iPSC-derived microglia carrying the TREM2 R47H/+ mutation are pro-inflammatory and promote synapse loss.
Genetic findings have highlighted key roles for microglia in the pathology of neurodegenerative conditions such as Alzheimer's disease (AD). A number of mutations in the microglial protein TREM2 (triggering receptor expressed on myeloid cells 2) have been associated with increased risk for developing Alzheimer's disease (AD), most notably the R47H/+ substitution. We employed gene editing and stem cell models to gain insight into the effects of the TREM2 R47H/+ mutation on human iPSC-derived microglia. We found transcriptional changes affecting numerous cellular processes, with R47H/+ cells exhibiting a pro-inflammatory gene expression signature. TREM2 R47H/+ also caused impairments in microglial movement and the uptake of multiple substrates, as well as rendering microglia hyper-responsive to inflammatory stimuli. We developed an in vitro laser-induced injury model in neuron-microglia co-cultures, finding an impaired injury response by TREM2 R47H/+ microglia. Furthermore, mouse brains transplanted with TREM2 R47H/+ microglia exhibited reduced synaptic density, with upregulation of multiple complement cascade components in TREM2 R47H/+ microglia suggesting inappropriate synaptic pruning as one potential mechanism. These findings identify a number of potentially detrimental effects of the TREM2 R47H/+ mutation on microglial gene expression and function likely to underlie its association with AD. Overall design: CRISPR/Cas9 gene editing was performed to generate TREM2 R47H/+ and control iPSC lines (2 clones of each). Cell lines were differentiated to iPSC-derived microglia and RNA-seq was performed to determine the effect of the TREM2 R47H/+ mutation under baseline conditions. A subsequent experiment involved treatment of the iPSC-derived microglia with pro-inflammatory compounds (LPS and IFNgamma) followed by RNA-seq to determine whether TREM2 R47H/+ microglia respond differently to pro-inflammatory stimuli than wild-type controls.