Connexin expression and gap junctional coupling in human cumulus cells: contribution to embryo quality

Gap junctional coupling among cumulus cells is important for oogenesis since its deficiency in mice leads to impaired folliculogenesis. Multiple connexins (Cx), the subunits of gap junction channels, have been found within ovarian follicles in several species but little is known about the connexins in human follicles. The aim of this study was to determine which connexins contribute to gap junctions in human cumulus cells and to explore the possible relationship between connexin expression and pregnancy outcome from in vitro fertilization (IVF). Cumulus cells were obtained from IVF patients undergoing intra-cytoplasmic sperm injection (ICSI). Connexin expression was examined by RT-PCR and confocal microscopy. Cx43 was quantified by immunoblotting and gap junctional coupling was measured by patch-clamp electrophysiology. All but 5 of 20 connexin mRNAs were detected. Of the connexin proteins detected, Cx43 forms numerous gap junction-like plaques but Cx26, Cx30, Cx30.3, Cx32 and Cx40 appeared to be restricted to the cytoplasm. The strength of gap junctional conductance varied between patients and was significantly and positively correlated with Cx43 level, but neither was correlated with patient age. Interestingly, Cx43 level and intercellular conductance were positively correlated with embryo quality as judged by cleavage rate and morphology, and were significantly higher in patients who became pregnant than in those who did not. Thus, despite the presence of multiple connexins, Cx43 is a major contributor to gap junctions in human cumulus cells and its expression level may influence pregnancy outcome after ICSI.

Experimental allergic encephalomyelitis in connexin 43-heterozygous mice

Alterations in the expression of gap junction proteins (connexins) have previously been observed in experimental allergic encephalomyelitis (EAE). Demyelinating lesions have significantly decreased Cx43, while recovering lesions have greatly increased Cx43 and increased glial fibrillary acidic protein-expressing astrocytes. This suggests an important role for gap-junctional intercellular communication in astrocytes in the recovery from CNS inflammation. To study the effects of decreased Cx43 expression during acute disease (21 days post-immunization) and in recovering spinal cord tissue (55 days post-immunization) we induced EAE in Cx43 heterozygous and wild-type mice. Mice showed signs of disease by day 10, and signs of recovery by day 25. There were no clinical or pathological differences between heterozygous and wild-type mice in the acute disease stage, except that wild-type male mice had fewer clinical signs of disease. Male mice that were heterozygous for Cx43, and therefore had decreased expression of Cx43, had increased EAE disease severity. All demyelinating lesions had reduced numbers of reactive astrocytes and a significant decrease in Cx43 expression. In the 55-day study, all heterozygous and wild-type mice were clinically improved, showed decreased pathological signs, and showed increased laminin expression, indicative of CNS recovery. Furthermore, all heterozygous mice showed a striking increase in Cx43 expression during recovery, suggesting that the regulatory factors affecting Cx43 expression are still present in mice that have only one wild-type Cx43 allele.

Functional significance of gap junctional coupling in preimplantation development

Gap junctional intercellular coupling allows cells to share low molecular weight metabolites and second messengers, thus facilitating homeostatic and developmental processes. Gap junctions make their appearance very early in rodent development, during compaction in the eight-cell stage. Surprisingly, preimplantation mouse embryos lacking the gap junction protein connexin 43 develop normally and establish full-term pregnancies despite severely reduced gap junctional coupling. It was suggested that this might be explained by the presence of at least five additional connexins known to be expressed in blastocysts. In the present study, we set out to clarify the number of connexins present in preimplantation rodent embryos and the role of gap junctional coupling, if any, in blastocyst development. We provide evidence from reverse transcription-polymerase chain reaction analysis that the genes encoding 3 additional connexins (connexin 30 or beta6, connexin 36 or alpha9, and connexin 57 or alpha10) are also transcribed in preimplantation mouse embryos. Furthermore, we show that multiple connexins are expressed in rat preimplantation embryos, indicating that multiplicity of connexin expression may be a common feature of early mammalian embryogenesis. We could detect no up-regulation of any of 3 coexpressed connexins examined in mouse embryos lacking connexin 43. Impaired intercellular coupling caused either by the loss of connexin 43 or by treatment of cultured embryos with the gap junctional coupling blocker 18alpha-glycyrrhetinic acid (AGA) had no discernable effect on either apoptosis or glucose utilization, parameters known to be affected by gap junctional coupling in other contexts. These results, taken together with the reported inability of AGA to perturb blastocyst formation, imply that gap junctional coupling is not essential during this developmental period. We propose that connexin expression and the assembly of multiple types of gap junction channels in preimplantation embryos facilitates the diversification of communication pathways that will appear during postimplantation development. New evidence of this diversification is presented using rat blastocyst outgrowths.

Failure of spermatogenesis in mice lacking connexin43

Connexin43 (Cx43), a gap junction protein encoded by the Gja1 gene, is expressed in several cell types of the testis. Cx43 gap junctions couple Sertoli cells with each other, Leydig cells with each other, and spermatogonia/spermatocytes with Sertoli cells. To investigate the role of this communication pathway in spermatogenesis, we studied postnatal testis development in mice lacking Cx43. Because such mice die shortly after birth, it was necessary to graft testes from null mutant fetuses under the kidney capsules of adult males for up to 3 wk. Grafted wild-type testes were used as controls. In our initial experiments with wild-type testes, histological examination indicated that the development of grafted testes kept pace with that of nongrafted testes in terms of the onset of meiosis, but this development required the presence of the host gonads. When excised grafts were stimulated in vitro with cAMP or LH, there was no significant difference in androgen production between null mutant and wild-type testes, indicating that the absence of Cx43 had not compromised steroidogenesis. Previous research has shown that Cx43 null mutant neonates have a germ cell deficiency that arises during fetal life, and our analysis of grafted testes demonstrated that this deficiency persists postnatally, giving rise to a "Sertoli cell only" phenotype. These results indicate that intercellular communication via Cx43 channels is required for postnatal expansion of the male germ line.

The mouse Na+-K+-ATPase gamma-subunit gene (Fxyd2) encodes three developmentally regulated transcripts

The Na(+)-K(+)-ATPase is understood to function as a hetero-oligomer of alpha- and beta-subunits, but a third subunit, gamma, has been proposed to influence the enzyme's catalytic function. Recently, two variants of the gamma-subunit have been described in kidney, raising the possibility of multiple gamma-subunits with diverse functions. We now report the cloning and sequencing of the mouse gamma-subunit gene (Fxyd2). Analysis of the structure of the gene shows that it encodes three mRNAs that have distinct NH(2)-terminal (extracellular) encoding sequences but common transmembrane and COOH-terminal-encoding sequences resulting from differential splicing and, probably, alternate promoter usage. The three mRNAs have tissue-specific expression patterns. The existence of three different extracellular domains of the gamma-variants and how they may interact with the sodium pump to alter its cation transport properties must now be taken into account for future understanding of the modulation of the Na(+)-K(+)-ATPase by its gamma-subunit.

Intercellular communication via connexin43 gap junctions is required for ovarian folliculogenesis in the mouse

The ovarian follicle in mammals is a functional syncytium, with the oocyte being coupled with the surrounding cumulus granulosa cells, and the cumulus cells being coupled with each other and with the mural granulosa cells, via gap junctions. The gap junctions coupling granulosa cells in mature follicles contain several different connexins (gap junction channel proteins), including connexins 32, 43, and 45. Connexin43 immunoreactivity can be detected from the onset of folliculogenesis just after birth and persists through ovulation. In order to assess the importance of connexin43 gap junctions for postnatal folliculogenesis, we grafted ovaries from late gestation mouse fetuses or newborn pups lacking connexin43 (Gja1(-)/Gja1(-)) into the kidney capsules of adult females and allowed them to develop for up to 3 weeks (this was necessitated by the neonatal lethality caused by the mutation). By the end of the graft period, tertiary (antral) follicles had developed in grafted normal (wild-type or heterozygote) ovaries. Most follicles in Gja1(-)/Gja1(-) ovaries, however, failed to become multilaminar, with the severity of the effect depending on strain background. Dye transfer experiments indicated that intercellular coupling between granulosa cells is reduced, but not abolished, in the absence of connexin43, consistent with the presence of additional connexins. These results suggest that coupling between granulosa cells mediated specifically by connexin43 channels is required for continued follicular growth. Measurements of oocyte diameters revealed that oocyte growth in mutant follicles is retarded, but not arrested, despite the arrest of folliculogenesis. The mutant follicles are morphologically abnormal: the zona pellucida is poorly developed, the cytoplasm of both granulosa cells and oocytes is vacuolated, and cortical granules are absent from the oocytes. Correspondingly, the mutant oocytes obtained from 3-week grafts failed to undergo meiotic maturation and could not be fertilized, although half of the wild-type oocytes from 3-week grafted ovaries could be fertilized. We conclude that connexin43-containing gap junction channels are required for expansion of the granulosa cell population during the early stages of follicular development and that failure of the granulosa cell layers to develop properly has severe consequences for the oocyte.

Intercellular communication in preimplantation development: the role of gap junctions

Gap junctions are sites where intercellular membrane channels are clustered that allow neighboring cells to pass small molecules directly between them. Gap junctional intercellular communication has been implicated in a variety of human diseases. Gap junction channels are assembled from a large family of proteins called connexins with each type of channel having some unique properties. Preimplantation mouse and rat embryos express multiple connexins and thus potentially contain many types of gap junction channels. Based on experiments focussing on connexin43, gap junction assembly in the mouse begins during compaction in the 8-cell stage and is post-translationally regulated. Gene targeting has been used to create mice lacking individual connexins that are expressed in preimplantation embryos, but none of these experiments has yet revealed a necessary role for any single connexin before implantation. Experiments with anti-connexin antibodies and pharmacological blockers of gap junctional coupling have provided conflicting evidence as to the importance of gap junctions for preimplantation development. However, connexin knockouts have revealed important roles for gap junctional coupling in early postimplantation development. It is proposed that expression of multiple connexins in the blastocyst could prepare the implanting conceptus for rapid diversification of cell types during gastrulation and development of the placenta.

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.

Assessment by differential display-RT-PCR of mRNA transcript transitions and alpha-amanitin sensitivity during bovine preattachment development

The objectives of this study were to compare patterns of mRNA expression, investigate the onset of transcription, and isolate stage-specific and alpha-amanitin-sensitive mRNAs during early bovine development by differential-display-reverse transcription-polymerase chain reaction (DD-RT-PCR). Embryos representing a preattachment developmental series from the 1-cell to the expanded/hatched blastocyst stage were cultured in synthetic oviduct fluid medium + citrate and amino acids (cSOFMaa) with and without alpha-amanitin (100 microg/mL) for 4 and 12 hr. mRNA profiles were displayed by DD-RT-PCR using 5' primers A and N. Total conserved cDNA banding patterns varied according to embryo stage with cDNA band numbers declining during early cleavage stages compared to oocyte values and then increasing in total number from the 6-8-cell stage through to the blastocyst stage. A cDNA banding pattern was established at the 8-16-cell stage that was largely unchanged through to the blastocyst stage. These findings with respect to cDNA banding patterns were conserved between oligo primer sets and experimental replicates. alpha-Amanitin sensitivity was first detected at the 2-5-cell stage but became predominant following the 6-8-cell stage of development to eventually affect the appearance of up to 40% of all cDNA bands by the blastocyst stage. A 12 hr alpha-amanitin treatment was required to effectively block (3)H-uridine incorporation into mRNA in blastocyst stage embryos. Several stage-specific and alpha-amanitin-sensitive cDNAs were isolated and they will be a focus for future studies. In conclusion, DD-RT-PCR is an effective tool for contrasting gene expression patterns and isolating uncharacterized mRNA transcripts during bovine early development. Mol. Reprod. Dev. 55:152-163, 2000.

Defects in the germ line and gonads of mice lacking connexin43

The connexins are a family of at least 15 proteins that form the intercellular membrane channels of gap junctions. Numerous connexins, including connexin43 (Cx43), have been implicated in reproductive processes by virtue of their expression in adult gonads. In the present study, we examined the gonads of fetal and neonatal mice homozygous for a null mutation in the Gja1 gene encoding Cx43 to determine whether the absence of this connexin has any consequences for gonadal development. We found that in both sexes at the time of birth, the gonads of homozygous mutants were unusually small. This appears to be caused, at least in part, by a deficiency of germ cells. The germ cell deficiency was traced back as far as Day 11.5 of gestation, implying that it arises during early stages of germ line development. We also used an organ culture technique to examine postnatal folliculogenesis in the mutant ovaries, an approach necessitated by the fact that Gja1 null mutant offspring die soon after birth because of a heart abnormality. The results demonstrated that folliculogenesis can proceed to the primary (unilaminar) follicle stage in the absence of Cx43 but that subsequent development is impaired. In neonatal ovaries of normal mice, Cx43 could be detected in the somatic cells as early as Day 1, when primordial follicles begin to appear, supporting the conclusion that this connexin is required for the earliest stages of folliculogenesis. These results imply that gap junctional coupling mediated by Cx43 channels plays indispensable roles in both germ line development and postnatal folliculogenesis.

Doubly mutant mice, deficient in connexin32 and -43, show normal prenatal development of organs where the two gap junction proteins are expressed in the same cells

The connexins are a family of proteins that form the intercellular membrane channels of gap junctions. Genes encoding 13 different rodent connexins have been cloned and characterized to date. Connexins vary both in their distribution among adult cell types and in the properties of the channels that they form. In order to explore the functional significance of connexin diversity, several mouse connexin-encoding genes have been disrupted by homologous recombination in embryonic stem cells. Although those experiments have illuminated specific physiological roles for individual connexins, the results have also raised the possibility that connexins may functionally compensate for one another in cells where they are coexpressed. In the present study, we have tested this hypothesis by interbreeding mice carrying null mutations in the genes (Gjb1 and Gja1) encoding connexin32 (beta 1 connexin) and connexin43 (alpha 1 connexin), respectively. We found that fetuses lacking both connexins survive to term but, as expected, the pups die soon thereafter from the cardiac abnormality caused by the absence of connexin43. A survey of the major organ systems of the doubly mutant fetuses, including the thyroid gland, developing teeth, and limbs where these two connexins are coexpressed, failed to reveal any morphological abnormalities not already seen in connexin43 deficient fetuses. Furthermore, the production of thyroxine by doubly mutant thyroids was confirmed by immunocytochemistry. We conclude that, at least as far as the prenatal period is concerned, the normal development of those three organs in fetuses lacking connexin43 cannot simply be explained by the additional presence of connexin32 and vice-versa. Either gap junctional coupling is dispensable in embryonic and fetal cells in which these two connexins are coexpressed, or coupling is provided by yet another connexin when both are absent.

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.

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.

Altered gap junctional communication, intercellular signaling, and growth in cultured astrocytes deficient in connexin43

Astrocytes are characterized by extensive intercellular communication mediated primarily by gap junction channels composed of connexin43. To examine this junctional protein in astrocytic functions, astrocytes were cultured from embryonic mice with a null mutation in the connexin43 gene (Reaume et al.: Science 267:1831-1834, 1995). Using anti-Cx43 antibodies, immunoblotting and immunostaining indicated that homozygous null astrocytes were devoid of Cx43. They are also deficient in intercellular dye transfer. Astrocytes cultured from heterozygous embryos express significantly lower Cx43 compared to wild type, and their dye coupling is reduced. Markers of glial differentiation, such as glial fibrillary acidic protein and S100, appeared similar in all genotypes. Measurement of intercellular calcium concentration following mechanical stimulation of confluent astrocytes revealed that the number of cells affected by a rise in intracellular calcium was reduced in homozygous cultures compared to wild type. In fact, the calcium response in homozygous astrocytes was similar to that observed in wild-type astrocytes in the presence of a gap junction blocker. The growth rate of astrocytes lacking Cx43 was reduced compared to wild-type astrocytes. These results suggest that gap junctional intercellular communication mediated by Cx43 is not critical for astrocyte differentiation but is likely involved in the regulation of intercellular calcium signaling and cell growth.

Evidence for the presence of sodium- and potassium-dependent adenosine triphosphatase alpha1 and beta1 subunit isoforms and their probable role in blastocyst expansion in the preattachment horse conceptus

The unusual hypotonicity of equine blastocyst fluid has prompted us to investigate the role of sodium- and potassium-dependent adenosine triphosphatase (Na+,K+-ATPase) in the process of fluid accumulation in the horse conceptus. Nine mares were used for the experiments. Reverse transcriptase polymerase chain reaction was conducted on two sets of five conceptuses recovered between 12 and 28 days (+/- 1 day) after ovulation. Messenger RNAs encoding the alpha1 and beta1 subunit isoforms of Na+,K+-ATPase were detected in all embryonic tissues examined. Western blot analysis showed that alpha1 and beta1 subunits are both present in Day 15 conceptuses. Trophoblast tissues from 19 conceptuses between 8 and 31 days after ovulation were stained immunohistochemically using primary antibodies against the alpha1 and beta1 subunit isoforms of the Na+,K+-ATPase. Both isoforms were detected in all sections. Trophoblastic vesicles, prepared from 6 conceptuses between 12 and 14 days after ovulation, were used to investigate the inhibition of blastocyst expansion with ouabain after collapse induced with cytochalasin D. In normal medium there was a mean 3-fold increase, and in ouabain (10(-6) M) a mean 3-fold decrease, in the volume of vesicles that had been partially collapsed with cytochalasin D. We therefore conclude that, despite the hypotonicity of the blastocyst fluid in the early horse conceptus, the Na+,K+-ATPase plays a role in its accumulation, as in other species.

Normal development of preimplantation mouse embryos deficient in gap junctional coupling

The connexin multigene family (13 characterized members in rodents) encodes the subunits of gap junction channels. Gap junctional intercellular coupling, established during compaction of the preimplantation mouse embryo, is assumed to be necessary for development of the blastocyst. One member of the connexin family, connexin43, has been shown to contribute to the gap junctions that form during compaction, yet embryos homozygous for a connexin43 null mutation develop normally, at least until implantation. We show that this can be explained by contributions from one or more additional connexin genes that are normally expressed along with connexin43 in preimplantation development. Immunogold electron microscopy confirmed that roughly 30% of gap junctions in compacted morulae contain little or no connexin43 and therefore are likely to be composed of another connexin(s). Confocal immunofluorescence microscopy was then used to demonstrate that connexin45 is also assembled into membrane plaques, beginning at the time of compaction. Correspondingly, embryos homozygous for the connexin43 null mutation were found to retain the capacity for cell-to-cell transfer of fluorescent dye (dye coupling), but at a severely reduced level and with altered permeability characteristics. Whereas mutant morulae showed no evidence of dye coupling when tested with 6-carboxyfluorescein, dye coupling could be demonstrated using 2′,7′-dichlorofluorescein, revealing permeability characteristics previously established for connexin45 channels. We conclude that preimplantation development in the mouse can proceed normally even though both the extent and nature of gap junctional coupling have been perturbed. Despite the distinctive properties of connexin43 channels, their role in preimplantation development can be fulfilled by one or more other types of gap junction channels.

Gap junctions promote the bystander effect of herpes simplex virus thymidine kinase in vivo

Transfer of the herpes simplex thymidine kinase gene (HSVtk) into tumor cells followed by the administration of ganciclovir (GCV) provides a potential strategy for the treatment of some malignancies. During GCV treatment, not only the cells that express the HSVtk gene are killed but also frequently neighboring tumor cells that are not genetically altered. This has been called the "bystander effect." Although the mechanism of the bystander effect in vivo remains elusive, our results suggest that gap junction formation between neighboring cells is an important contributing factor. The C6 rat glioma cell line, which exhibits a low level of intercellular communication by gap junctions and connexin43 (Cx43)-transfected clones of this cell line forming gap junctions from a moderate level (Cx43-12 and Cx43-14) to a high level (Cx43-13), were transduced with HSVtk. Transduced and nontransduced cells were mixed in various concentrations and then cultured in vitro or injected s.c. into C.B-17/SCID-beige mice followed by i.p. injections of GCV. Cx43-transfected clones showed a significant increase of the bystander effect compared with the less coupled C6 parental cell line. In 11 of 12 mice injected with cells of Cx43-transfected clones, no tumors were seen at the inoculation site when a mixture of 50% HSVtk-negative and HSVtk-positive cells was used. Moreover, in mice injected with cells of clone Cx43-13, which exhibits the highest intercellular communication, tumors were frequently undetectable at the inoculation site when using mixtures of 75% HSVtk-negative and 25% HSVtk-positive cells, and even mixtures containing 5% HSVtk-positive cells of Cx43-transfected clones showed tumor size reduction. All animals in control groups (n = 26) developed large tumors at every injection site. These results demonstrate that gap junctions are an important component in mediating the bystander effect in vivo.

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.

Development of astrocytes and neurons in cultured brain slices from mice lacking connexin43

Astrocyte and neuronal development was investigated in organotypic brain slice cultures from mouse fetuses with a null mutation in the connexin43 gene. Astrocyte morphology and electrical properties were indistinguishable in null mutant slices and control slices but at 18 days in vitro astrocyte density in the central regions of the null mutant slices was significantly higher than in control slices. Neuronal development assessed morphologically and electrophysiologically appeared normal in the mutant slices. These results suggest that intercellular communication mediated through connexin43 is not essential for the development of astrocytes and neurons but may play a role in regulating astrocytic migration.

Rat endometrial stromal cells express the gap junction genes connexins 26 and 43 and form functional gap junctions during in vitro decidualization

Gap junctions form between rat endometrial stromal cells as they undergo decidualization. We have examined the steady-state levels of the gap junction transcripts, connexins 26 and 43 (cx26 and cx43), during artificially induced decidualization in vivo and found that they have a temporal pattern similar to that observed in pregnancy. An in vitro model of decidualization was then used. Endometrial stromal cells from rat uteri sensitized for decidualization were cultured for 24, 48, or 72 h before total RNA was extracted and subjected to Northern blot analyses to determine the steady-state levels of cx26 and cx43 transcripts. The analyses revealed that cx26 transcript steady-state levels decreased, whereas those for cx43 increased, from 24 to 72 h. Using an anti-cx43 antibody, punctate immunofluorescent signals were observed around the periphery of the cells, suggesting that cx43 had been assembled into membrane plaques. The presence of functional gap junctions between the cells was determined in vitro by two dye-coupling methods: preloading and scrape-loading. Calcein (995 Da) and a membrane-bound dye, dioctadecyl-3,3,3',3-tetramethylindocarbocyanine perchlorate (933 Da), were preloaded into 5% of the endometrial stromal cells before plating. The percentage of preloaded cells that transferred calcein to adjacent cells increased from 10% at 3 h after plating to 40% at 6 h. To determine whether or not cells maintain the ability to dye-couple throughout the culture period, carboxyfluorescein (CF; 376 Da) and rhodamine dextran (9.3 kDa) were introduced into cells by scraping the cells with a scalpel, and the distribution of dyes was determined 20 min later. In cells cultured for 24, 48, or 72 h, only CF was transferred to cells distal to the scrape line. The results from these experiments suggest that stromal cells can dye-couple throughout the culture period (3-72 h) and indicate that functional gap junctions form between endometrial stromal cells as they undergo decidualization in vitro.

Role of germinal vesicle on protein synthesis in rat oocyte during in vitro maturation

To investigate the role of the germinal vesicle (GV) on in vitro maturation (IVM) of rat oocytes, we examined protein synthesis during IVM by comparing polypeptide patterns in control and enucleated oocytes using one and two-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). Separation of polypeptides extracted from the cytoplasm of GV by one-dimensional SDS-PAGE revealed that a 55 kDa polypeptide was present only in the GVs of rat oocytes. At 0, 12, 24, 36, and 44 hr after PMSG injection, prior to the initiation of maturation, enucleated oocytes synthesized the same major polypeptides as cumulus intact (CI) oocytes. During meiotic maturation, no major changes were detected in protein synthesis from prophase (GV stage) to prometaphase I (0-6 hr IVM). However, after entry into prometaphase I (7 hr IVM), striking changes were seen; a 24 kDa polypeptide disappeared and expression of a 34 kDa polypeptide became stronger. This pattern lasted until metaphase II. We detected no major differences in the pattern of protein synthesis between CI and enucleated oocytes using two-dimensional PAGE. These results indicate that protein synthesis in the maturing rat oocyte is controlled by cytoplasmic regulators rather than intrinsic nuclear components.

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

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

Temporal control of gap junction assembly in preimplantation mouse embryos

The de novo assembly of gap junctions during compaction in the 8-cell stage of mouse development is a temporally regulated event. We have performed experiments designed to explore the relationship between this event and DNA replication in the second, third, and fourth cell cycles after fertilization. Inhibition of DNA synthesis by continuous treatment with the DNA synthesis inhibitor, aphidicolin, during the third and fourth cell cycles had no effect on the establishment of gap junctional coupling during compaction. However, a delay of 10 hours in DNA synthesis during the second cell cycle caused by a transient aphidicolin treatment resulted in the failure of gap junctional coupling at the time of compaction. Thus the timing of establishment of gap junctional coupling, like the timing of compaction itself, is linked to DNA replication in the 2-cell stage. Immunofluorescence analysis showed that the failure of gap junctional coupling after aphidicolin treatment in the 2-cell stage is correlated with the failure of nascent connexin43 to be inserted into plasma membranes. We propose that the developmental ‘clock’ that controls gap junction assembly is set in motion by events surrounding the second cycle of DNA replication, and that this ‘clock’ ultimately controls the post-translational processing of connexin43.

Cardiac malformation in neonatal mice lacking connexin43

Gap junctions are made up of connexin proteins, which comprise a multigene family in mammals. Targeted mutagenesis of connexin43 (Cx43), one of the most prevalent connexin proteins, showed that its absence was compatible with survival of mouse embryos to term, even though mutant cell lines showed reduced dye coupling in vitro. However, mutant embryos died at birth, as a result of a failure in pulmonary gas exchange caused by a swelling and blockage of the right ventricular outflow tract from the heart. This finding suggests that Cx43 plays an essential role in heart development but that there is functional compensation among connexins in other parts of the developing fetus.

Expression of plasminogen activator genes and enzymatic activities in rat preimplantation embryos

Plasminogen activator has been implicated in tissue invasion and remodelling because of its role in the degradation of the extracellular matrix. Its activity can be detected in mouse embryos as early as day 6 of pregnancy, suggesting that plasminogen activator is involved in the process of implantation. The present study determined the time course of expression of the genes encoding tissue-type plasminogen activator (tPA) and urokinase-type plasminogen activator (uPA) during the preimplantation period in rats by the sensitive mRNA phenotyping procedure of reverse transcription-PCR. The tPA mRNA was present in rat oocytes and two-cell embryos, but was not detected between the four-cell and blastocyst stages. The uPA mRNA was first detected in two-cell rat embryos, and was present through to the blastocyst stage. In chromogenic assays, plasminogen activator activity was detected in oocytes and embryos between two-cell and blastocyst stages. Most plasminogen activator activity present in preimplantation embryos appeared to be uPA, as it could be inhibited by anti-uPA antibody and a specific uPA inhibitor, amiloride, but not by anti-tPA antibody. The present data demonstrate the expression of uPA gene and uPA activity in preimplantation rat embryos, suggesting that embryonic uPA may be involved in early embryo development and implantation.

Possible roles of insulin and insulin-like growth factors in rat preimplantation development: investigation of gene expression by reverse transcription-polymerase chain reaction

The sensitive mRNA phenotyping technique of reverse transcription-polymerase chain reaction was used to demonstrate that insulin receptor mRNA is present in rat embryos during the preimplantation period. In addition, mRNA encoding insulin-like growth factor (IGF) type I and type II receptors have also been detected in rat preimplantation embryos. IGF-I mRNA was not detected in preimplantation embryos but was found in oviducts and uteri of prepubertal and early pregnant rats. IGF-II mRNA was present in both embryos and in oviducts and uteri during the preimplantation period. These findings suggest that insulin and IGF-I could influence early embryo development in endocrine or in paracrine fashions, whereas IGF-II may have an additional autocrine mode of action in affecting preimplantation embryos in rats.

Regulation of Na+,K(+)-ATPase alpha subunit gene expression during mouse preimplantation development

Several lines of evidence support the hypothesis that a sodium flux, driven by Na+,K(+)-ATPase in the basolateral plasma membranes of mural trophectoderm, drives fluid transport during blastocoel formation in eutherians. In light of the importance of this enzyme for preimplantation development, attention has been focused on the regulation of expression of its alpha and beta subunits. Here we report on the spatial distribution and translation of the alpha subunit mRNA. Although this mRNA accumulates from the 2-cell stage onward the alpha subunit itself could not be detected by immunofluorescence prior to the late morula stage, after which it becomes concentrated in the mural trophectoderm. In the present study we have used a wholemount, fluorescent in situ hybridization technique that takes advantage of the optical sectioning capability of the confocal microscope to show that alpha subunit mRNA, in contrast to the alpha subunit itself, accumulates in all cells of the early blastocyst. This finding demonstrates that the spatial distribution of the alpha subunit is regulated post-transcriptionally. We have also examined the translational regulation of alpha subunit mRNA by preparing polyribosomal and subribosomal ribonucleoprotein fractions for mRNA assay by reverse transcription-polymerase chain reaction. We found that alpha subunit mRNA is in polyribosomes continuously from at least the 4-cell stage. Thus, the abrupt appearance of the alpha subunit in the late morula stage as revealed by immunofluorescence must be determined by post-translational events. In the Discussion, we consider the hypothesis that synthesis of the beta subunit of the enzyme is the rate limiting step in functional expression of the alpha subunit.

Alterations in the synthesis of insulin-like growth factor binding proteins and insulin-like growth factors in rat C6 glioma cells transfected with a gap junction connexin43 cDNA

When C6 glioma cells are stably transfected with a connexin43 cDNA and gap junctions are increased, the rate of cellular proliferation is decreased. To determine if this phenomenon is related to alterations in IGFBP and IGF synthesis, we have compared IGFBPs and IGFs in the conditioned media from primary rat astroglia, C6, and transfected C6 clones Cx43-13 (high expresser), and Cx43-12 and Cx43-14 (intermediate expressers). Primary astroglia produced IGFBP-2 (34 kDa) and IGFBP-3 (40-45 kDa). C6 cells synthesized high levels of IGFBP-3 and low levels of IGFBP-2, and a 24 kDa IGFBP (IGFBP-4). Cx43-13 cells did not synthesize IGFBP-3, but produced low levels of IGFBP-2 and high levels of IGFBP-4. Cx43-12 and Cx43-14 secreted IGFBP profiles similar to the parent C6 line, but with reduced levels of IGFBP-2. The lack of IGFBP-3 in Cx43-13 cells was not due to the presence of proteases. Northern analysis showed IGFBP-2 mRNA to be readily detectable only in the primary astroglia. IGFBP-3 mRNA was detected in the primary astroglia, C6, Cx43-12 and Cx43-14, but not in Cx43-13. In contrast, IGFBP-4 mRNA was readily detected only in the Cx43-13. IGF-II concentrations in the media were low to undetectable for both C6 and transfected cells. IGF-I concentrations were significantly lower in the media from transfected cells compared to the C6 cells. Stable mRNA levels for IGF-I were lower in transfected cells, with the lowest levels observed in the Cx43-13 cells. Although C6 cells did not respond mitogenically to exogenous IGF-I or IGF-II, Cx43-13 cells responded to IGF-I or IGF-II in a dose dependent manner. Conditioned media from Cx43-13 cells decreased the DNA synthesis of C6 cells, and this effect could be reversed by the addition of IGF-II. The decreased synthesis of the autocrine/paracrine growth factor IGF-I together with decreased levels of a positive modulator IGFBP-3, and the increased levels of a negative modulator IGFBP-4 in the extracellular milieu, may be responsible for the reduced proliferative capacity in cells expressing abundant connexin43.

Coexpression of gap junction proteins in the cumulus-oocyte complex

The connexins constitute a family of proteins that make up the intercellular membrane channels of gap junctions. We had previously reported the presence of two members of this protein family, connexins 32 and 43, in mouse one-cell zygotes (Barron et al., Dev Genet 10:318-323, 1989; Valdimarsson et al., Mol Reprod Dev 30:18-26, 1991), implying that both must be present in the mature oocyte and could be involved in mediating the intercellular coupling that occurs between the oocyte and cumulus granulosa during oogenesis. In the present report we provide evidence for this, based on an analysis of the cumulus-oocyte complex (COC) using reverse transcription-polymerase chain reaction (RT-PCR) and immunocytochemistry with a confocal microscope. Transcripts of both connexin32 (Cx32) and connexin43 (Cx43) were detected by RT-PCR in both components of the COC. Cx32 mRNA in the oocyte declined precipitously following human chorionic gonadotropin (hCG) stimulation of pregnant mare serum gonadotropin (PMSG)-primed ovaries, whereas there was no obvious change in Cx43 mRNA. Peptide-specific antibodies against both connexins provided diffuse cytoplasmic staining of oocytes as well as some punctate staining near the oocyte surface, which could not be unequivocally resolved as cumulus-oocyte gap junctions. However, the two antibodies did provide clear evidence of Cx32 and Cx43 in gap junction-like structures between cumulus cells. We could find no evidence of the incorporation of the oocyte's store of Cx32 into gap junctions during postfertilization development.(ABSTRACT TRUNCATED AT 250 WORDS)

Connexin trafficking and the control of gap junction assembly in mouse preimplantation embryos

Gap junction assembly in the preimplantation mouse embryo is a temporally regulated event, beginning a few hours after the third cleavage during the morphogenetic event known as compaction. Recently, we demonstrated that both mRNA and protein corresponding to connexin43, a gap junction protein, accumulate through preimplantation development beginning at least as early as the 4-cell stage. Using an antibody raised against a synthetic C-terminal peptide of connexin43, this protein was shown to assemble into gap junction-like plaques beginning at compaction (G. Valdimarsson, P. A. De Sousa, E. C. Beyer, D. L. Paul and G. M. Kidder (1991). Molec. Reprod. Dev. 30, 18–26). The purpose of the present study was to follow the fate of nascent connexin43 during preimplantation development, from synthesis to plaque insertion, and to learn more about the control of gap junction assembly during compaction. Cell fractionation and reverse transcription-polymerase chain reaction were employed to show that connexin43 mRNA is in polyribosomes at the 4-cell stage, suggesting that synthesis of connexin43 begins at least one cell cycle in advance of when gap junctions first form. The fate of nascent connexin43 was then followed throughout preimplantation development by means of laser confocal microscopy, using two other peptide (C-terminal)-specific antibodies. As was reported previously, connexin43 could first be detected in gap junction-like plaques beginning in the 8-cell stage, at which time considerable intracellular immunoreactivity could be seen as well. Later, connexin43 becomes differentially distributed in the apposed plasma membranes of morulae and blastocysts: a zonular distribution predominates between outside blastomeres and trophectoderm cells whereas plaque-like localizations predominate between inside blastomeres and cells of the inner cell mass. The cytoplasmic immunoreactivity in morulae was deemed to be nascent connexin en route to the plasma membrane since it could be abolished by treatment with cycloheximide, and redistributed by treatment with monensin or brefeldin-A, known inhibitors of protein trafficking. Treatment of uncompacted 8-cell embryos with either monensin or brefeldin-A inhibited the appearance of gap junction-like structures and the onset of gap junctional coupling in a reversible manner. These data demonstrate that the regulated step in the onset of gap junction assembly during compaction is downstream of transcription and translation and involves mobilization of connexin43 through trafficking organelles to plasma membranes.

Insulin-like growth factor binding protein-4 gene expression is induced by transfection of gap junction connexin43 gene in a C6 glioma cell line

When C6 glioma cells are stably transfected with a connexin43 cDNA and the gene overexpressed, the rate of cellular proliferation is decreased. To determine if this phenomenon is related to alterations in IGFBP synthesis, we have compared the conditioned media of primary rat astroglia, C6, clones Cx43-13 (high expresser of the transfected connexin43 gene), and Cx43-12 and Cx43-14 (intermediate expressers). Primary astroglia produced IGFBP-2 (M(r) 34 K) and IGFBP-3 (40-45 K). C6 cells synthesized high levels of IGFBP-3 and low levels of IGFBP-2, and a 24 K IGFBP (IGFBP-4). Cx43-13 cells did not synthesize IGFBP-3, but produced low levels of IGFBP-2 and high levels of IGFBP-4. Cx43-12 and Cx43-14 secreted IGFBP profiles similar to the parent C6 line, but with reduced levels of IGFBP-2. Northern analysis showed the changes in IGFBPs in the conditioned media to be correlated with alterations in stable mRNA levels. IGFBP-4, a inhibitor of IGF biological action, was produced in greater quantities by the slowly proliferating Cx43-13 cells. Alterations in IGFBP-4 synthesis may be responsible, at least in part, for the reduced proliferative capacity in cells with abundant connexin43.

Growth retardation in glioma cells cocultured with cells overexpressing a gap junction protein

To examine the role of gap-junctional intercellular communication in controlling cell proliferation, we have transfected C6 glioma cells with connexin 43 cDNA. The growth of transfected clones was dramatically reduced compared with nontransfected glioma cells. To further characterize the role of gap junctions in controlling proliferation, we have examined the growth of C6 cells cocultured with transfected cells overexpressing connexin 43. Although C6 cells grew at their normal rate when cocultured with nontransfected C6 cells, when cocultured with connexin 43-overexpressing cells they displayed a dramatic reduction in growth rate. Furthermore, a significant, dose-dependent reduction in cell proliferation was noted when C6 cells were cultured in medium conditioned by transfected cells. This effect correlated with the level of connexin 43 expression. These results suggest that the decreased cell proliferation rate of transfected cells and C6 cells cultured with them is due to the secretion of a growth inhibitory factor(s) and that the secretion of this factor may be linked to the level of gap junctional intercellular communication.

How to make a blastocyst

Several of the new reproductive technologies have been cultivated from our current understanding of the genetic programming and cellular processes that are involved in the major morphogenetic events of mammalian preimplantation development. Research directed at characterizing the patterns of gene expression during early development has shown that the embryo is initially under maternal control and later superseded by new transcriptional activity provided by the activation of the embryonic genome. Several embryonic transcripts encoding: (i) growth factors, (ii) cell junctions, (iii) plasma membrane ion transporters, and (iv) cell adhesion molecules have been identified as contributing directly to the progression of the embryo through the preimplantation interval of development. In this brief review, we have outlined the patterns of expression and the integral roles that these gene families play in the morphogenetic events of compaction and cavitation. Research of this type has greatly facilitate our understanding of the control processes that underlie preimplantation development and represent but one area of this exciting and vigorous field of research.

Intercellular calcium signaling via gap junctions in glioma cells

Calcium signaling in C6 glioma cells in culture was examined with digital fluorescence video microscopy. C6 cells express low levels of the gap junction protein connexin43 and have correspondingly weak gap junctional communication as evidenced by dye coupling (Naus, C. C. G., J. F. Bechberger, S. Caveney, and J. X. Wilson. 1991. Neurosci. Lett. 126:33-36). Transfection of C6 cells with the cDNA encoding connexin43 resulted in clones with increased expression of connexin43 mRNA and protein and increased dye coupling, as well as markedly reduced rates of proliferation (Zhu, D., S. Caveney, G. M. Kidder, and C. C. Naus. 1991. Proc. Natl. Acad. Sci. USA. 88:1883-1887; Naus, C. C. G., D. Zhu, S. Todd, and G. M. Kidder. 1992. Cell Mol. Neurobiol. 12:163-175). Mechanical stimulation of a single cell in a culture of non-transfected C6 cells induced a wave of increased intracellular calcium concentration ([Ca2+]i) that showed little or no communication to adjacent cells. By contrast, mechanical stimulation of a single cell in cultures of C6 clones expressing transfected connexin43 cDNA induced a Ca2+ wave that was communicated to multiple surrounding cells, and the extent of communication was proportional to the level of expression of the connexin43 cDNA. These results provide direct evidence that intercellular Ca2+ signaling occurs via gap junctions. Ca2+ signaling through gap junctions may provide a means for the coordinated regulation of cellular function, including cell growth and differentiation.

Characteristics of C6 glioma cells overexpressing a gap junction protein

1. C6 glioma cells transfected with connexin43 cDNA display a dramatic increase in the level of connexin43 mRNA and protein. 2. This overexpression of connexin43 is evident at the cellular level, as revealed with in situ hybridization and immunocytochemistry. Transfection with connexin43 cDNA also induced actin stress fibers in these glioma cells. 3. Although we observed up to a 50-fold increase in the level of connexin43 mRNA following transfection, virtually all of this mRNA was present in the polysomal fraction. 4. Overexpression of connexin43 mRNA did not appear to compete with other cellular mRNAs for access to the translational machinery. 5. It is likely that the reduced proliferation rate of the transfected cells, reported earlier, is due to enhanced connexin43 expression and intercellular coupling.

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.

Transfection of C6 glioma cells with connexin 43 cDNA: analysis of expression, intercellular coupling, and cell proliferation

C6 glioma cells express low levels of the gap junction protein connexin 43 and its mRNA and display very weak dye coupling. When implanted into the rat cerebrum, these cells quickly give rise to a large glioma. To investigate the role of gap junctions in the tumor characteristics of these cells, we have used Lipofectin-mediated transfection to introduce a full-length cDNA encoding connexin 43. Several transfected clones were obtained that exhibited various amounts of connexin 43 mRNA transcribed from the inserted cDNA. Immunocytochemical analysis revealed an increase in the amount of connexin 43 immunoreactivity in the transfected cells, being localized at areas of intercellular contact as well as in the cytoplasm. The level of dye coupling was also assessed and found to correlate with the amount of connexin 43 mRNA. When cell proliferation was followed over several days, cells expressing the transfected cDNA grew more slowly than non-transfected cells. These transfected cells will be useful in examining the role of gap junctions in tumorigenesis.

Expression of gap junction genes during postnatal neural development

The timing of appearance of mRNAs encoding gap junction proteins was examined during development of the rat and mouse brain. Complementary DNAs (cDNAs) specific for the mRNA for the liver-type gap junction protein, connexin32, and the heart-type gap junction protein, connexin43, were used to probe Northern blots of total RNA isolated from the forebrain and hindbrain of mice and rats at various times before and after birth. Prior to postnatal day 10, connexin32 mRNA is detectable only at low levels. By postnatal days 10 to 16, a sharp increase occurs in the level of this mRNA. This increase is detectable first in the hindbrain, and subsequently in the forebrain. In contrast, connexin43 mRNA is readily detectable at birth, and the level of this mRNA also increases during subsequent development. The developmental appearance of the gap junction proteins, connexin32 and connexin43, was similar to that of their respective mRNAs. These results indicate that the genes encoding connexin32 and connexin43 are differentially expressed during neural development.

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

Mouse preimplantation embryos were examined for the cell surface expression of epitopes that cross-react with antibodies to a 75-kDa subunit of a purified porcine renal brush border Na+/glucose cotransport system. A Na+ cotransport system is hypothesized to reside in the apical plasma membrane domain of mouse polar blastomeres and to be associated with the induction of their apical-basal polarity. Western blot analysis showed that unfertilized oocytes as well as preimplantation embryos contain a cross-reacting antigen with an apparent molecular weight of about 75,000. Embryos and their isolated blastomeres were double-labeled and assayed by indirect immunofluorescence (IIF) for the expression of epitopes (visualized by labeling with rabbit antiserum or mouse monoclonal IgG to cotransporter followed by the appropriate rhodamine-conjugated second antibodies) and for the development of cell surface polarity (visualized by the apical restriction of fluoresceinated succinylated concanavalin A binding; FS Con A). IIF did not detect these epitopes until after the second cleavage when 4-cell embryos expressed low-to-moderate levels. Although epitopes were expressed on all surfaces of 4-cell blastomeres, some blastomeres expressed more epitopes on their apical surfaces than on their basolateral ones. All precompaction 8-cell embryos expressed epitopes, with expression being greater apically on some blastomeres. The level of expression appeared to reach a maximum on morulae and to decline on cavitating embryos. Assays performed on isolated blastomeres from postcompaction embryos showed that by the 16-cell stage epitope expression appeared to become restricted to FS Con A-labeled apical plasma membrane domains and was no longer evident on basolateral domains. This apparent apical restriction of epitope expression was confirmed by electron microscopic examination of immunogold-labeled isolated polar 16-cell blastomeres. These results demonstrate that preimplantation mouse embryos contain an antigen(s) that is immunologically and structurally similar to a 75-kDa renal Na+/glucose cotransporter. The onset of cell surface expression of this antigen precedes development of the stable polar phenotype.

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.

Cell polarity and development of the first epithelium

In the 4 1/2 to 5 days between fertilization and implantation, the mouse conceptus must gain the abilities to implant and produce an embryo. Each of these is the sole developmental responsibility of one of two cell types forming the blastocyst, trophectoderm and inner cell mass (ICM), respectively. Trophectoderm is a polarized transporting epithelium while the ICM is an aggregate of non-epithelial pluripotent stem cells. These two cell types originate from the division of polar blastomeres when their cleavage furrows parallel their apical surfaces. Blastomeres polarize in response to asymmetric cell--cell contact, and understanding the mechanism of this induction is regarded as the key to understanding the origin of trophectoderm and ICM. Here we propose a model based on transcellular ion current loops for the induction of cell polarity during the development of the first epithelium, trophectoderm.

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

Na,K-ATPase is a plasma membrane enzyme that plays a critical role in eutherian blastocoel formation (cavitation) by pumping Na+ into the extracellular space enclosed by the trophectoderm. Previous experiments with the mouse had shown that the alpha (catalytic) subunit of the enzyme becomes detectable by immunocytochemistry in the late morula, just prior to the onset of cavitation. In the present study we have used cDNAs corresponding to three mRNA isoforms of the alpha subunit and a beta subunit to determine which genes are expressed during preimplantation development and to explore the timing of their expression. Of the three alpha subunit cDNAs tested by Northern blot hybridization with blastocyst RNA, only alpha 1 produced a hybridization signal, recognizing a single mRNA about 4 kb in length. This mRNA is relatively abundant in zygotes but barely detectable by the 2-cell stage and then accumulates steadily thereafter to reach its preimplantation maximum in blastocysts. The beta 1 cDNA detected mRNA of about 2.6-2.8 kb. This mRNA is present in zygotes but could not be detected in 2-, 4-, or 8-cell stages; it is present at a low level in late morulae and is abundant in blastocysts. The temporal profile of accumulation of beta 1 mRNA thus matches more closely than does alpha 1 the timing of appearance of the catalytic subunit. This suggests that the beta subunit may regulate production of the holoenzyme and hence the timing of cavitation.

Connexin32, a gap junction protein, is a persistent oogenetic product through preimplantation development of the mouse

Gap junctions appear de novo during compaction in the eight-cell stage of mouse development. This is a critical event in the life of the embryo, because gap junctional intercellular communication is an essential requirement for maintaining compaction and, hence, for development of the blastocyst. Recently, a family of genes encoding gap junction proteins (connexins) has been identified and cloned, and we have taken advantage of the availability of antibodies and cDNA probes to investigate the expression of these genes in early development. We found that a protein with antigenic and size similarity to the "liver" gap junction protein, connexin32, is present throughout preimplantation development from the zygote through the late morula. Connexin32 mRNA, however, could not be detected in any preimplantation stage. This, and the presence of connexin32 in zygotes before activation of embryonic transcription, leads us to conclude that this protein is inherited as an oogenetic product that persists well beyond the transition from the oogenetic to embryonic program of gene expression. Furthermore, we found that mRNA for another gap junction protein, connexin43, is fairly abundant in preimplantation embryos. We conclude that it is more likely connexin43, and not connexin32, that is used to assemble new connexons as the level of intercellular coupling increases after compaction.

Immunofluorescence assessment of the timing of appearance and cellular distribution of Na/K-ATPase during mouse embryogenesis

We have employed immunofluorescence with a rat kidney Na+/K+-ATPase polyclonal antibody to investigate the cellular distribution and timing of appearance of this enzyme during preimplantation development. The enzyme is first detected in the late morula within the cytoplasm of each blastomere. When cavitation begins this distribution changes dramatically to a ring encircling the blastocoel, restricted to the basolateral cell margins. Using this enzyme as a marker for cavitation, we examined its expression in embryos that had been treated with wheat germ agglutinin (WGA), which causes cleavage arrest and was reported to trigger premature compaction- and cavitation-like events in early cleavage stages (L. V. Johnson, 1986, Dev. Biol. 113, 1-9). Although WGA-treated 2-,4-, and 8-cell embryos quickly underwent compaction- and cavitation-like events, no Na+/K+-ATPase expression was observed. Thus the WGA effect does not likely involve acceleration of the developmental program for cavitation. Embryos arrested at the 8-cell stage but cultured overnight to Day 4, however, expressed the enzyme in the typical blastocyst pattern (around each fluid-filled cavity). We conclude that Na+/K+-ATPase expression is initiated or increases dramatically in the late morula and is independent of cytokinesis. The enzyme assumes a distribution during cavitation consistent with its presumed role in transtrophectodermal fluid transport.

Gap junction assembly in the preimplantation mouse conceptus is independent of microtubules, microfilaments, cell flattening, and cytokinesis

Gap junctions first appear during compaction in the eight-cell stage of mouse development. Their assembly can be initiated in the near absence of transcription and protein synthesis from the four-cell stage, indicating the existence of preformed precursors. We have investigated the temporal control of this event, focusing on the possible involvement of the cytoskeleton, cell flattening, and cytokinesis. Embryos in various cleavage stages were treated with cytochalasins, to disrupt microfilaments and block cell flattening, cytokinesis, or both, or nocodazole, to promote microtubule depolymerization. To assess their capacity to initiate gap junction assembly after such treatments, the embryos were then aggregated with communication-competent, compacted embryos that had been labeled with carboxyfluorescein diacetate. Passage of the fluorescent dye, carboxyfluorescein, from labeled to unlabeled embryo was taken as evidence that interembryonic junction formation had occurred. The capacity to assemble gap junctions was acquired at the normal time by embryos prevented by cytochalasin treatment from undergoing cell flattening or any cytokinesis from fertilization onward. Likewise, treatment with nocodazole beginning in the four-cell or early eight-cell stage did not interfere with gap junction assembly. Neither drug affected the inability of four-cell embryos to assemble gap junctions prematurely. We conclude that intact microfilament or microtubule networks are not required for gap junction assembly in this system, nor do they restrain junctional precursors from assembling prematurely. Furthermore, the timing of gap junction assembly is not linked to cell flattening, cytokinesis, or cell number.

Intercellular junctional coupling in preimplantation mouse embryos: effect of blocking transcription or translation

Gap junction formation in cleavage stage mouse embryos was examined by testing for ionic coupling or by observing the intercellular movement of the fluorescent dye, Lucifer yellow CH. Our results confirm that embryo-wide cell coupling, mediated by cell-to-cell membrane channels (gap junctions), is acquired in the 8-cell stage after compaction has begun. However, not all partially compacted embryos were found to be ionically or dye coupled, suggesting that the initiation of gap junction assembly is not necessarily triggered by the onset of cell flattening. The rate of fluorescent dye movement throughout the embryo was found to increase as embryos proceed through compaction and beyond, indicating that the number of gap junctional channels between blastomeres increases as development progresses. The inhibitors alpha-amanitin and cycloheximide were used to assess the requirement of new transcription and protein synthesis, respectively, for the onset of intercellular coupling and its progressive increase during compaction. Treatment conditions were chosen to bring about suppression of mRNA and protein synthesis within 2 hr. Ionic coupling was detected in almost all compacted 8-cell embryos treated with either inhibitor from the 4-cell stage. On the other hand, dye coupling was weak or undetectable in such embryos. We propose that a limited supply of junctional components is present by the 4-cell stage to serve as a pool of precursors for the first gap junctions to be assembled in the 8-cell stage. However, it is apparent that continued embryonic gene expression is required for the full extent of junctional coupling to be established.