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

Amit Jairaman,Stefano L Sensi,You Jung Kang,Michael D Cahalan,Shivashankar Othy,Ian Smith,Ian Parker,Jean Paul Chadarevian,Sunil Gandhi,Hansang Cho,Mathew Blurton-Jones,Amanda Mcquade,Alberto Granzotto

doi:10.7554/elife.73021

Abstract

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 Ca2+ signaling have been observed in several AD models, we investigated TREM2 regulation of Ca2+ 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 Ca2+ signals in response to purinergic agonists, such as ADP, that shape microglial injury responses. This ADP hypersensitivity, driven by increased expression of P2Y12 and P2Y13 receptors, results in greater release of Ca2+ from the endoplasmic reticulum stores, which triggers sustained Ca2+ influx through Orai channels and alters cell motility in TREM2 KO microglia. Using iPSC-microglia expressing the genetically encoded Ca2+ probe, Salsa6f, we found that cytosolic Ca2+ 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 Ca2+ using a P2Y12 receptor antagonist. Our results show that loss of TREM2 confers a defect in microglial Ca2+ response to purinergic signals, suggesting a window of Ca2+ signaling for optimal microglial motility.

Highlights

IntroductionAs the primary immune cells of the central nervous system, microglia survey their local environment to maintain homeostasis and respond to local brain injury or abnormal neuronal activity
Even before process extension was activated with ADP, cells treated with P2Y antagonists showed significantly fewer and shorter processes, suggesting that baseline purinergic signaling may regulate resting microglial process dynamics. These results demonstrate that activation of purinergic signaling through P2Y12 and P2Y13 receptors is required for ADP-driven microglial process extension and motility. 244 ADP-evoked changes in cell motility and process extension are enhanced in TREM2-KO microglia To characterize differences in motility characteristics between wild type (WT) and TREM2 KO microglia responding to ADP, we plotted mean squared displacement (MSD) vs time and compared cell track overlays which showed that ADP enhances motility in TREM2 KO cells to a greater extent than in WT microglia (Figure 5A, B)
Figure 2- source data 1: Higher sensitivity of TREM2 KO microglia to ADP is driven by increased purinergic receptor expression

Summary

Introduction

IntroductionAs the primary immune cells of the central nervous system, microglia survey their local environment to maintain homeostasis and respond to local brain injury or abnormal neuronal activity. Purinergic metabolites (ATP, ADP, UTP, UDP) in the brain constitute key signals driving microglial activation and chemotaxis, and are detected by microglial cells over concentrations 55 ranging from hundreds of nM to M 8–13. ADP is detected by P2Y purinergic receptors on microglia, causing IP3-dependent Ca2+ release from the endoplasmic reticulum (ER) lumen. Ca2+ depletion from the ER in turn activates ER STIM1 proteins to translocate proximally to puncta where closely apposed plasma membrane (PM) Orai[1] channels are activated. This mechanism underlies store-operated Ca2+ entry (SOCE) in many cell types 17, including microglia[18,19,20]

Results

Discussion

Conclusion