Astrocyte-Derived ATP Induces Vesicle Shedding and IL-1beta Release from Microglia

ATP has been indicated as a primary factor in microglial response to brain injury and inflammation. By acting on different purinergic receptors 2, ATP is known to induce chemotaxis and stimulate the release of several cytokines from these cells. The activation of purinergic receptors 2 in microglia can be triggered either by ATP deriving from dying cells, at sites of brain injury or by ATP released from astrocytes, in the absence of cell damage. By the use of a biochem. approach integrated with video microscopy expts., the authors investigated the functional consequences triggered in microglia by ATP released from mech. stimulated astrocytes, in mixed glial cocultures. Astrocyte-derived ATP induced in nearby microglia the formation and the shedding of membrane vesicles. Vesicle formation was inhibited by the ATP-degrading enzyme apyrase or by P2X7R antagonists. Isolation of shed vesicles, followed by IL-1? evaluation by a specific ELISA revealed the presence of the cytokine inside the vesicular organelles and its subsequent efflux into the extracellular medium. IL-1? efflux from shed vesicles was enhanced by ATP stimulation and inhibited by pretreatment with the P2X7 antagonist oxidized ATP, thus indicating a crucial involvement of the pore-forming P2X7R in the release of the cytokine. The data identify astrocyte-derived ATP as the endogenous factor responsible for microvesicle shedding in microglia and reveal the mechanisms by which astrocyte-derived ATP triggers IL-1? release from these cells.