Mitogen-activated protein kinase phosphorylation patterns in pig oocytes and cumulus cells during gonadotrophin-induced resumption of meiosis in vitro

Lab partners: oocytes, embryos and company. A personal view on aspects of oocyte maturation and the development of monozygotic twins

The present review addresses the oocyte and the preimplantation embryo, and is intended to highlight the underlying principle of the "nature versus/and nurture" question. Given the diversity in mammalian oocyte maturation, this review will not be comprehensive but instead will focus on the porcine oocyte. Historically, oogenesis was seen as the development of a passive cell nursed and determined by its somatic compartment. Currently, the advanced analysis of the cross-talk between the maternal environment and the oocyte shows a more balanced relationship: Granulosa cells nurse the oocyte, whereas the latter secretes diffusible factors that regulate proliferation and differentiation of the granulosa cells. Signal molecules of the granulosa cells either prevent the precocious initiation of meiotic maturation or enable oocyte maturation following hormonal stimulation. A similar question emerges in research on monozygotic twins or multiples: In Greek and medieval times, twins were not seen as the result of the common course of nature but were classified as faults. This seems still valid today for the rare and until now mainly unknown genesis of facultative monozygotic twins in mammals. Monozygotic twins are unique subjects for studies of the conceptus-maternal dialogue, the intra-pair similarity and dissimilarity, and the elucidation of the interplay between nature and nurture. In the course of in vivo collections of preimplantation sheep embryos and experiments on embryo splitting and other microsurgical interventions we recorded observations on double blastocysts within a single zona pellucida, double inner cell masses in zona-enclosed blastocysts and double germinal discs in elongating embryos. On the basis of these observations we add some pieces to the puzzle of the post-zygotic genesis of monozygotic twins and on maternal influences on the developing conceptus.

Mechanisms of FSH- and Amphiregulin-Induced MAP Kinase 3/1 Activation in Pig Cumulus-Oocyte Complexes During Maturation In Vitro

The maturation of mammalian oocytes in vitro can be stimulated by gonadotropins (follicle-stimulating hormone, FSH) or their intrafollicular mediator, epidermal growth factor (EGF)-like peptide—amphiregulin (AREG). We have shown previously that in pig cumulus-oocyte complexes (COCs), FSH induces expression and the synthesis of AREG that binds to EGF receptor (EGFR) and activates the mitogen-activated protein kinase 3/1 (MAPK3/1) signaling pathway. However, in this study we found that FSH also caused a rapid activation of MAPK3/1 in the cumulus cells, which cannot be explained by the de novo synthesis of AREG. The rapid MAPK3/1 activation required EGFR tyrosine kinase (TK) activity, was sensitive to SRC proto-oncogene non-receptor tyrosine kinase (SRC)-family and protein kinase C (PKC) inhibitors, and was resistant to inhibitors of protein kinase A (PKA) and metalloproteinases. AREG also induced the rapid activation of MAPK3/1 in cumulus cells, but this activation was only dependent on the EGFR TK activity. We conclude that in cumulus cells, FSH induces a rapid activation of MAPK3/1 by the ligand-independent transactivation of EGFR, requiring SRC and PKC activities. This rapid activation of MAPK3/1 precedes the second mechanism participating in the generation and maintenance of active MAPK3/1—the ligand-dependent activation of EGFR depending on the synthesis of EGF-like peptides.

Oocyte maturation-related gene expression in the canine oviduct, cumulus cells, and oocytes and effect of co-culture with oviduct cells on in vitro maturation of oocytes

PURPOSE

In contrast to most other mammals, canine oocytes are ovulated in an immature state and undergo oocyte maturation within the oviduct during the estrus stage. The aim of the study was to investigate whether oviduct cells from the estrus stage affect the maturation of oocytes and show gene expression patterns related to oocyte maturation.

METHODS

We analyzed MAPK1/3, SMAD2/3, and BMP6/15 expression in oviduct cells, cumulus cells, and oocytes from anestrus, estrus, and diestrus stages. Next, we investigated the effect of co-culture with oviduct cells derived from the estrus stage upon in vitro maturation (IVM) of canine oocytes.

RESULTS

There was significantly higher MAPK1/3 (1.42 ± 0.02 and 2.23 ± 0.06), SMAD2/3 (0.77 ± 0.03 and 2.39 ± 0.07), and BMP15 (2.21 ± 0.16) expression in oviduct cells at the estrus stage (P < 0.05). In cumulus cells, MAPK1 (1.26 ± 0.07), SMAD2/3 (0.82 ± 0.01, 1.04 ± 0.01), and BMP6 (13.09 ± 0.11) expression was significantly higher in the estrus stage (P < 0.05). In oocytes, significant upregulation of MAPK1/3 (14,960 ± 3121 and 1668 ± 253.4), SMAD3 (774.6 ± 79.62), and BMP6 (8500 ± 895.4) expression was found in the estrus stage (P < 0.05). After 72 h of IVM culture, a significantly higher maturation rate was observed in oocytes co-cultured with oviduct cells (10.0 ± 1.5%) than in the control group (3.2 ± 1.4%).

CONCLUSIONS

We demonstrate that oviduct cells at the estrus stage highly expressed MAPK1/3, SMAD2/3, and BMP15. Furthermore, canine oviduct cells from the estrus stage enhance the culture environment for canine oocyte maturation.

Regulation of mitogen-activated protein kinase 3/1 activity during meiosis resumption in mammals

In vivo, resumption of oocyte meiosis occurs in large ovarian follicles after the preovulatory surge of luteinizing hormone (LH). The LH surge leads to the activation of a broad signaling network in mural granulosa cells equipped with LH receptors. The signals generated in the mural granulosa cells are further augmented by locally produced peptides or steroids and transferred to the cumulus cell compartment and the oocyte itself. Over the last decade, essential progress has been made in the identification of molecular events associated with the final maturation and ovulation of mammalian oocytes. All new evidence argues for a multiple roles of mitogen-activated protein kinase 3/1 (MAPK3/1) in the gonadotropin-induced ovulation processes. However, the knowledge of gonadotropin-induced signaling pathways leading to MAPK3/1 activation in follicular cells seems limited. To date, only the LH-induced transactivation of the epidermal growth factor receptor/MAPK3/1 pathway has been described in granulosa/cumulus cells even though other mechanisms of MAPK3/1 activation have been detected in other types of cells. In this review, we aimed to summarize recent advances in the elucidation of gonadotropin-induced mechanisms leading to the activation of MAPK3/1 in preovulatory follicles and cultured cumulus-oocyte complexes and to point out a specific role of this kinase in the processes accompanying final maturation of the mammalian oocyte.

Bone morphogenetic protein-6 (BMP-6): mRNA expression and effect on steroidogenesis during in vitro maturation of porcine cumulus oocyte complexes

Oocyte secreted factors (OSFs) have emerged as important factors for follicular development. The present study investigated the effect of the potential OSF bone morphogenic protein (BMP)-6 on steroidogenesis in porcine cumulus oocyte complexes during in vitro maturation. Cumulus oocyte complexes (COCs), cumulus complexes (CCs) without oocytes and CCs with supplemented BMP-6 were cultured for 0, 5, 26 or 46 h. BMP-6 transcripts were detected in oocytes and cumulus cells at all time points. In both cell types the mRNA expression was most intense after 5h, and decreased during further maturation. After 26 and 46 h of culture, CCs secreted significantly less 17β-estradiol than COCs. This effect was reversed by adding BMP-6 to CCs cultures. In addition, a down-regulation of Cyp19A1, the rate-limiting enzyme of 17β-estradiol synthesis, was detected in CC cultures after 5h. As seen for 17β-estradiol secretion, the addition of BMP-6 caused a significant increase in Cyp19A1 mRNA levels after 5, 26 and 46 h of culture. Progesterone secretion and transcripts of steroidogenic marker proteins StAR and 3β-HSD were not affected considerably by oocyte removal or addition of BMP-6. Furthermore, BMP-6 did not affect the activity of the mitogen-activated protein kinase. The results indicated that BMP-6 is a potential OSF and is involved in the prevention of premature luteinisation in cumulus cells via enhancing 17β-estradiol synthesis.

Influence of nitric oxide during maturation on bovine oocyte meiosis and embryo development in vitro

The effect of s-nitroso-n-acetyl-l,l-penicillamine (SNAP, a nitric oxide donor) during in vitro maturation (IVM) on nuclear maturation and embryo development was investigated. The effect of increasing nitric oxide (NO) during prematuration or maturation, or both, on embryo development was also assessed. 10–3 m SNAP nearly blocked oocytes reaching metaphase II (MII) (7%, P < 0.05) while 10–5 m SNAP showed intermediate proportions (55%). For 10–7 m SNAP and controls (without SNAP), MII percentages were similar (72% for both, P > 0.05), but superior to the other treatment groups (P < 0.05). Blastocyst development, however, was not affected (38% for all treatments, P < 0.05). TUNEL-positive cells in hatched blastocysts (Day 9) increased when IVM included 10–5 m SNAP (8 v. 3 to 4 cells in the other treatments, P > 0.05), without affecting total cell numbers (240 to 291 cells, P > 0.05). When oocytes were prematured followed by IVM with or without 10–7 m SNAP, during either culture period or both, blastocyst development was similar (26 to 40%, P > 0.05). When SNAP was included during both prematuration and IVM, the proportion of Day 9 hatched embryos increased (28% v. 14 to 19% in the other treatments, P < 0.05). Apoptotic cells, however, increased when SNAP was included (6 to 10 cells) in comparison to prematuration and maturation without SNAP (3 cells, P < 0.05). NO may be involved in meiotic progression and apoptosis during embryo development.