Distinct effects of ectopic expression of Wnt-1, activin B, and bFGF on gap junctional permeability in 32-cell Xenopus embryos

Gap junctional signaling in pattern regulation: Physiological network connectivity instructs growth and form

Gap junctions (GJs) are aqueous channels that allow cells to communicate via physiological signals directly. The role of gap junctional connectivity in determining single-cell functions has long been recognized. However, GJs have another important role: the regulation of large-scale anatomical pattern. GJs are not only versatile computational elements that allow cells to control which small molecule signals they receive and emit, but also establish connectivity patterns within large groups of cells. By dynamically regulating the topology of bioelectric networks in vivo, GJs underlie the ability of many tissues to implement complex morphogenesis. Here, a review of recent data on patterning roles of GJs in growth of the zebrafish fin, the establishment of left-right patterning, the developmental dysregulation known as cancer, and the control of large-scale head-tail polarity, and head shape in planarian regeneration has been reported. A perspective in which GJs are not only molecular features functioning in single cells, but also enable global neural-like dynamics in non-neural somatic tissues has been proposed. This view suggests a rich program of future work which capitalizes on the rapid advances in the biophysics of GJs to exploit GJ-mediated global dynamics for applications in birth defects, regenerative medicine, and morphogenetic bioengineering. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 77: 643-673, 2017.

Gap junctional communication in morphogenesis

Gap junctions permit the direct passage of small molecules from the cytosol of one cell to that of its neighbor, and thus form a system of cell-cell communication that exists alongside familiar secretion/receptor signaling. Because of the rich potential for regulation of junctional conductance, and directional and molecular gating (specificity), gap junctional communication (GJC) plays a crucial role in many aspects of normal tissue physiology. However, the most exciting role for GJC is in the regulation of information flow that takes place during embryonic development, regeneration, and tumor progression. The molecular mechanisms by which GJC establishes local and long-range instructive morphogenetic cues are just beginning to be understood. This review summarizes the current knowledge of the involvement of GJC in the patterning of both vertebrate and invertebrate systems and discusses in detail several morphogenetic systems in which the properties of this signaling have been molecularly characterized. One model consistent with existing data in the fields of vertebrate left-right patterning and anterior-posterior polarity in flatworm regeneration postulates electrophoretically guided movement of small molecule morphogens through long-range GJC paths. The discovery of mechanisms controlling embryonic and regenerative GJC-mediated signaling, and identification of the downstream targets of GJC-permeable molecules, represent exciting next areas of research in this fascinating field.

Intercellular communication: the Drosophila innexin multiprotein family of gap junction proteins

Gap junctions belong to the most conserved cellular structures in multicellular organisms, from Hydra to man. They contain tightly packed clusters of hydrophilic membrane channels connecting the cytoplasms of adjacent cells, thus allowing direct communication of cells and tissues through the diffusion of ions, metabolites, and cyclic nucleotides. Recent evidence suggests that gap junctions are constructed by three different families of four transmembrane proteins: the Connexins and the Innexins found in vertebrates and in invertebrates, respectively, and the Innexin-like Pannexins, which were recently discovered in humans. This article focuses on the Drosophila Innexin multiprotein family, which is comprised of eight members. We highlight common structural features and discuss recent findings that suggest close similarities in cellular distribution, function, and regulation of Drosophila Innexins and vertebrate gap junction proteins.

Xenopus connexins: how frogs bridge the gap

Animal species use specialized cell-to-cell channels, called gap junctions, to allow for a direct exchange of ions and small metabolites between their cells' cytoplasm. In invertebrates, gap junctions are formed by innexins, while vertebrates use connexin (Cx) proteins as gap-junction-building blocks. Recently, innexin homologs have been found in vertebrates and named pannexins. From progress in the different genome projects, it has become evident that every class of vertebrates uses their own unique set of Cxs to build their gap junctions. Here, we review all known Xenopus Cxs with respect to their expression, regulation, and function. We compare Xenopus Cxs with those of zebrafish and mouse, and provide evidence for the existence of several additional, non-identified, amphibian Cxs. Finally, we identify two new Xenopus pannexins by screening EST libraries.

Overexpression of connexin43 alters the mutant phenotype of midgestational wnt-1 null mice resulting in recovery of the midbrain and cerebellum

The midbrain-hindbrain (MHB) junction plays a key role in the patterning of the embryonic neural tube and the formation of brain structures such as the cerebellum. The mitogen wnt-1 is critical for cerebellar development, as evidenced by the lack of MHB region and cerebellar formation in the wnt-1 null embryo. We have generated wnt-1 null embryos overexpressing the gap junction gene connexin43 by crossing wnt-1 null heterozygotes into the CMV43 mouse line. We have confirmed that these mice show an increase in gap junctional communication by dye coupling analysis. Two-thirds of wnt-1 null CMV43(+) mouse embryos at E18.5 have a cerebellum. In addition, changes in the wnt-1 null phenotype in mouse embryos overexpressing connexin43 are observed as early as E9.5. At this stage, one-quarter of wnt-1 null CMV43(+) embryos display extra or expanded tissue present at the MHB boundary (a wnt-1 null enlarged phenotype). In situ hybridization studies conducted on these embryos have indicated no changes in the expression of embryonic brain positional markers in this region. We conclude from these studies that overexpression of the connexin43 gap junction restores cerebellar formation by compensating for the loss of wnt-1.

Multiple connexins contribute to intercellular communication in the Xenopus embryo

To explore the role of gap junctional intercellular communication (GJIC) during Xenopus embryogenesis, we utilized the host-transfer and antisense techniques to specifically deplete Cx38, the only known maternally expressed connexin. Cx38-depleted embryos developed normally but displayed robust GJIC between blastomeres at 32-128 cell stages, suggesting the existence of other maternal connexins. Analysis of embryonic cDNA revealed maternal expression of two novel connexins, Cx31 and Cx43.4, and a third, Cx43, that had been previously identified as a product of zygotic transcription. Thus, the early Xenopus embryo contains at least four maternal connexins. Unlike Cx38, expression of Cx31, Cx43 and Cx43.4 continue zygotically. Of these, Cx43.4 is the most abundant, accumulating significantly in neural structures including the brain, the eyes and the spinal cord.

Modulation of mouse neural crest cell motility by N-cadherin and connexin 43 gap junctions

Connexin 43 (Cx43alpha1) gap junction has been shown to have an essential role in mediating functional coupling of neural crest cells and in modulating neural crest cell migration. Here, we showed that N-cadherin and wnt1 are required for efficient dye coupling but not for the expression of Cx43alpha1 gap junctions in neural crest cells. Cell motility was found to be altered in the N-cadherin-deficient neural crest cells, but the alterations were different from that elicited by Cx43alpha1 deficiency. In contrast, wnt1-deficient neural crest cells showed no discernible change in cell motility. These observations suggest that dye coupling may not be a good measure of gap junction communication relevant to motility. Alternatively, Cx43alpha1 may serve a novel function in motility. We observed that p120 catenin (p120ctn), an Armadillo protein known to modulate cell motility, is colocalized not only with N-cadherin but also with Cx43alpha1. Moreover, the subcellular distribution of p120ctn was altered with N-cadherin or Cx43alpha1 deficiency. Based on these findings, we propose a model in which Cx43alpha1 and N-cadherin may modulate neural crest cell motility by engaging in a dynamic cross-talk with the cell's locomotory apparatus through p120ctn signaling.

Cadherins and catenins, Wnts and SOXs: embryonic patterning in Xenopus

Wnt signaling plays a critical role in a wide range of developmental and oncogenic processes. Altered gene regulation by the canonical Wnt signaling pathway involves the cytoplasmic stabilization of beta-catenin, a protein critical to the assembly of cadherin-based cell-cell adherence junctions. In addition to binding to cadherins, beta-catenin also interacts with transcription factors of the TCF-subfamily of HMG box proteins and regulates their activity. The Xenopus embryo has proven to be a particularly powerful experimental system in which to study the role of Wnt signaling components in development and differentiation. We review this literature, focusing on the role of Wnt signaling and interacting components in establishing patterns within the early embryo.

Gap junctional communication in the early Xenopus embryo

In the Xenopus embryo, blastomeres are joined by gap junctions that allow the movement of small molecules between neighboring cells. Previous studies using Lucifer yellow (LY) have reported asymmetries in the patterns of junctional communication suggesting involvement in dorso-ventral patterning. To explore that relationship, we systematically compared the transfer of LY and neurobiotin in embryos containing 16-128 cells. In all cases, the junction-permeable tracer was coinjected with a fluorescent dextran that cannot pass through gap junctions. Surprisingly, while LY appeared to transfer in whole-mount embryos, in no case did we observe junctional transfer of LY in fixed and sectioned embryos. The lack of correspondence between data obtained from whole-mounts and from sections results from two synergistic effects. First, uninjected blastomeres in whole-mounts reflect and scatter light originating from the intensely fluorescent injected cell, creating a diffuse background interpretable as dye transfer. Second, the heavier pigmentation in ventral blastomeres masks this scattered signal, giving the impression of an asymmetry in communication. Thus, inspection of whole-mount embryos is an unreliable method for the assessment of dye transfer between embryonic blastomeres. A rigorous and unambiguous demonstration of gap junctional intercellular communication demands both the coinjection of permeant and impermeant tracers followed by the examination of sectioned specimens. Whereas LY transfer was never observed, neurobiotin was consistently transferred in both ventral and dorsal aspects of the embryo, with no apparent asymmetry. Ventralization of embryos by UV irradiation and dorsalization by Xwnt-8 did not alter the patterns of communication. Thus, our results are not compatible with current models for a role of gap junctional communication in dorso-ventral patterning.

Developmental regulation and asymmetric expression of the gene encoding Cx43 gap junctions in the mouse limb bud

The Gja1 gene encoding the gap junction connexin 43 (Cx43) is dynamically regulated during limb morphogenesis. Transcript expression is found in many regions of the limb bud known to be important in regulating limb growth and patterning. In the newly emerged limb bud, Gja1 transcripts are first expressed in the ventrodistal margin of the ectoderm, and later transcript expression is localized to the apical ectodermal ridge (AER). Interestingly, transcript expression in the ventrodistal ectoderm is initiated left/right asymmetrically, with some strain backgrounds showing reverse sidedness in the fore vs. hindlimb buds. In legless, a mouse mutant exhibiting both limb and left/right patterning defects, Gja1 transcripts could not be detected in this region. However, in the i.v./i.v. embryo, a mutant with randomization of body situs the same pattern of Gja1 asymmetry was found in the limb ectoderm regardless of body situs. This suggests that Gja1 transcript expression is not directly linked to signaling pathways involved in specification of the left/right axis. In addition to transcript expression in the apical ectodermal ridge, Gja1 transcripts were also found at high levels in the ventral ectoderm. In the limb bud mesenchyme, Gja1 transcripts were distributed in a posterior distal gradient, coincident with tissue known to have polarizing activity. With limb outgrowth and the initiation of limb mesenchyme condensation. Gja1 transcripts were localized in the presumptive progress zone, and in the condensing mesenchyme. In more proximal regions of the limb where mesenchyme differentiation has been initiated, Gja1 transcripts were expressed only in the outer mesenchymal cells comprising the presumptive perichondrium. Further analysis of transgenic mice ectopically expressing Wnt-1 in the limb mesenchyme revealed alterations in the pattern of Gja1 transcript expression in conjunction with the perturbation of limb mesenchyme condensation and differentiation. Together, these findings indicate that Cx43 gap junctions may mediate cell-cell interactions important in cell signaling processes involved in limb growth and patterning.

The effects of bone morphogenetic protein 2 and 4 (BMP2 and BMP4) on gap junctions during neurodevelopment

Nervous system deficits account for the third largest group of fatal birth defects (after heart and respiratory problems) in North America. Although considerable advance has been made in neuroscience research, the early events involved in neurogenesis remain to be elucidated. More specifically, the effects of signaling molecules on intercellular communication during neurodevelopment have not yet been studied. The development of the central nervous system is regulated, at least in part, by signaling molecules such as bone morphogenetic proteins (BMPs). In this study, we have used the embryonal mouse P19 cell line to examine the effects of BMP2 and BMP4 on gap junctional communication as well as neuronal and astrocytic differentiation. The undifferentiated P19 cells show high levels of the gap junction protein, connexin43 (Cx43), and functional intercellular coupling. However, Cx43 expression and dye coupling decrease as these cells differentiate into neurons and astrocytes. In contrast, cells treated with BMP2 or BMP4 lose their capacity to differentiate into neurons but not astrocytes, while they maintain extensive gap junctional communication. The very few neurons that remain in the BMP-treated cultures are coupled (a characteristic not seen in the control neurons). Together, our data suggest that BMPs may play a critical role in morphogenesis of P19 cells while they affect gap junctions.

Wnt-1 regulation of connexin43 in cardiac myocytes

Gap junction channels composed of connexin43 (Cx43) are essential for normal heart formation and function. We studied the potential role of the Wnt family of secreted polypeptides as regulators of Cx43 expression and gap junction channel function in dissociated myocytes and intact hearts. Neonatal rat cardiomyocytes responded to Li(+), which mimics Wnt signaling, by accumulating the effector protein beta-catenin and by inducing Cx43 mRNA and protein markedly. Induction of Cx43 expression was also observed in cardiomyocytes cocultured with Rat-2 fibroblasts or N2A neuroblastoma cells programmed to secrete bioactive Wnt-1. By transfecting a Cx43 promoter-reporter gene construct into cardiomyocytes, we demonstrated that the inductive effect of Wnt signaling was transcriptionally mediated. Enhanced expression of Cx43 increased cardiomyocyte cell coupling, as determined by Lucifer Yellow dye transfer and by calcium wave propagation. Conversely, in a transgenic cardiomyopathic mouse model that exhibits ventricular arrhythmias and gap junctional remodeling, beta-catenin and Cx43 expression were downregulated concordantly. In response to Wnt signaling, the accumulating Cx43 colocalized with beta-catenin in the junctional membrane; moreover, forced expression of Cx43 in cardiomyocytes reduced the transactivation potential of beta-catenin. These findings demonstrate that Wnt signaling is an important modulator of Cx43-dependent intercellular coupling in the heart, and they support the hypothesis that dysregulated signaling contributes to altered impulse propagation and arrhythmia in the myopathic heart.

Study on the formation of specialized inter-Sertoli cell junctions in vitro

An in vitro culture system using Sertoli cells was employed to assess the expression of component genes pertinent to occluding junctions (OJ) (such as zonula occludens-1, ZO-1), anchoring junctions (AJ) (such as N-cadherin and beta-catenin), and communicating gap junctions (GJ) (such as connexin 33, Cx33) when they are being formed in vitro. Freshly isolated Sertoli cells from 20-day-old rats with a purity of greater than 90% were cultured either at low- (2.5 x 10(4) cells/cm(2)) or high-cell density (0.6 x 10(6) cells/cm(2)) on Matrigel-coated dishes for 7 days in vitro to allow the establishment of specialized junctions. In low cell density Sertoli cell cultures, specialized OJ such as tight junctions did not form during the entire culture period when assessed by the transepithelial electrical resistance (TER). In high cell density cultures, there was an increase in ZO-1 expression in days 1 to 3 preceding the establishment of tight junctions by day 4. When Sertoli cells were cultured at both cell densities, there was a transient increase in Sertoli cell N-cadherin expression, which peaked by days 4-5, suggesting the time course for the establishment of AJ may overlap with the OJ. A significant increase in the expression of Sertoli cell beta-catenin was also detected by days 5-7 in the high but not low cell density cultures. The expression of Cx33 was also enhanced at days 4-5 in both high and low density cultures. These results suggest that OJ, AJ, and GJ are formed between Sertoli cells in high density cultures, whereas OJ cannot be formed in low density cultures. A full-length cDNA clone coding for rat testicular beta-catenin was also isolated. The deduced amino acid sequence of rat beta-catenin yielded a 781 amino acid polypeptide which displayed a 99.9% identity with the mouse homolog. Conditioned medium of germ cells induced a dose-dependent stimulation on Sertoli cell beta-catenin expression, suggesting germ cells may affect the N-cadherin/beta-catenin-mediated signal transduction pathway. In summary, this study illustrates several target genes can be used as molecular markers to monitor the inter-Sertoli junction formation. This system should be applicable to screen new male contraceptives in vitro targeted at the interference of junction formation by disrupting the timely expression of genes necessary for junction establishment and/or maintenance.

Gap junction-mediated transfer of left-right patterning signals in the early chick blastoderm is upstream of Shh asymmetry in the node

Invariant patterning of left-right asymmetry during embryogenesis depends upon a cascade of inductive and repressive interactions between asymmetrically expressed genes. Different cascades of asymmetric genes distinguish the left and right sides of the embryo and are maintained by a midline barrier. As such, the left and right sides of an embryo can be viewed as distinct and autonomous fields. Here we describe a series of experiments that indicate that the initiation of these programs requires communication between the two sides of the blastoderm. When deprived of either the left or the right lateral halves of the blastoderm, embryos are incapable of patterning normal left-right gene expression at Hensen's node. Not only are both flanks required, suggesting that there is no single signaling source for LR pattern, but the blastoderm must be intact. These results are consistent with our previously proposed model in which the orientation of LR asymmetry in the frog, Xenopus laevis, depends on large-scale partitioning of LR determinants through intercellular gap junction channels (M. Levin and M. Mercola (1998) Developmental Biology 203, 90–105). Here we evaluate whether gap junctional communication is required for the LR asymmetry in the chick, where it is possible to order early events relative to the well-characterized left and right hierarchies of gene expression. Treatment of cultured chick embryos with lindane, which diminishes gap junctional communication, frequently unbiased normal LR asymmetry of Shh and Nodal gene expression, causing the normally left-sided program to be recapitulated symmetrically on the right side of the embryo. A survey of early expression of connexin mRNAs revealed that Cx43 is present throughout the blastoderm at Hamburger-Hamilton stage 2–3, prior to known asymmetric gene expression. Application of antisense oligodeoxynucleotides or blocking antibody to cultured embryos also resulted in bilateral expression of Shh and Nodal transcripts. Importantly, the node and primitive streak at these stages lack Cx43 mRNA. This result, together with the requirement for an intact blastoderm, suggests that the path of communication through gap junction channels circumvents the node and streak. We propose that left-right information is transferred unidirectionally throughout the epiblast by gap junction channels in order to pattern left-sided Shh expression at Hensen's node.

Interactions between growth factors and gap junctional communication in developing systems

In the vertebrate limb bud fibroblast growth factor (FGF) 4 secreted by cells of the posterior apical ectodermal ridge controls digit pattern, which is directed by polarizing cells in the posterior mesenchyme at the tip of the bud. FGF4 also controls the expression of gap junctions in the limb. Both chick and mouse limb bud mesenchyme express connexin 32 (Cx32; beta 1) and Cx43 (alpha 1), although not in the same gap junction plaques. Quantitative analysis reveals two gradients of gap junctions: from posterior to anterior in the subapical mesenchyme and from distal to proximal along the bud. The highest gap junction density is associated with the polarizing region. Micromass cultures of chick and mouse posterior and anterior mesenchyme cells were used to assess the ability of FGF4 to modulate gap junctional communication. Posterior mesenchyme (polarizing region) cells express a population of gap junctions that are highly sensitive to FGF4, whereas gap junctions between anterior mesenchyme cells are completely insensitive to FGF4. FGF4 doubles gap junction density, intercellular communication and the polarizing capacity of posterior mesenchyme cells, restoring polarizing capacity to in vivo levels. We conclude that gap junctional communication and polarizing capacity are intimately linked. Interactions between signalling molecules and junctional communication may play an important role in controlling development.

Gap junctions are involved in the early generation of left-right asymmetry

Invariant left-right asymmetry of the visceral organs is a fundamental feature of vertebrate embryogenesis. While a cascade of asymmetrically expressed genes has been described, the embryonic mechanism that orients the left-right axis relative to the dorsoventral and anteroposterior axes (a prerequisite for asymmetric gene expression) is unknown. We propose that this process involves dorsoventral differences in cell-cell communication through gap junctions composed of connexin proteins. Global modulation of gap junctional states in Xenopus embryos by pharmacological agents specifically induced heterotaxia involving mirror-image reversals of heart, gut, and gall bladder. Greatest sensitivity was observed between st. 5 and st. 12, well before the onset of organogenesis. Moreover, heterotaxia was also induced following microinjection of dominant negative and wild-type connexin mRNAs to modify the endogenous dorsoventral difference in junctional communication. Heterotaxia was induced by either blocking gap junction communication (GJC) dorsally or by introducing communication ventrally (but not the reverse). Both connexin misexpression and exposure to GJC-modifying drugs altered expression of the normally left-sided gene XNR-1, demonstrating that GJC functions upstream of XNR-1 in the pathway that patterns left-right asymmetry. Finally, lineage analysis to follow the progeny of microinjected cells indicated that they generally do not contribute the asymmetric organs. Together with the early sensitivity window, this suggests that GJC functions as part of a fundamental, early aspect of left-right patterning. In addition, we show that a potential regulatory mutation in Connexin43 is sufficient to cause heterotaxia. Despite uncertainty about the prevalence of the serine364 to proline substitution reported in human patients with laterality defects, the mutant protein is both a mild hypomorph and a potent antimorph as determined by the effect of its expression on left-right patterning. Taken together, our data suggest that endogenous dorsoventral differences in GJC within the early embryo are needed to consistently orient left-right asymmetry.

Cx43 Gap Junctions in Cardiac Development

Studies utilizing knockout and transgenic mouse models revealed an important role for connexin 43 (Cx43) gap junctions in cardiac development. This may involve a quantitative requirement for gap junctions in modulating the development of cardiac crest cells. In addition, studies in humans and Xenopus indicate that Cx43 gap junctions also may play a role in regulating heart laterality. Together, these findings indicate that the perturbation of Cx43 function could play a significant role in specific congenital heart malformations.

Evidence that dorsal-ventral differences in gap junctional communication in the early Xenopus embryo are generated by beta-catenin independent of cell adhesion effects

Gap junctional communication (GJC) is regulated in the early Xenopus embryo and quantitative differences in junctional communication correlate with the specification of the dorsal-ventral axis. To address the mechanism that is responsible for regulating this differential communication, we investigated the function of beta-catenin during the formation of the dorsal-ventral axis in Xenopus embryos by blocking its synthesis with antisense oligodeoxynucleotides. This method has previously been shown to reduce the level of beta-catenin in the early embryo, prior to zygotic transcription, and to inhibit the formation of the dorsal axis (Heasman et al., 1994, Cell 79, 791-803). We show here that antisense inhibition of beta-catenin synthesis also reduces GJC among cells in the dorsal hemisphere of 32-cell embryos to levels similar to those observed among ventral cells. Full-length beta-catenin mRNA can restore elevated levels of dorsal GJC when injected into beta-catenin-deficient oocytes, demonstrating the specificity of the beta-catenin depletion with the antisense oligonucleotides. Thus, endogenous beta-catenin is required for the observed differential GJC. This regulation of GJC is the earliest known action of the dorsal regulator, beta-catenin, in Xenopus development. Two lines of evidence, presented here, indicate that beta-catenin acts within the cytoplasm to regulate GJC, rather than through an effect on cell adhesion. First, when EP-cadherin is overexpressed and increased adhesion is observed, embryos display both a ventralized phenotype and reduced dye transfer. Second, a truncated form of beta-catenin (i.e., the ARM region), that lacks the cadherin-binding domain, restores dorsal GJC to beta-catenin-depleted embryos. Thus, beta-catenin appears to regulate GJC independent of its role in cell-cell adhesion, by acting within the cytoplasm through a signaling mechanism.

Identification of connexin43 as a functional target for Wnt signalling

Wnt mediated signal transduction is considered to regulate activity of target genes. In Xenopus embryos, ectopic Wnt1 and Wnt8 expression induces gap-junctional communication. During murine brain formation, Wnt1 and the gap-junctional protein connexin43 (Cx43) are co-expressed at the mid/hindbrain border, while interference with Wnt1 or Cx43 expression during embryogenesis leads to severe brain defects in the mid/hindbrain region. In PC12 cells, Wnt1 expression leads to an apparent increase in cell-cell adhesion. We investigated the effects of Wnt1 overexpression on gap-junctional communication in PC12 cells. Wnt1 expressing clones displayed an increased electrical and chemical coupling. This coincides with an increased expression of Cx43 mRNA and protein, while other connexins, Cx26, Cx32, Cx37, Cx40 and Cx45, were not up-regulated. Also, induction of Wnt1 expression in a mammary epithelial cell line leads to an increase in gap-junctional communication and Cx43 protein expression. In transient transactivation assays in P19 EC cells we found that Wnt1 and Li+, an ion that mimics Wnt signalling, increased transcription from the rat Cx43 promoter, potentially via TCF/LEF binding elements, in a pathway separate from cAMP-induced Cx43 transactivation. The results demonstrate that Cx43 acts as a functional target of Wnt1 signalling, and Cx43 expression can be regulated by Wnt1 at the transcriptional level. Our data suggest that Wnt1-induced cell fate determination is likely to involve regulation of gap-junctional communication.

The role of gap junction membrane channels in development

In most developmental systems, gap junction-mediated cell-cell communication (GJC) can be detected from very early stages of embryogenesis. This usually results in the entire embryo becoming linked as a syncytium. However, as development progresses, GJC becomes restricted at discrete boundaries, leading to the subdivision of the embryo into communication compartment domains. Analysis of gap junction gene expression suggests that this functional subdivision of GJC may be mediated by the differential expression of the connexin gene family. The temporal-spatial pattern of connexin gene expression during mouse embryogenesis is highly suggestive of a role for gap junctions in inductive interactions, being regionally restricted in distinct developmentally significant domains. Using reverse genetic approaches to manipulate connexin gene function, direct evidence has been obtained for the connexin 43 (Cx43) gap junction gene playing a role in mammalian development. The challenges in the future are the identification of the target cell populations and the cell signaling processes in which Cx43-mediated cell-cell interactions are critically required in mammalian development. Our preliminary observations suggest that neural crest cells may be one such cell population.

Gravitational effects on the rearrangement of cytoplasmic components during axial formation in amphibian development

The spatial positioning of the dorsal-ventral axis in the amphibian, Xenopus laevis, can be experimentally manipulated either by tipping the embryo relative to Earth's gravitational force vector or by centrifugation. Experimental evidence suggests that certain cytoplasmic components are redistributed during the first cell cycle and that these components are, in part, responsible for the establishment of this axis. Further studies indicate that at least some of the cytoplasmic components responsible for establishing this axis may be RNA. Recombinant cDNA and PCR technology are utilized to isolate DNA clones for messenger RNA which becomes spatially localized to the dorsal side of the embryo. These clones are being used to study the mechanisms of spatial localization and the function of the localized RNA transcripts.

Regulation of Wnt5a mRNA expression in human mammary epithelial cells by cell shape, confluence, and hepatocyte growth factor

The Wnts are a family of genes with a role in cell fate and morphological development in numerous embryonic and adult tissues. In mouse mammary tissue a subset of the Wnts have a function in the normal development of the gland, and aberrant expression of Wnts normally silent in this tissue causes mammary carcinomas. We have previously shown that Wnt5a expression is elevated in the epithelial component of proliferative lesions of human breast and have therefore examined the regulation of Wnt5a mRNA expression in the human mammary epithelial cell line HB2, which has a luminal phenotype and thus represents the most commonly transformed cell type in human breast cancer. Wnt5a was up-regulated 30-fold at confluence. This up-regulation was induced specifically by confluence and not by the growth arrest that accompanied it. In addition, Wnt5a was down-regulated 3-fold by changes in cell shape associated with the transition from growth on a two-dimensional surface (flat cell morphology) to growth in three-dimensional gels (spherical cell morphology). Cytoskeletal disruption with non-toxic doses of colchicine also induced a spherical morphology and brought about a dose-dependent down-regulation of Wnt5a. Wnt5a was also down-regulated 10-fold during the hepatocyte growth factor-induced branching of HB2 cell aggregates in collagen gels. The down-regulation of Wnt5a preceded the branching process. A similar result was obtained with primary human breast epithelial populations and the breast cancer cell line MDA468. We conclude that regulation of Wnt5a expression is a down-stream effect of signaling by hepatocyte growth factor. These results are consistent with a role for Wnt5a in mammary epithelial cell motility and are in accord with Xwnt5a's function in embryonal cell migration. If Wnt5a's function in human mammary epithelial cells is similar to that of Xwnt5a, its up-regulation at confluence may be a mechanism for inhibition of cell migration beyond confluence.

Expression of a dominant negative inhibitor of intercellular communication in the early Xenopus embryo causes delamination and extrusion of cells

A chimeric construct, termed 3243H7, composed of fused portions of the rat gap junction proteins connexin32 (Cx32) and connexin43 (Cx43) has been shown to have selective dominant inhibitory activity when tested in the Xenopus oocyte pair system. Co-injection of mRNA coding for 3243H7 together with mRNAs coding for Cx32 or Cx43 completely blocked the development of channel conductances, while the construct was ineffective at blocking intercellular channel assembly when coinjected with rat connexin37 (Cx37). Injection of 3243H7 into the right anterodorsal blastomere of 8-cell-stage Xenopus embryos resulted in disadhesion and delamination of the resultant clone of cells evident by embryonic stage 8; a substantial number, although not all, of the progeny of the injected cell were eliminated from the embryo by stage 12. A second construct, 3243H8, differing from 3243H7 in the relative position of the middle splice, had no dominant negative activity in the oocyte pair assay, nor any detectable effects on Xenopus development, even when injected at four-fold higher concentrations. The 3243H7-induced embryonic defects could be rescued by coinjection of Cx37 with 3243H7. A blastomere reaggregation assay was used to demonstrate that a depression of dye-transfer could be detected in 3243H7-injected cells as early as stage 7; Lucifer yellow injections into single cells also demonstrated that injection of 3243H7 resulted in a block of intercellular communication. These experiments indicate that maintenance of embryonic cell adhesion with concomitant positional information requires gap junction-mediated intercellular communication.

Xwnt-11: a maternally expressed Xenopus wnt gene

We have isolated and characterized a novel Xenopus wnt gene, Xwnt-11, whose expression pattern and overexpression phenotype suggest that it may be important for dorsal-ventral axis formation. Xwnt-11 mRNA is present during oogenesis and embryonic development through swimming tadpole stages. Xwnt-11 mRNA is ubiquitous in early oocytes and is localized during mid-oogenesis. By late oocyte stages, Xwnt-11 mRNA is localized to the vegetal cortex, with some mRNA in the vegetal cytoplasm. After egg maturation, Xwnt-11 mRNA is released from the vegetal cortex and is found in the vegetal cytoplasm. This early pattern of Xwnt-11 mRNA localization is similar to another vegetally localized maternal mRNA, Vg1 (D. A. Melton (1987) Nature 328, 80–82). In the late blastula, Xwnt-11 mRNA is found at high levels in the dorsal marginal zone. As gastrulation proceeds, Xwnt-11 mRNA appears in the lateral and ventral marginal zone and, during tadpole stages, it is found in the somites and first branchial arch. Injection of Xwnt-11 mRNA into UV-ventralized embryos can substantially rescue the UV defect by inducing the formation of dorsal tissues. The rescued embryos develop somitic muscle and neural tube; however, they lack notochord and anterior head structures.

The Wnt-1 proto-oncogene induces changes in morphology, gene expression, and growth factor responsiveness in PC12 cells

The product of the Wnt-1 proto-oncogene is a secreted glycoprotein that is normally produced in regions of the embryonic neural tube. We show here that expression of mouse Wnt-1 cDNA in the rat PC12 pheochromocytoma cell line causes a dramatic conversion from a round to a flat cell morphology. In addition, PC12 cells expressing Wnt-1 (PC12/Wnt-1) fail to extend neurites after treatment with NGF, despite the presence and activation of high affinity NGF receptors encoded by the trk gene and the induction of early response genes. Furthermore, PC12/Wnt-1 cells fail to express several neuron- and chromaffin-specific genes, indicating that PC12/Wnt-1 cells have assumed a new phenotype. Although NGF and FGF utilize similar signal transduction pathways in PC12 cells, only FGF is capable of inducing a morphological response and synthesis of transin mRNA in PC12/Wnt-1 cells.

Genetic control of gastrulation in the mouse

In the past, understanding of the process of gastrulation in the mouse has primarily been based on morphological analyses. Recently, a number of molecules have been implicated in mesoderm induction and axis formation in Xenopus, and several of these exhibit unique patterns of expression during mouse gastrulation. These gene-expression data, together with fate mapping, ectopic expression experiments and mutational analysis, will now facilitate studies on the functional aspects of gastrulation in the mouse.

Molecular analysis of the Wnt-1 proto-oncogene in Ambystoma mexicanum (axolotl) embryos

To analyze Wnt-1 expression during neurulation in urodele embryos, we have isolated a Wnt-1 cDNA clone, Awnt-1, from an Ambystoma mexicanum (axolotl) neurula-stage cDNA library. Awnt-1 codes for a protein of 369 amino acids rich in cysteine residues, is preceded by a hydrophobic leader peptide sequence and contains four possible sites for N-linked glycosylation. The temporal expression profile of Awnt-1 was analyzed by reverse transcription-polymerase chain reaction (RT-PCR). Awnt-1 expression in the axolotl embryo is biphasic. Awnt-1 transcripts are found in early blastulae until gastrulation, are barely detectable during gastrulation, and are present again from neurulation until late embryogenesis. Transcripts are present before the midblastula transition, indicating that they might be of maternal origin. To localize Awnt-1 expression in embryos during the first phase of expression, early gastrulae were dissected by cutting along the animal-vegetal and future dorso-ventral axes and analyzed by RT-PCR. At the early gastrula stage Awnt-1 transcripts appear to be located in the future ventral region of the embryo. Hatching larvae no longer express Awnt-1. PCR reactions performed using cDNA library-phage DNA templates derived from whole neurulae versus embryos with the neuroectoderm removed suggest that, in the neurula, Awnt-1 transcripts are located in the neuroectoderm. This suggest that, as is the case for Wnt-1 in other vertebrates, Awnt-1 may be involved in neurogenesis. These results suggest that Wnt-1 has earlier roles in development than has been considered until now.

Activins are expressed in preimplantation mouse embryos and in ES and EC cells and are regulated on their differentiation

Members of the activin family have been suggested to act as mesoderm-inducing factors during early amphibian development. Little is known, however, about mesoderm formation in the mammalian embryo, and as one approach to investigating this we have studied activin expression during early mouse development. Activins are homo- or heterodimers of the beta A or beta B subunits of inhibin, itself a heterodimer consisting of one of the beta subunits together with an alpha subunit. Our results indicate that the oocyte contains mRNA encoding all three subunits, and antibody staining demonstrates the presence of both alpha and beta protein chains. From the fertilized egg stage onwards, alpha subunit protein cannot be detected, so the presence of beta subunits reflects the presence of activin rather than inhibin. Maternal levels of activin protein decline during early cleavage stages but increase, presumably due to zygotic transcription (see below), in the compacted morula. By 3.5 days, only the inner cell mass (ICM) cells of the blastocyst express activin, but at 4.5 days the situation is reversed; activin expression is confined to the trophectoderm. Using reverse transcription-PCR, neither beta A nor beta B mRNA was detectable at the two-cell stage but transcripts encoding both subunits were detectable at the morula stage, with beta B mRNA persisting into the blastocyst. We have also analyzed activin and inhibin expression in ES and EC cells. Consistent with the observation that activins are expressed in the ICM of 3.5-day blastocysts, we find high levels of beta A and beta B mRNA in all eight ES cell lines tested. F9 EC cells express only activin beta B, together with low levels of the inhibin alpha chain. When ES and EC cells are induced to differentiate, levels of activin fall dramatically. These results are consistent with a role for activins in mesoderm formation and other steps of early mouse development.

In pursuit of the functions of the Wnt family of developmental regulators: insights from Xenopus laevis

Wnts are a recently described family of secreted glycoproteins related to the Drosophila segment polarity gene, wingless, and to the proto-oncogene, int-1. Wnts are thought to function as developmental modulators, with signalling distances of only a few cell diameters. In Xenopus, at least six Wnts, including Xwnts-1, -3A, and -4, are expressed initially in the developing central nervous system, with some regions expressing multiple Xwnts. Xwnt-8 is expressed by mid-blastula stage, in ventral and lateral mesoderm. Xwnt-5A mRNAs are stored in the egg, and later are expressed throughout the embryo in both ectoderm and mesoderm, but with a pronounced enrichment in the head and tail. Recent studies in Xenopus have pursued the diverse roles of Xwnts in early development, the mechanisms by which Xwnts signal information between cells, and the cell physiological responses to Xwnt signals.

Isolation of cDNAs for two closely related members of the axolotl Wnt family, Awnt-5A and Awnt-5B, and analysis of their expression during development

To characterize molecular interactions between cells in the early amphibian embryo, we have isolated cDNAs for two members of the axolotl (Ambystoma mexicanum) Wnt family, Awnt-5A and Awnt-5B. The encoded proteins share 83% amino acid identity. Using a reverse transcription-polymerase chain reaction (RT-PCR) assay, we find that Awnt-5A transcripts are abundant in the blastula until gastrulation, barely detectable during gastrulation, and increase again during neurulation. They are detected throughout the remaining development and in hatched larvae. In contrast, transcripts for Awnt-5B are undetectable in the blastula. They appear with gastrulation, are present throughout neurulation and organogenesis, and decrease to barely detectable levels in hatched larvae. PCR reactions performed using cDNA library-phage DNA templates derived from whole neurulae versus embryos with the neuroectoderm removed suggest that, in the neurula, Awnt-5A transcripts are present in neuroectodermal as well as non-neuroectodermal tissues while Awnt-5B mRNAs are predominantly localized in the neuroectoderm. To localize Awnt-5A expression in embryos before gastrulation, early gastrulae were dissected by cutting along the animal-vegetal and future dorso-ventral axes and analyzed by RT-PCR. At this early stage, Awnt-5A transcripts appear to be predominantly localized in the dorso-vegetal region of the embryo. These results suggest that the two closely related Awnt-5 genes participate in different morphogenetic processes during early axolotl development.

Connexin 43 expression in the mouse embryo: localization of transcripts within developmentally significant domains

The expression of the gap junction gene, Cx43, during mouse embryogenesis was characterized by an in situ hybridization analysis of mouse embryos from gestation days 4.5 to 12.5. This analysis revealed that Cx43 transcripts are differentially expressed as a function of development beginning at the blastocyst stage. In many regions of the embryo, Cx43 transcripts were found in discrete spatially restricted domains. This was observed in conjunction with the development of the brain, neural tube, prevertebra, limb, and various aspects of organogenesis. In some cases, the differential localization of Cx43 transcripts is associated with developmental processes mediated by inductive interactions, such as that of the eye, otic vesicle, kidney, and the branchial arches. In addition, in the 10.5 day embryo, Cx43 transcripts appear to be distributed as a gradient in regions spanning the midbrain/hindbrain junction, in the telencephalon, and in the limb mesenchyme. Surprisingly, our results also suggest that neural crest and sclerotomal cells, i.e., cells that are presumably migratory, express high levels of Cx43 transcripts. Overall, these results suggest that gap junctions encoded by Cx43 may play a role in various aspects of mouse development, possibly including relaying second messengers emanating from signal transduction pathways that mediate inductive interactions.