Alzheimer's patient brain myeloid cells exhibit enhanced aging and unique transcriptional activation

Gene expression changes in brain from mouse models of Alzheimer9s disease (AD) are highly characterized and reflect specific myeloid cell activation states that could modulate AD risk or progression. While some groups have produced valuable expression profiles for human brain cells, the cellular clarity with which we now view transcriptional responses in mouse AD models has not yet been realized for human AD tissues due to limited availability of fresh tissue samples and technological hurdles of recovering transcriptomic data with cell-type resolution from frozen samples. We developed a novel method for isolating multiple cell types from frozen post-mortem specimens of superior frontal gyrus for RNA-Seq and identified 66 genes differentially expressed between AD and control subjects in the myeloid cell compartment. Myeloid cells sorted from fusiform gyrus of the same subjects showed similar changes, and whole tissue RNA analyses further corroborated our findings. The changes we observed did not resemble the damage-associated microglia (DAM) profile described in mouse AD models, or other known activation states from other disease models. Instead, roughly half of the changes were consistent with an enhanced human aging phenotype, whereas the other half, including the AD risk gene APOE, were altered in AD myeloid cells but not differentially expressed with age. We refer to this novel profile in [h]uman [A]lzheimer9s [m]icroglia/myeloid cells as the HAM signature. These results, which can be browsed at research-pub.gene.com/BrainMyeloidLandscape/reviewVersion, highlight considerable differences between myeloid activation in mouse models and human disease, and provide a genome-wide picture of brain myeloid activation in human AD.