Circadian Secretion of Glucagon-like Peptide-1

Circadian rhythms are 24-hour biological rhythms within organisms that have developed over evolutionary time due to pre-defined environmental changes, mainly the light-dark cycle. Interestingly, metabolic tissues, which have been found to express cell-autonomous clocks that are entrained by food intake, are largely responsible for establishing diurnal metabolic homeostasis. The incretin hormone glucagon-like peptide-1 (GLP-1) enhances glucose-stimulated insulin secretion following nutrient ingestion and has recently been shown to follow a circadian rhythm of secretion that parallels the expression of the core clock gene, Bmal1. However, the mechanisms by which the GLP-1 secretory rhythm is established and its role in maintaining diurnal glycemic homeostasis under normal and obesogenic conditions remain largely elusive. Thus, I hypothesized that western diet (WD)-feeding disrupts L cell clock (Bmal1) function thereby impairing rhythmic GLP-1 secretion and diurnal glycemic homeostasis. Use of Bmal1 knockout mice and ex vivo cultures demonstrated the essential role for the clock (Bmal1) in regulating GLP-1 secretion. Mass spectrometry analysis of murine (m) GLUTag L cells identified the SNARE-accessory protein STXBP-1 to be increased at peak GLP-1 secretion, potentially as a result of time-dependent binding of BMAL1 to its promoter region. Metabolic testing in normal and WD-fed mice throughout the 24-hour day demonstrated that insulin secretion is rhythmic and glycemia is normal in response to oral, and not intraperitoneal, glucose administration, implicating an important role for the incretin hormones. Identical injections of the incretin hormones at different timepoints revealed differential insulin responses in normal mice, while WD-fed mice maintained normoglycemia only upon GLP-1, and not glucose-dependent insulinotropic polypeptide, administration, suggesting GLP-1 is the key incretin in WD-conditions. Finally, isolation of primary L cells established time-of-day differences in primary L cell clock gene expression that were dampened by WD-feeding and studies in mGLUTag L cells demonstrated that the saturated fatty acid palmitate, a major component of the WD, impairs L cell BMAL1 expression and mitochondrial function, ultimately resulting in suppression of GLP-1 secretion. In combination, these studies provide novel insight into the regulation and role of circadian GLP-1 secretion that can, ultimately, help in the development of novel therapeutics for the treatment of metabolic disease.Ph.D