The earliest stages of follicular development: follicle formation and activation

Pituitary P62 deficiency leads to female infertility by impairing luteinizing hormone production

P62 is a protein adaptor for various metabolic processes. Mice that lack p62 develop adult-onset obesity. However, investigations on p62 in reproductive dysfunction are rare. In the present study, we explored the effect of p62 on the reproductive system. P62 deficiency-induced reproductive dysfunction occurred at a young age (8 week old). Young systemic p62 knockout (p62-/-) and pituitary-specific p62 knockout (p62flox/flox αGSUcre) mice both presented a normal metabolic state, whereas they displayed infertility phenotypes (attenuated breeding success rates, impaired folliculogenesis and ovulation, etc.) with decreased luteinizing hormone (LH) expression and production. Consistently, in an infertility model of polycystic ovary syndrome (PCOS), pituitary p62 mRNA was positively correlated with LH levels. Mechanistically, p62-/- pituitary RNA sequencing showed a significant downregulation of the mitochondrial oxidative phosphorylation (OXPHOS) pathway. In vitro experiments using the pituitary gonadotroph cell line LβT2 and siRNA/shRNA/plasmid confirmed that p62 modulated LH synthesis and secretion via mitochondrial OXPHOS function, especially Ndufa2, a component molecule of mitochondrial complex I, as verified by Seahorse and rescue tests. After screening OXPHOS markers, Ndufa2 was found to positively regulate LH production in LβT2 cells. Furthermore, the gonadotropin-releasing hormone (GnRH)-stimulating test in p62flox/flox αGSUcre mice and LβT2 cells illustrated that p62 is a modulator of the GnRH-LH axis, which is dependent on intracellular calcium and ATP. These findings demonstrated that p62 deficiency in the pituitary impaired LH production via mitochondrial OXPHOS signaling and led to female infertility, thus providing the GnRH-p62-OXPHOS(Ndufa2)-Ca2+/ATP-LH pathway in gonadotropic cells as a new theoretical basis for investigating female reproductive dysfunction.

Histomorphology and gene expression profiles during early ovarian folliculogenesis in duck and goose

In contrast to the later stages of follicle development, little is known about the characteristics and mechanisms associated with early folliculogenesis in avian species. The objectives of the present study were to examine and compare the histomorphological and molecular changes of primordial, primary, and secondary follicles from duck and goose ovaries during the first 6 post-hatching week. Morphological analysis showed that the length and width of both duck and goose ovaries increased steadily during weeks 1 to 5 but increased acutely at week 6, whereas a greater increment was observed in the ovarian length of ducks than that of geese during weeks 4 to 5. Furthermore, smaller diameters of the 3 categories of follicles were observed in ducks than those in geese at the first appearance, but they reached a similar size at week 6. More importantly, secondary follicles were found in the ovaries of ducks 1 wk earlier than in those of geese. These results indicated a more rapid growth rate for ovarian follicles in ducks than in geese during early post-hatching development. At the molecular level, it was found that the mRNAs encoding follicle stimulating hormone receptor (FSHR), anti-Müllerian hormone (AMH), B-cell leukemia/lymphoma 2, and cysteine-dependent aspartate specific protease 3 (CASPASE3) were ubiquitously expressed in all ovarian follicles of ducks and geese with different expression profiles in each follicular category during the first 6 post-hatching week. Notably, transcript levels of FSHR, AMH, and CASPASE3 changed differently between ducks and geese during weeks 5 to 6, which was postulated to be one of the mechanisms inducing more rapid growth of ovarian follicles in ducks rather than in geese. In conclusion, our results revealed, for the first time, differences in early folliculogenesis, including the rate of growth of each follicular category and the timing of transition of primary to secondary follicles, between ducks and geese, and these differences could result from different expression profiles of FSHR, AMH, and CASPASE3 during early post-hatching development.

Transforming growth factor-β is involved in maintaining oocyte meiotic arrest by promoting natriuretic peptide type C expression in mouse granulosa cells

Natriuretic peptide type C (NPPC) secreted by mural granulosa cells (MGCs) maintains oocyte meiotic arrest via the activation of guanylyl cyclase-linked natriuretic peptide receptor 2 (NPR2). Here, we investigated the effect of transforming growth factor (TGF)-β on NPPC expression in MGCs and oocyte maturation. TGF-β ligands (TGFB1 and TGFB3, but not TGFB2) and receptors (TGFBR1 and TGFBR2) were predominantly expressed in MGCs. The activation of the follicle-stimulating hormone (FSH) receptor by FSH/equine chorionic gonadotropin (eCG) increased the levels of TGFB1, TGFBR2, and TGF-β downstream SMAD proteins in MGCs, which were decreased following the activation of the luteinizing hormone (LH) receptor by human chorionic gonadotropin (hCG). TGF-β significantly increased the gene and protein levels of NPPC in cultured MGCs through SMAD3 binding to Nppc promoter regions. In the presence of FSH, TGF-β further increased NPPC levels and inhibited oocyte meiotic resumption of cumulus-oocyte complexes (COCs). Moreover, Tgfbr2-specific depletion in granulosa cells using Fshr-Cre mice reduced NPPC mRNA and protein levels, resulting in the weak maintenance of oocyte meiotic arrest within large antral follicles. Tgfbr2 depletion also impaired follicle development, ovulation, and female fertility. Taken together, TGF-β-promoted NPPC in MGCs is involved in maintaining oocyte meiotic arrest. FSH and LH could regulate NPPC levels in MGCs via TGF-β and then control the process of oocyte meiosis.

Culture and co-culture of mouse ovaries and ovarian follicles

The mammalian ovary is composed of ovarian follicles, each follicle consisting of a single oocyte surrounded by somatic granulosa cells, enclosed together within a basement membrane. A finite pool of follicles is laid down during embryonic development, when oocytes in meiotic arrest form a close association with flattened granulosa cells, forming primordial follicles. By or shortly after birth, mammalian ovaries contain their lifetime's supply of primordial follicles, from which point onwards there is a steady release of follicles into the growing follicular pool. The ovary is particularly amenable to development in vitro, with follicles growing in a highly physiological manner in culture. This work describes the culture of whole neonatal ovaries containing primordial follicles, and the culture of individual ovarian follicles, a method which can support the development of follicles from an immature through to the preovulatory stage, after which their oocytes are able to undergo fertilization in vitro. The work outlined here uses culture systems to determine how the ovary is affected by exposure to external compounds. We also describe a co-culture system, which allows investigation of the interactions that occur between growing follicles and the non-growing pool of primordial follicles.

Roles of Gremlin 1 and Gremlin 2 in regulating ovarian primordial to primary follicle transition

A network of extracellular signaling factors has previously been shown to act in concert to control the ovarian primordial to primary follicle transition. The current study was designed to investigate the roles of the endogenous bone morphogenetic protein (BMP) inhibitors Gremlin 1 (GREM1) and GREM2 in primordial follicle transition in the rat ovary. GREM1 and GREM2 treatments were found to reverse the effects of anti-Müllerian hormone (AMH) to inhibit follicle transition in a whole-ovary culture system. GREM1 reversed the effect of BMP4 to stimulate primordial follicle transition. Immunohistochemical studies showed that GREM2, but not GREM1, was present in primordial follicles suggesting that GREM2 may regulate primordial follicle transition in vivo. Co-immunoprecipitation studies indicated that GREM2 directly binds to AMH, as well as to BMP4. Transcriptome analyses of ovaries treated with GREM2 or GREM1 yielded negligible numbers of differentially expressed genes, suggesting that the immediate effects of GREM2 or GREM1 appear to be at the level of protein-protein interactions, rather than direct actions on the cells. A number of other ovarian growth factors were found to influence the expression of Grem2. Observations suggest that Grem2 is a part of the signaling network of growth factors that regulate the primordial to primary follicle transition. Insights into the regulatory networks affecting the pool of primordial follicles are important to understand the molecular basis for reproductive diseases such as primary ovarian insufficiency.

Triennial Reproduction Symposium: the ovarian follicular reserve in cattle: what regulates its formation and size?

The ovarian follicular reserve has been linked to fertility in cattle. Young adult cattle with low vs. high numbers of antral follicles ≥ 3 mm in diameter in follicular waves also have fewer preantral follicles and decreased fertility. This underscores the importance of understanding the factors that regulate early follicular development and establish the ovarian follicular reserve, but little is known about how the follicular reserve is first established. In ruminants and humans, follicles form during fetal life, but there is a gap (about 50 d in cattle) between the appearance of the first primordial follicles and the first growing, primary follicles. In this review we present evidence that in cattle, fetal ovarian steroids (i.e., estradiol and progesterone) are negative regulators of both follicle formation and of the acquisition by newly formed follicles of the capacity to activate (i.e., initiate growth). The results indicate that capacity to activate is linked to the completion of meiotic prophase I by the oocyte. The inhibitory effects of estradiol on follicle activation were found to be reversible and correlated with inhibition of the progression of meiotic prophase I. Fetal bovine ovaries produce steroid hormones and production varies considerably during gestation and in a pattern consistent with the hypothesis that they inhibit follicle formation and capacity of newly formed follicles to activate in vivo. However, little was known about how steroid production is regulated. In our studies, both LH and FSH stimulated progesterone and estradiol production by ovarian pieces in vitro. The addition of testosterone to the culture medium enhanced estradiol production, especially when FSH was also present, but inhibited progesterone production, even in the presence of gonadotropins. Evidence is also presented for effects of maternal nutrition and health and for potential effects of estrogenic endocrine-disrupting chemicals on the size of the ovarian follicular reserve established during fetal life. In summary, fetal ovarian steroids may be important regulators of the early stages of follicular development in cattle. Therefore, external factors that alter steroid production or action may affect the size of the ovarian follicular reserve.