The ubiquitin proteasome system in neuropathology

The ubiquitin proteasome system (UPS) orchestrates the turnover of innumerable cellular proteins. In the process of ubiquitination the small protein ubiquitin is attached to a target protein by a peptide bond. The ubiquitinated target protein is subsequently shuttled to a protease complex known as the 26S proteasome and subjected to degradative proteolysis. The UPS facilitates the turnover of proteins in several settings. It targets oxidized, mutant or misfolded proteins for general proteolytic destruction, and allows for the tightly controlled and specific destruction of proteins involved in development and differentiation, cell cycle progression, circadian rhythms, apoptosis, and other biological processes. In neuropathology, alteration of the UPS, or mutations in UPS target proteins may result in signaling abnormalities leading to the initiation or progression of tumors such as astrocytomas, hemangioblastomas, craniopharyngiomas, pituitary adenomas, and medulloblastomas. Dysregulation of the UPS may also contribute to tumor progression by perturbation of DNA replication and mitotic control mechanisms, leading to genomic instability. In neurodegenerative diseases caused by the expression of mutant proteins, the cellular accumulation of these proteins may overload the UPS, indirectly contributing to the disease process, e.g., sporadic Parkinsonism and prion diseases. In other cases, mutation of UPS components may directly cause pathological accumulation of proteins, e.g., autosomal recessive Parkinsonism and spinocerebellar ataxias. Defects or dysfunction of the UPS may also underlie cognitive disorders such as Angelman syndrome, Rett syndrome and autism, and muscle and nerve diseases, e.g., inclusion body myopathy and giant axon neuropathy. This paper describes the basic biochemical mechanisms comprising the UPS and reviews both its theoretical and proven involvement in neuropathological diseases. The potential for the UPS as a target of pharmacological therapy is also discussed

Changes in the ratio of free NEDD8 to ubiquitin triggers NEDDylation by ubiquitin enzymes

Ubiquitin and UBL (ubiquitin-like) modifiers are small proteins that covalently modify other proteins to alter their properties or behaviours. Ubiquitin modification (ubiquitylation) targets many substrates, often leading to their proteasomal degradation. NEDD8 (neural-precursor-cell-expressed developmentally down-regulated 8) is the UBL most closely related to ubiquitin, and its best-studied role is the activation of CRLs (cullin-RING ubiquitin ligases) by its conjugation to a conserved C-terminal lysine residue on cullin proteins. The attachment of UBLs requires three UBL-specific enzymes, termed E1, E2 and E3, which are usually well insulated from parallel UBL pathways. In the present study, we report a new mode of NEDD8 conjugation (NEDDylation) whereby the UBL NEDD8 is linked to proteins by ubiquitin enzymes in vivo. We found that this atypical NEDDylation is independent of classical NEDD8 enzymes, conserved from yeast to mammals, and triggered by an increase in the NEDD8 to ubiquitin ratio. In cells, NEDD8 overexpression leads to this type of NEDDylation by increasing the concentration of NEDD8, whereas proteasome inhibition has the same effect by depleting free ubiquitin. We show that bortezomib, a proteasome inhibitor used in cancer therapy, triggers atypical NEDDylation in tissue culture, which suggests that a similar process may occur in patients receiving this treatment

NUB1 modulation of GSK3β reduces tau aggregation

Abnormal phosphorylation of the microtubule-associated protein tau in neurodegenerative disorders, including Alzheimer’s disease (AD) and frontotemporal lobar degeneration, is associated with disrupted axonal transport and synaptic dysfunction ultimately manifesting as histopathological lesions of protein aggregates. Glycogen synthase kinase 3b (GSK3b) may be critical for the pathological hyperphosphorylation of tau. Here, we examined the role of the proteasome-associated protein Nedd8 ultimate buster 1 (NUB1) in the neuropathogenic phosphorylation and aggregation of tau. We reveal that NUB1 interacted with both tau and GSK3b to disrupt their interaction, and abolished recruitment of GSK3b to tau inclusions. Moreover, NUB1 reduced GSK3b-mediated phosphorylation of tau and aggregation of tau in intracellular inclusions. Strikingly, NUB1 induced GSK3b degradation. Deletion of the NUB1 ubiquitin-like (UBL) domain did not impair the interaction with tau and GSK3b, and the ability to suppress the phosphorylation and aggregation of tau was not affected. However, the UBL motif was necessary for GSK3b degradation. Deletion of the NUB1 ubiquitin-associated (UBA) domain abrogated the ability of NUB1 to interact with and degrade GSK3b. Moreover, the UBA domain was required to suppress the aggregation of tau. Silencing of NUB1 in cells stabilized endogenous GSK3b and exacerbated tau phosphorylation. Thus, we propose that NUB1, by regulating GSK3b levels, modulates tau phosphorylation and aggregation, and is a key player in neurodegeneration associated with tau pathology. Moreover, NUB1 regulation of GSK3b could modulate numerous signalling pathways in which GSK3b is a centrally important effector