G-protein-coupled receptors (GPCRs) represent a family with over 800 members in humans, and one-third of these are targets for approved drugs. A large number of GPCRs have unknown physiologic roles. Here, we investigated GPR27, an orphan GPCR belonging to the family of super conserved receptor expressed in the brain, with unknown functions. Cytosolic levels of L-lactate ([lactate]i), the end product of aerobic glycolysis, were measured with the Laconic fluorescence resonance energy transfer nanosensor. In single 3T3 wild-type (WT) embryonic cells, the application of 8535 (1 µM), a surrogate agonist known to activate GPR27, resulted in an increase in [lactate]i. Similarly, an increase was recorded in primary rat astrocytes, a type of neuroglial cell abundant in the brain, which contain glycogen and express enzymes of aerobic glycolysis. In CRISPR-Cas9 GPR27 knocked out 3T3 cells, the 8535-induced increase in [lactate]i was reduced compared with WT controls. Transfection of the GPR27-carrying plasmid into the 3T3KOGPR27 cells rescued the 8535-induced increase in [lactate]i. These results indicate that stimulation of GPR27 enhances aerobic glycolysis and L-lactate production in 3T3 cells and astrocytes. Interestingly, in the absence of GPR27 in 3T3 cells, resting [lactate]i was increased in comparison with controls, further supporting the view that GPR27 regulates L-lactate homeostasis.

The plasticity of astrocytes is fundamental for their principal function, maintaining homeostasis of the central nervous system throughout life, and is associated with diverse exposomal challenges. Here, we used cultured astrocytes to investigate at subcellular level basic cell processes under controlled environmental conditions. We compared astroglial functional and signaling plasticity in standard serum-containing growth medium, a condition mimicking pathologic conditions, and in medium without serum, favoring the acquisition of arborized morphology. Using optoÀ/electrophysiologic techniques, we examined cell viability, expression of astroglial markers, vesicle dynamics, and cytosolic Ca 2+ and cAMP signaling. The results revealed altered vesicle dynamics in arborized astrocytes that was associated with increased resting [Ca 2 + ] i and increased subcellular heterogeneity in [Ca 2+ ] i , whereas [cAMP] i subcellular dynamics remained stable in both cultures, indicating that cAMP signaling is less prone to plastic remodeling than Ca 2+ signaling, possibly also in in vivo contexts. K E Y W O R D S astrocyte, Ca 2 + , cAMP, confocal microscopy, electrophysiology, vesicles 1 | INTRODUCTION Astrocytes are morphologically and functionally heterogeneous glial cells in the central nervous system. Immunolabeling of glial fibrillary acidic protein (GFAP) reveals major processes with finer cellular parts remaining unstained (Connor & Berkowitz, 1985) making astrocytes appear as stellate cells (Wolfes et al., 2017; Wolfes & Dean, 2018).Advanced visualization techniques revealed that astrocytes exhibit a more complex, spongioform structure (Benediktsson et al., 2005;Bushong et al., 2004). The morphological complexity of astrocytes arguably correlates with their extended homeostatic roles. Astroglia assist neuro-and synaptogenesis, provide substrates to neurons, regulate blood flow and the blood-brain barrier, control uptake and recycling of neurotransmitters, and produce and secrete various neurotrophic factors to regulate memory formation (Araque et al., 1999;