A REAPPRISAL OF CELL-CLEARING MECHANISMS: THE AUTOPHAGOPROTEASOME HOSTING AUTOPHAGY AND UBIQUITIN PROTEASOME.

Autophagy (ATG) and ubiquitin proteasome system (UPS) are two quintessential clearing mechanisms widely involved in eukaryotic cell biology. The dysfunction of either pathway is associated with systemic diseases, tumor progression and neurodegeneration. Classic ATG occurs within a double-layered cytosolic vacuole named autophagosome, gifted with a rich enzymatic apparatus aimed at clearing various cell cargoes and waste compounds. In contrast, the classic UP pathway does not imply a well-defined cell organelle and it occurs within dispersed cytosolic domains. Here, UP subunits interact to recognize altered ubiquitinated substrates and provide proteolytic clearance. Although these pathways have been long considered as biochemically and morphologically independent, recent data suggest a functional interplay and reciprocal compensation between ATG and UPS. In this study, we demonstrated that ATG and UP components are hosted in the same organelle that we named autophagoproteasome. This novel organelle was characterized in a human glioblastoma U87MG cell line, in baseline conditions displaying defective ATG signaling, as well as following mTOR inhibition aimed at achieving a fine-tune modulation of both ATG and UPS. The quantitation carried out by confocal microscopy and ultrastructural morphometry shows that mTOR inhibition remarkably increases autophagoproteasomes. Nonetheless, within the autophagoproteasome, the relative amount for ATG compared with UP depends on the stimuli provided upon different experimental conditions. Noteworthy, upon strong m-TOR inhibition, the rich ATG compartment remains partly independent, while almost all cytosolic UP structures are sequestered within either early or late ATG vesicles, yielding a novel powerful clearing apparatus gifted with enriched catalytic pattern. Most notably, the occurrence of their co-immunoprecipitation strengthens the evidence for a reciprocal binding and a functional interplay in a newly integrated cell-clearing scenario, which is still under our scrutiny even in other experimental models. Our findings suggest that this ultimate organelle may be a specific target for drugs against neuronal degeneration.