Astrocyte-released cytokines induce ramification and outward K+ channel expression in microglia via distinct signalling pathways

Differentiation of microglial cells is characterized by transformation from ameboid into ramified cell shape and up-regulation of K+ channels. The pro cesses of microglial differentiation are controlled by astrocytic factors. The mechanisms by which astrocytes cause developmental changes in morpholog ical and electrophysiological properties of microglia have remained unclear . We show here that the cytokines transforming growth factor-beta (TGF-beta ), macrophage colony-stimulating factor (M-CSF) and granulocyte/macrophage colony-stimulating factor (GM-CSF) are released by astrocytes at concentrat ions sufficient to induce ramification and up-regulation of delayed rectifi er (DR) K+ channels in microglia. Transformation from ameboid into ramified morphology induced in microglia by exposure to astrocyte-conditioned mediu m (ACM) was inhibited by neutralizing antibodies against TGF-beta, M-CSF or GM-CSF, whilst ACM-induced DR channel expression was exclusively inhibited by antibodies against TGF-beta. Although both ramification and DR channel up-regulation occurred simultaneously, DR channel blockade by charybdotoxin failed to inhibit microglial ramification. The ACM-induced ramification of microglia was inhibited by the tyrosine kinase inhibitor genistein, wherea s DR channel up-regulation did not occur in the presence of the serine/thre onine kinase inhibitor H7. Our data suggest that astrocytes modulate proces ses of microglial differentiation in parallel but via distinct signalling p athways.