Specific antagonism of excitotoxic action of 'uncommon' amino acids assayed in organotypic mouse cortical cultures

Cycads: A comprehensive review of its botany, traditional uses, phytochemistry, pharmacology and toxicology

Cycads, which primarily consist of the families Cycadaceae and Zamiaceae, possess intrinsic therapeutic attributes that are prominently expressed across their morphological spectrum, including roots, leaves, flowers, and seeds. In Chinese traditional medicine, the leaves of cycads are particularly revered for their profound healing capabilities. This meticulous review engages with existing literature on cycads and presents insightful avenues for future research. Over 210 phytoconstituents have been isolated and identified from various cycad tissues, including flavonoids, azoxy metabolites, sterols, lignans, non-proteogenic amino acids, terpenoids, and other organic constituents. The contemporary pharmacological discourse highlights the antineoplastic, antimicrobial, and antidiabetic activities inherent in these ancient plants, which are of particular importance to the field of oncology. Despite the prevalent focus on crude extracts and total flavonoid content, our understanding of the nuanced pharmacodynamics of cycads lags considerably behind. The notoriety of cycads derived toxicity, notably within the context of Guam's neurological disease cluster, has precipitated an established emphasis on toxicological research within this field. As such, this critical review emphasizes nascent domains deserving of academic and clinical pursuit, whilst nested within the broader matrix of current scientific understanding. The systematic taxonomy, traditional applications, phytochemical composition, therapeutic potential, and safety profile of cycads are holistically interrogated, assimilating an indispensable repository for future scholarly inquiries. In conclusion, cycads stand as a veritable treasure trove of pharmacological virtue, displaying remarkable therapeutic prowess and holding vast promise for ongoing scientific discovery and clinical utilization.

Uptake of β-N-methylamino-L-alanine (BMAA) into glutamate-specific synaptic vesicles: Exploring the validity of the excitotoxicity mechanism of BMAA

The first mechanism of toxicity proposed for the cyanobacterial neurotoxin β-N-methylamino-L-alanine (BMAA) was excitotoxicity, and this was supported by numerous in vitro studies in which overactivation of both ionotropic and metabotropic glutamate receptors was reported. However, the excitotoxicity of BMAA is weak in comparison with other known excitotoxins and on par with that of glutamate, implying that to achieve sufficient synaptic concentrations of BMAA to cause classical in vivo excitotoxicity, BMAA must either accumulate in synapses to allow persistent glutamate receptor activation or it must be released in sufficiently high concentrations into synapses to cause the overexcitation. Since it has been shown that BMAA can be readily removed from synapses, release of high concentrations of BMAA into synapses must be shown to confirm its role as an excitotoxin in in vivo systems. This study therefore sought to evaluate the uptake of BMAA into synaptic vesicles and to determine if BMAA affects the uptake of glutamate into synaptic vesicles. There was no evidence to support uptake of BMAA into glutamate-specific synaptic vesicles but there was some indication that BMAA may affect the uptake of glutamate into synaptic vesicles. The uptake of BMAA into synaptic vesicles isolated from areas other than the cerebral cortex should be investigated before definite conclusions can be drawn about the role of BMAA as an excitotoxin.

Chronic BMAA exposure combined with TDP-43 mutation elicits motor neuron dysfunction phenotypes in mice

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease with an average age-of-onset of ∼60 years and is usually fatal within 2-5 years of diagnosis. Mouse models based upon single gene mutations do not recapitulate all ALS pathological features. Environmental insults may also contribute to ALS, and β-N-methylamino-L-alanine (BMAA) is an environmental toxin linked with an increased risk of developing ALS. BMAA, along with cycasin, are hypothesized to be the cause of the Guam-ALS epicenter of the 1950s. We developed a multihit model based on low expression of a dominant familial ALS TDP-43 mutation (Q331K) and chronic low-dose BMAA exposure. Our two-hit mouse model displayed a motor phenotype absent from either lesion alone. By LC/MS analysis, free BMAA was confirmed at trace levels in brain, and were as high as 405 ng/mL (free) and 208 ng/mL (protein-bound) in liver. Elevated BMAA levels in liver were associated with dysregulation of the unfolded protein response (UPR) pathway. Our data represent initial steps towards an ALS mouse model resulting from combined genetic and environmental insult.

Environmental Neurotoxin β-N-Methylamino-L-alanine (BMAA) as a Widely Occurring Putative Pathogenic Factor in Neurodegenerative Diseases

In the present review we have discussed the occurrence of β-N-methylamino-L-alanine (BMAA) and its natural isomers, and the organisms and sample types in which the toxin(s) have been detected. Further, the review discusses general pathogenic mechanisms of neurodegenerative diseases, and how modes of action of BMAA fit in those mechanisms. The biogeography of BMAA occurrence presented here contributes to the planning of epidemiological research based on the geographical distribution of BMAA and human exposure. Analysis of BMAA mechanisms in relation to pathogenic processes of neurodegeneration is used to critically assess the potential significance of the amino acid as well as to identify gaps in our understanding. Taken together, these two approaches provide the basis for the discussion on the potential role of BMAA as a secondary factor in neurodegenerative diseases, the rationale for further research and possible directions the research can take, which are outlined in the conclusions.

Neuropathological Mechanisms of β-N-Methylamino-L-Alanine (BMAA) with a Focus on Iron Overload and Ferroptosis

The incidence of neurodegenerative diseases and cyanobacterial blooms is concomitantly increasing worldwide. The cyanotoxin β-N-methylamino-L-alanine (BMAA) is produced by most of the Cyanobacteria spp. This cyanotoxin is described as a potential environmental etiology factor for some sporadic neurodegenerative diseases. Climate change and eutrophication significantly increase the frequency and intensity of cyanobacterial bloom in water bodies. This review evaluates different neuropathological mechanisms of BMAA at molecular and cellular levels and compares the related studies to provide some useful recommendations. Additionally, the structure and properties of BMAA as well as its microbial origin, especially by gut bacteria, are also briefly covered. Unlike previous reviews, we hypothesize the possible neurotoxic mechanism of BMAA through iron overload. We also discuss the involvement of BMAA in excitotoxicity, TAR DNA-binding protein 43 (TDP-43) translocation and accumulation, tauopathy, and other protein misincorporation and misfolding.

Western Pacific ALS-PDC: Evidence implicating cycad genotoxins

Amyotrophic Lateral Sclerosis and Parkinsonism-Dementia Complex (ALS-PDC) is a disappearing neurodegenerative disorder of apparent environmental origin formerly hyperendemic among Chamorros of Guam-USA, Japanese residents of the Kii Peninsula, Honshu Island, Japan and Auyu-Jakai linguistic groups of Papua-Indonesia on the island of New Guinea. The most plausible etiology is exposure to genotoxins in seed of neurotoxic cycad plants formerly used for food and/or medicine. Primary suspicion falls on methylazoxymethanol (MAM), the aglycone of cycasin and on the non-protein amino acid β-N-methylamino-L-alanine, both of which are metabolized to formaldehyde. Human and animal studies suggest: (a) exposures occurred early in life and sometimes during late fetal brain development, (b) clinical expression of neurodegenerative disease appeared years or decades later, and (c) pathological changes in various tissues indicate the disease was not confined to the CNS. Experimental evidence points to toxic molecular mechanisms involving DNA damage, epigenetic changes, transcriptional mutagenesis, neuronal cell-cycle reactivation and perturbation of the ubiquitin-proteasome system that led to polyproteinopathy and culminated in neuronal degeneration. Lessons learned from research on ALS-PDC include: (a) familial disease may reflect common toxic exposures across generations, (b) primary disease prevention follows cessation of exposure to culpable environmental triggers; and (c) disease latency provides a prolonged period during which to intervene therapeutically. Exposure to genotoxic chemicals ("slow toxins") in the early stages of life should be considered in the search for the etiology of ALS-PDC-related neurodegenerative disorders, including sporadic forms of ALS, progressive supranuclear palsy and Alzheimer's disease.

In Vitro Testing of Neurotoxicity

Many of the toxic compounds that are at large in the environment represent a risk to our neuronal functions. Chemicals may have a direct or indirect effect on the nervous system and they may interfere with general biochemical properties or specific neuronal structures and processes. In this review, a brief presentation of the major neurotoxicological targets is given, together with a discussion of some aspects of the use of different in vitro models for screening purposes and mechanistic studies. It is believed that in vitro methods offer special opportunities for the development of new neurotoxicological assays, and that this development will mainly involve cultured model systems. Therefore, a presentation of nerve and glia tissue culture methods is given, followed by an overview of how information on the action of mercury and mercurials, excitotoxins and acrylamide has been obtained through the use of cultured cell models. It is concluded that the developmental potential in cell neurotoxicology lies within the areas of separation and identification of cells representative for different structures in the nervous system, co-cultivation of different cell types, in vivo/in vitro (ex vivo) procedures, chemically defined media, metabolic competent cultures of human cells and improved physiological conditions for cultivation and exposure.

β-N-methylamino-L-alanine (BMAA) suppresses cell cycle progression of non-neuronal cells

β-N-methylamino-L-alanine (BMAA), a natural non-proteinaceous amino acid, is a neurotoxin produced by a wide range of cyanobacteria living in various environments. BMAA is a candidate environmental risk factor for neurodegenerative diseases such as amyotrophic lateral sclerosis and Parkinson-dementia complex. Although BMAA is known to exhibit weak neuronal excitotoxicity via glutamate receptors, the underlying mechanism of toxicity has yet to be fully elucidated. To examine the glutamate receptor-independent toxicity of BMAA, we investigated the effects of BMAA in non-neuronal cell lines. BMAA potently suppressed the cell cycle progression of NIH3T3 cells at the G1/S checkpoint without inducing plasma membrane damage, apoptosis, or overproduction of reactive oxygen species, which were previously reported for neurons and neuroblastoma cells treated with BMAA. We found no evidence that activation of glutamate receptors was involved in the suppression of the G1/S transition by BMAA. Our results indicate that BMAA affects cellular functions, such as the division of non-neuronal cells, through glutamate receptor-independent mechanisms.

Investigating β-N-Methylamino-l-alanine Misincorporation in Human Cell Cultures: A Comparative Study with Known Amino Acid Analogues

Misincorporation of β-N-methylamino-l-alanine (BMAA) into proteins has been proposed to be a mechanism of toxicity to explain the role of BMAA in neurodegenerative disease development. However, studies have shown that all detectable BMAA can be removed from proteins by SDS-PAGE purification and that the toxicity of l-canavanine cannot be reproduced in prokaryotes or in a rat pheochromocytoma cell line, strongly indicating that the misincorporation hypothesis of BMAA should be re-investigated. The aim of this study was therefore to determine if BMAA misincorporates into proteins in cells of human origin with subsequent misincorporation-type toxicity. Almost complete loss of viability in response to exposure to l-4-fluorophenylalanine and l-m-tyrosine was observed in all of the cell lines, corresponding to a concentration-dependent increase of the analogues in protein extracts from exposed cells. In contrast, BMAA exposure resulted in slight toxicity in one of the cell lines but the observed toxicity was not the result of misincorporation of BMAA into proteins, as no BMAA was detected in any of the SDS-PAGE purified protein extracts that were obtained from the cells following BMAA exposure. The results show that BMAA is not misincorporated into human proteins and that misincorporation is not a valid mechanism of toxicity.

50 years of research on α-amino-β-methylaminopropionic acid (β-methylaminoalanine)

The isolation of α-amino-β-methylaminopropionic acid from seeds of Cycas circinalis (now C. micronesica Hill) resulted from a purposeful attempt to establish the cause of the profound neurological disease, amyotrophic lateral sclerosis/parkinsonism/dementia, that existed in high frequency amongst the inhabitants of the western Pacific island of Guam (Guam ALS/PD). In the 50 years since its discovery the amino acid has been a stimulus, and sometimes a subject of mockery, for generations of scientists in a remarkably diverse range of subject areas. The number of citations of the original paper has risen in the five decades from a few to 120 within the decade 2007-2016 and continues at a high rate into the next decade. The reasons for this remarkable outcome are discussed and examples from the literature are used to illustrate the wide range of scientific interest that the original paper generated.

Neurotoxicity of the Cyanotoxin BMAA Through Axonal Degeneration and Intercellular Spreading

β-Methylamino-L-alanine (BMAA) is implicated in neurodegeneration and neurotoxicity, particularly in ALS-Parkinson Dementia Complex. Neurotoxic properties of BMAA have been partly elucidated, while its transcellular spreading capacity has not been examined. Using reconstructed neuronal networks in microfluidic chips, separating neuronal cells into two subcompartments-(1) the proximal, containing first-order neuronal soma and dendrites, and (2) a distal compartment, containing either only axons originating from first-order neurons or second-order striatal neurons-creates a cortico-striatal network. Using this system, we investigated the toxicity and spreading of BMAA in murine primary neurons. We used a newly developed antibody to detect BMAA in cells. After treatment with 10 μM BMAA, the cyanotoxin was incorporated in first-degree neurons. We also observed a rapid trans-neuronal spread of BMAA to unexposed second-degree neurons in 48 h, followed by axonal degeneration, with limited somatic death. This in vitro study demonstrates BMAA axonal toxicity at sublethal concentrations and, for the first time, the transcellular spreading abilities of BMAA. This neuronal dying forward spread that could possibly be associated with progression of some neurodegenerative diseases especially amyotrophic lateral sclerosis.

Methods for the Chemical Analysis of β-N-Methylamino-L-A lanine: What Is Known and What Remains to Be Determined

β-N-Methylamino-L-alanine (BMAA) is a non-canonical amino acid implicated as a cause for amyotrophic lateral sclerosis/parkinsonism dementia complex and potentially other neurodegenerative diseases. As interest in this molecule has increased, there has been a proliferation of methods along with a plethora of opinions as to the superiority of some methods over others. We analyzed the literature with reference to BMAA and its naturally occurring isomers, N-(2-aminoethyl) glycine (AEG) and 2,4 diaminobutyric acid (DAB). A comparison of methods, results, and critiques reveal that a single method has been approved by the AOAC but several different methods provide comparable BMAA quantification concentrations in similar tissues. We also describe a productive way to move forward as technology improves and changes.

Transport of BMAA into Neurons and Astrocytes by System xc

The study of the mechanism of β-N-methylamino-L-alanine (BMAA) neurotoxicity originally focused on its effects at the N-methyl-D-aspartate (NMDA) receptor. In recent years, it has become clear that its mechanism of action is more complicated. First, there are certain cell types, such as motor neurons and cholinergic neurons, where the dominate mechanism of toxicity is through action at AMPA receptors. Second, even in cortical neurons where the primary mechanism of toxicity appears to be activation of NMDA receptors, there are other mechanisms involved. We found that along with NMDA receptors, activation of mGLuR5 receptors and effects on the cystine/glutamate antiporter (system x_c-) were involved in the toxicity. The effects on system x_c- are of particular interest. System x_c- mediates the transport of cystine into the cell in exchange for releasing glutamate into the extracellular fluid. By releasing glutamate, system x_c- can potentially cause excitotoxicity. However, through providing cystine to the cell, it regulates the levels of cellular glutathione (GSH), the main endogenous intracellular antioxidant, and in this way may protect cells against oxidative stress. We have previously published that BMAA inhibits cystine uptake leading to GSH depletion and had indirect evidence that BMAA is transported into the cells by system x_c-. We now present direct evidence that BMAA is transported into both astrocytes and neurons through system x_c-. The fact that BMAA is transported by system x_c- also provides a mechanism for BMAA to enter brain cells potentially leading to misincorporation into proteins and protein misfolding.

Quantitative proteomics analysis of zebrafish exposed to sub-lethal dosages of β-methyl-amino-L-alanine (BMAA)

The non-protein amino acid β-methylamino-L-alanine (BMAA) is a neurotoxin present in microalgae and shown to accumulate in the food web. BMAA has been linked to the complex neurodegenerative disorder of Guam and to increased incidents sporadic ALS. Two main neurotoxic routes are suggested; an excitotoxic by acting as an agonist towards glutamate receptors and a metabolic by misincorporating into cellular proteins. We have used zebrafish, an increasingly used model for neurodegenerative diseases, to further identify signaling components involved in BMAA-induced toxicity. Zebrafish embryos were exposed to sub-lethal dosages of BMAA and a label-free proteomics analysis was conducted on larvae 4 days post fertilization. The exposed larvae showed no developmental abnormalities, but a reduced heart rate and increased expression of GSK3 isoforms. Search towards a reviewed database containing 2968 entries identified 480 proteins. Only 17 of these were regulated 2-fold or more in the exposed larvae. Seven of these proteins could be associated to glutamate receptor signaling and recycling. The remaining nine have all been linked to disturbance in protein homeostasis, reactive oxygen species (ROS) development or neuronal cell death. We also found that BMAA influenced the endocannabinoid system by up-regulation of fatty acid amide hydrolase (FAAH) and that FAAH inhibitor URB597 reduced the BMAA effect on heart rate and GSK3 expression.

BMAA extraction of cyanobacteria samples: which method to choose?

β-N-Methylamino-L-alanine (BMAA), a neurotoxin reportedly produced by cyanobacteria, diatoms and dinoflagellates, is proposed to be linked to the development of neurological diseases. BMAA has been found in aquatic and terrestrial ecosystems worldwide, both in its phytoplankton producers and in several invertebrate and vertebrate organisms that bioaccumulate it. LC-MS/MS is the most frequently used analytical technique in BMAA research due to its high selectivity, though consensus is lacking as to the best extraction method to apply. This study accordingly surveys the efficiency of three extraction methods regularly used in BMAA research to extract BMAA from cyanobacteria samples. The results obtained provide insights into possible reasons for the BMAA concentration discrepancies in previous publications. In addition and according to the method validation guidelines for analysing cyanotoxins, the TCA protein precipitation method, followed by AQC derivatization and LC-MS/MS analysis, is now validated for extracting protein-bound (after protein hydrolysis) and free BMAA from cyanobacteria matrix. BMAA biological variability was also tested through the extraction of diatom and cyanobacteria species, revealing a high variance in BMAA levels (0.0080-2.5797 μg g(-1) DW).

Acute β-N-Methylamino-L-alanine Toxicity in a Mouse Model

The cyanobacterial neurotoxin β-N-methylamino-L-alanine (BMAA) is considered to be an “excitotoxin,” and its suggested mechanism of action is killing neurons. Long-term exposure to L-BMAA is believed to lead to neurodegenerative diseases including Parkinson's and Alzheimer's diseases and amyotrophic lateral sclerosis (Lou Gehrig's disease). Objectives of this study were to determine the presumptive median lethal dose (LD₅₀), the Lowest-Observed-Adverse-Effect Level (LOAEL), and histopathologic lesions caused by the naturally occurring BMAA isomer, L-BMAA, in mice. Seventy NIH Swiss Outbred mice (35 male and 35 female) were used. Treatment group mice were injected intraperitoneally with 0.03, 0.3, 1, 2, and 3 mg/g body weight L-BMAA, respectively, and control mice were sham-injected. The presumptive LD₅₀ of L-BMAA was 3 mg/g BW and the LOAEL was 2 mg/g BW. There were no histopathologic lesions in brain, liver, heart, kidney, lung, or spleen in any of the mice during the 14-day study. L-BMAA was detected in brains and livers in all of treated mice but not in control mice. Males injected with 0.03 mg/g BW, 0.3 mg/g BW, and 3.0 mg/g BW L-BMAA showed consistently higher concentrations (P < 0.01) in brain and liver samples as compared to females in those respective groups.

Morphometric and neurochemical alterations found in l-BMAA treated rats

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease characterized by progressive muscle paralysis that reflects the motoneurons' degeneration. Several studies support the relationship between β-N-methylamino-l-alanine (l-BMAA), a neurotoxic amino acid produced by cyanobacteria and diatoms, and the sporadic occurrence of ALS and other neurodegenerative diseases. Therefore, the study of its neurotoxicity mechanisms has assumed great relevance in recent years. Recently, our research team has proposed a sporadic ALS animal model by l-BMAA administration in rats, which displays many pathophysiological features of human ALS. In this paper, we deepen the characterization of this model corroborating the occurrence of alterations present in ALS patients such as decreased muscle volume, thinning of the motor cortex, enlarged brain's lateral ventricles, and alteration of both bulbar nuclei and neurotransmitters' levels. Therefore, we conclude that l-BMAA treated rats could be a good model which mimics degenerative features that ALS causes in humans.

Analysis of β-N-methylamino-L-alanine (L-BMAA) neurotoxicity in rat cerebellum

Due to its structural similarity to glutamate, L-BMAA could be a trigger for neurodegenerative disorders caused by changes in the intracellular medium, such as increased oxidative stress, mitochondrial dysfunction, impaired synthesis and protein degradation and the imbalance of some enzymes. It is also important to note that according to some published studies, L-BMAA will be incorporated into proteins, causing the alteration of protein homeostasis. Neuronal cells are particularly prone to suffer damage in protein folding and protein accumulation because they have not performed cellular division. In this work, we will analyse the cerebellum impairment triggered by L-BMAA in treated rats. The cerebellum is one of the most important subcortical motor centres and ensures that movements are performed with spatial and temporal precision. Cerebellum damage caused by L-BMAA can contribute to motor impairment. To characterize this neurodegenerative pathology, we first carried out ultrastructure analysis in Purkinje cells showing altered mitochondria, endoplasmic reticulum (ER), and Golgi apparatus (GA). We then performed biochemical assays of GSK3 and TDP-43 in cerebellum, obtaining an increase of both biomarkers with L-BMAA treatment and, finally, performed autophagy studies that revealed a higher level of these processes after treatment. This work provides evidence of cerebellar damage in rats after treatment with L-BMAA. Three months after treatment, affected rats cannot restore the normal functions of the cerebellum regarding motor coordination and postural control.

Neuroprotective role of sphingosine-1-phosphate in L-BMAA treated neuroblastoma cells (SH-SY5Y)

Sphingosine-1-phosphate (S1P) is a bioactive lipid which regulates proliferation, cell migration, survival and differentiation by specific receptors activation. We studied its effects on L-BMAA treated neuroblastoma cells (SH-SY5Y), an amino acid that can trigger neurodegenerative diseases such as amyotrophic lateral sclerosis/Parkinson dementia complex (ALS/PDC). We found that S1P protects from necrosis and prevents the GSK3 increasing as long as the PI3K/AKT pathway is active. Moreover, GSK3 inhibition protects against neuronal death caused by L-BMAA.

Intrathecal infusion of BMAA induces selective motor neuron damage and astrogliosis in the ventral horn of the spinal cord

The neurotoxin beta-N-methylamino-l-alanine (BMAA) was first identified as a "toxin of interest" in regard to the amyotrophic lateral sclerosis-Parkinsonism Dementia Complex of Guam (ALS/PDC); studies in recent years highlighting widespread environmental sources of BMAA exposure and providing new clues to toxic mechanisms have suggested possible relevance to sporadic ALS as well. However, despite clear evidence of uptake into tissues and a range of toxic effects in cells and animals, an animal model in which BMAA induces a neurodegenerative picture resembling ALS is lacking, possibly in part reflecting limited understanding of critical factors pertaining to its absorption, biodistribution and metabolism. To bypass some of these issues and ensure delivery to a key site of disease pathology, we examined effects of prolonged (30day) intrathecal infusion in wild type (WT) rats, and rats harboring the familial ALS associated G93A SOD1 mutation, over an age range (80±2 to 110±2days) during which the G93A rats are developing disease pathology yet remain asymptomatic. The BMAA exposures induced changes that in many ways resemble those seen in the G93A rats, with degenerative changes in ventral horn motor neurons (MNs) with relatively little dorsal horn pathology, marked ventral horn astrogliosis and increased 3-nitrotyrosine labeling in and surrounding MNs, a loss of labeling for the astrocytic glutamate transporter, GLT-1, surrounding MNs, and mild accumulation and aggregation of TDP-43 in the cytosol of some injured and degenerating MNs. Thus, prolonged intrathecal infusion of BMAA can reproduce a picture in spinal cord incorporating many of the pathological hallmarks of diverse forms of human ALS, including substantial restriction of overt pathological changes to the ventral horn, consistent with the possibility that environmental BMAA exposure could be a risk factor and/or contributor to some human disease.

Maternal transfer of the cyanobacterial neurotoxin β-N-methylamino-L-alanine (BMAA) via milk to suckling offspring

The cyanobacterial neurotoxin β-N-methylamino-L-alanine (BMAA) has been implicated in the etiology of neurodegenerative disease and proposed to be biomagnified in terrestrial and aquatic food chains. We have previously shown that the neonatal period in rats, which in humans corresponds to the last trimester of pregnancy and the first few years of age, is a particularly sensitive period for exposure to BMAA. The present study aimed to examine the secretion of ¹⁴C-labeled L- and D-BMAA into milk in lactating mice and the subsequent transfer of BMAA into the developing brain. The results suggest that secretion into milk is an important elimination pathway of BMAA in lactating mothers and an efficient exposure route predominantly for L-BMAA but also for D-BMAA in suckling mice. Following secretion of [¹⁴C]L-BMAA into milk, the levels of [¹⁴C]L-BMAA in the brains of the suckling neonatal mice significantly exceeded the levels in the maternal brains. In vitro studies using the mouse mammary epithelial HC11 cell line confirmed a more efficient influx and efflux of L-BMAA than of D-BMAA in cells, suggesting enantiomer-selective transport. Competition experiments with other amino acids and a low sodium dependency of the influx suggests that the amino acid transporters LAT1 and LAT2 are involved in the transport of L-BMAA into milk. Given the persistent neurodevelopmental toxicity following injection of L-BMAA to neonatal rodent pups, the current results highlight the need to determine whether BMAA is enriched mother's and cow's milk.

β-N-methylamino-L-alanine induces changes in both GSK3 and TDP-43 in human neuroblastoma

β-N-methylamino-L-alanine (L-BMAA) is a neurotoxic amino acid produced by most cyanobacteria, which are extensively distributed in different environments all over the world. L-BMAA has been linked to a variety of neurodegenerative diseases. This work aims to analyze the toxicological action of L-BMAA related to alterations observed in different neurodegenerative illness as Alzheimer disease and amyotrophic lateral sclerosis. Our results demonstrate that neuroblastoma cells treated with L-BMAA show an increase in glycogen synthase kinase 3 β (GSk3β) and induce accumulation of TAR DNA-binding protein 43 (TDP-43) truncated forms (C-terminal fragments), phosphorylated  and high molecular weight forms of TDP-43, that appears frequently in some neurodegenerative diseases.

BMAA inhibits nitrogen fixation in the cyanobacterium Nostoc sp. PCC 7120

Cyanobacteria produce a range of secondary metabolites, one being the neurotoxic non-protein amino acid β-N-methylamino-L-alanine (BMAA), proposed to be a causative agent of human neurodegeneration. As for most cyanotoxins, the function of BMAA in cyanobacteria is unknown. Here, we examined the effects of BMAA on the physiology of the filamentous nitrogen-fixing cyanobacterium Nostoc sp. PCC 7120. Our data show that exogenously applied BMAA rapidly inhibits nitrogenase activity (acetylene reduction assay), even at micromolar concentrations, and that the inhibition was considerably more severe than that induced by combined nitrogen sources and most other amino acids. BMAA also caused growth arrest and massive cellular glycogen accumulation, as observed by electron microscopy. With nitrogen fixation being a process highly sensitive to oxygen species we propose that the BMAA effects found here may be related to the production of reactive oxygen species, as reported for other organisms.

Is neurodegenerative disease a long-latency response to early-life genotoxin exposure?

Western Pacific amyotrophic lateral sclerosis and parkinsonism-dementia complex, a disappearing neurodegenerative disease linked to use of the neurotoxic cycad plant for food and/or medicine, is intensively studied because the neuropathology (tauopathy) is similar to that of Alzheimer's disease. Cycads contain neurotoxic and genotoxic principles, notably cycasin and methylazoxymethanol, the latter sharing chemical relations with nitrosamines, which are derived from nitrates and nitrites in preserved meats and fertilizers, and also used in the rubber and leather industries. This review includes new data that influence understanding of the neurobiological actions of cycad and related genotoxins and the putative mechanisms by which they might trigger neurodegenerative disease.

The effect of β-N-methylamino-L-alanine (BMAA) on oxidative stress response enzymes of the macrophyte Ceratophyllum demersum

Cyanobacteria are known to produce bioactive secondary metabolites such as hepatotoxins, cytotoxins and neurotoxins. The newly recognized neurotoxin β-N-methylamino-L-alanine (BMAA) is a naturally occurring non-protein amino acid found in the majority of cyanobacterial genera tested. Evidence that exists for implication of BMAA in neurodegenerative disorders relies on bioaccumulation and biomagnification from symbiotic cyanobacteria. Uptake and accumulation of free BMAA by various non-symbiotic organisms, including aquatic macrophytes, has been documented but to date limited evidence of ecotoxicology exists. We therefore investigated the effect of BMAA on the oxidative stress responses of the macrophyte, Ceratophyllum demersum. Markers for oxidative stress in this study are the antioxidative enzymes superoxide dismutase, catalase, guaiacol peroxidase, glutathione peroxidase and glutathione reductase. We found that BMAA had an inhibitory effect on all the oxidative stress response enzymes tested in plants exposed to BMAA. However enzymes not related to oxidative stress response were not affected by BMAA in in vitro experiments. Binding studies in the presence of BMAA showed reduced enzyme specific activity over time compared to the control. This study shows that BMAA causes oxidative stress indirectly as it inhibits antioxidant enzymes required to combat reactive oxygen species that cause damage to cells. Further investigations are required to fully understand the inhibitory effect of BMAA on these enzymes.

Excitatory amino acid beta-N-methylamino-L-alanine is a putative environmental neurotoxin

The amino acid beta-N-methylamino-L-alanine (L-BMAA) has been associated with the amyotrophic lateral sclerosis/parkinsonismdementia complex in three distinct western Pacific populations. The putative neurotoxin is produced by cyanobacteria, which live symbiotically in the roots of cycad trees. L-BMAA was thought to be a threat only to those few populations whose diet and medicines rely heavily on cycad seeds. However, the recent discovery that cyanobacteria from diverse terrestrial, freshwater, and saltwater ecosystems around the world produce the toxin requires a reassessment of whether it poses a larger health threat. Therefore, it is proposed that monitoring L-BMAA levels in cyanobacteria-contaminated water supplies might be prudent.

β-N-methylamino-L-alanine induces neurological deficits and shortened life span in Drosophila

The neurotoxic non-protein amino acid, β-N-methylamino-L-alanine (BMAA), was first associated with the high incidence of Amyotrophic Lateral Sclerosis/Parkinsonism Dementia Complex (ALS/PDC) in Guam. Recently, BMAA has been implicated as a fierce environmental factor that contributes to the etiology of Alzheimer’s and Parkinson’s diseases, in addition to ALS. However, the toxicity of BMAA in vivo has not been clearly demonstrated. Here we report our investigation of the neurotoxicity of BMAA in Drosophila. We found that dietary intake of BMAA reduced life span, locomotor functions, and learning and memory abilities in flies. The severity of the alterations in phenotype is correlated with the concentration of BMAA detected in flies. Interestingly, developmental exposure to BMAA had limited impact on survival rate, but reduced fertility in females, and caused delayed neurological impairment in aged adults. Our studies indicate that BMAA exposure causes chronic neurotoxicity, and that Drosophila serves as a useful model in dissecting the pathogenesis of ALS/PDC.

Three phases of research on beta-N-methylamino-L-alanine (BMAA)--a neurotoxic amino acid

This paper discusses various aspects of the research that lead from the discovery of beta-N-methylamino-L-alanine (BMAA) to consider a variety of mechanisms that might explain the acute and chronic toxicities of this non-protein amino acid. Such is the fashion of science that current work represents the third phase of research on this compound over a period of more than 40 years. BMAA is now known to exist not only in the plant genus Cycas, where it is synthesized by symbiotic cyanobacteria in the coralloid roots of the plants, but to be widely distributed in the many sites at which free living cyanobacteria abound.

Mechanisms of beta-N-methylamino-L-alanine induced neurotoxicity

Since the initial discovery that the amino acid beta-N-methylamino-L-alanine (BMAA) was a neurotoxin, a great deal has been learned about its mechanism of action. However, exactly how it causes death of motor neurons, and how its actions may interact with other neurotoxins or pathological conditions, is not well understood. The focus of study on the mechanism of BMAA toxicity has been on its action as a glutamate receptor agonist. There is evidence that BMAA has effects on all of the main types of glutamate receptors: NMDA, AMPA/kainate, and metabotropic receptors. However, recent results suggest that BMAA may also act through other mechanisms to induce neuronal death. One such action is on the cystine/glutamate antiporter (system xc(-)). Through its effect of system xc(-), BMAA can induce oxidative stress and increase extracellular glutamate. This action of BMAA provides an attractive mechanism for the multiple neurological deficits that BMAA has been implicated in inducing.

Beyond Guam: the cyanobacteria/BMAA hypothesis of the cause of ALS and other neurodegenerative diseases

Excitement about neurogenetics in the last two decades has diverted attention from environmental causes of sporadic ALS. Fifty years ago endemic foci of ALS with a frequency one hundred times that in the rest of the world attracted attention since they offered the possibility of finding the cause for non-endemic ALS throughout the world. Research on Guam suggested that ALS, Parkinson's disease and dementia (the ALS/PDC complex) was due to a neurotoxic non-protein amino acid, beta-methylamino-L-alanine (BMAA), in the seeds of the cycad Cycas micronesica. Recent discoveries that found that BMAA is produced by symbiotic cyanobacteria within specialized roots of the cycads; that the concentration of protein-bound BMAA is up to a hundred-fold greater than free BMAA in the seeds and flour; that various animals forage on the seeds (flying foxes, pigs, deer), leading to biomagnification up the food chain in Guam; and that protein-bound BMAA occurs in the brains of Guamanians dying of ALS/PDC (average concentration 627 microg/g, 5 mM) but not in control brains have rekindled interest in BMAA as a possible trigger for Guamanian ALS/PDC. Perhaps most intriguing is the finding that BMAA is present in brain tissues of North American patients who had died of Alzheimer's disease (average concentration 95 microg/g, 0.8mM); this suggests a possible etiological role for BMAA in non-Guamanian neurodegenerative diseases. Cyanobacteria are ubiquitous throughout the world, so it is possible that all humans are exposed to low amounts of cyanobacterial BMAA, that protein-bound BMAA in human brains is a reservoir for chronic neurotoxicity, and that cyanobacterial BMAA is a major cause of progressive neurodegenerative diseases including ALS worldwide. Though Montine et al., using different HPLC method and assay techniques from those used by Cox and colleagues, were unable to reproduce the findings of Murch et al., Mash and colleagues using the original techniques of Murch et al. have recently confirmed the presence of protein-bound BMAA in the brains of North American patients dying with ALS and Alzheimer's disease (concentrations >100 microg/g) but not in the brains of non-neurological controls or Huntington's disease. We hypothesize that individuals who develop neurodegenerations may have a genetic susceptibility because of inability to prevent BMAA accumulation in brain proteins and that the particular pattern of neurodegeneration that develops depends on the polygenic background of the individual.

Toxicity of Non-protein Amino Acids to Humans and Domestic Animals

Non-protein amino acids are common in plants and are present in widely consumed animal feeds and human foods such as alfalfa ( Medicago sativa), which contains canavanine, and lentil ( Lens culinaris), which contains homoarginine. Some occur in wild species that are inadvertently harvested with crop species. Some nonprotein amino acids and metabolites can be toxic to humans, e.g. Lathyrus species contain a neurotoxic oxalyl-amino acid. Some potential toxins may be passed along a food chain via animal intermediates. The increased interest in herbal medicines in the Western countries will increase exposure to such compounds.

beta-N-methylamino-l-alanine induces oxidative stress and glutamate release through action on system Xc(-)

beta-N-methylamino-l-alanine (BMAA) is a non-protein amino acid implicated in the neurodegenerative disease amyotrophic lateral sclerosis/Parkinson-dementia complex (ALS/PDC) on Guam. BMAA has recently been discovered in the brains of Alzheimer's patients in Canada and is produced by various species of cyanobacteria around the world. These findings suggest the possibility that BMAA may be of concern not only for specific groups of Pacific Islanders, but for a much larger population. Previous studies have indicated that BMAA can act as an excitotoxin by acting on the NMDA receptor. We have shown that the mechanism of neurotoxicity is actually three-fold; it involves not only direct action on the NMDA receptor, but also activation of metabotropic glutamate receptor 5 (mGluR5) and induction of oxidative stress. We now explore the mechanism by which BMAA activates the mGluR5 receptor and induces oxidative stress. We found that BMAA inhibits the cystine/glutamate antiporter (system Xc(-)) mediated cystine uptake, which in turn leads to glutathione depletion and increased oxidative stress. BMAA also appears to drive glutamate release via system Xc(-) and this glutamate induces toxicity through activation of the mGluR5 receptor. Therefore, the oxidative stress and mGluR5 activation induced by BMAA are both mediated through action at system Xc(-). The multiple mechanisms of BMAA toxicity, particularly the depletion of glutathione and enhanced oxidative stress, may account for its ability to induce complex neurodegenerative diseases.

Beta-N-methylaminoalanine (BMAA): metabolism and metabolic effects in model systems and in neural and other tissues of the rat in vitro

The non-protein amino acid, beta-N-methylaminoalanine (BMAA), is neurotoxic and has been implicated in the amyotrophic lateral sclerosis-Parkinsonism-dementia (ALS-PD) complex of Guam. This concept remains controversial, in part because of the lack of a convincing animal model. The neuropharmacology of BMAA is well established, but little is known of its metabolism. This paper reports aspects of the metabolism, and metabolic effects, of BMAA in rat tissues. BMAA changed the distribution of taurine, glycine and serine between rat brain slices and their incubation medium; the glutamate/glutamine cycle between neurones and glia was also compromised. In model experiments BMAA reacted non-enzymatically with pyridoxal-5'-phosphate, releasing methylamine. Rat liver and kidney homogenates, but not brain homogenates, also formed methylamine and 2,3-diaminopropanoic acid when incubated with BMAA. These results provide evidence that several biochemical mechanisms are involved in the neurotoxicity of BMAA. The novel discovery that methylamine is formed from BMAA in rat liver and kidney preparations may be significant since chronic administration of methylamine to rats causes oxidative stress. The extent to which this reaction occurs in different animal species might be a decisive factor in selecting an animal model.

Augmentation and ionic mechanism of effect of beta-N-methylamino-L-alanine in presence of bicarbonate on membrane potential of Retzius nerve cells of the leech Haemopis sanguisuga

The role of neurotoxic non-protein amino acid beta-N-methylamino-L-alanine (L-BMAA) as a putative causative agent of Western pacific amyotrophic lateral sclerosis/Parkinsonism dementia complex (ALS/PDC) has recently been reinvigorated. In view of this data we have investigated the strength and mechanism of effect of L-BMAA in presence of 20 mmol/L bicarbonate (a cofactor for BMAA) on membrane potential of the Leech Haemopis sanguisuga. Our results show that L-BMAA has excitatory effect in bicarbonate containing solution, which is more potent than in nominally bicarbonate free solution. This potentiation by bicarbonate is L-BMAA specific, as it was not exhibited by beta-N-oxalylamino-L-alanine. The effect of L-BMAA was partially blocked by non-NMDA receptor antagonist CNQX. Application of L-BMAA caused a decrease in input membrane resistance, an increase of intracellular sodium activity, and a decrease of intracellular potassium activity. Present findings indicate that BMAA could initiate excitotoxicity through activation of non-NMDA ionotropic glutamate receptors.

TRPM7 and TRPM2-Candidate susceptibility genes for Western Pacific ALS and PD?

Recent findings implicating TRPM7 and TRPM2 in oxidative stress-induced neuronal death thrust these channels into the spotlight as possible therapeutic targets for neurodegenerative diseases. In this review, we describe how the functional properties of TRPM7 and TRPM2 are interconnected with calcium (Ca(2+)) and magnesium (Mg(2+)) homeostasis, oxidative stress, mitochondrial dysfunction, and immune mechanisms, all principal suspects in neurodegeneration. We focus our discussion on Western Pacific Amyotrophic Lateral Sclerosis (ALS) and Parkinsonism Dementia (PD) because extensive studies conducted over the years strongly suggest that these diseases are ideal candidates for a gene-environment model of etiology. The unique mineral environment identified in connection with Western Pacific ALS and PD, low Mg(2+) and Ca(2+), yet high in transition metals, creates a condition that could affect the proper function of these two channels.

Beta-N-methylamino-L-alanine enhances neurotoxicity through multiple mechanisms

The idea that the environmental toxin beta-N-methylamino-l-alanine (BMAA) is involved in neurodegenerative diseases on Guam has risen and fallen over the years. The theory has gained greater interest with recent reports that BMAA is biomagnified, is widely distributed around the planet, and is present in the brains of Alzheimer's patients in Canada. We provide two important new findings. First, we show that BMAA at concentrations as low as 10 muM can potentiate neuronal injury induced by other insults. This is the first evidence that BMAA at concentrations below the mM range can enhance death of cortical neurons and illustrates potential synergistic effects of environmental toxins with underlying neurological conditions. Second, we show that the mechanism of BMAA toxicity is threefold: it is an agonist for NMDA and mGluR5 receptors, and induces oxidative stress. The results provide further support for the hypothesis that BMAA plays a role in neurodegenerative diseases.

Excitotoxicity and amyotrophic lateral sclerosis

Since its description by Charcot more than 130 years ago, the pathogenesis of selective motor neuron degeneration in amyotrophic lateral sclerosis (ALS) remains unsolved. Over the years, many pathogenic mechanisms have been proposed. Amongst others these include: oxidative stress, excitotoxicity, aggregate formation, inflammation, growth factor deficiency and neurofilament disorganization. This multitude of contributing factors indicates that ALS is a complex disease and also suggests that ALS is a multifactorial disorder. Excitotoxicity is not the newest and most spectacular hypothesis in the ALS field, but it is undoubtedly one of the most robust pathogenic mechanisms supported by an impressive amount of evidence. Moreover, the therapeutic efficacy of riluzole, the only drug proven to slow disease progression in ALS, is most likely related to its anti-excitotoxic properties. In this review, we will give an overview of the arguments in favor of the involvement of excitotoxicity in ALS and of the possible mechanisms leading to motor neuron death. We will also summarize the intrinsic properties of motor neurons that render these cells particularly vulnerable to excitotoxicity and could explain the selective vulnerability of motor neurons in ALS. All this information could help to develop new and better therapeutic strategies that could protect motor neurons from excitotoxicity.

BMAA selectively injures motor neurons via AMPA/kainate receptor activation

The toxin beta-methylamino-l-alanine (BMAA) has been proposed to contribute to amyotrophic lateral sclerosis-Parkinsonism Dementia Complex of Guam (ALS/PDC) based on its ability to induce a similar disease phenotype in primates and its presence in cycad seeds, which constituted a dietary item in afflicted populations. Concerns about the apparent low potency of this toxin in relation to estimated levels of human ingestion led to a slowing of BMAA research. However, recent reports identifying potential new routes of exposure compel a re-examination of the BMAA/cycad hypothesis. BMAA was found to induce selective motor neuron (MN) loss in dissociated mixed spinal cord cultures at concentrations ( approximately 30 muM) significantly lower than those previously found to induce widespread neuronal degeneration. The glutamate receptor antagonist NBQX prevented BMAA-induced death, implicating excitotoxic activation of AMPA/kainate receptors. Using microfluorimetric techniques, we further found that BMAA induced preferential [Ca(2+)](i) rises and selective reactive oxygen species (ROS) generation in MNs with minimal effect on other spinal neurons. Cycad seed extracts also triggered preferential AMPA/kainate-receptor-dependent MN injury, consistent with the idea that BMAA is a crucial toxic component in this plant. Present findings support the hypothesis that BMAA may contribute to the selective MN loss in ALS/PDC.

The role of excitotoxicity in the pathogenesis of amyotrophic lateral sclerosis

Unfortunately and despite all efforts, amyotrophic lateral sclerosis (ALS) remains an incurable neurodegenerative disorder characterized by the progressive and selective death of motor neurons. The cause of this process is mostly unknown, but evidence is available that excitotoxicity plays an important role. In this review, we will give an overview of the arguments in favor of the involvement of excitotoxicity in ALS. The most important one is that the only drug proven to slow the disease process in humans, riluzole, has anti-excitotoxic properties. Moreover, consumption of excitotoxins can give rise to selective motor neuron death, indicating that motor neurons are extremely sensitive to excessive stimulation of glutamate receptors. We will summarize the intrinsic properties of motor neurons that could render these cells particularly sensitive to excitotoxicity. Most of these characteristics relate to the way motor neurons handle Ca(2+), as they combine two exceptional characteristics: a low Ca(2+)-buffering capacity and a high number of Ca(2+)-permeable AMPA receptors. These properties most likely are essential to perform their normal function, but under pathological conditions they could become responsible for the selective death of motor neurons. In order to achieve this worst-case scenario, additional factors/mechanisms could be required. In 1 to 2% of the ALS patients, mutations in the SOD1 gene could shift the balance from normal motor neuron excitation to excitotoxicity by decreasing glutamate uptake in the surrounding astrocytes and/or by interfering with mitochondrial function. We will discuss point by point these different pathogenic mechanisms that could give rise to classical and/or slow excitotoxicity leading to selective motor neuron death.