A non-canonical E-box within the MyoD core enhancer is necessary for circadian expression in skeletal muscle

The myogenic differentiation 1 (MyoD) gene is a master regulator of myogenesis. We previously reported that the expression of MyoD mRNA oscillates over 24 h in skeletal muscle and that the circadian clock transcription factors, BMAL1 (brain and muscle ARNT-like 1) and CLOCK (circadian locomotor output cycles kaput), were bound to the core enhancer (CE) of the MyoD gene in vivo. In this study, we provide in vivo and in vitro evidence that the CE is necessary for circadian expression of MyoD in adult muscle. Gel shift assays identified a conserved non-canonical E-box within the CE that is bound by CLOCK and BMAL1. Functional analysis revealed that this E-box was required for full activation by BMAL1/CLOCK and for in vitro circadian oscillation. Expression profiling of muscle of CEloxP/loxP mice found approximately 1300 genes mis-expressed relative to wild-type. Based on the informatics results, we analyzed the respiratory function of mitochondria isolated from wild-type and CEloxP/loxP mice. These assays determined that State 5 respiration was significantly reduced in CEloxP/loxP muscle. The results of this work identify a novel element in the MyoD enhancer that confers circadian regulation to MyoD in skeletal muscle and suggest that loss of circadian regulation leads to changes in myogenic expression and downstream mitochondrial function

The NKG2D Ligands RAE-1δ and RAE-1ε Differ with Respect to Their Receptor Affinity, Expression Profiles and Transcriptional Regulation

BACKGROUND: RAE-1 is a ligand of the activating receptor NKG2D expressed by NK cells, NKT, γδT and some CD8(+)T lymphocytes. RAE-1 is overexpressed in tumor cell lines and its expression is induced after viral infection and genotoxic stress. We have recently demonstrated that RAE-1 is expressed in the adult subventricular zone (SVZ) from C57BL/6 mice. RAE-1 is also expressed in vitro by neural stem/progenitor cells (NSPCs) and plays a non-immune role in cell proliferation. The C57BL/6 mouse genome contains two rae-1 genes, rae-1δ and rae-1ε encoding two different proteins. The goals of this study are first to characterize the in vivo and in vitro expression of each gene and secondly to elucidate the mechanisms underlying their respective expression, which are far from known. PRINCIPAL FINDINGS: We observed that Rae-1δ and Rae-1ε transcripts are differentially expressed according to tissues, pathological conditions and cell lines. Embryonic tissue and the adult SVZ mainly expressed Rae-1δ transcripts. The NSPCs derived from the SVZ also mainly expressed RAE-1δ. The interest of this result is especially related to the observation that RAE-1δ is a weak NKG2D ligand compared to RAE-1ε. On the contrary, cell lines expressed either similar levels of RAE-1δ and RAE-1ε proteins or only RAE-1ε. Since the protein expression correlated with the level of transcripts for each rae-1 gene, we postulated that transcriptional regulation is one of the main processes explaining the difference between RAE-1δ and RAE-1ε expression. We indeed identified two different promoter regions for each gene: one mainly involved in the control of rae-1δ gene expression and the other in the control of rae-1ε expression. CONCLUSIONS/SIGNIFICANCE: RAE-1δ and RAE-1ε differ with respect to their function and the control of their expression. Immune function would be mainly exerted by RAE-1ε and non-immune function by RAE-1δ

Real-time monitoring in three-dimensional hepatocytes reveals that insulin acts as a synchronizer for liver clock

Resetting the peripheral clock and understanding the integration between the circadian rhythm and metabolic pathways are fundamental questions. To test whether insulin acts as a synchronizer for the hepatic clock by cell-autonomous mechanisms, the phase-resetting capabilities of insulin were investigated in cultured hepatic cells. We provide evidence that three-dimensional (3D) cell culture conditions that preserve the differentiated state of primary hepatocytes sustained the robustness of the molecular clock, while this robustness rapidly dampened under classical monolayer cell culture conditions. Herein, we established a 3D cell culture system coupled with a real-time luciferase reporter, and demonstrated that insulin directly regulates the phase entrainment of hepatocyte circadian oscillators. We found that insulin-deficient diabetic rats had a pronounced phase advance in their hepatic clock. Subsequently, a single administration of insulin induced phase-dependent bi-directional phase shifts in diabetic rat livers. Our results clearly demonstrate that insulin is a liver clock synchronizer

Disturbed Clockwork Resetting in Sharp-1 and Sharp-2 Single and Double Mutant Mice

BACKGROUND: The circadian system provides the basis to anticipate and cope with daily recurrent challenges to maintain the organisms' homeostasis. De-synchronization of circadian feedback oscillators in humans causes 'jet lag', likely contributes to sleep-, psychiatric-, metabolic disorders and even cancer. However, the molecular mechanisms leading to the disintegration of tissue-specific clocks are complex and not well understood. METHODOLOGY/PRINCIPAL FINDINGS: Based on their circadian expression and cell culture experiments, the basic Helix-Loop-Helix (bHLH) transcription factors SHARP-1(Dec2) and SHARP-2(Stra13/Dec1) were proposed as novel negative regulators of the molecular clock. To address their function in vivo, we generated Sharp-1 and Sharp-2 single and double mutant mice. Our experiments reveal critical roles for both factors in regulating period length, tissue-specific control of clock gene expression and entrainment to external cues. Light-pulse experiments and rapid delays of the light-dark cycle (experimental jet lag) unravel complementary functions for SHARP-1 and SHARP-2 in controlling activity phase resetting kinetics. Moreover, we show that SHARP-1 and 2 can serve dual functions as repressors and co-activators of mammalian clock gene expression in a context-specific manner. This correlates with increased amplitudes of Per2 expression in the cortex and liver and a decrease in the suprachiasmatic nucleus (SCN) of double mutant mice. CONCLUSIONS/SIGNIFICANCE: The existence of separate mechanisms regulating phase of entrainment, rhythm amplitude and period length has been postulated before. The differential effects of Sharp-deficiency on rhythmicity and behavioral re-entrainment, coupled to tissue-dependent regulatory functions, provide a new mechanistic basis to further understand the complex process of clock synchronizations

SUMOylation of DEC1 Protein Regulates Its Transcriptional Activity and Enhances Its Stability

Differentiated embryo-chondrocyte expressed gene 1 (DEC1, also known as sharp2, stra13, or BHLHB2) is a mammalian basic helix-loop-helix protein that is involved in many aspects of gene regulation through acting as a transcription factor. Changes in DEC1 expression levels have been implicated in the development of cancers. Using COS-7 cell, we showed that DEC1 can be modified by the small ubiquitin-like modifiers, SUMO1, 2 and 3. Two major SUMOylation sites (K159 and K279) were identified in the C-terminal domain of DEC1. Substitution of either K159 or K279 with arginine reduced DEC1 SUMOylation, but substitution of both K159 and K279 abolished SUMOylation, and more protein appeared to be retained in the cytoplasm compared to wild-type DEC1. The expression of DEC1 was up-regulated after serum starvation as previously reported, but at the same time, serum starvation also led to more SUMOylation of DEC1. In MCF-7 cells SUMOylation also stabilized DEC1 through inhibiting its ubiquitination. Moreover, SUMOylation of DEC1 promoted its repression of CLOCK/BMAL1-mediated transcriptional activity through recruitment of histone deacetylase1. These findings suggested that posttranslational modification of DEC1 in the form of SUMOylation may serve as a key factor that regulates the function of DEC1 in vivo

A Large Scale shRNA Barcode Screen Identifies the Circadian Clock Component ARNTL as Putative Regulator of the p53 Tumor Suppressor Pathway

BACKGROUND: The p53 tumor suppressor gene is mutated in about half of human cancers, but the p53 pathway is thought to be functionally inactivated in the vast majority of cancer. Understanding how tumor cells can become insensitive to p53 activation is therefore of major importance. Using an RNAi-based genetic screen, we have identified three novel genes that regulate p53 function. RESULTS: We have screened the NKI shRNA library targeting 8,000 human genes to identify modulators of p53 function. Using the shRNA barcode technique we were able to quickly identify active shRNA vectors from a complex mixture. Validation of the screening results indicates that the shRNA barcode technique can reliable identify active shRNA vectors from a complex pool. Using this approach we have identified three genes, ARNTL, RBCK1 and TNIP1, previously unknown to regulate p53 function. Importantly, ARNTL (BMAL1) is an established component of the circadian regulatory network. The latter finding adds to recent observations that link circadian rhythm to the cell cycle and cancer. We show that cells having suppressed ARNTL are unable to arrest upon p53 activation associated with an inability to activate the p53 target gene p21(CIP1). CONCLUSIONS: We identified three new regulators of the p53 pathway through a functional genetic screen. The identification of the circadian core component ARNTL strengthens the link between circadian rhythm and cancer

Circadian Clock Genes Contribute to the Regulation of Hair Follicle Cycling

Hair follicles undergo recurrent cycling of controlled growth (anagen), regression (catagen), and relative quiescence (telogen) with a defined periodicity. Taking a genomics approach to study gene expression during synchronized mouse hair follicle cycling, we discovered that, in addition to circadian fluctuation, CLOCK–regulated genes are also modulated in phase with the hair growth cycle. During telogen and early anagen, circadian clock genes are prominently expressed in the secondary hair germ, which contains precursor cells for the growing follicle. Analysis of Clock and Bmal1 mutant mice reveals a delay in anagen progression, and the secondary hair germ cells show decreased levels of phosphorylated Rb and lack mitotic cells, suggesting that circadian clock genes regulate anagen progression via their effect on the cell cycle. Consistent with a block at the G1 phase of the cell cycle, we show a significant upregulation of p21 in Bmal1 mutant skin. While circadian clock mechanisms have been implicated in a variety of diurnal biological processes, our findings indicate that circadian clock genes may be utilized to modulate the progression of non-diurnal cyclic processes

Melatonin synthesis pathway: circadian regulation of the genes encoding the key enzymes in the chicken pineal gland and retina

The mRNAs encoding three enzymes of the melatonin synthesis pathway (tryptophan hydroxylase (TPH), arylalkylamine-N-acetyltransferase (AANAT) and hydroxyindole-O-methyl-transferase (HIOMT)) are expressed with a day/night rhythm in the chicken pineal gland and retina. TPH and AANAT mRNA levels reach their peak at night. HIOMT mRNA levels peak at night in the retina, but during the day in the pineal gland. In this tissue, the rhythm of TPH, AANAT and HIOMT mRNA levels persisted in constant darkness (DD), both in vivo and in vitro, indicating that the three genes are controlled by the circadian oscillator of the chicken pineal. In the retina, the rhythms of TPH and AANAT mRNA levels also persisted in DD in vivo, suggesting that they are driven by a circadian oscillator. In contrast, the rhythm of HIOMT mRNA in the retina appeared to be controlled only by light. The clones of chicken AANAT and HIOMT genes that we have isolated should help us to understand the molecular mechanisms of: 1) their transcriptional regulation by circadian oscillators and by light; 2) their tissue-specific expression in the pineal gland and the retina