Single-cell Spatial Transcriptomics Reveals Distinct Patterns of Dysregulation in Non-neuronal and Neuronal Cells Induced by the Trem2 Alzheimer's Risk Gene Mutation

The R47H missense mutation of the TREM2 gene is a known risk factor for development of Alzheimer's Disease. In this study, we analyze the impact of the Trem2 mutation on specific cell types in multiple cortical and subcortical brain regions in the context of wild-type and 5xFAD mouse background. We profile 19 mouse brain sections consisting of wild-type, Trem2, 5xFAD and Trem2; 5xFAD genotypes using MERFISH spatial transcriptomics, a technique that enables subcellular profiling of spatial gene expression. Spatial transcriptomics and neuropathology data are analyzed using our custom pipeline to identify plaque and Trem2-induced transcriptomic dysregulation. We initially analyze cell type-specific transcriptomic alterations induced by plaque proximity. Next, we analyze spatial distributions of disease associated microglia and astrocytes, and how they vary between 5xFAD and Trem2; 5xFAD mouse models. Finally, we analyze the impact of the Trem2 mutation on neuronal transcriptomes. The Trem2 mutation induces consistent upregulation of Bdnf and Ntrk2 across many cortical excitatory neuron types, independent of amyloid pathology. Spatial investigation of genotype enriched subclusters identified spatially localized neuronal subpopulations reduced in 5xFAD and Trem2; 5xFAD mice. Overall, our MERFISH spatial transcriptomics analysis identifies glial and neuronal transcriptomic alterations induced independently by 5xFAD and Trem2 mutations, impacting inflammatory responses in microglia and astrocytes, and activity and BDNF signaling in neurons.