Major loss of junctional coupling during mitosis in early mouse embryos

Emerging roles of gap junction proteins connexins in cancer metastasis, chemoresistance and clinical application

Connexin, a four-pass transmembrane protein, contributes to assembly of gap junctions among neighboring cells and thus facilitates gap junctional intercellular communication (GJIC). Traditionally, the roles of connexins were thought to mediate formation of hemichannels and GJIC assembly for transportation of ions and small molecules. Many studies have observed loss of GJIC, due to reduced expression or altered cytoplasmic localization of connexins, in primary tumor cells. Connexins are generally considered tumor-suppressive. However, recent studies of clinical samples suggested a different role of connexins in that expression levels and membrane localization of connexins, including Connexin 43 (Cx43, GJA1) and Connexin 26 (Cx26, GJB2), were found to be enhanced in metastatic lesions of cancer patients. Cx43- and Cx26-mediated GJIC was found to promote cancer cell migration and adhesion to the pulmonary endothelium. Regulatory circuits involved in the induction of connexins and their functional effects have also been reported in various types of cancer. Connexins expressed in stromal cells were correlated with metastasis and were implicated in regulating metastatic behaviors of cancer cells. Recent studies have revealed that connexins can contribute to cellular phenotypes via multiple ways, namely 1) GJIC, 2) C-terminal tail-mediated signaling, and 3) cell-cell adhesion during gap junction formation. Both expression levels and the subcellular localization could participate determining the functional roles of connexins in cancer. Compounds targeting connexins were thus tested as potential therapeutics intervening metastasis or chemoresistance. This review focuses on the recent findings in the correlation between the expression of connexins and patients’ prognosis, their roles in metastasis and chemoresistance, as well as the implications and concerns of using connexin-targeting drugs as anti-metastatic therapeutics. Overall, connexins may serve as biomarkers for cancer prognosis and as therapeutic targets for intervening metastasis and chemoresistance.

Redistribution of connexin 43 during cell division

Gap junction channels, once clustered into gap junction plaques, allow communication of essential metabolites between cells. Gap junction plaques have been reported to be lost from the cell surface during cell division. The mechanism involved in this loss of gap junction plaques during mitosis is unclear, but we hypothesize that an endoexocytotic mechanism that results in cytoplasmic double-membraned annular gap junction vesicles is involved. In this study, gap junction plaque changes in dividing cells were examined in SW-13 adrenocortical tumor cells. Endogenous gap junction protein, connexin 43 (Cx43), was detected with immunofluorescence, and live cell imaging was used to monitor green fluorescent protein-tagged Cx43 (Cx43-GFP). Mitotic stages were identified by Hoechst chromosomal staining. During interphase, large gap junction plaques were detected; however, the presence of these plaques decreased, whereas cytoplasmic puncta increased beginning with prophase. The cytoplasmic puncta were demonstrated with immunoelectron microscopy to be Cx43- positive annular gap junction vesicles. As gap junction plaques reformed at cleavage furrows between daughter cells, the number of annular gap junctions decreased during cytokinesis. The data are consistent with the mechanism of gap junction plaque loss during mitosis relying on an endoexocytotic process that results in annular gap junction vesicles formation. The rapid formation of gap junction plaques during cytokinesis points to the intriguing possibility of connexin recycling from annular gap junction vesicles to form gap junction plaques as mitosis is completed.

Position- and polarity-dependent Hippo signaling regulates cell fates in preimplantation mouse embryos

During the preimplantation stage, mouse embryos establish two cell lineages by the time of early blastocyst formation: the trophectoderm (TE) and the inner cell mass (ICM). Historical models have proposed that the establishment of these two lineages depends on the cell position within the embryo (e.g., the positional model) or cell polarization along the apicobasal axis (e.g., the polarity model). Recent findings have revealed that the Hippo signaling pathway plays a central role in the cell fate-specification process: active and inactive Hippo signaling in the inner and outer cells promote ICM and TE fates, respectively. Intercellular adhesion activates, while apicobasal polarization suppresses Hippo signaling, and a combination of these processes determines the spatially regulated activation of the Hippo pathway in 32-cell-stage embryos. Therefore, there is experimental evidence in favor of both positional and polarity models. At the molecular level, phosphorylation of the Hippo-pathway component angiomotin at adherens junctions (AJs) in the inner (apolar) cells activates the Lats protein kinase and triggers Hippo signaling. In the outer cells, however, cell polarization sequesters Amot from basolateral AJs and suppresses activation of the Hippo pathway. Other mechanisms, including asymmetric cell division and Notch signaling, also play important roles in the regulation of embryonic development. In this review, I discuss how these mechanisms cooperate with the Hippo signaling pathway during cell fate-specification processes.

Glomerular expression of connexin 40 and connexin 43 in rat experimental glomerulonephritis

BACKGROUND

Gap junctional intercellular communication is thought to play an important role in the maintenance of cell differentiation and homeostasis. Gap junctions connect glomerular mesangial cells to each other. In this study, we examined the glomerular expression of connexins (Cxs) 40 and 43 at both the protein and transcript levels in anti-Thy1.1 glomerulonephritis (GN).

METHODS

Anti-Thy1.1 GN was induced by intravenous injection of anti-Thy1.1 monoclonal antibody 1-22-3. Cx protein expression was examined by immunofluorescence, immunoelectron microscopy, and Western blotting. Changes in mRNA levels were detected by real-time reverse transcriptase-polymerase chain reaction.

RESULTS

Cx40 was detected in mesangial cells in normal rat glomeruli; its expression was reduced on days 3 and 7 and recovered to normal on day 14 following GN induction. Cx43 was detected in mesangial cells and podocytes in normal rat glomeruli, and its expression did not change during the disease course of GN. Expression of Cx40 and Cx43 was also detected in extraglomerular mesangial cells; this expression did not change during the disease course. Opposing patterns of expression between Cx40 and smooth muscle actin (SMA) were observed with double-immunofluorescence labeling. SMA is a differentiation marker of mesangial cells; it is often expressed during proliferation but not under physiological conditions.

CONCLUSION

These results suggest that Cx40 expression in mesangial cells is related to mesangial cell regeneration. Thus, Cx expression regulation could be a therapeutic target for glomerular diseases.

Biological role of connexin intercellular channels and hemichannels

Gap junctions (GJ) and hemichannels (HC) formed from the protein subunits called connexins are transmembrane conduits for the exchange of small molecules and ions. Connexins and another group of HC-forming proteins, pannexins comprise the two families of transmembrane proteins ubiquitously distributed in vertebrates. Most cell types express more than one connexin or pannexin. While connexin expression and channel activity may vary as a function of physiological and pathological states of the cell and tissue, only a few studies suggest the involvement of pannexin HC in acquired pathological conditions. Importantly, genetic mutations in connexin appear to interfere with GJ and HC function which results in several diseases. Thus connexins could serve as potential drug target for therapeutic intervention. Growing evidence suggests that diseases resulting from HC dysfunction might open a new direction for development of specific HC reagents. This review provides a comprehensive overview of the current studies of GJ and HC formed by connexins and pannexins in various tissue and organ systems including heart, central nervous system, kidney, mammary glands, ovary, testis, lens, retina, inner ear, bone, cartilage, lung and liver. In addition, present knowledge of the role of GJ and HC in cell cycle progression, carcinogenesis and stem cell development is also discussed.

CX43 expression, phosphorylation, and distribution in the normal and autoimmune orchitic testis with a look at gap junctions joining germ cell to germ cell

Spermatogenesis requires connexin 43 (Cx43).This study examines normal gene transcription, translation, and phosphorylation of Cx43 to define its role on germ cell growth and Sertoli cell's differentiation, and identifies abnormalities arising from spontaneous autoimmune orchitis (AIO) in mink, a seasonal breeder and a natural model for autoimmunity. Northern blot analysis detected 2.8- and a 3.7-kb Cx43 mRNA bands in seminiferous tubule-enriched fractions. Cx43 mRNA increased in seminiferous tubule-enriched fractions throughout development and then seasonally with the completion of spermatogenesis. Cx43 protein levels increased transiently during the colonization of the tubules by the early-stage spermatocytes. Cx43 phosphorylated (PCx43) and nonphosphorylated (NPCx43) in Ser368 decreased during the periods of completion of meiosis and Sertoli cell differentiation, while Cx43 mRNA remained elevated throughout. PCx43 labeled chiefly the plasma membrane except by stage VII when vesicles were also labeled in Sertoli cells. Vesicles and lysosomes in Sertoli cells and the Golgi apparatus in the round spermatids were NPCx43 positive. A decrease in Cx43 gene expression was matched by a Cx43 protein increase in the early, not the late, phase of AIO. Total Cx43 and PCx43 decreased with the advance of orchitis. The study makes a novel finding of gap junctions connecting germ cells. The data indicate that Cx43 protein expression and phosphorylation in Ser368 are stage-specific events that may locally influence the acquisition of meiotic competence and the Sertoli cell differentiation in normal testis. AIO modifies Cx43 levels, suggesting changes in Cx43-mediated intercommunication and spermatogenic activity in response to cytokines imbalances in Sertoli cells.

Connexins: sensors and regulators of cell cycling

It is nowadays well established that gap junctions are critical gatekeepers of cell proliferation, by controlling the intercellular exchange of essential growth regulators. In recent years, however, it has become clear that the picture is not as simple as originally anticipated, as structural precursors of gap junctions can affect cell cycling by performing actions not related to gap junctional intercellular communication. Indeed, connexin hemichannels also foresee a pathway for cell growth communication, albeit between the intracellular compartment and the extracellular environment, while connexin proteins as such can directly or indirectly influence the production of cell cycle regulators independently of their channel activities. Furthermore, a novel set of connexin-like proteins, the pannexins, have lately joined in as regulators of the cell proliferation process, which they can affect as either single units or as channel entities. In the current paper, these multifaceted aspects of connexin-related signalling in cell cycling are reviewed.

Combinational treatment of gap junctional activator and tamoxifen in breast cancer cells

Tamoxifen is a drug of choice for endocrine-responsive breast tumor patients. However, tamoxifen resistance has become a major concern for the treatment of breast cancer. Combinational therapies of tamoxifen and different drugs are being frequently studied. In this study, we tested the efficacy of substituted quinolines (code name=PQ1; a gap junctional activator) in combination with tamoxifen in T47D cells. Colony growth assay was performed using soft agar to measure the colony growth, whereas cell proliferation was measured by the MTT assay in T47D cells. The level of Ki67, survivin, and BAX was measured using confocal microscopy along with western blot analysis. Apoptosis-bromodeoxyuridine triphosphate labeling was also examined in the induced treatment of T47D cells. We observed a 55% decrease in the colony growth in the presence of combination of PQ1 and tamoxifen, whereas tamoxifen alone had little effect. A combination of 10 micromol/l tamoxifen and 200 or 500 nmol/l PQ1 resulted in only 16% cell viability compared with controls at 48 h in T47D cells by the MTT assay. We found a significant increase in BAX protein at 1 h in the presence of 500 nmol/l PQ1 alone, 10 micromol/l tamoxifen alone, and the combination of PQ1 and tamoxifen. A two-fold increase was observed in active caspase 3 in the presence of combinational treatment of 10 micromol/l tamoxifen and 200 or 500 nmol/l PQ1. In addition, flow cytometric analysis showed a 50% increase in the number of apoptotic cells in the presence of the combination of tamoxifen and PQ1 compared with the control. Furthermore, the results show that combinational treatment of tamoxifen and PQ1 significantly reduces the expression of survivin in T47D cells. Gap junction inhibitor studies with carbenexolone were also performed confirming the role of gap junctions in cell proliferation and cell death. The combinational treatment of PQ1 and tamoxifen has a significant increase in BAX expression, caspase 3 activation, and DNA fragmentation. Tamoxifen alone and in combination with PQ1 showed a decrease in the expression of survivin, whereas PQ1 alone was shown to be independent of the survivin-mediated pathway. This suggests that an increase in gap junction activity can potentiate the effect of tamoxifen. The combinational treatment of tamoxifen and PQ1 also showed a significant decrease in cell viability compared with tamoxifen treatment alone. The gap junction inhibitor carbenexolone was shown to increase cell proliferation by increased cyclin D1 expression, MTT assay, and Ki67 expression. It further decreased cell death. This study shows for the first time that combinational treatment of tamoxifen and PQ1 (a gap junctional activator) can be used to potentiate apoptosis of T47D human breast cancer cells. Thus, a gap junctional activator, PQ1, could potentially alter either the length or dose of tamoxifen clinically used for breast cancer patients.

Mechanisms of trophectoderm fate specification in preimplantation mouse development

During preimplantation mouse development, embryos establish two distinct cell lineages by the time of blastocyst formation: trophectoderm (TE) and inner cell mass (ICM). To explain the mechanism of this cell fate specification, two classical models, namely the inside-outside model and polarity model have been proposed based on experimental manipulation studies on embryos. This review summarizes recent findings on the molecular mechanisms of fate specification, and discusses how these findings fit into the classical models. TE development is regulated by a transcription factor cascade, the core transcription factors of which are Tead4 and Cdx2. The transcriptional activity of Tead4 is regulated by the position-dependent Hippo signaling pathway, thus supporting the inside-outside model. In contrast, several findings support the polarity model; some other findings suggest different mechanisms. We also discuss how the two classical models could be further developed in the light of recent molecular findings.

Inhibition of proliferation of a hepatoma cell line by fucoxanthin in relation to cell cycle arrest and enhanced gap junctional intercellular communication

Fucoxanthin is one of the most abundant carotenoids found in Undaria pinnatifida and has been shown to inhibit tumor proliferation in vitro. However, the mechanisms underlying the anti-cancer effects of fucoxanthin are unclear. In this study, we hypothesized that fucoxanthin may cause cell cycle arrest and enhance gap junctional intercellular communication (GJIC) in SK-Hep-1 human hepatoma cells. Data revealed that fucoxanthin (1-20microM) strongly and concentration-dependently inhibited the proliferation of SK-Hep-1 cells at 24h of incubation, whereas fucoxanthin facilitated the growth of a murine embryonic hepatic (BNL CL.2) cells at 24h of incubation and only slightly slowed the cell proliferation at 48h. In SK-Hep-1 cells, fucoxanthin caused cell cycle arrest at G0/G1 phase and induced cell apoptosis, as evidenced by increased subG1 cells and induction of DNA strand breaks. Using scrape loading-dye-transfer assay, fucoxanthin was found to significantly enhance GJIC of SK-Hep-1 cells without affecting that of BNL CL.2 cells. In addition, fucoxanthin significantly increased protein and mRNA expressions of connexin 43 (Cx43) and connexin 32 (Cx32) in SK-Hep-1 cells. Moreover, fucoxanthin markedly increased the concentration of intracellular calcium levels in SK-Hep-1 cells. Thus, fucoxanthin is specifically antiproliferative against SK-Hep-1 cells, and the effect is associated with upregulation of Cx32 and Cx43, which enhances GJIC of SK-Hep-1 cells. The enhanced GJIC may be responsible for the increase of the intracellular calcium level, which then causes cell cycle arrest and apoptosis.

Inactivation of aPKClambda reveals a context dependent allocation of cell lineages in preimplantation mouse embryos

Background

During mammalian preimplantation development, lineage divergence seems to be controlled by the interplay between asymmetric cell division (once cells are polarized) and positional information. In the mouse embryo, two distinct cell populations are first observed at the 16-cell stage and can be distinguished by both their position (outside or inside) and their phenotype (polarized or non-polarized). Many efforts have been made during the last decade to characterize the molecular mechanisms driving lineage divergence.

Methodology/Principal Findings

In order to evaluate the importance of cell polarity in the determination of cell fate we have disturbed the activity of the apical complex aPKC/PAR6 using siRNA to down-regulate aPKCλ expression. Here we show that depletion of aPKCλ results in an absence of tight junctions and in severe polarity defects at the 16-cell stage. Importantly, we found that, in absence of aPKCλ, cell fate depends on the cellular context: depletion of aPKCλ in all cells results in a strong reduction of inner cells at the 16-cell stage, while inhibition of aPKCλ in only half of the embryo biases the progeny of aPKCλ defective blastomeres towards the inner cell mass. Finally, our study points to a role of cell shape in controlling cell position and thus lineage allocation.

Conclusion

Our data show that aPKCλ is dispensable for the establishment of polarity at the 8-cell stage but is essential for the stabilization of cell polarity at the 16-cell stage and for cell positioning. Moreover, this study reveals that in addition to positional information and asymmetric cell divisions, cell shape plays an important role for the control of lineage divergence during mouse preimplantation development. Cell shape is able to influence both the type of division (symmetric or asymmetric) and the position of the blastomeres within the embryo.

Role of gap junctions in embryonic and somatic stem cells

Stem cells provide an invaluable tool to develop cell replacement therapies for a range of serious disorders caused by cell damage or degeneration. Much research in the field is focused on the identification of signals that either maintain stem cell pluripotency or direct their differentiation. Understanding how stem cells communicate within their microenvironment is essential to achieve their therapeutic potentials. Gap junctional intercellular communication (GJIC) has been described in embryonic stem cells (ES cells) and various somatic stem cells. GJIC has been implicated in regulating different biological events in many stem cells, including cell proliferation, differentiation and apoptosis. This review summarizes the current understanding of gap junctions in both embryonic and somatic stem cells, as well as their potential role in growth control and cellular differentiation.

The use of antibodies to gap junction protein to explore the role of gap junctional communication during development

Antibodies raised against the major 27 kDa protein electrophoretically eluted from isolated gap junctions and affinity purified against the antigen have been used to explore the role of communication through gap junctions in the early amphibian and mouse embryos. In both species, injection of the antibodies into one cell completely blocks both dye transfer and electrical coupling between cells connected by gap junctions. In the amphibian embryo the generation of a communication-incompetent clone of cells leads to patterning defects in the region derived from the antibody-injected cell. In the mouse embryo, blocking cell-to-cell communication leads to decompaction of the communication-incompetent cells. The possible significance of these findings in relation to development in general and to the organization of the first transporting epithelia to appear during development is discussed.

Connexin phosphorylation as a regulatory event linked to gap junction internalization and degradation

Gap junction proteins, connexins, are dynamic polytopic membrane proteins that exhibit unprecedented short half-lives of only a few hours. Consequently, it is well accepted that in addition to channel gating, gap junctional intercellular communication is regulated by connexin biosynthesis, transport and assembly as well as the formation and removal of gap junctions from the cell surface. At least nine members of the 20-member connexin family are known to be phosphorylated en route or during their assembly into gap junctions. For some connexins, notably Cx43, evidence exists that phosphorylation may trigger its internalization and degradation. In recent years it has become apparent that the mechanisms underlying the regulation of connexin turnover are quite complex with the identification of many connexin binding molecules, a multiplicity of protein kinases that phosphorylate connexins and the involvement of both lysosomal and proteasomal pathways in degrading connexins. This paper will review the evidence that connexin phosphorylation regulates, stimulates or triggers gap junction disassembly, internalization and degradation.

Lineage allocation and cell polarity during mouse embryogenesis

The first developmental lineage allocation during the generation of the mouse blastocyst is to outer trophoblast or to inner pluriblast (inner cell mass; ICM) cells. This allocation seems to be initiated at the 8-cell stage, when blastomeres polarise. Polarisation is followed by differentiative divisions at the subsequent two cleavage divisions to generate polar outer and non-polar inner 16- and 32-cells. The key events in polarisation are regulated post-translationally through a cell contact-mediated pathway, which imposes a heritable determinant-like organisation on the blastomere cortex. Two proteins in particular, E-cadherin and ezrin, are intimately involved in the generation and stabilisation of developmentally significant information. Transcriptional differences between lineages appear to follow and may coincide with the lineage commitment of cells.

Gap Junction Formation and Connexin Distribution in Pig Trophoblast before Implantation

This study describes the gap junctions in extraembryonic cell layers of the preimplantation pig embryo (trophectoderm and endoderm constituting the trophoblast). Using specific antibodies against connexins 31, 32 and 43, we found these connexins in embryos by immunodetection using Western blot and immunofluorescence analysis. By immunofluorescence, the first foci of connexin 31 were detected in the four-cell stage blastomeres, and the first diffuse gap junctions appeared at the eight-cell stage. Intercellular communication was observed with Lucifer yellow transfer to start also at the eight-cell stage around the onset of compaction. Typical gap junctions developed in the trophectoderm of blastocysts, as observed by transmission electron microscopy of thin sections and freeze-fracture replicas. Connexin proteins were differently expressed in time and space: connexin 31 was continuously present in trophectoderm, connexin 32 was essentially found in endoderm during elongation; connexin 43 was distributed in both trophectoderm and endoderm during blastulation and expansion. Connexin 43 was also found in two isoforms, phosphorylated or not, at day 14. Such developmentally regulated connexin expression may be essentially useful to control the exponential growth of trophoblast in preimplantation pig blastocysts.

Surface-expressed E-cadherin, and mitochondrial and microtubule distribution in rescue of mouse embryos from 2-cell block by aggregation

E-cadherin (uvomorulin)-mediated cell interactions are essential for preimplantation development in mammals. We observed that E-cadherin is expressed at contact sites between blastomeres of 2-cell mouse embryos of non-blocking genotype (CBA x C57BL F1) explanted at 32 h post human chorionic gonadotrophin (HCG) and cultured in vitro, while blastomere rounding and reduced zones of contact and E-cadherin-staining were observed in embryos of a blocking strain (MF1) arrested at the 2-cell stage. Embryos of MF1 strain can be rescued by aggregation with four 2-cell embryos of the non-blocking genotype. An early event in rescue is E-cadherin expression at contact zones between adjacent embryos of different genotype in aggregation chimeras. E-cadherin-mediated signalling appears important for the rescue (including formation of adherens-like contacts, cell polarization and morphogenetic processes) since there is no rescue when E-cadherin-specific antibodies are present during phytohaemagglutinin-mediated aggregation and subsequent culture. In blocked embryos, the distribution of microtubules is disturbed and concomitantly mitochondria cluster around the nucleus. Rescue by aggregation retains normal mitochondrial distribution in the presence of a dense microtubular lattice in all blastomeres. Therefore, E-cadherin-mediated signalling and its downstream effects on cytoskeletal organization are essential in the rescue of blocking embryos by aggregation. Normal preimplantation development appears to be dependent on the polarized expression of surface E-cadherin and the microtubule-mediated dispersal of mitochondria.

Intercellular communication in the immune system: differential expression of connexin40 and 43, and perturbation of gap junction channel functions in peripheral blood and tonsil human lymphocyte subpopulations

The distribution and function of connexins (integral membrane proteins assembled into gap junction intercellular communication channels) were studied in human lymphocyte subpopulations. The expression of mRNA encoding connexins in peripheral blood and tonsil-derived T, B and natural killer (NK) lymphocytes was examined. Connexin43 (Cx43) mRNA was expressed in peripheral blood and tonsil lymphocytes, but Cx40 mRNA expression was confined to tonsil-derived T and B lymphocytes; Cx26, Cx32, Cx37 and Cx45 were not detected by reverse transcription-polymerase chain reaction (RT-PCR). Western blot analysis also demonstrated the presence of Cx40 and Cx43 proteins in T and B lymphocytes in a manner coincidental to the mRNA detection. Stimulation in vitro of T and B lymphocytes with phytohaemagglutinin (PHA) and lipopolysaccharide (LPS), respectively, increased Cx40 and Cx43 protein expression. Flow cytometric analysis, using antibodies to extracellular loop amino acid sequences of connexins, confirmed the surface expression of connexins in all lymphocyte subpopulations. Assembly of connexins into gap junctions providing direct intercellular channels linking attached lymphocytes was demonstrated by using a dye transfer technique. The exchange of dye between lymphocytes was inhibited by a connexin extracellular loop mimetic peptide and alpha-glycyrrhetinic acid, two reagents that restrict intercellular communication across gap junctions. Dye coupling occurred between homologous and heterologous co-cultures of T and B lymphocytes, and was not influenced by their stimulation with PHA and LPS. The connexin mimetic peptide caused a significant decrease in the in vitro synthesis of immunoglobulin M (IgM) by T- and B-lymphocyte co-cultured populations in the presence or absence of stimulation by PHA. The results identify connexins as important cell surface components that modulate immune processes.

A role for Rho-like GTPases in the polarisation of mouse eight-cell blastomeres

Polarisation of cells during mouse preimplantation development first occurs within blastomeres at the eight-cell stage, as part of a process called compaction. Cell-cell contact mediated by the cell adhesion molecule uvomorulin (E-cadherin) and the activity of the microfilament cytoskeleton are important in the development of compaction, which is crucial for establishment of trophoblast and pluriblast (inner cell mass) lineages and for subsequent development. Members of the Rho family of p21 GTPases have been shown to regulate the organisation of the actin cytoskeleton and adhesion in other cell types. The potential role of these proteins in compaction was investigated. Inhibition of Rho with Clostridium botulinum C3-transferase disturbed intercellular flattening at compaction and prevented cytocortical microfilament polarisation of eight-cell blastomeres, in contrast to cytochalasin D which inhibited only adhesion. Microinjection of a constitutively activated recombinant Rho protein into four-cell blastomeres induced cortical microfilament disruption and apical displacement of nuclei associated with polarised clustering of microtubules. Interblastomere adhesion was reduced and E-cadherin was aberrently clustered at remaining cell-cell contacts. Similarly, activated Cdc42 protein induced nuclear displacement with additional cytoplasmic actin bundle formation between nucleus and cell-cell contacts. The effects produced by both of the activated GTPase proteins are indicative of prematurely induced but aberrently organised polarity. These results suggest that Rho family GTPases are involved in the polarisation of early mouse blastomeres.

Complete dissipation of coherent clonal growth occurs before gastrulation in mouse epiblast

Observations on chimaeric mice argue that there must be considerable dispersal and intermingling of the clonal descendants of epiblast founder cells at an early stage in the development of the tissue. However, it has not been established when or how this occurs. Here we have used a genetic marker that enables donor cells to be visualized in situ to examine the early postimplantation distribution of clones obtained by transplanting epiblast founder cells into host blastocysts. We have also determined the spatial relationship between sister cells in non-chimaeric postimplantation epiblast by ionophoretic injection of a fluorescent macromolecule. Both experimental approaches support the conclusion that breakdown of coherent clonal growth accompanies epithelialization of the epiblast and is essentially complete by the onset of gastrulation. Furthermore, the clonal analysis shows that descendants of different epiblast founder cells continue to intermingle extensively well into organogenesis. We suggest that this sustained intermingling of cells in the epithelial epiblast, which does not occur in the adjacent visceral endoderm, depends on cells losing contact with the basal lamina when they divide. These findings have implications both for patterning of the early amniote embryo and for the growth of tall columnar epithelia in general.

A mitosis-specific phosphorylation of the gap junction protein connexin43 in human vascular cells: biochemical characterization and localization

Western blotting studies revealed that connexin43 (Cx43), one of the major gap junction proteins in human vascular endothelial cells, is posttranslationally modified during mitosis. This mitosis-specific modification results in a Cx43 species that migrates as a single protein band and was designated Cx43(m). Cx43(m) was shown to be the result of additional Ser/Thr phosphorylation as indicated by: (a) the increased gel mobility induced by both alkaline phosphatase and the Ser/ Thr-specific protein phosphatase-2A (PP2A) and (b) the removal of virtually all (32)P(i) from Cx43(m) by PP2A. Immunofluorescent confocal microscopy of mitotic cells revealed that Cx43 is intracellularly located, while in nonmitotic cells Cx43 is located at regions of cell-cell contact. Dye coupling studies revealed that mitotic endothelial cells were uncoupled from each other and from nonmitotic cells. After cytokinesis, sister cells resumed cell coupling independent of de novo protein synthesis. The mitosis-specific phosphorylation of Cx43 correlates with the transient loss of gap junction intercellular communication and redistribution of Cx43, suggesting that a protein kinase that regulates gap junctions is active in M-phase.

The early blastocyst is bilaterally symmetrical and its axis of symmetry is aligned with the animal-vegetal axis of the zygote in the mouse

At least one polar body, almost invariably the second, persists intact to the early blastocyst stage in nearly two-thirds of mouse conceptuses of the PO strain. The distribution in early blastocysts of these surviving polar bodies was highly non-random. Most not only lay in the mid-region of the embryonic-abembryonic axis but, on discovering that early blastocysts are bilaterally rather than radially symmetrical about this axis, were found to align with the bilateral axis. Cell marking experiments failed to detect movement of polar bodies relative to the surface of the conceptus during either cleavage or blastulation. That the distribution of degenerating polar bodies and their presumed debris was similar to intact ones also argued against their motility, as did the finding that at all stages second polar bodies were attached to conceptuses by a thin, extensible, weakly elastic ‘tether’. Although the transfer of small fluorochromes between them was rarely observed beyond second cleavage, the second polar body and conceptus could remain coupled ionically up to the blastocyst stage. It is concluded that the second polar body normally remains attached to the conceptus through persistence of the intercellular bridge formed during its abstriction, and therefore provides an enduring marker of the animal pole of the zygote. Hence, according to the distribution of polar bodies, the axis of bilateral symmetry of the early blastocysts is normally aligned with the animal-vegetal axis of the zygote and its embryonic-abembryonic axis is orthogonal to it. Such relationships suggest that, at least in undisturbed development, specification of the axes of the blastocyst depends on spatial patterning of the zygote.

Functional analysis of amino acid sequences in connexin43 involved in intercellular communication through gap junctions

Gap junctions allow direct communication between cells without recourse to the extracellular space and have been widely implicated as important mediators of cell-cell signalling. They are constructed from the connexin proteins, which form a large family, and individual connexins show complex spatial and temporal variations in their expression patterns. Understanding how this variation contributes to the control of intercellular signalling, both in the adult and during embryonic development, is an important problem that would be aided by reagents that interfere with gap junctional communication through specific connexins. We have begun to address this issue by raising antibodies to peptides derived from connexin43 and connexin32. Connexin43 peptides were located in the amino terminus, cytoplasmic loop and carboxytail. Connexin32 peptides came from the cytoplasmic loop and the first extracellular loop. Immunoblotting and immunostaining properties of purified IgGs were characterized on mouse heart, liver and the 8- to 16-cell mouse embryo. Effects on transfer through gap junctions were assessed in the fully compacted 8-cell mouse embryo by co-injection with Lucifer Yellow or Cascade Blue. Embryos were maintained in culture to assess the developmental consequences of injection. Peptide competition was used to confirm the specificity of immunostaining and inhibition of dye transfer. All connexin specific antibodies recognized their parent connexin on immunoblots and showed no 43/32 cross-reactivity. The connexin32 extracellular loop antibody recognized both connexin 32 and 43 on immunoblots, as predicted by the amino acid sequence homology in this region, but did not immunostain intact gap junctions. Connexin specific antibodies that immuno-stained showed the predicted connexin specificity. Antibodies to either connexin43 amino acids (AA) 1–16 (amino terminus) or AA 101–112 (cytoplasmic loop) neither immunostained nor prevented functional communication through 8-cell embryo gap junctions. Antibodies to AA 123–136 and AA 131–142 in the cytoplasmic loop immunostained heart and 8-cell embryo gap junctions and blocked transfer through them with high efficiency. Fab' fragments were equally effective. Peptide competition showed that both antibodies contained epitopes within AA 131–136 of connexin43. Antibodies against AA 313–324 in the carboxytail immunostained heart and the 8-cell embryo and, as IgGs, prevented dye transfer. Fab' fragments were ineffective. All connexin43 antibodies that blocked gap junctional communication between cells of the 8-cell mouse embryo induced non-communicating cells subsequently to withdraw from compaction.(ABSTRACT TRUNCATED AT 400 WORDS)

Gap junction communication and cell adhesion in development

During the past decade growing evidence has suggested that cell-cell communication via gap junctions is crucial for early developmental processes (Warneret al., 1984; Guthrie & Gilula, 1989; Serraset al., 1989). It has been shown that embryos of mice (Kalimi & Lo, 1988), teleosts (Kimmelet al., 1984), insects (Warner & Lawrence, 1982; Ruangvoravat & Lo, 1992) and molluscs (Serraset al., 1989) become regionally organized into restricted domains of junctionally connected cells that share developmental potential. In the mouse gastrula, for example, dye-coupling experiments have demonstrated that cells within a developmental compartment have a high degree of coupling whereas cells across compartmental boundaries have reduced coupling (Kalimi & Lo, 1988). Classic experiments (Townes & Holtfretter, 1955; Steinberg, 1963) demonstrated that as cells begin to differentiate along common pathways, they develop selective adhesion properties and the ability to sort themselves from unlike neighbours (for review see Edelman 1988; Edelmanet al., 1990). More recently a multitude of specific cell adhesion molecules (CAMs) have been identified that mediate these processes and trigger the cytoplasmic events that drive further differentiation (Edelmanet al., 1990; Albelda, 1991; Geiger & Ayalon, 1992).

Cell adhesion and gap junction formation in the early mouse embryo are induced prematurely by 6-DMAP in the absence of E-cadherin phosphorylation

Compaction of the mouse embryo, which takes place at the 8-cell stage, is dependent upon the adhesion molecule E-cadherin (uvomurulin), but does not require protein synthesis, suggesting that post-translational modification(s) is (are) implicated in the setting up of this phenomenon. The demonstration recently that E-cadherin is phosphorylated at the 8-cell stage just before compaction supports this theory. In this work we used 6-dimethylaminopurine, a serine-threonine kinase inhibitor, to investigate the role of protein phosphorylation in compaction of mouse embryos. 6-dimethylaminopurine is able to induce cell flattening and gap junction formation prematurely at the 4-cell stage; however, it does not induce cell surface polarization, as occurs during normal compaction. 6-dimethylaminopurine-induced premature flattening is inhibited when the embryos are cultured in the presence of an anti-E-cadherin antibody or without extra-cellular Ca2+, demonstrating that this process requires functional E-cadherin; whereas cell flattening and gap junction formation take place in the absence of E-cadherin phosphorylation, suggesting that its phosphorylation is not required normally for these events. The relationship between E-cadherin-mediated cell flattening and gap junction formation during compaction is discussed.

Cell surface localisation and stability of uvomorulin during early mouse development

We have examined immunocytochemically the subcellular distribution of the cell adhesion molecule uvomorulin in cleavage stage mouse embryos using conventional and confocal microscopy, under a range of detergent extraction and fixation regimes. Only traces of uvomorulin were detectable on the surface of unfertilised oocytes, whereas between 6 and 11 h after activation detergent-resistant surface expression was evident. This shift correlates with previously demonstrated changes in the pattern of synthesis and accumulation of uvomorulin from precursor state in unfertilised oocytes to mature protein after fertilisation. Embryos at subsequent stages up to the 8-cell stage exhibited a uniform distribution of uvomorulin on free surfaces and its concentration in regions of contact between blastomeres. At the 8-cell stage, during compaction, there was increased intercellular adhesion with concomitant accumulation of uvomorulin at intercellular contacts, whilst free surface uvomorulin was reduced and became relatively more susceptible to detergent extraction. When compact 8-cell embryos were decompacted in calcium-free medium, uvomorulin at contacts decreased while free surface and cytoplasmic staining increased. Blastomeres disaggregated from 4- and 8-cell embryos showed traces or 'footprints' of anti-uvomorulin staining in regions previously in apposition. These footprints disappeared over 45-60 min, during which time uvomorulin distribution became uniform. Possible mechanisms underlying the rearrangements which take place both at fertilisation and during compaction and experimental decompaction are discussed.

Asymmetric patterns of gap junctional communication in developing chicken skin

To study the pattern of gap junctional communication in chicken skin and feather development, we injected Lucifer Yellow into single cells and monitored the transfer of the fluorescent dye through gap junctions. Dye coupling is present between cells of the epithelium as well as between cells of the mesoderm. However, dye transfer did not occur equally in all directions and showed several consistent patterns and asymmetries, including: (1) no dye coupling between mesoderm and epithelium, (2) partial restriction of dye coupling at the feather bud/interbud boundary during early feather bud development, (3) preferential distribution of Lucifer Yellow along the anteroposterior axis of the feather placode and (4) absence of dye coupling in some epithelial cells. These results suggest the presence of preferential pathways of communication that may play a role in the patterning of chicken skin.

Cytoskeletal organization of rat oocytes during metaphase II arrest and following abortive activation: a study by confocal laser scanning microscopy

In metaphase II arrested rat oocytes (M II), microtubules were found in the taper-shaped meiotic spindle and in the cytoplasm as asters and free microtubules. Whereas spindle microtubules were acetylated, those located in the cytoplasm were not. Cytoplasmic microtubules were also labile as assessed by mild cooling. In contrast to mouse oocytes, rat microtubule organizing centers (MTOCs) did not react with MPM-2 antibody by immunofluorescence despite the fact that this antibody reacts with several proteins as shown by immunoblot. However, cytoplasmic MTOCs in M II-arrested rat oocytes could be detected by their nucleating capacity in the presence of taxol, a drug that induced the formation of numerous cytoplasmic asters. In addition, taxol caused a change in the spindle shape and the formation of astral microtubules at the spindle poles. Meiotic spindles (as well as chromosomes devoid of microtubules after nocodazole-treatment) were overlaid by an actin-rich domain. Spontaneous abortive activation led to the extrusion of the second polar body followed by another metaphase arrest--metaphase III; however, normal spindles did not form and dispersed chromosomes surrounded by microtubules were observed. Electron microscopic studies confirmed these observations and revealed that the kinetochores, are located deep within the chromosomes in contrast to mouse kinetochores, and this might be responsible for the absence of a metaphase III spindle in the rat oocyte. Induced activation caused transition to interphase with the formation of a characteristic microtubule network. This study shows that there are several significant differences in the cytoskeletal organization of rat and mouse oocytes.

Role of ultrastructural studies in the analysis of cell lineage in the mammalian pre-implantation embryo

Ultrastructural studies have contributed significantly to our understanding of cell lineage differentiation in the mammalian pre-implantation embryo. Such studies have documented, and continue to document, morphological, biochemical, and physiological characteristics of the cell lineages established during the pre-implantation period in eutherian embryos, principally that of the mouse. This review evaluates these contributions and identifies areas of study in which ultrastructural analysis is most likely to have an important role in the future.

Cell-cell communication in the embryo of Caenorhabditis elegans

We have investigated the pattern of cell-cell communication in embryos of the free-living soil nematode Caenorhabditis elegans. For this, we have established a method for microinjection of tracer dyes into individual blastomeres. After iontophoresis of fluorescent dyes of different molecular weights (Lucifer yellow, LY, M(r) 457; rhodamine-labeled dextran, RD, M(r) 4000), we can visualize intercellular communication pathways. The dye-spread of LY, indicating communication via gap junctions, becomes first visible in the late 2-cell stage. From the 4-cell stage onward all cells appear to be well coupled by communication channels, which allow the free diffusion of LY. In contrast, RD remains restricted to the injected cell and its descendants. After the primordial germcell P4 has been generated in the 24-cell stage, dye-spread of LY into this cell and its somatic sister D is delayed. However, the restricted dye-coupling of D is only temporary. After a brief period it joins the somatic compartment. With the beginning of the morphogenesis phase the two existing germline cells (the daughters of P4) are completely uncoupled from the soma, while the latter still forms a single dye-coupling compartment. Only during the second half of embryogenesis different separate somatic communication compartments are established. We followed the pattern of intercellular communication in the alimentary tract and found a progressive restriction into smaller dye-coupling units. Our data are compared to those found in other systems and discussed with respect to cellular determination and differentiation.

Reduced cell-cell communication between mitotic and nonmitotic coupled cells

The effects of mitosis on gap junctional intercellular communication (GJIC) were quantified in a clonal cell line of spontaneously immortalized rat granulosa cells (SIGC) using a fluorescence recovery after photobleaching assay. Reduction of GJIC was associated with the process of mitosis and was first apparent at the onset of prophase. Resumption of GJIC between newly divided cells and surrounding cells occurred slowly, requiring several hours following cytokinesis before reestablishment of maximal rates. Mitotic rates of GJIC in SIGC were comparable to values obtained in interphase cells partially uncoupled by 0.5 mM octanol. Limited studies of other cell lines generalized the mitotic-associated reduction of GJIC observed in SIGC. The data suggest that mitosis is one process which alters GJIC. This could be of significance when there is a change in the rate of proliferation, such as in the acquisition of immortalization, an early stage of transformation.

c-mos proto-oncogene product is partly degraded after release from meiotic arrest and persists during interphase in mouse zygotes

Recently, it has been shown that the product of the c-mos proto-oncogene is a component of cytostatic factor, an activity present in unfertilized eggs from vertebrates that arrests the cell cycle in metaphase of the second meiotic division (metaphase II) possibly by stabilizing maturation-promoting factor (MPF). We have studied the behavior of the c-mos product in metaphase II mouse oocytes and soon after activation. The amount of c-mos in the oocyte was still very high after second polar body extrusion, when cyclin B has been degraded and MPF activity had decreased dramatically. Degradation of c-mos takes place later, during the G1 phase of the first cell cycle and a residual amount of c-mos is detectable during the first zygotic interphase. Our data show that the degradation of c-mos is not involved in the release from the metaphase arrest.

Developmental regulation of gap junction gene expression during mouse embryonic development

The expression of products from three different gap junction genes (alpha 1, beta 1 and beta 2) was studied in pre- and postimplantation mouse embryos, during organogenesis, during differentiation of F9 teratocarcinoma cells, and in cultured embryonic stem (ES) cells. In this analysis, the following results were obtained. 1) Pre- and postimplantation mouse embryos. The alpha 1 transcript was the earliest gap junction RNA detected (in the 4 cell stage embryo) and its abundance increased significantly throughout subsequent development. 2) Organogenesis. Evidence was obtained for developmental expression of these three different gap junction genes during early embryogenesis and throughout the late stages of organogenesis. The expression patterns for these genes may be related to differences in gap junctional communication requirements for fetal organ development versus neonatal and adult organ function, or the utilization of different genes by different cell types during organogenesis. 3) During the differentiation of F9 cells in culture, expression of these three genes was modulated. Thus, this is the first evidence for modulation of gap junction gene expression during the differentiation of a single cell type in culture. 4) In an ES cell culture line, alpha 1 was the only gap junction gene product detected. This is consistent with the findings of alpha 1 expression in the embryonic inner cell mass region and in undifferentiated teratocarcinoma cells.

Inhibition of gap-junctional intercellular communication and enhanced binding of fibronectin-coated latex beads by stimulation of DNA synthesis in quiescent 3T3-L1 cells

To clarify the modulation of intercellular communication via gap junctions, associated with the growth induction of quiescent 3T3-L1 cells, we investigated the gap-junctional intercellular communication in growth-stimulated cells that were able to bind fibronectin-coated beads. When quiescent 3T3-L1 cells were incubated with fibronectin-coated beads for the first 2 h after the addition of calf serum, 24.0% of the cells bound and phagocytosed beads. Among the cells with bound beads, the percentage of the cells labeled concurrently with bromodeoxyuridine was 63.7% when examined 13 h after the addition of calf serum. Transient reduction of dye-coupling, measured with Lucifer Yellow CH, was observed only in the cells with bound beads 2 h after addition of calf serum, but it was not observed in the cells without bound beads. When the quiescent cells were incubated with fibronectin-coated beads for 2 h from 4-6 h after the addition of calf serum, the percentage of cells with bound beads increased to 53.1%, but the decrease in dye-coupling among the cells with bound beads was slight. These results suggest that the induction of cell growth causes a transient reduction in gap-junctional intercellular communication in 3T3-L1 cells with bound fibronectin-coated beads.

A close relationship between microvilli and metabolic cooperation deficiency in embryonal carcinoma cells

A revertant clone has been isolated from the metabolic cooperation-deficient embryonal carcinoma clone PT2md1. In PT2md1, its cooperation-competent parent, and its cooperation-competent revertant an inverse correlation has been demonstrated between cooperation competence and the incidence of microvilli. This demonstration, together with a similar correlation previously reported in an independently isolated series of cell lines, establishes a close causal relationship between the incidence of microvilli and communication deficiency.

Roles of cell junctions in gametogenesis and in early embryonic development

Recent reviews of the role of cell junctions in development have focused primarily upon functions related to the relatively subtle physiological modulation of their subunits in relation to fundamental developmental processes in a wide variety of organisms. There is, however, considerable support from numerous laboratories that the more radical modulation of the presence and number of junctional subunits in many diverse tissues may play a pivotal role in a wide spectrum of developmental phenomena ranging from gametogenesis to organogenesis. Since a great deal of recent interest in this latter subject has concentrated upon vertebrate systems including mammals, this review will examine the functional significance of the modulation of gap junctions, tight junctions and desmosomes in a developing idealized mammalian system from gamete formation to tissue and organ differentiation during embryo-genesis.

Gap junctional communication and compaction during preimplantation stages of mouse development

The ability of gap junction antibodies to block dye transfer and electrical coupling was examined in the compacted 8-cell mouse zygote. In control zygotes, Lucifer yellow injected into 1 cell transferred to the rest of the embryo. When antibodies raised against the major protein extracted from gap junctions were co-injected with Lucifer yellow, dye transfer failed in 86% of the zygotes tested and electrical coupling was almost completely inhibited. Subsequently, the antibody-containing cells were extruded. When the antibodies were injected into 1 cell at the 2-cell stage, 82% of the zygotes divided normally to the 8-cell stage. Cells containing gap junction antibodies were uncompacted, but continued to divide. We conclude that these antibodies inhibit gap junctional communication in the early mouse zygote and that communication through gap junctions may be involved in the maintenance of compaction.

Major loss of the 28-kD protein of gap junction in proliferating hepatocytes

There is a reduction in the 28-kD gap junction protein detectable by immunofluorescence in livers of partially hepatectomized rats and in cultured hepatocytes stimulated to proliferate. By the coordinate use of antibodies directed to the hepatic junction protein (HJP28) and the use of a monoclonal antibody that recognizes bromodeoxyuridine (BrdU) incorporated into DNA, we have been able to study the relationship between detectable gap junction protein and cell division. Hepatocytes that label with BrdU in the regenerating liver and in cell culture show a significant reduction of HJP28. Cells that do not synthesize DNA, on the other hand, show normal levels and distribution of immunoreactive gap junction protein. We postulate that the quantitative changes in gap junction expression might play an important role in the control of proliferation in the liver.

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.

Redistribution of microtubules and pericentriolar material during the development of polarity in mouse blastomeres

The distribution of microtubules and microtubule organizing centers (MTOCs) during the development of cell polarity in eight-cell mouse blastomeres was studied by immunofluorescence and immunoelectron microscopy using monoclonal anti-tubulin antibodies and an anti-pericentriolar material (PCM) serum. In early eight-cell blastomeres microtubules were found mainly around the nucleus and in the cell cortex, whereas PCM foci were observed dispersed in the cytoplasm. During the eight-cell stage, microtubules disappeared from the area adjacent to the zone of intercellular contact and accumulated in the apical part of the cell while their number decreased in the basal domain. The PCM also relocalized to the apical domain of the cell, but this occurred after the redistribution of the microtubules by a mechanism that involved the microtubule network. The possible roles of both MTOCs and microtubules in establishing cell polarity are discussed.

The effect of inhibiting DNA replication in the one-cell mouse embryo

Inhibition of DNA replication in the 1-cell mouse embryo causes arrest in interphase. In such arrested cells the pattern of protein synthesis progresses as normal through the first cell cycle but does not show the characteristic changes that are associated with mitosis. Synthesis does occur of the 68/70 kd heat shock-like proteins, on the earliest detectable embryonic transcripts synthesised shortly after first cleavage in control embryos. However, the quantitative change in the pattern of protein synthesis, indicative of the major activation of the embryonic genome that normally occurs during G2 of the second cell cycle, is prevented.