Neurodegeneration and neuroinflammation are linked, but independent of alpha-synuclein inclusions, in a seeding/spreading mouse model of Parkinson's disease.

A key pathological process in Parkinson's disease (PD) is the transneuronal spreading of α-synuclein. Alpha-synuclein (α-syn) is a presynaptic protein that, in PD, forms pathological inclusions. Other hallmarks of PD include neurodegeneration and microgliosis in susceptible brain regions. Whether it is primarily transneuronal spreading of α-syn particles, inclusion formation, or other mechanisms, such as inflammation, that cause neurodegeneration in PD is unclear. We used a model of spreading of α-syn induced by striatal injection of α-syn preformed fibrils into the mouse striatum to address this question. We performed quantitative analysis for α-syn inclusions, neurodegeneration, and microgliosis in different brain regions, and generated gene expression profiles of the ventral midbrain, at two different timepoints after disease induction. We observed significant neurodegeneration and microgliosis in brain regions not only with, but also without α-syn inclusions. We also observed prominent microgliosis in injured brain regions that did not correlate with neurodegeneration nor with inclusion load. Using longitudinal gene expression profiling, we observed early gene expression changes, linked to neuroinflammation, that preceded neurodegeneration, indicating an active role of microglia in this process. Altered gene pathways overlapped with those typical of PD. Our observations indicate that α-syn inclusion formation is not the major driver in the early phases of PD-like neurodegeneration, but that microglia, activated by diffusible, oligomeric α-syn, may play a key role in this process. Our findings uncover new features of α-syn induced pathologies, in particular microgliosis, and point to the necessity for a broader view of the process of α-syn spreading.

Screening of innate immune receptors in neurodegenerative diseases: a similar pattern.

In Alzheimer's disease (AD), Parkinson's disease (PD), dementia with Lewy bodies (DLB) and amyotrophic lateral sclerosis (ALS), neuroinflammatory responses are considered to contribute to neuronal injury. Recently, the innate immune receptors, toll-like receptors (TLRs) and the LPS receptor (CD14) have been related to neurodegeneration. In this study, we systematically assessed the expression of most TLRs and CD14 in AD, PD/DLB and ALS using murine models of these diseases and human post-mortem brain tissues. A common upregulation of TLR2 and CD14 was found in all three animal models. While these two receptors could also be detected in AD patient tissues, they were absent from DLB and ALS tissues. This uniform pattern of innate immune response in animal models of neurodegenerative diseases clearly indicates that this response is part of a non-specific neuroinflammatory effector phase rather than a disease-specific event. The less dynamic disease progression in humans and the location (extracellular versus intracellular) of the aggregated proteins deposits might explain the divergent results seen between animal models and human tissues.