The bradykinesia, muscle rigidity, and resting tremor of Parkinson’s disease are associated with a loss of dopamine nerve terminals in the caudate nucleus and putamen of the basal ganglia. While levodopa initially restores brain dopamine, it becomes less effective with time and it also causes dyskinesias in many patients. In a rapidly evolving field of study, Parkinson’s disease researchers in several countries are suggesting that levodopa therapy is likely to be augmented or perhaps replaced by treatments that provide new dopaminergic neurons or that prevent their deterioration in the first place. These new approaches include transplantation of replacement neurons; the use of drugs to prevent neuron destruction; and the development of gene therapy to introduce neuroprotective proteins into vulnerable brain areas.

These strategies, along with deep brain stimulation and new surgical treatments, are expected to get a major boost from the National Institutes of Health’s “Parkinson’s Disease Research Agenda.” This initiative (see table) has been described as a “moon shot” targeted at a cure for Parkinson’s disease.

NEUROTRANSPLANTATION

The controversy surrounding the use of cellular therapy has less to do with the effectiveness of the treatment than with the source of cells for transplantation. Potential sources include fetal tissue, embryonic stem cells, adult stem cells, and cells from other animals (xenotransplants). According to Dr. Isacson, “To make transplantation for thousands of Parkinson’s disease patients, we need to develop a method so reliable that it is akin to a drug: totally predictable and totally safety tested. We don’t know which approach will work, so we need to explore all possible progenitor cells, including embryonic stem cells. This requires an open and free society that can do this research. And what is going to come out first will barely work, like the Wright brothers’ airplane.”

FETAL TISSUE TRANSPLANTS

Dr. Dunnett conceded that questions remain about the applicability of these animal models of selective dopaminergic degeneration to Parkinson’s disease in humans and whether there are alternative sources of transplantable cells. “We can repair experimental dopamine deficiency,” he said, “but this [animal model] is an acute rather than a chronic disease. Does functional repair in animals translate to more complex behavioral functions in humans? Fetal cell transplants can work with dramatic efficacy in some cases but can also go seriously wrong.”

The risk of things going “seriously wrong” hit the headlines earlier this year when the first double-blind, placebo-controlled clinical trial of fetal cell transplants for Parkinson’s disease was reported in the March 8 New England Journal of Medicine. Curt Freed, MD, and colleagues at the University of Colorado School of Medicine transplanted precursors of dopaminergic nerve cells that were obtained from mesencephalon isolated from 6- to 10-week-old human fetuses into the brains of patients with severe Parkinson’s disease. Each patient was injected with tissue from four embryos that had been obtained after elective abortions. Transplant patients were compared to patients who had sham surgery in which burr holes were drilled through the cranium but not into the brain. No immune suppression was used, as previous studies had shown that allogenic nerve-cell transplantation does not induce immune responses in humans or other primates.

The bad news is that 15% of patients who received transplants had recurrence of disabling dystonia and dyskinesias in the second or third years after surgery, and these problems occurred only in patients who had initially improved after treatment and who had been able to discontinue all levodopa, the most commonly used drug to treat Parkinson’s disease. The patients who developed dyskinesias and dystonia when off levodopa were all age 60 or younger at the time of surgery and all had had severe fluctuations in symptom severity before the procedure while taking levodopa. Although these dyskinesias have not been reported by European groups doing fetal tissue transplants, Dr. Freed’s group had seen them before. “In our first report of our first patient [Arch Neurol. 1990;47:505-512], we indicated that dyskinesias developed in the months after transplant,” Dr. Freed told Neurology Reviews. “In a 1992 New England Journal of Medicine paper describing seven transplant patients we pointed out that all patients who had benefit from the transplant also showed dyskinesias at some point in the first year. Cutting levodopa doses generally solved that problem. In the current study we point out that five of the 33 surviving transplant patients have developed dyskinesias that have persisted even after reducing or stopping all levodopa, suggesting that those patients either have unusual sensitivity to dopamine (four of the five had severe drug-induced dyskinesias prior to transplant) or that the transplants produced an absolute excess of dopamine. The latter explanation is unlikely since fluorodopa positron emission tomography scans show signals that do not exceed the normal range for patients without Parkinson’s disease.”

Dr. Freed suspects that these problems were perhaps caused by graft placement. He plans to reduce the amount of tissue transplanted in future studies by transplanting tissue in the dorsal portion of the putamen, and no longer placing tissue into more ventral areas that provide motor control to the head and upper extremities. Dr. Freed told Neurology Reviews that his group is currently transplanting cells to a new brain region, the substantia nigra, in addition to the putamen. “To reduce the chance of excessive dopamine effect, we have reduced the amount of tissue we are transplanting into the putamen and have changed the location of implantation somewhat,” he said. “There is no question that transplants can improve signs of Parkinson’s disease in appropriately selected patients,” Dr. Freed added. “The late-stage patients chosen for our study are extremely difficult to manage with medications. Because transplant surgery has proven to be relatively safe, even better results might be achieved by operating on patients with less far advanced disease.”

EMBRYONIC STEM CELLS

Some researchers have proposed that stem cells might be that alternative. Stem cells differ from neural fetal tissue cells in that they have the capacity to reproduce themselves and to differentiate into many cell types. Ron McKay, MD, and colleagues at the National Institute of Neurological Disorders and Stroke (NINDS) have done a great deal to advance the therapeutic development of stem cell transplantation.

“In rodent studies we learned that if you take either the morula or the blastula from the inner cell mass of a fertilized egg, you have a pluripotent cell that can become any cell in the body,” Dr. McKay said at a special symposium on Parkinson’s disease held at the annual meeting of the American Association for the Advancement of Science (AAAS). Studies in which stem cells were transplanted in rat models of Parkinson’s disease showed that these cells primarily differentiate into dopaminergic neurons but also to a lesser degree into serotonergic and GABAergic neurons.

Research reported by Vincent Tropepe, PhD, and colleagues in the April issue of Neuron supports the possibility that neural phenotypes are the “default” pathway for embryonic stem cell differentiation. Mouse embryonic stem cells cultured without serum and in low-density conditions without other growth signals “readily acquire a neural identity,” Dr. Tropepe wrote. The embryonic stem cells progressed first to a primitive neural stem cell that expressed neural precursor markers, generated neurons and glia in vitro, and could develop in vivo into either neural or non-neural tissues.

Mechanisms to control stem cell differentiation are needed because embryonic stem cell transplants tend to form teratomas unless induced to differentiate down the needed cell lineage before transplantation. “We are trying to improve the efficiency of differentiation to dopaminergic neurons using embryonic stem cells and as clean a preparation of cells as possible in animal studies. We need to make this technology routine. We also need to demonstrate that the cells we make will actually work in animals,” Dr. McKay said. However, Dr. McKay pointed out at the AAAS meeting that there have been no published studies showing that human embryonic stem cells give rise to dopaminergic neurons. Another potential problem is that human embryos have a higher incidence of aneuploidy than do those of other species. Stem cells from such an embryo would have an increased risk of neoplastic development.

ADULT STEM CELLS

Adult stem cells have been harvested from human cadavers and from nasal passages in adult mice. Theo D. Palmer, PhD, and colleagues report in the May issue of Nature that they have been able to take viable neural progenitor cells from tissue obtained from an adult human brain two hours after death. The researchers were able to persuade the cells to replicate upon exposure to conditioned medium from rat stem cells that had been genetically engineered to secrete basic fibroblast growth factor (FGF-2) and its stem cell cofactor. Cells from the adult brain grew for 30 doublings before losing their reproductive capacity with senescence. (By contrast, cells taken after death from an 11-week-old infant reached 70 doublings before senescence). The researchers did not report whether any of the resulting neurons produced dopamine.

At the Experimental Biology 2001 meeting in Orlando in April, Fred J. Roisen, PhD, reported that he and colleagues at the University of Louisville had managed to isolate neural stem cells from the nasal passage lining in mice. Dr. Roisen’s group was able to maintain the cells in culture for several months and to induce them to differentiate into neurons in vitro. Dr. Roisen’s group had previously isolated neural stem cells from the lining of the nasal passages of cadavers up to 18 hours after death.

A hybrid approach that uses nuclei from adult somatic cells to generate allogenic embryonic stem cells was reported by Teruhiko Wakayama, PhD, and colleagues at Rockefeller University and Memorial Sloan-Kettering Cancer Center in New York City, in the April 27 Science. The researchers derived 35 pluripotent embryonic stem cell lines by taking nuclei from adult mouse somatic cells, microinjecting them into denucleated mouse eggs, and cloning cells from the resulting blastocysts. Embryonic stem cells from these clones were able to differentiate into a variety of cell types, including dopaminergic and serotonergic neurons in vitro. Although the authors suggest that such “therapeutic cloning” could provide a source of differentiated cells for human autologous transplant therapy, this method would require transferring the patient’s somatic cell nucleus into a human egg, and human eggs are in short supply.

XENOTRANSPLANTS

Preventing rejection continues to be the main problem with xenotransplants for Parkinson’s disease. Dr. Isacson’s group is developing transgenic pigs that will express human complement inhibitor. Other researchers are working on tolerization methods for teaching the immune system to accept a foreign antigen without systemic immune suppression. Phase 2 and 3 studies of the effects of porcine fetal cells implanted into patients with advanced Parkinson’s disease are underway in study centers in Tampa, Atlanta, and Boston.

NEUROPROTECTANTS

Until then, a good bet for the “worried well” might be to drink a couple of cups of coffee each day. A Veterans Administration study of over 8,000 men reported last year showed that people who did not drink coffee were two to three times as likely to develop Parkinson’s disease than were those who drank two to three cups of coffee per day. Jiang-Fan Chen, PhD, and colleagues at Massachusetts General Hospital reported in the May 15 Journal of Neuroscience that mice who downed the mouse equivalent of the caffeine in two cups of coffee were protected from developing dopaminergic deficits in experimentally induced Parkinson’s disease. This protection was mediated by the effect of caffeine on adenosine A_2A receptors. Dr. Chen suggested that selective A_2A receptor antagonists might be even more effective than caffeine, which also blocks the adenosine A₁ receptor subtype.

Glial cell line–derived neurotrophic factor (GDNF) is a powerful neuroprotectant, but attempts to treat Parkinson’s disease patients with intraventricular administration of GDNF in solution did not prevent nigrostriatal degeneration or improve clinical function. Transferring a GDNF gene into the threatened brain areas may be more effective. Jeffrey H. Kordower, MD, and colleagues reported in the October 27, 2000, Science that treatment with a GDNF gene delivered by a lentiviral vector prevented Parkinson’s disease–like changes in aged monkeys. This gene therapy also reversed experimentally induced Parkinson’s disease–like changes in the younger monkeys, induced regeneration of nigrostriatal neurons, and reversed motor deficits in three of four animals treated. The lentivirus-GDNF preparation in this study was injected directly into the caudate nucleus, putamen, and substantia nigra.

NEW REGULATORY PROBLEMS

In the wake of the Wakayama “therapeutic cloning” study in mice, Sen. Sam Brownback (R-Kan) and Rep. Dave Weldon (R-Fla) introduced legislation to bar all human cloning. Meanwhile, human cloning studies are advancing in other countries. Britain passed legislation specifically allowing the cloning of human embryos (up to 14 days of age) for therapeutic research. A similar easing of the ban on human embryonic cloning is under consideration in France.

NR