Figure 4.

The intestinal epithelium displays a BMAL1-dependent circadian rhythm during normal homeostasis and during regeneration. (A) The BMAL1^+/+ intestine of animals on a standard 12-hour light/12-hour dark photoperiod shows rhythms in clock gene expression: PER2 (F = 7.267; P = .0055), BMAL1 (F = 30.67; P < .0001), and REV-ERBα (F = 18.33; P = .0002). BMAL1^-/- animals do not show these rhythms: PER2 (P = .7839), BMAL1 (P = .6397), and REV-ERBα (P = .6880). Expression of the gene TBP (TATA Binding Protein) has no rhythm in either genotype. There are significant differences between genotypes in PER2 (F = 3.41; P = .0242), BMAL1 (F = 219.0; P < .0001), and REV-ERBα (F = 87.27; P < .0001). (B) PER2 protein is present in the undamaged intestinal precursors (crypts are outlined to indicate the region where precursors are located). PER2 is present in higher levels at ZT16–20, and shows nuclear localization in the epithelial cells of the crypt at these times. At ZT0–8, expression is weaker and predominantly cytoplasmic. Images show single confocal sections of intestinal crypts. (C) The 24-hour rhythm in clock gene expression 4 days after irradiation is similar to the undamaged intestine, but at lower levels relative to GAPDH. The gene TBP is not affected by the clock, or stress, and has no circadian rhythm. There are significant differences between genotypes in PER2 (F = 120.8; P < .0001), BMAL1 (F = 225.7; P < .0001), and REV-ERBα (F = 74.65; P < .0001). IR, irradiation.