SUMMARY Circadian rhythms govern a large array of metabolic and physiological functions. The central clock protein CLOCK has HAT properties. It directs acetylation of histone H3 and of its dimerization partner BMAL1 at Lys537, an event essential for circadian function. We show that the HDAC activity of the NAD+-dependent SIRT1 enzyme is regulated in a circadian manner, correlating with rhythmic acetylation of BMAL1 and H3 Lys9/Lys14 at circadian promoters. SIRT1 associates with CLOCK and is recruited to the CLOCK:BMAL1 chromatin complex at circadian promoters. Genetic ablation of the Sirt1 gene or pharmacological inhibition of SIRT1 activity lead to disturbances in the circadian cycle and in the acetylation of H3 and BMAL1. Finally, using liver-specific SIRT1 mutant mice we show that SIRT1 contributes to circadian control in vivo. We propose that SIRT1 functions as an enzymatic rheostat of circadian function, transducing signals originated by cellular metabolites to the circadian clock.

Many metabolic and physiological processes display circadian oscillations. We have shown that the core circadian regulator, CLOCK, is a histone acetyltransferase whose activity is counterbalanced by the nicotinamide adenine dinucleotide (NAD+)-dependent histone deacetylase SIRT1. Here we show that intracellular NAD+ levels cycle with a 24-hour rhythm, an oscillation driven by the circadian clock. CLOCK:BMAL1 regulates the circadian expression of NAMPT (nicotinamide phosphoribosyltransferase), an enzyme that provides a rate-limiting step in the NAD+ salvage pathway. SIRT1 is recruited to the Nampt promoter and contributes to the circadian synthesis of its own coenzyme. Using the specific inhibitor FK866, we demonstrated that NAMPT is required to modulate circadian gene expression. Our findings in mouse embryo fibroblasts reveal an interlocked transcriptional-enzymatic feedback loop that governs the molecular interplay between cellular metabolism and circadian rhythms.

Coffin-Lowry Syndrome (CLS) is an X-linked mental retardation condition associated with skeletal abnormalities. The gene mutated in CLS, RSK2, encodes a growth factor-regulated kinase. However, the cellular and molecular bases of the skeletal abnormalities associated with CLS remain unknown. Here, we show that RSK2 is required for osteoblast differentiation and function. We identify the transcription factor ATF4 as a critical substrate of RSK2 that is required for the timely onset of osteoblast differentiation, for terminal differentiation of osteoblasts, and for osteoblast-specific gene expression. Additionally, RSK2 and ATF4 posttranscriptionally regulate the synthesis of Type I collagen, the main constituent of the bone matrix. Accordingly, Atf4-deficiency results in delayed bone formation during embryonic development and low bone mass throughout postnatal life. These findings identify ATF4 as a critical regulator of osteoblast differentiation and function, and indicate that lack of ATF4 phosphorylation by RSK2 may contribute to the skeletal phenotype of CLS.

and Molecular DNA around a histone octamer core particle containing one H3-H4 tetramer and two H2A-H2B dimers. The valid-Genetics Box 800733 ity of this model has been confirmed by determining the crystal structures of the histone octamer, and of the Charlottesville, Virginia 22908 † Institut de Ge ´ne ´tique et de Biologie Mole ´culaire nucleosome particle (Arents et al., 1991; Richmond et al., 1993; Luger et al., 1997). As revealed by these and et Cellulaire CNRS-INSERM-ULP, B. P. 163 other studies, the C-terminal histone-fold domains of core histones have similar conformations that are critical 67404 Illkirch, Strasbourg France for the assembly of nucleosomes by mediating histonehistone and histone-DNA interactions (reviewed in Wolffe, 1998). In contrast, the N-terminal tails of core histones are less structured and are not essential for Signal Transduction and Chromatin maintaining the integrity of nucleosomes since removal The ability to detect extracellular stimuli and execute the of these tails by trypsin treatment does not diminish appropriate response is crucial to all cellular functions. nucleosome stability (Whitlock and Simpson, 1977; Au-Upon receiving external signals, be it growth factor stimsio et al., 1989). Instead, histone tails are thought to ulation or exposure to stress, distinct pathways are actimake secondary and more flexible contacts with DNA vated that culminate in the induction or repression of and adjacent nucleosomes (Luger et al., 1997) that allow defined sets of genes. The transduction of signals from for dynamic changes in the accessibility of the underlycell surface to the nucleus often involves phosphorylaing genome. tion cascades that not only allow for rapid transmission, How do these N-terminal histone tails participate in but also serve to amplify the signal by activating multiple the modulation of chromatin architecture? One possible factors and genes. The finely tuned combination of these way is that they constitute targets for ATP-dependent signal-induced nuclear effects thus leads to integrated chromatin remodeling factors such as Swi/Snf and cellular responses such as proliferation, differentiation, NURF (Georgel et al., 1997; Lee et al., 1999; Krebs et and apoptosis. al., 2000). Readers interested in the function of these A central goal in the signaling field is the identification remodeling complexes are directed to other reviews of physiologically relevant targets of transduction pathmore focused on that topic (Kingston and Narlikar, 1999; ways. To that end, much attention has been focused Peterson and Workman, 2000). Another way is that these on the signal-induced activation of transcription factors tails are subjected to a diverse array of posttranslational and components of the transcription machinery. Howmodifications, such as acetylation and phosphorylation ever, chromatin, the physiological packaging structure (Figure 1), which may modulate the contacts between of histones and genomic DNA, has largely been nehistones and DNA. Because these modifications are regl...