Metabotropic Glutamate Receptor 8 Suppresses M1 Polarization in Microglia by Alleviating Endoplasmic Reticulum Stress and Mitochondrial Dysfunction

Background: Microglia-mediated neuroinflammation is a hallmark of neurodegeneration. Metabotropic glutamate receptor 8 (GRM8) has been reported to promote neuronal survival in neurodegenerative diseases, yet the effect of GRM8 on neuroinflammation is still unclear. Calcium overload-induced endoplasmic reticulum (ER)-mitochondrial miscommunication has been reported to trigger neuroinflammation in the brain. The aim of this study was to investigate putative anti-inflammatory effects of GRM8 in microglia, specifically focusing on its role in calcium overload-induced ER stress and mitochondrial dysfunction. Methods: BV2 microglial cells were pretreated with GRM8 agonist prior to lipopolysaccharide administration. Pro-inflammatory cytokine levels and the microglial polarization state in BV2 cells were then quantified. Cellular apoptosis and the viability of neuron-like PC12 cells co-cultured with BV2 cells were examined using flow cytometry and a Cell Counting Kit-8, respectively. The concentration of cAMP, inositol-1,4,5-triphosphate receptor (IP3R)-dependent calcium release, ER Ca2+ concentration, mitochondrial function as reflected by reactive oxygen species levels, ATP production, mitochondrial membrane potential, expression of ER stress-sensing protein, and phosphorylation of the nuclear factor kappa B (NF-κB) p65 subunit were also quantified in BV2 cells. Results: GRM8 activation inhibited pro-inflammatory cytokine release and shifted microglia polarization towards an anti-inflammatory-like phenotype in BV2 cells, as well as promoting neuron-like PC12 cell survival when co-cultured with BV2 cells. Mechanistically, microglial GRM8 activation significantly inhibited cAMP production, thereby desensitizing the IP3R located within the ER. This process markedly limited IP3R-dependent calcium release, thus restoring mitochondrial function while inhibiting ER stress and subsequently deactivating NF-κB signaling. Conclusions: Our results indicate that GRM8 activation can protect against microglia-mediated neuroinflammation by attenuating ER stress and mitochondrial dysfunction, and that IP3R-mediated calcium signaling may play a vital role in this process. GRM8 may thus be a potential target for limiting neuroinflammation.