The role of the ubiquitin-proteasome system in neurodegenerative disorders

Neurodegenerative disorders are a heterogeneous group of clinically and pathologically diverse diseases. The diseases are characterised by selective loss of neurons, in specific regions of the brain. The result is disruption of motor, sensory or cognitive systems, leading to severe disability of the patients. Despite the variability between the diseases, there are some striking similarities. A common feature in many of these diseases is the presence of aggregated proteins that are covalently linked to ubiquitin (Ub). The ubiquitin-proteasome system (UPS) is the main pathway in the cell for the elimination of aberrant or misfolded proteins. Nevertheless, in neurodegenerative diseases these proteins accumulate with disastrous consequences for neurons, eventually leading to cell death. In this thesis, the role of the UPS in neurodegeneration was investigated. These studies focus on the degradation of specific disease related proteins and the general status of the UPS under conditions of an excess of aberrant or misfolded proteins. To evaluate the capacity of the UPS to degrade disease related proteins, polyglutamine (polyGln) proteins were targeted for proteasomal degradation. These proteins were efficiently degraded independent of the length of the polyGln repeat. However, aggregation of the aggregationprone polyGln proteins prevented proteasomal degradation. Thus the formation of aggregates renders these toxic proteins resistant to proteasomal degradation and initiates the accumulation of polyGln proteins and polyGln-interacting proteins. A mutant form of Ub, UBB is another protein that can resist proteasomal degradation. UBB accumulates in neurons of patients with several neurodegenerative diseases. We show that UBB is a substrate of the proteasome but is too short to be efficiently degraded. The lack of UBB degradation causes an inhibitory effect on the UPS. The accumulation of misfolded proteins inside the endoplasmic reticulum (ER) causes ER stress which is found in many neurodegenerative disorders. Since the UPS is also responsible for the degradation of ER proteins we investigated the effect of ER stress on the functionality of the UPS. We found that ER stress compromises the UPS though not fully blocks its function. This suggests that the load of ER proteins and the ER environment may be important parameters for the gradual progressive accumulation of misfolded proteins in neurodegenerative diseases. In conclusion, the UPS is involved in the degradation of accumulated misfolded or aberrant proteins occurring in neurodegenerative diseases. However, in these diseases the UPS is compromised and some proteins might resist degradation. LIST OF PUBLICATIONS This thesis is based on the following papers that will be referred to in the text by their Roman numerals: I Lisette G. G. C. Verhoef, Kristina Lindsten, Maria G. Masucci and Nico P. Dantuma. Aggregate formation inhibits proteasomal degradation of polyglutamine proteins. (2002) Hum. Mol. Genet. 11(22):2689-2700. II Kristina Lindsten, Femke M. S. de Vrij, Lisette G. G. C. Verhoef, David F. Fischer, Fred W. van Leeuwen, Elly M. Hol, Maria G. Masucci and Nico P. Dantuma. Mutant ubiquitin found in neurodegenerative disorders is a ubiquitin fusion-degradation substrate that blocks proteasomal degradation. (2002) J. Cell. Biol. 157(3):417-427. III Lisette G. G. C. Verhoef and Nico P. Dantuma. Designed ubiquitin fusion degradation substrates reveal minimal substrate length for efficient proteasomal degradation. Manuscript. IV Victoria Menendez-Benito, Lisette G. G. C. Verhoef, Maria G. Masucci and Nico P. Dantuma. (2005) Endoplasmic reticulum stress compromises the ubiquitin-proteasome system. Hum. Mol. Genet. 14(19):2787-2799.