Cumulus Extracellular Matrix Is an Important Part of Oocyte Microenvironment in Ovarian Follicles: Its Remodeling and Proteolytic Degradation

The extracellular matrix (ECM) is an essential structure with biological activities. It has been shown that the ECM influences gene expression via cytoskeletal components and the gene expression is dependent upon cell interactions with molecules and hormones. The development of ovarian follicles is a hormone dependent process. The surge in the luteinizing hormone triggers ovulatory changes in oocyte microenvironment. In this review, we discuss how proteolytic cleavage affects formation of cumulus ECM following hormonal stimulation; in particular, how the specific proteasome inhibitor MG132 affects gonadotropin-induced cytoskeletal structure, the organization of cumulus ECM, steroidogenesis, and nuclear maturation. We found that after the inhibition of proteolytic cleavage, gonadotropin-stimulated oocyte-cumulus complexes (OCCs) were without any signs of cumulus expansion; they remained compact with preserved cytoskeletal F-actin-rich transzonal projections through the oocyte investments. Concomitantly, a significant decrease was detected in progesterone secretion and in the expression of gonadotropin-stimulated cumulus expansion-related transcripts, such as HAS2 and TNFAIP6. In agreement, the covalent binding between hyaluronan and the heavy chains of serum-derived the inter-alpha-trypsin inhibitor, essential for the organization of cumulus ECM, was missing.

Effect of epidermal growth factor-like peptides on pig cumulus cell expansion, oocyte maturation, and acquisition of developmental competence in vitro: comparison with gonadotropins

The aim of this work was to assess the FSH-stimulated expression of epidermal growth factor (EGF)-like peptides in cultured cumulus–oocyte complexes (COCs) and to find out the effect of the peptides on cumulus expansion, oocyte maturation, and acquisition of developmental competencein vitro. FSH promptly stimulated expression of amphiregulin (AREG) and epiregulin (EREG), but not betacellulin (BTC) in the cultured COCs. Expression ofAREGandEREGreached maximum at 2 or 4 h after FSH addition respectively. FSH also significantly stimulated expression of expansion-related genes (PTGS2,TNFAIP6, andHAS2) in the COCs at 4 and 8 h of culture, with a significant decrease at 20 h of culture. Both AREG and EREG also increased expression of the expansion-related genes; however, the relative abundance of mRNA for each gene was much lower than in the FSH-stimulated COCs. In contrast to FSH, AREG and EREG neither stimulated expression ofCYP11A1in the COCs nor an increase in progesterone production by cumulus cells. AREG and EREG stimulated maturation of oocytes and expansion of cumulus cells, although the percentage of oocytes that had reached metaphase II was significantly lower when compared to FSH-induced maturation. Nevertheless, significantly more oocytes stimulated with AREG and/or EREG developed to blastocyst stage after parthenogenetic activation when compared to oocytes stimulated with FSH alone or combinations of FSH/LH or pregnant mares serum gonadotrophin/human chorionic gonadotrophin. We conclude that EGF-like peptides do not mimic all effects of FSH on the cultured COCs; nevertheless, they yield oocytes with superior developmental competence.

Development of functional LH Receptors on pig cumulus-oocyte complexes cultured in vitro by a novel two-step culture system

We show in the present study that freshly isolated pig cumulus-oocyte complexes (COCs) display a limited response to LH, as assessed by the expression of hyaluronan synthase 2 (Has2) mRNA, activation of protein kinase A (PKA), production of hyaluronic acid (HA) and progesterone, cumulus cell expansion and resumption of meiosis. These data indicate that freshly isolated COCs do not possess a sufficient number of functional LH receptors (LHR). However, the expression of Lhr significantly increased during the culture of COCs in vitro in a medium supplemented with FSH. Assuming that the effect of FSH on LHR induction is mediated via cAMP signaling pathways, we developed a new culture system, in which the COCs were pre-cultured for 72 hr in a medium supplemented with dbcAMP. The pre-cultured COCs remained in the germinal vesicle stage, their cumulus investment underwent a dramatic increase in size and gap junctions between the cumulus cells were preserved. The stimulation of such COCs with either FSH or LH led to the resumption and completion of meiosis, activation of PKA, expression of Has2, synthesis of large amounts of HA and progesterone, and extensive expansion of cumulus cells. We conclude that the formation of functional LHR is stimulated in cumulus cells during the culture in vitro in a cAMP-dependent pathway. The dbcAMP-treated COCs thus represent a new model in which the resumption of meiosis and cumulus expansion can be induced exclusively by the action of recombinant LH.

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.

Inhibitory effect of cadmium and tobacco alkaloids on expansion of porcine oocyte-cumulus complexes

Studies aimed at the influence of smoking on reproductive functions have found out fertility disorders in smokers occurring at any stage of reproductive processes. In our experiments the role of cadmium, nicotine and anabasine was investigated in the expansion of oocyte-cumulus complexes (OCC) isolated from large antral porcine follicles. Suppression of FSH-induced cumulus expansion and significant inhibition of synthesis and accumulation of hyaluronic acid in the cell/matrix compartment of the OCC was observed in the presence of different concentrations of tested compounds. The suppressive effect of cadmium and tobacco alkaloids on the cumulus expansion was accompanied by decreased progesterone production by cumulus cells during 42 h incubation of the OCC with FSH.

Components of cigarette smoke inhibit expansion of oocyte-cumulus complexes from porcine follicles

The role of alkaloids in cigarette smoke was investigated in the cumulus expansion of oocyte-cumulus complexes (OCC) isolated from large antral porcine follicles. Suppression of the cumulus expansion stimulated by FSH was observed in the presence of different concentration of cadmium, anabasine and nicotine but not its metabolite cotinine. There were comparable inhibitory effects of cadmium and nicotine on the synthesis and accumulation of hyaluronic acid in the cell/matrix compartment of OCC. The inhibitory effect of tested compounds on the cumulus expansion was accompanied by decreased progesterone synthesis by cumulus cells during 42 h incubation of OCC with FSH. The results suggest that cigarette smoking may affect intrafollicular processes, which are responsible for normal ovulation and fertilization.

Gossypol inhibits follicle-stimulating hormone- and epidermal growth factor-stimulated expansion of oocyte-cumulus complexes from porcine preovulatory follicles

The role of gossypol in the cumulus expansion of oocyte-cumulus complexes (OCC) isolated from large antral porcine follicles was investigated. Marked suppression of cumulus expansion stimulated with follicle-stimulating hormone (FSH) and epidermal growth factor (EGF) was observed in the presence of different concentrations of gossypol. Comparable inhibitory effects were obtained in the presence of NO donor, S-nitroso-N-acetylpenicillamine or sodium nitroprusside, suggesting that the inhibitory effect of gossypol may be mediated via NO generation. The inhibitory effect of gossypol on cumulus expansion of OCC was accompanied by inhibition of progesterone secretion of OCC and the decrease of [125I]EGF binding to granulosa cells.

Effect of intraovarian factors on porcine follicular cells: cumulus expansion, granulosa and cumulus cell progesterone production

The role of granulosa cell conditioned media (CM) containing luteinization stimulator (LS), and the role of EGF in the cumulus expansion of oocyte-cumulus complexes (OCC) isolated from large antral follicles was investigated. The CM were prepared by incubation of granulosa cells isolated from large antral follicles. After 24h incubation, more than 61 or 64% of OCC expanded to the +3 and +4 stage in the presence of CM (50%) or EGF (10ng/ml), respectively. The stimulatory effect of LS and EGF on the cumulus expansion was accompanied by the enhanced hyaluronic acid synthesis. Complete suppression of cumulus expansion stimulated by LS and EGF was observed in the presence of 10 micromol/l genistein (tyrosine kinase inhibitor), in the presence of 10mmol/l LiCl (the inhibitor of inositol 1,4,5-trisphosphate metabolism), and 100 micromol/l gallopamil, verapamil and norverapamil (calcium channel blockers). Stimulatory effect of EGF on the cumulus expansion of OCC isolated from large follicles was accompanied by the increased cumulus cell progesterone production. However, EGF did not affect the progesterone production by OCC isolated from small follicles. To determine whether EGF could modulate the granulosa cell steroidogenesis also, the effect of EGF on granulosa cells isolated from large (LGC) and small (SGC) follicles was compared. EGF (10ng/ml) failed to affect the progesterone synthesis during 72h culture of SGC but significantly enhanced the LGC progesterone production. Our results indicate that luteinization factor stimulates the cumulus expansion and hyaluronic acid synthesis by the OCC isolated from large antral follicles. The mechanism of LS- and EGF-induced cumulus expansion may involve tyrosine kinase activation and calcium mobilization. In addition, these results indicate the different response of porcine cumulus and granulosa cells originating from small and large follicles on the stimulatory effect of EGF.

Secretion of paracrine factors enabling expansion of cumulus cells is developmentally regulated in pig oocytes

To demonstrate secretion of cumulus expansion-enabling factor (CEEF) by porcine oocytes, we used an interspecies testing system. Porcine oocytes were used to condition culture medium, and the presence of CEEF was tested using mouse oocytectomized complexes (OOX), which require CEEF for expansion. Follicle-stimulating hormone-stimulated expansion and synthesis of hyaluronic acid (HA) by mouse OOX were assessed after 18 h of culture in media conditioned by porcine oocytes: 1) at different stages of maturation and 2) in which maturation was inhibited with a specific inhibitor of cdk-kinases, butyrolactone I. Fully grown (GV-germinal vesicle), late-diakinesis (LD), metaphase I (MI), and metaphase II (MII) oocytes were prepared by culture of oocyte-cumulus complexes (OCC) for 0, 22, 27, and 42 h, respectively. To block GV breakdown, porcine oocytes were cultured for 27 h in medium supplemented with butyrolactone I (50 microM). Medium conditioned by oocytes in GV, LD, and after butyrolactone I block allowed full expansion of >90% of mouse OOX, whereas oocytes in MI and MII caused disintegration of mouse OOX without cumulus mucification. To measure synthesis of HA by cumulus cells, 25 mouse OOX were cultured in the conditioned media in the presence of 2.5 microCi of D-[6-(3)H]glucosamine hydrochloride. After 18 h, incorporation of the [(3)H]glucosamine into HA was determined either in complexes (retained HA) or in medium plus complexes (total HA). Total HA accumulation by mouse OOX was not different from that of intact OCC. However, oocytes in GV, LD, and after butyrolactone I treatment enabled mouse OOX to retain significantly more HA within the complex than oocytes in MI and MII. The results indicate that secretion of factors that promote the retention of HA within the complex is developmentally regulated during oocyte maturation.

Developmental regulation of effect of epidermal growth factor on porcine oocyte-cumulus cell complexes: nuclear maturation, expansion, and F-actin remodeling

Epidermal growth factor (EGF) efficiently stimulates expansion of mouse and rat oocyte-cumulus complexes (OCC). Contradictory data have been published by several laboratories about the ability of EGF to stimulate expansion of porcine OCC. We assumed that these contradictions may have resulted from heterogeneous conditions used for isolation, culture, and assessment of OCC. The present experiments were designed to test the hypothesis that porcine OCC acquire the ability to synthesize hyaluronic acid (HA) and undergo expansion following EGF-stimulation gradually during the growth of follicles. For this reason, we isolated OCC from follicles of different sizes and assessed quantity of produced HA and proportions of expanding OCC after stimulation by EGF. In addition, we assessed in those OCC changes in morphology of cumulus cells and assembly of F-actin microfilaments, which are necessary for expansion to occur. Finally, nuclear maturation of EGF-stimulated OCC was assessed and its relationship with occurrence of expansion was evaluated. In all experiments, OCC stimulated with FSH were used as positive controls. The results showed that EGF did not stimulate production of HA, rearrangement of F-actin and expansion in OCC isolated from small follicles (<4 mm in diameter). OCC isolated from large preovulatory follicles (6-7 mm in diameter and PMSG-stimulated follicles) underwent efficient expansion when stimulated by EGF (93% and 100%, respectively). EGF dramatically stimulated total production of HA in these OCC and its retention in extracellular matrix of the expanding cumulus. Cumulus cells of the large OCC underwent essential changes of their morphology and extensive rearrangement of F-actin microfilaments following stimulation with EGF. Interestingly, EGF enhanced nuclear maturation of OCC isolated from both small and large follicles, which suggest diversity of signaling pathways controlling maturation and expansion. FSH caused cumulus expansion, F-actin remodeling, and enhancement of oocyte nuclear maturation in OCC originated from both small and large follicles. We conclude that EGF can stimulate expansion of porcine OCC in vitro; however, only of those isolated from large follicles. This indicates that EGF may have a physiological role in regulation of porcine cumulus expansion in preovulatory follicles, presumably as a mediator of signals elicited by the LH surge.

Oocytectomy does not influence synthesis of hyaluronic acid by pig cumulus cells: retention of hyaluronic acid after insulin-like growth factor-I treatment in serum-free medium

Mouse oocytes secrete a factor that enables cumulus cells to undergo expansion in response to FSH (1 microg/ml), whereas expansion of the porcine cumulus oophorus has been shown to be independent of the oocyte. The aim of this study was to assess FSH-induced synthesis of hyaluronic acid (HA) by porcine cumulus cells before and after oocytectomy. In addition, we studied the effect of insulin-like growth factor-I (IGF-I) on the ability of cumulus cells to synthesize and retain HA in response to FSH in serum-free medium. Porcine oocyte-cumulus complexes and complexes from which the oocytes had been removed by oocytectomy were cultured for 24 h in the presence of 2.5 microCi of D-[6-(3)H]glucosamine hydrochloride, fetal calf serum (FCS, 5%), and FSH. After 24 h, incorporation of [(3)H]glucosamine into HA was measured either in complexes alone (retained HA) or in medium plus complexes (total HA). Specificity of incorporation of radioactivity into HA was confirmed by the sensitivity to highly specific Streptomyces hyaluronidase. Our results suggest that 1) the synthesis of HA by pig cumulus cells in vitro is stimulated by FSH and that oocytectomy does not change this synthesis; 2) oocytes do not influence retention of HA within the complex; 3) FSH-induced synthesis of HA by cumulus cells is decreased in medium with polyvinylpyrrolidone (PVP)-supplemented (total and retained HA) compared to FCS-supplemented medium; 4) IGF-I enabled cumulus cells to synthesize HA in response to FSH in PVP-supplemented medium in a manner similar to that observed when serum is present in the medium.

Secretion of cumulus expansion-enabling factor (CEEF) in porcine follicles

The objective of this study was to find out whether porcine cumulus and mural granulosa cells can secrete cumulus expansion-enabling factor (CEEF). Culture drops of M-199 medium were conditioned with denuded porcine oocytes (1 oocyte/microliter), cumulus cells from oocytectomized complexes (1 OOX/microliter), pieces of mural granulosa isolated from preantral to preovulatory follicles (1000 cells/microliter), or oviductal cells (1000 cells/microliter) for 24 hr. The production of CEEF was assessed by the addition of mouse OOX and follicle-stimulating hormone (FSH) (1 microgram/ml) to microdrops of the conditioned medium. After 16-18 hr, expansion of the mouse OOX was scored on a scale of 0 to 4 by morphologic criteria. Mouse OOX did not expand in nonconditioned FSH-supplemented medium. Immature porcine oocytes produced +3 to +4 expansion of the mouse OOX. Granulosa cells isolated from preantral and early antral follicles and cumulus cells isolated from all states of follicle development constitutively secreted CEEF under in vitro conditions. Mural granulosa cells of small, medium, and preovulatory (PMSG) follicles also secreted CEEF in vitro; however, FSH or leutenizing hormone (LH) stimulation was essential for this secretion. Hormonally induced secretion of CEEF was accompanied by expansion of the mural granulosa itself. Granulosa cells isolated from follicles of gilts 20 hr after PMSG and human chorionic gonadotropin (hCG) administration did not produce CEEF and did not expand in response to FSH and LH in vitro. CEEF activity also was found in the follicular fluid of small antral follicles, was reduced in medium follicles, and was not detectable in PMSG-stimulated follicles. However, CEEF activity was reestablished in the follicular fluid of preovulatory follicles by hCG injection, conceivably due to increased production of CEEF by cumulus cells. We conclude that (1) porcine cumulus and mural granulosa cells are capable of CEEF production in vitro and (2) autocrine secretion of CEEF by cumulus cells is involved in regulation of porcine cumulus expansion both in vitro and in vivo.

Is cAMP decrease essential for resumption of meiosis in mouse oocytes?

Resumption of meiosis was inhibited in mouse oocyte cumulus complexes (OCC) co-cultured with pig membrane granulosa (PMG). After 3 and 6 h of co-culture these oocytes possessed an intact nuclear envelope and their nucleolar surface was associated with granules approximately 80 nm in diameter. Preincubation of OCCs for 30, 45, 60 or 90 min followed by co-culture with PMG for 2 h of either OCCs or denuded oocytes resulted in germinal vesicle breakdown (GVBD) in approximately 0, 30, 70 and 100% mouse OCCs and in approximately 30, 60, 80 and 100% denuded oocytes, respectively. It seems that the inhibitory contact between mouse oocytes and PMG was established during the first h of co-culture. After isolation from antral follicles the oocytes contained 2.75 fmol cyclic adenosine 3', 5'-monophosphate (cAMP). When OCCs were co-cultured for 1, 2 or 3 h with PMG, the amount of cAMP per oocyte was 1.34, 1.33 and 1.51 fmol, respectively. After culture of OCCs in control medium the amount of cAMP was 1.21, 1.39 and 2.16 fmol, respectively. The present results suggest that the inhibitory activity of PMG is not species-specific. Moreover, PMG prevented resumption of meiosis in mouse oocytes in spite of the cAMP drop in oocyte cytoplasm characteristic of the resumption of meiosis.

Lack of effect of oocytectomy on expansion of the porcine cumulus

Oocyte-cumulus cell complexes (OCC) and complexes with an attached piece of membrana granulosa (C + P), isolated from prepubertal or cyclic gilts stimulated with pregnant mares' serum gonadotrophin, were cultured in media supplemented with follicle-stimulating hormone (FSH; 0.01-1.0 micrograms/ml) or forskolin (50-100 mumol/l) for 24 and 32 h. FSH and forskolin each induced dose-dependent cumulus and membrana granulosa expansion. After 2 h of culture, FSH (0.1 microgram/ml) or forskolin (100 mumol/l) increased the contents of intracellular adenosine 3',5'-phosphate (cAMP) in OCC from prepubertal gilts to almost 10 times that in unstimulated complexes. After 24 h of culture in media supplemented with FSH (0.1 microgram/ml) or forskolin (100 mumol/l), the oocytectomized OCC and C + P showed similar expansion to that of the control groups. The intracellular cAMP contents in intact and oocytectomized OCCs were similar in all groups except those treated with FSH, in which the intact OCCs had significantly higher contents than their oocytectomized counterparts (P less than 0.01). After hyaluronidase treatment, cumulus and membrana granulosa cells of intact and oocytectomized OCC and C + P were suspended, except for those of the innermost layers of the corona radiata. The results suggest that increases in cAMP contents and synthesis of an extracellular, hyaluronidase-sensitive mucus by pig OCC and C + P induced by FSH or forskolin are not dependent on the oocyte.