Stimulus-specific combinatorial functionality of neuronal c-fos enhancers.

The c-fos gene (also known as Fos) is induced by a broad range of stimuli and is a reliable marker for neural activity. Its induction mechanism and available reporter mouse lines are based exclusively on c-fos promoter activity. Here we demonstrate that multiple enhancers surrounding the c-fos gene are crucial for ensuring robust c-fos response to various stimuli. Membrane depolarization, brain-derived neurotrophic factor (BDNF) and forskolin activate distinct subsets of the enhancers to induce c-fos transcription in neurons, suggesting that stimulus-specific combinatorial activation of multiple enhancers underlies the broad inducibility of the c-fos gene. Accordingly, the functional requirement of key transcription factors varies depending on the type of stimulation. Combinatorial enhancer activation also occurs in the brain. Providing a comprehensive picture of the c-fos induction mechanism beyond the minimal promoter, our study should help in understanding the physiological nature of c-fos induction in relation to neural activity and plasticity.