Probing the Relative Importance of Molecular Oscillations in the Circadian Clock

Abstract

Circadian (24 hr) rhythms of behavior and physiology are driven by molecular clocks that are endogenous to most organisms. The mechanisms underlying these clocks are remarkably conserved across evolution and typically consist of auto-regulatory loops in which specific proteins (clock proteins) rhythmically repress expression of their own genes. Such regulation maintains 24-hr cycles of RNA and protein expression. Despite the conservation of these mechanisms, however, questions are now being raised about the relevance of different molecular oscillations. Indeed, several studies have demonstrated that oscillations of some critical clock genes can be eliminated without loss of basic clock function. Here, we describe the multiple levels at which clock gene/protein expression and function can be rhythmically regulated-transcription, protein expression, post-translational modification, and localization-and speculate as to which aspect of this regulation is most critical. While the review is focused on Drosophila, we include some discussion of mammalian clocks to indicate the extent to which the questions concerning clock mechanisms are similar, regardless of the organism under study. T HE light:dark cycle generated by the earth's rotation is the driving force of daily behavioral and physiological rhythms exhibited by most organisms. However, these daily (24 hr) rhythms are not just a passive response to the light:dark cycle; instead, an intrinsic timekeeping mechanism synchronizes physiological processes to the cyclic environment. The endogenous timekeeper is a self-sustained oscillator, termed the circadian clock, which can be entrained to environmental cues such as light and temperature (such environmental time signals are called zeitgebers), but more importantly, it free runs in constant conditions that lack environmental cues. In the past 20 years, genetic analysis of circadian rhythms in model organisms such as Drosophila, Neurospora, Arabidopsis, cyanobacteria, and mice has yielded considerable insight into the molecular mechanisms of circadian oscillators. Despite these advances, the question of how exactly a rhythm is generated is getting some attention again because a number of recent studies have challenged the simple models proposed initially. This review traces these developments in the field and then proposes a revised model that incorporates the old and new findings. While the focus is on the molecular mechanisms of the Drosophila melanogaster circadian clock, advances in other circadian systems will also be discussed to illustrate conserved mechanisms. Readers interested in circadian clock mechanisms of other organisms are encouraged to read recent reviews (Hastings and

Glucocorticoids and the circadian clock

“…Circadian rhythms and glucocorticoids oscillations to clock output (Sato et al 2006, Kadener et al 2008. Thus, despite the increasingly recognised importance of protein modifications in clock regulation, transcriptiontranslation feedback loop mechanisms still appear to be important players in the clock of all systems studied so far (Brunner & Schafmeier 2006, Zheng & Sehgal 2008.…”

The role of microRNAs (miRNA) in circadian rhythmicity

“…The daily cycling of light and temperature, generated by the earth's rotation, is one of the most important driving forces in the evolution of the circadian clock, allowing organisms to anticipate and adapt to their daily (and seasonally) changing environment (Zheng and Sehgal 2008). Although the molecular details of the circadian clock may vary somewhat in different taxa, the principle of a system of selfsustained transcriptional-translational feedback loops is well conserved (Dunlap 1999;Gallego and Virshup 2007;Zheng and Sehgal 2008): Transcription factors, such as the Figure 2.…”

“…Although the molecular details of the circadian clock may vary somewhat in different taxa, the principle of a system of selfsustained transcriptional-translational feedback loops is well conserved (Dunlap 1999;Gallego and Virshup 2007;Zheng and Sehgal 2008): Transcription factors, such as the Figure 2. Synthesis and function of miRNA.…”

“…This discrepancy indicates that posttranscriptional and posttranslational mechanisms (largely unknown) are important components of circadian rhythmicity. Additional evidence (reviewed in Zheng and Sehgal 2008) shows that cycling of clock transcripts and proteins is not entirely essential for clock function, indicating that the 'central dogma' of the transcriptional-negative-feedback loop is probably sufficient but not necessary, and posttranscriptional/translational mechanisms are an important part of the circadian clock.…”

RNA Surveillance: Molecular Approaches in Transcript Quality Control and their Implications in Clinical Diseases

“…To date, studies have focused on the understanding of the molecular mechanisms that underlie those rhythmic clocks, and genetic analyses have identified numerous clock genes in different species. Across evolution, the molecular circadian clock is self sustained and typically consists of autoregulatory loops regulated by specific proteins that are rhythmically translated in an integrated manner (158), and transcription is considered the most critical step in the control of such circadian rhythms (159,160). In mammals, the core molecular clock components are the period genes, Per (Per1, Per2, and Per3); cryptochrome genes, Cry (Cry1 and Cry2); two helix-loop-helix transcription factors (Clock and Bmal1 genes); casein kinase I epsilon (Csnk1e); and two nuclear hormone receptor genes (RevErbAa and Rora).…”