Toxoplasma-Induced Hypermigration of Primary Cortical Microglia Implicates GABAergic Signaling

Abstract

<i>Toxoplasma gondii</i> is a widespread obligate intracellular parasite that causes chronic infection and life-threatening acute infection in the central nervous system. Previous work identified <i>Toxoplasma</i>-infected microglia and astrocytes during reactivated infections in mice, indicating an implication of glial cells in acute toxoplasmic encephalitis. However, the mechanisms leading to the spread of <i>Toxoplasma</i> in the brain parenchyma remain unknown. Here, we report that, shortly after invasion by <i>T. gondii</i> tachyzoites, parasitized microglia, but not parasitized astrocytes, undergo rapid morphological changes and exhibit dramatically enhanced migration in 2-dimensional and 3-dimensional matrix confinements. Interestingly, primary microglia secreted the neurotransmitter γ-aminobutyric acid (GABA) in the supernatant as a consequence of <i>T. gondii</i> infection but not upon stimulation with LPS or heat-inactivated <i>T. gondii</i>. Further, microglia transcriptionally expressed components of the GABAergic machinery, including GABA-A receptor subunits, regulatory molecules and voltage-dependent calcium channels (VDCCs). Further, their transcriptional expression was modulated by challenge with <i>T. gondii</i>. Transcriptional analysis indicated that GABA was synthesized via both, the conventional pathway (glutamate decarboxylases GAD65 and GAD67) and a more recently characterized alternative pathway (aldehyde dehydrogenases ALDH2 and ALDH1a1). Pharmacological inhibitors targeting GABA synthesis, GABA-A receptors, GABA-A regulators and VDCC signaling inhibited <i>Toxoplasma</i>-induced hypermotility of microglia. Altogether, we show that primary microglia express a GABAergic machinery and that <i>T. gondii</i> induces hypermigration of microglia in a GABA-dependent fashion. We hypothesize that migratory activation of parasitized microglia by <i>Toxoplasma</i> may promote parasite dissemination in the brain parenchyma.