Peripheral circadian oscillators require CLOCK

In mammals, the circadian system is hierarchical — a brain pacemaker located within the suprachiasmatic nucleus (SCN) is responsible for regulating locomotor activity rhythms and for synchronizing peripheral oscillators [1,2]. Recent genetic evidence in mice indicates that the bHLH transcription factors CLOCK and NPAS2 have partially redundant functions within the SCN [3,4]. To further examine the roles of CLOCK and NPAS2, we generated CLOCK-deficient (Clock–/–), NPAS2-deficient (Npas2–/–) and double-mutant (Clock–/–;Npas2–/–) mice carrying the mPer2Luciferase reporter gene [5]. We monitored the bioluminescence rhythms of tissue explants in culture and found that while CLOCK or NPAS2 is able to maintain SCN bioluminescence rhythmicity (Supplemental Data) [4], peripheral oscillators are arrhythmic without CLOCK. Thus, there are fundamental differences between the clock machinery of different tissues. We determined CLOCK’s role in peripheral oscillators by focusing on liver and lung tissue. The circadian oscillator in the liver has been useful for understanding the biochemical interactions that comprise the mouse circadian clockwork, e.g., [6,7]. Moreover, the livers of CLOCK-deficient mice rhythmically express mPer1 and mPer2 mRNAs and proteins during the first day in constant darkness [3]. This suggested that the circadian clock in the liver might also be maintained in the absence of CLOCK due to the partially redundant function of NPAS2. Surprisingly, however, bioluminescence profiles of liver and lung explants from Clock–/– mice are arrhythmic (Figure 1A), N or m al iz ed lu ci fe ra se a ct iv ity Liver Lung