A toxicological analysis of the cyanotoxin β-methylamino-L-alanine at multiple neural levels and identification of novel modes of activity

Abstract

The cyanotoxin β-methylamino-L-alanine (BMAA) has long been associated with the elevated incidence of amyotrophic lateral sclerosis/Parkinsonism dementia complex (ALS/PDC). BMAA has been shown to be produced across the cyanobacterial order and its detection has been reported in a variety of aquatic and terrestrial environments worldwide, suggesting that it is ubiquitous. Various in vivo studies on rats, mice, chicks, monkeys and zebrafish have shown that BMAA can cause neurodegenerative symptoms such as ataxia and convulsions, as well as disruption to neural development. Previous in vitro studies using mice, rats and leeches have shown that BMAA acts predominantly on motor neurons. Observed increases in the generation of reactive oxygen species (ROS) and Ca2+ influx, coupled with disruption to mitochondrial activity and general neuronal death, indicate that the main mode of activity is via excitotoxic mechanisms. Data presented here indicate that BMAA induces increased intracellular Ca2+ influx, DNA damage, mitochondrial activity, lactate dehydrogenase (LDH) release and generation of ROS in human neurons. The amelioration of LDH release in the presence of the N methyl-D-aspartate (NMDA) receptor antagonist MK801 indicates that the neurotoxic effects of BMAA are mediated via NMDA receptor activation. Immunocytochemical stains show that BMAA induces the expression of neuronal nitric oxide synthase (nNOS) and caspase-3 indicating that it can stimulate apoptosis in human neurons, presumably via activation of NMDA receptors. Rat olfactory ensheathing cells (OECs) were also challenged with exogenous BMAA resulting in elevated cell death. Significant increases in Ca2+ influx, enhanced production of ROS, and disruption to mitochondrial activity were observed in OECs. This is the first study investigating BMAA toxicity using pure glial cells. These findings align BMAA with all proposed mechanisms of degeneration in ALS, those being non-cell autonomous death, excitotoxicity and mitochondrial dysfunction. Additionally, novel genes and proteins have been identified that were differentially expressed in BMAA treated OECs. Some of the identified genes/proteins of interest have previously been linked to Alzheimer's disease. Other genes/proteins identified have reported functions in the regulation of mitochondrial activity and cell cycle and apoptosis. Cell cycle analysis of OECs treated with BMAA resulted in cell cycle arrest at the G2/M phase.