THE ROLE OF PROTEIN QUALITY CONTROL SYSTEM IN REPEAT EXPANSION NEURODEGENERATIVE DISEASES

Repeat expansion neurodegenerative diseases (RENDS) are characterized by degeneration of specific neuronal populations. The presence of expanded repeat sequence in C9orf72 and in TATA-binding protein (TBP) genes are responsible for amyotrophic lateral sclerosis (ALS) and spinocerebellar ataxia 17 (SCA17). By repeat-associated non-ATG (RAN) translation, the G4C2 noncoding repeat expansions may also be translated in toxic di-peptide responsible for ALS. Intronic GGGGCC (G4C2) hexanucleotide repeat expansions within the human C9orf72 gene represents the most common cause of familial forms of amyotrophic lateral sclerosis (fALS) and frontotemporal dementia (FTD). Repeat-associated non-AUG (RAN) translation of resulting RNA leads to the production of neurotoxic dipeptide-repeat (DPR) proteins. DPR proteins aggregate into cytoplasm or nuclei of motor neurons, altering the proteotoxic response machinery. The protein quality control (PQC) system maintains protein homeostasis by re-folding or degradation (autophagy or proteasome) of misfolded proteins to counteract proteotoxic events. In parallel, mutations in the ubiquitin ligase STUB1, a gene involved in chaperone-assisted selective autophagy, explain the incomplete penetrance of SCA17 in patients carrying intermediate expansions contributing to misfolded protein accumulation. In C9 ALS models we identified i) forskolin (FSK, a cAMP-elevating compounds) as DPR protein levels enhancer, and ii) geldanamycin (GELD, an HSP90 inhibitor) and spironolactone (SPL, an aldosterone antagonist), as reducer of DPR protein levels. Interestingly, FSK-increased cAMP levels may activate PKA. We demonstrated that PKA blockage (by H89 treatment) or knockdown reduced translation efficiency (polyribosome profile) of DPRs in neuronal cells overexpressing DPR proteins, and in C9ALS/FTD patient-derived iPSC motor neurons with endogenous DPR protein levels. In neurons, we demonstrated that proteasome and autophagy pathways are responsible for proteins degradation in cells treated with GELD and with SPL suggesting that degradative systems and selective modulation of RAN translation can be molecular targets to reduce toxic protein in REND.