Neuroinflammation in Parkinson’s Disease: role in neuropathology and L-DOPA-induced motor complications.

Parkinson Disease (PD) is a neurodegenerative disorder characterized by the progressive dopaminergic loss in the Substantia Nigra (SN) and the presence of intracellular Lewy Bodies (LB) containing deposits of the protein α-synuclein (α-syn). Several studies identified the neuroinflammatory processes as important factors in the neuropathology of PD, involving mainly microglia cells. Indeed, in PD microglia lose their ability to autoregulate, sustaining a chronic pro-inflammatory environment in dopaminergic areas which exacerbates the neurodegenerative process. Moreover, recent pre-clinical studies have suggested a role of neuroinflammation in the pathophysiology of L-DOPA-induced dyskinesia, showing that the chronic administration of L-DOPA exacerbates the pre-existing inflammatory environment which may contribute to the altered neurotransmission associated with dyskinesia. The main component of LB is α-synuclein in its fibrillar form, but recent evidence suggests that the most toxic strain of α-synuclein are small soluble α-synuclein oligomers. Moreover, structure-based features have been suggested to mediate the toxicity of highly toxic α-synuclein oligomers. While the central role of α-synuclein in PD is generally acknowledged, the mechanisms underlying the neurotoxicity of α-synuclein oligomers, and their pathological interaction with Central Nervous System (CNS) immune-cells within PD-related neuroinflammation is still largely unknown. In the first part of our project (part I), we tested the neurotoxicity in vivo of α-synuclein oligomers previously recognized to hold a structure that confers high toxicity in in vitro models, and we focused on the inflammatory response elicited by these toxic α-synuclein oligomers and on their interaction with microglial cells. Thereafter, in the second part of our project (part II) we addressed the role of neuroinflammation in L-DOPA-induced dyskinesia. L-DOPA therapy is the gold standard for PD, however long-term administration leads to the onset of L-DOPA-induced abnormal involuntary movements named dyskinesia. Recent studies have suggested that neuroinflammatory processes play a pivotal role in dyskinesia, and the proinflammatory cytokine Tumor Necrosis Factor (TNF)-α may be a key player being also involved in synaptic strength mechanisms and in angiogenesis, another important component in the neuropathology of L-DOPA-induced dyskinesia (LID). Here, we tested whether the immunomodulatory drugs thalidomide (TLD) and its more potent derivative 3,6-dithiothalidomide (DTT), which specifically inhibit the synthesis of TNF-α, were effective in alleviating the L-DOPA-induced dyskinetic outcome in a rat model of PD. Oligomer infusion caused a gradual development of PD neuropathology. Microgliosis and increased levels of inflammatory cytokines were measured one month after oligomers-infusion, without any evidence of neurodegeneration and behavioral deficits. Notably, three months after the injection, rats displayed motor impairment, associated with 40% loss of dopaminergic neurons in the oligomers-treated SN, reaching a 50% cell loss after five months. Dopamine levels were significantly reduced by 40% in the striatum homolateral to α-synuclein infusion. An intense inflammatory response with reactive microglia and high levels of TNF-α immunoreactivity were detected in SNpc. Large deposits of p-αsyn were found within microglial cells three and five months post-infusion. TLD and DTT significantly reduced the severity of AIMs while not affecting the contralateral turning. Both drugs inhibited the L-DOPA-induced microgliosis and excessive TNF-α in the Str and SNpr, while restoring control levels of the anti-inflammatory cytokine interleukine (IL)-10 at striatal level. DTT inhibited L-DOPA-induced angiogenesis in SNpr and Str. GLUR1 analysis revealed that L-DOPA-induced an overexpression of this GLUR1 subunit in the Str, that was restored to normal levels by DTT.