Introduction: Common to many neurodegenerative diseases, including motor neurone disease (MND), is the cytosolic deposition of protein aggregates, ER stress, mitochondrial dysfunction, calcium dyshomeostasis and apoptotic death of motor neurons. Convergence of these pathways is likely to mediate disease onset and progression. The cyanobacterial non-protein amino acid BMAA has been implicated as an environmental trigger for MND. We have shown that incubation of neuroblastoma cells with BMAA results in the accumulation of cytosolic protein aggregates in cells and apoptotic cell death. Here we examine the mechanisms involved in BMAA toxicity to neuroblastoma cells in vitro.

Results: Misincorporation of BMAA into proteins in place of l-serine, activated the unfolded protein response (UPR), upregulated heat shock proteins and increased lysosomal protease activity in cells. These effects could be prevented by co-incubation with l-serine. Transcription of genes involved in the UPR were found to be significantly upregulated (ATF4, ATF6, Grp94 and Grp78) following BMAA treatment (500 uM for 4 hrs). Levels of ER stress proteins Grp78/BiP and calnexin, and cytosolic heat shock proteins, Hsp27 and Hsp90 were increased (2D gel electrophoresis and WB). The activity of lysosomal enzymes cathepsin B and L was increased. Cell lysates from BMAA treated cells also showed a global increase in protein ubiquitinylation suggesting increased levels of modified proteins.

Conclusion: BMAA-containing proteins induce proteotoxic stress by activation of the UPR, increasing translation of heat shock proteins, and selectively activating lysosomal enzymes cathepsin B and L. This mechanism is consistent with the slow toxicity reported in humans exposed to BMAA.