An intrinsic circadian clock of the pancreas is required for normal insulin release and glucose homeostasis in mice - PubMed (original) (raw)

An intrinsic circadian clock of the pancreas is required for normal insulin release and glucose homeostasis in mice

L A Sadacca et al. Diabetologia. 2011 Jan.

Abstract

Aims/hypothesis: Loss of circadian clocks from all tissues causes defective glucose homeostasis as well as loss of feeding and activity rhythms. Little is known about peripheral tissue clocks, so we tested the hypothesis that an intrinsic circadian clock of the pancreas is important for glucose homeostasis.

Methods: We monitored real-time bioluminescence of pancreas explants from circadian reporter mice and examined clock gene expression in beta cells by immunohistochemistry and in situ hybridisation. We generated mice selectively lacking the essential clock gene Bmal1 (also known as Arntl) in the pancreas and tested mutant mice and littermate controls for glucose and insulin tolerance, insulin production and behaviour. We examined islets isolated from mutants and littermate controls for glucose-stimulated insulin secretion and total insulin content.

Results: Pancreas explants exhibited robust circadian rhythms. Clock genes Bmal1 and Per1 were expressed in beta cells. Despite normal activity and feeding behaviour, mutant mice lacking clock function in the pancreas had severe glucose intolerance and defective insulin production; their isolated pancreatic islets had defective glucose-stimulated insulin secretion, but normal total insulin content.

Conclusions/interpretation: The mouse pancreas has an autonomous clock function and beta cells are very likely to be one of the pancreatic cell types possessing an intrinsic clock. The Bmal1 circadian clock gene is required in the pancreas, probably in beta cells, for normal insulin secretion and glucose homeostasis. Our results provide evidence for a previously unrecognised molecular regulator of pancreatic glucose-sensing and/or insulin secretion.

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Figures

Fig. 1

Fig. 1

Glucose intolerance and defective insulin secretion in mice selectively lacking an intrinsic circadian clock of the pancreas. a Presence of an autonomous circadian clock in the pancreas. Real-time bioluminescence output of an explanted pancreas from a Bmal1_-Luc circadian reporter mouse [7]. b Combined immunohistochemistry and fluorescence in situ hybridisation to pancreas sections showing co-presence of insulin protein (red) and circadian clock component mRNA Per1 and Bmal1 as labelled (both green) in pancreatic islets. Punctate appearance of fluorescence signal for mRNAs is typical. Hoechst stain (blue) shows positions of cell nuclei. Arrows, double-labelled cells. c Glucose tolerance test performed at Zeitgeber time (ZT) 4.5 h in mice selectively lacking circadian clocks in the pancreas (Panc_-Bmal1 −/−; C57BL/6-129 hybrid) and littermate control genotypes. Mice used: Bmal1 lox/lox, Bmal1 conditional allele homozygote without Cre; Pdx1-Cre, single copy of Cre transgene and wild-type Bmal1. p < 0.001 (ANOVA). d Insulin tolerance tests, as above (c). No significant difference between genotypes (ANOVA). e Serum insulin concentrations at the indicated times after glucose administration. p < 0.02 (ANOVA). f Circadian profile of glucose tolerance for the three genotypes (n = 9–10 per genotype). g Quantification of circadian glucose tolerance curves shown above (f). CT, circadian time for all groups as for dark grey bars. c–g Values are mean±SEM. Light grey, Pdx1-Cre; dark grey, Bmal1 lox/lox; black, Panc-Bmal1 −/−

Fig. 2

Fig. 2

Defective glucose-stimulated insulin secretion from pancreatic islets of mice selectively lacking an intrinsic circadian clock of the pancreas. a Normal general appearance of pancreatic islets in Panc-Bmal1 −/− mice vs Pdx-Cre. Immunofluorescence micrographs of pancreas sections were stained for insulin. b–d No significant differences were detected between Panc-Bmal1 −/− mice and littermate controls in variables reflecting the number, size or insulin expression of pancreatic islets (Student’s t test). e Defective insulin secretion from isolated islets of Panc-Bmal1 −/− mice under low and high glucose conditions, † p = 0.03, ‡ p = 0.002 (Student’s t test). f No significant difference in total insulin content of isolated islets from Panc-Bmal1 −/− mice and control littermates (Student’s t test). b–f Values are mean±SEM. Light grey, Pdx1-Cre; black, Panc-Bmal1 −/−

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