Ca2+ spiking activity caused by the activation of store-operated Ca2+ channels mediates INF-α release from microglial cells under chronic purinergic stimulation

Cytokines released from microglia mediate defensive responses in the brain, but the underlying mechanisms are obscure. One proposed process is that nucleotide leakage or release from surrounding cells is sensed by metabotropic (P2Y) and ionotropic (P2X) purinergic receptors, which may trigger long-term intracellular Ca2+ flux and tumor necrosis factor a (TNF-alpha) release. Indeed, 3 h of exposure to ATP was required to evoke TNF-alpha release from a murine microglial cell line (MG5). A Ca2+ chelator, ethylene glycol tetraacetic acid (EGTA), reduced ATP-induced TNF-alpha release, suggesting that intracellular Ca2+ is important in this response. Therefore, Ca2+ sensor genes (YC3.6) were transfected into MG5 cells to investigate the Ca2+ dynamics underlying ATP-induced TNF-alpha release. The results demonstrated ATP-induced biphasic Ca2+ mobilization mediated by P2Y (similar to 5 min) and P2X(7) receptors (5-30 min). Moreover, Ca2+ spiking activity in cell processes progressively increased with a reduction in P2X(7) receptor-mediated Ca2+ elevation during 3-h ATP stimulation. Increased Ca2+ spiking activity paralleled the reduction in thapsigargin-sensitive internal Ca2+ stores, dendrite extension, and expression of macrophage scavenger receptors with collagenous structure. The Ca2+ spiking activity was enhanced by a P2X(7) receptor antagonist (A438079), but inhibited by a store-operated channel antagonist (SKF96365) or by co-transfection of small interference ribonucleic acid (siRNA) targeted on the channel component (Orail). Furthermore, ATP-induced TNF-alpha release was enhanced by A438079 but was inhibited by SICF96365. Because store-operated channels (Stiml /Orail) were expressed both in MG5 and primary microglial cultures, we suggest that P2X(7) receptor signaling inhibits store-operated channels during ATP stimulation, and disinhibition of this process gates TNF-alpha release from microglial cells. (c) 2013 Elsevier B.V. All rights reserved.