Coordinated Transcription of Key Pathways in the Mouse by the Circadian Clock

AbstractIn mammals, circadian control of physiology and behavior is driven by a master pacemaker located in the suprachiasmatic nuclei (SCN) of the hypothalamus. We have used gene expression profiling to identify cycling transcripts in the SCN and in the liver. Our analysis revealed ∼650 cycling transcripts and showed that the majority of these were specific to either the SCN or the liver. Genetic and genomic analysis suggests that a relatively small number of output genes are directly regulated by core oscillator components. Major processes regulated by the SCN and liver were found to be under circadian regulation. Importantly, rate-limiting steps in these various pathways were key sites of circadian control, highlighting the fundamental role that circadian clocks play in cellular and organismal physiology

Early aging and age-related pathologies in mice deficient in BMAL1, the core componentof the circadian clock

Mice deficient in the circadian transcription factor BMAL1 (brain and muscle ARNT-like protein) have impaired circadian behavior and demonstrate loss of rhythmicity in the expression of target genes. Here we report that Bmal1(−/−) mice have reduced lifespans and display various symptoms of premature aging including sarcopenia, cataracts, less subcutaneous fat, organ shrinkage, and others. The early aging phenotype correlates with increased levels of reactive oxygen species in some tissues of the Bmal1(−/− )animals. These findings, together with data on CLOCK/BMAL1-dependent control of stress responses, may provide a mechanistic explanation for the early onset of age-related pathologies in the absence of BMAL1

BMAL1-dependent circadian oscillation of nuclear CLOCK: posttranslational events induced by dimerization of transcriptional activators of the mammalian clock system

Mammalian CLOCK and BMAL1 are two members of bHLH-PAS-containing family of transcription factors that represent the positive elements of circadian autoregulatory feedback loop. In the form of a heterodimer, they drive transcription from E-box enhancer elements in the promoters of responsive genes. We have examined abundance, posttranslational modifications, cellular localization of endogenous and ectopically expressed CLOCK and BMAL1 proteins. Nuclear/cytoplasm distribution of CLOCK was found to be under circadian regulation. Analysis of subcellular localization of CLOCK in embryo fibroblasts of mice carrying different germ-line circadian mutations showed that circadian regulation of nuclear accumulation of CLOCK is BMAL1-dependent. Formation of CLOCK/BMAL1 complex following ectopic coexpression of both proteins is followed by their codependent phosphorylation, which is tightly coupled to CLOCK nuclear translocation and degradation. This binding-dependent coregulation is specific for CLOCK/BMAL1 interaction, as no other PAS domain protein that can form a complex with either CLOCK or BMAL1 was able to induce similar effects. Importantly, all posttranslational events described in our study are coupled with active transactivation complex formation, which argues for their significant functional role. Altogether, these results provide evidence for an additional level of circadian system control, which is based on regulation of transcriptional activity or/and availability of CLOCK/BMAL1 complex