During the last few years, we have created new rodent Parkinsons disease (PD) models that mimic the progressive disease development in PD patients. With these models, we have elucidated the novel mechanisms of microglial activation that lead to inflammation-mediated neurodegeneration. For the last year, the major effort was the elucidation of signaling pathways leading to the progressive dopaminergic neuron degeneration. For the last year, our research focused on the reactive microgliosis is crucial for the maintenance of long-term microglial activation. After demonstrating that continuing neuronal death/damage is crucial for the maintenance of microglial activation, we tried to further understand the molecular mechanisms by studying the role of reactive microgliosis in the creation of the self-propelling cycle. Reactive microgliosis occurs when neurons are damaged and has been generally considered to serve a passive role in cleaning the dead or damaged neurons or debris by phagocytosis. Our recent studies provided convincing evidence indicating that reactive microgliosis play a critical and active role in the formation of a self-propelling cycle and the subsequent neuro-degeneration. We have shown that a spectrum of noxious endogenous compounds in extracellular milieu, generated following neuronal injury, can activate microglia leading to reactive microgliosis. These compounds include membrane breakdown products, abnormally processed, modified or aggregated proteins (e.g.a-synuclein and -amyloid), and leaked cytosolic compounds (e.g.a-synuclein and neuromelanin). It appears that the microglial response to these endogenous toxic signals resembles their response to invading microbes. The first direct evidence demonstrating that neuronal damage causes microglial activation and enhances neurotoxicity came from an experiment by using MPP+ to damage DA neuron cultures (N27, a DA neuron cell line), then the supernatant was added back to primary neuron-glia cultures. We found that DA neuron-conditioned media caused microglial activation and severe DA neurotoxicity and these effects were reduced in neuron-glia cultures prepared from either MAC1- or PHOX-deficient mice. Results from this experiment prompted us to determine more specifically the endogenous substances that leaked out from damaged DA neurons. To illustrate the molecular mechanisms underlying this reactive process, two of the most abundant, a-synuclein and neuromelanin will be used as examples. a-Synuclein is a major component of Lewy bodies in DA neurons. Neuromelanin contained in nigral DA neurons is a mixture of DA metabolites, lipids, peptides, metals and pesticides, which gives the dark color of the SN where DA neurons reside. The pathophysiological role of these substances in the DA degeneration is not clear. It is believed that both a-synuclein and neuromelanin can be leaked out to the extracellular space when DA neurons are damaged. Despite their difference in chemical composition, we showed that microglia greatly enhanced the DA toxicity of a-synuclein and neuromelanin in a similar fashion. The following data sets indicate critical roles of microglia in mediating DA neurotoxicity by these two substances: 1) DA neurotoxicity was dramatically enhanced by adding microglia, but not astroglia, back to neuron-enriched cultures;2) DA neurotoxicity was significantly prevented in microglia-depleted cultures. 3) Both a-synuclein and neuromelanin caused morphological activation of microglia and elevated gene expression and protein levels of a series of pro-inflammatory and neurotoxic factors. 4) The MAC1-PHOX axis is likely the site of action for these endogenous substances. Taken together, these two studies provided evidence supporting the possibility that endogenous substances from the damaged neurons, such as a-synuclein and neuromelanin, participated in the reactive microgliosis.
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