Mammalian circadian biology: elucidating genome-wide levels of temporal organization

A noncanonical E-box enhancer drives mouse Period2 circadian oscillations in vivo

The mouse Period2 (mPer2) locus is an essential negative-feedback element of the mammalian circadian-clock mechanism. Recent work has shown that mPer2 circadian gene expression persists in both central and peripheral tissues. Here, we analyze the mouse mPer2 promoter and identify a circadian enhancer (E2) with a noncanonical 5'-CACGTT-3' E-box located 20 bp upstream of the mPer2 transcription start site. The E2 enhancer accounts for most circadian transcriptional drive of the mPer2 locus by CLOCK:BMAL1, is a major site of DNaseI hypersensitivity in this region, and is constitutively bound by a transcriptional complex containing the CLOCK protein. Importantly, the E2 enhancer is sufficient to drive self-sustained circadian rhythms of luciferase activity in central and peripheral tissues from mPer2-E2::Luciferase transgenic mice with tissue-specific phase and period characteristics. Last, genetic analysis with mutations in Clock and Bmal1 shows that the E2 enhancer is a target of CLOCK and BMAL1 in vivo.

Reproductive performance in female ClockΔ19 mutant mice

The relationship between circadian rhythmicity and rodent reproductive cyclicity is well established, but the impact of disrupted clock gene function on reproduction has not been well established. The present study evaluated the reproductive performance of mice carrying the ClockΔ19 mutation that were either melatonin deficient (ClockΔ19/Δ19) or had the capacity to synthesise melatonin reinstated (ClockΔ19/Δ19+MEL). The ClockΔ19/Δ19 mice took 2–3 days longer to mate, and to subsequently deliver pups, than their control line. The melatonin-competent mutants had a smaller, but still significant (P < 0.05), delay. The ClockΔ19 mutation resulted in smaller median litter sizes compared with control lines (seven v. eight pups; P < 0.05), whereas melatonin proficiency reversed this difference. Survival to weaning was 84% and 80% for the ClockΔ19/Δ19 and ClockΔ19/Δ19+MEL lines, respectively, compared with 94–96% for the two control lines. The ClockΔ19/Δ19 mutants became behaviourally arrhythmic in constant darkness but, despite this, seven of seven became pregnant when paired with males after at least 14 days of constant darkness (five of seven within 4 days of pairing). In the ClockΔ19/Δ19+MEL mice, seven of 15 became arrhythmic in constant darkness but still became pregnant. The seven mice that free ran for at least 14 days in constant darkness with a period of 27.1 h also became pregnant. The present study has demonstrated that the ClockΔ19 mutation has significant, but subtle, effects on reproductive performance. The reintroduction of melatonin competency and/or other genes as a result of crosses with CBA mice reduced the impact of the mutation further. It would appear that redundancy in genes in the circadian system allows the reproductive cyclicity to persist in mice, albeit at a suboptimal level.