The ubiquitin-proteasome system during proteotoxic stress

Abstract: The dual function of the ubiquitin-proteasome system in protein quality control and as a master regulator of vital cellular processes places the system in a delicate position. This might be particularly relevant under patho- physiological conditions such as endoplasmic reticulum stress, which provoke the accumulation of aberrant proteins. This scenario, also referred to as proteotoxic stress, is associated with numerous devastating disorders, including a large number of neurodegenerative diseases like Alzheimer s disease, Parkinson s disease and polyglutamine diseases. The question is whether the ubiquitin-proteasome system is able to promptly and adequately respond to these challenging conditions without compromising its other functions. One of the difficulties approaching these questions is the lack of systems for monitoring the functionality of the ubiquitin-proteasome activity in vivo. We have generated fluorescent reporter substrates for monitoring the functionality of the ubiquitin-proteasome system in cell lines. In addition, we have developed a transgenic mouse model constitutively expressing one of these reporters. Finally, we have designed and characterized a novel fluorescent activity probe that permits specific labeling of proteasomes in vitro and in vivo. We have subsequently used these models to gain insight into the mechanisms contributing to the long term accumulation of deleterious proteins during proteotoxic stress. These studies revealed that proteotoxic stress conditions compromises the functionality of the ubiquitin-proteasome system. In these circumstances, ubiquitin-proteasomal degradation is still taking place but it is suboptimal. Detailed analysis of the dynamics of ubiquitylation in living cells suggests that ubiquitin is a rate limiting factor during stress conditions. Importantly, we found that whereas the ubiquitin-proteasome was able to remove the majority of the accumulated substrates once the cells have recovered from the stress condition, the cells were unable to clear accumulated aggregation-prone substrates. This observation might explain the preferential accumulation of such substrates in conformational diseases.