α-Synuclein and Noradrenergic Modulation of Immune Cells in Parkinson’s Disease Pathogenesis

In idiopathic Parkinson's disease massive cell death occurs in the dopamine-containing substantia nigra. A link between the vulnerability of nigral neurons and the prominent pigmentation of the substantia nigra, though long suspected, has not been proved. This possibility is supported by evidence that N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and its metabolite MPP+, the latter of which causes destruction of nigral neurons, bind to neuromelanin. We have directly tested this hypothesis by a quantitative analysis of neuromelanin-pigmented neurons in control and parkinsonian midbrains. The findings demonstrate first that the dopamine-containing cell groups of the normal human midbrain differ markedly from each other in the percentage of neuromelanin-pigmented neurons they contain. Second, the estimated cell loss in these cell groups in Parkinson's disease is directly correlated (r = 0.97, P = 0.0057) with the percentage of neuromelanin-pigmented neurons normally present in them. Third, within each cell group in the Parkinson's brains, there is greater relative sparing of non-pigmented than of neuromelanin-pigmented neurons. This evidence suggests a selective vulnerability of the neuromelanin-pigmented subpopulation of dopamine-containing mesencephalic neurons in Parkinson's disease.

To elucidate the role of the synaptic protein alpha-synuclein in neurodegenerative disorders, transgenic mice expressing wild-type human alpha-synuclein were generated. Neuronal expression of human alpha-synuclein resulted in progressive accumulation of alpha-synuclein-and ubiquitin-immunoreactive inclusions in neurons in the neocortex, hippocampus, and substantia nigra. Ultrastructural analysis revealed both electron-dense intranuclear deposits and cytoplasmic inclusions. These alterations were associated with loss of dopaminergic terminals in the basal ganglia and with motor impairments. These results suggest that accumulation of wild-type alpha-synuclein may play a causal role in Parkinson's disease and related conditions.

1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) damages the nigrostriatal dopaminergic pathway as seen in Parkinson's disease (PD), a common neurodegenerative disorder with no effective protective treatment. Consistent with a role of glial cells in PD neurodegeneration, here we show that minocycline, an approved tetracycline derivative that inhibits microglial activation independently of its antimicrobial properties, mitigates both the demise of nigrostriatal dopaminergic neurons and the formation of nitrotyrosine produced by MPTP. In addition, we show that minocycline not only prevents MPTP-induced activation of microglia but also the formation of mature interleukin-1beta and the activation of NADPH-oxidase and inducible nitric oxide synthase (iNOS), three key microglial-derived cytotoxic mediators. Previously, we demonstrated that ablation of iNOS attenuates MPTP-induced neurotoxicity. Now, we demonstrate that iNOS is not the only microglial-related culprit implicated in MPTP-induced toxicity because mutant iNOS-deficient mice treated with minocycline are more resistant to this neurotoxin than iNOS-deficient mice not treated with minocycline. This study demonstrates that microglial-related inflammatory events play a significant role in the MPTP neurotoxic process and suggests that minocycline may be a valuable neuroprotective agent for the treatment of PD.