The CLOCK transcription factor is a key component of the molecular circadian clock within pacemaker neurons of the hypothalamic suprachiasmatic nucleus. We found that homozygous Clock mutant mice have a greatly attenuated diurnal feeding rhythm, are hyperphagic and obese, and develop a metabolic syndrome of hyperleptinemia, hyperlipidemia, hepatic steatosis, hyperglycemia, and hypoinsulinemia. Expression of transcripts encoding selected hypothalamic peptides associated with energy balance was attenuated in the Clock mutant mice. These results suggest that the circadian clock gene network plays an important role in mammalian energy balance.

Circadian cycles affect a variety of physiological processes, and disruptions of normal circadian biology therefore have the potential to influence a range of disease-related pathways. The genetic basis of circadian rhythms is well studied in model organisms and, more recently, studies of the genetic basis of circadian disorders has confirmed the conservation of key players in circadian biology from invertebrates to humans. In addition, important advances have been made in understanding how these molecules influence physiological functions in tissues throughout the body. Together, these studies set the scene for applying our knowledge of circadian biology to the understanding and treatment of a range of human diseases, including cancer, and metabolic and behavioural disorders.

Circadian rhythms of cell and organismal physiology are controlled by an autoregulatory transcription-translation feedback loop that regulates the expression of rhythmic genes in a tissue-specific manner. Recent studies have suggested that components of the circadian pacemaker, such as the Clock and Per2 gene products, regulate a wide variety of processes, including obesity, sensitization to cocaine, cancer susceptibility, and morbidity to chemotherapeutic agents. To identify a more complete cohort of genes that are transcriptionally regulated by CLOCK and/or circadian rhythms, we used a DNA array interrogating the mouse protein-encoding transcriptome to measure gene expression in liver and skeletal muscle from WT and Clock mutant mice. In WT tissue, we found that a large percentage of expressed genes were transcription factors that were rhythmic in either muscle or liver, but not in both, suggesting that tissue-specific output of the pacemaker is regulated in part by a transcriptional cascade. In comparing tissues from WT and Clock mutant mice, we found that the Clock mutation affects the expression of many genes that are rhythmic in WT tissue, but also profoundly affects many nonrhythmic genes. In both liver and skeletal muscle, a significant number of CLOCKregulated genes were associated with the cell cycle and cell proliferation. To determine whether the observed patterns in cell-cycle gene expression in Clock mutants resulted in functional dysregulation, we compared proliferation rates of fibroblasts derived from WT or Clock mutant embryos and found that the Clock mutation significantly inhibits cell growth and proliferation.cell cycle ͉ circadian rhythms ͉ Clock mutation ͉ gene expression ͉ protein-encoding transcriptome M any organisms have Ϸ24-h rhythms in metabolism, physiology, and behavior that are driven by cell autonomous circadian pacemakers (1). These circadian rhythms allow organisms to coordinate a myriad of physiological processes with the changing environment. In mammals, the circadian pacemaker is composed of interlocked transcription-translation feedback loops: the primary loop is composed of the basic helix-loophelix transcription factors CLOCK and BMAL1, which drive transcription of the Period (Per1, Per2) and Cryptochrome (Cry1, Cry2) genes (1, 2). PER and CRY proteins form the negative limb of the feedback loop by inhibiting their own CLOCK: BMAL1-induced transcription; turnover of PER and CRY allows the cycle to begin anew. The interlocked loop consists of REV-ERB-␣ and ROR␣, which repress and activate the Bmal1 gene, thereby modulating its function (3, 4). Mutation or deletion of Clock (5), Bmal1 (6), Per1/2 genes (7, 8), or Cry1/2 (9, 10) genes results in behavioral arrhythmicity and disruption of the autoregulatory loop, whereas disruption of components of the secondary loop results in short period-length phenotypes (3, 4).The molecular components of the circadian clock are present in the majority of neurons in the suprachiasmatic nucleus (SCN), a bilateral body in the anterior hypot...

MyoD, a master regulator of myogenesis, exhibits a circadian rhythm in its mRNA and protein levels, suggesting a possible role in the daily maintenance of muscle phenotype and function. We report that MyoD is a direct target of the circadian transcriptional activators CLOCK and BMAL1, which bind in a rhythmic manner to the core enhancer of the MyoD promoter. Skeletal muscle of Clock Δ19 and Bmal1 −/− mutant mice exhibited ∼30% reductions in normalized maximal force. A similar reduction in force was observed at the single-fiber level. Electron microscopy (EM) showed that the myofilament architecture was disrupted in skeletal muscle of Clock Δ19 , Bmal1 −/− , and MyoD −/− mice. The alteration in myofilament organization was associated with decreased expression of actin, myosins, titin, and several MyoD target genes. EM analysis also demonstrated that muscle from both Clock Δ19 and Bmal1 −/− mice had a 40% reduction in mitochondrial volume. The remaining mitochondria in these mutant mice displayed aberrant morphology and increased uncoupling of respiration. This mitochondrial pathology was not seen in muscle of MyoD −/− mice. We suggest that altered expression of both Pgc-1α and Pgc-1β in Clock Δ19 and Bmal1 −/− mice may underlie this pathology. Taken together, our results demonstrate that disruption of CLOCK or BMAL1 leads to structural and functional alterations at the cellular level in skeletal muscle. The identification of MyoD as a clock-controlled gene provides a mechanism by which the circadian clock may generate a muscle-specific circadian transcriptome in an adaptive role for the daily maintenance of adult skeletal muscle.circadian clock | myofilaments | mitochondria A fundamental, evolutionarily conserved property of most organisms, from cyanobacteria to plants and animals, is the daily cycling of their internal physiology as well as certain behaviors in animals, such as sleep and feeding (1). The timing of these circadian rhythms is synchronized to the environment by external cues, with light and nutrient availability being two of the most salient entrainment cues (2). The synchronization of endogenous circadian rhythms with the daily cycles in the environment provides an adaptive mechanism for organisms to anticipate cyclic demands on cellular physiology and behavior (3, 4). At the molecular level, the circadian clock represents a well-defined gene regulatory network composed of transcriptional-translational feedback loops (5). The positive arm of the loop is composed of the transcription factors CLOCK and BMAL1, which heterodimerize and bind to E-box elements on target genes such as Per1 to drive the negative part of the feedback loop (5). More recently, the same molecular clock factors that have been identified in the central clock in the suprachiasmatic nucleus have been found to exist in most peripheral tissues (reviewed in ref. 6).We recently characterized the circadian transcriptome of adult skeletal muscle. These mRNAs exhibit significant oscillation in gene expression with a repeating period len...

Circadian rhythms are approximate 24-h behavioral and physiological cycles that function to prepare an organism for daily environmental changes. The basic clock mechanism is a network of transcriptional-translational feedback loops that drive rhythmic expression of genes over a 24-h period. The objectives of this study were to identify transcripts with a circadian pattern of expression in adult skeletal muscle and to determine the effect of the Clock mutation on gene expression. Expression profiling on muscle samples collected every 4 h for 48 h was performed. Using COSOPT, we identified a total of 215 transcripts as having a circadian pattern of expression. Real-time PCR results verified the circadian expression of the core clock genes, Bmal1, Per2, and Cry2. Annotation revealed cycling genes were involved in a range of biological processes including transcription, lipid metabolism, protein degradation, ion transport, and vesicular trafficking. The tissue specificity of the skeletal muscle circadian transcriptome was highlighted by the presence of known muscle-specific genes such as Myod1, Ucp3, Atrogin1 (Fbxo32), and Myh1 (myosin heavy chain IIX). Expression profiling was also performed on muscle from the Clock mutant mouse and sarcomeric genes such as actin and titin, and many mitochondrial genes were significantly downregulated in the muscle of Clock mutant mice. Defining the circadian transcriptome in adult skeletal muscle and identifying the significant alterations in gene expression that occur in muscle of the Clock mutant mouse provide the basis for understanding the role of circadian rhythms in the daily maintenance of skeletal muscle.

The basic helix-loop-helix (bHLH)-Per-Arnt-Sim (PAS) domain transcription factor BMAL1 is an essential component of the mammalian circadian pacemaker. Bmal1-/- mice lose circadian rhythmicity but also display tendon calcification and decreased activity, body weight, and longevity. To investigate whether these diverse functions of BMAL1 are tissue-specific, we produced transgenic mice that constitutively express Bmal1 in brain or muscle and examined the effects of rescued gene expression in Bmal1-/- mice. Circadian rhythms of wheel-running activity were restored in brain-rescued Bmal1-/- mice in a conditional manner; however, activity levels and body weight were lower than those of wild-type mice. In contrast, muscle-rescued Bmal1-/- mice exhibited normal activity levels and body weight yet remained behaviorally arrhythmic. Thus, Bmal1 has distinct tissue-specific functions that regulate integrative physiology.

We performed genome-wide chemical mutagenesis of C57BL/6J mice using N-ethyl-Nnitrosourea (ENU). Electroretinographic screening of the third generation offspring revealed two G3 individuals from one G1 family with a normal a-wave but lacking the b-wave that we named nob4.Th e mutation was transmitted with a recessive mode of inheritance and mapped to chromosome 11 in a region containing the Grm6 gene, which encodes a metabotropic glutamate receptor protein, mGluR6. Sequencing confirmed a single nucleotide substitution from T to C in the Grm6 gene. The mutation is predicted to result in substitution of Pro for Ser at position 165 within the extracellular, ligand-binding domain and oocytes expressing the homologous mutation in mGluR6 did not display robust glutamate-induced currents. Retinal mRNA levels for Grm6 were not significantly reduced, but no immunoreactivity for mGluR6 protein was found. Histological and fundus evaluations of nob 4 showed normal retinal morphology. In contrast, the mutation has severe consequences for visual function. In nob4 mice, fewer retinal ganglion cells (RGCs) responded to the onset (ON) of a bright full field stimulus. When ON responses could be evoked, their onset was significantly delayed. Visual acuity and contrast sensitivity, measured with optomotor responses, were reduced under both photopic and scotopic conditions. This mutant will be useful because its phenotype is similar to that of human patients with congenital stationary night blindness and will provide a tool for understanding both retinal circuitry and the role of ganglion cell encoding of visual information.

The ␣2-laminin subunit contributes to basement membrane functions in muscle, nerve, and other tissues, and mutations in its gene are causes of congenital muscular dystrophy. The ␣2 G-domain modules, mutated in several of these disorders, are thought to mediate different cellular interactions. To analyze these contributions, we expressed recombinant laminin-2 (␣ 2 ␤ 1 ␥ 1 ) with LG4 -5, LG1-3, and LG1-5 modular deletions. Wild-type and LG4 -5 deleted-laminins were isolated from medium intact and cleaved within LG3 by a furin-like convertase. Myoblasts adhered predominantly through LG1-3 while ␣-dystroglycan bound to both LG1-3 and LG4 -5. Recombinant laminin stimulated acetylcholine receptor (AChR) clustering; however, clustering was induced only by the proteolytic processed form, even in the absence of LG4 -5. Furthermore, clustering required ␣ 6 ␤ 1 integrin and ␣-dystroglycan binding activities available on LG1-3, acting in concert with laminin polymerization. The ability of the modified laminins to mediate basement membrane assembly was also evaluated in embryoid bodies where it was found that both LG1-3 and LG4 -5, but not processing, were required. In conclusion, there is a division of labor among LG-modules in which (i) LG4 -5 is required for basement membrane assembly but not for AChR clustering, and (ii) laminin-induced AChR clustering requires furin cleavage of LG3 as well as ␣-dystroglycan and ␣ 6 ␤ 1 integrin binding.The laminin ␣2-chain, a subunit of laminin-2 (␣ 2 ␤ 1 ␥ 1 ) and laminin-4 (␣ 2 ␤ 2 ␥ 1 ), is expressed in the basement membranes of skeletal muscle, peripheral nerve, brain, and placenta (1-3). ␣2-Laminins, similar in molecular morphology and functional attributes to ␣1-laminin, are thought to play important roles in basement membrane assembly and the maintenance of the neuromuscular axis (reviewed in Ref. 4). Mutations in the laminin ␣2-subunit are a cause of a human congenital muscular dystrophy typically characterized by early onset, severity, and involvement of peripheral nerve and brain (5, 6). Some of these dystrophies have mutations within the part of the gene coding for LG modules (7). The dystrophic phenotype in mouse models of the disease is one of defective basement membranes in muscle and nerve, muscle necrosis with poor regeneration, patchy peripheral nerve dysmyelination, and decreased complexity of infoldings and post-junctional membrane lengths of the NMJ motor endplate (6, 8 -12).Studies on cultured myotubes have revealed that ␣1-and ␣2-laminin polymerization and G-domain contributions drive laminin assembly on the cell surface and direct a redistribution of interacting cytoskeletal components (13). The ability of laminin to induce such cytoskeletal reorganizations is thought to reflect an important receptor-dependent property of laminin during early steps in assembly of a basement membrane. It has also been shown that laminins can induce the clustering of the acetylcholine receptor (AChR), 1 a property shared with agrin (14). However, while agrin is secreted by terminal ...