Interleukin-1β-dependent signaling between astrocytes and neurons depends critically on astrocytic calcineurin/NFAT activity

Interleukin-1β(IL-1β) and the Ca²⁺/calmodulin- dependent protein phosphatase, calcineurin, have each been shown to play an important role in neuroinflammation. However, whether these signaling molecules interact to coordinate immune/inflammatory processes and neurodegeneration has not been investigated. Here, we show that exogenous application of IL-1β (10 ng/ml) recruited calcineurin/NFAT (nuclear factor of activated T cells) activation in primary astrocyte-enriched cultures within minutes, through a pathway involving IL-1 receptors and L-type Ca²⁺ channels. Adenovirus-mediated delivery of the NFAT inhibitor, VIVIT, suppressed the IL-1β-dependent induction of several inflammatory mediators and/or markers of astrocyte activation, including tumor necrosis factor α, granulocyte/ macrophage colony-stimulating factor, and vimentin. Expression of an activated form of calcineurin in one set of astrocyte cultures also triggered the release of factors that, in turn, stimulated NFAT activity in a second set of "naive" astrocytes. This effect was prevented when calcineurin-expressing cultures co-expressed VIVIT, suggesting that the calcineurin/ NFATpathway coordinates positive feedback signaling between astrocytes. In the presence of astrocytes and neurons, 48-h delivery of IL-1β was associated with several excitotoxic effects, including NMDA receptor-dependent neuronal death, elevated extracellular glutamate, and hyperexcitable synaptic activity. Each of these effects were reversed or ameliorated by targeted delivery of VIVIT to astrocytes. IL-1β also caused an NFAT-dependent reduction in excitatory amino acid transporter levels, indicating a possible mechanism for IL-1β-mediated excitotoxicity. Taken together, the results have potentially important implications for the propagation and maintenance of neuroinflammatory signaling processes associated with many neurodegenerative conditions and diseases.