L-BMAA Induced ER Stress and Enhanced Caspase 12 Cleavage in Human Neuroblastoma SH-SY5Y Cells at Low Nonexcitotoxic Concentrations

The cyanobacterial β-N-methylamino-l-alanine (l-BMAA) is described as a low-potency excitotoxin, possibly a factor in the increased incidence of amyotrophic lateral sclerosis (ALS) and Parkinsonism-dementia complex (PDC) in Guam. The latter association is intensively disputed, as l-BMAA concentrations required for toxic effects exceed those assumed to occur via food. The question thus was raised whether l-BMAA leads to neurodegeneration at nonexcitotoxic conditions. Using human SH-SY5Y neuroblastoma cells, l-BMAA-transport, incorporation into proteins, and subsequent impairment of cellular protein homeostasis were investigated. Binding of l-BMAA to intracellular proteins, but no clear protein incorporation was detected in response to 14C-l-BMAA exposures. Nevertheless, low l-BMAA concentrations (≥ 0.1mM, 48h) increased protein ubiquitination, 20S proteasomal and caspase 12 activity, expression of the endoplasmic reticulum (ER) stress marker CHOP, and enhanced phosphorylation of elf2α in SH-SY5Y cells. In contrast, high l-BMAA concentrations (≥ 1mM, 48h) increased reactive oxygen species and protein oxidization, which were partially ameliorated by coincubation with vitamin E. l-BMAA-mediated cytotoxicity was observable 48h following ≥ 2mM l-BMAA treatment. Consequently, the data presented here suggest that low l-BMAA concentrations result in a dysregulation of the cellular protein homeostasis with ensuing ER stress that is independent from high-concentration effects such as excitotoxicity and oxidative stress. Thus, the latter could be a contributing factor in the onset and slow progression of ALS/PDC in Guam.