Pancreatic α- and β-cellular clocks have distinct molecular properties and impact on islet hormone secretion and gene expression

Abstract

A critical role of circadian oscillators in orchestrating insulin secretion and islet gene transcription has been demonstrated recently. However, these studies focused on whole islets and did not explore the interplay between α-cell and β-cell clocks. We performed a parallel analysis of the molecular properties of α-cell and β-cell oscillators using a mouse model expressing three reporter genes: one labeling α cells, one specific for β cells, and a third monitoring circadian gene expression. Thus, phase entrainment properties, gene expression, and functional outputs of the α-cell and β-cell clockworks could be assessed in vivo and in vitro at the population and single-cell level. These experiments showed that α-cellular and β-cellular clocks are oscillating with distinct phases in vivo and in vitro. Diurnal transcriptome analysis in separated α and β cells revealed that a high number of genes with key roles in islet physiology, including regulators of glucose sensing and hormone secretion, are differentially expressed in these cell types. Moreover, temporal insulin and glucagon secretion exhibited distinct oscillatory profiles both in vivo and in vitro. Altogether, our data indicate that differential entrainment characteristics of circadian α-cell and β-cell clocks are an important feature in the temporal coordination of endocrine function and gene expression.

Time zones of pancreatic islet metabolism

“…Indeed, these oscillators are not coupled even when cells are establishing tight physical contacts in confluent cell culture, as was demonstrated by co‐culture experiments between fibroblast cells bearing distinct oscillatory characteristics . In line with the reported cell‐autonomous nature of the peripheral oscillators, pronounced circadian oscillations were registered not only in isolated intact rodent and human pancreatic islets, but also in dispersed islet cells cultured in monolayer . These findings imply that physical interactions between neighbouring cells within the 3‐dimensional islet architecture may be dispensable for cellular oscillator function.…”

“…Peripheral oscillators operative in liver, skeletal muscle, adipose tissue, thyroid gland, endocrine pancreas and other organs orchestrate physiological functions within these organs, adjusting them on a daily basis to the rest‐activity and feeding‐fasting cycles of the body . Genome‐wide transcriptome profiling studies of peripheral oscillators indicate that the temporal orchestration of metabolism and xenobiotic detoxification is a major purpose of circadian clocks in peripheral tissues . It was recently suggested that in mouse liver circadian gene expression is controlled by rhythmic chromatin interactions between enhancers and promoters.…”

“… and references therein). Surprisingly, cell‐autonomous circadian oscillators operative in α‐cells bear distinct properties from those of their β‐cellular counterparts . Such phase difference between α‐ and β‐cellular clocks may contribute to the phase‐shift observed between insulin and glucagon secretion profiles measured in the blood .…”

“…Key functional genes exhibit similar or distinct temporal patterns in α‐ and β‐cells, comprising those encoding for glucose transporters, enzymes catalyzing glucose metabolism reactions (glycolysis, pyruvate metabolism, Krebs cycle) and genes responsible for granule trafficking and exocytosis. The distinct properties of α‐ and β‐cellular clocks, along with feeding‐fasting cycles, might contribute in orchestrating the different oscillating patterns of glucagon and insulin . Differential transcriptional regulation of functional α‐and β‐cell genes related to hormone granule assembly, trafficking and exocytosis, might represent a plausible link between the cellular oscillator and the phase of hormone secretion…”

The importance of being rhythmic: Living in harmony with your body clocks

“…31,54,61,[65][66][67][68][69][70][71][72][73][74][75][76][77] Interestingly, circadian phases of the core clock components, as well as of a high number of key functional genes, were not aligned between α-and β-cells within the pancreatic islet, which may play a role in fine-tuning of temporal profiles of insulin and glucagon secretion. [61][62][63][64] In agreement with rodent studies, emerging work in humans conducted through serial tissue biopsies collected across 24 hours, and in primary cells synchronized in vitro, suggest that a considerable portion of transcripts in these organs exhibit rhythmic expression. 31,41,[78][79][80] Circadian transcription reconstruction analysis through cyclic ordering by periodic structure (CYCLOPS) conducted by John Hogenesch and colleagues across 13 tissues derived from a cohort of over 600 human donors provides a valuable database for future studies of clock outputs in humans.…”