Distinguishing the cyanobacterial neurotoxin β-N-methylamino-L-alanine (BMAA) from other diamino acids

β-N-methylamino-L-alanine (BMAA) is produced by diverse taxa of cyanobacteria, and has been detected by many investigators who have searched for it in cyanobacterial blooms, cultures and collections. Although BMAA is distinguishable from proteinogenic amino acids and its isomer 2,4-DAB using standard chromatographic and mass spectroscopy techniques routinely used for the analysis of amino acids, we studied whether BMAA could be reliably distinguished from other diamino acids, particularly 2,6-diaminopimelic acid which has been isolated from the cell walls of many bacterial species. We used HPLC-FD, UHPLC-UV, UHPLC-MS, and triple quadrupole tandem mass spectrometry (UHPLC-MS/MS) to differentiate BMAA from the diamino acids 2,6-diaminopimelic acid, N-2(amino)ethylglycine, lysine, ornithine, 2,4-diaminosuccinic acid, homocystine, cystine, tryptophan, as well as other amino acids including asparagine, glutamine, and methionine methylsulfonium.

Discovery and partial characterization of primate motor-system toxins

beta-N-Oxalylamino-L-alanine (BOAA) and beta-N-methylamino-L-alanine (BMAA) are chemically related excitant amino acids isolated from the seed of Lathyrus sativus (BOAA) and Cycas circinalis (BMAA), consumption of which has been linked to lathyrism (an upper motor neuron disorder) and Guam amyotrophic lateral sclerosis (ALS), respectively. Both diseases are associated with degeneration of motor neurons. Experimentally, single doses of BOAA or BMAA induce seizures in neonatal mice and postsynaptic neuronal oedema and degeneration in explants of mouse spinal cord and frontal cortex. Preliminary studies show that these behavioural and pathological effects are differentially blocked by glutamate-receptor antagonists. In macaques, several weeks of daily oral doses of BOAA produce clinical and electrophysiological signs of corticospinal dysfunction identical to those seen in comparably well-nourished animals receiving a fortified diet based on seed of Lathyrus sativus. By contrast, comparable oral dosing with BMAA precipitates tremor and weakness, bradykinesia and behavioural changes, with conduction deficits in the principal motor pathway. BOAA and BMAA (or a metabolite thereof) are the first members of the excitotoxin family to have been shown to possess chronic motor-system toxic potential. These observations provide a rational basis for searching for comparable endogenous neurotoxins in sporadic and inherited forms of human motor neuron disease.

Neuroprotective effect of free radical scavengers on beta-N-oxalylamino-L-alanine (BOAA)-induced neuronal damage in rat hippocampus

The neurotoxin beta-N-oxalylamino-L-alanine (BOAA), found in Lathyrus sativus seeds, is thought to be the causative agent of neurolathyrism. We have investigated the neuroprotective effects of free radical scavengers on BOAA-induced toxicity following focal injection (1 microliter) of BOAA and comparing the pathological outcome with the effects of injections of alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionate (AMPA), kainate (KA) or N-methyl-D-aspartate (NMDA) into the dorsal hippocampus of male Wistar rats. Cellular damage was assessed histologically. BOAA (50 nmol) induced a highly selective pattern of hippocampal damage identical with that seen with AMPA (1 nmol). BOAA-induced neurotoxicity, but not AMPA, KA (0.5 nmol) or NMDA (25 nmol)-induced neurotoxicity, was prevented in a dose-dependent manner by focal co-injection of four potential free radical scavengers; dimethyl sulphoxide (DMSO) (1750-7000 nmol), dimethylthiourea (DMTU) (8000 nmol), dimethylformamide (DMF) (7000 nmol) and mannitol (1000 nmol). These findings suggest that hippocampal damage induced by BOAA involves an interaction between AMPA receptors and free radicals.

Neuronal damage induced by beta-N-oxalylamino-L-alanine, in the rat hippocampus, can be prevented by a non-NMDA antagonist, 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(F)quinoxaline

The neurotoxin beta-N-oxalylamino-L-alanine (BOAA), found in Lathyrus sativus seeds, is thought to be the causative agent of neurolathyrism. We have investigated the in vivo mechanism of action of BOAA by focal injection (1 microliter) in the dorsal hippocampus of male Wistar rats and comparing the pathological outcome with the effects of injections (1 microliter) of alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionate (AMPA), kainate (KA) or N-methyl-D-aspartate (NMDA). Cellular damage induced by the excitatory amino acids in the pyramidal (CA1-CA4) and dentate granule neurones (DG) was assessed histologically 24 h after the injection. The study shows that BOAA (50 nmol) induces hippocampal toxicity with a highly selective pattern of regional cellular damage. The CA1, CA4 and DG subfields show 70-90% neuronal injury whereas CA2 and CA3 show only minimal damage. This pattern of cellular damage is similar to that induced by AMPA (1 nmol) and NMDA (25 nmol) but not KA (0.5 nmol). BOAA-induced neurotoxicity is prevented in a dose-dependent manner by focal co-injection of the non-NMDA receptor antagonist 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(F)quinoxaline (NBQX) (1-25 nmol) but not by a dose of MK-801 (3 mg/kg i.p.) which is neuroprotective against an injection of NMDA. Delayed focal injections of NBQX (25 nmol) up to 2 h after the BOAA injection result in a significant protection of all pyramidal and granular cell regions. These results indicate that the in vivo hippocampal toxicity of BOAA is mediated by AMPA receptors rather than by KA or NMDA receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Induction of astrocyte glutamine synthetase activity by the Lathyrus toxin beta-N-oxalyl-L-alpha,beta-diaminopropionic acid (beta-L-ODAP)

beta-N-Oxalyl-L-alpha,beta-diaminopropionic acid (beta-L-ODAP) is thought to be the causative agent in lathyrism due to its neuroexcitatory and neurotoxic properties. We have recently reported that beta-L-ODAP is also gliotoxic at high concentrations (Bridges et al.: Brain Res 561:262, 1991). Evidence is now presented that low, subgliotoxic concentrations of beta-L-ODAP may alter the ability of astrocytes to regulate glutamate concentrations in the CNS by increasing astrocyte glutamine synthetase activity. When astrocytes cultured from rat cortex were exposed to 100 microM beta-L-ODAP for 24 h, the resulting glutamine synthetase activity was 155% of control levels. This effect was enantiomer- and isomer-specific, dose-dependent, and required protein translation as the induction was blocked with cycloheximide. The effect of beta-L-ODAP on glutamine synthetase was not mimicked by alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionate (AMPA) or kainate, suggesting that the induction was not transduced solely through activation of cell surface non-N-methyl-D-aspartate (NMDA) glutamate receptors. An intracellular site of action of beta-L-ODAP is proposed because its effect on glutamine synthetase activity could be blocked by the amino acid uptake blocker dihydrokainate.

Gliotoxic properties of the Lathyrus excitotoxin beta-N-oxalyl-L-alpha,beta-diaminopropionic acid (beta-L-ODAP)

beta-N-Oxalyl-L-alpha,beta-diaminopropionic acid (beta-L-ODAP) is an excitatory amino acid agonist found in the seeds of Lathyrus sativus that is believed to be the major causative agent in the pathology of human lathyrism. We have found that in addition to its previously recognized neurotoxic properties, beta-L-ODAP is also gliotoxic. When added to cultures of neonatal rat astrocytes, beta-L-ODAP induced a series of morphological changes (e.g., extensive vacuole formation, pale and swollen nuclei with obvious nucleoli, and cellular swelling) that led to the eventual lysis of the glial cells. If the beta-L-ODAP was removed prior to the lysis of the astrocytes, many of the early morphological changes appeared to be reversible. When quantitated by a loss of the lactate dehydrogenase activity, beta-L-ODAP lysed the astrocytes with an LD50 of 2.1 +/- 0.2 mM following 48 h of exposure. Lower concentrations of beta-L-ODAP were found to be more toxic if the duration of the exposure was increased. The results suggest that the overall impact of the toxin on the CNS may represent the cumulative action of beta-L-ODAP at a number of distinct points on both neurons and astrocytes. The potential that these multiple sites of action may affect the normal regulation of extracellular glutamate and, consequently, disturb the balance between its normal and pathological roles is discussed.

Complexes of zinc, copper, and nickel with the nonprotein amino acid L-alpha-amino-beta-methylaminopropionic acid: a naturally occurring neurotoxin

The non-protein amino acid L-alpha-amino-beta-methylaminopropionic acid (L-MeDAP) causes motor neuron dysfunction in macaques. The amino acid is a potent chelator of divalent metal ions such as copper and zinc. Binding constants nickel(II) for copper(II) and zinc(II) with L-MeDAP have been measured. Some copper(II) complexes of L-MeDAP have been synthesized and characterized by their electronic, infrared, and epr spectra. These results are used to comment on the possibility that metal ion complexation in the central nervous system is involved in the expression of the meurotoxicity of L-MeDAP.

The interaction of beta-N-methylamino-L-alanine with bicarbonate: an 1H-NMR study

The mode of action of the neurotoxic, non-protein amino acid beta-N-methylamino-L-alanine (L-BMAA) is unknown. We have shown, using 1H-NMR spectroscopy, that L-BMAA forms a stable adduct with bicarbonate (probably a carbamate). The properties of this adduct may explain the observation that L-BMAA and N-methyl-D-aspartic acid appear to act at the same central nervous system receptors.

Structure-function studies on N-oxalyl-diamino-dicarboxylic acids and excitatory amino acid receptors: evidence that beta-L-ODAP is a selective non-NMDA agonist

Excitatory amino acids and their receptors play an important role in both normal synaptic transmission and excitotoxic-mediated neuronal death. In the present investigation we have prepared a series of glutamate analogs and examined the pharmacological specificity with which they interact with excitatory amino acid receptors. Included within this group of compounds is a potent excitotoxic amino acid, beta-N-oxalyl-L-alpha, beta-diaminopropionic acid (beta-L-ODAP). This excitotoxin is of particular interest because it has been identified as a major causative agent of human neurolathyrism, a disease characterized by permanent spastic paralysis. The site of action of beta-L-ODAP was delineated with both electrophysiological recordings in hippocampal slices and radioligand binding assays in synaptic plasma membranes. We report that beta-L-ODAP is a potent agonist at the non-N-methyl-D-aspartate (NMDA) type of excitatory amino acid receptor. beta-L-ODAP interacts most potently with the quisqualate class of non-NMDA receptors (IC50 = 1.3 microM), less potently with the kainate receptor (IC50 = 17 microM), and very weakly with NMDA receptors. The specificity of this binding was consistent with physiological experiments that demonstrated that beta-L-ODAP-induced depolarizations were potently blocked by the newly identified non-NMDA receptor antagonist, CNQX, but were not affected by the NMDA antagonist D-AP5. These results extend recent studies that have focused on the contribution of NMDA receptors to excitotoxicity and highlight the potential involvement of non-NMDA receptors in excitotoxic-mediated cell death.

Guam amyotrophic lateral sclerosis-parkinsonism-dementia linked to a plant excitant neurotoxin

The decline in the high incidence of amyotrophic lateral sclerosis, parkinsonism, and Alzheimer-type dementia among the Chamorro population of the western Pacific islands of Guam and Rota, coupled with the absence of demonstrable viral and hereditable factors in this disease, suggests the gradual disappearance of an environmental factor selectively associated with this culture. One candidate is seed of the neurotoxic plant Cycas circinalis L., a traditional source of food and medicine which has been used less with the Americanization of the Chamorro people after World War II. Macaques were fed the Cycas amino acid beta-N-methylamino-L-alanine, a low-potency convulsant that has excitotoxic activity in mouse brain, which is attenuated by N-methyl-D-aspartate receptor antagonists. These animals developed corticomoto-neuronal dysfunction, parkinsonian features, and behavioral anomalies, with chromatolytic and degenerative changes of motor neurons in cerebral cortex and spinal cord. In concert with existing epidemiological and animal data, these findings support the hypothesis that cycad exposure plays an important role in the etiology of the Guam disease.

Stereospecific acute neuronotoxicity of 'uncommon' plant amino acids linked to human motor-system diseases

The L-isomer of beta-N-methylamino-L-alanine (BMAA), present in free form in seed of Cycas circinalis, elicits in spinal cord cultures a pattern of acute postsynaptic neuronal vacuolation comparable to that induced by beta-N-oxalylamino-L-alanine (BOAA), an excitotoxic amino acid of greater potency isolated from seed of Lathyrus sativus. The neuronotoxic properties of these compounds may be linked to the etiology of motor-system degenerative disorders (amyotrophic lateral sclerosis and lathyrism, respectively) found in human groups that have used these plant seeds for food.

Comparative toxicities of alpha- and beta-N-oxalyl-L-alpha, beta-diaminopropionic acids to rat spinal cord

alpha- and beta-N-oxalyl-L-alpha, beta-diaminopropionic acids, isolated from Lathyrus sativus seed, which is implicated in the pathogenesis of human neurolathyrism, were injected into rat lumbar cerebrospinal fluid. The alpha-isomer was neither acutely nor chronically toxic, whereas the beta-isomer was both an acute and a chronic neurotoxin in this test system. These results are relevant to the aetiology of neurolathyrism and suggest that the conversion of the beta- to the alpha-isomer that occurs during boiling of the seed prior to human consumption reduces the neurotoxicity of the seed.

Formation of glycine and aminoacetone from L-threonine by rat liver mitochondria

Threonine is a precursor of glycine in the rat, but the metabolic pathway involved is unclear. To elucidate this pathway, the biosynthesis of glycine, and of aminoacetone, from L-threonine were studied in rat liver mitochondrial preparations of differing integrities. In the absence of added cofactors, intact mitochondria formed glycine and aminoacetone in approximately equal amounts from 20 mM L-threonine, but exogenous NAD+ decreased and CoA increased the ratio of glycine to aminoacetone formed. In intact and freeze-thawed mitochondria, the ratio of glycine to aminoacetone formed was markedly sensitive to the concentration of L-threonine, glycine being the major product at low L-threonine concentrations. Disruption of mitochondrial integrity by sonication (1 min) decreased the ratio of glycine to aminoacetone formed, and in 20000 X g supernatant fractions from sonicated (3 min) mitochondria, aminoacetone was the major product. The main non-nitogenous two-carbon compound detected when intact mitochondria catabolized L-threonine to glycine was acetate, which was probably derived from deacylation of acetyl-CoA. These results suggest that glycine formation from L-threonine in rat liver mitochondria occurred primarily by the coupled activities of threonine dehydrogenase and 2-amino-3-oxobutyrate CoA-ligase, the extent of coupling between the enzymes being dependent upon a close physical relationship and upon the flux through the dehydrogenase reaction. In vivo glycine synthesis would predominate, and aminoacetone would be a minor product.

Metabolic homoeostasis of L-threonine in the normally-fed rat. Importance of liver threonine dehydrogenase activity

Threonine dehydratase, threonine aldolase and threonine dehydrogenase activities were assayed in livers of rats that had been normally-fed, starved for 72 h, fed a high-protein diet or normally-fed and injected with glucagon or cortisone. A modified continuous spectrophotometric assay for threonine aldolase overcame interference resulting from threonine dehydratase activity and revealed that threonine aldolase activity was very low in rat liver, irrespective of the metabolic state of the animal. The concentration of free threonine was determined in livers of animals subjected to the same treatments as described above. Using Michaelis-Menten kinetics to estimate enzyme activities in vivo at intracellular threonine concentrations it was calculated that in the normally-fed state, 87% of the threonine degraded was catabolized by threonine dehydrogenase. In other metabolic states (except in glucagon-treated animals) threonine dehydratase was the major enzyme catalysing threonine catabolism. It was concluded that threonine dehydrogenase activity plays a hitherto unrecognized role in the metabolic homoeostasis of threonine in the normally-fed rat and that this enzyme activity, in association with 2-amino-3-oxobutyrate CoA-ligase, accounts for the known rate of glycine formation from threonine in the rat.

The neurolathyrogen, beta-N-oxalyl-L-alpha,beta-diaminopropionic acid, is a potent agonist at 'glutamate preferring' receptors in the frog spinal cord

A neurotoxic amino acid, beta-N-oxalyl-L-alpha,beta-diaminopropionate (beta-ODAP), found in seeds of Lathyrus sativus and a possible causative agent of neurolathyrism, was equipotent with kainate as a depolarizing agent of frog spinal cord ventral roots. beta-ODAP and kainate appeared to act on a common receptor, as their actions could not be differentiated pharmacologically. These results could explain some of the symptoms of neurolathyrism.

High K+ content explains the abolition of the action potential in amphibian sciatic nerve in vitro by Lathyrus sativus seed extract

The action potential in amphibian sciatic nerve in vitro has been reported to be abolished by the topical application of Lathyrus sativus seed extract. We have confirmed this effect, but find that it is probably caused by the high K+ content of such seed extracts and that organic neurotoxins are not implicated.