Bidirectional communication between oocytes and ovarian follicular somatic cells is required for meiotic arrest of mammalian oocytes

Abstract

Coordinated regulation of oocyte and ovarian follicular development is essential for fertility. In particular, the progression of meiosis, a germ cell-specific cell division that reduces the number of chromosomes from diploid to haploid, must be arrested until just before ovulation. Follicular somatic cells are well-known to impose this arrest, which is essential for oocyte-follicle developmental synchrony. Follicular somatic cells sustain meiotic arrest via the natriuretic peptide C/natriuretic peptide receptor 2 (NPPC/ NPR2) system, and possibly also via high levels of the purine hypoxanthine in the follicular fluid. Upon activation by the ligand NPPC, NPR2, the predominant guanylyl cyclase in follicular somatic cells, produces cyclic guanosine monophosphate (cGMP), which maintains meiotic arrest after transfer to the oocyte via gap junctions. Here we report that both the NPPC/NPR2 system and hypoxanthine require the activity of inosine monophosphate dehydrogenase (IMPDH), the rate-limiting enzyme required for the production of guanylyl metabolites and cGMP. Furthermore, oocyte-derived paracrine factors, particularly the growth differentiation factor 9-bone morphogenetic protein 15 heterodimer, promote expression of Impdh and Npr2 and elevate cGMP levels in cumulus cells. Thus, although the somatic compartment of ovarian follicles plays an essential role in the maintenance of oocyte meiotic arrest, as has been known for many years, this function of the somatic cells is surprisingly regulated by signals from the oocyte itself. F ertility in mammals depends on the coordinated development of ovarian follicles and the oocytes contained within them. The ovulation of oocytes by Graafian follicles must coincide with oocyte meiotic progression. Meiosis is a germ cell-specific cell division that reduces the number of chromosomes from diploid to haploid. In mammals, oocytes are arrested at the diplotene stage (diploid) of meiosis until the surge of luteinizing hormone (LH) from the pituitary gland initiates the ovulatory process. The LH surge initiates the resumption of meiosis and its progression to the metaphase of the second meiotic division, and these haploid oocytes (eggs) are ovulated into the oviduct to await fertilization. Ever since the classic experiments of Pincus and Enzmann (1) in the 1930s, it has been widely accepted that follicular somatic cells of Graafian follicles maintain the fully grown oocytes in meiotic arrest until the LH surge: Removing them from follicles for culture in a supportive medium before the surge results in an LH-independent resumption of meiosis. Thus, isolated oocytes resume meiosis simply because they have been separated from the inhibitory effect of follicular somatic cells. The mechanisms by which these cells maintain oocyte meiotic arrest have been the subject of numerous studies over the last 75 y.Oocytes arrested at the diplotene stage are referred to as germinal vesicle (GV)-stage oocytes, and the dissolution of the oocyte nuclear envelope, often referred to as germinal vesic...

Oocyte-dependent activation of MTOR in cumulus cells controls the development and survival of cumulus-oocyte complexes

“…Although the actual concentration of GDF9 in follicular fluid is unlikely to be as high as the dose (500 ng/ ml) that we have observed here to be effective in regulating Ddit4l expression and MTOR activation, genetic studies clearly show that both GDF9 and BMP15 are present in follicular fluid and function in synergy . Interestingly, GDF9 and BMP15 have recently been found to form a heterodimer that strongly stimulates cumulus cell functions in vitro (Mottershead et al, 2015;Peng et al, 2013;Wigglesworth et al, 2013). We also observed here that mouse recombinant GDF9-BMP15 heterodimer suppressed Ddit4l expression in oocytectomized cumulus cells even at 1 ng/ml, which was 500 times as potent as GDF9 homodimer.…”

History, origin, and function of transzonal projections: the bridges of communication between the oocyte and its environment

“…3C, D). The function of the gap junctions that become assembled where the tips of the TZPs contact the oocyte plasma membrane has been extensively investigated and is well-understood (Phillips and Dekel, 1991;SelaAbramovich et al, 2006;Kidder and Vanderhyden, 2010;Wigglesworth et al, 2013;Winterhager and Kidder, 2015). Studies using radioactive tracers showed that sugars, amino acids and nucleotides were transferred from the granulosa cells to the oocyte (Eppig, 1979;Brower and Schultz, 1982).…”

Deconstructing the winding path to the recapitulation of mammalian oogenesis ex vivo

“…After coaxing mouse embryonic stem cells or induced pluripotent stem cells into a primordial germ cell-like state, the authors showed an age/stage-dependent requirement of the ovarian soma in order for the transplanted cells to become integrated into follicles capable of ovulating and producing oocytes with fertility potential, as shown by the birth of live young. These studies and others emphasize the critical nature of an ongoing dialogue between ovarian somatic cells that invokes a panoply of signaling mechanisms necessary to establish and maintain oocyte/follicle development to its ultimate state, ovulation (6,7). Indications from previous genetic manipulation studies identified key paracrine effectors at the interface between oocytes and somatic cells (8), but the extent to which these cues would be selfgenerating and operative in an in vitro culture system appeared to be problematic, given the enormous complexity at specific stages (6) and the many extrinsic factors now known to influence the dialogue in vivo (9).…”