Temperature as a universal resetting cue for mammalian circadian oscillators

Homeostastic and non-homeostatic functions of melanocortin-3 receptors in the control of energy balance and metabolism

The central nervous melanocortin system is a neural network linking nutrient-sensing systems with hypothalamic, limbic and hindbrain neurons regulating behavior and metabolic homeostasis. Primary melanocortin neurons releasing melanocortin receptor ligands residing in the hypothalamic arcuate nucleus are regulated by nutrient-sensing and metabolic signals. A smaller group of primary neurons releasing melanocortin agonists in the nucleus tractus solitarius in the brainstem are also regulated by signals of metabolic state. Two melanocortin receptors regulate energy homeostasis. Melanocortin-4 receptors regulate satiety and autonomic outputs controlling peripheral metabolism. The functions of melanocortin-3 receptors (MC3R) expressed in hypothalamic and limbic structures are less clear. Here we discuss published data and preliminary observations from our laboratory suggesting that neural MC3R regulate inputs into systems governing the synchronization of rhythms in behavior and metabolism with nutrient intake. Mice subjected to a restricted feeding protocol, where a limited number of calories are presented at a 24h interval, rapidly exhibit bouts of increased wakefulness and activity which anticipate food presentation. The full expression of these responses is dependent on MC3R. Moreover, MC3R knockout mice are unique in exhibiting a dissociation of weight loss from improved glucose homeostasis when subject to a restricted feeding protocol. While mice lacking MC3R fed ad libitum exhibit normal to moderate hyperinsulinemia, when subjected to a restricted protocol they develop hyperglycemia, glucose intolerance, and dyslipidemia. Collectively, our data suggest that the central nervous melanocortin system is a point convergence in the control of energy balance and the expression of rhythms anticipating nutrient intake.

Light-dependent and circadian clock-regulated activation of sterol regulatory element-binding protein, X-box-binding protein 1, and heat shock factor pathways

The circadian clock is phase-delayed or -advanced by light when given at early or late subjective night, respectively. Despite the importance of the time-of-day-dependent phase responses to light, the underlying molecular mechanism is poorly understood. Here, we performed a comprehensive analysis of light-inducible genes in the chicken pineal gland, which consists of light-sensitive clock cells representing a prototype of the clock system. Light stimulated expression of 62 genes and 40 ESTs by >2.5-fold, among which genes responsive to the heat shock and endoplasmic reticulum stress as well as their regulatory transcription factors heat shock factor (HSF)1, HSF2, and X-box-binding protein 1 (XBP1) were strongly activated when a light pulse was given at late subjective night. In contrast, the light pulse at early subjective night caused prominent induction of E4bp4, a key regulator in the phase-delaying mechanism of the pineal clock, along with activation of a large group of cholesterol biosynthetic genes that are targets of sterol regulatory element-binding protein (SREBP) transcription factor. We found that the light pulse stimulated proteolytic formation of active SREBP-1 that, in turn, transactivated E4bp4 expression, linking SREBP with the light-input pathway of the pineal clock. As an output of light activation of cholesterol biosynthetic genes, we found light-stimulated pineal production of a neurosteroid, 7α-hydroxypregnenolone, demonstrating a unique endocrine function of the pineal gland. Intracerebroventricular injection of 7α-hydroxypregnenolone activated locomotor activities of chicks. Our study on the genome-wide gene expression analysis revealed time-of-day-dependent light activation of signaling pathways and provided molecular connection between gene expression and behavior through neurosteroid release from the pineal gland.

Diurnal variations of mouse plasma and hepatic bile acid concentrations as well as expression of biosynthetic enzymes and transporters

Background

Diurnal fluctuation of bile acid (BA) concentrations in the enterohepatic system of mammals has been known for a long time. Recently, BAs have been recognized as signaling molecules beyond their well-established roles in dietary lipid absorption and cholesterol homeostasis.

Methods and Results

The current study depicted diurnal variations of individual BAs detected by ultra-performance liquid chromatography/mass spectrometry (UPLC/MS) in serum and livers collected from C57BL/6 mice fed a regular chow or a chow containing cholestyramine (resin). Circadian rhythms of mRNA of vital BA-related nuclear receptors, enzymes, and transporters in livers and ilea were determined in control- and resin-fed mice, as well as in farnesoid X receptor (FXR) null mice. The circadian profiles of BAs showed enhanced bacterial dehydroxylation during the fasting phase and efficient hepatic reconjugation of BAs in the fed phase. The resin removed more than 90% of BAs with β-hydroxy groups, such as muricholic acids and ursodeoxycholic acid, from serum and livers, but did not exert as significant influence on CA and CDCA in both compartments. Both resin-fed and FXR-null mouse models indicate that BAs regulate their own biosynthesis through the FXR-regulated ileal fibroblast growth factor 15. BA flux also influences the daily mRNA levels of multiple BA transporters.

Conclusion

BA concentration and composition exhibit circadian variations in mouse liver and serum, which influences the circadian rhythms of BA metabolizing genes in liver and ileum. The diurnal variations of BAs appear to serve as a signal that coordinates daily nutrient metabolism in mammals.

Crosstalk between components of circadian and metabolic cycles in mammals

In mammals, most metabolic processes are influenced by biological clocks and feeding rhythms. The mechanisms that couple metabolism to circadian oscillators are just emerging. NAD-dependent enzymes (e.g., Sirtuins and poly[ADP-ribose] polymerases), redox- and/or temperature-dependent transcription factors (e.g., CLOCK, NPAS2, and HSF1), nutrient-sensing transcriptional regulatory proteins (e.g., CREB-CBP-CRCT2, FOXO-p300, nuclear receptors, PGC-1, and SP1 family members) and protein kinases (e.g., AMPK), are plausible candidates for conveying a cell's metabolic state to the core clock circuitry. The intertwining between these acute regulators and circadian clock components is so tight that the discrimination between metabolic and circadian oscillations may be somewhat arbitrary.

Genetics of circadian rhythms in Mammalian model organisms

The mammalian circadian system is a complex hierarchical temporal network which is organized around an ensemble of uniquely coupled cells comprising the principal circadian pacemaker in the suprachiasmatic nucleus of the hypothalamus. This central pacemaker is entrained each day by the environmental light/dark cycle and transmits synchronizing cues to cell-autonomous oscillators in tissues throughout the body. Within cells of the central pacemaker and the peripheral tissues, the underlying molecular mechanism by which oscillations in gene expression occur involves interconnected feedback loops of transcription and translation. Over the past 10 years, we have learned much regarding the genetics of this system, including how it is particularly resilient when challenged by single-gene mutations, how accessory transcriptional loops enhance the robustness of oscillations, how epigenetic mechanisms contribute to the control of circadian gene expression, and how, from coupled neuronal networks, emergent clock properties arise. Here, we will explore the genetics of the mammalian circadian system from cell-autonomous molecular oscillations, to interactions among central and peripheral oscillators and ultimately, to the daily rhythms of behavior observed in the animal.

Circadian integration of metabolism and energetics

Circadian clocks align behavioral and biochemical processes with the day/night cycle. Nearly all vertebrate cells possess self-sustained clocks that couple endogenous rhythms with changes in cellular environment. Genetic disruption of clock genes in mice perturbs metabolic functions of specific tissues at distinct phases of the sleep/wake cycle. Circadian desynchrony, a characteristic of shift work and sleep disruption in humans, also leads to metabolic pathologies. Here, we review advances in understanding the interrelationship among circadian disruption, sleep deprivation, obesity, and diabetes and implications for rational therapeutics for these conditions.