Antibodies to a renal Na+/glucose cotransport system localize to the apical plasma membrane domain of polar mouse embryo blastomeres

Mechanisms of formation and functions of the early embryonic cavities

As the early mouse embryo develops, fundamental steps include the sequential formation of the first lumens in the murine conceptus. The first cavity established in the pre-implantation embryo is the blastocoel, followed by the emergence of the proamniotic cavity during the peri-implantation stages. The mouse embryo is a dynamic system which switches its modes of lumenogenesis before and after implantation. The blastocoel emerges in between the basolateral membranes, whereas the proamniotic cavity is formed on the apical interface. Defects in the sculpting of these luminal spaces are associated with developmental abnormalities and embryonic lethality. Here, we review the mechanisms by which these early embryonic cavities are formed and discuss the cavities in terms of their common and stage-specific principles of lumenogenesis and their functions.

Changing expression of chloride channels during preimplantation mouse development

Plasma membrane chloride channels (ClCs) play important roles in a broad range of cellular processes including cell volume regulation, proliferation, and transepithelial transport, all of which are critical during preimplantation embryonic development. In this study, the molecular and functional expression of voltage-gated ClCs was analyzed throughout preimplantation development of the mouse conceptus. mRNA transcripts for allClcngenes were detected. OnlyClcn1mRNA showed differential expression in the blastocyst, being detected in the trophectoderm but not in the inner cell mass. CLCN3 protein was detected at low levels in the cytoplasm and plasma membrane in 4-cell embryos and was localized to the apical plasma membrane of the trophoblasts in the blastocyst. Whole-cell patch-clamp recordings demonstrated the presence of a DIDS-sensitive, outwardly rectifying Cl−current throughout development, with this conductance being large at the 1-cell, morula and blastocyst stages. A second DIDS-insensitive Cl−current, which was inactivated by membrane depolarization, was present in cells differentiating into the trophoblast lineage and during blastocyst expansion. Inhibition of the DIDS-sensitive current and the DIDS-insensitive current, with 9-AC, prevented blastocyst expansion.

Glucose transporters in gametes and preimplantation embryos

The oocyte, sperm and preimplantation embryo have unique metabolic needs that must be met to ensure successful pregnancy. The family of facilitative glucose transporters (GLUTs) plays a major role in providing metabolic substrates to these tissues. The variety of GLUTs expressed in these tissues allows for flexibility to adapt to a changing environment. Alterations in glucose transport and metabolism at the earliest stages of development can impact fetal development. Research into the mechanisms of normal glucose transport into cells is critical for improving outcomes in the increasingly common diabetic maternal environment. Here, we review the current understanding in the distribution and role of glucose transporters in gametes and preimplantation embryos under normal and diabetic conditions.

Tight junctions containing claudin 4 and 6 are essential for blastocyst formation in preimplantation mouse embryos

The trophectoderm (TE) is the first epithelium to be generated during mammalian early development. The TE works as a barrier that isolates the inner cell mass from the uterine environment and provides the turgidity of the blastocyst through elevated hydrostatic pressure. In this study, we investigated the role of tight junctions (TJs) in the barrier function of the TE during mouse blastocyst formation. RT-PCR and immunostaining revealed that the mouse TE expressed at least claudin 4, 6, and 7 among the 24 members of the claudin gene family, which encode structural and functional constituents of TJs. When embryos were cultured in the presence of a GST-fused C-terminal half of Clostridium perfringens enterotoxin (GST-C-CPE), a polypeptide with inhibitory activity to claudin 4 and 6, normal blastocyst formation was remarkably inhibited; the embryos had no or an immature blastocoel cavity without expansion, and blastomeres showed a rounded shape. In these embryos, claudin 4 and 6 proteins were absent from TJs and the barrier function of the TE was disrupted; however the basolateral localization of the Na+/K+-ATPase alpha1 subunit and aquaporin 3, which are thought to be involved in blastocyst formation, appeared normal. These results clearly demonstrate that the barrier function of TJs in the TE is required for normal blastocyst formation.

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.

Role of gap junctions during early embryo development

Gap junctional communication plays a central role in the maintenance of cellular homeostasis by allowing the passage of small molecules between adjacent cells. Gap junctions are composed of a family of proteins termed connexins. During preimplantation development several connexin proteins are expressed and assembled into gap junctions in the plasma membrane at compaction but the functional significance of connexin diversity remains controversial. Although, many of the connexin genes have been disrupted using homologous recombination in embryonic stem cells to obtain unique phenotypes, none of these studies has demonstrated a specific role for connexins during preimplantation development in the null mutants. This review surveys evidence for the involvement of gap junctional communication during embryo development highlighting discrepancies in the literature. Although some evidence suggests that gap junctions may be dispensable during preimplantation development this is difficult to envisage particularly for the process of cavitation and the maintenance of homeostasis between the differentiated trophectoderm cells and the pluripotent inner cell mass cells of the blastocyst.

Na+ / H+ exchanger-3 is involved in mouse blastocyst formation

The mouse blastocyst consists of the trophectoderm, the inner cell mass, and a fluid-filled cavity, the blastocoel. Formation and subsequent expansion of this cavity is important for further differentiation of the inner cell mass and successful implantation. Previous work provided evidence that vectorial transport of Na+ and CL- ions through the trophectoderm into the blastocoel generates an osmotic gradient that drives fluid across this epithelium. As the activity of the Na+ / H+ exchanger (NHE) has been implicated as the exchanger responsible for facilitating the transtrophectodermal Na+ flux, the functional role of NHE in mouse blastocoel development was determined. Embryos were cultured in the presence of subtype-specific NHE inhibitors to examine the role of NHEs in blastocoel development. When 2-cell stage embryos were treated continuously with a specific inhibitor of NHE-1, cariporide, the embryos passed beyond the 8-cell stage and became blastocysts. However, in the presence of a specific inhibitor of NHE-3, S3226, the 2-cell stage embryos developed to the morula stage but formation of the blastocyst were inhibited in a dose-dependent manner. Cariporide did not inhibit the formation of the blastocoel cavity from the morula stage whereas S3226 did inhibit that process. S3226 also reduced the rate of re-expansion of blastocysts collapsed by cytochalasin D upon transfer to the control medium. An immunofluorescence study showed that NHE-3 was detected in the vicinity of the cell membrane of the trophectoderm, especially in the apical cell margins of the trophectoderm. These results suggest that NHE-3 is likely involved in blastocyst formation.

Requirements for glucose beyond sperm capacitation during in vitro fertilization in the mouse

In both the mouse and the human, it is a point of controversy whether glucose is necessary for in vitro fertilization. Some of this controversy has resulted from a failure to distinguish between requirements for glucose during sperm capacitation versus requirements during the multistage process of fertilization. Using the mouse as a model, we performed a series of experiments designed to identify specific processes that might require glucose. We observed a positive correlation between increasing glucose concentrations during capacitation and fertilization, and increasing fertilization of zona pellucida (ZP)-intact eggs. These data supported a requirement for glucose in the fertilization medium even when sperm were first capacitated in the presence of 5.5 mM glucose. This glucose requirement was observed for both ZP-intact and ZP-free eggs. During ZP-free in vitro fertilization, some binding and fusion between the plasma membrane of the sperm and egg occurred in the absence of glucose and at concentrations less than 1 mM, suggesting that this substrate is not absolutely required. However, glucose concentrations of 1 mM or higher greatly facilitated both binding and fusion under these conditions. These subtle distinctions suggest that during ZP-free in vitro fertilization, 1 mM glucose represents a threshold level that facilitates binding and fusion. Taken as a whole, the data suggest requirements for glucose during both capacitation and fertilization under normal physiologic conditions.

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.

Glucose transporter expression is developmentally regulated in in vitro derived bovine preimplantation embryos

Glucose is readily been taken up and utilized by preimplantation embryos from different species. However, a comprehensive analysis of the glucose transporter expression throughout preimplantation development is still missing. Here, we have investigated the expression of facilitative glucose transporters (Glut1-5 and 8) and sodium-dependent-glucose transporter (SGLT-I) in bovine oocytes and preimplantation embryos up to d16 of development, using RT-PCR and immunohistochemistry. The embryos were produced in vitro by IVM-IVF. Glut1, Glut3, Glut8, and SGLT-I were expressed in all stages studied. Glut4 transcripts were first detected at the blastocyst stage. Glut2 expression was restricted to the period of blastocyst elongation at d14 and d16. Transcription of the fructose transporter Glut5 started at the 8-/16-cell stage. Our results show a distinct expression pattern for glucose transporters during bovine embryo development in vitro indicating specialized functions for these isoforms at different developmental stages in bovine embryos. Mol. Reprod. Dev. 60:370-376,

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.

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.

Experimental manipulations of compaction and their effects on the phosphorylation of uvomorulin

Compaction of the eight-cell stage mouse embryo is a critical event in the generation of different cell types within the preimplantation embryo. Uvomorulin, a member of the cadherin family of cell adhesion molecules, is important during compaction and its phosphorylation increases early in the eight-cell stage, suggesting that this posttranslational modification may be important for compaction to proceed. We have assessed the importance of the phosphorylation of uvomorulin during compaction by preventing, reversing, or inducing adhesion prematurely. The only condition that affected the overall level of uvomorulin phosphorylation was the prevention of compaction through prolonged exposure of four-cell embryos to low Ca2+. This treatment reduced the level of uvomorulin phosphorylation in eight-cell embryos, and perturbed its localization to regions of cell-cell contact. Thus, whilst the phosphorylation of uvomorulin does not appear to regulate directly uvomorulin's adhesive function, it may be associated with the redistribution of uvomorulin during compaction.

IGF-I and insulin regulate glucose transport in mouse blastocysts via IGF-I receptor

The roles of glucose deprivation, insulin, and insulin-like growth factor I (IGF-I) in the regulation of glucose transport in the mouse blastocyst were examined. Glucose transport, measured by uptake of 3-0-methyl glucose (3-OMG), was increased by 19% (P < 0.01) in response to glucose deprivation. Both IGF-I and insulin stimulated uptake, but IGF-I was 1,000-fold more potent than insulin, increasing uptake by 51% at 1.7 pM (P < 0.001). These effects began to appear after 20 min of incubation with growth factors, and required the simultaneous presence of glucose. The relative potencies of insulin and IGF-I suggest that the actions of IGF-I and insulin were both mediated via the IGF-I receptor. The inactivity of a specific agonistic insulin receptor antibody (B10) confirms this and suggests that this action may be independent of signalling through IRS-1. Cycloheximide decreased growth factor-stimulated transport by about 40%, indicating that both protein synthesis and transporter recruitment from cytoplasmic stores are responsible for maximal stimulation. These characteristics are consistent with GLUT1-facilitated glucose uptake and suggest that GLUT1 is the regulatable transporter in mouse blastocysts. Stimulation of GLUT1 may be a ubiquitous feature of the autocrine/ paracrine activity of IGF-I in cell growth and proliferation.

Ion transport across the early chick embryo: II. Characterization and pH sensitivity of the transembryonic short-circuit current

The ectoderm of the one-day chick embryo generates dorsoventrally oriented short-circuit current (Isc) entirely dependent on extracellular sodium. At the dorsal cell membrane, the Isc was modified reversibly and in a concentration-dependent manner by: amiloride (60% decrease at 1 mM, with 2 apparent IC50S: 0.13 and 48 microM), phlorizin (0.1 mM) or removal of glucose (30% decrease, additive to that of amiloride), SITS (1 mM, 13% decrease). Acidification of alkalinization of the dorsal (but not ventral) superfusate produced, respectively, decrease or increase of Isc with a pH50 of 7.64. Ba2+ (0.1-1 mM) from either side of the ectoderm decreased the Isc by 30%. Anthracene-9-carboxylic acid, furosemide and inducers of cAMP had no effect on electrophysiological properties of the blastoderm. The chick ectoderm is therefore a highly polarized epithelium containing, at the dorsal membrane, the high and low affinity amiloride-sensitive Na+ channels, Na(+)-glucose cotransporter, K+ channels and pH sensitivity, and, at the ventral membrane, the Na+, K(+)-ATPase and K+ channels. The Na+ transport reacts to pH, but lacks the cAMP regulatory system, well known in many epithelia. The active Na+ transport drives glucose and fluid into the intraembryonic space, across and around the blastoderm which, in the absence of blood circulation, could secure renewal of extracellular fluid and disposal of wastes and thus maintain the cell homeostasis.

One-minute exposure of 4-cell mouse embryos to glucose overcomes morula block in CZB medium

One-cell mouse embryos that block at the 2-cell stage can progress to the morula stage in CZB medium, but fail to cavitate and then swell and lyse. A 1-min exposure to 27 mM glucose at the 4-cell stage (approximately 42 hr) will support a high frequency of development to the blastocyst stage (75%) in the same medium. A glucose exposure is beneficial anytime between 30 and 54 hr of culture (67-73% blastocysts). Of a group of additional sugars and glucose analogues tested for their ability to replace glucose, only galactose was equivalent in promoting embryo development to the blastocyst stage (64% blastocysts).

An unusual active hexose transport system in human and mouse preimplantation embryos

In a metabolic study of human and mouse preimplantation embryos (preembryos), we measured glucose uptake and phosphorylation with nonradioactive 2-deoxyglucose (DG) as tracer. Initial experiments indicated an active hexose transport capacity, a property thought to be restricted in mammals to intestinal villi and kidney tubules [Baly, D. L. & Horuk, R. (1988) Biochim. Biophys. Acta 947, 571-590]. Significant findings are as follows: (i) During a 60-min incubation with a low level of DG, mouse blastocyst DG rose to levels up to 30 times that of the medium. (The intestinal active system does not transport DG [Crane, R. K. (1960) Physiol. Rev. 40, 789-825].) (ii) Active preembryo transport was not blocked (as it would have been in the intestine) by phlorizin [Alvarado, F. & Crane, R. K. (1962) Biochem. Biophys. Acta 56, 170-172 and Sacktor, B. (1989) Kidney Int. 36, 342-350] or by replacement of Na+ with choline+ or K+ [Crane (1960) and Sacktor (1989)]. (iii) Transport of DG was blocked by cytochalasin B (which is not true for the intestinal transporter). We conclude that a distinct active hexose transporter and at least one facilitated transporter are present in preembryos, perhaps appearing in tandem on different membranes during formation of the increasingly complex preembryo structure.

Targeting of recombinant Na+/glucose cotransporter (SGLT1) to the apical membrane

A full-length Na+/glucose cotransporter cDNA (SGLT1) from rabbit intestine was subcloned into the pMAMneo mammalian expression vector and transfected by Ca2+ precipitation into Madin-Darby canine kidney (MDCK) cells. Stable MDCK transfectants isolated after clonal isolation and selection in G418 exhibited dexamethasone-inducible Na+/glucose cotransport activity under regulation of the MMTV promoter of the vector. Transfectants expressed the recombinant 75 kDa Na+/glucose cotransporter subunit as shown by Western blot, and SGLT1 mRNA as shown by Northern blot, but these were undetectable in untransfected MDCK cells. Over 93% of total recombinant transport activity was targeted to the apical membrane. This indicates that the primary amino acid sequence of SGLT1 contains the information necessary to target this transporter to the apical membrane.

Modulation of mouse preimplantation development by epidermal growth factor receptor antibodies, antisense RNA, and deoxyoligonucleotides

Two-cell mouse preimplantation embryos were cultured for 48 h in four different reagents to modulate epidermal growth factor (EGF) receptor function. These were rabbit polyclonal and mouse monoclonal antibodies to EGF receptor, EGF receptor antisense RNA, and EGF receptor antisense deoxyoligonucleotides. Embryos were scored for two endpoints: onset of cavitation as a measure of trophectoderm differentiation and mean embryo cell number as a measure of cell proliferation. The consistent observations were that cavitation was significantly accelerated by antibodies and delayed by antisense RNA and antisense deoxyoligonucleotides. None of these reagents exerted a significant effect on mean embryo cell number, with one exception, the polyclonal antibody. Our interpretation of these observations is that the antibody binding facilitated cavitation by mimicking natural ligand-receptor binding and inducing the signal transduction cascade that is typical for the EGF receptor. In the case of antisense RNA or deoxyoligonucleotide, we propose that they delayed onset of cavitation by interfering with EGF receptor production. We hypothesize that during this period of development, EGF receptor is concerned predominantly with the regulation of differentiation more than with cell proliferation.

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.

Epidermal growth factor receptor mRNA and protein increase after the four-cell preimplantation stage in murine development

Epidermal growth factor receptors (EGF-Rs) are expressed at increasing levels on mouse preimplantation embryos. Immunofluorescence assays were used to show that unfertilized eggs and 2-cell embryos have a very low level of reactivity to antimouse EGF-R antibodies, but by the 4-cell stage and later the reactivity increases. The synthesis of EGF-R protein was verified at the blastocyst stage by immunoprecipitation of a 170-kDa metabolically labeled protein. The EGF-R protein is expressed on cell plasma membrane surfaces, but after compaction at the 8-cell stage concentrates on the apical cell surfaces. We also find a low level of expression of EGF-R protein on inner cell mass cells; thus, all cell lineages express receptors from the beginning of gestation. The receptor protein synthesized by the 8-cell embryo and later is probably translated from embryonic transcription, since reverse transcription-polymerase chain reaction indicates increasing levels of mRNA starting after the 4-cell stage. However, we also detected maternal mRNA in zygotes and 2-cell embryos. The pervasive nature of EGF-R expression throughout development suggests important roles for these receptors which could include autocrine and paracrine stimulation.