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 Parkinson's disease. P arkinson's disease (PD) is a degenerative disorder characterized by a loss of midbrain dopamine (DA) neurons with a subsequent reduction in striatal DA (1). Pharmacological treatment with L-DOPA works initially, but reduced efficacy and development of motor complications requires treatment alternatives such as deep brain stimulation and fetal DA neuron transplantation (2). There is evidence both from animal models and clinical investigations showing that fetal DA neurons can produce symptomatic relief (3-6). Technical and ethical difficulties in obtaining sufficient and appropriate donor fetal brain tissue have limited the application of this new therapy.Previous work showed that DA neurons can be produced in vitro from ventral mesencephalic (VM) precursor cells (7). A problem using expanded fetal VM precursors (7) is the low in vivo survival rate of 3-5% of the grafted DA neurons, which eliminates the actual gain by in vitro cell number expansion compared with fresh (unexpanded) fetal day-12 VM (7,8).Embryonic stem (ES) cells have many characteristics required for an optimal cell source for cell-replacement therapy (9, 10). ES cells are self-renewing and multipotent cells derived from the inner cell mass of the preimplantation blastocyst (11). We have shown previously that mouse ES cells transplanted to normal mice or 6-hydroxydopamine (OHDA)-lesioned rats can differentiate spontaneously into tyrosine hydroxylase (TH)-positive and serotonin (5HT)-positive neurons. This differentiation is likely not caused by a specific inductive signal from the host brain, because similar neuronal differentiation occurs after placement in the kidney capsule (11). In our previous study (11), grafts frequently showed heterogeneous morphology and often became very large, disrupting the cytoarchitecture at the implantation site, which prevented the possibility for functional integration. Because neurons develop from ES cells even when implanted outside the central nervous system (11) and ectoderm develops into neural tissue when cellto-cell communication is disrupted by dissociation of the cells (12-14), we hypothesized that dilution of ES cells into single-cell suspension...

Environmental toxins have been implicated in the etiology of Parkinson's 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 2beta-carbomethoxy-3beta-(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, alpha-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.