Cell：昼夜节律基因Clock实际上是乙酰化转移酶

Circadian Rhythms

A fascinating string of studies have begun to reveal that throughout the animal kingdom there are widely conserved elements of a molecular “clock” that controls rhythmic behavior – sleeping and waking cycles in humans, eclosion and locomotor activity in Drosophila, and even rhythmic behavior in Neurospora and plants [summarized in 5,6,7,8].  Because of the extensive work that had already been done in Drosophila genetics, control of circadian rhythms is best understood in the fruit fly [9].  Now we are finding human homologs to some of the major Drosophila circadian genes.  Although there are differences in circadian regulation between Drosophila and vertebrates [10], the conservation of some of the key genes in fruit fly circadian rhythms provides us with an excellent framework for studying human sleep/wake cycles.  As Young [8] observes, “It is astonishing that genetic screens are identifying the same molecular determinants of time in creatures as different as mammals and fruit flies.”

A circadian pacemaking system contains a central clock, an input system (that influences the pacemaker), and an output system (that mediates the signals from the pacemaker to cause rhythmic behavior) [5,11].  Recent studies have revealed several of the key genes involved in Drosophila’s central clock.  Wilsbacher and Takahashi [9] compile these findings into a model of the Drosophila clock.  The period (PER) and timeless (TIM) genes are two of the key players in this cycle.  The CLOCK and BMAL1 proteins stimulate transcription of PER and TIM.  PER and TIM accumulate slowly in the cytoplasm until a threshold level is reached and TIM stabilizes PER.  This accumulation is controlled by light, which degrades TIM, and by the gene doubletime (DBT), which phosphorylates and degrades PER.  After the threshold is reached, the TIM/PER dimers enter the nucleus and negatively interact with CLOCK and BMAL1 to inhibit transcription of TIM and PER.  As the TIM and PER proteins turn over, transcription begins again in the morning [6,9,10].
 
 

Figure 1: Molecular model of circadian rhythm generation in Drosophila. Events A-F occur over ~24 hours. (A) CLOCK-BMAL1 heterodimers activate per and tim promoters and activate mRNA expression. (B) per and tim mRNA are transported to the cytoplasm and translated into PER and TIM protein. (C) Protein regulation occurs in two ways: DBT protein phosphorylates and destabilizes PER, and light destroys TIM. (D) PER and TIM levels accumulate slowly.  TIM stabilizes PER and promotes nuclear transport. (E) PER and TIM dimers enter the nucleus and inhibit CLOCK-BMAL-activated transcription. (F) Protein turnover allows per and tim mRNA expression to begin again.  Adapted from [9].

Some of the same genes that play key roles in Drosophila circadian regulation are conserved in humans, and there are striking similarities in the regulation of the central clock.  The human CKIe gene is a homolog of the Drosophila DBT gene.  In Drosophila, DBT phosphorylates and is thought to destabilize PER [12].  This form of regulation is very similar to that seen in humans, where CKIe phosphorylates the human homolog of PER [7].

Three human homologs of PER have been identified [7,11,13]  One of these, hPER2, has been implicated in Familial Advanced Sleep Phase Syndrome (FASPS).  People with FASPS are referred to as “morning larks” – they sleep for a normal amount of time, but sleep onset occurs around 7:30 p.m., and they awaken around 4:30 a.m.  FASPS has been shown to be caused by a mutation in the caseine kinase I epsilon (CKIe) binding region of hPER2 that leads to decreased phosphorylation of hPER2 by CKIe in vitro [7].

The conservation of two of the key players in Drosophila circadian rhythms shows the similarity in the regulation of this system in flies and humans.  Because of the wealth of genetic knowledge that we have about Drosophila, and the ease of doing genetic crosses in the lab, it is much easier to study the molecular basis of circadian rhythms in fruit flies than in humans.  We can then take the findings from Drosophila and look for homologous genes in humans.