GLYCINE AND L-SERINE ENHANCE MICROGLIAL CALCIUM FLUXES: A POSSIBLE ROLE FOR NEUTRAL AMINO ACID TRANSPORTERS

Abstract

Microglial cells are considered as the macrophages of the brain. Besides this immune function, recent data suggest an important role in neuronal development and synaptogenesis, as well as in different neuropathologies. Glycine is best known as the main neurotransmitter mediating fast inhibitory neurotransmission in the caudal part of the central nervous system. Glycine has also been proposed as an immunomodulatory signal for various immune cells. Several studies suggested that this modulation is dependent on glycine receptors (GlyRs), in general causing an inhibition of immune function. In microglia, it has been shown that astrocyte-derived glycine modulates microglial activity, although the mechanism is unknown. Our previous work showed that these modulating glycine effects were in fact GlyR-independent. The aim of this study was to study whether these effects were correlated with changes in calcium signalling, and, if so, to further elucidate the mechanism behind these effects. Two in vitro models were used: the mouse microglial BV-2 cell line and primary microglial cultures derived from newborn rats. Calcium fluxes were induced by ATP and thapsigargin and were measured in a fluorescent plate reader with the calcium indicator Fluo-3. We found a clear enhancement of calcium fluxes in millimolar concentrations of glycine or L-serine. This enhancement could be detected after a short time incubation with glycine or L-serine, arguing against a metabolic mechanism. The effects were not blocked by strychnine, confirming GlyR independency. AIB(2-amino-isobutyric acid), a blocker of neutral amino acid transporters could dose-dependently block the effects, suggesting that uptake of these amino acids is at least part of the process. By means of glycine immunolabelling an confocal microscopy we could confirm intensive glycine uptake under our experimental conditions. This study shows that the enhancing effects of glycine and L-serine on microglial activity are correlated with GlyR-independent enhancements of calcium fluxes. We hypothesize that these amino acids are taken up by microglial cells and somehow activate intracellular signalling cascades. Further research is necessary to unravel the precise mechanism.