Repression of latent NF-κB enhancers by PDX1 regulates β cell functional heterogeneity.

Interactions between lineage-determining and activity-dependent transcription factors determine single-cell identity and function within multicellular tissues through incompletely known mechanisms. By assembling a single-cell atlas of chromatin state within human islets, we identified β cell subtypes governed by either high or low activity of the lineage-determining factor pancreatic duodenal homeobox-1 (PDX1). β cells with reduced PDX1 activity displayed increased chromatin accessibility at latent nuclear factor κB (NF-κB) enhancers.

P2Y1 purinergic receptor identified as a diabetes target in a small-molecule screen to reverse circadian β-cell failure.

The mammalian circadian clock drives daily oscillations in physiology and behavior through an autoregulatory transcription feedback loop present in central and peripheral cells. Ablation of the core clock within the endocrine pancreas of adult animals impairs the transcription and splicing of genes involved in hormone exocytosis and causes hypoinsulinemic diabetes. Here, we developed a genetically sensitized small-molecule screen to identify druggable proteins and mechanistic pathways involved in circadian β-cell failure.

A role for alternative splicing in circadian control of exocytosis and glucose homeostasis.

The circadian clock is encoded by a negative transcriptional feedback loop that coordinates physiology and behavior through molecular programs that remain incompletely understood. Here, we reveal rhythmic genome-wide alternative splicing (AS) of pre-mRNAs encoding regulators of peptidergic secretion within pancreatic β cells that are perturbed in and β-cell lines. We show that the RNA-binding protein THRAP3 (thyroid hormone receptor-associated protein 3) regulates circadian clock-dependent AS by binding to exons at coding sequences flanking exons that are more frequently skipped in clock mutant β cells, including transcripts encoding () and ().

Transcriptional Basis for Rhythmic Control of Hunger and Metabolism within the AgRP Neuron.

The alignment of fasting and feeding with the sleep/wake cycle is coordinated by hypothalamic neurons, though the underlying molecular programs remain incompletely understood. Here, we demonstrate that the clock transcription pathway maximizes eating during wakefulness and glucose production during sleep through autonomous circadian regulation of NPY/AgRP neurons. Tandem profiling of whole-cell and ribosome-bound mRNAs in morning and evening under dynamic fasting and fed conditions identified temporal control of activity-dependent gene repertoires in AgRP neurons central to synaptogenesis, bioenergetics, and neurotransmitter and peptidergic signaling.

Requirement for NF-κB in maintenance of molecular and behavioral circadian rhythms in mice.

The mammalian circadian clock is encoded by an autoregulatory transcription feedback loop that drives rhythmic behavior and gene expression in the brain and peripheral tissues. Transcriptomic analyses indicate cell type-specific effects of circadian cycles on rhythmic physiology, although how clock cycles respond to environmental stimuli remains incompletely understood. Here, we show that activation of the inducible transcription factor NF-κB in response to inflammatory stimuli leads to marked inhibition of clock repressors, including the , , and genes, within the negative limb.

A day in the life of chromatin: how enhancer-promoter loops shape daily behavior.

Each spring, we get out of bed 1 h ahead of our biological wake-up time due to the misalignment of internal clocks with the light-dark cycle. Genetic discoveries revealed that clock genes encode transcription factors that are expressed throughout many tissues, yet a gap has remained in understanding the temporal dynamics of transcription. Two groups now apply circular chromosome conformation capture and high-throughput sequencing to dissect how "time of day"-dependent changes in chromatin drive core clock oscillations.

Circadian Clock Interaction with HIF1α Mediates Oxygenic Metabolism and Anaerobic Glycolysis in Skeletal Muscle.

Circadian clocks are encoded by a transcription-translation feedback loop that aligns energetic processes with the solar cycle. We show that genetic disruption of the clock activator BMAL1 in skeletal myotubes and fibroblasts increased levels of the hypoxia-inducible factor 1α (HIF1α) under hypoxic conditions. Bmal1 myotubes displayed reduced anaerobic glycolysis, mitochondrial respiration with glycolytic fuel, and transcription of HIF1α targets Phd3, Vegfa, Mct4, Pk-m, and Ldha, whereas abrogation of the clock repressors CRY1/2 stabilized HIF1α in response to hypoxia.

Circadian Transcription from Beta Cell Function to Diabetes Pathophysiology.

The mammalian circadian clock plays a central role in the temporal coordination of physiology across the 24-h light-dark cycle. A major function of the clock is to maintain energy constancy in anticipation of alternating periods of fasting and feeding that correspond with sleep and wakefulness. While it has long been recognized that humans exhibit robust variation in glucose tolerance and insulin sensitivity across the sleep-wake cycle, experimental genetic analysis has now revealed that the clock transcription cycle plays an essential role in insulin secretion and metabolic function within pancreatic beta cells.

Pancreatic β cell enhancers regulate rhythmic transcription of genes controlling insulin secretion.

The mammalian transcription factors CLOCK and BMAL1 are essential components of the molecular clock that coordinate behavior and metabolism with the solar cycle. Genetic or environmental perturbation of circadian cycles contributes to metabolic disorders including type 2 diabetes. To study the impact of the cell-autonomous clock on pancreatic β cell function, we examined pancreatic islets from mice with either intact or disrupted BMAL1 expression both throughout life and limited to adulthood.

Circadian clock NAD+ cycle drives mitochondrial oxidative metabolism in mice.

Circadian clocks are self-sustained cellular oscillators that synchronize oxidative and reductive cycles in anticipation of the solar cycle. We found that the clock transcription feedback loop produces cycles of nicotinamide adenine dinucleotide (NAD(+)) biosynthesis, adenosine triphosphate production, and mitochondrial respiration through modulation of mitochondrial protein acetylation to synchronize oxidative metabolic pathways with the 24-hour fasting and feeding cycle. Circadian control of the activity of the NAD(+)-dependent deacetylase sirtuin 3 (SIRT3) generated rhythms in the acetylation and activity of oxidative enzymes and respiration in isolated mitochondria, and NAD(+) supplementation restored protein deacetylation and enhanced oxygen consumption in circadian mutant mice.

Circadian measurements of sirtuin biology.

Many of our behavioral and physiological processes display daily oscillations that are under the control of the circadian clock. The core molecular clock network is present in both the brain and peripheral tissues and is composed of a complex series of interlocking transcriptional/translational feedback loops that oscillate with a periodicity of ~24 h. Recent evidence has implicated NAD(+) biosynthesis and the sirtuin family of NAD(+)-dependent protein deacetylases as part of a novel feedback loop within the core clock network, findings which underscore the importance of taking circadian timing into consideration when designing and interpreting metabolic studies, particularly in regard to sirtuin biology.

Genetic advances in ophthalmology: the role of melanopsin-expressing, intrinsically photosensitive retinal ganglion cells in the circadian organization of the visual system.

Daily changes in the light-dark cycle are the principal environmental signal that enables organisms to synchronize their internal biology with the 24-hour day-night cycle. In humans, the visual system is integral to photoentrainment and is primarily driven by a specialized class of intrinsically photosensitive retinal ganglion cells (ipRGCs) that express the photopigment melanopsin (OPN4) in the inner retina. These cells project through the retinohypothalamic tract (RHT) to the suprachiasmatic nuclei (SCN) of the hypothalamus, which serves as the body's master biological clock.

Circadian genes and insulin exocytosis.

The molecular clock controls 24-hour cycles of behavioral and physiological processes across the day-night cycle. Disruption of circadian rhythmicity has been implicated in the pathogenesis of several diseases, including the metabolic syndrome, although the role of clock genes in these disorders is still not well understood. Studies of the etiology of diabetes in circadian mutant mice have revealed a novel role for the clock in pancreatic β-cell insulin secretion, suggesting that a major cellular function of the circadian network involves control of protein exocytosis.

Circadian rhythms, sleep, and metabolism.

The discovery of the genetic basis for circadian rhythms has expanded our knowledge of the temporal organization of behavior and physiology. The observations that the circadian gene network is present in most living organisms from eubacteria to humans, that most cells and tissues express autonomous clocks, and that disruption of clock genes results in metabolic dysregulation have revealed interactions between metabolism and circadian rhythms at neural, molecular, and cellular levels. A major challenge remains in understanding the interplay between brain and peripheral clocks and in determining how these interactions promote energy homeostasis across the sleep-wake cycle.

Circadian disruption and metabolic disease: findings from animal models.

Social opportunities and work demands have caused humans to become increasingly active during the late evening hours, leading to a shift from the predominantly diurnal lifestyle of our ancestors to a more nocturnal one. This voluntarily decision to stay awake long into the evening hours leads to circadian disruption at the system, tissue, and cellular levels. These derangements are in turn associated with clinical impairments in metabolic processes and physiology.

Disruption of the clock components CLOCK and BMAL1 leads to hypoinsulinaemia and diabetes.

The molecular clock maintains energy constancy by producing circadian oscillations of rate-limiting enzymes involved in tissue metabolism across the day and night. During periods of feeding, pancreatic islets secrete insulin to maintain glucose homeostasis, and although rhythmic control of insulin release is recognized to be dysregulated in humans with diabetes, it is not known how the circadian clock may affect this process. Here we show that pancreatic islets possess self-sustained circadian gene and protein oscillations of the transcription factors CLOCK and BMAL1.

Circadian rhythms and metabolic syndrome: from experimental genetics to human disease.

The incidence of the metabolic syndrome represents a spectrum of disorders that continue to increase across the industrialized world. Both genetic and environmental factors contribute to metabolic syndrome and recent evidence has emerged to suggest that alterations in circadian systems and sleep participate in the pathogenesis of the disease. In this review, we highlight studies at the intersection of clinical medicine and experimental genetics that pinpoint how perturbations of the internal clock system, and sleep, constitute risk factors for disorders including obesity, diabetes mellitus, cardiovascular disease, thrombosis and even inflammation.

Clock genes and metabolic disease.

The circadian system is a key integrator of behavior and metabolism that synchronizes physiological processes with the rotation of the Earth on its axis. In mammals, the clock is present not only within the central pacemaker neurons of the hypothalamus, but also within extra-suprachiasmatic nucleus (SCN) regions of brain and nearly all peripheral tissues. Recent evidence suggests that the complex feedback networks that encompass both the circadian and metabolic systems are intimately intertwined and that disruption of either system leads to reciprocal disturbances in the other.

Circadian clock feedback cycle through NAMPT-mediated NAD+ biosynthesis.

The circadian clock is encoded by a transcription-translation feedback loop that synchronizes behavior and metabolism with the light-dark cycle. Here we report that both the rate-limiting enzyme in mammalian nicotinamide adenine dinucleotide (NAD+) biosynthesis, nicotinamide phosphoribosyltransferase (NAMPT), and levels of NAD+ display circadian oscillations that are regulated by the core clock machinery in mice. Inhibition of NAMPT promotes oscillation of the clock gene Per2 by releasing CLOCK:BMAL1 from suppression by SIRT1.

Age-associated loss of Sirt1-mediated enhancement of glucose-stimulated insulin secretion in beta cell-specific Sirt1-overexpressing (BESTO) mice.

The Sir2 (silent information regulator 2) family of NAD-dependent deacetylases regulates aging and longevity across a wide variety of organisms, including yeast, worms, and flies. In mammals, the Sir2 ortholog Sirt1 promotes fat mobilization, fatty acid oxidation, glucose production, and insulin secretion in response to nutrient availability. We previously reported that an increased dosage of Sirt1 in pancreatic beta cells enhances glucose-stimulated insulin secretion (GSIS) and improves glucose tolerance in beta cell-specific Sirt1-overexpressing (BESTO) transgenic mice at 3 and 8 months of age.

High-fat diet disrupts behavioral and molecular circadian rhythms in mice.

The circadian clock programs daily rhythms and coordinates multiple behavioral and physiological processes, including activity, sleep, feeding, and fuel homeostasis. Recent studies indicate that genetic alteration in the core molecular clock machinery can have pronounced effects on both peripheral and central metabolic regulatory signals. Many metabolic systems also cycle and may in turn affect function of clock genes and circadian systems.

The clockwork of metabolism.

The observation that cycles of sleep and wakefulness occur with a periodicity fixed in time to match the rotation of the Earth on its axis provided a key to unlock the first genetic code for a neurobehavioral pathway in flies and ultimately in mice. As a remarkable outcome of this discovery, we have gained an unprecedented view of the conserved genetic program that encodes a sense of time across all kingdoms of life. The tools are now in hand to begin to understand how important processes such as energy homeostasis and fuel utilization are coordinated to anticipate daily changes in environment caused by the rising and setting of the sun.