Systematic analysis of cellular cross-talk reveals a role for SEMA6D-TREM2 regulating microglial function in Alzheimer’s disease

Cellular cross-talk, mediated by membrane receptors and their ligands, is crucial for brain homeostasis and can contribute to neurodegenerative diseases such as Alzheimer's disease (AD). To find cross-talk dysregulations involved in AD, we reconstructed cross-talk networks from single-nucleus transcriptional profiles of 67 clinically and neuropathologically well-characterized controls and AD brain donors from the Knight Alzheimer Disease Research Center and the Dominantly Inherited Alzheimer Network cohorts. We predicted a role for TREM2 and additional AD risk genes mediating neuron-microglia cross-talk in AD. We identified a gene network mediating neuron-microglia cross-talk through TREM2 and neuronal SEMA6D, which we predicted is disrupted in late AD stages. Using spatial transcriptomics on the human brain, we observed that the SEMA6D-TREM2 cross-talk gene network is activated near Aβ plaques and SEMA6D-expressing cells. Using tissue immunostaining of human brains, we found that SEMA6D colocalizes with Aβ plaques and TREM2-activated microglia. In addition, we found that plaque-proximal SEMA6D abundance decreased with the disease stage, which correlated with a reduction in microglial activation near plaques. These findings suggest that the loss of SEMA6D signaling impairs microglial activation and Αβ clearance. To validate this hypothesis, we leveraged TREM2 knockout human induced pluripotent stem cell-derived microglia and observed that SEMA6D induces microglial activation and Aβ plaque phagocytosis in a TREM2-dependent manner. In summary, we demonstrate that characterizing cellular cross-talk networks can yield insights into AD biology, provide additional context to understand AD genetic risk, and find previously unknown therapeutic targets and pathways.