The Influence of the R47H Triggering Receptor Expressed on Myeloid Cells 2 Variant on Microglial Exosome Profiles

Variants in the triggering receptor expressed on myeloid cells 2 gene are linked with an increased risk of dementia, in particular the R47H triggering receptor expressed on myeloid cells 2 variant is linked to late-onset Alzheimer's disease. Using human induced pluripotent stem cells-derived microglia, we assessed whether variations in the dynamics of exosome secretion, including their components, from these cells might underlie some of this risk. We found exosome size was not altered between common variant controls and R47H variants, but the amount and constitution of exosomes secreted were different. Exosome quantities were rescued by incubation with an ATP donor or with lipids via a phosphatidylserine triggering receptor expressed on myeloid cells 2 ligand. Following a lipopolysaccharide or phagocytic cell stimulus, exosomes from common variant and R47H microglia were found to contain cytokines, chemokines, APOE and triggering receptor expressed on myeloid cells 2. Differences were observed in the expression of CCL22, IL-1β and triggering receptor expressed on myeloid cells 2 between common variant and R47H derived exosomes. Furthermore unlike common variant-derived exosomes, R47H exosomes contained additional proteins linked to negative regulation of transcription and metabolic processes. Subsequent addition of exosomes to stressed neurones showed R47H-derived exosomes to be less protective. These data have ramifications for the responses of microglia in Alzheimer's disease and may point to further targets for therapeutic intervention.