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 body's 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.