MOLECULAR MECHANISMS ORCHESTRATING LYSOSOME-MEDIATED DEGRADATION OF RIBOSOMES IN ANEUPLOID CELLS

Chromosome segregation errors lead to the generation of aneuploid daughter cells with unbalanced karyotypes. This condition has a profound impact on cell physiology, causing a plethora of cellular stresses. One of the most prominent effects of abnormal karyotypes is the development of proteotoxic stress, which is characterised by the aggregation of aberrant proteins in the cytoplasm. The disruption of protein homeostasis correlates with and depends on gene copy number changes and occurs at multiple levels in aneuploid cells. Protein folding, for instance, is known to be compromised and this feature together with the overwhelming of quality control mechanisms leads to the accumulation and aggregation of not-properly folded proteins in the cytoplasm. The pathway that is in charge for the degradation of extra, misfolded or defective proteins is autophagy, which, accordingly, is close to saturation in aneuploid cells. Unlike other aspects of aneuploidy, how exactly cells respond to the onset of proteotoxic stress is yet to be explored. The studies conducted within my PhD project revealed that, in aneuploid cells with random chromosome gains and losses, an impaired protein folding is strictly connected to the lysosome-mediated degradation of ribosomes. In detail, with my work I showed that, right after chromosome mis-segregation, cells face an increasing folding demand that challenges Hsp90 and Hsp70 chaperone families. The lack of an efficient folding machinery, in turn, causes the attenuation of the synthesis of new polypeptides, which is further exacerbated by the activation of the unfolded protein response (UPR). By addressing the molecular mechanisms that aneuploid cells activate to cope with folding and translation deficiencies, I have found that ribosomes are recognised by the E3-ligase ZNF598 and tagged for degradation. Their clearance is carried out by the autophagic pathway, in particular by their selective degradation mediated by lysosomes. This is indicated by the increased ribophagy I observed in aneuploid cells, right after chromosome mis-segregation. On the other hand, aneuploidy-driven genomic instability is known to fuel a vicious cycle leading to the generation of complex karyotypes. Accumulation of such chromosomal aberrations, in turn, exacerbate the aneuploidy-associated stresses and I demonstrated that this is also the case with proteotoxic stress. Importantly, I showed that ribosome degradation is increased under this condition, but the initial selectivity mediated by ZNF598 is lost, due to the uncontrolled increase in bulk autophagy. As a matter of fact, the accumulation of toxic aggregates and extra proteins boost bulk autophagy, leading to the random engulfment of cytosolic cargo in the autophagic structures, including ribosomes. Interestingly, by stratifying aneuploid human cancers by their aneuploidy score, I found that the highly aneuploid ones are negatively associated with ribosomal signatures and tend to overexpress ZNF598, which might enable them to keep under control the proteotoxic stress. Overall, this study shed light on a previously uncharacterised chain of events activated by aneuploid cells in response to the onset of proteotoxic stress. My results have the potential to indicate specific targets for cancer therapy, in contexts in which modulation of protein translation can be selectively targeted with the goal to interfere with cell proliferation.