β-Methylamino-L-alanine-induced protein aggregation in vitro and protection by L-serine

The cyanobacterial non-protein amino acid α-amino-β-methylaminopropionic acid, more commonly known as BMAA, was first discovered in the seeds of the ancient gymnosperm Cycad circinalis (now Cycas micronesica Hill). BMAA was linked to the high incidence of neurological disorders on the island of Guam first reported in the 1950s. BMAA still attracts interest as a possible causative factor in amyotrophic lateral sclerosis (ALS) following the identification of ALS disease clusters associated with living in proximity to lakes with regular cyanobacterial blooms. Since its discovery, BMAA toxicity has been the subject of many in vivo and in vitro studies. A number of mechanisms of toxicity have been proposed including an agonist effect at glutamate receptors, competition with cysteine for transport system x_c^_ and other mechanisms capable of generating cellular oxidative stress. In addition, a wide range of studies have reported effects related to disturbances in proteostasis including endoplasmic reticulum stress and activation of the unfolded protein response. In the present studies we examine the effects of BMAA on the ubiquitin-proteasome system (UPS) and on chaperone-mediated autophagy (CMA) by measuring levels of ubiquitinated proteins and lamp2a protein levels in a differentiated neuronal cell line exposed to BMAA. The BMAA induced increases in oxidised proteins and the increase in CMA activity reported could be prevented by co-administration of L-serine but not by the two antioxidants examined. These data provide further evidence of a protective role for L-serine against the deleterious effects of BMAA.