Revisiting the clastosome: a stress-induced nuclear proteolytic compartment of mammalian cells

The nuclear ubiquitin proteasome system (UPS) is fundamental to maintaining proteostasis and ensuring the quality control of nuclear proteins. Within the nucleus, proteasomes are distributed throughout the nucleoplasm and can aggregate into nuclear bodies. In 2002, our group provided the first description in mammalian cells of a specific subtype of nuclear body highly enriched in: (i) ubiquitin conjugates, (ii) the proteolytically active 20S and 19S regulatory complexes of the 26S proteasome, (iii) the molecular chaperone Hsp70, and (iv) proteasome substrates. We coined the term clastosome to define this nuclear proteolytic center of the UPS. Clastosomes exhibit dynamic behavior, and their formation is transiently and robustly induced in mammalian neurons during osmotic stress, coinciding with enhanced proteasomal activity. Subsequent studies have confirmed and mechanistically expanded our understanding of these nuclear proteolytic centers, which are now recognized as nuclear condensates formed via liquid-liquid phase separation mechanisms. Notably, PML nuclear bodies can establish interactions with clastosomes, thereby linking the PML protein and the SUMOylation pathway with UPS-mediated protein degradation. The association between PML nuclear bodies and clastosomes has been implicated in the clearance of toxic mutant proteins associated with certain neurodegenerative diseases. This review examines the structure, composition, and dynamic regulation of clastosomes, focusing on their modulation during stress responses and neurodegeneration. Given the essential role of nuclear proteolytic centers in proteostasis regulation, a better understanding of mechanisms that modulate their assembly may offer therapeutic strategies for neurodegenerative proteinopathies and other pathologies.