Elevation of Basal Intracellular Calcium as a Central Element in the Activation of Brain Macrophages (Microglia): Suppression of Receptor-Evoked Calcium Signaling and Control of Release Function

Microglia-brain macrophages are immune-competent cells of the CNS and respond to pathologic events. Using bacterial lipopolysaccharide (LPS) as a tool to activate cultured mouse microglia, we studied alterations in the intracellular calcium concentration ([Ca 2+]i) and in the receptor-evoked generation of transient calcium signals. LPS treatment led to a chronic elevation of basal [Ca 2+]i along with a suppression of evoked calcium signaling, as indicated by reduced [Ca 2+]i transients during stimulation with UTP and complement factor 5a. Presence of the calcium chelator BAPTA prevented the activation-associated changes in [Ca 2+]i and restored much of the signaling efficacy. We also evaluated downstream consequences of a basal [Ca 2+]i lifting during microglial activation and found BAPTA to strongly attenuate the LPS-induced release of nitric oxide (NO) and certain cytokines and chemokines. Furthermore, microglial treatment with ionomycin, an ionophore elevating basal [Ca 2+]i, mimicked the activation-induced calcium signal suppression but failed to induce release activity on its own. Our findings suggest that chronic elevation of basal [Ca 2+]i attenuates receptor-triggered calcium signaling. Moreover, increased [Ca 2+]i is required, but by itself is not sufficient, for release of NO and certain cytokines and chemokines. Elevation of basal [Ca 2+]i could thus prove a central element in the regulation of executive functions in activated microglia.