Daily variation of clock output gene activation in behaviorally arrhythmic mPer/mCry triple mutant mice.

The mammalian central pacemaker, driving circadian rhythms in behavior, physiology, and metabolism, is located in the suprachiasmatic nuclei (SCN) of the hypothalamus. At the molecular level circadian clocks are based on a system of transcriptional/translational feedback loops oscillating with a period of about 24h. In mammals the CLOCK/BMAL1 transcriptional activator complex regulates a set of central clock genes like mPer1, mPer2, mCry1, and mCry2. The corresponding gene products form protein complexes that translocate into the nucleus and inhibit CLOCK/BMAL1-driven transcription of their own genes and other E-box containing genes. To elucidate whether only one of these four genes of the negative feedback loop is sufficient to generate a 24h rhythm we generated mPer/mCry triple mutant mice. As could be expected on the basis of the arrhythmicity of mPer1/mPer2 and mCry1/mCry2 double mutant mice, we show that none of the triple mutants is able to maintain circadian rhythmicity in constant darkness. This indicates that a single mPer or mCry gene is not sufficient to drive circadian rhythms. Interestingly however, under light-dark conditions (LD) the oscillation of some output genes is persisting in these animals indicating that the LD cycle is able to partially drive rhythmic signalling to the body, through an hour-glass mechanism.