NMDA receptors as a possible component of store-operated Ca2+ entry in human T-lymphocytes

Elevation of intracellular Ca2+ in T-lymphocytes as a consequence of T cell antigen receptor activation triggers transcriptional programs resulting in effector cytokine secretion and immune response coordination. Increase of Ca2+ concentration in T-lymphocytes follows both the Ins(1,4,5)P3-dependent release from an intracellular store and subsequent influx from extracellular milieu. Flow cytometry and the fluorescent dye Fluo-4AM have been used to demonstrate that noncompetitive NMDA receptor antagonist (+)-MK801 inhibits Ca2+ influx in T cells induced by thapsigargin. Combination of thapsigargin and (+)-MK801 with following incubation does not affect Ca2+ mobilization from intracellular stores, while decreased Ca2+ entry was observed. Overall data indicate that the ion channel blocker (+)-MK801 is able to inhibit the Ca2+ influx and confirm our suggestion about involvement of NMDA receptor in the store-operated Ca2+ entry mechanisms in human T-lymphocytes. To identify the signal transduction pathways associated with NMDA receptors in mitogen-stimulated T-lymphocytes, the cells were incubated with (+)-MK801, then activity of key phosphorylated protein kinases of MAP-activated (pERK1/2, pSAPK/JNK, p-p38), Ca2+-dependent (pCaMKII), PI3/Akt-dependent (pGSK-3β), and PKC-activated (pPKCθ) pathways were detected. The data we obtained demonstrate that (+)-MK801 treatment leads to more prominent decrease in Ras-activated protein kinases pERK1/2 and Rac-activated proteins p-p38 and pSAPK/JNK, as compared to DAG-dependent pPKCθ and Ca2+-dependent pCaMKII. These results show that NMDA receptors are mainly involved in regulation of Ras/Rac-dependent signaling in T-lymphocytes.