The importance of maintaining a smooth-running
circadian clock becomes painfully evident whenever we suffer severe jet lag. Traveling through multiple time zones decouples our biological rhythms from the natural cycle of light and dark were used to. These lightdark cycles synchronize everything from basic metabolic processes to feeding behavior. People with total blindness, who cannot perceive these light cues because they lack functional retinal photoreceptors, have disrupted circadian rhythms. As mammals, we perceive light cues through retinal photoreceptors that relay the
signals to a cluster of some 16,000 neurons in the hypothalamus called the suprachiasmatic nucleus (SCN).
The Greenwich Mean Time of the circadian system, the SCN sets the cycle of the circadian clocks found in nearly every cell in the body. Phase adjustments in peripheral tissues make sure each clock follows the same schedule. Phase adjustments can be set indirectly, through biological rhythms that are themselves SCN-dependent, such as feeding cycles or body temperature. How these synchronizing cues operate at the molecular level remains obscure. Of particular interest is whether cyclically expressed
genes in peripheral tissues are controlled by local circadian clocks or by systemic cues that are directly or indirectly controlled by the SCN pacemaker.
In a new study, Benot Kornmann, Ueli Schibler, and colleagues investigated this question by focusing on circadian genes in the liver. To tell whether genes were under local or systemic control, they needed to shut down the local circadian oscillators (the molecular mechanisms responsible for cyclical gene
expression) in the liver. They solved that problem by generating a mouse strain with conditionally active liver clocks that they could turn on or off at will.
To create their model, the authors modified a transcription factor called REV-ERB that shuts down circadian
Gene expression by suppressing the core clock gene Bmal1. The modified gene harbored regulatory elements, called tetracycline-responsive elements (TREs), that respond to tetracycline and similar antibiotics. A mouse strain with this modified gene was bred with a strain carrying a gene construct that activates the elements specifically in liver cells. When the transgenic offspring, which carried both transgenes, ate the tetracycline-like drug doxycyline (Dox) in their food, Rev-erb remained silent in liver cells and the circadian oscillator functioned normally. In the absence of Dox, transgenic offspring overexpressed REV-ERB, and Bmal1 remained inactive throughout the day, effectively turning off the liver clocks.
When the authors analyzed the expression of genes targeted by BMAL1 in the presence and absence of Dox, they found arrhythmic expression of most of the targeted clock genes when the liver clocks stopped, as expected. But they were surprised to discover that this was not the case for the core clock gene
mper2: rhythmic expression of mPer2 transcript and protein levels continued even in the absence of BMAL1 activity. This result is particularly noteworthy because in brain tissue from transgenic mice lacking Bmal1, mPer2 transcripts in SCN neurons are barely noticeable during the course of a day.
While its possible that mPer2 transcription is more dependent on BMAL1 in the SCN than it is in the liver, the authors considered two other explanations for the observed mPer2 activity: either the mice that didnt eat Dox still harbored enough BMAL1 to drive cyclic mPer2 expression or oscillating systemic signals were responsible. To distinguish between these possibilities, they analyzed temporal mPer2 expression in isolation from master pacemaker signals by removing liver slices from mice carrying a fluorescent protein attached to mPer2. With this approach, luminescence signals showed a rhythmic profile that follows mPer2 activity.
The authors bred the mouse strain with the conditionally active clock (which nee
More abstracts about the Central and Peripheral Signals Set the Circadian Liver Clock