Motoneuron and muscle selective removal of ALS-related misfolded proteins.

Amyotrophic lateral sclerosis (ALS) occurs in clinically indistinguishable sporadic (sALS) or familial (fALS) forms. Most of the fALS-related mutant proteins identified so far, like mutant SOD1, TDP-43, FUS, etc., are prone to misfold; the product of the mutant C9ORF72 gene aberrantly codes for small highly hydrophobic dipeptides. Both misfolded proteins and hydrophobic peptides accumulate into insoluble proteinaceous material inside motoneurons. This material must be cleared away from cells with the assistance of the molecular chaperones. Chaperones may act on aberrant proteins either by assisting their refolding, or by directing them to degradation through the proteasome (UPS) or the autophagic system. Motoneurons are very sensitive to misfolded protein toxicity, but other cell types, such as astrocytes, oligodendrocytes, muscle cells could also be affected by their presence. Notably, muscle-restricted expression of mutant SOD1 (mutSOD1), responsible for some fALS, induces muscle atrophy and motoneurons death. We found that several genes are altered in mutSOD1 mice muscles. In fact, we observed up-regulation of typical muscle genes, such as MyoD, myogenin, but also of several components of cell response to proteotoxicity (atrogin-1, HspB8, Bag1, Bag3). Similar changes were found to occur in cultured ALS myoblasts. We then compared the potential mutSOD1 toxicity in motoneuron (NSC34) and muscle (C2C12) cells. Initially, we found that muscle ALS models possess much higher chimotryptic proteasome activity and autophagy power than motoneuron ALS models. The mutSOD1 molecular behaviour was also very different. MutSOD1 clearance was much higher in muscle than in motoneurons and the misfolded protein formed aggregates and impaired proteasome only in motoneurons. The motoneuronal cells were also more sensitive to superoxide-induced oxidative stress. In muscle cells, mutSOD1 remained soluble even after proteasome inhibition, possibly because of high mutSOD1 autophagic clearance. Finally, a N-terminal TDP-43 fragment accumulated in NSC34, but not in C2C12 cells. In the case of TDP-43, proteasome inhibition resulted in a large accumulation of both wt and N-terminal fragment of TDP-43. Therefore, our results suggest that muscle cells differentially manage misfolded mutSOD1 and TDP-43 and their toxicity in muscle may not directly depend on aggregation.