Dose-dependent inhibition of proteasome activity by a mutant ubiquitin associated with neurodegenerative disease

The ubiquitin-proteasome system is the main regulated intracellular proteolytic pathway. Increasing evidence implicates impairment of this system in the pathogenesis of diseases with ubiquitin-positive pathology. A mutant ubiquitin, UBB⁺¹, accumulates in the pathological hallmarks of tauopathies, including Alzheimer's disease, polyglutamine diseases, liver disease and muscle disease and serves as an endogenous reporter for proteasomal dysfunction in these diseases. UBB⁺¹ is a substrate for proteasomal degradation, however it can also inhibit the proteasome. Here, we show that UBB⁺¹ properties shift from substrate to inhibitor in a dose-dependent manner in cell culture using an inducible UBB⁺¹ expression system. At low expression levels, UBB⁺¹ was efficiently degraded by the proteasome. At high levels, the proteasome failed to degrade UBB⁺¹, causing its accumulation, which subsequently induced a reversible functional impairment of the ubiquitin-proteasome system. Also in brain slice cultures, UBB⁺¹ accumulation and concomitant proteasome inhibition was only induced at high expression levels. Our findings show that by varying UBB⁺¹ expression levels, the dual proteasome substrate and inhibitory properties can be optimally used to serve as a research tool to study the ubiquitin-proteasome system and to further elucidate the role of aberrations of this pathway in disease.