Regulation of cyclic AMP response element-binding protein during neuroglial interactions

Communications between neurons and glial cells play an important role in regulating homeostasis in the central nervous system. cAMP response element-binding protein (CREB), a transcription factor, is down-regulated by neurotoxins, which are known to be released by activated glial cells. To determine the role of CREB signaling in neuroglial interactions, we used three neuroglial coculture models consisting of human neuroprogenitor cell (NPC)-derived neurons and human microglia. Conditioned medium from the Abeta (Aβ)-activated microglia decreased CREB phosphorylation and brain-derived neurotrophic factor promoter activity (47%), whereas the same medium induced (p < 0.01) the promoter of CXCL10, a chemokine, in NPC-derived neuron-rich cultures. These effects were reversed when microglia were exposed to Aβ in the presence of minocycline, an anti-inflammatory agent. The expression of CREB targets, including brain-derived neurotrophic factor, synapsin-1, and BIRC3 decreased by 50-65% (p < 0.01) in neurons isolated by laser capture microdissection in close proximity of microglia in neuroglial mixed cultures. Neuronal survival actively modulated microglial behavior when neurons and microglia were cocultured side-by-side on semicircles of ACLAR membrane. Neuronal injury, caused by the over-expression of dominant negative form of CREB, exacerbated Aβ-mediated microglial activation, whereas CREB over-expression resulted in decreased microglial activation. Decreases in the levels of neuronal markers were observed when NPCs were differentiated in the presence of proinflammatory cytokines IL-1β, tumor necrosis factor α, or IL-6. Instead, the NPCs differentiated into a glial phenotype, and these effects were more pronounced in the presence of tumor necrosis factor α. Our findings suggest that CREB down-regulation is an important component of defective neuroglial communications in the brain during neuroinflammation. Neuroglial interactions were examined using coculture models of human neuroprogenitor cell-derived neurons and microglia isolated from human fetal brain. A novel coculture model of neurons and microglia cultured on ACLAR membranes in the same dish was also included. In this model, over-expression of the dominant negative mutant form of the transcription factor CREB in neurons induced neuronal apoptosis and microglial activation whereas expression of the wild type form of CREB resulted in protection of neurons and suppressed microglial activity, thereby suggesting that neurons play an active role in neuroglial interactions.