The proteasomal inhibition model of Parkinson's disease: “Boon or bust”?

To date, important breakthroughs and advances in our understanding of the pathogenesis of Parkinson’s disease (PD) and the expected subsequent development of effective neuroprotective therapy have been hampered by the lack of truly representative animal models. Although existing models have served many useful purposes, none of these models accurately reflects the progressive process that involves multiple areas of the central and peripheral nervous system and neurodegenerative changes dominated by the hallmark aggregation of -synuclein in remaining neurons and nerve processes. In 2004, McNaught and colleagues published a report describing a remarkable new model of PD using systemic exposure to proteasome inhibitors. This model pursued the hypothesis that environmental proteasome inhibitors are candidate toxins for inducing PD based on the initial observation that mutations in parkin, a ubiquitin E3 ligase, cause early-onset PD and on some (but not all) subsequent studies showing defects of the ubiquitin-proteasome system in sporadic PD. McNaught and colleagues treated adult rats over a period of 2 weeks with six subcutaneously administered doses of either the naturally occurring proteasome inhibitor epoxomicin or the synthetic proteasome inhibitor PSI. After a latency of 1 to 2 weeks, the animals developed progressive features of parkinsonism with bradykinesia, rigidity, tremor, and an abnormal posture, all of which improved in response to the potent dopamine agonist apomorphine or to L-dopa. A quantitative loss of striatal dopamine transporter was identified by Anna-Liisa Brownell at Massachusetts General Hospital, a pioneer in mini position emission tomography, using the dopamine transporter ligand carbon-11–labeled 2 -carbomethoxy-3 -(4-fluorophenyl) tropane (C-CFT). These studies provided evidence for loss of dopaminergic nerve terminals in the striatum due to degeneration of the nigrostriatal pathway. Postmortem analyses indicated striatal dopamine depletion and dopaminergic cell death with apoptosis and inflammation in the substantia nigra pars compacta (SNc). In addition, degeneration occurred in the locus ceruleus, dorsal motor nucleus of the vagus, and the nucleus basalis of Meynert. Within the degenerating neurons, it was observed that there were intracytoplasmic, eosinophilic, -synuclein/ubiquitin–containing inclusions resembling Lewy bodies. These observations brought great excitement to the PD community. They not only identified a new potential model of PD, but also pointed to pathogenetic mechanisms. As such, it was hoped that these toxins would provide a markedly improved model for testing novel therapeutics for PD, particularly those directed at modifying the disease process. Unfortunately, this model has subsequently become controversial. A number of laboratories have been unable to replicate the initial results. In this issue of Annals, there are a number of new reports that either claim to partially replicate or completely fail to replicate these initial findings. Given the extent of the controversy, some comments on the details of the research methodology are necessary. Almost all of these studies were done with PSI, which is a less costly proteasome inhibitor than epoxomicin. In the original study, the authors used Sprague–Dawley rats obtained from Taconic Farms (Germantown, NY). Twenty-seven rats were treated with 3.0mg/kg PSI subcutaneously, 3 with 6.0 mg/kg PSI subcutaneously and 27 with 70% ethanol subcutaneously as a control. In addition, six animals were injected with 1.5 mg/kg epoxomicin intraperitoneally and seven with 10% dimethylsulfoxide (DMSO) as a control. In each case, six injections were made over 2 weeks on a course of Monday, Wednesday, Friday, Monday, Wednesday, Friday. The rats were examined daily for the first 2 weeks, then weekly thereafter, for 21 weeks, to determine the presence, severity, and progression of motor dysfunction. The treatment with epoxomicin led to a significant (approximately 75%) depletion of dopamine and its metabolites. The positron emission tomography studies were done on rats at 17 to 19 weeks after completion of treatments with 3.0mg PSI or vehicle control (n 5 per group). These showed approximately a 50% depletion of the C-CFT binding in the PSI group ( p 0.01). In the series of articles in this issue of Annals, there is an update on further studies by McNaught and Olanow. The authors note that they have subsequently performed electron microscopy on tissues from the SNc. They demonstrated membranous whorls, some of which were associated with ribosomes and electrondense inclusion bodies with an internal structure, which immunoelectron microscopy stained positively for ubiquitin and -synuclein. They also note in their initial studies that proteasome enzyme activities were reduced in the ventral midbrain section of the SNc and the lower brainstem, but were upregulated in areas that did not degenerate, suggesting a compensatory process. The authors state that they have obtained similar behavioral and pathological changes after oral administration of epoxomicin in rats and after systemic administration of PSI in mice. However, these findings have not yet been reported.