Diurnal changes in the murine small intestine are disrupted by obesogenic Western Diet feeding and microbial dysbiosis.

Intestinal functions demonstrate circadian rhythms thought to be entrained, in part, by an organisms' intrinsic feeding and fasting periods as well as by the intestinal microbiome. Circadian disruption as a result of ill-timed nutrient exposure and obesogenic feeding poses an increased risk to disease. As such, the aim of this study was to assess the relationships between dietary timing, composition, and the microbiome with regard to rhythmic small intestinal structure and mucosal immunity. Rodent chow (RC)-mice exhibited time-dependent increases in small intestinal weight, villus height, and crypt depth as well as an increased proportion of CD8αα+ cells and concomitant decrease in CD8αß+ cells at the onset of the feeding period (p < 0.05-0.001). Western diet (WD)-animals displayed disrupted time-dependent patterns in intestinal structure and lymphocyte populations (p < 0.05-0.01). Antibiotic-induced microbial depletion abrogated the time- and diet-dependent patterns in both RC- and WD-mice (p < 0.05-0.001). However, although germ-free-mice displayed altered rhythms, fecal microbial transfer from RC-mice was generally unsuccessful in restoring structural and immune changes in these animals. This study shows that adaptive changes in the small intestine at the onset of the feeding and fasting periods are disrupted by WD-feeding, and that these changes are dependent, in part, on the intestinal microbiome.

Circadian GLP-1 Secretion in Mice Is Dependent on the Intestinal Microbiome for Maintenance of Diurnal Metabolic Homeostasis.

The incretin glucagon-like peptide 1 (GLP-1) is secreted by the intestinal L cell upon nutrient ingestion. GLP-1 also exhibits a circadian rhythm, with highest release at the onset of the feeding period. Similarly, microbial composition and function exhibit circadian rhythmicity with fasting-feeding. The circadian pattern of GLP-1 release was found to be dependent on the oral route of glucose administration and was necessary for the rhythmic release of insulin and diurnal glycemic control in normal male and female mice. In mice fed a Western (high-fat/high-sucrose) diet for 16 weeks, GLP-1 secretion was markedly increased but arrhythmic over the 24-h day, whereas levels of the other incretin, glucose-dependent insulinotropic polypeptide, were not as profoundly affected. Furthermore, the changes in GLP-1 secretion were shown to be essential for the maintenance of normoglycemia in this obesogenic environment. Analysis of the primary L-cell transcriptome, as well as of the intestinal microbiome, also demonstrated time-of-day- and diet-dependent changes paralleling GLP-1 secretion. Finally, studies in antibiotic-induced microbial depleted and in germ-free mice with and without fecal microbial transfer, provided evidence for a role of the microbiome in diurnal GLP-1 release. In combination, these findings establish a key role for microbiome-dependent circadian GLP-1 secretion in the maintenance of 24-h metabolic homeostasis.

Essential Role of Syntaxin-Binding Protein-1 in the Regulation of Glucagon-Like Peptide-1 Secretion.

Circadian secretion of the incretin, glucagon-like peptide-1 (GLP-1), correlates with expression of the core clock gene, Bmal1, in the intestinal L-cell. Several SNARE proteins known to be circadian in pancreatic α- and ß-cells are also necessary for GLP-1 secretion. However, the role of the accessory SNARE, Syntaxin binding protein-1 (Stxbp1; also known as Munc18-1) in the L-cell is unknown. The aim of this study was to determine whether Stxbp1 is under circadian regulation in the L-cell and its role in the control of GLP-1 secretion. Stxbp1 was highly-enriched in L-cells, and STXBP1 was expressed in a subpopulation of L-cells in mouse and human intestinal sections. Stxbp1 transcripts and protein displayed circadian patterns in mGLUTag L-cells line, while chromatin-immunoprecipitation revealed increased interaction between BMAL1 and Stxbp1 at the peak time-point of the circadian pattern. STXBP1 recruitment to the cytosol and plasma membrane within 30 minutes of L-cell stimulation was also observed at this time-point. Loss of Stxbp1 in vitro and in vivo led to reduced stimulated GLP-1 secretion at the peak time-point of circadian release, and impaired GLP-1 secretion ex vivo. In conclusion, Stxbp1 is a circadian regulated exocytotic protein in the intestinal L-cell that is an essential regulatory component of GLP-1 secretion.