Qian, Jingyi

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.

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.

Pancreatic islet failure is a characteristic feature of impaired glucose control in diabetes mellitus. Circadian control of islet function is essential for maintaining proper glucose homeostasis. Circadian variations in transcriptional pathways have been described in diverse cell types and shown to be critical for optimization of cellular function in vivo. In the current study, we utilized Short Time Series Expression Miner (STEM) analysis to identify diurnally expressed transcripts and biological pathways from mouse islets isolated at 4 h intervals throughout the 24 h light-dark cycle. STEM analysis identified 19 distinct chronological model profiles, and genes belonging to each profile were subsequently annotated to significantly enriched Kyoto Encyclopedia of Genes and Genomes biological pathways. Several transcriptional pathways essential for proper islet function (e.g., insulin secretion, oxidative phosphorylation), cell survival (e.g., insulin signaling, apoptosis) and cell proliferation (DNA replication, homologous recombination) demonstrated significant time-dependent variations. Notably, KEGG pathway analysis revealed “protein processing in endoplasmic reticulum - mmu04141” as one of the most enriched time-dependent pathways in islets. This study provides unique data set on time-dependent diurnal profiles of islet gene expression and biological pathways, and suggests that diurnal variation of the islet transcriptome is an important feature of islet homeostasis, which should be taken into consideration for optimal experimental design and interpretation of future islet studies.

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.

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 today's 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 .