Transcriptomically unique endolysosomal and homeostatic microglia populations in Alzheimer’s disease and aged human brain

Microglia-mediated neuroinflammation is hypothesized to contribute to disease progression in neurodegenerative diseases such as Alzheimer9s Disease (AD). Microglia demonstrate heterogeneous states in health and disease, with proposed beneficial, harmful, and disease specific subtypes. Defining the spectrum of microglia phenotypes is an important step in rational design of neuroinflammation modulating therapies. To facilitate improved phenotype resolution and group comparisons based on disease state we performed single-nucleus RNA-seq on more than 120,000 microglia nuclei from AD and control dorsolateral prefrontal cortex. We identify clusters of microglia enriched for biological pathways implicating defined myeloid roles. We detected several previously unrecognized microglia populations in human AD brain, including three internalization and trafficking subtypes that were heterogeneous in their metabolic and inflammatory signatures. One of these endolysosomal subtypes is larger in AD individuals and was uniquely enriched for genes involved in nucleic acid detection and activation of interferon signaling. This inflammatory endolysosomal cluster also differentially regulated expression of genes associated with AD risk by genome wide association studies. We also identified a cluster of microglia with upregulated cell cycle and DNA repair genes that is proportionately larger in control individuals. Within cluster comparisons demonstrate that in AD brain, homeostatic microglia subpopulations upregulate inflammatory gene expression. These results highlight the heterogenous nature of the microglia response to AD pathology and will inform efforts to target specific subtypes of microglia in the development of novel AD therapies.