Repopulation of the Microglia/Macrophage Niche in Experimental Lewy Body Disease - Lewy body disorders are a family of neurodegenerative conditions characterized by deposition of the abundant synaptic
protein, α-synuclein, in insoluble cytosolic aggregates. The two brain regions most vulnerable to α-synucleinopathy in early
disease stages are the olfactory bulb/anterior olfactory nucleus (OB/AON) of the rostral telencephalon and the dorsomotor
vagal nuclei of the caudal brainstem. We have infused preformed fibrils (PFFs) synthesized from α-synuclein into the rodent
OB/AON to induce aggregate formation in the limbic forebrain. Limbic α-synucleinopathy in our murine model is associated
with impairments in smell, affect, and spatial reference memory. A large body of experimental evidence supports the view
that α-synucleinopathy is associated with prion-like seeding and cell-to-cell spread of the aggregates through circuitry. Thus,
the intercellular transfer of aggregates may be prevented by engulfment of aggregates from the interstitial fluid or the
efferocytosis of neurons that bear early signs of Lewy-related pathology. Microglia/macrophages (MG/MΦ) are the
professional phagocytes of the brain, but they play complex roles in disease and may suffer phagocytic exhaustion when
faced with excess α-synuclein. Perhaps for these reasons, MG/MΦ are unable to fully prevent Lewy body disease. However,
repopulation of MG/MΦ can be jumpstarted with transient administration of the orally bioavailable CSF1R inhibitor,
PLX5622. In our PFF model, dietary PLX5622 killed ~60% of MG/MΦ, and upon PLX5622 withdrawal, there was vigorous
repopulation of the MG/MΦ niche and an 85% reduction (95% CI 0.02-27.53%) in α-synucleinopathic aggregates in male
mice by 20 months of age. Unexpectedly, transient PLX5622 also increased aggregate sizes in both sexes, and aggregate
sizes were positively correlated with spatial memory. Hence, larger aggregates may be associated with less prion-like
seeding and dispersal. We hypothesize that the probability of successful MG/MΦ engulfment of α-synuclein aggregates is
boosted after a depletion/repopulation event in the PFF model and associated with superior spatial memory. Pilot data
unexpectedly reveal that neurons (and not glia) house the most fluorescent PFFs after OB/AON infusions, while MG/MΦ
engulf neurons housing the PFFs. In Aim 1, we will identify the neural cell type with the highest probability of harboring
PFFs, determine if these cells can, in turn, be engulfed by MG/MΦ, and test if boosting MG/MΦ proliferation boosts their
engulfment capacities. In Aim 2, we will ascertain how transient PLX5622 exposure reduces α-syn aggregate burden in our
PFF model. Changes in the transcriptomes of depleted/repopulated MG/MΦ in the PFF model will be identified by RNAseq
on flow-sorted MG/MΦ, with a focus on genes involved in endocytic/lysosomal processing. The expression of candidate
genes will be suppressed by MG/MΦ cell-targeted AAV6/TM6-shRNA, and we will then test if MG/MΦ engulfment of α-syn
aggregates is lowered when lysosomal processing is impeded in vivo. Impact: If neurons house the most α-syn aggregates
and cannot be engulfed by MG/MΦ in sufficient numbers, this may explain why diseased neurons persist long enough to
propagate Lewy pathologies. This also implies that not all cell loss is detrimental. Further, if depleted/repopulated MG/MΦ
can engulf more α-syn aggregates, this may explain why they are able to alleviate aggregate burden in our PFF model, and
CSF1R blockade with various antagonists should continue to be exploited in the battle against Lewy body disorders.
