Wenyu Huang

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. In this Review, we evaluate how investigation of molecular timing may create new opportunities to understand and develop therapies for obesity and diabetes.

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

The clockwork of insulin release In healthy people, blood glucose levels are maintained within a narrow range by several physiological mechanisms. Key among them is the release of the hormone insulin by pancreatic β cells, which occurs when glucose levels rise after a meal. In response to insulin, blood glucose is taken up by tissues that need fuel, such as muscle. β cells can anticipate the bodys varying demand for insulin throughout the 24-hour day because they have their own circadian clock. How this clock controls insulin release has been unclear. Perelis et al. now show that the activity of transcriptional enhancers specific to β cells regulates the rhythmic expression of genes involved in the assembly and trafficking of insulin secretory vesicles (see the Perspective by Dibner and Schibler). Science, this issue p. 10.1126/science.aac4250; see also p. 628 Circadian control of insulin release is mediated by transcriptional enhancers active specifically in pancreatic β cells. [Also see Perspective by Dibner and Schibler] INTRODUCTION The circadian clock is a molecular oscillator that coordinates behavior and physiology in anticipation of the daily light cycle. Desynchrony of circadian cycles, through genetic or environmental perturbation, contributes to metabolic disorders such as cardiovascular disease, obesity, and type 2 diabetes. We previously showed that disruption of the clock transcription factors CLOCK and BMAL1 in the pancreas causes hypoinsulinemic diabetes in mice. The mechanism(s) linking clock dysfunction to pancreatic β cell failure and the means by which CLOCK and BMAL1 affect glucose metabolism in the whole organism are not well understood. RATIONALE The circadian system helps to maintain glucose homeostasis across the sleep-wake cycle. This system requires cross-talk between the master clock in the central nervous system, which coordinates feeding and sleep, and peripheral tissue clocks, which synchronize behavior with the storage, mobilization, and synthesis of glucose. Although it is clear that clocks within distinct organs participate in glucose turnover, the molecular basis for time-of-day variation in organismal glucose responsiveness is still not understood. Here, we combined genome-wide analyses with gene targeting in mice to study the impact of the cell-autonomous clock on β cell function. RESULTS We found that cell-autonomous expression of CLOCK and BMAL1 in pancreatic islets isolated from wild-type mice generates robust 24-hour rhythms of glucose- and potassium chloride–stimulated insulin secretion ex vivo. About 27% of the β cell transcriptome exhibited circadian oscillation. Many of these transcripts correspond to genes coding for proteins that are involved in the assembly, trafficking, and membrane fusion of vesicles that participate in insulin secretion. Chromatin immunoprecipitation sequencing revealed that CLOCK and BMAL1 regulate cycling genes in β cells by binding at distal regulatory elements distinct from those controlling the circadian transcription of metabolic gene networks within the liver. The regulatory sites of cycling genes in the β cell resided primarily within transcriptionally active enhancers that were also bound by the pancreatic transcription factor PDX1. Finally, we found that in islets from adult mice, Bmal1 ablation either in vivo or ex vivo abrogates nutrient-responsive insulin secretion, demonstrating clock control of pancreatic β cell function throughout adult life. CONCLUSION Our results show that local clock-driven genomic rhythms program cell function across the light-dark cycle, including the priming of insulin secretion within limited time windows each day. Cell type–specific transcriptional regulation by the clock localizes to rhythmic enhancers that are unique to the β cell. Thus, our findings uncover a transcriptional process through which the core clock aligns physiology with the light cycle, revealing pathways that are important in both health and disease states such as type 2 diabetes. β cell–specific enhancers control the rhythmic transcription of genes linked to insulin secretion. Peripheral clocks maintain glucose homeostasis across the sleep-wake cycle by gating β cell insulin secretion through genome-wide transcriptional control of the assembly and trafficking of insulin secretory vesicles. Clock transcription factors bind within cell type–specific enhancer neighborhoods of cycling genes, revealing the mechanisms that synchronize rhythmic metabolism at transcriptional and physiologic levels across the light-dark cycle. 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. We found pronounced oscillation of insulin secretion that was synchronized with the expression of genes encoding secretory machinery and signaling factors that regulate insulin release. CLOCK/BMAL1 colocalized with the pancreatic transcription factor PDX1 within active enhancers distinct from those controlling rhythmic metabolic gene networks in liver. We also found that β cell clock ablation in adult mice caused severe glucose intolerance. Thus, cell type–specific enhancers underlie the circadian control of peripheral metabolism throughout life and may help to explain its dysregulation in diabetes.

Ovarian Steroids Stimulate Adenosine Triphosphate-Sensitive Potassium (KATP) Channel Subunit Gene Expression and Confer Responsiveness of the Gonadotropin-Releasing Hormone Pulse Generator to KATP Channel Modulation

The ATP-sensitive potassium (K(ATP)) channels couple intracellular metabolism to membrane potential. They are composed of Kir6.x and sulfonylurea receptor (SUR) subunits and are expressed in hypothalamic neurons that project to GnRH neurons. However, their roles in regulating GnRH secretion have not been determined. The present study first tested whether K(ATP) channels regulate pulsatile GnRH secretion, as indirectly reflected by pulsatile LH secretion. Ovariectomized rats received sc capsules containing oil, 17beta-estradiol (E(2)), progesterone (P), or E(2)+P at 24 h before blood sampling. Infusion of the K(ATP) channel blocker tolbutamide into the third ventricle resulted in increased LH pulse frequency in animals treated with E(2)+P but was without effect in all other groups. Coinfusion of tulbutamide and the K(ATP) channel opener diazoxide blocked this effect, whereas diazoxide alone suppressed LH. Effects of steroids on Kir6.2 and SUR1 mRNA expression were then evaluated. After 24hr treatment, E(2)+P produced a modest but significant increase in Kir6.2 expression in the preoptic area (POA), which was reversed by P receptor antagonism with RU486. Neither SUR1 in the POA nor both subunits in the mediobasal hypothalamus were altered by any steroid treatment. After 8 d treatment, Kir6.2 mRNA levels were again enhanced by E(2)+P but to a greater extent in the POA. Our findings demonstrate that 1) blockade of preoptic/hypothalamic K(ATP) channels produces an acceleration of the GnRH pulse generator in a steroid-dependent manner and 2) E(2)+P stimulate Kir6.2 gene expression in the POA. These observations are consistent with the hypothesis that the negative feedback actions of ovarian steroids on the GnRH pulse generator are mediated, in part, by their ability to up-regulate K(ATP) channel subunit expression in the POA.

Acute diabetes insipidus mediated by vasopressinase after placental abruption.

CONTEXT Postpartum, diabetes insipidus (DI) can be part of Sheehans syndrome or lymphocytic hypophysitis in combination with anterior pituitary hormone deficiencies. In contrast, acute onset of isolated DI in the postpartum period is unusual. CASE PRESENTATION This patient presented at 33 weeks gestation with placental abruption, prompting a cesarean delivery of twins. Immediately after delivery, she developed severe DI. The DI could be controlled with the vasopressinase-resistant 1-deamino-8-D-arginine vasopressin (DDAVP), but not with arginine vasopressin (AVP), and it resolved within a few weeks. OBJECTIVE The aim of this study was to demonstrate that the postpartum DI in this patient was caused by the release of placental vasopressinase into the maternal bloodstream. METHODS AND RESULTS Cells were transiently transfected with the AVP receptor 2 (AVPR2) and treated with either AVP or DDAVP in the presence of the patients serum collected postpartum or 10 weeks after delivery. The response to the different treatments was evaluated by measuring the activity of a cAMP-responsive firefly luciferase reporter construct. The in vitro studies demonstrate that the patients postpartum serum disrupts activation of the AVPR2 by AVP, but not by the vasopressinase-resistant DDAVP. CONCLUSIONS Placental abruption can rarely be associated with acute postpartum DI caused by release of placental vasopressinase into the bloodstream. This clinical entity must be considered in patients with placental abruption and when evaluating patients presenting with DI after delivery.

Fasting-induced suppression of LH secretion does not require activation of ATP-sensitive potassium channels.

Reproductive hormone secretions are inhibited by fasting and restored by feeding. Metabolic signals mediating these effects include fluctuations in serum glucose, insulin, and leptin. Because ATP-sensitive potassium (K(ATP)) channels mediate glucose sensing and many actions of insulin and leptin in neurons, we assessed their role in suppressing LH secretion during food restriction. Vehicle or a K(ATP) channel blocker, tolbutamide, was infused into the lateral cerebroventricle in ovariectomized mice that were either fed or fasted for 48 h. Tolbutamide infusion resulted in a twofold increase in LH concentrations in both fed and fasted mice compared with both fed and fasted vehicle-treated mice. However, tolbutamide did not reverse the suppression of LH in the majority of fasted animals. In sulfonylurea (SUR)1-null mutant (SUR1(-/-)) mice, which are deficient in K(ATP) channels, and their wild-type (WT) littermates, a 48-h fast was found to reduce serum LH concentrations in both WT and SUR(-/-) mice. The present study demonstrates that 1) blockade of K(ATP) channels elevates LH secretion regardless of energy balance and 2) acute fasting suppresses LH secretion in both SUR1(-/-) and WT mice. These findings support the hypothesis that K(ATP) channels are linked to the regulation of gonadotropin-releasing hormone (GnRH) release but are not obligatory for mediating the effects of fasting on GnRH/LH secretion. Thus it is unlikely that the modulation of K(ATP) channels either as part of the classical glucose-sensing mechanism or as a component of insulin or leptin signaling plays a major role in the suppression of GnRH and LH secretion during food restriction.

Management of nonfunctioning pituitary adenomas (NFAs): observation

Clinically nonfunctioning pituitary adenomas (NFAs) range from those causing significant hypothalamic/pituitary dysfunction and visual field compromise due to their large size to those being completely asymptomatic. In the absence of hypersecretion, hypopituitarism or visual field defects, patients with NFAs may be followed by periodic surveillance using MRI to detect tumor enlargement. In some cases, endocrine tests are also needed during observation to discover new pituitary dysfunction. Enlargement of NFAs without treatment occurs in about 10% of microadenomas and 23% of macroadenomas. Growth of a pituitary incidentaloma, the development of visual field defects or the development of hypopituitarism are potential indications for surgery during follow up.

Enlarged thymus in a patient with dyspnea and weight loss

A 36-year-old woman reported experiencing shortness of breath for 1 month. She reported a 6.3-kg weight loss, tremor, diaphoresis, and palpitations over the course of 2 months. She denied eye symptoms or muscle weakness. Physical examination revealed tachycardia, tongue and hand tremors, no exophthalmos, a 2-fold diffusely enlarged thyroid gland, and brisk reflexes. Laboratory results showed a thyroid-stimulating hormone (TSH) level of 0.005 mIU/L (normal, 0.45-4.50 mIU/L), free thyroxine level greater than 7.77 ng/dL (100.01 pmol/L) (normal, 0.82-1.77 ng/dL [10.55-22.78 pmol/L]), free triiodothyronine level of 30.6 pg/mL (47.1 pmol/L) (normal, 2.0-3.9 pg/mL [3.08-6.01 pmol/L]), and thyroidstimulating immunoglobulin index of 5.5 (normal, <1.3). The patient began treatment with methimazole, 20 mg twice daily. A noncontrast computed tomography (CT) scan of the chest (Figure, left) and an 18F-fludeoxyglucose–positron emission tomography (FDG-PET)/CT scan (Figure, right) were completed to evaluate her dyspnea. Quiz at jama.com Figure. Left, Noncontrast computed tomography (CT) scan of the upper chest shown in cross section. Right, 18F-fludeoxyglucose-positron emission tomography/CT scan of the whole body. Note the increased uptake in the thymus indicating increased metabolic activity.

What Does This Retina Examination Show

A 50-year-old man was evaluated for a 5-month history of blurry vision. He had been diagnosed with the metabolic syndrome many years previously but has not seen his physician in years. He reported no current use of medications. On examination, he had a body mass index of 30.6 and a blood pressure of 160/102 mmHg. A dilated examination of the retina is shown in the Figure. Quiz at jama.com Figure. Left, Fundus photograph of the right eye. Right, Fundus photograph of the left eye.