THE INTERPLAY BETWEEN THE PROTEIN QUALITY CONTROL SYSTEM AND EXTRACELLULAR VESICLES IN THE DISPOSAL OF DISEASE-ASSOCIATED PROTEINS AND MIRNAS IN ALS AND FTD MODELS

Amyotrophic Lateral Sclerosis (ALS) and Frontotemporal dementia (FTD) are two neurodegenerative diseases characterized by the presence within neurons of toxic cytoplasmic inclusions containing the insoluble forms of the TAR DNA-binding protein of 43 KDa (TDP-43) and its C-terminal fragments (CTFs) of 35 (TDP-35) and 25 KDa (TDP-25). The accumulation of TDP-43 into insoluble cytoplasmic aggregates is toxic for cells that prevent their formation and/or promote their degradation through the protein quality control (PQC) system (i.e. chaperone and co-chaperone proteins, the ubiquitin proteasome system (UPS) and the autophagy). However, TDP-43 has been also found into extracellular vesicles (EVs), both large (LEVs) and small (SEVs), suggesting that EVs could cooperate with the PQC system for the disposal of TDP species. Moreover, TDP-43 is an RNA-binding protein that plays several functions in RNA metabolism, including miRNAs biogenesis. Its accumulation within the ALS/FTD-associated cytoplasmic aggregates prevents it from performing these functions causing an alteration in miRNAs biogenesis and possibly a deregulation of miRNAs secreted by the affected cells respect to the control condition. In my PhD project I investigated the crosstalk between EVs and PQC i) in the disposal of ALS and FTD-associated TDP species both in neuron and in muscle cells, and ii) in the secretion of miRNAs, both in physiological and in pathological (when the UPS and autophagy are blocked) conditions. By analyzing EVs obtained from neuronal (NSC34) and muscle (C2C12) cells and plasma of FTD patients and healthy controls, I demonstrated that both neurons and muscles cells secrete TDP-43, TDP-35 and TDP-25 (mainly in their insoluble form in LEVs) and that the Chaperone assisted selective autophagy (CASA) complex and/or the chaperone HSPB8 alone may assist TDP species secretion in EVs. I also found that the targeting of TDP species to EVs in neurons and muscle cells is increased mainly following the UPS and the autophagy blockade, respectively. This suggests that neurons degrade TDP species mainly via UPS, while muscle cells via autophagy and that the re-routing of TDP species into EVs may represent a protective compensatory mechanism for their disposal when the intracellular degradation is blocked. However, I demonstrated that these EVs were toxic to recipient cells, therefore they may contribute to the spreading of the disease. In line with this observation, I specifically detected the pro-aggregating TDP species TDP-35 in EVs derived from plasma of FTD patients. Finally, the analysis of EVs miRNA content demonstrated that LEVs and SEVs contain different miRNAs and that UPS and autophagy inhibition similarly deregulated the secretion of several miRNAs in SEVs. Interestingly, a number of SEVs deregulated miRNAs targets the prion disease pathway and thus probably affects cell survival. Taken together these data suggest that the secretion of TDP species and potentially harmful miRNAs into EVs normally happens in cell in basal condition, but the PQC inhibition, a typical ALS and FTD condition, increases the secretion of the TDP species and causes the deregulation of specific EVs miRNAs, contributing to the spreading of the disease to neighboring and/or more distant cells.