Résumé :
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Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease involving degeneration of the upper and lower motoneurons leading to progressive muscle atrophy. The best caracterized familial form of this disease is linked to mutations in the gene coding for the superoxide dismutase 1 (SOD1). We previously showed that cultured embryonic motoneurons expressing human mutant Superoxide Dismutase 1 (mSOD1) present an increased sensitivity to death induced by Nitric oxide (NO). In order to identify potential effectors in this death pathway, we realized a proteomic study which revealed a two-fold decrease in the expression of an ER chaperone/calcium storage protein, calreticulin, in mSOD1 motoneurons after NO treatment. Fluorescence intensity quantification studies confirmed a decrease of calreticulin in motoneurons expressing two different mutated SOD1 (mSOD1), but not in motoneurons overexpressing the human wildtype SOD1. We showed that the calreticulin decreased expression is cell type specific and occuring after an excitotoxic stress but not after a factor deprivation stress. Interestingly, in mSOD1mice, calreticulin expression is diminished in the lumbar and brachial but not thoracic motoneurons, at asymptomatic (60d), presymptomatic (90d) and symptomatic (112d) stages of the disease. We will now attempt to determine which role the decrease in calreticulin could play in disease progression and motoneuron death in ALS. Calreticulin is implicated in the folding of nascent proteins and also in Ca2+ storage. The accumulation of misfolded protein and/or Ca2+ depletion in ER might lead to ER stress and potentially to death. After overexpressing or silencing calreticulin in wildtype and mSOD1 motoneurons, we will study the consequences on survival, mSOD1 aggregation, Ca2+ pool changes and ER stress pathway activation. These experiments should indicate whether calreticulin could represent a good therapeutic target in ALS.
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