The pattern of neuronal loss and survival may reflect differential expression of proteasome activators in Parkinson's disease.

The basis of neuronal vulnerability, degeneration, and sparing in PD are unknown, but there is increasing evidence to suggest that the ubiquitin-proteasome system (UPS) plays an important role in the pathogenesis of the disorder. In this study, we employed an immunocytochemical approach to determine if the differential expression of key UPS components in various brain regions and cells might underlie the pattern of neuronal degeneration and survival seen in PD. We showed that the ubiquitin activating enzyme (E1), ubiquitin conjugating enzyme (E2), and 26/20S proteasome alpha- and beta-subunits, are abundantly expressed in the substantia nigra pars compacta (SNc) and in cultured dopaminergic neurons. Although the proteasome activator PA700 is expressed in the medial SNc, levels are low in the lateral region, and expression of the other proteasome activator, PA28, is near absent in the entire SNc. PA28 (but not PA700) was found to be poorly expressed in noradrenergic neurons in the locus coeruleus (LC) compared with adjacent cells in the mesencephalic nucleus. PA700 and PA28 are also poorly expressed in dopaminergic neurons compared with other cell types in culture. Inhibition of proteasomal function, generation of misfolded proteins, induction of oxidative stress or impairment of mitochondrial complex I activity, caused a compensatory upregulation in PA700 and PA28 in a variety of cells but not in dopaminergic neurons in culture. These findings are consistent with the demonstration that, in sporadic PD, proteasomal activity and levels of PA700/PA28 are reduced in the SNc but are markedly upregulated in regions/cells that are spared from the neurodegenerative process. Thus, the differential distribution and activity of proteasome activations could play a significant role in the pathogenesis of PD.

Proteasomal dysfunction in sporadic Parkinson's disease.

The cause and mechanism of neuronal death in sporadic Parkinson's disease (PD) continue to elude investigators. Recently, alterations in proteasomal function have been detected in the brain of patients with the illness. The biochemical basis of the defect and its relevance to the disease process are now being studied. The available results suggest that proteasomal dysfunction could underlie protein accumulation, Lewy body formation, and neuron death in PD. The cause of proteasomal dysfunction is unknown at present, but this could relate to gene mutations, oxidative damage, ATP depletion, or the actions of environmental toxins. It remains to be established if proteasomal dysfunction plays a primary or a secondary role in the initiation or progression of the neurodegenerative process in PD.

Generation and characterization of Dyt1 DeltaGAG knock-in mouse as a model for early-onset dystonia.

A trinucleotide deletion of GAG in the DYT1 gene that encodes torsinA protein is implicated in the neurological movement disorder of Oppenheim's early-onset dystonia. The mutation removes a glutamic acid in the carboxy region of torsinA, a member of the Clp protease/heat shock protein family. The function of torsinA and the role of the mutation in causing dystonia are largely unknown. To gain insight into these unknowns, we made a gene-targeted mouse model of Dyt1 DeltaGAG to mimic the mutation found in DYT1 dystonic patients. The mutated heterozygous mice had deficient performance on the beam-walking test, a measure of fine motor coordination and balance. In addition, they exhibited hyperactivity in the open-field test. Mutant mice also showed a gait abnormality of increased overlap. Mice at 3 months of age did not display deficits in beam-walking and gait, while 6-month mutant mice did, indicating an age factor in phenotypic expression as well. While striatal dopamine and 4-dihydroxyphenylacetic acid (DOPAC) levels in Dyt1 DeltaGAG mice were similar to that of wild-type mice, a 27% decrease in 4-hydroxy, 3-methoxyphenacetic acid (homovanillic acid) was detected in mutant mice. Dyt1 DeltaGAG tissues also have ubiquitin- and torsinA-containing aggregates in neurons of the pontine nuclei. A sex difference was noticed in the mutant mice with female mutant mice exhibiting fewer alterations in behavioral, neurochemical, and cellular changes. Our results show that knocking in a Dyt1 DeltaGAG allele in mouse alters their motor behavior and recapitulates the production of protein aggregates that are seen in dystonic patients. Our data further support alterations in the dopaminergic system as a part of dystonia's neuropathology.

Brainstem pathology in DYT1 primary torsion dystonia.

DYT1 dystonia is a severe form of young-onset dystonia caused by a mutation in the gene that encodes for the protein torsinA, which is thought to play a role in protein transport and degradation. We describe, for the first time to our knowledge, perinuclear inclusion bodies in the midbrain reticular formation and periaqueductal gray in four clinically documented and genetically confirmed DYT1 patients but not in controls. The inclusions were located within cholinergic and other neurons in the pedunculopontine nucleus, cuneiform nucleus, and griseum centrale mesencephali and stained positively for ubiquitin, torsinA, and the nuclear envelope protein lamin A/C. No evidence of inclusion body formation was detected in the substantia nigra pars compacta, striatum, hippocampus, or selected regions of the cerebral cortex. We also noted tau/ubiquitin-immunoreactive aggregates in pigmented neurons of the substantia nigra pars compacta and locus coeruleus in all four DYT1 dystonia cases, but not in controls. This study supports the notion that DYT1 dystonia is associated with impaired protein handling and the nuclear envelope. The role of the pedunculopontine and cuneiform nuclei, and related brainstem brainstem structures, in mediating motor activity and controlling muscle tone suggests that alterations in these structures could underlie the pathophysiology of DYT1 dystonia [corrected]