The transcriptional promoter of the cytomegalovirus (CMV) immediate early genes has been extensively exploited in mammalian vectors used for in vitro and in vivo transgene delivery. Transcriptional activation by the CMV promoter is dependent on the presence of certain cellular transcription factors, several of which are known to be altered in response to cellular stimuli. However, the stability of transgene expression in model systems that depend on coincident stimuli has not previously been addressed. Here we monitored transgene expression from two different CMV-based promoters in AAV vectors targeting primary cortical neurons in vitro and rodent brain in vivo after they were treated with various stimulatory compounds or the activation of G-protein coupled receptors. Our results indicate that a general increase in neuronal activity, whether through excess of excitatory amino acids, activation of G protein-coupled signaling, or pharmacological stimulation of dopaminergic signaling can have a strong activating effects on the CMV promoter. These results have important implications for studies where stable constitutive expression of a transgene from the CMV promoter is assumed. Given these findings, caution should be applied when using the CMV promoter to drive expression of effectors of neuronal activation.
