar atrophy (OPCA) was examined with the technique of quantitative autoradiography. Specific L-[3H]glutamate binding was higher in the molecular than in the granule cell layer of normal cerebellar tissue. A significant decrease of L-[3H]glutamate specific binding was observed in the molecular layer of all OPCA tissues. In the granule cell layer L-[3H]glutamate binding was decreased only in two patients who suffered from late-onset sporadic OPCA and in one patient who suffered from a form of OPCA inherited in a dominant manner. Quisqualate-sensitive binding sites were the most abundant binding sites in the molecular layer of normal cerebella, whereas N-methyl-D-aspartic acid (NMDA)-sensitive binding sites were the most abundant type in the granule cell layer. A significant decrease of quisqualate-sensitive and an increase in NMDA-sensitive binding sites were observed in the molecular layer of OPCA cerebellar tissues. No significant changes were observed in the granule cell layer of these tissues.

2-Amino-5-phosphonovalerate↗
PubMed1990

Glutamate dysfunction and selective motor neuron degeneration in amyotrophic lateral sclerosis: a hypothesis.

Recent studies provided evidence for a generalized defect in glutamate metabolism in patients with amyotrophic lateral sclerosis, associated with widespread alterations in the central nervous system levels of this excitatory amino acid putative transmitter. Present data support the hypothesis that altered presynaptic glutamatergic mechanisms may be responsible for a neuroexcitotoxic cell loss in this disorder. High local concentrations of glycine, released from glycinergic terminals, may disrupt adaptive processes contributing to abnormal potentiation of excitatory transmission mediated by glutamate receptors and resultant selective degeneration of motor neurons. These considerations offer new therapeutic strategies for amyotrophic lateral sclerosis.

Amyotrophic Lateral Sclerosis↗
PubMed1989

Characterization of glutamate dehydrogenase isoproteins purified from the cerebellum of normal subjects and patients with degenerative neurological disorders, and from human neoplastic cell lines.

Glutamate dehydrogenase (GDH) was purified to homogeneity from cerebellar tissue of three normal subjects and seven patients with four distinct types of degenerative neurological disorders. Nonequilibrium pH gradient gel electrophoresis showed that the purified enzyme consists of four major isoproteins designated GDH 1, 2, 3, and 4. With one exception, the relative abundance and isoelectric points of the GDH isoproteins decrease and the molecular weights increase progressively going from isoprotein 1 to isoprotein 4. The enzyme isolated from the brain of one patient with a variant form of multiple system atrophy displayed marked reduction of GDH isoprotein 1. The Km values of the patients' GDH for alpha-ketoglutarate, glutamate, NADH, and NADPH were significantly increased as compared to GDH obtained from normal and neurologic control subjects. In addition, glutamate levels were reduced markedly in the patient's cerebellum. Pulse-chase studies have shown that both the human hepatoma HepG2 and the human glioma U373 cell lines synthesize exclusively GDH isoprotein 2. The different GDH isoproteins do not have a precursor-product relationship and may represent products of different GDH mRNA species.

Adolescent↗
PubMed1989

L-glutamate binding sites of normal and atrophic human cerebellum.

The binding kinetics, pharmacologic properties, ontogeny and localization of L-glutamate binding sites were studied in membrane preparations and sections of normal and olivopontocerebellar atrophy (OPCA) human cerebellum. One binding component was found with a Kd value in the order of 150 x 10(-9) M. No significant changes of Kd values were observed with age, whereas the highest Bmax value was observed at the age of 1 year. L-Aspartate, ibotenate, quisqualate and L-homocysteic acid were potent inhibitors of L-[3H]glutamate binding. Quantitative densitometric measurements indicated the presence of L-glutamate sites in both the molecular and granule cell layer. In OPCA cerebella a very significant decrease of L-[3H]glutamate specific binding (Bmax) was observed, whereas Kd values were found unchanged. The pharmacologic properties of L-[3H]glutamate binding sites of OPCA cerebellar tissues were similar to those of normal cerebellum. [3H]quinuclidinyl benzylate binding, expressed in fmol/mg protein, did not show significant differences between normal and OPCA cerebella.

Adolescent↗
PubMed1988

Pilot trial of branched-chain aminoacids in amyotrophic lateral sclerosis.

22 patients with amyotrophic lateral sclerosis were entered into a double-blind, randomised, placebo-controlled trial of treatment with branched-chain aminoacids. 11 received daily 12 g L-leucine, 8 g L-isoleucine, and 6.4 g L-valine, by mouth, and the remainder received placebo. During the one-year trial, patients in the placebo group showed a linear decline in functional status consistent with the natural history of the disease. Those treated with aminoacids showed significant benefit in terms of maintenance of extremity muscle strength and continued ability to walk.

Amino Acids, Branched-Chain↗
PubMed1988

The neuroexcitotoxic amino acids glutamate and aspartate are altered in the spinal cord and brain in amyotrophic lateral sclerosis.

Because recent studies showed a systemic defect in glutamate metabolism in amyotrophic lateral sclerosis (ALS), we measured the levels of free amino acids in frontal and cerebellar cortex and two areas of spinal cord obtained at autopsy from 22 patients who died of this disease. Glutamate levels were significantly decreased (by 21 to 40% of control values) in all areas investigated; cervical and lumbar spinal cord showed the greatest change. Aspartate levels were also significantly reduced (by 32 to 35%) in the spinal cord only. A positive correlation was shown between the changes of glutamate and aspartate as well as a significant alteration in the glutamate to glutamine ratio in the spinal cord of patients with ALS. Although we cannot exclude the possibility that these abnormalities may partly result from neuronal cell loss, the data suggest the presence of a generalized defect that may affect the neurotransmitter and metabolic pool of glutamate. The defect may be expressed more severely in the spinal cord than in other central nervous system areas. These results, taken together with the previously shown systemic abnormality, raise the possibility that distribution of glutamate between the intracellular and extracellular pool may be altered in ALS and may mediate the neurodegeneration.

Aged↗
PubMed1988

Isolation and characterization of cDNA clones encoding human liver glutamate dehydrogenase: evidence for a small gene family.

We have isolated a series of human liver cDNA clones encoding glutamate dehydrogenase. The cDNA-derived protein sequence specifies a single 558-amino acid long polypeptide including a cleavable signal sequence of 53 amino acids. Blotting analysis of RNA from human, monkey, and rabbit showed that glutamate dehydrogenase mRNA is present in various amounts in all tissues tested. Glutamate dehydrogenase mRNAs are of four sizes and are found in different ratios in different tissues; the predominant ones are approximately 3.5 and approximately 2.9 kilobases. Blot hybridization of human genomic DNA to nonoverlapping cDNA fragments revealed multiple bands, many of which hybridize with two or more probes in a manner inconsistent with the existence of a single GLUD gene. Moreover, two separate 36-base synthetic oligonucleotides corresponding to the coding region hybridize to multiple genomic fragments, confirming the existence of more than one GLUD-related gene in human.

Adult↗
PubMed1987

Abnormal glutamate metabolism in amyotrophic lateral sclerosis.

Glutamate levels were determined in the fasting plasma of 22 patients with early-stage primary amyotrophic lateral sclerosis (ALS) and compared to those of healthy and diseased controls. There was a significant increase (by approximately 100%, p less than 0.0005) in the plasma glutamate of the ALS patients as compared with the controls. Oral glutamate loading (60 mg of monosodium glutamate per kilogram of body weight, taken orally after overnight fasting) resulted in significantly greater elevations in the plasma glutamate and aspartate levels in the ALS patients than in the controls. Glutamate, a potentially neuroexcitotoxic compound, is thought to be the transmitter of the corticospinal tracts and certain spinal cord interneurons. A systemic defect in the metabolism of this amino acid may underlie primary ALS.

Administration, Oral↗