Jingyi Qian

Disruption of Circadian Rhythms Accelerates Development of Diabetes through Pancreatic Beta-Cell Loss and Dysfunction

Type 2 diabetes mellitus (T2DM) is complex metabolic disease that arises as a consequence of interactions between genetic predisposition and environmental triggers. One recently described environmental trigger associated with development of T2DM is disturbance of circadian rhythms due to shift work, sleep loss, or nocturnal lifestyle. However, the underlying mechanisms behind this association are largely unknown. To address this, the authors examined the metabolic and physiological consequences of experimentally controlled circadian rhythm disruption in wild-type (WT) Sprague Dawley and diabetes-prone human islet amyloid polypeptide transgenic (HIP) rats: a validated model of T2DM. WT and HIP rats at 3 months of age were exposed to 10 weeks of either a normal light regimen (LD: 12:12-h light/dark) or experimental disruption in the light-dark cycle produced by either (1) 6-h advance of the light cycle every 3 days or (2) constant light protocol. Subsequently, blood glucose control, beta-cell function, beta-cell mass, turnover, and insulin sensitivity were examined. In WT rats, 10 weeks of experimental disruption of circadian rhythms failed to significantly alter fasting blood glucose levels, glucose-stimulated insulin secretion, beta-cell mass/turnover, or insulin sensitivity. In contrast, experimental disruption of circadian rhythms in diabetes-prone HIP rats led to accelerated development of diabetes. The mechanism subserving early-onset diabetes was due to accelerated loss of beta-cell function and loss of beta-cell mass attributed to increases in beta-cell apoptosis. Disruption of circadian rhythms may increase the risk of T2DM by accelerating the loss of beta-cell function and mass characteristic in T2DM.

Consequences of Exposure to Light at Night on the Pancreatic Islet Circadian Clock and Function in Rats

There is a correlation between circadian disruption, type 2 diabetes mellitus (T2DM), and islet failure. However, the mechanisms underlying this association are largely unknown. Pancreatic islets express self-sustained circadian clocks essential for proper β-cell function and survival. We hypothesized that exposure to environmental conditions associated with disruption of circadian rhythms and susceptibility to T2DM in humans disrupts islet clock and β-cell function. To address this hypothesis, we validated the use of Per-1:LUC transgenic rats for continuous longitudinal assessment of islet circadian clock function ex vivo. Using this methodology, we subsequently examined effects of the continuous exposure to light at night (LL) on islet circadian clock and insulin secretion in vitro in rat islets. Our data show that changes in the light–dark cycle in vivo entrain the phase of islet clock transcriptional oscillations, whereas prolonged exposure (10 weeks) to LL disrupts islet circadian clock function through impairment in the amplitude, phase, and interislet synchrony of clock transcriptional oscillations. We also report that exposure to LL leads to diminished glucose-stimulated insulin secretion due to a decrease in insulin secretory pulse mass. Our studies identify potential mechanisms by which disturbances in circadian rhythms common to modern life can predispose to islet failure in T2DM.

Circadian System and Glucose Metabolism: Implications for Physiology and Disease

The circadian system serves one of the most fundamental properties present in nearly all organisms: it generates 24-h rhythms in behavioral and physiological processes and enables anticipating and adapting to daily environmental changes. Recent studies indicate that the circadian system is important in regulating the daily rhythm in glucose metabolism. Disturbance of this circadian control or of its coordination relative to the environmental/behavioral cycle, such as in shift work, eating late, or due to genetic changes, results in disturbed glucose control and increased type 2 diabetes risk. Therefore, an in-depth understanding of the mechanisms underlying glucose regulation by the circadian system and its disturbance may help in the development of therapeutic interventions against the deleterious health consequences of circadian disruption.

Circadian Disruption and Diet-Induced Obesity Synergize to Promote Development of β-Cell Failure and Diabetes in Male Rats

There are clear epidemiological associations between circadian disruption, obesity, and pathogenesis of type 2 diabetes. The mechanisms driving these associations are unclear. In the current study, we hypothesized that continuous exposure to constant light (LL) compromises pancreatic β-cell functional and morphological adaption to diet-induced obesity leading to development of type 2 diabetes. To address this hypothesis, we studied wild type Sprague Dawley as well as Period-1 luciferase reporter transgenic rats (Per1-Luc) for 10 weeks under standard light-dark cycle (LD) or LL with concomitant ad libitum access to either standard chow or 60% high-fat diet (HFD). Exposure to HFD led to a comparable increase in food intake, body weight, and adiposity in both LD- and LL-treated rats. However, LL rats displayed profound loss of behavioral circadian rhythms as well as disrupted pancreatic islet clock function characterized by the impairment in the amplitude and the phase islet clock oscillations. Under LD cycle, HFD did not adversely alter diurnal glycemia, diurnal insulinemia, β-cell secretory function as well as β-cell survival, indicating successful adaptation to increased metabolic demand. In contrast, concomitant exposure to LL and HFD resulted in development of hyperglycemia characterized by loss of diurnal changes in insulin secretion, compromised β-cell function, and induction of β-cell apoptosis. This study suggests that circadian disruption and diet-induced obesity synergize to promote development of β-cell failure, likely mediated as a consequence of impaired islet clock function.

The islet circadian clock: entrainment mechanisms, function and role in glucose homeostasis.

Circadian regulation of glucose homeostasis and insulin secretion has long been appreciated as an important feature of metabolic control in humans. Circadian disruption is becoming increasingly prevalent in todays society and is likely responsible in part for the considerable rise in type 2 diabetes (T2DM) and metabolic syndrome worldwide. Thus, understanding molecular mechanisms driving the inter‐relationship between circadian disruption and T2DM is important in context of disease prevention and therapeutics. In this regard, the goal of this article is to highlight the role of the circadian system, and islet circadian clocks in particular, as potential regulators of β‐cell function and survival. To date, studies have shown that islet clocks respond to changes in feeding patterns, and regulate a multitude of critical cellular processes in insulin secreting β‐cells (e.g. insulin exocytosis, mitochondrial function and response to oxidative stress). Subsequently, either genetic or environmental disruption of normal islet clock performance compromises β‐cell function and leads to loss of glycaemic control. Future work is warranted to further unravel the role of circadian clocks in human islet function in health and contributions to pathogenesis of T2DM.

Development of diabetes does not alter behavioral and molecular circadian rhythms in a transgenic rat model of Type 2 Diabetes Mellitus

Metabolic state and circadian clock function exhibit a complex bidirectional relationship. Circadian disruption increases propensity for metabolic dysfunction, whereas common metabolic disorders such as obesity and type 2 diabetes (T2DM) are associated with impaired circadian rhythms. Specifically, alterations in glucose availability and glucose metabolism have been shown to modulate clock gene expression and function in vitro; however, to date, it is unknown whether development of diabetes imparts deleterious effects on the suprachiasmatic nucleus (SCN) circadian clock and SCN-driven outputs in vivo. To address this question, we undertook studies in aged diabetic rats transgenic for human islet amyloid polypeptide, an established nonobese model of T2DM (HIP rat), which develops metabolic defects closely recapitulating those present in patients with T2DM. HIP rats were also cross-bred with a clock gene reporter rat model (Per1:luciferase transgenic rat) to permit assessment of the SCN and the peripheral molecular clock function ex vivo. Utilizing these animal models, we examined effects of diabetes on 1) behavioral circadian rhythms, 2) photic entrainment of circadian activity, 3) SCN and peripheral tissue molecular clock function, and 4) melatonin secretion. We report that circadian activity, light-induced entrainment, molecular clockwork, as well as melatonin secretion are preserved in the HIP rat model of T2DM. These results suggest that despite the well-characterized ability of glucose to modulate circadian clock gene expression acutely in vitro, SCN clock function and key behavioral and physiological outputs appear to be preserved under chronic diabetic conditions characteristic of nonobese T2DM.

Differential effects of the circadian system and circadian misalignment on insulin sensitivity and insulin secretion in humans

Glucose tolerance is lower at night and higher in the morning. Shift workers, who often eat at night and experience circadian misalignment (i.e. misalignment between the central circadian pacemaker and the environmental/behavioural cycles), have an increased risk of type 2 diabetes. To determine the separate and relative impacts of the circadian system, behavioural/environmental cycles, and their interaction (i.e. circadian misalignment) on insulin sensitivity and β‐cell function, the oral minimal model was used to quantitatively assess the major determinants of glucose control in 14 healthy adults using a randomized, cross‐over design with two 8‐day laboratory protocols. Both protocols involved 3 baseline inpatient days with habitual sleep/wake cycles, followed by 4 inpatient days with the same nocturnal bedtime (circadian alignment) or with 12‐hour inverted behavioural/environmental cycles (circadian misalignment). The data showed that circadian phase and circadian misalignment affect glucose tolerance through different mechanisms. While the circadian system reduces glucose tolerance in the biological evening compared to the biological morning mainly by decreasing both dynamic and static β‐cell responsivity, circadian misalignment reduced glucose tolerance mainly by lowering insulin sensitivity, not by affecting β‐cell function.

Ghrelin is impacted by the endogenous circadian system and by circadian misalignment in humans

The human circadian system regulates hunger independently of behavioral factors, resulting in a trough in the biological morning and a peak in the biological evening. However, the role of the only known orexigenic hormone, ghrelin, in this circadian rhythm is unknown. Furthermore, although shift work is an obesity risk factor, the separate effects of the endogenous circadian system, the behavioral cycle, and circadian misalignment on ghrelin has not been systematically studied. Here we show—by using two 8-day laboratory protocols—that circulating active (acylated) ghrelin levels are significantly impacted by endogenous circadian phase in healthy adults. Active ghrelin levels were higher in the biological evening than the biological morning (fasting +15.1%, P = 0.0001; postprandial +10.4%, P = 0.0002), consistent with the circadian variation in hunger (P = 0.028). Moreover, circadian misalignment itself (12-h behavioral cycle inversion) increased postprandial active ghrelin levels (+5.4%; P = 0.04). While not significantly influencing hunger (P > 0.08), circadian misalignment increased appetite for energy-dense foods (all P < 0.05). Our results provide possible mechanisms for the endogenous circadian rhythm in hunger, as well as for the increased risk of obesity among shift workers.

Postnatal Ontogenesis of the Islet Circadian Clock Plays a Contributory Role in β-Cell Maturation Process

Development of cell replacement therapies in diabetes requires understanding of the molecular underpinnings of β-cell maturation. The circadian clock regulates diverse cellular functions important for regulation of β-cell function and turnover. However, postnatal ontogenesis of the islet circadian clock and its potential role in β-cell maturation remain unknown. To address this, we studied wild-type Sprague-Dawley as well as Period1 luciferase transgenic (Per1:LUC) rats to determine circadian clock function, clock protein expression, and diurnal insulin secretion during islet development and maturation process. We additionally studied β-cell–specific Bmal1-deficient mice to elucidate a potential role of this key circadian transcription factor in β-cell functional and transcriptional maturation. We report that emergence of the islet circadian clock 1) occurs during the early postnatal period, 2) depends on the establishment of global behavioral circadian rhythms, and 3) leads to the induction of diurnal insulin secretion and gene expression. Islet cell maturation was also characterized by induction in the expression of circadian transcription factor BMAL1, deletion of which altered postnatal development of glucose-stimulated insulin secretion and the associated transcriptional network. Postnatal development of the islet circadian clock contributes to early-life β-cell maturation and should be considered for optimal design of future β-cell replacement strategies in diabetes.