PROJECT SUMMARY
Parkinson's Disease (PD) is a complex and progressive neurodegenerative disorder that culminates in the
deposition of alpha synuclein (a-syn) containing protein aggregates, named Lewy bodies, and the
degeneration of nigrostriatal dopamine neurons. Evidence suggests that neuroinflammation plays a causative
role in the pathogenesis of PD. Post-mortem- and longitudinal PET imaging studies of the PD brain reveal an
increase in the number of activated microglia that occur early in the disease process and remain elevated
throughout the course of the disease. Activated microglia can convert astrocytes to a subtype of toxic “A1-
astrocyte” and these reactive astrocytes are observed in the PD brain. A1 astrocytes coordinate selective
neurodegeneration by releasing components of the complement system. The complement system is a division
of the innate immune system that coordinates the removal of pathogens and cell debris. However, the
complement system, and specifically complement component 3 (C3), is also used by the central nervous
system to tag vulnerable synapses and neurons for phagocytic clearance. Within the PD brain, toxic A1-
astrocytes dramatically increase the expression of C3. Taken together, we hypothesize that following an
initial immunogenic signal (pathological a-syn misfolding) C3 derived from A1 astrocytes is used to tag
inclusion-containing neurons for destruction by microglia. Supporting this idea, both midbrain dopamine
neurons and Lewy bodies label with activated C3 in the PD brain.
Neuroinflammation has been reported with virtually every model of PD. However the majority of PD models fail
to accurately recapitulate the defining features of PD pathology: The progressive aggregation of endogenous
a-syn and subsequent degeneration of nigrostriatal neurons. In contrast, intrastriatal injection of recombinant a-
syn pre-formed fibrils (PFFs) is able to accurately model these hallmarks of PD pathology, and a
neuroinflammatory response that mimics the human disease. We have extensively characterized this model
and found that injection of a-syn PFFs into the rat striatum results in peak a-syn aggregate formation within the
substantia nigra pars compacta (SNc) at 2 months post-injection, followed by significant nigral degeneration by
6 months. During the peak of a-syn aggregation we observe peak microglial activation as well as robust
astrogliosis. Genetic expression and morphological profiling of reactive astrocytes is consistent with the
presence of toxic A1-astrocytes. As such we hypothesize that C3 derived from A1 reactive astrocytes is used
to tag inclusion-containing nigral neurons for destruction. Supporting this idea, we observe a large increase in
the expression of C3 during this time point, and have localized C3 to reactive astrocytes. Importantly, the
presence of toxic A1-astrocytes and the increase in C3 expression occurs months prior to overt nigral
degeneration, suggesting astrocytic C3 may play a role in the degenerative process. Thus, preventing
release or activation of C3 may prevent the degeneration induced by a-syn aggregation.
In the present exploratory R21 application we propose to both characterize and directly test the role of the
complement system in the neurodegeneration induced following synucleinopathy. The overarching
hypothesis of this application is that C3 derived from A1-astrocytes, following a-syn aggregation, acts
to coordinate the selective degeneration of nigral neurons.