System-Driven and Oscillator-Dependent Circadian Transcription in Mice with a Conditionally Active Liver Clock

Abstract

The mammalian circadian timing system consists of a master pacemaker in neurons of the suprachiasmatic nucleus (SCN) and clocks of a similar molecular makeup in most peripheral body cells. Peripheral oscillators are self-sustained and cell autonomous, but they have to be synchronized by the SCN to ensure phase coherence within the organism. In principle, the rhythmic expression of genes in peripheral organs could thus be driven not only by local oscillators, but also by circadian systemic signals. To discriminate between these mechanisms, we engineered a mouse strain with a conditionally active liver clock, in which REV-ERBα represses the transcription of the essential core clock gene Bmal1 in a doxycycline-dependent manner. We examined circadian liver gene expression genome-wide in mice in which hepatocyte oscillators were either running or arrested, and found that the rhythmic transcription of most genes depended on functional hepatocyte clocks. However, we discovered 31 genes, including the core clock gene mPer2, whose expression oscillated robustly irrespective of whether the liver clock was running or not. By contrast, in liver explants cultured in vitro, circadian cycles of mPer2::luciferase bioluminescence could only be observed when hepatocyte oscillators were operational. Hence, the circadian cycles observed in the liver of intact animals without functional hepatocyte oscillators were likely generated by systemic signals. The finding that rhythmic mPer2 expression can be driven by both systemic cues and local oscillators suggests a plausible mechanism for the phase entrainment of subsidiary clocks in peripheral organs.

The role of clock genes and rhythmicity in the liver

“…A very elegant study by Kornmann and colleagues showed that rhythmic transcription of most genes in the liver depends on functional hepatocyte clocks (Kornmann et al, 2007). However, the transcription of a subset of genes oscillated in a robust fashion independent of a functional liver clock, and this rhythmic transcription appears to have been driven by systemic cues.…”

The daily rhythm of mice

“…The REV-ERBa protein bound to the promoter region of the Bmal1 gene in the phase of transcriptional repression [31]. This specific action of REV-ERBa was exploited to shut down specifically the circadian oscillator in the liver [42]. Inducible overexpression of this nuclear receptor completely abolished Bmal1 expression and consequently the rhythmicity of the circadian oscillator.…”

Forebrain oscillators ticking with different clock hands

“…Therefore, we cannot fully exclude that post-translational changes that would be specific to certain forebrain oscillators may impair respective binding of the antibodies to PER1 or PER2 used here, preventing immunoreactive signals in those structures. Moreover, in addition to the circadian control of CLOCK-BMAL1 described in the Introduction section, fine-tune regulation of Per1 (e.g., by glucocorticoids; Yamamoto et al, 2005) and Per2 (e.g., by heat shock proteins or acute temperature changes; Kornmann et al, 2007) may also account for differential expression of these genes in various cerebral oscillators. Nevertheless, our data on daily patterns of PER2 expression in the BLA and DG in mice fed ad libitum are consistent with previous results obtained in rats (Lamont et al, 2005;Waddington-Lamont et al, 2007).…”