CCL5 evokes calcium signals in microglia through a kinase-, phosphoinositide-, and nucleotide-dependent mechanism

Microglia, the resident macrophages of the CNS, are responsible for the innate immune response in the brain and participate in the pathogenesis of certain neurodegenerative disorders. Chemokines initiate activation and migration of microglia. The β-chemokine CCL5 induces an elevation in intracellular calcium concentration ([Ca²⁺]_i) in human microglia. Here, we examined the signal transduction pathway linking activation of chemokine receptor CCR5 to an elevation in [Ca²⁺]_i in cultured microglia by using pharmacological approaches in combination with Fura-2-based digital imaging. The CCL5-induced response required Janus kinase (Jak) activity and the stimulation of an inhibitory G protein. Multiple downstream signaling pathways were involved, including phosphatidylinositol 3-kinase (PI3K), Bruton's tyrosine kinase (Btk), and phospholipase C (PLC)-mediated release of Ca²⁺ from inositol 1,4,5-trisphosphate (IP₃)-sensitive stores. Activation of both the kinase and the lipase pathways was required for eliciting the Ca²⁺ response. However, the majority of the [Ca²⁺]_i increase was derived from sources activated by NAD metabolites. Cyclic ADP-ribose (cADPR) evoked Ca²⁺ release from intracellular stores, and ADPR evoked Ca²⁺ influx via a nimodipine-sensitive channel. Thus, a multistep cascade couples CCR5 activation to Ca²⁺ increases in human microglia. Because changes in [Ca ²⁺]_i affect chemotaxis, secretion, and gene expression, pharmacologic modulation of this pathway may alter inflammatory and degenerative processes in the CNS.