23 Jun 2004

This past week, a flurry of studies advanced Parkinson’s research on several different fronts. An experimental immunization using a drug developed for MS appears to reduce neurodegeneration in the MPTP model of the disease. A new animal model using proteasome inhibitors recapitulates surprisingly faithfully the clinical and pathologic picture of Parkinson's. The model hints at environmental toxins as a possible cause of some sporadic PD cases. Stem cells, homocysteine, and estrogen also made the news.

Immunize to Trigger Protective T Cells in Parkinson's?
In the 14 June early online edition of PNAS, a collaboration between the laboratories of Howard Gendelman of the University of Nebraska in Omaha and Serge Przedborski of Columbia University in New York City has produced a promising therapeutic approach to treating Parkinson's. Part of the appeal lies in the study’s use of glatiramer acetate (GA; copolymer-1 or Cop-1), a synthetic amino acid polymer that has shown a good safety profile in a number of multiple sclerosis trials. It appears to have some beneficial effects in MS, perhaps through instigating an antiinflammatory T-cell response.

Based on evidence that inflammatory processes might be involved in damaging substantia nigra cells in Parkinson's disease, first author Eric Benner and colleagues decided to see if GA could benefit neurons in the MPTP model of Parkinson's. (In contrast to the prominent AD animal models created with familial disease transgenes, the best-known Parkinson's models are created with toxins that kill substantia nigra neurons. The MPTP model was the first and remains an important tool for investigating the pathology of substantia nigra damage.)

Benner and colleagues could not directly immunize the mice in their study with GA because MPTP damage to the spleen interferes with T-cell function and would have prevented an appropriate immune response to GA. Instead they adopted a passive immunization approach, generating immune responses to GA in other mice, harvesting splenocytes (largely collections of T- and B-cells), and transferring these to MPTP-exposed mice.

The GA splenocytes reduced cell loss in the substantia nigra, as well as loss of dopamine and dopaminergic fibers in the striatum, the target of nigral dopaminergic neurons. This benefit appears to have been mediated by donor T-cells infiltrating the area of degeneration and modulating MPTP-induced microglial inflammatory responses, the authors report. The splenocyte transfer increased the levels of both antiinflammatory cytokines (IL-10 and IL-4) and glial cell line-derived neurotrophic factor (GDNF). This suggests a protective effect derived from a combination of an antiinflammatory Th2 response and an astrocyte-derived GDNF effect.

The authors note that given the extensive safety data on this drug, they see no apparent reason why an active immunization protocol (i.e., GA given directly to patients) could not be tried in Parkinson's disease. One limitation of the MPTP model, Przedborski told the Alzheimer Research Forum, is that the toxin produces only slight behavioral abnormalities, which are difficult to interpret. For that reason the researchers did not describe immunization effects on movement. Perhaps a new animal model will open up this line of inquiry?

New Model Points to Possible Cause of Parkinson's
On June 21, an advance online report in the Annals of Neurology describes a rat model of Parkinson's with significant improvements over current models caused by MPTP or the pesticide rotenone. The limitations of the MPTP model are well-known—it is not progressive, kills only nigral cells, and does not feature Lewy bodies. The model created by rotenone exposure does feature inclusions resembling Lewy bodies, but also kills cells in brain areas not affected by Parkinson's disease, besides killing more than a third of the experimental animals. Now enter the proteasome inhibitor.

Recently, Parkinson's researchers have become aware that there are problems in the degradation of proteins in PD, specifically in the ubiquitin proteasome system. Many of the major actors in PD—e.g., α-synuclein, parkin, ubiquitin C-terminal hydrolase L1—appear linked with proteasomal function. Kevin McNaught, Warren Olanow, and colleagues from Mount Sinai School of Medicine in New York City examined the effects of experimentally interfering with proteasomes in rats, using both synthetic and naturally occurring proteasome inhibitors.

About two weeks after receiving systemic injections of proteasome inhibitors, the rats began to show Parkinsonian symptoms, including bradykinesia, rigidity, and tremor. The symptoms gradually worsened over a period of months, and improved with apomorphine or levodopa. PET imaging studies of the animals' brains showed changes in a pattern identical to that seen in Parkinson's disease, with evidence of reduced dopamine in the striatum. Similarly, autopsy studies on the rats’ brains demonstrated a reduction in striatal dopamine and loss of dopaminergic cells in the substantia nigra. Markers of both inflammation and apoptosis were elevated in the substantia nigra. Consistent with Parkinson's disease, neurodegeneration was also detected in the locus coeruleus, dorsal motor nucleus of the vagus, and the nucleus basalis of Meynert. This was accompanied by Lewy body-like inclusions in the substantia nigra, locus coeruleus, and dorsal motor nucleus of the vagus.

The proteasome system changed in intriguing ways. Within the first week of proteasome inhibitor administration, proteasomal activity actually increased in most CNS regions, and this is consistent with reports in other models. In later weeks, proteasome activity remained high in the CNS overall but dropped in the ventral midbrain, the home of the substantia nigra. Similar findings were seen in PD patients. "These findings suggest that neurons in ventral midbrain and possibly lower brain stem have a relatively poor ability to mount or maintain compensatory proteolytic mechanisms, and this could contribute to their susceptibility to degenerate," write the authors.

Beyond the potential applications of this model for testing experimental therapies, the research may open up a new line of epidemiologic research. The authors note that epoxomicin, one of the most potent proteasome inhibitors known, is produced by the common actinomycetes bacteria, found in soil and well water throughout the world. "It's only speculation at this point, but the fact that living in rural areas and drinking well water has been reported to be associated with higher rates of Parkinson's disease could be related to higher levels of proteasome inhibitors found in these areas," said Olanow in a news release.

Stem Cells, Homocysteine, and Estrogen (Which Just Won’t Die)
In separate news, three studies just out in this month’s Archives of Neurology report findings relating to Parkinson’s disease. The first, led by Seth Pullman at Columbia University, New York, and Curt Freed at the University of Colorado, Denver, urges the field to take seriously the potential of fetal cell transplantation. Two of Freed’s earlier studies of PD patients who had received fetal stem cells to replace dying neurons in the substantia nigra had failed to show overall clinical improvement (see ARF related news story). However, there had been indications that the cells survived and perhaps even boosted dopamine levels in parts of the nigrostriatal system. Following up on some of these patients, first author Paul Gordon and colleagues report now that they do find some subtle motor improvements in the patients who received transplants. Archives editor Roger Rosenberg believes that these results should encourage continued exploration of transplant strategies, especially studies using genomic methods. "Embryonic neuronal implants and stem cell implants provide many potential therapeutic factors besides dopamine. These tissue implants secrete trophic factors, cytokines, interleukins, and even potential toxic inflammatory products," writes Rosenberg in an accompanying editorial.

The Archive’s second study, by Padraig E. O'Suilleabhain and colleagues at the University of Texas Southwestern Medical Center, Dallas, reports that homocysteine levels, elevated in part by L-dopa therapy in PD patients, are associated with depression and worse cognitive performance in a sample of 97 patients. This adds a new twist to the old connection between depression and PD.

Finally, the third PD study in this issue may help rekindle the fire of a separate debate. Estrogen replacement therapy appears to have gone up in flames with the dismal findings of the Women’s Health Initiative (WHI, see also upcoming comment by Sam Gandy on the latest development on this issue). While the field is still trying to read in the ashes, Lillian Currie and colleagues at the University of Virginia in Charlottesville present early data to suggest that postmenopausal estrogen exposure may protect against Parkinson’s disease. The study used questionnaire data from a small case-control population, generally considered one of the weaker study designs. Yet it fuels a question that remains unanswered for hormone replacement therapy and dementia, and that is whether HRT taken for only a few years during and after menopause—i.e., in women younger than those studied by the WHI—may protect the brain a decade or two down the road.—Hakon Heimer and Gabrielle Strobel

Comments

1. • Mark Cookson
     National Institute on Aging
   • Posted: 29 Jun 2004

   What a great group of papers...
   From my perspective, the most thrilling paper is the one from Kevin McNaught and colleagues. It is important because it tests the idea that proteasome dysfunction is central to cell damage and inclusion formation in PD and related disorders. A problem with this idea is that although dopaminergic neurons do appear to be susecptible to this type of damage, there are reports that decreased proteasome function also occurs in other diseases that may or may not have a parkinsonian component (e.g., the polyglutamine diseases or ALS). Therefore, unless one looks at the whole nervous system one cannot be sure that proteasome inhibition is a sufficient explanation for PD, and not a general facet of neurodegenerative diseases. The McNaught paper directly addresses this, and as a bonus may provide us with an easily replicable model that has both neuronal loss and inclusion formation - something that has been lacking to date. The challenge for the community will be to replicate the current methods and expand them into other areas - finding out whether this works, for example, in mice would be helpful to start testing ideas in transgenic and knockout animals.

   View all comments by Mark Cookson

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References

News Citations

1. Long-Term Follow-up of Parkinson's Patients Who Received Embryonic Cell 22 Nov 2002

Further Reading

Papers

1. McNaught KS, Perl DP, Brownell AL, Olanow CW. Systemic exposure to proteasome inhibitors causes a progressive model of Parkinson's disease. Ann Neurol. 2004 Jul;56(1):149-62. PubMed.