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Signaling pathways in the development of female germ cells
Umeå University, Faculty of Medicine, Department of Medical Biochemistry and Biophysics. (Kui Liu)
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Primordial follicles are the first small follicles to appear in the mammalian ovary. Women are born with a fixed number of primordial follicles in the ovaries. Once formed, the pool of primordial follicles serves as a source of developing follicles and oocytes. The first aim of this thesis was to investigate the functional role of the intra-oocyte signaling pathways, especially the phosphatidylinositol-3 kinase (PI3K) and mammalian target of rapamycin complex 1 (mTORC1) pathways in the regulation of primordial follicle activation and survival. We found that a primordial follicle remains dormant when the PI3K and mTORC1 signaling in its oocyte is activated to an appropriate level, which is just sufficient to maintain its survival, but not sufficient for its growth initiation. Hyperactivation of either of these signaling pathways causes global activation of the entire pool of primordial follicles leading to the exhaustion of all the follicles in young adulthood in mice. Mammalian oocytes, while growing within the follicles, remain arrested at prophase I of meiosis. Oocytes within the fully-grown antral follicles resume meiosis upon a preovulatory surge of leutinizing hormone (LH), which indicates that LH mediates the resumption of meiosis. The prophase I arrest in the follicle-enclosed oocyte is the result of low maturation promoting factor (MPF) activity, and resumption of meiosis upon the arrival of hormonal signals is mediated by activation of MPF. MPF is a complex of cyclin dependent kinase 1 (Cdk1) and cyclin B1, which is essential and sufficient for entry into mitosis. Although much of the mitotic cell cycle machinery is shared during meiosis, lack of Cdk2  in mice leads to a postnatal loss of all oocytes, indicating that Cdk2 is important for oocyte survival, and probably oocyte meiosis also. There have been conflicting results earlier about the role of Cdk2 in metaphase II arrest of Xenopus  oocytes. Thus the second aim of the thesis was to identify the specific Cdk that is essential for mouse oocyte meiotic maturation. We generated mouse models with oocytespecific deletion of Cdk1  or Cdk2  and studied the specific requirements of Cdk1 and Cdk2 during resumption of oocyte meiosis. We found that only Cdk1 is essential and sufficient for the oocyte meiotic maturation. Cdk1 does not only phosphorylate the meiotic phosphoproteins during meiosis resumption but also phosphorylates and suppresses the downstream protein phosphatase 1, which is essential for protecting the Cdk1 substrates from dephosphorylation.

Place, publisher, year, edition, pages
Umeå: Umeå universitet , 2014. , 41 p.
Series
Umeå University medical dissertations, ISSN 0346-6612 ; 1649
Keyword [en]
ovary, primordial follicle, activation, mTORC₁, PI₃K, oocyte maturation, MPF, Cdk₁
National Category
Biochemistry and Molecular Biology
Research subject
Medical Biochemistry
Identifiers
URN: urn:nbn:se:umu:diva-88309ISBN: 978-91-7601-056-3 (print)OAI: oai:DiVA.org:umu-88309DiVA: diva2:714940
Public defence
2014-05-23, KB3B1, KBC-huset, Linnaeus väg 6, Umeå, 09:30 (English)
Opponent
Supervisors
Available from: 2014-04-30 Created: 2014-04-30 Last updated: 2014-05-05Bibliographically approved
List of papers
1. Cdk1, but not Cdk2, is the sole Cdk that is essential and sufficient to drive resumption of meiosis in mouse oocytes
Open this publication in new window or tab >>Cdk1, but not Cdk2, is the sole Cdk that is essential and sufficient to drive resumption of meiosis in mouse oocytes
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2012 (English)In: Human Molecular Genetics, ISSN 0964-6906, E-ISSN 1460-2083, Vol. 21, no 11, 2476-2484 p.Article in journal (Refereed) Published
Abstract [en]

Mammalian oocytes are arrested at the prophase of meiosis I during fetal or postnatal development, and the meiosis is resumed by the preovulatory surge of luteinizing hormone. The in vivo functional roles of cyclin-dependent kinases (Cdks) during the resumption of meiosis in mammalian oocytes are largely unknown. Previous studies have shown that deletions of Cdk3, Cdk4 or Cdk6 in mice result in viable animals with normal oocyte maturation, indicating that these Cdks are not essential for the meiotic maturation of oocytes. In addition, conventional knockout of Cdk1 and Cdk2 leads to embryonic lethality and postnatal follicular depletion, respectively, making it impossible to study the functions of Cdk1 and Cdk2 in oocyte meiosis. In this study, we generated conditional knockout mice with oocyte-specific deletions of Cdk1 and Cdk2. We showed that the lack of Cdk1, but not of Cdk2, leads to female infertility due to a failure of the resumption of meiosis in the oocyte. Re-introduction of Cdk1 mRNA into Cdk1-null oocytes largely resumed meiosis. Thus, Cdk1 is the sole Cdk that is essential and sufficient to drive resumption of meiosis in mouse oocytes. We also found that Cdk1 maintains the phosphorylation status of protein phosphatase 1 and lamin A/C in oocytes in order for meiosis resumption to occur.

National Category
Medical and Health Sciences
Identifiers
urn:nbn:se:umu:diva-56192 (URN)10.1093/hmg/dds061 (DOI)000304053100008 ()
Available from: 2012-06-12 Created: 2012-06-12 Last updated: 2017-12-07Bibliographically approved
2. Tsc/mTORC1 signaling in oocytes governs the quiescence and activation of primordial follicles
Open this publication in new window or tab >>Tsc/mTORC1 signaling in oocytes governs the quiescence and activation of primordial follicles
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2010 (English)In: Human Molecular Genetics, ISSN 0964-6906, E-ISSN 1460-2083, Vol. 19, no 3, 397-410 p.Article in journal (Refereed) Published
Abstract [en]

To maintain the female reproductive lifespan, the majority of ovarian primordial follicles are preserved in a quiescent state in order to provide ova for later reproductive life. However, the molecular mechanism that maintains the long quiescence of primordial follicles is poorly understood. Here we provide genetic evidence to show that the tumor suppressor tuberous sclerosis complex 1 (Tsc1), which negatively regulates mammalian target of rapamycin complex 1 (mTORC1), functions in oocytes to maintain the quiescence of primordial follicles. In mutant mice lacking the Tsc1 gene in oocytes, the entire pool of primordial follicles is activated prematurely due to elevated mTORC1 activity in the oocyte, ending up with follicular depletion in early adulthood and causing premature ovarian failure (POF). We further show that maintenance of the quiescence of primordial follicles requires synergistic, collaborative functioning of both Tsc and PTEN (phosphatase and tensin homolog deleted on chromosome 10) and that these two molecules suppress follicular activation through distinct ways. Our results suggest that Tsc/mTORC1 signaling and PTEN/PI3K (phosphatidylinositol 3 kinase) signaling synergistically regulate the dormancy and activation of primordial follicles, and together ensure the proper length of female reproductive life. Deregulation of these signaling pathways in oocytes results in pathological conditions of the ovary, including POF and infertility.

Identifiers
urn:nbn:se:umu:diva-42927 (URN)10.1093/hmg/ddp483 (DOI)000273227200001 ()19843540 (PubMedID)
Available from: 2011-04-14 Created: 2011-04-14 Last updated: 2017-12-11Bibliographically approved
3. Oocyte-specific deletion of Pten causes premature activation of the primordial follicle pool
Open this publication in new window or tab >>Oocyte-specific deletion of Pten causes premature activation of the primordial follicle pool
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2008 (English)In: Science, ISSN 0036-8075, E-ISSN 1095-9203, Vol. 319, no 5863, 611-613 p.Article in journal (Refereed) Published
Keyword
1-Phosphatidylinositol 3-Kinase/metabolism, Animals, Female, Follicular Atresia, Mice, Mice; Transgenic, Oocytes/cytology/growth & development/*physiology, Organ Size, Ovarian Failure; Premature/physiopathology, Ovarian Follicle/cytology/*physiology, Ovary/anatomy & histology/physiology, Ovulation, PTEN Phosphohydrolase/genetics/*physiology, Phosphorylation, Protein Kinases/metabolism, Ribosomal Protein S6/metabolism, Signal Transduction
Identifiers
urn:nbn:se:umu:diva-9169 (URN)10.1126/science.1152257 (DOI)18239123 (PubMedID)
Available from: 2008-03-06 Created: 2008-03-06 Last updated: 2017-12-14Bibliographically approved

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