A clock ticks in pancreatic β cells

Abstract

Circadian rhythm is a pervasive feature of life, programmed by the conserved core clock machinery. A recent study in Nature (Marcheva et al, 2010) reveals that core clock components regulate β cell function and integrate insulin secretion into rhythmic metabolic regulation, providing new insight into the pathogenesis of diabetes.

Many fundamental biological processes, including the sleep-wake cycle, feeding, physical activity, and metabolism, are highly integrated into the 24-hr periodicity by the evolutionarily conserved molecular circadian clock machinery. In mammals, the core clock machinery resides within the central pacemaker neurons of the suprachiasmatic nuclei (SCN) of the hypothalamus. Among the well-characterized major clock regulators, CLOCK and BMAL1 are recognized as key constituents that heterodimerize and activate transcription of target genes, including Period (Per1, 2, and 3) and Cryptochrome (Cry 1 and 2). These core clock regulators function not only within the SCN but also within peripheral metabolic tissues, such as the liver and white adipose tissue, where they control diverse metabolic processes including glucose and lipid homeostasis (Green et al., 2008). Indeed, homozygous Clock mutant mice exhibit metabolic dysregulation (Turek et al., 2005), while liver-specific Bmal1 knockout mice show loss of rhythmic expression of metabolic genes in the liver and hypoglycemia in the fasting phase of the daily feeding cycle (Lamia et al., 2008). Furthermore, conditions of energetic excess, such as high-fat diet, significantly alter the circadian control of gene expression in the hypothalamus, liver, and adipose tissue (Kohsaka et al., 2007). In their recent study in Nature, Marcheva et al. (Marcheva et al., 2010) add a novel layer of interaction between circadian rhythm and metabolism, namely, the roles of core clock components in pancreatic β cell function.

The authors first examined whether the core clock machinery functions within pancreatic β cells, a central component of glucose homeostasis. They found that primary islets from Per2^Luc knock-in mice, which express a PERIOD2-luciferase fusion protein, display a robust and autonomous bioluminescent rhythm, whereas islets from Clock mutant mice lack this rhythmicity. In pancreatic islets, many genes involved in insulin signaling, glucose uptake and metabolism, cell cycle control, and β-cell growth and development exhibit rhythmic expression patterns, and Clock mutant islets show decreased and/or shifted expression of those genes. The observed transcriptional defects in Clock mutant islets appear to result in impaired insulin secretion in response to glucose and other secretagogues and decreased islet proliferation. Consequently, Clock mutant mice show impaired glucose tolerance with reduced glucose-stimulated insulin secretion and elevated blood glucose levels through the entire light/dark cycle. Interestingly, these phenotypes are more obvious during the dark period (Zietgeber time (ZT) 14) compared to the light period (ZT2).

To confirm the importance of the core clock machinery in pancreatic islets, the authors also examined Bmal1-deficient islets in vitro. These islets exhibit similar defects in insulin secretion and islet size, demonstrating that the core clock machinery plays a critical role in the regulation of β cell function and development. To determine in vivo relevance of the core clock machinery in pancreatic β cells, the authors generated islet-specific Bmal1-deficient mice and found that these mice show more profound phenotypes than the Clock mutant or whole-body Bmal1-deficient mice; that is, markedly impaired glucose tolerance with significantly reduced insulin secretion and elevated blood glucose levels throughout the day. Although Bmal1-deficient islets do not show defects in size and proliferation, they exhibit severe defects in insulin secretion in response to glucose and other secretagogues. These findings clearly demonstrate the physiological importance of the core clock machinery mediated by CLOCK and BMAL1 in the insulin-secreting function of pancreatic β cells (Figure 1).

Figure 1. Maintenance of glucose homeostasis by the pancreatic core clock machinery.

A scheme of CLOCK/BMAL1-mediated regulation of β-cell function is shown. The core clock mechanism mediated by CLOCK and BMAL1 regulates insulin secretion, likely at the step of insulin granule exocytosis, and proliferation in pancreatic islets. Lifestyle, dietary conditions, and other oscillators might modulate the CLOCK/BMAL1-mediated core clock mechanism in β cells. The disruption of this mechanism leads to type 2 diabetes due to the defect in rhythmic gene expression and hypoinsulinemia.

Numerous studies have reported that pancreatic islets display rapid (6-10 min), slow ultradian (~140 min), and circadian (~24 hr) oscillatory patterns of insulin secretion (Peschke and Peschke, 1998; Porksen et al., 2002). Glycolytic oscillation within β cells, intracellular communication/electrical coupling, and intra-pancreatic neural network have been suggested to generate these oscillatory patterns of insulin secretion, but molecular details remain unclear. The study by Marcheva et al. (Marcheva et al., 2010) provides the first molecular connection between the core circadian clock machinery and insulin secretion in pancreatic β cells. One possible function of the CLOCK/BMAL1-mediated circadian clock mechanism in β cells is to produce a circadian oscillatory gating or priming function that determines the capacity of insulin secretion. Given that Clock mutant mice exhibit impaired glucose tolerance during the dark period (ZT14), but not during the light period (ZT2), the presumed gating/priming mediated by CLOCK and BMAL1 might function more strongly during the dark period. Although exact CLOCK/BMAL1 target genes necessary for this circadian gating/priming function have not yet been identified, the lack of responsiveness of Clock mutant and Bmal1-deficient islets to a broad spectrum of stimuli implies that a defect lies in the process of insulin granule exocytosis. Thus, it will be of great interest to examine whether known critical components of insulin granule exocytosis, such as the SNARE complex, are regulated by circadian rhythm. Another possibility is that the CLOCK/BMAL1-mediated transcriptional machinery is simply necessary to maintain the normal insulin-secreting function of β cells and not necessarily linked to their circadian rhythm-regulatory function in β cells. Interestingly, unlike Clock mutant mice, islet-specific Bmal1-deficient mice show more severe phenotypes independent of the light/dark periods. As suggested previously (Porksen et al., 2002), there might be a neural regulatory component necessary for the production of the circadian oscillation of insulin secretion in vivo. Thus, it is possible that the actual oscillatory signal is mediated by a neural network to coordinate pancreatic islets. Alternatively, such a signal might be generated by another interlocking circadian feedback loop that also requires CLOCK and BMAL1. For example, it has recently been demonstrated that CLOCK and BMAL1 comprise a novel circadian regulatory feedback loop with the mammalian NAD-dependent deacetylase SIRT1 and the key NAD-biosynthetic enzyme NAMPT, generating the circadian oscillation of NAD (Nakahata et al., 2009; Ramsey et al., 2009). Because SIRT1 also plays an important role in insulin secretion in β cells, it is possible that oscillatory NAD biosynthesis functions as a systemic driver that could mediate a circadian oscillation in β cells (Imai, 2010).

Defective oscillatory insulin secretion has been suggested to be a possible primary dysfunction of β cells, leading to type 2 diabetes (Porksen et al., 2002). Given that energy-excess dietary conditions significantly alter circadian rhythm in major metabolic tissues (Kohsaka et al., 2007), sedentary life style or irregular work environment could cause serious dysfunction of the core clock machinery, including defective oscillatory insulin secretion, contributing to increasing incidences of type 2 diabetes. This study by Marcheva et al. (Marcheva et al., 2010) has shed new light into the fascinating connection between circadian rhythm and metabolism, and further investigation is eagerly awaited to help clarify these multi-layered circadian feedback loops regulating metabolic rhythmicity.