Reactive oxygen species released from astrocytes treated with amyloid beta oligomers elicit neuronal calcium signals that decrease phospho-Ser727-STAT3 nuclear content

Abstract

The transcription factor STAT3 has a crucial role in the development and maintenance of the nervous system. In this work, we treated astrocytes with oligomers of the amyloid beta peptide (A beta Os), which display potent synaptotoxic activity, and studied the effects of mediators released by A beta Os-treated astrocytes on the nuclear location of neuronal serine-727-phosphorylated STAT3 (pSerSTAT3). Treatment of mixed neuron-astrocyte cultures with 0.5 mu M A beta Os induced in neurons a significant decrease of nuclear pSerSTAT3, but not of phosphotyrosine-705 STAT3, the other form of STAT3 phosphorylation. This decrease did not occur in astrocyte-poor neuronal cultures revealing a pivotal role for astrocytes in this response. To test if mediators released by astrocytes in response to A beta Os induce pSerSTAT3 nuclear depletion, we used conditioned medium derived from A beta Os-treated astrocyte cultures. Treatment of astrocyte-poor neuronal cultures with this medium caused pSerSTAT3 nuclear depletion but did not modify overall STAT3 levels. Extracellular catalase prevented the pSerSTAT3 nuclear depletion caused by astrocyte-conditioned medium, indicating that reactive oxygen species (ROS) mediate this response. This conditioned medium also increased neuronal oxidative tone, leading to a ryanodine-sensitive intracellular calcium signal that proved to be essential for pSerSTAT3 nuclear depletion. In addition, this depletion decreased BCL2 and Survivin transcription and significantly increased BAX/BCL2 ratio. This is the first description that ROS generated by A beta Os-treated astrocytes and neuronal calcium signals jointly regulate pSerSTAT3 nuclear distribution in neurons. We propose that astrocytes release ROS in response to A beta Os, which by increasing neuronal oxidative tone, generate calcium signals that cause pSerSTAT3 nuclear depletion and loss of STAT3 protective transcriptional activity.