Differential stimulation of pluripotent stem cell-derived human microglia leads to exosomal proteomic changes affecting neurons

Microglial exosomes are an emerging communication pathway, implicated in fulfilling homeostatic microglial functions and transmitting neurodegenerative signals. Gene variants of the triggering receptor expressed myeloid cells-2 (TREM2) are associated with an increased risk of developing dementia. We investigated the influence of the TREM2 Alzheimer’s disease risk variant, R47H het , on the microglial exosomal proteome consisting of 3019 proteins secreted from human iPS-derived microglia (iPS-Mg). Exosomal protein content changed according to how iPS-Mg were stimulated. Thus lipopolysaccharide (LPS) induced microglial exosomes to contain more inflammatory signals, whilst stimulation with the TREM2 ligand phosphatidylserine (PS + ) increased metabolic signals within the microglial exosomes. We tested the effect of these exosomes on neurons and found that the exosomal protein changes were functionally relevant and influenced downstream functions in both neurons and microglia. Exosomes from R47H het iPS-Mg contained disease-associated microglial (DAM) signature proteins, and were less able to promote outgrowth of neuronal processes and increase mitochondrial metabolism in neurons compared with exosomes from TREM2 common variant iPS-Mg. Taken together these data highlight the importance of microglial exosomes in fulfilling microglial functions. Additionally, variations in the exosomal proteome influenced by the R47H het TREM2 variant may underlie the increased risk of Alzheimer’s disease associated with this variant. Graphical Abstract