Regulation of the CREB signaling cascade in the visual cortex by visual experience and neuronal activity

The cAMP-responsive element (CRE) regulatory pathway has been studied as a model of signal-regulated transcription and is critical for some forms of learning and adaptation. In cell culture systems, the extracellular-regulated kinase (ERK) and ribosomal S6 kinase (RSK) couple synaptic signals to CRE-mediated gene expression by modulating CRE-binding protein (CREB) phosphorylation. However, it is not known whether sensory experience regulates gene expression in the brain by this mechanism. In this study, we ask: Are activated forms of ERK, RSK, and CREB colocalized in the cortex and are they coordinately regulated by synaptic signals? We find that these three signaling components are regulated in distinct ways. First, cells that show CRE-lacZ reporter expression, primarily excitatory neurons, do not colocalize with cells containing phospho-ERK. Second, while phosphorylation of ERK and RSK are modulated by visual experience, phosphorylation of CREB at serines 133, 142, or 143 is detected constitutively and is unaffected by experience. This finding suggests that neural activity might not regulate CREB phosphorylation in vivo. To test this hypothesis, we blocked action potentials by injection of tetrodotoxin and found no effect on CREB phosphorylation. These in vivo data show that, in contrast to cell culture systems, cortical synaptic activity controls CRE-mediated gene expression without affecting CREB phosphorylation, possibly by modification of RSK and CREB-associated coregulators.