TREM2 regulates purinergic receptor-mediated calcium signaling and motility in human iPSC-derived microglia

The membrane protein TREM2 (Triggering Receptor Expressed on Myeloid cells 2) regulates key microglial functions including phagocytosis and chemotaxis. Loss-of-function variants of TREM2 are associated with increased risk of Alzheimer's disease (AD). Because abnormalities in Ca signaling have been observed in several AD models, we investigated TREM2 regulation of Ca signaling in human induced pluripotent stem cell-derived microglia (iPSC-microglia) with genetic deletion of TREM2. We found that iPSC-microglia lacking TREM2 (TREM2 KO) show exaggerated Ca signals in response to purinergic agonists, such as ADP, that shape microglial injury responses. This ADP hypersensitivity, driven by increased expression of P2Y and P2Y receptors, results in greater release of Ca from the endoplasmic reticulum stores, which triggers sustained Ca influx through Orai channels and alters cell motility in TREM2 KO microglia. Using iPSC-microglia expressing the genetically encoded Ca probe, Salsa6f, we found that cytosolic Ca tunes motility to a greater extent in TREM2 KO microglia. Despite showing greater overall displacement, TREM2 KO microglia exhibit reduced directional chemotaxis along ADP gradients. Accordingly, the chemotactic defect in TREM2 KO microglia was rescued by reducing cytosolic Ca using a P2Y receptor antagonist. Our results show that loss of TREM2 confers a defect in microglial Ca response to purinergic signals, suggesting a window of Ca signaling for optimal microglial motility.