Declining intracellular proteostasis capacity drives misfolded protein secretion in senescent human cells.

Healthy protein homeostasis ('proteostasis') relies on tightly-regulated protein quality-control (PQC) circuits that co-ordinate sequestration and clearance of potentially toxic aggregation-prone proteins, arising from various internal or external stress throughout an organism's lifespan. At the protein level, proteotoxic stress responses typically involve extensive poly-ubiquitylation and sequestration of aggregation-prone proteins and PQC factors into various protective cytoplasmic and nuclear granules. However, much of our current understanding regarding this aspect of stress responses in humans stems from research in proliferating cells--despite growing evidence that stress responses vary considerably at the transcriptional level across cell proliferation states. Here, we show that the senescent cellular state--considered a major contributor to ageing-associated degeneration due to a chronic inflammatory phenotype--re-wires PQC and expels the misfolded protein load to mitigate proteotoxic stresses. Starting with a multi-dimensional transcriptomics and proteomics approach for measuring levels of total, poly-ubiquitylated, and granule-forming proteins, we have discovered a clear point of divergence between senescent and proliferating or quiescent human cell states in their responses to proteotoxic stress. Although the proteins that were poly-ubiquitylated and degraded during stress were largely conserved across states, the stress-induced sedimentation of a large number of disease-associated RNA-binding proteins (including TDP-43) was impaired only in the senescent state. Strikingly, TDP-43, as well as several other misfolded proteins, were actively secreted through the endo-lysosomal system by a diverse range of senescent cells during acute or chronic stress, through a process that requires the vesicle-associated HSP70 co-chaperone DNAJC5--an established risk factor for several neurodegenerative diseases. Misfolded protein secretion could be rescued by increasing intracellular HSP levels in 'shallow' but not 'deep' senescence, suggesting that secretion is a proteostatic adaptation that becomes less reversible over time. Our findings reveal an unappreciated aspect of the senescent-cell secretory phenotype, which may have important consequences for the non-cell-autonomous impact of senescence at the level of tissue resilience and frailty.