The Cyanotoxin and Non-protein Amino Acid β-Methylamino-L-Alanine (L-BMAA) in the Food Chain: Incorporation into Proteins and Its Impact on Human Health

Neuronal control of microglia through the mitochondria

The microbial toxin β-N-methylamino-L-alanine (BMAA), which is derived from cyanobacteria, targets neuronal mitochondria, leading to the activation of neuronal innate immunity and, consequently, neurodegeneration. Although known to modulate brain inflammation, the precise role of aberrant microglial function in the neurodegenerative process remains elusive. To determine if neurons signal microglial cells, we treated primary cortical neurons with BMAA and then co-cultured them with the N9 microglial cell line. Our observations indicate that microglial cell activation requires initial neuronal priming. Contrary to what was observed in cortical neurons, BMAA was not able to activate inflammatory pathways in N9 cells. We observed that microglial activation is dependent on mitochondrial dysfunction signaled by BMAA-treated neurons. In this scenario, the NLRP3 pro-inflammatory pathway is activated due to mitochondrial impairment in N9 cells. These results demonstrate that microglia activation in the presence of BMAA is dependent on neuronal signaling. This study provides evidence that neurons may trigger microglia activation and subsequent neuroinflammation. In addition, we demonstrate that microglial activation may have a protective role in ameliorating neuronal innate immune activation, at least in the initial phase. This work challenges the current understanding of neuroinflammation by assigning the primary role to neurons.

Selection, characterization, and biosensing applications of DNA aptamers targeting cyanotoxin BMAA

Scientists have established a connection between environmental exposure to toxins like β-N-methylamino-l-alanine (BMAA) and a heightened risk of neurodegenerative disorders. BMAA is a byproduct from certain strains of cyanobacteria that are present in ecosystems worldwide and is renowned for its bioaccumulation and biomagnification in seafood. The sensitivity, selectivity, and reproducibility of the current analytical techniques are insufficient to support efforts regarding food safety and environment monitoring adequately. This work outlines the in vitro selection of BMAA-specific DNA aptamers via the systematic evolution of ligands through exponential enrichment (SELEX). Screening and characterization of the full-length aptamers was achieved using the SYBR Green (SG) fluorescence displacement assay. Aptamers BMAA_159 and BMAA_165 showed the highest binding affinities, with dissociation constants (Kd) of 2.2 ± 0.1 μM and 0.32 ± 0.02 μM, respectively. After truncation, the binding affinity was confirmed using a BMAA-conjugated fluorescence assay. The Kd values for BMAA_159_min and BMAA_165_min were 6 ± 1 μM and 0.63 ± 0.02 μM, respectively. Alterations in the amino proton region studied using solution nuclear magnetic resonance (NMR) provided further evidence of aptamer-target binding. Additionally, circular dichroism (CD) spectroscopy revealed that BMAA_165_min forms hybrid G-quadruplex (G4) structures. Finally, BMAA_165_min was used in the development of an electrochemical aptamer-based (EAB) sensor that accomplished sensitive and selective detection of BMAA with a limit of detection (LOD) of 1.13 ± 0.02 pM.

Cell cycle of microalga Isochrysis galbana arrested by neurotoxin β-N-methylamino-l-alanine and corresponding molecular mechanisms

The phycotoxin β-N-methylamino-l-alanine (BMAA) has attracted attention due to its risks to marine organisms and human health. In this study, approximately 85 % of synchronized cells of the marine microalga Isochrysis galbana were arrested at the cell cycle G1 phase by BMAA at 6.5 μM for a 24-h exposure. The concentration of chlorophyll a (Chl a) gradually decreased, while the maximum quantum yield of PSII (F_v/F_m), the maximum relative electron transport rate (rETR_max), light utilization efficiency (α) and half-saturated light irradiance (I_k) reduced early and recovered gradually in I. galbana exposed to BMAA in 96-h batch cultures. Transcriptional expression of I. galbana analyzed at 10, 12, and 16 h disclosed multiple mechanisms of BMAA to suppress the microalgal growth. Production of ammonia and glutamate was limited by the down-regulation of nitrate transporters, glutamate synthase, glutamine synthetase, cyanate hydrolase, and formamidase. Diverse extrinsic proteins related to PSII, PSI, cytochrome b6f complex, and ATPase were influenced by BMAA at transcriptional level. Suppression of the DNA replication and mismatch repair pathways increased the accumulation of misfolded proteins, which was reflected by the up-regulated expression of proteasome to accelerate proteolysis. This study improves our understanding of the chemical ecology impacts of BMAA in marine ecosystems.

Seeing Neurodegeneration in a New Light Using Genetically Encoded Fluorescent Biosensors and iPSCs

Neurodegenerative diseases present a progressive loss of neuronal structure and function, leading to cell death and irrecoverable brain atrophy. Most have disease-modifying therapies, in part because the mechanisms of neurodegeneration are yet to be defined, preventing the development of targeted therapies. To overcome this, there is a need for tools that enable a quantitative assessment of how cellular mechanisms and diverse environmental conditions contribute to disease. One such tool is genetically encodable fluorescent biosensors (GEFBs), engineered constructs encoding proteins with novel functions capable of sensing spatiotemporal changes in specific pathways, enzyme functions, or metabolite levels. GEFB technology therefore presents a plethora of unique sensing capabilities that, when coupled with induced pluripotent stem cells (iPSCs), present a powerful tool for exploring disease mechanisms and identifying novel therapeutics. In this review, we discuss different GEFBs relevant to neurodegenerative disease and how they can be used with iPSCs to illuminate unresolved questions about causes and risks for neurodegenerative disease.

Non-Proteinogenic Amino Acid β-N-Methylamino-L-Alanine (BMAA): Bioactivity and Ecological Significance

Research interest in a non-protein amino acid β-N-methylamino-L-alanine (BMAA) arose due to the discovery of a connection between exposure to BMAA and the occurrence of neurodegenerative diseases. Previous reviews on this topic either considered BMAA as a risk factor for neurodegenerative diseases or focused on the problems of detecting BMAA in various environmental samples. Our review is devoted to a wide range of fundamental biological problems related to BMAA, including the molecular mechanisms of biological activity of BMAA and the complex relationships between producers of BMAA and the environment in various natural ecosystems. At the beginning, we briefly recall the most important facts about the producers of BMAA (cyanobacteria, microalgae, and bacteria), the pathways of BMAA biosynthesis, and reliable methods of identification of BMAA. The main distinctive feature of our review is a detailed examination of the molecular mechanisms underlying the toxicity of BMAA to living cells. A brand new aspect, not previously discussed in any reviews, is the effect of BMAA on cyanobacterial cells. These recent studies, conducted using transcriptomics and proteomics, revealed potent regulatory effects of BMAA on the basic metabolism and cell development of these ancient photoautotrophic prokaryotes. Exogenous BMAA strongly influences cell differentiation and primary metabolic processes in cyanobacteria, such as nitrogen fixation, photosynthesis, carbon fixation, and various biosynthetic processes involving 2-oxoglutarate and glutamate. Cyanobacteria were found to be more sensitive to exogenous BMAA under nitrogen-limited growth conditions. We suggest a hypothesis that this toxic diaminoacid can be used by phytoplankton organisms as a possible allelopathic tool for controlling the population of cyanobacterial cells during a period of intense competition for nitrogen and other resources in various ecosystems.

Motor dysfunction in Drosophila melanogaster as a biomarker for developmental neurotoxicity

Adequate alternatives to conventional animal testing are needed to study developmental neurotoxicity (DNT). Here, we used kinematic analysis to assess DNT of known (toluene (TOL) and chlorpyrifos (CPS)) and putative (β-N-methylamino-L-alanine (BMAA)) neurotoxic compounds. Drosophila melanogaster was exposed to these compounds during development and evaluated for survival and adult kinematic parameters using the FlyWalker system, a kinematics evaluation method. At concentrations that do not induce general toxicity, the solvent DMSO had a significant effect on kinematic parameters. Moreover, while TOL did not significantly induce lethality or kinematic dysfunction, CPS not only induced developmental lethality but also significantly impaired coordination in comparison to DMSO. Interestingly, BMAA, which was not lethal during development, induced motor decay in young adult animals, phenotypically resembling aged flies, an effect later attenuated upon aging. Furthermore, BMAA induced abnormal development of leg motor neuron projections. Our results suggest that our kinematic approach can assess potential DNT of chemical compounds.

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Alternatives to mammalian testing are needed to detect developmental neurotoxicity

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The pesticide chlorpyrifos causes partial lethality and motor dysfunction

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Non-lethal levels of BMAA induce motor dysfunction in a dose-dependent manner

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Kinematic profiling of adult Drosophila can identify developmental neurotoxicity

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.

1,2-Dehydropyrrolizidine Alkaloids: Their Potential as a Dietary Cause of Sporadic Motor Neuron Diseases

Sporadic motor neuron diseases (MNDs), such as amyotrophic lateral sclerosis (ALS), can be caused by spontaneous genetic mutations. However, many sporadic cases of ALS and other debilitating neurodegenerative diseases (NDDs) are believed to be caused by environmental factors, subject to considerable debate and requiring intensive research. A common pathology associated with MND development involves progressive mitochondrial dysfunction and oxidative stress in motor neurons and glial cells of the central nervous system (CNS), leading to apoptosis. Consequent degeneration of skeletal and respiratory muscle cells can lead to death from respiratory failure. A significant number of MND cases present with cancers and liver and lung pathology. This Perspective explores the possibility that MNDs could be caused by intermittent, low-level dietary exposure to 1,2-dehydropyrrolizidine alkaloids (1,2-dehydroPAs) that are increasingly recognized as contaminants of many foods consumed throughout the world. Nontoxic, per se, 1,2-dehydroPAs are metabolized, by particular cytochrome P450 (CYP450) isoforms, to 6,7-dihydropyrrolizines that react with nucleophilic groups (-NH, -SH, -OH) on DNA, proteins, and other vital biochemicals, such as glutathione. Many factors, including aging, gender, smoking, and alcohol consumption, influence CYP450 isoform activity in a range of tissues, including glial cells and neurons of the CNS. Activation of 1,2-dehydroPAs in CNS cells can be expected to cause gene mutations and oxidative stress, potentially leading to the development of MNDs and other NDDs. While relatively high dietary exposure to 1,2-dehydroPAs causes hepatic sinusoidal obstruction syndrome, pulmonary venoocclusive disease, neurotoxicity, and diverse cancers, this Perspective suggests that, at current intermittent, low levels of dietary exposure, neurotoxicity could become the primary pathology that develops over time in susceptible individuals, along with a tendency for some of them to also display liver and lung pathology and diverse cancers co-occurring with some MND/NDD cases. Targeted research is recommended to investigate this proposal.

The neuromicrobiology of Parkinson's disease: A unifying theory

Recent evidence confirms that PD is indeed a multifactorial disease with different aetiologies and prodromal symptomatology that likely depend on the initial trigger. New players with important roles as triggers, facilitators and aggravators of the PD neurodegenerative process have re-emerged in the last few years, the microbes. Having evolved in association with humans for ages, microbes and their products are now seen as fundamental regulators of human physiology with disturbances in their balance being increasingly accepted to have a relevant impact on the progression of disease in general and on PD in particular. In this review, we comprehensively address early studies that have directly or indirectly linked bacteria or other infectious agents to the onset and progression of PD, from the earliest suspects to the most recent culprits, the gut microbiota. The quest for effective treatments to arrest PD progression must inevitably address the different interactions between microbiota and human cells, and naturally consider the gut-brain axis. The comprehensive characterization of such mechanisms will help design innovative bacteriotherapeutic approaches to selectively shape the gut microbiota profile ultimately to halt PD progression. The present review describes our current understanding of the role of microorganisms and their endosymbiotic relatives, the mitochondria, in inducing, facilitating, or aggravating PD pathogenesis.

Cyanobacteria, Cyanotoxins, and Neurodegenerative Diseases: Dangerous Liaisons

The prevalence of neurodegenerative disease (ND) is increasing, partly owing to extensions in lifespan, with a larger percentage of members living to an older age, but the ND aetiology and pathogenesis are not fully understood, and effective treatments are still lacking. Neurodegenerative diseases such as Alzheimer’s disease, Parkinson’s disease, and amyotrophic lateral sclerosis are generally thought to progress as a consequence of genetic susceptibility and environmental influences. Up to now, several environmental triggers have been associated with NDs, and recent studies suggest that some cyanotoxins, produced by cyanobacteria and acting through a variety of molecular mechanisms, are highly neurotoxic, although their roles in neuropathy and particularly in NDs are still controversial. In this review, we summarize the most relevant and recent evidence that points at cyanotoxins as environmental triggers in NDs development.

Food web biomagnification of the neurotoxin β-N-methylamino-L-alanine in a diatom-dominated marine ecosystem in China

The neurotoxin β-N-methylamino-L-alanine (BMAA) reported in some cyanobacteria and eukaryote microalgae is a cause of concern due to its potential risk of human neurodegenerative diseases. Here, BMAA distribution in phytoplankton, zooplankton, and other marine organisms was investigated in Jiaozhou Bay, China, a diatom-dominated marine ecosystem, during four seasons in 2019. Results showed that BMAA was biomagnified in the food web from phytoplankton to higher trophic levels. Trophic magnification factors (TMFs) for zooplankton, bivalve mollusks, carnivorous crustaceans and carnivorous gastropod mollusks were ca. 4.58, 30.1, 42.5, and 74.4, respectively. Putative identification of β-amino-N-methylalanine (BAMA), an isomer of BMAA, was frequently detected in phytoplankton samples. A total of 56 diatom strains of the genera Pseudo-nitzschia, Thalassiosira, Chaetoceros, Planktoniella, and Minidiscus isolated from the Chinese coast were cultured in the laboratory, among which 21 strains contained BMAA mainly in precipitated bound form at toxin concentrations ranging from 0.11 to 3.95 µg/g dry weight. Only 2,4-diaminobutyric acid (DAB) but not BMAA or BAMA was detected in seven species of bacteria isolated from the gut of gastropod Neverita didyma, suggesting that this benthic vector of BMAA may have accumulated this compound via trophic transfer.

Is Exposure to BMAA a Risk Factor for Neurodegenerative Diseases? A Response to a Critical Review of the BMAA Hypothesis

In a literature survey, Chernoff et al. (2017) dismissed the hypothesis that chronic exposure to β-N-methylamino-L-alanine (BMAA) may be a risk factor for progressive neurodegenerative disease. They question the growing scientific literature that suggests the following: (1) BMAA exposure causes ALS/PDC among the indigenous Chamorro people of Guam; (2) Guamanian ALS/PDC shares clinical and neuropathological features with Alzheimer's disease, Parkinson's disease, and ALS; (3) one possible mechanism for protein misfolds is misincorporation of BMAA into proteins as a substitute for L-serine; and (4) chronic exposure to BMAA through diet or environmental exposures to cyanobacterial blooms can cause neurodegenerative disease. We here identify multiple errors in their critique including the following: (1) their review selectively cites the published literature; (2) the authors reported favorably on HILIC methods of BMAA detection while the literature shows significant matrix effects and peak coelution in HILIC that may prevent detection and quantification of BMAA in cyanobacteria; (3) the authors build alternative arguments to the BMAA hypothesis, rather than explain the published literature which, to date, has been unable to refute the BMAA hypothesis; and (4) the authors erroneously attribute methods to incorrect studies, indicative of a failure to carefully consider all relevant publications. The lack of attention to BMAA research begins with the review's title which incorrectly refers to BMAA as a "non-essential" amino acid. Research regarding chronic exposure to BMAA as a cause of human neurodegenerative diseases is emerging and requires additional resources, validation, and research. Here, we propose strategies for improvement in the execution and reporting of analytical methods and the need for additional and well-executed inter-lab comparisons for BMAA quantitation. We emphasize the need for optimization and validation of analytical methods to ensure that they are fit-for-purpose. Although there remain gaps in the literature, an increasingly large body of data from multiple independent labs using orthogonal methods provides increasing evidence that chronic exposure to BMAA may be a risk factor for neurological illness.

Microbial BMAA elicits mitochondrial dysfunction, innate immunity activation, and Alzheimer's disease features in cortical neurons

BACKGROUND

After decades of research recognizing it as a complex multifactorial disorder, sporadic Alzheimer's disease (sAD) still has no known etiology. Adding to the myriad of different pathways involved, bacterial neurotoxins are assuming greater importance in the etiology and/or progression of sAD. β-N-Methylamino-L-alanine (BMAA), a neurotoxin produced by some microorganisms namely cyanobacteria, was previously detected in the brains of AD patients. Indeed, the consumption of BMAA-enriched foods has been proposed to induce amyotrophic lateral sclerosis-parkinsonism-dementia complex (ALS-PDC), which implicated this microbial metabolite in neurodegeneration mechanisms.

METHODS

Freshly isolated mitochondria from C57BL/6 mice were treated with BMAA and O₂ consumption rates were determined. O₂ consumption and glycolysis rates were also measured in mouse primary cortical neuronal cultures. Further, mitochondrial membrane potential and ROS production were evaluated by fluorimetry and the integrity of mitochondrial network was examined by immunofluorescence. Finally, the ability of BMAA to activate neuronal innate immunity was quantified by addressing TLRs (Toll-like receptors) expression, p65 NF-κB translocation into the nucleus, increased expression of NLRP3 (Nod-like receptor 3), and pro-IL-1β. Caspase-1 activity was evaluated using a colorimetric substrate and mature IL-1β levels were also determined by ELISA.

RESULTS

Treatment with BMAA reduced O₂ consumption rates in both isolated mitochondria and in primary cortical cultures, with additional reduced glycolytic rates, decrease mitochondrial potential and increased ROS production. The mitochondrial network was found to be fragmented, which resulted in cardiolipin exposure that stimulated inflammasome NLRP3, reinforced by decreased mitochondrial turnover, as indicated by increased p62 levels. BMAA treatment also activated neuronal extracellular TLR4 and intracellular TLR3, inducing p65 NF-κB translocation into the nucleus and activating the transcription of NLRP3 and pro-IL-1β. Increased caspase-1 activity resulted in elevated levels of mature IL-1β. These alterations in mitochondrial metabolism and inflammation increased Tau phosphorylation and Aβ peptides production, two hallmarks of AD.

CONCLUSIONS

Here we propose a unifying mechanism for AD neurodegeneration in which a microbial toxin can induce mitochondrial dysfunction and activate neuronal innate immunity, which ultimately results in Tau and Aβ pathology. Our data show that neurons, alone, can mount inflammatory responses, a role previously attributed exclusively to glial cells.

Living near waterbodies as a proxy of cyanobacteria exposure and risk of amyotrophic lateral sclerosis: a population based case-control study

BACKGROUND

Epidemiological studies highlighted the possibility that exposure to cyanotoxins leads to the development of the neurodegenerative disease amyotrophic lateral sclerosis (ALS).

METHODS

We devised a population-based case-control study in two Italian populations. We used residential proximity of the residence to water bodies as a measure of possible exposure to cyanotoxins.

RESULTS

Based on 703 newly-diagnosed ALS cases and 2737 controls, we calculated an ALS odds ratio (OR) of 1.41 (95% CI: 0.72-2.74) for current residence in the vicinity of water bodies, and a slightly lower estimate for historical residence (OR: 1.31; 95% CI: 0.57-2.99). Subjects <65 years and people living in the Northern Italy province of Modena had higher ORs, especially when historical residence was considered.

CONCLUSIONS

Overall, despite some risk of bias due to exposure misclassification and unmeasured confounding, our results appear to support the hypothesis that cyanotoxin exposure may increase ALS risk.

Mechanisms of L-Serine-Mediated Neuroprotection Include Selective Activation of Lysosomal Cathepsins B and L

L-serine is a naturally occurring dietary amino acid that has recently received renewed attention as a potential therapy for the treatment of amyotrophic lateral sclerosis (ALS), Alzheimer's disease (AD), hereditary sensory autonomic neuropathy type I (HSAN1), and sleep induction and maintenance. We have previously reported L-serine functions as a competitive inhibitor of L-BMAA toxicity in cell cultures and have since progressed to examine the neuroprotective effects of L-serine independent of L-BMAA-induced neurotoxicity. For example, in a Phase I, FDA-approved human clinical trial of 20 ALS patients, our lab reported 30 g L-serine/day was safe, well-tolerated, and slowed the progression of the disease in a group of 5 patients. Despite increasing evidence for L-serine being useful in the clinic, little is known about the mechanism of action of the observed neuroprotection. We have previously reported, in SH-SY5Y cell cultures, that L-serine alone can dysregulate the unfolded protein response (UPR) and increase the translation of the chaperone protein disulfide isomerase (PDI), and these mechanisms may contribute to the clearance of mis- or unfolded proteins. Here, we further explore the pathways involved in protein clearance when L-serine is present in low and high concentrations in cell culture. We incubated SH-SY5Y cells in the presence and absence of L-serine and measured changes in the activity of proteolytic enzymes from the autophagic-lysosomal system, cathepsin B, cathepsin L, and arylsulfatase and specific activities of the proteasome, peptidylglutamyl-peptide hydrolyzing (PGPH) (also called caspase-like), chymotrypsin, and trypsin-like. Under our conditions, we report that L-serine selectively induced the activity of autophagic-lysosomal enzymes, cathepsins B and L, but not any of the proteasome-hydrolyzing activities. To enable comparison with previous work, we also incubated cells with L-BMAA and report no effect on the activity of the autophagic lysosomes or the proteasomes. We also developed an open-source script for the automation of linear regression calculations of kinetic data. Autophagy impairment or failure is characteristic of many neurodegenerative disease; thus, activation of autophagic-lysosomal proteolysis may contribute to the neuroprotective effect of L-serine, which has been reported in cell culture and human clinical trials.

Microbial BMAA and the Pathway for Parkinson's Disease Neurodegeneration

The neurotoxin β-N-methylamino-L-alanine (BMAA) is a natural non-proteinogenic diamino acid produced by several species of both prokaryotic (cyanobacteria) and eukaryotic (diatoms and dinoflagellates) microorganisms. BMAA has been shown to biomagnify through the food chain in some ecosystems, accumulating for example in seafood such as shellfish and fish, common dietary sources of BMAA whose ingestion may have possible neuronal consequences. In addition to its excitotoxic potential, BMAA has been implicated in protein misfolding and aggregation, inhibition of specific enzymes and neuroinflammation, all hallmark features of neurodegenerative diseases. However, the exact molecular mechanisms of neurotoxicity remain to be elucidated in detail. Although BMAA is commonly detected in its free form, complex BMAA-containing molecules have also been identified such as the paenilamicins, produced by an insect gut bacterial pathogen. On the other hand, production of BMAA or BMAA-containing molecules by members of the human gut microbiota, for example by non-photosynthetic cyanobacteria, the Melainabacteria, remains only hypothetical. In any case, should BMAA reach the gut it may interact with cells of the mucosal immune system and neurons of the enteric nervous system (ENS) and possibly target the mitochondria. Here, we review the available evidence and hint on possible mechanisms by which chronic exposure to dietary sources of this microbial neurotoxin may drive protein misfolding and mitochondrial dysfunction with concomitant activation of innate immune responses, chronic low-grade gut inflammation, and ultimately the neurodegenerative features observed across the gut-brain axis in Parkinson's disease (PD).