The genetic program for preimplantation development

An integrated genome-wide multi-omics analysis of gene expression dynamics in the preimplantation mouse embryo

Early mouse embryos have an atypical translational machinery that consists of cytoplasmic lattices and is poorly competent for translation. Hence, the impact of transcriptomic changes on the operational level of proteins is predicted to be relatively modest. To investigate this, we performed liquid chromatography–tandem mass spectrometry and mRNA sequencing at seven developmental stages, from the mature oocyte to the blastocyst, and independently validated our data by immunofluorescence and qPCR. We detected and quantified 6,550 proteins and 20,535 protein-coding transcripts. In contrast to the transcriptome – where changes occur early, mostly at the 2-cell stage – our data indicate that the most substantial changes in the proteome take place towards later stages, between the morula and blastocyst. We also found little to no concordance between the changes in protein and transcript levels, especially for early stages, but observed that the concordance increased towards the morula and blastocyst, as did the number of free ribosomes. These results are consistent with the cytoplasmic lattice-to-free ribosome transition being a key mediator of developmental regulation. Finally, we show how these data can be used to appraise the strengths and limitations of mRNA-based studies of pre-implantation development and expand on the list of known developmental markers.

Non-destructive monitoring of mouse embryo development and its qualitative evaluation at the molecular level using Raman spectroscopy

Current research focuses on embryonic development and quality not only by considering fundamental biology, but also by aiming to improve assisted reproduction technologies, such as in vitro fertilization. In this study, we explored the development of mouse embryo and its quality based on molecular information, obtained nondestructively using Raman spectroscopy. The detailed analysis of Raman spectra measured in situ during embryonic development revealed a temporary increase in protein content after fertilization. Proteins with a β-sheet structure—present in the early stages of embryonic development—are derived from maternal oocytes, while α-helical proteins are additionally generated by switching on a gene after fertilization. The transition from maternal to embryonic control during development can be non-destructively profiled, thus facilitating the in situ assessment of structural changes and component variation in proteins generated by metabolic activity. Furthermore, it was indicated that embryos with low-grade morphology had high concentrations of lipids and hydroxyapatite. This technique could be used for embryo quality testing in the future.

Maternal embryonic leucine zipper kinase (MELK): a novel regulator in cell cycle control, embryonic development, and cancer

Maternal embryonic leucine zipper kinase (MELK) functions as a modulator of intracellular signaling and affects various cellular and biological processes, including cell cycle, cell proliferation, apoptosis, spliceosome assembly, gene expression, embryonic development, hematopoiesis, and oncogenesis. In these cellular processes, MELK functions by binding to numerous proteins. In general, the effects of multiple protein interactions with MELK are oncogenic in nature, and the overexpression of MELK in kinds of cancer provides some evidence that it may be involved in tumorigenic process. In this review, our current knowledge of MELK function and recent discoveries in MELK signaling pathway were discussed. The regulation of MELK in cancers and its potential as a therapeutic target were also described.

Adiponectin and its receptors in preimplantation embryo development

Adiponectin can play an important role in regulating the female reproductive function and embryo development and can affect the embryo at very early stages of pregnancy--during the preimplantation period. Disturbances in the maternal adiponectin system are associated with several diseases, including diabetes type 2, obesity, and some female reproductive disorders. Adiponectin receptors are expressed in oocytes and preimplantation embryos and can be activated by adiponectin produced by maternal adipose tissue or organs of the female reproductive tract. Adiponectin can affect proliferation and survival of cells in preimplantation embryos, and these effects are isoform dependent. Experimental results suggest involvement of various protein kinases, including mitogen-activated protein kinases, in the regulation of these processes by adiponectin. Actions of adiponectin on lipid and glucose metabolism can increase the energy supply to the embryo, and final targets of adiponectin signaling are metabolic enzymes, glucose transporters, and fatty acid transporters. The involvement of several signaling molecules, such as AMPK/PRKA, PI3K, or AKT/PKB, in the regulation of metabolic processes by adiponectin has been demonstrated in preimplantation embryos. In summary, adiponectin produced in an endocrine/paracrine/autocrine manner can significantly influence preimplantation embryo development, uterine receptivity, and embryo implantation.

Gene expression differences in oocytes derived from adult and prepubertal Japanese Black cattle during in vitro maturation

The present study was carried out to compare the gene expression profiles in oocytes derived from adult and prepubertal Japanese Black cattle during in vitro maturation (IVM) using microarray gene chips (Bovine genome array containing 24,072 probe sets representing over 23,000 transcripts). Microarray experiments were conducted using total RNA isolated from immature [germinal vesicle (GV)] and in vitro matured [metaphase II, (MII)] oocytes derived from adult and prepubertal animals. A total of 333 (1.4%) and 549 (2.3%) genes were differentially expressed between prepubertal vs adult bovine GV and MII stages oocytes, respectively. Of these, 176 and 312 genes were up-regulated, while 157 and 237 were down-regulated in prepubertal when compared with adult GV and MII oocytes, respectively. It was also observed that 695 (2.9%) and 553 (2.3%) genes were differentially expressed between GV vs MII stage oocytes in the adult and prepubertal groups, respectively. Gene ontological classification of the differentially expressed genes revealed that up-regulated genes in adult oocytes were involved in signal transduction, transcriptional control and transport. Quantitative reverse transcription-PCR validated the expression profile of some selected transcripts and confirmed differences in the expression levels of transcripts between adult vs prepubertal groups in both GV and MII stages oocytes as identified by microarray data analysis. This study indicated for the first time that significant number of genes were differentially expressed (>2-fold, p < 0.01) between oocytes derived from adult and those from prepubertal Japanese Black cattle, and this difference increased during IVM.

The use of embryonic stem cell derived bioactive material as a new protein supplement for the in vitro culture of bovine embryos

Embryonic stem (ES) cells are expanded versions of the inner cell mass cells that compose the early mammalian blastocyst. Components derived from ES cells may contain various bioactive materials (BM) helpful for early preimplantation embryo growth. In this study, we examined the effect of human ES cell derived BM (hES-BM) on in vitro culture of bovine embryos. When bovine parthenogenetic day 2 embryos were cultured in 10% hES-BM, a significantly higher embryo development rate (44.3%) and increased cell numbers were observed relative to control medium containing 3 mg/ml BSA (19.5%; P<0.01). Among the various concentrations (5, 10 and 15%) and days of treatment (2 or 4 days) tested, 10% hES-BM treatment for 4 days provided the best culture environment to support the growth of bovine embryos in vitro (P<0.05). Little difference was observed between 10% hES-BM and 10% FBS treatment in the examined parthenogenetic or in vitro fertilized embryos, although the hES-BM group developed at a slightly better rate. However, the ICM cell numbers were significantly higher in the hES-BM group in irrespective of embryo origin (P<0.05). In addition, the relative levels of pluripotency (Oct4, × 1.8 fold; Nanog. × 3.3 fold), embryogenesis (Stat3, × 2.8 fold; FGF4, × 18.8 fold; E-cad, × 2.0 fold) and growth (Glut5, × 2.6 fold) genes were significantly higher in the 10% hES-BM group than in the 10% FBS group (P<0.05), while the levels of other genes (Bax, Bcl2, MnSOD and Connexin43) were not different. This is the first report examining the positive effects of hES-BM on bovine embryo development in vitro. Based on our results, we conclude that hES-BM can be used as a new protein supplement for bovine preimplantation embryo development.

Expression of mRNA, before and after freezing, in bovine blastocysts cultured under different conditions

Production methods and culture systems have been shown to affect blastocyst mRNA expression and cryopreservability, which may serve as sensitive indicators of embryo quality and developmental competence. In the present study, the impact of four established culture conditions for producing bovine blastocysts (in vitro production, IVP; gamete intra-fallopian transfer, GIFT; transfer of cleaved stages into the oviduct, CLVT; multiple ovulation embryo transfer, MOET) was assessed, in terms of both cryosurvival and levels of mRNA expression of several selected genes (occludin, desmocollin 2, solute carrier family 2 member 3, BAX, BCL-XL, heat shock protein 1A, aquaporin 3, DNA methyltransferase 1a) detected with RT-qPCR. At 24 hours post-thawing, blastocysts derived from in vitro production showed a significantly higher re-expansion rate compared to the other groups. At later times, this difference was no longer significant. Before freezing, embryos of the MOET group showed significantly more desmocollin 2 mRNA compared to embryos produced using other culture methods. After freezing, significant upregulation was found in transcripts of heat shock protein 1A in embryos of all groups; of solute carrier family 2 member 3, only in IVP derived embryos; of BAX, BCL-XL, occludin, desmocollin 2, only in the MOET and IVP groups. Aquaporin 3 and DNA methyltransferase 1a were neither up- nor downregulated in blastocysts of any group. In conclusion, these findings suggest that, after freezing, embryos seem to have switched on mRNA synthesis, an active metabolism, operational cell connections, and are prepared for hatching and beyond.

Bidirectional communication between oocytes and follicle cells: ensuring oocyte developmental competence

Female fertility is determined to a large extent by the quality (developmental competence) of the oocyte as reflected in its ability to undergo meiosis, be fertilized, and give rise to a healthy embryo. Growth of the mammalian oocyte is coordinated with that of the follicle that encloses it by the actions of signals that pass in both directions between the germline and somatic components. This review summarizes what is known about the roles played by 2 different modes of intrafollicular signalling in oogenesis: paracrine factors activating receptors on the opposite cell type, and direct sharing of small molecules throughout the follicle via gap junction channels. Recent evidence indicates that these 2 modes of signalling interact to regulate oocyte growth and granulosa cell proliferation and that defects in either can contribute to female infertility.

Effects of culture medium and protein supplementation on mRNA expression of in vitro produced bovine embryos

Numerous studies have reported aberrant gene expression levels attributed to suboptimal in vitro culture conditions. This study investigated the effects of different culture systems and protein sources on the developmental competence of in vitro production (IVP) embryos measured by cleavage and blastocyst rates, cell number, and relative abundance of POU5F1 (OCT4), nanog, GJA1 (connexin 43), and SLC2A1 (GLUT1) transcripts when compared to in vivo embryos. Experiment 1 compared IVP embryos cultured in either synthetic oviductal fluid (SOFaa) or potassium simplex optimized medium supplemented with amino acids (KSOMaa). Experiment 2 compared the same two culture systems with and without the addition of calf serum (CS). Results from both experiments indicated that despite similar developmental rates, significant differences were observed at the mRNA level. In Experiment 1, OCT4 was the only transcript to have a mean abundance level significantly higher in KSOMaa blastocysts when compared with both SOFaa and in vivo embryos. The same pattern of upregulation of OCT4 mRNA was noted in Experiment 2. There were no significant alterations of the ICM specific transcript nanog in either experiment. In contrast to reports by others, connexin 43 mRNA was not expressed at detectable levels in in vivo embryos analyzed in our studies. Blastocysts cultured in SOFaa with CS or KSOMaa had a significant upregulation of GLUT1 mRNA when compared with other treatments and in vivo embryos. Until differences between IVP and in vivo embryos are minimized, aberrations in IVP will continue to arise.

Expression and functional analyses of circadian genes in mouse oocytes and preimplantation embryos: Cry1 is involved in the meiotic process independently of circadian clock regulation

In mammals, circadian genes, Clock, Arntl (also known as Bmal1), Cry1, Cry2, Per1, Per2, and Per3, are rhythmically transcribed every 24 h in almost all organs and tissues to tick the circadian clock. However, their expression and function in oocytes and preimplantation embryos have not been investigated. In this study we found that the circadian clock may stop in mouse oocytes and preimplantation embryos. Real-time PCR analysis revealed the presence of transcripts of these genes in both oocytes and preimplantation embryos; however, their amounts did not oscillate every 24 h in one- to four-cell and blastocyst-stage embryos. Moreover, immunofluorescence analyses revealed that CLOCK, ARNTL, and CRY1 were localized similarly in the nuclei of germinal vesicle (GV) oocytes and one-cell- to four-cell-stage embryos. Because CRY1 is known to interact with the CLOCK-ARNTL complex to suppress transcription-promoting activity of the complex for genes such as Wee1, Cry2, Per1, Per2, and Per3 in cells having the ticking circadian clock, we hypothesized that if the circadian clock functions in GV oocytes and one-cell- to four-cell-stage embryos, CLOCK, ARNTL, and CRY1 might suppress the transcription of these genes in GV oocytes and one-cell- to 4-cell-stage embryos as well. As a result, knockdown of CRY1 in GV oocytes by RNA interference did not affect the transcription levels of Wee1, Cry2, Per1, Per2, and Per3, but it reduced maturation ability. Thus, it seems that circadian genes are not involved in circadian clock regulation in mouse oocytes and preimplantation embryos but are involved in physiologies, such as meiosis.

Messenger RNA in oocytes and embryos in relation to embryo viability

Messenger RNA (mRNA) expression techniques have become a powerful tool to analyze the relative abundance of transcripts related to oocyte and/or embryo quality. Numerous efforts to identify candidate genes for the developmental competence of bovine oocytes and embryos have been made employing different strategies. The preimplantation bovine embryo is initially under the control of maternal genomic information that is accumulated during oogenesis. Soon, the genetic program of development becomes dependent upon new transcripts derived from activation of the embryonic genome. The early steps in development including maturation, fertilization, timing of first cleavage, activation of the embryonic genome, compaction, and blastocyst formation can be affected by the culture media and conditions as well as the production procedure itself. These perturbations can possibly result in a dramatic decrease of the quality of the resulting blastocysts, and may even affect the viability of offspring born after transfer.

Physiological Roles of Clathrin Adaptor AP Complexes: Lessons from Mutant Animals

Clathrin-associated adaptor protein (AP) complexes play a key role in the transport of proteins, by regulating the formation of transport vesicles as well as cargo selection, between organelles of the post-Golgi network, namely, the trans-Golgi network (TGN), endosomes, lysosomes and the plasma membrane. Evidence has been accumulating for the physiological importance of AP complexes. Deficiency in AP-1A or AP-2 results in embryonic lethality in mice, indicating that these AP complexes are essential for normal development of embryos in mammals. In contrast, mutations in the genes encoding subunits of AP-3A cause an autosomal recessive disorder, Hermansky-Pudlak syndrome in human and its disease models in mice. Knockout mice for the neuron-specific AP-3B suffer from epileptic seizure. Further studies on the physiological and pathological aspects of AP complexes will not only be beneficial for better understanding of developmental biology and medical sciences, but also deepen our insight into the molecular mechanisms of vesicular traffic.

Molecular control of mitochondrial function in preimplantation mouse embryos

Mitochondria play a key role in a number of physiological events during all stages of life, including the very first stages following fertilization. It is, therefore, important to understand the mechanisms controlling mitochondrial activity during early embryogenesis to determine their role in development outcome. The objective of this study was to investigate the molecular control of mitochondrial transcription and mitochondrial DNA (mtDNA) replication in mouse preimplantation embryos. We estimated the mtDNA copy number and characterized the expression patterns of two mitochondrial genes and several nuclear genes that encode mitochondrial transcription and replication factors throughout preimplantation development. Mitochondrial gene transcripts were present in larger quantities in morula and blastocyst stage embryos relative to other stages. A significant increase in the amount of mRNA for nuclear genes encoding mtDNA transcription factors was observed in eight-cell stage embryos. Although a similar increase in the mRNA levels of nuclear genes encoding mtDNA replication factors was observed in morula and blastocyst stage embryos, the number of mtDNA molecules remained stable during preimplantation stages, suggesting that nuclear-encoded mitochondrial transcription factors are involved in the regulation of mtDNA transcription during early development. Although transcripts of replication factors are abundant at the morula and blastocyst stage, mtDNA replication did not occur until the blastocyst stage, suggesting that the inhibition of mtDNA replication is controlled at the post-transcriptional level during early embryogenesis.

Messenger RNA expression patterns in bovine embryos derived from in vitro procedures and their implications for development

The preimplantation bovine embryo is initially under the control of maternal genomic information that is accumulated during oogenesis. The genetic programme of development soon becomes dependent on new transcripts derived from activation of the embryonic genome. The early steps in development, including the timing of the first cleavage, activation of the embryonic genome, compaction and blastocyst formation, can be affected by the culture media and conditions, as well as the production procedure itself. These perturbations can possibly result in a marked decrease in the quality of the resulting blastocysts and may even affect the viability of offspring born after transfer. In vitro procedures such as in vitro production and somatic nuclear transfer of bovine embryos have been shown to be correlated with significant up- or downregulation, de novo induction or silencing of genes critical for undisturbed fetal and neonatal development. These alterations are likely to be caused by epigenetic modifications, such as DNA methylation and histone modifications. Analysis of perturbed epigenetic reprogramming and of the related phenomena, such as genomic imprinting and X-chromosome inactivation, in bovine embryos is promising for understanding the underlying mechanisms of developmental abnormalities, such as large offspring syndrome.

Temporal/spatial expression and efflux activity of ABC transporter, P-glycoprotein/Abcb1 isoforms and Bcrp/Abcg2 during early murine development

ABC transporters pump out from cells a large number of endo- and xenobiotics including signal molecules and toxins; they are molecular markers of stem/progenitor cells as well. Here, we present the study of temporal/spatial patterns of Abcb1 isoforms and Abcg2 transporter expression and efflux activity in pre- and early postimplantation murine embryos. We found in 2-cell embryos abcb1a, abcb1b and abcg2 mRNAs which were believed to be maternally inherited. The expression of abcb1b and abcg2 genes was found in blastocysts and in 7 days postcoitum (dpc) embryos, while in 9dpc embryos beside of abcb1b/abcg2, the abcb1a gene was expressed. The abcb2 mRNA was detectable neither in pre- nor in postimplantation embryos. Moreover, we analysed temporal/spatial patterns of rhodamine 123/Hoechst 33342 efflux, which mirrors the ABC transporter phenotype, from individual cells of pre- and postimplantation murine embryos. The blastomeres of 2-, 4- and 8-cell embryos had efflux-inactive phenotype. Single, efflux-active cells emerged first in the morulae and their number increased in blastocyst inner cell mass. In 6 and 7 dpc embryos, all embryonic cells hold the efflux-active phenotype. Proximal embryonic endoderm of 6-8 dpc embryos contained two sub-domains: one consisted of efflux-active cells and another one of efflux-inactive cells reflecting polarity of an embryo. Between 7 and 8 dpc, at the onset of organogenesis, the vehement surge of efflux-inactive embryonic cells occurred, and their number increased in 9 dpc embryos, which consequently contained few efflux-active cells.

Na+/K+-ATPase regulates tight junction formation and function during mouse preimplantation development

Research applied to the early embryo is required to effectively treat human infertility and to understand the primary mechanisms controlling development to the blastocyst stage. The present study investigated whether the Na(+)/K(+)-ATPase regulates tight junction formation and function during blastocyst formation. To investigate this hypothesis, three experimental series were conducted. The first experiments defined the optimal dose and treatment time intervals for ouabain (a potent and specific inhibitor of the Na(+)/K(+)-ATPase) treatment. The results demonstrated that mouse embryos maintained a normal development to the blastocyst stage following a 6-h ouabain treatment. The second experiments investigated the effects of ouabain treatment on the distribution of ZO-1 and occludin (tight junction associated proteins). Ouabain treatment (up to 6 h) or culture in K(+)-free medium (up to 6 h) resulted in the appearance of a discontinuous ZO-1 protein distribution and a loss of occludin immunofluorescence. The third set of experiments examined the influence of ouabain treatment on tight junction function. Ouabain treatment or culture in K(+)-free medium affected tight junction permeability as indicated by an increase in the proportion of treated embryos accumulating both 4 kDa and 40 kDa fluorescein isothiocyanate (FITC)-dextran into their blastocyst cavities. The results indicate that the Na(+)/K(+)-ATPase is a potent regulator of tight junction formation and function during mouse preimplantation development.

Molecular biology of preimplantation embryos: primer for philosophical discussions

This article is based on a presentation at the First International Conference on Ethics, Science and Moral Philosophy of Assisted Human Reproduction. The goal is to provide scientific background for the discussion of philosophic issues. Recent advances in the systematic molecular analysis of preimplantation embryos are summarized, including the molecular identification of nearly all genes involved in preimplantation development and their detailed expression patterns. Notwithstanding a quantum leap in molecular understanding of preimplantation embryos, molecular evidence seems to provide no decisive definition of a threshold for the beginning of human life during preimplantation development.

Fully grown mouse oocyte contains transcription inhibiting activity which acts through histone deacetylation

Mouse ovarian oocytes remain transcriptionally active during the entire period of oocyte growth. In the fully grown antral oocytes, RNA synthesis is terminated before initiation of maturation. The mechanism involved in this process remains unknown. It was proposed that some signals provided by companion granulosa cells, and accumulating in the cytoplasm of the oocyte, play a role in the termination of RNA synthesis. It seems possible that under influence of these signals, the oocyte cytoplasm becomes transcriptionally nonpermissive. In the present study, transcriptionally active, single blastomeres of the 8-cell mouse embryo were fused with nontranscribing fully grown oocytes, and RNA synthesis was analyzed in the resulting hybrids. We show that after 27 hr of culture of hybrid cells, transcription in the blastomere nuclei was finally silenced. This process occurs without nuclear envelope breakdown and is accompanied by progressive condensation of the chromatin in the blastomere nuclei. We show also the involvement of core histone deacetylation in the modification of chromatin configuration and termination of RNA synthesis in the blastomere nuclei.

Male infertility and DNA damage in Doppel knockout and prion protein/Doppel double-knockout mice

The prion protein (PrP) and Doppel (Dpl) have many structural and biochemical properties in common, leading to the suggestion that the lack of an obvious phenotype in PrP-deficient mice maybe because of compensation by Dpl. To test this hypothesis and also investigate the function of Dpl we have generated Prnd(-/-) and Prnp(-/-)/Prnd(-/-) mouse lines. Both develop normally and display an identical male sterility phenotype that differs from that reported for another Prnd(-/-) mouse line. Sperm from both our mutant lines were present at normal concentrations, had normal motility, and no morphological abnormalities. Despite only rarely fertilizing oocytes in vivo, because of an inability to perform the acrosome reaction, mutant sperm were capable of fertilization in vitro, albeit at reduced rates compared to wild type. Elevated levels of oxidative DNA damage were found in both types of mutant sperm and resulting embryos failed at an early stage. Therefore we found no evidence that Dpl compensates for the loss of PrP function in mutant mouse lines, but it does have an important anti-oxidant function necessary for sperm integrity and male fertility.

Cytoplasmic transfer in the mouse in conjunction with intracytoplasmic sperm injection

Cytoplasmic transfer (CT) from a donor to a recipient oocyte has been used clinically to facilitate human pregnancies. Data reported here describe the first characterization of CT coincident with intracytoplasmic sperm injection in the mouse system. Sibling oocytes were used to transfer 2, 4, or 6 pl of ooplasm to a recipient egg along with a sperm head using piezo-actuated injection. Survival and fertilization after CT were comparable to controls at 2 pl and 4 pl, but survival was significantly reduced with 6 pl volumes. Development to the blastocyst stage was also inversely related to CT volume, with some decline beginning with the 4 pl CT group. However, some blastocysts did develop in all of the groups. The results are in contrast with human eggs, which tolerate larger CT volumes. Results indicate that the mouse system can be used to characterize the transfer of exogenous materials concomitant with sperm injection, provided that the CT volume is not excessive.

A role for tachykinins in female mouse and rat reproductive function

Tachykinins may be involved in reproduction. A reverse transcription-polymerase chain reaction assay was used to analyze the expression of tachykinins and tachykinin receptors in different types of reproductive cells from mice. The preprotachykinin (PPT) genes, PPT-A, PPT-B and PPT-C, that encode substance P/neurokinin A, neurokinin B, and hemokinin-1, respectively, and the genes that encode the tachykinin NK1, NK2, and NK3 receptors were all expressed, at different levels, in the uterus of superovulated, unfertilized mice. The mRNA of neprilysin (NEP), the main enzyme involved in tachykinin metabolism, was also expressed in the uterus. Isolated cumulus granulosa cells expressed PPT-A, PPT-B, PPT-C, and NEP and low levels of the tachykinin NK1 and NK2 receptors. Mouse oocytes expressed PPT-A and -B mRNA transcripts. A low expression of the three tachykinin receptors was observed but PPT-C and NEP were undetectable. Two- and 8- to 16-cell mouse embryos expressed only a low-abundance transcript corresponding to the NK1 receptor. However, the mRNAs of PPT-B, PPT-C and NEP appeared in blastocyst-stage embryos. A low-abundance transcript corresponding to the NK2 receptor was the only target gene detected in mice sperm. Female mice or rats treated neonatally with capsaicin showed a reduced fertility. A reduction in litter size was observed in female rats treated in vivo with the tachykinin NK3 receptor antagonist SR 142801. These data show that tachykinins of both neuronal and nonneuronal origin are differentially expressed in various types of reproductive cells and may play a role in female reproductive function.

Display of different modes of transcription by the promoters of an early embryonic gene, Zfp352, in preimplantation embryos and in somatic cells

We have previously reported a Krupple-like finger protein gene, Zfp352, which is expressed temporarily in two- to eight-cell mouse embryos. The Zfp352 gene is intron-less in the coding region but carries a solitary 4.3-kb intron in the 5'-untranslated region. In this study, we have analyzed the Zfp352 promoter activity in early embryos and in somatic cells. We determined that the major Zfp352 promoter, designated P1 and located upstream of exon 1, is utilized in both early embryos and in somatic cells. A TATA-like box and a transcription initiator element are discernible in the P1 promoter. We uncovered an alternative promoter, designated P2, in the intron. 5'-Rapid amplification of cDNA ends and real-time RT-PCR experiments indicated that the P2 promoter is weak and is probably fortuitous in early embryos. In somatic cells, however, transfection experiments showed that P2 is as active as P1 as a promoter. Furthermore, P2 appears to be composed of two different subdomains used differentially for transcription initiation in embryos and in somatic cells. Our observations may bear relevance in explaining developmental deficiencies associated with somatic cell cloning experiments.

Epigenetic mechanisms for primary differentiation in mammalian embryos

This review examines main developments related to the interface between primary mammalian cell differentiation and various aspects of chromosomal structure changes, such as heterochromatin dynamics, DNA methylation, mitotic recombination, and inter- and intrachromosomal differentiation. In particular, X chromosome difference, imprinting, chromosomal banding, methylation pattern, single-strand DNA breaks, sister chromatid exchanges (SCEs), and sister chromatid asymmetry are considered. A hypothesis is put forward which implies the existence of an epigenetic asymmetry versus mirror symmetry of sister chromatids for any DNA sequences. Such epigenetic asymmetry appears as a result of asymmetry of sister chromatid organization and of SCE and is a necessary (not sufficient) condition for creating cell diversity. The sister chromatid asymmetry arises as a result of consecutive rounds of active and passive demethylation which leads after chromatin assembly events to chromatid difference. Single-strand DNA breaks that emerge during demethylation trigger reparation machinery, provend as sister chromatid exchanges, which are not epigenetically neutral in this case. Taken together, chromatid asymmetry and SCE lead to cell diversity regarding their future fate. Such cells are considered pluripotent stem cells which after interplay between a set of chromosomal domains and certain substances localized within the cytoplasmic compartments (and possibly cell interactions) can cause sister cells to express different gene chains. A model is suggested that may be useful for stem cell technology and studies of carcinogenesis.

Degradation of maternal cdc25c during the maternal to zygotic transition is dependent upon embryonic transcription

To gain a better understanding of the molecular differences that may contribute to cleavage arrest and the poorer development associated with laboratory produced embryos, a series of experiments were conducted to quantitate the message levels of the cell cycle controller cdc25c, over the maternal to zygotic transition (MZT) in 4-cell in vivo- and in vitro-derived porcine embryos. The experiments were designed to measure both maternal and embryonic derived cdc25c transcripts by quantitative reverse transcription-competitive polymerase chain reaction (RT-cPCR), determine the point of the transition to zygotic genome activation, and study the interaction between initial embryonic transcription and maternal cdc25c degradation. Analysis of in vivo- and in vitro-derived embryos revealed no difference in cdc25c message level for any of the times P4CC (P > 0.05). Comparison of control embryos from 5- to 33-hr P4CC revealed a reduction in transcript quantities in the 10-hr P4CC group that was maintained at later time points (P < 0.05). Embryos cultured in the RNA polymerase inhibitor, alpha-amanitin, from cleavage to 5-, 10-, 18-, 25-, or 33-hr P4CC displayed no difference in cdc25c levels when compared to controls at similar time points (P > 0.05). However, if embryos were first exposed to alpha-amanitin after 18-hr P4CC with this treatment continuing to 33 hr, the levels of cdc25c transcript were reduced (P < 0.04) when compared to those embryos that were first exposed to the inhibitor at either 5- or 10-hr P4CC. This finding and the comparison of these same embryos to the 0-33-hr alpha-amanitin and control groups allowed us to conclude that cdc25c transcription began between 10- and 18-hr P4CC, with the degradation of maternal cdc25c message dependent on transcriptional initiation.

Of mice, frogs and flies: generation of membrane asymmetries in early development

Embryonic development begins with cleavage of the fertilized egg. Cleavage comprises two major processes: cytokinesis and formation of a polarized epithelial cell layer. The focus of this review is comparison of the generation of membrane polarity during embryonic cleavage in three different developmental model systems. In mammalian embryos, as exemplified by analysis of the mouse, generation of distinct membrane domains is uncoupled from cleavage divisions and is initiated in a specific developmental phase, called compaction. In Xenopus laevis embryos, generation of polarized blastomeres occurs simultaneously with cytokinesis. The origin of specific membrane domains of X. laevis polar blastomeres, however, can be traced back to oogenesis. Finally, in Drosophila melanogaster, generation of polarized cells occurs at cellularization. The relevance of cell adhesion, cell junctions and cytocortical scaffolds will be discussed for each of the model systems. Despite enormous morphologic differences, the three models share many common features; in particular, many important molecular interactions are conserved.

In silico mining of EST databases for novel pre-implantation embryo-specific zinc finger protein genes

Progress in the understanding of early mammalian embryo development has been severely hampered by scarcity of study materials. To circumvent such a constraint, we have developed a strategy that involves a combination of in silico mining of new genes from expressed sequence tags (EST) databases and rapid determination of expression profiles of the dbEST-derived genes using a PCR-based assay and a panel of cDNA libraries derived from different developmental stages and somatic tissues. We demonstrate that in a random sample of 49 independent dbEST-derived zinc finger protein genes mined from a mouse embryonic 2-cell cDNA library, more than three-quarters of these genes are novel. Examination of characteristics of the human orthologues derived from these mouse genes reveals that many of them are associated with human malignancies. Expression studies have further led to the identification of three novel genes that are exclusively expressed in mouse embryos before or up to the 8-cell stage. Two of the genes, designated 2czf45 and 2czf48 (2czf for 2-cell zinc finger), are zinc finger protein genes coding for a RBCC protein with a RFP domain and a protein with three C2H2 fingers, respectively. The third gene, designated 2cpoz56, codes for a protein with a POZ domain that is often associated with zinc finger proteins. These three genes are candidate genes for regulatory or other functions in early embryogenesis. The strategy described in this report should generally be applicable to rapid and large-scale mining of other classes of rare genes involved in other biological and pathological processes. Mol. Reprod. Dev. 59:249-255, 2001.

Mitochondrial DNA instability and peri-implantation lethality associated with targeted disruption of nuclear respiratory factor 1 in mice

In vitro studies have implicated nuclear respiratory factor 1 (NRF-1) in the transcriptional expression of nuclear genes required for mitochondrial respiratory function, as well as for other fundamental cellular activities. We investigated here the in vivo function of NRF-1 in mammals by disrupting the gene in mice. A portion of the NRF-1 gene that encodes the nuclear localization signal and the DNA-binding and dimerization domains was replaced through homologous recombination by a beta-galactosidase-neomycin cassette. In the mutant allele, beta-galactosidase expression is under the control of the NRF-1 promoter. Embryos homozygous for NRF-1 disruption die between embryonic days 3.5 and 6.5. beta-Galactosidase staining was observed in growing oocytes and in 2. 5- and 3.5-day-old embryos, demonstrating that the NRF-1 gene is expressed during oogenesis and during early stages of embryogenesis. Moreover, the embryonic expression of NRF-1 did not result from maternal carryover. While most isolated wild-type and NRF-1(+/-) blastocysts can develop further in vitro, the NRF-1(-/-) blastocysts lack this ability despite their normal morphology. Interestingly, a fraction of the blastocysts from heterozygous matings had reduced staining intensity with rhodamine 123 and NRF-1(-/-) blastocysts had markedly reduced levels of mitochondrial DNA (mtDNA). The depletion of mtDNA did not coincide with nuclear DNA fragmentation, indicating that mtDNA loss was not associated with increased apoptosis. These results are consistent with a specific requirement for NRF-1 in the maintenance of mtDNA and respiratory chain function during early embryogenesis.

Identification and characterization of human Wee1B, a new member of the Wee1 family of Cdk-inhibitory kinases

BACKGROUND

In eukaryotic cells, the kinase activity of the mitosis-promoting complex composed of cyclin B and Cdc2 (Cdk1) is negatively regulated by the phosphorylation of Cdk1 on threonine or tyrosine residues within its ATP binding domain.

RESULTS

We identified human Wee1B by searching a sequence database. The predicted human Wee1B protein comprises 561 amino acids. Northern blot analysis revealed that human Wee1B mRNA is particularly abundant in testis. Interestingly, RT-PCR using early embryos revealed that the Wee1B product was readily detectable at the mature oocyte, but abruptly disappeared at embryonic day 2.5, suggesting that the amount of Wee1B mRNA is dependent on the maternal expression. GFP-Wee1B showed a predominantly nuclear localization in HeLa cells. Human Wee1B was able to rescue the lethal phenotype of the fission yeast wee1-50Deltamik1 mutant, and over-expression of the human protein in these cells resulted in cell elongation as a result of arrest of the cell cycle at the G2-M transition. Recombinant Wee1B effectively phosphorylated cyclin B-associated Cdk1 on tyrosine-15, resulting in an inactivation of the kinase activity of Cdk1.

CONCLUSION

We identified human Wee1B as a novel Cdk1-inhibitory kinase. The identification of this new member of the Wee1 family suggests that inhibition of Cdk1 is mediated at multiple levels in mammals.

Molecular genetics of implantation in the mouse

Embryo implantation is a complex developmental process requiring precise coordination between mother and offspring to ensure success. Implantation failure is clinically relevant to in vitro fertilization programs and to an understanding of diseases of pregnancy like preeclampsia. Basic and clinical research have identified a number of proteins involved in peri-implantation development, but an understanding of the implantation process and its cellular and molecular components is just beginning. This review will focus on the implantation and development of the murine embryo and placenta. The significance of ectopic expression and targeted mutagenesis models to these processes will be discussed.

Large-scale cDNA analysis reveals phased gene expression patterns during preimplantation mouse development

Little is known about gene action in the preimplantation events that initiate mammalian development. Based on cDNA collections made from each stage from egg to blastocyst, 25438 3′-ESTs were derived, and represent 9718 genes, half of them novel. Thus, a considerable fraction of mammalian genes is dedicated to embryonic expression. This study reveals profound changes in gene expression that include the transient induction of transcripts at each stage. These results raise the possibility that development is driven by the action of a series of stage-specific expressed genes. The new genes, 798 of them placed on the mouse genetic map, provide entry points for analyses of human and mouse developmental disorders.

Expression of Melk, a new protein kinase, during early mouse development

A new gene named maternal embryonic leucine zipper kinase, Melk, has been recently identified (Heyer et al. [1997] Mol. Reprod. Dev. 47:148-156). As a basis for further study of the function of the gene, we have examined the expression of Melk across a wide range of embryonic stages, from the ovulated egg and 2-cell embryo through the gastrulation and early organogenesis stages, by in situ hybridization and immunohistochemistry. Melk is expressed in a spatially and temporally specific pattern during mammalian embryogenesis. The strongest expression was detected during maturation of oocytes and preimplantation development. Given its expression pattern, Melk may play an important role during preimplantation embryonic development. Dev Dyn 1999;215:344-351.

Alterations in the relative abundance of gene transcripts in preimplantation bovine embryos cultured in medium supplemented with either serum or PVA

In preimplantation bovine embryos, the relative abundance of various developmentally important gene transcripts was determined by a semi-quantitative RT-PCR assay to analyze the effects of two medium supplements, serum or polyvinyl alcohol (PVA). Development to morula, blastocyst, and hatched blastocyst stages was higher (P < or = 0.05) in medium supplemented with serum than in medium supplemented with PVA. Connexin43 mRNA expression virtually disappeared from the 8-16 cell stage onward, but reappeared in the hatched blastocyst in serum-supplemented medium, whereas it was detected in PVA-derived embryos throughout development. No differences were found for plakophilin mRNA between both culture groups. Desmocollin II mRNA showed a sharp increase at the blastocyst stage in both groups with a higher transcription level in PVA-generated embryos. A significant difference in desmocollin III transcripts was detectable at the 8-16-cell stage between serum- and PVA-derived embryos. Transcripts for desmoglein 1 and desmocollin I were not detected at any preimplantation stage, irrespective of medium supplementation. The relative abundance of glucose-transporter-1 mRNA was significantly increased at the 8-16-cell stage in embryos produced in medium supplemented with PVA, but not serum. Heat shock protein and poly(A)polymerase mRNA were continuously expressed during preimplantation development in both culture groups. Although poly(A)polymerase mRNA was significantly elevated in PVA- over serum-derived embryos, heat shock protein mRNA expression was significantly enhanced in serum-generated embryos over PVA-derived embryos. Interferon tau mRNA showed a significant increase at the hatched blastocyst stage only in PVA-supplemented medium. These data suggest that alterations in mRNA expression are associated with culture environment. Timing and magnitude of the alterations varied among the different transcripts and were significantly affected by the presence of exogenous protein in a stage-specific manner, predominantly at critical developmental time points.

Early embryonic death of mice deficient in gamma-adaptin

Intracellular protein transport and sorting by vesicles in the secretory and endocytic pathways requires the formation of a protein coat on the membrane. The heterotetrameric adaptor protein complex 1 (AP-1) promotes the formation of clathrin-coated vesicles at the trans-Golgi network. AP-1 interacts with various sorting signals in the cytoplasmic tails of cargo molecules, thus indicating a function in protein sorting. We generated mutants of the gamma-adaptin subunit of AP-1 in mice to investigate its role in post-Golgi vesicle transport and sorting processes. gamma-Adaptin-deficient embryos develop until day 3.5 post coitus and die during the prenidation period, revealing that AP-1 is essential for viability. In heterozygous mice the amount of AP-1 complexes is reduced to half of controls. Free beta1- or micro1 chains were not detectable, indicating that they are unstable unless they are part of AP-1 complexes. Heterozygous mice weigh less then their wild-type littermates and show impaired T cell development.

Contributions of Na+/H+ exchanger isoforms to preimplantation development of the mouse

Previous work provided evidence of Na+/H+ exchanger activity in the apical domain of mouse trophectodermal plasma membranes that provides a route for entry of extracellular Na+ (Manejwala et al., 1989). This activity was hypothesized to contribute to the trans-trophectodermal Na+ flux that is required for blastocoel expansion. In the present work, we have used reverse transcriptase-polymerase chain reaction (RT-PCR) and immunocytochemistry to identify members of the Na+/H+ exchanger (NHE) family that are likely to participate in this process. When cDNA preparations from ovulated oocytes and several stages of preimplantation development were tested with PCR primers specific for the NHE-1, -2, -3, and -4 isoforms of the exchanger, only amplicons representing the NHE-1 and NHE-3 isoforms were detected. The identity of these amplicons was confirmed by direct sequencing. NHE-1 mRNA is present in oocytes and in all preimplantation stages, increasing threefold on a per embryo basis between the 4-cell and blastocyst stages. NHE-3 mRNA, on the other hand, was only detected in oocytes. Immunocytochemical analysis of blastocysts revealed that NHE-1 is localized in the basolateral domain of the trophectoderm, whereas NHE-3 is localized in the apical domain, a situation like that in epithelia of adult organs. We conclude that NHE-3, an oogenetic product that persists into the blastocyst stage, is the Na+/H+ exchanger isoform most likely to be involved in blastocoel expansion.

Connections with connexins: the molecular basis of direct intercellular signaling

Adjacent cells share ions, second messengers and small metabolites through intercellular channels which are present in gap junctions. This type of intercellular communication permits coordinated cellular activity, a critical feature for organ homeostasis during development and adult life of multicellular organisms. Intercellular channels are structurally more complex than other ion channels, because a complete cell-to-cell channel spans two plasma membranes and results from the association of two half channels, or connexons, contributed separately by each of the two participating cells. Each connexon, in turn, is a multimeric assembly of protein subunits. The structural proteins comprising these channels, collectively called connexins, are members of a highly related multigene family consisting of at least 13 members. Since the cloning of the first connexin in 1986, considerable progress has been made in our understanding of the complex molecular switches that control the formation and permeability of intercellular channels. Analysis of the mechanisms of channel assembly has revealed the selectivity of inter-connexin interactions and uncovered novel characteristics of the channel permeability and gating behavior. Structure/function studies have begun to provide a molecular understanding of the significance of connexin diversity and demonstrated the unique regulation of connexins by tyrosine kinases and oncogenes. Finally, mutations in two connexin genes have been linked to human diseases. The development of more specific approaches (dominant negative mutants, knockouts, transgenes) to study the functional role of connexins in organ homeostasis is providing a new perception about the significance of connexin diversity and the regulation of intercellular communication.

Multiple members of the connexin gene family participate in preimplantation development of the mouse

The connexin gene family, of which there are at least 12 members in rodents, encodes the protein subunits intercellular membrane channels (gap junction channels). Because of the diverse structural and biophysical properties exhibited by the different connexins, it has been proposed that each may play a unique role in development or homeostasis. We have begun to test this hypothesis in the preimplantation mouse embryo in which de novo gap junction assembly is a developmentally regulated event. As a first step, we have used reverse transcription-polymerase chain reaction (RT-PCR) to determine the connexin mRNA phenotype of mouse blastocysts, and have identified transcripts of connexins 30.3, 31, 31.1, 40, 43, and 45. Quantitative measurements indicated that all six of these connexin genes are transcribed after fertilization. They can be divided into two groups with respect to the timing of mRNA accumulation: Cx31, Cx43, and Cx45 mRNAs accumulate continuously from the two- or four-cell stage, whereas Cx30.3, Cx31.1, and Cx40 mRNAs accumulate beginning in the eight-cell stage. All six mRNAs were found to co-sediment with polyribosomes from their time of first appearance, indicating that all six are translated. The expression of Cx31.1 and Cx40 was examined by confocal immunofluorescence microscopy; whereas both could be detected in compacting embryos, only Cx31.1 could be seen in punctate membrane foci indicative of gap junctions. Taken together with other results (published or submitted), our findings indicate that at least four connexins (Cx31, 31.1, 43 and 45) contribute to gap junctions in preimplantation development. The expression of multiple connexin genes during this early period of embryogenesis (when there are only two distinct cell types) raises questions about the functional significance of connexin diversity in this context.

Expression of the gap junction proteins connexin31 and connexin43 correlates with communication compartments in extraembryonic tissues and in the gastrulating mouse embryo, respectively

We have characterized the pattern of connexin expression in embryonic and extraembryonic tissues during early mouse development. In the preimplantation blastocyst, at 3.5 days post coitum (dpc), immunofluorescent signals specific for connexin31 and connexin43 proteins were present in both the inner cell mass and the trophectoderm, as shown by confocal laser scan microscopy. Immediately after implantation at 6.5 dpc, however, we find complete compartmentation of these two connexins: connexin31 mRNA and protein are expressed exclusively in cells derived from the trophectoderm lineage, whereas connexin43 mRNA and protein are detected in cells derived from the inner cell mass. This expression pattern of connexin31 and connexin43 is maintained at 7.5 dpc when the axial polarity of the mouse embryo is established. It correlates with the communication compartments in extraembryonic tissues and the gastrulating mouse embryo, respectively. The communication boundary between those compartments may be due to incompatibility of connexin31 and connexin43 hemichannels, which do not communicate with each other in cell culture.

Epithelial cell polarity in early Xenopus development

The Xenopus blastula consists of two morphologically distinct cell types. Polarized epithelial cells build up the embryonic surface and fence off an inner non-polarized cell population. We examined the establishment of this early functional cell diversification in the embryo by single cell analysis, in vitro cell culture, and transplantation experiments. Single blastomeres from a 64-cell embryo (1/64 cells) exhibit several features of polarized cells. The plasma membrane of 1/64 cells consists of an apical domain, which is inherited from the original egg membrane, and a basolateral domain derived from newly formed membrane during cleavage. These are inherent, cell-autonomous properties of the blastomeres, as they form and are maintained in blastomeres raised in the absence of any cell interactions in calcium free medium. Upon in vitro culture a single 1/64 cell gives rise to an aggregate of two different cell types. Cells carrying a part of the former egg membrane domain differentiate into polarized epithelial cells, whereas cells lacking this membrane domain are not polarized. These results demonstrate that the inclusion of the egg membrane, rather than external signals related to the position of a cell in the intact embryo, is required for the apical/basolateral differentiation of the surface epithelium. This view is supported by cell transplantation studies. A single 1/64 cell was implanted into the blastocoel of a stage 8 blastula embryo. The progeny of the implanted cell proliferate within the host embryo and split into two morphologically distinct populations with different cell behaviours. Cells incorporating a part of the egg membrane form coherent patches of polarized epithelial cell sheets in the interior of the host embryo. In contrast, cells lacking egg membrane do not exhibit any characteristics of polarized cells and eventually spread into different regions of the host embryo. Our results show that the egg membrane and/or components of the submembrane cortex play a determinative role in the formation of the blastula epithelium.

DNA-strand breaks in chromosomes of early mouse embryos as detected by in situ nick translation and gap filling

The nick translation and gap filling procedures, without external addition of nicking enzymes, were performed in situ on fixed chromosomes of mouse preimplantation and postimplantation embryos and of bone marrow in order to detect possible DNA single-strand breaks (nicks and (or) gaps). All chromosome preparations were made using the same technique. Nick translation of chromosomal DNA with DNA polymerase I (Pol I) or gap filling with the Klenow fragment of Pol I in the presence of biotinylated-dUTP, demonstrated a regular absence of label on chromosomes of postimplantation embryos and bone marrow. No difference in sensitivity was found between the holoenzyme and the Klenow fragment. In preimplantation embryos, the chromosome reactivity in nick translation was highest at the blastocyst stage and varied according to cleavage divisions of the zygote.

Expression of SRY transcripts in preimplantation human embryos

We have examined the expression of SRY mRNA in individual in vitro fertilized preimplantation human embryos; because of ethical constraints, these studies were confined to embryos with one and three pronuclei. Using a sensitive reverse transcriptase-polymerase chain reaction (RT-PCR) assay, we observed SRY mRNA at the one-cell through the blastula stages but not in spermatozoa. These results indicate that the de novo transcription of this sex-specific gene occurs at a developmental time considerably earlier than that of gonadal differentiation. Our results also indicate that in vitro fertilized embryos with one pronucleus are likely to be diploid.