Expression of Na,K-ATPase alpha and beta subunit genes during preimplantation development of the mouse

Free fatty acid treatment of mouse preimplantation embryos demonstrates contrasting effects of palmitic acid and oleic acid on autophagy

Treatment of mouse preimplantation embryos with elevated palmitic acid (PA) reduces blastocyst development, while co-treatment with PA and oleic acid (OA) together rescues blastocyst development to control frequencies. To understand the mechanistic effects of PA and OA treatment on early mouse embryos, we investigated the effects of PA and OA, alone and in combination, on autophagy during preimplantation development in vitro. We hypothesized that PA would alter autophagic processes and that OA co-treatment would restore control levels of autophagy. Two-cell stage mouse embryos were placed into culture medium supplemented with 100 μM PA, 250 μM OA, 100 μM PA and 250 μM OA, or KSOMaa medium alone (control) for 18 - 48 h. The results demonstrated that OA co-treatment slowed developmental progression after 30 h of co-treatment but restored control blastocyst frequencies by 48 h. PA treatment elevated LC3-II puncta and p62 levels per cell while OA co-treatment returned to control levels of autophagy by 48 h. Autophagic mechanisms are altered by non-esterified fatty acid (NEFA) treatments during mouse preimplantation development in vitro, where PA elevates autophagosome formation and reduces autophagosome degradation levels, while co-treatment with OA reversed these PA-effects. Autophagosome-lysosome co-localization only differed between PA and OA alone treatment groups. These findings advance our understanding of the effects of free fatty acid exposure on preimplantation development, and they uncover principles that may underlie the associations between elevated fatty acid levels and overall declines in reproductive fertility.

Metabolic activity of human blastocysts correlates with their morphokinetics, morphological grade, KIDScore and artificial intelligence ranking

STUDY QUESTION

Is there a relationship between blastocyst metabolism and biomarkers of embryo viability?

SUMMARY ANSWER

Blastocysts with higher developmental potential and a higher probability of resulting in a viable pregnancy consume higher levels of glucose and exhibit distinct amino acid profiles.

WHAT IS KNOWN ALREADY

Morphological and morphokinetic analyses utilized in embryo selection provide insight into developmental potential, but alone are unable to provide a direct measure of embryo physiology and inherent health. Glucose uptake is a physiological biomarker of viability and amino acid utilization is different between embryos of varying qualities.

STUDY DESIGN, SIZE, DURATION

Two hundred and nine human preimplantation embryos from 50 patients were cultured in a time-lapse incubator system in both freeze all and fresh transfer cycles. A retrospective analysis of morphokinetics, morphology (Gardner grade), KIDScore, artificial intelligence grade (EmbryoScore), glucose and amino acid metabolism, and clinical pregnancies was conducted.

PARTICIPANTS/MATERIALS, SETTING, METHODS

ICSI was conducted in all patients, who were aged ≤37 years and previously had no more than two IVF cycles. Embryos were individually cultured in a time-lapse incubator system, and those reaching the blastocyst stage had their morphokinetics annotated and were each assigned a Gardner grade, KIDScore and EmbryoScore. Glucose and amino acid metabolism were measured. Clinical pregnancies were confirmed by the presence of a fetal heartbeat at 6 weeks of gestation.

MAIN RESULTS AND THE ROLE OF CHANCE

Glucose consumption was at least 40% higher in blastocysts deemed of high developmental potential using either the Gardner grade (P < 0.01, n = 209), KIDScore (P < 0.05, n = 207) or EmbryoScore (P < 0.05, n = 184), compared to less viable blastocysts and in blastocysts that resulted in a clinical pregnancy compared to those that failed to implant (P < 0.05, n = 37). Additionally, duration of cavitation was inversely related to glucose consumption (P < 0.05, n = 200). Total amino acid consumption was significantly higher in blastocysts with an EmbryoScore higher than the cohort median score (P < 0.01, n = 185). Furthermore, the production of amino acids was significantly lower in blastocysts with a high Gardner grade (P < 0.05, n = 209), KIDScore (P < 0.05, n = 207) and EmbryoScore (P < 0.01, n = 184).

LIMITATIONS, REASONS FOR CAUTION

Samples were collected from patients who had ICSI treatment and from only one clinic.

WIDER IMPLICATIONS OF THE FINDINGS

These results confirm that metabolites, such as glucose and amino acids, are valid biomarkers of embryo viability and could therefore be used in conjunction with other systems to aid in the selection of a healthy embryo.

STUDY FUNDING/COMPETING INTEREST(S)

Work was supported by Virtus Health. D.K.G is contracted with Virtus Health. The other authors have no conflict of interest to declare.

TRIAL REGISTRATION NUMBER

N/A.

Treatment with AICAR inhibits blastocyst development, trophectoderm differentiation and tight junction formation and function in mice

STUDY QUESTION

What is the impact of adenosine monophosphate-activated protein kinase (AMPK) activation on blastocyst formation, gene expression, and tight junction formation and function?

SUMMARY ANSWER

AMPK activity must be tightly controlled for normal preimplantation development and blastocyst formation to occur.

WHAT IS KNOWN ALREADY

AMPK isoforms are detectable in oocytes, cumulus cells and preimplantation embryos. Cultured embryos are subject to many stresses that can activate AMPK.

STUDY DESIGN, SIZE, DURATION

Two primary experiments were carried out to determine the effect of AICAR treatment on embryo development and maintenance of the blastocoel cavity. Embryos were recovered from superovulated mice. First, 2-cell embryos were treated with a concentration series (0-2000 μM) of AICAR for 48 h until blastocyst formation would normally occur. In the second experiment, expanded mouse blastocysts were treated for 9 h with 1000 μM AICAR.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Outcomes measured included development to the blastocyst stage, cell number, blastocyst volume, AMPK phosphorylation, Cdx2 and blastocyst formation gene family expression (mRNAs and protein measured using quantitative RT-PCR, immunoblotting, immunofluorescence), tight junction function (FITC dextran dye uptake assay), and blastocyst ATP levels. The reversibility of AICAR treatment was assessed using Compound C (CC), a well-known inhibitor of AMPK, alone or in combination with AICAR.

MAIN RESULTS AND THE ROLE OF CHANCE

Prolonged treatment with AICAR from the 2-cell stage onward decreases blastocyst formation, reduces total cell number, embryo diameter, leads to loss of trophectoderm cell contacts and membrane zona occludens-1 staining, and increased nuclear condensation. Treatment with CC alone inhibited blastocyst development only at concentrations that are higher than normally used. AICAR treated embryos displayed altered mRNA and protein levels of blastocyst formation genes. Treatment of blastocysts with AICAR for 9 h induced blastocyst collapse, altered blastocyst formation gene expression, increased tight junction permeability and decreased CDX2. Treated blastocysts displayed three phenotypes: those that were unaffected by treatment, those in which treatment was reversible, and those in which effects were irreversible.

LARGE SCALE DATA

Not applicable.

LIMITATIONS, REASONS FOR CAUTION

Our study investigates the effects of AICAR treatment on early development. While AICAR does increase AMPK activity and this is demonstrated in our study, AICAR is not a natural regulator of AMPK activity and some outcomes may result from off target non-AMPK AICAR regulated events. To support our results, blastocyst developmental outcomes were confirmed with two other well-known small molecule activators of AMPK, metformin and phenformin.

WIDER IMPLICATIONS OF THE FINDINGS

Metformin, an AMPK activator, is widely used to treat type II diabetes and polycystic ovarian disorder (PCOS). Our results indicate that early embryonic AMPK levels must be tightly regulated to ensure normal preimplantation development. Thus, use of metformin should be carefully considered during preimplantation and early post-embryo transfer phases of fertility treatment cycles.

STUDY FUNDING AND COMPETING INTEREST(S)

Canadian Institutes of Health Research (CIHR) operating funds. There are no competing interests.

Time-lapse monitoring reveals that vitrification increases the frequency of contraction during the pre-hatching stage in mouse embryos

Contraction during the blastocyst stage is observed during embryonic development of various mammals, including humans, but the physiological role of this process is not well understood. Using time-lapse monitoring (TLM), we studied the influence of vitrification and contractions on embryonic development in mice. Mouse embryos were cultured at the 2-cell stage. At the 8-cell stage, embryos were randomly divided into a fresh group (FG) and vitrified group (VG) and observed for up to 144 h. Strong contractions (i.e., contractions causing a decrease in volume of more than 20% and expansion of the perivitelline space) occurred significantly more often in unhatched embryos than hatching embryos in both groups. Regarding hatching embryos, contractions in the pre-hatching stage were significantly more frequent in the VG than the FG. Furthermore, mRNA expression levels of genes related to contractions were determined at three time points, the 8-cell stage, early blastocyst stage, and 20 h after blastocoel formation, with quantitative reverse transcription-polymerase chain reaction. There was no significant difference in Hspa1a expression between the FG and VG, but Hspa1a overexpression was observed just after thawing and tended to decrease gradually thereafter in some blastocysts. Furthermore, in the VG, Atp1a1 tended to show higher expression in the strong contraction group than in the weak contraction group. Overall, vitrification is an excellent method for cryopreservation but could increase contractions in the pre-hatching stage and may increase energy demands of the embryo. Observation of contraction by TLM may improve the evaluation of embryo quality.

Fe(III) Is Essential for Porcine Embryonic Development via Mitochondrial Function Maintenance

Iron is an important trace element involved in several biological processes. The role of iron in porcine early embryonic development remains unknown. In the present study, we depleted iron (III, Fe³⁺) with deferoxamine (DFM), a specific Fe³⁺ chelator, in cultured porcine parthenotes and monitored embryonic development, apoptosis, mitochondrial membrane potential, and ATP production. Results showed biphasic function of Fe³⁺ in porcine embryo development. 0.5 μM DFM obviously increased blastocyst formation (57.49 ± 2.18% vs. control, 43.99 ± 1.72%, P < 0.05) via reduced (P < 0.05) production of reactive oxygen species (ROS), further increased mitochondrial membrane potential and ATP production in blastocysts (P < 0.05). 0.5 μM DFM decreased mRNA expression of Caspase 3 (Casp3) and increased Bcl-xL. However, results showed a significant reduction in blastocyst formation in the presence of 5.0 μM DFM compared with the control group (DFM, 21.62 ± 3.92% vs. control, 43.99 ± 1.73%, P < 0.05). Fe³⁺ depletion reduced the total (DFM, 21.10 ± 8.78 vs. control, 44.09 ± 13.65, P < 0.05) and increased apoptotic cell number (DFM, 11.10 ± 5.24 vs. control, 2.64 ± 1.43, P < 0.05) in the blastocyst. An obvious reduction in mitochondrial membrane potential and ATP level after 5.0 μM DFM treatment was observed. Co-localization between mitochondria and cytochrome c was reduced after high concentration of DFM treatment. In conclusion, Fe³⁺ is essential for porcine embryonic development via mitochondrial function maintenance, but redundant Fe³⁺ impairs the function of mitochondria.

p38 MAPK regulates cavitation and tight junction function in the mouse blastocyst

Blastocyst formation is essential for implantation and maintenance of pregnancy and is dependent on the expression and coordinated function of a series of proteins involved in establishing and maintaining the trans-trophectoderm ion gradient that enables blastocyst expansion. These consist of Na/K-ATPase, adherens junctions, tight junctions (TJ) and aquaporins (AQP). While their role in supporting blastocyst formation is established, the intracellular signaling pathways that coordinate their function is unclear. The p38 MAPK pathway plays a role in regulating these proteins in other cell types and is required for embryo development at the 8–16 cell stage, but its role has not been investigated in the blastocyst.

Preimplantation embryo programming: transcription, epigenetics, and culture environment

Preimplantation development directs the formation of an implantation- or attachment-competent embryo so that metabolic interactions with the uterus can occur, pregnancy can be initiated, and fetal development can be sustained. The preimplantation embryo exhibits a form of autonomous development fueled by products provided by the oocyte and also from activation of the embryo's genome. Despite this autonomy, the preimplantation embryo is highly influenced by factors in the external environment and in extreme situations, such as those presented by embryo culture or nuclear transfer, the ability of the embryo to adapt to the changing environmental conditions or chromatin to become reprogrammed can exceed its own adaptive capacity, resulting in aberrant embryonic development. Nuclear transfer or embryo culture-induced influences not only affect implantation and establishment of pregnancy but also can extend to fetal and postnatal development and affect susceptibility to disease in later life. It is therefore critical to define the basic program controlling preimplantation development, and also to utilize nuclear transfer and embryo culture models so that we may design healthier environments for preimplantation embryos to thrive in and also minimize the potential for negative consequences during pregnancy and post-gestational life. In addition, it is necessary to couple gene expression analysis with the investigation of gene function so that effects on gene expression can be fully understood. The purpose of this short review is to highlight our knowledge of the mechanisms controlling preimplantation development and report how those mechanisms may be influenced by nuclear transfer and embryo culture.

Na/K-ATPase beta1 subunit expression is required for blastocyst formation and normal assembly of trophectoderm tight junction-associated proteins

Na/K-ATPase plays an important role in mediating blastocyst formation. Despite the expression of multiple Na/K-ATPase alpha and beta isoforms during mouse preimplantation development, only the alpha1 and beta1 isoforms have been localized to the basolateral membrane regions of the trophectoderm. The aim of the present study was to selectively down-regulate the Na/K-ATPase beta1 subunit employing microinjection of mouse 1 cell zygotes with small interfering RNA (siRNA) oligos. Experiments comprised of non-injected controls and two groups microinjected with either Stealthtrade mark Na/K-ATPase beta1 subunit oligos or nonspecific Stealthtrade mark siRNA as control. Development to the 2-, 4-, 8-, and 16-cell and morula stages did not vary between the three groups. However, only 2.3% of the embryos microinjected with Na/K-ATPase beta1 subunit siRNA oligos developed to the blastocyst stage as compared with 73% for control-injected and 91% for non-injected controls. Na/K-ATPase beta1 subunit down-regulation was validated by employing reverse transcription-PCR and whole-mount immunofluorescence methods to demonstrate that Na/K-ATPase beta1 subunit mRNAs and protein were not detectable in beta1 subunit siRNA-microinjected embryos. Aggregation chimera experiments between beta1 subunit siRNA-microinjected embryos and controls demonstrated that blockade of blastocyst formation was reversible. The distribution of Na/K-ATPase alpha1 and tight junction-associated proteins occludin and ZO-1 were compared among the three treatment groups. No differences in protein distribution were observed between control groups; however, all three polypeptides displayed an aberrant distribution in Na/K-ATPase beta1 subunit siRNA-microinjected embryos. Our results demonstrate that the beta1 subunit of the Na/K-ATPase is required for blastocyst formation and that this subunit is also required to maintain a normal Na/K-ATPase distribution and localization of tight junction-associated polypeptides during preimplantation development.

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.

Roles of Na,K-ATPase in Early Development and Trophectoderm Differentiation

Before implantation into the uterine wall, the mammalian embryo undergoes a period of cell division, cell shape change, and cell differentiation leading to the formation of an outer epithelium, the trophectoderm. The trophectoderm is the part of the embryo that initiates uterine contact and, after transformation to become the trophoblast, uterine invasion. Similar to the kidney nephron, the trophectoderm is a transporting epithelium with distinct apical and basolateral membrane domains; its function is to facilitate transepithelial Na+ and fluid transport for blastocoel formation. That transport is driven by Na,K-adenosine triphosphatase (ATPase) localized in basolateral membranes of the trophectoderm. Preimplantation embryos express multiple alpha and beta subunit isoforms of Na,K-ATPase, potentially constituting multiple isozymes, but the basolaterally located alpha1beta1 isozyme appears to function uniquely to drive fluid transport. Embryos unable to express alpha1 subunits because of targeted deletion of the gene are able to form a blastocoel, but they fail to maintain their integrity and expire during the peri-implantation period. Preimplantation embryos also express the gamma subunit, a modulator of Na,K-ATPase activity, but targeted deletion of that gene did not reveal an essential developmental role. The preimplantation embryo offers a unique model for understanding the roles of Na,K-ATPase subunit isoforms in epithelial development and transepithelial transport.

Deletion of the Na/K-ATPase alpha1-subunit gene (Atp1a1) does not prevent cavitation of the preimplantation mouse embryo

Increases in Na/K-ATPase activity occur concurrently with the onset of cavitation and are associated with increases in Na(+)-pump subunit mRNA and protein expression. We have hypothesized that the alpha1-isozyme of the Na/K-ATPase is required to mediate blastocyst formation. We have tested this hypothesis by characterizing preimplantation development in mice with a targeted disruption of the Na/K-ATPase alpha1-subunit (Atp1a1) using embryos acquired from matings between Atp1a1 heterozygous mice. Mouse embryos homozygous for a null mutation in the Na/K-ATPase alpha1-subunit gene are able to undergo compaction and cavitation. These findings demonstrate that trophectoderm transport mechanisms are maintained in the absence of the predominant isozyme of the Na(+)-pump that has previously been localized to the basolateral membranes of mammalian trophectoderm cells. The presence of multiple isoforms of Na/K-ATPase alpha- and beta-subunits at the time of cavitation suggests that there may be a degree of genetic redundancy amongst isoforms of the catalytic alpha-subunit that allows blastocyst formation to progress in the absence of the alpha1-subunit.

Embryotrophic factor-3 from human oviductal cells enhances proliferation, suppresses apoptosis and stimulates the expression of the β1 subunit of sodium–potassium ATPase in mouse embryos

BACKGROUND

Embrytrophic factor-3 (ETF-3) from human oviductal cells enhanced the development of mouse preimplantation embryos. This report studied the embryotrophic mechanisms of the molecule.

METHODS AND RESULTS

Mouse embryos were incubated with ETF-3 for 24 h at different stages of development. ETF-3 treatment between 96 and 120 h post-HCG increased the cell count of blastocysts, whilst treatment between 72 and 96 h post-HCG enhanced the expansion and hatching of the blastocysts. ETF-3 increased the cell number of the embryos by suppressing apoptosis and increasing proliferation as determined by TUNEL and bromodeoxyuridine uptake assays, respectively. Real-time quantitative PCR showed that the in vivo developed and ETF-3-treated blastocysts had a significantly higher mRNA copy number of Na/K-ATPase-beta1, but not of hepsin, than that of blastocysts cultured in medium alone. The former gene was associated with cavitation of blastocysts while the latter was related to hatching of blastocyst. The beneficial effect of ETF-3 on blastocyst hatching was also seen when ETF-3-supplemented commercially available sequential culture medium for human embryo culture was used to culture mouse embryos.

CONCLUSIONS

ETF-3 improves embryo development by enhancing proliferation, suppressing apoptosis and stimulating expression of genes related to blastocyst cavitation. Supplementating human embryo culture medium with ETF-3 may improve the success rate in clinical assisted reproduction.

Molecular regulation of blastocyst formation

Preimplantation development encompasses the interval from insemination until embryo implantation and thus includes the 'freeliving' period of oviduct and uterine development. Formation of the blastocyst is required for implantation and establishment of pregnancy, and is a principal determinant of embryo quality prior to embryo transfer. Development through this period is regulated by the expression of specific gene families that encode for cell polarity, cell junctional, cytoskeletal, ion transporter, and water channel gene products that direct the acquisition of cell polarity and differentiation of the outer cells of the early embryo. This results in the formation of the trophectoderm, which is the first epithelium of development. This review considers the roles of each of these gene families in trophectoderm differentiation and blastocyst formation. The principal hypothesis under investigation is that blastocyst formation is regulated by a Na/K-ATPase-generated trans-trophectoderm ion gradient that promotes the accumulation of water across the epithelium. This, combined with the formation of the tight junction seal controlling paracellular movement of water between adjacent trophectoderm cells, results in the formation of a fluid-filled blastocyst cavity and the expansion of the blastocyst. Results from recent experiments, however, have cast some doubt on the role of Na/K-ATPase in mediating these events and have defined water channels or Aquaporins (AQPs) as physiological mediators of fluid movement across the trophectoderm. In addition, studies have now implicated mitogen-activated protein kinase (MAPK) signaling as an important mediator of development to the blastocyst stage. Such studies define the physiology of blastocyst formation and serve to support the application of assisted reproductive technologies (ART) to both human and animal species.

Na+, K+, ATPase activity in the human and bovine preimplantation embryo

Blastocyst formation is associated with a marked increase in ATP production, much of which is thought to be associated with the active transport of ions across the trophectoderm mediated by the sodium pump (Na+, K+, ATPase) resulting in the vectorial transport of water into the blastocoel. In this study, the biochemical activity of the sodium pump was measured directly in single human and bovine embryo extracts by monitoring the conversion of ATP to ADP in the presence and absence of ouabain. ATP and ADP were assayed by HPLC. In both species, there was a transient, significant increase in sodium pump activity while the blastocyst was actively expanding. The oxygen consumption of single human blastocysts was measured in order to estimate the proportion of total ATP used by the Na+, K+, ATPase. The results suggest that approximately 60 and 36% of the ATP produced is used by the sodium pump during blastocoel expansion in the human and bovine blastocyst, respectively.

Trophectoderm development and function: the roles of Na+/K(+)-ATPase subunit isoforms

Preimplantation development is a period of cell division, cell shape change, and cell differentiation leading to the formation of an epithelium, the trophectoderm. The trophectoderm is the part of the conceptus that initiates uterine contact and, after transformation to become the trophoblast, uterine invasion. Thus, trophectoderm development during preimplantation stages is a necessary antecedent to the events of implantation. The preimplantation trophectoderm is a transporting epithelium with distinct apical and basolateral membrane domains that facilitate transepithelial Na+ and fluid transport for blastocoel formation. That transport is driven by Na+/K(+)-ATPase localized in basolateral membranes of the trophectoderm. Preimplantation embryos express multiple alpha and beta subunit isoforms of Na+/K(+)-ATPase, potentially constituting multiple isozymes, but the basolaterally located alpha1beta1, isozyme uniquely functions to drive fluid transport. They also express the gamma subunit, which is a modulator of Na+/K(+)-ATPase activity. In the mouse, two splice variants of the gamma subunit, gammaa and gammab, are expressed in the trophectoderm. Antisense knockdown of gamma subunit accumulation caused a delay of cavitation, implying an important role in trophectoderm function. The preimplantation trophectoderm offers a unique model for understanding the roles of Na+/K(+)-ATPase subunit isoforms in transepithelial transport.

mRNAs encoding aquaporins are present during murine preimplantation development

The present study was conducted to investigate the mechanisms underlying fluid movement across the trophectoderm during blastocyst formation by determining whether aquaporins (AQPs) are expressed during early mammalian development. AQPs belong to a family of major intrinsic membrane proteins and function as molecular water channels that allow water to flow rapidly across plasma membranes in the direction of osmotic gradients. Ten different AQPs have been identified to date. Murine preimplantation stage embryos were flushed from the oviducts and uteri of superovulated CD1 mice. Reverse transcription-polymerase chain reaction (RT-PCR) methods employing primer sets designed to amplify conserved sequences of AQPs (1-9) were applied to murine embryo cDNA samples. PCR reactions were conducted for up to 40 cycles involving denaturation of DNA hybrids at 95 degrees C, primer annealing at 52-60 degrees C and extension at 72 degrees C. PCR products were separated on 2% agarose gels and were stained with ethidium bromide. AQP PCR product identity was confirmed by sequence analysis. mRNAs encoding AQPs 1, 3, 5, 6, 7, and 9 were detected in murine embryos from the one-cell stage up to the blastocyst stage. AQP 8 mRNAs were not detected in early cleavage stages but were present in morula and blastocyst stage embryos. The results were confirmed in experimental replicates applied to separate embryo pools of each embryo stage. These results demonstrate that transcripts encoding seven AQP gene products are detectable during murine preimplantation development. These findings predict that AQPs may function as conduits for trophectoderm fluid transport during blastocyst formation.

Effects of X chromosome number and parental origin on X-linked gene expression in preimplantation mouse embryos

Diploid androgenetic mouse embryos, possessing two sets of paternally inherited chromosomes, and control fertilized embryos were used to examine the relative effects of X chromosome number and parental chromosome origin on androgenone viability and X-linked gene expression. A significant difference in efficiency of blastocyst formation was observed between XX and XY androgenones in some experiments, but this difference was not uniformly observed. Significant effects of both X chromosome number and parental origin on X-linked gene expression were observed. Male and female control embryos expressed the XIST: RNA initially. This expression was followed by a preferential reduction in XIST: RNA abundance in male embryos, indicating that dosage compensation for the X chromosome may normally require the downregulation of XIST: RNA expression in male embryos, in conjunction with the production of stable XIST: transcripts in female embryos. By the late blastocyst stage, XX control embryos expressed significantly more XIST: RNA than did XY embryos. Unlike their normal counterparts, XX androgenones did not express significantly more XIST: RNA than did XY androgenones at the late blastocyst stage. Androgenones exhibited severe repression of the Pgk1 gene, but during development to the late blastocyst stage Pgk1 mRNA expression increased in XX androgenones and decreased in XY androgenones. Thus, the initial repression of the Pgk1 gene in XX androgenones was lost as the XIST: RNA declined in abundance, and this loss was correlated with a failure of XX androgenones to express significantly more XIST: RNA than did XY androgenones. These results indicate that androgenones may lack a factor that is expressed from the maternal genome and required for dosage compensation in preimplantation embryos. The results also indicate that early dosage compensation in preimplantation embryos may normally be reversible, thus providing flexibility to meet different developmental requirements of the embryonic and extraembryonic lineages.

Genetic variation in trophectoderm function in parthenogenetic mouse embryos

The developing oocyte constitutes the source of a unique and essential molecular legacy that supports embryo metabolism for a substantial period after fertilization and that also directs important epigenetic events that prepare the embryonic genome for transcription and faithful execution of the developmental program. Parthenogenetically activated embryos provide a useful tool with which to examine how maternally derived factors contribute to early development. They also provide a means for evaluating genetic effects on the maternal genomic imprinting process. We report here that the genetic background of the oocyte affects trophectoderm function at the blastocyst stage. Parthenogenetic embryos obtained from activated (B6D2)F1 oocytes hatch efficiently in culture, whereas parthenogenones from C57BL/6 oocytes hatch less efficiently. Fertilized embryos of both strains hatch efficiently. The (B6D2)F1 parthenogenones also undergo blastocoel re-expansion after treatment with cytoskeletal inhibitors more rapidly than do C57BL/6 parthenogenones and exhibit a moderately greater abundance of the Na+, K(+)-ATPase alpha 1 subunit mRNA. Surprisingly, parthenogenones of both strains undergo blastocoel re-expansion more rapidly than do their normal fertilized counterparts. Parthenogenones of both types are able to attach efficiently in culture after removal of the zona pellucida. These observations indicate that significant genetic effects of maternal genotype on trophectoderm function are revealed in the absence of a paternal genetic contribution and that trophectoderm function also differs between parthenogenetic embryos and fertilized embryos. The differences observed between parthenogenetic and fertilized embryos indicate a likely role for one or more imprinted genes in the development of hatching and blastocoel expansion ability. The effect of maternal genotype on parthenogenetic embryo phenotype is consistent with possible differences in maternal genome imprinting or differences in ooplasm composition that have long-term effects on development. The specific differences in hatching and blastocoel re-expansion between parthenogenones of the two strains may be the result of differences in the activity or expression of a hatching enzyme or other molecules that affect fluid accumulation within the blastocyst, such as components of junctional complexes or proteins that regulate Na+, K(+)-ATPase activity.

Differential involvement of Na(+),K(+)-ATPase isozymes in preimplantation development of the mouse

Na(+),K(+)-ATPase plays an essential role in mammalian blastocoel formation (cavitation) by driving trans-epithelial sodium transport. Previously, the alpha1 and beta1 subunit isoforms of this enzyme were identified in preimplantation mouse embryos and were assumed to be responsible for this function. Here we show that mRNAs encoding an additional alpha subunit isoform (alpha3) and the remaining two beta subunit isoforms are also present in preimplantation embryos. Whereas alpha3 mRNA accumulates between the four-cell and the blastocyst stages and thus results from embryonic transcription, the same could not be demonstrated for beta2 and beta3 mRNAs. Immunoblot analyses confirmed that these subunits are present in cavitating embryos. Using confocal immunofluorescence microscopy we found that alpha1 and beta1 subunits are concentrated in the basolateral membranes of the trophectoderm while being equally distributed in plasma membranes of the inner cell mass. In contrast, alpha3, beta2, and beta3 subunits were not detected in plasma membranes. Our current assessment, therefore, is that as many as six isozymes of Na(+),K(+)-ATPase could be involved in preimplantation development although it is primarily the alpha1beta1 isozyme that is responsible for blastocoel formation. Our findings imply that the regulation of sodium transport within the preimplantation mouse embryo is more complex than had been appreciated.

Translation of maternal messenger ribonucleic acids encoding transcription factors during genome activation in early mouse embryos

Embryonic genome activation (EGA) in mice is sensitive to treatment with cycloheximide, indicating that protein synthesis plays an important role in mediating EGA. We hypothesized that regulated maternal mRNA recruitment may control the time of EGA by controlling the time of appearance of certain transcription factors (TFs). We also hypothesized that synthesis of other TFs may contribute to EGA independently of controlling the timing of EGA. To test these hypotheses, we used sucrose density gradient fractionation coupled to a quantitative reverse transcription-polymerase chain reaction method to compare polysomal mRNA abundances of specific TF mRNAs between metaphase II oocytes, 1-cell-stage embryos, and 2-cell-stage embryos. We observed a 2-cell-stage-specific increase in polysomal abundance of mouse TEA DNA binding domain 2 (mTEAD-2) mRNA, coincident with the first appearance of mTEAD activity in the early embryo. The mRNAs encoding Sp1, TATA binding protein, and cyclic AMP response element binding protein did not undergo translational recruitment, but exhibited differences in polysomal abundance. We also observed a continuous, high proportion in the polysomal fraction for the mRNA encoding ribosomal protein L23 mRNA, which contrasted with the patterns observed for other maternal transcripts. These observations are consistent with the hypothesis that regulated recruitment of maternal TF mRNAs may control the time of activation of some genes during EGA, and that continuous synthesis of other TFs, like Sp1, may facilitate EGA.

Modulation of mouse preimplantation embryo development by acrogranin (epithelin/granulin precursor)

Preimplantation mammalian embryos in culture secrete autocrine growth factors into the surrounding medium that, in turn, stimulate the development of the embryos. The full complement of these factors is unknown. Since one hallmark of embryo development is the formation of an epithelium, the trophectoderm, we tested the hypothesis that one such embryo-derived growth factor is acrogranin (epithelin/granulin precursor), a factor that possesses growth-regulatory activities principally toward epithelial cells. We found that acrogranin mRNA was expressed in preimplantation mouse embryos with the transcript levels rising to their highest point in blastocysts, coincident with the appearance of the trophectoderm. Indirect immunofluorescence confocal microscopy of preimplantation mouse embryos at different developmental stages revealed that acrogranin immunostaining was most concentrated in the trophectoderm of blastocysts. Immunoblotting and immunoprecipitation experiments demonstrated that the embryos secreted acrogranin into the surrounding medium. To determine how altering the levels of acrogranin in the culture medium surrounding the embryos might affect embryonic growth and development, acrogranin protein levels in the culture medium were decreased with a function-blocking antibody or increased by adding the purified acrogranin to the medium. In both a concentration-dependent and a reversible manner, affinity-purified anti-acrogranin antibody significantly inhibited the development of eight-cell embryos to the blastocyst stage compared to controls (no added immunoglobulin or nonspecific IgG). Furthermore, embryo cell numbers were significantly decreased in the presence of the highest concentrations of acrogranin antibody compared to control embryos. Exogenous acrogranin added to cultures of eight-cell embryos accelerated the time for the onset of cavitation, as well as stimulating the rate of blastocoel expansion and increasing the number of trophectoderm cells compared to controls. These results indicate that acrogranin can regulate the appearance of the epithelium in the developing mouse blastocyst, the growth of the trophectoderm, and/or the function of the embryonic epithelium.

Embryo metabolism during the expansion of the bovine blastocyst

Embryo metabolism was evaluated during re-expansion of in vitro produced bovine blastocysts collapsed with cytochalasin D (CCD) and incubated in the presence and absence of ouabain, a specific inhibitor of the Na+, K+ pump. Day 8 expanded blastocysts were treated for 2 to 4 hr with 20 microg/ml CCD. Four conditions were tested: untreated embryos and embryos collapsed with CCD and allowed to re-expand for 4 hr in the presence of 0 M, 1 nM, or 1 microM ouabain. Incubation of collapsed embryos for 4 hr in the presence of 1 nM or 1 microM ouabain significantly inhibited blastocyst re-expansion. Glucose, pyruvate, and amino lactate uptake/release were not significantly affected by ouabain treatment and did not correlate with the degree of blastocyst re-expansion. Few variations in the uptake/release of amino acids by the embryos were observed. Ouabain treatment significantly decreased oxygen uptake which directly correlated with the degree of blastocyst re-expansion. For embryos allowed to re-expand in the presence or absence of ouabain, a direct correlation was observed between the uptake of oxygen and of glucose. One mM cyanide or 2,4 dinitrophenol inhibited blastocyst re-expansion although 0.01 and 0.1 mM were ineffective. This study indicates a role for oxidative metabolism in providing the energy necessary for blastocoel expansion in the bovine. Nevertheless, blastocyst expansion is relatively insensitive to inhibition of oxidative phosphorylation indicating the ability of the bovine blastocyst to adapt to hypoxic conditions.

Gene expression regulating blastocyst formation

Development of embryos to the blastocyst stage is a critical event in the early lives of all eutherian mammalian species. Blastocyst formation is essential for implantation and is the principal morphological determinant of embryo quality prior to embryo transfer. The physiological events and roles of specific gene families that regulate blastocyst formation are subjects of intense research Recent findings have demonstrated that bovine embryos express multiple members of the Na/K-ATPase ion transporter gene family. Two members of this family have been co-localized to bovine trophectoderm, but each becomes largely confined to opposing cell membrane margins. Bovine blastocysts display a greater sensitivity to ouabain (potent inhibitor of the Na/K-ATPase) than murine blastocysts, and enzyme activity (ouabain sensitive 86Rb+ uptake) undergoes a 9-fold increase from the bovine morula to the blastocyst stage. Disruption of Na/K-ATPase gene expression by antisense oligodeoxynucleotide inhibition abolishes blastocyst formation. These results have implicated the Na/K-ATPase as a key regulator of bovine blastocyst formation and have provided insights necessary for the production of healthy bovine embryos by the application of in vitro maturation, in vitro fertilization and in vitro culture methods.

Na/K-ATPase-mediated 86Rb+ uptake and asymmetrical trophectoderm localization of alpha1 and alpha3 Na/K-ATPase isoforms during bovine preattachment development

This study evaluated Na/K-ATPase alpha 1- and alpha 3-subunit isoform polypeptide expression and localization during bovine preattachment development. Na/K-ATPase cation transport activity from the one-cell to blastocyst stage was also determined by measuring ouabain-sensitive 86Rb+ uptake. Both alpha1- and alpha 3-subunit polypeptides were detected by immunofluorescence to encircle the entire cell margins of each blastomere of inseminated zygotes, cleavage stage embryos, and morulae. Immunofluorescent localization of alpha1-subunit polypeptide in bovine blastocysts revealed an alpha1 immunofluorescence signal confined to the basolateral membrane margins of the trophectoderm and encircling the cell periphery of each inner cell mass (ICM) cell. In contrast, alpha 3-subunit polypeptide immunofluorescence was localized primarily to the apical cell surfaces of the trophectoderm with a reduced signal present in basolateral trophectoderm regions. There was no apparent alpha 3-subunit signal in the ICM. Analysis of 86Rb+ transport in vitro demonstrated ouabain-sensitive activity throughout development from the one-cell to the six- to eight-cell stage of bovine development. 86Rb+ uptake by morulae (day 6 postinsemination) did not vary significantly from uptake detected in cleavage stage embryos; however, a significant increase was measured at the blastocyst stage (P < 0.05). Treatment of embryos with cytochalasin D (5 micrograms/ml) did not influence 86Rb+ uptake in cleavage stage embryos. Cytochalasin D treatment however was associated with a significant rise in ion transport in morulae and blastocysts (13.49 and 61.57 fmol/embryo/min, respectively) compared to untreated controls (2.65 and 22.83 fmol/embryo/min, respectively). Our results, for the first time, demonstrate that multiple Na/K-ATPase alpha-subunit isoforms are distributed throughout the first week of mammalian development and raise the possibility that multiple isozymes of the Na/K-ATPase contribute to blastocyst formation.

Analysis of variation in relative mRNA abundance for specific gene transcripts in single bovine oocytes and early embryos

Variation in the abundance of a specific gene transcript was assessed in single bovine oocytes and in vitro-derived blastocysts. Transcripts encoding the Na+,K(+)-ATPase alpha 1 subunit were detected by reverse-transcription polymerase chain reaction (RT-PCR) and quantified relative to an exogenously supplied rabbit alpha-globin mRNA using laser-induced fluorescence capillary electrophoresis (LIF-CE). The precision of this relative abundance (RA) calculation was predicted and shown to resolve 2-fold differences in transcript abundance between individual blastocysts and predicted in oocytes to resolve 3-fold differences. The RA of the alpha 1 subunit transcript differed by 2- to 3-fold among blastocysts, and 3- to 6-fold among oocytes. Comparison of a general population of oocytes with blastocysts revealed little overlap in RA values between the two groups, with a 8- to 14-fold increase in the mean RA for each group with development observed in two successive experiments (P < or = 0.05). In contrast, oocytes selected for their developmental competence on the basis of morphologic criteria exhibited only a 1.6- to 1.7-fold developmental increase when the assay was performed on cDNA generated from either embryo pools (n = 6 versus 6) or individuals (n = 7 versus 7), respectively. These results provide the first characterization of the degree of heterogeneity in the abundance of a specific mRNA transcript among individual mammalian oocytes and preimplantation embryos and demonstrate that transcript relative abundance can be correlated with bovine oocyte morphology.

Temporal patterns of embryonic gene expression and their dependence on oogenetic factors

Successful development of a fertilized egg beyond early cleavage divisions requires the de novo initiation and subsequent regulation of embryonic transcription. The egg provides the specialized environment within which the newly formed zygotic nucleus initiates its developmental program and as a result plays an obligatory role in its regulation. Although the precise timing of the onset of embryonic transcription in mammals varies during early cleavage divisions, several common elements exist. In the present essay we review the current literature on the timing and control of embryonic gene expression in mammals, and discuss recent findings from our laboratory on gene expression patterns in bovine embryos and their relation to other species, and zygotic gene activation (ZGA). Lastly, we discuss the putative role of maternally inherited factors in conferring developmental competence to the blastocyst stage, and a method to identify such factors present in oocytes as mRNA.

Embryonic expression of the putative gamma subunit of the sodium pump is required for acquisition of fluid transport capacity during mouse blastocyst development

The sodium/potassium pump, Na+,K+-ATPase, is generally understood to function as a heterodimer of two subunits, a catalytic alpha subunit and a noncatalytic, glycosylated beta subunit. Recently, a putative third subunit, the gamma subunit, was cloned. This small protein (6.5 kD) coimmunoprecipitates with the alpha and beta subunits and is closely associated with the ouabain binding site on the holoenzyme, but its function is unknown. We have investigated the expression of the gamma subunit in preimplantation mouse development, where Na+, K+-ATPase plays a critical role as the driving force for blastocoel formation (cavitation). Using reverse transcriptase-polymerase chain reaction, we demonstrated that the gamma subunit mRNA accumulates continuously from the eight-cell stage onward and that it cosediments with polyribosomes from its time of first appearance. Confocal immunofluorescence microscopy revealed that the gamma subunit itself accumulates and is localized at the blastomere surfaces up to the blastocyst stage. In contrast with the alpha and beta subunits, the gamma subunit is not concentrated in the basolateral surface of the polarized trophectoderm layer, but is strongly expressed at the apical surface as well. When embryos were treated with antisense oligodeoxynucleotide complementary to the gamma subunit mRNA, ouabain-sensitive K+ transport (as indicated by 86Rb+ uptake) was reduced and cavitation delayed. However, Na+, K+-ATPase enzymatic activity was unaffected as determined by a direct phosphorylation assay ("back door" phosphorylation) applied to plasma membrane preparations. These results indicate that the gamma subunit, although not an integral component of Na+,K+-ATPase, is an important determinant of active cation transport and that, as such, its embryonic expression is essential for blastocoel formation in the mouse.

Ouabain sensitivity and expression of Na/K-ATPase alpha- and beta-subunit isoform genes during bovine early development

The fluid movements that arise during blastocyst formation (cavitation) are, at least in part, driven by the Na/K-ATPase. In this study, the reverse transcriptase-polymerase chain reaction (RT-PCR) was used to survey bovine pre-attachment embryos for transcripts encoding known isoforms of the Na/K-ATPase alpha- and beta-subunits, including isoforms not previously detected during the first week of mammalian development. Transcripts encoding the Na-K-ATPase alpha 1, alpha 2, alpha 3 and beta 2 isoforms were detected throughout bovine preattachment development. This is the first indication that alpha 2, alpha 3 and beta 2 mRNAs are expressed during this early developmental interval. As in the mouse, beta 1-subunit transcripts were not detected until the morula stage and were also present in blastocysts. Thus, in two mammalian species an increase in abundance of beta 1 isoform transcripts in the morula stage is coincident with the onset of cavitation. Transcripts encoding the recently characterized alpha 4 isoform were not detected. The sensitivity of bovine blastocysts to ouabain (a potent inhibitor of Na/K-ATPase) was determined by assessing the ability of bovine blastocysts to recover in ouabain supplemental culture medium following cytochalasin-induced blastocyst collapse. Re-expansion of bovine blastocysts was inhibited in all ouabain concentrations down to 10(-9) M. Mouse blastocysts, in contrast, were sensitive to ouabain at or above 10(-3)M. These results have established that transcripts encoding multiple isoforms of both the alpha and beta subunits of the Na/K-ATPase are expressed throughout early bovine development and that bovine blastocysts display a greater sensitivity to ouabain than murine blastocysts. Future analysis will determine the possible individual and collective roles of these isoforms during blastocyst formation.

Translational regulation of Na,K-ATPase alpha1 and beta1 polypeptide expression in epithelial cells

To investigate the regulation of the Na,K-ATPase, we have studied the expression of the Na,K-ATPase polypeptides in several mammalian cell lines using the vaccinia virus/T7 RNA polymerase expression system. Infection of several fibroblast-like cell lines with viral recombinants containing the Na,K-ATPase alpha and beta isoforms, the glucose transporters, GLUT 1 and GLUT 4, or the capsid protein of the Sindbis virus all result in the production of the appropriate protein products. However, all epithelial cell lines tested fail to synthesize the Na,K-ATPase viral recombinants, yet they efficiently express the other virally directed polypeptides. While Madin-Darby canine kidney (MDCK) epithelial cells infected with the Na,K-ATPase alpha1 or beta1 recombinant viruses produce both mRNAs, the messages are inefficiently translated. Furthermore, the RNA from infected MDCK cells does not direct the in vitro synthesis of the beta1 polypeptide, whereas the message from infected fibroblast-like BSC 40 cells is efficiently translated both in vivo and in vitro. Moreover, the synthesis of the H,K-ATPase alpha subunit is also limited in MDCK cells, although the H,K-ATPase beta subunit is efficiently expressed. Expression of chimeras constructed between the Na+ pump beta1 isoform and the H,K-ATPase beta subunit indicates that sequences in the 5' coding region of the beta1 message have an inhibitory effect; however, the stringent translational regulation of the beta1 isoform in MDCK cells requires the 5' and 3' regions of the coding sequence. The ability of the polarized cell lines to limit the synthesis of the Na+ pump polypeptides while expressing other vaccinia recombinants at high levels suggests that the polarized cells possess a stringent mechanism for the specific translational regulation of a select set of messages.

Inhibitory effects of ouabain on early heart development and cardiomyogenesis in the chick embryo

Pericardial cavity formation and epithelialization of the cardiac precursor cell population constitute a critical developmental period that precedes stable cardiac cell commitment and differentiation. These events delineate the myocardial and endocardial precursor population in the embryo. Restriction of Na/K-ATPase (the sodium pump) expression to the pre-cardiomyocyte lateral membranes coincides with these events. Na/K-ATPase has been implicated developmentally in cavitation and in maintaining membrane potential. Experiments were undertaken to determine if the effects of perturbing sodium pump activity will affect pericardial cavity formation and, in turn, whether heart formation and/or cardiac cell commitment will be affected. We incubated whole chick embryos in vitro between stages 5 to 8 in the presence of the highly specific Na/K-ATPase inhibitor ouabain. Exposure of whole embryos to 10 microM ouabain (10(-5) M) demonstrated that heart development and precardiomyocyte differentiation are inhibited principally between stage 5 through stage 7. In each stage the degree of inhibition follows a rostrocaudal gradient as development proceeds along the anterior to posterior axis. After stage 8 ouabain no longer affects heart development or cardiomyogenesis. The inhibition is concentration- and developmental stage-dependent. The inhibition is reversible by elevating the outside potassium ion concentration [Ko] in the culture medium or by transferring the embryos into normal medium minus ouabain even after 20 hr of ouabain exposure. The results also suggest that the regulation of the formation of the three-dimensional organ is independent from regulation of myogenesis.

Developmental cell-specific regulation of Na(+)-K(+)-ATPase alpha 1-, alpha 2-, and alpha 3-isoform gene expression

Na(+)-K(+)-activated adenosine triphosphatase (Na(+)-K(+)-ATPase) is the integral membrane protein that maintains the Na(+)-K(+) electrochemical gradient across the plasma membrane. Because of the importance of the Na(+)-K(+) electrochemical gradient to fundamental and specialized cell functions, we investigated the cell-specific modulation of Na(+)-K(+)-ATPase alpha-subunit isoform (alpha 1, alpha 2, and alpha 3) gene expression in different stages of postimplantation mouse embryos and neonatal rat tissues by in situ hybridization with use of isoform-specific rat-derived antisense RNA probes. At early organogenesis (9.5-10.5 days postcoitus), we demonstrated generalized coexpression of alpha 1- and alpha 2-isoforms throughout the mouse embryo with greater levels in the developing but already functional heart, in contrast to the distinct spatially restricted alpha 3-isoform gene expression in the early developing neural tube. At midorganogenesis (15.5-16.5 days postcoitus), differential spatial variation in alpha 1-, alpha 2-, and alpha 3-isoform gene expression was already evident in all organs. Interestingly, region-specific expression patterns within single cell types were noted throughout development and were exemplified by 1) alpha 3-isoform gene expression in marginal cells of the 10.5-day-postcoitus developing neural tube; 2) alpha 1-, alpha 2-, and alpha 3-isoform gene expression in cerebellar granular cells of the 4-day-old rat brain; and 3) alpha 1- and alpha 3-isoform gene expression in 4-day-old rat ventricular cardiomyocytes. These isoform-specific changes in cellular and regional Na(+)-K(+)-ATPase alpha-isoform gene expression may play an active role in development and specialized cell functions.

The role of exogenous energy substrates in blastocoele fluid accumulation in the rat

Preimplantation mammalian development culminates in the formation of a fluid-filled cavity, the blastocoele, which is a prerequisite for successful implantation and further development. The blastocoele is enclosed by a single layer of polarised cells, the trophectoderm, which is the first epithelium formed in development. In embryos of the mouse and the rabbit, a basolaterally located Na+/K(+)-ATPase hydrolyses ATP to drive the vectorial transport of ions, which is responsible for the accumulation of blastocoele fluid. Using non-invasive assays of energy substrate consumption and blastocoele fluid accumulation, experiments were carried out on single preimplantation rat embryos, to establish: (1) the roles of the Na+/K(+)-ATPase and exogenous energy substrates, and (2) the relationship between the consumption and metabolism of energy substrates and fluid accumulation, during blastocoele cavity formation in this species. Ouabain 0.5 mM and energy-substrate-free medium both caused an inhibition in the number of embryos forming a blastocoele in culture, and also reduced the rate of fluid accumulation by day 5 blastocysts collapsed in cytochalasin-D and allowed to re-expand. Ouabain also reduced the consumption of glucose (but not pyruvate) and the production of lactate by re-expanding blastocysts. In the absence of the inhibitor, a direct relationship was seen between fluid accumulation and both glucose (but not pyruvate) consumption and lactate production. However, ouabain had no effect on intact, expanded blastocysts.(ABSTRACT TRUNCATED AT 250 WORDS)

Polyadenylation of Na(+)-K(+)-ATPase beta 1-subunit during early development of Xenopus laevis

In fully grown Xenopus oocytes, the synthesis of beta-subunits is limiting for the formation of functional Na(+)-K(+)-adenosinetriphosphatase alpha/beta-complexes (Geering, K. FEBS Lett. 285: 189-193, 1991). In the present study, we show that during oocyte growth (from stage I to stage VI) alpha 1-, but not beta 1- or beta 3-isoform, mRNAs accumulate. In addition, beta-mRNAs are apparently sequestered in an untranslated pool in fully grown oocytes (stage VI). From fertilization to morulation, the total pools of alpha 1-, beta 1-, or beta 3-mRNAs vary little. Whereas polyadenylated [poly(A)+] alpha 1- and beta 3-isoform mRNAs did not change significantly, poly(A)+ beta 1-mRNA abundance increased three- to fourfold at morulation, accompanied by a parallel increase in beta 1-protein synthesis. After midblastula transition (i.e., at early gastrula) and during neurulation, poly(A)+ alpha 1- and beta 3-mRNAs accumulated rapidly, whereas poly(A)+ beta 1-mRNA accumulation was delayed by approximately 2 h, beginning only at early neurula. Our results indicate that 1) the abundance of poly(A)+ beta 1-mRNA is rate limiting during embryonic development for the assembly of alpha 1/beta 1-heterodimers, shown to be involved in the vectorial transport of sodium in kidney cells, and 2) the polyadenylation of beta 1-mRNA is a rate-limiting factor during morulation for the synthesis and assembly of new sodium pumps at the time of blastocoel fluid formation. The 3'-untranslated region of beta 1-mRNA (but not of alpha 1-mRNA) expresses cytoplasmic polyadenylation elements (CPEs) with the consensus sequence AXX-AUUUU(A/U)(A/U)(A/U). A role of CPE in the differential polyadenylation of alpha 1- and beta 1-mRNA is proposed.

Molecular maturation of cell adhesion systems during mouse early development

During cleavage, the mouse embryo expresses a variety of cell adhesion systems on its cell surfaces. We have reviewed biogenetic and assembly criteria for the formation of the uvomorulin/catenin, tight junction and desmosome adhesion systems as the trophectoderm differentiates. Each system reveals different mechanisms regulating molecular maturation. Adhesion processes contribute to the generation of distinct tissues in the blastocyst by modifying the expression pattern of blastomeres entering the non-epithelial inner cell mass lineage. Cell adhesion also influences the spatial organisation, but rarely the timing of expression, of proteins involved in trophectoderm differentiation.

Effect of sodium and betaine in culture media on development and relative rates of protein synthesis in preimplantation mouse embryos in vitro

Results of recent experiments indicate that the improved development of mouse embryos in medium containing a low NaCl concentration (85 mM) or the inclusion of the organic osmolyte betaine in a medium containing a high NaCl concentration (125 mM) is correlated with the maintenance of intracellular sodium concentrations that more closely approximate those found in freshly isolated embryos (Biggers et al., 1993, Mol Reprod Dev 34:380-390). We examined the effect of these different culture media on the relative rates of protein synthesis since increased levels of intracellular sodium inhibit protein synthesis; a reduced rate of protein synthesis could therefore account for the differences in development in the different media, since cell division requires protein synthesis. We observe that the ability of these media to support development and to maintain more physiological concentrations of intracellular sodium is correlated with their ability to support increased relative rates of protein synthesis. Reducing the NaCl concentration from 125 mM to 85 mM leads to a greater fraction of the embryos developing from the 2-cell stage to the 8-cell stage after 1 day of culture and a substantially improves extent of development to the morula stage after 2 days of culture. This reduction in NaCl concentration also leads to a 2.4-fold increase in the relative rate of protein synthesis in 4-cell embryos. Moreover, addition of betaine to medium containing 125 mM NaCl increases the relative rate of protein synthesis. This finding provides an explanation, at least in part, for the increase in development to the blastocyst stage exhibited by mouse embryos cultured in these media.(ABSTRACT TRUNCATED AT 250 WORDS)

The cell biology of blastocyst development

Preimplantation development encompasses the "free"-living period of mammalian embryogenesis, which culminates in the formation of a fluid-filled structure, the blastocyst. Cavitation (blastocyst formation) is accompanied by the expression of a novel set of gene products that contribute directly to the attainment of cell polarity with the trophectoderm, which is both the first epithelium of development and the outer cell layer encircling the inner cell mass of the blastocyst. Several of these gene products have been identified and include the tight junction (ZO-1), Na/K-ATPase (alpha and beta subunits), uvomorulin, gap junction (connexin43), and growth factors such as transforming growth factor-alpha (TGF-alpha) and epidermal growth factor (EGF). This review will examine the role(s) of each of these gene products during the onset and progression of blastocyst formation. The trophectodermal tight junctional permeability seal regulates the leakage of blastocoel fluid and also assists in the maintenance of a polarized Na/K-ATPase distribution to the basolateral plasma membrane domain of the mural trophectoderm. The polarized distribution of the Na/K-ATPase plays an integral role in the establishment of a trans-trophectoderm Na+ gradient, which drives the osmotic accumulation of water across the epithelium into the nascent blastocoelic cavity. The cell adhesion provided by uvomorulin is necessary for the establishment of the tight junctional seal, as well as the maintenance of the polarized Na/K-ATPase distribution. Growth factors such as TGF-alpha and EGF stimulate an increase in the rate of blastocoel expansion, which could, in part, be mediated by secondary messengers that result in an increase in Na/K-ATPase activity. Insight into the mechanism of cavitation has, therefore, directly linked blastocyst formation to trophectoderm cell differentiation, which arises through fundamental cell biological processes that are directly involved in the attainment of epithelial cell polarity.

Gene expression in pre-implantation mammalian embryos

The pre-implantation mammalian embryo is initially under the control of maternal informational macromolecules that are accumulated during oogenesis. Subsequently, the genetic program of development becomes dependent upon new transcription derived from activation of the embryonic genome. Several embryonic transcripts including those that encode growth factors, cell junction components and plasma membrane ion transporters are required for normal progression of the embryo to the blastocyst stage. The pattern of genes expressed and the overall program of development is subject to the influences of genomic imprinting as well as external influences encountered by the embryo within the maternal reproductive tract.

U2 small nuclear RNA localization and expression during bovine preimplantation development

This study describes the localization of the U2 small nuclear RNA (snRNA) and the major U snRNA group ribonucleoproteins (snRNPs) during bovine preimplantation development. In vitro maturation, fertilization, and oviductal epithelial cell coculture methods were employed to produce several developmental series totalling over 2,000 preimplantation-stage bovine oocytes and embryos. These oocytes and preimplantation embryos were processed for in situ hybridization, immunofluorescence and Northern blotting methods. The U2 snRNA and the major U group snRNPS were localized initially over the germinal vesicle (GV) of preovulatory oocytes but following GV breakdown were released throughout the ooplasm. They subsequently reassociated with both pronuclei during fertilization. From the two-cell to the blastocyst stages, the U2 snRNA and U snRNPs were localized to the interphase nucleus of each blastomere. The levels of U2 snRNA throughout bovine preimplantation development were determined by probing a Northern blot containing total RNA isolated from the following preimplantation bovine embryo stages: one to two cell, eight to 16 cell, early morula (greater than 32 cell), and late morula/early blastocysts. The levels of U2 snRNA remained constant between the one-cell and eight- to 16-cell bovine embryo stages but increased 4.4-fold between the eight- to 16-cell stage and the late morula/early blastocyst stages. The results suggest that a maternal pool of snRNAs is maintained in mammalian preimplantation embryos regardless of the duration of maternal control of development.

The genetic program for preimplantation development

This review summarizes information on accumulation profiles of individual gene transcripts in preimplantation development. Most of the information is from the mouse, but some data from other species are reviewed as well. The principal finding is that the transcription of most genes is not temporally linked with any of the three morphogenetic transitions (compaction, cavitation, and blastocoel expansion) that characterize this period. Most genes that are expressed during preimplantation development of the mouse are already being transcribed in the 4-cell stage, and some clearly begin as early as the 2-cell stage. Once activated, a gene continues to be transcribed at least into the blastocyst stage, resulting in continuous mRNA accumulation. Thus the pattern of gene transcription established at the time of genomic activation in the 2-cell stage is perpetuated into the blastocyst, with a few additions along the way. This information is interpreted in light of previous findings concerning the sensitivity of morphogenetic transitions to inhibition of gene expression. The lack of a clear relationship between the timing of expression of most genes and the schedule of morphogenesis leads one to conclude that temporal regulation is imposed downstream of transcription and translation. This conclusion is substantiated by a consideration of factors controlling the events of compaction.

Zygotic expression of the connexin43 gene supplies subunits for gap junction assembly during mouse preimplantation development

De novo assembly of gap junctions begins during compaction in the eight-cell stage of mouse development, and intercellular coupling mediated by gap junctions appears to be required for maintenance of the compacted state. We have begun to explore the expression of the family of genes encoding the connexins, the proteins that form the gap junction channels. We recently reported that a protein with antigenic and size similarity with connexin32, the rat liver gap junction protein, is inherited as an oogenetic product by the mouse zygote, but its gene appears not to be transcribed prior to implantation (Barron et al., Dev Genet 10:318-323, 1989). Here we report that another member of this gene family, connexin43, is transcribed by the embryonic genome from shortly after the time of genomic activation. As revealed by Northern blotting, connexin43 mRNA is absent from ovulated oocytes, becomes detectable in the 4-cell stage, and accumulates steadily thereafter to reach a maximum in blastocysts. In contrast, no transcripts of connexin26 could be detected in any preimplantation stage. A protein with antigenic and size similarity with connexin43 from rat heart was found by Western blotting to accumulate from the four-cell stage onward. Immunofluorescence analysis with embryo whole mounts was used to demonstrate that this protein is incorporated into punctate interblastomeric foci during compaction, consistent with its assembly into gap junction plaques. We conclude that connexin43 is one member of the connexin gene family whose zygotic expression is critical for preimplantation morphogenesis.

Ouabain-sensitive Rb+ uptake in mouse eggs and preimplantation conceptuses

The results of histochemical and immunocytochemical studies have been used elsewhere to support the hypothesis that Na+/K(+)-ATPase expression is initiated or increases dramatically in preimplantation mouse conceptuses just before they begin to cavitate. Moreover, localization of the enzyme in the inner membrane of the mural trophoblast is thought to be involved directly in formation and maintenance of the blastocyst cavity. Presumably, Na+/K(+)-ATPase extrudes the cation, Na+, and therefore water into the cavity. The cation transporting activity of the enzyme can be determined by measuring ouabain-sensitive Rb+ uptake by cells. Therefore, we measured Rb+ uptake in mouse eggs and preimplantation conceptuses at various stages of development. 86Rb+ uptake by conceptuses increased linearly with time for at least 60 min in medium containing 0.7 mM total Rb+ plus K+ in the absence or presence of 1.0 mM ouabain, and ouabain inhibited more than 70% of 86Rb+ uptake. The ouabain concentration at 1/2 of maximum inhibition of the ouabain-sensitive component of 86Rb+ uptake was about 10-20 microM in eggs and conceptuses at all stages of preimplantation development. Moreover, ouabain-sensitive Rb+ uptake had a twofold higher Vmax value in blastocysts than in eggs or conceptuses at earlier stages of development (i.e., approximately 173 vs 70-100 fmole.conceptus-1.min-1), although the total cell surface area also was probably about two times greater in blastocysts than in eggs or other conceptuses. Ouabain-sensitive Rb+ transport in eggs and conceptuses may have occurred via a single ouabain-sensitive Rb+ transporter with a Hill coefficient of 1.5-1.8 (Hill plots). When it was assumed that the Hill coefficient had a value of 2.0, however, eggs and conceptuses appeared to contain at least two forms of Na+/K(+)-ATPase activity. These studies are the first to show that the cation transporting activity of Na+/K(+)-ATPase can be measured quantitatively in mammalian eggs and preimplantation conceptuses. Inclusion of this assay in experiments designed to determine how Na+/K(+)-ATPase activity is controlled in oocytes and conceptuses should yield further insight into the role of this enzyme in oogenesis and preimplantation development.

Differentiation of an epithelium: factors affecting the polarized distribution of Na+,K(+)-ATPase in mouse trophectoderm

Na+,K(+)-ATPase is a marker of the basolateral plasma membrane domain of polarized epithelial cells, including the mural trophectoderm of the mammalian blastocyst (Watson and Kidder (1988). Dev. Biol. 126, 80-90). We have used this marker to explore the factors governing the establishment and maintenance of apical/basolateral polarity during differentiation of trophectoderm. A polyclonal antiserum (anti-GP80) against human cell-CAM 120/80, a homolog of the mouse cell-cell adhesion protein, uvomorulin, was used to prevent cell flattening (compaction) and formation of the epithelial junctional complex. The majority of treated embryos failed to develop a blastocoel; instead their blastomeres developed fluid-filled cavities that expanded while untreated control embryos were cavitating. Immunocytochemistry revealed that the catalytic subunit of Na+,K(+)-ATPase was contained within the membranes lining these cavities, as well as within numerous punctate foci in the cytoplasm. The down-regulation of expression of the enzyme that normally occurs in the ICM and polar trophectoderm did not take place, since the immunoreactivity remained equally strong in all blastomeres. The enzyme could not be detected in plasma membranes. We conclude that uvomorulin-mediated cell adhesion is involved in spatially restricting the expression of the catalytic subunit and is a prerequisite for the insertion of enzyme-laden vesicles into plasma membranes, but not for expression of the catalytic subunit gene. When fully developed blastocysts were treated with cytochalasins to disrupt the epithelial junctional complex, the catalytic subunit shifted from the basolateral to the apical plasma membrane. This finding suggests a primary role for the apical plasma membrane in the process of polarization, and implies that tight junctions are a manifestation of polarity that serve to maintain the separation between apical and basolateral markers.