Human microglia reduce alpha-synuclein aggregation and are neuroprotective in adult mouse brain

Microglia, brain-resident immune cells, are involved in pathophysiology of several neurodegenerative diseases, including Parkinson's disease. Given significant species-specific differences in microglia gene expression, particularly in disease-risk genes, as well as the highly reactive nature of these cells, studying human microglia in a whole brain environment is essential. Here, we established a humanized mouse model by transplanting human induced pluripotent stem cell-derived hematopoietic progenitor cells into the striatum of immunodeficient adult mice and injected human alpha-synuclein preformed fibrils to model Parkinson's disease pathology. Transplanted human cells engraft, mature into microglia and maintain their phenotype for at least three months post-transplantation. These human microglia interact with alpha-synuclein, significantly limiting its propagation from the striatum to the substantia nigra and further reducing local small aggregates; they also mildly protect tyrosine hydroxylase neurons there. Transcriptomic profiling reveals 56 differentially expressed genes in human microglia in response to alpha-synuclein preformed fibrils, while host mouse cells show 202 gene expression changes, including an upregulation of gene Hcrt (fold change = 7.77, p = 0.0015). Immunohistochemistry analysis further confirms the preservation of hypocretin-positive neurons in the hypothalamus of the transplanted mice (p = 0.0079). The findings highlight the neuroprotective role of human microglia and establish a more disease-relevant in vivo model for investigating alpha-synuclein aggregation and therapeutic interventions in Parkinson's disease.