Hyunjung Lim

Multiple Female Reproductive Failures in Cyclooxygenase 2–Deficient Mice

Cyclooxygenase (COX) is the rate-limiting enzyme in the synthesis of prostaglandins (PGs) and exists in two isoforms, COX-1 and COX-2. In spite of long-standing speculation, definitive roles of PGs in various events of early pregnancy remain elusive. We demonstrate herein that the targeted disruption of COX-2, but not COX-1, in mice produces multiple failures in female reproductive processes that include ovulation, fertilization, implantation, and decidualization. Using multiple approaches, we conclude that these defects are the direct result of target organ-specific COX-2 deficiency but are not the result of deficiency of pituitary gonadotropins or ovarian steroid hormones, or reduced responsiveness of the target organs to their respective hormones.

Formin-2, polyploidy, hypofertility and positioning of the meiotic spindle in mouse oocytes

Successful reproduction in mammals requires a competent egg, which is formed during meiosis through two assymetrical cell divisions. Here, we show that a recently identified formin homology (FH) gene, formin-2 (Fmn2), is a maternal-effect gene that is expressed in oocytes and is required for progression through metaphase of meiosis I. Fmn2−/− oocytes cannot correctly position the metaphase spindle during meiosis I and form the first polar body. We demonstrate that Fmn2 is required for microtubule-independent chromatin positioning during metaphase I. Fertilization of Fmn2−/− oocytes results in polyploid embryo formation, recurrent pregnancy loss and sub-fertility in Fmn2−/− females. Injection of Fmn2 mRNA into Fmn2-deficient oocytes rescues the metaphase I block. Given that errors in meiotic maturation result in severe birth defects and are the most common cause of chromosomal aneuploidy and pregnancy loss in humans, studies of Fmn2 may provide a better understanding of infertility and birth defects.

ERM is required for transcriptional control of the spermatogonial stem cell niche

Division of spermatogonial stem cells produces daughter cells that either maintain their stem cell identity or undergo differentiation to form mature sperm. The Sertoli cell, the only somatic cell within seminiferous tubules, provides the stem cell niche through physical support and expression of surface proteins and soluble factors. Here we show that the Ets related molecule (ERM) is expressed exclusively within Sertoli cells in the testis and is required for spermatogonial stem cell self-renewal. Mice with targeted disruption of ERM have a loss of maintenance of spermatogonial stem cell self-renewal without a block in normal spermatogenic differentiation and thus have progressive germ-cell depletion and a Sertoli-cell-only syndrome. Microarray analysis of primary Sertoli cells from ERM-deficient mice showed alterations in secreted factors known to regulate the haematopoietic stem cell niche. These results identify a new function for the Ets family transcription factors in spermatogenesis and provide an example of transcriptional control of a vertebrate stem cell niche.

Regulation of Cyclooxygenase-2 Induction in the Mouse Uterus During Decidualization AN EVENT OF EARLY PREGNANCY

The infertility phenotype of cyclooxygenase-2 (Cox-2)-deficient female mice establishes the important role of Cox-2 in pregnancy. Cox-2 deficiency results in defective ovulation, fertilization, implantation, and decidualization; the latter of which can be restored in part by the prostacyclin analog carbaprostacyclin. Uterine Cox-2 expression during early pregnancy shows distinct localization and kinetics in the uterine luminal epithelium and underlying stromal cells, suggesting that expression is tightly regulated. Several intracellular signaling cascades including ERK, p38, and JNK are implicated in vitro as critical components of regulated Cox-2 expression in response to mitogens, growth factors, and cytokines. We investigated the involvement of these signaling pathways during Cox-2 induction in vivo by monitoring uterine kinase activity after intraluminal application of a deciduogenic stimulus. Our results show that the ERK and p38 pathways are activated in uterine preparations as early as 5-min post-stimulation. ERK activation was sustained for several hours with a return to baseline levels by 4 h. p38 activation was rapid with a peak at 5-min post-stimulation and returned to near baseline levels after 45 min. Systemic administration of a MEK inhibitor completely inhibited ERK activation, but did not affect early (2 h) luminal epithelial or late (24 h) stromal Cox-2 expression and only modestly affected decidualization. In contrast, administration of a p38 inhibitor modestly inhibited early Cox-2 expression in the luminal epithelium, while dramatically diminishing late stromal expression. In parallel, induced stromal peroxisomal proliferator activated receptor-δ (PPARδ) expression is blunted by p38 inhibition. p38 inhibition also significantly inhibited decidualization. These results suggest that p38, but not ERK, activation is required for induced Cox-2 and PPARδ expression during decidualization. In addition, inhibition of p38 led to decreased decidualization suggesting that an intracrine prostanoid pathway consisting of Cox-2, prostacyclin, and PPARδ is required for maintenance of early pregnancy.

Loss of Nkx3.1 leads to the activation of discrete downstream target genes during prostate tumorigenesis

The expression of NKX3.1, a transcriptional regulator and tumor suppressor gene in prostate cancer, is downregulated during early stages of prostate tumorigenesis. However, little is known of the alterations in gene expression that occur as a result of this event. We combined laser capture microdissection and gene expression profiling to analyse the molecular consequences of Nkx3.1 loss during prostate cancer initiation using Nkx3.1-deficient mice. This analysis identified a cohort of genes (loss-of-Nkx3.1 signature) that are aberrantly overexpressed during loss-of-Nkx3.1-driven tumor initiation. We studied the expression of these genes in independent loss-of-Pten and c-myc overexpression prostate adenocarcinoma mouse models. Nkx3.1 expression is lost in prostate epithelial proliferation in both of these mouse models. However, Nkx3.1 loss is an early event of tumor development in the loss-of-Pten model, whereas it occurs at later stages in c-myc transgenic mice. A number of genes of the loss-of-Nkx3.1 signature, such as clusterin and quiescin Q6, are highly expressed in prostatic hyperplasia and intraepithelial neoplasia (PIN) lesions that also lack Nkx3.1 in the Pten-deficient prostate, but not in similar lesions in the c-myc transgenic model. Meta-analysis of multiple prostate cancer gene expression data sets, including those from loss-of-Nkx3.1, loss-of-Pten, c-myc overexpression and constitutively active Akt prostate cancer models, further confirmed that genes associated with the loss-of-Nkx3.1 signature integrate with PTEN–AKT signaling pathways, but do not overlap with molecular changes associated with the c-myc signaling pathway. In human prostate tissue samples, loss of NKX3.1 expression and corresponding clusterin overexpression are co-localized at sites of prostatic inflammatory atrophy, a possible very early stage of human prostate tumorigenesis. Collectively, these results suggest that the molecular consequences of NKX3.1 loss depend on the epithelial proliferative stage at which its expression is lost, and that alterations in the PTEN–AKT–NKX3.1 axis are important for prostate cancer initiation.

Etv5, an ETS transcription factor, is expressed in granulosa and cumulus cells and serves as a transcriptional regulator of the cyclooxygenase-2

Etv4, Etv1, and Etv5 are members of Etv4 subfamily of E26 transformation-specific (Ets) transcription factors that are known to influence a host of biological processes. We previously showed that Etv5, expressed in Sertoli cells, plays a crucial role in maintaining spermatogonial stem cell niche in the mouse testis. However, it is not yet known whether Etv4 family members are expressed in the ovary or play any role in ovarian functions. Here, we show that Etv5 and Etv4 are expressed in mouse ovaries in granulosa and cumulus cells during folliculogenesis. Both Etv5 and Etv4 mRNAs are also detected in cumulus-oocyte complexes (COCs) and denuded oocytes. Notably, Etv4 is highly expressed in the cumulus cells of ovulated COCs at 16-h post-human chorionic gonadotropin. Cyclooxygenase-2 (PTGS2), a rate-limiting enzyme for prostaglandin synthesis, is critical for oocyte maturation and ovulation. Since several putative Ets-binding sites are present in the PTGS2 promoter, we examined whether Etv5 influences Ptgs2 transcriptional activity. Indeed, we found that addition of Etv5 increases the transcriptional activity of the 3.2-kb mouse Ptgs2 promoter by 2.5-fold in luciferase reporter assays. Collectively, the results show that Etv4 and Etv5 are expressed in granulosa and cumulus cells during folliculogenesis and ovulation, suggesting that they influence cellular events in the ovary by regulating downstream genes such as Ptgs2.

Utilization of DR1 as true RARE in regulating the Ssm, a novel retinoic acid-target gene in the mouse testis

Various nuclear receptors form dimers to activate target genes via specific response elements located within promoters or enhancers. Retinoid X receptor (RXR) serves as a dimerization partner for many nuclear receptors including retinoic acid receptor (RAR) and peroxisome proliferator-activated receptor (PPAR). Dimers show differential preference towards directly repeated response elements with 1-5 nucleotide spacing, and direct repeat 1 (DR1) is a promiscuous element which recruits RAR/RXR, RXR/RXR, and PPAR/RXR in vitro. In the present investigation, we report identification of a novel RAR/RXR target gene which is regulated by DR1s in the promoter region. This gene, namely spermatocyte-specific marker (Ssm), recruits all the three combinations of nuclear receptors in vitro, but in vivo regulation is observed by trans-retinoic acid-activated RAR/RXR dimer. Indeed, chromatin immunoprecipitation experiment demonstrates binding of RARbeta and RXRalpha in the promoter region of the Ssm. Interestingly, expression of Ssm is almost exclusively observed in spermatocytes in the adult mouse testis, where RA signaling is known to regulate developmental program of male germ cells. The results show that Ssm is a RAR/RXR target gene uniquely using DR1 and exhibits stage-specific expression in the mouse testis with potential function in later stages of spermatogenesis. This finding exemplifies usage of DR1s as retinoic acid response element (RARE) under a specific in vivo context.

Deficiency in DGCR8-dependent canonical microRNAs causes infertility due to multiple abnormalities during uterine development in mice

DGCR8 is an RNA-binding protein that interacts with DROSHA to produce pre-microRNA in the nucleus, while DICER generates not only mature microRNA, but also endogenous small interfering RNAs in the cytoplasm. Here, we produced Dgcr8 conditional knock-out mice using progesterone receptor (PR)-Cre (Dgcr8d/d) and demonstrated that canonical microRNAs dependent on the DROSHA-DGCR8 complex are required for uterine development as well as female fertility in mice. Adult Dgcr8d/d females neither underwent regular reproductive cycles nor produced pups, whereas administration of exogenous gonadotropins induced normal ovulation in these mice. Interestingly, immune cells associated with acute inflammation aberrantly infiltrated into reproductive organs of pregnant Dgcr8d/d mice. Regarding uterine development, multiple uterine abnormalities were noticeable at 4 weeks of age when PR is significantly increased, and the severity of these deformities increased over time. Gland formation and myometrial layers were significantly reduced, and the stromal cell compartment did not expand and became atrophic during uterine development in these mice. These results were consistent with aberrantly reduced stromal cell proliferation and completely failed decidualization. Collectively, we suggest that DGCR8-dependent canonical microRNAs are essential for uterine development and physiological processes such as proper immune modulation, reproductive cycle, and steroid hormone responsiveness in mice.

CHAPTER 4 – Implantation

Infertility and rapid population growth are two pressing global reproductive health issues. The processes of preimplantation embryo development and uterine preparation for implantation are two major determinants of the reproductive success. Basic and clinical research to better understand these events will help alleviate problems of female infertility, improve fertility regulation in women, and lead to the development of new and improved contraceptive methods. Interactions between the major uterine and embryonic cell types with respect to endocrine, paracrine, juxtacrine, and autocrine factors during implantation are extremely complex. Strategies comparing global gene expression profiles between the implantation and interimplantation sites have identified novel genes in the implantation process. Thus, a genome-wide screening approach coupled with functional assays will help elucidate these complex signaling pathways. In addition, experiments should be pursued to compare global gene expression patterns between wild-type and gene-deleted mouse uteri and blastocysts under defined physiological experimental conditions. The results obtained from these experiments may help uncover new signaling molecules and pathways not previously identified. The application of proteomics is also likely to provide information regarding interactions among various molecular pathways relevant to implantation.

Epidermal Growth Factor Signaling in Embryo-Uterine Interactions During Implantation

The word ‘growth factor’ refers to diverse families of molecules that promote cell growth in specific contexts. The term ‘cytokines’ is at times used interchangeably with growth factor, but with a more immunological inclination. Among the diverse growth factors, epidermal growth factor (EGF)-like factors are clearly distinct as mediators of molecular events in periimplantation uterine biology and embryo implantation. Involvement of certain growth factors in early events of pregnancy have also been explored in various animal models. These animal models include, but are not limited to, rodents, domestic animals, and primates. Human samples are the frequent subjects of comparative investigation. In this article, we focus on one notable mediator of the embryo-uterine interaction during implantation, the heparin-binding EGF-like growth factor. It will help readers grasp how the idea was developed and experimentally executed to establish the function of the growth factor in embryo-uterine interactions during implantation. Expansive descriptions of studies of EGF in uterine biology of other species could not be included because of page limitations, but further readings are suggested at the end of this text.