The tyrosine kinase inhibitor AG126 restores receptor signaling and blocks release functions in activated microglia (brain macrophages) by preventing a chronic rise in the intracellular calcium level

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Abstract
We recently reported that lasting activation of mouse microglial cells with bacterial lipopolysaccharide (LPS) chronically elevated the basal intracellular calcium concentration ([Ca2+]i). This correlated to an attenuated calcium signaling of complement (C5a) and purinergic (UTP) receptors as well as to the capacity for effective production of cytokines–chemokines. Here, we demonstrate that these adjustments in the [Ca2+]i regulation require a critical protein tyrosine kinase (PTK) function – even in varying stimulation scenarios. Changes in basal [Ca2+]i and calcium signaling are not restricted to Gram‐negative bacterial confrontation. Pneumococcal cell wall (PCW) modelling Gram‐positive infection causes virtually the same effects. Moreover, decreases in calcium signaling efficacy are neither associated with altered receptor expression, nor mediated by autocrine loops. Administration of microglial release products, transfer of conditioned supernatant or presence of a radical scavenger during LPS or PCW treatments have no consequence. However, both the elevation in basal [Ca2+]i as well as the suppression of C5a‐ and UTP‐evoked calcium signals are selectively and dose‐dependently reversed by tyrphostin AG126, a PTK inhibitor that, moreover, blocks inducible nitric oxide and cytokine–chemokine release. The findings suggest that the AG126‐sensitive PTK critically controls both sensory and executive features of the microglial activation process via sustained up‐regulation of basal [Ca2+]i.