RESEARCH HIGHLIGHTS Nature Reviews Neuroscience | AOP, published online 5 March 2014; doi:10.1038/nrn3712
CIRCADIAN RHYTHMS
J. Vallis/NPG
Methylation mediates clock plasticity exposure to a 22 h light–dark cycle caused long-lasting changes in the SCN transcriptome by altering DNA methylation
The control of circadian behaviour by the suprachiasmatic nucleus (SCN) in the hypothalamus involves intricate loops of transcriptional and post-translational regulation of clock gene expression. This process is adjusted each day (entrained) by the light–dark cycle. The mechanisms underlying this adjustment are not well understood, but they can be investigated by experimentally manipulating the cycle. Azzi et al. now show that a transient change in the duration of the light–dark cycle has long-lasting effects on DNA methylation in the SCN that correlate with behavioural activity changes. The authors exposed mice to either normal day lengths of 12 h light and 12 h darkness (control mice) or shortened day lengths of 11 h light and 11 h darkness (22 h mice) for 4 weeks, and then assessed how the preceding light–dark cycle influenced their internal (or ‘free running’) behavioural rhythm during a week spent in darkness. As previously shown, shortening the light–dark cycle resulted in a backwards shift in the timing of behavioural activity. Transcriptome sequencing of SCN tissue taken from 22 h mice and control mice after the week in darkness revealed that day-length shortening had changed the expression level — measured 4 h after the mice fell
asleep (known as circadian time 4 (CT4)) — of clock genes and of many non-circadian genes, including some genes involved in DNA methylation. Could changes in DNA methyl ation in the SCN underlie the transcriptome changes induced by a shorter light–dark cycle? A comparison of SCN methylomes of 22 h mice and control mice revealed that almost 1,300 regions were differentially methylated at CT4; these included regions containing clock genes and genes involved in synaptogenesis, axon guidance and hormone signalling. Promoter hypomethylation and hypermethylation of genes in the SCN of 22 h mice (compared with control mice) were associated with increased and decreased SCN expression of these genes, respectively. This suggested that exposure to a 22 h light–dark cycle caused long-lasting changes in the SCN transcriptome by altering DNA methylation. Infusion of a DNA methyl transferase inhibitor near the SCN in mice during entrainment to a 22 h light–dark cycle not only decreased global SCN methylation levels but also reduced the shift in ‘free running’ behavioural activity compared with vehicle-treated 22 h mice. Thus, entrainment to a shorter light–dark cycle caused changes in DNA methyl ation in the SCN, which in turn
NATURE REVIEWS | NEUROSCIENCE
regulated gene expression in this nucleus and, consequently, behavioural adjustment to the light–dark cycle. The changes were relatively stable, lasting for weeks after the entrainment. However, they were also reversible: when the authors re-entrained 22 h mice to a 24 h light– dark cycle, they found that both SCN methylation levels and the timing of behavioural activity reverted back to those observed in control mice. This study showed that the period length of the circadian clock in the SCN can be modulated by changes in DNA methylation, which presumably enables animals to adjust to annual fluctuations in daylight duration. Whether light itself directly regulates the expression or activity of enzymes involved in DNA methylation is a topic for future research. Leonie Welberg ORIGINAL RESEARCH PAPER Azzi, A. et al. Circadian behavior is light-reprogrammed by plastic DNA methylation. Nature Neurosci. http://dx.doi.org/10.1038/nn.3651 (2014) FURTHER READING Masri, S. & Sassone-Corsi, P. The circadian clock: a framework linking metabolism, epigenetics and neuronal function. Nature Rev. Neurosci. 14, 69–75 (2013)
VOLUME 15 | APRIL 2014 © 2014 Macmillan Publishers Limited. All rights reserved
CIRCADIAN RHYTHMS
J. Vallis/NPG
Methylation mediates clock plasticity exposure to a 22 h light–dark cycle caused long-lasting changes in the SCN transcriptome by altering DNA methylation
The control of circadian behaviour by the suprachiasmatic nucleus (SCN) in the hypothalamus involves intricate loops of transcriptional and post-translational regulation of clock gene expression. This process is adjusted each day (entrained) by the light–dark cycle. The mechanisms underlying this adjustment are not well understood, but they can be investigated by experimentally manipulating the cycle. Azzi et al. now show that a transient change in the duration of the light–dark cycle has long-lasting effects on DNA methylation in the SCN that correlate with behavioural activity changes. The authors exposed mice to either normal day lengths of 12 h light and 12 h darkness (control mice) or shortened day lengths of 11 h light and 11 h darkness (22 h mice) for 4 weeks, and then assessed how the preceding light–dark cycle influenced their internal (or ‘free running’) behavioural rhythm during a week spent in darkness. As previously shown, shortening the light–dark cycle resulted in a backwards shift in the timing of behavioural activity. Transcriptome sequencing of SCN tissue taken from 22 h mice and control mice after the week in darkness revealed that day-length shortening had changed the expression level — measured 4 h after the mice fell
asleep (known as circadian time 4 (CT4)) — of clock genes and of many non-circadian genes, including some genes involved in DNA methylation. Could changes in DNA methyl ation in the SCN underlie the transcriptome changes induced by a shorter light–dark cycle? A comparison of SCN methylomes of 22 h mice and control mice revealed that almost 1,300 regions were differentially methylated at CT4; these included regions containing clock genes and genes involved in synaptogenesis, axon guidance and hormone signalling. Promoter hypomethylation and hypermethylation of genes in the SCN of 22 h mice (compared with control mice) were associated with increased and decreased SCN expression of these genes, respectively. This suggested that exposure to a 22 h light–dark cycle caused long-lasting changes in the SCN transcriptome by altering DNA methylation. Infusion of a DNA methyl transferase inhibitor near the SCN in mice during entrainment to a 22 h light–dark cycle not only decreased global SCN methylation levels but also reduced the shift in ‘free running’ behavioural activity compared with vehicle-treated 22 h mice. Thus, entrainment to a shorter light–dark cycle caused changes in DNA methyl ation in the SCN, which in turn
NATURE REVIEWS | NEUROSCIENCE
regulated gene expression in this nucleus and, consequently, behavioural adjustment to the light–dark cycle. The changes were relatively stable, lasting for weeks after the entrainment. However, they were also reversible: when the authors re-entrained 22 h mice to a 24 h light– dark cycle, they found that both SCN methylation levels and the timing of behavioural activity reverted back to those observed in control mice. This study showed that the period length of the circadian clock in the SCN can be modulated by changes in DNA methylation, which presumably enables animals to adjust to annual fluctuations in daylight duration. Whether light itself directly regulates the expression or activity of enzymes involved in DNA methylation is a topic for future research. Leonie Welberg ORIGINAL RESEARCH PAPER Azzi, A. et al. Circadian behavior is light-reprogrammed by plastic DNA methylation. Nature Neurosci. http://dx.doi.org/10.1038/nn.3651 (2014) FURTHER READING Masri, S. & Sassone-Corsi, P. The circadian clock: a framework linking metabolism, epigenetics and neuronal function. Nature Rev. Neurosci. 14, 69–75 (2013)
VOLUME 15 | APRIL 2014 © 2014 Macmillan Publishers Limited. All rights reserved
