Circadian oscillations in mammalian physiology and behavior are regulated by an endogenous biological clock. Here we show that loss of the PAS protein MOP3 (also known as BMAL1) in mice results in immediate and complete loss of circadian rhythmicity in constant darkness. Additionally, locomotor activity in light-dark (LD) cycles is impaired and activity levels are reduced in Mop3-/- mice. Analysis of Period gene expression in the suprachiasmatic nucleus (SCN) indicates that these behavioral phenotypes arise from loss of circadian function at the molecular level. These results provide genetic evidence that MOP3 is the bona fide heterodimeric partner of mCLOCK. Furthermore, these data demonstrate that MOP3 is a nonredundant and essential component of the circadian pacemaker in mammals.

SUMMARY The microbiome is being characterized by large-scale sequencing efforts, yet it is not known whether it regulates host metabolism in a general versus tissue-specific manner or which bacterial metabolites are important. Here, we demonstrate that microbiota have a strong effect on energy homeostasis in the colon compared to other tissues. This tissue specificity is due to colonocytes utilizing bacterially-produced butyrate as their primary energy source. Colonocytes from germfree mice are in an energy-deprived state and exhibit decreased expression of enzymes that catalyze key steps in intermediary metabolism including the TCA cycle. Consequently, there is a marked decrease in NADH/NAD+, oxidative phosphorylation, and ATP levels, which results in AMPK activation, p27kip1 phosphorylation, and autophagy. When butyrate is added to germfree colonocytes, it rescues their deficit in mitochondrial respiration and prevents them from undergoing autophagy. The mechanism is due to butyrate acting as an energy source rather than as an HDAC inhibitor.

Disruption of the murine Mop3 (also known as Bmal1 or Arntl) locus results in a loss of behavioral and molecular circadian rhythms. Although Mop3 null mice do not display anomalies in early development, they do display reduced activity as they age. In an effort to explain this decreased activity, we characterized the physiological and anatomical changes that occurred with age. We observed that Mop3 null mice display an increased mortality after 26 weeks of age and a phenotype best described as a progressive noninflammatory arthropathy. Although little pathology is observed prior to 11 weeks of age, by 35 weeks of age essentially all Mop3 null animals develop joint ankylosis due to flowing ossification of ligaments and tendons and almost complete immobilization of weight-bearing and nonweight-bearing joints. This pathology appears to explain the decreased activity of Mop3 null mice and suggests that MOP3 is an inhibitor of ligament and tendon ossification.

The Ah receptor (AHR) mediates the metabolic adaptation to a number of planar aromatic chemicals. Essential steps in this adaptive mechanism include AHR binding of ligand in the cytosol, translocation of the receptor to the nucleus, dimerization with the Ah receptor nuclear translocator, and binding of this heterodimeric transcription factor to dioxin-responsive elements (DREs) upstream of promoters that regulate the expression of genes involved in xenobiotic metabolism. The AHR is also involved in other aspects of mammalian biology, such as the toxicity of molecules like 2,3,7,8-tetrachlorodibenzo-p-dioxin as well as regulation of normal liver development. In an effort to test whether these additional AHR-mediated processes require a nuclear event, such as DRE binding, we used homologous recombination to generate mice with a mutation in the AHR nuclear localization/DRE binding domain. These Ahr nls mice were found to be resistant to all 2,3,7,8-tetrachlorodibenzo-p-dioxin-induced toxic responses that we examined, including hepatomegaly, thymic involution, and cleft palate formation. Moreover, aberrations in liver development observed in these mice were identical to that observed in mice harboring a null allele at the Ahr locus. Taken in sum, these data support a model where most, if not all, of AHR-regulated biology requires nuclear localization. The aryl hydrocarbon receptor (AHR)1 regulates an adaptive metabolic response to a variety of planar aromatic chemicals that are widely dispersed in the environment. Over the last 20 years, the mechanistic details of this adaptive signaling pathway have been well characterized (1-4). The AHR is a basic helix-loop-helix-PAS (bHLH-PAS) transcription factor. Upon binding agonists, the AHR translocates from the cytoplasm to the nucleus, where it forms a heterodimer with another bHLH-PAS protein known as the aryl hydrocarbon nuclear translocator (ARNT). This heterodimeric complex interacts with dioxin-responsive elements (DREs) within the genome and upregulates the transcription of a battery of xenobiotic metabolizing enzymes (XMEs). These regulated XMEs include the cytochrome P450s Cyp1a1, Cyp1b1, and Cyp1a2 and the phase II enzymes Gst-a1 and Ugt1-06 (reviewed in Refs. 2 and 3).In addition to regulating an adaptive metabolic response, the AHR also mediates toxic responses to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and plays an important role in normal development. Early genetic and pharmacological experiments provided evidence that the AHR mediates toxic responses to TCDD and related pollutants (5). Highly reproducible toxic endpoints in rodent species include thymic involution, hepatomegaly, epithelial hyperplasia, and teratogenesis. More recently, generation of null alleles at the Ahr locus in mice revealed that the AHR also plays an important role in normal mammalian development (6 -9). Across laboratories, the most reproducible phenotype associated with the homozygous null allele is a smaller liver. We have proposed that smaller liver size is the result of the pers...

The Ah receptor nuclear translocator (ARNT) is the dimeric partner of hypoxia-inducible factors and thus plays a pivotal role in cellular adaptation to low oxygen environments. ARNT is also a dimeric partner for the Ah receptor (AHR), and this complex is essential in regulating the adaptive metabolic response to polycyclic aromatic hydrocarbons. Because of the essential role of ARNT in hypoxia-driven developmental events, it has been difficult to study the physiological significance of AHR⅐ARNT heterodimers in vivo. To address this issue, we developed a hypomorphic Arnt allele that displayed normal development and allowed the examination of the role of ARNT in AHR biology. In this regard, the AHR is also known to mediate two additional biological processes: the toxicological response to compounds such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (dioxin) and the developmental closure of a fetal vascular structure known as the ductus venosus. Although the mechanism of the adaptive pathway has been well described, the mechanism of AHR-mediated signal transduction in the toxic and developmental pathways is not well understood. Liver perfusion studies demonstrated that ARNT hypomorphs have a patent ductus venosus, identical to that observed in the Ahr null mice. Parallel dioxin toxicity studies demonstrated that the ARNT hypomorphs exhibited resistance to the end points of dioxin exposure. Moreover, we observed that toxicity could be segregated from the classical adaptive responses such as P4501A induction. Taken in sum, these experiments demonstrate that ARNT is an essential component of AHR developmental signaling and shed light on the mechanism of dioxin toxicity.The aryl hydrocarbon receptor nuclear translocator (ARNT) 1 and the Ah receptor (AHR) are founding members of the PAS superfamily of transcriptional regulators (1). These proteins were originally identified as the result of their involvement in the regulation of an adaptive metabolic response to certain xenobiotics, such as polycyclic aromatic hydrocarbons (PAHs) (2, 3). In this pathway, PAH molecules bind to the AHR, which then translocates to the nucleus and heterodimerizes with its transcriptional partner, ARNT. The AHR⅐ARNT complex then interacts with specific response elements to up-regulate a battery of xenobiotic metabolizing enzymes that include the cytochrome p450 enzymes, Cyp1a1, Cyp1a2, Cyp1b1 as well as the phase II enzymes Gst-a1 and Ugt1-06 (2, 4). Given that each of the up-regulated enzymes metabolize PAHs, we refer to this process as the "adaptive pathway" of AHR.In addition to its role in the adaptive metabolism of xenobiotics, the AHR also mediates two other biological pathways that we refer to as "toxic" and "developmental." In the toxic pathway, exposure to potent agonists such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (dioxin) results in a response that includes end points such as hepatotoxicity, thymic involution, epithelial hyperplasia, and cleft palate (5). A compelling body of genetic and pharmacological evidence has demonstrated that the...