The activation of Mucolipin TRP channel 1 (TRPML1) protects motor neurons from L-BMAA neurotoxicity by promoting autophagic clearance

Cellular clearance mechanisms including the autophagy-lysosome pathway are impaired in amyotrophic lateral sclerosis (ALS). One of the most important proteins involved in the regulation of autophagy is the lysosomal Ca2+ channel Mucolipin TRP channel 1 (TRPML1). Therefore, we investigated the role of TRPML1 in a neuronal model of ALS/Parkinson-dementia complex reproduced by the exposure of motor neurons to the cyanobacterial neurotoxin beta-methylamino-L-alanine (L-BMAA). Under these conditions, L-BMAA induces a dysfunction of the endoplasmic reticulum (ER) leading to ER stress and cell death. Therefore we hypothesized a dysfunctional coupling between lysosomes and ER in L-BMAA-treated motor neurons. Here, we showed that in motor neuronal cells TRPML1 as well as the lysosomal protein LAMP1 co-localized with ER. In addition, TRPML1 co-immunoprecipitated with the ER Ca2+ sensor STIM1. Functionally, the TRPML1 agonist ML-SA1 induced lysosomal Ca2+ release in a dose-dependent way in motor neuronal cells. The SERCA inhibitor thapsigargin increased the fluorescent signal associated with lysosomal Ca2+ efflux in the cells transfected with the genetically encoded Ca2+ indicator GCaMP3-ML1, thus suggesting an interplay between the two organelles. Moreover, chronic exposure to L-BMAA reduced TRPML1 protein expression and produced an impairment of both lysosomal and ER Ca2+ homeostasis in primary motor neurons. Interestingly, the preincubation of ML-SA1, by an early activation of AMPK and beclin 1, rescued motor neurons from L-BMAA-induced cell death and reduced the expression of the ER stress marker GRP78. Finally, ML-SA1 reduced the accumulation of the autophagy-related proteins p62/SQSTM1 and LC3-II in L-BMAA-treated motor neurons. Collectively, we propose that the pharmacological stimulation of TRPML1 can rescue motor neurons from L-BMAA-induced toxicity by boosting autophagy and reducing ER stress.