Kevin St. P. McNaught

Embryonic stem cells develop into functional dopaminergic neurons after transplantation in a Parkinson rat model

Although implantation of fetal dopamine (DA) neurons can reduce parkinsonism in patients, current methods are rudimentary, and a reliable donor cell source is lacking. We show that transplanting low doses of undifferentiated mouse embryonic stem (ES) cells into the rat striatum results in a proliferation of ES cells into fully differentiated DA neurons. ES cell-derived DA neurons caused gradual and sustained behavioral restoration of DA-mediated motor asymmetry. Behavioral recovery paralleled in vivo positron emission tomography and functional magnetic resonance imaging data demonstrating DA-mediated hemodynamic changes in the striatum and associated brain circuitry. These results demonstrate that transplanted ES cells can develop spontaneously into DA neurons. Such DA neurons can restore cerebral function and behavior in an animal model of Parkinsons disease.

Altered Proteasomal Function in Sporadic Parkinson's Disease

Parkinsons disease (PD) is characterized pathologically by preferential degeneration of the dopaminergic neurons in the substantia nigra pars compacta (SNc). Nigral cell death is accompanied by the accumulation of a wide range of poorly degraded proteins and the formation of proteinaceous inclusions (Lewy bodies) in dopaminergic neurons. Mutations in the genes encoding alpha-synuclein and two enzymes of the ubiquitin-proteasome system, parkin and ubiquitin C-terminal hydrolase L1, are associated with neurodegeneration in some familial forms of PD. We now show that, in comparison to age-matched controls, alpha-subunits (but not beta-subunits) of 26/20S proteasomes are lost within dopaminergic neurons and 20S proteasomal enzymatic activities are impaired in the SNc in sporadic PD. In addition, while the levels of the PA700 proteasome activator are reduced in the SNc in PD, PA700 expression is increased in other brain regions such as the frontal cortex and striatum. We also found that levels of the PA28 proteasome activator are very low to almost undetectable in the SNc compared to other brain areas in both normal and PD subjects. These findings suggest that failure of the ubiquitin-proteasome system to adequately clear unwanted proteins may underlie vulnerability and degeneration of the SNc in both sporadic and familial PD.

Systemic exposure to proteasome inhibitors causes a progressive model of Parkinson's disease

Environmental toxins have been implicated in the etiology of Parkinsons disease. Recent findings of defects in the ubiquitin‐proteasome system in hereditary and sporadic forms of the illness suggest that environmental proteasome inhibitors are candidate PD‐inducing toxins. Here, we systemically injected six doses of naturally occurring (epoxomicin) or synthetic (Z‐lle‐Glu(OtBu)‐Ala‐Leu‐al [PSI]) proteasome inhibitors into adult rats over a period of 2 weeks. After a latency of 1 to 2 weeks, animals developed progressive parkinsonism with bradykinesia, rigidity, tremor, and an abnormal posture, which improved with apomorphine treatment. Positron emission tomography demonstrated reduced carbon‐11‐labeled 2β‐carbomethoxy‐3β‐(4‐fluorophenyl)tropane (CFT) binding to dopaminergic nerve terminals in the striatum, indicative of degeneration of the nigrostriatal pathway. Postmortem analyses showed striatal dopamine depletion and dopaminergic cell death with apoptosis and inflammation in the substantia nigra pars compacta. In addition, neurodegeneration occurred in the locus coeruleus, dorsal motor nucleus of the vagus, and the nucleus basalis of Meynert. At neurodegenerative sites, intracytoplasmic, eosinophilic, α‐synuclein/ubiquitin–containing, inclusions resembling Lewy bodies were present in some of the remaining neurons. This animal model induced by proteasome inhibitors closely recapitulates key features of PD and may be valuable in studying etiopathogenic mechanisms and putative neuroprotective therapies for the illness. Ann Neurol 2004

Failure of the ubiquitin–proteasome system in Parkinson's disease

Parkinsons disease (PD) is a neurodegenerative movement disorder characterized by degeneration of dopamine-containing neurons in the midbrain. In cases of familial PD, mutations that lead to failure of the ubiquitin–proteasome system (UPS) have been identified. These genetic abnormalities do not occur in sporadic PD, but we propose that impairment of the UPS could also contribute to neurodegeneration in this disorder. We discuss evidence that failure of the UPS is a common aetiopathogenic factor that underlies the development of familial and sporadic PD, an idea that might help to explain clinical and pathological differences and similarities in these disorders.

Impairment of the ubiquitin-proteasome system causes dopaminergic cell death and inclusion body formation in ventral mesencephalic cultures

Mutations in α‐synuclein, parkin and ubiquitin C‐terminal hydrolase L1, and defects in 26/20S proteasomes, cause or are associated with the development of familial and sporadic Parkinsons disease (PD). This suggests that failure of the ubiquitin–proteasome system (UPS) to degrade abnormal proteins may underlie nigral degeneration and Lewy body formation that occur in PD. To explore this concept, we studied the effects of lactacystin‐mediated inhibition of 26/20S proteasomal function and ubiquitin aldehyde (UbA)‐induced impairment of ubiquitin C‐terminal hydrolase (UCH) activity in fetal rat ventral mesencephalic cultures. We demonstrate that both lactacystin and UbA caused concentration‐dependent and preferential degeneration of dopaminergic neurons. Inhibition of 26/20S proteasomal function was accompanied by the accumulation of α‐synuclein and ubiquitin, and the formation of inclusions that were immunoreactive for these proteins, in the cytoplasm of VM neurons. Inhibition of UCH was associated with a loss of ubiquitin immunoreactivity in the cytoplasm of VM neurons, but there was a marked and localized increase in α‐synuclein staining which may represent the formation of inclusions bodies in VM neurons. These findings provide direct evidence that impaired protein clearance can induce dopaminergic cell death and the formation of proteinaceous inclusion bodies in VM neurons. This study supports the concept that defects in the UPS may underlie nigral pathology in familial and sporadic forms of PD.

Proteasome inhibition causes nigral degeneration with inclusion bodies in rats

Structural and functional defects in 26/20S proteasomes occur in the substantia nigra pars compacta and may underlie protein accumulation, Lewy body formation and dopaminergic neuronal death in Parkinsons disease. We therefore determined the pathogenicity of proteasomal impairment following stereotaxic unilateral infusion of lactacystin, a selective proteasome inhibitor, into the substantia nigra pars compacta of rats. These animals became progressively bradykinetic, adopted a stooped posture and displayed contralateral head tilting. Administration of apomorphine to lactacystin-treated rats reversed behavioral abnormalities and induced contralateral rotations. Lactacystin caused dose-dependent degeneration of dopaminergic cell bodies and processes with the cytoplasmic accumulation and aggregation of &agr;-synuclein to form inclusion bodies. These findings support the notion that failure of the ubiquitin-proteasome system to degrade and clear unwanted proteins is an important etiopathogenic factor in Parkinsons disease.

Aggresome‐related biogenesis of Lewy bodies

Neurodegenerative disorders such as Parkinsons disease (PD) and ‘dementia with Lewy bodies’ (DLB) are characterized pathologically by selective neuronal death and the appearance of intracytoplasmic protein aggregates (Lewy bodies). The process by which these inclusions are formed and their role in the neurodegenerative process remain elusive. In this study, we demonstrate a close relationship between Lewy bodies and aggresomes, which are cytoplasmic inclusions formed at the centrosome as a cytoprotective response to sequester and degrade excess levels of potentially toxic abnormal proteins within cells. We show that the centrosome/aggresome‐related proteins γ‐tubulin and pericentrin display an aggresome‐like distribution in Lewy bodies in PD and DLB. Lewy bodies also sequester the ubiquitin‐activating enzyme (E1), the proteasome activators PA700 and PA28, and HSP70, all of which are recruited to aggresomes for enhanced proteolysis. Using novel antibodies that are specific and highly sensitive to ubiquitin–protein conjugates, we revealed the presence of numerous discrete ubiquitinated protein aggregates in neuronal soma and processes in PD and DLB. These aggregates appear to be being transported from peripheral sites to the centrosome where they are sequestered to form Lewy bodies in neurons. Finally, we have shown that inhibition of proteasomal function or generation of misfolded proteins cause the formation of aggresome/Lewy body‐like inclusions and cytotoxicity in dopaminergic neurons in culture. These observations suggest that Lewy body formation may be an aggresome‐related event in response to increasing levels of abnormal proteins in neurons. This phenomenon is consistent with growing evidence that altered protein handling underlies the etiopathogenesis of PD and related disorders.

Lewy-body formation is an aggresome-related process: a hypothesis.

Parkinsons disease (PD) is an age-related neurodegenerative disorder that is associated with the formation of intracytoplasmic protein aggregates (Lewy-body inclusions) in neurons of the substantia nigra pars compacta and other brain areas. These inclusions were discovered over 90 years ago, but the mechanism underlying their formation and their relevance to the neurodegenerative process are unknown. Recent studies have begun to shed light on the biogenesis of Lewy bodies and suggest that they are related to aggresomes. Aggresomes are cytoprotective proteinaceous inclusions formed at the centrosome that segregate and facilitate the degradation of excess amounts of unwanted and possibly cytotoxic proteins. The concept of Lewy bodies as aggresome-related inclusions fits well with ongoing discoveries suggesting that altered protein handling might contribute to the neurodegenerative process in familial and sporadic forms of PD.

Selective loss of 20S proteasome α-subunits in the substantia nigra pars compacta in Parkinson's disease

The proteolytic activities of 26/20S proteasomes are impaired in the substantia nigra pars compacta (SNc) in sporadic Parkinsons disease (PD). In the present study, we examined the structural integrity of the proteasome by determining the levels of the beta- and alpha-subunits which together normally constitute the catalytic core of 26/20S proteasomes. Western blot analyzes and immunohistochemical staining revealed a major and selective loss of alpha-subunits in dopaminergic neurons of the SNc but not in other brain regions in sporadic PD. This defect is known to cause the proteasome to become unstable and prevents its assembly with resultant impairment of enzymatic activity. Thus, structural and function defects in 26/20S proteasomes may underlie protein accumulation, formation of proteinaceous Lewy bodies and dopaminergic neuronal death in the SNc in sporadic PD.

Proteolytic stress: A unifying concept for the etiopathogenesis of Parkinson's disease

The etiopathogenesis of Parkinsons disease (PD) has been elusive. Recently, several lines of evidence have converged to suggest that defects in the ubiquitin‐proteasome system and proteolytic stress underlie nigral pathology in both familial and sporadic forms of the illness. In support of this concept, mutations in α‐synuclein that cause the protein to misfold and resist proteasomal degradation cause familial PD. Similarly, mutations in two enzymes involved in the normal function of the ubiquitin‐proteasome system, parkin and ubiquitin C‐terminal hydrolase L1, are also associated with hereditary PD. Furthermore, structural and function defects in 26/20S proteasomes with accumulation and aggregation of potentially cytotoxic abnormal proteins have been identified in the substantia nigra pars compacta of patients with sporadic PD. Thus, a defect in protein handling appears to be a common factor in sporadic and the various familial forms of PD. This hypothesis may also account for the vulnerability of the substantia nigra pars compacta in PD, why the disorder is age related, and the nature of the Lewy body. It has also facilitated the development of experimental models that recapitulate the behavioral and pathological features of PD, and hopefully will lead to the development of novel neuroprotective therapies for the disorder. Ann Neurol 2003;53 (suppl 3):S73–S86