Numerous studies have documented risk variants in immune genes and increased microglial inflammatory markers in the parenchyma and biofluids of Parkinson’s disease (PD) patients. Recently our lab has characterized a new rat model of PD induced by injection of α-syn preformed fibrils (α-syn PFFs). The PFF model more faithfully recapitulates key features of idiopathic PD: namely early development of Lewy body-like pathology in widespread, PD-relevant brain regions under the context of normal levels of endogenous α-syn and protracted nigrostriatal degeneration over the course of 6 months. The distinct stages afforded by this model allow for investigation of neuroinflammation at different stages of synucleinopathy without the confound of α-syn pathology and degeneration occurring concurrently. First, I histologically examined the time course of synucleinopathy, microgliosis and nigral degeneration at monthly intervals. Microglia in the vicinity of Lewy body-like inclusions display significantly increased cell body area and observable differences in extent and thickness of branching at 2 months post-injection, months prior to degeneration. Interestingly, major-histocompatibility complex-II (MHC-II; present on antigen-presenting microglia) was significantly increased in PFF-injected animals compared to controls 3 months prior to degeneration and relatively absent during the interval of degeneration. Moreover, the number of microglia expressing MHC-II at 2 months was positively correlated with the number of Lewy body-like inclusions in the substantia nigra, similar to observations documented in human PD tissue. I next investigated the temporal profile of inflammatory cytokine expression in cerebrospinal fluid and plasma in the context of naïve aging animals and in PFF-injected animals. In the context of normal aging, tumor necrosis factor (TNF) and keratinocyte chemoattractant (KC/GRO) were significantly increased in aged animals compared to young and young-adult animals. In the synucleinopathy cohort when α-syn burden in the SN is greatest, I observed significant correlations between number of nigral α-syn inclusions and CSF levels of interferon-gamma (IFN-γ) were observed. At 4 months I continued to observe correlations between α-syn burden and CSF IFN-γ and TNF, and significantly elevated interleukin-6 in PFF animals compared to controls. During the interval of degeneration, significantly increased levels of interleukin-5 (IL-5), keratinocyte chemoattractant (KC/GRO), and TNF were observed, though cytokines did not correlate with magnitude of degeneration at this time point. These results suggest that a certain threshold of α-syn burden must be present in order for deviations in cytokine levels to be detected in early stages of disease and that overt differences in cytokines between PD patients and controls require the effect of pathology and time. Collectively, our results suggest that inflammatory mechanisms, specifically antigen presentation by microglia, in the substantia nigra have the potential to contribute to degeneration. Moreover, deviations in pro-inflammatory cytokines that occur in early disease stages are closely associated with α-syn burden within the SN. Importantly the combination of biofluid sampling and measurement of α-syn within the brain may represent a biomarker for early disease detection.
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