NCBI Summary:
The protein encoded by this gene plays a central role in the regulation of circadian rhythms. The protein encodes a transcription factor of the basic helix-loop-helix (bHLH) family and contains DNA binding histone acetyltransferase activity. The encoded protein forms a heterodimer with ARNTL (BMAL1) that binds E-box enhancer elements upstream of Period (PER1, PER2, PER3) and Cryptochrome (CRY1, CRY2) genes and activates transcription of these genes. PER and CRY proteins heterodimerize and repress their own transcription by interacting in a feedback loop with CLOCK/ARNTL complexes. Polymorphisms in this gene may be associated with behavioral changes in certain populations and with obesity and metabolic syndrome. Alternative splicing results in multiple transcript variants. [provided by RefSeq, Jan 2014]
General function
DNA binding, Transcription factor
Comment
Circadian clocks in the ovary. Sellix MT et al. Clock gene expression has been observed in tissues of the hypothalamic-pituitary-gonadal (HPG) axis. Whereas the contribution of hypothalamic oscillators to the timing of reproductive biology is well known, the role of peripheral oscillators like those in the ovary is less clear. Circadian clocks in the ovary might play a role in the timing of ovulation. Disruption of the clock in ovarian cells or desynchrony between ovarian clocks and circadian oscillators elsewhere in the body might contribute to the onset and progression of various reproductive pathologies. In this paper, we review evidence for clock function in the ovary across a number of species and offer a novel perspective into the role of this clock in normal ovarian physiology and in diseases that negatively affect fertility.
Cellular localization
Nuclear
Comment
Ovarian function
Ovulation, Steroid metabolism
Comment
Circadian Clock genes Per2 and clock regulate steroid production, cell proliferation, and luteinizing hormone receptor transcription in ovarian granulosa cells. Shimizu T et al. Circadian Clock genes are associated with the estrous cycle in female animals. Treatment with Per2 and Clock siRNAs decreased the number of granulosa cells and LHr expression in follicle-stimulating hormone FSH-treated granulosa cells. Per2 siRNA treatment did not stimulate the production of estradiol and expression of P450arom, whereas Clock siRNA treatment inhibited the production of estradiol and expression of P450arom mRNA. Per2 and Clock siRNA treatment increased and unchanged, respectively, progesterone production in FSH-treated granulosa cells. Similarly, expression of StAR mRNA was increased by Per2 siRNA and unchanged by Clock siRNA. Our data provide a new insight that Per2 and Clock have different action on ovarian granulosa cell functions.
A circadian egg timer gates ovulation.
Sellix MT, Yoshikawa T, Menaker M.
Since the pioneering work of Everett and Sawyer, the idea that pituitary gonadotrophins provide the critical timing cue for ovulation has remained unquestioned . It is widely accepted that the timing of ovulation depends entirely on the timing of luteinizing hormone (LH) secretion, itself driven by neuroendocrine releasing factors controlled by the circadian clock in the suprachiasmatic nucleus (SCN) (2,3). As a consequence, there has been little investigation of a role for the ovary in this process. However, we and others have demonstrated the presence of endogenous circadian clocks in the rat ovary (4,5,6). Here we describe a circadian rhythm of ovarian sensitivity to LH that determines the ovulatory response to gonadotrophins. It is plausible that the circadian clock in the ovary may set the responsiveness of the ovarian follicle to the LH surge. Our results significantly alter the classic view that gonadotrophins provide the only timing cue for ovulation. They suggest that the ovary itself plays a major role in the process and provide a new perspective that will inform future research on infertility and ovarian physiology. PMID: 20334830
Circadian clock gene regulation of steroidogenic acute regulatory protein gene expression in pre-ovulatory ovarian follicles. Nakao N et al. It is now known that circadian clocks are localized not only in the central pacemaker but also in peripheral organs. An example of a clock-dependent peripheral organ is the ovary of domestic poultry where ovulation is induced by the positive feed-back action of ovarian progesterone on the neuroendocrine system to generate a pre-ovulatory release of LH during a daily 6-10 hr 'open period' of the ovulatory cycle. It has previously been assumed that the timing of ovulation in poultry is controlled solely by a clock-dependent mechanism within neuroendocrine system. Here, we question this assumption by demonstrating the expression of the clock genes, Per2 and 3, Clock and Bmal1 in pre-ovulatory follicles in laying quail. Diurnal changes in Per2 and 3 expression were seen in the largest pre-ovulatory follicle (F1), but not in smaller follicles. We next sought to identify clock-driven genes in pre-ovulatory follicles focusing on those involved in the synthesis of progesterone. One such gene was identified, encoding steroidogenic acute regulatory protein (StAR), which showed 24 hr changes in expression in the F1 follicle coinciding with those of Per2. Evidence that StAR gene expression is clock-driven was obtained by showing that its 5' flanking region contains E-box enhancers, which bind to CLOCK/BMAL1 heterodimers to activate gene transcription. We also showed that LH administration increased the promoter activity of chicken StAR. We therefore suggest that the timing of ovulation in poultry involves a LH- responsive F1 follicular clock that is involved in the timing of the pre-ovulatory release of progesterone.
Expression regulated by
LH, Steroids
Comment
Expression pattern of circadian genes and steroidogenesis-related genes after testosterone stimulation in the human ovary. Chen M et al. (2016) Previous studies have shown that circadian genes might be involved in the development of polycystic ovarian syndrome (PCOS). Hyperandrogenism is a hallmark feature of PCOS. However, the effect of hyperandrogenism on circadian gene expression in human granulosa cells is unknown, and the general expression pattern of circadian genes in the human ovary is unclear. Expression of the circadian proteins CLOCK and PER2 in human ovaries was observed by immunohistochemistry. The mRNA expression patterns of the circadian genes CLOCK, PER2, and BMAL1, and the steroidogenesis-related genes STAR, CYP11A1, HSD3B2, and CYP19A1 in cultured human luteinized granulosa cells were analyzed over the course of 48 h after testosterone treatment by quantitative polymerase chain reaction. Immunostaining of CLOCK and PER2 protein was detected in the granulosa cells of dominant antral follicles but was absent in the primordial, primary, or preantral follicles of human ovaries. After testosterone stimulation, expression of PER2 showed an oscillating pattern, with two peaks occurring at the 24th and 44th hours; expression of CLOCK increased significantly to the peak at the 24th hour, whereas expression of BMAL1 did not change significantly over time in human luteinized granulosa cells. Among the four steroidogenesis-related genes evaluated, only STAR displayed an oscillating expression pattern with two peaks occurring at the 24th and 40th hours after testosterone stimulation. Circadian genes are expressed in the dominant antral follicles of the human ovary. Oscillating expression of the circadian gene PER2 can be induced by testosterone in human granulosa cells in vitro. Expression of STAR also displayed an oscillating pattern after testosterone stimulation. Our results indicate a potential relationship between the circadian clock and steroidogenesis in the human ovary, and demonstrate the effect of testosterone on circadian gene expression in granulosa cells.//////////////////
Circadian Clock Gene Expression in the Ovary: Effects of Luteinizing Hormone. Karman BN et al. A molecular device that measures time on a daily, or circadian, scale is a nearly ubiquitous feature of eukaryotic organisms. A core group of clock genes, whose coordinated function is required for this timekeeping, is expressed both in the central clock and within numerous peripheral organs. We examined expression of clock genes in the rat ovary. Transcripts for core oscillator elements (Arntl, Clock, Per1, Per2, Cry1) were present in the ovary as indicated by quantitative real-time RT-PCR. Rhythmic expression patterns of Arntl and Per2 transcripts and protein products were out-of-phase with respect to the central oscillator and in complete anti-phase to each other. Expression of Arntl was significantly elevated after the LH surge on the day of proestrus. Finally, human chorionic gonadotropin (hCG) treatment induced cyclic expression of both Arntl and Per2 gene products in hypophysectomized, immature rats primed with pregnant mares serum gonadotropin (eCG). Collectively, these data suggest that the core underpinnings of the transcriptional/translational feedback loop that drives circadian rhythmicity is present in the rat ovary. Furthermore, the study identifies luteinizing hormone (LH) as a potential regulator of circadian clock gene rhythms in the ovary.
Ovarian localization
Oocyte, Granulosa
Comment
Is the aging human ovary still ticking?: Expression of clock-genes in luteinized granulosa cells of young and older women. Brzezinski A et al. (2018) It has been shown - mostly in animal models - that circadian clock genes are expressed in granulosa cells and in corpora luteum and might be essential for the ovulatory process and steroidogenesis. We sought to investigate which circadian clock genes exist in human granulosa cells and whether their expression and activity decrease during aging of the ovary. Human luteinized granulosa cells were isolated from young (age 18-33) and older (age 39-45) patients who underwent in-vitro fertilization treatment. Levels of clock genes expression were measured in these cells 36 h after human chorionic gonadotropin stimulation. Human luteinized granulosa cells were isolated from follicular fluid during oocyte retrieval. The mRNA expression levels of the circadian genes CRY1, CRY2, PER1, PER2, CLOCK, ARNTL, ARNTL2, and NPAS2 were analyzed by quantitative polymerase chain reaction. We found that the circadian genes CRY1, CRY2, PER1, PER2, CLOCK, ARNTL, ARNTL2, and NPAS2, are expressed in cultured human luteinized granulosa cells. Among these genes, there was a general trend of decreased expression in cells from older women but it reached statistical significance only for PER1 and CLOCK genes (fold change of 0.27 ± 0.14; p = 0.03 and 0.29 ± 0.16; p = 0.05, respectively). This preliminary report indicates that molecular circadian clock genes exist in human luteinized granulosa cells. There is a decreased expression of some of these genes in older women. This decline may partially explain the decreased fertility and steroidogenesis of reproductive aging.//////////////////
Expression analysis of circadian genes in oocytes and preimplantation embryos of cattle and rabbits. Amano T et al. We previously showed that circadian genes clock, bmal1, cry1, cry2, per1, and per2 are expressed and function as maternal mRNA regulating events in the oocytes and preimplantation embryos of mice. Recent evidence indicates however that either or both expression profiles of circadian genes in some tissues, and transcript sequences of circadian genes, differ to generate the physiological differences between diurnal and nocturnal species. We therefore investigated the expression profiles of circadian genes in oocytes and preimplantation embryos of species other than mice, namely cattle and rabbits, representing diurnal and nocturnal species, respectively, and determined the protein sequences of circadian genes in these species. Quantitative real-time PCR revealed that all circadian genes considered in this study were present in the oocytes and preimplantation embryos of both species, and the transcript amounts of clock, cry1 and per1 contained in oocytes were significantly higher than in preimplantation embryos of both species. The transcripts of clock, cry1, and per1 of cattle and rabbits were determined by primer walking, and functional domains in the estimated amino acid sequences were compared between cattle and rabbits and with those of humans and mice. The sequences of clock, cry1, and per1 in cattle and rabbits closely resembled those in mice (85-100% homologies), and no difference based on diurnality or nocturnality was observed. These findings suggest that circadian genes in the oocytes and preimplantation embryos of mammals fulfill the same functions across species as maternal mRNA.