Functional role of circadian clock system in steroidogenesis and cell death pathways during corpus luteum regression...

The corpus luteum (CL) is an ovarian structure that secretes progesterone (P4) following ovulation, playing a crucial role in regulating the estrous cycle and maintaining pregnancy. Luteolysis, the structural and functional degradation of the CL, occurs through apoptosis and autophagy. Recent studies suggest that the circadian clock (CC) system, particularly the gene NR1D1, is involved in these processes. This study investigated the role of NR1D1 in bovine CL regression using an ex vivo model treated with prostaglandin F_2α (PGF_2α), the NR1D1 agonist GSK4112, and the antagonist SR8278. CL samples were classified into four estrous cycle stages based on ovarian morphology and analyzed for P4 secretion, as well as gene and protein expression related to steroid synthesis, the CC system, autophagy, and apoptosis. P4 levels, steroid synthesis-related genes, and CC system-related genes, including NR1D1, were highly expressed in the mid and late stages of the CL, whereas autophagy- and apoptosis-related genes peaked during regression. Western blotting and immunofluorescence revealed increased expression of NR1D1 and BMAL1 in the mid and late stages, while LC3 and CTSB were most prominent during regression. PGF_2α treatment reduced NR1D1 and BMAL1 expression, along with decreased P4 levels and increased apoptosis markers. GSK4112 suppressed steroid synthesis while upregulating autophagy- and apoptosis-related genes. Conversely, SR8278 reversed PGF_2α-induced luteal regression, restoring P4 and steroidogenic gene expression while suppressing CTSB. These findings suggest that NR1D1 interacts with PGF_2α to regulate CL regression, highlighting the CC system as a potential target for improving reproductive efficiency in cattle.