br Conflict of Interest br Acknowledgements Marques F
Conflict of Interest
Acknowledgements Marques F. was supported by the Foundation for Science and Technology (FCT), Portugal, Grant UMINHO/BI/35/2012. This work was supported by FCT by the Project PTDC/AGR-AAM/70418/2006.
Introduction β-methylamino-l-alanine (BMAA) exposure has long been associated with neurodegenerative diseases, particularly on Guam where high rates of amyotrophic lateral sclerosis (ALS) and Parkinson’s disease-like dementia complex (ALS-PDC) have been reported (Reed and Brody, 1975, Bradley and Mash, 2009). Such conditions have been linked to consumption of foodstuffs with high BMAA levels, typically flying foxes and other animals that bioaccumulate BMAA in their tissues as a result of eating cycad seeds (Bradley and Mash, 2009, Cox et al., 2003). The increasing westernisation of the diet on Guam has decreased consumption of these BMAA rich dietary sources and this has been linked to decreases in the incidence of ALS and ALS-PDC on Guam (Plato et al., 2003). These observations are consistent with BMAA being neurotoxic in humans. BMAA is present in cycad seeds through synthesis by symbiotic cyanobacteria. While numerous cyanobacteria species can produce BMAA (Cox et al., 2005), it is currently unclear to what extent humans may be exposed e.g. via cyanobacterial blooms (Faassen, 2014). However, BMAA has been found in the brains of ALS and other patients not only from Guam but also North America and this does suggest a more widespread exposure than initially considered (Murch et al., 2004a, Pablo et al., 2009). In vitro studies have shown that BMAA is toxic to neurons (Chiu et al., 2011, Lobner, 2009). BMAA can react with bicarbonate ions to form a β-carbamate that is a structural analogue of glutamate and hence can bind to various glutamate receptors including the N-methyl-d-aspartate and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic WWL 70 of receptors leading to excitotoxicity. Receptor activation leads to a chain of events that involves increased concentrations of intracellular Na+ and Ca2+ and decreased K+ levels, altered membrane permeability, mitochondrial dysfunction and increased reactive oxygen species (ROS) production, and ultimately leads to cell death (Chiu et al., 2011, Lobner, 2009). BMAA can also act as a substitute for L-serine and be incorporated into human (Dunlop et al., 2013), though not bacterial proteins (van Onselen et al., 2015) nor proteins obtained from the brains of BMAA treated cynomologus monkeys (Spencer et al., 2016). BMAA incorporated into proteins can cause protein misfolding and aggregation (Dunlop et al., 2013) a process often linked to neurodegenerative disease (Rodgers, 2014). Furthermore, protein-bound BMAA may act as a reservoir that releases free BMAA as proteins are degraded (Murch et al., 2004b). Numerous in vivo studies have shown that high dose acute and chronic BMAA exposure can have neurotoxic effects that are typically motor function deficits (Chiu et al., 2011, Karamyan and Speth, 2008, de Munck et al., 2013). For example, a motor system disorder, similar to ALS/PDC was observed in macaques treated with between 0.1–0.3g/BMAA/kg/day by gavage for up to 12 weeks (Spencer et al., 1987). These doses may be much higher than would be observed in human populations (Duncan, 1992), and so the association between BMAA and ALS/PDC remains controversial. In addition, the cycad nut also contains cycasin, whose active metabolite, methylazoxymethanol (MAM) is toxic to neurons and can affect brain development in experimental animal models (Kisby et al., 2009, Kisby et al., 2013) with DNA repair inhibition resulting in more pronounced effects suggesting a DNA damage mediated mechanism (Kisby et al., 2009, Kisby et al., 2013). MAM is a methylating agent, one of a class of agents that is toxic, mutagenic and carcinogenic largely as a result of the formation of specific methylated nucleobase adducts (Kaina et al., 2007). Interestingly DNA alkylating agents can also be formed by the N-nitrosation of natural amino acids (Garcia-Santos et al., 2001, Shephard et al., 1991) and hence may have similar properties to MAM.