Misfolded proteins toxicity in motor neuron diseases

Motor neuron diseases as spinal and bulbar muscular atrophy (SBMA) or amyotrophic lateral sclerosis (ALS) are characterized by the presence of misfolded proteins that form aggregate and possibly developing toxicity in motor neuron. Misfolded androgen receptor (AR) and superoxide dismutase 1 (SOD1) or TAR DNA-binding protein 43 (TDP-43) in ALS are responsible for SBMA or ALS respectively and are managed by protein quality control (PQC) system: chaperones, autophagy and ubiquitin proteasome system (UPS) which are regulated by co-chaperones like BAG1 and BAG3, in order to prevent their aggregation and/or to increase their degradation. Here, alteration in PQC can be one of the pathologic triggering events and autophagy facilitation restores misfolded proteins clearance. To be degraded by autophagy misfolded proteins are transported via dynein-mediated transport to the microtubule organization centre (MTOC). In preliminary studies performed in NSC34 cell line (murine motor neuron-like cells) we showed that dynein silencing drastically alters the LC3 and SQSTM1/p62 autophagic markers distribution. Surprisingly, dynein silencing reduced the total amount of misfolded proteins retained in filter retardation assays (FRA). In order to reduce toxic effects of dynein silencing we have evaluated the chemical inhibition of dynein ATPase activity by EHNA compound. We have observed that EHNA increased the solubility of mutant AR in PBS or Triton X100-containing buffer and reduced the levels of PBS insoluble fraction of mutant AR, mutant SOD1 and mutant TDP-43 retained in FRA. Also, the inhibition of dynein ATPase activity increased the mRNA levels of the co-chaperone BAG1 vs. BAG3 thus promoting UPS degradation. Then, BAG1 overexpression restored the UPS activity and reduced misfolded proteins aggregation. Collectively, all these data indicate that misfolded protein transport plays a central role in MNDs and dynein activity inhibition restores the PQC equilibrium.