Metabolism of the neurotoxic amino acid ß-N-methylamino-L-alanine in human cell culture models.

Human dietary exposure to the environmental neurotoxin ß-N-methylamino-L-alanine (BMAA) has been implicated in an increased risk of developing sporadic neurodegenerative diseases like Alzheimer's and amyotrophic lateral sclerosis. Evidence suggests that humans are exposed to BMAA globally, but very little is known about BMAA metabolism in mammalian systems, let alone in humans. The most plausible, evidence-based mechanisms of BMAA toxicity rely on the metabolic stability of the amino acid and that, following ingestion, it enters the circulatory system unmodified. BMAA crosses from the intestinal lumen into the circulatory system, and the small intestine and liver are the first sites for dietary amino acid metabolism. Both tissues have substantial amino acid metabolic needs, which are largely fulfilled by dietary amino acids. Metabolism of BMAA in these tissues has been largely overlooked, yet is important in gauging the true human exposure risk. Here we investigate the potential for BMAA metabolism by the human liver and small intestine, using in vitro cell systems. Data show that BMAA metabolism via common proteinogenic amino acid metabolic pathways is negligible, and that in the presence of other amino acids cellular uptake of BMAA is substantially reduced. These data suggest that the majority of ingested BMAA remains unmodified following passage through the small intestine and liver. This not only supports oral BMAA exposure as a plausible exposure route to toxic doses of BMAA, but also supports previous notions that protein deficient diets or malnutrition may increase an individual's susceptibility to BMAA absorption and subsequent toxicity.

The Evaluation of BMAA Inhalation as a Potential Exposure Route Using a rat Model.

Chronic inhalation of aerosolized ß-N-methylamino-L-alanine (BMAA) could serve as potenital route for exposure to this cyanobacterial neurotoxin implicated in the development of neurodegenerative disease. We investigated environmental aerosol BMAA loads and the fate of inhaled isotopically labeled aerosolized BMAA in adult male Sprague Dawley rats, with doses corresponding to chronic aerosolized environmental BMAA exposure of over 65 days and up to 266 years. Environmental BMAA aerosol concentrations ranged from 6-39 pg L¯1. No clinical signs of toxicity were observed in rats exposed to aerosol containing BMAA at concentrations far exceeding the maximum recorded environmental BMAA aerosol load. Surprisingly, no labeled BMAA was observed in the brain, liver or lung tissues of exposed rats. However, a dose-dependent reduction in the Gln:Glu ratio was observed in brain and liver tissues together with an increase in 2,3 diaminopropanoic acid,15N2, the demethylated L-BMAA-4,4,4-d3,15N2 product, in liver tissues. This confirmed both BMAA uptake and distribution throughout the body. The increase in 2,3 diaminopropanoic acid,15N2 did however not account for the total loss of administered L-BMAA-4,4,4-d3,15N2 and thus, the absence of detectable L-BMAA-4,4,4-d3,15N2 in tissues and feces, together with the absence of other known BMAA catabolites, N-acetylated BMAA and methylamine, additional metabolic reactions are indicated. Significant biochemical responses to BMAA were only observed in doses corresponding to an unrealistic chronic exposure timeframe, suggesting that the inhalation of environmental levels of aerosolized BMAA might not be sufficient to elicit a biochemical response in adults.

Human Scalp Hair as an Indicator of Exposure to the Environmental Toxin ß-N-Methylamino-l-alanine.

Dietary or aerosol exposure to the environmental neurotoxin ß-N-methylamino-l-alanine (BMAA) is a putative risk factor for the development of sporadic neurodegenerative disease. There are many potential sources of BMAA in the environment, but BMAA presence and quantities are highly variable. It has been suggested that BMAA in human hair may serve as an indicator of exposure. We sought to evaluate the use of the BMAA content of human scalp hair as an indicator of exposure, as well as the correlation between specific lifestyle or dietary habits, reported as hypothesised exposure risk factors, and BMAA in hair. Scalp hair samples and questionnaires were collected from participants in a small residential village surrounding a freshwater impoundment renowned for toxic cyanobacterial blooms. Data suggested a positive correlation between hair BMAA content and consumption of shellfish, and possibly pork. No statistically significant correlations were observed between hair BMAA content and residential proximity to the water or any other variable. Hair BMAA content was highly variable, and in terms of exposure, probably reflects primarily dietary exposure. However, the BMAA content of human hair may be affected to a great extent by several other factors, and as such, should be used with caution when evaluating human BMAA exposure, or correlating exposure to neurodegenerative disease incidence.

Β-N-Methylamino-L-Alanine (BMAA) Toxicity Is Gender and Exposure-Age Dependent in Rats.

Cyanobacterial ß-N-methylamino-L-alanine (BMAA) has been suggested as a causative or contributory factor in the development of several neurodegenerative diseases. However, no BMAA animal model has adequately shown clinical or behavioral symptoms that correspond to those seen in either Alzheimer's Disease (AD), Amyotrophic Lateral Sclerosis (ALS) or Parkinson's Disease (PD). We present here the first data that show that when neonatal rats were exposed to BMAA on postnatal days 3, 4 and 5, but not on gestational day 14 or postnatally on days 7 or 10, several AD and/or PD-related behavioral, locomotor and cognitive deficits developed. Male rats exhibited severe unilateral hindlimb splay while whole body tremors could be observed in exposed female rats. BMAA-exposed rats failed to identify and discriminate a learned odor, an early non-motor symptom of PD, and exhibited decreased locomotor activity, decreased exploration and increased anxiety in the open field test. Alterations were also observed in the rats' natural passive defense mechanism, and potential memory deficits and changes to the rat's natural height avoidance behavior could be observed as early as PND 30. Spatial learning, short-term working, reference and long-term memory were also impaired in 90-day-old rats that had been exposed to a single dose of BMAA on PND 3-7. These data suggest that BMAA is a developmental neurotoxin, with specific target areas in the brain and spinal cord.

A Single Neonatal Exposure to BMAA in a Rat Model Produces Neuropathology Consistent with Neurodegenerative Diseases.

Although cyanobacterial ß-N-methylamino-l-alanine (BMAA) has been implicated in the development of Alzheimer's Disease (AD), Parkinson's Disease (PD) and Amyotrophic Lateral Sclerosis (ALS), no BMAA animal model has reproduced all the neuropathology typically associated with these neurodegenerative diseases. We present here a neonatal BMAA model that causes ß-amyloid deposition, neurofibrillary tangles of hyper-phosphorylated tau, TDP-43 inclusions, Lewy bodies, microbleeds and microgliosis as well as severe neuronal loss in the hippocampus, striatum, substantia nigra pars compacta, and ventral horn of the spinal cord in rats following a single BMAA exposure. We also report here that BMAA exposure on particularly PND3, but also PND4 and 5, the critical period of neurogenesis in the rodent brain, is substantially more toxic than exposure to BMAA on G14, PND6, 7 and 10 which suggests that BMAA could potentially interfere with neonatal neurogenesis in rats. The observed selective toxicity of BMAA during neurogenesis and, in particular, the observed pattern of neuronal loss observed in BMAA-exposed rats suggest that BMAA elicits its effect by altering dopamine and/or serotonin signaling in rats.

The metabolism of the non-proteinogenic amino acid ß-N-methylamino-L-alanine (BMAA) in the cyanobacterium Synechocystis PCC6803.

The neurotoxic amino acid ß-N-methylamino-L-alanine (BMAA) is produced by cyanobacteria under nitrogen starvation conditions and its metabolism is closely associated with cellular nitrogen control. Very little is known regarding the metabolism or biosynthesis of this amino acid in the producing organisms and current knowledge is limited to the spontaneous formation of carbamate adducts in the presence of aqueous carbon dioxide, the rapid removal of free cellular BMAA upon the addition of ammonia to nitrogen-starved cyanobacterial cultures, and the link between cellular nitrogen status and BMAA synthesis. Data presented here show that exogenous BMAA is readily metabolised by cyanobacteria during which, the primary amino group is rapidly transferred to other cellular amino acids. Furthermore, data suggest that BMAA is metabolised in cyanobacteria via a reversible transamination reaction. This study presents novel data on BMAA metabolism in cyanobacteria and provides the first proposed biosynthetic precursor to BMAA biosynthesis in cyanobacteria.

The fate of the cyanobacterial toxin ß-N-methylamino-L-alanine in freshwater mussels.

The cyanobacterial neurotoxin, ß-N-methylamino-l-alanine (BMAA) has been suggested as a causative agent for certain neurodegenerative diseases. This cyanotoxin bioaccumulates in an array of aquatic organisms, in which it occurs as both a free amino acid and in a protein-associated form. This study was intended to investigate the environmental fate of BMAA by examining the metabolism of isotopically labeled BMAA in four freshwater mussel species. All species showed substantial uptake of BMAA from the culture media. Data showed no significant evidence for BMAA catabolism in any of the animals but did suggest metabolism via the reversible covalent modification of BMAA in freshwater mussels, a process that appears to be variable in different species.