Cumulus expansion initiates physical and developmental autonomy of the oocyte

Intraovarian control of selective follicular growth and induction of oocyte maturation in mammals

In newborn mammals, most of the germ cell population rests in a pool of quiescent small follicles in the ovaries. Regularly throughout adulthood, a small percentage of these oocytes and follicles grows to a certain stage of development and then either degenerates or matures and ovulates. This entire process is under both exogenous and endogenous control. Recent work, including my laboratory's, has clarified that cytokines and glycosaminoglycans are involved as exogenous and endogenous factors in ovarian follicular development, atresia, and maturation in mammals. The present article describes our contribution regarding the cytokines and ovarian glycosaminoglycans that act as intraovarian regulators of follicular development and oogenesis, including oocyte maturation, in mammals.

Lipopolysaccharide initiates inflammation in bovine granulosa cells via the TLR4 pathway and perturbs oocyte meiotic progression in vitro

Infections of the reproductive tract or mammary gland with Gram-negative bacteria perturb ovarian function, follicular growth, and fecundity in cattle. We hypothesized that lipopolysaccharide (LPS) from Gram-negative bacteria stimulates an inflammatory response by ovarian granulosa cells that is mediated by Toll-like receptor (TLR) 4. The present study tested the capacity of bovine ovarian granulosa cells to initiate an inflammatory response to pathogen-associated molecular patterns and determined subsequent effects on the in vitro maturation of oocytes. Granulosa cells elicited an inflammatory response to pathogen-associated molecular patterns (LPS, lipoteichoic acid, peptidoglycan, or Pam3CSK4) with accumulation of the cytokine IL-6, and the chemokine IL-8, in a time- and dose-dependent manner. Granulosa cells responded acutely to LPS with rapid phosphorylation of TLR signaling components, p38 and ERK, and increased expression of IL6 and IL8 mRNA, although nuclear translocation of p65 was not evident. Targeting TLR4 with small interfering RNA attenuated granulosa cell accumulation of IL-6 in response to LPS. Endocrine function of granulosa cells is regulated by FSH, but here, FSH also enhanced responsiveness to LPS, increasing IL-6 and IL-8 accumulation. Furthermore, LPS stimulated IL-6 secretion and expansion by cumulus-oocyte complexes and increased rates of meiotic arrest and germinal vesicle breakdown failure. In conclusion, bovine granulosa cells initiate an innate immune response to LPS via the TLR4 pathway, leading to inflammation and to perturbation of meiotic competence.

Ultrastructural and cytochemical characterization of follicular cell types in bovine (Bos taurus) cumulus-oocyte complexes aspirated from small and medium antral follicles during the estrus cycle

Stereological quantitative methods have revealed the presence of three distinct follicular cell populations (C1-C3) in bovine cumulus-oocyte complexes. Type C3 cells became largely predominant from metestrous to proestrous, with a simultaneously large decrease in the other two cell types. To further characterize these cumulus cell types, cumulus-oocyte complexes from small (1-4mm) and medium (4-8mm) antral follicles (category 1: with a compact multilayered cumulus and a homogeneous ooplasm) were aspirated from ovaries of Holstein-Friesian cows and processed for electron microscopy, ultrastructural cytochemical detection of glycogen and glycoproteins, and immunogold localization of chondroitin sulfate. Each follicular cell type displayed the same ultrastructural characteristics independently of the size of the follicle and the stage of the estrous cycle. Type C1 cells showed morphological characteristics of undifferentiated cells and progressively transformed into type C2 cells. Type C2 cells were characterized by cell extensions, polarized nuclei with evident nucleolar fibrilar centers, steroidogenic characteristics (numerous large lipid droplets, large endoplasmic reticulum vesicles and vacuoles), and synthesis of glycoproteins and chondroitin sulfate by the Golgi apparatus and endoplasmic reticulum. Type C3 cells presented morphologic features of fully differentiated and luteinized cumulus cells. They were characterized by an increase in cytoskeleton filaments, loss of cell extensions and of intercellular junctions, depletion of lipid and glycogen stores, and initiation of glycoprotein and chondroitin sulfate exocytosis. In conclusion, the present study suggests that bovine cumulus cells from small and medium antral follicles follow a complete dynamic functional differentiation process, in which the three cell types seem to correspond to a functional continuum. We identified undifferentiated cells, cells strongly engaged in glycoprotein and proteoglycan synthesis, and cells fully differentiated and secreting.

Influence of epidermal growth factor supplementation during in vitro maturation on nuclear status and gene expression of canine oocytes

This study evaluated the effect of epidermal growth factor (EGF) supplementation during in vitro maturation on the meiotic status and the expression of EGF receptor (EGFr), luteinizing hormone receptor (LHr) and gap junction protein α 5 (GJA5) in canine cumulus-oocyte-complexes (COCs). COCs of ≥110 μm diameter, exhibiting dark pigmentation and completely surrounded by three or more layers of cumulus cells collected from anestrus stage ovaries in natural cycle were matured in TCM-199 supplemented with 10% fetal bovine serum, 0.57 mM cysteine, 10 μg/ml LH and FSH, and different concentrations of EGF (0, 10 and 30 ng/ml). Oocytes cultured for 72 h were fixed to assess the nuclear maturation. Expression of EGFr, LHr and GAJ5 was assessed by immunocytochemistry and real-time PCR. Proportion of metaphase II status of oocytes cultured in in vitro maturation (IVM) medium supplemented with 10 ng/ml EGF for 72 h was significantly (P<0.05) higher than 0 and 30 ng/ml EGF supplemented IVM medium (9.8% vs. 6.5% and 5.2%). In both cumulus cells and oocytes, EGFr protein was undetectable, LHr protein level of expression was low and a strong expression of GJA5 protein was observed. The relative abundance (RA) of EGFr transcript revealed low levels and the LHr expression decreased steadily with addition of EGF. However it did not vary among different concentrations of EGF supplementation. The RA of GJA5 transcript exhibited lower level at 10 ng/ml EGF supplementation. In conclusion, the supplementation of 10 ng/ml EGF in IVM media exerted a positive influence on the progression of maturation to MII phase and the expression level of GJA5 at 72 h, but did not demonstrate any stimulatory role on the expression of EGFr and LHr during the maturation of the canine IVM oocytes.

Higher expression of hyaluronan binding protein 1 (HABP1/p32/gC1qR/SF2) during follicular development and cumulus oocyte complex maturation in rat

Ovulation is a complex process of releasing a fertilizable oocyte and depends on the proper formation of an extracellular hyaluronan rich matrix by the cumulus oocyte complex (COC). The formation of a HA rich matrix is dependent on the synthesis and organization of HA in the presence of several biomolecules that mediate its crosslinking. To gain an insight into the follicular maturation and COC expansion, we have studied the expression of hyaluronan binding protein 1 (HABP1), which is known to interact specifically with hyaluronan. The level of HABP1 increased markedly during ovulation after gonadotropin stimulation, and the overexpression was seen in mural granulosa cells, expanding cumulus cells and follicular fluid. However, HABP1 could not be detected in the luteal cells of corpus luteum after ovulation. Such increased expression of HABP1 was observed both during in vivo and in vitro conditions of COC expansion. The level of HABP1 transcript was upregulated up to fivefold after COC expansion as compared to compact COC. Immunofluorescence analysis showed HABP1 to be localized in the cytoplasm and extracellular matrix, suggesting its role in ECM organization. The cultured expanded COC treated with hyaluronidase for different time periods showed the gradual dispersion of COC, which coincide with the loss of HABP1 from the matrix suggesting that HABP1 is bound to hyaluronan. These results indicate that HABP1 expressed in rat COCs during maturation may facilitate the formation of the HA matrix in the extracellular space around the oocyte with cumulus expansion during maturation.

Molecular mechanisms of insulin-like growth factor 1 promoted synthesis and retention of hyaluronic acid in porcine oocyte-cumulus complexes

The purpose of the present study was to elucidate signaling pathways by which insulin like-growth factor 1 (IGF1) promotes FSH-stimulated synthesis and retention of hyaluronic acid (HA) in pig oocyte-cumulus complexes (OCCs) cultured in serum-free medium. We found that IGF1 had no effects on FSH-stimulated production of cAMP and activation of protein kinase A in the OCCs. Immunoblotting with phospho-specific antibodies showed that FSH moderately phosphorylated v-akt murine thymoma viral oncogene homolog (AKT) and mitogen-activated kinase 3 and 1 (MAPK3/1) in cumulus cells. The exposure of OCCs to both FSH and IGF1 resulted in a significant (P < 0.05) increase in AKT and MAPK3/1 phosphorylation. An inhibitor of phosphoinositide-3-kinase (PIK3), LY 294002, significantly (P < 0.05) reduced the IGF1-enhanced phosphorylation of AKT, and inhibitors of AKT (SH6) and MAPK3/1 (U0126) significantly (P < 0.05) decreased the synthesis and retention of HA stimulated by concomitant exposure of OCCs to both FSH and IGF1. The IGF1-promoted synthesis of HA was not accompanied by an increase in the relative abundance of hyaluronan synthase 2 (HAS2) mRNA in the cumulus cells. We conclude that IGF1 promotes the FSH-stimulated synthesis and retention of HA in pig OCCs by PIK3/AKT- and MAPK3/1-dependent mechanisms.

Molecular mechanisms of ovulation: co-ordination through the cumulus complex

Successful ovulation requires that developmentally competent oocytes are released with appropriate timing from the ovarian follicle. Somatic cells of the follicle sense the ovulatory stimulus and guide resumption of meiosis and release of the oocyte, as well as structural remodelling and luteinization of the follicle. Complex intercellular communication co-ordinates critical stages of oocyte maturation and links this process with release from the follicle. To achieve these outcomes, ovulation is controlled through multiple inputs, including endocrine hormones, immune and metabolic signals, as well as intrafollicular paracrine factors from the theca, mural and cumulus granulosa cells and the oocyte itself. This review focuses on the recent advances in understanding of molecular mechanisms that commence after the gonadotrophin surge and culminate with release of the oocyte. These mechanisms include intracellular signalling, gene regulation and remodelling of tissue structure in each of the distinct ovarian compartments. Most critical ovulatory mediators exert effects through the cumulus cell complex that surrounds and connects with the oocyte. The convergence of ovulatory signals through the cumulus complex co-ordinates the key mechanistic processes that mediate and control oocyte maturation and ovulation.

Expression of connexin 43 and gap junctional intercellular communication in the cumulus-oocyte complex in sheep

To evaluate the effects of FSH, LH and/or cAMP on expression of connexin 43 (Cx43) in the ovine cumulus-oocyte complex (COC) and gap junctional intercellular communication (GJIC) of cumulus cells, two experiments were carried out. In experiment 1, Cx43 was immunodetected in the COC, before or after maturation, obtained from non-treated or FSH-treated ewes. The expression of Cx43 in the COC was greater (P < 0.01) on day 16 than on day 15 of the estrous cycle. In vivo FSH treatment decreased (P < 0.02) Cx43 expression on day 16 but not on day 15 of the estrous cycle. In experiment 2, intact COCs or isolated cumulus cells obtained from small and large follicles from FSH-treated ewes were cultured with or without FSH, LH or cAMP agonist and evaluated for GJIC by laser cytometry. For large follicles, the basal rate of GJIC was greater (P < 0.01) for cumulus cells in intact COCs than for isolated cumulus cells. FSH increased (P < 0.04) GJIC in cumulus cells in intact COCs and tended to increase (P < 0.1) GJIC in isolated cumulus cells from small follicles but decreased (P < 0.01) GJIC in cumulus cells in intact COCs from large follicles. LH also increased (P < 0.01) GJIC in isolated cumulus cells from small follicles but decreased GJIC in intact COCs (P < 0.01) and isolated cumulus cells (P < 0.02) from large follicles. cAMP increased (P < 0.01) the GJIC in both intact COCs and cumulus cells from small and large follicles. These results indicate that day of estrous cycle, stage of maturation and duration of FSH treatment affect expression of Cx43 in ovine COCs. In intact COCs, GJIC in cumulus cells was enhanced, probably due to the presence of the oocyte. In addition, the effects of FSH and LH, but not cAMP, on GJIC of cumulus cells depended on the stage of follicular development and on the presence of the oocyte.

Cumulus-oocyte complex interactions during oocyte maturation

In most mammals, the oocyte in the Graafian follicle is surrounded by tightly packed layers of cumulus cells, forming the cumulus-oocyte complex. During the preovulatory period, cumulus cells change from a compact cell mass into a dispersed structure of cells for the synthesis and deposition of a mucoid intercellular matrix, a process referred to as cumulus expansion. Cumulus expansion is thought to influence a variety of fundamental developmental changes during oocyte maturation. Volumetric expansion of the cumulus-oocyte complex correlates, at least in pig, with the outcome of oocyte maturation, fertilization, and embryo development. Therefore, detailed functional studies of cumulus expansion seem to be required to elucidate the mechanism of oocyte maturation. We summarize the current knowledge about (1) morphological changes of cumulus-oocyte complexes during oocyte maturation, (2) follicle factors inducing cumulus expansion, (3) the role of cumulus expansion in oocyte maturation, (4) cytoplasmic regulators of oocyte maturation, and (5) possible roles of cumulus expansion.

Identification of Hyaluronic Acid-Binding Proteins and Their Expressions in Porcine Cumulus-Oocyte Complexes During In Vitro Maturation1

Hyaluronic acid-binding proteins (HABPs) are necessary for expansion of the cumulus-oocyte complex (COC) during oocyte maturation. In this study, to obtain the detailed information of HABPs during cumulus expansion, we examined the expression of HABPs in porcine COCs during in vitro maturation (IVM). After maturation culture, proteins were extracted from porcine COCs and separated by SDS-PAGE and then transferred to polyvinylidene fluoride membranes. After transfer, the membranes were subjected to ligand blotting with biotinylated hyaluronic acid (bHA) or fluorescein isothiocyanate-labeled hyaluronic acid (FITC-HA). Furthermore, the extracted proteins were subjected to immunoprecipitation, Western blotting, and immunofluorescence analysis to dissect the HABPs. Ligand blotting with FITC-HA could detect HABPs. Using this ligand-blotting method, 13 and 14 bands of HABPs were detected in porcine COCs after 0 and 48 h in culture, respectively. Of these, the level of expression of 85-kDa HABP increased with cumulus expansion during IVM and was newly detected after culture. Immunoprecipitation, Western blotting, and immunofluorescent analysis confirmed that the 85-kDa HABP corresponded to CD44 and that it existed on/in the membrane of cumulus cells. The present results indicated that HABP expressed in porcine COCs during IVM, particularly CD44, may form a network of the matrices in the extracellular space of the oocyte with cumulus expansion during IVM.