The clearance of aggregated TDP-43 responsible for ALS/FTD diseases

Increasing biochemical and genetic evidence have suggested that Amyotrophic Lateral Sclerosis (ALS) and Frontotemporal Dementia (FTD) are two related neurodegenerative diseases, with a continuous clinical spectrum. Even if the neuronal population affected may differ between ALS and FTD, a common feature is the mislocalization and aggregation of the TAR DNA binding protein 43 (TDP-43), in affected cells. TDP-43 aggregates contain C-terminal TDP-43 fragments of 35 kDa (TDP-35) and 25 kDa (TDP-25), that are highly aggregation-prone, and are thought to be neurotoxic. Therefore, TDP-35 or TDP-25 fragments must be efficiently cleared from cells to prevent their aggregation and sequestration of other important neuronal components. The clearance of aberrantly folded or misfolded proteins is mediated by the intracellular protein quality control (PQC) system. The PQC system is composed of chaperone/co-chaperone proteins, which recognize and bind to aberrant proteins for their refolding, but when this fails, they may target them to the degradative systems, like the ubiquitin proteasome system and the autophagy. Also the extracellular secretory pathway, mainly exosomes (EXOs) and microvesicles (MVs), might be involved in misfolded proteins clearance from affected cells. Using neuronal and muscle cell models and flies we have investigated the role of PQC and extracellular vesicles in the clearance of TDP-43 and its C-terminal fragments. We have demonstrated that all TDP-43 species are cleared by proteasome, but TDP-25 impairs autophagy. We found that the routing of TDP fragments to proteasome, by overexpressing the co-chaperone BAG1, or to autophagy, by overexpressing the neuroprotective small heat shock proteins HSPB8 or its co-chaperone BAG3 decreased the accumulation of both TDP43 fragments, possibly reducing their toxicity. Finally, our preliminary results show that all TDP-43 species are secreted in EVs, mainly in MVs. Very interestingly, we found that also HSPB8 is present in EVs, suggesting an interplay between PQC and EVs. GRANTS: Fondazione Cariplo, Italy (n. 2017_0747); Fondazione AriSLA, Italy (n. ALS_HSPB8; ALS_Granulopathy; MLOpathy; Target-RAN), Università degli Studi di Milano e piano di sviluppo UNIMI - linea B; Agenzia Italiana del Farmaco (AIFA) (Co_ALS); Italian Ministry of Health (n. GR-2011- 02347198); Fondazione Regionale per la Ricerca Biomedica (FRRB) (Regione Lombardia, TRANS_ALS, project nr. 2015-0023).