GRM2 and Alzheimer disease: Several GPCRs have been associated with this imbalance including i) muscarinic ACh receptors [158], for which the number is reduced in AD, leading to enhanced phosphorylation of tau; ii) the CXCR2 and CC3 chemokine receptors, for which activation is implicated in the inflammatory response [367] and tau phosphorylation [369] and which are upregulated in AD [370]; and iii) the metabotropic glutamate receptor 2 (mGluR2) that activates the ERK pathway [371] and is overexpressed in AD leading to tau phosphorylation [367,372].