TREM2 regulates purinergic receptor-mediated calcium signaling and motility in human iPSC-derived microglia
Amit Jairaman1, Amanda McQuade2,3,4,5, Alberto Granzotto2,6,7
1Department of Physiology and Biophysics, University of California, Irvine, Irvine, United States.
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.
Insights
Loss of Triggering Receptor Expressed on Myeloid cells 2 (TREM2) in microglia causes exaggerated calcium signals, impairing directed movement crucial for Alzheimer's disease pathology. Restoring calcium balance improves microglial chemotaxis.
Area of Science:
- Neuroscience
- Immunology
- Cell Biology
Background:
- Triggering Receptor Expressed on Myeloid cells 2 (TREM2) is vital for microglial function and linked to Alzheimer's disease (AD) risk.
- Calcium (Ca2+) signaling dysregulation is implicated in AD pathogenesis.
- Microglia, the brain's immune cells, rely on precise signaling for their roles.
Purpose of the Study:
- To investigate how TREM2 influences Ca2+ signaling in human iPSC-derived microglia.
- To understand the impact of TREM2 deficiency on microglial responses to purinergic agonists.
- To explore the relationship between Ca2+ signaling, motility, and chemotaxis in TREM2-deficient microglia.
Main Methods:
- Utilized human induced pluripotent stem cell-derived microglia (iPSC-microglia) with genetic deletion of TREM2 (TREM2 KO).
- Measured Ca2+ signals in response to purinergic agonists like ADP using genetically encoded Ca2+ probes (Salsa6f).
- Assessed microglial motility and directional chemotaxis along ADP gradients; effects of P2Y12 receptor antagonism were evaluated.
Main Results:
- TREM2 KO iPSC-microglia exhibited exaggerated Ca2+ responses to ADP, driven by increased P2Y12/P2Y13 receptor expression.
- This hypersensitivity led to enhanced Ca2+ release from the endoplasmic reticulum and sustained influx via Orai channels, altering motility.
- While TREM2 KO microglia showed increased displacement, their directional chemotaxis was reduced, but this defect was rescued by P2Y12 antagonism.
Conclusions:
- Loss of TREM2 disrupts microglial Ca2+ signaling responses to purinergic stimuli.
- Altered Ca2+ dynamics in TREM2-deficient microglia impact cell motility and chemotaxis.
- Adequate Ca2+ signaling is essential for optimal microglial chemotactic function, highlighting a potential therapeutic target for AD.
