Liver receptor homolog 1 (LRH-1) regulates follicle vasculature during ovulation in mice.

In brief: It is well-established that liver receptor homolog 1 (LRH-1/NR5A2) regulates the ovarian function and is required for ovulation and luteinization in mice. In the present experiment, we showed that LRH-1 is required to control vascular changes during ovulation, a novel mechanism of action of this orphan nuclear receptor. Abstract: Liver receptor homolog 1 (LRH-1/NR5A2) is a key regulator of ovarian function, and recently, it has been suggested that it may regulate changes in follicular angiogenesis, an important event during the ovulatory process and luteal development. In the present experiment, the objective was to determine whether conditional depletion of LRH-1 in mice granulosa cells modified vascular changes during the periovulatory period and to explore the possible mechanisms of this modification. We generated mice (22- to 25-day-old) with specific depletion of LRH-1 in granulosa cells by crossing Lrh1 floxed (Lrh1 f/f) mice with mice expressing Cre-recombinase driven by the anti-Müllerian type II receptor (Amhr2-cre; conditional knockout or cKO mice). We showed that preovulatory follicles of LRH-1 cKO mice had a reduced number of endothelial cells in the theca cell layer at 8 h after human chorionic gonadotropin treatment compared with control (CON) mice. Additionally, mRNA and protein expression of leptin receptor (LEPR), a protein that stimulates angiogenesis in a vascular endothelial growth factor-A (VEGFA)-dependent manner, and teratocarcinoma-derived growth factor-1 (TDGF1), which may directly stimulate endothelial cell function, were reduced in LRH-1 cKO mice as compared to CON after the LH surge. These results showed that LRH-1 is necessary for the correct vascular changes that accompany ovulation in mice and that this effect may be regulated through VEGFA-dependent and VEGFA-independent pathways mediated by LEPR and TDGF1.

Orphan nuclear receptors in angiogenesis and follicular development.

Orphan nuclear receptors (ONRs) are a subset of the nuclear receptor family that lacks known endogenous ligands. Among 48 nuclear receptors identified in humans, 25 are classified as ONRs. They function as transcription factors and control the expression of a wide range of genes to regulate metabolism, fertility, immunity, angiogenesis, and many other functions. Angiogenic factors are essential during ovarian follicle development, including follicle growth and ovulation. The correct development of blood vessels contributes to preantral and antral follicular development, selection of the dominant follicle or follicles, follicular atresia, and ovulation. Although progress has been made in understanding the molecular mechanisms that regulate follicular angiogenesis, the role of ONRs as regulators is not clear. Based on their functions in other tissues, the ONRs NR1D1 (REV-ERBß), NR2C2 (TR4), NR2F2 (COUP-TF-II) and NR3B1, 2, and 3 (ERRα, ERRß and ERRγ) may modulate angiogenesis during antral follicle development. We hypothesize that this is achieved by effects on the expression and function of VEGFA, ANGPT1, THBS1, and soluble VEGFR1. Further, angiogenesis during ovulation is expected to be influenced by ONRs. NR5A2 (LRH-1), which is required for ovulation, regulates angiogenic genes in the ovary, including VEGFA and the upstream regulator of angiogenesis, PGE2. These angiogenic molecules may also be regulated by NR5A1 (SF-1). Evidence from outside the reproductive tract suggests that NR2F2 and NR4A1(NUR77) promote VEGFC and PGF, respectively, and NR4As (NUR77, NOR1) seem to be necessary for the angiogenic effects of VEGFA and PGE2. Together, the data suggest that ONRs are important regulators of follicular angiogenesis.

Nuclear receptors: Key regulators of somatic cell functions in the ovulatory process.

The development of the ovarian follicle to its culmination by ovulation is an essential element of fertility. The final stages of ovarian follicular growth are characterized by granulosa cell proliferation and differentiation, and steroid synthesis under the influence of follicle-stimulating hormone (FSH). The result is a population of granulosa cells poised to respond to the ovulatory surge of luteinizing hormone (LH). Members of the nuclear receptor superfamily of transcription factors play indispensable roles in the regulation of these events. The key regulators of the final stages of follicular growth that precede ovulation from this family include the estrogen receptor beta (ESR2) and the androgen receptor (AR), with additional roles for others, including steroidogenic factor-1 (SF-1) and liver receptor homolog-1 (LRH-1). Following the LH surge, the mural and cumulus granulosa cells undergo rapid changes that result in expansion of the cumulus layer, and a shift in ovarian steroid hormone biosynthesis from estradiol to progesterone production. The nuclear receptor best associated with these events is LRH-1. Inadequate cumulus expansion is also observed in the absence of AR and ESR2, but not the progesterone receptor (PGR). The terminal stages of ovulation are regulated by PGR, which increases the abundance of the proteases that are directly responsible for rupture. It further regulates the prostaglandins and cytokines associated with the inflammatory-like characteristics of ovulation. LRH-1 regulates PGR, and is also a key regulator of steroidogenesis, cellular proliferation, and cellular migration, and cytoskeletal remodeling. In summary, nuclear receptors are among the panoply of transcriptional regulators with roles in ovulation, and several are necessary for normal ovarian function.