Résumé :
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Spinal Muscular Atrophy (SMA) is characterised by the degeneration of motor neurons in the spinal cord and results from a loss of function (usually a deletion) of the SMN1 (Survival Motor-Neuron 1) gene. The highly similar SMN2 gene partly compensates for this defect, but, due to a predominant skipping of its exon 7, produces only low levels of functional SMN protein. It is still unclear if SMA is due to a deficiency in the best characterised SMN function - snRNP assembly - or in another, motoneuron-specific function of this ubiquitously expressed protein. To improve our understanding of SMA pathogenesis, we are comparing the proteomes of neuronal and non-neuronal cells expressing different levels of SMN. Preliminary results from cells depleted of SMN by RNA interference reveal reduced levels of proteins involved in DNA damage and repair, protein biosynthesis, folding and transport, cell cycle control, or different aspects of RNA metabolism. In particular, effectors of stress granules seem to be affected. In a second approach, we have isolated motoneurons from spinal cord sections from a severe SMA mouse model by laser-dissection microscopy. cDNA generated from these motoneurons was hybridised to exon junction microarrays. The results have just been obtained and are still being evaluated.While these two approaches yield complementary information on gene expression changes in response to low levels of SMN, we are also directly analysing the neuromuscular junctions (NMJs) of SMA mice. Interestingly, in the diaphragm, but not the soleus muscle, mitochondria degenerate on both sides of the NMJs, and perisynaptic Schwann cells (PSCs) as well as endoneurial fibroblasts show profound alterations. In particular the involvement of PSCs is likely to be relevant for human SMA pathogenesis. Based on these findings, we have initiated immunofluorescence studies of the proteins involved in the regulatory signalling between PSCs, the axon terminal and the muscle fibre. By this approach, we hope to identify possible reasons for the degeneration of NMJs in SMA. In combination, these studies are expected to reveal new therapeutic targets that may be amenable to a small drug therapy for SMA.
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