SOD1 PREVENTS MOTOR NEURON DEATH INDUCED BY THE CYCAD NEUROTOXIN BETA-METHYLAMINO-L-ALANINE BY MODULATING PLASMA MEMBRANE TARGETS INVOLVED IN INTRACELLULAR Ca2+ HOMEOSTASIS

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease characterized by the loss of motor neurons. Although the incoming knowledge on the genetic aspects of the disease, the pathogenesis remains unknown. It has been hypothesized that ALS may be the result of a conspiracy of genes, environment and time. The environmental basis is supported by the high incidence of ALS in Guam and in the Trust Territories of the Pacific, where the exposure to the cycad neurotoxin beta-methilamino-L-alanine (L-BMAA) causes ALS/Parkinson-Dementia Complex (ALS/PDC). L-BMAA induces neurodegeneration by determining a dysfunction in intracellular Ca2+ concentration ([Ca2+]i) within endoplasmic reticulum (ER) and mitochondria. In this respect, we have recently shown that, in motor neurons exposed to L-BMAA, SOD1 prevents ER stress and cell death through the Ca2+/Akt/ERK1/2 pathway independently from catalytic activity. Indeed, SOD1 triggers a rapid and transient [Ca2+]i increase, in part derived from ER Ca2+ release and in part due to the entry of Ca2+ via uncharacterized plasma membrane targets. This [Ca2+]i increase is determinant for neuroprotection, as demonstrated by the lack of the effect of recombinant SOD1G93A on [Ca2+]i. To identify the molecular target of SOD1, we dissect the two components involved in [Ca2+]i modulation and we investigate the involvement of several targets, including Na+/Ca2+ exchanger (NCX), purinergic P2X7 receptor and cADP-ribose pathway. We showed that, in a subset of neuronal cells, SOD1-induced [Ca2+]i rise was prevented by CB-DMB, a pan inhibitor of NCX, but not by A430879, a P2X7 receptor specific antagonist, and 8-bromo-cADPR, a cell permeant antagonist of cADP-ribose. Furthermore, by siRNA strategy, we identify the NCX isoform mainly involved in the neuroprotective action of SOD1 against L-BMAA. Collectively, our data indicate that SOD1 exerts a neuroprotective effect in ALS/PDC in a subset of motor neurons by modulating [Ca2+]i through NCX.