Abstract
<jats:p>GPR27 (SREB1) is a highly conserved orphan class A G-protein coupled receptor implicated in insulin production, metabolic regulation, tumour biology, neurodegeneration and L-lactate homeostasis, but its immediate second-messenger signalling remains poorly defined. We used single-cell Foerster resonance energy transfer nanosensors to monitor cytosolic Calcium and cAMP in wild-type 3T3 MEF cells, CRISPR-Cas9 GPR27-knockout cells (GPR27KO) and GPR27-knockout cells transiently re-expressing FLAG-tagged GPR27 (GPR27-rescued). The GPR27 surrogate agonist 8535n increased intracellular calcium in wild-type and rescued cells but not in GPR27-knockout cells and produced no significant cAMP response in wild-type cells. Basal Calcium and cAMP levels were unaffected by GPR27 deletion. Extracellular L-lactate (2 mM) induced a GPR27-dependent increase in calcium and cAMP in wild-type and rescued cells, but not in knockout cells, raising the possibility that L-lactate acts as an endogenous ligand or modulator of GPR27.</jats:p>