Inhibition of dynein-mediated retrograde transport reduces aggregation of misfolded protein responsible for motoneuron diseases

Motor neuron diseases, like spinobulbar muscular atrophy (SBMA) and amyotrophic lateral sclerosis (ALS) are characterized by the presence of inclusions o aggregates of proteinaceous materials. In SBMA, aggregates contain mutant androgen receptors (AR) with an elongated polyglutamine tract (ARpolyQ), responsible for disease, while in ALS aggregates contain TDP43, ubiquilin, optineurin, etc. Exceptions are familial ALS forms linked to superoxide dismutase 1 (SOD1) mutations, in which aggregates are composed of mutant SOD1. In general, protein aggregation is due to generation of aberrant protein conformations (misfolding). Thus, in neuronal cells, the protein quality control (PQC) system may be insufficient to correctly remove the misfolded proteins. The PQC system requires the activities of efficient chaperones and of the degradative systems ubiquitin-proteasome (UPS) and autophagy. After misfolded protein recognition by chaperones, the dynein motor complex plays a crucial role to efficiently remove these species via autophagy transporting them to autophagosome and assisting autophagosome-lysosome fusion. We found that in immortalized motoneuronal NSC34 cells treated with trehalose to induce autophagy, a selective inhibitor of dynein (EHNA) drastically reduced activated autophagy. In addition, in NSC34 cells expressing ARpolyQ dynein was sequestered into ARpolyQ aggregates. When we perturbed dynein function with EHNA in NSC34 cells expressing ARpolyQ, mutant SOD1 or a truncated TDP43 form, we unexpectedly found a great reduction of mutant protein aggregates, even in presence of an autophagy inhibitor (3-MA), but not of a proteasome inhibitor (MG132). In fractionation studies we found that EHNA increased the ARpolyQ levels in PBS and Triton-X100 fractions. Surprisingly, we found that ENHA effects were paralleled by an increased expression of BAG1, a co-chaperone which routes misfolded proteins to UPS, but not of BAG3, required for autophagy. The decreased BAG3/BAG1 ratio is suggestive of a prevalence of UPS functions when dynein activity is impaired. Thus, dynein blockage results in autophagy alteration, but also to a reduced aggregation of misfolded proteins,, a phenomenon that may occurs via an increase in their solubility and the induction of UPS functions.