Résumé :
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At the neuromuscular junction (NMJ) individual muscle fibers are first contacted by many motoneurons, later in development synapse elimination and denervation followed by branch pruning occurs, so that, only one motoneuron innervates a same muscle fiber. In motoneuron diseases as ALS, one of the first signs of the diseases is also the denervation of neuromuscular junctions. Indeed, it is known that neurodevelopment and pathological neurodegeneration converge. The aim of this study is to identify the mechanisms involved in motoneurons pathological degeneration using as a model the axonal dismantling of drosophila NMJ during development In this study: 1) We have performed a precise description of the NMJ dismantling during drosophila development using immunohistochemistry, and shown that muscles degenerate before motoneurons. Thus, after muscle degeneration signs are observed, cell adhesion molecules are lost and the motoneuron cytoskeleton pulls-back by a retraction mechanism. 2) We have evidenced cellular (p35-mediated apoptosis) and genetic (ecdysone) pathways involved in NMJ dismantling. Accordingly, we have been able to stop NMJ dismantling by overexpressing or blocking specific ecdysone regulated nuclear receptors and by expressing apoptosis inhibitor proteins in muscle. Together these results suggest that muscle degeneration is the trigger of the dismantling process since : -muscle degeneration takes place before motoneuron pruning. -Blocking muscle degeneration stops motoneuron pruning. However, blocking motoneuron pruning do not stop muscle lost. We are actually trying to decipher the molecular nature of the message sent from the muscle to the motoneuron which induce motoneuron pruning. For this we are using a gene candidate screening approach. Importantly, we have set up an in vivo developmental assay of NMJ dismantling in wich denervation can be stopped. The assay is ready to test the potential motoneuron or muscle degeneration-protective human genes, and can be used as a first test before gene therapy.
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