The circadian regulation of pancreatic β-cell function and identity is critical for the maintenance of glucose homeostasis. The circadian clock is entrained by rhythmic environment cues (e.g., light/dark and feeding/fasting cycles) that coordinate expression of the molecular circadian machinery. Nevertheless, the relative contribution of the aforementioned components in maintaining circadian β-cell identity remains unclear. Therefore, we set out to test the hypothesis that the molecular clockwork is required for the circadian regulation of β-cell transcriptional and epigenetic identity. To address this question, we performed diurnal RNA sequencing (RNA-seq) and assay for transposase-accessible chromatin using sequencing (ATAC-seq) in pancreatic islets isolated from adult wild type (WT) and β-cell-specific Bmal1 (requisite circadian clock transcription factor) deficient (β-Bmal1-/-) mice. Time-dependent RNA-seq of β-Bmal1-/- islets revealed >95% decrease in the number of rhythmic transcripts compared to WT islets. Of the remaining rhythmic genes in β-Bmal1-/- islets, they predominately encoded for GO pathways regulating protein folding (FDR<.05). Correspondingly, ATAC-seq analysis found that there was >50% decrease in rhythmic chromatin accessibility in β-Bmal1-/- islets compared to WT. In line with transcriptomic findings, the remaining rhythmically accessible regions in β-Bmal1-/- islets annotated to regions regulating protein folding and insulin processing (FDR<.05). Interestingly, these regions were enriched for transcription factor motifs critical for insulin processing and biosynthesis including RFX and FOXA1. This study suggests that although the circadian clockwork is critical for entrainment of circadian β-cell identity, environmental stimuli (e.g., feeding) may maintain rhythmicity of insulin biosynthesis and protein folding in lieu of the circadian clock.
M. Brown: None. A. Matveyenko: None.
National Institutes of Health (DK098468 to A.M.), (DK123834 to M.B.)