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      Circadian Rhythms Within the Female HPG Axis: From Physiology to Etiology

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          Abstract

          Declining female fertility has become a global health concern. It results partially from an abnormal circadian clock caused by unhealthy diet and sleep habits in modern life. The circadian clock system is a hierarchical network consisting of central and peripheral clocks. It not only controls the sleep–wake and feeding–fasting cycles but also coordinates and maintains the required reproductive activities in the body. Physiologically, the reproductive processes are governed by the hypothalamic–pituitary–gonadal (HPG) axis in a time-dependent manner. The HPG axis releases hormones, generates female characteristics, and achieves fertility. Conversely, an abnormal daily rhythm caused by aberrant clock genes or abnormal environmental stimuli contributes to disorders of the female reproductive system, such as polycystic ovarian syndrome and premature ovarian insufficiency. Therefore, breaking the “time code” of the female reproductive system is crucial. In this paper, we review the interplay between circadian clocks and the female reproductive system and present its regulatory principles, moving from normal physiology regulation to disease etiology.

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          Most cited references 125

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          The orphan nuclear receptor REV-ERBalpha controls circadian transcription within the positive limb of the mammalian circadian oscillator.

          Mammalian circadian rhythms are generated by a feedback loop in which BMAL1 and CLOCK, players of the positive limb, activate transcription of the cryptochrome and period genes, components of the negative limb. Bmal1 and Per transcription cycles display nearly opposite phases and are thus governed by different mechanisms. Here, we identify the orphan nuclear receptor REV-ERBalpha as the major regulator of cyclic Bmal1 transcription. Circadian Rev-erbalpha expression is controlled by components of the general feedback loop. Thus, REV-ERBalpha constitutes a molecular link through which components of the negative limb drive antiphasic expression of components of the positive limb. While REV-ERBalpha influences the period length and affects the phase-shifting properties of the clock, it is not required for circadian rhythm generation.
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            A functional genomics strategy reveals Rora as a component of the mammalian circadian clock.

            The mammalian circadian clock plays an integral role in timing rhythmic physiology and behavior, such as locomotor activity, with anticipated daily environmental changes. The master oscillator resides within the suprachiasmatic nucleus (SCN), which can maintain circadian rhythms in the absence of synchronizing light input. Here, we describe a genomics-based approach to identify circadian activators of Bmal1, itself a key transcriptional activator that is necessary for core oscillator function. Using cell-based functional assays, as well as behavioral and molecular analyses, we identified Rora as an activator of Bmal1 transcription within the SCN. Rora is required for normal Bmal1 expression and consolidation of daily locomotor activity and is regulated by the core clock in the SCN. These results suggest that opposing activities of the orphan nuclear receptors Rora and Rev-erb alpha, which represses Bmal1 expression, are important in the maintenance of circadian clock function.
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              Kisspeptin directly stimulates gonadotropin-releasing hormone release via G protein-coupled receptor 54.

              We have recently described a molecular gatekeeper of the hypothalamic-pituitary-gonadal axis with the observation that G protein-coupled receptor 54 (GPR54) is required in mice and men for the pubertal onset of pulsatile luteinizing hormone (LH) and follicle-stimulating hormone (FSH) secretion to occur. In the present study, we investigate the possible central mode of action of GPR54 and kisspeptin ligand. First, we show that GPR54 transcripts are colocalized with gonadotropin-releasing hormone (GnRH) neurons in the mouse hypothalamus, suggesting that kisspeptin, the GPR54 ligand, may act directly on these neurons. Next, we show that GnRH neurons seem anatomically normal in gpr54-/- mice, and that they show projections to the median eminence, which demonstrates that the hypogonadism in gpr54-/- mice is not due to an abnormal migration of GnRH neurons (as occurs with KAL1 mutations), but that it is more likely due to a lack of GnRH release or absence of GnRH neuron stimulation. We also show that levels of kisspeptin injected i.p., which stimulate robust LH and FSH release in wild-type mice, have no effect in gpr54-/- mice, and therefore that kisspeptin acts directly and uniquely by means of GPR54 signaling for this function. Finally, we demonstrate by direct measurement, that the central administration of kisspeptin intracerebroventricularly in sheep produces a dramatic release of GnRH into the cerebrospinal fluid, with a parallel rise in serum LH, demonstrating that a key action of kisspeptin on the hypothalamo-pituitary-gonadal axis occurs directly at the level of GnRH release. The localization and GnRH release effects of kisspeptin thus define GPR54 as a major control point in the reproductive axis and suggest kisspeptin to be a neurohormonal effector.
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                Author and article information

                Journal
                Endocrinology
                Endocrinology
                endo
                Endocrinology
                Oxford University Press (US )
                0013-7227
                1945-7170
                August 2021
                14 June 2021
                14 June 2021
                : 162
                : 8
                Affiliations
                [1 ] Obstetrics and Gynecology Hospital, Fudan University , Shanghai, 200011, China
                [2 ] The Shanghai Key Laboratory of Female Reproductive Endocrine-Related Diseases , Shanghai, 200011, China
                Author notes
                Correspondence: Dr. Ying Zhang, Obstetrics and Gynecology Hospital of Fudan University, Fangxie Road 419, Huangpu District, Shanghai, 200011, China. Email: 13818218839@ 123456163.com .

                These authors contributed equally to this work.

                Article
                bqab117
                10.1210/endocr/bqab117
                8256628
                34125877
                © The Author(s) 2021. Published by Oxford University Press on behalf of the Endocrine Society.

                This is an Open Access article distributed under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs licence ( http://creativecommons.org/licenses/by-nc-nd/4.0/), which permits non-commercial reproduction and distribution of the work, in any medium, provided the original work is not altered or transformed in any way, and that the work is properly cited. For commercial re-use, please contact journals.permissions@oup.com

                Page count
                Pages: 12
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                Categories
                Mini-Reviews
                AcademicSubjects/MED00250

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