Stimulation of group I metabotropic glutamate receptors evokes calcium signals and c-jun and c-fos gene expression in human T cells

To study if the activation of group I mGlu receptors in human T cells modifies intracellular Ca2+ concentration ([Ca2+]i) and cell function, we measured [Ca2+]i on cell suspensions (spectrofluorimetric method) or single cell (digital Ca2+ imaging system) using fura-2 as indicator. Early-inducible gene (c-jun and c-fos) expression was studied by reverse transcriptase-polymerase chain reaction assay as representative of Ca2+-sensitive gene expression. (1S,3R)-ACPD (100 μM), the selective mGlu receptor agonist, evoked a significant increase (34.1 ± 4.9%) of [Ca2+]i, pharmacologically characterized as mediated by group I mGlu receptors, since both (S)-3,5-DHPG (100 μM), a selective group I mGlu receptor agonist and CHPG (1 mM), the specific mGlu5 receptor agonist, reproduced the effects, that were abolished by AIDA (1 mM), a selective group I mGlu receptor antagonist. (S)-3,5-DHPG-induced a rapid [Ca2+]i rise (initial phase) followed by a slow decrease (second phase) to the baseline. Both extracellular Ca2+ and Ca2+ released from intracellular stores contribute to the [Ca2+]i increase which depend on PLC activation. In a Ca2+-free buffer, the second phase rapidly return to the baseline; LaCl3 (1-10 μM), an inhibitor of extracellular Ca2+ influx, significantly reduced the second phase only; thapsigargin (1 μM), by discharging intracellular Ca2+ stores, U 73122 (10 μM) and D609 (300 μM), by inhibiting PLC activity, prevented both phases. In our system, PTX pre-treatment increased (S)-3,5-DHPG effects, demonstrating that PXT-sensitive Gi/o proteins are involved. Finally, specific stimulation of these receptors in Jurkat cells upregulates c-jun and c-fos gene expression, thus activating multiple downstream signalling regulating important T cell functions.